THREE-PHASE TRANSFORMER

BANK DIAGRAMS

OBJECTIVE

You will complete various combinations of three-phase transformer bank diagrams for:

WYE-WYE

WYE - DELTA

DELTA - DELTA

DELTA - WYE

THREE-PHASE TRANSFORMER BANK DIAGRAMS

As a lineman, you will often install, maintain, and trouble-shoot three-phase transformer banks. Understanding how the bank works helps you install it to standard and fix it should trouble occur.

You already know the characteristics of both a Wye system and Delta system. In this module you will review basic transformer banks you learned in a previous module plus connect single phase services from 3 phase banks.

First, let's review the information you must have when connecting distribution transformers single phase or three phase. This information is found on the Name Plate, namely:

1. Primary or High Voltage - a transformer is rated to operate on a given primary voltage. If it is supplied with other than it's rating, it will not give the rated secondary voltage.

2. Secondary or Low Voltage - is the voltage the transformer is rated to deliver if the primary voltage is supplied as rated on the nameplate.

3. If the transformers in a bank have tap changers, all transformers must be on the same percentage tap position to ensure the same output voltage on all three phases.

4. The impedances of banked transformers must be within 1% of one another. If not, the one with the lowest impedance (most efficient) will tend to hog the load or overwork.

When these four conditions are met, the transformers are ready to bank.

Most distribution systems used today are Wye systems with three phases and a neutral. Let's look at banking three transformers into a Wye primary.

A Wye system has the following characteristics:

1. Three phase, four wire

2. Phases may be said to have positive polarity

3. Neutrals are negative polarity and are grounded

4 There are two voltage levels, phase to ground and phase to phase, with the phase to phase value being 1.73 times the phase to neutral (ground) value.

As you see, by following the characteristics of the particular system we deal with, and observing polarities, it is simple to connect three-phase transformer banks.

SUMMARY

Figure 1 shows three transformers in position to be connected into a three-phase bank. C.S.A. states Hi bushing is always on the left when looking from the secondary side. This bushing is indicated on each transformer nameplate. The number 2400 on the transformer is the transformer' s rated primary voltage. For the transformer to operate properly, it must have the primary bushings (and primary winding) connected across lines giving that particular voltage.

Figure 1

Notice the size of the transformers in KVA has not been mentioned. The size of the transformers banked is usually the same, but not necessarily. The main criteria for banking is the transformers must have the same primary voltage and secondary voltage ratings. The importance of impedance matching was covered in a previous module.

Figure 2 shows the three transformers connected into the Wye system. Notice each transformer is connected to a different primary line. Remember, we are connecting to make a three-phase bank.

Figure 2

In the diagram, each transformer has been connected between a phase and the neutral. This is a phase to ground (neutral) connection. It supplies the transformer with its rated voltage, 2400 volts, since 2400 volts is the phase to ground voltage of this particular Wye system.

The lead from a primary phase to the transformer high voltage bushing does not always have to connect to Hi bushing. As a result, polarity markings are very helpful to ensure we connect the bank correctly. Figure 3 shows the phase lead for two of the transformers connected to the H2 bushing. One is connected to the third transformer's Hi bushing. All three transformers will operate. In this case, the correct voltage, 2400 volts, is still being applied to the primary windings. Notice the position of the polarity markings.

Figure 3

With the polarity markings being different on the primary bushing, it is especially important to pay attention to hooking up the secondaries. The polarity of Hi determines the polarity of X1. Remember, there are two possible locations for X1 secondary bushing. It can be diagonally opposite H1 on an additional transformer, or directly opposite H1 on a subtractive transformer. Checking the transformer nameplate tells you the location of X1 bushing and the polarity of the transformer.

Keep in mind: Hi and X1 bushing on a transformer always have the same polarity. From these examples you see connecting a Wye primary is relatively simple. But, you must keep the characteristics of the Wye system in mind.

1. Connect the transformer primary winding across the leads that supply the correct voltage the transformer is rated for.

