EmI Unit-2.pptx

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1. Prepared by V.Dharani POTENTIOMETERS AND INSTRUMENT TRANSFORMERS UNIT-2

2. What is Potentiometer?           A potentiometer is an  instrument  used to measure the  potential  in a circuit.A   potentiometer  is an instrument designed to measure an unknown  voltage  by comparing it with a known voltage.             The known voltage may be supplied by a standard cell or any other known voltage reference source. Measurements using comparison methods are capable of a high degree of accuracy because the result obtained does not depend upon the actual deflection of a pointer, as is the case in deflectional methods, but only upon the accuracy with which the  voltage  of the reference source is known.

3.  Another advantage of the  potentiometers  is that since a potentiometer makes use of a balance or null condition, no current flows and hence no power is consumed in the circuit containing the unknown emf when the  instrument  is balanced. Thus the determination of voltage by a potentiometer is quite independent of the source resistance.           Since a potentiometer measures voltage, it can also be used to determine current simply by measuring the  voltage  drop produced by the unknown current piling through a known standard resistance.The   potentiometer  is extensively used for a calibration of voltmeters and ammeters and has, in fact, become the standard for the calibration of these  instruments .For the above mentioned advantages, the potentiometer has become very important in the field of  electrical measurements  and calibration.

4. Construction of DC Potentiometer: The  working principle of potentiometers  is based on the figure shown below, which shows the  circuit diagram of the basic slide wire potentiometer

5. Working Principle of basic dc Potentiometer With switch 'S' in the "operate" position and the galvanometer key K open, the battery supplies the "working current" through the rheostat R and the  slide wire. The   working  current through the slide wire may be varied by changing the rheostat setting.The method of measuring the unknown voltage, E, depends upon finding a position for the sliding contact such the galvanometer shows zero deflection, i.e., indicates a null condition, when the galvanometer key, K. is closed.Let us now discuss the  working principle of basic dc potentiometer . 

6. Zero galvanometer deflection or a null means that the unknown  voltage , E, is equal to the voltage drop E1, across portion ac of the slide wire. Thus the determination of the value of unknown voltage now becomes a matter of evaluating the voltage drop E1 along the portion ac of the slide wire.The slide wire has a uniform cross-section and hence uniform resistance along its entire length.A calibrated scale in cm and fractions of cm is placed along the slide wire so that the sliding contact can be placed accurately at any desired position along the slide wire.                 Since the resistance of  slide wire  is known accurately, the voltage drop along the slide wire can be controlled by adjusting the value of working current in the  basic dc potentiometer .The process of adjusting the working current so as to match the  voltage  drop across a portion of sliding wire against a standard reference source is known as " Standardisation ". 

7.  It is very important that internal thermoelectric EMFs in a  potentiometer  are minimum.The use of manganin resistors helps in this direction.It is desirable that all the parts work at the same temperature.Therefore , all the parts are covered in a single case.This has the added advantage of protecting the contacts from fumes and dust which may cause corrosion and appearance of voltaic EMFs at the joints.         Potentiometers  designed especially for thermocouple measurements have copper terminals.In order to prevent leakage, all the parts must be enclosed, so as to protect them from moisture.The   working  parts are normally mounted on ebonite or Keramot panels.

8. Advantages A potentiometer is highly sensitive It is a highly accurate instrument because it uses the comparing method for measurements, where the voltage of a reference source is known It has a wide range of measurement Disadvantages Its operation is very time consuming

9. Applications of dc potentiometer Calibration of voltmeter Calibration of voltmeter requires a suitable stable DC supply of voltage. If there are little change occurs in the supply voltage, it can affect the calibration process of the voltmeter. Arrangement for calibration of a voltmeter by potentiometer is shown below:

10. Calibration of voltmeter

11. The network consists of a potential divider network consisting of 2 rheostats. One is for coarse adjustments and the other is for fine adjustments. Both the rheostats are connected to the stabilized supply voltage. With the help of these rheostats, it is possible to adjust the supply voltage so that the pointer coincides exactly with a major division of the voltmeter. The voltage across the voltmeter is stepped down to a suitable value with the help of a voltage-ratio box. For accuracy of measurements, it is required to measure voltage near the maximum range of the potentiometer. And if the potentiometer reading does not match with the reading of the voltmeter. A positive or a negative error is indicated.

12. Calibration of ammeter A standard resistor S with a high current carrying capacity is connected in series with an ammeter for a test. The voltage drop across the standard resistor is measured by the potentiometer. Circuit diagram for calibration of an ammeter by potentiometer is shown below:

13. Calibration of ammeter

14. Calibration of ammeter Now the current through the resistor S can be computed I = V s /S Where, V s  = voltage drop across the resistor S          S = resistance of the resistor Now, by comparing the ammeter reading with the current found by calculation, a positive or negative error can be indicated if they do not match. This method of calibration is very accurate because the resistance of the resistor S is exactly known and the current across the S is calculated.

