Comparison of two MMC-based HVDC Links with dierent Energy Control Strategies

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Advantages of VSC-HVDC over LCC-HVDC

The VSC-HVDC technology has several advantages over LCC-HVDC, the main dierence being the higher controllability of the former. Here is a detailed list of potential advantages [14]:
ˆ Simpler circuitry.
ˆ Considerably smaller footprint (i.e. occupying less space) than an equivalent LCC-HVDC station.
ˆ No need of reactive power support via shunt banks or series capacitors, since the PWM allows the VSC to control both active and reactive power independently.
The reactive power can even be used for voltage control for weak or islanded AC systems;
ˆ Need of only harmonic lters, which can be non switchable.
ˆ A VSC transmission has no minimum DC current limits.
ˆ Easier power reversal: in an LCC, the current cannot be inverted through the thyristors, thus an active power transfer reversal supposes to shut-down the whole system and start it up again but with opposite voltage polarity.
ˆ The voltage polarity on the DC side is always the same. DC cables are always exposed to the same voltage polarity, which makes the VSC technology more suitable for meshed MTDC systems.
ˆ Possibility of black start for weak, islanded or passive AC systems.
ˆ No risks of commutation failures, hence more suitable for oshore wind-farm connection: with LCC, commutation failures can happen when a valve cannot be extinguished (i.e. the owing current is not returning to zero); e.g. case of an AC grid weakened by a short circuit.
ˆ Possibility of operating the VSC for a very small short-circuit ratio of the connected AC grid. The least SCR which has been practically experienced by the end of year 2004 is 1.3 [31].
However, similarly to the LCC, the standard two-level VSC oers crude basic AC waveforms that often need to be ltered. More importantly, a VSC station also suers from increased losses by comparison to its corresponding LCC station (primarily due to higher switching frequency, and secondly because of the increased number of semiconductor switches in a VSC than an LCC valve of the same rating), hence the lack of high rated power VSC-based HVDC transmissions. Additionally, DC line faults require opening of the VSC AC circuit breakers at both ends of a scheme in order to clear the DC fault, unless appropriate DC breakers are provided in the scheme.

MMC-HVDC Technology

Over the last 15 years, VSC-based HVDC has become a mature technology for HVDC transmission schemes [47]. The Modular Multilevel Converter (MMC) represents the recent development among the diverse available topologies of VSC and is allegedly the most promising solution today [69]. In fact, the MMC topology oers signicant benets compared to the traditional two-level VSC, such as lower losses, distributed storage of capacitive energy, improved scalability to higher voltage ratings, a modular design, low total harmonic distortion and, hence, the potential lack of passive lters on the AC-side of the converter [90].

Components of MMC-based HVDC transmissions

An MMC-based HVDC system consists of converters with thousands of cascaded bidirectional chopper-cells and their respective oating DC capacitor [58], transformers, phase reactors and transmission lines. In opposition to LCC-HVDC or even 2-level VSC-HVDC, there are neither AC nor DC lters.
The thousands of IGBTs are distributed within three legs. Each leg consists of an upper and a lower arm, and each arm consists of an inductor Larm, which limits arm-current harmonics and fault currents, and N SMs. Finally, each SM consists of a half-bridge unit that includes two IGBTs with anti-parallel diodes (switches S1 and S2) and a oating DC capacitor C (see Figure 2.12). The switching of these IGBTs allows a selective connection and disconnection of the oating capacitors to the AC grid. Thus, the N SMs per arm can produce a lineto- neutral voltage waveform of N+1 levels. Figure 2.13 shows the voltage waveform of a 9-level MMC (i.e. 8 SMs per arm) where the red curve corresponds to the reference voltage and the blue curve corresponds to the actual output AC voltage of the MMC.A higher number of the voltage waveform levels means less harmonics on the AC side. The number of levels of an MMC depends on the manufacturer, and they generally attain several hundreds.

Modelling and Control of a VSC-HVDC Station

A large number of publications deal with the modelling and control of standard 2-level VSCs. On one hand, the cascaded control of the converters is usually preferred and the inner control (the current control loop) is generally the same. On the other hand, the outer control can greatly dier from one publication to another, depending on the situation of the studied systems. While some of them correspond to very specic circumstances, such as using the VSC to support the main grid frequency [59] or to support weak AC systems [118], many are correlated with the control strategy of MTDC systems [38, 20, 54, 25, 43] that will be detailed in the next chapter.
The VSC model derived in this section is an AVM and its control is similar to the one detailed in [117]: it is realised in the dq0 reference frame, implying an inner current controller that decouples the grid currents between them, and outer controllers that ensure the proper tracking of the references. The obtained model is standardised and can be used for most simulations involving a 2-level VSC.

