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Test technology for UPFC
CHAPTER
Test technology for UPFC
7
7.1 TEST TECHNIQUES FOR THE MAIN EQUIPMENT The UPFC (unified power flow controller), which mainly includes converter valves, a thyristor bypass switch (TBS), parallel transformers, and series transformers, must be tested before and during operation to ensure its safety and reliability. The quality of the equipment can be guaranteed if this practice is adopted in all processes, such as design, manufacture, transportation, installation, and operation. The tests can be divided into two categories: the insulation test and the characteristics test. The insulation test is mainly refers a test of the electrical properties of the insulation system. It includes the insulation characteristics test and the insulation withstand test. The insulation characteristics test is operated with a lower voltage pertaining and causes no damage to the insulation of the equipment; so this is called nondestructive testing. To pass the insulation withstand test, the equipment withstand capacities of power frequency, AC voltage, DC voltage, lightningimpulse voltage, as well as switching impulse voltage, should undergo a test which may have either of two results: tolerance or breakdown. Therefore it is called a destructive test. The insulation withstand test should be operated after the insulating characteristics test has been passed, in order to avoid an extra consumption. Most tests, except for insulation tests, are characteristics tests. Electrical or mechanical performance are mainly tested during these processes. The characteristics tests mainly consist of the transformer voltage ratio test, polarity test, no-load test, load test, and zero sequence impedance measurement test. The major test components of the UPFC are the converter valve, the TBS, parallel transformer, series transformer, MOA, DC isolation switch, grounding switch, start-up resistor, and bridge-arm reactor, which may be described as follows.
7.1.1 CONVERTER VALVE The converter valve transfers power between AC and DC systems. It has the ability to withstand normal operating voltage and current, as well as the impulse
Unified Power Flow Controller Technology and Application. DOI: http://dx.doi.org/10.1016/B978-0-12-813485-6.00007-8 Copyright © 2017 China Electric Power Press Ltd. Published by Elsevier Inc. All rights reserved.
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voltage and current result from the malfunction of the valve trigger system or system failure. The main test items are shown in Table 7.1. As it is mentioned above, the elementary wiring diagrams of the AC voltage withstand test, DC voltage withstand test, impact overvoltage withstand test, conducting and overcurrent breaking test and short-circuit current test are as follows.
Table 7.1 Converter Valve: Main Test Items No.
Test Types
Test Items
Test Purposes
1
Insulation test
AC voltage withstand test of valve support DC voltage withstand test of valve support Lightning impulse test of valve support Operation impulse test of valve support DC voltage withstand test of valve unit AC voltage withstand test of valve unit Lightning impulse test of valve unit Operation impulse test of valve unit Insulation test of valve unit
Test the voltage withstand characteristic of valve support
2 3 4 5 6 7 8 9
10
Characteristic test
Minimum DC voltage test
11
Maximum continuous running load test
12
The conducting and overcurrent breaking test Maximum transient overload ability test Short-circuit current test
13 14
15
Electromagnetic compatibility test
Test the voltage withstand characteristic between the valve units and to the ground
Test the voltage withstand characteristic of valve and the working states of the damping circuit and the control circuit Test the electronic equipment performance in getting power from DC capacitance Test the effect of a power device and related circuit on the current, voltage, and temperature Test the validity of the valve design Test the transient overload ability of the valve Test the performance of the power device and related circuit in circuit conditions Test the ability to resist electromagnetic interference in the model
7.1 Test Techniques for the Main Equipment
R
T1 ∼
C1
r
C2
G
Ty
Cx TV V A TA
FIGURE 7.1 Diagram of AC withstand voltage test. Ty—voltage regulator; T1—intermediate testing transformer; R—current-limiting resistance; C1 and C2—high and low arms of capacitive voltage divider; TV—voltmeter; TA—ammeter; R—sphere gap protection resistance; G—ball clearance; Cx—sample.
T1
Ty
∼ v
VD
TA
R
TV1
μA C
v TV2
Cx
FIGURE 7.2 Diagram of DC withstand voltage test. Ty—voltage regulator; T1—intermediate testing transformer; R—resistance protector; TV1, TV2—voltmeters; TA—ammeter; Cx—sample; C—filter capacitor.
7.1.1.1 AC voltage withstand test Fig. 7.1 shows the wiring diagram of the AC voltage withstand test. The voltage withstood by samples was boosted by the test transformer. The two main terminals were shorted in the test. The test AC voltage was applied between the shorted terminals and ground.
7.1.1.2 DC voltage test Fig. 7.2 shows the wiring diagram of the DC voltage withstand test. The voltage withstood by samples was obtained by a half-wave rectifier circuit. As in the AC voltage withstand test, the two main terminals were shorted and the test DC voltage was applied between the shorted terminals and ground.
7.1.1.3 Lightning and switching impulse tests Fig. 7.3 shows the wiring diagram of the lightning impulse overvoltage withstand test and the operation impulse overvoltage withstand test. The DC high voltage is generated by rectifier devices, and C1 gets charged by this DC high voltage.
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T1
Ty
VD
R0
∼
U0
G C1
R1
R2
Cx
u2
FIGURE 7.3 Diagram of lightning and switching impulse test. Ty—voltage regulator; T1—intermediate testing transformer; VD—high-voltage silicon rectifier; R0—current-limiting resistance; G—sphere clearance; R1—wave front resistance; R2—discharge resistance; Cx—sample.
R
LR
A
CT
B
C
Operating condition circuit
D
E
Heating circuit
Uniform controller for overcurrent shutdown test
FIGURE 7.4 Diagram of conducting and overcurrent breaking test. A—High-voltage source with low current; B—Resonance control link; C, D—Isolation control links; E—High-current source with low voltage.
When the voltage increases, the charge of capacitor C1 is C1U0 before the sphere gap breaks down, However, once the sphere gap has broken down, the capacitor C1 discharges to the test samples. The waveform of the lightning impulse overvoltage withstand test has three main parameters of characteristics: the peak, the wave front time, and the semipeak time. The wave front time of a standard waveform is 1.2 µs and the semipeak time is 50 µs. In accordance with the International Electrotechnical Commission IEC 60060.1-2010 “high-voltage test technology” and Chinese Standard GB/T 16927.1-2011 “high-voltage test technology,” the impulse voltage waveform parameters are 250 (1 6 20%)/2500 (1 6 60%) µs.
7.1.1.4 Conducting and overcurrent breaking test Fig. 7.4 shows the schematic wiring diagram of conducting and the overcurrent breaking test. The test circuit consists of two power supply sections. One is the
7.1 Test Techniques for the Main Equipment
stress resource, which has a low-current high-voltage power supply, largecapacity DC capacitance CT, and tunable inductors LR. The other is a heating source and it contains low-voltage high-current power supply sections and adjusting resistance R. The left portion of the sample valve is a condition circuit, consisting of the stress sources and an isolation control section. It is constructed to simulate a second zero-input response circuit, which is equivalent to similar real conditions of a straight arm. The right section of the sample valve is a heating circuit, which consists of a heating source and isolation areas. It can provide the required test temperature and heating currents that the sample valve needs during the overcurrent shutdown before the test. The left part of the sample valve is the condition circuit, which is composed of the stress sources and the isolation control section. It simulates a real bridge arm in the same conditions as in the second zero-input response circuit. The right section of the sample valve is a heating circuit, which is composed of a heating source and isolation areas. It can provide high current to be used for heating so as to satisfy the test temperature requirement before the sample valve is put through the overcurrent shutdown test.
7.1.1.5 Short-circuit current test This current is composed of an exponentially decaying current and a synthesized sinusoidal current as short circuit current, as shown in Fig. 7.5. The lefthand section is an attenuated current source circuit, which consists of a high-voltage lowcurrent power supply, capacitance C, a decaying oscillation circuit, and a control valve 1. The righthand part of the sample is a sinusoidal current source circuit, which consists of a low-voltage high-current power supply, a current limiting resistor R, and a control valve 2.
C
D
R
Submodule 1
A
C
B
Valves for test
Submodule 2 Submodule n
FIGURE 7.5 Diagram of short-circuit current test. A—High-voltage source with low current; B—Attenuation oscillation circuit; C, D—Control valve; E—High-current source with low voltage.
E
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Table 7.2 Main Test Items of TBS No.
Test Types
Test Items
Test Purposes
1
Insulation test
AC voltage withstand test of valve port Lightning impulse test of valve port Operation impulse test of valve port AC voltage withstand test between valve ports Lightning impulse test between valve ports Operation impulse test between valve port Minimum AC voltage test
Insulation level test of TBS from valve port to ground
2 3 4 5 6 7
8
Characteristic test
Cycle trigger and quench test
Insulation level test of TBS from between valve ports and ground
Test the correct running of the TCR valve trigger system under prescribed minimum AC voltage and operating conditions Verify the capacity of the thyristor valve under severe current strength and conducting and overcurrent breaking capacity
7.1.2 THYRISTOR BYPASS SWITCH TBS TBSs are closed to isolate valves from an AC line converter when line failure or inverter failure occurs (less than 5 ms). The TBS prevents interaction between the AC system and the broken-down valve zone. The main test items are shown in Table 7.2.
7.1.3 TRANSFORMER The transformers in a UPFC, can be divided into two types: parallel transformers and series transformers. Both have the same interconnect function, structure, and experimental contents. Their insulation of wire to ground is also the same, but the insulation tests are different because series transformers have the full insulation structure. The main test content is shown in Table 7.3. In the above experiment, the basic schematic winding diagrams of the induction withstand test, winding deformation test, and short-circuit withstand test are as follows.
