Analysis and Preventive Measures for an Abnormity of 220kV Isolating Switch

With the development of the power grid, the cutter brake is a kind of main equipment used cooperatively in the transmission and distribution line of the substations with the circuit breaker in the operation and maintenance of the electric power system. Its main role is to ensure the safety of the staff during the maintenance of high voltage equipment. The cutter brake forms a clear disconnection point to provide the staff with a sufficiently large insulation interval between the equipment in need of repair and other energized parts. In the long-term operation, various reasons may lead to various failures of the cutter brake, causing great security risks to the safe and stable operation of the power grid and personal safety. This paper is mainly focused on analyzing the abnormal phenomenon of the poor contact of the dynamic and static contact fingers of B phase of I cutter brake of Xinlang Line 1 in the 220kV substation, which causes most of the current to flow through the B phase of Xinlang Line 2. Future management and preventive measures for this type of cutter brake are also proposed in this paper.


Introduction
Isolating switch is the most commonly used electrical equipment in the power grid.Its main role is to cooperate with the operation mode switching, isolation of power supply in maintenance and cut-off of small load electrical equipment.In recent years, power grid security problems caused by the quality of isolating switch products are becoming more and more prominent, endangering the safe and stable operation of the power grid [1].The working principle and structure of the isolating switch itself are relatively simple, but its usage is large and its working reliability requirements are high.Therefore, it has a great impact on the design, establishment and safe operation of substations and power plants.The main feature of the isolating switch is that it has no arc extinguishing ability, and can only open and close circuits when there is no load current.The isolating switch is used at all levels of voltage to change the circuit connections or isolate the lines or equipment from the power supply.It has no current-breaking capability and can only be operated after disconnecting the line with other equipment.Generally, there is an interlock device to prevent misoperation when the switch is under load.Sometimes a pin is required to prevent the switch from being disconnected under the magnetic force of a large fault.
GW22A-252D(W) Type III single-column vertical fracture single-arm telescopic clamp outdoor three-phase high-voltage AC isolating switch is used in the power supply system with rated frequency of 50Hz and rated voltage of 252kV for high-voltage lines to be changed under no load, and for the electrical isolation between the high voltage busbar in maintenance, circuit breaker and other high voltage electrical equipment and live high voltage line.Meanwhile, when the operation mode of the power grid system needs to be changed, under the premise of constantly opening or closing the load current, the charging current within the specified range of the busbar transfer current, voltage transformer, arrester, etc. is carried out during the switching operation.
Xinlang Line 1 I cutter brake and Longdong Line 1 I cutter brake are single-arm cutter brakes produced by Taikai Group Shandong Taikai Isolating Switch Co., Ltd.The model is GW22A-252(W) II, the production date is October 21, 2008, and the operation date is September 28, 2009.The last inspection test time of Xinlang Line 1 I cutter brake was June 25, 2016, and the inspection content was cleaning and inspection of the cutter brake.The last inspection time of Longdong Line 1 I cutter brake was June 25, 2016, and the inspection content was the cleaning and inspection of the cutter brake [2].The routine inspection cycle of Langtou Substation was changed into once a week, and during the Spring Festival, according to the State Grid Dandong Power Supply Company 2023 Spring Festival Power Supply Guaranteed Operation and Maintenance Plan, the frequency of inspections was increased, and special patrol was carried out every day during the Spring Festival.The last inspection time was January 23, and the equipment of the whole substation was inspected and the temperature of primary equipment contact was measured, and no abnormity was found.

Equipment Structure and Operation Principle
GW22A-252D(W) Type III isolating switch is a single-column vertical retractable isolating switch, consisting of three unipoles.It mainly includes base, pillar insulator, rotary insulator, main cutter brake, static contact, ground switch, operating mechanism, etc.
The mechanical interlock of the ground switch and the isolating switch is realized by the interlock installed on the main and ground cutter rotating shaft of the phase base, and the interlock device has sufficient mechanical strength.When the isolating switch is in the closing position, the interlock in the bottom frame makes the ground switch unable to close; Conversely, when the ground switch is in the closing position, the isolating switch can also be prevented from closing by interlock.The mechanical interlocking principle is shown in figure 1.

