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PER UNIT SYSTEM OBJECTIVE TYPE QUESTIONS
Ans: (b)
Ans: (c)
Ans: (c)
Ans: (c)
Ans. None of the above
Per unit (pu) system is equivalent to normalizing the quantities use and has special importance in the area of power system and machines, which involve the use of transformers. The per unit system values can also be expressed as per cent values.
Pu value = actual value in any unit/ base or reference value in the same unit
How to choose base or reference values?
For a single machine or apparatus, the base value chosen is often the rated values of the machine and therefore in pu system their values become 1pu each. However, when several machine or transmission lines are involved the common value chosen should be such that the calculated pu values should not be too large or too small compared to unity. This helps in improving numerical accuracy in many involved computations.
How many independent base quantities could there be in a power system?
They could be two. Usually, base VA (or MVA_{b}), and base voltage (or kV_{b}).
Others are derived quantities, such as,
I_{b}, Z_{b}, R_{b}, X_{b}, P_{b} etc.
Base quantities in three phase circuits
The numerical values of base voltage or base current may differ depending upon whether it refers to phase quantities or line quantities, but pu quantities remain unchanged whether expressed on line to line basis or phase basis.
For three phase circuit s, one may define
Ib = VA_{b}/(Ö 3 V_{b})
Where VA_{b} is threephase value, V_{b} is line value and I_{b} is line current.
But base impedance is always defined on a per phase basis:
Z_{b} + V_{b} (LL) /(Ö 3 I_{b}) = Vph/Iph
= V_{b}^{2}/ VA_{b} = (base line voltage)^{ 2}/ base VA
= (Base line voltage in kV) ^{2 }/base MVA
Often the impedances of the transformers are calculated on their own ratings i.e. they have a pu value based on their own ratings and then may be required to be changed to another base value. In such cases,
Zpu (new) = Zpu (old) Vb(old) ^{2} VA_{b} (new)/[V_{b} (new)^{ 2}VA_{b} (old)]
Prove the above expression.
Inclusion of transformers in transmission circuit