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Product Overview
Application
The REL 356 is a dual-microprocessor based, composite sequence filter, current differential protection system. The
REL 356 operates on the principles inherited from previous successful current differential systems (LCBII); but adapted
and improved using numerical techniques.
The REL 356 is a pilot system, utilizing a communication channel with wide choice of analog and digital communication
options.
The REL 356 is a high speed relaying system and is suitable for application to any voltage level. Its principle of operation
makes it ideal for short lines and tapped lines with a power transformer, where traditional distance protection are not
practical.
The REL 356 is a current-only system and provides all benefits associated with relaying systems not needing potential
transformers such as:
· Unaffected by CCVT transients
· Unaffected by power swings
· Not affected by mutuals on parallel lines
An optional distance relaying system has been included to provide back-up for loss of communication channel. This
back-up system consists of a two zone distance unit and logic for non-pilot relaying system. Phase and ground distance
units are included.
The current differential protection is inherently immune to system swings and the relay will block the tripping. However,
if power swing trip is desired, blinders have been provided for detecting this condition. OST (Out-of-Step Trip) is included
in the optional distance back-up system.
An overcurrent tripping function is also included in the relaying system. The high set overcurrent function activates
instantaneous trip when the phase (IPH) or the ground (IGH) threshold units detect currents above the settings. These
units may be supervised by the directional units. The phase units are supervised by FDOP (Forward Directional Overcurrent
Phase) and the ground unit is supervised by FDOG (Forward Directional Overcurrent Ground). Connection of external
voltage transformers is required to activate the directional units.
The REL 356 also requires the connection to voltage transformers for out-of-step trip, distance protection, fault location,
loss-of-potential and loss-of-current detection.
Because of the setting ranges available in the relaying system, it is possible to accommodate different ct ratios at the
two terminals of the transmission line.
The REL 356 relay has the capability, through its channel modem, to accurately measure the communication channel
delay. A continuous channel delay measurement with delay compensation is provided.
REL 356 System Operation
The basic operation of the REL 356 relay system performs a true differential comparison of line current flowing through
each terminal of the protected line. A pilot channel, either fiber optic or audio tone, is used to bring in the remote terminal
signals for comparison to the local signals. The unique methods used to represent the three phase current and securely
transmit the remote signal produce the application flexibility and fidelity of the REL 356.
Current only systems, like the REL 356, compare the currents measured at the terminals of the transmission line. In a
current differential system, the phasor relationship determines whether the condition is external or internal.
Product Overview and Specifications
ABB REL 356 Current Differential Protection
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