Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 63

There is a correlation between the rating of the switchgear incoming breakers and the incoming transformers ratings.

For an even larger plant primary distribution system, the use of 33 kV would be likely, with the transformer incoming and bus tie breakers rated at 2000 A. These 33 kV breakers can accommodate 60/80 MVA, ONAN/ONAF incoming transformers at 80% of their breaker nameplate ratings. The switchgear bus can also be selected as 2000 A, while the plant feeders to the individual plant load centers will be any of those available: 1200, 1600, or 2000 A breakers. In this instance, we can standardize on a continuous rating of 1200 A, which can be loaded up to 960 A (55 MVA at 33 kV), i.e. 80% of the breaker nominal rating and 100% of the breaker nameplate rating for a temporary emergency usage.

Here is the switchgear for a large plant either at 13.8 or 33 kV, metal‐clad or 33 kV gas insulated switchgear (GIS) with vertical sections having single breaker positions. In summary, the two switchgear assemblies are rated as follows:

Nominal voltage	13.8 kV	33 kV
BIL, indoors/outdoors	95 kV/110 kV	170 kV
Phasing	3 ph, 3 w	3 ph, 3 w
Interrupting capacity	40 kA r.m.s. symmetrical	31 kA r.m.s. symmetrical
Construction	Metal‐clad	Metal‐clad or GIS
Incoming and tie breakers	2000 A	2000 A
Normal operation, incomers	37.6 MVA at 80%	72 MVA at 80% rating
Maximum loading, incomers	47 MVA at 100%	92 MVA at 100% rating
Feeder breakers	1200 A	800, 1200 A

The breaker 2000 A rating for the incoming and tie breakers at the 13.8 kV switchgear is more than adequate for both normal and emergency duty. The branch feeders on the switchgear are usually positioned in such a way to minimize the power flow across the bus tie breaker.

Evidently, 13.8 kV voltage can be used for the plants up to 50 MVA. Certainly, 20 or 33 kV voltage can be equally applied and used for this 40 MVA plant as well as in even larger industrial facilities as the primary plant distribution voltage. The economies of these options would have to be worked out during the design phase of the project, when the largest plant loads are confirmed.