Droop control has been widely applied in DC microgrids for regulating the power sharing of battery storage. However, conventional droop control needs to select appreciate droop coefficients and large droop coefficients could have the potential to change the system quiescent operating point, thus causing instability. To solve such concern, this paper proposes a novel droop control realization with a natural droop coefficient. A reconstructed control block is used to achieve this purpose. The reconstructed control block also generates a controllable variable for integrating with hierarchical control scheme. Besides, this variable enhances the stability performance of converter due to variable load-adjusted bandwidth of controller. Numerical analysis is provided to visually inspect to the relationship between the bandwidth of controller and load power. A Matlab/simulink model is used to validate this novel reconstructed droop control.
A method that can adjust the input impedance of loads in DC microgrids is proposed. This method can be applied to loads such as close-loop-controlled Buck converters followed by DC load, close-loop-controlled inverters, inverters with motor loads, etc. This method can improve the stability of DC microgrids by improving the amplitude-frequency characteristics of loads. Close-loop-controlled inverter is taken as an example to explain how this method works, and simulation model is built in SIMULINK to verify the effectiveness of the proposed method.
To avoid mutual interference between the converter least power point tracking (CLPPT) algorithm and the photovoltaic maximum power point tracking (PMPPT) algorithm in the photovoltaic differential power processing (DPP) architecture, the converter-least-photovoltaic-maximum power point tracking (CL-PMPPT) algorithm is proposed in this paper. The proposed algorithm eliminates wrong response between CLPPT and PMPPT by using PMPPT as the control inner loop and CLPPT as the outer loop. Moreover, by adding the steady-state zero-oscillation algorithm and the approximate calculation of the optimal main string current, the proposed algorithm eliminates the steady-state three-level oscillation and improves the tracking speed. Compared with conventional DPP control algorithm, the proposed algorithm achieves faster tracking speed, better steady-state performance and higher efficiency. Followed by theoretical analysis, the simulation verifies the claimed advantages of the proposed CL-PMPPT algorithm.
Over the past years, there has been an increasing scientific interest in LVDC microgrids as an alternative for AC microgrids. Ongoing research mainly focusses on the two-wire unipolar architecture and, although the bipolar structure may offer several advantages, it is often regarded as just a double version of its unipolar equivalent. This paper compares the behaviour of unipolar and bipolar LVDC microgrids under fault conditions to demonstrate that this generalisation is not always valid. The differences in the fault behaviour of a unipolar and bipolar structure are first introduced by calculating the first fault transient in a fundamental example of both structures. Subsequently, PSCAD simulations of faults in a unipolar and bipolar LVDC microgrid are compared to demonstrate the differences between the two structures in a realistic LVDC microgrid context. These simulations are then applied to show the potential impact of generalising conclusions from one structure to the other on the protection algorithm of the LVDC grid. It is shown that a protection algorithm designed for one of the structures does not necessarily work in the other structure. Finally, an experimental validation of the differences observed regarding the fault behaviour of both structures is presented.
This research focuses on establishing a secure data communication system among power electronic converters connected to a common voltage bus in a dc microgrid. As a proof-of-concept, a buck and two boost dc-dc converters, sharing a 24 V dc bus, have been used for simulation in Matlab® Simulink®. Digital bits are generated at 10 kbps rate at the three converters which are modulated by frequency shift-keying (FSK) algorithm using three different frequency bands with two distinct frequencies to represent bits ‘0’ and ‘1’. Active band-pass filters are used to accurately extract useful data signal from the dc voltage waveform for the three communication channels having different frequency bands. A chaotic dynamic system is adopted for each power converter to encrypt and decrypt the data stream. The encrypted data in the communication channel is secure unless the attacker can perfectly synchronize with the chaotic dynamic system, which is very unlikely. Data in various frequency bands can be simultaneously demodulated and decrypted at the receiver with very good accuracy. The simulation result suggests that the bit-error-ratio (BER) is less than 0.01% even during system transients such as load change and/or due to transition between on-off states of other power converter(s).
