Conference Paper

Conversion of Existing AC into DC Cable Links in Distribution Grids: Benefits and Challenges

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Abstract The need for higher transmission capacities even in low (LV) and medium voltage (MV) distribution grids motivates to add direct current (DC) links to homogenous alternating current (AC) grids. The AC2DC project takes up the challenges associated with this transition. The project aims to develop the necessary concepts and guidelines for the system integration of DC links as well as to derive the requirements for the electrical equipment. This paper presents the projects first results, including an overview of the present day research work, the evaluation of a survey regarding cable types in distribution grids, an analytic derivation of the transmission capacities of DC cable systems as well as the accentuation of research gaps in the field of power electronics and test procedures for the electrical equipment.

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... Basierend auf den theoretischen Voruntersuchungen in [8] sowie den vielversprechenden Ergebnissen der Belastungsuntersuchungen im vorangegangenen Kapitel können praxisnahe Netzkonfigurationen abgeleitet werden. Nachfolgend werden Kabel-und Netzkonfigurationen für verschiedene Einsatzzwecke vorgestellt. ...
... An dieser Stelle versucht das Forschungsprojekts AC2DC ein Beitrag zu leisten, indem Umnutzungskonzepte von AC-Kabeln und die DC-Anbindung von EZAs im Bereich der Niederspannung (NS) und Mittelspannung (MS) untersucht werden. Dabei widmet sich das Projekt im Detail  der Erhöhung der Übertragungskapazität von AC-Kabeln durch einen DC-Betrieb,  den damit einhergehenden experimentellen Untersuchungen zur Spannungsfestigkeit und Strombelastbarkeit von DC-belasteten AC-Kabeln,  der Netzkonfiguration im Punkt-zu-Punkt-und Multiterminalbetrieb sowie  der Wirtschaftlichkeit am Beispiel eines Windparks (WPs) mit DC-Anschluss.Grundlegende Untersuchungen zur übertragbaren Leistung und den Herausforderungen bei der DC-Umnutzung von AC-Kabeln wurden bereits in[8] veröffentlicht und werden in diesem Beitrag fortgeführt. In Kapitel 2 werden Ergebnisse aus den experimentellen Belastbarkeitsuntersuchungen präsentiert, während im darauffolgenden Kapitel resultierende DC-Systemkonzepte abgeleitet werden. ...
Conference Paper
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Im Rahmen des Forschungsprojekts AC2DC werden Konzepte sowie deren Umsetzbarkeit zum Betrieb von DC-Verteilnetzen auf Basis konventioneller Nieder- und Mittelspannungs-AC-Kabeltechnik untersucht. Notwendige Systemkonzepte wurden auf Basis physikalischer und technischer Rahmenbedingungen festgelegt sowie umfangreiche Studien zur Strom- und Spannungsbelastbarkeit neuer und betriebsgealterter Kabel durchgeführt. Die Ergebnisse der Belastungsstudien zeigen ein enormes Potential für den Betrieb herkömmlicher AC-Kabel unter DC. Bei einem abschließenden Vergleich des DC-Konzepts mit einem 110-kV-AC-Netz konnten die großen Vorteile eines DC-Netzes herausgestellt werden. Die zu-gehörige Wirtschaftlichkeitsanalyse beweist die Zukunftsfähigkeit der DC-Netzkonzepte.
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Die Gleichstromübertragung kann bei der Lösung der bestehenden Herausforderungen im Mittelspannungsnetz einen Beitrag leisten und zu einem sicheren Netzbetrieb beitragen. MGÜ-Anwendungen können dabei unter anderem für die Erhöhung der Übertragungskapazität von einzelnen Leitungen (Freileitung, Kabel) und eine flexible Lastflusssteuerung sorgen. Es besteht die Möglichkeit, bereits in Betrieb befindliche extrudierte AC-Kabelsysteme auf DC-Betrieb umzurüsten. Dabei sind zahlreiche technische Aspekte zu betrachten, die auch für einen Einsatz von neuen extrudierten AC-Standard-Kabeln für die MGÜ gelten. Die in Anlehnung an die CIGRE TB 496 bisher durchgeführten experimentellen Untersuchungen zeigen, dass ein handelsübliches 12/20-kV-VPE-AC-Kabelsystem den DC-Beanspruchungen des LCC-Präqualifikationstests für ein extrudiertes 55-kV-DC-Kabel standhalten kann.
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Transfer capacities of urban distribution networks need to be increased to fulfill the increasing load demands and to accommodate distributed generation (DG). However, there are limited spaces to build new substations and lines, and curtailment of DGs may happen due to voltage violation during DG outputs fluctuation. This paper proposes and analyzes various methods to convert some existing AC MV lines to DC lines in order to form a hybrid AC/DC distribution network, based on which transfer capacities of lines can be increased, and flexible power shift can be achieved through a voltage source converter (VSC) between two lines. The increases of transfer capacities are quantified. Optimal operation to fully utilize the increased capacities is achieved, in which losses are minimized in day-ahead scheduling, and node voltages are regulated real-time within security ranges based on limited measurement. Not only reactive power but also real power optimization are designed to maximize load supply and DG accommodation. A sensitivity method is proposed considering relatively large r/x ratio of a MV distribution network, which is effective for the real-time voltage regulation. Simulations are performed to verify the proposed method. OAPA
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It is anticipated that with the thrust towards use of clean energy resources such as electric vehicles, future distribution grids will face a steep increase in power demand, forcing the utility operators to invest in enhancing the power delivering capacity of the grid infrastructure. It is identified that the critical 5– 20 km medium voltage (MV) underground ac distribution cable link, responsible for bulk power delivery to the inner urban city substation, can benefit the most with capacity and efficiency enhancement, if the existing infrastructure is reused and operated under dc. Quantification of the same is offered in this paper by incorporating all influencing factors like voltage regulation, dc voltage rating enhancement, capacitive leakage currents, skin and magnetic proximity effect, thermal proximity effect and load power factor. Results are presented for three different ac and dc system topologies for varying cable lengths and conductor cross-sections. The computed system efficiency is enhanced with use of modular multilevel converters that have lower losses due to lower switching frequency. A justified expectation of 50–60% capacity gains is proved along with a generalized insight on its variations that can be extrapolated for different network parameters and configurations. Conditions for achieving payback time of 5 years or lower due to energy savings are identified, while the socio-economic benefits of avoiding digging and installing new cable infrastructure are highlighted. The technical implications of refurbishing cables designed for ac to operate under dc conditions is discussed in terms of imposed electric fields, thermal profile and lifetime. A novel opportunity of temperature dependent dynamic dc voltage rating to achieve additional capacity and efficiency gains is presented.
Conference Paper
ANGLE DC is an innovative project which will convert two existing AC distribution circuits from the island of Anglesey to the Welsh mainland to operate as DC circuits. At present, there is a double circuit 33kV AC connection between the Llanfair PG sub-station on Anglesey and the Bangor 33kV sub-station on the mainland. For the ANGLE DC project, the 3-phases of one AC circuit will be connected in parallel to form the positive conductor and the three phases of the other AC circuit will form the negative conductor. At each end of the circuit, an AC - DC converter will be installed. The use of Voltage Source Converter (VSC) technology for the converter inherently provides reactive power control capability on both AC networks, which will improve the voltage profile in the local region and hence reduce the operating losses in the networks. A key feature of the project is the retention of the existing AC cables and overhead line; hence attention was paid to the choice of DC voltage and DC current to avoid over-stressing the existing asset, parts of which are in excess of 50 years old. As discussed in the paper, the voltage was chosen to be ±27kV and the current was chosen to be 556A. This gave a scheme power rating of 30.5MW, which is a 23% increase in transmitted power capacity from the existing operational limit of 24.8MW. Initial voltage testing of the cable at 38kV indicated that operation under DC stress was possible and that no significant partial discharge was measured. To provide condition monitoring on the cable, an on-line partial discharge detection system will be installed when the circuits are converted for DC operation. The ANGLE DC project will demonstrate the viability of converting existing AC assets to DC operation and the up-lift in power transmission which can be achieved. For future applications, with less concern about the age of the assets, higher power up-lift may be anticipated.
The development of new technologies in power generation and application, together with the demand for the reliability and capability of power supply has drawn increasing attention to DC distribution and micro-grid technology. This paper introduces the renovation project for a single-circuit XLPE cable line being converted into DC operation after its second failure under 35 kV AC. This is the first time ever in China for this type of system conversion, in which the existing XLPE cable line was put into operation in a bipolar voltage source converter (VSC) DC system of 10 kV voltage and 150 A current to maintain its 3 MW capacity. The test methods for cable performance check under the designed DC scheme were put forward and conducted on sectioned cable samples. After the cable test and equipment installation, the line was put into operation under the VSC DC topology in 2011. It has now been running for 4 years without any interruption. Compared with other alternatives for AC operation, this proposal shows advantages in aspects of economy, reliability and engineering. Besides, the theoretical evaluation suggests that the system capacity can be further increased by 4 times, and the first step is underway for that goal.
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