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
Insulation disks are used to allow the current to flow from the transmission line to the ground
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
Phase shift: wye delta
-30 degrees
Wye leads delta
VR NL
IR = 0
VR NL = VS/A
