Why did KVA increase while KW decreased during furnace melting process?

In summary, the conversation was about a melting furnace melting aluminum and the changes made to the PID control. The KW seemed to drop while the KVA value increased after changing the PID sampling time. The person also offered to share a graph of the results. There was a discussion about the reason for the KVA jump and the possibility of closed loop oscillation due to too much gain or time delay. The conversation also touched on the thyristor control and the use of an Elster meter for measuring fundamental frequency quantities.
  • #1
roro36
27
1
Hi Guys,

So we have a melting furnace that is melting Aluminium. After 2 days of holding a melt to see how the furnace performs in a holding situation, I changed the PID control to have a longer sampling time and hence smoother control. What I noticed was the KW seemed to drop, although the average usage was probably the same. The KVA value had a definite increas in the average value. I can post a graph of the results if anyone is interested?

Can this be explained? The elements a SiC and are controlled by a thyristor card by current setting.

Thanks for any help.
 
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  • #2
Do post your results.
 
  • #3
Graph

This is using an Elster 3 phase meter, integrating over 5 minutes. The KVAh term is just me looking at what the KVA has been doing over the previous hour.
 

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  • #4
You can see the kW has stabalised, but using the same average. The KVA value however at the exact time I changed the PID sampling time shot up.

I can only think that because the drive is now staying on constantly and never turning off there is constant switching to keep the power at 20%. Before it could turn on to 100% where it uses the power more efficiently and then off again, meaning the capacitance introcuded into the system was not as great? I'm not really sure though.
 
  • #5
What is "PID sampling time?"

Your graph looks like a classic case of closed loop oscillation from way too much gain or time delay...

If oscillation is around 2/3 of integral time try lengthening that setting
if it's faster try less proportional gain
try switching derivative all the way out and add back as required to reduce overshoot in response to step change of setpoint

Do some reading on control system tuning. Old line companies like Fisher Controls and Bailey Meter pretty much wrote the books.
 
  • #6
Hi Jim. Thanks for the reponse. Unfortunately, that was not the answer I was looking for. As it's an aluminium melting furnace, with 2 tons of Aluminium inside, I believe the time delay factor is out of my hands, as it honestly takes ages for even full power to adjust the temperature ever so slightly.
My question was actually on the reasoning behind the KVA jump when the system had changed to a larger sampling time and more stable?
 
  • #7
Are the thyrisistors fired at zero crossing, ie on and off for interger numbers of cycles, or are they phase angle controlled?

I suspect it's a matter of metering - how does that Elster meter work?

reason i ask is

The harmonics shall be filtered out while measuring Wh, Var
and VArh, and only fundamental frequency quantities
shall be measured/computed.
from http://nrldc.in/Commercial/calculations/SEM_Hand_book.pdf
which i got from a search on Elster

is this it?
http://www.elstersolutions.com/assets/products/products_elster_files/A1700_31052012.pdf
 
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  • #8
Hi Jim,

Yes it's Phase Angle Controlled and I am using the A1140 meter.
 

Related to Why did KVA increase while KW decreased during furnace melting process?

1. What is KVA jump with KW decrease?

KVA jump with KW decrease is a phenomenon that occurs in electrical systems when the KVA (kilovolt-ampere) increases while the KW (kilowatt) decreases. This can happen when there is a change in the power factor of the system or when there is a sudden increase in the reactive power demand.

2. Why does KVA increase while KW decreases?

This happens because KVA is a measure of the total electrical power in the system, including both real power (KW) and reactive power. When there is an increase in the reactive power demand, it causes the KVA to increase while the KW decreases, resulting in a KVA jump with KW decrease.

3. How does KVA jump with KW decrease affect the performance of the system?

This phenomenon can lead to a decrease in the overall power quality of the system, as the increase in reactive power can cause voltage fluctuations and other issues. It can also result in a decrease in the efficiency of the system, as the reactive power demand needs to be compensated for by the power source.

4. Can KVA jump with KW decrease be prevented?

Yes, KVA jump with KW decrease can be prevented by implementing measures to improve the power factor of the system and reduce the reactive power demand. This can include using power factor correction equipment or optimizing the use of reactive power in the system.

5. What are the potential consequences of ignoring KVA jump with KW decrease?

If KVA jump with KW decrease is ignored, it can lead to equipment failures, increased energy costs, and other issues with the electrical system. It can also affect the reliability and safety of the system, potentially causing power outages or other disruptions.

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