CIRED
17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
ACTIVE NETWORKS FOR THE ACCOMMODATION OF DISPERSED GENERATION Vaughan ROBERTS; Alan COLLINSON, Andrew BEDDOES EA Technology - United Kingdom
[email protected];
[email protected];
[email protected]
SUMMARY Passive electricity distribution networks will have to evolve, both technically and economically, into actively managed networks. Such networks, with a new range of paid-for system services, should prove to be the best tool to facilitate embedded generation in a deregulated electricity market. In this paper, EA Technology describe their research on active networks conducted within the Strategic Technology Programme, a co-operative research programme for electricity distribution companies funded by European distribution network operators. INTRODUCTION Modern network planning and control techniques need to take account of the technical and economic impact of embedded generation by adapting existing networks and developing new network extensions to optimise the performance of the power system as a whole. Key elements of modern network planning and control strategies for network capacity and security include management of network voltages, fault levels and protection to provide customers with the desired quality of supply and power quality. For example, planning techniques can be introduced to ensure that network voltages remain within statutory limits and fault levels remain within plant ratings whilst still accommodating high levels of embedded generation. It is also important to determine how best to meet network and generator protection requirements. New technologies also have an important role to play in accommodating high levels of embedded generation and by applying new network operation and management techniques it is possible to minimise network re-inforcement costs. The paper will describe how new technologies and new network planning and control techniques can be brought together to provide flexible distribution networks capable of accommodating significant levels of embedded generation in a costeffective manner. BACKGROUND The story of electricity networks begins a century ago, when electrification was first introduced. In the beginning, local entrepreneurs, followed later by municipal authorities, built their own generating stations and built an infrastructure to distribute the electricity across their city.
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As soon as the service was more or less established, issues of economy and of security of supply received more attention, and this led to interconnections between power stations within municipal areas, and later also between neighbouring cities. The same happened within the distribution system and the first move from radial distribution to an actual network structure became visible. In the meantime the equipment industry matured, and it became possible to increase the unit size of a power station, thereby improving the fuel efficiency and reducing the cost per kilowatt installed. Smaller units were removed and the interconnection network, which started off as an emergency backup system only, was used to transmit power from the larger stations to the distribution networks - see Figure 1. However, distances were still reasonably short and the whole system was still managed by one vertically integrated utility.
Municipal Power Station
Municipal Power Station
Figure 1: Early Distribution Network Configurations
The next step in this evolution is predictable. Having optimised the cost of the power stations, the next step in the optimisation was to build them in locations where space was more affordable, bulk fuel transport was available and where the environmental impact was acceptable. This meant that power had to travel over larger distances and a transmission network was needed to collect all the power generated and take it to the distribution networks - see Figure 2. At this point in time, the logical relationship between power generation and distribution was finished and many governments chose to concentrate generation and distribution in separate, specialised companies.
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Transmission Network
Figure 2: Early Transmission and Distribution Networks
With hindsight, it is clear that all these evolutions are quite logical and can be explained from external drivers. It is very important to keep that in mind when considering what the next step in the evolution could be. PROBLEMS OF DISTRIBUTED GENERATION What we have observed as a next step is deregulation of the electricity industry and the liberalisation of electricity markets, with governments stepping back from the industry believing that market-driven companies will provide the same and more services more efficiently than governmentowned companies can. But in order to protect the interest of society, regulation is re-introduced in the natural monopoly business (ie electricity, transmission and distribution) so that a government can ensure that its public service objectives are still met. However, this paper is not about regulation, although regulation has to be mentioned, because in the evolution described it seems to be not an external driver but an external brake. In many countries, regulation has fixed the existing structure of the power industry as if that is the only possible structure. It has done so in such a manner that innovation like those shown in our short history is very strongly disincentivised. That is not what we need when at the same time we believe that renewable generators and local CHP systems are preferred options to preserve our environment. Many distribution networks haven’t seen generators at levels of 33 kV or less for half a century, but they are returning and at an increasing pace. In order to understand why something is a problem where it was a normal practice long ago, let us examine the difference between the past and present situations. Please note that this list is by no means exhaustive.
