What is the definition of low voltage and medium voltage VFD?
We all probably know that variable frequency drives (VFD) can be classified into two groups based on their voltage: low voltage VFD and medium voltage VFD. But what voltage do we talk about? Let’s shortly explain…
IEC 61800 distinguishes power drive systems up to 1’000 V AC or 1’500 V DC (group 1) and power drive systems above 1’000 V AC or 1’500 V DC up to 36 kV AC or 54 kV DC (group 2). These two groups are normally what we simply call “low voltage” and “medium voltage” drives/VFDs/VSDs and drive systems. Still, the question is which voltage is meant here? Input voltage? Internal DC voltage? Output voltage?
The answer may somewhat differ depending on the VFD topology. However, in almost all cases it is the inverter output voltage that matters for the classification.
Relevant for the classification is the inverter output voltage. If the nominal inverter output voltage exceeds 1’000 V then the drive is classified as medium voltage. Decisive is the inverter output voltage, not the motor voltage (as there can be a step-up output transformer in between).
Typical input and output voltages of popular VFD topologies
The best way to illustrate the classification of low voltage and medium voltage VFD is to look at couple of examples of popular VFD topologies.
a) 3-level neutral point clamped (NPC) inverter
3-level NPC is a very popular platform among voltage source inverters. Almost every manufacturer of medium voltage VFDs has this platform in its portfolio. The inverter provides 3 levels in its phase voltage and 5 levels in the line to line machine voltage.
Figure 1 shows the VFD topology with IGBT devices. However, it is very common to use IGCT semiconductors as well (see comparison here).
Most 3-level NPC drives have nominal output voltage of approx. 3…3.3 kV. There is often also a 4.16 kV variant to match a typical voltage level in North America (used also elsewhere). In some cases, a variant with 2.3 kV nominal output voltage is available as well.
The input voltage depends on the type of rectifier. Diode front end (DFE) drives use a multi-pulse rectifier: 12-pulse or 24-pulse. Typical nominal input voltage of the 12-pulse rectifier bridge is about 1.8 to 2.4 kV. In case of 24-pulse rectifier it depends on the internal connection. Series-parallel connection will have the same voltages as the 12-pulse. When all 4 rectifier bridges are connected in series, then the nominal input voltage is one half, i.e. approximately 1 kV.
Active front end (AFE) has an input voltage almost identical to the nominal output voltage, i.e. 3.0/3.3/4.16 kV.
b) Cascaded H-bridge
Another widespread topology is a cascaded H-bridge. Also this platform is part of the portfolio of almost every medium voltage VFD manufacturer. The building block of this VFD is a power cell. Such unit consists of a 6-pulse diode rectifier, dc capacitor and single phase full bridge inverter. On the grid side the cells are connected in parallel and supplied from a multi-winding phase shifting transformer. Each cell is supplied from a dedicated transformer winding. Thus, the number of transformer secondary windings is equal to the number of power cells. On the machine side the cells in each phase are connected in series to achieve the required machine voltage.
Cascaded H-bridge has a modular design. The power cells are standardized and the VFD output power is scaled with voltage. Typical cell output AC voltage is in the range 600…800 V. The corresponding cell input voltage is in a similar range, i.e. some 630…850 V.
Cascaded H-bridge is a nice example of medium voltage VFD with (multi-phase) low voltage supply (< 1’000 V).
c) Load-commutated inverter (LCI)
Third topology in our example is a load-commutated inverter (LCI). This platform is based on conventional thyristors and despite its long history still popular in the high-power range. The grid side converter consists of a thyristor bridge and also the machine side converter is built of one or more thyristor bridges.
LCI scales the power with voltage. In this regard the LCI is similar to the cascaded H-bridge (although of course historically older). Instead of putting number of cells in series the LCI stacks the thyristor devices directly.
LCI soft starter often consists of just a 6-pulse rectifier and 6-pulse inverter. That is the simplest LCI configuration. An output transformer is used to adapt the inverter output voltage to the motor voltage. While the converter input and output nominal voltage is relatively low, it generally exceeds 1’000 V and thus belongs to medium voltage category.
LCI drive is usually in a 12-12 pulse configuration. In such symmetrical configuration the input and output nominal voltage are of similar value. As said, the power is scaled with the voltage.
Summary
As a rule of thumb the inverter output voltage is the quantity to determine whether VFD belongs to low voltage or medium voltage category. Cascaded H-bridge is an example of a VFD with low voltage input (600…800 VAC cell input voltage) but medium voltage output (at least 3 cells series connected).
Below table shows few typical examples of VFD input and output voltages. LCI is not shown as the design is rather project-specific.
References
[1] VFD basics (series), https://mb-drive-services.com/category/vfd_basics/
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What is the difference between dv/dt filter and sine filter? - MB Drive Services · October 28, 2023 at 11:30 pm
[…] example is a 3-level NPC converter (see e.g. here). Equipped with a dv/dt filter this drive supports motor cables with a length of several hundreds […]
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