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Context and state of the arts for HVDC DC/DC converters
Background
According to the International Energy Agency (IEA) statement, global energy demand grew by 2.1% in 2017, which is more than twice the growth rate in 2016. It reached an estimated 14 gigatonnes of oil equivalent energy (Mtoe), in which fossil fuels are the principal growth (70%) [1], [2]. At the same time, IEA states also that global energy related CO2 emission grew by 1.4% in 2017 than 2016, reaching a historical high of 32.5 gigatonnes, which contrasts with the demand of Paris Agreement established in December 2015.
However, it is hopeful to see that emissions dropped in some countries in 2017, such as United States, United Kingdom, Mexico and Japan, thanks to coal-to-gas switching, higher renewable based electricity generation and nuclear generation [1]. Moreover, China, which is the “first” country for carbon emissions, has increased its emission increased just by 1% in 2017 than their 2014 level, thanks to a continued renewable deployment and a coal-to-gas switching [1].
These reports confirm positively that renewable energy generation is possible and efficient to reduce carbon emissions. But there are still a lot of works and efforts to do to cope with climate change and meet the Paris Agreement.
The renewable energy mentioned above includes solar/hydro/biomass/wind/ocean/ geothermal energy [2]. The thesis focuses on High Voltage Direct Current (HVDC) transmission, which is the technology trend to integrate renewable energy in power systems.
AC transmission versus DC transmission
Alternative Current (AC) transmission (Fig 1) is the most used transmission technique. The three-phase alternative current allows the direct use of electrical machine as power generator and transformers ensuring voltage adaptation in a low losses and low cost way.
However, the limitation of AC transmission is linked to the capacitance value of long cable in great power transmissions [3] creating reactive power, which greatly increases cable currents and definitely limits power transmission. It can be solved by two methods explained in [3]: using shunt inductive reactors along the cable line to compensate reactive power or using DC transmission instead of AC transmission.
Fig 2 shows AC line performance with and without compensation. Transmission distance decreases when delivered power and line voltage increase. To transmit more power in high voltage, inductive compensation is mandatory to extend the transmission distance. However with reactors, a higher cost is necessary. This solution is also ineffective for undersea transmissions where numerous offshore substations are needed.
Thereby, DC transmission (Fig 3) is a solution to deliver the high power from remote generator plants without reactive power compensators.
Some advantages of DC transmission over AC are described as follows:
• Low costs for long distance:
DC transmission shows lower costs over the critical distances (Fig 4), according to power and voltage (approx underground: 600-800km, undersea: 50km), than AC even though DC terminal stations are more expensive [5]. It is linked to the huge cost of compensators increasing according to the line distance.
• Interconnection of asynchronous grids:
If two AC grids need to be connected, their frequency, voltage and phase must be identical. However, the interconnection can be realized by DC transmission thanks to AC/DC and DC/AC converters [6].
• No skin effect:
Skin effect exists in AC conductors where current frequency is high. DC transmission avoid the skin effect using direct current which reduces conductor losses compared to AC transmission.
These advantages make DC transmission the preferred solution for long distance, high voltage and great power transmission. However, several disadvantages should be also taken into account to minimize costs and losses:
• Expensive:
Fig 4 shows DC transmission economic benefits for long distance. However, it is also true that DC/AC and AC/DC converter stations are more expensive than AC substations. Therefore, converter design criteria has small footprint and low cost.
• Harmonics:
Power converters are sources of harmonics [7]. These harmonics spread to DC and AC grids, impacting power quality. To reduce high-frequency harmonics, filters are needed to improve power quality which increases costs and weight, especially for high voltage and high power applications. Therefore, efficient high power converters are necessary.
• DC short circuit protection:
Unlike AC grids, DC grids are more vulnerable for short circuit at DC sides. Then, most of DC grids are protected actually from the AC side by AC breaker or protected by control algorithms. A research trend is now to develop DC circuit breakers demonstrators to protect directly DC grid at DC side.
Despite the disadvantages described above, a hundred of DC lines ( around 139 projects reported by ABB and Siemens in 2019 [8]-[11] ) have been commissioned.
