Wednesday, March 19, 2014

Power System Protection Course - Relays




RELAYS



CONTENTS
RELAYS - GENERAL.........................................................................................
TYPES AND APPLICATIONS.............................................................................
CLASSIFICATIONS..............................................................................................
TYPES AND CHARACTERISTICS.....................................................................
Attracted Armature Control Relays..........................................................................
Protection Relays.....................................................................................................
RELAYS  -  ELECTROMECHANICAL...........................................................
INDUCTION  RELAYS.........................................................................................
TYPICAL  APPLICATIONS...............................................................................
Wattmetric  Relay...................................................................................................
kVAr Relay.............................................................................................................
Phase-angle-compensated Relay............................................................................
Overcurrent Relay..................................................................................................
Over- or Undervoltage Relay.................................................................................
ATTRACTED-ARMATURE RELAYS...............................................................
TYPICAL APPLICATIONS................................................................................
All-or-nothing Relays.............................................................................................
Measuring Relays...................................................................................................
MOVING- COIL RELAYS..................................................................................
THERMAL  RELAYS..........................................................................................
MEASUREMENT.................................................................................................
Single Quantity Measurement................................................................................
Product Measurement............................................................................................
TIMING RELAYS................................................................................................
Short-time Relays...................................................................................................
Medium-value Accurate-time Delays.....................................................................
Long-time Relays....................................................................................................
DESIGN................................................................................................................
Simplicity................................................................................................................
High Operating Force............................................................................................
High-Contact Pressure...........................................................................................
Contact Circuit Voltage..........................................................................................
Contact-making Action...........................................................................................
Minimum Size of Wire............................................................................................
Enclosures..............................................................................................................
STATIC RELAYS..............................................................................................


RELAYS - GENERAL

In any sizeable electrical installation the control systems incorporate, apart from simple switches, various devices - mostly electrically actuated switching devices - which are required to carry out automatic, remote or other functions without the benefit of manual control.  Traditionally these have been electromechanical devices, but there is now an increasing tendency for 'solid-state' techniques to be used for such purposes.

The kinds of device commonly found in power installations - onshore and offshore - are described below, together with some account of the functions they normally perform.

TYPES AND APPLICATIONS

Relays operate contacts in response to an electrical input of relatively low power (the term is also used, though inaccurately, for devices controlled by other inputs, such as temperature).  Typical uses are remote control, electrical isolation between control circuits, protection of equipment against potentially damaging conditions, and the interlocking of switchgear to prevent inadvertent misuse.

CLASSIFICATIONS

A relay may be classified in three ways:
·         Type. 
Nearly all relays are operated either electro-magnetically or electronically.  The most common electromagnetic types are attracted-armature, induction disc and reed.
·         Characteristics. 
Relays can be divided into those which have no precisely defined characteristics and simply operate 'instantaneously' when an input is applied, and those which are calibrated, in terms either of operating level (current, voltage, etc.), or of operating time, or of both.  If both, the level and time calibrations may be independent, or may be related by an inverse-time characteristic.  Different character­istics may be combined within one relay or relay unit.
·         Application. 
Any type of relay may be used, within limits, for many different purposes, although it is usual to employ the simplest type which meets the require­ments.  The basic function implied by the word 'relay', and the simplest in practice, is to repeat in one circuit the effect of a signal in another circuit - for the purpose of isolation or interlocking, for example.  More complicated functions, calling for special calibrated relays, include the many different forms of protection.  These relays are designated according to their specific purpose - e.g.  overcurrent relay, undervoltage relay, negative phase-sequence relay etc.

TYPES AND CHARACTERISTICS

For the purposes of description, relays may be grouped here as:
·         Control relays:
those that perform no specific function other than operating their contacts in response to an input, with or without a specific time delay (often referred to as 'auxiliary relays').
·         Protection relays:
those that are calibrated to operate in accordance with closely defined characteristics for specific purposes.

Attracted Armature Control Relays

Control relays are mostly of the simple attracted-armature type, having two states: energised or de-energised.  The control supply is typically direct current at 24V or 110V.  Figure 3.1 shows a typical construction.

