Approach for flexible and adaptive distribution and transformation design in rural electrification and its implications

Energy for Sustainable Development 54 (2020) 101e110

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Approach for flexible and adaptive distribution and transformation design in rural electrification and its implications Jimmy Ehnberg a, *, Helene Ahlborg b, c, Elias Hartvigsson d a

Department of Electrical Engineering, Chalmers University of Technology, Gothenburg, Sweden Department of Technology Management and Economics, Chalmers University of Technology, Gothenburg, Sweden c School of Global Studies, University of Gothenburg, Gothenburg, Sweden d Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 March 2019 Received in revised form 23 October 2019 Accepted 23 October 2019

Microgrids have an important role to play in achieving current international targets of electrifying poor rural communities around the world. In the East-African context, microgrid developers face challenges related to dispersed settlement patterns and high poverty levels that prevent many rural citizens from affording grid connections. Contextual factors influence demand for electricity, leading to uncertainties regarding development of consumption in newly electrified areas. Developers struggle with the sizing of microgrids and often initially oversize the system in anticipation of growing demand, which leads to significant investment costs and economic risk in case projected growth fails to appear. Our focus in this paper is to introduce an approach for flexible and adaptive distribution design e a process that can reduce initial investment cost and still be able to meet the long-term variations of the load in a controlled manner, thereby removing an entry barrier related to microgrid development. We exemplify the usefulness of this design approach in three different application areas: distribution capacity, transformation capacity and level of protection systems. Each application area consists of a number of steps based on mature technologies that correspond to change in capacity. The steps can be taken in sequence or in part, to achieve a system configuration adaptive enough to handle changes in electricity consumption, both increasing and, in some cases, also decreasing. Considerations on how steps would impact on system operation, power transfer capacity and demands on local technical expertise and maintenance are included. Importantly, the technical discussion details socio-economic aspects and the consequences for end-users as well as the utility. We exemplify the feasibility of the approach and provide a context for the discussion using real-world examples from East Africa. © 2019 International Energy Initiative. Published by Elsevier Inc. All rights reserved.

Keywords: Load flow Distribution system Lines Compensation Smart grids Microgrid Flexibility Adaptiveness Rural electrification

1. Introduction As of today, one billion people lack access to electricity around the world. Roughly half of these people live in sub-Saharan Africa, and a large majority of them live in rural areas (International Energy Agency, 2017). In order to provide electricity access, within the foreseeable future, to the people in rural communitiesdwho now commonly rely on kerosene and candles for illumination, diesel generators to power machinery and batteries to power radiosdoff-grid solutions are needed (International Energy Agency, 2017; Tenenbaum, Greacen, Siyambalapitya, & Knuckles, 2014). For low-income rural communities, off-grid solutions come in various sizesdranging from a few W for small Solar Home

* Corresponding author. E-mail address: [email protected] (J. Ehnberg).

Systems (SHS), to diesel generator sets of a few kW, and microgrids with generation capacity of several hundreds of kW or a few MW. Here, we are primarily interested in the potential for reliable, affordable and sustainable electricity provision from microgrids (ranging between hundreds to thousands of customers). Projections suggest that by 2030, 30% of the new connections will be provided by microgrids (International Energy Agency; International Renewable Energy Agency; United Nations; World Bank Group; World Health Organization, 2018). Microgrids are large enough to supply productive activities with power. The capital investment required for constructing the production and distribution system can be significant and difficult to finance for local investors. A large-scale dissemination of microgrids has been hindered by several challenges associated with off-grid electrification, including for example: weak customer demand due to low household incomes and dispersed population; lack of sufficient

https://doi.org/10.1016/j.esd.2019.10.002 0973-0826/© 2019 International Energy Initiative. Published by Elsevier Inc. All rights reserved.

