GTO overcurrent protection

GTO is mainly used in large-capacity choppers, inverters and switching circuits. The most concerning problem is the short-circuit overcurrent phenomenon caused by various reasons; therefore, it seriously threatens the safety of the device and even the entire equipment. Therefore, it is necessary to study the causes of overcurrent and how to take measures to protect GTO from damage under current conditions.

1. Generation of overcurrent

Overcurrent includes overload and short circuit. Overload current caused by excessive load can generally be protected by negative feedback control method, which will not be discussed here. Here we mainly discuss short circuit overcurrent. There are roughly three reasons for the occurrence of short circuit overcurrent.

( 1 ) Inverter arm short circuit In an inverter composed of GTOs , if two GTOs on the same arm are turned on at the same time, a bridge arm short circuit phenomenon will occur , also known as a bridge arm direct fault. At this time, the rising rate of the short-circuit current and the surge impact current will be very large, causing the GTO to burn out. There are generally two reasons for the bridge arm short circuit . One is unreasonable logic design. There is not enough time interval (i.e. dead zone) between the turn-off and turn-on of the two GTOs . When one GTO is not completely turned off, the other GTO is already turned on, causing the two GTOs to be turned on at the same time; the other is false triggering due to various reasons. When one GTO in the bridge arm is turned on, the other GTO will also cause a bridge arm short circuit due to false triggering . The cause of false triggering generally comes from the gate circuit, such as the appearance of various burr signals, external electromagnetic interference, or excessive du/dt due to improper buffer circuit design .

( 2 ) Line short circuit at the output end If a line short circuit occurs at the output end , the short circuit current will flow through the GTO of the corresponding branch , and the short circuit current will be quite large. If the load of the inverter is a motor, the current impact will also be large when the motor starts. Although it is not a short circuit current at this time, the current rise rate will also be quite large, causing damage to the GTO .

( 3 ) Output line short circuit to ground If one phase of the inverter AC output is grounded, the short-circuit current flows through the GTO anti-parallel diode , filter capacitor , rectifier diode and AC side fuse. The magnitude of the short-circuit current is related to the voltage on the filter capacitor. In the most serious case, when the filter capacitor voltage is discharged to a value lower than the peak value of the voltage between the busbar and the neutral point, a large fault current will flow. Proper selection of the capacity of the anti-parallel diode and fuse can achieve reasonable protection.

2. Overcurrent characteristics of GTO

The overcurrent characteristics of GTO are different from those of GTR . When the gate is in forward bias, GTO is turned on, and there are only restrictions on the effective value of the current, surge current and I2t ^value , and no provisions for the safe operating area. When the gate is in reverse bias, GTO is in the shutdown process. At this time, there is a limit on the maximum anode current that can be turned off, and there is a reverse bias safe operating area RB-SOA . It can be seen that the overcurrent protection of GTO is more complicated than that of GTR .

Overcurrent protection can be considered from two aspects: one is thermal protection, GTO cannot exceed the specified junction temperature due to overload; the other is electrical protection. When overcurrent occurs, it is hoped that the gate shutdown signal will be quickly removed when the overcurrent does not exceed the maximum allowable anode current that can be turned off, and the surge capacity will be used for protection, or the GTO will be quickly turned off within the range of the maximum anode current that can be turned off .

In view of the above problems, various measures can be taken to protect GTO from overcurrent.

3. Overcurrent protection of inverter

The overcurrent protection of the inverter is the final protection measure of the GTO equipment, which can prevent the further expansion of the accident. There are three specific protection methods .

1) Fuse protection method

Since GTO has surge capability, it can be protected by fast fuses under certain conditions. The process of using fuses to protect GTO inverters is: detect overcurrent → limit current with reactor → cut off GTO shutdown signal → fuse cuts off current.

The internal inductance of the GTO inverter is very small, usually only a few microhenries. Once a short circuit occurs, the current rises very quickly and the peak value is very large. Therefore, a reactor should be used to limit the current and the shutdown signal should be removed as soon as possible. Then, the fuse should cut off the fault current within the surge capacity range. Obviously, it is difficult to protect this overcurrent with ordinary power fuses or AC/DC electromagnetic switches, and fast fuses must be used for protection.

The breaking time of a fast fuse is within 10ms , and the arc voltage generated when it is cut off is very small. When using a fast fuse to protect a GTO , the following two points must be noted.

(1) An overcurrent detection circuit must be set up. When the GTO approaches the limit of its shutoff capability, the GTO shutdown signal is cut off, that is, when the GTO overcurrent exceeds the limit of the anode current that can be shut off, the gate signal must never be used to shut it off. The GTO has the same surge capability as the SCR , and this feature can be used for protection with a fast fuse.

(2) The I ² t values ​​of the GTO and the fast-acting fuse must be reasonably matched. When selecting a fast-acting fuse, the allowable I ² t of the GTO should be greater than the I ² t value of the fast-acting fuse .

The short-circuit overcurrent protection of large-capacity GTO inverters cannot use the GTO self-shutdown method. Because the non-repetitive shutoff current of high-power GTOs is smaller than that of medium and low-power GTOs , the gate pulse forced shutdown method of GTOs cannot be used in the case of short-circuit overcurrent . On the contrary, the opening signal of all GTOs should be sent to make the inverter fully turned on, and the GTOs should be protected from overcurrent by fuses.

2) Crowbar Protection

The crowbar protection method is also called the non-fuse protection method. It is widely used in medium and large capacity GTO circuits. Its working principle is shown in Figure 1. As can be seen from Figure 1 , when the inverter has a short circuit fault due to false triggering or other reasons, the discharge current of the filter capacitor increases sharply, and the signal of the capacitor discharge current is detected by the sensor , and then the following actions are generated.

(1) Trigger the parallel thyristor T to shunt the GTO inverter . _A small inductor L3 is connected in series with T to limit the di/ dt of T.

(2) Block the shutdown signal of GTO and trigger all GTOs , so that the short-circuit current is borne by all GTOs , reducing the current concentration phenomenon.

(3) The AC side circuit breaker trips. In Figure 1 , L1 _and L2 are inductors that limit the rising speed of the short-circuit current, and _Ld is a DC filter reactor _.

In the application of large-capacity GTO , a combination of fuse and crowbar can also be used. The protection process is: short-circuit overcurrent → reactor current limiting → crowbar current shunting → fuse current breaking.

This protection method takes advantage of the strong T surge capability of the crowbar circuit and has a good effect on short-circuit overcurrent protection of GTO inverters, so it has been used to a certain extent.

3) Self-shutdown protection method

The self-shutdown capability of GTO can also be used as overcurrent protection. The anode shutoff capability of GTO includes two parameters: the shutoff on-state repetitive peak current and the shutoff on-state non-repetitive peak current. 200A The following small capacity GTO can turn off the non-repetitive peak current in the on-state, which is much larger than the repetitive peak current in the on-state. This characteristic can be used for overcurrent protection. This method is called self-shutdown protection method.

4. Overcurrent protection of gate circuit

When the GTO is damaged, the anode and the gate are often in a short-circuit state, and the external high voltage is introduced into the gate circuit through the anode, causing damage to the gate circuit. For this reason, a protection link for the gate circuit should be set. The specific measures are as follows:

(1) Connect a fast fuse to the output end of the gate circuit to achieve overcurrent protection so that the gate circuit can be disconnected from the GTO gate terminal as quickly as possible;

(2) Connect a Zener diode to the output end of the gate circuit to clamp the gate circuit within the safe voltage range.