ABB Relion 670 Series Technical Reference Manual page 540

Section 8
Voltage protection
534
An analog overexcitation relay would have to evaluate the following integral
expression, which means to look for the instant of time t = t
equation 142.
t
op
( ( ) )
ò
2
- ³
M t Pickup1 dt
0
ANSIEQUATION2300 V1 EN
A digital, numerical relay will instead look for the lowest j (that is, j = n) where it
becomes true that:
n
å
(
D × -
t M( j) PUV / Hz
=
j k
EQUATION1636 V1 EN
where:
is the time interval between two successive executions of OEXPVPH (24) and
Dt
Pickup1 is the relative excitation at (time j) in excess of the normal (rated) excitation which is given
M(j) -
as Vn/fn.
As long as M > Pickup1 (that is, overexcitation condition), the above sum can only be
larger with time, and if the overexcitation persists, the protected transformer will be
tripped at j = n.
Inverse delays as per figure 273, can be modified (limited) by two special definite
delay settings, namely t_MaxTripDelay and t_MinTripDelay, see figure 272.
×
0.18 TD
)
2
³ ×
0.18 TD
1MRK505222-UUS C
according to
op
(Equation 142)
(Equation 143)
Technical reference manual