11-09-2013, 02:39 PM
Transmission Line Protection Principles
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Introduction
Transmission lines are a vital part of the electrical distribution
system, as they provide the path to transfer power between
generation and load. Transmission lines operate at voltage levels
from 69kV to 765kV, and are ideally tightly interconnected for
reliable operation.
Factors like de-regulated market environment, economics, right-
of-way clearance and environmental requirements have pushed
utilities to operate transmission lines close to their operating
limits. Any fault, if not detected and isolated quickly will cascade
into a system wide disturbance causing widespread outages for a
tightly interconnected system operating close to its limits.
Transmission protection systems are designed to identify the
location of faults and isolate only the faulted section . The key
challenge to the transmission line protection lies in reliably
detecting and isolating faults compromising the security of the
system.
Factors Influencing line Protection
The high level factors influencing line protection include the
criticality of the line (in terms of load transfer and system stability),
fault clearing time requirements for system stability, line length, the
system feeding the line, the configuration of the line (the number
of terminals, the physical construction of the line, the presence
of parallel lines), the line loading, the types of communications
available, and failure modes of various protection equipment.
GE Multilin Application Advantages
Before considering using a GE Multilin relay for a specific
transmission line protection application, it is important to
understand how the relay meets some more general application
requirements for simplicity, security, and dependability. GE Multilin
relays provide simplicity and security for single pole tripping,
dependability for protection communications between line
terminals, security for dual-breaker line terminals, and simplicity
and dependability of redundant protection schemes.
Single-Pole Tripping
Single pole tripping using distance protection is a challenging
application. A distance relay must correctly identify a single-
phase fault, and trip only the circuit breaker pole for the faulted
phase. The relay also must initiate the recloser and breaker failure
elements correctly on the fault event. The distance elements
protecting the unfaulted phases must maintain security during
the open-pole condition and any reclosing attempts.
The D90Plus Line Protection System and D60 Line Distance
Relay use simple, dedicated control logic for single pole tripping
applications. This control logic uses a Phase Selector, Trip Output
and Open Pole Detector in conjunction with other elements as
shown in the simplified block diagram.
Communications
Often transmission lines are protected by using schemes
that require communications with relays located at other line
terminals. The reliability of the communications obviously
impacts the reliability of the protection system. GE Multilin relays
include features that maintain reliable operation of the protection
communications during power line faults, communications
channel delays, communications channel switching, and
communications channel dropout.
Redundancy Considerations to Enhance
Reliability
The reliability of transmission system protection is dependent on
the reliability of the protection scheme used and the individual
components of the protection scheme. Transmission protection
systems typically use redundancy to increase the dependability
of the system. There are two general methods of implementing
redundancy. One method is to use multiple sets of protection
using the same protection scheme. The other method is to use
multiple sets of protection using different protection principles.
Depending on the voltage class, either method of redundancy
may involve using 2 or 3 sets of protection. In both cases, the goal
is to increase dependability, by ensuring the protection operates
for a fault event. Security may be improved through the use of so-
called voting schemes (e.g. 2-out-of-3), potentially at the expense
of dependability.