Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 66

A simplified one‐line diagram of one of the LV switchgear is shown in Figure 2.10. It is mostly used to distribute power to plant MCCs. The switchgear breakers are rated from 800 to 3200 A frame sizes. The breaker trip unit can be lower than the frame sizes to suit the load. This is shown as 800AF/600AT, as frame and trip rating for the breaker. L and S are the characteristics for the long and short breaker sensor trip element, respectively.

Figure 2.10 Part of LV switchgear.

The common LV switchgear voltages are as follows:

 In USA: 480 V, 3 ph, 60 Hz. The lowest voltage used is 120 V, 1 ph.

 Canada: 600 V, 3 ph, 60 Hz. The lowest voltage is 120 V, 1 ph.

 Europe, Asia, South America, Australia: 400 V, 3 ph, 50 Hz. The lowest plant voltage is 231 V, 1 ph, which is the line to ground voltage from 400 V, 3 ph.

These voltages are used to feed low voltage, three‐phase motors up to 200 kW, and auxiliary feeders.

Smaller motors are fed at 1 ph, 120 V in USA/Canada, or 231 V in the IEC countries.

Larger motors up to 500 kW can also be fed at LV (400–600 V), if controlled by VFDs or SoftStarts (see Chapter 15).