Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 60

In our earlier discussion, we have proposed the voltage levels for the plant. Here, we will confirm the plant voltage levels for the primary and secondary plant distribution and the plant loads. The primary switchgear (Figure 2.8) is located at the main substation several kilometers away from the process plant with convenient routes to feed all the plant buildings with overhead distribution lines.

For the primary distribution, we can apply either 13.8 or 20, or even 33 kV voltages. The switching equipment and cables at 20 and 33 kV are considerably more expensive compared to 13.8 kV equipment. However, higher voltages may be needed if the distances are over 10 km and if there is a need for transfer of larger blocks of power. Since the project requires a large number of short feeders, it does not seem to be cost‐effective to distribute minor load at higher voltages.

The distances to the plant load centers are not large (<6 km) and 13.8 kV can be employed without significant voltage drops (<5%), which is to be verified by system studies.

Therefore, 13.8 kV voltage seems to be the most appropriate for the primary distribution throughout the site for this 40 MVA plant.

Figure 2.8 Plant MV switchgear.

We have already noted that a single transformer must be able to carry the full plant load of 40 MVA for an emergency condition of having one transformer out of service. The maximum incoming current without overloading from a 30/40 MVA transformer is 1675 A at 13.8 kV and 700 A at 33 kV. The standard 13.8 kV switchgear breakers go up to 4000 A. Forced air or water cooled breakers up to 5000 A are also available, but considered less reliable. Since we do not wish to have water complications in the switchgear, we can limit ourselves to lower breaker ampacities.

A good engineering practice dictates that breakers can be loaded to up to 80% of their nominal ratings. For instance 960 A for a 1200 A frame breaker. However, in case of an emergency like having a failure of one incoming transformer, the incoming breaker can be loaded closer to its nameplate rating.

Since the maximum incoming current at 13.8 kV is expected to be 1675 A (40 MVA), we can choose 2000 A incoming breakers. The breaker maximum loading is calculated to be 84% of the expected maximum rated current. The switchgear bus will be of the same rating, as the incoming breakers.

The most common types of breakers at 4.16 and 13.8 kV are vacuum type breakers.