Phase-locked loop (PLL) is commonly used for three-phase grid-connected inverters to obtain the information of grid synchronization, and PLL dynamics are the key factors for stable operation of the inverters. Under weak grid conditions, the coupling between PLL and grid impedance can result in harmonic resonance, or even instability in the system. In this paper, the effect of PLL dynamics and grid impedance on the stability of three-phase grid-connected inverters is studied with the d – q impedance model. Besides, the variable transfer relationship is used to analyze the influence of PLL perturbations on output current under weak grid conditions. To suppress low-order harmonics of the network current caused by PLL perturbations under weak grid conditions, a novel feedforward control method is proposed to compensate PLL perturbations and revise the output impedance, where the operation of the inverter and PLL dynamics have been taken into account in the design process. By analyzing the impedance characteristic of the system, it can be demonstrated that the proposed method can enhance the stability of grid-connected inverters under weak grid conditions and reduce the impact of PLL perturbations on grid-connecting current. The experimental results indicate that the low-order harmonics of the network current can be suppressed effectively, which verifies the analysis.

An Improved Feedforward Control Method Considering PLL Dynamics to Improve Weak Grid Stability of Grid-Connected Inverters

A subsynchronous oscillation (SSO) phenomenon has been observed in a modular multilevel converter-based high-voltage dc (MMC-HVDC) transmission system for wind farm integration in the real world, which is independent of the type of wind turbine generator. This kind of oscillation appears different from those in doubly fed induction generator-based wind farm with series-compensation line or wind farm integration through two-level voltage-source converter-HVDC transmission system, because the internal dynamics of the MMC may have significant impact on the oscillation. By far, however, very few papers have reported it. In this paper, the generation mechanism of the SSO phenomenon in an MMC-HVDC transmission system for wind farm integration is revealed from an impedance point of view. The harmonic state-space modeling method is applied to model the multifrequency behavior of the MMC, based on which, the ac-side small-signal impedance of the MMC is analytically derived according to harmonic linearization theory. As a general rule, the controller parameters of the wind power inverter and the HVDC converter are designed separately, to meet the performance requirements of the single converter under ideal conditions, but this practice does not guarantee the stability of the interconnected system. Therefore, an optimal design method for controller parameters is proposed in this paper in order to guarantee the small-signal stability of the interconnected system from a system point of view. Finally, time-domain simulations validate the effectiveness of the theoretical analysis and the proposed optimal design method.

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Optimal Design of Controller Parameters for Improving the Stability of MMC-HVDC for Wind Farm Integration

For three-phase LCL -type grid-connected converter, when it is attached to weak grid, current control interacts with phase-locked loop (PLL) via point of common coupling voltage. Consequently, PLL dynamic might deteriorate grid current control and even result in system instability. However, the conventional design method of current controller neglects the impact of PLL, and therefore, it is hard for the current controller to mitigate the negative effect of PLL. In this paper, an improved design of current controller, i.e., proportional–integral controller and capacitor-current-feedback active damping, is proposed to reduce the negative effect of PLL on current control. First, a small-signal impedance model is developed to analyze the impact of PLL on current control. Then, the effect of current controller parameters on the converter output impedance is analyzed, and a design guideline to improve the current controller parameters is presented. With the improved parameters, in precondition of satisfying system stability margin under both stiff and weak grid, the negative effect of PLL on current control can be effectively mitigated without employing additional control strategies. Furthermore, the current control has a strong robustness against wide-range variation of grid impedance. Finally, the experiment demonstrates the effectiveness of the proposed design method.

An Improved Design of Current Controller for LCL -Type Grid-Connected Converter to Reduce Negative Effect of PLL in Weak Grid

The premise that the virtual synchronous generator (VSG) can actively support the weak grid is that the VSG can operate stably. In this paper, the small-signal sequence impedance models of the voltage-controlled VSG and the current-controlled VSG are built and the sequence impedance characteristics of these two types of VSGs are compared and analyzed. The sequence impedance of the voltage-controlled VSG, being consistent with the grid impedance characteristics, is generally inductive. By contrast, the sequence impedance of the current-controlled VSG is mainly negative impedance in the middle- and high-frequency area and the impedance amplitude is quite low. Based on the built sequence impedance models and the impedance stability criterion, the influence of the weakness of the grid on the stability of these two types of VSG grid-connected system is analyzed. The results of stability analysis show that the current-controlled VSG is prone to instability while the voltage-controlled VSG remains stable. Therefore, the voltage-controlled VSG is more suitable than the current-controlled VSG for grid-connected renewable energy generation in an ultraweak grid from the point of view of system stability. Finally, experiment results validate the stability analysis.

Sequence Impedance Modeling and Stability Comparative Analysis of Voltage-Controlled VSGs and Current-Controlled VSGs

When LCL -type converter is attached to weak grid, its current control and phase-locked loop (PLL) will interact with each other, via the point of common coupling voltage. Unfortunately, the conventional PLL controller design methods are mainly for PLL independent systems, regardless of the aforesaid interaction. As a consequence, PLL dynamic might deteriorate the grid current control and even leads to instability problem. For this issue, this article proposes an improved design of PLL controller parameters to mitigate the negative effect of PLL on the current control. First, the small-signal model of the current control system considering PLL effect is established, and the system stability with the conventional PLL controller design method is analyzed. Then, an improved design of PLL controller parameters is proposed, and the design guideline is given in detail. With the method, not only the dynamic and static response performance of PLL independent system can be maintained, but also the negative influence of PLL dynamic on the current control can be effectively reduced in weak grid. Moreover, the grid current control has a strong robustness to the grid impedance variation. Finally, the proposed method is validated by the simulation and experiment.

