op
2
2
0.18 0.18
t
M
1
PUV Hz
TD TD
overexcitation
× ×
=
-
=
æ ö
ç ÷
è ø
ANSIEQUATION2298 V2 EN-US (Equation 191)
where:
M the relative excitation
Pickup1
is maximum continuously allowed voltage at no load, and rated frequency, in pu and
TD is time multiplier for inverse time functions, see figure 348.
Parameter TD (“time delay multiplier setting”) selects one delay curve from the family of curves.
The relative excitation M is calculated using equation
192
measured
measured rated
measured
measured
rated
M
V
V f
f
VBase
VBase f
f
= = ×
æ ö
ç ÷
è ø
æ ö
ç ÷
è ø
ANSIEQUATION2404 V1 EN-US (Equation 192)
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 193.
( )( )
opt
2
0
M t Pickup1 0.18dt TD- ³ ×
ò
ANSIEQUATION2300 V1 EN-US (Equation 193)
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-US (Equation 194)
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.
Section 9 1MRK 502 066-UUS B
Voltage protection
642
Technical manual
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