Curtailment of energy can be defined as involuntary reduction in the output of the power plant from what it could produce in normal conditions. Curtailment of energy generation is generally observed in non-dispatchable renewable sources like wind, solar and wave. This issue can be regarded as one of the biggest disadvantages of renewable energy supply systems, so this is significant to address such problem and try to find solutions.
When these non-dispatchable renewable energy sources are considered, solar energy tends to form ‘embedded generation’ at local level and is well integrated into the distribution network. Wind is by far the most extensive new large-scale renewable energy resource in UK, so in UK the energy curtailment mostly occurs in wind energy generation.
Grid operators command wind generators to reduce their outputs for two main reasons; first to minimize transmission congestion and secondly to prevent penetration of oversupply into the grid. Transmission congestion generally occurs with wind farms that are located far from cities or towns. For those places transmission lines are generally weak, because the small local population required comparatively weak transmission lines to be installed before the construction of wind farm. Thus, during the times of full capacity generations from the wind farm, they may be asked to reduce their output to prevent overload and any damage to the transmission system. In the other case, when the demand is low or base load generators’ minimum generation thresholds are enough to cover demand, wind farms are asked to reduce their output to prevent oversupply because of frequency or voltage balance or interconnection issues. The oversupply curtailment generally occurs during night time when there is a abundance of wind resource available, but the demand is considerably lower than in daytime. Apart from these main curtailment reasons, environmental reasons such as birds and bats in migration, unusual meteorological conditions and similar issues might cause curtailment.
In order to understand curtailed wind energy in UK, as a first step ‘Transmission and Distribution’ system should be understood. The main duty of the transmission system in UK is to deliver generated electricity from large generation systems to the distribution networks. Transmission and distribution systems are connected at a point which is known as a ‘Grid Supply Point’ (GSP), so each distribution system is known as a GSP group. GSP groups and their distribution over regions are as shown in Map-1. Smaller power sources such as combined heat and power, solar power and some wind turbines (approximately one third of the total installed capacity) are connected to the Distribution Network, in other words the low voltage network. Roughly two thirds of the total installed wind capacity in UK consists of large wind farms and these are connected to the Transmission Network, i.e. the high voltage network. Nuclear, gas and coal fired power stations are other participants of the Transmission Network. Transmission Network participants are members of a trading system of National Grid which is known as the ‘Balancing Mechanism’ (BM).
Curtailment of wind generation is arranged by the BM. As wind energy generation increases, curtailment is becoming more widespread. Curtailment affects the energy output of the wind farm therefore it affects the revenue and related financial liabilities of the wind farm. To compensate wind farms for losses caused by curtailment, National Grid makes payments to the wind farms. These payments are known as ‘Constrained Payments’. National Grid has been making constrained payments to the wind farms since 2010. Before that time, gas and coal power stations might be called for output reduction. Renewable Energy Foundation (REF) records data from the BM which includes wind farm constraint payments and volume of constrained wind generation from 2010 to date. The data is sourced variously from Renewables Obligation Certificates, Renewable Energy Guarantees of Origin and in the case of some municipal waste generation, from Climate Change Levy Exemption Certificates. According to REF, the annual data of curtailed wind energy is as shown in Table-1 below,
|Cost||Volume of Curtailed Wind Generation (MWh)||Average Price|
Note that constraint volumes and constraint payments shared above only include the trades carried out as a part of the BM. There might be further constraints that are based on private contracts between National Grid and the generators which are not published or available to the public. As can be seen understood, the amount of curtailed wind generation has increased with the increase in installed capacity. Table-2 shows the comparison of total produced wind energy and the volume of curtailed wind generation.
|Total Produced Wind Energy (GWh)||Volume of Curtailed Wind Energy (GWh)||Curtailment Ratio (%)|
The ratio of volume of curtailed wind generation and produced wind energy varies over the years. Figure-1 shows the change in ratio of produced vs curtailed wind energy according to years.
Figure-1 shows that the increase in wind energy penetration into the grid resulted in an increase in curtailed wind energy. The main point is that curtailed wind energy increases faster than the increase in capacity of generation, so it is seen as the increase in ratio. Future estimation of curtailment ratio is a challenging task, because supply and demand are two dynamic parameters which are also the main drivers of the curtailment. However, it is possible to make reasonable comments on the likely behaviour of the curtailment ratio in future.
 Due to aggregation of data and rounding of calculated results there may be small apparent inconsistencies
 There are missing data (Months of Jan, Feb and Mar) for 2011