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This paper mainly concentrates on design of improved controller and its implementation based on single phase synchronous reference frame theory (SRFT) for Dynamic Voltage Restorer (DVR) compensating voltage sag particularly for nonlinear load. In case of single phase distribution line with nonlinear load, the complexity of controller’s design becomes more serious issue. The present single phase and/or three phase theories applicable to DVR shows poor response to restore voltage sag in case of nonlinear load due to presence of harmonics. Hence restoration of voltage sag in single phase nonlinear load connected system has been a serious concern. Therefore, new controller for DVR has been proposed incorporating effective design concept for fundamental component extraction in case of nonlinear load. The single phase SRFT based main controller for DVR works on two separate closed path viz. feed forward path for quick transient response and feedback path for reducing the steady state error. Moreover, pre-sag mitigation strategy of DVR has been adapted through these two aforementioned paths. Complete design of proposed controller is based on phasor analysis. It also consist of proportional integral (PI) controller to reduce the error in the DC-link voltage during compensation time. The controller performance has been verified in MATLAB Simulink for both types (linear and nonlinear) of load. The results obtained indicates that the proposed controller is effective in its performance.

In distribution system power electronics based custom power devices plays a major role in present modern industries for upgrading the power quality during production process. Power quality issue has major impact on sensitive industrial and utility end loads in distribution line. Major power quality problem amongst voltage sag, voltage swell, harmonic distortion, flicker, spikes etc. is the single phase unbalance voltage sag. Moreover, compensation of unbalance sag in three phase system makes the design process complex. The conventional theories which are generally used in compensation of unbalance voltage sag with the utilization of custom power devices gives inefficient performance in case of nonlinear load. As nonlinear load generates the harmonic components, the detection and correction of voltage at load end becomes improper.

The characteristic of voltage sag is sudden decrease in rms voltage magnitude which remains in between half a cycle and few seconds [

and _{f} − C_{f} − L_{f}) filter.

Dynamic Voltage Restorer (DVR) equipment used in distribution line, penetrates the voltage in series with system voltage, providing the most cost effective resolution for mitigation of voltage sag with improved power quality required by utility customer [

The basic aim of the DVR is compensate required load voltage in which the compensating voltage is in series with the supply voltage to regulate the load terminal voltage. DVR could be the effective device to overcome some of the major power quality problems such as voltage sag, voltage swell, distortion and unbalance in supply and load with the injecting active and/or reactive power into the system.

Due to occurrence of fault (balance or unbalance) or starting of Induction motor or change in load, supply voltage

In general, the active and reactive power flows are controlled by the angle between the voltage that is injected in series with the line and the line current. For example, if the injected voltage is in phase with the current, only active power is changing with the line. Otherwise, if the injected voltage is in quadrature with the current, nothing more than reactive power will change with the line, moreover minimum active power will be required if the power factor of supply is unity. Voltage compensation by DVR will be in quadrature with the load current in case of no ESS but in that case large injected voltage magnitude is required to mitigate the voltage sag. In addition, reactive power compensation is only effective for small voltage sag. So injection of voltage at the same phase instant for deeper and longer voltage sag or swell and voltage with phase jump requires active power for which the energy storage device is necessary.

The minimization of voltage amplitude can be realized with this method where DVR compensate load voltage with a minimum voltage injection. The voltage is compensated in phase to post sag value of PCC voltage i.e. the phase change in PCC voltage is not compensated. Hence by this method injected voltage magnitude can be minimized. But in most of the cases, a voltage sag occurs with a phase change, therefore this method produces distortions on to the load voltage leading to transients and circulating currents. Moreover for sensitive load, in phase compensation is hardly beneficial as it could lead to tripping of the load. Note that, to realize this compensation strategy, the phase-locked loop (PLL) has to be synchronized to the grid voltage itself, and therefore, must not be locked to the pre-sag grid voltage during the compensation.

The most prominent solution for voltage sag compensation is to maintain the same amplitude and the phase of the voltage before the sag. This solution is provided by pre-sag compensation through which load voltage is exactly restored.

