(e.g., wind turbines and mini hydro units) usually employ asynchronous generators, and the waveform of the generated voltage is not necessarily synchronized with the rotation of the generator. This may create flicker and reactive power quality problems if not properly designed and controlled.
The voltage in the power generation stage is stepped up by a transformer, normally to a much higher voltage. At that HV, the generator connects to the grid in a substation and electricity starts its journey toward the Consumption Points.
1.2.2.1.1.2 Distributed Generation/Distributed Energy Resources
DG refers to the “utilization of small (0 to 5 MW), modular power generation technologies dispersed throughout a utility's distribution system” [7], i.e., small‐size generation that connects into the Distribution part of the system, as opposed to conventional centralized power generation systems. DER (sometimes shortened as DR or Distributed Resources) is literally “a source of electric power that is not directly connected to a bulk power system [… and] includes both generators and energy storage technologies capable of exporting active power to an electric power system” [8].
Although DG, DER, and DR are modern acronyms, the concepts they represent stem from the past [9] and refer to a basic smaller scale generation closer to Consumption Points. The novelty nowadays is that DG/DER/DR intends to be connected to the conventional grid as well and has the potential to be widely adopted across it. This wide adoption is based on their capability to reduce system losses and improve power quality and reliability, thus deferring Transmission and Distribution grid improvements [7].
US EPA (United States Environmental Protection Agency) reinforces the idea and points out the environment factors that the concept, when fully developed, involves [10]: “DG refers to a variety of technologies that generate electricity at or near where it will be used, such as solar panels and combined heat and power. Distributed generation may serve a single structure, such as a home or business, or it may be part of a microgrid (a smaller grid that is also tied into the larger electricity delivery system), such as at a major industrial facility, a military base, or a large college campus. When connected to the electric utility's lower voltage distribution lines, distributed generation can help support delivery of clean, reliable power to additional customers and reduce electricity losses along transmission and distribution lines.” Under this definition, solar photovoltaic panels, small wind turbines, natural‐gas‐fired fuel cells, combined heat and power systems, biomass combustion or cofiring, municipal solid waste incineration, and even Electric Vehicles (EV) may be included.
The role of DG/DER/DR in the electric power system is increasingly relevant, and despite their existing challenges in operational and regulatory terms, “for the many benefits of DG to be realized by electric system planners and operators, electric utilities will have to use more of it” [9]. Moreover, the technical improvements in cost and efficiency terms will make small‐size generators come closer to the performance of large power plants.
1.2.2.1.2 Transmission
Power from Generation is connected to the Transmission part of the grid, with transmission lines that carry electric power at various HV levels. The Transmission grid is the backbone of the electric power system covering long distances to connect large and geographically scattered generation plants to demand hubs where Distribution system starts.
A Transmission system corresponds to a web‐like structure achieving the back‐up of every substation of the grid by all the others. It is a networked, meshed topology connecting generation plants and substations together into a grid that usually is defined at 100 kV or more. The electricity flows over HV transmission lines to a series of substations where the voltage is stepped down by transformers to levels appropriate for Distribution systems.
Transmission power lines are sturdy, durable, and efficient conductors, usually supported by towers. The design of the system needs to be based on mechanical (weight of the conductors, safety distances between conductors, tower and ground depending on the voltage, etc.) and electrical considerations. Transmission lines are typically deployed with three wires along with a ground wire. The conductors are attached to the towers that support them by an assembly of insulators. The towers may support several power lines in the same route, to optimize costs. The system includes sophisticated measurement, protection, and control equipment to prevent its malfunctioning in case of faults (e.g., short‐circuits, lightning, dispatch errors, or equipment failure).
Although not common due to higher costs, in congested areas within cities, underground cables are alternatively used for electric energy transmission. The technology to be used is more sophisticated and applies to the lower voltage ranges. These underground Transmission systems are preferable from the environment perspective.
Although most Transmission systems are AC, a mention needs to be made to DC systems. DC transmission systems require expensive converter stations to convert to the regular AC systems. They are used because they present some advantages (namely, no reactive power flows, higher transmission capacity, lower losses, and lower voltage drops for the same voltage and size of the conductors, controllability of the flow, no frequency dependence, reduced stability problems) over AC Transmission systems in applications such as long distances, submarine cables, and the interconnection systems with different security standards or system frequency.
1.2.2.1.3 Distribution
Distribution segment is widely recognized as the most challenging part of the grid due to its ubiquity. Distribution networks are more subject to failures than Transmission networks. They have HV, MV, and LV levels. Further than the formal definition of voltage levels, HV usually comprises 132 (110 in some places), 66, and 45 kV; MV 30, 20, 15, 10, 6.6 kV, etc.; and LV, levels below 1 kV.
The Distribution network1 consists of power lines connecting primary substations (PSs) and secondary substations (SSs), the former in charge of transforming voltage from HV to MV and the latter from MV to LV. The parts of the Distribution network with the higher complexity are the MV and LV grids. MV has concentrated the attention of grid infrastructure evolution in technical and technological terms in recent years; on the contrary, the LV has witnessed less evolution. Thus, LV grids present more complex and heterogeneous topologies than MV grids.
MV grid topologies (Figure 1.3) can be classified in three groups, although their operation is radial:
Radial topology. Radial lines are used to connect PSs with SSs, and the SSs among them. These MV lines (often named “feeders”) can be used exclusively for one SS or can reach several of them. Radial topologies show a tree‐shaped configuration when they grow in complexity.
Ring topology. A ring topology is an improved evolution of the radial topology, connecting SSs to other MV lines to create redundancy, and from there to a PS to close the ring. This topology is fault‐tolerant and overcomes the weakness of radial topology when one element of the MV line gets disconnected. The elements in the MV circuit need to be maneuvered to reconfigure the grid and connect SSs.
Networked topology. Networked topology consists of PSs and SSs connected through multiple MV lines to provide a variety of distribution alternatives. In the event of failure, many alternative solutions may be found to reroute electricity.
