EE ELECTRICAL MEASUREMENTS AND INSTRUMENTATION SYSTEMS MODULE I

Transcription

1 EE ELECTRICAL MEASUREMENTS AND INSTRUMENTATION SYSTEMS MODULE I Prepared by: Muhammedali Shafeeque K kshafeeq007@gmail.com onlinemas.weebly.com Reference: A.K. Sawhney, A course in Electrical and Electronics Measurements and Instrumentation, Dhanpat Rai and sons Indicating Instruments: Indicating Instruments are those instruments which indicate the magnitude of a quantity being measured. They generally make use of a dial and a pointer for this purpose. Ordinary voltmeters, ammeters and watt meters belong to this category. The analog indicating instruments may be divided into two groups: (i) electromechanical instruments,. (ii) electronic instruments. Electronic instruments are constructed by addition of electronic circuits to electromagnetic indicators in order to increase the sensitivity and input impedance. Recording Instruments give a continuous record of the quantity being measured over a specified period. The variations of the quantity being measured are recorded by a pen (attached to the moving system of the instrument; the moving system is operated by the quantity being measured) on a sheet of paper carried by a rotating drum. For example, we may have a recording voltmeter in a sub-station which keeps record of the variations of supply voltage during the day. Principles of Operation: Instruments may be classified according to the principle of operation they utilize. The effects they utilize are : (i) magnetic effect (ii) heating effect (iii) electrostatic effect (iv) electromagnetic effect (v) hall effect. 1. Magnetic Effect: Consider a current carrying conductor of Fig.1(a), it produces a magnetic field in the anticlockwise direction. We now have a uniform magnetic field as shown in Fig.1(b). Let the current carrying conductor be placed in this magnetic field. The resultant field is as shown in Fig. 1 (c). This results in distortion of magnetic field causing a force to act from left to right, The reversal of direction of the current will cause a force in the opposite direction, i.e., from right to left, subject to the condition that the direction of the existing field remains the same.

2 Fig.1 If we form the conductor into a coil, the magnetic field produced by each of the coil will add up and the coil will behave as an imaginary magnet as shown in Fig 2. Fig.2 Force of Attraction or Repulsion: Consider a current carrying coil as shown in Fig.2. It produces an imaginary bar magnet When a piece of soft iron which has not been previously magnetized is brought near the end of the coil, it will be attracted by the coil. Therefore, if we pivot the soft iron on a spindle between two bearings and a coil is mounted near it, the iron piece will swing into the coil when the latter is carrying current. The effect is utilized in the attraction type of moving iron instrument. If we have two pieces of soft iron placed near the coil the two will be similarly magnetized and there will be force of repulsion between them. This effect is utilized in repulsion type moving iron instruments. Force between a Current Carrying coil and a Permanent Magnet: Consider the coil of Fig.3. It produces an imaginary bar magnet when carry a current. When a permanent magnet is brought near it, there will be either a force of attraction or repulsion. If the coil is mounted on a spindle between bearings, there will be a movement of the coil. This effect is utilized in permanent magnet moving coil instruments.

3 Fig.3 Fig.4 Force between Two Current Carrying Coils: Consider two current carrying coils shown in Fig 4.For the direction of current shown, the two coils produce unlike poles near each other and thus there is a force of attraction and if one of the coils is movable and the other is fixed, there will be a motion of the movable coil. This effect is utilized in the dynamometer type of instruments. 2. Thermal Effect:The current to be measured is passed through a small element which heats it. The temperature rise is converted to an emf by a thermocouple attached-to the element. A thermo-couple consists of lengths of two dissimilar electric conductors joined at ends to form a closed loop. If the junctions of the two dissimilar metals are maintained at different temperatures, a current flows through the closed loop. This current can be measured and is indicative of the r.m.s. value of the current flowing through the heater element. 3. Electrostatic Effect: When two plates are charged, there is a force exerted between them. This force is used to move one of the plates. The instruments working on this principle are called electrostatic instruments and they are usually voltmeters. 4. Induction Effect: When a non-magnetic conducting pivoted disc or a drum is placed in a magnetic field produced by a system of electromagnets excited by alternating currents, an emf is induced in the disc or drum. If a closed path is provided, the emf forces a current to flow in the disc-or drum. The force produced by the interaction of induced currents and the alternating magnetic fields makes the disc move. The induction effect is mainly utilized for a.c. energy meters. 5. Hall Effect: If a strip of conducting material carries current in the presence of a transverse magnetic field as shown in Fig. 5, an emf is produced between two edges of conductor. The magnitude of the voltage depends upon the current, flux density and a property of conductor called "Hall Effect Co-efficient".. Fig.5

