Multiple Voltage Disturbance Compensation in Distribution Systems using DVR

The subject of power quality is gaining much interest nowadays. With the sophisticated sensitive electrical devices available in the market, power stations should ensure qualified and eminent power to the point of utilization. Bad voltage conditions in a power system can cause a malfunction at the point of utilization. Dynamic voltage restorer (DVR) is a reliable power electronic offsetting device which when engaged in the power system is effectual in the operation of compensating voltage sag/swell. In this research work, a DVR for offsetting multiple voltage disturbances in a power system is presented and a novel control strategy to trigger power switches in DVR is proposed. The proposed novel control methodology is compared with Synchronous Reference Frame (SRF) theory-controlled DVR. The simulation model is designed and the analysis with MATLAB/SIMULINK is presented. Keywords-Voltage disturbance; sag; swell; DVR; novel control


INTRODUCTION
The power quality topic is of high importance to the power sector as the loads are extremely responsive to power quality disturbances. Huge economic losses can occur due to bad power quality. High grade sensitive non-linear power electronic equipment is employed at the load points causing deformation of system parameters or degradation of the system [1][2][3][4]. The knowledge regarding the different forms in which power quality is deteriorated presents the solution methodology. This information is also important in identifying the origin of device failure. Power system quality is often reduced due to voltage noise. Voltage sags/swells are said to have major effects on power systems [1]. Voltage sags/swells can reduce the life time of a device or can even damage the equipment. Voltage sags can be classified as symmetrical and unsymmetrical depending on the nature of the signal [4][5]. Phase to ground faults might generate voltage sags in the power system. On the other hand, swells in voltage can be generated by abrupt switching of capacitor banks or reactive power generating devices. Voltage quality is ensured by using controllers in series which compensate voltage issues. The circuit diagram of a series controller in a power system is shown in Figure 1. FACTS (Flexible AC Transmission System) controllers are developed to mitigate the power quality issues in the power system. DVR (Dynamic Voltage Restorer) [6][7][8] is a series connected FACTS controller which addresses voltage issues in a power system [9][10]. This paper presents a DVR for the compensation of multiple voltage disturbances in the power system. A novel control strategy to trigger power switches in DVR is proposed and compared with SRF theory-controlled DVR. The Dynamic Voltage Restorer (DVR) is a power electronic component placed in series with the electrical transmission or distribution system. The series compensating DVR device guards the perceptive loads from voltage issues in the power system. The DVR operates on the principle of inducing selective frequency-based voltage magnitude essential to reinstate the load voltage when the source voltage is fed with sag/swell/disturbance nature. The DVR can also be stated as solid state power electronic switching device that induces synchronous voltage in series to power system line. The basic DVR configuration in a power system is shown in Figure 2. DVR comprises of an energy source and an IGBT based power  DVR in a power system