7.1.3.1 Induced voltage withstand test The basic schematic winding diagram of the transformer-induced overvoltage withstand test setup is shown in Fig. 7.6. In the induction overvoltage withstand test, AC voltage is usually applied between the low-voltage transformer terminal, while the other windings are switched off to ensure their waveforms are sine waves as much as possible. In order to prevent the test excitation current from
7.1 Test Techniques for the Main Equipment
Table 7.3 Transformer Main Test Items No.
Test Types
Test Items
Test Purposes
1
Insulation test
Insulation resistance and absorption ratio measurement test Dielectric loss factor and capacitance measurement test
Check whether the transformer is affected with damp, dirt, or other penetrating agents overall or in part, or on its surface Check whether the transformer is affected with damp overall, or degraded by the effects of oil, sludge, or serious local defects, etc. Verify the winding terminal when it is vertical to ground, and check winding and winding insulation resistance strength
2
3 4 5 6
Characteristic test
Power frequency withstand voltage test Lightning and operation impulse test Induced voltage withstand test Winding resistance measurement test
7
Voltage ratio measurement and connection group designation test
8
Load test
9
No-load test
10
Temperature rise test
11
Short-circuit withstand test
12
Sound-level measurement test
13
Winding deformation test
14
On-load transformer tap switch test Three-phase transformer zero sequence impedance measurement test
15
Check the welding quality of the winding, whether the tap-changer is good contact, and whether there is any short circuit between the winding layers Test whether the winding voltage is qualified. Confirm that the numbers of windings of each tap parallel coils or line are the same, and judge that each connection of the lead and tap-changer is correct Test whether short-circuit impedance and load loss tests are qualified. Judge the winding deformation or short circuit between stocks Examine the partial or whole defect conditions of a magnetic circuit, and judge the short-circuit conditions of winding interterms Examine local overheating conditions of tank, structure, and the joints between leads and drive pipes, as well as leads and tap-changers Test withstand ability of transformer mechanical strength under the condition of heavy current Test transformer-rated run-time sound level and sound power level, to prevent noise pollution Test the winding deformation during transportation, installation, and operation Verify switch control performance, insulation level, and reliability Provide a theoretical basis for setting safeguard measures
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FIGURE 7.6 Diagram of transformer-induced voltage withstand test.
U1
L
L
L
K
K
K
Rs Us
C
C
K
C
C
R
U2
FIGURE 7.7 Frequency response analysis of basic detection circuit.
being too large during the test, the frequency of the power supply should be appropriately turned up past the rated frequency.
7.1.3.2 Detection test for transformer winding deformation The detection test of transformer winding deformation is achieved by testing winding amplitude-frequency response characteristics, and comparing the detection results vertically or horizontally. The wiring diagram is shown in Fig. 7.7. In the diagram, Us denotes the voltage source, Rs denotes source impedance, and R denotes measuring resistance; U1 and U2 are voltage signals of the winding terminals; L, K, and C are respectively the distributed inductance, the inter-turn distributed capacitance, and the ground distributed capacitance of the transformer winding.
7.1.3.3 Short-circuit current duration test Though the transformer short-circuit current duration is very short, it is a severe test. It gives the short-circuit dynamic stability and heat-resistant ability. The dynamic stability is verified through the short-circuit test. The test principle is shown in Fig. 7.8.
7.1 Test Techniques for the Main Equipment
NOB1
X
NOB 2
T1
T
TA U
u
V
v
W
w
A
3~
A A
FIGURE 7.8 Diagram of short-circuit withstand test. X—adjustable reactor; NOB1, NOB2—protection and closing switch; T1—intermediate transformer; TA—current transformer; T—test transformer.
Table 7.4 Main Test Items of Metal Zinc Oxide Lightning Arrester No.
Test Item
Purpose
1
Insulation resistance measuring test
2
Measurement test of DC voltage under 1 mA and leakage current under the above DC voltage
3
Leakage current measurement test under continuous operating voltage
4
Discharge counter action condition test
Preliminary test to show whether the interior of the arrester is affected by damp Test whether the ZnO valve becomes damp and determine if its movement characteristics and long-term working current are qualified Test the aging conditions of valve internal insulation and damp lightning arrester when it is affected by damp, severe damage and surface dirt To avoid discharge improperly resulting from counter rust due to bad sealing and damp
7.1.4 METAL OXIDE SURGE ARRESTERS The metal zinc oxide lightning arrester is the main item of equipment for limiting overvoltage in the UPFC. Its insulation test contents are shown in Table 7.4.
7.1.5 DC DISCONNECTOR AND GROUNDING SWITCH The main test contents of DC disconnector switch and earth switch are shown in Table 7.5.
7.1.6 STARTUP RESISTOR The startup resistor is set to the AC converter valve in series, and has great significance for the safe operation of the UPFC. The test includes mainly:
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Table 7.5 DC Isolation Switch and Earth Switch Main Test Items No.
Test Types
Test Items
Test Purposes
1
Insulation test
AC voltage withstand test
Check the insulation performance of the disconnector and grounding switch under DC voltage Check the insulation performance of the disconnector and grounding switch under lightning voltage Check the circuit contact of the DC disconnector or grounding switch
2
3
Impulse voltage withstand test Characteristic test
4
5
Primary loop resistance measurement test Porcelain ultrasonic flaw detection test Infrared temperature measurement test
Check the interior defects of the DC disconnector or grounding switch post insulator Check the defects of terminals, active contactors, and static contactors
7.1.6.1 Insulation test The power frequency voltage withstand test measures the insulation level between the terminals of the startup resistor. It is tested to ensure the safe operation of resistance at startup. In the test, the resistance network and outer cover should be disconnected. Meanwhile, the test voltage is applied between startup resistance terminals and its insulation bracket. The operating time is one minute. The test voltage shall be determined according to the following formulas: Utest 5 Utest 5
U Uk ðThe midpoint of the rheostat is connected to the housingÞ 2n
U Uk ðThe midpoint of the rheostat is not connected to the housingÞ n
(7.1) (7.2)
where U is the power frequency voltage of the entire resistor; n is the total number of series-connected resistors; k is the voltage nonuniformity factor between modules.
7.1.6.2 Characteristic test The resistance measurement test measures the starting resistance under power frequency, and during it the temperature effect should be considered for correction, in order to verify whether the startup resistance meets technical standards.
7.1.7 BRIDGE-ARM REACTOR The bridge-arm reactor is put into operation on the AC side of the converter. It has the functions of power control, wave filtering, and suppression of current fluctuation on the AC side when it works with the transformer.
7.2 Closed-Loop Test Techniques
FIGURE 7.9 Diagram of pulse reactor turn-to-turn insulation oscillating voltage method.
The bridge-arm reactor uses the entire insulation design. It is a kind of drytype air-core reactor, whose insulation test includes transformer-related items. In addition, the pulse oscillating voltage method is often used because of the possibility of reactor turn-to-turn insulation defects. The pulse oscillating voltage method makes use of reactor turn-to-turn insulation defects, and the reactor turnto-turn insulation condition is usually judged by the fact that inductance value changes according to frequency. The basic principle is shown in Fig. 7.9. In the diagram, C is the charging capacitor; G is the ball gap; L is a dry-type reactor; C1 and C2 are the high-voltage arm and the low-voltage arm in the capacitive voltage divider, respectively. In the test, if the reactor has an inter-turn short circuit, the following phenomena will appear: its inductance value will become small, its oscillation frequency will increase; the current between the short-circuit turns will increase, the energy loss will increase, and the oscillation will decay faster.
7.2 CLOSED-LOOP TEST TECHNIQUES FOR CONTROL AND PROTECTION SYSTEMS OF UNIFIED POWER FLOW CONTROLLER To carry out the feasibility demonstration of a UPFC project, comprehensively validate the physical control protection system performance and function, and analyze the impact that the UPFC has on relay protection, providing technical support for the operation and maintenance of the project, it is necessary to carry out closed-loop testing of control and protection devices based on a real-time digital simulation system (RTDS) and the actual engineering.
7.2.1 CLOSED-LOOP TEST SYSTEM 7.2.1.1 System overview When building a closed-loop test platform for the UPFC control and protection system, the purpose and nature of the test should be considered, since different application purposes correspond to different software and hardware systems.
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Power grid
Power grid
Simulation system MMC Analog output GTAO
MMC
FPGA(ML-605)
Digital input
TBS triggering pulse
Control command
Operating command for breakers and isolators
Analog I/O
States of breakers and isolators
DC current and voltage
AC current and power
Simulation system for valve control
for power system Digital input Digital output GTDI GTDO
Digital output
UPFC control and protection system FIGURE 7.10 Schematic diagram of overall structure of simulation system.
Generally, for a UPFC to carry out a control protection system closed-loop test in practice, it needs to be based on the actual needs of the project and construct a system model by using a RTDS. The UPFC control and protection system has the same configuration as the actual project site control protection device and constitutes a closed-loop system with the RTDS. A complete closed-loop test system of the UPFC control protection system should include grid and converter simulation equipment and a secondary control protection system. It should be noted that in the actual project the valve control equipment must be matched with the flow valve. The valve control equipment is generally provided by the supplier, and the communication between valve control and the converter valve triggering unit is based on a private agreement. Therefore, to carry out the UPFC control protection system closed-loop test, we need to develop the corresponding valve control simulation system to ensure the normal high-speed communication with the FPGA (ML-605) of the RTDS (to exchange the information of the submodule capacitor voltage and the trigger control word) (Fig. 7.10).
