MODULE 3 MEASUREMENT OF RESISTANCE,...

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  • MODULE 2 MEASUREMENT OF RESISTANCE, POWER,

    ENERGY

    1

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  • Measurement of resistance

    Measurement of low resistance

    (upto 1 ohm)

    Measurement of medium resistance

    (1 to 0.1M )

    Measurement of high resistance

    (greater than 0.1M )

    Measurement of earth resistance

    2

  • Measurement of low resistance

    Measurement of low resistance

    Ammeter Voltmeter method

    Potentiometer method

    Kelvin Double Bridge method

    Ohm meter method

    Series type

    Shunt type

    Crossed coil Ohm meters

    Fixed magnet moving coil Ohmmeter

    Crossed coil moving magnet Ohmmeter

    3

  • Measurement of medium resistance

    Measurement of medium resistance

    Ammeter Voltmeter method

    Substitution method

    Wheatstone bridge method

    Carey Foster Bridge method

    Ohmmeter method

    66

  • Measurement of high resistance

    Measurement of high resistance

    Direct deflection method

    Megger method

    Loss of charge method

    Mega Ohm bridge method

    99

  • Direct deflection method

    108

    Measurement of insulation resistance of a cable

  • Direct deflection method

    Case (a) cables with metal sheath

    Galvanometer G measures the current between core and metal sheath

    Leakage currents over the surface of insulating material are carried by the guard wire wound on the insulation and does not flow through the insulation

    The ration of voltage applied between the core and metal sheath and current flowing between them(galvanometer deflection) gives insulation resistance of the cable

    109

  • Direct deflection method

    Case (b)

    Cable is immersed in water for at least 24hrs, so that it enters pores of the cable

    Initially galvanometer should be shunted, if possible it should be connected in series with a high resistance(MegaOhms)

    Leakage current flows through guard wire

    Ration of voltmeter reading to galvanometer deflection gives the value of insulation resistance

    110

  • Direct deflection method

    Limitations

    Galvanometer should be highly sensitive

    Galvanometer should be prevented from initial inrush of currents

    Battery should be at least 500V and its emf should remain constant

    111

  • Loss of charge method

    124

  • Loss of charge method

    Step 1: Capacitor C is charged by battery- by keeping switch in position 1

    Step 2: Capacitor C is discharged via Rx and Rleak

    125

  • Loss of charge method

    Step 3: Time (t) taken for the potential difference to fall from V1 to V2 is noted during discharge.

    =

    +

    126

  • Loss of charge method

    At the time of discharge:

    =

    =

    =

    127

  • Loss of charge method

    =

    =

    128

  • Loss of charge method

    =

    0

    21

    2

    1=

    2

    1= ( )

    From the above

    expression Reff can be determined

    129

  • Loss of charge method

    The test is then repeated with Rleak only.

    So value of Rleak is found and from the expression of resistance RX that is unknown value of resistance is found.

    130

  • EARTHING AND MEASUREMENT OF EARTH RESISTANCE

    What is meant by earthing?

    How will you measure earth resistance?

    136

  • EARTHING

    The connection of electrical machinery/equipment to a general mass of earth, with a conducting material of low resistance is called earthing or grounding

    The conducting material used is known as earth electrode

    137

  • ADVANTAGES OF USING EARTH ELECTRODE

    All parts of the electrical equipment will be at zero potential.

    Leakage current flows through low resistance path provided by the earth electrode so human protection

    Voltage spikes/ current spikes due to lightning or short circuits or other faults will easily get dissipated to earth.

    138

  • ADVANTAGES OF USING EARTH ELECTRODE

    In the case of three phase system, neutral is earthed which helps to maintain line voltage constant

    For telephone and traction work, earthing acts as return path. So cost of cable and cast of such cable is avoided.

    Earth electrode ensures low resistance path and hence able to carry leakage currents without deterioration.

