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
142.
( )( )
opt
2
0
M t Pickup1 0.18dt TD- ³ ×
ò
ANSIEQUATION2300 V1 EN (Equation 142)
A digital, numerical relay will instead look for the lowest j (that is, j = n) where it
becomes true that:
( )
n
2
j k
t M( j) PUV / Hz 0.18 TD
=
D × - ³ ×
å
EQUATION1636 V1 EN (Equation 143)
where:
Dt is the time interval between two successive executions of OEXPVPH (24) and
M(j) -
Pickup1
is the relative excitation at (time j) in excess of the normal (rated) excitation which is given
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.
Section 8 1MRK505222-UUS C
Voltage protection
534
Technical reference manual
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