Supply device for power supply to an electronic unit in a semiconductor valve in a shunt-connected thyristor-switched capacitor

A supply device (FD1, FD2) for power supply to an electronic unit (EU1, EU2) for a controllable semiconductor element (T1, T2) in a semiconductor valve in a shunt-connected thyristor-switched capacitor (CA). The capacitor being intended to carry an alternating current with a known period (T), the semiconductor valve comprising a snubber circuit (SC) with a first and a second terminal (CS1 and CS2, respectively) only. The supply device has an energy storage (C1, C2) for storing electrical energy, a valve terminal (J13, J23), a snubber terminal (J12, J22), a supply terminal (J11, J21) connected to the energy storage and a first current path from the snubber terminal to the supply terminal. The energy storage is designed to store an amount of energy which is larger than the energy requirement of the electronic unit during one cycle of the alternating current but smaller than its energy requirement during two cycles.

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 N1. 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 T1, T2 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, Ake Ekstrom: 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.degree., its amplitude is zero and increasing
 in a positive direction, under steady-state conditions these sign
 reversals take place at the electrical angles 90.degree. and 270.degree..
 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.degree. to 270.degree..
 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.degree. to 450.degree..
 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 T1, T2 in antiparallel connection, a snubber
 circuit SC with a snubber capacitor CS and a snubber resistor RS in series
 connection. Supply devices FD1 and FD2, respectively, are adapted to
 supply electronic units (not shown in the figure) for the thyristors T1,
 T2, respectively, with electrical energy. Each one of the supply devices
 comprises an energy storage in the form of a capacitor, in the figure
 designated C1 and C2, respectively. The voltage across the capacitors, in
 the figure designated UF1 and UF2, 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 alternating current
 through the thyristor-switched capacitor flows, has a number of separate
 secondary windings, two of which, designated S1 and S2, respectively, are
 shown in the figure. The supply device FD1 further comprises diodes Da1
 and Da2. When current flows through the secondary winding S1, a current
 path through the supply device FD1 is closed via the diode Da2, the
 capacitor C1, and via a Zener diode Zc in a supply device FD2', which is
 adapted for power supply of an electronic unit (not shown) for a thyristor
 T2', connected in series with the thyristor T2. In the event that the
 supply device FD2' does not exist, the current path is instead closed via
 a Zener diode Za' in the supply device FD1. The capacitor C1 is thus
 supplied with energy via the current through the secondary winding S1.
 The thyristor T1 has one anode terminal TA1 and one cathode terminal TC1.
 When no current flows through the current transformer, that is, when the
 semiconductor elements are in a non-conducting state, and when the voltage
 between the anode and cathode terminals exhibits a positive time rate of
 change, a small amount of energy is supplied to the capacitor C1 through a
 current path from the anode terminal TA1 via a diode Db1 in the supply
 device FD2, the snubber circuit SC and a diode D11. Conventionally, the
 energy storage is designed to contain energy sufficient for the safe
 function of the electronic unit for a plurality of cycles of the
 alternating current, which, however, also implies that a plurality of ac
 cycles are required for supplying, via the snubber circuit, an amount of
 energy which is large enough for the energy storage to attain a voltage
 level and an energy content sufficient for the safe function of the
 electronic unit. Thus, this solution presupposes that the energy
 requirement of the electronic unit is ensured via supply from a current
 transformer, which component, of course, complicates and renders more
 expensive the system for energy supply to the electronic units.
 FIG. 3 illustrates a known system for energy supply to electronic units of
 a corresponding kind in a semiconductor valve included in a converter for
 conversion between alternating current and high-voltage direct current. A
 thyristor T1 included in the semiconductor valve has a snubber circuit SC
 connected between the anode terminal TA1 and the cathode terminal TC1. In
 this case, the snubber circuit comprises a first series connection of a
 resistor RS1 and a capacitor CS1, which in turn is connected in series
 with a second series connection of a resistor RS2 and a capacitor CS2. A
 third series connection of a capacitor CS3 and a resistor RS3 is connected
 between the point of connection between the above-mentioned capacitors and
 a supply device FDH for energy supply of an electronic unit (not shown in
 the figure) for the thyristor T1. When the voltage between the anode and
 cathode terminals exhibits a positive time rate of change, a current path
 is formed from the anode terminal via the first and third series
 connections, a diode D11 in the supply device and an energy storage in the
 form of a capacitor C1h to the cathode terminal. The voltage across the
 capacitor, designated UF1h in the figure, is supplied to the electronic
 unit. In this case, the energy storage is designed to be charged during
 each cycle, via the current path mentioned, with an amount of energy which
 is large enough for the energy storage to attain a voltage level and an
 energy content sufficient for the safe function of the energy unit during
 one cycle of the alternating current. In this case, the snubber circuit is
 designed as a voltage-divider circuit, which implies that only part of the
 current through the snubber circuit is supplied to the supply device.