A.C.  relays are also used with a slightly modified magnetic structure, including a shading ring around part of the pole face to reduce vibration due to the alternating flux.


FIGURE 3.1
A CONTROL (INTERPOSING) RELAY ATTRACTED ARMATURE TYPE WITH TWO NC AND TWO NO CONTACTS

Individual contacts may be arranged to be opened ('normally closed') or closed ('normally open') by the energising of the operating coil (see Figure 3.2); the types of contact may be mixed on one relay.  They may also be combined to act as changeover contacts.


FIGURE 3.2
RELAY CONTACTS SHOWN IN THE DE-ENERGISED STATE

It is important to note that the designations normally open' (NO), 'normally closed' (NC) and 'changeover' (CO) define the types of contact fitted to the relay and refer to the states of the contacts when the relay is de-energised, or in its 'shelf' state.  They do not necessarily relate to the usual state of the relay (which might be energised or de-energised) when it is in use in a circuit.  A corresponding convention applies to circuit diagrams, in which contacts should always be shown for the de-energised condition of the relay regardless of its normal function.  Normally open contacts are sometimes referred to as 'make' contacts, and normally- closed as 'break' contacts, indicating what happens when the relay becomes energised.

Usually, when a relay is energised, the normally cIosed contacts open before the normally- open contacts close ('break-before-make').  For special requirements they may be specified as make-before-break, so that for a very short period as the relay operates all the contacts are closed simultaneously.

'Instantaneous' operation in a relay means that it is not deliberately designed to introduce a delay, the operating time being normally a few tens of milliseconds.  For particular purposes it is possible to increase the operating time by large amounts by simple expedients such as mounting a copper slug' on the magnetic core.

A control function may in some cases require a much longer time delay than the operating time of a simple relay; possibly it may need to be adjustable.  Time-delay relays operate on a variety of principles, including thermal elements, clockwork escapements, induction discs, dash pots, pneumatic cylinders and synchronous motors.  Modern relays increasingly make use of electronic timing circuits.

Relays are often provided with 'flags', which indicate clearly when the relay has operated and remain showing, even though the relay is subsequently de-energised, until reset by hand.  It is very important that, when some mishap in a system has resulted in a trip, dropped flags should not be reset until a written record has been made of which flags have fallen, so avoiding a loss of valuable information.  This applies especially to protection relays - see below.

Protection Relays

A control relay, as described above, operates with an 'on-off', or 'digital', input of sufficient magnitude to actuate its contacts.  By contrast a protection relay for use against overcurrent or other potentially damaging conditions in an electrical machine or system responds accurately to the level of its operating signal and is actuated when the signal exceeds, or falls short of, a preset value.  Such a relay is sometimes referred to as a 'measuring relay'.  Depending upon whether it responds to an excess or a shortfall, it is termed an 'overcurrent ('overvoltage', 'overfrequency  etc.) relay or an  undercurrent' ('undervoltage', 'under- frequency') relay.

Many relays of this type provide a time delay, which may be fixed (definite) or 'inverse'.  With an inverse-time characteristic the delay decreases as the input signal increases, so that protection becomes more rapid as the severity of a fault increases.

Most protective relays are fitted with flags which indicate when they have operated and remain showing until they are reset by hand, even though the relays themselves revert to their normal states as soon as the fault is removed.

Protection relays are mostly of three basic types:

Attracted-armature. 

This type is used when 'instantaneous' operation is required, and it can be energised by either direct or alternating current.  It is fundamentally similar to the attracted-armature control relay referred to earlier, but unlike the control relay it is calibrated in terms of operating current or voltage.  The cali­bration depends upon the restoring force applied to the armature by gravity or by a spring.  The operating current level is set by an adjusting screw at the top of the relay which controls the armature backstop or adjusts the control spring - see Figure 3.3.


