# Controlled Fault Current Interruption Scheme for Improved Fault Prediction Accuracy
## 4 Controlled Fault Current Interruption Sequence  
### Double-Sample Time Interruption Sequence  
In the case of non-phase-controlled interruption, the circuit breaker triggers the opening action after the relay protection time, the contacts begin to break after the breaker tripping time, and the arc is extinguished at current zero after the arcing time. In conventional controlled fault current interruption sequences, after a fault occurs, the relay protection system is required to calculate the time coordinates of the target zero  
Figure 7: Recovered waveform of fault recording data. crossing point within its response time [18]. Subsequently, the necessary waiting time is determined by combining the circuit breaker's opening time and the minimum arc ignition time. However, this sequence can lead to a waiting time of up to about 10 ms, resulting in wasted sampling data during the waiting time. In view of this, this study proposes a controlled fault current interruption sequence based on double-sampling. The sequence is based on the results of one calculation of the traditional sequence, using the waiting time with the remaining relay protection response time to continue optimizing the target zero calculation accuracy and reset the waiting time to complete the CFI. The specific steps are as follows:  
1. Set the first sampling time \(t_{\text{sam1}}\) within the response time of the relay protection, and use the data during this time to complete the calculation of the first target zero-crossing point \(t_{\text{zero1}}\). 2. In order that the second calculation will not result in a negative waiting time, combine the results of the batch simulation to obtain the maximum error \(t_{\text{error}}\) of the first target zero-crossing point calculation, while considering the algorithm's time margin \(t_{\text{mg}}\), to calculate the time interval \([t_{\text{sample1}}\ t_{\text{zero1}}-t_{\text{mg}}-t_{\text{error}}]\) of the supplementary data required for the second calculation. 3. Based on the results of the first calculation, use the supplementary data to complete the second calculation before the calculation time threshold \(t_{\text{deadline}}=t_{\text{zero1}}-t_{\text{error}}\), to obtain the final confirmation of the target over-zero point \(t_{\text{zero2}}\). The upper limit of the time interval of the supplementary data to the second waiting time between the signals issued by the controlled switching device improves the accuracy of the calculation of the target over-zero point. 4. If the first calculation fails, use the supplementary data to calculate the maximum error \(t_{\text{wait2}}\) of the first target over-zero point calculation. 5. If the first calculation fails, issue a switching command at the end of the relay protection response time. If the second calculation fails, input \(t_{\text{zero1}}\) to the controlled switching device to realize controlled switching. The sequence comparison of double-sampling time interruption with conventional controlled fault current interruption and non-phase-controlled interruption is shown in Figure 8.  
The figure assumes that the target zero point calculated by the conventional controlled fault current interruption sequence and double-sampling time interruption sequence are error-free; then, the waiting time \(t_{\text{wait2}}\) after the completion of the second calculation in the double-sampling time CFI sequence is equal to the sum of \(t_{\text{error}}\) and \(t_{\text{mg}}\). Under the actual working condition, the double sampling time CFI sequence will calculate a more accurate zero coordinate \(t_{\text{zero2}}\) than the traditional sequence, and then wait time \(t_{\text{wait2}}=t_{\text{zero2}}-(t_{\text{sam1}}+t_{\text{sam2}})\); after the upper limit of the supplementary data interval \(t_{\text{zero1}}-t_{\text{mg}}-t_{\text{error}}\), wait for \(t_{\text{wait2}}\) to issue a gate command to complete the controlled fault current interruption function.