if the pickup value (parameter Threshold) of the stage is set to 49.8 Hz and the Dropout differential
to 100 mHz, the stage will drop off at 49.9 Hz.
Setting example of the underfrequency protection for generators
Underfrequency protection provides an additional protection for the turbine. It has the task of ensuring the
demand of the power plant system by a timely decoupling from the electrical power system. After the decou-
pling from the electrical power system, the turbo set controller sets the generator unit to the rated speed.
Turbogenerators can operate permanently at as little as 95 % of the rated frequency. This requires the appa-
rent power to be reduced by the same factor. A decrease of the frequency leads not only to a higher current
consumption by the inductive loads but is also a hazard to stable operation.
This enables the supply of power plant's own system to continue at rated frequency. The decoupling from the
electrical power system is initiated without delay at 0.95 f
rated
. After the decoupling, the generating unit has to
recover. If it does not, the turbine is stopped for its own protection. In the turbine, underfrequency can lead to
resonances and inadmissible strains of the turbine blades. You can delay the tripping in order to give the
speed controllers sufficient time for a response.
The frequency value to be set depends on the specifications of the power plant operator or the turbine manu-
facturer (speed monitoring). It is approx. 0.94 f
rated
.
For this application, 3 stages of the underfrequency protection are used. The following table shows a setting
suggestion.
Stage Caused by Setting Values
at f
rated
= 50 Hz at f
rated
= 60 Hz Delay
f1< Warning 49,80 Hz 59,80 Hz 10.00 s
f2< Decoupling from power system 47,50 Hz 57,00 Hz 0.00 s
f3< Shutdown 47,00 Hz 56,40 Hz 10.00 s
NOTE
This table shows one example of a possible frequency protection setting. The setting values can differ
depending on the application and the turbine.
Settings
Addr.
Parameter C Setting Options Default Setting
General
_:2311:101 General:Minimum voltage 3.000 V to 175.000 V 37.500 V
_:2311:109 General:Dropout differen-
tial
20 mHz to 2000 mHz 20 mHz
Stage 1
_:61:1 Stage 1:Mode
•
off
•
on
•
test
off
_:61:2 Stage 1:Operate & flt.rec.
blocked
•
no
•
yes
no
_:61:3 Stage 1:Threshold 40.00 Hz to 70.00 Hz 1.00 Hz
_:61:6 Stage 1:Operate delay 0.00 s to 600.00 s 10.00 s
Stage 2
_:62:1 Stage 2:Mode
•
off
•
on
•
test
off
_:62:2 Stage 2:Operate & flt.rec.
blocked
•
no
•
yes
no
7.19.5
Protection and Automation Functions
7.19 Underfrequency Protection
832 SIPROTEC 5, High-Voltage Bay Controller, Manual
C53000-G5040-C015-9, Edition 11.2017
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