Frequency control capability of VSC-HVDC transmission system A.S. Elansari*, S.J. Finney*, J. Burr*, M.F. Edrah* *Strathclyde University, Institute of Energy and Environment, Glasgow, UK
[email protected]
Keywords: DFIG, Frequency controller, VSC-HVDC.
control,
inner
High voltage direct current (HVDC) transmission has been efficiently used to transfer higher amounts of power over long distances, and to connect unsynchronized systems with different frequencies. The first generation of HVDC transmission systems is Line-commuted converters (LCC) which controls power by controlling the switching of thyristor devices [5-7]. The performance of HVDC systems has been improving since the emergence of the second generation of HVDC system, which employs voltage source converter (VSC) technology. A wind park of 400 MW was the first project of VSC-HVDC wind connection, which was commissioned in 2010 in Germany[5]. Being independently able to control active and reactive power at both sides, voltage and frequency control has become achievable in VSC-HVDC. Voltage source converter based HVDC systems have sophisticated control features compared with current source converter (CSC) HVDC systems. For instance, independent control of active and reactive power allows feeding of passive networks, continuous control of voltage and frequency, black-starting and quick reversal of power flow. On the other hand, CSC-HVDC has higher power rating and lower power losses compared with VSCHVDC [5, 8-10]. VSC-HVDC systems employ a simple structure of robust PI controllers that include current inner-loop and outer loop control [11]. Reference control values are hierarchically passed from the outer controller into the faster acting inner current control loop. VSC-HVDC links are typically designed to control active power in the sending end converter, and dc voltage is controlled at the receiving end converter. Reactive power at both sides is controlled in order to maintain the AC voltage and reactive power within desired levels [12]. Many cases of frequency control implementation in conventional LCC-HVDC are reported in [8, 13-16]. However, few cases have been reported in the literature regarding VSC based HVDC frequency control [9, 17]. Although quality of voltage and frequency are important, reference current limit strategy gives higher priority to the ac voltage control because of higher voltage sensitivity of induction equipment [9]. VSC-HVDC frequency control systems may include proportional control of steady state frequency droop, or proportional integral (PI) controller to eliminate steady state error [10]. Frequency control can be achieved by including frequency control loop either in the active power controller or the dc voltage controller[8]. This paper studies the possibility of frequency performance improvement by the features available in VSC-HVDC systems. The two possible frequency control strategies of active power and dc voltage control loops are discussed to determine and compare the effectiveness of these strategies.
current
Abstract High voltage direct current (HVDC) systems have been used as an effective solution to transmit power over long distances. Performance of HVDC systems has significantly improved with the emergence of voltage source converter based HVDC (VSC-HVDC). Increased controllability provides independent control of active and reactive power which facilitates controlling of voltage and frequency. This paper investigates the frequency control capability of VSC-HVDC to determine the improvement extent that VSC HVDC systems provide for the frequency recovery and system inertia. Two frequency control strategies using dc voltage and active power outer loops are investigated. The analysis is carried out using a time domain simulator, where the tie line in the Kundur test system is replaced by VSC-HVDC. Simulation results have shown that frequency control capability of VSC-HVDC dramatically improves the frequency performance, especially in isolated power systems.
1
Introduction
Power industry across the world has developed dramatically in all levels including generation, transmission and distribution. However, this development is often at the expense of the environment because of the generated gases from the burning of fossil fuels. This encourages energy companies to invest in renewable energy [1]. Offshore wind farms have been given significant consideration because of potential high wind availability and its lower environmental impact compared with onshore wind farms. However, integration of large scale wind power is a challenge because of the unpredictable nature of wind, and strict requirements of voltage and frequency grid codes, especially in the case of isolated and offshore power systems[2]. The Doubly-Fed Induction Generator (DFIG) based Wind turbine is the most widespread type of variable speed wind turbine because of its contribution to reactive power and voltage control. Furthermore, the DFIG converter is cheaper than a full power converter based wind turbine of the same rating, as about 70% of the power passes through the induction generator. However, fully rated converter type wind turbines have better fault ride-through (FRT) capability as they have full control of the AC output current during the AC faults [3, 4].
