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Internally fused capacitor bank
Function of power capacitor bank
1.Deliver capacitive reactive power compensation
2.power factor correction
Structure of capacitor bank (Understanding the working principle of internal fuse)
The large power capacitor is made of many capacitor elements which connect in series and parallel. The capacitor elements are made of aluminum foil and insulation medium like polypropylene film. Multiple capacitor elements are connected in series to form a parallel group, with the aim of increasing the capacity; By connecting the capacitor elements in parallel to increase the voltage withstand ability. The internal fuse will be installed on the lead wire of each elements, connect itself with capacitor element in series.
The working principle of internal fuse in capacitor bank (Step by step)
1.Firstly, Working in normal
When all the capacitor elements undertake the normal rated voltage, the current goes through all the circuit in normal,the fuse will be safe and intact.
2.Secondly, When some elements happen to breakthrough
The breakthrough happens because of aging of dielectric medium, over-voltage or some special circumstances, then the insulation of the capacitor element damaged, further more the breakthrough happened. It is like short-circuited between the electrode of the element. Therefore, the breakdown element become the weak spot where the current will flood into in priority which is the nature of the electric diagram. The current comes from the stored electric of other element that being connected in series.
3.Thirdly, Internal fuse action (fusing)
A huge discharge current: Also known as discharge inrush current will flood into the breakdown element. So the huge discharge current will generate extremely high heat on the internal fuse that connected with element in series. Thus the fusing happens in less than one millisecond. Therefore cut off the breakdown element.
4.Forth, fault isolation and the capacitor continues to operate
After the fusing, the breakdown element is isolated from the remaining capacitor elements. Since the power capacitor unit is made up of multiple capacitor elements, so losing one would decrease its capacity slightly. The capacitor unit is capable of operating in normal in such condition.
Key technical requirement of Internal fuses
1.Sensitivity and speed ability: Response to the huge discharge current very fast when breakthrough occurs.
2.Accurate energy calculation: The fusing performance is designed through a very accurate calculation, to ensure the fuses are capable of handling the faulty discharge current meanwhile withstand the normal discharge current and external electromagnetic interference.
3.Strong Short-circuit breaking capacity： cut-off the fault current and extinguish the arc
4.Strong voltage withstand ability: After the fusing, the ends of fuses will undertake the voltage which is supposed to go through the breakdown elements, and the possible over-voltage is very risk to cause the breakthrough. So a strong voltage withstand ability is necessary.
What effect does the fusing bring?
Change in capacitor capacitance: Each time a internal fuse blown, it means that the capacitor element is out of operation, so the total capacity of power capacitor will be decreased step by step.
How to monitor the fusing status and protect the capacitor?
There will be a protection design like unbalance protection ( Bridge differential current protection, voltage differential protection etc). When a amount of fault elements breakdown, the unbalance protection will take action (like the differential current or voltage exceed the set value).
In general, the internal fuse is a protective design that sacrifices the individual for the greater good of power capacitor bank. It greatly enhance the operational reliability and service life of capacitors banks.
