PRINCIPLES
OF DISCRIMINATION
CONTENTS
INTRODUCTION.................................................................................................
ECONOMIC CONSIDERATION.........................................................................
DISCRIMINATION..............................................................................................
THE NEED..............................................................................................................
THE APPLICATION..............................................................................................
DISCRIMINATION BETWEEN FUSES, MCCBs AND MCBs.........................
BACKUP FUSES....................................................................................................
Types of Protection.............................................................................................
The Fuse...................................................................................................................
Inverse Definite Minimum Times (I.D.M.T.)
Relay..................................................
Tripping Batteries...................................................................................................
INDUCTION DISC..............................................................................................
Inverse-time Overcurrent (OCIT)...........................................................................
Other Single-quantity Relays..................................................................................
ELECTRONIC RELAYS.....................................................................................
INTRODUCTION
Protection is needed to remove 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 consumer’s premise and even to other
consumers of the Board, with resultant loss of protection and interruption of
vital processes.
It is the function of protective equipment,
in association with the automatic switch fuse or circuit breaker to avert those
effects.
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.
If we consider some typical electrical
layouts the need for discrimination will become clear.
Figure 5.1:- 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
5.1 -
11KV OIL FUSE SWITCH
Figure 5.2 shows a Radial System. A fault as indicated must be interrupted by a
circuit breaker at C, even though the fault is on the LV side of one
transformer. The Board owned circuit
breaker must not trip.
FIGURE
5.2 RADIAL SYSTEM WITH CIRCUIT BREAKERS
If the protection has discriminated
correctly supply will remain via the two transformers controlled by A and B.
Repairs will, of course, be required before
supply can be restored to the remaining two transformers.
Figure 5.3 shows a Closed Ring Main
System. A fault as indicated must be
cleared by circuit breaks at A and B so that all supplies remain uninterrupted.
FIGURE
5.3 -
CLOSED RING MAIN SYSTEM
Figure 5.4 shows a more economic Ring Main
System using non-automatic oil switches.
A fault as indicated must, however, be cleared by a circuit breaker at
the main substation thereby causing loss of supply to substation No. 1. This supply can be restored by manual
switching before repairs commence.
FIGURE
5.4 OPEN RING MAIN SYSTEM
ECONOMIC CONSIDERATION.
The type of layout will depend upon how
important the supply of electricity is, the load required, and whether, in the
circumstances, the capital outlay can be justified considering that OCB’s with
relay protection may cost over 4 times as much as a fuse switch or contactor
with fuse backup.
The cost of protection, however, can be
likened to a premium for insurance against damage to plant and loss of
supply. The cost will normally be small
compared to the amount of capital protected.
With your insurance it is not sufficient to
await a disaster before reading the small print to see if you are covered. So it is with Protection that you must pay
close attention to detail, before a fault occurs, to ensure that the maximum
benefit is obtained.
DISCRIMINATION
THE NEED
It has already
been stated that one of the main purposes of protection was to remove a faulty
equipment or circuit from the electrical system so that as much as possible of
the system could continue to function normally.
It is therefore desirable that any particular fault should be cleared by
that protection device which will perform the service with the least effect on
healthy parts, and not by some device further upstream which would disconnect
an unnecessarily large section of the system.
For example, if a fault occurs on one of a number of circuits fed from
one transformer, it is better to isolate that particular circuit by its own
circuit breaker or fuse than that the transformer should be disconnected from
the supply by its primary overcurrent protection or the generator tripped. This preferential or selective operation of
protection devices is known as 'discrimination'.
THE APPLICATION
Almost all
switchgear is fitted with overcurrent protection of some sort. If a fault develops low down in the system,
fault current will flow right through the network from the supply generator,
through every intervening switch, down to the fault point itself. All these overcurrents will be detected by
the relays of each individual switch, and, if no steps were taken to prevent
it, all might trip together, so shutting down the whole system for what might
have been a purely local fault.
FIGURE 5.5 SIMPLE DISCRIMINATION
The overall
protection system is therefore developed so that the breaker (or fuse) nearest
the fault operates first, thereby isolating only the fault itself. If this does not clear it, the breaker
nearest upstream of the fault operates next, thereby isolating only the minimum
number of consumers. If this one does
not clear, the next upstream breaker operates, and this continues until the
generator breaker trips, but only as a last resort. Each breaker backs up the one below it.
