A Novel Bidirectional DC-DC Converter for Dynamic Performance Enhancement of Hybrid AC/DC Microgrid
Abstract
:1. Introduction
- (i)
- Usually, one controller is used to control the converter in both modes, whereas two converters (Buck and Boost) exhibit two distinctive characteristics that cannot be controlled with one controller, since the controller usually designed based on one converter structure to operate in one mode cannot properly handle grid voltage and load disturbances in both modes.
- (ii)
- The bidirectional operation does not have symmetrical voltage gain. This asymmetry originates from the different circuit structures in both modes. Therefore, the converter should work with different duty ratios in both modes, resulting in an asymmetric and relatively slow control response during power flow direction changeover. This issue is addressed to some extent by selecting a small enough battery voltage compared to the grid voltage, resulting in a high voltage difference between two sides; otherwise, the Buck operation would not be as effective. However, this potential difference results in high current peaks in both switching operations, leading to high current ripples, particularly in heavy load levels. This issue is usually tackled by selecting a large inductor, thereby incurring a higher capital cost.
- (iii)
- Both step-up and step-down switches are simultaneously modulated in both modes, resulting in increased converter switching losses that restrict the converter to low power density levels.
- (i)
- It employs two Boost converters in back-to-back topology and has a minimum additional component compared with other proposed converter configurations.
- (ii)
- Compared to the conventional type, the proposed converter has three more diodes and one more inductor and capacitor. Since there is no Buck operation and the battery voltage can be raised to higher levels, the converter inductors have less inductance and current capacity due to the reduced battery current.
- (iii)
- The proposed configuration has two voltage levels in the battery side in two modes of operation. The converter exhibits similar dynamic characteristics in both modes of operation, hence, one single controller can be designed and implemented.
2. Proposed DC-DC Converter for BESS System
2.1. Principle of Operation
- Works symmetrically with the same dynamic properties in both directions;
- Has equally high DC gain in both discharge and charge modes;
- Does not have much power loss compared with the conventional type;
- Has a robust yet straightforward controller that fits both modes.
2.2. Converter Transfer Functions
2.3. Estimation of the Equivalent Load Resistance
2.4. Power Stage Adjustment
3. Control System
3.1. Proposed Converter Controller
3.1.1. Current Control Mode
3.1.2. Voltage Control Mode
3.2. Conventional BDC Controller
4. Simulation Results
- Case I—Grid-connected HMG: In this state, the DC grid is connected to the main grid through a grid-following interlinking converter (ILC). In the voltage control mode, the BDC controls the DC grid voltage, while ILC tracks the reference powers. In the current control mode, this scheme switches between BDC and ILC.
- Case II—Islanded HMG: In this state, Case I is modified to form an islanded HMG. In the voltage control mode of BDC, the ILC is a grid-forming voltage source converter (VSC), and DER1 is a grid-following generation source to supply reference active/reactive powers. All control systems of VSCs in ILC and DER1 are well-established structures adopted from [30].
5. Conclusions
Author Contributions
Funding
Conflicts of Interest
References
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Proposed | Conventional | [15] | [18] | [13]/[19] | [20] | [16] | |
---|---|---|---|---|---|---|---|
Voltage gain in discharge mode | |||||||
Voltage gain in charge mode | |||||||
Symmetrical scheme | ✓ | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ |
Control complexity | low | low | high | low | medium | low | high |
Component count: | - | - | - | - | - | - | - |
Inductors | 2 | 1 | 3 | 2 | 3/3 | 2 | 1 |
Capacitors (2) | 0 | 0 | 3 | 2 | 2/2 | 1 | 2 |
Switching devices | 4 | 2 | 4 | 3 | 2/4 | 4 | 3 |
Diodes (3) | 3 | - | - | - | - | - | 3 |
Coupled inductors | - | - | - | - | 1/1 | - | 1 |
1 | 1 | 1 | |||
Battery: | Capacity | Type | Li-ion |
PI1: | |||||
PI2: | - | ||||
PI3: | - | ||||
Battery: | Capacity | Type | Li-ion | - |
460 V | DER1 | 250 kW | DER2 | 307 kW | |
DC Load | AC Load | ILC | 250 kW |
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Keshavarzi, M.D.; Ali, M.H. A Novel Bidirectional DC-DC Converter for Dynamic Performance Enhancement of Hybrid AC/DC Microgrid. Electronics 2020, 9, 1653. https://doi.org/10.3390/electronics9101653
Keshavarzi MD, Ali MH. A Novel Bidirectional DC-DC Converter for Dynamic Performance Enhancement of Hybrid AC/DC Microgrid. Electronics. 2020; 9(10):1653. https://doi.org/10.3390/electronics9101653
Chicago/Turabian StyleKeshavarzi, Morteza Daviran, and Mohd Hasan Ali. 2020. "A Novel Bidirectional DC-DC Converter for Dynamic Performance Enhancement of Hybrid AC/DC Microgrid" Electronics 9, no. 10: 1653. https://doi.org/10.3390/electronics9101653
APA StyleKeshavarzi, M. D., & Ali, M. H. (2020). A Novel Bidirectional DC-DC Converter for Dynamic Performance Enhancement of Hybrid AC/DC Microgrid. Electronics, 9(10), 1653. https://doi.org/10.3390/electronics9101653