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1 HubNet Position Paper Series Power Electronics in Distribution System Management Title Authors Power Electronics in Distribution System Management TC Green, RW Silversides and T Lüth Author Contact Version Control Version Date Comments Status Published 30/04/ /02/ /07/ /01/ /4/2014 6/6/ /6/14 11/12/14 5/1/15 Initial Draft by R W Silversides Edited by T C Green Edited by R W Silversides with comments by C M Johnson Edited by R W Silversides Edited by T Lüth Edited by TC Green Version for peer review Edited in response to peer review comments Final version for publication Date Issued 5/1/15 Available from HubNet Position Paper Power Electronics in Distribution System Management Page 1 of 21

2 About HubNet HubNet is a consortium of researchers from eight universities (Imperial College and the universities of Bristol, Cardiff, Manchester, Nottingham, Southampton, Strathclyde and Warwick) tasked with coordinating research in energy networks in the UK. HubNet is funded by the Energy Programme of Research Councils UK under grant number EP/I013636/1. This hub will provide research leadership in the field through the publication of in- depth position papers written by leaders in the field and the organisation of workshops and other mechanisms for the exchange of ideas between researchers, industry and the public sector. HubNet also aims to spur the development of innovative solutions by sponsoring speculative research. The activities of the members of the hub will focus on seven areas that have been identified as key to the development of future energy networks: Design of smart grids, in particular the application of communication technologies to the operation of electricity networks and the harnessing of the demand- side for the control and optimisation of the power system. Development of a mega- grid that would link the UK's energy network to renewable energy sources off shore, across Europe and beyond. Research on how new materials (such as nano- composites, ceramic composites and graphene- based materials) can be used to design power equipment that is more efficient and more compact. Progress the use of power electronics in electricity systems through fundamental work on semiconductor materials and power converter design. Development of new techniques to study the interaction between multiple energy vectors and optimally coordinate the planning and operation of energy networks under uncertainty. Management of transition assets: while a significant amount of new network equipment will need to be installed in the coming decades, this new construction is dwarfed by the existing asset base. Energy storage: determining how and where storage brings value to operation of an electricity grid and determining technology- neutral specification targets for the development of grid scale energy storage. The HubNet Association is a free- to- join grouping of researchers and research users. Join via the HubNet Registration tab at to get access to working document versions of positions papers, an archive of workshop and symposium presentations and to receive notification of future events. HubNet Position Paper Power Electronics in Distribution System Management Page 2 of 21

3 Power Electronics in Distribution System Management 1 Introduction Power electronics as a means of conditioning and controlling electrical power has essentially completely displaced electromechanical means (such as rotary motor- generator sets) and simple resistive controllers in several end- use areas of electricity because of the benefits in making the overall system efficient and controllable. Such examples include the increased use of power electronics in transmission networks in the form of HVDC, as well as in variable speed generation, notably wind- turbines, as the control means used to maximise utilisation of the energy source. Although power electronics is a widespread enabling technology, its adoption in different application areas sometimes progresses in surprising isolation from other applications. Nonetheless, it seems reasonable to expect that the control flexibility that power electronics offers would find ready application in local electricity distribution networks but it appears little deployment has actually happened. The notion of using power electronic controllers for enhancing the operation of the power system itself gained currency during the 1990s under the term FACTS (flexible AC transmission systems) [1]. Some of the candidate FACTS controller such as SVCs and STATCOMs have gained acceptance and been used, in reasonable numbers, in practice. High Voltage DC (HVDC) systems have continued to grow in numbers and ratings [2]. Some FACTS controllers such as the UPFC (unified power quality controller) have yet to become used in earnest. Examination of the future role of power electronics in power systems, e.g. [3], has tended to concentrate on the interfacing of non- traditional generation (wind turbines, photovoltaics, fuel cells etc.), transmission system use (HVDC and FACTS) or niche applications such as microgrids. There are various devices on offer for use on distribution networks at the customer interface, such as active power filters and dynamic voltage restorers. There are also many potential uses of distribution versions of the FACTS controllers (sometimes known as D- FACTS) and proposals for power electronic transformers [4] [5]. Until recently, there was little interest from the distribution network operators (DNO) and little evidence of their use except where exceptional power quality problems exist. However, with the advent of the Low Carbon Network Fund (LCNF) established by OFGEM in 2010 [6], the need for innovative solutions to the problem of network reinforcement in a short time scale has led the DNOs in the UK to start to evaluate power electronic solutions to address their network issues. This has then led to DNOs procuring a number of power electronic devices, in particular storage interfaces, in small numbers in order to trial them on their networks. In the UK, research in this area is being carried out as part of the HubNet and Top and Tail projects amongst other researchers. The work also ties in with the recently launched Power Electronics Centre of Excellence ( However, it is in the interest of the power electronics community to ensure that these trials are a success. It is vital that the devices and converters developed are tested against a range of criteria, not limited to their functionality but also covering their business case and compatibility with leading technical standards. The network interventions developed should not just provide