In all electrical systems, the production and the consumption should always be in close conformity as it would otherwise be physically impossible to keep the system frequency within allowable tolerances. This balance is the main reason why continuous production planning is necessary.
In the EU white paper on Renewable energy, 40000 MWe installed capacity is targeted by 2010 ( delivering about 80 TWh/y ). If 25% should be delivered by 10000 MWe capacity from large wind farms, preferentially offshore, then this would lead to an annual production of over 20 TWh/y. Concentration of such large production capacity could lead to a large penetration in the existing network and the problems of balancing production and consumption becomes then important.
Load forecast aims at an economic and reliable adjustment of the production to the load, from seasonal, daily down to 15 minutes level. Load forecast handles prediction of the load with all stochastic aspects involved. Analysis tools include probabilistic generation simulation and generation costing models and reliability analyses in generation/transmission based on Monte Carlo simulations.
The temporal production from wind parks, with a substantial stochastic character, will require new sophisticated methods to forecast the production capacity, and to mobilise conventional generation capacity to continuously meet the consumption. What is the state of the art in short term resource forecast for offshore wind parks?
Prognosis tools: At European level, an annual wind turbine production of about 20 TWh in 2010 would require a number of changes in the EMS practices and tools. Especially the prognosis tools currently available for this variable production concept need to be further developed and optimised. When the local penetration of wind energy is large, it is very important ?technically and financially ?to be able to forecast the expected production from such a large installed capacity.?In some electricity markets (e.g. the UK NETA balancing market system) it is also very important for individual wind farms. Furthermore, the capacity cannot be equally distributed throughout Europe as the wind resources and the available sites are concentrated in certain areas, e.g. Northern Europe. Experience has demonstrated that the uncertainty of the present prognostic tools is in the vicinity of 30-40% for a 36-hour forecast. The accuracy of the prognostic tools should be improved to less than 10% to reduce the costs for regulating power to an acceptable level. There is another issue related to?forecasting periods (�look-ahead? period).?In conventional systems, it is relatively easy to forecast generation and demand for periods of a day or more ahead.?It may be?suspected that look-ahead periods of 24 hours and more are chosen for administrative convenience rather than real need.?The costs this imposes on the system or on generators is small in conventional systems, but is much higher when there are variable sources such as wind in the generation mix.?There is a need to establish the costs and benefits of longer look-ahead periods, in order to determine the optimum.?This optimum is likely to be different for different systems.
In spite of the improvements made in the prognoses tools, it will remain difficult to forecast the power gradients arising in the wind power production within a quarter of an hour. The Transmission System Operation (TSO)will be under an obligation (i.e. the grid code) to keep the Area Control Error (ACE) within limits to avoid penalties for too large imbalances and to ensure that these power gradients are compensated for via the secondary control, either by central production facilities or cross-border exchange. This gives rise to a number of important questions, for instance: Who will be establishing and financing data acquisition and remote control facilities as such? Who will be paying for the lost production and other system costs? Who will be refunding the loss if the production margin is lowered before a particular time of operation ?resulting in the wind turbine owner being unable to deliver the production offered to the exchange? How will the priority between several wind farms be administered ?whose production is going to be restricted??Is this need best met by requiring wind farm operators to install more expensive equipment in order to appear more like conventional generators?
Demand Side Management . An improved agreement between consumption and production would also improve the real-time operation.
Energy storage : Means to face imbalance between consumption and production: operation and EMS in situations with imbalance between consumption and production within a certain area must be investigated in-depth in order to ensure operation with a high penetration of wind energy. A number of solutions must be developed, such as:
-Electricity storage facilities .?Regenerative fuel cells, pumped storage are important technologies for storing large quantities of electrical energy. While pumped storage is already been fully exploited for peak shaving with limited possibilities for extension, the regenerative fuel cells would offer a flexible means for storing energy. If this technology could reach technical and commercial maturity this would significantly improve the real-time operation in systems with high penetration of fluctuating wind energy.
-Energy Conversion. The feasibility for conversion of (surplus) electricity into Hydrogen should be investigated.
-Possible modifications on conventional power plants . The present control possibilities including response time for the existing thermal power plants should be analysed.
Grid integration of large-scale offshore wind wind farms may be constrained by the technical limits of state-of-the-art grid connection equipment.?The number of (parallel) cables between the wind farm and the onshore grid connection point will often be limited for economic or environmental reasons.?
Operating conditions :?Rapid technological progress is made in the areas of sea cable technology and offshore electrical equipment.?Questions to be answered are : What are the maximum (power, voltage, ? ratings for state-of-the-art sea cables, transformers and switchgear.
Maximum distance from shore : ?or offshore wind farms at a large distance from the shore losses and reactive power production in the sea cable(s) may become important.?A question to be answered is : what is the maximum distance from the shore for which grid connection using current technology remains technically and economically feasible ?
