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SSTI and its mitigation in wind farms connected with an HVDC line
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ScienceDirect Available online at www.sciencedirect.com Procedia Computer Science 00 (2015) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Procedia Computer Science 111 (2017) 399–405
8th International Conference on Advances in Information Technology, IAIT2016, 19-22 December 2016, Macau, China
SSTI and its mitigation in wind farms connected with an HVDC line Wenhui Qu1,a* and Jianguo Jiang1 1 1
School of Electronic, Information and Electrical Engineering, Shanghai Jiao Tong University, 200240 Shanghai, People’s Republic of China
Abstract Frequent oscillation occurs at the wind farms of Santanghu District, which are connected to a ±800 kV high voltage direct current line (HVDC). After analysis of the oscillation frequency and its characteristics, it is concluded that the oscillation occurs because of subsynchronous torsional interaction (SSTI) caused by the interaction between the wind turbine generators and the HVDC link. To suppress the SSTI phenomenon, an additional SSTI stabilizer is proposed and installed in Wind Farm 1 of Santanghu. The additional SSTI stabilizer exhibits good performance in mitigating oscillation, with the advantage of low cost and a low additional area footprint. © 2015 The Authors. Published by Elsevier B.V. © 2017 The Authors. Published by B.V. committee of the 8th International Conference on Advances in Information Peer-review under responsibility of Elsevier the organizing Peer-review under responsibility of the organizing committee of the 8th International Conference on Advances in Information Technology. Technology Keywords: subsynchronous torsional interaction, high voltage direct current, wind farms, stability
1. Introduction As the fastest growing renewable energy resource, wind power has attracted much concern because of its effect on grid stability 1. Because high-capacity wind energy bases are always far from the load centre, e.g., the Santanghu wind power base discussed in this paper, long transmission lines are necessary to supply electricity. However, series capacitors in the ac line and controllers in the dc line may bring instability issues such as subsynchronous oscillation (SSO) to the power system.
* Corresponding author. E-mail address: [email protected]
1877-0509 © 2015 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of the 8th International Conference on Advances in Information Technology.
1877-0509 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of the 8th International Conference on Advances in Information Technology 10.1016/j.procs.2017.06.040
Wenhui Qu et al. / Procedia Computer Science 111 (2017) 399–405 Wenhui Qu et al./ Procedia Computer Science 00 (2015) 000–000
400
SSO is a general term that defines two parts of an electric system exchanging energy with each other at one or more of the natural frequencies of the combined system below the fundamental frequency of the system 2. According to the different elements involved, SSO is classified into three categories: subsynchronous resonance (SSR), subsynchronous control interaction (SSCI) and subsynchronous torsional interaction (SSTI) 3. Table 1 presents the different SSO categories and their relationship with different electrical or mechanical elements. SSR negatively affects generators near a series capacitor when the mechanical mass resonates with the effective impedance of the system. SSR has been a recognized concern because of the shaft failures at the Mohave power plant in the 1970s 4. SSCI is a condition in which a power electronic device interacts with a series compensated system. SSCI was first documented in 2009 in Texas when a line fault and subsequent outage left a large wind farm connected radially into a series compensated high voltage line. The frequency of SSCI is not fixed but is based on the configuration of the controls as well as the electrical transmission system 5. SSTI is an interaction between a power electronic controller and the mechanical mass system of a generator. Any controller that can respond rapidly to power variations in the subsynchronous frequency range can be the source of negative damping 3. Controllers in HVDC links are the most common type that lead to SSTI 6, 7. For example, the first reported SSTI arose from the coupling between the turbogenerator at Square Buttez and the classical HVDC controller in1977 6. The oscillation described in this paper also belongs to this category. Table 1. The composition of various SSO
Generator
Series Capacitor-Compensated Line
SSR
√
√
SSTI
√
SSCI
Power Electronic Controller
√ √
√
This paper is organized as follows. Section 2 introduces the oscillations that occurred at Santanghu wind power base as well as the structure of the wind farm distribution and the connected hybrid transmission network. Section 3 analyses the broad characteristics of the oscillation at Santanghu. In Section 4, an additional SSTI stabilizer designed according to the IEI mitigation method is presented and reports of a field test in Wind Farm 1 are summarized. 2. Background Owing to the rich availability of wind energy, many wind farms have been built or are being planned at Santanghu wind power base. Nevertheless, the local energy demand is not high because of limited economic opportunities. Thus, it is necessary to transmit the electric power to more energy hungry central regions. Since a ±800 kV high voltage direct current (HVDC) link between South Hami and Zhengzhou was put into operation in 2014, wind-based electrical energy in Santanghu has been delivered to the load centre through an ac and dc hybrid network, as shown in Fig. 1. The Santanghu wind power base at present consists primarily of five wind farms with a total capacity of 297 MW. The capacity of Wind Farm 2 is 99 MW, whereas it is 49.5 MW for other wind farms. The wind farms consist primarily of doubly fed induction generators (DFIGs) except for Wind Farm 3, which consists of permanent-magnet direct-drive generators (PMDDGs). All wind farms have been configured with reactive power compensation devices such as the static synchronous compensator (STATCOM) in Wind Farm 1. The power from the wind power base collects at T2 and is delivered to the receiving end through the hybrid network. The HVDC link from South Hami to Zhengzhou is the primary power transmission route from Santanghu to centre regions, with a capacity of 8,000 MW.
