Low Voltage Ride-Through Capability Improvement of Wind Power ...

RECENT ADVANCES in ENERGY & ENVIRONMENT

Low Voltage Ride-Through Capability Improvement of Wind Power Generation Using Dynamic Voltage Restorer NAOHIRO HASEGAWA, TERUHISA KUMANO Department of Electronics and Bioinformatics, School of Science & Technology Meiji University 1-1-1 Higashi-mita, Tama-ku, Kawasaki-shi, Kanagawa 214-8571 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 Ride-Through (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 energy storage capacity. Key-Words: - Wind power generation, Fixed-speed induction generator, Fault Ride-Through, Low Voltage RideThrough, Voltage sag, Dynamic Voltage Restorer, Energy storage turbine and the other is to increase electrical output of wind generator during fault and after fault clearance. The method to reduce mechanical input is represented by pitch angle controlling [2]. The methods to increase electrical output are represented by using mechanically switched capacitor [3], Static Var Compensator (SVC) [4], STATic synchronous COMpensator (STATCOM) [5]-[7], Unified Power Quality Conditioner (UPQC) [6],[7], Dynamic Voltage Restorer (DVR) [8], and Series braking resistor [9] etc. Pitch angle controlling can enhance LVRT capability and has advantage in the cost. But, the response of change in the pitch angle is slow in general, so that this technique has a possibility not enough to enhance LVRT capability. Using mechanically switched capacitor has also cost-effective. However, the ability to supply reactive power declines in proportion to the square of the voltage, thus it may degrade the contribution of the capacitor to enhance LVRT capability. The same thing can be said to the capacitor-based SVC. Reactive power output from STATCOM during low voltage is larger than SVC or capacitor, but STATCOM can not output during voltage sag in order to avoid injection of additional fault current into the power system. Therefore, STATCOM has to be operated after fault clearance. UPQC and DVR have good performance of LVRT enhancement. However, both techniques often require

1 Introduction In recent years, the total capacity of wind generation connected to the power system has been increased significantly due to its low environmental cost and low installation cost compared with other renewable energy. In this situation, the sudden disconnection of wind power generation due to the power system disturbance may collapse power balance between the power supply and the power demand. In response to this problem, transmission system operators have revised grid codes in many countries, and they require Fault Ride-Through (FRT) capability [1]. FRT is to keep connection of the wind power generator to the power system when power system disturbance (e.g. voltage sag and swell, over and under frequency etc.) occurs. In FRT, the case of voltage sag is called Low Voltage Ride-Through (LVRT). However, sudden voltage sag may cause unstable generator speeding because of an unbalance between input power (mechanical) and output power (electrical). In order to meet the LVRT, the stabilization of the generator and voltage recovery are needed. But it is very challenging for wind power generation, especially Fixed-Speed Induction Generator (FSIG) type wind power plants because it can not control its active and reactive power outputs. There are two methods to enhance LVRT capability of FSIG. One is to reduce mechanical input of wind

ISSN: 1790-5095

166

ISBN: 978-960-474-159-5


high capital cost because UPQC and DVR need twin inverter and energy storage device respectively. Series braking resister is cost-effective but it does not work effectively after the generator accelerates greatly. This paper studies LVRT capability enhancement of FSIG using DVR device, and proposes the method for 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 the simulation model. Section 4 shows numerical simulation results using EMTP-ATP, where two simulation cases are described. The first simulation examines the stabilization effects of the wind generator when Automatic Voltage Regulator (AVR) 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 in this work.

UPS used for outage compensation or STATCOM. This conventional role is modified to a wind generator protection in this work. General configuration of DVR consists of the series transformer, the harmonic filter, the voltage source converter and the energy storage device.

2 Application of DVR to Wind Power System

One-line diagram of the FSIG type wind farm and the power system with DVR is shown in Fig.1. In this figure, DVR boosts up the generator side voltage Vr regulated by the DVR output voltage VDVR in the event of supply side voltage Vs sags. By this voltage insertion, DVR can absorb the excess power that cannot be exported into the power system from the generator, and inject necessary reactive power. Block diagram of DVR controller in this work is presented in Fig. 2. DVR has the function of AVR because it aims to keep constant voltage usually. Although there are some methods of voltage insertion, In-Phase Compensation (IPC) is adopted in AVR considering that wind power system is robust against phase jump. IPC is the method that the injected DVR voltage is in phase with the supply side voltage regardless of current and the pre-fault voltage.

