1. Introduction
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
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.
2. HVDC Converter Topologies, a Brief Review
This section introduces each of the three converter topologies in consideration and provides a background overview on their operation. Common topologies for each converter as well as operating characteristics are provided. These characteristics are explored further and compared in Section 3. While the review here is brief, and focuses on the salient features from the view of a comparative evaluation, a more detailed discussion on functioning installations of these representative technologies may be found elsewhere [12]. Notably, for a more comprehensive discussion on CSCs in wind applications, the readers may be refer to recent works [15–21]. Similarly, a detailed discussion on the operational features of VSCs in wind generation applications may be found in [8,20–28], while a description and operational features of the BoBC/MMLCs may be found in [13,29–35].
2.1. Current Sourced Converter (CSC)
Since its inception in the 1950’s the current sourced converter (CSC) has been the workhorse of HVDC transmission systems. Despite a gradual evolution of valve designs and harmonic suppression techniques, the conversion process has remained unchanged. Generation voltage is increased with a step-up transformer operating at the power frequency and rectified to feed a current stiff dc bus as shown in Figure 2(a), consisting of 6-pulse CSCs. In order to maintain the comparative evaluation to be of reasonable complexity while preserving the essential structural elements, the CSC illustration shown in Figure 2(a), along with sub-module realization Figure 2(b) is considered in this study. The results may be suitably modified to study alternative realizations if desired.
To improve system harmonics CSCs typically use transformers with multiple secondary windings phase shifted from one another to drive independent thyristor bridges. A series or parallel connection of the 6-pulse thyristor bridges results in a higher pulse frequency converter for enhanced performance.
The switching device of each of the six arms of the rectifier bridge is made up of N sub-modules connected in series to obtain the desired voltage blocking rating. Each sub-module contains a single semiconductor switch, usually a thyristor or an SCR. The rectified output is filtered using an inductor, which may be a discrete component or the transmission line’s inherent inductance, which gives the converter its “current stiff” property. Current stiffness combined with phase controlled rectification make the CSC robust against HVDC line faults.
Although the CSC is capable of bidirectional power flow, this requires a voltage reversal at the dc terminals when the bridges are realized with thyristors that conduct current in one direction. In practice, bi-directional power flow for CSCs may be achieved by advancing the firing angle to reverse polarity of the output voltage while maintaining current direction thus reversing power flow. However, one should exercise caution in relying on this technique in multi-terminal networks, a simple voltage reversal of the dc terminals alone may not fulfill all the requirements of power flow management across each of the terminals.
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
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.
2. HVDC Converter Topologies, a Brief Review
This section introduces each of the three converter topologies in consideration and provides a background overview on their operation. Common topologies for each converter as well as operating characteristics are provided. These characteristics are explored further and compared in Section 3. While the review here is brief, and focuses on the salient features from the view of a comparative evaluation, a more detailed discussion on functioning installations of these representative technologies may be found elsewhere [12]. Notably, for a more comprehensive discussion on CSCs in wind applications, the readers may be refer to recent works [15–21]. Similarly, a detailed discussion on the operational features of VSCs in wind generation applications may be found in [8,20–28], while a description and operational features of the BoBC/MMLCs may be found in [13,29–35].
2.1. Current Sourced Converter (CSC)
Since its inception in the 1950’s the current sourced converter (CSC) has been the workhorse of HVDC transmission systems. Despite a gradual evolution of valve designs and harmonic suppression techniques, the conversion process has remained unchanged. Generation voltage is increased with a step-up transformer operating at the power frequency and rectified to feed a current stiff dc bus as shown in Figure 2(a), consisting of 6-pulse CSCs. In order to maintain the comparative evaluation to be of reasonable complexity while preserving the essential structural elements, the CSC illustration shown in Figure 2(a), along with sub-module realization Figure 2(b) is considered in this study. The results may be suitably modified to study alternative realizations if desired.
To improve system harmonics CSCs typically use transformers with multiple secondary windings phase shifted from one another to drive independent thyristor bridges. A series or parallel connection of the 6-pulse thyristor bridges results in a higher pulse frequency converter for enhanced performance.
The switching device of each of the six arms of the rectifier bridge is made up of N sub-modules connected in series to obtain the desired voltage blocking rating. Each sub-module contains a single semiconductor switch, usually a thyristor or an SCR. The rectified output is filtered using an inductor, which may be a discrete component or the transmission line’s inherent inductance, which gives the converter its “current stiff” property. Current stiffness combined with phase controlled rectification make the CSC robust against HVDC line faults.
Although the CSC is capable of bidirectional power flow, this requires a voltage reversal at the dc terminals when the bridges are realized with thyristors that conduct current in one direction. In practice, bi-directional power flow for CSCs may be achieved by advancing the firing angle to reverse polarity of the output voltage while maintaining current direction thus reversing power flow. However, one should exercise caution in relying on this technique in multi-terminal networks, a simple voltage reversal of the dc terminals alone may not fulfill all the requirements of power flow management across each of the terminals.
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