Why Thyristor used in Power Factor Correction Panel?

Automatic Power Factor Control(APFC) Panel

Hey, in this article we are going to discuss a very important and interesting topic which is, Why Thyristor or SCR is used in the Automatic Power Factor Control(APFC) Panel. Here we will discuss also the advantages of Thyristor Based Power Factor Panel compared to others such as Contactor Switched APFC Panel.
Actually, the thyristor-based power factor control panel is mainly known as the Real Time Power Factor Correction(RTPFC) Panel, because here power factor correction happens within a few microseconds.

What is the Automatic Power Factor Control(APFC) Panel?

The APFC panel or automatic power factor improvement panel is a compact electrical panel consisting of circuit breakers, relays, capacitor banks, microcontrollers, and thyristors that automatically control or improve the power factor when the power factor becomes low due to a large inductive load in an electrical system.

Generally, all companies having inductive loads have an APFC panel in their main LT Panel Control room. The APFC panel is always connected in parallel to the main incoming of the whole electrical circuit.

There are mainly two types of APFC panels that are commonly used,

1. Contactor Switched APFC Panel
2. Thyristor Based APFC Panel

Contactor Switched APFC panels are built with special capacitor switching contactors which are connected to capacitor banks and act as a valve for the capacitor bank. When the power factor goes low contactor will be turned on and connect the capacitor bank to the load circuit.

Thyristor-based APFC Panels are built with Thyristor Switching Modules(TSM). They also act as a valve of the capacitor bank to connect or disconnect with the load circuit.

Thyristor Based APFC Panel

Now, we will discuss why thyristors are used in automatic power factor correction panels. Mostly SCR or TRIAC is used for these types of panel design.

So, thyristors are used to make a switch or valve between the capacitor bank and the load circuit. Suppose in an APFC panel ten numbers of Capacitor bank is there, so we do not need to connect all of them at a time to the load circuit.
When the power factor goes very low then we required to connect more capacitor banks to the load, when the power factor is near to the unity then we required to connect few capacitor banks to the load circuit.

Thyristor Based APFC Panel block diagram

So there are microcontrollers, CT, and relay circuits to sense the reactive current, and they continue monitoring the power factor of the whole electrical system. According to the power factor value or relative power demand, the microcontroller will send the signals to the gate terminal of the thyristors to trigger them. Once the thyristor gets triggered, they will connect the capacitor bank with the load circuit to compensate the reactive power or to maintain the power factor.

The thyristor gate triggering pulses are generated using Zero Cross Over cathodic firing techniques. Zero Cross Over technique means they use the alternating current at zero points (which comes between a positive and a negative half cycle) where voltage is completely zero. When the voltage across thyristor is zero, that time only they will be fired by gate pulses.  This technique is used to prevent the generation of transients, voltage spikes, and harmonics during switching action.

Advantages of Thyristor Based APFC Panel

Now, let's go to know what are the advantages of using thyristors or SCRs in Automatic Power Factor Control(APFC) Panels.

1. Thyristor provides solid-state switching, so connecting and disconnecting of the capacitor bank with the load circuit can be possible within a few micro-seconds. So thyristor-based APFC panel provides a real-time power factor correction facility.

2. As thyristor provides very fast switching, so these panels are most suitable for compensating loads.

3. Thyristor Based APFC Panels are especially suitable for fast variable loads, where Contactor Switched APFC Panels fails to provide desired results.

4. As there is no moving part, so no special arrangement required to prevent sparking during switching action with a very high current.

5. Thyristor Switching Modules(TSM) are very compact, reliable, and naturally cooled.

6. Thyristor Based APFC Panels are mostly reliable for low maintenance.

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Why Thyristor used in Power Factor Correction Panel? Why Thyristor used in Power Factor Correction Panel? Reviewed by Author on October 02, 2020 Rating: 5
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