solutions to immediate problems but also open up opportunities for non- conventional approaches, anticipating future trends in the industry. Fulfilling all these requirements will make them significantly more likely to be readily adopted by the target industries. The purpose of this position paper is to explore the slow uptake of power electronics in distribution applications prior to LCNF, to identify the change in innovation thinking among DNOs that has led to this new interest and to identify the most promising use- cases for power electronics in the expected evolution of distribution networks over the next two decades. The paper goes on to recommend areas in which further research should be focused in order to either reduce the barriers to the use of power electronics or to increase the functionality. The objective is to make the control flexibility that is believed to be present in power electronics play a useful role in adapting existing distribution networks to their perceived future use. This position paper is presently in draft form and has been written following workshops held at the EPE 2011 conference in Birmingham and the HubNet SmartGrid Symposium 2011 in London. The intention is to seek reactions to the points raised here and refine the observations before release of a final text to the HubNet community and then more widely still. HubNet Position Paper Power Electronics in Distribution System Management Page 3 of 21

4 2 Characteristics of Distribution Networks In Great Britain the transmission network operates at 400 kv and 275 kv with 132 kv used for some transmission routes and for sub- transmission. Distribution networks operate at 33 kv and 11 kv (with some new 20 kv and legacy use of 66 kv and 22 kv) with the final distribution to domestic and commercial customers operating at 400 V. There is some debate over whether these voltages should be reviewed and whether a level could be eliminated but most of the network is in the form described. With some exceptions, distribution networks use radial lines (double circuit at 33 kv and single circuit at 11 kv) with normally- open and normally- closed points available to switch connections in order to accommodate planned and unplanned outages. Recent design of the distribution network was guided by the assumption that the network would almost exclusively service loads with virtually no generation connected at these voltages. Voltage regulation in distribution networks involved adjusting for variations in voltage drop along feeders as load increased or decreased. Primary substations (33 kv to 11 kv) were equipped with on- load tap- changers (OLTC) and this remains the principal means of regulating voltage. Although a shift to remotely controlled switching actions has led to some of these tap changers being upgraded, a significant number operate according to local, and relatively simple, line- drop compensation. For a general introduction to power systems with examples from UK practice the reader is referred to [7], distribution system practice is covered in [8] and the issues raised by distributed generation are covered in [9]. 3 Challenges in Distribution Networks There are a number of challenges facing the distribution networks as identified by the distribution network operators: a. Increased penetration of renewable energy sources (RES) connected directly to the distribution network may lead to a number of issues for existing distribution networks. Distributed generation (DG) offsets the real power load of a feeder (but perhaps adds to the reactive power draw) and with sufficient clustering of distributed generation, this can lead to power flow reversal on some feeders. In rural, upland and island areas this may be caused by wind- farms and individual wind turbines. In urban areas this may be caused by roof- top photovoltaic panels. b. Increased demand, particularly anticipated demand from heat pumps and electric vehicles (EV) [10], will require more capacity to be made available on the network c. A significant portion of the distribution assets are nearing the end of their effective lifetime and will require replacement soon [11]. d. Heat pumps, EVs and RES can cause additional harmonic current flow on the feeders and threaten to breach limits on harmonic voltages. e. Reduced power flows during the night- time may no longer happen to the same extent and undermine the use of the cyclic rating of transformers and other equipment. f. Increased penetration of DG can make the correct detection of faults on the distribution network more problematic. In rising to these challenges, nothing must occur that undermines the high level of reliability of the network or adds unduly to network use costs. Further to this, from a technical point of view these challenges imply that the distribution network will have to adapt to accommodate certain operational changes, for instance, power flow reversals in feeders could become common place and all assumptions of outward flow of power in designing control, operation and protection will need to eliminated. The following sub- sections outline the technical challenges in more detail. 3.1 Under- Voltage Condition The classic approximation for the voltage difference across a line impedance is shown in equation (1) HubNet Position Paper Power Electronics in Distribution System Management Page 4 of 21

5 ΔV = V S V R RP R + XQ R V R (1) Distribution lines and cables have a relatively low X:R ratio (X/R 3 at 33 kv; 1 at 11 kv and 0.3 at 400 V) meaning that the flow of real power has a significant effect on node voltages, which is in contrast to the case of transmission networks. Heavy loading of a feeder will cause a large voltage drop which to some extent can be corrected by tapping- up at the primary substation On- Load Tap Changer (OLTC). However, the OLTC setting affects more than one feeder and must respect the maximum voltage at the near end of the feeder as well as the minimum voltage at the far end. Further notes on the X:R ratio can be found in appendix A. Heavy loading due to EV and heat- pump deployment is expected to cause some feeders to become limited by the voltage drop along their length. Short, very heavily loaded feeders may become thermally limited before voltage- drop limited but a recent study [12] that analysed a large number of real feeders for a UK DNO indicated that 75% of feeders in medium density urban areas are likely to be voltage- drop limited rather than thermally limited under heavy loading conditions. Traditional means of over- coming a voltage drop limit include re- conductoring with larger gauge cables and lines, and shortening feeders by splitting and installing substations at a greater density. It is possible to replace manual tap changers at secondary substation (11 kv to 400 V) with OLTC to give additional voltage control although this is rare in UK practice. 