What is the optimal pulse number of a diode rectifier?
Among variable frequency drives (VFD), the voltage source inverter with a diode rectifier is the most common all-rounder. However, there are several arrangements of the diode rectifier varying in the number of pulses. Is a 12-pulse rectifier sufficient? Do I need 24-pulses? Or shall I better go for 36-pulse rectifier? In this post we look at the optimal pulse number of a diode rectifier.
6-pulse bridge as building block
Basic building block of a diode rectifier is 3-phase full bridge. Such block consists of six diodes in two diode groups – cathode and anode. As there are six diodes, the rectified voltage contains a ripple of frequency equal six times the line supply frequency. We talk about “6-pulse bridge”. On the AC side the dominant harmonic frequencies are “plus minus one” with respect to the DC ripple, i.e. 5th and 7th harmonic order.
Higher rectifier pulse number
In medium voltage drives, 6-pulse diode bridge is normally not acceptable due to high harmonic content and resulting grid harmonic distortion. The rectifier connections therefore combine several 6-pulse bridges into a rectifier with higher pulse number (integer multiple of 6). That is how we get 12-pulse, 18-pulse, 24-pulse, 30-pulse rectifier.
What is the optimum rectifier pulse number?
The combination of higher pulse number together with a phase shifting transformer helps to eliminate (or at least minimize) certain harmonic orders and to reduce overall harmonic distortion. At the same time, higher pulse number is linked with higher cost of the rectifier, more complex input transformer (design and protection), more power cables between transformer and rectifier etc. Thus, the original question: What is the optimum pulse number of a diode rectifier?
Fortunately or unfortunately, there is no simple answer. Harmonics present a system topic and also the selection of an optimal rectifier pulse number depends on system parameters. For example, a relatively small VFD connected to a strong grid ¹ can have just a 12-pulse rectifier and the harmonic distortion is going to be a ‘no-brainer.’ On the other hand, for multiple VFDs of considerable total installed power connected to a network of medium or low short circuit power, higher rectifier pulse number is recommended ².
Some rules of thumb put in relation the ratio of grid short circuit power to the VFD installed power with the recommended minimum pulse number to fulfil a specific standard, e.g. IEEE 519. For example, one could say that 24-pulse diode rectifier almost always fulfils the limits of IEEE 519 while 12-pulse diode rectifier is only compliant for a ratio grid short circuit power and VFD installed power to be above certain value (e.g. S_sc/S_VFD > 25).
¹ Strong grid in the sense of high short circuit power / low short circuit impedance
² Either on VFD level or achieved as a “quasi system” by phase-shifting the input transformers of individual VFDs against each other
Rectifier pulse number and transformer design
The number of transformer windings increases with the rectifier pulse number. Each 6-pulse bridge needs its individual supply from a dedicated transformer secondary winding. Thus, a 12-pulse rectifier requires two secondary windings, 18-pulse rectifier is supplied from three secondary windings etc. The number of transformer secondary windings can be even higher than the rectifier pulse number – this is the case when not all windings have a phase shift.
Pulse number and VFD topology
Pulse number of a diode rectifier can also be linked with a VFD topology. It means that the VFD model comes with one of more options on the rectifier configuration. However, it may not be possible to select an arbitrary rectifier pulse number.
For example, some drives might have the option of 12-pulse or 24-pulse rectifier. Others may come either with 18-pulse or 36-pulse rectifier. Cascaded H-bridge type of VFD scales the voltage (and power) with the number of series connected cells. The number of cells is then linked with the number of transformer windings and thus also the total pulse number. Thus, for a given motor voltage the rectifier pulse number cannot be freely selected.
On the other hand, VFDs with a “central” DC link usually offer some flexibility on rectifier pulse number. Higher pulse number can be selected in case of strict limits on harmonics, low grid short circuit power or presence of sensitive loads to name few reasons.
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References
[1] VFD harmonic spectrum, https://mb-drive-services.com/network_harmonics_vfd_harmonic_spectrum/
[2] Selection of a variable frequency drive, https://mb-drive-services.com/selection-of-a-variable-frequency-drive/
