2.16 Frequency Protection
263
7UT613/63x Manual
C53000-G1176-C160-2
2.16 Frequency Protection
The frequency protection function detects abnormally high and low frequencies. If the
network frequency lies outside the admissible range, appropriate actions are initiated.
For generators, e.g. the machine is separated from the network. Network decoupling
or load shedding can be initiated in networks.
A frequency decrease occurs when the system experiences an increase in real power
demand or sub-networks that cannot (or not fast enough) be compensated by addi-
tional performance generation. Real power demand has to be decreased by load
shedding. A faulty frequency, or speed regulation, can also be the cause in the power
station sector. Frequency decrease protection is also applied for generators which
operate (temporarily) to an island network. This is due to the fact that the reverse
power protection cannot operate in case of a drive power failure. The generator can
be disconnected from the power system using the frequency decrease protection.
An increase in system frequency occurs when large blocks of load are removed from
the system, or again when a malfunction occurs with a generator governor or AGC
system. For rotating machines this increased speed means an increased mechanical
loading. There is also a risk of self-excitation for generators feeding long lines under
no-load conditions.
Frequency protection consists of four frequency elements. Each stage is independent
and can initiate different control functions. Three frequency stages are designed for
decreased frequency detection (f<, f<<, f<<<), the fourth is an overfrequency stage
(f>).
It can also be used for three-phase protective objects. This implies that the device is
connected to a voltage transformer. This is therefore only possible for 7UT613 and
7UT633. As the frequency protection only gets its measuring information from the con-
nected voltage measurement, it leaves the assignment of currents to one side or a
measuring location for the function without coating. For the setting of the minimum
voltage for the frequency measurement: If the line protection for the frequency protec-
tion is assigned to a specific side of the protective object or to the three-phase busbar,
the voltage threshold is to be set as relative value (U/UN). The value is set to second-
ary in volts when assigned to a measuring location.
2.16.1 Function Description
The frequency protection in 7UT613/63x uses the positive sequence system from the
fundamental harmonic of the connected phase-to-earth voltages. Missing phase volt-
ages or phase-to-phase voltages have no negative affect, as long as the positive se-
quence system of the voltages is available in sufficient magnitude. If the measurement
voltage drops below a settable value U MIN, frequency protection is disabled because
precise frequency values can no longer be calculated from the signal.
The frequency protection cannot work if voltage or frequency are outside the working
range of the frequency protection (see Technical Data). If a frequency stage picks up
frequencies at >66 Hz (or >22 Hz at 16,7 Hz nominal frequency), the pickup is main-
tained. If the frequency increases and exceeds the operational range, or if the positive
phase-sequence voltage of 8.6 V as phase-phase voltage or 5 V as non-interlinked
voltage is undershot, the pickup is maintained and a trip on overfrequency is thus en-
abled.
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