INTRODUCTION
CONTENTS
Introduction.................................................................................................
The Reasons for Protection..........................................................................
Role of Protection.........................................................................................
Protection Principles....................................................................................
Discrimination................................................................................................
Faults and Fault Levels................................................................................
Overview of Protection Components.........................................................
Fuses...............................................................................................................
Thermal Trip Units.........................................................................................
Electromechanical Relays...............................................................................
Static relays.....................................................................................................
Current and Voltage Transformers.................................................................
Overview of Protection Testing and Maintenance...............................
Why Protection Relays need Maintenance....................................................
Importance of Protection Relay Setting Data...............................................
Frequency of Maintenance...........................................................................
INTRODUCTION
Any consideration of an Electrical
Distribution System is not complete without some thought being given to
Protection.
Electrical Equipment which is correctly
installed and maintained is normally very reliable, but the consequence when
this equipment does become faulty can be out of all proportion in terms of
danger, extensive damage and loss of production unless it is adequately
“protected”.
The function of ‘protection’ in this sense
is not as the name implies preventive but it is the ambulance at the foot of
the cliff rather than the fence at the top.
This manual is designed to enhance the
basic knowledge including the examination of the need for protection,
protection components and basic protection testing. Design of new protection systems is not
covered but sample existing systems are studied. Design of a protection system requires more
advanced information and knowledge than can be given in this manual.
By the end of the manual and the associated
practical exercises all readers will have increased their knowledge of the
theory and practice of protection systems.
The Reasons for Protection.
Electrical
plant, machines and distribution systems must be protected against damage which
may occur through abnormal conditions arising.
Abnormal
conditions may be grouped into two types:
·
Operation
outside the designed ratings due to overloading or incorrect functioning of the
system.
·
Fault
conditions due usually to breakdown of some part of the system.
The first
condition is usually 'chronic' - that is, it may persist for some time and is
often acceptable for a limited period.
It may give rise to temperatures outside the design limit of the machines
and equipment, but, unless these are very excessive or very prolonged, it
seldom causes sudden or catastrophic failure.
It can usually be corrected before it leads to breakdown or a fault
condition.
The second
condition on the other hand is 'acute' and arises from electrical or mechanical
failure which, once established, produces a condition beyond control. It usually gives rise to very severe excess
currents which will quickly cause catastrophic failure of other electrical and
mechanical plant in the system unless the fault is rapidly isolated. It may be caused by a breakdown of insulation
due to a material failure or overheating or to external conditions such as
weather, or it may be due to physical damage to an item of plant or cable.
Automatic protection
against these conditions is possible in electrical installations because it is
easy to measure various parameters, to detect abnormalities, and to set in
motion the protective action the instant an abnormality arises.
An electrical network normally operates
within its designed rating. Generators,
transformers, cables, switchboards, busbars and connected apparatus are each
designed to carry a certain maximum current.
Most can carry a moderate overload for a short time without undue
overheating.
However, if a fault should develop
somewhere in the system, that is to say a phase to phase short circuit or a
phase to earth breakdown, then all connected generators will feed extremely
high currents into that fault, which will be limited only by the impedance of
the complete circuit from generator to fault.
Fault currents can be ten to twenty times the normal full load current.
Such currents will quickly cause intense
overheating of conductors and windings, leading to almost certain breakdown
unless they are quickly disconnected; they will also give rise to severe
mechanical forces between the current carrying conductors or windings. All such apparatus must be manufactured to
withstand such forces.
The purpose of automatic protection is to
remove the fault from the system and so break the fault current as quickly as
possible. Before this can be achieved,
the fault current will have flowed for a finite, if small, time, and much heat
energy will have been released. Also the
severe mechanical forces referred to above will already have occurred and
subjected all conductors to intense mechanical stress.
Role of Protection.
Protection is needed to remove, from the
system, as speedily as possible any part of the equipment in which a fault has
developed. So long as it is connected
the whole system is in jeopardy from three main effects of the fault, namely:
·
a risk of
extended damage to the affected plant.
·
a risk of
damage to healthy plant.
