Research on abnormal analysis and treatment methods of isolating switches in substations

introduction

The main function of the isolating switch, or knife switch, is to isolate the energized and non-energized parts during equipment maintenance, creating an obvious disconnection point to isolate the equipment under maintenance from the power system to ensure the safety of the staff and equipment. In substation operation and maintenance, the isolating switch is the main tool for changing the operation mode, which directly affects the reliability and safety of the system power supply. During normal operation, the isolating switch rarely has abnormal conditions, but during the switching operation, equipment abnormalities and operation abnormalities often occur. Therefore, the operation and maintenance personnel must master the basic common abnormality handling methods of the isolating switch to improve the efficiency of the switching operation.

1 Analysis of the electric operation principle of the isolating switch and the treatment method of operation failure

The control circuit of the electric operation of the disconnector is shown in Figure 1, and the primary wiring diagram of the 220kV interval is shown in Figure 2. The connection of the 1G circuit must meet the following conditions: the disconnector is in the open position (the three phases of the phase-splitting switch are all in the open position), the 2G knife switch is in the open position, and the 1GD and 3GD1 grounding knife switches are in the open position: or the 2G knife switch is in the closed position, and the 1GD and 3GD1 grounding knife switches are in the open position. After the error prevention logic judgment of the measurement and control device, the "local/remote" is selected by the switch. When the "local" position is selected, 1oK1-2 and 1oK5-6 are connected. When the "remote" position is selected, 1oK3-4 and 1oK7-8 are connected. After the remote control opening and closing or local opening and closing contacts are connected, the opening and closing contactor in the knife switch mechanism box is connected to perform the opening and closing operation of the 1G knife switch. YK1-2 is the contact for releasing the electrical interlock.

Remote operation closing circuit: AC power supply A phase 2 Anti-mistaken locking contact 2 in the measurement and control device Remote and local control selection switch 1oK3-4 contacts (1oK is set to "remote") 2 Opening contactor KM2 moving contact 2 Closing contactor KM1 coil 2 Closing stroke switch SL1 2 Thermal relay moving contact 2 Emergency stop button SB2 2 AC power supply N. When the closing contactor KM1 coil is energized and actuated, its moving contact closes, realizing the self-holding of the KM1 coil and completing the closing. After the isolating switch is closed, the closing stroke switch SL1 is disconnected, cutting off the closing circuit, and the closing stops. The opening circuit is similar to this.

Common operating abnormalities of the isolating switch include the operating mechanism refusing to open or close, the opening and closing of the switch not being in place, and the failure of the electric operation. When the electric operation of the isolating switch fails, the following items should be checked: (1) Is the motor power supply normal? Is the power fuse blown or in poor contact? (2) Is the control power supply normal? Is the control power switch turned on? (3) Are the remote and local control modes in the control box corresponding? (4) Is the motor thermal relay actuated but not reset? (5) Are the switch auxiliary, limit contacts, and locking circuit contacts locked properly?

(6) Check whether the electrical components are damaged.

1.1 Poor contact of auxiliary contacts causes the disconnector to be unable to operate electrically

At 07:00 on 2017-03-06, during the on-site operation of the xx transformer, the 7143 switch of the xx line and the 7023 switch of the No. 2 main transformer could not be operated. After testing, the switch control power supply and the motor power supply were found to have power.

Interlock detection: The xx transformer site does not have the measurement and control interlocking conditions, but only has electrical interlocking and five-protection interlocking conditions. Therefore, it is judged that the 7143 and 7023 switches cannot be operated because the electrical interlocking conditions are not met.

