DAI Jie， MA Zhao， SHANG Yu-wei， ZHANG Wei, ZHOU Li-mei

(1. Scottish and Southern Electricity Networks, Reading RG1 3JH, UK； 2. China Electric Power Research Institute, Beijing 100192, China)

Abstract: This article aims to provide an overview on UK renewable distribution generation (DG) market and to present the progress and improvement made by distribution network operators (DNOs). New relevant government regulation and incentive mechanisms are briefly explained. The typical DG connection cycle is introduced. The main challenges faced by UK distribution network operators as a result of high level DG penetration and innovative solutions are discussed with project examples. To overcome common issues caused by DG like higher fault level, voltage rise, and reverse power flows, solutions are proposed such as fault current limiter, FACTS device, energy storage and DC interlinks. Also by equipping real time system analysis and planning tools, network restrains can be relieved and more network capacity can also be released.

OverviewofrenewabledistributiongenerationinUKandDNOprogress

DAI Jie1， MA Zhao2， SHANG Yu-wei2， ZHANG Wei2, ZHOU Li-mei2

(1. Scottish and Southern Electricity Networks, Reading RG1 3JH, UK； 2. China Electric Power Research Institute, Beijing 100192, China)

Abstract: This article aims to provide an overview on UK renewable distribution generation (DG) market and to present the progress and improvement made by distribution network operators (DNOs). New relevant government regulation and incentive mechanisms are briefly explained. The typical DG connection cycle is introduced. The main challenges faced by UK distribution network operators as a result of high level DG penetration and innovative solutions are discussed with project examples. To overcome common issues caused by DG like higher fault level, voltage rise, and reverse power flows, solutions are proposed such as fault current limiter, FACTS device, energy storage and DC interlinks. Also by equipping real time system analysis and planning tools, network restrains can be relieved and more network capacity can also be released.

renewable energy; energy storage; power distribution network; DC distribution network; active network management (ANM)

Supported by the State Grid Corporation of China (SGCC) “Thousand Talents program” special support project (EPRIPDKJ (2014)2863)

Abouttheauthors: DAI Jie, born in 1982 in Chongqing, male, senior engineer, Ph D. Research directions include power system analysis, substation design and project management, etc. MA Zhao, born in 1957 in Shanxi, male, National Distinguished Expert of “1000 Elite Program”, Chief Expert-Smart Power Distribution Networks for China Electric Power Research Institute (CEPRI), Ph D. Research directions include smart distribution network planning and asset management, intelligent T&D equipment, MVDC and Energy Internet, etc.

1 Introduction

British government has committed itself to transfer UK permanently into alow-carbon economy and meet 15 percent renewable energy target by 2020 and 80 percent carbon reduction target by 2050.

The power network has been delivering secure and affordable electricity since the market was privatized in 1980s. However it cannot meet the challenges of the future low carbon, more diverse, more sustainable electricity mix[1].

In this decade around a quarter of existing capacity, mainly coal and nuclear power stations, has been or will be closed. The electricity market was reformed to help renewable generation competing with conventional power stations and the market share of renewable generation will expand rapidly as shown in Fig.1.

Fig.1 UK renewable generation growth[2]

At distribution level, embedded generation or distributed generations will make a large contribution to meeting the renewable and low carbon targets.

Future power distribution network is required to be able to integrate a high volume of renewable DGs. The power network reinforcement is unavoidable and the challenge is to meet the future in cost effective way with innovative and smarter approaches.

2 Market incentives and regulations

UK government has introduced financial incentives to encourage investment on small scale renewable generations less than 5MW, in parallel with new regulations superimposed on Distribution Network Operators for a better connection services.

2.1Financialincentives

(1)Feed-in tariffs

The name of feed-in tariffs (FITs) was originally applied to the system in Germany. The tariffs are payments to anyone who owns a renewable electricity system, for every kilowatt hour they generate.

FITs were first introduced in the 2008 Energy Act in UK and became payable in April 2010. The price was set so that investors should earn back the capital cost at least two to three times over the duration of the tariffs, which is 20 to 25 years[3].

