Electricity: power supply or regulation systems – For reactive power control – Using impedance
Reexamination Certificate
2000-10-02
2001-07-03
Berhane, Adolf Deneke (Department: 2838)
Electricity: power supply or regulation systems
For reactive power control
Using impedance
Reexamination Certificate
active
06255806
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a supply device for power supply to an electronic unit for a controllable semiconductor element in a semiconductor valve in a shunt-connected thyristor-switched capacitor, the capacitor being intended to carry an alternating current with a known period, the semiconductor valve comprising a snubber circuit for transient protection of the semiconductor element with a first and a second terminal only.
BACKGROUND ART
It is known to connect, to electric power networks in shunt connection, static compensators for compensation of the reactive power consumption of the power network and of equipment connected to the power network. One type of such compensators comprises at least one and usually a plurality of thyristor-switched capacitors (TSC). A thyristor-switched capacitor substantially comprises a capacitor in series connection with a controllable semiconductor valve. In addition thereto, an inductive element, an inductor, is usually arranged in series connection with the capacitor to limit the rate of change of the current through the capacitor when the capacitor is connected to the power network and to avoid resonance phenomena with inductive components located in the power network.
The controllable semiconductor valve comprises at least two controllable semiconductor elements, usually thyristors, arranged in anti-parallel connection. By bringing the semiconductor elements in a conducting state, that is, by controlling their firing time relative to the phase position of the voltage of the ac network, the capacitor may be coupled to the power network for generating reactive power. It is to be understood that, in this application, the concept capacitor comprises also those cases where the capacitor is composed of a plurality of mutually connected capacitive elements, sub-capacitors, which are all commonly coupled by the controllable semiconductor valve. Further, it is to be understood that the semiconductor valve may comprise a plurality of mutually series-connected, and then usually pair-wise antiparallel-connected, semiconductor elements, which are each controlled by a firing order. A control device generates individual firing pulses for the semiconductor elements included in the semiconductor valve.
FIG. 1
illustrates a static compensator of the kind described above, which is connected via a transformer TR to an ac network N
1
. The compensator comprises three capacitors CA, CB, CC, each being shunt-connected to a common voltage busbar BB via a controllable semiconductor valve VA, VB, VC, respectively, and an inductor LA, LB, LC, respectively. The semiconductor valves are schematically illustrated in the figure with two semiconductor elements T
1
, T
2
in antiparallel connection. Control equipment CEQ supplies firing orders COA, COB, COC, respectively, to the semiconductor valves.
For a general description of thyristor-switched capacitors and control thereof, reference is made to, for example, {dot over (A)}ke Ekström: High Power Electronics HVDC and SVC, Stockholm 1990, in particular pages 10-1 to 10-7.
Since the current through the thyristor-switched capacitor in steady state has a phase position
90
electrical degrees in advance of the voltage across the same, the two antiparallel-connected semiconductor elements of the semiconductor valve should be given firing orders alternately at the times when the time rate of change of the fundamental tone for the voltage across the thyristor-switched capacitor changes sign from a positive value to a negative value, and inversely. If the phase position of the voltage is defined such that, at 0°, its amplitude is zero and increasing in a positive direction, under steady-state conditions these sign reversals take place at the electrical angles 90° and 270°. When the above-mentioned time rate of change changes sign from a positive to a negative value, a firing order should be given to that of the semiconductor elements, the conducting direction of which coincides with the expected current direction in the next interval, that is, with the above-mentioned convention, in the interval 90° to 270°. When the mentioned time rate of change again changes signs, a firing order is given to the other semiconductor element, the conducting direction of which coincides with the expected current direction in the interval which is then to follow, that is, with the above-mentioned convention, in the interval 270° to 450°.
When the generation of firing orders is brought to an end, for example in dependence on a voltage control system for maintaining the voltage in the ac network or the voltage busbar BB constant, the current through the semiconductor valve will cease at the next zero crossing of the current. The voltage of the capacitor thus remains at a level determined by the voltage of the power network when the current through the capacitor was forced to cease. When a firing order is again generated, according to the criterion mentioned above, and the voltage of the voltage busbar has remained unchanged, the connection of the capacitor occurs, in principle, without any transition phenomena in current and voltage.
Usually, each semiconductor element is associated with an electronic unit with an indicating device which, in some manner known per se, generates indicating signals, indicating that an off-state voltage exists across the semiconductor elements, in the respective conducting direction of the semiconductor elements. Typically, an indicating signal is generated when the off-state voltage amounts to about 50 V across a semiconductor element in the form of a thyristor. These indicating signals are usually transferred from the potential of the semiconductors via light guides to the control equipment arranged at ground potential.
Likewise, in some manner known per se, the control equipment generates, in dependence on received indicating signals, firing orders and supply these to the electronic units, also usually via light guides. In general, therefore, the electronic units comprise circuits with components, for example photodiodes, for transforming the firing order in the form of light into electrical firing signals for each of the semiconductor elements.
To limit current and voltage stresses on the semiconductor elements in connection with a change of their conducting state, a transient protection circuit, a so-called snubber circuit, is usually arranged in parallel connection with the semiconductor elements, this circuit comprising a series connection of resistive and capacitive components.
The above-mentioned functions of the electronic units require electrical energy and the electronic units must therefore have access to a power supply. This power supply should be galvanically separated from ground potential and the electric power should thus be supplied from that ac network to which the thyristor-switched capacitor is connected.
The electronic units usually also comprise a gate circuit which forwards, to the semiconductor elements, firing orders received from the control equipment for firing the respective semiconductor element in dependence on the voltage level of the supply voltage.
A known way of arranging this power supply for thyristor-switched capacitors is illustrated in FIG.
2
. The figure schematically illustrates parts of a semiconductor valve of the kind described above, which comprises two thyristors T
1
, T
2
in antiparallel connection, a snubber circuit SC with a snubber capacitor CS and a snubber resistor RS in series connection. Supply devices FD
1
and FD
2
, respectively, are adapted to supply electronic units (not shown in the figure) for the thyristors T
1
, T
2
, respectively, with electrical energy. Each one of the supply devices comprises an energy storage in the form of a capacitor, in the figure designated C
1
and C
2
, respectively. The voltage across the capacitors, in the figure designated UF
1
and UF
2
, is supplied to the respective electronic units. A current transformer—not shown in its entirety in the figure—with a primary winding, through which the alternati
Broo Björn
Seppanen Ari
ABB AB
Berhane Adolf Deneke
Connolly Bove Lodge & Hutz
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