It has already
been shown that most protective devices, such as overcurrent relays and fuses,
have an inverse-time characteristic as shown in the middle column of Figure
5.5. This causes the tripping time to
vary inversely as the magnitude of the fault current. It has also been shown that in relays the
characteristic curve can be altered by adjustment of the relay current and time
settings. For fuses the characteristic
cannot be altered, but a different characteristic can be obtained by choosing a
different fuse.
In Figure 5.5 it
has been assumed that relay settings have been chosen and applied:
·
for the
generator circuit-breaker (breaker C),
·
for the HV feeder circuit-breaker (breaker B),
·
for the LV feeder overcurrent device (breaker
or fuse A),
as shown in the
characteristic curves of the middle column.
For the purposes of direct comparison the three curves have been drawn
to the same scales of time and current referred to a common base voltage.
All three curves
are superimposed on the right. If the
settings have been properly chosen, the curves should appear as shown, each
clear of the other at all points. Since
these curves are subject to tolerance (a relay accuracy of +7% is usual, and
there will be other errors), the curves should all be well clear of each other.
If a fault of
current value F (adjusted to a common base voltage) appears at point P on the
network, the fault current flows through all the breakers A, B and C. Characteristics of A, B and C show that this
current would trip (or blow the fuse) A in time T1 , B in time T2 and C in time T3. Provided that A does trip or blow in time T1 , the fault will be removed and B and C will not trip at all and all
the other consumers on both boards will remain in service.
Should A fail to
trip or blow, or if the fault were at point Q higher in the network, the first
breaker to trip would be B in time T2, but C would remain closed. More consumers would be lost, but the
generator would remain on-line feeding all others. Only if both A and B failed to clear would C
trip and take the generator itself off-line.
It should be
noted that the time delay increases as the tripping point moves nearer the
supply source (in this case the generator).
For this reason generators and their HV switchgear have to have a
3-second through-fault rating under British and European rules, calling in
general for heavier copperwork, whereas distribution switchgear normally has
only a 1-second through-fault rating.
(The 3-second rating does not apply in the US.)
Restricted earth
fault and differential protection, it should be noted, which are instantaneous
and cover only faults within the protected zone, do not form part of a
discriminating protective system. They
may however be used together with one.
If, for example, a fault occurred within a transformer, the differential
protection would deal with it instantly without waiting for the time-delayed
transformer HV breaker to trip.
DISCRIMINATION BETWEEN FUSES, MCCBs AND MCBs
To achieve
adequate discrimination between two fuses of similar type, it is usual to give
the major fuse about three times the normal current rating of the minor
fuse. Between a moulded case
circuit-breaker and a minor fuse the ratio can be reduced to about two.
Because Moulded
Case Circuit-breakers (MCCBs) and Miniature Circuit-breakers (MCBs) have
instantaneous trips in addition to their normal thermal trips, they will not
discriminate with each other at the higher currents. For this reason it is bad practice to install
two MCCBs or MCBs in series, even though they may have different trip units.
BACKUP FUSES
Contactors,
MCCBs and MCBs are not described as part of this manual and although they can
all close onto a fault and carry it for a very short time, their breaking
capacities are strictly limited and are far below those of conventional
circuit-breakers.
When used to
control equipment in networks, their breaking capacities are usually much lower
than the fault levels of the system at the points where they are
installed. For example, a high-voltage
contactor with a maximum breaking capacity of, say, 180MVA at 6.6kV is often
used on a high-voltage switchboard where the fault level may be 500MVA. Similarly low voltage contactors with a
maximum breaking capacity of 15MVA at 440V, or an MCCB with a breaking capacity
of l5MVA, may well be installed in an LV system where the fault level is 3lMVA,
or even 50MVA. If any one of these was
ever called upon to break such fault currents, it would undoubtedly fail and
probably cause a fire. To remove this
risk contactors, MCCBs and MCBs are where necessary backed up by HRC fuses in
series. Such fuses would be chosen with
a breaking current rating to suit the fault levels of the system at the
switchboard in which they are used. LV fuses used on offshore or onshore
installations have a maximum breaking current rating up to an equivalent of
61MVA.
FIGURE
5.6 DISCRIMINATION BY BACKUP FUSE
An LV back-up
fuse and its contactor are shown (in single-line) in Figure 5.6 (a). The fuse, and the contactor (or MCCB) in
series with it, both pass the same fault current. The characteristics of most HRC fuses, which
are thermal devices and therefore of the inverse-time form, are generally of a
somewhat different shape from those of the overcurrent relay protecting the
contactor or of the MCCB tripping device.