2. Each transformer connects to a different phase.

3. Phases have positive polarity, neutrals are negative and grounded.

In Figure 4, secondary bus wires, transformer secondary coils, and secondary bushings are shown. Also shown is the location of the X1 bushing. From the location of X1 bushing, these must be additive transformers.

The secondary coil and its voltage rating, 347 volts, are shown. We will always show the secondary coil or coils, as some transformers have more than one inside to be dealt with.

On the X2 secondary bushing is shown a ground strap. This connects the X2 bushing to the transformer case. It also keeps the bushing grounded via the transformer case ground.

Figure 4A

There are four secondary wires shown. Three are marked as phases and one as the neutral. This indicates the secondary will be connected Wye. Also, two voltages are shown, 347/600 volts. If 347 is multiplied by 1.73, the answer is 600. Therefore, this is a Wye system where 347 volts is the secondary phase to ground voltage and 600 volts is the secondary phase-to-phase voltage. Each transformer secondary output is rated 347 volts. Each transformer will supply one secondary phase. When the three transformers have been connected, the phase-to-phase voltage is calculated as follows:

Transformer output (347) x 1.73 = phase to phase voltage

Each transformer output (secondary voltage) must equal the phase to ground voltage required on the secondary bus.

Figure 4B

In the Wye system, phases are considered positive; neutrals are negative and grounded. In Figure 4, H11 connected to a primary phase is positive; therefore, X1 is also positive. The positive secondary phase leads come from the positive X1 secondary bushing. The X2 secondary bushing is negative (opposite polarity from X1) and connects to the secondary neutral. Neutrals are grounded. This is indicated by the ground symbol on the neutral line and by the individual grounded tank straps.

The secondary lead from each transformer X1 bushing connects to a secondary phase. It will read 347 volts (the transformer output) to neutral. Since each transformer connects to a different phase, we have a three-phase system. Voltage phase to phase in this system is the phase to ground voltage, 347 volts, multiplied by 1.73 to give 600 volts as indicated by the diagram.

Another common Wye secondary voltage is 120/208 volts. This service is used where 208 volts phase to phase is required to supply three-phase motors and 120 volts is required to run small drills, power hand saws, clocks, radios, light bulbs, etc.

Let's examine this transformer bank connection. The transformers used in standard-single phase 120/240 volt household distribution transformers.

Figure 5a

Now we connect the secondary. Each transformer output must match the phase to ground reading required by the service. In this case, we need 120 volts, so the two secondary coils in each transformer must be put in parallel for 120 volt output.

Do you remember how this is accomplished? Two connections are shown in Figures 5a and 5b, involving the two secondary coils in a transformer. One connection is a series connection (voltages add) and the output will be 120/240 volts. The other connection is a parallel connection (voltage stays the same) and the output is 120 volts. Polarity markings have been shown to help in making these connections. For a parallel connection, you connect positive-to-positive and negative-to-negative.

Figure 5a Figure 5b

In our 120/208 Wye secondary, each transformer output must match the services phase to ground voltage, 120 volts. Each transformer will have its secondary coils connected in parallel inside the transformer for an output of 120 volts. When the transformers are banked three phases, the phase-to-phase voltage will be: 120 x 1.73 = 208

Figure 6

Figure 6 shows the internal transformer coils put in parallel so each transformer output will be 120 volts. The positive lead comes out the X1 bushing. The negative lead comes out the grounded X2 bushing to become the neutral. For 120 volt output, you use the grounded center bushing, X2, and the X1 bushing.

As a lineman, you rely on what you see to help determine many situations. For instance, if you see a standard distribution transformer with three secondary leads, one from the center bushing and one from each of the other bushings, you may assume that transformer is wired for 120/240 volt output. If you see one bushing taped up without a lead from it and leads coming from the other two bushings, you may assume the transformer is wired for 120 volts. If you see two leads, one from each outside bushing, and the center bushing taped up, AND the tank strap removed, you may assume the transformer is wired for 240 volt output (Figure 7).

