Power Supply for Electric Arc Gas Heater

Abstract

This invention concerns power supplies suitable for electric arc gas heaters such a plasma torches. It more particularly relates to the dimensioning of the inductor in the switched-mode DC to DC converter used for feeding the torch. The invention concerns in particular a DC power supply for driving a non-transferred electric arc gas heater, comprising: an AC to DC rectifier providing a potential U.sub.0; a DC to DC switching converter having a switching frequency f.sub.S; a current control loop having a latency Formula (I); and, a ballast inductor having an inductance L; characterized in that inductance L is such that Formula (II) and Formula (III). Such a design ensures the stability of the current control loop, while also ensuring a sufficient amount of current ripple to spread out the erosion zone on the electrodes of the torch.

[00001]$\begin{array}{cc};& \left(I\right)\\ L>\left(\frac{U_0}{1500}\right).Math.,& \left(\mathrm{II}\right)\\ L<\frac{1}{f_s}.Math.\left(\frac{U_0}{200}\right).& \left(\mathrm{III}\right)\end{array}$

Claims

1-6. (canceled)

7. A DC power supply for driving a non-transferred electric arc gas heater, comprising: an AC to DC rectifier providing a potential U.sub.0; a DC to DC switching converter having a switching frequency f.sub.S; a current control loop latency ; and, a ballast inductor having an inductance L; wherein $L>\left(\frac{U.Math..Math.0}{1500}\right).Math.,\mathrm{and}.Math..Math.L<\frac{1}{f_s}.Math.\left(\frac{U.Math..Math.0}{200}\right).$

8. A DC power supply according to claim 7, wherein the DC to DC converter is a buck converter.

9. A DC power supply according to claim 7, wherein U.sub.0>3000 V.

10. A DC power supply according to claim 7, wherein the power delivered to the gas heater is between 1 and 10 MW.

11. A DC power supply according to claim 7, wherein the electric gas heater is a non-transferred segmented plasma torch with hollow electrodes.

12. . A method of operating a non-transferred electric arc gas heater, characterized in that the heater is fed with a current of more than 500 A RMS, the current comprising a DC component and an AC component, the AC component having a peak to peak amplitude between 50 A and 20% of the DC component.

Description

[0028] FIG. 1 illustrates the invention. Are shown: [0029] (1) the AC to DC rectifier producing a DC voltage of U.sub.0; [0030] (2) the pulse-width modulated chopper, operating at frequency f.sub.S; [0031] (3) the ballast inductor with inductance L; [0032] (4) the fly-back diode, part of the buck converter topology; [0033] (5) the sensor reporting the instantaneous torch current; [0034] (6) the desired torch current or set-point value; [0035] (7) the current regulator, comparing the instantaneous torch current with the set-point value; [0036] (8) the unit driving the pulse-width modulation of the chopper based on the output of the regulator; [0037] (9) the plasma torch.

[0038] The following example illustrates an apparatus according to the invention. A 4 MW power supply comprises a rectifying unit delivering a voltage under nominal load of 3000 V (U.sub.0), and a chopper unit equipped with IGBT switching devices operating at 2 kHz (f.sub.S).

[0039] A ballast inductor is placed in series with an electric arc heater having a nominal power rating of 2.5 MW. The current to the load is measured using a Hall-probe and the value is fed to a PID regulator. A current set-point of 1000 A is chosen, which, for this particular torch, corresponds to a potential of about 1450 V. The duty cycle (D) of the chopper is thus about 48%.

[0040] The digital PID regulator induces a delay of 1 ms, and the chopper adds a further mean delay of 0.5 ms. A control loop latency of 1.5 ms () is thus considered. According to the invention, a minimum inductance of 3 mH is needed to ensure the stability of the control loop.

[0041] The maximum inductance is calculated according to the invention as 7.5 mH. This indeed ensures the desired peak to peak current ripple of 50 A.

[0042] To maximize electrode lifetime and power supply robustness, a value of 4 mH is chosen for this specific installation.