Application - ABB RELION REX640 Technical Manual

1MRS759142 C
4.5.10.6
REX640
Technical Manual
immediately reset. The function resets when the calculated average
RMS value drops below Start value.
Blocking logic
There are three operation modes in the blocking function. The operation modes are
controlled by the BLOCK input and the global setting in Configuration/System/
Blocking mode which selects the blocking mode. The BLOCK input can be controlled
by a binary input, a horizontal communication input or an internal signal of the
protection relay's program. The influence of the BLOCK signal activation is
preselected with the global setting Blocking mode.
The Blocking mode setting has three blocking methods. In the "Freeze timers" mode,
the operation timer is frozen to the prevailing value, but the OPERATE output is not
deactivated when blocking is activated. In the "Block all" mode, the whole function
is blocked and the timers are reset. In the "Block OPERATE output" mode, the
function operates normally but the OPERATE output is not activated.

Application

Overvoltage in a network occurs either due to the transient surges on the network or
due to power frequency overvoltages. Surge arresters are used to protect the network
against the transient overvoltages; power frequency overvoltage of magnitude greater
than 15...20% of nominal voltage is normally handled by traditional overvoltage
protection. However, slow voltage variations are usually quantified by calculating the
RMS value of the voltage. To assess the RMS voltage, measurements are taken over
a relatively long period of time to avoid any instantaneous effect caused by switching
transients or faults.
It is important to monitor such slow voltage variation which can cause sub-optimal
operation of the equipment. EN 50160 quantifies slow voltage variation using 10 min
mean RMS value. CEI EN 50160 standard indicates that the RMS value of the voltage
calculated over a period of 10 min should not exceed 110% of the nominal voltage; if
the voltage exceeds this level, the generator must be disconnected from the network
within 3 s. PHVPTOV provides overvoltage protection by calculating the RMS value
over a period of 10 min.
A sudden change in the magnitude of the supply voltage is not immediately detected
by the RMS calculation; hence this is mainly used as an indication of voltage quality
supplied by the utility.
Section 4
Protection functions
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