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基于改进锁频环的并网变流器惯性增强控制方法
Inertia Enhancement Control Strategy Based on Frequency-Locked Loop for Grid Connected Converter
【目的】由于电力电子变流器的并网特性，电力系统在新能源发电占比不断提高的背景下，整体惯性显著降低。为了增强电力系统的频率稳定性和抗扰动能力，要求并网变流器主动提供惯性支撑。【方法】在功率分配控制策略基础上，深入分析了基于常规锁相环（phase locked loop， PLL）控制结构的零极点分布及其在并网控制中的稳定性下降问题，提出了一种适用于并网变流器的基于改进锁频环（frequency locked loop， FLL）控制结构的惯性增强控制结构，将频率导数信号与参考有功功率成比例结合，以解决功率振荡问题。并且，采用一种改进的模型预测控制代替传统电流内环，以改善动态性能。通过频率阶跃响应的对比，展示了在弱电网条件下基于改进锁频环控制结构在并网变流器频率稳定性方面的优势。给出了基于改进锁频环架构的有功环控制策略，阐述了其惯量增强的原理，并对惯性参数进行了调优设计。【结果】通过硬件在环（hardware in loop， HIL）实验验证了所提策略的有效性。【结论】该方法避免了因微分操作引入的高频噪声和锁相环导致的不稳定问题，显著改善了系统的频率响应特性，实现了系统惯性增强的控制目标。
[Objective] With the increasing penetration of renewable energy sources， traditional rotating machines have been replaced with power electronic converters， causing a considerable decline in the overall inertia of power systems. This reduction in inertia poses serious challenges to the frequency stability and system disturbance rejection. To overcome these challenges， grid-connected converters that can actively provide synthetic inertial support are required. [Methods] First， based on a power-sharing control strategy， this study analyzes the pole-zero distribution characteristics of conventional phase-locked loop （PLL）-based control structures and highlights the associated stability degradation under weak grid conditions. Next， an inertia enhancement control scheme based on an improved frequency-locked loop （FLL） structure is proposed to address these problems. By proportionally integrating the frequency-derivative signal with the reference active power， the proposed method effectively mitigates power oscillations and enhances the inertial response of the converter. Furthermore， a modified model predictive control strategy is employed to replace the conventional inner current loop， significantly improving the transient performance of the system. Further， the effectiveness of the proposed control strategy is validated through hardware-in-the-loop simulations.Comparative studies of frequency step responses demonstrated the superiority of the proposed FLL-based structure in maintaining frequency stability under weak grid conditions. The active power control strategy under the improved FLL framework is detailed， along with the principle of inertia emulation and tuning of the associated parameters. [Results] The frequency response of the system is substantially improved and synthetic inertia enhancement is achieved. [Conclusions] The proposed method prevents high-frequency noise issues caused by the direct differentiation of frequency signals and eliminates stability problems typically introduced by PLL.
并网变流器 / 功率振荡 / 锁频环（FLL） / 惯性增强技术
grid connected inverters / power oscillation / frequency locked loop （FLL） / inertial enhancement technology
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随着新型电力系统建设持续推进，直流微电网将成为配电网的重要组成部分。直流微电网接入交流负载时，振荡型功率会进入直流系统，影响分布式储能系统功率分配。为此，给出了储能系统功率分配综合原则，并以此提出一种无互联通信网络的功率分配方法。该方法以荷电状态作为“信息载体”，各储能单元仅需本地荷电状态(state of charge，SOC)信息即可完成自适应调整，在实现SOC均衡控制的同时，还能够让振荡型功率合理分配。此外，从等效输出阻抗的角度出发，对不同控制算法的分配效果展开了详细的分析讨论，表明了所提控制算法可以满足综合原则的要求。最后，通过实验验证了所提分布式储能控制策略的有效性。
As the construction of new power systems, DC microgrids will become an important part of the distribution network. As AC load connecting to DC microgrids, the oscillating power will enter the DC system, which affects the power distribution of energy storage system. To solve this issue, the comprehensive principle of power distribution of distributed energy storage system was given, and a wireless power sharing method was proposed. This method takes the charge state as the “information carrier”, each energy storage unit can realize state of charge (SOC) balancing and oscillating power sharing at same time. In addition, from the perspective of equivalent output impedance, this paper gived a detailed analysis and discussion on the distribution effect of different control algorithms, which showed the proposed control algorithm could meet the requirements of the comprehensive principle. Finally, the effectiveness of the proposed distributed energy storage control strategy was validated by experiments. |
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模块化多电平直流变压器(modular multilevel direct current transformer，MMDCT)为实现电动汽车充放电集群高效稳定并入城市直流配电网提供了一种可行的解决方案，但其启动过程中存在电容充电和变压器励磁涌流造成的冲击电流。为此，提出了一种基于复合频率控制的快速预充电策略，利用直流量和中频量分别独立MMDCT的原边和副边充电。原边充电采用双闭环控制，副边充电采用变步长移相的峰值电流控制，解决了充电速度和冲击电流之间的矛盾。进一步对多种预充电策略的冲击电流进行了量化对比分析。通过仿真验证了所提出预充电策略的有效性和可行性，结果表明，所提出的方法有效控制了冲击电流大小和充电时间。
