• Document: Multi-Terminal DC System line Protection Requirement and High Speed Protection Solutions
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21, rue d’Artois, F-75008 PARIS LUND 2015 http : //www.cigre.org 331 Multi-Terminal DC System line Protection Requirement and High Speed Protection Solutions Jianping Wang, Bertil Berggren, Kerstin Linden, ABB AB Sweden Jiuping Pan, Reynaldo Nuqui, ABB Inc. USA SUMMARY For long distance and high power transmission, high-voltage direct current (HVDC) transmission is a preferred solution due to its technical and economic advantages. A multi-terminal HVDC (MTDC) transmission system either based on voltage source converters (VSC) or line-commutated converters (LCC) has also been introduced recently. With the development of fast and high breaking capability HVDC circuit breakers, the collapse of the MTDC network voltage resulting from DC line faults can be prevented and the disturbances to the connected AC grid can be minimized by using related protection systems. Therefore, it is required to have a fast and selective DC line fault detection and location methods using initial transient fault signatures. This paper describes the basic requirements of a line protection system in a VSC based MTDC network such as speed, selectivity, sensitivity and security, etc. with consideration of the expected breaking capability of HVDC circuit breakers. The transient phenomena of DC fault currents and DC voltages for different types of faults are analysed together with the protection requirements for an example four- terminal bi-pole MTDC transmission system. Based on the fault analysis and high speed operational requirements, fast fault detection algorithms using traveling waves measured at the MTDC terminal stations are proposed. The proposed fault detection algorithms are based on single end measurement. To verify the proposed fault detection algorithms, the example four-terminal bi-pole MTDC network is used to check the performance of the fault detection functions for different types of DC faults, such as pole-to-pole faults and pole to ground faults, at different locations. Finally, conclusions are provided based on the fault analysis and fault detection testing results. KEYWORDS DC Grid Protection, Multi-terminal HVDC System, Power transmission. 1 INTRODUCTION Technology development always brings new innovations in the application areas. One important technology progress in power transmission domain is voltage source converter (VSC) based HVDC transmission during last decade. One of advantages of VSC based HVDC transmission system is the possibility to perform power reversal through current reversal in which an HVDC link could be used similar as an AC link [1]. There are several MTDC systems in advanced planning stage of development in Europe, North America and a couple of practical MTDC systems have been put into operation in Asia [1]. In particular, MTDC systems have been considered technically and economically attractive for Jianping.wang@se.abb.com 1 integration of large-scale offshore wind farms and for reinforcement of interconnected regional power grids over AC transmission solutions. For a VSC-based MTDC system, there might be different grounding schemes which could influence the fault detection and protection solutions. For high impedance grounded system, the voltage stress during the pole to ground fault period implies higher demand on the insulation systems while the low impedance grounded system might cause high fault current during the fault period [1]. This paper will focus on the fault detection and protection solutions for bi-pole VSC-based MTDC systems with low impedance grounding scheme. Considering various grid system expansion requirements, a MTDC transmission system could be configured with different network topologies such as radial, meshed or a combination of both. Figure 1 below shows an example four-terminal MTDC system which is connected with four asynchronous AC grids. In the shown MTDC network, each converter station is connected to the other three converter stations by DC cable systems. Each DC cable system has three parallel cables: positive pole cable, negative pole cable and a metallic return. For MTDC network protection purpose, HVDC circuit breakers (not shown) are installed at both ends of the DC power cables. Figure 1- An Example of MTDC Transmission System 2 DC LINE PROTECTION REQUIREMENTS 2.1 Protection Strategies for Handing DC Line Faults One critical design requirement for any MTDC system is the need of reliable protection strategy, especially fast fault detection and selective protection system for DC lines. When a short circuit fault occurs in any DC line, fault propagation on the MTDC network is very fast and power transmission in all converter stations may become infeasible. It is thus critical to isolate the faulted DC line quickly so that power tran

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