P54x/EN FD/La4 Firmware Design
(FD) 9-10
MiCOM P543, P544, P545 & P546
Note:
These relay contacts are POLARITY SENSITIVE. External wiring
must comply with the polarity requirements described in the external
connection diagram to ensure correct operation.
This board contains a hybrid of MOSFET solid state devices (SSD) in parallel with high
capacity relay output contacts. The MOSFET has a varistor across it to provide protection
which is required when switching off inductive loads as the stored energy in the inductor
causes a reverse high voltage which could damage the MOSFET.
When there is a control input command to operate an output contact the miniature relay is
operated at the same time as the SSD. The miniature relay contact closes in nominally
3.5 mS and is used to carry the continuous load current; the SSD operates in <0.2 ms and is
switched off after 7.5 mS. When the control input resets to open the contacts, the SSD is
again turned on for 7.5 mS. The miniature relay resets in nominally 3.5 ms before the SSD
and so the SSD is used to break the load. The SSD absorbs the energy when breaking
inductive loads and so limits the resulting voltage surge. This contact arrangement is for
switching dc circuits only. As the SSD comes on very fast (<0.2 ms) then these high break
output contacts have the added advantage of being very fast operating.
3.5ms + contact bounce
load current
relay contact
databus
control input
MOSFET reset
MOSFET operate
on
7ms
on
3.5ms
closed
on
7ms
off
FD
Figure 3 High break contact operation
2.5.3.1 High break contact applications
1) Efficient schem
e engineering
In traditional hardwired scheme designs, high break capability could only be achieved
using external electromechanical trip relays. External MVAJ tripping relays can be used
or the new high break contacts inside MiCOM relays can be used - reducing panel
space.
2) Accessibility of CB auxiliary contacts
Common practice is to use circuit breaker 52a (CB Closed) auxiliary contacts to break
the trip coil current on breaker opening, thus easing the duty on the protection contacts.
In cases such as operation of disconnectors, or retrofitting, it may be that 52a contacts
are either unavailable or unreliable. High break contacts can be used to break the trip
coil current in these applications.
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