Modular multilevel direct current transformer (MMDCT) provides a feasible solution to realize the efficient and stable integration of electric vehicle charging and discharging cluster into urban DC distribution network. However, there are impulse currents caused by capacitor charging and transformer inrush current in its starting process. Therefore, a fast pre-charging strategy based on compound frequency control was proposed. The independent control of direct current component and intermediate frequency component was used to realize simultaneous charging of the primary and secondary sides. The double closed-loop control was adopted in the primary side, while the variable-step phase shift peak current control was adopted in the secondary side, which solved the contradiction between charging speed and inrush current. Furthermore, a quantitative comparative analysis of various pre-charging strategies was carried out. The effectiveness and feasibility of the proposed pre-charging strategy were verified through simulation. The results show that the magnitude of the inrush current and the charging time are effectively controlled by the proposed method. |
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With the rapid development of renewable energy sources, virtual synchronous generators (VSG) have garnered significant attention as a key technology for enhancing the stability of power systems. However, severe power-oscillation issues are inherent to multi-VSG systems. In this study, a control strategy was investigated for a multi-VSG parallel operation. A distributed adaptive parameter method is proposed to suppress power oscillations. First, by integrating the virtual impedance, frequency rate of change, and adaptive inertia, this method effectively reduces the initial power overshoot and improves the stability and robustness of the system. Second, in the case of non-uniform communication delays in transmission line communication, an operation strategy is introduced, wherein centralized and distributed controls serve as mutual backup protection, further enhancing system reliability. Finally, the simulation results demonstrate the robustness and high delay tolerance of the method considering non-uniform communication delays, promoting the practical application of multi-VSG systems and enhancing the frequency response performance of generator clusters. |
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目的 针对弱电网背景下直驱风机网侧电流内环PI控制器带宽影响并网系统稳定性，诱发次同步振荡(subsynchronous oscillation，SSO)现象，提出改进线性状态误差反馈(linear state error feedback，LSEF)控制律的线性自抗扰控制器(linear active disturbance rejection control，LADRC)替换电流内环的PI控制器去抑制SSO。 方法 首先，对LADRC控制器的LSEF进行改进，并对其跟踪误差、抗扰性能进行分析。然后，推导出计及频率耦合影响的系统阻抗模型，并且利用Nyquist判据分析线路阻抗值对于网侧变流器并网系统稳定性的影响。最后，通过PSCAD/EMTDC仿真软件进行分析验证。 结果 相较于传统LADRC，改进LADRC使直流侧电压波动范围减少了85%，有功功率波动范围减少了89%。 结论 与传统LADRC控制器相比，改进LADRC控制器可以更好地缩小功率波动范围及直流侧电压波动范围，减小跟踪误差。
Objectives In response to the impact of the bandwidth of the current inner loop PI controller on the stability of the grid connected system of direct drive wind turbines under the background of weak electricity network, which leads to the induction of sub synchronous oscillation (SSO) phenomenon, a linear active disturbance rejection control (LADRC) was proposed to replace the current inner loop PI controller with an improved linear state error feedback control law (LSEF) to suppress SSO. Methods Firstly, the LSEF of the LADRC controller was improved, and the tracking errors and anti-interference capabilities were studied. Moreover, an impedance model for the system considering frequency coupling effects was developed, and the Nyquist criterion was employed to assess the impact of line impedance values on the stability of the grid-connected converter system. Finally, the analysis and validation were performed using PSCAD/EMTDC simulation software. Results Compared with traditional LADRC, the improved LADRC can reduce the DC side voltage fluctuation range by 85% and the active power fluctuation range by 89%. Conclusions Compared with traditional LADRC controllers, improved LADRC controllers can better reduce the range of power fluctuations, DC side voltage fluctuations, and tracking errors. |
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