Wenhui Qu et al. / Procedia Computer Science 111 (2017) 399–405 Wenhui Qu et al./ Procedia Computer Science 00 (2015) 000–000 WF2 99MW
WF3 49.5MW
±800kV HVDC
WF1
49.5MW
401
T2 WF4 49.5MW
T4 South Hami Station
T3
Zhengzhou Station
WF5 49.5MW 0.69/35kV
RSC
GSC
T1 SVG 33 × 1.5MW
Figure 1. The structure of Santanghu wind base and the connected hybrid network
However, with wind turbines of different types and models installed, oscillations occur frequently in this area, knocking generators off line and damaging the stability of the electrical system. In contrast to conventional SSTI, oscillations at Santanghu exhibit a lower frequency, and this frequency changes along with the operational mode and the speed of the wind farm. These changes in oscillation frequency make it challenging to suppress SSTI at Santanghu. 3. Analysis Wind turbine generators are classified into two categories: constant-speed constant-frequency wind turbine generators and variable-speed constant-frequency wind turbine generators. The former type is generally a squirrel cage induction generator (SCIG), and the latter contains DFIG and PMDDG. The DFIG is the most commonly used type of wind turbine generator at present because the technology is relatively more mature 8. Recently, PMDDG is becoming increasingly popular because of its better low voltage ride-through capability and greater reliability. Compared with a steam turbine, the natural frequency of a wind turbine is much lower. In a practical configuration, the compensation level is rarely high enough to couple with the torsional mode of the turbine shaft. Thus, when a series-compensated ac line is connected with a wind turbine, SSR is essentially absent 9. Both DFIG and PMDDG contain converters that can enable SSCI when connected with a series capacitor. The rotors in PMDDG are decoupled with the series-compensated line by the back-to-back converters. Therefore, DFIG is the only type of wind turbine generator susceptible to SSCI 10. Further, this is the most common type of SSO found in existing systems, e.g., the SSCI accident in South Texas 3, 5, 11. As mentioned, rotors in PMDDG cannot interact with the utility grid, and thus only SCIG and DFIG can resonate with the adjacent controller and cause SSTI. So far, SSTI has been reported only between steam turbines and converters in HVDC lines. And SSTI phenomenon for hydroelectric turbine generator units with small generator-to-turbine inertia rations interconnected to an HVDC system has been investigated 12.No SSTI involving wind turbines and controllers in HVDC lines has yet been documented. However, controllers in HVDC lines can exhibit negative damping at lower frequency that match the operational parameters of wind turbines. Therefore, with the increasing application of both wind energy and HVDC technology, SSTI and its mitigation are of great interest. The waveform of uab on a 35kV bus in Wind Farm 1 in an oscillation accident is displayed in Fig. 2, in which a significant harmonic component in uab can be observed. The harmonic frequency of uab, as analysed by Fast Fourier Transform Algorithm, is presented in Fig. 3. The dominant oscillation frequency of uab is approximately 2.4Hz. In fact, Wind Farm 1 consisted of 33 DFIGs whose natural frequency was 2.5 Hz at the time (vwind=11 m/s). In addition, the records of the oscillations in Wind Farm 1 indicate that the value of the oscillation frequency does not remain unchanged. It varies in a range, from 1.5 Hz to 2.6 Hz, along with changes of the rotor speed and the operational mode of the HVDC link. These changes exactly agree with the variation in the natural mode of the DFIG with the external factors 13, 14. Additionally, no significant series capacitor is coupled to this power system, and the long-distance power delivery here depends primarily on the ±800 HVDC line. Consequently, the oscillations occurring in Wind Farm 1 falls into the aforementioned SSTI caused by the interaction between DFIGs and an HVDC link.
