Protection circuits for power electronic devices

Protection circuits for power electronic devices

In power electronic circuits, in order to make the power electronic devices can be used normally without damage, in addition to the reasonable selection of the parameters of the power electronic devices and the careful design of the driving circuit, necessary protection measures must be taken against the damage of overvoltage, overcurrent, du/dt and di/dt.

1. Over voltage protection

When it exceeds normal operation, the maximum peak voltage that power electronic devices should withstand is called overvoltage. There are two types of reasons for overvoltage in power electronic devices: external overvoltage and internal overvoltage.

External overvoltage mainly comes from external causes such as lightning strikes and operating processes in the system:

1) Lightning strike overvoltage: Overvoltage caused by lightning strike.

2) Operational overvoltage: overvoltage caused by switching operations such as opening and closing. When the circuit is closed and the power is turned on, the high voltage on the grid side is directly transmitted to the secondary side power electronic converter through the distributed capacitance between the primary and secondary windings of the transformer. When the circuit is opened and the transformer is disconnected, the overvoltage caused by the excitation current on the primary side of the transformer is suddenly cut off, which will be induced to the secondary side, causing the switching devices of the power electronic converter to withstand the operating overvoltage.

The internal overvoltage mainly comes from the switching process of the internal devices of the power electronic device, mainly including commutation overvoltage and turn-off overvoltage:

1) Commutation overvoltage: The thyristor or the diode in anti-parallel with the fully controlled device cannot resume blocking immediately after commutation, so a large reverse current flows, when the blocking ability is restored. The reverse current decreases sharply, which will induce a large self-inductive back electromotive force on the line inductance. After the back electromotive force is added to the power supply voltage, it may act on both ends of the device, which may cause the device to be damaged by overvoltage.

2) Turn-off overvoltage: When the fully-controlled device is turned off, the forward current decreases rapidly, and a high induced voltage is generated on the line inductance.

Figure 1 shows possible overvoltage suppression measures and their configuration in a power electronic device.

Protection circuits for power electronic devices
Figure 1 – Overvoltage suppression measures and their configuration

When the lightning overvoltage enters from the power grid, the arrester F discharges to the ground to prevent the lightning from entering the transformer. C is the electrostatic induction overvoltage suppression capacitor. When S is closed, the high voltage of the power grid is applied to the transformer, and the AC high voltage of the power grid is directly transmitted to the secondary side through the coupling capacitor of the transformer. Since the capacitor C is large enough to absorb the overvoltage, the subsequent switching devices are protected from the overvoltage of the closing operation.

Resistor-capacitor RC overvoltage suppression circuit is the most common and effective protection method for overvoltage. It absorbs overvoltage by using the characteristic that capacitor voltage cannot suddenly change, and resistor consumes absorbed energy and suppresses the oscillation of the loop. The position of RC is different, and the focus of protection is different. The connection method of RC2 and RC3 is the direct protection of the device. Its typical connection method is shown in Figure 2. A reverse blocking RC overvoltage suppression circuit is used in large-capacity power electronic devices, as shown in Figure 3. This circuit effectively suppresses the surge peak voltage generated by overvoltage and capacitor discharge. For the relevant parameters of the protection circuit, please refer to the relevant engineering manual.

Protection circuits for power electronic devices
Figure 2 – RC voltage suppression circuit connection

Rv is a metal oxide varistor, which is a nonlinear overvoltage protection element, and its volt-ampere characteristics are shown in Figure 4. During normal operation, the leakage current is microampere; when the surge voltage comes, thousands of amps can pass through, so the component can absorb the surge overvoltage on the AC side well.

Protection circuits for power electronic devices
Figure 3 – Reverse blocking RC overvoltage suppression circuit

2. Overcurrent protection

Overcurrents can occur when power electronics and control systems do not operate properly or fail. The overload capacity of power electronic devices is low, and overcurrent will cause permanent damage to power electronic devices. Overcurrent is usually caused by a short circuit or overload. The overcurrent protection measures that may be used in the power electronic control system are shown in Figure 5.

Protection circuits for power electronic devices
Figure 5 – Overcurrent protection measures commonly used in power electronic control systems

A current transformer is set on the AC side of the power electronic device to detect the line current, the overcurrent signal is sent to the trigger circuit through the switch circuit, so that the trigger pulse is stopped instantaneously or the pulse is moved backward, so as to turn off the power electronic device and achieve the purpose of suppressing the overcurrent; the overcurrent signal can also pass the overcurrent relay to make the AC circuit breaker. The contacts are disconnected, and the overcurrent relay is set to act when overloaded. The DC fast switch can act before the fast fuse when overcurrent occurs, and can be used in high-power converters and high-demand occasions with more short-circuit possibilities.

Fast fuse (referred to as fast fuse) is the most effective and widely used overcurrent protection measure in power electronic devices. When choosing a fast melt, consider:

1) The rated voltage of the fast fuse should be determined according to the actual voltage of the fast fuse after the fuse.

2) The current capacity of the fast fuse should be determined according to the way it is connected in the main circuit and the connection form of the main circuit.

3) The I2t value of the fast fuse should be less than the allowable I2t value of the protected device.

For example, in the thyristor rectifier circuit, the rated current IRD of the fast fuse connected in series with the thyristor should be less than the rated current RMS of the protected thyristor, 1.57IT(AV), and at the same time, it should be greater than the actual maximum RMS value ITM flowing through the thyristor. which is

1.57IT(AV)≥IRD≥ITM      (1-1)

3. Static electricity protection

Power MOSFETs and IGBTs are both voltage-controlled devices. Due to their extremely high input impedance, it is difficult to discharge charges in situations with high static electricity, which is easy to cause electrostatic breakdown. To prevent electrostatic breakdown, attention should be paid to:

1) Devices should be stored in antistatic packaging bags, conductive material bags or metal containers.

2) During installation or testing, the workbench, soldering iron and measuring instruments should be well grounded, and the staff should be well grounded through the wrist strap.