FIGURE 3.3
INSTANTANEOUS OVERCURRENT RELAY



For overcurrent protection - a common application - the relay is normally fed from a current transformer at a nominal current of 1A or 5A.  In 3-phase systems three relays are assembled in one unit, or two in a 3-wire circuit (see manual 'Electrical Protection'), each coil being fed by a separate current trans­former.  Like other 'instantaneous' relays, the instantaneous overcurrent relay takes a finite time to operate, usually not more than about 0.2 seconds, and its overall characteristic is shown, somewhat idealised, in Figure 3.3.  Other common uses are for undercurrent, undervoltage, overvoltage and earth-fault protection.

Induction Disc. 

The induction disc relay functions by the interaction of the magnetic flux which is generated by an energising coil and passed through the disc, and of the eddy currents which are produced in the disc by the same flux or by a second coil.  The mechanism is described in the manual 'Fundamentals of Electricity 3' in relation to instruments, such as the integrating kilowatt-hour meter (e.g.  the domestic 'meter').  For protection purposes this type of relay has the advantages that its operating time can be controlled over a wide range by means of eddy- current braking magnets and that a wide variety of functions can be obtained by using different arrangements of operating magnets and coils.  The actual operating current level can be varied by adjusting a light restraining hairspring.

Electronic Relays. 

To a considerable extent protection relays of the electro-magnetic type, in which a moving armature or disc is actuated by some kind of electromagnet, are being superseded by electronic types.  In these the functions of signal detection and processing are carried out by entirely static circuits, and only the final operation of contacts is done by electromechanical relays, which can be of any suitable but simple control type.  The advantages of this technique include a greater flexibility in providing virtually any desired function, however complex, better accuracy, ease of adjustment, and the usual benefits of static circuits with regard to reliability and freedom from regular servicing requirements.


RELAYS  -  ELECTROMECHANICAL

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 of the devices covered in Chapter 1 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.

INDUCTION  RELAYS

Induction relays operate on the same principle as the induction motor.  Torque is produced by subjecting a moving conductor to two alternating fields which are displaced in both space and time.  The moving conductor is typically a metal disc which is pivoted so as to be free to rotate between the poles of two electromagnets.  Torque is produced by the interaction of upper electromagnet flux and eddy currents induced in the disc by the lower electromagnet flux, and vice versa.  The torque produced is proportional to the product of upper and lower electromagnet fluxes and the sine of the angle between them.

T µ FaFb sin A.

This means that maximum torque is produced when the angle between the fluxes is 90° and as Fa and Fb are proportional to Ia and Ib

T µ IaIb sinA. 



FIGURE 3.4  -  ELECTROMAGNETIC INDUCTION SYSTEM

Consider the system shown in Figure 3.4 (a) and let Ia and Ib be in quadrature.  This would be the condition if the upper coil, which is inductive, was supplied from system voltage and the lower coil with system current at unity power factor.
Fa and Fb the upper and lower electromagnetic fluxes are phase with Ia and Ib respectively.  Figure 3.4 (b) shows the vector diagram and Figure 3.4(c) shows the displacement in space of the relay pole faces.  1 and 5 are the outer poles of the upper electromagnet: 3 is the central pole of this magnet and 2 and 4 are the poles of the lowest electromagnet.

At the moment of time shown by the vector diagram, Ia is a maximum in the positive direction and if pole 3 is assumed to be N then poles 1 and 5 are S.  Ib = 0 and therefore poles 2 and 4 have no polarity.  One-quarter cycle later Ia = 0 and poles 1, 3 and 5 have no polarity.  Ib is a maximum in the negative direction and if pole 2 is assumed to be S then pole 4 is N.  This condition is shown in Figure 3.1(d).  Figures.  3.4(e), 3.4(f), and 3.4(g) show the conditions at ¼ cycle intervals.  From this diagram it can be seen that a sliding flux is produced which causes disc movement from left to right.  Reversal of polarity of either electromagnet will result in disc movement in the opposite direction.

