Power System Protection Course - Introduction

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.