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In DC microgrids, constant power loads (CPLs) would lead to a destabilizing effect that may cause the main bus voltage oscillation. Hence, most researcher are exploring the stabilization methods for DC/DC buck converters feeding CPLs. In practical applications, buck converters are always cascaded with LC input filters to prevent the switching harmonics from returning into the source. With the addition...
The stability analysis of multi-stage DC-DC converters system is a big challenge, since the interaction among the individual converters can lead to an instability of the whole system. This paper applied the stability analysis method based on Floquet theory for a three-stage DC-DC converters system. In this paper, the stability analysis method based on Floquet theory is reviewed at the beginning. Following,...
This paper addresses the harmonic current control for LCL-filtered Voltage-Source Converters (VSCs) connected to ultra-weak (high-impedance) grids. It is shown that the harmonic current controllers tend to be unstable as the Short-Circuit Ratio (SCR) of the system reduces. An active stabilizing control scheme is thus proposed by feeding back the filter capacitor voltage and the converter-side current...
Solid state transformers (SSTs) include dc subsystems to enable plug-and-play support of dc loads, generation sources, and energy storage. Dual active bridge (DAB) converters are a suitable topology for both the primary energy conversion and load interface applications in an SST. This study considers an SST that uses DAB converters for both applications, and describes a method of generating a full...
Frequency domain passivity theory is used to evaluate harmonic resonance instabilities in a system with multiple grid-tied voltage source converters (VSCs), and subsequently, a stabilizing controller using predictive current control (PCC) method is proposed to prevent such instabilities. The input admittance of VSCs using PCC can achieve passivity almost up to the Nyquist frequency. This research...
Because of the differences in line impedances, parallel inverters with conventional droop control will have huge circulating current, which does not flow through the loads. To mitigate the circulating current, the research in this paper finds out that parallel-operated inverters should have the impedance in inverse proportion to their capacities, which is difficult to realize in practice. To meet...
To avoid the stability problem of grid connected inverter caused by the conventional phase locked loop (PLL), the direct power control of voltage source inverter in virtual synchronous reference frame without PLL is investigated considering grid voltage disturbance in the paper. Based on the small signal analysis methods, comparison between conventional PLL and no PLL is made. It can be found that...
As a basic building block, the phase-locked loop (PLL) has been widely used in the grid-connected voltage source inverter (VSI). But it has been shown recently that PLL has adverse impact on stability of grid-inverter system. In a weak grid, the PLL may cause unexpected harmonic, or even instability. In order to overcome this problem, this paper proposes a q-axis voltage feedforward control method...
Small-signal models of impedances for grid-tied converters have recently attracting researchers' attention and shown of great importance in stability analysis. This paper proposes the impedance model in d-q frame for STATCOMs, including dynamics from synchronization, current and voltage loops and reveals the significant features compared to other types of grid-tied converters that the 1) grid impedance...
The high R/X ratio of distributed lines in low voltage microgrid causes the coupling between the active and reactive power control. Forcing the output impedance to achieve the decoupling through additional virtual inductor impedance loop heavily causes the voltage amplitude drop at the point of common coupling. Such voltage drop deteriorates the microgrid voltage stability. To avoid the problem of...
The paper proposes a hybrid droop control strategy to enhance the stability and increase maximum constant power loads (CPLs) capability of DC microgrids in a realistic scenario. By capturing the detailed model of inner control loops and hybrid droop control and general dc MG topology, a thorough comparative study is conducted between traditional droop control and hybrid droop control. Enhanced control...
In this paper, an eigenvalue-based harmonic stability analysis method for offshore wind farm is proposed. Considering the internal cable connection layout, a component connection method (CCM) is adopted to divide the system into individual blocks as current controller of converters, LCL filters, collection system cable impedance, transmission cable impedance and grid impedance. By using this method,...
Grid-connected converters have been widely used in distributed generations. However, the resonance brought by converters in weak grid has been a great threat to system stability. Impedance-based method provides a powerful tool for stability analysis. Active damping is also widely applied to improve system stability. This paper derives the impedance model of grid-connected converter system and analyzes...
In this paper, a new distributed cooperative control for voltage unbalance compensation in an islanded microgrid is proposed. In order to decrease the negative impacts of impedance of distributed lines upon conventional negative-sequence reactive power-conductance droop control, a cooperative control component is added to accomplish the trade-off between the accuracy of negative-sequence reactive...
Transparency and stability issues have been a major concern for haptic tele-operation. Researchers in this area have already shown that a minimum refresh rate of 1 kHz is required to achieve smoothness for human perception in force-feedback experiments. This rate requires the highest priority for real-time applications to achieve transparent haptic tele-presence. As a result, this rate leads to a...
For loosely coupled wireless power transfer systems, strongly magnetic coupling is becoming a viable scheme to realize power transfer over medium distances. The method using DC-DC circuit is regarded as the most promising way to realize impedance matching for such systems. However, due to the nonlinear nature of rectifying circuit, it is difficult to track the optimal load accurately and the robust...
In the VSC-HVDC system supplying power to passive network, the dc-side of the system is prone to oscillation and even instability, which is resulted from the constant power characteristic of the inverter station and the interactions of the dc impedance between converter stations. In view of this, firstly, the output impedance model of the rectifier station, dc cable impedance model and the input impedance...
DC distribution technology has become the new choice and the trending technology of shipboard power systems for its advancement over its AC counterpart. In DC shipboard power systems, the bus voltage stability is a critical issue. The presence of tightly controlled high-power constant power load can induce system-level voltage instability. To mitigate such a problem, a novel compensation method based...
In DC microgrid, it is common that constant power loads (CPLs) exhibit negative impedance characteristics, which degrades the stability margin of the whole system. This paper focuses on the stability problem of isolated DC microgrid with multiple DC voltage control units and constant power loads. Taking controllers into account, the detailed impedance model of DC microgrid is established, and it shows...
Digital-analog hybrid simulation method combines the advantages of both real-time digital simulation and physical simulation, and will be an effective means for the simulation and analysis of future power system. However, the ideal transformer model (ITM), as the most widely used interface algorithm, is easy to encounter instability issues. A novel impedance compensation algorithm based on ITM is...
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