How to build a 3 phase solar inverter
PN171Table of Contents
This quick-start guide builds a 3 phase solar inverter using imperix high-end control and power equipment for power electronics. It is specifically made to accompany users who would want to get familiar with imperix’s solutions and build a more advanced converter with the B-Box RCP using the Simulink blockset. The converter is built using an imperix power electronic bundle.
The power electronic bundle, thanks to its flexibility, allows for the implementation of numerous power converter topologies. This page will focus on the practical aspects of implementing a grid-tied 3 phase solar inverter, which is one of the many possible applications. For a more theoretical approach to this application, please refer to the note Three-phase PV inverter for grid-tied applications. Also, for a more basic introduction on how to assemble Imperix power converters, you may refer to the pages How to build a buck converter (PN119) and How to build a 3 phase inverter.
A 3 phase solar inverter, as shown by the diagram below, is basically a DC/DC stage in the form of a boost converter connected to a DC/AC stage in the form of a 3 phase inverter.
3 phase solar inverter using the power electronic bundle
The content of the power electronic bundle, listed below, includes all the components needed for the realization of a 3 phase solar inverter.
- Programmable controller (B-Box RCP)
- ACG SDK toolbox for automated generation of the controller code from Simulink or PLECS
- 6x phase-leg modules (PEB8038)
- Passives filters box
- Grid connection panel with switchgear and precharge circuit
- 6x voltage sensors
- 4x current sensors
- All necessary RJ45 and fiber optic cables
- Laboratory safety cables (similar to this one)
Using the aforementioned components, this power converter is rated for up to 20kW. On top of that, some additional material, listed below, is required to test the system before operating the full converter.
- Laboratory DC power supply (rated for at least 100V, 5A)
- 3x power resistors (10-100Ω) to test the inverter stage
- 1x power resistor (50-500Ω) to test the boost stage
Obviously, for operating the full system, a PV panel or a programmable power supply that is able to emulate a PV panel is also needed.
Wiring of the power stage
The schematic of the grid-tied solar inverter is presented here. The photovoltaic panel is connected, through a relay, to a boost stage that charges the DC bus of the power modules. Connected to this DC bus is a three-phase inverter tied to the grid.
The illustration below details the wiring corresponding to the schematic above. The solar panel is connected to the first power module through an inductor from the passive filter box. The DC buses of the modules are connected together. The middle points of the three other modules are connected through 3 inductors and the EMC filter to the grid-side panel.
Wiring of the control stage
Considering the measurements, the three grid voltages and the solar panel voltage are measured with external sensors mounted at the back of the converter. All the currents and the DC bus voltage are measured using the modules’ internal sensors.
The two following schematics illustrate the connections for the measurements (in red) and for the PWM signals (in blue).
Front panel configuration of the B-Box RCP
To ensure that the ratings of the power converter are never exceeded, the hardware protection limits of the B-Box RCP need to be configured properly. A detailed explanation of how to compute these limits is given on the page Analog front-end configuration on B-Box RCP. Here the limit for the solar panel current and voltage are set to 20A and 600V respectively. These limits correspond to analog signals of 4V and 5.9V. The DC bus voltage is limited to 800V. The grid voltage and current are limited to 350V and 20A, respectively. The table below gives the complete configuration of the analog front-end of the B-Box according to the aforementioned limits.
| Measured signal | Input channel number | Low impedance | Gain | Filter | Limit high [V] | Limit low [V] | Disable safety |
|---|---|---|---|---|---|---|---|
| \(I_{pv}\) | 0 | no | x4 | no | 0.2 | -4 | no |
| \(V_{pv}\) | 1 | no | x4 | no | 5.9 | -0.2 | no |
| \(V_{dc}\) | 2 | no | x2 | no | 7.9 | -0.2 | no |
| \(V_{g,a,b,c}\) | 3,4,5 | no | x4 | no | 3.5 | -3.5 | no |
| \(I_{g,a,b,c}\) | 6,7,8 | no | x4 | no | 4 | -4 | no |
Test procedure for the 3 phase solar inverter
Before operating the system, it is always a good idea to test that everything is properly wired and configured. With the help of the Simulink model below, the following test procedure can be used to check the behavior of the 3 phase solar inverter. The test will be performed in two steps: first checking the correct operation of the boost stage, and then the correct operation of the inverter stage.
This basic model is designed to operate the boost converter and three-phase inverter individually and in open loop. The ADC blocks retrieve the analog measurements. The PWM blocks, along with the tunable parameters, send PWM signals with the desired duty cycle to the power converter.
Testing the boost converter stage
The boost converter is wired according to the following schematic. For more information on boost converters, please refer to the page Step-up boost converter.
