Impedance measurement is an effective method to obtain the impedance characteristic of the renewable power generator (RPG). According to the measurement results, the equivalent transfer function of the impedance can be obtained for stability analysis by fitting algorithm. For accurate impedance measurement and model fitting, existing studies have proposed design methods for the amplitudes, frequency bins and phases of the wideband perturbation signal. However, the wideband signal with excessive peak value may result in over-modulation of the RPG under measurement, leading to inaccurate measurement results, which is not considered in the above methods. In order to fill this gap, this paper proposes a comprehensive design method for frequency bins, amplitudes and phases considering peak-value constrain. The evaluation method for over-modulation risk is proposed to judge whether the peak value of designed signal is excessive. The method can ensure the accuracy of impedance measurement and model fitting while using wideband signal for impedance measurement. Finally, the experiments based on Control-hardware-in-Ioop (CHIL) are conducted to verify the effectiveness of proposed method.

The transient stability of grid-following (GFL) converters is very important for power systems. This article combines the dynamics of DC-link voltage control (DVC) and phase-locked loop (PLL), and establishes a large-signal model for GFL converters. Considering bidirectional power flow, the existence and stability of the equilibrium point (EP) in the system are analyzed, and the power transmission range that enables the system to have a stable equilibrium point (SEP) is obtained. The analysis results indicate that DVC significantly alters the distribution characteristics of the EP in the system, resulting in bidirectional differences in the system's capacity to transmit power. Even if the power is within the power transmission range, the system may still lose synchronization with the power grid due to sudden changes in power. In addition, when power flows from the power grid to the converter, it may generate a “negative capacitance effect”, causing the original SEP to become unstable and weakening the power transmission capacity of the system. As a result, guidelines for designing control parameters that prevent the “negative capacitance effect” are proposed.

The doubly fed induction generators (DFIG) has the resonance issues under inductive weak grid due to the phase locked loop (PLL). Direct power control (DPC) without PLL is a widely studied control strategy, which can remove the PLL. By the equivalent single-input and single-output (SISO) impedance model, it can be found that the phase margin of DFIG system is always sufficient even if the parameters of DFIG and grid change. Then the influence of different reference frames and control targets are both analyzed, and the key influencing factor to improve the fundamental frequency stability for DPC without PLL is revealed, that the power control can abandon the Park transformation and reduce one grid voltage disturbance path. The experiments established in the control-hardware-in-loop (CHIL) validate the above conclusion.

Since only one inverter voltage vector is applied during each duty cycle, traditional model predictive direct power control (MPDPC) for grid-connected inverters (GCIs) results in serious harmonics in current and power. Moreover, a high sampling frequency is needed to ensure satisfactory steady-state performance, which is contradictory to its long execution time due to the iterative prediction calculations. To solve these problems, a novel dead-beat MPDPC strategy is proposed, using two active inverter voltage vectors and one zero inverter voltage vector during each duty cycle. Adoption of three inverter vectors ensures a constant switching frequency. Thus, smooth steady-state performance of both current and power can be obtained. Unlike the traditional three-vector based MPDPC strategy, the proposed three vectors are selected based on the power errors rather than the sector where the grid voltage vector is located, which ensures that the duration times of the selected vectors are positive all the time. Iterative calculations of the cost function in traditional predictive control are also removed, which makes the proposed strategy easy to implement on digital signal processors (DSPs) for industrial applications. Results of experiments based on a 1 kW inverter setup validate the feasibility of the proposed three-vector based dead-beat MPDPC strategy.

This paper presents a multiple target implementation technique for a doubly fed induction generator (DFIG) under unbalanced and distorted grid voltage based on direct power control (DPC). Based on the mathematical model of DFIG under unbalanced and distorted voltage, the proportional and integral (PI) regulator is adopted to regulate the DFIG average active and reactive powers, while the vector PI (VPI) resonant regulator is used to achieve three alternative control targets: (1) balanced and sinusoidal stator current; (2) smooth instantaneous stator active and reactive powers; (3) smooth electromagnetic torque and instantaneous stator reactive power. The major advantage of the proposed control strategy over the conventional method is that neither negative and harmonic sequence decomposition of grid voltage nor complicated control reference calculation is required. The insensitivity of the proposed control strategy to DFIG parameter deviation is analyzed. Finally, the DFIG experimental system is developed to validate the availability of the proposed DPC strategy under unbalanced and distorted grid voltage.

We investigate two different kinds of resonant current regulators for a doubly fed induction generator (DFIG) under distorted grid voltage conditions: proportional integral resonant (PIR) regulator with traditional resonant part and vector proportional integral (VPI) regulator with VPI resonant part. Based on the mathematical model of DFIG under distorted grid voltage, the transfer function and frequency response characteristics of the two current regulators are analyzed and compared. The superiority of the VPI current regulator over the PIR regulator is pointed out, and the influence of discretization methods on the performance of the resonant current regulator is studied. All the results are validated by MATLAB simulation and experiments.