# Nonlinear Control Method of Superconducting Energy Storage considering Voltage Imbalance in Distribution Network

Authors: Tuo Xin; Fei Li
Journal: Security and Communication Networks
 (2022)

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## Abstract

A new nonlinear control approach of superconducting energy storage is devised under the condition of addressing the voltage imbalance of the distribution network in order to obtain more precise control of superconducting energy storage devices (SMEs). The model of the superconducting energy storage device is built on the basis of determining the voltage imbalance signal of the distribution network. Aiming at the situation that the voltage of the device will change at the same time when the structure of the device changes, a nonlinear controller is designed to estimate the discontinuous interference state of the nonlinear controller device and combined with the nonlinear control algorithm to realize the nonlinear control of superconducting energy storage. The simulation test results demonstrate that the design method can accomplish a more accurate signal tracking cut and that switching voltage generation estimation is performed well. It shows that the design method can meet the actual nonlinear control requirements of a superconducting energy storage device.


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## Body

## 1. Introduction

Various nonlinear control technologies are introduced into the superconducting energy storage controller, especially state feedback linearization, backstepping control, fuzzy logic control, neural network, passivity control, and sliding mode control [1]. These methods have proved their effectiveness in reducing total harmonic distortion and response time and eliminating DC bus voltage overshoot.The optimal control model is optimized by using the latest prediction algorithm [2] in the literature. The proposed control is used to design load frequency control (LFC) installed in interconnected systems including different renewable energy sources (RESs). This method is used to identify the optimal parameters of MPC to minimize the integral time absolute error (ITAE) of frequency and tie line power deviation. The analysis is carried out on three multi-interconnected systems. The first system includes two thermoelectric and photovoltaic units with maximum power point tracking (MPPT). Others include three linear/nonlinear interconnected systems with/without superconducting magnetic energy storage (SMEs). The nonlinear model is realized by considering generation rate constraint (GRC) and governor dead band (GDB). In addition, MPC-LFC under system parameter uncertainty is also studied. In addition, the random wind speed and load disturbance of multi-interconnected systems are analyzed. Xu et al. [3] introduced a frequency tunable high-Q superconducting resonator made of niobium nitride and niobium-titanium nitride films. The resonant frequency is tuned by applying a magnetic field perpendicular to the hole structure in the inductance line of the resonator, and its dynamic inductance is modified by wirelessly induced DC overcurrent. Continuous in situ frequency tuning of more than 300 MHz can be achieved in a 10 GHz resonator with a medium magnetic field of 1.2 Mt. The design and noncontact tuning scheme of the planar resonator greatly reduce the manufacturing complexity and can be widely used in many hybrid systems to couple microwave modes with other forms of excitation such as photons, phonons, and magnons, and spins. The above two methods provide a basis for the study of nonlinear control in this paper. Jin et al. [4] proposed superconducting magnetic energy storage (SME) device based on a parallel active power filter (SAPF), which is used to suppress harmonics and unbalanced currents in photovoltaic microgrids and suppress power fluctuation. The AC side of SAPF is connected to the common coupling point (PCC), and its DC link integrates DC/DC converter and energy storage superconducting coil (SC). The multi-objective control technology based on the improved IPIQ method and hysteresis SVPWM is adopted to realize the dual functions of active filter and power fluctuation suppression. A fuzzy logic control (FLC) method for DC/DC converter is proposed to stabilize the DC link voltage and reduce the discharge depth of the SME. The single and combined performance of SAPF-based SMEs in many environments is demonstrated through a series of comparisons of conventional SAPF-based and SAPF-based battery energy storage (BES). The effect of voltage imbalance on control performance, on the other hand, is not taken into account in this method. Aseem and Kumar [5] introduced the power flow control of solar-wind renewable energy systems (RESs). The control technology of the fractional-order PID (FOPID) controller is used to soften the power change. Superconducting magnetic energy storage system (SME) adopts the second generation superconducting material, which has a large irreversible field and critical current density and is used as an energy storage device. Compared with traditional PID controller, FOPID controller has a shorter rise time, less oscillation or overshoot, and stronger robustness. The FOPID controller is compared with the conventional PID controller to verify the superiority of this technology. The results show that although the load has a sudden change and the power generation has increased, the method does not consider the power exchange between supply and demand, resulting in the control performance being further improved.In order to solve the problem of poor switching control stability of superconducting energy storage devices, this paper studies the smooth switching control between grid-connected operation and island operation in a superconducting energy storage device for the purpose of the optical storage controller, so as to realize the nonlinear control of superconducting energy storage.This paper is organized as follows. Section2 determines the voltage imbalance signal of the distribution network. Section 3 proposes the design of the nonlinear control method for superconducting energy storage device. Section 4 defines the simulation experiment. Finally, the paper ends with a conclusion in Section 5.


