Functions
2.13 Startup Overcurrent Protection (ANSI 51)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
98
2.13 Startup Overcurrent Protection (ANSI 51)
Gas turbines can be started by means of a startup converter. A controlled converter feeds a current into the
generator creating a rotating field of gradually increasing frequency. This causes the rotor to turn and thus drive
the turbine. At approx. 70 % of rated speed, the turbine is ignited and further accelerated until it attains rated
speed. The startup converter is thereby switched off.
2.13.1 Functional Description
Startup procedure
The following figure shows the characteristic quantities during startup. Please note that all quantities are scaled
to rated values.
Figure 2-26 Characteristic quantities during startup of a gas turbine (S
N
= 150 MVA; U
N
= 10.5 kV; P
Startup
Converter
= 2.9 MW)
Assuming that a short-circuit can occur in the generator during startup, a short-circuit protection is necessary
over the entire frequency range.
The automatic adaptation of sampling frequency to current generator frequency implemented in the 7UM62
offers great advantages for this purpose since sensitivity remains the same over the entire frequency range.
This adaptation starts on transition from 10 Hz to 11 Hz. As a result, all short-circuit protection functions, such
as time-overcurrent protection, impedance protection and differential protection are active with the same sen-
sitivity as with nominal frequency.
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