- Email: [email protected]
Interfacing wind energy conversion equipment with utility systems
Entm~ Vol. 7, No. I, pp. E-29. Printed in Great Britain.
1982
0360-5442/82/010015-15~3.0010 Pcrgamon Press Ltd.
INTERFACING WIND ENERGY CONVERSION EQUIPMENT WITH UTILITY SYSTEMS R. C. MEIERand S. L. MACKLIS Advanced Energy Programs Department, General Electric Company, Philadelphia, PA 19101, U.S.A.
Abstract-Technical interface issues between utility systems and solar technology devices are receiving increasing attention from government and industry. These issues arise from the intermittent power generation characteristics of solar systems, in contrast to most utility generation equipment. This paper explores the interface issues for wind energy systems, a solar technology likely to achieve early commercialization. Described are government- and industry-sponsored assessments of the impact of wind energy devices on industry operations, controls, and protective subsystems. The conclusions drawn indicate that: there does not appear to be any major technical interface problem in applying wind energy devices to utility power systems; current utility control and protection design methods are adequate for interfacing wind energy units with a utility system; and, the cost of control and protection for large (over 1 Mw) wind machines appears acceptable but may prove prohibitive for small machines. Several advanced technology options with the potential for reducing the costs for protection and control are also described, for both the wind machines and the utility system.
INTRODUCTION
A wide range of technical interface issues confronts gas and electric utilities as they seek to both accommodate and utilize emerging solar energy technologies in their production and delivery systems. These interface issues arise because solar technologies present technical characteristics that run counter to the trends of the utility industry for the past 50 years: (1) Solar system units tend to be small installations relative to most utility generation facilities. (2) Solar system units will probably be widely dispersed over a utility’s service area. (3) Solar unit contributions to a utility’s energy production are intermittent and not predictable. (4) Some solar installations may be under the partial or complete control of the end-user. The technical interface issues presented by solar technologies can be solved using current utility design practices and equipment. The experience gained in integrating small hydro and gas turbine peaking generation units provides the technical know-how for integrating solar units into a utility system. However, using this base of experience may add unnecessary costs to large solar installations and may place an unacceptable cost burden on smaller solar units. These costs can be reduced through the innovative application of current utility design practices and equipment for the operation, control, and protection of solar units. Further reductions of interface costs can be realized in the future through the use of advanced control and protection technologies. The purpose of this discussion is to examine the most important technical interface requirements between a representative solar technology and an electric utility, and then to explore the economic solution to these requirements. Each of the many solar technologies under development presents unique interface requirements to a utility system. This discussion will examine wind energy conversion systems (WECS) and address those interface issues considered most demanding to the WECS unit(s) and its utility system. Wind energy systems have been chosen for several reasons: (1) Wind energy will probably be the first solar electric technology to be commercialized for utility power generation. (2) WECS can span a wide range of power ratings, from a few kilowatts (residential/farm applications) to multi-megawatt utility wind farms. IS
16
R.E.MEIERandS.L.MACKLIS
(3) WECS represent the solar technology with the most demanding siting requirements. (4) Of all solar generation technologies, WECS will represent the fastest temporal variations to an electric utility. Thus, wind energy systems serve as an excellent example to evaluate the technical integration of solar technologies into utility systems. WIND ENERGY SYSTEMS IN UTILITY APPLICATIONS
Early in the Federal Wind Program (1976) a major study was undertaken by General Electric (GE) under the sponsorship of the Department of Energy (DOE) (then the Energy Research and Development Administration, or ERDA).’ Its objectives were to: examine the effects of wind variations of the WECSlutility electrical system; determine any unique characteristics or interfaces with respect to stability and operational control and protection; and, provide engineering design information for WECS manufacturers and utilities. A specific goal was to determine the feasibility and constraints of installing numerous WECS, either in a group or dispersed, throughout a power system, as would be required for largescale applications of this generation technology. Major ingredients in the study were wind characteristics, WECS machine characteristics, protection and control for both the WECS and the power system, and representative power system characteristics. The initial part of the project characterized the inputs and variations to be examined for their effect on WECS stability. The primary variable is the wind, which was considered on three scales-local gusts, gust fronts, and large regional weather disturbances. The WECS units were characterized over a wide range of sizes (100, 1500 and 3000 kW) and design parameter variations. Control and protection techniques were ejtamined and a system defined for detailed examination. The second part of the program analyzed WECS unit operating stability, first as a single unit connected to an infinite bus, then as fully configured WECS clusters again connected to the infinite bus. Both synchronous and induction machines were examined. Stability was tested by the application of wind variability as previously characterized and modeled. In the third part of the program, the utility system was defined and modeled using an existing large-scale simulation program. The effects of wind variability and other parametric variations were examined to evaluate interactions between WECS units and the effects of WECS on overall power system stability, including fault behavior. Wind variability A difficult task in assessing the technical interface issues between solar technologies and utilities is to adequately represent the variability of the energy input. For the WECS analysis, the characteristics of the wind variability were identified and investigated for three scale levels: local spatial gusts, storm front gusts, and synoptic weather or regional storms. Only the local gusts showed the potential for serious disturbance to the WECS transient stability. It was found that although storm fronts and regional storms reached very high wind levels with marked turbulence, the rate of onset to the WECS unit was relatively slow (tens of minutes rise time) when compared with the response time of the WECS unit speed control system (5 set), so that the effects of even very severe gust fronts and storms were well within the control capability of the WECS. For local spatial gusts, meaningful results were obtained by using actual time histories. Representative gusts were selected from the 100 worst gusts of recorded gust data. The irregular and random nature of these records indicated that no simple analytical representation or other limited frequency content simulation could adequately test the stability response of the WECS unit. A quantitative measure of the effect of wind variability on system performance was developed using the sample of 100 gusts of varying severity and probability of occurrence, retaining the geometry, structure, and statistical properties of the original ensemble. The representation is shown in Fig. 1 and can be used to estimate the down time or forced outage due to severe wind conditions for any given WECS and site location. Single WECS response-infinite bus Although a number of different wind turbine configurations are under development, the horizontal axis, propeller type was chosen for evaluation because of its high efficiency potential and because it is farthest along in development for utility applications. The specific WECS
Interfacing wind energy conversion equipment
17
25 (55.91
WIND SPEED M/S (MPH)
15 (33.6
10 (22.4: .4
-3
-2
-1
0
1
2
3
4
5
T (SEC)
Fig. 1. Simulated wind gusts and probability of occurrence
evaluated uses a two-blade, constant-speed rotor, positioned downwind of the tower, driving either a synchronous or induction generator. Rated power levels covered the range from 100 to 3000 kW when operated in a wind regime of 18 mph (8 m per second) median speed. Both the wind speed and a number of design details were varied to obtain parametric results. The WECS units were characterized both by steady-state performance and dynamic parameters (rotational and structural) to establish transient electric response to gusts. The important electrical variables, in terms of system stability, are the power angle and terminal voltage. If either parameter goes beyond. set limits during the transient caused by a gust passage, the unit will be dropped off the line, causing some perturbation of line regulation as well as a loss of energy capture until the unit is resynchronized to the line. The response of these variables was studied by subjecting computer-modeled WECS units to specific gust environments. Figures 2 and 3 are examples of the behavior of a single unit connected to an infinite bus and responding to a specific wind gust time history, in addition to the other input disturbances of wind shear and tower shadow. The outputs shown are terminal voltage (dotted line) and power angle (solid line). The power angle, also referred to as “delta (8) angie” represents the angular displacement of the synchronous generator rotor from its desired position for perfect synchronism with the power line. These analyses, covering a wide range of WECS and gust parameters, showed that in most locations shutdown of individual units from gusts would occur only occasionally from very severe gusts-much less often than from calms or low wind speed. The analyses also showed that synchronous generators are preferable to induction generators for utility networks. The induction units exhibited unacceptable voltage dips caused by relatively moderate disturbances with no economically practical correction technique identified. Multi-unit WECS application-infinite bus In synthesizing a cluster of WECS to form part of a utility network, the wind regime and siting requirements are major considerations. The wind regime and application wiil establish the desirable size of the individual units, whereas site-specific characteristics will establish the cluster configuration and minimum spacing.
