3.1 GENERAL
Switchgear is required to enable
power sources to be connected to and disconnected from the low-voltage
distribution system. This switching is
necessary both for normal operational purposes and also for the rapid and
automatic disconnection of any circuit that becomes faulty. The switchgear also allows any circuit to be
isolated from the live system and for that circuit to be made safe so that work
may be carried out on the equipment connected to it.
This chapter deals with switching
devices as applied to low voltage (typically 415V or 440V). Three types of LV switchgear are considered:
(a) Circuit-breakers
Circuit-breakers are used to
control inputs from transformers, section breakers on switchboard sections,
interconnectors between LV
switchboards and inputs from auxiliary LV
generators. All LV circuit-breakers are of the air-break
type.
(b) Contactors
Contactors are used to control
mainly motor circuits. They are always
of the air-break type and are usually enclosed in the individual cubicles which
form that part of a low-voltage switchboard referred to as a ‘Motor Control
Centre’
(MCC).
Contactors
are designed only to make and carry fault current for a short time, not to
break it. Where the system fault level
exceeds the limited breaking capacity of the contactor, fuses are inserted in
series with the contactor contacts.
Contactors are designed for remote operation and to undergo repeated and
frequent operation without undue wear.
(c) Moulded Case and Miniature Circuit-breakers
These form a special class of
lightweight compact circuit-breakers for mounting onto or behind panels. They are designed for hand operation only but
have built-in protective tripping arrangements.
3.2 CONSTRUCTION
3.2.1 Main
Air Break Circuit-breakers (ACB)
The main low-voltage
circuit-breakers are always of the air-break type whose construction and
operation are similar to those for high-voltage ACBs described in Chapter 2 and
shown in Figure 2.2. Being designed for
low-voltage systems their insulation levels are of course lower, but, by the
same token, their normal rated currents and their short-circuit current ratings
are considerably higher. This leads to
generally heavier copperwork, to large arc chutes and especially to heavy
switching contacts and isolating contacts.
Like their HV equivalents, LV circuit-breakers are
horizontally isolated, with similar interlocks to ensure the correct sequence
of operations when being withdrawn or reinserted.
Being smaller in size they are
usually mounted in pairs, one above the other in an LV switchboard, presenting a dead-front panel
face.
Most LV
systems are 4-wire. Some main
circuit-breakers are 4-pole, but most are 3-pole with an unswitched neutral
connecting link.
LV circuit-breakers are rated from 800A to 4 000A normal current. They come in standard ranges of breaking
capacity, which in British Standards is 35kA, 43kA and 70kA rms
symmetrical. These currents at 415V are
equivalent to 25MVA, 31MVA and 50MVA respectively, or at 440V are equivalent
respectively to 27MVA, 33MVA and 53MVA.
Because of the heavier normal and short-circuit currents found in LV
systems, the circuit-breakers usually have much heavier breaking and isolating
contacts than those of the HV types.
The circuit-breaker closing mechanism may be operated by
solenoid or motor/spring. Descriptions
of both these methods will be found in Chapter 2, para. 2.3. Tripping is by a separate shunt-trip coil,
always powered from an independent battery-supported d.c. supply.
The basic circuit-breaker control circuits are
essentially the same as those described in Chapter 2 for high-voltage
switchgear. Some circuit-breakers are
equipped with an additional release device in series with the main circuit
which trips the circuit-breaker instantaneously if it is closed onto a fault;
it does not operate under any other circumstance. This series tripping release is part of the
circuit-breaker, not a protective relay, and requires to be reset by hand after
operation. Where this device is fitted,
the anti-pumping circuit is unnecessary and is omitted.
Closing and tripping of low-voltage circuit-breakers is
usually carried out by operating switches on a remote electrical control
panel. In some cases this remote control
facility is not provided, and switching is done locally at the switchboard.
The procedure for disconnecting a circuit-breaker and
removing it from its panel depends on the type of switchgear; the maker’s
instructions must be consulted in each case.
To avoid personal injury, the closing spring must always be discharged
before removing a spring-closed circuit-breaker from its switchgear panel.
3.2.2 Contactors
All contactors form a part of the individual
distribution cubicles which make up an MCC.
Almost all are unlatched.
