Inverters not converting direct (DC) to alternating current (AC) from a battery bank but directly from the DC generated by solar panels are called grid-tied inverters. Inverters are designed to feed the AC generated into the power grid via a net metering device.
Solar inverters convert DC of variable output to clean AC suitable for being fed into the power grid. Inverters must be tested to comply with UL 1741 stringent tests to ensure that they do not cause disturbance to the power grid and will shut off in case of a power outage.
Inverters designed for grid-tied applications must be tested to ensure that they comply with the following functional conditions:
- The solar inverter must meet the grid-connection design requirements;
- The solar inverter must be properly sized for the solar array output;
- The solar inverter must meet operational failsafe functions
Before connecting to the power grid, let’s review how solar inverters are tested during production and after installation.
How Are Solar Inverters Tested Before Connection To The Grid?
The research authorities developed a standardized test protocol for implementation by Underwriters Laboratories (UL) for the compliance testing of solar inverters designed for grid-tied applications. UL1741 is the test standard applicable.
Grid-tied inverters are the most common type of residential installation to convert solar power DC to the grid in the form of AC of the correct voltage and operational frequency with a Total Harmonic Distortion of less than 3%.
Connecting the hundreds or thousands of grid-tied solar inverters to the power grid cannot be done without prior verification of the performance and safety characteristics of the inverter.
The test protocol is performed by Underwriters Laboratories on all Solar Inverters that are designed for connection to the power grid to feed the AC generated from the solar generation into the grid.
The testing laboratory uses a simulated DC input generator to recreate the variations typical of solar-generated DC. As such, the following aspects are measured:
Steps To Test Your Inverter
- DC input power
- AC output to a simulated utility grid
- The Maximum Power Point Voltage (V)
- The Maximum Power Point Current (I)
- The Maximum Continuous Output Power (W)
- The Conversion Efficiency (%)
- The distortion in the AC output (THD – %)
- Response tests to grid power outage
All solar inverter manufacturers must have their inverters tested and certified by Underwriters Laboratory (UL) to authorize the sale of their products in the US.
At the point of installation, qualified installers must be used to complete the installation, testing, and connection to the power grid. Before installation, homeowners must obtain approval from the local power utility company to install a grid-tied system.
Can An Off-Grid Solar Inverter Supply AC Without A Battery?
Solar inverters are balanced to suit AC loads if the input voltage from the solar panel array is sufficient to generate a stable AC output to power the connected load. The system will only be operational during the peak solar production hours each day.
Testing such a system will require that the solar panels are connected in series to provide the maximum voltage input to the inverter.
- Firstly connect the series solar array to the inverter with cables of adequate gauge.
- Test the voltage and current input to the inverter with a multi-meter to determine the power input in Watt (Voltage V x Current I).
- Calculate the output of the inverter by reducing the input power by 80% to account for conversion inefficiency.
- Connect an AC load below the calculated power output and switch the device on. If the inverter powers the device without an under-voltage or over-voltage disconnect, you are good to proceed.
Note: such a setup will only operate as long as the solar power generated is sufficient to prevent the inverter from shutting down due to a low voltage input as the sun’s intensity diminishes.
It is an interesting experiment but ultimately not efficient due to the limited hours it can be operational. A backup battery or grid-tied solution is required for a more reliable system with the flexibility of powering loads during off-peak solar production.
Inverters Are UL-Rated By Manufacturers To Comply
Underwriters Laboratories (UL) and the California Public Utilities Commission (CPUC) issued testing standards for PV and grid-interactive solar electric inverters (and other power conversion equipment).
All inverters sold in the US must be tested and certified by Underwriters Laboratory (UL) to comply with the standards for safe operation and output wattage according to UL1741.
UL 1741 is a product safety standard that defines the manufacturing and product testing requirements to produce inverters capable of managing irregularities in the grid supply, ensuring the continued safety and reliability of the grid.
Inverters must automatically disconnect from the grid during a power outage, preventing utility technicians from current sources other than the primary grid power stations.
As solar power generation grows to the extent that it forms part of the base grid load, this arrangement will have to prevail.
Tests required by UL 1741 to certify inverters as suitable for grid-tied installations are:
- Low/High Voltage Output Stabilization
- Low/High-Frequency Output Stabilization
- Must Trip Test in case of a power outage
- Ramp Rate for switching into the grid at startup
- Specified Power Factor must be sufficient
- Voltage Output Variation Mode
- Frequency of Power Output
- Output Voltage and Power
- Anti-Islanding shut off
Inverter manufacturers are mandated to implement these test protocols in the quality assurance in conjunction with blind product testing performed by UL. Only certified manufacturers who pass the UL testing may apply the UL1471 compliance sticker to their inverters.
As more homeowners and businesses install grid-tied solar systems, the power supply from such systems may prevent an individual inverter from detecting a power outage at a utility power station.
Utility companies must manage the signals to solar inverter producing inverters to shut down to protect the repair technicians. This is called “Anti-Islanding,” which is required by UL1741 to ensure that inverters can be disengaged from the grid in the event of a grid outage.
The voltage and frequency output stability and ramp-up rate test the inverter’s ability to feed AC of the required voltage and operational frequency into the power grid. The inverter must be able to sense changes in the standard frequency of the grid and not disconnect from the grid.
Disconnecting large solar array inputs to the grid due to a change in the grid standard operating frequency may cause even more instability and exacerbate the grid’s disruption.
The ramp-up rate control is required to allow grid utility operators to balance the power supplied from traditional power stations with solar-generated power output and blend the power sources into a stable power grid.
Inverter manufacturers in compliance with UL 1741 are:
- Enphase
- Fronius
- SMA
- SolarEdge
- Yaskawa – Solectria
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