A Dynamic Voltage Restorer with Voltage Sag
Compensation at Medium Voltage Level Using
PI Control Scheme

M. Swathi Priya; Dr.T.Venkatesan

A Dynamic Voltage Restorer with Voltage Sag Compensation at Medium Voltage Level Using PI Control Scheme

M. Swathi Priya¹ and Dr.T.Venkatesan²

PG Student, Dept. of EEE, K.S.Rangasamy College of Technology, Namakkal, Tamilnadu, India
Professor, Dept. of EEE, K.S.Rangasamy College of Technology, Namakkal, Tamilnadu, India

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Abstract

The increment of voltage – sensitive load equipment has made the industrial process must more susceptible to degradation in the power quality. Voltage deviation, often on the form of voltage sag, can cause severe process, result in economical loss. Among the Custom Power Device (CPD), the application of Dynamic Voltage Restorers (DVRs) in the distribution system is the recent invention. DVR’s are used to protect sensitive loads from the effects of voltage sags on the distribution feeder. DVR normally installed between the source voltage and critical or sensitive load. This paper describes DVR using Proportional Integral (PI) control technique. The simulations are performed using Matlab/Simulink’s SimPowersystem Toolbox.

Keywords

Dynamic Voltage Restorer (DVR), Power Quality (PQ), Proportional Integral (PI), voltage sag.

INTRODUCTION

Power Quality problems like voltage sag, voltage swell and harmonic are major concern of the industrial and commercial electrical consumers due to enormous loss in terms of money and time. For high power sensitive loads, the DVR shows promise in providing more cost effective solution than the energy storage capabilities of Uninterrupted Power Supply (UPS) [1]-[4]. The DVR can be implemented both in Low Voltage (LV) level [5]-[9] as well as Medium Voltage (MV) level [10]-[15] and DVR protect high power application from voltage sag. A DVR uses series-connected topology; it injects a voltage on the system in order to compensate any disturbance affecting the load voltage. This aims to protect critical loads against voltage sag/swell.

Voltage sag/swell that occur more frequently than any other power quality phenomena is known as the most important power quality problem in power distribution system. Voltage sag is defined as a sudden reduction of supply voltage down 90% to 10% of normal. On the other hand, voltage swell is defined as a sudden increasing of supply voltage up 110% to 180% in rms voltage at the fundamental frequency with duration from 10ms to 1 minute. According to IEEE 519-1992 and IEEE 1159-1995 standards, a typical duration of voltage sag and swell is 10ms to 1 minute [16]. Faults at distribution level causes voltage sag or swell, which can cause sensitive equipment to fail as well as create a large current unbalance that could blow fuses or trip breakers.

Table 1 shows the voltage variations events in 230 kV bus at Mettur Thermal Power Station (MTPS) [17]. From the table, it is observed that there is voltage sag will be observed during summer season and voltage swell will be observed during winter season. The voltage variation in 230 kV bus is reflecting on 230 V residential distribution system in the same way. The voltage sags and swells are normally caused by starting of large induction motors, energizing a large capacitor bank and short circuit faults in power network [18], [19] such as single line to ground fault, three phase to ground fault, double line to ground fault on the power distribution system. Voltage sag and swell in power systems produce an important effect on the behaviour of sensitive loads. The ensuing adverse consequences are a reduction in the energy transfers of electric motors and the disconnection of sensitive equipment and industrial processes brought to a standstill. This will result in loss of time and production, or damaged equipment may cause significant economical losses. To solve the above problems a new method is proposed in this paper.

In general, the voltage injection from DVR compensates the voltage sag, swell and outage. However, it needs a high capacity DC storage system. In the proposed DVR design, Proportional Integral (PI) controller is used to boost the DC storage system to compensate the voltage sag.

DYNAMIC VOLTAGE RESTORER (DVR)

Dynamic voltage restorer was first built in U.S for the Electric Power Research Institute (EPRI) by Westinghouse. To protect an automated yarn manufacturing and weaving factory it was first installed in 1996 on Duke Power Company grid system. DVR is a series connected solid state device that is used for mitigating voltage disturbances in the distribution system. It is used to regulate the load side voltage by injecting voltage into the system.

It is used to compensate the load voltage at a nominal magnitude and phase by compensating the voltage sag/swell, voltage unbalance and voltage harmonics presented at the point of common coupling [20]. Its main function is to rapidly boost up the load side voltage in the event of a disturbance in order to avoid any power disruption to load.

It is generally installed in a distribution system between the supply and the critical load feeder at the point of common coupling (PCC). Fig. 1 shows the location of the DVR. The general configuration of DVR consists of an injection/ booster transformer, a harmonic filter, a Voltage Source Converter (VSC), DC charging circuit and a control and protection system.

In most sag correction techniques, the DVR is required to inject active power into the distribution line during the period of compensation. Hence, the capacity of the energy storage unit can become a limiting factor in the disturbance compensation process especially for sags for long duration. Voltage sags caused by unsymmetrical line-to line, line to ground, double-line-to-ground and symmetrical three phase faults is affected to sensitive loads.

