Why Does Transformer Impedance Descrive in Percentage?

Transformer Impedance Descrives in Percentage
Fig-A Typical Transformer

Describe Why Does Transformer Impedance  Measured in Percentage

The rated impedance of a transformer is the voltage drop on full load because of the winding obstruction and spillage reactance communicated as a level of the evaluated voltage.

It is additionally the level of the typical terminal voltage at one side required to flow full-stack current under short out conditions on the opposite side.

The impedance of a transformer majorly affects framework blame dimensions. It decides the most extreme estimation of current that will stream under blame conditions. 

It is anything but difficult to compute the most extreme current that a transformer can convey under symmetrical blame conditions. By the method, for instance, consider a 2 MVA transformer with an impedance of 5%. The most extreme blame dimension accessible on the auxiliary side is: 

2 MVA x 100/5 = 40 MVA 

also, from this figure, the proportional essential and auxiliary blame flows can be determined. 

A transformer with a lower impedance will prompt a higher blame dimension (and the other way around). 

The figure determined above is the most extreme. Practically speaking, the real blame dimension will be decreased by the source impedance, the impedance of links and overhead lines between the transformer and the blame, and the blame impedance itself.

How do I calculate the impedance of the transformer?

We have Short circuit test to figure the impedance of the transformer. 

For this, the low voltage side of the transformer is shortcircuited, an autotransformer and voltmeter is associated with higher voltage side. 

An ammeter is associated with the lower voltage side(short-circuited side). 

At first, the autotransformer is giving ZERO voltage. 

Presently voltage is expanded bit by bit. 

The ammeter in the auxiliary side ( Low voltage side for our situation) will detect some current. Presently voltage is expanded by changing the autotransformer till the ammeter on the opposite side demonstrates the typical full load current. 

Presently this is where we can compute the impedance. 

Let us have a model.- 

Transformer rating 10 KVA 

Single-stage essential voltage-480 V. 

Auxiliary voltage - 240 V 

Full load current - (10 kVA X 1000)/(240 V)- 41.67 A 

Presently the investigation Assume the essential voltage infused is 24 V at which the ammeter peruses 41.7 A (Transformer full load current). 

Impedance is proportion of 24 V to 480 V, 

i.e. Impedance Z = 24/480 =0.05. 

Be that as it may, the impedance of the transformer is communicated in %, 
So the % impedance is 0.05 X 100 =5%.

What is the significance of percentage impedance of a Transformer?

Increasing percentage or % Z impedance reduces the short circuit current and;

High Value of percentage impedance or %Z causes more voltage drop and poor voltage regulation. Finally, the transformer delivers less efficiency;

The %Z is used to design a protection switchgear. Typically, it is used to calculate maximum level of fault current. So that, we can design a switch gear to withstand that maximum level of fault current.

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