Torque applied to a disc without control would, of course continually accelerate the disc to a speed limited only by friction and windage.  Control is provided in two ways:

·         By a permanent magnet whose field passes through the disc and produces a braking force proportional to disc speed.  This controls the time characteristic of the relay.
·         By a control spring which produces a torque proportional to disc angular displacement.  This controls disc speed at low values of torque and determines the relay setting.



Figure 3.5  The Effect of a Quad Loop on the Upper Electromagnet Flux

From Figure 3.5 it can be seen that disc speed is dependent on torque, and as disc travel over a fixed distance is inversely proportional to time.


1
t
 
 
IaIb sinA µ

which is an inverse time characteristic.

TYPICAL  APPLICATIONS

Wattmetric  Relay

The upper coil is supplied by voltage and the lower coil by current.  The voltage coil is very inductive and the voltage coil current, I1 lags the voltage by about 80°.  At unity power factor the phase angle between upper and lower coil fluxes would be 80°.  For a wattmetric relay it is necessary for maximum torque to be produced at unity power factor and therefore the fluxes must be 90° apart.  To modify the upper coil flux a quadrature compensating loop, known as a quad loop, is used.  This is merely a short-circuited turn of wire around the centre limb of the electromagnet.  An e.m.f.  is generated in the loop proportional to the rate of change of upper electromagnet flux.  This e.m.f., which lags the upper electromagnet flux by 90°, produces a current 'I' which in turn produces a flux 'F', both flux and current are in phase with the e.m.f.  The net effect is to produce a secondary flux, lagging the main flux by 90°, which modifies the upper electromagnet flux so that it lags the voltage by 90°.  Where accurate measurement is required, e.g.  kWh meters, the quad loop is made adjustable.  The relay torque is therefore:

T µ IaIb sin(A + 90) µ VI sinA. 

kVAr Relay

For a wattmetric relay the correct phase angle is produced with say, R/N voltage and R current.  If R current was associated with Y/B voltage then the voltage phase shift is -90° and the relay torque is

T µ IaIb sin(A + 90 - 90) µ VI sinA. 


Phase-angle-compensated Relay

From the above it can be seen that relays having maximum response to any chosen phase angle can be produced.  For example, Figure 3.6 shows a relay with 45° compensated connections.  Maximum torque is produced when the current lags the voltage by 45° by associating R current with Y - B voltage and connecting a resistor in series so that the voltage coil circuit current lags the voltage by 45° then:

T µ IaIb sin(A + 90 - 45) µ VI sin(A + 45)

Figure 3.6 (b) shows the vector diagram for this connection.



Figure 3.6  -  Compensated Induction Relay
(Producing Maximum Torque at a System Phase Angle of 45°)

Overcurrent Relay

In an overcurrent relay a transfornmer connection is used.  The upper electromagnet carries two windings, a primary which is fed from the current transformers and a secondary which feeds the lower electromagnet winding.  As rthe secondary current is dependant on the priimary current and the phase angle between these is fioxed, the relay torque is T µ I2

Over- or Undervoltage Relay

This is similar to the overcurrent relay but the upper electromagnet winding is connected to the voltage supply.  In the case of an undervoltage relay the contacts are arranged so that they make when the relay resets.  T µ V2

In last two applications where only one quantity is to be measured then an electromagnet as shown in Figure 3.7 may be used.



Figure 3.7  Induction Relay for Single Quantity Measurement

The short circuited turn produces a phase displacement in the fluxes in adjacent poles causing movement of the disc.  The torque is proportional to the square of the current.

Further application using this type of electromagnet are where the relay is required to respond to the sum of more than usually the difference between two quantities, for example when used in a biased differential scheme where the vector sum is compared to the vector difference of two currents.



Figure 3.8  Principle of Biased Differential Protection


The simple explanation of the use of this type of protection is as follows.  It is based on discrimination by comparison.  If current flowing into the generator is the same as current flowing out then there is no fault and the relay should not operate.

If the currents are not the same then there is a fault and the relay should operate.

Coil A produces a torque in the disc in the direction to close the contact.  The current in this coil is the vector sum of the input and output current - zero if there is no fault - whilst coil B produces a torque to open the contact.  The current in this coil is the vector difference - maximum when there is a fault in the generator.