3.2 Over- Voltage Condition The introduction of significant amounts of domestic PV could cause a reversal of power flow on a feeder which will cause a voltage rise, and not the traditional voltage drop, along the feeder and the possibility of a breach of the upper limit on feeder voltage at some nodes. Reports have started to emerge in Germany, where domestic PV installation is very high, that such voltage rise effects have already started to occur [13]. The Orkney Islands have a large installed capacity of wind turbines which can cause power reversals in feeders under certain conditions and for which active network management has been installed which constrains some generators when conditions such as over- voltage limits are reached or nearly reached [14]. Clearly, technologies that allow renewable generation to continue operating in such cases would have benefits. 3.3 Thermal Limit As the demand for electricity increases in response to the change in the type of demand that is to be supplied, the power flow in the distribution networks will increase. Eventually, the thermal limit of the cables or transformers will be reached, even if voltage limitations have been addressed. The absence of meshing or the ability to control power flow through particular routes means that the problem cannot be directly managed by a network operator. Demand management has been suggested as a solution to this problem, limiting the power flow along the lines at times of high demand or high power injection. Thermal limits are not necessarily simple fixed limits. There may be different limits for different seasons (given the different ambient temperatures) and some equipment, notably large substation transformers have cyclic load ratings that assume periods of cooling overnight. The use of dynamic ratings that account for actual or estimated equipment temperature is gaining favour as a way to release network capacity [15]. 3.4 Reverse Power Flow In principle, the lines and substation transformers of a distribution network can accommodate reverse power flow provided that the voltage rise is within limits. In practice, some control and protection equipment will have been designed assuming unidirectional power flow. This can apply to the line drop compensation element of the controller of an OLTC and some lines, particularly double circuit routes at 33 kv, may be fitted with directional over- current protection that assumes reverse power flow is a fault. It might be that any power flow management introduced to make better use of the thermal limits of equipment could also manage reverse power flow [15]. HubNet Position Paper Power Electronics in Distribution System Management Page 5 of 21

6 3.5 Phase Imbalance Domestic and some commercial customers (in the UK at least) have single- phase connections and so imbalance in three- phase equipment is expected. This may be structural because of uneven distribution of connections between phases or time- varying because of differences in consumption patterns of consumers. Unbalanced loads cause higher conduction losses than balanced loads of the same power and give rise to greater difficulties with under- and over- voltage. The lack of monitoring of individual phases means the extent of the problem is often not known and estimation of the problem can be difficult if records of phase connections are not reliable. Imbalance can reduce network utilisation and static and dynamic balancing solutions would be useful. A number of LCNF projects cite phase imbalance as a major cause of capacity limitation [16]. 3.6 Fault Level Networks are designed to be able to survive and indeed interrupt significant fault current. Many distribution feeders already have very high fault levels [17] which cannot be increased as this would endanger the network infrastructure, in particular the circuit breakers that are present to isolate the faulted equipment. Analysis has shown that distributed generation based on induction machines or synchronous machines contributes to the fault level in the network [18]. PV and other inverter connected generators present less of a problem as their control systems limit their output under fault conditions. However, DNOs are aware that large numbers of small, inverter connected DG will also increase fault levels in these cluster areas [17]. Traditional methods of controlling fault level would involve splitting substations so that loads are fed through a single transformer; however, this has an effect on system security and can lead to increased load on some transformers. 3.7 Waveform Distortion and Harmonics Loads and generation with DC/AC power conversion are prone to causing waveform distortion perhaps as low- order harmonic distortion or as injection of high frequency components related to Pulse Width Modulation (PWM) switching. Low- order harmonics, especially triplen harmonics can cause additional losses in lines and transformers. Higher frequency terms may pose interference risks [19]. 4 The Role of Power Electronics in Future Distribution Networks Whilst DNOs have recognised the need for asset replacement and reinforcement, [20], it can take a significant amount of time before such investments are fully implemented due to their scale. Some electrical problems on the network however may require a more immediate response It was noted in section 1 that DNOs are aware that traditional reinforcement techniques is unlikely to deliver the additional network capacity they need in the short term in order to supply the evolving loads and the additional connection of DG to their networks. In section 3.1 it was noted that the traditional approach to enhancing a network to overcome an under- voltage condition would be to replace the line or cable with one of a higher cross- sectional area or perhaps to split the feeder to reduce its length by adding a substation. This approach could also apply to over- voltage conditions and thermal rating breaches. A number of DNOs have outlined that they are considering these solutions. If, however, they were applied widely, they would take time to deliver because of planning, land purchase and lead- time issues and cause significant disruption during the works. Also, these interventions establish a large amount of additional capacity in one go and so are under utilised in their early use. That investment and disruption can be deferred for several years by smaller scale interventions that use flexible control to release latent capacity in existing traditional assets. This is the beginning of the case for deploying power electronic interventions. Further, in some cases a reinforcement with traditional assets may be unfeasible due to space limitations. This may happen in densely populated urban areas, where acquiring additional land can come at a forbidding financial cost or may simply take too long due to legislative and administrative hurdles. The flexibility in the design of the power electronic intervention might allow some system aspects to be tailored to be particularly space effective. HubNet Position Paper Power Electronics in Distribution System Management Page 6 of 21

