Vahidreza Nasirian

Distributed Cooperative Control of DC Microgrids

A cooperative control paradigm is used to establish a distributed secondary/primary control framework for dc microgrids. The conventional secondary control, that adjusts the voltage set point for the local droop mechanism, is replaced by a voltage regulator and a current regulator. A noise-resilient voltage observer is introduced that uses neighbors data to estimate the average voltage across the microgrid. The voltage regulator processes this estimation and generates a voltage correction term to adjust the local voltage set point. This adjustment maintains the microgrid voltage level as desired by the tertiary control. The current regulator compares the local per-unit current of each converter with the neighbors and, accordingly, provides a second voltage correction term to synchronize per-unit currents and, thus, provide proportional load sharing. The proposed controller precisely handles the transmission line impedances. The controller on each converter communicates with only its neighbor converters on a communication graph. The graph is a sparse network of communication links spanned across the microgrid to facilitate data exchange. The global dynamic model of the microgrid is derived, and design guidelines are provided to tune the systems dynamic response. A low-voltage dc microgrid prototype is set up, where the controller performance, noise resiliency, link-failure resiliency, and the plug-and-play capability features are successfully verified.

Distributed Adaptive Droop Control for DC Distribution Systems

A distributed-adaptive droop mechanism is proposed for secondary/primary control of dc microgrids. The conventional secondary control that adjusts the voltage set point for the local droop mechanism is replaced by a voltage regulator. A current regulator is also added to fine-tune the droop coefficient for different loading conditions. The voltage regulator uses an observer that processes neighbors data to estimate the average voltage across the microgrid. This estimation is further used to generate a voltage correction term to adjust the local voltage set point. The current regulator compares the local per-unit current of each converter with the neighbors on a communication graph and, accordingly, provides an impedance correction term. This term is then used to update the droop coefficient and synchronize per-unit currents or, equivalently, provide proportional load sharing. The proposed controller precisely accounts for the transmission/distribution line impedances. The controller on each converter exchanges data with only its neighbor converters on a sparse communication graph spanned across the microgrid. Global dynamic model of the microgrid is derived with the proposed controller engaged. A low-voltage dc microgrid prototype is used to verify the controller performance, link-failure resiliency, and the plug-and-play capability.

Droop-Free Distributed Control for AC Microgrids

A cooperative distributed secondary/primary control paradigm for AC microgrids is proposed. This solution replaces the centralized secondary control and the primary-level droop mechanism of each inverter with three separate regulators: voltage, reactive power, and active power regulators. A sparse communication network is spanned across the microgrid to facilitate limited data exchange among inverter controllers. Each controller processes its local and neighbors information to update its voltage magnitude and frequency (or, equivalently, phase angle) set points. A voltage estimator finds the average voltage across the microgrid, which is then compared to the rated voltage to produce the first-voltage correction term. The reactive power regulator at each inverter compares its normalized reactive power with those of its neighbors, and the difference is fed to a subsequent PI controller that generates the second-voltage correction term. The controller adds the voltage correction terms to the microgrid rated voltage (provided by the tertiary control) to generate the local voltage magnitude set point. The voltage regulators collectively adjust the average voltage of the microgrid at the rated voltage. The voltage regulators allow different set points for different bus voltages and, thus, account for the line impedance effects. Moreover, the reactive power regulators adjust the voltage to achieve proportional reactive load sharing. The third module, the active power regulator, compares the local normalized active power of each inverter with its neighbors and uses the difference to update the frequency and, accordingly, the phase angle of that inverter. The global dynamic model of the microgrid, including distribution grid, regulator modules, and the communication network, is derived, and controller design guidelines are provided. Steady-state performance analysis shows that the proposed controller can accurately handle the global voltage regulation and proportional load sharing. An AC microgrid prototype is set up, where the controller performance, plug-and-play capability, and resiliency to the failure in the communication links are successfully verified.

Team-Oriented Load Sharing in Parallel DC–DC Converters

A distributed networked method for load sharing of parallel converters is proposed. Using consensus-voting protocols, the need for a master converter or a central controller is eliminated. The proposed modular structure does not require a priori knowledge of the number of active converters, which makes it a viable option for a plug-and-play operation. The voltage regulation at the desired set point and the consensus of the per-unit currents are analytically proven for the steady-state conditions. Moreover, in the absence of a centralized controller, measuring output voltages of each converter individually can lead to a measurement mismatch. The effect of this voltage mismatch on the controller performance is analyzed, and a solution is provided. Experimental results verify the proposed distributed control method using a parallel four-converter system and show its efficacy in response to changes in operational conditions and its resiliency against the loss of converters or communication links.

Distributed adaptive droop control for DC microgrids

Conventional droop controls have been widely studied for load sharing in dc Microgrids. However, unlike ac Microgrids, transmission line impedances can undermine the controllers performance, and it might fail to provide regulated rated voltage and proportional load sharing. Herein, a distributed secondary controller is introduced that includes two modules; a voltage regulator and a current regulator. A sparse cyber network is also spanned through the converters for data exchange wherein, each converter is in contact with a few others as its neighbors. A cooperative algorithm is introduced and used in a voltage observer that estimates the average voltage across the Microgrid. This estimation is further used in the voltage regulator to locally adjust the voltage set point, compensating the voltage drop caused by the droop mechanism. Simultaneously, the current regulator compares local per-unit current with the neighbors per-unit currents and, accordingly, adjusts the droop virtual impedance to balance the per-unit supplied currents. Thus, the voltage and current regulators together provide the best choice of voltage set point and virtual impedance that leads to the global voltage regulation and tight load sharing. Simulation studies on a low-voltage dc Microgrid verify the performance of the proposed control methodology.

