Frequency Protection 81 O/U; Description - Siemens siprotec SJ62 User Manual

2 Functions
2.9

Frequency Protection 81 O/U

Applications
2.9.1

Description

Detection of Fre-
quency
Underfrequency
and Overfrequency
Protection
Operating Ranges
171
The frequency protection function detects abnormally high and low frequencies in the
system or in electrical machines. If the frequency lies outside the allowable range, ap-
propriate actions are initiated, such as load shedding or separating a generator from
the system.
• A decrease in system frequency occurs when the system experiences an increase
in the real power demand, or when a malfunction occurs with a generator governor
or automatic generation control (AGC) system. The frequency protection function is
also used for generators, which (for a certain time) operate 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, e.g. when large blocks of load (island net-
work) are removed from the system, or again when a malfunction occurs with a gen-
erator governor. This entails risk of self-excitation for generators feeding long lines
under no-load conditions.
The frequency is detected from the phase–to–phase voltage V
device. If the amplitude of this voltage is too small, one of the other phase–to–phase
voltages is used instead.
With the applications of filters and repeated measurements, the frequency evaluation
is free from harmonic influences and very accurate.
Frequency protection consists of four frequency elements. To make protection flexible
for different power system conditions, theses stages can be used alternatively for fre-
quency decrease or increase separately, and can be independently set to perform dif-
ferent control functions.
The parameter setting decides for what purpose the particular element will be used:
• Set the pickup threshold lower than nominal frequency if the element is to be used
for underfrequency protection.
• Set the pickup threshold lower than nominal frequency if the element is to be used
for overfrequency protection.
• If the threshold is set equal to the nominal frequency, the element is inactive.
The frequency can be determined if for three-phase voltage transformer connections
the positive frequency component of the voltages or for single-phase voltage trans-
former connections the corresponding voltage is present and of sufficient magnitude.
If the measured voltage drops below a settable value Vmin, the frequency protection
is blocked since a precise frequency value can no longer be calculated from the signal
under these conditions.
applied to the
A-B
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015