Two typical characteristics, for the fuse and for the contactor relay or
MCCB, are shown in Figure 5.6(b).
The contactor
relay or MCCB settings and the HRC fuse
ratings are so chosen that their characteristics cross just below the limiting
breaking current (for example 20kA at 440V) of the contactor or MCCB. Suppose the curves cross at point P,
corresponding to the maximum permissible fault current F for the contactor or
MCCB, then for a fault current F1 less than F, the
contactor or MCCB will be the first to open in time T1 , and it will be well within its rating. For a fault current F2 greater than F which could damage the contactor or MCCB, the fuse
will operate first in time T2, so protecting the contactor or MCCB which will then open on a
'dead' circuit. Fuses can even be used
to back up a main circuit-breaker where the fault level is near to, or exceeds,
its rated breaking capacity. (This can
happen, for example, when the generating capacity of a network is extended
after the switchgear has been installed.)
This use of
fuses as a back-up for both HV and LV switchgear is very common on offshore
installation systems. Unlike
circuit-breakers or contactors they cannot be reclosed but must be physically
replaced after blowing.
It should be
noted that the back-up fuse selected is chosen solely for its characteristic
curve and not for its normal current rating.
It is not intended as overload protection, which is catered for by the
contactor. It is there only to protect
the contactor itself against heavy short-circuits. The actual normal current rating of such a
fuse may seem to bear little relation to the load on the circuit in which it is
used, and it must always be replaced by an identical fuse, not one with a
normal rating apparently more suited to the circuit. If this is not done the whole back-up
protection is lost.
Types of Protection
The Fuse
The simplest form of protection is the fuse,
which is mostly used on L.V. systems. On
the H.V. systems it is commonly used for transformer protection and
occasionally as back-up for motor protection.
When used in H.V. Fuse switches, the blowing
of a fuse is arranged to trip the switch and disconnect all three phases.
Inverse Definite Minimum Times (I.D.M.T.) Relay
The I.D.M.T. relay will give much more
reliable settings and discrimination, and it usually used with a tripping
battery, and D.C. trip coil on thecircuit breaker.
The time of operation of the relay varies
inversely with the current in the operating coil, with a definite minimum time
of operation.
The construction of the relay is shown in
Figure 5.7, where it can be seen that tappings are brought out to give various
current settings. For overcurrent relays
these settings will usually be 50% to 200% rated current in 25% steps. Earth fault relays usually have settings
between 20% and 80% in 10% steps.
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FIGURE
5.7 -
INVERSE DEFINITE MINIMUM TIME (I.D.M.T.) RELAY
In addition to a variety of current settings,
adjustment of the starting point of the disc is possible. This is known as the time setting multiplier,
usually calibrated from 0 to 1.
Both these settings will be applied to the
basic operating curve, Figure 5.8.
Figure 5.8 -
Basic Current/Time curve of IDMT Relay
An I.D.M.T. relay will give discrimination in
terms of current and time, and the connections shown in Figure 5.7 will allow one relay to discriminate as to
type of fault, i.e. earth fault.
Earth fault settings can usually be lower,
since operation on load current does not have to be avoided.
I.D.M.T. relays are used which can also
discriminate as to the direction of the fault.
These directional I.D.M.T. relays contain a nearly normal I.D.M.T. relay
element, but its operation is permitted or prevented by an additional
directional element. The direction
element compares the current to the voltage to determine the direction of flow.
Directional I.D.M.T. relays are particularly
concerned with fault current, and the directional element is usually arranged
to be subjected to maximum torque at very low power factors.
This is different from a reverse power relay,
usually associated with generators, which usually compares voltage and current
directly on one disc, and maximum torque is arranged to occur at high power
factors.
Tripping Batteries
These vary in type, but all have on common
feature. Failure of tripping battery
causes all the associated protection to be ineffective.
INDUCTION DISC
This type of relay is used in a number of forms; the principal ones
are as follows:
Inverse-time Overcurrent (OCIT).
This has a single shaded-pole driving magnet
energised by alternating current from the associated current transformer,
producing a torque which varies with the square of the current. When the
current exceeds a predetermined value the driving torque overcomes the
resistance of the restraining spring and the disc starts to rotate until
eventually a moving contact attached to the spindle (or actuated by it) strikes
a fixed contact (Figure 5.9).
