ABB Relion, Техническое руководство

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9.4.7.1 Measured voltage
A check is made if the Selected voltage signal is higher than 70% of the rated phase-to-
ground voltage. When below this value, OEXPVPH (24) exits immediately and no
excitation is calculated.
The frequency value is received from the pre-processing block. The function operates for
frequencies within the range of 33-60 Hz and of 42-75 Hz for 50 Hz and 60 Hz
respectively.
• OEXPVPH (24) can be connected to any power transformer side, independent from
the power flow.
• The side with a load tap changer must not be used, since the tap changer can change
the relative excitation (M)
9.4.7.2 Operate time of the overexcitation protection
The operate time of OEXPVPH (24) 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
PUV Hz
TD TD
overexcitation
× ×
=
-
=
æ ö
ç ÷
è ø
ANSIEQUATION2298 V2 EN
(Equation 89)
where:
M the relative excitation
Pickup1 Operate level of over-excitation function at no load in % of (/f
rated
)
TD is time multiplier for inverse time functions, see figure 140 .
The relative excitation M is calculated using equation 90
Section 9
1MRK 502 048-UUS A
Voltage protection
304
Technical manual