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.
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.
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.
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').
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.
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.
|
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
|
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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