Hello

I've got 2 separate solar panel systems with 2 inverters.

System A is the bigger one with more space available on the inverter for adding extra panels.system a got a better feed in tariff.

System B is the smaller one with a lower feed in tariff.

I want to be able take about 10 panels from system B and automatically (programmed by hours) connect them to system A during the morning and also the after noon.

Then i will maximise the production of system A that got a higher tariff.

The inverter in system A is big enough to handle the extra panels.

Is someone can provide with a diagram and the necessary parts for the system?

Thanks

Arthur

You might want to draw some pictures

22 hours ago, terkazarian said:

Hello

I've got 2 separate solar panel systems with 2 inverters.

System A is the bigger one with more space available on the inverter for adding extra panels.system a got a better feed in tariff.

System B is the smaller one with a lower feed in tariff.

I want to be able take about 10 panels from system B and automatically (programmed by hours) connect them to system A during the morning and also the after noon.

Then i will maximise the production of system A that got a higher tariff.

The inverter in system A is big enough to handle the extra panels.

Is someone can provide with a diagram and the necessary parts for the system?

Thanks

Arthur

Switching the high voltage of DC and also disconnecting them from an inverter is never a good idea. DC contractors are also quite expensive. 

Doing what you want entails switching the DC from the panels while under load from both inverters and drawing a larger arc/flame. 

The circuit is not a problem if members can come up with suitable rated switching relays/contractors. 

My personal preference would be to 1st switch off the inverters before doing such a switch over. 

While considering the options perhaps you can supply the make/model of the inverters and panels. 

Our electronic guys can perhaps come up with ideas using FETs + relays. 

Also indicate how many panels do you have and how they are now connected and how after switching. Strings? Series/parallel etc. 

@TaliaB give your input please. 

Edited May 17, 20242 yr by Scorp007

Not sure if interrupting dc voltage to the mppt under load would be a problem for the inverter maybe our electronic specialists @BritishRacingGreen and @Coulombcan give advice.

Switching dc under load is not a problem if correct switchgear is used but like @Scorp007has mentioned it would be pricey. Albright Dc contactor with blow out magnets would be a safe bet.

 

4 minutes ago, TaliaB said:

Not sure if interrupting dc voltage to the mppt under load would be a problem for the inverter maybe our electronic specialists @BritishRacingGreen and @Coulombcan give advice.

Switching dc under load is not a problem if correct switchgear is used but like @Scorp007has mentioned it would be pricey. Albright Dc contactor with blow out magnets would be a safe bet.

 

What is the going rate for these contractors? 

5 minutes ago, Scorp007 said:

What is the going rate for these contractors? 

~R 1950 available from Aztec electronics in Elandsfontein importers of heavy duty dc switchgear. 

This heavy duty change over  ~R3.5k

 

7 hours ago, TaliaB said:

Not sure if interrupting dc voltage to the mppt under load would be a problem for the inverter

Buck types that charge the battery directly (e.g. 145 V max PV types) should be fine. But these days most are the high PV voltage models, and for those, I think it's a bad idea. You are mechanically interrupting current through a large inductor, and that produces a large kick-back EMF that is very hard on the contacts, and also the electronics. Even if there is a movistor to absorb the transient, those lose efficacy with each transient, so it's not a good idea to do something like this that regularly switches under load.

In my humble opinion.

Thanks for this information @Coulomb

11 hours ago, TaliaB said:

Not sure if interrupting dc voltage to the mppt under load would be a problem for the inverter maybe our electronic specialists @BritishRacingGreen and @Coulombcan give advice.

Switching dc under load is not a problem if correct switchgear is used but like @Scorp007has mentioned it would be pricey. Albright Dc contactor with blow out magnets would be a safe bet.

 

I based my comment about switching the inverter off from this post.

 

Edited May 18, 20242 yr by Scorp007

10 hours ago, Scorp007 said:

Thanks for this information @Coulomb

I based my comment about switching the inverter off from this post.

 

Hi 007 , the context of that statement was when installing. The main reason is that if you inadvertently touch screw driver to earth while connecting PV when inverter is on , then you risk damaging the DC-AC bridge.

Regarding the context of this thread, I don't think removing the PV will damage the inverter , but if that PV is under load then the abrupt removal will ripple thru the power chain and cause all sorts of metastable conditions , I think which should be avoided.

The OP does not give details regarding the configuration of the PV strings and the type and model of inverters. @Scorp007 and @TaliaB has pretty much covered important issues here. 

The cost of such a selector system will be rather high , say between ZAR10k - ZAR15k . Which is already the cost of half the panels at stake. I would have just added the the other half and installed panels statically. However the OP may have the situation where roof space is just not available and hence its not an option. If so , I assume that the OP has done the math in order to make sure that the net result may be not close to stealing from Peter to pay Paul . 

if such a selector  then becomes a viable option , then a number of issues will be important to address. And these issues are not trivial.

As @TaliaB pointed out , DC contactors are expensive. And further to this , the roving solar panel string needs to be fully double cut , both its negative and positive leg must be switched. It is imperative in a transformer-less inverter design , that the MPPT(s) of and inverter should be in isolation to MPPT(s) of other inverters. Neglecting it can affect serious damage to both inverters , mainly because the Live and neutrals are not isolated from PV- and PV+ .  And using a dc contactor with a changeover contact set is not good enough , unless they are specifically rated as break-before-make. So in practice you can find yourself using up to 4 dc contactors to perform this function. And on top of this you need a contactor with an auxiliary contact so you can read and prove that a contactor has been de-energized , before an attempt is made to energize the alternate contactor.

