Study Report

Protection Relay tripping due to harmonic distortion

Cogeneration power plants are increasingly facing operation disruption. Its protection relay trips on grid's electrical fault or with voltage fluctuation. A 6MW, 11kV steam power plant had reported frequent nuisance tripping with its protection relay system. The problem appeared after it had replaced the legacy steam powered mill drive with DC thyristor drive in sugar mills and variable frequency IGBT drives (VFD drive) in process and boiler auxiliary loads in 2010. This was done to achieve higher energy efficiency.

Grid Failure protection

Causes of Power Failure: Grid incomer relay intercepts blackout in synchronized power generating stations. It isolates faulty grid & safely islands the power plant with home load. It's catastrophic if power goes out that fails completely. Grid incomer relay prevents such an emergency in the event of grid failure. It lightens the severity of the power outage and mitigates a possible blackout condition with complete power failure into a brownout state with partial power outage. The major cause for grid failures are storm, rain and thunderstorm.

Transformer Failure protection

Transformer Failure protection relays & arrestors ensure that it's withstand limits on short-circuit current and power frequency voltages are not exceeded. Impulse over-voltage will happen if lightning stroke falls on an overhead line near the transformer installation. Power frequency over-voltage could occur from the opening of a highly loaded circuit breaker. Transformer insulation protection against these two types of voltage surges are ensured by connecting ZnO type surge arrestor near HV bushing.

Generator Failure Protection

Major generator failure concerns are over-excitation during load-rejection wherein V/F ratio could be excessive and dead generator excitation protection. Increase in voltage/frequency ratio subjects a generator to higher flux-density level than it's designed for. Sudden load throw-off in the grid increase voltage at the generator terminal. The increased V/F ratio could cause saturation and inducement of stray flux in the generator's magnetic core. In turn, it leads to overheating and winding insulation failure.