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In theory it ought to be simple. There arewell-proven electronics that will take thedirect current or 'wild' alternating current

output from your PV, small wind, microtur-bine, biomass or other device, and convert itto alternating current at the precise frequen-cy and voltage required for grid matching.Electronics will also synchronise phases andmake the connection, first checking that the

renewables output is sufficient to justify'spilling' surplus electricity onto the publicnetwork. Commercial inverter systems (so-called because they convert DC to AC, theinverse of what is often required in powersystems) are readily available, and you mightthink that grid connection should be as easyas plugging your generator output into the'black box' and connecting the output fromthat to the grid. Ergo, you should then haveachieved small system embedded generator(SSEG) status.

Unfortunately, it can never be as simple as'plug and play' and the DNOs, who haveresponsibility for the grid locally, introducecomplications. Whereas you would like openaccess to the grid so that you could readilyfeed in your contribution of power and bepaid for it, DNOs are alarmed by theprospect of thousands of minimally regulat-ed, scarcely controlled generators comingonto the grid, increasing risks of instability,outages and extra expense. Network opera-tors, who must consider the quality of thepower supplied plus the safety of the publicand their own personnel, have understand-ably been loath to compromise. But, maybeimpressed by the possibility of illegal connec-tions being made on a substantial scale (wit-ness, for example, the 'guerilla solar' move-ment in the United States), they have been

conferring with renewables interests andrelaxing their stance a little, as we shall illus-trate. First, though, it is worth consideringjust what the complications are.

ComplicationsConnection to the grid opens up the possi-bility that the SSEG, or more likely a num-ber of them together, can adversely affect thequality of power delivered to consumers.Faulty generators can degrade key parame-ters and destabilise the network. They canalso threaten the safety of personnel workingon the grid and, in the event of a self-fault,create a potential point of power leakage.Conversely, grid issues could create problemsfor SSEGs, possibly damaging their equip-ment. Therefore, grid connection can neverrely on an inverter alone; there must also besufficient control and protection. The extentand nature of that protection depends on thejurisdiction you are operating under and therequirements of its grid and network author-ities. A universal requirement is that thecore inverter is well engineered, but specifi-cations for robustness, fault tolerance anddurability vary between territories. In gener-al, grid connection inverters must be type-approved in accordance with national andregional requirements.

Other requirements concern the protectiontechnology overlay that the inverter must haveto safeguard the grid and general consumerinterests. Network operators require that volt-age, frequency, phase and power factor param-eters must stay within set limits and that, ifthey do not, the generator senses the departureand disconnects itself from the network. Thisrequires a sensing and control system. This sys-tem must also limit current and voltage tran-sients created when a generator either connectsto or disconnects from the network, since large

Perhaps you are a small-scale generator of electric power fromrenewables and have a surplus which you'd like to export to thepublic electricity grid. But you are put off by the associated costsand bureaucracy. Does it all, you wonder in frustration, have tobe so complex and pricey? Perhaps not, and the admission gateis gradually opening, but the district network operators (DNOs)have to look after their interests - which are very different fromyours. George Marsh reports.

LOWERING THEBARRIERS TO REGrid connection of small renewables

The Solar PV electricity generation processPhoto: solar century

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transients can adversely affect power qualityand network stability. Furthermore, DNOsrequire that inverters feed only alternating cur-rent onto the grid, not direct current. DC canlead to system losses and affect the settings ofrelays and switches used for system protection.

A crucial stipulation is that, should therebe a mains/grid failure, the SSEG must dis-connect. A highly visible and lockable manu-al isolator switch must also be present asback-up. Secure disconnection is needed pri-marily so that DNO personnel working onbranches of the network can do so safely, withthe assurance that the branch has been dis-connected from any power source. However,it is also to avoid the possibility of 'islanding'.This situation arises when a small local sec-tion of the network continues to operateindependently of the failed main grid, theresulting 'island' of supply being powered bylocal embedded generators. This can be ben-eficial when controlled, but DNOs fear thatcontrol may become tenuous where there aremultiple assorted and unfamiliar SSEGs.

A further control requirement comes where,as in most cases, the SSEG is parallel-connect-ed to the operator's own electrical apparatus aswell as to the grid. In this case, a battery back-up may be needed if the power required fordomestic purposes could exceed the ratedcapacity of the generator. This is to ensure fullpower continuity in the event of a utility out-age. The presence of a battery bank will call foradditional control circuitry, with a charge con-troller also being needed if the utility supply isto be utilised for battery charging.

