MAHATMA GANDI INST. OF TECHNICAL EDUCATION & RESEARCH CENTRE, NAVSARI. Instrument transformer and their application in the extension of instrument range. PREPARE BY: ENRL NO. MARVADI PRITESH R. 140333109008 PATEL KRUTIKA D. 140333109015 PATEL SHIVANI P. 140333109017 PRAJAPATI PARTH P. 140333109022 SUBJECT: EMMI GUIDED BY: MAHATMA GANDI INST. OF TECHNICAL EDUCATION & RESEARCH CENTRE, NAVSARI.

Instrument transformer Introduction: Instrument transformers are special transformers used in conjunction with the relevant instruments such as, voltmeter, ammeter, wattmeter and energy meter for the measurement of voltage, current, power and energy. Such transformers are of two types: 1 Current Transformer (CT): Current transformers are used in conjunction with ammeter or wattmeter current coil when the magnitude of AC currents exceeds the safe value of current of measuring instruments.

Instrument Transformer as CT Circuit diagram Instrument Transformer as CT

Instrument Transformer as PT Circuit diagram 2. Potential Transformer (PT): Potential transformers are used in conjunction with voltmeter when the voltage of an AC circuit exceeds 750 V as it is not possible to provide adequate insulation on measuring instruments for voltage more than this. Instrument Transformer as PT

continue Advantages of Instrument Transformer: 1. The measuring instruments can be placed for away from the high voltage side by connecting long wires to the instrument transformer. This ensures the safety of instruments as well as the operator. 2. These instrument transformers can be used to extend the range of measuring instruments like ammeters and voltmeters up to any required value. 3. The power loss in instrument transformers is very small as compared to power loss due to the resistance of shunts and multipliers. Disadvantages of Instrument Transformer: 1. The only main drawback is that these instruments can’t be used in DC circuits.

Theory of current transformer Equivalent circuit of CT

continue Various symbols are : n=turns of ratio=secondary turns/primary turns = Ns/Np Rp=Resistance of primary winding xp=Reactance of primary winding Xs= Reactance of secondary winding Rs= Resistance of secondary winding Xe= Reactance of external burden i.e. load on secondary Re= Resistance of external burden i.e. load on secondary Ep= Primary induced voltage Es= Secondary induced voltage Vs= Secondary terminal voltage Ip= Priimary current

continue Is= Seconadry current I0= No load current Ic= coreloss ccomponant of I0 Im= Magnetizing componant of I0

Phasor diagram of CT

continue Resultant primary current Ip is given by: Hence, transformation ratio R is given by:

Equivalent circuit of PT Circuit diagram Equivalent circuit of PT

Phasor diagram of PT

continue Is - Secondary current. Es - Secondary induced emf. Vs - Secondary terminal voltage. Rs - Secondary winding resistance. Xs - Secondary winding reactance. Ip - Primary current Ep - Primary induced emf. Vp - Primary terminal voltage. Rp - Primary winding resistance Xp - Primary winding reactance. KT - Turns ratio = Numbers of primary turns/number of secondary turns. I0 - Excitation current Im - Magnetizing component of I0.

Frequency measurement Types of frequecy meters: 1. Mechanical resonance (reed) type frequency meter 2. Electrical resonance type frequency meter a) Ferro-dynamic type frequency meter b) Electrodynamometer type frequency meter 3. Weston frequency meter 4. Ratiometer type frequency meter 5. Saturable core type frequency meter

Mechanical response type frequency meter

continue The vibrating reed type of frequency meter is one of the simplest devices for indicating the frequency of an ac source. The current whose frequency is to be measured flows through the coil and exerts maximum attraction on the soft iron armature twice during each cycle. Reeds of suitable dimensions to have natural vibration frequencies of 110, 112, 114, and so forth up to 130 cps are mounted on the bar. When the coil is energized with a current having a frequency between 55 and 65 cps, all the reeds are vibrated slightly; but the reed having a natural frequency closest to that of the energizing current (whose frequency is to be measured) vibrates through larger amplitude.

Electrical resonance type frequency meter: (a) Ferro-dynamic type frequency meter: This type of instrument operate on the principle of electrical resonance, when inductive reactance XL and capacitive reactance Xc become equal, electrical resonance said to be occurred. The moving coil is further connected to a suitable capacitor “C” whereas the magnetizing coil is connected across the supply terminal whose frequency is to be measured. If we neglect the resistance of the coil and the iron losses in the core, flux ɸ is in phase with current I. Since flux ɸ is an alternating flux, it will induce e.m.f. in the moving coil.

Power factor measurement Types of Power Factor Meter: 1. Electrodynamometer type a) Single phase b) Three phase 2. Moving iron type a) Rotating field type moving iron power factor meter b) Alternating field type moving iron power factor meter Single phase electrodynamometer type power factor meter: The general circuit diagram of single phase electrodynamometer power factor meter is given below.

figure

continue The pressure coil is spitted into two parts one is purely inductive another is purely resistive as shown in the diagram by resistor and inductor. At present the reference plane is making an angle 90o - θ with coil A. And the angle between both the coils A and B is 90°. Scale of the meter is properly calibrated shown the value values of cosine of angle θ. Let us mark the electrical resistance connected to coil A be R and inductor connected to coil B be L. Now during measurement of power factor the values of R and L are adjusted such that R=ωL so that both coils carry equal magnitude of current. Therefore the electric current passing through the coil B is lags by 90° with reference to electric current in coil A as coil B path is highly inductively in nature.

continue .Advantages of Electrodynamic Type Power Factor Meters: 1. Losses are less because of minimum use of iron parts and also give less error over a small range of frequency as compared to moving iron type instruments. 2. They high torque is to weight ratio. Disadvantages of Electrodynamic Type Power Factor Meters 1. Working forces are small as compared to moving iron type instruments. 2. Scale is not extended over 360°. 3. Calibration of electrodynamometer type instruments are highly affected by the changing the supply voltage frequency. 4. They are quite costly as compared to other instruments.

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