Power electronic transformer with cascaded H-bridge (CHB-PET) can realize the flexible interconnection of AC/DC microgrid and renewable energy sources such as energy storage devices and DC loads. In a practical project, CHB-PET is connected to different bus segments through two short leads that are standby for each other. Considering the fault occurring AC side of CHB-PET, the spare short lead needs to be put into operation quickly to realize the rapid recovery of power supply. However, the conventional scheme cannot distinguish whether there is a fault before the spare short lead is put into operation, and there is a risk of connecting the faulty short lead. To solve this problem, this paper proposes a scheme based on the cooperative control of CHB-PET and DC microgrid, which achieves safe input of short leads by actively injecting characteristic voltage.

Firstly, CHB is blocked to achieve fault isolation after fault occurring. The energy storage device is switched to DC voltage control mode, maintaining the DC bus voltage and ensuring the normal operation of DC microgrid load and other equipment. Secondly, CHB is unlocked and switched to the U/f control. CHB-PET can inject characteristic voltage into the spare line to detect whether there is a fault point in the line. Furthermore, a fault detection method considering the current imbalance factor is proposed to reduce the time required for fault detection, so as to realize the rapid and safe investment of spare short lead.

The verification results on PSCAD/EMTDC simulation platform show that the scheme based on characteristic voltage injection can give accurate detection results when the spare line is fault-free. When three-phase fault, phase-phase fault, double-phase to ground fault and single-phase to ground fault occur in the spare line, the transition resistance of the phase-to-phase fault is 200 Ω, and the transition resistance of the ground fault is 300 Ω. The proposed scheme can also give accurate detection results. Through the simulation verification and experimental verification of the proposed scheme in different scenarios, the consistent results are obtained, which confirms the accuracy of the simulation model and the feasibility of the proposed scheme.

Through the simulation analysis, the following conclusions can be drawn: (1) The scheme uses the CHB-PET ontology to inject characteristic voltage with controlled amplitude and frequency into the standby line. According to the difference of electrical characteristics of the standby line in different scenarios, proposed scheme can accurately detect whether there is a fault in the standby line. (2) In three stages of the proposed scheme, the collaborative control of CHB-PET and DC microgrid energy storage device is used to ensure the normal operation of each equipment in the DC microgrid after the AC short lead fault, which is conducive to the rapid recovery after the fault. (3) In the fault detection stage of spare short lead, the fault detection criterion is constructed by using the product of the characteristic current imbalance factor and the current integral value. Compared with the direct use of the current integral value, the fault detection time under high resistance fault is reduced and the protection action speed is improved.