CN110198055A - Based on the microgrid bi-directional inverter control method of virtual synchronous machine and stability analysis - Google Patents
Based on the microgrid bi-directional inverter control method of virtual synchronous machine and stability analysis Download PDFInfo
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Abstract
一种基于虚拟同步机的微网双向换流器控制方法及稳定性分析，通过在转子运动方程阻尼环节增加积分器，反馈补偿角频率偏差值，实现微网离网模式下交流子网频率的无差控制。考虑到换流器的双向传输特性，通过改变虚拟机械有功功率参考值给定方式，将控制目标从交流频率实时切换为直流电压，实现直流母线电压的稳定控制。通过建立功率内环和电压/频率外环小信号模型，分别求取换流器频率控制和直流电压控制模式下的传递函数，并进行了稳定性分析。本发明能够很好的实现离网模式下频率的无差调节以及直流电压的稳定控制。
A microgrid bidirectional inverter control method and stability analysis based on virtual synchronous machine, by adding an integrator to the damping link of the rotor motion equation, feedback compensation angular frequency deviation value, to realize the AC sub-network frequency in the microgrid off-grid mode. No difference control. Considering the bidirectional transmission characteristics of the converter, by changing the given method of the virtual mechanical active power reference value, the control target is switched from the AC frequency to the DC voltage in real time, so as to realize the stable control of the DC bus voltage. By establishing power inner loop and voltage/frequency outer loop small-signal models, the transfer functions of the converter in frequency control and DC voltage control modes are obtained respectively, and the stability analysis is carried out. The invention can well realize the no-difference regulation of the frequency and the stable control of the DC voltage in the off-grid mode.
Description
技术领域technical field
本发明涉及一种交直流混合微网双向换流器。特别是涉及一种基于虚拟同步机的微网双向换流器控制方法及稳定性分析。The invention relates to an AC-DC hybrid micro-grid bidirectional converter. In particular, it relates to a control method and stability analysis of a microgrid bidirectional converter based on a virtual synchronous machine.
背景技术Background technique
随着当今世界环境和能源问题日益严重，各种分布式电源(光伏、风力等)得到相关学者的高度重视。微网最早是由美国学者R.H.Lasseter教授提出的，能够为分布式接入配电网提供有效的途径。随着分布式电源种类的增加和数量的增长、直流负荷的普及，配电网结构的复杂多样，交流微网难以全面满足日益增长的供电需求。为保证新能源及可再生能源的高效利用，以及更好地满足用户多元化电力需求，交直流混合微网应运而生。交直流混合微网兼具交流微网和直流微网的优点，属于当下研究热点。交流区域和直流区域通过微网双向换流器连接，对维持交直流混合微网内功率平衡起着至关重要的作用。With the increasingly serious environmental and energy problems in today's world, various distributed power sources (photovoltaic, wind power, etc.) have been highly valued by relevant scholars. Microgrid was first proposed by American scholar Professor R.H.Lasseter, which can provide an effective way for distributed access to distribution network. With the increase in the types and quantity of distributed power sources, the popularization of DC loads, and the complexity and variety of distribution network structures, it is difficult for AC microgrids to fully meet the growing demand for power supply. In order to ensure the efficient use of new energy and renewable energy, and to better meet the diversified power needs of users, the AC-DC hybrid microgrid came into being. The AC-DC hybrid microgrid has the advantages of both the AC microgrid and the DC microgrid, and belongs to the current research hotspot. The AC area and the DC area are connected through the microgrid bidirectional converter, which plays a crucial role in maintaining the power balance in the AC/DC hybrid microgrid.
