Percentage Impedance of Power Transformer

"The percentage impedance of a transformer is the volt drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage."

"It is also the percentage of the normal terminal voltage at on side required to circulate full-load current under short circuit conditions on other side."

The impedance of a transformer has a major effect on system fault levels. It determines the maximum value of current that will flow under fault conditions.

It is easy to calculate the maximum current that a transformer can deliver under symmetrical fault conditions. By way of example, consider a 2 MVA transformer with an impedance of 5%. The maximum fault level available on the secondary side is:

2 MVA x 100/5 = 40 MVA  and from this figure the equivalent primary and secondary fault currents can be calculated.

A transformer with a lower impedance will lead to a higher fault level (and vice versa).


the percentage impedance of a transformer is defined as the percentage of the drop in voltage to the at full load to the rated voltage of the transformer.  This drop in voltage is due to the winding resistance and leakage reactance.

Alternatively, the percentage of a transformer can be described as the percentage of the nominal voltage in the primary that is required to circulate the rated current in the secondary.
The impedance of a transformer can be measured by means of a short-circuit test.
The secondary of the transformer whose percentage impedance is to be measured is shorted.  The voltage on the primary is gradually increased from zero till the secondary current reaches the transformer's rated value. 
The percentage impedance of the transformer is calculated as
Z%= (Impedance Voltage/Rated Voltage)*100
Thus a transformer with a primary rating of 110V which requires a voltage of 10V to circulate the rated current in the short-circuited secondary would have an impedance of 9%.
The percentage impedance of a transformer a crucial parameter when operating transformers in parallel. It also determines the fault level of a system during faults. 


Importance of Percentage Impedance (%Z):

1. It decides suitability of transformer to the system
2. It limits Short Circuit Current (Inversely)
3. It causes voltage drop (Directly)

For transformers up to 2 MVA it is around 5%, Above 2 MVA it varies up to 25% also.

Note: %Z is depends on Resistance value (%Z = V R^2 + X^2). Resistance value may affect with temperature, so, all resistance and Impedance values must convert in to standard temperature of 75%. 

What are V and inverted V curves ?

V Curve- Excitation vs Armature current


V curve is the graph showing the relation of armature current as a function of field current in synchronous machines. The purpose of the curve is to show the variation in the magnitude of the armature current as the excitation voltage of the machine is varied.


Inverted V Curve- Excitation vs Power Factor


The synchronous motor “V Curves” shown above illustrate the effect of excitation (field amps) on the armature (stator) amps and on system power factor. There are separate “V” Curves for No-Load and Full-Load and sometimes the motor manufacturer publishes curves for 25%, 50%, and 75% load. Note that the Armature Amperage and Power Factor “V” Curves are actually inverted “V’s”.

Assume it is desired to determine the field excitation which will produce unity power factor operation at full motor load. Project across from the unity power factor (100%) operating point on the Y-Axis to the peak of the inverted Power Factor “V” Curve (blue line). From this intersection, project down (red line) from the full-load unity power factor (100%) operating point to determine the required field current on the X-Axis.

In this example the required DC field current is shown to be just over 10 amps. Note at unity power factor operation the armature (stator) full-load amps is at the minimum value.

Increasing the field amps above the value required for unity power factor operation will cause the machine to run with a leading power factor, while field weakening caused the motor power factor to become lagging. When the motor runs either leading or lagging, the armature (stator) amps increases above the unity power factor value.

More Electrical Question

PARALLEL OPERATION OF ALTERNATORS

Alternators are connected in parallel to
(1) increase the output capacity of a system beyond that of a single unit,
(2) serve as additional reserve power for expected demands, or
(3) permit shutting down one machine and cutting in a standby machine without interrupting power distribution. When alternators are of sufficient size, and are operating at different frequencies and terminal voltages, severe damage may result if they are suddenly connected to each other through a common bus. To avoid this, the machines must be synchronized as closely as possible before connecting them together. This may be accomplished by connecting one generator to the bus (referred to as bus generator), and then synchronizing the other (incoming generator) to it before closing the incoming generator’s main power contactor. The generators are synchronized when the following conditions are set:

1. Equal terminal voltages. This is obtained by adjustment of the incoming generator’s field strength.

2. Equal frequency. This is obtained by adjustment of the incoming generator’s prime-mover speed.

3. Phase voltages in proper phase relation.