Types of Bypass Technologies in Variable Frequency Drives
Power cells constitute the most intensively operated parts of any variable frequency drives. As a general rule, this section is the one that suffers the most from a crucial damage caused by factors unfriendly to equipment such as heat generation, current load, voltage, load response. A bypass function is applied to preserve the efficiency of the variable frequency drive during performance of critical technological processes with unacceptable emergency situations. This mode is provided by a set of software and hardware tools in such a way that the problem cell shuts down while the variable frequency drive continues operation.
Currently there are several bypass technologies employed by electrical engineering manufacturers:
- Transistor bypass
A transistor bypass is a time-proven solution considered to be traditional. However, the technology has limitations on a type of accidents. This is due to capabilities and physics of the IGBT transistors operation within the scope of functioning in critical modes. The transistor bypass proves itself in situations such as an accident of power tongs (including insulation breakdowns, failures, but without a closed-circuit fault) and a driver malfunction (also without the closed-circuit fault). In some cases, this approach just does not work.
- Thyristor bypass
A thyristor bypass is widely used by many manufacturers, but, at the same time, is characterized by an over-reaction sometimes. A sensitivity to du/dt (or voltage growth rate) often leads to an inadvertent opening of the bypass. Activation of the thyristor bypass during regular operation of the variable frequency drive results in an emergency situation that can even lead to a failure of the power cell. To prevent such situations, quite a lot of different approaches are used, but there is no perfect solution at the moment, so that in some cases the inadvertent triggering occurs all the same.
- Electromechanical bypass
An electromechanical bypass is a technology applied rather rarely, but is distinguished by its reliability and versatility. Regardless of a mode of emergency processes occurring in the cell or in a DC link, the electromechanical bypass is capable to endure almost anything. This type of the bypass has an enormous overload capacity, on the one hand, and an absence of the du/dt sensitivity, on the other, which makes this solution the most optimal way to keep the equipment in operation. At the same time, a closing speed constitutes a relative complexity of this technology.
Engineers of Triol Corporation are in a continuous search for solutions capable to increase a reliability of the equipment. To the present day, they have developed the electromechanical bypass technology to make an efficient respond in several milliseconds. This amount of time is enough to avoid overvoltage in the DC link, when the inverter unit transistors close by virtue of an emergency and only reverse diodes, providing energy from an electric machine into capacitors for accumulation, start operating.
There is a similar offer for electromechanical bypass technology involving exploders is developed by other manufacturers and introduced to the market. But implementation of the exploders requires a thorough inspection of equipment for any damages caused by any small explosion. In addition, the spent exploders should be replaced after each operation. The electromechanical bypass applied in Triol AT27 variable frequency drives is distinguished by a usage of an electromagnet instead of the exploders, which makes this Triol technology much safer and excludes a need for a subsequent elements replacement.
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