Abnormity Discovery Process
At 19:50, January 24, when the monitoring personnel toured to the Xinlang Line 1 interval, they found that the B-phase current was displayed as 0A, the A-phase and C-phase currents were 114A and 111A respectively, and the three-phase voltage was normal; and they checked Xinlang Line 2 (in parallel operation with Xinlang Line 1) and found that the A-phase current was 129A, the B-phase current was 209A, and the C-phase current was 129A.The B-phase current was increased, and three-phase voltage was normal.
The monitoring personnel immediately informed the operation and maintenance personnel to check the background machine, protection device and measurement and control device of Langtou Substation and Xinxing Substation.The operation and maintenance personnel confirmed that the B-phase current of Xinlang Line 1 of Langtou Substation was 0A, the B-phase current of Xinxing Substation was 13A, the B-phase current of Xinlang Line 2 of Langtou Substation was 208A, and the B-phase current of Xinxing Substation was 208A.
At 20:20, they immediately notified the substation maintenance, testing, relaying protection, metering and other professionals to prepare for emergency repair.The transmission area began to inspect the Xinlang Line 1 tensioning tower drainage wire, current and voltage of Xinlang Line 1 and Xinlang Line 2 of Langtou Substation.

Abnormity Processing
On January 24, no abnormalities were found in the appearance inspection, electrical inspection and infrared temperature measurement on the unit interval primary and secondary equipment of Xinlang Line 1 and Xinlang Line 2 of Langtou Substation and Xinxing Substation.At 22:17, Xinlang Line 1 was powered off.In the process of power outage, the operation and maintenance personnel monitored the equipment of Xinlang Line 1 and Xinlang Line 2. At 22:18, when the operation and maintenance personnel opened the switch of Xinlang Line 1 of Xinxing Substation, they found that there was a discharge between the dynamic and static contacts of the I cutter brake of Xinlang Line 1 of Langtou Substation (the switch of Xinlang Line 1 was not opened).At 22:23, after separating the switch of Xinlang Line 1 of Langtou Substation, the discharge phenomenon disappeared.Then, the power of Xinlang Line 1 of Langtou Substation was cut off.The arc phenomenon and damage diagram of the I cutter brake of Xinlang Line 1 are shown in figure 2. At 9:17 on January 25, during the period of 220kV I section busbar outage operation by the operation and maintenance personnel, the I cutter brake A phase of Langdong Line 1 could not be opened (manual operation), which was reported to the dispatcher, and the operation was suspended.The picture that the I cutter brake of A phase of Langdong Line 1 of Langtou Substation can not be separated is shown in figure 3. From March 29 to March 30, power outage maintenance was carried out on the I cutter brake of Langdong Line 1, and the three phases of the cutter brake could operate normally during the switching operation.
It was found that the reset spring lubricating grease was whiter than other normal phase after disassembling the I busbar A phase cutter brake of Langdong Line 1.The sealing cover at the bottom of the ejector rod was intact, but the ejector rod inside the sealing cover had obvious rust stains, as shown in figure 4. From March 31 to April 1, power outage maintenance was carried out on the I cutter brake of Xinlang Line 1. Through the on-site disassembly and inspection of the conductive part of the B phase cutter brake of Xinlang Line 1, it was found that the insulation part of the clamping spring that fixed the contact finger had melted on the side of the burned contact finger, as shown in figure 5.The number and size of the insulation pads fixed on both sides of the contact fingers are inconsistent.After the contact fingers are clamped, the two sides of the contact fingers are not in the same plane, as shown in figure 6.On April 1, after the replacement and maintenance of the conductive part of the I cutter brake and the static contact seat of Xinlang Line 1 and Langdong Line 1, the 220kV system resumed its original operation mode.

