Power transformers increase or decrease voltage and current magnitude in a power system. This transformation occurs because of Faraday’s induction principle and the variation in ampere-turns (or winding turns). Do note, the power transferred remains the same (minus few core and copper losses).
A power transformer contains 6 key components.
- Load tap changer
As a power engineer, understanding the component design means you can specify transformers correctly.
Load tap changer design
Tank design is where you get creative, to support location and project requirements. You can specify bushings on any side, install cooling systems, reduce sound using a unique tank panel, choose isolated phase bus ducts – segregated or non-segregated bus ducts, etc.
Another critical design decision is to pick a three 1-phase or one 3-phase transformer. Generator step-up transformers at large power stations, transformers at EHV substations go the three 1-phase route.
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.
Cooling system design
Transformer Winding Connection
Once the coils are in place, the three primary windings and three secondary windings can be tied either as a delta or a wye (or star). One such setup is shown below.
Although it may seem you are short-circuiting by tying one end of the coil to neutral-ground (in a star) and by tying one coil to another (in a delta), this is not the case. These connections work because of Lenz’s law.
The use of any one combination: delta-star, star-delta, star-star, or delta-delta makes a huge impact on the design of the power system. So the choice of connection is critical.
Wye-ground Wye-ground transformer advantages
- Provides insulation savings, leading to cost savings on the transformer.
- Simplified phasing i.e. no phase shift occurs – simplifies transformer paralleling.
Wye-ground Wye-ground transformer disadvantages
- Harmonics (unwanted frequencies) propagate through the transformer, potentially causing radio interference.
- The zero sequence current flows through the transformer.
- External line-to-ground faults will trip the transformer (if neutral connection allows fault current back-in, then in a differential protection zone, the current entering is not the same as current leaving).
- There’s a possibility to load the phases differently leading to an unbalanced high voltage system.
Delta Wye-ground transformer advantages
- Because the delta winding traps zero sequence current, the upstream relay on the delta-wye transformer can be assumed to pick up for only high-side ground faults. This allows for very sensitive pick-up settings. In contrast, the wye-wye combination allows zero-sequence current through – making it difficult to assess the location of the fault. In short, relay protection is improved.
Delta Wye-ground transformer disadvantages
- Because of the phase shift associated with these transformers, closer attention needs to be paid to the design. Potential error traps occur during paralleling and CT wiring.
- High insulation cost leading to an expensive transformer.
Additional details on the pros and cons of various winding configurations can be found in General Electric’s paper titled The Whys of the Wyes.
To capture the pros of each combination, a power transformer can be fabricated with three sets of winding (instead of just two), typically primary-wye, secondary-wye, and tertiary-delta.
Delta Tertiary and Its Application
In a three winding wye-wye-delta transformer, the delta tertiary winding allows for connecting a:
- Capacitor bank – for voltage or power factor correction
- Reactors – to prevent voltage from bulging (Ferranti effect) on EHV lines during lightly loaded conditions.
- Station service transformer – AC power for equipment inside the substation
- From the protection and control standpoint, it traps zero-sequence (ground fault) current. If you insert a CT in this tertiary winding, you can measure this current. Because this winding also traps 3rd harmonics, it is called a stabilizing winding.
- Delta tertiaries induce a current in one direction only, regardless of where the fault occurs – high side or low side. Thus, a directional relay can be polarized using the delta tertiary CT’s.
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 – 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 (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 (phase-to-ground) voltages. The transmission system too will have a lower insulation requirement. These provide tremendous cost savings in the design and construction of the transmission system.
There is, however, a disadvantage in grounding the transformer neutral. When one line or all three lines on the star side short-circuits to the ground, the grounded neutral of the transformer serves as a return path for the fault current. These fault currents, when not cleared in fractions of a second, can severely damage the transformer and all the equipment connected to it. The ground fault currents are also rich in third harmonic currents. The third harmonics on the transmission line disrupts all communication channels (for instance, power line carrier – pilot relaying) in the vicinity.
But all is not lost with the star-delta/delta-star combination (because of neutral grounding). The delta connection offers high impedance to third harmonics and traps the ground fault current, thereby preventing it from propagating one side to another.
- Delta-star transformers: Applied at generating stations and load centers.
- Star-star-delta transformers: Applied at transmission substations (765kV, 500kV, 345kV).
- Grounding the neutral provides higher ground fault currents however the cost savings realized by lower insulation requirements makes neutral grounding acceptable.