Figure 7

Note: You should NEVER go on assumption. Always check the nameplate and take a voltage reading to be sure your prediction is correct. Assumptions can be deadly!

As we are dealing with Wye systems, let's connect another system you may encounter. It is a 240/416 Wye secondary, often used to supply apartment buildings. Notice the phase to ground (neutral) voltage of this bank. What does each transformer output have to be?

If you said 240 volts, you are correct. Where, then, does the 416 volts come from? Since 416 is the phase to phase voltage in this Wye system, 416 must equal 240 x 1.73. It does! This is a characteristic of all Wye systems.

Figure 8 shows this transformer bank, a Wye/Wye bank, because is fed from a Wye system and the secondary is a Wye system.

Figure 8

Note: The X2 bushing is "alive" at 120 V to ground.

Notice how all the polarity markings are used to advantage. They help us to place the internal transformer secondary coils in series for 240 volt secondary output. They also identify which bushing of each transformer is positive and a phase lead, and which is negative and neutral lead. If the ground or tank strap was long enough, it could be removed from X2 and repositioned on X3. However, you will find in the field these straps are only long enough to connect to X2 bushings.

Figure 9 illustrates how one secondary coil would short circuit if the ground strap was left on X2 and X3 was connected to neutral.

Figure 9

In the previous examples of the Wye primary and Wye secondary banks (often referred to as Wye/Wye banks), all transformers used were additive polarity. In Figure 10, two subtractive transformers have been banked with one additive transformer. By using polarity markings throughout, you see the bank still exhibits all the characteristics of a Wye system.

Figure 10

As you know, each primary lead will always have a fused cutout and lighting arrester for line and transformer protection respectively, even though we have not drawn them in the diagrams.

WHAT TO DO NOW

If you have any questions about the Wye primary, Wye secondary connections, ask your supervisor before progressing.

In previous examples of the Wye primary and Wye secondary (often referred to as Wye/Wye banks) banks, all transformers used were additive polarity. In Figure 11, two substractive transformers have been banked with one additive transformer. By using polarity markings throughout, you see the bank still exhibits all the characteristics of a Wye system.

Figure 11

DELTA CONNECTED TRANSFORMER BANKS

Delta-connected primary systems are quickly fading from distribution systems. As you remember from the previous module, there is no neutral or ground connection. A transformer connected in this system must be connected between two phases to operate. As it is connected this way, both primary leads must have their own cutout switch and lightning arrester. This makes the Delta primary connected transformer more costly to install. Also, since there is no ground connection in the Delta system, an energized phase in contact with ground does not always cause circuit protection devices to open. This is illustrated in Figure 1.

Figure 1

A Delta schematic is shown on the left and a Wye schematic on the right. A ground is shown on a phase in each system. Since electricity always seeks the quickest return path to the source of supply, only the fuse on the grounded Wye phase will operate. The Delta system has no electrical connection with ground. Consequently, the grounded phase has no entry into the system for current to flow through. A Wye system reacts quickly to ground faults, since it is a grounded system.

Before we look at connecting the Delta primary and Delta secondary, let's review the characteristics of Delta systems. As with the Wye connections, we use the Delta characteristics to ensure we make our connections correctly:

The Delta system has:

- Three phases, three wires

- One voltage, phase to phase

- Phases have positive negative polarity due to "bridging" of coil ends

- No grounds in this system

Figure 2

30 3-Wire Delta System

Figure 3 shows three transformers ready, to be banked into this Delta system. Notice the transformer primary coil rating shown. This indicates each transformer wants 4160 volts primary to make it work properly. Where do we have 43.60 volts in this system? Between any two phases.

Figure 3

It is easy to connect the transformer primaries. Just make sure each transformer is connected between two different phases, i.e., no transformer connected between the same two phases as another.

Figure 4 shows this connection. Notice the polarity marks used. The transformer on the left gets its 4160 voltage between the positive portion of the top phase and the negative part of the middle phase. The center transformer, energized 1200 after the one on the left, gets its voltage between the positive of the middle phase and the negative of the bottom phase. The last transformer, on the right, gets its voltage between the positive part of the bottom phase and the negative part of the top phase.

