# A Review on Frequency Stability Enhancement and Effective Energy Storage through Various Optimization Techniques

Authors: Rashmi Mishra; Shimpy Ralhan; Md. Khaja Mohiddin
Journal: Mobile Information Systems
 (2022)

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## Abstract

In recent years, the applicability of these DG units to electrical Microgrids (MGs) has grown rapidly, enabling them to contribute a large percentage of the installed generating capacity. However, the fluctuating and intermittent nature of renewable generation can adversely affect electric grid stability and operations. Conventionally, to overcome these problems, batteries are employed. Nevertheless, the quick charging and discharging cycle reduces the battery life span, resulting in an economic burden and environmental damage. To resolve these problems, short-term Distributed Energy Storage (DES) systems based on advanced technologies, such as Superconducting Magnetic Energy Storage (SMES) and Supercapacitor Energy Storage (SCES), are emerging as potential alternatives. A supplementary regulator which includes storing energy as well as a flexible AC transmission system is designed to boost the minute signal reliability. The precise optimization of multiple energy device characteristics is needed for efficient performance. The artificial intelligence methods and Particle Swarm Optimization (PSO) are used to obtain the optimum parameters in the micro-hydro system. This research examines the many Energy Storage Systems (ESSs) in power systems, particularly microgrids, and demonstrates their critical role in improving the quality of electrical systems. As a result, the ESSs were divided into several technologies based on the energy storage form and the most important technological features. In this review paper, the most common classifications are introduced/presented, summarized, and compared according to their characteristics.


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## Body

## 1. Introduction

Energy is a valuable resource in today’s world. Renewable energy sources are increasingly being used to generate electricity in countries throughout the world. As a result, the Microgrid (MG) idea has been introduced. A microgrid device is a grid-connected or off-grid energy system that can operate separately or in collaboration with additional microgrids [1]. This involves generating electricity from a one source or a number of sources like solar and wind energy. Mainly, subplant systems sustaining use of wind power generation require the addition of an energy storage cell. For grid network, unreliability of wind power due to its extremely fluctuating nature can occasionally cause power system outages. For remote off-grid installations, this constraint becomes even more of a problem [2]. Furthermore, wind power generation must adhere to grid synchronization regulations. As a result, Wind Energy Conversion System (WECS) must be capable of sustaining swings in power and voltage while also regulating injected power. Also, one more typical challenge in a real system is stability, especially when disturbance occurs. Small changes in load, for example, low frequency fluctuation because the system will not be stable [26, 27].A frequency band of around 0.1–2 Hz is found in minimal oscillation and it focuses on either local or global problems in electromechanical mode [4, 5]. If the size of this oscillation is not properly damped, it may continue to develop till the system's synchronism has been dropped. Flexible AC Transmission System (FACTS) devices may assist in reducing oscillations. FACTS devices, however, cannot solve low-frequency oscillation problems by themselves because of the load's uncertainty [28]. As a result, energy storage deployment is becoming increasingly important [39]. Energy Storage Systems (ESSs) are important in WECS because they control the output of wind power plant and to provide the power system auxiliary services, allowing wind power to be used more efficiently. In today’s world, some examples for storage of energy are compressed air storing, hydroelectricity from pumped reservoir, lithium-ion battery storage, batteries with a redox flow, storage of capacitive energy, supercapacitor, and magnetic energy conservation in superconductors. By maintaining the similar energy density as traditional battery technologies, the battery, supercapacitor, and SMES HESS brings technical advantages of high-power density [3, 5]. For optimization, the PSO method is used in the power system. When it comes to determining the maximum density of energy storage devices in a microgrid system, PSO performs admirably. The researchers demonstrated that SMES and FACTS devices can improve small signal stability through minimizing the power system's oscillatory condition. These studies also revealed that PSO can solve optimization problems quickly, with simple models and accurate results [29].As a result, for small signal stability enhancement, this research idea encompasses different devices, namely, FACTS devices and energy storage (SMES, battery, and supercapacitor) [6, 7]. The FACT device improves transmission line tiny signal stability, while energy storage contributes damping by instantly supplying power back to the grid [40]. Also, HESS is concerned with determining the impact of WECS power variations on the duration of the battery throughout a charging cycle [30]. Moreover, this research contributes to how FACTS devices (TCSC) and energy storage (SMES) can work together to alleviate oscillatory conditions on the power system induced by load fluctuations utilizing one of the intelligent strategies known as PSO [9, 10]. A new design framework for system frequency regulation using HESS (coupled battery and SC) is presented as shown in Figure 1 [48]. One of the most important issues in HESS applications is to determine the appropriate storage capacity. Various methods have been proposed for storage capacity sizing. Some methods are developed to determine the HESS capacity of a particular technology, and some others, regardless of technology, can be used for sizing all types of storage. Authors in [49] reviewed battery sizing methods and their applications in various RESs. In the HESS sizing procedure, total cost and the reliability of the system should be considered [50]. The HESS sizing methods based on the purpose of HESS application may be different. Storage capacity sizing techniques can be classified into Analytical Methods (AM), Statistical Methods (SM), Search-Based Methods (SBM), Pinch Analysis Method (PAM), and Ragone Plot Method (RPM).Figure 1
Energy storage technology.The remaining of the article is as following: Segment 2 explains the suggested WECS scheme is characterized. Section3 provides a brief overview of SMES. Section 4 describes about the supercapacitor. Section 5 explains battery’s charging and discharging properties. Concepts of PSO are explained in Section 6. Section 7 presents the conclusion.


