The transformer short-circuit load test, also known as the load-loss and impedance test, is a routine factory and field test that determines the load losses and short-circuit impedance of power and distribution transformers. Performed in accordance with IEC 60076-1, EN 60076-1, and GB/T 1094.1, the test provides critical data for efficiency evaluation, voltage regulation analysis, fault current calculation, and compliance verification against guaranteed values. This article explains the technical principles, sample types, measured parameters, test procedures, and practical applications of the short-circuit load test.
Technical Overview
The transformer short-circuit load test measures the load losses (copper losses) and short-circuit impedance of a transformer by simulating loaded operating conditions in a controlled manner. During the test, one winding (typically the low-voltage winding) is solidly short-circuited with a low-resistance conductor, while a reduced single-phase or three-phase voltage at rated frequency is applied to the other winding. The applied voltage is gradually increased until rated current flows through the windings. Under these conditions, the input power recorded by precision wattmeters or power analyzers equals the total load loss of the transformer, and the applied voltage expressed as a percentage of rated voltage is the short-circuit impedance (impedance voltage, uk%).
Because the excitation voltage is only a small fraction of rated voltage (typically 4% to 20% depending on transformer rating and design), the core is barely magnetized and no-load losses are negligible. The measured losses therefore consist of the DC resistance (I²R) losses of the windings, eddy-current losses in the winding conductors, and stray losses induced in the tank, core clamps, bushings, and structural steel parts. The short-circuit impedance obtained from this test is a fundamental design parameter used for calculating fault currents, setting protective relays, evaluating voltage regulation, and confirming that parallel-operating transformers are properly matched.
The test is standardized internationally and forms part of the routine test sequence for virtually every power and distribution transformer leaving the factory. Applicable standards include IEC 60076-1, EN 60076-1, GB/T 1094.1, IEC 60076-11 and GB/T 1094.11 for dry-type units, GB/T 6451 for oil-immersed power transformers in China, UL 1561 and UL 1562 for dry-type transformers in North America, and JIS C 4304 and JIS C 4306 for distribution transformers in Japan. Accredited third-party laboratories and manufacturer test bays alike perform this test to demonstrate conformity with contractual and regulatory requirements, including energy-efficiency limits such as those in GB 20052 and IEC 60076-20.
Testing Samples
Oil-Immersed Power Transformers
Generator step-up transformers, transmission interconnection transformers, and station transformers from 10 kV up to 500 kV and beyond are subjected to the short-circuit load test as a routine acceptance test. For very large units where the test bay cannot supply full rated current, the measurement is performed at a reduced current of at least 50% of rated value in accordance with IEC 60076-1, and the results are extrapolated to rated conditions.
Dry-Type Transformers
Cast-resin and open-wound dry-type transformers, tested per IEC 60076-11 and GB/T 1094.11, undergo the same test with reference temperatures adapted to the insulation class (for example, 120 °C for Class F). These units are widely used in commercial buildings, data centers, underground installations, and renewable energy applications where flammable oil must be avoided.
Distribution Transformers
Pole-mounted, pad-mounted, and compact substation transformers in the 10 kV to 35 kV class, including amorphous-alloy core energy-saving models, are tested in high volumes. Their load loss and impedance values directly determine energy-efficiency grades under regulations such as GB 20052, making accurate measurement commercially significant for manufacturers and utilities.
Special and Custom Transformers
Rectifier transformers, furnace transformers, traction transformers, isolation transformers for variable frequency drives, and autotransformers are also tested, often with impedance requirements specified for each winding pair or tap position due to their non-standard duty cycles and connection configurations.
Testing Items
Depending on the specification, contract, and applicable standard, the short-circuit load test determines the following quantities, all corrected to the reference temperature and referred to rated current and rated frequency:
- Total load losses, comprising winding DC resistance (I²R) losses, winding eddy-current losses, and stray losses in metallic structural parts
- Short-circuit impedance (impedance voltage, uk) expressed as a percentage of rated voltage at rated current
- The resistive component (load loss) and reactive component (leakage reactance) of the impedance
- Impedance and load loss at the principal tapping as well as at the extreme tapping positions, where required
- Rated impedance referred to a specified reference temperature (75 °C for oil-immersed transformers per IEC 60076-1; insulation-class-dependent temperatures for dry-type transformers)
- Correction data supporting total loss summation, total owning cost (TOC) evaluation, and energy-efficiency classification
Testing Methods
Test Circuit and Connection
The test circuit comprises a variable voltage source (regulator or dedicated test generator) at rated frequency, precision current and voltage transformers with accuracy class 0.1 or better, and low-power-factor wattmeters or a calibrated power analyzer. Because the power factor of the input under short-circuit conditions is very low, typically between 0.02 and 0.15, instruments must be specifically suited for low power factor measurement. One winding is short-circuited with a copper or aluminum busbar of ample cross-section, kept as short as possible to minimize additional resistance and reactance in the measurement loop. For three-phase transformers, either the two-wattmeter or three-wattmeter method is applied, and phase symmetry is verified before recording data.