7.2.1.2 The composition of the closed-loop test system The UPFC closed-loop test system mainly includes: a control and protection system which is like that used in engineering practice, a real-time simulation system
7.2 Closed-Loop Test Techniques
RTDS workstation
Server
OWS/EWS
GPS clock
Station LAN
ACC
CCP1 A&B
CCP2 A&B
Protection 2
2 out of 3 VBC1 A&B
MMC1
2 out of 3
VBC2 A&B AMI(ASI) A&B
Switch value analog value
Protection 1 Protection 2
DMI(DSI) A&B
Field bus
Switch value analog value MMC2
Power system RTDS model
Power grid
Power grid
FIGURE 7.11 Block diagram of the closed-loop test system of the UPFC.
of using simulation of primary equipment and a real-time simulation system provided by the RTDS that can simulate the MMC (modular multilevel converter) converter valve equipment and power system. The UPFC closed-loop test system block diagram is shown below (Fig. 7.11). The whole simulation system includes the manmachine interface, the control and protection of the host computer and primary equipment simulated by the simulation system. The manmachine interface mainly includes the RTDS workstation, the server, the workstation, etc. The control and protection equipment includes the actual converter control and protection host computer and valve control and protection host computer. The primary system uses the RTDS simulation platform. The RTDs workstation was used to build and modify the primary system and to monitor the primary system. The MMC model arithmetic uses FPGA
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implementation. In order to simulate the actual engineering as realistically as possible, real-time simulation system of the AC system needs to be as close as possible to the real power grid. The simulation system is configured with a SCADA operator workstation, which can be used for data collection of the manmachine interface and station monitoring during UPFC operation and can achieve control protection program compilation, HiBug debugging, and so on. The communication between the operator workstation and the control and protection system is based on the TCP/IP protocol. Through the LAN network station, the system receives normal operation monitoring and operating instructions regarding the UPFC control station from operating personnel or the remote control center/centralized control center, and then monitors and processes faults and abnormal conditions, records event and alarms, synchronizes the secondary system, and adjusts the DC system parameters (active power instructions, reactive power instructions, etc.). The main device in the control protection system is the converter control protection host (CCP) and the valve control host (VBC). The CCP is connected by optical fiber with the VBC, and the VBC is connected to the real-time simulation device FPGA by optical fiber and an MMC converter valve; the CCP is connected to the AC system and the DC-line real simulation device through the optical fiber or cable. The control protection device and the simulation system interface of the simulation system are also included: the state of the switch and the control switch, the AC/DC analog signal, etc. In order to achieve a good simulation, the AC system of the real-time simulation system should be as close as possible to the real power system.
7.2.2 CLOSED-LOOP TEST OF CONTROL AND PROTECTION FUNCTION 7.2.2.1 Closed-loop test of the UPFC control system 7.2.2.1.1 Tap control test The UPFC system with tapping adjustment function needs to verify the transformer tap control strategy. In the automatic control and in the manual control mode, we need to make sure that the joint action is right the command event and status events are right, and that the operator interface displays the status correctly.
7.2.2.1.2 Sequence control and interlock test The sequence control and interlock function can effectively reduce the number of staff required to operate the equipment and avoid misoperation. The main purpose of the experiment is to verify that the control system is able to protect the knife switch equipment with interlocking function as well as to realize smooth startup and shutdown of the system, that the various operating modes can switch to normal and that all sequential event records display correctly.
7.2 Closed-Loop Test Techniques
7.2.2.1.3 STATCOM function test The parallel side uses the STATCOM (static synchronous compensator) operation start, setting for a constant voltage/no power control mode and set voltage/reactive power reference value. The STATCOM automatically outputs inductive reactive power. Special attention should be paid to ensuring the bus voltages are within the limit and the STATCOM is not overloaded. Reactive power/voltage control modes should be switched during operation of the STATCOM. The AC bus voltage, AC line current, DC voltage, submodule voltage, and bridge-arm current should be inspected and a record kept.
7.2.2.1.4 SSSC function test The series side uses SSSC (static synchronous series compensator) operating mode to start, and sets it as the active control method so as to control active power on the line according to requirements and control the DC voltage. For the operation of a multiloop SSSC, a multiloop series converter should be started to control active power on the line and the DC voltage. It is possible to switch between multiloop active coordination control and single-loop active control modes while ensuring that active power on the line changes smoothly to the intended target. During the inspection, AC bus voltage, AC line current, DC voltage, submodule voltage, bridge-arm current, and so on should also be recorded.
7.2.2.1.5 Circulation inhibition test Start up and exit the circulation inhibition function, and verify the suppression of the bridge-arm circulation and the number of submodules for each side.
7.2.2.1.6 UPFC startup and shutdown test The start-up and shutdown control of the UPFC system is one of the key problems. For every mode of operation, the UPFC start-up and shutdown need verification tests. During the tests, manual switching function of the verification system will cause little disturbance to the AC system.
7.2.2.1.7 Control mode conversion test For the control mode that can be switched on line, this can be done without disturbing voltage, current, active power, reactive power, and so on.
7.2.2.1.8 Steady state check and power range test During steady state operation, the system can control the electric quantity to order value, and the steady state deviation should meet the requirements of the technical specification. The control system’s ability to control active power and reactive power should be validated in various operating modes. According to the set rate, set smoothly the control of active power and reactive power values as instructed and validate
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the operating range of system active power and reactive power to meet the designed value.
7.2.2.1.9 Dynamic performance test To verify the dynamic performance of the control system. The step response of active power and reactive power should be fast; it does not impact the AC system.
7.2.2.1.10 Additional control test Additional control tests include overvoltage/undervoltage control, active power control (power lifting, power back down), AC and DC coordinated control device function tests, etc. By doing additional control experiments, the optimization performance of additional control to the system can be checked.
7.2.2.1.11 Operation mode conversion test In order to improve the reliability of the UPFC system, at present the project uses multiple operating modes of UPFC topology, such as STATCOMs, UPFCs, SSSCs, and many kinds of operational mode, with a converter mutual standby function. By doing an operating mode conversion test, we can verify that function.
7.2.2.1.12 Transient test This is to verify the fault ride-through capability of the UPFC system in the case of a transient fault of the AC system. Single-phase, two-phase, and three-phase grounding fault tests are carried out on the parallel side and the AC circuit of the network, and the UPFC can cross the fault or avoid a line fault through the fault restart function.
7.2.2.2 UPFC protection function closed-loop test The UPFC protection test includes a UPFC protection function test and a fault test. The protection function test verifies the control system in normal operation (e.g., starting and stopping, running mode switching, lifting power), and checks that the protection of the whole protection system is working well. The protection function test involves the protection of the whole protection system, including AC field protection, converter area protection, DC field protection, etc. By setting faults in each region in the RTDS, control and protective action can be examined. The fault point is set as shown in Fig. 7.12 and Table 7.6, and it is required that the fault can be quickly cleared immediately after it occurs in the UPFC system. By simulating a fault in the line and a fault in the outside system, coordination between UPFC protection and protection in the system can be evaluated and it is important that no maloperation occurs during the experiment.
7.3 Onsite Debugging Test Techniques in the UPFC Project
Power grid
Power grid f10 f9
f7
f8 f1
f11
f6
f2
f3
f5 f4
FIGURE 7.12 Failure point diagram of protection function test for a UPFC control and protection system.
Table 7.6 Fault Point Setting of Closed-Loop Test for UPFC Protection Function Fault Name
Fault Location
Converter valve area fault DC side fault Series transformer fault Parallel transformer fault AC bus fault Line fault Out of system fault
f1, f5 f2, f3, f4 f7 f9 f6, f8 f10 f11
7.3 ONSITE DEBUGGING TEST TECHNIQUES IN THE UPFC PROJECT 7.3.1 THE PURPOSE AND CONDITIONS OF THE SYSTEM DEBUGGING TEST The purpose of system debugging is to check the performance of all equipment, subsystems and the overall system in the UPFC project. Through system debugging, coordinating, and optimizing the cooperation between system and equipment, we can improve the overall performance of the UPFC system and the performance of the whole power system after the operation of the UPFC. Data and parameters that are essential to ensure the safe and stable operation of the HVDC (high voltage direct current) system should be tested and collected and production management and operation personnel can also be trained.
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To carry out on-site debugging tests for a UPFC, the following basic conditions are required: 1. Subsystem debugging has ended. 2. The AC access part of the circuit and the substation bay on the other side have been charged, and have the ability to transmit power. 3. All AC/DC protection has been set correctly and can be put into live debugging of the test system. 4. Emergency handling measures and plans have been drawn up during the debugging of the system. 5. After checking the sequence of events, there is no related alarm signal and the system is working properly. 6. The AC system protection and other specific protection configurations are in accordance with the scheduled requirements. 7. The corresponding UPFC system transformer protection and UPFC converter protection have been put into operation.
