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Fig-Transformer Tap Changers Line Diagram
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What is the Principal of Transformer Tap Changers?
We will try to understand the principle of transformer tap changer andtry to find the answer of question- Can
a transformer be tapped when it is on a load? Why
is a tap changer used in an HV side?
A tap changer is a mechanism in transformers which allows for
variable turn ratios to be selected in discrete steps. Transformers with this
mechanism obtain this variable turn ratio by connecting to a number of access
points known as taps along either the primary or secondary winding.
A tap is a connection point along a transformer winding that
allows a certain number of turns to be selected. This means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the output.
The tap selection is made with a tap changer mechanism.
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Can a transformer be tapped when it is on a load?
Yes.
Yes, on load tap changers are in use to adjust the output
voltage of power transformers since long. OLTC are attached to the power
transformers. Control cables run from OLTC to panel in the control room and it
can be operated auto /manual.
Variable autotransformers like Variacs and Power-stats have a
sliding contact that allows them to effectively change the tap point by turning
a knob while they are under load.
Many power utility systems have large, high power tap
changers that can change the tap point of a distribution transformer under load
to compensate for varying demand conditions.
There are two (perhaps three) types of tap changer available,
and the difference lies in the question you ask.
Off-load tap changer, sometimes called offline. Which
requires load disconnection before the contacts move. There is usually a
downstream breaker to interrupt the load during tap changing.
On load tap changer (make before break) . The contacts are
designed to have a shorting piece, and to make contact with the next tap,
before relinquishing contact with the first. The shorting piece has to be
resistive, to avoid a shorted turn.
On load tap changer (break before make). This is not really
onload, but the break during tap changing is very short indeed. The contacts
very nearly bridgeacross two taps but not quite. No resistance is needed.
Interruptions of 0.1 hallf cycles can easily be achieved.
The first type is rarely automated, the second two normally
are.
The construcion of each is very different, so they cannot be
used ith swopped duty.
Why is a tap changer used in an HV side?
In this type the tap changer circuit is placed in primary side or supply side. As we know;
Turns ratio = secondary winding turns (Ns)/ primary winding turns (Np)= Ns/Np.
Secondary voltage = (supply voltage or primary voltage) / Turns ratio.
It is mainly due to the current magnitude. The HV side has
higher voltage than the LV side but lower current than the LV side.
You want as low current as possible to be handled by your
tapchanger and,hence,the logical choice of installing the tapchanger on the
winding side with the lower current (i.e HV side).
Transformer Tap changer on Primary Side or HV side:
1) In normal operation the tap changer will be at 0% position to provide required designed secondary voltage.
2) If the supply voltage increases or load current decreases there will be an increase in supply voltage which is not desirable. At this case the tap position in the primary winding will rise towards positive direction i.e. +2.5%, and hence decreases the Np. This will increases the turns ratio (Ns/Np) further decreases the secondary voltage.
Transformer Tap changer on Primary Side or HV side:
1) In normal operation the tap changer will be at 0% position to provide required designed secondary voltage.
2) If the supply voltage increases or load current decreases there will be an increase in supply voltage which is not desirable. At this case the tap position in the primary winding will rise towards positive direction i.e. +2.5%, and hence decreases the Np. This will increases the turns ratio (Ns/Np) further decreases the secondary voltage.
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