4 The emf may be measured after amplification. They can also be used for sensing of current. ELECTROMECHANICAL INDICATING INSTRUMENTS: Operating Forces.: Three types of forces are needed for the satisfactory operation of any indicating instrument. These are : (i) Deflecting force (ii) controlling force, and (iii) damping force. 1. Deflecting Force: The deflecting or operating force is required for moving the pointer -from its zero position. The system producing the deflecting force is called "Deflecting System" or "Moving System". The deflecting force can be produced, by utilizing any of the effects mentioned earlier. Thus the deflecting system of an instrument converts the electric current or potential into a mechanical force called deflecting force. The deflecting system thus acts as the prime mover responsible for deflection of the pointer. 2. Controlling Force: This force is required in an indicating instrument in order that the current produces deflection of the pointer proportional to its magnitude. The system producing a controlling force is called a "Controlling System". The functions of the controlling system are : (i) to produce a force equal and opposite to the deflecting force at the final steady position of pointer in order to make the deflection of the pointer definite for a particular magnitude of current, In the absentee of a controlling system, the pointer will shoot (swing) beyond the final steady position for any magnitude of current and thus the deflection will be indefinite. (ii) to bring the moving system back to zero when the force causing the instrument to deflect is removed. In the absence of a controlling system the pointer will not come back to zero when current is removed. Controlling force is usually provided by springs. 3. Damping Force: When a deflecting force is applied to the moving system, it deflects and is should come to rest at a position where the deflecting force is balanced by the controlling force. The deflecting and controlling forces are produced by systems which have inertia and, therefore, the moving system cannot immediately settle at its final position but overshoots or swings ahead of it. Consider Fig. 6. Suppose 0 is the equilibrium or final steady position and because of inertia the moving system moves to position a. Now for any position 'a' beyond the equilibrium position the controlling force is more than the deflecting force and hence the moving system swings back. Due to inertia it cannot settle at '0' but swings to a position say 'b' behind the equilibrium position. At b, the, deflecting force is more than the controlling force and hence the moving system again swings ahead. Fig.6 The pointer thus oscillates about its final steady (equilibrium) position with decreasing amplitude till its kinetic energy (on account of inertia) is dissipated in friction and therefore, it will settle down at its final steady position. If extra forces are not provided to "damp" these oscillations,

5 the moving system will take a considerable time to settle to the final position and hence time consumed in taking readings will be very large. Therefore, damping forces are necessary so that the moving system comes to its equilibrium position rapidly and smoothly without any oscillations. Control Systems: The deflecting system of most of the commercial instruments is mounted on a pivoted spindle, the quantity being measured producing a deflecting torque proportional 3 its magnitude. There are two types of control systems which are used for such a mounted system : (i) Gravity control. (ii) Spring control. 1. Gravity Control: In this type of control, a small weight is placed on an arm attached to the moving system. The position of this weight is adjustable. This weight produces a controlling torque due to gravity.fig.7 shows the pointer at zero position. In this case the control torque is zero. Suppose he system deflects through an angle θ as shown in Fig. Fig 7 The weight acts at a distance l from the centre, the component of weight trying to restore the pointer back to zero position is W sin θ. Therefore, controlling torque is: Thus the controlling torque is proportional to sine of angle of deflection of moving system. The controlling torque can be varied by simply adjusting the position of control weight upon the arm which carries it. It is obvious that the instruments employing gravity control must be used in vertical position in order that the control may operate. The instruments must be mounted in level position otherwise there will be a very serious zero error. For these reasons, gravity control is not suited for indicating instruments in general and portable instruments in particular. The system is obsolete now. 2. Spring Control: A hair spring attached to the moving system (Fig. 8) exerts a controlling torque. The essential requirements for instrument springs are : (i) They should be non-magnetic. (ii) They should be proof from mechanical fatigue.(iii) Where springs are used to lead current into moving system they should have a small resistance, their cross-sectional area must be sufficient to carry the current without a temperature rise which effects their constant.. They should also have a low resistance temperature co-efficient.

6 A number of non-magnetic materials like silicon bronze, hard rolled silver or copper, platinum silver, platinum-irridium and German silver have been used but have not been found satisfactory owing to some reason or the other. For most applications phosphor bronze has been the most suitable material except in instruments of low resistance (like millivoltmeters). In this case some special bronze alloyshaving low resistance may be used with some sacrifice in mechanical quality. Flat spiral springs are used in almost all indicating instruments as the space required by these springs is less than for other types. One form of control spring mounting is shown in Fig. 9.The inner end of the spring is attached to the spindle and the outer end carries a spigot which engages in a circular disc surrounding the jewel screw. This disc carries an arm which is slotted and splayed out at the end. The purpose of slotted extension arm is to allow the spring to be coiled or uncoiled slightly, so that the pointer may be set at zero. The slotted arm is actuated by a set screw mounted at the front of instrument and, therefore, zero setting of the instrument can be done without removing the cover. By making the number of turns large, the deformation per unit length is kept small on full scale deflection. The controlling torque is thus made proportional to the angle of deflection of the moving system. For a flat spiral spring, the controlling torque developed by deflection is : where E=Young's modulus of spring, material; b = width of spring ; m, t=thickness of spring ; m, l=length of spring ; m, θ= angular deflection ;rad. These all are constant for a particular spring. So, T C =Kθ Where K= constant called spring constant Comparison between Spring and Gravity Control: The disadvantages of gravity control have been mentioned earlier. Its advantages, when compared with spring control, are : (i) It is cheap.(ii) The control is independent of temperature variations.(iii) It does not deteriorate with time., Let us see the effect of the two types of controls on shape of the scale. Consider an instrument in which the deflecting torque is directly proportional to current being measured. Therefore, deflecting torque T d =K d I where I=current being measured and K d = a constant. If the instrument is spring controlled, the controlling torque T C =where, θ = angle of deflection and K= spring constant. At the equilibrium position, the deflecting torque is equal to the controlling torque or Tc =Td Kθ=K d I θ=(k d /K)I Therefore, the deflection is directly proportional to current (the quantity being measured) throughout the whole range of scale. If the instrument is gravity controlled Tc=K g sin θ, Then, θ =sin -1 {(K /K g ) I}

