Functions
2.20 Frequency Protection (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
382
The frequency protection automatically selects the largest of the phase-to-phase voltages. If all three voltages
are below the operating range of 65 % · U
N
(secondary), the frequency cannot be determined. In that case the
indication 5215
„Freq UnderV Blk“
is displayed. If the voltage falls below this minimum value after a fre-
quency stage has picked up, the picked up element will drop out. This implies also that all frequency stages will
drop out after a line has been switched off (with voltage transformers on line side).
When connecting a measuring voltage with a frequency outside the configured threshold of a frequency stage,
the frequency protection is immediately ready to operate. Since the filters of the frequency measurement must
first go through a transient state, the command output time may increase slightly (approx. 1 period). This is
because a frequency stage picks up only if the frequency has been detected outside the configured threshold
in five consecutive measurements.
The frequency range is from 25 Hz to 70 Hz. If the frequency leaves this operating range, the frequency stages
will drop out. If the frequency returns into the operating range, the measurement can be resumed provided that
the measuring voltage is also inside the working range. But if the measuring voltage is switched off, the picked
up stage will drop out immediately.
Power swings
In interconnected networks, frequency deviations may also be caused by power swings. Depending on the
power swing frequency, the mounting location of the device and the setting of the frequency stages, power
swings may cause the frequency protection to pickup and even to trip. In such cases out-of-step trips cannot
be prevented by operating the distance protection with power swing blocking (see also Section 2.6). Rather, it
is reasonable to block the frequency protection once power swings are detected. This can be accomplished via
binary inputs and binary outputs or by corresponding logic operations using the user-defined logic (CFC). If,
however, the power swing frequencies are known, tripping of the frequency protection function can also be
avoided by adapting the delay times of the frequency protection correspondingly.
Pickup/tripping
Figure 2-179 shows the logic diagram for the frequency protection function.
Once the frequency was reliably detected to be outside the configured thresholds of a stage (above the setting
value for f> stages or below for f< stages), a pickup signal of the corresponding stage is generated. The deci-
sion is considered reliable if five measurements taken in intervals of
1
/
2
period yield one frequency outside the
set threshold.
After pickup, one delay time per stage can be started. When the associated time has elapsed, one trip
command per stage is issued. A picked up stage drops out if the cause of the pickup is no longer valid after five
measurements or if the measuring voltage was switched off or the frequency is outside the operating range.
When a frequency stage drops out, the tripping signal of of the corresponding frequency stage is immediately
terminated, but the trip command is maintained for at least the minimum command duration which was set for
all tripping functions of the device.
Each of the four frequency stages can be blocked individually by binary inputs. The blocking takes immediate
effect. It is also possible to block the entire frequency protection function via binary input.