7 Other application areas offer a more permanent role for power electronics: for example electronic on- load tap changers or low- voltage power electronics for Soft Open Points [21]. Solid- state fault current limiters are already under consideration by at least one DNO [22] for deployment in A range of power electronic devices was assessed by Parsons Brinckerhoff for the Carbon Trust [23]; the study examined 6 technologies: 1. Active in- line voltage regulators; 2. All- electronic power transformers; 3. Flywheel energy storage; 4. Solid state fault current limiters; 5. Low voltage switching devices; and 6. On- load tap changers for secondary substation transformers. With the exception of energy storage, the devices considered by The Carbon Trust are, or could be, based on power electronics or a hybrid of power electronics and another means. Energy storage as a means of releasing network capacity has already gained some momentum among DNOs, with a number of trials outlined [22] [24]. However, energy storage will not form part of this discussion because the power electronic element of it is largely a facilitator for the energy storage and any additional functions (such as reactive power provision, or phase balancing) are present in other devices. The possible power electronic interventions in distribution networks will now be discussed by function. 4.1 Voltage Control If a feeder is experiencing a voltage rise or fall problem then this could be tackled either at the substation, part- way along the feeder or at the point- of- load. The ability of tap changers to affect system voltage is well- established; however, 400V transformers (in UK practice) rarely have on load tap changers (OLTC) and are often installed in small spaces that would not allow for addition of an OLTC. Electronic transformers [4] or transformers with electronic or hybrid on- load tap changers [25] could, in principle, be installed in any of the three locations identified above but in practice would be subject to space constraints. Traditional tap changers are typically installed on the HV side of a transformer and adjust all the outgoing feeders in concert. In contrast electronic transformers could be configured to control the voltage levels on individual feeders. An alternative method of voltage control would involve the installation of Unified Power Quality Controllers (UPQC a small UPFC); the advantage of this architecture is that it only processes a small percentage of the load and hence these devices can be installed in a mid- feeder location in a link box or LV pillar in urban streets. However, existing solutions [26] typically contain large 50 Hz transformers so a more compact solution using high- frequency power electronics would need to be sought. Reactive power compensation on distribution networks is currently being pursued by at least one DNO [22] using STATCOMs. Their intention is to connect these devices primarily to lines operated at 11 kv and 33 kv. The efficacy of providing reactive compensation very much depends on the characteristics of the network in question. Distribution networks tend to employ cables more than overhead lines in most UK urban areas and under- ground cables tend to have a smaller X:R ratio than overhead lines. LV lines have a lower X:R anyway. A device dedicated to reactive power compensation on 400 V feeders may have little value but reactive power as an additional feature of a device with another primary purpose may add some value. Voltage problems could be addressed by adding further voltage control measures such as OLTC in secondary substations where the OLTC may be implemented (at least partially) through power electronic means. 4.2 Power Flow Control To facilitate an increase in power flows through existing distribution networks (to support EV charging etc.) the stress on feeders that are power flow constrained can be relieved by interconnecting (meshing) what is otherwise a radial network to make use of capacity on less heavily loaded routes. This is recognised in the developments plans of several DNOs e.g., [11] [20]. A step toward this is to close some of the existing normally HubNet Position Paper Power Electronics in Distribution System Management Page 7 of 21

8 open points (NOPs) that are provided for post- fault supply restoration. However, closing the NOP may lead to excessive fault current or violation of assumptions in the design of protection grading. The difficulties of meshing a previously radial system can be overcome to some extent by soft meshing, that is, replacing normally- open points with so called soft open points (SOP) comprising, for instance, back- to- back power converters [21]. These allow control of real power flows, fault current blocking and reactive power manipulation. A note on meshed networks and summary of findings of [13] can be found in appendix A Phase Rebalancing Mid- feeder compensators (of the UPQC style), SOPs, DVRs (dynamic voltage restorers) and STATCOMs usually contain a common DC bus. This allows exchange of instantaneous power between the phases that can be used to rebalance current flow on a feeder. This capability could also be included as part of a power electronic or hybrid substation to assist in the reduction of losses in the 11 kv network. DNOs in the UK have stated that they would consider a power electronic solution to this problem notwithstanding a general lack of experience with power electronics since few other means exist to mitigate this problem. 