RECQL5, another DNA helicase potentially involved in increased breast cancer susceptibility

A large proportion of AI electricity is projected to be coming from

non-synchronous sources by 2020, as discussed in Section 3.1.1 this is in the

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PENETRATION 3.2 Transmission System Operations

region of 32-37%. There is no precedent for a system of this size to have such a level of non-synchronous generation without AC links to neighbouring systems. Ireland’s AC isolation is discussed in Section 3.2.2. However it is necessary for a AC system to require some amount of conventional

synchronous generation on-line at all times in order to maintain overall frequency stability as well as local voltage stability as discussed in Section 3.2.3. The majority of wind turbines being double-fed induction generators (DFIG’s) or full-converter generators which are non-synchronous are not capable of providing sufficient inertia [82] to the system. However it is

recognised that a number of TSOs of high wind penetration systems view that inertial response from wind turbines will become a grid code requirement in the future [83]. Modern wind turbines do also produce reactive power but at a lower quality than conventional generators [84], though it is assumed that in the future wind turbines will be capable of providing more reactive power [85].

As wind energy penetration begins to reach the technical limits of what is possible on present-day electricity systems it is becoming evident that more research is needed in relation to allowing higher levels of non-synchronous sources of electricity on to the system.

Previous studies have included SOCs in the form of a minimum conventional generation requirement [15, 16, 18, 19, 22] and studies that have not included these constraints have recognised their potential impacts on results

[17, 20, 23, 24, 25, 30, 86]. So far, the only study that has assessed the impact of relaxing these constraints in terms of wind curtailment and costs is [16], which looked at such effects on the NI system. It has also been shown in [22]

that SOCs in the AI system will have a dramatic effect in terms of wind curtailment and generator dispatch in the future.

It was shown in [16] that relaxing the NI constraint requiring three large generators to be on-line at all times to two generators on-line results in wind curtailment dropping from 7.5% to the region of 1.5-5% and also indicates possible increases in OCGT generation. An unspecified minimum number of large base load generators were required to be on-line at all times in the AI model of [15] in order to maintain sufficient inertia and reactive power on the system. For NI, an examination of the effects of variable generation on

conventional generators is shown in [25] and there is also a recommendation made for further research into the effects of the requirement for three large

Investigation of factors driving the costs of operating the 2020 Irish power system with large-scale wind generation.

25 Edward V. Mc Garrigle

generators to be on-line at all times. While a constraint for a minimum number of on-line generators was not included in [23, 24], it is stated in [24]

that such constraints would increase wind curtailment. In [23] it was stated that the exact minimum required number of on-line generators was not obvious and therefore was neglected but recognised that its inclusion would increase wind curtailment.

From a wider international perspective, in [18] it was assumed that a

minimum of 400MW of conventional generation was required on the Western Denmark system, but by 2025 it was assumed that 300MW would be

sufficient due to stronger interconnection with neighbouring regions. This assumption was taken from [19] where the year 2008 was examined to find the lowest instantaneous level of conventional generation during periods of excess wind energy generation, which was estimated to be 415MW. This was then assumed as a minimum technical feasible state of system operation.

However, in 2012 wind generation in Western Denmark has been allowed to exceed demand through the use of interconnectors to export surplus

generation [20]. It should be noted that due to AI’s AC isolation as discussed in Section 3.2.2, care should be taken when comparisons are made between countries that are part of a larger synchronous system, i.e. Western Denmark [19] due to the system’s use of synchronous compensators as well as its strong AC interconnection to its neighbours, thus providing stability support.

Systems with high percentages of installed capacities of CHP’s and wind mimic the same problem as SOCs in the form of must-run units. In [87] wind curtailment is shown to increase from a minimum generation requirement made up of the CHP plants on the system. In Spain it is recognised that a minimum amount of conventional generation to be kept synchronised to the system will have to calculated and this will increase wind curtailment

estimates [30]. In studies of the GB system it is recognised that a minimum amount of conventional plant running at all times will be necessary to

provide frequency response and also due to inflexible must-run units such as nuclear plants which will result in wind curtailment [17]. The modelling of the AI Single Electricity Market (SEM) includes a inertia constraint requiring a minimum number of conventional generators to be on-line at all times [26], however such a constraint on the GB system is not included in the same study. The Hawaiian electricity system has similar wind targets to that of AI, albeit on a small system. Problems related to system stability have recognised as important issues in the coming years [28, 29].

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