In recent researches on inverter-based distributed generators, disadvantages of traditional grid-connected current control, such as no grid-forming ability and lack of inertia, have been pointed out. As a result, novel control methods like droop control and virtual synchronous generator (VSG) have been proposed. In both methods, droop characteristics are used to control active and reactive power, and the only difference between them is that VSG has virtual inertia with the emulation of swing equation, whereas droop control has no inertia. In this paper, dynamic characteristics of both control methods are studied, in both stand-alone mode and synchronous-generator-connected mode, to understand the differences caused by swing equation. Small-signal models are built to compare transient responses of frequency during a small loading transition, and state-space models are built to analyze oscillation of output active power. Effects of delays in both controls are also studied, and an inertial droop control method is proposed based on the comparison. The results are verified by simulations and experiments. It is suggested that VSG control and proposed inertial droop control inherits the advantages of droop control, and in addition, provides inertia support for the system.

Comparison of Dynamic Characteristics Between Virtual Synchronous Generator and Droop Control in Inverter-Based Distributed Generators

Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded modes Microgrids are attracting considerable attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted Therefore, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized in this paper for active power sharing Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method, and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control Finally, the future research trends on islanded microgrids are also discussed in this paper

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Review of Active and Reactive Power Sharing Strategies in Hierarchical Controlled Microgrids

A phase-locked loop (PLL) is a nonlinear negative-feedback control system that synchronizes its output in frequency as well as in phase with its input PLLs are now widely used for the synchronization of power-electronics-based converters and also for monitoring and control purposes in different engineering fields In recent years, there have been many attempts to design more advanced PLLs for three-phase applications The aim of this paper is to provide overviews of these attempts, which can be very useful for engineers and academic researchers

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Three-Phase PLLs: A Review of Recent Advances

Virtual synchronous generator (VSG) control is a promising communication-less control method in a microgrid for its inertia support feature. However, active power oscillation and improper transient active power sharing are observed when basic VSG control is applied. Moreover, the problem of reactive power sharing error, inherited from conventional droop control, should also be addressed to obtain desirable stable state performance. In this paper, an enhanced VSG control is proposed, with which oscillation damping and proper transient active power sharing are achieved by adjusting the virtual stator reactance based on state-space analyses. Furthermore, communication-less accurate reactive power sharing is achieved based on inversed voltage droop control feature ( V–Q droop control) and common ac bus voltage estimation. Simulation and experimental results verify the improvement introduced by the proposed enhanced VSG control strategy.

Enhanced Virtual Synchronous Generator Control for Parallel Inverters in Microgrids

In this paper, a generalized droop control (GDC) is proposed for a grid-supporting inverter based on a comparison between traditional droop control and virtual synchronous generator (VSG) control Both the traditional droop control and VSG control have their own advantages, but neither traditional droop control nor VSG control can meet the demand for different dynamic characteristics in grid-connected (GC) and stand-alone (SA) modes at the same time Rather than using a proportional controller with a low-pass filter, as in a traditional droop control, or fully mimicking the conventional synchronous generator parameters in a VSG control, the active power control loop of the GDC can be designed flexibly to adapt to different requirements With a well-designed controller, the GDC can achieve satisfactory control performance; unlike a traditional droop control, it can provide virtual inertia and damping properties in SA mode; unlike a VSG control, the output active power of an inverter with GDC can follow changing references quickly and accurately, without large overshoot or oscillation in the GC mode Moreover, given specific controller parameters, the GDC can function as both a traditional droop control and a VSG control The GDC's controller parameter design is more intuitive and flexible, and this paper provides a distinct design process Finally, the effectiveness of the proposed control method is validated by the simulation and experimental results

A Generalized Droop Control for Grid-Supporting Inverter Based on Comparison Between Traditional Droop Control and Virtual Synchronous Generator Control

A renewable energy-based dc micro-grid with hybrid energy storage, consisting of battery and ultracapacitor, is investigated. To achieve high penetration depth of renewable sources into the utility grid, a novel system operation strategy and the corresponding energy management method is proposed. In the operation strategy, the ultracapacitor unit works as the sole voltage source of the micro-grid to support the dc link in both connected and islanding mode. The micro-grid is controlled to deliver/absorb predefined amount of power to/from the utility grid during connected mode and zero during islanding mode. This design will certainly simplify the power dispatching algorithm of the power system and increase the possibility of including large quantities of micro-grids into the utility grid. The energy management method is dedicated to conducting the net power of the micro-grid effectively. The net power is separated into high- and low-frequency components. The high-frequency power is suppressed by the ultracapacitor automatically and the low frequency power is shared by the battery and an adjustment unit. A small-scale dc micro-grid structure with a single dc link is considered for investigation. MATLAB/Simulink simulation results are presented to validate the proposed system operation strategy and the energy management method.

