power quality improvement for wind connected...

International Journal of Computers Communication Networks and Circuit Systems (IJCCN) Vol 1, Issue 1 April 2015

RMK COLLEGE OF ENGINEERING AND TECHNOLOGY 427

POWER QUALITY IMPROVEMENT FOR WIND CONNECTED SYSTEM WITH NONLINEAR

LOAD

J. PARDHA SARADHI, R. SRINIVASA RAO, V.GANESH.

Department of EEE, Bapatla Engineering College,

Department of EEE,JNTU Kakinada,

Department of EEE, JNTU Ananthapur, Pulivendula.

[email protected]

[email protected]

[email protected]

Abstract— Now a day’s wind power plants playing a major role

in reducing the power deficiency. But the main concern with the

wind power plants it is generating the harmonics, and causes

power quality problems, and it forms weak grid. This paper

mainly uses CUSTOM power devices i.e., UPQC (unified power

quality conditioner) and STATCOM (Static compensator) for the

improvement of the system’s power quality, by reducing the

harmonics which are injected into a source. The proposed control

strategy decrease THD in source current and also improves

voltage profile. Overall power system performance can be

improved under the influence of wind power plant. Simulation

results show the performance of system, and the performance of

UPQC is compared with STATCOM. Which shows power

quality by using UPQC is better than STATCOM in terms of

THD and voltage profile. The results show comparative study of

the system with UPQC and STATCOM, without any

compensation technique. IndexTerm s— UPQC, STATCOM, THD, CUTOM power

devices, weak grid. I. INTRODUCTION

With the ever increasing power demand the fear of

extinction of the fuels for the thermal power is also going to its peak. But to ensure sustainable growth economic and social

progress, the requirement for the energy has to be met. In order to ensure the fuel for future generations utilizing the non-conventional energy resources like wind, biomass, hydro, co-generation etc. are to be encouraged. In modern energy

system, conservation of power and the use of renewable source are the key paradigm. In addition the basic responsibility to minimize the environmental and economic impacts on conventional power plants, the requirement of

integration of renewable energy like wind energy into power system raised. Particularly in India we are dependent on wind power, due to its exclusive advantages compared to other non conventional devices like

1. The wind is free with modern technology it can be captured efficiently.

2. Operating cost of wind mill is negligible.

Unfortunately the source of power is non steady and

unreliable. Also the varying wind speed creates problem in

case of wind plants used for electric power generation. The need for controlling of power flow in the

transmission lines and to enhance quality of power being distributed[8][9], the technological development in high power electronic devices has led to implementation of electronic equipment suited for the present day electric power systems, with fast response compared to the line frequency.

In the first section of discussion proposed scheme Unified Power Quality Conditioner (UPQC) control system is connected and simulated using MATLAB/SIMULINK in power system block set along with a wind generator system connected to a grid to mitigate the power quality issues. The effectiveness of this scheme notably reduced the main supply source harmonics and it will balance the load. UPQC performs well even in the environment of nonlinear loads and wind generators, which are producing harmonics. Power quality at grid side is ensured by connecting the UPQC. Voltage profile at source also increased.

In the second section a scheme of static compensator (STATCOM) is proposed to be connected at a point of common coupling to mitigate the power quality issues. Here also STATCOM control scheme connected to a grid connected wind energy generation system for power quality improvement is simulated using MATLAB/SIMULINK in power system block set. This scheme also reduced harmonics from the main supply along with the reactive power demand of the load and the induction generator. It is also having capability of load balancing. It performs even improving the power factor.

In the third section simulation results are analyzed for the system with UPQC, with STATCOM and without any compensating device. The performances of UPQC, STATCOM are compared when wind power plant is connected to grid in the environment of non-linear loads.

Test results shows effectiveness of UPQC in mitigating the harmonics and improving the voltage profile at grid.



Fig1 shows study case power system which consists of a non linear load and a wind power plant. Compensators are connected at point of common coupling.

