Jafar Mohammadi

This paper proposes a practical and reliable decentralized load shedding strategy to protect the integrity of the direct-current (DC) microgrid. The proposed strategy utilizes time delays that automatically adapt to the DC microgrid operating conditions through continuous evaluation of the bus voltage variations, without depending on remote communication. It is evaluated in comparison with the conventional timer-based load shedding strategy. The studies are performed in the PSCAD software environment, on a detailed model of a DC microgrid that contains various types of loads and distributed energy resources. The results of the investigations show that the proposed adaptive time delay strategy (i) effectively restores the balance between the power demand and supply in the DC microgrid by quickly shedding the necessary amount of loads, (ii) is able to prioritize the non-critical loads by coordinating the load shedding steps based on a pre-determined order, (iii) prevents the steady-state values of the DC microgrid voltages from falling below a predetermined lower limit, (iv) significantly limits the voltage sags caused by power deficit in the microgrid, (v) is highly expandable and is able to coordinate a large number of load shedding steps, and (vi) improves the reliability of the electrical power provided to the DC microgrid loads, by avoiding inessential load shedding.

The direct current (DC) microgrid requires a fast load shedding scheme that prevents instability and voltage collapse when the distributed energy resources are unable to meet the power demand. The load shedding scheme is also expected to prevent unnecessary service interruptions caused by over-shedding of loads. This paper proposes an adaptive voltage-based load shedding scheme utilizing voltage thresholds that are automatically adjusted depending on the rate of change of locally measured bus voltages. The performance of the proposed load shedding scheme is investigated and compared with that of the conventional voltage-based load shedding scheme, using a realistic and detailed study system, under various disturbances. The comprehensive simulation studies that are conducted using the PSCAD software indicate that the adaptive load shedding scheme (i) effectively maintains the DC microgrid power balance through fast and coordinated load shedding, (ii) prevents the DC bus voltages from falling below acceptable levels, (iii) ensures that the critical loads do not experience excessive steady-state voltage deviations, (iv) minimizes the durations and magnitudes of the temporary voltage drop caused by sudden disturbances, and (v) improves power supply reliability.

A comprehensive knowledge of the available grounding strategies and their effects is essential for design, operation, and protection of the dc microgrid. This paper investigates and compares different dc microgrid grounding strategies that involve the choice of grounding configurations and grounding devices. The impacts of different grounding strategies on line-to-ground (LG) fault detection and protection, transient LG fault current magnitude, leakage current level, common mode voltage, personnel/equipment safety, system reliability, service continuity, and insulation requirements are thoroughly investigated. LG fault response of a low-voltage dc microgrid with different grounding devices is studied by detailed simulation of a realistic study system modeled in the PSCAD software. The study results indicate that: 1) the ungrounded, bipolar solidly grounded, unipolar parallel resistance grounded, and bipolar resistance grounded dc microgrids enable LG fault ride through, due to the low fault current; 2) the unipolar solidly grounded, bipolar solidly grounded, diode-grounded, and thyristor-grounded dc microgrids experience significantly large transient discharge currents under LG faults; and 3) the unipolar solidly grounded, diode-grounded, thyristor-grounded, and unipolar low-resistance-grounded dc microgrids provide considerable steady-state LG fault current which necessitates fast detection and clearance of faults.

The mode-adaptive droop control (MADC) strategy enables bus voltage regulation and power sharing between distributed energy resources (DERs) in the direct current (DC) microgrid without communication systems. The conventional MADC strategy may fail to provide acceptable voltage regulation and power sharing performance in large DC microgrids where the voltage drops across the DC lines are not negligible. This paper proposes an improved MADC strategy for the DC microgrid. The proposed control strategy minimizes the adverse effects of the aforementioned voltage drops on the bus voltage regulation and the power sharing between the DERs in the DC microgrid. The performance of the proposed control strategy is investigated under various operating conditions and disturbance scenarios, using a detailed and realistic DC microgrid study system that is modelled in the PSCAD/EMTDC software environment. The study results indicate that the proposed control strategy (i) effectively maintains the power balance in the DC microgrid, (ii) accurately regulates the DC bus voltages under various operating conditions, (iii) improves power sharing between the DERs without using communication systems, (iv) significantly reduces the circulating currents between the DERs in the islanded microgrid, and (v) enhances the DC microgrid reliability, flexibility, modularity and scalability.

