Integrating distributed renewable energies into the grid
Today, the new renewable energies are a global reality, no longer dependent on the support from individual countries. But the approach of connecting renewable energies to the existing systems is too shortsighted. Instead, electric power supply systems must be further developed to integrate new sources on a larger scale.
More than 10 years ago the new renewable sources of electric energy — sun and wind — began to make their way into the electric power supply system. At that time, they were seen as two additional primary energy sources that could be connected to the existing systems without making any fundamental changes. Today, these new renewable energies have, in some countries, become the largest generation subsector.
Countries from all regions are active, and some of the early pioneers — recognisable by their high installed capacities — have been overtaken by other countries. The strongest driver of this change is photovoltaics, which — after the significant cost reductions at the end of the last decade — has reached or fallen below grid parity in a number of countries. That is, photovoltaics has achieved competitive end-consumer prices in low-voltage grids.
Photovoltaics is an economical option for meeting the demand of individual households, provided that the grid usage fee is largely energy based. This makes it independent from direct subsidiaries for a large scope of applications as long as it reduces the owner’s own demand.
The figure below shows the five leading countries in the world in terms of installed capacity and new wind and solar capacity in 2013.
New renewable energy sources and system integration
New renewable energies have three main features that fundamentally change the electric power supply system: remote generation, distributed generation and volatility.
Remote generation
The share of remote generation of renewable energy is much higher than with power plant systems in which a regional balance of generation and demand is preferred for both economic and technical reasons. This development is mainly driven by the heavily location-dependent sources of wind and water and can lead to very large generation units or clusters.
Distributed generation
The growth of distributed generation is primarily driven by photovoltaics and combined heat and power generation (CHP). For photovoltaics, this is mainly due to the relatively low economies of scale in terms of costs combined with economic performance, relative to the end-consumer prices in a low-voltage grid. CHP must be distributed in order to provide the heat close to the consumer. Very small PV systems in particular can lead to a considerable share of the generation being covered by a very large number of smaller units feeding energy into the distribution networks.
Volatility
Volatility is mainly introduced to the electric power supply system by wind and solar energy, both of which lead to faster, larger and, especially in the case of wind energy, less predictable fluctuations than before.
Remote generation, distributed generation and volatility affect all areas of electric power supply and utilisation. The figure below gives an overview of these areas, including the influence of new loads as drivers for change.
Conventional provision of electric power
The rising share of renewable energies is influencing the operation of conventional power plants. The increased frequent use of power plants originally intended as base-load plants for loads following operation with steep power output gradients poses a great technical challenge.
Another factor influencing the operation of conventional power plants is that, as wind and solar energy have no variable costs, they will always be placed at the lower end of the merit order in an energy-only market. This means they displace conventional generation, reducing the utilisation of conventional power plants and making fixed-cost coverage more difficult.
These economic effects mean that building and operating conventional power plants is no longer attractive. But as conventional generating capacity is indispensable both as backup for periods of low renewable power output and for power system control, suitable adaptations of the market design are now being discussed. ABB is deeply involved in the discussions and is helping to shape the modern electric power supply system.
Transmission level
In transmission networks, remote generation leads to increased capacity requirements. Additionally, the volatility of the generation — particularly in combination with the low number of full-load hours of the renewable energies — increases transmission requirements. Expanding the interconnected power system represents the most cost-efficient option to match volatile generation and consumption. The benefit of regional expansion for the integration of a very high share of renewable energies into the electric power supply is illustrated in the figure below, using the expansion of the European interconnected power system to North Africa and the Middle East as an example.
Distribution level
The changes occurring in the distribution networks are manifold. In many cases, an increase in distributed generation requires a reinforcement of the grids. However, especially in rural grids with relatively long transmission lines, voltage support problems occur first. As this is not caused by the one load situation the network has been designed for, but by the multitude of operating conditions between feeding and extracting power, the traditional solution of manually adapting the transformation ratio of the local distribution transformer is no longer sufficient. In such cases, the often significantly more expensive grid reinforcement can be postponed or even entirely avoided by installing a voltage regulator such as a voltage-controlled distribution transformer (see, eg,[1,2]).