R. E. MEEK and S. L. MACKLIS
PER UNIT
r
0.01
0.5
1.0
1.5
2.0
I
0.10
: Y
:
” E L
A
0.39
46615.12 TIME SECONDS
.15.52 M/SEC
I 0.40 TIME (SECONDS1
I 0.50
I 0.59
WlND VELOCITY METERS PEFI SECOND/IO.
POWER ANGLE-RADIANS
46595.12
, 0.70
Fig. 3. Time response to severe wind gust above rated power.
EAN
I 0.20
-5 46565.12
0
: ci
:
0.80
VI,
LOSS OF SYNCH
I 0.90
20
R. E. MEIERand S. L.
MACKLIS
Major parameters affecting the stability of interconnected WECS units were identified, and nominal and extreme values expected in practical utility applications were established. The models thus developed were then simplified and parameters lumped, where justified, to facilitate the analysis of steady-state load flow conditions and dynamic stability, including both nominal and extreme or boundary conditions. In performing the stability analysis, a linear analysis technique was used to obtain both frequency and time domain information. Effort was specifically aimed at synthesis of a WECS cluster and the evaluation of the impact of the cluster on the stability of the WECS units individually and collectively. This analysis led to the conclusion that stability of the WECS should not be a limiting factor so long as the connection reactance between the WECS and the utility transmission system is less than 0.4 per unit on the base of the WECS rating. This reactance would correspond, for example, to connecting a 3 MVA WECS through about 5 miles of 5 kV feeder or a 30 MVA WECS through about 40 miles of 34.5 kV feeder. It was also determined that this reactance criterion is valid over a range of reactance to resistance ratios typical of utility distribution and subtransmission line design. Also, under steady wind conditions, no intracluster problems were encountered with either load sharing or stability for a small WECS cluster when the connection reactance criterion was observed. WECS application in typical networks The optimum approach for installing WECS into a specific network will require the same depth of analysis applied to any generation equipment. However, a number of basic conclusions, applicable to a broad spectrum of situations, were established by evaluating WECS in four representative systems: a small city subtransmission system, a large rural distribution system, a typical distribution feeder, and a remote site subtransmission system. These systems, based on actual systems in Vermont, span a range of applications that are representative of the types of primary distribution and subtransmission systems encountered throughout the rural and suburban portions of the U.S. interconnected power system where WECS installations are most likely. The small city subtransmission system is shown in Fig. 4, and the distribution feeder is depicted in Fig. 5. To permit an accurate analysis of system stability when WECS are incorporated into a utility power system, the various types of conventional units and their controls were modeled for a realistic simulation of power system transients resulting from both wind variability and system faults. A large-scale transient stability program used to analyze power system performance was utilized for the analysis. To represent the wind turbine, a new prime mover model consistent with others used in the program was developed. Standard models were already available for the generator, the excitation system, the power system stabilizer, and other standard utility system components. Results of the analyses indicated that in general: (1) The synchronous generator is preferred to the induction type for utility class WECS because of superior voltage stability under moderate to severe wind gust conditions. (2) Very severe, low probability of occurrence (0.1%) gusts can cause large voltage variations or even loss of synchronism; however, the annual outage time due to disconnection is expected to be well within acceptable utility limits. (3) Practical WECS clusters can be configured using existing power equipment and utility practices and by observing guidelines for connecting line resistance and reactance. (4) Transient stability response to faults was good due to the high inertia of the WECS unit relative to conventional generating units. When WECS were analyzed for installation on a distribution feeder, the results also indicated the following: (1) The application of two WECS units to the same distribution feeder produced only a small compounding of the voltage fluctuations. No additional intermachine modes of oscillation were indicated. (2) In the presence of severe gusts, voltage fluctuations as high as f. 10%can be expected. Such fluctuations would certainly be objectionable if sustained for several cycles or if frequent in occurrence. (3) Application of WECS to distribution feeders appears feasible but will require careful
Fig. 4. Small city 46 kV subtransmission
system.
R. E.
MEIER
and S. L.
MACKLIS
\(Y 1.5 MW ?’ +WTG2 ‘8 t f 12.5 KV - , - W75 4 KV-
1.5 MW T\
; 0.5 MI #2 I ‘8
w30
w20
w40 1 MI 4/O
k ,
w50
,
W60
w70
we0
1
_-I 1 MI 4/O
1 MI Ii0
1 MI 110
1 MI 110 0.970
1.028 Wlcf
w15
3
10 MVA
// / / v = 1.10
1.046 t
0.963 46 KV
1 MVA 0.92 pf
0.92 pf
NOTE: FEEDER
NO. 2
0.92
VOLTAGES
0.92 pf
SHOWN ARE WITHOUT
1 MVA 0.92 pf
WECS UNITS
5 MVA -0.92 pf
1.& 12.5 KV
Fig. 5. Representativedistribution feeder.
coordination of voltage regulating devices as well as protective equipment for the WECS and the distribution system. These results indicate that there should be no major problems in applying WECS to utility power systems but that design of the WECS and utility control and protective subsystems must receive adequate attention to ensure safe, cost-effective use of this type of generation equipment. WECS CONTROL
AND STABILIZATION
IMPROVEMENT
The typical WECS systems analyzed in the study used existing control and stabilization techniques and were lightly damped with long settling times. Although the WECS exhibited stable performance with steady wind and under moderate gusts including wind shear and tower shadow effects, it showed large voltage and power angle oscillations with severe gusts. Results of the study indicated that improved WECS control systems could provide greater damping.for natural rotor oscillations. Several approaches were investigated for improved stabilization of WECS in a gust environment. The prime considerations were ability to prevent unacceptable oscillations or shutdowns, energy efficiency, development status, and life cycle cost. Only techniques with adequate stabilization capacity to prevent shutdowns were considered. Evaluation of the options was therefore based on the remaining factors. One approach, energy storage, does not appear promising for enhanced control at this time, based on the best understood electrical and mechanical devices-lead-acid batteries and advanced flywheels. Their high cost is partially due to requirements for rapid sensing, high-energy discharge rates, and the relatively short life due to the large number of operational cycles required. Even if limited to responding only to moderately severe gusts (l%), over 40,000 cycles per year could be expected in a typical 18 mph wind site. This is approximately the cyclic life of the best of presently available relays or of clutches that must start and stop large amounts of electrical or mechanical energy. Several advanced control techniques or improvements in present techniques were identified that might be applied to improve system damping and transient response to wind gusts. These include the following: Shaft dumper. Connecting the rotor and generator through a high loss coupling could provide isolation of the electrical system from wind disturbances. Use of this device has been proven to be effective; however, it also implies a continuous loss and lower system efficiency. Optimized power system stabilizer. Based on work performed for the NASA/DOE MOD-l Wind Turbine Generator, it appears to be possible to obtain significant increases in damping by redesign of the conventional power system stabilizer. The stabilizer operates to phase match the transfer function from speed-to-voltage reference-to-electrical torque through the stabilizer and excitation system. This closed loop drivetrain exhibits an increase in damping over open
Interfacing wind energy conversion equipment
23
loop behavior. A responsive excitation system is generally necessary, and only the fundamental on-line drivetrain frequency damping is modified. Wind feed fonoard, This concept utilizes wind speed measurement upstream of the rotor to provide an anticipatory gust sensing control signal. A sensor ahead of the rotor measures the transient speed increase (characteristics of gusts) before the main force of the gust reaches the rotor. It operates outside the closed loop power control and ideally would control the rotor wind-to-torque gain to provide constant torque into the drivetrain. Practically, lags are involved in the rotor torque control mechanism and wind sensing means, and the wind sensed is not exactly what the rotor sees. The lags can be compensated for by sensor location, and significant reduction in variational torques from the rotor can be achieved. These approaches offer the promise of improved WECS unit control and utility system protection through the development of control devices for the WECS unit itself. However, there are many control and protection issues facing the application of WECS to utility networks, particularly on distribution feeders, where the presence of WECS may represent a significant source of local generation capacity. Control and protection of the WECS unit is therefore only part of the larger challenge of control and protection of the WECS/utility system. PROTECTION
ISSUES
FOR WECS ON UTILITY
DISTRIBUTION
SYSTEMS
To
