The American Standards Association (ASA) has developed a standard system of marking transformer leads. The high-voltage winding leads are marked H and H . The low-voltage winding leads are marked X and X . The H lead is always located on the left-hand side when the transformer is faced from the low-voltage side. When H is instantaneously positive, X is also instantaneously positive.
Transformers with subtractive (buck) and additive (boost) polarities are shown in Figure 13–6. In a transformer with subtractive polarity, the H and X leads are adjacent to or directly across from each other. The H and X leads of a transformer with additive polarity are diagonally across from each other. The arrows in the figure indicate the instantaneous directions of the voltage in the windings.
Standard Test Procedure
Transformer leads normally have identifying tabs or tags marked H1, H2 and X1 , X2 .
However, because it may be impossible to identify the leads because the tags are missing
or disfigured, a standard test procedure can be used.
Figure 13–7A shows a test being made on a transformer with additive (boost) polarity. In this test, a jumper lead is temporarily connected between the high-voltage lead (H ) and the low-voltage lead directly across from it. A voltmeter is connected between the other high-voltage lead (H ) and the low-voltage lead directly across from it. If the voltmeter reads the sum of the primary input voltage and the secondary voltage, the trans- former has additive (boost) polarity. The sum is 2400 V + 240 V = 2640 V. When H1 is instantaneously positive, 240 V is induced in the secondary winding. The input voltage (2400 V) is applied to X through the temporary jumper connection. This value adds to the 240 V so that the potential difference is 2640 V, as indicated on a voltmeter connected from X to H . Note that the path from X to H has the same direction as the voltage arrows. As a result, the two voltages are added.
There is a hazard involved in making the previous test at high voltage values. Thus, a test using a relatively low voltage was developed to determine transformer polarity. For example, in Figure 13–7B, 240 V is used as the test voltage. This potential is usually available in the laboratory or repair shop. By impressing 240 V on the 2400-V winding, a voltage of 24 V is induced in the secondary winding of the transformer.
The voltage ratio is 10:1. The voltmeter is connected between H and the low-voltage lead X1. The reading on the voltmeter is 240 V + 24 V = 264 V. This means that an ac voltmeter, with a range of 0 to 300 V, can be used to determine the polarity of the transformer. The voltmeter is connected between the high-voltage lead (H ) and the low-voltage lead directly across from it. The meter indicates the sum of the primary and secondary voltages. A transformer with this type of polarity markings is an additive polarity type.
In Figure 13–8A, the same polarity test connections are used for a transformer with subtractive (buck) polarity. The 240 V induced in the secondary winding opposes the 2400 V entering X from the temporary jumper connection. The voltmeter is connected
between H2 and X2 and indicates a value of 2400 V - 240 V, or 2160 V. Figure 13–8B shows a polarity test using a low voltage of 240 V. In this case, the voltmeter indicates the difference between the primary and secondary voltages. This difference is 240 V - 24 V = 216 V.
The direction from X to H opposes the voltage arrow from X to X and is the same as the voltage arrow from H to H . Therefore, the X , X voltage is subtracted from the H , H voltage.
ASA AND NEMA STANDARDS
The American Standards Association (ASA) and the National Electrical Manufacturers Association (NEMA) developed the following standards that relate to the polarity of transformers:
1. Additive polarity shall be standard for single-phrase transformers up to 200 kVA, and having voltage ratings not in excess of 9000 V.
2. Subtractive polarity shall be standard for all single-phase transformers larger than 200 kVA, regardless of the voltage rating.
3. Subtractive polarity shall be standard for all single-phase transformers in sizes of 200 kVA and below, having high voltage ratings above 9000 V.
The polarity of a single-phase transformer must be known before it can be connected in parallel with other single-phase transformers or in a three-phase bank. This information normally is provided on the transformer lead tags on the nameplate of the machine. However, when such information is not available, the standard polarity test just explained should be used to determine the polarity.
TRANSFORMERS IN PARALLEL
Single-phase transformers often must be operated in parallel. Several conditions must be satisfied to ensure that the current outputs of the transformers will be in proportion to the kVA capacity of the transformers. These conditions are as follows: (1) the transformers must have the same secondary terminal voltages; (2) the transformer polarities must be correct; and (3) each transformer must have the same percent impedance.
Two stepdown transformers are shown in Figure 13–9. If these transformers have the same voltage ratings, percent impedance values, and additive polarity, they can be connected in parallel. The following steps are used to connect the transformers:
1. The high-voltage leads (H ) of both transformers are connected to one line wire. The other two primary high-voltage leads (H ) are connected to the other line wire.
