Automatic Power Direction Control of Dual Active Bridge/Triple Active Bridge Converter in Emergency Energy Supply for Sustainability

Abstract

With their multidirectional power flow capability, dual active bridge (DAB) and triple active bridge (TAB) converters find application in energy routers as DC/DC transfer components for emergency energy supply during significant power outages. These converters ensure stable sustainable power transmission across various energy sources while enabling high-power conversion. However, controlling power direction poses a challenge in DAB/TAB converters for emergency energy supply, typically a rapid change of power direction of any port of DAB/TAB converters. To address this problem, this study proposes a novel automatic power direction control method for DAB/TAB converters, enabling bidirectional power transmission without manual intervention based on the state of charge (SOC) of battery for emergency energy supply. This method realizes the change of power direction of each port in DAB/TAB converter automatically according to different situations, even in emergencies. Given the now widespread shortage of emergency energy and the higher cost of labor regulation, this approach simplifies operations and enhances system safety and sustainability by eliminating the need for human supervision. A well-implemented automatic control method ensures efficient and consistent power transfer within the system by change the direction in about 3 s, whenever power direction adjustment is necessary.

1. Introduction

The necessity for emergency energy arises from myriad unpredictable events, including natural disasters [1], inadvertent power supply system failures [2], demand surpassing supply [3], and deliberate malicious acts [4] all of which can lead to the incapacitation of urban power grids, profoundly affecting people’s lives. Electricity serves as a fundamental component across various industries, powering essential equipment required for daily operations [5]. The repercussions of large-scale power outages are severe, posing a critical issue: the cessation of all electrical equipment functionality during unexpected incidents. This disruption can impede regular work activities, resulting in economic losses and jeopardizing lives and safety. A sustainable grid requires a stable supply of electricity to avoid power outages [6]. To address this challenge, two common emergency power distribution methods—distributed power [7] and emergency power vehicles [8,9]—have been extensively employed in daily life. Additionally, a novel approach to emergency power distribution, namely, the energy router [10], has been proposed and validated for its feasibility. Equipped with features such as high energy utilization, versatility in power forms, energy storage capabilities, and grid monitoring functionalities, the energy router emerges as a suitable power supply solution for smart grids and microgrids [5].

In an energy router designed for emergency supply in urban settings, achieving energy balance among diverse electric power sources is paramount. The realization of dynamic and dynamic energy control can significantly help to reduce costs and to detect the real-time state of the grid [11]. This necessitates effective control of energy flow within the energy router, which is a critical aspect for managing various emergencies in urban environments [12]. Specifically, in the context of the DC power transfer component within the energy router, this paper employs DAB and TAB converters due to their inherent advantages [13]. DAB/TAB converters possess several advantageous features. Firstly, they can automatically adjust bidirectional power flow and are capable of adapting to rapid changes in power flow direction [14]. Moreover, they offer a wide voltage conversion gain range [15]. Additionally, DAB/TAB converters are equipped with zero-voltage switching (ZVS) capability [16], enabling them to achieve high efficiency through power control [17,18]. In a study referenced in [19], it was demonstrated that DAB and TAB converters exhibit high power efficiency, reaching up to 97.6%.

Power exchange among DAB/TAB converters is facilitated through a phase-shift pulse width modulation (PWM) mechanism [20]. Leveraging the lag/lead relationship of two or three pulse width waveforms, ports on DAB/TAB converters enable power transmission. The direction of power transmission varies based on the different lag/lead relationships, as each port of the DAB/TAB converter can facilitate bidirectional power flow [21]. Many control methods such as single-phase shift (SPS), dual-phase shift (DPS), and triple-phase shift (TPS) are used in DAB/TAB converters to realize the high power efficiency and bidirectional power flow [22,23,24,25,26]. However, for emergency power supply, there is not a proper power direction control method dealing with various emergencies such as power outages [27] to ensure the stability of DAB/TAB converters and normal operation for loads of clients.

