Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 46

This is the system subtransient impedance Z″ representing the generating capacity of the utility at the POI. It also includes the impedance of the interconnecting transmission line. The source impedance is derived from the short‐circuit level at the plant as advised by the utility. The figure given will likely be based on a present and future generation planned by the utility. This value will be used as the base for determining the interrupting ratings of the plant circuit breakers that connect to the transmission line and the voltage regulation and capability of the plant large motors to start properly.

We have to determine the source impedance for two different extreme cases, the maximum and the minimum values, as follows:

 The maximum source impedance (minimum fault level) when the utility is operating on light load with a minimum generating capacity connected to the grid. This source impedance will be used for voltage regulation calculations and large motor starting duty. If the supply network is weak (low short‐circuit level), soft, or variable frequency starting may be required for starting large motors in order to satisfy the utility flicker requirement and to minimize the impact on other nearby customers connected to the grid.

 The minimum source impedance (maximum fault level) is when the utility is operating on high load with maximum generating capacity. This impedance will be used to determine the short‐circuit interrupting duty of the plant circuit breakers.

For our system studies and calculations, we will use MVA_b = 30 MVA figure as our per unit MVA base. This is the base rating of our main incoming transformers: 30/40 MVA, 230 to 13.8 kV.