Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-41
AP
2.11 Overvoltage protection
The inclusion of the two stages and their respective operating characteristics allows for a
number of possible applications;
Use of the IDMT characteristic gives the option of a longer time delay if the overvoltage
condition is only slight but results in a fast trip for a severe overvoltage. As the voltage
settings for both of the stages are independent, the second stage could then be set lower
than the first to provide a time delayed alarm stage if required.
Alternatively, if preferred, both stages could be set to definite time and configured to
provide the required alarm and trip stages.
If only one stage of overvoltage protection is required, or if the element is required to
provide an alarm only, the remaining stage may be disabled within the relay menu.
This type of protection must be co-ordinated with any other overvoltage relays at other
locations on the system. This should be carried out in a similar manner to that used for
grading current operated devices. The measuring mode (ph-N or ph-ph) and operating mode
(single phase or 3 phase) for both stages are independently settable.
2.12 Compensated overvoltage protection
Tempo
rary overvoltages in the order of seconds (even minutes) which may originate from
switching or load rejection.may damage primary plant equipment. In particular, this type of
overvoltage protection is applied to protect long transmission lines against Ferranti effect
overvoltages where the transmission line is energized from one end only. The following
graph shows the ferranti overvoltages calculated for a 345 kV and 765 kV transmission line
for different line lengths based on the formulas as in chapter (P54x/EN/OP)
Figure 11 Calculated ferranti voltage rise on 345 kV and 765 kV lines
The two stage compensated overvoltage element can be applied as alarming or trip
elements. Both stages’ time delays should be set not to pick-up for transient overvoltages in
the system with a typical time delays of 1-2 seconds upwards being adequate for most
applications. In the example above for a 345 kV transmission line of 400 km line length, the
alarm threshold (stage 1) can be set to 105% and the trip threshold set to 110% for example.
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