2. The low-voltage leads (X ) of both transformers are connected to one secondary line wire. The other two low-voltage leads are connected to the other low-voltage line wire.
These two transformers satisfy the three conditions listed previously. As a result, they will both deliver secondary currents to the load in proportion to their kVA ratings.
Transformers of Unknown Polarities
When transformers are supplied by different manufacturers and it is not known whether they have additive or subtractive polarity, the following test may be used. It is assumed that one transformer operates as a stepdown transformer to supply energy to the 120-V bus bars. This transformer is called transformer 1. Transformer 2 is to be paralleled with the first transformer. Transformer 2 has the same voltage ratings and percent impedance as transformer 1, but its polarity is not known.
Figure 13–10 shows that transformer 1 has additive polarity. Regardless of the polarity of transformer 2, its H lead is always on the left-hand side when viewed from the low-voltage side of the transformer. This means that the H lead is connected to the same high-voltage line wire as the H lead of transformer 1. Thus, the H lead of transformer is connected to the other side of the high-voltage line. One of the low-voltage leads of transformer 2 is connected to one side of the 120-V secondary. A voltmeter is connected between the other side of the 120-V secondary and that unconnected secondary lead of transformer 2. If transformer 2 has subtractive polarity, the voltmeter reading is twice the secondary coil voltage. In this case, the voltmeter indicates 240 V.
The instantaneous voltage directions are shown in Figure 13–10. The reason why the voltmeter indicates 240 V is evident by reviewing these instantaneous voltage directions. Assume that the X lead of transformer 2 is connected to the secondary line wire
where the voltmeter is already connected. There will be a potential difference of 240 V at the connection point, resulting in a short circuit.
In Figure 13–11, the low-voltage lead X of transformer 2 is reconnected to the other secondary line wire. The voltmeter now shows a zero potential because the secondary leads of both transformers have the same instantaneous polarity. The voltmeter can be removed and the final connections made without fear of a short circuit.
When stepdown transformers are operated in parallel, as in Figure 13–9, it may be necessary to remove one of the transformers from service for repairs. To do this, the low-voltage side of the transformer is always disconnected from the 120-V line wires before the primary fuses are opened. Remember that the 120-V line wires are still ener- gized by the other transformer. If the primary fuses are opened but the low-voltage transformer leads are still connected to the 120-V line, there will be a serious safety hazard. The low-voltage winding will become a high-voltage secondary. A worker may be electrocuted if it is assumed that the high-voltage winding is deenergized because the primary fuses are open. Although the primary fuses are open, there is still 4800 V across the terminals of the high-voltage winding. As a result, a sign reading “DANGER— FEEDBACK” must be placed at each primary fuse to minimize this hazard.
THE DISTRIBUTION TRANSFORMER
A typical distribution transformer is shown in Figure 13–12. The transformer has two high-voltage windings that are rated at 2400 V each. These windings are connected to a terminal block. The block is located slightly below the level of the insulating oil in the transformer case. Small metal links are used to connect the two high-voltage windings either in series, for a 4800-V primary service, or in parallel, for a 2400-V input.
The two 2400-V primary coils shown in Figure 13–12 are connected in series. A metal link connects terminals B and C for 4800-V operation. Operation at 2400 V is obtained by connecting a metal link between terminals A and B. A second link is used to connect terminals C and D to place the two 2400-V coils in parallel. Although the distribution transformer has two high-voltage coils, note that there are only two external high-voltage
leads. These leads are marked with the standard designations H and H . These leads are permanently connected to the terminal block with lead H attached to terminal A and lead attached to terminal D.
There are four low-voltage leads. When lead H is instantaneously positive, leads X and are also instantaneously positive. At the same time, leads X and X are instantaneously negative. If leads X2 and X3 are commoned together, and leads X and X are commoned together, then the low-voltage coils are connected in parallel to supply an output of 120 V.
If leads X and X are connected together, the two low-voltage coils are connected in series. The resulting output is 240 V across leads X1 and X4 .
If there is a requirement for a 120/240-V, single-phase, three-wire service, then the following connections must be made: The two 120-V secondary windings are connected in series and a grounded neutral wire is connected between leads X2 and X3 . These connections are shown in Figure 13–12. The resulting service provides 120 V for lighting and small-appliance loads and 240 V for heavy-appliance and single-phase, 240-V motor loads.