A winding error can be determined from the ratio error or from the voltage difference between the declared and actual value when using the voltmeter method. The faulty phase can be found by testing phase per phase. Using the known calculated winding voltage, the winding fault can be determined as follows:
where:
Uact = measured actual voltage (low voltage) Udecl= declared value (low voltage)
Uw = winding voltage Nf = winding error
4. Measuring voltage ratio 4. Measuring voltage ratio
If the winding error is positive, the low voltage has too many turns, or the high voltage too few turns. If the winding error is negative, the low voltage has too few turns, or the high voltage too many turns. The above formula is primarily valid for voltage measurement.
For the test using the measuring bridge the following holds:
When reading the error directly:
where:
ract= actual voltage ratio rN = declared voltage ratio N = number of turns (HV or LV) f = ratio error in %
For calculation of the ratio error f refer to section 4.4.
Calculation of the winding error can be carried out in the same way for high and low voltage windings, because the source of the error is not yet known. To determine the faulty winding the two suspected windings are compared to a winding which has no faults: e.g. for three-legged types the adjacent winding is used. The winding on the third leg must be shorted, or an auxiliary winding must be used for comparison. Depending on the number of turns in the winding to be tested, the auxiliary winding should have from 10 to 20 turns. The comparison can be made in a similar way when using the voltmeter method.
An apparent shortage of turns can also be caused by a short circuit between turns. This is the easiest to prove by applying rated voltage for 15 to 30 minutes. – Important: for oil-cooled active parts in air, the voltage must be reduced in accordance with the insulation, in order to subsequently check the winding for hot spots. The location of the winding fault can usually be found easily due to significant heating. This method can however, only be recommended for small and mid-sized transformers up to about 10 MVA. For power transformers any faults must be localized by checking the individual winding segments.
If the transformer has series-parallel reconnectable windings, the two winding halves must be exactly symmetrical. A small asymmetry in the number of turns is difficult to detect using ratio measurement. For this reason, no-load losses must be measured for series and parallel connection types. If it is determined that there are significant differences in the losses, an asymmetry in the number of turns must be assumed. To localize the error the separated winding halves on all three limbs must be compared with one another.
5. Measuring short-circuit voltage / impedance and load loss 5. Measuring short-circuit voltage / impedance and load loss
Testing of
Power Transformers
5. Measuring the short-circuit
voltage impedance and
the load loss
5.1 References / Standards
• IEC 60076-1 (2000) clause 10.1: “General requirement for routine, type and special tests”, clause 10.4: “Measurement of short-circuit impedance and load loss” [1]
• IEC 60076-8 (1997), clause 10: “Guide to the measurement of losses in power transformers” [6]
• IEEE Std C57.12.90-1999, clause 9: “Load losses and impedance voltage” [51]
Note:
Measuring the short-circuit voltage, the short-circuit impedance and the load loss are routine tests according to IEC Standard [1]
and to IEEE Standard [50].
5.2 Purpose of the test
Transformer short-circuit voltage and load loss are guaranteed by the manufacturer and are verified for the customer during the acceptance test. Exact knowledge of the load loss is important not only for capitalization of losses but is also important for the safe operation of large power transformers.
A comparison of the calculated and measured values gives an indication about the eddy losses caused by leakage flux in the mechanical parts and the tank wall. Furthermore, it is essential to know the short-circuit voltage and load loss to carry out the temperature rise test, see section 11. For transformers with tapped windings the short-circuit voltage has to be measured in the two extreme tap positions in addition to the principal tap position. Knowledge about short-circuit voltage in extreme tap position is important for parallel operation.
5.3 General
The definition of short-circuit voltage is:
The AC voltage that must be connected to one pair of terminals of a transformer with another pair of terminals shorted, which causes rated current to flow on the two sides of the transformer. The absorbed active power corresponds to the transformer load loss.
In reality a component of the no-load losses of the transformer is also measured but it can be neglected most of the time, since the short-circuit voltage is minimal compared to the rated voltage.
R1 = resistance of winding 1 R2 = resistance of winding 2 X1 = stray resistance of winding 1 X2 = stray resistance of winding 2 RFe = equivalent resistance of iron XH = main reactance
UCC = short-circuit impedance voltage
5. Measuring short-circuit voltage / impedance and load loss 5. Measuring short-circuit voltage / impedance and load loss
Figure 5.2: Short-circuit vector diagram