ANALYSIS OF VARIOUS COMPENSATION DEVICES FOR POWER QUALITY IMPROVEMENT IN WIND ENERGY SYSTEM

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International Journal of Electrical Engineering & Technology (IJEET)

Volume 7, Issue 3, May–June, 2016, pp.25–39, Article ID: IJEET_07_03_003

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© IAEME Publication

ANALYSIS OF VARIOUS COMPENSATION

DEVICES FOR POWER QUALITY

IMPROVEMENT IN WIND ENERGY

SYSTEM

M. Thirupathaiah and P. Venkata Prasad

Department of EEE, Chaitanya Bharathi Institute of Technology,

Gandipet, Hyderabad, India

V. Ganesh

Department of EEE, JNTUA College of Engineering,

Pulivendula, Y.S.R District, Andhra Pradesh, India

ABSTARCT

In recent trend, the renewable source of energy is increasingly used in the

electric power generation, which leads to certain power quality issues. Hence

some of the supplementary devices like capacitors, compensators or reactive

power injection devices are added to the compensation system. With the

advancement in power electronics, compensating devices such as Distribution

Static Compensator (DSTATCOM), Unified Power Quality Conditioner

(UPQC), Dynamic Voltage Restorer (DVR), Static Var Compensator (SVC)

etc. are used. In this paper, the characteristics of three important

compensating devices DVR, UPQC and DSTATCOM are analysed. Initially

these devices are modelled and their performance is analysed with common

power quality problems such as voltage sag, swell and harmonics. The overall

outcome suggests that the performance of the DSTATCOM is comparatively

better than that of the other two devices, which become the most suitable

device for power quality improvement in wind energy system.

Key words: Distribution Static Compensator (DSTATCOM), Dynamic

Voltage Restorer (DVR), Power Quality, Unified Power Quality Conditioner

(UPQC), Wind Energy System.

Cite this Article: M. Thirupathaiah, P. Venkata Prasad and V. Ganesh,

Analysis of Various Compensation Devices For Power Quality Improvement

In Wind Energy System. International Journal of Electrical Engineering &

Technology, 7(3), 2016, pp. 25–39

http://www.iaeme.com/ijeet/issues.asp?JType=IJEET&VType=7&IType=3

M. Thirupathaiah, P. Venkata Prasad and V. Ganesh


1. INTRODUCTION

Today’s technological world completely depends on electricity; however the

availability of electric sources are low. The deficiency of electricity becomes the

breaking point for developing countries like India. Hence the electric utilities are

finding a suitable solution for providing uninterruptable electricity. In this situation

the usage of renewable energy sources are the better solution, so these renewable

energy sources are encouraged for electricity production [1, 2]. In India, the most

available renewable source is wind and solar and the research on this area is under

progress [3, 4]. The wind based energy acquisition is most encouraging research area

because of its low complexity in installation and maintenance. In wind energy

acquisition the wind turbine is used [5]. The technical challenges that a power system

integrated with a wind power requires the analysis of power quality issues such as

voltage regulation and stability [6, 7]. The wind turbine produces a continuous

variable output. In wind power system, the wind turbine has a great importance to the

power quality issues in the power system [8, 9]. The variations in the power output

are caused due to wind groom and the disturbances in the power system [10].

Therefore, the system has to manage such variations so that the power quality issues

such as voltage sag, voltage swell, flickers and harmonics can be considered w.r.to the

generation, transmission and distribution systems of wind power [11].

But, the wind generation produces turbulence into the network. An induction

generator directly connected to the grid system can be used to run a wind generation

system [12]. Due to the variations in the wind, the active power generated by an

induction generator varies which affects the reactive power absorbed by the induction

generator and its terminal voltage [13]. In order to provide the control over the active

power produced by the induction generator, the wind generation system should work

with a précised control technique [14]. At point of common coupling (PCC), a

compensator device such as Distribution Static Compensator (DSTATCOM), Unified

Power Quality Conditioner (UPQC), Dynamic Voltage Restorer (DVR) & Static Var

Compensator (SVC) etc. can be connected for improving the power quality which can

manage the challenges of wind turbines [15].

