August 2017 – APSEEE

Month: August 2017

Power Transformer and Distribution Transformer

Power Transformer and Distribution Transformer

Transformer plays an important role in the area of electrical engineering. Transformer is used either for raising or lowering the voltage of ac supply. It is used at electrical energy generating station where basically it raise the voltage level of an ac supply and it is also used at substations where it is used to lowering the voltage at suitable level. In power engineering it occupies a very important place.In this article, we will study about power transformer and distribution transformer. The power transformer are located at power generating stations whereas the distribution transformers are installed in the localities of the city.

Uses

The power transformers are installed at the sending and receiving end of the high voltage transmission lines whereas the distribution transformers are installed at the near to the load centre to provide utilization at the consumers premises. Distribution transformers are basically pole mounted.

Load

Power transformers are usually operated at full load or nearly full load in simple words we can say that the power transformer gives high efficiency at full load or nearly full load. On the other hand distribution transformers are operates at light load during major part of the day.

Rating

The rating of a power transformer is very high in the orders of MVA and power transformer generally rated above 100MVA. The rating of distribution transformer is smaller and distribution transformer are generally less than 100MVA.

Insulation

The insulation required in case power transformer is more in comparison to the distribution transformer because the power transformer generally operates at very high voltage in order of 400KVA, 220KVA, 132KVA etc.

Size

The power transformer is basically larger in size as comparatively distribution transformer.

Copper Losses and Iron Losses

The power transformers are operated at nearly full load so iron losses are less so the power transformer are designed to have low copper losses. However, the load on the distribution transformer vary time to time so these transformer designed to have low iron losses.

 

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Transformer on No-Load

Transformer on No-Load

Transformer on No-Load transformer, there is no iron losses and copper losses. The ideal transformer is not practically possible. In case actual transformer these losses are present. When an actual transformer put on load, there is iron loss in the core and copper loss in the windings. When secondary of the transformer is kept open and primary winding is connected to alternating source, the transformer is called transformer on no-load.

When primary is connected to source and transformer is on no load, the transformer draws some current which is not wholly reactive. The primary input current under no load conditions has to supply iron losses in the core and copper losses in the windings (both primary and secondary).

 

When transformer is connected to source. It draws no load current IO at voltage VI. The current lags the voltage by angle φO which is less than 900. No load current IO has two components. One component is called Active or Working Component and second component is called Reactive or Magnetizing Component.

transformer on no-load

Active or Working component IW is in phase with input voltage V1. Active component supplies the iron loss and small quantity of copper loss. This component is also known as wattfull component or iron loss component.

phasor diagram of transformer on no load

IW = IO Cos φO

Reactive or Magnetizing component is in quadrature with VI. The Function of reactive component is to sustain the alternating flux in the core. This component is also known as wattless component because it does not consume energy.

      Im = IO Sin φO           

Conclusions

  1. The no-load primary current IO is very small in value as compared to the full load primary current. It value may be 2 or 3% of the full load primary current.
  2. The value of input primary current IO is very small. Hence, copper loss is negligibly small. It means input power is equal to the iron loss in the transformer.

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Electric Heating

Electric Heating



An electrical energy at generating station then transmitted to a load centers. The electrical energy use for various applications such as heating, lighting, welding etc. The conversion of electrical energy into other suitable for is very simple and convenient. When an electric current passed through a high resistance wire heat is produced. In simple words we can say that when heat is produced by flow of current though a wire having high resistance, the production of heat in such a way that is called a Electric Heating. This is the definition of electric heating. This is the most suitable methods over other methods of heating.

Advantages 

Cleanliness

In the absence of dust and ash of the fuel, charge never gets contaminated.

Absence of Flue Gases

There is a absence of flue gases in case of this type of heating that is why atmosphere is not polluted.

Ease of Control

We can easily control the temperature of appliances either manually as made fully automatic.

Better Working Conditions

Radiation losses are low. The operation of heating furnace is noiseless.

High Efficiency of Utilization

In electric heating, source of heat can be brought directly to the point where heat is required thereby reducing losses and increasing efficiency.

Uniform Heating

In this method, material can be heated uniformly. Uniformity of heating is closely achieved by electric heating only.

Heating of Non-Conducting Materials

It is possible. Only with electric heating to get non-conducting materials heated very uniformly through out the section.

Different Methods of Electric Heating

different methods of Electric Heating

 

Requirements of a Good Heating Material

  • Heating material should have high resistance so that small length of wire may be required to produce given amount of heat.
  • It should have high meting point.
  • It should have low temperature coefficient of resistance.
  • Heating material should be free from oxidation. It means if should not oxidize at the temperature of the furnace.

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Induction Type Wattmeter

Induction Type Wattmeter

Induction type wattmeter can be used to measure as power only. In last article we have studied about dynamometer type wattmeter which can be used to measure dc as well as ac power.

