Author: Damien Tholomier, AREVA T&D Automation, Canada
Protection relay failure may be the result of component failure, operating principle, measuring transformer failure, etc. Relays are also only one of the components of a fault clearing system. As can be seen from Figure 1, a simple protection system consists of:
Failure of any of the components will result in the failure of the fault clearing system to operate as designed. To determine the failure rate of the fault clearing system in Figure 1, we can use an example of a "fault tree analysis" based on the assumed failure rate of the individual components of the system. The result is shown in Figure 2.
If we analyze the numbers in Figure 2, we will notice the significant impact of the failure of a breaker or a battery. Even if we provide local backup protection in a substation with a single battery, the failure of the battery can not be overcome. Remote backup protection is required in such case to provide fault clearance by remote relays. Unfortunately, remote backup protection can not be always applied without any challenges.
Power system protection obviously requires a very reliable power supply in order to ensure the availability of protective relays when a fault occurs. Because of that the utilities pay a lot of attention to the design, commissioning and maintenance of the DC system in the substations. In very critical substations the utilities install two battery systems and separate the primary and backup protection systems completely by adding also two trip coils to the breakers. However, the large number of substations have a single battery subjected to different fault and operating conditions. There are existing solutions that will reduce the probability for a complete loss of DC in the substation or even if such an event occurs, it will ensure adequate fault clearing for any fault condition in the substation.