Therefore, an automatic power direction control method in this paper can adjust power direction automatically according to the SOC of battery or EEPS at the port of DAB/TAB converters typically for emergency energy supply. This eliminates the need for manual monitoring of equipment status and ensures that the equipment can automatically adjust power direction to address different emergencies, thereby saving manpower.

This paper is arranged as follows. Firstly, the principle of power direction of DAB/TAB converters is briefly described to show the bidirectional power flow of DAB/TAB converters. Secondly, the automatic power direction control method is demonstrated to realize the automatic and rapid power direction control of DAB/TAB converters for sustainability so that the cost of manual monitoring can be eliminated. Thirdly, a simulation model that contains a DAB/TAB converter is implemented in MATLAB/Simulink 2021B, and a brief conclusion is provided at the end of this paper.

2. Principle of Power Direction of DAB/TAB Converters

Figure 1 illustrates the topologies of both the DAB and TAB converters. Each topology features two or three ports, with each port comprising an active bridge consisting of four switches. In the DAB topology, there is a single inductor L and a high-frequency transformer N. On the other hand, each port of the TAB converter includes an inductor L_i (where i = 1, 2, 3) and a transformer with winding N_i. Additionally, a power source B_i is connected to the terminal of each port. For the energy router designed for emergency energy supply in urban areas, B₁ can be considered as the DC bus of the urban power grid, while B₂ represents any source capable of serving as an emergency electric power source (EEPS). B₃ corresponds to the energy storage (ES) unit within the energy router. In this configuration, power is transferred among ports through the magnetic coupling of the windings.

2.1. DAB Converter

DAB operates primarily through SPS control [26], meaning that energy flow is managed by adjusting the lead/lag relationship between port 1 and port 2. Figure 2 illustrates an example of forward energy transmission in DAB, where S₁, S₂, S₃, and S₄ represent the signals controlling the MOSFETs at port 1, while Q₁, Q₂, Q₃, and Q₄ represent the signals controlling the MOSFETs at port 2, lagging behind S₁, S₂, S₃, and S₄ by the phase shift angle φ₁. V₁ denotes the voltage at port 1, V₂ denotes the voltage at port 2, and i_Lc represents the current through the inductance of port 1. According to Figure 1, within one switching period T_sp, there are four different modes of inductor current.

When t₀ ≤ t ≤ t₁,

$$i_{Lc}\left(t\right)={\displaystyle \frac{V_1+V_2}{L}}\left(t-t_0\right)+i_{Lc}\left(t_0\right)$$

(1)

where t₁ = t₀ + (φ₁T_sp)/2π.

When t₁ ≤ t ≤ t₂,

$$i_{Lc}\left(t\right)={\displaystyle \frac{V_1-V_2}{L}}\left(t-t_1\right)+i_{Lc}\left(t_1\right)$$

(2)

where t₂ = t₀ + T_sp/2.

When t₂ ≤ t ≤ t₃,

$$i_{Lc}\left(t\right)={\displaystyle \frac{-V_1-V_2}{L}}\left(t-t_2\right)+i_{Lc}\left(t_2\right)$$

(3)

where t₃ = t₂ + (φ₁T_sp)/2π.

When t₃ ≤ t ≤ t₀ + T_sp,

$$i_{Lc}\left(t\right)={\displaystyle \frac{-V_1+V_2}{L}}\left(t-t_3\right)+i_{Lc}\left(t_3\right)$$

(4)

From Figure 2, the equations $\left\{\begin{array}{c}{i}_{Lc}\left(t_0\right)=-i_{Lc}\left(t_2\right)=-i_{Lc}\left(t_0+{\displaystyle \frac{T_{sp}}{2}}\right)=-{\displaystyle \frac{\left(V_1-V_2\right)\pi +2V_2{\phi }_1}{4\pi f_{sp}{L}_c}}\\ i_{Lc}\left(t_1\right)=-i_{Lc}\left(t_3\right)=-i_{Lc}\left(t_1+{\displaystyle \frac{T_{sp}}{2}}\right)=-{\displaystyle \frac{\left(V_2-V_1\right)\pi +2V_1{\phi }_1}{4\pi f_{sp}{L}_c}}\end{array}\right.$ can be obtained which are explained in detail in [28]. Through integral, forward power P₊ and reverse power P₋ can be circulated.

