5.5. Control functions
5.5.1. Automatic voltage regulator AVR (90)
Automatic voltage regulator (AVR) is used for secondary voltage control in transformers which have on-
load tap changer (LTC). Voltage regulator raises or lowers the secondary voltage based onto
measurements of the bus voltage. Actual controlling takes place in LTC which either increases or
decreases the secondary winding thus causes increase or decrease of the transformer output voltage.
Transformer secondary voltage / bus voltage may vary based onto the load change, load power factor
changes, transmission system variations, R and X of the load changes and LTC changes. From these
mentioned quantities the LTC changes. Aim of using automatic voltage regulator is to maintain stable
secondary voltage so that distribution voltage does not rise dangerously high or fall unusably low.
For the acceptable voltage range utilities have to follow regional or governmental regulations. For
example, in Finland regulation (SFS-EN 50160) requires that distribution voltage phase to ground is
230V. Voltage quality measurement is done with 10-minute average, within this measurement 95% of
the measured voltages must be ±10% of nominal voltage and all measured voltages must be +10% …
-15% of the nominal voltage. This voltage is normally gotten from 20/0.4 kV distribution transformers
on the medium voltage overhead line (rural areas) and cable networks (urban areas) so the 20kV
medium voltage is the place where the voltage has to be controlled for all distribution transformers
behind the feeding transformer by controlling the LTC. This control model is commonly called “Bus
regulation”.
Other uses for voltage control are for example reactive power control and optimization in the
transmission lines.
AVR features and con guration
AVR features separate de nite and inverse operating time voltage raise and lower windows, instant
overvoltage lower, undervoltage blocking and inbuilt overcurrent blocking functions. Target voltage as
well as operating settings for the voltage windows can be changed via setting group changes. Tap
changer location is monitored with mA, RTD or digital input channel voltage measurement. Tap
changer position can be controlled with automatic control and manual control. AVR monitors bus
phase to phase voltage. Operation of the AVR can be blocked by external commands by blocking
either control algorithm completely or blocking the control outputs only.
Con guring of the AVR is presented in following examples.
General settings
General settings include the measurement reference voltage selection. In this part the measured phase
to phase voltage has to be selected as well as the measurement input in case if U4 input is used for
voltage measurements.
AQ-T257
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
206