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subsidies and lack of supporting policies and institutions (Ahlborg & Hammar, 2014; Brent & Rogers, 2010). Matching electricity generation with current and future demand is a central and challenging issue in energy and grid planning in sub-Saharan Africa (Riva, Tognollo, Gardumi, & Colombo, 2018). The complexity of electricity demand growth makes future projections uncertain (Riva, Ahlborg, Hartvigsson, Pachauri, & Colombo, 2018), thus making the sizing of electricity generation in rural electrification difficult. Predictions often rely on interview data to construct load profiles, which have shown to be uncertain (Blodgett, Dauenhauer, Louie, & Kickham, 2017; Hartvigsson & Ahlgren, 2018). As a consequence, developers typically either oversize the system in anticipation of growing demand or undersize the system to reduce unused capacity. Oversizing leads to significant investment costs and economic risk in case projected growth fails to appear. Under-sizing can lead to reduced reliability, causing long-term viability issues (Greacen, 2004; Hartvigsson, 2018). This dilemma is a contributory cause why many utilities have struggled to reach cost-recovery, making microgrids economically unattractive for investors (Kihedu & Kimambo, 2006; Kirubi, Jacobson, Kammen, & Mills, 2009; Levin & Thomas, 2014; Schnitzer et al., 2014) and thus presenting a barrier for expanding electricity access in poor rural communities. In this paper, we propose a new approach to reducing financial barriers and risk associated with microgrid development that focus on building flexible and adaptive power systems that can handle both short and long-term uncertainties in demand and, thereby, income. The approach increases the likelihood for achieving the two objectives of: (1) making microgrids economically viable investments and; (2) maintaining reliable and affordable service provision over time, to the benefit of local communities. Whereas both ‘flexibility’ and ‘adaptiveness’ take on various meanings in the technical literature on microgrid design (often from the viewpoint of discussing flexible loads and adaptive control), we here use it with specific reference to uncertainties in the societal and environmental context. Flexibility here refers to the power system withstanding temporary, short-term shocks, whereas adaptiveness refers to the system being reconfigured to accommodate more long-term pressures and/or a changing environment. These two concepts are developed and exemplified together with the issue of reliability of power supply to customers. The ambition is that by focusing on flexibility and adaptiveness as design principles we can inspire creative ways of tackling uncertain future demand through interdisciplinary dialogue. In order to realize the advantages of flexibility and adaptiveness, these qualities are needed in all parts of the electric power system: production, distribution and user dynamics. In the following, we limit the discussion to distribution, and thus focus on flexibility and adaptiveness in the power capacity (i.e. power transfer and transformation capacity) and protection. This voltage level is selected for discussion because, at this level, there are large variations in load and its development over time, which suggests this is where these qualities are needed the most. It also has the highest probability of implementation due to high cost and relatively low numbers, compared to low voltage lines. The following discussion on flexibility and adaptiveness starts with description of the design principles and then exemplifies how to apply these in terms of: distribution line capacity; transformation; and protection systems. Conventional microgrid design is based on expected load development within the technical lifetime of the system, around 40 years. The location of renewable energy sources (e.g. a river site, wind site or placing of a solar PV park) respective to the load clusters (e.g. villages, industries) conditions possible designs, often requiring multiple voltage levels, with distribution of electricity at higher voltage levels. Apart from distribution from generation site to load clusters, larger sized microgrids supplying larger areas often

need medium voltage lines to appropriately transmit power to different load clusters. Since most of the electricity in rural areas is consumed at low voltage level, 400 V, each load cluster requires a low-voltage grid. The initial investments and selected system structure tend to steer all later investments in the system, especially given that many components are expensive. When a component breaks (e.g. a transformer) it is replaced, but the overall grid design is rarely changed. Initial design decisions and investments are thus important and have lasting effects. The key argument in this paper is that there are technical solutions available to build more flexible and adaptive grids to handle the uncertainties described, but a design approach to this end is missing. Alzola et al. (2009) provide an interesting starting point for us, with their context-sensitive methodology for design of electrification kits in rural Senegal. Whereas they highlight the issue of uncertain demand and need for modularity, we wish to go more in depth with how to design grids given these uncertainties. The technologies we propose are used around the world, although currently underutilized in sub-Saharan Africa, and have potential to reduce entry barriers (related to oversizing systems) and deal with short and long-term uncertainties in demand. The challenge of adopting novel distribution design is therefore not primarily a technical one, but one of breaking long-established conservative approaches to power system sizing. The target groups of this paper are donors, project managers and design engineers for microgrids. The specific purpose of this paper is to present an approach of flexible and adaptive microgrid development, and to discussdacross disciplinesdthe potential applications and consequences of such flexibility and adaptiveness for the utility and the customers. The paper is outlined accordingly. In section 2, we present our approach. This is followed by descriptions of its application to distribution line capacity (section 3), transformation capacity (section 4) and protection systems (section 5). In these sections we also discuss potential impacts on the utility. In section 6, we discuss impacts on the customers, followed by conclusions (section 7). 2. Flexible and adaptive grids: method and design principles for the approach The approach we are about to present is based on a synthesis conducted among the three authors who have worked together on the topic of how to build sustainable off-grid renewable energy systems in the East African region. The work is interdisciplinary and bridges between electric power engineering, systems engineering and social science energy and development studies. The synthesis was carried out during joint workshops that we undertook over the course of a couple of years, drawing on our own previous studies and existing literature in combination with the first author's knowledge of new and emerging power technologies. The starting point for discussion was the question of how to achieve reliable, affordable and sustainable microgrids, in contexts where load development is uncertain and demand starts from very low levels. Our empirical knowledge of the contextual challenges and multiplicity of factors that threaten the sustainability of small-scale local power generation and distribution in poor rural communities inspired us to rethink conventional design approaches and envision design solutions that are technically possible but not currently utilized in the field. The goal is a grid that is reliable in providing stable and highquality power distribution on a daily basis. Given the fluctuating daily loads and the very uncertain development of demand over time, the microgrid has to respond well to daily, weekly and seasonal changes, but also have the possibility to be adjusted to different and changing electricity consumption. Following Stirling (2011) we differentiate between the need for a grid to be reliable to handle short-term variations and durable to handle long-term