Treatment: After on-site inspection, it was found that the auxiliary contacts of the grounding knife of line xx 7144 and the grounding knife of main transformer 2 7024 were not in place, resulting in the electrical interlocking conditions of the two knives not being met and unable to operate. The auxiliary contacts of the grounding knife of 7144 and 7024 were reset on site and then restored to normal operation. The auxiliary contacts of the knife are shown in Figure 3. The knife operation link follower device drives the auxiliary contact switch to move the remote operation closing circuit: AC power supply A phase 2 Anti-mistake locking contact 2 in the measurement and control device Remote control selection switch 1oK3-4 contacts (1oK is set to "remote") 2 Opening contactor KM2 moving contact 2 Closing contactor KM1 coil 2 Closing travel switch SL1 2 Thermal relay moving contact 2 Emergency stop button SB2 2 AC power supply N. When the closing contactor KM1 coil is energized and actuated, its moving contact is closed, realizing the self-holding of the KM1 coil and completing the closing. After the isolating switch is closed, the closing travel switch SL1 is disconnected. When the slave device is loose and cannot follow the action, the auxiliary contact of the grounding knife cannot be accurately changed, resulting in the contact knife being open, but the normally closed contact cannot be closed, so that the main knife electrical interlock cannot be satisfied and the main knife operation circuit is disconnected. When encountering similar abnormalities on site, you can consider manually resetting the auxiliary contact switch and resuming operation while ensuring safety. After the operation is completed, notify the maintenance to handle it and make a record in the work log.

1.2 The incorrect position of the travel switch causes the switch to be unable to be operated electrically

At 14:30 on 2017-05-09, during the on-site acceptance of the xx transformer, the 45733 line switch could not be operated electrically on site, but the switch control power supply and the motor power supply were found to be energized after testing.

Interlocking detection: The electrical interlocking conditions and the five-defense interlocking conditions are met. After on-site inspection, it was found that the limit switch position in the 45733 line switch mechanism box was incorrect, resulting in failure to operate normally.

Processing: Use the operating handle on site to turn the actual green open switch to match the open switch position marked in the figure, then resume normal operation.

As shown in Figure 4, the auxiliary contacts of the 45733 knife travel switch are connected in series to the knife control circuit. When the travel switch is not in the correct position, its auxiliary contacts will disconnect the knife control circuit, causing the knife to be unable to operate electrically. It is recommended to replace this type of knife travel switch as a whole.

2. Key points for inspection of abnormal switch blades and methods for handling heat generation

The abnormal conditions that may occur during the operation of the disconnector include: (1) Overheating of the contact part. (2) Damage or breakage of the insulator, and cracks in the wire clamp. (3) Flashover, discharge, and grounding breakdown due to severe contamination or overvoltage. (4) The insulator cover falls off due to poor quality and natural aging of the glued part of the pillar insulator.

On the evening of February 12, 2017, the operation and maintenance personnel conducted a lights-out inspection and infrared temperature measurement at the 220kV xx transformer and found that the pile head of the C phase main transformer side of the 7033 switch of the No. 3 main transformer was heated, as shown in Figure 5. The abnormal point temperature: C phase was 94.3℃, and the ambient temperature was 10℃. The normal point temperature is shown in Figure 6, and the normal point temperature: B phase was 23.4℃.

Figure 7 shows the appearance of the pile head under visible light.

After comprehensively considering the on-site situation and confirming that the pile head was indeed overheated, the defect was reported to the team leader and operation specialist as soon as possible according to the abnormal handling process of the substation operation and maintenance team. The team leader immediately communicated with the monitoring center, and then the dispatcher ordered the No. 3 main transformer to be shut down and the maintenance personnel to eliminate the defect. After passing the acceptance, the operation and maintenance personnel contacted the dispatcher to resume power supply. After the power supply was completed, all monitoring data were normal, and there were no remaining alarm signals in the background, and the abnormal handling was completed.

The hidden danger was discovered in time, which avoided an incident in which a short-circuit fault near the medium-voltage side of the 220kV main transformer caused damage to the main transformer, ensuring the safe and healthy operation of the equipment.

In infrared temperature measurement, special attention should be paid to high-current equipment that is more prone to abnormal temperature rise. In particular, when abnormal temperature rise occurs in equipment that is easily damaged by heat, infrared temperature measurement should be combined with appearance observation and careful analysis of the equipment's defects.

3 Conclusion

Disconnectors are important electrical equipment in power systems. They can perform switching operations and have the ability to isolate voltages and connect and disconnect small currents. Since most of the electric switches in substations are outdoor, equipment abnormalities and abnormal operations often occur. In addition, there is currently a lack of technical monitoring methods for the disconnectors themselves and the electric operation process in substations, and it is impossible to make timely judgments on abnormal switches. In combination with post-maintenance acceptance and daily inspection work, this article diagnoses and analyzes common abnormal conditions of disconnectors, and proposes corresponding treatment methods, which provides a reference for the on-site application, daily maintenance, and abnormal maintenance of disconnectors.