Aspects on Wind Turbine Protections and Induction Machine Fault Current Prediction
This thesis presents detailed modelling of the induction machine with the main objective being accurate fault current prediction. The thesis focus mainly on mains connected induction machines and the response due to severe disturbances in the supply voltage, such as three-phase short-circuit faults. Both the squirrel-cage induction machine as well as the doubly-fed induction machine are treated.
In order to accomplish accurate results, dynamic parameters of the induction machine is needed, and accordingly, extensive measurement series has been accomplished in order to identify these. A large number of locked-rotor tests and no-load tests with varying voltages and frequencies have been performed to find out the varying nature of the parameters due to skin effect and saturation. Regarding saturation, the effect of leakage flux saturation has been investigated.
To incorporate for the skin effect and the effect of leakage flux path saturation in the calculations, a conventional dynamic mathematical description of the induction machine 5th order Park model) is modified, which makes it applicable for various operating conditions.
The fault-current prediction of the detailed model is compared with the prediction of the conventional fifth order model and a dynamically equivalent three-phase model, and also with measurements of symmetrical and asymmetrical short circuit faults, starts and locked-rotor tests. The different models presented are also discussed with respect to wind power as the main application.
The proposed model shows excellent agreement, and also indicates the essentiality to incorporate for the saturation of the leakage flux path and the skin effect as the response due to severe faults is to be determined and if a high accuracy result is the objective. Incorporation of the main flux saturation, is of little importance for the short-circuit fault currents.
Regarding the selection of a two- or three phase model, the most appropriate model as well as the more simple model, is generally the two axis model. The disadvantage is the inability to account for a zero sequence component. The possible choice of using a three phase model is however only an issue if the generator is wye connected and if the neutral point of the generator is connected. Wind turbine generators are mainly connected in delta which imply no neutral point and if they are connected in wye, the neural point is however not connected.
In the thesis the fault current determination for the doubly-fed induction generator system and the fixed speed system are treated. The fault current determination for the full power converter system is however not dealt with since this type of system have full control of the fault currents and can control these to a desired level.
Short circuit currents
Slip ring Generator