GB2300080A

GB2300080A - Method of driving the converter valves of parallel resonant circuit inverters connected in series on the direct-current side

Info

Publication number: GB2300080A
Application number: GB9608160A
Authority: GB
Inventors: Robert Ibach; Jan Fabianowski
Original assignee: ABB Patent GmbH
Current assignee: ABB Patent GmbH
Priority date: 1995-04-20
Filing date: 1996-04-19
Publication date: 1996-10-23
Anticipated expiration: 2016-04-19
Also published as: DE19514537A1; DE19514537B4; GB2300080B; GB9608160D0

Description

4 2300080 Method of driving the converter valves of Darallel resonant

circuit inverters connected in series on the direct-current side DescriDtio

The invention relates to a method of driving the converter valve of parallel resonant circuit inverters connected in series on the directcurrent side, according to the precharacterizing clause of independent Claims 1 and 6.

Such a method of driving the converter valves of parallel resonant circuit inverters connected in series is on the direct-current side in disclosed by DE 42 09 645 Al and DE 44 03 078 Al. Then documents describe systems In which two or more parallel resonant circuit inverters (each of which In associated with an individual induction furnace) are fed from a common direct-current source with only one rectifier for converting a mains alternating voltage into a direct voltage. The instantaneously produced rectifier power corresponds to the sum of all instantaneously consumed parallel resonant circuit Inverter powers. In this case it is, for example, possible to dimension the installed rectifier power according to the installed power of a parallel resonant circuit inverter. This has the advantage of great cost savings compared to the conventional technique, in which each parallel resonant circuit inverter requires its own rectifier with power matched to the parallel resonant circuit inverter. It is possible to set the Instantaneously consumed parallel resonant circuit inverter powers continuously and independently of one another, it being of course necessary for the sum of all instantaneously required inverter powers not to exceed the installed rectifier is power. The object of the invention is to provide a method, of the type mentioned at the outset, for driving the converter valves of parallel resonant circuit inverters connected in series on the direct-current side, which permits individual parallel resonant circuits to be switched off and on during operation without the other parallel resonant circuits needing to be switched off beforehand. With regard to switching a resonant circuit off, this object in achieved according to the invention in conjunction with the features of the precharacterizing clause by the features specified in the characterizing part of Claim 1. With regard to switching a resonant circuit on this object is achieved according to the invention in conjunction with the features of the precharacterizing clause by the features specified in the characterZzing part of Claim 6. 20 The advantages obtainable with the invention reside, in particular, in that, in the case of systems consisting of at least two parallel resonant circuit inverters connected in series on the direct-current side, individual parallel resonant circuits can be switched off and on without interruption, without having to interfere with the operation of the other parallel resonant circuits. Nevertheless, expedient intervention can be taken on the operation of a direct-current source and/or the operation of the other parallel resonant circuits in order always to ensure optimum exploitation and operational control of the system.

The invention will be explained below with the aid of the illustrated embodiments represented in the drawing, in which:

Figure 1 shown a system having four parallel resonant circuit inverters fed by a direct-current source, with a control device for power setting and power distribution, Figure 2 shows a parallel resonant circuit fed via an is inverter.

Figure 1 shown a system having f our parallel resonant circuit inverters fed by a direct-current source, with a control device for power netting and power distribution. A rectifier 1 is shown which is connected on the alternating-voltage side to a three-phase mains 2 and, on the directcurrent side, feeds an intermediate circuit with a smoothing reactor Ld The direct voltage of the intermediate circuit has the reference U and the direct current in 1d Four parallel resonant circuit inverters 5, 6, 7, 8 are connected In series on the direct-current side between the positive pole 3 and the negative pole 4 of the intermediate circuit or of the direct-current source.

