WO1999005765A1

WO1999005765A1 - Device for compensating voltage fluctuations at the terminals of a load with unstable impedance

Info

Publication number: WO1999005765A1
Application number: PCT/FR1998/001622
Authority: WO
Inventors: Xavier Lombard; Mohamed Zouiti
Original assignee: Electricite De France - Service National
Priority date: 1997-07-25
Filing date: 1998-07-22
Publication date: 1999-02-04
Also published as: FR2766635B1; FR2766635A1

Abstract

The invention concerns a device for compensating voltage fluctuations at the terminals of a load (12) with unstable impedance connected to an electrical power supply line (14) comprising an inverter circuit (26) connected on said line (14) and supplied by a direct voltage source (C1). The inverter circuit (26) is controlled by a control unit (28) for injecting into said line (14) an alternating voltage compensating voltage fluctuations generated by the load (12) impedance variations.

Description

"Device for compensating for voltage fluctuations at the terminals of a load with unstable impedance".

The present invention relates to a device for compensating for voltage fluctuations at the terminals of a load with unstable impedance, such as an arc furnace.

Arc furnaces, and in general electrical equipment, the impedance of which may vary rapidly, are the source of disturbances propagating on the electrical energy supply networks which supply them. These disturbances are mainly in the form of rapid voltage variations at frequencies ranging from 1 Hz to 30 Hz.

Currently, these disturbances are compensated for by having on the load supply line a static reactive power compensator (CSPR) making it possible to supply or absorb reactive power.

This type of compensation device is however unsatisfactory. Indeed, the compensation carried out is imperfect. In addition, the dimensioning, in terms of power, of these devices is relatively large and often exceeds the power of the load itself.

The object of the invention is to overcome these drawbacks and to provide a device for compensating for voltage fluctuations at the terminals of a load with unstable impedance, of relatively low cost and capable of effectively reducing the disturbances caused by the load, while increasing the performance of the latter. It therefore relates to a device for compensating for voltage fluctuations at the terminals of a load with unstable impedance, such as an arc furnace, connected to a polyphase electrical energy supply line of a supply network, comprising an inverter circuit connected to said line and supplied by a DC voltage source, said inverter being controlled by a unit control device for injecting into said line an alternating voltage compensating for the voltage fluctuations generated by the variations in load impedance, characterized in that it further comprises a reactive power exchange circuit with said supply line in electrical energy;

The compensation device according to the invention may also include one or more of the following characteristics: - the device further comprises, connected to the control unit, means for measuring at least one quantity characteristic of electrical energy supplying the load, said control unit comprising means for calculating the voltage to be inserted in series in said line for the purpose of compensating for said variations, on the basis of the values delivered by said measuring means;

- The DC voltage source includes an energy storage means supplied by a DC voltage applied to its terminals; - The device further comprises, connected to the terminals of said energy storage means, a superconductive coil with inductive storage associated with a chopper for supplying said energy storage means with direct voltage; - The device comprises means for withdrawing current from said supply line, connected to a conversion circuit, of said withdrawn current, in direct current, said conversion circuit being connected to said energy storage means for its supply ; - Said current draw means consist of a transformer whose primary winding is connected in parallel to said supply line and whose secondary winding is connected to said conversion circuit; - said inverter circuit is connected to said supply line via a galvanic decoupling transformer; said inverter circuit comprises a set of switching cells controlled by said control unit and comprising a switching element which can be switched on opening and closing and a diode connected in anti-parallel mode to said switching element; said reactive power exchange circuit comprises a reactive power exchange inverter circuit supplied by at least a second DC voltage source and controlled by said control unit, said inverter circuit being connected to said supply line by the through a galvanic decoupling transformer;

- Said at least a second DC voltage source is connected to the energy storage means of the inverter circuit for compensating for voltage fluctuations with a view to charging it; and - said at least one second DC voltage source is connected to the energy storage means via a galvanic decoupling transformer.

Other characteristics and advantages will emerge from the following description, given solely by way of example and made with reference to the appended drawings in which:

- Figure 1 is a diagram illustrating the structure of a compensation device according to the invention, according to a first embodiment;

- Figure 2 is a curve showing the variations of active and reactive power in an arc furnace; FIG. 3 illustrates a second embodiment of the compensation device according to the invention; FIG. 4 shows a third embodiment of the compensation device according to the invention; FIG. 5 represents a fourth embodiment of the compensation device according to the invention; and - Figure 6 illustrates a fifth embodiment of the fluctuation compensation device according to the invention.

