5 Pro Tips For Impactful Parallel Operation Of Three-Phase Transformers.

three-phase transformers December 19, 2022

Three-phase transformers are used to generate, transmit and balance three-phase voltages and current. In the power system, these transformers are used to step-up and step-down the voltage level as per the requirement. The three phase transformers are deployed as per the rating of the demand. However, the need keeps fluctuating, and to fulfill this, one can either replace the existing one with a higher capacity transformer or create a parallel operation by adding another transformer to the existing one.

The known fact is that the parallel operation of a three-phase transformer is more advantageous than creating a large unit. Two or more three-phase transformers can be connected parallelly, satisfying the condition of identical angular displacement to obtain additional capacity. Parallel operation is prevalent in three-phase power generation for distribution and transmission purposes. It offers flexibility for operation and maintenance too. So, hereby we are sharing mandatory conditions and a few tips for an impactful operation.

Why Parallel Operation Of Three-Phase Operation?

  1. If the demand for power supply increases and you don't want to create a bigger unit and replace the existing one, add additional transformers to increase the capacity.
  2. At the time of maintenance, it helps maintain the continuity of supply without any interruption. In this way, it increases the reliability of a system.
  3.  It avoids the intrusion of power. As there is some fault in one transformer, one can continue the power supply with another one.

Advantages of Transformer Parallel Operation

  • Maximize the Efficiency - It enhances the efficiency of the electrical system in both conditions. If the load is minimum, one can switch on one transformer to complete the power supply demand by running nearer to its complete load rating for that time. If the load is maximum, one can switch on transformers connected in parallel to meet the total demand. In both conditions, one can run the system with maximum efficiency.
  • Provides Reliability To system - Due to the parallel operation of several transformers, electrical systems are more readily available. One can turn off any of them without disturbing the power supply if any transformer requires maintenance.
  • Avoid Interruption of the power supply - It avoids interruption of the power supply. If any of the transformers get tripped due to a fault, other transformers in the parallel system will share the load and supply power without any interruption.
  • Offer flexibility to the system - It provides flexibility to the electrical system to meet increased future demand. It is always easier to install a transformer in parallel operation than to establish a large-rated single transformer for the forecasted increased future demand. If there is less demand, it is easy to remove from the system and balance the capital investment and its return.

Conditions to Encounter - 

Although, the process does not require large infrastructure and time but needs to fulfill some basic conditions. For parallel operation of three-phase transformers, one should ensure the following conditions -

  1. Primary windings should be connected to the source bus bar, while secondary windings of the load bus bars. The primary windings of both transformers should be designed for the frequency and the supply system.
  2. All transformers should have equal per-unit leakage impedance.
  3. The line voltage ratio must be the same. The voltage rating of the primary and secondary winding must be the same. In the absence, parallel operation of the transformer will not be possible but some amount of circulating current will keep flowing in no-load conditions, creating unequal loading conditions. 
  4. Polarity markings must be followed and must connect with the same polarity. If the polarity is not the same, there is a chance of short-circuiting.
  5. If both transformers have different KVA ratings, the equivalent impedance should be inversely proportional to individual KVA ratings. 

5 Pro Tips For Impactful Parallel Operation Of Three-Phase Transformers

  1.  If primary and secondary winding impedance results are given separately, then before sending to secondary impedance, the transference of primary impedance should send after its multiplication of with (transformation ratio).
  2. The impedance triangle of both transformers must be the same to avoid circulating current. In absence of the condition, both transformers will work on separate power factors.
  3. If transformers are working on separate KVA ratings then the equivalent impedance is inversely proportional to the KVA rating of individuals while the circulating current is avoided.
  4. There should be equality in phase displacement in primary and secondary voltages of transformers.
  5. The voltage ratio must be utilized to examine the reference of the terminal voltage on the primary and secondary sides.
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