Wenhui Qu et al. / Procedia Computer Science 111 (2017) 399–405 Wenhui Qu et al./ Procedia Computer Science 00 (2015) 000–000
402
36.2 36.0 35.8 35.6 35.4 35.2 35.0
) V k ( b
ua
0
5
10
t(s)
15
25
20
Figure 2. uab on the high voltage side of T1 when oscillation occurs
The natural frequencies of wind turbine generators, in the range of 0~3 Hz for SCIG and DFIG and 0~10 Hz for PMDDG, are much lower than those of steam turbine generators which are generally vulnerable to subsynchronous oscillations. Thus, the oscillations of wind turbine generators are at lower frequencies; e.g., the oscillation frequency recorded in Fig. 2 is 2.4 Hz. Although the frequency of this type of oscillation does not exactly match the conventional definition of subsynchronous which should be in the range of the 3 Hz~ the fundamental frequency. From the perspective of the mechanism, however, it is caused by the coupling between the controllers in an HVDC link and the wind turbine generators. Therefore, in a broad sense the type of oscillation at Santanghu can be considered SSTI. ) % ( n o i t a r c i n o m r a H
2.0 1.5 1.0
0.5 0.1
5.2
10.4
f(Hz)
15.6
20.8
Figure 3. Frequency analysis of uab on the high voltage side of T1 when oscillation occurs
4. Additional SSTI stabilizer 4.1. IEI mitigation method Under active power flow from the wind farms to the receiving network, reactive power is also exchanged in the lines. The reactive power can be expressed as: (1) q = ud iq - uq id where ud and uq are the output voltages of the generator in the dq reference frame and id and iq represent the dq-axis output currents of the generator. When the oscillations occur in the power system, iq can be given by (2) iq = iq,f0 + iq,fr where iq, f0 and iq fr are the reactive current at the fundamental frequency and subsynchronous frequency, respectively. When the generator output voltage is in phase with the d-axis, the reactive power can be represented as (3) q = ud iq,f0 + ud iq,fr = qf0 + qfr where qf0 and qfr are the reactive powers based on f0 and fr, respectively. The latter, qfr, is the inductive energy from oscillation and should be suppressed. The IEI mitigation method involves injecting inverse energy to the electrical system to cancel out oscillations. By eliminating the energy based on fr, the system will regain stability. 4.2. Control strategy The crucial part of the IEI mitigation method is to generate inverse energy at high accuracy and fast response
Wenhui Qu et al. / Procedia Computer Science 111 (2017) 399–405 Wenhui Qu et al./ Procedia Computer Science 00 (2015) 000–000
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speed. Considering that the No.1 wind farm at Santanghu District is already augmented with STATCOMs, we propose to add an SSTI stabilizer to the existing STATCOMs. The additional SSTI stabilizer identifies and suppresses the SSTI energy in the utility grid according to the IEI mitigation method. 0.6/35kV DFIG
us _
PWM
SPLL
Amplifer Mode Identificator
Phase Compensator Amplifer
+
uref
umain + +
uadd
Phase Compensator
Additional SSTI Stabilizer
Figure 4. The control strategy of STATCOM with additional SSTI stabilizer
Fig. 4 demonstrates the control strategy of the reformed STATCOM; the block in the dotted box is the additional component that suppresses SSTI. The modal signal is identified from the terminal voltage us. Then, the amplifier magnifies the isolated modal signal and the compensator corrects the phase bias. We add the uadd obtained from the additional SSTI stabilizer to the original control signal umain to determine the new voltage reference uref. We compare uref with us and send the result to the pulse-width modulation (PWM) block as the modulating signal. Pulses are sent from the PWM block to the STATCOM to trigger at every electronic switch. Suppressing SSTI through an additional block in the existing STATCOM has the advantage of fast respond speed, low cost and almost no additional area footprint. 4.3. Performance To mitigate SSTI in Wind Farm 1 of Santanghu District, a set of additional SSTI stabilizers has been added to the STATCOM which had been installed on the 35 kV side of T1. The additional SSTI stabilizer generates inverse energy to cancel out the oscillation energy and help the system regain stability.
