Full H7 compliant trip circuit supervision using numerical relays

Protection of power systems requires a high degree of reliability. Whenever a protection is called upon to operate, it must do so. Failure to do so may result in costly damage to plant, loss of supply to many consumers, and potentially, danger to life. Protection relays are designed in such a way as to ensure reliability of operation. Very often, the main threat to reliable operation is the circuit connecting the operating contact and the circuit breaker trip coil itself. Thus, it is usual to monitor these elements using trip circuit supervision (TCS). The aim of our work in this area was by investigation, discussion, reasoning, calculation and testing, to devise a straightforward solution to strictly meet the ENA “H7” specification by utilisation of opto-isolator inputs in numerical relays. This paper details the success achieved with this.