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Table of contents :

List of gures
List of tables
1 Introduction 
1.1 Electrication History – The War of Currents
1.2 Today’s Incentives for HVDC Systems
1.3 Motivations behind Multi-Terminal HVDC Systems – Example of the North Sea
1.4 Objective of the Thesis and Main Contributions
1.5 Outline of the Thesis
1.6 List of Publications Derived from This Work
2 HVDC Systems State of the Art 
2.1 Chapter Introduction
2.2 LCC-HVDC Technology
2.2.1 Components of LCC-HVDC
2.2.2 LCC-HVDC Congurations
2.3 VSC-HVDC Technology
2.3.1 Components of VSC-HVDC
2.3.2 Conguration and Operation of VSC-HVDC
2.3.3 Advantages of VSC-HVDC over LCC-HVDC
2.4 MMC-HVDC Technology
2.4.1 Components of MMC-based HVDC transmissions
2.4.2 Advantages of the MMC
2.5 Chapter Conclusion
3 Modelling and Control of VSC-based Converter Stations 
3.1 Chapter Introduction
3.2 Modelling and Control of a VSC-HVDC Station
3.2.1 Modelling of the VSC
3.2.2 Control of the VSC
3.3 Modelling and Control of an MMC-HVDC Station
3.3.1 Modelling of the MMC
3.3.2 Control of the MMC
3.4 Chapter Conclusion
4 State-Space Representation and Modal Analysis of an HVDC Link 
4.1 Chapter Introduction
4.2 State-Space Representation of the VSC Model
4.2.1 Linearisation of the VSC Model
4.2.2 State-Space Representation of the Linear VSC Model
4.2.3 Validation of the VSC Models
4.3 State-Space Representation of the MMC Model
4.3.1 Linearisation of the simplied MMC Model
4.3.2 State-Space Representation of the Linear MMC Model
4.3.3 Validation of the MMC Models
4.4 Modal Analyses of an HVDC Link
4.4.1 HVDC link with VSCs
4.4.2 HVDC link with MMCs
4.4.3 Comparison of the HVDC Systems
4.4.4 Comparison of two MMC-based HVDC Links with dierent Energy Control Strategies
4.5 Chapter Conclusion
5 SVD Analysis of a 5-Terminal MMC-Based MTDC System 
5.1 Chapter Introduction
5.2 State-Space Representation of the MTDC System
5.2.1 Control Strategy of an MTDC System
5.2.2 State-Space Representation of the MTDC System
5.2.3 Modal Analysis of the 5-Terminal MTDC System
5.3 Singular Value Decomposition Analysis of the 5-Terminal MTDC System
5.3.1 Motivation Behind the SVD Tool
5.3.2 Design of the Voltage-Droop Gain using the SVD Tool
5.4 Impact of the DC Cable Model on the SVD Results
5.4.1 Wideband Model Reference
5.4.2 DC Cable Models Used in this Study
5.4.3 Comparison of the SVD Results for the 5 DC grid Models
5.5 Chapter Conclusion
6 A Frequency Droop Technique for the MTDC to Support AC Grid Frequency 
6.1 Chapter Introduction
6.2 MTDC Systems to Support the AC grids Frequency Regulation
6.2.1 Proposition for a Frequency-Droop Controller
6.2.2 Voltage Droop and Frequency Droop: a Dual Controller
6.2.3 Interactions between Voltage and Frequency Droop
6.2.4 Correction of the Frequency-Droop Parameter
6.2.5 Impact of the Droop Controller Limits
6.3 EMT Simulations
6.3.1 Considered System
6.3.2 Eect of an AC Frequency Variation
6.3.3 Eect of an AC Frequency Variation when a Controller Reaches its Limit
6.4 Experimental Tests
6.4.1 Presentation of the L2EP Lille 5-Terminal Mock-up
6.4.2 Experimental Conditions
6.4.3 Experimental Results
6.5 Chapter Conclusion
7 Conclusions and Future Perspectives 
7.1 Conclusions
7.2 Future Work
Bibliography
Appendix A Parameters or the HVDC Transmission Systems Used Throughout this Thesis 
A.1 HVDC Link with VSCs
A.2 HVDC Link with MMCs
A.3 MTDC with VSCs
A.4 MTDC with MMCs
Appendix B Direct-Quadrature-Zero (or dq0) Transformation 
Appendix C Controller Tuning 
C.1 Inner Current Loops Controller Tuning
C.2 DC Voltage Loop Controller Tuning
Appendix D State-Space Association Routine 
D.1 State-Space Association Theoretical Principle
D.2 Routine Methodology for State-Space Interconnection

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