There is currently little experience with high-capacity transformers and switchgear installed on offshore platforms.?The environmental conditions in an offshore environment may significantly reduce equipment reliability ( e.g. marine corrosion).? Access for maintenance will not always be possible.?Design changes to improve reliability and maintainability may yield significant benefits for the development of large-scale offshore wind farms.
The same issue becomes even more important when it is considered to install power electronics (frequency converters, ? offshore.?
Information is required regarding the behaviour of electrical equipment in a highly aggressive offshore environment, and regarding developments aimed at improving reliability and maintainability.
Whereas most of the current offshore wind energy projects use an AC link for the grid connection, the possibility of using DC links has never been excluded, and due to technological progress in the field of AC/DC conversion technology, DC links may become the preferred choice for future offshore wind farms.
Questions to be answered are : What performance can be expected from state-of-the-art AC/DC conversion technology (in particular, capital cost and electrical losses)?
Economic Break-Even Distance for DC connection. ?nother important question to be addressed is?: From what distance may DC links be considered as being economically more interesting than AC links, taking into account the current state-of-the-art in AC/DC conversion technology.?This distance will be a function of MW capacity.
Ease of connection
HVDC may offer control benefits which allow connection to a weaker part of the network, so saving costs.?If underground cable is required for the onshore section, for environmental reasons, HVDC may be cheaper than AC.
Redundant grid connection systems . By increasing the redundancy in the grid connection system it is expected to improve the availability and the reliability of the system.?On the other hand, increasing redundancies?will also?increase the system complexity and cost.?Given the limited available information on the reliability of offshore electrical equipment,?it is not excluded that increasing redundancy will effectively decrease the overall system reliability.? Some questions to be answered are therefore : What is the optimal degree of redundancy to be used in LSOWE grid connection systems ??How must emergency (back-up) power for equipment in the wind farm be provided ??
Internal Wind Farm Grid Lay-Out
Many different designs have been considered for the internal grid of large-scale wind farms. The lay-out adopted for the internal wind farm grid may have an important impact on the global wind farm availability and on investment costs. A question to?be answered is therefore : What is the optimal internal grid lay-out ?
New wind turbine concepts have been proposed which might significantly alter the cost and the feasibility of grid connection of large scale offshore wind farms.?For instance, systems using DC generation in the wind turbines, combined with IGBT-based DC/AC conversion onshore have been announced.?The impact of these new designs on grid connection must be analysed.
Various factors contribute to voltage fluctuations at the terminals of a wind turbine generator: aerodynamic phenomena (wind turbulence, tower shadow effect, etc) short circuit power at the grid connection point,(?), number of wind turbines, and the type of wind turbine control systems. Under particular connection conditions, this may result in a significant flicker level. As a consequence, some limitations for installed power could be recommended in case of a weak network or particular polluting devices. This is especially valid for offshore wind farms, as the grid connection point may be a weak point of the grid and the building of a reinforced transmission line may not be feasible for environmental reasons.
Required grid characteristics (Installed power versus short circuit power)
Impact of long-distance power cable to shore on power stability
Suitability of existing guidelines for Power Quality Assessment
However typical power quality issues, like flicker, harmonics, voltage fluctuations and variations (during normal operation and during switchings of the wind turbines) is a less problem for LSOWE due to the soothing effect of the number of wind turbines within the wind farm and due to the improved power quality behaviour of today's wind turbines. This problem will be treated in chapter 2.3.1.
The impact on voltage control depends primarily on the connection point and the generating plant power output. Present day onshore wind parks have a relative low power output and are connected to the Medium Voltage grid system, which means they rarely have a significant impact on voltage control. But in the event of substantial power increase or wide-scale connection to the High Voltage grid, existing specifications might be changed to account for the impact on voltage control.
??Until some years ago, generator technology for wind turbines used to be mainly based on fixed speed induction generators.?For several years however, variable speed induction generators (using IGBT rectifier - DC link - inverter technology) have consistently won an increasing share of the market.? ). The main advantages of the variable speed wind operation are to reduce drivetrain requirements and to optimise the energy conversion. The power quality such as flicker, harmonics, voltage and frequency variation can be controlled by variable speed wind turbine generators using a power electronics interface. The type of interface used for connecting the wind park unto the network has a determining impact on harmonic interference. Thyristor technology for inverters generates low frequency harmonics (250 Hz to 1 kHz), whereas IGBT technology generates high frequency harmonics (1 kHz to 1 MHz) depending on interface power rating.
The large installed capacity combined with long transmission distances to the net may create problems of instabilities and excessive reactive current transmission. It may be advisable to perform dynamic analyses to understand the nature of the unbalance and to correct the situation.
Incident conditions (short circuits, voltage dips,…) may have to be simulated with models which incorporate the interface technology (direct coupling, inverter interface, power electronics interface,…) since the interface technology appears to have a determining impact on the system behaviour under incident conditions.?For fixed-speed wind turbines, the drive-train characteristics must also be simulated. What is the state-of-the-art in dynamic grid stability analysis tools??Are suitable models available?