Fig: 1.Tested power system

S. No Parameter Rating

1 Grid voltage 3-phase,415v,50hz

2 Induction machine 3.35KVA,415v,50hz,four

pole, Ls=0.12,Lr=0.06

3 Line series 0.05mH

inductance

Table.1 represents test system data

II. UPQC IN POWER QUALITY IMPROVEMENT TOPOLOGY

The shunt converter of UPQC at point of common coupling (PCC) injects current, whereas series converter is develops the voltages, for the improvement of the voltage profile. The power electronic converters i.e. VSI (voltage source inverter) or CSI (current source inverter) which were used produce the three phase voltages for the operation. Generally VSI is used due to its lower linking power losses and quicker response during the operation.

UPQC control is implemented here in a rotating frame of dq0 using park’s transformation.

In this transformation it is allowed to the align a rotating reference frame with the positive sequence of the point of common coupling voltages space vector. For this to happen, a reference angle synchronized with the PCC positive sequence fundamental voltage space vector is calculated using a Phase Locked Loop (PLL) system. An ―instantaneous power theory‖ based PLL has been implemented in or work [10].

In this synchronous reference frame under balanced

conditions voltage and current vectors are constant quantities. This feature is useful for analysis and decoupled control.

A.UPQC CONTROL STRATEGY i) Series controller: Series converter in the UPQC is controlled to maintain the

terminal voltage of the wind power plant at rated value and this can be used to compensate the PCC voltage variations .So that voltage disturbances comes from the grid cannot spread

to other part of the system.

Fig.2: series controller B) Shunt controller:

The active and reactive pulsations generated by the wind power plant are filtered out through the use of shunt converter of UPQC.Therefore power injected into the grid is free from power fluctations.The controller generates both voltage commands base upon power fluctuations .The difference of are the mean power from the instantaneous power measured in PCC are calculated by subtraction. The means of active and reactive components are calculated by low pass filtering. The band width of these filters are to be selected in such a way that power fluctuation components selected for compensation have to fall into flicker band.

Fig.3.Shunt controller



III. TOPOLOGY OF STATCOM FOR POWER

QUALITY IMPROVEMENT

A. Structure of STATCOM: A basic STATCOM (Static Compensator) with a

two-level Voltage Source Converter (VSC), a DC energy

storage device like a battery, a coupling transformer connected in shunt to the distribution network through a coupling

transformer is used. The main function of VSC is to convert

the dc voltage across the storage device into a set of three-phase ac output voltages which is nothing but Inverter action.

These 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 these voltages’s

phase and magnitude at the output of the STATCOM allows to effectively controlling the exchange of active and reactive

power between the STATCOM and the grid. This topology

allows the STATCOM to generate controllable active and reactive power or absorb the same. But, there are several

factors those are to be considered while designing the STATCOM and its associated circuits for its control. In

relation to the power circuit the following issues are of major

importance: Capacitor size at the DC link Ratio of Coupling transformer reactance and transformation Equipment requirement of output filter

regulates the voltage at DC link while the other PI controller regulates the PCC’s terminal voltage. Power factor correction of load is the responsibility of the STATCOM’s in-phase reference currents components and the regulation of the AC system voltage at PCC is done by quadrature components of supply reference currents.

Fig 4: Schematic diagram of a STATCOM

Also, there are several other factors which are to be

considered when designing the STATCOM and its associated control circuits. In relation to the power circuit .The following are the issues of major importance in relation to power circuit:

The VSC connected in shunt with the ac system provides a multifunctional topology which can be used for up to three quite distinct purposes:

1. power factor correction 2. Compensation of reactive power and there by the

voltage regulation 3. Current harmonics elimination B. Control Circuit Modeling:

The control algorithm of STATCOM with two PI controllers is shown in fig: 5.Of the two PI controllers, one

Fig.5: control circuit

The output of PI controller over the DC bus voltage

is considered as the amplitude of the in-phase component of supply reference currents and the output of PI controller over AC terminal voltage is considered as the amplitude of the quadrature component of supply reference currents. By adding

the in-phase supply reference currents to the quadrature supply reference currents the instantaneous reference currents are obtained. once the reference supply currents are generated, a carrier less hysteresis PWM controller is employed over the sensed supply currents and instantaneous reference currents to

generate gating pulses to the IGBTs of STATCOM. The controller controls the STATCOM currents to maintain supply currents in a band around the desired reference current values.



