RECENT ADVANCES in ENERGY & ENVIRONMENT

Low Voltage Ride-Through Capability Improvement of Generation Using Dynamic Voltage Restorer

NAOHIRO HASEGAWA, TERUHISA KUMANO Department of Electronics and Bioinformatics, School of Science & Technology Meiji University 111 Higashimita, Tamaku, Kawasakishi, Kanagawa 2148571 JAPAN [email protected], [email protected]

Abstract: - Recently, the total amount of generation from wind power plants has been increased all over the world. In this situation, a large amount of disconnection of wind generation may give a serious influence in the power system. Consequently, Low Voltage RideThrough (LVRT) is now required for wind power plants in many countries. This paper studies LVRT capability enhancement using Dynamic Voltage Restorer (DVR), especially it purposes to reduce DVR capacity. It shows that limit of DVR output and only reactive power output achieves to reduce device MVA rating capacity and capacity.

Key-Words: - Wind power generation, Fixedspeed , Fault RideThrough, Low Voltage Ride Through, Voltage sag, Dynamic Voltage Restorer, Energy storage

1 Introduction turbine and the other is to increase electrical output of wind generator during fault and after fault clearance. In recent years, the total capacity of wind generation The method to reduce mechanical input is represented connected to the power system has been increased by pitch angle controlling [2]. The methods to increase significantly due to its low environmental cost and low electrical output are represented by using installation cost compared with other renewable mechanically switched capacitor [3], Static Var energy. In this situation, the sudden disconnection of Compensator (SVC) [4], STATic synchronous wind power generation due to the power system COMpensator (STATCOM) [5][7], Unified Power disturbance may collapse power balance between the Quality Conditioner (UPQC) [6],[7], Dynamic power supply and the power demand. In response to Voltage Restorer (DVR) [8], and Series braking this problem, transmission system operators have resistor [9] etc. revised grid codes in many countries, and they require Pitch angle controlling can enhance LVRT Fault RideThrough (FRT) capability [1]. FRT is to capability and has advantage in the cost. But, the keep connection of the wind power generator to the response of change in the pitch angle is slow in general, power system when power system disturbance (e.g. so that this technique has a possibility not enough to voltage sag and swell, over and under frequency etc.) enhance LVRT capability. Using mechanically occurs. In FRT, the case of voltage sag is called Low switched capacitor has also costeffective. However, Voltage RideThrough (LVRT). However, sudden the ability to supply reactive power declines in voltage sag may cause unstable generator speeding proportion to the square of the voltage, thus it may because of an unbalance between input power degrade the contribution of the capacitor to enhance (mechanical) and output power (electrical). In order to LVRT capability. The same thing can be said to the meet the LVRT, the stabilization of the generator and capacitorbased SVC. Reactive power output from voltage recovery are needed. But it is very challenging STATCOM during low voltage is larger than SVC or for wind power generation, especially FixedSpeed capacitor, but STATCOM can not output during Induction Generator (FSIG) type wind power plants voltage sag in order to avoid injection of additional because it can not control its active and reactive power fault current into the power system. Therefore, outputs. STATCOM has to be operated after fault clearance. There are two methods to enhance LVRT capability UPQC and DVR have good performance of LVRT of FSIG. One is to reduce mechanical input of wind enhancement. However, both techniques often require

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high capital cost because UPQC and DVR need twin UPS used for outage compensation or STATCOM. inverter and energy storage device respectively. Series This conventional role is modified to a wind generator braking resister is costeffective but it does not work protection in this work. General configuration of DVR effectively after the generator accelerates greatly. consists of the series , the harmonic filter, This paper studies LVRT capability enhancement of the voltage source converter and the energy storage FSIG using DVR device, and proposes the method for device. decreasing the energy storage and inverter capacity of DVR. Section 2 describes how to use DVR to FSIG in order to enhance LVRT capability. Section 3 explains FSIG TR2 the simulation model. Section 4 shows numerical simulation results using EMTPATP, where two Infinite bus TR1 simulation cases are described. The first simulation FSIG TR2 VDVR examines the stabilization effects of the wind Vr Vs VPCC generator when Automatic Voltage Regulator (AVR) TR_DVR of DVR is operated with its output limitation. The second simulation shows the case that DVR is operated after fault clearance. Section 5 is conclusions DVR in this work. Fig.1 Fixedspeed with DVR