2. POWER QUALITY ISSUES IN WIND ENERGY SYSTEM

The guidelines for measurement of power quality of wind turbine are developed by

The International Electro-technical Commission (IEC) in coordination with the

Technical Committee-88.This commission explained the methodologies for

measuring the power quality characteristics of a wind turbine [16]. For grid

connection, the base for the analysis is provided by the data sheet with electrical

characteristic of wind turbine.

2.1. Power Quality Issues

2.1.1. Voltage Variations

In wind generation system, the velocity of wind and the torque developed by the

generator results in the variations in the voltage. This voltage variation results in real

and reactive power variations. The fluctuations in the wind power occur during the

normal operation of the wind turbine. The magnitude of these fluctuations depends on the impedance, phase angle, power factor of wind turbine and the strength of the grid.

Various types of voltage variations are given as follows:

Analysis of Various Compensation Devices For Power Quality Improvement In Wind Energy

System


Voltage sag.

Voltage swells.

Voltage flicker.

Short interruptions.

Long duration voltage variation.

2.1.2. Harmonics

The harmonics in the wind power system is due to the usage of power electronic

equipment. At the connection point of the wind turbine to the system, the harmonic

voltages and currents should be within their limits. To ensure the voltage harmonics

within the limit, the current harmonics should contribute in a limited manner.

2.1.3. Wind Turbine Location & Self-Excitation of Wind Turbine

The quality of the power generated by the wind energy system depends on the way of

connecting the generation system into the network. Thus the location of the wind

turbine in power system influences the operation of the power system. In general, the

Wind Turbine Generating System (WTGS) is provided with a capacitor which results

in the risk of self excitation. The capacitor connected to the generator provides the

reactive power compensation. In Wind Turbine Generating System (WTGS), the self

excitation is provided by a synchronous generator immediately after disconnecting the

Wind Turbine Generating System (WTGS) with the load. But the major disadvantages

of self excitation are the imbalance between real and reactive power and the safety

[17].

2.2. Consequences

The issues mentioned above are the major causes to reduce the power quality of the

grid. The voltage variations such as voltage sag, swells, flickers, short and long

interruptions and harmonics causes the disoperation of the programmable logic

controllers and microprocessor based control systems. Also it results in tripping of

protective equipment such as circuit breakers, relays and contactors. Since the control

system consists of sensitive equipments like computers, microprocessors and PLCs,

the variation in the voltage leads to malfunction and sometimes even damage to this

sensitive equipment. Finally, due to this disoperation and malfunctioning of the

equipment the process may get stopped.

2.3. Grid Coordination

After the blackout in the United States in August 2003, the grid code was developed

by The American Wind Energy Association (AWEA) at the distribution level for

stable grid operation. The guidelines of grid operation of wind generating system at

the distribution system are defined as-per IEC-61400-21. The organization and

operation of interconnected system is governed by the operator of transmission grid

[18].

2.3.1. Voltage Swell/Rise

At the Point of Common Coupling (PCC), the voltage rise can be approximated as a

function of maximum apparent power maxS of the turbine, the phase angle and the

grid impedances R and X at the point of common coupling [19], given in eqn. (1).



2

max sincos

U

XRSu

(1)

Where u - Voltages rise

maxS - Maximum apparent power,

- Phase difference,

U - Nominal voltage of grid.

The limiting voltage rise value is < 2 %

2.3.2. Voltage Dip/Sag

The voltage dip or sag is a sudden reduction in the voltage due to the start of wind

turbine. It is mentioned in relative percentage voltage. The decrease in voltage is

given in eqn. (2).

K

nu

S

SKd

(2)

Where d - Relative voltage change

uK - Sudden voltage reduction factor

nS - Rated apparent power

KS - Short circuit apparent power.

The limiting value of voltage dips is %3

2.3.3. Flicker

The voltage flicker measurements are done with more number of switching operations

of wind turbine, as given in eqn. (3).