## 2. Wind Energy Conversion System (WECS)

Mechanical and electrical elements make up the WECS. The wind turbine and gearbox are mechanical components. The generator, control, and other related components make up the electrical part [13, 14]. Because it converts mechanical energy into electrical energy, the generator is an essential component of a wind turbine [31].In wind turbines, various types of electrical generators are commonly employed. Depending on the configuration, asynchronous and synchronous generators are commonly employed in WECS. A permanent magnet synchronous generator otherwise a doubly fed induction generator is used in the majority of WECS [3, 17]. To charge the battery energy storage, a rectifier is widely employed, and also a PM generator is often used, since it does not necessitate the use of a gearbox. Wind energy sources that are quite far away from populated regions have the capability of small-scale power output, for require storing of energy as shown in Figure 2 [32, 33]. Installing energy storage technologies into WECS transforms such power sources into MG’s [41]. Such types of sources are attached to energy storage devices either directly or through a cascaded mix of rectifier or DC-DC converters [21, 25]. Because the wind turbine induces the output voltage which is modified by the 3-phase rectifier and rectifier's output is erratic DC voltage, to modulate Voltage level, a DC-DC converter is employed as shown in Figure 1 [34].Figure 2
Structure of wind energy.


## 3. Superconducting Magnetic Energy Storage (SMES)

An apparatus that stores as well as discharges lot of energy at the same time called SMES. It saves energy by storing it inside a magnetic flux which is established on flowing DC current through the coils of superconductor as well as cooling it with a cryogenic [35, 36]. For a few years, the SMES system has been employed to get better quality of power and to supply fine voltage management when voltage fluctuation occurs as shown in Figure 3 [41]. It takes only a few minutes to recharge, and the charge and discharge phases can be repeated thousands of times without degrading the magnet's strength. Depending on the system's capacity, the recharging time can be shortened to satisfy specified parameters [15, 23].Figure 3
Block diagram of SMES.The SMES system is a DC current storage device that uses a strong magnetic field to store energy [1]. Superconductors are used to create the magnetic field. Because regular conductors or normal coils have minor amounts of resistance, where the major aim is to remove the resistive property from the normal conductor and create a superconductor [37]. A coolant is used in the elimination process [2]. Because the coolant lowers the conductor's temperature, the resistance of the conductors drops to zero at a specific low temperature, and the conductors behave like superconductors [1, 3]. Cryogenic temperature is the name given to the temperature. A cryostat, refrigerator, or Dewar containing helium or liquid nitrogen gas maintains that temperature. DC power is required to charge the superconductors. A converter is used to provide the DC power. To supply electricity to an AC load in the form of AC, an inverter is used [8]. The SMES can help to improve the power system's overall reliability. It is useful in the power system for load levelling, dynamic voltage support, and dynamic stability; FACTS improves power quality, increases transmission line capacity, and provides frequency support during power outages [11].