2 Application of DVR to Wind Power Oneline diagram of the FSIG type wind farm and System the power system with DVR is shown in Fig.1. In this figure, DVR boosts up the generator side voltage Vr There are two influences that the short circuit in the regulated by the DVR output voltage VDVR in the event power system exerts on FSIG type wind generator. of supply side voltage Vs sags. By this voltage Firstly, the generator accelerates during voltage sag insertion, DVR can absorb the excess power that caused by short circuit, so that it will be disconnected cannot be exported into the power system from the by the overspeed relay if it exceeds the maximum generator, and inject necessary reactive power. tolerable speed. This phenomenon results from the Block diagram of DVR controller in this work is fact that the active power output from IG declines by presented in Fig. 2. DVR has the function of AVR the square of the terminal voltage, while the because it aims to keep constant voltage usually. mechanical input from the is almost Although there are some methods of voltage insertion, constant. The maximum speed of generator depends InPhase Compensation (IPC) is adopted in AVR on the residual voltage, the inertia of generator and considering that wind power system is robust against turbine, input wind (mechanical) power and the phase jump. IPC is the method that the injected DVR duration of the fault. Secondly, huge amount of voltage is in phase with the supply side voltage absorption of reactive power by IG after fault regardless of current and the prefault voltage. clearance may disturb terminal voltage recovery. As a Vr_ref (AVR) result, the generator is further accelerated, and it will Vr_d VDVR_d + VDVR abc － PI dq be tripped by overspeed or undervoltage relays. For Vr dq PI abc these two reasons, voltage compensation is a good Vr_q VDVR_q － + solution in order to avoid disconnection of wind Vr_d Vr_q 0 Eq.1 generator (i.e. improvement of LVRT capability). θ Vs PLL Eq.2 Therefore, the authors use DVR as voltage sag compensator. φ abc amp I tan1(Iq/Id) DVR is a series solid state device that connects dq ψ power system in order to regulate the load side voltage. It has been introduced for the purpose of protecting Eq. 1: Vd=amp*cosθ, Vq=amp*sinθ [IPC] sensitive load such as semiconductor fabrication plant Eq. 2: Vd=amp*cos(ψ+φ+θ), Vq=amp*sin(ψ+φ+θ) from power system disturbance (e.g. voltage sag, swell, harmonics, fault current etc.). It can compensate for voltage sag by low device capacity compared with Fig.2 DVR control model

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“Vr_ref” in this figure is the reference value of the occurred. Subsection 1 presents the results in case of generator side voltage Vr, Vr is adjusted by using PI using AVR for its original purpose (to keep terminal controller in AVR for Vr_ref. In case of using AVR, voltage to be constant during fault). In particular, VDVR changes depending on Vs and Vr. In addition to relation between DVR capacity and generator AVR, this control model can set “amp” (amplitude of stabilizing effect for various compensation voltage (by VDVR) and “ψ” (phase angle between current and changing Vr_ref in Fig. 2) is studied. Subsection 2 VDVR). In case of setting “amp” and “ψ”, it can control presents the case that DVR output inserts after fault active and reactive power independently (Eq.2 in this clearance without compensation during fault. This figure). Eq.1 is used in case of using IPC as constant subsection analyzes the influence when the DVR voltage. These schemes are showed in Fig.3 by phasor output voltage phase (ψ in Fig. 2) changes under the diagram. arbitrary voltage insertion. The generator delivers nominal power (0.877 p.u. [IPC (AVR:variable VDVR, Eq.1: constant VDVR)] 11.4MVA base) to the power system under constant I nominal wind in both simulations. A voltage sag φ Vr occurs at infinite bus during t=1.0 to 1.1 s, which

Vs VDVR simulates threephase balanced short circuit. All simulation cases are carried out numerically using EMTPATP. [ψ = 0 degree] [ψ = 90 degree] (only active power) (only reactive power) 4.1 Stabilizing effects using AVR Ψ= 90 I Vr I Fig. 4 shows the simulation results in case of using

φ φ VDVR φ Vs AVR. It compares the three cases; (1) no control, (2)