15. Calibration of wattmeter A standard resistor is connected in series with a current coil of wattmeter. The current coil is supplied with a low voltage current and the current through the current coil is measured by measuring the voltage drop across the standard resistor divided by the value of the standard resistor. The potential coil of the wattmeter is supplied from normal supply through the potential divider. The voltage across the potential coil is measured directly by the potentiometer. Then the power is calculated by P = VI Where, V = voltage across the potential coil I = current through the current coil of the wattmeter Now, the wattmeter reading can be compared with the calculated value.

16. Measurement of current The unknown current I,  whose value is to be measured is passed through a resistor R. The value of the resistor is such that voltage drop across it may not exceed the range of the potentiometer. The circuit diagram of current measurement by potentiometer is shown below: So, the value of the unknown current is the voltage drop across the resistor divided by the value of the resistor. I = V/R

17. Measurement of current

18. Measurement of resistance An unknown resistance is connected in series with a standard resistance S. A rheostat controls the current in the circuit. A two-pole double throw switch is also used in the circuit. The circuit is shown below:

19. Measurement of resistance

20. Measurement of resistance When the two poles double throw switch is put in position 1, the unknown resistance is connected to the potentiometer. Let the reading of the potentiometer be V R V R  = IR                              ( i ) Now the switch is put in position 2, this connects the standard resistor S to the potentiometer. Let the reading of potentiometer be Vs V s  = IS                             (ii) From i and ii V R /V s  = IR/IS R = (V R /V s )*S The value of R is calculated accurately.

21. Measurement of power In the measurement of power, 2 measurements are made. One is across the resistor S connected in series with the load and the other is across the output terminals of voltage—ratio box. The load current is calculated from the voltage drop across the standard resistor

22. Measurement of power

23. Measurement of power The voltage drop across the load is calculated by the potentiometer reading across the output terminal of the voltage-ratio box Load current I = V S /S Where V S  is the voltage drop across the standard resistor The voltage drop across the load Vl = kV R Where, k = multiplying factor of the voltage-ratio box. So, the power consumed, P = V L I P = K*V R *(V S /S)

24. AC POTENTIOMETER The potentiometer is an instrument that is used for the measurement of potential differences across a known resistance between two terminals of a circuit. Potentiometers are of two types DC potentiometer and AC potentiometer. The working principle of both the potentiometer is the same except for one difference

25. What is AC potentiometer? Alternating current (AC) potentiometer is the potentiometer in which the magnitude and the phase angle of unknown emf are to be compared with the known emf to obtain balance. The working principle of the AC potentiometer is the same as the DC potentiometer. But there is a difference between both the potentiometer, that is in the  DC potentiometer  only the magnitude of unknown voltage is compared with the known. On the other side, in the AC potentiometer, the magnitude and the phase angle of unknown emf are to be compared.

26. Thus a DC potentiometer can’t be used for AC measurements. So, some modifications and additions have to be made for ac measurements. The following points must be considered The slide wire and the resistance coil of an ac potentiometer should be non-inductive, this is to be done to avoid errors in reading. The AC supply source should be free from harmonics, because balance may not be achieved in presence of harmonics. The reading is affected by the external magnetic field, so they must be eliminated in the time of measurements. The AC supply source should be sinusoidal.

27. Types of ac potentiometer Polar potentiometer In this type of potentiometer, the unknown emf is measured in polar form. This means that the unknown emf is measured in terms of its magnitude and its relative phase. The magnitude is measured by one scale and the phase is indicated by another scale. There is provision for reading phase angle up to 360 degrees Coordinate potentiometer In this type, the unknown emf is measured in cartesian form. It has two different scales to read the in-phase V1 and the other is quadrature V2. There is provision is made in this potentiometer to read both positive and negative values of voltages and cover all angles up to 360degree

28. What is a polar potentiometer? A polar potentiometer measures the unknown voltage in the polar form. Polar form’ refers to the indication of unknown voltage in magnitude and the relative phase of the quantity. One scale of the potentiometer indicates the magnitude while another scale indicates the phase w.r.t . some reference axis. Drysdale polar potentiometer is an example of a polar potentiometer.