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variations. This is achieved mainly through design measures that help control well-known disturbances, such as fuses that prevent system shut-down due to faults or customer misconduct, and highquality materials that withstand termites, wear and tear. However, in the context of rural East Africa (as elsewhere), there are also disturbances and pressures on technical systems that are more uncertain and difficult, or impossible, for the operator to control. These include occasional short-term shocks such as storms and lightning, trees falling across power lines, IT crashes, or theft of equipment. We suggest that the grid needs to be flexible in the face of such short-term shocks, meaning that it can withstand sudden events that are not possible to prevent, but that one needs to respond to. In addition, there are the long-term stresses that are both uncertain and generally outside the operator's control, such as economic fluctuations, the arrival of the national grid or changes in national regulations regarding tariffs or licenses for small power producers and distributors. We propose the concept of adaptiveness to signify the situation in which the grid structure is adapted to changing conditions, including the possibility to respond to unforeseen load development and changing environmental conditions. These four concepts: reliability; durability; flexibility; and adaptiveness are interlinked and overlapping rather than mutually exclusive. Fig. 1 illustrates this and exemplifies the short-term and long-term shocks that can affect microgrids in East Africa. The concepts emphasize different qualities of a sustainable energy system and provide a lens through which we can analyze different types of vulnerabilities. Conventional grid design generally revolves mainly around reliability, durability, and flexibility. Our definition of adaptiveness highlights more long-term uncertainties that we may still respond to through careful design. Strategies for responding to changing conditions are difficult given the path dependency of built infrastructure, but important to consider. We assume a situation where the initial demand is very low and development of demand over time is uncertain. Hence, in order to reduce initial investment costs, the starting point is that the design needs to allow for stepwise expansion of the grid and increase in distribution and transmission capacity. We consider three issues related to this expansion that influence whether or not the system

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is adaptive. First, unless capacity restrictions are continuously adapted to demand, such systems can suffer from reliability issues. Second, there is the risk of creating lock-in situations based on technical limitations, the steps therefore have to be designed to minimize these risks. Third, it should also be possible to reduce capacity in order to: use the installation for other purposes; to lower operation costs; reduce losses; or extend the expected lifetime of a certain component. These requirements will put new demands on utilities, and on funders as such adaptations may require new business models. Fig. 2 shows the structure of flexible and adaptive grids. It shows three application areas representing distribution line capacity, transformation capacity and protection systems. Each subsystem contains at least one conventional technology and, in most cases, several technologies to make the grid more flexible and adaptive. For a grid to achieve these qualities, one or more application areas must be introduced with one or more flexible or adaptive qualities. In practice, the approach allows for initial minimizing of costs and avoidance of a lock-in situation, but the system functionality may decrease and there can be negative impact on customers. These trade-offs are discussed as part of results. For each area of application, the design approach is based on arrangement of well-established electricity distribution technologies, here called steps. Some of the steps include some minor modifications from conventional applications in order to avoid lock-in (i.e. to promote adaptiveness). The steps are arranged in an order of increasing capacity (for distribution and transformation) or reliability (for protection systems). It is not necessary to advance the steps in order, and some can be skipped if considered unsuitable. The steps are also arranged such that they minimize the impact on customers. Some of the transitions between steps are reversible (enhancing adaptiveness) but not all. For each step, we describe the increase in capacity/reliability and incremental cost for associated equipment. Capacity/reliability and cost are estimated in relation to a reference case, i.e. where we assume that a threephase system is built from the beginning, which can be considered as the conventional case. For the cost estimations, we assume a constant number of customers, but later discuss the option of an increasing number of customers for each application. 3. Application 1: development in distribution line capacity The first application area is distribution line capacity. Distribution line capacity refers to the transport capacity of electricity from a network node to a customer. Conventionally, distribution lines are constructed as three phase systems, requiring three conductors, and have the ability to transport high levels of power. Table 1 presents the seven steps for this application, where each corresponds to a development in distribution line capacity. Most of the steps are reversible. For each step there is a short technical description below (see Table 2). These steps can be considered a part of a flexible and adaptive design because the system has a lower initial investment cost and allows a controllable cost development by allowing the capacity development to be done in the mostly reversible steps desired or wanted by the operator. 3.1. Description of steps D1-D7

Fig. 1. Design strategies for sustainability of electric microgrids. The overlapping area in the middle of the four qualities represents a sustainable microgrid that combines control and response strategies in the face of both short-term and long-term pressures, Based on Stirling, 2011.

The most basic step (D1) to achieve flexible and adaptive distribution line capacity, is based on batteries being transported between a charging station and the customer. Such a system is described in (Barnes et al., 2004) and its cost is dependent on the amount of electricity used by the customers. For calculation purposes we are assuming a system of 100 batteries being charged every second day, and only the cost of batteries is included in the

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Fig. 2. Overview of the structure of the grid design approach. A flexible and adaptive grid consists of different area of applications. All application areas can be realized in a conventional way but if at least one application area is introduced with flexibility and/or adaptiveness the complete grid can be considered such. The vertical dotted lines mean that there could be additional solutions that are not yet developed for each application but could be introduced into the approach. The red shape and the horizontal dotted line indicate a placeholder for more application areas to be introduced in the approach. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Table 1 The different steps and their impact on capacity of a distribution line, incremental equipment cost compared with the cost for reaching step D4 directly, utility and customer usage. The system perspective is only summarized in the table. No. Description

Distribution line capacitya [ ]

Costa [ ]