The pre-sag compensation does not change the phase during compensation period which leads to less distortion at the load end resulting in no transients and circulating current. In this case PLL must be synchronized with the PCC voltage. At the instant of disturbance, PLL must be locked so as to restore the phase angle [

In this strategy the injected voltage is in phase quadrature with the load current. In other wards the DVR will provide as much reactive power as possible resulting in minimization of active power needed from DC link. The main concept of this compensation is to draw as much active power from the grid as possible. Also the phase of the load voltage is shifted resulting in steady state error causing tripping of critical nonlinear load. Beside the enormous advantages of not requiring active power and unlimited compensation time this strategy has in most cases two major disadvantages. First is phase change occur and second is requirement of injected voltage amplitude is quite high. Furthermore, the compensation with pure reactive power is only possible for shallow sags. If a deep sag occurs, a large amount of active power is also needed with this strategy [

Hence from overall analysis of different compensation methods, the most prominent pre-sag compensation method has been considered for the controller design for DVR. Because this method minimizes the transient and steady state error.

To deal with the power quality problems in single phase system, several control methods are available preferably for shunt compensation viz. active power filter [

In the proposed controller with the existing phase (considered α phase), other orthogonal fictitious phase is created which is considered as β phase. This orthogonal β phase generation has been done with the quarter cycle time delay method. Generation of α-β quadrature phases resembles the Clarke’s transformation in three phase theory. Accordingly this α and β instantaneous values are transformed into synchronous reference frame (SRF) based d-q components. Here Clarke and then Park transformation is implemented where the reference angle is obtained through single phase PLL as shown in

MAF consist of following blocks: integration block, transport delay block, subtraction block and division block as presented in

number. The moving average of two input values calculated over the time interval T/n gives the output of MAF filter.

The fundamental positive sequence reference load voltage is restored by phasor analysis based Load voltage controller. This controller works on two different sub-controller loops, one as feed-forward loop for faster transient response and other as feedback loop for zero steady state error. Each of the aforementioned phasor control loop is comprised of magnitude control loop and phase angle control loop. Both these loops are decoupled with each other. The phasor analysis based proposed controller incorporating single phase SRF d-q theory is shown in

voltage reference signal. This signal is compared with the triangular carrier signal so as to get the pulses required for the switching of inverter switches to get the injected voltage.

The considered single phase system for simulation under MATLAB Simulink environment [

For further judgment half bridge diode rectifier is connected across load to make it nonlinear. The

time instants as discussed for linear load. With the application of proposed modified controller for DVR, results for injected voltage and restored load voltage are shown in

case nonlinear load there is no delay observed in reference load voltage restoration.

From the overall observation and discussion of results, it can be further stated that the load voltage is maintained at its rated RMS value. The load voltage is observed to be satisfactory due to exact voltage injection by DVR resulting in exact restoration of load voltage to its reference value of 200 V. The DC-link voltage is maintained constant to its reference value of 300V in

The generalized single phase SRFT based d-q controller for DVR has been analyzed and simulated successfully for nonlinear load. The controller design is completely based on phasor analysis of pre-sag compensation method. Also no freezing of PLL has been carried out to get the information of phase value prior to occurrence of fault. This reduces the complexity of the controller. The extensive simulation and investigation has been carried out. The performance of proposed controller for DVR has been found better restoring exact voltage at sensitive load. Simulation results shows that the controller design from phasor analysis of pre-sag compensation method for DVR has effective in mitigating voltage sag for nonlinear load.

The Author is thankful to all the coauthors of the paper and G.H.R.C.E. as research Centre for valuable guidance and help for this work.

Katole, D.N., Daigavane, M.B., Gawande, S.P. and Daigavane, P.M. (2017) Modified Single Phase SRF d-q Theory Based Controller for DVR Mitigating Voltage Sag in Case of Nonlinear Load. Journal of Electromagnetic Ana- lysis and Applications, 9, 22-33. https://doi.org/10.4236/jemaa.2017.92003

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