Are surge arresters suitable for protection of VFD fed motors?
Surge arresters are commonly used to protect electric equipment against excessive overvoltages. At the same time, many voltage source inverters produce non-sinusoidal voltages that stress the machine insulation. Thus, one could ask whether surge arresters are suitable means to protect the motors supplied from variable frequency drives?
Overvoltage protection in general
Overvoltages can be long term, temporary or just a short transient (surge). Surge or spike is a short-term high voltage typically much more than 110% of the nominal voltage. Transient overvoltage can have atmospheric origin (lightning strike) or a switching transient in the grid. A popular and effective protection against transient overvoltages is the use of surge arresters¹. These devices have highly non-linear impedance as a function of applied voltage. In normal operation (below the threshold voltage) the surge arresters have very high impedance and only negligible leakage current is flowing through the arrester. When the voltage exceeds the threshold the impedance drastically drops and the surge arrester creates a path for the surge current. Surge arresters are typically applied phase to ground, between phases or a combination of both.
¹ The spelling can be both surge arrester or surge arrestor. Another naming is surge protection device (SPD).
Insulation stress of inverter fed motors
Basic considerations about protection of variable speed motors were stated in [1]. An inverter supplies the motor with a voltage waveform that is mostly not of the same quality as when the motor is directly connected to the grid (DOL). The waveform is characterized by voltage steps with rate of change (dv/dt) up to 10 kV/μs. Moreover, there is a common mode voltage – a zero sequence voltage appearing between neutral point and ground – that superimposes to the phase voltage of the machine. The common mode voltage “jumps” as the inverter is modulating. The peak line to line voltage depends on the dc link level and is typically some 5 to 10% higher than a peak voltage of ideal sine wave. Finally, there can be voltage overshoots and high frequency ringing as a result of the interaction in the inverter-cable-motor system [2].
The motor must have adequately graded stator insulation system to cope with the inverter voltage waveform without reducing the motor lifetime or even causing a premature failure. The reinforced insulation increases the cost of the motor and can impact the performance as well (e.g. cooling).
This blog post just scratches the surface…
Do you want to know more? There are many possibilities:
- Purchase our premium subscription
- Sign up for a specific webinar
- Purchase one of our publications
- Request a customized drive training
Are surge arresters suitable for protection of VFD fed motors?
A frequent question is whether the surge arresters could be used to protect a VFD fed motor against overvoltages that appear in the normal operation? May it be a low-hanging fruit? The short answer is ‘No’. Most drive systems with a voltage source inverter will not benefit from surge arresters. In opposite, installation of surge arresters would likely cause an issue.
A more detailed reasoning is available for our premium subscribers. There you learn:
- Why surge arrester does not provide an adequate protection
- Difference between application on grid side and machine side
- What are the risks of having surge arresters on a VFD fed motor
- Few exceptions when a surge arrester may be a viable option
References
[1] Protection of VFD-fed motors, https://mb-drive-services.com/protection-of-vfd-fed-motors/
[2] Long motor cables, https://mb-drive-services.com/long-motor-cables/
[3] Hitachi Energy surge arresters offering, https://www.hitachienergy.com/products-and-solutions/surge-arresters
Do you wish to explore more about this subject and many other topics related to variable frequency drive systems? Purchase our premium subscription or one of our specialized webinars.