Wenhui Qu et al. / Procedia Computer Science 111 (2017) 399–405 Wenhui Qu et al./ Procedia Computer Science 00 (2015) 000–000
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) V k ( ) 0 2 2 ( b a
u
249 243 237 233 0
5
10
0
5
10
t(s) (a)
15
20
25
15
20
25
125 105 87
) A ( a
i
68 50 30
t(s) (b)
Figure 5. Performance of reformed STATCOM
(a) uab on the 220kV-bus (b) ia generated by STATCOM
Field tests have been performed to assess the effect of the additional SSTI stabilizer in Wind Farm 1, and a typical case is recorded in Fig. 5. Fig. 5a shows the waveform of uab at the 220 kV side of T2; Fig. 5b presents the waveform of ia generated by STATCOM in Wind Farm 1. As shown in Fig. 5, oscillations at 2.2 Hz appeared at the Santanghu District at 0 ~ 2 s. However, before 2 s the STATCOM in Wind Farm 1 regulated the voltage only, and the main component of ia was at the fundamental frequency. At 2 s, the additional SSTI stabilizer was put into operation, and the SVG began emitting both existing reactive power to regulate the voltage and inverse reactive power based on 2.2 Hz. With the inverse energy injected into the grid by the SVG, oscillation in uab(220) was suppressed and the system regained stability at 12 s. After the oscillation was suppressed, the SSTI modal signal could no longer be identified, and thus the SVG ceased generating inverse energy and operated only as a voltageregulator again. Thus, the additional SSTI stabilizer provides an outstanding improvement by mitigating the oscillations occurring at the DFIG-based wind farm connected to an HVDC line. In addition, it works in coordination with the voltage-regulating block of the STATCOM. 5. Conclusion Connected to an ac and dc hybrid network, the wind farms at Santanghu face frequent SSO. Through analysis of the characteristics and mechanisms of different SSO, the problem at Santanghu is concluded to generally consist of SSTI between DFIGs and converters in the HVDC line in a broad sense. Additionally, an additional SSTI stabilizer attached at the existing STATCOM, which was originally intended for voltage regulation, is proposed and tested efficiently in Wind Farm 1. In 2015, the wind power capacity increased by 63 GW and 31 GW worldwide and in China, respectively. Because over 38％ of the new wind generators in China are installed in the Northwest, far away from the load centre, long distance transmission technology is necessary. In addition, as of 2016, a large wind power capacity is expected to be added over the next five years in China. Consequently, more wind turbine generators will face the threat of SSTI. Successful application of the additional SSTI stabilizer in this paper provides an economic and feasible solution.
Wenhui Qu et al. / Procedia Computer Science 111 (2017) 399–405 Wenhui Qu et al./ Procedia Computer Science 00 (2015) 000–000
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6. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (grant no. 2014AA052602) and the Major State Basic Research Development Program of China (grant no. 2014CB046306). References 1 L. P. Kunjumuhammed, B. C. Pal, C. Oates, K. J. Dyke, Electrical Oscillations in Wind Farm Systems: Analysis and Insight Based on Detailed Modeling, IEEE Transactions on Sustainable Energy, 7 (2016) 51-62. 2 L. C. Gross, Sub-synchronous grid conditions: new event, new problem, and new solutions, Proc.Western Protective Relay Conf., (2010) 1-5. 3 G. D. Irwin, A. Isaacs, D. Woodford, Simulation requirements for analysis and mitigation of SSCI phenomena in wind farms, Transmission and Distribution Conference and Exposition (T&D), 2012 IEEE PES, (2012) 1-4. 4 J. W. Ballance, S. Goldberg, Subsynchronous resonance in series compensated transmission lines, Power Apparatus and Systems, IEEE Transactions on, PAS-92 (1973) 1649-1658. 5 G. D. Irwin, A. K. Jindal, A. L. Isaacs, Sub-synchronous control interactions between type 3 wind turbines and series compensated AC transmission systems, Power and Energy Society General Meeting, 2011 IEEE, (2011) 1-6. 6 C. T. Wu, K. J. Peterson, R. J. Piwko, M. D. Kankam, D. H. Baker, The intermountain power project commissioning-subsynchronous torsional interaction tests, Power Delivery, IEEE Transactions on, 3 (1988) 2030-2036. 