Power Quality Comparison of Grid Connected Wind Energy System Using STATCOM & UPQC

Resincap Journal of Science and Engineering

Volume 4, Issue 3, March 2020 ISSN: 2456-9976

906

Power Quality Comparison of Grid Connected Wind Energy System Using STATCOM & UPQC

Ms. Swapnshila Sunil Gaikwad PG Student

Electrical Engg.Dept.

MSS’s college of Engineering &

Technology Jalna,

[email protected]

Prof. K Chandra Obula Reddy HOD

Electrical Engg.Dept.

MSS’s college of Engineering &

Technology Jalna, [email protected]

ABSTRACT

Renewable energy sources like solar, wind, tidal and hybrid each contribute major amount of power to generate electricity.

Earlier days fossil fuels are used largely to extract electricity.

Today due to shortage of fuels and environmental pollution caused by greenhouse gases, renewable energy has come to an effect. So, wind power penetration is increased worldwide in present days. Due to the combination of penetration level of wind generation in to existing power grid has important impact on the ability system operation. The connection of large wind farms to power grid may result in power quality problems. Power quality problems such as current and voltage harmonics, voltage sag / swell, voltage unbalance etc. have become the major reasons causing the malfunction of the electrical equipment. Poor power quality may cause various serious impacts on electrical loads and ultimately results in huge economic losses. Mitigation of voltage dips and swells and harmonics in a grid-connected wind energy conversion systems (WECS) employing multiple permanent magnet synchronous generator (PMSG) will be investigated (unified power quality conditioner) UPQC. So UPQC plays a major role to overcome the power quality problems. The performance of the designed custom power devices like DVR, STATCOM, UPQC etc. in the grid to which the wind energy systems are connected and evaluated using the MAT LAB /SIMULINK software. Power quality problems such as voltage dip, voltage swell and harmonic contents are mitigated in a grid to which a WECS is connected. The results shows that the quality of power is increased with CPD.

Keywords

1. INTRODUCTION

Now a days energy demand is increasing rapidly, due to the growth in population and economic development in the world leading to increase in environmental impact on conventional plants. Hence renewable energy resources must be employed in order to meet the energy demand and have communal development and prolong growth. In recent years, among the other renewable energy sources, wind energy is gaining ever increasing attention as a clean, safe and economical resource.

Thus to exploit wind power effectively its grid connection is necessary so as to realize its potential to significantly mitigate present day problems like energy demand along with atmospheric pollution. But amalgamation of wind power to grid introduces power quality issues, which predominantly consist of voltage regulation and reactive power

compensation. The power quality is a crucial customer- focused measure and is of prominent importance to the wind turbine. The main power quality issues are voltage, current and frequency that results in mal-operation of customer end equipment’s. Wind turbine produces a continuously variable output power during its normal operation because of wind variation, effect of tower shadow, wind shear, turbulence.

Voltage sag, swell, flickers, harmonics etc. are the power quality issues which are more harmful to wind generation, transmission and distribution network i.e. for grid. In wind power based generation, mostly induction generators are used because of its cost effectiveness and robustness. Induction generators draw reactive power from the grid for magnetization to which they are connected. The active power generated by induction generated is varied due to fluctuating nature of wind and this variation can prominently affect the absorbed reactive power and terminal voltage of induction generator. Integration of wind energy into grid affects the power quality of system.The devices used for mitigation of power quality problems are known as Customer Power Devices (CPDs). The generalized compensating devices are:

Dynamic Voltage Regulator (DVR), Static VAR Compensator (SVC), Static Synchronous Compensator (STATCOM), and Unified Power Quality Conditioner (UPQC). Among all these devices performance of STATCOM with battery energy storage system (BESS) is very good and user friendly to mitigate the power quality issues.

• Support the reactive power to wind turbine and non-linear load from STATCOM .

• For fast dynamic response bang-bang controller is implemented in STATCOM

• Minimize the THD percentage at the PCC waveform For improving the power quality of wind generating system as powered STATCOM based control technology has been proposed. A STATCOM is connected at common coupling point (PCC) along with solar power feed battery energy storage system (SBESS) to make the source current harmonic free and to improve the system performance.

There are different control techniques available for STATCOM operation like, p-q theory, SPWM, d-q theory etc.

In d-q theory reference currents are directly obtained from load currents without consideration of source voltage so that reference signals are not affected by voltage unbalance or distortion. This will increase the compensation robustness and performance. The proposed STATCOM along with hysteresis current control scheme for grid connected wind energy generation for improving the power quality.

