Introduction to 6KV overvoltage protection for variable frequency drive VFD/IGBT inverter

In all industrial control systems, variable frequency drives (VFDs)/inverters are usually installed in front of the motor to regulate speed and save energy. According to different input voltage requirements, inverters are usually divided into low voltage (110, 220 and 380V), medium voltage (690, 1140 and 2300V) or high voltage (3, 3.3, 6, 6.6 and 10kV).

Since VFDs/inverters are key components in providing power, their operation, performance, and reliability are critical to maintaining uninterrupted power. This application note focuses on TVS diodes that can help ensure the reliability and operation of VFDs even under adverse conditions.

Since VFDs/inverters are key components in providing power, their operation, performance, and reliability are critical to maintaining uninterrupted power. This application note focuses on TVS diodes that can help ensure the reliability and operation of VFDs even under adverse conditions.

In the figure above, the orange blocks highlight the AC line fuses to provide overcurrent or short circuit protection. The blue boxes represent transient suppressors, which are used to prevent damage from lightning-induced surges or harmonic voltage interruptions on the power line. Metal oxide varistors (MOVs) are widely used in high voltage AC circuits due to their high power performance.

The required MOV rating will depend on the expected level of surge current and the AC line circuit topology involved. For example, if the AC 380V power supply is connected as shown in Figure 1, in order to meet the 3kA, 8/20µs lightning surge requirement, three 20D391K (250V) high surge current radial lead varistors can be used for line-to-line (differential mode) protection,

Three MOVs are connected in a star configuration and grounded separately from another GDT. This will provide both line-to-line and line-to-ground protection. When selecting MOVs, the clamping voltage must also be considered. For this case, when two 20D391K varistors are connected in series to provide differential protection, they produce a clamping voltage of approximately 1400V.

For rectifier diodes, capacitors and IGBTs (insulated gate bipolar transistors), the rated voltage should be greater than this value to provide adequate voltage protection for the entire system. Sometimes, it is difficult to provide voltage protection for the entire system, especially for high-power line voltage applications of 600V or more. For these cases, Leimao Electronics recommends using additional (or secondary) high-power TVS diodes, which have precise low-voltage clamping capabilities to provide differential protection for rectifier diodes, capacitors and IGBTs.

As shown in the pink box in the figure above, two AK3-380C TVS diodes can be added to provide differential protection during surge currents up to 3kA, 8/20µs. The AK3-380 is a bidirectional high-power TVS diode rated at 3kA (8/20µs) with an isolation voltage of 380V. The maximum clamping voltage of two AK3-380C TVS diodes is 1040V, which is much lower than MOVs.

As a result, they provide excellent surge protection for the 6-pack IGBTs while keeping the circuit under normal operating conditions. Adding these TVS diodes gives designers greater flexibility in selecting rectifier diodes and IGBTs.

Leimao Electronics EMC Laboratory provides free lightning surge testing.