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Why are research and development needed for Distributed Generation?
To pave the way to a sustainable energy future based on a large share of DG, there is a clear need to prepare the European electricity system for the large-scale integration of both renewable and other distributed energy sources. To this end research on the key technologies will allow a transition towards interconnected grids using common European planning and operational systems.
The research will assist in removing barriers relating to finance, policies, technologies and technology standards and RTD actions aimed at the adaptation of technical grid infrastructures, the establishment of necessary institutions and the harmonisation of related regulatory frameworks and market conditions need to be undertaken.
The challenges that need to be addressed to achieve a broad and sustainable future European energy service network, can be summarized as follows:
1.- Power Reliability and Quality
When power is injected into the network at the distribution level flows of electricity are changed and this leads to technical issues affecting the stability of the network and the power quality.
A number of electrical parameters are used to characterise power quality - or more accurately voltage quality - at any point in the system. Maintaining a steady state voltage is very important - a fairly wide tolerance is acceptable for long term ‘static’ voltage but fast small variations can cause problems especially in remote areas where networks are ‘weak’.
Short circuit power level is a measure of the ability of the network to absorb disturbances - effectively describing its relative ‘strength’ or ‘weakness’ at any point. Depending on the electrical equipment installed with DG they may be able to operate in weak conditions.
Voltage flicker and variation caused by fluctuating loads or production are the most common cause of complaint on power quality. Harmonics are a measure of the distortion of the voltage sine-wave and are becoming more important to power quality. They are produced by many types of electrical equipment including power electronics such as linear drive motors and personal computers and affect both supply and demand sides.
Reactive power is produced in capacitive components of a network (e.g. cables) and consumed by inductive components (e.g. motors, transformers). High reactive currents lead to higher losses in power transmission and cause voltage instability in networks, therefore TSOs and DSOs work to minimise them. Depending on the type of generation, DG may supply or consume reactive power. DGs connected via power electronic interfaces can support the network voltage given appropriate market incentives.
Mains electricity supply in Europe has a fixed frequency of 50 Hz. Increasing electrical load tends to reduce the frequency and frequency control systems act to bring the system back to equilibrium. DGs can provide frequency responsive spinning reserve or support the network operation participating in the secondary frequency reserve arrangements, if the appropriate ancillary service markets encourage such participation.
Network connection of any DG is constrained by power quality considerations. Appropriate technical standards are being developed that ease the wider uptake of DG but do not affect acceptable power quality standards.
For example, fixed speed Wind Turbines connected at weak feeders, can produce flicker because of periodic changes in the Wind Turbine output. The technical requirements for the connection of WTs however limit this possibility. In regions where there is a very high WT penetration grid operators can face large changes in the power supplied from combined WT capacity, as the wind varies. The current operating practice is to maintain power capacity in a standby mode (called spinning reserve) to ensure stability. Large scale energy storage systems can efficiently compensate for the intermittent nature of wind and other RES based DG sources in combination with new and flexible energy management tools equipped with improved wind power forecasting functions to enable efficient management of the spinning reserve.
2.- Power Systems technologies
New technologies and concepts for the operation and exploitation of the networks which are able to cope with the integration of RES and other DG will encompass new command and control systems and algorithms and standards for generator and storage dispatch to match instantaneous supply with demand in a predictive and cost effective manner.
The new concepts and strategies for control and supervision must ensure secure operation of a Unified European Electricity Grid and the proper utilisation of the increasingly complex grid structure with new intermittent energy sources. The implementation of interactive service networks will require novel control and supervisions schemes.
The development of many intermittent and grid-connected RES will need new ways of securing energy services to end-users. New common planning methods will need to be developed that take into account the interchangeable roles of DG energy producer and user. The future grid has to be able to handle situations where the consumer of energy services becomes producer when there is a surplus of local supply.
Technologies bundled into the DG system will increasingly include interfaces for connection to local supervisory control and data acquisitions (SCADA), distributed control systems (DCS) and possibly internet systems. Other technologies that are necessary or complete a system include developments in: metering; protection and control; automated (decentralised) dispatch and control; and site optimisation of electrical/ thermal outputs.
SCADA systems have only become feasible with the development of modern communication methods. SCADA systems are tending to increase the degree of automatic control in networks. In parallel, following the establishment of liberalised electricity markets, Plan And Data Acquisition (PANDA) systems have been developed to allow exchange of information on production and consumption schedules, measurements of actual production and consumption and to allow settlement of traded contracts and balancing of power.
It is logical to combine these two parallel systems into one integrated control/ market information system that has open architecture and the ability to handle the massive information flow that the future network will produce.
3.- Enabling technologies
Key enabling technologies will facilitate the development of interactive energy networks with high power quality and security of service. To build the new type of grid structure it is essential to bring to market low cost technologies that can efficiently bridge together local networks to form a truly pan-European network with the capability of integrating significant RES input.
These technologies include:
Power line communications
4.- Commercial and regulatory challenge
The use of novel technical solutions would reduce the costs of connection and operation, but also lead to the need to more fully utilize and/or share the existing infrastructure where adjustments are made to both generation and demand in relation to overall system dynamic needs.
Such arrangements, as well as being technically more challenging, call for new ways of charging for use of networks and ancillary services, and incorporate market signals and recompense when consumers or generators deviate from an otherwise unconstrained pattern.
Markets, regulators and tariffs do not exists for such arrangements, except at transmission level, and there are significant organization challenges in applying this logic to distribution networks.
Early moves to allow the development of new approaches are essential.