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.
The Canadian standard for grid interconnected inverters is CAN/CSA C22.2 No. 107.1-01 “General Use Power Supplies.” While UL 1741 is written specifically for inverters, the CSA standard is broader in scope, with sections dedicated to inverters including a section with specific requirements for grid connected inverters. The constructional requirements in the US and Canadian standards are very similar as are the majority of the tests. Given the similarities in the regulatory frameworks and requirements of the inverter standards, it’s relatively simple for manufacturers to satisfy both simultaneously.
In Europe, inverters need to comply with the Low Voltage and EMC directives. The typical route for demonstrating compliance with these directives is evaluation of the equipment to EN standards such as EN 62109 “Safety of Power Converters for use in Photovoltaic Power Systems – Part 1: General Requirements,” or EN 50178 “Electronic Equipment for use in Power Installations.” While compliance with these standards, including the additional standards they reference for EMC compliance, is sufficient for CE marking, there also are separate standards that cover grid interconnect requirements for EU countries. EN 50438 has general requirements as well as national deviations in Annex A. The national deviations may be detailed in the Annex or they can take the form of references to country-specific grid interconnect standards such as VDE 0126-1-1 for Germany and others. The choice of safety and grid interconnect standards will depend on the application (PV, wind, etc) and the inverter power output level. For example, EN 62109 applies only to equipment used in PV applications with DC input voltage up to 1500 VDC and output voltage up to 1000 VAC. EN 50438 is limited to single or multiphase equipment at 230/400 V and up to 16 A per phase. There also are country specific requirements to qualify for rebates or feed-in-tariffs such as G83 or G59 in the UK. These requirements are commercial in nature rather than regulatory.
As the distributed energy market grows, new standards and compliance requirements are being developed. For example:
- IEC and other standard writing committees are writing standards for Smart-Grid technology. This new technology will allow the electric utility to communicate with the inverter or other equipment and control how and when inverters energize the grid.
- UL 6171 “Proposed First Edition of the Standard for Wind Turbine Converters and Interconnection Systems Equipment” is in development.
- IEEE 1547.8 is a draft standard establishing a common technical platform for distributed resources interconnection applications.
- Requirements are in development for LVRT. IEC 61400-21 “Wind turbines - Part 21: Measurement and assessment of power quality characteristics of grid connected wind turbines” includes methods for testing LVRT for wind turbine converters. Similar efforts are underway for large PV inverters that also require this functionality.
With all of the international, national, regional, and local regulations it becomes increasingly complex for the manufacturer to interpret the regulations and build a product that will be compliant worldwide or even regionally. Accredited third party agencies, like Intertek, can help manufacturers understand and interpret these regulations resulting in product certification for the markets of their choosing. As new standards are developed and changes to existing standards occur, manufacturers can spend significant resources trying to keep up.
Understanding of the global requirements, where they overlap and where they differ, is key to developing a plan for compliance that is integrated with the product design, enables access to global markets, and reduces overall time and cost.