## 4. Supercapacitor

A supercapacitor, like a regular capacitor, contains two electrodes that are separated. The electrodes are comprised of a metal that has been covered with a porous substance, such as powdered, activated charcoal, to provide them a larger surface area for holding a greater amount of charge. Consider electricity as water: where a regular capacitor likes a cloth that can only mop up a tiny spill, a superporous capacitor’s plates transform it into a thick sponge that can absorb many times more. Supercapacitor plates with a porous surface act as electrical absorbers as shown in Figure4.Figure 4
Principle of a supercapacitor.The energy is supplied by a supercapacitor like in the type of a static electric field. The supercapacitor's energy storage (E) is proportional to the square of the voltage applied. The following equation quantifies this relationship:(1)E=12CV2.The energy storage method in a supercapacitor differs from that of a battery. A supercapacitor is ideal in order to generate energy to apply in which a high current cycle of charging/discharging occurs often and for short periods of time [12]. Batteries, on the other hand, are better suited to systems with rare charging/discharging cycles and require longer charge/discharge than supercapacitors [38]. Combining the two to create a composite form fits the power and energy requirements of wind energy conversion system while also reduces battery stress, resulting in an increased battery life. The following equation governs the discharge process of a supercapacitor.(2)Vt=VC0e−t/RPC.In this,VC(0) refers to the initial voltage of the supercapacitor; RP and C refer to the parallel resistance and capacitance, respectively.The proportion of useable power (P) to the entire power (P) is the super efficiency. Capacitor’s entire power consists of both useable and inherent lost electricity Pw. The below formula indicates the supercapacitor’s efficiency.(3)η=PP+PW×100%.


## 5. Battery

### 5.1. Charging/Discharging Properties of a Battery

The charging and discharging habits of a battery have a substantial influence on its existence. Electrode, a liquid, and terminals make up a lead-acid cell. A grid and energetic material are used to make the terminals. The grid supports the active medium physically and dissipates the current relevant to charging and discharging. In a battery, an organic process takes place, converting chemical energy to electrical energy. The electrolyte substance is soaked into the electrodes, allowing leading electricity; ion exchange is being used [19]. The discharge process in lead-acid batteries converts electrodes to crystals of lead sulphate, whereas the sulfuric acid (electrolyte) is transformed to water as shown in Table 1. The charging technique of a battery is an important ingredient which influences the battery’s lifespan, as it determines the charging duration, temperature, and charging status. To extend the battery's life, the charging system should be able to safeguard it from under- and overcharging.Table 1
Comparison of energy storage technologies.
Characteristics/SpecificationsStorage technologies for high power stationLiFly-wheelSCSMESMass of energy (Wh/kg)71–20010–5105–6.01–11Density of energy (kWh/m3)200–60521–1005.0–10.00.4–2.5Density of power (W/kg)151–500510–40001000–10000500–2000Density of power (MW/m3)0.4–2.01.0–2.50.4–10.01.0–4.0Efficiency%90–9790–9590–9795–98Discharging time2 to 8 hr (approx)10second- 15 minute1Milli sec -1.5 hrUpto 35 minWorking temperature (⁰C)−30–70−45–45−45–70–55–65Rate of emission (%/day)0.1–0.325–9525–4514–18Life time (years)7–1414–2111–3525–35Life period (revolution)1200 to 1200030000 to 22000+11000 to 51000+110000+


