To Improve the Quality of power in Hybrid systems using Active control Abstract A high power Quality is one of the major part in power system which is nothing but a distribution and transmission sector

To Improve the Quality of power in Hybrid systems using Active control
Abstract A high power Quality is one of the major part in power system which is nothing but a distribution and transmission sector . In the renewable energy generation side (wind ,solar ,hydro energy ) etc to created various type of problem in the grid. The source of a power quality problem often cause a disturbance or power quality variation .Need for solving these power quality disturbance .To mitigate these problem with the help of passive filter and active filter are used. The FACTS (Flexible AC Transmission systems) device like Static Synchronous Compensator (STATCOM) and Dynamic Voltage Restorer (DVR) are used for the system. With the help of DVR to compensate voltage drop by injecting active and reactive power. STATCOM is a device to regulate the reactive power in the system. In this paper will design the STATCOM and DVR for voltage control and harmonics filter out. Separately control technique used for the STATCOM and DVR.The simulation has been made by MATLAB/SIMULINK and the result of these system are combine.

Index terms- Active filter , STATCOM ,DVR ,Harmonics, Power quality ,FACTS
Wind energy.

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INTRODUCTION
Flexible AC Transmission systems (FACTS) are a new technology. It has the principal role of the enhancing controllability and power transfer capability in a AC systems. With the increasing demand of power ,the power system networks are becoming more complex from the point of view of operation and control. The existing networks are mostly mechanically controlled. Microelectronics, computers and high speed communication are widely used for protection and control of transmission system. A number of control devices were develop under the term FACTS technology. FACTS technology opens up new opportunities for controlling power and enhancing the usable capacity of existing and new line. Power quality as anything that affects the voltage, current and frequency of the power being supplied to the end user. The power quality problem solved is very essential these affect the entired system. Now a days there are many technique to solved these problem. The ability to define and understand the various types of the power quality problems the necessary background needed to prevent and solved those problems. The power quality characteristic, of the disturbance identifies the type of power quality problem. The nature of variation in the basic components of the sine wave distortion. The most common problem are voltage sag, swell, voltage fluctuation, voltage dip are occurred. 5,6,7 . Voltage sag (voltage dip )are most common type of power quality problem. Utilities and end user and cause voltage sag on the transmission line. The increase use of non liner equipment have cause harmonics to become more common. Also another more common problem is voltage unbalance are rapid change in voltage within the allowable limits.

Power Quality Standards
The organization responsible for developing power quality standards in the United States include the following Institute of Electrical and Electronics Engineer (IEEE), American National Standards Institute (ANSI),
National Fire Protection Association (NFPA) etc.

International Electro technical Commission (IEC)
It has since evolved into an organization with membership from 43 countries. The IEC council heads the IEC and oversees 200 technical committees, subcommittees and working groups. They refer to power quality standards as so called electromagnetic compatibility (EMC) standards. When wind turbine is involved for determine power quality characteristics.

American National Standards Institute (ANSI)
Five engineering societies and three government agencies founded
ANSI .It is a private nonprofit organization with member organizations form the private and public sectors.

Purpose of power quality standards
The purpose of power quality standards is to protect utility and end user equipment from failing or misoperating when the voltage, current and frequency deviates from normal. power quality standards provide this protection by setting measurable limits as to how far the voltage, current or frequency can deviate from normal. By setting these limits power quality standards helps utility and their customers gain agreement as to what are acceptable level of service.

C Harmonics

Mostly harmonics are created due to the device of power electronic switching in the system and increase the use of Non linear equipment. The harmonics standard for the United States, IEEE 519-1992 recommended practice and requirements for Harmonics control in electrical power system
Recognizes that the primary source of harmonics current is non liner load located on the end user side of meter.IEEE -519-1992 set current limit at the point of common coupling (PCC) .Due to the Harmonics created electronic equipment to malfunction.

Voltage variation
Generally change in the voltage variation which affected on the active and reactive power. Mostly voltage variation problem are voltage sag , swell, voltage dip, unbalance , Interruption (complete loss of power) .In wind frame voltage variation occurs due to torque of induction generator and wind speed. The nature of voltage variation are sine wave distortion that is voltage, current and frequency.The non linear load include all the type of power electronics equipments that use switch mode power suppies, rectifiers converting ac to dc ,arc welders ,adjustable speed drive etc.

Wind turbine Induction Generator (WTIG)
A large amount of reactive power is need for its operation.The power must be feed for operation of wind turbine induction generator whether from capacitor bank or synchronous grid. When the wind turbine initially connect to a grid it need some reactive power for generation of energy in that case the voltage to be drop at pcc.

Static Synchronous Compensator (STATCOM)
Function of STATCOM
The primary objective of STATCOM are used to improve the power quality issue which is nothing but to removed Harmonics in the desired system and also to improve power t transmission capacity. To improve the system voltage as a shunt compensator for the combination of WTIG. This device which is nothing but a capacitor are shunted connect across voltage source converter (VSC).