$$\left\{\begin{array}{c}{P}_{+}={\displaystyle \frac{1}{T_{hs}}}{\int }_0^{T_{hs}}{V}_1{i}_{Lc}dt={\displaystyle \frac{{\phi }_1(\pi -{\phi }_1)V_1{V}_2}{2{\pi }^2{f}_{sp}L}}\\ P_{-}={\displaystyle \frac{1}{T_{hs}}}{\int }_0^{T_{hs}}{V}_1{i}_{Lc}dt={\displaystyle \frac{{\phi }_1(\pi +{\phi }_1)V_1{V}_2}{2{\pi }^2{f}_{sp}L}}\end{array}\right.$$

(5)

where f_sp = 1/T_sp represents the switching frequency. D is the phase shift ratio ($D={\phi }_1/180°$) and T_hs is the half of the switching period.

Figure 3 depicts the relationship between D and the power P. It is evident that when D falls within the range of −0.5 and 0.5, there exists a positive correlation between P and D. Therefore, D is typically set within the range of − 0.5 and 0.5, indicating that φ₁ varies between − 90° and 90°.

2.2. TAB Converter

The phase-shift PWM method employed in the DAB converter is also utilized to control power flow direction in the TAB converter. In Figure 4, the control signals of switches S₁ and S₂ serve as references, while the control signals of switches S₅ and S₆, and switches S₉ and S₁₀ are lagged by φ₂ and φ₃ radians, respectively. Here, −π < φ₂ < π and −π < φ₃ < π. Moreover, each switch operates with a duty ratio half of the switching cycle. Additionally, Figure 4 illustrates the current waveforms of L₁, L₂, and L₃ for i_L1, i_L2, and i_L3, respectively, in a traditional phase-shift TAB converter. The turns ratio of the transformer N₁: N₂: N₃ is set to 1:1:1.

The power flow characteristics of a phase-shift TAB converter can be derived from those of a DAB converter, and the expression in (5) can be extended to (6) by analyzing the various operating states of the TAB converter.

$$\left\{\begin{array}{c}{P}_{12}=V_1{I}_1={\displaystyle \frac{{\phi }_2(\pi -\left|{\phi }_2\right|)V_1{V}_2}{2{\pi }^2{f}_{sp}(L_1+L_2)}}\\ P_{13}=V_3{I}_3={\displaystyle \frac{{\phi }_3(\pi -\left|{\phi }_3\right|)V_1{V}_3}{2{\pi }^2{f}_{sp}(L_1+L_3)}}\\ P_{23}=V_2{I}_2={\displaystyle \frac{{({\phi }_3-\phi }_2\left)\right(\pi -\left|{{\phi }_{3-}\phi }_2\right|)V_1{V}_2}{2{\pi }^2{f}_{sp}(L_2+L_3)}}\end{array}\right.$$

(6)

In this expression, V₁, V₂, and V₃ refer to the voltages of B₁, B₂, and B₃, respectively, and P₁₂, P₁₃, and P₂₃ are the power flow from port 1 to port 2, from port 1 to port 3, and from port 2 to port 3, respectively. Furthermore, f_sp is the switching frequency.

Suppose each inductor has the same inductance, and the sum of any two inductors is denoted as Le. Then, the port power can be derived from

$$\left\{\begin{array}{c}{P}_1=P_{12}+P_{13}\\ ={\displaystyle \frac{{\phi }_2\left(\pi -\left|{\phi }_2\right|\right)V_1{V}_2+{\phi }_3\left(\pi -\left|{\phi }_3\right|\right)V_1{V}_3}{2{\pi }^2{f}_{sp}{L}_e}}\\ P_2={-P}_{12}+P_{23}\\ ={\displaystyle \frac{-{\phi }_2\left(\pi -\left|{\phi }_2\right|\right)V_1{V}_2+({\phi }_3-{\phi }_2)\left(\pi -\left|{\phi }_3-{\phi }_2\right|\right)V_2{V}_3}{2{\pi }^2{f}_{sp}{L}_e}}\\ P_3={-P}_{13}-P_{23}\\ ={\displaystyle \frac{-{\phi }_3\left(\pi -\left|{\phi }_3\right|\right)V_1{V}_3-({\phi }_3-{\phi }_2)\left(\pi -\left|{\phi }_3-{\phi }_2\right|\right)V_2{V}_3}{2{\pi }^2{f}_{sp}{L}_e}}\end{array}\right.$$

(7)

where P₁, P₂, and P₃ represent the total power of port 1, port 2, and port 3, respectively.

Therefore, the direction of power transmission is determined by both the port voltages and the phase shift angles of the switching gate signals. For instance, when the difference between port voltages is small or nearly nonexistent, Figure 5 illustrates four typical power flow scenarios that can satisfy the needs of the TAB converter for emergency energy supply. Figure 5a shows power transfers from port 1 to port 2 and port 3, which means that the main power grid is normal and the ES and EEPS ports are charging. When the energy at EEPS is beyond the needs, the TAB converter will be shifted into Figure 5b. This mode will reduce pressure on main grid transmission and is more economic for the clients. In Figure 5c, when the energy of the ES and EEPS ports are all beyond the needs, the ES and EEPS ports can transfer power to the main power grid, which realizes the peer-to-peer trading [29]. When an emergency occurs, the TAB converter will be shifted into Figure 5d, the main power grid will be off the grid, and the EEPS port and ES port will be the power source to the loads and clients, and will maintain the normal working of loads until the main grid paralysis is resolved.

3. Automatic Power Direction Control Method of DAB/TAB Converter

To address the need for automatic power direction adjustment without manual intervention, this paper proposes an automatic power direction control method for DAB/TAB converters. In the context of emergency energy supply, numerous unforeseen accidents may occur, making it difficult for individuals to swiftly change the power direction of DAB/TAB converters to address each unexpected situation. An automatic power direction control method can assist in adjusting the power direction according to preset values, thereby enhancing the responsiveness and effectiveness of the system in managing unexpected events. Compared with the automatic modulation methods in [30], this method is easier and more flexible for clients to control the power direction according to their own needs because the power direction is mainly controlled by the SOC of ES and EEPS.

3.1. DAB Converter

Based on the preceding explanation of the power direction of the DAB converter, it is evident that when −90° < φ < 0°, power transfers from port 2 to port 1, whereas when 0° < φ < 90°, power transfers from port 1 to port 2. Figure 6 illustrates the proposed automatic power direction control method. In this method, two variables, SOC₂ and MS, are employed to control the power direction of the DAB converter. When MS is equal to 0, −90° < φ < 0°, and power transfers from port 2 to port 1. Conversely, when MS equals 1, 0° < φ < 90°, and power transfers from port 1 to port 2. Here, max represents the maximum setting value, and min represents the minimum setting value.

According to Figure 6, the process involves two “if” judgments. The first “if” judgment checks if SOC₂ is greater than the maximum setting value and if MS equals 1. If both conditions are met, then MS is set to 0, indicating a change in power direction from forward to reverse. Otherwise, the power direction remains forward. The second “if” judgment examines if SOC₂ is less than the minimum setting value and if MS equals 0. If both conditions are satisfied, then MS is set to 1, signifying a change in power direction from reverse to forward. Otherwise, the power direction remains reverse. With this automatic power direction control method, the power direction of the DAB converter is regulated by the SOC of Port 2 SOC₂ to ensure bidirectional automatic power transmission within a specified range.

