Heat accumulated in critical parts during a period of overexcitation will be reduced gradually
when the excitation returns to the normal value. If a new period of overexcitation occurs after
a short time interval, the heating will start from a higher level, therefore, OEXPVPH must have
thermal memory. A fixed cooling time constant is settable within a wide range.
The general experience is that the overexcitation characteristics for a number of power
transformers are not in accordance with standard inverse time curves. In order to make
optimal settings possible, a transformer adapted characteristic is available in the IED. The
operate characteristic of the protection function can be set to correspond quite well with any
characteristic by setting the operate time for six different figures of overexcitation in the
range from 100% to 180% of rated V/Hz.
When configured to a single phase-to-phase voltage input, a corresponding phase-to-phase
current is calculated which has the same phase angle relative the phase-to-phase voltage as
the phase currents have relative the phase voltages in a symmetrical system. The function
should preferably be configured to use a three-phase voltage input if available. It then uses the
positive sequence quantities of voltages and currents.
Analog measurements shall not be taken from any winding where a load tap
changer is located.
Some different connection alternatives are shown in figure
.
G
U/f>
24
U/f>
24
U/f>
24
en05000208.vsd
IEC05000208 V1 EN-US
Figure 301: Alternative connections of an Overexcitation protection OEXPVPH(Volt/Hertz)
9.4.3
Setting guidelines
IP15039-1 v1
9.4.3.1
Recommendations for input and output signals
M6496-75 v4
Recommendations for Input signals
M6496-79 v5
Please see the default factory configuration.
BLOCK: The input will block the operation of the Overexcitation protection OEXPVPH, for
example, the block input can be used to block the operation for a limited time during special
service conditions.
1MRK 505 343-UEN B
Section 9
Voltage protection
479
Application manual
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