The traditional urban rail vehicle power supply system is complex and affects the beauty of the city. The contactless modern urban rail transit power supply method has attracted widespread attention.
As a new energy tram, fuel cell hybrid trams use proton exchange membrane fuel cell power generation to provide energy for the system. It can operate without a grid and has no exhaust emissions during operation, independent of the catenary power supply system. It has broad development prospects, because it eliminates the need for contact nets and other devices, shortens the construction period of the project, and reduces the investment cost of the project at the same time. However, the transient response speed of the fuel cell is slow, and the vehicle cannot accurately provide the required energy during rapid start, emergency acceleration, or climbing. It is usually necessary to add an energy storage system to improve vehicle performance. Therefore, designing a reasonable energy management method to make the energy distribution reasonably is the key to ensuring the system is in stable and efficient operation.
This paper proposes an instantaneous equivalent minimum hydrogen consumption method for a fuel cell/super capacitor hybrid power system. This method converts the electrical energy of the ultra super capacitor into hydrogen energy equivalently to achieve optimal control of the overall hydrogen consumption of the system. As a real-time optimization control method, the output performance of the fuel cell system is controlled by real-time calculation of the optimal output power of the ultra super capacitor under the current state of charge (SOC) and combined with the required power. By building the RT-LAB semi-physical hardware-in-the-loop platform, it is verified that the proposed method can significantly optimize the improvement in hydrogen consumption and super capacitor SOC, and effectively reduce the hydrogen storage cost and operating costs of fuel cell/super capacitor hybrid trams.
⑴ Super capacitor hybrid power system
The fuel cell/super capacitor hybrid power system for urban mass transit studied in this paper is mainly composed of fuel cell, one-way DC/DC converter, supercapacitor ultracapacitor, two-way DC/DC converter, control unit, traction inverter, motor and braking resistors. The fuel cell is connected to the DC bus through a DC/DC converter, and the super capacitor is connected to the system DC bus through a bidirectional DC/DC converter. The one-way DC/DC converter in the entire locomotive system is a key component to realize the energy management strategy of the locomotive, and it is mainly used to control the power output of the fuel cell.
Figure 1 Hybrid power system topology
⑵ Modeling of instantaneous equivalent hydrogen consumption
The tram is mainly powered by a fuel cell system and an low esr super capacitor system. The total instantaneous hydrogen consumption C of the system is composed of the instantaneous hydrogen consumption of the fuel cell and the instantaneous hydrogen consumption of the CFC super capacitor Cuc:
According to the experimentally measured fuel cell stack efficiency data, the instantaneous hydrogen consumption rate curve of the fuel cell system is obtained, and the instantaneous hydrogen consumption of the super capacitor is calculated:
Aiming at the efficiency model of the super capacitor system, this article equivalents the internal resistance of the super capacitor to a constant due to its small internal resistance, so the charge and discharge efficiency model is as follows:
In the formula, Puc is the charge and discharge power of the super capacitor, R is the internal resistance of the super capacitor, and Uocv is the terminal voltage of the super capacitor. The resulting efficiency surface of the ultracapacitor is shown in Figure 2.
Figure 2 Super capacitor charging and discharging efficiency surface
⑶ Solving optimization problems
The instantaneous equivalent minimum hydrogen consumption method aims at the minimum instantaneous hydrogen consumption of the system. The algorithm flow chart is shown in Figure 3. The optimization problem is as follows:
In the formula, PFCmax is the maximum output power of the fuel cell, ∆PFC is the fluctuation of the fuel cell, and Iucmax and Iucmin respectively represent the maximum limit of the charge and discharge of the super capacitor. By solving the above optimization problem, the analytical solution of the hydrogen consumption cost function is obtained as shown in equation (8):
Figure 3 Algorithm flow chart
⑷ Experimental verification and conclusion
According to the structural parameters of the fuel cell hybrid tram system, through the RT-LAB semi-physical system, the energy management controller is designed to connect with the RT-LAB target machine, and the hardware-in-the-loop test system is built to realize the real-time simulation operation of the system. Monitor the entire running process of the system through the Windows upper computer. The experiment platform is composed of DSP TMS28335 controller, OP5600, and host computer monitoring computer. Among them, the fuel cell system, super capacitor system, DC/DC converter, braking resistor, and load model are used to generate C code, which is processed by the field programmable gate array (FPGA) in the real-time digital simulator, and drives the analog I/ The O board transmits the corresponding signal to the DSP controller for sampling. After processing by the DSP control algorithm, the control signal is sent to the RT-LAB. The system structure diagram of the experimental platform built is shown in Figure 4.
Figure 4 System structure diagram of HIL experiment platform
In order to verify the effectiveness of the instantaneous equivalent minimum hydrogen consumption method proposed in this paper, different strategies were programmed using the CCS compiler, based on the RT-LAB semi-physical hardware-in-the-loop platform, based on the low fuel cell/super capacitor hybrid power measured operating condition data of floor trams compares the method in this paper with the power following method. The power distribution results and hydrogen consumption of each power source are shown in Figure 5 and Figure 6.
Table 1 Performance comparison of different control methods
As shown in Table 1, under the same test conditions, the total hydrogen consumption of the method in this paper is 743.1g, and the total hydrogen consumption of the power following method is 796.4g. Compared with the power following method, the hydrogen consumption of this method is reduced by 6.69%. It can be seen that the method in this paper has obvious advantages in terms of total hydrogen consumption of the system and super capacitor SOC.
Figure 5 Instantaneous equivalent minimum hydrogen consumption power allocation results
Figure 6 System total hydrogen consumption curve
Aiming at the practical application of hydrogen energy in the field of rail transit, this paper proposes an instantaneous equivalent minimum hydrogen consumption method suitable for fuel cell/super capacitor hybrid power systems. According to the equivalent hydrogen consumption theory, the equivalent of hydrogen energy and electric energy conversion is realized , by building the RT-LAB semi-physical hardware-in-the-loop platform, the effective distribution of the output power of each power source based on the measured operating conditions of the tram has been completed. Through comparative analysis, it is proved that the hydrogen consumption of the method in this paper is compared with the power follow method reduces by 6.69%, and the super capacitor SOC changes in a small range, which effectively reduces the hydrogen storage cost and operating cost of the fuel cell/super capacitor hybrid tram.
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