Application Scope and Test Procedures of Transformer Ratio Tester

Transformer ratio tester is an important testing device in power systems, transformer manufacturing plants and maintenance departments. Its core function is to measure the voltage ratio (ratio), ratio error and connection group designation of transformers and instrument transformers.
Application scope
The application of transformer ratio testers covers the entire life cycle of transformers, including manufacturing, installation, maintenance and fault diagnosis.
Power transformers
Three-phase transformers: It can measure the ratio of various common connection groups such as Yyn0, Yd11, Dyn11, etc.
Single-phase transformers: It can measure the ratio and polarity of single-phase transformers.
Tap changer testing: It can test the ratio of on-load tap-changing transformers at different tap positions and check the accuracy and contact state of the tap changer.
Special transformers
Rectifier transformers: It can verify the phase-shifting angle and voltage transformation relationship of rectifier transformers with phase-shifting function.
Grounding transformers: It can measure the ratio of Z-type grounding transformers and verify the neutral point lead-out capability.
Electric furnace transformers: It can measure the ratio relationship on the low-voltage high-current side.
Instrument transformers
Voltage transformers: It can measure the ratio and polarity of electromagnetic voltage transformers.
Current transformers: Some ratio testers with corresponding functions can measure the ratio of current transformers.
Production and maintenance scenarios
New installation acceptance: After the transformer is installed, it verifies whether the winding turns ratio meets the design requirements and checks whether the windings have shifted or short-circuited during transportation.
Preventive tests: During regular maintenance, the ratio test is used to determine whether there are inter-turn short circuits, open circuits or poor contact in the windings.
Post-overhaul testing: After transformer core lifting maintenance, winding replacement or rewinding, the ratio test is used to verify the repair quality.
Fault diagnosis: When the transformer differential protection operates or there are doubts about internal faults, the ratio test is used to determine whether the windings are damaged.
Test procedures and judgment basis
Measurement principle
The common methods include low-voltage short-circuit, high-voltage application or reverse application. The ratio is calculated by measuring the induced voltage or current on the high-voltage side and the low-voltage side. Modern instruments mostly use the ratio bridge method or digital sampling calculation method.
Accuracy requirements
The accuracy of the instrument itself is usually 0.1 grade (error ±0.1%) or higher grades (0.05 grade).
Judgment basis for qualification
Rated tap: The ratio compared with the manufacturer’s nameplate data should have an allowable deviation of ±0.5%. For specific equipment such as voltage transformers, the requirements are more stringent.
Other taps: The ratio compared with the nameplate data should have a deviation within 1/10 of the impedance voltage value (%), generally ±0.5% or a slightly relaxed standard, and should comply with the tap position change rule.
Three-phase unbalance: The difference in ratios of each phase of three-phase transformers should generally not exceed 0.5%.
Connection group designation: The connection group designation displayed by the instrument should be completely consistent with the nameplate marking.