Working Principle 

Induction type wattmeter works on induction principle.

Construction of induction type wattmeter

Induction type wattmeter consists of two laminated electromagnets. One is called series magnet and other is called shunt magnet. Series magnet is connected in series with the supply and carries the load current. Series magnet is made highly non-inductive so that angle of lag or lead is wholly determined by the load. Shunt magnet carries the current which is proportional to load voltage. This magnet is made highly inductive. So that the current in it lags behind the supply voltage by 90O.

A thin aluminum disc mounted on the spindle of the induction type wattmeter. It is mounted in such a way that it cuts the fluxes of both magnets. Hence, two eddy currents are produced in the disc. The controlling torque is provided by spiral springs. In this wattmeter, electro magnetic damping torque is provided. Shading rings are provided on the central limb of the shunt magnet.

induction type wattmeter

Working

When induction type wattmeter is connected in the circuit to measure ac power, the current start flowing through both magnets. The shunt magnet carries the current proportional to the voltage across load while series magnet carries the load current. The fluxes produced by the series and shunt magnets induced eddy currents in the aluminum disc. The deflection torque is produced due to interaction of these eddy current and inducing fluxes. The deflecting torque on the disc causing the pointer connected to the moving system to move over the scale. The pointer comes to rest at a position where deflection torque becomes equal to controlling torque provided by spiral spring.

Deflecting Torque

Td α VI cos Φ

Applications

This is used to measure ac power.

Advantages

Induction type wattmeter free from the effects of story field and have good damping.

Induction type wattmeter has fairly long scales.

 

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Classifications of Overhead Transmission Lines

Classifications of Overhead Transmission Lines

An electrical energy transmitted from generating power stations to substations or grid through overhead transmission lines. A transmission lines has three constants resistance R, inductance L and capacitance C. These constants are distributed uniformly along the whole length of the transmission line.

The line resistance cause voltage drop (IR) and power loss (I2R) in the line, the inductance also cause voltage drop due to inductive reactance (IXL). The resistance and inductance form the series impedance. The capacitance produces charging current (2∏fcv) in the line, which quadrature with the voltage. This constant existing between conductor and neutral for single phase and or three phase line forms shunt path through out the length of the line. The capacitance effects make the calculations complex. The capacitance effects are more predominant in case of underground cables. The overhead transmission lines are classified as.

  • Short transmission lines
  • Medium Transmission lines
  • Long transmission lines

Short transmission Lines

Transmission lines having length about 50km and operating voltage is comparatively low about 20KV, it is usually considered as a short transmission lines. Due to smaller distance and lower operating voltage, the capacitance effects are extremely small and hence can be neglected. Hence, performance of short transmission lines depends upon resistance and inductance only. Therefore, while studying the performance of such lines, only resistance and inductance of the transmission line are taken into account. In short transmission lines, Constants are assumed to be lumped at one phase.

Medium Transmission Lines

Transmission lines having length is about 50 to 150km and operating voltage is more than 20KV and less than 100KV, it is considered as medium transmission line. Due sufficient length and operating voltage of the line, capacitance effects are taken into account. The capacitance is distributed uniformly over the entire length of the line. It is assumed to be shunted across the line at one or more points.

Long Transmission Lines

Transmission lines having length more than 100km and operating voltage more than 100KV (that is very high), it is considered as a long transmission lines. In case of long transmission lines constants are considered uniformly distributed over the entire length of the transmission line.

 

 

 

 

 

 

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Generation of Electrical Energy

Generation of Electrical Energy

An electrical energy is produced or generated by generator. The generator is coupled with prime mover prime mover is a mechanical rotating device that rotates the generated. Prime mover takes energy from different kinds of sources. The name of generating station is depends upon types of sources used.

generation of electrical energy

The different generating stations are

  • Diesel power generating station
  • Thermal power generating station
  • Hydro electric power generating station
  • Nuclear power generating station

Diesel Power Generating Station

In such type of power stations diesel engine is used as the prime mover for the generation of electrical energy. These power stations are used where other sources of energy is not available (such as, coal water etc). Diesel power stations are finding favour at places where demand of electric power is less.

Advantages of Diesel Power Station

  1. The design is quite simple.
  2. It occupies less space.
  3. It can be located at any place.
  4. It can be started quickly and pick up load in short time.
  5. Its cost is very small as comparatively other stations.

Disadvantages

  1. The running cost of this plant is very high.
  2. The maintenance charges are generally high.

Thermal Power Generating Station

In such power station coal is used as fuel in boiler and steam is produced which is used to rotate the turbines. An alternator is coupled with turbines or prime mover which also rotates and electric power is generated.

Advantages

  1. The fuel used is cheap in cost.
  2. It requires less space as compared to hydroelectric power station.
  3. The cost of generation of electric energy is less as compare to diesel power station.