## 2. Determining the Voltage Imbalance Signal of the Distribution Network

In this section, we discuss the model construction of superconducting energy storage device and design voltage imbalance signal detection algorithm.


### 2.1. Model Construction of Superconducting Energy Storage Device

The superconducting energy storage system can be used to adjust the peak and valley of the power system (for example, store the excess electric energy when the power grid is running at its trough and send the stored electric energy back to the power grid when the power grid is running at its peak). It can also be used to reduce or even eliminate the low-frequency power oscillation of the power grid, so as to improve the voltage and frequency characteristics of the power grid. At the same time, it can also be used to adjust reactive power and power factors to improve the stability of the power system [6, 7]. In this study, in order to better complete the nonlinear control design of superconducting energy storage, firstly, the superconducting energy storage device model is constructed to provide a platform for the subsequent technology proposal and application. The basic framework of the superconducting energy storage device is shown in Figure 1.Figure 1
Overall structure of superconducting energy storage device.According to this basic framework, the output voltage of a superconducting energy storage device can be obtained as follows:(1)u=e+ki−Ri+αkii′,where e represents capacitance; ki represents output current signal; R represents resistance; i represents current; α is the transformation coefficient; and i′ represents the reference current of the reactive current loop. Through this formula, it can be seen that there is variable coupling in the superconducting energy storage device, so it is difficult to control it in practice. Therefore, during normal operation, it is necessary to install a current controller in it, so the control equation of the superconducting energy storage device can be expressed as(2)u′=−lis+liiue,where lis represents the proportional coefficient of the current inner loop and lii represents the integral coefficient in the control process. The dual control of voltage outer loop and the current inner loop is a control structure often used in the grounding process of optical storage DC superconducting energy storage device [8]. In order to analyze the current change during the grounding of the superconducting energy storage device, set the equivalent gain of the three-phase rectifier bridge as G and the delay time of the superconducting energy storage device itself as τ; then, the current loop transfer function of the superconducting energy storage device can be expressed as(3)τ=Glis+lii/ukiR/u.According to this formula, the closed-loop transfer function of a superconducting energy storage device can be obtained. The specific formula is(4)τ′=Gkiτ×kiRu.By sorting out the above formula, the current transfer calculation formula of the superconducting energy storage device can be obtained. By substituting the relevant data of the superconducting energy storage device into this formula, the current transfer function in the grounding process of superconducting energy storage device can be obtained. According to this function, the model of the superconducting energy storage device is constructed, which provides an environmental basis for subsequent research.


### 2.2. Designing Voltage Imbalance Signal Detection Algorithm

A huge number of voltage imbalance signals will be generated during the grounding process of the superconducting energy storage device, which will alter the output result of the power signal. As a result, a voltage imbalance signal detecting method is built using the superconducting energy storage device concept described above. After analyzing a large number of signal detection algorithms, the PSO algorithm [9, 10] is selected as the design basis of the voltage imbalance signal detection algorithm in this study. All of the signal’s parameters are translated into the form of birds, and the best parameters are determined using the fitness and optimization functions. Set all signal parameter particles in the superconducting energy storage device as ai and the initial velocity of each parameter particle as vi; then, the signal particle update formula during the grounding process of the superconducting energy storage device can be expressed as(5)ai′=ait+1+vit,where vit represents the propagation speed of the ith signal particle. After the inertia weight coefficient ω and acceleration coefficient ϕ are introduced into this calculation formula, the signal propagation speed formula can be optimized as follows:(6)vit=ωvit+1+ϕai′.According to the above formula, in the process of voltage imbalance signal capture, the acceleration coefficient, inertia weight coefficient, and the propagation speed of signal particles have a great impact on the signal capture algorithm [11]. To improve the algorithm’s dependability, the signal in the superconducting energy storage device should be sampled on a wide scale if the signal particle diameter is d. However, when the signal particle sampling rate is large, the signal capture algorithm’s computing efficiency suffers, resulting in a lengthy and time-consuming calculating procedure. To solve this problem, in this study, the learning factor is introduced to optimize formula (6), and the inertia weight coefficient ω is dynamically adjusted through the linear function. After optimization, ω can be expressed as(7)ω=ωmax−ωmax−ωminkmax,where ωmax and ωmin represent the maximum and minimum values of inertia weight coefficient ω, respectively, and kmax represents the highest iteration of the calculation process. When processing optical DC superconducting energy storage devices, the value is usually 1–1.5. Use the value result of formula (7) to complete the voltage imbalance signal detection and identify the voltage imbalance signal type and transmission range if it is within the specified range.