7.3.2 DEBUGGING TEST PROJECT AND TECHNOLOGY OF UPFC The purpose of the UPFC debugging test is to comprehensively assess the function of all the equipment in the project that checks the performance of the UPFC system after the series side and the parallel side are combined and to check the interaction between AC and DC systems, ensuring safe and reliable operation. The test scope includes: control and protection, flow control valve, valve control, water cooling system, ACDC field, etc. Starting from this stage, the primary device will be energized, the overall performance of the system will be assessed. System test items include:
7.3.2.1 Noncharged sequential control The test is about checking the order of the UPFC system and related operation when the primary devices are not energized. This test is to test the operating sequence, check whether electric interlocking can be performed correctly and verify that the DC equipment remains safe when a failure occurs during the execution of a sequence of operations. Noncharged sequential operations include: 1. Station operation, in the operator station for sequential operation; 2. Remote operation, in the dispatching institution for sequential operation. Noncharged control mode: in the manual/automatic control mode to test operation and implementation: a. In manual control mode, when implementing single-step operation, the correct operation should be performed and the wrong operation should be rejected; b. In automatic control mode, sequential operation should be executed automatically following the integrated order. If it is normal, the operation
7.3 Onsite Debugging Test Techniques in the UPFC Project
should be completed in accordance with the provisions of the order. If the sequence is not completed, the appropriate alarm information should be given, and the corresponding equipment should be able to return to the previous defined state, or to enter the next defined state.
7.3.2.2 Noncharged protection tripping The purpose of the tripping test (the final tripping test) is to check the tripping function for the protection system of the UPFC or a certain element. The main equipment includes the control and protection system for the UPFC AC station cabinet, control cabinet, cabinet interface, etc. The tripping test is carried out before the UPFC station is charged or after the control protection circuit (including software) is modified. All test items in the final tripping test should be carried out at least once. Select one protection item or more to start the tripping to ensure that the circuit action is totally correct, including the protection tripping matrix circuit. The test utilizes the debugging function of the logic software for the control protection system to simulate protection action by setting numbers with the software and verifies the correctness of the tripping circuit.
7.3.2.3 Charging test of shunt transformer and converter When the parallel transformer is charged, the peak value of the inrush current and the operating overvoltage should be within the limit, and the resonance should be fully damped. Check that the transformer ratio is correct and the related parallel transformer protection does not go into action. Ensure that the position of the tap and the start process of the transformer fan meet the designed requirements. In the converter charging test, the charging resistor should be connected first, then the low voltage side of the transformer circuit breaker is connected and the converter is charged. Next, verify whether the voltage phase, amplitude and transformer ratio on the transformer valve side are correct, and that the relevant protection is not active. Pay attention to the influence of the zero-sequence current on transformer and reactor. Check valve-side current, VBC return voltage, arm current, state signals and the DC-side voltage polarity ratio.
7.3.2.4 Charging trigger for converter valve After the transformer circuit breaker on the network side is connected, the converter valve under blocking state is charged by starting the resistor. attention must be paid to the influence of the zero-sequence current on the transformer and the reactor. When the charging current on the AC side is zero, the AC circuit breaker should be disconnected and the converter valve should be unlocked. Then the AC voltage phase sequence and phase angle difference at both ends of circuit breaker are measured. The phase sequence triggered by the valve group and the correctness of the phase lock link of the control and protection system are checked. When the delay of the control system is measured, the phase difference caused by
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the delay in the data processing of the control and protection system should be compensated.
7.3.2.5 Charging and starting of series transformer A charging test is performed for the series transformer. In the test, the series transformer will be charged from the high and low voltage sides separately so as to have a comprehensive assessment of the insulation of the series transformer to the ground, insulation of high and low voltage windings, and the TBS signal. The voltage phase related to the series transformer protection system is then checked. The load test for the protection of the series transformer and related TA loop of the UPFC protection system is performed by examining the closed-loop line load current after closing the TBS on the low voltage side of the series transformer.
7.3.2.6 STATCOM operation test The STATCOM operational test is carried out for the parallel converter.
7.3.2.6.1 Steady-state performance test Reactive-power control function test. Start the converter in STATCOM operational mode. Set the reactive power control mode and the reactive power reference value (including capacitive reactive and inductive reactive power) incrementally from the minimum to the rated value. The reactive-power/voltage control mode switching test is carried out for the STATCOM operation converter, which makes the converter control mode switch from reactive power control mode to voltage control mode. Voltage control function test. Set a few typical voltage reference values lower than the operating voltage in the system. The STATCOM automatically outputs inductive reactive power. Special attention should be paid to ensuring the bus voltages are within the limit and the STATCOM is not overloaded. Set the voltage reference value higher than a few typical values of operating voltage in the system, and repeat.
7.3.2.6.2 Dynamic performance test A dynamic performance test tests the system’s ability to quickly adjust the command value when facing a larger disruption. The faster the speed of adjustment and the smaller the corresponding oscillation, the better the system’s dynamic performance will be. Test items include DC voltage, parallel reactive power, AC voltage, etc. It should be ensured that the bus voltage is within the limit and the STATCOM is not overloaded.
7.3.2.6.3 Circulation inhibition test Enter and exit the circulation inhibition function to verify the suppression of the bridge-arm circulation and the number of submodules.
7.3 Onsite Debugging Test Techniques in the UPFC Project
7.3.2.7 Operation test of SSSC The SSSC operation test is carried out for the series converter.
7.3.2.7.1 Steady-state performance test Starting with the SSSC in operating mode, choose the power control mode first, then set the reference values in turn, finally check and record the AC bus voltage, A- line current, DC voltage, submodule voltage, bridge-arm current, etc. For SSSC operational mode with parallel double circuit lines, first start the double loop converter and choose double circuit active power control mode, then set the reference values of active power in turn, finally switch between doublecircuit and single-circuit power control mode.
7.3.2.7.2 Dynamic performance test A dynamic performance test tests the system’s ability to quickly adjust the command value when facing a larger disruption. The faster the speed of adjustment and the smaller the corresponding oscillation, the better the system’s dynamic performance will be. Test items include DC voltage, parallel reactive power, AC voltage, etc. It should be ensured that the bus voltage is within the limit and the STATCOM is not overloaded.
7.3.2.7.3 Circulation inhibition test Enter and exit the circulation inhibition function to verify the suppression of the bridge-arm circulation and the number of submodules.
7.3.2.8 UPFC initial operation test The initial operation test tests and evaluates the UPFC’s ability to smoothly lock and unlock as well as the conventional/emergent start-and-stop logic function for the control protection system of the UPFC under various modes and operating conditions. The test includes a conventional start-and-stop test of the UPFC, a manual switching test of redundant equipment, a control position shift test, and an emergent shutdown test.
7.3.2.9 Protection trip test The purpose of the test is to check whether the protection function of the system can act correctly. By performing an energized tripping test covering all the protected areas and different types of protective action, several protection functions should be tested on both sides. The test will be carried out on series and parallel sides to test whether the time sequence of trip blocking in different locations is correct and whether the time sequence of protective action is normal. The debugging function of the control protection software is controlled, and thus the software is used to simulate the protective action.
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7.3.2.10 Redundant equipment switching test The purpose of this test is to verify that when the control equipment fails, the system can smoothly switch to redundant equipment without affecting the operation of the UPFC. The system control protection function is checked, system function under power failure and bus fault monitored, and faults of the duty system controlled.
7.3.2.11 UPFC performance test 7.3.2.11.1 Steady-state performance test This test is to check that all UPFC system equipment is able to achieve the designed operating value when the AC system voltage, frequency, and shortcircuit situations reach a certain level. A check is made on whether the converter/ AC system characteristics meet the designed requirements. The tests mainly include: (a) power take-off and landing; (b) control-system switching during the power take-off and landing process; (c) shift in control position during the power take-off and landing process; (d) control mode switching; (e) tap control of parallel transformer.
7.3.2.11.2 Dynamic performance test The dynamic performance test tests the system’s ability to quickly adjust the command value when facing a larger disruption. The faster the speed of adjusting and the smaller the corresponding oscillation, the better the system’s dynamic performance will be. The dynamic performance test is used to optimize the controller in the step-response test, and to test whether the dynamic response time meets requirements. The test includes an AC voltage step, a reactive power step, a line active power step, a line reactive power step, and a DC voltage step test. During the test, it should be ensured the bus voltage is within the limit and the UPFC is not overloaded. The AC bus voltage, AC-line current, DC voltage, voltage of the submodule, and bridge-arm current should be checked and recorded.
7.3.2.12 Additional control function test When additional control function tests are performed, the performance optimization of additional controls to the system can be checked.
7.3.2.13 Overload test The test mainly includes a long-term overload test (without a backup cooling system and with parallel side 1 p.u. and series side 1.05 p.u.), a 2h overload test (with back-up cooling system, 1.1 p.u.), and a special load test (equivalent disturbing current preliminary detection, AC harmonic, radio interference measurement, audible noise, and system power loss measurement).
7.3 Onsite Debugging Test Techniques in the UPFC Project
7.3.2.14 Disturbance test of AC system The test is to verify the fault ride-through capability of the UPFC system under transient fault condition. Owing to the significant risk of test failure, it is necessary to develop a detailed test plan and emergency plans, according to the situation of the power grid.
7.3.2.14.1 Single-phase to ground fault in the parallel-side AC system The test is to verify whether the UPFC is able to recover within a specified time after a transient fault occurs in the parallel system. It is also to ensure that there is no overvoltage situation that is beyond the designed requirement when a fault happens.
7.3.2.14.2 Single-phase to ground fault in the series-side AC system The test is to verify whether the UPFC is able to recover within a specified time after a transient fault occurs in the series system. It is also to ensure that there is no overvoltage situation that is beyond the designed requirement when a fault happens.