Renewable generations larger than 5MW remain eligible to earn renewables obligation certificates (ROC) within the existing renewables obligation quota mechanism. To prevent companies from moving large scale (for example big wind) projects from the ROCs to the Feed-in tariff program, a number of anti-gaming provisions have been inserted in the policy design; this should avoid the breaking up of bigger projects into several small ones, to fit within the 5MW energy size cap.

FITs compensation will reduce per annual following a mechanism called “degression” as the costs of low carbon technologies are expected to reduce and gradually become more competitive to conventional generations[4].

(2)Losses reduction reward

DGs have the advantage of feeding local loads and reducing network losses compared to conventional power station generations which have to transport electricity through transmission and distribution network to end users.

Financial reward is offered by DNOs to DG owners for the losses reduction they contributed to the distribution network. This is true in most cases where the local demands are larger than DG infeed.

Technically the losses reduction is quantified by the calculated loss adjustment factor (LAF), which is later developed as line lose factor (LLF). The methodologies are published by DNOs and regularly audited by regulatory body ELEXON.

Generic LLF is assigned to non-EHV customers who are connected to 11kV networks and below. Site specific LLF is calculated for each large EHV customers who are normally either connected to EHV network or directly onto 6.6kV or 11kV busbar.

Most DNOs adopt a method developed by EA technology to calculate the generic LLF for their network at each voltage level. The formula is as below[5]:

(1)

WherePinis the power into voltage level from higher voltage level;Poutis the power out of voltage level into lower voltage level;fis the fixed loss constant for voltage level;vis the variable loss constant for voltage level;Lis the metered sales at voltage level;Gis the metered generation at voltage level.

The site specific LLF calculation can vary among different DNOs. The logic to calculate site specific LLF is generally simple, which is determined by comparing losses of the whole network under a grid supply point (GSP) with or without the DG. This can be carried out on a power system modelling tool. For example, under a defined load scenario a power flow study is carried out and the Grid Infeed-Total Load=System Losses. Another power flow study will be carried out adding one unit power (1kW or 1MW) input to the DG connection point to obtain System Losses again. The reduction of system losses per unit power DG input can be then easily calculated.

2.2Guaranteedstandard

In 2010 UK electricity regulatory body OFGEM revised the license conditions for DNOs and the guaranteed standard for DG connection is added to license condition section 15A.

DNOs are required to answer enquiries, provide quotations in time, commence and finish work per offer to DG applicants. Failure to meet the guaranteed standards will incur financial penalties as Tab.1 for examples.

Tab.1 DG connection guaranteed standard and

Notes:0MW means maximum working days.1MP means maximum period to provide quotation and penalty of failure.2LP means late payment per working day of delay.3PiFC means penalty payable per day if failed to commence work at agreed date.4PiFM means penalty if failed to make connection at agreed date.

DNOs will be also measured by customer satisfaction level. A high score will be financially rewarded by OFGEM each year.

3 DG connection process and challenges

3.1DGconnectioncycle

Before contacting a DNO, early stage research can be carried out by DG developers based on available information published by DNOs like long term development statement (LTDS), which also includes useful network technical data.

A few DNOs have facilitated a mapping system on their website to indicate network restrains for new connections. This not only immediately provides price signals to intended DG developers, but also potentially saves DNOs’ staff time providing quotations that will not be financially attractive.

Developers can ask for extra information such as budget estimation and feasibility studies from DNOs with fees to pay. DNOs have the obligation to respond following Guaranteed Standard.

The work and activities required for a DG connection can be either contestable or non-contestable work. Non-contestable work generally refers to network alternation directly involve DNO existing assets and will normally be carried out by DNO itself. Contestable work like DG site substation and power line to DNO connection point can be carried out by independent connection provider (ICP) hired by DG developer. Contestable work can be carried out by DNO itself as well if requested by customers.

DG developer can discuss with DNO to decide who to construct the infrastructure and make formal application. Once the application is received, DNOs have to make offer within certain period. The offer letter normally breaks down the work required for network alternation or reinforcement. The price and estimated connection date should also be included.

Following the construction stage the commissioning of DG can be made on the agreed date. The requirements and tests for the commissioning DGs are set out in EREC G59 (Section 12). The DNO can choose to witness commissioning test with a charge. In this case DG owner have to submit a scope, and the date and time of commissioning tests at least 15 days before the commissioning date.

