Abstract
Impedance/admittance models (IM/AMs) have been widely used to analyze the small-signal stability of grid-tied power electronic devices, such as the voltage source converter (VSC). They can be either derived from theoretical analysis under white-box conditions, where all parameters and control structures are fully known, or measured based on experiments under black-box conditions, where the topology and parameters of the controllers are completely unknown. As the IM/AMs depend on specific operating conditions, it is highly desirable to develop fast algorithms for IM/AM prediction (or estimation) under the black-box and variable-operating-point conditions. This article extends the nearly-decoupled AM method for sequence AMs proposed recently by Liu <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al</i> to fit any unknown control structure, including not only grid-following VSC, but also grid-forming VSC. It is, therefore, referred to as the fully-decoupled IM (FDIM) method. Furthermore, a curve fitting method for the transfer function is proposed to expedite the algorithm, based on the information of a few disturbance frequencies only. Finally, the algorithm is verified by wide simulations and experiments under different situations, including the direct-drive wind turbine generator. The whole approach is expected to be broadly applicable to the stability analysis of power-electronic-based power systems under variable operating conditions.
Highlights
Due to increasing pressure of environmental protection and energy resource, a great quantity of solar and wind powers with the form of distributed sources have been connected to grid recently [1], and the traditional power system has being gradually transformed into a semiconducting power system [2]–[5]
We have been faced with various types of serious stability and oscillation problems in system level caused by power electronic devices, and accurate mathematical modeling and analysis of voltage source converter (VSC) is highly desirable [6]–[8]
To test the fully-decoupled IM (FDIM) algorithm, we have studied several major control forms, such as the phase-locking loop (PLL) combined with alternating current control (ACC), DC-link voltage control (DVC), and terminal voltage control (TVC), the PLL combined with ACC and power flow control, the droop control, the virtual synchronous generators control, etc., as shown in Figs. 1 and 3
Summary
Due to increasing pressure of environmental protection and energy resource, a great quantity of solar and wind powers with the form of distributed sources have been connected to grid recently [1], and the traditional power system has being gradually transformed into a semiconducting power system ( called power-electronic-based power system) [2]–[5]. There the IM/AM can be obtained through experiments by frequency sweeping measurement via either series voltage or shunt current injection, under black/gray-box conditions [12]–[15], where usually we cannot get all the structure and parameters of devices for the sake of commercial secrecy Based on these IM/AM results from either theoretical modeling or measurement, the small-signal stability analysis can be further studied based on the generalized Nyquist stability criterion [16]–[19]. After performing small-signal derivations on the PLL in Fig. 1(b) and the four different controllers, we find that ∆θ can always be written in a unified form, These eight variables including fdin, fdid, fqin, fqid, fdvn, fdvd, fqvn, and fqvd are intermediate variables generated by linearization.
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