Transformers Flashcards
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?
- LV (5-10% of rated voltage)
- Real power, voltage and rated current
- Applied Voltage
- Series (Impedance triangle) Ze = Z1 + a^2 Z2
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?
- HV
- Nothing (Open circuit)
- Rated voltage applied, real power and measured current
- Parallel, formula same as series but with fraction (shunt resistance)
Per unit test (1 phase)
- steps
- voltages are L-N or L-L?
- base units?
- Pick Sbase
- Pick a voltage base for each different voltage level
- Calculate the impedance base Z base = (Vbase)^2 / Sbase
- Calculate current base Vbase/Zbase
L-N voltage
3 phase per unit conversion?
- Impedance base
- Current base
refer to C6 Slide 37 & 38
L-L to L-N?
- ROOT 3
3 phase apparent power formula?
root 3 * VL * IL
Transformer purpose and benefits
- Transfers energy between different voltage levels
- Inexpensively change voltage levels in AC systems
Ideal transformer
No real power losses
No leakage flux
Magnetic core has infinite permeability
Secondary impedance effect on pri side
Z2’ = Z2 = a^2 * Z2
Primary impedance effect on secondary side
Z1’= 1/a^2 * Z1
Real power losses
Eddy current : change in core flux (heat)
Hysteresis : produces heat
Resistance parallel to ideal transformer
Series Impedance
Resistive loss and flux linkage
Loss in 2 windings
Found using SC test
Shunt Admittance
Iron core loss and magnetising susceptance
Overcurrent test
Per Unit Calculation Theory
Quantity in per unit = Actual quantity / base value of quantity
Actual = measured value
Base = rated value
Transformer can step-up or step-down the power between the primary and secondary sides
False. Transformers maintain the same power on both windings. Only step up/ step down the voltages according to internal windings
Ideal three-phase transformers have zero power loss
True
Open-circuit test is used to find the winding/series impedances of a transformer.
False. Open-circuit test is used to find the shunt admittance. For series impedance, short-circuit test is used.
Circulating current in a transformer core increases when using laminated core than single solid core
False
Circulating current in transformer core = eddy current
No lamination = higher eddy current
with lamination = low eddy currents
Per unit values are normalized to a common base, and are always dimensionless.
True
A transformer is inductive in nature
True
Nameplate data
gives rated voltages and power
Three phase transformer
- Y-Y , D-D
- Turns ratio
- Voltage and current in primary and secondary are in-phase
- Ratio of voltage and current follows the turn ratio of the transformer
Wye-Delta Transformer
- introduces magnitude variation and phases shift due to VL-L
Hysteresis Losses
- Proportional to area of BH curve and frequency
- Losses are reduced by using material with a thin BH curve
