Voltage Regulation in Power Distribution System using Dynamic Voltage Restorer

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Introduction
In Power system network the power quality is vital term.In this power quality the term power quality incorporates voltage droop and swell.The voltage list and swell are happening 80% in the dissemination frame work according to the IEEE principles the RMS voltage at framework recurrence the voltage hang is 0.1 to 0.9 p. u with the span of 1min in the half cycle.The reasons of the voltage sags are like sudden changes of load, short circuits, starting large motors.Voltage sag is a transient phenomenon it is decided by the nature of the voltage when sudden changes occur in voltage, the consequences are divided in to transient with low and medium frequency.In this modern days different compensation techniques are used for the industries to reduce the voltage sag one of the modern technique is to enhance the power quality and compensate load fluctuations is DVR.To know the performance of DVR various experiments are done on the DVR finally different control strategies are found, those control strategies are based on the persistence of using DVR, here the main persistence of using the DVR is to detect and reduce the voltage sag with injecting the minimum amount of active power into the DVR, and also in-phase compensation for voltage sag and swell mitigation [1].
To control more than a one line multiline DVR is used by eliminating the battery in DVR structure.the control of DVR underneath variable frequency and harmonic reductions are for the medium voltage levels are research has been done.To control the DVR the simple method developed by taking consideration of load voltage, feedback current with closed loop control.The Posicast and P+ Resonant controllers are utilized to enhance the transitory reaction and to nullify the stable state error in DVR.The Posicast regulator is a kind of stair work with two segments and is recycled to work on the oscillating of the transient movements headed at the starting second from the voltage droop, The P+Resonant controller contains of a comparative function plus a resonant function and it nullifies the error of steady state voltage tracing.The different control strategies of DVR are state feed forward and input techniques, even segments estimation, hearty control, and wavelet change have additionally been proposed.In the Proposed system, a multipurpose control strategy has been introduced in which if the shunt feeder creates a disturbance.DVR can protect the load voltage by utilizing different control schemes called Posicast and P+Resonant controllers.Load voltage with outer feedback and filter current with an inner feedback loop are the basic of this control strategy in which the fault current did not pass through the DVR.The dynamic stability of load voltage has been improved by utilizing the feed-forward loop.Posicast and P+ Resoanat controllers are used to enhance transient response and steady-state error, respectively.Dynamic voltage restorer is utilized as a series voltage regulator based on a flux control algorithm, injecting voltage in the opposite direction when a short circuit current passes through it.[2]- [5].A distribution system connected with the Dynamic voltage restorer is shown in fig. 1 where the DVR comprises of basically an arrangement associated with an isolation transformer, voltage-source converter, an converter yield channel, and a vitality stockpiling gadget that is associated with the dc connect.Before infusing the inverter yield to the framework, it must be separated with the goal that music because of exchanging capacity in the converter are disposed of.In operation of DVR, the bypass switch and injected transformers are connected in antiparallel(Fig.1).The isolation transformer will be short circuited when there is no variations in load voltage.by this change to limit losses and boost cost viability.Likewise, this modification can be as two equal thyristors, as they have operated very fast.A money related appraisal of voltage hang occasions and utilization of adaptable air conditioning transmission frameworks (FACTS) gadgets, for example, DVR, to relieve them [6].

Basic Principle Operation Of DVR and Its Components
The fact that the out put of DVR can be flexibly depends on accuracy of switching of PWM and control strategy.The production of PWM has been done by comparison of sinusoidal signals with saw tooth signal which is utilized for switching of converter.

Fig:1.
A distribution system connected with the DVR.

Basic working of Dynamic Voltage Restorer
As shown in Fig. 2 normal DVR which is connected in where the DVR power system which is a combination of DC storage device long with Voltage source converter fed with filter, which reduces the harmonics created by switching function of converters and connected in cascaded with injection transformer.In general DVR and injected transformer are connected in shunt manner with a bypass switch.The phasor representation in Fig. 2, explains the electrical situations during voltage drop for one phase.V1, V2, and Vdvr represents the Voltages at supply-side, load side and the DVR injected voltage, respectively.It must be referred to that notwithstanding the in-stage injected strategy, another procedure, explicitly "the piece increment voltage compensation method" is furthermore utilized.Essential advantage of this strategy is that least active power would transferred from distribution system to storage system which gives the better compensation for large voltage sag and swells [7]- [10]. .To increase the transitory response in the system posicast regulator has been employed.Fig. 4 indicates a classic control block schematic of the DVR.Here as a part of normal practice, we are utilizing Note that because in real states, we are distributing with numerous feeders connected to PCC.Which is called as "the Point of Common Coupling".Here onwards to make the system normalized Voltages V1 and V2 can be replaced with VPCC.As explained in the figure 4, the necessary injected potential difference can be drawn by comparing the Source Voltage and load Voltage with help of open-loop control [11]- [15].
It is a very much easy method to generate the error signal and give to PWM inverter in DVR.The disadvantage of this method is that at damping in transient oscillations could not eliminate completely [5].The Posicast regulator has been connected in front signal to transferred to the PWM inverter connected as DVR.The transfer function of regulator can be derived as

