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-SHORT CIRCUIT CURRENT GOES THROUGH HERE 41 (1.20) Asymmetrical short-circuit current Asymmetrical short-circuit kva = (symmetrical current) (multiplying factor) = (symmetrical kva) (multiplying factor) DIAGRAMS One-line Diagram. The first step in making a short-circuit study is to prepare a one-line diagram showing all sources of short-circuit current, i.e., utility ties, generators, synchronous motors, induction motors, syn- chronous condensers, rotary converters, etc., and all significant circuit elements, such as transformers, cables, circuit breakers, etc. (Fig. 1.28). The second step is to make an impedance or reactance diagram showing all significant react- ances and resistances (Pig. 1.29). In the following pages this will be Make an Impedance or Reactance Diagram. C UTILITY SYSTEM I TRANS D GENERATOR GENERATOR CABLE E SHORT CIRCUIT LARGE CABLE J MOTOR FIG. 1.28 e diagram c 480 VOLT MOTORS , typical large industrial power system. FIG. 1.29 Reactonce diagram of system shown in Fig. 1.28. 42 SHORT-ClRCUIT.CURRENT CALCULAltNG PROCEDURES referred to as an impedance diagram, recognizing of course that only reactances will be used in many diagrams. The circuit element,s and machines considered in the impedance diagram depend upon many factors, i.e., circuit voltage, whether momentary or interrupting duty are to be checked, etc. The foregoing discussion and Table 1.2 explain when motors are to be considered and what motor reactances are to he used for checking the dut,y on a given circuit breaker or fuses of a given voltage class. There are other problems, i.e., (1) selecting the type and location of the short circuit in the system, (2) determining the specific reactance of a given circuit element or machine, and (3) deciding whether or not circuit resistance should be convidered. SELECTION OF TYPE AND LOCATION OF SHORT CIRCUIT Three-phase Short Circuits Generally Considered. I n most indus- trial systems, the maximum short-circuit current is obtained when a three-phase short circuit occurs. Short-rircuit-current magnitudes are generally less for line-to-neutral or line-to-line short circuits than for the three-phase short circuits. Thus, the simple three-phase short-circuit- current calculations will suffice for application of short-circuit protective devices in most industrial systems. In some very large systems where the high-voltage-system neutral is solidly grounded, maximum short-circuit current flows for a single phase-to-ground short rircuit. Such a system might be served from a large delta-Y trans- former bank or directly from the plant generators. Hence the only time that single-phase short-circuit-current calculations need be made is on large high-voltage systems (2400 volts and above) with solidly grounded generator neutrals or where main transformers that supply a plant from a utility are ronnected in delta on the high- voltage side (incoming line) and in Y with solidly grounded neutrals on the low-voltage (load) side. The calculations of unbalanced short-circuit currents in large power systems can best be done by symmetrical components, see Chap. 2. Normally, generator and large delta-Y transformer secondaries are grounded through a reactor or resistor to limit the short-circuit current for a single line-to-ground short circuit on the system to letis than the value of short-circuit current for a three-phase short circuit. Several tests have been made to evaluate the effect of arc drop at the point of short circuit in reducing the short-circuit-current magnitude. It was felt by some engineers that the current-limiting effect of the arc was pronounced. These tests showed, however, that for circuit voltages as low as 300 volts Unbalanced Short Circuits in Large Power Systems. Bolted Short Circuits Only Are Considered. SHORT-CIRCUIT-CURRENT CALCULATING PROCEDURES 43 there may be no substantial difference in the current that flows for a bolted short circuit and when there is an arc of several inches of length. These test,s also confirmed modern calculating procedure as an accurate method of estimating the short-circuit-current magnitude in systems of 600 volts and less. .4rcs cannot be counted on to limit the flow of short-circuit currents even in louvoltage circuits; so short-circuit-current calculations for all circuit voltages are made on the basis of zero impedance at the point of short circuit, or, in other words, a bolted short circuit. This materially simplifies calculation because all other circuit impedances are linear in magnitude, whereas arcs have a nonlinear impedance characteristic. At What Point in the System Should the Short Circuit Be Considered to Occur? The maximum short-circuit current will flow through a cir- cuit breaker, fuse, or motor starter when the short circuit occurs at the 4160V. I I I $? MAX.SHORT CIRCUIT DUTY ON $- $EW:RS FOR SHORT CIRCUIT BREAKERS ON THIS BUS 1 T ?; A&?? Y T T - 3 & + * + r y r-x MAX. DUTY FOR THESE BREAKERS OCCURS FOR SHORT CIRCUIT HERE FIG. 1.30 Location of faults for maximum Short-circuit duty on circuit breakers. 44 SHORT-CIRCUIT-CURRENT CALCULATING PROCEDURES terminals of the circuit breaker, etc. (Fig. 1.30). These devices, if properly applied, should be capable of opening the maximum short- circuit current that can flow through them. Therefore, only one short- circuit location (at the terminal of the device) need be considered for checking the duty on a given circuit breaker, fuse, or motor starter. DETERMINING REACTANCES AND RESISTANCES OF CIRCUITS AND MACHINES Typical reactances of circuit elements and machines are given at the end of this chapter. These tables may be used as a basis for assigning values to the various elements of the impedance diagram. The reactances and resistances are all line- to-neutral values for one phase of a three-phase circuit. Where the reactances of a specific motor, generator, or transformer are known, these values should he used in lieu of the typical reactances in this chapter. The following is a guide to general practice in selecting and representing reactances. Whether or not the reactance of a certain circuit element of a system is significant depends upon the voltage rating of the system where the short circuit occurs. In all cases, generator, motor, and transformer reactances are used. In systems rated above 600 volts, the reactances of short bus runs, current transformers, disconnecting switches, circuit breakers, and other circuit elements of only a few feet in length are so low that they may be neglected without significant error. In circuits rated 600 volts or less, the reactances of low-voltage current transformers, air circuit breakers, disconnecting switches, low-voltage bus runs, etc., may have a significant hearing on the magnitude of total short- circuit current. As a general guide, the reactance of the low-voltage secondary-switch- gear section in load-center unit substations with closely coupled trans- formers and secondary switchgear is not significant for all voltages of 600 volts and below. However, where there are several transformers or generators paralleled on one bus, or connections several feet long between a single transformer and its switchgear, reactances of the bus connections will generally be significant and should be considered in short-circuit calculations. In systems of more than about 1000 kva on one bus a t 208Y/120 or 240 volts, reactance of all circuit components such as short bus runs, current transformers, circuit breakers, etc., should be included in the short-circuit study. I n systems of more than about 3000 kva on one bus a t 480 volts or 600 volts, reactances