## 2.1. Model Construction of Superconducting Energy Storage Device

The superconducting energy storage system can be used to adjust the peak and valley of the power system (for example, store the excess electric energy when the power grid is running at its trough and send the stored electric energy back to the power grid when the power grid is running at its peak). It can also be used to reduce or even eliminate the low-frequency power oscillation of the power grid, so as to improve the voltage and frequency characteristics of the power grid. At the same time, it can also be used to adjust reactive power and power factors to improve the stability of the power system [6, 7]. In this study, in order to better complete the nonlinear control design of superconducting energy storage, firstly, the superconducting energy storage device model is constructed to provide a platform for the subsequent technology proposal and application. The basic framework of the superconducting energy storage device is shown in Figure 1.Figure 1
Overall structure of superconducting energy storage device.According to this basic framework, the output voltage of a superconducting energy storage device can be obtained as follows:(1)u=e+ki−Ri+αkii′,where e represents capacitance; ki represents output current signal; R represents resistance; i represents current; α is the transformation coefficient; and i′ represents the reference current of the reactive current loop. Through this formula, it can be seen that there is variable coupling in the superconducting energy storage device, so it is difficult to control it in practice. Therefore, during normal operation, it is necessary to install a current controller in it, so the control equation of the superconducting energy storage device can be expressed as(2)u′=−lis+liiue,where lis represents the proportional coefficient of the current inner loop and lii represents the integral coefficient in the control process. The dual control of voltage outer loop and the current inner loop is a control structure often used in the grounding process of optical storage DC superconducting energy storage device [8]. In order to analyze the current change during the grounding of the superconducting energy storage device, set the equivalent gain of the three-phase rectifier bridge as G and the delay time of the superconducting energy storage device itself as τ; then, the current loop transfer function of the superconducting energy storage device can be expressed as(3)τ=Glis+lii/ukiR/u.According to this formula, the closed-loop transfer function of a superconducting energy storage device can be obtained. The specific formula is(4)τ′=Gkiτ×kiRu.By sorting out the above formula, the current transfer calculation formula of the superconducting energy storage device can be obtained. By substituting the relevant data of the superconducting energy storage device into this formula, the current transfer function in the grounding process of superconducting energy storage device can be obtained. According to this function, the model of the superconducting energy storage device is constructed, which provides an environmental basis for subsequent research.


## 2.2. Designing Voltage Imbalance Signal Detection Algorithm

A huge number of voltage imbalance signals will be generated during the grounding process of the superconducting energy storage device, which will alter the output result of the power signal. As a result, a voltage imbalance signal detecting method is built using the superconducting energy storage device concept described above. After analyzing a large number of signal detection algorithms, the PSO algorithm [9, 10] is selected as the design basis of the voltage imbalance signal detection algorithm in this study. All of the signal’s parameters are translated into the form of birds, and the best parameters are determined using the fitness and optimization functions. Set all signal parameter particles in the superconducting energy storage device as ai and the initial velocity of each parameter particle as vi; then, the signal particle update formula during the grounding process of the superconducting energy storage device can be expressed as(5)ai′=ait+1+vit,where vit represents the propagation speed of the ith signal particle. After the inertia weight coefficient ω and acceleration coefficient ϕ are introduced into this calculation formula, the signal propagation speed formula can be optimized as follows:(6)vit=ωvit+1+ϕai′.According to the above formula, in the process of voltage imbalance signal capture, the acceleration coefficient, inertia weight coefficient, and the propagation speed of signal particles have a great impact on the signal capture algorithm [11]. To improve the algorithm’s dependability, the signal in the superconducting energy storage device should be sampled on a wide scale if the signal particle diameter is d. However, when the signal particle sampling rate is large, the signal capture algorithm’s computing efficiency suffers, resulting in a lengthy and time-consuming calculating procedure. To solve this problem, in this study, the learning factor is introduced to optimize formula (6), and the inertia weight coefficient ω is dynamically adjusted through the linear function. After optimization, ω can be expressed as(7)ω=ωmax−ωmax−ωminkmax,where ωmax and ωmin represent the maximum and minimum values of inertia weight coefficient ω, respectively, and kmax represents the highest iteration of the calculation process. When processing optical DC superconducting energy storage devices, the value is usually 1–1.5. Use the value result of formula (7) to complete the voltage imbalance signal detection and identify the voltage imbalance signal type and transmission range if it is within the specified range.


