Energy storage is an essential part of any physical process, because without storage all events would occur simultaneously; it is an essential enabling technology in the management of energy. An electrical power system is an interconnected network designed for electrical energy generation and delivery from producers to consumers. It consists of generating plants that produce electrical energy, high-voltage transmission lines that carry energy from distant sources to demand centres and distribution lines that connect individual consumers to these centres. The Central Electricity Generation Board (CEGB) standardised the United Kingdom electricity supply and established the first synchronised AC grid, running at 132 kV and 50 Hz. Electric power grids of industrialised countries had become highly interconnected for economic and reliability reasons, with thousands of 'central' generation power stations delivering power to major load centres via high capacity power lines. Interconnection allows energy to be purchased from large, efficient sources. It also allows regions to have access to cheap bulk energy by receiving power from different sources. The relatively low utilisation of these peaking generators (generally gas turbines are used due to their relatively lower capital cost and faster start-up times), together with the necessary redundancy in the electricity grid, resulted in high costs to the electricity companies, which were passed on consumers in the form of increased tariffs.

The diversity of applications of electricity and particularly the fact that some of its uses, such as lighting and space heating, are subject to substantial seasonal variation makes the economic ideal of supply for constant throughout the year unrealistic. There should be an intermediate unit between producer and customer that can coordinate them. This intermediate unit therefore has to be able to separate partly or completely the processes of energy generation and in the power system. Secondary energy storage in a power system is any installation or method, usually subject to independent control, with the help of which it is possible to store energy, generated in the power system, keep it stored and use it in the power system when necessary.

A typical electricity bulk supply power system consists of central generating stations (supply side) connected to a power transmission system. This bulk supply system is connected to a distribution system comprising a sub -transmission system of primary distribution feeders and secondary circuits (demand side). Distributed energy sources might be connected either to distribution feeders or to secondary circuits.

Thermal energy storage (TES) is efficient due to the high specific melting heat of water. One metric ton of water, just one cubic metre, can store 334 MJ (317 k BTU, 93 kWh or 26.4 ton -h). No matter what the technology of ice production is (whether ice was produced by modern anhydrous ammonia chillers or hauled in by horse-drawn carts - ice was originally transported from mountains to cities for use as a coolant), a sufficiently small storage facility can hold enough ice to cool a large building for a day or a week. Originally, the defmition of a 'ton' of cooling capacity was the heat to melt 1 ton of ice every 24 h. This defmition has since been replaced by less archaic units: 1 ton HVAC capacity equals to 12 000 BTU/h.

Storing energy in the form of mechanical kinetic energy (for comparatively short periods of time) in flywheels has been known for centuries, and is now being considered again for a much wider field of utilisation, competing with electro chemical batteries. In inertial energy storage systems, energy is stored in the rotating mass of a fly wheel. In ancient potteries, a kick at the lower wheel of the rotating table was the energy input to maintain rotation. The rotating mass stored the short energy input so that rotation could be maintained at a fairly constant rate. Flywheels have been applied in steam and combustion engines for the same purpose since the time of their invention. The application of flywheels for longer storage times is much more recent and has been made possible by developments in materials science and bearing technology. The energy capacity of flywheels, with respect to their weight and cost, has to date been very low, and their utilisation was mainly linked to the unique possibility of being able to deliver very high power for very short periods (mainly for special machine tools).

Pumped hydro storage uses gravity to store energy generated by the power system raising water to a higher altitude, thus storing potential energy in an upper basin. This potential energy is then used to generate electricity when the water returns to its original level in a lower basin, passing through a turbine on the way down. Stored energy capacity can be increased either by using more water involved in the process or by increasing the 'head' - height difference between upper and lower basins.

Citywide compressed air energy systems have been built since 1870. Cities such as Paris, Birmingham, Offenbach, Dresden in Germany and Buenos Aires in Argentina installed such systems. Victor Popp constructed the first systems to power clocks by sending a pulse of air every minute to change the pointer. They quickly evolved to deliver power to homes and industry. As of 1896, the Paris system had 2.2 MW of generation distributed at 550 kPa in 50 km of air pipes for motors in light and heavy industry. Usage was measured in metres. The systems were the main source of house-delivered energy in these days and also powered the machines of dentists, seamstresses, printing facilities and bakeries. The application of elastic energy storage in the form of compressed air storage for feeding gas turbines has long been proposed for power utilities; a compressed air storage system with an underground air storage cavern was patented by Stal Laval in 1949. Since that time, only two commercial plants have been commissioned; Huntorf CAES, Germany, and Mcintosh CAES, Alabama, USA.The compressed air energy storage (CAES) concept involves a thermodynamic process in which the major energy flows are of work and heat, with virtually no energy stored in the compressed air itself. The performance of a CAES plant depends on the precise details of both the compression process and the expansion process.

