ABB ret650, Applications Manual

Page: 197 / 392

11.2.2 Application
SEMOD159067-1 v2
SEMOD159053-5 v5
When the load in a power network is increased the voltage will decrease and vice
versa. To maintain the network voltage at a constant level, power transformers are
usually equipped with on-load tap-changer. This alters the power transformer ratio
in a number of predefined steps and in that way changes the voltage. Each step
usually represents a change in voltage of approximately 0.5-1.7%.
The voltage control function is intended for control of power transformers with a
motor driven on-load tap-changer. The function is designed to regulate the voltage
at the secondary side of the power transformer. The control method is based on a
step-by-step principle which means that a control pulse, one at a time, will be
issued to the tap changer mechanism to move it one position up or down. The
length of the control pulse can be set within a wide range to accommodate different
types of tap changer mechanisms. The pulse is generated whenever the measured
voltage, for a given time, deviates from the set reference value by more than the
preset deadband (degree of insensitivity).
The voltage can be controlled at the point of voltage measurement, as well as at a
load point located out in the network. In the latter case, the load point voltage is
calculated based on the measured load current and the known impedance from the
voltage measuring point to the load point.
The automatic voltage control can be either for a single transformer, or for parallel
transformers. Parallel control of power transformers can be made in three
alternative ways:
• With the master-follower method
• With the reverse reactance method
• With the circulating current method
Of these alternatives, the first and the last require communication between the
function control blocks of the different transformers, whereas the middle
alternative does not require any communication.
The voltage control includes many extra features such as possibility to avoid
simultaneous tapping of parallel transformers, hot stand by regulation of a
transformer within a parallel group, with a LV CB open, compensation for a
possible capacitor bank on the LV side bay of a transformer, extensive tap changer
monitoring including contact wear and hunting detection, monitoring of the power
flow in the transformer so that for example, the voltage control can be blocked if
the power reverses and so on.
The voltage control function is built up by two function blocks which both are
logical nodes in IEC 61850-8-1:
• Automatic voltage control for tap changer, TR1ATCC for single control and
TR8ATCC for parallel control.
• Tap changer control and supervision, 6 binary inputs, TCMYLTC and 32
binary inputs, TCLYLTC
1MRK 504 169-UEN A
Section 11
Control
Transformer protection RET650 2.2 IEC 191
Application manual