Active and Reactive Power Control for Grid Connected Wind Energy System with Statcom

Active and Reactive Power Control for Grid Connected Wind Energy System with Statcom

G. V. S. Babu1

PG Scholar, Power Electronics Department of Electrical and Electronics Engineering,

Sri Vasavi Engineering College, Tadepalligudem (A.P),India.

N. Sri Hareesh M.Tech, 2

Assistant Professor

Department of Electrical and Electronics Engineering,

Sri Vasavi Engineering College, Tadepalligudem (A.P),India

CH. Rambabu, M.Tech, (p.hd)3

Head of the Department,

Department of Electrical and Electronics Engineering,

Sri Vasavi Engineering College, Tadepalligudem (A.P),India.

Abstract-Grid connected wind energy system affects the power quality. The specification of a power electronic interface is subject to requirements related not only to the renewable energy source itself but also to its effects on the power-system operation. In this paper, new trends presented for the integration of wind energy and power electronics. It is desire to measure and assess power quality in wind turbines in accordance with the International Electro technical Commission (IEC) 61400-21 Standard. In the paper, the impact of static compensator (STATCOM) to facilitate the integration of a wind generation (WG) into the power system is studied. In this proposed scheme STATIC COMPEN- SATOR (STATCOM) is connected at a point of com- mon coupling with a battery energy storage system (BESS) to mitigate the power quality issues. Finally, effectiveness of the proposed method is verified by si- mulation.

Index Terms International electro-technical commis- sion (IEC), wind generating system (WGS), static com- pensator (STATCOM).

1. INTRODUCTION

The increasing number of renewable energy sources and distributed generators requires new strategies for the operation and management of the electricity grid in order to maintain or even to improve the power- supply reliability and quality. To have sustainable growth and social progress, it is necessary to meet the energy need by utilizing the renewable energy re- sources like wind, biomass, hydro, co-generation, etc. In sustainable energy system, energy conservation and the use of renewable source are the key para- digm. The need to integrate the renewable energy like wind energy into power system is to make it possible

to minimize the Environmental impact on conven- tional plant. The integration of wind energy into ex- isting power system presents a technical challenges and that requires consideration of voltage regulation, stability, power quality problems. The power quality is an essential customer-focused measure and is greatly affected by the operation of a distribution and transmission network. The issue of power quality is of great importance to the wind turbine. There has been an extensive growth and quick development in the exploitation of wind energy in recent years. The individual units can be of large capacity up to 2 MW, feeding into distribution network, particularly with customers connected in close proximity. Today, more than 28,000 wind generating turbines are successfully operating all over the world. In the fixed-speed wind turbine operation, all the fluctuation in the wind speed are transmitted as fluctuations in the mechani- cal torque, electrical power on the grid and leads to large voltage fluctuations. During the normal opera- tion, wind turbine produces a continuous variable output power. These power variations are mainly caused by the effect of turbulence, wind shear, and tower-shadow and of control system in the power system. Thus, the network needs to manage for such fluctuations. The power quality issues can be viewed with respect to the wind generation, transmission and distribution network, such as voltage sag, swells, flickers, harmonics etc. However the wind generator introduces disturbances into the distribution network. The proposed STATCOM control scheme for grid connected wind energy generation for power quality improvement has following objectives.

Active power support from battery energy storage system.

• Unity power factor at the source side.

• Reactive power support only from STATCOM to wind Generator and Load.

• For fast dynamic response from STATCOM con- nected to bang-bang controller.

  The paper is organized as fallows. The Section II introduces the grid coordination rule for grid quality

  voltage reduction factor. The acceptable voltage dips limiting value is %.

  1. Flicker: The measurements are made for maxi- mum number of specified switching operation of wind turbine with 10-min period and 2-h period are specified, as given in (3)

     limits. The Section III introduces the power quality standards, issues and its consequences of wind tur-

     bine. The Section IV describes the topology for pow- er quality improvement. The Sections V, VI, VII de- scribes the proposed configurations, system perfor- mance and conclusion respectively.

     1. GRID COORDINATION RULE

        The American Wind Energy Association (AWEA) led the effort in the united state for adoption of the grid code for the interconnection of the wind plants to the utility system. The first grid code was focused on the distribution level, after the blackout in the United State in August 2003. The United State wind energy industry took a stand in developing its own grid code for contributing to a stable grid operation. The rules for realization of grid operation of wind generating system at the distribution network are defined as-per IEC-61400-21. The grid quality characteristics and limits are given for references that the customer and the utility grid may expect. According to Energy- Economic Law, the operator of transmission grid is responsible for the organization and operation of in- terconnected system [6].