Summary of system impact

D1 Manual transport

0,005 (Barnes & Foley, 2004) ~0.33 (Widmer & Arter, 1992) ~0.57 (Widmer et al., 1992) 1 (Widmer et al., 1992) 1.1

0.05 (Barnes et al., 2004)

Low investment cost; relatively simple management and technical skills required; low loads limit incomes, no possibilities for productive use. Capacity increase: load monitoring required; need for training of operators; still limited incomes for utility, low possibilities for productive use. Additional cost for little extra capacity; requires training of operators. Still low possibilities for productive use Capacity increase; allows higher loads and productive uses; can generate new income and productive use. Capacity increase; allows for more customers; improved stability of voltage

D2 R-SWER D3 Two phase D4 Three phase D5 Reactive power compensation D6 DLR D7 DSM a b

0.68 (Widmer et al., 1992) 0.86 (Widmer et al., 1992) 1.05 (Widmer et al., 1992)

1.054 (Energimarknadsinspektionen, 2015) Up to 2 (Michiorri 1.054b Capacity increase; requires additional monitoring, load management plan and training et al., 2015) b Better use of available capacity; requires agreement and enforcement of rules and behavior 1.33 1.054 change of customers

Normalized costs in relation to the standard technical solution, in this case direct construction of a three-phase line, like D4. The additional costs are very small or unknown because they are highly dependent on available load.

Table 2 The different steps and their impact on transformation capacity, equipment cost and utility usage. The normalization is only valid for the transformation capacity study. No. Step

Transformation capacity** [ ]

Cost** [ ]

T1 MVC, 1LVC, 1MFT T2 MVC, 1LVC, 2MFT T3 MVC, 2LVC, 2MFT T4 MVC, 2LVC, 3MFT T5 DSM

0.33 (Huber et al., 2014) 0.75 (Huber et SEEDT, 2008) 0.50 (Huber et al., 2014) 0.86 (Huber et SEEDT, 2008) 0.66 (Huber et al., 2014) 1.04 (Huber et SEEDT, 2008) 1 (Huber et al., 2014) 1.15 (Huber et SEEDT, 2008) 1.33 1.15a

Summary of system impact al., 2014; al., 2014; al., 2014; al., 2014;

Need for training of operators; low loads; limits incomes for utility and low possibility for productive use Little capacity increase; still limiting incomes; increased reliability; low possibility for productive use Additional cost for little extra capacity; possibility to separate low voltage grid and possibility for productive use Full capacity; can generate more income and possibility for productive use Better use of available capacity; requires agreement and enforcement of rules; possibility for productive use

*Normalized costs in relation to the standard technical solution, in this case direct construction of an SST, like T4. a The additional costs are very small or unknown because they are highly dependent on available load. The equipment needed is the same for D7 so if that step is used, no extra costs will be incurred.

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estimated equipment cost (Barnes et al., 2004). The lifetime of this kind of system is short due to the relatively short lifetime of batteries in rural electrification (Lujano-Rojas et al., 2016). If the utility expands the system from step D1 to D2, the batteries may not be used. Still, the relatively low cost of batteries, their short lifetime and the possibility of a second-hand market imply that D1 does not require significant investment, nor creates a lock-in or high extra cost for the utility, even if it lasts only for a short period of time. As a battery failure only affects one customer, the reliability of a battery charging system is likely not a significant issue (Dung, Anisuzzaman, Kumar, & Bhattacharya, 2003). For the utility, this step can enhance the knowledge on electricity uses in the area without large infrastructure investments and thus be used to evaluate electricity use and demand, improving assessments of future demand. Also, the system can be rapidly built and continuously expanded depending on demand. Special competence on battery management and maintenance are required to avoid early decay of the battery and thus ensure durability. The second step (D2) utilizes the concept of Single Wire Earth Return (SWER). SWER is a technology to transfer electricity using only one conductor (the current is returned through the earth). SWER provides low cost solutions to power transfer at distribution level (Hosseinzadeh & Rattray, 2008) when limited power is needed. A problem with SWER is that it relies on specialized equipment that is not easily upgradable (few of the components used in a SWER system can be reused). Investing in SWER can thus create a lock-in situation. To avoid lock-in, we suggest a modified version of SWER, named Reinforced SWER (R-SWER). In R-SWER, the system is built in a way that allows for upgrading and where components can be reused. The cost for a R-SWER is approximately 50% higher than a conventional SWER system (Jarret, Maung Than Oo, & Harvey, 2012), but it does not have the same risk for lock-in as SWER. The reliability of R-SWER can be considered lower than D1 due to its dependency on a single power line supplying electricity to multiple customers. However, conductor or pole failures are relatively easy to solve since it is easy to detect failures and the replacement requires a small work effort and simple tools. The power capacity is relatively small and R-SWER can only support limited productive use, which often constitutes a significant share of a utility's income. From the utility's perspective, a R-SWER system requires monitoring and special training for the operators, while providing limited income. R-SWER is thus vulnerable to shocks and changing conditions, but may still be a relevant step for a period. The third step (D3) utilizes the infrastructure of the R-SWER system but is upgraded with an additional conductor. It is expensive and only has a minor positive impact on the distribution line capacity, and it may only be of interest under certain conditions such as when no further load growth can be expected. The reliability for a two-phase system is the same as for SWER since any single conductor failure can be solved in a relatively short time. Two-phase systems are unusual and therefore requires special training for technicians. The fourth step (D4) is three-phase, the conventional technology used for electricity transfer in distribution systems (Widmer et al., 1992). It allows for high and stable transfer of power and can supply any type of load. It requires three conductors (one for each phase) plus an additional conductor for grounding and is therefore more expensive than two-phase. Most electric power systems are three-phase and since equipment and training are standardized these are easily accessible, which is an advantage for small utilities. The fifth step (D5) utilizes technology to optimize the flow of power in the existing power lines. This is achieved by reducing the reactive power that is transmitted. Appliances mainly use active power and transfer of reactive power is therefore unnecessary. In