In Figure 1, there is shown the general structure of a voltage fluctuation compensation device according to the invention, according to a first embodiment.

This device, designated by the general reference 10, is intended to compensate for voltage fluctuations generated by a load with unstable impedance 12, constituted by an arc furnace connected to a line 14 of polyphase electrical energy supply of a power network.

The supply network is shown diagrammatically in the form of an alternating voltage source 16. Its impedance is shown diagrammatically by an inductor L1 and a resistor RI. The characteristic impedance of the arc furnace 12 is shown diagrammatically by an inductance L2 and a resistor R2.

As previously mentioned, the arc furnace 12 and, generally, the charges having an unstable impedance which fluctuates rapidly is the source of a certain number of disturbances at the level of the network 16 which supplies it. These disturbances are of different natures.

First, an arc furnace generates large voltage fluctuations, relatively spaced in the time, for example due to single-phase or three-phase short-circuits appearing at the start of the fusion. This type of disturbance has a very low repetition frequency, for example less than 0.3 Hz. Furthermore, an arc furnace generates on the network voltage fluctuations of low amplitude and whose frequency is between approximately 5 and 25 Hz.

In addition, arc furnaces are important consumers of reactive energy and considerably degrade the power factor of the network.

To compensate for these different types of disturbance, the compensation device according to the invention, in the embodiment shown in FIG. 1, comprises a first active power compensation module 18 connected to the supply line 14 by l 'Intermediate of a galvanic decoupling transformer 20 and a second reactive power compensation module 22 also connected to the supply line 14 via a second galvanic decoupling transformer 24.

It will be noted that in FIG. 1, the compensation modules 18 and 22 are represented in the tπfilaire form while the supply network 16 and the supply line 14 are represented in a single line form.

The first active power compensation module 18 comprises an inverter circuit 26 constituted by a DC voltage-to-AC voltage converter circuit, the AC side of which is connected to line 14 by means of the corresponding transformer 20 and the DC side of which is connected to a DC voltage source comprising an energy storage means, for example a capacitor C1, as shown in FIG. 1. The inverter circuit 26 comprises a set of switching cells, the switching of which is controlled by a control unit 28. Each switching cell comprises a switching element, such as 30, which can be switched on opening and closing, for example a GTO, at the terminals of which an anti-parallel diode, such as 32, is connected.

The DC voltage source, constituted by the capacitor C1, is itself supplied by a DC voltage applied to its terminals, delivered by a current source, constituted by a superconductive coil L3 with inductive storage (SMES) with the interposition of a chopper 34 ensuring the conversion of the current delivered by the coil L3 into a corresponding direct voltage. It will be noted that the current source in fact constitutes an active power source intended to be injected into line 14 for the compensation of voltage fluctuations of low amplitude and frequency between 5 and 25 Hz. The second module 22 for compensation of reactive power includes a static reactive power compensator (CSPR) circuit designated by the reference 36 and of conventional type. It consists of three inductors, L4, L5 and L6 respectively, each inductor being placed in series with two thyristors, such as 38, mounted head to tail, these thyristors being controlled by a second control unit 40.

It will be noted that the inductors L4, L5 and L6 are coupled in a triangle in order to allow the priming of the thyristors at any time during the half period of the supply voltage.

It will be noted that the reactive power absorbed by the coils is controlled by acting on the starting angle of the thyristors so that the reactive power absorbed by the whole of the furnace and the inductances is substantially constant. This reactive power is then compensated by a capacitor bank C2 connected between the connection point of the transformer 24 and of the CSPR 36 and the earth. The device represented in FIG. 1 is supplemented by sensors 42 and 44 for measuring at least one quantity characteristic of the electrical energy supplying the oven 12 connected to line 14 and connected to the control units 28 and 40. These sensors are of conventional type and will therefore not be described in detail below. They make it possible to measure the voltage, the intensity or the phase of the electrical energy delivered by the network with a view to piloting, by the control units 28 and 40, the switching elements of the inverter 26 and of the CSPR 36, for example by comparison with reference values supplied as input to the control units.

As mentioned previously, the first compensation module 18 compensates for the fluctuations in active power absorbed by the furnace 12 to counter rapid fluctuations and small amplitudes of the voltage present at its terminals and thus compensate for fluctuations in the value of its resistance. .

