Carl Barker

High-Frequency Operation of a DC/AC/DC System for HVDC Applications

Voltage ratings for HVdc point-to-point connections are not standardized and tend to depend on the latest available cable technology. DC/DC conversion at HV is required for interconnection of such HVdc schemes as well as to interface dc wind farms. Modular multilevel voltage source converters (VSCs), such as the modular multilevel converter (MMC) or the alternate arm converter (AAC), have been shown to incur significantly lower switching losses than previous two- or three-level VSCs. This paper presents a dc/ac/dc system using a transformer coupling two modular multilevel VSCs. In such a system, the capacitors occupy a large fraction of the volume of the cells but a significant reduction in volume can be achieved by raising the ac frequency. Using high frequency can also bring benefits to other passive components such as the transformer but also results in higher switching losses due to the higher number of waveform steps per second. This leads to a tradeoff between volume and losses which has been explored in this study and verified by simulation results with a transistor level model of 30-MW case study. The outcome of the study shows that a frequency of 350 Hz provides a significant improvement in volume but also a penalty in losses compared to 50 Hz.

Energy diverting converter topologies for HVDC transmission systems

Grid codes imposed by utilities regulate the operation of Voltage Source Converter - High Voltage Direct Current (VSC-HVDC) interconnected offshore wind farms. Fault ride-through (FRT) specifications require the adoption of specific measures to avoid over-voltages of the HVDC link during faults in order to protect the HVDC equipment. Implementing Energy Diverting Converters (EDC), for instance Dynamic Braking Resistor (DBR) circuits, at the DC link is an established method to comply with the grid codes, where the excess energy of the wind farm is diverted into the parallel circuit during the fault. In this paper an evaluation of three different state-of-the-art DBR circuits is performed in order to establish the advantages and disadvantages of each circuit. The evaluation has shown that although the three solutions meet the FRT requirements, the modular topologies generate reduced slope current and voltage step changes during their operation, while being larger in size and requiring a higher number of semiconductors as compared to the traditional DC chopper circuit employing hard switched series connected semiconductor arrangements.

Scalable shunt connected HVDC tap using the DC transformer concept

This paper describes the concept of a recently proposed scalable DC/DC converter topology applied to a shunt connected HVDC tap. This solution offers some advantages with respect to previously proposed solutions. Two different configurations of the circuit are proposed for interconnection with different HVDC systems. The internal structure of the modular DC/DC converter is presented and a suitable strategy to control the system is investigated in order to achieve soft-switching of the converter semiconductors. The operation is validated using computer simulations, which include a rough estimation of the DC/DC converter power losses.

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.

Choice of AC operating voltage in HV DC/AC/DC system

Demand for DC/DC conversion in HV applications is expected to rise because of the increasing number of HVDC links using different DC voltage levels. This paper presents a DC/AC/DC system consisting of two VSCs connected through an inductor. The two VSCs are Alternate Arm Converters (AAC). Since the two AACs share the same AC voltage level, they cannot be operated at their respective “sweet-spots” at the same time. This results in an energy drift in the valves which is tackled by additional balancing currents. However, the choice of the AC voltage level remains critical as it determines the required amount of balancing current, the number of devices and the cell topology, influencing greatly the total efficiency and volume of the obtained DC/DC converter. A study on a scale-down converter highlights the trade-offs affecting the AC voltage choice.

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.

Practical Management of Variable and Distributed Resources in Power Grids

Direct current power transmission was at the vanguard of the electrical industry. Whilst in the early part of the twentieth century transmission of electricity by direct current was supplanted by alternating current transmission new technologies are enabling the benefits of direct current. With the changes brought about to the electricity network as a consequence of the integration of renewable sources there will be important benefits to integrating direct current transmission into the electrical network. In this chapter the basic concept of electrical transmission via direct current will be introduced along with modern day developments.