7 R. H. Song, J. M. Lin, L. G. Ban, Z. T. Xiang, Study on the SSO damping characteristic and damping control of Mongolia-China HVDC transmission system, 2010 International Conference on Power System Technology, (2010) 1-5. 8 X. Hailian, L. Bin, C. Heyman, M. M. d. Oliveira, M. Monge, Subsynchronous resonance characteristics in presence of doubly-fed induction generator and series compensation and mitigation of subsynchronous resonance by proper control of series capacitor, IET Renewable Power Generation, 8 (2014) 411-421. 9 F. Lingling, M. Zhixin, Mitigating SSR Using DFIG-Based Wind Generation, Sustainable Energy, IEEE Transactions on, 3 (2012) 349-358. 10 H. A. Mohammadpour, E. Santi, Sub-synchronous resonance analysis in DFIG-based wind farms: Definitions and problem identification — Part I, Energy Conversion Congress and Exposition (ECCE), 2014 IEEE, (2014) 812-819. 11 H. A. Mohammadpour, A. Ghaderi, E. Santi, Analysis of sub-synchronous resonance in doubly-fed induction generator-based wind farms interfaced with gate-controlled series capacitor, Generation, Transmission & Distribution, IET, 8 (2014) 1998-2011. 12 M. S. Annakkage, C. Karawita, U. D. Annakkage, Frequency Scan-Based Screening Method for Device Dependent Sub-Synchronous Oscillations, Power Systems, IEEE Transactions on, PP (2015) 1-7. 13 F. Mei, B. Pal, Modal Analysis of Grid-Connected Doubly Fed Induction Generators, IEEE Transactions on Energy Conversion, 22 (2007) 728-736. 14 L. Yang, Z. Xu, J, x00D, stergaard, Z. Y. Dong, K. P. Wong, X. Ma, Oscillatory Stability and Eigenvalue Sensitivity Analysis of A DFIG Wind Turbine System, IEEE Transactions on Energy Conversion, 26 (2011) 328-339.
ScienceDirect Available online at www.sciencedirect.com Procedia Computer Science 00 (2015) 000–000
ScienceDirect
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Procedia Computer Science 111 (2017) 399–405
8th International Conference on Advances in Information Technology, IAIT2016, 19-22 December 2016, Macau, China
SSTI and its mitigation in wind farms connected with an HVDC line Wenhui Qu1,a* and Jianguo Jiang1 1 1
School of Electronic, Information and Electrical Engineering, Shanghai Jiao Tong University, 200240 Shanghai, People’s Republic of China
Abstract Frequent oscillation occurs at the wind farms of Santanghu District, which are connected to a ±800 kV high voltage direct current line (HVDC). After analysis of the oscillation frequency and its characteristics, it is concluded that the oscillation occurs because of subsynchronous torsional interaction (SSTI) caused by the interaction between the wind turbine generators and the HVDC link. To suppress the SSTI phenomenon, an additional SSTI stabilizer is proposed and installed in Wind Farm 1 of Santanghu. The additional SSTI stabilizer exhibits good performance in mitigating oscillation, with the advantage of low cost and a low additional area footprint. © 2015 The Authors. Published by Elsevier B.V. © 2017 The Authors. Published by B.V. committee of the 8th International Conference on Advances in Information Peer-review under responsibility of Elsevier the organizing Peer-review under responsibility of the organizing committee of the 8th International Conference on Advances in Information Technology. Technology Keywords: subsynchronous torsional interaction, high voltage direct current, wind farms, stability
1. Introduction As the fastest growing renewable energy resource, wind power has attracted much concern because of its effect on grid stability 1. Because high-capacity wind energy bases are always far from the load centre, e.g., the Santanghu wind power base discussed in this paper, long transmission lines are necessary to supply electricity. However, series capacitors in the ac line and controllers in the dc line may bring instability issues such as subsynchronous oscillation (SSO) to the power system.
* Corresponding author. E-mail address: [email protected]
1877-0509 © 2015 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of the 8th International Conference on Advances in Information Technology.
