Dynamic behaviour of generators during short-circuits and its impact on directional protection systems

In medium-voltage systems, busbars are often protected by directional short-circuit functions (ANSI 67) with reverse blocking, as differential protection is impractical due to the high number of outgoing bays. However, the reliability of such protection can be compromised when generators are connected. During near short-circuits, generators accelerate due to a power imbalance caused by the drop in terminal voltage, potentially altering current phase angles and reversing fault direction evaluation. This study assessed the reliability of directional busbar protection under these conditions. Laboratory tests were conducted with a DC machine emulating a turbine, a synchronous generator, and an RLC network to replicate realistic scenarios. The results revealed unexpected behaviour: rather than a reduction in active power during the voltage drop, the generator experienced a power surge of up to three times nominal power within the first 10–50 ms. This transient response caused initial rotor deceleration, followed by acceleration. The initial power spike, often overlooked in fault ride-through calculations, significantly impacts directional protection reliability. The findings highlight a real risk of protection failure for busbars and generator feeders. To mitigate this, alternative busbar protection systems or modifications to directional protection are recommended. A potential modification to improve reliability is also proposed.