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addition, reactive power increases the losses and reduces voltage for the customer. The calculated capacity and cost in Table 1 are based on a shunt capacitor compensation to unity from a power factor of 0.9. The cost of the reactive power compensation might be lower than presented, or even negative (i.e. the utility makes a profit) since the compensation reduce losses in the distribution system. This step is reversible since the shunt capacitor can be removed. The reliability of the system will likely decrease as additional components are introduced. For the utility, the initial investment can be smaller than the gains from reducing losses in the distribution system and increasing capacity. It requires monitoring, either automatic or manual, in order to avoid overcompensation or over voltage. The added operating knowledge are relatively low. The sixth step (D6) uses Dynamic Line Rating (DLR) to increase the transferred capacity in power lines without replacing the actual power lines. All components in a power system have a limit of how much capacity they can transport during worst possible conditions. As these conditions rarely happen (thus defined as temporary shocks), the power lines can often transfer higher capacity than they are rated for. Using DLR, climate conditions are monitored and the allowed capacity on a power line is set accordingly. DLR requires additional equipment such as measuring devices, load controlling equipment and load scheduling. The estimated cost of the DLR includes measuring equipment, load controlling equipment and compensation to customers. This step is reversible if the power demand decreases. The compensation to customers can stop and the load controlling equipment may be removed. By utilizing dynamic line rating the system flexibility (to respond to weekly and seasonal conditions) increases but reliability can decrease as additional components are required. For the utility, DLR requires that action plans are put in place for situations (shocks) when the loads need to be restricted, i.e. it needs to develop new operating procedures. Hence, both flexibility and adaptiveness are both a precondition and an outcome for DLR. The seventh step (D7) increases the power transfer capacity of a power line by controlling the load. This is achieved using Demand Side Management (DSM). DSM involves both regulatory design measures and technical solutions. It reduces the uncertainty of the load by improving the load factor of single customers, and thereby increases available capacity in a power line. For calculation purposes, the values in Table 1 are based the load situation described in (Hartvigsson et al., 2018). The added costs of DSM are mainly load controlling equipment. The step is thus reversible (adaptive) if the power demand decreases. The compensation to customers can be stopped and the load controlling equipment may be moved. DSM has very similar implications for the utility as the DLR, but more focus is put on load control. The business model also needs to be developed or revised for a functioning DSM. The order of DLR and DSM may be different depending on local conditions. When utilizing DLR, the system becomes more sensitive as it depends on the correlation between time periods of high load and good weather conditions (e.g. windy). DSM can to some extent reduce this vulnerability by improving load management and consumption patterns. 4. Application 2: development in transformation capacity The step-wise process for flexible and adaptive transformation capacity is based on Solid State Transformers (SST). One of the main characteristics of solid state transformers is that they can be made Modular Solid State Transformer (MSST) (Huber & Kolar, 2014). MSST is a new technology and thus expensive when compared to traditional transformers. However, as MSST are scalable and modular their use can be modified to suit a changing demand, unlike traditional transformers that require replacement. The MSST

