How to analyse the rootcause when switchgear cabinet accident?

Here we can check base on 1 actual example happend recently.

It's located in Longgang District, Shenzhen Dongjiang water pumping station, with 6 units in total ZS1 type armored metal switchgear (ZS1 switchgear) is selected for 10 kV switchgear, which is developed and provided with technology transfer by calor Emag schaltanlagen Ag company of Germany of ABB group and produced by Xiamen ABB switch Co., Ltd.

The switchgear accident is as below:

The switch cabinet is equipped with an excellent performance withdrawable VD4 Vacuum circuit breaker handcart, and the secondary circuit of the switch cabinet is equipped with ABB advanced and reliable control and protection elements.The main switch is connected with the busbar chamber and the cable chamber respectively through the static contacts in the static contact box. The static contact box is separated from each other and is cast with epoxy resin.The valve of the main switch is made of metal plate. When the main switch enters the working position from the test position, the valve opens automatically; when the main switch moves from the working position to the test / move position, the valve automatically closes and locks.

How the accident process was studied?

(1) Investigation before and after the accident.Before the switch cabinet was burnt out, two external line trips occurred. The staff of the pump station thought that it was caused by the unstable line voltage, so they switched on directly.Among them, after tripping, closing, tripping again and closing again, 

the tripping time interval is several hours, and the equipment smoke is found after the third trip.The switch did not trip, only external bus voltage loss trip.

(2) Field study.Push out the trolley switch and detect the bus insulation, the result is 0. 

What's the possible causes of failure?

According to the analysis, the accident is mainly caused by the insulation reduction of the switch cabinet. The reasons may be: 

(1), The insulation reduction caused by the aging of the insulation parts.

The bad environment and high pollution air have adverse effects on the insulation parts, especially the high temperature conditions, which are more likely to cause deterioration and aging of organic insulation parts, forming insulation weakness and causing accidents.

But if the accident happens, you need to have enough stock to replace parts in a certain date even it's not break out, AB Power can provide ABB parts , Bussmann medium voltage Fuse, and  other electrical parts in very short leadtime.

(2), Atmospheric overvoltage or switching overvoltage.

Under the action of over-voltage, such as the over-voltage caused by breaking off the motor and three-phase simultaneous breaking off overvoltage, the vacuum circuit breaker causes insulation breakdown and causes accidents.

(3), Operation not in accordance with the operating procedures or misoperation by operators.

(4), The locking mechanism fails.

(5), The contact surface of the main circuit is corroded or the connecting bolt is loose, resulting in heating.

(6), When small animals enter the switch cabinet or maintenance, objects are left in the cabinet.

(7), The insulation leakage distance of electrical equipment is too small.

The distance between conductor to ground and phase to phase in switch cabinet is too small,

 which makes it easy to cause accident in case of over-voltage.

(8) The contact is poor.

The surface of epoxy casting transformer and phenolic epoxy glass insulator is rough and easy to accumulate dust and damp.Sometimes creepage occurs under normal operating voltage, and finally develops into flashover accident.

(9) Insulation pollution flashover.

The pollution on the insulation components is affected by moisture, forming a dielectric film, and the leakage current increases accordingly. Under the adverse weather conditions such as rain, fog and thunderstorm, the porcelain bottle flashover and grounding accidents, namely pollution flashover, are caused.

  Fault analysis

According to the analysis of the field situation and the successful treatment of the fault, the possible factors causing the failure are eliminated, and the main cause of the fault can be obtained.

(1)The equipment insulation is reduced due to air humidity.

Rex Lake pumping station is located in the south, with humid and hot climate, and close to the reservoir. Although all switch cabinets are installed indoors, they are not completely isolated from the outside, so the switchgear is easily affected by the external climate conditions.When the accident happened, it was a rainy day, the air humidity was high, and the pump station was relatively humid. During the operation of the equipment, it was easy to be affected by moisture and produce condensation on the insulation surface, so that the attached pollution reduced the insulation.

In the end, the insulation of the external contact of Ť A and ා B ᦇ, and the insulated contact of Ť B ᦇ switch was tripped due to phase to phase insulation of Ť B and Ť B  switch, and the insulation of Ť A and Ť B ᦇ switch was broken due to phase to phase insulation.

(2) The operator made mistakes.

The operator did not carry out strict troubleshooting after the external line trip, so the external line was switched on; when the external line was powered on, the bus was switched on with load, and the equipment flash-over discharge was caused by repeated closing, which caused the short circuit accident and burned the switch cabinet.This kind of load operation will produce terrible high-temperature electric arc, which will seriously burn equipment and hurt people. The switching operation regulations in the electrical industry safety regulations are specially designed to prevent this situation.

Maintenance and rectification measures of switch cabinet

(1) The operation time of pump group is not much, the heat of electrical equipment is not enough, and it can not effectively remove moisture.Dehumidification equipment should be installed to keep the air dry.Formulate detailed operation and maintenance measures, and regularly check the insulation of switch cabinet.

(2) The humidity in Shenzhen is high, and the temperature fluctuation speed is fast and the range is large. The switch equipment is in danger of condensation in this climate condition.Therefore, when the switchgear is in standby and operating state, the heater should be put into operation all day long. Only when the actual load current of the switchgear reaches or exceeds 1250 a, the heater can be out of operation.

(3) The design of inlet switch protection and external line protection of pump house bus-bar does not match.In case of short circuit, only the external line switch trips, while the incoming line switch of pump house bus-bar does not trip.The fault filter of the incoming line switch protection device has detected over-current and quick break faults for three times, but no tripping command has been issued. This is because the tripping protection time is different from the actual demand, and the protection setting value should be smaller than that of the external line. Therefore, it is necessary to contact the design unit for correction.

Conclusion

Now there are many popular automatic correction systems, such as the use of global GPS positioning system, and the use of proximity switches to measure rigid legs and flexible

The action of the proximity switch on the side of the sexual leg first reflects the action of the door leg, and the real-time detection system of the cart composed of the total station and the single-chip microcomputer are also used in the actual operation, but the cost is too high or the fault points are too many.The scheme of measuring whether the two legs of gantry crane are parallel by distance measuring pull type sensor is simple, 

intuitive and with high accuracy. At the same time, the system not only has a vector closed-loop control system composed of frequency converter, motor and incremental encoder, but also regulates and protects the system by detecting the torque and speed signals fed back by the frequency converter,In this way, the accuracy and safety of the system are enhanced, and it is easy to operate and maintain.

 Frequent tripping of DC fast switch

In the actual operation, we found that the operational amplifier element in the over-current detection (composed of an integrated operational amplifier comparison circuit) is damaged, which makes the fault level of the over-current detection circuit output, resulting in the DC fast switch misoperation.

At first, the interval was longer, and then it became more and more frequent, In the end, the DC fast switch tripped immediately as soon as it was on.To deal with this fault, the operational amplifier can be replaced.

It is common that the parameters of simulation system change over time.In order to reduce the occurrence of faults, it is necessary to calibrate the parameters of the speed control system regularly according to the actual situation.