System Operation and Energy Management of a Renewable Energy-Based DC Micro-Grid for High Penetration Depth Application

Unlike the typical microgrid with a common ac bus, networked microgrid always suffers more serious reactive power sharing issues due to its complex inner structure. In such case, the system reactive power sharing error cannot be easily evaluated and eliminated. So, this paper proposes a wireless control strategy that employs optimized virtual impedance controllers and local load measurements for the reactive power sharing in networked microgrid. First, from the modeling of microgrid network, an estimation method for network reactive power sharing error is derived. Through the estimation method-based network feature analyses, corresponding design for virtual impedance controller is presented. Then, by introducing genetic algorithm, virtual impedance controller parameters of each distributed generation unit are optimized, which aims to minimize the microgrid global reactive power sharing error. The parameter optimization process is performed offline in microgrid configuration stage. By using these optimized virtual impedance controllers, the reactive power sharing performance of a networked microgird can be greatly improved. Finally, the accuracy of the estimation method is validated by MATLAB simulation results, and the feasibility of the proposed virtual impedance optimization method is verified through real power experiments.

A Virtual Impedance Optimization Method for Reactive Power Sharing in Networked Microgrid

This paper proposes a novel three-phase phase-locked loop (PLL) algorithm, which focuses on the reforming of the primary signals before grid synchronization rather than improving the phase estimation methodologies. The unbalanced signals are reformed to balanced ones without damage to the phase angle, through which the negative sequence of the grid voltages is removed. This eliminates the estimation errors of conventional synchronous reference frame PLL and enhances its response speed with a higher bandwidth. The reforming process is supposed to be carried out at every zero-crossing point of the three-phase voltages and choose one phase as reference to balance the other two. Coefficients for the signal reforming are calculated at one zero-crossing point and updated until the next comes. In implementation, a certain phase is chosen as the reference all along and the reforming process will be suspended when it just crosses the zero line. This PLL algorithm has a fast and precise character to reform the three-phase grid voltages and is flexible for application. Under heavily distorted grid conditions, it can still perform effectively even with multiple zero-crossings. Comprehensive experimental results from a digital signal processor-based laboratory prototype are provided to validate the performance of this PLL algorithm.

A Three-Phase PLL Algorithm Based on Signal Reforming Under Distorted Grid Conditions

To solve the reactive power sharing issue in droop control application, many solutions have been developed based on the basic wireless manner However, existing wireless methods cannot eliminate reactive power sharing errors effectively, especially in load change situations In this paper, a wireless reactive power sharing method that employs feeder current sensing and adaptive virtual impedance control is proposed for islanded microgrid To improve reactive power sharing accuracy of virtual impedance method, an equivalent feeder concept is introduced, which can reflect the mismatch in connecting circuits equivalently Through fast feeder current sensing, distributed generation units can calculate their respective equivalent feeders in real time With the cooperation between real-time calculation and virtual impedance control, the proposed method achieves both accurate and fast performance in reactive power sharing Compared with communication-based methods, the proposed wireless control provides the same high accuracy in reactive power sharing; meanwhile, its response speed to load change is much faster than that of other methods Moreover, the proposed method can work as normal even though load changes during the transient process Matlab simulation and real-time digital simulator test are used to validate the feasibility of this method

A Wireless Load Sharing Strategy for Islanded Microgrid Based on Feeder Current Sensing

Different from the traditional microgrid with a common ac bus, networked microgrid always suffers more serious reactive power sharing problems due to its complex inner configurations. In such a situation, the reactive power sharing errors among distributed generation (DG) units can't be eliminated effectively. Thus, an advanced reactive power sharing strategy that employs communication and virtual resistance control is hereby proposed for networked microgrid. First of all, the relation between reactive power output and virtual impedance regulation (VRR) is discussed. After that, communication is introduced to assign reactive power reference to each DG unit for their respective VRR, which is able to compensate the mismatch in network. The method is immune to the load change during the regulation stage, and also to the time delay in communication channels. From the small signal analysis, it also can be seen that in the designed regulation range, VRR has no significant effect on the system stability. The feasibility and effectiveness of the proposed strategy are validated by the simulation and real time digital simulator (RTDS) test results from a 50 kVA networked microgrid system.

Baoquan Liu

Papers

System Operation and Energy Management of a Renewable Energy-Based DC Micro-Grid for High Penetration Depth Application

A Virtual Impedance Optimization Method for Reactive Power Sharing in Networked Microgrid

A Three-Phase PLL Algorithm Based on Signal Reforming Under Distorted Grid Conditions

A Wireless Load Sharing Strategy for Islanded Microgrid Based on Feeder Current Sensing

A virtual resistance based reactive power sharing strategy for networked microgrid