Why do VFDs sometimes trip during grid faults?

Grid disturbances and faults appear now and then. And it is desirable that the VFD does not trip every time such event occurs. Thus certain immunity against grid disturbances is desired. In fact, many products claim to have a capability to withstand such disturbances. But how does the reality look like? And why do VFDs trip during grid faults?

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Operational experience shows that a considerable number of VFD trips is associated with grid disturbances. Thus, an increased robustness against such disturbances would significantly increase the availability of the drive system. As a result, the end user would benefit from higher productivity and improved profitability. This article aims to explain the potential root cause of VFD trips along with some proposals for improvement.

Types of grid faults

Classification of grid faults can be as follows:

A. According to symmetry

– Symmetrical (balanced) faults

– Asymmetrical (unbalanced) faults

B. According to the location

– Three-phase grid fault

– Three phase to ground grid fault

– Phase-to-phase grid fault

– Two phase to ground grid fault

– Phase-to-ground grid fault

C. According to distance

– Close faults

– Far faults

D. According to duration

– Transients

– Temporary

– Long term

Grid disturbance may generally be somewhat less severe than faults. These may include voltage dips and sags caused by across the line starting of larger motors or energizing of transformers. Some loads are inherently asymmetric and may lead to certain unbalance in the grid. Finally, there is the phenomenon of voltage distortion due to presence of harmonics.

Duration of grid disturbance

Although some grid disturbances can be foreseen, most of them have a stochastic character. It means that they appear “randomly” and their duration varies. Despite of that we can say that most disturbances are quite short with duration of less than 200 ms. In fact, most grid faults statistically appear for some 60 to 100 ms.

VFD response during grid faults

For majority of VFD applications the expectation on the grid fault performance can be formulated simply as “avoid tripping”. That is the foremost requirement. It is frequently called ride through capability – the VFD is supposed to ride through the grid fault and resume the normal operation when the grid recovers. The performance during the grid fault can be limited. Most common reaction is to ramp the torque down to zero during the ride through operation. Another option is to still provide some portion of the rated torque.

In case the VFD is not just a consumer but supplies power to the grid as well (bi-directional power flow), the grid code may become applicable. Then the expectations on the VFD increase. It shall not just stay online during the fault but shall also provide grid supporting functions, for example injection of defined amount of reactive power. Whether the grid code is applicable or not and how strict the requirements are usually depends on the power rating of the drive system and the nominal voltage of the grid.

Immunity of the driven load

There are few key factors that decide how robust is the driven load against the grid disturbances. Some loads are inherently more immune against temporary torque reduction while others are more sensitive. And inertia is just one of those factors…