Voltage Equalization Control Method for ships and warships Supercapacitor Charging

A DC-DC converter voltage balancing controller with added voltage anti overcharging protection device is proposed to address the voltage balancing control problem of supercapacitors used in Ships and warships. A simulation model is built in the PSIM environment, and through simulation experiments, this balancing controller can improve the voltage balancing rate, reduce space occupation, and prevent overcharging phenomena. The simulation results show that the improved DC-DC voltage balance controller has a detailed improvement effect on the charging voltage balance of ship supercapacitors.


Introduction
Ships and warships require energy storage systems to improve energy efficiency and management, especially in the face of increasing pulse loads.Traditional batteries have a slow response time and cannot meet instantaneous energy demands, making supercapacitors a powerful supplement to ship energy storage systems.However, due to the large voltage differences among supercapacitor cells, it is crucial to control voltage balancing to ensure the stability and safety of Ships and warships systems.
The voltage of supercapacitors is relatively low, and most of them have a voltage of only 1-3V.In high-power applications used on large ships, supercapacitors are usually used in series.Due to differences in the manufacturing process and materials of supercapacitors [1] , the initial voltage and current parameters inside the capacitor are different.As the parameter discreteness increases, it has a serious impact on voltage sharing.During design, efforts are made to maintain voltage consistency and complete the charging of supercapacitors at the same time [2-3] .
Scholars have conducted in-depth research on voltage balancing for supercapacitors.Literature [4][5] divides voltage balancing circuits into two categories: energy consumption type and feedback type.Literature [6] proposed a voltage balancing method using a voltage regulator, but it has problems with energy loss, among others.Literature [7] focused on the forward and reverse converter method for voltage balancing, but the control is complex.Meanwhile, literature [8][9] summarizes various methods for feedback voltage balancing.Feedback methods have faster balancing rates and less energy loss, making them the main direction of research.

Principle of DC/DC converter
The DC/DC converter method utilizes the DC variation ability of the converter to transfer energy from high voltage capacitors to low voltage capacitors to achieve energy complementarity.The commonly used bidirectional Buck/Boost converter has a simple structure and high conversion efficiency.This article conducts research based on the bidirectional Buck/Boost DC converter shown in Figure 1.The same applies in reverse, depending on Q 1 and reverse diode D 2 , and independent of Q 2 .Therefore, when using a traditional bidirectional Buck/Boost DC converter for charging and voltage equalization, there may be issues such as redundant switches, increased costs, and increased control difficulty.

Improved supercapacitor voltage equalization circuit
Based on the above analysis, on the basis of traditional DC-DC converters, diodes are used to replace some MOSFETS, and the improved DC-DC converter method equalization topology is shown in Figure 3.In Figure 3, the number of MOSFET is reduced by half, which is more cost-effective than traditional methods.The use of a converter for unidirectional cyclic energy transfer can also gradually equalize the voltage at both ends of each individual capacitor.However, the isolation transformer installed at the head and end will take up more space.If the transformer is reduced, the capacitors C n and C 1 will be directly connected in series, which will cause a short circuit in the system.
Therefore, based on the topology structure in Figure 3, a secondary optimization is carried out by adding MOSFET switch transistor C to the circuit where the capacitor Q is located, forming a loop to avoid the occurrence of short circuits, as shown in Figure 4.In Figure 4, the difference between the capacitor bank and the one with isolation transformer is that in Figure 3, the capacitance C is transferred to the capacitance C ,while in Figure 4, the capacitanceC is transferred to the series capacitance C ~C module.