With the help of STATCOM to mitigate the harmonics and uses a Hysteresis current control method to inject a current in the system to measure a harmonics created by the non linear load connect in the system.

Fig 1 Basic STATCOM Model
Reference current generation
In these paper With the help of these formula to calculates reference current in the system. In STATCOM control to use in a reference current we need topology grid coordination. If we take the three phase RMS voltages (Va,Vb,Vc) and is expressed, as total voltage as peak voltage, as in (1).

Vtotal ={2/3 (Va2+Vb2+Vc2)}
The unit vector are generate from the source as shown in equation number 2
Then reference current shown by following equation
This method is easy to generate reference current and also it is simple and robust to other method.

Hysteresis Current Controller Hysteresis controller is used as it is simple, it has fast transient Response and good accuracy. Hysteresis current controller is used for producing switching signal by comparing the error present in the current in a fixed tolerance band. Here comparison is done between the actual current & reference current within a fixed tolerance band. It is different for different phases. Hysteresis current controller is used to compare current so to generate switching signals. This error is compared by a relay that gives an signal for the IGBT if the error is higher or lower .

When ???<(???*???/2) then the upper switch is ON and lower switch is OFF and the current is allowed to decay in phase ? it is similar for phase ?&?. When ???>(??? *+??/2) then upper switch is OFF and lower switch is ON.

Because of advancement in solid state power devices and microprocessors,
switching power converters are used widely

Fig2. Hysteresis Controller
For phase A we have the following switching operation.

if Isa – Is?a <?hb , then to inject a current and Sa =1
if Isa – Is?a >hb , then to absorb current then Sa=0.

Where hb = hysteresis band.

Phase B and phase C work in the same way for these switching logic. If smaller the hysteresis band then less the error and hysteresis band is the high then large the error.

Dynamic Voltage Regulator (DVR)
1. Basic DVR operation
The power quality problem is occurring generally in transmission and distribution side to overcome this problem by using custom power devices. In the family of FACTS devices DVR is the most efficient and effective modern device used in the power system.DVR are connected between the critical load feeder and the supply side at a point of common coupling (PCC).15Due to the fault in the system the voltage variation problem occurred in the system with the help of that device to mitigate the effect in the system and stable the voltage .

In basic construction of DVR is connecting to an injection transformer that is mounted in series with the three phase nonlinear load in which an inverter connect itself. The depth of voltage sag (dip) DVR can compensate these voltage by injecting active and reactive power which is depend upon the system requirement.

Fig. 3 Dynamic voltage restorer (DVR)
2. DVR control method

Various control strategy used in DVR device but in these paper to use a Synchronous Reference Frame Theory .By using park or dqo transformation to make this algorithm. With the help of these transformation to convert current or voltage in rotating component into phase component or vice versa.

Fig. 4Control Algorithm for SRF Theory
Consider the following equations.

Fundamental quantities become d-q quantities which can be separated easily through filtering. The Fundamental component become a Constant which can be low pass filter to leave the high frequency component. Low pass filter of DC component does not cause any phase error in a signal if the issue occur then high pass filter are used. The cutoff frequency 50 Hz selected in low pass second order filter is butter worth .

PLL circuit provides the rotation speed of the rotating reference frame these are open loop controller.

Fig 5.MATLAB /SIMULINK Model System
System operation
STATCOM Operation
When the Wind turbine work on the operation and also non linear load are connected in that case voltage and current are separated out in the PCC and need to filtering. In that time the STATCOM (active filter ) to mitigate a harmonics by injected a current is generated by Hysteresis control technique.
In fig 6 shows the current at PCC when STATCOM is OFF position .Fig 7 to compensate the current at the PCC after the STATCOM , to get a sinusoidal wave as compare the device is OFF. Fig 8 shows the value of current injects form the STATCOM to cancel the effect of harmonics. In Fourier analysis to show the current THD with and without STATCOM connected in the fig 9 and 10.

Table THD analysis
THD % Before connecting STATCOM THD % After connecting STATCOM
DVR Operation
At a starting when wind turbine is connected to the grid it want reactive power for its operation. In that case to create a voltage drop at the PCC, with the help of DVR to compensate these voltage drop by inject a voltage at the coupling Transformer. In fig 6 shows the when wind turbine is running to cause a voltage drop at PCC. In fig 7 voltage at load bus when DVR compensate the voltage. In fig 7 DVR compensate the voltage.

CONCLUSION
This entire system was modified in the MATLAB /simulink.

STATCOM and DVR are used separate controller . To control STATCOM with the help of Hysteresis control technique and also by using Synchronous reference theory .The power quality issues are solve such as harmonics are STATCOM and DVR to mitigate the voltage drop respectively. This device work as a active filter when nonlinear load connected in the system.

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Volume 6, Issue 8 August 2017

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