Abstract— Power quality is the major concern in today’s electrical system. Mainly, the power qualityproblems are endlessly observed in distributed power system. The voltagequality is the major feature in distributed electrical system. Voltage qualitycan be adequately improved in distributed systems by using some custom powerelectronic devices such as Dynamic Voltage Restorer (DVR).
The Dynamic VoltageRestorer (DVR) is fast, flexible and professional solution to voltage sag andvoltage swell crisis. The DVR is a power electronic based device that providesthree-phase controllable voltage output, which adds to the provider voltageduring sag event, to restore the load voltage to pre-sag states and whenvoltage swell occurs DVR injects 1800 out of phase voltage into thepower line. The simulation is based on designed PI controller for mitigatingthe voltage sag and swell. The transfer function is first obtained to design PIcontroller. Two 11 KV feeders are used. DVR can also be designed for protectingthe whole plant with loads in some MVA range.
The DVR can be able to restorethe load voltages within few milliseconds. Keywords — custom power device, PIcontroller, dynamic voltage restorer, voltage sag, voltage swell.Ashish DoorwarAsst. Professor Dept.
ofElectrical Engineering, AITSRajkot, [email protected] voltage sags are not compensated because of thelimited time period of mechanical switching. The transformer tap changing canbe used but tap changing under load is very much complex and pricey.
Here,power electronics technology had played an essential role in power flow controlof an electrical energy. Also for the electrical power system, the FACTSdevices are more often developed because of its extraordinary performances and thesedevices have the ability to mitigate power quality problems such as voltagesag, swell, harmonics, current quality etc. The definitions of voltage sagand voltage swell can be highlighted in the following table 5. TABLE I VOLTAGE SAGS AND SWELLS CRITERIA Disturbance Type Voltage Duration Voltage sag 0.1 – 0.9 pu 0.5 cycles- 1 min Voltage swell 1.
1 – 1.8 pu 0.5 cycles – 1 min I. INTRODUCTIONA s an electrical energy plays a fundamental role in today’s modern life.It is very important to handle this immense energy with supreme care.Especially, the thermal power plant, nuclear power plant and hydro power plant took the responsibility to supplythis immense electrical energy. This electrical energy can be well utilized byindustrial, customer, commercial, military, etc.
applications and the demand ofthis energy is increasing gradually. Thus, the control and use of this energy must be limited by certain limits. Actually, the electrical devicesare very much sensitive and became less tolerant towards the power disturbancessuch as voltage sags, swells, harmonics etc.
voltage sags/dips are consideredto be the most severe disturbances when industrial equipments are considered.Some of the industrial equipments are very sensitive that these are unable tohandle a very low deviation in voltage. Voltage sags can cause brutal damage inthese equipments. Now, the voltage support at the load end is very muchrequired and it can be achieved by injecting the reactive power at the load pointof common coupling.
Some of the useful conventionalmethod is to utilize the mechanically switched capacitors – SVC – which areconnected in shunt. The reactive power can be injected by mechanical switchingthe capacitors as per the requirement of the electrical systems. But thedisadvantage is that some of the high speed transients cannot be compensated bythis way and also someFor example, the FACTS fortransmission systems, the term custom power pertains to use of powerelectronics controllers in particular distribution systems in order to mitigatevarious power quality problems. FACTS devices can boost the power transfercapabilities and improve the stability of the system. The custom power ensuresthat the customers get pre-specified power quality and reliability of the powersupply.
The pre-specified power quality may include the followingspecifications of the parameters: low phase unbalance, low flicker at loadpoint, no power interruptions, low THD (total harmonic distortions), andovervoltage and under- voltage within specified limits etc. There are many types of CustomPower devices. Such as : Active Power Filters (APF) 7, 11, Battery EnergyStorage Systems (BESS), Distribution static synchronous compensators (DSTATCOM)7, Distribution Series Capacitors (DSC), Dynamic Voltage Restorer (DVR) 7,Surge Arresters (SA), Super conducting Magnetic Energy storage(SMES), StaticElectronic Tap Changers (SETC), Solid-State Transfer Switches (SSTS), SolidState Fault Current Limiter (SSFCL), Static VAR Compensator (SVC), ThyristorSwitched Capacitors (TSC),and Uninterruptible Power Supplies (UPS) 1. II. DVR BASIC CONFIGURATION DVR is a series connected custom power electronicdevice.
It has other features such as to compensate for fault current,harmonic reduction etc. Especially for thedistribution electrical set-ups, voltage profile is so much important So it isnecessary to improve the voltage profile mainly in distribution networks. TheDVR is an excellent in mitigating the voltage sags and swells in very precisemanner. It has main five components. a) Thebooster transformerb) Theharmonic filter (L-C filter) unitc) Voltagesource inverter d) DC energystorage unite) Controlunit Fig.
1 Basic DVR Power Circuit Fig. 1 1 shows the basicprinciple setup for DVR in which control block is the main concern fordesigning DVR. A. The booster transformer The designing oftransformer is very crucial because it faces saturation, overrating,overheating, cost and performance which are very important. The injectedvoltage may consist of fundamental, desired harmonics, Switching harmonics anddc voltage components 2. The transformer is designed as per the requirementi.
e. this well designed transformer is mainly used in distribution electricalnetworks. If the transformer is not designed properly, the injected voltage maysaturate the transformer core which can cause the improper operation of the DVR1.