INTRODUCTION 3.1 Instrument transformers are used in connection with measurement of voltage, current, energy and power in ac circuits. There are principally two reasons for use of instrument transformers in measurement: first, to extend (multiply) the range of the measuring instrument and second, to isolate the measuring instrument from a high-voltage line. In power systems, levels of currents and voltages handled are very high, and, therefore, direct measurements with conventional instruments is not possible without compromising operator safety, and size and cost of instrument. In such a case, instrument transformers can be effectively used to step down the voltage and current within range of the existing measuring instruments of moderate size. Instrument transformers are either (a) current transformer or CT, or (b) voltage or potential transformers or PT. The former is used to extend current ranges of instruments and the latter for increasing the voltage ranges. Instrument transformers have their primary winding connected to the power line and secondary windings to the measuring instrument. In this way, the measuring instruments are isolated from the high power lines. In most applications, it is necessary to measure the current and voltage of large alternators, motors, transformers, buses and other power transmission equipments for metering as well as for relaying purposes. Voltages in such cases may range from 11,000 to even 330,000 V. It would be out of question to bring down these high-voltage lines directly to the metering board. This will require huge insulation and pose great danger for operating personnel otherwise. In such a case, instrument transformers can greatly solve this problem by stepping down the high voltage to safe levels for measurement. ADVANTAGES OF INSTRUMENT TRANSFORMERS 3.2 Shunt and multipliers used for extension of instrument ranges are suitable for dc circuits and to some extent, for low power, low accuracy ac circuits. Instrument transformers have certain distinguishing characteristics as compared to shunts and multipliers, as listed below. 1. Using shunts for extension of range on ammeters in ac circuits will require careful designing of the reactance and resistance proportions for the shunt and the meter. Any deviation from the designed time constants of the shunt and the meter may lead to errors in measurement. This problem is not present with CT being used with ammeter.
2. Shunts cannot be used for circuits involving large current; otherwise the power loss in the shunt itself will become prohibitably high. 3. Multipliers, once again, due to inherent leakage current, can introduce errors in measurement, and can also result in unnecessary heating due to power loss. 4. Measuring circuits involving shunts or multipliers, being not electrically isolated from the power circuit, are not only safe for the operator, but also insulation requirements are exceedingly high in high-voltage measurement applications. 5. High voltages can be stepped down by the PT to a moderate level as can be measured by standard instruments without posing much danger for the operator and also not requiring too much insulation for the measuring instrument. 6. Single range moderate size instruments can be used to cover a wide range of measurement, when used with a suitable multi-range CT or PT. 7. Clamp-on type or split-core type CT’s can be very effectively used to measure current without the need for breaking the main circuit for inserting the CT primary winding. 8. Instrument transformers can help in reducing overall cost, since various instruments, including metering, relaying, diagnostic, and indicating instruments can all be connected to the same instrument transformer. CURRENT TRANSFORMERS (CT) 3.3 The primary winding of a current transformer is connected in series with the line carrying the main current. The secondary winding of the CT, where the current is many times stepped down, is directly connected across an ammeter, for measurement of current; or across the current coil of a wattmeter, for measurement of power; or across the current coil of a watt-hour meter for measurement of energy; or across a relay coil. The primary winding of a CT has only few turns, such that there is no appreciable voltage drop across it, and the main circuit is not disturbed. The current flowing through the primary coil of a CT, i.e., the main circuit current is primarily determined by the load connected to the main circuit and not by the load (burden) connected to the CT secondary. Uses of CT for such applications are schematically shown in Figure 3.1. Figure 3.1 CT for (a) current, and (b) power measurement
One of the terminals of the CT is normally earthed to prevent any accidental damage to the operating personnel in the event of any incumbent insulation breakdown. When a typical name plate rating of a CT shows 500/1 A 5 VA 5P20 it indicates that the CT rated primary and secondary currents are 500 A and 1 A respectively, its rated secondary burden is 5 VA, it is designed to have 5% accuracy and it can carry up to 20 times higher current than its rated value while connected in line to detect fault conditions, etc. THEORY OF CURRENT TRANSFORMERS 3.4 Figure 3.2 represents the equivalent circuit of a CT and Figure 3.3 plots the phasor diagram under operating condition of the CT. Figure 3.2 Equivalent circuit of a CT Figure 3.3 Phasor diagram of a CT VP = primary supply voltage EP = primary winding induced voltage VS = secondary terminal voltage
ES = secondary winding induced voltage IP = primary current IS = secondary current I0 = no-load current IC = core loss component of current IM = magnetising component of current rP = resistance of primary winding xP = reactance of primary winding rS = resistance of secondary winding xS = reactance of secondary winding RC = imaginary resistance representing core losses XM = magnetising reactance re = resistance of external load (burden) including resistance of meters, current coils, etc. xE = reactance of external load (burden) including reactance of meters, current coils, etc. NP = primary winding number of turns NS = secondary winding number of turns n = turns ratio = NS /NP φ = working flux of the CT θ = the “phase angle” of the CT δ = phase angle between secondary winding induced voltage and secondary winding current (i.e. phase angle of total burden, including secondary winding) β = phase angle of secondary load (burden) circuit only α = phase angle between no-load current I0 and flux φ The flux φ is plot along the positive x-axis. Magnetising component of current IM is in phase with the flux. The core loss component of current Ic, leads by IM 90°. Summation of IC and IM produces the no-load current I0, which is α angle ahead of flux φ. The primary winding induced voltage EP is in the same phase with the resistive core loss component of the current IC. As per transformer principles, the secondary winding induced voltage ES will be 180° out of phase with the primary winding induced voltage EP. The secondary current IS lags the secondary induced voltage ES by angle δ.