Preliminary Analysis of the Abnormal Phenomenon of the I cutter Brake of Xinlang Line 1 of Langtou Substation
Before the power outage of Xinlang Line 1, Xinlang Line 1 and Xinlang Line 2 are running together.The preliminary analysis is that the contact of the dynamic and static contact fingers of the I cutter brake B phase of Xinlang Line 1 is poor, resulting in most of the current flowing through the B phase of Xinlang Line 2, so there will be no overheating and discharge between the I cutter brake and busbar of the B phase of Xinlang Line 1 before the side switch of the Xinlang Line 1 of Xinxing Substation is opened.When the side switch of the Xinlang Line 1 of Xinxing Substation is opened, the contact point of the dynamic and static contacts of the side I cutter brake of Langtou Substation produces voltage difference, which leads to discharge.When the switch of Xinlang Line 1 of Langtou Substation is opened, the discharge phenomenon disappears immediately.Meanwhile, in order to further check the health status of the I cutter brake of Xinlang Line 1, the testing personnel carried out high-voltage test on it, and no abnormity was found.It's shown in table 1

Analysis on no TA Disconnection Alarm on Xinlang Line 1 of Langtou Substation
After consulting the protection model instructions of Line 1 and Line 2, the alarm criterion of TA disconnection is that there is zero sequence current but no zero sequence voltage, and the zero sequence current is greater than 0.1Ie (Ie is 5A), and TA disconnection is generated after 10 seconds.Because 0.47A is smaller than 0.5A, it does not meet the alarm criterion of TA disconnection, so Xinlang Line 1 TA disconnection was not reported during the abnormal period.

Preliminary Analysis on the Abnormity of the I Cutter Brake of Langdong Line 1 of Langtou Substation
During the separating operation of the I cutter brake of Langdong Line 1 of Langtou Substation, the dynamic and static contact fingers of the cutter brake A phase was not separated normally, and the small pull rod in the upper conductive arm did not have displacement changes.It was suspected that the conductive tube of the cutter brake was poorly sealed, the conductive arm was affected by moisture, and the reset spring was corroded and out of work, and the contact fingers could not be separated and reset, and still maintained the clamping state.Similarly, the testing personnel carried out high-voltage test on the I cutter brake of Langdong Line 1, and no abnormity was found [3].

Preventive Measures and Related Suggestions
First of all, there are currently 18 groups of the cutter brake of that model, and it is necessary to transform the single-arm vertical telescopic type into scissors type to prevent similar isolating cutter brake failures [4].Secondly, we should do a good job of upgrading the function of the isolating cutter brake, and jointly discuss and improve the three-phase current of the circuit with the D5000 system manufacturers.When the three-phase current difference is large, it is necessary to have the alarm function.Meanwhile, in the construction of a new generation of centralized control system in the future, it is required to be equipped with relevant alarm functions and issue analysis reports to avoid the operation and maintenance personnel from analyzing the reason for the failures [5].
Finally, in accordance with the 18 counter-measures, we should strengthen the quality supervision of the whole process of the cutter brake, do a good job of factory inspection and acceptance, and prevent the defective cutter brakes from leaving factory and being put into use.

Conclusion
In this paper, through the analysis of an abnormity treatment of the I cutter brake in 220kV substation, the author preliminarily analyzes that the reason of the abnormity is that the poor sealing in the conductive tube of the upper part of the cutter brake causes the moisture in the conductive arm and serious corrosion of the reset spring, resulting in failure of normal working.In the process of abnormity treatment, the professionals give rigorous and professional judgment, which shortens the time of power outage, ensures the reliability of power supply, and improves the working quality and efficiency.Meanwhile, the author puts forward the suggestions for the future operation and maintenance of GW22A-252D(W) Type III isolating switch and the preventive measures to prevent such incidents, which has accumulated valuable experience for the abnormity treatment of such equipment.In the next step, we will focus on the operation of the cutter brake of the same models in other substations.In addition, we will discuss and analyze with manufacturers and experts in related fields to avoid the recurrence of such abnormity [6].

Figure 2 .
Figure 2. The arc phenomenon and damage of the I cutter brake of Xinlang Line 1 of Langtou Substation.

Figure 3 .
Figure 3. Wave change east line II cutter A phase cannot be opened.The I cutter brake A phase of Langdong Line 1 of Langtou Substation can not be separated.

Figure 4 .
Figure 4. Disassembling structure of A phase cutter brake.

Figure 5 .
Figure 5.The insulation part of the clamping spring that fixed the contact finger was burned and damaged.

Figure 6 .
Figure 6.Insulation pads fixed on both sides of the contact fingers.

Table 1 .
and table 2 as follows.Insulation resistance measurement results.

Table 2 .
Measurement results of conductive circuit resistance.