HK1161030B - Power supply assembly - Google Patents

Description

The present invention relates to a power supply device for supplying a load connected to an output of the power supply device with a rectangular current, in particular as a power supply device in an arc furnace for generating a arc.

In the light arc furnaces, electrical energy is used to melt steel scrap. The energy is converted into thermal energy, which leads to the melting of the steel scrap. The heat generated by the light arc between the electrode and the steel scrap is mainly transmitted to the steel scrap by radiation.

A transformer in a power supply system of a arc furnace adjusts the energy supplied by a supply network to the current and voltage values necessary for the operation of the furnace.

This problem is solved by a power supply device of the invention, which has a transformer and a bridge circuit. The transformer has at least two primary side taps, which form an input to the power supply device. It also has several secondary side taps. The bridge circuit includes several first half-bridges, a second half-bridge and a bridge branch. The first half-bridges have power-reversing valves and each has a first connection to the bridge circuit. The second half-bridge has power-reversing valves and a second connection to the bridge circuit.

Each first connection of the bridge circuit is connected to one of the secondary side connections of the transformer. The second connection of the bridge circuit is connected to the output of the circuit assembly. The bridge circuit can thus be connected in series with the output of the power supply. From the output, the power supply arrangement forms a power source that supplies a current through the second connection of the bridge circuit. The current is preferably rectangular and has the same frequency circuit as the voltage at the input of the power supply arrangement.

Such a power supply may be operated by a process according to the invention with a load connected to the output in such a way that a control device induces the first half-bridge actuator valves to switch in succession during a half-life of the network so that the current is transferred through the output of the power supply to a graduated current approaching the current flow of the network voltage through the input of the power supply.

The transformer's translation ratio can be determined by switching the first half-bridge's current-reversing valves in succession and selecting the secondary side-up of the transformer. By choosing the transformer's translation ratio, the current constant during a half-life of the network, which is absorbed by the throttle in the bridge branch, can be transformed by the second connection into a desired current through the E of the power supply system. The current through the input can thus be adjusted to the voltage flow at the input of the power supply system. This can achieve a high power factor, as the surface waveform of the input current can be reduced.

It is also possible that during the second half of the network half-periods, i.e. during the secondary side-voltage decreasing in volume, the rectifier valves are switched off successively. For this purpose, IGBTs and GTOs are suitable. It is particularly advantageous if switching on and off during the first half of the network is done. Further developments such as the IGCT and the IGGBT are also suitable.

The first half-bridges of a power supply device according to the invention may have two AC valves. In each first half-bridge, an anode of a first AC valve of the two AC valves and a cathode of a second AC valve of the two AC valves may be connected to the first connection of the bridge circuit arranged in this first half-bridge. The cathodes of the first AC valves of the first half-bridges may be connected to a first node of the bridge circuit and the anodes of the second AC valves of the first half-bridges may be connected to a second node of the split-circuit.

The bridge branch is conveniently located between the first and second nodes.

The second half-bridge of a power supply device according to the invention may have two AC valves. Each of the two AC valves may have a first AC valve connected to its anode and a second AC valve of the two AC valves may be connected to the second connection of the AC circuit by its cathode. The cathode of the first AC valve of the second half-bridge may be connected to the first node of the AC circuit and the anode of the second AC valve of the second half-bridge may be connected to the second node of the AC circuit.

A power supply device of the invention may have at least one control device for controlling the rectifier valves.

The first AC valve and the second AC valve of the second half-bridge shall preferably be actuated in reverse and synchronous mode to an AC voltage at the input of the power supply device for switching; the first AC valves of the first half-bridges shall preferably be actuated in succession during a first half-period of AC voltage at the input; and the second AC valves of the first half-bridges shall preferably be actuated in succession during a second half-period of AC voltage at the input.

The control device may be subordinated to a device for generating pulses to switch the AC valves, and the pulse generator may be subordinated to a device for transmitting pulses connected to a control electrode of the AC valves.

The current-displacement valves are preferably thyristors, but other controllable switching elements, in particular other power semiconductors, may also be used.

A light arc furnace has the advantage of having three power supply devices according to the invention. The primary sides of the transformers of the power supply devices are conveniently connected in a triangle and an arc furnace electrode is connected at the output of each power supply device. An arrangement of two light arc furnaces may have a total of three power supply devices according to the invention, which are alternately supplied with electric current by the power supply devices.

Further features and advantages of the present invention are illustrated by the following description of an example of an embodiment, with reference to the accompanying illustrations. Fig. 1a simplified diagram of a two-arc system with three power supplies in total,Fig. 2a part of the system as shown in Fig. 1 with a more detailed representation of a bridge circuit of one of the three power supplies andFig. 3the flow of current through the input, current through the output and voltage at the input of one of the power supplies in schematic representation.

Figure 1 shows three transformers TU, TV, TW, which are connected on the primary side in a triangular circuit to three phase conductors U, V, W of a rotary power supply network. On the secondary side, the transformers TU, TV, TW are arranged in a star circuit.

