Features, advantages and application analysis of silicon controlled rectifiers

Thyristor rectifier is a kind of power control electric appliance based on thyristor (power Electronic power device) and intelligent digital control circuit as the core. It has many advantages such as high efficiency, no mechanical noise and wear, fast response speed, small size, light weight and so on. “NX70 series thyristor rectifier” achieves precise temperature control through precise control of voltage, current and power. And by virtue of its advanced digital control algorithm, the efficiency of electric energy usage is optimized. Played an important role in saving electric energy.

Thyristor rectifier is a kind of power control electric appliance based on thyristor (power electronic power device) and intelligent digital control circuit as the core. It has many advantages such as high efficiency, no mechanical noise and wear, fast response speed, small size, light weight and so on. “NX70 series thyristor rectifier” achieves precise temperature control through precise control of voltage, current and power. And by virtue of its advanced digital control algorithm, the efficiency of electric energy usage is optimized. Played an important role in saving electric energy.

1. Rectifier element (thyristor)

Simply put: the rectifier is a single-phase or three-phase sinusoidal alternating current through the rectifier element into a stable and adjustable unidirectional direct current. Its realization condition mainly depends on the rectifier, thyristor and other components to realize through rectification. In addition, there are many rectifier devices, such as: turn-off thyristor GTO, reverse conducting thyristor, bidirectional thyristor, rectifier module, power module IGBT, SIT, MOSFET, etc., only the thyristor is discussed here.

Thyristor is also called thyristor, usually people call it thyristor. It is a power semiconductor device. Because of its high efficiency, good control characteristics, long life, and small size, it has been developed rapidly since the sixties of the last century and has formed an independent discipline. “Thyristor AC Technology”. The thyristor has been developed to this day, and it has been very mature in technology, better in quality, greatly improved in yield, and is developing towards high voltage and high current. At present, the maximum rated current of domestic thyristors can reach 5000A, and it is even larger abroad. All of my country’s Shaoshan electric locomotives are equipped with high-power thyristors developed by our country.

Application of thyristor:

1. Controllable rectification

Just like diode rectification, it can rectify AC to DC, and when the AC voltage is unchanged, the rectification can be controlled by conveniently controlling the size of the DC output voltage to achieve AC-variable DC

2. AC voltage regulation and power regulation

Use the switching characteristics of the thyristor to replace the old-fashioned contact voltage regulator, inductive voltage regulator and saturation reactor for voltage regulation. In order to eliminate the high-order harmonics generated by the AC voltage regulation of the thyristor, a zero-crossing trigger has appeared, which realizes the stepless adjustment of the AC power of the load, that is, the thyristor power regulator. Communication-variable communication.

Three, inverter and frequency conversion

DC transmission: The three-phase high-voltage AC is rectified into high-voltage DC, and the high-voltage DC is transmitted over a long distance to reduce losses and increase the stability of the power grid. Then, the inverter converts the high-voltage DC to 50HZ three-phase AC.DC-AC

Frequency conversion of intermediate frequency heating and AC motors such as variable frequency speed regulation and series excitation speed regulation, AC-variable frequency AC

4. Chopper voltage regulation (pulse voltage regulation)

Chopper voltage regulation is a conversion between DC and variable DC. It is used in city trams, electric locomotives, battery carriers, forklifts (forklifts), electric cars, etc., and high-frequency power supplies are used for EDM.

5. Non-contact power static switch (solid state switch)

As a power switching element, it can replace contactors and relays for high switching frequency occasions

Thyristor conduction conditions:

After the positive anode voltage is applied to the thyristor, an appropriate positive gate voltage is applied to the gate, and the conduction process of the thyristor is called trigger. Once the thyristor is triggered and turned on, the gate loses its control. Usually, only a positive pulse voltage is added to the gate, which is called the trigger voltage. The gate can trigger the thyristor to turn on under certain conditions, but it cannot turn it off. To restore the blocking of the conducting thyristor, reduce the anode voltage or increase the load resistance to reduce the anode current flowing through the thyristor to the holding current (IH) (when the gate is disconnected, the thyristor is switched from a larger The state current drops to just the minimum anode current required to keep the thyristor conducting, called the maintenance current), the current will suddenly drop to zero, and then increase the voltage or reduce the load resistance, the current will no longer increase, indicating that the thyristor has restored resistance Off.

