Over Fluxing in Transformer

How the transformer works?

The transformer has two coils wound around the common magnetic core. When an alternating voltage is applied to the primary coil, the current flows through it. The current flowing in the primary coil produces the magnetic flux.

The magnetic flux produced in the primary coil is proportional to the ratio of the applied voltage to the frequency. According to Faraday’s law of electromagnetic induction, the EMF is induced in the primary and as per the Lenz’ law, the induced EMF always opposes the primary current that is responsible for begetting the EMF.

The voltage induced in the coil can be expressed by the following mathematical expression.

E= 4.44 Φf N

Where,

E = Induced EMF in the primary coil
Φ = The magnetic flux
f = Frequency
N = Turns /Phase

The flux travels through the magnetic core and it gets linked to the secondary coil. Practically, all the flux produced in the primary does not link to the secondary. Some parts of the magnetic flux links to the primary coil and other parts of the transformer. The flux that does not link to both primary and secondary coils is known as the leakage flux. The losses in the transformer increase with an increase in the leakage flux. The useful flux that links to the secondary coil induces a voltage in the secondary according to the Faraday’s Law of Electromagnetic Induction. The voltage induced in the secondary coil is

Es= -N dΦ/dt

The secondary voltage remains constant if the rate of change of the flux is constant. It is desirable that the flux in the transformer must remain constant.

The Flux density of the CRGO Core:

The transformer designer first checks the rated flux density of the colled rolled grain oriented(CRGO) core. The maximum flux density of the CRGO core is about 1.9 Tesla. If the flux density of the transformer is above 1.9 Tesla, the core of the transformer gets saturated, and it will lead to insulation failure of the laminated core. The designed maximum flux density of the core must be below the maximum rated flux density of the core. The core must not get saturated in any case. The magnetization curve of the different material is as given below.

magnetic field vs flux density in transformer

The flux density of the core can be controlled during the design stage of the transformer. The flux density of the core can be controlled by adjusting the cross-sectional area of the core during transformer designing. The flux through the core is the product of the flux density and the cross-sectional area of the core(Φ=B*A). The flux density of the core can be reduced by increasing the cross-sectional area of the core.

The voltage induced in the primary when the sinusoidal voltage is applied is

E=4.44 ΦfN
Φ=E/4.44f N

The Number of turns in the primary is constant for a given transformer

Φ=K* E/f

The induced voltage is approximately proportional to the applied voltage if the primary impedance is ignored.

Φ=K*V/f ———-(1)

The flux density in the core of the core is as given below.

B=Φ / A

Where A is the cross section area of the core that is also constant.

B=K/A*V/f
B=K1*V/f ———–(2)
B ∝ V/f

Thus the flux density in the core is directly proportional to the ratio of V/f if the number of turns of the primary is fixed. For on load tap changer transformer, the number of turns is not fixed but changes according to the output voltage requirement so we will also discuss the flux density in the core with the tap changer is not at the center tap position.

The over fluxing in the transformer happens with an increase in the supply voltage or decrease of the supply frequency. If the transformer has tap changer and the number of turns is reduced to increase the secondary voltage the flux density in the core will increase. The designer always takes the margin of increase in the flux due to tap changer operation. Also, the margin of allowable increase of voltage and decrements of the frequency is taken into consideration while designing the transformer.

Reasons for transformer Over fluxing :

The Increase of the supply voltage due to sudden load through.
The receiving end voltage can be more than the sending end voltage due to the Ferranti effect if the line is lightly loaded.
The transients and overshoots in the electrical network

Why important to keep the flux constant in the core?

The rated maximum flux density of the CRGO core is 1.9 Tesla. The transformer core gets saturated if the flux density is more than 1.9 Tesla. The transformer is designed at the flux density of 1.73 Tesla in order to have the 10 % margin in the flux density. If the flux density in the core is below 1.9, the flux is confined in the core because the permeability of the core is more than the permeability of surrounding materials.

What happens if the flux density in the core goes above the saturation point of the CRGO core?

The flux remains confined in the core if the flux density is below the rated flux density of the core. If the flux density exceeds beyond the maximum rated flux density, the flux starts linking to other parts of the transformer through the air. Beyond the saturation point, the flux density of the core becomes constant, and the additional flux starts linking to the steel parts and induces the eddy current in the loops. The eddy current heats up the structural parts and it will raise the temperature of the core. The core of the transformer can get damaged if the over fluxing is allowed for a longer period.

Moreover, if the core of the transformer is over excited it operates in the non-linear region of the B-H curve. The exciting current is no more sinusoidal and generates harmonics. The 5th order harmonics is produced when the core of the transformer gets saturated.

The higher frequency current tends to flow at the outer surface of the winding conductor because of the skin effect. The effective area of the conductor gets reduced and its resistance increases. This all causes the higher I^2*R loss, and cause additional heat loss in the winding. Also, the higher frequency current cause temperature rise of the transformer core on account of eddy current as the eddy current loss is proportional to the square of the frequency. If the transformer is operated on over fluxing for a longer period of the time, the laminated stamping insulation is apt to fail. Therefore, the transformer must not be operated under over fluxing.

The over fluxing in the transformer can be judged by inspecting the transformer during the maintenance. The over fluxing cause temperature rise of the core, winding and the transformer parts. The temperature rise adversely affects the useful life of the transformer life. The transformer oil properties get changed with temperature increase. The dissolved gas analysis (DGA) must be performed once in a year to check the health of the transformer. The heating cause discoloring of the paint of the tank, carbonized material in the transformer oil, discoloring of the insulation of the winding and blistering of the paint.

Permissible flux density:

The saturation flux density of the Colled rolled grain oriented(CRGO) core is 1.9 Tesla. As per the present design practice, the peak rated value of the flux density is kept about 1.7 Tesla which is about 0.9 times of the rated value. The design margin of 10 % in the flux density is kept for taking care of increased flux density with an increase in the system voltage or with a decrease in the system frequency and the thermal time constant of transformer heated parts.

The maximum over fluxing in transformer shall not exceed 110%. the transformer can be continuously operated at 110 % of the designed flux density. However, the operation of the transformer above 110% and up to 130 % of the flux density can be allowed for a shorter period of the time. If the flux density increases to 140 % the transformer shall be tripped instantaneously to avoid the permanent damage.

The below-given table shows the permissible over fluxing of the transformer.