29. Drysdale polar potentiometer n the Drysdale polar potentiometer, the phase-shifting transformer indicates the phase of unknown voltage, and the position of the slide wire indicates the magnitude. A Diagram of the Drysdale polar potentiometer is shown below:

30. Drysdale polar potentiometer

31. Construction Ammeter is an electrodynamometer-type ammeter because it works both on AC and DC. D’arsonval galvanometer is used for standardization of the potentiometer, and later during the measurement of the unknown voltage vibration galvanometer is used. The slidewire should be non-inductive. The polar potentiometer also consists of 2 stator windings and rotor winding

32. Working Drysdale potentiometer is  an AC potentiometer  i.e. it uses AC supply for the measurements. First, the potentiometer should be calibrated or standardized. The standardization of polar type ac potentiometer is done by DC supply and a standard cell. For standardization, a standard cell is connected with slide wires through the d’arsonval galvanometer. The slide wire is fixed at the same voltage as the standard cell (1.0186V). On the other side, an ammeter is connected with a rheostat through a dc supply. Now the rheostat should be adjusted in such a way that the galvanometer indicates zero reading. At this point, the value of standard current at ammeter should be noted. The phase shifter circuit remains constant for unknown supplied voltage while the phase can be varied through 360°.

33. The phase shifter consists of 2 stators and rotor winding. Some air gap is present in between the windings. 90° phase shift is present between both stator windings. If necessary, the capacitor and the resistor should be adjusted to keep the 90° phase shift between both stator windings. These are being adjusted until the ammeter shows an exact similar current value which it shows during standardization for all positions of the rotor. These adjustments of slide wire and the rotor helps in achieving a balance of the potentiometer. After standardizing the potentiometer and tuning the phase shifter, the unknown voltage should be measured. The unknown voltage is measured by adjusting the slide wire and the rotor. By adjusting the slide wire, the vibration galvanometer indicates the zero reading which gives the magnitude of the unknown voltage. And slight adjustment in the rotor dial gives the phase of the unknown voltage.

34. Polar type AC Potentiometer - Construction & Working The Drysdale Tinsley AC Potentiometer is a polar-type potentiometer, which measures the magnitude (V) in one scale and phase (θ) in another scale. The complete connection diagram of the Drysdale Tinsley ac potentiometer is shown below. Where,T.I . - Transfer instrument (precision type electro-dynamometer ammeter) DPDT - Double pole double throw SPDT - Single pole double throw G - D'Arsonval galvanometer VG - Vibration galvanometer B - Standard battery POS 1 - position 1 POS 2 - position 2

35. When an ac voltage measurement is done by taking a reference ac voltage supply, the conditions that must be satisfied are,Both the voltages should have same frequency. Their phases should be same. Their magnitudes should also be same at all the instants. It is very difficult to satisfy all three conditions if we use a separate reference source. Hence, in this instrument we connect the unknown ac voltage to a phase-shifting transformer whose one stator winding is connected directly to the unknown supply and the other stator winding is connected to the same supply through a variable resistor and a capacitor. By varying the resistance and capacitance of the second winding, the current through it can be made exactly in quadrature with the supply. This results in the production of a rotating magnetic field (RMF) (i.e., due to phase splitting) which links with the rotor winding to induce an emf in it with the same frequency as that of supply and whose phase angle can be selected by changing the rotor position. Hence, the phase angle of the unknown voltage can be measured against this reference rotor position.

36. For the measurement of magnitude with a normal dc potentiometer, all the resistors and the slide wire are replaced by standard non-inductive resistors and slide wire. So that its resistance does not vary with frequency and waveform. Procedure for the Measurement : To measure an unknown ac voltage using this potentiometer, first, the meter is standardized. For the standardization, all the three DPDT switches are thrown to position 1 (POS 1), and the current through ammeter (A) for which the D Arsonval galvanometer (G) gives null deflection is noted. Now, the DPDT switches are thrown to position 2 (POS 2) which connects the rotor terminals of phase-shifting transformer to supply terminals of the potentiometer, vibration galvanometer to detector terminals, and the unknown ac voltage to potentiometer test terminals. Now, the current through the ammeter is made equal to the current through it when dc supply was connected by varying the standard resistor R and the balance is obtained in the vibration galvanometer by changing the slide wire contact position and the phase shifter's rotor position. Hence, the magnitude and phase of the unknown ac voltage are obtained from the slide wire position and rotor position readings respectively.

