Active network management
The Orkney Automated Network Management scheme (Orkney ANM) was developed by Scottish and Southern Electricity Networks (SSEN) then SSE Power Distribution, and its contractor Smarter Grid Solutions Ltd, between around 2005 and 2009 when the first generator to be connected under the control of the scheme was installed.
The Orkney ANM scheme was needed because of the demand for additional renewable generation connections within Orkney. Generation was accepted within Orkney on an N-1 basis from 1998 when Orkney had a second 33,000-volt (33kV) circuit installed connecting it to the Great Britain (GB) grid at Thurso. At the time Thurso was a 132kV to 33kV substation. On an N-1 basis the amount of generation which could be connected was based on the larger circuit installed in 1998 being out of service. Therefore, only around 17MW could be exported and to this was added the islands minimum load of 6MW. This gave a capacity to accept generation not under SSENs direct control of 23MW of which 10.5MW was already taken up by the gas turbine power station at the oil terminal on Flotta. This gave a further 12.5MW of available capacity under the N-1 process. Of this 4.8MW of wind turbines at Burgar Hill and 7MW of capacity for a wave energy test site at Billia Croo took up the majority. A medium sized machine at Burray of 0.85MW and an upgrade of one of the Burgar Hill machines by 0.75MW took the total to 13.4MW. Kirkwall power station was ignored in these calculations, it is only licenced to provide essential supplies to Orkney, is not run on a commercial basis and is under the direct control of SSEN. At this point it was decided that no further connections could be made using what was at the time the standard N-1 basis.
However, with the two circuits to Thurso in operation it would be possible to export 34MW under most circumstances. It was decided to allow additional generation to be connected on an interruptible basis. This new generation was called Non-Firm and the existing generation was called Firm. (The terms Firm and Non-Firm, when referring to generation in Orkney, do not directly equate to the normal GB electricity supply industry definitions of Firm and Non-Firm, but within Orkney and SSEN when referring to the Orkney network, they are now well established and understood. Under normal GB terminology all generation connections except for Kirkwall Power station would be regarded as having Non-Firm connections.)
Further renewable generation was connected as Non-Firm generation which totalled 20.1MW before it was decided that no more generation could be accepted under the rules for Non-Firm generation. Network security was obtained by installing inter-trip circuits from Scorradale to the Non-Firm generation sites. These were arranged to trip the generation on the loss of either of the two interconnector circuits from Thurso. The inter-trip circuits are also set up to trip generation if there is an overload, where power flow was from Orkney back towards Thurso, on three of the internal 33kV circuits which emanate from Scorradale. Somewhere along the way it looks like too much generation was allowed if following a strictly numerical analysis of the network capacity. However, this has never been a problem as the gas turbines at the oil terminal, wave and tidal test sites show a large amount of diversity. In practice the Orkney-GB interconnector circuits never came anywhere near being fully loaded on export. 2.7MW of wind turbines on Stronsay connected under this scheme, proved to be problematic and have now been removed leaving 17.4MW of generation capacity in the Non-Firm category.
Between the Firm and Non-Firm generation Orkney was able to export at times and in June 2008 it achieved its first net export month where exports exceeded imports during the month.
There was however demand for further connection capacity and considering the actual spare capacity the problem was researched by SSEN in conjunction with Strathclyde University. An offshoot company of Strathclyde University, Smarter Grid Solutions used the research undertaken by the University and SSEN and developed the Orkney ANM scheme on behalf of SSEN. Funding for the development work was provided from SSENs share of innovation funding allowed by OfGEM.
The Orkney ANM scheme operates using live network monitoring at pinch points on the network which network modelling has shown to be the critical points in terms of network loading. Reducing generation beyond the pinch points will always result in reduced current flow at the pinch points for those points within Orkneys internal 33kV network. Two pinch points are on the Orkney end of the Orkney-GB interconnectors and these are directional so that they only operate when power is been sent from Orkney to GB. Operationally any generation beyond the pinch point could be reduced to achieve the desired effect. However, all the existing generation both Firm and Non-Firm already had contracts which did not include requirements to reduce their output for an intact system. Therefore, only new generation connected under the ANM scheme is required to reduce output.
While there were different options open to the designers of the scheme to decide which generation would be reduced the scheme chosen was called Last On, First Off or LOFO. The first generator to contract was allocated first place in what is called the LOFO stack, the next generator is given the second place and so on. When a network constraint limit at a pinch point is reached then the generator with the highest number on the stack and which is located beyond the pinch point is requested to reduce its output. If it fails to reduce its output, then a signal is sent to trip the generator off the grid.
Its important to note that beyond a pinch point that the connected generators might be number 5, 7, 11 and 18. Generators 19 and 20 might well not be beyond the pinch point which is experiencing a constraint and so will be able to continue to operate while generator 18 is curtailed. If further curtailment is required, then the next relevant generator to be curtailed will be number 11 and generators 12 through 17 will be able to continue to operate.
Initially, the scheme was set up with 5 zones, a central area covering the three circuits connecting Scorradale with Kirkwall and including Stromness and Kirkwall sub stations this was called the core zone and its pinch points were the two interconnectors to GB. There were then 4 numbered zones, 1 to 4. Zone 1 covers the Burgar Hill substation and the substations on Rousay, Westray and two substations on Eday. Zone 2 covers the sub stations at Shapinsay, Stronsay and Sanday. Zone 3 covers the sub stations at North Hoy, Lyness and Flotta. Finally Zone 4 covers the sub station at St Marys.
As well as 11kV connections through the named substations certain sections of 33kV line are included in each zone and are relevant for several generation sites connected at 33kV rather than 11kV.
However as more generation was connected under the Orkney ANM scheme Zones 1 and 2 had to be split up because additional pinch points were recognised so there are now Zone 1A and 1B along with zone 2A and 2B. Potentially there are several other pinch points which may appear as more generation is added to the network.
There is not at present sufficient generation connected to justify establishing a pinch point measurement device for zone 4 as the connected generation in the zone is much less than the capacity of the circuit. Therefore, operationally zone 4 is treated in the same way as the core zone.
Because of the way power flows are distributed unevenly across the feeders into Scorradale substation an additional pinch point has been established on the circuit to Stromness which may overload if one of the circuits to Kirkwall is not in service. This acts on the core zone to produce curtailment and this will always operate sequentially starting at the highest numbered generator in the stack.
Initially the scheme was successful, however problems started to occur around 2010 when a boom in small scale generators of <50kW which were not required to join the ANM scheme took place in Orkney. This was driven by the feed in tariff available from April 2010 which provided for a larger per kWh tariff for smaller machines. This led to around 6MW of small-scale generation being connected within Orkney over the next two and a half years, this was mostly wind with some PV. The sub 50kW generation connected in the core Zone, and Zones 3 and 4 did not have much of a direct impact on curtailment of generators under the control of the ANM scheme within these zones, mainly because the firm generation at Flotta and the wave test centre at Billiacro were often not producing anywhere near their contracted generation capacity, during the period that the small-scale wind turbines and PV were being connected.
However, this was not the case with Zones 1 and 2. Because most of the small-scale generation was wind power its maximum output generally coincided with the maximum output of the larger machines. Curtailment events had been expected to take place under the ANM scheme at maximum output. These would be larger at times when the load was less than the maximum. However, for each kW of small-scale wind connected additional curtailment was imposed on the generators operating under the ANM scheme. Zone 1 and its sub zones were severely affected by unexpected curtailment as was Zone 2.
The connection of small-scale generation of <50kW though was not the only thing contributing to higher-than-expected curtailment. There were two other things which contributed. First only the older turbines in the firm and non-firm categories had known histories in terms of output over time. Over time manufacturers produced more efficient turbines in terms of MWh produced per year. Simply scaling output by MW capacity did not give an accurate estimate for the new turbines. It did not make a significant difference to the first connectees under the ANM scheme as their forecast curtailment periods were based on known outputs from older machines. But for second comers using data from older machines scaled to the MW size of the new machines were used in the forecasting models the higher output of the new machines did cause problems. It was found that the first ANM machines produced more at the shoulder times just below full output and this fed into additional curtailment for second and subsequent machines in a zone. This had the effect of increasing curtailment for second and subsequent turbines. For the first connected turbines the increased output compensated for some of the additional curtailment they saw, because of the sub 50kW generation.
The second additional contribution to increased curtailment for some generators also came from the output from the earlier connectees to the ANM scheme because they just had higher average wind speeds than the older sites which had been used to scale from. This would not happen in all cases and in some average wind speeds would be lower reducing curtailment for later generation connectees but is quite possible and would need to be considered in designing any future schemes.
While those generators who were earlier connectees were not severely affected by additional curtailment above the forecasted figures those lower down the stack were with some generators seeing curtailment of more than 50% of expected output in several months each year. The graph enclosed shows the curtailment from the wind turbine in Eday. This site has 7 wind turbines at 3 different sites with higher priority, connected in zone 1 under the ANM scheme.
Curtailment in zone 1 could have been worse than it has been as long outages of some of the older machines connected as firm or non-firm within zone 1 have allowed ANM generators to operate when they otherwise would not have been able to do so.
While curtailment has been more than expected for some, and has led to financial problems, the use of curtailment has ensured that Orkney can produce significantly more renewable energy than it would have been able to do if it had stopped at the firm or non-firm limits. Since 2013 when the second to last connection was made under the ANM scheme Orkney has consistently produced more electricity than it has consumed and is a net exporter.
Because of issues with curtailment the full potential of Orkney to produce renewable power has not yet been realised. Growth has been slow since the end of 2012. This has forced Orkney to innovate in other ways and producing different load management schemes and encouraging the use of EVs to replace fossil fuel cars.
While the Orkney ANM scheme has been a success there have been several lessons learnt. If looking at ANM in another area, then a different approach which would produce a more equitable sharing of curtailment between generators would enable more generation to be connected.