Measured Voltage - ABB REG650 Technical Manual

Section 9
Voltage protection
9.4.7.1
9.4.7.2
276
overexcited as long as the relative excitation is M ≤ Pickup1, Pickup1 expressed in %
of Vn/fn.
It is assumed that overexcitation is a symmetrical phenomenon, caused by events such
as loss-of-load, etc. It will be observed that a high phase-to-ground voltage does not
mean overexcitation. For example, in an ungrounded power system, a single phase-to-
ground fault means high voltages of the "healthy" two phases-to-ground, but no
overexcitation on any winding. The phase-to-phase voltages will remain essentially
unchanged. The important voltage is the voltage between the two ends of each winding.

Measured voltage

A check is made if the Selected voltage signal is higher than 70% of 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)
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 transformers' overexcitation capability characteristics. They
can match the transformer core capability well.
The inverse-square law is according to equation 86.
1MRK 502 043-UUS B
Technical Manual