VFD protection reaction
Reliable protection is key for long term successful operation of a variable frequency drive (VFD). There is a wide range of situations outside of the normal operation to what the drive system shall respond. In this blog post we briefly explore the VFD protection reaction.
Introduction
VFD protection shall provide the basic requirements put on any protection system, such as:
- Reliability
- Selectivity
- Sensitivity
- Speed/Reaction time
- Safety
Reliability
VFD protection function shall be reliable with regards to the detection mechanism, robustness against disturbance etc. Sometimes, the same type of fault can be detected by multiple independent protection functions. One of them serves as a primary protection function (more reliable, faster etc) while the other one provides a backup protection. On the control hardware level a (partial) redundancy may be provided.
Selectivity
The protection is active within its protection zone and is coordinated with upstream protection. Applicable protection zone covers the VFD itself and the downstream system, which is normally the electric motor.
Sensitivity
Sensitivity is about the threshold level where the protection is activated. In VFD applications, the sensitivity can change with the operation point of the VFD (e.g. earth fault detection on the motor side).
Speed
Fast detection and response time help to minimize the hardware damages. That is especially important for power electronic converters due to their very short thermal time constant and very limited overloadability. Short duration of fault reduces the amount of energy dissipated into the fault (e.g. during an arc fault). That turns into a lower risk for the personnel (see safety below).
Safety
Personal safety is a key priority and lot of effort is made to minimize any risk for the people being in the vicinity of the VFD. Various protection functions are utilize to provide personal safety. They range from safety earthing and electromechanical interlocking of medium voltage compartments up to arc fault detection.
Self protection and protection of other system components
The primary aim of the VFD is to protect the personnel and the VFD hardware itself. However, VFD is able to detect abnormalities and faults also outside of the VFD itself. Such protection can be used as primary protection function or as a backup protection in case the main protection functions fails to detect the fault and react correspondingly.
Protection concept
The protection concept is generally quite similar among various VFD topologies and models. However, some differences exist. One specific difference depends on the type of power semiconductor. While IGBT can switch off a short-circuit current, classical thyristor and IGCT can withstand the short-circuit current for a defined period of time. That results in a different VFD reaction in case of a short-circuit event.
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Classification of faults
Understanding the type and nature of fault is key to decide appropriate VFD reaction. The faults have different origin, character and severity. They are usually sorted in several groups. For each group there is a clearly defined VFD reaction.
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Protection reaction
Principally, three basic reactions of VFD with respect to fault occurrence can be expected:
- VFD issues an alarm and remains in operation
- VFD trips by stopping the inverter modulation → no active power delivered to the motor anymore
- VFD trips by stopping the inverter modulation and sending opening command to incoming circuit breaker → no active power delivered to the motor, disconnection of drive system from power supply, discharge of the dc link
Remark 1: An active short-circuit may be part of protection reaction. Although it sounds harsh at first, it is an effective field-proven method to distribute the fault current into multiple parallel paths. Moreover, creating a symmetrical short circuit has a very positive impact on motor or transformer compared to asymmetrical fault.
Remark 2: Reaction to certain faults can be parameterized by the user while other reactions are hard-coded. For more information purchase our premium subscription or a dedicated webinar.
Severity of faults
Obviously, some faults are more severe than the others. While in one case it is sufficient for the VFD to stop the inverter modulation and cut the power to the motor, other faults require isolation from the supply grid by opening the incoming circuit breaker and discharging the dc bus.
Examples of severe faults
- Overcurrent (rapid increase of current, high magnitude)
- Overvoltage in dc link
- Excessive semiconductor junction temperature (if monitored)
- Failure of the VFD main cooling system
Examples of moderate faults
- Slow increase of conductivity of cooling water finally reaching a trip level
- Unbalance detection (in dc bus or to the motor)
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