The fault analysis and countermeasures of switchgear

What is a switchgear?

Switchgear consists of one or more low-voltage switchgear and related control, measurement, signal, protection, regulation and other equipment, with the manufacturer responsible for all internal electrical and mechanical connections, a complete assembly of structural components together.The main function of switch cabinet is to open and close, control and protect electrical equipment in the process of power generation, transmission, distribution and electric energy conversion.The components in the switch cabinet mainly include circuit breaker, disconnecting switch, load switch, operating mechanism, mutual inductor and various protective devices.

The fault analysis and countermeasures of switchgear
12 ~ 40.5kV switchgear equipment is the largest number of substation equipment in the power grid system. In recent years, switchgear accidents have occurred frequently, resulting in economic losses, casualties and other bad social impact.
The hidden danger of accidents and inherent defects mainly focus on wiring mode, internal arc release capacity, internal insulation, heat and anti-mis-lock, etc. Through the formulation of targeted countermeasures, the number of switchgear and ring network cabinet accidents are greatly reduced, and the reliability of power network operation is steadily improved.

1. Hidden trouble in wiring mode
1.1. Hidden Danger type
1.1.1 The arrester in the TV cabinet is directly connected to the bus
According to the typical design specification requirements, TV ark arrester must be approved by gap handcart connection bus, TV rack position arrangement, connection mode and various, some TV ark arrester through isolation handcart connected to the bus, when the TV repair, away isolation handcart, lightning arrester is still charged, bring into the warehouse operation staff get an electric shock risk.The arrester in the TV cabinet mainly has the following wiring forms, as shown in Figure 2:

The switchgear connection mode is hidden

1, wiring mode one: TV cabinet lightning arrester and TV installed in the rear warehouse, fuse installed on the car, lightning arrester is directly connected with the bus, TV through the isolation hand and bus connected;
2, wiring mode two: TV cabinet lightning arrester installed in the bus room, directly connected with the bus, TV and fuse installed on the car;
3, wiring mode three: TV cabinet lightning arrester installed separately in the back warehouse or the front warehouse, directly connected with the bus, TV and fuse installed on the car.
4, wiring mode four: TV and fuse installed in the XGN series fixed cabinet compartment, arrester installed separately in another compartment, directly connected with the bus;
5, wiring mode five: lightning arrester, TV and fuse are installed in the rear warehouse, the lightning arrester is directly connected with the bus, TV is connected with the bus through the isolation hand car;
6, wiring mode six: lightning arrester, fuse and TV are installed in the same hand car, lightning arrester is connected to the fuse after the stage.
This arrangement belongs to the wrong wiring, once the fuse is broken in operation, the equipment will lose the protection of the arrester.

1.1.2 The lower cabinet of the switch cabinet is not completely isolated from the rear cabinet
The lower cabinets and rear cabinets of some KYN series switch cabinets, such as the main transformer in-line switch cabinets, female coupling switch cabinets, and feeder switch cabinets, are not completely isolated. When the personnel enters the lower cabinets, they may accidentally touch the bus or the live part of the cable head, resulting in electric shock.
The hidden danger is not isolated between the lower cabinet and the rear cabinet of the switch cabinet, as shown in Figure 3:

Figure 3 No hidden danger is isolated between the lower cabinet and the rear cabinet of the switch cabinet

1.2, countermeasures
The switch cabinet with hidden wiring mode should be reformed once.
The schematic diagram of switch cabinet wiring mode transformation is shown in Figure 5:

FIG. 5 Schematic diagram of transformation of switchgear wiring mode

1.2.1 Technical reform scheme for wiring mode of lightning arrester in TV cabinet
1, for the wiring mode one, remove the lightning arrester in the compartment, THE TV wiring mode is unchanged, the original bus room through the wall hole is blocked, the lightning arrester is converted into a fuse arrester hand car on the hand car, and the lightning arrester is in parallel with the fuse and TV circuit.
2. For wiring mode two, remove the lightning arrester in the bus compartment, move the lightning arrester to the mobile car and refit it into a fuse and lightning arrester, add the installation plate of the lower contact box, the baffle of the contact box and the valve mechanism, install the TV in the back bin, and connect it to the lower contact of the isolation hand car through the lead.
This scheme can be implemented on the original hand car, but also can consider replacing the new hand car.
3. For the wiring mode three, remove the lightning arrester of the original compartment, move the lightning arrester to the mobile car and refit it into a fuse and lightning arrester, close the wall hole of the original bus room, add the installation plate of the lower contact box of the hand car, the baffle of the contact box and the valve mechanism, install the TV in the back compartment and connect it to the lower contact through the lead wire.
This scheme can be implemented on the original hand car, but also can consider replacing the new hand car.

