Today’s Topics 1- The per unit system 2-Transformer Voltage Regulation
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1 Today’s Topics 1- The per unit system 2-Transformer Voltage Regulation 3- Transformer Efficiency4- Transformer Taps5- Transformer Rating and Related Problems
2 The Per Unit SystemIn power systems there are so many different elements such asMotors, Generators and Transformers with very different sizesand nominal values.To be able to compare the performances of a big and a smallelement, per unit system is used.In per unit system, each electric quantity is measured as a decimal fraction of some base levelPer Unit Value =Actual valueBase valuee.g. in a synchronous generator with 13.8 Kv as its nominal voltage,Instead of saying the voltage is Kv, we say the voltage is 0.9 p.u.
3 How Are the Base Values Defined For an electric element, we have : Power, Voltage, Current and ImpedanceUsually, the nominal apparent power (S) and nominal voltage (V)are taken as the base values for power and voltageThe base values for the current and impedance can be calculated
4 Example 1:A load takes 20 A at a power factor 0.9 lead when connected to a 600 Voltage. If the base voltage and apparent power are 600 and 6000 VA, determine the variables and impedance in per unit system
7 Example: Given the transformer in the previous example rated at 20 kVA, 8000/240, calculate the equivalent circuit, with per unit values
8 The equivalent circuit in per unit values Referred to the primary or secondary
9 Transformer Voltage Regulation Fact: As the load current is increased, the voltage (usually) drops.Transformer voltage regulation is defined as:
10 Other useful formulasPrefer small VRFor an ideal transformer : VR=0Note: If there were only resistive loads, the VR would not be much large in real transformers, but because loadsnormally have different power factors, VR may be considerably large in real transformers.
16 Transformer Taps The voltage in a distribution line is not constant. It may be 1.05 p.u. at generator terminal and 0.95 at the load side.Depending on the place the transformer is used, we may need to adjust the transformer ratio to get similar load voltage. That’s why we need TapIn Tap Changing Under Load (TCUL) transformer, the tap changes automatically to keep the output voltage constant
17 Transformer Rating and Related Problems Transformers have three major ratings:Voltage, Apparent power (current) and frequencyThe parameters of equivalent circuit are given in p.u.As long as a transformer is working under rated conditions, there should be (is) no problemWhat should we consider if we want to use a transformer under de-rating conditions?
18 Voltage ratingThe insulation has been designed to stand up to a particular voltage10% increase in voltage in the linear part of B-H curve would result in 10% increase in magnetization current, but in the saturation part, the magnetization current may be unacceptable
19 Frequency ratingExactly the same problem.If a 60 Hz transformer is going to be used in a 50 Hz system, the applied voltage should be reduced by a factor 5/6.Apparent power (current) ratingEach coil has limit for current magnitude and also the higher current results in higher power loss (which is converted to heat) and may damage the insulation. This has a dramatic effect on transformer life.If the voltage rating has been reduced, the apparent power should be reduced by the same factor.
20 Current InrushIf a transformer is switched to a system, there may be some current with large magnitude (Current Inrush), in the first few milliseconds after the switch is closed.The reason behind the current inrush is transient
21 TransientsTransient in electrical systems is the severe changes in current and voltage after a switch is closed or opened