Open Circuit and Short Circuit test of Transformer: A Complete Practical Guide !
Open Circuit and Short Circuit Tests are carried out during the Transformer Commissioning and when there is something wrong with the Transformer, like say the Transformer trips on some Internal Fault.
Open Circuit Test:
1) To ensure healthiness of winding,
2) to get Iron/no load/Constant losses of the Transformer
3) to verify the efficiency
4) to verify the voltage regulation
5) to verify the percentage impedance
Short Circuit Test:
1) To ensure healthiness of winding,
2) to verify the copper losses
3) to verify the efficiency
4) to verify the voltage regulation
5) to verify the percentage impedance
This test should be done before DC measurements of winding resistance to reduce the effect of the residual magnetism , otherwise Magnetizing current readings may be affected
Open Circuit Test (No-load Test) Procedure:
- Apply Three phase 4 wire Low voltage to HV Winding and note down the current flowing in the HV winding at No Load on all the Taps.
| Tap No. | HV Vtg.(V) | LV Vtg(V) | HV Current (mA) | ||||||
| VRN | VYN | VBN | Vrn | Vyn | Vbn | IR0 | IY0 | IB0 | |
| 9 | 220 | ||||||||
| 220 | |||||||||
| 220 | |||||||||
Interpretation of Test Results
- Unequal current shall indicate possible S.C. in winding.
- If the measured Exciting Current value is 50 times higher than the value measured during pre-commissioning checks, there may be a fault in the winding.
- If the results compared to previous tests taken under the same conditions vary more than 5%, there may be an internal fault which needs further analysis.
Short Circuit Test Procedure:
- Disconnect jumpers first then Apply Three phase 4 wire Low voltage to HV Winding and short the LV Winding using proper size cable and note current in LV and in the neutral.
- Also measure turret CT current on normal tap to determine its ratio connected.
| HV VTG. APPLIED=220 V | ||||||
| Tap No. | HV Current | LV Current | ||||
| IRSC | IYSC | IBSC | IrSC | IySC | IbSC | |
Interpretation of Test Results
1. If the readings in all the three phases are not equal and there is a flow of current in the Neutral, It indicates a problem in the Winding.
2. Short Ckt Current measured can be used to calculate the Percentage impedance for the concerned Tap and the measured Percentage Impedance can be cross-checked with the Percentage Impedance given on the NamePlate. Percentage Impedance is defined as Percentage of Rated Voltage of the Primary Winding required to be applied across the Primary Windings of the Transformer to cause Rated Full Load current to flow through this Winding when Secondary Winding is Shorted.
Z% = Applied Voltage X HV Full Load Current Short Sckt. Current X HV Rated Voltage
It controls extreme Fault current during faults.