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Dynamic performance of HVDC system according to exciter characteristics of synchronous compensator in a weak AC system
Electric Power Systems Research 63 (2002) 203 /211 www.elsevier.com/locate/epsr
Dynamic performance of HVDC system according to exciter characteristics of synchronous compensator in a weak AC system Chan-Ki Kim a, Gilsoo Jang b,*, Byung-Mo Yang a a
b
Korea Electric Power Research Institute, Munji-dong, Yusung-gu, Daejon, Republic of Korea NPTC/School of Electrical Engineering, Korea University, Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea Received 31 October 2001; received in revised form 28 March 2002; accepted 15 May 2002
Abstract This paper analyses the dynamic performance of HVDC system connected to a weak AC system for various exciter characteristics of synchronous machines connected at the converter bus. Conventionally capacitors are used to supply reactive power requirement at a strong converter bus. But the installation of a synchronous machine is essential in an isolated weak network to re-start after a shutdown of HVDC and to increase system strength. The dynamic performance of a synchronous machine depends on the characteristics of its exciter. In this paper, several exciter types are used to investigate their effect on the dynamic performance of the HVDC system, and modifications to standard exciter topologies are suggested to mitigate observed problems. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Exciter; HVDC; Synchronous condenser (SC)
1. Introduction Over long distances bulk power transfer can be carried out by HVDC connection than by a long distance AC transmission line. Also, bulk power of HVDC transmission scheme may be transmitted through very long cables or across borders where the two AC systems are not synchronized or operate at different frequencies. HVDC converters (both rectifiers and inverters) draw lagging reactive power from the AC system in the amount of about 60% of the real power. Usually the reactive power is fully compensated at the converter bus to improve system regulation and reduce transmission losses. Variation in DC power changes the reactive power requirement of the converter, hence it tends to cause change of AC voltage. For strong AC system, these fluctuations are tolerable and we can use
* Corresponding author. Tel.: /82-2-3290-3246; fax: /82-2-9210544 E-mail address: [email protected] (G. Jang).
fixed capacitors to provide reactive power. However, these changes are particularly significant if the AC system is weak enough to lead to system instability. Hence we must use dynamically adjustable compensator schemes. Static Var Compensator (SVC), Static Condenser (STATCON) or Synchronous Condenser (SC) are the commonly used dynamic compensator schemes. Nyati et al. have shown that static compensators provide a much faster system response compared to the synchronous compensators. Gole et al. have shown that the use of a proper mix of SVCs and synchronous compensators is the best choice for HVDC systems connected to a very weak AC system. For the extreme case of a receiving system of zero inertia (no generation), a conventional inverter cannot start after even a momentary interruption of DC power. A synchronous machine in the receiving system is generally the only practical solution in such situations [1 /7]. Lee and Kundur have shown that torsional mode instability through excitation control may be caused by a terminal voltage limiter which uses feedback of terminal voltage to control excitation through a very high gain [8].
0378-7796/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 7 7 9 6 ( 0 2 ) 0 0 1 1 6 - 5
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C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
Fig. 1. Conventional AC excitation system.
Fig. 2. Conventional static excitation system.
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
Fig. 3. Test system based on the CIGRE model.
Fig. 4. DC voltage of HVDC according to exciter type with VRmax of 6 p.u.
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C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
206 Table 1 Static excitation system parameters
Table 2 Rotating excitation system parameters
KA TA TB ? Tdo vc
KA TA TEX ? Tdo vc
150 0.1 (s) 0.375 (s) 2 (s) 10 (dB)
The normal operation of a synchronous machine with a static exciter does not usually pose any major problems. However, during AC line-faults or in the case of connection to a weak AC network, a synchro-
37.5 0.01 (s) 0.2 (s) 2 (s) 10 (dB)
KEX ? TgA TF KF VAmax
4 1 (s) 0.1 (s) 0.01 (s) 1.0 (p.u)
nous machine with a fast field forcing bus-fed static exciter has a drawback, which may cause system instability. This is because the output voltage of the static exciter is dependent on the generator terminal
Fig. 5. DC voltage of HVDC according to exciter type with VRmax of 8 p.u.
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
voltage that is controlled by the static exciter. This paper investigates the problem represented above and proposes a solution. Also, a new excitation scheme is proposed in order to eliminate a low frequency oscillation of AC output voltage. The investigation is performed by time domain digital simulation using EMTDC program.