A further type of relay is the induction cup relay.  The four-pole electromagnet has an iron core and a copper cylinder which is free to rotate in the air gap between the pole faces and the core.  This arrangement produces a high torque and is used mainly in high-speed protection schemes.  As the air gap is small a high degree of accuracy is required in matching which makes it an expensive relay to manufacture.

The relay can be used as a simple product relay, e.g.  VIcosA, VIsinA, etc., or in the eight-pole version as a polyphase device.

ATTRACTED-ARMATURE RELAYS




FIGURE 3.9  -  ATTRACTED ARMATURE RELAY

The attracted-armature relay comprises an iron-cored electromagnet which attracts an armature which is pivoted, hinged or other wise supported to permit motion in the magnetic field.

The force exerted on the armature is given by the equation


B2s newtons
2m0
 
 
Force F =

where B is the flux density in Wb/m2, s is the effective area of the magnetic pole in m2, and m0 is the permeability of free space = 4 px 10-7.

The magnetic circuit can be represented in a similar manner to an electric circuit.  Figure 3.9, using magneto-motive force (m.m.f.) in ampere-turns applied to the reluctance of the iron and air gap in series - represented by resistance - which causes a flux F to flow in the circuit.  The permeability of the iron is about 5000 times that of air which means that most of the m.m.f. will be used to magnetise the air gap.  When the relay starts to operate, the length of the air gap, and therefore the reluctance, decreases which causes the flux, and the force to increase.  The effect of this in practical terms is that when the current in the coil reaches a value which produces sufficient force to move the armature - movement of the armature itself causes the flux and the operating forces to increase.  So that once the armature moves it accelerates with increasing force until it is fully closed.  This is the reason that contractors are very successful because once the contractor starts to move positive contact making is assured.

The snap action which is beneficial from the point of positive operation is sometimes a drawback in that the relay will not drop out until the flux density is reduced to below the pick-up value.  As the magnetic circuit reluctance has been decreased by the closing of the armature a large reduction in ampere-turns is required to decrease the flux density to its original value, i.e. the relay has a low drop-off/pick-up ratio.  In some applications this can be inconvenient and in these instances the ratio can be improved by reducing the change in reluctance by not allowing the armature to close completely.  In fact the ratio can be controlled by adjustment of the final air gap.  An increase in drop-off/pick-up ratio reduces contact rating and operating speed.

In the simple case the moving contacts are carried by the armature or the armature is arranged to operate the contacts by means of a rod which pushes the contacts together (or apart if normally closed).

Control is generally by gravity assisted to a small extent by the contact spring pressure although in some cases spring control is used.  Relays for use in a.c. circuits tend to vibrate no matter how large the operating quantity as the flux must pass through zero every half-cycle and during this period the armature tends to release,  To eliminate this it is usual to split the electromagnetic pole face and surround one-half by copper loop.  The current induced in this loop compared with that passing through the other half of the pole and therefore the net flux is never zero.  Alternatively vibration can be prevented by supplying the relay through a rectifier.  In this case coil inductance maintains the flux during the idle portions of the cycle.

In d.c.  operated relays residual flux is the problem and may prevent release of the armature.  In order to reduce it to a low value the armature should not bed entirely on both poles of the electromagnet in the closed position but should always have a non-magnetic stop, to ensure that there is a small air gap.
In general attracted -armature relays are used.
·         as auxiliary repeat relays and for flag indicators.  These are known as “all-or-nothing relays”.
·         as measuring relays where a drop-off/pick-up ratio of less than 90% can be tolerated

TYPICAL APPLICATIONS

All-or-nothing Relays

Tripping relays.

These are multi-contact relays designed to be energised for a short time.  The coil power is high resulting in an operating time of approx.  0.01 s.  The relay can be self-resetting or of the latching type which are reset by hand or, with the addition of a second coil, electrically reset.

Auxiliary relays.

