Synchronization is the key of AC power system operation, but its mechanism becomes unclear with the large-scale integration of voltage-source converters (VSCs). As an important feature distinguishing VSCs from synchronous generators (SGs), the current-source characteristics of VSCs have hardly been focused from the perspective of synchronization. In this letter, a synchronization mechanism between power-synchronized voltage-source (VS) and PLL-controlled current-source (CS) is clarified. Based on this mechanism, negative damping oscillations of SGs caused by VSCs are disclosed.

Virtual synchronous generators (VSGs) have attracted widespread attention due to their advantage in supporting voltage and frequency of power systems. However, relevant studies have shown that a VSG has similar low-frequency oscillation as synchronous generators, which is more likely to occur under strong grid conditions. In this paper, the linearized mathematical model of a VSG is established by using small-signal analysis; based on this, the physical process of low-frequency oscillation of a VSG is explained from the perspective of vector motion. Firstly, the amplitude and phase motion of the current vector of a VSG under small disturbance are analyzed, then the mechanism of negative damping caused by terminal voltage control is revealed, and the reason why a VSG is more prone to instability under strong grid conditions is explained. Based on these, the influence of control and grid strength on the low-frequency oscillation of a VSG is analyzed. Studies show that the amplitude motion of the output current is the main cause of negative damping, and the oscillation can be suppressed by optimizing the value of key parameters.

Voltage source converter (VSC)-interfaced renewable power plants are typically far from load centers, the weak grid condition presents significant stability risks. Extensive research has been conducted to analyze oscillations within the DC voltage control (DVC) timescale. However, most studies focus on scenarios where generators are connected to the grid through long transmission lines. More general scenarios, where loads are distributed along these transmission lines, have not received sufficient attention, potentially leaving undiscovered oscillation mechanisms. This paper finds a small-signal stability issue, which is an unexpected oscillation that may occur if local loads are connected near the VSC-based power plants. To comprehensively analyze this phenomenon, this paper takes the phase-locked loop (PLL) as the main object and analyzes the interaction between the terminal voltage control (TVC), DVC, and PLL. Initially, the complex coupling relationship is clarified by applying series expansion to the interaction paths between TVC and DVC. It is found that the load effect contributes to forming a crucial path that introduces the negative damping torque caused by TVC to PLL. Subsequently, analyses show that undamped oscillation risks exist under normal control parameters. A targeted control strategy is proposed for oscillation suppression. Verifications are conducted using MATLAB/Simulink and RT-LAB.

The dynamic behavior of large-scale systems containing diverse devices is a crucial focus for system operators, particularly in power systems where grid-connected devices generate AC electrical signals through various modulated methods. One of the main objectives of power systems is to transmit power. To this end, time-varying amplitude/frequency rotating vectors are used to describe different AC voltage and current signals, and then the active/reactive power separation mechanisms and characteristics for different signal-modulated power devices are explored. These mechanisms and characteristics are analyzed and verified through time-domain simulations. The theoretical contribution of this paper is that it thoroughly clarifies the misconception in current power theories by demonstrating that active power and reactive power naturally arise as inherent physical quantities rather than being solely mathematically defined. In terms of practicality, this paper can provide physically grounded insights for the power calculation methods and offer guidance for the design of power measurement in actual power system dynamic processes. Through the analysis presented in this study, the analysis, measurement, and control of the active/reactive components in renewable energy equipment based on the instantaneous reactive power calculation method or the traditional sinusoidal steady-state power calculation method do not need to be updated.