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A control strategy to overcome LVRT issues in wind turbine driven squirrel cage induction generator.

INTRODUCTION

The utilization of renewable energy over power generation is becoming more popular because of pollution free environment. Hence this energy can be transformed into electricity with the help of power plants [1]. Wind energy is one among the major sources of renewable energy which employs wind turbine technology to produce electricity. Nowadays wind turbines are associated with three different generators namely Squirrel Cage Induction Generator (SCIG) [2], Doubly Fed Induction Generator (DFIG), Permanent Magnet Synchronous Generator (PMSG).

SCIG is a fixed or constant speed wind turbine whereas DFIG and PMSG are variable speed wind turbines. Most of the existing wind power plants in India are equipped with SCIG. The main reason for adopting SCIG is because of it's to low cost, robustness and simplicity in construction. The wind power plants using SCIG will have to face some of the challenges like voltage collapse, reactive power compensation and power factor correction [3]. In the recent studies it is observed that LVRT is becoming a major issue. These wind turbines due to LVRT issue leads to grid voltage collapse [4].

When the system voltage drops below 10%-20%, the wind turbine gets disconnected from the grid. Voltage drop is noticed at the point of common coupling (PCC) between the wind farm and the electricity network. Hence this voltage drop is due to the consumption of reactive power by the SCIG. Therefore many countries developed their own grid code requirements regarding wind turbine low voltage ride through. The technical requirements that should be satisfied for proper operation of any power system are termed as grid cods. Grid codes are issued by the transmission system operators (TSO) which vary from country to country according to the requirement. Recently in 2014 the Taiwan government reveals the effect of LVRT in first offshore wind farm planning which is analyzed by using various FACTS devices according to their Grid Code requirement [5]. Grid codes are initially developed by Germany in the year 2004 [6].

The implementation of LVRT requirement needs some of the modification in system design by installing some additional hardware or by improving the control strategy of power electronic equipment in case of DFIG. Hence, an FACTS device called Static Synchronous Compensator (STATCOM) is employed to overcome this issue. STATCOM is a shunt connected device based on a power electronic voltage source converter which converts the DC power into AC power of variable magnitude and phase angle [7]. It is mainly used to supply or absorb reactive power into the grid. STATCOM is also called regulating device that regulates the flow of reactive power flow into the system. This control strategy can technically manage the power levels associated with the wind turbines [8]. It supports the electricity network to maintain power factor near to unity so that the system balance will be maintained throughout the operation. This paper provides a simulation analysis of wind turbine driven SCIG with LVRT protection using STATCOM.

The paper is organized as follows: Section II deals with the overall system description. The LVRT issue is discussed in Section III. Section IV describes the principle of operation of STATCOM whereas the simulation analysis and results are presented in Section V.

System Description:

The above fig shows the overall operation of Existing wind turbine technology. It consist of various components namely Wind Turbine Rotor, Gear Box, Squirrel Cage Induction Generator, Transformer and Grid. The wind turbine can run at three different wind speeds namely, (i) Cut in wind speed, (ii) Base wind speed, (iii) Cut out wind speed. The maximum power output is obtained between base and cut out wind speed. Gear box is connected between low speed shaft and high speed shaft to match the speed of the turbine rotor with the generator. SCIG is an asynchronous generator that works on the principle of faradays law of electromagnetic induction. It consists of two modes of operation. They are (i) Motoring Mode and (ii) Generating Mode which means that it can either act as a motor or generator. In motoring mode, the induction machine consumes electrical energy from the grid at sub synchronous speed with positive slip. Ultimately in generating mode the induction machine produce electricity at super synchronous speed with negative slip. SCIG is associated with 2% slip.

Motor ([N.sub.r] < [N.sub.s]), Slip = [[N.sub.s] - [N.sub.r]]/[N.sub.s] [right arrow] Positive (1)

Generator ([N.sub.r] > [N.sub.s]), Slip = [[N.sub.s] - [N.sub.r]]/[N.sub.s] [right arrow] Negative (2)

Induction generator requires some of the external reactive power to drive the generator which is taken from the utility grid. Whenever the reactive power is consumed there occurs a voltage dip in the grid which results in disconnection of wind turbine from the grid is termed as LVRT. To compensate this issue STATCOM is used.

The above block diagram shows the basic operation of the proposed model. STATCOM plays a major role in the operation of proposed system which is used for reactive power compensation. When a Squirrel Cage Induction Generator is about to consume reactive power from the grid during operation STATCOM supplies reactive power to maintain grid stability so that the power factor can be maintained.

III. Low Voltage Ride Through:

The Indian power system is a group of number of agencies which includes generation, transmission, distribution activities [9]. Each and every customer connected to the electricity board either power consumer or producer will have certain requirements. These technical requirements are termed as grid codes.