7 Hence a gravity control, taken on its merit, gives a cramped (compressed or crowded) scale at the lower end. This is a disadvantage when the pointer lies at the lower scale values, as these will not be accurately read. Damping Systems:The damping torque should be of such a magnitude that the pointer quickly comes to its final steady position without overshooting. If the instrument is underdamped, the moving system will oscillate about the final steady position with a decreasing amplitude and will take some time before it comes to rest. When the moving system moves rapidly but smoothly to its final steady position, the instrument is said to be critically damped or dead beat. If the damping torque is more than what is required for critical damping, the instrument is said to be overdamped. In an overdamped instrument, the moving system moves slowly to its final steady position in a lethargic fashion. The readings are very tedious to take in this case. Fig. 8' illustrates the way an underdamped, an overdamped and a critically damped system moves to its final steady position. Fig 8 The damping device should be such that it produces a damping torque only while the moving system is in motion. To be effective "the damping torque should be proportional to the velocity of the moving system but independent of the operating current. It must not effect the controlling torque or increase the static friction. The methods for producing damping torque are: (i) air friction damping, (ii) fluid friction damping.(iii) eddy current damping and (iv) electromagnetic damping. 1. Air Friction Damping: Two types of air friction damping devices are shown in Fig. 9. The arrangement consists of a light aluminium piston which is attached to the moving system. This piston moves in a fixed air chamber which is closed at one end. The clearance between piston and chamber walls is uniform throughout and is very small. When there are oscillations the piston moves into and out of an air chamber. When the piston moves into the chamber, the air' inside is compressed and the pressure of air, thus built up, opposes the motion of piston and hence of whole of the moving system. When the piston moves out of air chamber, pressure in the closed space falls, and the pressure on the open side of piston is greater than on the other side. Thus there is again an opposition to motion.

8 Fig 9 The arrangement of Fig. 9(b), consists of an aluminium vane which moves in a quadrant (sector) shaped air chamber. This air chamber is a recess cast in a bakelite moulding or diecasting. The chamber is completed by providing a cover plate at the top. The aluminium piston should be carefully fitted so that it does not touch the wall otherwise a serious error will be caused in readings. 2.Eddy current damping: Eddy current damping is the most efficient form of damping. It is very convenient to use in instruments where a metallic disc or a former and a permanent magnet already form part of the operating system. For these reasons this method is used in hot wire, moving coil and induction type instruments. This method cannot be used in instruments where introduction of a permanent magnet required for producing eddy currents will distort the existing magnetic field. Fig 10 There are two common forms of damping devices: (i) A metal former which carries the working coil of the instrument. (ii) A thin aluminium disc attached to the moving system of the instrument, This disc moves in the field of a permanent magnet. Fig 10 shows a metallic former moving in the field of a permanent magnet. Permanent Magnet Moving Coil Instruments (PMMC) The permanent magnet moving coil instrument is the most accurate type for dc. measurements. Instrument is provided with a pointer and a scale.

9 Construction : The general constructional features of this instrument are shown in fig.11 Moving Coil.:The moving coil is wound with many turns of enamelled or silk covered copper wire. The coil is mounted on a rectangular aluminium former which is pivoted on jewelled bearings. The coils moves freely in the field of a permanent magnet. Most voltmeter coils are wound on metal frames to provide the required electro-magnetic damping. Most ammeter coils, however, are wound on nonmagnetic formers, because coil turns are effectively shorted by the ammeter shunt. The coil itself, therefore, provides electro-magnetic damping. Magnet Systems:There has been considerable development in materials for permanents magnets and, therefore, magnet assemblies have undergone a lot of change in the recent past. Old style magnet systems consisted of a relatively long U shaped permanent magnets having soft iron pole pieces. Owing to development of materials like Alcomax and Alnico, which have a high co-rcive force, it is possible to use smaller magnet lengths and high field intensities. Thus in small instruments it is possible to use a small coil having small number of turns and hence a reduction in volume is achieved. Alternatively in instruments having a large scale length it is possible to increase the air gap length to accommodate large number of turns. The movement of the coil is restricted in the above design. This is because no actual part of the coil is allowed to reach the extreme positions near the pole tips where, there is fringing field. (Owing to fringing the flux density near the pole tips is smaller than that at the centre and also the field is not radial). Thus the angular span of scale is restricted to In order to obtain longer movement of the pointer and a longer angular swing of the coil a concentric magnet construction as shown in Fig. 12 is used. Since the magnet is concentric type it produces a radial flux pattern which extends over 250 or more. This type of construction is used for many panel type instruments and some portable instruments.