Synthetic fuels are considered to be substitutes for natural gas or oil and are made from biomass, waste, coal or water. Production of these fuels demands energy, which can be obtained from base -load power plants during off-peak hours. Therefore, synthetic fuels are a type of energy storage since it is possible to use them instead of oil or gas for peak energy generation. The fuels themselves are only a type of medium (e.g. hydrogen is simply a method to store and transmit energy); as with any other storage concept, a power transformation system and central store are also required. Storage media have to be produced during off-peak hours in a chemical reactor or electrolyser - this has to be considered as a part of a power transformation system used during the charge regime. During the discharge regime, the storage media have to be converted into electrical energy, using any kind of thermal plant with an appropriate combustion chamber. As mentioned in Chapter 7, CAES is among the possible consumers of synthetic fuels. The storage media have to be stored in a special containing device - a central store. The use of synthetic fuels does impose some problems of safety and container material, but it is not very different from the infrastructure of storage and distribution systems involving natural gas and oil fuels.

The most traditional of all energy storage devices for power systems is electrochemical energy storage (EES), which can be classified into three categories: primary batteries, secondary batteries and fuel cells. The common feature of these devices is primarily that stored chemical energy is converted to electrical energy. The main attraction of the process is that its efficiency is not Carnot-limited, unlike thermal processes. Primary and secondary batteries utilise the chemical components built into them, whereas fuel cells have chemically bound energy supplied from the outside in the form of synthetic fuel (hydrogen, methanol or hydrazine). Unlike secondary batteries, primary batteries cannot be recharged when the built-in active chemicals have been used, and therefore strictly they cannot be considered as genuine energy storage. The term 'batteries', therefore, will only be applied for secondary batteries in this chapter.

Energy can also be stored in the form of an electrostatic field. Let us consider an electrical capacitor, that is, a device that can collect electric charge which is establishing an electric field and hence storing energy. The capacitance C of a capacitor is defmed by the amount of charge q it can take up and store per unit of voltage. The properties of this medium may be described by the constant k called the permittivity, which is measured in F/m since both A and d have metres as the basic unit. The electric field E is homogeneous inside the plate capacitor only when the distance between the plates is small. In order to keep the charges at the plates divided, and thereby to maintain the electrostatic fi eld, the dielectric medium must have a low electronic conductivity; so we are looking for dielectric materials with high permittivity.

A number of companies in the United Kingdom, the United States, Germany, France, Japan and Russia started SMES R&D work in the early 1970s. Since that time, many SMES projects have been proposed, but only some have been put into practice. The leading roles belong to the United States, Russia and Japan. As reported by the Soviet Academy of Sciences, the first Russian experimental SMES of 10⁴ J energy capacity and with a rated power of 0.3 MW was connected, through a six -pulse thyristor inverter, to the Moscow power system in the 1970s. This experimental SMES was constructed by the High Temperature Institute (IVTAN), which has subsequently been involved in a number of other SMES projects. Since 1989, this work has been done within the framework of the Russian State Scientific 'High Temperature Superconductivity' Programme. IVTAN's latest achievement is a 100 MJ 30 MW SMES installed in the Institute's experimental field, and is connected to the nearby 11/35 kV substation owned by Moscow Power Company. The electrical proximity of the 22 MW and 100 MW synchronous generators, as well as a specially designed load simulator, provides possibilities to conduct full-scale experiments on an SMES's influence on power system behaviour under normal and fault conditions.

The paper is chapter 12 of Energy storage for power systems, 3rd edition. It has two sections. Section 12.1 is about power system as a flyhwheel and Section 12.2 discusses the topic of interconnected supergrid.

Energy storage usage is being restrained by a lack of regulation to guide utilities and investors. There is no overall strategy of the incorporation of energy storage technique and methods into the power systems particularly in those with separation of transmission and generation. Energy storage benefits both generation and transmission companies, distributed energy storage benefits end users and this immediately leads to confusion concerning the question of energy storage control. This situation leaves utilities and investors uncertain as to how investments in energy storage should be treated and how costs could be recovered, so neither is willing to invest. The technology should be given more financial and institutional support - alike renewable energy technologies - in order to encourage the construction of storage plant. A slightly simplified rule states that storing heat saves energy while storing electricity saves capital investment. Since the energy industry on a global basis faces a resource crisis, both in respect of primary energy and capital investment, there are good reasons for investigating the storage of low -quality as well as high quality energy.