Evolving guidelinesEvolving regulations and guidelines are theresult of on-going debate, sometimes vigorous,between the grid authorities and DNOs on oneside, and renewable/SSEG interests on theother. Some heat in the debate is understand-able given that the supply authorities are beingasked to accept a situation that may appear tothem retrograde, harking back to the morechaotic early years of public electricity supply.Many view the prospect of a move from ratio-nalised central generation to a dispersed supplysituation, in which thousands of SSEGs couldeventually be connected, with suspicion.Enlightened executives, however, recognise thatchanges are necessary - not just to the tech-nology of the grid, but to the entire electricitysupply model and its management culture.Authorities in developed countries, sensing thepolitical momentum building up behindrenewables, are beginning to adapt. Even in theUK, a country where innate conservatism tem-pers technical ingenuity, there has been notice-able progress. In particular, the technical stan-dards that SSEGs are expected to adhere tohave been relaxed.

British generators that are large in renewableterms - those that pump power into the gridat 'high pressures' of 11kV and above - have,justifiably, to meet higher standards thanSSEGs feeding a few kVA to local networks atlow voltages. The Department of Trade andIndustry's G59/1and 2 engineering guidelineshave long been the 'bible' of good practice inthese cases. But, with the emergence of the PVindustry in particular, the safeguards enshrinedin G59 came to be considered too burdensomefor the small renewables sector. The DTI there-fore funded a task force to develop a new guide-line, G77/1. Its best practice recommenda-tions, framed primarily for PV suppliers, werenevertheless applicable for other small renew-able generators feeding up to 5kVA per phaseonto the public low-voltage network in parallelwith their own applications. An associated typetest procedure developed by SouthamptonUniversity was adopted to ensure that commer-cial inverter systems offered for network con-nection were of a 'grid-worthy' standard.

G77/1 required that inverters should deliverpower of good quality, and specified voltageand frequency limits accordingly. It also speci-fied a power factor of near unity (>0.95). Thismeans that virtually all the power fed to thenetwork must be usable, lacking the unusable

reactive component that often accompaniesalternating current generation, augmentinglosses and reducing system capacity. This inturn requires that the delivered voltage and cur-rent are in phase. An inverter must be con-trolled electronically to achieve this. Anotherrole for this electronic overlay is to monitor theinverter's output to check for any direct currentcontent and counter it if necessary. (The unde-sirable effects of DC on the system were men-tioned earlier. G77 required that DC contentshould be negligible - less than 5mA.) Analternative countermeasure is to feed theinverter's output to the network via a trans-former, an accepted DC isolation device thatpasses AC only.

G77/1 also required the presence of a maindisconnect relay to isolate the generator in theevent of a mains (grid) failure. Sensors thatcan detect tell-tale step changes in voltage,current or frequency are used to trigger thisrelay. Other relays are needed to protectagainst over and under-voltage, and frequen-cies that stray outside specified limits. In prac-tice, full protection therefore calls for a com-bination of three relays, any one of which candisconnect the inverter system via a contactoror circuit breaker. Regulations usuallydemand that this is backed up by an isolationswitch, placed between the SSEG meter andthe network connection point, that requirespositive manual operation and can be locked.Although G77/1 improved prospects for

Installation of a domestic Solar PV system. Photo: solar century

The PV inverter in a domestic setting. Photo: Mastervolt

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SSEGs in the UK, the small-renewables sectorstill saw it as restrictive and adding unduecost. It therefore lobbied for further changes,along with discussion of ways and means forachieving the standards set. As a result,G77/1 was recently superseded byEngineering Recommendations G83/1 2003.This publication (available from the EnergyNetworks Association - info@energy net-works.org) introduces a number of changeshelpful to the UK's SSEG interests.