Improved Design of PLL Controller for LCL -Type Grid-Connected Converter in Weak Grid

The invention provides an axial half-section LL-shaped non-contact power supply slip ring. The magnetic core structure of the axial half-section LL-type non-contact power supply slip ring takes an axial half-section basic LL shape as a foundation, the axial half-section of a primary/secondary magnetic core is L-shaped, the axial half-section of a secondary/primary magnetic core is inverted L-shaped, and primary and secondary windings are coaxially shunt-wound and positioned between a primary side magnetic core and a secondary side magnetic core. The vertical distances of air gaps between windings and between magnetic cores are maintained to be unchanged, and the air gaps are moved to the secondary/primary side by cutting one triangle and/or an arc-shaped part of the primary/secondary side magnetic core, so that the effective area of a coupling magnetic circuit between the primary side magnetic core and the secondary side magnetic core is increased, the magnetic resistance is reduced, and the slip ring is also applicable for the treatment of other transformer magnetic cores; and the primary side magnetic core and the secondary side magnetic core can extend along the axial direction respectively. The primary and secondary side windings are coaxially wound, and can be coaxially wound in parallel, coaxially wound externally or coaxially wound in a cone-shaped manner; and the axial half-section LL-shaped non-contact power supply slip ring can be combined to form any number of channels or phases. The axial half-section LL-shaped non-contact power supply slip ring is applicable for non-contact electric energy transmission occasions needing rotation on a power supply side or a power receiving side.

Axial half-section LL-shaped non-contact power supply slip ring

The invention provides a multi-coil excitation magnetic field coupling type wireless charging platform, comprising two-part coils. In the first part, the geometrical centers of the coils are connected to form a regular polygon, meaning that an enclosure is formed at a spatial position; and through the pump-in of AC excitation of different phases, a traveling magnetic field with enclosed movement path is formed, which solves the problem of the uneven distribution of the magnetic fields caused by the end effect. The second-part coils are located in the center of a charging area, and through the AC excitation, the problem of induction blind area arising in the central area due to mutual cancellation of magnetic fields can be resolved. With the invention, the magnetic field distribution in an entire charging area is even and no induction blind area exists, which makes the platform suitable for most non-contact power transmitting occasions.

Multi-coil excitation magnetic field coupling type wireless charging platform

For grid-connected inverters, phase-locked loop (PLL) is usually adopted for the injected grid current to achieve phase tracking of the grid voltages, which means the PLL will have an inevitable effect on the output impedance characteristics and the impedance stability of grid-connected inverters. Hence, the harmonic linearization method was firstly used to derive the positive- and negative-sequence output impedance models of the three-phase grid-connected inverter with and without PLLs. ond, two kinds of the commonly used PLL circuits that are synchronous reference frame phase locked loop (SRF-PLL) and dual second order generalized integrator frequency locked loop (DSOGI-FLL) were employed to compare and analyze their frequency characteristics. Finally, the simulation and experimental results were given to verify the impedance models of the grid-connected inverter with different PLLs.

Impedance Influence Analysis of Phase-Locked Loops on Three-Phase Grid-Connected Inverters

The traditional fundamental harmonic approximation (FHA) method is not applicable any more in high coupling coefficient WPT system. Instead, the harmonic input impedance and currents are derived and analyzed. Soft switching missing phenomenon may occurs when the coupling coefficient is designed to be in the zones (0.89∼0.93), and (0.99∼1). Simple serial-connected LC filters are introduced to correct the undesired phenomenon. Simulation results verified the validity of the theoretical analysis and the correction method.

Analysis, and correction of soft switching missing phenomenon in high coupling coefficient WPT system

Asymmetric control and dynamic characteristic of DC bus will cause frequency coupling effects which turn converter into a single input double output (SIDO) system. The interaction between two SIDO system is more complicated than the interaction between converter and passive grid impedance. This work focus on impacts of coupling effects on the wind farm integration through MMC-HVDC system. The paper firstly presents impedance modeling wind farm and MMC-HVDC considering the coupling effects separately, which are used to explain the coupling effects on the frequency characteristics and system stability analysis results. Then the developed model is used to illustrate the effect of the bandwidth of PMSG PLL controller, MMC-HVDC voltage controller and the degree of unbalance on impedance characteristic and system stability. The system stability analysis results are validated by numerical simulation of a PMSG-based wind farm integration through MMC-HVDC to prove the correctness of the theoretical analysis.

Xin Chen

Papers

Axial half-section LL-shaped non-contact power supply slip ring

Multi-coil excitation magnetic field coupling type wireless charging platform

Impedance Influence Analysis of Phase-Locked Loops on Three-Phase Grid-Connected Inverters

Analysis, and correction of soft switching missing phenomenon in high coupling coefficient WPT system

Impacts of Coupling Effects on Stability Analysis of PMSG-based Wind Farm Integration through MMC-HVDC