Performance Evaluation of Three Different Inverter Configurations of DVR for Mitigation of Voltage Events

The voltage events namely voltage sags and voltage swells represent the most common, frequent and important power quality events in today’s power system. Dynamic voltage restorer (DVR) is one of the key components used to mitigate the supply voltage quality disturbances in terms of voltage sags and swells in the distribution system. It consists of an energy storage unit, a voltage source inverter, a filter, a coupling transformer and the control system. This paper presents three different inverter configurations of dynamic voltage restorer (DVR) for mitigation of voltage events such as voltage sags and swells with sudden addition or removal of the nonlinear load. These three configurations are voltage source inverter based DVR (VSI-DVR), current source inverter based DVR (CSI-DVR) and impedance or Z-source inverter based DVR (ZSI-DVR). The d-q control technique is used to control the operation of the DVR. The response of ZSI-DVR for mitigation of voltage sags and swells are investigated and compared with VSIDVR and CSI-DVR using MATLAB/SIMULINK environment.


Introduction
In recent years, many researchers have given their focus on power quality.Discussing the quality of power is a complicated issue and is measured in terms of such things as line interruptions, voltage sags, voltage swells, flickers, harmonics, and distortion.However, among all disturbances, the voltage sags and voltage swells represents the most common, frequent and important power quality disturbances in today's power system [1,2].Voltage sags are commonly known phenomenon in supply systems.A voltage dip (sag) is a disturbance where the RMS value of the line voltage is reduced for a period ranging from one-half cycle of the voltage to 500 ms [3].A typical cause of voltage sags is the starting of large induction motors that normally draw 5 to 7 times their rated currents during start-up.Short circuits in the other branches of the supply are also a common origin of voltage sags.The sudden addition of a larger load and also loose or defective wiring can cause voltage sags [1,4].Swell is opposite of sag.It is the short duration phenomenon of increase in RMS source voltage between 1.1 to 1.8 pu and duration of the event ranging from 0.5 cycles to 1 minutes.A voltage swell can occur due to a fault, energizing a larger capacitor bank, switching off of a heavy load [5][6][7].
Traditionally the passive filters are used for the enhancement of power quality.But currently new kinds of network configuration type custom power devices namely series connected Dynamic Voltage Restorer, shunt connected Distributed Static Compensator [8,9] and seriesshunt connected Unified Power Quality Conditioner [10,11] have been commonly used due to their high performance to improve the controllability of power system.DVR is the key component used to compensate long duration voltage events such as voltage sags and voltage swells [12][13][14][15].Generally, the DVRs consists of voltage source inverter based DVR (VSI-DVR), current source inverter based DVR (CSI-DVR) [16] and impedance source inverter based DVR (ZSI-DVR) [17][18][19][20][21].The limitation of VSI-DVR is their buck (step-down) type output voltage characteristics thereby the maximum output voltage is limited by DC link voltage.The upper and lower IGBT switches of each leg of VSI cannot be fired on simultaneously, so a condition of shoot-through would appear and damage the IGBT switches.The shoot-through is a forbidden switching state for the VSI.The CSI-DVR is a boost type so the voltage at output level is greater than the DC voltage level.For the utilization where a large voltage is desirable, an additional DC-AC boost converter is required.The additional power conversion levels increase system cost and lower efficiency.At least one of the upper IGBT switches and one of the lower IGBT Otherwise, an open circuit of the DC inductor would appear and damage the CSI.ZSI is an emerging type of converter which has unique characteristics so that it can overcome the disadvantages of VSI and CSI.The unique feature of the ZSI is that the output AC voltage can be any value between zero and infinity regardless of the DC voltage.That is, the ZSI is a buck-boost inverter that has a wide range of obtainable voltage.Unlike a VSI and CSI, the shoot-through state is not harmful and actually has been utilized in ZSI [17][18][19][20][21].In this paper, three different inverter configurations based on DVR such as VSI-DVR, CSI-DVR, and ZSI-DVR are discussed and compared.The VSI-DVR, CSI-DVR and ZSI-DVR is simulated using MATLAB\SIMULINK platform for mitigation of commonly occur voltage events such as voltage sags and voltage swells under nonlinear load condition.Comparative analysis of simulation results for the alleviation of voltage sags and swells under nonlinear load are also presented.

Research Method
DVR can be implemented by a three-phase voltage source, current source and Zsource inverters as depicted in Figure 1, Figure 2, and Figure 3.The structure of voltage sag/swell compensator contains a bank of three-single phase voltage source inverters.Each voltage source inverter group is associated with the network through three-phase an isolating transformer which provides isolation between the converters.Lf represents the inductance of each transformer as well as an additional interfacing inductance and is used to filter highfrequency components of injecting voltages.