The hysteresis controller generates appropriate switching pulses for six IGBTs of the VSI working as STATCOM. VI. RESULTS AND DISCUSSIONS

A. When UPQC is connected: The power generated by wind form system is connected to

the grid having a nonlinear load. The performance of this system is observed by switching a UPQC ON in the system. The mitigation of harmonics induced into the grid starts without change in any other load condition parameters when UPQC is ON. Voltage profile at source side also observed. The three phase current from source, load current, wind form’s output current and compensator currents are shown in Figure.6.The test system is simulated for 0.3 sec.

Fig 6: Three phase source, load, compensator and wind form currents with

UPQC ON .

B. Power quality improvement: Power quality is mainly affected due to affects of

nonlinear load and wind generator, there by power quality is affected on source and load sides. Initially the system is tested without UPQC .Then it is observed that THD is high and its value is 27.04%.FFT analysis block is used to find THD.FFT is analyzed at 0.13 seconds per one cycle. Which shows that power quality is reduced?

Fig8: THD analysis of source current without UPQC

The power quality is compared with and without UPQC.

Harmonics are drastically reduced by connecting with

UPQC.THD obtained by connecting UPQC is 1.88%.THD is

measured for one cycle at 0.13 seconds. The amount of THD

is in within limits of IEEE standards of power quality.

Fig7: Voltage current relation at source side, when UPQC is ON observed at grid

Voltage current relation when UPQC is connected is observed at grid side, and it is observed that voltage profile is increased compared to STATCOM and without any compensator.

Fig9: THD analysis of source current with UPQC (THD=1.8%)



C .When STATCOM is ON: A STATCOM connected to a weak grid and its

performance is studied for power factor correction and load balancing capability. The variations of performance variables such as supply voltages, supply currents, load currents STATCOM currents is analyzed.

Fig: 10 shows the currents of source, load, compensator and the wind form. The STATCOM is turned on at 0.1 seconds and the comparative performance is presented.

Fig10: Three phase source, load, compensator and wind form currents with

and without STATCOM

Fig11: Simulation results for power factor with Non linear

Load

Simulations are carried to calculate the voltage profile

and power factor, which shows better power factor, which is unity.

FFT analysis is done, and harmonics are calculated by

using FFT analysis. Circuit Breaker is used here to

differentiate the performance of system with and without

compensator. Circuit Breaker is kept in open position up to

0.1sec and it kept

closed at 0.1 sec.Therefore the performance of the system is studied with and without controller which shows that THD is reduced to 2.9% when CB is closed from 27.04% when CB is open.

Fig12: THD analysis with STATCOM at source current (THD=2.89%)

V. COMPARISION BETWEEN UPQC AND STATCOM

PERFORMANCE IN POWER QUALITY IMPROVEMENT.

UPQC and STATCOM performance in terms of power

quality is compared here. Which shows harmonics in source will be reduced much extent by using UPQC, voltage profile at grid side also improved due to UPQC action. Whereas STATCOM performs well in improving the power factor and it is performs well in reducing the harmonics up to the mark.

TABLE.2

Compensating THD at source side Terminal voltage

Device (%) of generator(V)

UPQC 1.8 560

STATCOM 2.89 340

Without 27.04 340

Compensator



VI. CONCLUSION

The UPQC and STATCOM-based control schemes

are proposed for power quality improvement in grid connected wind generating system along with non linear load. The

consequences of power quality issues on the consumer and electric utility are presented. On the simulation of the control

system developed for the UPQC & STATCOM in

MATLAB/SIMULINK for maintaining the power quality both the compensators proved that they have their uniqueness in the

capability to cancel out the harmonic components of the load

current. Individually, STATCOM is good in maintaining the source side voltage and current in in-phase and supply the

reactive power demanded by the wind generator and load at the point of common coupling in the interconnected system,

and it provides an opportunity to enhance the utilization factor

of transmission line. Whereas UPQC performs better than STATCOM in reducing harmonics and improving the voltage

profile at source side the integrated wind generation system

and grid with a UPQC have shown the outstanding improved performance. And the present scheme of connection in the

grid connected system fulfills the power quality norms as per the IEC standard 61400-21.

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