Electricity: power supply or regulation systems – For reactive power control – Using saturable inductor
Patent
1983-02-25
1985-03-05
Wong, Peter S.
Electricity: power supply or regulation systems
For reactive power control
Using saturable inductor
323210, G05F 170
Patent
active
045033803
DESCRIPTION:
BRIEF SUMMARY
This invention relates to static reactive compensators for reactive power compensation and voltage control in alternating current (a.c.) transmission systems.
The use of static reactive compensators is now well established, using one of the two alternative key components, i.e., saturated reactors with massive iron cores or air-cored linear reactors controlled by phase-controlled thyristors.
FIG. 1 shows the basic circuit, voltage/current characteristic and waveforms of a typical thyristor-controlled reactor (TCR) system. A linear reactor 1, that is, one with an air rather than an iron core, is connected in series with a thyristor switching arrangement 2 between an a.c. high voltage line 3 and a neutral point 4. The switching arrangement comprises thyristors in inverse parallel to provide conduction in both directions, several thyristors being connected in series in each path to withstand the voltage.
The line voltage V is sensed by a control circuit 5 by way of a stepdown transformer 6, the control circuit adjusting the firing angle a of the thyristors so as to increase the conduction period as the line voltage increases.
FIG. 1(b) shows the voltage/current characteristic for the circuit of FIG. 1(a). The broken line OC is the characteristic (of constant reactance) that would obtain if the thyristors were fully conducting permanently. However by retarding the firing angle from 90.degree. at point C to 180.degree. at point A the reactance is effectively increased as the current falls and the characteristic AC is obtained. The thyristors may, however, cater for more than the full rated current I.sub.o so that normal operation is on the portion AB. At the point B, a constant current control would operate, producing the characteristic BD, up to the voltage corresponding to point D above which the characteristic shifts to the constant reactance line at and above the point E. FIG. 1(c) shows the voltage and current waveforms for the three points A, B and C.
A known arrangement such as that of FIG. 1 has the advantage over saturated reactors of cheapness but has the considerable disadvantages of a large harmonic current content and thyristors which have to carry fault currents and which therefore have to be down-rated, making them correspondingly more expensive.
An alternative to, and in fact a forerunner of, the above TCR compensator is the saturated reactor compensator shown in FIG. 2. A saturated reactor SR is connected in series with a slope correcting capacitor 7 between the output of a transformer (6) and a neutral point, the transformer being connected to the supply line and having tap-changing facilities. The saturated reactor has a non-linear characteristic as shown in FIG. 2(b) the main part of which consists of a constant positive slope commencing at a knee point which gives the minimum operating voltage or reference voltage V.sub.s. The slope reactance (i.e., the incremental reactance) is reduced still further, towards zero, by the slope correcting capacitor 7 whose negative reactance is chosen to just cancel the positive slope reactance. The solid line shows the uncorrected characteristic and the broken line the corrected version.
Since the saturated reactor is essentially a single voltage device it is necessary to provide a tap changer on the supply transformer to cater for voltages above and below the basic voltage V.sub.s. Two such additional characteristics are shown in FIG. 2(b).
The saturated reactor has the advantage of substantially instantaneous and automatic reaction to supply voltage changes. It can also sustain large overloads and can be designed to draw a relatively small proportion of current harmonics. Examples of such harmonically compensated reactors are given in UK Patent Specification Nos. 1303634 and 1545491.
One disadvantage of the saturated reactor is the above mentioned necessity for transformer tap changers to vary the operating voltage range. Another disadvantage is the inability of the associated slope correcting capacitor to sustain overload currents. The capacitor would th
REFERENCES:
patent: 3195038 (1965-07-01), Fry
patent: 3222592 (1965-12-01), Kellogg
patent: 3551799 (1970-12-01), Koppelmann
patent: 3842342 (1974-10-01), Friedlander et al.
patent: 4210860 (1980-07-01), Rosa et al.
Associated Electrical Industries Limited
Wong Peter S.
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