Figure 4

Both top and bottom diagrams in Figure 4 are the same electrically. In the bottom diagram, the 112 of the left transformer has been connected to the H1 of the middle transformer, and a single lead is taken up to the center phase. This has also been done between the middle and right-hand transformers. You often do this in the field to save a bit of wire and eliminate two cutouts and arresters. Notice in both diagrams how no two transformers are connected between the same two phases, and we have a three—phase connection.

Figure 5 shows these transformers connected to the secondary lines to provide a 600-volt Delta secondary. Each transformer output (secondary coil rating) is 600 volts, since, in the Delta connection, each end of a coil supplies two different phases.

Polarity markings have been affixed to the secondary bushings. The positive of one transformer has been connected to the negative of the next transformer. Each phase lead comes from the bridged connection and will have positive negative polarity. Remember back in the Delta schematic -the positive of one coil was connected, or bridged, to the negative of the next. From this connection came the positive negative phase lead (Figure 6)

The secondary wires exhibit all the characteristics of the Delta system:

- three phases, three wires

- phases have positive and negative polarity

- no ground connection

- one voltage, phase to phase

Figure 5

Figure 6

We mentioned earlier the visual identification you use to help you. Specifically, we looked at the connection of the secondary leads coming from distribution transformers, i.e., 120/240 volt, 120 volt, and 240 volt with three (or four) secondary bushings, and whether or not the ground ~trap on the center bushing was connected.

To expand on this idea, consider: of the most widely-used distribution transformers, those with only two secondary bushings (indicating only one secondary coil inside) are either wired for 347 volt or 600 volt output. The transformer wired for 347 volt output, since it is used to supply Wye secondaries, usually has a tank or ground strap connected to one secondary bushing.

A transformer wired (or wound) for 600 volt output will not have a ground strap connected to either secondary bushing. This transformer will always have two primary bushings.

Note: Most new transformers designed to work on Wye primary systems are built with only one high voltage bushing. This helps reduce the cost of the transformer. A new transformer wired for 600 volt output always has two primary bushings, even though it will connect to a Wye primary. This is because it often uses a connection called a "Floating Neutral". We look at this connection in a later module.

Another visual you will use is the way the secondary transformer leads connect to the secondary bus or service wires. If one live lead from each transformer feeds only one leg of the bus or service, this is a Wye-connected secondary with the other transformer lead connected to the service neutral. If there "bridging" between transformer secondary bushings from which a lead connects to the bus or service leads, this is a Delta-connected secondary. Always check your predictions by looking at the transformer nameplate or by using a voltmeter.

We have stated in a Delta there are no ground connections. Most household voltage (120/240) distribution transformers are equipped with a ground strap connected. These same transformers are used to supply 120 volt Delta 240 V loads. The following diagrams show how this is done eliminating the ground strap connection.

Figure 7

In Figure 7, the secondary voltage wanted was 120 volt Delta. The transformer internal coils were put in parallel (positive to positive, negative to negative) so each transformer output would be 120 volts. The positive lead from one transformer was "bridged", or connected, to the negative lead of the next transformer. A lead was brought, from this connection, down to the secondary wires. The ground straps were removed from the X2 bushing of each transformer.

Figure 8 shows the connection for a 240 volt Delta secondary.

Figure 8

In Figure 7, the internal secondary transformer coils have been connected in series to provide 240 volt output. The positive lead from one transformer has been bridged to the negative lead of the next transformer. A lead has been brought down, from this bridge, to the secondary wire. The ground straps were removed from the X2 bushings.

Each transformer output is 240 volts. The X1 lead feeds to one secondary wire and the X3 lead feeds another, A a result, a voltage of 240 volts appears across the secondary wires.

Let's look at the reason for removing the ground straps. To illustrate, Figure 9 has been drawn in Delta configuration, showing the transformer secondary coils.