37. Functions of Transfer Instrument and Phase Shifting Transformer : The function of the phase-shifting transformer is, To produce the rotating magnetic field which passes through the air gap between its stator and rotor and induces an emf in the rotor winding. To provide the required phase shifting of the rotor induced emf by adjusting the rotor position. The rotor position can be adjusted by adjusting the rotor angle with respect to the null pointer. Now, the induced emf in the rotor windings due to two stator windings is given by, E 1  = KI sinωt cosθ E 2  = KI sin( ωt + 90) cos (θ + 90)

38. Therefore, the resultant emf is given by, E = E 1  + E 2 E = KI [ sinωt cosθ + sin( ωt + 90) cos (θ + 90)] We know that sin( ωt + 90) = cosωt and cos (θ + 90) = - sinθ . ∴ E = KI sinωt ( ωt - θ) From the above, it is clear that the rotor emf has constant amplitude and the phase angle is given by the rotor deflection θ.

39. Gall-Tinsley Coordinate Type Potentiometer - Construction & Working A coordinate type potentiometer is a combination of two potentiometers. One of the potentiometers carries a current in-phase with the supply voltage and it is called an 'In-phase Potentiometer'. The other potentiometer carries the current in quadrature with supply voltage and it is called a ' Quadrature Potentiometer'. Gall-Tinsley Coordinate Type Potentiometer : The connection diagram of this potentiometer is shown in the below figure. T 1  and T 2  are the two step-down transformers fed from a single-phase supply. The supply to T 2  is obtained through the series combination of variable capacitor C s  and variable resistor R s  for splitting the phase. The exact quadrature in phase is obtained by adjusting R s  and C s . Here, ab and cd are sliding contacts of In phase and Quadrature potentiometer respectively, and rheostats R 1  and R 2  are used for current adjustments.

40. COORDINATE AC POTENTIOMETER

41. VG is a vibrational galvanometer tuned to the supply frequency. The ammeter A (reflecting electro-dynamometer type) ensures current in both In-phase and Quadrature Potentiometers slide wire at standard value. Similarly, the reversing switches RS 1 and RS 2 of two potentiometers are used to reverse the direction of the unknown emf across its slide wires. S 2  is a selector switch for placing unknown voltages to be measured in the circuit. The In-phase potentiometer measures the component of unknown voltage which is in phase with its slide wire current. Let its value be V 1  and the component which is in phase with the Quadrature potentiometer current is measured on it and it is the quadrature component of unknown voltage. Let its value be V 2 .

42. Then the magnitude of the unknown voltage and phase angle with respect to supply voltage is given by,

43. Standardization of Potentiometer :

44. Standardization of Potentiometer : The dc standardization of the in-phase potentiometer is done by connecting the battery B through the switch S 1  and changing the multiple circuit switch S 2  to position 1-1. The vibrational galvanometer is replaced by a galvanometer for this purpose. The electro-dynamometer ammeter is tuned to zero position on direct current and this setting is left untouched. The switches S 1  and S 2  are again brought back to the initial position. The alternating current is adjusted in the in-phase potentiometer by rheostat R 1  to give zero deflection of the milli -ammeter. The magnitude and phase of the quadrature potentiometer's current are adjusted by the mutual inductor M.

45. The switch S 2  is brought to position 3-3. The dial settings of the in-phase potentiometer are done to read a value of M i (∵ i is the primary current, emf induced in the secondary winding = 2π f Mi). Where i is the standard alternating current in the in-phase potentiometer. The magnitude and phase of the current in the quadrature potentiometer are adjusted through rheostat R 2  and variable resistance R s  of the phase splitting device to obtain the exact balance which is indicated by the vibration galvanometer. The switch S 2  is again brought to position 2-2. In this position, two slide circuits and a vibration galvanometer are in series with the unknown voltage. Now, the potentiometer is ready to measure the two components of unknown voltage. The balance is obtained by adjusting the settings of sliding contact a and c together with the reversing switches RS 1 and RS 2 if necessary.

46. Applications of AC Potentiometer The AC potentiometer has numerous applications. The few of them are explained below in details. 1. Voltmeter Calibration  – The AC potentiometer directly measures the low voltages up to 1.5V. The higher voltage is measured by either using the volt box ratio or two capacitors in series with the potentiometer. The circuit diagram for the calibration of the voltmeter is shown below. It consists of a stabilized ac supply, rheostats, voltmeter, voltage ratio box, and potentiometer. The basic and important requirement of the circuit is that the input ac supply must not have any fluctuations. Any fluctuation in the supply will have a corresponding change in the calibration of the voltmeter.

47. Hence, to avoid this a stabilized ac supply must be used. Two rheostats Rh 1  and Rh 2  are used to have a very precise control so that the voltmeter accurately coincides with the major divisions. A voltage-ratio box is used to reduce the voltage across the voltmeter and applied to the potentiometer.

