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Research on Storage Technology of Super Capacitor
Jul 13, 2018

Supercapacitor is a research hotspot of new energy devices in recent years. It is different from conventional capacitors, and its capacity can reach the Faraday scale or even tens of thousands of Farah, and can not be punctured when the voltage of the electrode terminal exceeds the rated voltage. As an ideal new energy device, its specific power and specific energy between the conventional capacitor and rechargeable batteries, in many applications to make up for the conventional energy storage devices unilateral defects. In addition, it also has a small internal resistance, high charge and discharge efficiency (90% to 95%), long cycle life (tens of thousands to 100,000 times), low temperature performance, pollution-free and other unique advantages [1]. These advantages make the supercapacitor very suitable for short-term high-power output applications.
The pursuit of stable operation, reliable power supply system in recent years also introduced a super capacitor as a new energy storage equipment, so that the power system voltage fluctuations, short-term power supply interruption, such as rapid charge and discharge, so as to ensure the safety and reliability of the system Sex. At present, the application of super capacitor in the power system has been set off a boom, practical super capacitor charge and discharge control is not yet mature. In this paper, the two-way DC-DC converter is applied to the charging and discharging process of the supercapacitor. The control strategy of the two-way control model and the control strategy of the PWM are used to install the PI and PID compensation links respectively for the different charge and discharge conditions, To meet the vast majority of the need for fast charge and discharge of super capacitors occasions.
1 bidirectional DC-DC converter model
The bi-directional DC-DC converter is a cyclical switching control between the DC bus and the supercapacitor. Its purpose is to change the voltage supplied to the supercapacitor and actually operate as a voltage regulation system.
In order to meet the needs of the use of the converter should be used to reverse the current two-quadrant converter, when the capacitor discharge, DC-DC converter in a state of boost, while the capacitor charging, current feedback, DC-DC conversion The device is in a buck state.
System design converter can be half-bridge, by the control transistor IGBT: S1 and S2, and freewheeling diodes D1 and D2, to protect the super capacitor diode D3 and D4, inductance L composition, see Figure 1.
When S1 is in operation, S2 is turned off, S1 and D2 form step-down chopper circuit, in which case bi-directional DC-DC converter is in Buck state. In a switching period Ts, when S1 is closed (ie 0 <t <ton, where t is the time variable), the diode D2 is subjected to a reverse voltage, the current is charged backwards to the capacitor, the energy is charged directly from the DC bus to the supercapacitor, At the same time the inductance L stores part of the energy; when S1 is off (ie ton <t <Ts), the diode D2 is subjected to forward bias for the inductor L to release energy to form the path and charge the capacitor. When the circuit is in buck buck state, the relationship between the two ends of the voltage:

2 Two-way DC-DC converter control model
The system measures the voltage of the DC bus and the supercapacitor to determine whether the bidirectional DC-DC converter is operating in the Boost state or the Buck state. When the DC bus voltage increases rapidly, adjust the bi-directional DC-DC converter to work in the Buck state, the system energy transfer to the super capacitor. Prior to this, the system control must make the capacitor voltage in a low state. On the contrary, when the DC bus voltage is rapidly reduced, it will control the two-way DC-DC converter into Boost state, the super capacitor stored energy released to the system. The voltage across the bi-directional DC-DC converter is adjusted by controlling the duty cycle of S.
Control of 2.1Buck Type Bidirectional DC-DC Converter

2.2Boost type bidirectional DC-DC converter control
In Figure 2 shows the circuit Vi side regulator, the Boost type of DC chopper. Set the duty cycle of S2 in Figure 2 to D, then the duty cycle of D1 (ie, S2 break time duty cycle) is (1-D).

2.3 Control block diagram of bi-directional DC-DC converter
By the knowledge of the automatic control theory, for the bi-directional DC-DC converter in the super capacitor charging process in the Buck working state, through the proportional integral (PI) compensation link correction can make the system has a certain phase angle margin, so that the system closed loop stable. PI transfer function model:

3 simulation verification
3.1 Constant current charging simulation
In the PSCAD / EMTDC power system simulation software, a constant current charging simulation model of super capacitor with current as control is established. The simulation parameters are shown in Table 1. The simulation figures are shown in Figure 6 and Figure 7.

As can be seen from Fig. 6, the supercapacitor is charged with a constant high current, and its voltage rises approximately linearly until saturation, and this voltage rises significantly over time, indicating that the supercapacitor can absorb high energy in a short time. As can be seen from Fig. 7, a relatively constant charging current of only 5% pulsation can be obtained with the PI compensation link, while the system is stable when the system is open.
Single current charging current pulse, consider the use of multiple circuits to reduce the charge current ripple, will continue to charge the super capacitor until saturation. In this paper, double charging, for example, the two bi-directional DC-DC converter in parallel to charge the super capacitor.
In the PSCAD / EMTDC power system simulation software to establish its simulation model, the simulation parameters with the single-channel constant current charging parameters, simulation. The simulation diagram is shown in Figure 8.

As can be seen from Fig. 8, a relatively constant charging current with a decrease in the current ripple with respect to Fig. 7 can be obtained. It can be seen that a double circuit can reduce the charge current ripple, which can be extended to multiple circuits.
3.2 Constant pressure discharge simulation
In the PSCAD / EMTDC power system simulation software, a constant voltage discharge simulation model of super capacitor with voltage as control is established. The simulation parameters are shown in Table 2. The simulation diagram is shown in Figure 9.

As can be seen from Figure 9, in the super capacitor discharge using PID compensation, DC bus voltage stability in the reference value. Therefore, the installation of PID compensation link can make the system more open loop stability, to meet the stability requirements of the power system.
4 Summary
In this paper, the two-way DC-DC converter to the super capacitor charge and discharge, according to the different working conditions of the converter to choose a different control, driven by PWM control TGBT, fast and stable. Using PSCAD / EMTDC power system simulation software to super-capacitor single-circuit charge and discharge simulation, comprehensive analysis of super-capacitor charge and discharge control has the following characteristics:
(1) PI and PID compensation can be used to make the super capacitor charge and discharge process is stable, to meet the stability requirements of the power system;
(2) in the other parameters of the circuit under the premise of the corresponding relationship between the choice of inductance L, you can control the current pulse is set value;
(3) in order to ensure the same parameters under the premise of the circuit, in order to reduce the charge current ripple, can be further use multiple charging mode. The simulation results show that the proposed method is feasible.
references:
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[3] Zhang Fanghua, Zhu Chenghua, Yan Yangguang. Control model of bidirectional DC-DC converter [J]. Proceeding of the CSEE, 2005,25 (11): 46-49.
(1) - Circuit Principle and Control Strategy [J]. Journal of Electrical Engineering, 2006,21 (10): 31-37 (in Chinese with English abstract) [J]. Journal of Electrical Engineering, 2006,21 (10): 31-37 The
[5] JANG S, LEE T, LEE W, et al. I-directional DC-DC converter for fuel cell generation system [J]. IEEE, 2004 (6): 4722-4728.
[6] JAN L, PAVOL B, PETR B, et al. Bi-directional DC-DC converters for supercapacitor based energy buffer for electrical gensets [J] .IEEE, 2007,9 (2/5): 1-10.
[7] SHIGENORI I, HIROFUMI A.A bi-directional DC-DC converter for an energy storage system [J]. IEEE, 2007,22 (2): 761-767.

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