Each contactor is rated according to the service which
it feeds, which may vary from a fractional horsepower motor drawing one ampere
to a large 250kW motor drawing over 400A.
Consequently the contactor cubicle may vary in size from ‘one tier’ deep
up to ‘seven tiers’ deep (see para. 3.3.2).
Each contactor operating coil is supplied, through its
control circuits, from the switchboard busbar either direct at 415V or 440V, or
through a small step-down transformer.
This ensures that, if busbar power fails, all connected contactors ‘drop
off’ and keep their motors disconnected until each can be individually
restarted.
Every contactor is backed up by a set of high rupturing
capacity (HRC) fuses housed in the same cubicle. These are rated according to the fault level
at the switchboard (typically 31MVA or 50MVA, equivalent to 43kA or 70kA at
415V). The correct size of fuse is
chosen so that, in the event of a fault in the feeder circuit which exceeds the
ability of the contactor to clear it, the fuse will blow first, leaving the
contactor to open on a dead circuit. The
choice of back-up fuse is further discussed in the manual ‘Electrical
Protection’.
Facilities are provided for testing the contactor while
isolated from the busbar. While so
isolated a separate test supply can be applied to the contactor coil to check
its operation without actually starting the motor.
3.2.3 Moulded
Case Circuit-breakers
A type of low-voltage
circuit-breaker widely used in most installations is the Moulded Case
Circuit-breaker, or MCCB for short.
Shown in Figure 3.1, it consists of a moulded plastic case containing a switching element which is operated manually by an external handle or ‘dolly’. Because the original design was American, the dolly position is down for ‘Off’ and up for ‘On’. MCCBs can be used for switching either a.c. or d.c. circuits. They are usually mounted, when used on distribution panels, behind the panel, and only the dolly shows. Other arrangements however, such as surface mounting, are also found.
FIGURE 3.1
MOULDED CASE CIRCUIT-BREAKER
Most of the MCCBs used onshore and
offshore are 3-pole, but very occasionally a 4-pole version is fitted. They are also supplied as 2-pole (for example
for d.c. switching), but this is usually a 3-pole type with one pole omitted.
MCCBs are very compact and have a
high breaking capacity for their small size.
Where the system fault level at the point where an MCCB is used exceeds
its fault-breaking capacity, separate HRC back-up fuses must be used in series,
as described above for
contactors. MCCBs are made by a number of manufacturers, and different makes and sizes are used in installations. The following description, therefore, can be no more than very general.
contactors. MCCBs are made by a number of manufacturers, and different makes and sizes are used in installations. The following description, therefore, can be no more than very general.
A MCCB as used in onshore and offshore installations is
normally fitted with two separate overcurrent devices. One is a thermal element in each pole having
an inverse-time characteristic, and the other an instantaneous ‘high-set’
electromagnetic element in two of the three poles; this operates
instantaneously but only on the highest fault currents and then overrides the
thermal element. Both trip the
circuit-breaker when the current reaches the set operating level in any of the
poles.
Typical MCCBs used in installations have normal current
ratings of either 125A or 250A, according to the circuits they control. The breaking capacities of these two sizes
are given below.
|
Normal Current
|
Breaking Capacity to BS
Rules
|
||
|
MVA (3-phase, 440V)
|
Equiv. kA (a.c.)
(rms symmetrical)
|
kA (d.c. 250V)
|
|
|
125A
250A
|
7.5 MVA
15 MVA
|
10kA
20kA
|
18kA
25kA
|
The maximum currents that can be handled by these two
sizes of MCCB are therefore 125A and 250A, but they can be arranged to trip at
lower currents. This is achieved by
fitting a separate ‘trip unit’ to the breaker.
In the 250A size the trip units are interchangeable, but in the 125A
size they must be fitted at the time of ordering and cannot thereafter be
changed; if a different trip setting is needed, the whole 125A MCCB must be
replaced by one with the new setting.
Sizes larger than 250A are manufactured but are not installed in
platforms.
