Voltage unbalance protection VT and relay errors effect on unbalanced voltage detection; Considerations for system relaying, stability and developments

In a power system network, an unbalanced voltage condition can occur due to various reasons. Excessive voltage unbalance has become an issue of concern in power quality at network level, leading to numerous problems. This calls for a stringent management of unbalanced. There are several codes of practice to advise and to enforce network operating conditions to keep system unbalance within specific limits. Analysis of system unbalance voltage and power system control is carried out through Voltage Transformers and protective relays. The accuracy of Voltage Transformers from primary to secondary varies according to the type (Class of VT). Also, the accuracy of Relays is of the concern to the same extent. This poses a limitation on detection of actual system unbalance. This study gives an overview of fundamentals of sequence components, unbalanced voltage definitions and use of different types of VTs and relays to analyse unbalanced voltage. This paper considers detection of low level voltage unbalance equivalent to maximum limits allowed in national grid codes in order to initiate system to generate tripping/alarm. This article endeavours to summarise deviational effect of accuracy levels of VT and relays. The study provides a reference of selection of VTs and Relays to detect known % unbalance in the system. Coverage has been extended to discuss mitigation techniques of voltage unbalance and deviational error minimisation. The emphasis is to initiate a thought process for requirement of improved methodologies to eliminate errors in analysing. Therefore this study aims in building a better power system.