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what is impedance of transformer?
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Answer Posted / montu
Percentage Impedance (Z%)
The impedance of a transformer is marked on most
nameplates - but what is it and what does the Z% figure
mean?
Definition
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
required to circulate full-load current under short circuit
conditions
Measuring Impedance
The impedance is measured by means of a short circuit
test. With one winding shorted, a voltage at the rated
frequency is applied to the other winding sufficient to
circulate full load current - see below:
The percentage impedance can then be calculated as follows:
Z% = Impedance Voltage x 100
Rated Voltage
Changing the Impedance Value
The most economical arrangement of core and windings leads
to a 'natural' value of impedance determined by the leakage
flux. The leakage flux is a function of winding ampere
turns and the area and length of the leakage flux path.
These can be varied at the design stage by changing the
volts per turn and the geometric relationship of the
windings.
The Effect of Higher and Lower Impedances
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 figure calculated above is a maximum. In practice, the
actual fault level will be reduced by the source impedance,
the impedance of cables and overhead lines between the
transformer and the fault, and the fault impedance itself.
As well as fault level considerations, the impedance value
also:
determines the volt drop that occurs under load - known
as 'regulation'
affects load sharing when two or more transformers operate
in parallel
Sequence Impedance (Z1 Z2 Z0)
The calculation above deals with a balanced 3-phase fault.
Non-symmetrical faults (phase-earth, phase-phase etc) lead
to more complex calculations requiring the application
symmetrical component theory. This in turn involves the
use of positive, negative and zero sequence impedances
(Z1, Z2 and Z0 respectively).
As with all passive plant, the positive and negative
sequence impedances (Z1 and Z2) of a transformer are
identical.
However, the zero sequence impedance is dependent upon the
path available for the flow of zero sequence current and
the balancing ampere turns available within the
transformer. Generally, zero sequence current requires a
delta winding, or a star connection with the star point
earthed. Any impedance in the connection between the star
point and earth increases the overall zero sequence
impedance. This has the effect of reducing the zero
sequence current and is a feature that is frequently put to
practical use in a distribution network to control the
magnitude of current that will flow under earth fault
conditions.
regards
dhruv shah
G.T.P.L
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