Thyristor Controlled Heater vs Diesel Genset

[Peter Mather]

I have a 226kW heater that's controlled through a 12 pulse thyristor unit.
This has caused system instability on our small network causing the main power generator (Gas Cat with Woodward Electronic Governor) to hunt.

Changed the control card in the thyristor unit to proportional control.
Connected this string to a dedicated 500kW diesel set with similar result.

I suspect that the genset governor is struggling to keep track of the constant load/unload cycle of 226kW as the heater ramps up to set point and plan to split the heater bank into 150kW constant load with 76kW thyristor controlled.
From the Cat literature I read it seems that whilst this may experience resonance at certain duty points the reduced demand/capacity ratio from 0.45:1 to 0.152:1 should allow the governor to react more effectively.
If anyone has experience of this or similar genset/heater application please let me know.

Thanks

 

[waross]

I am aware of an issue with electric heating and generators.
The issue had been solved before I arrived on site, so details are sketchy.
A large construction project was running on diesel generators while a 140 kV transmission line was being constructed to supply the mill.
The engine of a new diesel generator failed after several weeks of service.
Several weeks later a second generator engine failed.
The issue was determined to be the electric heating in the onsite office building.
As i understood the issue, the heating was proportional control with zero point switching.
The heating would be on for a number of cycles and off for a number of cycles.
The on off nature of the switching was taking out the bearings in the engines.
The electric heating was converted to simple thermostatic control and the issues went away.
Many heating loads are very forgiving of control methods, however some process heating loads need closer control than thermostatic, on-off control.
Thinking out of the box, can you consider voltage control?
Rather than controlling between the generator and the heaters, use a dedicated generator and control the output voltage by controlling the input to the brushless exciter of a dedicated gen-set.
PID control of the exciting current in place of AVR control should allow close control of the heaters while providing a generator friendly sine wave output current.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[djs]

Another issue may be harmonic distortion due to phase control of the thyristor bank. I have had experience where this caused problems not with the thyristor unit, but with other loads connected to the system. How quickly does the heater change power? How big is the generator? You might try slowing down the thyristor controller so that the generator load does not quickly change.

 

[jraef]

There were issues with instability of generators when using thyristor controls decades ago, because the noise and harmonics created by phase controlled firing (voltage control) interacted with the AVR in the generator. Cat used to sell a retrofit kit for this, again, decades ago, before they fixed it in their design, so it hasn’t been an issue since the late 90s. But if your generator is from the 80s and never upgraded, it may be what you were seeing. Changing to proportional control means you went to zero-cross firing, which would not have that issue.


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