Pit of medium voltage switchgear in foreign projects

Many foreign customers will request domestic manufacturers to provide prototypes for type testing abroad. If the products are provided completely according to domestic conventional testing methods, the probability of failure is very high.

17.5kV is the IEC standard voltage level used in power systems with voltages of 13.2/13.8/15kV. IEC 17.5kV metal enclosed switchgear, rated voltage 17.5kV, power frequency withstand voltage 38kV, lightning impulse withstand voltage 95kV. Compact indoor installed switchgear in accordance with IEC 62271-1, 62271-200, and 62271-100.

More and more foreign projects require 17.5kV switchgear. 24kV switchgear is commonly used as a substitute in China, which is not only 40% larger in size than conventional 17.5kV switchgear abroad, but also 30% more expensive (of the same product grade and quality) than 17.5kV switchgear abroad, and cannot meet customers' requirements for space occupation and price.

Replacing 17.5kV with 24kV is not a problem, but it is important to note the insulation withstand voltage level. The insulation withstand voltage parameters for technical response are not recommended to be filled in according to national standards. They can be based on the IEC standard 17.5kV power frequency 36kV BIL lightning impulse 95kV, or the IEC standard 24kV power frequency 50kV lightning impulse withstand voltage 125kV.

If according to the national standard of 24kV power frequency withstand voltage 65kV, this parameter is determined based on the neutral point ungrounded system, and the parameter value is relatively high, while in foreign countries, the neutral point firmly grounded system is generally used, and the power frequency withstand voltage value is relatively low. The 65kV power frequency withstand voltage test is not easy to pass, especially for many 24kV contact boxes and wall bushings that do not have internal and external voltage shielding. For high current copper bars and large contact diameters, the air gap is small, and the voltage division due to capacitance leads to excessive air gap division, resulting in high electric field strength and easy breakdown. Therefore, the 50kV power frequency withstand voltage is relatively easy to pass the test.

There is also a rated current. For 60Hz applications, a 50Hz continuous current test is required, with a margin of over 5% for temperature rise. In this case, it is not recommended to directly fill in the temperature rise parameters at 1.1 times the domestic current. Some customers may require a temperature rise test. If the temperature rise test exceeds the standard due to the 1.1 times rated current test, it is difficult to explain. Therefore, it is relatively easy to directly test at 50Hz according to the rated current and calculate the margin. The larger the current, the faster the temperature rise and heat generation will increase, making it difficult to pass the test.

Foreign projects generally require switchgear to have pressure release devices (ARS) and arc release systems. This is because the ceiling height of the switchgear installation room in foreign projects is generally relatively small, and more attention is paid to the safety of operation and maintenance personnel. Therefore, ARS can reduce the harm to the operators in front and behind the cabinet when internal arc faults occur, especially the pressure release channel, which directly releases the internal arc outdoors without causing financial damage to indoor personnel.

To meet this requirement, some designs install buffer compartments at the top of the cabinet and stack arc blocking plates inside. If there are too many arc blocking plates, the pressure release of the switch cabinet will not be smooth, which will in turn affect the switch cabinet door and cover plate. At this time, the pressure will be much higher than that in China without ARS testing, and the cabinet door will not be able to withstand it, resulting in opening.

In addition, if the customer does not require it, for switch cabinets that require an internal arc rating, especially those with a rating of 40kA or above, it is recommended to use bolt locks instead of conventional single handle door locks in China. Conventional single handle door locks cannot withstand the destructive force of internal arcs, so do not blindly believe that there is no problem and dig a hole for yourself.