A brief review on hardware structure of AC electric spring

ABSTRACT


INTRODUCTION
In modern electric grid, the integration of power into grid is obtained by using renewable energy sources, electric and hybrid vehicles, different types of linear and non-linear devices and power electronics converters. This integration provides a path for up-gradation of the existing electric grid without making any drastic changes and makes it a smart grid. A smart grid should fulfil the following requirements: good power quality, advanced control, real time control, optimal operation, communication, and advanced protection [1].
spring works similarly as a flexible AC device or an active filter, but it uses the input voltage control instead of the output voltage control. It controls the balance between the generation and the load demand. This characteristic of the electric spring makes it a suitable candidate for further research in the electric grid to have more utilization of renewable energy sources.
In technical literature of the electric spring, several research lines are highlighted regarding: its hardware structure [6], control strategies [7], their steady state [8] and dynamic characteristics [9]- [11], optimization [12]- [17] and application in electrical distribution network [18]. Due to its new features, different method of controlling voltage, variety of applications, it is required to review the electric spring ES thoroughly to recognize the potential application in electric grid with the intermittent renewable energy source [19]. The available literature of the ES helps to understand, how it evolved by applying simple analogy of the mechanical spring in [20]. The objective of this paper is to review all the available ES topologies in the AC system to solve the power quality issues which arises due to the penetration of renewable energy sources. Robert Hooke in 1660 discovered the law of elasticity and proposed a concept of mechanical spring which exhibits elastic behavior; mathematically it is expressed as (1) [20].

=
(1) Where: F is the applied Force; X is the displacement in length; K is constant whose value depends on shape and dimension of elastic material. Potential energy associated with mechanical spring can be mathematically expressed as (2): Where: P.E. is the potential energy.
The Mechanical spring provides the mechanical strength, reduces the mechanical oscillations, and stores the mechanical energy. The concept of the Electric spring is based on the Hooke's law which was discovered in [5] with its practical realization. An analogy of the mechanical spring and the electric spring, under three different conditions such as: voltage boosting, neutral and the voltage suppression are elaborated in the Figure 1.
The Electric spring is analogous to a mechanical spring. It provides voltage support, reduces electric oscillation, and stores electrical energy. The analogous equation to (1) can explain the function the electric spring which can be expressed as (3)-(5): Where: q is the stored charge in capacitor having capacitance C; Va is the voltage across capacitor; i is the current flowing through capacitor having capacitance C.
In (3) and (4) indicates, the voltage enhancing and the voltage suppression function of the electric spring are controlled by the stored charge, and the (5) indicates that the charge can be controlled by utilizing the controlled current source. Thus, the electric spring is addressed as a current-controlled voltage source. -Problem statement Electrical energy generation using non-renewable energy sources causes the emission of greenhouse gases such as carbon dioxide, sulfur dioxide, and mercury. These emissions are responsible for the coastal flood, heat waves, acid rain, soil contamination, and so on. The cost to generate power from non-renewable sources is also increasing and the dwindling availability of such sources is causing economic crises and conflicts. Hence, it is needed to gradually increase power generation by using abundant renewable energy sources such as solar, and wind. The use of RES decreases electricity costs for all types of electricity consumers, increases job opportunities in the power generation sector, and is healthier for the general environment. Despite all these advantages, renewable energy sources are intermittent which causes issues like power quality deterioration, fluctuations in voltage, and power factor reduction. To overcome these problems a power electronics-based instantaneous power balancing device was invented by Dr. Ron Hui named an Electric spring which uses input voltage control and reduces issues arising due to the use of RES. -Objectives i) To encourage the use of renewable energy sources which are: better for the environment, better economic development of economies dependent on imported power, and healthier flora and fauna. ii) To discourage the use of fossil fuels that causes pollution in the environment and protection of fragile ecologies. iii) To settle all problems caused due to nonrenewable energy sources by electric spring in connection with the non-sensitive electric load

PROPOSED SOLUTION
The encouragement for the use of renewable energy sources is to reduce their intermittency. Hence the concept of an electric spring is introduced [5] in the power distribution network to reduce voltage fluctuations caused by renewable energy sources. The Figure 2 shows the structure of electric spring for describing its working principle. Electric spring has arisen in [5] as a unique demand side management technique which uses input voltage control. To regulate voltage of sensitive load, ES injects voltage perpendicular to current flowing through the non-sensitive load. While doing this non-critical load has to sacrifice power stability. To overcome this the concept of the switchable smart load was introduced [21] using indirect voltage control. The electric spring does not require, a communication network but with use of a communication network with ES can drastically reduce the power instability [22]. In microgrids for unplanned islanding operations, ES stabilizes the voltage across critical loads [23]. The ES uses energy storage devices such as capacitors, batteries, and PV systems. For different energy storage devices, the compensation range of ES is also different. The use of energy management system [19] and use of controller such as PID, Fuzzy, and ANN with ES is used to reduce voltage fluctuation [24].

ELECTRIC SPRING TOPOLOGIES
Electric spring is one of the advance reactive power compensator in electrical power distribution network having presence of the intermittent renewable energy sources. Topologies of ES are classified, on the basis of: type of energy storage devices used, type of supply (single phase/three phase), type of power converter used, type of converter topologies (VSI/CSI/ZSI) used and utilization of distributed generation [25].

Hardware structure of single-phase electric spring (SPES)
Electric spring has emerged as a facility provider to resolve the problems happening due to the intermittent nature of the renewable energy sources. The type of power compensation depends on hardware structures of the electric spring. In a power grid, ES is used with a capacitor; it provides reactive power compensation only, ES used with battery provides both the active and reactive power compensation and ES used with back-to-back converter strictly regulates the grid voltage. The ability of ES to compensate both active and reactive power makes it unique.
The Figure 3 shows the schematic for the SPES. From this diagram it can be observed that ES is connected in series with the non-critical load in the presence of renewable energy sources. A low pass filter used with power converter which forms the structure of the ES. This filter reduces the harmonics [26] as well as keeps the fundamental frequency of output voltage of ES. This filter may be L, LC or LCL type depends upon application of ES. Non-critical load can be: resistive, capacitive, inductive or customer loads like water heater [26], air conditioning unit [27], and thermal storage [28]. It can tolerate the voltage fluctuation. The critical load is a load which cannot tolerate voltage fluctuations for example: medical instrument and other sensitive equipments. The controller is the heart of the ES; it provides the pulses for the operation of ES and it defines the operating mode of the ES depending upon the version of the ES. According to the research studies, four versions of ES are available i.e., SPES with capacitor, battery, back-to-back converter, photovoltaic system, combination of battery and capacitor for single phase ac system. For three phase AC systems, there are two versions of ES available i.e., TPES with capacitor and TPES with battery. The different topology can be applied for ES by using different types of power converter, different types of low pass filter, controller and control strategies according to its configuration. The working principle of ES remains same in every combination among these.

Single phase electric spring with capacitor (SPES-C)
Single phase electric spring with capacitor is a special type of reactive power compensator first proposed in [5]. It uses PWM inverter in series with non-critical load and capacitor as energy storage device. It is best suitable for reactive power compensation. Different topologies of SPES-C are proposed based on protection for switching device used in power converter, types of inverters, type of application and type of material used for capacitor.
SPES-C with undeland snubber circuit is proposed in [29] or providing current protection and reducing switching losses of power converter. The Figure 4(a) SPES-C uses half bridge inverter and has two switches and two capacitors for power supply. The SPES-C using full bridge inverter is shown in Figure 4(b). It uses four switches & four diodes and it gets energy from the capacitor. It uses PWM commutation techniques to produce two or three voltage levels. Modified five level packed U-cell topology of SPES-C is shown in Figure 4(c) [30] to improve the THD of the ES output without using any additional controller. Modified seven level packed U cells is shown in Figure 4(d) [31]. This topology of SPES-C is an advancement of modified five level packed U-cell topology of SPES-C which improves the power density of electric spring by applying finite control set model predictive current (MPC) control. It has been observed that by replacing the electrolytic capacitor of the SPES-C with DC link film capacitor of less capacitance value [32], range of DC link voltage can be increase and provides a reliable operation. The SPES-C is used with the fuel cell [33]to reduce the load transients, it provides voltage regulation and increases efficiency of system. Five level neutral point clamped based SPES-C is shown in Figure 4(e) [34]. It is used with the IPD modulation to reduce the THD to 1.84%. Figure 4(f) shows the diode clamped MLI based SPES-B [35]. It uses the carrier-based modulation technique of SPWM to reduce THD to 1.27%. The Table 1shows summary of the single-phase ES with the capacitor considering parameters like: modification in hardware structure features and hardware implementation.  The Figure 5 shows the construction and the control structure of the SPES-C with renewable energy source 'Ir'. It consists of four switches S1, S2, S3 and S4 for full bridge inverter, one capacitor as energy source and a low pass filter as shown in Figure 5(a). There are two operating modes of SPES-C: inductive and capacitive shown in the Figures 5(b) and 5(c) SPES-C operates only in these modes, hence providing reactive power compensation. The Figure 5(b) shows the ES voltage leads the current through NCL by 90° in inductive mode. Figure 5(c) shows the ES voltage lags the current through NCL by 90° in capacitive mode. Voltage across NCL is decreases in inductive mode and increases in capacitive mode. It is observed that for the reactive power compensation, the ES voltage must be perpendicular to the current flowing through the NCL.

Single phase electric spring with battery (SPES-B)
The single-phase electric spring with battery has the same structure as SPES-C but it uses battery as energy source instead of capacitor. This version of ES is illustrated in number of research articles. It is experimented in [36], [37] with half bridge and full bridge inverter. SPES-B provides both the active and the reactive power compensation. It is configured with: different MLIs, split storage capacitor, multiport transformer and without non-critical load.
The Figure 6(a) shows SPES-B with three level cascaded H bridge inverter which consist of four switches S1, S2, S3 and S4 with Filter parameter Lf and Cf. ,it reduces the THD with 1.64% [38]. In MLI, the number of levels and the THD are inversely proportional to each other. Increasing levels demands more complex control. Hence the seven level MLI with reduced switch count is proposed in [39] and shown in Figure 6(b) .It reduces THD to 0.44% in main voltage. The packed E cell based SPES-B is shown in Figure 6(c), it uses the artificial neural network (ANN) based controller with features like: reduced number of components, more switching states and spectacular dynamic performance during transients [40]. The Figure 6(d) shows the SPES-B without the non-critical load. It is also called the fractional order ES which is integrated with the resonant filter. The key feature of this ES is that its operating range does not depend on the NCL in improving the power factor but it shows poor performance in the voltage regulation of critical load during high power applications [41].
Different topologies of the SPES-B are available based on number of ports used for the single-phase transformer. The SPES-B with the two port transformer is shown in the Figure 6(e) [42]. It uses the refined delta control to change the voltage of the non-critical load in the direction of the line voltage. The Figure 6(f) and the Figure 6 The Figure 7 shows the construction and the control structure of the SPES-B. The use of battery makes it capable to provide: real and reactive power compensation, power factor improvement and voltage regulation. SPES-B operates in eight operating modes, namely: capacitive, inductive, resistive, negative resistive, inductive plus resistive, capacitive plus resistive, inductive plus negative resistive and capacitive plus negative resistive. To understand the working principle of SPES-B it is assumed that non-critical load is purely resistive and supply voltage is constant for all operating modes.  Table 2 shows summary of single-phase ES with the battery considering parameters like: modification in hardware structure features and hardware implementation.

Single phase electric spring with back-to-back converter (SPES-B2B)
The Figure 9 shows the construction and the control structure of single phase electric spring with back to back converter [46]. It consists of two converters; shunt ES and series ES. The shunt ES is connected to grid via Lsh1, Lsh2 and Csh1 and the series ES is connected to grid via Lse1 and Cse1 via transformer. This version of ES does not use the non-critical load in series connection as used in SPES-C and SPES-B. The controller of SPES-B2B uses reference DC voltage and supply voltage as the input signals.
The Figure 10 shows the operating modes of the SPES-B2B. To understand the operating mode, it is assumed that noncritical load is resistive. The Figure 10  The Figure 11 shows the SPES-B2B without the use of transformer which provides: independent control for DC link voltage, active and reactive power [48]. The Figure 12 shows the SPES-B2B with the additional converter in [49] to help in the active power compensation provided by the series converter without need of the energy storage device. Its structure resemble the structure of UPQC except one converter of it, is connected in series with the non-critical load and another converter is connected parallel to supply for maintaining DC link voltage [19]. The Table 3 shows summary of single-phase ES with back-to-back converter considering parameters like: modification in hardware structure features and hardware implementation.

Photovoltaic single phase electric spring (PV-SPES)
The photovoltaic single phase electric spring is another version of the electric spring which has same structure as that of the SPES-B only difference is that the photovoltaic system is added on the DC bus as shown in the Figure 13. The operation of PV-SPES is based on the radial chordal method proposed in [50] shows that the power consumed by smart load is not depend on the variations in the photovoltaic power. The Figure 14 shows the phasor diagram for the operation of the PV-SPES with the radial component of ES voltage as Vesr and the chordal component as Vesc. The current through the smart load and the non-critical load is same. The Vesr is parallel to the Vnc and Vesc is perpendicular to Vnc. The operation shows that the power consumed by the smart load is not depend on the power variations in the photovoltaic system.

Single phase ES with current source inverter (SPES-CSI)
The single-phase ES based on current source inverter is shown in the Figure 14. It consists of DC current source as an energy storage, four switches, and single capacitor as a filter. The working of the SPES-CSI is similar to the active power filter. The voltage control strategies used for the recent VSI based ES, reduces the total harmonic distortion. It can be again improve with the SPES-CSI using the direct current control method [51].

Single phase ES with output voltage control (SPES-OVC)
This version of ES has structure similar to the SPES-C only difference is; it uses the output voltage control. ES with the input voltage control is a peculiarity of it. In the core concept of ES, it regulates the voltage across the critical load by sacrificing voltage at the non-critical load but in the practical application of it, the majority of customers were not satisfied. Hence the SPES with output voltage control is implemented in [52]. The output voltage control is divided in three sections: the DC voltage regulation, the voltage regulation at load and the selection of operating mode. While regulating the voltage across critical load, it maintains the voltage at the non-critical load also. The SPES-OVC requires less reactive power as compared to the traditional SPES, this leads to more economical solution.

