Help:Solar Pump

[abicash]

Hi Northguy
Today it worked till 5.30pm
Voltage was gradually falling but mppt adjusted it to max power always.
Someone had commented that some solar pump works till 7pm.
But I find that ridiculous, since here in india, twilight occurs at 6pm and quite darkish at 6.30pm
I measured solar power at 5.20pm and I found it to be 400w.
So I think this guy just wants to push my boundaries.
Although one thing I observed yesterday, if I shut it down and start at 5'ish in evening, the mppt didn't stabilise.today I did not restart, and it worked til 5.30pm
So maybe starting mechanism is flawed (?) or mppt goes in wrong direction while starting, but if was started in healthy light, stays.
Anyways I am quite happy with todays result

 

[NorthGuy]

Voltage was gradually falling but mppt adjusted it to max power always.

That is imossible. To get MPPT, voltage should decrease only a little. Say, if you get 500V instead of 600V, you're below MPPT and voltage must go higher to get more power.

Someone had commented that some solar pump works till 7pm. But I find that ridiculous, since here in india, twilight occurs at 6pm and quite darkish at 6.30pm

Even if you get some production close to sunset, it'll only be a fraction of full production and only for a short time. Say, you get another 300W during 20 minutes, it's 0.1kWh, compared to your daily production of 30kWh+. 0.3% - not something worth chasing.

So maybe starting mechanism is flawed (?) or mppt goes in wrong direction while starting, but if was started in healthy light, stays.

May be. Adjust your algorithm to keep panel voltage between 550 and 580V and see if it keeps working longer.

 

[abicash]

Hello NorthGuy

Voltage was gradually falling but mppt adjusted it to max power always

What i meant here is that due to reduced sunlight, power is falling gradually.So MPPT is adjusting to such a voltage at that particular solar insolation where power was maximum.So that motor is constantly running and pulling water because of VFD.

0.3% - not something worth chasing.

I agree.

I will check keeping panel voltage to 600V and see what happens.

Although i am pretty sure that a similar outcome may be observed.

Apart from all this i seem to have missed your post about your installation of 24 panels.Silly of me !!!
I would love to hear about it....can you elaborate?

 

[abicash]

Hello

Related to this project I have a question regarding dry running of motor which i have posted here

I know you said you are not an expert in water pumps...but nevertheless

 

[NorthGuy]

Apart from all this i seem to have missed your post about your installation of 24 panels.Silly of me !!!
I would love to hear about it....can you elaborate?

That's not much to elaborate. I live off-grid with 6kW array and batteries. All the wiring is AC. I do have a water pump - 240V single phase, but we don't use much water.

What may be of interest to you is that my cut-off time is one hour before astronomical sunset. At this time, my controller considers that it will be no more energy for the day and decides whether a generator must run or not. I do have trees. Without them, I probably could go 30-40 minutes longer, but there's definitely nothing after that.

 

[abicash]

Hello NorthGuy

Today it was a bit cloudy and i saw that it could not correct shading and the voltage falls rapidly.
so i implemented the voltage correction loop.keeping voltage to 600v.
And hurray....it works marvellously.
Changing clouds don't affect it at all.
Pump draws water till 15 min before sunset (5.45)
Although there isn't any pressure.
Watts measured was 600w , pump is 5hp

 

[NorthGuy]

so i implemented the voltage correction loop.keeping voltage to 600v.
And hurray....it works marvellously.

Great.

Now you can add a slow MPPT tracking component to it.

E.g. you run it at 600V. Every minute or two you do something like that:

- Measure average power for 2 sec
- Run 2 sec at lower voltage (e.g. 595V) and measure average power
- Run 2 sec at higher voltage (e.g. 605V) and measue average power
- Find the highest power of three runs.
- Switch to the voltage that gave you the higest power.

More formally you have a target voltage Vt. Then you do two perturbations - Vt-dV and Vt+dV and choose the max power of the three (Vt-dV, Vt, Vt+dV). The one that gave you the most power becomes your new Vt, which you keep for the next minute or so.

It's important to keep it very slow, so that the noise wouldn't cause a voltage drift.

 

[abicash]

But I don't see the need for it.
Rather I dont understand what is going to happen in +/-5v ?
If a 1 minute loop is run , how will it take care of clouds?
I have a vfd in background, since if a cloud covers the sun, voltage drops, but motor is running at earlier set frequency.and if mismatched, will draw more current, leading to decadence of voltage and eventually within next minute, failure.
I have seen this earlier..am I interpreting this incorrectly?

 

[NorthGuy]

It will not take care of clouds. Your system is handling clouds fine now. So, you keep your voltage following part as is.

The perturbation part is independent and will take care of slow changes it the MPPT point, which mostly depends on the temperature of the panels. It can be 600V, or it may be 550V. If you fix at any single voltage, it'll produce less power than it could. It doesn't change fast, so you need to do it slowly.

Say, your target is 600V, but the MPPT point at the moment is 570V.