48. The potentiometer reads the true value of the voltage. If this value is matched with the voltmeter readings, then the error is zero. However, if these two readings do not match, an error is encountered. This error may be positive or negative depending on the relative value of the voltmeter and the potentiometer. In order to have greater accuracy, the voltage should be measured near the maximum range of the potentiometer. A voltage ratio box or two capacitances in series are used along with an ac potentiometer if the voltage to be measured is of medium or high magnitude. Otherwise, an ac potentiometer can be used directly for voltages under 1.5 V.

49. 2. Ammeter Calibration  – The measurement of the alternating current may be measured by the use of non-inductive standard resistor with the potentiometer. The connection diagram for the calibration of the ammeter is shown in the figure below. The circuit consists of a stabilized ac supply, variable resistor, ammeter, a standard resistor, and a potentiometer. The ammeter is connected in series with the standard, non-inductive resistance of known value R. The potentiometer is connected across the standard resistor. Let V R  be the potentiometer reading which corresponds to the voltage across R. Then the current through R is given by, I R  = V R /R. Let I be the ammeter reading. The two values i.e., I R , I will be the same since the ammeter and standard resistor are in series. Since the value of standard resistor is accurately known, accurate calibration of ammeter can be obtained by this method.

50. Calibration of Ammeter

51. 3. Wattmeter and Energy Meter Testing  – The testing circuit of the Wattmeter and the energy meter is same as that of the DC measurements. The phase shifting transformer is connected to the potentiometer to vary the phase of the voltage on the current. Thus, the voltage and current may vary at different power factor. The connection diagram for the calibration of the wattmeter using a polar-type ac potentiometer is shown below.

52. Calibration of Wattmeter

53. The current coil of the wattmeter is connected in series with a standard resistor, a variable resistor (or rheostat), an ammeter, and a mutual inductance and it is energized with the supply through a step-down transformer. The pressure coil is energized through the secondary of a variable transformer whose primary winding is connected to the rotor terminals. The current through the current coil and the voltage across the pressure coil is varied using a variable transformer. The purpose of voltmeter (V) and ammeter (A) is not to measure the voltage (V) and current (I) but only to ensure that the voltage across and current through the wattmeter are within its range. The voltage across R is measured with the potentiometer and the current (I) through CC is calculated as I = V/R. The voltage across the pressure coil (V) is measured by the potentiometer with the help of a volt ratio box. The phase angle φ between the voltage and current can be varied by changing the position of the rotor. Hence, the power to be measured by wattmeter is VI cos  φ. The value obtained by the potentiometer is compared with the deflection of the wattmeter and the wattmeter is calibrated for different combinations of V, I, and φ. The mutual inductance M in the current coil circuit is to check the accuracy of wattmeter when φ = 90° i.e., at zero power factor.

54. 4. Measurements of Self Reactant of a Coil  – The standard reactance is placed in series with the coil whose reactance is to be measured.

55. INSTRUMENT TRANSFORMER What is an Instrument Transformer : Instrument Transformers  are a type of transformer used in an AC system to measure electrical quantities as voltage, current, power, energy, power factor, frequency. Instrument transformers are also equipped with protective relays to protect the power system. Instrument transformers  have the basic function of reducing the AC System voltage and current. The current and voltage level of the power system is relatively high. It is very difficult and costly to design the measuring instruments to measure such high-level current and voltage. Generally, measuring instruments are designed for 110 V and 5 A The measurement of such very large electrical quantities can be made possible by using the Instrument transformers equipped with these small rating measuring instruments. Thus, these instrument transformers are very well-known in modern power systems.

56. Advantages of Instrument Transformers A small rating measuring instrument can be used to measure the large current and voltage of the AC Power system i.e., 5 A, 110 – 120 V. Measuring instruments can be standardized by using instrument transformers. Which results in the reduction in the cost of measuring instruments. If the measuring instruments are damaged, they can be replaced easily by healthy standardized measuring instruments. Instrument transformers provide electrical isolation between measuring instruments and high voltage power circuits, which reduces the electrical insulation requirement for protective circuits and measuring instruments and also assures the safety of operators. Several measuring instruments can be linked through a single transformer to a power system. Due to the low current and voltage levels in measuring and protective circuits, there is low power consumption in measuring and protective circuits.

