Download Proposed Rev: Tap Changer Position during Induced Test

New proposed text for paragraph 10.8.1:
Each Class II power transformer shall receive an induced-voltage test with the required test levels induced
in the high-voltage winding. The tap connections shall be chosen, when possible, so that the voltage developed
in the other windings during the 1 hour test are x times their nominal system voltages, as specified in ANSI
C57.12.00, Table 5, where x is the ratio of the test voltage on the high-voltage winding to its nominal system
voltage, also referred to as overvoltage factor in the text below.
For a transformer built with a single magnetic core holding all windings, all windings are excited at a unique
induction level, often referred to as "Volts-per-turn". During induced test, with the transformer connected and
excited as in service, all windings are excited at the same overvoltage factor, regardless of what tap is selected.
Each winding turn receives the same voltage. In such cases, the selection of the tap changer position is guided
by the required voltage to be applied at regulated terminals. The position is selected to match the required
voltage on every terminal as close as possible.
The situation is quite different when transformers are equipped with auxiliary devices with separate magnetic
cores, such as preventive autos, series injectors, series regulators... Different magnetic cores can be excited at
different levels during operation or testing. In certain tap positions, these auxiliary devices do not have their
core excited at all. No voltage appears across their windings.
The selection of the tap changer position shall be guided by the following principle: "During an induced
overvoltage test, all windings on auxiliary magnetic cores shall receive the same overvoltage factor as windings
on the main transformer core." One exception to this principle is when such auxiliary devices are not meant to
be excited on a permanent basis but rather used only as transitional devices.
For transformers equipped with a series (booster) transformer, preventive auto, or any other device, the
selected tap position of the LTC shall be the one that will produce the highest voltage across the windings of
the series (booster) transformer, preventive auto, and other auxiliary devices as applicable. There can be a
conflict of choosing such tap position when more than one such device are present. In such case the selected
tap position of the LTC should be the best compromise so that all devices are tested with overvoltage. One
common example is the case where a series (booster) transformer and preventive auto are both present. In this
case, the tap selected shall be the one that is closest to the position that produces the highest voltage across the
windings of the series (booster) transformer and simultaneously excites the preventive auto, which is typically
a bridging position (not applicable when the preventive auto is energized only during transition). In order to
test the series (booster) transformer, preventive auto and other devices, the voltage developed on the terminals
of other windings may exceed the one hour level mentioned in IEEE C57.12.00, Table 5.
For certain types of devices such as series reactors used as current limiting devices, there is no voltage
developed across their windings during the induced voltage test as these devices are only excited when current
flows in their windings. This must be accepted as is.
The following examples can serve as a guide to select the LTC tap position for transformers having series
(booster) transformer and/or preventive autos in the main tank. These are only examples and do not intend to
cover all possible applications of auxiliary devices in transformers.
Transformers with series (booster) transformer only.
a) 33 Tap positions: 16R to 16L, Type of LTC connection: Linear or Coarse Fine. Selected Tap position:
16R
b) 33 Tap positions: 16R to 16L, Type of LTC connection: Reversing (Buck/Boost). Selected Tap
position: 16R or 16L, to be agreed with purchaser.
Transformers with preventive auto only.
a) 33 Tap positions: 16R to 16L, Type of LTC connection: Linear, Coarse Fine or Reversing
(Buck/Boost): the tap position which provides the highest voltage across the PA.
b) 16 Tap positions: 8R to 8L, Type of LTC connection: Linear, Coarse Fine or Reversing (Buck/Boost),
assuming the PA is used in bridging mode during transition only. Selected Tap Positions: as if there
was no PA.
Transformers having series (booster) transformer as well as preventive auto.
a) 33 Tap positions: 16R to 16L, Type of LTC connection: Linear or Coarse Fine. Selected Tap position:
15R if PA is excited only in bridging position, 16R otherwise.
b) 33 Tap positions:16R to 16L, Type of LTC connection: Reversing (Buck/Boost). Selected Tap position:
15R or 15L if PA is excited only in bridging position, 16R or 16L otherwise.
c) 16 Tap positions: 8R to 8L, Type of LTC connection: Linear or Coarse Fine, assuming the PA is used
in bridging mode during transition only. Selected Tap position: 8R
d) 16 Tap positions: 8R to 8L, Type of LTC connection: Reversing (Buck/Boost) , assuming the PA is
used in bridging mode during transition only. Selected Tap position: 8R or 8L.
In general this test can be performed for most power transformers. However, in some extreme cases of large
LTC reactors the demand for current can exceed the induced voltage supply capacity. In such a case the
manufacturer should explore all possibilities, including, but not limited to, changing system configuration,
adding compensating capacitors, or changing the test frequency until the required test voltage is induced . If
all the efforts to reach the test level fail, the test method and voltages should be agreed upon by the
manufacturer and purchaser, e.g. by accepting a combination of the test results at a full test voltage for a nonbridging tap position with the results at a lower voltage for a bridging tap position.
The manufacturer and purchaser can agree on a tap position different from the ones recommended above if
circumstances justify.