4.4 Active Power Filtering There is a wealth of literature on active power filters [27] [28] [29] covering many application examples, circuit types and control schemes. They have seen some application with railway traction equipment and within customer premises but not as network elements compensating a group of consumers. Given the tendency of low- order harmonics to appear with approximately similar phase angles (through synchronisation to the fundamental voltage) and simply add in magnitude 1, there may be a value for compensation at network level. DNOs have started to analyse their networks in locations with a high penetration of PV and some LCNF project descriptions mention the need for filtering in networks with power electronic compensation devices. It is possible that even if all customer equipment on a feeder meets the relevant product standard (e.g., IEC ) their cumulative effect would cause voltage harmonics beyond the network planning standard (G5/4-1, G59/3, IEEE 519) which would require the DNO to take action to reduce the supply impedance or filter/compensate the harmonic currents. 4.5 Associated benefits The introduction of power electronic converters into distribution systems to solve a particular difficulty, such as a voltage control problem, is likely to bring with it additional sensing, monitoring and communication infrastructure to the locality. This acts as a bridgehead for tacking other problems which could not have carried the full cost of this infrastructure. This is an additional benefit of the first installation. Further, the power electronic converter may well have capacity and flexibility to tackle other problems (such as phase imbalance). The need for dynamic control systems within the converters means that they must carefully monitor the local network. This information could be made available for wider network control purposes and this would allow the converter to assist with demand management and also the mitigation of the problems caused by reverse power flows by providing additional protection functionality without additional investment in equipment and communications. 5 Meeting the Business Case for Power Electronics in Distribution Networks The preceding sections have discussed the challenges facing distribution networks and suggested ways in which power electronics could help to overcome the problems or at least reduce or defer the cost of a longer term solution. However important it is to meet the technical challenges faced by the DNOs though, power electronic inventions will not be deployed unless the business case for the investment is sound, a point that was strongly made in workshops and interviews conducted as preparation for this paper. DNOs list the 1 (Note that diode rectifiers tend to drawn pulses of current at the voltage peak and the low- order (3 rd, 5 th and 7 th ) harmonics align to these peaks have similar phase angles across various loads. Current controlled devices can exhibit an harmonic alignment effect also. [40] HubNet Position Paper Power Electronics in Distribution System Management Page 8 of 21

9 pressure to keep costs low as one of their primary challenges. The perceived risk stemming from the lack of track record of power electronics in distribution applications means that a risk premium is sometimes built into the business case making the case more difficult. A consensus view in workshops was that shaping power electronics design to make a good business case was a greater challenge than solving technical and control issues. 5.1 Costs of power electronics DNOs will weigh up the cost and the perceived benefits of any new device as part of the process of deciding for or against its installation. As many of the power electronic devices provide an alternative to a traditional reinforcement solution, their cost can be compared against them to determine when it becomes favourable to deploy power electronics. The annual cost (C!" ) can be calculated as shown in (2) as the sum of an annualised cost of the capital investment plus the operational and maintenance costs. C!" = K! + E A! K! + K! (2)!!,! where: A!!,! = annutity factor for device lifetime and cost of capital rate (r) T! = lifetime of device in years E! = electrical losses per unit time K! = cost per unit electrical loss K! = mechanical and maintenance cost per unit time K! = investment cost The annuity factor can be calculated as described in (3). 1 A!!,! = r!! r (3) Each of the factors in the cost equation is important and is discussed in further detail in the following sub- sections Investment Costs It is clear that the investment cost is a key part of the business case and that semiconductor technologies are relatively expensive compared to electromagnetic technologies. However, there are further important considerations. One such is the rating of the power electronics relative to the network impact being sought. The doubly- fed induction generator (DFIG) serves as a good illustration of this. The DFIG became popular in variable- speed wind turbines because the relatively narrow speed range meant that a pair of power converters rated at about 30% of the generator output could have control influence over all of the power flow. Thus the DFIG was more cost effective than a full- converter solution [30]. The UPFC is similar in some ways in that a relative low rating of series converter, of say 0.2 p.u., can have influence over the total power flow in the line. Applying these ideas, variously known as thin power electronics or hybrid power electronics, is a route to driving down costs other than direct cost reduction in the underlying technology. An important part of the investment cost can be the cost of housing the equipment. Power converters in the megawatt range are substantial items of equipment and distribution network substations are often limited in space, especially in urban environments. The cost of purchasing or renting space and the cost of pursuing agreements can be substantial. Thus there is a pressure to reduce the footprint, volume or mass of the power electronics. Increasing the power density of power electronic devices is possible through device as well as circuit innovations. Circuits that utilise an internal AC connection can be operated at higher frequencies to minimise passive and magnetic components. New materials can allow circuit components to operate at higher temperatures reducing the volume of cooling equipment required. HubNet Position Paper Power Electronics in Distribution System Management Page 9 of 21

