Functions
156
7UM62 Manual
C53000-G1176-C149-3
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
customers 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
ensures the power station’s own demand by disconnecting it from the power system
on time. The turbo regulator regulates the machine set to the nominal speed.
Consequently, the station’s own demands can be continuously supplied at nominal
frequency.
Under the assumption that the apparent power is reduced to 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
does not only mean an enhanced current input but also endangers stable operation.
For this reason, only a short-time frequency reduction down to about 48 Hz (for f
N
=
50 Hz) or 58 Hz (for f
N
= 60 Hz) is permissible.
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.
Setting example:
Delays
The time delays (definite time)
to
entered at addresses
and
) allow the device to prioritize or sort corrective actions based on the degree
to which the actual system frequency departs (upward or downward) from the nominal
system frequency. The set times are additional time delays not including the operating
times (measuring time, drop-out time) of the protective function.
Minimum Voltage
Address
is used to set the minimum voltage. If the positive sequence
voltage is less than this value, the frequency protection is blocked. The recommended
value is approx. 65 % U
N
. The minimum voltage limiting can be deactivated by means
of the”
0
” setting.
Stage
Cause
Setting Values
for f
N
= 50 Hz
for f
N
= 60 Hz
Delays
f1
Disconnection from the
network
48.00 Hz
58.00 Hz
1.00 s
f2
Shutdown
47.00 Hz
57.00 Hz
6.00 s
f3
Alarm
49.50 Hz
59.50 Hz
20.00 s
f4
Alarm or tripping
52.00 Hz
62.00 Hz
10.00 s