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allows for possibilities to split customers into different subsystems, thus limiting the impact of disturbances such as faults or repairs, which should improve reliability and flexibility. In addition, MSST modules can: reduce the physical size of transformers; provide additional information, regulation and control in the system; and compensate reactive power transfer. The number of modules and their placement in a grid determines the functions and the capacity they provide. The transformation steps are defined by which, and how many, modules that are used in each MSST. The following modules are considered from (Huber et al., 2014): Medium Voltage Converters (MVC), Low Voltage Converters (LVC) and Medium Frequency Transformers (MFT). The connections between the modules and the surrounding equipment are initially fixed and sized, thus reducing lock-in. For calculation purposes it is assumed that the MVCs have a full capacity while the LVC and MFT has respectively only one half and one third of the full capacity. Apart from the last step (T5), all steps are based on the minimum use of modules and a transition to the next step involves adding an additional LVC or MFT module. All steps are technically, but not all economically, reversible (it is assumed that modules cannot be sold or traded). By reversing to a previous step, operation can be restored after a grid fault, but with reduced capacity. Steps can also be reversed since modules are mobile, making it possible to increase transformer capacity in areas of high demand by reducing it in areas with low demand. This increases the flexibility and adaptiveness of the system. Table 1 presents the steps in transformation capacity. The system impact of each step is described. The transformation capacity and incremental cost for equipment are estimated in relation to the reference case (T4). The direct implications of each step for the utility are summarized in the table. 4.1. Technical description of steps T1-T5 The first step (T1) is a minimum design with one MVC, one LVC and one MFT. However, the interconnections between the modules have full capacity (three phase) to reduce the risk of a lock-in. At this step, the MFT is the limiting factor. In the second step (T2) a second MFT is added. The upgrade is simple and can be done with minimal disruption to the system. However, this assumes that the MFTs have compatible ratings1 (for voltage and impedance). Adding an MFT also increase the reliability by providing a certain level of redundancy. At this step, the LVC is the limiting factor. In the third step (T3) a second LVC is added. The added LVC is either connected in parallel with the existing module or to divide the grid on the secondary side. Dividing the grid increases the reliability of the system and limits the spread of disturbances in the grid. However, a divided grid is more difficult to balance and thus requires careful consideration of the load supplied. At this step the MFT is the limiting factor. In the fourth step (T4) another MFT is added. The procedure and benefits are similar to that in step two (T2). The last step (T5) uses DSM to increase the available capacity through better load control. It is identical to DSM applied to the seventh step in distribution capacity (D7) and therefore shares the same benefits and drawbacks. For the utility, the additional cost of MSST should be weighed against the importance of reliability, reactive power compensation and monitoring. If modules break, repairs are mainly done by replacement and can be done relatively easy by local technicians. Transport of modules is easier than for a standard transformer since

1 Other ratings can be considered (but are outside the scope of our paper) as long as the internal impedance compensates for the mismatch.

all individual modules can be carried by one person and therefore transported on a regular bus or in car. This adds flexibility to the system in case of transformer breakdown, specifically during the rainy season, when roads are often in very poor condition. The reversibility of the steps minimizes the need of local repair because some capacity will mostly be available even if some parts are removed. Also, modules can be rearranged across the grid to minimize impact of damages, giving adaptive qualities. A durability challenge is related to the control of the converters. There will be need for updates and adjustment of the software. This can be done remotely given that there is a network in the area and thus there is only little need for computer skills. Reparametrization of the LVC will be needed when shifting between the different steps as to avoid overload but this can either be done remotely or via a switch on site. However, the MVCs have shorter life time than transformers and since the technology is new it can be more difficult to access spare parts. Finally, MSSTs have the advantage that they do not require any oil for cooling which reduces their environmental impact and the risk of oil theft. 5. Application 3: protection system The third application provides a process to ensure increasing levels of technical reliability associated with the numbers of breakers and disconnectors. What level of reliability that is deemed necessary depends on the type and degree of vulnerabilities that can be expected and tolerated. The protection strategy is based on replacing one or several breakers in the system with disconnectors. Disconnectors cost around half compared to breakers and provide the same function, i.e. they can separatedand thus protectdthe main system from a fault in the grid. Their main drawback is that the distribution system shuts down in case of a disturbance. How long the interruption lasts depend on whether the system monitoring is manual or IT based. Using an IT based system, the disconnector can quickly be reset, thus minimizing the interruption. However, the localization function of a breaker will be needed even though it is replaced with a disconnector in order to keep the outage times low. There might be issues with settings but that is system specific and needs to be investigated system wise. To compare the reliability impacts for different number of breakers and disconnectors, Fig. 3 exemplifies a simple grid. The investigated test case is a four-level tree structured grid that is fed from one feeder and each level represents a three-way split. Table 3 presents the results from this test case in terms of estimated costs and consequences for the customers. Each power line is 2 km with a fault probability of 0.1 fault/km/ year. All customers are located at the end of the fourth level of feeders where 100 customers per end feeder is assumed. Based on the network in Fig. 2, the total number of customers is 2700. The system is evaluated based on SAIDI2 and SAIFI.3 Mean repair time is assumed to be 5 h and separation by use of the disconnectors is assumed to be 5 min (SAIDI5), which is reasonable for a system with motor-controlled equipment. If no motor-controlled system is installed, then a mean time of 60 min (SAIDI60) is assumed. Faults that are cleared by a breaker are assumed to not impact the customer up stream and therefore do not contribute to neither SAIDI nor SAIFI. The material costs for the breaker and disconnector (Cost), including motor control for distant operation (Costmotor), are presented in relation to the full breaker cost without any motor control possibility (see Table 3). These steps can be considered a part of a flexible and adaptive design because the system has a controllable cost by allowing the

2 3

System average interruption duration index. System average interruption frequency index.

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Fig. 3. Tree structure of the lines that are used for the test case. Each box represents a location where either a breaker or a disconnector can be installed.

