Commutation Techniques of Thyristor SCR

What is commutation techniques of SCR?

Or

What is thyristor commutation technique?

A thyristor is turned on by applying a signal to its gate-cathode circuit. For the purpose of power control or power conditioning,for by understanding a conducting thyristor must be turned-off as desired.

The turn-off of a thyristor means bringing the device from forward-conduction state to forward-blocking state. 

The thyristor turn-off requires that

1. Its anode current falls below the holding current and 
2. A reverse voltage is applied to thyristor for a sufficient time to enable it to recover to blocking state.

Commutation is defined as the process of turning-off a thyristor. Once thyristor starts conducting, gate loses control over the device, therefore, external means may have to be adopted to commutate the thyristor. Several commutation techniques have been developed with the sole objective of reducing their turn-off (or commutation) time.

Classification of Thyristor Commutation Techniques

The classification of thyristor commutation techniques varies among different authors. However, the classification is primarily based on how the anode current is reduced to zero and the configuration of the commutating circuits. Resonant LC or underdamped RLC circuits are commonly used in thyristor commutation techniques to force the current and/or voltage of a thyristor to zero, thus turning off the device.These circuit configurations are also employed in various power-electronic converters. Studying the different commutation techniques provides an introduction and better understanding of the transient phenomena that occur in power-electronic converters during switching conditions.

CLASS A COMMUTATION: LOAD COMMUTATION

Load, or class-A, commutation is prevalent in thyristor circuits supplied from a dc source. The nature of the circuit should be such that when energized from a dc source, current must have a natural tendency to decay to zero for the load commutation to occur in a thyristor circuit. Load commutation is possible in dc circuits and not in ac circuits. Class A, or load, commutation is also called resonant commutation or self-commutation.

Class A commutation refers to the natural or inherent turn-off process of a thyristor, where the current flowing through the device naturally decreases below the holding current level, causing the thyristor to turn off without requiring any external commutation circuitry.

In Class A commutation, the load circuit is designed in such a way that the current naturally decreases below the holding current when the desired turn-off condition is met. This can be achieved by using appropriate load and circuit parameters, such as the inductance, capacitance, and load resistance.

When the thyristor is conducting, the load current flows through the thyristor and the load circuit. As the current reduces below the holding current level, the thyristor turns off by itself, without any external commutation circuitry.

It's important to note that Class A commutation is suitable for specific applications where the load and circuit parameters are carefully designed to achieve the desired turn-off behavior. It may not be applicable or optimal for all thyristor-based circuits, and different commutation techniques, such as Class B or Class C, may be used depending on the specific requirements of the application.

CLASS B COMMUTATION: RESONANT-PULSE COMMUTATION

Class B commutation, also known as resonant-pulse commutation, is a method used in electronic circuits, particularly in power electronics. It involves the use of a Class B amplifier configuration, where an LC resonant circuit is connected in parallel in the case of Class B, while in Class A, it is connected in series.

In Class B commutation, when an input voltage (Vs) is applied, the capacitor in the circuit starts charging up to the input voltage. The thyristor, which acts as a switching element, remains reverse biased until a gate pulse is applied. When the gate pulse is applied, the thyristor turns on, allowing current to flow in both directions.

However, due to the presence of a constant load current flowing through the resistance and inductance connected in series, the large reactance of the circuit causes a sinusoidal current to flow through the LC resonant circuit. This current charges the capacitor with reverse polarity, resulting in a reverse voltage appearing across the thyristor.

The reverse voltage across the thyristor causes a commutating current (Ic) to oppose the flow of the anode current (IA). As a result, when the anode current becomes lower than the holding current, the thyristor turns off. This opposing commutating current plays a crucial role in turning off the thyristor during the commutation process.

Class C: Complementary Commutation

Class C commutation, also known as complementary commutation. It involves the use of two thyristors in parallel, one as the main thyristor and the other as the auxiliary thyristor.

Initially, both thyristors are in the OFF state, and the voltage across the capacitor is zero. When a gate pulse is applied to the main thyristor (T1), current starts flowing through two paths: one through R1-T1 and the other through R2-C-T1. As a result, the capacitor begins to charge, reaching a peak voltage equal to the input voltage, with plate B positive and plate A negative.

When a gate pulse is applied to the auxiliary thyristor (T2), it turns ON, causing a negative polarity of current to appear across the main thyristor (T1). This negative current causes T1 to turn OFF. Consequently, the capacitor starts charging with reverse polarity, where plate A becomes positive and plate B becomes negative.

In this configuration, the switching action of T1 and T2 is complementary. When T1 turns ON, it turns OFF T2, and when T2 turns ON, it turns OFF T1. This complementary switching ensures that the current flows through the desired path, and the capacitor charges and discharges correctly in each half-cycle of the input voltage.

Class C commutation is commonly used in certain types of inverters and high-frequency power applications, where it allows for efficient voltage conversion and control.