·
a risk of
extending the outage to other plant on the system, with resultant loss of
protection and interruption of vital processes.
It is the function of protective equipment,
in association with the automatic switch fuse, contactor or circuit breaker to
avert those effects.
Protection Principles
Protection of an
electrical system is provided for one or more of the following principles:
·
To
maintain electrical supplies to as much of the system as possible after a fault
has been isolated.
·
To protect
the generators and other plant against damage due to abnormal conditions and faults.
·
To protect
the consumer equipment against damage due to abnormal conditions (e.g. overload).
·
To isolate
faulty equipment to limit the risk of fire locally.
·
To limit
damage to the cable system resulting from a fault.
These principles
will determine the type of protective equipment fitted in any
installation. It will be noted that the
first principle conflicts with the other requirements to some extent. For example, the best way to protect a
generator against damage by fault currents is to disconnect it, but it would
not then be available to supply other consumers.
Where continuity of supply is considered
essential alternative feeds are necessary.
But, if full advantage is to be gained from this additional capital
outlay, the protection must be highly ‘selective’ in its function.
For this it must possess the quality known
as ‘discrimination’ whereby it is able to select and disconnect only the faulty
element leaving all others in normal operation so far as it is possible.
Discrimination
If we consider a simple typical electrical
layout the need for some form of discrimination will become clear.
Figure 1.1 shows an 11kV Oil fuse switch
(OFS) controlling a transformer beyond which there are a bank of Low Voltage
(LV) fuses. Clearly a fault as indicated
must be interrupted by fuse A so that supply may continue to the other
circuits. The 11kV OFS must not trip.
Figure
1.1 - 11kV Oil Fuse Switch
Faults and Fault Levels
Before selecting a protection system we
must consider the kind of fault which may occur.
The principal types are:
·
3 Phase
(with or without earth)
·
Phase - to
- phase
·
Phase - to
-Earth
·
Double
Phase - to - Earth
Sometimes there are open-circuits
involved. Transformers and motors are
also subject to short circuits between turns of the same winding.
Only the 3 phase short circuit is a
balanced condition. The others are
unbalanced and require a knowledge of the method known as ‘symmetrical
components’ before they can be fully analysed.
This analysis is necessary if the amount of
fault current that will flow is to be correctly predicted but is beyond the
scope of this course.
However, if a
fault should develop somewhere in the system, that is to say a phase-to-phase
short-circuit or a phase-to-earth breakdown, then all connected generators will
at first feed extremely high currents into that fault, which will be limited
only by the impedance of the complete circuit from generator to fault. Fault currents can be ten or more times the
normal full-load current.
Such currents
will quickly cause intense overheating of conductors and windings, leading to
almost certain breakdown unless they are quickly disconnected. They will also give rise to severe mechanical
forces between the current-carrying conductors or windings. All such apparatus must be manufactured to
withstand these forces.
A fault current of 50000A
(rms) flowing in two busbars 3 inches apart will produce between them a peak
mechanical force of nearly half a ton-force per foot run of bars.
The purpose of
automatic protection is to remove the fault from the system and so break the
fault current as quickly as possible.
Before this can be achieved, however, the fault current will have flowed
for a finite, if small, time, and much heat energy will have been
released. Also the severe mechanical
forces referred to above will already have occurred and will have subjected all
conductors to intense mechanical stress.
Overview of Protection Components
Fuses
Modern H.R.C. fuse-links are manufactured to
the highest standard to ensure reliability.
The ceramic cartridge usually holds a fuse element of pure silver
surrounded by powdered quartz. The
quartz filler is required to condense the metal vapour, produced by the element
under short circuit conditions, as quickly as possible. In addition, due to the high temperatures
produced by the fault condition, the quartz forms a glass insulating barrier
between the contact points. The greater
the surface area in contact with the filler the faster is the heat
dissipation. This is why the element is
made up of fine wires or strips. By
modifying the shape of these strips the speed at which a fuse will blow with
any particular current flowing can be controlled. This relationship between speed or time and
current is known as the time- current characteristic. In selecting a fuse you must ensure
that:
·
The fault
level of the circuit does not exceed the fuse ‘Breaking Capacity’
·
The
maximum load will not exceed the Fuse Rating.
