ELECTROLYSIS CELL RECTIFIER HARMONICS 5

[lansford]

At a new installation, a rectifier is feeding a DC electrolytic cell making sodium hypochlorite (bleach). The rectifier is rated 110 KW input three phase, 480 volt 60 Hz.It has a tranformer input and its design at rated load is 90% power factor with an IEEE 6 pulse type harmoinic current distribution. The source is a 480/277 volt grounded wye system.
At about full load, the 5th harmonic current is about twice the expected value and the 7th is about 1.5 X times the design value. The power factor at about full load is 60% and it remins the same at lower loads.
To make a long story short, the indications are that the rectifer is OK and there is now suspicion that the electrolysis cell is not behaving as a typical R or XL type load and that there is some other dynamic prcoeess occurring causing these strange conditions.
Does any one have any insight into this?
Thank you JIM

 

[magoo2]

You say the 5th harmonic current is about twice the expected value. Are you assuming 20% 5th harmonic or something else?

Same comments for the 7th harmonic. Are you assuming 14% (i.e., 1/7) 7th harmonic or something else?

To make sense of this, it would be good to know the source impedance, both R and X, at the 480 V bus. This would help with the determination of whether the observed power factor makes sense or not.

 

[Compositepro]

An electrolytic cell does not behave as a simple resistor. Voltage must exceed a certain value (~1.2 volt) before any curent will flow.

 

[sibeen]

lansford, is this a thyristor (SCR) controlled rectifier, ie: is the DC voltage at the ouput of the rectifier a controlled voltage or does it vary with the incoming supply?

If it is a controlled DC (controlled rectifier) the input power factor will vary with the firing control angle to the thyristors.

Is there some form of harmonic filter at the input of the rectifier. This could be an in-line choke or a inductor/capacitor trap.

*hand waving and approximate equations to follow*

For a standard 6 pulse rectifier (Graetz bridge) the output DC voltage = 1.35*Vac (line -line). If the rectifier is controlled the equation becomes:

VDC = 1.35*Vac*cos(a), where (a) is the firing angle of the rectifier.

The input P.F is directly proportional to the firing angle, so if the firing angle of the rectifier was 30 degrees (cos 30 = 0.866) The the input P.F will be 0.866.

With this equation it also shows that the P.F will also vary with the incoming voltage (if the DC is to be kept constant). The higher the input voltage the lower the P.F.

This doesn't take into account factors like the commutation andle of the firing.

Another part of this is that you need to take into account the magnetising current of the input transformer which will also make the input P.F worse.

I'd be putting and oscilloscope on the input lines, measuring current, and check that the input current has the normal 'double hump' that a rectifer is supposed to draw.

Other quick thoughts:

1. Is the incoming voltage symetrical?

2. Check the input voltage for distortion at the point of common coupling.

3. is the phase rotation correct. For an uncontrolled rectifier (diode) it doesn't matter, but for a controled rectifier (thyristor) it does.

 

[Skogsgurra]

The short answer is: Yes, you are right. A cell does not behave like a linear load (resistor or inductor). It is only when the rectifier output voltage exceeds cell voltage that current flows and that makes exact calculations of THDi a little more complicated than it would be for a thyristor controlled rectifier with normal (linear) load.

What can be said for sure is that THDi always is higher than expected (from school-book assumptions) and that it has to be taken into account when designing an electrolysis plant. All serious suppliers of electrolysis equipment know how to calculate and design rectifiers for "their" electrolyte and the THDi usually is not any concern.

What sometimes is a concern is that DC current ripple in the bath may be too high and cause unwanted secondary effects. But that should not be a problem in a pure electrolysis plant. It is usually only in plating and alactrophoresis plants you have to worry about current ripple.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricuwe]

Judging from the poor power factor it looks like rectifier no load voltage and voltage requirement are not properly matched. This forces the control to operate the SCRs at a large delay angle resulting in poor power factor and high harmonic content.

- Check if the transfomer has taps to reduce the output volatge, this will provide a small improvement, but will not fully resolve the issue
- If there are not taps and you are sure that the volatge required will stay that low as it actually is, have the transformer rewound with a higher number of turn on the primary
- adding a choke on the DC-side will reduce harmonics

 

[lansford]

MORE ON THE INSTALLATION
The input 480 volt side of the rectifers have a thyristor bridge for the DC voltage control.
The input voltage is well balanced and the source is strong.
I don't think phase rotation (phase sequence) was given any thought in wiring the units.
thanks
JIM