2.1 3-PHASE CONNECTIONS
In Chapter 1 was described how an
a.c. generator is usually equipped with three separate windings, disposed at
equal 120° space intervals around the stator.
These windings produce identical alternating voltages, but each is timed
to peak one-third of a cycle (120° time degrees)
after its predecessor. The three
windings are distinguished by calling them ‘red’, ‘yellow’ and ‘blue’, or
sometimes A, B and C or U, V and W. The
red-yellow-blue notation will be used in this manual.
FIGURE 2.1
STAR AND DELTA CONNECTIONS
Each winding has two ends, called R
& R’, Y & Y’ and B & B’.
Normally each winding supplies one element of a 3-phase load. If the three loads are identical - that is,
if they each draw the same current - the load is said to be ‘balanced’. Unless otherwise stated in the description
which follows, loads will always be assumed to be balanced.
The simplest way of connecting the
generator windings to the loads is to arrange that each is connected to its own
load independently of the others, as shown in Figure 2.1(a). Since each winding has two terminals, this
will require six wires to convey the power from all three phases.
As will be shown later, such a form
of connection, though perfectly correct, is wasteful of copper and cable material
and is seldom, if ever, used.
2.2 STAR CONNECTION
What is done is to make each phase
share one conductor with the others. One
way of doing this is to connect the inner ends of all three phase windings
together - that is to say, terminals R’, Y’ and B’ are commoned - and a fourth
wire is taken from this common point to a similar common point of the three
loads; this is shown in Figure 2.1(b).
The wires connected to the three outer terminals R, Y and B each carry
the outward phase currents into their respective loads, but all use the common
return path to R’, Y’ and B’. Such an
arrangement is called a ‘star connection’ (American ‘wye’). The common point is called the ‘star point’,
and the fourth or common wire is called the ‘neutral’ conductor.
Star connection of generators and
transformer secondaries is widely used both ashore and on platforms.
2.3 NEUTRAL CURRENT
Look now more closely at the current
which flows in the neutral wire; it is the sum of the red, yellow and blue
return currents.
FIGURE 2.2
NEUTRAL CURRENT
In a balanced system the currents in
the three phases are equal in magnitude but are displaced one-third of a cycle,
or 120° time degrees, from each other.
Three such balanced phase currents are shown in the upper part of Figure
2.2, and it will be easily seen that yellow current lags 120° on red, and blue current 120° on yellow. It follows that,
on completion of one full cycle, that red once more lags 120° on blue.
Since the neutral return current is
the sum of all three phase currents at all instants, the neutral current wave
can be obtained by adding the three values of the phase currents at every
instant, taking account of their signs.
Suppose we take the instant t1
where blue is at a negative peak (B). At
that moment red and yellow are both positive at half peak value, yellow rising
(Y) and red falling (R). So the sum of
the three is -1 +½ +½, which is zero. If
another arbitrary point is taken, say at t2,
and the three values of current at that instant are measured taking account of
their signs (NR + NY + NB), it will be found that, once again, they add up to
zero.
The surprising conclusion is that,
although the neutral wire is carrying the sum of the three phase currents, it
is actually carrying nothing at all.
(Note that it was assumed that the loads were balanced; if they had not
been, this conclusion would be no longer valid.)
2.4 3- AND 4-WIRE SYSTEMS
If the neutral conductor carries no
current, why have it at all, or waste money on expensive cable?
FIGURE 2.3
STAR CONNECTION 3- AND
4-WIRE SYSTEMS
It is in fact usually left out, at
least in high-voltage systems where the loads are always regarded as nearly
balanced and also in some low-voltage systems where balance may be assumed (for
example motors). The neutral conductor
is entirely dispensed with, and all the power is transmitted by the three phase
conductors only. Such a system is known
as ‘3-wire’ distribution and is shown in Figure 2.3(a).
Where balance cannot be assumed,
particularly in low-voltage systems where there may be many single-phase loads,
the neutral current is not zero, and the neutral wire must be retained. This is shown in Figure 2.3(b) and is known
as ‘4-wire’ distribution. It is
generally used on platform and shore-side low-voltage distribution systems.
Where there are many single-phase
loads which cause unbalance (for example lighting circuits), they are connected
between one phase and the neutral which is available on a 4-wire system. Every effort is made at the design stage to
distribute the single-phase loads as evenly as possible between each of the
phases and neutral so as to reduce to a minimum any unbalance caused. As a result, although the neutral does not
carry zero current and therefore cannot be dispensed with, the current which it
does carry is relatively small compared with the phase currents. If the cables from a transformer feeding a
low-voltage system are examined, it will usually be found that, although each
phase may require perhaps four cables in parallel for each phase to carry the
large phase currents, there may be only two, or even one, neutral cable.
2.5 DELTA CONNECTION
Referring to Figure 2.1(c) it will
be seen that there is another way by which the generator windings can share
conductors. In this case, instead of
sharing a common return conductor as with star connection, each winding shares
a conductor with its neighbour at both ends.
That is to say, R is commoned with Y’, Y with B’ and B with R’. There are only three conductors leaving the
generator and carrying power to the loads.
Because, for convenience of drawing, such an arrangement is usually
shown in a triangular form, it is called a ‘delta connection’ (American
‘mesh’). There is no star-point in this
case and therefore no possibility of any neutral connection. Distribution from a delta-connected source or
to a delta-connected load must therefore always be 3-wire.
Figure 2.1 shows the loads
all star-connected, even when supplied from a delta-connected generator. The loads themselves however may equally well
have been delta-connected, although this is unusual. In that case there would be no neutral
conductor.
From Figure 2.1(b) it will be seen
that, with star connection, the line current from generator to load must be the
same as the generator winding’s phase current, since they are in series. With a delta connection however (Figure
2.1(c)) the line current is divided between two phases. This gives a slight cost advantage where
heavy currents are involved and the copper section is large.
From Figure 2.1(c) it will be seen
that, in a delta-connected circuit, the voltage between lines is the same as
the voltage across one winding of the generator, since they are in parallel.
For various reasons, chiefly because
of the availability of the neutral for earthing purposes (see manual
‘Electrical Power Systems’), star connection is almost always used with
generators and with the secondary sides of distribution transformers (the
primaries however are usually delta-connected).
One notable exception is the
generator system of many platform drilling installations where they are
separate from the platform system. They
are seldom earthed, and drilling practice customarily uses delta-connected
generators and transformer secondaries.
The relationships between the phase
and line voltages, and between the phase and line currents, in star- and
delta-connected systems are explained in Chapter 7.
2.6 DRAWING OF STAR- AND DELTA-CONNECTED APPARATUS
In Figures 2.1 and 2.3, star
connection was shown by a ‘star’ or ‘Y’ arrangement of windings or loads, and
delta connection by a triangular arrangement.
This is an advantage for instructional purposes, but for distribution
drawings another way is used for showing these connections (Figure 2.4).
FIGURE 2.4
PRESENTATION OF STAR AND
DELTA CONNECTIONS
IN DISTRIBUTION DIAGRAMS
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