Research on Low-Frequency Controlled Fault Interruption Algorithm Based on FFRLs

As a new type of AC power transmission technology, flexible low-frequency power transmission has the advantages of low line impedance and high transmission capacity. However, in the event of a short circuit fault in the low-frequency power transmission line, the interval between current zero crossings becomes longer and the short-circuit current value is large. It is not conducive to the breaking of circuit breakers. Phase-controlled technology enables the control of circuit breakers to break at the target phase. This reduces the arcing time of circuit breakers and enhances their breaking capacity. A low-frequency short-circuit fault model is established in this paper. The fault current parameter estimation and zero-crossing prediction method based on forgetting factor recursive least square are applied. The hypothesis test is used for fault initiation time judgment and current parameter validity verification. The influence of current signal-to-noise ratio and sampling frequency on algorithm accuracy is studied. Multiple sets of simulations are conducted within the 40dB to 80dB SNR range. The results show that higher SNR leads to smaller errors. When the SNR is above 60dB, the algorithm converges quickly. And the zero-crossing prediction error can be maintained within ±0.5ms. The comparison simulation is performed for sampling frequencies between 4kHz and 10kHz. It reveals that the sampling frequency has minimal impact on algorithm accuracy. The convergence speed of the algorithm at different sampling frequencies is not significantly different. The low-frequency controlled fault interruption algorithm based on forgetting factor recursive least square can meet the requirements of low-frequency short-circuit breaking.