FIGURE
5.9 -
INVERSE TIME OVERCURRENT RELAY
The motion of the disc is opposed by the drag
exerted by the permanent braking magnet, and this gives rise to an appreciable
and consistent time delay. The greater the coil current relative to the minimum
operating current, the faster the disc has to rotate before the braking torque
balances the driving torque, and the shorter is the operating time. This
results in the kind of inverse-time characteristic illustrated in Figure 5.9,
with a long delay at currents barely greater than the minimum operating current
but only a relatively short delay at high overcurrents.
Adjustment of both the operating current and
the delay time-scale is provided for in order to enable a standard relay to
accommodate variations in current transformer (CT) ratios and line currents and
to facilitate discrimination in regard to other protection devices in the
system. Current adjustments are made by
selecting taps on the driving coil, usually by moving a plug between a number
of holes at the front of the relay; typically the range covered is from 50% to
200% of the normal operating current (1A or SA). The time delay is adjusted by
moving the 'fixed' contact, or by altering the starting position of the disc, and
so altering the travel of the disc necessary to close the contacts; this means
that a particular adjustment alters all times on the inverse-time
characteristic in the same ratio.
Figure 5.10 shows a typical resulting family
of characteristics scaled in terms of multiples of the current selector plug
setting and the time multiplier set by the contact adjustment.
FIGURE
5.10 -
TYPICAL RELAY SETTING CURVES
An inverse-time relay may be equipped with an
additional instantaneous element in the same casing set to a high current
value, referred to as a 'High Set' element. This gives it the feature of a
combined 'inverse-time and high-set instantaneous relay, the instantaneous
feature overriding the time delay only on the most severe faults. An example of
this additional feature is shown dotted in Figure 5.9. The modification to the time/current
characteristic is indicated in that figure by the dotted section of the curve.
Very Inverse and Extremely Inverse Overcurrent.
There are two variations of the inverse-time
overcurrent relay: they are referred to as 'very inverse' and extremely
inverse'. The differences lie mainly in the shape of the time! current
characteristic, and examples of each are shown in Figure 5.11, where they are
compared with the characteristic of a normal type. There are no recognised
special abbreviations for these variations of OCIT relays.
Both these variations have characteristics
which are steeper than that of the normal inverse-time type. Advantage is taken
of this when there is a long chain of circuit-breakers with inverse-time relays
and it is desired to achieve sufficient discrimination between their tripping
times for a given fault current. This is indicated in principle in Figure 5.12,
although the full explanation is more complicated.
FIGURE
5.11
VERY
INVERSE AND EXTREMELY INVERSE OVERCURRENT RELAY CHARACTERISTICS
FIGURE
5.12
COMPARISON
OF DISCRIMINATION USING NORMAL AND VERY INVERSE OVERCURRENT RELAYS
The two thick curves are the characteristics
of two adjacent relays in the chain, with the same time settings and with their
current plug settings appropriate to the fault levels at those points. The two
thinner curves are the corresponding characteristics of two very inverse-time
relays installed at those points in place of the normal relays.
It can be seen that, for a given fault
current, the difference (t2) in operating time between the two very
inverse relays is greater than the difference (t1) between the two
normal inverse-time relays. Therefore, if the discrimination time between
circuit-breakers in a distribution time is 'tight', the use of very inverse
overcurrent relays could offer a solution. For example, this might occur where
there is a relatively long chain and the tripping time delay at the supply end
would otherwise be unacceptable.
The further variation of the 'extremely
inverse' relay merely exaggerates this feature. It is often employed when it is
necessary to discriminate with a fuse, which also has a steep characteristic at
the lower current. The longer time delay at the lower end also permits large
'switching-in' currents such as might occur when reclosing a circuit which has
loads still connected. For example heaters or refrigerators may remain
connected even after a prolonged interruption of supply. The in-rush currents
of large transformers can be similarly passed.
FIGURE
5.13
INVERSE
DEFINITE MINIMUM TIME OVERCURRENT RELAY CHARACTERISTIC
Inverse Definite Minimum Time Overcurrent (OCIDMT).
The exact shape of the inverse-time
characteristics is controlled to some extent in design by appropriate design of
the driving electromagnet. A common variation is the Inverse Definite Minimum
Time characteristic, shown in Figure 5.13, in which a lower limit is set to the
delay time as the current increases.
Other Single-quantity Relays.
