For switching and regulating high-power voltage, thyristors are used, which are a kind of semiconductor devices. Today, switches of various capacities are used to ensure the correct operation of the power grid. It is only necessary to correctly select the CVC (current-voltage characteristic) of the thyristor, which eliminates equipment breakdowns, ensuring its correct functioning.
Content
- Features of semiconductors
- Principle of operation
- Main varieties
- Switching methods
- Varieties of forced control schemes
- Use in powerful circuits
- Correct connection and protection
Features of semiconductors
The main purpose of thyristor switches is to transfer electric current in the forward direction. In the closed state, the semiconductor traps forward and reverse voltage, thereby providing power grid regulation.
The thyristor structure includes three outputs:
- Control electrode.
- Cathode.
- Anode.
All semiconductors have their own volt-ampere characteristics, by which one can judge the purpose and state of this element. Powerful switches are capable of operating at a voltage of 5000 volts, and the maximum allowable current is 5000 amperes.
Principle of operation
The principle of operation of a thyristor is extremely simple: it is turned on by supplying powerful current pulses to the output. Such signals with respect to the cathodes must be positive. The operation of the thyristor is affected by the temperature of the semiconductor and a method of applying voltage and current to the switches used in the circuit.
In an electrical circuit where thyristors are used, a high rate of voltage rise is excluded, which can lead to spontaneous switching on of the element. Therefore, additional diodes and circuits are installed that provide voltage equalization, preventing parasitic surges. One of the features of using keys is the presence in the circuit of the steepness of the control signal characteristics, which is necessary for their correct operation.
Main varieties
Today there are several main types of semiconductors, which differ in their design, switching principle and a number of other parameters. The most widespread are the following types of thyristors:
-
Optical switches designed to control the flow of light. - Thyristors with field-effect transistor control.
- Inverter semiconductors with high switching speed.
- Symmetrical modifications make it possible to replace two semiconductors connected in antiparallel.
- Diodes become conductive when the voltage peaks are exceeded.
The parameters and I - V characteristics of thyristors, depending on their type, differ significantly. Accordingly, by choosing one or another variety, it will be possible to ensure the correct functioning of the electrical circuits, simplifying the equipment execution scheme.
Switching methods
The operation of the keys is controlled using the appropriate switching signals, which allow you to open and close the inputs, while ensuring the correct operation of the electrical equipment.
It is customary to distinguish two methods of switching:
- Forced.
- Natural.
Natural commutation of conductors occurs when the switch is used with alternating current. Redirection occurs when the current falls to zero. This method of controlling devices has not received proper distribution, since when using it it is difficult to ensure the correct operation of the electrical circuit, significantly reducing the functionality of the thyristors.
For forced switching, additional capacitors are required, which are pre-charged by pressing the key button. An LC circuit is additionally included in the used control circuit, in which a charged capacitor is a prerequisite. Powerful current fluctuations occur during the transition in the load circuit, which makes it possible to switch thyristors. To date, it is forced control with semiconductors that is most widespread, which is explained by its versatility, simplicity and maximum reliability.
Varieties of forced control schemes
Various types of forced switching can be used to control the operation of the keys. Most often, a circuit with a switching capacitor with reverse polarity is used. Such a diode is connected to the circuit using an additional auxiliary thyristor, which ensures the formation of a charge on the working semiconductor.
The capacitor current is directed towards the current from the main switch, which makes it possible to reduce the voltage in the network, up to the fall of this parameter to zero. When the current decreases, the thyristor is turned off, after which the cycle is repeated, which makes it possible to correctly control the operation of the entire electrical circuit and its individual elements in particular.
It is also possible to use a forced commutation circuit where LC circuits are connected. At the beginning of switching, the current from the LC-circuit is directed towards the operating voltage, they are quickly equalized and the thyristor is turned off.
From the oscillatory circuit, the electric current flows through the switch into the semiconductor diode. A corresponding voltage is applied to the thyristors, which in modulus is equal to the voltage drop across the diode.
Use in powerful circuits
The main purpose of thyristors is to organize the correct operation of a powerful circuit. By including semiconductors in the circuit, you can perform the following operations:
-
Vary the average current to help regulate control signals. - Disconnect or enable an electrical circuit with an active and resistive load.
A feature of thyristor switches is their ability to conduct current exclusively in one direction. Therefore, using them in alternating current circuits, it is necessary to ensure parallel connection. The average indicators of the electric current at the time the signals are applied to the thyristors can change, which forces the use of additional capacitors for the correct organization of the circuit.
The phase control method of operation with forced switching allows the load to be regulated by changing the voltage amplitude between phases. Such artificial switching is performed using special circuits or the installation of additional lockable keys. The phase control method is used in chargers where it is required to regulate the current strength, taking into account the level of accumulated energy in the battery.
Pulse width control technology is often referred to as current PWM. When the thyristor opens, a control signal is given. In the transition phase, the voltage becomes zero, which is a signal to close the key. The current curve when using phase control will not be sinusoidal, but completely dependent on the shape of the supply voltage signals. Pulse-width control has a complex implementation scheme, therefore this switching method is used in industrial equipment and powerful power supplies.
Correct connection and protection
Power thyristors are critical to the rate of rise of the current. The value of the electric current when it flows back through the switch can drop to zero, which leads to overvoltage of the semiconductors. To protect the keys, additional diodes and various circuits are used to protect devices in dynamic modes.
The use of such a circuit allows switches to be switched on in parallel, which prevents the reverse current and overvoltage of semiconductors from falling to zero. Today there are many options for schematic modifications of circuits, which are used depending on the parameters of thyristors in various conditions and modes.