Vs VDVR voltage control with the reference value “Vr_ref” 1.0, and (3) 0.7 p.u. is used as the reference value. In case Vr of 0.7 p.u. it is active only during fault (t= 1.0 to 1.1 s). Fig.3 Phasor diagram of DVR control method In “No control” case, voltage oscillation and unsuccessful voltage recovery can be observed (see (a), 3 Model Configuration (b)), and the generator reaches overspeed limit after t=3 s (see (c)). This oscillatory behavior of generator This section describes simulation model. The speed can be explained by the mechanical elasticity studied system is shown in Fig.1. It is 11.4 MVA (10 between the turbine and the generator. Active power MW) wind farm composed of 10 squirrel cage output from generator is reduced greatly during the induction generators with a rating of 1.14 MVA (1 fault, which causes generator overspeeding in MW). Each generator is connected to DVR by 1.2 consequence (see (d)). In contrast, both case of using MVA transformer (TR2:690V/6600V). Shunt AVR can compensate terminal voltage, so that active capacitors are adjusted so that the generator terminal power output of the generator increases during fault voltage becomes 1.0 p.u. at nominal output power and generator speed is stabilized within one second operation. Ratings of DVR and the series transformer though some speed increase is noted. As a result of this, are 11.4 MVA. These are connected to the power the generator does not reach overspeed limit. The system by 11.4 MW transformer (TR1: effects of reference value setting on the resultant 6600V/66000V). Wind farm is finally connected to acceleration cannot be observed too much. DVR infinite bus through double circuit transmission line. output (apparent power) is momentarily exceeded 2.0 These parameters are presented in Appendix (Table 1, p.u. (in case of Vr_ref=1.0) immediately after the fault 2 and 3.). E.ON LVRT requirement [1] and 1.1 p.u. clearance because of over voltage due to PI controller generator speed limit are assumed. delay (see (e)). This problem is expected to be mitigated by adjustment of PI parameter. Energy storage capacity of DVR is shown in (f). Though it is 4 Simulation Results true that the energy storage capacity in the case of Vr_ref=0.7 p.u. is lower than the case of 1.0 p.u., it In this section the simulation results are shown does not increase simply by a factor of 0.7 because of concerning the influence of DVR given to the power taking time to stabilize generator. system and the wind generator when voltage sag

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1.1 1.5 1.4 1.2 1.0 1.08 1.2 0.8 0.6 0.4

1.06 0.9 voltage[pu] No control Vref=1.0 0.2 Maximum Vref=0.7 speed 0.0 1.04 Energy 0.6 0 0.5 1 1.5 2 2.5 3 3.5 4 (t=1.1s) [MJ]Energy Time [s] Energy (t=4s) (a) DVR generator side voltage Vr

Maximumgeneratorspeed[pu] 1.02 0.3 1.2

1.0 1 0 0.8 0 0.2 0.4 0.6 0.8 1 0.6 Vr_ref [pu] 0.4

voltage[pu] No control Vref=1.0 Fig.5 Maximum generator speed and energy storage 0.2 Vref=0.7 FRT(E.ON) capacity of DVR in case that voltage reference of 0.0 AVR is changed 0 0.5 1 1.5 2 2.5 3 3.5 4 Time [s] Fig. 5 shows the relation between the values of (b) voltage at PCC Vr_ref, maximum generator speed and energy storage 1.20 capacity of DVR. Vr_ref is chosen from 0.4 p.u. to 1.0 1.15 p.u. , energy capacity is measured at t=1.1 s (just after 1.10 fault clearance) and t=4.0 s. All these cases can meet 1.05 LVRT. Though the maximum speed is decreased and 1.00 0.95 the absorbed energy at 1.1 s is increased as Vr_ref No control Vref=1.0 Vref=0.7 speed limit GeneratorSpeed[pu] 0.90 increases, the absorbed energy at 4.0 s in case of low 0 0.5 1 1.5 2 2.5 3 3.5 4 Vr_ref (0.4 and 0.5 p.u.) is more than the case of Time [s] Vr_ref=0.6. This result is caused by the fact that it (c) Generator speed takes time to stabilize due to low compensation 4.0 No control Vref=1.0 voltage. 3.0 Vref=0.7 By these results, it can be concluded that slightly 2.0 lowering compensation voltage leads to the energy 1.0 storage capacity reduction. 0.0 -1.0

-2.0 generatoractivepower [pu] 4.2 Stabilizing effects in case of post-fault 0.9 1 1.1 1.2 1.3 1.4 1.5 Time [s] initiation of DVR (d) Generator active power output