According to the volt-ampere characteristics of the thyristor anode, it can be concluded:

1. When the gate is disconnected, the forward leakage current of the thyristor is larger than the reverse leakage current of a general silicon diode, and it increases as the forward anode voltage of the tube increases. When the anode voltage rises high enough, the thyristor will be turned on, which is called positive turning or “hard turn-on.” Multiple hard openings will damage the tube.

2. After the positive anode voltage is applied to the thyristor, a trigger voltage must be added and sufficient trigger current must be generated to turn the thyristor from blocking to conducting. When the trigger current is not enough, the tube will not turn on, but the forward leakage current increases significantly with the increase. The thyristor can only work stably in the two states of off and on, without an intermediate state, and has bistable switching characteristics. It is an ideal non-contact power switching element.

3. Once the thyristor is triggered and turned on, the gate completely loses control. To turn off the thyristor, the anode current must be kept current. For resistive loads, the anode voltage of the tube must be reduced to zero. In order to ensure the reliable and rapid shutdown of the thyristor, a reverse voltage is usually applied for a certain period of time after the anode voltage of the tube drops to zero.

Main characteristic parameters of thyristor

1. Forward and reverse repetitive peak voltage-rated voltage (VDRM, VRRM, whichever is the smaller)

2. Rated on-state average current IT (AV)-rated current (average value of sine half-wave)

3. Gate trigger current IGT, gate trigger voltage UGT, (subject to temperature changes)

4. On-state average voltage UT (AV) is the tube pressure drop

5. Holding current IH and holding current IL

6. Turn on and turn off time

The choice of thyristor:

The overload capacity of the thyristor is poor. According to the actual maximum current, it must be multiplied by 1.5 to 3 times, that is, the current margin. It is usually selected according to the average current IT (AV), and the effective value ITe (ie the root mean square value) of the rated current is 1.57 times the average current.

Form factor Kf=Ite/IT(AV)=1.57

The rated voltage should be 2 to 3 times the maximum voltage possible during actual work, that is, the voltage margin.

At the same time, necessary protective measures must be added.

Gate trigger current: dozens of mA~several hundreds of mA, it may trigger by mistake or be difficult to trigger if it leaves this range

Gate trigger voltage: about 3V

The countertop is divided into concave and convex, and the radiator is related to this

2. Main circuit type and multi-phase rectification

One, single phase

1. Single-phase half-wave controllable rectifier circuit

2. Single-phase full-wave controllable rectifier circuit (double half-wave)

3. Single-phase half-controlled bridge controllable rectifier circuit

4. Single-phase bridge controllable rectifier circuit

Two, three phase

1. Three-phase half-wave controllable rectifier circuit

2. Three-phase bridge fully controlled rectifier circuit

3. Three-phase bridge half-controlled rectifier circuit

4. Three-phase fully controlled bridge in-phase and anti-parallel rectifier circuit

5. Double anti-star controllable rectifier circuit with balanced reactor

6. Double anti-star with balanced reactor full circuit in-phase anti-parallel controllable rectifier Δ/Y┻Y+ Y┻Y

Three, multi-phase rectification

Multi-phase rectification can greatly reduce high-order harmonic currents and reduce pollution to the power grid.

Whether it is a three-phase bridge or a double reverse star circuit, it can form a multi-phase rectifier. Such as 12, 24, 36, 48 pulse waves, that is, the number of DC pulses in an AC cycle. Generally speaking, the structure of transformers with more than 24 pulses is relatively complicated, and there are more copper bars on the side of the rectifier valve, which brings some other difficulties. Large-capacity units can use multiple rectifier units to achieve multi-phase rectification through phase shifting and parallel operation.

