Inverter Testing and Certification for Global Markets
- Friday, 02 September 2011
- Troy Hewitt and Anthony G. Steiner, Intertek -
Inverters/converters are a key element of most renewable energy systems where the power generated is not suitable to either be used locally or to provide power directly to the electric utility. Many renewable sources such as photovoltaics generate DC power, while some renewable sources such as wind turbine systems generate AC, but not at the same voltage and/or frequency required by the utility.
As renewable energy sources have developed and the market for them has grown, the technology for power and the need for conversion has also developed. Today, inverters/converters are solid-state devices with increasingly precise control over the output power waveform, high efficiencies, and added functionality for increased safety and performance. One of the challenges facing inverter manufacturers is the ability to design for and comply with the differing regulatory requirements for safety and interconnection to the electric utility in global markets.
Two of the largest markets for inverters/converters are North America and Europe. Each market has its own regulatory framework that defines the requirements for equipment safety and for interconnection to the electric utility, as well as how compliance with these requirements must be demonstrated. Complying with these requirements affects the construction and functionality of the inverters. Factors such as the inverter output power level, input types, and the application also affect the regulatory requirements. Inverters can be part of a small system, owned and installed by homeowners, or may be part of a commercial system professionally installed and maintained. Utility interconnected systems can employ microinverters as small as a hundred watts, while commercial systems utilize inverters up to a few megawatts. All utility interconnected inverters – large, mid-range, or micro – must meet applicable local safety requirements and grid interconnection rules.
The U.S. and Canada have similar regulatory systems for electrical equipment. Both require certification of equipment such as inverters to national product standards. Certification is the process of having a product checked against appropriate standards that ensure they comply with safety, performance, and compliance of local, national, and international standards. Certification must be performed by an accredited third party agency. In the U.S,. Nationally Recognized Testing Laboratories accredit by OSHA perform certification, while in Canada this is done by Certification Organizations and Testing Organizations accredited by Standards Council of Canada. In Europe, compliance with the appropriate European Union (EU) directives is required. Manufacturers may self-declare or have a third party evaluate and test their equipment. The manufacturer then issues a Declaration of Conformity and CE Marks (Communauté Européenne: European Community) the equipment.
For residential and mid-range inverters, the U.S. safety requirements are in UL 1741 “Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources.” The grid interconnection requirements are incorporated in UL 1741 through a requirement that the inverter complies with IEEE 1547 “Standard for Interconnecting Distributed Resources with Electric Power Systems.” These standards apply to distributed generation installations where the expectation is that the inverter will cease energizing the electricity grid in the event of an abnormal condition. The abnormal condition may be that the voltage or frequency is out of the specified range or that the grid voltage has disappeared completely. This latter situation is called “islanding” and specific tests are required to show that the inverter is capable of detecting an island and will cease energizing the grid within the required time limit.
Larger commercial megawatt scale wind parks or solar installations typically fall under grid transmission rules that are not compatible with the requirements of IEEE 1547. In these cases, the installation is expected to support the grid during voltage sags – a function called Low Voltage Ride-Through (LVRT) – or other abnormal events, and cannot simply cease energizing the grid. In these cases, electrical safety certification may be done to a different standard such as UL 508C for “Power Conversion Equipment.” The utility may accept manufacturer’s testing to verify functionality such as LVRT or require third party verification.