A89- Negative Wattmeter Reading during Transformer Loss Measurement

1.? Transformer loss measurements are done at the factory test bed using three wattmeters, one in each phase. Sometimes, the wattmeter reading in one of the phases may show a negative reading during no-load loss measurement. Customer engineers may raise doubts about test circuits and ask the test engineer to explain why a phase-watt meter shows negative readings.

1.1???The three-wattmeters will exhibit differing figures when testing losses of large three-phase units. It is even possible for one wattmeter reading to be negative. The actual input power is the algebraic sum of the readings of the three-watt meters, like 75 -5 +12= 82 kW, where the V phase was showing a negative meter reading.

1.2?? ??In the case of no-load loss measurements, the magnetic asymmetry of the iron core causes asymmetrical no-load currents. Depending on the flux density in the core, the phase displacement between current and voltage in one phase can be greater than 90° (making the leading power factor), which will be seen as a negative power in one of the phases (i.e. watt meters) (Testing of Transformers- Book by ABB, 2003 page 89 – No Load Loss Measurement) Also refer to the text, "Electrical measurements and Measuring Instruments" by E W Golding ed5.0,1962 (Measurement of Power Page 775)

1.3 ???This current asymmetry between phases depends on flux density in the core (increase with higher flux density), core limb height-to-width ratio, quality of CRGO used, etc. This phenomenon was analysed for the first time in 1954 by Dr Brailsford, a professor at University College, London (earlier worked at Metropolitan -Vickers Electrical Co, UK- an old transformer manufacturer). Refer to his paper Paper No. 1550 Measurements Section in IEE Proc., Vol 101, Part2, UK 1954 "Current and Power relationships in the measurement of iron losses in 3 limb transformer Core."? “if a three-wattmeter method is employed, the magnetic inequality of the three phases also causes two of the watt meters to have high readings, one of them being negative at higher flux densities.”

1.4 ??The above can be analysed from the zero sequence components (third harmonics) in the no-load current. The higher the magnetising asymmetry, the higher the magnitude of the zero sequence component. As per fig-2, the no-load current in each phase results from the respective positive sequence phase current and the zero-sequence current (Ih). The resultant current vectors of the phase current and zero sequence current are shown in the figure. As can be seen, the phase angle between respective voltages and resultant currents of the U & V phase remains less than <90 degrees, but in the W phase, the angle is more than 90o, thereby making the power negative in that phase. This negative reading is rarely noted, with lower flux densities in the 1.6T or lower range, probably due to reduced harmonics.

1.5? ????No load loss asymmetry between phases:

[1] F.Brailsford,”Current and power relationship in the measurement of iron losses in a three-limb transformer core “, IEE Paper no.1550, Measurements Section, pp.409-416,1954

[2] J. I. Parion, “Magnetizing unsymmetry in three-phase core-type transformers,” Elect. Energy, pp. 330–336, Jul. 1957.

[3] C. S. Walker, “The excitation requirement of 3-phase core-type 3-legged y-connected transformer,” AIEE Trans., pp. 1113–1119, Dec. 1957.

[4] A. S. Nene, S. V. Kulkarni, and K. Vijayan, “Asymmetrical magnetising phenomenon of three-phase core-type transformers,” in Proc. Int. Conf. Transformers, TRAFOTECH, Jan. 1994, vol. 15, pp. IV 21–26.

[5] S. V. Kulkarni and S. A. Kharparde, Transformer Engineering: Design and Practice. New York: Marcel Dekker, May 2004, pp.46

2. ?????The load losses in a transformer are also usually measured by a three-wattmeter method. It is generally found that the three readings are appreciably different, even though the total losses (addition of the three readings) are near the design value. This asymmetry can raise doubts in the minds of transformer users, and test engineers find it difficult to explain the phenomenon convincingly.

2.1?? The stray loss may form an appreciable part of the total load loss in power transformers. Hence, the simplest explanation that may normally be given is that the stray loss for each phase could be different due to the asymmetry of the tank and other structural parts. However, this explanation is marginally true. The major reason contributing to the phenomenon is the asymmetry of the mutual magnetic couplings between phases.

2.2? ??Apart from unequal mutual impedances between the three phases of the transformer, slight unbalanced phase differences in the three-phase source test voltage can also create asymmetry in power readings between phases.

2.3?? Asymmetry load in loss measurements between phases:

[1] R. Escarela-Perez, S. V. Kulkarni, N. Kodela, and J. Olivares-Galvan, “Asymmetry during load-loss measurement of three-phase three-limb transformers,” IEEE Trans. Power Del., vol. 22, no. 3, pp. 1566–1574, Jul. 2007.

[2] R. Escarela-Perez, S.V. Kulkarni, J. Alvarez-Ramirez and K Kaushik, “Analytical description of the load loss asymmetry phenomenon in three-phase three-limb transformers” IEEE Trans. Power Del., vol.24, no.2, pp. 695-702, Apr.2009.