57. Types of Instrument Transformers There are 2 types of instrument transformers as follows: Current Transformer (C.T.) Potential Transformer (P.T.) Current Transformer (C.T.) A current transformer  is a type of transformer used to reduce the current of a power system to a lower level to make it feasible to be measured by a small rating Ammeter (i.e. 5A ammeter). A typical connection diagram of a current transformer is shown as follows. Primary of C.T. has very few turns. bar primary is sometimes also used. Primary is linked in series with the power circuit. Therefore, sometimes it is also called a  series transformer . The secondary is having a large no. of turns. The secondary is linked directly to an ammeter. As the ammeter is having very small resistance. Hence, the secondary current transformer works almost in short-circuited conditions. One terminal of the secondary is earthed to avoid the large voltage on the secondary with respect to the earth. Which in turn reduces the chances of insulation breakdown and also protects the operator against high voltage. Furthermore, before disconnecting the ammeter, the secondary is short-circuited through a switch ‘S’ as shown in the above figure to avoid the high voltage build-up across the secondary.

58. Current Transformer

59. Potential Transformer (P.T.) The potential transformer is a type of transformer used to reduce the voltage of the power system to a lower level to make it feasible to be measured by a small rating voltmeter i.e. 110 – 120 V voltmeter. A typical connection diagram of a potential transformer is showing in the following figure. Primary of P.T. has large no. of turns. Primary is linked across the line (generally between on earth and line). Therefore, it is sometimes also called the  parallel transformer . Secondary of P.T. has few turns and is linked directly to a voltmeter. As the voltmeter has a large resistance. Thus, the secondary of a P.T. works almost in open-circuited condition. One earthed terminal of secondary of P.T. is to maintain the secondary voltage with respect to earth.

60. What are the differences between C.T. and P.T? Sl. No. Current Transformer (C.T.) Potential Transformer (P.T.) 1 linked in series with a power circuit. linked in Parallel with the Power circuit. 2 The secondary is linked to Ammeter. The secondary is linked to Voltmeter. 3 Secondary operates almost in short-circuited conditions. Secondary operates almost in open-circuited conditions. 4 The primary current is subject to on power circuit current. The primary current is subject to secondary burden. 5 Primary current and excitation vary over a wide range with a change of power circuit current Primary current and excitation variations are restricted to a small range. 6 One terminal of the secondary is earthed to avoid the insulation break down. One terminal of secondary can be earthed for safety. 7 The secondary is never open-circuited. Secondary can be used in open circuit conditions.

61. Current Transformer (CT) – Construction and Working Principle A current transformer (CT) is a type of transformer that is used to measure AC current. It produces an alternating current (AC) in its secondary which is proportional to the AC current in its primary. Current transformers, along with voltage or potential transformers are Instrument transformer. Current transformers are designed to provide a scaled-down replica of the current in the HV line and isolate the measuring instruments, meters, relays, etc., from the high voltage power circuit.

62. The large alternating currents which can not be sensed or passed through the normal ammeter, and current coils of  wattmeters , energy meters can easily be measured by use of current transformers along with normal low range instruments.

63. Current Transformer Symbol / Circuit Diagram

64. A  current transformer  (CT) basically has a primary coil of one or more turns of heavy cross-sectional area. In some, the bar carrying high current may act as a primary. This is connected in series with the line carrying high current. The secondary of the current transformer is made up of a large number of turns of fine wire having a small cross-sectional area. This is usually rated for 5A. This is connected to the coil of normal range ammeter.

65. Working Principle of Current Transformer These transformers are basically step-up transformers i.e. stepping up a voltage from primary to secondary. Thus the current reduces from primary to secondary.  So from the current point of view, these step down transformer, stepping down the current value considerably from primary to secondary.  Let, N 1  = Number of Primary Turns N 2  = Number of Secondary Turns I 1  = Primary Current I 2  = Secondary Current For a transformer, I 1 /I 2   = N 2 /N 1 As N 2  is very high compared to N 1 , the ratio I 1  to I 2  is also very high for current transformers. Such a current ratio is indicated for representing the range of the current transformer.  For example, consider a 500:5 range then it indicates that C.T. steps down the current from primary to secondary by a ratio 500 to 5.  I 1 /I 2   = 500/5 Knowing this current ratio and the meter reading on the secondary, the actual high line current flowing through the primary can be obtained. 

66. Types of Current Transformer On the basis of their applications in the field, current transformers can be broadly classified into two types, Indoor current transformers Outdoor current transformers Indoor Current Transformers Current transformers designed for mounting inside metal cubicles are known as Indoor Current Transformers. Depending upon the method of insulation, these can further be classified as: Tape insulated Cast resin (epoxy, polyurethane or polycrete )

67. In terms of constructional aspects, Indoor Current Transformers can be further classified into the following types: Bar Type CT : The CTs having a bar of suitable size and material used as primary winding are known as bar-type CT s’. The bar may be of rectangular or circular cross-section. Slot/ Window/ Ring Type CT : CTs having an opening in the center to accommodate a primary conductor through it is known as ‘ring-type’ (or ’slot/ window type’) CT. Wound Type CT : A CT having a primary winding of more than one full turn wound on the core is known as wound type CT. The connecting primary terminals may be similar to those of a bar type CT or rectangular pads can be provided for this purpose.