The inverter 5 feeds a parallel resonant circuit consisting of a coil 9 of an induction furnace and a compensation capacitor Cl. Similarly, the Inverters 6; 7; 8 supply parallel resonant circuits consisting of coils 10; 11; 12, respectively, of induction furnaces and compensation capacitors C2.. CV Q&, respectively. At a rated power of 100% for each parallel resonant circuit inverter 5 to 8, the rectifier 1 in designed, for example, for 200% of this power, it then being possible, for example, optimally to operate two induction furnaces simultaneously at 90% rated power each (melting operation) and two induction furnaces at 10% rated power each (temperature holding operation).

A control and regulation device 13, including the trigger pulse generator for the converter valves of the inverters and of the rectifier, is used for control/regulation of the system. The powers P,. P2t P31 P41 to be adjusted, of the Individual parallel resonant circuits or parallel resonant circuit inverters, and therefore of the coils 9, 10, 11, 12 of the induction furnaces, are applied on the input side to this control and regulation device 13. Using a current detection device 14 the direct current Id flowing in the intermediate circuit can be detected and communicated to the control and regulation device 13. An an alternative, it is also possible to detect the current on the alternating-voltage side, instead of the direct current.

on the outward side, the control and regulation device 13 drives the converter valves of the Inverters 5 to 8 and of the rectifier 1 by means of corresponding pulses.

The control and regulation device 13 furthermore drives start devices 15; 16; 17; 18, which are assigned to the individual inverters 5; 6; 7; 8, respectively.

By way of example, Figure 2 shown a parallel resonant' circuit fed via an inverter. The inverter 5 consisting of four converter valves T1 to T4 can be seen, the anodes of the valves T1, T2 being connected to the positive pole of the intermediate circuit and the cathodes of the converter valves T3 and T4, respectively, being connected to the junction point with the further inverter 6. This junction point is simultaneously the negative pole for the inverter 5. The junction point of the cathode of the valve T1 with the anode of the valve T3 forwin the first load connection, and the junction point of the cathode of the valve T2 with the anode of the valve T4 forms the second load connection. The parallel resonant circuit consisting of a compensation capacitor Cl and the coil 9 of the induction furnace in connected between the two load connections, a commutation inductor LX1 being arranged between one load connection and the parallel resonant circuit. The current flowing through the commutation inductor LK1 has the reference M. The coil 9 has an inductance L01 and an equivalent ohmic resistance ROl. The voltage across the compensation capacitor has the reference UC1.

The start device 15, consisting of a start capacitor CS, a charging circuit 19 for the start capacitor CS and a converter valve TS in arranged in parallel with the compensation capacitor Cl. The cathodes of the valve TS and the froo connection of the capacitor CS i.e. the one not connected to the anode of the valve TS, are connected to the connections of the compensation capacitor Cl.

The other inverters of the system, which have parallel resonant circuits, are constructed in the same way as described with reference to Figure 2. Moreover, Figure I is based, purely by way of example, on a system having four inverters which are connected in series and have parallel resonant circuits. it is also possible to use systems having two, three, five, etc. inverters connected in series. For the mode of operation of the system or of the individual inverters, reference should be made to the detailed embodiments according to DE 42 09 645 Al and DE 44 03 078 Al. It is in general true for parallel resonant circuit inverters that a current flow from the direct-current source via the parallel resonant circuit connected to an inverter is set by triggering two diagonally opposite converter valves of is this inverter in each case.

DE 42 09 645 Al proposes a method of driving the converter valves of two or more load-commutated parallel resonant circuit inverters which are fed from a common direct-current source and are connected in series on the direct-current side, in which two diagonally opposite converter valves of an inverter are in each case triggered with a time delay, so that a "short-circuit current" is temporarily produced through two converter valves of this inverter which are connected directly in series. The power drawn by each parallel resonant circuit is set by varying the time delay on triggering of two diagonally opposite converter valves independently of each other and continuously.

DE 44 03 078 Al proposes a method for control /regulation of the converter valves of two or more load- commutated parallel resonant circuit inverters which are fed from a common direct-current source or directvoltage source and are connected in series on the directcurrent side, in which the load phase angles of the parallel resonant circuit inverters are set in such a way that, on the one hand, the arbitrarily preset individual powers of the parallel resonant circuit result, account being taken of the maximum power that the direct-current source or direct-voltage source can output, and, on the 6 - other hand, the preset ratio between the individual powers of the parallel resonant circuits in obtained.