1
The two area Kundur test system, shown in Figure 5 in section 4, is used to carry out time domain simulation. The AC tie line is replaced by a VSC-HVDC link, and one of the synchronous generators is replaced by aggregated DFIG based wind farm. A detailed model of the AC system, HVDC link, DFIG wind farm, and their associated crowbar and converter controllers are considered in the analysis. This paper is organized to discuss frequency grid code requirements of wind farm connection in the first section, control systems of VSC-HVDC in the second section and frequency control strategies are discussed in detail in the third section. A case studies and the conclusion are discussed in the final two sections.
2
Vconv 2 Vconv .Vnet .cos I (3) X As shown in Figure 1, considering fundamental frequency, the voltage droop across the coupling transformer and AC filter can be given by [12]: § di · (4) VR VS L ¨ ¸ R © dt ¹ The inductor current iL in (4) is also a function of the dc current (idc), and the derivative of dc voltage change in the capacitor as formulated in the equation below; § dV · (5) C ¨ DC ¸ idc iL © dt ¹ Using dq synchronous reference results from the Park transformation of equation(4), neglecting the losses, the active and reactive power yield (6) and(7). More details are in [12]; 3 Pac Vconvq .iq (6) 2 3 Qac Vconvq .id (7) 2 It is clear from (6) and (7) that P and Q are independently controlled by adjusting iq and id respectively. Q
Operating control theory of VSC-HVDC
2.1 VSC-HVDC operating principle
2.2 Voltage source converter HVDC control The VSC-HVDC control system is formed of outer and inner controllers. Vac and Q outer controller calculates the reference current Idref of the inner current controller which defines the required Id current. on the other hand, the required Iq current is determine by the inner current controller using reference current Iqref obtained from the active power or the dc voltage outer controller. 2.3 Inner current controller Figure 2 describes the inner current control loop of a VSCHVDC system. This controller consists of four parts [12, 18]: ki i. PI compensator ( G1inn ( S ) kpinn inn ). S 1 1 ii. System model ( G 2inn ( S ) ). . R 1 S L / R
Figure 1: Block diagram of VSC-HVDC with frequency control strategies VSC-HVDC can be considered as a fast controllable synchronous machine connected to an AC system through an AC filter as shown in Figure 1. This system provides efficient control of active and reactive power independently. Equation (1) explains that the output voltage of the voltage source converter is a function in the modulation index (M) and the phase shift (Ø) of the AC voltage [18]: 1 Vconv (1) M .Vdc sin(Zt I ) 2 VSC control is achieved using vector control method with synchronous rotating dq reference frame. This method provides independent control of active and reactive power exploiting PWM control features that allow determination of P and Q by effectively adjusting id and iq references to obtain the required values of Vconv and Ø (see (2) and(3)). Vconv .Vnet .sin I P (2) X
iii.
iv.
1 ), where Ta is 1 Ta .S PWM response time Ta=Ts/2. 1 Measurement circuit ( G 4inn ( S ) ), where Ts 1 Ts .S is the sampling time of the inner controller.
PWM converter ( G3inn ( S )
Figure 2: block diagram of inner control loop in dq axes
2
In order to include the effect of VSC-HVDC on system inertia and power-frequency characteristic, assuming that the losses in the HVDC system are neglected [20], the relationship between rate of change of frequency (ROCOF), and change of dc power PDC is expressed by: § df · (10) PDC PDES K DC ¨ ¸ © dt ¹ Where, KDC is dc power-frequency droop, PDES is a desired power ordered by the HVDC controller. From (9) and (10), the general equation of the frequency dynamic including the contribution of HVDC power yields; K · df § Pm Pe PDES 2 ¨ H DC ¸ (11) 2 ¹ dt ©
2.4 DC voltage control Dc voltage control in VSC-HVDC is performed by inner and outer control loops in an integrated operation. Dc voltage control system is formed of four parts as shown in Figure 3. ki i. PI controller ( G1vdc ( S ) kpvdc vdc ). S 1 ii. Inner current controller G 2vdc ( S ) , where 1 Teq .S iii. iv.
