Chapter 7. Instrument Transformers

Chapter 7. Instrument Transformers - 7 Instrument...

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7 Instrument Transformers Randy Mull ikin Kuhlman Electric Corporat ion 7.1 Overv iew . ............................................................................. 7 -1 7.2 Transformer Basics . ............................................................. 7 -2 Core Design . Burdens . Relative Polarity . Industr y Standards . Accuracy Classes . Insulation Systems . Thermal Ratings . Primar y Winding 7.3 Voltage Transformer . ......................................................... 7 -13 Overvoltage Ratings . VT Compensation . Short-Circuit Operation . VT Connections . Ferroresonance . VT Construction . Capacitive-Coupled Voltage Transformer . Optical Voltage Transducer 7.4 Current Transformer . ........................................................ 7 -18 Saturation Curve . CT Rating Factor . Open-Circuit Conditions . Over voltage Protection . Residual Magnetism . CT Connections . Construction . Optical Current Transducers . Proximity Effects . Linear Coupler . Direct-Current Transformer . Slipover CT Installations . Combination Metering Units . Primar y Metering Units . New Horizons This section covers the fundamental basics and theor y of operation of instrument transformers. Common ty pes of instrument transformers and construction hig hlig hts w ill be discussed. Application features and characteristics of instrument transformers w ill be covered w ithout prov iding details of three-phase circuit fundamentals, fault analysis, or the operation and selection of protective dev ices and measuring instruments. Thoug h incomplete, this section covers the common practices used in industr y over the last 30 years. 7.1 Overview Instrument transformers are primarily used to prov ide isolation between the main primar y circuit and the secondar y control and measuring dev ices. This isolation is achieved by magnetically coupling the two circuits. In addition to isolation, levels in magnitude are reduced to safer levels. Instrument transformers are divided into two categories: voltage transformers ( VT) and current trans- formers (CT). The primary winding of the VT is connected in parallel with the monitored circuit, while the primary winding of the CT is connected in series ( see Figure 7.1 ). The secondary windings proportionally transform the primar y levels to typical values of 120 V and 5 A. Monitoring devices such as wattmeters, power-factor meters, voltmeters, ammeters, and relays are often connected to the secondar y circuits. ß 2006 by Taylor & Francis Group, LLC.
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7.2 Transformer Basics An ideal transformer (see Figure 7.2) magnetically induces from the primary circuit a level exactly proportional to the turns ratio into the secondary circuit and exactly opposite in phase, regardless of the changes occurring in the primary circuit. A review of the general relationships of the ideal case yields the transformation ratio V P = V S ¼ N P = N S (7 : 1) and the law of conservation of energy yields V P I p ¼ V S I S (7 : 2) I P N P ¼ I S N S (7 : 3) where V P ¼ primary-terminal voltage V S ¼ secondary-terminal voltage I P ¼ primary current I S ¼ secondary current N P ¼ primary turns N S ¼ secondary turns V VT VT CT CT H1 H2 X2 X2 X1 X1 H1 H2 f A f B f A f B A R R W A R R V W FIGURE 7.1 Typical wiring and single-line diagram.
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This note was uploaded on 10/19/2010 for the course ENGINEERIN ELEC121 taught by Professor Tang during the Spring '10 term at University of Liverpool.

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Chapter 7. Instrument Transformers - 7 Instrument...

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