# Modeling and Simulation of Autonomous DC Microgrid with Variable Droop Controller
## 3 Proposed Variable Droop Controller for DC Microgrid  
The proposed system shown in Figure 2 uses parallel connected boost converters with its own droop control system and have identical droop characteristics. All the n units have the same structure, but the current values depend on the droop resistor values for each converter. The converter with the highest droop value will contribute to the maximum current share. In the proposed system, identical PWM pattern is assumed for all units to simplify control coordination and accurate current sharing between the units. The DC bus voltage is 300 V. Each unit comprises a source, a PEC, and a droop controller. The DCs are modeled as constant voltage sources of 100 V, with the droop equation applied to define the droop characteristics. The block schematic of the proposed droop control scheme for a single unit is shown in Figure 3. The droop controller models the resistive behavior, resulting in a decrease in the converter output voltage as the source current increases. A low-pass filter is employed to reduce fluctuations at the DC bus. The filtered bus voltage is compared to the reference and the resulting error is processed by a proportional (P) controller. The output of the controller is multiplied by the DC bus voltage (\(V_{DC}\)) and divided by the source voltage to determine the required source current to meet the demand. The inner current control loop is regulated by a hysteresis controller, implemented using an SR flip-flop, which compares the reference current signal from the voltage control unit with the actual source current. Figure 3: Proposed droop control. Figure 2: Proposed system. Figure 4 illustrates the proposed droop control scheme combined with hysteresis current control. In this work, the hysteresis current controller is used in the inner current control loop, which offers extremely fast and effective current tracking for microgrid applications. Compared to the classical PI, hysteresis control relies heavily on precise system modeling to a lesser extent and is inherently robust to variations in system parameters. Hysteresis current control is a straightforward technique often employed in power converters to generate an output current that closely follows a reference current waveform. The hysteresis band, defined by upper and lower tolerance limits, is set at \(\pm 5\) A, based on the current ratings of both the source and the load. The switching signal is generated by comparing the error signal with the hysteresis band, ensuring that the output current remains within the specified tolerance. Equation (1) denotes the reference power generated by the voltage droop controller. \[P_{ref}=G(s)(V_{ref}-V_{DC,filtered})\cdot V_{DC} \tag{1}\]  
\[I_{ref}=\frac{P_{ref}}{V_{source}} \tag{2}\]  
where \(P_{ref}\), \(V_{ref}\), and \(I_{ref}\) are the reference power, voltage, and current, respectively. \(G(s)\) is the proportional controller transfer function as given in Equation (3). \[G(s)=\frac{1}{R_{D}} \tag{3}\]  
where \(R_{D}\) is the droop coefficient. The relationship between terminal voltages and current sharing as a function of the droop values in a parallel connected converter system is depicted in Figure 5. Two sets of droop resistors are considered, characterized by different droop coefficients, \(R_{d1}\) and \(R_{d2}\), where \(R_{d1}\) is smaller than \(R_{d2}\). The nominal bus voltage is denoted as \(V_{dcn}\). The resulting current sharing difference is represented by \(I_{2}\)-\(I_{1}\) A for the droop coefficient \(R_{d1}\) and \(I_{2}^{\prime}\)-\(I_{1}^{\prime}\) A for \(R_{d2}\). The current sharing performance improves as the droop gain increases. To investigate the effect of droop gain on terminal voltage, consider a specific load current, \(I_{0}\). For a smaller droop gain \(R_{d1}\), the terminal voltage drops to \(V_{dc1}\), while for a larger droop gain \(R_{d2}\), the terminal voltage drops to \(V_{dc2}\). As the droop gain increases, voltage regulation deteriorates.