Teq=2 Ta (see [18] for more details). 3 1 vd , where C is the dc The system G3vdc ( S ) . . 2 C.S vdc capacitor. Measurement circuit as described earlier.
Equation (11) illustrates that a change in dc power influences system frequency. Therefore, proper coordination between wind generation and the HVDC link can improve system frequency performance. It can be seen from (11) that (ROCOF) is proportional to the change of power ∆P, which includes dc power. Hence steady power-frequency characteristic droop (R) can be written as; (12) f f 0 R( P P0) Where, (f-f0) is the deviation from nominal frequency and is caused by (P-P0) the change in power (∆P).
Figure 3: Dc voltage control loop in dq axes 2.5 Active and reactive power control
3.2 Fixed frequency control strategy
The structure and transfer functions of active and reactive power control are similar to that of dc voltage control described in section 2.4. The system consists of active and reactive power PI controller , equivalent of inner current controller, power transmission relationship and measurement circuits as shown in Figure 4 [12].
Fixed frequency control strategy is performed by the HVDC converted to maintain the frequency at its nominal value (f0). This strategy is used when the isolated system does not include any frequency control, and the frequency control is only performed by the HVDC link. Frequency control is achieved by supplying a fixed frequency f0 to the output voltage of the HVDC converter as depicted in Figure 1(frequency control at position 1) [8]. Any change in the power demand is reflected as an error in the frequency controller, which in turns brings the frequency back into its nominal value.
Figure 4: Active and reactive power control loop in dq axes
3
3.3 Vdc frequency control strategy
VSC-HVDC frequency control
Vdc frequency control depends on the dc voltage change, which is used to establish a new reference frequency. Equation (13) illustrates the relationship between Vdc and the electrical energy (Wdc)in the dc capacitor of the HVDC system [8]. 1 Wdc (13) Cdc .Vdc2 2
3.1 Dynamic of system frequency System frequency depends on the amount of rotating inertia in the system which determines the power-frequency characteristic during and after a disturbance. The rotating inertia of the system is given in per unit by[19]: § dZ · 2 H ¨ t ¸ Tm Te (8) © dt ¹ Where ωt is angular speed, Tm and Te are mechanical and electromagnetic torques and H is system inertia. Equation (8) can be expressed as the relationship between frequency deviation , electrical and mechanical powers (Pe and Pm) as: § df · 2 H ¨ ¸ Pm Pe (9) © dt ¹
Neglecting the losses in the converter, change in the dc power (∆P) is accompanied by electrical energy change in the dc capacitor according to the equation: d 1§ d 2· 'P Wdc (14) ¨ Cdc . Vdc ¸ dt 2© dt ¹ Based on the frequency droop given in(12), equation (14) can be written to include the frequency droop as:
3
out with and without HVDC frequency control strategies at the rectifier converter side of the HVDC link. It can be seen in Figure 6-a that the frequency has increased in the island due to power and generation unbalance. It is clear that without HVDC frequency control, system frequency reaches a relatively higher steady state frequency as the contribution of the synchronous generator is not enough to bring the frequency back to its nominal value. On the other hand, both frequency control strategies have effectively brought the frequency back to its nominal value by increasing the exported power via the HVDC link. Consequentially, the control strategies cause a slight difference in the dc power and dc voltage as shown in Figure 6-b and Figure 6-c.
V 0 2 V 1dc 2 · 1§ (15) R ¨ Cdc . dc ¸ 2© T ¹ Where V0dc, V1dc are the change of the dc capacitor voltage from V0 to V1 at time T. it can be seen that (-R.Cdc/2T) represents the droop of dc voltage-frequency characteristic (Rdc), which can be used as a proportional gain of the frequency controller based on the equation as shown in Figure 1(position 2): (16) f f 0 Rdc V 0dc 2 V 1dc 2 f f0
3.4 Power-frequency control strategy The third strategy of the VSC-HVDC frequency control is based on power-frequency droop control characteristic described in section 3.1. This controller may include proportional control of steady state frequency droop, or proportional integral (PI) controller to eliminate steady state error. The phase locked loop (PLL) system in the converter estimates the frequency at the HVDC PCC. The estimated frequency is used as a frequency feedback signal for the frequency controller, which includes a low-pass filter to get rid of harmonics and impulse noise as shown in Figure 1(position 3).