Table 3 The different steps in the development of the reliability and related cost for the example of the third application. No. Levels w breakers

SAIDI60/ SAIDI5 [min]

SAIFI [No]

Cost***/ Summary of system impact Costmotor*** [pu]

R1 1st

672/276

8

R2 1st 2nd

360/250

2.6

R3 1st 3rd

276/243

1.8

0.54/2.0 (EBR, 2018) 0.54/2.1 (EBR, 2018) 0.64/2.15 (EBR, 2018) 0.69/2.2 (EBR, 2018) 0.86/2.35 (EBR, 2018) 0.89/2.39 (EBR, 2018) 0.96/2.46 (EBR, 2018) 1/2.5 (EBR, 2018)

R4 1st 2nd 3rd 262/242

1

R5 1st 4th

360/250

2,8

R6 1st 2nd 4th 262/242

1,2

R7 1st 3rd 4th

264/242

1,2

R8 1st 2nd 3rd 240/240 4th

0,8

All faults effect all customers and there is no back-up in the system. The system is divided into three parts with a small common part. This enables operators to restart undamaged parts of the grid while isolating the faulty part. The system is divided into nine parts with a larger common part. The system is divided into nine parts with a small common part. Not cost effective Not cost effective Not cost effective Breakers in all standard places.

*Normalized costs in relation to the standard technical solution in this case, direct construction of the system to R8.

operator to choose the suitable level based their circumstances, like reliability requirements and financial situation. 5.1. Technical descriptions of steps The first step (R1) utilizes only one breaker. This is the minimum number of breakers a system requires in order to protect the generating device. As there is no back-up protection in the system a fault occurring in the outskirts of the grid will result in shutdown of the entire system. In the second step (R2) three breakers divide the system into three parts and thus also provide a back-up protection system. The split of the system reduces the impact of disturbances, e.g. a tree falling over a line, significantly as can be seen from both the SAIDI and SAIFI values. Hence, the flexibility increases. The third step (R3) divides the system into nine parts. This further reduces the impacts from faults and improves SAIDI and SAIFI. However, the reduction in SAIDI and SAIFI is lower than in the second step (R2).

The fourth step (R4) places breakers on the first three levels. This is similar to traditional protection design of the grid. The decrease in SAIDI is comparably small while there is a significant reduction in SAIFI. The impact on SAIFI will decrease more because short interruptions are stopped, thus increasing the reliability substantially. The fifth (R5) to the seventh (R7) step are possible but do not provide any additional benefits. However, the cost for each step is higher than for previous steps. In the eight step (R8), each level is equipped with breakers. Having a full set of breakers provides the most reliable and flexible system and the impact of faults is minimized. The main motivation for local utilities to adopt a strategy with fewer breakers is to reduce cost in the system while maintaining acceptable levels of reliability. Due to the often large and inaccessible areas covered by microgrids, transport to the fault locations can be problematic and time consuming. Commonly, in the case of shut down, technicians must visit all disconnectors to identify the fault and manually reconnect the disconnectors. Distance

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monitoring can reduce the time from shut down to restart from some hours to a few minutes. A shift to disconnectors instead of breakers might require additional training of staff and more planned work, with a protocol for fault localization and restart. 6. Implications for the customers In the following, we consider the consequences from the proposed applications from an East African rural customer perspective. Since implications for the customers are mostly related to capacity and protection, the implications of steps D and T relating to distribution lines and transformation can be considered the same. 6.1. Development of capacity 6.1.1. Very low distribution line capacity (D1) A system with battery charging stations provides for small-scale uses such as charging of mobile phones, illumination, TV and radio. Loads that require constant power supply or high capacity to power machinery are not possible. Since the system uses low voltage DC and many appliances require AC, additional converting equipment may be needed. The need for transport to and from charging stations requires time and may result in extra costs for users. Step D1 allows users to access electricity at a small cost, benefit from cost savings and get used to electricity (Ulsrud, Winther, Palit, & Rohracher, 2015). Thereby, this step lays the ground for the introduction of a more extensive system of higher capacity. The step also gives the customers opportunities to discover and learn how they could use electricity. This will allow the customer to be able to estimate more correctly her/his demand on the future system and, thereby, minimize unnecessary investments. 6.1.2. Low distribution line and transformation capacity (D2-D3, T1-T3) These steps provides similar loads as step D1, but allow for uses that require constant availability of electricity, e.g. refrigerators. Already at this stage, electric safety will be a larger issue. More capacity will be available in the system, allowing for more customers to be connected; there may be some capacity for productive uses (at a small scale). In terms of reliability, these steps can improve the service to customers by delimiting disturbances to the affected part of the grid, e.g. to customers connected to one transformer, while maintaining services in the rest of the system. With the possibility to arrange modules such that they provide backup, power can also be switched on again relatively fast, and service provision can continuedalthough at a lower capacitydwhile repairs are undertaken. This flexibility may be very important from customers’ point of view and improve the customer satisfaction. 6.1.3. Standard distribution line and transformation capacity (D4, T4) Three phase provides more capacity and customers can use electricity for productive uses, such as electric machinery in workshops and mills. There are cases where the utility or other actors offer various support to local entrepreneurs, resulting in €stedt, 2015; UNDP, positive economic development (Ahlborg & Sjo 2004). 6.1.4. Improved efficient use of distribution line and transformation capacity (D5-D7, T5) These steps increase capacity, which allows for more customers to be connected, or for existing customers to increase their use. The steps also improve the stability of voltage supplied which means less risk for equipment failure and need for expensive voltage stabilizers.

