An Examination of AC/HVDC Power Circuits for Interconnecting Bulk Wind Generation with the Electric Grid

[tweetmeme Daniel Ludois and Giri Venkataramanan of the Department of Electrical and Computer Engineering, University of Wisconsin – Madison have issued an article titled „An Examination of AC/HVDC Power Circuits for Interconnecting Bulk Wind Generation with the Electric Grid.“  The application of high voltage dc (HVDC) transmission for integrating large scale and/or off-shore wind generation systems with the electric grid is attractive in comparison to extra high voltage (EHV) ac transmission due to a variety of reasons. While the technology of classical current sourced converters (CSC) using thyristors is well established for realization of large HVDC systems, the technology of voltage sourced converters (VSC) is emerging to be an alternative approach, particularly suitable for multi-terminal interconnections. More recently, a more modular scheme that may be termed ‘bridge of bridge’ converters (BoBC) has been introduced to realize HVDC systems. While all these three approaches are functionally capable of realizing HVDC systems, the converter power circuit design trade-offs between these alternatives are not readily apparent. This paper presents an examination of these topologies from the point of view of power semiconductor requirements, reactive component requirements, operating losses, fault tolerance, multi-terminal operation, modularity, complexity, etc. Detailed analytical models will be used along with a benchmark application to develop a comparative evaluation of the alternatives that maybe used by wind energy/bulk transmission developers for performing engineering trade-off studies.

The introduction to this good article reads:

Increasing share of wind energy systems has prompted a concomitant attention to their integration into major electrical transmission systems, i.e., the grid. Particularly, the promise for large scale generation in offshore and remote locations due to the meteorological consistency of the wind in such locations make bulk power transmission from generation centers to load locations a critical aspect of the emerging future. Despite this promise, the issue of wind turbine interconnection and bulk power transmission to the existing distribution networks has not yet been solved with certainty. High voltage direct current (HVDC) systems have been proposed by numerous authors [1–10] as a solution for integrating renewable and existing sources of energy together in configuration similar to Figure 1. Here, several power converters are used to interface multiple generation sources and load locations in a multi-terminal HVDC interconnection with power converters that may be integrated with each turbine (Cluster 1), or integrated with a group of turbines (Cluster 2). The subject of this paper is an examination of alternative power converter topologies that may be applied in the HVDC interconnection in a centralized configuration as in Cluster 2, although the results may be extended to their application in a distributed configuration as in Cluster 1.

Figure 1. A simplified single line diagram of a sample HVDC interconnection of wind generation sources and ac grids.

Although the HVDC power converters may generally be used in either sending or receiving mode, this paper will concentrate on sending power from a generation source using one of the two canonical power converter topologies, current sourced converters (CSCs) or voltage sourced converters (VSCs). In general VSC technologies appear to be favored against the CSC to realize future HVDC installations for a variety of reasons. But a more critical analysis is necessary to establish this generalization in a definitive manner. Additionally, a new HVDC power conversion approach has recently emerged which can be potentially transformerless and utilizes a modular multi-level converter (MMLC). This converter belongs to the aptly named “bridge of bridge” converter (BoBC) family, and holds promise to be a competitive solution in the future of HVDC [11–13]. The performance trade-offs between the three types of converters have not been definitively presented in the literature, particularly in view of the application to bulk power transmission in regard to utility integration of wind power.

To be sure, a comparative evaluation of particular solutions for a given application may be made on the basis of several features. Salient power circuit features include: harmonics of waveforms, operating losses, ratings of power converters, reactive component requirements, transformer kVA requirements, and complexity of control. Given the degree of variability based on the application a definitive evaluation appears to be a formidable task. Therefore, in order to maintain a focus in the evaluation, a particular benchmark application is considered in this paper. Furthermore, the evaluation is limited to solutions that feature superior waveform quality arising from high frequency or high pulse number switching with nearly sinusoidal line current waveforms.

A focused analytical modeling and design study of a candidate application using the different approaches is performed in order to evaluate their performance. The comparison criteria used for the evaluation include voltage, current and power throughput ratings of the main power circuit components (including transformers, capacitors, and semiconductors), quality of terminal voltage and current waveforms in terms of harmonics, and losses in power semiconductors. Although the trade-offs of complete systems using these alternative approaches may be a complex function of market trends, economic factors and engineering development, and would change considerably with respect to time and location, a preliminary estimate of these metrics together provide a basis for making a first order trade-off among these approaches.

In today’s state of the art, doubly fed induction generators operating in the low voltage regime (480/690 V) are most commonly used to realize wind turbine installations. As turbine power levels steadily increase into the 5 MW+ levels, low voltage machine designs become impractical from an efficiency perspective [14]. Following this trend, wind turbine manufacturers may be expected to migrate to medium voltage generators that may be tied to the electric grid via a single power converter. The focus of this paper is to call attention to the properties of the CSC, VSC, and BoBC and compare them in a benchmark application in following this trend.

A brief background discussion of each converter is provided in Section 2 and a detailed comparison including a benchmark design follows in Section 3. Section 4 provides a summary of the conclusions.

Read the full article here in PDF format.

Citation: Ludois, Daniel; Venkataramanan, Giri. 2010. “An Examination of AC/HVDC Power Circuits for Interconnecting Bulk Wind Generation with the Electric Grid.” Energies 3, no. 6: 1263-1289.

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  1. […] This post was mentioned on Twitter by txtgreen, Professor Paul. Professor Paul said: Interconnecting Bulk #Wind Generation with the Electric Grid #Energy #Sustainability #Green […]

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