Conditions for Parallel Operation of Transformers

Conditions for Parallel Operation of Transformers

When two or more transformers are run in parallel their primary windings are connected to supply bus bars and secondaries are connected to load bus bar.When connecting two or more transformers in parallel certain  conditions has to be met for satisfactory performance. These are the conditions for parallel operation of transformers.
a) same polarity.
b) same voltage ratio of transformer
c) same percentage impedance.
d)Same Phase Sequence

Same Polarity
                            Transformers should be properly connected with regarding to its polarity.The polarity of the transformers connected in parallel should be same otherwise it may lead to dead short circuit.When connecting two or more transformers in parallel it is essential that their terminals of similar polarities should joined to same bus bar.bar. Otherwise emfs induced in the secondaries will be in opposite polarities and they acts as a short circuit even for no load.Polarity of transformer means the direction of induced emf in secondary. If the directions of induced secondary emf in two transformers are opposite to each other when same input power is fed to the both of the transformers, the transformers are said to be in opposite polarity. If the directions of induced secondary emf in two transformers are same when same input power is fed to the both of the transformers, the transformers are said to be in same polarity.

Same Voltage Ratio
                               The voltage ratio of the  transformers*( turns ratio) which are connected in parallel should be kept equal in order to avoid losses occurring in transformers due to load circulating currents.If two transformers connected in parallel have a slight different in their voltage ratio ,the parallel operation is possible.But due to the difference in volt ratio even the primary supply voltage is identical there will be a difference in secondary voltages even in no load condition.So there will be a circulating current between secondaries and therefore between primaries also.This circulating current makes unequal loading conditions,so transformers in parallel system may be overloaded. As the internal impedance of transformer is small, a small voltage difference may cause sufficiently high circulating current causing unnecessary extra I2R loss.

Same Percentage Impedance
                              If the impedance of transformers which are connected in parallel are not same, then the parallel operation is possible but the power factor at which two transformer operates will be different from the common load .So the transformers will not share load in proportional to their KVA ratings.
                             In other words,The current shared by two transformers running in parallel is strictly depends on  their KVA ratings. As we know electric current carried by these transformers are inversely proportional to their internal impedance. From these two statements it can be said that impedance of transformers running in parallel are inversely proportional to their KVA ratings.So if the impedance are not identical ,load sharing will also be different.
                      Identical percentage impedance of two different transformers is difficult to achieve practically that is why the transformers run in parallel may not have exactly same percentage impedance but the values would be as nearer as possible.

Same Phase Sequence
                           The phase sequence or the order in which the phases reach their maximum positive voltage, must be identical for two parallel transformers. Otherwise, during the cycle, each pair of phases will be short circuited.

Advantages of Parallel Operation of Transformers.

 Advantages of Parallel Operation of Transformers.

1)To maximize the efficiency of electrical power system :
         Power transformer gives the maximum efficiency when they works on full load. By connecting numbers of transformers in parallel, we can switch on only those transformers which will give the total demand by running nearer to its full load rating for that time. When load increases we can switch on more transformers one by one that are connected in parallel to meet the total demand similarly when load decreases we can turn off  transformers one by one. In this way we can run the system with maximum efficiency.

2)To maximize electrical power system availability:
            If numbers of transformers run in parallel we can take shutdown any one of them for maintenance purpose. Other parallel transformers in system will meet the total demand of the system.

3)To maximize power system reliability:
             If any one of the transformers run in parallel, is tripped due to fault other parallel transformers is the system will share the load.So electrical system  not interrupted by tripping of one transformer ,if the shared loads do not make other transformers over loaded.

4)To maximize electrical power system flexibility:
             By using parallel connection we can connect a number of transformers in addition to existing system to meet our required load very easily.If the load is increased in future more than the available capacity,just connect more transformers in parallel.If the demand is decrease

Parallel operation of transformers

Why Parallel Operation of Transformers is required ?
   
                        When load demand increases we have to use more transformers. ie,for meet a load which is more than the rating of the existing transformer,a second transformer connected in parallel with it.

 Advantages of Parallel Operation of Transformers.

1)To maximize the efficiency of electrical power system :
         Power transformer gives the maximum efficiency when they works on full load. By connecting numbers of transformers in parallel, we can switch on only those transformers which will give the total demand by running nearer to its full load rating for that time. When load increases we can switch on more transformers one by one that are connected in parallel to meet the total demand similarly when load decreases we can turn off  transformers one by one. In this way we can run the system with maximum efficiency.

2)To maximize electrical power system availability:
            If numbers of transformers run in parallel we can take shutdown any one of them for maintenance purpose. Other parallel transformers in system will meet the total demand of the system.

3)To maximize power system reliability:
             If any one of the transformers run in parallel, is tripped due to fault other parallel transformers is the system will share the load.So electrical system  not interrupted by tripping of one transformer ,if the shared loads do not make other transformers over loaded.

4)To maximize electrical power system flexibility:
             By using parallel connection we can connect a number of transformers in addition to existing system to meet our required load very easily.If the load is increased in future more than the available capacity,just connect more transformers in parallel.If the demand is decreased remove a transformer from the system.