Single phase ES with battery and capacitor (Hybrid ES)
The hybrid ES uses the battery and split storage capacitor with renewable energy generation. The Figure 15 shows the SPES-B with the split storage capacitor [45]. This configuration of electric spring provides cost effective solution for industrial use, it prevents the charging and discharging of the battery by changing the non-critical load.

Hardware structure of three phase electric spring (TPES)
The power fluctuations are more common in the three-phase system due to unbalance loads. It impacts on the power quality and give rise to the power quality issues like unbalanced line current, excessive and neutral current. To provide the solution for these issues, the three-phase electric spring is implemented in [53] to reduce the three-phase power imbalance. The Figure 16 shows the schematic of the three-phase electric spring (TPES). Similar to the SPES, every ES of three phase system is connected in series with its respective the non-critical load to form the smart load for regulating voltage across the critical load of each phase. On the basis of energy storage devices, the TPES is classified as TPES with the capacitor and TPES with the battery.

Three phase ES with capacitor (TPES-C)
The Figure 17 shows the three phase ES with the capacitor. It consists of six switches, three legs with group of inductors and capacitors connected to the grid via a three-phase transformer. This topology of ES provides the balance and equal voltage to the critical load, it requires large requirement of voltage injection for a very small change in grid voltage. This in turn increases the rating of the switches. Thus nine switch converter topology of TPES-C is proposed in [54]. It performs all power compensation for all operating modes without the need of additional energy storage requirement. It provides the active power transfer from the grid to the ES and the ES to grid hence provides full control for the phase angle of injected voltage. The unified topology of the TPES-C is proposed in [55] operates as: series, series to shunt and shunt to compensator. It consists of combination of series and shunt converter. The LCL filter is connected on the  The Figure 18 shows the operating modes of TPES-C with phasor diagram. The Figure 18(a) shows Vpcc is at the reference value. The voltage across the non-critical load coincides the voltage at point of common coupling (PCC). Here the ES is bypassed hence voltage across the non-critical load and the critical load is same. The Figure 18(b) shows the unbalance between the power generation and the power consumption which changes the voltage at PCC. The ES voltage leads the non-critical load voltage to regulate the voltage at PCC. The Figure 18(c) shows the generation is more than the consumption. This changes the voltage at PCC and it goes beyond the reference value. The voltage of ES lags the voltage of the noncritical load to regulate the voltage at PCC.

Three phase electric spring with battery (TPES-B)
Three phase electric spring has a structure similar to the TPES-C, only difference is; it uses battery instead of the capacitor. This version of the TPES has the capability to convert the smart load into an adaptive load and can transform them to fulfill all power compensation requirements during the transient state. It can simultaneously compensate the grid voltage and the current flowing through the critical load [56]. It is used in smart buildings to convert conventional thermal load into an adaptive load to reduce voltage and the frequency deviation [28]. It reduces voltage and load imbalance by using the independent delta control [57]. It reduces the total harmonic distortion of the main voltage [58]. The TPES-B performs multiple functions like: voltage regulation at PCC, frequency regulation, active and reactive power compensation, power factor improvement at the same time using instantaneous power theory [59]. In unplanned island type grid, the TPES-B regulates the voltage across the critical load without neutral connection [23]. It provides a more practical application and changes the modes of operation by the addition of extra transformer [60]. It has eight operating modes similar to SPES-B, namely: capacitive, inductive,  Table 4 provides summary of all single phase and three phase electric spring considering parameters such as: energy source, characteristic, power controllability, type of compensation, different types of controllers, advantages and disadvantages.

RESULT AND DISCUSSION
After reviewing the literature on the AC electric spring, it has been observed that this concept is applied to both three-phase and single-phase AC systems. It is applied for improving power factor, regulating voltage across sensitive loads, reducing power imbalance, and reducing neutral current mitigation using different converter topologies such as VSI, CSI; different types of low filters, and using different control strategies with different controllers such as PI, PR, hysteresis, fuzzy, and ANN for single ES and droop control, and consensus control. for multiple ES using energy storage elements such as a capacitor, battery, and photovoltaic system. Results among all these combinations of ES show that the electric spring has more potential as a reactive power compensator as compared to other devices such as active power filter and FACT devices like SSSC, and STATCOM and reduce the power quality issues.
A theoretical analysis shows that AC electric spring can become an advanced demand-side management technique in electric power distribution networks with distributed generation. This requires: the ES to be used with a well-structured communication network, the use of a low-cost & compact design of an inverter associated with it, and the use of control strategies that are simple and cost-effective.