Dynamic Model Development and Variable Switching-Frequency Control for DCVM Cúk Converters in PFC Applications

A Cúk converter operating in discontinuous capacitor voltage mode (DCVM) is an inherent power factor correction converter. A reduced-order switch-network model of a Cúk converter in DCVM is developed, which is valid for both dc-dc and ac-dc applications. The model can be used for controller analysis and design. Moreover, a fast dynamic response controller is proposed, which is an integration of a switching-frequency control unit and a proportional-integral (PI) controller. The PI controller is in charge of duty ratio adjustment. In the proposed controller, unlike conventional PI controllers, switching frequency is no longer fixed and varies as load changes. The switching-frequency control unit reacts much faster than the PI controller and instantly compensates for the output voltage in case of drastic load change, thus significantly improving the transient response. Furthermore, the proposed control scheme keeps the switch voltage stress constant, which eases switch selection and improves converter reliability.

Team-oriented adaptive droop control for autonomous AC microgrids

This paper proposes a distributed control strategy for voltage and reactive power regulation in ac Microgrids. First, the control module introduces a voltage regulator that maintains the average voltage of the system on the rated value, keeping all bus voltages within an acceptable range. Dynamic consensus protocol is used to estimate the average voltage across the Microgrid. This estimation is further utilized by the voltage regulator to elevate/lower the voltage-reactive power (Q-E) droop characteristic, compensating the drop caused by the droop mechanism. The second module, the reactive power regulator, dynamically fine-tunes the Q-E coefficients to handle the proportional reactive power sharing. Accordingly, locally supplied reactive power of any source is compared with neighbor sources and the local droop coefficient is adjusted to mitigate and, ultimately, eliminate the load mismatch. The proposed controllers are fully distributed; i.e., each source requires information exchange with only a few other sources, those in direct contact through the communication infrastructure. A Microgrid test bench is used to verify the proposed control methodology, where different test scenarios such as load change, link failure, and inverter outage are carried out.

High-Fidelity Magnetic Characterization and Analytical Model Development for Switched Reluctance Machines

This paper proposes a new experimental procedure for magnetic characterization of switched reluctance machines. In the existing methods, phase voltage and current data are captured and further processed to find the flux linkage. Conventionally, assuming zero initial flux value, the flux linkage can be found by integrating the corresponding voltage term. However, the initial flux value is usually unknown, e.g., it can be nonzero when the current is zero due to the residual flux effect, and, thus, imposes error in magnetic characterization. The proposed method addresses this issue by considering an additional equation in steady state. This method injects a low-frequency sinusoidal current to one of the phase windings when the rotor is blocked at a given position. Since the phase is excited by a sinusoidal current, the averaged flux over an excitation cycle is zero, even though the residual flux and core loss exist. This additional equation together with the voltage integration make it possible to avoid errors associated with the core nonidealities and accurately solve for the magnetic flux. Furthermore, an analytical expression is proposed that precisely fits the magnetic curves. The proposed characterization methodology and analytical model are verified using the experimental results from a 3-phase 12/8 switched reluctance machine.

Output power maximization and optimal symmetric freewheeling excitation for Switched Reluctance Generators

Space constraint is a limiting factor in widespread commercial adaptation of Switched Reluctance Generators (SRG). Maximizing the output power of the SRG, and minimizing the DC bus filter size, are among possible solutions. The output power profiles of a typical SRG are determined by applying various excitation angles. Excitation for maximum output power is obtained while considering the maximum RMS value of the phase current as a constraint. DC bus current of an SRG drive, operating in single pulse mode, is highly distorted by low frequency ripples. Introducing a freewheeling angle in the excitation pattern can lower the current ripple factor. A novel transform is proposed to establish a correspondence between the conventional and freewheeling excitation patterns. Based on which, optimal symmetric freewheeling excitation is proposed that achieves the optimal freewheeling angle and minimizes the ripple factor of the DC bus current, while preserving the maximum output power. Numerical simulation and hardware measurements verify the proposed methodology.

A Multi-Functional Fully Distributed Control Framework for AC Microgrids

This paper proposes a fully distributed control methodology for secondary control of ac microgrids. The control framework includes three modules: 1) voltage regulator; 2) reactive power regulator; and 3) active power/frequency regulator. The voltage regulator module maintains the average voltage of the microgrid distribution line at the rated value. The reactive power regulator compares the local normalized reactive power of an inverter with its neighbors’ powers on a communication graph and, accordingly, fine-tunes Q-V droop coefficients to mitigate any reactive power mismatch. Collectively, these two modules account for the effect of the distribution line impedance on the reactive power flow. The third module regulates all inverter frequencies at the nominal value while sharing the active power demand among them. Unlike most conventional methods, this controller does not utilize any explicit frequency measurement. The proposed controller is fully distributed; i.e., each controller requires information exchange with only its neighbors linked directly on the communication graph. Steady-state performance analysis assures the global voltage regulation, frequency synchronization, and proportional active/reactive power sharing. An ac microgrid is prototyped to experimentally validate the proposed control methodology against the load change, plug-and-play operation, and communication constraints such as delay, packet loss, and limited bandwidth.