Potential transformer

                                                  Potential Transformer 

• The Potential transformer is another type of instrumentation transformer. It is used to measure high voltages.
• The construction of a potential transformer (P.T) is same as that of the normal transformer.

The connection of a potential transformer in a circuit to measure voltage is shown in fig.

Description :

• To measure high voltage, the potential voltage transformer which is a step down transformer is connected across high voltage terminals and voltmeter is connected on low voltage side.
• A potential transformer is an ordinary two winding transformer with high turns ratio, designed for low current carrying capacity.
• Due to P.T., voltage across voltmeter gets reduced by a factor equal to the turns ratio of P.T. 
• The scale of thr voltmeter is suitably calibrated by taking in to account the turns ratio of P.T.
• So if the ratio (N2/N1) is known and V2 is measured then we can obtain the voltage V1 accurately

Sources of error :

          There are two types of errors that are likely to occure in the potential transformer :

           1. Ratio error      2. phase angle error.

1. Ratio error :

          To ensure accurate measurement, the turns ratio of the P.T. should be very accurately adjusted. The error introduced due to incorrect ratio is called as the ratio error.

2. Phase angle error :

           A little deviation in the phase angle between voltage and current can affect the voltmeter reading considerably. Such an error is called as phase angle error.

Current transformer

                                                              Current Transformer 

• When a large current carried by a conductor is to be measured, it is advisable to avoid the connection of ammeter in this high current circuit.
• For this purpose a current transformer is used. 
• A current transformer (C.T) is nothing but a coil wound on a toroidal core.
• The construction of a current transformer is shown in fig. and its equivalent circuit is shown in fig.
  

Construction of Current Transformer :

• Construction of C.T. is shown in fig.
• C.T. has a primary coil which has a primary coil of one or more turns. In Fig . the bar acts as the primary.
• The primary of C.T. carries the large current I(1) which is to be measured. so it is of large cross sectional area.
• The secondary of a C.T. made up pf large number of turns. It is wound on a core. The secondary winding is a low current winding hence its cross sectional are is small.
• An ammeter of small range (typically 0-5A) is connected across the secondary as shown in fig.

Operation :

• C.T. is basically a step up transformer. Hence the secondary is a high voltage low current winding. The current is stepped down.
• The current I(2) is measured by the ammeter. so if we know the turns ratio N1/N2 then it is possible to measured I(1) 
• The ammeter can be calibrated directly to measure the current I(1), if the turns ration is known. thus a C.T. can be used to measure a high current without actually connecting the ammeter directly in series with the high current.

Transformer Cooling Method

                                               Transformer Cooling Method

(1) Air Natural Type transformer Cooling :

•  This method is used for small dry type transformer. The air in the surrounding vicinity of the transformer is used for cooling. This method is suitable for transformer up to a rating of 25 KVA 

(2) Air Forced Type Transformer Cooling : 

• This cooling method also is suitable for the dry type transformer of slightly higher ratings. The air is forced upon the tank surface to increase the rate of heat dissipation.
• Fans are turned on automatically to facilitate forced air cooling whenever the winding temperature rises beyond the permissible limit.

(3) Oil Natural Air Natural Type Cooling :

• This type of cooling is used for oil filled transformer up to a rating of about 30MVA. As the oil in the tank gets heated due to heated up core and winding it ( oil ) flows upwards and the cold oil comes down as per the principle of convection.
• The heat is transferred from various parts of the transformer to the oil and the heated oil is cooled by the natural air.
• It is possible to increase the cooling are by providing the cooling tubes.

(4) Oil Natural Air Forced Cooling :

• In this method. the transformer of heat from various parts of a transformer takes place naturally as explained for the O.N.A.N. type cooling.
• However the hot oil is now cooled by means of fans mounted near the transformer 
• The forced air from the fans is directed to the cooling tubes to increase the rate of cooling.
• A thermostat is used to automatically sense the temperature of oil and winding and turns the fans on when it increase above the permissible limit.
  

(5) Oil Forced Air Forced Cooling :

• This type of cooling is  preferred for the transformer of ratings above 60 MVA.
• A separate cooler is mounted away from the transformer tank . This cooler is connected to the transformer with pipes at the bottom and top.
• The Oil is circulated from the transformer to the cooler with the help of an oil pump. This oil is then subjected to forced air cooling with the help of fans installed the cooler.
• The O.F.A.F. type of cooling is used for the big transformer used at the substations and Power stations.
  

(6) Oil Forced, Water Forced Cooling :

• O.F.W.F. type cooling for large transformer. This type of cooling needs a heat exchanger.
• Inside the heat exchanger the heat from thr oil is transformer to the cooling water.
• The cooling water is taken away and cooled in separate coolers. The oil is forced to circulate through the heat exchanger by using a pump.
• This type of cooling is useful for transformer having ratings of few MVA ( generating transformer. )
  

Construction of a Transformer

                                                         Construction of a transformer

• The most important parts of a transformer are the winding and the core 
• However for the large capacity transformers, some other parts such as suitable tank, conservator, Buchholz relay bushings, breather, explosion vent etc, are also used alongwith the core and winding.
• 
  
  
  
  
• Laminated steel core
•       The material used for the construction of the transformer core is silicon steel. It is used for the high permeability and low magnetic reluctance. due to this the magnetic field produced in the core is very strong 

                The core is in the form of stacks of laminated thin steel which are electrically isolated from          each other. The core is assembled in such a way. that the assembly provides a continuous path            for the magnetic flux, with a minimum air gap.

