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A generation unit that is poorly damped or has periodical mechanical power outputs from its prime mover can become a source of oscillation in a bulk power system. Choosing the proper electric quantity to extract the oscillation characteristics is crucial for the location and mechanism analysis of the oscillation source. This paper proposes the concept of oscillation phasor to characterize a type of sustained power system oscillations that originate from power plants. On this basis, a two-stage scheme is presented to locate one type of the sustained oscillation source and discover its mechanism: first, Prony analysis and the first wave algorithm are applied to extract the oscillation phasors on active power and angular frequency; second, the phase relationships between the oscillation-source generator and other generators are derived to guide the location of the source; finally, damping estimation on these oscillation phasors help distinguish a forced oscillation from a poorly damped oscillation. The proposed scheme is first demonstrated in detail on a two-area four-machine system and then tested using historical oscillation event data.

This paper presents an active disturbance rejection control (ADRC) method to mitigate subsynchronous resonance (SSR) in a doubly-fed induction generator (DFIG)-based wind farm connected to series compensated transmission lines. First, the frequency of the SSR and its influencing factors are analyzed, and the limitation of the traditional damping controller based on phase compensation is discussed. Then, a damping controller is designed using the ADRC method, which is adapted to the uncertainty SSR frequency of wind farms. The stator current of the DFIG that contain apparent SSR frequency is adopted as the input signals of the ADRC damping controller, and the outputs are added to the current inner loop of the rotor side converter (RSC), which has fast response speed. The performances of the ADRC damping controller are tested in different compensation levels, numbers of DFIG in wind farm, and parameters of the converter control. The simulation results indicate that the ADRC damping controller is superior to the traditional damping controller for mitigating the SSR because it can automatically estimate and compensate the uncertainty disturbances.

Subsynchronous oscillation (SSO) phenomena caused by interactions between controllers of Doubly-fed induction generators (DFIGs) and series capacitor compensators has been well recognized. However, SSO can also happen on a DFIG-based wind farm without series compensation as reported in a recent real event. This paper firstly analyzes the recorded voltage waveform during this SSO event. Then an equivalent model of the system including the wind farm, static synchronous compensator (STATCOM) and the power grid was established. The paper derives the subsynchronous frequency response of the DFIG converter together with the STATCOM in detail and gives the criterion for SSO to occur, which is verified by both time domain simulation and spectrum analysis. The methodology adopted in this paper can be generalized to analyze the causes of a type of SSO that involves DFIGs and STATCOM but not series compensation. The paper also provides practical suggestions on adjustments of control parameters with the DFIG and STATCOM to prevent this type of SSO.

A new compact microstrip tunable bandpass filter with continuous control of center frequency and bandwidth is proposed in the paper. The proposed design is based on short parallel-coupled lines, which are tuned by properly loading varactors. The structure exhibits a pair of transmission poles and one-side passband edge transmission zero that can be flexibly adjusted for required positions. A thorough theoretical analysis is derived to estimate the performance of the proposed filter and verify the initial values of design parameters. To verify the design concept, a prototype is fabricated and measured. The fabricated circuit has an extreme compact size (smaller than 0.03λg × 0.1λg). The simulated and measured results agree well with the derived theory, and indicate wide tunable center frequencies (0.56-1.15 GHz) and 1-dB bandwidth (65-180 MHz).