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What is Distributed Generation?
Much of the energy generated today is produced by large-scale, centralised power plants using fossil fuels (coal, oil and gas), hydropower or nuclear power, with energy being transmitted and distributed over long distances to consumers. In this paradigm, power flows only in one direction: from the central power station to the network and to the consumers.
There are a number of drawbacks to such a system, such as the high level of dependence on imported fuels, the environmental impact of greenhouse gases and other pollutants, transmission losses and the necessity for continuous upgrading and replacement of transmission and distribution facilities.
In contrast, in a power system composed of distributed energy resources, much smaller amounts of energy are produced by numerous small, modular energy conversion units, which are often located close to the point of end use. These units can be stand-alone or integrated into the electricity grid.
Architecture of the future electricity systems
The models for future architecture of electricity systems recognise the fundamental fact that with increased levels of distributed generation penetration, the distribution network can no longer be considered as a passive appendage to the transmission network - the entire system has to be designed and operated as an integrated unit. In addition this increased complex operation must be undertaken by a system under multiple management.
Three conceptual models have been envisaged: Micro (or Mini) Grids, Active Networks supported by ICT and an ‘Internet’ model - all of which could have application depending on geographical constraints and market evolution.
New business roles
The advent of new distribution paradigms and increased DG penetration in a single market bring new business opportunities. The communication systems required to operate the energy market will be open systems and an effective energy ‘stock market’ will be enabled. Such systems will require that uniform energy and information interfaces are established, probably using internet-based information networks.
Access to this information will allow new roles for energy brokers, the establishment of “virtual power plants” and variable energy tariffs. Trading in energy futures and other financial instruments will be much more wider than today.
Micro-Grids are small electrical distribution systems that connect multiple customers to multiple distributed sources of generation and storage. Micro-grids are typically characterised by multipurpose electrical power services to communities with populations ranging up to 500 households with overall energy demands ranging up to several thousand kWh per day and are connected via low voltage networks. These hybrid systems have the potential to provide reliable power supply to remote communities where connection to transmission supply is uneconomic. A number of demonstration projects have been undertaken in the Greek islands with this type of system.
Active networks are envisaged as a possible evolution of the current passive distribution networks and may be technically and economically the best way to initially facilitate DG in a deregulated market. Active networks have been specifically conjectured as facilitators for increased penetration of DG and are based on a recognition that new ICT technology and strategies can be used to actively manage the network.
The model employs two novel concepts. The first is that the primary role of the network is to provide connectivity. That is the network is a highway system that provides (multiple) links between points of power supply and demand. The second is that the network must interact with the consumer. The current system essentially provides an ‘infinite’ system in that the network itself remains virtually unaffected whatever is happening on the supply or demand side. If a customer requires such a supply then they should pay for this ‘premium’ service.
The structure of this model is based on increased interconnection as opposed to the current mostly linear / radial connections, relatively small local control areas and the charging of system services based on connectivity. The active network has some analogies to telephone networks and requires active management of congestion unlike conventional passive systems that rely on Ohm’s law to determine power routing. With increased distribution of power input nodes due to DG, bi-directional energy flow is possible and new technologies are emerging that can enable direct routing of electricity. New power electronics systems offer ways to control the routing of electricity and also provide flexible DG interfaces to the network. Flexible AC Transmission Systems (FACTS) and Custom Power Devices at lower voltages offer the potential manage routing of power supply in an active manner.
Each node whether a gigawatt natural gas power station or a single solar photovoltaic panel needs to be controlled and the necessary number of combined control tasks multiply. Application of FACTS, or similar technology, increases the number of control parameters. Accurate information on the state of the network and coordination between local control centres is essential using state of the art ICT.
Electricity transmission in the system is not dependent on a single route so failure due to a single component problem is reduced. However an inherent risk of interconnected networks is a domino effect - that is a system failure in one part of the network can quickly spread. Therefore the active network needs appropriate design standards, fast acting protection mechanisms and also automatic reconfiguration equipment to address potentially higher fault levels.
The greatest change in the active network model is at the local control area level where each defined area has its own power control system managing the flow of power across its boundaries. The system would be ICT-based with management enabled by remote actuators controlling the system. The central area control computer would ‘negotiate’ with neighbouring areas on exchange of power. If an area was isolated then the system would react by disconnecting enough load or generation to maintain the correct power balance. This could lead to considerable improvements in the reliability of the supply system as a whole. This model requires relatively little further investment in infrastructure, except to reinforce some areas of the network to provide increased interconnection and investment in automated switch gear.
The internet model effectively takes the active network to the global scale but distributes control around the system. The flow of information around the world wide web/ internet uses the concept of distributed control where each node, web host computer, email server or router, acts autonomously under a global protocol. In the analogous electricity system every supply point, consumer and switching facility corresponds to a node.
The internet enables many new opportunities. A conventional power station generates electricity in one location, using (usually) one type of generating technology and is owned by one legal entity. A virtual power station is a multi-fuel, multi-location and multi-owned power station.
Both stations supply energy reliably at predetermined times. Today this means making a power supply contract for each hour of the next day. The power stations must be able to change their power output quickly and sell this capability as ancillary services to the grid operator.
For a grid operator or energy trader, purchasing energy or ancillary services from a virtual power station is equivalent to purchasing from a conventional station. The concept of a virtual power station is not itself a new technology but a method of organising decentralised generation and storage in a way that maximises the value of the generated electricity to the utility. Virtual power stations using DG, RES and energy storage have the potential to replace conventional power stations step by step until a sustainable energy mix has developed. Extending this concept to a virtual utility merely extends the services available.