The following trip units are available:
125A size 250A size
125A 250A
100A 200A
75A 160A
50A 125A
25A 100A
15A 60A
100A 200A
75A 160A
50A 125A
25A 100A
15A 60A
There is normally no adjustment of current setting in
any of the thermal units, but the electromagnetic elements have settings
adjustable in five steps, numbered 1 to 5, by means of a control knob on the
trip unit. The range of adjustment of the
high-set instantaneous overcurrent trip element is typically from 6 to 13 times
the full-load rating of the trip unit (note, not of the MCCB itself).
When a closed MCCB self-trips due to operation of either
of its overcurrent trip elements, the main contacts open fully, but the dolly
goes to a mid, ‘half-cock’ position where it shows a white line in the dolly
window (see Figure 3.1). This indicates
which of a number of MCCBs has tripped and enables it to be distinguished from
those which were already open. Before
the MCCB can be reclosed by hand, the dolly must first be moved to the ‘Off’
position.
MCCBs are ‘latched’ breakers in that they do not, like
contactors, fall out automatically if the service voltage disappears; they must
be individually opened by hand. However
the 250A size (only) can be fitted with an undervoltage release, but none is so
fitted on offshore installations.
Although MCCBs are self-tripping,
they are not normally remote controlled.
However, the 250A size (only) can be provided with a shunt-trip release
which enables it to be electrically tripped from a remote point. It cannot be remotely closed without an
additional operating mechanism. Neither
of these features is employed in most installations except for the shunt trip
on certain offshore d.c. distribution boards.
Many MCCBs are used in onshore and
offshore installations as incomer isolators for sub-distribution boards such as
lighting or sundries panels. When used
in this way they are pure isolators and do not have a protective function. Such MCCBs usually have their trip units
removed and can only be opened by hand.
3.2.4 Miniature
Circuit-breakers
 |
The range of MCCBs already described extends down to a series of smaller breakers of ratings up to 70A maximum. These are known as Miniature Circuit-breakers (MCBs). (The term MCB should not be confused, as it often is, with MCCB for the moulded-case type.)
FIGURE 3.2
MINIATURE CIRCUIT-BREAKER
A typical single-pole MCB is shown
in Figure 3.2. In operation it is
generally similar to, though physically smaller than, the moulded-case
design. The MCB has a moulded plastic
case and is manually operated by an external dolly. It is manufactured as a 1-pole, 2-pole,
3-pole or 4-pole unit, but the commonest types in most installations are the
1-pole and
2-pole models which are used in
single-phase-and-neutral sub-distribution panels. Such panels usually mount the MCBs
horizontally in blocks of 6 or 12. Other
forms of mounting are also found.
Sometimes where the 3-phase
switching is desired, instead of using a 3-pole MCB, three 1-pole units are
physically ganged together by a bar joining the dollies.
Each pole of an MCB is protected by
two separate overcurrent devices. One is
a thermal element having an inverse-time characteristic, and the other an
instantaneous ‘high-set’ electromagnetic element; this operates only on the
highest fault currents and then overrides the thermal element. Both trip the circuit-breaker when the
current reaches the set operating level in any of the poles.
The size ranges of MCBs vary from
one manufacturer to another, but only a limited number are used in most
installations. Two typical sizes have
normal current ratings of 70A and 32A, but they may be fitted with one of
several different trip units, as follows:
|
70A size
|
32A size
|
|||
|
|
|
|
|
|
|
3A
|
25A
|
1A
|
10A
|
|
|
6A
|
32A
|
2A
|
15A
|
|
|
10A
|
40A
|
3A
|
20A
|
|
|
16A
|
50A
|
5A
|
25A
|
|
|
20A
|
70A
|
|
32A
|
|
The trip unit fitted in any given
MCB is usually denoted by a figure on the dolly. The unit is fitted during manufacture and
cannot be changed. (Figure 3.2 shows a 6A trip unit fitted.)
The fault current breaking capacity
of an MCB differs according to the number of poles, the working voltage and, in
some cases, the trip unit fitted. The
maximum breaking currents are as follows:
70A size
|
Trip Unit
|
Breaking Current (kA)
|
|||
|
250V a.c.
|
440V a.c
3-pole
|
125V d.c.
