Torbjörn Thiringer

Temporary Primary Frequency Control Support by Variable Speed Wind Turbines— Potential and Applications

This paper quantifies the capability of providing a short-term excess active power support of a commercial multi-megawatt variable speed wind turbine (VSWT) and generalizes the findings by considering different wind turbine (WT) physical parameters in a wider range from the example case. The paper also identifies some possible applications of it, in particular, in a hydro dominated system. To be able to quantify the system characteristic, a delay model of the studied hydro system is developed. Due to the fact that the initial power surge of a hydro turbine is opposite to that desired, the short-term extra active power support from a wind farm (WF) could be beneficial for a hydro dominated system in arresting the initial frequency fall, which corresponds to an improvement in the system temporary minimum frequency (TMF). The improvements in the TMF are calculated by using both the developed delay model and a detailed model, and the results show good agreement. It is shown that the WT under consideration can provide a 0.1 pu extra active power support for 10 s quite easily which is twice the Hydro-Quebec requirement.

Modeling of wind turbines for power system studies

The modeling of wind turbines for power system studies is investigated. Complexities of various parts of a wind turbine model, such as aerodynamic conversion, drive train, and generator representation, are analyzed. The results are verified by field measurements made on a stall-regulated fixed-speed wind turbine. The modeling focuses on deriving a representation that is suitable for use in grid simulation programs.

Control by variable rotor speed of a fixed-pitch wind turbine operating in a wide speed range

Variable rotor speed control of a fixed-pitch wind turbine is investigated on a system consisting of a wind turbine which can operate in a wide speed range, from 0 to 38 RPM. It produces any desired output from the rated (20 kW) to no-load, providing there is enough wind. A special technique is used to determine the operating point of the wind turbine by using the measured rotor speed and power. A difficult problem with this type of wind turbine control is to make the upper speed limit reasonably high to capture as much energy as possible but still low enough to avoid power peaks. >

Wind Farms as Reactive Power Ancillary Service Providers—Technical and Economic Issues

This paper examines the possibility of providing reactive power support to the grid from wind farms (WFs) as a part of the ancillary service provisions. Detailed analysis of the WF capability curve is carried out considering maximum hourly variation of wind power from the forecasted value. Different cost components are identified, and subsequently, a generalized reactive power cost model is developed for wind turbine generators that can help the independent system operator (ISO) in managing the system and the grid efficiently. Apart from the fixed cost and the cost of loss components, a new method is proposed to calculate the opportunity cost component for a WF considering hourly wind variations. The Cigre 32-bus test system is used to demonstrate a case study showing the implementation of the developed model in short-term system operations. A finding is that higher wind speed prediction errors (a site with high degree of wind fluctuations) may lead to increased payments to the WFs for this service, mainly due to the increased lost opportunity cost (LOC) component. In a demonstrated case, it is found that 2340

Modeling and experimental verification of grid interaction of a DFIG wind turbine

/h is paid to the WF as the LOC payment only, when the wind prediction error is 0.5 per unit (p.u.), whereas 54

Variable Speed Wind Turbines for Power System Stability Enhancement

/h is the expected total payment to the WF when the prediction error is 0.2 p.u. for its reactive power service.

Power quality measurements performed on a low-voltage grid equipped with two wind turbines

The response of the doubly fed induction generator (DFIG) wind turbine system to grid disturbances is simulated and verified experimentally. The results are compared to the response that a fixed-speed wind turbine would have given. A voltage sag to 80% (80% remaining voltage) is handled very well, which is not the case for a fixed-speed wind turbine. A second-order model for prediction of the response of DFIG wind turbines is derived, and its simulated performance is successfully verified experimentally. The power quality impact by the DFIG wind turbine system is measured and evaluated. Steady-state impact, such as flicker emission, reactive power, and harmonic emission, is measured and analyzed. It is found that the flicker emission is very low, the reactive power is close to zero in the whole operating range, and the current THD is always lower than 5%.

Comparison between stator-flux and grid-flux-oriented rotor current control of doubly-fed induction generators

This paper investigates possible improvements in grid voltage stability and transient stability with wind energy converter units using modified P/Q control. The voltage source converter (VSC) in modern variable speed wind turbines is utilized to achieve this enhancement. The findings show that using only available hardware for variable-speed turbines improvements could be obtained in all cases. Moreover, it was found that power system stability improvement is often larger when the control is modified for a given variable speed wind turbine rather than when standard variable speed turbines are used instead of fixed speed turbines. To demonstrate that the suggested modifications can be incorporated in real installations, a real situation is presented where short-term voltage stability is improved as an additional feature of an existing VSC high voltage direct current (HVDC) installation

Periodic pulsations from a three-bladed wind turbine

The power quality of a low-voltage grid with two wind turbines is investigated. Slow voltage variations as well as transients and harmonics are measured and analysed. Furthermore, the spectrum of the power is determined so that the presence of periodic power components can be investigated. Although periodic power fluctuations reaching 10% of the rated power are registered, voltage variations are lower than the prescribed IEC flicker limit at steady-state operation. As the turbines are put on-line, the voltage level is lowered by 3%, which exceeds the flicker limit. The risk for flicker increases if the X/R ratio of the grid is low and if turbines which have a tendency to produce large periodic power fluctuations are used.

Comparison of reduced-order dynamic models of induction machines

Grid-flux (or grid-voltage)-oriented rotor current control of the doubly-fed induction generator is investigated in this paper. It is shown that by using a grid-flux-oriented system, the stability and damping of the system is independent of the d component of the rotor current, in contrast to a stator-flux-oriented system. Also for the stator-flux-oriented system, a certain value of the d component of the rotor current causes the system to become unstable. Hence, with a grid-flux-oriented system it is possible to produce as much reactive power as desired from a stability point of view. The theoretical results are verified experimentally with good result.