 SUMMARY OF THE INVENTION
 The object of the invention is to achieve an improved supply device or the
 kind mentioned in the introductory part of the description, which
 eliminates the need of energy supply via a current transformer, utilizes
 in full the current through the snubber circuit for energy supply to the
 supply device and to the electronic unit, and which, through the design of
 its circuitry, contributes to a simple, reliable and economically
 advantageous design.
 According to the invention, this is achieved by arranging the snubber
 circuit of the semiconductor element with only one first and one second
 terminal, the supply device with an energy storage for storing electrical
 energy, a valve terminal, a snubber terminal, a supply terminal connected
 to the energy storage, and with a first current path from the snubber
 terminal to the supply terminal, the valve terminal for connection to the
 cathode terminal, the snubber terminal for connection to one of the
 terminals of the snubber circuit, and the supply terminal for connection
 to the electronic unit, the snubber circuit for connection such that a
 second current path, for carrying a charge current to the energy storage
 in a direction from the snubber terminal to the cathode terminal, is
 formed from the anode terminal through the snubber circuit to the snubber
 terminal, and therefrom via the energy storage to the cathode terminal,
 and by designing the energy storage to store an amount of energy which is
 larger than the energy requirement of the electronic unit during one cycle
 of the alternating current but smaller than its energy requirement during
 two cycles.
 In this way, the current transformer may be eliminated and a current path
 may be formed for carrying the whole current through the snubber circuit
 direct to the electronic unit and to the energy storage of the supply
 device.
 Advantageous improvements of the invention will become clear from the
 following description and claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 4 shows an embodiment of supply equipment according to the invention.
 A semiconductor valve comprises two semiconductor elements in the form of
 thyristors T1, T2 in antiparallel connection, each one with an electronic
 unit EU1, EU2, respectively. The electronic units are of the kind
 described above and form indicating signals IP1, IP2 which are supplied to
 control equipment CEQ. The control equipment forms firing orders FO1, FO2,
 as described above, and supply these to the electronic units. The signal
 transmission takes place via light guides LIC1, LIC2. Each of the
 thyristors has an anode terminal TA1, TA2, respectively, and a cathode
 terminal TC1, TC2, respectively. The anode terminal TA1 and the cathode
 terminal TC2 are connected to an inductor LA and the anode terminal TA2
 and the cathode terminal TC1 are connected to a capacitor CA. The
 inductor, in its turn, is connected to an ac circuit (not shown in the
 figure) with a known period T, for example as illustrated in FIG. 1.
 Further, the semiconductor valve comprises a snubber circuit SC with a
 series connection of a resistor RS and a capacitor CS and with only a
 first terminal CS1 and a second terminal CS2.
 A first supply device FD1 for power supply to the electronic unit EU1
 comprises an energy storage in the form of a capacitor C1 for storing
 electrical energy, a valve terminal J13, a snubber terminal J12, and a
 supply terminal J11. The capacitor C1 is connected between the valve
 terminal and the supply terminal. The valve terminal is intended for
 connection to the cathode terminal of the semiconductor element, the
 snubber terminal for connection to one of the terminals of the snubber
 circuit, in this case to the terminal CS1, and the supply terminal for
 connection to the electronic unit. The supply device comprises a first
 current path from the snubber terminal via a resistor R1 and a diode D11
 to the supply terminal such that a current may flow from the snubber
 terminal to the supply terminal but not in the reverse direction. For
 power supply to the electronic unit EU2, a second supply device FD2 of the
 same kind is provided, the components included in the supply devices and
 the terminals of the same kind being designated in the figure with the
 corresponding designations, whereby, for the second supply device, FIG. 1
 in the first figure of the reference numerals is replaced by FIG. 2.
 The snubber circuit is connected such that a second current path, for
 carrying a charge current to the energy storage in a direction from the
 snubber terminal to the valve terminal, is formed from the anode terminal
 through the snubber circuit to the snubber terminal, and from this via the
 energy storage and the valve terminal to the cathode terminal. The supply
 devices further comprise a third current path, parallel with the second
 current path, from the respective valve terminals via diodes D12 and D22,
 respectively, to the respective snubber terminals, for carrying a parallel
 current in a direction from the valve terminal to the snubber terminal and
 further to the snubber circuit, but not in the reverse direction. The
 snubber circuit is thus connected as a series circuit between the snubber
 terminal J12 in the first supply device FD1 and the snubber terminal J22
 in the second supply device FD2.
 A breakdown diode in the form of a Zener diode Z11 is connected between the
 supply terminal and the valve terminal for limitation of the voltage UF1
 of the supply terminal.
