Wind Turbine Characterization by Voltage Source Converter Based Test Equipment

Doktorsavhandling, 2016

In countries where wind power has become a relevant part of the total generated electrical power, transmission system operators (TSOs) have included in their Grid Codes specific technical requirements for interconnection of wind power plants. Some Grid Codes demand specific testing procedures in order to verify the fulfilment for their requirements. These tests are today carried out using an impedance-based test equipment, which is limited to voltage dips and swells. For this reason, many of the requirements remain untested by field experiment. The use of a fully controllable converter system operated as test equipment allows for a wide variety of tests that can be carried out, in order to assess for successful grid integration of the generating unit. In this regard, a different approach for testing of multi-megawatt wind turbines by fully-rated converter system is presented in this thesis. The investigated testing setup consists of a 4 MW full power converter (FPC) based wind turbine and an 8 MW testing equipment constituted by a set of two voltage source converters (VSCs) in back-to-back configuration. In particular, this thesis focuses on the use of the testing equipment in order to obtain a full characterization of the behavior of the wind turbine system in both steady-state and dynamic condition of the grid. A detailed description of the technical requirements for grid interconnection of wind power plants included in the selected Grid Codes is given. The control algorithms that govern both the testing equipment and the wind turbine are derived in detail, with special focus on the control scheme of each VSC. The risk for poorly damped resonances and possible interactions between the testing equipment and the tested object are investigated through small signal stability analysis. In this regard, the characteristic impedance of the wind turbine unit is derived and verified by frequency scan using the testing device. The testing methodology is verified through time-domain simulations. Moreover, the ability of the testing equipment in reproducing a low voltage ride through (LVRT) profile that is typically included in Grid Codes is analysed in detail, including other features such as the emulation of the short-circuit impedance of the grid at the connection point. In addition, wind turbine testing of active power curtailment during grid-frequency deviation is also investigated in this thesis. Laboratory experiments comprising of a low-voltage prototype of the testing equipment and a reduced model of the FPC wind turbine equipped with the derived control features are carried out to verify the investigated methodology. Finally, field tests carried out on a full-size testing facility in Gothenburg, Sweden, are included in this thesis. The obtained results demonstrate the flexibility of the investigated testing equipment in controlling the voltage at the wind turbine terminals, which allows for a full characterization of the generating unit.