ABB Relion REG670, Technical Manual

Page: 528 / 1298

• OEXPVPH 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
The operate time of OEXPVPH 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 127 .
op
2 2
0.18 0.18
t
M
1
V Hz>
k k
overexcitation
× ×
=
-
=
æ ö
ç ÷
è ø
IECEQUATION2298 V2 EN
(Equation 127)
where:
M
the relative excitation
V/Hz> is maximum continuously allowed voltage at no load, and rated frequency, in pu and
k
is time multiplier for inverse time functions, see figure 251 .
Parameter k (“time multiplier setting”) selects one delay curve from the family of curves.
The relative excitation M is calculated using equation 128
measured
measured rated
measured
measured
rated
M
U
U f
f
UBase
UBase f
f
=
= ×
æ ö
ç ÷
è ø
æ ö
ç ÷
è ø
IECEQUATION2404 V1 EN
(Equation 128)
An analog overexcitation relay would have to evaluate the following integral
expression, which means to look for the instant of time t = t
op
according to
equation 129 .
Section 9
1MRK502052-UEN B
Voltage protection
522
Technical manual