Chapter 3: Q4MCQ (page 140)
An ideal transformer has no real or reactive power loss.
(a) True
(b) False
Therefore, the correct option is (a): True
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Figure 3.39 shows a oneline diagram of a system in which the three-phase generator is rated 300 MVA , 20kV with a sub transient reactance of 2.0 per unit and with its neutral grounded through a 0.4 V reactor. The transmission line is 64 km long with a series reactance of . The three-phase transformer is rated 350 MVA , 230/20kV with a leakage reactance of 0.1 per unit. Transformer is composed of three single-phase transformers,each rated 100 MVA , 127/13.2 kV with a leakage reactance of 0.1 per unit. Two 13.2kV motors and with a sub transient reactance of 0.2 per unit for each motor represent the load. has a rated input of 200 MVA with its neutral grounded through a 0.4 V current-limiting reactor. has a rated input of 100 MVA with its neutral not connected to ground. Neglect phase shifts associated with the transformers. Choose the generator rating as base in the generator circuit and draw the positive-sequence reactance diagram showing all reactance in per unit.
Consider the adopted per-unit system for the transformers. Specify true or false for each of the following statements:
(a) For the entire power system of concern, the value ofis not the same.
(b) The ratio of the voltage bases on either side of a transformer is selected to be the same as the ratio of the transformer voltage ratings.
(c) Per-unit impedance remains unchanged when referred from one side
of a transformer to the other.
For an ideal transformer, the efficiency is
(a) 0%
(b) 100%
(c) 50%
Using the transformer ratings as base quantities, work Problem 3.14 in per-unit.
A balanced Y-connected voltage source with is applied to a balanced-Y load in parallel with a balanced- load where and . The Y load is solidly grounded. Using base values of and, calculate the source current in per-unit and in amperes.
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