Due to the different ordering technical requirements and standards, especially the different transformation ratio combination requirements of current transformers, the setting value of switch action current cannot be fully considered during installation, resulting in the mismatch between the switch setting and the current of the protected line. The consequences are as follows:

① When the action current value selected by the switch is less than the maximum load current of the line, it is easy to cause misoperation of the line switch;

② When the action current value selected by the switch is large, it cannot form a ladder coordination with the relay protection setting value of the substation, which is easy to cause a skip trip, expanding the line patrol range and increasing the difficulty of fault finding.

③ When adjusting the protection action value of the switch on the column, it is difficult to adjust due to the limitation of its structure and installation position, power failure disassembly and adjustment, using more machinery, manpower, and long power failure time, resulting in reduced power supply reliability.

1. Status Investigation

According to the requirements of State Grid Corporation of China, the power supply reliability rate of 10kV distribution lines is 99.75%. The principle of our bureau's high-quality service is to avoid large-scale blackouts and ensure reliable power supply. We have investigated the following aspects:

Investigation I: According to the investigation of the vacuum switch (ZW8 type) installed on the 10kV line in the county, the secondary lead of the vacuum switch current transformer is usually equipped with 600/5, 400/5 and 200/5 taps. The purpose of adjusting the protection setting value is achieved by changing the terminal position in the switch operation box; When the user has no requirements, the factory protection setting value is 600A. As the vacuum switch (ZW8 type) can be pulled and closed with load, and the primary side is equipped with an isolation switch, the operation is stable and reliable, and there are obvious disconnection points during power outage maintenance to ensure the safety of the maintenance line. Therefore, it is widely used in 10kV distribution lines.

Conclusion: ZW8 vacuum switch has superior performance and can replace other outdoor switches.

Investigation 2: Through the investigation of the 10kV lines under our jurisdiction, we found that at present, there are 65 vacuum switches operating in 47 10kV lines of our bureau. Only some line switch protection devices in the east and west urban areas can operate correctly, with a reliability rate of only 60%. The reliability rate of 10kV line vacuum switches in other villages and towns is almost zero, which can only be used as load breaking and closing under normal conditions, without the purpose of protecting the lines.

Conclusion: Most vacuum switches installed on the circuit can not play their due role, and the action reliability is extremely low.

2 Cause analysis

We found out from various aspects that the main factors affecting the low operation reliability of 10kV line vacuum switch are:

2.1 The fixed value of vacuum switch protection is too large: confirmed through field investigation. QC team members found that the 65 switches installed in the county were all 600/5, 400/5 and 200/5 switches of the same model and large protection transformation ratio. Except for the urban areas, most of the switch settings were inconsistent with the actual needs, and no statistical calculation was made according to the protected lines and load conditions, and the switch settings were set, which caused that the vacuum switch protection settings were too large to cooperate with the substation protection ladder, resulting in low reliability of the switch action.

2.2 The technical parameters of the switch purchase are unreasonable: after investigation and analysis, it is confirmed that some line vacuum switches are rural power grid transformation projects, and the selected conductors, transformers and vacuum switches are designed and tendered according to the long-term long-term long-term long-term load planning, and some are purchased with funds raised by the county bureau. In particular, when ordering, the actual protection setting range of the protected line was not indicated, the setting value of the future switch action current, the maximum load current of the controlled line operation and the actual short-circuit current of the grid under the maximum and minimum operation modes were not fully considered, which resulted in unreasonable technical parameters for the purchase of switches, resulting in the mismatch between the switch setting value and the short-circuit current of the actual protected line, As a result, the switch action reliability is low.

2.3 Inconvenient adjustment of switch protection settings: after investigation, analysis and confirmation, it is found that the current adjustment of various outdoor 10kV vacuum switch protection settings is achieved by changing the tap of the secondary coil of current transformer. Since the tap terminal is installed in the box, once installed on the tower, its base bracket is inclined to the outside of the tower and can not stand, so it is inconvenient to open the box on the tower and there are many wire terminals, The adjustment method is not obvious. In the absence of instructions, ordinary personnel cannot adjust correctly. When adjustment is needed, the power supply reliability is reduced due to power failure, disassembly and adjustment, use of machinery, more manpower and long power failure time.

The secondary factors affecting the low operation reliability of 10kV line vacuum switch include:

2.3.1 Inadequate personnel business training: according to the training plan, our bureau timely conducted business training for rural power system employees. 2.3.2 Insufficient lubrication of transmission part: wipe dirt to ensure good external contact of switch and flexible mechanical moving parts. 2.3.3 The switch position is unreasonable. 2.3.4 The operating environment of the vacuum switch is dusty: the environmental dust of the fully enclosed vacuum switch device has no impact on the switch action. Add lubricating oil to key parts and clean them in time

3 Countermeasure implementation

Implementation I: Improve the adjustment method of switch protection setting value.

According to the voltage regulation structure and principle of 10kV transformer, we have formulated the following improvement scheme: connect the secondary tap to the rotary switch and install it at the bottom outside the box.

That is, the secondary tap wire of the current transformer in the original operating mechanism box is connected through a rotary switch, installed at the outer bottom of the operating box, and marked with three setting positions (100/5, 50/5, 30/5)

Implementation effect: the modified vacuum switch protection setting value can be adjusted intuitively and conveniently, and can be easily implemented on the tower when the protection setting value needs to be adjusted. It saves time and labor, reduces power outage time, and improves power supply reliability.

Implementation II: Technical parameters of switch procurement are not uniform

The statistics of vacuum switches to be installed on 10kV lines in the county are made, and the basic statistical data are provided by each power supply station; According to the current situation of each line load and considering the future load development, the rural power work department will make a site investigation to determine reasonable technical parameters, correctly select the switch model and capacity, so that the adjustment range of switch protection setting value matches with the actual situation, and ensure that the switch action is correct and reliable in case of line fault.

Implementation effect: fundamentally solve various refusal phenomena of vacuum switch caused by unreasonable switch technical parameters.

Implementation 3: The switch protection setting is too large

As the vacuum switch protection setting value is too large, the load current of the line under its jurisdiction shall be statistically calculated every quarter, and the protection setting value of the line shall be calculated according to the high and low load operation of each line, and then the notice of vacuum switch protection setting value shall be issued according to the calculation results.

Implementation effect: each power supply station timely adjusts the protection setting value of vacuum switch according to the protection setting value adjustment notice, which eliminates the vacuum switch inaction caused by excessive switch protection setting value, and ensures the reliable and accurate action of vacuum switch.

4 Effect inspection

According to the actual needs of some lines, we have planned to install 27 improved ZW8 vacuum switches of protection setting external adjustment type for ten power supply substations, including Siwan, Jingguan, Dashiqiao, Shengwan, Maotang, etc. Through nearly four months of operation practice, the effect is good, and the reliability rate of vacuum switch action reaches 99.9%.