Grid codes vary from country to country according to the requirement which depends on the power system and the protection employed. The wind generators are not allowed to remain connected to the grid when the voltage dips below 80%. It is forced to get disconnected from the grid even when the fault occurred far away from the wind farm.

Once the wind turbine is disconnected from the grid the recovery voltage also increases because of reactive power lagging [4]. As of wind turbine generators Grid code demand involves LVRT, reactive power compensation and power factor correction.

LVRT is the imbalance in the voltage levels produced between the active power generated from the generator and the active power to be supplied to the grid. LVRT requirement is described by voltage vs. time characteristics shows that the wind turbine is not allowed to get disconnected over a period of 0 ms to 3000 ms i.e. above the dark line. LVRT requirements expects fast active and reactive power restoration to the pre fault values even after the system returns to the normal operating levels.

IV. Statcom:

STATCOM is otherwise called Static Synchronous Condenser (STATCON). It is a shunt connected FACTS device with a power electronic voltage source converter, coupling transformer and a source or sink. The coupling transformer is used to allow AC power and block the DC power flow into the system. The energy stored in the reactive elements is called reactive power.

STATCOM regulates the reactive power flow into the system. Fig.4. shows the single line diagram of STATCOM. It acts as a voltage compensation device which can maintain constant voltage across the transmission lines. A capacitor is used to supply DC power to the voltage source converter.

The voltage at the capacitor should be as high as enough for the STATCOM to supply reactive power into the network. When the diodes are conducting converter operation takes place. The response time of STATCOM is less compared to SVC because of the fast switching of Insulated Gate Bipolar Transistor (IGBT) switches used in Voltage Source Converter [10].

The voltage source converter consists of six IGBT switches which denote the three arm bridge circuit. Top three switches are called positive group switches and the bottom three switches are called negative group switches. When the IGBT switches are conducting the inverter operation takes place similarly

The reactive power compensation is explained with the help of power exchange graph is shown in Fig. 6. Reactive power exchange in any system is controlled by controlling the amplitude of output voltage. If the amplitude of STATCOM voltage is higher than the line voltage then the STATCOM supplies reactive power into the line similarly if the amplitude of line voltage is higher than the STATCOM voltage then it absorbs the reactive power. In both cases IGBT switches are energized.

V. Simulation And Results:

The wind turbine runs at a base wind speed of 12 m/s is shown in Fig. 8. Even for the step change in wind speed the output of the wind turbine is as same as the output obtained at base wind speed except for wind speeds between cut-in and base values. The step change in wind speed is shown in Fig. 9. The power curve shows that the wind turbine is designed with a power output of 2 MW.

The three phase grid voltage and current under normal operating condition is shown in Fig. 11. and Fig. 12. The voltage and currents are mentioned in pu. Initially the starting current is high over a period of Oms to 0.1ms.

Fig. 13. and Fig. 14. shows the three phase output voltage and current under LVRT condition. The voltage dip is introduced with the help of three phase fault over a period of 0.4ms to 0.6ms. The voltage dip indicates the LVRT condition in the wind turbine technology. It is observed that during voltage dip there will be more current.

Fig. 17. and Fig. 18. shows the three phase output voltage and current waveform of STATCOM. The output voltage of the grid is measured and given as a feedback to the pulse generator. Whenever the dip occurs pulses are generated to activate the IGBT switches to generate the reactive power to balance the LVRT condition.

Conclusion:

In this paper the impact of LVRT in wind turbine driven SCIG is analyzed and illustrated. The analysis proves that the LVRT capability of SCIG is very poor which leads to the disconnection of wind turbines from the grid. As per the grid code requirements these wind turbines have to satisfy certain demands like reactive power compensation to improve the LVRT capability of SCIG. Hence this demand can be satisfied with the help of an FACTS device called STATCOM. This device would provide high amounts of reactive power during faults, to effectively support the terminal voltage and therefore compensate the magnitude of the voltage dip experienced by the wind turbines.

REFERENCES

[1.] Xiangwu Yan, Liming Yang, Bo Zhang, Lei Sun and Yuzhao Liang, 2013. "The Transition Process Of Wind Turbine Based On Squirrel Cage Induction Generator When Voltage Sag and Its Low Voltage Ride Through Method," in IEEEproc. Pow electron.

[2.] Shrikant Mali, Steffy James and Ishwari Tank, 2014. "Improving Low Voltage Ride Through Capabilities for Grid Connected Wind Turbine Generator," in Elsevier Proc. Science Direct., pp: 530-540.

[3.] Tsili, M., Ch. Patsiouras and S. Papathanssiou, 2011. "Grid Code Requirements for Large Wind Farms: A Review of Technical Regulations and Available Wind Turbine Technologies," in proc. EWEC, pp: 1-11.

