What is a capacitor switching switch?

What is a capacitor? What is its working principle? Many people will feel unfamiliar with the capacitor switching switch. In fact, it is not difficult. This article explains it through overview, classification, and how it is different from ordinary ones. Now let’s start the lesson on capacitor switching!

01 What is a capacitor switching switch?

In fact, it is a device similar to a switch. When the capacitor wants to work, the switching switch connects the capacitor to the system; when the capacitor exits the working state, the switching switch disconnects the capacitor from the system.

02 What are the common types of capacitor switching switches in low-voltage reactive power compensation systems?

Common switching switches include three categories: special contactors for capacitor switching, thyristor switches, and composite switches. From the perspective of price, it is optimal to choose a contactor dedicated for capacitor switching; from the perspective of switching speed and frequent switching requirements, thyristor switches are optimal.

03 What is the difference between a special contactor for capacitor switching and a conventional contactor?

First, let's look at the contactor.

The special contactor for capacitor switching has one more contact per phase than the conventional contactor. This contact has a resistance and inductance loop in series. When the capacitor is switched in, the auxiliary contact is closed first, and the inductance in the loop is passed through. and resistance effectively reduce the inrush current when the capacitor is put in. Then the main contact of the contactor is closed, the auxiliary contact is opened, and the capacitor is connected to the system and works normally.

Therefore, we can see that the special contactor for capacitor switching can effectively reduce the inrush current through special design. Reducing inrush current can ensure the safety of equipment and systems. However, in terms of the effect of effectively suppressing inrush current, product engineers felt that it was not perfect enough; at the same time, this contactor was not suitable for frequent and fast switching requirements, so another product was born, that is, the "thyristor switch".

04 What is the difference between a thyristor switch and a special contactor for capacitor switching?

Thyristor switches have many names, some are called "thyristor switches", some are called "regulators", and some are called "contactless switches". It is a product designed based on thyristors. Through the control of thyristors, the "input" and "removal" of capacitors are realized, thereby realizing "zero-crossing switching", that is, "input at zero-crossing of voltage" and "input at zero-crossing of current". "Zero crossing point" is cut off.

Each cycle of the 50Hz sine wave voltage is 20ms. Therefore, there will be a zero-crossing point during each cycle, so that the thyristor switch can be switched quickly and the inrush current can be limited to a very low range. How low? Below 3 times the rated current, if it does not meet this standard, it will not pass the type test. And the action time is very fast, and the action can be completed within 20ms.

05 For sales engineers, what needs to be remembered is:

First, the thyristor switch has extremely fast switching speed and can be completed within 20ms (different brands of products have differences, but this 20ms is a guaranteed figure), which can meet the application scenarios of rapid load changes and enable the reactive power compensation system to respond to load changes. Fast compensation, the compensation effect is better; secondly, the inrush current suppression effect is very good, it can suppress the inrush current to less than 3 times the rated current, ensuring the safety of equipment and systems. From these two aspects, thyristor switches are superior to special contactors for capacitor switching.

What are the disadvantages of thyristor switches? One word: "hot". The thyristor switch is a product that generates a lot of heat. At the same time, it is very sensitive to heat. Overheating will cause damage. Therefore, the thyristor switch is equipped with a cooling system (including cooling fans, heat sinks, overheating protection, etc.). From this point of view, contactors are better than thyristors. Is there a way to combine the advantages of thyristor switches and contactors? At this time, the composite switch was born.

06 What kind of product is a composite switch?

The composite switch is composed of a thyristor and a mechanical contact in parallel. It uses the thyristor to realize the "zero-crossing switching" of the capacitor. After the capacitor is put into the system, the mechanical contact closes to ensure the normal connection between the capacitor and the system, thereby reducing the heat and heat of the switch. power consumption. Therefore: the composite switch can realize "zero-crossing switching", which is very effective in suppressing inrush current; at the same time, the composite switch has low power consumption and low heat generation. Compared with the special contactor for capacitor switching, the composite switch has better inrush current suppression effect and lower power consumption; compared with the thyristor switch, the power consumption is smaller while also effectively suppressing the inrush current.

However, because the composite switch has one more set of mechanical contacts than the thyristor switch, and there are more coordination links in its operation, the time efficiency of the "input-removal-re-input" of the composite switch is worse than that of the thyristor switch; that is, Composite switches are not suitable for reactive power compensation applications where the load changes rapidly. The above is the working principle of the capacitor switching switch, which requires engineers to continuously accumulate experience in practice.