Introduction of DLR increases capacity during certain times. The load must be controlled occasionally, by prioritizing loads or disconnecting some users. In order to maintain trust between the customers and the utility, it requires well established agreements between both parties. DSM offers a possibility to increase the capacity without adding installation costs, but it requires regulations and agreements between utility and users. Users must comply with rules or risk sanctions. DSM can be flexible in responding to temporary shocks and users have possibilities to influence regulations. Strong public support for DSM can also help minimize free-riding and facilitate enforcement (Ahlborg et al., 2015). As number of customers increases over time, the trade-off between social impact and economic viability can become more articulated, especially when there are many low-usage customers who generate little income for the utility. DSM may not be feasible in response to evening peak load due to customers cooking and turning on lights. DSM can be however effective for fewer and larger loads. For example, in the Mawengi hydropower project in Tanzania, prolonged drought in 2013 and low water table in the river supplying the hydropower station created a situation where the utility sat in meetings with milling machine owners (the largest loads) to introduce voluntary load control. The measure successfully prevented overload and removed the need for load shedding (Ahlborg et al., 2015). 6.2. Protection systems The protection system strategy provides different degrees of reliability and flexibility. For customers, there is a trade-off between affordability and reliability. Higher reliability requires additional equipment and measures, that might result in higher tariffs or connection charges. Even if initial investments in equipment are donated, as is often the case in donor-funded development projects providing electricity, the need for maintenance, replacement of components and technical knowledge increases in more technically advance systems, which can result in higher operation costs. What is an appropriate level depends not only on initial funding arrangements, but also on the local capacity to pay, which varies significantly within and between communities. The costs charged by local utilities depend among other things on the national regulatory environment for independent power producers and distributors, and the level of tariffs that they are allowed to charge their customers. In some of the places where we have carried out studies, we have seen that short and infrequent blackouts are often not a major problem for customers, although these may cause frustration. Losing power for longer time periods can however disrupt business activities causing customers to acquire backup options, often at a significant cost. For the utility, shorter blackouts do not need to have a significant impact on electricity sales. However, if blackouts happen frequently or for longer time periods there can be both economic losses (reduced sales) for the utility and for customers (who cannot conduct business or have to use costly backup systems). Frequent and/or longer blackouts can have a significant negative impact on trust between utility staff and customers and lead to collapse of the microgrid (Hartvigsson, 2018). Customers who can afford to buy solar PV may defect from a local grid that is considered too unreliable. If the national grid arrives, it may be considered a better option resulting in local customers abandoning the microgrid. There are indications that trust plays an important role for maintaining or losing adaptiveness in the face of changing €ng, 2018; Gollwitzer, Ockwell, Muok, conditions (Ahlborg & Bora Ely, & Ahlborg, 2018). Finally, there might be customers that require higher reliability, e.g. hospitals where unplanned blackouts can cause serious harm if these occur during critical treatments requiring electric equipment. Hospitals may keep backup systems,

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but utilities may want to prioritize the placement of breakers such that hospitals or other key customers experience as few power cuts as possible. 7. Conclusions This paper has addressed the challenge of how to size microgrids in contexts where it is hard to predict development of demand. This has been achieved by initiating an in-depth discussion on the available technical and managerial options to reduce initial financial barriers and handle uncertain load development by a more flexible and adaptive design process. We argue for creative thinking around development of capacity and exemplify the approach with three grid applications: distribution line design, transformation and protection systems. The paper demonstrates the technical feasibility of these alternatives that render the power system's capacity flexible in the face of short-term shocks and adaptive to changing conditions. This is important in order to deal with uncertain and changing demand. Using new technologies is a challenge since new skills are required. However, with the new technologies come also new opportunities. The approach is based on a stepwise process with reversibility between most steps and possibilities to skip steps that are deemed unnecessary. The main implication for the utility is that initial investments can be smaller, and the power system's capacity can better adapt to demand. As capacity levels are initially lower, productive uses are limited. The technical skills required by the operators at various steps differ and can be gradually acquired as the system is developed. For users, this approach provides a stepwise and more affordable electricity access, with capacity that can expand. Hopefully, the contextual trade-offs between affordability, reliability and level of technical complexity in the system come out clearly and can be considered and communicated between stakeholders before new investments are made. All steps are technically possible, but have economic, managerial and social consequences for funders, utilities and customers. In particular, we consider impacts on economic viability for the utility, affordability and reliability for the customers. The attention placed on affordability and reliability, in combination with making use of dynamic properties of technical systems, can hopefully increase customers’ trust in the system and sustain it even during periods of higher stress. Further work is required to address other aspects related to flexibility and adaptiveness in the other parts of electric power systems, i.e. the production system and user behavior, before we can assess the overall implications of adopting fully this innovative approach to microgrid design. Conflicts of interest The authors declare no conflict of interest and the founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results. Acknowledgements For the first and third authors, no direct financial support was received for this work. It was done as a part of our employments according to the affiliations. The second author acknowledges the funding from the Swedish research agency Formas and grant number 2017-01012, which supported her during the work on the article. The writing of the article is a joint effort were all authors have contributed with the writing within their specific research field. Jimmy Ehnberg has been responsible for the design of the method, technical descriptions and utility implications. Helene Ahlborg has

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