Although HVDC transmission systems have been available since mid-1950s, almost all installations worldwide are point-to-point systems. In the past, the lower reliability and higher costs of power electronic converters, together with complex controls and need for fast telecommunication links, may have prevented the construction of multiterminal DC (MTDC) networks. The introduction of voltage-source converters for transmission purposes has renewed the interest in the development of supergrids for integration of remote renewable sources, such as offshore wind. The main focus of the present work is on the control and operation of MTDC networks for integration of offshore wind energy systems. After a brief introduction, this paper proposes a classification of MTDC networks. The most utilized control structures for VSC-HVDC are presented, since it is currently recognized as the best candidate for the development of supergrids, followed by a discussion of the merits and shortcomings of available DC voltage control methods. Subsequently, a novel control strategy-with distributed slack nodes-is proposed by means of a DC optimal power flow. The distributed voltage control (DVC) strategy is numerically illustrated by loss minimization in an MTDC network. Finally, dynamic simulations are performed to demonstrate the benefits of the DVC strategy.

A Novel Distributed Direct-Voltage Control Strategy for Grid Integration of Offshore Wind Energy Systems Through MTDC Network

Papers published over a 17 year period, on the concept of extending 2-terminal HVdc transmission to multiterminal systems, are reviewed. In classifying the contributions, the design and performance expectations are discussed and the prospects for applications are evaluated. Reference is also made to the implications of dc interrupting switches. A comprehensive bibliography is included.

Multiterminal HVDC Power Systems

Any configuration of multiterminal HVdc system is accommodated in a generalized Newton dc load-flow program. It readily interfaces with any ac load flow program. Pract ical operating requirements have been given particular attention to permit aspects such as converter transformer tap-limits, pole and bipolar dc outages, emergency pole-paralleling and detailed control and scheduling strategies. Examples are provided of typical solutions incorporating realistic dc and ac systems, and accomplished in conjunction with an established modern ac load flow program.

Versatile load flow method for multiterminal HVDC systems

When large amounts of electricity need to be transported for long distances, or when underground or submarine cables are involved, using direct current high-voltage transmission systems is more efficient and cost effective than using traditional high-voltage alternating current transmission. Therefore, the main thesis objective is to study to what extent can multi-terminal dc networks provide an optimal platform to foster the integration of remotely located renewable resources, with particular focus on the integration of offshore wind farms in the North Sea. In this thesis, five main challenges were identified before high-voltage multi-terminal dc networks – which can promote the inclusion of remotely located renewable sources while strengthening the existing ac power system networks – can finally become widespread: system integration, power flow control, dynamic behaviour, stability and fault behaviour. These challenges are investigated through a comprehensive literature review, a series of detailed simulation models, and an experimental laboratory setup of a three-node multi-terminal dc network. A novel strategy to control the power flow in multi-terminal dc networks – called Distributed Voltage Control – was developed in this thesis. In the experimental setup, three voltage-source converters were successfully operated in a parallel-radial multi-terminal dc network with a symmetric monopolar configuration. Lastly, a real-time digital simulator was used to emulate the behaviour of an offshore wind farm. Real measurements from the Dutch offshore wind farm Egmond aan Zee were used to validate the Distributed Voltage Control strategy, which successfully controlled the power flow inside the three-terminal low-voltage dc network with high overall precision, while providing the complete system with a fast dynamic response.

Multi-Terminal DC Networks: System Integration, Dynamics and Control

This paper describes an innovative HVDC tap design based on series connections of current-source capacitor-commutated thyristor modules. Besides immunity from commutation failure and autonomous frequency control, the tap has independent active and reactive power control. There is potential of cost reduction because the number of thyristors is reduced by one-third. Switching loss is minimum as the modules switch at 2 pulses per ac cycle. Harmonic elimination by “time shifting and zig-zag transformers” achieves a current total harmonic distortion of about 4%.

Tap for Classical HVDC Based on Multilevel Current-Source Inverters

The feasibility of a series connection of stations in a multiterminal HVDC transmission system is demonstrated. Techniques were developed to control this system, and the operation of the system was studied on a large power system simulator. The performance of the system was shown to be similar to conventional point to point systems with most control functions being carried out by local controls. An overall system control is shown to be necessary only for optimizing load distributions. Given adequate tap changer ranges on the converter transformers the increase in valve damping and system reactive power requirements are shown to be moderate. The series connected multitermimal system is shown to have a number of advantages over the alternative parallel connected system for certain applications.

R.L. Vaughan

Papers

The control and performance of a series connected multiterminal HVDC transmission system