2-pole
|
||
|
1-pole
|
2-pole
|
|||
|
3A
6A
10A
16 to 70A
|
1kA
1kA
5kA
5kA
|
1kA
1kA
8kA
8kA
|
1kA rms symm (=0.75MVA)
1kA rms symm (=0.75MVA)
5kA rms symm (=3.8MVA)
3kA rms symm (=2.3MVA)
|
1kA
1kA
10kA
10kA
|
32A size
|
Trip Unit
|
Breaking Current (kA)
|
|||
|
250V a.c.
|
440V a.c
3-pole
|
125V d.c.
2-pole
|
||
|
1-pole
|
2-pole
|
|||
|
1 to 5kA
10 to 32A
|
6kA
6kA
|
10kA
8kA
|
3kA rms symm (=2.3MVA)
3kA rms symm (=2.3MVA)
|
5kA
5kA
|
The electromagnetic trips in the 70A size operate at 7
to 12 times the current rating of the thermal trip unit fitted (note, not of
the MCB itself). In the case of the 32A
size the range is 6 to 9 times.
Both sizes of MCB are also manufactured in an
alternative ‘high breaking capacity’ version giving approximately 50% higher
breaking current, but these are not used on offshore or onshore installations.
If the system fault level at the point where the MCB is
used exceeds its fault-breaking capacity given in the tables opposite, a
properly selected back-up fuse of correct characteristic must be placed in
series, as described in the manual ‘Electrical Protection’.
MCBs are ‘latching’ devices and do not, like contactors,
fall out automatically if the service voltage disappears; they must be
individually opened by hand. They cannot
be fitted with an undervoltage release, nor can they be remote-controlled.
3.2.5 Earth
Leakage Circuit-breakers
A special type of miniature circuit-breaker, very
sensitive to small earth-leakage currents, is described in the manual
‘Electrical Protection’, Chapter 5.
3.3 LOW VOLTAGE SWITCHBOARDS
3.3.1 Centre
(Incomer) Section
An LV switchboard is usually supplied from one or two
step-down transformers fed from the HV system. LV operating voltages are
normally 415V onshore and 440V offshore.
FIGURE 3.3
TYPICAL ONSHORE 415V SYSTEM
Figure 3.3 shows diagrammatically such an onshore LV
system with two transformer incomers, a bus-section breaker, a heavy feeder and
two grouped distributing sections, left and right, also called Motor Control
Centres (MCCs). (Note: This is a
somewhat misleading term, as by no means all the feeder circuits supply power
to motors.)
The circuit-breakers usually form the centre section,
with power being passed to left and right by the busbars. Heavy-current feeders and the larger interconnectors
feeding power to or from other LV switchboards sometimes require
circuit-breaker protection and are then brought into the centre section.
FIGURE 3.4
PART OF TYPICAL OFFSHORE
440VSWITCHBOARD
Part of a typical offshore 440V switchboard is shown in
Figure 3.4. The part shown consists of
five panels mounted side by side; the centre three panels contain cubicles for
the incoming feeder, bus-section and heavy feeder circuit-breakers; the
associated protective relays, control switches and indication equipment are
mounted on the fronts of each panel.
On each side of the circuit-breaker panels are MCC
panels, one of which is shown in Figure 3.4 on each side of the centre
panels. Further MCC panels are added as
required. Each contains a number of motor
control contactor cubicles and fuse-switch cubicles mounted one above the other
to control the outgoing circuits. The
fuse-switch cubicles control those circuits not associated with motors such as
sub-distribution boards or welding sockets.
Additional MCC panels are mounted on each side of the
centre section to house the feeder cubicles necessary to meet the requirements
of the system concerned. A large
switchboard may include as many as 30 or more MCC panels.
The arrangement of the busbars and circuit connections
is shown diagrammatically in colour on Figure 3.4; the main busbars are shown
in red for the phases and blue for neutral and run through busbar chambers at
the top and to the rear of the panels; they are connected through the length of
each section of switchboard. Power is
supplied to each outgoing feeder cubicle by a set of dropping busbars (also
shown in red and blue) housed in a vertical enclosure at the rear of each MCC
panel.
On many switchboards each incoming switchgear panel has
provision for earthing the neutral busbar through a bolted link, shown in
black. It is not switched with the
breaker. When the incoming supply is from a transformer, this link is closed,
earthing both the neutral busbar and the star-point of the transformer; this
provides an earth for that particular part of the LV system. On systems where the transformer star-point
is earthed direct, this feature is not provided at the switchboard.
