Automatic control for tap changer, parallel control TR8ATCC (90)
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Parallel control of power transformers means control of two or more power transformers
connected to the same busbar on the LV side and in most cases also on the HV side. Special
measures must be taken in order to avoid a runaway situation where the tap changers on the
parallel transformers gradually diverge and end up in opposite end positions.
Three alternative methods can be used for parallel control with Automatic control for tap changer,
parallel control TR8ATCC (90):
• master-follower method
• reverse reactance method
• circulating current method.
Parallel control with the master-follower method
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In the master-follower method, one of the transformers is selected to be master, and will regulate
the voltage in accordance with the principles Automatic voltage control for a tap changer.
Selection of the master is made by activating the binary input FORCMAST in the TR8ATCC (90)
function block for one of the transformers in the group.
The followers can act in one of two alternative ways selected by a setting parameter:
1. Raise and lower commands (VRAISE and VLOWER) generated by the master, initiates the
corresponding command in all follower TR8ATCCs (90) simultaneously, and consequently they
will blindly follow the master commands irrespective of their individual tap positions.
2. The followers read the tap position of the master and adapt to the same tap position or to a
tap position with an offset relative to the master. In this mode, the followers can also be time
delayed relative to the master.
Parallel control with the reverse reactance method
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In the reverse reactance method, the LDC (Line voltage drop compensation) is used. The purpose
of which is normally to control the voltage at a load point further out in the network. The very
same function can also be used here but with a completely different objective. Whereas the LDC,
when used to control the voltage at a load point, gives a voltage drop along a line from the busbar
voltage V
B
to a load point voltage V
L
, the LDC, when used in the reverse reactance parallel control
of transformers, gives a voltage increase (actually, by adjusting the ratio X
L
/R
L
with respect to the
power factor, the length of the vector V
L
will be approximately equal to the length of V
B
) from V
B
up towards the transformer itself.
When the voltage at a load point is controlled by using LDC, the line impedance from the
transformer to the load point is defined by the setting
Xline
. If a negative reactance is entered
instead of the normal positive line reactance, parallel transformers will act in such a way that the
transformer with a higher tap position will be the first to tap down when the busbar voltage
increases, and the transformer with a lower tap position will be the first to tap up when the busbar
voltage decreases. The overall performance will then be that a runaway tap situation will be
avoided and that the circulating current will be minimized.
Parallel control with the circulating current method
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This method requires extensive exchange of data between the TR8ATCC (90) function blocks (one
TR8ATCC (90) function for each transformer in the parallel group). The TR8ATCC (90) function
block can either be located in the same IED, where they are configured in PCM600 to co-operate,
or in different IEDs. If the functions are located in different IEDs they must communicate via
GOOSE interbay communication on the IEC 61850 communication protocol.
1MRK 502 066-UUS B Section 14
Control
871
Technical manual
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