achieve economic, reliable, and safe operation of utility distribution systems, utility planners regularly address system protection and control issues. Design strategies have evolved around a radial design, where any point on a distribution feeder is supplied from only one direction. Should trouble occur, protective equipment isolates the system from the trouble. Because the system is radial, this isolation can be accomplished by interrupting at one point only, thus isolating as few customers as possible. Utility system designers have achieved high levels of service reliability on distribution systems utilizing an approach of coordination and integration of several equipments with the characteristics of the radial design. Integration of WECS units (or any dispersed power source) into existing electric distribution circuits places additional requirements on the protection and control subsystem due to the siting of the WECS on the distribution system. Transmission and subtransmission networks can readily accept generation at many locatidns. Distribution systems are, by contrast, not designed to accommodate power sources due to the unidirectional flow of electricity and the protective and control systems designs based on that principle of radial design. In the immediate future, early demonstration projects will require that existing protection hardware be utilized to integrate WECS in a safe and reliable manner. In the long run, utility designers need to focus on protection and control technology options that will provide economic and safe ways to fully integrate WECS into distribution systems while maximizing utilization of WECS potential capabilities. Designing effective protection subsystems for WECS on utility systems represents a relatively straightforward technical task. The cost of such protective subsystems for large WECS (1 MW or larger) appears acceptable. However, protection subsystem costs for small residential and commercial class machines may be high relative to the unit cost itself. Automated distribution systems appear to offer means to more effectively control, protect, and operate distribution systems containing WECS with low-cost comprehensive communication and control systems. Fortunately, automated distribution system technology programs are aggressively exploring ways to provide this protection and control economically using microprocessor-based digital technology. The principal protection and safety issues that must be addressed for integrating WECS into utility distribution systems are summarized in Table 1. The most important of these issues are discussed below, along with descriptions of protection subsystem designs utilizing current equipment. Overcurrent protection Except for network systems, time overcurrent, radial, coordinated protection systems have been the most common design used on electric utility distribution systems. This radial configuration requires the coordination of several types of equipment, including breakers, reclosers, sectionalizers, and fuses. Since a distribution system has neither the need nor the
R. E. MEIERand S. L. MACKLIS
24
Table 1. Distribution protection and safety issues for WECS. overcurrent
Coordination equipment
Fault
Fault current interruption Bidirectional flow Ground fault current Short circuit duty
current
WBCS machine duty Energizing circuit Coordination with h'ECS protection
Reclosing
Dead
circuit
Voltage
of protective
energization
protection
Transients--lightning or switching surges Machine voltage transients VAR supply switching Ferroresonance Machine transients Stability
Frequency
Self-excited
Islanding Converter
Safety Start-up Synchronizing
control
Current
operation
and voltage
control
Synchronizing
Frequency on/off Machine duty Inrush currents
cycling
Harmonics
Relay operation Telephone interference Filtering
capability to sense the direction of overcurrent flow, only the magnitude and time are measured and used as a basis for circuit interruption to isolate the faulted line section. When a dispersed generating device such as a WECS is sited on a distribution feeder, it provides a source of electric power and voltage that may be located downstream from a potential fault. A one-line diagram of a simple distribution circuit with a WECS installation is shown in Fig. 6. In this case, current would be provided to the fault from more than one direction, thus violating the normal utility distribution protection design principles of radial design and unidirectional flow. Early WECS installations will probably require that the unit automatically (with manual backup) disconnect itself from the utility system and lock out whenever fault current is sensed at its terminals. Since service reliability at the feeder and safety to the crew dominate the individual service and energy produced by a WECS, utility practices will probably require that the system be protected from the WECS. More expensive schemes such as transfer tripping may be necessary. Longer term solutions involving automated distribution systems, which would incorporate smart remote terminal units on distribution feeders, will allow maintenance of safety and reliability while minimizing the time and automating the process of the restoration of generation by the WECS after the fault is cleared. Fault current Utility systems with WECS will require detection of all faults, either line to line or line to ground, and interruption at the unit. Normal utility system design practices will continue to require the ability to interrupt all faults at the distribution sutstation or wherever the normal feeder protective equipment is placed. Of particular significance will be single phase-to-ground faults near a WECS that is connected to the distribution system via a delta-wye transformer. A single line-to-ground fault will be detected by an upstream protective device, but unless special steps are taken at the WECS (i.e., transfer tripping coordination with substation breakers), the fault will not be sensed at the unit because of the ungrounded primary of the delta-wye transformers. If the upstream device that cleared the fault recloses, WECS out-of-phase synchronization might occur. If the
Interfacing
l-
--
wind energy conversion equipment
-
1 I
I
I I
I
I
I I
Cl
I I I
‘4.16
I I I I---_.
#I
I
WECS
I
KV
!
CJ __.
I
I I I I
C7 -_
DISTRIBUTION SUBSTATION
I I
=
I I
2s
-_
#2 --
t
---
-I
WECS
3 - PHASE PRIMARY
1
1 -PHASE
LATERAL_ DISTRIBUTION TRANSFORMER *
r------i
SERVICES
_--_-
Fig. 6. Sample distribution circuit with WECS.
fault is permanent and the upstream device remains open, the circuit will remain energized by the WECS, creating a possible safety hazard to crewmen. While today’s distribution system equipments are nondirectional current detecting, the size and number of WECS on a distribution circuit may require that more expensive directional protective equipments be specified. A typical protection subsystem design for a distribution system with large WECS is shown in Fig. 7. This type of equipment is commonplace on transmission lines but would represent a cost increase when applied to distribution systems. Reclosing Many utility distribution systems are now designed for automatic reclosing sequences after an initial fault has been detected and interrupted. Since many faults on distribution systems are temporary, automatic reclosing provides a reliable technique that will frequently restore customer service after a momentary outage. Not uncommonly, a circuit may be automatically reclosed three times after initial fault interruption before the circuit breaker locks out, requiring a crew to investigate before service can be restored. Should WECS be out on the circuit at the time a fault occurs, the distribution substation feeder breaker will interrupt the circuit and then could automatically reclose out of synchronism with the WECS. This reclosing onto a synchronous machine, an induction generator, or an a.c./d.c. converter could place a severe duty cycle on the WECS. In the case of the synchronous machine, large mechanical torques can be exerted upon the WECS rotor, possibly causing damage to the rotating machinery. Significant inrush currents might occur, be interrupted as fault currents after reclosing, and cause the same or another protective device to operate. While the service primary feeder breaker is open, the WECS keep the feeder energized, thus sustaining the fault current flow. Reclosing of the source primary feeder breaker will be unsuccessful unless the fault current has been interrupted by completely deenergizing
R. E.
MEIER andS. L. MACKLIS DISTRIBUTION
LEGEND 51 51N 67 67N
-
PHASE GROUND PHASE GROUND FAULT RELAY
TIME-OVERCURRENT RELAY TIME-OVERCURRENT RELAY DIRECTIONAL TIME-OVERCURRENT DIRECTIONAL TIME-OVERCURRENT CURRENT TRIPPING DIRECTION
RELAY RELAY
Fig. 7. Typical directional overcurrent relaying design.
the primary feeder section. This requires opening of the source primary feeder breaker plus simultaneously disconnecting all WECS. As indicated above, present utility practices generally require that the WECS be automatically disconnected from the system from the time the circuit breakers at the substation initially open to clear a fault and before the circuit breaker is scheduled to automatically reclose. Approaches to meet this requirement, although more expensive, may involve synch check or synchronizing relays and transfer tripping via direct communication from the WECS to the substation protective equipments. Energizing dead circuits To assure crew safety, the most significant requirement by electric utilities is that the WECS be prevented from improperly energizing a dead distribution circuit. If the WECS were to be reenergized and connected into the system without proper authorization, hazardous conditions might result. This safety consideration will probably require automatic lock-out switches with manual backup- to electrically separate the WECS from the distribution circuit. Voltage protection and frequency control In general, utility distribution systems are designed to protect equipment in the face of any abnormal voltage on the distribution circuits, either on the primary or the secondary. One voltage-related protection issue arises from rotating machinery such as induction generators on large wind machines, as discussed before. They are susceptible to voltage fluctuations as a result of shaft torque transients caused by wind gusting or other sporadic power inputs to the WECS. Even in the case where the voltage swing magnitude may be low, if the frequency of these voltage excursions is excessive, then customer service will be degraded due to voltage flicker. If the voltage swing is excessive, then equipment and safety problems may result. As discussed before, synchronous generators are preferred for large WECS utility installations. However, improvements in WECS control systems to minimize voltage fluctuation magnitude and frequency are needed.