Disadvantages

  1. It pollutes the atmosphere.
  2. Running cost is high as compare to hydroelectric power station.

Hydroelectric Power Generating Station

A generating station in which potential energy of water is used to run trubines is called hydroelectric power plants. These plants are constructed where water is available in abundance. These plants are located in hilly areas.

Advantages of Hydroelectric Power Generating Station

  1. The useful life of this plant is around 50 year.
  2. Running cost is low.
  3. There is no stand by loss.
  4. These plants are free from air pollution

Disadvantages

  1. High initial cost
  2. Generation of electrical energy is used to generation of electrical energy is called nuclear power generating station.

Nuclear power generating station

In this type of plant, nuclear fuel such as uranium is used as fuel to generate electrical energy.

Advantages

  1. The amount of fuel required is small.
  2. A nuclear power plant requires less space.
  3. The operating cost is low.
  4. It can be located near the load centre.

Disadvantages

  1. Skilled personnel are required.
  2. The fuel used is expensive.
  3. The disposal of radioactive material is big problem.

 

 

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Three Phase Transformers

Three Phase Transformers

For Generation of electrical power three-phase alternators are employed at the power generating station. Usually large power is generated at high voltage. Transmission is generally accomplished at higher voltage of 132KV, 220KV, 400KV, and 765KV. For which purpose 3-phase transformers are necessary to step up the generated voltage to that of the transmission line. At load centre, these voltages are stepped down to 66KV, 33KV, or 11KV, for distribution purpose. For distribution purpose 3-phase transformer also required. So that three phase transformers plays very important role in transmission and distribution of electric power.

Advantages of Three Phase Transformers

  • It is light in weight.
  • It occupies less floor space.
  • Its cost is less than 3 single phase transformers of equal rating.

Disadvantages of Three Phase Transformer

The main disadvantage of this transformer is that one of the phases becomes faulty, and then whole of transformer is to be replaced.

Three Phase Transformers Connections

The windings of three phase transformers may be connected in Y(star) or Δ(delta) in the same manner as for three single phase transformers. There are various type of 3-phase transformers available according to arrangement of connections. The most common connections are given below.

  1. Star – Star (Y – Y) connection.
  2. Delta – Delta (Δ – Δ) connection.
  3. Star – Delta (Y – Δ) connection.
  4. Delta – Star (Δ – Y) connection.
  5. Open – Delta (V – V) connection.
  6. Scott connection or T-T connection.

Star – Star (Y –Y) connection

This connection is most economical for small, high-voltage transformers. The insulation required is minimum for these connections. It given line voltage  times phase voltage. There is no phase shift between primary and secondary voltage.

Delta – Delta (Δ – Δ) connection

This connection is used for large current and low voltage. The conductor is required of smaller x-section as the phase current is 58% of the line current.

Star – Delta (Y – Δ) connection

This connection is used at the substation end of the transmission line where the voltage is to be stepped down. There is a 300 shift between primary and secondary line voltages.

Delta – Star (Δ – Y) connection

This connection is used at generating station because voltage is generated at low level and it need to be stepped up for transmission purpose. So that, it is used at the beginning of the high tension transmission system. This connection is also used in distribution because it give neutral point at secondary side. It is useful for 3-phase 4-wire service. It can be used to serve both the 3-phase machine and single phase lighting circuits.

three phase transformers connections delta star

Open – Delta or V – V connection

If one of the phase is removed and 3 – phase supply is connected to the primary of the transformer. In this case 3 – phase voltage will appear at secondary. The arrangement is called open-delta or V-V connection.

open delta connections

Scott Connections

The conversion of three phase transformer into single phase or two phase such available transformer is called scott connection transformer.

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Permanent Magnet Moving Coil Instruments

Permanent Magnet Moving coil  Instruments (PMMC)

Permanent Magnet Moving coil  Instruments  are  moving coil instruments. These instruments are very sensitive and accurate. DC voltage or current can be measured by these instruments.

Principle of Operation

The operation of PMMC instrument is based on the principle when a current carrying conductor placed in a magnetic field, a mechanical force experienced by the conductor.

Construction of Permanent Magnet Moving coil Instruments

It consists of a permanent shoe magnet. Light rectangular coil of having many turns of fine wire wound on a light aluminum former. The coil acts as a moving element. This coil is mounted on a spindle. Two phosphor bronze spiral hair springs are attached to the spindle. The springs provide the controlling torque as well as they act as incoming and outgoing leads for the current. Eddy current damping is used and it is provided by the aluminum former.

permanent magnet moving coil instruments (PMMC)

Working

When the instrument is connected in the circuit. The current flows through coil which is mounted on the spindle. Magnetic flux is set up in the coil. This magnetic field or flux interact with the field produced by permanent magnets, a force is exerted on the current carrying conductors of the coil which produces deflecting torque. This results pointer moves over the calibrated scale.

