Heater - Thyristor driven

[pamungkas]

all,
I am in the middle of a project which involving fuel gas heater selection. The specification said that it should be controlled by thyristor either zero crossover or phase vector control in all range. Anyone can help me detail explanation (advantage or disadvantage)?

I also understand that SCR could produce harmonic. these heaters will be driven by Thermal engine generator which limit in capacity. Could you guide me, how to justify minimum spare capacity of the generator to handle harmonic?

Thank you for the enlightenment.

-p-

 

[Skogsgurra]

Zero cross does give you more flicker. And phase control gives you harmonics. Which one is the worst problem in your plant? I think you have to decide that first. The see how you can mitigate the problems.

Never heard of phase vector control in thyristors . How does that work?

Gunnar Englund

http://www.gke.org

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

 

[itsmoked]

I am not clear on what you are going to actually be running with the the switching element. Fuel gas heating is not going to cause much of any kind of loading. Not like an electric heater. I am surprised you don't just supply a contact to tell the system to go on or off.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

'Smoked: I think the application is pre-heating of fuel gas prior to combustion. It's quite often a necessity with heavier gases like propane or butane, particularly if they have been through pressure let-down which results in cooling of the gas.


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Sometimes I only open my mouth to swap feet...

 

[itsmoked]

Eh Scotty; Ah.. Never considered that. Would that be electric heat? Certainly be the easiest way to heat gas to a controlled temp.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

pamungkas
First you need to describe the heating devices, power topography and the precision level at which you want to provide control. In other words, is this a precision continuous control situation where you are using analog outputs from a PID controller to continually maintain a temperature? Or are you thinking of just turning a heater element on for a few minutes, then off for a few minutes once it reaches temperature, then back on when it drops too low again (also called Hysteresis control)? Then as far as power topography, are you supplying the same voltage as what your heater device is rated for, or are you using a transformer to power them?

In a nutshell: [ul]
[li]if you are doing simple Hysteresis control (On - off), then all you need is a solid state contactor, which will be Zero Cross. By its very nature, Zero Cross means it will NOT present significant harmonics or noise to the line.[/li]
[li]If you want continuous analog control, i.e. from a PID controller, the Zero cross becomes a special version called Variable Time Base Zero Cross Control. This method is essentially On-off control, but is done at the sine wave level. For instance, if you want 50% output, the controller turns on the SCRs for 10 cycles, then off for 10, then on for 10, then off for 10 etc. etc. But because the On parts still always take place at the Zero cross, again, no harmonics or noise. This is the least expensive analog control method and is the one most widely used. The only drawback is that it does not work well with inductive loads, i.e. transformers. That's why I asked that question earlier on the power topography. If you are feeding it with 208V for instance, but want to run the heaters at 480V to keep the current down, then Zero Cross is not a good idea, you need to go with phase angle control.[/li]
[li]Phase Angle Control is a more complicated method of analog SCR control involving gating the SCRs at whatever part of each and every sine wave cycle is necessary to accomplish the variable voltage output. It works great, can be highly accurate, allows for ramping (which is good for heater elements that have high coefficient of temperature ratios) and works with transformers. The drawbacks are comparably higher cost over zero cross, plus it creates harmonics when operating at anything less than 100% output. It's impossible to quantify the THD because it varies with firing angle, but it can be quite substantial at lower output levels (albeit at a lower demand level). This is one of the reasons many people choose not to use it.[/li][/ul]


Keith, they often do it with heat tape on the piping or heating elements in a receiver vessel, so yes, it's electric heating.

 

[itsmoked]

Variable Time Base Zero Cross Control

Also called "Distributed Zero Crossing."

Whichever you want to call it they use the standard "zero crossing" SSRs.

Phase angle control can be done with non-zero crossing SSRs. Also called Random SSRs. This is because zero crossing SSRs limit turn on to a few degrees from the power sine wave zero point, but random units allow turn-on anytime.

pamungkas; What is a "Thermal engine generator"?


Thanks Jeff.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[pamungkas]

I am sorry for too long response your valuable help.

itsmoke,
"thermal engine generator is liquified Natural Gas drive an engine generator. I use plant maintenance people jargon.

jraef,
Thank you for your explanation. We agree to use variable Time base zero cross control(VTB zero cross ctrl) and it supply to heater directly, but I concern with flicker issue. The generation system is 85% loaded (neglecting loads peak start) and I shall design the control stages required by the heater to prevent flicker. for example 4 stages 100 kW electrical heater control,it means to achieve constant temperature output, there are 4 regions, each region cover 25 kW heating elements. Usually, 1 block is modulating and 3 are fixed using contactors.

when I use VTB zero cross cntr, I let repeat flicker (25 kW)in system when thyristor switching on-off, that why I asked about sizing generators or sizing the stages of thyristor control.

Thank you.

-p-