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Transformer Turns Ratio Test: Some Unknown Facts

Transformer Turns Ratio Test: Some Unknown Facts

NETA, Powertest Conference 2020

Transformer Turns Ratio Testing: Lesser known facts that are affecting your Results

Overview

Transformer Turns Ratio (TTR) Measurement is a diagnostic field test performed on a transformer to assess winding and core condition. The traditional method of performing this test has been limited to applying an AC voltage across each phase of the HV winding and measuring the AC voltage induced on the corresponding LV winding phase. The measured ratios are compared with calculated ratios from the nameplate voltages and as per international standards the calculated % error needs to be within ±0.5%. A number of factors such as core permeability, mutual and leakage flux, excitation losses, and winding configuration can influence the % error obtained. Through field testing it has been found that accurate results are obtained by energizing all the three phases simultaneously from the LV winding side and measuring the voltage induced on the HV winding.

Authors Daniel Carreno and Dinesh Chhajer
Published: 10 September 2020
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Challenge

The following factors influence the accuracy of the TTR results

  1. Voltage dependence – When lower test voltages (e.g. < 120 V) are applied on an HV winding, the % error varies with the test voltage level.
  2. Winding on which the voltage is applied (HV excitation vs LV excitation) – Higher % errors can result from HV winding excitation because of weak coupling between HV and LV windings.

Transformer configuration – When using a single-phase test source to excite HV phase winding of a transformer with a delta connected LV winding, , the corresponding LV phase winding gets loaded by the other 2 phase windings connected to it (in a delta configuration) resulting in a higher % error.

Figure 1 – Step-up turns ratio diagram – Single-phase & three phase LV winding excitation (Dyn1)

Solution

Step up mode (LV winding excitation) testing and simultaneous three-phase excitation as shown in Figure 1 greatly improves the TTR results accuracy. When all 3 phases are excited simultaneously from LV winding, it results in better flux distribution and higher magnetic permeability. The coupling between the windings is increased and hence more accurate turns ratio is achieved thereby reducing % errors. The measurements are less sensitive to the voltage being applied as volts/turn requirement to setup flux is less with step up mode. It also results in time savings as all the phases are measured simultaneously (avoiding any phase by phase switching) thereby reducing the test duration to one third of the time. It is to be noted that one should be cautious of applying too high of a voltage to the LV winding as that would induce a very high voltage on the HV winding. Instruments with automatic regulation of applied test voltage is recommended.