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The first point is the notion of a stiff network. We are used to having a network nowadays with a very low supply impedance and correspondingly high fault level, where the voltage and frequency hardly respond to considerable changes in electrical load. Compare this to the electrical system on a ship and you will notice the difference. A low impedance network is good from the viewpoint of loads, but leads to problems when connecting generators. Is the impedance of the generator part of the network impedance, or does it have to be counterbalanced with even less network impedance? Technically, this leads to a spiral where fault levels, and thus costs, go sky high. Many generators claim that their contribution to the network is that upstream investments can be postponed and that they increase the downstream security of supply. However existing reliability models are much too conservative to reward those contributions correctly and the contractual mechanisms are not yet in place to ensure that the reliability added by the generator is tied to a similar time frame as the normal planning horizon for network reinforcements. Finally, the methodology used for voltage control in networks is very much based on the assumption that all power comes from upstream. Downstream generation can lead to unacceptable voltage excursions or to an incorrect response from automatic voltage controllers. This is an issue that needs to be addressed with great urgency. OVERALL SOLUTION – DIFFERENT CONCEPT We believe that problems like these cannot be solved by local patchwork. That is not efficient and it could lead to an unmanageable network. We propose a structural solution, based on: • • •
Interconnection - as opposed to dominantly radial networks. local areas (cells) - using automation to support relatively small local control areas System services as specified attributes of a connection - which are charged to individual customers.
Before addressing these concepts in more detail, two fundamental messages mark the transition from the present to the future. First, the network is not, and must no be considered as a power supply system. The network is a highway system that provides connectivity between points of supply and consumption. Second, a network interacts with its customers. A network that remains virtually unaffected whatever loads or generators are doing, is a notion of the past. If you as a customer require an infinite network, you may have to pay for that service.
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The question of why an electricity network is called a network where there are almost no meshes is bound to disappear. Interconnection is going to be the rule on all voltage levels. The advantages of that approach can be observed in the telecommunications business.
Th e Cell Con cep t Pr i m a r y Cell
In an interconnected network, energy transport is never dependent upon a single path, so that the vulnerability to component failures is significantly reduced. If some paths run a risk of being over-stressed, power can be re-routed. Please note that this calls for some form of active network management because in purely passive networks Ohm’s law, alone, would determine the routing of power. An inherent risk of interconnected networks is the domino effect. If the system falls over somewhere, the fault could propagate over a very large area. However, appropriate protection mechanisms, not too much different from what is being used already, can ensure that the fault is correctly isolated and the rest of the system operates normally. Apart from the protection systems needed, network design engineers have to address the issues of fault levels which tend to increase in interconnected networks; and automatic reconfiguration actions may lead to a shift from infrequent but long supply interruptions to more frequent but short supply interruptions. This is an issue that requires further research. CELL STRUCTURE The most revolutionary change that we propose is the introduction of the 'cell' concept, based on local monitoring, communication and control, see Figure 3. The cell concept does not have a large impact on the topology of the power network - at least as a short-term measure. The difference is in the control hierarchy.
Barcelona, 12-15 May 2003
Secon d a r y Cells
Figure 3 Network split into cells acting as 'independent islands'
We are going to add one level to that, and that is the level of power flow control. Each cell will eventually have its own power control system, essentially computer based, that manages the flow of power across the cell’s boundaries. This management is enabled by all sorts of actuators helping the control system achieve its objectives. Typical examples of such actuators are: • • • • •
Voltage and reactive power controllers Fault current limiters Storage devices FACTS and UPFC devices Remotely controlled loads and generators.
In a more distant future this means that control systems of adjacent cells will negotiate in real time how much power needs be transferred over their mutual interconnection. Software agents for such processes are under development already.
In the existing operational control hierarchy of a distribution network we normally see three levels. The protection systems and interlocks form the lowest level. They operate very quickly, completely autonomously, and serve only to protect the network from propagating faults and to protect the public from unsafe situations.
Power flow control also means that, if all connections with neighbouring cells are lost, the cell may still be able to remain powered, by simply disconnecting enough load or generation to obtain a power balance. This could lead to considerable improvements in the reliability of the electricity supply system as a whole.
The second level is automatic control systems, which are limited in practice to automatic voltage control relays operating on transformer tap changers. The third level is network reconfiguration either by local or remote operation of switchgear. In virtually all cases this requires human intervention.