1877-0509 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of the 8th International Conference on Advances in Information Technology 10.1016/j.procs.2017.06.040
Wenhui Qu et al. / Procedia Computer Science 111 (2017) 399–405 Wenhui Qu et al./ Procedia Computer Science 00 (2015) 000–000
400
SSO is a general term that defines two parts of an electric system exchanging energy with each other at one or more of the natural frequencies of the combined system below the fundamental frequency of the system 2. According to the different elements involved, SSO is classified into three categories: subsynchronous resonance (SSR), subsynchronous control interaction (SSCI) and subsynchronous torsional interaction (SSTI) 3. Table 1 presents the different SSO categories and their relationship with different electrical or mechanical elements. SSR negatively affects generators near a series capacitor when the mechanical mass resonates with the effective impedance of the system. SSR has been a recognized concern because of the shaft failures at the Mohave power plant in the 1970s 4. SSCI is a condition in which a power electronic device interacts with a series compensated system. SSCI was first documented in 2009 in Texas when a line fault and subsequent outage left a large wind farm connected radially into a series compensated high voltage line. The frequency of SSCI is not fixed but is based on the configuration of the controls as well as the electrical transmission system 5. SSTI is an interaction between a power electronic controller and the mechanical mass system of a generator. Any controller that can respond rapidly to power variations in the subsynchronous frequency range can be the source of negative damping 3. Controllers in HVDC links are the most common type that lead to SSTI 6, 7. For example, the first reported SSTI arose from the coupling between the turbogenerator at Square Buttez and the classical HVDC controller in1977 6. The oscillation described in this paper also belongs to this category. Table 1. The composition of various SSO
Generator
Series Capacitor-Compensated Line
SSR
√
√
SSTI
√
SSCI
Power Electronic Controller
√ √
√
This paper is organized as follows. Section 2 introduces the oscillations that occurred at Santanghu wind power base as well as the structure of the wind farm distribution and the connected hybrid transmission network. Section 3 analyses the broad characteristics of the oscillation at Santanghu. In Section 4, an additional SSTI stabilizer designed according to the IEI mitigation method is presented and reports of a field test in Wind Farm 1 are summarized. 2. Background Owing to the rich availability of wind energy, many wind farms have been built or are being planned at Santanghu wind power base. Nevertheless, the local energy demand is not high because of limited economic opportunities. Thus, it is necessary to transmit the electric power to more energy hungry central regions. Since a ±800 kV high voltage direct current (HVDC) link between South Hami and Zhengzhou was put into operation in 2014, wind-based electrical energy in Santanghu has been delivered to the load centre through an ac and dc hybrid network, as shown in Fig. 1. The Santanghu wind power base at present consists primarily of five wind farms with a total capacity of 297 MW. The capacity of Wind Farm 2 is 99 MW, whereas it is 49.5 MW for other wind farms. The wind farms consist primarily of doubly fed induction generators (DFIGs) except for Wind Farm 3, which consists of permanent-magnet direct-drive generators (PMDDGs). All wind farms have been configured with reactive power compensation devices such as the static synchronous compensator (STATCOM) in Wind Farm 1. The power from the wind power base collects at T2 and is delivered to the receiving end through the hybrid network. The HVDC link from South Hami to Zhengzhou is the primary power transmission route from Santanghu to centre regions, with a capacity of 8,000 MW.
Wenhui Qu et al. / Procedia Computer Science 111 (2017) 399–405 Wenhui Qu et al./ Procedia Computer Science 00 (2015) 000–000 WF2 99MW
WF3 49.5MW
±800kV HVDC
WF1
49.5MW
401
T2 WF4 49.5MW
T4 South Hami Station
T3
Zhengzhou Station
WF5 49.5MW 0.69/35kV
RSC
GSC
T1 SVG 33 × 1.5MW
Figure 1. The structure of Santanghu wind base and the connected hybrid network
