Immersion Heater - PV electricity

[scoobiedoo]

Tryed to send a massage to see if you are still suppling your immersion units but get this massage (echase has exceeded their stored private messages quota and cannot accept further messages until they clear some space.)!

 

[pmcalli]

Ah sorry, should have read pmcalli's post properly - yes, the high current wires disappear under the control unit, so presumably there is a filter there. I notice in the wiring diagram that you can't have your PV connected to the existing consumer unit - but then if the consumer unit is going to be full of relays it'll need replacing anyway.

The emma unit uses a PID function in the cylon controller to balance the outputs of the two CT's which measure the power from the solar panels against the power consumed in the house including the variable load immersion heater. If the PV system is wired into a spare way in the consumer unit they recommend that it is seperated by fitting a henley block and a second consumer unit. This is not stricly necessary as there is a sneaky way to install the CT's without adding a second consumer unit. If you have 16mm2 meter tails then you can clip the CT around both the incoming meter tail and the PV wire. As the meter tail is carrying house load - pv gen putting both wires through the CT results in house load -pv +pv = house load. If you have 25mm2 tails then you may need to buy the bigger CT's.

 

[suntrap]

Someone on the openenergymonitor forums gave me that idea until I decided that one CT on the meter tail with a sniff of the voltage to determine the direction of current flow (not to mention give more accurate readings) would be a more elegant solution!

 

[pmcalli]

Accurate power readings are not necessary in a two ct set up as you are only make two things the same. the control function for this mode is very simple to implement. If you only sense the composite current you are trying to reduce the value to zero. current transformers are very inaccurate a low levels and the function is non trival to implement.

 

[suntrap]

Good points, although I was really considering a more sophisticated system based on a microprocessor so that I could log things. Maybe just keep it simple, although it might be difficult to stabilise the loop. It does require the solar feed T&E cable to be split.

 

[pmcalli]

I moved the RCBO for the solar feed to be next to the main incomer in the cs unit and then put the CT around the the incoming meter tail and the solar feed live which are now right next to each other. second CT just goes around the solar feed live all nicely contained in the CS. My CS is a large MK sentry so there is plenty of room.
I am also thinking of using a micro controller type microprocessor and implement the PID function in code there are plenty of code examples for PID loops. I use them at work regularly and finding the right loop values can be done by a simple set of experiments. I am also looking at using an off the shelf PID controller and the crydom unit some of them take CT inputs directly. An expensive solution but only needs wiring up. I did try to see if i could get hold of the controller emma uses but cylon wouldn't supply details just referred me to their partner integrators

 

[inie meanie]

emma circuit diagram shows a filter which appears to be mounted under the cylon controller. the crydom incorporates a PWM controller which switches at a high rate. this is necessary both to comply with the regs and to make sure that the inverter has enough power storage (hold up) to supply the 3kw load even though it might be generating a few watts on average.

Hi guys
Just to let you know the 10PCV2425 is not controlled by PWM. It is a triac and as such, as soon as fired will continue to conduct until zero cross over. the EMMA uses this unit and therefore definitely does not use pwm. This has been confirm to me today by an application engineer at Crydom.

 

[suntrap]

Good stuff - please keep us posted!

One of the things that baffles me is the apparent lack of heatsink in the EMMA unit. I'd have thought the phase controller would have to dissipate about 15W going full tilt.

Edit: thanks inie meanie - your post crossed mine.

 

[pmcalli]

Data sheet I had on the crydom said it incorporated a proportional power control which I incorrectly interpreted as being a PWM. Sorry for the bum steer. I just downloaded the full data sheet from USA site and it doesn't make pleasant reading. voltage drop is specd at 1.6V which I make 20W dissipation at full load. Looking at EMMA pics it appears to be mounted on a DIN rail. The EMMA case looks like it is stainless steel. using the derating curves from the data sheet and some guesswork on the watts per degree C of such a mounting it will run hot but within max temps. This might explain why the EMMA unit has vents in the case top and bottom of the crydom. If it was mounted directly on the metalwork it would run stone cold. Looks like they could do with some lessons on thermal design. I am surprised that the solar inverters have enough hold up to support simple phase control. The data sheet specifies a minimum current of .15 A but the graphs show it going to zero have to assume the .15 minimum means that this is the lowest controllable current and after that it goes straight off meaning lowest usable power control is 36 W not really a problem.

