FTEEE-1698 An Offshore Wind Generation Scheme With a High-Voltage Hybrid Generator, HVDC Interconnections, and Transmission – IEEE EEE Project 2016-2017

FTEEE1698-An-Offshore-Wind-Generation-Scheme-With-a-High-Voltage-Hybrid-Generator-HVDC-Interconnections-and-Transmission-IEEE-EEE-Project-2016-2017

FTEEE-1698 An Offshore Wind Generation Scheme With a High-Voltage Hybrid Generator, HVDC Interconnections, and Transmission – IEEE EEE Project 2016-2017 

ABSTRACT:

A new off-shore high voltage DC (HVDC) wind generation scheme is proposed in this paper. The scheme implements a high voltage hybrid generator (HG), HVDC interconnection and transmission systems. The turbine power-train of the proposed system is compared with a typical system installed in a commercial wind farm. The analyses demonstrate improvements in system losses and hence efficiency, power-train hardware including cable system mass and, importantly, a reduction in major component count and installed power electronics in the nacelle and turbine tower, features that lead to reduced capital cost and maintenance. The resulting power conversion system is much simplified and more amenable to higher voltage implementation since it is not constrained by existing state-of-art power electronic voltage source converter (VSC) structures. Voltage control is facilitated via DC/DC converters located away from the turbine tower. To demonstrate the HG operational concept, measured results from a low power laboratory prototype HG system are compared with analytical results and show good agreement.

OFF-SHORE wind generation has attracted considerable research and industrial attention over the past decade. To-date studies have investigated many areas related to wind generation including, but not limited to, the control and stability of wind farms, power electronic converters for the turbine-generator power-trains and DC grids for wind generation. The high voltage DC (HVDC) system proposed reduces the number of voltage source converters (VSC) in the system by using a higher rated VSC for a group of wind turbines. However, such a scheme results in lower (electrical) control functionality at the individual turbines.

The preceding section sets out a benchmark system against which the proposed HV wind generation scheme is compared. The HV wind generation scheme comprises of a high voltage HG, passive rectification, DC/DC converters, and HVDC interconnection and transmission systems. This scheme potentially results in reduced system cost due to reduced component count and complexity, in particular the HG and passive rectifier replace the existing industry practice of induction or PM generator, back-to-back VSC’s and tower mounted transformer, yielding a much simplified hardware scheme.

The proposed HV HG system is analysed and compared with the Walney wind farm keeping the geometrical topology of the wind farm i.e. distances, site layout, turbine arrangements etc. unchanged. The single line diagram of the proposed system. The analysis studies are carried out with following assumptions:  The passive rectifier is integral to the generator and installed in the nacelle, hence the distance between the HG and rectifier is neglected.  At full-load the voltage is at its rated value and the voltage transfer ratio of the DC/DC converters at the off-shore substation is nominally 1:4. As the wind velocity changes and the voltage drops the DC/DC voltage transfer ratio increases.

STEPS:

  1. Turbine Characteristic
  2. System Electrical Scheme and Main Component Details
  3. Hybrid Generator (HG)
  4. Report Generation

Turbine Characteristic:

During wind farm operation the wind velocity is a highly variable parameter having both steady-state and dynamic (gust) components. The output power capability of a wind turbine design is prescribed by the Betz criteria and the cube of wind velocity. For the Walney scheme the published turbine power versus wind velocity characteristic. The turbine generator output is controlled from the turbine cut-in speed at a minimum wind velocity of 3-4 m/s. From cut-in speed the turbine maximum power point is tracked, to maximize wind-to-mechanical energy conversion. At a wind velocity of around 13-14 m/s the turbine/generator speed is controlled to maintain a constant power (maximum of 3.6 MW) with varying wind velocity (13- 25 m/s).