Measurement Procedure
- Stabilize the transformer at essentially ambient temperature and measure winding DC resistance beforehand to establish the average winding temperature at the time of test
- Short-circuit the low-voltage winding (or the winding designated by the specification) with the low-resistance busbar
- Apply a low initial voltage to the supply winding and gradually raise it until rated current, or an agreed reduced current not below 50% of rated, flows through the windings
- Record applied voltage, current, frequency, and input power simultaneously once readings are steady
- Repeat the measurement for each required winding pair and each required tap position
Correction and Calculation
Measured load losses are separated into the DC resistance loss component and the stray loss component before temperature correction, because the two components vary with temperature in opposite directions. For copper windings, the resistance loss is multiplied by the ratio (235 + t2)/(235 + t1) and the stray loss by (235 + t1)/(235 + t2), where t1 is the winding temperature during the test and t2 is the reference temperature; the constant 235 is replaced by 225 for aluminum windings. Losses measured at reduced current are scaled by the square of the current ratio, while the impedance voltage is scaled linearly. The percentage short-circuit impedance is calculated as uk = (Us / Ur) × 100%, where Us is the applied voltage at rated current and Ur is the rated voltage of the excited winding.
Accuracy and Safety Considerations
All shorting connections must be mechanically robust and tightly bolted, since test currents can reach thousands of amperes; loose joints introduce errors and create burn hazards. Instrument transformer polarity and ratio must be verified, and cable routing should minimize stray coupling. The windings should not be energized long enough to heat appreciably; if repeated measurements are needed, winding resistance is rechecked to confirm temperature stability. Results are finally compared with guaranteed values, applying the tolerances of IEC 60076-1 or GB/T 1094.1, which typically allow up to +15% on load loss and on the order of ±10% on short-circuit impedance at the principal tapping, with wider tolerances at other taps.
Applications
The short-circuit load test delivers data that underpins transformer design, procurement, and operation across the entire product life cycle:
- Routine factory acceptance testing and type testing of new transformers before shipment
- Verification of contractual guaranteed loss and impedance values, including loss-capitalization and total owning cost evaluations by utilities
- Energy-efficiency classification and regulatory compliance under GB 20052, IEC 60076-20, and comparable national programs
- Short-circuit fault current calculation, protective relay setting, and switchgear rating verification in system design
- Voltage regulation and parallel operation studies, where matched impedances are essential for load sharing
- Condition assessment of service-aged or fault-exposed transformers, since a measurable shift in impedance can indicate winding deformation or displacement
- Research and development validation of new designs, including amorphous-core and high-efficiency product platforms
FAQ
What is the difference between the short-circuit load test and the short-circuit withstand test?
The short-circuit load test is an electrical measurement performed at rated current with reduced voltage to determine load losses and impedance. The short-circuit withstand test, governed by IEC 60076-5, is a type test that applies actual fault currents to verify the mechanical and thermal strength of the windings. The two tests serve entirely different purposes and should not be confused.
Why is only a small voltage needed for the test?
With the secondary short-circuited, the current is limited almost entirely by the internal leakage impedance of the transformer. Since typical impedance values range from about 4% to 20% of the base impedance, only the same small percentage of rated voltage is required to drive rated current through the windings.
Which winding should be short-circuited?
Conventionally, the low-voltage winding is short-circuited and supply is connected to the high-voltage winding, because this allows lower test currents and safer, more convenient voltage levels. The measured impedance is fundamentally a property of the winding pair and can be referred to either side.
At what temperature are the results reported?
Results are corrected to a standardized reference temperature: 75 °C for oil-immersed transformers per IEC 60076-1, and a value corresponding to the insulation class plus rated conditions for dry-type transformers per IEC 60076-11, such as 120 °C for Class F.
Can the test be performed in the field or at reduced current?
Yes. IEC 60076-1 permits measurement at a minimum of 50% of rated current with extrapolation to rated conditions, which accommodates both very large factory units and field diagnostics using portable test sets operated by accredited third-party laboratories.
How are the test results used in fault calculations?
The per-unit short-circuit impedance directly determines the prospective fault current: a transformer with 6% impedance will deliver a symmetrical fault current of approximately 1/0.06, or about 16.7 times rated current at its terminals, which is essential input for protection coordination and equipment rating studies.