However, with wind turbines of different types and models installed, oscillations occur frequently in this area, knocking generators off line and damaging the stability of the electrical system. In contrast to conventional SSTI, oscillations at Santanghu exhibit a lower frequency, and this frequency changes along with the operational mode and the speed of the wind farm. These changes in oscillation frequency make it challenging to suppress SSTI at Santanghu. 3. Analysis Wind turbine generators are classified into two categories: constant-speed constant-frequency wind turbine generators and variable-speed constant-frequency wind turbine generators. The former type is generally a squirrel cage induction generator (SCIG), and the latter contains DFIG and PMDDG. The DFIG is the most commonly used type of wind turbine generator at present because the technology is relatively more mature 8. Recently, PMDDG is becoming increasingly popular because of its better low voltage ride-through capability and greater reliability. Compared with a steam turbine, the natural frequency of a wind turbine is much lower. In a practical configuration, the compensation level is rarely high enough to couple with the torsional mode of the turbine shaft. Thus, when a series-compensated ac line is connected with a wind turbine, SSR is essentially absent 9. Both DFIG and PMDDG contain converters that can enable SSCI when connected with a series capacitor. The rotors in PMDDG are decoupled with the series-compensated line by the back-to-back converters. Therefore, DFIG is the only type of wind turbine generator susceptible to SSCI 10. Further, this is the most common type of SSO found in existing systems, e.g., the SSCI accident in South Texas 3, 5, 11. As mentioned, rotors in PMDDG cannot interact with the utility grid, and thus only SCIG and DFIG can resonate with the adjacent controller and cause SSTI. So far, SSTI has been reported only between steam turbines and converters in HVDC lines. And SSTI phenomenon for hydroelectric turbine generator units with small generator-to-turbine inertia rations interconnected to an HVDC system has been investigated 12.No SSTI involving wind turbines and controllers in HVDC lines has yet been documented. However, controllers in HVDC lines can exhibit negative damping at lower frequency that match the operational parameters of wind turbines. Therefore, with the increasing application of both wind energy and HVDC technology, SSTI and its mitigation are of great interest. The waveform of uab on a 35kV bus in Wind Farm 1 in an oscillation accident is displayed in Fig. 2, in which a significant harmonic component in uab can be observed. The harmonic frequency of uab, as analysed by Fast Fourier Transform Algorithm, is presented in Fig. 3. The dominant oscillation frequency of uab is approximately 2.4Hz. In fact, Wind Farm 1 consisted of 33 DFIGs whose natural frequency was 2.5 Hz at the time (vwind=11 m/s). In addition, the records of the oscillations in Wind Farm 1 indicate that the value of the oscillation frequency does not remain unchanged. It varies in a range, from 1.5 Hz to 2.6 Hz, along with changes of the rotor speed and the operational mode of the HVDC link. These changes exactly agree with the variation in the natural mode of the DFIG with the external factors 13, 14. Additionally, no significant series capacitor is coupled to this power system, and the long-distance power delivery here depends primarily on the ±800 HVDC line. Consequently, the oscillations occurring in Wind Farm 1 falls into the aforementioned SSTI caused by the interaction between DFIGs and an HVDC link.
Wenhui Qu et al. / Procedia Computer Science 111 (2017) 399–405 Wenhui Qu et al./ Procedia Computer Science 00 (2015) 000–000
402
36.2 36.0 35.8 35.6 35.4 35.2 35.0
) V k ( b
ua
0
5
10
t(s)
15
25
20
Figure 2. uab on the high voltage side of T1 when oscillation occurs
The natural frequencies of wind turbine generators, in the range of 0~3 Hz for SCIG and DFIG and 0~10 Hz for PMDDG, are much lower than those of steam turbine generators which are generally vulnerable to subsynchronous oscillations. Thus, the oscillations of wind turbine generators are at lower frequencies; e.g., the oscillation frequency recorded in Fig. 2 is 2.4 Hz. Although the frequency of this type of oscillation does not exactly match the conventional definition of subsynchronous which should be in the range of the 3 Hz~ the fundamental frequency. From the perspective of the mechanism, however, it is caused by the coupling between the controllers in an HVDC link and the wind turbine generators. Therefore, in a broad sense the type of oscillation at Santanghu can be considered SSTI. ) % ( n o i t a r c i n o m r a H
2.0 1.5 1.0
0.5 0.1
5.2
10.4
f(Hz)
15.6
20.8
Figure 3. Frequency analysis of uab on the high voltage side of T1 when oscillation occurs
4. Additional SSTI stabilizer 4.1. IEI mitigation method Under active power flow from the wind farms to the receiving network, reactive power is also exchanged in the lines. The reactive power can be expressed as: (1) q = ud iq - uq id where ud and uq are the output voltages of the generator in the dq reference frame and id and iq represent the dq-axis output currents of the generator. When the oscillations occur in the power system, iq can be given by (2) iq = iq,f0 + iq,fr where iq, f0 and iq fr are the reactive current at the fundamental frequency and subsynchronous frequency, respectively. When the generator output voltage is in phase with the d-axis, the reactive power can be represented as (3) q = ud iq,f0 + ud iq,fr = qf0 + qfr where qf0 and qfr are the reactive powers based on f0 and fr, respectively. The latter, qfr, is the inductive energy from oscillation and should be suppressed. The IEI mitigation method involves injecting inverse energy to the electrical system to cancel out oscillations. By eliminating the energy based on fr, the system will regain stability. 4.2. Control strategy The crucial part of the IEI mitigation method is to generate inverse energy at high accuracy and fast response