·
The fuse
will rupture with an earth fault at the remotest point of the apparatus
controlled.
Discrimination will be obtained with
protection both preceding and following the fuse.
Thermal Trip Units
Lower levels of fault current from a little
over the rated load current of the circuit-breaker up to about 10x load current
setting the protection is by a thermal tripping device. This consists of a bimetallic strip which is
deflected by the heat generated in the strip by the fault current and
eventually trips the circuit- breaker or contactor. The arrangement provides a time delay which
is inversely proportional to current
These are normally used on Low Voltage motors
up to approx 50 kW. They are usually an integral part of the contactor and
cause tripping by breaking the contractor holding coil circuit. The heaters are connected one in each phase
in the main circuit and carry the actual motor current and only for large
motors would current transformers be used.
The indirectly heated device usually has a heater winding wrapped around
the bimetal whereas the bimetal itself is shaped so that it becomes the path
for the current in the case of a directly heated device.
Electromechanical Relays
When two
protection devices are required to discriminate the chosen settings will depend
on how closely the devices can be guaranteed to conform to their characteristic
curves. Most devices have fairly
generous tolerances in both operating levels and time and therefore if close
discrimination is required then protection relays would have to be used.
A relay is a
device which makes a measurement or receives a signal which causes it to
operate and to effect the operation of other equipment.
A protection
relay is a device which responds to abnormal conditions in an electrical power
system to operate a circuit-breaker to disconnect the faulty section of the
system with the minimum interruption of supply.
Many designs of
relay elements have been produced but these are based on a few basic operating
principles. The great majority of relays
are in one of the following groups.
·
Induction
Relays
·
Attracted-armature
relays.
·
Moving-coil
relays.
·
Thermal
relays.
·
Timing
Relays.
Static relays
Relays based on
electronic techniques offer many advantages over the more usual
electromechanical type. Apart from the
obvious advantage of no moving parts the power requirements are low and
therefore smaller current and voltage transformers can be used to provide the
input. Additional benefits are improved
accuracy and a wider range of characteristics.
The invention of
the transistor and the microprocessor has allowed the development of static
relays but difficulties were experienced because the high voltage substation
proved to be a very hostile environment to the device. The close proximity of high voltage heavy
current circuits produces conditions which could damage the transistor because
of its low thermal mass or cause mal-operation of the relay because of the
electromagnetic or electrostatic interference.
A lot of
research and development has taken place and commercial relays which meet very
exacting standards have been produced.
Electromechanical relays will still representa large proportion of
relays remaining in service. However as
new equipment and systems are designed it is likely that there will be a
change-over to static relays and most of the future development in protection
will be in static relays.
The large
application potential of the digital integrated circuit has led to enormous
expenditure on research and development which has resulted in microprocessors
with spectacular computing capabilities at a low cost. It is fairly certain that microprocessors
will ultimately dominate protection and control systems.
The
utilisation of microprocessors in the
field of protection means that the logic part of the relay can be replaced by a
programme held in the microprocessor memory.
This enables a relay function to be specified by software which widens
the scope of the relay and allows a single relay to be provided with a number
of characteristics.
Experience has
been gained with microprocessors in high voltage substations over a number of
years by using them for voltage control, automatic switching and reclosing and
other control functions. Therefore the
difficulties which arise in this environment have been overcome.
Current and Voltage Transformers
Current transformers
The current
transformer is well established but it is generally regarded as merely a device
which reproduces a primary current at a reduced level. A current transformer designed for measuring
purposes operates over a range of current up to a specific rated value, which
usually corresponds to the circuit normal rating, and has specified errors at
that value. On the other hand, a
protection current transformer is required to operate over a range of current
many times the circuit rating and is frequently subjected to conditions greatly
exceeding those which it would be subjected to as a measuring current
transformer. Under such conditions the
flux density corresponds to advanced saturation and the response during this
and the initial transient period of short-circuit current is important.