III. DVR CONTROL STRATEGY
A. Conventional SRF Thoery for Controling DVR SRF theory generates current reference signals by transforming current signals to rotating coordinates and then again back to the original format. Load currents are transformed from the stationary 'abc' frame to the rotating 'dq' frame using (1) which generates currents in 'dq' coordinates [12]: The loss component is calculated from the PI controller where the inputs are the actual voltage and the reference DC voltage level of the DVR energy source. The rotating 'dq' frame coordinates are again inverse-transformed to obtain the reference current signals in the 'abc' coordinate system as shown in (2): The generated reference current signals in the 'abc' coordinate system are compared with the three-phase actual current wave to setup control signals to the power switches of DVR. This theory involves multiple mathematical and arithmetic operations while generating reference current signals making the control system complex [13]. The overall power system consisting of the DVR controlled with SRF theory is shown in Figure 3. Overall power system DVR controlled with SRF theory B. Proposed Novel Control Theory for DVR Figure 4 illustrates the overall power system with a DVR controlled with the proposed novel control methodology which generates control pulses to DVR. The novel control methodology senses the line voltage of the system and is split into three individual phases: V a , V b and V c . The three voltage error signals V ae , V be and V ce from each phase to the pulse width modulation generator are: also the transformations blocks that are utilized in SRF theory are not used. This prevents complex problematic calculations and makes the control technique facile [14][15][16]. The data of V a , V b , V c are obtained from the source voltage. The product of V a with V a gives V α 2 . This signal is added to V β 2 signal which is obtained by adding a 90 0 delay to V a . Square-root is applied to the obtained signal. The resultant signal is the maximum value. The maximum value is correlated with 1pu value. This resultant magnitude is multiplied with sinusoidal information obtained from the phase locked loop. The resultant signal is the reference signal for V a . Similarly, this arithmetic procedure is followed for phase B and phase C with delay of 120 o and 240 o respectively. The three phase error signals are processed through the PWM generator to produce gate pulses to the power switches of DVR [17][18][19]. Figure 5 illustrates the control strategy of the novel control theory. Control strategy for DVR with the novel control theory.
IV. RESULT DISCUSSION AND ANALYSIS The system parameters considered with the SRF theory and the novel control method are represented in Table I. The overall power system with DVR is shown in Figure 6. The Simulink model for control strategy for DVR with the novel control theory is shown in Figure 7.  Simulink model DVR control strategy with the novel control theory A. Case 1: Sag in One Phase 1) DVR Controlled with SRF Theory Figure 8 illustrates the supply voltage, the voltage injected by DVR, and the voltage across the load sag in one phase of the source voltage. 2) DVR Controlled with Novel Control Theory Figure 11 illustrates the supply voltage, the voltage injected by DVR, and the voltage across the load. One phase of the source voltage contains sag and compensating signals are injected by DVR and thus the voltage magnitude across the load remains constant.  The harmonic distortion in supply and load voltage is shown in Figures 12 and 13 respectively. The source voltage is distorted by 22.51% and the load voltage by 1.08%. Figure 14 illustrates the supply voltage, the voltage injected by DVR, and the voltage across load with a system consisting of swell in one phase of the source voltage crossing the limit pu value.  Figure 17 illustrates the supply voltage, the voltage injected by DVR, and the voltage across load with a swell in one phase of the supply voltage.   Figures 18 and 19 respectively. The supply voltage is distorted by 11.25% and the load voltage 4.44%.  Figure 23 illustrates the supply voltage, the voltage injected by DVR, and the voltage across the load.  Figures 24 and 25 respectively. The source voltage is distorted by 11.08% and load voltage is distorted by 3.01%.  Figure 29 illustrates the supply voltage, the voltage injected by DVR, and the load voltage. Compensating signals are injected by DVR and thus the voltage magnitude across the load remains constant. The harmonic distortion in the source voltage and the load voltage is shown in Figure 30 and 31 respectively. The source voltage is distorted by 11.67% and the load voltage by 3.09%.  Figure 32 illustrates the supply voltage, the voltage injected by DVR, and the voltage across the load.  Figure 33 and 34 respectively. The source voltage is distorted by 11.21% and the load voltage by 4.15%. Figure 35 illustrates the supply voltage, the voltage injected by DVR, and the voltage across the load. Compensating signals are injected by DVR and the voltage across the load remains constant. The harmonic distortion in source and load voltage is shown in Figure 36 and 37 respectively. The supply voltage is distorted by 11.08% and load voltage is distorted by 2.20%.  Figure 38 illustrates the supply voltage, the voltage injected by DVR, and the voltage across the load.  Figure 39 and 40 respectively. Supply voltage is distorted by 14.79% and load voltage is distorted by 4.17% for swell three phases using SRF 2) DVR Controlled with the Proposed Novel Control Theory Figure 41 illustrates the supply voltage, the compensating voltage injected by DVR, and the voltage across the load. Compensating signals are injected by DVR and the voltage magnitude across the load is kept constant. The harmonic distortion in supply and voltage is shown in Figure 42 and 43 respectively. The source voltage is distorted by 11.67% and the load voltage by 2.03%.

G. Result Comparison
The comparison between the THD in source and load voltages for all cases is tabulated in Table II. It is clear that the THD in load voltage is less when the DVR is controlled with the proposed novel control method than when it is controlled with the SRF theory.
V. CONCLUSION Dynamic Voltage Restorer is one of the forefront devices utilized for reducing voltage quality issues and for improving electrical power quality in a power system. The DVR stabilizes the load voltage irrespective of the condition in source voltage signal. This research work presents a DVR controlled with a novel control technique for compensating sag and swell types of voltage disturbances in a power system ensuring that the load receives stabilized voltage. The DVR is controlled using a novel unit vector control methodology and the results are compared with SRF theory-controlled DVR. The results indicate that the harmonic distortion in load voltage is less when the DVR is controlled with the novel control method compared to SRF theory-controlled DVR.