3.2. TAB Converter

Considering the similarities between the DAB converter and the TAB converter, this automatic power direction control method can indeed be applied to the TAB converter as well. However, as depicted in Figure 5, the TAB converter presents four different typical power directions determined by the lagging/leading relationship between φ₂ and φ₃. Additionally, scenarios such as Figure 5d illustrate instances where power flow occurs solely between port 2 and port 3, with no power transmission at port 1. Therefore, relying solely on two variables may not suffice for the automatic power direction control of the TAB converter.

To address this, three additional controlling variables are proposed for the automatic power direction control of the TAB converter: the voltage of battery 1 V₁, the SOC of port 3 SOC₃, and the SOC of port 2 SOC₂, as illustrated in Figure 7. Incorporating these additional variables ensures more comprehensive and accurate control of power direction in the TAB converter.

Among these controlling variables, the direction of power transmission is ultimately determined by the value of MS. The relationship between the direction of power transmission and MS is as follows.

• MS = 1, power transfers from port 1 to port 2 and port 3.
• MS = 2, power transfers from port 1 and port 2 to port 3.
• MS = 3, power transfers from port 2 and port 3 to port 1.
• MS = 4, power transfers from port 2 to port 3 and there is no power transmission at port 1.

According to Figure 7, the process involves five “if” judgments to accommodate the four different types of power flow in the TAB converter. Assume that the initial value of MS is equal to 1, which determines the power flow as shown in Figure 5a. Here is how the process unfolds. If V₁ is not within the acceptable voltage range between V_min and V_max, indicating an anomaly at port 1, then MS is set to 4, and the power flow is as depicted in Figure 5d. After the first “if” judgment, if SOC₂ exceeds the maximum setting for SOC₂, MS is set to 2, and the power flow is as shown in Figure 5b. Otherwise, MS remains equal to 1. In the third “if” judgment, if SOC₃ surpasses the maximum setting for SOC₃, MS is set to 3, and the power flow is as illustrated in Figure 5c. The fourth “if” judgment checks if SOC₃ falls below the minimum setting for SOC₃. If this condition is met, MS is changed to 2, and the power flow is as depicted in Figure 5b. Otherwise, MS remains at 3. During the final “if” judgment, if SOC₂ is lower than the minimum setting for SOC₂, MS is reverted to 1, and the power flow is as shown in Figure 5a. Similar to the automatic power direction control method of the DAB converter, this method for the TAB converter can automatically adjust the power direction to handle different situations based on the states of each port. This approach significantly reduces the need for manual intervention and ensures the safe and stable operation of the TAB converter.

4. Simulation Results

Two simulation models of the DAB converter and the TAB converter were developed in MATLAB/Simulink to evaluate the feasibility of the proposed automatic power direction control method. These simulations aim to validate the direction of power transmission at each port by monitoring the SOC, MS, and current waveforms at each port. Furthermore, by analyzing the timing of changes in waveform values, it is possible to confirm that the change in the direction of power transmission aligns with the proposed method. The simulation models are constructed according to the diagrams presented in Figure 1. Through these simulations, we aimed to assess the efficacy and reliability of the automatic power direction control method in ensuring the safe and stable operation of both the DAB and TAB converters.

4.1. DAB Converter

Considering that the DAB converter only contains two ports, it is necessary to validate the performance of the proposed automatic power direction control method in two directions between two ports of the DAB converter.

The system configurations of the DAB converter model are exhibited in

Table 1. Parameters of the DAB simulation model.

Parameter Name	Parameter Value
V1/V2 (Battery pack voltages)	180/200 V
SOC1/SOC2 (Initial SOC)	50%
N (Transformer turns ratio)	1
fsp (Switching Frequency)	50 kHz
L (Series inductance)	82 μH
C1/C2 (Series capacitance)	50 μF
SOC2max/SOC2min (Maximum and minimum SOC)	60/40%
Initial value of MS	1

, where the voltages of the two battery packs are set to 180 V and 200 V, respectively, with an initial SOC of 50%. Moreover, the transformer turns ratio is N = 1 and the initial value of MS is 1. The range of SOC₂ is set between 60% and 40% to obtain the simulation results more quickly.

