Current, voltage and power scaling of VFD
Variable frequency drives (VFD) became unthinkable part of industrial motor drives. They cover voltages from some 120 V up to 13.8 kV (special designs go even higher) and power range from fraction of kW until 100 MW and beyond. Enabler of such wide range is the scalability of the products. In this post we will review commonly used topologies in medium and high power range with regards to their scalability. This article taks you through the power scaling of VFDs.
Scalability and modularity
In order to cover a range of motor voltages and shaft power it is important to have a scalable power hardware platform. Increase of VFD power rating beyond the capability of base power module can be achieved by:
(a) Paralleling
(b) Series connection
(c) Series-parallel combination
Power scaling of VFD
A. Paralleling
Parallel connection of semiconductors or modules allows higher current rating beyond the capability of a single device.
The challenge in paralleling of multiple semiconductors is to ensure equal current sharing. As a matter of fact the current will not distribute perfectly equally. Switching device with lower internal impedance draws more current. The unbalance needs to be properly assessed in the design of the VFD. Only semiconductors of same accuracy class are paralleled and a derating factor shall be applied.
A more convenient way is to parallel whole building blocks or complete inverters. It is much easier to handle the current sharing in this way. Distribution of the current is influenced by the hardware design and often dynamically compensated by VFD control.
B. Series connection
Purpose of series connection is to increase the output voltage. it is used to match the required machine (or grid) voltage and to scale the power with voltage.
Series connection also has its challenges. Most important is to control the voltage distribution across the series connected devices, both in steady state as well as dynamically. When one device switches on a bit earlier (change from off state to on state) it does not block the voltage anymore, but the rest of the chain still must block the full voltage. Hence, voltage stress across each device would increase. Series connected devices typically use a snubber circuit that improves the voltage distribution. The switching pulses are usually transferred optically to minimize any delays.
Similarly as for paralleling, it is possible to make a series connection of power semiconductors or power modules/cells.
C. Series-parallel combination
Both paralleling and series connection have their pros and cons. In order to reach high power a combination of both might be necessary.
In case of paralleling the total current at high power becomes cumbersome to handle. It would negatively impact the inverter-machine connection (cable or busbar) as well as the machine itself.
Series connection has its limits as well. Industrial motors are available up to certain rated voltage. Too high voltages are not supported as the insulation system is the limiting factor.
From above reasons a series-parallel connection sometimes becomes inevitable. Some VFD topologies are even based on such a combination.
Of course, there is also the possibility to use a transformer between inverter and machine. While such solution helps to adapt the voltage, it is mostly not the preferred one. Moreover, it might solve the voltage mismatch, but our primary focus here is on power scaling.
Some topologies are suited for easy paralleling, others are determined for series connection. In the next section we will look at the most common VFD topologies from perspective of scalability.
Load-commutated inverter
For our frequent readers we don’t need to introduce the load-commutated inverter (LCI). There is a whole series of articles dedicated to LCI and its comparison with voltage source inverters [1]. LCI is a typical example of VFD topology using the series connection. Multiple thyristors are connected in series to achieve the desired output voltage. RC snubber circuit supports the voltage distribution across the thyristor stacks. Parallel connection is not used. Thyristors are inherently suitable for quite high currents.
The manufacturers of LCI drives usually utilize 2-3 thyristor models with different current rating (e.g. 1’500 A, 2’200 A and 3’000 A). The power dimensioning is then basically a selection of one thyristor model and determination of number of series connected devices. In high power range it is very common to use 12-pulse inverter in combination with dual three-phase machine.
Neutral point clamped inverters
Neutral point clamped (NPC) is a popular and widely used topology within voltage source inverters. There are few variants, such as classical NPC, active neutral point clamped (ANPC), neutral point piloted (NPP). Majority of these VFDs is of a 3-level topology with regards to output waveform. Voltage in the dc link is around 5 kV resulting in output voltage of ~ 3 kV. To reach higher power the inverter modules are paralleled. Therefore, power is scaled with current while the rated output voltage is kept constant or can be adapted in rather narrow range (by changing nominal dc link level). There are also products using 3-level NPC for 6 kV output voltage.
NPC topology as such is a typical example of paralleling. Instead of paralleling devices (semiconductors) it is preferred to parallel rectifier or inverter modules.
Cascaded H-bridge
This is another widely used voltage source inverter topology. As the name already indicates it is based on series connection of cells. Although used in medium voltage the cells themselves are usually low voltage based with nominal voltage in the range 600 – 750 V. Depending on the number of cells the output voltage can be scaled. To match the current rating few semiconductor models with different current ratings can be used. In this sense the basic philosophy of power scaling is actually quite similar like in LCI (but the semiconductor is very different with all its pros and cons). For medium/high power a series-parallel combination can be used.
Neutral point clamped with H-bridge
This topology evolved in order to use well-proven neutral point clamped (NPC) scheme for higher output voltage and more voltage levels. Basically three 3-level NPC modules, each having single phase output towards the machine, are connected in an H-bridge. Power scaling is the same as for classical NPC described above, i.e. power is scaled with current by paralleling of modules.
Modular multilevel converter
Modular multilevel converter (abbreviated as MMC, M2C or MMLC) is a quickly evolving platform for power electronics based converters. Similar to cascaded H-bridge it is based on a series connection of cells. The number of cells is determined by the required voltage on machine and/or grid side. There might be several cell designs with regard to the nominal cell voltage allowing some additional flexibility in selection of number of series connected cells and associated voltage quality. Few different semiconductor models can be standardized in order to have certain degree of freedom in current rating.
In medium voltage VFD applications the MMC drive is based on low voltage cells (like the cascaded H-bridge). MMC based converters are used also in higher power/higher voltage applications, such as e.g. STATCOMs or grid interties [4], where medium voltage cells are utilized.
Power scaling of VFD: Summary
Power scaling is an important feature of a VFD topology to cover wide range of applications and support the trend moving from fix speed towards variable speed applications.
Basic principles for power scaling are paralleling, series connection or a combination of both. Each principle has its advantages as well as challenges. There is no “one topology fits all” and the leading manufacturers have several VFD platforms to select from. This article described common VFD topologies that use paralleling, series connection or a combination in order to reach the required power.
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References
[1] LCI versus VSI series, https://mb-drive-services.com/category/lci-versus-vsi/
[2] Current source and voltage source inverter, https://mb-drive-services.com/current-source-and-voltage-source-inverter/
[3] Medium voltage AC drive portfolio, https://new.abb.com/drives/medium-voltage-ac-drives
[4] Static frequency converters (SFC) for grid interconnection, https://www.hitachiabb-powergrids.com/offering/product-and-system/facts/static-frequency-converters-sfc
1 Comment
Selection of a variable frequency drive - MB Drive Services · November 21, 2020 at 1:33 pm
[…] the power/current/voltage rating influences the selection of VFD [3]. There are different VFD topologies and solutions when we talk about low power, medium power or […]
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