Interconnecting a weak AC system to an HVDC link with a hybrid inverter

Electric Power Systems Research, 14 (1988) 121 - 128

121

Interconnecting a Weak AC System to an HVDC Link with a Hybrid Inverter K. S. TAM

Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (U.S.A.) W. F. LONG and R. H. LASSETER

University of Wisconsin, Madison, WI 53706 (U.S.A.) (Received September 8, 1987)

SUMMARY

To overcome the weaknesses o f the conventional HVDC converter for weak A C system applications, the concept o f the hybrid inverter has been introduced recently. Because o f its unique characteristics, the hybrid inverter not only can implement the existing control methods but also can support the development o f more innovative control strategies. This paper reports the major results o f a study that investigated the performance o f an H V D C link interconnected to a weak A C system with a hybrid inverter using an unconventional control scheme. The results show that the hybrid inverter improves the overall D C / A C s y s t e m behavior and is effective in operating an HVDC link into a weak A C system.

INTRODUCTION

The use of DC systems for electric power transmission has grown significantly in recent years because of their functional, economic and environmental advantages. The growth of HVDC is n o t only in terms of an increase in the a m o u n t of gigawatts but also in terms of a wider range of applications. Besides being used for long distance transmission, submarine transmission and back-to-back links, applications such as the use of HVDC technology in urban areas, tapping energy from HVDC lines to supply relatively small power systems and multiterminal HVDC systems have been seriously considered. There is an increasing number of cases in which the capacity of the DC system is comparable with that of the AC system. These 'weak' AC 0378-7796/88/$3.50

systems, usually characterized by large system impedances ' make the operation of DC/AC conversion difficult for the conventional converter. The conventional converter has other shortcomings as well, such as the requirement of a considerable a m o u n t of reactive power and the coupling between real power and reactive power. In view of the potential of DC transmission and the inadequacy of the conventional converter, the concept of the hybrid converter has been introduced recently [1]. Basically, a hybrid converter consists of a naturally c o m m u t a t e d converter (NCC) and an artificially c o m m u t a t e d converter (ACC). A hybrid converter allows independent control of real and reactive power within certain operating boundaries. This capability can be utilized to improve the AC/DC system performance via two control strategies. The first strategy, which is implemented in this project, is to reduce the reactive power demanded by the DC/AC converter to zero. In the absence of reactive power, the overall DC/AC system performance is improved and the operation tends to be more stable. Rapid changes in real power or power modulation can be carried out w i t h o u t worrying about the effects of the corresponding changes in reactive power. There is no need for VAR compensation. The filters are no longer required to supply reactive power and to carry current at the fundamental frequency. This will result in lower cost and losses, and will contribute to the reduction of dynamic overvoltages. Another control strategy will be to utilize the reactive power to regulate the AC bus voltage. Since the real power and reactive power can be changed independently, the © Elsevier Sequoia/Printed in The Netherlands

122 reactive p o w e r required to keep the bus voltage constant can be predetermined such that the same bus voltage can be maintained throughout a substantial change in real power. With its flexibility of reactive power control, the hybrid converter is capable of interconnecting a DC system to a weak AC system without auxiliary equipment such as a synchronous condenser or a static V A R compensator. At the current state of technology, the NCC is best implemented by the six-pulse Graetz bridge using thyristors as the switching elements. The conventional converter, which is a twelve-pulse converter, consists of two NCCs fed by a w y e - w y e and a w y e - d e l t a transformer, respectively. A survey of power electronics suitable for HVDC applications reveals that the capacitively c o m m u t a t e d current-sourced converters are either technically or economically better than the other alternatives for the implementation of the ACC [2]. Compared with the NCC, an ACC implemented with a capacitively c o m m u t a t e d circuit generally results in higher equipment ratings and higher harmonics. It was shown that the major equipment cost of a hybrid inverter could be comparable to that of the conventional converter with a synchronous condenser [2]. Among the capacitively comm u t a t e d circuits, two circuits can fulfill certain performance criteria with the lowest cost [2]. Both are candidates for the implementation of the ACC at the current state of technology. This paper reports the major results of a further study on interconnecting a weak AC system to an HVDC link with a hybrid inverter. The impact of the hybrid inverter on the overall DC/AC system interactions is the main aim of this investigation. Although the hybrid converter can operate into AC systems with very low short-circuit ratios {SCRs), the degree of system 'weakness' as indicated by the SCR value is not important at this point because the main goal is to examine the improvement in system performance. In this study, the AC system has an SCR of 1.78. CONTROL STRATEGY Direct current control and extinction angle control are the basic ingredients of