10 The equation for the equivalent annual cost (see (2)) presently assumes zero salvage value at the end of the lifetime and zero disposal cost. To increase the accuracy of the comparisons these items could also be taken into account, but would require knowledge of salvageable values for all solutions which are to be compared. The value of power electronics at the end of their lifetime may be less clear than that of an old transformer which contains a significant amount of reusable metals Lifetime A factor that will greatly affect the business case of the power electronic device is its expected lifetime because it has a direct impact on the annualised cost of the capital investment. In practice this would be seen in terms of the cost of replacing short lifetime equipment more frequently than long lifetime equipment. Traditional distribution network assets have had expected lifetimes of 50 or more years [11] which contrasts with presently expected lifetimes of 20 to 25 years for power electronic devices [22]. Regardless of whether designing for temporary (reinforcement deferral) or permanent (replacement avoidance) interventions, the lifetime will be a key factor in the business case. Uncertainty in lifetime will have a negative impact by introducing a risk premium. The technologies used for distribution network equipment are mature and as such, the capital cost is well understood. It is supposed that DNO planners consider power electronics to be expensive, but also that their depreciation model is not well understood. Given that the lifetime of a new converter, especially one that is using the latest technologies, cannot be guaranteed, it is difficult to plan the purchase and replacement of such equipment Electrical Power Losses The dominant term in the operation cost of most network equipment is the cost of the power losses incurred. When asked about the potential of power electronics in their networks, most DNOs have cited losses as a major consideration for them. In transmission- level projects, notably HVDC links, the expected converter losses, determined over the lifetime of the equipment, are often capitalised and added to the system cost, giving an incentive to maximise efficiency. To illustrate the difference between power electronics and traditional equipment, presently available DC/AC converters with a power rating suitable for distribution networks of 230 kva, have a quoted full load efficiency of 95.9 % [31]. This figure includes not only the semiconductor losses but also the peripheral devices associated, like PWM filters and line reactors. This compares with a rated full load efficiency of 98.9 % for a distribution transformer rated at 250 kva [32]. This is only a single comparison but illustrates that there is some way to go before power electronics will be directly competitive on power losses. However, it may be that somewhat higher equipment power losses can be tolerated within the business case provided there is a greater benefit to the system, possibly even in terms of reducing losses in other network equipment through better management of power flows. The current round of LCNF projects contain several examples of power electronic demonstrations and so it is clear that there is a belief that the relatively higher power losses can be outweighed by the benefit to the network or that power losses can be reduced by further refinement of the equipment as deployment gathers pace Mechanical and Maintenance Costs Two key indicators of performance for Distribution Network Operators are the number of customer interruptions (CI) and the number of customer minutes lost (CML) [33]. This emphasis on quality of supply to end consumers means that reliability of individual pieces of equipment is very important. The inspection and maintenance regime of the power electronics could be a major cost item. If certain components are known to be a failure risk (DC bus capacitors are an often quoted example) then periodic replacement of these components in the field may be necessary. This would contrast strongly with most other equipment in distribution (11kV to 400V) substations where an annual visual inspection and some vegetation control may be the only maintenance cost. Uncertainty in failure modes and in required maintenance regimes may lead to a very conservative approach and high costs. Unplanned outages which cause customer interruptions will have a high attributed cost. HubNet Position Paper Power Electronics in Distribution System Management Page 10 of 21

11 5.2 Other barriers to the introduction of new PE devices As well as meeting the hard business case requirements for new power electronics devices to be used on the distribution network, a number of soft requirements have to be met Training and common guidelines The engineers that work on distribution networks are not used to working with power electronic converters and the introduction of such devices would necessitate an expensive training process. Further to this, network planners may require additional tools to incorporate power electronics into future networks. Neither the current distribution code nor the BSI documentation specifies any guidelines for power electronic devices on the distribution network. A common set of guidelines and requirements would help to focus the research and design efforts We don t need it There may also be a feeling amongst distribution network planners that they can solve their network problems without power electronics. The DNOs are aware of the devices used in the transmission system, but distribution networks have very different challenges and they possibly do not value the broader benefits of power electronics. The LCNF (and in future the innovation incentives under RIIO- ED1 [34]) is an excellent opportunity for the power electronics community to promote and demonstrate power electronic interventions to the DNOs. The DNOs have recognised this in some of the LCNF submissions. Of the four projects accepted for full submission to the Second Tier projects LCNF in May 2014, one included proposals for the use of power electronics on distribution networks [35]. 5.3 Comments The preceding sections list a number of factors in the business case of power electronic network interventions which are considered to need considerable further attention before the deployment of power electronics in distribution networks could become commonplace. The building of acceptance of power electronics solutions is likely to need at least as much effort on proving that the cost factors have been well characterised as in proving that the benefits can be realised. Using the LCNF submissions as an indicator, the overriding concern for DNOs is the lack of capacity on their networks for the anticipated future growth of load and generation. Increasing network infrastructure is certainly a way of accommodating this growth but the cost of this approach is high and therefore alternative approaches are being sought and explored. Power electronic solutions will compete not just against traditional reinforcement but also against demand management, dynamic line ratings and energy storage. The controllability and versatility of power electronic interventions can bring a wide range of benefits but the benefit/cost ratio is what really matters. Power electronic equipment may have a particular role in deferring