Technical Article

T and Zigzag Transformer Connections

December 21, 2021 by Alex Roderick

Learn about both T- and Zigzag- transformer wiring connections in relation to synchronous converters.

T-Connections

A T-connection is a low-cost technique of wiring two transformers in order to provide three-phase power. This method is an alternative to the open delta connection for providing low-cost power to a circuit with only two transformers. A T-connection maintains a balanced phase relation better than an open delta.

One of the transformers is called the main transformer. The main transformer is one of the transformers in a T-connected transformer bank and is provided with a 50% voltage tap. The other transformer is called the teaser transformer. A teaser transformer is one of the transformers in a T-connected transformer bank and is connected to the 50% tap of the main transformer.

The capacity of the T-connected transformers is similar to two transformers in the open delta. The advantage is that the T-connection with the teaser transformer operates at reduced flux and therefore operates with reduced iron loss and at a higher efficiency.

 

Wiring Connections

There are several common arrangements of T-connections that give useful output voltages. For example, if a source is 3-phase 2400 V, two transformers can be connected to give 3-phase 240 V at the secondaries. The teaser is connected to the 50% tap of the main transformer. The vector diagram shows 207.8 V across the teaser secondary. See Figure 1.

 

Figure 1. In a T-connection, one of the units is the main transformer and is provided with a 50% tap to which a teaser transformer is connected.

 

277 V and 208Y/120 V

A teaser transformer that is supplied with an 86.6% tap can also be used to develop 277 V from 480 V lines or to develop a 208Y/120 V source (see Figure 2). The transformer has 416 V from H2 to the center tap on the main transformer. The voltage between the H1, H2, and H3 taps is 480 V.

 

Figure 2. A teaser transformer with an 86.6% tap can be used to develop 277 V from a 480 V source or to develop a 208Y/120 V source.

 

The voltage from H2 to the 86.6% tap is 277 V with 138.5 V to the center tap on the main transformer. The two secondaries are connected for 3-phase 208Y/120 V. The voltage between the X1, X2, and X3 taps is 208 V. The 86.6% tap is used as the zero point, and the voltage from X0 to the secondary taps is 120 V. Special T-connected transformers may be available for this purpose, and both transformers are contained in the same case.

 

Zigzag Connections

There are many applications where a zigzag connection is used. A zigzag connection is a method of wiring transformers where the windings are divided over several legs of the transformer core. Zigzag connections are commonly used to create a neutral or to cancel unwanted fluxes, such as from harmonics or synchronous converters.

 

Wiring Connections

There are several variations on the zigzag wiring of transformers. In some situations, the primary windings are in a standard configuration, while the secondary windings are wired in series from one leg to another. This is a common configuration for transformers used to mitigate harmonics. Another common configuration is to use a standard 480 V/240 V transformer wired as a zigzag to create a neutral to derive 277 V for lighting systems from a 480 V source. Synchronous motors can also use a zigzag configuration to reduce or eliminate flux distortion.

277V for Lighting

Industrial lighting systems often use 277 V. This is typically derived from the line-to-neutral voltage on a 480 V system. However, there is no fourth wire for a neutral in plants that have a 3-wire, 480 V ungrounded, or corner-grounded delta.

When there is no neutral, there are two common ways to create a 277 V source for the lighting. The simplest is to purchase a transformer to step down the 480 V source to 277 V. This is not a common size, and such a transformer would be a special order and would be expensive. Another method is to use a standard transformer with a 480 V primary and a 240 V secondary. The transformer can be wired in a zigzag to create a neutral that can be used to derive 277 V for the lighting (see Figure 3).

 

Figure 3. A zigzag transformer can be used to develop a neutral from a 3-wire, 480 V source.

 

Synchronous Converters

A synchronous converter is a rotating machine that produces a DC current from an AC source or AC current from a DC source. A characteristic of synchronous converters is that the transformer feeding the converter can have a problem with flux distortion in the windings due to an imbalance of DC current flowing in the neutral. This neutral is the return path for the DC circuit.

In the case of a 3-phase synchronous converter, the input power is from a bank of transformers that are connected delta on the primary side and interconnected wye on the secondary side.

Note:

The single-phase impedance of a transformer with a zigzag secondary is less than the impedance indicated on the nameplate. Fault currents must be calculated using the lower impedance of the zigzag. Failure to calculate properly can result in serious injury and equipment damage from arc flash or explosion.

 

Interconnected Wye

In a 3-phase system, the flux distortion caused by a synchronous converter can be eliminated with an inter-connected wye connection in the secondary. An interconnected wye connection is a technique of wiring transformers where two separate windings are interconnected on each phase in a zigzag manner. See Figure 4. The unbalanced current from the synchronous converter flows into the neutral of the interconnected wye windings and divides equally through each of the three legs. The unbalanced current flows in opposite directions in each secondary.

 

Figure 4. An interconnected wye secondary can be used to eliminate any flux distortion caused by a synchronous converter.

 

For example, the direction of the current flow in the half-section a-b is opposite to the flow of the current in the adjacent half-section c-d. The magnetizing action of the unbalanced current is neutralized because the two half-sections, a-b, and c-d form the two half-sections of the secondary winding of the first transformer.

The low-voltage side operates at only 86.6% of its normal capacity (if operated in a straight wye connection) because the two half-sections of each transformer secondary winding are connected in different phases. Therefore, transformers wired for interconnected wye operation are larger than those wired for straight wye connection.

 

Double Delta

Some synchronous converters also operate on 6-phase power. A 6-phase synchronous converter uses a transformer with a dual secondary wired in a double-delta connection to feed the synchronous converter. A double-delta connection is a method of wiring transformers where both sets of secondary windings are connected in a delta connection, but one is reversed with respect to the other. As a result, two delta connections displaced 180° from each other are formed (see Figure 5). The high-voltage windings are usually wired in a delta connection.

 

Figure 5. A double-delta connection can be used to create a 6-phase source for a synchronous converter

 

The synchronous converter is tapped at 60° intervals to receive the 6 phases from the source. A synchronous converter with 6-phase input is more efficient than the 3-phase equivalent.