DG developers usually race to commission its generation before government low carbon tariff price change each year, which has significant financial implication. The pressure form DG developers will be passed to the ICP and DNO making the connections.

3.2ChallengesimposedbyDGconnections

The financial incentives as discussed above have attracted many developers to invest in renewable generations. A great number of domestic customers also benefit from installing solar panels on their roofs.

UK DNOs have been receiving high volume of DG connection applications since 2010. The increased work load, together with guaranteed standards, has mounted certain pressure on most DNOs. In RIIO ED1, a new 8 year long price period, more incentives have been introduced to improve DNO service provided to customers including DG connections. A DNO could lose a few million pounds per year for low standard of service provided to connection costumers. On the other hand extra cash in the scale of a couple of million pounds per year can be awarded if a DNO out performs OFGEM benchmark. Many DNOs have chosen to offer quicker response and connection to DG applicants and higher payment in case of delay.

In the progress of DG penetration, it has encountered more and more network restrains and technical challenges.

Conventional distribution power system looks downwards from GSPs to customer metering point circuitbreakers or fuse cut out. Large volume of high infeed of DGs has changed the power flow and cause problem in mainly following aspects:

(1)DGs contribute to fault current and push the distribution system fault levels higher, which forces many circuit breaker replacement or reinforcement in power networks.

(2)Reverse power flow could be where DG infeed surplus demand. It is most obvious when the demand is at its lowest and the DG inputs are at their highest, typically in a sunny and windy summer day. It is not helped by the case that some of the UK’s old transformer on-load tap changers are only rated to 50% of the full rating under reverse power flow due to damping impedance switching sequence. Accumulated DGs can trigger a 132kV transformer replacement which will cost around 1.5 million pounds.

(3)DGs cause system voltage rise and step changes. It is most onerous at weak and remote network with high DG density. When the voltage cannot be regulated by transformer tapping range, action is required.

(4)Problem of power quality issues like transient voltage variations and harmonic distortions can also be introduced by DGs.

(5)Connected DGs increase network complexity and difficulties in network management and protections.

When the network reinforcement is triggered, in addition to the connection fees, the DG owner will have to also pay a proportion of cost of system upgrade due to its DG connection. The rest of cost will be taken by the DNO but eventually transferred to all network users and customers.

In general, renewable DGs and other green technologies add costs to the system and contribute to the rise of electricity bill. OFGEM therefore encourage smarter solutions and more efficient investment. The industry has made collaborating effort, trying to mitigate the issues caused by DGs and provide cost effective solutions.

4 Reinforcement options and innovative solutions examples

The most commonly seen issues caused by DG connections that result in expensive network reinforcement are fault level increase, voltage rise and reverse power flow.

Alternative solutions are available, other than costly transformer and switchgear replacement, to mitigate issues above and release network capacity restrains.

OFGEM has been encouraging innovative approaches to solve distribution network issues by funding DNO R&D or trial projects. Through Low Carbon Network Fund (LCNF), DNOs in UK have been actively exploring new ideas and adopting new technologies to release more DG connection capacities.

4.1Faultlevelmitigation

It is a common practice when the DGs pushes the system fault level higher than 95% rating of the switchgear a reinforcement scheme should take place.

The obvious answer is to replace the existing with higher rated ones. Although the cost of modern switchgear themselves has significantly reduced, changing switchgear could still be costly involving installation work and system interruption.

Choice can be made to split busbar into sections so that the fault level form the busbar downward is immediately reduced but it is at the expense of network security and not recommended.

When it is financially viable, installing a fault current limiter (FCL) at critical point will be preferred than switchgear replacement. Different types of FCLs are readily available in the market. Superconducting and saturable-core FCLs are most common.

Superconducting fault-current limiters, resistive or reactive, normally operate with low impedance and are “invisible” components in the electrical system. In the event of a fault, the limiter develops impedance and significantly reduces the fault current.

Saturable-core fault current limiters (SFCLs) utilize the dynamic behavior of the magnetic properties of iron to change the inductive reactance on the AC line. SFCLs are made similar to a transformer with iron core pre-saturated. Under fault conditions, the fault current force the core out of saturation, resulting in increased line impedance during part of each half cycle[7].