On some installations where dry-type encapsulated
transformers are used, the transformers themselves form part of the LV
switchboard, installed behind a panel and with their LV terminals connected
directly onto the copperwork of the incomer panel.
3.3.2 MCC
Section
|
Each outgoing circuit (other than interconnectors) is controlled by a feeder cubicle on one of the MCC ‘wings’ of the switchboard. These cubicles are of different types, depending on
the manufacturer.
The following description is typical and is widely used in both onshore
and offshore installations.
The MCC feeder cubicles occupy the full width of an MCC
panel, but their vertical height depends upon the rating and function of the
unit. The smallest unit occupies one
module of height - a ‘one-tier’ cubicle - and there is space for ten of
them. The largest is seven tiers
high. In practice a panel usually
contains a mixture of cubicles of different heights to suit the particular
distribution requirement.
In the smallest (one-tier) feeder cubicles the HRC fuse
bases are permanently fixed to the busbar droppers and, although they are
shrouded, care is needed when withdrawing or replacing them. Such feeder cubicles have a simple rotary
isolating switch on the hinged panel-front door, as shown in Figure 3.5(a).
Larger sizes of switch panel are provided with a fuse-switch
for isolation. This is operated by an
isolating handle on the door of the unit with a mechanical drive to the fixed
fuse-switch through a dog-clutch which is engaged only when the door is closed. The fuses are dead when the fuse-switch is
off. This is shown in Figure 3.5(b).
Both isolating switches and fuse-switches are
interlocked with the doors of their associated switch unit so that the door
cannot be opened unless the switch is off.
All MCC feeder cubicles used for motor control have a
contactor in the circuit following the isolator switch or fuse-switch; the two
types are shown in Figures 3.5(c) and 3.5(d).
It is possible to test the contactors without actually
starting the motor. When the cubicle
door is opened, the isolator having first been opened, a switch inside can be
closed to provide an alternative supply to the contactor coil. The contactor can then be operated while its
main contacts are isolated from the mains.
A small cubicle at the top of the MCC panel provides the test supply through
small distribution fuses; it is labelled ‘TEST’ or ‘CONTROL’.
Motors are normally started and stopped by remote
control from the control room or the motor site. Starting pushbuttons or switches at those
points cause the contactor at the MCC to close.
It is very rare for provision to be made to start a motor at the MCC
cubicle itself (some ventilation fans are exceptions). However each motor cubicle at the MCC has an
emergency stop pushbutton.
3.4 FUSES
Fuses are used with low-voltage
switchgear:
(a) as back-up for
distribution contactors, or
(b)
for
various control and instrumentation circuits.
In all cases
they are of the HRC type.
When used as back-up the fuses are
inside the individual distribution cubicles on the MCC section of the switchboard. In one design they are either direct on the
busbars (for one-tier units) or embodied in the isolating switch as a
‘fuse-switch’ in larger units - see Figure 3.5.
In this case protection against accidental contact is afforded by the
cubicle enclosure itself. Access to a
fuse-switch is only possible after the fuse-carrying blades of the switch have
been put in the isolated (open) position and the door opened.
Great care is needed with the busbar
fuses in one-tier cubicles. Although the
door cannot be opened until the isolating switch has been opened, the fuses
themselves are still
connected to the live busbar, though not carrying current - see Figure 3.5(a) and (c). Although the fuse links are well shrouded, caution should be shown when removing or replacing them.
connected to the live busbar, though not carrying current - see Figure 3.5(a) and (c). Although the fuse links are well shrouded, caution should be shown when removing or replacing them.
Low-voltage control and instrument fuses are usually panel-mounted in their own carriers. Their physical size is determined by their normal current rating. Their breaking capacity is determined by the fault level current of the circuit in which they are connected
FIGURE 3.6
COMPLETE LV FUSE UNIT (TYPICAL)
A typical low-voltage fuse assembly
is shown in Figure 3.6. The replaceable
ceramic cartridge with its metal terminal caps is known as the ‘fuse
link’. It is held in an insulated ‘fuse
carrier’ which completely shrouds all live metal. The carrier is supported on an insulated
‘fuse base’, where it is firmly fixed by various mechanical means, among them
tongue contacts, butt contacts held by insulated screw pressure, or wedge contacts
pressed in by insulated screws. A
tongue-contact type is shown in Figure 3.6.
