How to Connect two Solar Inverters in Parallel ?

In the PV inverter application scenario, if the load demand for power is relatively high, a single inverter may not be able to meet the user's needs, and multiple inverters need to be connected in parallel to provide energy for the load together. However, due to the inverter common start will cause a relatively large inrush current, in the project will generally asynchronous start two inverters, first by an inverter for the load to provide energy, and then the other in parallel, to deliver energy.

Inverter Topology

Realization Methods

The two basic conditions for grid paralleling are equal phase and equal amplitude of output voltage. When two inverters are started asynchronously, their respective output voltage magnitude and phase may be different, and forcing them to be connected to the grid will cause more damage to both devices. Therefore, it is necessary to achieve equal amplitude and phase of the inverter output voltage in the synchronization link.

Phase Adjustment

To achieve phase synchronization, the basis is to be able to obtain the phase of the main network or the main inverter, so as to be able to adjust the phase of the other device, and then meet the conditions of grid connection.

Phase locking is carried out by means of a single synchronized coordinate system, obtaining the angular frequency of the first-start inverter and integrating to obtain the angle.

The main process is described as follows according to the principle of single synchronous coordinate system phase locking, through the coordinate transformation to get the dq-axis component of the inverter output voltage, if the phase is consistent, the d-axis and the reference vector of the output voltage is in the same direction, and the q-axis component is zero. The q-axis component is zero. After the difference between the q-axis component and zero, PI operation is carried out to get the angular frequency and the target angular frequency, and then the angle can be obtained by integrating.

Output Voltage Amplitude Adjustment

When there is voltage at the output of the first inverter, it means that the first one has begun to work, at this time, the output voltage of the first inverter is collected, the amplitude is obtained, and after dq transformation, it is used as the target value of the latter inverter, and then the power regulation is carried out after the system is stabilized.

Steps to Connect Two Solar Inverters In Parallel

Connecting two solar inverters in parallel allows you to expand your system's capacity or share the load efficiently. This step-by-step guide integrates advanced details from a practical video demonstration.

Step 1: Prepare for the Parallel Connection

1. Set Up Master and Slave Configuration:

   • Determine which inverter will act as the MASTER and which as the SLAVE.
   
   • Configure the dip switches:
     • MASTER: Set SW1-1 and SW1-2 to ON (up position).
     • SLAVE: Set SW1-1 to OFF (down position) and SW1-2 to ON (up position).
     

2. Remove Protection Plaques:

   • Carefully take off the protection plaques from both inverters to access connection ports.

Step 2: Connect DC Inputs

1. Connect Battery or Power Supply to Inverters:
   • Attach the J1 (-Vin) and J2 (+Vin) inputs on both inverters to the battery terminals or DC power supply.
   • Ensure proper polarity to avoid damage to the inverters.

Step 3: Configure AC Output Connections

1. Connect Phases:

   • For 3-phase loads, include the neutral phase (N), even if your load does not explicitly require it.
   • Use the following wires:
     • N (Neutral phase): Blue wire.
     • L1, L2, L3 (Live phases): Match live phases across all inverters.

2. Load Placement:

   • The load (black wire) can be connected to either the MASTER or SLAVE inverter.

Step 4: Connect Communication Ports

1. Set Up Communication Between Inverters:

   • Use RJ45 FTP Cat 6 (or higher) cables to connect the MASTER and SLAVE units:
     • Plug one cable into J6B of the first inverter (MASTER) and connect it to J6A of the second inverter (SLAVE).
     • For a ring connection, link the last inverter’s J6B back to J6A of the first inverter.
   

2. Verify Communication LEDs:

   • On the MASTER inverter, ensure two green LEDs indicate proper operation.
   • On the SLAVE inverter, verify the DC input LED is illuminated.

Step 5: Test and Monitor the System

1. Power On the System:

   • Turn on the DC supply and AC breakers.
   • Observe system synchronization as inverters share the load.

2. Check Load Distribution:

   • The system should supply load evenly:
     • For example, if the total load requires 1250A per phase, each inverter might deliver approximately 674A per phase.
     

3. Output Verification:

   • Confirm the total power output is as expected (e.g., 8.6 kW in the demonstration indicates correct parallel operation).

Step 6: Maintenance and Troubleshooting

1. Routine Inspections:

   • Check connections regularly to ensure cables are secure and free from corrosion.
   • Monitor the LEDs and displays for fault indicators.

2. Adjust Load Capacity:

   • If necessary, rebalance the load between the MASTER and SLAVE inverters for optimal performance.

Additional Notes

• Always refer to your specific inverter model’s user manual for exact configurations.
• Ensure compliance with local electrical codes when setting up the system.
• If uncertain, consult a professional electrician or solar technician to minimize risks.

By following these enhanced steps, you'll be able to configure two inverters in parallel efficiently and safely, ensuring your solar system operates at maximum capacity.

If you have any questions or needs, you can contact Shielden, we are the solar inverter factory.