Figure 9

You can see with ground straps left on the X2 bushings, now ground connections, short out between all three-transformer windings. One ground would cause no problem, as there would be no return path through ground back to the system. However, with more than one ground, as illustrated, short circuit current has access back into the system. Remember, no arounds in the Delta system.

All the transformers used as examples in this section have been additive polarity. Figure 10 shows a 120 volt Delta secondary connected using one subtractive and two additive transformers. By using the polarity markings, the proper connection can easily be made. Follow the connections and see how they are made.

Figure 10

DELTA/WYE TRANSFORMER BANKS

Let's look at another transformer bank connection. This has a Delta-connected primary and a Wye-connected secondary. This type of bank is commonly referred to as a Delta/Wye bank.

You already know how to connect the primary of the transformer into a Delta system and how to connect secondaries into a Wye system. The Delta/Wye bank is a combination of the two.

Figure 11 shows such a connection. The primary is 8320 volts Delta and the secondary is 120/208 Wye. Polarity markings are shown to facilitate the paralleling of internal transformer coils and for connecting the phase leads.

Figure 11

Since the primary voltage is 8320 V phase to phase, transformers have been selected with a primary coil rating matching this voltage. The primary is connected between phases. No two transformers have been connected between the same two phases. The primary is connected Delta. The transformers will supply their rated secondary voltage.

Follow the individual polarity markings. The internal transformer secondary coils have been out in parallel so the transformer output will be 120 volts. This matches the required phase to ground voltage of the secondary service. The secondary leads have been connected to the four wire service. This ensures the service takes on the characteristics of the Wye system: three phase, four wire; phases have positive polarity, neutrals are negative and grounded; there are two voltages, phase to ground (neutral) of 120 volts, phase to phase of 120 x 1.73 = 208 volts. There is no real concern the grounded secondary neutral will have any adverse effect on the Delta primary, as there is no electrical connection between the primary circuit and the secondary circuit. Remember, the secondary is energized by Induction between the primary winding and the secondary winding, not by an electrical connection.

Figure 12, shows another Delta/Wye bank connected to supply 347/600 volts secondary. There has been one subtractive transformer used, as a reminder the only difference between it and the two additive ones is the location of the X1 bushing. This information is on the nameplate. In the diagram we use positive/negative polarity markings to connect correctly.

Figure 12

As you can see, by following the characteristics of the particular system we deal with, and observing polarities, it is simple to connect three-phase transformer banks.

WYE/DELTA TRANSFORMER BANKS

You have already covered many of the standard three-phase banks used throughout the province. You know that during connection and operation banks exhibit definite Wye or Delta characteristics, depending on the secondary voltage required.

A common secondary voltage in large factories is 600 volt three-phase Delta. Its voltage level is five times higher than 120 volts. Consequently, it can supply power at a far lower current level than a single-phase 120 volt system. Since 600 volt Delta does not have a neutral in its connection, there are small transformers (called dry transformers) inside the factory to convert from the 600 volts to supply 120/208 volts for lights and small tools.

You have connected the 600 volt Delta secondary fed from a Delta primary. Now, we look at this secondary connection fed from a Wye primary. This connection could be as a standard Wye primary and Delta secondary. Some utilities do connect this way. However, most utilities and Ontario Hydro connect this bank with what is called a "Floating Neutral".

In the Wye/Delta bank, the H2 high voltage bushings are interconnected and left floating - they are connected to one another but not connected to the system or primary neutral or ground. This prevents possible bank burnout when one sectionalizing device (line fuse, reclosor, etc) between the station and the bank opens.

The Hi bushing of each transformer is connected to an individual phase conductor. The H₂bushings are interconnected but not tied to the system neutral, i.e., not grounded (Figure 1).

Figure 1

The following two Wye/Delta banks are examples of using the "floating neutral". We could also add this statement to our list of characteristics of Wye and Delta systems:

- Wye/Delta banks have a floating neutral

Figure 2

Figure 3

A detailed account of how a "floating neutral" function to provide transformer protection is given in the next module.

If there are any questions concerning this module, ask your Foreman/supervisor. If not, move on to the Skill Check.