Peter Jones, Head of Technology for ABB Power UK, outlines the key elements that comprise ABB’s approach to smart grids
The UK’s traditional power grid is based on large, centralized power stations that supply end-users via long-established, transmission and distribution systems. Over the years it has performed very well in delivering secure and reliable power. But times are changing. The UK Government has introduced a blueprint for 15 per cent of the UK’s energy requirement to be met from renewable resources sources by 2020.
These challenging targets will require the existing transmission and distribution networks to operate in ways for which they were not originally designed. Energy will be generated from large offshore windfarms in the North Sea or the North of Scotland or from community based district heating schemes. Small embedded onshore windfarms and energy storage will be more common within the distribution system. Increasingly, we will need an intelligent grid that can receive power of all qualities from all sources – both centralized and distributed – and deliver reliable supplies, on demand, to consumers of all kinds. In other words, what we need is a smart grid.
While there is a great deal of discussion in the media about smart grids, it is a term that can mean many different things to different people. However in ABB’s view, a smart grid is an infrastructure that puts the emphasis firmly on active rather than passive control. A good analogy is in the control of traffic on a busy stretch of motorway. During off-peak periods, cars can drive freely with no speed restrictions other than the maximum speed limit. But in the rush-hour the warning signs on the overhead gantries are used to impose speed limits on specific lanes.
So by restricting the speed of movement of individual streams, congestion is avoided, optimizing the flow of all traffic. More intelligent control of power flows in the transmission and distributions systems will allow higher utilisation even during high demand periods.
ABB’s smart grid philosophy comprises four key elements: HVDC Light; FACTS; WAMS; and distribution network management:
HVDC light®
HVDC Light® is ABB’s patented high voltage direct current technology based on voltage source converters (VSCs) and extruded DC cables. It is a well-proven and environmentally-friendly way to design a power transmission system for a submarine link, an underground cable system or network interconnection.
HVDC Light® can rapidly control both active and reactive power independently of each other, to keep the voltage and frequency stable. This gives total flexibility in the location of the converters in the AC system, since the short-circuit capacity requirements of the connected AC network are low. An HVDC Light transmission system can do much more than just transmit power between two points – it can also improve conditions in the connected AC networks.
HVDC Light® converter station design is based on a modular concept. For DC voltages up to ±150kV, most of the equipment is installed in enclosures at the factory. For the highest DC voltages, the equipment is installed in buildings. The site areas needed for converter stations is also minimal. All equipment except the power transformers is indoors. Well-proven and tested equipment at the factory make installation and commissioning quick and efficient. The stations are designed to be unmanned. They can be operated remotely or could even be automatic, based on the needs of the interconnected AC networks.
Potential HVDC Light® applications include:
· Connecting wind farms
· Underground power links
· Powering islands
· Oil & gas offshore platforms; power from shore
· Asynchronous grid connection
· City centre in-feed
FACTS
Flexible AC Transmission Systems (FACTS) covers a number of technologies that enhance the security, capacity and flexibility of power. transmission systems. FACTS solutions can help increase the capacity of existing transmission networks while maintaining or improving the operating margins necessary for grid stability. As a result, more power can reach consumers with a minimal impact on the environment, after substantially shorter project implementation times, and at lower investment costs – all compared with the alternative of building new transmission lines or power generation facilities.
Components include:
Series compensation
Series compensation (SC) is used primarily to reduce transfer reactances, most notably in bulk transmission corridors. The result is a signifcant increase in the stability of the transmission system transient and voltage.
Thyristor Controlled Series Capacitors add another controllability dimension, as thyristors are used to dynamically modulate the ohms provided by the inserted capacitor. This is used primarily to provide inter-area damping of prospective low frequency electromechanical oscillations, but it also makes the whole Series Compensation scheme very tolerant to Subsynchronous Resonance (SSR).
Dynamic shunt compensation
Dynamic shunt compensation is based on Static Var Compensator (SVC) or STATCOM (STATic COMpensator) devices. Both use power semiconductors to control the exchange of MVArs with the grid, over a shunt connection. Thanks to cycle-by-cycle controllability, SVC and STATCOM can counteract event the most rapid voltage transients and consequently reduce the risk of serious voltage depressions and/or voltage collapse.
Furthermore, SVCs and STATCOMs can, under steady state conditions, control the grid voltage profile according to a given optimal characteristic. The benefits for the grid owner/operator are that the voltage profile will be more controlled and the stability limit dictated by system voltage will be raised. So the grid capacity can be enhanced while making its behaviour more robust, flexible and predictable.
Possible FACTS applications include:
· boosting transmission capacity in new and existing transmission lines
· improving long-distance power transfer
· relieving transmission bottlenecks quickly
· improving power quality
· controlling load flows
· improving dynamic stability of grids
· minimizing the risk of blackouts
· facilitating integration of renewable energy sources.
WAMS
When taking active control of grid power flows it is essential to monitor them very carefully and to develop a complete understanding of how the system will behave under all circumstances. This is where ABB’s wide area monitoring system (WAMS) technology comes in.
Transmission system engineers have always wanted to monitor grid operations in real time. But until recently, lack of computing power, combined with the substantial difficulties in collecting, coordinating and synchronizing the grid data made this impossible.
However, the situation has changed through new developments in technology such as phasor measurement units (PMU's). A PMU is an electronic device installed in the generating stations and substations located throughout a power grid. Using global positioning system (GPS) satellites to provide highly accurate time stamping, the data supplied by each PMU is synchronized to the same time base.
By monitoring the data in real time, operators can use WAMS as an early warning system – one that provides enough time to take the kind of corrective action needed to first limit the scope and impact of system disturbances and then prevent major blackouts from occurring. Utilities can also use the data to improve grid security and safely operate their system closer to maximum capacity during periods of high electrical demand.
The benefits of WAMS technology for grid operators include the capability to:
· safely optimize the design capacity of their transmission assets,
· eliminate or reduce revenue losses due to transmission system outages
· optimize new equipment investment.
· enable more rapid and co ordinated recovery after outages.
Distribution network management
While the previous three elements of the smart grid focus mainly on the transmission network, the variable nature of the new renewable energy sources will require some form of distributed energy storage, such as large batteries, flywheels, compressed air and so on to help maintain the security of supply.
In addition, there are calls for the government to adopt feed-in-tariff schemes that will actively encourage small scale renewable generation – for example, farmers might use their land and existing 11kV connections to install a small array of three 6kW wind turbines.
So, as well as distributed energy storage, we need to anticipate the contribution of many diverse renewable energy projects – single wind turbines, wind farms, solar PV on houses, waste to energy and biomass etc – sitting alongside each other and connecting at multiple points along the network. This means that we need to change the way we look at power distribution networks to accommodate more fluctuations in power quality and two-way power flow, while becoming more responsive to changes in consumer demand.
The management of such a complex system will depend on real-time and secure communications and highly adaptive control systems. These will provide utilities and their customers with real-time information from across the network on the performance of grid installations, power flow and consumer demand. They will allow intelligent automated devices to react to imbalances in the system and also improve asset management by enabling improved predictive maintenance programmes and faster emergency response times.
The incorporation of enterprise-wide information systems and customer response management tools will improve utility operations, enabling better customer relations and the provision of tailored services.
Summary
Smart grids will grow through evolution rather than revolution. We expect to see a gradual transformation of the systems that have served us for many years into a more intelligent, more effective and environmentally sensitive network to provide for the UK’s future needs. ABB has the expertise and experience to provide integrated, adaptable solutions now, and the vision and technical know-how to meet the coming challenges.
In future articles we will look at the individual elements of the smart grid in more detail.