TR2

FSIG

TR2

Infinite bus TR1

VDVR

VPCC

Vs

Vr

TR_DVR

DVR

Fig.1 Fixed-speed wind farm with DVR

There are two influences that the short circuit in the power system exerts on FSIG type wind generator. Firstly, the generator accelerates during voltage sag caused by short circuit, so that it will be disconnected by the over-speed relay if it exceeds the maximum tolerable speed. This phenomenon results from the fact that the active power output from IG declines by the square of the terminal voltage, while the mechanical input from the wind turbine is almost constant. The maximum speed of generator depends on the residual voltage, the inertia of generator and turbine, input wind (mechanical) power and the duration of the fault. Secondly, huge amount of absorption of reactive power by IG after fault clearance may disturb terminal voltage recovery. As a result, the generator is further accelerated, and it will be tripped by over-speed or under-voltage relays. For these two reasons, voltage compensation is a good solution in order to avoid disconnection of wind generator (i.e. improvement of LVRT capability). Therefore, the authors use DVR as voltage sag compensator. DVR is a series solid state device that connects power system in order to regulate the load side voltage. It has been introduced for the purpose of protecting sensitive load such as semiconductor fabrication plant 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

ISSN: 1790-5095

FSIG

(AVR)

Vr_ref Vr_d abc

Vr

－

+

－

+

VDVR_d PI

dq

VDVR_q

Eq.1 Eq.2

θ PLL

φ

abc

I

Vr_q

Vr_d

0

Vs

abc

PI Vr_q

dq

amp

tan-1(Iq/Id)

ψ

Eq. 1: Vd=amp*cosθ, Vq=amp*sinθ [IPC] Eq. 2: Vd=amp*cos(ψ+φ+θ), Vq=amp*sin(ψ+φ+θ)

Fig.2 DVR control model

167

VDVR

dq

ISBN: 978-960-474-159-5


occurred. Subsection 1 presents the results in case of using AVR for its original purpose (to keep terminal voltage to be constant during fault). In particular, relation between DVR capacity and generator stabilizing effect for various compensation voltage (by changing Vr_ref in Fig. 2) is studied. Subsection 2 presents the case that DVR output inserts after fault clearance without compensation during fault. This subsection analyzes the influence when the DVR output voltage phase (ψ in Fig. 2) changes under the arbitrary voltage insertion. The generator delivers nominal power (0.877 p.u. 11.4MVA base) to the power system under constant nominal wind in both simulations. A voltage sag occurs at infinite bus during t=1.0 to 1.1 s, which simulates three-phase balanced short circuit. All simulation cases are carried out numerically using EMTP-ATP.

“Vr_ref” in this figure is the reference value of the generator side voltage Vr, Vr is adjusted by using PI controller in AVR for Vr_ref. In case of using AVR, VDVR changes depending on Vs and Vr. In addition to AVR, this control model can set “amp” (amplitude of VDVR) and “ψ” (phase angle between current and VDVR). In case of setting “amp” and “ψ”, it can control active and reactive power independently (Eq.2 in this figure). Eq.1 is used in case of using IPC as constant voltage. These schemes are showed in Fig.3 by phasor diagram. [IPC (AVR:variable VDVR, Eq.1: constant VDVR)] I Vr

φ

Vs

VDVR

[ψ = -90 degree] (only reactive power)

[ψ = 0 degree] (only active power)

4.1 Stabilizing effects using AVR I φ

φ

VDVR

φ

Fig. 4 shows the simulation results in case of using AVR. It compares the three cases; (1) no control, (2) voltage control with the reference value “Vr_ref” 1.0, and (3) 0.7 p.u. is used as the reference value. In case of 0.7 p.u. it is active only during fault (t= 1.0 to 1.1 s). In “No control” case, voltage oscillation and unsuccessful voltage recovery can be observed (see (a), (b)), and the generator reaches over-speed limit after t=3 s (see (c)). This oscillatory behavior of generator speed can be explained by the mechanical elasticity between the turbine and the generator. Active power output from generator is reduced greatly during the fault, which causes generator over-speeding in consequence (see (d)). In contrast, both case of using AVR can compensate terminal voltage, so that active power output of the generator increases during fault and generator speed is stabilized within one second though some speed increase is noted. As a result of this, the generator does not reach over-speed limit. The effects of reference value setting on the resultant acceleration cannot be observed too much. DVR output (apparent power) is momentarily exceeded 2.0 p.u. (in case of Vr_ref=1.0) immediately after the fault clearance because of over voltage due to PI controller 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 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 does not increase simply by a factor of 0.7 because of taking time to stabilize generator.

