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Future LV distribution network design.pdf

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1 Abstract—The current distribution networks are going to be challenged by new developments on the side of generation and simultaneously also on the side of consumption. Therefore this paper provides an overview of LV network design aspects with emphasis on the implications of the future challenges on LV networks. The attention is especially given to the future aspect of the network design, substation automation, protection and mitigat
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   1    Abstract   —The current distribution networks are going to be challenged by new developments on the side of generation and simultaneously also on the side of consumption. Therefore this paper provides an overview of LV network design aspects with emphasis on the implications of the future challenges on LV networks. The attention is especially given to the future aspect of the network design, substation automation, protection and mitigation of the issues accompanying distributed generation. The current operational and power quality measurements undertaken in LV distribution networks are presented as well as new additional measurements. The need to increase flexibility and assets utilization of LV networks is discussed together with the technical means enabling them. The outcomes related to the future LV network design are presented in conclusions.  Index Terms  —Power distribution, Power system planning, Substation automation, Smart grids I. I  NTRODUCTION  OR a long time, the production, distribution and consumption of electrical energy has been organized in the same manner; electricity has been generated mainly in large central power plants and those power plants were coupled by a transmission network, which transports the generated electricity at a high voltage level across the country. At different substations, the power is brought to a lower voltage level. From these substations, the power is then distributed at a medium voltage (MV) level and subsequently at low voltage (LV) level suitable for the end users. The electricity networks have been designed to move electricity from large power plants, generating electricity mostly from fossil fuels, to end users. Mostly due to the environmental goals and due to the concerns about the security of supply, alternative energy sources are being introduced (i.e. renewables). As a result of this development, an increasing  penetration of small-scale distributed generators (DG) connected directly at the distribution level is envisioned in the future. DGs will alter the power flows in the network and will require more attention and different operational strategies Petr Kadurek is a PhD candidate in the Electrical Energy Systems (EES) group of the Eindhoven University of Technology, The Netherlands, (corresponding author, e-mail: p.kadurek@tue.nl). J. F. G. Cobben is currently working at Alliander, one of the Dutch distribution network operators and as a part-time professor at the EES group of the Eindhoven University of Technology, The Netherlands, (e-mail:  j.f.g.cobben@tue.nl). W. L. Kling is currently a full time professor and the chairman of the EES group of the Eindhoven University of Technology, The Netherlands, (e-mail: w.l.kling@tue.nl).  from the network operator [1]. Several challenges lay ahead for Distribution System Operators (DSOs) in the future, on the one hand they will have to accommodate the increasing penetration of DG among the distribution network. And on the other hand, they will have to  provide sufficient connection capacity for increase in customers’ loading, especially for envisaged transitions towards electro mobility and electrical heating ( i.e.  heat  pumps) [2]. The primary task of the DSO is to guarantee economic, safe, reliable and efficient energy distribution to end users. Therefore, they have to install new networks, maintain network assets and monitor them. The scope of the DSO operation is restricted due to the unbundling of electricity utilities and current legal framework. However, this legal framework also inhibits DSOs in deploying some applications, which can help to improve network operation ( inter alia  application of energy storage). The execution of DSO’s operational tasks will be more complicated in future. The network operators will have to investigate how the safety, reliability and efficiency of the electricity supply can be warranted under the changing circumstances, how to provide sufficient connection capacity to the customers with adequate quality of supply and how can the transition towards Smart Grids be achieved [3]. The development towards the future distribution network will bring the extension of functionality of components like electricity meters and distribution substations, which will be enhanced with measurements and communication  possibilities. The monitoring of distribution networks will be crucial in the future to precisely evaluate the current state of the distribution network as well as to enable their higher utilization and to timely plan reinforcements or maintenance of them.  