System Electrical Scheme and Main Component Details:

The Walney system is required to be operated at constant voltage, i.e. at around the nominal rated voltage at all times and for all wind speed conditions. This is to ensure operation of the back-to-back VSC’s and ancillary components in the system including monitoring, measurements and maintenance equipment. For the proposed HV scheme the system voltage is allowed to vary. Two control methods could be considered for the HG output DC-link voltage fixed voltage and variable (but controlled) voltage. In the fixed voltage approach the rectified output voltage of the HG is kept around its nominal DC value at all wind speed conditions i.e. from cut-in to cut-out speed.

Walney 1 wind farm has been modelled using equivalent models for the induction generator, converters, DC-link, transformer and cables as discussed previously. Different loading conditions were studied and analysis conducted with the following assumptions:  The distance between the induction generator and Converter 1 in the nacelle is neglected.  The distance between Converter 2 and the transformer at the bottom of the tower is negligible. All the circuits connect to the off-shore substation with the same power factor.

Hybrid Generator (HG)

The HG is presented in this section. However, the detailed design and analysis of the HG is out of the scope of this paper. The HG is a multiphase, high voltage generator that uses two rotor sections, namely a PM rotor and a WF rotor, to provide the total machine excitation. Hence, the name hybrid refers to the combination of these two rotors. The PM and WF rotor sections exist on one rotor assembly inside one machine housing. Thus, the rotors rotate with the same speed. A schematic view of the HG is depicted. Therefore, the HG combines the output voltage due to a fixed field from the PM rotor and a controlled variable voltage due to the variable field of the WF rotor.

Simulation results of the HG phase voltage and current, and DC-link output voltage and current for varying system DC-link voltage, The HG machine concept is verified by test results taken from a low power prototype HG designed, built and tested in the laboratory, further details of which are discussed by the authors presents measured and simulated results for the HG WF excitation current versus DC-link voltage variation at nominal speed when delivering a fixed power of 2 kW. Results are shown for a wide range of voltage variation since this is theoretically possible and hence testable via the prototype.

Report Generation:

A new wind generation scheme based on a high voltage HG, HVDC interconnection and transmission is proposed in this paper. The new system is compared with a commercial system, Walney 1, chosen as a representative benchmark of existing industry practice. The analysis results show that the proposed system results in 3.43% higher energy conversion efficiency assuming all turbines are operating at, or near, their full-load thermal rating. Results for the proposed wind generation scheme at other loads have also been assessed but are not included here since the primary aim of this paper is to assess the system envelope rating specification requirements. However, the results at lower loads show similar efficiency gains. The study highlights the potential reduction of system plant within the nacelle and turbine tower, features that will improve the serviceability and maintenance of the wind farm over time.

REFERENCE:

[1] H. Liu, J. Sun, “Voltage Stability and Control of Offshore Wind Farms with AC Collection and HVDC Transmission,” IEEE Journal of Emerging and Selected Topics in Power Electronics, vol.2, no.4, pp.1181-1189, Dec. 2014.

[2] D. Yoon, H. Song, G. Jang, S. Joo, “Smart Operation of HVDC Systems for Large Penetration of Wind Energy Resources,” IEEE Trans. Smart Grid, vol.4, no.1, pp.359-366, March 2013.

[3] R. Feldman, M. Tomasini, E. Amankwah, J.C. Clare, P.W. Wheeler, D.R. Trainer, R.S. Whitehouse, “A Hybrid Modular Multilevel Voltage Source Converter for HVDC Power Transmission,” IEEE Trans. Industry Applications, vol.49, no.4, pp.1577-1588, July-Aug. 2013.

[4] D. Jovcic, N. Strachan, “Offshore wind farm with centralised power conversion and DC interconnection,” IET Generation, Transmission & Distribution, vol.3, no.6, pp.586-595, 2009.

[5] E. Veilleux, P.W. Lehn, “Interconnection of Direct-Drive Wind Turbines Using a Series-Connected DC Grid,” IEEE Trans. Sustainable Energy, vol.5, no.1, pp.139-147, Jan. 2014.