Ψ= -90

I

Vr

Vs VDVR

Vs Vr

Fig.3 Phasor diagram of DVR control method

3 Model Configuration This section describes simulation model. The studied system is shown in Fig.1. It is 11.4 MVA (10 MW) wind farm composed of 10 squirrel cage induction generators with a rating of 1.14 MVA (1 MW). Each generator is connected to DVR by 1.2 MVA transformer (TR2:690V/6600V). Shunt capacitors are adjusted so that the generator terminal voltage becomes 1.0 p.u. at nominal output power operation. Ratings of DVR and the series transformer are 11.4 MVA. These are connected to the power system by 11.4 MW transformer (TR1: 6600V/66000V). Wind farm is finally connected to infinite bus through double circuit transmission line. These parameters are presented in Appendix (Table 1, 2 and 3.). E.ON LVRT requirement [1] and 1.1 p.u. generator speed limit are assumed.

4 Simulation Results In this section the simulation results are shown concerning the influence of DVR given to the power system and the wind generator when voltage sag

ISSN: 1790-5095

168

ISBN: 978-960-474-159-5


1.1

1.4

1.5

1.2

1.06

0.9 Maximum speed Energy (t=1.1s) Energy (t=4s)

1.04

1.02

0.6

volta ge [pu]

1.2

Energy [MJ]

Maximum generator speed [pu]

1.0

1.08

0.8 0.6 0.4 No control

0.2

Vref=1.0

Vref=0.7

0.0 0

0.5

1

1.5

2 Time [s]

2.5

3

3.5

4

(a) DVR generator side voltage Vr

0.3 1.2 1.0

1

0 0.2

0.4

0.6

0.8

0.8

1

voltage [pu]

0

Vr_ref [pu]

Fig.5 Maximum generator speed and energy storage capacity of DVR in case that voltage reference of AVR is changed

0.4 0.2

No control

Vref=1.0

Vref=0.7

FRT(E.ON)

0.0 0

Fig. 5 shows the relation between the values of Vr_ref, maximum generator speed and energy storage capacity of DVR. Vr_ref is chosen from 0.4 p.u. to 1.0 p.u. , energy capacity is measured at t=1.1 s (just after fault clearance) and t=4.0 s. All these cases can meet LVRT. Though the maximum speed is decreased and the absorbed energy at 1.1 s is increased as Vr_ref increases, the absorbed energy at 4.0 s in case of low Vr_ref (0.4 and 0.5 p.u.) is more than the case of Vr_ref=0.6. This result is caused by the fact that it takes time to stabilize due to low compensation voltage. By these results, it can be concluded that slightly lowering compensation voltage leads to the energy storage capacity reduction.

0.5

1

1.5

2 Time [s]

2.5

3

3.5

4

(b) voltage at PCC 1.20

Generator Speed [pu]

1.15 1.10 1.05 1.00 0.95

No control

Vref=1.0

Vref=0.7

speed limit

0.90 0

0.5

1

1.5

2 Time [s]

2.5

3

3.5

4

(c) Generator speed genera tor a ctive power [pu]

4.0

4.2 Stabilizing effects in case of post-fault initiation of DVR

No control

3.0

Vref=1.0

Vref=0.7

2.0 1.0 0.0 -1.0 -2.0 0.9

1

1.1

1.2 Time [s]

1.3

1.4

1.5

(d) Generator active power output

This subsection studies the case that DVR is activated after fault clearance and does not use AVR. Compensation is started at t=1.12 s (1cycle delay after fault clearance), and DVR voltage amplifier “amp” (see Fig.2) is decreased from 0.1 p.u. to 0 p.u. in proportion to the elapsed time from control beginning to t=4.0 s in order to prevent over voltage after stabilization. DVR voltage phase “ψ” (see Fig. 2) is set 0 degree and -90 degree, and uses IPC (it follows supply side phase). The case of 0 and -90 degree correspond to active power absorption and reactive power injection respectively, which are defined active power compensation (APC) and reactive power compensation (RPC) respectively in this paper. Simulation results are showed in Fig. 6. It can be observed that the voltage is decreased greatly and generator speed increases temporarily in all cases