The second compensation module 22 mainly compensates for fluctuations in reactive power exchanged between the network and the furnace 12. It also makes it possible to balance the load, for example in the case of the breaking of one of the electrodes during the phase of fusion, and to correct the power factor of the network likely to be degraded by the furnace 12.

In FIG. 2, the variation of active power P is shown, as a function of the reactive power Q exchanged between the arc furnace 12 and the supply network 16. In this figure, An corresponds to the nominal operating point of the oven, Ace corresponds to the short-circuit operating point and A∞ corresponds to the operating point for a very long arc. During normal oven operation, the oven operating point moves between the two points Ain and A2n. It can be seen that, for this displacement, the variation in reactive power ΔQ is greater than that of the active power ΔP. This variation in active power P and reactive Q results in a fluctuation in the voltage across the arc furnace.

This voltage fluctuation, generated by a corresponding fluctuation in the value of the resistance of the oven is compensated by the first compensation module 18 which injects into the supply line 14 an active power P making it possible to compensate for the active power absorbed by the oven , in order to keep constant the amplitude of the active component of the current absorbed by it. To do this, the first module 18 injects a voltage in phase opposition with the supply voltage of the furnace 12, so that the algebraic sum of these two voltages gives rise to a voltage of fixed amplitude. It should be noted that this type of compensation may be purely an active power compensation if the main cause of the voltage fluctuation is a variation in the resistance of the furnace. It can also be an asset and reagent compensation if the voltage fluctuation is due to a variation in the impedance of the oven.

As mentioned above, to perform this active power compensation, the inverter 26 is controlled by the control unit 28 in which is stored a conventional calculation algorithm allowing the determination ^" the amplitude of the voltage fluctuations to be compensated as a function of their frequency, from the values of the characteristic quantities delivered by the corresponding measurement sensor 42. The voltage injected in series in line 14 thus makes it possible to compensate for the voltage fluctuations present at the terminals of the arc furnace 12 in order on the one hand to reduce the disturbances generated on the electrical network 16 and, on the other hand, to increase the efficiency of the furnace In addition, the second module 22 of reactive power compensation provides compensation malfunctions accompanied by large amplitude voltage variations, for which the operating point An of the furnace moves between Ace and A∞. It will be noted that for this operating regime, the variation in reactive power is, in this case, of the same order as that of the active power. The second compensation module 22 then directly compensates for the reactive power fluctuations e Q in order to check the voltage at the connection point of the oven 12.

As mentioned previously, this second compensation module 22 is a compensation module of the conventional type, generally fitted to arc furnaces. It will therefore not be described in more detail. Note however that it is associated with a capacitor bank C2 allowing the correction of the power factor of the network.

It will also be noted that the dimensioning of the first compensation module 18, in terms of power, is almost 10 times less than that of the second power compensation module 22. This reduced dimensioning makes it possible to use a technology of components that are faster at switching, such as IGBTs (bipolar transistors with insulated gate), which makes it possible to reduce the response times of the compensation device. Furthermore, the first module 18 being relatively fast, it can serve as an active filter of the harmonics circulating on the power supply network 16. In the embodiment shown in the figure

1, the voltage source, constituted by the capacitor C1 is supplied by the coil L3.

It is however possible, as shown in FIG. 3, to supply this energy storage means with a current drawn from the supply line 14.

In this FIG. 3, the elements identical to those of FIG. 1 have the same reference numbers.

It can be seen in this figure that the capacitor C1 is connected to a three-phase rectifier constituted by an alternating current-to-direct current converter, designated by the reference numeral 46, of conventional type, and controlled by the control unit 28, this converter being connected to the supply line 14, via a transformer 48 ensuring the current draw and the primary winding of which is connected in parallel to the supply line and the secondary winding of which is connected to the converter 46.

It will be appreciated that the first compensation module 18 in fact constitutes an alternating current to alternating current converter. The active power necessary to counter variations in the resistance of the arc is supplied to the capacitor C1 by means of the three-phase rectifier 46 ensuring the conversion of the current drawn to direct current.

