Phasor Based Analysis and Design of Single Phase SRFd-q Controller for Dynamic Voltage Restorer

This paper proposes an effective control concept for single phase dynamic voltage restorer (DVR) and overall analysis is performed based on the restoration of load voltage without phase change. The proposed controller is based on conventional proportional integral (PI) controller to compensate single phase voltage sag by synchronous reference frame (SRF) theory incorporating d-q concept. A detailed phasor analysis of voltage injection identical to pre-sag compensation strategy has been carried out with the consideration of unlock phase lock loop (PLL) at the sag initiation point and accordingly a controller is designed. The digital simulation has been performed using MATLAB Simulink to prove the effectiveness of the proposed control. The simulation results for linear and nonlinear load shows that this generalized proposed method can compensate single phase voltage sag effectively.


Introduction
Most of the power quality (PQ) problems are related to voltage such as voltage sag/swell, distorted supply etc. [1]. Voltage sag is generally considered as the most common and costly PQ problem. Voltage sag is characterized by momentary decrease in rms voltage magnitude lasting between half a cycle and several seconds. It is caused mainly by upstream faults at the feeder connected parallel to common coupling point (PCC). Custom power devices such as dynamic voltage restorer (DVR) plays major role in medium and low power system for power quality improvement [2]. Sags are often nonsymmetrical and accompanied by a phase jump. Most of the three phase control strategies for DVR have been addressed in [2] to [4] which tightly compensate voltage sag but with a disadvantage of more losses in switching due to unsymmetrical fault. Control method based on synchronous reference frame theory (SRFT) [4], instantaneous symmetrical component theory [5], instantaneous p-q theory [6], etc. are reported in various literatures. This paper mainly concentrates on SRFT d-q based controller for single phase system. The controller is based on conventional Proportional integral (PI) with decoupled feed-forward and feedback loop. The successful implementation of DVR for mitigating the voltage sag proves the effectiveness of the controller.

II. DVR Structure and Operation
The injected power can be minimized by maintaining unity power factor on supply side.
The aim of DVR is to maintain load voltage magnitude to its desired value say 1pu despite voltage sag ( sag V ) at supply end. It Using (2), (3), and (4) the minimum injected active power can be written as From (5) it is clear that when Otherwise, it needs the support of active power to restore the load voltage. To handle power quality problems in single phase, several control strategies are available in case of shunt compensation without energy storage device like active power filter [6] but less reported in case of series compensation. The DVR effectively supply reactive power but active power is fulfilled by energy storage device [7]. In general, the active and reactive power flow are controlled by regulating the angle between injected voltage and the line current. The effective solution for maintaining exact magnitude of voltage at load without any phase change, as like a voltage before sag is pre-sag compensation method. This method leads to less distortion at the load side resulting in no transients and circulating currents. Most of the controllers earlier designed, are based on this method where locking of PLL is necessary at sag initiation point. Whereas in this paper a phasor analysis of pre-sag method has been done to obtain the magnitude and angle of injected voltage without locking the PLL. The proposed controller based on this analysis has been discussed below.

III.Proposed control system
The complete phasor analysis for the design of controller is based on SRFT as shown in Fig. 2. Generation of SRF d-q component in single phase system needs creation of one fictitious phase which is considered as â phase along with the originally present á phase as given in [6]. Accordingly this á and â instantaneous values are transformed into SRF d-q components [3]. Constant d-q component in the steady state of system representing dc value, becomes variable in na ture due to harmonic contents in nonlinear load. The variable value of d-q components of voltage can achieve constant value with the application of moving average filter (MAF) [8].
The phasor controller is based on combined feed -forward and feed-back strategy to obtain better transient and steady state response. The block diagram of the proposed phasor based controller f or DVR is shown in Fig.3 Here, C larke's and then park transformation is implemented on single phase where the reference angle pre q is obtained through single phase phase lock loop ( PLL

IV.Simulation Results and Discussion
The single phase system considered consists of a source, a bus, and two parallel loads as shown in Fig. 4 for simulation in MATLAB Simulink environment [20]. To verify the performance of aforementioned controller, the LG fault is created in other feeder parallel to sensitive nonlinear load, resulting in voltage sag at PCC. The extensive simulations are performed, results of which are shown in Figs. 5-6 and in Figs. 7-8 for linear and nonlinear load respectively. The parameters under consideration for simulation is given in appendix I. Due to fault the PCC voltage experience sag of 58% from instant 0.05sec to instant 0.30sec which nearly covers up to 12-13 cycles as shown in Fig. 5(a). It can be clearly stated from Fig. 5(b) that the DVR is restoring the load voltage to its desired value without any phase change represented in Fig. 5(c). It is clearly noted that the controller is initiated with approximately one cycle delay because of quarter cycle delay in b component generation. From the overall scenario of results, it can be further stated that the load voltage is tightly restored to its reference value. The DClink voltage is maintained constant to its reference value given in Fig. 6(a), load current and generation of fundamental component from it has been shown in Figs. 6 (b-c) for linear load. For further judgment half bridge diode rectifier is connected across load to make it nonlinear. The Fig. 8(b) indicates the nonlinear nature of load current. The a b component of nonlinear load current has been generated by quarter cycle time delay method represented in Fig.  8(c). Fig. 7(a) depicts approximately 12 cycles sag generation for the same 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 Fig. 7(b) and Fig. 7(c). It can be clearly notified that 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 Fig.  8(a), generation of fundamental a b component of load current from nonlinear current is shown in Fig. 8(c). Fig. 4. System under consideration.

V. Conclusion
The single phase SRF theory based d-q controller has been proposed, analyzed, designed and simulated for linear load and nonlinear load. The proposed controller validates the mitigation of voltage sag at the load end. In this controller no freezing of PLL is required to get the information of phase value prior to occurrence of fault. This reduces the complexity of the controller. The performance of proposed controller of DVR has been found better which tightly compensate sensitive load against voltage sag.
The results demonstrated proves the efficacy of the proposed controller. The controller shows the several benefits over the other existing controllers such as • The proposed controller can be used in three phase system if phase sequence is properly taken care by the PLL.
• It energizes only one phase which reduces the switching losses and increases the time of compensation.