ABB ret650, Technical Manual

Page: 349 / 670

• master-follower method
• reverse reactance method
• circulating current method.
Parallel control with the master-follower method
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 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 (URAISE and ULOWER) generated by the master,
initiates the corresponding command in all follower TR8ATCCs 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
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 UB to a load point voltage
U
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 U
L
will be approximately equal to the length of
UB) from UB 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
This method requires extensive exchange of data between the TR8ATCC function
blocks (one TR8ATCC function for each transformer in the parallel group). The
TR8ATCC 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 504 135-UEN A
Section 12
Control
343
Technical manual