ISSN: 1790-5095

0.6

2.5 Vref=1.0

Appa rent Power [pu]

2.0

Vref=0.7

1.5 1.0 0.5 0.0 0

0.5

1

1.5

2 Time [s]

2.5

3

3.5

4

3

3.5

4

(e) DVR apparent power

Energy [MJ]

1.40 1.20

Vref=1.0

1.00

Vref=0.7

0.80 0.60 0.40 0.20 0.00 -0.20 0

0.5

1

1.5

2 Time [s]

2.5

(f) DVR energy strage capacity (positive: absorb energy)

Fig.4 Response of the wind turbine and DVR using AVR

169

ISBN: 978-960-474-159-5


because DVR is activated after fault clearance (see (a), (b) and (c)). However, in all cases of activating DVR increases voltage after fault clearance, so that it finally successfully stabilizes generator speed. The effects of voltage compensation and rotation stabilization are the biggest in IPC, then RPC. They are smallest in APC. It is thought that this result, in which RPC has a better performance than APC, arises from the characteristic of induction generator. In high generator speed situation compared with nominal operation point, induction generator absorbs a large amount of reactive power, while it cannot generate active power too much. This is showed in Fig. 7. Therefore, the lack of reactive power supply from DVR causes voltage drop because APC cannot supply reactive power at all. Apparent power outputs from DVR (see (d)) of the three cases are almost the same, and they become obviously low capacity compared with the case of using AVR though simple comparison might be misleading because of difference in voltage output. Although they have almost the same apparent power output, active power outputs are not the same as shown in (e). APC absorbs the largest active power, while IPC is the second. RPC does not absorb or inject active power except for the small oscillation. Consequently, energy storage capacity of DVR in case of using RPC is zero, which result may be very helpful because energy storage device such as battery or electric double-layer capacitor are expensive now. In addition, the cases of IPC and APC need bigger energy storage at t=4 s compared with the case of using AVR because long compensation time is necessary. By these results, we conclude that it is possible to stabilize only by handling reactive power in case of activating DVR after fault clearance. But, this method cannot be used in the case that generator reaches speed limit during fault.

1.2 1.0 volta ge [pu]

0.8 0.6 0.4 0.2

No control

APC

RPC

IPC

0.0 0

0.5

1

1.5

2 Time [s]

2.5

3

3.5

4

(a) DVR generator side voltage Vr 1.2 1.0

volta ge [pu]

0.8 0.6 0.4 No control RPC FRT(E.ON)

0.2

APC IPC

0.0 0

0.5

1

1.5

2 Time [s]

2.5

3

3.5

4

(b) voltage at PCC 1.20

Genera tor Speed [pu]

1.15 1.10 1.05 1.00 0.95

No control

APC

RPC

IPC

speed limit

0.90 0

0.5

1

1.5

2 Time [s]

2.5

3

3.5

4

3

3.5

4

3

3.5

4

(c) Generator speed 0.50 APC

Apparent Power [pu]

0.40

RPC IPC

0.30 0.20 0.10 0.00 0

0.5

1

1.5

2 Time [s]

2.5

(d) DVR apparent power 0.40 APC

Active Power [pu]

0.30

3

RPC

0.20

IPC

0.10 0.00 -0.10 0

0.5

1

1.5

2

2.5

-0.20

Reactive Power 2

Time [s]

Active Power

(e) DVR active power (positive: absorb power)

1.5 2.50 APC

2.00

1 Energy [MJ]

Generator Output [pu]

2.5

0.5 0 1

1.05

1.1 1.15 1.2 Generator speed [pu]

