Grid Code Testing of Wind Turbines by Voltage Source Converter Based Test Equipment

Licentiate thesis, 2015

Wind energy is expected to play a crucial role in the energy mix in the future society with limited access to fossil fuels. Wind turbines with larger power rating are being installed every year, making possible to extract more energy from wind. In countries where wind power has become a relevant part of the total generated electrical power production, transmission system operators (TSOs) have included in their grid codes specific technical requirements for interconnection of wind power plants. Today, grid code requirements are tested by using an impedance-based testing equipment which is limited to voltage dips and swells. For this reason, many of the requirements remain unverified. The use of fully controllable converter systems operated as test equipment allows for a wide variety of tests that can be carried out on the generating unit. For this reason, a different approach of grid code testing methodology is investigated in this thesis. The investigated testing setup consists of a 4 MW wind turbine and an 8 MW testing equipment constituted by a set of voltage source converter (VSC) in back-to-back configuration. In particular, this thesis focuses in the Low Voltage Ride Through (LVRT) test of full-power converter (FPC) wind turbines. In this work, a detailed description of the technical requirements included in grid codes for interconnection of the wind power plant with the grid is given. The control algorithm that governs 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 interaction between the testing equipment and the tested object is investigated through small signal analysis. The grid code testing methodolgy is then validated through time domain simulation where all the sub-systems that constitute the testing setup are integrated in one simulation model. The obtained results demonstrate the flexibility of the proposed approach in controlling the voltage at the wind turbine terminals, including the ability in emulating the short-circuit impedance of the grid at the connection point. Furthermore, laboratory experiments are carried out in order to verify the investigated methodology. Finally, field test of the actual testing facility in Gothenburg, Sweden are included in this thesis.