Power Factor Improvement for Thyristor Controlled Heater (Furnace)

Convert it to multiple-element mode, by allocating one thyristor to one heating element, multiplied by however many elements there are. When the turndown in heating is required, switch off 30-60% or more of the thyristors. Each individual energised circuit will then have near-unity power factor.

Thank you for your reply but currently i am not allowed to make any changes to the electronics or controls of the furnace.

How about installing power factor improvement capacitors with a power factor controller like we do in case of motor loads to improve the power factor? Would it work in this case too ?

Maybe, though if low power factor isn't being charged on the tariff, it won't achieve a return on the investment.

An energised feeder with no load on it has a power factor of almost zero.

<...not allowed to make any changes to the electronics or controls...>

So adding power factor correction is out, then? That would rather limit the possible solutions.

I am not allowed to amend any controls of the machine itself, but adding power factor capacitors at the input side (supply) is fine.

You will not be able to fix this by adding PFC caps. A resistive load never has a reactive component. The poor power factor you are reading is likely DISTORTION power factor caused by the harmonic distortion from the phase angle control of the thyristors, not displacement power factor. Adding capacitors will set up a race between the thyristors being damaged by the rapid rate of change in capacitor charging current, or the failure of the capacitors because of the added harmonic current. You pretty much have to live with it, or change the control strategy. But first you need to assess whether or not this is really something that needs addressing at all. Most Power Utility VAR meters will not read distortion power factor. They may have another requirement for limiting THD, but that is not directly the same.

As mentioned in #5 , there is lot of distortion due to firing angle change of thyristors. Fourier series suggests that this consists of many harmonics. So it is not just adding capacitors in parallel, but placing a low pass input filter may help. This filter will consist of series inductor and parallel capacitor. But these will be extremely bulky and inefficient too.

I have seen active high frequency switched PF controller made by a German company. It is a 2 terminal box connected across the mains. Since power rating of the furnace is not mentioned- I cannot comment. For old furnaces - where one is not allowed to change the controls- placing active PF controls will definitely improve PF to near unity. Active PF controllers are made using switched MOSFETs or IGBTs, microprocessors with embedded intelligent software. This will correct for distortion and displacement factors simultaneously.

The best adapted solution is to use an Active Harmonic Filter. The AccuSine range by Schneider-Electric is providing both Harmonic cancellation and reactive power compensation.

The resulting Power Factor can be close to 1.