207
2. An excitation system review The IEEE has classified excitation systems currently in use into 12 types. From a system characteristics point of view, excitation system is classified as three types */ DC, AC and static type. Because the DC-type exciters are gradually being phased out, this paper discusses only AC and static type exciters. 2.1. AC rotating excitation system Fig. 1 shows an AC rotating excitation system that consists of a Permanent Magnet Generator (PMG) with controlled rectifier and exciter generator with noncontrolled rectifier. In this system, the exciter is on the same shaft as the turbine generator. The AC output of exciter system is rectified by either a controlled or non-controlled rectifier to produce the direct current needed for the generator field. The rectifiers may be stationary or rotating. This system is more stable than a static excitation system because exciter power is not supplied by the generator terminal. 2.2. Static excitation system The excitation system shown in Fig. 2 consists of a synchronous generator and a controlled converter. The excitation power is supplied through a transformer from the generator terminals. Some of the advantages of this system include small inherent time constant, lower cost and easy maintenance. However, during system-fault conditions, available excitation system ceiling voltage is diminished because the exciter output voltage is dependent on the AC system input voltage. 2.3. Controller design of static excitation system [9] The controller design procedure of static excitation shown in Fig. 2(b) is as follows: The steady state gain (KA) of the exciter, is designed as per Eq. (1). The secondary voltage of excitation transformer is designed as per Eq. (2). 100 o Vf0 p(V Vf0 VfD ) V2 pffiffiffi P 3 2(cos a 0:5 ZT ) KA ]
Fig. 6. AC voltage of AC network according to exciter type.
VfN Vf0
(1) (2)
where, VfN is full-load field voltage, Vfo is no-load field voltage, ZT is transformer impedance, VfD is thyristor voltage drop and o is a voltage variation rate (%). The excitation system controller has a lead-lag compensator which has the time constants Tb and Tc. The transient gain KT of this controller expressed in terms of KA, Tb and Tc is as given by Eqs. (3) /(6) using
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
208
Fig. 7. DC voltage of HVDC with static exciter fed synchronous compensator.
gain margin and phase margin method of analysis. These equations are given below: KT KA
TC TB
dynamic performance of the excitation system. The controller design of AC rotating exciter is similar to the static exciter method.
(3)
KT ]VRmax =DVg
(4)
KT =T do ? vc
(5)
1=Tc 5vc =n (where n]2)
(6)
As can be seen from these equations, the transient gain is also a function of the ceiling voltage VRmax. This ceiling voltage is a significant factor in determining the
3. HVDC with synchronous compensator in weak AC system 3.1. Study system Fig. 3 shows the inverter side of the study system. The DC controls of the study system are identical to the
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
209
Fig. 8. Modified and proposed excitation system.
system in the CIGRE benchmark model. The study system represents a 1000 MW, 500 kV, 12 pulse, monopolar HVDC system connected to a weak AC system. The Short Circuit Ratio (SCR) of inverter side is 2.0 Ú//808 and Effective SCR (ESCR) is 1.5 Ú//608. Fixed capacitors provide 500 MVAr at rated voltage and the remaining reactive power requirement of the converter is met by the synchronous condenser of rating /165//300 MVAr.
3.2. Analysis of system characteristics Fig. 4 shows DC voltage waveforms with static exciter and rotating exciter. The effective ceiling voltage for the exciter in both cases is 6 pu. Other control parameters are given in Tables 1 and 2 for both exciters. In both cases the system is running in steady state. However, with static exciter the DC voltage shows oscillations (near fundamental frequency) of considerable magni-
210
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Fig. 9. DC voltage waveform of HVDC for the proposed static exciter.
tude as a result of fluctuations in the AC voltage caused by the exciter of the synchronous compensator. If the AC system is strong, the effect of interaction is negligible. In a weak system, these interactions could easily cause system instability as shown in Fig. 6. The high frequency oscillations in these waveforms are present in both cases, and those are commutation notch characteristics of 12-pulse HVDC system. Fig. 5 shows the same system as in Fig. 4 except that the effective ceiling voltages of the exciters are changed
to 8 pu. In this case, TC(0.08) and TB(0.2553) time constants of the static exciter are adjusted as per Eqs. ? and vc are the same as those for (3) /(6) so that KA, Tdo the cases shown in Fig. 4. Note that the system with static exciter is unstable even during steady state operation whereas the system with rotating exciter response is not affected. To understand the phenomena further, the AC system voltage waveforms for the case shown in Figs. 4 and 5 are shown in Fig. 6. Note that as shown in Fig. 6 (a) with static exciter, the AC system
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
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voltage has subharmonic modulation as a result of the interaction of the excitation system with the terminal voltage. These oscillations are absent in the Fig. 6(b) with rotating exciter. Fig. 6(c) is the enlarged one of (a) to show the low frequency oscillation in AC voltage. Now that we have seen that the conventional static exciter is unstable with HVDC systems connected to a weak AC system, the same tests were performed with differently tuned gains for the HVDC controller (gamma controller) and the exciter itself. Fig. 7(a) shows the DC voltage of HVDC when HVDC controller gain is tuned and Fig. 7(b) when the exciter gain is tuned. From Fig. 7, the improvement, if any, was very insufficient to change the unstable behavior of the static exciter at higher field forcing limits.