68. Outdoor Current Transformer These current transformers are designed for outdoor application. They use  transformer oil  or any other suitable liquid for insulation and cooling. A liquid-immersed CT which is sealed and does not communicate with the atmosphere is known as a hermetically sealed CT. Outdoor oil-filled CTs are further classified as live tank type CT dead tank type CT Most of the outdoor current transformers are high voltage current transformers. Based on the application they are further classified into : Measurement Current Transformer Protection Current Transformer

69. Construction of Current Transformer As we discussed above, there are three types of constructions used for the indoor current transformers which are, Wound Type CT Toroidal (Window) Type CT Bar Type CT Wound Type Current Transformer  – The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turn’s ratio of the transformer.  Toroidal (Window) Type Current Transformer  – These do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the toroidal transformer. Some current transformers have a “split core” which allows it to be opened, installed, and closed, without disconnecting the circuit to which they are attached. Bar-type Current Transformer  – This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current-carrying device.

70. Errors in Current Transformer : Before learning about errors in the current transformer. Let us see the current and turn ratio of a current transformer. Turn Ratio (n) :      For a current transformer, if N 1  and N 2  are the number of turns in the primary and secondary windings. Then turn ratio is defined as the ratio of number of secondary turns N 2  to the primary turns N 1 .

71. Actual Current Ratio (R) :      It is the ratio of the magnitude of current in the primary to the secondary windings. It is denoted by 'R'. The actual current ratio is the transformation ratio of a current transformer. Nominal Current Ratio ( K n ) :       The nominal current ratio of a current transformer can be obtained from the data mentioned on nameplate details. It is the ratio of the rated primary winding current to the secondary winding current.

72.  The relation between actual and nominal current ratio is given by the 'Ratio Correction Factor (RCF)’. R = RCF × K n      Owing to this turn, actual, and nominal ratios of a current transformer. There are two types of errors in a current transformer. They are ratio error and phase angle error.

73. Ratio Error :      The ratio error of a current transformer is due to a change in the actual current ratio from the turn ratio. We know that for a current transformer the current ratio must be equal to the turn ratio i.e., I 1 /I 2  = N 2 /N 1 . But due to magnetizing and cross loss components of the primary winding current and power factor of the seconding winding. The current ratio I 1 /I 2  will differ from the turn ratio N 2 /N 1 .      Thus the actual current ratio will not be constant and depends upon the load current, and power factor of the load or burden connected to secondary. Due to this change in actual current ratio, the current in the primary cannot be determined exactly and causes an error called ratio error. The formula for percentage ratio error is given as,

74. Phase Angle Error :      In practice, the secondary current of the CT must be in exact phase opposition with the primary current i.e., exactly by 180 phase difference. At the time of power measurement, there exists a difference in the phase angle between primary and secondary currents.      This is due to fact that the primary current has to supply core loss and magnetizing components of the CT for which it losses some phase angle. Due to which the secondary current wouldn't be in exact phase opposition. This difference or loss in phase angle causes an error called 'Phase Error or Phase Angle Error' denoted by angle θ. The phase angle error can understand by below phasor diagram of a current transformer.

75. Where,I p  = Primary current I s  = Secondary current n = Turn ratio I o  = Excitation currrent I c  = Core loss component I m  = magnetising component E p  = Primary induced EMF E s  = Secondary induced EMF φ  = Flux develop θ = Phase angle error α = Burden Angle β = Angle between flux and excitation current

76. In the above phasor diagram, by taking flux as the reference. If load connected is to be lagging power factor. The secondary current I s  lags behind the secondary emf E s . The primary has to supply excitation current components i.e., I m  and I c . The secondary current can be referred to as primary by multiplying with turns ratio i.e., nI s . The vector sum of nI s  and I o  gives the primary current. The phase angle error is given by,

77. In practice, most of the loads ( relays or instrument or pilot lights ) connected across secondary are inductive type. For inductive, δ is positive and very small. Therefore, sine δ ≈ 0 and cos δ ≈ 1. By substituting in the above equation we get,

78. POTENTIAL TRANSFORMER What is a Potential Transformer? A potential transformer (also known as voltage transformer) is a type of instrument transformer. It is a step-down voltage transformer that reduces the high-level voltage to safer low levels. The output voltage of the potential transformer can be measured by connecting an ordinary voltmeter.  Potential Transformer Construction  Potential transformer or PT can have the same construction as any normal transformer. It has primary & secondary winding. The number of turns in primary windings is greater than the number of turns in the secondary winding because it is a step-down transformer .