然而，越来越多的新能源并入电网，使得电力系统中各种分布式电源的渗透率越来越高，相应的传统同步发电机电源在整个电力系统中占比逐渐减少。以电力电子器件为接口的分布式电源缺乏传统电机所具有的惯性和阻尼，当系统出现功率波动或发生故障时，难以抑制电网频率的快速波动。虚拟同步机(virtual synchronous generator，VSG)控制技术的出现，使得电力电子器件能够模拟同步发电机的特性，即具有惯性和阻尼等特性，有效的解决了电力系统因分布式电源接入而惯性阻尼不足问题，近些年得到了广泛的关注。交直流混合微网离网模式运行时，应维持系统电压和频率稳定，并且并离网切换过程中应保持较小的暂态过程。但传统虚拟同步机控制存在频率偏差，属于有差调节。However, as more and more new energy sources are integrated into the power grid, the penetration rate of various distributed power sources in the power system is getting higher and higher, and the proportion of the corresponding traditional synchronous generator power sources in the entire power system is gradually decreasing. The distributed power generation interfaced with power electronic devices lacks the inertia and damping of traditional motors, and it is difficult to suppress the rapid fluctuation of the grid frequency when the system has power fluctuations or failures. The emergence of virtual synchronous generator (VSG) control technology enables power electronic devices to simulate the characteristics of synchronous generators, that is, has the characteristics of inertia and damping, which effectively solves the problem of inertial damping of power systems due to the access of distributed power sources. The problem of insufficiency has received extensive attention in recent years. When the AC-DC hybrid microgrid operates in the off-grid mode, the system voltage and frequency should be kept stable, and a small transient process should be maintained during the on-off-grid switching process. However, the traditional virtual synchronous machine control has frequency deviation, which belongs to differential regulation.
发明内容SUMMARY OF THE INVENTION
本发明所要解决的技术问题是，提供一种能够实现微网交流母线频率无偏差调节的基于虚拟同步机的微网双向换流器控制方法及稳定性分析。The technical problem to be solved by the present invention is to provide a control method and stability analysis of a microgrid bidirectional converter based on a virtual synchronous machine, which can realize no-bias adjustment of the AC busbar frequency of the microgrid.
本发明所采用的技术方案是：一种基于虚拟同步机的微网双向换流器控制方法，是在已有虚拟同步机控制中的转子运动方程阻尼环节上并联一个积分环节，将转子运动方程由一阶方程变为二阶方程，所述的积分环节为其中，s为拉普拉斯算子，Ki为积分环节的积分系数，由确定取值范围，其中，ξ表示有功功率环闭环传递函数的阻尼比，D为阻尼系数，Xs是引入虚拟阻抗后的微网双向换流器总的输出电抗，E0表示稳态运行情况下微网双向换流器输出电势，Ue0为稳态运行情况下微网双向换流器输出端口电压，ω0为电网同步角速度，J为同步发电机的转动惯量。The technical scheme adopted in the present invention is: a micro-grid bidirectional inverter control method based on virtual synchronous machine, which is to connect an integral link in parallel with the damping link of the rotor motion equation in the control of the existing virtual synchronous machine, and combine the rotor motion equation From a first-order equation to a second-order equation, the integral link is Among them, s is the Laplace operator, and K i is the integral coefficient of the integral link, which is given by Determine the value range, where ξ represents the damping ratio of the closed-loop transfer function of the active power loop, D is the damping coefficient, X s is the total output reactance of the microgrid bidirectional converter after introducing the virtual impedance, and E 0 represents the steady-state operation. The output potential of the lower microgrid bidirectional converter, U e0 is the output port voltage of the microgrid bidirectional converter under steady-state operation, ω 0 is the grid synchronous angular velocity, and J is the moment of inertia of the synchronous generator.
所述的转子运动二阶方程为：The second-order equation of rotor motion is:
其中，Ki为积分环节的积分系数，J为同步发电机的转动惯量，ω0为电网同步角速度，ω为电网实际的角速度，d为微分算子，t为时间，Pm表示微网双向换流器输出的虚拟机械有功功率，Pe表示微网双向换流器输出的有功功率，D为阻尼系数，s为拉普拉斯算子。Among them, K i is the integral coefficient of the integral link, J is the moment of inertia of the synchronous generator, ω 0 is the synchronous angular velocity of the grid, ω is the actual angular velocity of the grid, d is the differential operator, t is the time, and P m is the bidirectional microgrid The virtual mechanical active power output by the converter, Pe represents the active power output by the microgrid bidirectional converter, D is the damping coefficient, and s is the Laplace operator.