10 Fig 12 Fig 13 Fig 14 Fig 15 An -air cored coil offset from the axis of rotation is used as shown in Fig. 14 the scale length or the instrument can be increased from 120deg to 240deg or even 300deg thereby giving better resolution of reading for the same scale range.improved magnetic materials like Alnico, is also used to design a magnetic system in which the magnet itself serves as the core as shown in Fig. 15. The moving coil moves over the magnet. The active sides of the moving coil are located in the uniform radial field between pole pieces and the steel yoke. This arrangement has the obvious advantage of being relatively unaffected by the external magnetic fields. It also eliminates the magnetic shunting effects in steel panel construction, where several meters operating side by side may affect each others readings. The need for magnetic shielding in the form of iron cases, is also eliminated by core magnet construction. Control: When the coil is supported between two jewel bearings the control torque is provided by two phosphor bronze hair springs. These springs also serve to lead current in and out of the coil. The control torque is provided by the ribbon suspension. This method is comparatively new and is claimed to be advantageous as it eliminates bearing friction.

11 Damping: Damping torque is produced by movement of the aluminium former moving in the magnetic field of the permanent magnet. Pointer and Scale: The pointer is carried by the spindle and moves over a graduated scale. The pointer is of light-weight construction and, apart from those used in some inexpensive instruments, has the section over the scale twisted to form a fine blade. This helps to reduce parallax errors in the reading of the scale. In many instruments such errors may be reduced further by careful alignment of the pointer blade and its reflection in the mirror adjacent to scale. The weight of the instrument is normally counter balanced by weights situated diametrically opposite and rigidly connected to it. Torqne Equation:

12 As the deflection is directly proportional to the current passing through the meter, we get a uniform (linear) scale for the instrument. PMMC D.C. Ammeter Coil of PMMC winding is small and light and can carry only small current. When heavy currents are to be measured, the major part of the current is bypassed through a low resistance called a "shunt". Fig. 8 6 shows the basic movement (meter) and its shunt to produce an ammeter. When heavy currents are to be measured, the major part of the current is bypassed through a low resistance called a "shunt". Fig. 16 shows the basic movement (meter) and its shunt to produce an ammeter. Fig 16 Let Rm =internal resistance of PMMC (i.e. the coil),rsh= resistance of the shunt, Im=Ifs=full scale deflection current of movement, Ish=shunt current, I=current to be measured. Since the shunt resistance is in parallel with the meter movement, the voltage drops across shunt and movement must be the same. or Ish Rsh=ImRm So, Rsh=ImRm / Ish

13 Construction of Shunts: The general requirements for shunts are : (i) the temperature co-efficient of shunt and instrument should be low and should be as nearly as possible the same ; (ii) the resistance of shunts should not vary with time; (iii) they should carry the current without excessive temperature rise ; (iv) they should have a low thermal electromotive force. Manganin' is usually used for shunts of d.c. instruments as it gives low value of thermal emf with copper although it is liable to corrosion and is difficult to solder. 'Constantan' is a useful material for a c. circuits since its comparatively high thermal emf, being unidirectional, is ineffective on these circuits. Multi-range Ammeters. The current range of a dc. ammeter may be further extended a by a number of shunts, selected by a range switch. Such meter is called a multirange ammeter. Fig.17 shows a schematic diagram of multirange ammeter. The circuit has four shunts Rsh1,Rsh2,... which can be put in parallel with the meter movement to give four different current ranges I1,I2... Fig 17 PMMC D.C. Voltmeter: Voltmeter Multipliers. A basic PMMC meter is converted into a voltmeter by connecting a series resistance with it. This series resistance is known as a multiplier. The combination of the meter movement and the multiplier is put across the circuit whose voltage is to be measured. (See Fig. 18).The multiplier limits the current through the meter so that it does not exceed the value for full scale deflection and thus prevents the movement from being damaged. Fig 18

14 Constrnction of Multipliers: The essential requirements of multipliers are : (i) their resistance should not change with time ; (ii)0 the change in their resistance with temperature should be small; (iii they should be non-inductively wound for a.c. meters. The resistance materials used for multipliers are manganin and constantan. Moving Iron Instruments: A plate or vane of soft iron or of high permeability steel forms the moving element of the system. This iron vane is so situated that it can move in a magnetic field produced by a stationary coil. The coil is excited by the current or voltage under measurement. When the coil is excited, it becomes an electromagnet and the iron vane moves in such a way so as to increase the flux of the electromagnet. This is because the vane tries to occupy a position of minimum reluctance. Thus the force (or torque) produced is always in such a direction as to increase the inductance of the coil (As inductance increases as reluctance decreases). General Torque Equation: An expression for the torque of a moving iron instrument may be derived by considering the energy relations when there is a small increment in current supplied to the instrument. When this happens there will be a small deflection dθ and some mechanical work will be done.