        1. Voltage Rise (u): The voltage rise at the point of common coupling can be approximated as a function of maximum apparent power of the turbine, the grid impedances R and X at the point of common coupl- ing and the phase angle [7], given in (1)

           u= smax (1)

           Where u voltage rise, smax max. apparent power, phase difference, u is the nominal voltage of grid. The Limiting voltage rise value is

           %

        2. Voltage Dips (d): The voltage dips is due to start up of wind turbine and it causes a sudden reduction of voltage. It is the relative %voltage change nominal voltage change is given in (2).

        d

        (2)

        Where is d relative voltage change, rated appar- ent , short circuit apparent power, and sudden

        (3)

        Where Long term flicker. Flicker coefficient calculated from Rayleigh distribution of the wind speed. The Limiting Value for flicker coef- ficient is about %. , for average time of 2 h [8].

        1. Harmonics: The harmonic distortion is assessed for variable speed turbine with a electronic power converter at the point of common connection [9]. The total harmonic voltage distortion of voltage is given as in (4):

           (4)

           where is the nth harmonic voltage and is the funda- mental frequency (50) Hz. The THD limit for 132 KV is %.THD of current is given as in (5)

           (5)

           where is the nth harmonic current and is the funda- mental frequency (50) Hz. The THD of current and limit for 132 KV is %.

        2. Grid Frequency: The grid frequency in India is specified in the range of 47.551.5 Hz, for wind farm connection. The wind farm shall able to withstand change in frequency up to 0.5[9].

     2. POWER QUALITY STANDARDS, ISSUES AND ITS CONSEQUENCES

        A International Electro Technical Commission Guidelnes

        The guidelines are provided for measurement of power quality of wind turbine. The International standards are developed by the working group of Technical Committee-88 ofthe International Electro- technical Commission (IEC), IECstandard 61400-21, describes the procedure for determining the power quality characteristics of the wind turbine [4].

        The standard norms are specified.

        1. IEC 61400-21: Wind turbine generating system, part-21.Measurement and Assessment of power qual- ity characteristic of grid connected wind turbine

        2. IEC 61400-13: Wind Turbinemeasuring proce- dure in determining the power behavior.

        3. IEC 61400-3-7: Assessment of emission limit for fluctuating load IEC 61400-12: Wind Turbine per- formance. The data sheet with electrical characteristic of wind turbine provides the base for the utility as- sessment regarding a grid connection.

        B. Voltge Variation

        The voltage variation issue res results from the wind velocity and generator torque. The voltage variation is directly related to real and reactive power varia- tions. The voltage variation is commonly classified as under:

• Voltage Sag/Voltage Dips.

• Voltage Swells.

• Short Interruptions.

• Long duration voltage variation.

The voltage flicker issue describes dynamic varia- tions in the network caused by wind turbine or by varying loads. Thus the power fluctuation from wind turbine occurs during continuous operation. The am- plitude of voltage fluctuation depends on grid strength, network impedance, and phase-angle and power factor of the wind turbines. It is defined as a fluctuation of voltage in a frequency 1035 Hz. The IEC 61400-4-15 specifies a flicker meter that can be used to measure flicker directly.

C Wind Turbine Location in Power System

The way of connecting the wind generating system into the power system highly influences the power quality. Thus the operation and its influence on pow- er system depend on the structure of the adjoining power network

1. Self Excitation of Wind Turbine Generating Sys- tem

   The self excitation of wind turbine generating sys- tem(WTGS) with an asynchronous generator takes place after disconnection of wind turbine generating system (WTGS) with local load. The risk of self exci- tation arises especially when WTGS is equipped with compensating capacitor. The capacitor connected to induction generator provides reactive power compen- sation. However the voltage and frequency are de- termined by the balancing of the system. The disad- vantages of self excitation are the safety aspect and balance between real and reactive power [5].

2. Harmonics

   The harmonic results due to the operation of power electronic converters. The harmonic voltage and cur-

   rent should be limited to the acceptable level at the point of wind turbine connection to the network. To ensure the harmonic voltage within limit, each source of harmonic current can allow only a limited contri- bution, as per the IEC-61400-36 guideline. The rapid switching gives a large reduction in lower order har- monic current compared to the line commutated con- verter, but the output current will have high frequen- cy current and can be easily filter-out.

3. Consequences of the Issues

The voltage variation, flicker, harmonics causes the malfunction of equipments namely microprocessor based control system, programmable logic controller; adjustable speed drives, flickering of light and screen. It may leads to tripping of contractors, tripping of protection devices, stoppage of sensitive equipments like personal computer, programmable logic control System and may stop the process and even can dam- age of sensitive equipments. Thus it degrades the power quality in the grid.

1. TOPOLOGY FOR POWER QUALITY IM- PROVEMENT

   The STATCOM based current control voltage source inverter injects the current into the grid in such a way that the source current are harmonic free and their phase-angle with respect to source voltage has a de- sired value. The injected current will cancel out the reactive part and harmonic part of the load and induc- tion generator current, thus it improves the power factor and the power quality. To accomplish these goals, the grid voltages are sensed and are synchro- nized in generating the current command for the in- verter. The proposed grid connected system is im- plemented for power quality improvement at point of common coupling (PCC), as shown in Fig. 1.

