Smart Grid Telecommunications. Ramon Ferrús

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СКАЧАТЬ system operations and investment planning meet in system planning. System planning is part of the grid operations, and its consequence is the identification of the investment needed. System planning, as a process, intends to understand and forecast load location and needs to adapt the grid consequently. System planning considers the generic elements mentioned earlier and considers a short‐ and a long‐term scope. Long term requires a system model and considers load evolution and associated changes needed in the existing system (e.g., new power lines or substations, refurbishment of existing infrastructure: feeders, transmission capacity) along with relevant constraints (e.g., budget) to develop different prospective scenarios and adapt to them. Short‐term planning implies detailed analysis of the existing infrastructure, both Transmission and Distribution segments. Different topological and performance data feed studies to analyze voltage drops (to identify weak points in the grid), sectionalizing options (to minimize outages), conductor adequacy (based on power required), etc. It will ultimately affect the evolution of all system components.

Protection and control are electric power system functions that are transversal to all grid management areas. System operations depend on a combination of automated or semiautomated control and actions requiring direct human intervention. Operations are assisted by electromechanical grid elements and recently enhanced with the support of ICTs:

       Protection. This function ensures the safety of the system, its elements, and the people. Protection schemes must act in real time when there is a condition that might cause personal injuries or equipment damage. However, protection cannot avoid disturbances in the system. Faulty conditions (faults) are detected and located based not only on grid voltage and current measurements but also on some other parameters. A fundamental part of the operation of a power system is to quickly detect and clear faults, rapidly and selectively disconnecting faulty equipment and automatically reclosing for supply recovery in case of transient failures.

       Control. Power system operators manage their grids from Utility Control Centers (UCCs). Different UCCs exist, dealing with different grid domains (grid segment and/or territory). Most routine operations of a well‐designed system should not require any human intervention; however, a number of manual operations are needed. Power system data are constantly and automatically collected for the required analysis of system performance for planning and contingency analysis.

      Producing the power needed in the system is the task for the Generation segment in a traditional electric power system. In traditional monopolistic environments, vertically integrated utilities knew when, where, and how much electricity was going to be needed and scheduling of energy production was relatively easy. More recent market‐based decentralized approaches have increased the complexity of the task for the sake of the system efficiency. Short‐term markets (bidding mechanisms) and bilateral agreements between producers and consumers need to be coordinated. Eventually, energy producers offer their production capacity and get it awarded in, with a price assigned per MW to the different generation sources.

      Technical aspects of the different conventional generation sources are taken into consideration together with the economic mechanisms. Their different constraints (e.g., costs, speed of startup and shutdown, capacity factor, forecast of day availability) determine unit commitments [11]. These are assignments of a production rate and temporal slot some time in advance of the real need. To cope with unexpected contingencies, the reserves (i.e., the “back‐up” generation units) are planned as well.

      Frequency stability is a key aspect of power system operation. It starts in the Generation segment. Traditional production of electricity (hydro, fuel, nuclear) involves mechanical elements (e.g., water, steam, or gas flowing through a turbine) and has an effect on the rotational speed of the turbine that consequently determines the exact frequency of the electricity signal. If rotational speed is higher, frequency is higher too. Loads also affect system frequency: when load is heavy, the turbine will tend to rotate more slowly, and the output frequency will be lower. This effect needs to be compensated on the generator side (with the mechanical resources) to keep the frequency as close as possible to the 50 or 60 Hz nominal value.

      Voltage levels are the other key aspect that needs to be controlled. Loads in the system exhibit a reactive behavior and, if the consumption of reactive power is excessive, the generated output power will not be efficiently used. The grid (Transmission and Distribution segments) takes care of compensating loads to keep reactive consumption low and maximizes the real power flowing in the system. As a last resource, generators may need to act.

Transmission systems support the electricity transported in the power line. These systems need to be highly reliable, resilient, and able to dynamically adapt to physical limitations and tolerances of the cables (e.g., thermal) to minimize system losses. Control of the reactive part of the load is done with Volt‐Ampere Reactive (VAR) regulation elements (inductors, capacitors, and semiconductor switches) deployed in the grid; other losses are highly dependent on weather conditions (humidity and temperature). Geomagnetical‐induced currents must also be taken into account [12], as they might cause damage specially in long conductors (more common in North America than in Europe). Transmission is highly coordinated with Generation, as network capacity expansion must be coordinated with any new generation plant in the grid, and the hubs it must reach depend on where the electricity is needed.

      Distribution systems are the most significant in terms of territory coverage and have a fundamental role in supply availability and quality control. Distribution operations take care of the control and the voltage regulation (i.e., LV levels within limits in both the unloaded and full‐load conditions), power factor (i.e., the reactive part of the load), harmonics (degradation of the waveform frequency, more common due to the growing presence of solid‐state switching devices in the grid), and voltage unbalance among phases in multiphase systems. Literature [4] specifically identifies concepts such as supply outages (supply interruptions of different duration), voltage drops (dips in supply voltage), overvoltage (voltage increases caused by network events), voltage wave harmonics (deviations from the fundamental frequency), and flicker (low‐frequency fluctuations in voltage amplitude frequency).

      1.2.4 The Grid Assets

      Grid assets are the elements of the electric power system. This section will focus on the network part of it, i.e., Transmission and Distribution segments.

      All the network elements are important not only because they are the elements making the grid work but also because they are the elements to be controlled, the ones needing telecommunication services, and the ones that can be used to support the means to develop telecommunication networks (e.g., optical fiber cables on the power lines).

      1.2.4.1 Substations

      Substations are the grid nodes where voltage level is transformed. PSs transform voltage from HV to MV and SSs from MV to LV. Both PSs and SSs use transformers for this purpose, and as voltage is reduced, current is increased. Voltage transformation takes place in several stages and at several substations in sequence. These substations are connected through power lines.

      Other than the transformer as the main component of a substation, substations consist of a complex set of circuit breaking, and control equipment arranged to direct the flow of electric power. Substations perform several functions, namely, safety (isolating parts of the system in case an electric fault occurs), operation (minimization of energy losses; separation of parts of the network when maintenance or network upgrades are needed), and interconnection (if different electric networks with diverse voltages need to be connected).

PSs (Figure 1.5) are designed for several specific functions such as regulating voltage to compensate for system changes, switching transmission and distribution circuits, providing lightning protection, measuring power quality and other parameters, hosting communication, protection, СКАЧАТЬ