Study material for SSE/JE examination by RRB on Electrical Engineering Part II

Transformer

Transformer is the most important application of electro-magnetic induction and transfer of power from one independent circuit to another through a magnetic media. There is a primary and secondary winding on a common core. The primary produces magnetic flux equal to number of turns which induces emf in the secondary current in proportion to the number of turns in secondary winding. Now by balancing volt-ampere and ampere-turns of primary with that of secondary, assuming no losses and an ideal transformer, we get

N_p*I_p=N_s*I_s and V_p*I_p=V_s*I_s; and this derives the very fundamental transformer equation

N_p/ N_s =I_s/ I_p = V_p/ V_s

The turns ration is inversely proportional to the current ratio but directly proportional to the voltage ratio.

Transformer Losses

Losses in the transformer are simply a waste of energy into heat than its management to dissipate  is an important factor to consider for selection of transmission voltages. For energy conservation, handling transformer losses is a priority item.

There are two types of losses namely Hysteresis, Eddy and load current joules heating losses.

Hysteresis Losses: During sinusoidal application of voltage, the magnetic field (H=NI/l) varies with time. This magnetic field induces flux in the core. During reversal of the magnetic field, the reduction in the flux does not follow the same path and retains some flux called retentivity or remanence. There is a loss of energy in a cycle and the area demonstrates the loss of energy per cycle. When H is in AT/m, B in Wb/m² than work done due to hysteresis is in joule/m³/cycle as per the diagram shown below. Alternatively, hysteresis losses is also given by W_h = k_h* B_max^1.6 *f*v watt (where B is the maximum flux density, f is frequency and v is the volume  of core). The hysteresis loop of three materials has been shown of which the loop 1 is hard steel suitable for permanent magnet, loop 2 is wrought iron and cast steel for core of electromagnets and loop 3 is of alloyed sheet steel suitable for making transformer and motor core.

Eddy Current Losses: When there exists magnetic field in a core, the induction takes place between two winding but also inside the core making closed loops of tiny current around flux lines. There is heating loss due to the flow of these current paths. There is very complex function to evaluate the eddy losses and is given by W_e = k_e *B_max² f²*t² Wattt (where f is frequency and t thickness of the core).   It is for this reason that the core is made of  very small thickness stamping  which are insulated with each other to break the path of the current.

Load Current Joule Heating Losses: This is arising due to the passing of no load and load current through the primary and load current through secondary winding. Total losses due winding resistance is =  I_p²R_p + I_s²R_s. 

Frictional Heating: The core is made of stampings and the varying magnetic field results in expansion and contraction, the effect called magnetostriction. This is also the reason for producing humming sound in a transformer.

Mechanical Losses: Mechanical losses are not only in motor but in transformer arising due to fluctuating forces between primary and secondary winding inciting vibrations within nearby metalwork.

Stray Losses: Leakage inductance does not contribute to losses but when it intercepts nearby conductive material such as its tank etc., it give rises to eddy current losses, though very small.

Transformer is a device which converts electrical energy into electrical energy, and therefore, all losses in primary as well secondary is mainly on account of joule heating by a current irrespective of active or reactive. For this reason, transformers is rated as kVA and not kW.

The important parameters finds mentioning on the name plate of a transformer are No. of phases (mainly 3 but in some cases one), kVA or MVA, frequency (50 Hz), Primary and Secondary voltage, Tap voltages if provided, Connection diagram, type of transformer i.e. star, delta or scott , type of insulating oil, phasor diagram etc.

Delta connections are generally used for transformer used in transmission and star at distribution point of the transformer. Delta connection in transmission line also saves on running fourth conductor.  Scott connection converts three phase into two phase, very important application in traction power supply system.

Methods of cooling Transformer: The efficiency of transformer varies from 98 to 95 % depending on it’s rating. 1000 kVA transformer with 98% efficiency will use 20 kVA for heating. Just imagine a heater of 20 kVA capacity in a closed chamber, the temperature it will produce. This temperature is harmful to its insulation, and therefore, methods of removing this heat is very important subject in designing of transformers. The transformer cooling design depends on the capacity, efficiency etc. The various methods are

ONAN: Oil natural and air natural  ONAF:  Oil natural and air forced  OFAF:   Oil forced and air forced  OFWF (This system is used for cooling in Electric Locomotive):  Oil forced and water forced and ODWF: Oil directed and water forced.

Moisture in transformer: The paper insulation and oil media for insulation and cooling is having tendency to absorb moisture. The moisture ingress results in loss of break down voltage of transformer oil. During variation of load on the transformer, its temperature also changes along with volume. In order to accommodate, this volume changed, a conservator is provided. The air volume above the conservator also changes, during which the air exchanges with environment which may have moisture. A silica gel breather is used which absorbs moisture of the air before it exchanges with the conservator air.

Moisture reduces dielectric strength of paper and oil and mechanical strength of paper, Temperature reduces life of oil and oxygen results in oxidation of paper insulation thus increasing the acidity of transformer oil. Therefore, moisture, oxygen and temperature are enemy of transformer.

Transformer Protection:  The two very important protection of transformer are

1. Buchholz Relay: This relay acts on sudden increase of oil pressure. The oil pressure will increase if there is excessive heating due to over loading or flow of excessive fault current in the transformer. This also protects the transformer from explosion.
2. Differential Relay: The differential relay monitors the difference of primary and secondary current taking into consideration the transformer ratio.

Condition Monitoring of Transformer: It is not possible to open and check the condition of transformer very frequently. Therefore, checking the acidity, breakdown voltage and Dissolved gas analysis (DGA) are the common methods followed to assess the inside condition of transformer.

Types of Transformer: The main function of a transformer is to transform voltage through electro-magnetic induction. But there are many types of transformers depending upon application. These are

1. Transmission transformer: High power rating and the load current does not vary widely
2. Distribution Transformer: Medium rating but the load current varies widely
3. Traction Transformer: Medium rating of 3 MVA to 8 MVA with very wide current variation, high harmonics etc.
4. Current Transformer: It is used for high current measurement and for circuit protection. A safety measure is necessary for current transformer where the secondary is not made to open circuit which may result very high voltage.
5. Potential Transformer: Similar to Current Transformer, Potential transformers is for voltage measurement and protection towards over and under voltage.

% Impedance: Percentage impedance is an important term used understand the behavior of transformer during short circuit conditions. Percentage impedance is defined as the percent voltage required to maintain  full load under short circuit condition of secondary.  On base values , it can also be written as %Z = (kV_b)² /   (kVA _b)²    and short circuit current is equal to (I_l * 100/%Z ), so therefore, %Z defines the level of fault current. It does not mean that we design transformer for high percentage impedance, because it will be at the cost of efficiency of transformer.     So somewhere a optimization of transformer losses and its impedance is  done. This calls for Short circuit analysis in association with codes and standards of a country.

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