IN-PHASE VOLTAGE COMPENSATION METHOD

In general, there are three techniques such as presag, in-phase and minimal energy injection techniques are utilized to calculate the injection voltage of DVR. In this paper, in-phase compensation technique is used to calculate the injection voltage of DVR due to its simple implementation and fast response in calculating the compensating voltage. A DVR can compensate the voltage drop across a load by injecting a voltage through a series injection transformer in-phase with the source voltage [21]. The injected voltage across the secondary of the series injection transformer is in-phase with supply voltage, as shown in Fig. 2.

In normal condition, the supply voltage (Vpresag) is equal to the load voltage with zero phase angle. During the voltage sag/swell, the supply voltage decreases or increases to a value less than or greater than its nominal value. The DVR reacts to the sag/swell events and injects the compensating voltage Vinj in-phase with the supply voltage to restore the voltage at nominal value.

(4)

CONTROL OF DVR

Voltage sag is created at load terminal via a three-phase fault. Load voltage is sensed and passed through a sequence analyzer. The magnitude component is compared with reference voltage (Vref). Pulse width modulation (PWM) control technique is applied for inverter switching to produce a three-phase 50 Hz sinusoidal voltage at load terminals [22]. Chopping frequency is in the range of few KHz. The IGBT inverter is controlled with PI controller in order to maintain 1 per unit voltage at the load terminals.

PI controller is a closed loop controller, which drives the plant to be controlled with a weighted sum of the error and the integral of that value. An advantage of a proportional plus integral controller is that the integral term in a PI controller causes the steady state error to be zero for a step input. PI input is an actuating signal which is the difference between the Vref and Vin output of the controller which is shown in Fig. 3.

(5)

The modulated signal Vinv_ref is compared against a triangular signal in order to generate the switching signals for the VSC valves as shown in Fig. 4. The main parameters of the sinusoidal PWM scheme are the amplitude modulation index of signal and the frequency modulation index of the triangular signal. The VSC switching strategy is based on PWM techniques that offer simplicity and good response[23]-[26]. During sag condition, the correct voltage must be injected so that the load voltage become normal again.

SIMULATION RESULTS AND DISCUSSION

The overall simulation diagram of the proposed Dynamic Voltage Restorer with PI controller is shown in Fig. 5.In this Simulink model we have a system in which two parallel feeders are shown. In second feeder further load is connected in series. In one feeder DVR is connected with line. PI controller is used for control purpose. Here DVR system is connected to the distribution system using a injection transformer.

The test system Table 2 employed to take out the simulations regarding the DVR actuation. The system composed by a 13KV, 50 Hz generating system, feeding two transmission lines through a three winding transformer connected using above system parameters.

The proposed DVR responds to the sag within one cycle, and injects the appropriate amount of missing voltage during the sag event. On detection of voltage recovery, the DVR switches off to keep conduction losses to minimum.

The simulation is carried out by using DVR with PI controller. The three phase to ground fault is applied to the system at the point with fault resistance of 0.66 ohm which result voltage sag as shown in Fig. 6 the above graph describes the event of sag takes place for the time duration of 0.2s to 0.4s. When there is a sudden interruption, the nominal voltage reduced from 1pu to 0.8pu also it gets constant up to some time period; here the fault is set for 2s. The injected voltage of DVR from 0.2s to 0.4s is shown in Fig. 7.The proposed DVR using PI controller responds to this sag within one cycle, and injects the appropriate amount of missing voltage during the sag event t=0.2s to 0.4s as shown in Fig. 8. The above graph shows the voltage magnitude at load point, whereas the time period from 0.2s to 0.4s, the voltage sag is mitigated and the voltage level is boosted up to the few extend level.

CONCLUSION

The DVR is considered to be the best choice to protect the industrial facilities from voltage sag. This paper aims to propose a PI controller for the DVR for voltage mitigation in the distribution utilities. The reference load terminal of the DVR has been extracted by obtaining the reference voltages. The performance of DVR has been observed to be satisfactory for various power quality problems like voltage sag, voltage swell in supply voltage. Moreover, it is able to provide self-supported dc bus of the DVR through power transfer from ac line at fundamental frequency. These results also shows that the DVR compensation is fast and the source voltage fault can be compensated by series voltage injection transformer and it can be examined through simulation using MATLAB software along with Simulink and power system block set toolboxes.

ACKNOWLEDGMENT

A project of this nature needs co-operation and support from many persons for the successful completion. In this regard, I am fortunate to express my heartfelt thanks to Professor S. SathishKumar, Surya Engineering College for his effective leadership, encouragement and guidance in the project.

Tables at a glance


Table 1	Table 2

Figures at a glance


Figure 1	Figure 2	Figure 3	Figure 4


Figure 5	Figure 6	Figure 7	Figure 8

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