Do you know how the current limiter works?

The current limiter, as its name implies, limits the current flow. When the current exceeds the limit current, the current limiter is automatically disconnected to achieve the purpose of power-off; and when the current is less than the limit current of the current limiter, the current limiter will automatically open. At present, there are many types of current limiters, including superconducting current limiters, magnetic element current limiters, PTC resistance current limiters, and solid current limiters.

With the development of economy, the demand for electric power is increasing day by day, and the scale of the electric power system is gradually expanding. Therefore, various electrical equipments in the electric network are facing more stringent requirements. Limiting the short-circuit current in the power system becomes a problem that needs to be solved urgently. However, different types of current limiters do not work in the same way.

The solid-state current limiter is a new type of short-circuit current limiter developed using power electronics technology. It has the advantages of fast operation speed, multiple allowable actions and easy control. With the rapid improvement of power electronics technology, flexible AC transmission systems (FACTS) are becoming increasingly sophisticated. As a member of FACTS components, solid-state short-circuit current limiters have begun to receive more and more attention.

When the traditional fluorescent lighting power supply is turned on, the fluorescent lamp is in a high-impedance state. The power supply is applied to both ends of the starter through the ballast and the filament. The helium in the helium bubbles is ionized to generate a glow discharge. The heat of the discharge makes the bimetal. The sheet is heated and deformed to contact the contacts. The current in the circuit starts heating the two cathodes of the fluorescent lamp, emitting a large amount of electrons, increasing the mercury vapor in the lamp, and completing the preheating of the fluorescent tubes.

At the same time, due to the closure of the two electrodes in the helium bubble, the glow discharge stops, the temperature in the tube decreases, the bimetal cools, and the bimetal automatically resets and disconnects after 1-3 seconds. When the two poles are disconnected, the circuit current is suddenly interrupted, and a very high induced voltage (V=L*di/dt) of about 1 ms will be generated across the ballast. According to the type of lamp, this voltage is In the 600V-1500V range, the induced voltage is superimposed on the power supply voltage and applied to both ends of the fluorescent tube. Under this high voltage, a large amount of electrons emitted when the filament is warmed up collide with argon molecules in the tube at a high speed, rapidly ionizing it. Argon ionizes and generates heat, the heat causes mercury in the tube to produce steam, and then the mercury vapor is also ionized and causes an arc discharge, emitting ultraviolet rays with a wavelength of λ=253.7 nm. The fluorescent powder on the inner wall of the lamp radiates intense visible light under the excitation of ultraviolet rays. Fluorescent tubes enter normal operating conditions.

After the lamp is ignited, the current in the circuit will generate a large voltage drop on the ballast, and the voltage across the lamp rapidly decreases, thereby causing the starter parallel to the lamp to stop working due to the terminal voltage being too low. At this time, the ballast mainly acts to limit the lamp current, and at the same time, a phase difference of 55°-65° is generated between the lamp current and the power supply voltage, so as to maintain the secondary start of the lamp and enable the lamp to work more stably. If the starter did not start the lamp successfully, the above process will be repeated until the lamp starts.

In parallel with the xenon lamp, there is a paper capacitor with a few thousand picofarads, which can eliminate the spark generated when the contacts are separated, so as not to burn the contacts. At the same time, it can also reduce high-frequency interference to nearby radio equipment.

From the above summary, we can see that the current limiter basically fulfills the requirements of the power system for the short-circuit current limiter, can greatly limit the short-circuit current, and the reaction speed is fast, and it will not excite the oscillation in the power grid; when the automatic reclosing, It can also avoid the impact of large current impact caused by permanent short circuit on the power grid, and improve the stability of the system operation.