1- Power transformers: transformers used in transmission of electrical power for long distances in high voltage ranges.
2-Distribution transformers: transformers used in distribution of electrical energy to users for short distances in medium and low voltage ranges ( note that traditionally , we use the term distribution transformers for only transformers that transform power to the domestic user at 400 V or Less).
In contrary, the IEC didn’t distinguish between power and distribution transformers; it deals with them as transformers only used to transmit power from one voltage to another.
1- Power Transformers: see fig.1
These transformers usually have primary voltages up to and including 800 KV, these types of transformers have many difficulties in their design due to some challenges which they need to overcome.
fig (1): Power Transformer 400 MVA
Power transformers design challenges:
1- Transient Overvoltage :
Some overvoltage conditions may be applied to the terminals of transformer windings ( like lightning strikes) resulting in dielectric stresses on the transformer windings insulation which must have ability to withstand these overvoltage and this ability depend on peak value of the overvoltage and its duration.
2- The magnetic leakage field :
The magnetic field increases with the power rating and the short circuit impedance of the transformer resulting in harmful heating in windings and other transformer parts, and excessive heating may cause gas bubbles in the oil. If such bubbles rise through the dielectric high stress area in the transformer, breakdown may occur.
3- Mechanical forces :
The power transformers should able to withstand mechanical forces that arise during short circuits through the windings and this need suitable selection of winding conductor cross section especially for lager power transformers.
4- Power rating limitations :
The transformer power rating was restricted by the transport problems due to the height of large power transformers , the transforer designers usually use the 5-limb cores instead of 3-limb core to reduce the transformer hieght to overcome this problem.
Types of power transformers:
there are different types of power transformers used for different applications as follows:
- Generator step-up transformers.
- Step- down transformers.
- System intertie transformers.
- Industrial transformers.
- Traction transformers.
fig (2): Generator step-up transformer 10 MVA
These transformers are used to step up the generator voltage level up to 800 KV.
such transformers are usually Ynd-connected,using delta connection for the low voltage side for the following reasons:
- The delta connected winding keep the zero sequence impedance of the transformers reasonably low.
- The line current in delta connection equal to the line current divided by √3 which makes the winding smaller by using smaller bundle of wiring conductors
Tappings placed at the neutral end of the high voltage winding to ensure the exchange of reactive power between the generator and the power system network.
Transformer high voltage connection to the network:
Circuit breaker on the high voltage side is used and will be connected to the transformer by a cable , when energizibng the transformer fronm high voltage side , high frequency oscillatioons arise on the terminals due to travelling waves are redlected back and forth in the cable.
If one of these frequencies oscillations coincide with one of the critical resonance frequencies of the transformer winding resulting in high internal overvoltages in the transformer windings. Nut this can be avoided by energizing the transformer from the generator side and then synchronising the generator with the system by means of the circuit breaker at the high voltage side.
Transformer low voltage connection to the generator: see fig.3 &4
1- Connection through a circuit breaker.
2- Fixed connection by cables:
The generator and the transformer act as one unit, in case of failure of the transformer, relays must quicklt trigger the circuit breaker on the high voltage side of the transformer to disconnect the unit from the system.
This ssudden load rejection may cause higher voltage on the generator terminals and consequently to overexcitation of the transformer. The magntiude and duration of this overexcitation depend of the characteristics of the generator and its excitation system which must be considered during transformer design.
fig (3): Generator power plant with generator step-up transformers
fig (4): Site Photo for Generator power plant in fig (3) above
3- Fixed connection by bus duct:
Using the fixed connection by bus duct will minmize the risk of short circuits between the phase conductors in case of using cables
Over voltage protection:
Over voltage protection for the low voltage side is needed Because of the often large difference in voltage and insulation levels on the the transformer two sides and the magnitude of transferred transient and temporary overvoltages from the high voltage side to the low voltage side
Surghe arresters should be installed between each low voltage terminal and earth and also between terminals of different phases.
Capacitors should be installed between phase terminal and earth (typical capcitance that has been used is 0.25µF)
In the next topic, other types from power transformers will be explained.