A. Main contribution This paper addresses the operational and quality of supply aspects of the future LV distribution networks. With regard to those aspects, the appropriate measurements in the LV networks are proposed to enable reliable and flexible operation of LV networks in the future. The placement of the measuring devices among the distribution and their sampling frequency are discussed. The main functionalities, responsibilities of DSO and the overview of current LV network design are described in section II. The developments and future challenges related to the LV distribution networks are presented in section III and the implications for the operation and power quality of future LV networks are Future LV distribution network design and current practices in the Netherlands P. Kadurek, Student     Member  ,  IEEE  , J. F. G. Cobben, W. L. Kling,  Member, IEEE    F   2 assessed in section IV. The design and experiences with future substation design in the Netherlands are presented in section V. The recommendations and conclusions are presented in section VI. II. C URRENT LV    N ETWORKS    A. Functionality of LV networks and DSO’s responsibility In the past, the large electricity companies in Europe were vertically integrated, mostly covering the distribution, generation and supply under one holding. The European Parliament and Council put into force the directive related to unbundling of electricity companies in European Union in 2003 and a replacing directive in 2009 [4]. In the Netherlands, the vertically integrated structure of the electricity companies has been also divided. The distribution network companies have completely separated their activities from the vertically integrated holdings (ownership unbundling in the  Netherlands), with the aim to enhance competition, guarantee access to new market entrants and improve transparency [5]. In this legal framework, the DSO has defined competences, especially to be responsible for operation, maintenance and development of the distribution networks to ensure the long term ability of them to meet reasonable electricity demands and facilitate distributed production. For those tasks, DSO should take into account the economic conditions, security, reliability, environmental aspects and energy efficiency of electricity distribution [5], [6].  B. Network topology The majority of the distribution networks have been designed decades ago to meet the customer needs at that time and to provide reliable connection to end users. The networks have been designed for predicable loads of the customers connected and for a relative small annual load increase (2 %) over the planned life time (40 years). Simultaneously, the network design has been based on low load factor (0.1), where the load factor is the ratio of average peak load to maximal  peak load for all consumers connected. The generic MV (10 kV) and LV (0.4 kV) networks in the  Netherlands are entirely underground cable network mostly radially operated. The typical MV/LV substation (usually with 400 kVA MV/LV transformer) is supplying 200 - 240 LV customers on average (single phase or 3 phase connections equally distributed). The LV customers are connected via several outgoing feeders from the MV/LV substation up to 500 meters far away from the substation. The typical distribution networks in the Netherlands have been planned as an inevitable part of the urban planning of larger residential areas. An example of LV network topology, taking into account also the future presence of DGs, advanced metering and monitoring infrastructure, is depicted in Fig. 1. The LV residential customers have usually relatively high rating of the main circuit breaker (typically 40 A single phase connections, 25 A three phase connections). Together with the low load factor, this enables LV customer high connection flexibility to simultaneously draw high power from the LV network without interruption at customer’s POC. Fig. 1. Schematic topology of the future LV network in the Netherlands, which will be in future integrated with DGs, advanced measurement (1,2) and smart metering (3,4). The LV in the Netherlands is an entirely underground cable network supplying about 240 LV customers on average, equally distributed among three phases. III. F UTURE C HALLENGES R  EGARDING LV    N ETWORKS  The current distribution networks have been designed to effectively meet the predicted demands at that time, when they were designed (see section II). This section discusses the most important implications and future challenges for the future distribution networks, considering the common aspects of the networks in European Union and United States [7].  A. Changing role of the electricity customers The passive role of the electricity customers is going to change. Traditionally, the customers have been only passive users of electricity, but in the future they are expected to actively participate in the power systems. It is envisioned that the customers are going to be actively involved in electricity generation, demand response and grouped in means of virtual  power plant with high reliability [8]. Where the DSO will have to enable the accommodation of the different customer appliances and mitigate the local restrictions to fully access their future potential [9]. The project like the “  Powermatching    city ” in the  Netherlands investigates the customers participation on electricity markets [10] or like in the case of Norway the availability of residential demand response [11]. Especially the demand response and customers response to the price signals is envisioned to have significant impact on power systems by reshaping the demand rather than the supply side or participate when the system reliability is jeopardized in the future [12], [13].  