Directional protection and directional zone selectivity

Overview

Directional protection D works like a definite-time S function but with two delays, t7FW (forward) and t7BW (backward), applied once threshold I7 is exceeded; it needs no auxiliary supply or dedicated wiring. Direction is decided from reactive power: the phase reactive power must exceed 2% of nominal phase power, otherwise the relay uses the shorter of t7FW and t7BW.

Directional zone selectivity requires zone selectivity S and G to be off and a 24 V DC auxiliary supply; each relay has DFin/DBin inputs and DFout/DBout block outputs. Breakers bounding the faulted section trip at t7s, and if one fails to open the next upstream breaker opens after about a further 100 ms. Without auxiliary power, directional zone selectivity degrades to plain directional protection with t7FW/t7BW times, which the designer must allow for.

The paper shows how lock signals between relays isolate a faulted section in ring or meshed networks. Worked examples cover two generators on a common busbar, a double-transformer substation with bus tie, and a board fed by LV generators in parallel with transformers, followed by a practical commissioning guide.

Key points

1. D works like a definite-time S with two delays (t7FW forward, t7BW backward) above threshold I7; no auxiliary supply or dedicated wiring needed.
2. Direction comes from reactive power; phase reactive power must exceed 2% of nominal phase power, otherwise the shorter of t7FW and t7BW is used.
3. Directional zone selectivity needs S and G zone selectivity off and a 24 V DC auxiliary supply; DFin/DBin inputs and DFout/DBout outputs.
4. Breakers bounding the faulted section trip at t7s; if one fails, the next upstream opens about 100 ms later.
5. Without auxiliary power it degrades to plain directional protection with t7FW/t7BW times.
6. Examples: two generators on one busbar, MV/LV substation with bus tie, LV generators in parallel with transformers.

Using it in design

• For main switchboards with two transformers plus bus tie, or generator paralleling boards, flag that conventional time selectivity cannot discriminate fault direction and propose directional or directional-zone selectivity on incomers and bus tie.
• Capture per breaker whether it supports D/ZSI-D and whether auxiliary supply is present.
• Warn that loss of 24 V DC auxiliary reverts to slower directional times and include that in the selectivity study.

Who should read it

Electrical engineers designing selectivity for multi-source LV boards with parallel transformers or generators.

Related standards

• IEC 60947-2
• IEC 60947-1

Publication details

Authors

ABB

Publisher

ABB

Year

2008

Document number

1SDC007401G0201

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation