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Large offshore wind farms will require an extensive sub-sea power network to provide internal interconnection. Present solutions are based around conventional medium-voltage AC architectures. This paper proposes an alternative DC collection network based around modular DC/DC converters with input-parallel-output-series (IPOS) connection. Small-signal analysis of the converter is presented, to assist...
Circuit breakers for High Voltage Direct Current (HVDC) are seen as a required technological step in the development of HVDC grids. The development of several new HVDC circuit breaker prototypes indicate that this step may be achieved in the years to come. In order to facilitate the installation of HVDC circuit breakers, appropriate standards and tests need to be developed. These tests and standards...
One of the most challenging aspects regarding realization of meshed HVDC systems are DC line faults. On the one hand new fault clearing equipment needs to be developed. On the other hand it is very important to define suitable fault detection methods. This is necessary since proven methods for AC grids cannot be applied in DC grids since they are too slow to cope with DC fault current phenomena. To...
This paper deals with the technical feasibility of low frequency alternating current (LFAC) for offshore wind power transmission. As is well-known, the charging current presents itself as the main limitation for ac power transmission using 50/60Hz. By using LFAC, the reach of ac transmission can be extended and the investment cost that would have been necessary for VSC HVDC based transmission reduced...
This paper compares the DC breaker requirements for different DC breaker protection schemes for high voltage direct current (HVDC) grids with half bridge converters. DC breakers are essential for DC grid protection as they can provide a selective and fast protection. The DC breakers experience challenging operation conditions which results in an increased cost to meet the breaker requirements. The...
High Voltage Direct Current (HVDC) transmission has been expanding due to rapid development of power electronics technology and by the need for connection of offshore/remote wind farms and large hydro power generators. An HVDC grid will be required to operate the healthy lines continuously, even if a voltage collapse occurs at the remote end. Rapid fault clearing is essential for DC Circuit Breaker...
HVDC and HVAC systems at the same support structure are an important issue to increase the transmission capacity of a corridor. But due to the near vicinity of the different systems influences can occur. This paper describes the induced voltages on HVDC systems. These will be determined with a decoupled method for inductive and capacitive coupling of different tower structures to get a first impression...
This paper evaluates the use of a back-to-back modular multilevel converter (B2B-MMC) in offshore low frequency (LF) AC transmission. The scheme is compared to two alternative transmission technologies: DC and 50Hz AC. The MMC topology is further compared to a cycloconverter. A high level design of a ±120kVdc, 250MW B2B-MMC system is described, with the total volume and weight of submodule capacitors...
A new 525 kV DC cable system with a power rating range up to powers above 2 GW has been developed for both subsea and underground applications. The extruded HVDC cable system technology is appropriate when power needs to be efficiently delivered through populated or environmentally sensitive areas, or in coastal and open-sea applications. HVDC cable links are essential components of sustainable energy...
This publication presents the results of a series of laboratory investigations on the DC ion current coupling between nearby AC and DC conductors, which is an important consideration in the design of hybrid AC/DC transmission towers. The corona activity depends on the surface condition of the conductor. Whether or not the conductors produced corona was controlled by selectively wetting them. This...
Nowadays, development of multi-terminal HVDC systems is driven by aim to connect remote loads, offshore wind power plants and increase of energy trading. Operation of meshed multi-terminal systems is hard to imagine without fault clearing devices. The aim of this paper is to investigate operation of a hybrid circuit breaker under simulated system fault conditions. A simulation of the HVDC systems...
For its notable advantages, the modular multilevel converter (MMC) has become a hot topic in high-voltage direct current (HVDC) research and construction. Based on the existing MMC configuration and double-loop control strategy, there's a coupling between AC/DC voltage-offset, which brings a series of problems in different engineering background. This paper proposes solutions to decouple this coupling...
The impact of technology and topology selection on the success of a HVDC interconnector project is significant. Rapid technology advances in power electronics, especially Voltage Source Converters (VSC), can offer new opportunities for the commercial success of projects. However, new technology can also bring risk to the end user if it is unproven. It is critical that engineers involved in the research,...
In order to predict the stability of cable-based offshore Voltage Source Converter High Voltage Direct Current (VSC HVDC) systems, an accurate cable model is needed to represent the characteristics of the transmission system. This paper presents a model consisting of cascaded pi-sections with multiple longitudinal parallel branches, allowing for a detailed representation in the frequency domain. Furthermore,...
The past decade has seen VSC-HVDC systems deployed to establish transmission schemes both onshore and offshore, often for interconnectors utilizing underground or subsea cables but also a few systems designed to operate on long overhead transmission lines (OHTLs) [1]. In specific literature, it is referred to the capability of VSC schemes being able to feed power into passive AC grids, hence with...
MVDC is starting to be considered as an option for enhancing transfer capacity and providing increased power quality at distribution networks. There is a term starting to be used of “soft open-point” which can provide controlled power transfer between two 11kV or 33kV distribution groups, without affecting short-circuit levels, voltage differences, loop flows or limitations due to phase-angle differences...
With the increased levels of offshore wind power penetration into power systems, the impact of offshore wind power on stability of power systems require more investigation. In this paper, the effects of a large scale doubly fed induction generator (DFIG) based offshore wind farm (OWF) on power system stability are examined. The OWF is connected to the main onshore grid through a voltage source converter...
The main objective of this paper is to evaluate alternative DC power flow control strategies applied to Multi-Terminal HVDC (MTDC) grids comprising island systems with high Renewable Energy Source (RES) penetration. A statistical assessment is first performed by calculating the expected steady-state DC voltage and power flow variations of the MTDC grid, induced by stochastic variations of the imported...
Several processes have been initiated for the development of technical concepts, guidelines and common requirements of HVDC Grid Systems. For implementing extended HVDC Grid Systems on a multi-vendor basis consolidated and agreed functional requirements for equipment and solutions are needed. Providing such requirements is the role of international standardisation bodies. With the primary focus on...
Hybrid HVDC combines the advantages of line commutated converter (LCC) and modular multilevel converter (MMC). In this paper the topology and basic control strategy of hybrid HVDC are described. Then the power reversal of hybrid HVDC is analysed. For LCC-HMMC (MMC adopting half bridge sub-module), the sequence of power reversal after stopping operation is proposed while for LCC-FMMC (MMC adopting...
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