    139

  • MEASUREMENT OF EARTH RESISTANCE

    Measurement of earth resistance

    Fall of potential method

    Megger earth tester

    140

  • MEASUREMENT OF EARTH RESISTANCE

    Measurement of earth

    resistance

    Fall of potential

    method

    Megger earth tester

    141

  • FALL OF POTENTIAL METHOD

    142

  • FALL OF POTENTIAL METHOD

    Potential E is applied

    Current I circulates through the E and Q

    Voltage between E and P is noted

    143

  • FALL OF POTENTIAL METHOD

    Pattern of current flow through earth:

    144

  • FALL OF POTENTIAL METHOD

    Current diverge from E

    Current converge at Q

    Current density is high near E and Q

    Near electrodes, voltmeter reads high, where as between the electrodes

    145

  • FALL OF POTENTIAL METHOD

    The potential V rises near E and Q due to current density

    In the middle section V remains constant

    146

  • FALL OF POTENTIAL METHOD

    Value of earth resistance is given by:

    =

    147

  • FALL OF POTENTIAL METHOD

    The measurement of VEP is done at various points between E and Q

    The potential variation curve is shown in fig

    Resistance RE is determined when the potential curve is absolutely flat To get accurate reading,

    distance between P and Q should be large

    148

  • FALL OF POTENTIAL METHOD

    Variation of resistance with distance is shown

    149

  • MEASUREMENT OF EARTH RESISTANCE

    Measurement of earth

    resistance

    Fall of potential method

    Megger earth tester

    150

  • MEGGER EARTH TESTER

    151

  • MEGGER EARTH TESTER

    It consists of

    DC generator

    Current reverser

    Rectifier

    Current coil

    Potential coil

    Electrodes E,P and Q

    152

  • MEGGER EARTH TESTER

    Current reverser and rectifier have L type commutators

    These are mounted on the shaft and rotated with handle

    Two brushes of commutator are arranged so that it they make contact alternately with each segment of the commutator

    153

  • MEGGER EARTH TESTER

    Other two brushes of commutator are placed in such away that they always make contact with the commutator

    154

  • MEGGER EARTH TESTER

    Earth tester consists of the terminals P1, C1, P2,C2

    P1 andd C1 is shorted and connected to common point E

    P2 and C2 are connected to auxiliary electrodes P and Q respectively

    155

  • MEGGER EARTH TESTER

    The ratio of voltage sensed by voltage coil and current passing through current coil, directly gives the value of earth resistance RE

    Deflection of the pointer gives RE

    It can be used for dc purposes only, but to measure ac reverser and rectifier is used

    156

  • MEGGER EARTH TESTER

    AC current through soil prevents back emf in the soil due to electrolytic action

    157

  • MEASUREMENT OF POWER

    Measurement of power is done by wattmeters

    Wattmeter is a combination of Ammeter and Voltmeter.

    So it contains current coil and voltage coil(pressure coil)

    158

  • MEASUREMENT OF POWER

    159

    Wattmeters

    Dynamometer type

    Induction type

    Electrostatic type

  • Wattmeter

    160

    Current coil carries load current

    Pressure coil carries current proportional to voltage

    Inductance of pressure coil should be minimum to avoid phase lag between current and voltage

  • MEASUREMENT OF POWER

    161

    Wattmeters

    Dynamometer type

    Induction type

    Electrostatic type

  • Dynamometer type wattmeter

    162

  • Dynamometer type wattmeter

    Fixed coil current flowing is proportional to load current

    Moving coil- current flowing is proportional to load voltage

    163

  • Dynamometer type wattmeter

    Strength of magnetic field- proportional to currents through two coils

    164

  • Dynamometer type wattmeter

    165

    V- supply voltage

    I load current

    R- resistance of moving

    coil circuit

  • Dynamometer type wattmeter

    Fixed coil current: =

    Moving coil current:

    =

    Deflecting torque:

    =

    166

  • Dynamometer type wattmeter

    For dc circuit deflecting torque is proportional to power

    For ac circuit deflecting torque is proportional to voltage, current and power factor

    =

    167

  • Dynamometer type wattmeter

    1

    2

    M- mutual inductance between

    two coils

    Instantaneous torque is given by:

    = 12

    168

  • Dynamometer type wattmeter

    Instantaneous voltage in pressure coil is:

    = 2

    Instantaneous current through pressure coil is:

    =2

    = 2

    - resistance of

    pressure coil

    169

  • Dynamometer type wattmeter

    Current through current coil lags voltage by an angle

    = 2 I sin

    170

  • Dynamometer type wattmeter

    Instantaneous torque is given by:

    171

  • Dynamometer type wattmeter

    Average deflecting torque is given by:

    172

  • Dynamometer type wattmeter

    Control torque is =

    K- spring constant

    - final steady state deflection

    At balance position

    = ()

    173

  • Dynamometer type wattmeter

    At balance condition:

    174

  • Shape of scale of dynamometer wattmeter

    175

  • Shape of scale of dynamometer wattmeter

    Deflection is proportional to power measured

    and scale is uniform since

    is constant.

    Wattmeters are designed such that

    remains over 40 to 50 degree on each side

    of zero mutual inductance position.

    M varies linearly in this zone with respect to

    176

  • Shape of scale of dynamometer wattmeter

    If zero mutual inducatnce position is kept in the middle then M varies linearly for deflection upto 80 to 100 degrees

    177

  • Shape of scale of dynamometer wattmeter

    178

  • Dynamometer type wattmeter

    Ranges:

    Current coil: 0.25A to 100A

    Pressure coil : 5V to 750V

    179

  • Dynamometer type wattmeter

    Dynamometer type wattmeter

    Suspended coil torsion type

    Pivoted coil indicating type

    180

  • Suspended coil torsion type dynamometer wattmeter

    181

  • Suspended coil torsion type dynamometer wattmeter

    The moving, or voltage, coil is suspended from a torsion head by a metallic suspension which serves as a lead to the coil.

    This coil is situated entirely inside the current or fixed coils and the winding in such that the system is a static.

    Errors due to external magnetic fields are thus avoided. The torsion heads carries a scale, and when in use, the

    moving coil is bought back to the zero position by turning this head; the number of divisions turned through when multiplied by a constant for the instrument gives the power.

    Eddy currents are eliminated as far as possible by winding the current coils of standard wire and by using no metal parts within the region of the magnetic field of the instrument.

    182

  • Suspended coil torsion type dynamometer wattmeter

    The mutual inductance errors are completely eliminated by making zero position of the coil such that the angle between the planes of moving coil and fixed coil is 90 degree. i.e. the mutual inductance between the fixed and moving coil is zero.

    The elimination of pivot friction makes possible the construction of extremely sensitive and accurate electrodynamic instruments of this pattern.

    183

  • Pivoted coil indicating type dynamometer wattmeter

    184

  • Pivoted coil indicating type dynamometer wattmeter

    In these instruments, the fixed coil is wound in two halves, which are placed in parallel to another at such a distance, that uniform field is obtained.

    The moving coil is wound of such a size and pivoted centrally so that it does not project outside the field coils at its maximum deflection position.

    The springs are pivoted for controlling the movement of the moving coil, which also serves as currents lead to the moving coil.

    185

  • Pivoted coil indicating type dynamometer wattmeter

    The damping is provided by using the damping vane attached to the moving system and moving in a sector-shaped box.

    The reading is indicated directly by the pointer attached to the moving system and moving over the calibrated scale.

    The eddy current errors, within the region of the magnetic field of the instrument, are minimized by the use of non-metallic parts of high resistivity material.

    186

  • Electrodynamometer type wattmeter

    Advantages:

    1) In dynamometer type wattmeter, the scale of the instrument is uniform (because deflecting torque is proportional to the true power in both DC as well as AC and the instrument is spring controlled.)

    2) High degree of accuracy can be obtained by careful design; hence these are used for calibration purposes.

