Voltage Sag Mitigation by Network Reconfiguration

The electric power distribution system must be designed to operate and supply acceptable level of electrical energy to customers. Power utilities must ensure that the power supply to customers is with voltage magnitude within standard levels. Other features like minimal interruptions and minimal system power loss also must be considered. Hence, the quality and reliability of supply must be maintained in an acceptable level even during contingencies. Voltage magnitude is one of the parameters that determine the quality of power supply. A decrease in voltage magnitude may result in voltage sag which is currently considered as one of the main power quality problems. Voltage sag is defined as a decrease in magnitude between 0.1 and 0.9 pu in rms voltage at a power frequency of duration from 0.5 cycle to 1 min (IEEE Std 1159, 1995). Voltage sag may cause sensitive equipment to malfunction and process interruption and therefore are highly undesirable for some sensitive loads, especially in high-tech industries. However, loads at distribution level are usually subjected to frequent voltage sags due to various reasons. Voltage sag can be treated as a compatibility problem between equipment and power supply. When installing a new piece of equipment, a customer needs to compare the equipment sensitivity with the performance of the supply. There are various engineering solutions available to eliminate, correct or reduce the effects of power quality problems (Kusko &Thomson, 2007). Currently, a lot of research works are under way to solve the problem of voltage sag in distribution systems. Most of these research works focus on installing voltage sag mitigation devices (Sensarma et al., 2000). Other researchers focus on improving the immunity level of customer equipment by installing custom power devices to improve the voltage sag ride through capability (Shareef et al., 2010). Some other research works focus on utility efforts in finding feasible solutions to mitigate voltage sag problem. Since system faults are considered as main causes of voltage sags, utilities try to prevent faults and modify the available fault clearing practice in power systems. Normally, voltage sag assessment at a particular site in the network consists of determining the frequency of sags of specified sag magnitude and duration over a period of interest (Conrad & Bollen, 1997). It is also dependent on the utility fault performances, the way the fault affects propagation of disturbance in the system, and the customer’s service quality requirements (Shen et al., 2007). For voltage sag assessment, voltage sag characteristics has to be