## 5.1. Charging/Discharging Properties of a Battery

The charging and discharging habits of a battery have a substantial influence on its existence. Electrode, a liquid, and terminals make up a lead-acid cell. A grid and energetic material are used to make the terminals. The grid supports the active medium physically and dissipates the current relevant to charging and discharging. In a battery, an organic process takes place, converting chemical energy to electrical energy. The electrolyte substance is soaked into the electrodes, allowing leading electricity; ion exchange is being used [19]. The discharge process in lead-acid batteries converts electrodes to crystals of lead sulphate, whereas the sulfuric acid (electrolyte) is transformed to water as shown in Table 1. The charging technique of a battery is an important ingredient which influences the battery’s lifespan, as it determines the charging duration, temperature, and charging status. To extend the battery's life, the charging system should be able to safeguard it from under- and overcharging.Table 1
Comparison of energy storage technologies.
Characteristics/SpecificationsStorage technologies for high power stationLiFly-wheelSCSMESMass of energy (Wh/kg)71–20010–5105–6.01–11Density of energy (kWh/m3)200–60521–1005.0–10.00.4–2.5Density of power (W/kg)151–500510–40001000–10000500–2000Density of power (MW/m3)0.4–2.01.0–2.50.4–10.01.0–4.0Efficiency%90–9790–9590–9795–98Discharging time2 to 8 hr (approx)10second- 15 minute1Milli sec -1.5 hrUpto 35 minWorking temperature (⁰C)−30–70−45–45−45–70–55–65Rate of emission (%/day)0.1–0.325–9525–4514–18Life time (years)7–1414–2111–3525–35Life period (revolution)1200 to 1200030000 to 22000+11000 to 51000+110000+


## 6. Flexible AC Transmission Systems (FACTS)

FACTS are becoming standard in transmission lines. It controls the power flow by providing parameter correction in a transmission line. This device can boost the system's security by controlling the impedance of line, phase angle, and magnitude of the voltage at the channel's end [14]. These FACT devices for transmission lines can be used for both series and shunt compensation. In series compensation, the line impedance is changed, resulting in lower net impedance and a higher active power is to be transmitted. A device called shunt compensation involves injecting reactive current into the line to adjust the voltage at the connecting end. Expansion of the power transmission network poses a number of challenges. As a result, there is a growing interest in maximizing the capacity of current power networks by incorporating advance technologies like Flexible AC Transmission Systems (FACTS). This will be minimized power flow in dense lines that have been loaded; as a result of this, load capacity is increased, minimal system failure, improved network reliability, and lower manufacturing costs [42].There are two main causes for the rising interest in these devices. First, recent trends in high-power electronics have reduced the cost of these devices; second, increased power system loading, in conjunction with competition the industry of power, encourages the utilization of regulating power as a low-cost method of routing certain energy activities [22, 52]. By utilizing reliable and high-speed power electronic controllers for greater utility efficiency, the technique provides five options that are as follows:(i)
Enhanced power control to ensure that power flows along the designated transmission pathways.(ii)
Ensuring that transmission lines are loaded to levels that are closer to their maximum temperature.(iii)
Increased capacity to shift across restricted zones,(iv)
The avoidance of a chain reaction of breakdowns,(v)
Distortion within the power system is suppressed.


## 7. Particle Swarm Optimization (PSO)

PSO is an approach for enhancing transformative algorithms. The system initiates with a count of arbitrary solutions. A particle is the name given to each conceivable solution. As it travels via the problem space, an individual particle has a different velocity [16, 20]. Memory is in-built within the particles, and each one remembers its prior best position (known as Pbest) and the fitness associated with it. Each particle in the swarm has many Pbests, with the swarm’s global best being the particle with the highest fitness (Gbest) as shown in Figure 5.Figure 5
Flowchart of PSO.The key stages in the PSO and selection criterions are as follows:(i)
In the dimensions of the problem space, fly a population of particles with arbitrary locations and speed.(ii)
Compute the swarm’s fitness for each particle.(iii)
For each loop, match individual particle’s fitness to its preceding best fitness (Pbest). In case, the present value is more effective than Pbest; set Pbest to the present value and Pbest area to the present area in the proportional space.(iv)
Particle Pbest values can be compared, and the swarm’s global best position including best fitness is modified Gbest.(v)
Alter the particle's location and speed.(vi)
At this point, redo steps (i) through (v) up-till convergence is achieved relevant to a solo or many criterions.Velocity of particle is updated as shown below:Area is updated as follows:(4)Xi+1=Xi+Vi+1,where “ω” is the inertia weight and C1 and C2 are the learning factors.