4. For wiring mode four, remove the arrester in other compartmentalparts, move the arrester to the fuse and TV compartmentalparts, connect it to the disconnecting switch break, and connect it in parallel with the fuse and TV circuit.
5, for the wiring mode five, lightning arrester, TV installation position unchanged, the original lightning arrester lead is directly connected to the isolation hand car contact, the original bus room through the wall hole.
6. For the connection mode 6, the layout mode belongs to the wrong connection. Once the fuse is fused in operation, the equipment will lose the protection of the arrester.
Remove the lightning arrester and fuse in the original hand car, change the wiring position, make the lightning arrester connected to the superior of the fuse, and in parallel with the fuse and TV circuit.

1.2.2 Precautions for incomplete isolation between the lower cabinet and the rear cabinet of the switch cabinet
Because this kind of switch cabinet product structure is fixed, if the partition plate is installed in the transformation, its internal structure form and space distribution will be changed, and the internal protection performance of the product can not be guaranteed. Therefore, it is necessary to confirm the main transformer 10kV side maintenance and the main transformer switch maintenance before the work can be carried out.

2. Insufficient internal arc release capacity
2.1 Types of hidden dangers
In the actual operation, the metal closed switch cabinet itself has defects, coupled with the bad operating conditions caused by insulation performance deterioration or misoperation and other reasons, will cause the internal arc fault.
The arc generated by short circuit has high temperature and large energy. The arc itself is a very light plasma gas. Under the action of electric power and hot gas, the arc will move at high speed in the cabinet and cause the rapid expansion of the fault range.
Gasification in this case, insulation materials, metal melting, switch cabinet internal temperature and pressure surge, if it is not designed or installed qualified pressure release channel, great pressure will cause the cabinet put oneself in another's plate, door plank, hinges, window serious deformation and fracture, arc produced by high temperature air cabinet put oneself in another's position, cause near equipment operation maintenance personnel serious burns,
Even life-threatening.
At present, there are some problems such as no pressure relief channel is set, unreasonable pressure relief channel is set, the internal arc release capacity is not tested and verified, and the assessment is not strict during the test.

2.2, countermeasures
[Selection] Switch cabinet internal fault arc performance should be IAC level, internal arc allowed duration should not be less than 0.5s, the test current is rated short-term withstand current.
For products with rated short circuit breaking current above 31.5kA, internal fault arc test can be carried out according to 31.5kA.
[Modification] Add or change the pressure relief channel, and conduct the internal arc test and verification in strict accordance with the type test standard requirements.
[Protection] Appropriate compression of the main transformer protection level difference, reduce the continuous failure time of fault arc.

3, internal insulation problem
3.1 Hidden danger type
In recent years, the volume of switch cabinet products has been reduced, the insulation performance of the cabinet defects, faults increased.
Main performance in: climb distance and air clearance is not enough, especially the hand cabinet, now many manufacturers in order to shorten the size of the cabinet, greatly reduce the circuit breaker installed in the cabinet, isolation plug and the distance between the ground, but did not take effective measures to ensure the insulation strength;
Poor assembly process, due to poor assembly quality, a single component in the switch cabinet can pass the pressure test, but the whole switch cabinet can not pass after assembly;
Contact capacity is insufficient or bad contact, when the contact capacity is insufficient or bad contact, local temperature increase, insulation performance decline, cause to the ground or phase flasticity;
Condensation phenomenon, the built-in heater is easy to damage, can not work normally, in the switch cabinet condensation phenomenon, reduce the insulation performance;
Poor insulation performance of supporting accessories.
In order to reduce the cost, some manufacturers adopt lower insulation level of supporting accessories, reduce the overall insulation performance of the switch cabinet.

