WTG grid compliance testing and validation part 1: grid-converter test rig measurement and verification based on the IEC 61400-21-4

Grid compliance testing and validation of Wind Turbine Generators (WTG) has typically been performed on a full-scale proto-type. However, with the industry being confronted with more demanding and numerous grid requirements, difficulties associated with testing increasingly larger WTGs, and the greater reliance on validated simulation models for wind farm design studies earlier in the design phase, manufacturers are turning to other means for performing WTG validation, such as test rigs. The use of test rigs also enables expanded test capabilities, which are not possible in the field due to limitations when directly connected to the public grid. With an increasing share of WTG validation tests being performed on test rigs, there is a growing need for clear guidelines on when it is appropriate and how measurements should be performed and assessed on test rigs. The IEC 61400-21-4 will be the first such technical specification that specifies how to use test rigs to measure and assess the electrical characteristics of WTG components and subsystems. This paper presents the experience of a WTG manufacturer in applying the IEC 61400-21-4 to a converter test rig with a Type IV wind turbine in cooperation with a measurement institute and certification body. Firstly, the design and implementation of the test rig are explained, which is followed by results to establish the grid emulator capability. This is followed by tests which examine the generator and mechanical simulation in the hardware in the loop system, where the results are compared against prototype measurements. Furthermore, grid compliance tests, including FRT, Power Quality, and steady-state capability, performed on both the test rig and prototype are presented. The objective of this comparison is to assess the ability of the test rig to accurately represent a full-scale WTG and to understand the source of the differences. An evaluation of the IEC61400-21-4 test methodologies and the use of a converter test rig for the different test types is then provided. Finally, a strategy for future WTG validation is proposed, which effectively combines subsystem and component testing with full-scale measurements.