K

nKlt

S

SCP (3)

Where KC - Flicker coefficient

ltP - Long term flicker

The Limiting Value for flicker coefficient is 4.0 [20]

2.3.4. Harmonics

At the point of common coupling, the harmonic distortion is measured for variable

speed turbine with an electronic power converter [21]. The total harmonic voltage

distortion of voltage is given as in eqn. (4):

%10040

2 1

2

h

nTHD

V

VV

(4)

Where, Vn - nth harmonic voltage The THD limit for voltage is < 3 % The THD of current THDI is given as in eqn. (5):


System


%1001

I

II n

THD

(5)

Where, nI - nth

harmonic current

The THD limit for current is < 2.5 %

2.3.5. Grid Frequency

In India, the frequency of the grid is maintained in the range 0f 47.5 to 51.5 Hz for

wind power systems.

3. POWER QUALITY IMPROVEMENT IN WIND ENERGY

SYSTEM

The power quality problem can arise in many ways in the wind turbine system,

however any of the suitable compensator is well enough to solve the problems and

improve the power quality in wind based power system. The grid connected system

for improving the power quality at Point of Common Coupling (PCC) is shown in

Fig. 1. Mainly it consists of a compensation device such as Distribution Static

Compensator (DSTATCOM), Unified Power Quality Conditioner (UPQC), Dynamic

Voltage Restorer (DVR) etc., induction generator, source and a non-linear load. The

control system of the compensation device injects a harmonic free current into the

grid system. To improve the power factor, the reactive part of the load current and the

harmonics in the induction generator current are cancelled out by injecting the

inverter output current of the compensating device, thus improving the power quality

of the grid. To achieve these, the grid voltages are synchronised while generating the

current command for the inverter in the control system of the compensating device.

Figure 1 Grid connected system for improving Power Quality

Because of the cost effectiveness, robust and simplicity the induction generator is

used in this method. It can be used for constant and variable loads and also has a

natural protection against short circuits. It is assumed that the wind generators in this

configuration are working based on constant speed topography and pitch control

methods for turbine.

(DVR/UPQC/

STATCOM)

Vs, Is

Vi, Ii

VL, ILNon linear load

Induction Generator

Source

Compensation Device

3-Phase 415 V, 50 Hz



3.1. Dynamic Voltage Restorer (DVR)

Dynamic Voltage Restorer (DVR) is a series compensating device, which can protect

a sensitive load from the distortions. The basic principle of Dynamic Voltage Restorer

(DVR) involves the injection of the voltage of required magnitude and frequency

which restores the load side voltage to the desired magnitude. The Dynamic Voltage

Restorer (DVR) employs GTO thyristors power electronic switches with pulse width

modulated (PWM) inverter structure. The Dynamic Voltage Restorer (DVR) injects a

set of three phase output voltages in series with the distribution feeder, which is made

of a solid state converter. At the load side the DVR can generate or absorb the real

and reactive power independently. The magnitude and the phase angle of the voltages

injected by DVR can be variable which allows the reciprocation of the real and

reactive power between the Dynamic Voltage Restorer (DVR) and the distribution

feeder system. The dc input terminal of a dynamic voltage restorer is connected to an

energy source or an energy storage device of proper capacity.

VS P+jQ

V LoadI Load

Sou

rce RT

jXT VDVR

Transformer

Low pass LC filterINVERTER

DC

Sto

rage +

-

By Pass Switch

Figure 2 Dynamic Voltage Restorer

The transfer of reactive power between the DVR and the distribution feeder

generated internally by the DVR without any AC passive reactive components. An

external energy source or an energy storage system is used for the real power

exchange at the DVR output ac terminals. The DVR structure comprises of rectifier,

inverter, filter and coupling transformer. PWM technique is used to control variable

voltage. Filter is used for eliminating harmonics generated from high switching

frequency in PWM technique. DVR system is connected in series with the distribution

feeder in the power system that supplies a sensitive load shown in Fig. 2.

