networks
The energy transition towards a sustainable electricity supply system leads to a growth of decentralised production of electricity connected to the distribution net- works and to an increase in electricity usage for new appliances. These developments lead to two fundamental changes that affect the electricity distribution networks:
• due to the strong increase in DG, the electricity flows are no longer known; • there is an increase in flexibility in the electricity demand that can be used. This has consequences for the operation and design of electricity networks. How this affects the electricity distribution networks, is discussed in this section.
2.3.1
Distributed generation
In the traditional power system the electricity flows top down from centralised gen- eration units connected to the HV grid to the consumers connected to the MV and
2.3. Fundamental changes in electricity distribution networks 13
LV grid. In this system the distribution networks play passive roles as they just receive the power from the transmission networks and deliver it to the loads (cus- tomers) and the power flow is mostly mono-directional. The infrastructure of the current distribution networks, their protection and monitoring devices, as well as their control systems are all designed to operate in this passive environment [36]. Integration of large-scale DG into the distribution networks changes the networks into active distribution networks with bi-directional power flows [22].
The capacities of the generators determine whether they are connected to the MV or LV grid. Current praxis shows that available technology for DG and the typical available size varies widely [8]. Also the location of the generators is arbitrary and not always close to the demand. This is especially true of wind power, which is usually generated in remote areas far from the more populated regions. Wind turbines are often concentrated in wind parks and connected to the MV grids. At the LV grids, in residential areas, electricity can be generated by photovoltaic panels which are placed at roof tops of houses and office buildings, by micro combined heat and power systems (small, domestic systems that generate heat for space and tap water heating and simultaneously deliver electricity back to the grid) that may replace conventional boilers, or even by small city wind mills.
The connection of DG to the distribution networks can lead to operating issues; for instance, DG can affect voltage control or distribution grid protection [9, 28, 119]. Besides this, especially the intermittent and fluctuating character of renewable distributed generation poses some additional difficulties to their integration into the distribution networks. For instance, solar power is dependent on the abundance of sun and the absence of clouds. This makes the amount of solar power difficult to predict. Wind can be predicted more accurately, but can fluctuate dramatically. This not controllable nature makes it fairly difficult to use these resources optimal and pose obstacles to their integration into the power system. It makes it difficult to match the available electricity to the local electricity demand, and can cause large variations in power flows in the distribution network. Without any changes to the networks and the way DG is operated, a high penetration of DG can only be achieved by major network reinforcements [32].
One way to support a higher level of penetration of DG is to invest in the ca- pacity of the networks, but there are also other solutions which can be thought of. These assume continuously monitoring and controlling the grid and the generators. With active management, the penetration grade can be much higher [151]. Simul- taneously, distributed electricity storage and controllable loads can be incorporated into the grids by applying more active network management. These technologies make it possible to shift demand for electricity in time or, more precisely, to shift the transport of electricity in time and use the intermittent distributed generation locally. Distributed electricity storage can store the energy produced by DG when the source is abundant and demand is low, and release the power during peak peri- ods. Management of controllable loads would make it possible to shift the demand in time. In this way, the electricity grids can be used more efficiently, energy loss
due to the transportation of electricity is reduced and the integration of distributed renewable energy sources into the electric power system can be supported without requiring major network reinforcements in the first place.
2.3.2
Flexible electricity demand
The demand for electricity is expected to grow, especially due to an increase in demand for heat pumps and electric transport. This does not inherently mean that it is difficult for the grid to cope with this additional load. The reason for that is that an important characteristic of these loads is that the exact moment at which the demand is fulfilled is less important than for regular loads like electric lighting or microwaves. For example, a house has the capacity to hold the thermal energy within its walls for some time, and cars may be charged during night. The electricity demand for these loads is less time dependent than for most other types of loads. The flexibility in these demands brings with it the opportunity to shift demand in time and apply load management without any discomfort for the consumer. There can be various reasons to use this flexibility in residential electricity demands.
As mentioned in Section 2.2, the electricity networks are designed to meet peak demands. However, shifting a part of the electricity demand by for instance directing flexible loads to off-peak periods or by applying demand response programmes to stimulate the consumers to use domestic appliances at other moments of the day, makes it possible to shift the loads, allowing more electricity to be transported without increasing the network capacity to the level of high peak demands. As was already mentioned in the previous section, another advantage of shifting the use of electricity in time is that it can contribute to a better integration of DG.
From the broader perspective of the electricity supply system, a third advantage of shifting demand in time would be to match electricity demand with supply in order to optimise the electricity production capacity and maintain the power bal- ance in the system. The fulfilment of this overall, system-wide requirement can be achieved by large-scale storage technologies, such as pumped hydro or compressed air energy storage, but also by distributed electricity storage or load management. An example of applying load management for this goal will be demonstrated at the island of Bornholm in Denmark where more than 50% of the electricity consump- tion is supplied by renewable energy sources. In this project a real-time market concept is developed to give end users of electricity and owners of small-scale DG units new options (and potential economic benefits) for offering additional balanc- ing and ancillary services to the system operator. Of a total of 28,000 customers on Bornholm, approximately 2000 residential consumers will participate with flexible demand response to real-time price signals [6].
One should realise that, in a liberalised electricity sector, the aforementioned objectives for load management are in the interest of different parties. Maintaining the balance between demand and supply of electricity is primarily the responsibility of transmission system operators and commercial electricity suppliers. However, the