This subsection studies the case that DVR is 2.5 activated after fault clearance and does not use AVR. Vref=1.0 2.0 Compensation is started at t=1.12 s (1cycle delay after Vref=0.7 1.5 fault clearance), and DVR voltage amplifier “amp” 1.0 (see Fig.2) is decreased from 0.1 p.u. to 0 p.u. in 0.5

proportion to the elapsed time from control beginning Apparent[pu] Power 0.0 to t=4.0 s in order to prevent over voltage after 0 0.5 1 1.5 2 2.5 3 3.5 4 stabilization. DVR voltage phase “ψ” (see Fig. 2) is set Time [s] 0 degree and 90 degree, and uses IPC (it follows (e) DVR apparent power 1.40 supply side phase). The case of 0 and 90 degree 1.20 Vref=1.0 correspond to active power absorption and reactive 1.00 Vref=0.7 0.80 power injection respectively, which are defined active 0.60 power compensation (APC) and reactive power 0.40

Energy[MJ] 0.20 compensation (RPC) respectively in this paper. 0.00 Simulation results are showed in Fig. 6. It can be -0.20 0 0.5 1 1.5 2 2.5 3 3.5 4 Time [s] observed that the voltage is decreased greatly and (f) DVR energy strage capacity (positive: absorb energy) generator speed increases temporarily in all cases Fig.4 Response of the wind turbine and DVR using AVR

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because DVR is activated after fault clearance (see (a), 1.2 (b) and (c)). However, in all cases of activating DVR 1.0 increases voltage after fault clearance, so that it finally 0.8 successfully stabilizes generator speed. The effects of 0.6 0.4

voltage compensation and rotation stabilization are the voltage[pu] No control APC 0.2 biggest in IPC, then RPC. They are smallest in APC. It RPC IPC 0.0 is thought that this result, in which RPC has a better 0 0.5 1 1.5 2 2.5 3 3.5 4 Time [s] performance than APC, arises from the characteristic (a) DVR generator side voltage Vr of induction generator. In high generator speed 1.2 situation compared with nominal operation point, 1.0 induction generator absorbs a large amount of reactive 0.8 power, while it cannot generate active power too much. 0.6 This is showed in Fig. 7. Therefore, the lack of 0.4 No control APC

voltage[pu] 0.2 RPC IPC reactive power supply from DVR causes voltage drop FRT(E.ON) because APC cannot supply reactive power at all. 0.0 0 0.5 1 1.5 2 2.5 3 3.5 4 Apparent power outputs from DVR (see (d)) of the Time [s] three cases are almost the same, and they become (b) voltage at PCC obviously low capacity compared with the case of 1.20 using AVR though simple comparison might be 1.15 misleading because of difference in voltage output. 1.10 Although they have almost the same apparent power 1.05 1.00 output, active power outputs are not the same as 0.95 No control APC RPC IPC speed limit shown in (e). APC absorbs the largest active power, GeneratorSpeed[pu] 0.90 while IPC is the second. RPC does not absorb or inject 0 0.5 1 1.5 2 2.5 3 3.5 4 Time [s] active power except for the small oscillation. (c) Generator speed

Consequently, energy storage capacity of DVR in case 0.50 APC of using RPC is zero, which result may be very helpful 0.40 RPC because energy storage device such as battery or 0.30 IPC electric doublelayer capacitor are expensive now. In 0.20 addition, the cases of IPC and APC need bigger energy 0.10

storage at t=4 s compared with the case of using AVR Apparent[pu] Power 0.00 because long compensation time is necessary. 0 0.5 1 1.5 2 2.5 3 3.5 4 By these results, we conclude that it is possible to Time [s] stabilize only by handling reactive power in case of (d) DVR apparent power activating DVR after fault clearance. But, this method 0.40 APC 0.30 cannot be used in the case that generator reaches speed RPC 0.20 limit during fault. IPC 0.10

3 0.00

Active Power ActivePower [pu] -0.10 0 0.5 1 1.5 2 2.5 3 3.5 4 2.5 Reactive Power -0.20 Time [s] 2 Active Power (e) DVR active power (positive: absorb power) 1.5 2.50 APC 2.00 1 RPC GeneratorOutput[pu] 1.50 IPC 0.5 1.00

0.50 0 Energy[MJ] 1 1.05 1.1 1.15 1.2 1.25 1.3 0.00 Generator speed [pu] -0.50 0 0.5 1 1.5 2 2.5 3 3.5 4 Time [s] Fig.7 An example of generator output – speed curve (f) DVR energy strage capacity (positive: absorb energy)

Fig.6 Response of the wind turbine and DVR in case of starting operation after fault clearance