Interfacing wind energy conversion equipment
?I
Many small WECS units now being considered for use require inverters to convert the d.c. power output to the 60Hz power of the utility distribution system. The commutation of a.c. inverters requires a supply of VARs sized to the needs of the WECS. Should the WECS unit be off line, either due to low wind or mechanical failure, the VARs being supplied locally by capacitors would remain connected on the distribution system, and the voltage control provided by substation voltage regulators, load tap changing, or switched capacitors might be disrupted. If these capacitors are not disconnected from the system when the WECS is, this may interfere with the operation of the voltage regulating equipments on the distribution system. And with the presence of WECS, in particular with induction generators, the conditions exist for a possible resonant circuit between the machine inductance and system capacitors. Overvoltage transients would then occur when switching those capacitors as part of feeder voltage control. Utility concern with voltage control on distribution feeders may require that existing equipment, such as surge arresters, be increased in their ratings or possibly that special voltage control requirements be placed on the WECS unit to assure satisfactory and safe voltage control. The protection of utility distribution systems from off-nominal frequency operation may be complicated on distribution systems that have WECS units with synchronous generators. Where several synchronous machines are operating in parallel, the potential for machine instability or hunting may exist. Protection against hunting may also be required. Islanding and self-excitation When a substation breaker interrupts a distribution feeder from the system, it may create an electrical island that is energized by the WECS devices on that feeder. If sufficient system capacitance exists, induction machines will self-excite and continue to operate at off-nominal voltage and frequency. Depending upon the specific amount of system capacitance and machine inductance, the frequency and voltage will rise or fall from their nominal values. Therefore, when a circuit is deenergized at the substation, all induction machines, no longer having line excitation, would be tripped off line. Since the safety and operational concern with off nominal voltage on the distribution circuit dominates, utility system practices will require protective devices to prevent self-excited operation of the induction machines or installation of line commutated inverters. Power conditioning and harmonic control The characteristics of the power conditioner will often be as important in their effects on the power system as the characteristics of the WECS itself. There are a wide variety of circuit designs available for converting the d.c. potential of small WECS to the a.c. potential required by utility systems. Converters may be line commutated or self-commutated, and the amount of harmonic filtering is often critical. Short circuit currents, harmonic content of the output, and technical features of the converters will have a great bearing on the requirements of the distribution protective and control system. D.c./a.c. inversion associated with some WECS technologies means the use of inverter devices that produce harmonics of 60 cycle. Should harmonic voltages and currents be excessive, a potential exists for the misoperation of relays as well as, the propagation of electromagnetic noise on the distribution system. To the extent these problems interfere with the quality of service or cause misoperation of system protective devices, the presence of harmonics caused by inverters must be designed for. Current utility practices for high-voltage d.c. applications or arc furnaces use extensive harmonic filtering equipments to trap the harmonics generated by the inverter. Utility systems may require that similar protective equipment be put in place to interrupt a.c. inverters producing excessive harmonics, or possibly that the WECS unit include harmonic filters as an integral part of the generating device. ADVANCED
PROTECTION
TECHNOLOGIES
FOR USE WITH WECS
Present protection and control approaches for interconnection of dispersed generation and storage sources, such as WECS, to distribution feeders are based on local sensing of conditions, utilizing groupings of component relays. Such protective equipment experience is based on industrial and commercial installations of generators driven by diesel engines or small gas- or steam-driven turbines. Each component relay utilizes electromechanical or solid-state analog
R. E. MEIERand S. L. MACKLIS
28
technology. Compared to digital techniques, these component relay technologies have limitations in sensitivity, speed, memory, and logic capability. Technical and economic advances in digital electronics technology are making possible new opportunities for integrated protection, control, and monitoring of the distribution system. These new technologies offer potential advantages such as improved utilization of power system facilities and cost effectiveness, improved system operation and lower operating costs, more complete and timely information for system planning, and reduced outage duration. Automated distribution application areas include the distribution substation, feeder, and user level, and distribution communications. Automated feeder sectionalizing for fault isolation and service restoration is one of a number of distribution functions being implemented in automated distribution systems. Improved utilization of substation and feeder facilities with opportunities for deferring new system investment offers the potential for significant economic savings through improved monitoring and control. Load management and dispersed storage and generation offer opportunities for significant savings in possible deferral of new generation and/or transmission capacity. A functional diagram of several WECS units is shown in Fig. 8, with the components of an integrated digital control/protection subsystem. The control/protection subsystem would feature digital sensing; advanced analog relay techniques; digital relaying; and communication of data, status, and control commands. Examples of the applications of these advanced technologies include (1) digital measurement of directional power flow between the WECS installation and the utility, compared with status changes of circuit breakers and sectionalizing devices on associated feeders and substations; (2) transfer tripping of the WECS when the connection to the utility distribution system is interrupted by opening of circuit breakers and
00
Generating Plant Utility Energy and Operational Management Systems (EMS)
Step-up Transformers
L______U_______-__-_-------_----__---_-A
Subtransmission System Distribution Substation
1
/
I I I 1
I
I
-
**
Large WECS Unit (l-5 MW)
v Feeder Sectionalizing Switches
Primary Circuits WECS Cluster (3-25 MW)i L--______
Distribution Control Unit (DCUI
iPhase Primary Main
_______._; 1-Phase Laterals Distribution Secondaries J
+ Services
1, 22ov 19
Fig. 8. Functional diagram of WECS units integrated into a distribution system.
Interfacing wind energy conversion equipment
29
sectionalizing devices; (3) communications between WECS units, changing protective characteristics as a function of total WECS capability connected to a feeder (or substation) vs load actually being served by the associated feeder (or substation); (4) sensitive measurement and trend change of unbalanced line-to-ground voltage conditions where WECS units connect to a supply feeder; (5) use of an automated distribution system to locate, isolate, and restore service after a feeder fault, including reconnection of unfaulted feeder sections to appropriate alternate feeder sources; (6) disconnection of WECS units via transfer trip over communication channels when one or more remote distribution switching devices opens the utility supply to feeder connection; (7) isolate WECS connections, or inhibit reclosing, when a sensed change in 63 Hz occurs at the point of utility connection, indicating that frequency is being determined by the WECS rather than the utility. The amount of control command information to be transmitted from the control center to the WECS location and the data to be returned from the WECS to the control center will vary with the size of the WECS unit and the individual utility practices. The kind of communication equipment that can be utilized includes the communication link with the Distribution Control Center (DCC), the interface and control equipment located at the DCC that enables the distribution dispatcher to interact with the remote WECS, the interface and control equipment which is located at the WECS unit and interfaces the communication link and the WECS local control equipment, and the system/WECS power protection interface equipment that enables the WECS power equipment to operate compatibly with the utility distribution network. CONCLUSIONS
Based on results of studies conducted to date, the following conclusions have been drawn in this paper: (1) There does not appear to be any major technical interface problem in applying WECS to utility power systems. (2) Significant deviations in WECS voltage and frequency due to severe wind gusts will probably require automatic disconnection of the WECS unit from its utility connection. However, the occurrences of disconnection and resynchronization will be few compared to those due to low wind conditions and will represent a negligible loss of annual energy output. (3) Application of existing utility design practices and equipment can provide necessary operation, control, and protection functions for the WECS and the utility. However, the cost of providing these functions for smaller WECS installations may be prohibitive. (4) Improved WECS unit controls and integrated utility control and protection systems utilizing digital electronics technology offer improved economic potential for all classes of WECS installations. For small, user-controlled machines, these developments could be critical to their economic success. Thus, it appears that the technical interfaces between WECS units and utility systems present problems that are solvable today. As more solar technologies approach commercial status and become significant factors in utility planning, the experience provided by earlier technologies such as WECS will help to smooth their entry into utility operations. Acknowledgements-The authors would like to thank Mr. Robert Moisan of GE’s Digital Systems Operation for his several contributions to the discussion on WECS protection and control subsystems, and to Mr. Roy Schaeffer of GE’s Advanced Energy Programs Department for his assistance in reviewing this paper.
REFERENCE 1. Department of Energy, “Systems Dynamics of Multi-Unit Wind Energy Conversion Systems Application”, Rep. DSE-2332-T1,2, 3.