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7
7.1 TEST TECHNIQUES FOR THE MAIN EQUIPMENT The UPFC (unified power flow controller), which mainly includes converter valves, a thyristor bypass switch (TBS), parallel transformers, and series transformers, must be tested before and during operation to ensure its safety and reliability. The quality of the equipment can be guaranteed if this practice is adopted in all processes, such as design, manufacture, transportation, installation, and operation. The tests can be divided into two categories: the insulation test and the characteristics test. The insulation test is mainly refers a test of the electrical properties of the insulation system. It includes the insulation characteristics test and the insulation withstand test. The insulation characteristics test is operated with a lower voltage pertaining and causes no damage to the insulation of the equipment; so this is called nondestructive testing. To pass the insulation withstand test, the equipment withstand capacities of power frequency, AC voltage, DC voltage, lightningimpulse voltage, as well as switching impulse voltage, should undergo a test which may have either of two results: tolerance or breakdown. Therefore it is called a destructive test. The insulation withstand test should be operated after the insulating characteristics test has been passed, in order to avoid an extra consumption. Most tests, except for insulation tests, are characteristics tests. Electrical or mechanical performance are mainly tested during these processes. The characteristics tests mainly consist of the transformer voltage ratio test, polarity test, no-load test, load test, and zero sequence impedance measurement test. The major test components of the UPFC are the converter valve, the TBS, parallel transformer, series transformer, MOA, DC isolation switch, grounding switch, start-up resistor, and bridge-arm reactor, which may be described as follows.
7.1.1 CONVERTER VALVE The converter valve transfers power between AC and DC systems. It has the ability to withstand normal operating voltage and current, as well as the impulse
Unified Power Flow Controller Technology and Application. DOI: http://dx.doi.org/10.1016/B978-0-12-813485-6.00007-8 Copyright © 2017 China Electric Power Press Ltd. Published by Elsevier Inc. All rights reserved.
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voltage and current result from the malfunction of the valve trigger system or system failure. The main test items are shown in Table 7.1. As it is mentioned above, the elementary wiring diagrams of the AC voltage withstand test, DC voltage withstand test, impact overvoltage withstand test, conducting and overcurrent breaking test and short-circuit current test are as follows.
Table 7.1 Converter Valve: Main Test Items No.
Test Types
Test Items
Test Purposes
1
Insulation test
AC voltage withstand test of valve support DC voltage withstand test of valve support Lightning impulse test of valve support Operation impulse test of valve support DC voltage withstand test of valve unit AC voltage withstand test of valve unit Lightning impulse test of valve unit Operation impulse test of valve unit Insulation test of valve unit
Test the voltage withstand characteristic of valve support
2 3 4 5 6 7 8 9
10
Characteristic test
Minimum DC voltage test
11
Maximum continuous running load test
12
The conducting and overcurrent breaking test Maximum transient overload ability test Short-circuit current test
13 14
15
Electromagnetic compatibility test
Test the voltage withstand characteristic between the valve units and to the ground
Test the voltage withstand characteristic of valve and the working states of the damping circuit and the control circuit Test the electronic equipment performance in getting power from DC capacitance Test the effect of a power device and related circuit on the current, voltage, and temperature Test the validity of the valve design Test the transient overload ability of the valve Test the performance of the power device and related circuit in circuit conditions Test the ability to resist electromagnetic interference in the model
7.1 Test Techniques for the Main Equipment
R
T1 ∼
C1
r
C2
G
Ty
Cx TV V A TA
FIGURE 7.1 Diagram of AC withstand voltage test. Ty—voltage regulator; T1—intermediate testing transformer; R—current-limiting resistance; C1 and C2—high and low arms of capacitive voltage divider; TV—voltmeter; TA—ammeter; R—sphere gap protection resistance; G—ball clearance; Cx—sample.
T1
Ty
∼ v
VD
TA
R
TV1
μA C
v TV2
Cx
FIGURE 7.2 Diagram of DC withstand voltage test. Ty—voltage regulator; T1—intermediate testing transformer; R—resistance protector; TV1, TV2—voltmeters; TA—ammeter; Cx—sample; C—filter capacitor.
7.1.1.1 AC voltage withstand test Fig. 7.1 shows the wiring diagram of the AC voltage withstand test. The voltage withstood by samples was boosted by the test transformer. The two main terminals were shorted in the test. The test AC voltage was applied between the shorted terminals and ground.
7.1.1.2 DC voltage test Fig. 7.2 shows the wiring diagram of the DC voltage withstand test. The voltage withstood by samples was obtained by a half-wave rectifier circuit. As in the AC voltage withstand test, the two main terminals were shorted and the test DC voltage was applied between the shorted terminals and ground.
7.1.1.3 Lightning and switching impulse tests Fig. 7.3 shows the wiring diagram of the lightning impulse overvoltage withstand test and the operation impulse overvoltage withstand test. The DC high voltage is generated by rectifier devices, and C1 gets charged by this DC high voltage.
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T1
Ty
VD
R0
∼
U0
G C1
R1
R2
Cx
u2
FIGURE 7.3 Diagram of lightning and switching impulse test. Ty—voltage regulator; T1—intermediate testing transformer; VD—high-voltage silicon rectifier; R0—current-limiting resistance; G—sphere clearance; R1—wave front resistance; R2—discharge resistance; Cx—sample.
R
LR
A
CT
B
C
Operating condition circuit
D
E
Heating circuit
Uniform controller for overcurrent shutdown test
FIGURE 7.4 Diagram of conducting and overcurrent breaking test. A—High-voltage source with low current; B—Resonance control link; C, D—Isolation control links; E—High-current source with low voltage.
When the voltage increases, the charge of capacitor C1 is C1U0 before the sphere gap breaks down, However, once the sphere gap has broken down, the capacitor C1 discharges to the test samples. The waveform of the lightning impulse overvoltage withstand test has three main parameters of characteristics: the peak, the wave front time, and the semipeak time. The wave front time of a standard waveform is 1.2 µs and the semipeak time is 50 µs. In accordance with the International Electrotechnical Commission IEC 60060.1-2010 “high-voltage test technology” and Chinese Standard GB/T 16927.1-2011 “high-voltage test technology,” the impulse voltage waveform parameters are 250 (1 6 20%)/2500 (1 6 60%) µs.
7.1.1.4 Conducting and overcurrent breaking test Fig. 7.4 shows the schematic wiring diagram of conducting and the overcurrent breaking test. The test circuit consists of two power supply sections. One is the
7.1 Test Techniques for the Main Equipment
stress resource, which has a low-current high-voltage power supply, largecapacity DC capacitance CT, and tunable inductors LR. The other is a heating source and it contains low-voltage high-current power supply sections and adjusting resistance R. The left portion of the sample valve is a condition circuit, consisting of the stress sources and an isolation control section. It is constructed to simulate a second zero-input response circuit, which is equivalent to similar real conditions of a straight arm. The right section of the sample valve is a heating circuit, which consists of a heating source and isolation areas. It can provide the required test temperature and heating currents that the sample valve needs during the overcurrent shutdown before the test. The left part of the sample valve is the condition circuit, which is composed of the stress sources and the isolation control section. It simulates a real bridge arm in the same conditions as in the second zero-input response circuit. The right section of the sample valve is a heating circuit, which is composed of a heating source and isolation areas. It can provide high current to be used for heating so as to satisfy the test temperature requirement before the sample valve is put through the overcurrent shutdown test.
7.1.1.5 Short-circuit current test This current is composed of an exponentially decaying current and a synthesized sinusoidal current as short circuit current, as shown in Fig. 7.5. The lefthand section is an attenuated current source circuit, which consists of a high-voltage lowcurrent power supply, capacitance C, a decaying oscillation circuit, and a control valve 1. The righthand part of the sample is a sinusoidal current source circuit, which consists of a low-voltage high-current power supply, a current limiting resistor R, and a control valve 2.
C
D
R
Submodule 1
A
C
B
Valves for test
Submodule 2 Submodule n
FIGURE 7.5 Diagram of short-circuit current test. A—High-voltage source with low current; B—Attenuation oscillation circuit; C, D—Control valve; E—High-current source with low voltage.
E
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Table 7.2 Main Test Items of TBS No.
Test Types
Test Items
Test Purposes
1
Insulation test
AC voltage withstand test of valve port Lightning impulse test of valve port Operation impulse test of valve port AC voltage withstand test between valve ports Lightning impulse test between valve ports Operation impulse test between valve port Minimum AC voltage test
Insulation level test of TBS from valve port to ground
2 3 4 5 6 7
8
Characteristic test
Cycle trigger and quench test
Insulation level test of TBS from between valve ports and ground
Test the correct running of the TCR valve trigger system under prescribed minimum AC voltage and operating conditions Verify the capacity of the thyristor valve under severe current strength and conducting and overcurrent breaking capacity
7.1.2 THYRISTOR BYPASS SWITCH TBS TBSs are closed to isolate valves from an AC line converter when line failure or inverter failure occurs (less than 5 ms). The TBS prevents interaction between the AC system and the broken-down valve zone. The main test items are shown in Table 7.2.
7.1.3 TRANSFORMER The transformers in a UPFC, can be divided into two types: parallel transformers and series transformers. Both have the same interconnect function, structure, and experimental contents. Their insulation of wire to ground is also the same, but the insulation tests are different because series transformers have the full insulation structure. The main test content is shown in Table 7.3. In the above experiment, the basic schematic winding diagrams of the induction withstand test, winding deformation test, and short-circuit withstand test are as follows.