is important to select proper exciter systems to achieve optimum performance. Static exciter has a faster response compared to the AC rotating type exciter but can be less stable than the AC rotating type exciter. By properly adjusting the negative field forcing limits, it is possible to achieve a fast and stable excitation system. Modification to the conventional excitation system included a controlled inverter with an independent power supply (battery). The improved performance was verified through digital simulation using electromagnetic transient simulation program EMTDC.
3.3. System response with modified static exciter systems
The authors would like to convey their thanks to Professor A.M. Gole for his contribution to this work. Also, this work was sponsored by Next-Generation Power Technology Center supported by Ministry of Science and Technology and Korea Science and Engineering Foundation.
In order to solve the mentioned problems above, the following method is proposed. The static exciter is modified as shown in Fig. 8(a) by replacing the thyristors in the lower arm of the rectifier with diodes. This modification makes VRmin equal 0. To show how it affects the HVDC system behavior, an AC single phase fault case was simulated. Fig. 9(a) shows the DC voltage response with modified exciter shown in Fig. 8(a). As shown in Fig. 9(a), the modification clearly improved the steady state performance (0.5 /1.0 s) but the peak over-voltage (1.5 pu) during recovery is very high. This is mainly attributable to diminished negative field forcing ability. Another change to the exciter as shown in Fig. 8(b) was also studied. This change includes an independent power supply and a fully controlled inverter system. The system response is shown for a single phase to ground fault in Fig. 9(b). Fig. 8(c) shows the block diagram of the proposed excitation system. This scheme has a boost switch to control capacitor voltage, and it can make the proposed excitation system not disturbed by generator voltage variation. This excitation system shows improved steady state and post fault response. But, in Fig. 9(b), an oscillation due to the rotor dynamics of the synchronous machine is observed. Adjusting the negative ceiling voltage to /4 pu provided a better control of overvoltage as shown in Fig. 9(c).
4. Conclusion Although synchronous compensator improves the system strength and is essential in weak AC systems, it
Acknowledgements
References [1] O.B. Nayak, A.M. Gole, et al., Dynamic performance of static and synchronous compensators at HVDC inverter bus a very weak AC system, IEEE Transactions on Power Systems 9 (3) (1994) 1350 / 1358. [2] P.S. Bodger, G.D. Irwin, D.A. Woodford, Controlling harmonic instability of HVDC links connected to weak AC system, IEEE Transactions on Power Delivery 5 (4) (1990) 2039 /2046. [3] Y. Zhuang, R.W. Menzies, O.B. Nayak, et al., Dynamic performance of STACON at HVDC inverter feeding a very weak AC system, IEEE Transactions on Power Delivery 11 (2) (1996) 958 / 964. [4] C.V. Thio, J.B. Davies, New synchronous compensator for the Nelson River HVDC system-planning requirements and specification, IEEE Transactions on Power Delivery 6 (2) (1991) 922 /928. [5] N. Kaul, R.M. Mathur, Solution to the problem of low order harmonic resonance from HVDC converter, IEEE Transactions on Power Systems 5 (4) (1990) 1160 /1166. [6] The IEEE Special Stability Controls Working Group and the Dynamic Performance and Modeling of DC Systems Joint Working Group, HVDC controls for system dynamic performance, IEEE Transactions on Power Systems, 6 (2) (1991) 743 /752. [7] N. Mohan, T.M. Undeland, W.P. Robbins, Power Electronics: Converters, Applications and Design, second ed., Wiley, 1995. [8] D.C. Lee, P. Kundur, Advanced Excitation Controls for Power System Stability Enhancement, CIGRE paper 38-01, 1986. [9] IEEJ Technical Report (in Japanese), Specification of Exciters of Synchronous Generator, 1995, pp. 44 /53.
