Monitoring and controlling the electrical distribution system for real time is becoming very important to improve its operating performance after transition to active distribution systems. So, many sensors are needed to monitor all parts in the systems. But if sensors are installed at all buses, investment cost becomes huge. To reduce the number of sensors, state estimation approach can be used to estimate the voltage of buses, which do not have sensors. State Estimation (SE) algorithms are broadly classified into Static State Estimator (SSE) and Dynamic State Estimator (DSE). This chapter classifies most algorithms used in active distribution networks, also State estimation in unbalanced distribution systems, and Role of PMU in Distribution System State Estimation.
TopIntroduction
Distribution systems are the link between the transmission system and the end-users. While electricity travels in transmission lines at a high voltage, end-users consume electricity at a low voltage, figure 1 illustrate typical transmission and distribution structure with the corresponding voltage levels.
Figure 1. Typical transmission and distribution structure with the corresponding voltage levels
Distribution networks have been affected by the changes in technology, changes in the values of society, changes in economy and in environment. Examples of changes are
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The liberalization of the energy market
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The increase of distributed generation DG, mainly renewable energy sources
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New Distributed Energy Resources (DER) such as Electric Vehicle
Thus, system security, operation safety, environmental protection, power quality, cost of supply and energy efficiency need to be examined in new ways in response to changing requirements in a liberalized market environment. Technologies should also demonstrate reliability, sustainability and cost effectiveness.
The evolution of electricity grids is referred to smart grid. Which can be defined According to the European Technology Platform of Smart Grids, as “an electricity network that can intelligently integrate the actions of all users connected to it – generators, consumers and those that assume both roles – in order to efficiently deliver sustainable, economic and secure electricity supplies”. A smart grid employs innovative products and services together with intelligent monitoring, control, communication and self-healing technology.
The appearance of smart grid has contributed of the transformation from the passive distribution network into a very complex and active network, where passive networks designed to accept bulk power from transmission system and distribute to customers, real time control problem resolved at planning stage, and the principle “fit and forget” will cause to have no control over DER and limiting capacity of DER to be absorbed by the existing networks (Hatziargyriou, 2014; Alhelou et al., 2019; Makdisie et al., 2018; Alhelou et al., 2018; Alhelou et al., 2016; Haes Alhelou et al., 2019; Njenda et al., 2018).
It is worth noting that power systems have always been smart, especially at the transmission network level. However, the distribution network is now testing a development that needs more intelligence for:
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Facilitate the integration of highly distributed generation based on renewable energy sources (RESs)
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Enable local energy demand management, interacting with end-users through smart measurement systems.
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Utilizing technologies already applied in transmission grids such as dynamic control techniques to offer high level of safety, quality and reliability of energy.
In summary, the transition from passive distribution networks into active one, meaning that control and decision-making have been distributed, and power flow has become bi-directional (Khan et al., 2019; Kifle et al. 2018; Khan et al., 2018; Khan et al., 2017; Anteneh et al., 2019; Molla et al., 2019, Molla et al., 2018, Jariso et al. 2018).
The main function of an active distribution network is to link the efficiency of power generation with consumer demands, allowing the best way to work in real time. The assessment of power flow, voltage control and protection requires cost-competitive technologies and new communications systems as well as information and communication technology (ICT) playing a key role (Hatziargyriou, 2014).