OVERVIEW OF DVR FOR POWER QUALITY IMPROVEMENT

Transcription

1 OVERVIEW OF DVR FOR POWER QUALITY IMPROVEMENT Shyam V. Alaspure 1, Snehal G. Vinchurkar 2, Swapnil D. Raut 1 Electronics & Telecommunication 2 Electronics & Power 3 Computer 1 Lecturer, G.H. Raisoni Polytechnic Amravati 2 J.E. Maharastra state transmission company Ltd 3 Lecturer, G.H. Raisoni Polytechnic Amravati. 1 shyamalaspure@gmail.com ABSTARCT: Power quality is a very important issue due to its impact on electricity suppliers, equipment manufactures and customers. Power quality is described as the variation of voltage, current and frequency in a power system. It refers to a wide variety of electromagnetic phenomena that characterize the voltage and current at a given time and at a given location in the power system Both, electric utilities and end users of electrical power are becoming increasingly concerned about the quality of electric power. Power quality in the distribution system can be improved by using a custom power device Dynamic Voltage Restorer for voltage disturbances such as voltage sags, swells, harmonics, unbalanced voltage and etc. The DVR has been developed by Westinghouse for advance distribution. The Dynamic Voltage Restorer (DVR) is a device that detects the sag or swells and connects a voltage source in series with the supply voltage in such a way that the load voltage is kept inside the established tolerance limits. The function of the DVR is a protection device to protect the precision manufacturing process and sophisticate sensitive electronic equipments from the voltage fluctuation and power outages. This paper explains all the techniques for minimization of different voltage issues in power quality. Keywords: Dynamic, sags, harmonics. 1. INTRODUCTION Sensitive loads such as computers, programmable logic controllers (PLC), variable speed drives (VSD) -etc. need high quality supplies. Power distribution systems, should ideally provide their customers with an uninterrupted flow of energy with a smooth sinusoidal voltage at the contracted magnitude level and frequency. However, in practice, power systems, especially distribution systems, have numerous nonlinear loads, which significantly affect the quality of the power supply. As a result of these nonlinear loads, the purity of the supply waveform is lost in many places. This ends up producing many power quality problems. An important percentage of all power quality problems are of the voltage-quality type where what matters is the deviation of the voltage waveform from its ideal form. The best known disturbances of the voltage waveform are voltage sags and swells, harmonics, inter harmonics and voltage imbalances. Distribution system is mainly affected by voltage sag and swell power quality issue. Short circuits, lightning strokes, faults and inrush currents are the causes of voltage sags. Start/stop of heavy loads, badly dimensioned power sources, badly regulated transformers, single line to ground fault on the system lead to voltage swells. 2. NECESSITY OF DYNAMIC VOLTAGE RESTORER Power quality in the distribution system can be improved by using a custom power device DVR for voltage disturbances such as voltage sags, swells, harmonics, unbalanced voltage. There are many different methods to mitigate voltage sags and swells, but the use of a custom power device is considered to be the most efficient method, e.g. FACTS for transmission systems which improve the power transfer capabilities and stability margins. The term custom power pertains to the use of power electronics controller in a distribution system. especially, to deal with various power quality problems. Custom power assures customers to get pre-specified quality and reliability of supply. This pre-specified quality may contain a combination of specifications of the following: low phase unbalance, no power interruptions, low flicker at the load voltage, and low harmonic distortion in load voltage, magnitude and duration of over voltages and under voltages within specified limits, 33