There are also other types of FCLs but their technologies are less mature than above two types.

UK Power Network (UKPN), the DNO looks after power distribution in London area, hascommissioned a SFCL into service in May 2013 at a main substation in Newhaven, as shown in Fig.2[8].

Fig.2 Saturable-core fault current limiters in UKPN Newhaven substation[8]

In order to facilitate more renewable generation connections, Western Power Distribution (WPD) also plan to install FCLs in five primary substations as trial in Birmingham area, which is the second largest city in UK. Both superconducting and saturable-core technologies will be used. Other DNOs are also carrying out trial projects on FCLs[9].

4.2Voltageregulation

Voltage rise is another major issue caused by DGs. It is another key factor that restricts the amount of DG capacity that can be connected to rural or remote distribution networks. In those places the land and renewable resources like wind are largely available however the power network is the weakest due to long distance and lower population density.

Network design and planning engineer in distribution business normally study the network voltage limitations under worst scenarios i.e. minimum system demand and maximum DG outputs underN-1 circuit outage. Network reinforcement will be suggested if any busbar voltage exceeds statutory voltage limits and it cannot be corrected by transformer AVC and tap changing actions.

FACTS devices which were normally used in transmission system are now become economical to be used in distribution systems. WPD recently connected a 3.75MVar FACTS device onto East Lincolnshire substation 33kV busbar where significant amount of low carbon generations are connected down stream. The device is as shown in Fig.3, which automatically export or absorb reactive power and has fault ride through ability[10].

Fig.3 “DStatcom” FACTS device installed by WPD in East Lincolnshire Low Carbon Hub[8]

4.3Energystorage

Energy storage like batteries provide an attractive solution to locally store DG infeed surplus and provide a layer of buffer to distribution network.

UK DNOs have installed much “utility scale” energy storage system as part of LCNF projects including Northern Power Grid’s “Customer-Led Network Revolution”, WPD’s “BRISTOL” and “Falcon”, UKPN’s and SSE’s Orkney and Shetland projects. A number of electrical energy storage technologies have been, or are being trialed. These include: sodium sulphur; zinc bromine flow cells; sodium nickel chloride; gel-filled lead acid; and various Li-ion chemistries including “second-life” EV batteries.

As shown in Fig.4, in 2013 there was 5.1MW and 6.4MW·h commissioned with an additional 7.2MW and 13.8MW·h either under construction or being planned[11].

Fig.4 Locations of UK distribution energy storage systems[12]

4.4MVDCandLVDCnetwork

One and half centuries after Nikola Tesla won the “War of the Currents” with Thomas Edison, a new debate is arising over AC versus DC: should DC power delivery systems displace or augment the AC distribution system in buildings or other small, distributed applications?

DC network becomes increasingly attractive again because:

(1)DC can now be transported at a high voltage thanks to power electronic technologies and become more efficient than AC.

(2)DC provide solution to inter AC grid connection and provide flexibility in power flow management.

(3)More equipment nowadays operates on DC like data centre and energy storage batteries.

(4)Many generations produce DC power like PV farms and some wind turbines.

(5)At lower voltage level DC system can isolate fault faster than AC system with solid state switch.

(6) The price of power electronics becomes more competitive.

HVDC interlink is not new to us in power transmission level for long distance power transport. This technology can also be adopted by distribution network thanks to price reduction of power electronic devices.

Distribution grid supply points (GSPs) which could not be interconnected due to various reasons like circulating current can be bridged by medium voltage DC (MVDC) interlink circuits. This will offer great flexibility on distribution network power flow management and largely mitigate the burden from DGs in certain concentrated areas.

WPD has kick started a project in early 2015 to build a 20MV·A 33kV DC link in Somerset and Devon area areas with back to back power electronic convertor (AC-DC-AC). It is estimated that this relatively small DC link will be able to release up to 36MW of capacity for DG connections based in the Trials area. The system can be installed faster, up to 12 months less, than the conventional system reinforcement scheme[13].

Scottish Power Energy Networks(SPEN) pioneered in the “Angle-DC” project will convert an existing 33kV AC circuit to DC operation in order to create network capacity headroom for low carbon generations in Anglesey and North Wales area[14]. Additional benefits of converting existing AC cable into DC lies in two folds, firstly the cost laying new cable is avoided, secondly the rating of cable can be increased theoretically to the AC peak voltage.

