I. Electric Double-Layer Capacitors
(I) Working principle of double-layer capacitors
Double-layer capacitors are a novel type of capacitor based on the interfacial double-layer theory proposed by German physicist Helmholtz. It is well known that when a metal electrode is inserted into an electrolyte solution, excess charges of opposite signs appear on the surface of the electrode and on either side of the liquid surface, creating a potential difference between the phases. If two electrodes are simultaneously inserted into the electrolyte and a voltage less than the decomposition voltage of the electrolyte solution is applied between them, the positive and negative ions in the electrolyte will rapidly move towards the electrodes under the influence of the electric field, forming a dense charge layer, or double layer, on the surfaces of the two electrodes. This double layer is similar to the polarization charge generated by the dielectric in a conventional capacitor under an electric field, thus producing a capacitance effect. The dense double layer approximates a parallel-plate capacitor; however, because the spacing between the dense charge layers is much smaller than the distance between charge layers in a conventional capacitor, it has a larger capacitance than a conventional capacitor.
Compared to aluminum electrolytic capacitors, double-layer capacitors have a higher internal resistance, allowing them to be charged directly without a load resistor. In the event of overvoltage charging, the double-layer capacitor will open-circuit without damage, a characteristic different from the overvoltage breakdown of aluminum electrolytic capacitors. Furthermore, compared to rechargeable batteries, double-layer capacitors can be charged without current limitations and can be charged more than 10^6 times. Therefore, double-layer capacitors possess characteristics of both capacitors and batteries, making them a novel and special type of component that falls between the two.
The basic principle is as follows: when the electrode is charged, the surface charge of the electrode in an ideally polarized state will attract oppositely charged ions from the surrounding electrolyte solution, causing these ions to attach to the electrode surface and form a double charge layer, thus constituting a double-layer capacitor. Because the distance between the two charge layers is very small (generally less than 0.5 nm), and with the use of a special electrode structure, the electrode surface area is increased by tens of thousands of times, thereby generating an extremely large capacitance.
(II) Characteristics of Double-Layer Capacitors
(1) High power density
It can reach 102-104 W/kg, which is far higher than the power density level of batteries.
(2) Long cycle life
After 500,000 to 1,000,000 high-speed deep charge-discharge cycles lasting only a few seconds, the characteristics of the double-layer capacitor change very little, with capacitance and internal resistance decreasing by only 10% to 20%.
(3) Operating temperature range
Because the adsorption and desorption rates of ions in a double-layer capacitor do not change significantly at low temperatures, its capacity change is much smaller than that of a battery. Commercial double-layer capacitors can operate in a temperature range of -40℃ to +80℃.
II. Differences between intelligent capacitors and ordinary capacitors
Compared with traditional capacitors, smart capacitors have the following advantages:
1. Modular Structure Intelligent Capacitors feature a modular structure, resulting in small size, simple on-site wiring, and convenient maintenance. The reactive power compensation system can be expanded simply by increasing the number of modules.
2. High-quality capacitors use self-healing low-voltage compensation capacitors with built-in temperature sensors to reflect the degree of internal heating and achieve over-temperature protection.
3. Embedded Switching Module: The intelligent capacitor incorporates a built-in switching module. This module consists of a thyristor, a magnetic latching relay, a zero-crossing trigger circuit, and a thyristor protection circuit, achieving "zero-switching" of the capacitor and ensuring no inrush current or operational overvoltage during the switching process. The switching module features a fast response time and can be operated frequently.
4. Comprehensive protection design: Intelligent capacitors have functions such as power outage protection, short circuit protection, voltage phase loss protection, and capacitor over-temperature protection, effectively ensuring capacitor safety and extending equipment life.
5. Advanced control technology: The control physical quantity is reactive power. Reactive power flow prediction and delayed multi-point sampling technology are used to ensure oscillation-free switching. Under heavy load, reactive power is fully compensated.
6. The anti-switching oscillation technology adopts a unique design principle to prevent uncompensated or overcompensated situations caused by controller crashes, and to prevent capacitor switching oscillations.
7. Automatic reactive power compensation: Intelligent capacitors automatically switch on and off based on the magnitude of reactive power in the load, dynamically compensating for reactive power and improving power quality. Intelligent capacitors can be used individually or in series.
8. The user-friendly interface displays equipment operating parameters such as current, voltage, and reactive power. It also displays switching status, composite switch module fault status, and communication status. Furthermore, it facilitates switching between debugging and operating states, as well as manual and automatic operation functions.
