Electricity: power supply or regulation systems – For reactive power control
Reexamination Certificate
1999-09-30
2001-06-12
Wong, Peter S. (Department: 2838)
Electricity: power supply or regulation systems
For reactive power control
C363S086000
Reexamination Certificate
active
06246218
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for stabilizing a power supply network against fluctuations of the reactive power of an electrical system as claimed in the preamble of patent claim
1
, and to a control device of an electrical system as claimed in the preamble of patent claim
6
. The electrical system is, in particular, an arc furnace.
2. Discussion of Background
A general problem in power electronics is the product of a wattless component, which produces a reaction on the public power network and interferes with other subscribers to the same network. This is particularly the case when the reactive power varies with time, so that network voltage fluctuations occur which can be seen, for example, as so-called flicker (light fluctuations on luminaries). Such voltage fluctuations occur, for example, in arc furnaces as a result of irregular and uncontrolled changes in the arc.
Widely differing devices are therefore known for stabilizing a power supply network, which attempt to stabilize fluctuations in the reactive load. One conventional method is to vary the triggering angle as a function of the changing load, in order in this way to compensate for the reactive power.
U.S. Pat. No. 5,463,653 describes, for example, such a static VAR compensator (SVC), in which each secondary coil is connected to a thyristor and a freewheeling control loop. The thyristor is triggered with a variable triggering angle in order to obtain a reactive power that is as constant as possible, independently of the changing impedance of the load.
In contrast, U.S. Pat. No. 5,155,740 discloses a flicker compensation device for DC arc furnaces, in which the reactive power is intended to be controlled as a function of a detected reactive power of the arc furnace.
Such compensation devices are complex, and thus relatively expensive.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a novel method and a control device for stabilizing a power supply network against fluctuations in the reactive power of an electrical system, in order to prevent such interference reactions on the network, with little complexity.
This object is achieved by a method having the features of patent claim
1
, and by a control device having the features of patent claim
6
.
A further object of the invention is to reduce the size of the transformer, and thus the costs for its production. This object is achieved by an apparatus having the features of patent claim
8
.
In the method according to the invention, switching sequences are used which allow the fundamental-frequency power factor to be kept at least approximately equal to one at all times, as a function of the network current. This means that the fundamental-frequency reactive power is thus always at least approximately zero and fluctuations have scarcely any effect on the network.
A network current is in this case switched on and off at least approximately symmetrically with respect to its extreme values. Voltage segments which each extend as far as an extreme value of the network voltage are applied to primary windings of a transformer. In this case, each voltage segment of a positive or negative cycle is repeated in the next cycle with the opposite mathematical sign.
In a preferred variant of the method, a three-phase system is used, in which the voltage segments are applied to the at least one primary winding, phase-shifted through 120° with respect to one another. An addition of all the voltage segments preferably results in 100% of all the voltage values of one phase.
In a further preferred variant of the method, the voltage segments which are applied via the primary windings are pulse-width modulated. This prevents relative harmonics, which would otherwise cause negative reactions on the supply network and to sensitive equipment connected to it.
In one preferred embodiment of the control device according to the invention for carrying out the method, the primary windings are composed of three primary coils or coil systems, each of which can be connected to in each case one phase of the supply network. Each coil or each coil system is preferably connected to a freewheeling control loop.
In another preferred embodiment, only one primary coil or one primary coil system is provided, which has three phases applied to it, with each phase input being provided with a switch element. Furthermore, means are provided to prevent interference with the individual phases. Such a means is, for example, a network commutator.
In one preferred embodiment of the invention, all the primary windings are connected to the same secondary windings. These secondary windings may in this case comprise a single coil or a coil system. This allows the size of the furnace transformer to be reduced, as a result of which its production costs can be greatly reduced.
It is furthermore advantageous that the method according to the invention can be used for both DC and AC arc furnaces.
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patent: 3955133 (1976-05-01), Schroder et al.
patent: 3968422 (1976-07-01), Waldmann
patent: 4068159 (1978-01-01), Gyugyi
patent: 4698581 (1987-10-01), Shimamura et al.
patent: 5155740 (1992-10-01), Ao et al.
patent: 5412557 (1995-05-01), Lauw
patent: 5463653 (1995-10-01), Du Parc et al.
patent: 3040330A1 (1982-06-01), None
patent: 37 33077 A1 (1989-04-01), None
patent: 19617191A1 (1997-11-01), None
patent: 0 707369 A1 (1996-04-01), None
Asea Brown Boveri AG
Burns Doane Swecker & Mathis L.L.P.
Laxton Gary L.
Wong Peter S.
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