3.2, countermeasures
We should not blindly pursue the miniaturization of switchgear. We should buy appropriate switchgear according to the project situation, substation layout, operation and maintenance, equipment overhaul and other factors.
For equipment using air or air/insulating material as insulating medium, the thickness, design field strength and aging of insulating material should be considered, and the manufacturer should conduct condensation test according to standard requirements;
For the parts such as the wall sleeve in the switch cabinet and the ring network cabinet, mechanical valve, and the bend of the bus bar, if the net air insulation distance is less than 125mm (12kV) and 300mm (40.5kV), the conductor should be equipped with insulation sheath.
Measures such as chamfering and polishing should be taken to prevent electric field distortion in the parts where the field strength is concentrated, such as the inlet and outlet bushing, mechanical valve and the corner of the bus.
The busbar in the cabinet supports some equipment whose insulation crawl distance can not meet the antifouling conditions, such as porcelain bottles. Spray RTV insulation coating to improve the technical conditions of the old equipment operation.

4. Fever defect
4.1 Types of hidden dangers
Loop connection point contact is bad, contact resistance increases, heating problem is prominent, such as poor contact isolation contact;
Metal armored cabinet vent design is not reasonable, air is not convection, heat dissipation ability is poor, heating problems in the cabinet are more;
Wall casing, current transformer and other installation structures form electromagnetic closed loop, resulting in eddy current, causing some insulation baffle material heating phenomenon is serious;
Partial closed switch cabinet dry equipment (cast type current transformer, cast type voltage transformer, dry type transformer) selected winding wire diameter is insufficient, casting process control is not strict, easy to overheating damage.
4.2, countermeasures
Strengthen the heat dissipation of the switch cabinet, and install blower and induced draft fan;
In combination with power failure, the contact pressure of dynamic and static contacts should be checked and replaced if necessary. At the same time, the fatigue contact spring should be replaced.
Increase the research on temperature measurement technology inside the cabinet, and apply new technologies such as wireless temperature measurement to solve the difficult problem of temperature measurement.

5, prevent error locking is not perfect
5.1 Potential hazards
Most of the switch cabinets are equipped with anti-error locking device, but its comprehensive and mandatory anti-error locking does not meet the requirements.
Part of armored switch cabinet on the rear door can be opened, no mistake-proof locking, no double isolation baffle, opened after the live parts can be directly touched, and the screws are ordinary hexagonal screws, easy to open the door into the live cabinet electric shock accident;
Part of the main transformer, female, TV, the transformer and other switch without grounding switch, after the following cabinet door and grounding switch did not form a mechanical lock, can remove the screw directly open after the door, not closed in the case of the door can also close the power, easy to cause maintenance personnel mistakenly open, enter the electric interval, personnel shock accident;
The upper and lower parts of the back door of some switch cabinets cannot be locked independently, and the upper door is locked by the lower door.
When the outlet grounding switch is closed, the lock of the lower cabinet door is removed, and the rear cabinet door can also be opened, easy to cause electric shock accident.
Such as KYN28 switchgear;
After some switchgear handcars are dragged out, the insulation isolation block can be easily pushed up. Without preventing accidental locking, the charged body is exposed, and the staff is prone to open the static contact valve baffle of the switch by mistake, resulting in electric shock accident.

5.2, countermeasures
For the switch cabinet anti-error function is not perfect, for the back of the cabinet door can be opened, and open can directly touch the live parts of the high voltage switch cabinet installed mechanical padlock, configure computer anti-error program lock locking;
Install interlock between ground switch and rear cabinet door on switch cabinet such as GG1A and XGN, and install live display device to lock ground switch operation.
Check the reliability of the anti-error device regularly, and check the mechanical latching device between the handcar and the grounding switch, the disconnecting switch and the grounding switch by the opportunity of power failure.

6, the concluding
Switch cabinet equipment is an important primary substation equipment in the power grid. In order to ensure its stable operation, the control should be strengthened in all aspects such as design, material, process, test, type selection, operation and maintenance.
In strict accordance with the typical design requirements, combined with national and industrial standards, put forward design technical requirements, fundamentally eliminate wiring hidden dangers;
According to the national and industrial standards, as well as anti-accident measures, formulate strict requirements of equipment bidding documents, to prevent unqualified products into the network operation;
Strengthen on-site manufacturing supervision, strictly witness the key points of production and factory test, and resolutely forbid the unqualified products from leaving the factory;
Actively carry out switch cabinet defect management, strengthen the implementation of anti-accident measures;
Improve the switch cabinet anti-error function, strengthen the management of anti-error locking device, install the live display device, and cooperate with the "five prevention" system, to ensure the comprehensive and mandatory anti-error locking.