Secondary control is the system-wide adjustment of the production in the neighbouring zone to a new operating situation to maintain balance between production and consumption with a time constant of the orders of 10-15 minutes. The introduction of LSOWE may have an impact on the required dispatchable power.?How can this be done ? With hydro power or pumped storage? What is the additional cost to guarantee the needed dispatchable power? In a free market, will this cost increase the cost of ancillary services ? Can the wind farm or wind turbines be controlled satisfactorily to control power, power gradients, and voltage??What does a TSO really need?
Ancillary services: are the services needed to transmit the energy from generation plants to end users with guarantees concerning power system dependability. The main ancillary services concern active power and frequency regulation, reactive power and voltage regulation and system restoration after collapse (blackstart capability). We may notice the fact that in terms of quantities, wind turbine generators are expected to take a large part of renewable generation in the future (EU target 11.9 % of?the total Renewable energy production in 2010). As a consequence, we should pay great attention to the ancillary services capability of this energy production.?Is it sufficient to rely on a market approach, or are firm technical requirements necessary?
In a fully liberalised market, the power utility context moves from a monopolistic structure, with technology driven developments, towards an open production competition with market driven developments. The collegial interaction between former geographical monopolies disappears completely.
Superimposed on this trend are some policy driven developments in the field of Renewable Energy, which cannot be handled by the open market as such.
The TSO will remain in hands of geographical monopolies, however subject to �strong? national supervision.
This new situation poses a series of challenges in the power system planning for satisfactory operation of the system as a whole and in particular for the large connecting large offshore wind parks , such as:
Impact of the grid code (connection code) on generating investments (and profitability). The grid code contains the national requirements for the user of the network with strict procedures for connection and information exchange.
Technical requirements for small scale generation and impact in case of substantial penetration of these small scale units. The criteria have been fixed as for conventional onshore wind farms and may not be flexible enough to handle large offshore wind park connections.( e.g. Belgium : Operational reserve should at least be 10% of the total production of the park, with the possibility to recover lost capacity within 15 minutes).
Impact of geographically concentrated generation (particularly large wind parks) on national and interconnected grid development. Special attention should go to investigate the grid capacity along the coastline. Note also that the coastal areas are mostly located at the end of the transmission line, which is not conceived to transmit power in the reverse direction.
Stability of the context (ruling) in order to perform reasonably long term planning, particularly for large offshore wind parks, which is necessarily policy ( and not market) driven. The EU proposal to force priority access for Wind energy to the grid?is an example.
Contrary to onshore wind projects, the offshore technology is not in an advanced state to evaluate the total investment budget with enough precision. Indeed the foundation costs and the interconnection costs, which can easily exceed the cost of the wind turbines, contain some large unknowns and may vary considerably from site to site.
LSOWE contain considerable risk elements that can have a large impact on the production, and hence on the revenues. There is not enough experience with offshore wind parks to evaluate the technical availability, due to inaccessibility for repairs. Advances in technology ( based on past experience) may possibly increase the technical availability and hence the production capacity which is a considerable risk factor to reckon with. Operation and maintenance costs are very difficult to predict. No guarantees can be given regarding lifetime of wind farm equipment in harsh offshore conditions.
Financing institutions are currently prepared to invest in offshore wind energy projects. Nevertheless, these projects are considered as high-risk investments. Financing conditions (e.g. minimum equity versus loan, rates,…) may therefore be higher than for conventional, and even onshore technologies.
Important investments in LSOWE will only be possible if the inherent investment risk can adequately be insured. Therefore, it should be examined to what extend and under what conditions insurance companies are ready to insure offshore wind farms.
Under the current liberalised market conditions, Renewable Energy technologies, face significant barriers to be widely used such as
High capital cost
Lack of network infrastructure
Lack of confidence in these new technologies
Technical problems associated with the geographical distribution of available potential, and the stochastic nature of the primary energy (Wind)
Legislative barriers for obtaining construction and operating licenses.
Electricity trading mechanisms which inequitably penalise unpredictability.
Support mechanisms :
The most critical policy issue towards the EU white paper targets concerns the support mechanisms to be established for Renewable energy (including LSOWE): Across Europe, there exist a wide range of support mechanisms?such as :
Fixed feed-in tariffs : (e.g. Germany) : not market based, but highly effective for promoting local industry
Quota system (with or without penalties) : Competition based mechanisms ensure that the quota are obtained with the cheapest technologies.(e.g. Belgium)
Public tender approach (cfr former NFFO in the UK) :
Green certificates (Denmark, Netherlands) : A market based approach where the Wind park generates kWh and certificates which are both handled separately and traded. This requires however a large enough trading area (e.g. European) to be effective and stable. However, this presupposes harmonisation rules at the European level.