B. Distributed generation and challenges for the DSO Thanks to the subsidies and feed-in schemes from the governments, the customers are installing DG and  participating on electricity generation [14]. With the cost of the DGs falling, the wide-spread of DGs is further envisioned after reaching the competitive prices with electricity supplied from the distribution network [15]. The customers will in future also supply the electricity generated back to the distribution network. The large numbers of small-scale DGs will be connected to the distribution network without commissioning procedure. However, the DSO will have to   3 still accommodate the increasing penetration of DGs and  provide electricity connection with certain power quality in accordance to the standard of supplied voltage [16]. The issues accompanying the implementation of DG among the LV network are becoming real challenges for DSOs (for instance voltage rise [17]). Those issues will be further emphasized with increasing penetration of electric vehicles or heat pumps simultaneously operating with those DG, which can lead even to a deterioration of network components [18]. C. Increasing demand The electricity consumption is predicted to increase in the future, the forecast increase for European Union (EU 27) is 1.5 % annually and about 1 % for US [12], [19]. In the  Netherlands, the transmission network operator estimates the scenario with an anticipated increase up to 3 % by the year 2030 [20]. The switch towards electro-mobility with electric vehicles (EV) can further significantly increase the load in the distribution networks.  D. Measuring infrastructure The design approach of the future distribution networks will have to change; the lack of monitoring in distribution network can create barriers for implementation of network supervision and automation. Therefore, the advanced measuring and metering infrastructure should take a common place among the distribution networks in the future. The monitoring infrastructure should enable more accurate estimation of the current state of those networks, i.e.  advanced data analysis [21], [22], power quality analysis [23], and fraud detection [24]. IV. O PERATIONAL A  ND P OWER Q UALITY A SPECTS OF F UTURE LV    N ETWORKS  Until recently, only few monitoring and active devices existed in the electric distribution system. Network monitoring has been conducted on random basis or as follow up to complaints of the customers [25]. This passive approach for LV networks was acceptable because the whole distribution system was relatively static. However, this is not going to be acceptable any more in the future, as discussed in previous sections. Based on the challenges presented in section III, the implications for the operational and power quality aspects of future LV networks are presented in this section.  A. Advanced and overloading protection schemes The current protection scheme of the LV networks is based on the presumption of unidirectional power flows in the networks. The genuine protection scheme applies the  protection at the customers’ point of connection (POC), at the substation to protect the main feeders and to protect the MV/LV transformer to guarantee protection selectivity. At each POC, single- phase customers are protected by a circuit  breaker or fuse, usually with rated current of 40 A (there-phase customers with 25 A per phase). The customers are connected to the main feeder, which is protected in the MV/LV substation (200 - 250 A fuses per phase) and the secondary side of MV/LV transformer is protected by the circuit breaker or fuses in accordance to the transformer size. However, the presence of DGs and the increasing loading will make the power flows in LV network more complex and the current protection scheme won’t be suitable to protect the network assets any more ( inter alia with high penetration of DG and simultaneous high loading like charging of EVs). This situation is schematically demonstrated in Fig. 2, where the overloading of network component is depicted. Fig. 2. The schematic model of a generic LV network and overloaded network element. The protection devices for each feeder at the LV side of MV/LV substation, DG and loads among one feeder are depicted. The overloading of the network components (and even deterioration) can occur without tripping of the protection devices at POCs or in the MV/LV substation. In addition, the overloading can occur even without violating the voltage level for customers supplied. Additional measures have to be taken to adequately protect network assets and to avoid interruptions. Therefore, additional protection should be applied, utilizing distributed power flow estimation and measurements from smart meters, to evaluate each part of the network separately for overloading. The local processing of the data and network modeling can assist to tackle the overloading issues in the future LV networks, as presented in [18].  B. Voltage level control The current LV networks are supplied by MV/LV transformer with off-lad tap changer, to adjust the voltage for the supplied LV network and to offset the voltage drop in MV network. The tap changer can be adjusted offline during the installation or after topology changes in the MV network. However, due to the presence of DGs, the voltage conditions is going to vary more during the normal network operation. The voltage variations due to DGs will be present in both MV and LV networks. The smart transformer is a MV/LV transformer equipped with a power electronic tap changer, which is capable of adjusting the secondary voltage on-line [26]. The capabilities offered by smart transformer can be utilized in future distribution networks in several ways; to mitigate the MV voltage fluctuations [27] or to mitigate the voltage level  problems due to increasing penetration of DGs in the LV