the conventional DC control system. A low voltage dependent current limit (VDCL) may be added to improve recovery performance following AC system faults at the inverter end. An inverter operating with minimum extinction angle control exhibits a negative impedance characteristic. The weaker the AC system the more severe is this negative impedance effect which contributes to instability. Stability is significantly improved if the inverter exhibits a positive impedance characteristic under normal operating conditions. A positive impedance characteristic may be obtained by implementing constant reactive current control at the inverter [ 3 ]. The basic characteristics of a DC converter may be extended to provide performance improvement. The converter can act like a localized AC voltage controller by regulating the reactive power consumed in response to the AC voltage variations [4]. The operating range can be extended into the third quadrant by the addition of capacitive reactive power sources at the AC busbar [5]. With the damping provided by real p o w e r modulation, the amount of real power transfer can be increased [6]. Coordinated modulation of both the real and reactive power would compensate for the undesirable effects caused b y the coupling between the real and the reactive power. Control schemes are closely related to circuit characteristics. Any of the existing strategies can be implemented b y the hybrid inverter. However, since the hybrid inverter has some unique characteristics, it allow the development of more innovative control strategies. One simple strategy is called the 'zero Q control'. According to this control scheme, the reactive power demanded by the NCC is always supplied by the ACC. The DC system imposes no reactive p o w e r requirement on the AC system under all operating conditions. Problems associated with reactive power consumption are eliminated. Real power modulation can be effectively carried out. The AC filters are no longer required to provide reactive power at the fundamental frequency and consequently the cost and losses of the AC filters can be reduced. The effects caused by these filters during transient conditions are reduced as well.

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Fig. 2. Schematic diagram of the hybrid inverter control.

Besides zero Q control, the hybrid inverter also regulates the DC voltage at the inverter end. The characteristic of DC voltage control is shown in Fig. 1. During normal operating conditions, the rectifier performs constant current control and the hybrid inverter performs constant voltage control which, with a zero impedance characteristic, contributes to stable operation. The control scheme for the hybrid inverter is shown in Fig. 2. Under normal operating conditions, the hybrid inverter performs DC voltage control. When the rectifier cannot perform current control, the hybrid inverter will give up voltage control and assume current control. A selection circuit determines which control the hybrid inverter will assume b y generating a signal which closes the switch on either path. The error signal from either the voltage control path or the current control path is the input to a pair o f integrators, the o u t p u t s of which are the firing angles for the NCC and the ACC. The o u t p u t of each integrator is b o u n d e d b y a pair o f limits that determine the range of operation. Zero Q control is implemented b y delay firing the NCC and advance firing the ACC

by the same firing angle. Any change in the firing angles for the ACC and the NCC would be equal in magnitude. Thus, the integrators have gains that are equal in magnitude b u t are opposite in sign. Unless a waveform synthesis technique is employed, an ACC can operate at unity displacement factor at only one operating point, for a certain DC current level. Waveform synthesis, which has been realized by high frequency switching for relatively low voltage and low power applications, is difficult to implement for HVDC applications at the current state of technology. Implementing the zero Q control by advance firing the ACC and delay firing the NCC b y the same amount is a simple b u t approximate approach because the effects of commutation overlap on circuit operation are not included. The effects of c o m m u t a t i o n on the ACC are that its AC current o u t p u t magnitude is reduced b u t its phase angle with respect to the bus voltage is increased. The effect of c o m m u t a t i o n on the NCC is that the phase angle of its AC current o u t p u t is increased. The c o m m u t a t i o n effects on the ACC compensate for the c o m m u t a t i o n effects on the NCC, although the degree of cancellation depends on the operating condition. In the simulation study discussed below, the displacement factor at the rated condition is 0.9996 leading. Displacement factors very close to unity are also found at other operating points. Thus, although it is an approximate approach, implementing the zero Q control by advance firing the ACC and delay firing the NCC by the same amount is effective in practice. Current or voltage control are effective in handling steady-state operation. Start-up and fault recovery are handled b y manipulating the integrator limits. For example, upon the detection o f a transient situation the upper limit would be set equal to the lower limit so that the firing angle can be controlled instantaneously b y assigning values to the limits. A set of limits is prepared for each event that calls for fast control action.

SYSTEM MODEL

With the advent of c o m p u t e r technology, digital simulation has become a convenient

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Fig. 3. Schematic diagram of an HVDC/AC system i n t e r c o n n e c t i o n with a hybrid inverter.

