Typical layout of American standard 27/38kV switchgear

Different switch cabinet standards and application habits result in different layout of the system's primary scheme. For conventional mid mounted cabinets, the primary scheme is relatively fixed. Similarly, for American standard switch cabinets, there are also conventional typical primary schemes to achieve different system functions. Unit modular design, flexible layout.

The typical layout of a 15kV switchgear allows for the installation of two auxiliary function handcarts in the upper (or lower) compartment, which can be either bus side or line side voltage transformers, power transformers CPT, or fuse handcarts.

The American standard 27kV air insulated metal armored switchgear meets the IEEE C37.20.2 standard. The 27kV North American voltage standard can also be used for Canada's 27.6/29.8kV, power frequency withstand voltage of 60kV, isolation fault 66kV, lightning impulse withstand voltage of 125kV, and isolation fault 138kV.

The current and breaking capacity of the American standard 27kV switchgear are the same regardless of size. The width of the switchgear is 36 inches and 914mm, and the phase spacing is generally 10 inches and 254mm.

It can be seen that the American standard 27kV switchgear and 15kV have the same design, with a double-layer circuit breaker scheme and a single-layer circuit breaker+auxiliary function handcart scheme. However, only one auxiliary function handcart can be installed, unlike 15kV where two functional handcarts can be installed on the upper (or lower) level.

The 27kV power transformer CPT and fuse handcart need to be separately arranged in the upper or lower compartment in front of the cabinet. Considering the large size and weight of the power transformer CPT, it is generally placed below.

The power frequency withstand voltage of the 38kV switchgear is 80kV, and the lightning impulse withstand voltage is 150kV. For metal valves, there is no need for isolation fracture withstand voltage testing. For non-metallic valves, the isolation fracture withstand voltage value is 1.1 times the standard value. Similarly, a 600V through type current transformer is installed on the primary contact box of the circuit breaker compartment. TOC and MOC are installed on the circuit breaker compartment without using a grounding switch. The rear cabinet is simple, with only the main busbar and copper bars connecting the incoming and outgoing lines.

It can also be a single-layer circuit breaker handcart with a single-layer voltage transformer compartment. For the power transformer CPT, a separate switchgear is generally required, with the fuse handcart installed in the front compartment of the cabinet and the power transformer CPT fixedly installed behind the cabinet.

The voltage transformer handcart is placed on the upper part of the circuit breaker handcart compartment and connected to the busbar through cables.

The instruments and indicator lights are installed on the door panel of the circuit breaker compartment. The front unit of the circuit breaker compartment has a depth of 12 inches, which means there is still a 300mm depth space in the front when the circuit breaker is in the open position. Secondary circuits such as terminals and control switches can be installed on both sides and the upper and lower parts, arranged together with the circuit breaker. Current transformers, circuit breaker handcart position auxiliary switches, circuit breaker opening and closing position auxiliary switches, etc. are all installed in this compartment to achieve local control and protection.

For power transformers, a fuse handcart is also required, installed in the front compartment of the cabinet. Due to high voltage and large insulation distance requirements, the entire cabinet needs to be used as the power transformer cabinet. As a device providing operational power, it needs to be on the incoming side, so it is led from the incoming cabinet to the adjacent cabinet through a cable. The lower end of the fuse handcart and the upper end lead out to the power transformer CPT fixedly installed behind the cabinet.

Compact and miniaturized 38kV circuit breaker compartment, upper voltage transformer compartment, and a 500mm high meter compartment can be left in the middle without the need for instruments to be installed on the door of the circuit breaker handcart compartment.