These are for operation from auxiliary d.c.  supply and are used as repeat contactors to provide additional contacts and/or flag indicators with induction relays, moving coil relays or mechanical devices such as thermometers, buchholz relays etc.

Measuring Relays.

The relay is suitable  for all single quantity measurements, i.e.  voltage, current, etc.  Such relays usually have a range of adjustment by altering the number of effective turns in the coil in the case of current measuring relays, by changing the resistance in series with the coil in the case of voltage measuring relays or by adjustment of a spring so that the force required to pick up the relay can be changed.

MOVING- COIL RELAYS

The moving-coil relay consists of a light coil which when energised, moves in a strong permanent magnet field.  The coil can either be pivoted between bearings as in the usual moving-coil instrument (D’Arsonval movement, Figure 3.10) or suspended in the magnet field in the manner of the moving-coil loudspeaker (axial movement, Figure 3.11),


FIGURE 3.10  - D’ARSONVAL MOVING-COIL RELAY


In both cases the movement is very sensitive, that is, very little energy is required to produce operating force.  It is for this reason, coupled with its ability to withstand high overloads, that it is almost invariably used in modern high-speed protection schemes.

The force produced is proportional to the product of the permanent magnet flux and the coil current.  But, as the permanent magnet flux in any one relay is constant over the range of coil movement, the force is proportional to the coil current.  The relay is polarised by the permanent magnet and must be used with a rectifier for all a.c.  applications.

In the axial moving-coil relay the coil movement is essentially small whereas this need not be the case with the D’Arsonval relay.  The latter, whilst it does not have a short contact travel in its high-speed applications, can have a contact movement of up to 80°.

Movement damping is accomplished in both types by use of a metal coil former which acts as a shorted turn which will have a current induced in it, in such a direction as to oppose motion when the coil moves.  In long travel relays the effect can be also used to introduce a time delay.

Control in both types of relay is by spring; leaf springs in the axial relay and a spiral spring in the D’Arsonval type.  Current is conveyed to the coil and the moving contact carried by the coil by ligaments which in the D’Arsonval type are light spiral springs.  The D’Arsonval movement is extremely sensitive, “galvanometer class” sensitivity is obtainable for special applications with a setting power as low as 20 x 10-6 watts.  As the movement is proportional to current the contact differential ratio is nominally zero but on account of pivot friction, contact adhesion, etc., it is nominally 2%, i.e.  drop-off/pick-up ratio = 98%.

FIGURE 3.11  - AXIAL MOVING-COIL RELAY

In the long-travel relay it is usual to provide a calibrated scale along which the fixed contact can be set.  In addition a 3-1 spring wind-up is allowable to widen the scale over the required operating range, e.g.  an overvoltage relay could have a scale of say, 100% or 150%.  Two independently adjustable fixed contacts can be provided for use as low and high contacts with a side zero relay or forward and reverse with a centre zero relay.

The axial relay is less sensitive but is very robust..  It has the advantage of having no bearings but on the other hand is affected by gravity if the relay case is not correctly aligned on the panel.  In general moving-coil relays are used
·         where a sensitive (low energy) relay is required,
·         to provide a high drop-off/pick-up ratio,
·         where the relay can be subjected to a continuous overload of many times its setting,
·         in high-speed protection schemes.

The importance of a sensitive relay with a high overload capability can be appreciated when the conditions of operation of a protection scheme are considered.

A relay may be required to have a setting of, say, 10% normal current and yet be capable of carrying, say, 50 times normal current, which means that the relay must be capable of carrying 500 times setting current or 500² times setting power.  With the moving-coil relay with a setting of 20mW the power at maximum fault condition is only 5W.

THERMAL  RELAYS

These are relays in which the operating quantity generates heat in a resistance winding and so affects some temperature-sensitive component.  Most protective relays of the thermal type are based upon the expansion of metal, a typical example being the use of bimetal material.