7.1.3.1 Induced voltage withstand test The basic schematic winding diagram of the transformer-induced overvoltage withstand test setup is shown in Fig. 7.6. In the induction overvoltage withstand test, AC voltage is usually applied between the low-voltage transformer terminal, while the other windings are switched off to ensure their waveforms are sine waves as much as possible. In order to prevent the test excitation current from
7.1 Test Techniques for the Main Equipment
Table 7.3 Transformer Main Test Items No.
Test Types
Test Items
Test Purposes
1
Insulation test
Insulation resistance and absorption ratio measurement test Dielectric loss factor and capacitance measurement test
Check whether the transformer is affected with damp, dirt, or other penetrating agents overall or in part, or on its surface Check whether the transformer is affected with damp overall, or degraded by the effects of oil, sludge, or serious local defects, etc. Verify the winding terminal when it is vertical to ground, and check winding and winding insulation resistance strength
2
3 4 5 6
Characteristic test
Power frequency withstand voltage test Lightning and operation impulse test Induced voltage withstand test Winding resistance measurement test
7
Voltage ratio measurement and connection group designation test
8
Load test
9
No-load test
10
Temperature rise test
11
Short-circuit withstand test
12
Sound-level measurement test
13
Winding deformation test
14
On-load transformer tap switch test Three-phase transformer zero sequence impedance measurement test
15
Check the welding quality of the winding, whether the tap-changer is good contact, and whether there is any short circuit between the winding layers Test whether the winding voltage is qualified. Confirm that the numbers of windings of each tap parallel coils or line are the same, and judge that each connection of the lead and tap-changer is correct Test whether short-circuit impedance and load loss tests are qualified. Judge the winding deformation or short circuit between stocks Examine the partial or whole defect conditions of a magnetic circuit, and judge the short-circuit conditions of winding interterms Examine local overheating conditions of tank, structure, and the joints between leads and drive pipes, as well as leads and tap-changers Test withstand ability of transformer mechanical strength under the condition of heavy current Test transformer-rated run-time sound level and sound power level, to prevent noise pollution Test the winding deformation during transportation, installation, and operation Verify switch control performance, insulation level, and reliability Provide a theoretical basis for setting safeguard measures
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FIGURE 7.6 Diagram of transformer-induced voltage withstand test.
U1
L
L
L
K
K
K
Rs Us
C
C
K
C
C
R
U2
FIGURE 7.7 Frequency response analysis of basic detection circuit.
being too large during the test, the frequency of the power supply should be appropriately turned up past the rated frequency.
7.1.3.2 Detection test for transformer winding deformation The detection test of transformer winding deformation is achieved by testing winding amplitude-frequency response characteristics, and comparing the detection results vertically or horizontally. The wiring diagram is shown in Fig. 7.7. In the diagram, Us denotes the voltage source, Rs denotes source impedance, and R denotes measuring resistance; U1 and U2 are voltage signals of the winding terminals; L, K, and C are respectively the distributed inductance, the inter-turn distributed capacitance, and the ground distributed capacitance of the transformer winding.
7.1.3.3 Short-circuit current duration test Though the transformer short-circuit current duration is very short, it is a severe test. It gives the short-circuit dynamic stability and heat-resistant ability. The dynamic stability is verified through the short-circuit test. The test principle is shown in Fig. 7.8.
7.1 Test Techniques for the Main Equipment
NOB1
X
NOB 2
T1
T
TA U
u
V
v
W
w
A
3~
A A
FIGURE 7.8 Diagram of short-circuit withstand test. X—adjustable reactor; NOB1, NOB2—protection and closing switch; T1—intermediate transformer; TA—current transformer; T—test transformer.
Table 7.4 Main Test Items of Metal Zinc Oxide Lightning Arrester No.
Test Item
Purpose
1
Insulation resistance measuring test
2
Measurement test of DC voltage under 1 mA and leakage current under the above DC voltage
3
Leakage current measurement test under continuous operating voltage
4
Discharge counter action condition test
Preliminary test to show whether the interior of the arrester is affected by damp Test whether the ZnO valve becomes damp and determine if its movement characteristics and long-term working current are qualified Test the aging conditions of valve internal insulation and damp lightning arrester when it is affected by damp, severe damage and surface dirt To avoid discharge improperly resulting from counter rust due to bad sealing and damp
7.1.4 METAL OXIDE SURGE ARRESTERS The metal zinc oxide lightning arrester is the main item of equipment for limiting overvoltage in the UPFC. Its insulation test contents are shown in Table 7.4.
7.1.5 DC DISCONNECTOR AND GROUNDING SWITCH The main test contents of DC disconnector switch and earth switch are shown in Table 7.5.
7.1.6 STARTUP RESISTOR The startup resistor is set to the AC converter valve in series, and has great significance for the safe operation of the UPFC. The test includes mainly:
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Table 7.5 DC Isolation Switch and Earth Switch Main Test Items No.
Test Types
Test Items
Test Purposes
1
Insulation test
AC voltage withstand test
Check the insulation performance of the disconnector and grounding switch under DC voltage Check the insulation performance of the disconnector and grounding switch under lightning voltage Check the circuit contact of the DC disconnector or grounding switch
2
3
Impulse voltage withstand test Characteristic test
4
5
Primary loop resistance measurement test Porcelain ultrasonic flaw detection test Infrared temperature measurement test
Check the interior defects of the DC disconnector or grounding switch post insulator Check the defects of terminals, active contactors, and static contactors
7.1.6.1 Insulation test The power frequency voltage withstand test measures the insulation level between the terminals of the startup resistor. It is tested to ensure the safe operation of resistance at startup. In the test, the resistance network and outer cover should be disconnected. Meanwhile, the test voltage is applied between startup resistance terminals and its insulation bracket. The operating time is one minute. The test voltage shall be determined according to the following formulas: Utest 5 Utest 5
U Uk ðThe midpoint of the rheostat is connected to the housingÞ 2n
U Uk ðThe midpoint of the rheostat is not connected to the housingÞ n
(7.1) (7.2)
where U is the power frequency voltage of the entire resistor; n is the total number of series-connected resistors; k is the voltage nonuniformity factor between modules.
7.1.6.2 Characteristic test The resistance measurement test measures the starting resistance under power frequency, and during it the temperature effect should be considered for correction, in order to verify whether the startup resistance meets technical standards.
7.1.7 BRIDGE-ARM REACTOR The bridge-arm reactor is put into operation on the AC side of the converter. It has the functions of power control, wave filtering, and suppression of current fluctuation on the AC side when it works with the transformer.
7.2 Closed-Loop Test Techniques
FIGURE 7.9 Diagram of pulse reactor turn-to-turn insulation oscillating voltage method.
The bridge-arm reactor uses the entire insulation design. It is a kind of drytype air-core reactor, whose insulation test includes transformer-related items. In addition, the pulse oscillating voltage method is often used because of the possibility of reactor turn-to-turn insulation defects. The pulse oscillating voltage method makes use of reactor turn-to-turn insulation defects, and the reactor turnto-turn insulation condition is usually judged by the fact that inductance value changes according to frequency. The basic principle is shown in Fig. 7.9. In the diagram, C is the charging capacitor; G is the ball gap; L is a dry-type reactor; C1 and C2 are the high-voltage arm and the low-voltage arm in the capacitive voltage divider, respectively. In the test, if the reactor has an inter-turn short circuit, the following phenomena will appear: its inductance value will become small, its oscillation frequency will increase; the current between the short-circuit turns will increase, the energy loss will increase, and the oscillation will decay faster.
7.2 CLOSED-LOOP TEST TECHNIQUES FOR CONTROL AND PROTECTION SYSTEMS OF UNIFIED POWER FLOW CONTROLLER To carry out the feasibility demonstration of a UPFC project, comprehensively validate the physical control protection system performance and function, and analyze the impact that the UPFC has on relay protection, providing technical support for the operation and maintenance of the project, it is necessary to carry out closed-loop testing of control and protection devices based on a real-time digital simulation system (RTDS) and the actual engineering.
7.2.1 CLOSED-LOOP TEST SYSTEM 7.2.1.1 System overview When building a closed-loop test platform for the UPFC control and protection system, the purpose and nature of the test should be considered, since different application purposes correspond to different software and hardware systems.
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Power grid
Power grid
Simulation system MMC Analog output GTAO
MMC
FPGA(ML-605)
Digital input
TBS triggering pulse
Control command
Operating command for breakers and isolators
Analog I/O
States of breakers and isolators
DC current and voltage
AC current and power
Simulation system for valve control
for power system Digital input Digital output GTDI GTDO
Digital output
UPFC control and protection system FIGURE 7.10 Schematic diagram of overall structure of simulation system.
Generally, for a UPFC to carry out a control protection system closed-loop test in practice, it needs to be based on the actual needs of the project and construct a system model by using a RTDS. The UPFC control and protection system has the same configuration as the actual project site control protection device and constitutes a closed-loop system with the RTDS. A complete closed-loop test system of the UPFC control protection system should include grid and converter simulation equipment and a secondary control protection system. It should be noted that in the actual project the valve control equipment must be matched with the flow valve. The valve control equipment is generally provided by the supplier, and the communication between valve control and the converter valve triggering unit is based on a private agreement. Therefore, to carry out the UPFC control protection system closed-loop test, we need to develop the corresponding valve control simulation system to ensure the normal high-speed communication with the FPGA (ML-605) of the RTDS (to exchange the information of the submodule capacitor voltage and the trigger control word) (Fig. 7.10).