1982
0360-5442/82/010015-15~3.0010 Pcrgamon Press Ltd.
INTERFACING WIND ENERGY CONVERSION EQUIPMENT WITH UTILITY SYSTEMS R. C. MEIERand S. L. MACKLIS Advanced Energy Programs Department, General Electric Company, Philadelphia, PA 19101, U.S.A.
Abstract-Technical interface issues between utility systems and solar technology devices are receiving increasing attention from government and industry. These issues arise from the intermittent power generation characteristics of solar systems, in contrast to most utility generation equipment. This paper explores the interface issues for wind energy systems, a solar technology likely to achieve early commercialization. Described are government- and industry-sponsored assessments of the impact of wind energy devices on industry operations, controls, and protective subsystems. The conclusions drawn indicate that: there does not appear to be any major technical interface problem in applying wind energy devices to utility power systems; current utility control and protection design methods are adequate for interfacing wind energy units with a utility system; and, the cost of control and protection for large (over 1 Mw) wind machines appears acceptable but may prove prohibitive for small machines. Several advanced technology options with the potential for reducing the costs for protection and control are also described, for both the wind machines and the utility system.
INTRODUCTION
A wide range of technical interface issues confronts gas and electric utilities as they seek to both accommodate and utilize emerging solar energy technologies in their production and delivery systems. These interface issues arise because solar technologies present technical characteristics that run counter to the trends of the utility industry for the past 50 years: (1) Solar system units tend to be small installations relative to most utility generation facilities. (2) Solar system units will probably be widely dispersed over a utility’s service area. (3) Solar unit contributions to a utility’s energy production are intermittent and not predictable. (4) Some solar installations may be under the partial or complete control of the end-user. The technical interface issues presented by solar technologies can be solved using current utility design practices and equipment. The experience gained in integrating small hydro and gas turbine peaking generation units provides the technical know-how for integrating solar units into a utility system. However, using this base of experience may add unnecessary costs to large solar installations and may place an unacceptable cost burden on smaller solar units. These costs can be reduced through the innovative application of current utility design practices and equipment for the operation, control, and protection of solar units. Further reductions of interface costs can be realized in the future through the use of advanced control and protection technologies. The purpose of this discussion is to examine the most important technical interface requirements between a representative solar technology and an electric utility, and then to explore the economic solution to these requirements. Each of the many solar technologies under development presents unique interface requirements to a utility system. This discussion will examine wind energy conversion systems (WECS) and address those interface issues considered most demanding to the WECS unit(s) and its utility system. Wind energy systems have been chosen for several reasons: (1) Wind energy will probably be the first solar electric technology to be commercialized for utility power generation. (2) WECS can span a wide range of power ratings, from a few kilowatts (residential/farm applications) to multi-megawatt utility wind farms. IS
16
R.E.MEIERandS.L.MACKLIS
(3) WECS represent the solar technology with the most demanding siting requirements. (4) Of all solar generation technologies, WECS will represent the fastest temporal variations to an electric utility. Thus, wind energy systems serve as an excellent example to evaluate the technical integration of solar technologies into utility systems. WIND ENERGY SYSTEMS IN UTILITY APPLICATIONS
Early in the Federal Wind Program (1976) a major study was undertaken by General Electric (GE) under the sponsorship of the Department of Energy (DOE) (then the Energy Research and Development Administration, or ERDA).’ Its objectives were to: examine the effects of wind variations of the WECSlutility electrical system; determine any unique characteristics or interfaces with respect to stability and operational control and protection; and, provide engineering design information for WECS manufacturers and utilities. A specific goal was to determine the feasibility and constraints of installing numerous WECS, either in a group or dispersed, throughout a power system, as would be required for largescale applications of this generation technology. Major ingredients in the study were wind characteristics, WECS machine characteristics, protection and control for both the WECS and the power system, and representative power system characteristics. The initial part of the project characterized the inputs and variations to be examined for their effect on WECS stability. The primary variable is the wind, which was considered on three scales-local gusts, gust fronts, and large regional weather disturbances. The WECS units were characterized over a wide range of sizes (100, 1500 and 3000 kW) and design parameter variations. Control and protection techniques were ejtamined and a system defined for detailed examination. The second part of the program analyzed WECS unit operating stability, first as a single unit connected to an infinite bus, then as fully configured WECS clusters again connected to the infinite bus. Both synchronous and induction machines were examined. Stability was tested by the application of wind variability as previously characterized and modeled. In the third part of the program, the utility system was defined and modeled using an existing large-scale simulation program. The effects of wind variability and other parametric variations were examined to evaluate interactions between WECS units and the effects of WECS on overall power system stability, including fault behavior. Wind variability A difficult task in assessing the technical interface issues between solar technologies and utilities is to adequately represent the variability of the energy input. For the WECS analysis, the characteristics of the wind variability were identified and investigated for three scale levels: local spatial gusts, storm front gusts, and synoptic weather or regional storms. Only the local gusts showed the potential for serious disturbance to the WECS transient stability. It was found that although storm fronts and regional storms reached very high wind levels with marked turbulence, the rate of onset to the WECS unit was relatively slow (tens of minutes rise time) when compared with the response time of the WECS unit speed control system (5 set), so that the effects of even very severe gust fronts and storms were well within the control capability of the WECS. For local spatial gusts, meaningful results were obtained by using actual time histories. Representative gusts were selected from the 100 worst gusts of recorded gust data. The irregular and random nature of these records indicated that no simple analytical representation or other limited frequency content simulation could adequately test the stability response of the WECS unit. A quantitative measure of the effect of wind variability on system performance was developed using the sample of 100 gusts of varying severity and probability of occurrence, retaining the geometry, structure, and statistical properties of the original ensemble. The representation is shown in Fig. 1 and can be used to estimate the down time or forced outage due to severe wind conditions for any given WECS and site location. Single WECS response-infinite bus Although a number of different wind turbine configurations are under development, the horizontal axis, propeller type was chosen for evaluation because of its high efficiency potential and because it is farthest along in development for utility applications. The specific WECS
Interfacing wind energy conversion equipment
17
25 (55.91
WIND SPEED M/S (MPH)
15 (33.6
10 (22.4: .4
-3
-2
-1
0
1
2
3
4
5
T (SEC)
Fig. 1. Simulated wind gusts and probability of occurrence
evaluated uses a two-blade, constant-speed rotor, positioned downwind of the tower, driving either a synchronous or induction generator. Rated power levels covered the range from 100 to 3000 kW when operated in a wind regime of 18 mph (8 m per second) median speed. Both the wind speed and a number of design details were varied to obtain parametric results. The WECS units were characterized both by steady-state performance and dynamic parameters (rotational and structural) to establish transient electric response to gusts. The important electrical variables, in terms of system stability, are the power angle and terminal voltage. If either parameter goes beyond. set limits during the transient caused by a gust passage, the unit will be dropped off the line, causing some perturbation of line regulation as well as a loss of energy capture until the unit is resynchronized to the line. The response of these variables was studied by subjecting computer-modeled WECS units to specific gust environments. Figures 2 and 3 are examples of the behavior of a single unit connected to an infinite bus and responding to a specific wind gust time history, in addition to the other input disturbances of wind shear and tower shadow. The outputs shown are terminal voltage (dotted line) and power angle (solid line). The power angle, also referred to as “delta (8) angie” represents the angular displacement of the synchronous generator rotor from its desired position for perfect synchronism with the power line. These analyses, covering a wide range of WECS and gust parameters, showed that in most locations shutdown of individual units from gusts would occur only occasionally from very severe gusts-much less often than from calms or low wind speed. The analyses also showed that synchronous generators are preferable to induction generators for utility networks. The induction units exhibited unacceptable voltage dips caused by relatively moderate disturbances with no economically practical correction technique identified. Multi-unit WECS application-infinite bus In synthesizing a cluster of WECS to form part of a utility network, the wind regime and siting requirements are major considerations. The wind regime and application wiil establish the desirable size of the individual units, whereas site-specific characteristics will establish the cluster configuration and minimum spacing.