In addition to the secondary side sockets 2UN, 2VN, 2WN, the secondary sides of the TU, TV, TW transformers have three other sockets 2U1, 2U2, 2U3, 2V1, 2V2, 2V3, 2W1, 2W2, 2W3. These secondary side sockets 2U1 , 2U2, 2U3, 2V1, 2V2, 2V3, 2W1, 2W2, 2W2, 2W3 are connected to BU, BV, bridge circuits and the secondary side sockets of the TU BW transformer are connected to the BU bridge circuit, the secondary side sockets of the TV BW transformer to the BV bridge circuit and the secondary side sockets of the transformer to the BW T circuit. The BW, BV, BW and BW bridge circuits are identical and consist of a series of thyristor.

The bridge circuits have outputs A20 connected by an intermediate circuit of two C1 , C2 switches with switch contacts CU1, CU2, CV1, CV2, CW1, CW2 to the electrodes of the arc furnaces K1 , K2. The C1 , C2 switches are controlled by a controller S2, whereby the controller S2 ensures that the arc furnaces cannot be operated simultaneously.

For controlling the AC valves of the BU, BV, BW bridge circuits, the controls SU, SV, SW and a superior control S1 are assigned to the BU, BV, BW bridge circuits.

The bridge circuit BU is illustrated in Figure 2: The bridge circuit BU has three first half-bridges 1 1 , 12 , 13 which are preferably identical in structure. Each first half-bridge 1 1 , 12 , 13 has a first thyristor V11, V13, V15 and a second thyristor V12, V14, V16 as a current rectifier valve. The cathodes of the first thyristors V11, V13, V15 are connected at a first node BK1 and the anodes of the second thyristors V12, V14, V16 are connected at a second node BK2. Furthermore, in each branch 1 1 , 12, 13 the anode of the first thyristor and the cathode of the second thyristor are connected to a first node A11, A12, A13 of the bridge circuit.

The first connections A11, A12, A13 of the BU bridge circuit are connected to a secondary socket 2U1, 2U2, 2U3 of the TU transformer, respectively.

The bridge branch connects the first node BK1 and the second node BK2. It comprises a L1 throttle. Due to its inductivity, the throttle drives a current through the bridge branch when the power supply is running.

The second half-bridge 14 has a first thyristor V17 and a second thyristor V18 as rectifier valves. It is sufficient if these thyristors are only switchable thyristors. The cathode of the first thyristor V17 is connected to the first node BK1. The anode of this thyristor V17 is connected to a second junction A20 which is connected to the output A of the power supply device. The cathode of the second thyristor V18 is also connected to the second junction A20, while the anode of the second thyristor is connected to the second junction BK2.

The electrical current is passed through the secondary coil of the transformer. The current through the secondary coil of the transformer TU corresponds to the load current through the output of the power supply. It can only flow if either one of the first thyristors V11,

V13, V15 of the first half-bridge 1 1 , 12, 13 and the second thyristor V18 or one of the second thyristors V12, V14, V16 of the first half-bridge 1 1 , 12, 13 and the first thyristor V17 of the second half-bridge are conductive.

The input voltage of the power supply unit is transformed by the transformer TU to the secondary side. A voltage then falls between one of the secondary 2U1, 2U2, 2U3 taps of the transformer TU and the secondary 2UN. This secondary voltage also falls through one of the first sockets A11 , A12, A13, one of the thyristors V11 , V12, V13, V14, V15, V16, the bridge branch with the L1 throttle and one of the thyristors V17, V18 of the second half-bridge 20, the A20 connection or source A of the power supply, and the secondary current is driven by the current, i.e. the load current is determined by the load load load of the L1 throttle without interruption.

The thyristors V17, V18 of the second half-bridge are operated in counter-clockwise grid synchrony.

By controlling the thyristors V11, V12, V13, V14, V15, V16, the first bridge branch leading the secondary current can be determined. This also determines the secondary side socket 2U1, 2U2, 2U3 leading the secondary current. Selecting the socket 2U1, 2U2, 2U3 simultaneously determines the translation ratio that transforms the secondary current to the primary side. Thus, by selecting the first bridge branch leading the current, it is possible to adjust the primary current of the transformer.

The ability to adjust the primary current allows the primary side to adjust a current that leads to the highest possible power factor at the specified primary voltage. This is achieved at a sine wave primary voltage U1 by bringing the primary current I1 closer to a sine path by selecting the first bridge branch that carries the current, thereby reducing the overhead wave ratio. This is shown in Figure 3, in which the secondary current I2 is also shown.

If only switched-on thyristors are used instead of on- and off-mode thyristors in the first bridge branches, a graduated flow of primary current could be achieved only in the first half of each half-life of the network.

