Phase shift and phase polarity between two windings of a single-phase transformer depends on how the windings are wound on the core. Transformer phase shift and transformer polarity needs to be considered for many applications some of which are:

• Forming a 3-phase transformer using single phase transformers
• Parallel operation of transformers
• Voltage transformer connections for metering
• Voltage transformers for synchronism check between two sources, protection etc.

In this article basics of transformer polarity is discussed. A method to test potential transformer polarity (PT or VT) is discussed and actual test results provided.

Learn 3 Phase transformer, star delta, delta star connection diagram & working concept in this video tutorial. What is the three phase transformer connection. Related Post: How to Find the Rating of Single Phase & Three Phase Transformer in kVA? Apart from the electrical connection between the primary & secondary, there is an energy flow through induction.

Transformer Phase Shift

There are four different ways in which single phase transformers can be connected to form three phase banks. These are:

Wye-Wye and Delta-Delta transformers do not cause any phase shift from primary to secondary. Delta-Wye transformers have a 30-degree phase shift which is discussed below.

Delta Wye or Wye Delta Transformer Phase Shift

We know that across a delta-wye (star) or wye- delta transformer there will a 30-degree phase shift between line voltages. With this there are two options: delta could lead the wye side by 30 degree or wye side could lead the delta by 30 degrees.

What determines transformer phase shift and which side of delta-wye transformer leads or lags?

Answer: The way the delta is ‘closed’ determines which side leads or lags. There are two possible combinations which are discussed below:

1. Delta Closing- Type DAB

This is one method of closing the delta triangle. In this connection, the polarity side of A phase is connected to the non-polarity side of B phase. Three phase transformer connection diagram using this method is shown below.

The figure above shows a delta wye connection with ‘DAB’ connection. In this case delta side will lead the wye side by 30⁰. This is the normal connection for delta wye transformer with delta on the primary. Per north American standards the primary side leads the secondary low voltage side by 30⁰.

2) Delta Closing- Type DAC

This is another method of closing the delta triangle. In this the polarity side of A phase is connected to the non-polarity side of C phase. Three phase transformer connection diagram using this method is shown below.

Delta Closing- Type DAC

The figure above shows a delta wye connection with ‘DAC’ connection. In this case delta side will lag the wye side by 30⁰. Or in other words the wye side will lead the delta side by 30⁰. This is the normal connection for wye delta transformer with wye on the primary.

Note that these phase angles are referring to positive sequence voltages. A method to derive the polarity by looking at winding connections is given in ref [1].

Transformer Polarity

There are two polarity standards for transformers. These are subtractive and additive as shown below. Polarity markings are indicated by ‘X’.

Single phase power transformers (in North America) could be additive or subtractive depending on the kVA and the voltage class. Other regions of the world too could have a mix of additive and subtractive polarity transformer in use. Two rules of transformer polarity are:

1. Current flowing ‘in to’ polarity marking of one winding flows ‘out of’ the polarity mark of the other winding. Both currents will be in phase.
2. Voltage drop from polarity to non-polarity across one winding is essentially in phase with the voltage drop from polarity to non-polarity across the other winding.

Additive Polarity: For power distribution transformers that fall in to the category as noted in the IEEE standard below has additive polarity. These are mostly single-phase distribution transformers.

IEEE Std C57.12.00-2000 Standard for liquid immersed distribution, power and regulating transformers states that “Single phase transformers in sizes of 200kVA and below and having high-voltage rating of 8,660V and below (winding voltage) shall have additive polarity. All other single-phase transformers shall have subtractive polarity”.

Subtractive Polarity: Large power transformers and instrument transformers usually have subtractive polarity.

Polarity marking is indicated by a dot or ‘X’ or it can be indicated by standardized terminal markings. Below is another way of indicating transformer polarity. The secondary polarity is determined by the location of ‘X1’ relative to ‘H1’. If H1 and X1 are on the same side then the transformer has subtractive polarity and vice versa.

Here is an instrument transformer with subtractive polarity. Note that in addition to having a white ‘dot’ indicating the polarity, it also has H1 and X1 markings. The schematic for this VT or PT will be the same as the figure shown above for subtractive polarity.

Potential Transformer [Square D]

How to check polarity of a transformer?

