7SIPROTEC 4/5, Injection Unit 7XT71, Product Information
C53000-B1174-C128-7, Edition 09.2016
5 Application
Large generators require a high sensitivity of the rotor ground fault protection. The
influence of the rotor ground capacitance must largely be eliminated, and the sig-
nal-to-noise ratio to the harmonics (for example 6
th
harmonic) of the excitation de-
vice must be increased.
This type of machine protection is implemented in the SIPROTEC family using the
Injection Unit 7XT71 in combination with the Resistor Unit 7XR6004 and the Multi-
functional Machine Protection 7UM62/Generator Protection 7UM85. This combina-
tion of devices continuously monitors the rotor ground resistance to ensure the
sensitive rotor ground fault protection. From hardware version /DD, the
Injection Unit 7XT71 can be connected to up to 2 devices 7UM62 or 7UM85. This
enables the protection function to be assigned to 2 protection groups increasing the
functional redundancy.
The square-wave generator of the Injection Unit 7XT71 generates a low-frequency
square-wave voltage of approximately ±50 V that can be adjusted in the device via
a DIP switch in increments of 0.5 Hz, from 0.5 Hz to 4.0 Hz. The Resistor Unit
7XR6004 injects a square-wave voltage between the rotor winding of a generator
and the ground brushes.
The square-wave voltage injected in the rotor winding causes a permanent charge
reversal of the rotor ground capacitance. The ground current enters the measuring
circuit of the Injection Unit 7XT71 where it is galvanically separated and injected
into the measuring input of the 7UM62 or 7UM85. If no fault is present (R
ground
infinite), the rotor ground current approximates zero after charging the ground ca-
pacitance. If a fault is present, the ground resistance and the series resistance of
the Resistor Unit 7XR6004 and the supply voltage determine the stationary current.
Simultaneously, the Injection Unit 7XT71 acquires the polarity reversal caused by
the square-wave voltage (square-wave voltage and square-wave frequency) and
injects it as galvanically separated control signals into the control input of the
7UM62/7UM85.
The 7UM62/7UM85 monitors the rotor ground circuit for interruptions by evaluating
the rotor ground current during the polarity reversals.
The sensitivity of the rotor ground fault protection can detecting fault resistances of
up to 80 k.
An AC/DC wide-range power supply unit processes the auxiliary voltage for the de-
vice modules in the Injection Unit 7XT71.
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