Effects of Harmonics on Transformers - Electrical Volt

Effects of Harmonics on Transformers


The transformer does not generate harmonics.The transformer has primary and secondary winding which are inductive in nature. The inductive load draws current linearly with the applied voltage. However, the transformer generates harmonics in two conditions. 

When transformer is energized the magnetizing current drawn by it is non linear and it generates harmonics.This is because, when transformer is energized the back emf is zero and transformer draws huge magnetizing current which is not sinusoidal.The magnetizing current is rich in the 2nd order harmonic current. 
The second condition is the over excitation of the transformer. The over excitation means the transformer is operated at increased flux density than its rated flux density. Under over excitation of transformer the 5th order harmonic current is produced. The above two conditions happen for a short period of time and thus the 2nd and 5th order harmonics is not a serious problem.
The transformer primary function is to feed power to the load.The nature of load decides whether the load current waveform has harmonics or not. If the load is linear, no harmonics is produced and if the load is non linear the harmonics are produced in the electrical network. 
The power electronics are nowadays widely used for better control of the drives and moreover, it is used for the energy saving. If the converter and /or inverter used are of 6 pulse configuration the lower order harmonics 5th and 7th are predominant and it  harms the power system because the magnitude of the lower order harmonics is more in comparison with magnitude of the higher order harmonics. Whatever harmonic order current produced by the load flows in the primary and secondary of the transformer.
The transformer is supposed to feed the sinusoidal current at its rated kVA. However, if the current flowing through the transformer winding it adversely affects the transformer performance. If the magnitude of the harmonic current is more, it also distorts the input voltage of the transformer. The harmonics cause following adverse effects on the transformer performance.

Effect of Harmonics on Iron Loss 

The core or iron loss of the transformer is the loss occurs due to eddy current loss and hysteresis loss. The hysteresis loss is directly proportional to the frequency and, eddy current loss is directly proportional to the square of the frequency. Thus, it is clear that the iron loss gets increased with increase in frequency.The harmonic current is the integral multiples of the fundamental frequency and harmonic current may have frequency of 100 Hz, 150Hz, 200Hz, 250 Hz and so on. Thus the iron loss of the transformer gets increased if harmonic rich current flow through the transformer.

Effect of Harmonics on Copper Loss 

The tendency of flowing of current at the outer surface of the conductor is known as skin effect. The higher frequency current tends to flow at the outer surface of the conductor. When the current flows at the outer surface of the conductor, the effective cross section area of the conductor gets reduced. The reduction in effective cross section area leads to increase in the conductor resistance. The higher resistance of the conductor cause the more copper loss(I^2R) in the transformer. Thus, the copper loss of the transformer gets increased with increase in the harmonic current.

Effect of Harmonics on Efficiency of Transformer

 The efficiency of the transformer is defined as the ratio of output power to input power.
Efficiency  =  Output Power/ Input Power
                  = Input power- Losses/ Input Power
                  = 1 – Losses/Input Power
With increase of losses in the transformer due to harmonics, the efficiency of the transformer gets decreased. 

Harmonics Cause De-rating of Transformer

If the transformer is operated at X kVA when harmonic current is absent needs to be operated at lower kVA if transformer is operated with harmonic current to control the temperature rise of the transformer.
In a nutshell, we can say that the transformer needs to be de-rated if the transformer is to be operated in harmonic rich current.
If the load characteristics is known to be non linear and total harmonic distortion(THD) of load is known, the transformer of inverter duty must be selected for reliable operation of the transformer.
The harmonic current cause the voltage distortion at the input side of the transformer. The harmonic distortion in voltage cause following adverse effects on the performance of the transformer.
1. Increased dielectric stress on insulation
2. Resonance between the winding reactance and feeder capacitance
3. EMI and RFI interference with communication circuit

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