Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 74

Earlier in this section, we chose the main (grid) transformers to have YNd1 winding configurations. We did not explain the reasons for our specific choice. Let us clarify it. The method of system grounding is a critical issue of every electrical installation and it must be defined for each voltage level. The transformer winding configuration plays an essential role in this matter. The system grounding (equipment neutrals) has evolved over the years by using different methods of high resistance to no resistance grounding. In the last 20 years, the industry has basically standardized on the following methods:

Transmission level, 69 kV and higher	Solid (zero resistance) → No fault limitation
MV 3.3–33 kV	Resistance → Limitation to 100 A
LV 400–600 V	Solid (zero resistance) → No Limitation
Generators, 5 kV and higher	High resistance → Limitation to 5–10 A

The zero resistance (solid) grounding is now officially called “effective grounding.”

In some parts of the world, the LV systems for manufacturing plants are left ungrounded or resistance grounded to allow the plant operation to continue, but to clear the fault at some convenient time. A phase to phase fault occurs if the plant receives another fault on a different phase. In that case, the affected part of the plant is tripped.

Brief reasoning for the above methods of grounding and standardization are given below. For further discussion on the above standardization (see Chapter 5).