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5 Conclusions [2] L. Holdsworth, I. Charalambous, J.B. Ekanayake and N. Jenkins, “Power System Fault Ride This paper analyzes LVRT enhancement of FSIG Through capabilities of induction generator based based wind farm using DVR by numerical simulation. wind turbines”, Wind Engineering, Vol. 28, No. 4, It simulates two DVR control methods, one is to use pp. 399412 (2004) AVR by limiting output, while the other is to control [3] A. Kehrli, M, Ross, “Understanding frid voltage phase of DVR which is activated after fault integration issues at wind farms and solution using clearance. This study concludes the following points. voltage source converter FACTS technology”, (1) The stabilizing effect using AVR has good IEEE Society General Meeting, performance, but DVR capacity and energy vol. 3, pp. 18221828 (2003) storage capacity tend to become large. [4] T. Ahmed, O Noro, E. Hiraki, and M. Nakaoka, (2) Limiting output using AVR can reduce DVR “Terminal Voltage Regulation Characteristics by capacity and energy storage capacity, but the Static Var Compensator for a ThreePhase Self required energy capacity might increase in case Excited Induction Generator”, IEEE Trans. of low compensation voltage on the contrary. Industry Applications, Vol. 40, No. 4, pp. 978988 (3) The method in which DVR is deactivated during (2004) fault can also stabilize. In particular the method [5] L. Qi, J, Langston, and M. Steurer, “Applying a with only reactive power injection has advantage STATCOM for Stability Improvement to an because of small storage capacity. Existing Wind Farm with FixedSpeed Induction Generators”, IEEE Power and Engergy Society 6 Appendix General Meeting, pp. 16 (2008) [6] M. F. Farias, M. G. Cendoya and P. E. Battaiotto,

Table 1 Wind generator parameters (1.14MVA base) “Wind Farms in Weak Grids Enhancement of Quantity Value RideThrough Capability Using Custom Power Nominal apparent power 1.14[MVA] Systems”, IEEE/PES Transmission and Nominal power 1.0[MW] Distribution Conference and Exposition Latin Nominal Voltage 690[V] America 2008, pp. 15 (2008) Nominal slip 0.0091[pu] [7] N.G.Jauamto, M. Basu, M.F. Conlon and K. Stator resistance/reactance 0.0063/0.089[pu] Rotor resistance/reactance 0.0095/0.092[pu] Gaughan, “Rating requirements of the unified Magnetizing reactance 2.85[pu] power quality conditioner to integrate the Generator/Turbine inertia 0.5/3.0[s] fixedspeed induction generatortype wind Spring constant 0.55[pu] generation to the grid”, IET Renewable Power Generation, Vol. 3, pp. 133143 (2009) Table 2 Transformer parameters (self base) [8] H. Gaztanaga, I. Etxeberria Otadui, S. Bacha and Quantity Value D. Roye, “FixedSpeed Wind Farm Operation [TR1] Primary/secondary voltage 66/6.6[kV] [TR1] apparent power 11.4[MVA] Improvement by Using DVR Devices”, IEEE [TR1] resistance/reactance 0.008/0.08[pu] International Symposium on Industrial Electronics [TR2] Primary/secondary voltage 6.6/0.69[kV] 2007 (ISIE 2007), pp. 26792684 (2007) [TR2] apparent power 1.2[MVA] [9] Andrew Causebrook, David J. Atkinson and Alan [TR2] resistance/reactance 0.008/0.08[pu] G. Jack, “Fault RideThrough of Large Wind [TR_DVR] Primary/secondary voltage 6.6/0.44[kV] [TR_DVR] apparent power 11.4[MVA] Farms Using Series Dynamic Braking Resistors [TR_DVR] resistance/reactance 0.008/0.08[pu] (March 2007)”, IEEE Trans. on Power Systems, Vol. 22, No. 3, pp. 966975 (2007) Table 3 Grid parameter (1000MVA base) [10] S. S. Choi, B. H. Li, and D. M. Vilathgamuwa, Quantity Value “Dynamic Voltage Restoration with Minimum Line resistance/reactance 0.286/3.217[pu] Energy Injection”, IEEE Trans. on Power Systems, Vol.15, No.1, pp. 5157 (2000) References: [11] Bharat Singh, and S. N. Singh, “Wind Power [1] J. Schlabbach, “Low Voltage Fault Ride Through Interconnection into the Power System: A Review Criteria for Grid Connection of Wind Turbine of Requirements”, The Electricity Generators”, 5th International conference on Journal, vol. 22, Issue 5, pp. 5463 (2009) European Electricity Market 2008, pp. 14 (2008)

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