In the Simulink simulation, we set the initial value of MS to 1, and the initial phase-shift angle φ lags the port 1 bridge waveform, as shown in Figure 2. Consequently, power flows from B₁ to B₂, resulting in an increase in SOC₂. Once SOC₂ reaches the maximum setting value, MS will be changed to 0, and φ₁ will lead the port 1 bridge waveform, following the automatic power direction control method depicted in Figure 6. During this phase, SOC₂ will decrease, and the power flow will reverse until SOC₂ reaches the minimum setting value. Subsequently, MS will revert to 1, and the power flow will return to being forward. Additionally, the current waveform of port 2 i₂ is displayed in Figure 8b. This simulation scenario allows us to observe the dynamic behavior of the system and validate the effectiveness of the automatic power direction control method.

In Figure 8a, the variation of SOC for B₂ throughout the entire simulation process is depicted. It can be observed that B₂ is initially charging, and its charge and discharge status alternate each time SOC₂ reaches the peak point, consistent with the analysis in the previous sections. Furthermore, in Figure 8b, during the steady state, the current of port 2 switches between positive and negative values, reflecting the power direction of the DAB converter. The timing of the forward and reverse directions aligns perfectly with the conclusions drawn from the SOC₂ waveform analysis. These observations validate the effectiveness of the automatic power direction control method and demonstrate its ability to regulate power flow in the DAB converter accurately and consistently.

4.2. TAB Converter

Different from the DAB converter, the TAB converter has three ports, but as a sustainable multiport converter for emergency energy supply, only four types of power flow need to be verified according to Figure 5. Among them, in the Figure 5d condition, port one needs to be simulated separately because of its failure, so the TAB converter simulation is divided into two parts.

4.2.1. When V₁ Is within Acceptable Voltage Range

The first simulation of the TAB converter is run when the voltage of B₁ is normal, which means that there is no power outage in the power station. MS will be shifted between 1, 2, 3, and 4 and will not be equal to 5. This simulation shows the automatic change of the TAB converter power transmission direction according to the SOC status and power requirements of the individual ports automatic power direction in the absence of a sudden power failure.

The system configurations of the TAB converter model when V₁ is within the acceptable voltage range are exhibited in

Table 2. Parameters of the TAB simulation model when V₁ is within the acceptable voltage range.

Parameter Name	Parameter Value
V1/V2/V3 (Port voltages)	180/180/180 V
SOC2/SOC3 (Initial SOC)	50%
N1:N2:N3 (Transformer turns ratio)	1:1:1
L1/L2/L3 (Series inductance)	300 μH
fsp (Switching Frequency)	50 kHz
Maximum2 (Maximum SOC of port 1)	60%
Minimum2 (Minimum SOC of port 1)	5%
Maximum3 (Maximum SOC of port 2)	91%
Minimum3 (Minimum SOC of port 2)	77%
Initial value of MS	1

. In Table 2, the voltages of three ports are all 180 V and the initial SOC of two battery packs are all 50%. Moreover, the transformer turns ratio N₁:N₂:N₃ is 1:1:1 and the initial value of MS is 1. To obtain simulations results more quickly, four peak values of SOC are set 60%, 5%, 91%, and 77% respectively.

In the TAB Simulink simulation, the initial value of MS is 1 and the initial values of the phase-shift angles φ₂ and φ₃ are lagging the port 1 bridge waveform and |φ₂| < |φ₃| shown in Figure 3. Therefore, power is transferred from B₁ to B₂ and B₃, which means that SOC₂ and SOC₃ is increasing. After SOC₂ reaches the maximum setting value, MS will be changed into 2, φ₂ will lead the port 1 bridge waveform, and φ₃ will still lag the port 1 bridge waveform with |φ₂| < |φ₃| according to the automatic direction control method shown in Figure 7. During this period, SOC₂ decreases while SOC₃ increases until the SOC₃ arrives at the maximum setting value. After that, MS will be changed into 3 and φ₃ will also lead the port 1 bridge waveform with φ₂ < φ₃ < 0. During MS = 3, SOC₂ and SOC₃ decrease at the same time until the SOC₃ arrives at the minimum setting value. Then, MS will be changed back into 2. In this period, SOC₃ increases with SOC₂ decreasing until SOC₂ reaches the minimum setting value. For this reason, MS will be back to 1 and φ₂ will be back to lag the port 1 bridge waveform with |φ₂| < |φ₃| so that SOC₂ can increase. Additionally, the waveform of MS is exhibited in Figure 9.