将虚拟同步机控制中的虚拟机械有功功率的给定方式由单向选择改变为双向选择，具体是，当交流子网出现功率缺额时，微网双向换流器传输功率为正，此时控制目标为交流子网频率；当直流子网出现功率缺额时，微网双向换流器传输功率为负，此时控制目标为直流母线电压。Change the given mode of virtual mechanical active power in virtual synchronous machine control from one-way selection to two-way selection. Specifically, when there is a power shortage in the AC sub-network, the transmission power of the microgrid two-way converter is positive, and the control The target is the frequency of the AC sub-network; when there is a power shortage in the DC sub-network, the transmission power of the microgrid bidirectional converter is negative, and the control target is the DC bus voltage.
所述的交流子网频率公式如下：The stated AC subnet frequency formula is as follows:
Pm＝Pref+kf(fref-f)P m =P ref +k f (f ref -f)
其中，Pm表示微网双向换流器输出的虚拟机械有功功率，Pref和fref分别为微网双向换流器输出的有功功率参考值和系统频率参考值，kf为频率调节系数；Among them, P m represents the virtual mechanical active power output by the micro-grid bidirectional inverter, P ref and f ref are the active power reference value and the system frequency reference value output by the micro-grid bi-directional inverter, respectively, and k f is the frequency adjustment coefficient;
所述的直流母线电压公式如下：The DC bus voltage formula is as follows:
其中Udc表示直流母线电压，Udc_ref表示直流母线电压参考值，kpu和kpi分别表示直流母线电压PI调节器的比例系数和积分系数。Wherein U dc represents the DC bus voltage, U dc_ref represents the DC bus voltage reference value, k pu and k pi respectively represent the proportional coefficient and integral coefficient of the DC bus voltage PI regulator.
一种基于虚拟同步机的微网双向换流器控制方法的稳定性分析，是通过建立微网双向换流器输出功率的小信号模型，对微网双向换流器控制方法进行稳定性分析，具体如下：The stability analysis of a microgrid bidirectional inverter control method based on a virtual synchronous machine is to analyze the stability of the microgrid bidirectional inverter control method by establishing a small signal model of the output power of the microgrid bidirectional inverter. details as follows:
1)将转子运动二阶方程、微网双向换流器输出的有功功率Pe和无功功率Qe以及微网双向换流器输出电势方程中的变量表示为稳态量和扰动量之和，即1) The second-order equation of rotor motion, the active power P e and reactive power Q e output by the microgrid bidirectional inverter, and the output potential equation of the microgrid bidirectional inverter The variable in is expressed as the sum of the steady state quantity and the disturbance quantity, namely
其中，Kq表示微网双向换流器输出电势微分系数，E表示微网双向换流器输出电势，Qe和Qref分别为微网双向换流器输出无功功率和输出无功功率参考值，Ue和Uen分别为微网双向换流器输出端口电压和输出端口电压额定值，Dq为电压调节系数；E0表示稳态运行情况下微网双向换流器输出电势，表示微网双向换流器输出电势扰动量，δ为功角，δ0为稳态运行情况下功角，为功角扰动量，Pe0和Qe0分别为稳态运行情况下微网双向换流器输出的有功功率和无功功率，和分别为微网双向换流器输出的有功功率扰动量和无功功率扰动量，Pm表示微网双向换流器输出的虚拟机械有功功率，Pm0表示稳态运行情况下微网双向换流器输出的虚拟机械有功功率，表示微网双向换流器输出的虚拟机械有功功率扰动量，ω为电网实际的角速度，ω0为电网同步角速度，为电网实际的角速度的扰动量，Ue0为稳态运行情况下微网双向换流器输出端口电压，为微网双向换流器输出端口电压扰动量。Among them, K q represents the differential coefficient of the output potential of the microgrid bidirectional inverter, E represents the output potential of the microgrid bidirectional inverter, and Q e and Qref are the output reactive power and output reactive power reference of the microgrid bidirectional inverter, respectively. U e and U en are the output port voltage of the microgrid bidirectional converter and the rated value of the output port voltage respectively, D q is the voltage regulation coefficient; E 0 represents the output potential of the microgrid bidirectional converter under steady-state operation, represents the output potential disturbance of the microgrid bidirectional converter, δ is the power angle, δ 0 is the power angle under steady-state operation, is the power angle disturbance, P e0 and Q e0 are the active power and reactive power output by the microgrid bidirectional converter under steady-state operation, respectively, and are the active power disturbance and reactive power disturbance output by the microgrid bidirectional inverter, respectively, P m represents the virtual mechanical active power output by the microgrid bidirectional inverter, and P m0 represents the microgrid bidirectional commutation under steady-state operation. The virtual mechanical active power output by the device, represents the virtual mechanical active power disturbance output by the microgrid bidirectional converter, ω is the actual angular velocity of the grid, ω 0 is the synchronous angular velocity of the grid, is the disturbance of the actual angular velocity of the power grid, U e0 is the output port voltage of the microgrid bidirectional converter under steady-state operation, is the voltage disturbance at the output port of the microgrid bidirectional converter.
