Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-7
AP
1. INTRODUCTION
1.1 Protection of overhead line, cable, and hybrid circuits
Overhead lines, typically ranging from 10 kV distribution lines to 800 kV transmission lines,
are probably the most fault susceptible items of plant in a modern power system. It is
therefore essential that the protection associated with them provides secure and reliable
operation.
For distribution systems, continuity of supply is of paramount importance. The majority of
faults on overhead lines are transient or semi-permanent in nature. Multi-shot auto-reclose
cycles are therefore commonly used in conjunction with instantaneous tripping elements to
increase system availability. For permanent faults it is essential that only the faulted section
of plant is isolated. As such, high speed, discriminative fault clearance is often a
fundamental requirement of any protection scheme on a distribution network.
The requirements for a transmission network must also take into account system stability.
Where systems are not highly interconnected the use of single phase tripping and high
speed auto-reclosure is often required. This in turn dictates the need for very high speed
protection to reduce overall fault clearance times.
Many line configurations exist which need to be addressed. Transmission applications may
typically consist of 2 or 3 terminal applications, possibly fed from breaker and a half or mesh
arrangements. Lower voltage applications may again be 2 or 3 terminal configurations with
the added complications of in zone transformers or small teed load transformers.
Charging current may also adversely affect protection. This is a problem particularly with
cables and long transmission lines. Both the initial inrush and steady state charging current
must not cause relay maloperation and preferably should not compromise protection
performance.
Physical distance must be taken into account. Some EHV transmission lines can be up to
several hundred kilometers in length. If high speed, discriminative protection is to be
applied, it will be necessary to transfer information between line ends. This not only puts the
onus on the security of signaling equipment but also on the protection in the event of loss of
this signal.
Back-up protection is also an important feature of any protection scheme. In the event of
equipment failure, such as signaling equipment or switchgear, for example, it is necessary to
provide alternative forms of fault clearance. It is desirable to provide back-up protection
which can operate with minimum time delay and yet discriminate with both the main
protection and protection elsewhere on the system.
Transmission systems are essential to route power from the point of generation to the region
of demand. The means of transport is generally via overhead lines, which must have
maximum in-service availability. The exposed nature of overhead lines make them fault-
prone, and protection devices must trip to initiate isolation of any faulted circuit. In addition
to fast fault clearance to prevent plant damage, the requirements for a transmission network
must also take into account system stability. Where systems are not highly interconnected
the use of single phase tripping and high speed auto-reclosure is often required. This in turn
dictates the need for very high speed protection to reduce overall fault clearance times.
The MiCOM P54x provides fast, highly selective protection, to trip for genuine line faults.
The current differential principle easily detects intercircuit, evolving and cross country faults
amongst others as the relay works on a per phase basis. It is also immune to voltage
measurement problems such as CVT transients and power swings on the system and the
most important benefit of all; differential principle offers the most selective line protection.
A combination with a full scheme distance protection and aided directional earth fault (DEF)
makes the relay a complete and versatile solution for line protection. Differential and
distance protection can be set to operate to work separately or simultaneously. Distance can
also be set to work upon failure of the relay protection communications. These options allow
the user to set different protection schemes such as Differential as main 1 and Distance as
main 2 or vice versa, Differential as main 1 and Distance as backup, etc.
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