How to choose a medium voltage VFD:
Engineering support
Engineering support from VFD manufacturer significantly influences how smooth the execution of your project goes and how successful it will be in the end. It also directly impacts the quality of the system solution. Some manufacturers focus on selling their standard ‘catalog products’ with limited or no engineering support during execution phase. Others can provide you with full set of engineering services. Yes, you can also hire a third party company for the engineering. However, this company will need inputs from VFD manufacturer as well and if he is not able to support then the issue is still unsolved.
Let’s talk about few areas of engineering support:
Engineering documentation
It all starts with documentation as a valuable source of engineering details. The VFD datasheet shall contain relevant technical information, such as type of VFD and its topology, load profile, input and output voltages and currents, output frequency range, continuous output rating and short time overloadability, ambient conditions, cooling requirements, heat losses (for liquid cooled VFDs the split between losses into liquid and losses into ambient air), auxiliary and control supply information, protection requirements, dimensions and weight (or a reference to outline drawing), details about enclosure, list of all included options, list of spare parts (if applicable) etc. The outline drawing shall include information about overall dimensions and weight, for larger VFDs information about transport sections and their individual dimensions, center of gravity, cable connections and terminations, cooling inlet and outlet and space requirements for installations and maintenance. Wiring diagram provides clear information about accessories and available analog and digital I/Os.
Besides datasheet and outline drawing of the VFD you need specifications for other system components, detailed user manual, installation manual, maintenance schedule for service and so on.
There will be cables between transformer and rectifier (unless it is a drive with integrated transformer) and between inverter and motor. What is the required insulation voltage of cables? Do you need electrostatic shield or not? What is the maximum cross section of cables? Shall you select single core or three core cables? How about equipotential bonding? VFD manufacturer shall provide you with cable specification. Same applies for auxiliary and signal cables.
Is the transformer external (i.e. not integrated in drive enclosure) and not in scope of supply of VFD manufacturer? Then you shall receive a specification for such transformer with fundamental electrical requirements (mechanical design to be specified by you considering installation, ambient conditions, space requirements etc). In most cases normal distribution transformer is not suitable. You might need a multi-winding design, increased insulation, specific impedance, harmonic spectrum for thermal design etc. All this shall be written in transformer specification received from VFD supplier.
Motor specification is required to make sure that motor fits with the selected type of VFD and is properly designed. The specification shall include information about inverter output voltage and current waveform and their spectra, voltage rise time (dv/dt), common mode voltages, torque ripple etc on top of project requirements (shaft power, speed range, cooling,…).
The manufacturer of driven equipment might also need the information about torque pulsations in normal operation (spectrum with magnitudes and frequencies) as well as transient torques during fault conditions as he needs to perform a torsional analysis of the whole shaft string. VFD manufacturer shall provide required inputs for the torsional analysis and also inform if there are any specific features for passive or active damping of torsional vibration.
How is the VFD protected? If fuses are part of protection concept you shall receive a detailed specification of the fuse characteristic. Fuses are aging so you would need to replace them in certain intervals. Those intervals shall be defined by manufacturer. If the VFD has fuseless protection (better option in our view) then there will likely be specific requirements on the incoming main circuit breaker (e.g. opening time) and the interface between the breaker and the VFD.
Software is a mighty tool. It is also an area where the VFD manufacturers can significantly differ. You might purchase more or less the same hardware topology from several suppliers, but their control software features might be very different! It is essential to know what software functionality is included and how powerful the brain of the specific VFD is. Having flexible software with several non-standard add-on functions is always a good choice. There should be a clear description of the software functions and parameters including their default values (factory settings), simple and understandable rules for tuning including examples etc.
Besides all the specifications and standard documents it is very helpful if drive manufacturer can share engineering guidelines on selected topics describing good engineering practice and hints. Such documents are valuable source of information as you don’t need to repeat all the mistakes done by your predecessors.
System calculations and studies
System calculations and system studies are other services that you might need. You would like e.g. to check the compliance with the grid code at the connection point. The drive manufacturer shall be able to provide you with the harmonic calculation at the specified point of common coupling (PCC). He shall be flexible to update the calculation in case the grid parameters change. In complex grid configurations with potential resonances a network harmonic study is recommended. Unlike the simplified harmonic calculation the network harmonic study models the grid much more precisely and takes the resonances into account. On the motor side project specific simulations might be needed to qualify a motor for specific inverter model. The interaction between drive system and driven load is important. The manufacturer of the load machine might request information about pulsating torques and eventually also control algorithms since he is obliged to perform a torsional analysis of the entire shaft string. Immunity against grid voltage dips is crucial for a robust VFD operation. If the ride through capability is based on kinetic buffering the VFD manufacturer shall calculate how long the VFD can bridge the voltage dip. He shall also be able to simulate the VFD behavior when a specific grid disturbance (representative for given project) is provided. Other small calculations or checks are for instance time to trip in case of loss of cooling, start-up calculation for specific load profile or start-up time from hot standby. There can be many more calculations – above examples are just for illustration. Some typical calculations are listed below:
– harmonic calculation (with simplified network representation)
– network harmonic study (with detailed network representation)
– other network studies (load flow, load rejection, fault study)
– system efficiency calculation (for arbitrary load point)
– energy savings calculation and payback time
– cable dimensioning
– start-up (and braking) calculation
– ride through calculation or simulation
– pulsating torque calculation or simulation
– loss of cooling -> time to trip
– virtual commissioning (for demanding projects)
The manufacturer shall be in possession of verified analytic and simulation tools to support you with calculations and studies around VFD and its integration into a larger system.
VFD customization (engineered design)
Ideal VFD manufacturer is capable of providing a balance between standardized and proven VFD design on one hand and flexibility for project customization on the other hand. The idea is to keep the standard design when possible (verified and field proven), but sometimes engineering adaptations are unavoidable.
One of the typical tasks is an engineered layout. Standard layout might not fit due to space limitations (e.g. existing building) and manufacturer can alternatively offer layouts such as L-shape, U-shape or Back-to-Back configurations. For some VFDs these layouts might be semi- or fully standardized (modular concepts).
Frequent engineering task is to integrate some additional project components into the VFD lineup, e.g. transformer protection relay, power quality meter, input or output isolation switch (disconnector) etc. Customer might require additional sensors or e.g. thermographic windows for maintenance purpose. Some customers wish to have analog meters on the cabinet doors in addition to standard displays on control panels and trend windows. They might also want extra signalization lamps in line with their internal guidelines (all drives have similar look for the operator).
Other area is the cooling circuit. For air cooled VFDs the customer might wish to have an air duct instead of standard solution with roof-mounted fans. For liquid cooled VFDs it might be an engineered heat exchanger (e.g. for aggressive water), redundant heat exchanger etc. Engineering adaptations might be required for a VFD working in extreme environmental conditions (very high ambient temperature, presence of dust etc).
Integration into E-house typically also requires certain customization and engineering support. Special connections (e.g. bus duct instead of cables) usually requires modifications from mechanical engineering. Motor cables exceeding standard length are usually supported as engineered option, too. It typically implies certain modifications at the inverter output.
Even if your application appears simple, we always recommend to select a VFD supplier that can provide the full range of engineering services. In this way you have a backup in case something unexpected pops up.
Engineering support: Summary
There are many manufacturers on the market, but obviously not each of them can provide you with the same level of engineering support. Keep it in mind when selecting a medium voltage VFD.
References
[1] How to choose a medium voltage VFD: System integration, https://mb-drive-services.com/selecting-mv-vfd-system-integration/
