AU2010291895A1

AU2010291895A1 - Dynamic VAR compensation system and method for AC furnace

Info

Publication number: AU2010291895A1
Application number: AU2010291895A
Authority: AU
Inventors: Frederik Petrus Greyling; Hendrik Willem Greyling
Original assignee: GREYLING FREDERIK; GREYLING HENDRIK
Current assignee: GREYLING FREDERIK; GREYLING HENDRIK
Priority date: 2010-04-15
Filing date: 2010-11-10
Publication date: 2011-11-03
Anticipated expiration: 2030-11-10
Also published as: AU2010291895B2; AU2010291895B8; ZA201102398B; WO2011128730A1

Abstract

A furnace power supply system 10 comprises at least one furnace power supply voltage step-down transformer 12 for stepping down a first voltage V1 to a second voltage V2. A secondary winding 12.2 of the voltage step-down transformer is connectable to at least one electrode 14 of the furnace 16. The power supply system comprises a compensation system 18 connected to the secondary winding 12.2 of the step-down transformer. The compensation system 18 comprises a plurality of capacitors 20.1 to 20.n in respective parallel branches 22.1 to 22.n. At least some of the branches comprise a respective switch element 24.1 to 24.n. A controller 28 for operating the switch elements comprises an input 29 for a signal representative of a power factor to be controlled and the controller being configured to open and close the switch elements in response to the signal, thereby to control the power factor in real time.

Description

1 DYNAMIC VAR COMPENSATION SYSTEM AND METHOD FOR AC FURNACE INTRODUCTION AND BACKGROUND 5 This invention relates to an electrically operable furnace and more particularly to an electrical power supply for an AC furnace and a method of controlling a furnace parameter, such as power factor. It is well known to use capacitors on a primary side of a furnace step-down 10 transformer of an AC furnace system to improve the power factor of the system. However, due to a reactive component of the furnace load as seen by the transformer, the furnace still operates at a lagging power factor of typically between 70% and 85%. Hence, these capacitors do not improve the supply capacity of the furnace power supply system or 15 contribute to the furnace production capacity. OBJECT OF THE INVENTION Accordingly, it is an object of the present invention to provide an alternative furnace power supply system, associated method of controlling 20 a power factor and electrically operable furnace with which the applicants believe the aforementioned disadvantages may at least be alleviated or which may provide a useful alternative for the known furnace power supply systems, methods of controlling power factor and electrically operable furnaces.

2 SUMMARY OF THE INVENTION According to the invention there is provided a furnace power supply system comprising: 5 - at least one furnace power supply voltage step-down transformer for stepping down a first voltage to a second voltage; - a secondary winding of the voltage step-down transformer being connectable to at least one electrode of the furnace; 10 - a compensation system connected to the secondary winding of the step-down transformer, the compensation system comprising a plurality of capacitors in respective parallel branches, at least some of the branches comprising a respective switch element; and 15 - a controller for operating the switch elements; - the controller comprising an input for a signal representative of a power factor seen by the secondary winding of the step down transformer and the controller being configured to open and close the switch elements in response to the signal, 20 thereby to control the power factor. The switch elements may be solid-state switch elements.

3 The controller may be configured to operate the switch elements at suitable times, so as to reduce transient effects over the capacitors. For example, the controller may be configured to operate the switch elements when the instantaneous value of the second voltage is equal to the voltage 5 across the capacitors. The controller may be configured automatically to generate switching signals to operate selected switch elements, thereby to connect or disconnect capacitors selected from the plurality of capacitors to or from 10 the secondary winding, thereby to improve the power factor in real time. The controller may be configured to operate the switch elements thereby 15 to connect or disconnect harmonic filter elements comprising at least some of the capacitors and/or reactors and/or resistors to or from the secondary winding, thereby to filter out unwanted harmonics. Hence, a reactor, such as a coil and/or a resistor may also be connected in at least some of the branches. 20 The first voltage may be between 11 kV and 33kV and the second voltage may be between 200V and 600V.

4 A first connection may be provided between the secondary winding of the at least one voltage step-down transformer and the at least one electrode and the first connection may comprise a high current furnace bus tube. 5 A second connection may be provided between the secondary winding of the at least one voltage step-down transformer and the compensation system and the second connection may comprise a high current connection. 10 The compensation system may comprise a three-phase system. In other embodiments the compensation system may comprise an arrangement of three single phase systems. Also included within the scope of the present invention is an AC furnace 15 system comprising a furnace power supply system as herein defined and/or described. Yet further included within the scope of the present invention is a method of controlling a power factor in an AC furnace, the method comprising the 20 steps of: - utilizing a compensation system connected to a secondary winding of a power supply step-down transformer for stepping down a first supply voltage to a second lower 5 voltage, the second winding being connected to at least one electrode of the furnace; - receiving a signal representative of a power factor as seen by the secondary winding of the step-down transformer; and 5 - causing the compensation system to connect capacitors to the secondary winding to control the power factor in real time. BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS 10 The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein: figure 1 is a basic circuit diagram of a furnace power supply system according to the invention; figure 2 is a basic diagram of a first embodiment of the furnace and 15 power supply system; figure 3 is a similar diagram of a second embodiment of the furnace and power supply system; figure 4 is a similar diagram of a third embodiment of the furnace and power supply system; and 20 figure 5 is a vector diagram illustrating an improvement in active power delivered to the furnace and hence an improvement in capacity of the furnace.