15 Moving iron instruments are of two types: (i) Attraction type (ii)repulsion type. Attraction Type: Fig. 19 shows the constructional details of an attraction type moving iron instrument. The coil is flat and has a narrow slot like opening. The moving iron is a flat disc or a sector eccentrically mounted. When the current flows through the coil, a magnetic field is produced and the moving iron moves from the weaker field outside the coil to the stronger field inside it or the moving iron is attracted in. The controlling torque is provided by spring but gravity control can be used for panel type of instruments which are vertically mounted.damping is provided by air friction usually by a vane moving in a sector shaped chamber.

16 Fig 19 Repulsion Type.; In the repulsion type, there are two vanes inside the coil one fixed and other movable. These are similarly magnetised when the current flows through the coil and there is a force of repulsion between the two vanes resulting in the movement of the moving vane. Fig 20 Two designs are in common use : (i) Radial Vane Type. In this type, the vanes are radial strips of iron. The strips are placed within the coil as shown in Fig.20. The fixed vane is attached to the coil and the movable one to the spindle of the instrument. (ii) Coaxial Vane Type. In this type, the fixed and moving vanes are sections of co-axial cylinders. The fixed and moving vanes have shapes, similar to those shown in Fig 20, when developed. Reason for Use on both AC. and D.C: From deflection equation,whatever may be the direction of the current in the coil of the instrument, the iron vanes are so magnetised that there is always a force of Attraction in the attraction type and repulsion in the repulsion type of instruments. Thus

17 moving iron instruments are unpolafised instruments i.e they are independent of the direction in which the current passes. Therefore, these instruments can be used on both a.c. and d.c. Electrodynamometer type Instruments: Electrodynamometer instruments are those which can be used for both ac and dc. Fig 21 Electrodynamometer type of instruments are used as a.c. voltmeters and ammeters both in the range of power frequencies and lower part of the audio frequency range. They are used as wattmeters, varmeters and with some modification as'power factor meters and frequency meters. Operating Principle. We can have an idea of the working principle of this instrument by taking up a permanent magnet moving coil instrument and considering how it would behave on a,c. It would have a torque in one direction during one half of the cycle and an equal effect in the opposite direction during the other half of the cycle. If the frequency were very low,the pointer would swing back and forth around the zero point. However, for an ordinary meter the inertia so great that on power frequencies the pointer does not go very far in either direction but merly stays (Vibrates slightly) around zero. If however, we were to reverse the direction of field flux each time the current through the movable coil reverses, the torque would be produced in thezzz direction for both halves of the cycle. The field can be made to reverse simultaneously with the current in the movable coil if the field coil is connected in series with movablecoil. Construction: Fixed Coils: The field is produced by a fixed coil This coil is divided into two sections to give a more uniform field near the centre and to allow passage of the instrument shaft. The instrument as shown in Fig. 8'40may be a milliammeter, or may become a voltmeter by the addition of a series resistance. The fixed coils are wound with fine wire for such applications. Fixed coils are usually wound with heavy wire carrying the main current in ammeters and wattmeters. The wire is stranded where necessary to reduce eddy current losses in conductors. The coils are usually varnished and baked to form a solid assembly. These are then clamped in place against the coil supports. This makes the construction rigid so that there is no shifting or change in dimensions which might effect the calibration. The mounting supports are preferably made out of ceramic, as metal parts would weaken the field of the fixed coil on account of eddy currents.

18 Moving Coil. A single element instrument has one moving coil. The moving coil is wound either as a self-sustaining coil or else on a non-metallic former. A metallic former cannot be used as eddy currents would be induced in it by the alternating field. Light but rigid construction is used for the moving coil. It should be noted that both fixed and moving coils are air cored. Control The controlling torque is provided by two control springs. These springs act as leads to the moving coil. Moving System. The moving coil is mounted on an aluminium spindle. The moving system also carries the counter weights and truss type pointer. Sometimes a suspension may be used in case high sensitivity is desired. Damping. Air friction damping is employed for these instruments and is provided by a pair of aluminium vanes, attached to the spindle at the bottom. These vanes move in sector shaped chambers Shielding: The field produced by the fixed coils is somewhat weaker than in other types of instruments. In d.c. measurements even the earth's magnetic field may affect the readings. Thus it is necessary to shield an electrodynamometer type instrument from the effect of stray magnetic fields. Air cored electrodynamometer type instruments are protected against external magnetic fields by enclosing them in a casing of high permeability alloy. This shunts external magnetic fields around the instrument mechanism and minimizes their effects on the indication. Double casing is highly effective in the case of precision instruments. The outer casing is made up of a material of high saturation density and low coercive force, while the inner casing is made up of a material having high initial permeability. Torque Equation:

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21 Electrodynamometer Ammeters: Fig. 22 shows that the arrangement of coils of an electrodynamometer ammeter. In this case the fixed and moving coils are connected in series and, therefore, carry the same current (a) Fig 22 (b) Electrodynamometer Voltmeters: The electrodynamometer movement is used as a voltmeter by connecting the fixed and moving coils in series with a high non-inductive resistance. Current Transformer (CT) and Potential Transformers(PT): Ratios related to CT and PT: Transformation Ratio: It is the ratio of the magnitude of the primary phasor to the secondary phasor. Transformation ratio R= primary phasor secondary phasor primary current = = secondary current primary voltage secondary voltage for PT for CT Nominal Ratio: It is the ratio of rated primary current (or voltage) to the rated secondary current (or voltage). Kn= rated primary voltage rated secondary voltage for PT

22 = rated primary current rated secondary current for CT number of turns Of secondary winding Turns Ratio: Turns Ratio n= = number of turns Of primary winding number of turns Of primary winding number of turns Of secondary winding for C.T. for P.T. Ratio Correction Factor (RCF): The ratio correction factor of a transformer is the transformeration ratio divided by nominal ratio. Transformation ratio=ratio correction factor x nominal ratio ie R=RCF X Kn The ratio marked on the transformers is their nominal ratio. Burden: It is convenient to express load across the secondary terminals as the output in voltampere at the rated secondary voltage permissible without errors exceeding the limits for particular accuracy. Total secondary burden = secon dar y induced vo ltage impedance of secondary circuit including impedance of secondary winding 2 =secondary current 2 X impedance of secondary circuit including secondary winding Secondary burden due to load = secon dar y terminal vo ltage impedance of load on secondary winding =secondary current 2 X impedance of load on secondary winding 2 Current transformers: The current transformer is used with its primary winding connected in series with line carrying the current to be measured and, therefore, the primary current is not determined by the load on the current transformer secondary. The primary consists of very few turns and, therefore, there is no appreciable voltage across it. The secondary of the current transformer has larger number of turns, the exact number being determined by the turns ratio. The ammeter, or wattmeter current coil, is connected directly across the secondary terminals. Thus a current transformer operates its secondary nearly under short circuit conditions. One of the terminals of the secondary winding is earthed so as to protect equipment and personnel in the vicinity in the event of an insulation breakdown in the current transformer. Fig. 9'3 shows a circuit for measurement of current and power with a current transformer. Theory: Fig. 23 represents the equivalent circuit & phasor diagram of a current transformer. The diagrams are same as for any other transformer.

23 Fig 23

24 Construction : The current transformers may be classified as: (i) Wound type. A current transformer having a primary winding of more than one full turn wound on core. (ii) Bar type. A current transformer in which the primary winding consists of a bar of suitable size and material forming an integral part of transformer Figs. 24 and 25 shows wound type and bar type transformers respectively.

25 The simplest form any current transformer can take is the ring type or window types; three commonly shapes are used i.e., stadium, circular and rectangular orifices(fig 26). The core, if of a nickel-iron alloy or an oriented electrical steel is almost certainly of the continuously wound type. But current transformer using hot rolled steel will consist of stack of ring stampings. Before putting secondary winding on the core, the latter is insulated by means of end collars and circumferential wraps of elephantide or presspahn. These pressboards, in addition to acting as insulating medium, must also protect the secondary winding conductor from mechanical damage due to sharp corners. The secondary winding conductor is put on the core by a toroidal winding machine although hand winding is still frequently-adopted if the number of secondary turns is small. Fig 24 Fig 25 After the secondary, winding has been placed on the core, the ring type transformer is completed by exterior taping with or without first applying exterior and rings and circumferential insulating wraps. A near relative of the ring type current transformer is the so called bushing type transformer. This is, in fact, indistinguishable from the ordinary ring type but the term is used when the current transformer fits over a fully insulated primary conductor such as over the oil end of a terminal bushing of a power transformer or oil circuit breaker.