Resincap Journal of Science and Engineering

Volume 4, Issue 3, March 2020 ISSN: 2456-9976

907 2. INDUCTION GENERATOR

Induction generator (IG) is a type of AC generator to produce power to the system. When induction motor runs more than synchronous speed (Ns) i.e. negative slip acts as an IG [2]. In rotor operation stator flux induces the rotor current which creates the rotor with polarity opposite to stator. The slip of rotor and stator becomes equal. The configuration of grid connected wind system is shown in Fig. 1.

Fig .No 1. Grid Connected Wind System

In generating mode, the prime mover and engine drives the rotor above the Ns. The opposing rotor flux is cutting the stator coils as stator still induces currents in the rotor that’s how active current is generated in stator and motor operate as generator. In standalone IG, the magnetizing flux is provided by the capacitor bank connected in parallel with induction generator and it draw magnetizing current from the grid in grid connected induction generator. The value of frequency and voltage of the machine in standalone system is very small compared to the grid connected system. Two schemes are developed for the operation of WECS with IG:

 Fixed speed drive scheme -the shaft speed is kept constant for whole range of wind speed by pitch control mechanism. This type of schemes is limited for low rating machines. SCIG is the example of fixed speed drive scheme.

 Variable speed drive scheme- variable speed is maintained by use of power electronics devices.

Variable V & variable F output from generator is first rectified and then converted to fixed V and F by use of an inverter [3]. WRIG, DFIG are examples of variable speed drive.

3. POWER QUALITY ISSUES

Since poor power quality results in terms of economic loss to industry and also causes damages to the equipment. The main PQ problems are voltage sags, small interruptions, long interruptions, voltage spikes, harmonic distortion, voltage fluctuations, voltage flickering and voltage swells [4]. Since wind flow is not constant, the integration of wind with grid is very complex and we need to maintain the PQ of system without affecting the efficiency of the system. Facts devices, reactive power compensation methods and UPQC controllers can be used for the mitigation of PQ issues in WECS.

3.1voltage Variations/ Voltage Flickering

Voltage variation is directly proportional to the P and Q power of the system. Difference in voltage is result from the varying wind speed and generator torque output. Switching

ON/OFF of wind turbine generator also causes variation in the system voltage [5]. Induction generator also starts consuming reactive power from the grid during any fault instead of supplying power which leads to PQ problems. Voltage sag/Voltage dips are defined as decreased in the supposed voltage level at the system power frequency for 1/2 of the cycle. Causes of voltage sags are sudden change in load interruption in operation of turbines, faults on transmission and distribution networks. Causes of voltage swells are sudden change of heavy loads, faults on the network, deregulated transformer Operation. Both voltage sags and swells cause damage of sensitive equipment, mal-operation of relays and circuit breakers, flickering of light and screen etc.

In grid connected wind system main reasons for voltage flickering are repeated switching actions, power variations and poor performance of sensitive electronic components.

3.2Harmonics

These are sinusoidal current or voltages with frequencies which are multiples of the actual frequency (fundamental). It results from the operation of power electronics converters, use of non- linear loads, outage of waveform of current and voltage, mismatching of system and equipment frequency etc.

Harmonics have degraded effect on operation of protective equipment, mal-operation of transformer, increases losses in the system; reduce efficiency of the system, overheating of motors and capacitor bank etc.

3.3 Transients

Transients are mainly due to the two reasons, one of them is internal faults and other is external faults. Internal faults consist of switching operation of devices like ON/OFF of circuit breakers, relays, power electronic devices etc. External faults include lightning faults which results a peaky current flow on the system. The protection of equipment from lightning is done by the use of lightning arrestor before the equipment.

4. STATCOMIN POWER QUALITY IMPROVEMENTTOPOLOGY

STATCOM(Static Compensator) consists of one SVC with capacitor on dc side and other with transformer[6]. A basic STATCOM with Voltage Source Converter (VSC) is connected in shunt to the system through a coupling transformer. The three phase voltages are in-phase and synchronized with an AC system which is considered as grid through the reactance of a coupling transformer. The adjustment of phase and magnitude of these voltages at the output of the STATCOM allows to effectively controlling the exchange of active and reactive power between the STATCOM and the grid. STATCOM acts as interlink between renewable resources and grid system and also for real power exchange between them. STATCOM is an electronic device having no inertia so it is better than synchronous condenser. When load requires Q power STATCOM takes active part in delivering the Q power to the load. When Q current component of the STATCOM is lags by 90º, it’s in absorbing mode when it is leads by 90º, and it’s in delivering mode. STATCOM operates in dual mode which gives both inductive and capacitive compensation. The inductive compensation is more important as it provides compensation when over compensation occurs due to capacitor banks.