## 3. Design of Nonlinear Control Method for Superconducting Energy Storage Device

In this part, we define the device modeling, design controller, and design switching synovial controller.


### 3.1. Device Modeling

Build the model of superconducting energy storage device. The specific formula is as follows:(8)x˙1=f1t+g1x2,x˙2=f2t+g2ht+d,y=x1,where x2 refers to the device state vector attitude angular rate; x1 is the device state vector attitude; y is the device output vector; d refers to unknown external interference; ht refers to the saturation function of the input value of the device [12], as shown in equation (9); and g1, g2, f1t, and f2t refer to the functions that the device is fully in a smooth state. Constant value of switchgear function vector of x˙1 and x˙2 devices [13].(9)ht=tmaxt≥tmaxttmin<t<tmaxtmint≤tmin,where tmax refers to the maximum value of control input and tmin is the minimum value of the control input.


### 3.2. Designing Controller

Aiming at the situation that the voltage will change at the same time when the structure of the device changes [14], a nonlinear controller is designed to estimate its discontinuous interference.The design of nonlinear switching interference observation is as follows.For the device of formula (8), let(10)x˙2=f2t+g2ht.When the unknown external disturbanced can satisfy the following assumptions:For the unknown time-varying interferenced=d1,d2,d3T of the device, the interference satisfies the following formula:(11)dn=∑i=1nbdn−idi,where bdn−i refers to unknown time-varying interference parameters.Introduce intermediate variables, as shown in the following formula:(12)zd0=v0=x2zd1=v1−bd1v0...zdn=vn−∑i=1nbdn−idi,where zdn refers to the intermediate variable introduced [15].Through the intermediate variable after derivation, the form of disturbance observer is designed, as shown in the following formula:(13)zdn′=−bdnft+uθzd0−zd0′,where zdn′ refers to the estimated value of these intermediate variables; zd0′ refers to the constituent term of the disturbance observer; u refers to the stable design parameter of parameter function; and θ refers to the switching time design parameter.


### 3.3. Designing Switching Synovial Controller

Combined with the designed controller, a switching synovial controller is designed by the backstepping method. According to the scalar nonlinear characteristics, a synovial surface is designed [16], and a nonlinear controller algorithm is designed to make the superconducting energy storage device meet the actual accessibility conditions of the designed synovial surface, complete the design of switching synovial controller, and realize the synovial control of the device.The designed synovial surface is as follows:(14)sk,i=∑l=k0+1kQiAi+Bigxl,where sk,i refers to the designed synovial surface; Qi is the synovial design function; Ai, Bi represent the ideal synovial function; and g⋅ refers to the nonsingular synovial threshold function.The designed nonlinear controller algorithm can make the trajectory of superconducting energy storage device reach the synovial surface in limited time under the designed switching synovial controller. The details are as follows:(15)pk,i=Qigxl−ρsk,i,Dsk,i=−sk,iω,sk,i≤ω,signsk,i,sk,i>ω,where pk,i refers to the scalar nonlinear characteristic function [17]; ρ is a positive definite function; and Dsk,i is the left-right multiplication function of the device trajectory. So far, the nonlinear control of the superconducting energy storage device is realized.


## 3.1. Device Modeling

Build the model of superconducting energy storage device. The specific formula is as follows:(8)x˙1=f1t+g1x2,x˙2=f2t+g2ht+d,y=x1,where x2 refers to the device state vector attitude angular rate; x1 is the device state vector attitude; y is the device output vector; d refers to unknown external interference; ht refers to the saturation function of the input value of the device [12], as shown in equation (9); and g1, g2, f1t, and f2t refer to the functions that the device is fully in a smooth state. Constant value of switchgear function vector of x˙1 and x˙2 devices [13].(9)ht=tmaxt≥tmaxttmin<t<tmaxtmint≤tmin,where tmax refers to the maximum value of control input and tmin is the minimum value of the control input.