If current in the coil is reversed, the direction of deflecting torque will be reversed because field produced by the permanent magnets does not change.

Advantages

  • These instruments have uniform scale.
  • These instruments require low power for their operation.
  • High torque/weight ratio.
  • No hysteresis loss as the former is of aluminum
  • Reliable damping torque.
  • These instruments are very accurate and reliable.

Disadvantages

  1. The permanent magnet moving coil instruments cannot be used for ac.
  2. These instruments are costlier.

Errors in PMMC instruments

  1. Change of resistance of the moving coil with temperature.
  2. Due to ageing effects, wakening the stiffness of springs.

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Electrical Instrument

Electrical Instruments

Electrical instruments are used to measure electrical quantities such as current, voltage, power, energy, frequency, power factor, resistance etc. To measure these quantities several instruments such as ammeter, voltmeter, wattmeter, energy meter are used. In simple words we can say that the instruments which are used to measure electrical quantities are known as electrical instruments.

Classifications of Electrical Instruments

These instruments are divided into two types

Absolute Instruments

Secondary Instruments

  1. Absolute Instruments

Absolute instruments are the instruments which give the value of the quantity to be measured in terms of the constants. Such instruments do not require any previous calibration. Tangent galvanometer is the example of absolute instruments.

  1. Secondary Instruments

Secondary instruments are the instruments which determine the electrical quantities to be measured directly in terms defection.

Types of Secondary Instruments

The secondary instruments are further divided into three types.

Indicating Instruments

Integrating Instruments

Recording Instruments

  1. Indicating Instruments

These are the instruments that indicate the magnitude of electrical quantity being measured instantaneously. In such instruments, a pointer moves over the calibrated scale. Ammeter, voltmeter, wattmeter etc are the example of indicating instruments.

  1. Integrating Instruments

The instruments that add up the electrical quantity. Energy meter are the example of such instruments because these instruments measure the total energy (in KWH) in a given period.

  1. Recording Instruments

These are the instruments that give a continuous record of the variations of the electrical quantity being measure. ECG is the example of such instruments.

Essentials of Indicating Instruments

Indicating instruments are those instruments in which pointer moves over the calibrated scale to indicate the magnitude of electrical quantity which to be measured. The torques required for operation of indicating instruments are

Deflecting Torque

Controlling torque

Damping torque

  1. Deflecting Torque

It is a torque due to which pointer of instruments moves from its zero position in an indicating instrument is called deflecting torque.

  1. Controlling Torque

It is a torque which allow the pointer to deflect in accordance to the magnitude of electrical quantity or which brings the pointer to zero position when the instrument is disconnected from the circuit.

The controlling torque is provided by following methods.

  1. Spring Control
  2. Gravity Control

3. Damping Torque

It is the torque which suppresses the under oscillations of the pointers and brings the pointer to its final position quickly. The damping torque is provided by following methods.

  1. Air friction damping
  2. Fluid friction damping
  3. Eddy current damping

 

 

 

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Dynamo Type Wattmeter

Dynamo Type Wattmeter

A Dynamo type wattmeter variably used for measurement of ac power as well as dc power.

Working Principle

It works on the dynamo meter principle. According to this principle,  a mechanical force exists between two current carrying conductors when current passed through the conductors.

Construction

There are two coils are used in dynamo type wattmeter. One is called current coil and other is called potential coil. Current coil is a fixed coil which is connected in series with the load and carries load current. While potential coil is a moving coil connected across the load through a series resistance R and carries a current which is proportional the voltage across the load. The current coil or fixed coil is splitted into two parts. The controlling torque is provided by two spiral springs which also lead the current into and out of moving coil. Damping is provided by light aluminium vanes moving in an air dash pot. A pointer is attached to the movable coil that moves over a calibrated scale.

construction of dynamotype wattmeter

Working of dynamo type wattmeter

When the wattmeter is connected in the circuit to measure power. Current coil carries the load current I1 and produce a magnetic filed. Potential coil carries current (I2) proportional to the load voltage and produces another magnetic field. The magnetic fields of the current and potential coils react on one another causing the movable coil moves the pointer over the scale. The pointer comes to rest at a position where deflecting torque is equal to the controlling torque.

working of dynamo type wattmeter

The change of direction of current reverse current reverse current in both the current coils and potential or movable coil. So that, the direction of deflecting torque remains unchanged. Hence, these instrument suitable for the measurement of dc as well as ac power.

Deflecting Torque

Deflecting torque Td α I1 I2

Deflecting torque, Td α VI1 α load power

Advantages 

  1. High degree of accuracy
  2. Uniform scale

Disadvantages

  1. Errors due to stray field acting on the potential coil.
  2. Error due to inductance of potential coil.
  3. It produce errors due to eddy currents.

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