Furnace Power Factor

The PF correction needs to be fitted on the furnace side of the transformer. It will take some of the stress off the transformer. The capacitors will have to be switched in and out in banks to correct the PF at the time. I've only worked on induction furnaces but that is where the PF correction was fitted.

Guys this is a strange one, since elements use every watt they get to heat up, and capacitors specifically take care of the "wattless" or reactive currents in appliances, like induction motors where you need excitaion before a rotaing fiels can be setup. How will pf correction improve a furnace efficiency? Or have i misunderstood the post as to the type of elements/probes used, or even the methon of heating used?

I post this "explanation" from somehwere on the web.

Power factor compares the real power (watts) being consumed to the apparent power (Volts-Amps) of the load. A purely resistive load (incandescent lights, electric heating elements.) would have a power factor of 1.0.

Such a low pf is indeed detrimental. However, you can't have a fixed capacitor all the time since the system pf will go leading when the furnace pf goes higher. You must have a good compensating system with a number of smaller capacitors which can be switched into the system as the pf varies. Induction furnaces always have them. The switching of capacitors is a very severe duty, and is done by special capacitor contactors in LV systems.

I believe, the best technical solution for such a situation is a stepless static compensation which can respond dynamically to the power factor fluctuations and avoid the loss of life of severe switching duty on the contactors and capacitors alike. If the penalties of power factor non-compliance is significant, that can be brought into bounds effectively. It will also address voltage flicker that can have several secondary consequences and provide a healthy power quality in the system.

Disclaimer: We are in the field of providing such solutions. Would be happy to provide more info on it to interested users.

but I have submerged arc furnace sir where connecting capacitor bank in the secondary is difficult

It may be difficult but if you want something to ease the stress on the transformer as well as improve general PF, it is where it will have to be fitted.

Running with 0.2 ,0.3 furnace PF with rated current will harm the transformer ?

Not if it has been installed and maintained in accordance with local codes, with all required protective devices under that code in good order.

Apologies, i did not realize that we were talking about an EAF and not induction heaters/elements. however, there seems to be e4nough knowledgable posters on the problem already.

I would be very careful at adding the capacitor on the load side. The transformer is probably acting as a ballast to limit the short circuit current. It acts as a large inductor! The PF compensation is very likely to be done at the primary side.

While the load should provide enough dampening, one has to check for resonances. All the harmonics, sub-harmonics, and in between frequencies will be produced by the arc. The control will need (most likely) to work independently on each single phase.

You need a fast capacitor switch (semiconductors) and controllers for the capacitor banks as the load changes very rapidly (~within 20mS). This is especially true at the beginning of the cycle when the furnace is loaded with coarse material. This period is usually characterized by the electrodes "dancing" up and down.

These people may be able to help you.

http://www.arteche.com/web/frontoffice/Index.aspx?Idioma=2

Good luck.

Good point. And a GA

My experience is with induction furnaces.

It didn't enter my head about the transformer acting a choke.

Please approach Siemens for solution

Can you check what is the average time of a certain PF value ? If it is long enough, than you can put step PF at the secondary side of the transformer.One thing you have to remember that your process is the same as short circuiting the transformer secondary dide, that's why the transformer vibrates. I have the same experience with such steel arc furnace, we use a 5 MVA transformer 70 kv / 20 kV, to supply the arc furnace transformer, this transformer has been designed to be able to withstand the short circuit current shock. So I believe that your arc furnace transformer should be able to handle it too.

Of course using Static VAR Compensator is the best way, but as I know it's very expensive. please try with conventional step capacitor but with oversized contactor.

regards,

Stanyadji

I would be surprised if you had a capacitor bank at the secondary of your transformer. Can you confirm?

Dear Marcot,

Yes we have the capacitor in the secondary side of the 70 kV/ 20 kV transformer, but the furnace has its own step down transformer. Our transformer is the public Utility transformer which supply the steel factory. I believe it's a misunderstanding, the capacitor isn't connect to the furnace transformer secondary side.

regrds

Whatever the solution you will finally arrive at, you also need to take into consideration any harmonics that can be generated by the furnace and injected into your power distribution network.

Furnces are among the leading generators of harmonics in power distribution networks. Therefore, you will need to install harmonics filters along with any power factor correction capacitors. If you do not install the filters, then your capacitors may blow within a few hours of operation. Morever these harmonics could also cause havoc in other equipment within you power distribution network (if you have not yet experineced it already, that is)

The professional approach I would recommend is that you first carry out a power line analysis including for harmonics (this may require a consultant). The results of the analysis will give a clear picture of trends of voltage, power consumption, power factor and harmonics (you may need to specify what you need in terms of reference for the analysis). Any power factor correction equipment manufacturer can use the power line anlysis results to design the suitable filters for your capacitors. You will also need to specify what value you need to bring your power factor to so as to determine the KVAr capacity for the capacitors you need.

Contact the furnace manufacturer