The transformer secondary voltage and bus voltage may vary based on changes and variations in the
load, the load power factor, the transmission system, and the resistance and reactance of the load. The
aim of using an automatic voltage regulator is to maintain a stable secondary voltage and thus make
sure that the distribution voltage does not rise dangerously high or fall unusably low.
Utilities have to follow the regional, national and international regulations that specify the acceptable
voltage range. For example, in Finland regulations (SFS-EN 50160) require that the distribution voltage
is 230 V (phase-to-earth). Voltage quality measurement is done on a 10-minute average: 95 % of the
measured voltages must be ±10 % of the nominal voltage and all measured voltages must be
+10…-15 % of the nominal voltage. This measurement is usually taken from 20/0.4 kV distribution
transformers on MV overhead lines (rural areas) and cable networks (urban areas) so the 20kV
medium voltage is the side where the voltage has to be controlled for all distribution transformers
behind the feeding transformer by controlling the load tap changer. This control model is commonly
called bus regulation.
Other uses for voltage control are, for example, reactive power control and optimization of the
transmission lines.
Features and configuration
The automatic voltage regulator features separate operating windows for voltage raise and lower
commands. Both raise and lower commands have two operating stages with different operation voltage
levels and operation times. First stage of both voltage and raise commands have common definite time
delay for operation. Second stage Voltage raise and lower commands have a common definite and
inverse operating time, whereas undervoltage the in-built overcurrent function blocks all commands to
raise or lower the voltage. The target voltage as well as the operating settings for the voltage windows
can be changed by editing the setting groups. The tap changer's location is monitored with mA, RTD,
or digital input channel voltage measurement. The position of the tap changer can be controlled
automatically and manually. The AVR monitors the phase-to-phase voltage of the bus. External
commands can block the operation of the AVR either by completely blocking the control algorithm, or
by only blocking the control outputs.
The following examples present how to configure the automatic voltage regulator.
General se
General setttings
tings
General settings include the selection of the measurement reference voltage. Additionally, the
measured phase-to-phase voltage and the measurement input (if U4 is used for voltage
measurements) must be selected as well.
The image below two connection options for voltage measurement.
A
AQ
Q-T215
-T215
Instruction manual
Version: 2.04
262
