Functions
2.35 Sensitive Rotor Earth Fault Protection with 1 to 3 Hz Square Wave Voltage Injection (ANSI 64R - 1 to 3 Hz)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
245
2.35 Sensitive Rotor Earth Fault Protection with 1 to 3 Hz Square Wave
Voltage Injection (ANSI 64R - 1 to 3 Hz)
The rotor earth fault protection detects high and low resistance earth faults in the excitation circuit of synchro-
nous generators. An earth fault in the excitation winding itself causes no direct damage. If however a second
earth fault occurs, this results in a winding short-circuit in the excitation circuit. The resulting magnetic imbal-
ances can cause extreme mechanical forces which may destroy the machine. The following protection function
differs from the function described in Section 2.34 in that it is far more sensitive; it is used for large generators.
2.35.1 Functional Description
Basic Principle
The rotor earth fault protection works with a direct voltage of approx. 50 V, the polarity of which is reversed
between 1 and 4 times per second, depending on the setting. This voltage U
g
injected into the rotor circuit is
generated in the 7XT71 series device. The voltage passes through a resistor unit 7XR6004 (or 7XR6003) and
is symmetrically coupled to the excitation circuit via high-resistance resistors, and at the same time connected
to the earthing brush (potential to earth) via a low-resistance measuring shunt R
M
(see also Appendix). The
voltage taken at the measuring shunt and the control voltage are fed into the protection device via measuring
transducers. The control voltage is proportional to the injected 50 V voltage Ug in terms of amplitude and fre-
quency. The earth current flowing in the rotor is reflected by the measurement voltage.
Every time the polarity of the direct voltage U
g
is reversed, a charging current I
g
is driven across the resistor
unit into the earth capacitance of the excitation circuit. This current causes a proportional voltage drop U
Meas
in
the measuring shunt. Once the rotor earth capacitance is charged, the charging current drops to zero. If a rotor
earth fault is present a continuous earth current is driven. The amplitude depends on the fault resistance.
The use of a low-frequency square-wave voltage as displacement voltage eliminates the influence of the earth
capacitance and ensures at the same time a sufficient margin against interference signals from the excitation
system.
Figure 2-110 Connection Schematic of Voltage Injection into the Rotor Winding
CE Rotor-earth capacitance
Rs Series resistor
Ug Square-wave voltage from 7XT71
Ig Current flowing from the 7XT71 through the rotor into the earth
fg Square-wave frquency of the 7XT71
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