Reactive current control scheme is used in STATCOM. The three phase line voltage is used to compute the reference

Resincap Journal of Science and Engineering

Volume 4, Issue 3, March 2020 ISSN: 2456-9976

908

angle. Quadrature component is compared with reference which is desired. The error obtained is amplified through error amplifier which produces angle α. Quadrature component of converter is either positive or negative. Positive when STATCOM acts as inductive reactance and negative when STATCOM acts as capacitive reactance. The control system shows inner the reactive current flow through STATCOM leaving line voltage. Q power is compensated by injecting current. The current injected is from the capacitor bank which is produced by absorbing P power by AC system.

5. UPQC IN POWER QUALITY IMPROVEMENT

In recent years, UPQC is employed to produce sensible quality of power to the shoppers. UPQC could be a combination of series and shunt compensator through a typical dc link electrical capacitor to limit the harmonics content within limit obligatory by IEEE-519 normal. The series a part of UPQC is referred as Dynamic Voltage refinisher (DVR), which is employed to take care of balanced, distortion less nominal voltage at the load end. DSTATCOM is shunt part of UPQC which is employed to compensate load Q power, harmonics and balance the load I by maintain the supply current balanced and harmonic free with unity power.

General diagram of UPQC is shown in Fig. 2

Fig. No 2: General block diagram of UPQC They inject series voltage and shunt currents to the system.

These devices atone for power quality (PQ) disturbance like current harmonics and voltage sag and swell to guard sensitive load moreover on improve service dependableness.

There are several strategies to alleviate voltage sags and swells; however, the employment of custom power (CP) device is taken into account to be the foremost economical methodology to serve for different purposes. CP devices are planned for enhancing the standard and reliability of the electrical power. Custom Power suggests that no power interruptions, low harmonic distortion in load voltage, acceptance of fluctuations underneath voltage among such as

limits, low section unbalance, low flicker at the section voltage, magnitude and period of over voltage and poor power issue hundreds while not vital error on the terminal voltage. In series control scheme the active series filter is providing for the voltage recompense. It generates the recompense voltage i.e. made by the PWM converter and insert in sequence with the provide voltage to force the voltage of Point of Common Coupling (PCC) to suit fair and sinusoidal. In shunt control scheme the active shunt power filter provides current and Q power (if the system requires) compensation. It would act as a controlled current generator that compensates the load current to power the source currents exhausted from the system to be unbiased which is in phase and sinusoidal with the positive sequence system voltage.

6. SIMULATION MODEL OF GRID CONNECTED WIND ENERGYSYSTEM WITH STATCOM

Fig. 3 shows the internal structure of STATCOM.

STATCOM connected in parallel with the system and injects current in it to compensate for reactive power. Hysteresis control technique is used which gives correct switching for STATCOM. The control system shows inner the reactive current flow through STATCOM leaving line voltage. Q power is compensated by injecting current. The current injected by STATCOM which is produced by absorbing P power by AC system. STACOM is designed with the help of IGBT/diode power electronics devices.

Fig. No 3 Internal Structure of STATCOM Fig. 4 shows complete model of grid connected wind system using STATCOM. Here STATCOM is connected with the grid connected wind energy system. The STATCOM controlled scheme is used for improving the power factor, voltage and current. Here the input value is set for wind i.e. 10 as there will be any change in the wind speed the feedback is given to the STATCOM which will compensate the desired voltage or current required by the system which improves the power factor the system.

Resincap Journal of Science and Engineering

Volume 4, Issue 3, March 2020 ISSN: 2456-9976

909

system using STATCOM

7. SIMULATION MODEL OF GRID CONNECTED WIND ENERGY SYSTEM WITH UPQC

Fig. 5 and Fig. 6 show the internal structure of UPQC and complete model of grid connected wind system using UPQC respectively. It is a combination of series and shunt APF(active power filter). Both series and shunt APF are consist of 6-thyristors conFig.d together in each module & a dc link is used in between series &shunt APF. Thyristors are fired sequentially at fixed interval to control the electrical parameters. To further improve the power quality of the system, UPQC is used here in place of STATCOM. UPQC compensates for power quality disturbances to protect sensitive loads as well as to improve the reliability of the system. Fig. 6 shows complete model of grid connected wind system using UPQC.