    187

  • Electrodynamometer type wattmeter

    Disadvantages:

    1) The error due to the inductance of the pressure coil at low power factor is very serious (unless special features are incorporated to reduce its effect)

    2) In dynamometer type wattmeter, stray field may affect the reading of the instrument. To reduce it, magnetic shielding is provided by enclosing the instrument in an iron case.

    188

  • Errors in dynamometer type wattmeter

    Errors

    Due to pressure coil inductance

    Due to pressure coil capacitance

    Due power loss in pressure coil and current coil

    Due to Eddy current

    Due to friction

    Due to temperature

    Due to stray fields

    189

  • Errors in dynamometer type wattmeter

    Errors

    Due to pressure coil inductance

    Due to pressure coil capacitance

    Due power loss in pressure coil and current coil

    Due to Eddy current

    Due to friction

    Due to temperature

    Due to stray fields

    190

  • ENERGY

    261

  • ENERGY

    262

  • ENERGY METER TYPES

    263

    Energy meter

    Single phase

    Three phase

  • SINGLE PHASE ENERGYMETER CONSTRUCTION

    264

  • SINGLE PHASE ENERGYMETER CONSTRUCTION- PARTS

    265

    Driving system

    Moving system

    Braking system and

    Registering system.

  • Driving system

    consists of two electromagnets, called shunt magnet and series magnet, of laminated construction.

    266

  • Driving system

    A coil having large number of turns of fine wire is wound on the middle limb of the shunt magnet.

    This coil is known as pressure or voltage coil and is connected across the supply mains.

    This voltage coil has many turns and is arranged to be as highly inductive as possible.

    In other words, the voltage coil produces a high ratio of inductance to resistance.

    This causes the current, and therefore the flux, to lag the supply voltage by nearly 900.

    267

  • Driving system

    An adjustable copper shading rings are provided on the central limb of the shunt magnet to make the phase angle displacement between magnetic field set up by shunt magnet and supply voltage is approximately 90degrees.

    The copper shading bands are also called the power factor compensator or compensating loop

    268

  • Driving system

    The series electromagnet is energized by a coil, known as current coil which is connected in series with the load so that it carry the load current.

    The flux produced by this magnet is proportional to, and in phase with the load current.

    269

  • Moving system

    The moving system essentially

    consists of a light rotating aluminium disk mounted on a vertical spindle or shaft.

    The shaft that supports the aluminium disk is connected by a gear arrangement to the clock mechanism on the front of the meter to provide information that consumed energy by the load

    270

  • Moving system

    The time varying (sinusoidal) fluxes produced by shunt and series magnet induce eddy currents in the aluminium disc.

    The interaction between these two magnetic fields and eddy currents set up a driving torque in the disc.

    The number of rotations of the disk is therefore proportional to the energy consumed by the load in a certain time interval and is commonly measured in killowatt-hours (Kwh).

    271

  • Braking system

    Damping of the disk is provided by a small permanent magnet, located diametrically opposite to the a.c magnets.

    The disk passes between the magnet gaps.

    272

  • Braking system

    The movement of rotating

    disc through the magnetic field crossing the air gap sets up eddy currents in the disc that reacts with the magnetic field and exerts a braking torque.

    By changing the position of the brake magnet or diverting some of the flux there form, the speed of the rotating disc can be controlled.

    273

  • Registering or Counting system

    The registering or counting system essentially consists of gear train, driven either by worm or pinion gear on the disc shaft, which turns pointers that indicate on dials the number of times the disc has turned.

    274

  • Registering or Counting system

    The energy meter thus determines and adds together or integrates all the instantaneous power values so that total energy used over a period is thus known.

    Therefore, this type of meter is also called an integrating meter

    275

  • Working/operation of single phase energy meter

    276

  • Working/operation of single phase energy meter

    Induction instruments operate in alternating-current circuits and they are useful only when the frequency and the supply voltage are approximately constant

    The rotating element is an aluminium disc, and the torque is produced by the interaction of eddy currents generated in the disc with the imposed magnetic fields that are produced by the voltage and current coils of the energy meter.