……………….. (1)
The notations  and Td represents the peak overshoot and time period.The Posicast regulator works via post eliminating and suitable principles of its boundaries is vital.Therefore, it's sensitive to inaccurate data from the device when measuring the recurrence of damping noise.To limit this responsiveness, as is explained in Fig. 5, the open-circle regulator can be changed to a shut circle regulator via including a multiloop comments course corresponding to the current feed ahead way [16].
As the name suggests, the 2-DOF regulator provides two degrees of freedom to ensure rapid and active tracking through the feed forward path, while disturbances can be eliminated by utilizing the feedback path.. To nullify the error produced by steady state voltage tacking VDR fed with outer Voltage feedback with inner current loop.Generally P+resonant controller utilized as outer loop controller and this compensator has been analytically derives as …………………….(4) where KP and KI are regulator gain constants.Hypothetically, the thunderous regulator repays through presenting a boundless achieve at the resounding recurrence of 50 Hz (Fig. 6) to constrain the consistent state voltage mistake to nothing.The best thunderous regulator, nonetheless, behaves like a local area with an endless wonderful variable, which is presently not feasible by and by.An additional a reasonable (nonideal) compensator is therefore utilized here, and is communicated as

Proposed Multifunctional DVR
Not with standing the previously mentioned abilities of DVR, it can be utilized to safeguard the customers who are dealing with medium voltage levels as indicated in figure 7. The increased amount of current can be flows through DVR when a fault was occurred in this group.This type of problem can be eliminated by protecting equipment by limiting the faulty current and loads in shunt connected feeders up to operation of breaker, which will disconnects the faulty feeder [17].
. Fig. 7. Medium-voltage level distribution system fed with DVR Bu the generation of large fault currents voltages at PCC has been dropped which causes the overloading of other feeder connected to same bus.If this fault current doesnot controlled properly it will tends to mall functioning of DVR.That instead of reducing the voltage sag it will inject the additional voltage in to the system.Flux charge strategy of DVR has been employed to limit the faulty current which will work as perfect inductance which does not draw any real power from the system.It will protect the input side storage devices like Battery and DC capacitor.As the PCC voltage was not mentioned in regulator loop, which indicates the fixed values of injection transformer winding and practical inductance value of DVR which was very much disadvantage of this model.In this paper we are utilizing main reference quantity is voltage at PCC and DVR was acting like flexible inductance.This will tends to damage the battery and DC link Capacitor.To address this problem we are incorporating a resistance in series with inductance which is connected in shunt with DC link capacitor.This DC link capacitor isolated from the supply.A diode was connected in series with the battery which will protect the battery from fault currents.Huge oscillations in the current has been eliminated mainly by utilizing the inductance present in the circuit and active nature of power also can be absorbed [18]- [20].

Conclusion
In this paper, a multipurpose DVR is introduced, to enhance the damping ratio and transient response a closed loop control strategy has been utilized for DVR.Further more enhancement in transient stability and reduction in steadt state error has been done.This can be achieved by flux charge method which limits the load side fault currents with the of control equipment.Which protects the voltages at PCC by acting as variable impedance.To address this problem we are incorporating a resistance in series with inductance which is connected in shunt with DC link capacitor.This DC link capacitor isolated from the supply.A diode was connected in series with the battery which will protect the battery from fault currents.The simulation results confirm the efficiency and competence of the planned DVR , which reveals that there is a improvement in voltage sags which caused because fault currents and starting of large induction machine .which has been eliminated by the limitation of faulty currents.Which has also improved the voltages at PCC.

,Fig. 2 .
Fig. 2. Phasor representation of the electrical environment during a voltage droop.

Fig. 3 .Fig. 4 .
Fig.3.Block diagram of DVR connected to Distribution Syatem By injecting the voltage in opposite direction through DVR the load voltage can be compensated as shown in figure 3. Load resistance and inductances are R1 and L1 respectively.as stated in figure 1.Here the bypass switch has not shown in this figure.The DVR fed with harmonic filter which includes Rf, Lf and Cf and also overall resistance of the injection transformer was indicated with Rt having leakage inductance Lf and having the transformation ratio as n:1