Yunfeng Li

The interaction between the negative damping inductive characteristics of MMC-based converter station in the high-frequency band and the capacitive characteristics of the AC system is the reason for the high-frequency resonances (HFRs). Existing damping controls and voltage feedforward filtering methods cannot suppress the HFRs completely because the negative damping of MMC in the high frequency range cannot be eliminated, resulting in the persistent HFR risk. To address this problem, the requirements of voltage feedforward loop for shifting the mentioned negative damping frequency range is revealed based on the simplified MMC impedance. A voltage feedforward filter composed of a notch filter and a first-order low-pass filter is presented accompanied by the parameters calculated and optimized. Furthermore, a coordinated suppression strategy integrating voltage feedforward filtering with a passive damping filter is proposed based on the principle of impedance reshaping. This strategy can eliminate the negative damping of the HVDC station completely in the high-frequency range while simultaneously reducing the parameters of passive damping filter. By doing so, the comprehensive performance of the coordinated suppression scheme is significantly improved. Time-domain simulations validate the effectiveness and robustness of the proposed method under various operating conditions.

In the modular multilevel converter (MMC) based high voltage direct current (HVDC) practical projects, the existing voltage feedforward control (VFC) method presents challenges in addressing the frequency shift of high-frequency resonance (HFR) caused by the changes in the interconnected AC system's operating conditions. In this paper, the mathematical characterization function of the impedance real part is used to reveal the mechanism why the existing VFC cannot simultaneously reshape the multiple HFR risk regions into the positive damping characteristics. Therefore, a frequency divided voltage feedforward control (FDVFC) based on the second-order band-pass filters (BPFs) is proposed to simultaneously reshape the multiple HFR risk regions into the positive damping characteristics. The proposal includes the selection of the number of BPFs and how to design their parameters. The parameter design principles and parameter selection ranges of the BPFs are presented one by one using the simplified model of the MMC in high-frequency band. Finally, the effectiveness and correctness of the FDVFC and its analytical calculation expressions for parameter selection are verified by time-domain simulations.

There is a risk of sub-synchronous resonance (SSR) when a permanent magnet synchronous generator-based wind farm is connected to a power grid through a high-voltage direct current (HVDC) system. To study such SSR, the linear model of the integrated system has been established using the state space method firstly. The root locus and modal analysis methods are adopted to study the participation variables and participation factors of SSR. It shows that the variables in the q-axis in both wind farms and MMC with high power levels dominate the resonance. Secondly, the simplified impedance models are derived. The simplified minimum loops L^dd and L^qq are used to carry out the parameter calculations. An active damping control strategy with virtual series impedance is proposed by feeding back AC current into the outer loop of MMC. Thirdly, three types of virtual series impedance are presented, considering the structure of damping controller. In addition, the parameter selection ranges and principles for those damping controllers are presented using analytical calculation methods. Finally, the effectiveness and correctness of proposed methods, as well as the robustness of parameters are verified by simulation models. And the feasible structure of the damping controller is suggested.

DC chopper used to absorb the surplus power to prevent the devices from been damaged by the dc overvoltage is one of the important equipment for the offshore wind farm grid connection via the modular multilevel converter-HVdc system. A hybrid dc chopper structure composed of cascaded submodules and a lumped resistor is proposed in which the distributed resistor is deployed inside each submodule. Due to the cascaded structure, a hysteresis control strategy with dc bus voltage coordinated is proposed to reduce the large charging power of the dc chopper. By doing so, the capacitor in each submodule is only charged during the inserting and bypassing stages giving an opportunity to greatly reduce the charging energy and capacitance of capacitor. As a result, the switching frequencies and power losses of insulated gate bipolar transistors (IGBTs) as well as the thermal energies of the distributed resistors are reduced by 95%, 99%, and 97.7%, respectively, compared to that of the original control strategy. Consequently, the press-packed IGBTs can be replaced with welded-type ones resulting in air-cooling device instead of water-cooling device. Finally, an experimental prototype is established for the validation of the proposed method.