A full description of how an HRC
fuse operates to interrupt current is given in the manual ‘Electrical
Protection’.
3.5 BUSBAR BRACING – SHORT-CIRCUIT FORCES
Figure 3.7 shows two busbars, A and
B, installed side by side, supported on post insulators and carrying a
single-phase current.
FIGURE 3.7
FORCES ON A BUSBAR
This current, at the instant shown,
is assumed to be flowing down (into the paper) in the left-hand bar A, and up
(from the paper) in the right-hand bar B, as indicated in the figure.
By Oersted’s rule the current in bar
A will give rise to a circular magnetic field around it in a clockwise
direction (corkscrew motion), whereas the current in bar B will give rise to a
circular magnetic field in the anti-clockwise direction. These fields are shown in the figure, and
each field system cuts the other busbar.
By Fleming’s Left-hand Rule (see the
manual ‘Fundamentals of Electricity 1’) the interaction between the current in
A and the magnetic field from B which cuts it is to produce on bar A a
mechanical force F to the left.
Similarly the interaction between the current in B and the magnetic
field from A produces on bar B an equal mechanical force F to the right.
Half a cycle later both the currents
and both the magnetic fields are reversed, so that the mechanical force on
each, produced according to Fleming’s Left-hand Rule, is unaltered in
direction.
Thus both bars are subjected to
outward mechanical forces trying to push them apart. They are resisted only by the post insulators
which undergo a shearing and cantilever stress tending to break them. This force alternates in magnitude as the
current cycles, but not in direction. It
reaches a peak value twice each cycle: once when the currents peak as shown in
the figure, and again half a cycle later when they peak in the opposite
direction.
The magnitude of the force on a bar
depends on the strength of the current in the bar and also on the strength of
the magnetic field cutting it, which itself depends on the strength of the
equal and opposite current in the other bar.
The force is thus proportional to the current squared. It also depends on the distance between the
bars, becoming less as the spacing is increased.
The following calculation gives some idea of the scale
of these forces, particularly under conditions of short-circuit when the
currents may be very large indeed. If
the currents were, say, 50 000A d.c. in each bar, and if the bars were
spaced 3 inches (7.5cm) apart, the outward force on each bar would amount to no
less than 670 kgf per metre run of bars, or nearly ¼ ton-force per foot run.
If the current were 50 000A alternating (rms), the peak current would be Ö 2 times this, namely over 70 000A, and the peak forces would
be doubled to nearly ½ ton-force per foot run.
It does not end there: on switching on, or at the onset of a fault, the
current might be 100% asymmetrical (see Chapter 5 of the manual ‘Fundamentals
of Electricity 3’), in which case the current could peak to 2.55 times the rms
value, namely to nearly 130 000A, which would give momentary forces of
over 1½ tons-force per foot run.
It can easily be seen that such enormous forces would
instantly break up the busbar system unless the bars were firmly braced at
close intervals against lateral movement.
The forces due to one single asymmetrical peak could cause the damage
which would initiate a catastrophic break-up.
Short-circuit currents of the order of 50 000A rms are not uncommon
in LV systems.
Care must be taken in switchboards to brace not only the
busbars themselves, but also any ‘droppers’ or tee-offs through which a
short-circuit current may pass.
In 3-phase switchboards, where three busbars are usually
arranged side by side, the currents, being 120° apart in time, are not equal and opposite as described above for
single phase. In different parts of the
cycle the currents in adjacent bars will sometimes be in opposite directions
and sometimes in the same direction.
Therefore the mechanical force on any bar is at one instant acting to
blow it outwards, and at the next to draw it inwards. If the spacing is not sufficient (bolt-heads
can be a source of trouble) or if the bar or bracing is not stiff enough, a bar
might even touch its neighbour, so transferring the original external
short-circuit into the switchboard itself, with a high risk of fire. Possible mechanical resonance of the bars or
of any droppers to the 50Hz or 60Hz supply must also not be overlooked.
