In this paper, the inherent limitations of the conventional droop control scheme are revealed. It has been proven that parallel-operated inverters should have the same per-unit impedance in order for ...them to share the load accurately in proportion to their power ratings when the conventional droop control scheme is adopted. The droop controllers should also generate the same voltage set-point for the inverters. Both conditions are difficult to meet in practice, which results in errors in proportional load sharing. An improved droop controller is then proposed to achieve accurate proportional load sharing without meeting these two requirements and to reduce the load voltage drop due to the load effect and the droop effect. The load voltage can be maintained within the desired range around the rated value. The strategy is robust against numerical errors, disturbances, noises, feeder impedance, parameter drifts and component mismatches. The only sharing error, which is quantified in this paper, comes from the error in measuring the load voltage. When there are errors in the voltage measured, a fundamental tradeoff between the voltage drop and the sharing accuracy appears. It has also been explained that, in order to avoid errors in power sharing, the global settings of the rated voltage and frequency should be accurate. Experimental results are provided to verify the analysis and design.
Power systems are going through a paradigm change from centralized generation to distributed generation and further on to smart grids. In this paper, it is shown that future power systems will be ...power electronics based, instead of electric machines based, with a huge number of incompatible players and that the fundamental challenge behind this paradigm change is how to make sure these players could work together and maintain system stability. Then, a lateral architecture based on the synchronization mechanism of synchronous machines (SM), which has underpinned the growth and operation of power systems for over 100 years, is proposed to unify the integration and interaction of these players with the grid by operating power electronic converters to behave like virtual synchronous machines (VSM), which are coined cyber synchronous machines (CSM) here. Thus, all the suppliers and the majority of loads can follow the same mechanism to regulate system stability. This paves the way for autonomous operation of future power systems. Moreover, two technical routes, one based on the synchronverter technology and the other based on the robust droop control technology, are proposed to implement the architecture. Real-time simulation results are presented to illustrate the operation of such a system.
The uncertainty and disturbance estimation (UDE)-based robust control strategy is able to achieve good robust performance by estimating and compensating the uncertainty and disturbance in a system ...with a filter having the appropriate frequency characteristics. However, how to systematically design the filter and the reference system in a UDE-based control system to achieve asymptotic reference tracking and disturbance rejection is still missing in the literature. In this paper, this is solved by applying the well-known internal model principle. The conditions to guarantee the closed-loop system stability and to achieve asymptotic disturbance rejection and reference tracking are derived. Experimental results on a servo system are presented as an example to demonstrate its excellent performance, which can actually reach the hardware resolution limit, with comparisons to the disturbance-observer-based control and the active disturbance rejection control.
In this paper, the idea of operating an inverter to mimic a synchronous generator (SG) is motivated and developed. We call the inverters that are operated in this way synchronverters. Using ...synchronverters, the well-established theory/algorithms used to control SGs can still be used in power systems where a significant proportion of the generating capacity is inverter-based. We describe the dynamics, implementation, and operation of synchronverters. The real and reactive power delivered by synchronverters connected in parallel and operated as generators can be automatically shared using the well-known frequency- and voltage-drooping mechanisms. Synchronverters can be easily operated also in island mode, and hence, they provide an ideal solution for microgrids or smart grids. Both simulation and experimental results are given to verify the idea.
In this paper, a new robust control strategy based on a predictor and the uncertainty and disturbance estimator is developed for a class of uncertain nonlinear systems with input/output delays. The ...closed-loop system is analyzed and sufficient stability conditions are derived based on Lyapunov analysis. The proposed strategy is applied to the particular case of quadrotor systems and validated through extensive simulations to evaluate performance and robustness. The controller is also implemented in a quadrotor prototype and validated in flight tests.
Droop control is a well-known strategy for the parallel operation of inverters. However, the droop control strategy changes its form for inverters with different types of output impedance, and so ...far, it is impossible to operate inverters with inductive and capacitive output impedances in parallel. In this paper, it is shown that there exists a universal droop control principle for inverters with output impedance having a phase angle between -(π/2) rad and (π/2) rad. It takes the form of the droop control for inverters with resistive output impedance (R-inverters). Hence, the robust droop controller recently proposed in the literature for R-inverters actually provides one way to implement such a universal droop controller that can be applied to all practical inverters without the need of knowing the impedance angle. The small-signal stability of an inverter equipped with the universal droop controller is analyzed, and it is shown to be stable when the phase angle of the output impedance changes from -(π/2) rad to (π/2) rad. Both real-time simulation results and experimental results from a test rig consisting of an R-inverter, an L-inverter, and a C-inverter operated in parallel are presented to validate the proposed strategy.