The increasing variety of operating conditions in the distribution networks increases the information requirements. This leads to an at least partial automation of the distribution substations, which thus far have been minimally monitored or remotely controlled. Distributed generation as well as e-mobility (due to the mobile nature of the consumers) will lead to an insufficient capacity of distribution networks in some situations. This means that measurement and control will be required — and as every technical system, including measurements, can be faulty, the solution will be to transfer well-known approaches from the transmission networks, such as state estimation, to the distribution level and into the secondary distribution systems.
If the grid is unable to offer sufficient capacity for all situations, possible congestion must be proactively detected and resolved — a task that is not new in the electric power supply domain. In fact, it is common practice in the coordination between (large-scale) power plants and system operators. Hence, the solutions for this electric power supply area must be largely standardised and automated. An example of predictive distribution network operation, which also takes the requirements of the deregulated market into account, has been developed and successfully taken into operation within the scope of the MeRegio E-energy project in Germany[3].
Consumption
Due to the volatile power output associated with renewable energies, the short-term demand response is gaining in importance. Demand-response measures, particularly those involving loads with inherent storage, may contribute to this. The figure below shows the requirements associated with the balancing of loads and generation for different time domains, the solutions commonly used today and the solutions expected in the future. This clearly shows that demand response can make an important contribution especially in the first 15 min. This is an important period because it is sufficiently long enough to ramp up power plants with fast start-up capability when generation capacity is suddenly lacking. Whether demand response can help in the very short time frame in which the rotating mass of power plants has a stabilising effect today depends on whether an autonomous reaction of the load to imbalances between generation and consumption can be achieved. After 15 min the use of demand response is only realistic for selected applications.
Demand response is particularly suitable for heating and cooling applications as thermal energy storage can, in most cases, be implemented at a relatively low cost. Hence, a holistic approach considering the supply of electric energy as well as of heating and cooling is essential for the utilisation of demand-side flexibility options.
Storage
Storage is another important building block for the integration of renewable energies. But due to the variety of applications and available solutions it is a highly complex topic, which requires a separate discussion.
The road ahead
The transition from an electricity supply based on thermal power plants to a supply using new renewable energies as its main source has technical implications in all areas of electric power supply and utilisation, and will lead to a fundamental redesign of power systems.
Future conventional generation will require plants that can be operated economically even at low loads and in frequently and fast-changing load situations. The transmission networks will have to take over more long-distance transmission tasks with strongly varying load flow situations compared with the past. To compensate for the volatility of the new renewable sources, wide-area interconnected systems can be an option.
The consequences of the integration of distributed generation into the distribution networks will be particularly far-reaching, both quantitatively and qualitatively. First of all, an increase of grid capacity will be inevitable in many cases. As the combination of extracting power from and feeding power into the grid leads to a larger range of operating conditions, additional voltage monitoring and regulation will often be required. And finally, it will no longer be sensible to design the distribution networks for rare extreme situations — this is mainly due to the low number of full-load hours associated with solar energy and because of e-mobility. Thus monitoring and control down to the secondary distribution level will be necessary.
Balancing loads and generation will become more difficult in systems with a strongly varying primary energy supply that is not storable. Besides the proven but landscape-profile-dependent pumped storage plants, battery storage facilities can contribute in the short term, eg, for frequency stabilisation and peak shaving. In the long term, ie, mainly for the compensation of seasonal variations, the system boundaries will likely be expanded by extending the interconnected systems or interconnecting other systems such as heat and gas supply.
The most significant changes in the system management will be the integration of a very large number of distributed units on both the generation and the consumption side, as well as achieving frequency control with a decreasing number of rotating masses acting as stabilising elements.
The greatest challenges in the necessary further development of the systems are — from a more organisational perspective — the coordination of the required measures in all system areas and — from a technical perspective — the development of suitable storage, the operation of the system without rotating masses and the integration of large numbers of distributed units into the system management. With its commitment to innovation, ABB continues to drive the growth of renewables, paving the way for the new electric power supply system.
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