Standards and grid codes – the latest developments and trends
The increased penetration of renewable energy requires changes to the standards and grid codes to ensure that power grids remain reliable and robust.
In this blog post we will give a short overview of current developments and trends.
CENELEC, the European Committee for Electrotechnical Standardization, is currently working on CLC/TS 50549-2, a technical specification (TS) intended for generating plants connected to medium voltage distribution power grids. Its comprehensive scope covers aspects of normal frequency and voltage operating ranges, rate of change of frequency, low and high voltage fault ride-through, voltage support, power quality, and reconnection and synchronization after disconnection. It remains to be seen how this specification will impact the wind industry as its content overlaps with the NC RfG legislation, national grid code standards and their respective certification regimes.
The Institute of Electrical and Electronics Engineers (IEEE) has published IEEE Standard 519-2014, a revision to the IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems. It supersedes the IEEE Standard 519-1992 revision. The main approach of the standard is that users are responsible for limiting harmonic currents and system owners/operators are responsible for managing voltage quality at the point of common coupling (PCC). The main changes are related to measurement techniques, time-varying harmonic limits, low voltage harmonic limits (<1 kV) and interharmonic limits. In general, it is recommended that the measurement techniques specified in IEC 61000‐4‐7 are followed, and both the “very short (3 sec.)” and “short (10 min.)” value methods can be used in accordance with IEC 61000‐4‐30.
The Chinese National Energy Administration (NEA) has issued a notice about stepping up testing and certification to guarantee the quality of wind power equipment. All wind turbines and main turbine components, such as rotor blades, gearboxes, generators, converters, controllers and bearings, used in newly built on-grid projects, including distributed power projects, should pass type testing in accordance with GB/Z25458-2010 Wind Turbine Certification Rules and Procedures. Certification has to be done by a certification body approved by the CNCA (Certification and Accreditation Administration of China). All related organizations have to be ready for type testing since the standard became effective on July 1, 2015. Wind power developers should only publish tenders for equipment with a type testing certificate, as products without a certificate are excluded from the bidding.
Grid code requirements can be roughly divided into two subcategories: static requirements relating to the steady state behavior of wind farms, and dynamic requirements relating to the temporary behavior of wind farms, or sometimes individual wind turbines during fault conditions. Even though there has also been changes to the static requirements, most of the changes concern dynamic requirements, namely fault ride-through, transient overvoltage and grid supporting events. Generally speaking, many of the recent changes have been associated with high voltage fault ride-through (HVRT) requirements in the 120…130% range, and with the introduction of positive sequence grid voltage support on similar lines to the SDLWindV requirements in force in Germany. These requirements are typically defined at the point where a wind park is connected to the grid (point of common coupling), but can sometimes also be defined at the point where a single wind turbine is connected to the grid (point of connection).
Europe: Progress has been made towards grid code harmonization in the form of network code NC RfG, which has been a binding EU regulation since May 2016 and is in force under EU law. This “umbrella” network code has been developed by the European Network of Transmission System Operators for Electricity (ENTSOE), which has members from 35 countries and includes 42 transmission system operators (TSOs). While the NC RfG contains many common grid interconnection requirements, a large percentage of these are “non-exhaustive”, meaning that the detailed specifications will be set at a national level. This national implementation phase of the RfG is now underway within EU member states and is expected to be completed by 2018.
Germany: A new grid code (VDE-AR-N 4120) for onshore connections at 110 kV (or above) came into force on January 1, 2015 with a transition period of 2 years. This replaced the Transmission Code 2007 and introduced clearer definitions and methods to be followed by grid users. The basic changes concern the capability of wind turbines to support the grid voltage during unbalanced faults with both positive and negative sequence reactive short-circuit current injections. Fault ride-through and grid voltage support requirements for both low voltage ride-through (LVRT) and high voltage ride-through (HVRT) as well as consecutive grid faults are included. The corresponding generation unit and plant certification guidelines TR 3,4, and 8 are being updated by FGW during 2016. A similar update of the BDEW MV grid code is currently being undertaken within VDE FNN and is expected to be published during 2016 as VDE-AR-N 4110.
China: Grid code developments in China normally follow Europe, and the latest grid code revision is GB/T 19963-2011. One of the aspects that has been most widely discussed is related not to low voltage ride-through (LVRT) performance but to the high voltage ride-through (HVRT) capability of wind turbines. These requirements are typically considered to be fulfilled at wind farm rather than at wind turbine level. The trend in China, however, is that this is clearly a turbine level requirement. In general, wind turbines must be capable of remaining grid connected if there is a temporary overvoltage at the turbine’s point of grid interconnection (1.30 pu / 0.2 sec.; 1.25 pu / 0.8 sec.; 1.15 pu / 8.0 sec.).
North America: There is no grid code that applies to all territories in North America. The North American Electric Reliability Corporation (NERC) has four isolated interconnection territories: Western, Eastern, ERCOT (Electric Reliability Council of Texas) and Quebec Interconnections. In addition, there are nine independent system operators (ISOs): Alberta, Ontario, Midcontinent, Southwest Power Pool, California, PJM Interconnection, ERCOT, New York, and New England.
In the US there are several regulatory bodies that influence and shape national and local grid code requirements. In order 661A the Federal Energy Regulatory Commission (FERC) lays down site-specific reactive power requirements for TSOs that must be followed by site-specific wind farms. NERC enforces standards that apply to voltage and reactive power control to ensure reliable operation. The ISO/RTO (Regional Transmission Organization) stipulates grid specific requirements such as low voltage ride-through, voltage regulation, dynamic reactive power control and reactive power requirements. In addition to these requirements, a local utility may specify its own interconnection requirements. Last but not least, each state’s Public Utilities Commission (PUC) approves renewable power contracts for state utilities, sets renewable targets for the state, and reviews rate cases. The recent trend has been to close down coal-fired power plants and add more renewable generation like wind and solar. As a result, ISOs are evaluating technologies like energy storage to provide frequency regulation due to increased mismatches between power supply and demand. As for the future, it is difficult to forecast how grid codes will evolve in the US.
How are standards and grid codes reflected in wind turbine technology?
The significant expansion of renewable energy raises a number of technical and economic issues that will have to be resolved. It may even prove necessary to develop new technologies. For the wind power industry, it is clear that wind farms will have to make their own contribution to stable – and therefore safe – grid operation. The technical evolution of wind turbines has been closely linked to the development of international standards and grid codes, and this will continue to be the case in the future.
At ABB, we work closely with the standards organizations, industry, and our customers to find the optimum solutions to today’s challenges. We can support customers in the demanding wind turbine certification processes which play an important role in the industry. If you would like to learn more about how you can ensure that your wind turbines and wind farm will meet the latest grid codes, please read our next blog or meet our experts at events like China Wind Power, which is held on October 19-21, 2016 in Beijing. We look forward to discussing with you how ABB can help you achieve a better wind economy for your wind farm.