FIG. 4 shows a third embodiment of the device for compensating for voltage fluctuations at the terminals of the arc furnace 12 according to the invention. It can be seen in this figure that, in this embodiment, the second module 22 for reactive power compensation consists of a three-phase inverter with GTO thyristors (switches controllable in opening and closing) controlled by the control unit 28, and supplied by a DC voltage source, constituted by a capacitor C3 and by decoupling inductors, constituted by the leakage inductors of the transformer 24 which connects the inverter to the line 14. It can also be seen that the storage means for energy

C1 of the first active energy compensation module 18 is supplied by the capacitor C3, by means of a DC-DC bridge constituted by a pulse transformer 49 of the conventional type. This conversion bridge performs a galvanic decoupling between the voltage of the capacitor C1 of the first module 18 and the voltage of the capacitor C3 of the second module 22, which makes it possible to reduce the dimensioning of the first module 18. In this embodiment, the second module 22, which performs a reactive power compensation, also performs the function of taking the active power necessary for the operation of the first module 18. The latter therefore comprises a reduced number of components.

The presence of the galvanic decoupling transformer 49 therefore makes it easier to control the first module 18.

Note, however, that this transformer 49 is optional and that it can be deleted if necessary.

FIG. 5 shows another embodiment, in which the galvanic decoupling means between the first 18 and second 22 compensation modules have been eliminated. In the embodiments described with reference to FIGS. 1, 3, 4 and 5, the voltage fluctuation compensation device can be placed in an arc furnace already equipped with the reactive power compensation module 22.

Thus, the first active power compensation module reinforces the compensation performed by the second module 22, in particular to compensate for voltage fluctuations of low amplitude and frequency varying between approximately 5 and 25 Hz.

The invention however makes it possible to produce a device for compensating for voltage fluctuations forming a unitary assembly for compensating the active power, as shown in FIG. 6. This embodiment does not differ from the embodiment described with reference to FIG. 1 that by the absence of the second reactive power compensation module 22.

As with the embodiments described with reference to FIGS. 1 and 3, this compensation module 18 is completely autonomous and its control by the control unit 28 and therefore considerably simplified.

Claims

1. Device for compensating for voltage fluctuations at the terminals of a load (12) with unstable impedance, such as an arc furnace, connected to a polyphase electric power supply line (14) of a network (16 ) power supply, comprising an inverter circuit (26) connected to said line (14) and supplied by a DC voltage source (Cl), said inverter (26) being controlled by a control unit (28) for injecting into said line (14) an alternating voltage for compensating for the voltage fluctuations generated by the variations in load impedance (12), characterized in that it further comprises a circuit (22) for reactive power exchange with said line supply (14) of electrical energy.

2. Device according to claim 1, characterized in that it further comprises, connected to the control unit, measuring means (42) of at least one quantity characteristic of the electrical energy supplying the load (12 ), said control unit (28) comprising means for calculating the voltage to be inserted in series in said line (14) with a view to compensating for said variations, from the values delivered by said measuring means (42).

3. Device according to one of claims 1 and 2, characterized in that the DC voltage source comprises an energy storage means (Cl) supplied by a DC voltage applied to its terminals.

4. Device according to claim 3, characterized in that it further comprises, connected to the terminals of said energy storage means (Cl), a superconductive coil (L3) with mductive storage associated with a chopper (34) for the supply of said energy storage means (Cl) with direct voltage.

5. Device according to claim 3, characterized in that it comprises means (48; 24) for withdrawing current from said supply line, connected to a circuit (46; 22) for converting said withdrawn current, into direct current, said conversion circuit being connected to said energy storage means (Cl) for its supply.

6. Device according to claim 5, characterized in that said current drawing means consist of a transformer (48, -24) whose primary winding is connected in parallel on said supply line (14) and whose l the secondary winding is connected to said conversion circuit (46; 22).

7. Device according to any one of claims 1 to 6, characterized in that said inverter circuit (26) is connected to said supply line by means of a transformer (20) of galvanic decoupling.

8. Device according to any one of claims 1 to 7, characterized in that said inverter circuit (26) comprises a set of switching cells controlled by said control unit (28) and comprising a switching element (30) switchable on opening and closing and a diode (32) connected in anti-parallel to said switching element (30).

9. Device according to any one of claims 1 to 8, characterized in that said circuit (22) for reactive power exchange comprises an inverter circuit for reactive power exchange supplied by at least a second source (C3) of direct voltage and controlled by said control unit, said inverter circuit being connected to said supply line by means of a galvanic decoupling transformer (24).

10. Device according to claims 3 and 9, characterized in that said at least one second DC voltage source (C3) is connected to the energy storage means (Cl) of the inverter circuit (26) for compensating for voltage fluctuations for loading.

11. Device according to claim 10, characterized in that said at least one second DC voltage source is connected to the energy storage means by means of a transformer (49) of galvanic decoupling.