Voltage Fluctuation

First of do learn the reason for fluctuations. Actually, the voltage fluctuations in power systems can cause a number of harmful technical effects, resulting in disruption to production processes and substantial costs. But flicker, with its negative physiological results, can affect worker safety as well as productivity. Humans can be sensitive to light flicker caused by voltage fluctuations. Generally speaking, flicker can significantly impair our vision and cause general discomfort and fatigue. The physiology of this phenomenon is complex. In general terms, flicker affects our vision process and brain reaction, almost always producing discomfort and deterioration in work quality. In some situations, it can even result in workplace accidents because it affects the ergonomics of the production environment by causing operator fatigue and reduced concentration levels.Probably now you might get your issue resolved.

Get an ideal voltage source.If it is not possible increase the transformer capacity as much as you can afford.You can limit voltage fluctuation.

I'm sorry to say that I cannot make any sense of the numbers you're providing us. I'll assume that your voltage drop is from 400V to 300V. This is a 25% drop in your voltage from applying your load. This means that your load impedance magnitude is only three times larger than your source impedance magnitude. (Ignoring for the time being your power factor numbers.) I would expect this to be considered a short circuit condition that should open circuit breakers or fuses somewhere. You then state that your welding shop draws a megawatt of power with a reasonable power factor. Assuming that you are getting three phase 400V power then your current draw will be on the order of 1E6/(400*.8*√3)≈1,800 amperes.

It depends on the magnitude and duration of the fluctuations. If the duration is small (the fluctuations are simply voltage spikes) then filtering will work, if however the fluctuations are due to voltage sags (and surges) due to large switching loads inside or outside the factory then basic line filtering probably won't work.

For voltage sags of this magnitude I would suggest performing a study to locate the cause and if the high voltage transmission line voltage to the transformers powering your factory is also fluctuating, if it isn't then simply increasing the size of your factory distribution transformer(s) may be the best option (your loads are too big for your existing supply), if it is then your power provider is supplying outside the regulated limits and it is up to them to fix the problem (country dependant of course).

Again a study is needed to find the causes, especially for a project of this size and potential cost. Nothing worse than guessing and finding that the expensive power quality solution doesn't fix the problem.

In general, good results are obtained by adding fast KVAR controllers on the feeder or even better, directly at the offensive loads. The objective is to rapidly compensate for the kvars generated by the welders when they turn on and off. This must be done using semiconductor switches with specially tuned capacitor banks to prevent resonances. Here is an example.

Here is the link to their site in Mexico:

http://www.arteche.com/web/frontoffice/empresasdelgrupoficha.aspx?idioma=2&id_empresa=7

Good luck.