Robert Whitehouse

A Hybrid Modular Multilevel Voltage Source Converter for HVDC Power Transmission

HVDC transmission systems are becoming increasingly popular when compared to conventional ac transmission. HVDC voltage source converters (VSCs) can offer advantages over traditional HVDC current source converter topologies, and as such, it is expected that HVDC VSCs will be further exploited with the growth of HVDC transmission. This paper presents a novel modular multilevel converter hybrid VSC intended for the HVDC market. The concept of the converter operation is described based on steady-state ac-dc power balance. Techniques for dynamic voltage control, enabling the active and reactive powers exchanged with the grid to be controlled, are introduced. Simulation results further illustrate the theory of operation of the converter and confirm the viability of the proposed control approaches. Detailed predictions of the semiconductor losses confirm the potential to achieve very high efficiencies with this topology. Experimental results are provided to validate the presented converter operation.

Experimental validation of autonomous converter control in a HVDC grid

Much research has been done regarding the control and coordination of VSC converters in a Multiterminal HVDC network or grid. Amongst those, one underlying concept is the most common- the DC voltage droop control. In this paper, the control concept has been further developed to use alternative droop characteristics on each converter. This approach allows precise converter current regulation during normal operation while stabilizes DC voltage during power disturbance. Control algorithms of alterative droop characteristics are provided and interactions of different control characteristics are also analyzed. This concept is validated using both digital simulation (PSCAD/EMTDC) and physical modelling of a HVDC grid using a 4-terminal VSC Physical Simulator. Results obtained from the two simulation platforms are compared and show good agreement. The feasibility and advantage of using alternative droop characteristics on each single converter are also validated.

Protecting a HVDC link against accidental isolation from its receiving AC system

When a high-voltage DC (HVDC) scheme is isolated from its receiving AC system, the inverter may continue to operate, generating its own AC bus voltages; this is defined as islanding. If islanding is allowed to continue unrestricted, main circuit components may in some conditions be damaged. It is therefore necessary to provide a suitable protection system. The author describes a protection scheme developed for an HVDC link that prevents damage due to islanding while still permitting the link to automatically restart on reclosure of the isolating breaker. Oscillograms showing the protection in operation on both the GEC ALSTHOM HVDC simulator and during tests carried out as part of the commissioning of the McNeill HVDC link are included. >

Progressive Fault Isolation and Grid Restoration Strategy for MTDC Networks

A multiterminal dc (MTDC) grid has a number of advantages over traditional ac transmission. However, dc protection is still one of the main technical issues holding back the expansion of point-to-point dc links to MTDC networks. Most dc protection strategies are based on dc circuit breakers; however, DCCBs are still under development and their arrival to the market will come at an unclear time and cost. Conversely, ac circuit breakers (ACCBs) are readily available and represent a more economic alternative to protect dc networks. Following this line, a protection strategy for MTDC grids is proposed in this paper. This uses ACCBs for dc fault current clearing and fast dc disconnectors for fault isolation. The faulty link is correctly discriminated and isolated while communication links are not required. This strategy contributes to a reduced network outage period as the nonfaulty links are out of operation for a relatively short period of time and are restored in a progressive manner. The effectiveness of the proposed strategy is tested in PSCAD/EMTDC for pole-to-ground and pole-to-pole faults.

Double modulation control (DMC) for dual full bridge current flow controller (2FB-CFC)

Power flow control in multi-terminal DC grids can be accomplished through the connection in series of a controlled voltage source converter to a line. Having finite energy, the voltage source requires a means for either power recycling or additional source of power to regulate its voltage. An interline dual full bridge current flow controller (CFC) provides the capability to recycle power from one line to another through the voltage source in addition to controlling the power flow on either line. This paper proposes a control methodology that fulfils both control objectives. A double modulation controller (DMC) implements one modulating signal and two control signals to control the duty ratio of the converters simultaneously. The first control signal is proportional to the required reduction in the line current. The second control signal is implemented to regulate the voltage to either a set average value or to a value that is proportional to the amount of reduction in current.