14/1/98 Page 2 Issue H
1. Introduction
The application of powerful microprocessors opens a new chapter for power system protective
relaying. The digital processing of measured values and the ability to perform complex arithmetic
and logic operations, give digital protection relays significant performance and flexibility
improvements over their traditional analogue counterparts. Additional advantages - very small power
consumption, adaptability, self-supervision, fault diagnosis through fault data recording, smaller
physical construction and selectable relay characteristics - all combine to allow the implementation
of accurate and highly reliable protection schemes at a significantly reduced financial burden.
The development of microprocessor based protective relays and their introduction into the market
has been stimulated by the recent trend to replace analogue with digital equipment. This modern
trend has prompted the development of a new P&B protective relay family - the MR relay series.
This comprehensive family of protection relays can satisfy the demands of even the most complex
protection schemes:
MRI - Overcurrent Relay (Independent time/I.D.M.T + earth + directional facilities)
MRI-V - Voltage Dependent Overcurrent Relay
MREF - Restricted Earth Fault Relay
MRMF - Mains Failure Relay
MRNS - Negative Sequence Relay
MRRP - Power Relay
MRCS - Check Synchronising Relay
MRFF - Field Failure Relay
The superiority of digital protective relaying over traditional analogue devices, as embodied by the
MR relay family, is summarised by the following features:
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• Integration of many protective functions in a single compact case
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• High accuracy owing to digital processing
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• Digital relay setting with very wide setting ranges and fine setting steps
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• Comfortable setting procedure through extensive human - relay dialogue
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• Measured values and fault data indication by means of alpha-numeric display
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• Data exchange with DCS/SCADA by means of RS485
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• Operational reliability through self-supervision
A similar but simplified range, with reduced functions and without display, is also available. The
MIRI - overcurrent and earth fault relays, and the MIRV - undervoltage, overvoltage and neutral
voltage displacement relays. To complement the MR series, a range of Auxiliary, Timing and
Tripping devices are also available.
The MPR range of motor protection relays embodies the developments in technology of the MRI,
and MIRI as well as previous electronic motor protection relays, with the traditional knowledge and
expertise developed over the long tradition of motor protection within P & B Engineering.
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General