Harmonic filters for the .5th, 7th, l l t h , 13th and high frequency harmonics are provided on the inverter AC bus. These filters are specially designed so that they do not carry current at the fundamental frequency. The series L - C branches carry out the filter function. The parallel L - C branch forms an infinite impedance path at the fundamental frequency. The DC link is supplied through a w y e wye and a w y e - d e l t a transformer on the rectifier side for twelve-pulse operation. For a hybrid inverter implementing the zero Q control, the phase angles between the AC current outputs from the ACC and the NCC are not necessarily 30 °. The 30 ° phase shift between a w y e - w y e and a w y e - d e l t a transformer would not achieve twelve-pulse operation but would add unnecessary complexity to the control. The hybrid inverter is fed from two w y e - w y e transformers.

START-UP

and inexpensive tool for the studies of power systems including HVDC links. A comparison of the test results from digital simulation with those from the actual system shows that digital simulation is a valid technique for the study of DC/AC system interaction [7]. In this study, the Electro-Magnetic Transients Program (EMTP) has been used as the simulation tool. Figure 3 shows the inverter end of the system being modelled. The AC system, represented b y its Th~venin equivalent, has a short-circuit ratio (SCR) of 1.78 and a damping angle of 80 °. The AC system on the rectifier side is a strong system. The DC system is a back-to-back monopolar link with a rating of 450 MW (180 kV, 2500 A). The ACC is implemented b y circuit 6 mentioned in ref. 2. It consists of a main circuit which is a six-pulse Graetz bridge and an auxiliary circuit which has two pairs of thyristors and a capacitor. Commutation in the main circuit is performed b y transferring the current between the main circuit and the auxiliary circuit with the commutation voltage supplied by the capacitor. The control system and its interactions with the power system are simulated b y using the TACS (Transient Analysis of Control Systems) feature of EMTP.

The start-up procedure is employed during the initial system start-up and the recovery from severe faults such as DC faults and threephase faults at the AC bus. The injection of current into the AC system affects the AC bus voltage, both in the magnitude and phase angle, and consequently affects the commutation process of the NCC. Since weak systems are easily disturbed, the start-up o f a DC link into a weak AC system is a challenge. The philosophy of zero Q control is implanted in the start-up procedure. This implies that the ACC needs to be started in the third quadrant directly. The start-up process is carried out at a low power level. After the current is established and the c o m m u t a t i o n capacitor is properly charged, the DC current and the DC voltage are then brought up to their rated values as fast as the AC system allows. Figure 4 shows the start-up process. The system is brought up smoothly from zero power to the rated steady-state condition in about 80 milliseconds. A set of limits on the firing angles is prepared for the start-up process. The charging of the commutation capacitor impedes the current build-up process, especially if the capacitor carries no initial charge. To facil-

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Figure 5 shows the system response to a step reduction of 0.2 p.u. in the rectifier DC current order. The DC current changes smoothly to the new steady state. The DC voltage is kept constant by the inverter control. The change in DC current does not introduce appreciable AC voltage disturbances. The system response to a 5070 drop in AC bus voltage on the rectifier side is shown in Fig. 6(a). The drop in rectifier voltage causes a drop in DC current as well as DC voltage. When the DC current drops below 2070 of the rated level and the DC voltage drops below 1570 of the rated value, the hybrid inverter is signalled to switch to current control. The DC current is restored smoothly to the level determined by the inverter current order (in this case, 0.9 p.u.). Zero Q control is maintained throughout the transient conditions. The current injected into the AC system is reduced. System operation is stable and system response is fast and smooth. In Fig. 6(b), the rectifier-side AC bus voltage recovers at t = 0.25 s and consequently the DC voltage and the DC current begin to increase. When the AC voltage rises above a certain level, the rectifier begins to exercise some control. When the inverter DC voltage increases to within 1570 of the rated level and the DC current level is b e y o n d the rated value, the hybrid inverter is switched back to voltage control and the rectifier completely takes over the current control.

FAULTS

Figure 7 shows the system response to a DC line to ground fault across the inverter terminal. Upon the detection of this t y p e of fault, the corresponding set of firing angle limits would be issued. The hybrid inverter is blocked and the rectifier is driven into the inverter mode. After the DC current drops

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to zero, the rectifier is blocked. During this blocking stage, the integrator limits are reset to values prepared for restart. After the fault is cleared, the whole startup procedure is repeated. When the ACC is blocked, the commutating capacitor is not discharged and there" is a residual charge left on the capacitor during the fault. The partially charged capacitor enhances the commutation process and the restart behavior is improved.