2 acceptance of fluctuations, and poor factor loads without significant effect on the terminal voltage. There are different types of Custom Power devices used in electrical network to improve power quality problems. Each of the devices has its own benefits and limitations. A few of these reasons are as follows. The SVC pre-dates the DVR, but the DVR is still preferred because the SVC has no ability to control active power flow. Another reason include that the DVR has a higher energy capacity compared to the SMES and UPS devices. Furthermore, the DVR is smaller in size and cost is less compared to the DSTATCOM and other custom power devices. Based on these reasons, it is no surprise that the DVR is widely considered as an effective custom power device in mitigating voltage sags. In addition to voltage sags and swells compensation, DVR can also add other features such as harmonics and Power Factor correction. Compared to the other devices, the DVR is clearly considered to be one of the best economic solutions for its size and capabilitiesajor headings should be typeset in boldface with the words capitalized. 3. LITERATURE OVERVIEW Power Quality in electric networks is one of today's most concerned areas of electric power system. The power quality has serious economic implications for consumers, utilities and electrical equipment manufacturers. The impact of power quality problems is increasingly felt by customers - industrial, commercial and even residential. Some of the main power quality problems are sag, swell, transients, harmonic, and flickers etc. This interest in the practice of power quality devices (PQDs) arises from the need of growing power quality levels to meet the everyday growing sensitivity of customer needs and expectations. 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. Its application includes lower cost, smaller size, and its fast dynamic response to the disturbance. Several research papers and reports addressed the subject of improving power quality in distribution system by the use of custom power devices. The followings present a brief review of the work undertaken so far. Paisan Boonchiaml presents detailed analysis of load voltage compensation for dynamic voltage restorer (DVR) that used for enhancing power quality. A technique of determining the accurate amount of voltage injection necessary to correct a specific voltage reduction with least power injection is described. Systematic expressions for both magnitude and angle of the injected voltage are also derived. It has been shown that a voltage reduction and power factor should be analyzed before compensating the voltage H.P. Tiwari, presents dynamic voltage restorer against voltage sag. A dynamic voltage restorer (DVR) is a custom power device used to correct the voltage sag by injecting voltage as well power into the system. The mitigation capability of these devices is generally influence by the maximum load; power factor and maximum voltage dip to be compensated. Voltage Dip on a feeder is an main task for DVR system operation and appropriate desired voltage sag compensation. This paper is intended to assimilate the amount of DC energy storage depends on voltage dip. It is available in a convenient manner for DVR power circuit. 4. DYNAMIC VOLTAGE RESTORER Dynamic voltage restorer is a static var device that has applications in a variety of transmission and distribution systems. It is a series compensation device, which protects sensitive electric load from power quality problems such as voltage sags, swells, unbalance and distortion through power electronic controllers that use voltage source converters (VSC). The first DVR was installed in North America in a kv system located in Anderson, South Carolina. Since then, DVRs have been applied to protect critical loads in utilities, semiconductor and food processing. Today, the dynamic voltage restorer is one of the most effective PQ devices in solving voltage sag problems. The basic principle of the dynamic voltage restorer is to inject a voltage of required magnitude and frequency, so that it can restore the load side voltage to the desired amplitude and waveform even when the source voltage is unbalanced or distorted. Generally, it employs a gate turn off thyristor (GTO) solid state power electronic switches in a pulse width modulated (PWM) inverter structure. The DVR can generate or absorb independently controllable real and reactive power at the load side. In other words, the DVR is made of a solid state DC to AC switching power converter that injects a set of three phase AC 34

3 output voltages in series and synchronism with the distribution line voltages. Dynamic voltage restorer is a series connected device designed to maintain a constant RMS voltage across a sensitive load. Fig. 1. Basic Representation of DVR Basic component of DVR DVR consist of following components:- Series Voltage Injection Transformers Energy Storage Passive Filters Voltage Source Inverter (VSI) By Pass Switch Sections Fig. 2 Power circuit of DVR 35

