Question

The magnetising current in a transformer is rich in ______________ harmonic?
a. 3rd
b. 5th
c. 7th
d. 13th
please answer this question and specify the reason...

Answer 1

3rd

It is true that transformers produce a third order harmonic in their exciting (magnetizing) current. When this exciting current flows in the primary winding of a utility's downline distribution transformer, third order harmonic voltage drops will develop on the source side lines which feed the distribution transformer. Harmonic voltage drops will also develop in the secondary windings in the upline substation transformer which feeds the distribution transformer.

If there are enough downline distribution transformers that are fed off of the upline substation transformer, the collective exciting current of all the distribution transformers could add up enough to cause an excessive and unacceptable third order harmonic voltage distortion. This distortion would be a part of the total voltage energizing all the distribution transformers and would cause 3rd harmonics to flow into any plant facility load.

All loads that are connected to the substation transformer's secondary, are energized with these harmonic voltage distortions and will thus draw distorted harmonic currents. However this is usually insignificant to the power loads that any transformer handles and is therefore not normally a concern.]

When harmonics flow through transformers, the transformers will usually heat up more than you would expect from the same RMS value of a purely fundamental 60 Hz current.

The reason for this extra heat comes from mainly one source. The "eddy current" losses in the transformer COIL WINDINGS (not the eddy currents in the iron core - see below) produce the major portion of excess heat. The higher frequency current running through the coils causes eddy currents to flow more readily in each of the coil windings' individual conductors. It is these excess eddy currents in the coil windings' conductors that are responsible for the excessive heat and for the derating of the transformers' capability and KVA rating.

Some additional heating occurs due to the additional AC resistance in the coils when higher frequency current flows through them. Remember, AC resistance increases with frequency due mainly to skin effect. However, this additional resistance is a smaller part of the overall overheating of transformers when they supply harmonic currents to non linear loads.

What does NOT SIGNIFICANTLY add to the overheating of transformers due to the flow of harmonic currents, is the core losses that occur due to the flow of harmonic magnetic flux in the iron core of a transformer.