 The first current path comprises a branch point, which in this embodiment
 of the invention consists of the supply terminal J11, such that the first
 and second current paths coincide with each other between the snubber
 terminal and the branch point mentioned. The resistor R1, which
 constitutes a current-limiting element, and the diode D11, are arranged in
 the coinciding part of the two current paths.
 When, for example, the thyristor T1 is to assume the current through the
 capacitor CA, the condition for firing of the same is that the off-state
 voltage in its forward direction exceeds a predetermined level, which is
 transferred to the control equipment CEQ from the electronic unit EU1 via
 an indicating signal. In dependence on known quantities in the
 installation, the control equipment generates, in some manner known per
 se, firing orders FO1, which are supplied to the electronic unit when a
 corresponding indicating signal has been received.
 The build-up of the off-state voltage across the thyristor T1 implies that
 the time rate of change of the voltage between the anode terminal and the
 cathode terminal will have a positive sign, that the third current path in
 the second supply device FD2 is brought to a conducting state and that a
 charge current will flow through the snubber circuit SC in a direction
 from the terminal CS2 to the snubber terminal J12 in the first supply
 device. The third current path in the first supply device is in a
 nonconducting state, so the charge current in the first supply device
 flows through the coinciding part of the first and second current paths
 and, at the branch point between them, is divided into a current which
 flows directly to the electronic unit and a current which charges the
 capacitor C1. As long as the voltage UF1 at the supply terminal J11, that
 is, across the capacitor C1, is below the breakdown voltage of the Zener
 diode Z11, the capacitor is charged with the charge current. When the
 mentioned voltage reaches the breakdown voltage, the charge current will
 flow through the Zener diode.
 When the thyristor T1 fires, the voltage across the same returns to a value
 near zero, whereby a current of short duration flows through the snubber
 circuit in a direction from the cathode terminal via the diode D12, via
 the snubber circuit, and in the second supply device FD2 via the resistor
 R2 and the diode D21 to the capacitor C2 and hence provides a charge
 addition thereto.
 The function in case the thyristor T2 is to assume the current through the
 capacitor CA is completely analogous to the one described above, whereby,
 in the description, the second supply changes places with the first one.
 In an advantageous improvement of the invention, a shunt regulator,
 comprising a controllable switching member in the form of an auxiliary
 thyristor TX1, is arranged between the snubber terminal and the valve
 terminal. By means of a breakdown diode, in this embodiment a Zener diode
 Z12, connected between the snubber terminal and the gate of the auxiliary
 thyristor, the voltage US1 on the snubber terminal is sensed, which
 voltage thus constitutes a comparison voltage for the predetermined
 breakdown voltage UZ12 of the Zener diode. When the comparison voltage
 exceeds the breakdown voltage, the auxiliary thyristor is brought to a
 conducting state by the current through the breakdown diode and then
 closes a fourth current path from the snubber terminal via the auxiliary
 thyristor to the valve terminal for carrying the charge current past the
 capacitor C1. The voltage at the snubber terminal depends on the sum of
 the voltage at the supply terminal and the voltage across the resistor R1,
 whereby the shunt regulator intervenes in dependence on the voltage at the
 supply terminal as well as the amplitude of the current in the coinciding
 part of the first and second current paths.
 Typical component values for a supply device according to the invention are
 C1=1 .mu.F, R1=1 .OMEGA., RS=10 .OMEGA. and CS=2 .mu.F. The breakdown
 level of the Zener diode Z11 is typically 25 V and of the Zener diode Z12
 typically 47 V. In the known embodiment of a supply device for the same
 purpose, described with reference to FIG. 2, the capacitor C1 is usually
 designed to have a capacitance value of typically 20 to 30 .mu.F, that is,
 typically of an order of magnitude greater than in the embodiment
 according to the invention.
 When designing the supply device according to the invention, which is done
 with knowledge of the energy and voltage requirements of the electronic
 unit, the energy storage, that is the capacitor C1, is designed to store
 an amount of energy which is larger than the energy requirement of the
 electronic unit during one cycle of the alternating current but smaller
 than its energy requirement during two cycles.
 This implies that the energy storage is practically emptied in connection
 with the thyristor T1 being brought to a conducting state. At the next
 current pulse through the snubber circuit, part of this current pulse may
 be passed directly to the electronic unit via the first current path. By
 this series connection of the snubber circuit and the mentioned design of
 the energy storage, a simple and inexpensive design of the supply
 equipment, which utilizes in full the current through the snubber circuit,
 is obtained.
 It is to be understood that, in those cases where the semiconductor valve
 comprises a plurality of mutually series-connected thyristors, each of
 these is equipped with a supply device of the kind described above and
 connected over thyristors, which are antiparallel-connected in pairs, in
 the way illustrated in FIG. 4.
 The thyristor-connected capacitor may, of course, also be connected, for
 example, between two phases in a three-phase ac network.