Alstom SHPM 101, Service Manual

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SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 3 of 74
Section 3. MECHANICAL LAYOUT
In order to ensure that the distance protection is unaffected by conditions of severe
high frequency interference, such as can occur in a high voltage substation, certain
precautions have been taken in designing the physical layout. These are:
a) separation of interface modules from measuring and control modules.
The interface modules occupy the right hand side of the subrack and provide
isolation between all connections with the outside world and the measuring and
control modules which occupy the left hand side of the subrack (see Appendix A
Figure 1).
The interface modules provide galvanic isolation to 5kV peak and filter out high
frequency common-mode and transverse-mode noise signals. The measuring
and control modules therefore operate in a relatively quiet electrical
environment. The need for galvanic isolation of circuits in the quiet space is
eliminated and so the relay internal dc supply centre rail is connected to the
case. The case earth stud is in turn connected to the relay room earth.
b) within the interface modules, isolating transformers have screens to minimise
primary to secondary capacitance coupling, thereby attenuating common-mode
interference. The efficiency of any screen is defined more by the electrical
strength of the connection to the common rail or earth than by the design of the
screen itself, so the larger transformers are mounted on metal plates and the
transformer screens are connected directly to the plates by short thick wires.
A special 4mm heavy-duty plug and socket arrangement allows each plate to
be connected to the earth stud of the case by multi-strand wire of thick cross
section (50/0.2mm wire). This ensures a low inductance between the screens
and earth. A low inductance is more important than low resistance because of
the high rates of rise of currents that exist in screen connections under
interference conditions.
c) three different categories of currents flow to earth. These are signal currents,
power supply currents and screen surge currents. To eliminate cross-coupling
effects, these are conducted by separate wires to the case earth stud. For
convenience, the signal and power supply earths are connected to a common
ground plane on the four layer pcb backplane which is employed to provide
interconnection of module signals. The ground plane also affords good
screening of the intermodule connections.
The ground plane is connected to the relay case at each mounting point and is
also connected to the case earth stud by a short thick wire.
Figure 2 shows the basic schematic of the relay earthing and screening
arrangements and paths of current flow for some typical common mode surges.