Voltage Source Inverter (VSI), Current Source Inverter (CSI) and Impedance Source Inverter (ZSI)
Figure 4 shows the 3-phase VSI.It is a DC-AC buck inverter with a capacitor on the DC side and works as a voltage source [22].The usual converter switching devices are insulated gate bipolar transistor (IGBTs) and anti-parallel diodes to provide bidirectional current flow and unidirectional voltage blocking potential.A three-phase VSI basically has a six active modes and two zero modes.A zero state is produced when upper three or lower three devices are fired on as well as short-circuiting the output terminals.The two upper and lower IGBT switches of VSI cannot be fired on at the same period because a condition of shoot-through appears and damage the inverter.VSI has the following limitations: (i) VSI is a DC-AC buck inverter.That means the obtained AC output voltage is lower than the DC bus voltage.For applications where a large range of AC output voltage is required, an additional DC-AC buck converter is used to obtain a desired AC output voltage.Due to additional converter stage increases a cost of the system and lower efficiency.(ii) shoot-through would appear and damage the devices.
Figure 5 shows the three-phase CSI.It is a DC-AC boost inverter with an inductance in the DC side.CSI has the following limitations: The CSI is a boost or step-up inverter so its voltage at output level is greater than the input DC voltage level.For the utilization where large voltage range is required an additional DC-AC step-down (step-up) inverter is used.The additional conversion levels increase system cost and lowers the efficiency.(ii) At least one of the upper devices and one of the lower devices have to be fired on and maintained on at any time because an open circuit across common inductor would appear and damage the inverter.
ZSIs are recently proposed inverter topologies that can perform both buck/boost function as a single unit.The VSI and CSI cannot provide such feature.The ZSI overcomes the above-mentioned problems of the VSI and CSI and provides a new concept of power conversion.The general configuration of three-phase ZSI is shown in Figure 6.The ZSI consists of two inductors L1 and L2 and capacitors C1 and C2 to form a unique impedance network to protect the IGBT switching devices when the devices are in a shoot-through state.Which is one of the important and significant feature of the ZSI.

Operating Modes Of Z-Source Inverter
The ZSI has three operating modes: (i) normal mode (ii) zero-state mode (iii) shootthrough mode.Figure 7 shows active and zero states of ZSI.In these states diode is conducting at the DC-link side.The voltage across the inductors is And Vi =VDC.The input voltage of the converter during active and zero states during interval t1 is The mean voltage of the inductor over one switching interval t should be zero in steady state.
Figure 8 shows the shoot-through mode of the ZSI where two switches of three legs turned on simultaneously.In a shoot-through mode, the diode at source side is off due to reverse bias and capacitors C1 and C2 charge the inductors L1 and L2.
The peak DC-link Voltage across the inverter bridge is Where  is the boost factor resulting from the shoot-through state.The output peak phase voltage from the inverter can be expressed as Where Mi is the modulation index.The voltage across the capacitors can be expressed as

D-Q Control Technique
The control strategy plays a most important role in any distributed flexible AC transmission (D-FACTS) devices.The performance of DVR system solely depends on its control technique for generation of reference signals.In this paper d-q control technique is used   9 shows the d-q control strategy used to generate the reference signals of three inverter configurations of DVR.
In order to maintain the DC bus voltage of the capacitor or inductor, a PI controller is used at the DC bus voltage of the DVR and output is considered as the voltage loss (VLoss). where denotes the error between the reference load terminal voltage ( and actual load terminal voltage (   Ln V ) amplitudes at the nth sampling instant.Kp2 and Ki2 are the proportional and the integral gains of the PI controller.The reference quadrature axes (qaxes) voltage is The reference load voltages ( ) in abc frame are obtained from the reverse Parks transformation as in equation ( 15) The errors between the sensed load voltages (

,,
La Lb Lc V V V ) and reference load voltages are used in the PWM controller to generate gate pulses for the VSC of the DVR.

Simulation of DVR
Figures 1-3 shows the three different inverter configurations of the test system used to carry out the simulation with the associated control technique.These DVR models are simulated under nonlinear load condition with simulation period 0.25 s.

