Session Chair:
Jef Beerten
University of Leuven (KU Leuven), Belgium and Norwegian University of Technology (NTNU), Norway
Towards HVDC Grids – Challenges for the Future Power System
● Dr. Yousef Pipelzadeh, Imperial College, UK
- The Future of Offshore HVDC Grids in the UK: Big Challenge, Big Opportunity
● Kerstin Lindén, ABB Inc., Sweden
- Power Flow control & DC Voltage Control in HVDC Grids - Results of CIGRE WG B4-58
● Prof. Staffan Norrga, Royal Institute of Technology, Sweden
- Power Electronics for HVDC Grids
● Prof. Dirk Westermann, Technische Universität Ilmenau, Germany
- Operation Methods for Meshed HVDC Grids

The projected strong increased penetration of intermittent renewable energy sources in the transmission system is posing challenges to transmission system operators worldwide. The transmission grid updates required to accommodate this massive amount of renewable energy sources in the system go well beyond standard system reinforcements used in the past: in the coming decades, an HVDC-based North- Sea grid interconnecting various offshore wind farms is expected to be built in Europe. This grid can gradually evolve into a European overlay supergrid connecting the offshore wind resources with the demand centers on the continent. In this session, we will address the latest developments in the DC grids area and discuss the various challenges on the road towards using Voltage Source Converter (VSC) technology in a meshed DC transmission system.

Session Lecture 1:
Power Electronics for HVDC Grids
Prof. Staffan Norrga, Royal Institute of Technology, Sweden

HVDC grids have significant benefits and will likely play an important role in the future energy system. However, protection and fault handling aspects will be different from those of current ac transmission grids and point-to-point HVDC connections. Therefore, new power electronic solutions will be required. Different fault handling strategies are being discussed and the required converter and breaker hardware to implement these is identified. Also different converter topologies with the needed properties, mainly dc-side short-circuit handling capability, are described and compared.

Session Lecture 2:
Power Flow Control & DC Voltage Control in HVDC Grids - Results of CIGRE WG B4-58
Kerstin Lindén, ABB Inc., Sweden

In an HVDC grid, contingencies such as AC network faults, converter trips and uncoordinated load changes give rise to a current imbalance that is reflected in the DC node voltages in the grid. This imbalance has to be corrected for by changing the currents flowing in and out of the HVDC system. The correction needs to be done rapidly, since an HVDC system has a low inherent inertia. This presentation deals with this issue, which is now the subject for Cigré WG B4-58: ”Power flow control and direct voltage control in a meshed HVDC Grid”. The control of load flow and AC voltage control in HVAC systems is well understood. Similar technologies will be needed in HVDC grids, and the WG is looking at different control solutions by describing and testing different HVDC node voltage control strategies for the imbalance correction. Furthermore, the WG is looking at a coordinated system controller, taking into account the desired set-points for power exchange at all AC/DC connections together with the constraints on HVDC node voltages and flows on the different lines. For a better utilization of the HVDC Grid elements, power flow controlling devices, ranging from switchable resistors to DC/DC converters are described and investigated.

Extended abstract can be downloaded here.

Session Lecture 3:
Operation Methods for Meshed HVDC Grids
Prof. Dirk Westermann, Technische Universität Ilmenau, Germany

The optimal operation of meshed HVDC grids is a heavily discussed question. So far the operation of point-to-point connections is state of the art but differs in many aspects from that of meshed HVDC grids. The challenges in operating HVDC grids increase with an increasing size of the network as well as with an increasing number of involved TSOs especially if the HVDC grid is not owed by a single organization.

This contribution gives a short introduction in major challenges regarding the design of operation management methods and gives an excerpt of actual research activities especially regarding:
- local methods for automatic participation of the HVDC grid in unscheduled AC power flow changes
- centralized method for optimal converter operation point calculation using mixed AC/DC optimal power flow
- local method for converter operation point calculation with multiple involved TSOs in the HVDC grid

Additionally analogies between AC and DC systems regarding their stability and security categories are introduced.

Session Lecture 4:
The Future of Offshore HVDC Grids in the UK: Big Challenge, Big Opportunity
Dr. Pipelzadeh, Yousef, Imperial College London, Great Britain

The UK electricity supply network is set to experience a period of major change, as old infrastructure is replaced, and grid connections are made to offshore wind farms and into continental Europe. The introduction of offshore HVDC grids brings with it major challenges, and opportunities that need to be examined in depth.
As an example, with increasing penetration of offshore wind installation, system support features, such as inertial support and frequency regulation usually demanded from synchronous generators will need to be rapidly sourced from wind generators. In fact, such services have been included as a requirement in the generic unified grid code for wind power plants proposed by the EWEA.
This presentation will cover the future of offshore HVDC grids in the UK, the challenges and requirements for HVDC connected offshore wind farms (i.e. DC faults and loss-of-infeed, ancillary services, transients and harmonics issues, etc.) and finally outline case studies demonstrating potential solutions (i.e. DC converters topologies with fault-blocking capability, extracting inertia from HVDC converters and wind turbines, power oscillation damping from remote offshore DC grids etc).