4 4.1.1 Series Voltage Injection Transformers In a three-phase system, either three single-phase transformer units or one three phase transformer unit can be used for voltage injection purpose. The injection transformer comprises of two side voltages namely the high voltage side and low voltage side. Normally the high voltage side of the injection transformer is connected in series to the distribution system while power circuit of the DVR can be connected at the low voltage side. The basic function of the injection transformer is to increase the voltage supplied by the filtered VSI output to the desired level while isolating the DVR circuit from the distribution network. The transformer winding ratio is pre-determined according to the voltage required in the secondary side of the transformer (generally this is kept equal to the supply voltage to allow the DVR to compensate for full voltage sag). A higher transformer winding ratio will increase the primary side current, which will adversely affect the performance of the power electronic devices connected in the VSI. Three single phase or three-phase voltage injection transformers can be used for a three-phase DVR. In this case the high voltage of the injection transformer is connected to the distribution line and for single phase DVR one single-phase injection transformer can be connected. The single phase transformers can be used to inject the compensating voltages separately when three phase inverter is used. To evaluate the performance of the DVR the rating of the injection transformer is an important factor that need to be considered due to the compensation ability of the DVR is totally depend on its rating. The DVR performance is totally depend on the rating of the injection transformer, since it limits the maximum compensation ability of the DVR inverter topology is used in DVR allowing the direct connection of the DVR to the distribution system without using injection transformer Energy Storage The DVR needs real power for compensation purposes during voltage disturbances in the distribution system. In this case the real power of the DVR must be supplied by energy storage when the voltage disturbances exit. The energy storage such as a battery is responsible to supply an energy source in DC form. Energy storage consists of two types form. One using stored energy to supply the delivered power and the other having no significant internal energy storage but instead energy is taken from the faulted grid supply during the sags. A shunt converter or the rectifier is the main sources of the direct energy storage which is supplied to DVR. Flywheels, batteries, superconducting magnetic energy storage (SMES) and super capacitors can be used as energy storage devices. It is supplies the real power requirements of the system when DVR is used for compensation. The application of the energy storage in DVR is depending on the designed rating required and total cost is also must be considered.flywheels, batteries, superconducting magnetic energy storage (SMES) and super capacitors can be used as energy storage devices. It is supplies the real power requirements of the system when DVR is used for compensation lead acid batteries are popular among the others owing to its high response during charging and discharging. But the discharge rate is dependent on the chemical reaction rate of the battery so that the available energy inside the battery is determined by its discharge rate Storage systems with auxiliary supply is used to increase the system performance when the grid of DVR is weak. Flywheel Energy Storage as a preferred energy storage device, the system utilizes a single AC/AC power converter for the grid interface as opposed to a more conventional AC/DC/AC converter, leading to higher power density and increased system. However the suitable of the type of energy storage depend on the DVR designed in term rated power and the total cost factor L.C. Fliter Basically filter unit consists of inductor (L) and capacitor (C).. In DVR, filte rs are used to convert the inverted PWM waveform into a sinusoidal waveform. This can be achieved by eliminating the unwanted harmonic components generated by the VSI action. Higher orders harmonic components distort the compensated output voltage. The unnecessary switching harmonics generated by the VSI must be removed from the injected voltage waveform in order to maintain an acceptable Total Harmonics Distortion (THD) level. The switching frequencies of the VSI are usually up to several khz for medium power level. The passive filters can be placed either in the high voltage or in low voltage side winding of the series injection transformer.if the filter is installed at the low voltage side it has the advantage of being closer to the harmonic source thus high order harmonic currents are avoided to penetrate into the series injection transformer. Harmonics currents will circulate into the series injection transformer if the filtering scheme is placed at the high voltage. 36

5 4.1.4 Voltage Source Inverter(VSI) The function of an inverter system in DVR is used to convert the DC voltage supplied by the energy storage device into an AC Voltage source inverter (VSI) of low voltage and high current with step up injection transformer is used for this purpose in the DVR compensation technique Generally Pulse-Width Modulated Voltage Source Inverter (PWMVSI) is used transformer is used. Thus a VSI with a low voltage rating is sufficient. There are two basic three phase inverter topologies, the popular two-level inverter as shown in Figure 3. multilevel inverters have recently emerged as an attractive alternative to PWM schemes so that the losses associated with fast switching can be eliminated. The implementation of the PWM in the two level inverter is simpler and its cost is cheaper than a multilevel inverter. Fig 3. Two level switch mode inverter By Pass Switch Fault current causes by faults in the downstream will flow through the inverter circuit of the DVR. Therefore to avoid high currents flowing to the inverter, a protection device namely by-pass (crowbar circuit) switch is used, which is incorporated to by-pass the inverter circuit. Normally the by-pass switch will be in active mode and senses the current flowing in the distribution circuit and if the current flowing over than the inverter current rating limit, the circuit bypasses the DVR circuit components in order to protect the inverter from over currents. The bypass switch will become in inactive when the source current is in rated value or in normal condition. 4.2 Operation of DVR Circuit diagram of DVR is as shown in fig.the basic function of the DVR is to inject a dynamically controlled voltage VDVR generated by a forced commutated converter in series to the bus voltage by means of a booster transformer. The momentary amplitudes of the three injected phase voltages are controlled such as to eliminate any detrimental effects of a bus fault to the load voltage VL. This means that any differential voltages caused by transient disturbances in the ac feeder will be compensated by an equivalent voltage generated by the converter and injected on the medium voltage level through the booster transformer. The DVR works independently of the type of fault or any event that happens in the system, provided that the whole system remains connected to the supply grid, i.e. the line breaker does not trip. For most practical cases, a more economical design can be achieved by only compensating the positive and negative sequence components of the voltage disturbance seen at the input of the DVR. This option is Reasonable because for a typical distribution bus configuration, the zero sequence part of a disturbance will not pass through the step down transformer because of infinite impedance for this component A typical DVR as shown in Figure 4. is used for voltage compensation in the distribution line. 37