other measures through temporary installation of a device. This can be used to provide some capacity release pending a reinforcement when a more substantial capacity increase is indicated. This is helpful where reinforcement would initially provide a much large capacity increase than need. In generation technologies, end- use and transmission (notably HVDC), power electronics has a clear business case for its use and is widely accepted. Until recently, power electronics was not considered for deployment in distribution networks. It is evident now that field trials and demonstration projects are underway and while that does not say that the case has been made, it does indicate an appetite to test and understand the benefits and costs. It appears as if research in power electronics for distribution networks may now enter a new phase in which speculative development of circuit ideas progresses to a detailed attack on the underlying issues determining cost, density, efficiency and reliability which if improved would decisively tip the business cases in favour of use. 6 Recommendations The aim of this document was to review the problems presently faced by the distribution network operators and review the potential roles power electronic devices can play in mitigating these problems in the short and long term. Section 5 outlined the requirements for any new device to become feasible from the DNO s point of view. This section summarises the resulting research questions. HubNet Position Paper Power Electronics in Distribution System Management Page 11 of 21

12 6.1 Primary Research Effort The systems listed in section 4 can be created using circuit topologies, design methods and control schemes well- established in other application fields. Therefore, the focus of the research effort in power electronics for distribution system use should be on the barriers to the application of power electronics as listed in section 5. Figure 1 illustrates the design goals that need to be fulfilled to make power electronic devices more attractive and feasible for use on distribution networks. Figure 1: Device properties to be improved by research: red illustrates the current state and green the goal of the research Power Density If a device is to be installed in a pavement box or an LV pillar in the street, up a pole or in a transformer chamber, a key physical characteristic is its power density. This is a measure of how much power the converter can process compared with its volume (kw/m 3 ) or in some contexts its mass (kw/kg). Power electronic converters in transmission system applications tend to be low density because of the large clearance distance needed at EHV. In industrial drives the density is also somewhat low because of the use air- cooled systems. In aerospace density is a very important factor but may be emphasised too strongly in relation to cost for the solutions to be applied in distribution networks. As EV technology matures one might see some of the impressive power densities [36] become available at costs appropriate for network use. The previously mentioned distribution transformer [32] achieves an effective power density of 194 kva/m 3 at an efficiency of 98.9 %. Comparing this with a power density of 388 kva/m 3 and efficiency of 95.9 % for the DC/AC converter [31] of a similar power rating, illustrates the trade- off that in many cases has to be made: an increase in power density is sometimes at the expense of efficiency and often with the burden of a more demanding cooling system. A better understanding of the underlying reasons for these trends could significantly help in tailoring power electronic solutions to particular applications Thermal Management Increasing the power density of converters makes the problem of cooling them much more challenging as there is less space for the cooling medium to circulate around the components. Another research area that is of great importance is therefore examining new cooling techniques and allowing the components to operate at higher temperatures. It is recognised that getting the heat out of the components is only part of the problem; this waste heat must be dissipated and this dissipation is another area that needs to be investigated. This point could become significantly more important as the thermal cycling of existing transformer assets is disrupted by non- conventional power flows during the cooling- off periods. HubNet Position Paper Power Electronics in Distribution System Management Page 12 of 21

13 6.1.2 Durability The lifetime, or durability of the device, was seen to be an important factor in the feasibility assessment section 5.1. Existing distribution network assets like transformers and cables have extremely long lifetimes compared with conventional power electronics. Research to extend the life of power electronic alternatives would make them more attractive for future applications Efficiency As the annualised cost of an item of power electronic equipment will have a significant contribution from the power losses, it will be important to reduce those losses. There is a secondary benefit in that reduced losses places a lower requirement on the cooling system (which may in turn help to improve the power density). An important factor for this will be reducing the losses in the converter and this emphasizes the importance of the investigation of new types of component (e.g. SiC and GaN semiconductors). Similarly new magnetic materials will help reduce losses in specialised transformers, particularly as several of the approaches to high- density power converters make use of high frequency internal AC links requiring magnetic components. The pursuit of thin power electronics (see section 5.1.1) with partially rated converters has a benefit in efficiency as well as in investment cost and new opportunities to pursue this approach should be sought Reliability It is important to both work to raise reliability of power converters and work to accurately characterise the reliability to avoid unnecessary conservatism. Improved reliability can be tackled through component reliability, operating regime (particularly the control of thermal cycling) and the use of redundancy (as is seen in the modular multi- level converter). There are likely to be trade- offs between cost, density and reliability that need to be characterised and optimised. It must be recognised that reliability is not just an issue of cost. The power converters in question will most likely be much closer to the public than for transmission systems examples and failure of the devices must be contained and not lead to danger to those in the vicinity Condition Monitoring Related to