Siemens SIPROTEC 7SD80 Manual page 134

Functions
2.9 Frequency Protection 81 (Optional)
Operating Ranges
Frequency evaluation requires a measured quantity that can be processed. This implies that at least a suffi-
ciently high voltage is available and that the frequency of this voltage is within the working range of the frequen-
cy protection.
The frequency protection automatically selects the largest of the phase-to-phase voltages. If all three voltages
are below the operating range of 65 % · V
indication 5215 „81 UnderV Blk" is displayed. If the voltage falls below this minimum value after a frequency
element has picked up, the picked up element will drop out. This implies also that all frequency elements 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 ele-
ment, 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 ele-
ments will drop out. If the frequency returns into the operating range, the measurement can be resumed pro-
vided that the measuring voltage is also inside the operating range. But if the measuring voltage is switched
off, the picked up element will drop out immediately.
Power Swings
In interconnected systems, 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 elements, power
swings may cause the frequency protection to pick up and even to trip. In these cases it is reasonable to block
the frequency protection once power swings are detected. This can be accomplished via binary inputs and
binary outputs (e.g. power swing detection of an external distance protection) or by corresponding logic oper-
ations 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 time delays of the frequency protection cor-
respondingly.
Pickup / Tripping
Figure 2-52 shows the logic diagram for the frequency protection function.
Once the frequency was reliably detected to be outside the configured thresholds of an element (above the
setting value for f> elements or below for f< elements), a pickup signal of the corresponding element is gener-
ated. The decision is considered reliable if 5 measurements taken in intervals of
outside the set threshold.
After pickup, a time delay per element can be started. When the associated time has elapsed, one trip
command per element is issued. A picked up element 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 element drops out, the tripping signal of the corresponding frequency element is im-
mediately 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 elements 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.
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(secondary), the frequency cannot be determined. In that case the
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E50417-G1140-C474-A1, Release date 09.2011
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/ period yield one frequency
2
SIPROTEC, 7SD80, Manual