AU2010291894A1

AU2010291894A1 - High voltage secondary power factor correction system and method for AC furnaces

Info

Publication number: AU2010291894A1
Application number: AU2010291894A
Authority: AU
Inventors: Frederik Petrus Greyling; Hendrik Willem Greyling
Original assignee: GREYLING FREDERIK; GREYLING HENDRIK
Current assignee: GREYLING FREDERIK; GREYLING HENDRIK
Priority date: 2010-01-14
Filing date: 2010-10-26
Publication date: 2011-07-28
Anticipated expiration: 2030-10-26
Also published as: ZA201102396B; WO2011086429A1; AU2010291894B2

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 12 is connectable to at least one electrode 14 of the furnace 16. The power supply system comprises at least one voltage step-up transformer 18, a primary winding 18.1 of which is connected to the secondary winding 12.2 of the voltage step-down transformer 12, to step up the second voltage V2 to a third voltage V3. At least one power factor correction reactive component 20 is connected to the secondary winding 18.2 of the voltage step-up transformer.

Description

1 INTRODUCTION AND BACKGROUND This invention relates to an electrically operable furnace and more particularly to an electrical power supply for an AC furnace and a method of power factor correction. 5 It is well known to use capacitors on a primary side of a furnace step-down transformer of an AC furnace system to improve the power factor of the system. However, due to the reactive component of the furnace load as seen by the transformer, the furnace still operates at a lagging power 10 factor of typically between 70% and 85%. Hence, these capacitors do not improve the supply capacity of the furnace power supply system or contribute to the furnace production capacity. OBJECT OF THE INVENTION 15 Accordingly, it is an object of the present invention to provide an alternative furnace power supply system, associated method of power factor correction 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 20 power supply systems, methods of power factor correction and electrically operable furnaces. SUMMARY OF THE INVENTION 2 According to the invention there is provided 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; - at least one voltage step-up transformer, a primary 10 winding of which is connected to the secondary winding of the voltage step-down transformer, to step up the second voltage to a third voltage; and - at least one power factor correction reactive component connected to the secondary winding of the voltage step 15 up transformer. The power factor correction component may comprise a capacitor. A switch may be provided between the secondary winding of the step-up 20 transformer and the at least one power factor correction reactive component. The first voltage may be higher than the third voltage, which is higher than the second voltage.

3 The first voltage may be between 11 kV and 33kV, the second voltage may be between 200V and 600V and the third voltage may be between 6.6kV and 22kV. 5 The connection between the secondary winding of the at least one step down transformer and the at least one electrode may comprise high current furnace bus tubes. 10 The connection between the secondary winding of the at least one step-up transformer and the at least one power factor correction reactive component may comprise high voltage cable. Also included within the scope of the present invention is an AC furnace 15 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 power factor correction in an AC furnace comprising the steps of: 20 - on a secondary side of a power supply step-down transformer for stepping down a first supply voltage to a second lower voltage connected to at least one electrode 4 of the furnace, stepping up the second lower voltage to a third voltage; and - utilizing a reactive component in a circuit connected to the third voltage, to improve a power factor as seen by 5 the step-down transformer. The reactive component may comprise a capacitor. 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 more detailed diagram of a first embodiment of the 15 system; figure 3 is a more detailed diagram of a second embodiment of the system; and figure 4 is a vector diagram illustrating an improvement in active power delivered to the furnace and hence an improvement in 20 capacity of the furnace.