    277

  • Working/operation of single phase energy meter

    Let us consider a sinusoidal flux (t) is acting perpendicularly to the plane of the aluminium disc, the direction of eddy current (Ie) by Lenzs law is indicated in figure

    278

  • Working/operation of single phase energy meter

    279

    =0 since reactance of Al dsc is zero

  • Working/operation of single phase energy meter

    So in all induction type meters, two eddy currents are there so that resultant torque will be there

    280

  • Working/operation of single phase energy meter

    281

  • Working/operation of single phase energy meter

    Current coil produces two fluxes in opposite directions

    So torques produced by the interaction of eddy current due to voltage and current coil is opposite at two points.

    Hence the disc will start to rotate

    282

  • Derivation of Torque equation

    Phasor Diagram

    283

  • Derivation of Torque equation

    284

  • Derivation of Torque equation

    285

  • Derivation of Torque equation

    286

  • Derivation of Torque equation

    287

  • ENERGYMETER CONSTANT

    288

  • SOURCES OF ERRORS IN SINGLE PHASE ENERGYMETER

    Incorrect magnitude of fluxes due to abnormal voltages and load currents

    Incorrect phase relation of fluxes due to defective lagging, abnormal frequencies, changes in iron loss

    Unsymmetrical magnetic structure disc rotates when pressure coils alone is excited

    289

  • SOURCES OF ERRORS IN SINGLE PHASE ENERGYMETER

    Changes in resistance of disc due to change in temperature

    Changes in strength of drag magnets due to temperature and ageing

    Phase angle errors due to lowering of power factor

    Abnormal deflection of moving parts

    Badly distorted waveform

    Changes in retarding torque of the disc

    290

  • ERRORS IN SINGLE PHASE ENERGY METER

    Erro

    rs

    Phase error

    Frictional Error

    Creeping

    Speed Error

    Temperature error

    Overload compensation

    Voltage compensation

    291

  • Phase error

    An error due to incorrect adjustment of the position of shading band results an incorrect phase displacement between the magnetic flux and the supply voltage (not in quadrature)

    This is tested with 0.5 p.f. load at the rated load condition

    292

  • Compensation for phase error

    By adjusting the position of the copper shading band in the central limb of the shunt magnet this error can be eliminated.

    293

  • Frictional Error

    Frictional forces at bearings and registering mechanism give rise to unwanted braking torque on the disc

    So additional driving torque is required

    295

  • Creeping Error

    In some meters a slow but continuous rotation is seen when pressure coil is excited but with no load current flowing.

    This slow revolution records some energy.

    This is called the creep error.

    This slow motion may be due to

    (a) incorrect friction compensation,

    (b) stray magnetic field

    (c) for over voltage across the voltage coil.

    297

  • Compensation for creeping error

    This can be eliminated by drilling two holes or slots in the disc on opposite side of the spindle.

    When one of the holes comes under the poles of shunt magnet, the rotation being thus limited to a maximum of 180 degrees

    298

  • Compensation for creeping error

    In some cases, a small piece of iron tongue or vane is fitted to the edge of the disc.

    When the position of the vane is adjacent to the brake magnet, the attractive force between the iron tongue or vane and brake magnet is just sufficient to stop slow motion of the disc with full shunt excitation and under no load condition.

    299

  • Speed Error

    Due to the incorrect position of the brake magnet, the braking torque is not correctly developed.

    This can be tested when meter runs at its full load current alternatively on loads of unity power factor and a low lagging power factor

    300

  • Temperature Error

    Energy meters are almost inherently free from errors due to temperature variations.

    Temperature affects both driving and braking torques equally (with the increase in temperature the resistance of the induced-current path in the disc is also increases) and so produces negligible error.

