2.6.1 The UK’s Offshore Wind Farms
Section 2.4.1 highlighted the move in wind turbine technology towards increasing use of power electronics. These power converters involve extremely fast control loops that are flexible to react rapidly to desired changes. Fast control is also essential because of power electronic switches’ sensitivity to over current and the need to rapidly reduce currents in the event of a fault on the output. The impact of the fast control of a power converter is to limit the in-feed current in the case of a fault to a much lower level than might be the case with a traditional synchronous generator.
Generating
Motoring Effect of frequency decrease
The power converters are also deliberately designed to decouple the rotational frequency of the blades from the grid frequency so as to maximise the energy capture from the wind. Yet this removes the inertia to frequency changes that synchronous machines inherently provide to stabilise the grid. A frequency change on the grid side is rectified to DC and not automatically seen by the machine side. Hence, the kinetic energy of the blades does not automatically stabilise the grid.
In response to supply shortfalls causing a frequency change, a wind turbine’s output is limited by the available wind power. Whereas a conventional synchronous plant could increase its output by burning more fuel or steam raising, the wind turbine has no equivalent extra resource. This means that for a wind turbine to provide low frequency response, it would have to deliberately operate at a reduced output. This involves spilling wind energy in order to hold a margin and have the potential to increase its output in response to a falling frequency, but does not have associated fuel savings.
These three differences, which are inherent to the design of wind turbines, and provide them with exceptional controllability, mean that power converter interfaced wind turbines behave very differently to grid disturbances to the familiar synchronous generators. Section 2.4.2 highlighted the trends in grid connection which will further distance offshore wind farms from the grid of today. The increasing use of DC transmission and potentially distribution will lead to isolation of the offshore wind farms from the behaviour of the onshore grid. This provides not only technical challenges, to ensure the continued security of the GB grid, but also regulatory ones.
Today the effects of rising wind power levels are already beginning to be felt, with the National Grid having to act to constrain wind in 2011 to ensure the security of the GB system according to the Renewable Energy Foundation [28]. With ever growing levels of wind power, the challenges of managing the system will continue to grow and challenges other than constraints will emerge. These challenges, combined with the technological differences of wind farms will mean that the grid in 2020 with 33GW of offshore wind is likely to have to look very different to the grid of today. The challenge is to deliver this whilst protecting the security of supply and grid stability experienced today.
2.6.2 The Grid’s Frequency Challenge
Major frequency deviations on a power system usually occur as the result of a large generator or load tripping and disconnecting. Such events are currently extremely rare; however, the GB grid was shown to be susceptible to sizeable frequency deviations by the
events of 27th May 2008. On this occasion, the UK’s largest nuclear plant, Sizewell B, tripped offline shortly after a coal plant of 345MW. This led to a supply shortfall of around 1582MW having to be picked up by responsive generators. This is just within the 1600MW maximum loss that the GB grid is typically secured against.
Figure 2.22: GB System Frequency 27th May 2008
After the loss of these two generators, the subsequent frequency fall, whilst initially arrested, then accelerated again and was ultimately only stopped by the activation of automatic demand disconnection at 48.8Hz. This further frequency fall is not totally explained, however, the official report, from National Grid [29], found that “The unexpected loss of a significant amount of small embedded generation resulted in a total loss of some 1993MW in 3.5 minutes”. This embedded generation was outside of the scope of the transmission system Grid Code and therefore was subject to G59, which the National Grid [30] review found at the time set recommended frequencies at which generation should trip; this is the reverse philosophy to that applied in Grid Code. It brought forward the activation of automatic demand disconnection.
Immediately after the loss of each generator, the inertia of the large number of synchronous machines on the system helped to slow the Rate of Change of Frequency (ROCOF) such that the maximum ROCOF was 0.073Hz. This provided time for other plant to increase their output powers to compensate for the lost plant and prevented the level of demand disconnection being worse. This event demonstrates the importance of synchronous machines’ inherent inertia in securing the grid.
In the run up to 2020 the UK may have single generation connections of 2GW as a result of the offshore wind development plans. Partly as a consequence of this, the power system will be secured against a loss of 1.8GW by National Grid [31] and will also have to deal with connections from wind turbine technologies with very different grid interfaces to conventional generation. In a small islanded system, where frequency can already see significant deviations, this thesis addresses what future offshore wind farms can do to support the grid’s frequency stability and whether energy storage is ready to provide a more robust solution.
2.6.3 Power System Oscillation
Ashton et al. [32] have shown that the UK transmission system currently experiences a number of power system events due to circuit switching as well as a significant major oscillation between the generators of Scotland and those of England and Wales. Currently limited capacity across the North/South boundary does not help this situation. Figure 2.5 shows that the UK’s best wind resources are located in the North, beyond the constraint boundary. Development of these resources will lead to increased stress of the system and is part of the cause of National Grid’s installation of a wide area monitoring system based on phasor measurement unit installations at some substations.
The installed monitoring system has been shown by Ashton to have measured the time delay as a large frequency deviation rippled through the system following a loss of a generator. A time delay of 0.65s was observed between the frequency deviation occurring at the closest substations to the lost generator and those furthest away. The monitoring system provides a large amount of real time data which will in future enhance the system operator’s visibility of events such as that in Figure 2.22, the challenge will be to use that data to secure the system’s stability in the face of these multiple challenges.