 

[echase]

Tryed to send a massage to see if you are still suppling your immersion units but get this massage (echase has exceeded their stored private messages quota and cannot accept further messages until they clear some space.)!

Sorry, have now deleted some.

 

[inie meanie]

Data sheet I had on the crydom said it incorporated a proportional power control which I incorrectly interpreted as being a PWM. Sorry for the bum steer. I just downloaded the full data sheet from USA site and it doesn't make pleasant reading. voltage drop is specd at 1.6V which I make 20W dissipation at full load. Looking at EMMA pics it appears to be mounted on a DIN rail. The EMMA case looks like it is stainless steel. using the derating curves from the data sheet and some guesswork on the watts per degree C of such a mounting it will run hot but within max temps. This might explain why the EMMA unit has vents in the case top and bottom of the crydom. If it was mounted directly on the metalwork it would run stone cold. Looks like they could do with some lessons on thermal design. I am surprised that the solar inverters have enough hold up to support simple phase control. The data sheet specifies a minimum current of .15 A but the graphs show it going to zero have to assume the .15 minimum means that this is the lowest controllable current and after that it goes straight off meaning lowest usable power control is 36 W not really a problem.

I have been using this module and have it bolted to a sealed case with only a slight perceptible rise in temperature. I think the EMMA uses a heat sink on the rear or the triac that then clips to the din rail. I have been driving it hard as I switch between immersion heaters on different tanks automatically via a contactor change over. I have monitored current down to 10 w when the control voltage is 2.4 vdc

 

[bradburts]

You can tune a PID using:
https://controls.engin.umich.edu/wiki/index.php/PIDTuningClassical
Usually enough just to have a PI system.
I think that in this case you should be careful how you tune though. I would be more interested in not importing electricity than having a control system which does not oscillate. The sun goes in & your immersion power should drop sharply is more important than slowly integrating to a fine balence.
I would have thought that a simplier control system would be more relevant. Suppose you could use an integral to approach ideal but when the sun goes in the integral will have you importing with a true PID.
Perhaps EMMA use a PID because thats what comes with the DIN rail controller.
Is CT accuracy much of an issue? I would be happy using all but 200W or 100W as a design aim, thats quite a small percentage 'loss' after all. 200W is 1 Amp which should be easy enough to measure?
I have not used CTs before, always used shunts. What range of accuracies would you expect?

 

[inie meanie]

You can tune a PID using:
https://controls.engin.umich.edu/wiki/index.php/PIDTuningClassical
Usually enough just to have a PI system.
I think that in this case you should be careful how you tune though. I would be more interested in not importing electricity than having a control system which does not oscillate. The sun goes in & your immersion power should drop sharply is more important than slowly integrating to a fine balence.
I would have thought that a simplier control system would be more relevant. Suppose you could use an integral to approach ideal but when the sun goes in the integral will have you importing with a true PID.
Perhaps EMMA use a PID because thats what comes with the DIN rail controller.
Is CT accuracy much of an issue? I would be happy using all but 200W or 100W as a design aim, thats quite a small percentage 'loss' after all. 200W is 1 Amp which should be easy enough to measure?
I have not used CTs before, always used shunts. What range of accuracies would you expect?

I have calibrated my system which is totally analogue to export slightly all the time. I too did not want any import to be used as I seem to have plenty of available power. The CTs do have a slight curve at the lower end but I have managed to just export 10 or so watts at these points but then I have to export a 100 or so when power is in the 1 to 2 kw range and I was quite happy with that. The control function is only proportional as I would need to go digital to introduce and integral action. I think I will be investigating a digital setup in the future, now I have everything running really well, I can relax a bit. :- )

 

[pmcalli]

You can tune a PID using:
https://controls.engin.umich.edu/wiki/index.php/PIDTuningClassical
Usually enough just to have a PI system.
I think that in this case you should be careful how you tune though. I would be more interested in not importing electricity than having a control system which does not oscillate. The sun goes in & your immersion power should drop sharply is more important than slowly integrating to a fine balence.
I would have thought that a simplier control system would be more relevant. Suppose you could use an integral to approach ideal but when the sun goes in the integral will have you importing with a true PID.
Perhaps EMMA use a PID because thats what comes with the DIN rail controller.
Is CT accuracy much of an issue? I would be happy using all but 200W or 100W as a design aim, thats quite a small percentage 'loss' after all. 200W is 1 Amp which should be easy enough to measure?
I have not used CTs before, always used shunts. What range of accuracies would you expect?