R. E. MEEK and S. L. MACKLIS
PER UNIT
r
0.01
0.5
1.0
1.5
2.0
I
0.10
: Y
:
” E L
A
0.39
46615.12 TIME SECONDS
.15.52 M/SEC
I 0.40 TIME (SECONDS1
I 0.50
I 0.59
WlND VELOCITY METERS PEFI SECOND/IO.
POWER ANGLE-RADIANS
46595.12
, 0.70
Fig. 3. Time response to severe wind gust above rated power.
EAN
I 0.20
-5 46565.12
0
: ci
:
0.80
VI,
LOSS OF SYNCH
I 0.90
20
R. E. MEIERand S. L.
MACKLIS
Major parameters affecting the stability of interconnected WECS units were identified, and nominal and extreme values expected in practical utility applications were established. The models thus developed were then simplified and parameters lumped, where justified, to facilitate the analysis of steady-state load flow conditions and dynamic stability, including both nominal and extreme or boundary conditions. In performing the stability analysis, a linear analysis technique was used to obtain both frequency and time domain information. Effort was specifically aimed at synthesis of a WECS cluster and the evaluation of the impact of the cluster on the stability of the WECS units individually and collectively. This analysis led to the conclusion that stability of the WECS should not be a limiting factor so long as the connection reactance between the WECS and the utility transmission system is less than 0.4 per unit on the base of the WECS rating. This reactance would correspond, for example, to connecting a 3 MVA WECS through about 5 miles of 5 kV feeder or a 30 MVA WECS through about 40 miles of 34.5 kV feeder. It was also determined that this reactance criterion is valid over a range of reactance to resistance ratios typical of utility distribution and subtransmission line design. Also, under steady wind conditions, no intracluster problems were encountered with either load sharing or stability for a small WECS cluster when the connection reactance criterion was observed. WECS application in typical networks The optimum approach for installing WECS into a specific network will require the same depth of analysis applied to any generation equipment. However, a number of basic conclusions, applicable to a broad spectrum of situations, were established by evaluating WECS in four representative systems: a small city subtransmission system, a large rural distribution system, a typical distribution feeder, and a remote site subtransmission system. These systems, based on actual systems in Vermont, span a range of applications that are representative of the types of primary distribution and subtransmission systems encountered throughout the rural and suburban portions of the U.S. interconnected power system where WECS installations are most likely. The small city subtransmission system is shown in Fig. 4, and the distribution feeder is depicted in Fig. 5. To permit an accurate analysis of system stability when WECS are incorporated into a utility power system, the various types of conventional units and their controls were modeled for a realistic simulation of power system transients resulting from both wind variability and system faults. A large-scale transient stability program used to analyze power system performance was utilized for the analysis. To represent the wind turbine, a new prime mover model consistent with others used in the program was developed. Standard models were already available for the generator, the excitation system, the power system stabilizer, and other standard utility system components. Results of the analyses indicated that in general: (1) The synchronous generator is preferred to the induction type for utility class WECS because of superior voltage stability under moderate to severe wind gust conditions. (2) Very severe, low probability of occurrence (0.1%) gusts can cause large voltage variations or even loss of synchronism; however, the annual outage time due to disconnection is expected to be well within acceptable utility limits. (3) Practical WECS clusters can be configured using existing power equipment and utility practices and by observing guidelines for connecting line resistance and reactance. (4) Transient stability response to faults was good due to the high inertia of the WECS unit relative to conventional generating units. When WECS were analyzed for installation on a distribution feeder, the results also indicated the following: (1) The application of two WECS units to the same distribution feeder produced only a small compounding of the voltage fluctuations. No additional intermachine modes of oscillation were indicated. (2) In the presence of severe gusts, voltage fluctuations as high as f. 10%can be expected. Such fluctuations would certainly be objectionable if sustained for several cycles or if frequent in occurrence. (3) Application of WECS to distribution feeders appears feasible but will require careful
Fig. 4. Small city 46 kV subtransmission
system.
R. E.
MEIER
and S. L.
MACKLIS
\(Y 1.5 MW ?’ +WTG2 ‘8 t f 12.5 KV - , - W75 4 KV-
1.5 MW T\
; 0.5 MI #2 I ‘8
w30
w20
w40 1 MI 4/O
k ,
w50
,
W60
w70
we0
1
_-I 1 MI 4/O
1 MI Ii0
1 MI 110
1 MI 110 0.970
1.028 Wlcf
w15
3
10 MVA
// / / v = 1.10
1.046 t
0.963 46 KV
1 MVA 0.92 pf
0.92 pf
NOTE: FEEDER
NO. 2
0.92
VOLTAGES
0.92 pf
SHOWN ARE WITHOUT
1 MVA 0.92 pf
WECS UNITS
5 MVA -0.92 pf
1.& 12.5 KV
Fig. 5. Representativedistribution feeder.
coordination of voltage regulating devices as well as protective equipment for the WECS and the distribution system. These results indicate that there should be no major problems in applying WECS to utility power systems but that design of the WECS and utility control and protective subsystems must receive adequate attention to ensure safe, cost-effective use of this type of generation equipment. WECS CONTROL
AND STABILIZATION
IMPROVEMENT
The typical WECS systems analyzed in the study used existing control and stabilization techniques and were lightly damped with long settling times. Although the WECS exhibited stable performance with steady wind and under moderate gusts including wind shear and tower shadow effects, it showed large voltage and power angle oscillations with severe gusts. Results of the study indicated that improved WECS control systems could provide greater damping.for natural rotor oscillations. Several approaches were investigated for improved stabilization of WECS in a gust environment. The prime considerations were ability to prevent unacceptable oscillations or shutdowns, energy efficiency, development status, and life cycle cost. Only techniques with adequate stabilization capacity to prevent shutdowns were considered. Evaluation of the options was therefore based on the remaining factors. One approach, energy storage, does not appear promising for enhanced control at this time, based on the best understood electrical and mechanical devices-lead-acid batteries and advanced flywheels. Their high cost is partially due to requirements for rapid sensing, high-energy discharge rates, and the relatively short life due to the large number of operational cycles required. Even if limited to responding only to moderately severe gusts (l%), over 40,000 cycles per year could be expected in a typical 18 mph wind site. This is approximately the cyclic life of the best of presently available relays or of clutches that must start and stop large amounts of electrical or mechanical energy. Several advanced control techniques or improvements in present techniques were identified that might be applied to improve system damping and transient response to wind gusts. These include the following: Shaft dumper. Connecting the rotor and generator through a high loss coupling could provide isolation of the electrical system from wind disturbances. Use of this device has been proven to be effective; however, it also implies a continuous loss and lower system efficiency. Optimized power system stabilizer. Based on work performed for the NASA/DOE MOD-l Wind Turbine Generator, it appears to be possible to obtain significant increases in damping by redesign of the conventional power system stabilizer. The stabilizer operates to phase match the transfer function from speed-to-voltage reference-to-electrical torque through the stabilizer and excitation system. This closed loop drivetrain exhibits an increase in damping over open
Interfacing wind energy conversion equipment
23
loop behavior. A responsive excitation system is generally necessary, and only the fundamental on-line drivetrain frequency damping is modified. Wind feed fonoard, This concept utilizes wind speed measurement upstream of the rotor to provide an anticipatory gust sensing control signal. A sensor ahead of the rotor measures the transient speed increase (characteristics of gusts) before the main force of the gust reaches the rotor. It operates outside the closed loop power control and ideally would control the rotor wind-to-torque gain to provide constant torque into the drivetrain. Practically, lags are involved in the rotor torque control mechanism and wind sensing means, and the wind sensed is not exactly what the rotor sees. The lags can be compensated for by sensor location, and significant reduction in variational torques from the rotor can be achieved. These approaches offer the promise of improved WECS unit control and utility system protection through the development of control devices for the WECS unit itself. However, there are many control and protection issues facing the application of WECS to utility networks, particularly on distribution feeders, where the presence of WECS may represent a significant source of local generation capacity. Control and protection of the WECS unit is therefore only part of the larger challenge of control and protection of the WECS/utility system. PROTECTION
ISSUES
FOR WECS ON UTILITY
DISTRIBUTION
SYSTEMS
To