Dynamic performance of HVDC system according to exciter characteristics of synchronous compensator in a weak AC system Chan-Ki Kim a, Gilsoo Jang b,*, Byung-Mo Yang a a
b
Korea Electric Power Research Institute, Munji-dong, Yusung-gu, Daejon, Republic of Korea NPTC/School of Electrical Engineering, Korea University, Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea Received 31 October 2001; received in revised form 28 March 2002; accepted 15 May 2002
Abstract This paper analyses the dynamic performance of HVDC system connected to a weak AC system for various exciter characteristics of synchronous machines connected at the converter bus. Conventionally capacitors are used to supply reactive power requirement at a strong converter bus. But the installation of a synchronous machine is essential in an isolated weak network to re-start after a shutdown of HVDC and to increase system strength. The dynamic performance of a synchronous machine depends on the characteristics of its exciter. In this paper, several exciter types are used to investigate their effect on the dynamic performance of the HVDC system, and modifications to standard exciter topologies are suggested to mitigate observed problems. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Exciter; HVDC; Synchronous condenser (SC)
1. Introduction Over long distances bulk power transfer can be carried out by HVDC connection than by a long distance AC transmission line. Also, bulk power of HVDC transmission scheme may be transmitted through very long cables or across borders where the two AC systems are not synchronized or operate at different frequencies. HVDC converters (both rectifiers and inverters) draw lagging reactive power from the AC system in the amount of about 60% of the real power. Usually the reactive power is fully compensated at the converter bus to improve system regulation and reduce transmission losses. Variation in DC power changes the reactive power requirement of the converter, hence it tends to cause change of AC voltage. For strong AC system, these fluctuations are tolerable and we can use
* Corresponding author. Tel.: /82-2-3290-3246; fax: /82-2-9210544 E-mail address: [email protected] (G. Jang).
fixed capacitors to provide reactive power. However, these changes are particularly significant if the AC system is weak enough to lead to system instability. Hence we must use dynamically adjustable compensator schemes. Static Var Compensator (SVC), Static Condenser (STATCON) or Synchronous Condenser (SC) are the commonly used dynamic compensator schemes. Nyati et al. have shown that static compensators provide a much faster system response compared to the synchronous compensators. Gole et al. have shown that the use of a proper mix of SVCs and synchronous compensators is the best choice for HVDC systems connected to a very weak AC system. For the extreme case of a receiving system of zero inertia (no generation), a conventional inverter cannot start after even a momentary interruption of DC power. A synchronous machine in the receiving system is generally the only practical solution in such situations [1 /7]. Lee and Kundur have shown that torsional mode instability through excitation control may be caused by a terminal voltage limiter which uses feedback of terminal voltage to control excitation through a very high gain [8].
0378-7796/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 7 7 9 6 ( 0 2 ) 0 0 1 1 6 - 5
204
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
Fig. 1. Conventional AC excitation system.
Fig. 2. Conventional static excitation system.
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
Fig. 3. Test system based on the CIGRE model.
Fig. 4. DC voltage of HVDC according to exciter type with VRmax of 6 p.u.
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C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
206 Table 1 Static excitation system parameters
Table 2 Rotating excitation system parameters
KA TA TB ? Tdo vc
KA TA TEX ? Tdo vc
150 0.1 (s) 0.375 (s) 2 (s) 10 (dB)
The normal operation of a synchronous machine with a static exciter does not usually pose any major problems. However, during AC line-faults or in the case of connection to a weak AC network, a synchro-
37.5 0.01 (s) 0.2 (s) 2 (s) 10 (dB)
KEX ? TgA TF KF VAmax
4 1 (s) 0.1 (s) 0.01 (s) 1.0 (p.u)
nous machine with a fast field forcing bus-fed static exciter has a drawback, which may cause system instability. This is because the output voltage of the static exciter is dependent on the generator terminal
Fig. 5. DC voltage of HVDC according to exciter type with VRmax of 8 p.u.
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
voltage that is controlled by the static exciter. This paper investigates the problem represented above and proposes a solution. Also, a new excitation scheme is proposed in order to eliminate a low frequency oscillation of AC output voltage. The investigation is performed by time domain digital simulation using EMTDC program.