6 Fig. 4. Dynamic Voltage Restorer. The basic function of the DVR is to inject a dynamically controlled voltage VDVR generated by a forced commutated converter in series to the bus voltage by means of a booster transformer. The momentary amplitudes of the three injected phase voltages are controlled such as to eliminate any detrimental effects of a bus fault to the load voltage VL. This means that any differential voltages caused by transient disturbances in the ac feeder will be compensated by an equivalent voltage generated by the converter and injected on the medium voltage level through the booster transformer. The DVR works independently of the type of fault or any event that happens in the system, provided that the whole system remains connected to the supply grid, i.e. the line breaker does not trip. For most practical cases, a more economical design can be achieved by only compensating the positive and negative sequence components of the voltage disturbance seen at the input of the DVR. This option is Reasonable because for a typical distribution bus configuration, the zero sequence part of a disturbance will not pass through the step down transformer because of infinite impedance for this component A typical DVR as shown in Figure4. is used for voltage compensation in the distribution line. 5. Operating Modes of DVR The operation of the DVR can be categorized into three operation mode as follows; Protection mode Standby mode Injection mode 5.1 Protection mode If the current on the load side exceeds a permissible limit due to a short circuit on the load or large inrush current, the DVR will be isolated from the systems by using the bypass switches as shown in Figure 5, S2 and S3 will open and S1 will be closed to provide an alternative path for the load current. 38

7 Fig 5. Protection Mode (creating another path for the load current) 5.2 Standby Mode: (VDVR = 0) In the standby mode the booster transformer s low-voltage winding is shorted through the converter as shown in Figure No switching of semiconductors occurs in this mode of operation and the full load current will pass through the transformer primary. 5.3 Injection/Boost Mode: (VDVR 0) Fig 6. Standby mode In the Injection/Boost mode the DVR is injecting a compensating voltage through the booster transformer after the detection of a disturbance in the supply voltage. 6. COMPENSATION TECHNIQUE IN DVR FOR POWER QULAITY DEVELOPMENT There are four different methods of DVR voltage injection which are 39

8 6.1 Pre Sag/Dip Compensation method 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 as shown in figure 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. The voltage of DVR is given below: VDVR = Vprefault Vsag 6.2 In Phase Compensation Fig 7. Pre-sag 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 as shown in figure 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. The load voltage is given below: VL = Vprefault 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. Practical application of this method is in non sensitive loads to phase angle jump. Fig 8. In-Phase compensation method 6.3 In phase Advanced compensation method 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 40

9 one of the most expensive parts of DVR. The 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. 6.4 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 7. Conclusion Fig. 9. Voltage tolerance method with minimum energy injection Comprehensive study of a DVR as a powerful custom power device has been presented in this paper. The main function of a DVR is the protection of sensitive loads from voltage disturbances coming from network The types of voltage disturbances such as voltage sags or swells, transients, unbalanced voltage and harmonics DVR. can mitigate the some types of power quality disturbances such as voltage sags/swells, voltage harmonics and unbalances. The main advantages of DVR are low cost, simpler implementation; require less computational efforts and its control is simple as compared to other methods. This study also gives useful knowledge for the researchers to develop a new design of DVR for voltage disturbances in electrical system. From the literature survey of DVR applications, this work concluded that the trends of DVR through the years are still assumed as a powerful area of research References [1] Rosli Omar, N.A. Rahim and Marizan Sulaiman, Dynamic Voltage Restorer Application for Power Quality Improvement in Electrical Distribution System: An Overview, Australian Journal of Basic andapplied Sciences, 5(12): , ISSN , [2] M.Sharanya, B.Basavaraja, M.Sasikala An Overview of Dynamic Voltage Restorer for Voltage Profile Improvement International Journal of Engineering and Advanced Technology (IJEAT) ISSN: , Volume-2, Issue-2, December 2012 [3] Shazly A. Mohammed1, Aurelio G. Cerrada2, Abdel-Moamen M. A1, Hasanin3 Dynamic Voltage Restorer (DVR) System for Compensation of Voltage Sags, State- of- the-art Review International Journal Computational Engineering Research (ijceronline.com) Vol. 3 Issue. 1 41