The type of relay described above for
inverse- time overcurrent protection is also applied to a variety of other
functions for which an inverse-time characteristic is appropriate, such as
earth-fault and overvoltage protection. Voltage relays have higher-resistance
windings, in some cases with series resistors for adjustment, and are normally
fed from voltage transformers.
Voltage-restrained Overcurrent Relay.
The voltage-restrained OCIT relay is designed
to overcome a difficulty encountered in protecting generators, namely that even
a severe fault causes a relatively low overcurrent after the initial transient
period, because of the high synchronous reactance which is normal in
synchronous generators. Such a fault is
therefore not cleared before an undesirably long delay has elapsed. In this
modification an additional driving element is coupled to the disc (or a
separate disc may be mounted on the same spindle) energised by the line voltage
and arranged to produce a restraining torque in opposition to that produced by
the current coil. When the fault causes a drop in system voltage, this
additional restraining torque is weakened, so making the relay more sensitive
and reducing the time delay. Allowance is made in the current setting for the
increased restraining torque and for the consequent longer time delay occurring
when operating at normal voltage.
Voltage-controlled Overcurrent.
This relay achieves a similar effect to that
of the voltage-restrained type, but by a different method. The normal solid-
copper shading rings in the pole faces of the driving magnet are replaced by
windings which are normally connected across resistors, resulting in a moderate
torque and long delay times. If a fault is sufficiently severe to reduce the
system voltage appreciably, an
instantaneous (attracted-armature) undervoltage relay short-circuits the
shading windings; the consequent increase in torque then reduces the delay time
to something more compatible with the characteristics of other protective
devices in the system.
FIGURE
5.14
ELECTROMAGNETIC
SYSTEM OF A WATTMETRIC INDUCTION DISC RELAY
Power
Power relays are 'two-quantity' relays (i.e.
voltage and current) which incorporate the type of driving magnet structure
used for induction watt- meters and kilowatt-hour meters, in which the torque
exerted on the disc depends upon the product of the currents in the two
energising coils, and upon the phase-angle between them. Figure 5.14
illustrates the basic structure. Because power flow is directional, power
relays are also used to give a directional bias to other relays.
The sensitivity of this arrangement to the
phase-angle between the inputs gives the relay directional properties; a flow
of power in one direction generates torque in the direction required to close
the relay contact, while the only effect of a power flow in the opposite
direction is to produce a thrust against the back-stop. The principal use for
such a relay is in reverse- power protection.
ELECTRONIC RELAYS.
FIGURE 5.15 - ELECTRONIC INVERSE TIME OVERCURRENT RELAY
To a considerable extent protection relays of the electromagnetic
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.
The diversity of functions and principles to be found in static
protection relays is such that no comprehensive discussion is possible here.
The various characteristics and adjustments established in electromagnetic
relay practice are readily reproduced electronically. Figure 5.15, without
exactly representing any actual apparatus, illustrates as an example the
application of analogue principles to inverse-time overcurrent protection. (An
analogue system is one in which continuously variable internal signals are used
to represent external quantities such as current and time.)
In Figure 5.10(a) the input from the line current transformer is fed
through a small matching transformer to a low-pass filter R1/C1 which
suppresses transient voltage surges. A voltage proportional to the input
current is developed across the current-setting potentiometer R2. This voltage
is applied to the bridge rectifier.
The d.c. output voltage, which is proportional to the line current,
is used to charge the capacitor C2 through the potentiometer R5. The setting of
this potentiometer determines the rate at which the voltage across C2 increases
and hence the timing of the inverse-time operating characteristic of the relay.
When the voltage across C2 reaches a predetermined value, the detector circuit
operates to switch the electromechanical relay RLA through the output amplifier
and power transistor T2.
'Instantaneous' operation is obtained by applying the output voltage
of the bridge rectifier directly to the input of the amplifier through R4.
Thus, for higher values of fault current, the inverse-time delay circuit is
bypassed.
The power supply for the solid-state circuits is applied through D3
and R6. It is stabilised by zener diode DZ1, and spike protection is afforded
by R7 and C3. The diode B3 guards against reversed polarity of the d.c. power
supply.
Figure 5.15(b) shows the corresponding circuit in block form.
The flexibility and scope of present-day electronics enables a very
wide variety of characteristics to be created with relative ease. While a
simple analogue overcurrent circuit has been described above for the purpose of
illustration, digital techniques have latterly been adopted very widely as a
result of the availability of microprocessors and other digital integrated circuits.
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