Currently, the definition of system services is more or less restricted issues for which the transmission system operator has responsibility. However the most obvious system service for a distributor is the balance of reactive power as in many cases it is more cost-effective to ensure that balance locally. This means that the distribution network operator will also provide that as a service. Ensuring that the system voltage is within agreed limits is also a duty that each network operator needs to fulfil. Symmetry between phases has never received much attention but may become more of an issue when networks are operated at higher impedance levels. Which brings us to the issue of network impedance. The value of that
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impedance, with the impact on voltage regulation on the one side and issues of fault level at the other side. This is a system service and needs to be quantified accordingly. The optimal value will be determined by a trade off between requirements of voltage stability and absorption of flicker phenomena on one hand, and limitations on fault level on the other hand. Accepting fault level contributions from customers, in particular generators, is considered a service as well and customers may have to choose between paying for that service or limiting the fault contribution on their side of the meter. Finally, the security of supply or connectivity is a system service, where the option of being controlled or even disconnected could result in a discount on your network charges. Each system service will have its price and these prices apply to generators as well as to loads. For example, reactive power could be charged in a very similar way to active power. There is a standing charge for availability plus a certain charge per unit actually exchanged with the system. In terms of phase symmetry, exceeding certain limits of homopolar or negative sequence currents, which is now simply forbidden could be allowed, at an appropriate price. As the variable cost of compensating these currents will be relatively low, the customer would probably pay only a standing charge.
Barcelona, 12-15 May 2003
certain amount of controllable negative-sequence current. When it comes to network impedance, generators with fast-acting voltage controllers could contribute considerably to the dynamic admittance of the network. Again power electronic controllers have an advantage because they can react extremely rapidly without increasing the fault level. Finally any load or generator that can respond to remote control can be contracted by the network operator in order to ensure power balance in case of islanding or restricted availability of external supply. CONTROLS WITHIN CELLS The concept of “Active Networks” and the proposed cell structure is considered to be a necessary development for distribution systems, especially if significant levels of Embedded Generation are to be integrated and accommodated into the distribution network. As a result of the evolutionary process, there will be various levels of active networks. The starting point for this is the present network, and a ‘Basic’ level of control and interaction can be established. This is illustrated in Figure 4.
Primary
When it comes to network impedance, we propose that the network operators guarantee a dynamic impedance of, for example, 100 times the contracted power. The customer can then exercise the option of lower supply impedance at a cost. In the same manner, fault level contribution can be charged in the form of a standing charge for each kVA contributed. These charges are all based on the cost that the network operator has to make either to reduce his own fault level contribution, or to accept a higher fault level by upgrading part of the network.
SCADA Master Station
Information Flow X X
SCADA Out Station
X X
X
X
X
Moni tor: Breaker state, Powe r output and Voltage l l
Figure 4: “Basic” cell control and interaction
And now, because services have received a price, a network operator has a basis of purchasing those services from his customers if that is more attractive than producing them within his network. Again, for example, a generator providing controllable reactive power production may receive a premium dependent upon the degree of controllability. The compensation he receives may be composed of a standing charge for availability and per unit compensation for the amount actually provided.
This arrangement is a “selective” extension to existing monitoring. Although no “real time” interaction is present, a generator however may have two operational modes, perhaps Winter and Summer. At the other end of the spectrum, a fully active arrangement can be introduced, with specific control feedback and local control and intelligence. This is illustrated in Figure 5.
Generators using power electronic inverters are perfectly positioned to provide balancing between phases. The network operator would contract the availability of a
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But this can be done in a phased manner - the only issue is to have a clear vision of where we want to arrive in the end.
Primary SCADA
Master Station
Information Flow X X
Control
X X
X
X
SCADA Out
X Intellige SCADA Out
Control
AVR Governor X
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SCADA Out
THE R&D AGENDA This is new thinking and currently there are no distribution companies operating in this way. There are many technical and some regulatory barriers to overcome before we can do this. However, we now know what we need. We only need some bright engineers and economists to process those questions and translate them into applicable solutions.
Figure 5: “Fully active” cell control and interaction
Some of this work is being planned for academic research programmes in various counties. More practical issues are considered within the Strategic Technology Programme, a co-operative research programme for electricity distribution companies managed by EA Technology.