## 3.2. Designing Controller

Aiming at the situation that the voltage will change at the same time when the structure of the device changes [14], a nonlinear controller is designed to estimate its discontinuous interference.The design of nonlinear switching interference observation is as follows.For the device of formula (8), let(10)x˙2=f2t+g2ht.When the unknown external disturbanced can satisfy the following assumptions:For the unknown time-varying interferenced=d1,d2,d3T of the device, the interference satisfies the following formula:(11)dn=∑i=1nbdn−idi,where bdn−i refers to unknown time-varying interference parameters.Introduce intermediate variables, as shown in the following formula:(12)zd0=v0=x2zd1=v1−bd1v0...zdn=vn−∑i=1nbdn−idi,where zdn refers to the intermediate variable introduced [15].Through the intermediate variable after derivation, the form of disturbance observer is designed, as shown in the following formula:(13)zdn′=−bdnft+uθzd0−zd0′,where zdn′ refers to the estimated value of these intermediate variables; zd0′ refers to the constituent term of the disturbance observer; u refers to the stable design parameter of parameter function; and θ refers to the switching time design parameter.


## 3.3. Designing Switching Synovial Controller

Combined with the designed controller, a switching synovial controller is designed by the backstepping method. According to the scalar nonlinear characteristics, a synovial surface is designed [16], and a nonlinear controller algorithm is designed to make the superconducting energy storage device meet the actual accessibility conditions of the designed synovial surface, complete the design of switching synovial controller, and realize the synovial control of the device.The designed synovial surface is as follows:(14)sk,i=∑l=k0+1kQiAi+Bigxl,where sk,i refers to the designed synovial surface; Qi is the synovial design function; Ai, Bi represent the ideal synovial function; and g⋅ refers to the nonsingular synovial threshold function.The designed nonlinear controller algorithm can make the trajectory of superconducting energy storage device reach the synovial surface in limited time under the designed switching synovial controller. The details are as follows:(15)pk,i=Qigxl−ρsk,i,Dsk,i=−sk,iω,sk,i≤ω,signsk,i,sk,i>ω,where pk,i refers to the scalar nonlinear characteristic function [17]; ρ is a positive definite function; and Dsk,i is the left-right multiplication function of the device trajectory. So far, the nonlinear control of the superconducting energy storage device is realized.


## 4. Simulation Experiment

In this portion, we have to define the simulation platform, steady-state simulation experiment analysis, experimental results of switching from grid-connected operation mode to island operation mode, experimental results of switching from island operation mode to grid-connected operation mode, voltage fluctuation, load voltage of superconducting energy storage device, and voltage stabilizing control accuracy in detail.


### 4.1. Simulation Platform

In order to verify the reliability and stability of the control method in this paper, real-time digital simulation (RTDS) is used to simulate a local superconducting energy storage device, and a simulation platform is built. The platform structure is shown in Figure2.Figure 2
Structure diagram of simulation platform.The composition of the simulation platform includes energy storage control device, photovoltaic control device, battery management device, energy management monitoring device, and microgrid central controller (MGCC). Set the capacity of each piece of equipment to 25% of the real capacity of the superconducting energy storage device and design the simulation platform using RTDS, according to the structure of the superconducting energy storage device in Figure1. In the RTDS device, the main topology of a photovoltaic inverter and energy storage converter operated by an external photovoltaic device and energy storage device is also modeled. The monitoring function, power generation, and load forecasting are realized by the energy management monitoring device. In order to realize the control method in this paper, MFCC is used.


### 4.2. Steady-State Simulation Experiment Analysis

In order to verify the stability of the control method in this paper under steady-state control, the simulation experiment is realized on the simulation platform, and the RTDS simulation diagram of tie line control is obtained according to the energy management monitoring device, as shown in Figure3.Figure 3
RTDS simulation results.It can be seen from Figure3 that when the tie line power is within the set range and the plan command changes, the actual power of the tie line also changes, which has good response performance. In the above figure, when the mode is switched in 0.5 s, 1.0 s, 2.0 s, and 2.5 s, the output power can be quickly balanced under the control of this method, and the control effect is good.


