2. Traditional Droop Control
2.1. State-of-the-Art of DC Microgrids
- Control [11,12]: DC microgrids have simpler models and control since there is no phase angle, frequency or reactive power, while synchronization, reactive power flow and harmonics have to be considered for AC systems which leads to more complicated control system. Moreover, DC is chosen over AC because it facilitates integrating most modern electronic loads, energy storage devices, and DG technologies—all of them inherently DC.
- Economical operation [13,14,15]: Economical operation in DC microgrids can be achieved without complex and computation-intensive optimization algorithms and as pointed out in the existing literature, the total cost of ownership, infrastructure, equipment, maintenance and operation are lower in DC microgrids.
- Efficiency [16,17,18]: The system efficiency becomes higher due to the reduction of conversion losses of inverters between DC output sources and loads. DC microgrids already have a fault-ride-through capability of their own due to the stored energy of the DC capacitor and the voltage control of the AC/DC converter.
2.2. Configuration of DC Microgrids
- Equipment costs become lower.
- Distribution capabilities are enhanced.
- Sustainability is improved due to reduced copper use.
2.3. Drawbacks of Traditional Droop Control
- Deviation of bus voltage is inevitable.
- There is trade-off between power-sharing accuracy and voltage deviation.
3. Analysis of Droop Control Issues
3.1. Mesh Configuration
3.2. Radial Configuration
4. Proposed Approach
5. Simulations and Experimental Results
|Reference of DC output voltage||vdc||380 V|
|Load resistance||RL||19 Ω|
|Power sharing proportion (Converter #i=1,2,3)||ki||1|
|LPF cutting frequency||fc||20 Hz|
|Communication delay||τ||0.1 s or 1 s|
|Averaging voltage controller proportion coefficient||kpv||2 × 10−3|
|Averaging voltage controller integral coefficient||kiv||4.5 × 10−2|
|Averaging power controller proportion coefficient||kpp||2.3 × 10−3|
|Averaging power controller integral coefficient||kip||5.5 × 103|
|Mesh configuration||Radial configuration|
|Case No.||r1L||r12||r23||r3L||τ||Case No.||r1L||r2L||r3L||τ|
|1||0.8 Ω||1.0 Ω||1.0 Ω||1.2 Ω||0.1 s||4||0.8 Ω||1.0 Ω||1.2 Ω||0.1 s|
|2||0.8 Ω||1.0 Ω||1.0 Ω||1.2 Ω||1 s||5||0.8 Ω||1.0 Ω||1.2 Ω||1 s|
|3||0.6 Ω||1.2 Ω||1.2 Ω||1.8 Ω||1 s||6||0.6 Ω||1.2 Ω||1.8 Ω||1 s|
5.1. Simulation Results for Mesh Configuration
5.2. Simulation Results for Radial Configuration
5.3. Experimental Verification
- It was demonstrated that the feasibility and stability of the control system can be guaranteed with both mesh and radial configurations.
- Local DC output voltage deviation can be eliminated and the load power sharing accuracy can be enhanced with the modified droop control method.
- The proposed control method only relies on the data of voltage and current sampled from two adjacent converters, which reduces the stress on the communication system.
- The control system is also stable considering the longer communication delay time and larger mismatch of line resistance.
Conflicts of Interest
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