How to choose a medium voltage VFD:
Protection concept
A variable speed drive system with gaps in protection philosophy is a ticking bomb. It might have a huge impact on plant availability and potentially also on personal safety. Therefore, no compromises shall be accepted. Other topics in this series are definitely important as well, but this one shall be taken very seriously! Let’s have a closer look.
VFD as part of a drive system is the component wit most ‘brain power’. Logically, apart from control and supervision, VFD is predestined for protection purpose. But not for entire protection and not every VFD provides the same level and quality of protection functions.
A well designed VFD shall fully protect itself and the downstream system which is in most cases directly the electric motor. Some of the leading manufacturers have fuseless protection concept (medium voltage current limiting fuses are not required). Exceptions are usually VFDs with integrated transformers in case the solution shall be arc certified. The fuseless concept normally includes the incoming circuit breaker (ICB) that must fulfill certain characteristics. The VFD manufacturer shall provide you with a specification for the ICB where the requirements are clearly stated.
Before ramping up the motor
Most voltage source inverters have a pre-charging circuit that charges the capacitors in intermediate dc bus before the incoming main circuit breaker is closed. This sequence eliminates large inrush current when switching on the medium voltage to empty capacitors. It shall not be allowed to close the main circuit breaker if the dc bus voltage did not reach certain minimum level (e.g. 90% of nominal voltage). Liquid cooled drives shall have a sophisticated supervision of cooling system. It shall not be allowed to switch on medium voltage when the conductivity of cooling medium is too high (that is typically the case after longer period of standstill). Some VFDs perform a semiconductor test during start-up sequence. They basically check whether all semiconductors are healthy before the motor starts to ramp up the speed.
During normal operation
In normal operation mode the VFD shall monitor all key parameters such as dc voltage (VSI type), inverter output current, internal temperatures inside cabinets, temperature of cooling medium, pressure of cooling medium etc. Liquid cooled drives also monitor the conductivity of cooling liquid and active front end (AFE) drives supervise the supplying grid and active rectifier.
It is a standard feature that VFD also fully protects the driven electric motor against overload, overcurrent, overvoltage, stalling, underfrequency etc. Basically a good VFD shall fully replace a motor protection relay and shall provide the same level of motor protection. This is of course the preferred variant as it reduces the complexity. Moreover, not many motor protection relays support variable frequency operation and the inverter duty with non-sinusoidal waveforms. VFD usually also supervises the temperatures of transformer and motor (Pt100 sensors located in windings, bearings etc).
Special care shall be given to VFDs with output sine filter or with very long motor cables where at certain motor speed the self excitation phenomena might appear. This is typically not a problem when motor is driven by VFD, but the risk is there if motor e.g. coasts down after a trip and passes through the self-excitation speed.
During fault condition
The faults are obviously the most critical scenarios. The protection must detect the fault fast and reliable, act fast and bring the VFD back into safe mode. First priority is personal safety, second priority is elimination or limitation of hardware damages.
When it comes to personal safety no compromises shall be allowed. Medium voltage compartments shall be interlocked and never accessible to personnel when the VFD is energized. Robust electro-mechanical door interlocking is a frequent solution. In addition, some customers require purely mechanical key interlocking system where the VFD might also be interlocked with incoming switchgear or other components.
Arc fault is an actual topic and most manufacturers can offer an ‘arc proof design’. However, some manufacturers offer solutions based on cabinet structure (reinforced cabinet with special overpressure flaps – everything inside gets destroyed, but personnel is not endangered) or a combination or reinforced cabinet structure and fuses or fused contactor. Some manufacturers promote separated control cabinet so that the personnel does not come even close to the medium voltage cubicles. Such solution might require overall more space and, same as previous, does not prevent complete damage of the hardware. More advanced solutions are based on fast arc detection and elimination. This method has several optical sensors inside the cabinets. When an electric arc is detected, all controlled semiconductors are turned on (inverter switches as well as crowbar). The energy gets distributed in multiple paths and the arc is cleared within few milliseconds. Such solution provides not only personal safety, but also minimizes or completely eliminates hardware damages. The benefits are obvious.
The faults can be classified according their severity into few categories. Less severe fault might cause inverter to stop the modulation and display a fault on the panel. Severe faults require quick reaction of the VFD. Such faults are mainly overvoltage and overcurrent cases.
The VFD might also protect the input isolation transformer – at least the converter side (secondary) windings. Protection of multi-winding converter transformers (rectifier duty transformers) requires special attention and we will issue a dedicated post on that specific topic. At this stage we mention at least few things to be careful about: detectability of short circuit when only one secondary winding has a fault, asymmetric fault and corresponding dynamic forces (telescope effect), diode fault (‘arc back’) or fast transient overvoltages triggered by a vacuum circuit breaker.
Check carefully what protection functions the VFD includes – both hardware and software based protection. It directly impacts the protection scheme of the complete drive system. VFD with poor protection concept might require additional protection devices resulting in higher cost (equipment cost, but also engineering) and complexity and potentially lower reliability. Consider also what it means for the operator to adopt the protection philosophy. Be sure that there are no white spots in your protection concept.
References
Few references on the topic… You can find lot of information on the internet, although seldom going deeper into detail. A great source is the IEEE Xplore library (if you have the access). Setup a direct discussion with manufacturer if you need more information. And ask for certificates in order to get more confidence.
1. ABB MV Drives – Managing safe applications
2. ABB’s ACS5000 medium voltage VFD (6.0 – 6.9 kV, up to 36 MVA+)
https://new.abb.com/drives/medium-voltage-ac-drives/acs5000
3. ABB’s ACS6000 medium voltage VFD (3.0 – 3.3 kV, up to 36 MVA)
https://new.abb.com/drives/medium-voltage-ac-drives/acs6000
4. Eaton – Arc resistant medium voltage VFD
5. Rockwell Automation – Arc shield (PowerFlex 7000)
6. How to choose a medium voltage VFD – entire series, https://mb-drive-services.com/category/choose-mv-vfd/