5 DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION A furnace power supply system according to the invention for an electrically operable AC furnace is generally designated by the reference 5 numeral 10 in the figures. Referring to figure 1, the furnace power supply system 10 comprises at least one furnace power supply step-down transformer 12 for stepping down a first voltage V1 to a second voltage V2. A secondary winding 12.2 10 of the step down transformer 12 is connectable to at least one electrode 14 of the furnace 16. The system 10 further comprises at least one voltage step-up transformer 18. A primary winding 18.1 of the voltage step-up transformer is connected to the secondary winding 12.2 of the voltage step-down transformer 12, to step up the second voltage V2 to a third 15 voltage V3. At least one power factor correction reactive component in the form of a capacitor 20 is connected to the secondary winding 18.2 of the step-up transformer 18. In figure 2, there is shown a first embodiment of the system according to 20 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 supply voltage V1 (typically 11kV to 33kV) to a second and lower furnace operating voltage 6 V2 (typically 200V to 600V). Electrodes 14.1 and 14.2 are connected in known manner by high current furnace bus tubes 22 to the secondary winding 12.2 of the step-down transformer 112.1. In the same manner, electrodes 14.2 and 14.3 are connected to the secondary winding 12.2 of 5 the transformer 112.2 and electrodes 14.3 and 14.1 to the secondary winding of the transformer 112.3. A primary winding 18.1 of a first voltage step-up transformer 118.1 is connected across the secondary winding 12.2 of step-down transformer 112.1 to step up the second voltage to a third voltage V3 (typically 6.6kV to 22kV). A power factor correction capacitor 10 arrangement 20.1 forming part of a high voltage capacitor bank 120 is connected by high voltage cables 24 and optionally, but preferably, a high voltage switch arrangement 26, to the secondary winding 18.2 of first voltage step-up transformer 118.1. The switch arrangement 26, which may be in the form of a circuit breaker or contactor, in use, serves selectively to 15 isolate the capacitor arrangement 20.1 or bank 120 from the load in the form of the electrode furnace 16. In the same manner, power factor correction capacitor arrangements 20.2 and 20.3 are connected to the secondary windings 18.2 of second and third voltage step-up transformers 118.2 and 118.3, respectively. 20 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 7 to the secondary windings of the voltage step-down transformers 118.1 to 118.3, respectively, as shown in figure 3. The vector diagram in figure 4 illustrates that for an apparent power S for 5 the furnace step-down transformer 12 (shown in figure 1) of say 63 MVA, with a prior art power factor 01 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 reduce the reactive power 10 component as seen by the transformer 12 to Q2, thereby improving the power factor to 02 of about 90%, 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. Hence, it is believed that while utilizing existing furnace transformers, more 15 active power is supplied to the furnace, thus resulting in improved production capacity. The function of the step-up transformers is to step up the second voltage V2 to a substantially higher third voltage V3, to enable the use of high-voltage switchgear, cables and capacitor banks. Hence, standard high-voltage capacitor banks designed to improve the power 20 factor, as hereinbefore described, may be connected to the step-up transformers 118.1 to 118.3, thus allowing additional power to be supplied to the load 16 from existing or conventional furnace power supply step down transformers 112.1 to 112.3.

8 Standard and proven reliable high voltage capacitor banks 120 may be used for the power factor correction. Because conventional high voltage cables 24 are used between the secondary winding 18.2 of the step-up 5 transformers 118.1 to 118.3 and the power correction capacitor bank 120, the capacitor bank 120 may be located remote from the step-up transformers 118.1 to 118.3 and hence outside the furnace building (not shown). The high voltage switch 26 allows for disconnection of the capacitor bank 120 from the furnace step-down transformers 112.1 to 10 112.3, ensuring that the secondary power factor correction system does not affect furnace availability. The capacitor bank 120 may be switched out during switching operation of the furnace transformers 112.1 to 112.3. Only after switching on of the furnace power supply transformers, is the capacitor bank 120 connected. Switching surges and power system 15 disturbances may thereby be avoided.

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; - at least one voltage step-up transformer, a primary 10 winding of which is connected to the secondary winding of the voltage step-down transformer, to step up the second voltage to a third voltage; and - at least one power factor correction reactive component connected to the secondary winding of the voltage step 15 up transformer.

2. A furnace power supply system as claimed in claim 1 wherein the at least one reactive component comprises a capacitor. 20

3. A furnace power supply system as claimed in claim 1 or claim 2 comprising a switch connected between the secondary winding of the voltage step-up transformer and the at least one reactive component. 10

4. A furnace power supply system as claimed in any one of claims 1 to 3 wherein the first voltage is higher than the third voltage and 5 wherein the third voltage is higher than the second voltage.

