Why harmonics in 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. 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 Hz, Hz, Hz, Hz and so on. Thus the iron loss of the transformer gets increased if harmonic rich current flow through the transformer.

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. Thus, the copper loss of the transformer gets increased with increase in the harmonic current.

Moreover, these transformers are designed to withstand the additional overheating caused by harmonic currents. The quality and reliability of the electrical system can be improved considerably by using harmonic mitigating transformers. Below are a few examples of such transformers along with a description of their respective use.

The primary of the transformer has a delta connection and the secondary has a special double winding connection. Although there is only one secondary three-phase output, the electromagnetic effect of its secondary winding with a zigzag connection ends up cancelling the 3 rd , 9 th and 15 th harmonic currents. Features of the Harmonic Mitigating Transformers include:.

The 5 th and 7 th harmonic currents are cancelled in the electrical supply common Point of Common Coupling to both transformers and the voltage distortion is reduced. This reduces the 5 th and 7 th harmonics in the system. The primary of the transformer has a delta connection and its secondary has a Dual-output winding connection with two coils each rated at half of the total kAV. This means a dual-output HMT is effective at canceling harmonics from both single and three phase non-linear loads.

The power factor is the ratio between the active power W and the apparent power VA. Electricity supplied by Utilities has a sinusoidal voltage wave of 60 Hz. If the current and voltage curves are not aligned, the efficiency of the electrical system is diminished and the apparent power exceeds the active power.

In an inductive system, the voltage curve leads the current curve. In a capacitive system, the current curve leads the voltage curve. In general, when speaking of the power factor, we are actually referring to the displacement factor.

For the past few years, because of the increase in nonlinear loads, we have had to take into account the effect of harmonics in electrical systems and modify certain mathematical equations. The power factor is now defined as:. There are two elements that combine to deteriorate the power factor; inductive or capacitive loads. These affect the displacement factor and the harmonic currents of the non-linear loads, which affects the distortion factor.

Reducing the level of harmonic currents in a system therefore improves the systems power factor. The harmonic wave increases the core and copper loss of the transformer and hence reduces their efficiency.

It also increases the dielectric stress on the insulation of the transformer. In a three-phase transformer, the non-sinusoidal nature of magnetising current produces sinusoidal flux which gives rise to the undesirable phenomenon. The phase magnetising currents in transformer should contain third harmonics and higher harmonics necessary to produce a sinusoidal flux. If the phase voltage across each phase is to remain sinusoidal, then the phase magnetising currents must be of the following form.

It is seen from equation 1 , 2 , and 3 that the third harmonics in the three currents are co-phase, that is they have the same phase. The fifth harmonics have different phases. The line currents can be found by subtracting two phases current. For examples,. The third harmonic present in the phase magnetising current of three phase transformer is not present in the line current.

The third harmonic components are co-phase and hence cancel out in the line.