### 4.3. Experimental Results of Switching from Grid-Connected Operation Mode to Island Operation Mode

Set the operation time to 0.5 s, switch the operation mode of the superconducting energy storage device to island operation, and the energy storage controller causes the grid-connected inverter to switch from grid-connected control strategy to island control strategy. Generally, the main network provides reactive power to the superconducting energy storage device, and the superconducting energy storage device outputs active power to the main network. See Table1 for the change in device stability during switching.Table 1
Device stability results under different methods.
Voltage (V)Device stability (%)The control method in [4]The control method in [5]The control method in this paper400788297500828398600858495700848097800828295900837994Mean value82.3081.6796Table1 shows that the stability of superconducting energy storage device is different under different control strategies. When the voltage is 500 V, the superconducting energy storage device in [4] is 82% stable, the superconducting energy storage device in [5] is 83% stable, and the superconducting energy storage device in this paper is 98% stable. When the voltage is 900 V, the stability of the superconducting energy storage device of the control method in [4] is 83%, the stability of the superconducting energy storage device of the control method in [5] is 79%, and the stability of the superconducting energy storage device of the control method in this paper is 94%. This method has high stability after control. In the process of switching the superconducting energy storage device operating in grid-connected mode to island operation mode, according to the simulation results, the optical storage controller ensures that the grid-connected inverter can quickly complete the smooth switching of the superconducting energy storage device from grid-connected control mode to island control mode. The simulation experiments show that this method can effectively ensure the stability of the power supply.


### 4.4. Experimental Results of Switching from Island Operation Mode to Grid-Connected Operation Mode

When the setting time is 1.0 s, the operation of the superconducting energy storage device is switched from island mode to grid-connected mode. The optical storage controller causes the grid-connected inverter to change from an island control strategy to a grid-connected control strategy. The results are shown in Table2.Table 2
Device stability results under different methods.
Voltage (V)Device stability (%)The control method in [4]The control method in [5]The control method in this paper400737996500777595600728192700777993800788397900808495Mean value76.1680.1694.60Table2 shows that the stability of superconducting energy storage devices is different under different control strategies. When the voltage is 600 V, the stability of the superconducting energy storage device of the control method in [4] is 72%, the stability of the superconducting energy storage device of the control method in [5] is 81%, and the stability of the superconducting energy storage device of the control method in this paper is 92%. The average stability of the superconducting energy storage device used in the control method in [4] is 76.16%, that of the superconducting energy storage device used in the control method in [5] is 80.16%, and that of the superconducting energy storage device used in this paper is 94.6%. After control, this approach offers a high level of stability. Change the superconducting energy storage device’s operation mode from island to grid-connected once more. The optical storage controller first implements synchronous control and then switches the island control to grid-connected control. During the switching process, the optical storage controller ensures that the grid-connected inverter is smoothly switched from island operation to grid-connected operation. In the grid-connected operation mode, the optical storage controller can accurately and quickly track and monitor the specified power to ensure the stability of the superconducting energy storage device.


### 4.5. Voltage Fluctuation

Based on the above simulation results, in order to verify the voltage fluctuation at different times after the application of this method and compare it with that without this method, the simulation results are shown in Figure4.Figure 4
Voltage fluctuation.According to the analysis of Figure4, when the method in this paper is not applied, the voltage fluctuation is large when the time gradually increases. After the method in this paper is applied, the voltage fluctuation is small with the increase of time, and the fluctuation is between 22 V–24 V. Simulation results show that this method can effectively suppress voltage fluctuation.


### 4.6. Load Voltage of Superconducting Energy Storage Device

The maximum drop value and minimum steady-state value of load voltage after the application of the three methods are compared. That is, input the three methods into SPSS software, respectively, and output the maximum drop value of load voltage of superconducting energy storage device after the application of different methods. The comparison results are shown in Figure5.Figure 5
Maximum drop value of load voltage in different methods.It can be seen from Figure5 that the maximum load voltage drop value of the control method in [4] and the control method in [5] is higher than that of this method, and the maximum load voltage drop value of this method is the lowest, only 52 V. The simulation results show that this method uses the nonlinear control method to determine the voltage fluctuation value in the superconductive energy storage device. It can effectively prevent the load voltage of superconducting energy storage device from falling greatly, and the voltage stabilizing performance is good.The change of the lowest steady-state value of different methods at different times is shown in Figure6.Figure 6
Minimum steady-state value at different times.It can be seen from Figure6 that since the output voltage of each traction substation is set to be 24 kV, the minimum steady-state value under the control of each method fluctuates around 24 kV at different times. The minimum steady-state value of the control methods in [4, 5] fluctuates below 24 kV, but this method can effectively ensure that the substation’s minimum steady-state value fluctuates slightly around 24 kV. The lowest steady-state value of the superconducting energy storage device with voltage stabilizing control according to the method in this paper is close to each substation’s set output voltage.


