ABB Relion 670 series, Technical Manual

Page: 647 / 1432

remain essentially unchanged. The important voltage is the voltage between the two ends of each
winding.
9.4.7.1 Measured voltage
M5854-39 v8
If one phase-to-phase voltage is available from the side where overexcitation protection is
applied, then Overexcitation protection OEXPVPH (24) shall be set to measure this voltage,
MeasuredV. The particular voltage which is used determines the two currents that must be used.
This must be chosen with the setting MeasuredI.
It is extremely important that
MeasuredV and MeasuredI are set to same value.
If, for example, voltage Vab is fed to OEXPVPH(24), then currents Ia, and Ib must be applied. From
these two input currents, current Iab = Ia - Ib is calculated internally by the OEXPVPH (24)
algorithm. The phase-to-phase voltage must be higher than 70% of the rated value, otherwise the
protection algorithm exits without calculating the excitation. ERROR output is set to 1, and the
displayed value of relative excitation V/Hz shows 0.000.
If three phase-to-ground voltages are available from the side where overexcitation is connected,
then OEXPVPH (24) shall be set to measure positive sequence voltage and current. In this case the
positive sequence voltage and the positive sequence current are used by OEXPVPH (24). A check is
made if the positive sequence voltage is higher than 70% of rated phase-to-ground voltage, when
below this value, OEXPVPH (24) exits immediately, and no excitation is calculated. ERROR output is
set to 1, and the displayed value of relative excitation V/Hz shows 0.000.
• OEXPVPH (24) can be connected to any power transformer side, independent from the power
flow.
• The side with a possible load tap changer must not be used.
9.4.7.2 Operate time of the overexcitation protection
M5854-52 v8
The operate time of OEXPVPH (24) is a function of the relative overexcitation.
Basically there are two different delay laws available to choose between:
• the so called IEEE law, and
• a tailor-made law.
The so called IEEE law approximates a square law and has been chosen based on analysis of the
various transformers’ overexcitation capability characteristics. They can match the transformer
core capability well.
The square law is according to equation 191 .
1MRK 502 066-UUS B Section 9
Voltage protection
641
Technical manual