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.
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.
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.
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.
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.)
If the neutral conductor carries no current, why have it at all, or waste money on expensive cable?
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.
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.
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).
PRESENTATION OF STAR AND DELTA CONNECTIONS
IN DISTRIBUTION DIAGRAMS