Figure 10 presents the variation of SOC for the B₂ and B₃ in the whole simulation process, from which it can be observed that SOC₂ and SOC₃ changes correspond to the proposed analysis, and power direction changes correspond to the proposed automatic power direction control method, which is consistent with the analysis in the previous sections. In this whole TAB simulation process, a completed automatic direction control loop is finished and a whole loop can be divided into four modes, which are 0 − t₁, t₁ − t₂, t₂ − t₃, and t₃ − t₄. In Figure 9, MS can also be different values in these four modes, and the period of each mode is consistent with that in Figure 10. Through Figure 11, during the steady state, the current waveforms of port 1 i₁, port 2 i₂, and port 3 i₃ also switch between positive and negative values, which also shows the power direction of the TAB converter. When the current is negative, this port is the input port. Conversely, when the current is positive, this port is the output port. Comparing Figure 10 with Figure 11, the power direction obtained from the SOC status of each port is the same as the power direction obtained from the current value of each port.

4.2.2. When V₁ Is Not within Acceptable Voltage Range

In this scenario, port 1 exhibits an abnormal voltage, indicating an issue that prevents it from effectively carrying out power transmission duties in the TAB converter. Given that port 1 represents the main power grid, an abnormal voltage suggests that the main power grid is experiencing significant disruptions and is unable to function normally. Prior to resolving the issue with port 1, it is imperative that the TAB converter operates similarly to a DAB converter. This entails transferring power exclusively between port 2 and port 3 while ensuring that no power transmission occurs at port 1. This temporary operational mode allows the TAB converter to maintain critical power flow functionalities within the system until the problem with port 1 is rectified.

The system configurations of the TAB converter model when V₁ is not within the acceptable voltage range are exhibited in

Table 3. Parameters of the TAB simulation model when V₁ is not within the acceptable voltage range.

Parameter Name	Parameter Value
V1/V2/V3 (Port voltages)	100/180/180 V
SOC2/SOC3 (Initial state of charge)	50%
N1:N2:N3 (Transformer turns ratio)	1:1:1
L1/L2/L3 (Series inductance)	300 μH
fsp (Switching Frequency)	50 kHz
Initial value of MS	1

. In Table 2, the voltages of battery packs of port 2 and port 3 are both 180 V, with the initial SOC of them all 50%, while the voltage of port 1 is 100 V. Moreover, the transformer turns ratio N₁:N₂:N₃ is 1:1:1 and the initial value of MS is 1.

Although the initial value of MS is 1, because of the abnormal value of V₁, MS is rapidly changed into 5. This causes φ₂ to lead the port 1 bridge waveform and φ₃ to lag the port 1 bridge waveform at the same time with |φ₂| = |φ₃|. With this relationship between φ₂ and φ₃, power only transfers from B₂ to B₃ and there is no power transmission at B₁. This mode will last until the problem of port 1 is fixed. After that, MS will be back to 1 so that the TAB converter can work in its own role again and continue the power transmission loop shown in Figure 12. Additionally, the waveform of MS is exhibited in Figure 13.

