ABB REG650 ANSI, Technical Manual

Page: 286 / 728

9.4.7.2 Operate time of the overexcitation protection
The operate time of OEXPVPH is a function of the relative overexcitation.
The so called IEEE law approximates an inverse-square law and has been chosen
based on analysis of the various transformer overexcitation capability characteristics.
They match the transformer core capability well.
The inverse-square law is according to equation 89 .
op
2
2
0.18 0.18
t
M
1
V Hz>
k k
overexcitation
× ×
=
-
=
æ ö
ç ÷
è ø
IECEQUATION2298 V2 EN
(Equation 89)
where:
M
the relative excitation
V/Hz> Operate level of over-excitation function at no load in % of (UBase/f
rated
)
k is time multiplier for inverse time functions, see figure 140 .
The relative excitation M is calculated using equation 90
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 90)
Inverse delays as per figure 140 , can be modified (limited) by a special definite delay
setting tMin , see figure 139 .
Section 9
1MRK 502 048-UEN A
Voltage protection
280
Technical manual