Fig. No 5. Internal Structure of UPQC

Fig. No 6 Complete model of grid connected wind system using UPQC

7.1 Simulation Results Of Grid Connected Wind Energy System With STATCOM and UPQC

 Simulation results of grid connected wind energy system with STATCOM

Fig. No 7 Output Voltage waveform with STATCOM

Resincap Journal of Science and Engineering

Volume 4, Issue 3, March 2020 ISSN: 2456-9976

910

STATCOM

Fig. No 9 Output Real power with STATCOM

Fig. No 10 Output Real power of WECS

Fig. 11 Graph showing the THD value of grid connected WECS with STATCOM

 Simulation results of grid connected wind energy system with UPQC

Fig. No 12 Output Voltage waveform with UPQC

Fig. No 13 Output Current waveform with UPQC

Resincap Journal of Science and Engineering

Volume 4, Issue 3, March 2020 ISSN: 2456-9976

911

Fig. No 14 Output Real Power waveform with UPQC

Fig. No 15 Graph showing the THD value of grid connected WECS with UPQC

8. RESULT & CONCLUSION

for power production. By using UPQC the PQ of grid connected wind system is improved having nonlinear load.

The power quality issues are described briefly which is to be improved. Compensation of reactive power is most important for the power quality improvement. STATCOM and UPQC are used in proposed work for PQ improvement in grid connected WECS. STATCOM maintain the source voltage and current in phase. STATCOM provides control of reactive power only whereas UPQC provides control of both real and reactive power. Here the input value is set for wind i.e. 10 as there will be any change in the wind speed the feedback is given to the controller which will compensate the desired voltage or current required by the system which improves the power factor the system without any losses. Therefore, the overall efficiency of the system will increase. The STATCOM and UPQC connected wind system is designed in MATLAB/SIMULINK for improving the system power factor. Based on analysis of simulation results, it is verified that system shows an excellent performance using UPQC as compared to STATCOM. The system fulfills the requirement of power quality and it maintains the system from any distortion of voltage and harmonics. From this table, it can be concluded that THD value of grid connected system to the

UPQC is much lesser than the system connected to the STATCOM. From the result, it is verified that system shows an excellent performance using UPQC as compared to STATCOM.

TABLE No 1. Total Harmonics Distortion Values Of Grid Connected Wind System.

With STATCOM 4.16 %

With UPQC 0.52%

ACKNOWLEDGEMENT

I greatly Indebted for forever to my Guide, to my HOD and all teaching and non-teaching staff who supported directly and indirectly to complete my work. I sincerely thankful to my principal Dr. S.K.Biradar for continues encouragement and active interest in my progress throughout the work. I am grateful being a M.E Electrical Power System student of Matshyodhari Shikshan Sanstha’s College of Engineering and Technology, Jalna, Maharastra.

REFERENCES

[1] T. Ackermann, Ed., Wind Power in Power Systems, 2005, Chichester, England: John Wiley Sons, Ltd.

[2] E. Muljadi, C.P. Butterfield (2004), “Wind Farm Power System Model Development”, World Renewable Energy Congress VIII, Colorado, Aug-Sept 2004 [3] Ahmed F Zooba and Ramesh Bansal, ”Handbook of

Renewable energy sources”, (2011) World Scientific Publishing Co. Ptv. Ltd., ISBN-13 978-981-4289-06-1.

[4] Sharad W. Mohod and Mohan V.Aware, Aug 2011,

“PQ issues and its improvement in wind energy generation interface to grid system.

[5] S. Rajesh Rajan , “Power Quality Improvement In Grid Connected Wind Energy System Using UPQC”, International Journal of Research in Engineering &

Technology (IJRET),Vol. 1, Issue 1, June 2013, 13-20.

[6] LokeshVitonde, SurbhiShrivastava, AlkaDharwal,

“Power Quality Improvement Of Wind Energy By Statcom”, international Research Journal Of Engineering And Technology (IRJET), Volume: 03 Issue: 07 , July-2016360.

ABOUT AUTHOR

1]Ms. Swapnshila Sunil Gaikwad

is pursuing ME degree in the stream of Electrical Power Systems From MSS’s college of Engineering & Technology Jalna, Dr. BAMU University Aurangabad.

Resincap Journal of Science and Engineering

Volume 4, Issue 3, March 2020 ISSN: 2456-9976

912

2]Prof. K Chandra Obula Reddy

Received Mtech in Power Electronics From The Oxford college of Engineering Bangalore VTU University Belgaum Btech in Electrical and Electronics Engineering From SVCET,Chitoor JNTU University Anantapur A.P He is currently working as HOD and Professor in Department of Electrical Engineering , MSS’s Collage of Engineering &

Technology Jalna.