Existing and future installation equipment for HVDC grids
Existing installation equipment
A DC link is created by the connection of two AC/DC converter stations to AC generators transmitting power in DC cables (Fig 5). Regarding the converter stations, two main types of stations exist: Line Commutated Current Source Converter (LCC or CSC) and Voltage Source Converter (VSC).
The LCC is the converter topology after mercury-arc valve industrially confirmed to be suitable for high voltage and power application thanks to high blocking voltage of Thyristor [12].
Table of contents :
Introduction
I Context and state of the arts for HVDC DC/DC converters
I.1 Background
I.1.1 AC transmission versus DC transmission
I.1.2 Existing and future installation equipment for HVDC grids
I.1.3 Multi-Terminal DC transmission
I.1.4 CIGRE test case analysis
I.1.5 Problem statement
I.2 State of the arts for DC/DC converters
I.2.1 Classical DC converters
I.2.2 Modular DC converters
I.3 Conclusion
I.4 References
II DC-DC Modular Multilevel Converter (M2DC)
II.1 Introduction
II.2 Topology of the DC-DC Modular Multilevel Converter
II.3 Submodule topologies
II.3.1 Half Bridge Submodule
II.3.2 Full-Bridge Submodule
II.4 Average model of a string of submodules
II.4.1 Voltage relations for a string of submodules
II.4.2 Current relations of a submodule string
II.4.3 Energy relations of a string of submodules
II.5 Steady state analysis
II.5.1 Static analysis of an M2DC leg
II.5.2 DC buses currents
II.5.3 Operating principle
II.6 Model of M2DC Converter and parameters design
II.6.1 DC and AC behavior modeling
II.6.2 Degrees of freedom
II.7 AC voltage components analysis
II.7.1 Choices of angle 𝜃 and amplitudes of AC voltages
II.7.2 Limits of operation
II.8 AC currents analysis with 𝑳𝒔≫𝒍
II.8.1 Minimized AC currents
II.8.2 Analysis of arm currents
II.9 Arm inductance design and Capacitance design
II.9.1 Arm inductance design
II.9.2 Capacitance design
II.10 Conclusion
II.11 References
III Converter control strategy and dynamics
III.1 Introduction
III.2 Control architecture
III.3 Decoupling average current model
III.3.1 Decoupled average model
III.3.2 Steady state analysis of variables
III.4 Current loop design
III.4.1 Continuous time transfer functions
III.4.2 PI controller and closed current loop
III.4.3 Static and dynamic analysis of the closed current loop
III.5 Energy model
III.5.1 Energy definition
III.5.2 Energy model
III.5.3 Steady state analysis of energy stability
III.6 Energy loop design
III.6.1 Equivalent Low pass filter
III.6.2 Continuous time transfer function and closed energy loop
III.6.3 Static and dynamic analysis of the energy loop
III.7 Validation of AC components using average model
III.7.1 Validation of frames of reference
III.7.2 Validation of limitations of AC components
III.7.3 M2DC global simulation results of CIGRE test case
III.8 Conclusion
III.9 References
IV Implementation: Laboratory-based Real-Time simulation and Control Hardware-In-the-Loop (HIL) simulation
IV.1 Introduction
IV.2 Description of test scenarios
IV.2.1 Selection of secondary inductor and DC voltages
IV.2.2 AC voltage limitation
IV.2.3 DC current leg limitation
IV.2.4 AC frequency selection
IV.2.5 Selection of transmission power
IV.3 Control development methodology: definitions and simulation development
IV.3.1 Real-time simulation definition
IV.3.2 HIL simulation definition
IV.3.3 Simulations development methodology
IV.4 Instantaneous model of an arm
IV.5 Low-level control
IV.6 Description of simulation devices
IV.6.1 Description of real-time simulation devices
IV.6.2 Presentation of HIL setup
IV.7 Comparison and validation
IV.7.1 Simulation results in the steady state
IV.7.2 Dynamic state: case 1
IV.8 Conclusion
IV.9 References
V General conclusion and perspectives