Simulation Results and Discussion
In this section, simulation results of three different inverter topologies based on DVR for mitigation of voltage sag and swell under nonlinear load condition is presented.Load1 is considered as fixed resistive load (R-load) and load2 is considered as a nonlinear load.The nonlinear load is realized by three-phase diode-rectifier with R-C load.A three-phase breaker is used to control the connection of a nonlinear load to the distribution network.Initially, both the loads are connected to the network, but after a certain period of time load2 are switched on and off by opening the breaker.Due to the sudden change of the heavy load, voltage sag and swell Because of sudden addition or removal of the heavy nonlinear load, system encounters three-phase balanced voltage sag of magnitude 26% of the normal voltage which starts at t=0.01s and ends at t=0.05s and three-phase voltage swell of magnitude 18% of the normal voltage which starts at t=0.15s and ends at t=0.2s.For balanced voltage sag/swell, the supply voltage signal before compensation, the rms supply voltage, the compensation voltage, the load voltage after voltage sag/swell compensation and variation of DC link voltage are depicted in Figures 10(a)-(e).DVR does not produce any voltage during normal condition, but during voltage sag and swell condition, it generates voltage with required magnitude and polarity and injects to the system as depicted in Figure 10b.So that load voltage becomes sinusoidal as depicted in Figure 10c.

Voltage Sag and Swell Mitigation by CSI-DVR under Nonlinear Load
In the period of a 0.05-0.10sheavy non-linear load is connected to the system by closing the breaker2.Due of sudden addition of non-linear load, the system may encounters voltage sag of magnitude 25% of the normal level and then the source voltage signal recovers to its normal levels.The system may also encounter a voltage swell of magnitude 14% of the normal voltage due to the sudden removal of a heavy non-linear load at 0.15-0.20s.For balanced voltage sag/swell, the supply voltage before compensation, the rms supply voltage, the DVR injecting missing voltage, the load voltage after voltage sag/swell compensation and variation of DC link voltage are depicted in Figures 12(a    The first simulation contains without ZSI-DVR and it is observed that, the system experience voltage sag and swell.The second simulation is carried with ZSI-DVR.When ZSI-DVR is connected to the system, it produces the proper amount of compensating voltage so that voltage sag and swell is completely eliminated as shown in Figure 15.  1. Table 2 shows the comparative analysis of three configurations of DVR.

Conclusion
In this work three different inverter topologies such as VSI-DVR, CSI-DVR and ZSI-DVR are tested using MATLAB/SIMULINK platform under nonlinear load condition for mitigation of supply voltage disturbances like voltage sags and swells in a distribution system.It is found that ZSI-DVR shows a superior performance to mitigate the most significant power quality events such as voltage sags and swells in the supply voltage compared to VSI-DVR and CSI-DVR.

6 Figure 4 .
Figure 4. Basic Configuration of Voltage Source Inverter

Figure 7 .
Figure 7. Active and Zero State of Z-Source Inverter

Figure 9 .
Figure 9. Block Diagram of D-Q Control Technique Performance Evaluation of Three Different Inverter Configurations of DVR … (Miska Prasad) the nth sampling instant.KPI and Ki1 are the proportional and the integral gains of the PI controller.Therefore, reference direct axes (d-axes) load voltage is the load terminal voltage ( L V ) is controlled by its reference voltage ( * L V ) using another PI controller.The output of PI controller is considered as the reactive component of voltage ( qr V ) for voltage regulation of load terminal voltage.

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ISSN: 2089-3272 IJEEI Vol. 4, No. 4, December 2016 : 225 -239 234 occurs in the source voltage.The objective of the simulation is to study three different performance aspects: (i) Voltage sag and swell mitigation, by VSI-DVR under nonlinear load (ii) Voltage sag and swell mitigation by CSI-DVR under nonlinear load and (iii) Voltage sag and swell mitigation by ZSI-DVR under nonlinear load 7.1.Voltage Sag and Swell Mitigation by VSI-DVR under Nonlinear Load

Figure 10 .Figure 11 .
Figure 10.Simulation Results of VSI-DVR (a) Supply Voltage (b) RMS Supply Voltage (c) Injected Voltage of VSI-DVR (d) Load Voltage and (e) DC-link Voltage )-(e).CSI-DVR is activated only during sag or swell events and produces compensating voltage with required magnitude, frequency and polarity and injects to the three-phase distribution network as depicted in Figure12b.It is clearly observed that VSI-DVR and CSI-DVR are injected the same amount of compensating voltage during sag and swell.

Figure 12 .Figure 13
Figure 12.Simulation Results of CSI-DVR (a) Supply Voltage (b) RMS Supply Voltage (c) Injected Voltage of CSI-DVR (d) Load Voltage and (e) DC-link Voltage

Figure 13 .
Figure 13.Load Voltage with and without CSI-DVR

Figure 14 .
Figure 14.Simulation Results of ZSI-DVR (a) Supply Voltage (b) RMS Supply Voltage (c) Injected Voltage of ZSI-DVR (d) Load Voltage and (e) DC-link Voltage

Figure 15 .
Figure 15.Load Voltage with and without ZSI-DVR

Table 2 .
Comparative Analysis of VSI, CSI AND ZSI