In order to maintain the load voltage, reactive power must be injected by DVR

system. Upon the occurrence of the fault which may be a short circuit current flow, a

line-line-ground fault which leads to reduction in the voltage magnitude at the Point

of Common Coupling (PCC).

3.2. Unified Power Quality Conditioner (UPQC)

There are lots of problems integrated with the power system with the advancement in

complex electronics industries, and it has become mandatory to provide a dynamic

solution with fast speed of response and high degree of accuracy in order to mitigate

the power quality issues. The active power filtering has appeared as one of the best


System


solutions for mitigation of major power quality problems [22].The UPQC which is an

integration of shunt and series APF is one of the most appropriate as well as effective

device in this concern [23]. A comprehensive review on the UPQC to enhance the

electric power quality for various type of power generation system at distribution and

transmission levels has been given in [24].

LOAD

Series

Inverter

Shunt

Inverter

UPQC

Figure 3 UPQC System Configuration

The main motive of UPQC is to solve the problems coming from both source side

and load side, such as voltage sag, voltage swell, harmonic reactive currents,

distortion in the supply voltage, etc., [25]. Using a common dc bus capacitor the

components of UPQC series and shunt inverter connected back to back. The general

block diagram representation of a UPQC based system is shown in Fig. 3.

3.3. Distribution Static Compensator (DSTATCOM)

Fig. 4 shows a typical DSTATCOM configuration. DSTATCOM is a Multiple Input

Multiple Output (MIMO) system. Thus a multivariable control approach is needed for

the DSTATCOM control design. There is one powerful tool for studying balanced

three phase system, which transform the three phase voltages and currents into

orthogonal components in a synchronous rotating frame by Park’s Transform but it is

not possible to totally decouple the system variables.



Figure 4 DSTATCOM system configuration

For the decoupling method the MIMO system will be simplified. The active and

reactive components are simply the orthogonal components in the rotating frame. The

proposed approach for PID controller design and synthesis will be applied for the

decoupled control variable.

4. PERFORMANCE ANALYSIS

In this paper, the performance of various compensation devices to rectify the power

quality issues is compared. A power system is modelled using Matlab/Simulink and

the system parameter is given in Table I. The execution of the proposed wind energy

system is dissected in view of two cases: voltage sag and swell. Fig. 5 shows the pre-

fault conditions, in which the aggregate limit of generating voltage of wind turbine is

350V and the load voltage is 300V.

TABLE I System Parameters

Parameter Range

Grid Voltage 3-Phase, 300 V, 50Hz

Induction Generator

3.35 kVA, 300V, 50 Hz, P = 4

Speed = 1440rpm, Rs= 0.01 Ω,

Rr = 0.015Ω, Ls = 0.06H,

Lr = 0.06H

Series Line Inductance 0.05mH

Inverter Parameters

DC Link Voltage = 800 V

DC link Capacitor =100 µF

Switching frequency = 2 kHz

IGBT Rating

Collector Voltage = 1200 V

Forward Current = 50 A

Gate voltage = 20 V

Load parameter Non-linear Load 25kW

Vdc

C

iL

Vtc

Vtb

Vta

ia

ib

ic

LR

VscVsa Vsb

Linear/ Non linear/

Unbalanced Loads

PWM Current Controller

Control Algorithm

N

Three

Phase

Supply

Ripple Filter

Rf Cf


System


(a) Generation Side Voltage (b) Load Side Voltage

Figure 5 Voltage measure without fault

In this paper, a nonlinear load is used; which varies independent of time, raising

the power quality problems in system. In fig. 6, the load side voltage at fault condition

is shown, in which during the time period 0 to 0.1 sec. the load voltage raises to

around 350 V, which is a voltage swell. Then in the time interval 0.1 and 0.2 sec. the

voltage level is 300 V, which is the normal level. Then in the next period from 0.2 to

0.25 sec., there is voltage sag, where the voltage falls to around 230 V.

Figure 6 Load Voltage with fault condition

However, identifying the most suitable device for the compensation of power

quality is not clearly mentioned in the previous research. Thus the proposed paper

motivated to identify the most suitable device for the compensation of power quality

issues in the wind based power system by integrating the compensation devices DVR,

UPQC and DSTATCOM into the power system and analyzing the performance of

those in order to maintain the voltage level to its normal value.