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Single line to ground faults in the AC system may be handled in several ways. The DC system may be blocked and restarted. The AC/DC system may be designed to ride through the fault (i) by changing the control from equidistant firing to individual phase control; or (ii) by the action of the VDCL control which reduces the current to a lower level during the fault and, after the fault is cleared, restores the current to the original level at a rate determined by a built-in time constant; or (iii) by reducing the firing angle to maintain circuit operation. A combination of the third approach and a modified first approach is employed by the hybrid inverter. Figure 8 shows the system response to a single-phase (phase A) line to ground fault at the AC bus on the inverter side. The fault causes commutation failures in the NCC, a surge in DC current, and substantial overcurrent in the faulted phase. Since the NCC is out, zero Q control can no longer be carried out. Upon recognition of this fault, the limit generating circuit provides the corresponding sets of limits which reduce the DC voltage and the AC currents in the healthy phases. System operation is maintained at a reduced power level until the fault is cleared (in this case, after 3 cycles). Shortly after the fault is cleared, the limit generating circuit restores the limits to their steady-state values. After some disturbances, the bus voltage in the faulted phase gradually

127

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Fig. 8. System response to a single line to ground fault (phase A) on the inverter side.

recovers. The system steadily settles down to the original steady-state operating condition. Performance comparisons among different systems are difficult because the system characteristics are different and t h e y may be operated under different constraints. An observation is made here. It can be seen from Fig. 8 t h a t recovery to 90% of the prefault power occurs around 70 ms after the removal of the fault. No c o m m u t a t i o n failure is observed. During a commissioning test for the 200 MW Blackwater back-to-back system, recovery to 90% power from a single line to ground inverter fault required 300 ms [8]. In this case, the short-circuit capacity at the inverter is 600 MVA, and so the SCR is 3.0. The response of the Blackwater inverter is slowed to maintain voltage regula-

The cases reported in this paper are a representative subset of a series of tests designed for the investigation of the interactions between a hybrid HVDC inverter and a weak AC system. The results demonstrate that the hybrid inverter can operate an HVDC link stably into a weak AC system and that the overall system performance is improved. Besides its technical advantages, the use of a hybrid inverter also reduces the requirement for the filters, eliminates the need for auxiliary equipment such as synchronous condensers or static VAR devices, and therefore contributes to reduction in system costs and losses. The zero Q control strategy used by the hybrid inverter is the main reason for the stable operation. The artificially c o m m u t a t e d converter can operate w i t h o u t reactive power interchange with the AC system at only one operating point and thus, by itself, cannot perform zero Q control over the entire operating region. Although the artificially commutared converter, because of its self-commutated capability, can always maintain proper circUit operation a n d is quite invulnerable to the AC system disturbances and transients, the results of a simulation study show that with the artificially c o m m u t a t e d converter operated at a nonzero displacement factor, it takes longer for the system to settle down and the overall operation is less stable. Although zero Q control is found to be effective, other modes of operation can also be performed by the hybrid inverter. For example, the hybrid inverter could be operated in the voltage control mode to regulate the AC bus voltage at various power levels [1]. There is the possibility of using different control modes to handle different situations such that the overall system operation can be optimized. Making use of its capability to control real and reactive power independently, control or modulation schemes can be designed to improve AC system performance.

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The hybrid inverter has some shortcomings. The use of the naturally commutated converter reduces cost and losses but additional control needs to be provided to accommodate its vulnerability to AC bus voltage disturbances. The commutation capacitor voltage has an important impact on the overall circuit performance and equipment ratings. A larger capacitor voltage is needed for a weaker system. Capacitor overvoltages can occur, especially during transient conditions when the DC current changes quickly, and some manner of voltage limiting becomes necessary. The hybrid inverter also generates higher levels of AC and DC harmonics. It is important to differentiate between the concept of hybrid inverter and its implementation. Most of the shortcomings of the hybrid inverter can be removed by improving the circuit implementation, especially that of the ACC. This may be done by using switches other than thyristors or different converter circuit designs. The hybrid inverter is promising for HVDC applications but more research and development work needs to be done.

ACKNOWLEDGEMENT

This study was conducted at the University of Wisconsin-Madison and was supported by Martin Marietta Energy Systems/Oak

Ridge National Laboratory for the Division of Electric Energy Systems of the United States Department of Energy.

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