7.2.1.2 The composition of the closed-loop test system The UPFC closed-loop test system mainly includes: a control and protection system which is like that used in engineering practice, a real-time simulation system
7.2 Closed-Loop Test Techniques
RTDS workstation
Server
OWS/EWS
GPS clock
Station LAN
ACC
CCP1 A&B
CCP2 A&B
Protection 2
2 out of 3 VBC1 A&B
MMC1
2 out of 3
VBC2 A&B AMI(ASI) A&B
Switch value analog value
Protection 1 Protection 2
DMI(DSI) A&B
Field bus
Switch value analog value MMC2
Power system RTDS model
Power grid
Power grid
FIGURE 7.11 Block diagram of the closed-loop test system of the UPFC.
of using simulation of primary equipment and a real-time simulation system provided by the RTDS that can simulate the MMC (modular multilevel converter) converter valve equipment and power system. The UPFC closed-loop test system block diagram is shown below (Fig. 7.11). The whole simulation system includes the manmachine interface, the control and protection of the host computer and primary equipment simulated by the simulation system. The manmachine interface mainly includes the RTDS workstation, the server, the workstation, etc. The control and protection equipment includes the actual converter control and protection host computer and valve control and protection host computer. The primary system uses the RTDS simulation platform. The RTDs workstation was used to build and modify the primary system and to monitor the primary system. The MMC model arithmetic uses FPGA
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implementation. In order to simulate the actual engineering as realistically as possible, real-time simulation system of the AC system needs to be as close as possible to the real power grid. The simulation system is configured with a SCADA operator workstation, which can be used for data collection of the manmachine interface and station monitoring during UPFC operation and can achieve control protection program compilation, HiBug debugging, and so on. The communication between the operator workstation and the control and protection system is based on the TCP/IP protocol. Through the LAN network station, the system receives normal operation monitoring and operating instructions regarding the UPFC control station from operating personnel or the remote control center/centralized control center, and then monitors and processes faults and abnormal conditions, records event and alarms, synchronizes the secondary system, and adjusts the DC system parameters (active power instructions, reactive power instructions, etc.). The main device in the control protection system is the converter control protection host (CCP) and the valve control host (VBC). The CCP is connected by optical fiber with the VBC, and the VBC is connected to the real-time simulation device FPGA by optical fiber and an MMC converter valve; the CCP is connected to the AC system and the DC-line real simulation device through the optical fiber or cable. The control protection device and the simulation system interface of the simulation system are also included: the state of the switch and the control switch, the AC/DC analog signal, etc. In order to achieve a good simulation, the AC system of the real-time simulation system should be as close as possible to the real power system.
7.2.2 CLOSED-LOOP TEST OF CONTROL AND PROTECTION FUNCTION 7.2.2.1 Closed-loop test of the UPFC control system 7.2.2.1.1 Tap control test The UPFC system with tapping adjustment function needs to verify the transformer tap control strategy. In the automatic control and in the manual control mode, we need to make sure that the joint action is right the command event and status events are right, and that the operator interface displays the status correctly.
7.2.2.1.2 Sequence control and interlock test The sequence control and interlock function can effectively reduce the number of staff required to operate the equipment and avoid misoperation. The main purpose of the experiment is to verify that the control system is able to protect the knife switch equipment with interlocking function as well as to realize smooth startup and shutdown of the system, that the various operating modes can switch to normal and that all sequential event records display correctly.
7.2 Closed-Loop Test Techniques
7.2.2.1.3 STATCOM function test The parallel side uses the STATCOM (static synchronous compensator) operation start, setting for a constant voltage/no power control mode and set voltage/reactive power reference value. The STATCOM automatically outputs inductive reactive power. Special attention should be paid to ensuring the bus voltages are within the limit and the STATCOM is not overloaded. Reactive power/voltage control modes should be switched during operation of the STATCOM. The AC bus voltage, AC line current, DC voltage, submodule voltage, and bridge-arm current should be inspected and a record kept.
7.2.2.1.4 SSSC function test The series side uses SSSC (static synchronous series compensator) operating mode to start, and sets it as the active control method so as to control active power on the line according to requirements and control the DC voltage. For the operation of a multiloop SSSC, a multiloop series converter should be started to control active power on the line and the DC voltage. It is possible to switch between multiloop active coordination control and single-loop active control modes while ensuring that active power on the line changes smoothly to the intended target. During the inspection, AC bus voltage, AC line current, DC voltage, submodule voltage, bridge-arm current, and so on should also be recorded.
7.2.2.1.5 Circulation inhibition test Start up and exit the circulation inhibition function, and verify the suppression of the bridge-arm circulation and the number of submodules for each side.
7.2.2.1.6 UPFC startup and shutdown test The start-up and shutdown control of the UPFC system is one of the key problems. For every mode of operation, the UPFC start-up and shutdown need verification tests. During the tests, manual switching function of the verification system will cause little disturbance to the AC system.
7.2.2.1.7 Control mode conversion test For the control mode that can be switched on line, this can be done without disturbing voltage, current, active power, reactive power, and so on.
7.2.2.1.8 Steady state check and power range test During steady state operation, the system can control the electric quantity to order value, and the steady state deviation should meet the requirements of the technical specification. The control system’s ability to control active power and reactive power should be validated in various operating modes. According to the set rate, set smoothly the control of active power and reactive power values as instructed and validate
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the operating range of system active power and reactive power to meet the designed value.
7.2.2.1.9 Dynamic performance test To verify the dynamic performance of the control system. The step response of active power and reactive power should be fast; it does not impact the AC system.
7.2.2.1.10 Additional control test Additional control tests include overvoltage/undervoltage control, active power control (power lifting, power back down), AC and DC coordinated control device function tests, etc. By doing additional control experiments, the optimization performance of additional control to the system can be checked.
7.2.2.1.11 Operation mode conversion test In order to improve the reliability of the UPFC system, at present the project uses multiple operating modes of UPFC topology, such as STATCOMs, UPFCs, SSSCs, and many kinds of operational mode, with a converter mutual standby function. By doing an operating mode conversion test, we can verify that function.
7.2.2.1.12 Transient test This is to verify the fault ride-through capability of the UPFC system in the case of a transient fault of the AC system. Single-phase, two-phase, and three-phase grounding fault tests are carried out on the parallel side and the AC circuit of the network, and the UPFC can cross the fault or avoid a line fault through the fault restart function.
7.2.2.2 UPFC protection function closed-loop test The UPFC protection test includes a UPFC protection function test and a fault test. The protection function test verifies the control system in normal operation (e.g., starting and stopping, running mode switching, lifting power), and checks that the protection of the whole protection system is working well. The protection function test involves the protection of the whole protection system, including AC field protection, converter area protection, DC field protection, etc. By setting faults in each region in the RTDS, control and protective action can be examined. The fault point is set as shown in Fig. 7.12 and Table 7.6, and it is required that the fault can be quickly cleared immediately after it occurs in the UPFC system. By simulating a fault in the line and a fault in the outside system, coordination between UPFC protection and protection in the system can be evaluated and it is important that no maloperation occurs during the experiment.
7.3 Onsite Debugging Test Techniques in the UPFC Project
Power grid
Power grid f10 f9
f7
f8 f1
f11
f6
f2
f3
f5 f4
FIGURE 7.12 Failure point diagram of protection function test for a UPFC control and protection system.
Table 7.6 Fault Point Setting of Closed-Loop Test for UPFC Protection Function Fault Name
Fault Location
Converter valve area fault DC side fault Series transformer fault Parallel transformer fault AC bus fault Line fault Out of system fault
f1, f5 f2, f3, f4 f7 f9 f6, f8 f10 f11
7.3 ONSITE DEBUGGING TEST TECHNIQUES IN THE UPFC PROJECT 7.3.1 THE PURPOSE AND CONDITIONS OF THE SYSTEM DEBUGGING TEST The purpose of system debugging is to check the performance of all equipment, subsystems and the overall system in the UPFC project. Through system debugging, coordinating, and optimizing the cooperation between system and equipment, we can improve the overall performance of the UPFC system and the performance of the whole power system after the operation of the UPFC. Data and parameters that are essential to ensure the safe and stable operation of the HVDC (high voltage direct current) system should be tested and collected and production management and operation personnel can also be trained.
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To carry out on-site debugging tests for a UPFC, the following basic conditions are required: 1. Subsystem debugging has ended. 2. The AC access part of the circuit and the substation bay on the other side have been charged, and have the ability to transmit power. 3. All AC/DC protection has been set correctly and can be put into live debugging of the test system. 4. Emergency handling measures and plans have been drawn up during the debugging of the system. 5. After checking the sequence of events, there is no related alarm signal and the system is working properly. 6. The AC system protection and other specific protection configurations are in accordance with the scheduled requirements. 7. The corresponding UPFC system transformer protection and UPFC converter protection have been put into operation.