Bimetal is available in strips which are formed by welding two bars of different metals together throughout their length and then rolling out the composite bar to form a thin sheet.  When a strip of this material is heated the difference in expansion rates of the two metals cause the strip to bend into a curve.  The amount of motion of the end of the strip being magnified compared with the actual expansion of the individual metals.  Relays can be constructed using straight pieces of bimetal or a longer strip may be coiled into a spiral thereby producing a large amount of motion in a constant space.  The bimetal strip can be heated directly by passing current through it.  In this case it is usually split longitudinally except at the extreme end so forming an elongated U.  The two divided ends are clamped to a support and current is fed through the loop.  This results in the bimetal becoming heated and causing motion of the tip through a proportionate angle.


MEASUREMENT

Single Quantity Measurement

This classification covers al simple relays such as those detecting current or voltage levels.  The choice depends on the characteristic required.

Product Measurement

This subject has been partially discussed under induction relays where it was shown that the induction relay can  readily be used to measure the product of two alternating quantities.  The typical example of this is in power and directional types of relay.

It is interesting to note that other types of element can make a product measurement if the applied quantities are first mixed.  For example, a beam relay is a natural amplitude comparator.  If, however, two alternating signals A and B are first summated and then applied to the relay so that one coil is energised by the sum A + B, whilst the other coil is energised with A - B then the relay becomes a phase comparator as the forces will only equal when there is a phase difference of 90° between A and B.  Such an arrangement has been used as a directional relay.

On the other hand, if the two signals are summated as before and applied to the two windings of a power-type induction relay, then this combination will become a simple amplitude comparator because (A + B) and (A - B) have the same polarity only if A is greater than B.

These concepts are useful in designing new schemes with complicated response functions.  When dealing with simple measurements it must be realised that some elements are fundamentally more suitable for amplitude or phase comparison than others, since notwithstanding the above algebraic conversions, errors must also be considered and these sometimes limit the range of application of an apparently suitable arrangement.

TIMING RELAYS

In some circumstances a time delay is required in conjunction with protection relays.  These fall into three distinct groups.

Short-time Relays

A short-time lag can be easily imposed using an attracted-armature type element, by fitting a solid copper cylinder to occupy a portion of the normal winding space.  The “copper Slug” may be placed at either end of the core, but it is most  powerful when situated at the armature end.  In this position it delays both operate and release functions of the relay by virtue of the eddy-currents induced in it which resist a change in the core flux.  Time relays of the order of 50ms in the operate sense and 200ms for release are possible.

Medium-value Accurate-time Delays

For this application a more elaborate mechanism is employed.  The relay is powered by a solenoid or attracted armature element, either of which compress a spring.  The other end of the spring drives a train of gears and an eddy-current brake system comprising a disc or drum rotating in a permanent-magnet field.  The spring shaft also carries a contact arm which rotates as the gears run and ultimately makes contact at the end of its travel.  A ratchet is usually fitted so that the relay can reset instantly when the coil is de-energised.  This type of relay can give a maximum time delay in the range of 1.0 to 30s and can be adjustable for any one value over a 10 to 1 range.

Long-time Relays

Relays of this class are usually of the motor-operated type.  The motor may be d.c.  or a.c., either synchronous or induction, and will drive through gearing of such ratio that the operating time is achieved.  The operating range extends from a few seconds up to hours, there being in principal no upper limit.  When the gear ratio is high it is usual to incorporate a friction clutch in the drive chain, to avoid excess stress being built up should the motor continue to operate after the contact has completed full travel.

DESIGN

Many other designs of relays are possible and a great many other arrangements have been used and.  providing that the necessary operating function is obtained, it only remains to say that the only other essential requirement is absolute reliability.
The protection relay, as distinct from a control relay, may remain inoperative for long periods but when operation is called for the response must be both immediate and accurate.  For example, a busbar protection relay may operate under fault conditions perhaps only once in its normal span of life.  If on this occasion should the relay be incapable of performing its function owing to some deterioration which has taken place, then its provision has been in vain.  Furthermore, the very fact that it has remained inactive for a long period is the condition which is liable to lead to the mechanism becoming stuck so as to be inoperative.  Hence protective relays are designed with certain principles in mind.

