Power Quality Improvement Using STATCOM in Grid Connected Wind Energy System

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Power Quality Improvement Using STATCOM in Grid Connected Wind Energy System

Pradeep Kumar Dept. of Electrical Engineering DPG Institute of technology and

management, Gurgaon India

e-mail: [email protected]

Deepak Lakra Dept. of Electrical Engineering

Delhi Technical Campus Bahadurgarh, Haryana

India

Sandeep Dhundhara

Department of Electrical and Electronics Engineering

UIET, Panjab University, Chandigarh, India

Abstract— The global electrical energy consumption is rising and there is steady increase of the demand on power generation.

So in addition to conventional power generation units a large no.

of renewable energy units is being integrated into the power system. A wind electrical generation system is the most cost competitive of all the environmentally clean and safe renewable energy sources in world. The recent evolution of power semiconductors and variable frequency drive technology has aided the acceptance of variable speed generation systems. Both fixed-speed squirrel-cage induction generator and variable speed double fed induction generator are used in wind turbine generation technology. Therefore, a detailed model of induction generator coupled to wind turbine system is presented in the paper. Modeling of induction machine and grid connected Self Excited Induction Generator and simulation done in MATLAB/SIMULINK platform. The significant result of the analysis is also shown and being compared with the existing literature to validate approach.

Index Terms— Self-excited induction generator, STATCOM, Power quality issues.

I. INTRODUCTION

SEIG–wind turbine is an integrated part of distributed generation system. Therefore, any abnormalities i.e. under voltage problem associates with grid are going to affect the system performance considerably. Non-linear load connected to system injects harmonics in the system which effect other device on the system at PCC. For improvement of voltage level and reduction of harmonics at PCC we used STATCOM control scheme connect at PCC to improvement the quality of power. Taking this into account, the performance of grid connected self-excited induction generator (SEIG) fixed speed wind turbine, a non –linear load and STATCOM connected to grid or at the PCC is studied in this paper. To have sustainable growth and social progress, it is necessary to meet the energy need by utilizing the renewable energy resources like wind, biomass, hydro, co-generation, etc. In sustainable energy

system, energy conservation and the use of renewable source are the key paradigm. The need to integrate the renewable energy like wind energy into power system is to make it possible to minimize the environmental impact on conventional plant. 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 turbine is 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 mechanical torque, electrical power on the grid and leads to large voltage fluctuations. During the normal operation, wind turbine produces a continuous variable output power. A proper control scheme in wind energy generation system is required under normal operating condition to allow the proper control over the active power production. In the event of increasing grid disturbance, a battery energy storage system for wind energy generating system is generally required to compensate the fluctuation generated by wind turbine. A STATCOM based control technology has been proposed for improving the power quality which can technically manages the power level associates with the commercial wind turbines[1-4].

The proposed STATCOM control scheme for grid connected wind energy generation for power quality improvement has following objectives :

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

2. Reduction of Harmonics due to non-linear load at Point of Common Coupling.

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II. SELFEXCITEDINDUCTIONGENERATOR The use of squirrel-cage induction machines in wind generation is widely accepted as a generator of choice. The squirrel-cage induction machine is simple, reliable, cheap, lightweight, and requires very little maintenance. Generally, the induction generator is connected to the utility at constant frequency. With a constant frequency operation, the induction generator operates at practically constant speed (small range of slip). The wind turbine operates in optimum efficiency only

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within a small range of wind speed variation. The variable- speed operation allows an increase in energy captured and reduces both the torque peaks in the drive train and the power fluctuations sent to the utility[10].

A. Modeling of SEIG

The voltage equations of a squirrel cage induction generator in the d – q (direct – quadrature) reference frame, using the generator convention, are as follows:

ds

ds s ds s qs

v R i d

dt

  

   

in which s is the slip, u is the voltage, i is the current, R is the resistance, and



is the flux. All quantities are in per unit.

(8) In these equations, is flux linkage and L is the inductance. The indices m, r and



^stand

for mutual, rotor and leakage, respectively. By inserting above flux equation in voltage equation while neglecting the stator transients, in agreement with the assumptions discussed above, the voltage current relationships become:

 

ds s ds s s m qs m qr

v   R i     L

_

 L i  L i  

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 

qs s qs s s m ds m dr

v   R i     L

_

 L i  L i  

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 

0

^dr

dr r dr s r m qr m qs

v R i s L L i L i d



dt

   

        

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 

0

^qr

qr r qr s r m dr m ds

v R i s L L i L i d



dt

 

          

(12) The electrical torque, Te, is given by:

e qr dr dr qr

T   i   i

(13) The equations for active power generated, P, and the reactive power consumed, Q, are:

s ds ds qs qs

P  v i  v i

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s qs ds ds qs

Q  v i  v i

(15) Because only the stator winding is connected to the grid, generator and grid can exchange active and reactive power only through the stator terminals. Therefore, the rotor does not need to be taken into account.