4
Case studies
The test system shown in Figure 5 is used in this paper to evaluate the frequency control strategies described in sections 3.4. The analysis includes evaluation of the system frequency during sudden changes in load demand and during a sudden trip of G3. The right side of the HVDC link is an isolated system that includes; an aggregated model of a 400 MW offshore wind farm with its associated controllers, 100 MW local loads with 0.95 lagging power factor and a 121 MVA onshore synchronous generator with voltage and frequency regulators. Most of the generated wind power is exported to an onshore grid (1.8 GVA) via a ± 200 kV VSC-HVDC link of 200 km length (DCS1 section in Cigre B4 HVDC test system)[21] [22].NEPLAN power system analysis tool is used in the network modelling and simulations[23].
(6-a)
(6-b)
Figure 5: Two areas Kundur test system with HVDC link and DFIG wind farm 4.1 An excessive power generation case study
(6-c)
In this case study, a sudden load shedding of 5% in load demand in the island power system is simulated assuming a negligible change in the wind speed. The analysis is carried
Figure 6: Simulation results for a sudden load shedding of 5% at island side.
4
4.2 Wind speed fluctuation case study
4.3 Isolated wind farm with HVDC link case study
Wind speed decrease causing a generation deficit in the island network is simulated in this case. Figure 7-a shows a fluctuation in wind production for a one minute time frame due to change of wind speed. Frequency results in Figure 7-b show that both described frequency control strategies recover the frequency to its nominal value. On the other hand, for the case with no HVDC frequency control support, fluctuation of wind speed causes poor frequency performance in the island. It is important to mention in this case that the PI controller adopted by the power-frequency control shows better performance than the proportional controller adopted by the dc voltage-frequency control strategy (Figure 7-b and 7-c).
The effectiveness of the described frequency control strategies during isolated DFIG wind farm with HVDC link is examined in this case. Line B10-B11 in Figure 5 is tripped, to clear a 100 ms three phase short circuit fault at the middle of the line, forming an isolated wind farm connected to the HVDC link with almost 50 MW generation deficit.
(8-a)
(7-a)
(8-b)
(7-b)
(8-c)
Figure 8: Simulation results for isolated operating condition of offshore wind farm connected to HVDC link. Results of this case reveal the importance of frequency control on the wind farm side of the HVDC link. Figure 8-a illustrates that loss of rotating inertia with the absence of any sort of frequency control leads to an un-damped oscillatory operating mode for the system. However, in reality the HVDC systems are designed to control system frequency during isolated mode. In contrast, presence of any sort of frequency control, even in the case of inertialess systems,
(7-c)
Figure 7: Simulation results for wind speed fluctuation
5
dramatically improves the frequency performance as the HVDC power is adapted to rebalance generation and demand in the island as shown in Figure 8-b and Figure 8-c.
5
[10]
Conclusion
[11]
This paper has presented frequency control strategies of a voltage source converter based HVDC system. Simulation results have shown that both dc voltage-frequency control and power-frequency control strategies dramatically improve the frequency performance as they effectively bring the frequency back to its nominal value by adjusting the HVDC exported power. The results have also shown that both frequency control strategies recover system frequency even in the case of an inertialess isolated DFIG power system. Powerfrequency control has shown better frequency performance than dc voltage-frequency control strategy because of improved control capability of the PI controller included in the power-frequency controller.
