Dynamic Voltage Restorer for Mitigation of Voltage Sags

Dynamic Voltage Restorer for Mitigation of Voltage Sags

Dynamic Voltage Restorer for Mitigation of Voltage Sags

S.Ganesh,

Assistant Professor, Chandy College of engineering.

Tuticorin, Tamilnadu, Gauti.ganeshs@gmail.com

S.Muthukumar,

UG Scholar,

Chandy college of engineering, Tuticorin, Tamil nadu.

Muthukumar635@gmail.com

1. Vijaya kumar,

   UG Scholar,

   Chandy college of engineering, Tuticorin, Tamil nadu.

   Vijayakumarssrv@gmail.com

   Abstract Voltage sags are the most common power quality disturbance in the distribution system. It occurs due to the fault in the electrical network or by the starting of a large induction motor and this can be solved by using the custom power devices such as Dynamic Voltage Restorer (DVR). DVR was a power electronics device that was able to compensate voltage sags on critical loads dynamically. The DVR consists of VSC, injection transformers, passive filters and energy storage (lead acid battery). By injecting an appropriate voltage, the DVR restores a voltage waveform and ensures constant load voltage. The simulation and experimental results of a DVR using MATLAB software showed clearly the performance of the DVR in mitigating voltage sags.

   Keywords dynamic voltage restorer, voltage sags, power quality, injection methods

   1. Power quality is a very important issue for electricity suppliers, equipment manufactures and customers as sensitive equipment and non-linear loads are becoming common in both the industrial sectors and the domestic environment. So, both, electric utilities and end users of electrical power are becoming concerned about the quality of electric power. Power quality problems are related to voltage as well as current. The major power quality problems in the power system are poor load power factor, DC offset on load voltages, harmonic contents in loads, voltage sag & voltage swell. These power quality problems cause malfunctioning of sensitive equipments; protection and relay system .Faults at either the transmission or distribution level may cause transient voltage sag or swell in the entire system or a large part of it. Also, under heavy load conditions, a significant voltage drop may occur in the system. Voltage sag is one of the most common power quality problems faced nowadays. Voltage sag is a decrease of the normal voltage level between

      10 and 90% of the nominal rms voltage at the power frequency, for durations of 0.5 cycle to 1 minute[1].

      Sabin presents voltage deviations, commonly in the form of voltage sags, can cause severe process disruptions and result in substantial production loss. Several recent surveys attribute that 92% of the all disturbances in electrical power distribution systems are due to voltage

      sags[2].

      Gosh and Ledwich, has been proposed the Dynamic Voltage Restorer (DVR) to protect sensitive loads from such voltage sags. The DVR is connected in series with the sensitive load or distribution feeder and is capable of injecting real and reactive power demanded by the load during voltage sag compensation. The output of the DVR inverter is usually provided with an output LC filter to attenuate the harmonic contents appearing in injected voltage. The filter parameters are designed according to certain design aspects such as depth of the sag to be mitigated and the load voltage [3].

      Carl N.M.Ho, Henery and S.H. Chaung proposed a method, when a disturbance occurs (abnormal condition) and supply voltage deviates from nominal value, DVR supplies voltage for compensation of sag and is said to be in transient state. The DVR is connected in series between the load and the supply voltage [4].

      Kasuni Perera, Daniel Salomon son and Arulampaiam presented a DVR basically supplies the voltage difference (difference between the pre sag and sag voltage) to transmission line and maintains the pre sag values condition in the load sides [5].

      Yun Wei Li.Poh Chiang Loh et al. proposed a use of DVR is proposed in low and medium voltage distribution network to protect sensitive load from sudden voltage dips/sag [6].

      V.K.Ramachandramurthy et al. presents a pulse width modulated inverter is used to vary the amplitude and the phase angle of the injected voltages, thus allowing the control of both real and reactive power exchange between the distribution system and the load [7].

      D.Mahinda et al. proposed a method for proper voltage sag compensation, it is necessary to derive suitable and fast control scheme for inverter switching. The general requirement of a control scheme is to obtain an ac waveform with minimum total harmonic distortion (THD) and best dynamic response against supply and load disturbance when the DVR is operated for voltage sag compensation [8].

   2. Among the power quality problems (sags, swells, harmonics etc) voltage sags are the most severe disturbances. In order to overcome these problems the concept of custom power devices is introduced recently. One of those devices is the Dynamic Voltage Restorer (DVR), which is the most efficient and effective modern custom power device used in power distribution networks. DVR is a recently proposed series connected solid state device that injects voltage into the system in order to regulate the load side voltage. It is normally installed in a distribution system between the supply and the critical load feeder at the point of common coupling (PCC). Other than voltage sags and swells compensation, DVR can also added other features like: line voltage harmonics compensation, reduction of transients in voltage and fault current limitations [9]. DVR consists of rectifier, energy storage device, PWM inverter, filter and injection transformer.

      Fig. 1 Block diagram of DVR

      1. Rectifier

         The process of converting AC supply into DC supply is known as rectification. A device which is used for rectification is known as rectifier. The AC voltage cannot be able to store directly in a storage device. Hence, the rectifier circuit is used in the DVR.

      2. Energy Storage Device

         Energy storage device stores the converted DC voltage from the rectifier. It is very important device in the DVR. The energy storage device is DC capacity, batteries, super- capacitor, super conducting magnetic energy storage and flywheels.

      3. PWM inverter

         The DC supply from the storage device is passed to the PWM inverter. The PWM inverter generates the voltage with required magnitude and frequency. It converts the DC voltage into AC voltage. The PWM inverter is connected between the energy storage device and filter circuit.

      4. Filter

         Filter circuit is used to remove the unwanted noise signals or harmonics in the generated voltage from the PWM inverter. The LC filter is used in the DVR to improve the quality of power.