1.25

1.50

RPC IPC

1.00 0.50 0.00

1.3

-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

ISSN: 1790-5095

170

ISBN: 978-960-474-159-5


[2] L. Holdsworth, I. Charalambous, J.B. Ekanayake and N. Jenkins, “Power System Fault Ride Through capabilities of induction generator based wind turbines”, Wind Engineering, Vol. 28, No. 4, pp. 399-412 (2004) [3] A. Kehrli, M, Ross, “Understanding frid integration issues at wind farms and solution using voltage source converter FACTS technology”, IEEE Power Engineering Society General Meeting, vol. 3, pp. 1822-1828 (2003) [4] T. Ahmed, O Noro, E. Hiraki, and M. Nakaoka, “Terminal Voltage Regulation Characteristics by Static Var Compensator for a Three-Phase Self Excited Induction Generator”, IEEE Trans. Industry Applications, Vol. 40, No. 4, pp. 978-988 (2004) [5] L. Qi, J, Langston, and M. Steurer, “Applying a STATCOM for Stability Improvement to an Existing Wind Farm with Fixed-Speed Induction Generators”, IEEE Power and Engergy Society General Meeting, pp. 1-6 (2008) [6] M. F. Farias, M. G. Cendoya and P. E. Battaiotto, “Wind Farms in Weak Grids Enhancement of Ride-Through Capability Using Custom Power Systems”, IEEE/PES Transmission and Distribution Conference and Exposition Latin America 2008, pp. 1-5 (2008) [7] N.G.Jauamto, M. Basu, M.F. Conlon and K. Gaughan, “Rating requirements of the unified power quality conditioner to integrate the fixed-speed induction generator-type wind generation to the grid”, IET Renewable Power Generation, Vol. 3, pp. 133-143 (2009) [8] H. Gaztanaga, I. Etxeberria Otadui, S. Bacha and D. Roye, “Fixed-Speed Wind Farm Operation Improvement by Using DVR Devices”, IEEE International Symposium on Industrial Electronics 2007 (ISIE 2007), pp. 2679-2684 (2007) [9] Andrew Causebrook, David J. Atkinson and Alan G. Jack, “Fault Ride-Through of Large Wind Farms Using Series Dynamic Braking Resistors (March 2007)”, IEEE Trans. on Power Systems, Vol. 22, No. 3, pp. 966-975 (2007) [10] S. S. Choi, B. H. Li, and D. M. Vilathgamuwa, “Dynamic Voltage Restoration with Minimum Energy Injection”, IEEE Trans. on Power Systems, Vol.15, No.1, pp. 51-57 (2000) [11] Bharat Singh, and S. N. Singh, “Wind Power Interconnection into the Power System: A Review of Grid Code Requirements”, The Electricity Journal, vol. 22, Issue 5, pp. 54-63 (2009)

5 Conclusions This paper analyzes LVRT enhancement of FSIG based wind farm using DVR by numerical simulation. It simulates two DVR control methods, one is to use AVR by limiting output, while the other is to control voltage phase of DVR which is activated after fault clearance. This study concludes the following points. (1) The stabilizing effect using AVR has good performance, but DVR capacity and energy storage capacity tend to become large. (2) Limiting output using AVR can reduce DVR capacity and energy storage capacity, but the required energy capacity might increase in case of low compensation voltage on the contrary. (3) The method in which DVR is deactivated during fault can also stabilize. In particular the method with only reactive power injection has advantage because of small storage capacity.

6 Appendix Table 1 Wind generator parameters (1.14MVA base) Quantity Nominal apparent power Nominal power Nominal Voltage Nominal slip Stator resistance/reactance Rotor resistance/reactance Magnetizing reactance Generator/Turbine inertia Spring constant

Value 1.14[MVA] 1.0[MW] 690[V] -0.0091[pu] 0.0063/0.089[pu] 0.0095/0.092[pu] 2.85[pu] 0.5/3.0[s] 0.55[pu]

Table 2 Transformer parameters (self base) Quantity [TR1] Primary/secondary voltage [TR1] apparent power [TR1] resistance/reactance [TR2] Primary/secondary voltage [TR2] apparent power [TR2] resistance/reactance [TR_DVR] Primary/secondary voltage

[TR_DVR] apparent power [TR_DVR] resistance/reactance

Value 66/6.6[kV] 11.4[MVA] 0.008/0.08[pu] 6.6/0.69[kV] 1.2[MVA] 0.008/0.08[pu] 6.6/0.44[kV] 11.4[MVA] 0.008/0.08[pu]

Table 3 Grid parameter (1000MVA base) Quantity Line resistance/reactance

Value 0.286/3.217[pu]

References: [1] J. Schlabbach, “Low Voltage Fault Ride Through Criteria for Grid Connection of Wind Turbine Generators”, 5th International conference on European Electricity Market 2008, pp. 1-4 (2008)

ISSN: 1790-5095

171

ISBN: 978-960-474-159-5