Fault current considerations for integrated transformer of CHB drive
Introduction
Cascaded H-bridge (CHB) variable frequency drives (VFD) have the input transformer as an integral part of the VFD. This transformer has high number of secondary windings (equal to the number of VFD power cells). Such multi-winding transformer represents a challenge with respect to effective protection. As the number of secondary windings increases, the relative fault current on the transformer primary side gets lower. Thus, the effective protection of such a transformer becomes a challenging task. This post lists some considerations related to the fault current of CHB transformer and illustrates them on several calculation examples.
Cascaded H-bridge
CHB is a very popular topology of VFD. It was originally developed by company called Robicon which was later purchased by Siemens (now Innomotics). In China, a company called Leader and Harvest started to manufacture CHB drives in a big scale. That company was later purchased by Schneider Electric. Nowadays, practically every major VFD manufacturer has a product in his portfolio that is based on CHB platform.
Selected VFD manufacturers and their CHB models
- ABB: ACS580MV
- Hyundai Electric: N5000
- Innomotics (Siemens): Sinamics GH180
- Nidec: Silcovert TH
- Rockwell Automation: PowerFlex 6000
- Schneider Electric: Altivar 1200
- WEG: MVW3000
Power cell
The topology is also called “multi-cell” VFD as there are multiple power cells connected in series per each motor phase. The power cell consists of a 6-pulse diode rectifier, cell dc capacitor and H-bridge inverter.
Rated dc voltage of a power cell is around 1’000 V.
Scalability
Cascaded H-bridge scales the output power with the voltage (the higher the required power the higher design voltage is selected). The required voltage is achieved by connecting multiple power cells in series. The smallest amount of power cells per motor phase is typically 3 (total 9 power cells in VFD). The highest number of power cells per phase can go up to 9 or 10 (total 27 or 30 power cells in VFD).
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Integrated transformer
Each power cell is supplied from a dedicated transformer secondary winding. It means that number of power cells is equal the number of transformer secondary windings. As the inverter power and voltage go up, the number of transformer windings increases as well.
- Number of secondary windings = 3 * number of series connected power cells
- Number of secondary windings = Total number of power cells inside CHB drive
Faults and fault currents
In our considerations we will reflect two basic types of faults:
- Three-phase short circuit
- Phase-to-phase short circuit
Location of the fault can be inside the transformer secondary winding, at the winding terminals or along the power cables connecting transformer and power cell.
The fault current is a function of the supply voltage and the impedance of the path. Neglecting the impedance of the fault itself, the impedance that limits the fault current is given by the short-circuit impedance of the multi-winding transformer and impedance of the supplying grid.
Below formulas express the fault currents for three-phase fault and phase-to-phase fault.
Three-phase fault
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Phase-to-phase fault
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Primary side current
Finally, we come to the point how much current is seen on the primary side of the CHB multi-winding transformer in fault case. It is of interest to analyze the situation for different loading of the VFD.
Assumptions
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Calculation examples
The current seen on the primary side of the integrated multi-winding input transformer is illustrated for following cases (always in per unit of the nominal current):
- 1.5 MVA transformer, 9 secondary windings / 9-cell CHB drive
- 2.5 MVA transformer, 15 secondary windings / 15-cell CHB drive
- 4.0 MVA transformer, 24 secondary windings / 24-cell CHB drive
- 5.0 MVA transformer, 30 secondary windings / 30-cell CHB drive
The rated power of each secondary winding is 167 kVA for all configurations. Grid short cicuit power is 100 MVA. Results are shown for transformer short-circuit impedance varying between 5% and 8%.
Summary
Cascaded H-bridge is a widely used and popular VFD platform. This topology comes with an integrated multi-winding transformer. Number of transformer secondary windings corresponds to the number of power cells inside the VFD. The more secondary windings the lower the primary current in case of a fault in one secondary system. The current level is so low (in many cases below 1 pu) that standard overcurrent protection does not provide a reliable protection in the whole operation range. It is illustrated on several examples of CHB drive configurations
References
[1] CHB drives, available online, https://www.chbdrives.com/news.html
[2] VFD transformers: Protection in general, MB Drive Services, March 2020, available online, https://mb-drive-services.com/protection_of_vfd_transformers/
[3] VFD transformers: Differential protection, MB Drive Services, September 2020, available online, https://mb-drive-services.com/differential-protection-of-vfd-transformers/