You run at 600V. After the first minute you also try 595V and 605V for couple of seconds. You will find out that 595V is better, so you'll run a minute at 595V. After that you'll test 590V and 600V. You'll see that 590V is better, so you'll run a minute at 590V, at the end of the minute you'll try 585V and 595V. And so on until you arrive to 570V (and it'll take 6 minutes to get there). While running at 570V, you'll be trying 565V and 575V every minute and you will see that either 565V or 575V is inferior to 570V, so you will continue at 570V.

Later in the day, it may get hot, so the MPPT point may drift from 570 to 560V. Your algorithm will slowly follow. That's your perturb-and-observe.

Makes sense?

 

[abicash]

Yes
Implementing
Will get back with results

 

[abicash]

Hello NorthGuy

Implemented and testing the new algorithm.
It works splendidly in a dense cloud cover (which we have in the area right now)
Every Minute it perturbs higher for 10 seconds, then lower for 10 seconds and acquires highest wattage among the 3,
Works at this voltage for another minute and the cycle continues.
The tracking is going fine.
I find this may have cracked it

Thanks a lot for supporting...maybe i could buy you a beer if i were to visit Canada sometime

P.S. : will trouble you some more if there's a problem

 

[NorthGuy]

You're welcome. Feel free to trouble me more

You can further tweak parameters such as wait time (1 minute) and sample time (10 sec) to make sure it is stable. The biggest danger with the algorithm is that it may become unstable when conditions are changing directionally. Say, if it light conditions are constantly improving, your last sample of three may be better then the first simply because there is more light, not because it is a better MPPT point. When this continues for a long time, the algorithm may drift away. If such things happen, tweaking parameters should take care of most of such effects.

 

[abicash]

But how does the algorithm understand that that's the problem.
Can u hint a way to what you are saying?

 

[abicash]

Somewhat unrelated but heres another issue.
as I said earlier, duty adjustment means adjusting inverter duty.
Now I am keeping voltage at a Vmp point.So the output voltage never goes above 350vac.since duty determines output voltage.
I am not sure how to explain this better

 

[NorthGuy]

Somewhat unrelated but heres another issue.
as I said earlier, duty adjustment means adjusting inverter duty.
Now I am keeping voltage at a Vmp point.So the output voltage never goes above 350vac.since duty determines output voltage.
I am not sure how to explain this better

The power drawn by a pump depends on both voltage and current. With everything else being equal, increasing duty will increase voltage and decrease current, keeping the total power the same (equal to the power produced by the panel less inverter losses). So, voltage alone cannot tell you if it's more or less power. The power cannot disappear (unless something is overheating), so the more power at the panel the more power at the pump.

Also, every day is different and 10-15% variation in solar power depending on the atmospheric condition is normal. I sometimes get up to 7kW from my array, but other day, with seemingly full sun, it may be as low as 5kW.

The important thing is to make sure that you get max power from the panel. Do you have a provision for disabling the slow part of the algorithm and setting the target panel voltage manually. If you do, try to increase/decrease it and see if you get better power at a different voltage. If you do, there's probably a coding bug somewhere.

On my previous post. Mediocre performance in varying conditions is an inherent flow of P&O algorithm because it assumes constant conditions. By keeping its rate slow, compared to the time between measurements, you partly alleviate this problem. It might work fine, but if it doesn't, you can introduce some tweaks. For example, you can alternate the sampling order. Say, you do higher voltage first then lower one, but for next perturbation you do it in reverse order. Or, you can do multiple perturbations and see if you get consistent directional signal before you decide to change your target voltage - it's better to be slow with changes than wonder away from the good point. But I wouldn't worry about this unless there's a problem.

 

[abicash]

Do you have a provision for disabling the slow part of the algorithm and setting the target panel voltage manually. If you do, try to increase/decrease it and see if you get better power at a different voltage.
.

I had already checked this part.It does not improve o/p ac voltage or the power.
Although i have another control, which will change the o/p frequency within a factor of 0-2 of existing frequency.
When i reduce this (currently i am not able to actually measure the voltage at this pot,since it is placed at the bottom of the enclosure, below all high voltage parts...yesterday i suffered a 700V shock from the panel, and now i am a bit spooked of delving deeper ) i am not measuring,but know that the ratio is reducing(pot moved ccw), frequency is reducing, and the o/p voltage increases to a limit , i think i measured 400Vac. Don't remember exact , but def above 400.

Regarding the P&O part, currently am unable to see any problem as such.But i understood what you are saying here.
I will implement that and get back.

On a different note, i had posted the dry run problem as another thread and cowboybob says

I would say that Northguy has way more knowledge and experience on this subject than I do.
.

 

[NorthGuy]

I wanted to install all-serial array, but I didn't want to get stung with 600V, so I went with 8 parallel strings I also didn't have time to build my own controller, so I went with commercial ones. I now regret this, and I believe my own controller would do better

Apparently, when the speed of the pump increases (assuming the same water pressure), the current through the pump should increase too, so you should expect the voltage to go down. The pump voltage going down will require you to decrease your duty cycle (otherwise panel voltage would go down too). The decreased duty cycle will produce more current (and less voltage) to the pump. So, it is possibe to regulate pump speed and keep MPPT voltage at the panel at the same time.

When optimizing the speed of the pump, you should worry about the efficiency, safety, and other things. I really don't know much about this part. I'm bad with motors