( )( )
opt
2
0
M t V Hz 0.18dt k- > ³ ×
ò
IECEQUATION2300 V1 EN (Equation 129)
A digital, numerical relay will instead look for the lowest j (that is, j = n) where it
becomes true that:
D
t M(j) V/Hz>–
( )
2
j k=
n
å
×
0.18 k
׳
EQUATION906 V1 EN (Equation 130)
where:
Dt
is the time interval between two successive executions of OEXPVPH and
M(j) -
V/Hz>
is the relative excitation at (time j) in excess of the normal (rated) excitation which is given
as Ur/fr.
As long as M > V/Hz> (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 251, can be modified (limited) by two special definite
delay settings, namely tMax and tMin, see figure
250.
0
M
max
- V/Hz>
M
max
tMin
V/Hz>
E
max
E (only if f = fr = const)
tMax
inverse delay law
overexcitation
under -
excitation
delay in s
99001067.vsd
Overexcitation M-V/Hz>
Excitation M
M=V/Hz>
IEC99001067 V1 EN
Figure 250: Restrictions imposed on inverse delays by tMax and tMin
A definite maximum time, tMax, can be used to limit the operate time at low degrees
of overexcitation. Inverse delays longer than tMax will not be allowed. In case the
inverse delay is longer than tMax, OEXPVPH trips after tMax seconds.
1MRK502052-UEN B Section 9
Voltage protection
523
Technical manual
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