III. WINDCONNECTEDINDUCTIONGENERATOR A. Principle of operation

Induction generators and motors produce electrical power when their rotor is rotated faster than the synchronous frequency. For a typical four-pole motor (two pairs of poles on stator) operating on a 60 Hz electrical grid, synchronous speed is 1800 rotations per minute. Similar four-pole motor operating on a 50 Hz grid will have synchronous speed equal to 1500 rpm. In normal motor operation, stator flux rotation is faster than the rotor rotation. In generator operation, a prime mover (turbine, engine) drives the rotor above the synchronous speed. Stator flux still induces currents in the rotor, but since the opposing rotor flux is now cutting the stator coils, active current is produced in stator coils, and motor is now operating as a generator, and sending power back to the electrical grid [7].

B. Fixed speed grid connected wind turbine generator The structure and performance of fixed-speed wind turbines as shown in Fig. 1 depends on the features of mechanical sub- circuits, e.g., pitch control time constants etc[1].

Fig 1: Fixed speed wind turbine with directly grid connected squirrel-cage induction generator.

The reaction time of these mechanical circuits may lie in the range of tens of milliseconds. As a result, each time a burst of

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wind hits the turbine, a rapid variation of electrical output power can be observed.

C. Variable speed grid connected wind turbine generator A way to make more convenient turbines is variable speed turbines. Variable speed turbines have become the most dominating type of the yearly installed wind turbines as they can store some of the power fluctuations due to turbulence by increasing the rotor speed, pitching the rotor blades, these turbines can control the power output at any given wind speed.

Fig.2 shows a variable speed turbine connected to a Squirrel- Cage Induction Generator SCIG. Although these direct-online systems have been built up to 1.5 MW, but presence of power inverter causes lots of disadvantages such as [5,6]:

a) This power converter, which has to be rated at 1 p.u. of total system power, is expensive.

b) Converter efficiency plays an important role in total system efficiency over the entire operating range.

Fig 2: variable speed wind turbine with squirrel-cage induction generator.

Another way is using Doubly Fed Induction Generator DFIG, as shown in Fig.3. It consists of a stator connected directly to grid and a rotor – via slip rings – is connected to grid through four-quadrant ac-to-ac converter based on insulated gate bipolar transistors (IGBTs)

This system offers the following advantages:

1. Reduced inverter cost, because inverter rating is typically 30% of total system power.

2. Improved system efficiency.

3. Power-factor control can be implemented at lower cost.

4. It has a complete control of active and reactive power.

Fig 3: Variable speed wind turbine with doubly-fed induction generator.

IV. POWERQUALITYIMPROVEMENTUSINGSTATCOM A. Power quality

Power quality can also be defined as a set of electrical boundaries that allows a piece of equipment to function in its intended manner without significant loss of performance or life expectancy. Power distribution systems, ideally, should provide their customers with an uninterrupted flow of energy at smooth sinusoidal voltage at the contracted magnitude level and frequency. However, in practice, power systems, especially the distribution systems, have numerous nonlinear loads, which significantly affect the quality of power supplies. As a result of the nonlinear loads, the purity of the waveform of supplies is lost. This ends up producing many power quality problems. Power quality can be simply defined as shown in the interaction diagram Figure 4[2].

Electrical Grid Utility

Loads Cons um ers Voltage

Quality

Current Quality

Power Quality

Figure 4 : The Power Quality Diagram

Delivering a certain level of voltage stability and sinusoidal quality should be the main concern for designers of the utility electrical grid. When electrical distribution/utilization system is interconnected, electric loads and their profile, grid design, utility operation including the electric load degree of nonlinearity, all together affect and influence the power quality [11,12].

The main reasons for concern with power quality (PQ) are as following:

(a) End user devices become more sensitive to PQ due to many microprocessor based controls.

(b) Large computer systems in many businesses facilities.

(c) Power electronics equipment used for enhancing system stability, operation and efficiency. They are major source of bad Power Quality.

(d) Continuous development of high performance equipment:

Such equipment is more susceptible to power disturbances.

The users always demand higher power quality. Some basic criterions for power quality are constant rms value, constant frequency, symmetrical three-phases, pure sinusoidal wave shape and limited THD.

B. STATCOM

Recently, FACTS-based devices have been used for power flow control and for damping power system oscillations. They can also be used to increase transmission line capacity; steady state voltage regulation; provide transient voltage support to prevent system collapse; and damp power oscillations. FACTS devices can be used in wind power systems to improve the

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transient and dynamic stability of the overall power system.

