Frequency Protection; Functional Description - Siemens SIPROTEC 4 7UT6 Series Manual

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 additional 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 connected 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 protection 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 secondary in volts when assigned to a measuring location.

Functional Description

2.16.1
The frequency protection in 7UT613 / 7UT633 uses the positive sequence system from the fundamental
harmonic of the connected phase-to-earth voltages. The lack of phase voltages or phase-to-phase voltages
thus has no negative impact as long as the positive sequence voltage is present and of sufficient magnitude. If
the positive sequence voltage drops below a settable value U MIN, frequency protection is disabled because
precise frequency values can no longer be calculated from the measured quantity.
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 maintained. 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 enabled.
Maintaining the pickup is ended if the frequency measurement reads again frequencies <66 Hz (or <22 Hz) or
the frequency protection is blocked via the indication >FQS. Each frequency stage has a set delay time.
Each of the four frequency elements can be blocked individually by binary inputs. The entire frequency protec-
tion can be blocked via a binary input. A corresponding command is signalled after the delay time.
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
Functions
2.16 Frequency Protection
239