79. Potential Transformer Working The working of PT is similar to any conventional transformer. The electrical energy is transferred between the primary & secondary winding through magnetic induction. The alternating voltage at the primary generates alternating magnetic flux in the transformer core. Since both windings use the same core, this alternating flux induces a voltage in the secondary winding. Thus current starts to flow in the secondary winding. Since the primary has a greater number of turns compared to fewer secondary turns, the voltage induced in the secondary is very low. The secondary voltage is measured by using a standard low voltage voltmeter. Using the turn ratio equation of the transformer, we can calculate the primary voltage. V P /V S  = N P /N S Where V P  = Primary Voltage V S  = Secondary Voltage N P  = No. of Turns in Primary N S  = No. of Turns in Secondary Since the voltmeter has very high impedance, very low current flow through the secondary windings of the PT. for the same reason, the PT has very low VA ratings around 200VA.

80. Connection of Potential Transformer The potential transformer is connected in parallel with the circuit as opposed to CT that is connected in series. The primary of the PT is directly connected to the power line whose voltage is being measured. While the secondary is connected to the voltage measuring instrument like a voltmeter, wattmeter, etc. Since the voltage in the secondary is very low, an ordinary voltmeter can be used to measure it. The primary & secondary of the PT are magnetically coupled through mutual induction where the primary voltage is reduced based on the turn ratio of the transformer. The primary voltage can be up to several thousand volts while the secondary voltage falls below 110v. Both windings are electrically isolated but for safety reasons, the secondary winding is grounded at its one end.

81. Errors in Potential Transformer In an ideal transformer, the primary & secondary voltage are in exact proportion as its turns ratio & they are both in-phase. But practically, there is a voltage drop at primary due to its reactance which creates voltage ratio error & phase-shift error. Here are some of the errors that may occur in PT. Ratio Error The ratio error is the change in the voltage ratio due to the variation in load. Varying load changes the magnetizing current & the core losses that affect the secondary voltage of the PT. In simple words, its nominal ratio differs from its actual ratio. Ratio error is given by Ratio Error = (Nominal Ratio – Actual Ratio) / Actual Ratio Ratio Error = ( K n  – R)/R % Ratio Error = {( K n  – R)/R} x 100 Where K n  = Nominal Ratio (Rated Ratio) R = Actual primary to secondary voltage Ratio The nominal ratio is the ratio of rated primary voltage to rated secondary voltage.

82. Voltage Ratio Error The voltage ratio error is the difference between the ideal voltage & the practical or actual voltage. Here is the formula to find the voltage ratio error Voltage Ratio Error = (V P  – K n V S )/ V P % Voltage Ratio Error = {(V P  – K n V S )/ V P } x 100 Where K n  = Nominal Ratio (Rated Ratio) V P  = Actual Primary Voltage V S  = Actual Secondary Voltage Phase Angle Error The phase angle error is the difference between the phase of primary voltage & the reversed secondary voltage. Ideally, the primary voltage is in phase with the secondary voltage in reverse. But practically, there is the reactance of the windings that shifts the phase of the secondary voltage creating phase angle error. Phasor Diagram of Potential Transformer The phasor diagram for the potential transformer is given below. This phasor diagram shows the primary current I P , primary voltage V P , secondary current I S  & secondary voltage V S .

83. Where V P  = primary voltage E P  = Primary Induced EMF R P  = Primary Winding Resistance X P  = Primary Winding Reactance β = Phase angle Error. I P  = primary current I o  = Excitation current I m  = Magnetizing current (part of I o ) I w  = core loss current (part of I o ) K n  = Turn Ratio of Transformer Φ m  = Main Flux V S  = Secondary voltage E S  = Secondary Induced EMF R S  = Secondary Winding Resistance X S  = Secondary Winding Reactance I S  = Secondary Current

84. The reference of the given phasor diagram is the main flux Φ m . The primary induced voltage is achieved by the subtraction of losses due to the primary winding resistance R P , & reactance X P . The voltage drop due to primary windings is I P R P , & the reactance of the windings is I P X P . The excitation current I o  is the vector sum of magnetizing current I m   & core loss current I W . The vector sum of excitation current I o  & the reversal secondary current I S  multiplied by turn ratio 1/ K n   results in the primary current I P . Due to mutual induction, the primary emf will transform into the secondary emf E S  in the secondary windings. The secondary voltage V S  that appears at the output of the secondary windings is derived by subtracting the voltage drops due to the secondary windings resistance R S  & reactance X S

85. Capacitive Voltage Transformers Indoor Potential Transformers