207
2. An excitation system review The IEEE has classified excitation systems currently in use into 12 types. From a system characteristics point of view, excitation system is classified as three types */ DC, AC and static type. Because the DC-type exciters are gradually being phased out, this paper discusses only AC and static type exciters. 2.1. AC rotating excitation system Fig. 1 shows an AC rotating excitation system that consists of a Permanent Magnet Generator (PMG) with controlled rectifier and exciter generator with noncontrolled rectifier. In this system, the exciter is on the same shaft as the turbine generator. The AC output of exciter system is rectified by either a controlled or non-controlled rectifier to produce the direct current needed for the generator field. The rectifiers may be stationary or rotating. This system is more stable than a static excitation system because exciter power is not supplied by the generator terminal. 2.2. Static excitation system The excitation system shown in Fig. 2 consists of a synchronous generator and a controlled converter. The excitation power is supplied through a transformer from the generator terminals. Some of the advantages of this system include small inherent time constant, lower cost and easy maintenance. However, during system-fault conditions, available excitation system ceiling voltage is diminished because the exciter output voltage is dependent on the AC system input voltage. 2.3. Controller design of static excitation system [9] The controller design procedure of static excitation shown in Fig. 2(b) is as follows: The steady state gain (KA) of the exciter, is designed as per Eq. (1). The secondary voltage of excitation transformer is designed as per Eq. (2). 100 o Vf0 p(V Vf0 VfD ) V2 pffiffiffi P 3 2(cos a 0:5 ZT ) KA ]
Fig. 6. AC voltage of AC network according to exciter type.
VfN Vf0
(1) (2)
where, VfN is full-load field voltage, Vfo is no-load field voltage, ZT is transformer impedance, VfD is thyristor voltage drop and o is a voltage variation rate (%). The excitation system controller has a lead-lag compensator which has the time constants Tb and Tc. The transient gain KT of this controller expressed in terms of KA, Tb and Tc is as given by Eqs. (3) /(6) using
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
208
Fig. 7. DC voltage of HVDC with static exciter fed synchronous compensator.
gain margin and phase margin method of analysis. These equations are given below: KT KA
TC TB
dynamic performance of the excitation system. The controller design of AC rotating exciter is similar to the static exciter method.
(3)
KT ]VRmax =DVg
(4)
KT =T do ? vc
(5)
1=Tc 5vc =n (where n]2)
(6)
As can be seen from these equations, the transient gain is also a function of the ceiling voltage VRmax. This ceiling voltage is a significant factor in determining the
3. HVDC with synchronous compensator in weak AC system 3.1. Study system Fig. 3 shows the inverter side of the study system. The DC controls of the study system are identical to the
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
209
Fig. 8. Modified and proposed excitation system.
system in the CIGRE benchmark model. The study system represents a 1000 MW, 500 kV, 12 pulse, monopolar HVDC system connected to a weak AC system. The Short Circuit Ratio (SCR) of inverter side is 2.0 Ú//808 and Effective SCR (ESCR) is 1.5 Ú//608. Fixed capacitors provide 500 MVAr at rated voltage and the remaining reactive power requirement of the converter is met by the synchronous condenser of rating /165//300 MVAr.
3.2. Analysis of system characteristics Fig. 4 shows DC voltage waveforms with static exciter and rotating exciter. The effective ceiling voltage for the exciter in both cases is 6 pu. Other control parameters are given in Tables 1 and 2 for both exciters. In both cases the system is running in steady state. However, with static exciter the DC voltage shows oscillations (near fundamental frequency) of considerable magni-
210
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
Fig. 9. DC voltage waveform of HVDC for the proposed static exciter.
tude as a result of fluctuations in the AC voltage caused by the exciter of the synchronous compensator. If the AC system is strong, the effect of interaction is negligible. In a weak system, these interactions could easily cause system instability as shown in Fig. 6. The high frequency oscillations in these waveforms are present in both cases, and those are commutation notch characteristics of 12-pulse HVDC system. Fig. 5 shows the same system as in Fig. 4 except that the effective ceiling voltages of the exciters are changed
to 8 pu. In this case, TC(0.08) and TB(0.2553) time constants of the static exciter are adjusted as per Eqs. ? and vc are the same as those for (3) /(6) so that KA, Tdo the cases shown in Fig. 4. Note that the system with static exciter is unstable even during steady state operation whereas the system with rotating exciter response is not affected. To understand the phenomena further, the AC system voltage waveforms for the case shown in Figs. 4 and 5 are shown in Fig. 6. Note that as shown in Fig. 6 (a) with static exciter, the AC system
C.-K. Kim et al. / Electric Power Systems Research 63 (2002) 203 /211
211
voltage has subharmonic modulation as a result of the interaction of the excitation system with the terminal voltage. These oscillations are absent in the Fig. 6(b) with rotating exciter. Fig. 6(c) is the enlarged one of (a) to show the low frequency oscillation in AC voltage. Now that we have seen that the conventional static exciter is unstable with HVDC systems connected to a weak AC system, the same tests were performed with differently tuned gains for the HVDC controller (gamma controller) and the exciter itself. Fig. 7(a) shows the DC voltage of HVDC when HVDC controller gain is tuned and Fig. 7(b) when the exciter gain is tuned. From Fig. 7, the improvement, if any, was very insufficient to change the unstable behavior of the static exciter at higher field forcing limits.
