Application - Siemens SIPROTEC 4 7XT71 Manual

Introduction

1.1 Application

1.1 Application
Machine Protection
For large generators, a high sensitivity is necessary for the rotor ground-fault protection. The influence of the
rotor ground capacitance must be eliminated as far as possible degree and the signal-to-noise ratio with
respect to the harmonics (for example, the 6th harmonic) of the excitation machine must be increased.
For this type of machine protection, the injection unit 7XT71 is used in the SIPROTEC family, in connection
with the resistor unit 7XR6004 or 7XR8004 and the multi-functional machine protection 7UM62 or generator
protection 7UM85. This combination of devices ensures sensitive rotor ground-fault protection by conti-
nuously monitoring the rotor ground resistance. Starting with hardware version /DD, up to 2 devices 7UM62
or 7UM85 can be connected to the injection unit 7XT71. In this way, the protection function can be assigned
to 2 protection groups and a greater functional redundancy can be achieved.
Square-Wave Generator
The square-wave generator of the injection unit 7XT71 produces a low-frequency square-wave voltage of
about ±50 V. This voltage can be set from 0.5 Hz to 4.0 Hz in increments of 0.5 Hz in the device using a DIP
switch. The square-wave voltage is supplied via the resistor unit 7XR6004 or 7XR8004 between the rotor
winding of a generator and the grounding brushes.
The square-wave voltage injected into the rotor winding results in a constant charge reversal of the rotor
ground capacitance. The ground current is detected via the measuring circuit in the injection unit 7XT71 and
injected, galvanically separated, into the measuring input of the 7UM62 or 7UM85. If the machine is operating
normally and without fault (R
is charged. In case of an error, the fault resistance and the series resistance of the resistor unit 7XR6004 or
7XR8004, as well as the supplying voltage, determine the stationary current.
At the same time, the polarity reversal (square-wave voltage and frequency) generated by the square-wave
voltage in the injection unit 7XT71 is detected and fed to the control input of the 7UM62/7UM85 in the form
of control signals after being galvanically separated.
Monitoring
The rotor ground circuit is monitored for interruptions in the 7UM62/7UM85 by evaluation of the rotor ground
current during polarity reversals. The sensitivity of the rotor ground-fault protection is designed so that fault
resistances up to 80 kΩ are detected.
The auxiliary voltage for the printed circuit board assemblies of the device is processed using an AC/DC wide-
range power supply unit in the injection unit 7XT71.
NOTE
i
i
7UM62:
You can find a detailed description of the method of measurement in connection with the 7UM62 in the
SIPROTEC manual 7UM62, order number C53000-G1100-C149.
7UM85:
You can find a detailed description of the method of measurement in connection with the 7UM85 in the
SIPROTEC manual 7UM85, order number C53000-G5000-C027.
10
≈ infinite), the rotor ground current is virtually 0 after the ground capacitance
gnd
SIPROTEC 4/5, Injection unit 7XT71, Manual
C53000-B1174-C128-8, Edition 05.2020