Although busbar forces have been described here in
relation to low-voltage switchgear, they occur equally in high-voltage
switchgear. However the current levels
there, even short-circuit levels, are generally much lower and the busbar
spacing greater; consequently the problem is most acute on low-voltage
switchboards.
3.6 HOLLOW BUSBARS - SKIN EFFECT
If a conductor, such as the one shown in Figure 3.8, is
carrying a current, that current will normally make use of the conductor’s
whole cross-section area. If the cross-section
is regarded as made up of a number of equal, thin elements each carrying some
of the total current, then all these elements are in parallel and have the same
resistance for a given length of conductor.
The current will divide equally between them - that is to say, it will
distribute itself uniformly over the whole cross-section.
This is certainly true of direct currents and also of
alternating currents at power frequencies if they are not too large.
FIGURE 3.8
SKIN EFFECT - ELEMENT OF CONDUCTOR
To examine how the conductor behaves
when carrying an alternating current, consider a solid conductor of circular
section as shown in Figure 3.8. Take a
short length of this conductor, for example between points A and D. Assume the current (I) to be flowing upwards as indicated, the direction being taken at
a certain instant of time during the alternating cycle when the current is
rising.
Consider now a rectangular element
ABCD within the conductor, the side DA along the axis, AB and DC in a radial direction,
and the side BC inside the conductor
and parallel with AD. The thin strip of
conducting material around the sides of the rectangle forms a closed conducting
loop.
That part of the main current I flowing
through the centre part DA of the conductor causes a circular magnetic field
around it, anti-clockwise as seen from above and as shown in the figure, and in
phase with the current causing it. This
alternating flux passes through the conducting loop ABCD, causing an emf to be
generated in it by Faraday’s Law, that emf being anti-clockwise at the instant
shown in the figure, but lagging 90° in phase on the
flux, and therefore on the conductor’s main current.
The emf induced in the closed
rectangular element ABCD causes a current i
to flow round it, again anti-clockwise at the instant shown. Because the loop is mainly inductive this
current will lag almost 90° on the emf causing it, which, as shown
above, itself lags 90° on the flux and so on the main
current. Therefore the loop current lags a total of 180° on the main current - that is to say, it is anti-phase with it.
FIGURE
3.9
SKIN EFFECT - CURRENT IN A CONDUCTOR
Although Figure 3.8 suggests that
the current flow i in the loop is
anti-clockwise at the instant shown, in fact, because of its 180° phase lag, it is opposite in sign, and the actual directions at
that instant are as shown in Figure 3.9.
This figure shows clearly that the
loop current opposes the main current at the centre but adds to it towards the
outside. The current density at the
centre is therefore reduced, but it is increased as one goes outwards. The effect is dependent on the magnitude and
on the rate of change of the flux. The
former is directly proportional to the conductor’s main current, and the latter
to its frequency. The effect is also
greater with large diameter conductors.
The result is an increasing
concentration of the main current towards the outer layers and a decrease at
the centre. This phenomenon is known as
‘skin effect’. It is not present at all
with d.c.,
since the circular magnetic field does not alternate and so produces no
emf. With normal levels of a.c. power
currents the effect, though present, is negligible. But when currents of several thousand amperes
are flowing, the internal magnetic field is large and the skin effect becomes
pronounced.
By the time currents of the order of
3 000 to 4 000A are reached, the effect is so strong that the bulk of
the current is flowing in the outer parts of the conductor and very little in
the centre. The centre, in fact, is
hardly being used and is a waste of expensive copper. With currents above these levels it is not
uncommon to install hollow busbars, which are in effect a continuous skin
having no centre. A further advantage of
using hollow busbars is that
cooling air or other medium may be passed along their full length. There is no need for the hollow busbar to be circular; it may be a rectangular extrusion or made up of plates.
cooling air or other medium may be passed along their full length. There is no need for the hollow busbar to be circular; it may be a rectangular extrusion or made up of plates.
Although LV switchboards with busbar
ratings up to 4 000A are found on some offshore platforms, hollow busbars
have so far not been used. They may
however be found on shore installations of very large current capacity.
The phenomenon of skin effect is
well known in the radio world where high frequencies up to many megahertz are
used, but it is not generally realised that it also occurs at power
frequencies.
No comments:
Post a Comment