BASIC INSTRUMENT TRANSFORMER INFORMATIONS AND DISCUSSIONS TAKEN FROM STANDARD HANDBOOK FOR ELECTRICAL ENGINEERS

BASIC INSTRUMENT TRANSFORMER INFORMATIONS AND DISCUSSIONS TAKEN FROM STANDARD HANDBOOK FOR ELECTRICAL ENGINEERS
How does instrument transformers being use in the power system?

The discussion that follows is a short summary of information on instrument transformers as measurement elements. For more extensive information, consult American National Standard C57.13, Requirement for Instrument Transformers; American National Standards Institute; American National Standard C12, Code for Electricity Metering; Electrical Meterman’s Handbook, Edison Electric Institute; manufacturer’s literature; and textbooks on electrical measurements.

AC range extension beyond the reasonable capability of indicating instruments is accomplished with instrument transformers, since the use of heavy-current shunts and high-voltage multipliers would be prohibitive both in cost and power consumption. Instrument transformers are also used to isolate instruments from power lines and to permit instrument circuits to be grounded.

The current circuits of instruments and meters normally have very low impedance, and current transformers must be designed for operation into such a low-impedance secondary burden. The insulation from the primary to secondary of the transformer must be adequate to withstand line-to-ground voltage, since the connected instruments are usually at ground potential. Normal design is for operation with a rated secondary current of 5 A, and the input current may range upward to many thousand amperes. The potential circuits of instruments are of high impedance, and voltage transformers are designed for operation into a high-impedance secondary burden. In the usual design, the rated secondary voltage is 120 V, and instrument transformers have been built for rated primary voltages up to 765 kV.

With the development of higher transmission-line voltages (350 to 765 kV) and intersystem ties at these levels, the coupling-capacitor voltage transformer (CCVT) has come into use for metering purposes to replace the conventional voltage transformer, which, at these voltages, is bulkier and more costly.

The metering CCVT, shown in Fig. 3-11, consists of a modular capacitive divider which reduces the line voltage V1 to a voltage V2 (10–20 kV), with a series-resonant inductor to tune out the high impedance and make available energy transfer across the divider to operate the voltage transformer which further reduces the voltage to VM, the metering level. Required metering accuracy may be 0.3% or better.

Instrument transformers are broadly classified in two general types: (1) dry type, having molded insulation (sometimes only varnish-impregnated paper or cloth) usually intended for indoor installation, although large numbers of modern transformers have molded insulation suitable for outdoor operation on circuits up to 15 kV to ground; and (2) liquid-filled types in steel tanks with high-voltage primary terminals, intended for installation on circuits above 15 kV. They are further classified according to accuracy: (1) metering transformers having highest accuracy, usually at relatively low burdens; and (2) relaying and control transformers which in general have higher burden capacity and lower accuracy, particularly at heavy overloads. This accuracy classification is not rigid, since many transformers, often in larger sizes and higher voltage ratings, are suitable for both metering and control purposes.

Another classification differentiates between single and multiple ratios. Multiple primary windings, sometimes arranged for series-parallel connection, tapped primary windings, or tapped secondary windings, are employed to provide multiple ratios in a single piece of equipment. Current transformers are further classified according to their mechanical structure: (1) wound primary, having more than one turn through the core window; (2) through type, wherein the circuit conductor (cable or busbar) is passed through the window; (3) bar type, having a bar, rod, or tube mounted in the window; and (4) bushing type, that is, through type intended for mounting on the insulating bushing of a power transformer or circuit breaker.

Current transformers, whose primary winding is series connected in the line, serve the double purposes of (1) convenient measurement of large currents and (2) insulation of instruments, meters, and relays from high-voltage circuits. Such a transformer has a high-permeability core of relatively small cross section operated normally at a very low flux density. The secondary winding is usually in excess of 100 turns (except for certain small low-burden through-type current transformers used for metering, where the secondary turns may be as low as 40), and the primary is of few turns and may even be a single turn or a section of a bus bar threading the core. The nominal current ratio of such a transformer is the inverse of the turns ratio, but for accurate current measurement, the actual ratio must be determined under loading corresponding to use conditions. For accurate power and energy measurement, the phase angle between the secondary and reversed primary phasor also must be known for the use condition. Insulation of primary from secondary and core must be sufficient to withstand, with a reasonable safety factor, the voltage to ground of the circuit into which it is connected; secondary insulation is much less, since the connected instrument burden is at ground potential or nearly so.

The overload capacity of station-type current transformers and the mechanical strength of the winding and core structure must be high to withstand possible short circuits on the line. Various compensation schemes are used in many transformers to retain ratio accuracy up to several times rated current. The secondary circuit—the current elements of connected instruments or relays—must never be opened while the transformer is excited by primary current, because high voltages are induced which may be hazardous to insulation and to personnel and because the accuracy of the transformer may be adversely affected.

Voltage transformers (potential transformers) are connected between the lines whose potential difference is to be determined and are used to step the voltage down (usually to 120 V) and to supply the voltage circuits of the connected instrument burden. Their basic construction is similar to that of a power transformer operating at the same input voltage, except that they are designed for optimal performance with the high-impedance secondary loads of the connected instruments. The core is operated at high flux density, and the insulation must be appropriate to the line-to-ground voltage.

Standard burdens and standard accuracy requirements for instrument transformers are given in this LINK.

Accuracy. Most well-designed instrument transformers (provided they have not been damaged or incorrectly used) have sufficient accuracy for metering purposes. See Sec. 10 for typical accuracy curves. Where higher accuracy is required, see Appendix D of ANSI C12, The Code for Electricity Metering. Another comparison method uses a “standard” transformer of the same nominal rating as the one being tested. Accuracies of 0.01% are attainable. Commercial test sets are available for this work and are widely used in laboratory and field tests. Commercial test sets based on the current-comparator method and capable of 0.001% accuracy are also available. For further details, see ANSI Standard C57.13.

source: Standard Handbook for Electrical Engineers