We frequently get asked, “What happens if you connect a single phase solar inverter with a three phase net metered grid connection?”. So we at Sunkalp Energy, feel it’s important to break it down.
Technical Impact of connecting single phase inverter to a three phase connection
Connecting a three phase inverter with a three phase grid connection is always the first choice because a three phase inverter will push the same uniform amount of power down each of the phases and there is no risk of introducing any imbalance. This is almost always the preferred choice in large or industrial connections where the consumer is charged for kVA consumption and balancing each of the phases is important.
However, techncially a single phase solar inverter will also easily synchronize with one of the phases in a three phase connection. It will typically not cause any issues with smaller residential connections, sub 5 kW, where this scenario mostly comes up. Residential connections have both single phase and three phase loads, and adding a single phase power source will not result in an technical concerns.
Design wise, the installer must ensure that the single phase solar output is connected to the phase which has the maximum load. In the rare case where the loads are highly imbalanced and solar is connected to the the phase with lower loads it may cause a further imbalance which may eventually affect grid voltage and hence in tripping/ de-syncrhonization.
Another question that comes up is, “Will a three phase meter give me benefit for single phase generation?”. There will be no difference in billing in the case of smaller residential connections where the client is billed for kW consumed. The meter will simply add the power consumed on each of the three phases and subtract the power generated by solar, like it would in other cases. However, with larger three phase connections which are billed for kVA consumption, it is extremely important to balance the PF or preferable to go for a three phase solar connection.
Commercial Impact of connecting single phase inverter to a three phase connection
This one is simple. Commercially, single phase inverters are cheaper than three phase inverters of the same power rating, simply because the protections and electronics for managing single phase power is cheaper. This makes a per watt cost impact which becomes significant for smaller capacities sub 10 kW.
Policy Provisions for connecting single phase inverter to a three phase connection
MNRE guidelines (quoted below) are fairly ambiguous about the injection levels, leaving the decision whether to approve a single phase solar connection to a three phase connection, to the judgement of the approving project officer.
In grid connected rooftop or small SPV system, the DC power generated from SPV panel is converted to AC power using power conditioning unit and is fed to the grid either of 33 kV/11 kV three phase lines or of 440/220 Volt three/single phase line depending on the capacity of the system installed at institution/commercial establishment or residential complex and the regulatory framework specified for respective States.
21. What are the grid connectivity levels for such systems? The Projects under these guidelines fall within two broad categories i.e.(a) the projects connected to HT voltage at distribution network (i.e. below 33 kV) (b) the projects connected to LT voltage i.e. 400/415/440 volts (3-phase) as the case may be or 230 volts (1-phase). Accordingly, the projects may be under the following two categories.
Category 1: Projects connected at HT level (below33kV) of distribution network The Projects with proposed installed capacity of minimum 50 kW and upto 500 kW and connected at below 33kV shall fall with in this category. The projects will have to follow appropriate technical connectivity standards in this regard.
Category 2: Projects connected at LT level (400 Volts-3 phase or 230 Volts-1 phase) The Projects with proposed installed capacity of less than100kW and connected of the grid at LT level (400/ 415/ 440 volts for 3-phase or 230V for1-phase) shall fall within this category.
Our recommendations are:
- Where consumer has a single phase grid connection, a single phase inverter is mandatory
- For solar systems which have capacity 8 kW and above, where the client has a three phase connection, a three phase solar inverter should be preferred.
- For sub 8 kW grid connections, where the client has a three phase connection but is metered for kW consumption, the installer should audit the client’s phase wise load consumption. A single phase connection can be suggested to be connected on the phase which has the highest load, in the condition that it isn’t imbalance the system.
- It’s always good practice to take prior approval on your SLD from your local DISCOM and nodal agency.
Feel free to comment and add your experiences!