is important to select proper exciter systems to achieve optimum performance. Static exciter has a faster response compared to the AC rotating type exciter but can be less stable than the AC rotating type exciter. By properly adjusting the negative field forcing limits, it is possible to achieve a fast and stable excitation system. Modification to the conventional excitation system included a controlled inverter with an independent power supply (battery). The improved performance was verified through digital simulation using electromagnetic transient simulation program EMTDC.
3.3. System response with modified static exciter systems
The authors would like to convey their thanks to Professor A.M. Gole for his contribution to this work. Also, this work was sponsored by Next-Generation Power Technology Center supported by Ministry of Science and Technology and Korea Science and Engineering Foundation.
In order to solve the mentioned problems above, the following method is proposed. The static exciter is modified as shown in Fig. 8(a) by replacing the thyristors in the lower arm of the rectifier with diodes. This modification makes VRmin equal 0. To show how it affects the HVDC system behavior, an AC single phase fault case was simulated. Fig. 9(a) shows the DC voltage response with modified exciter shown in Fig. 8(a). As shown in Fig. 9(a), the modification clearly improved the steady state performance (0.5 /1.0 s) but the peak over-voltage (1.5 pu) during recovery is very high. This is mainly attributable to diminished negative field forcing ability. Another change to the exciter as shown in Fig. 8(b) was also studied. This change includes an independent power supply and a fully controlled inverter system. The system response is shown for a single phase to ground fault in Fig. 9(b). Fig. 8(c) shows the block diagram of the proposed excitation system. This scheme has a boost switch to control capacitor voltage, and it can make the proposed excitation system not disturbed by generator voltage variation. This excitation system shows improved steady state and post fault response. But, in Fig. 9(b), an oscillation due to the rotor dynamics of the synchronous machine is observed. Adjusting the negative ceiling voltage to /4 pu provided a better control of overvoltage as shown in Fig. 9(c).
4. Conclusion Although synchronous compensator improves the system strength and is essential in weak AC systems, it
Acknowledgements
References [1] O.B. Nayak, A.M. Gole, et al., Dynamic performance of static and synchronous compensators at HVDC inverter bus a very weak AC system, IEEE Transactions on Power Systems 9 (3) (1994) 1350 / 1358. [2] P.S. Bodger, G.D. Irwin, D.A. Woodford, Controlling harmonic instability of HVDC links connected to weak AC system, IEEE Transactions on Power Delivery 5 (4) (1990) 2039 /2046. [3] Y. Zhuang, R.W. Menzies, O.B. Nayak, et al., Dynamic performance of STACON at HVDC inverter feeding a very weak AC system, IEEE Transactions on Power Delivery 11 (2) (1996) 958 / 964. [4] C.V. Thio, J.B. Davies, New synchronous compensator for the Nelson River HVDC system-planning requirements and specification, IEEE Transactions on Power Delivery 6 (2) (1991) 922 /928. [5] N. Kaul, R.M. Mathur, Solution to the problem of low order harmonic resonance from HVDC converter, IEEE Transactions on Power Systems 5 (4) (1990) 1160 /1166. [6] The IEEE Special Stability Controls Working Group and the Dynamic Performance and Modeling of DC Systems Joint Working Group, HVDC controls for system dynamic performance, IEEE Transactions on Power Systems, 6 (2) (1991) 743 /752. [7] N. Mohan, T.M. Undeland, W.P. Robbins, Power Electronics: Converters, Applications and Design, second ed., Wiley, 1995. [8] D.C. Lee, P. Kundur, Advanced Excitation Controls for Power System Stability Enhancement, CIGRE paper 38-01, 1986. [9] IEEJ Technical Report (in Japanese), Specification of Exciters of Synchronous Generator, 1995, pp. 44 /53.