## 4 Fault Protection Strategy
### Current Protection  
\(\bullet\) Bus over-current protection  
In extreme cases, such as starting with a heavy load or stalling in a short time, the peak of bus-current may increase to a very large value. The system requires instantaneous protective action under these circumstances, which is called bus over-current transient protection. In contrast, the protective action dealing with a current increase caused by long-time overload is called bus over-current locked protection. In order to reduce switching noise and loss, the protection circuit should not be under frequent operation.  
Figure 8a shows the bus over-current transient protection logic. _BOC_1 stands for hardware input protection signal, which is \"0\" under a normal state and \"1\" when the DC-link current is over 100 A. The protection measure varies with \(t_{s}\), which is the duration of _BOC_1. It can be divided into three cases: (1) when \(t>t_{s}>t_{r}\), switch \(T8\) shuts off instantly and keeps this state until \(t\); (2) when \(t_{s}\leq t_{r}\), the signal is considered as a spike and the protection action will not be triggered; (3) when \(t_{s}\geq t\), switch \(T8\) shuts off until _BOC_1 is back to \"0\" level. In this work, \(t\) and \(t_{r}\) are taken to be 5 and 1 ms, respectively.  
Bus over-current locked protection logic is shown in Figure 8b. _BOC_2 represents the hardware input protection signal, which is \"0\" under a normal state and \"1\" when the DC-link current is over 80 A. The protection measure can be divided into five categories varying with \(t_{p}\), the duration of _BOC_2: (1) when \(t_{p}\leq t_{r}\), it is considered to be an interference; (2) when \(2t\geq t_{p}\geq t\), the signal shows again after the interval time \(t_{s}\) (\(t_{s}>t_{r}\)), and if its duration \(t_{p}\leq t_{r}\), the protection does not take effect; (3) when \(2t\geq t_{p}\geq t\), the protection does not carry on; (4) when \(t_{p}\geq t\), the signal shows again after the interval time \(t_{n}\) (\(t_{n}\leq t_{r}\)), but if its duration \(t_{p}\geq t\), switch \(T8\) shuts off until the signal returns to \"0\"; and (5) when \(t_{p}\geq 2t\), switch \(T8\) shuts off until _BOC_2 returns to \"0.\" Here, \(t\) and \(t_{r}\) are taken to be 5 and 1 ms, respectively.  
\(\bullet\) Phase over-current protection  
Short-circuit faults of machine windings may result in phase over-current faults. Their protection logic is shown in Figure 9, including two cases: (1) if the peak of phase-current exceeds \(i_{r}\), \(T8\) shuts off to cut off the drive system from the power source. When the signal is normal, \(T8\) turns on after a duration \(t_{r}\). The purpose of the delay is to prevent excessive loss and influence on the rapidity of the system; (2) if the peak of phase-current continuously exceeds \(i_{s}\) but is below \(i_{r}\) for at least \(t_{s}\), \(T8\)  
Figure 8: Bus over-current protection logic. (**a**) Transient protection; (**b**) locked protection.  
Figure 7: Bus under-voltage protection logic.  
will turn off and lock the state. However, if the over-current state lasts no longer than \(t_{s}\), the signal is negligible. In this work, \(t_{s}\), \(t_{r}\), \(i_{s}\), and \(i_{r}\) are taken to be 5 ms, 10 ms, 150 A, and 190 A, respectively.