achieve economic, reliable, and safe operation of utility distribution systems, utility planners regularly address system protection and control issues. Design strategies have evolved around a radial design, where any point on a distribution feeder is supplied from only one direction. Should trouble occur, protective equipment isolates the system from the trouble. Because the system is radial, this isolation can be accomplished by interrupting at one point only, thus isolating as few customers as possible. Utility system designers have achieved high levels of service reliability on distribution systems utilizing an approach of coordination and integration of several equipments with the characteristics of the radial design. Integration of WECS units (or any dispersed power source) into existing electric distribution circuits places additional requirements on the protection and control subsystem due to the siting of the WECS on the distribution system. Transmission and subtransmission networks can readily accept generation at many locatidns. Distribution systems are, by contrast, not designed to accommodate power sources due to the unidirectional flow of electricity and the protective and control systems designs based on that principle of radial design. In the immediate future, early demonstration projects will require that existing protection hardware be utilized to integrate WECS in a safe and reliable manner. In the long run, utility designers need to focus on protection and control technology options that will provide economic and safe ways to fully integrate WECS into distribution systems while maximizing utilization of WECS potential capabilities. Designing effective protection subsystems for WECS on utility systems represents a relatively straightforward technical task. The cost of such protective subsystems for large WECS (1 MW or larger) appears acceptable. However, protection subsystem costs for small residential and commercial class machines may be high relative to the unit cost itself. Automated distribution systems appear to offer means to more effectively control, protect, and operate distribution systems containing WECS with low-cost comprehensive communication and control systems. Fortunately, automated distribution system technology programs are aggressively exploring ways to provide this protection and control economically using microprocessor-based digital technology. The principal protection and safety issues that must be addressed for integrating WECS into utility distribution systems are summarized in Table 1. The most important of these issues are discussed below, along with descriptions of protection subsystem designs utilizing current equipment. Overcurrent protection Except for network systems, time overcurrent, radial, coordinated protection systems have been the most common design used on electric utility distribution systems. This radial configuration requires the coordination of several types of equipment, including breakers, reclosers, sectionalizers, and fuses. Since a distribution system has neither the need nor the
R. E. MEIERand S. L. MACKLIS
24
Table 1. Distribution protection and safety issues for WECS. overcurrent
Coordination equipment
Fault
Fault current interruption Bidirectional flow Ground fault current Short circuit duty
current
WBCS machine duty Energizing circuit Coordination with h'ECS protection
Reclosing
Dead
circuit
Voltage
of protective
energization
protection
Transients--lightning or switching surges Machine voltage transients VAR supply switching Ferroresonance Machine transients Stability
Frequency
Self-excited
Islanding Converter
Safety Start-up Synchronizing
control
Current
operation
and voltage
control
Synchronizing
Frequency on/off Machine duty Inrush currents
cycling
Harmonics
Relay operation Telephone interference Filtering
capability to sense the direction of overcurrent flow, only the magnitude and time are measured and used as a basis for circuit interruption to isolate the faulted line section. When a dispersed generating device such as a WECS is sited on a distribution feeder, it provides a source of electric power and voltage that may be located downstream from a potential fault. A one-line diagram of a simple distribution circuit with a WECS installation is shown in Fig. 6. In this case, current would be provided to the fault from more than one direction, thus violating the normal utility distribution protection design principles of radial design and unidirectional flow. Early WECS installations will probably require that the unit automatically (with manual backup) disconnect itself from the utility system and lock out whenever fault current is sensed at its terminals. Since service reliability at the feeder and safety to the crew dominate the individual service and energy produced by a WECS, utility practices will probably require that the system be protected from the WECS. More expensive schemes such as transfer tripping may be necessary. Longer term solutions involving automated distribution systems, which would incorporate smart remote terminal units on distribution feeders, will allow maintenance of safety and reliability while minimizing the time and automating the process of the restoration of generation by the WECS after the fault is cleared. Fault current Utility systems with WECS will require detection of all faults, either line to line or line to ground, and interruption at the unit. Normal utility system design practices will continue to require the ability to interrupt all faults at the distribution sutstation or wherever the normal feeder protective equipment is placed. Of particular significance will be single phase-to-ground faults near a WECS that is connected to the distribution system via a delta-wye transformer. A single line-to-ground fault will be detected by an upstream protective device, but unless special steps are taken at the WECS (i.e., transfer tripping coordination with substation breakers), the fault will not be sensed at the unit because of the ungrounded primary of the delta-wye transformers. If the upstream device that cleared the fault recloses, WECS out-of-phase synchronization might occur. If the
Interfacing
l-
--
wind energy conversion equipment
-
1 I
I
I I
I
I
I I
Cl
I I I
‘4.16
I I I I---_.
#I
I
WECS
I
KV
!
CJ __.
I
I I I I
C7 -_
DISTRIBUTION SUBSTATION
I I
=
I I
2s
-_
#2 --
t
---
-I
WECS
3 - PHASE PRIMARY
1
1 -PHASE
LATERAL_ DISTRIBUTION TRANSFORMER *
r------i
SERVICES
_--_-
Fig. 6. Sample distribution circuit with WECS.
fault is permanent and the upstream device remains open, the circuit will remain energized by the WECS, creating a possible safety hazard to crewmen. While today’s distribution system equipments are nondirectional current detecting, the size and number of WECS on a distribution circuit may require that more expensive directional protective equipments be specified. A typical protection subsystem design for a distribution system with large WECS is shown in Fig. 7. This type of equipment is commonplace on transmission lines but would represent a cost increase when applied to distribution systems. Reclosing Many utility distribution systems are now designed for automatic reclosing sequences after an initial fault has been detected and interrupted. Since many faults on distribution systems are temporary, automatic reclosing provides a reliable technique that will frequently restore customer service after a momentary outage. Not uncommonly, a circuit may be automatically reclosed three times after initial fault interruption before the circuit breaker locks out, requiring a crew to investigate before service can be restored. Should WECS be out on the circuit at the time a fault occurs, the distribution substation feeder breaker will interrupt the circuit and then could automatically reclose out of synchronism with the WECS. This reclosing onto a synchronous machine, an induction generator, or an a.c./d.c. converter could place a severe duty cycle on the WECS. In the case of the synchronous machine, large mechanical torques can be exerted upon the WECS rotor, possibly causing damage to the rotating machinery. Significant inrush currents might occur, be interrupted as fault currents after reclosing, and cause the same or another protective device to operate. While the service primary feeder breaker is open, the WECS keep the feeder energized, thus sustaining the fault current flow. Reclosing of the source primary feeder breaker will be unsuccessful unless the fault current has been interrupted by completely deenergizing
R. E.
MEIER andS. L. MACKLIS DISTRIBUTION
LEGEND 51 51N 67 67N
-
PHASE GROUND PHASE GROUND FAULT RELAY
TIME-OVERCURRENT RELAY TIME-OVERCURRENT RELAY DIRECTIONAL TIME-OVERCURRENT DIRECTIONAL TIME-OVERCURRENT CURRENT TRIPPING DIRECTION
RELAY RELAY
Fig. 7. Typical directional overcurrent relaying design.
the primary feeder section. This requires opening of the source primary feeder breaker plus simultaneously disconnecting all WECS. As indicated above, present utility practices generally require that the WECS be automatically disconnected from the system from the time the circuit breakers at the substation initially open to clear a fault and before the circuit breaker is scheduled to automatically reclose. Approaches to meet this requirement, although more expensive, may involve synch check or synchronizing relays and transfer tripping via direct communication from the WECS to the substation protective equipments. Energizing dead circuits To assure crew safety, the most significant requirement by electric utilities is that the WECS be prevented from improperly energizing a dead distribution circuit. If the WECS were to be reenergized and connected into the system without proper authorization, hazardous conditions might result. This safety consideration will probably require automatic lock-out switches with manual backup- to electrically separate the WECS from the distribution circuit. Voltage protection and frequency control In general, utility distribution systems are designed to protect equipment in the face of any abnormal voltage on the distribution circuits, either on the primary or the secondary. One voltage-related protection issue arises from rotating machinery such as induction generators on large wind machines, as discussed before. They are susceptible to voltage fluctuations as a result of shaft torque transients caused by wind gusting or other sporadic power inputs to the WECS. Even in the case where the voltage swing magnitude may be low, if the frequency of these voltage excursions is excessive, then customer service will be degraded due to voltage flicker. If the voltage swing is excessive, then equipment and safety problems may result. As discussed before, synchronous generators are preferred for large WECS utility installations. However, improvements in WECS control systems to minimize voltage fluctuation magnitude and frequency are needed.