26 Fig 26 At very high voltages, the insulation of the current carrying conductor from, the measuring circuit becomes an expensive problem. At 750 kv, cascaded current transformers are used or alternatively a coaxial shunt is used to modulate a radio frequency signal that is transmitted from the shunt placed in the high voltage line to receiving equipment on the ground, thereby overcoming the insulation problem. However, this type of system has severe limitation in its power output which has to be amplified in order to operate relays etc. In a split core current transformer, the core is split, each half having two finely ground.or lapped gap faces. These current transformers are assembled on to the primary conductor "on site' for either permanent or temporary duty. In a bar type current transformer, the core and secondary windings are the same as in a ring type transformer but the fully insulated bar conductor constituting the single turn primary is now an integral part of the current transformer. The insulation on the primary conductor may be bakelized paper tube or a resin directly moulded on the bar. In a low voltage wound type current transformers the secondary winding is wound on a bakelite former or bobbin and the heavy primary conductor is either wound directly on top of secondary, suitable insulation being first applied over the secondary winding or the primary is wound entirely separately, taped with suitable insulating material and then assembled with the secondary winding on the core. In the manufacture of current transformers the assembly of lamination stacks demands somewhat greater care than ordinary transformers in order to keep down the reluctance of the interleaved corners as low as possible so as to minimize the magnetizing current. Sometimes cut cores are used. Whenever possible secondary windings should utilize the whole available winding length on the core, the secondary turns being suitably spaced to accomplish this and the insulation between secondary winding and core and earth must be capable of withstanding the high peak voltages caused if the secondary winding is open circuited when primary current is flowing. In the case of a large number of secondary turns, requiring more than one winding layer, the frequently adopted technique is to sectionalize the secondary winding so as to considerably reduce the peak voltage between. layers.

27 With wound primary current transformers this particular problem is rarely met but it is of importance to try to obtain good relative positioning of primary and secondary coils, thus minimizing the axial forces on both coils caused by primary short circuit currents. Windings. The windings should be close together to reduce the secondary leakage reactance as the leakage reactance increases the ratio error. Round copper, wire of about:3 mm 2 area is. frequently used for secondary windings rated at 5 A. Copper strip is used for primary winding, the dimensions of which depend upon the primary current.when using bar primary, the external diameter of the tube must be large enough to keep the voltage gradient, in the dielectric at its surface, to an acceptable value in order to avoid. corona effect. The windings must be designed to withstand, without damage, the large short circuit forces that are caused when a short circuit takes place on the system in which the current transformer is connected.the'windings are separately wound, and are insulated by tape and varnish for small line. voltages. For voltages above 7 kv the transformers arc oil immersed or compound filled. Effect of Secondary Open Circuit: Current transformers are always used with the secondary circuit closed through ammeters, wattmeter current coils, or relay coils. A precaution which should always be observed in using current transformers is the following ; Never open the secondary circuit of a current transformer while its primary is energised. Failure to observe this precaution may lead to serious consequences both to be operating personnel' and to the transformer. This is clear from the following : The difference between a power transformer and a current transformer is that in a power transformer the current flowing in the primary winding is largely the reflection of that flowing in the secondary circuit, whereas in the current transformer, the primary winding is connected in series with the line, whose current is being measured or indicated and this current is no way controlled or determined by the conditions of the secondary circuit of the current transformer. Under normal operating conditions both primary and secondary windings produce mmfs which act against each other. The secondary mmf is slightly less than the primary mmf and consequently the resultant mmf is small. This resultant mmf is responsible for production of flux in the core and as this mmf is small, the fiux density is quite low under normal operating conditions and hence a small secondary voltage is induced. If winding is open circulated when the primary is carrying current, the primary mmf remains the sarne while the opposing secondary mmf reduces to zero. Therefore the resultant mmf is equal to the primary mmf which is very large. This large mmf produces a large flux in the core till it saturates. This large flux linking the turns of the secondary wjnding, would induce a high secondary voltage which could be dangerous to the transformer insulation (although, modern CTs are designed to withstand this voltage) and to th person who has opened the circuit. Also the eddy current and hysteresis losses would be very high under these conditions and due to this the transformer may be overheated and completely damaged. Even if it does not happen, the core may become permanently magnetized and this gives appreciable ratio and phase angle errors". Many current transformers are provided with a short circuiting link or a switch at the secondary terminals. If such a link is provided, it should always be closed before any change is made in the secondary circuit with primary excited. The secondary winding can safely be short-circuited since when used for measurement it is practically short-circuited as the impedance of the burden i.e. an ammeter or a current coil of a wattmeter is very small.

