One way to protect a transformer from harmonics is to limit the amount of load placed on it. This is called “derating” the transformer.
The most rigorous derating method is described in ANSI/IEEE standard C57.110-1986. It is somewhat impractical because it requires extensive loss data from the transformer manufacturer plus a complete harmonic spectrum of the load current.
The Computer and Business Equipment Manufacturers Association (CBEMA) has recommended a second method that involves several straightforward measurements that you can get with commonly available test equipment.
It appears to give reasonable results for 415/240 V receptacle transformers that supply low frequency odd harmonics (third, fifth, seventh) commonly generated by computers and office machines operating from single-phase branch circuits.
Derating factor
To determine the derating factor for the transformer, take the peak and true-rms current measurements for the three phase conductors.
If the phases are not balanced, average the three measurements and plug that value into the following formula:
HDF = Harmonic derating factor
This formula generates a value between 0 and 1.0, typically between 0.5 and 0.9. if the phase currents are purely sinusoidal (undistorted) the instantaneous peaks are 1.414 times the true-rms value and derating factor is 1.0.
However, with harmonics present the transformer rating is the product of the nameplate kVA rating times the HDF.
kVA derated = (HDF) x kVA nameplate
For example: Assume transformer rated at 300 kVA.
The results indicate that with the level of harmonics present the transformer should be derated to 72.3% of its rating to prevent overheating.
At FullSpectrum Energy solutions, we assist organizations enhance productivity and profits by mitigating harmonics and other Power Quality issues prevalent in today’s commercial and industrial workplace.
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Victor Oyedu, FNSE, FNIEEE, CPQ.
Power Quality and Energy Management Specialist.
Publisher at Afrienergyonline.com
CEO, FullSpectrum Energy Solutions Limited, Nigeria.