Claims (14)

1. Power supply arrangement for supplying a squarewave current (12) to a load which is connected to an output of the power supply arrangement, in particular in the form of a power supply arrangement in an arc furnace for generating an arc, comprising

   - a transformer (TU)

   - having at least two primary-side taps (1U1, 1U2) which form an input of the power supply arrangement, and

   - having a plurality of secondary-side taps (2U1, 2U2, 2U3, 2UN),

   characterized by

   - a bridge circuit (BU)

   - having a plurality of first half-bridges (11, 12, 13) which have converter valves (V11, V12, V13, V14, V15, V16) and a respective first connection (A11, A12, A13) of the bridge circuit,

   - having a bridge arm with an inductor (L1) and

   - having a second half-bridge (14) which has converter valves (V17, V18) and a second connection (A20) of the bridge circuit (BU),

   - wherein each first connection (A11, A12, A13) is connected to one of the secondary-side taps (2U1, 2U2, 2U3) of the transformer (TU),

   - wherein the second connection (A20) is connected to the output.
2. Power supply arrangement according to Claim 1, characterized in that the first half-bridges (11, 12, 13) have two converter valves (V11, V12; V13, V14; V15, V16), wherein in each first half-bridge an anode of a first converter valve (V11, V13, V15) of the two converter valves and a cathode of a second converter valve (V12, V14, V16) of the two converter valves are connected to the first connection (A11, A12, A13) of the bridge circuit (BU), which is arranged in this first half-bridge.
3. Power supply arrangement according to Claim 2, characterized in that the cathodes of the first converter valves (V11, V13, V15) of the first half-bridges (11, 12, 13) are connected to a first node (BK1) of the bridge circuit, and the anodes of the second converter valves (V12, V14, V16) of the first half-bridges (11, 12, 13) are connected to a second node (BK2) of the bridge circuit (BU).
4. Power supply arrangement according to Claim 3, characterized in that the bridge arm is situated between the first node (BK1) and the second node (BK2).
5. Power supply arrangement according to one of Claims 1 to 4, characterized in that the second half-bridge (14) has two converter valves (V17, V18), wherein in each case a first converter valve (V17) of the two converter valves (V17, V18) is connected to the second connection (A20) of the bridge circuit (BU) by way of its anode, and a second converter valve (V18) of the two converter valves (V17, V18) is connected to the second connection (A20) of the bridge circuit (BU) by way of its cathode.
6. Power supply arrangement according to Claim 5, characterized in that the cathode of the first converter valve (V17) of the second half-bridge (14) is connected to the first node (BK1) of the bridge circuit (BU), and the anode of the second converter valve (V18) of the second half-bridge (20) is connected to the second node (BK2) of the bridge circuit (BU).
7. Power supply arrangement according to one of Claims 1 to 6, characterized in that the power supply arrangement has at least one control means (S2) for controlling the converter valves (V11, V12, V13, V14, V15, V16).
8. Power supply arrangement according to Claim 7, characterized in that the first converter valve (V17) and the second converter valve (V18) of the second half-bridge (14) can be driven, so as to form a connection, in push-pull fashion and synchronously to an AC voltage which is applied to the input (U, V) of the power supply arrangement.
9. Power supply arrangement according to Claim 7 or 8, characterized in that the first converter valves (V11, V13, V15) of the first half-bridges (11, 12, 13) can be driven, so as to form a connection, in succession over a first half-period of the AC voltage which is applied to the input, and the second converter valves (V12, V14, V16) of the first half-bridges (11, 12, 13) can be driven, so as to form a connection, in succession over a second half-period of the AC voltage which is applied to the input.
10. Power supply arrangement according to one of Claims 7 to 9, characterized in that a means (E) for generating pulses for connecting the converter valves (V11, V12, V13, V14, V15, V16, V17, V18) is arranged downstream of the control means (S1).
11. Power supply arrangement according to Claim 10, characterized in that pulse-transmitting means (Z11, Z12, Z13, Z20) are arranged downstream of the pulse-generating means (E), the said pulse-transmitting means being connected to a control electrode of the converter valves (V11, V12, V13, V14, V15, V16, V17, V18).
12. Arc furnace having three power supply arrangements, characterized in that the three power supply arrangements are designed in accordance with one of Claims 1 to 11, the primary sides of the transformers (TU, TV, TW) are connected in delta, and in that an electrode is connected to the output of each power supply device, wherein an arc burns between the electrodes during operation of the arc furnace.
13. Arrangement comprising two arc furnaces according to Claim 12, characterized in that the arc furnaces can be supplied with electric current by the power supply arrangements alternately.
14. Method for operating a power supply arrangement according to one of Claims 7 to 11 having a load which is connected to the output, characterized in that the control means (S1) drive the converter valves (V11, V12, V13, V14, V15, V16) of the first half-bridges (11, 12, 13), so as to form a connection, in succession such that the rectangular profile of the current (12) through the output of the power supply arrangement is converted into a stepped current (11) through the input of the power supply arrangement, the said current approximating the profile of the input voltage (U1).