5th WSPLC Fifth Workshop on Power Line Communications. Power Line Communication Concept for the LVDC SmartGrid

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1 5th WS 2011 Fifth Workshop on Power Line Communications Power Line Communication Concept for the LV SmartGrid A. Pinomaa, J. Ahola, and A. Kosonen Introduction The topic of the presentation is about a work package of our on-going Finnish national research program SGEM (Smart Grids and Energy Markets), where smart grid concept a low-voltage direct current (LV) distribution network is researched. The research is mainly carried out in Lappeenranta University of Technology (LUT). In the LV concept, smart grid functionalities, such as smart metering, grid monitoring and protection are implemented with communications. Moreover, distributed intelligent with distributed small scale generation units and energy storages are implemented and controlled with customer interfaces at every customer in the grid. Hence, communication network for data flows for the applications is required on the LV grid. Power-line communication () is commonly known attractive alternative for smart grids and automatic networks, and it is also considered to be a feasible data transmission method for the LV system. The on-going research, the latest research results and publications with further studies in the future are presented. SGEM Research Program SGEM is a national research program, which started in Finland in 2010 (5 year program, yearly budget around M ). The research partners are Finnish universities and industrial companies, such as ABB, Vattenfall, Fortum, Aidon, okia Siemens etworks, etc. The goal of the SGEM program is to develop future electricity distribution grids to be 1) more reliable with improved quality of AC voltage distribution to customers, and 2) more intelligent, such as the future smart grid could be a market place for the electric energy related services. The topic of this presentation, Lowvoltage direct current (LV) distribution system concept, is included in SGEM WP2.2 (work package) Power Electronics in Electricity Distribution and LV. Concept of an LV Electricity Distribution The LV system is the main result of Power electronics in electricity distribution project, which took place, and was carried out in LUT between It was mainly funded by the Finnish Funding Agency for Technology and Innovation (TEKES), and by several companies involved in the project. In Finland, the distribution grids has been developed from 20/0.4 kv MV/LV AC grids through 20/1/0.4 kv AC grids to the LV grids. First the LV system was proposed for rural areas, where there are long distribution distances and only few customers. In addition, in such areas the traditional 20/1/0.4 kv distribution system is not the most economical one. This is, among other things, because of the high costs (investment, maintenance and outage) of over-head MV branch lines, which are prone to weather related risks.

2 The main characteristics of the LV system are the following. An LV distribution system comprises power electronic converters and underground connection between the converters The low-voltage network and MV branches are converted to LV. Over-head low-voltage AC lines and MV branches are replaced with an LV grid implemented with underground cabling. The medium voltage AC is transformed, rectified, and filtered to low-voltage and distributed to customers. Each customer is connected through a /AC inverter. There are two basic solutions for an LV system: a unipolar (1500 V, 0 V) and a bipolar method (+750 V, 0 V, 750 V) [1]. Compared with traditional AC distribution system, the LV system provides at least the following advantages. The highest low-voltage ; 1500 V [LVD 2006/95/EC] is utilized in the LV system. Savings in grid investments. Low voltage underground cables are more than less expensive than MV cables. Furthermore, with 1500 V, power transfer capability is times as high as capability of traditional 400 VAC distribution. According to [1], the total costs of the LV grid compared with traditional AC MV/LV distribution grids are decreased significantly. Operational costs are decreased significantly with underground cabling. The number of overhead lines of the MV grid is mitigated, and thus the reliability of the distribution is improved. Safe and reliable electric energy transmission from the MV network to the LV customers, and constantly good-quality voltage supply for customers. An easy-to-control connection point for small-scale generation units and electric energy storages (because of, no synchronization for these is required). A ready-to-use platform for smart metering, demand side management and network control with other smart functionalities. Low constructing and operating costs of the distribution network. Concept of Interactive Customer Gateway Concept of interactive customer gateway (ICA project) was researched and developed in LUT from 2008 to 2010, when it was integrated into one work package of SGEM. It is closely related to, and developed with the LV concept. However, active customer gateway is independent form the type of electricity distribution network. Interactive customer gateway includes and acts as a platform for developing and implementing applications for smart grids. The studied smart grid concept including LV grid and active customer interfaces are illustrated in Fig. 1. Every customer in the distribution grid is equipped with active customer gateway (embedded PC in this case), which is responsible for; Continuous system supervision and active power quality supervision Automatic meter reading and metering infrastructure (AMR/AMI) Grid side power demand control and control of EV charging/discharging Market information exchange Bidirectional data flows with all actors and for all smart applications related to the distribution grid.