### 4.7. Voltage Stabilizing Control Accuracy

To compare the voltage stabilizing control accuracy after the application of the three approaches, control methods in [4, 5] were chosen as comparative methods. The comparison results of voltage stabilizing control accuracy output by SPSS software are shown in Figure 7.Figure 7
Voltage stabilizing control accuracy of different methods.It can be seen from Figure7 that the voltage stabilizing control accuracy of the control method in [4] and the control method in [5] is lower than that of this method, and the voltage stabilizing control accuracy of this method is up to 98%. The simulation results show that this method effectively realizes the voltage stabilizing control of rail transit multi-flow traction superconducting energy storage devices.


## 4.1. Simulation Platform

In order to verify the reliability and stability of the control method in this paper, real-time digital simulation (RTDS) is used to simulate a local superconducting energy storage device, and a simulation platform is built. The platform structure is shown in Figure2.Figure 2
Structure diagram of simulation platform.The composition of the simulation platform includes energy storage control device, photovoltaic control device, battery management device, energy management monitoring device, and microgrid central controller (MGCC). Set the capacity of each piece of equipment to 25% of the real capacity of the superconducting energy storage device and design the simulation platform using RTDS, according to the structure of the superconducting energy storage device in Figure1. In the RTDS device, the main topology of a photovoltaic inverter and energy storage converter operated by an external photovoltaic device and energy storage device is also modeled. The monitoring function, power generation, and load forecasting are realized by the energy management monitoring device. In order to realize the control method in this paper, MFCC is used.


## 4.2. Steady-State Simulation Experiment Analysis

In order to verify the stability of the control method in this paper under steady-state control, the simulation experiment is realized on the simulation platform, and the RTDS simulation diagram of tie line control is obtained according to the energy management monitoring device, as shown in Figure3.Figure 3
RTDS simulation results.It can be seen from Figure3 that when the tie line power is within the set range and the plan command changes, the actual power of the tie line also changes, which has good response performance. In the above figure, when the mode is switched in 0.5 s, 1.0 s, 2.0 s, and 2.5 s, the output power can be quickly balanced under the control of this method, and the control effect is good.


## 4.3. Experimental Results of Switching from Grid-Connected Operation Mode to Island Operation Mode

Set the operation time to 0.5 s, switch the operation mode of the superconducting energy storage device to island operation, and the energy storage controller causes the grid-connected inverter to switch from grid-connected control strategy to island control strategy. Generally, the main network provides reactive power to the superconducting energy storage device, and the superconducting energy storage device outputs active power to the main network. See Table1 for the change in device stability during switching.Table 1
Device stability results under different methods.
Voltage (V)Device stability (%)The control method in [4]The control method in [5]The control method in this paper400788297500828398600858495700848097800828295900837994Mean value82.3081.6796Table1 shows that the stability of superconducting energy storage device is different under different control strategies. When the voltage is 500 V, the superconducting energy storage device in [4] is 82% stable, the superconducting energy storage device in [5] is 83% stable, and the superconducting energy storage device in this paper is 98% stable. When the voltage is 900 V, the stability of the superconducting energy storage device of the control method in [4] is 83%, the stability of the superconducting energy storage device of the control method in [5] is 79%, and the stability of the superconducting energy storage device of the control method in this paper is 94%. This method has high stability after control. In the process of switching the superconducting energy storage device operating in grid-connected mode to island operation mode, according to the simulation results, the optical storage controller ensures that the grid-connected inverter can quickly complete the smooth switching of the superconducting energy storage device from grid-connected control mode to island control mode. The simulation experiments show that this method can effectively ensure the stability of the power supply.