4.1. Analysis with DVR

Figure 7 Simulation Diagram for DVR

Figure 8 Load Voltage after compensation using DVR

The simulation diagram of DVR for improving the power quality in wind power

system is shown in Fig. 7 and Fig. 8 demonstrates the load voltage after compensation

by using DVR. It was observed that the voltage swell is decreased to 325 V and the

sag is compensated to 250 V, and the average load voltage is around 285 V, which is

not much satisfactory in view of restoring the voltage to the normal value.

Figure 9 Harmonic Analysis using DVR


System


Fig. 9 shows the harmonic analysis using DVR. The Total Harmonic Distortion

(THD) is found to be 2.851%.

4.2 Analysis with UPQC

Figure 10 Simulation Diagram for UPQC

Figure 11 Load Voltage after compensation by UPQC

The simulation diagram of UPQC is shown in Fig. 10 and Fig. 11 shows the load

voltage after compensation using UPQC. In this case, the voltage swell is decreased to

318 V and the sag is compensated to 265 V, and the average load voltage is around

290 V. After compensation there is a slight reduction of issues, but however still there

exists a voltage swell and sag, which needs to be improved further.



Figure 12 Harmonic Analysis using UPQC

Fig. 9 shows the harmonic analysis using UPQC and the Total Harmonic

Distortion (THD) in this case is found to be 2.281%.Here it was observed that the

voltage profile is slightly increased when compared with DVR compensation and also

the THD value is reduced slightly.

4.3. Analysis with DSTATCOM

Fig. 13 shows the simulation diagram of DSTATCOM integrated into the wind power

system and Fig. 14 shows the load voltage after compensation using DSTATCOM.

Here, it was seen that the voltage swell is decreased to 310 V and the sag is

compensated to 290 V, and the average load voltage is around 300 V, which is almost

reached to the nominal value of the load voltage. Also it has been observed that the

performance of DSTATCOM is very much satisfactory in restoring the load voltage

to its value when compared to DVR and UPQC. Also the THD value obtained is

1.426 %, which is very much reduced comparing with the previous two cases.

Figure 13 Simulation Diagram for DSTATCOM


System


Figure 14 Load Voltage after compensation by DSTATCOM

Figure 15 Harmonic Analysis using DSTATCOM

The performance of various devices for the power quality improvement is shown.

The analysis clearly shows that the performance of the DSTATCOM is comparatively

better than the other two devices in terms of improving the voltage profile to its

nominal values and reduction in harmonic levels. This performance variation clearly

shows that the DSTATCOM have better performance than the other two devices DVR

and UPQC and hence DSTATCOM is the suggested device for the power quality

improvement in the distributed power system. The numerical results of the various compensation devices are given in the Table II.

TABLE II Performance Comparison of Devices

PQ Issue At Fault

Condition

DVR

Performance

UPQC

Performance DSTATCOM

Performance

Voltage Swell 345 V 325 V 318 V 310 V

Voltage Sag 240 V 250 V 265 V 290 V

Avg. Load Voltage 270 V 285 V 290 V 300 V

THD 4.028% 2.851% 2.281% 1.426%



5. CONCLUSION

The power quality improvement in the distribution system integrated with a wind

turbine is one of the recent research trends. There are many devices used for the

power quality improvement, so it is tough to identify the most suitable device for the

power quality improvement in the distribution system. Hence, this paper presented the

analysis on three major compensation devices for power quality improvement, DVR,

UPQC and DSTATCOM. These three devices are analysed on a common problem in

wind based distribution system. The power quality is analysed based on the voltage

sag and swell measures, the implementation results and its analysis clearly shows the

betterment of the DSTATCOM. So, the DSTATCOM is the most suitable device

which can be used for wind energy system for power quality improvement. In future

scope, the modification of this DSTATCOM can be done to further enhance the

performance for improving the power quality in distribution systems.

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