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Measuring Turns Ratio of Star-Delta Transformers with Low Voltage Ratio

Introduction to Measuring Turns Ratio

The turns ratio of a transformer is the ratio number of turns in a higher voltage winding to that in a lower voltage winding. The test which determines the turns ratio of a transformer is one of the basic tests of transformers.

Measuring turns ratio can be used to verify the manufacturing quality and design specifications, but also, to detect a wide range of problems with transformers such as incorrect turns ratio due to short circuits between the windings, winding, and ground, incorrect operations of tap changers, etc.

The measurement is standardized, and it is described in various national and international standards, such as the IEC 60076-1 and the IEEE Std. 62-1995. If the transformer has load taps, the turns ratio should be determined for all these taps. Due to safety reasons, the transformer should be excited from the high voltage winding. Care should be taken during the application of voltage and during the measurement.

Most of today’s commercially available measuring instruments measure turns ratio in the range of 0,8 to 50 000. This measuring range guarantees that the measurement is possible for the majority of the transformers that are being used. However, there are some transformers with a turns ratio close to 1. This can create a measurement problem for combinations of delta-wye, and wye-delta transformers.

The calculated turns ratio value for certain vector groups (wye-wye, delta-delta) is the same as the nameplate ratio meaning the three-phase voltage ratio, indicated on the transformer nameplate, is equal to the ratio of the number of turns of the high voltage to the low voltage windings. For some other vector groups, the ratio of winding turns is not the same as the nameplate ratio. For example, for wye-delta groups turns ratio is √3 times lower than the nameplate ratio, while for delta-wye groups it is √3 times higher. When the TRT instrument calculates a deviation between the measured and nameplate turns ratio, these √3 factors are automatically taken into consideration.

YNd11 Transformer Turns Ratio Measurement

The analysis of the measuring turns ratio on a three-phase YNd11 15/11 kV transformer will be elaborated. De-energized tap changer with 9 taps (±4 x 2,5%) is mounted on the high-voltage side of the transformer.

If the standard sequential three-phase measurement is performed by applying single-phase test voltages from the HV side, the expected turns ratio results are given in Table 1. The connection principle is shown in Figure 1.

It can be seen from Table 1 that modern turns ratio testers won’t be able to measure the ratio in positions 1-5 since the ratio is lower than 0,8. TRT instrument would display an error “Turns ratio too low” in these cases.

Turns Ratio Measuring Results for YNd11
Table 1. Nameplate data for the YNd11 15/11 kV transformer 
Connection of TRT to YNd11 Transformer
Figure 1. Connection of TRT to YNd11 transformer 

To resolve this issue, the sequential three-phase turns ratio test must be performed from the 11 kV side of the transformer. In this case, the transformer needs to be tested with reversed connections – H cables of TRT should be connected to the transformer LV side, and X cables of TRT should be connected to the transformer HV side. Even though the line-to-line voltage on the 15 kV side is higher than the line-to-line voltage on the 11 kV side, we can use the fact that the phase-to-phase voltage on the 11 kV side is higher than the phase-to-phase voltage on the 15 kV side, due to higher physical number of turns on the 11 kV side. This connection is shown in Figure 2.

This means that for the sequential turns ratio test, this transformer needs to be treated as Dyn1 11/15 kV transformer. In this case, the expected turns ratio for all taps is shown in Table 2.

Turns Ratio Measuring Results for Dyn1
Table 2. Nameplate data for the Dyn1 11/15 kV transformer 
Connection of TRT to Dyn1 Transformer
Figure 2. Reversed connection of TRT to YNd11 transformer 

If nameplate voltages are entered, TRT will calculate the ratio deviation. To enter nameplate voltages correctly for this case, a user should: 

  • Select DETC on the LV side
  • Insert 9 tap positions 
  • Enter 11 000 V as the nameplate voltage of the HV side 
  • Input 13 500 V as the nameplate voltage of the first tap of the LV side 
  • Input 16 500 V as the nameplate voltage of the last tap of the LV side
Screenshot of DV-Win Software
Figure 3. Test settings in DV-Win Software 

The turns ratio results obtained with the TRT device on such YNd11 15/11 kV transformer are shown in Figure 4 below.

Turns Ratio Measurement Results
Figure 4. Turns ratio results when testing the YNd11 transformer from the 11 kV side 

If the user wants to measure voltage ratio using a simultaneous three-phase test available in TRT devices, the transformer should be treated as it is – YNd11 15/11 kV. For that test, H cables from TRT should be connected to the transformer 15 kV side, and X cables from TRT should be connected to the transformer 11 kV side.

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March 29, 2022

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