Frequency Protection (Optional); Functional Description - Siemens SIPROTEC 7SD610 Manual

Functions

2.12 Frequency Protection (optional)

2.12 Frequency Protection (optional)
The frequency protection function detects overfrequencies or underfrequencies in the system or in electrical
machines. If the frequency is outside the permissible range, appropriate actions are initiated such as load shed-
ding or separating the generator from the system.
Underfrequency is caused by increased real power demand of the loads or by a reduction of the generated
power e.g. in the event of disconnection from the network, generator failure or faulty operation of the power
frequency control. Underfrequency 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 by means of the underfrequency protection.
Underfrequency also results in increased reactive power demand of inductive loads.
Overfrequency is caused for instance by load shedding, system disconnection or malfunction of the power fre-
quency control. There is also a risk of self-excitation for generators feeding long lines under no-load conditions.
2.12.1

Functional Description

Frequency stages
Frequency protection consists of the four frequency stages f1 to f4 Each stage can be set as overfrequency
stage (f>) or as underfrequency stage (f<) with individual thresholds and time delays. This enables the stages
to be adapted to the particular application.
• If a stage is set to a value above the rated frequency, it is automatically interpreted to be an overfrequency
stage f>.
• If a stage is set to a value below the rated frequency, it is automatically interpreted to be an underfrequency
stage f<.
• If a stage is set exactly to the rated frequency, it is inactive.
Each stage can be blocked via binary input and also the entire frequency protection function can be blocked.
Frequency measurement
The largest of the 3 phase-to-phase voltages is used for frequency measurement. It must amount to at least
65 % of the nominal voltage set in parameter 204, Unom SECONDARY. Below that value frequency measure-
ment will not take place.
Numerical filters are used to calculate a virtual quantity from the measured voltage. This quantity is proportional
to the frequency and is practically linear in the specified range (f
ensure that the frequency measurement is free from harmonic and phase jumps influences.
An accurate and quick measurement result is obtained by considering also the frequency change. When
changing the frequency of the power system, the sign of the quotient
repeated measurements. If, however, a phase jump in the measured voltage temporarily simulates a frequency
deviation, the sign of
are quickly discarded.
The dropout value of each frequency element is approximately 20 mHz below (for f>) or above (for f<) of the
pickup value.
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.
172
Δf
/ will subsequently reverse. Thus the measurement results corrupted by a phase jump
dt
± 10 %). Filters and repeated measurements
N
Δf / remains unchanged during several
dt
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011