   The grid connected system in Fig. 1, consists of wind energy generation system and battery energy storage system with STATCOM.

   1. Wind Energy Generating System

      In this configuration, wind generations are based on constant speed topologies with pitch control turbine. The induction generator is used in the proposed scheme because of its simplicity, it does not require a separate field circuit, it can accept constant and vari- able loads, and has natural protection against short circuit.

      The available power of wind energy system is pre- sented as

      Under in (6).

      (6)

      Where (kg/m ) is the air density and A ) is the

      area swept out by turbine blade, is the wind speed in mtr/s. It is not possible to extract all kinetic energy of wind, thus it extract a fraction of power in wind, called power coefficient Cp of the wind tur- bine, and is given in (7).

      Fig. 1. Grid connected system for power quality improvement.

      (7)

      Where Cp is the power coefficient, depends on type and operating condition of wind turbine. This coeffi- cient can be express as a Function of tip speed ratio and pitch angle. The mechanical power produce by wind turbine is given in (8)

      (8)

      Where R is the radius of the blade (m).

   2. BESS-STATCOM

      The battery energy storage system (BESS) is used as an energy storage element for the purpose of voltage regulation. The BESS will naturally maintain dc ca- pacitor voltage constant and is best suited in STAT- COM since it rapidly injects or absorbed reactive power to stabilize the grid system. It also control the distribution and transmission system in a very fast rate. When power fluctuation occurs in the system, the BESS can be used to level the power fluctuation by charging and discharging operation. The battery is connected in parallel to the dc capacitor of STAT-

      COM [10][14]. The STATCOM is a three-phase voltage source inverter having the capacitance on its DC link and connected at the point of common coupl- ing. The STATCOM injects a compensating current of variable magnitude and frequency component at the bus of common coupling.

   3. System Operation

   The shunt connected STATCOM with battery energy storage is

   connected with the interface of the induction genera- tor and non-linear load at the PCC in the grid system. The STATCOM compensator output is varied ac- cording to the controlled strategy, so as to maintain the power quality norms in the grid system. The cur- rent control strategy is included in the control Scheme that defines the functional operation of the STATCOM compensator in the power system. A single STATCOM using insulated gate bipolar tran- sistor is proposed to have a reactive power support, to the induction generator and to the nonlinear load in the grid system.

   Fig. 2. System operational scheme in grid system.

2. CONTROL SYSTEM

   The control scheme approach is based on injecting the currents into the grid using bang-bang control- ler. The controller uses a hysteresis current con- trolled technique. Using such technique, the control- ler keeps the control system variable between boun- daries of hysteresis area and gives correct switching signals for STATCOM operation. The control system scheme for generating the switching signals to the STATCOM is shown in Fig. 3.

   Fig. 3. Control system scheme.

   The control algorithm needsthe measurements of several variables such as three-phase source current, DC voltage, inverter current with the help of sensor. The current control block, receives an input of refer- ence current and actual current are subtracted so as to activate the operation of STATCOM in current con- trol mode [16][18].

   1. Grid Synchronization

      In three-phase balance system, the RMS voltage source amplitude is calculated at the sampling fre-

      quency from the source phase voltage ) and is expressed, as sample template Vsm, sampled peak voltage, as in (9).

      The in-phase unit vectors are obtained from AC sourcephase

      Voltage and the RMS value of unit vector as shown in (10).

      This method is simple, robust and favorable as com- pared with other methods [18].

   2. Bang-Bang Current Controller

   Bang-Bang current controller is implemented in the current control scheme. The reference current is gen- erated as in (10) and actual current are detected by current sensors and are subtracted for obtaining a current error for a hysteresis based bang-bang con- troller. Thus the ON/OFF switching signals for IGBT of STATCOM are derived from hysteresis controller [19].The switching function for phase a is ex- pressed as (12).

   (12)

   Where HB is a hysteresis current-band, similarly the switching function can be derived for phases b and

   c.

3. SYSTEM PERFORMANCE

   The system performance of proposed system under dynamic condition is also presented. In this configu- ration, wind generations are based on constant speed topologies with pitch control turbine.

   S. N O	PARAMETERS	RATINGS
   1	Grid Voltage	3-phase,415V,50Hz
   2	Induction Gene- rator	3.35KVA,415V,50Hz,p=4,N= 1500rpm,Rs=20,Rr=20,Ls =0.06H,Lr=0.06H
   3	Line Series in- ductance	0.058mH
   4	Inverter Parame- ters	Dc link voltage=800v Dc link capacitance=100µF Switching frequency=2KHz
   5	Load Parameters	Non-linear load

   (10)

   The in-phase generated reference currents are derived using in-phase unit voltage template as, in (11)

   (11)

   Where is proportional to magnitude of filtered source voltage for respective phases. This ensures that the source current is controlled to be sinusoidal. The unit vectors implement the important function in the grid connection for the synchronization for STATCOM.

   1. Voltage Source Current ControlInverter Opera- tion

      The three phase injected current into the grid from STATCOM will cancel out the distortion caused by the nonlinear load and wind generator. The IGBT based three-phase inverter is connected to grid through the transformer. The generation of switching signals from reference current is simulated within hysteresis band of 0.08. The choice of narrow hyste- resis band switching in the system improves the cur- rent quality. The control

      Signal of switching frequency within its operating band as Shown in Fig. 4.

      The choice of the current band depends on the operat- ing voltage and the interfacing transformer imped- ance. The compensated current for the nonlinear load and demanded reactive power is provided by the in- verter. The real power transfer from the batteries is also supported by the controller of this inverter. The three phase inverter injected current are shown in Fig. 5

      Fig.5.Threephase injected inverter Current.

   2. STATCOMPerformance under Load Variations

      The wind energy generating system is connected with grid having the nonlinear load. The performance of the system is measured by switching the STATCOM at t=0.7time s in the system and how the STATCOM responds to the step change command for increase in additional load at 1.0 s is shown in the simulation. When STATCOM controller is made ON, without change in any other load condition parameters, it starts to mitigate for reactive demand as well as har- monic current. The dynamic performance is also car- ried out by step change in a load, when applied at 1.0

      s. This additional demand is fulfill by STATCOM compensator. Thus, STATCOM can regulate the available real power from source. The result of source current ,load current are shown in Fig. 6(a)

      and (b) respectively. While the result of injected cur- rent from STATCOM are shown in Fig. 6(c)

      the generated current from wind generator at PCC. are depicted in Fig. 6(d).

      Fig. 6. (a) Source Current. (b) Load Current. (c) Inverter Injected Current. (d) Wind generator (Induction generator) current.

      The DC link voltage regulates the source current in the grid system, so the DC link voltage is maintained constant across the capacitor as shown in Fig. 7(a). The current through the dc link capacitor indicating the charging and discharging operation as shown in Fig. 7(b)

      Fig. 7. (a) DC link voltage. (b) Current through Capacitor.

   3. Power Quality Improvement

   It is observed that the source current on the grid is affected due to the effects of nonlinear load and wind generator, thus purity of waveform may be lost on both sides in the system. The inverter output voltage under STATCOM operation with load variation is shown in Fig. 8.

   Fig. 8. STATCOM output voltage.

   The dynamic load does affect the inverter output vol- tage. The source current with and without STAT- COM operation is shown inFig.9.

   Fig. 9. Supply Voltage and Current at PCC.

   This shows that the unity power factor is maintained for the source power when the STATCOM is in oper- ation.

   The current waveform before and after the STAT- COM operation is analyzed. The Fourier analysis of this waveform is expressed and the THD of this source current at PCC without STATCOM is 5.34%, as shown in Fig. 10.

   Fig. 10. (a) Source Current. (BALANCED LOAD)

   Fig. 10. (b) FFT of source current.

   The power quality improvement is observed at point of common coupling, when the controller is in ON condition. The STATCOM is placed in the operation at 0.7 sec the THD of this source current at PCC with STATCOM is 0.35%. source current waveform is shown in Fig. 11.

   .

   Fig. 11 (a) Source Current (BALANCED LOAD)

   Fig. 11 (b) FFT of source current.

   with its FFT it is shown that the THD has been de- creased considerably and within the norms of the standard. The above tests with proposed scheme has not only power quality improvement feature but it also has sustain capability to support the load with the energy storage through the batteries.

   Type of load	Without STAT- COM	With STATCOM
   Balanced	5.34%	0.35%
   Unbalanced	3.40%	0.29%

   THD Comparison

   The current waveform before and after the STAT- COM operation is analyzed. The Fourier analysis of this waveform is expressed and the THD of this source current at PCC without STATCOM for UN- BALANCED LOAD is 3.40%, as shown in Fig. 12.

   Fig. 12. (a) Source Current. (UNBALANCED LOAD)

   Fig. 12 (b) FFT of source current.

   The power quality improvement is observed at point of common coupling, when the controller is in ON condition. The STATCOM is placed in the operation at 0.7 sec the THD of this source current at PCC with STATCOM is 0.29%. source current waveform is shown in Fig. 11. The above tests with proposed scheme has not only power quality improvement fea- ture but it also has sustain capability to support the load with the energy storage through the batteries.

   Fig. 12. (a) Source Current. (UNBALANCED LOAD)

   Fig. 11. (a) Source Current. (b) FFT of source current.

CONCLUSION