28 Potential Transformers: Potential transformers are used to operate voltmeters, the potential coils of wattmeters and relays from high voltage lines. The primary winding of the transformer is connected across the lines carrying the voltage to be measured and the voltage circuit is connected across the secondary winding. The design of a potential transformer is quite similar to that of a power transformer but the loading of a potential transformer is always small, sometimes only a few volt-ampere. The secondary winding is designed so that voltage of 100 to 120 V is delivered to the instrument load. The normal secondary voltage rating is 110.V. Difference between CT. and P.T.: There are a few differences in the operation of a current transformer and a potential transformer. (i) The potential transformer may be considered as a parallel' transformer with its secondary nearly under open circuit conditions whereas the current transformer may be thought as a 'series transformer under virtual short circuit conditions. Thus the secondary of a P.T. can be opencircuited without any damage being caused either to the operator or to the transformer. (ii) The primary current in a C.T. is independent of the secondary circuit conditions while the primary current in a P.T. certainly depends upon the secondary burden. (iii) In a potential transformer, full line voltage is impressed upon its terminals whereas a C.T. is connected in series with one line and a small voltage exists across its terminals. However the C.T. carries the full line current. (iv) Under normal operation the line voltage is nearly constant and, therefore, the flux densitj and hence the exciting current of a potential transformer varies only over a restricted range whereas the primary current and excitation of a C,T. vary over wide limits in normal operation. Theory.: The theory of a potential transformer is essentially the same as that of a power transformer. The main point of difference is that the power loading of a P.T. is very small and consequently the exciting current is of the same order as the secondary current while in a power transformer the exciting current is a very small fraction of secondary load current. Figs. 27 show the equivalent circuit of a potential transformer. Fig27 Construction of Potential Transformers: The design and construction of potential transformers are basically the same as those of power transformers but there are few major points of difference: (i) Power transformers are designed keeping in view the efficiency, regulation and cost. The cost being reduced by using small core and conductor sizes. In designing a potential transformer,

29 economy in material is not a big consideration and the transformers are designed to give desired, performance ie., constancy of ratio and smallness of phase angle. Compared to a power transformer, a potential transformer has larger core and conductor sizes. Economic designs may lead to large ratio and phase angle error which are very undesirable features. (ii) The output of a voltage transformer is always small and the size is quite large. Therefore, the temperature rise is small and hence there are no thermal problems caused by overloads as in power transformers. In fact, the loading on a potential transformer is limited by accuracy considerations while in a power transformer the load limitation is on heating basis. Actually, the potential transformers are able to carry loads on a thermal basis many times their rated loads. These loads range from 2-3 times for low voltage potential transformers and upto 30 or more times for some high voltage transformers. Core : The core may be of shell or core type of construction. Shell type construction is normally only used for low voltage transformers; Special precautions should be taken to assemble and interleave so that the effect of air gaps at the joints may be minimized. Windings: The primary and secondary windings are coaxial to reduce the leakage reactance to minimum. In order to simplify the insulation problems, the low voltage winding (secondary) is put next to the core. The primary winding may be a single coil in low voltage transformers but must be subdivided into a number of short coils in high voltage transformers in order to reduce the insulation needed between coil layers. Insulation: Cotton tape and varnished cambric are used as insulation for coil construction. Hard fibre separators are used between coils. At low (voltages, the transformers are usually filled without compound but potential transformers for use at voltages above 7000 volt are oilimmersed. Dry type, porcelain insulated transformers have been developed in the continent for use upto 45 kv. Bushings: Oil filled bushings are usually used for oil filled potential transformers as this minimizes the overall size of the transformer. Two bushings are used when neither side of the line is at ground potential. Some potential transformers, connected from line to neutral of grounded neutral systems, have, only one high voltage. bushing., It is pertinent to point out here that a current transformer needs only one bushing as leads from the two ends of the primary winding are brought through the same insulator since there is only a small voltage between them, thus saving the expense of another high voltage insulator. High Voltage Potential Transformers: Conventional type potential transformers used for high voltages of 100 kv and above, are very large in size and costly to build because of insulation requirements. For example, a 110 kv potential transformer has an overall height of about 7.5 metre and weighs nearly 5 tonnes. This is a very unwieldy size and also the materials utilized in the transformer construction are very uneconomically utilized.

30 Recently there has been a development in the design and construction of P.Ts which has resulted in considerable reduction in size and cost of transformers.two designs have been developed which eliminate the high voltage lead in bushings The elimination of bushings reduces the size and cost of transformers. These designs are intended to measure line to ground voltages in a three phase system. The designs employ : (i) Insulated Casing. The transformer is built entirely in an oil filled high voltage insulator. This results economy in space and material. (ii) Moulded Rubber Potential Transformer. Recent developments in the synthetic industry have introduced the moulded rubber potential transformer replacing the insulating porcelain bushings for some applications. This unit is less expensive than the conventional oil filled P.T., and since the bushings are made of moulded rubber, the difficulties caused due to porcelain breakage are eliminated (iii) Cascaded Transformers. In a cascaded arrangement, the voltage is divided among a number of transformers. Suppose the voltage is divided among N transformers and, therefore will take 1/N of total voltage. In this way each requires insulation corresponding to the with a consequent saving in space and material. The chief application of cascade arrangement to increase voltage rating of dry type units. Protection. Potential transformers can be continuously operated at 1.2 times the rated Voltage. A short circuit on the secondary side of a potential transformer can lead to complete of the transformer. In order to protect the power system against short circuits in the potential transformers, fuses are used on the primary (high voltage) side. Fuses are used in the secondary side to protect the P.T against faulty switching and defective earthing.