Distribution Transformers play a vital role in the system which delivers electricity to the end user. It is the final part of the transmission system from the power plant to the consumer. Distribution Transformers step down the MV power, usually 11kv into the domestic LV, 440 V supply.
Distribution Transformers are a critical part of the distribution network. These transformers are always online throughout the year. Hence, design of the distribution transformer is made considering the high iron losses. Besides, the transformer is sized to have high efficiency at 70% of the load as the power output varies through the day as per the load cycle.
Distribution transformer are protected by fuses in the HV side. They are also designed to withstand unbalanced loading. They have ONAN cooling (Oil Natural, Air Natural ).
Distribution Transformer are usually of the vector Dyn11. While, designers are not particular about any particular vector group , most systems will standardize on one particular vector group, usually the Dyn11. Some systems also use the Dyn1. These vector groups have a difference of 30 degrees between the primary and secondary vectors which is unavoidable in delta to star conversion.
Single Phase Pole Mounted Distribution Transformers
Single Phase Pole Mounted Transformers are usually used in rural areas where three phase power may not be required. These Transformers reduce the voltage from the line voltage 11kV to a single phase voltage usually 230V.
The secondary of the Pole mounted Distribution is usually connected between the two phases of a MV line. The secondary voltage is a single phase voltage which is fed to the house. These transformers can be easily installed and do not require extensive mounting Structures.
Pole mounted Transformers come in sizes up to 500 kVA.
These transformers have a fuse to protect against faults inside the transformer. They are also equipped with an interrupting device. These Transformers are tested to withstand the impulse of lightning. In the US, The secondary of these transformers has a centre tapping and therefore has three terminals.
The voltage between the end terminal and the centre tapping will provide 120V while the voltage between two end terminals will be 220 volts. These Single Phase Pole Mounted transformers can also be used to provide three phase LV supply.
Three pole mounted transformers are connected in wye or delta to get the desired connection. Besides, small size three phase transformers which can be mounted on poles are also available. Video Showing the Manufacture of Single Phase Transformers
Amorphous Metal Transformers
The chief losses in a transformer are the losses that occur in the core. These are the the eddy current loss and the hysteresis losses. About 1 to 4 percent of the power which passes through a transformer is lost due to these losses.
The losses in the distribution transformers constitute nearly 20% of the total losses in the distribution system. Since transformers are online continually, the no load losses of the transformers is constant throughout the day regardless of the load.
The Amorphous Metal Transformer is fast emerging as an efficient alternative to the conventional transformer. The Amorphous Metal transformer has a core which is made of ferromagnetic materials such as Iron or Cobalt in a glass former such as phosphorous, silicon or boron.
Metglas, as this substance is known, has high susceptibility, low coercivity and high resistance. The low coercivity reduces the hysteresis losses while the high resistance greatly reduces the eddy current losses.
By using Amorphous Metal Transformers, it is estimated that many millions of units of electricity can be saved. Amorphous Metal Transformers are widely used in developing economies such as India and China in an effort to bring down the distribution losses.
Some of the other advantages of Amorphous Metal Transformers are the lower operating temperature, higher overloading capability, slower ageing of the winding insulation and better performance when subjected to harmonics.
Grounding Transformers
Grounding Transformers are used in Ungrounded systems to provide a earth point. Grounding Transformers are classified into two types
1) Zig Zag Transformers and
2) The Star-Delta Grounding Transformer with secondary unloaded.
We had looked at the Zig-zag Transformer in an earlier Post (Click here).
Let us now look at the Star-Delta Grounding Transformer
The Star-delta grounding Transformer has a primary which is star connected and a delta secondary. The phases of the star primary are connected to the busbar while the neutral is grounded. The secondary of the transformer which is delta connected is usually left unloaded, though it can also be used to supply power. The delta serves to provide a return flux path for unbalanced loads.
During an earth fault, the zero sequence currents can flow through the grounded neutral of the transformer. If the current is to be limited, a resistor can be added in series to the neutral of the transformer primary.
What is a Rectifier Transformer
Rectifiers Transformers are Transformers which have an in-built Rectifier in the secondary side. The Diodes are mounted inside the Transformer Assembly itself. The Rectifier Transformer is used in applications which require high DC power such as in DC Traction Systems and in processes such as electrolysis and smelting.
Voltage regulation is achieved using Tap Changers in the HV side.
Rectifier Transformers are sometimes also known as Rectiformers.
Rectifier Transformers are usually connected to a bank. Each Rectifier-Transformer is designed with an inherent phase shift such that the transformers attain the peak voltage one after another. This ensures a steady supply at peak voltage. Thus a system with five banks of rectifier transformers with twelve pulses at a phase shift of +12°, – 6°, 0°, + 6° and +12° will result in a system with 60 pulses .