3 Figure 1. Studied smart grid concept including bipolar LV grid and active customer gateways. Concept for LV Distribution etwork Communication is needed for applications integrated in the LV grid, and bi-directional data flows are required by those between rectifier substation and customer gateways at inverters. The communication network in the LV system is centralized. Centralizer unit is located at the rectifier, and every customer in the grid is equipped with customer gateway implemented with embedded industrial PC. Smart grid applications, integrated into the LV grid bring at least the following challenges and requirements to the communication network and technology. Certain throughput is required for AMR (automatic meter reading) applications and DSM (demand side management), especially, when power consumption data are gathered from the customers first to centralizer unit in the grid (to the rectifier in the case of LV grid), and from there to the remote databases. Latency is the most critical requirement in grid protection and monitoring applications implemented with communications between rectifier and inverters in the LV distribution network. For example, in a case of short circuit fault in the grid, the information has to be sent and received to the rectifier in 10 ms after the fault. The structure of the concept is defined by the dimensions and architecture of the LV grid, for example communication range and repeating interval has to be defined and known. Furthermore, customer inverters generate harmonics and impulsive noise to the grid, which pose also challenges to. A bipolar LV system with the proposed concept, and the contents of the over-ground cable connection cabinet are presented in Fig. 2.

4 Medium voltage 20 kv AC/ Backhaul W 20/1 kv AC/ Rectifier +750V 0V _ 750V. Segment 1 Segment 3 /AC 230/400 V Segment 2 Cable connection cabinet Segment V AXMK cable _ 750V HP Ethernet Switch HP 750 V 325 V +750V _ 750V Figure 2. Proposed concept on the LV distribution system. The contents of over ground cable connection cabinet. is shown to be viable for the LV smart grid concept [2], [3]. An IP-based concept is designed and implemented on the LV system. The concept utilizes commercial industrial compliant HF-band modems, commercial Ethernet router and switches, embedded PCs in the customer end interfaces. The signal coupling method used in the concept is commercial inductive couplers, and modems are coupled differentially between short-circuited neutral conductors of the AXMK cable used for power transfer between the rectifier and customer inverters. Thus, segmented communication network is achieved [2]. Commonly, narrow-band (in Europe CEELEC band) is proposed for the applications of smart grids, and automation networks. It provides low data transmission rates with longer communication distances between two modems than broad-band in high frequency band. The reason for ending up to HF-band in the LV grid was the increasing number of power electronic devices, such as small scale generation units, electric energy storages, fluorescent light bulbs etc. connected to the grids. Such power electronic devices generate harmonics and non-stationary noise components, such as impulsive noise to the grids, and the total noise power and impedance variations are increased remarkably compared with traditional AC grids. Furthermore, HF-band provides sufficient throughput for smart applications. Research results show that communication ranges of 500 m between two HF-band modems are achievable with data rates of 2-4 Mbps, which is sufficient for the proposed grid structure. AXMK power cables are delivered in cable reels of 500 m. (also repeater interval). Segmented communication network is developed for the system, and its operation is verified [2], [3]. Furthermore, CEELEC band technologies, with higher communication distances between modems is not an advantage in the proposed concept.

5 Further Research & Future Work Research of concept for the LV will be carried out in the near future focusing on the following cases: Practical environment research platform, LV pilot system is built in electricity distribution grid. For researching and developing LV technology with interactive customer gateway functionalities Co-effort of Suur-Savon Sähkö Ltd. and LUT started in 2010 Installation environment Developing recreational dwelling and detached house area in first phase 4 customers, fed by 3 customer inverters Optic fibre implemented with underground power cablings for communication netwok modems will also be tested in some grid branches to study the suitability of the proposed concept in real environment Furthermore, the channel characteristics are studied in the pilot system (noise voltages and power) Channel Modeling for the LV distribution system [4] will be researched for studying the channel characteristics more precisely; to support and verify the practical measurements carried out in an LV laboratory setup, and in real LV distribution environment. References [1] T. Kaipia, P. Salonen, J. Lassila, and J. Partanen, Application of low voltage distribution system a techno-economical study, CIRED 2007, In proc. of 19th Int. Conf. on Electricity Distribution, Vienna, Austria, May [2] A. Pinomaa, J. Ahola, A. Kosonen, Power-Line Communication-Based etwork Architecture for Low Voltage Direct Current Distribution System, 15th IEEE International Symposium on Power Line Communications and Its Applications 2011, IEEE IS, from April 3 to April , Udine, Italy. [3] A.Pinomaa, J. Ahola, A. Kosonen, Concept for LV Distribution Systems, IEEE Communications Magazine Feature Topic Issue on Power Line Communications for Automation etworks and Smart Grids, (in press) Forthcoming. In the process: [4] A. Pinomaa, A. Kosonen, and J. Ahola, Simulation Models in the Analysis of Channel Characteristics for in the LV Distribution Systems, (IEEE Transactions on Power Delivery, 2011)