Class D: Impulse Commutation

Class D commutation is also called as Impulse Commutation or Voltage Commutation.Class D commutation is actually a specific type of commutation used in chopper circuits, which are used for DC-DC voltage conversion.

In Class D commutation, the main thyristor (T1) and the auxiliary thyristor (T2) are triggered alternately to control the current flow through the load and the inductor. When T1 is triggered, current flows through the load and the inductor, and the capacitor starts charging with plate A negative and plate B positive.

When T2 is triggered, it turns OFF T1 and redirects the current path through the inductor, causing the energy stored in the inductor to discharge to the load. The capacitor, in this case, plays a role in filtering and smoothing the output voltage.

The main difference between Class C and Class D commutation lies in their applications and operating principles. Class C commutation is used in certain types of inverters and high-frequency power applications, while Class D commutation is used in chopper circuits for DC-DC voltage conversion.

It's essential to use the appropriate commutation method for the intended application, as the behavior and performance of the circuit can vary significantly based on the commutation method used.

CLASS E COMMUTATION: EXTERNAL PULSE COMMUTATION

In this type of commutation, a pulse of current is obtained from a separate voltage source to turn off the conducting SCR. The peak value of this current pulse must be more than the load current.

shows in figure a circuit using external-pulse commutation. Here Vs is the voltage of the main source and V₁ is the voltage of the auxiliary supply. Thyristor T1 is conducting and load is connected to source Vs. When thyristor T3 is turned on at t=0; V₁, T3, L and C form an oscillatory circuit. Therefore, C is charged to a voltage +2V₁ with upper plate positive at t=π√LC as shown and as oscillatory current falls to zero.

For turning off the main thyristor T1, thyristor T2 is turned on.With T2 on, T1 is subjected to a reverse voltage equal ,Vs-2V₁ and T1 is therefore turned off.After T1 is off,capacitor discharges through the load.

CLASS F COMMUTATION: LINE COMMUTATION

This type of commutation is also known as natural commutation. This method of commutation is applied to phase-controlled converters, line-commutated inverters, ac voltage controllers and step-down cycloconverters.

The thyristor carrying the load current is reverse biased by the ac source voltage and the device is turned-off when anode current falls below the holding current (assumed nearly zero). A single-phase half-wave (or one-pulse) controlled converter employing line commutation is shown in Fig.

In this figure, thyristor T is fired at firing angle equal to zero i.e when ωt=0, Vs = 0. Load is resistive in nature. With zero degree firing-delay angle, the thyristor be- haves like a diode.

During the positive half-cycle, Vc = Vs and wave shape of load current Io is identical with the wave shape of Vo for a resistive load.

At ωt = π, Vs=0, Vo= 0 and Io= 0 therefore T gets turned off at this instant. From ωt=π to ωt=2π, T is reverse biased for a period tc=π/ω sec, longer than the thyristor turn-off time tq. Here tc is called the circuit turn-off time.

Uses an external AC source or an additional SCR to force the current through the SCR to decrease.

Another method of classification of thyristor commutation technique is as under:

1. Line commutation: class F
2. Load commutation: class A
3. Forced commutation: class B, C and D
4. External-pulse commutation: class E.

In line, or natural, commutation, natural reversal of ac supply voltage commutates the conducting thyristor. As stated before, line commutation is widely used in ac voltage controllers, phase-controlled rectifiers and step-down cycloconverters.

In load commutation, L and C are connected in series with the load or C in parallel with the load such that overall load circuit is under damped. Load commutation is commonly employed in series inverters.

In forced commutation, the commutating components L and C do not carry load current continuously. So class B, C and D commutation constitute forced commutation techniques. As stated before, in forced commutation, forward current of the thyristor is forced to zero by external circuitry called commutation circuit. Forced commutation is usually employed in de choppers and inverters.

Importance of Commutation Techniques of Thyristor

Efficient commutation techniques are vital for achieving optimal performance and reliability in SCR circuits. Proper commutation ensures smooth transfer of current between SCRs, preventing excessive power loss, voltage spikes, and unwanted stress on the components. By effectively controlling the turn-off timing, commutation techniques help minimize harmonic distortions and improve overall system efficiency.

Additionally, the choice of commutation technique depends on the specific application requirements, such as power rating, load characteristics, and desired control parameters. Each technique offers distinct advantages and disadvantages, requiring careful consideration during the design and implementation phases.

Conclusion of Commutation Thyristor

Commutation techniques play a crucial role in the efficient and reliable operation of thyristors, particularly SCRs. Natural commutation is suitable for AC circuits, while forced commutation techniques are employed in DC circuits and high-power applications. Forced commutation techniques include resistance commutation, capacitance commutation, external voltage commutation, and GTO commutation. Each technique offers advantages and considerations, and the choice depends on the specific application requirements. By understand sanding these commutation techniques, engineers can design SCR circuits that meet the needs of various power electronic applications.

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