## 4.4. Experimental Results of Switching from Island Operation Mode to Grid-Connected Operation Mode

When the setting time is 1.0 s, the operation of the superconducting energy storage device is switched from island mode to grid-connected mode. The optical storage controller causes the grid-connected inverter to change from an island control strategy to a grid-connected control strategy. The results are shown in Table2.Table 2
Device stability results under different methods.
Voltage (V)Device stability (%)The control method in [4]The control method in [5]The control method in this paper400737996500777595600728192700777993800788397900808495Mean value76.1680.1694.60Table2 shows that the stability of superconducting energy storage devices is different under different control strategies. When the voltage is 600 V, the stability of the superconducting energy storage device of the control method in [4] is 72%, the stability of the superconducting energy storage device of the control method in [5] is 81%, and the stability of the superconducting energy storage device of the control method in this paper is 92%. The average stability of the superconducting energy storage device used in the control method in [4] is 76.16%, that of the superconducting energy storage device used in the control method in [5] is 80.16%, and that of the superconducting energy storage device used in this paper is 94.6%. After control, this approach offers a high level of stability. Change the superconducting energy storage device’s operation mode from island to grid-connected once more. The optical storage controller first implements synchronous control and then switches the island control to grid-connected control. During the switching process, the optical storage controller ensures that the grid-connected inverter is smoothly switched from island operation to grid-connected operation. In the grid-connected operation mode, the optical storage controller can accurately and quickly track and monitor the specified power to ensure the stability of the superconducting energy storage device.


## 4.5. Voltage Fluctuation

Based on the above simulation results, in order to verify the voltage fluctuation at different times after the application of this method and compare it with that without this method, the simulation results are shown in Figure4.Figure 4
Voltage fluctuation.According to the analysis of Figure4, when the method in this paper is not applied, the voltage fluctuation is large when the time gradually increases. After the method in this paper is applied, the voltage fluctuation is small with the increase of time, and the fluctuation is between 22 V–24 V. Simulation results show that this method can effectively suppress voltage fluctuation.


## 4.6. Load Voltage of Superconducting Energy Storage Device

The maximum drop value and minimum steady-state value of load voltage after the application of the three methods are compared. That is, input the three methods into SPSS software, respectively, and output the maximum drop value of load voltage of superconducting energy storage device after the application of different methods. The comparison results are shown in Figure5.Figure 5
Maximum drop value of load voltage in different methods.It can be seen from Figure5 that the maximum load voltage drop value of the control method in [4] and the control method in [5] is higher than that of this method, and the maximum load voltage drop value of this method is the lowest, only 52 V. The simulation results show that this method uses the nonlinear control method to determine the voltage fluctuation value in the superconductive energy storage device. It can effectively prevent the load voltage of superconducting energy storage device from falling greatly, and the voltage stabilizing performance is good.The change of the lowest steady-state value of different methods at different times is shown in Figure6.Figure 6
Minimum steady-state value at different times.It can be seen from Figure6 that since the output voltage of each traction substation is set to be 24 kV, the minimum steady-state value under the control of each method fluctuates around 24 kV at different times. The minimum steady-state value of the control methods in [4, 5] fluctuates below 24 kV, but this method can effectively ensure that the substation’s minimum steady-state value fluctuates slightly around 24 kV. The lowest steady-state value of the superconducting energy storage device with voltage stabilizing control according to the method in this paper is close to each substation’s set output voltage.


## 4.7. Voltage Stabilizing Control Accuracy

To compare the voltage stabilizing control accuracy after the application of the three approaches, control methods in [4, 5] were chosen as comparative methods. The comparison results of voltage stabilizing control accuracy output by SPSS software are shown in Figure 7.Figure 7
Voltage stabilizing control accuracy of different methods.It can be seen from Figure7 that the voltage stabilizing control accuracy of the control method in [4] and the control method in [5] is lower than that of this method, and the voltage stabilizing control accuracy of this method is up to 98%. The simulation results show that this method effectively realizes the voltage stabilizing control of rail transit multi-flow traction superconducting energy storage devices.


## 5. Conclusion

The disturbance observer is used to study the nonlinear control of a superconducting energy storage device, a nonlinear controller is designed to estimate the device’s discontinuous disturbance, and a switching sliding film controller is designed to realize the device’s nonlinear control. The research results refer to the modeling of superconducting energy storage devices, enabling accurate estimation of device discontinuity disturbances, the design method enables more accurate tracking of cut-in signals and shows good switching voltage generation estimation performance. However, in the research, due to the limitations of time, manpower, and material resources, there are still some problems that have not been solved, such as the idealization of simulation experiments. More detailed research will be carried out in the future.



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*Source: 9301911-2022-05-12.xml*