At lower voltage level down to 400V it is technically viable to build DC local “micro-grid” so that new low carbon technologies can be more effectively integrated including electrical vehicle, solar panels, battery storage and data centre.

WPD’s “Project Solar BRISTOL” carries out trials to install DC system in homes and offices with solar panels and battery systems. This project has provided valuable learning points to all DNOs[15].

To take it to another level, a part of distribution network can be built to operate under DC and runs in parallel with existing AC system with AC-DC converter interface. This may become a reality in a future nearer than we expect today.

4.5Softwareimprovement

Efforts were also made by UK DNOs to invest on more sophisticated planning tools and software. It helps DNO to safely run the high voltage network closer to its limits, by moving away from conservative, ‘worst case’ assumptions. More network capacity can be therefore released.

In planning stage improvement could be made on desk top studies if more realistic data and more detailed analysis are carried out. Substation loads and generator exports profiles can be created using available historical data, weather corrected forecast profiles for demand and existing generations. More realistic power flow analysis can be carried out. Conventional safety margin used by planners can therefore be confidently squeezed, which in return delays any network reinforcement trigger and gives more head room for low cost DG connection acceptance.

5 More dynamic and smarter distribution network

As previously mentioned high level of DG penetration increases distribution network complexity and the intermittent nature of green energies like solar and wind added more dynamics into the system. Besides, the distribution network will see higher integration of other green technologies like heat pump, electrical vehicle, battery storages and so on.

The traditional ‘passive’ role of the DNO must evolve such that the distribution system becomes more actively managed, taking advantage of opportunities afforded by flexible demand, distributed generation and energy storage to support efficient network operation. This will require network operators deploy the necessary technology to allow them to better manage supply and demand at the local level[12].

Higher level of engagement with customer through demand side management and smart meter will help the power consumption and generation balance.

The distribution power system itself has to be equipped with intelligence to manage network in real time. It will involve dynamic asset capacity rating, high level of system automation, large quantity of data collection, and fast process and decision making.

For example, to maximize overhead line capacity, live weather conditions is taken into considerations such as wind speed, wind direction and ambient temperature to assess its rating in real time as opposed to static ratings using conservative environmental conditions[16]. In a trial project by SPEN, on line monitoring sensors are installed in cable to monitor cable temperature and maximize wind farm output[17].

Effort can be made to improve the network model accuracy to narrow down uncertainties and safety margins in fault level studies. At lower voltage level like 11kV and below, real time fault level monitoring is helpful to allow flexible network management. It is to introduce a fast artificial fault through an impedance. Knowing the current and voltage, the system source impedance and hence real time system fault level at that point can be calculated. This allows accurate evaluation and gives confidence in decision making. Real time network management is achievable by switching in or break open interlinked or parallel system accordingly[9].

UK DNOs also invests on active network management (ANM) to manage generation and loads on the network in real time. UK Power Networks (UKPN) launched a “Flexible Plug and Play” project in late 2013. It carries out real time monitoring of the network from the field devices and is able to configure a number of application thresholds at which it can take pre-determined actions. Once the threshold is breached, the ANM solution automatically issues a power export curtailment instruction to the associated generators as agreed by UKPN and the flexible generation customers[18].

UKPN is also looking at building a tool to help outage planning and post fault action taken process so that the disconnection of existing DGs can be shorten and less delay for new DG connections. It will involve data exchange with National Grid and distribution demand and generation analysis. A software package will be developed to automatically analyze outage scenarios[19]. Similar tool already exists in some transmission network operators. Investment on smarter and more sophisticated design/planning tools in distribution network will be worthwhile with growing system complexity.

6 Conclusion

New energy market legislation and regulatory rules were implemented by UK government, which successfully excited growth of low carbon generations especially at distribution level. The growth of low carbon market and smart grid development have also contributed to the economy and created new jobs.

It is a good sign that the new connection department has been busy in DNOs since 2010. In this welcoming environment, new low carbon technology firms appear and ICP business thrives.

