Power generated at the generating stations need to be transmitted over long distance. In order to reduce I2R losses↗, we have to reduce current. Hence voltage is stepped up during the transmission of power by using using step up transformers. Step up transformers used in the transmission network are known as power transformers. Distribution transformers on the other hand are used between local substation and consumer for distribution of electrical power at lower voltage. In this article, we will discuss the differences between power transformer and distribution transformer in various factors.
Power transformers are used in the transmission networks of higher voltages generally 33kv and higher (66KV, 110KV, 200KV,400KV). These transformers are rated above 200 MVA.
Distribution transformers on the other hand operated at less voltages generally below 33KV (11KV,6.6KV, 3.3KV,440V,230V). They are rated less than 200 MVA.
Power transformers are much larger in size as compared to distribution transformers.
Windings of power transformers are provided with high insulation since they have to handle high voltages. Distribution transformers on the other hand have low insulation level.
Distribution transformers are connected directly to the load. Since load at the consumer side is not constant due to which these transformers experience high load fluctuations. On the other hand, voltage level of transmission line is almost constant. Hence power transformers doesn’t face load fluctuations.
Power transformers are used to transfer the whole power that is being generated at the generating station. Hence they are designed to achieve maximum efficiency at rated 100% load.
Distribution transformer on the other hand doesn’t operate at full load all the time since its load depend on the distribution demand. Due to unknown distribution of load, these transformers are designed to achieve maximum efficiency at 60 % to 70% of the rated load.
Since maximum efficiency in a transformer is achieved when iron loss becomes equal to the copper loss i.e. Iron loss = Copper loss. Hence in power transformer, Iron loss= Copper loss when it is operating at full rated 100% load. In case of distribution transformer, iron loss becomes equal to the copper loss at 60-70% of the rated load.
Power transformers are larger in size. Due to their large heavy sizes, cooling fans are also provided along with radiator (cooling tubes) for fast cooling. Distribution transformers on the other hand are much smaller in size due to which radiators are sufficient for their cooling.
Power transformer operates at full load all the time due to which both copper and iron losses are constant in these transformers. Moreover, power transformers are designed in such a way to keep the copper losses minimum at full load.
Distribution transformers on the other hand doesn’t operates at full load all the time. Iron losses in distribution transformers remains same at all the time( 24Hrs a Day) for any load or even at no load. Copper losses on the other hand depend on the load cycle. In case of distribution transformers, steps are been taken to minimize the iron losses.
Core Material :
Power transformers handle large voltages. They are designed to utilize the core to its maximum. Hence core of power transformer is made up of CRGO (cold rolled grain oriented) steel core since its carry more flux. On the other hand, amorphous CRGO core material is used in distribution transformers.
Flux density of power transformer is high as compared to the distribution transformer.
Power transformers are connected with on load tap changer (OLTC) ↗ . In case of OLTC, output voltage of power transformer can be controlled (adjusted) without isolating the transformer from the supply. Distribution transformers on the other hand is fitted with de-energized tap changer( DETC) which can alter the voltage only when the transformer is de-energized ( off load).
Delta -Delta connections or star-delta connections are used in power transformers. Delta–Delta type connections are preferred in transformers which are used for transmission purposes since in that case neutral is not required. Distribution transformers on the other hand has always delta-star connections.