Figure 12 presents the variation of SOC for B₂ and B₃ in the whole simulation process, from which it can be observed that SOC₂ and SOC₃ changes correspond to the proposed analysis, and power direction is stable from port 2 to port 3, corresponding to the proposed automatic power direction control method, which is consistent with the analysis in the previous sections. In Figure 13, MS is also constant at 1, which corresponds to the SOC waveforms in Figure 12. In this whole TAB simulation process, the TAB converter is working as a DAB converter considering the abnormal performance of port 1. Through Figure 14, during the steady state, the current of port 1 is zero, and the currents of port 2 and port 3 are constant positive and negative values, respectively, which also shows that the power direction is only from B₂ to B₃. Comparing Figure 12 with Figure 14, the power direction obtained from the SOC status of each port is the same as the power direction obtained from the current value of each port.

4.3. Discussion

4.3.1. DAB Converter

Based on the two panels in Figure 8, particularly Figure 8b, it is evident that the duration of the power direction change is relatively short. The transition from a positive steady-state value to a negative steady-state value for i₂ takes only 3 s, which ensures the stability of the system because the interval between two steady states of the port is short. This demonstrates that the proposed automatic power direction control method can swiftly and effectively adjust the power direction without significantly disrupting the normal operation of the DAB converter. Thus, this approach saves manual supervision time and ensures the seamless functioning of the converter for emergency energy supply. Compared with other automatic power direction control methods [30,31] used for batteries or PV energy, this proposed method is easier and more flexible to use for emergency energy supply according to the different needs of clients.

4.3.2. TAB Converter

Based on the three panels in Figure 11, it is apparent that the duration of the power direction change at each port is also relatively short, similar to the behavior observed in the DAB converter. The transition from a positive steady-state value to a negative steady-state value for the current at each port takes only 3 s, which ensures the stability of the system because the interval between two steady states of each port is short. This confirms that the proposed automatic power direction control method can swiftly and effectively adjust the power direction in the TAB converter without significantly disrupting its normal operation. Consequently, this approach saves manual operation time while ensuring the smooth functioning of the converter. Similar to the automatic power direction control method for the DAB converter, the automatic power direction control method for the TAB converter can also save manual supervision time and ensure the seamless functioning of the converter for emergency energy supply.

In the second simulation of the TAB converter, it is obvious that there will be no power transfer at port 1, which means that when the main power grid is paralyzed, the connection between the main power grid and other equipment will be cut off immediately. In addition, the disconnection of the mains grid during this process does not require human intervention, and the client and other power supplies can still operate normally in the system, ensuring system stability and sustainability.

5. Conclusions

To realize the automatic and rapid power direction change of DAB/TAB converters in emergency energy supply for sustainability, this paper introduces an automatic power direction control method for DAB/TAB converters, designed to facilitate energy balancing and power direction automation within an energy router system for emergency energy supply to realize a sustainable power grid. This proposed method ensures the energy management among various electric power sources, which increases the efficiency of power generation and usage. This can be a great help for sustainability. In addition, through this method, more clients can store power and decrease the reliability of the main power grid. Even when the emergency power outages occur, clients can still work and live with stored power and have no need to temporary call for power at a long distance which would be more costly in terms of human, financial, and environmental resources than a sustainable program. Power conversion is accomplished through phase-shift PWM, while direction control is managed by the proposed automatic power direction control method. The power flow direction across the three ports is regulated by the lead/lag states of the converter switches and the SOC of the ES port and EEPS port. In addition, in practice, the proposed method offers a swift directional control solution and leads to significant cost savings by reducing the need for manual monitoring and regulation of the energy router’s status when an emergency occurs. Consequently, emergencies within the grid can be promptly addressed and resolved. To validate the feasibility of the automatic power direction control method in DAB/TAB converters, several simulation models implemented in MATLAB/Simulink have been employed. Furthermore, the direction change is solely based on the SOC status of the individual ports of the DAB/TAB converter, enabling customers to make informed decisions about their power consumption plans depending on various situations. This approach ensures a more flexible and efficient utilization of energy resources, reducing reliance on the main power grid for power supply. In addition, with more energy routers containing this kind of DAB/TAB converters connecting together to form an emergency energy power system, peer-to-peer trading can be realized for sustainable power grids.