Action is taken in the control/regulation of the direct-current source (for example consisting of a direct-voltage source and inductor) only when one of the inverter load phase angles to be set reaches the minimum valve, in particular dependent on the valve hold-of f interval, in order then to reduce the direct current (or the direct voltage), whereas if the inverter load phase angle to be set is above the minimum valve, then the direct-current source or direct-voltage source is operated fully.

If the parallel resonant circuit of an inverter needs to be switched off during operation of the system, there are three different method variants possible for this, which will be explained below with the aid of Figure 2. In all three variants, it will be assumed that a current flow through the valves Tl-T4 occurs shortly before the parallel resonant circuit is switched off.

This current flow is maintained for a certain period of time, i. e. the valves T2, T3 on the other diagonal are not triggered, but instead the valves Tl, T4 which were on last are left on and a current IW1 flows through the valves Tl, T4 and the parallel resonant circuit. A plurality of damped oscillations of the parallel resonant circuit (damping by Rol) then result, af ter which the voltage UC1 (amplitude value) across the compensation capacitor C1, i.e. the load voltage of the inverter, approaches the value 0.

There are two possibilities for determining the exact time t. at which the load voltage approaches the value 0, i.e., for example, has fallen to approximately I... 3% of the rated value. The f irst possibility consists in determining the load voltage by measurements and comparing this current load voltage with a preset threshold value. The second possibility consists in presetting a fixed period during which the two diagonally opposite converter valves must be on.

According to the first variant, the valve T3 is triggered at time t., as a result of which a current flow through TI-T3 is then set up and the current IW1 through the load circuit as well an the current flow through T4 are quenched. According to the second method variant, the valve T2 is triggered at time t., an a result of which a current flow through T2-T4 is then not up and the current IW1 as well as the current flow through T, are quenched. According to the third method variant, the valves T2 and T3 are triggered at time Ta, as a result of which current flows through TI-T3 and T2-T4 are then set up and the current IW1 is quenched.

The third method variant is particularly advantageous since it distributes the current 'd over two parallel valve paths after the load circuit has been is switched off, as a result of which each valve path is loaded only with ld/2. As a result, however, in all three variants the inverter of the switched-off parallel resonant circuit is short-circuited, while the other series-connected parallel resonant circuit inverters are still in operation. The position of the parallel resonant circuit in the inverter series circuit is, of course, of no importance here.

It is furthermore possible to switch off, for example, two or more parallel resonant circuits simultaneously or sequentially, while the other parallel resonant circuit inverters of the system still remain in operation.

Switching off is carried out using the control and regulation device 13 by presetting to zero the power of the parallel resonant circuit to be switched off. In this case, it is particularly important when switching off parallel resonant circuits with relatively high set power that, in coordination with the switching off, the power output by the rectifier 1 is also reduced, i.e. altered control signals are applied to the valves of the rectifier, so that the direct current Id (for example by reducing the direct voltage U) is correspondingly reduced for the power reduction. As an alternative to this, it is also possible, when switching off a parallel resonant - a - circuit, simultaneously to increase, in coordinated f a8hion by means of the control and regulation device 13, the power of those parallel resonant circuits that remain in operation, as a result of which it is not necessary to alter the rectifier power. The latter came is expedient in particular whenever the rectifier was already operated at rated power before switching off a parallel resonant circuit and it is desired to increase the power of the further parallel resonant circuits that remain in operation.

