Voltage Sag Mitigation Methods Analysis

Chapter 3
VOLTAGE SAG/SWELL MITIGATION USING DYNAMIC VOLTAGE RESTORER (DVR)

3.1 Voltage Sag Mitigation Methods
Power quality problems cannot be avoided for the consumers (end users) who are directly connected together by the interconnected systems. Therefore, consumers must protect themselves from PQ problems with mitigation or power conditioning devices. It is researched by EPRI that 90 % of all power quality problems are voltage sags in the USA and it is assumed that the European power network exhibits similar attributes. In Turkey, the problem should be worse as the power grid is weak and frequent daily interruptions are not unusual. Therefore, the voltage sag correction solutions are the most important issue for consumers.

Present day
The DVR shown in Fig. 3.1 is based on an inverter system that has energy storage for supplying active power, an output filter to make more sinusoidal voltage, and a step up transformer. The DVR is one of the FACTS devices that use the power electronics technology, especially inverter technology and is configured as a series-connected voltage controller. To control the output voltage of the DVR, the inverter supplies the missing load voltage using self-commutable electronic switches such as a gate turn-off thyristor (GTO), an insulated gate bipolar transistor (IGBT), or an insulated gate commutated thyristor (IGCT). The DVR injects the missing voltage in a series. Therefore, it can be called a series voltage controller, but the term DVR is commonly used now. DVRs have a same configuration of SSSC. The DVRs can be operated with a relatively small capacitor to exchange reactive power or can supply active powers to loads with energy storage. The large capacitor bank, flywheel, superconducting magnetic device, and battery can be used for the energy storage. The DVR, located between the supply and critical loads, has demonstrated excellent dynamic capability for mitigating voltage sags or swells. Each phase can be controlled independently, and the DVR can adjust the
A relatively small capacitor is present on dc side of the converter. The voltage over this capacitor is kept constant, by exchanging energy with the energy storage reservoir. The required output voltage is obtained by using a pulse width modulation switching pattern. As the controller will have to supply active as well as reactive power, some kind of energy storage is needed. The amount of energy storage depends on the maximum duration of the sag. The controller is typically designed for certain maximum sag duration and a certain minimum sag