Siemens siprotec SJ62 User Manual page 175

2 Functions
Time Delays
173
be used for underfrequency protection. Set the pickup threshold higher than nominal
frequency if the element is to be used for overfrequency protection.
Note
If the threshold is set equal to the nominal frequency, the element is inactive.
If underfrequency protection is used for load shedding purposes, then the frequency
settings relative to other feeder relays are generally based on the priority of the cus-
tomers served by the protective relay. Normally a graded load shedding is required
that takes into account the importance of the consumers or consumer groups.
Further application examples exist in the field of power stations. The frequency values
to be set mainly depend, also in these cases, on the specifications of the power
system / power station operator. In this context, the frequency decrease protection
safeguards the power station's own demand by disconnecting it from the power
system on time. The turbo governor regulates the machine set to the nominal speed.
Consequently, the station's own demands can be continuously supplied at nominal fre-
quency.
Under the assumption that the apparent power is reduced by the same degree,
turbine-driven generators can, as a rule, be continuously operated down to 95% of the
nominal frequency. However, for inductive consumers, the frequency reduction not
only means an increased current input, but also endangers stable operation. For this
reason, only a short-term frequency reduction down to about 48 Hz (for f
58 Hz (for f = 60 Hz) is permissible.
N
A frequency increase can, for example, occur due to a load shedding or malfunction
of the speed regulation (e.g. in an island network). In this way, the frequency increase
protection can, for example, be used as overspeed protection.
The time delays (definite time) 81-1 DELAY to 81-4 DELAY entered at addresses
5405, 5408, 5411 and 5414 allow the device to prioritize or sort corrective actions
based on the degree to which the actual system frequency departs (upward or down-
ward) from the nominal system frequency, e.g. for load shedding equipment. The set
times are additional time delays not including the operating times (measuring time,
drop-out time) of the protective function.
= 50 Hz) or
N
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015