it is also possible to switch on a switched-off parallel resonant circuit during operation of the system without interrupting the operation of the other resonant circuits. in order to explain the switching-on, it will be assumed, for example, that a current flow through the valves Tl-T3 of the short-circuited inverter occurs before the switching-on. It should furthermore be presupposed that the start capacitor CS has been charged by means of the charging circuit 19. In a first step of the method for switching on, the control and regulation device 13 drives the converter valve TS in such a way that the capacitor CS in discharged to the compensation capacitor C1. In coordination herewith, the valve T2 is also driven and the valve Tl is turned off. A current flow through T2-T3 and the parallel resonant circuit (current IWl through LKI) then results. In accordance with the load voltage UCl which is set up across the compensation capacitor Cl (oscillation), the valves Tl-T4 or T2-T3 are driven in the manner known from 30 DE 42 09 645 Al or DE 44 03 078 Al. Switching on can be carried out in equivalent fashion if a current flow through the valves T2-T4 or through both valve branches Tl-T3 and T2-T4 of the shortcircuited inverter occurs before the switching-on. The position in the inverter series circuit of the parallel resonant circuit to be switched on in once again of no importance. It is furthermore possible to switch on two or more parallel resonant circuits simultaneously or sequentially while the other inverters of the system remain in uninterrupted operation. Switching-on is generally carried out using the control and regulation device 13 by presetting the power of the parallel resonant circuit to be switched on. In this case it is import- s ant, in particular when switching on inverters with relatively high act power, that, in coordination with the switching-on, the power output by the rectifier 1 is also increased, i.e. altered control signals are applied by the control and regulation device 13 to the valves of the rectifier, so that the direct current Id is kept at a constant value or increased (for example by increasing the direct voltage U) in spite of the fact that inverters are switched on. An an alternative to this it is also possible, when switching on parallel resonant circuits, is to reduce, in coordinated fashion using the control and regulation device 13, the power of other parallel resonant circuits that are in operation, if, directly before the switching-on, for example the rectifier 1 was already operated at rated power and no further increase in the rectifier power in therefore possible.

Claims

Patent claims is Method of driving the converter valves of two or more

parallel resonant circuit inverters with one induction furnace in each case which are fed from a common direct-current source and are connected in series on the direct-current side, a current flow from the direct-current source via the parallel resonant circuit connected to an inverter being sat by triggering two diagonally opposite converter valves of this inverter in each case, characterized in that, in order to switch off a parallel resonant circuit, two diagonally opposite converter valves are left on until the load voltage of the parallel resonant circuit has approximately zero value and at least one further converter valve located in series on the direct-current side with a valve in the on state in then triggered.
2. Method according to Claim 1, characterized in that the exact triggering time of the further converter valve to he triggered in determined from the current value of the load voltage.
3. Method according to Claim 1, characterized in that the triggering time of the further converter valve to be triggered in formed by presetting a fixed period during which the two diagonally opposite converter valves must be on.
4. Method according to one of Claims 1 to 3, characterized In that, in coordination with the switching-off of a parallel resonant circuit, the power output by the direct-current source is correspondingly reduced.
5. Method according to one of Claims 1 to 3, characterized in that, In coordination with the switching-off of a parallel resonant circuit, the power of at least one further parallel resonant circuit is 35 increased.
6. Method of driving the converter valves of two or more load-commutated parallel resonant circuit inverters with one induction furnace in each case which are fed is from a common direct-current source and are connected in series on the direct-current side, a current flow from, the direct-current source via the parallel resonant circuit connected to an inverter being set by triggering two diagonally opposite converter valves of this inverter in each case, characterized in that at least one parallel resonant circuit in In the off state and a current flow via at least two converter valves located in series on the direct-current side consequently results, and in that, in order to switch on a parallel resonant circuit in the of f state, a converter valve (TS) of a start device (17) and, in coordination herewith, at least one converter valve, in the of f state, of the Inverter are triggered, as a result of which a current f low between two diagonally opposite converter valves results af ter charging of a compensation capacitor (Cl) of the parallel resonant circuit.
7. Method according to Claim 4, characterized in that, in coordination with the switching on of the parallel resonant circuit, the power output by the direct-current source in increased.
8. Method according to Claim 4, characterized in that, in coordination with the switching on of the parallel resonant circuit, the power of at least one further parallel resonant circuit in reduced.