4.Modeling and simulation
Consider using four Maxwell 3V/3KF supercapacitors for simulation.The charging methods for supercapacitors include constant current charging, constant voltage charging, and combination charging [10-12] .Fast response to constant current charging is the best choice for fast charging of ships.After the supercapacitor is connected in series and parallel, the current of the DC power supply is 1.5KA, the internal resistance is 20Ω, the voltage logic comparator value is 3V, and the switching frequency of MOSFET tubes Q 1 ~Q4 is 20KHz.It should be noted that when C n transfers energy to C 1 ~Cn-1 , the output end of the converter is connected at both ends of C 1 to C n-1 , so it is necessary to change the duty cycle of the last switch tube at both ends to change [13] .The formula for selecting the duty cycle D n is： n 1 (1) The duty cycle of Q n calculated by the formula is 0.8.Display the initial voltage and duty cycle of each individual capacitor in Table 1.
Table 1 Voltage and corresponding duty cycle of supercapacitors Supercapacitor monomer Initial Voltage/V Duty cycle  The working principle of its backup protection: The four voltage logic comparators on the far right are used as backup protection, and each comparator is preset with a rated voltage value of 3V for the supercapacitor.Set up four voltage detectors for it.Once any one of V 1 ~V4 exceeds the rated voltage, the comparator will turn off the main switch Q 5 through logic and gate control.Cut off the DC power supply to prevent overcharging of the supercapacitor.The simulation results are shown in Figure 6, and the voltage balance state is reached at approximately 0.86 seconds.To verify the effect of the protection device, we set the voltage values of C 1 -C 4 to 2.95V, 2.85V, 2.75V, and 1V, and observed the simulation results,as shown in Figure 7.

Figure 7 Backup Protection Verification
According to the protection verification results shown in Figure 7, it can be seen that when the voltage value is only 0.05V away from the rated voltage, the isolated transformerless circuit with the added protective device transitions rapidly from high voltage to the low voltage capacitor, effectively preventing overcharging of the supercapacitor after achieving voltage balance between them.

5.Conclusion
This article proposes an improved equalization circuit that performs secondary improvements on the traditional DC-DC converter equalization method.The first improvement reduces the number of switch devices required for the circuit by half compared to the traditional equalization circuit.The second improvement adopts a circuit structure without an isolated transformer to reduce the occupation of space on ships and adds a protective device to avoid overcharging of the supercapacitor.The feasibility of the improvement is verified through PSIM simulation, and therefore, this circuit has practical and theoretical value for the equalization of supercapacitor charging in Ships and warships.

Figure 1
Figure 1 Bidirectional Buck/Boost DC Converter The bidirectional Buck/Boost DC converter has two main operating modes for bidirectional energy transfer.As shown in Figure 2 (a) and (b), the variation of C 1 voltage when it is greater than C 2 voltage is divided into two stages.The first stage is Q 1 conduction, consisting of Q 1 and L f forming a current circuit, with L f in the energy storage state; In the second stage, Q 1 is turned off, and the current loop is composed of L f and the reverse freewheeling diode D 2 , with the inductance L f releasing the energy state.This causes the C 1 terminal voltage to decrease and the C 2 terminal voltage to increase, completing the energy transfer from the high voltage terminal to the low voltage terminal.

Figure 2 (
Figure 2 (a) Phase 1 Figure 2 (b) Phase 2From Figure2(a)(b), Energy flows from high voltage capacitor C 1 to low voltage capacitor C 2 , depending on the switch Q 2 and reverse diode D 1 , and independent of Q 1 .The same applies in reverse, depending on Q 1 and reverse diode D 2 , and independent of Q 2 .Therefore, when using a traditional bidirectional Buck/Boost DC converter for charging and voltage equalization, there may be issues such as redundant switches, increased costs, and increased control difficulty.

Figure 3
Figure 3 Equalization topology of DC-DC converter with isolation transformer

Figure 4
Figure 4 Voltage Equalizing Circuit without isolation transformer backup protection concept added in reference[14] includes a protection device in Figure4, and the overall structure diagram is shown in Figure5.

Figure 5
Figure 5 Simulation Diagram of PSIM Circuit without isolation transformer

Figure 6
Figure 6 Simulation results of optimized charging voltage sharing for supercapacitors