Evaluation of overcurrent protection performance and application on the Eskom shunt capacitors during system disturbances.
MetadataShow full item record
This dissertation report began as an investigation into an overcurrent relay protection operation on a shunt capacitor bank (SCB) at ESKOM’s Westgate substation. Westgate substation has two SCBs, both of which were in service at the time of the 2007 incident. However, only the overcurrent protection scheme applied on SCB No.2 operated due to an external feeder fault on the Eltro feeder at Westgate substation. In 2004, SCB No.2 had tripped also on an overcurrent relay protection operation for an external fault. The difference identified in the otherwise identical SCBs was the relay technology employed by the overcurrent protection schemes i.e. electromechanical and electronic overcurrent relays were utilised. Therefore an investigation was initiated to determine any difference in the performance and reliability of overcurrent relay technologies in the SCB environment. The purpose of this work is to present the performance of the different technologies of overcurrent relays (electromechanical, electronic and digital) as applied to an ESKOM SCB during system disturbances and to compare their operation and behaviour. MatLAB and DigSILENT simulation packages were used to conduct preliminary fault studies to determine overcurrent relay performance, for a definite time overcurrent setting. These simulation results indicated that the simple electromechanical and electronic overcurrent relay could operate incorrectly in the SCB environment, during system disturbances. Practical laboratory tests were also conducted. This comprised of injecting DigSILENT simulations, comprising of system switching events and external faults, into three technologies of overcurrent relays. These Omicron injection tests found that the Westgate electronic relay would operate incorrectly for certain fault events in the SCB environment. Due to the results observed, further frequency response tests were conducted. These results suggested that the electronic and electromechanical overcurrent relays were susceptible to harmonics i.e. harmonics impact both the pick-up current setting and operating time of electronic and electromechanical overcurrent relays. The digital relay did not exhibit this vulnerability. Finally, recommendations were made to address the incorrect operation of the Westgate electronic relay in its SCB application. These recommendations could be applied in other ESKOM SCB overcurrent protection schemes, to prevent incorrect operation for system disturbances.