Evaluation of possible Winding & Core damage through Transformer Turns Ratio Measurement (TTRM)

In a previous blog post, we shared our views on the importance of transformer testing and discussed: a) the principles of winding resistance measurement, b) OLTC condition analysis and c) our thoughts on the ideal kit for transformer testing.

In this blog post, we will share our knowledge on assessment of possible winding & core damage through transformer turns ratio measurement.

Main Reasons of Transformer Winding & Core Failure

To begin with, let us review the main causes of transformer winding & core failure. The major causes of transformer winding and core failure which can strongly lead to calamitous failure are the following:

  • Dielectric faults/stresses due to turn to turn insulation breakdown. Insulation breakdown occurs due to the high current and high voltage which are high above the rated voltage values. The insulation breakdown which results in the flashover in winding turns and causes a short circuit
  • Mechanical Faults/stresses can deform the windings, causing the variation of the turn ratio and short circuit impedance values that can decrease the performance of the transformer. Mechanical faults are the loosening or displacement of the windings, distortion, Core Overheating, damage of Core. Typically, reasons for such faults include: a) improper repair/maintenance, b) corrosion, c) manufacturing deficiencies and d) mechanical movement in the transformer
  • Thermal stresses can occur since windings are usually of copper material. Copper line resistance causes thermal losses. Thermal losses cause hotspots in the winding due to improper maintenance.  The resultant wear and tear further causes decrease in physical strength with breaking of the winding a possible consequence

Assessment of possible Winding & Core Damage

To evaluate possible winding/core damage due to above mentioned issues, the following tests need to be conducted: a) turns ratio/voltage ratio, b) %error in ratio, c) phase angle deviation, d) excitation current measurement, e) vector group/polarity test and f) magnetic balance test.

Need and Importance for Transformer Turns Ratio Measurement:

The need for transformer turns ratio measurement is underscored by the following:

  • To verify the transformer has the right ratio corresponding on its rated voltage in primary and secondary winding
  • To verify the design specifications & manufacturing process quality
  • To ensures the inductance property of the transformer
  • To confirm the transformer is balanced or not (all three phases should have the same turns ratio)
  • To find out whether the overheating or overcurrent conditions shorted the turns
  • This measurement is also important for instrument transformers, i.e., current & potential transformer are connected to protective relays for the protection/metering purpose
  • Inter turns Shorted in transformers can lead to big issues, with this case protective relays can operate abnormally

Additionally, turns ratio testing of transformers needs to conform to the following recommended standards: a) IEC 60076-1 Clause 10.3, b) IS 2026-1 Clause 10.3 and c) IEEE C57.12.90-2010 Clause 6 & 7/ ANSI STD C57

Need & Importance of the Excitation Current Test

  • Analysis of shorted turns and open turns in the winding
  • Tracking Magnetic core structure, shifting or de shaping of windings
  • The effective reluctance of the magnetic circuit can be erroneous, thus affecting the current required to establish flux in the core
  • If the core is magnetized an irregular pattern (high/ medium/ low readings) will be present and you will be unable to compare results effectively, therefore the core should be demagnetized and the test should be repeated for the effective results

Phase Angle Deviation Test

  • Phase angle deviation is the difference between the angle of applied HV winding voltage and angle of LV winding voltage. Typically, this difference is very small. However, it can be change drastically if the transformer core is damaged
  • Phase angle deviation of LV winding voltage and w.r.t its HV winding voltage is measured and displayed in the range of Positive cycle ( 0 to 180°)  and negative cycle (0 to -180°)

Vector Group Test

  • Vector group is an imperative property of three phase transformers for the parallel operation of transformers
  • The phase sequence or the order in which phases reach their maximum voltage and current must be identical for parallel operation to avoid short circuits in parallel operation
  • This test is important to ensure a customer specified vector group of transformers

Polarity Test

  • Polarity test is required to ensure that we connect the same polarity windings and not the opposite ones during parallel operation of transformer with this we can be able to avoid short-circuits
  • In case of a single phase transformer, voltage is applied to primary side. Accordingly, voltage will be generated in secondary side winding. So phase of secondary voltage will be the same or out of phase; i.e.,  it will be either 0 degrees or 180 degrees

Magnetic Balance Test

  • Magnetic balance concerns checking the flux distribution in the magnetic circuit. This test is applicable for 3 phase transformers
  • This test is useful for the identification of inter turn faults as well as magnetic imbalance in the circuit due to magnetic core shifting/de shaping of windings, wrong interleaving connections/joints in windings
  • This test is not applicable in case of 1 phase transformers
  • It is important to note that the voltage should be applied on one phase and measured in the remaining two phases of the winding. This process should be repeated for each of the three phases

Interpretation of Results of Transformer Testing

  • Excitation Current
    • Oftentimes, there may be similar readings on two outer limb phases on the transformer core; with one lower reading on the center limb phase
    • As per standard practice, result within 30% of the measured exciting current with the previous test is normally considered satisfactory
    • It is important to always ensure that while comparing readings to previous test results, it should be ensured that the same voltage should be used for both tests
  • Turns Ratio/Voltage Ratio & Error
    • Results of the transformation turns/ voltage ratio may be compared with the specified values measured during factory/manufacturing stage testing.
    • The acceptance criteria is that the measured values should be within 0.5% of the specified values for all windings.
    • We can also consider the trend of voltage ratio values with reference to the ratio values measured during the commissioning/Installation tests.
    • For star/delta type connection type transformers ratio testing we need to consider conversion factor for converting into turns ratio from voltage ratio as per specified below,
Type of Transformer Turns Ratio
Star-Star =Voltage Ratio
Star-Delta=Voltage Ratio /√3
Delta-Star= Voltage Ratio* √3
Delta-Delta= Voltage Ratio
  • Magnetic Balance Test
    • The voltage induced in the center phase shall be 60 to 90% of the applied voltage
    • The voltage induced in the remaining phase shall be 10 to 40% of the applied voltage
    • However, when the center phase is excited then the voltage induced in the outer phases shall be 40 to 60% of the applied voltage
    • Identical results confirm lack of damage on account of transposition
    • Zero voltage/very negligible voltage induced in the other two windings need to be delve

Conclusion

In this manner, possible winding & core damage is done through transformer turns ratio measurement. SCOPE manufactured TTRM range instruments are capable of conducing all tests mentioned above. With our instruments, we can ratio test any type of transformer including current transformers and voltage transformers.

To know more about our transformer testing capabilities, please visit https://www.scopetnm.com/test-and-measurements/transformer-testing-equipment/transformer-turns-ratio-meter  or write to us at marketing@scopetnm.com 

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