Selection and application of thyristors in high voltage soft start

1 Introduction

With the rapid development of power electronics technology, thyristor soft start devices have emerged. The start-stop technology of three-phase asynchronous motors has undergone epoch-making changes. Thyristor soft start products have only been available for about 30 years, but their main performance is much better than traditional soft start methods such as magnetic control soft start and liquid resistance soft start. It has a small size, compact structure, almost maintenance-free, complete functions, good starting repeatability, and comprehensive protection. It has become a leader in the field of soft start.

2 Thyristor Introduction

Thyristor is the abbreviation of thyristor, also known as silicon-controlled rectifier, formerly referred to as thyristor; in 1957, General Electric Company of the United States developed the world's first thyristor product and commercialized it in 1958; thyristor is a pnpn four-layer semiconductor structure, it has three poles: anode, cathode and gate; the working conditions of thyristor are: positive voltage is applied and there is trigger current at the gate. Its derivative devices include: fast thyristor, bidirectional thyristor, reverse thyristor, light-controlled thyristor, etc. It is a high-power switching semiconductor device, represented by the text symbols "v" and "vt" in the circuit (in the old standard, it is represented by the letter "scr").

Thyristors have the characteristics of silicon rectifier devices, can operate under high voltage and high current conditions, and their working process can be controlled. They are widely used in electronic circuits such as controlled rectification, AC voltage regulation, contactless electronic switches, inverters and frequency converters.

3 Thyristor parameter description

In order to correctly select and use thyristors, you should have some understanding of their main parameters so that you can select them correctly. The main parameters of thyristors are:

(1) Off-state repetitive peak voltage udrm

It refers to the maximum peak voltage allowed to be applied between a and k when the thyristor is forward blocked. As shown in Figure 1, a is the anode of the thyristor, k is the cathode of the thyristor, and g is the gate of the thyristor. This voltage is about 90% of the non-repetitive peak voltage udsm.

(2) Reverse repetitive peak voltage urrm

When the control electrode is disconnected, the reverse peak voltage that is allowed to be repeatedly applied to the thyristor is called the reverse blocking peak voltage. This voltage is about 90% of the non-repetitive peak voltage ursm.

Udrm and urrm are generally similar in value and are collectively referred to as the blocking peak voltage of the thyristor. The smaller value is usually taken as the rated voltage value of the device of that model.

Since transient overvoltage can also damage the thyristor, the rated voltage of the thyristor should be selected to be 2 to 3 times the normal operating peak voltage to ensure safety.

(3) Rated average forward current if

Under the specified standard heat dissipation conditions and ambient temperature (40°C), the average value of the industrial frequency sinusoidal half-wave current allowed to pass continuously between the anode and cathode of the thyristor is called the rated forward average current.

Since the overload capacity of the thyristor is small, the rated forward average current of the thyristor should be at least 1.5 to 2 times greater than the normal operating average current to leave a certain margin.

(4) Maintaining current ih

At room temperature, when the control electrode is open, the minimum current required to keep the thyristor on is called the holding current. When the forward current is less than the ih value, the thyristor turns off automatically. The ih value is generally tens to more than one hundred milliamperes.

(5) Control electrode trigger voltage vg, trigger current ig

At room temperature, when the anode is applied with a forward voltage of 6V DC, the minimum control electrode voltage and current required to change the thyristor from blocking to conducting are called control electrode trigger voltage and trigger current. Vg is generally 3.5~5V, and IG is about tens to hundreds of mA. In practical applications, the trigger voltage and trigger current added to the control electrode should be slightly larger than the rated value to ensure reliable triggering.

(6) Voltage rise rate dv/dt

When the thyristor is blocked, the anode and cathode are equivalent to a junction capacitor. When the anode voltage is suddenly applied, a charging capacitor current will be generated. This current may cause the thyristor to be mis-conducted. Therefore, the maximum forward voltage rise rate of the tube must be limited. Generally, a resistive-capacitive absorption element is connected in parallel at both ends of the thyristor to limit it.