network [28]. With the smart transformer, the voltage at the LV side is no longer directly linked to the loading in the network or to the voltage in the MV network and thus new   4 control and accommodation options are available in the networks also on the LV level. The advantages of the smart transformer are presented in a case study [28], where the impact on the genuine LV network in the Netherlands is investigated. The smart transformer can mitigate the voltage level problems in the LV network and significantly increase the amount of DGs accommodated among LV network. The worst case scenario, as a scenario featuring a high DG penetration with net generation up to the transformer rating and with DGs (in this case intermittent  photovoltaic generators) connected only to 3 out of 4 LV feeders, is presented and evaluated in [28]. The voltage level  profiles for all LV customers connected are depicted as box  plots for the situation with traditional transformer and with smart transformer, see Fig. 3. The central marks of the boxes are the median values, the edges of the boxes are the 25 th  and 75 th  percentiles and the outliers are plotted individually. In accordance to the standard for the supply voltage EN50160, the voltage limits ( U  n ± 10 % ) are plotted in figure as vertical dashed lines. Fig. 3. The voltage level profiles for all customers connected in the LV network accommodating significant amount of DGs. Two cases are displayed; with smart transformer (ST) and without ST. C. Fault localization The fault protection scheme in LV networks should react fast and selectively disconnect only the smallest possible number of customers connected. The majority of faults in LV networks are due to excavation works, but faults can srcinate also spontaneously due to the cable aging. LV networks in the  Netherlands are mostly radially operated networks. When the fault occurs in the LV network, the whole feeder will be disconnected in the substation and all customers connected to this feeder will experience interruption. In radial networks with no opportunity for network reconfigurations, the customers will be disconnected until the fault will be repaired. In current networks, the repair crew is dispatched after the interruption is reported by a customer. However, the fault localization can be complicated when the fault occurs spontaneously without visual signs, which usually assist the repair crew to easily identify the fault location. Therefore, one of the important aspects for future LV networks will be the speed of fault location. The fast fault location can significantly reduce the interruption time for the customers supplied. Better fault localization will be available for the future LV networks, where the fault current can be noticed with measurements conducted in the MV/LV substation or can be localized utilizing the measurements from smart metering infrastructure. The smart metering infrastructure has to provide frequent information, which availability can assist the DSO in observation of the fault in the LV network. In the Netherlands this function is defined in the standard for smart metering, which requires 15 minutes measurements [29]. However, smart meters measure also the instantaneous values which can be used for more effective fault localization. The smart meters should send the voltage measurements from the time of the fault, to enable a more accurate and quick fault location. The smart meters are connected as three-phase meters also for single-phase customers. Therefore, even if the  power line communication is used, not faulted phases can be utilized for data transfer to the data aggregator. The fault localization utilizing smart meter measurements is presented here. In a typical LV network topology, a fault in a feeder with 40 single-phase customers has been investigated. The customers are equally distributed over three phases, where each third POC is connected to the same phase. A single phase fault was simulated in the main feeder near POC 20. The fault occurs at t = 2 s and is switched off in the distribution substation at t = 2,06 s. The r.m.s. voltages at all POCs on the faulted phase during the fault are depicted in Fig. 4. 1.9922.012.022.032.042.052.062.072.08050100150200250Time [s]    V  o   l   t  a  g  e   [   V   ]   POC 1POC 4POC 7POC 10POC 13POC 16POC 19POC 22POC 25POC 28POC 31POC 34POC 37POC 40   Fig. 4. The voltage level profiles of customers (POCs) connected at the faulted phase of the LV feeder during the time, when the fault occurs (t = 2 s). The r.m.s. voltage profile for all POCS on the faulted phase during the fault (t = 2,03 s) is depicted in Fig. 5. From results depicted in Fig. 4 and Fig. 5, the conclusion can be derived about the fault location, which has taken place  between POC-19 and POC-22. This information will provide a repair crew with a good approximation of fault location in the network and will speed up the power restoration. In the simulation, the fault was switched off after 0,06 s, but currently in real situation, a fault may be disconnected faster if the feeder is protected by fuses. Nevertheless, more (remotely controlled) circuit breakers will be used to provide protection of future LV feeders and those circuit breakers will be operated at the speed as presented in this simulation. One of the characteristics of the smart meters in future should be also the possibility to measure and communicate the voltage measurements during the faults enabling a quick fault location and reduction of interruption times for the customers connected.
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