3. Protection

The thyristor itself has prepared a large current and voltage margin when selecting it. In order to make the rectifier work reliably, various protections must be added. Overcurrent, current limiting, overvoltage, fast fusing, component damage, abnormal water pressure, high water temperature, lack of phase, under-branch, bridge arm overheating, lightning protection, control circuit overcurrent and overvoltage out of control, etc.; reforming light and heavy gas , The oil temperature is abnormal.

(1) Thyristor turn-off overvoltage (commutation overvoltage, hole accumulation effect overvoltage) and protection

When the thyristor is turned on to off, the line inductance (mainly the transformer leakage inductance LB) releases energy to generate overvoltage. Since the thyristor is conducting, the carriers fill the inside of the component. During the turn-off process, when the forward current drops to zero under the reverse action of the tube, carriers remain inside the component. Under the action of the reverse voltage, these carriers have a large reverse current instantaneously, which makes the remaining carriers disappear quickly. At this time, the reverse current decreases, that is, the diG/dt is extremely large, and the induced potential is very large. The electric potential is connected in series with the power supply and applied in the reverse direction to the component that has been restored and blocked, which can cause the thyristor to reverse breakdown. This is the shutdown overvoltage (commutation overvoltage). The value can reach 5-6 times the working voltage. Protective measures: connect the resistance-capacitance absorption circuit in parallel at both ends of the thyristor.

(2) AC side overvoltage and its protection

Due to the transient process of the AC side circuit when it is turned on or off, an operating overvoltage will be generated. At the moment of high-voltage closing, due to the distributed capacitance between the primary and the secondary, the primary high-voltage is capacitively coupled to the secondary, and instantaneous overvoltage appears. Measures: Connect an appropriate capacitor in parallel between the secondary star midpoint of the three-phase transformer and the ground to significantly reduce this overvoltage. When other loads connected in parallel with the rectifier are cut off, an overvoltage of induced potential is generated due to the inductance of the power circuit. When the transformer is no-load and the power supply voltage crosses zero, the primary is switched on. Due to the sudden change of the transformer excitation current, a high instantaneous voltage is induced in the secondary. This voltage spike can reach more than 6 times the working voltage. If the AC power grid is struck by lightning or the power grid invades and interferes with over-voltage, that is, occasional surge voltage, a resistance-capacitance absorption circuit must be added for protection.

(3) DC side overvoltage and protection

When the load is disconnected or quickly blown, the magnetic field energy stored in the transformer will generate overvoltage. Obviously, the RC absorption protection circuit on the AC side can suppress this overvoltage, but the energy stored when the transformer is overloaded is higher than that when the transformer is under no load. To be large, it cannot be completely eliminated. Measures: Pressure-sensitive absorption can often be used for protection.

(4) Overcurrent protection

Generally, a fast fuse is used for protection. In fact, it cannot protect the thyristor, but protects the transformer coil.

(5) Limitation of voltage and current rise rate

4. Current sharing and thyristor selection

If the current sharing is not good, it is easy to burn out the components. In order to solve the current sharing problem, the current sharing reactor was used in the past, which was very noisy and the effect was not good. The comparison was made one by one, and the screws were tightened, which was blind, the effect was poor, the noise was large, and the energy consumption. The method we adopted is: use computer program software to perform dynamic parameter screening and matching, numbering, and assembling according to the order of numbers during assembly, which is very time-to-order. Each component is engraved with words for reference when replacing it. In this way, the current sharing coefficient can reach above 0.85. In order to reduce parallel connection, use large components. This can further improve the current sharing and reduce losses, because each component has a voltage drop, which is also the main loss of the rectifier.

5. Trigger control circuit

At present, there are many thyristor trigger circuits: analog IC integrated trigger circuits, including domestic IC KJ004 (KJ009); imported ICs have TCA785, 787 circuits; digital trigger (a logic chip) analog control, which can be synchronized and locked; single-chip microcomputer Trigger control circuits are also more and more widely used, all of which can be adjusted by PI. Can meet the triggering requirements of SCR.

The Links:   6MBI25L-120 LM201U05-SLL1

Author: Yoyokuo