(7) Current rise rate di/dt

When the thyristor is turned on, the current starts to conduct from the cathode near the gate area and then gradually expands to the entire cathode area until it is fully turned on. This process takes a certain amount of time. If the anode current rises too quickly, the current will not have time to expand to the entire pn junction surface of the tube, causing the cathode near the gate to have excessive current density and heat too concentrated on the pn junction. The junction temperature will quickly exceed the rated junction temperature and burn out the thyristor. Therefore, the current rise critical value di/dt of the thyristor must be limited. Generally, an inductor or ferrite magnetic ring is connected in series in the bridge arm.

[page]4 Thyristor working conditions

Since the thyristor has only two working states, on and off, it has a switching characteristic, which requires certain conditions to be transformed. The conditions are shown in the attached table.

5 Application of thyristors in high voltage soft start

With the rapid development of the national economy, the number of high-voltage motors is increasing. Since the current of a large motor when it is directly started is 5 to 7 times the rated current, and the starting torque is only 0.4 to 1.6 times the rated torque. It can be started directly when the grid conditions (the grid voltage drop when the motor is started is less than 10%) and the process conditions (the starting torque is satisfied) allow. However, excessive starting current, too small starting torque and too long starting time cause great harm to the motor and the grid. It often leads to an increase in grid voltage and harmonic voltage fluctuations, and even tripping of the front stage, which greatly increases the burden on the grid and grid pollution, and seriously affects the safe operation of the grid. At the same time, it also causes great harm to itself. Therefore, a soft starter must be connected in series between the power supply and the motor to solve these problems.

The emergence of thyristor motor soft starter has solved the above problems very well. It makes up for the various deficiencies of traditional soft starters, effectively reduces the starting current of the motor, reduces the power distribution capacity, and prolongs the service life of the motor and related equipment. The starting parameters can be adjusted according to the load, which is easy to maintain.

5.1 Working Principle of Thyristor Motor Soft Starter

The application of thyristors in high-voltage motor soft starters is an application of using thyristors for AC voltage regulation. The voltage can be adjusted by changing the phase angle of the thyristor conduction through phase control.

The thyristor phase-shift soft starter changes the waveform of the sinusoidal AC voltage into a non-sinusoidal pulsed AC by adjusting its duty cycle, as shown in Figure 2.

Notes:

(1) α: Control angle. Refers to the time when the trigger pulse is added.

(2) q: conduction angle. The conduction angle of the thyristor in each half cycle. The larger the control angle, the smaller the conduction angle, and their sum is a constant α+q=p. It changes the average voltage of the alternating current, and its average voltage is controllable and changes smoothly.

5.2 Thyristor selection

Thyristor is the most critical power device in the motor soft starter. Whether the whole device works reliably is closely related to the correct selection of thyristor rated current, rated voltage and other parameters. The principle of selection should first consider the working reliability, that is, the current and voltage must have sufficient margin multiples. Secondly, the economy should be considered, that is, the cost performance, and finally the installation should be beautiful and the volume should be minimized.

For 6kv and 10kv high voltage motors, due to the high voltage, the thyristors need to be connected in anti-parallel and then in series. 6kv requires 6 thyristors per phase (2 anti-parallel, 3 groups in series), and 10kv requires 10 thyristors per phase (2 anti-parallel, 5 groups in series). In this way, the voltage borne by each thyristor is about 2000v, so the forward and reverse non-repetitive rated voltages Vdsm and VRSM of the selected thyristors should be above 6500v.

The selection of the thyristor rated current must take into account the rated operating current of the motor. Generally speaking, the thyristor current should be 3 to 4 times the rated current of the motor.

In the thyristor high-voltage motor soft starter, two independent thyristor devices are connected in anti-parallel to form an AC phase-controlled voltage regulator. One thyristor works in each positive and negative half-cycle, so the consistency of the parameters of the two anti-parallel devices is required to be high. Thyristor trigger parameters and holding current parameters are also required to be selected as consistent as possible. Try to make the positive and negative half-waves symmetrical, otherwise there will be a DC component current flowing through the motor. Since the motor winding load is inductive, too high a DC component will cause the motor stator to heat up severely, and even burn the motor winding, thus making the motor scrapped.