6 DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION A furnace power supply system according to the invention for an electrically operable AC furnace 16 is generally designated by the 5 reference numeral 10 in the figures. Referring to figure 1, the furnace power supply system 10 comprises at least one furnace power supply voltage step-down transformer 12 for stepping down a first voltage V1 to a second voltage V2. A secondary 10 winding 12.2 of the step down transformer 12 is connectable to at least one electrode 14 of the furnace 16. A dynamic VAR compensation system 18 is connected to the secondary winding 12.2 of the step-down transformer 12. The compensation system comprises a plurality of capacitors 20.1 to 20.n in parallel branches 22.1 to 22.n in a bank 20. At 15 least some of the capacitors in the bank are in series with a respective switch element. In some embodiments, the switch elements form part of a switching module 24. In some embodiments, at least some of branches 22.1 to 22.n comprise 20 one capacitor 20.1 to 20.n and a respective switch element 24.1 to 24.2 in series. In other embodiments, at least some branches comprise serial or parallel groups of capacitors connected in series with a respective switch element for the branch.

7 In the embodiment shown, the switch elements 24.1 to 24.n are solid-state switch elements. 5 Also in the embodiment shown, a reactor in the form of a coil 26.1 to 26.n is connected in at least some of the branches 22.1 to 22.n. The VAR compensation system 18 comprises a controller 28 for generating switching signals on output lines 28.1 to 28.n to operate the 10 switch elements 24.1 to 24.n, thereby to connect or disconnect the capacitors 20.1 to 20.n to or from the secondary winding 12.2. The controller 28 has an input 29 for a signal representative of a power factor as seen by the secondary winding of the step-down transformer, to 15 be controlled. The controller is configured to operate selected ones of the switch elements 24.1 to 24.n, to connect or disconnect capacitors selected from the plurality of capacitors to or from the secondary winding thereby to improve the power factor of the load 14 as seen by the secondary winding 12.2 of the step-down transformer in real time. Optimally, in addition, the 20 switch elements 24.1 to 24.n connect or disconnect branches to or from the secondary winding thereby to filter out unwanted harmonics caused by the at least one electrode 14.

8 Hence, a signal or data relating to the power factor as seen by the secondary winding is supplied to the controller 28 via input 29 and the controller 28 is configured, by means of the high speed switch elements 24.1 to 24.n, to add or subtract capacitors from the circuit connected to the 5 secondary winding, thereby, in real time, reducing the reactive load and hence power factor and optionally tuning harmonic filters on the secondary side. The controller is preferably configured to cause the switch elements 24.1 to 24.n to operate at suitable times, to reduce transient effects over the 10 capacitors. The first voltage may be between 11kV and 33kV and the second voltage may be between 200V and 600V. 15 The connection 30 between the secondary winding 12.2 of the at least one step-down transformer and the at least one electrode 14 comprises high current furnace bus tubes. The connection 32 between the secondary winding of the at least one 20 step-down transformer and the dynamic VAR system 18 comprises a high current connection.

9 In figure 2, there is shown a first embodiment of the furnace system according to the invention. The furnace 16 comprises three electrodes 14.1, 14.2 and 14.3. Three similar furnace power supply step-down transformers 112.1 to 112.3 are provided to step down a first and high 5 supply voltage V 1 (typically 11kV to 33kV) to a second and lower furnace operating voltage V 2 (typically 200V to 600V). Electrodes 14.1 and 14.2 are connected in known manner by high current furnace bus tubes 30 to the secondary winding 12.2 of step-down transformer 112.1. In the same manner, electrodes 14.2 and 14.3 are connected to the secondary winding 10 12.2 of the transformer 112.2 and electrodes 14.3 and 14.1 to the secondary winding of transformer 112.3. Single phase capacitor banks 20.1 to 20.3 of a dynamic VAR system 18 as hereinbefore described are connected to the secondary windings 12.2 of the transformers 112.1 to 112.3 respectively. 15 The embodiment in figure 3 is similar to the embodiment of figure 2, except that the furnace comprises six electrodes 14.1 to 14.6, of which electrodes 14.1 and 14.2, 14.3 and 14.4, and 14.5 and 14.6 are connected to the secondary windings of the voltage step-down transformers 112.1 to 20 112.3, respectively. Single phase banks 20.1 to 20.3 of a dynamic VAR system 18 as hereinbefore described are connected to the secondary windings 12.2 of the transformers 112.1 to 112.3 respectively.

