Design of power converters for HVDC applications

Introduction to HVDC

Historically the main use of High Voltage Direct Current (HVDC) systems has been the bulk transmission of power over long distances and inter-connecting neighbouring asynchronous AC systems. Recently new applications have been found, most prominently the interconnection of large offshore wind-farms to onshore grids.

HVDC stations are significantly more expensive that the AC equivalent transformer. However as the transmission distance increases, HVDC becomes the more economic choice over AC due to its lower incremental cost with distance. This lower incremental cost is due to the lower losses within a DC transmission line or cable, a reduced conductor size requirement, as well as reduced right of way requirements in transmission corridors.

This is illustrated in Figure 1.

Power converters, capable of processing power at a Gigawatt scale with operating voltages in the several hundreds of kilovolts, are the key technology which enables HVDC systems.

The converters themselves are physically large, complex devices, containing thousands of power electronic devices, control equipment, filtering elements, cooling systems and ancillary equipment.

A converter has additional losses, lower reliability and reduced availability in comparison to the AC transformers. HVDC does however have several key benefits over AC transmission which can make it the correct techno-economic choice in certain applications.

Key benefits over AC transmission:

1. The transmission distances possible are not limited by the reactive power requirement of the conductor system, or by power angle stability limitations.
2. It can connect asynchronous AC areas together.
3. Power flow through the converter station can be directly controlled.
4. It has a higher power density per conductor cross section vs. AC systems, and improved utilisation of a line or cable’s voltage insulation design. This results in reduced right of way and reduced conductor requirement for a given power rating.

Modular HVDC voltage source converters

This thesis investigates the design of modular voltage source converters for High Voltage Direct Current (HVDC) applications. The first half of the thesis focuses on the design of existing multilevel HVDC technology. A design methodology for sizing modular converters for a given grid code specification, and with given design constraints in terms of peak sub-module voltage rating and capacitor size, is developed and used as the basis of comparing converter designs.

The design of the Hybrid MMC, which combines half- and full-bridge sub-modules in the design in order to achieve DC fault tolerance, is then investigated using the same design methodology, an advantage of which is that the optimum modulation index can be determined, rather than assumed.

Results show that the highest efficiencies may be achievable if the converter is operated at a modulation index of 1.2.

The power-loss and thermal properties of several converters are then analysed. The Alternate Arm Converter (AAC) and over-modulating Hybrid MMC show the greatest efficiencies, though the AAC suffers from relatively high junction temperatures within its director switches.

The potential of designing overload capability into MMCs, to enable them to provide system support services such as frequency response is then investigated.

The second half of this thesis focuses on a brand new multilevel thyristor-augmented structure called a power-group, which has the potential to allow voltage source converters that are tolerant to faults on both the AC and DC network to be constructed, while having efficiencies similar to those achievable with Current Source Converter (CSC) technology.

Results show that this is possible while also retaining high quality current waveforms and independent control of real and reactive power.

Results throughout the thesis are backed up by a combination of simulation and experimental work using a lab-scale multilevel converter that was constructed during the project.