    A flux level in the brake magnet decreases with increase in temperature and introduces a small error in the meter readings

    302

  • Voltage compensation

    When supply voltage varies, energymeter causes errors

    This is due to:

    Non linear magnetic characteristics of shunt magnet core

    Braking torque is proportional to square of supply voltage

    307

  • ADVANTAGES OF INDUCTION TYPE ENERGYMETER

    309

  • DISADVANTAGES OF INDUCTION TYPE ENERGYMETER

    310

  • THREE PHASE ENERGYMETER

    Three phase system

    Four wire Three element energy meter

    Three wire Two element energy meter

    311

  • THREE PHASE THREE ELEMENT ENERGYMETER

    312

  • THREE PHASE THREE ELEMENT ENERGYMETER

    It consists of three elements

    Each element is similar to that of single phase energy meter

    Pressure coils are P1,P2 and P3

    Current coils are C1, C2 and C3

    All elements are mounted in a vertical line in common case and have a common spindle, gearing and recording mechanism

    313

  • THREE PHASE THREE ELEMENT ENERGYMETER

    The coils are connected in such a manner that the net torque produced is equal to sum of torques produced by each element

    These are employed for three phase four wire system, where fourth wire is the neutral wire

    Current coils are connected in series with the lines where as pressure coils are connected in parallel across line and neutral

    314

  • THREE PHASE THREE ELEMENT ENERGYMETER

    One unit of three phase energy meter is cheaper than three individual units

    Due to interaction of eddy currents between all elements, errors are produced which are reduced by suitable adjustments

    315

  • THREE PHASE TWO ELEMENT ENERGYMETER

    316

  • THREE PHASE TWO ELEMENT ENERGYMETER

    It is provided with two discs for an element

    Shunt magnet is carrying pressure coil

    Series magnet is carrying current coil

    Pressure coils are connected in parallel

    Current coils are connected in series

    Torque is produced in same manner as that of single phase energy meter

    The total torque on registering mechanism is sum of torques on two discs

    317

  • ELECTRONIC ENERGY METER

    318

  • ELECTRONIC ENERGY METER

    Average power = mean product of instantaneous voltage across the load and instantaneous current through load

    Potential divider- for making voltage to required level

    Voltage is scaled in the required range using voltage scaling device

    Current scaling device scales load voltage which is proportional to load current

    319

  • ELECTRONIC ENERGY METER

    Both scaled voltages are connected to voltage and current multiplier unit

    Voltage and current multiplier unit outputs current as a result of product of ac voltage and current

    The current is proportional to instantaneous power applied to voltage controlled oscillator

    VCO works on the principle of constant current charging capacitor

    320

  • ELECTRONIC ENERGY METER

    VCO basically voltage to frequency converter Output of VCO is square wave The frequency of square wave is proportional to

    output current of VCO So power dependent current and frequency

    dependent current decides the value of consumed energy

    ADC (analog to digital converter) converts analog signal to digital signal

    Display unit displays energy in watt-hour

    321

  • ADVANTAGES OF ELECTRONIC ENERGYMETER

    High sensitivity

    No frictional losses

    Less loading effect

    Low load, full load, creeping adjustments are not required

    High frequency range

    High accuracy of 1%

    322

  • TOD METER

    Electric company supplies electricity to various loads such as domestic, industrial and commercial purposes

    These loads varies over various time periods

    For some time load is maximum and for sometime load is minimum

    The hours in which load is maximum is called peak load hours

    330

  • TOD METER

    The hours in which load is minimum is called off-peak hours

    During on-peak hours company has to generate more power to supply for the demand.

    This causes difficulties

    331

  • TOD METER

    Time of delay rate is special service offered by electric company that allows consumer to take advantage of lower electricity price during a certain time period

    Consumer can save money being on TOD rate

    To lessen the load during a particular time of a day, company offers special rate to the consumers who are willing shift the load or portion of the load to off-peak hours

    332

  • TOD METER

    A special metering arrangement is done to measure energy consumption during different time zones of the day including on-paek and off-peak hours

    Trivector meter itself is provided with capability required

    It is provided with a time of delay registser (TOD register) which is capable of being progarammed during off-peak and on-peak hours

    333

  • TOD METER

    TOD meters are time- of delay meter which is suitable of recording and indicating consumption during specific time periods of the day

    Trivector meter with such an arrangement is called TOD meter

    334

  • TOD METER

    335