Domestic Grid-Tied Inverters should have ant-islanding feature and during power outage, the PV arrays become useless to the Home Owner. Therefore, it would be great if the same PV array could be connected to an Off-Grid Inverter which is also in the same electrical installation at home. My question is whether we could use Contactors to switch the PV array from one inverter to the other, based on the situation?
Yes you can do it. A grid-tied inverter is working based on grid-feeding strategy and supplies a synchronized current to the grid. The off-line inverter is working based on grid-forming strategy and like a ups regulates its output voltage and frequency. If the starting current (we assume there is no problem for supplying the steady state current) of the load can be tolerated by the inverter and its dc-bus (pv and batteries) this can be done easily by a STS (Static Transfer Switch).
Thank you Farhad Yazdi for your reply. What we have built is a Smart Control Unit (SCU) that disconnects Grid (when Grid is down) and provide power to the load though the Off-Grid Inverter and batteries. Right now, there is a separate PV array for the Off-Grid Inverter and that is a pure power back-up arrangement. We have deployed contactors to switch the Grid to back-up inverter so that the load is powered by the back-up inverter when the Grid is down. When the Grid is down, so is the Grid-Tied inverter and at that time, the PV array connected to the Grid-Tied Inverter is at OCV. My intention is to switch the PV array connected to the Grid-Tied inverter at the moment to the Off-Grid Inverter as and when the Grid is down so that I do not have to add more PV panels. I understand that the PV array should match both Grid-Tied and Hybrid Inverter and that can be easily achieved by matching the capacities of both Inverters. Please let me know whether you have any experience in doing this kind of installation or is your answer based on theory.
Jeremy depending on your premises load profile, there is another design solution that may be more appropriate to suit your needs. DC Couple the Solar PV Array and the Battery Storage to create a dual power source DC Bus (the Solar PV when available charges the Batteries). Connect an Off Grid Standalone Inverter to this DC Bus and the Off Grid Inverter AC output as Input 1 to an (AC Changeover Switch). Connect the Changeover Switch AC Output to the premises loads main switch board. Connect the Grid Supply as Input 2 to the AC Changeover Switch and implement some control logic for the AC Changeover Switch that suits your load management circumstances.
This is a simple and effective system design solution. It provides for efficient DC Coupled PV/Battery charging. It requires only one inverter (Off Grid) and never wastes power (Solar PV and Battery) by paralleling with the grid. It is easily and precisely controlled by the users own logic definitions in managing the Dual AC Power Sources via the AC Change Over Switch to service the premises loads.
Lawrence Coomber
Thanks to all who shared their views on this subject. I have been able to connect the same PV Array to an on-grid and off-grid inverter and the solution is working fine. This is a good solution for all on-grid systems as they do not have to change the inverter to add battery backup. During power outages, the off-grid system works well with the same PV array and islanding is established with the on-grid inverter, which is accepted by the Utility. Since the DC Transfer is close to the PV on-grid inverter, Power Line Communication (PLC) became handy to do the switching, without drawing additional wires.
Lawrence Coomber I am aware of the DC and AC Coupled solutions. The problem I am solving here is the power distribution to critical and non-critical loads that are separated through the Main Electrical Panel (MDB). In our region, cooling load is almost 60% of the total load and it is also powered by grid power 230VAC. This is different to other countries such as Australia, UK, USA, Canada and EU where heating load is powered by gas. Managing non-critical loads using EPS is a challenge, unless it is managed in collaboration with an home automation system and smart Plug Sockets or power supplies that could disconnect non-critical loads when running on storage batteries.
Difficult to visualise your system schematic through words alone Jeremy. But what you are searching for is easily dealt with via system logic controls. Try posting a schematic image and all will be revealed my friend.
Lawrence Coomber
Dear Jeremy, I'm from Colombo and I'm also looking for the same solution to divert PV arrays from the on-grid system to my off-grid backup inverter during grid power failures. If you can share the system diagram with me or if your company provides system designing with complete implementation, please let me know. I'm more than happy to have it in my place.
Dear Anoma, Glad to e-meet you. I still haven't done this as a commercial product, although I have done it on bench as an R&D exercise. I may do this very soon in the field, if the current situation permits. However, please get in touch with me on +94773124844 or [email protected] for us to discuss this further.
This can be done using a DC changeover with DC isolators in each string. Not commonly used though
Have you done this practically anywhere? The requirement is to transfer panels from On-Grid to Off-Grid system when there is a power failure. If you have some experience doing so, I would like to discuss..