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Standards Development
IEC 61400-27-4 contains normative specification for validation of EMT models of converter-based units, subsystems or components.
The model validation procedure specified in IEC 61400-27-4 is applicable to converter-based units which are used in renewable power plants, i.e. active power generation units like wind turbines and PV systems, energy conversion units like battery and electrolyzer systems, and units with reactive power capability such as STATCOMs.
The specifications in IEC 61400-27-4 includes definitions, specification of model structure, specification of a model validation procedure, and specification of limits in the applicability of the models.
The model structure defines the model interface in terms of input parameters to and output parameters from the model, and the point of connection to the grid.
The model validation procedure provides quantitative measures for the model accuracy based on comparison of model outputs to measured values.
The first edition of IEC 61400-27-4 does not include validation procedure for EMT models at plant level, but good practices for how to use validated unit models and plant controller models in plant level modelling will be described in an informative annex.
IEC 61400-27-4 does not specify generic EMT models of units but specifies the model structure in terms of inputs and outputs of a black box EMT model.
IEC 61400-27-4 does not specify test procedures for wind turbines or renewable power plants but refers to tests specified in the IEC 61400-21 series.
IEC 61400-27-4 does not specify procedures for conformity assessment of model validation. Conformity assessment for validation of electrical, mechanical and other models is harmonized in IECRE, based on model validation procedures like the one specified in IEC 61400-27-4.
IEC 61400-27-4 does not specify procedures for grid compliance evaluation of renewable power plants. Grid compliance evaluation of renewable power plants is in the scope of IEC TC8, IEC SC8A and other legal entities
The purpose of the proposed standard is to ensure properly validated EMT models of renewable power plants. Traditionally, grid operators have analysed power system stability based on quasi-static and dynamic fundamental frequency models.
But today, EMT models are requested by several grid operators to be able to assess the impact of converter based renewable generation on the stability and protection coordination in converter dominated power systems. Thus, ENTSO-E’s Expert Group Interaction Studies and Simulation Models (EG ISSM) concludes the need for validation of mainly 3 types of models: RMS time domain models, EMT time domain models (real time and non-real time), and frequency dependent harmonic impedance models.
The global energy transition is expected to increase rapidly the share of converter based renewable generation in power systems. Therefore, the current request from grid operators for validated EMT models is expected to propagate rapidly internationally and become a generally accepted requirement to renewable plant developers for obtaining grid connection. With the present and expected future request for validated EMT models in mind, standardization is an efficient mean to reduce the financial resources and manpower involved to ensure the credibility and ease the applicability of the models.
IEC 61400-27-4 defines normative generic software interface for EMT models. This definition must be coordinated with the definitions of the informative Annex G of IEC 61400-27-2 which defines an interface which applies to RMS (fundamental frequency) models as well as EMT models. IEC 61400-27-4 is proposed by TC88 to complete the existing IEC 61400-27 electrical simulation model series for wind energy generation systems. WG27 includes experts from wind power industry and academia with most advanced experience on modelling of converter-based wind turbines and other converter-based units. Other technology units such as STATCOMs to extend reactive power capability and storage units to enhance generation flexibility and reserve capability are already used in several wind power plants. Hybrid power plants combining wind, solar and storage technologies is also a growing solution which enables connection of more renewable to the existing grids. The growing grid operator request for frequency domain models and EMT models suggest supplementing the existing IEC 61400-27 series standards for fundamental frequency models with the proposed IEC 61400-27-4 standard for validation of EMT models and a proposed IEC 61400-27-3 standard for validation of frequency domain models.
However, other technical committees are working with other converter-based technologies than wind turbines, and the standard is relevant for conformity assessment in IECRE and for power system stability in IEC TC8. Therefore, IEC 61400-27-4 is horizontal and relevant for the following committees:
• IEC TC8 System aspects of electrical energy supply
• IEC SC8A Grid Integration of Renewable Energy Generation
• IEC TC82 Solar photovoltaic energy systems
• IEC TC120 Electrical Energy Storage (EES) Systems
• IECRE System for Certification to Standards Relating to Equipment for Use in Renewable Energy Applications.
Standardization and best practice for EMT modelling is also ongoing outside IEC. The following working groups will be considered:
CIGRE JWGB4.82/IEEE Guidelines for Use of Real-Code in EMT Models for HVDC, FACTS and Inverter based generators in Power Systems Analysis
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