Sometimes it is required to test the polarity of a single-phase transformer or a voltage transformer (VT or PT) for testing or troubleshooting purposes. One way to test the VT with known voltage transformation ratio is to connect an AC source as shown in the figure below.

Testing Transformer/PT Polarity Schematic (top) Simplified Test Circuit (bottom)

Note: Care should be taken in connecting the voltage as dangerous voltage could appear depending on the voltage rating and the terminals at which connections are made. Connection at 120VAC or smaller to be applied at the high voltage terminals and not at low voltage terminals.

In the figure above +, – are for illustration purposes identifying the terminals at same potential at any given time and do not represent DC voltage.

For additive and subtractive polarity winding, terminals H1 and X1 will always be at the same polarity. This knowledge will help create the figure above. In the example test above, the transformer ratio is 120V/12V. If the Voltage Transformer (VT) has additive polarity then 132V will be read on the multimeter. If the VT has subtractive polarity then 108V will be read on the multimeter.

Voltage Transformer Polarity Testing

Below is the test set up for testing a Voltage Transformer or Potential Transformer polarity testing. The test leads are connected as detailed in the section above. The specs of the VT are:

Primary 480V

Secondary 120V

Transformation Ratio= 480/120= 4

H1 and X1 are on the same side of the transformer (similar to the picture of the VT shown above). Hence the VT is of subtractive polarity. After making connections as shown in the schematic above. The voltage across H2 and X2 is measured to be 90V.

This confirms the polarity of VT is subtractive. The voltage applied across H1 H2 is 120V. Based on the transformation ratio 120/4=30V will be induced across X1 X2. Since the windings are connected for subtractive polarity, the net voltage measured across H2 X2 is 120-30=90V. This is exactly what is measured.

Voltage waveforms from primary and secondary are shown below. For subtractive polarity the voltage waveforms across H1 H2 and X1 X2 has same phase angle. In other words the potential of H1 and X1 rise and fall in unison.

For additive polarity the voltage waveforms across H1 H2 and X1 X2 has phase angle difference of 180 degree.

Additional reading:

Ref [1]: Power System Analysis and Design J Duncan Glover, S. Sarma, Thomas Overbye

The three-phase system is used to generate, transmit, and distribute electrical power. It generates power on a large scale to meet the needs of industries and commercial establishments. Three identical single-phase transformers are connected suitably or combined on a single core to form a three-phase system. Based on various types of industrial needs, the step-up and step-down transformers are employed for generating, transmission, and distributing the electric power. The building of a three-phase transformer unit is economical as it consumes less material compared to connecting three individual single-phase transformers. Additionally, the three-phase system transfers AC power instead of DC and is simple to construct.

What is a Three-Phase Transformer?

As known, a single-phase transformer is a device that is capable of transferring electrical energy from one circuit to one or more circuits based on the concept of mutual induction. It comprises two coils – a primary and a secondary coil, which helps to transform the energy. The primary coil is connected to a single-phase supply, while the secondary is connected to a load.

Similarly, a three-phase transformer consists of three primary coils and three secondary coils and is represented as 3-phase or 3ɸ. A three-phase system can be constructed using three individual identical single-phase transformers, and such a 3-phase transformer is known as the bank of three transformers. On the other hand, the three-phase transformer can be built on a single core. The windings of a transformer can be connected in either delta or wye configurations. The working of the 3-phase system is similar to a single-phase transformer, and they are normally employed in power generation plants.

Three-Phase Transformer Construction

The diagram of a three-phase transformer is shown in the figure below.

A three-phase transformer of a single unit is used widely because it is lighter, cheaper and occupies less space than the bank of three single-phase transformers. The three-phase transformer construction is of two types: Core type and Shell type.

Core Type Construction

Three Phase Auto Transformer Connection Diagram

In this type of construction, there are three cores and two yokes. Each core has both primary and secondary windings wounded spirally as shown in the figure. Each leg of the core carries high voltage as well as low voltage windings. The core is laminated to minimize eddy current losses on core and yoke. As it is easier to laminate low voltage (LV) winding than the high voltage (HV) winding. The LV windings are positioned near the core with appropriate insulation and oil ducts in between them whereas, the HV windings are placed above the LV windings with appropriate insulation and oil ducts between them.

