By hvhipot 
Main Test Items (Core Functions)
A Transformer Test Bench typically integrates all or most of the following test functions:
1.Winding DC Resistance Test:
Purpose: To check the quality of winding connections, the contact integrity of tap changers, and for possible shorted turns.
Principle: Applies a direct current and measures the resistance.
2.Turns Ratio and Vector Group Test:
Purpose: To verify the voltage transformation ratio matches the design specifications, check the correctness of the winding turns ratio, and determine the transformer’s connection vector group (e.g., Dyn11, Yyn0).
Principle: Measures the voltages on the high-voltage and low-voltage sides simultaneously to calculate the ratio and phase difference.
3.Insulation Resistance Test (Absorption Ratio, Polarization Index):
Purpose: To evaluate the dryness and overall condition of the transformer’s complete insulation system.
Principle: Applies a high DC voltage and measures the leakage current flowing through the insulation to calculate the resistance.
4.Dielectric Loss Factor (tan δ) Test:
Purpose: A sensitive indicator to determine if the transformer’s insulation (mainly oil-paper insulation) is damp, aged, or deteriorated.
Principle: Measures the ratio of power lost in the insulating material under an AC electric field.
5.No-Load Loss and No-Load Current Test:
Purpose: To measure the magnetic performance and core loss (iron loss) of the transformer core, checking for defects in the core material, design, or manufacturing.
Principle: One winding is left open, and the rated voltage is applied to the other winding;
the losses and current are measured.
6.Load Loss and Impedance Voltage Test (Short-Circuit Test):
Purpose: To measure the copper losses in the windings, calculate the transformer’s operating efficiency and temperature rise, and obtain the crucial impedance voltage parameter.
Principle: One winding is short-circuited, and a current is applied to the other winding to bring it to rated current;
the losses and voltage are measured.
7.Power Frequency Withstand Voltage Test (Applied Voltage Test):
Purpose: To assess the strength of the transformer’s main insulation and verify its ability to withstand overvoltages in operation.
Principle: Applies an AC power frequency voltage significantly higher than the rated voltage for a specified duration to check if the insulation breaks down.
8.Induced Withstand Voltage Test (Inter-turn, Layer Insulation Test):
Purpose: To assess the strength of the transformer’s longitudinal insulation (insulation between turns, layers, and sections).
Principle: Applies a voltage at a frequency higher than twice the rated frequency to induce a test voltage that is a multiple of the rated voltage.
9.Partial Discharge Test:
Purpose: To detect the presence of partial discharges within the transformer, which is a critical sign of early-stage insulation deterioration.
Principle: Uses high-frequency sensors to detect tiny discharge signals within the insulation.