Power transformers are widely used in a power system to transform voltage. The most common type, a two winding transformer, has one winding connected to a high voltage – low current circuit while the other winding is connected to a low voltage – high current circuit.
Transformers rely on Faraday’s induction principle and ampere-turns to induce voltage from primary winding to the secondary.
The transformer’s core is made of laminated sheets of metal. It is constructed either as a shell type or a core type. See figure 1. With the ubiquitous application of three phase power, these cores are then wound and connected using conductors to form three 1-phase or one 3-phase transformer. Three 1-phase transformers have each bank isolated from the other and thereby offer continuity of service when one bank fails. A single 3-phase transformer, whether core or shell type, will not operate even with one bank out of service. This 3-phase transformer however is cheaper to manufacture, has a smaller footprint, and operates relatively with higher efficiency.
The transformer’s core with its windings is immersed in an fire retardant insulating oil inside a tank. The conservator on top of the tank allows for the expanding oil to spill into it. The load tap changer on the side of the tank changes the number of turns on the high voltage-low current winding for better voltage regulation. The bushings on top of the tank allow for conductors to safely enter and exit the tank without energizing the outer shell.
The transformers power capability is limited by thermal rating. This means the transformer can be operated beyond its MVA rating as long as the temperature of its top oil stays within the 65ºC temperature rise above ambient temperature (See IEEE C57.91-1995 standard). For instance, if the ambient temperature is 45ºC then the transformer can be pushed to a value less than 45ºC + 65ºC = 110ºC. Since temperature is usually the limitation, the radiators on the transformer are fitted with fans so they can force cool the oil flowing through the radiator fins. Prolonged overloading of the transformer is not recommended on account of saturating it’s core (higher losses), loss of life expectancy, and deterioration of winding insulation.
Transformer Winding Connection
In a three phase core type transformer, for each phase, the primary winding and the secondary winding are wound on the same leg. The windings are ofcourse insulated from each other. The low voltage secondary winding is typically on the inside while the high voltage winding is on the outside. This is done to minimize the leakage flux during the whole induction process.
To enable the conduction of currents, the windings are wound and connected either as a delta or a star. These shapes form as a result of the way the three conductors inside the transformer get connected. See figure 2. The use of these connections delta-star, star-delta, star-star, or delta-delta make a huge impact on the design of power system. So the choice of connection is critical.
How Transformer Grounding Affects Power System Design
Without going into a lot of detail, for cost savings and safety, the star connection is the preferred connection for high voltage transmission. In this scenario, the common point in the star, also called the neutral, is grounded or earthed. Doing this causes the phase to neutral voltage or phase to earth voltage to be reduced by a factor of 1/sqrt(3). You will not get this reduction with a delta or any ungrounded connection.
It only makes sense to use a delta-star transformer near the generating station where the delta is connected to the generator terminals and the star is connected to the high voltage transmission lines. With grounded star connection on the high voltage side, the transformer winding can be insulated for lower voltages. The transmission system too will have a lower insulation requirement. These provide for huge cost savings in the design and construction of a high voltage power system.
There is, however, a disadvantage in grounding the transformer. When one of the lines or all three lines on the star side gets short-circuited to the ground, the grounded neutral in the transformer serves as a return path for the current. These currents are pretty high in magnitude and if not cleared in fractions of a second, it can severely damage the transformer and all the equipment connected to it. These ground fault currents are also rich in third harmonic currents. Third harmonics disrupt telecommunication network which, by the way, is used to implement pilot relaying in a power system.
But all is not lost with grounding the transformer. The delta connection on the primary winding helps here. It offers high impedance to third harmonics and traps the ground fault current in the delta thereby isolating the ground faults to the secondary system.
Now, you would think the delta-star configuration of the transformer is pretty awesome and that it is installed everywhere in the system. However, it is not. To retain the advantage of a star connected system, few bulk power stations have a star-star connected three winding transformer, the third winding being a delta tertiary. With this three winding transformer, the primary star connection keeps the primary system solidly grounded while the grounded secondary star connection extends the cost savings into the secondary system.
Delta Tertiary and Its Application
A star-star connected transformer is rarely applied in the power system. It is susceptible to third harmonics and voltage transients when left ungrounded. However to incorporate the design advantage of a star winding and those of delta winding, a third winding – a delta tertiary is built into the two winding star-star transformer. The delta in a star-star-delta transformer not only traps ground fault currents and offers high impedance to third harmonics, it also allows for connecting a:
- Capacitor bank – for voltage or power factor correction
- Reactors – for limiting ground fault currents (resonant grounding)
- Resistors – for limiting ground fault currents
- Station service transformer – AC power for equipment inside the substation
On a final note, transformers should be specified with the following information among others for proper selection or analysis:
- Size of transformer in MVA (nominal and full load)
- Primary and secondary voltage. If supplied by load tap changer, then available voltage taps
- Primary and secondary winding connection
- Per unit impedance (%Z)
- Delta-star transformers : Useful at generation and load centers.
- Star-star-delta transformers : Useful at transmission substations (765kV, 500kV, 345kV).
- Grounding the neutral provides higher ground fault currents however the cost savings realized by lower insulation requirements makes grounding viable.