One such is the easing of the over-voltageceiling from 253V to 264V, intended toalleviate problems of nuisance tripping.Also welcome are a slight increase in powerfactor tolerance and a widening of DCinjection tolerance from 5mA to 20mA.Trip levels on protection relays can be setaccordingly. These changes mean that agreater number of commercially availableinverter systems qualify for type testing,potentially reducing the need for moreexpensive or bespoke systems. Slightchanges in type test procedure are intro-duced for new inverters, though units previ-ously approved under G77/1 are by defaultcertified to G83/1. The new recommenda-tions are framed around a nominal 16A perphase limit (3.68kW single phase, 11.04kWthree-phase) for SSEGs. Although thiseffectively represents a reduction on the5kVA included in G77/1 for single phase,DNOs can choose to apply the G83/1 rec-ommendations if they consider these to bemore appropriate than the G59 guidelinesused for larger generators. Thus scopemight, for instance, be extended to includea 5kVA PV array or even a 10kVA wind tur-bine, while appropriate inverters of greaterthan 16A capacity can be submitted for typetesting. (UK readers please note that this is, asyet, new and relatively untested territory.)

Other changes are procedural. For example,whereas G77 required an application for a sin-gle SSEG to be made to the DNO before con-nection, G83/1 allows the operator simply toinform the DNO on the day of connection,then give full details on a commissioning con-firmation form (contained in a G83/1 appen-dix) within 30 days. An IEEE BS7671 installa-tion test certificate must also be attached. Priornotification is, though, still required for multi-ple installations located within a small area.Dual-supply warning labels, in an approvedformat, must be attached to the apparatus.

Removing barriersChanges such as those the UK has madereduce the technological and economic bar-riers to SSEG implementation by enablingcompliant inverter systems to be producedand qualified at lower cost than previously.Producers of inverters that are initially non-compliant can add provision ranging fromrelays (to IEC 255 standard) and isolationtransformers to full electronic controllers inorder to meet official and client require-ments. Renewable interests, especially thePV sector, are keen to substitute semicon-ductor switches for mechanical relays sincesolid-state devices can be cheaper and aremore easily integrated with PV (or otherdevice) electronics. Some countries havemoved in this direction, but so far BritishDNOs have preferred to retain the morefamiliar technology with its long trackrecord.

Differences also remain over the level andnature of the required mains break protec-tion. For example, should automatically-resetting circuit breakers (as distinct frommanual reset) be mandatory? Are the risks ofislanded operation over-stated? How obvi-ously visible should the manual isolator beand who should have keys to the lock? Howcan the system best be policed? Other discus-sions have focused on the trade-off betweenprotection device sensitivity and the risk ofnuisance tripping, the ability of sensors todiscriminate between different types of fault,speeds of fault disconnection and reconnec-tion, the adjustment and on-going verifica-tion of relay settings, the 'tamper-proofing'of those settings, plus the likelihood of sys-tems drifting out of tolerance over time andhow often they should therefore be re-inspected.

Finally, to illustrate a representative exam-ple of grid-interactive inverter technologywe cite the UK's Scolar Programme, an ini-tiative under the government's ForesightChallenge that involved installing solar PVat nearly 100 schools. This programme stim-ulated the design of large and small inverterssuitable for PV/ SSEG application. One ofthese externally comprises a 600mm by600mm by 250mm cabinet with a smallmeter and control panel. The cabinet pro-tects the electronics inside against moistureingress and other environmental hazards, toIP54 standard. The unit can withstand tem-

perature extremes of -20 to +40 degrees C.Lightning surge protection is incorporated.Wiring complies with BS 7671 wiring regu-lations, and switch gear meets the appropri-ate BS EN standards.

The system accepts DC inputs of 48 to 70volts, floating (not earthed), and delivers upto 700W of power at 90% efficiency (fullload) to the local network. Output voltageand frequency are held within G59/1 limitsby the inverter's microprocessor-based con-trol system. This also ensures that powerquality and protection meet G77 require-ments (The unit was designed before theadvent of G83/1). The system is pro-grammed to run at unity power factor andhas high immunity to voltage spikes andradio frequency 'noise'. The system was typetested to verify that key performance param-eters are met. DC and AC elements are seg-regated, being in two separate compart-ments. An isolation transformer ensures thatDC is isolated from the network. Alarms areprovided for conditions including AC out-put and output frequency out of limits, over-load, inverter failure and earth fault. A self-monitoring facility provides display ofalarms and data via a fibre-optic cable and asocket on the front panel. Data can also belogged continuously.

There will be other matters for aspiringSSEG operators to consider, not least thenecessary two-way, power export/importmetering. But the core inverter and its safe-guards will always be the heart of the matter.On-going evolution of the requirements andstandards surrounding this grid connectiontechnology will continue to lower the barri-ers to SSEG implementation.

Close up of a PV Inverter. The Sunny Mini CentralSMC 6000 from SMA Technologie AG, Germany.