Interfacing wind energy conversion equipment
?I
Many small WECS units now being considered for use require inverters to convert the d.c. power output to the 60Hz power of the utility distribution system. The commutation of a.c. inverters requires a supply of VARs sized to the needs of the WECS. Should the WECS unit be off line, either due to low wind or mechanical failure, the VARs being supplied locally by capacitors would remain connected on the distribution system, and the voltage control provided by substation voltage regulators, load tap changing, or switched capacitors might be disrupted. If these capacitors are not disconnected from the system when the WECS is, this may interfere with the operation of the voltage regulating equipments on the distribution system. And with the presence of WECS, in particular with induction generators, the conditions exist for a possible resonant circuit between the machine inductance and system capacitors. Overvoltage transients would then occur when switching those capacitors as part of feeder voltage control. Utility concern with voltage control on distribution feeders may require that existing equipment, such as surge arresters, be increased in their ratings or possibly that special voltage control requirements be placed on the WECS unit to assure satisfactory and safe voltage control. The protection of utility distribution systems from off-nominal frequency operation may be complicated on distribution systems that have WECS units with synchronous generators. Where several synchronous machines are operating in parallel, the potential for machine instability or hunting may exist. Protection against hunting may also be required. Islanding and self-excitation When a substation breaker interrupts a distribution feeder from the system, it may create an electrical island that is energized by the WECS devices on that feeder. If sufficient system capacitance exists, induction machines will self-excite and continue to operate at off-nominal voltage and frequency. Depending upon the specific amount of system capacitance and machine inductance, the frequency and voltage will rise or fall from their nominal values. Therefore, when a circuit is deenergized at the substation, all induction machines, no longer having line excitation, would be tripped off line. Since the safety and operational concern with off nominal voltage on the distribution circuit dominates, utility system practices will require protective devices to prevent self-excited operation of the induction machines or installation of line commutated inverters. Power conditioning and harmonic control The characteristics of the power conditioner will often be as important in their effects on the power system as the characteristics of the WECS itself. There are a wide variety of circuit designs available for converting the d.c. potential of small WECS to the a.c. potential required by utility systems. Converters may be line commutated or self-commutated, and the amount of harmonic filtering is often critical. Short circuit currents, harmonic content of the output, and technical features of the converters will have a great bearing on the requirements of the distribution protective and control system. D.c./a.c. inversion associated with some WECS technologies means the use of inverter devices that produce harmonics of 60 cycle. Should harmonic voltages and currents be excessive, a potential exists for the misoperation of relays as well as, the propagation of electromagnetic noise on the distribution system. To the extent these problems interfere with the quality of service or cause misoperation of system protective devices, the presence of harmonics caused by inverters must be designed for. Current utility practices for high-voltage d.c. applications or arc furnaces use extensive harmonic filtering equipments to trap the harmonics generated by the inverter. Utility systems may require that similar protective equipment be put in place to interrupt a.c. inverters producing excessive harmonics, or possibly that the WECS unit include harmonic filters as an integral part of the generating device. ADVANCED
PROTECTION
TECHNOLOGIES
FOR USE WITH WECS
Present protection and control approaches for interconnection of dispersed generation and storage sources, such as WECS, to distribution feeders are based on local sensing of conditions, utilizing groupings of component relays. Such protective equipment experience is based on industrial and commercial installations of generators driven by diesel engines or small gas- or steam-driven turbines. Each component relay utilizes electromechanical or solid-state analog
R. E. MEIERand S. L. MACKLIS
28
technology. Compared to digital techniques, these component relay technologies have limitations in sensitivity, speed, memory, and logic capability. Technical and economic advances in digital electronics technology are making possible new opportunities for integrated protection, control, and monitoring of the distribution system. These new technologies offer potential advantages such as improved utilization of power system facilities and cost effectiveness, improved system operation and lower operating costs, more complete and timely information for system planning, and reduced outage duration. Automated distribution application areas include the distribution substation, feeder, and user level, and distribution communications. Automated feeder sectionalizing for fault isolation and service restoration is one of a number of distribution functions being implemented in automated distribution systems. Improved utilization of substation and feeder facilities with opportunities for deferring new system investment offers the potential for significant economic savings through improved monitoring and control. Load management and dispersed storage and generation offer opportunities for significant savings in possible deferral of new generation and/or transmission capacity. A functional diagram of several WECS units is shown in Fig. 8, with the components of an integrated digital control/protection subsystem. The control/protection subsystem would feature digital sensing; advanced analog relay techniques; digital relaying; and communication of data, status, and control commands. Examples of the applications of these advanced technologies include (1) digital measurement of directional power flow between the WECS installation and the utility, compared with status changes of circuit breakers and sectionalizing devices on associated feeders and substations; (2) transfer tripping of the WECS when the connection to the utility distribution system is interrupted by opening of circuit breakers and
00
Generating Plant Utility Energy and Operational Management Systems (EMS)
Step-up Transformers
L______U_______-__-_-------_----__---_-A
Subtransmission System Distribution Substation
1
/
I I I 1
I
I
-
**
Large WECS Unit (l-5 MW)
v Feeder Sectionalizing Switches
Primary Circuits WECS Cluster (3-25 MW)i L--______
Distribution Control Unit (DCUI
iPhase Primary Main
_______._; 1-Phase Laterals Distribution Secondaries J
+ Services
1, 22ov 19
Fig. 8. Functional diagram of WECS units integrated into a distribution system.
Interfacing wind energy conversion equipment
29
sectionalizing devices; (3) communications between WECS units, changing protective characteristics as a function of total WECS capability connected to a feeder (or substation) vs load actually being served by the associated feeder (or substation); (4) sensitive measurement and trend change of unbalanced line-to-ground voltage conditions where WECS units connect to a supply feeder; (5) use of an automated distribution system to locate, isolate, and restore service after a feeder fault, including reconnection of unfaulted feeder sections to appropriate alternate feeder sources; (6) disconnection of WECS units via transfer trip over communication channels when one or more remote distribution switching devices opens the utility supply to feeder connection; (7) isolate WECS connections, or inhibit reclosing, when a sensed change in 63 Hz occurs at the point of utility connection, indicating that frequency is being determined by the WECS rather than the utility. The amount of control command information to be transmitted from the control center to the WECS location and the data to be returned from the WECS to the control center will vary with the size of the WECS unit and the individual utility practices. The kind of communication equipment that can be utilized includes the communication link with the Distribution Control Center (DCC), the interface and control equipment located at the DCC that enables the distribution dispatcher to interact with the remote WECS, the interface and control equipment which is located at the WECS unit and interfaces the communication link and the WECS local control equipment, and the system/WECS power protection interface equipment that enables the WECS power equipment to operate compatibly with the utility distribution network. CONCLUSIONS
Based on results of studies conducted to date, the following conclusions have been drawn in this paper: (1) There does not appear to be any major technical interface problem in applying WECS to utility power systems. (2) Significant deviations in WECS voltage and frequency due to severe wind gusts will probably require automatic disconnection of the WECS unit from its utility connection. However, the occurrences of disconnection and resynchronization will be few compared to those due to low wind conditions and will represent a negligible loss of annual energy output. (3) Application of existing utility design practices and equipment can provide necessary operation, control, and protection functions for the WECS and the utility. However, the cost of providing these functions for smaller WECS installations may be prohibitive. (4) Improved WECS unit controls and integrated utility control and protection systems utilizing digital electronics technology offer improved economic potential for all classes of WECS installations. For small, user-controlled machines, these developments could be critical to their economic success. Thus, it appears that the technical interfaces between WECS units and utility systems present problems that are solvable today. As more solar technologies approach commercial status and become significant factors in utility planning, the experience provided by earlier technologies such as WECS will help to smooth their entry into utility operations. Acknowledgements-The authors would like to thank Mr. Robert Moisan of GE’s Digital Systems Operation for his several contributions to the discussion on WECS protection and control subsystems, and to Mr. Roy Schaeffer of GE’s Advanced Energy Programs Department for his assistance in reviewing this paper.
REFERENCE 1. Department of Energy, “Systems Dynamics of Multi-Unit Wind Energy Conversion Systems Application”, Rep. DSE-2332-T1,2, 3.