Transformers

Question 1

Short Circuit Test
- HV or LV side?
- What is measured on the HV side?
- What is measured on the LV side?
- How to find equivalent impedance?

Answer 1

• LV (5-10% of rated voltage)
• Real power, voltage and rated current
• Applied Voltage
• Series (Impedance triangle) Ze = Z1 + a^2 Z2

Question 2

Open Circuit Test
- HV or LV side?
- What is measured on the HV side?
- What is measured on the LV side?
- How to find equivalent impedance?

Answer 2

• HV
• Nothing (Open circuit)
• Rated voltage applied, real power and measured current
• Parallel, formula same as series but with fraction (shunt resistance)

Question 3

Per unit test (1 phase)
- steps
- voltages are L-N or L-L?
- base units?

Answer 3

1. Pick Sbase
2. Pick a voltage base for each different voltage level
3. Calculate the impedance base Z base = (Vbase)^2 / Sbase
4. Calculate current base Vbase/Zbase

L-N voltage

Question 4

3 phase per unit conversion?
- Impedance base
- Current base

Answer 4

refer to C6 Slide 37 & 38

Question 5

L-L to L-N?

Answer 5

• ROOT 3

Question 6

3 phase apparent power formula?

Answer 6

root 3 * VL * IL

Question 7

Transformer purpose and benefits

Answer 7

• Transfers energy between different voltage levels
• Inexpensively change voltage levels in AC systems

Question 8

Ideal transformer

Answer 8

No real power losses
No leakage flux
Magnetic core has infinite permeability

Question 9

Secondary impedance effect on pri side

Answer 9

Z2’ = Z2 = a^2 * Z2

Question 10

Primary impedance effect on secondary side

Answer 10

Z1’= 1/a^2 * Z1

Question 11

Real power losses

Answer 11

Eddy current : change in core flux (heat)
Hysteresis : produces heat

Resistance parallel to ideal transformer

Question 12

Series Impedance

Answer 12

Resistive loss and flux linkage
Loss in 2 windings
Found using SC test

Question 13

Shunt Admittance

Answer 13

Iron core loss and magnetising susceptance
Overcurrent test

Question 14

Per Unit Calculation Theory

Answer 14

Quantity in per unit = Actual quantity / base value of quantity

Actual = measured value
Base = rated value

Question 15

Transformer can step-up or step-down the power between the primary and secondary sides

Answer 15

False. Transformers maintain the same power on both windings. Only step up/ step down the voltages according to internal windings

Question 16

Ideal three-phase transformers have zero power loss

Answer 16

True

Question 17

Open-circuit test is used to find the winding/series impedances of a transformer.

Answer 17

False. Open-circuit test is used to find the shunt admittance. For series impedance, short-circuit test is used.

Question 18

Circulating current in a transformer core increases when using laminated core than single solid core

Answer 18

False
Circulating current in transformer core = eddy current
No lamination = higher eddy current
with lamination = low eddy currents

Question 19

Per unit values are normalized to a common base, and are always dimensionless.

Answer 19

True

Question 20

A transformer is inductive in nature

Answer 20

True

Question 21

Nameplate data

Answer 21

gives rated voltages and power

Question 22

Three phase transformer
- Y-Y , D-D
- Turns ratio

Answer 22

• Voltage and current in primary and secondary are in-phase
• Ratio of voltage and current follows the turn ratio of the transformer

Question 23

Wye-Delta Transformer

Answer 23

• introduces magnitude variation and phases shift due to VL-L

Question 24

Hysteresis Losses

Answer 24

• Proportional to area of BH curve and frequency
• Losses are reduced by using material with a thin BH curve

Question 25

Insulation disks are used to allow the current to flow from the transmission line to the ground

Answer 25

False. Insulation disks are used to protect the line conductors to prevent leakage current from the conductors to the Earth

• Number of disks are dependent on voltage

Question 26

Phase shift: wye delta

Answer 26

-30 degrees
Wye leads delta

Question 27

VR NL

Answer 27

IR = 0
VR NL = VS/A