Solar inverter manufacturers must test products effectively to meet safety, grid connection, performance and certification requirements, as well as reliability expectations.
By Fred Zhu, TÜV Rheinland
A PV inverter undergoing the circuits analysis and single fault testing.
Inverter reliability is integral to smooth and dependable solar operation. The inverter, consisting of hundreds or even thousands of electric and electronic components, is at the heart of the PV power system. Failure of any inverter component will cause it to stop working.
It’s important that inverter manufacturers have a general idea about the standard requirements from the beginning. If they design an inverter without compliance in mind, they may need to change the design late in the development process.
Safety Standards
In the United States, the American National Standards Institute (ANSI), UL, and the Institute of Electrical and Electronics Engineers (IEEE) are responsible for safety standards. The testing requirements for inverters are specified in the UL 1741 standard.
The Canadian Standards Association (CSA) is the governing body for safety testing in Canada, which mandates that inverters meet the requirements of CSA C22.2 No. 107.1.
The International Electrotechnical Commission (IEC) oversees the requirements for the international community. It mandates the IEC 62109-1 and IEC 62109-2 standards for testing inverters internationally, including in Europe and Asia.
Even though different standard systems are used, product safety requirements for various countries are similar. It is especially true in the case of the United States and Canada. With the exception of some minor documentation differences, the two countries’tests are much alike. This allows certification providers to combine tests for the two markets and save costs for manufacturers.
Inverter manufacturers should perform safety tests to ensure all built-in components are within their applied temperature limits to avoid fire hazards. Testing of spacing and insulation can prevent electric shock hazards tooperators or service personnel. Testing the response to abnormal conditions should determine whether there are any risks due to extreme working conditions or foreseeable components failures.
A PV inverter tested for grid connection.
Grid Connection Standards
Regulatory and testing requirements differ among various countries, and inverter manufacturers have to design different versions of the control system — software and (possibly) hardware — for inverters intended for different countries. In the United States, the UL-1741 standard refers to IEEE 1547 and IEEE 1547.1 standards for the grid connection and protection requirements.
Evaluating inverter output quality, such as harmonics and synchronization, ensures that the product meets the minimum power quality requirements set by the national standards and utility companies.
Testing the response to abnormal grid conditions, such as abnormal voltage, frequency, lost phase, and anti-islanding, allows the manufacturer to ensure safe operation and maintenance of the whole grid.
Other Lab Tests
Solar inverter manufacturers should also test for performance and certification.
Performance tests, such as tests for efficiency measurement, are an important parameter for inverters. These include Maximum Power Point Tracking (MPPT) efficiency testing per EN 50530 (testing the efficiency and ability to track the maximum power point of the PV array under different sunlight conditions, including static and dynamic efficiency measurements), power conversion efficiency per IEC 61683, and the California Energy Commission’s (CEC) guidelines.
Tests for certification purposes can be done either in an inverter testing lab, or at a manufacturer’s facility, especially for inverters rated at power levels of 100 kW and higher. The requirements for testing in a lab and onsite are the same, and so are the testing procedures.
The only difference when testing at a manufacturer’s plant is that a third-party engineer does not operate the testing equipment, but checks the set-up, confirms conditions per applied standards, and gathers and validates the data during a witness test. Manufacturers’internal tests are considered reliable, provided the manufacturer kept a thorough record of testing conditions, methods, and the original testing data to allow for replicate testing. This is essential because manufacturers often declare their warranties based on their internal reliability testing data.
Design engineers will do well to read the standards for targeted countries so their products comply with requirements — from specifying correct cable sizes for internal and external connections to choosing correct safety-related components. If a manufacturer does not have such capability in-house, engineers can consult a testing lab that offers this type of service. Such consultations can save time and costs of re-designing and repeated testing if an inverter turns out to need modifications thanks to anon-compliance issue exposed during testing.
An inverter tested for electromagnetic compatibility
Testing For Reliability
Reliability is a measure of the product’s performance. There is not yet a specific standard to test it. Manufacturers are using some widely accepted testing methods, such as Highly Accelerated Life Test (HALT) and Highly Accelerated Stress Screen (HASS) to estimate product life of the key components and units, as well as to identify problems or weaknesses.
Many manufacturers, well aware that their success depends on the reliability of their products, have refined their inverters by selecting high-quality components, despite higher costs. That, coupled with the application of a quality control system in the production line, as well as professional installations, should provide for a good performance and long, reliable service life
There is ongoing work to develop a standard measure of reliability. For example, TÜV Rheinland is working on developing a reliability standard for module-level power electronics, such as micro-inverters, together with a number of inverter and module manufacturers, national laboratories, and U.S. universities. The project is funded by the U.S. Department of Energy.
New technology and designs, including higher DC input voltage, DC-side arc fault protection, and interruption and transformer-less inverters for better power conversion efficiency, are being introduced to the international markets. TÜV Rheinland’s worldwide network of inverter testing laboratories allows the organization to engage local engineers and testing labs to make the certification process easier and quicker. Nine facilities are located in Australia, Germany, Greater China (Shanghai and Taiwan), Hungary, India, Italy, Japan, and the United States.
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