The concept of the virtual synchronous generator (VSG) is emerging as an attractive solution for controlling the grid-connected inverter when the renewable energy has a high penetration level into ...the grid. This paper focuses on the small-signal modeling and parameters design of the power loop of the VSG, and points out that the bandwidth of the power loop should be far less than twice the line frequency for the purpose of avoiding the VSG output voltage to be severely distorted. Consequently, the line-frequency-averaged small-signal model of the VSG is derived for system analysis and parameters design. Based on the model, the decoupling conditions between the active power loops (APLs) and the reactive power loops (RPLs) of the VSG are given. Finally, a step-by-step parameters design method is proposed to facilitate the design of the control parameters of the VSG. A 10-kVA prototype is built and tested in the laboratory, and the experimental results are given to verify the effectiveness of the theoretical analysis and the proposed parameters design method.
A current-limiting droop controller is proposed for single-phase grid-connected inverters with an LCL filter that can operate under both normal and faulty grid conditions. The controller introduces ...bounded nonlinear dynamics and, by using nonlinear input-to-state stability theory, the current-limiting property of the inverter is analytically proven. The proposed controller can be operated in the set mode to accurately send the desired power to the grid or in the droop mode to take part in the grid regulation, while maintaining the inverter current below a given value at all times. Opposed to the existing current-limiting approaches, the current limitation is achieved without external limiters, additional switches, or monitoring devices and the controller remains a continuous-time system guaranteeing system stability. Furthermore, this is achieved independently from grid voltage and frequency variations, maintaining the desired control performance under grid faults as well. Extensive experimental results are presented to verify the droop function of the proposed controller and its current-limiting capability under normal and faulty grid conditions.
In this paper, the load and/or grid connected to an inverter is modeled as the combination of voltage sources and current sources at harmonic frequencies. As a result, the system can be analyzed at ...each individual frequency, which avoids the difficulty in defining the reactive power for a system with different frequencies because it is now defined at each individual frequency. Moreover, a droop control strategy is developed for systems delivering power to a constant current source, instead of a constant voltage source. This is then applied to develop a harmonic droop controller so that the right amount of harmonic voltage is added to the inverter reference voltage to compensate the harmonic voltage dropped on the output impedance due to the harmonic current. This forces the output voltage at the individual harmonic frequency to be close to zero and improves the total harmonic distortion (THD) of the output voltage considerably. Both simulation and experimental results are provided to demonstrate that the proposed strategy can significantly improve the voltage THD.
Instability problem is an important issue for dc/dc conversion cascaded systems (Cascaded system in short). Though most of the existing stabilization methods can stabilize the whole system very well, ...they may ignore their impacts on the dynamic performance of the original cascaded system. Unfortunately, these impacts are negative to some extent. Recently, an adaptive-series-virtual-impedance (ASVI) control strategy has been reported to address the above problem. It not only can stabilize the cascaded system via shaping the load input impedance, but also can reduce its impact on the original load converter. However, though the ASVI control strategy has already greatly reduced its impact on the load converter, its remaining impact is negative. To solve this problem, this paper moves the ASVI from the load side to the source side via a proposed source-side series-virtual-impedance (SSVI) control strategy for the source converter. This SSVI control strategy not only has the same stabilization function and adaptive characteristics as the ASVI control strategy, but also improves the performance of the source converter. In addition, since the SSVI control strategy is realized by changing the control block of the source converter, the performance of the load converter is not affected. Therefore, the SSVI control strategy can be treated as a supplement and expansion of the ASVI control strategy. Moreover, depending on the method of realization, the SSVI control strategy can be divided into the source stabilization methods of the cascaded system. Finally, a 100 W 48 V-32 V-24 V cascaded system has been fabricated to validate the proposed control strategy.
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