It will be
appreciated, therefore that the method of specification of current transformers
for measurement purposes is not necessarily satisfactory for those for
protection. In addition an intimate knowledge
of the operation current transformers is required in order to predict the
performance of the protection.
Current
transformers have two important qualities:
·
They
produce the primary current conditions at a much lower level so that the
current can be carried by the small cross-sectional area cables associated with
panel wiring and relays.
·
They
provide an insulating barrier so that relays which are being used to protect
high voltage equipment need only be insulated for a nominal 600V.
Current transformers
are usually designed so that the primary winding is the line conductor which is
passed through an iron ring which carries the secondary winding. They are mostly of this type and are known as
bar-primary or ring-wound current transformer.
Voltage transformers
The voltage
transformer in use with protection has to fulfil only one requirement, which is
that the secondary voltage must be an accurate representation of the primary
voltage in both magnitude and phase.
To meet this
requirement, they are designed to operate at fairly low flux densities so that
the magnetising current, and therefore the ratio and phase angle errors, is
small. This means that the core area for a given output is larger than that of
a power transformer, which increases the overall size of the unit. In addition,
the normal three- limbed construction of the power transformer is unsuitable as
there would be magnetic interference between phases. To avoid this interference a five-limbed
construction is used, which also increases the size. The nominal secondary voltage is sometimes
110V but more usually 63.5V per phase to produce a line voltage of 110V.
Overview of Protection Testing and Maintenance
Why Protection Relays need Maintenance.
Protective relays are normally inactive,
although energised and possibly under some degree of stress, or can possibly be
subject to continuous low amplitude vibration.
Over a long period the effects of environmental conditions such as
temperature, humidity and atmospheric pollution can deteriorate parts of the
relays.
A relay may be only be called upon to `work'
at very infrequent intervals, as normally failures of modern electrical
equipment are comparatively rare, but a long period of inactivity may allow
`sticking effects' to develop due to the forming of sticky substances
evaporating from the insulating materials used in the relay's construction.
Many other conditions can affect the response
of a relay when it detects a system fault.
A short list of the more common conditions follows:-
·
Continuous
vibration at power frequency can cause some contact adhesion.
·
Corrosion
can also cause considerable problems in some types of relays.
·
Silicon
oils can be the cause of problems in protection relays and may contaminate the
contacts.
·
Dust can
be a hazard, as it acts as an insulating material and can cause contact failure
if allowed to enter a relay case.
·
Relay
coils are subject to corrosion from electrolytic action. (due to leakage
current with respect to earth)
All parts of a relay may be affected by these
causes, but the two most vulnerable parts are the contacts and coils. In polluted atmospheres, silver contacts can
become coated with black tarnish or silver sulphide which may in time cause
protection failure due to high contact resistance.
Over-maintenance of protection relays should
be avoided, particularly by inexperienced personnel. Relay adjustment requires personnel with the
experience of this type of work.
Importance of Protection Relay Setting Data.
Complete
schedules of all the original commissioning data supplied by the
manufacturer/contractor shall, where available, be given to the personnel who
are to carry out the preventative maintenance of the protection equipment. This shall include the protective relay
settings, the function and details of all such settings and any basic fault
calculations used.
Frequency of Maintenance
The recommended
frequency for maintaining different types of protection equipment is to be
found in the manufacturers information.
However, individual companies will also have to take into account their
own operational experience and the importance of the electrical equipment
`protected' by the overall protection scheme, when deciding on how often to
maintain relays, etc.
The following is
a suggested list of inspections and maintenance activities that should be
carried out at differing pre-determined intervals :-
·
Inspection
of relays and checking of relay settings.
·
Trip tests
including intertripping tests.
·
Insulation
resistance checks on all small wiring, etc.
·
Secondary
injection tests on all protection relays such as overcurrent and earth fault
induction type, but possibly less frequently on thermal or electronic units.
·
Inspection
and testing of transformer's Buchholz Relays.
·
Inspection
and testing of transformer's Winding or Oil temperature instruments.
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