The turns ratio of a power transformer—the ratio of the number of turns between the high-voltage and low-voltage windings—directly determines whether its voltage transformation capability meets design requirements. The turns ratio test involves applying a test voltage to one side of the transformer under test, measuring the induced voltage on the other side, calculating the measured turns ratio, and comparing it with the nameplate rated value. Although this seemingly simple measurement actually serves multiple diagnostic functions: verifying correct winding turn counts, checking that tap changers are in the intended position, detecting inter-turn short circuits or connection errors, and assessing symmetry among three-phase windings.
From the perspective of standardization systems, the turns ratio test is not an isolated item. In GB 50150-2016, it is listed as the third test item for power transformers, immediately following direct resistance measurement and preceding winding connection group verification. This sequence is not coincidental: direct resistance reflects winding continuity, turns ratio reflects winding turn relationships, and connection group reflects phase relationships—forming a progressive diagnostic logic from “continuity” to “ratio” and then to “phase.” DL/T 596-2021 also includes turns ratio testing within the core framework of preventive transformer tests.
This paper systematically elaborates on the standard positioning, technical criteria, and engineering interpretation methods of turns ratio testing, primarily based on these two standards and supplemented by relevant provisions from IEEE Std C57.152-2013.
Standard Positioning and Purpose of Turns Ratio Testing
Position in Commissioning Tests
Clause 8.0.1 of GB 50150-2016 explicitly lists 15 test items for power transformers, with the third being “checking the voltage ratio at all taps.” This statement carries two implications: first, the test applies to “all taps,” not just the rated tap, meaning load-tap-changing transformers must be tested at each tap position; second, the nature of the test is “checking,” not merely “measuring,” indicating that the intent is verification against nameplate data to determine deviations.
The standard also specifies applicability of test items according to transformer capacity and type. Voltage ratio checks are required for oil-immersed power transformers up to 1600 kVA, dry-type transformers, converter and rectifier transformers, and electric furnace transformers. Notably, for phase-separate tap changers, the standard requires completion of the tap changer body inspection test before conducting the transformer turns ratio test, to confirm that each phase’s tap position is correct.
Position in Preventive Tests
DL/T 596-2021, as a guiding document for preventive testing in the power industry, lists turns ratio testing as a routine item for assessing the condition of operating transformers in Chapter 6 on “Power Transformers.” Unlike the “verification” role in commissioning tests, preventive turns ratio testing emphasizes “trend-based” judgment—by comparing results with previous test data or factory data, early signs of winding deformation can be detected.
Core Objectives of the Test
Integrating both standards and engineering practice, the objectives of turns ratio testing can be summarized into four levels:
Verifying design compliance. Confirming that the measured turns ratio matches the nameplate rating ensures the transformer has the expected voltage transformation capability.
Assessing tap changer status. Measuring the turns ratio at each tap position verifies proper contact of tap changer contacts and accurate position indication. If a particular tap shows abnormal ratio while adjacent taps are normal, the issue typically lies in contact or lead connections at that specific tap.
Detecting winding faults. Inter-turn short circuits directly alter the effective number of turns, causing a shift in the turns ratio. Connection errors (such as reversed polarity or disconnection of a phase in delta winding) will similarly manifest as abnormal ratios.
Evaluating three-phase balance. For three-phase transformers, the turns ratios of each phase at the same tap position should be essentially consistent; excessive deviation may indicate internal winding asymmetry or connection issues.
Based on Standard Turns Ratio Error Criteria
Voltage Level-Dependent Tolerance Limits
The allowable range for turns ratio error is the core criterion. GB 50150-2016 and related acceptance standards specify tiered requirements:
- Transformers with voltage level ≤35 kV and voltage ratio <3: allowable deviation ±1%
- All other transformers at rated tap: allowable deviation ±0.5%
- All other transformers at non-rated taps: not exceeding 1/10 of the impedance voltage value, and no more than ±1%
This tiered logic is noteworthy: a slightly relaxed tolerance of ±1% for low-voltage, small-ratio transformers accounts for their fewer turns—where a manufacturing deviation of ±1 turn has a relatively larger impact on the turns ratio, and strict ±0.5% could lead to unnecessary false alarms. Conversely, high-voltage, large-capacity transformers are subject to tighter ±0.5% limits because they have many turns, and early signs of winding deformation often first manifest as slight deviations in turns ratio.
In engineering practice, some references further differentiate: transformers up to 35 kV allow ±0.5% deviation, while those above 35 kV require ±0.25%. This stricter standard is commonly applied during handover acceptance, reflecting higher safety margins required for high-voltage equipment.
Three-Phase Balance Criteria
In addition to single-phase turns ratio errors, three-phase transformers must also be evaluated for balance among phases. IEEE Std C57.152-2013 specifies that differences in turns ratios between phases should generally not exceed 0.5%. For star-connected windings, individual phase measurements can be taken; for delta-connected windings, line-to-line turns ratios are measured.
Typical causes of three-phase imbalance include inter-turn short circuits in one phase, poor contact in a tap changer’s phase contact, or disconnection of a phase lead in a delta connection. Engineering experience indicates that if the turns ratio deviation of one phase exceeds twice that of the other two phases, a fault in that phase winding should be strongly suspected.
Additional Criteria for Tap Changers
DL/T 596-2021 specifies detailed requirements for turns ratio testing of tap changers: for load-tap-changing transformers, turns ratios must be measured at all tap positions; for no-load tap-changing transformers, measurements should be performed at least at the rated tap and an intermediate tap. If the variation trend of turns ratios across taps shows non-monotonic abrupt changes, the switching sequence of the tap changer should be checked for correctness, and contacts should be inspected for burnout or poor contact.
Comprehensive Interpretation of Test Results
Correlation with DC Resistance Testing
Turns ratio testing and DC resistance testing complement each other in diagnostic logic. DC resistance reflects the electrical continuity of windings and is sensitive to open circuits, poor connections, and welding defects; turns ratio reflects the winding turn relationships and is sensitive to inter-turn short circuits and wiring errors. The basic principle for combined interpretation is:
- Abnormal turns ratio + normal DC resistance → suspect inter-turn short circuit or incorrect tap changer position
- Normal turns ratio + abnormal DC resistance → suspect poor lead contact or welding defect
- Both abnormal → high likelihood of severe winding fault
Coordination with Connection Group Testing
Turns ratio testers typically feature automatic identification of connection groups. GB 50150-2016 lists “checking the three-phase connection group of transformers and polarity of single-phase transformer leads” as the fourth test item. Joint testing of turns ratio and connection group can be completed in a single wiring setup. A connection group error will directly result in turns ratio measurements deviating from expectations. For example, a transformer nameplate indicating Yd11 but actually wired as Yd1 would trigger a group error report by the tester, and the turns ratio values might also show abnormal patterns due to incorrect phase correspondence.
Consideration of Temperature and Test Conditions
Clause 3.0.6 of GB 50150-2016 stipulates that insulation tests should be conducted under favorable weather conditions, with the temperature around the test object and instruments not lower than 5°C and the relative humidity of the air not exceeding 80%. Although ratio testing does not fall within the scope of insulation tests, the slight effect of temperature on winding dimensions should not be neglected in precision measurements. For precise diagnostic purposes, it is recommended to record either the top oil temperature or ambient temperature to ensure traceability of data.
Post time: Oct-09-2026