05/10/2026
Interview Technical Question: Power Transformer Stability Test
Question: How is a Stability Test performed on a power transformer on-site, and what is the purpose of this test?
Answer & Technical Explanation:
Test Procedure: The stability test on power transformers is conducted by applying a short-circuit across either the High Voltage Side (HV Side) or the Low Voltage Side (LV Side) windings, while injecting a low test voltage (typically 380 Volts AC) from the opposite side of the transformer.
Purpose of the Test: The main objective of this test is to verify the correct polarity and wiring direction of the Current Transformers (CTs) connected to the Differential Protection Relay. This ensures that no false or unwanted tripping occurs when the transformer is initially energized or subjected to initial loading.
Practical Engineering Example: To clearly illustrate this, let us consider a practical example featuring a 50 Megavolt-Amperes (50 MVA) power transformer with a rated voltage ratio of 132 kilovolts by 13.8 kilovolts (132 / 13.8 kV). In this setup, we will inject the voltage through the Low Voltage Side (LV Side) and apply a short-circuit across the High Voltage Side (HV Side).
Nameplate Data & Impedance (Z PU): From the transformer Nameplate, we can extract the short-circuit impedance percentage (Z percentage) for different tap positions:
At Tap Position 11 (representing the middle/nominal tap): High Voltage Side = 132 kV, Low Voltage Side = 13.8 kV, and Impedance (Z percentage) = 21.6 percentage.
At Tap Position 1 (representing the first tap): High Voltage Side = 148.5 kV, Low Voltage Side = 13.8 kV, and Impedance (Z percentage) = 23.1 percentage.
At Tap Position 27 (representing the final tap): High Voltage Side = 105.6 kV, Low Voltage Side = 13.8 kV, and Impedance (Z percentage) = 19.6 percentage.
Step-by-Step Current Calculations for Tap 11: We will now calculate the expected currents at Tap 11 of the power transformer, which are expected to be read on the differential protection relay. (An Excel Sheet is attached at the end of the article containing all calculation formulas where you only need to enter the transformer data, but knowing these important formulas is essential):
Per-Unit Voltage (Vpu): Voltage Per-Unit equals applied voltage divided by rated Low Voltage Side voltage. Expressed as: Vpu = 380 divided by 13800 equals 0.0276 PU.
Per-Unit Current (Ipu): Current Per-Unit equals Per-Unit Voltage divided by Per-Unit Impedance. Expressed as: Ipu = Vpu divided by Zpu equals 0.0276 divided by 21.6 percentage equals 0.128 PU.
Current at Low Voltage Side (IF LV): Low Voltage Side Fault Current equals Per-Unit Current multiplied by rated Low Voltage Side full load current. Expressed as: Current at Low Voltage Side IF = 0.128 multiplied by 2091.85 equals 267.8 Amperes.
Current at High Voltage Side (IF HV): High Voltage Side Fault Current equals Low Voltage Side Current multiplied by the ratio of Low Voltage Side to High Voltage Side rated voltages. Expressed as: Current at High Voltage Side IF = IF multiplied by LV SIDE divided by HV SIDE equals 267.8 multiplied by 13800 divided by 132000 equals 28 Amperes.
Field Ex*****on & Safety Guidelines:
It is expected to repeat the previous steps for Tap Position 1 and Tap Position 19 (or Tap 27) and add them to the Test Sheet.
We must select appropriate cables capable of withstanding these high currents. If a suitable high-current cable is not available, the short-circuit can alternatively be applied on the Low Voltage Side while injecting from the High Voltage Side.
During each test step, we must record the Differential current reading and ensure it is strictly zero, alongside recording the phase currents on both transformer windings.
Reference Tool Link: Excel Sheet used in the test: https://lnkd.in/dGHC68pf