消除扰动量二次分量和稳态量，得到微网双向换流器输出功率的小信号模型：By eliminating the secondary component of the disturbance quantity and the steady-state quantity, the small-signal model of the output power of the microgrid bidirectional converter is obtained:
式中：X为微网双向换流器等效输出电抗，D为阻尼系数，J为同步发电机的转动惯量，Ki为积分环节的积分系数；where X is the equivalent output reactance of the microgrid bidirectional converter, D is the damping coefficient, J is the moment of inertia of the synchronous generator, and K i is the integral coefficient of the integral link;
在计算微网双向换流器输出功率的小信号模型时，设定：When calculating the small-signal model of the output power of the microgrid bidirectional converter, set:
Ue＝E，sinδ0＝δ0，cosδ0＝1；U e =E, sinδ 0 =δ 0 , cosδ 0 =1;
2)不考虑有功环和无功环的耦合，通过微网双向换流器输出功率的小信号模型求出有功功率环的闭环传递函数公式如下：2) Without considering the coupling of the active loop and the reactive power loop, the closed-loop transfer function formula of the active power loop is obtained through the small signal model of the output power of the microgrid bidirectional converter as follows:
式中，G(s)为有功功率环闭环传递函数，和分别为复频域下微网双向换流器输出的有功功率扰动量和输出的虚拟机械有功功率扰动量，Xs是引入虚拟阻抗后的微网双向换流器总的输出电抗，s表示拉普拉斯算子；K为比例常数，ωn和ξ分别表示有功功率环闭环传递函数的自然振荡角频率和阻尼比，由下式求得：In the formula, G(s) is the closed-loop transfer function of the active power loop, and are the output active power disturbance of the microgrid bidirectional converter and the output virtual mechanical active power disturbance in the complex frequency domain, respectively, X s is the total output reactance of the microgrid bidirectional converter after introducing the virtual impedance, s represents the pull Plath operator; K is the proportionality constant, ω n and ξ represent the natural oscillation angular frequency and damping ratio of the closed-loop transfer function of the active power loop, respectively, and are obtained from the following equations:
3)根据有功功率环的闭环传递函数和交流子网频率公式，得到微网双向换流器控制目标为交流子网频率下开环传递函数表示如下：3) According to the closed-loop transfer function of the active power loop and the frequency formula of the AC sub-network, it is obtained that the control target of the microgrid bidirectional converter is the open-loop transfer function at the frequency of the AC sub-network, which is expressed as follows:
式中，Gf(s)为微网双向换流器控制目标为交流子网频率下的开环传递函数，kf为频率调节系数。In the formula, G f (s) is the open-loop transfer function of the microgrid bidirectional converter control target at the frequency of the AC sub-grid, and k f is the frequency adjustment coefficient.