Continued growth of renewable DG capacity has imposed great challenges to power network operators. To overcome issues introduced by DG like higher fault level, voltage rise, and reverse power flows, UK DNOs have been actively exploring innovative solutions mostly through trial schemes partly funded by government. Those project returns good value to the whole industry and the learning points are shared. It is recommended from UK experiences that the fault current limiter, FACTS device, energy storage, DC interlinks, and real time system analysis tools may help relieve network restrains and release network capacity.

Along with the encountered challenges we also see an exciting future to embrace state of art technologies and transformer existing power grid to a greener, smarter and more efficient system.

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[2] Digest of UK energy statistics [EB/OL]. https://www.gov.uk/government/collections/digest-of-uk-energy-statistics-dukes, 2013-07.

[3] Department of Energy and Climate Change. Feed-in Tariffs: Get money for generating your own electricity [R]. 2013-08.

[4] Feed-in Tariffs Limited. Tariff digression [R]. 2012-03.

[5] Western Power Distribution. Statement of loss adjustment factor methodology for Western Power Distribution plc Electricity Distribution Networks [R]. 2013-10.

[6] Office of the Gas and Electricity Markets (OFGEM). Standard licence condition 15A - Distributed generation standards direction guidance document [R]. 2015.

[7] Electric Power Research Institute. Superconducting fault current limiters technology watch 2009[R]. 2010.

[8] UK Power Networks. GridON’s Fault Current Limiter successfully suppresses multiple network faults during first year in service [EB/OL]. www.gridon.com, 2014-07.

[9] Western Power Distribution. FlexDGrid project six monthly progress report [R]. 2015-12.

[10] Western Power Distribution. DStatcom project technical report [R]. 2014-07.

[11] Energy Storage Operators’ Forum. ESOF White Paper: State of charge of GB [R]. 2013-11.

[12] Department of Energy & Climate Change, Smart Grid Forum. Smart grid vision and route map [R]. 2014-02.

[13] Western Power Distribution. Network equilibrium six monthly progress report [R]. 2015-06.

[14] Scottish Power Energy Networks. Angle submission AC cable for DC - Electricity network innovation competition [R]. 2015.

[15] Western Power Distribution. Project Solar BRISTOL - Six monthly progress report [R]. 2015-12.

[16] UK Power Networks. Long term development statement (LTDS) network summary UK Power Networks plc [R]. 2013-11.

[17] Scottish Power Energy Networks. Temperature monitoring wind farm cable circuits, SPEN LCN Fund Close Down Report [R]. 2015-07.

[18] UK Power Networks. Flexible Plug and Play- Implementation of active voltage and active power flow management within FPP Trial area - SDRC 9.6 [R]. 2014-12.

[19] UK Power Networks. Kent Active System Management (KASM) project handbook V1.0D [R]. 2015-02.

英國可再生分布式發(fā)電市場進展和配電網(wǎng)應(yīng)對方案

戴 潔1， 馬 釗2， 尚宇煒2， 張 偉2， 周莉梅2

(1. 蘇格蘭和南方電網(wǎng)公司，英國 雷丁 RG1 3JH; 2. 中國電力科學(xué)研究院，北京 100192)

本文介紹了英國可再生分布式發(fā)電市場現(xiàn)狀和英國配電網(wǎng)(132kV～11kV)所取得的相關(guān)進展。簡述了英國分布式發(fā)電相關(guān)政府法規(guī)、市場補貼機制和分布式電源入網(wǎng)規(guī)程。分析了高比例分布式發(fā)電對配電網(wǎng)造成的困難，并用實際工程案例闡述了應(yīng)對方案。進而，為解決分布式發(fā)電帶來的故障電流增高、電壓升高和反向潮流等技術(shù)問題，列舉了英國各地區(qū)試點項目應(yīng)用的故障電流限值器、FACTS裝置、儲能設(shè)備和中壓直流輸電等技術(shù)方案。介紹了英國實時網(wǎng)絡(luò)分析軟件和配電系統(tǒng)優(yōu)化技術(shù)，應(yīng)用該技術(shù)可進一步釋放配電網(wǎng)容量。

可再生能源； 儲能； 配電網(wǎng)； 直流配電； 主動配電網(wǎng)管理

： 1003-3076(2017)09-0043-08

： TM73

Apr.15, 2017

10.12067/ATEEE1704056