7.3.2 DEBUGGING TEST PROJECT AND TECHNOLOGY OF UPFC The purpose of the UPFC debugging test is to comprehensively assess the function of all the equipment in the project that checks the performance of the UPFC system after the series side and the parallel side are combined and to check the interaction between AC and DC systems, ensuring safe and reliable operation. The test scope includes: control and protection, flow control valve, valve control, water cooling system, ACDC field, etc. Starting from this stage, the primary device will be energized, the overall performance of the system will be assessed. System test items include:
7.3.2.1 Noncharged sequential control The test is about checking the order of the UPFC system and related operation when the primary devices are not energized. This test is to test the operating sequence, check whether electric interlocking can be performed correctly and verify that the DC equipment remains safe when a failure occurs during the execution of a sequence of operations. Noncharged sequential operations include: 1. Station operation, in the operator station for sequential operation; 2. Remote operation, in the dispatching institution for sequential operation. Noncharged control mode: in the manual/automatic control mode to test operation and implementation: a. In manual control mode, when implementing single-step operation, the correct operation should be performed and the wrong operation should be rejected; b. In automatic control mode, sequential operation should be executed automatically following the integrated order. If it is normal, the operation
7.3 Onsite Debugging Test Techniques in the UPFC Project
should be completed in accordance with the provisions of the order. If the sequence is not completed, the appropriate alarm information should be given, and the corresponding equipment should be able to return to the previous defined state, or to enter the next defined state.
7.3.2.2 Noncharged protection tripping The purpose of the tripping test (the final tripping test) is to check the tripping function for the protection system of the UPFC or a certain element. The main equipment includes the control and protection system for the UPFC AC station cabinet, control cabinet, cabinet interface, etc. The tripping test is carried out before the UPFC station is charged or after the control protection circuit (including software) is modified. All test items in the final tripping test should be carried out at least once. Select one protection item or more to start the tripping to ensure that the circuit action is totally correct, including the protection tripping matrix circuit. The test utilizes the debugging function of the logic software for the control protection system to simulate protection action by setting numbers with the software and verifies the correctness of the tripping circuit.
7.3.2.3 Charging test of shunt transformer and converter When the parallel transformer is charged, the peak value of the inrush current and the operating overvoltage should be within the limit, and the resonance should be fully damped. Check that the transformer ratio is correct and the related parallel transformer protection does not go into action. Ensure that the position of the tap and the start process of the transformer fan meet the designed requirements. In the converter charging test, the charging resistor should be connected first, then the low voltage side of the transformer circuit breaker is connected and the converter is charged. Next, verify whether the voltage phase, amplitude and transformer ratio on the transformer valve side are correct, and that the relevant protection is not active. Pay attention to the influence of the zero-sequence current on transformer and reactor. Check valve-side current, VBC return voltage, arm current, state signals and the DC-side voltage polarity ratio.
7.3.2.4 Charging trigger for converter valve After the transformer circuit breaker on the network side is connected, the converter valve under blocking state is charged by starting the resistor. attention must be paid to the influence of the zero-sequence current on the transformer and the reactor. When the charging current on the AC side is zero, the AC circuit breaker should be disconnected and the converter valve should be unlocked. Then the AC voltage phase sequence and phase angle difference at both ends of circuit breaker are measured. The phase sequence triggered by the valve group and the correctness of the phase lock link of the control and protection system are checked. When the delay of the control system is measured, the phase difference caused by
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the delay in the data processing of the control and protection system should be compensated.
7.3.2.5 Charging and starting of series transformer A charging test is performed for the series transformer. In the test, the series transformer will be charged from the high and low voltage sides separately so as to have a comprehensive assessment of the insulation of the series transformer to the ground, insulation of high and low voltage windings, and the TBS signal. The voltage phase related to the series transformer protection system is then checked. The load test for the protection of the series transformer and related TA loop of the UPFC protection system is performed by examining the closed-loop line load current after closing the TBS on the low voltage side of the series transformer.
7.3.2.6 STATCOM operation test The STATCOM operational test is carried out for the parallel converter.
7.3.2.6.1 Steady-state performance test Reactive-power control function test. Start the converter in STATCOM operational mode. Set the reactive power control mode and the reactive power reference value (including capacitive reactive and inductive reactive power) incrementally from the minimum to the rated value. The reactive-power/voltage control mode switching test is carried out for the STATCOM operation converter, which makes the converter control mode switch from reactive power control mode to voltage control mode. Voltage control function test. Set a few typical voltage reference values lower than the operating voltage in the system. The STATCOM automatically outputs inductive reactive power. Special attention should be paid to ensuring the bus voltages are within the limit and the STATCOM is not overloaded. Set the voltage reference value higher than a few typical values of operating voltage in the system, and repeat.
7.3.2.6.2 Dynamic performance test A dynamic performance test tests the system’s ability to quickly adjust the command value when facing a larger disruption. The faster the speed of adjustment and the smaller the corresponding oscillation, the better the system’s dynamic performance will be. Test items include DC voltage, parallel reactive power, AC voltage, etc. It should be ensured that the bus voltage is within the limit and the STATCOM is not overloaded.
7.3.2.6.3 Circulation inhibition test Enter and exit the circulation inhibition function to verify the suppression of the bridge-arm circulation and the number of submodules.
7.3 Onsite Debugging Test Techniques in the UPFC Project
7.3.2.7 Operation test of SSSC The SSSC operation test is carried out for the series converter.
7.3.2.7.1 Steady-state performance test Starting with the SSSC in operating mode, choose the power control mode first, then set the reference values in turn, finally check and record the AC bus voltage, A- line current, DC voltage, submodule voltage, bridge-arm current, etc. For SSSC operational mode with parallel double circuit lines, first start the double loop converter and choose double circuit active power control mode, then set the reference values of active power in turn, finally switch between doublecircuit and single-circuit power control mode.
7.3.2.7.2 Dynamic performance test A dynamic performance test tests the system’s ability to quickly adjust the command value when facing a larger disruption. The faster the speed of adjustment and the smaller the corresponding oscillation, the better the system’s dynamic performance will be. Test items include DC voltage, parallel reactive power, AC voltage, etc. It should be ensured that the bus voltage is within the limit and the STATCOM is not overloaded.
7.3.2.7.3 Circulation inhibition test Enter and exit the circulation inhibition function to verify the suppression of the bridge-arm circulation and the number of submodules.
7.3.2.8 UPFC initial operation test The initial operation test tests and evaluates the UPFC’s ability to smoothly lock and unlock as well as the conventional/emergent start-and-stop logic function for the control protection system of the UPFC under various modes and operating conditions. The test includes a conventional start-and-stop test of the UPFC, a manual switching test of redundant equipment, a control position shift test, and an emergent shutdown test.
7.3.2.9 Protection trip test The purpose of the test is to check whether the protection function of the system can act correctly. By performing an energized tripping test covering all the protected areas and different types of protective action, several protection functions should be tested on both sides. The test will be carried out on series and parallel sides to test whether the time sequence of trip blocking in different locations is correct and whether the time sequence of protective action is normal. The debugging function of the control protection software is controlled, and thus the software is used to simulate the protective action.
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7.3.2.10 Redundant equipment switching test The purpose of this test is to verify that when the control equipment fails, the system can smoothly switch to redundant equipment without affecting the operation of the UPFC. The system control protection function is checked, system function under power failure and bus fault monitored, and faults of the duty system controlled.
7.3.2.11 UPFC performance test 7.3.2.11.1 Steady-state performance test This test is to check that all UPFC system equipment is able to achieve the designed operating value when the AC system voltage, frequency, and shortcircuit situations reach a certain level. A check is made on whether the converter/ AC system characteristics meet the designed requirements. The tests mainly include: (a) power take-off and landing; (b) control-system switching during the power take-off and landing process; (c) shift in control position during the power take-off and landing process; (d) control mode switching; (e) tap control of parallel transformer.
7.3.2.11.2 Dynamic performance test The dynamic performance test tests the system’s ability to quickly adjust the command value when facing a larger disruption. The faster the speed of adjusting and the smaller the corresponding oscillation, the better the system’s dynamic performance will be. The dynamic performance test is used to optimize the controller in the step-response test, and to test whether the dynamic response time meets requirements. The test includes an AC voltage step, a reactive power step, a line active power step, a line reactive power step, and a DC voltage step test. During the test, it should be ensured the bus voltage is within the limit and the UPFC is not overloaded. The AC bus voltage, AC-line current, DC voltage, voltage of the submodule, and bridge-arm current should be checked and recorded.
7.3.2.12 Additional control function test When additional control function tests are performed, the performance optimization of additional controls to the system can be checked.
7.3.2.13 Overload test The test mainly includes a long-term overload test (without a backup cooling system and with parallel side 1 p.u. and series side 1.05 p.u.), a 2h overload test (with back-up cooling system, 1.1 p.u.), and a special load test (equivalent disturbing current preliminary detection, AC harmonic, radio interference measurement, audible noise, and system power loss measurement).
7.3 Onsite Debugging Test Techniques in the UPFC Project
7.3.2.14 Disturbance test of AC system The test is to verify the fault ride-through capability of the UPFC system under transient fault condition. Owing to the significant risk of test failure, it is necessary to develop a detailed test plan and emergency plans, according to the situation of the power grid.
7.3.2.14.1 Single-phase to ground fault in the parallel-side AC system The test is to verify whether the UPFC is able to recover within a specified time after a transient fault occurs in the parallel system. It is also to ensure that there is no overvoltage situation that is beyond the designed requirement when a fault happens.
7.3.2.14.2 Single-phase to ground fault in the series-side AC system The test is to verify whether the UPFC is able to recover within a specified time after a transient fault occurs in the series system. It is also to ensure that there is no overvoltage situation that is beyond the designed requirement when a fault happens.
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