Differing from a two-level voltage source converter (TL-VSC), the internal dynamics of a modular multilevel converter (MMC) are much more complicated. Since the fluctuation of submodule capacitor voltage is unavoidable when transmitting power, the MMCs output voltages cannot track the references perfectly. The interaction between the time-varying capacitor dynamics and controllers may lead to self-instability of the MMCs. In order to address this self-instability issue, this paper first establishes the small-signal closed loop transfer function matrices (TFMs) of the external and internal control loop by the harmonic linearization method. Further, a novel design principle of MMC controllers is proposed. Then, the critical impact factors on the stability of circulating current controller and AC voltage controller are investigated based on the generalized Nyquist criterion(GNC). The proposed models and analysis methods are validated by time-domain simulations in PSCAD/EMTDC.

The high-frequency resonances (HFRs) around 700 Hz and 1.8 kHz have occurred in Yu ^′ E project. Mechanism studies indicate that HFRs are caused by the interactions between the negative damping inductance characteristics of the MMC and the capacitive characteristics of the AC system. In this paper, two optimized methods regarding the current control loop are presented to suppress the HFRs. The first one is to use a round(x) function inserted into voltage feed-forward loop instead of low pass filter (LPF). The proposed method has less negative impact on transient characteristics of an MMC. The second one is to employ a second-order damping controller, including the parameters design, to reshape the MMC impedance around the multiple risky regions. The core of HFRs suppression is that the real part of the MMC impedance near the multiple intersection frequencies should be positive according to impedance stability theory. Combined with the proposed control methods and core of HFRs suppression, the simplified model of MMC is adopted to design the parameters using the analytical expressions. Moreover, the general steps of parameters design are presented. Finally, the optimized methods to suppress HFRs are verified by time-domain simulations.

High frequency resonance (HFR) is a key issue of power electronics based systems. Two solutions can be applied for addressing HRFs, including 1)active damping method and 2)passive damping method. For modular multilevel converters (MMCs), most of existing harmonic resonance suppression methods are focused on active methods, including active filtering and virtual resistance. However, the active filtering may lead to negative damping in its attenuation band and the virtual resistance may become negative in high frequency range when influenced by control delay. In order to avoid the negative effects of control delay and increase the damping of MMCs in high frequency range, this paper reconsiders the arm inductor in MMCs and proposes a passive damper design method for MMCs. With the coordinated design between active controller and passive damper, the negative damping of MMCs in high frequency range can be effectively eliminated and the system stability can be improved. Also, the proposed passive damper has little impacts on the steady-state operation and increases few losses of the MMC. Effectiveness of the proposed passive damper is validated by electromagnetic transient (EMT) simulations in PSCAD/EMTDC.

It is vital of practical engineering significance to connect the isolated station into the DC grid smoothly without affecting other stations or triggering protective systems. To this end, an active charging method with damping control of the isolated station is proposed and will be used in a 500 kV‐level DC grid in China. First, the analytical expression of starting current is calculated. The key factors influencing the starting current are analysed to provide theoretic evidence for simplifying the calculation model. Second, the selecting expression of start‐up resistor is derived by simplifying the main circuit parameters, DC line parameters, control processes and so on. Third, the principle of active charging method is clarified, the selecting principle of decreasing slope and parameters of damping controller are presented, and the required time of charging process is calculated. This method does not need the start‐up resistor any more but still ensure no oscillation and impulse during the active charging process. Meanwhile, it is also faster than the traditional method with a better economy. Finally, electromagnetic transient simulation models are built to verify the correctness of the resistor selecting expression and the effectiveness of the active charging strategy with damping control.

This study presents a generic linearised modelling method and a virtual parallel resistor damping control for a voltage source converter‐based high‐voltage direct current (HVDC) system including a direct current (DC) grid. The generic modelling method is composed of DC network's equivalent node‐admittance matrix and the Thévenin equivalent matrix of all converters. A simplified two‐terminal HVDC system is used to identify the key factors influencing the stability of the DC grid easily. Those key factors are proved by two methods, namely, root locus and participation factors analysis. The parameters of the virtual parallel resistor damping control are designed using the simplified HVDC system. The performance of the proposed damping control is studied employing the linearised model of the DC grid. Time simulations such as instability suppressing, power‐controlled station N  −   1 fault, and DC system faults are carried out to validate the effectiveness of the damping control.