10 The embodiment in figure 4 comprises a three-phase furnace voltage step down transformer 12. Each of the phases of the secondary winding 12.2 is connected to a respective electrode 14.1 to 14.3 of the furnace 16. A three-phase dynamic VAR system 18, generally similar to the dynamic 5 VAR systems as hereinbefore described, is connected to the secondary of the transformer 12. The vector diagram in figure 5 illustrates that for an apparent power S for the furnace step-down transformer 12 (shown in figure 1) of say 63 MVA, 10 with a prior art power factor $1 of between 70% and 85% as referred to in the introduction of this specification, there is an active power component P1 of about 50.3 MW and a reactive power component of Q1. The aforementioned capacitor arrangements of the dynamic VAR system 18 reduce the reactive component as seen by the transformer 12 continually 15 and in real time to Q2, thereby improving the power factor to 02 of about 96%, so that for the same apparent power S, additional active power (up from P1 to P2 of about 56.7 MW) is supplied to the furnace 16, thus increasing the production capacity of the furnace. 20 Hence, it is believed that while utilizing existing furnace transformers, more active power is supplied to the furnace, thus resulting in improved production capacity. Secondary or electrode voltage varies in furnaces, mainly due to substantial changes in the reactive load. By reducing the 11 reactive component on a real time basis as hereinbefore described, it is expected that a more constant voltage may be maintained at the electrodes. Furthermore, the power factor is improved on the secondary side of the furnace transformer 12, thus increasing the capacity of the 5 existing power supply to provide active power to the furnace and improving the power factor on the primary side as well and hence reducing electricity bills. Conventional switching of capacitor banks normally cause transients, whereas it is believed that with a dynamic VAR system 18 as herein described, transients may be reduced, thus inhibiting degradation of 10 furnace transformers. Still furthermore, furnaces, such as open arc scrap melters, generate high levels of unwanted harmonics which are conventionally filtered out with filter banks on the primary side of the furnace transformer 12. With the system according to the invention, the dynamic VAR system 18 may comprise harmonic filter components and 15 hence it is believed that the total harmonic distortion (THD) may be reduced on the secondary side of the furnace transformer 12, thereby reducing furnace system losses.

Claims

1. A furnace power supply system comprising: - at least one furnace power supply voltage step-down transformer for stepping down a first voltage to a second 5 voltage; - a secondary winding of the voltage step-down transformer being connectable to at least one electrode of the furnace; - a compensation system connected to the secondary 10 winding of the step-down transformer, the compensation system comprising a plurality of capacitors in respective parallel branches, at least some of the branches comprising a respective switch element; and - a controller for operating the switch elements; 15 - the controller comprising an input for a signal representative of a power factor seen by the secondary winding of the step-down transformer and the controller being configured to open and close the switch elements in response to the signal, thereby to control the power 20 factor. 13

2. A furnace power supply system as claimed in claim 1 wherein the controller is configured to operate the switch elements at suitable times, so as to reduce transient effects over the capacitors. 5

3. A furnace power supply system as claimed in claim 2 wherein the controller is configured to operate the switch elements when the instantaneous value of the second voltage is equal to the voltage across the capacitors. 10

4. A furnace power supply system as claimed in any one of claims 1 to 3 wherein the controller is configured automatically to generate switching signals to operate selected switch elements, thereby to connect or disconnect capacitors selected from the plurality of 15 capacitors to or from the secondary winding, to improve the power factor in real time.

5. A furnace power supply system as claimed in any one of claims 1 to 4 wherein the controller is configured to operate the switch 20 elements thereby to connect or disconnect capacitors selected from the plurality of capacitors to or from the secondary winding, to filter out unwanted harmonics.

6. A furnace power supply system as claimed in any one of claims 1 to 5 comprising a coil in at least some of the branches. 14

7. A furnace power supply system as claimed in any one of claims 1 to 5 6 wherein the first voltage is between 11kV and 33kV and the second voltage is between 200V and 600V.

8. A furnace power supply system as claimed in any one of claims 1 to 7 comprising a first connection between the secondary winding of 10 the at least one voltage step-down transformer and the at least one electrode and wherein the first connection comprises a high current furnace bus tube. 15

9. A furnace power supply system as claimed in any one of claims 1 to 8 comprising a second connection between the secondary winding of the at least one voltage step-down transformer and the compensation system and wherein the second connection comprises a high current connection. 20

10. A furnace power supply system as claimed in any one of claims 1 to 9 wherein the compensation system comprises a three-phase system.

11. A furnace power supply system as claimed in any one of claims 1 to 25 9 wherein the compensation system comprises an arrangement of three single phase systems. 15

12. An AC furnace system comprising a furnace and a furnace power supply system as claimed in any one of claims 1 to 11. 5

13. A method of controlling a power factor in an AC furnace, the method comprising the steps of: - utilizing a compensation system connected to a secondary winding of a power supply step-down transformer for 10 stepping down a first supply voltage to a second lower voltage, the secondary winding being connected to at least one electrode of the furnace; - receiving a signal representative of a power factor to be controlled as seen by the secondary winding of the step 15 down transformer; and - causing the compensation system to connect capacitors to the secondary winding, to control the power factor in real time.