5. A furnace power supply system as claimed in claim 4 wherein the first voltage is between 11 kV and 33kV, the second voltage is between 200V and 600V and the third voltage is between 6.6kV 10 and 22kV.

6. A furnace power supply system as claimed in any one of claims 1 to 5 comprising a first electrical connection between the secondary 15 winding of the at least one voltage step-down transformer and the at least one electrode and wherein the first electrical connection comprises a high current furnace bus tube.

7. A furnace power supply system as claimed in any one of claims 1 to 20 6 comprising a second electrical connection between the secondary winding of the at least one voltage step-up transformer and the at least one power correction reactive component and wherein the second electrical connection comprises a high voltage cable. 11

8. An AC furnace comprising at least one electrode and a power supply system for the at least one electrode, the 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 the at least one electrode of the furnace; at least one voltage step-up transformer, a primary winding of which is connected to the secondary winding of the voltage step-down transformer, to step up the second voltage to a third voltage; and at 10 least one power factor correction reactive component connected to a secondary winding of the at least one voltage step-up transformer.

9. An AC furnace as claimed in claim 8 comprising first, second and 15 third electrodes; first, second and third step-down transformers; first second and third step-up transformers, wherein primary windings of the first, second and third step-down transformers are connected to the first voltage, wherein a secondary winding of the first step-down transformer is connected to the first and second electrodes, wherein 20 a secondary winding of the second step-down transformer is connected to the second and third electrodes, wherein a secondary winding of the third step-down transformer is connected to the third and first electrodes, wherein a primary winding of the first step-up 12 transformer is connected to the secondary winding of the first step down transformer, wherein a primary winding of the second step-up transformer is connected to the secondary winding of the second step-down transformer, wherein a primary winding of the third step 5 up transformer is connected to the secondary winding of the third step-down transformer and wherein the secondary winding of each of the first second and third step-up transformers are connected to a respective power factor correction capacitor arrangement.

10 10. An AC furnace as claimed in claim 8 comprising first, second, third, fourth, fifth and sixth electrodes; wherein the power supply system comprises first, second and third step-down transformers and first, second and third step-up transformers, wherein primary windings of the first, second and third step-down transformers are connected to 15 the first voltage, wherein a secondary winding of the first step-down transformer is connected to the first and second electrodes, wherein a secondary winding of the second step-down transformer is connected to the third and fourth electrodes, wherein a secondary winding of the third step-down transformer is connected to the fifth 20 and sixth electrodes, wherein a primary winding of the first step-up transformer is connected to the secondary winding of the first step down transformer, wherein a primary winding of the second step-up transformer is connected to the secondary winding of the second 13 step-down transformer, wherein a primary winding of the third step up transformer is connected to the secondary winding of the third step-down transformer and wherein a secondary winding of each of the first, second and third step-up transformers is connected to a 5 respective power factor correction capacitor arrangement.

11. An AC furnace as claimed in claim 9 or claim 10 wherein a switch is provided between each of the secondary windings of the first, second and third step-up transformers and the respective power 10 factor correction capacitor arrangement.

12. An AC furnace as claimed in claim 11 wherein the secondary 15 windings of the first, second and third step-up transformers are connected to the respective power factor correction capacitor arrangements by high voltage connection means.

13. An AC furnace as claimed in claim 12 wherein the respective power 20 factor correction capacitor arrangements are located remote from the first, second and third step-up transformers. 14

14. An AC furnace as claimed in claim 13 wherein the respective power factor correction capacitor arrangements are located outside of a building housing the furnace. 5

15. A method of power factor correction in an AC furnace comprising the steps of: - on a secondary side of a power supply step-down transformer for stepping down a first supply voltage to a second lower voltage connected to at least one electrode 10 of the furnace, stepping up the second lower voltage to a third voltage; and - utilizing a reactive component in a circuit connected to the third voltage, to improve a power factor as seen by the step-down transformer.