freely across the network, as it is physics (Kirchhoff's laws) that determines, depending on the impedances in the power lines and the rest of the grid elements, where electricity flows. Thus, the current distribution cannot easily be forced to take any given route, and alternative routes in the grid are highly interdependent.
From an operational perspective, deviations from normal operation may cause the instantaneous reconfiguration of power flows that may have substantial effects on facilities (e.g., substations, power lines, etc.) in the grid and propagate almost instantaneously across the entire system.
Finally, electric power consumption is sensitive to the technical properties of the electricity supply, to the extent that devices may malfunction or simply cease to operate unless the voltage wave is stable over time within certain parameters including shape (sinusoidal), frequency (cycles per second), and value (voltage). The system must have mechanisms to react (detect and respond) instantly to unexpected situations and avoid degradations in service quality.
1.2.1.1 Frequency and Voltage
The frequency of the electricity signal in the different world regions is either 50 or 60 Hz. The waveform adopted by Europe, Asia, Africa, many countries in South America, Australia, and New Zealand for their electricity systems is 50 Hz. North America, some parts of northern South America, Japan, and Taiwan, opted for a frequency of 60 Hz [4, 5].
In contrast, the voltage levels that can be seen in the different parts of the grid span a much larger range of options. A widely accepted, though loosely precise, definition of the voltage levels is:
Low Voltage (LV), defined as “a set of voltage levels used for the distribution of electricity and whose upper limit is generally accepted to be 1000 V for alternating current” (IEV 601‐01‐26 [6]).
High Voltage (HV), defined as either “the set of voltage levels in excess of low voltage” or “the set of upper voltage levels used in power systems for bulk transmission of electricity” (IEV 601‐01‐27 [6]).
Medium Voltage (MV), defined as “any set of voltage levels lying between low and high voltage” (IEV 601‐01‐28 [6]).
The International Electrotechnical Commission (IEC) has standardized three‐phase AC rms voltage levels internationally in IEC 60038:2009 within the following ranges:
Having a highest voltage for equipment exceeding 245 kV: 362 or 420 kV; 420 or 550 kV; 800 kV; 1100 or 1200 kV highest voltages.
Having a nominal voltage above 35 kV and not exceeding 230 kV: 66 (alternatively, 69) kV; 110 (alternatively, 115) kV or 132 (alternatively, 138) kV; 220 (alternatively, 230) kV nominal voltages.
Having a nominal voltage above 1 kV and not exceeding 35 kV: 11 (alternatively, 10) kV; 22 (alternatively, 20) kV; 33 (alternatively, 30) kV or 35 kV nominal voltages (there is a separate set of values specific for North American practice).
Having a nominal voltage between 100 and 1000 V inclusive: 230/400 V is standard for three‐phase, four‐wire systems (50 or 60 Hz) and also 120/208 V for 60 Hz. For three‐wire systems, 230 V between phases is standard for 50 Hz and 240 V for 60 Hz. For single‐phase three‐wire systems at 60 Hz, 120/240 V is standard. Practically, LV consumers within most 50 Hz regions will eventually be delivered 230 Vac, and 110 Vac in 60 Hz regions.
Thus, it can be said that while LV is clearly below 1 kV, the boundary between HV and MV is commonly placed at 35 kV.
1.2.2 The Grid
The “grid,” the power grid, the electric power system, or the electricity supply system, is defined by the IEC as “all installations and plant provided for the purpose of generating, transmitting and distributing electricity.”
The power grid is a hierarchical infrastructure comprising a large set of interconnected elements to provide electricity service to its end‐customers. The interconnected elements can be grouped in different building blocks, as shown in Figure 1.2. Traditional grids deliver the energy produced by the Generation systems to the Consumption Points, through Transmission and Distribution systems. Generation and consumption are matched in real time.
In a traditional conception of the power system, Generation is conceived as the big power plants where energy transformation into electricity happens. Transmission steps generated voltage levels up, to transport it over long distances with the minimum energy losses. Distribution drives electric energy to all the disperse locations where it is consumed. Finally, Consumption Points are the locations where energy is ultimately delivered.
In practical and intuitive terms, Generation is the block with the big thermal, nuclear, and hydro plants. Transmission grid transports electricity with the costly HV power lines acting as the highways of the energy. Distribution grid is the heterogeneous mix of pervasive electricity assets reaching everywhere (the assets in Transmission and Distribution can be simplified in two, substations and power lines). And Consumption Points conceptually gather the different electricity users and their loads, from commercial and industrial customers to residential ones. Last but not least, Distributed Generation (DG) and/or DER have started to play a relevant role in the power generation closer to end‐customers.
Figure 1.2 Building blocks of traditional electric power systems.
Thus, the power grid is usually understood as a very large network connecting power plants (large or small) to loads, by means of an electric grid that spans countries, with international interconnections. These are referred to as “full power systems,” autonomous in their operation.
1.2.2.1 The Grid from a Technical Perspective
Although the grid concept has converged over the past century toward a similar structure and configuration in different world regions and countries, there are however many differences in the details of the infrastructure deployed. These differences can even be found in regions within the same country and even within the same company, exhibiting different implementations of the same concepts depending on a variety of factors. This situation poses difficulties from the equipment standardization and evolution perspective, as it adds complexity to the long lifecycles of grid elements.
1.2.2.1.1 Generation
1.2.2.1.1.1 Traditional Power Generation
Power plants convert the potential energy of existing resources such as renewable energies (water, wind, solar, etc.) and fuel (coal, oil, natural gas, enriched uranium, etc.) into electric energy.
Traditional centralized power plants generate AC power from synchronous generators. These generators provide in fact three‐phase electric power; the voltage source is a combination of three AC voltage sources, i.e., three voltage phasors separated by phase angles of 120°. The frequency of the electricity waveform (i.e. 50 or 60 Hz) is a multiple of the rotation speed of the machine. Voltage is usually no more than 6–40 kV, being determined by the current in the rotating winding (i.e. the rotor) of the generator. The output is taken from the fixed winding (i.e. the stator).
More
