This paper addresses the frequency oscillation problem in a parallel-inverter-based grid-connected system. Angular frequency interactions between inverters and the grid exhibit various numbers of complicated characteristics that seriously threaten the connected power system's stable operation. This paper proposes a unified frequency oscillation method analysis to investigate the oscillation problem in a multi-VSG grid-connected system. In this method, the power control model of each grid-connected inverter is equivalent to a two-terminal network by analogy method, which can be described as a “current source” connected in parallel with a reciprocal of “admittance” and then joined in series with a joint of “admittance.” Next, considering the influence of grid impedance, the model of the whole grid-connected system is established to reveal the mechanism of multi-VSG angular frequency oscillation. Subsequently, the influence factors of frequency oscillation and its variation law are analyzed through transfer functions, which provide an essential theoretical method for solving the frequency oscillation problem in the current multi-inverter grid-connected engineering application. Finally, we validate the effectiveness and correctness of the small-signal model and the frequency oscillation characteristics by building the MATLAB/Simulink simulation.

This paper discusses the sensitivity of programmable logic controllers (PLCs) to voltage sags based on mechanistic analysis and numerous tests. First, the sag sensitivity caused by different values of rated working voltage is shown through an analysis of the power supply and discrete input/output modules of PLCs. Second, a detailed sag-tolerance experimental scheme is proposed, and PLC sensitivity curves that differ from early curves are established through several experimental tests. Experiments are conducted with nine typical PLCs. The sensitivity of the power supply and discrete input modules are researched first, and then, the effect of the key characteristics of voltage sag, namely, magnitude, duration, points-on-wave of sag initiation, odd harmonics, multiple sags and continuous sags, are tested. The uncertain area of the PLC sag tolerance is identified after intensively processing the extensive experimental data and curve fitting. A more accurate sag influence assessment for PLCs can be made with the given curves compared to the early curves. Furthermore, experiments have revealed that certain PLCs may work in an uncertain status during voltage sags. The reason and the method of elimination with the proper choice of DC bus capacitance for PLCs are also analyzed.

Harmonic impedance estimation and suitable evaluation index selection are the key steps of harmonic contribution evaluation. Most of the traditional harmonic impedance estimation methods are only applicable to the scenario, where the background harmonic is stable and the utility impedance is constant. However, this scenario will change in many cases due to the fluctuation of harmonic, the change of system operation mode and so on. In order to improve the estimation accuracy for harmonic impedance, a harmonic impedance estimation method is proposed in this paper based on similarity measure and ordering points to identify the clustering structure (OPTICS). The total harmonic contribution index and harmonic comprehensive contribution index are proposed based on subjective analytic hierarchy process to simplify the evaluation results. Simulation analysis and their comparison with the traditional methods reveal that the proposed harmonic impedance estimation method can reduce the influence of background harmonic voltage fluctuation and utility impedance change, making the harmonic impedance estimation result more accurate. Besides, the proposed total harmonic contribution index and harmonic comprehensive contribution index can effectively simplify contribution evaluation results, which provide a new methodology for the evaluation of harmonic contribution.

This study presents a new approach to the optimal placement of voltage sag monitors considering the uncertainties associated with transition resistance. The influence of transition resistance on the magnitude of voltage sags triggered by symmetrical and unsymmetrical faults is analyzed. Then the transition resistance interval set array for voltage sags is established, on the basis of which, a random vector model on voltage sag observability is proposed and related observability indices are defined in the form of conditional probability. The optimal placement model is established by taking the available number of monitors as the constraint condition and the maximum sag global observability index as the objective function. Binary particle swarm optimization (BPSO) is implemented to obtain the optimal placement results. Finally, simulation is carried out on IEEE 30-bus system, and it is shown that the proposed optimal monitor placement method is more applicable compared with the traditional MRA method.

This paper proposes a new method, called PLSR (partial least-squares regression), which is used for assessing harmonic impedance and the harmonic emission level at the point of common coupling (PCC). Utilizing the signals of harmonic voltage and harmonic current measured synchronously at PCC, regression coefficients are worked out through partial least-squares algorithm. By separating and filtering the measured data, the comprehensive variables that can explain dependent variables most effectively are distilled, and then the useful information and noise are identified from the measured data. Therefore, the disadvantage of variable dependence in establishing of the system model is overcome. After that, the supply harmonic impedance is got, consequently the harmonic emission level of customer is calculated. The simulation analysis and the analytic result of practical engineering road-testing data indicate that the proposed method is valid and feasible.