      5. Injection Transformer

         The injection transformer is connected with the transmission line in series. When the voltage level is decreased in the transmission line then the transformer inject the voltage with required magnitude and frequency. The supply to the transformer is given from the filter circuit.

   3. Injection Methods

      Voltage injection or compensation methods by means of a DVR depend upon the limiting factors such as; DVR power ratings, various conditions of load, and different types of voltage sags. Some loads are sensitive towards phase angel jump and some are sensitive towards change in magnitude and others are tolerant to these. Therefore the control strategies depend upon the type of load characteristics. There are four different methos of DVR voltage injection which are

      1. Pre-sag compensation method

      2. In-phase compensation method

      3. In-phase advanced compensation method

      4. Voltage tolerance method with minimum energy injection

      1. Pre-Sag/Dip Compensation:

         The pre-sag method tracks the supply voltage continuously and if it detects any disturbances in supply voltage it will inject the difference voltage between the sag or voltage at PCC and pre-fault condition, so that the load voltage can be restored back to the pre-fault condition. Compensation of voltage sags in the both phase angle and amplitude sensitive loads would be achieved by pre-sag compensation method. In this method the injected active power cannot be controlled and it is determined by external conditions such as the type of faults and load conditions [10].

         Fig. 2 Pre-Sag Compensation

      2. In phase Compensation method:

         This is the most straight forward method. In this method the injected voltage is in phase with the supply side voltage irrespective of the load current and pre-fault voltage. The phase angles of the pre-sag and load voltage are different but the most important criteria for power quality that is the constant magnitude of load voltage are satisfied. One of the advantages of this method is that the amplitude of DVR injection voltage is minimum for certain voltage sag in comparison with other strategies [10].

         Fig. 3 In-phase compensation method

      3. In Phase advanced compensation:

         In this method the real power spent by the DVR is decreased by minimizing the power angle between the sag voltage and load current. In case of pre-sag and in-phase compensation method the active power is injected into the system during disturbances. The active power supply is limited stored energy in the DC links and this part is one of the most expensive parts of DVR. Minimization of injected energy is achieved by making the active power component zero by having the injection voltage phasor perpendicular to the load current phasor .In this method the values of load current and voltage are fixed in the system so we can change only the phase of the

         sag voltage. IPAC method uses only reactive power and unfortunately, not al1 the sags can be mitigated without real power, as a consequence, this method is only suitable for a limited range of sags [10].

         Fig. 4 In-phase advance compensation method

      4. Voltage tolerance method with minimum energy injection:

         A small drop in voltage and small jump in phase angle can be tolerated by the load itself. If the voltage magnitude lies between 90%-110% of nominal voltage and 5%-10% of

         nominal state that will not disturb the operation characteristics of loads. Both magnitude and phase are the control parameter for this method which can be achieved by small energy injection [10].

         Fig. 5 Voltage tolerance method with minimum energy injection

   4. In this section the various results obtained after simulation are analysed and discussed. The test system comprises of 580V distribution network and the system has been examined under different fault conditions such as three phase fault, single line to ground fault and line to line fault.

      1. Test Results

      The simulation time for the model is taken as 0.5 sec. The first simulation was done without creating any fault at the network where supply is 580V with frequency 50 Hz. Fig.6 shows the waveforms of input voltage without fault. Y- Axis shows the magnitude of voltage and X-axis shows the simulation time. Fig.7 shows the waveform of load voltage without fault. Hence from the input voltage is obtained to be 580V and it is found that the magnitude of both the input and the load voltage is almost same.

      Fig. 6 Simulation model without fault

      Fig. 7 Input voltage without fault

      Fig. 8 Load voltage without fault

      The second simulation is done by applying three-phase to ground fault with fault resistance of 4.6 for a time duration of 100 ms i.e. from 0.2s to 0.3s and the ground resistance is 0.1. Supply is 580V, with frequency 50 Hz.

      Fig. 9 Simulation model with fault

      Waveforms for the load voltages (without compensation) are given below. Fig.10 shows the waveform of the input voltage with L-G fault and without DVR. Fig.11 shows the waveform of the input voltage with L-L fault and without DVR. Fig.12 shows the waveform of the input voltage with three phase fault and without DVR. Even after the fault is created the input voltage remains almost same as before while load voltage experiences a huge change. With the application of the fault to the circuit the magnitude of the load voltage decreases at the fault period from 580V to 250V. This voltage dip is needed to be compensated to get the desired voltage at the load.

      Fig. 10 Input voltage with L-G fault

      Fig. 11 Input voltage with L-L fault

      Fig. 12 Input voltage with three phase fault

      The third simulation is carried out at the same scenario as above but the DVR is now introduced at the load side to compensate the voltage sag occurred due to the three phase fault applied. Fig.14 shows the injected voltage generated by the dynamic voltage restorer to compensate the voltage sag. The waveform obtained from the test model is shown in Fig.15. It is clearly observed that the voltage waveform that is obtained after connection of DVR in series is almost similar to the supply voltage i.e. the DVR we installed is working efficiently.

      Fig. 13 Test model of DVR

      Fig. 14 Injected voltage

      Fig. 15 Compensated voltage by DVR

The modeling and simulation of a DVR using MATLAB has been presented. The simulation results showed clearly the performance of the DVR in mitigating voltage sags. The DVR handled fault conditions without any difficulties and injected the appropriate voltage to keep the load voltage balanced and constant at the nominal value. In this study, the DVR has shown the ability to compensate for voltage sags at the distribution side; this can be proved through simulation and experimental results. The efficiency and the effectiveness in voltage sags compensation showed by the DVR makes it an interesting power quality device compared to other custom power devices.

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