[4.] Wen-tsan Liu, Yuan-kang Wu, Ching-yin Lee and Chao-rong Chen, 2011. "Effect of Low Voltage Ride Through Technologies On The First Taiwan Offshore Wind Farm Planning," in IEEE trans.sustain.energy., 2(1): 78-86.

[5.] Marta Molinas, Jon Are Suul, Tore Undeland, 2008. "Low Voltage Ride Through Of Wind Farms With Cage Generators: STATCOM Versus SVC," IEEE trans. pow electron., 23(3): 1104-1117.

[6.] Nima Saadat, S.S., Choi and D. Mahinda Vilarhgamuwa, 2015. "A Statistical Evaluation of The Capability of Distributed Renewable Generator-Energy-Storage System in Providing Load Low-Voltage Ride-Through," IEEE Trans. Power Del., 30(3): 1128-1136.

[7.] Dionisio Ramirez, Sergio Martinez, Carlos A. Platero, Francisco Blazquez and Rosa M. de Castro, 2011. "Low-Voltage Ride-Through Capability for Wind Generators Based on Dynamic Voltage Restorers," IEEE Trans. Energy Convers., 26(1): 195-203.

[8.] George, C., Konstantopoulos and Antonio T. Alexandridis, 2014. "Full-Scale Modeling, Control, and Analysis of Grid-Connected Wind Turbine Induction Generators with Back-To-Back AC/DC/AC Converters," IEEE Trans. Emerg. Sel. Topics Power Electron., 2(4): 739-748.

[9.] Andres E. Leon, Marcelo F. Farias, Pedro E. Battaiotto, Jorge A. Solsona and Maria Ines Valla, 2011. "Control Strategy of a DVR to Improve Stability in Wind Farms Using Squirrel-Cage Induction Generators," IEEE Trans. Power Syst., 26(3): 1609-1617.

[10.] Sharad W. Mohod and Mohan V. Aware, 2010. "A STATCOM-Control Scheme for Grid Connected Wind Energy System for Power Quality Improvement," IEEE Syst Jounl., 4(3): 346-352.

(1) M. Bijo Merlin, (2) Dr. P. S. Mayurappriyan, (3) M. Saravanan, (4) A. Subashini

(1) PG scholar, KCG College of Technology Chennai, Tamil Nadu

(2) Professor KCG College of Technology Chennai, Tamil Nadu

(3) Assistant Director-Technical, National Institute of Wind Energy

(4) PG scholar, KCG College of Technology Chennai, Tamil Nadu

Received 7 June 2016; Accepted 12 October 2016; Available 20 October 2016

Address For Correspondence:

M. Bijo Merlin, PG scholar, KCG College of Technology Chennai, Tamil Nadu

E-mail: bijo.merlin@gmail.com

Caption: Fig. 1: Existing system

Caption: Fig. 2: Proposed system

Caption: Fig. 3: LVRT requirements

Caption: Fig. 4: STATCOM

Caption: Fig. 5: Voltage source converter

Caption: Fig. 6: Power exchange graph

Caption: Fig. 7: V-I Characteristics of STATCOM

Caption: Fig. 8: Base wind speed

Caption: Fig. 9: Step change in wind speed

Caption: Fig. 10: Power curve

Caption: Fig. 11: Output voltage

Caption: Fig. 12: Output current

Caption: Fig. 13: Output voltage under LVRT

Caption: Fig. 14: Output current under LVRT

Caption: Fig. 15: Rotor speed

Caption: Fig. 16: Generator torque
Table 1: Technical specification

Quantity                          Value

Rated Output Power                2 MW
Cut in wind speed                 3.5 m/s
Cut out wind speed                25 m/s
Base Wind Speed                   12 m/s
Rotor Radius                      50 m
Air Density                       1.225 kg/[m.sup.3]
Rated Line-to-line Voltage        690 V
Rated Phase Voltage               398.4 V
Rated Frequency                   50 HZ
Number of Pole Pairs              2
Stator Winding Resistance, Rs     0.01965 pu
Rotor Winding Resistance, Rr      0.01909 pu
Stator Leakage Inductance, Lls    0.0397 pu
Rotor Leakage Inductance, Lr      0.0397 pu
Magnetizing Inductance, Lm        1.354 pu
Inertia Time Constant, H          0.952
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Article Details
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Title Annotation:low voltage ride through
Author:Merlin, M. Bijo; Mayurappriyan, P.S.; Saravanan, M.; Subashini, A.
Publication:Advances in Natural and Applied Sciences
Article Type:Report
Geographic Code:9INDI
Date:Sep 15, 2016
Words:2355
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