微网双向换流器传输功率为负时，微网双向换流器传输的有功功率为：When the transmission power of the microgrid bidirectional converter is negative, the active power transmitted by the microgrid bidirectional converter is:
式中，Req为直流子网等效电阻，Udc为直流母线电压，Pe1为微网双向换流器传输功率为负时输出的有功功率，C为直流侧电容，d为微分算子。考虑小信号干扰下，有功功率/直流电压传递函数表示如下：In the formula, Req is the equivalent resistance of the DC sub-network, U dc is the DC bus voltage, P e1 is the active power output when the transmission power of the microgrid bidirectional converter is negative, C is the DC side capacitance, and d is the differential operator . Considering the small signal interference, the active power/DC voltage transfer function is expressed as follows:
式中，GP-U(s)为微网双向换流器传输功率为负时的有功功率/直流电压传递函数，和分别为稳态运行情况下直流母线电压扰动量和微网双向换流器传输功率为负时输出的有功功率扰动量。In the formula, G PU (s) is the active power/DC voltage transfer function when the transmission power of the microgrid bidirectional converter is negative, and are the DC bus voltage disturbance under steady-state operation and the active power disturbance output when the transmission power of the microgrid bidirectional converter is negative, respectively.
根据有功功率环的闭环传递函数、直流母线电压公式和微网双向换流器传输功率为负时的有功功率/直流电压传递函数，得到微网双向换流器控制目标为直流母线电压下开环传递函数表示如下：According to the closed-loop transfer function of the active power loop, the DC bus voltage formula and the active power/DC voltage transfer function when the transmission power of the microgrid bidirectional converter is negative, it is obtained that the control target of the microgrid bidirectional converter is the open loop under the DC bus voltage. The transfer function is expressed as follows:
其中：GU(s)为微网双向换流器控制目标为直流母线电压下的开环传递函数，kpu和kpi分别表示直流母线电压PI调节器的比例系数和积分系数。Among them: G U (s) is the open-loop transfer function of the microgrid bidirectional converter control target under the DC bus voltage, k pu and k pi represent the proportional coefficient and integral coefficient of the DC bus voltage PI regulator, respectively.
4)根据微网双向换流器控制目标为交流子网频率和直流母线电压下开环传递函数进行稳定性分析。所述的分析包括：4) According to the control objective of the microgrid bidirectional converter, the stability analysis is carried out for the open-loop transfer function under the frequency of the AC sub-grid and the voltage of the DC bus. The analysis described includes:
根据微网双向换流器控制目标为交流子网频率和直流母线电压的开环传递函数，利用根轨迹法绘制参数J、D、Ki变化时，闭环传递函数极点在s平面上的变化轨迹，分析可得：According to the open-loop transfer function of the AC sub-network frequency and the DC bus voltage as the control target of the microgrid bidirectional converter, the root locus method is used to draw the change trajectory of the pole of the closed-loop transfer function on the s-plane when the parameters J, D, and K i change. , the analysis can be obtained:
转动惯量J越大，闭环传递函数极点越靠近虚轴，系统越不稳定，转动惯量过大会引起超调量、调节时间的增大，从而造成功率振荡，系统越不容易稳定；随着阻尼系数的增大，系统稳定性增强；随着积分环节的积分系数的增大，闭环极点远离逐渐实轴，超调增大，但始终保持在虚轴左侧，系统稳定性不变。The larger the moment of inertia J is, the closer the pole of the closed-loop transfer function is to the imaginary axis, and the more unstable the system is. If the moment of inertia is too large, the overshoot and adjustment time will increase, resulting in power oscillation, and the system will be less stable. With the increase of , the system stability is enhanced; with the increase of the integral coefficient of the integral link, the closed-loop pole is gradually moved away from the real axis, and the overshoot increases, but it always remains on the left side of the imaginary axis, and the system stability remains unchanged.
当微网双向换流器控制目标为直流母线电压时，开环传递函数由比例环节、一阶微分环节、积分环节、二阶振荡环节和一阶惯性环节构成。为使系统保持稳定且具有较好的动态特性，截止频率fc位于中频段，且满足kpi/kpu＜fc＜ωn。When the control target of the microgrid bidirectional converter is the DC bus voltage, the open-loop transfer function consists of a proportional element, a first-order differential element, an integral element, a second-order oscillation element, and a first-order inertia element. In order to keep the system stable and have better dynamic characteristics, the cut-off frequency f c is located in the middle frequency band and satisfies k pi /k pu <f c <ω n .