[page]6 Thyristor protection

The thyristor has a poor ability to withstand overvoltage and overcurrent, which is its main disadvantage. The thermal capacity of the thyristor is very small. Once an overcurrent occurs, the temperature rises sharply, which may burn the pn junction and cause a short circuit or open circuit inside the component. For example, when a 100A thyristor has an overcurrent of 400A, it is only allowed to last for 0.02s, otherwise it will be damaged by overheating; the thyristor has a very poor ability to withstand overvoltage. When the voltage exceeds its reverse breakdown voltage, it is easy to be damaged even for a very short time. If the forward voltage exceeds the breakover voltage, the thyristor will be mis-conducted, and the current after conduction is large, causing damage to the device.

6.1 Thyristor Overvoltage Protection

Connect an RC resistor-capacitor absorption circuit in parallel at both ends of the thyristor, as shown in Figure 3, and use capacitors to absorb overvoltage. Its essence is to convert the energy that causes overvoltage into electric field energy and store it in the capacitor, and then release it to the resistor for consumption.

When the thyristor switches from on to off, like the switching circuit, overvoltage will be generated due to the release of energy from the line inductance (mainly the transformer leakage inductance lb). Since the carriers fill the inside of the thyristor during the on period, when the forward voltage drops to zero during the off process, there are still carriers inside. These accumulated carriers will instantly produce a large reverse current under the action of the reverse voltage, causing the accumulated carriers to disappear quickly. At this time, the reverse current disappears very quickly, that is, di/dt is extremely large. Therefore, even if the line inductance l connected in series with the element is very small, the induced potential l (di/dt) value generated by the inductance is still very large. This potential is connected in series with the power supply voltage and applied in reverse to the element that has been restored to block, which may cause the reverse breakdown of the thyristor. This overvoltage caused by the thyristor turning off is called the turn-off overvoltage, and its value can reach 5 to 6 times the peak value of the operating voltage, so suppression measures must be taken.

The capacitor in the RC absorption circuit converts the electromagnetic energy of the overvoltage into electrostatic energy for storage, and the resistor prevents the capacitor and inductor from resonating, limiting the thyristor turn-on loss and current rise rate. This absorption circuit can suppress the overvoltage generated when the thyristor is turned on and off, effectively preventing the thyristor from being broken down.

The installation position of the RC absorption circuit should be as close as possible to the main terminal of the module, that is, the lead should be short. It is best to use non-inductive resistors to achieve better protection effects.

6.2 Thyristor Overcurrent Protection

Since semiconductor devices are small in size and have small heat capacity, especially high voltage and high current power devices such as thyristors, the junction temperature must be strictly controlled, otherwise they will be completely damaged. When a current greater than the rated value flows through the thyristor, the heat cannot be dissipated in time, causing the junction temperature to rise rapidly, which will eventually cause the junction layer to burn out.

There are many reasons for overcurrent, for example, damage to the thyristor of the converter itself, failure of the trigger circuit, failure of the control system, etc., as well as AC power supply voltage being too high, too low or missing phase, load overload or short circuit, and the impact of adjacent equipment failure.

The most commonly used method for thyristor overcurrent protection is fast fuse. Since the fusing characteristics of ordinary fuses are too slow, the thyristor will be burned out before the fuse is blown; therefore, it cannot be used to protect thyristors. Fast fuses are made of silver fuse wire buried in quartz sand, and the fusing time is extremely short, so they can be used to protect thyristors.

Compared with ordinary fuses, fast fuses are specially used to protect semiconductor power devices from overcurrent. They have the characteristics of fast fusing, and their fusing time is less than one cycle (20ms) of 50Hz AC when 6 times the rated current flows through them. Generally speaking, the effective value of the rated current of the fast fuse should be less than the rated effective value of the protected thyristor, and at the same time, it should be greater than the actual effective value flowing through the thyristor.

6.3 Thyristor Overheat Protection

When current passes through the thyristor, a certain voltage drop will be generated, and the existence of the voltage drop will generate a certain amount of power consumption. The greater the current, the greater the power consumption and the greater the heat generated. If the heat is not quickly dissipated, the thyristor chip will be burned out. Therefore, when using a thyristor module, a heat sink must be installed.

The quality of heat dissipation is an important factor affecting whether the module can work safely. Good heat dissipation conditions can not only ensure the reliable operation of the module and prevent the module from overheating and burning, but also improve the current output capacity of the module.

7 Conclusion

The application of thyristors in high-voltage soft start has brought revolutionary changes to soft start, and it will leave a strong mark in the history of soft start development.