Modeling and Analysis of the Common Mode Voltage in a Cascaded H-Bridge Electronic Power Transformer
Abstract
:1. Introduction
- Due to the long power cables connecting to the grid or motors, the traveling high-rise-rate CMV wave reflected by the terminals of cables will further result in overvoltage for terminal devices and cause bearing currents that reduce the life of the motors.
- High may cause leakage currents through the parasitic capacitances, which also damage the insulation of elements in the devices, e.g., power cables; in addition, it also creates electromagnetic interference problems, which will interfere with the control systems of devices.
2. Description of the CMV in a CHB-EPT
2.1. CHB-EPT System
2.2. The CMV at the High-Voltage Side in the EPT System
2.2.1. Voltages Generated by the HVPC
2.2.2. The CMV at Neutral Point
- Normal grid: For the three-phase balanced power grid, the voltage between the virtual grid neutral g (see the location in Figure 4) and ground can be assumed as zero so the neutral point of the grid can be chosen as the reference ground directly; then we have .
- Single-phase to ground fault: The voltage between the neutral point g of the grid and ground will shift up to phase voltage when a single-phase to ground fault occurs. For a balanced EPT, the CMV at neutral point N with respect to ground will be shifted synchronously. Then we have:
2.2.3. Voltages with Respect to Ground in HVPCs
3. Analytical Calculation of Voltages in EPT
3.1. Principle and Method to Calculate Voltages in EPT System
- (1)
- Calculate the CMV at neutral point N with respect to ground, i.e., .
- (2)
- Calculate the voltage between and N, that is:
- (3)
- Finally, obtain the voltage potential with respect to ground in HVPC L based on Equation (5).
3.2. Derivation and Calculation of the CMV at Neutral Point and Voltage Potential of HVPCs
3.2.1. The CMV with Respect to Ground at a Neutral Point
3.2.2. The Voltage with Respect to N at Point
3.2.3. Voltage Potential of HVPCs
3.3. Analysis of the Voltages
3.3.1. Analysis of the CMV with Respect to Ground at Neutral Point
- (1)
- The relationship between the amplitude of the CMV harmonics component and the modulation ratio M
- (2)
- The relationship between the amplitude of CMV harmonics and its order
3.3.2. Analysis of the Voltage with Respect to Ground at Midpoint
3.4. Effects of SPWM Dead Time on the CMV
4. Simulation Results
5. Conclusions
- The CMV at the neutral point N under normal and balanced grid voltage conditions mainly consists of high-order harmonics with relatively high voltage stress for the high-voltage side of the EPT. The EPT has to sustain a higher fundamental component of the CMV, i.e., the voltage of the fault phase, when it operates under a fault grid condition (single-phase to ground fault).
- The voltage potential at each equivalent midpoint of the HVPC with respect to ground, not only includes the high-order harmonics components, but also contains the line-frequency fundamental component. Furthermore, the magnitude of the latter will increase with an increase of the sequence number of the HVPC. Obviously, the highest electrical stress will appear in the top HVPC, which provides a theoretical guide for the structural design and further power density optimization of the EPT.
- SPWM dead time has little effect on the magnitude of the CMV. In addition, it will not bring a fundamental component to the CMV at the neutral point.
Acknowledgments
Author Contributions
Conflicts of Interest
References
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Parameter | Value |
---|---|
Number of cascaded H-bridges | 6 per phase |
Number of paralleled H-bridges | 6 per phase |
Rated high voltage dc-link | 1500 V |
Rated low voltage dc-link | 360 V |
Capacitance in one high-voltage dc-link | 2200 |
Capacitance in one low-voltage dc-link | 56 mF |
Inductance of rectifier | 30 mH |
Filter inductance of the inverter | 0.2 mH |
Filter capacitance of the inverter | 250 |
Rated ratio of the MFIT | 4.17:1 |
Switching frequency at high-voltage side | 1 kHz |
Switching frequency at low-voltage side | 4 kHz |
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Yang, Y.; Mao, C.; Wang, D.; Tian, J.; Yang, M. Modeling and Analysis of the Common Mode Voltage in a Cascaded H-Bridge Electronic Power Transformer. Energies 2017, 10, 1357. https://doi.org/10.3390/en10091357
Yang Y, Mao C, Wang D, Tian J, Yang M. Modeling and Analysis of the Common Mode Voltage in a Cascaded H-Bridge Electronic Power Transformer. Energies. 2017; 10(9):1357. https://doi.org/10.3390/en10091357
Chicago/Turabian StyleYang, Yun, Chengxiong Mao, Dan Wang, Jie Tian, and Ming Yang. 2017. "Modeling and Analysis of the Common Mode Voltage in a Cascaded H-Bridge Electronic Power Transformer" Energies 10, no. 9: 1357. https://doi.org/10.3390/en10091357