A tripping and power outage occurred on the 20kV Fo72 Fukang Line of a substation. The overcurrent I protection of the incoming line bay of the primary switching station operated and the switch tripped, resulting in a full-line power outage ranging from the No.1 Ring Main Unit to the No.8 Ring Main Unit of the 20kV Fukang Line. Wuhan Haomai Zhengwei assisted the Maintenance and Operation Department of the power supply company in carrying out emergency repair work. Since the feeder bays of the ring main units are equipped with REJ603 protection relays, the current transformers must be the ones matched with the ring main units. However, most feeder bays are equipped with conventional transformers, which makes it impossible to cut off faults when faults occur in the feeder bays of the line, thus causing the main line to trip and power outage, and making it impossible to accurately determine the faulty bay during fault detection.

The REJ603 protection relay is a type of feeder protection relay (dial-code type protection). It provides selective phase-to-phase short-circuit protection and grounding protection for the secondary side of feeder bays in the distribution network, ensures the reliable operation of feeder bays, avoids overstep tripping to the main line and prevents large-area power outages.

△ No.8 Ring Main Unit

△ REJ603 Protection Relay

After conducting a detailed analysis and investigation one by one, the cause of this line tripping was determined to be the cable fault in the 04 bay of the No.8 Ring Main Unit of the 20kV Fukang Line, which led to overstep tripping. The specific analysis is as follows:

The capacity of the 04 bay of the No.8 Ring Main Unit of the 20kV Fukang Line is 2000kVA, the current transformer ratio is 200/5, the zero-sequence current transformer ratio is 100/5, the overcurrent I setting value is 7.1A/0s, the overcurrent II setting value is 1.8A/0.3s, and the current transformer model is AMZ-0.5W5 (CT5), corresponding to a current range of 128A–448A. The selection of the CT size of this type of transformer depends on the rated current of the transformer, which is calculated as 2000kVA/√3/20kV = 57.8A.

△ Current Range Corresponding to Transformer

The rated current of the transformer connected to the 04 bay of the No.8 Ring Main Unit of the 20kV Fukang Line is 57.8A, while the current range of the actual on-site current transformer is 128A–448A. The selected CT size is much larger than the CT size corresponding to the capacity of this bay. Therefore, when a fault occurs in this bay, the protection device fails to operate, resulting in overstep tripping of the line.

According to the figure above, the 04 bay of the No.8 Ring Main Unit of the 20kV Fukang Line should be equipped with the current transformer CT2 with a current range of 16A–56A. It is necessary to replace the current transformer with the model AMZ-0.5W2 (CT2) on site. The current transformer ratio of this bay is 200/5, the zero-sequence current transformer ratio is 100/5, the overcurrent I setting value is 7.1A/0s, the overcurrent II setting value is 1.8A/0.3s, and the CT current size should be selected as 56A within the CT2 range.

Correspondingly, the S1 dial switch in the figure above (switches 1–4 of S1 are CT switches) should be set to ON, ON, ON, ON; the S2 dial switch (switches 1–4 of S2 are the time setting of overcurrent II) should be set to OFF, ON, ON, OFF; the S3 dial switch (switches 1–5 of S3 are the current setting of overcurrent II) should be set to OFF, OFF, OFF, ON, OFF; the S5 dial switch (switches 1–4 of S5 are the current setting of overcurrent I) should be set to OFF, OFF, ON, OFF; the S5 dial switch (switches 5–8 of S5 are the time setting of overcurrent I) should be set to OFF, OFF, OFF, OFF.

Due to the grounding mode of the substation, the grounding protection is not put into operation. The default grounding protection setting value and time of the protection device are both the minimum values. To prevent the misoperation of the feeder bay switch, both the grounding protection setting value and time should be set to the maximum values. Correspondingly, the S2 dial switch in the figure above (switches 5–8 of S2 are the time setting of zero-sequence II) should be set to ON, ON, ON, ON; the S4 dial switch (switches 1–5 of S4 are the current setting of zero-sequence II) should be set to ON, ON, ON, ON, ON; the S6 dial switch (switches 1–4 of S6 are the current setting of zero-sequence I) should be set to ON, ON, ON, ON; the S6 dial switch (switches 5–8 of S6 are the time setting of zero-sequence I) should be set to ON, ON, ON, ON.

On this occasion, Wuhan Haomai Zhengwei actively cooperated with the line inspection work of the power supply company, conducted a comprehensive inspection of the switching stations and ring main units under its jurisdiction, verified and modified the current transformers of the ring main units equipped with REJ603 protection relays in the high-voltage cabinet feeder bays and the protection setting values of each feeder bay. This avoids similar accidents that may cause large-area power outages in the later period, further ensures the safe and stable operation of the line, and improves the power supply reliability.