                                                                   Transformer Tank

•      The whole assembly of large size transformer is placed in a sheet metal tank. Inside the tank the assembly of the transformer is immersed in oil which acts as an insulator as well as a coolant 

     The oil will take out the heat produced by the transformer winding and core and transfer it to the surface of the transformer tank.

                                                                  Function of transformer oil

•       The construction of the transformer should be such that the heat generated at the core and at the winding should be removed efficiently.
• Moreover in order to avoid the insulation deterioration. the moisture should not be allowed to creep into the insulation.
• Both these objectives can be achieved by immersing the built uo transformer in a closed tank filled with non inflammable insulating oil called transformer oil.
• IN order to increase the cooling surface exposed to ambient, tubes or fins are provided on the outside of tank walls. 

                                                                  Conservator

• In large transformer, some empty space is always provided above the oil level. This space is essential for letting the oil expand or contract due to the temperature changes.
• When the oil temperature increases, it expands and the air will be expelled out form the conservator. Whereas when the oil cools, it contracts and the outside air gets sucked inside the conservator. This process is called as the breathing of the transformer.
•  However, the outside air which has being drawn in can have the moisture content. When such an air comes in contact with the oil, the oil will absorb the moisture content and loses its insulating properties, to some extent. This can be prevented by using a Conservator.

The conservator is a cylindrical shaped air tight metal drum placed on the transformer tank. The conservator is connected to the tank by a pipe

                                                                 Breather

• The apparatus though which breathing of the transformer takes place is called as "Breather"

The air goes in or out through the breather . To reduce the moisture content of this air, Some drying agent such as silica gel or calcium chloride is used in the breather. The dust particles present in the air are also removed by the breather .

                                                               Buchholz Relay

• There is a pipe connecting the tank and conservator . On this pipe a protective devise called Buccholz relay mounted.
• When the transformer is about to be faulty and draws large currents, the oil becomes very hot and decomposes.

During this process different types of gases are liberated. The Buchholz relay get operated by these gases and gives an alarm to the operator. If the fault continues to persist. then the relay will trip off the main circuit breaker to protect the transformer.

                                                              Explosion vent

• An explosion vent or relif valve is the bent up pipe fitted on the main tank.
• The explosion vent consists of a glass diaphragm or aluminium foll. When the transformer becomes faulty, the cooling oil will get decomposed and various types of gasses are liberated. 
• If the gas pressure reaches a certain level then the diaphragm in the explosion vent will burst to release the pressure.
• This will save the main tank from getting damaged.

Construction of a DC Machines

                                         Construction of DC Machines                                                                                                                                                                                                                                                                                                                                                                                                                                                                            Important Parts of a DC machines:
  (1) Yoke
  (2) Field winding
  (3) poles
  (4) Armature
  (5) commutator
  (6) Bearings.


( 1) Yoke:
               - Yoke is Also called as frame. It provides protection to the rotating and other parts of the machine from moisture, dust etc.
               - Yoke is an iron body which provides the path for the flux. This is essential to complete the magnetic circuit.
               - It provides the mechanical support for the poles.
               - Material used for yoke are basically the low reluctance materials such as cast iron, silicon steel, rolled steel, cast steel etc.

( 2) Field winding
                -The coils wound around the poles cores are called field coils.
               - The field coils are connected in series to form the field winding
                -Current is passed through the field winding in a specific direction, to magnetize the poles and pole shoes. The magnetic Flux is thus produced in the air gap between the poles shoes and armature.
                -The field winding is also called as exciting winding.
                -The material used for the field conductor is copper.
               - Due to the current flowing through the field winding alternate N and S poles are produced.
which pole is produced at a particular core is decided by the right hand thumb rule for a current carrying circular conductor.

( 3) Poles
             - A pole of a generator is an electromagnet. The field winding is wound over the poles.
             - Poles produced the magnetic flux when the field winding is excited.
             - Pole shoe is an extended part of a pole. Due to its typical shape, it enlarge the area of the pole.
            - Due to this enlarge area, more flux can pass through the air gap to armature.
            - A low reluctance magnetic material such as cast steel or cast iron is used for the construction of a pole or pole shoe.
            - The construction of poles is done using the lamination's of particular shape. This is to reduce the power loss due to eddy currents.

(4) Armature
            - Armature core is a cylindrical drum mounted on the shaft.
            - It is provided with a large number of slots all over its periphery.
            - All these slots are parallel in these slots.
            - Armature core provided a low reluctance path to the flux produced by the field winding.
            - High permeability,low reluctance materials which helps in cooling the armature core.
            - The laminated construction is used to produce the armature core to minimize the eddy current losses.

(5) commutator
           - A commutator is s cylindrical drum mounted on the shaft along with the armature core.
           - It is made of a large number of wedge-shaped segment of hard-drawn copper.
           - The segments are insulated from each other by thin layers of mica.
           - The armature winding is tapped at various points and these taping s are successively connected to various segments of the commutator
          functions of a commutator:
           - It convert the alternating current to Direct current
           - It collects the current from the armature conductors and passes it to the external load via-brushes.
           - For dc motors, it helps to produce a unidirectional torque.
         Material used:
           - The commutator segments are made of copper and the insulating material between the segments is mica.