Smart Grid and Enabling Technologies. Frede Blaabjerg

Figure 1.1 The fundamentals of electric power system. Adapted from Ref Num [5].

      1.2.2 Electric Power Transmission

The transmission system has the highest voltage rating and is used to transmit the energy from the bulk generation plant to the distribution networks through interconnecting substations. The transmission system may include overhead lines, underground and under water cables. This transmission system could be high voltage alternating current (HVAC) or high voltage direct current (HVDC). Traditionally, an HVAC system is mostly used, however, the HVDC is rapidly gaining popularity due to reduced losses and cost particularly over large distances. The electric power could also be transported and distributed via underground cables. Construction of an underground transmission system normally costs 4 to 10 times that of an equal distance overhead line [7]. The overall amount of power and distance that the power needs to be transported, overdetermine the essential design of the transmission and distribution systems. Hence, the greater the distance and the more power to be transferred, the higher the rated voltage is, and the higher the cost of the system will be as shown in Figure 1.2 [8]. This shows that distributed renewable energy generation allows for the reduction of the cost not only of the central power generation plants, but also of the transmission and distribution infrastructures. This is one of the drivers for the SG energy paradigm.

      1.2.3 Electric Power Distribution

The distribution network is the last stage in electric power delivery responsible for carrying electricity from the transmission and sub‐transmission systems to end users. There are four main arrangements used in electric power distribution: radial, parallel feeders, ring main, and interconnected (mesh) systems as shown in Figure 1.3 [9]. Distribution networks typically have a radial topology “star network,” with merely a single power flow path between the distribution substation and a certain load (unidirectional power flow). Distribution networks rarely implement a ring or loop topology, with two power flow paths between the distribution substation and the load [10]. Moving toward renewable energy systems and introducing distributed generators (DGs) enforces the use of bidirectional power flows which causes many challenges for traditional distribution systems. A transformation to the SG paradigm includes smart elements to allow for bidirectional power flow and ES technologies. ES systems may include pumped hydro, compressed air, electrochemical batteries, flow batteries, compressed air, superconducting magnetic ES, supercapacitors, and flywheels.

Figure 1.2 Selection of rated voltage for three‐phase AC transmission line. Ref [8]. Reproduced with permission from John Wiley & Sons.

Figure 1.3 Main types used in electric power distribution, (a) Redial feeder system. (b) Ring main feeder. (c) Meshed system. (d) Parallel feeder system. Adapted from Ref Num [9].

1.3 Limitations of the Traditional Power Grid

The first AC generating system was built more than 130 years ago as a centralized and unidirectional system [11]. The traditional power network consists of various elements such as conductors, transformers, switches, relays, etc. for safely delivering electricity to end consumers. The traditional power grid is a centralized control and management system that uses a supervisory control and data acquisition (SCADA) as shown in Figure 1.4.

      Developing real‐time control, supervision, and monitoring systems with a smart protection system is essential to optimize the production and consumption of electricity, to improve the overall efficiency, and to ensure the grid's reliability. The challenge is that several new generation sources must randomly connect and disconnect seamlessly with the distribution grid. Moreover, controlling a large number of different sources with different characteristics is of the utmost importance due to the possibility of conflicting requirements and limited communication resources [12]. Those are essential challenges for transforming the current grid into an SG. Other challenges before implementing the SG could be as follows:

      1.3.1 Lack of Circuit Capacity and Aging Assets

      In many locations worldwide, the power system has extended widely since the 1950s and the distribution and transmission equipment are now beyond the expected lifetime and require replacement. The capital costs of like‐for‐like change are very high. A typical large‐scale utility may require hundreds of billions of dollars to fully modernize the whole grid's infrastructure, which is an overrated cost and should decrease gradually over time. The practical solution is to transform this overstrained infrastructure into an SG, which may even take 20 years or more.

      1.3.2 Operation Constraints

Every power system should operate within predefined voltage and frequency limits. The capacity of traditional distribution circuits is restricted by power limits and the changes in voltage and frequency that exist between maximum and minimum loads. Different actions should be taken to vary the output of generations according to the demand, as well as to reconfigure and optimize the network to prevent system collapse. Increasing the output of generation as per demand at different points in the network is an acceptable solution in case the overloads are small and infrequent [13]. If constraints continue to exist, other solutions should be considered such as advancing the capacity of current power lines, including new lines, or producing separate electricity “highways” to avoid the affected parts, which is still of high cost. SG paradigm can achieve the energy management in a cheaper way by engaging end users in managing their consumption levels which protects the grid without the need to add new expensive infrastructure.

Figure 1.4 Traditional power grid.

      1.3.3 Self‐Healing Grid

      Nowadays, society needs a growing reliable electricity supply as crucial loads are increasingly connected. The conventional approach is to add more redundant systems, at a considerable environmental impact and capital cost. No action has been needed to maintain supply after a fault other than disconnecting the faulty circuit. The SG paradigm allows for distributed renewable energy generation and intelligent post‐fault reconfiguration and self‐healing for sustained electricity supply at significantly reduced cost [14]. Better utilization of assets can be achieved with SG at fewer redundant circuits, lower cost, and higher efficiency.

      1.3.4 Respond to National Initiatives

      Many agencies and policy makers are promoting SG initiatives as cost‐effective solutions for modernizing their power systems while activating the deployment of low‐carbon energy resources. Advancements of the SG can be noticed in a number of countries. Governments are aware that the SG principle is able to mitigate various