Conditions for Parallel Operation of Transformers

When two or more transformers are run in parallel their primary windings are connected to supply bus bars and secondaries are connected to load bus bar.When connecting two or more transformers in parallel certain  conditions has to be met for satisfactory performance. These are the conditions for parallel operation of transformers.
a) same polarity.
b) same voltage ratio of transformer
c) same percentage impedance.
d)Same Phase Sequence

Same Polarity
                            Transformers should be properly connected with regarding to its polarity.The polarity of the transformers connected in parallel should be same otherwise it may lead to dead short circuit.When connecting two or more transformers in parallel it is essential that their terminals of similar polarities should joined to same bus bar.bar. Otherwise emfs induced in the secondaries will be in opposite polarities and they acts as a short circuit even for no load.Polarity of transformer means the direction of induced emf in secondary. If the directions of induced secondary emf in two transformers are opposite to each other when same input power is fed to the both of the transformers, the transformers are said to be in opposite polarity. If the directions of induced secondary emf in two transformers are same when same input power is fed to the both of the transformers, the transformers are said to be in same polarity.

Same Voltage Ratio

                               The voltage ratio of the  transformers*( turns ratio) which are connected in parallel should be kept equal in order to avoid losses occurring in transformers due to load circulating currents.If two transformers connected in parallel have a slight different in their voltage ratio ,the parallel operation is possible.But due to the difference in volt ratio even the primary supply voltage is identical there will be a difference in secondary voltages even in no load condition.So there will be a circulating current between secondaries and therefore between primaries also.This circulating current makes unequal loading conditions,so transformers in parallel system may be overloaded. As the internal impedance of transformer is small, a small voltage difference may cause sufficiently high circulating current causing unnecessary extra I2R loss.

Same Percentage Impedance
                              If the impedance of transformers which are connected in parallel are not same, then the parallel operation is possible but the power factor at which two transformer operates will be different from the common load .So the transformers will not share load in proportional to their KVA ratings.
                             In other words,The current shared by two transformers running in parallel is strictly depends on  their KVA ratings. As we know electric current carried by these transformers are inversely proportional to their internal impedance. From these two statements it can be said that impedance of transformers running in parallel are inversely proportional to their KVA ratings.So if the impedance are not identical ,load sharing will also be different.
                      Identical percentage impedance of two different transformers is difficult to achieve practically that is why the transformers run in parallel may not have exactly same percentage impedance but the values would be as nearer as possible.

Same Phase Sequence
                           The phase sequence or the order in which the phases reach their maximum positive voltage, must be identical for two parallel transformers. Otherwise, during the cycle, each pair of phases will be short circuited.

Humming sound of transformers

      The humming of transformer is due to the phenomenon known as magnetostriction.This is the phenomenon which cause a piece of magnetic sheet to extend itself when magnetized.When the magnetization is taken away it goes back to its original shape.
   In a transformer the magnetic field is produced by the applied alternating voltage,hence the magnetic field set up by this alternating voltage also alternating in nature.So for a full cycle of this magnetization, the sheet laminations of core will expand and contract.The magnetization at different points on the sheets are different hence the expansion and contraction is not uniform.Transformer core is made up of large number of sheet laminations to reduce eddy current losses.Each laminations have this expansion and contraction.We cant see the variations with our naked eyes, but they are enough to make vibrations and produce noise(hum).Applied voltage produces the magnetic flux,this flux produces humming sound in transformer.Hence the level of humming is depend on the applied voltage.

What will happen if the primary of a transformer connected to D.C supply?

                   If the primary of the transformer is connected to D.C supply,the primary will draw a steady current and hence produce constant flux.,so no back emf will be produced.The primary winding will draw excessive current due to low resistance in primary.The result is that the primary will overheat due to this heavy current and burnout.So the primary of the transformer should not connect to the D.C supply.

Why transformer rating in KVA?

        As we know Copper loss in the transformer is depends on the current and core loss in the transformer is depends on the voltage.Hence total transformer loss is depends on Volt Ampere  (VA) and not on the phase angle between the current and voltage. ie, It is not depend on the load power factor.This why transformer rating in KVA and not in KWh.
       There is also another saying,If we consider an electric motor we can predict its output power.Beyond that particular power it doesn't work.The transformers are used in generation and distribution side so we cant predict the load on the transformer as that in the case of an electric motor.The load on the transformer may be lagging,leading or unity power factor.Hence we use KVA rating for transformer.
       The rated KVA of the primary winding and the secondary winding are equal.Hence we can easily calculated the rated current from the rated KVA and the corresponding rated voltage.

Working principle of transformer

                    Transformer consists of two inductive coils which are magnetically linked through a core of low reluctance but the coils are electrically isolated.The two coils have high mutual induction. If one of the coils is connected to an alternating voltage source,an alternating current will flow through that winding and this current set up an alternating flux in the core.Most of this flux is linked with the other coil and induces mutually induced emf in the other winding. This is according to the Faraday's law of electromagnetic induction.If the second coil circuit is closed,an induced current flows through it.So electrical energy is transferred from one circuit to another by magnetically.

What are the applications of transformer ?

  ¥ It can rise or lower voltage or current in an electrical circuit.
  ¥ It can acts as impedance transferring device .
  ¥ It can isolate two electrical circuits.
  ¥ It can be used to prevent direct current passing from one circuit to another .

Define transformer ?

   The transformer is a stationary device which transfer electrical power in one circuit into other circuit without change in frequency .It can raise or lower the voltage in a circuit with corresponding decrease or increase in current.The working principle is mutual induction between two circuits linked by a common magnetic flux .

transformer