[12]
[13]
[14]
References [1]
[2]
[3]
[4]
[5]
[6] [7]
[8]
[9]
D. Jinxu and A. Somani, "Reduction of green house gas emission by clean power trading," in Energy Conversion Congress and Exposition (ECCE), 2010 IEEE, 2010, pp. 4588-4592. M. Tsili and S. Papathanassiou, "A review of grid code technical requirements for wind farms," Renewable Power Generation, IET, vol. 3, pp. 308332, 2009. I. Erlich, et al., "Dynamic Behavior of DFIG-Based Wind Turbines during Grid Faults," in Power Conversion Conference - Nagoya, 2007. PCC '07, 2007, pp. 1195-1200. M. Mohseni, et al., "Emergency control of DFIGbased wind turbines to meet new European Grid Code requirements," in Innovative Smart Grid Technologies (ISGT), 2011 IEEE PES, 2011, pp. 1-6. E. Prieto-Araujo, et al., "Methodology for Droop Control Dynamic Analysis of Multiterminal VSCHVDC Grids for Offshore Wind Farms," Power Delivery, IEEE Transactions on, vol. 26, pp. 24762485, 2011. N. G. Hingorani and L. Gyugyi, Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems: Wiley, 2000. M. R. Hasan, et al., "Generic high level VSC-HVDC grid controls and test systems for offline and real time simulation," in Electric Power Quality and Supply Reliability Conference (PQ), 2014, 2014, pp. 57-64. C. Du, et al., "Comparison of Different Frequency Controllers for a VSC-HVDC Supplied System," Power Delivery, IEEE Transactions on, vol. 23, pp. 2224-2232, 2008. C. Du, et al., "A New Control Strategy of a VSC–HVDC System for High-Quality Supply of Industrial Plants," Power Delivery, IEEE Transactions on, vol. 22, pp. 2386-2394, 2007.
[15]
[16] [17]
[18]
[19]
[20]
[21] [22] [23]
6
C. Du and M. H. J. Bollen, "Power-frequency control for VSC-HVDC during island operation," in AC and DC Power Transmission, 2006. ACDC 2006. The 8th IEE International Conference on, 2006, pp. 177-181. Z. Chengyong, et al., "Parameters Optimization of VSC-HVDC Control System Based on Simplex Algorithm," in Power Engineering Society General Meeting, 2007. IEEE, 2007, pp. 1-7. W. Liying and N. Ertugrul, "Selection of PI compensator parameters for VSC-HVDC system using decoupled control strategy," in Universities Power Engineering Conference (AUPEC), 2010 20th Australasian, 2010, pp. 1-7. M. Davies, et al., "Latest control and protection innovations applied to the Basslink HVDC interconnector," in AC and DC Power Transmission, 2006. ACDC 2006. The 8th IEE International Conference on, 2006, pp. 30-35. F. Mazzoldi, et al., "Adaptation of the control equipment to permit 3-terminal operation of the HVDC link between Sardinia, Corsica and mainland Italy," Power Delivery, IEEE Transactions on, vol. 4, pp. 1269-1274, 1989. M. Sampei, et al., "Operating experience of Hokkaido-Honshu high voltage direct current link," Power Delivery, IEEE Transactions on, vol. 12, pp. 1362-1367, 1997. J. Chand, "Auxiliary power controls on the Nelson River HVDC scheme," Power Systems, IEEE Transactions on, vol. 7, pp. 398-402, 1992. C. Du, et al., "Use of VSC-HVDC for Industrial Systems Having Onsite Generation With Frequency Control," Power Delivery, IEEE Transactions on, vol. 23, pp. 2233-2240, 2008. C. Bajracharya, et al., "Understanding of tuning techniques of converter controllers for VSCHVDC," in Nordic Workshop on Power and Industrial Electronics (NORPIE/2008), June 9-11, 2008, Espoo, Finland, 2008. Z. Chengyong, et al., "A novel coordinated control strategy for improving the stability of frequency and voltage based on VSC-HVDC," in Electric Utility Deregulation and Restructuring and Power Technologies, 2008. DRPT 2008. Third International Conference on, 2008, pp. 2202-2206. M. Zhixin, et al., "Wind Farms With HVDC Delivery in Inertial Response and Primary Frequency Control," Energy Conversion, IEEE Transactions on, vol. 25, pp. 1171-1178, 2010. P. Kundur, Power system stability and control: Tata McGraw-Hill Education, 1994. T. K. Vrana, et al., "The CIGRE B4 DC Grid Test System," System, vol. 500, p. D1, 2013. C. P. A. B. Busarello. NEPLAN, power system analysis software. Available: www. neplan. ch