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speed. Considering that the No.1 wind farm at Santanghu District is already augmented with STATCOMs, we propose to add an SSTI stabilizer to the existing STATCOMs. The additional SSTI stabilizer identifies and suppresses the SSTI energy in the utility grid according to the IEI mitigation method. 0.6/35kV DFIG
us _
PWM
SPLL
Amplifer Mode Identificator
Phase Compensator Amplifer
+
uref
umain + +
uadd
Phase Compensator
Additional SSTI Stabilizer
Figure 4. The control strategy of STATCOM with additional SSTI stabilizer
Fig. 4 demonstrates the control strategy of the reformed STATCOM; the block in the dotted box is the additional component that suppresses SSTI. The modal signal is identified from the terminal voltage us. Then, the amplifier magnifies the isolated modal signal and the compensator corrects the phase bias. We add the uadd obtained from the additional SSTI stabilizer to the original control signal umain to determine the new voltage reference uref. We compare uref with us and send the result to the pulse-width modulation (PWM) block as the modulating signal. Pulses are sent from the PWM block to the STATCOM to trigger at every electronic switch. Suppressing SSTI through an additional block in the existing STATCOM has the advantage of fast respond speed, low cost and almost no additional area footprint. 4.3. Performance To mitigate SSTI in Wind Farm 1 of Santanghu District, a set of additional SSTI stabilizers has been added to the STATCOM which had been installed on the 35 kV side of T1. The additional SSTI stabilizer generates inverse energy to cancel out the oscillation energy and help the system regain stability.
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) V k ( ) 0 2 2 ( b a
u
249 243 237 233 0
5
10
0
5
10
t(s) (a)
15
20
25
15
20
25
125 105 87
) A ( a
i
68 50 30
t(s) (b)
Figure 5. Performance of reformed STATCOM
(a) uab on the 220kV-bus (b) ia generated by STATCOM
Field tests have been performed to assess the effect of the additional SSTI stabilizer in Wind Farm 1, and a typical case is recorded in Fig. 5. Fig. 5a shows the waveform of uab at the 220 kV side of T2; Fig. 5b presents the waveform of ia generated by STATCOM in Wind Farm 1. As shown in Fig. 5, oscillations at 2.2 Hz appeared at the Santanghu District at 0 ~ 2 s. However, before 2 s the STATCOM in Wind Farm 1 regulated the voltage only, and the main component of ia was at the fundamental frequency. At 2 s, the additional SSTI stabilizer was put into operation, and the SVG began emitting both existing reactive power to regulate the voltage and inverse reactive power based on 2.2 Hz. With the inverse energy injected into the grid by the SVG, oscillation in uab(220) was suppressed and the system regained stability at 12 s. After the oscillation was suppressed, the SSTI modal signal could no longer be identified, and thus the SVG ceased generating inverse energy and operated only as a voltageregulator again. Thus, the additional SSTI stabilizer provides an outstanding improvement by mitigating the oscillations occurring at the DFIG-based wind farm connected to an HVDC line. In addition, it works in coordination with the voltage-regulating block of the STATCOM. 5. Conclusion Connected to an ac and dc hybrid network, the wind farms at Santanghu face frequent SSO. Through analysis of the characteristics and mechanisms of different SSO, the problem at Santanghu is concluded to generally consist of SSTI between DFIGs and converters in the HVDC line in a broad sense. Additionally, an additional SSTI stabilizer attached at the existing STATCOM, which was originally intended for voltage regulation, is proposed and tested efficiently in Wind Farm 1. In 2015, the wind power capacity increased by 63 GW and 31 GW worldwide and in China, respectively. Because over 38％ of the new wind generators in China are installed in the Northwest, far away from the load centre, long distance transmission technology is necessary. In addition, as of 2016, a large wind power capacity is expected to be added over the next five years in China. Consequently, more wind turbine generators will face the threat of SSTI. Successful application of the additional SSTI stabilizer in this paper provides an economic and feasible solution.
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