FEEDER PROTECTION
COORDINATING DEVICES FOR FEEDER CONDUCTOR PROTECTION
• Types Of Feeder Conductor Circuits
• Types Of Protection
• Factors Affecting Protection
• Cable Damage Criteria
• Problem 5
• Problem 6
ABB REF 601 FEEDER PROTECTION RELAY
ABB REF 541 FEEDER TERMINAL
PILOT WIRE PROTECTION
• Internal Faults
• External Faults
DISTANCE RELAYING
• Principles of Distance Relays
• Relay Performance
• Electromechanical/Static Distance Relays
• Digital/Numerical Distance Relays
• Zones of Protection
• Distance Relay Characteristics
• Distance Relay Implementation
• Distance Protection Setting Example
ABB REL521 DISTANE PROTECTION TERMINAL
COORDINATING DEVICES FOR CONDUCTOR ROTECTION
Types of Conductor Circuits
Power cables (conductors) are divided into two voltage classes: low voltage (600 volts
and below) and medium voltage (above 600 volts).
Low Voltage: In general, NEC Article 240-3 requires that conductors that are rated 600 volts or less be protected in accordance with their ampacities; however, there are
exceptions to the basic rule, such as motor and motor control circuits and tapped
conductors. The NEC should be reviewed for these special cases (exceptions). MCCBs
and LVPCBs are used to provide both overload and short circuit protection for low voltage
conductors.
Medium Voltage: Protection of medium voltage conductors is further separated into two sub-categories: aerial lines and cables in conduit.
Aerial Lines: Although aerial lines usually sustain more faults over their life than do
cables in conduit, aerial line faults are also usually self-clearing, which means that the
faults were caused by high winds, lightning strikes, or animals (e.g., birds and squirrels).
Distribution fuses (e.g., K or T links), NEMA Type E power fuses, and relays (medium
voltage power circuit breakers) are used to protect aerial lines.
Cables in Conduit: NEC Article 240-100 only requires that feeders have short circuit
protection in each ungrounded conductor. The same article sets limits on the protective
device’s maximum ratings that are in excess of the conductor’s full load (continuous)
current ratings. In general, the NEC is specifying relay (breaker) and fuse ratings to
protect the medium voltage cables from phase and ground faults and they (the NEC) leave
overload protection of the cables to the designer’s preference (choice).
Types of Protection
Conductors must be protected against overloads, phase, and ground fault conditions that may range from 100-250% of the conductor’s continuous current ratings for overloads, and anywhere from 500-2000% of the conductor’s continuous current ratings for faults.
Overloads: The bimetalic thermal elements of an MCCB, the long time functions (LTPU
and LTDB) of an LVPCB, the overcurrent time-delay relay (ANSI Device No. 51), and up to
approximately 250% of a fuse’s continuous current rating (Icontinuous) provide overload
protection for conductors.
Phase Faults: The magnetic (instantaneous) element of an MCCB, the short time (STPU
and STDB) and instantaneous (IT) functions of an LVPCB, an instantaneous relay (ANSI
Device No. 50), and the fuse (>250% of Icontinuous) provide phase fault (short circuit)
protection for conductors.
Ground Faults: Ground fault protection of conductors requires separate devices. To trip
a MCCB on ground faults requires a special shunt-trip attachment or a separate ground
sensing (e.g., ANSI Device No. 50GS) relay. LVPCBs have separate built-in ground fault
functions (GFPU and GFT). Ground fault relays are available in both time-delay (ANSI
Device No. 51G) and instantaneous (ANSI No. Device 50G) models, which are basically
the same models as their phase fault counterparts except that the tap settings are much more sensitive (e.g., 0.5 A versus 5 A). Fuses provide very poor to no protection for
ground faults
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