The STATCOM is from the family of FACTS devices that can be used effectively in wind farms to provide transient voltage support to prevent system collapse. In other words a STATCOM is an electronic generator of reactive power [12].

Transmission of power „S‟

 ^P ^ ^jQ 

over a power line with impedance „Z‟

 ^R ^ ^jX 

results in a voltage drop (ΔV ) R.P X.Q

V V

   (16)

For larger wind farms connected to transmission systems X>>R and, from equation (16), ΔV is directly proportional to the reactive power (Q) transferred. From equation (16), it is clear that for efficient voltage control an effective reactive power strategy is required. FACTS devices can provide dynamic and steady state support. They can improve dynamic and transient stability, control dynamic overvoltage‟s and under voltages and also support against frequency and voltage collapses [14,16].

The STATCOM control scheme for the grid connected wind energy generation system for power quality improvement is simulated using MATLAB/SIMULINK in power system block set. The effectiveness of the proposed scheme relives the main supply source from the reactive power demand of the load and the induction generator. The development of the grid co-ordination rule and the scheme for improvement in power quality norms as per IEC-standard on the grid has been presented. The shunt connected STATCOM with battery energy storage is connected with the interface of the induction generator and non-linear load at the PCC in the grid system.

The STATCOM compensator output is varied according to the controlled strategy, so as to maintain the power quality norms in the grid system. The current 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 transistor is proposed to have a reactive power support, to the induction generator and to the nonlinear load in the grid system. The main block diagram of the system operational scheme is shown in Fig.

5[3].

Fig.5. System operational scheme in grid system.

V.SIMULATIONANDRESULTS

The matlab simulation model comprise of four major components as show in the fig 6. with

(a) Power Source of Voltage line to line 415 V , 50 Hz (b) Grid connected SEIG of same output voltage as above (c) A non-linear load

(d) A STATCOM

Fig 6. MATLAB Simulation model.

The grid connected SEIG decrease the voltage after connecting to the PCC because of absorption of reactive power from the Source i.e SEIG deliver real power and draw reactive power. The non-linear load connected at PCC introduce the harmonics in the system which effect the function of other device connected to system due to poor power quality. Hence grid connected SEIG and non-linear load connected at PCC effect the power quality.

For the improvement of quality at PCC we introduced a STATCOM at PCC which improve the voltage by providing reactive power to the system and also act as filter for reducing harmonics level coming from non-linear load. For proper controlling the STATCOM a control scheme is provided which is space vector pulse width modulation.

Various result will be shown i.e source output when there is no wind and non-linear connected and output after both connected at PCC.After this STATCOM will applied for improving the voltage level as well as harmonics reduction at PCC.

(a) Voltage Output of source bus and at the PCC

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Fig 7 Output of source voltage. (b) Voltage at Pcc

Fig 8 voltage at pcc

(c) Voltage at pcc when non- linear load connected.

Fig 9 voltage at pcc when only non-linear load connected

As shown in the fig above after connecting of non-linear load wavefom distorted hence harmonics present in the system.

(d) Voltage at pcc when both wind and non-linear connected.

Fig 10 voltage waveform when wind and non-linear load connected at PCC.

As shown the wind connected induction generator connected at 0.1 sec after this there is voltage dip. Hence due to interconnection of non-linear load and SEIG voltage at PCC id distorted and decrease.

(e) Voltage at PCC when STATCOM connected (1) Output of STATCOM

fig 11(a) voltage output of STATCOM ( line to ground)

Fig 11(b) Voltage ( line to line )

(2) Voltage at PCC after STATCOM connected

Fig 12 Voltage At pcc after STATCOM connected

As shown in the fig above the statcom is connected at 0.3 sec voltage is increased and also shape of waveform improve i.e reduction of harmonics at PCC.

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(f) FFT analysis

Fig 13FFT analysis when non-linear connected at pcc.

Fig 14 FFT analysis when STATCOM connected at PCC

Above fig show the FFT analysis at PCC which shows that when non-linear load connected there is THD = 8.83 which is large according to grid coordination rule. When STATCOM connected at 0.3 sec the THD = 2.78 which is low as compared to without STATCOM and according to grid coordination rule. Hence harmonics level reduced at pcc by the STATCOM.

VI. CONCLUSION

In this paper, simulation studies show that the power quality is improved with the use of a STATCOM in Grid connected SEIG and Non-linear load. STATCOM based power conditioner shows effectiveness to improve the quality of power at PCC, which reduce the harmonics presented to system by Non-linear load and maintain the voltage reduced by grid connected SEIG by providing reactive power support to the system. The performance of system is analyzed, which shows that proposed technique maintain the voltage and harmonics distortion level at PCC within permissible limits.

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Power Quality Improvement Using STATCOM in Grid Connected Wind Energy System