风电、光伏等新能源发电一般通过逆变器与电网相连，而逆变器属于电力电子设备，不具备旋转惯量。随着新能源发电在电力系统中所占的供电比例越来越高，电网的惯性水平和阻尼特性也随之减弱，其频率控制问题也就显得更为严峻。相对于传统电网而言，此类电网可被称为弱电网。根据对新能源利用方式的不同，弱电网有多种存在形式，比如微电网、与HVDC系统相连的交流电网或与MTDC相连的交流电网等。虽然不同类型的弱电网面临的频率问题不尽相同，但其控制目标大体是一致的，即增强电网惯性水平，以减小扰动后频率变化率和频率偏差。虚拟同步发电机技术能够模拟同步发电机的运行机制，为系统提供惯性支撑。本文以虚拟同步发电机技术为主要手段，对上述三种典型的弱电网的频率进行控制和调节，主要工作如下：（1）建立了适用于新能源友好接入的虚拟同步发电机模型。通过对传统同步发电机的电磁暂态和机电动态特性的模拟，得出了虚拟同步发电机的控制方法，其主要功能包括惯性响应、一次调频和一次调压，并讨论了关键参数的设计方法。（2）提出了改善微电网频率特性的虚拟同步发电机模型预测控制方法。通过对微电网内频率问题进行分析，建立了虚拟同步发电机的预测模型。将预测模型化为标准的二次规划问题，并将在线求解的结果用于虚拟同步发电机的输入功率控制，以使其能够根据微电网频率变化情况迅速调整自身出力，从而减弱频率的波动。同时，对模型预测算法的稳定性进行分析，并给出了关键参数的设计依据。（3）提出了基于虚拟同步发电机的HVDC系统参与电网调频的直流电压协调控制策略。通过分析传统同步发电机的惯性响应和一次调频的机理，设计了HVDC参与调频的有功参考值设定方法，并通过控制两端直流电压实现传输功率对参考值的跟踪；建立了计算电压参考值的优化模型，并利用虚拟同步发电机技术实现了直流电压的无差控制；最后，采用伯德图对关键参数的选取进行了分析。（4）提出了基于虚拟同步发电机和自适应P-V下垂的MTDC系统中换流站控制方法。该方法能够为所接交流电网提供惯性支撑，以减弱其扰动后频率变化率；同时，MTDC系统的不平衡功率在各交流电网之间分配时能根据各个电网的频率裕度大小而自适应调整分摊比例，有效地提升了交流电网的频率特性。

Generally, the renewable energy, e.g. wind and photovoltaic power generation, are connected to the power networks by inverters. The inverters belong to power electric devices, which do not possess rotation part. With the penetration of renewable energy in power networks constantly increases, the inertia and damping of the power system decrease, and the frequency control will become more severe. Compared with the traditional power networks, these power networks are called weak networks, and they can be divided into many kinds according to the utilization mode of renewable energy. Such as the microgrid, the power networks connected to high voltage direct current transmission (HVDC) system, and the power networks connected to multi-terminal DC (MTDC) system. Different types of weak networks are facing different problems, but their control objectives are nearly consistent, which is to promote the inertia of the network, and reduce the frequency change rate as well as frequency deviation when disturbance occurs. The virtual synchronous generator (VSG) technique, which could mimic the operate mechanism of synchronous generator, can provide inertial support for the weak networks. In this paper, the frequency of above three typical weak networks are controlled basing on the virtual synchronous generator technology technique, and the main work is as follows: (1) A control model of VSG is introduced for the integration of renewable energy in AC networks. The control method of virtual synchronous generator is obtained by mimicking the electromagnetic transient and electromechanical dynamic characteristics of the traditional synchronous generator, including the inertial response, primary frequency and primary voltage regulation, and the design method of control parameters is also discussed. (2) A VSG-based model predictive control method is proposed to improve the frequency dynamics of microgrid. Based on the analysis of the frequency in the microgrid, the VSG prediction model is established, which is transformed into a standard quadratic programming problem. The result of the on-line solution is applied to the input power control of the VSG, which makes it adjust its output power quickly depending on the frequency variation of the microgrid to attenuate frequency fluctuation. At the same time, the stability of the model prediction algorithm is analyzed, and the tuning method of the key parameters is given.(3) A VSG-based coordinated DC voltage control strategy is proposed for the VSC-HVDC system participating frequency regulation. By analyzing the inertial response and the primary frequency regulation process of the traditional synchronous generator, the active reference of the HVDC for frequency regulation is set. The active power tracking is realized by controlling the DC voltage at both ends. The optimal model of the voltage reference calculation is established, and the DC voltage is controlled with the VSG algorithm. Finally, the selection method of the key parameters is analyzed through the Bode diagram.(4) A VSG-based control method is proposed for MTDC to provide inertial support for the AC networks, and an adaptive droop method of active power and DC voltage is designed. With the proposed control strategy, the VSC stations could provide inertial support for the AC networks, and reduce the frequency change rate after disturbances. At the same time, the sharing of unbalanced power from the MTDC system among AC networks could be adaptively adjusted depending on the frequency margin of each AC network, which improves the frequency characteristic of the AC networks effectively.