Design of American standard medium voltage switchgear

Product development requires a solid and serious approach. Firstly, the customer's demand is for authentic American standard and IEEE C37.20.2 standard metal armored switchgear. What is authentic is the world's first 15kV, 2-layer height, horizontal vacuum circuit breaker switchgear PowerVac developed by GE in 1976.

Afterwards, various American companies produced medium voltage switchgear according to this standard, and the IEEE C37.20.2 standard was also developed based on this product. That is to say, by this year, this switchgear is exactly 50 years old.

The American standard switchgear has its unique features, especially the highly integrated circuit breaker and its compartment. The primary and secondary are deeply integrated, and the current transformer is mounted on the contact box. The auxiliary switches for opening and closing the circuit breaker and the circuit breaker handcart position are installed on the circuit breaker compartment. The secondary device and instrument microcomputer protection are installed on the door of the circuit breaker compartment. One compartment is an integrated input and output circuit for operation, measurement, and protection.

Therefore, double-layer circuit breakers or voltage transformers and other functional handcarts, similar to low-voltage switchgear, have more outgoing circuits and more auxiliary functions in one switchgear.

The 15kV/27kV American standard switchgear is arranged with double-layer circuit breakers. Regardless of the current and short-circuit current specifications, the width of the switchgear is 36 inches and 914mm, with a phase spacing of 10 inches and 254mm.

The requirement for American standard circuit breakers is to increase the roller for circuit breaker handcarts to roll on the ground. The external dimensions of the primary contact, especially the high current 3000A contact, need to be small, and the circuit breaker needs to release the spring energy storage to extract the compartment.

The design of the circuit breaker compartment is the core, consisting of a static contact and a contact box. The through type 600V voltage transformer is mounted on the contact box, and the outer diameter of the contact box is required to be small. For 38kV, the phase spacing is generally 280mm, and the outer diameter of the contact box cannot exceed 176mm. However, the outer diameter of the 3000A primary contact is generally greater than 120mm. Therefore, the wall thickness of the contact box is only 28mm, and an inner and outer pressure equalization shielding cover must be used.

The 15/27kV switchgear can also adopt a single-layer circuit breaker handcart design, but the core functional compartment must meet the requirements of C37.20.2 standard. The C37.20.2 standard requires that there should be no ventilation holes between compartments, the thickness of partitions should not be less than 3mm, and the thickness of door panels and cover plates should not be less than 1.9mm. Large or heavy cover plates require hinges or lifting devices.

The protection level indoors is generally NEMA 1 IP2X. For ventilation holes, it is necessary to ensure sufficient distance from live objects such as busbars. Otherwise, small-sized mesh plates are needed to block them. The top of the cabinet needs to have dust-proof baffles.

For the arc cabinet, it can also be rated according to NEMA 1, simply quickly close the cover plate, open the ventilation during normal operation, and quickly close it when an internal arc fault occurs to prevent arc discharge.

The switchgear needs to have mechanical interlocking. American standard switchgear does not use grounding switches, so there is no corresponding mechanical interlocking. For example, for the rear cabinet door panel, there is no need to say that only the grounding switch can be closed to open the cover plate. In addition, there is no need for an electromagnetic lock with a live display to forcibly lock the rear cover plate. It only needs to be tightened with bolts based on tool opening.

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