I agree PI with D set to zero will be fine. if you don't use the I term there will be an error to the set point. This error could be above or below the set point depending on what direction you have aproached from so its important to use the I term even if its set very low to give a rapid response. We do not know if EMMA uses a PID controller its just a guess. the maker of the controller only releases the information to integration partners.
CT's are usually designed for a current range but this can be adjusted by modifying the so called burden resistor ( load ). if the burden resistor is high you get a higher o/p voltage which helps with the low values but it becomes non linear at high currents. For the immersion requirement I would reccomend a high burden value as this gives the resoultion you want at low values. if you are using a ct on both house and pv the high current non linearity is the same for both and so has no effect if you use a PI controller. you need to be careful about o/p voltage at high currents on the house CT and i would suggest that clamp diodes or a zener is used to ensure that the i/p to the controller is not exceeded. the clamp should be fitted for saftey reasons anyway and some CT's have them built in.

 

[pmcalli]

I have calibrated my system which is totally analogue to export slightly all the time. I too did not want any import to be used as I seem to have plenty of available power. The CTs do have a slight curve at the lower end but I have managed to just export 10 or so watts at these points but then I have to export a 100 or so when power is in the 1 to 2 kw range and I was quite happy with that. The control function is only proportional as I would need to go digital to introduce and integral action. I think I will be investigating a digital setup in the future, now I have everything running really well, I can relax a bit. :- )

You don't need to go digital to introduce the I term. A simple change to your amplifier to put a second order term in with give you a PI loop. If you have used an op amp as the gain element the feedback resistor just needs replacing with resistor+ capacitor so that it acts like and integrator with the slew rate limited by the resistor.

 

[suntrap]

More good stuff. pmcalli, have you seen the openenergy monitor forums? A lot of what you say, such as about clamping diodes and burden resistor selection, is discussed there. inie: I presume you're keeping your circuit diagram to yourself. :smiley2:

 

[inie meanie]

I have been using the AT20B10 coils the same as the EMMA, well no point in reinventing the wheel The good thing is they give a 0 to 10 vdc output for the specified range. No worries about a peek voltage. A bit more expensive but it makes things very easy. So far the slope I have achieved is so close it's not worth worring about the off set. As stated Integral can cause an over shoot and I dont want any of that :smiley2:

 

[pmcalli]

More good stuff. pmcalli, have you seen the openenergy monitor forums? A lot of what you say, such as about clamping diodes and burden resistor selection, is discussed there. inie: I presume you're keeping your circuit diagram to yourself. :smiley2:

just had a look very interesting. The only fear I would have with some of the circuits suggested is there is very little protection and no filtering none of them would pass CE. As to circuits I have been experimenting for a while with various options from expensive like the EMMA to cheap using entirely analogue control loop and discrete thyristor. My conclusion based on the prototypes and entry's on this forum is to be lazy and go for the AT20B10/ AT50B10 sensors, crydom phase controller, i/p filter, packaged power supply and an off the shelf PID controller ( not yet selected any suggestions welcome ). This is very expensive but I don't have the hassle of designing and building a pcb. The analogue cts were std stuff. I used precision rectifiers for the ct interface and then a simple type 2 op amp integrator

 

[pmcalli]

I have been using the AT20B10 coils the same as the EMMA, well no point in reinventing the wheel The good thing is they give a 0 to 10 vdc output for the specified range. No worries about a peek voltage. A bit more expensive but it makes things very easy. So far the slope I have achieved is so close it's not worth worring about the off set. As stated Integral can cause an over shoot and I dont want any of that :smiley2:

EMMA uses AT20B10 for the PV sense and AT50B10 for the house sense. these sensors are only rated for continuous overload of 120% so peak voltage is a problem

 

[pmcalli]

I have been using the AT20B10 coils the same as the EMMA, well no point in reinventing the wheel The good thing is they give a 0 to 10 vdc output for the specified range. No worries about a peek voltage. A bit more expensive but it makes things very easy. So far the slope I have achieved is so close it's not worth worring about the off set. As stated Integral can cause an over shoot and I dont want any of that <img src="http://www.electriciansforums.net/images/smilies/smiley2.gif" alt="" title="Smiley2" smilieid="329" class="inlineimg" border="0">

EMMA uses AT20B10 for the PV sense and AT50B10 for the house sense. these sensors are only rated for continuous overload of 120%