## 8. Cuckoo Search Algorithm (CSA)

The CSA is a newly developed metaheuristic optimization technique for solving optimization problems. Levy flights random walks and brood parasitism in various cuckoo species are used in this nature-inspired metaheuristic algorithm [18, 19]. To boost the chances of their eggs hatching, cuckoos of certain varieties hurl eggs from the actual parent at the nest. It could lead to a quarrel between the two. Both the swarm and the cuckoo birds are present when it deposits its egg. As a result, the bird swarms either discard its egg, else abandon its nest and thereafter tossing out the new one [35, 51]. Parasites of additional organism’s cuckoo eggs hatch when the cuckoo hatches. Usually hatching before the eggs of the host bird, the cuckoo's unhatched eggs were released a place for kids to receive extra meals as shown in Figure 6 [24, 53].Figure 6
Cuckoo search algorithm process.Three idealized rules underpin the CS algorithm:(i)
Individual cuckoo puts each egg eventually which is placed in a nest chosen at arbitrary.(ii)
The best nests with finest eggs (solutions) would be gone through further generations.(iii)
The quantity of accessible swarm nests is permanent, and a swarm has a chance of pa€ [0, 1] of determining an alien egg. In this situation, the swarm bird must choose between rejecting the egg or else departing the nest and starting over elsewhere.


## 9. Conclusion

This research presents a comparison of energy storage optimization techniques for effective low frequency stability in freestanding microgrids as shown in Table2. In this work, various types of energy storage techniques like SMES, SCES, Battery, Fly-Wheel, WECS have been compared and proved that the hybrid model of battery-SCES-SMES will be performing better in accordance with the optimization techniques such as PSO and CSA. In this paper, a summary of energy storage is presented, along with a preliminary guide to choosing the suitable technology. The study in [43] also elaborates the functionality of supercapacitor, superconductors, and battery device energy [44] storage devices [45]. Furthermore, the study provides a way to integrate different [46] systems into a single [47] microgrid through the optimization of different devices using FACTS and PSO and techniques. Recently, ESS is seen as the key enabling technology for the integration of RES into the existing grid as it can provide instantaneous power.Table 2
Comparison of energy storage techniques with respect to merits and demerits.
Energy storageMeritsDemeritsSuperconducting magnetic energy storage(i) Its ability to absorbing as well as delivering a significant quantity of power.(ii) It has an excellent performance (more than 90%).(iii) It has a very rapid response (less than 100 ms).(iv) Maintenance of SMES is very less.(v) Its structure is fully stable and reliable.(vi) Any capacity can also be maintained by SMES(i) To sustain the coils at a low temperature, a lot of electricity is required.(ii) There is an issue of mechanical characteristics.(iii) The initial installation takes a lot of money.Supercapacitor(i) Amongst different energy storage systems, it has the maximum impulsive power.(ii) It stores energy that can be controlled.(iii) Exploitation is simple.(iv) Its ability to have high charging capacity.(v) This device has a longer span of time(i) It has a decreased density of energy among various energy storage devices.(ii) The clamps of a supercapacitor have a changing voltage value.Battery(i) It has minimal, easy-to-manufacture, and has less cost per KWh.(ii) Self-discharge rate of batteries is less.(iii) Specific power of battery is high and also it has the ability to carry the discharge current.(iv) It has an excellent performance at both cold and hot temperatures(i) The process of charging is slow and tries to fully charge in approximately 14–16 hours.(ii) In battery to avoid sulfation, it must be kept in a charged state.(iii) charge and discharge rates of battery are less, it affects the battery life.(iv) It is not eco-friendly.



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