reliability, is the ability to monitor the converters and to understand how they degrade over time. This information can be very useful in preventing failures and ensuring that the DNOs get the best value for money out of the capital expenditure. Research is needed into the way in which the new devices behave and how their performance varies with age Network Protection Changes in the way distribution networks operate may affect the operation of the protection systems presently installed. Research is needed into ways in which the monitoring systems in power electronic converters can be applied to new protection systems for the distribution network Cost Density and Cost- Benefit Analysis A recurring theme of workshops and discussions and therefore of this paper has been the need to focus attention on the cost- benefit case, not just bring forward benefits. Further, it is important to have all aspects of cost of ownership in mind and important to be able to quantify benefits in monetary terms. There are broad alternatives to power electronic solutions that form a competitive case. This does not mean that the focus of progress is simply commercial; there are research challenges in making a fundamental attack on costs and analytical challenges in quantifying benefits. A key potential benefit of power electronic interventions is the avoidance or deferment of extensive investment in traditional network reinforcement. DNOs have indicated plans to upgrade their existing assets (transformers and cables) to the next highest capacity to cope with additional load and power flows [11] [37]. The additional cost of installing an up- rated transformer, for example, has been estimated to be only an additional 6% addition to the cost of a like- for- like replacement ( [37], section 2.102). Since a significant portion of the existing assets are due to be replaced in any case, a lot of this kind of capacity upgrading can be expected in the near future. It is important that the costs and benefits of power electronic solutions need be quantified for direct comparison of alternative upgrade paths. This will require models of the networks with HubNet Position Paper Power Electronics in Distribution System Management Page 13 of 21

14 and without the power electronic systems to be created that allow the value of these new converter systems to be quantified. 7 Conclusion It is evident from the LCNF projects running in the UK that network operators envisage that capacity shortages will emerge in distribution networks as efforts to decarbonise the electricity system progress. The two concerns are the growth of clusters of PV installations and growth of EV charging and perhaps heat pumps. The specific capacity constraints are under- or over- voltage conditions cause by increased power flow for some feeders and thermal limitations caused by increased power flow on other feeders. Both difficulties can be exacerbated by unbalance that causes a voltage limit to apply on one phase before others or by causing higher heating effect. The overall question is whether there is a research effort needed to bring power electronics to bear on these problems. Power electronic devices, principally the STATCOM, are recognised as effective and cost- effective solutions to voltage control challenges in transmission systems and could be applied to the EHV sections of distribution networks. At lower voltage levels, particularly at LV (400 V), the fact that the feeders are resistance dominated (low X:R ratio) makes reactive power compensation much less effective. Series voltage injection with real power manipulation (via a UPQC or similar) is indicated instead. Power electronic controllers (which generally have a shared DC bus) can be effective at correcting current and voltage imbalances alongside other actions. Realising an increase in power transfer through a previously voltage limited feeder with a power electronic device is a realistic prospect. Relieving power flow constraints requires options for routing power flow which are generally not available in radial networks other than through switching options. However, placing power flow control devices across existing open- points (the so called soft open- point) or creating new points to do this appears to be a useful way to make the network more meshed without changing fault current levels. These facilities can allow relief of power flow limits by an additional and controlled flow through an unconstrained route. Power electronic interventions might be temporary measures to defer a more extensive upgrade by several years or may be a long- term solution in their own right. These technical opportunities are known and the subject of research work and UK demonstration projects within LCNF. The technical barriers to wide deployment are really issues of proving that benefits can be realised in practice through field trials (which is a motivation for an LCNF project) and finding a control and communications arrangement that is suitable (which is really part of a wider debate on approaches to active network management). The LCNF projects will also have been built on a business case that the cost- benefit of such measures would, at a future date, support adoption as a routine method of enhancing a network. However, the costs may not yet be at that point and need further work, and the benefits may need refinement. A number of fundamental engineering research challenges have been identified which aim to improve the cost- benefit analysis in favour of power electronic solutions..this covers capital cost of equipment, capital cost of installation (related to space use), lifetime (for annualised cost of investment), cost of maintenance, cost of operation (which is losses) and cost of unplanned outages. Although these issues are a matter of industrial engineering refinement they can also be the subject of a research effort to achieve a major shift in life- time, reliability, efficiency and power density. Here materials science, semiconductor devices, packaging, circuit topology innovation and control could all be brought to bear. It was also noted that over recent years the knowledge of what power electronics can offer has grown inside distribution network operators and also that more of the power electronics community is aware of the challenges that distribution networks community faces. However, these are still two communities of engineers who would benefit from a deepened mutual understanding of the challenges and opportunities for radical redesign of distribution networks through power electronic interventions. HubNet Position Paper Power Electronics in Distribution System Management Page 14 of 21