Investigation on Performance Improvement & Efficient Utilization of Unified Power Quality Conditioner

Abstract

In an era of automation and computation, sophisticated equipments with their sophisticated controllers, demand a current profile which is unenviable for the supply lines. Generally, the controllers for proper controlling of equipment and mechanical devices are based on power electronic systems which undesirably introduce harmonics in the supply system. This in turn, affects the voltage being available for other sensitive loads. Also, abrupt variations in the supply voltage for a short duration is a common phenomenon in a distribution system due to sudden load changes. Moreover, the loads with heavy reactive power demand are also responsible for voltage variations in the line due to its continuous switching operation. The electrical and electronic equipments irrespective of its sensitivity might suffer due to degrading power quality.

The use of power electronic based custom power devices (CPD) such as active power filter (APF) has become more prevalent than the passive elements in the past few decades for power quality improvement. Due to continuous and rigorous research work in the power quality domain, an integrated model of shunt and series APF came into existence with time, which was universally identified as unified power quality conditioner (UPQC). For addressing all the issues simultaneously (i.e. both current and voltage related), UPQC is an obvious choice of selection and due to its all-round performance, has been a topic for continued research in recent years. Many interesting topologies and control mechanisms of UPQC have been in the limelight in these years and belong to a good area of research. Out of these UPQC-S has found special attention due its applicability of power angle control (PAC) concept along with other regular compensating ability. With the unbalanced supply voltage condition, SRF method has always exhibited superior performance for determining the reference current and voltage signal as it deals with only current component for shunt APF, independent of supply voltage.

In this work, the PAC concept is effectively implemented with an SRF method for efficient utilization of UPQC by introducing reactive power sharing feature in a single-phase and three-phase distribution system. It is also validated for unbalanced loading condition by using a four leg inverter topology for shunt APF. However, with conventional approach of reactive power sharing between shunt and series APF, the series APF role of handling reactive power comes into picture only when the load reactive power demand exceeds a particular limit. This idea motivates for application of two APFs with the same rating and sharing an equal amount of compensating reactive power. Therefore, with further analysis, power angle estimation is based on equal reactive power sharing feature, thus performing a successful implementation of the PAC concept under the unbalanced supply with efficient utilization of both APF.

The reverse arrangement of most popular UPQC configuration (i.e. UPQC-R) is termed as UPQC-L where shunt APF is on the left side of UPQC and series APF on right side and can be used specifically for special applications. Thus, a new SRF based power angle estimation approach is presented for a specific UPQC-L arrangement with two different points of common coupling (PCC). In the proposed work, a rating based comparison is performed between the two stated UPQC topologies.

Apart from the above mentioned power quality problems, voltage interruption due to various unavoidable reasons is another factor that affects the entire distribution system or an important and sensitive load site, which suffers heavily due to these short voltage disruption issues. In this work, a fuel cell based UPQC system with linear and non-linear load handling features, is implemented in a single-phase distribution system. The fuel cell of PEMFC type is used to supply the load demand via both the inverters at the time of supply cut off, thus making it an efficient system. A fuel cell stack system offers a portable power source which, along with a boost converter and DC to AC inverter system can be comfortably used to meet the load power demand at the time of voltage outages.

The proposed controllers for the discussed topologies of UPQC in this research work are precisely described by mathematical and diagrammatic descriptions. The practicality of the proposed configurations is authenticated by their simulation and real-time results.