Shell Type Transformer

The three-phase shell type transformer is generally constructed by stacking three individual single-phase transformers. Three phases of a shell-type transformer are independent than the core-type transformer, while each phase has an individual magnetic circuit. These magnetic circuits are parallel to each other and flux induced by each winding is in phase. Shell type transformer is highly preferred as the voltage waveforms are less distorted.

Working of Three-Phase Transformers

The figure below shows the three-phase transformer, wherein three cores are placed at 120˚ from each other. This figure is simplified to show only primary windings and their connection to the three-phase power supply. As soon as the three-phase supply is excited, the currents IR, IY, and IB are carried by the primary windings and thus inducing the fluxes ɸR, ɸY, and ɸB individually in each core. The center leg will carry the sum of all the fluxes, and the center leg combined all the legs of a core.

For instance, if the sum of the currents IR+IY+IB is zero in a three-phase system, then the sum of all the three fluxes also becomes zero, resulting in the center leg carrying no flux. Therefore, removing the center leg makes no difference for other transformer conditions.

Three-Phase Transformer Connections

Various three-phase transformer connection is described below.

Wye and Delta configurations are applied for three-phase transformers because Wye connections provide the options to have multiple voltages, whereas delta configurations offer high reliability. The phase diagram of Wye and Delta is given below. For Wye connection, either all the minus or all the plus points of windings shall be tied together. However, in delta connection, polarities of winding are connected in a converse way. The phase difference between any two phases is 120˚.

Wye-wye Connection

The diagram of Y-Y connected transformers is shown below. It can serve both single-phase and three-phase loads. In this connection, all the windings ending with dots are connected to phases A, B, and C, while non-dots endings are connected to become the centers of “Y” configuration.

Wye-Delta Connection

The Y-Delta connection shown in the figure below shows that the secondary windings (which are at the bottom in the figure) are connected to form a chain. The windings with dot connection on one side are connected with the non-dot connection of the other side to form the “Delta” loop.

Delta-Wye Connection

The connection of Delta-Y is shown in the figure below. This type of configuration allows wye-connected secondary to connect multiple voltages such as line-to-line or neutral. As the delta-wye configuration presents a 30˚ phase shift between primary and secondary, it cannot be used to connect in parallel with delta-delta and Y-Y configurations.

Delta-Delta Connection

The diagram of the delta-delta connection is shown below. These connections can be made either with three identical single-phase transformers or one three-phase transformer. The delta-delta configuration is preferred due to its inherent reliability.

Advantages/Disadvantages of a Three-Phase Transformer

The advantages and disadvantages of a three-phase transformer are discussed below.

Advantages of a three-phase transformer

Three Phase Autotransformer Connections Wiring

• Needs less space to install and it is easier to install
• Less weight and reduced size
• Higher efficiency
• Low cost
• Transportation cost is low

Disadvantages of a three-phase transformer

• The entire unit shuts down in case of fault or loss occurs in any one unit of a transformer as a common core is shared by all three units.
• Repair costs are higher
• Cost of spare units are high

FAQs

1). Mention the applications of 3-phase transformer

Three-phase transformers are used in electrical grids, power transformer, and as distribution transformers

2). What are the types of 3-phase transformer?

The four types of 3-phase transformers include: Delta-Delta (Dd), Star-Star (Yy), Star-Delta (Yd), and Delta-Star (Dy)

3). What happens if a 3-phase motor loses a phase?

If a 3-phase motor loses a phase during operation, the motor continues to operate at less speed and experiences vibrations. The current also increases abruptly in other phases leading to internal heating of the components of a motor.

4). Under which condition delta/wye works satisfactorily?

The wye-delta connection works satisfactorily with large unbalanced and balanced loads. It can handle third harmonic components because of the circulating currents in the delta.

5). For the Wye-Wye connection, what is the phase shift?

The phase shift is 0 degrees.

Three Phase Autotransformer Schematic

Although a single-phase transformer is preferred by most industries, it is not suitable for large power distribution. Therefore, 3-phase systems are used by large industries to generate power on a large scale.

In this article, we discussed various benefits and a few disadvantages offered by a 3-phase transformer. We also focused on a three-phase transformer and its construction and various configurations. Here is a question for you, what is the function of the three-phase transformer?