Short circuit voltage

FIGURE 17.4 External short circuit voltage data fora 14[Pg.392]

It is easy to notice, that the protection against a short-circuit failure in the X-ray tube circuit implements due to the "soft" outer characteristic of the apparatus main circuit. The overvoltage protection at emergencies in the control system happens due to the redistribution of the magnetie flow, created by power winding I, between the 3,6 control yokes. Therefore the voltage on the X-ray apparatus anode drops approximately two times. 

The magnetic regulators allow to synthesize in one module of the X-ray apparatus main cir-euit commutator, form converter, noncontact smooth ac voltage amplitude regulator, para-metrie stabilizer, ac supply filter, fimetional protection against the short-circuit in the X-ray tube and protection against emergencies in the control circuits. 

There are several methods to determine and compare the resistance to partial discharges. Some tests are done on finished cables, such as the U-bend test, and others are done on laboratory samples molded from the insulation, that are subjected to partial discharges created by sharp objects, such as needles under high voltages. The tests compare either the energy required or the length of time required to erode or fail (short circuit) samples of similar thickness. 

Under both short-circuit and open-circuit conditions, a solar cell produces no electric power, the power is consumed internally in the cell and is dissipated as heat. When a resistive load is connected to a cell in sunlight, a photogenerated voltage, F, is induced across the load and a current flows through it. The existence of requites that the flow of majority carriers be reduced from that in the open-circuit condition there must be a higher battier potential than in the open-circuit case (Fig. 2d). This higher barrier potential (V6 — ) indicates a smaller reduction from Since the photogenerated... 

The photogenerated current is in the same direction as /, but is always less than because the battier potential under load conditions is always less than F, which results in a larger flow of majority carriers than that in a short-circuited cell. Thus, when a solar cell is under load, the current and voltage are always less than and lU, respectively this condition is the curve-factor loss. Depending on the characteristics of the particularp—n junction and on the cell operating conditions, there is an optimal load resistance that maximizes the power output of the cell, ie, the product of its current and voltage. 

An electrorefining plant may operate with either an acid or an alkaline bath. The acid bath contains stannous sulfate, cresolsulfonic or phenolsulfonic acids (to retard the oxidation of the stannous tin in the solution), and free sulfuric acid with P-naphthol and glue as addition agents to prevent tree-like deposits on the cathode which may short-circuit the cells. The concentration of these addition agents must be carefliUy controlled. The acid electrolyte operates at room temperature with a current density of ca 86—108 A/m, cell voltage of 0.3 V, and an efficiency of 85%. Anodes (95 wt % tin) have a life of 21 d, whereas the cathode sheets have a life of 7 d. Anode slimes may be a problem if the lead content of the anodes is high the anodes are removed at frequent intervals and scmbbed with revolving bmshes to remove the slime (7). 

Tia, as well as some other metals, can undergo a phenomenon where tiny metal filaments, called whiskers, form randomly on parts used ia electrical apphcafions. In low voltage, miniature circuitry, whiskers can cause short circuits. Alloys having 2% lead minimum or 0.5% bismuth or heat treatments are said to overcome the problem. A specification for electroplated fin coafings is available (133). 

Features common to most high-voltage dc powder supplies include rev ersible polaritv, short-circuit and current-limiting protection, and automatic residual-charge dissipation to ground. 

High-voltage contactor-type motor controls depend on power fuses for short-circuit protection. The fuses are coordinated with the overload relays to protect the motor circuit over the full range of fault conditions from overload conditions to solid maximum-current short circmts. 

These reactances are measured by creating a fault, similar to the method discussed in Section 14.3.6. The only difference now is that the fault is created in any of the phases at an instant, when the applied voltage in that phase is at its peak, i.e. at Vni- so that the d.c. component of the short-circuit current is zero and the waveform is symmetrical about its axis, as shown in Figure 13.19,... 

The p.f. during a short-circuit as noted in Section 13.4.1 is quite low, and is normally of the order of 0.1. The current will now lag the voltage by nearly 84° (Figure... 

When the short-circuit occurs at a current zero, i.e., when the applied voltage is almost at its peak, the voltage and current waves will follow Figure 13.19, the cuiTent lagging the voltage by almost 84°. The current will now be almost symmetrical. 

Supposing the current and the voltage waves both have some value on their respective wave forms at the instant of short-circuit. The current will again tend to become somewhat asymmetrical but not fully. The content of asymmetry will depend upon the instant at which the short-circuit condition occurs on the current wave and the p.f. of the faulty circuit (Figure 13.27). The higher the recovery voltage at the instant of fault, the lower will be the asymmetry (at l/, , the d.c. component will be zero) and vice versa (at Fq. the d.c. component will be the maximum). 

Figure 13.27 Approximate illustration of a short-circuit condition occurring when both voltage and current waves are not at their natural zeros. Current shifting its zero axis from A, Aj to B B2 to rise from zero again at the instant of short-circuit...

To isolate the circuit after the lest is over and to also interrupt the test in between, in case the test piece fails. The breaker must possess an instantaneous capacity of more than the test current and the short-circuit MVA of the feeding generator. To achieve the desired voltage it must be suitable to perform the duties of repeated short-circuit tests. 

For a switching device (which has not been previously tested for a short-circuit test). This should be closed and held in the normal service position. The test voltage (that would generate the required level of fault current) may be applied on one set of terminals, the other terminals being shorted. The test may be continued until the short-circuit device operates to clear the fault, but in no case for less than 10 cycles. In LT assemblies the point where the short-circuit is created should be 2 0.4 m from the nearest point of supply. 

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