With this arrangement, real time interaction with the network and generator is used and local control possible via the generators AVR and governor.
Finally we hope that representatives from regulatory bodies will take their responsibilities in facilitating innovative solutions for tariffs and system service charges.
SYSTEM SERVICES
WAY FORWARD (TOOLS FOR TRANSITION)
There has to be an economic case for introducing new operating and management techniques in place of the current conventional methods. The most obvious advantage is that the changes the authors propose ask for virtually no physical reinforcement. Without any changes in operating techniques these reinforcements will be unavoidable if huge amounts of embedded generation are to be accommodated that the market, encouraged by governments, is trying to impose on network operators.
In general, we have seen that network planning and control procedures in most European countries are influenced more by historical and local factors rather than a strong vision for any new perceived role for distribution networks in the future. This is a reflection of the fact that large amounts of distribution network already exist and it is difficult to change what is already there.
Monitor: Breaker state, Power output, Voltage
The form of network automation that the authors envisage allows us to operate transformers much closer to their physical limits and would thereby reduce the need for new transformers considerably. This not only saves money, it also means that DNOs can save themselves the trouble of obtaining land and permission to expand substations. Part of the money saved on these items will be spent on more switchgear and control systems. A flexible network requires as many options for reconfiguration as possible. Moreover the switches must be remotely controllable because of the need to exclude manual human intervention as much as possible. The biggest investment, not necessarily in money but certainly in engineering effort, would be the introduction of automation to support this. Much work has been going on already in establishing what kind of communications technology is available and applicable for various functions. A lot of work will have to go in design and implementation of control strategies.
Typically, distribution networks grow and evolve “organically” and it is recognised that there is limited scope for network operators to significantly influence future network development. As a consequence, most research activity involves the development of small incremental improvements for existing networks. However, current network planning and control techniques are clearly inadequate and lack the necessary flexibility required to accept significant levels of embedded generation. The problem is compounded by a lack of fundamental reasearch aimed at developing strategic models for the development of future distribution networks capable of accepting significant levels of embedded generation. Given the nature of established distribution networks, the distribution network of the future will have to evolve using small incremental steps. However, the thinking behind any future network design concept is unlikely to be carried out incrementally – it will require a paradigm shift in the way distribution networks are conceived.
However, a gradual transition from present situation
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CIRED
17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
should be possible, addressing issues of network reliability, maintainability and safety. The primary aim is to achieve increased flexibility of the network; from ‘distribution network’ to ‘electrical highway’. There is a need to build a theoretical framework to support the system design and engineering process. We believe that the “Cell Concept” provides this framework. This will involve the development of the “tools for transformation” to enable the change to the new design concept. It will also be necessary to quantify the savings made possible by any new approach (less overrating, less redundancy, lower fault levels) and show to what extent these pay for the additional investments in controls and actuators. Finally, there is a need to re-define the concept of "security of supply" taking into account the fact that not all supply comes from upstream. REFERENCES 1.
P. D Stowell, 1957, “ A Review of Some Aspects of Electricity Distribution”, IEE 2. N. Jenkins, et al, 2000, “Embedded Generation”, IEE Power Energy series 3. J.T. Hampson, 1997, “Embedded Generation Review of Voltage Regulation and Control”, EA Technology 4. EPRI, 1998, , “Substation Communications and Protocols”, Conference on Substations and Equipment Diagnostics Conference 5. G.V. Roberts, 1999, “Communications for Substation Intelligent Electronic Devices” EA Technology Course on Plant and Substation Management 6. EA Technology Seminar, 2001, “Embedded Generation- Balancing Future Risk and Rewards” 7. J. T. Hampson, 2001, “Urban Network Development”, IEE Power Engineering Journal 8. A. Collinson, 2001, “Practicalities of re-inforcing the network for embedded generation without prohibitive costs” IIR Conference on Profitable Embedded Generation, London, UK 9. A. Beddoes, 2001, “The Future of Embedded Generation”, EA Technology Course on Networks with Embedded Generators 10. M. I. Lees, 2001, “ The strategic technology programme - a model for high value and focussed collaborative technology development”, Cired 2001
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