A DC power supply (\(V_{in}\) on the schematic) replaces the solar panel and a resistor is connected to the DC bus of the module. It is first advised to test the system at low power with, for instance, an input voltage in the range of 50V and a load resistor of 100Ω along with a duty cycle of 0.5. This would theoretically result in an output voltage of 100V with a 1A current through the resistor and therefore an output power of 100W. The illustration below shows the required connections on the power electronic bundle.
With the circuit being built, the control code of the open-loop test model given above can be uploaded on the B-Box RCP. More details on how to proceed are given in Getting started with BB Control.
To turn on the converter, switch on the power supply, set the variable activate_boost to 1, activate_inverter to 0 and boost_dutycycle to 0.5 and enable the PWM outputs. Check that the measured values for the input and output voltages match the expected values.
Testing the 3 phase inverter stage
An easy way to test the inverter stage is to operate it in open loop on a passive load. The 3 phase inverter is wired according to the following schematic. For more information on 3 phase inverters, please refer to Three-phase Voltage Source Inverter (VSI).
A DC power supply is connected to the DC bus of the three power modules and three resistors in a star configuration are connected on the AC side of the converter, as shown in the diagram below. As before, it is advised to operate the converter at low power in the first place. As a guideline, a 100V of DC input voltage would result in 1A of output current on a resistor of 8.5Ω with a modulation index of 0.25.
The last steps are to upload the code on the B-Box RCP, turn on the power supply, and set the variable activate_inverter to 1, and activate_boost to 0. The modulation index variable m_ref can be left to its default value of 0.25. Note that the current goes through the grid-side panel, so the circuit breaker needs to be manually closed. To let power flow, set the variables precharge_relay and bypass_relay (corresponding respectively to relays \(K_1\) \(K_2\) on the schematic) to 1 and enable the PWM outputs. Again, check that the measured values for the input and output voltages match the expected values.
Operation of the 3 phase solar inverter
With all the tests completed, the 3 phase solar inverter can be wired according to its aforementioned wiring diagram. The Simulink model for the converter is given below.
Now, the main objective of this section is to detail how to connect the converter to the grid, which is one of the most important aspects of operating the 3 phase solar inverter and grid-tied converters in general.
Precharging the DC bus
An obvious prerequisite of grid-tied operation is that all necessary precautions are used in order to make the flow of uncontrolled currents through the inverter diodes impossible. This notably requires that the DC bus is pre-charged by suitable means before the connection to the grid. Useful information regarding how to pre-charge the DC bus in grid-tied applications is given in DC bus pre-charging techniques. In practice, the DC bus voltage needs to be higher than the rectified AC voltage, meaning that \(V_{DC,min} = \sqrt{3}\sqrt{2}V_{RMS}\).
In this example, the DC bus will be precharged from the grid, through the precharge resistors of the grid-side panel. The corresponding procedure is detailed below.
Using a variable autotransformer
It is advised not to connect the converter directly to the grid voltages but to add, if possible, a variable autotransformer in between the converter and the grid. This way, the converter could be first tested at lower voltages.
Operating the 3 phase inverter
In the given model, the operation of the precharge and bypass relay is handled by a state machine in the Simulink model. Therefore, the PWM outputs can be enabled from Cockpit, and then, setting the model variable activate to 1 will automatically precharge the DC bus and start the converter. Setting activate back to 0 stops the converter and discharges the DC bus.
However, if the relays are manually switched, it is very important to follow the steps below to safely operate a grid-tied converter:
To activate the converter:
- Make sure the relays are open
- Close the breaker
- Close the precharge relay
- When the DC bus is precharged (around 560V) close the bypass relay
- Now that the DC bus is precharged, the DC side PV relay can be closed
- Enable PWM outputs
To deactivate the converter:
- Disable PWM outputs (to ensure no power flow)
- Open the DC side PV relay
- Open the bypass relay
- Open the precharge relay
- Open the breaker
- Enable the PWM outputs (the switching losses will discharge the DC bus)
As already mentioned, the DC bus needs to be precharged to avoid currents flowing through the inverter’s diodes. Then, the precharge relay is closed first so that currents from the grid are limited by the three resistors. Finally, the bypass relay must be closed to fully connect the converter to the grid (the precharge resistors are not rated for the full converter current).
When followed carefully, these instructions will prevent damage to the power semiconductors during the connection of the converter to the grid.
Now that the converter is running, the last step is to set the DC bus voltage to the desired value (higher than the rectified grid voltage of course). One could use 700V for instance. Monitoring the converter and exporting data can then be done thanks to the Cockpit monitoring software. A getting started guide for the software can be found here.
To go further…
With the practical aspects of the 3 phase solar inverter out of the way, one could dive deeper into the control strategy for such a converter by referring to the page: Three-phase PV inverter for grid-tied applications.