Energy storage is required because the demand side in a power utility is characterised by hourly, daily and seasonal variations, whereas the installed capacity of the supply side is fixed. To facilitate this varying demand at minimum cost and acceptable reliability, the utilities plan and operate their generation resources to match the load characteristics. During the decision-making process of planning, information regarding the effect of an energy storage unit on power system reliability and economics is required before it can be introduced as a decision variable in the power system model. The main objectives of introducing energy storage to a power utility are to improve the system load factor, achieve peak shaving, provide system reserve and effectively minimise the overall cost of energy production. Constraints of various systems must also be satisfied for both charge and discharge storage regimes. The impact of distributed energy storage integrated within the system has to be considered, including the effects of distributed units on system stability and spinning reserve requirements. The economics of storage devices have an influence on both the initial capital investment in the system and the operating and maintenance costs.

Interconnected grids are supposed to supply electric power at principally constant voltages. This has to be achieved using generation, distribution and transmission equipment that is not perfectly reliable with varying demand and variable reactive nonlinear loads. All generators must run at the same frequency and must be in phase with each other on the grid. A local governor regulates the driving torque of rotating generators, preserving constant speed while the load changes. Electrical energy is stored in the immediate short term by the rotational kinetic energy of the turbine -generators sets. Power balance across the entire grid has to be maintained because energy is consumed at the same time as it is generated. Droop speed control ensures that multiple parallel generators share load changes in proportion to their rating. As load increases, the frequency slows below the nominal frequency and local governors adjust their generators so that more electricity is generated. As load decreases grid frequency runs above the nominal frequency, and this is an indication for automated generation control (AGC) systems across the power grid that generators has to reduce their output. Small deviations from the nominal system frequency are very important in regulating individual generators and assessing the equilibrium of the grid as a whole. In addition, central control changes the parameters of the AGC systems over a minute timescale to adjust the regional grid power fl ow and operational frequency of the grid. For timekeeping purposes, to prevent line -operated clocks from gaining or losing significant time, the nominal frequency is allowed to vary to balance out transient deviations.

The situation when the total rated power and energy capacity of the energy storage are used in a compulsory regime is quite rare, since the storage unit parameters have been chosen under 'cover the winter peak demand and summer trough demand' conditions. It may be considered as an unplanned regime when all the reserve capacity is already used and there is still a shortage of generating power. An optimal regime arises when part of the rated power and energy capacity of the storage is not used for compulsory load coverage. Usage of the rest of installed capacity of the energy storage unit allows us to change the load on generating units in an optimal way, making it possible to minimise the fuel cost for the energy consumed. Unused or spare energy storage capacity may also be used as spinning reserve. Use of the reserve regime provides fuel saving whenever there is a need for spinning reserve.

Distributed energy generation (DEG) systems are small-scale power generation units usually in the range of 1-10 000 kW without any special siting requirements that might be connected to the grid at the demand side. The concept is not brand new. For example, there were plenty of the so-called local hydro power stations in the rural areas of the Soviet Union in the beginning of 1950. The renaissance of DEG systems coincided with small wind generators and photovoltaic solar cells introduction in electricity markets of some western countries. In the so-called free market economy, it was obvious to encourage DEG development. So, since the beginning of 2000 many small generators were allowed to sell electricity back to the grid for the same price they would pay to buy it. The major investors in small-scale DEG projects have been unregulated independent power producers that sell their generated electric power to utilities usually under state -guaranteed long-term contracts.

Renewable energy sources - in particular, wind, tidal and solar - will play a significant role in the supply side structure of future power systems. Their intermittent nature may be smoothed partly by reserve capacity of the power system, partly by fmding a special place for them in the load demand curve and partly by using secondary storage. Wind and tidal sources are expected to participate in base generation while solar sources are only suited to the intermediate zone of generation curve.

Electric motors typically have on-board efficiencies of around 80% at converting electrical energy into driving a vehicle. Electric motors do not consume energy while freewheeling or idling. Moreover, modern plug-in electric cars can recharge their on-board batteries using regenerative braking and reuse most of the energy normally lost during braking. Electric vehicle requires electricity to power its motor either directly or via a battery. Hybrid electric car generates the required energy by an on -board ICE mechanically connected to electric generator which feeds electricity to a motor and may charge an on -board battery. Plug in hybrid electric car is an example of distributed energy source with storage. So, electric vehicle might be an alternative to an ICE -driven one and it is not surprising that as of September 2018, there were over 4 million all -electric and plug-in hybrid cars in use all over the world.

By installing storage systems for operation with coal and nuclear base-load plants in the short term, the utilities will directly support the introduction of intermittent renewable power generation in the longer term. Storage is a `greening' technology, because it amplifies existing renewable energy units and allows optimisation of the whole system. The problem of choice of storage parameters appears to be the first to be solved. In order to define the requirements for storage units, power system analysis should be carried out on the following topics: Different types of energy storage means in operation at the design stage of the supply side of power utility expansion planning; and Operating experiences and criteria in electricity power systems with storage plants.