 Furthermore  the selector will require that the 10 panels be configured such that it will be in parallel with the static panels , so that when the string is removed by contactor the inverter MPPT will not see an open circuit. This is true for both solar panel clusters. 

You will then need an industrial Programmable Logic Controller  (PLC) , example  the DELTA DVP range . The PLC must be fitted with a real time clock (RTC) . The PLC will be responsible for the timekeeping as well as the safety cycle involved to switch a string out of circuit and in to circuit.  It will also be recommended that the plc be fitted with a small control panel , with a 3 way selector switch (FORCE A - AUTO - FORCE B ) , plus indications to show to which cluster the string is switched into.

 

*** The Delta PLC as indicated above has standard Modbus R485 , which may be used to control the selector remotely as well , using algorithms that are bases on power demand criteria etc.

 

The budgetary quote of 10k-12k excludes installation , cables,wiring , 24VDC power supplies , enclosures , testing , commisioning costs.

 

Edited May 18, 20242 yr by BritishRacingGreen

27 minutes ago, BritishRacingGreen said:

The OP does not give details regarding the configuration of the PV strings and the type and model of inverters. @Scorp007 and @TaliaB has pretty much covered important issues here. 

The cost of such a selector system will be rather high , say between ZAR10k - ZAR15k . Which is already the cost of half the panels at stake. I would have just added the the other half and installed panels statically. However the OP may have the situation where roof space is just not available and hence its not an option. If so , I assume that the OP has done the math in order to make sure that the net result may be not close to stealing from Peter to pay Paul . 

if such a selector  then becomes a viable option , then a number of issues will be important to address. And these issues are not trivial.

As @TaliaB pointed out , DC contactors are expensive. And further to this , the roving solar panel string needs to be fully double cut , both its negative and positive leg must be switched. It is imperative in a transformer-less inverter design , that the MPPT(s) of and inverter should be in isolation to MPPT(s) of other inverters. Neglecting it can affect serious damage to both inverters , mainly because the Live and neutrals are not isolated from PV- and PV+ .  And using a dc contactor with a changeover contact set is not good enough , unless they are specifically rated as break-before-make. So in practice you can find yourself using up to 4 dc contactors to perform this function. And on top of this you need a contactor with an auxiliary contact so you can read and prove that a contactor has been de-energized , before an attempt is made to energize the alternate contactor.

 Furthermore  the selector will require that the 10 panels be configured such that it will be in parallel with the static panels , so that when the string is removed by contactor the inverter MPPT will not see an open circuit. This is true for both solar panel clusters. 

You will then need an industrial Programmable Logic Controller  (PLC) , example  the DELTA DVP range . The PLC must be fitted with a real time clock (RTC) . The PLC will be responsible for the timekeeping as well as the safety cycle involved to switch a string out of circuit and in to circuit.  It will also be recommended that the plc be fitted with a small control panel , with a 3 way selector switch (FORCE A - AUTO - FORCE B ) , plus indications to show to which cluster the string is switched into.

 

*** The Delta PLC as indicated above has standard Modbus R485 , which may be used to control the selector remotely as well , using algorithms that are bases on power demand criteria etc.

 

The budgetary quote of 10k-12k excludes installation , cables,wiring , 24VDC power supplies , enclosures , testing , commisioning costs.

 

https://www.deltaww.com/en-US/products/PLC-Programmable-Logic-Controllers/252

1 hour ago, BritishRacingGreen said:

Hi 007 , the context of that statement was when installing. The main reason is that if you inadvertently touch screw driver to earth while connecting PV when inverter is on , then you risk damaging the DC-AC bridge.

Regarding the context of this thread, I don't think removing the PV will damage the inverter , but if that PV is under load then the abrupt removal will ripple thru the power chain and cause all sorts of metastable conditions , I think which should be avoided.

Thanks for clarification. I just thought with sys A bigger than B and adding 10 panels from B over to A it brings A to well over 20 panels which will then in the afternoon be disconnect under full load. 

Not being sure if it is save I rather mentioned an alert for members to give their input. 

Edited May 18, 20242 yr by Scorp007

What brand inverters do you have installed? Maybe one can look at some sort of AC coupled approach?

Thanks for all answers, i'm a bit lost here with all the answers.

Here is some basic diagram to explain the idea.

i need to know what parts do i need to make it work

Drawing1.pdf

Interesting, I have a flat and a house and had thought of 2 separate installations but later just decided to combine the 2 inverters and have them paralleled so that inefficiencies are accommodated for especially as the flat is not always in use or busy. 
I think a summary of the comments suggests;

1. It is quite expensive to do it if you have roof space available rather spend the cash on add permanent extra panels to the smaller system. 
2. if you don’t have roof space and that is the only way it might also have long term effects on your system. it should be avoided if you can not a efficient solution.
3. If you still feel you have to do it (because you can or you have no choice) consider some alternative ways to make it safer.

Edited May 22, 20242 yr by Kory