An improved under frequency load shedding scheme in multi-machine power system based on the rate of change of frequency

Under frequency load shedding (UFLS) scheme is a common practice for power system utilities for preventing frequency drop in power system after disturbances causing dangerous imbalance between the load and generation. The traditional UFLS scheme has a weak self-adaptive ability and set some certain permanent thresholds decided off-line, on the base of experience and simulations, which makes the act of loads shedding speed relatively slow. This paper gives a detailed new improved UFLS scheme based on the rate of change of frequency and applies it to a simulation on multi-machine model. Another simulation on a multi-region system shows that this new UFLS scheme will not cause overload problem on transmission lines. The new UFLS scheme overcomes all the disadvantages, mentioned above in traditional schemes and is especially suitable for modern large-scale complex power systems.