Fast transient and smooth steady state performance of wind energy conversion systems (WECSs) is essential for sustained power conversion and grid code fulfilment including low voltage ride through capability. In this paper, an analysis of permanent magnet synchronous generator (PMSG) based WECSs under vector control and direct torque control provides the basis for combining salient features of the two control methods into a reliable and effective control system to provide enhanced machine performance. The system includes two hysteresis current controllers in connection with an inverter-switching table. The proposed control provides fast transient and smooth steady state performance for machine side converter of PMSGs, which controls DC-link voltage under grid fault conditions. Extensive simulation results on a 2 MW PMSG based WECS demonstrate the desired performance of the proposed control system under both grid-normal and -fault conditions.

This study proposes a versatile decentralised control strategy for the direct current (DC) microgrid based on the DC bus signalling method. Performance of the proposed control strategy is investigated under various generation and load disturbances in both grid‐connected and islanded operation modes of the DC microgrid. The studies are conducted using a realistic DC microgrid study system simulated in the PSCAD/EMTDC software. The study results indicate that the proposed control strategy: (i) effectively maintains the power balance in the DC microgrid, (ii) effectively limits the DC bus voltage deviations under large disturbances, (iii) improves stability and dynamic response, (iv) does not curtail renewable power generation unless it is necessary, (v) does not require excessively large grid‐tied converter and battery energy storage systems, and (vi) enhances system reliability, flexibility, modularity, and expandability.

This study proposes a combined control method based on vector control (VC) and virtual flux direct power control (VFDPC) for grid‐side converter of doubly fed induction generator (DFIG)‐based wind energy conversion systems (WECSs). VC gives lower power ripple with a slower dynamic response, while VFDPC provides a faster dynamic response, but higher power ripple. So, an analogy between VC and VFDPC is proved first and then used to propose a combined control method that takes the advantages of VC and VFDPC in an integrated control system. In the combined control method, the grid currents are directly controlled using hysteresis controllers and optimal switching table. It has several advantages compared to VC including faster power/current dynamic response, robustness to grid filter parameter variation, lower computation, and simple implementation. On the other hand, its advantages compared to VFDPC include less current harmonic distortion, lower power ripple, and robustness to measurement noise. To demonstrate the effectiveness and robustness of the combined control method, simulation results on a 1.5 MW DFIG‐based WECS are provided and compared with both VC and VFDPC under different steady‐state and transient conditions. The simulation results verify the superiority of the proposed method over either VC or VFDPC.

Grounding strategy of a DC microgrid affects the stray current level, the common-mode voltage, the energy supply reliability, personnel/equipment safety and protection system design. Therefore, a comprehensive knowledge of the available grounding strategies and their effects is essential for design and operation of the DC microgrid components and especially its protection. This paper investigates and compares characteristics of different DC microgrid grounding strategies that involve the choice of grounding configurations and grounding devices. The study includes the grounding strategy impacts on line-to-ground (LG) fault detection and protection, transient LG fault current magnitude, stray current level, common mode voltage, personnel/equipment safety, system reliability, service continuity, and insulation requirements. Finally, the strengths and weaknesses of the various grounding strategies are discussed.

Grounding strategy of an AC microgrid affects its Line-to-Ground (LG) fault response, personnel/equipment safety, service continuity, insulation requirements, and protection criteria. Therefore, a comprehensive knowledge of the available grounding strategies and their effects is essential for design and operation of the microgrid components and especially its protection. In this paper, characteristics of different AC distribution system grounding devices, i.e., grounding impedance types, are investigated and compared. Subsequently, different AC distribution system grounding configurations including three-wire ungrounded, three-wire uni-grounded, four-wire uni-grounded, and four-wire multi-grounded systems are compared. Finally, AC microgrid grounding requirements are identified based on the unique characteristics and constraints of microgrids.