Simplicity

In so far as this is compatible with achieving the necessary measurements, simplicity is the most desirable characteristic.  Any reduction in number of components reduces the possible caused of difficulty and simplicity in operation assists the maintenance staff and generally results in higher standard of maintenance.

High Operating Force

Relays are designed with as high a working force as possible to minimise the effects of friction so that should it vary during the life of the relay the overall effect on the performance is negligible.

High-Contact Pressure

This is closely related to the working force but is also governed by the contact shape.  To this end domed or cylindrical form contacts are used so that the contact-making area is small with the result that a given force corresponds to a high pressure.

Contact Circuit Voltage

For general purposes contacts are made from silver which is excellent in its general characteristic.  In bad atmospheres, however, it is liable to form surface layers of oxide or sulphide, which are not a great detriment unless the layer is excessively thick.  In general contact difficulties are encountered
·         where there is a bad atmosphere,
·         where the tripping voltage is low (30V or less),
·         with low torque relays, i.e.  where contacts make on resetting or with thermal relays.

Contact-making Action

Contacts should close together with a certain amount of wiping or scraping action in order to help in breaking down the surface films of oxide or other contaminants and should be designed so that they do not bounce apart or chatter after first closing.  It is very difficult to ensure that the impact between the contact tips on making does not result in a rebound but the effect can be minimised by suitable design.  Many complex arrangements have been evolved, but for normal purposes the main requirement is to ensure that the moving contact has a lower natural frequency than the fixed one.  It is also important to ensure that the rest of the element and moving system does not generate excessive vibration which can be passed on to the contact.  Any chattering from such a source might lead to excessive burning of the contact tips.

Minimum Size of Wire

It is desirable that protective relay coils should not be wound with a wire which is thinner than 0.1mm to guard against the risk of mechanical fracture.  An even more serious problem is that of corrosion.  It is most important that all the insulating materials with which the coil is wound should be absolutely neutral and incapable of releasing even small traces of substances with corrosive tendencies.  Even when this is done coil corrosion can occur if the coil is allowed to assume a positive potential relative to earth.  Should this be the case the wire is an anode and any small leakage current will deposit copper from the wire which in time will be corroded away by this electrolytic action.  Even with moderate potentials and quite high insulation resistance to earth the wire can be completely severed by this action within a short time.  Hence, it is desirable that all d.c.  coils should be connected to the negative pole of the battery or maintained at a negative potential relative to earth by some other means.

Enclosures

Even robust relays have to be regarded as precision measuring instruments and although they may work well when first produced they will not maintain this quality if exposed to accumulations of dust and other deposits from the atmosphere.  Therefore the relay should be enclosed in a substantial and tight-fitting case which is made as dust-proof as possible by the fitting of gaskets although it should still be possible for the relay to breathe slightly.  under these circumstances the relay should remain in good condition for long periods.

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.


FIGURE 3.12  -  MICROPROCESSOR-BASED OVERCURRENT RELAY

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 represent the bulk of relays manufactured and it is unlikely that there will be a sweeping change-over to static relays particularly where the electromechanical relay is adequate   However, most of the current development in protection is 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 difficulties which arise in this environment have been overcome.

An example of the versatility of the microprocessor is demonstrated in one of the first protection applications.  This is an overcurrent relay which has a setting range of 10% to 200%, an extremely wide range made possible by the low power requirements of the relay, and a choice of five different characteristics.  Figure 3.13 shows the block diagram of the relay.

The CT current is reduced to a more suitable level by an interposing current transformer in the relay.  The current is rectified and passed through a resistance network which produces a voltage output which is proportional to current.  The network provides the current setting control by switches mounted on the front of the relay and its output is fed into the analogue-digital converter which is part of the micro-processor.

Three other banks of switches are mounted on the front of the relay.  The switches are connected to separate input ports on the microprocessor and control the time multiplier setting, the high-set relay setting and selection of the type of characteristic required, i.e.  normal, very or extremely inverse, long time inverse or definite time.



FIGURE 3.13  -  SIMPLIFIED BLOCK DIAGRAM

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