Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 47

We received the conductor (name) information from the utility. It will be a single Hawk ACSR (aluminum conductor steel reinforced) conductor. The overhead conductors are symbolically called by the names of birds. The data for the Hawk conductor can be obtained from online sources. The best source of data for the conductors is the old T&D Westinghouse handbook, from which we find the following data:

 HawkType: ACSR, 477 kcmilStranding: 26/7Ampacity: 660 AResistance: 0.135 Ω/kmInductive reactance: 0.24 Ω/kmCapacitive reactance: 0.188 Ω/km

The Hawk line carrying capacity is well in excess of our plant requirements. Utilities like to build lines with sufficient capacity for future expansions. The maximum expected current from the plant at 230 kV is 100 A at 40 MVA. The line length from the plant to the local utility source of generation is estimated at 120 km.

 Calculate the line parameters in pu for the system studies:Now, calculate line characteristics in pu(2.1)

 Convert line characteristics (pu) from one MVA base to another:(2.2) (2.3)

 Convert line characteristics (Ω) from one system voltage kV base to another:(2.4) (2.5)

The utility “informed” us that fault level at our plant bus, projected for the future with possible expansion is 10 kA at 230 kV. This is calculated approximately as: MVA_sc = 4000 MVA.

Therefore, we can now calculate the source impedance at our 230 kV bus as follows:

Or pu source impedance if kA fault interrupting current from the source (power utility) is given:

This source impedance will be used in our computer studies to represent the utility at our plant point of interface. It is a conservative value that will provide plenty of margin in our calculation.

However, for our quick hand calculations and for our interrupting ratings of the switchgear, we will rate our equipment on a conservative basis of an infinite fault level from the utility (zero source impedance). Therefore, the utility can now be called an Infinite bus at the point of interface with our plant.

If we now calculate the fault MVA_sc from the Utility on our selected MVA_b base, it is: