Increasing distributed (or dispersed) generation (DG), mainly from renewable energies, coupled with the early installation of DG units non-compliant with current standards and disconnection requirements are reducing the security of Continental Europe’s interconnected network, the European Network of Transmission System Operators for Electricity (ENTSO-E) has advised. Whereas in 2002 the probability of using all the frequency containment reserve (FCR) was 1 event in 32.5 years, in 2011 it was 1 event in 20.9 years – close to the 1 event in 20-year level where the risk of facing a partial or total blackout is real.
Growing penetration of renewable energies
In a new position paper, ENTSO-E outlines the two factors involved.
In the first years of installation, high capacities of DG units were installed with frequency disconnection settings in the range between 49.5Hz and 50.5Hz. The standard disconnection limits of generation units are between 47.5Hz and 51.5Hz. Subsequently the rapid growth of renewables has also led to a reduction of inertia because, unless otherwise required, the power generating modules used by DG provide no inertia or contribution to the FCR.
With large penetration of renewable energies, combined with increasing imbalances resulting of variable exchanges, this situation has led to higher frequency deviation than in the past and a significant decrease in the security of operation of Continental Europe’s interconnected synchronous network, states ENTSO-E.
Grid frequency deviations
ENTSO-E emphasizes that the system has been designed to withstand frequency deviations originating from the reference case having 50Hz as starting point, but in normal operation, the frequency often deviates from this value. For the last few years, practically all synchronous areas of ENTSO-E (similarly to other synchronous systems in the world) have been experiencing significant frequency deviations corresponding to sudden and simultaneous changes of generation due to standardized time intervals introduced by market schedules. These frequency deviations activate a significant share of FCR, which is initially intended and dimensioned for large generation and load outages, hence increasing the overall probability of contingency events exceeding the designed assumptions of the reference case.
For Continental Europe, if the frequency reaches 49.0Hz, load shedding of customer consumption is mandatory, and at least 5% of reference load has to be shed. In the range 49.0Hz to 48.0Hz, in total 30-50% of reference load will be shed. The reference FCR is 3,000MW.
ENTSO-E also notes that an insufficiency of FCR does not mean in itself that load shedding will occur. However, running out of FCR means entering into frequency areas where an uncoordinated disconnection of a high amount of DG will lead the frequency to drop down to the load shedding level.
Retrofitting non-compliant DG units
ENTSO-E states that in the future, DG will need to be compliant with the Requirements for Generators Network Code (RfG NC) stipulating broader frequency disconnection ranges for these units.
However, in the meantime in order to re-establish the system security, the non-compliant DG capacity has to be reset, ENTSO-E continues. Therefore, a retrofit programme involving all countries which contribute to the risk is considered urgent. Germany and Italy, the two countries in which most of the non-compliant DG units are installed, have started large programmes to retrofit the main parts of the existing non-compliant units. This retrofit programme must be extended to all countries which contribute to the risk.
DG retrofit costs and benefits
Based on the costs of the Italian/German retrofit of €3.5 million per GW retrofitted, ENTSO-E estimates the overall cost of retrofitting all non-compliant DG units in Continental Europe (46GW) at €161 million. Considering the benefits of retrofit measures to stem mainly from avoiding load shedding (or black out) in case of large frequency deviations, the benefit of retrofit against a 4-hour blackout is estimated as high as €1,760 million. As the duration of the historical major blackouts is within a range of a few hours up to a few days, and such an event may occur in the next 20 years or so, it is clear that the benefits of the retrofit programme are significantly higher than its costs, states ENTSO-E.
ENTSO-E also notes that increasing the FCR is not an option as it cannot cover all the situations in which frequency deviations exceed 200mHz. For example, in case of large generation outages, the frequency deviation occurs too rapidly for the FCR to be effectively activated and prevent the frequency to reach values below 49.8Hz, thus triggering the disconnection of retrofitting DG with inadequate frequency disconnection settings. At best, increasing the FCR may be considered as a potential temporary mitigation measure until the retrofit programme is completed. The cost is also significant with a required 120MW increase of FCR in the range of €31 million per year.