本发明的基于虚拟同步机的微网双向换流器控制方法及稳定性分析,实现了微网交流母线频率无偏差调节，并从理论推导证明其正确性，同时增大了微网系统的惯性，在负荷发生波动或发生故障时，能够响应系统频率的快速波动。考虑到换流器的双向传输特性，改变虚拟机械有功功率给定方式，实现交流频率和直流电压控制目标的实时切换。The control method and stability analysis of the micro-grid bidirectional converter based on the virtual synchronous machine of the present invention realizes the adjustment of the frequency of the micro-grid AC bus without deviation, and proves its correctness from theoretical derivation, and at the same time increases the inertia of the micro-grid system , when the load fluctuates or fails, it can respond to the rapid fluctuation of the system frequency. Considering the bidirectional transmission characteristics of the converter, the virtual mechanical active power given mode is changed to realize the real-time switching of the AC frequency and the DC voltage control target.
附图说明Description of drawings
图1是本发明基于虚拟同步机的微网双向换流器控制方法第一实例的控制框图；Fig. 1 is the control block diagram of the first example of the microgrid bidirectional inverter control method based on virtual synchronous machine of the present invention;
图2是本发明基于虚拟同步机的微网双向换流器控制方法第二实例的控制框图；Fig. 2 is the control block diagram of the second example of the microgrid bidirectional inverter control method based on virtual synchronous machine of the present invention;
图3是本发明中主电路等效电路；Fig. 3 is the main circuit equivalent circuit in the present invention;
图4是本发明中有功无功功率环小信号模型；Fig. 4 is the active and reactive power loop small signal model of the present invention;
图5是本发明中频率控制模式下控制框图；Fig. 5 is the control block diagram under the frequency control mode in the present invention;
图6a是同步发电机的转动惯量变化时闭环传递函数极点变化轨迹；Figure 6a shows the change trajectory of the poles of the closed-loop transfer function when the moment of inertia of the synchronous generator changes;
图6b是阻尼系数变化时闭环传递函数极点变化轨迹；Figure 6b shows the change trajectory of the poles of the closed-loop transfer function when the damping coefficient changes;
图6c是积分环节的积分系数变化时闭环传递函数极点变化轨迹；Figure 6c shows the change trajectory of the poles of the closed-loop transfer function when the integral coefficient of the integral link changes;
图7是本发明中直流母线电压控制模式下系统简化模型图；7 is a simplified model diagram of the system under the DC bus voltage control mode of the present invention;
图8是本发明中直流母线电压控制模式下控制框图；8 is a control block diagram under the DC bus voltage control mode in the present invention;
图9是本发明中实施例中传统VSG控制和本方法频率控制模式下交流频率波形；Fig. 9 is the AC frequency waveform under the traditional VSG control in the embodiment of the present invention and the frequency control mode of this method;
图10是本发明中实施例中频率控制模式下交流母线电压波形；Fig. 10 is the AC bus voltage waveform under the frequency control mode in the embodiment of the present invention;
图11是本发明中实施例中直流母线电压控制模式下直流母线电压波形；Fig. 11 is the DC bus voltage waveform under the DC bus voltage control mode in the embodiment of the present invention;
图12是本发明中实施例中直流母线电压控制模式下交流频率波形。FIG. 12 is an AC frequency waveform in the DC bus voltage control mode in the embodiment of the present invention.
具体实施方式Detailed ways
下面结合实施例和附图对本发明的基于虚拟同步机的微网双向换流器控制方法及稳定性分析做出详细说明。The control method and stability analysis of the microgrid bidirectional converter based on the virtual synchronous machine of the present invention will be described in detail below with reference to the embodiments and the accompanying drawings.
在交直流混合微网中，微网双向换流器协调交流子网和直流子网功率分配，承担着维持交流母线频率和直流母线电压的作用。传统虚拟同步机控制模拟同步发电机的稳态下垂特性和暂态惯性与阻尼，具有与同步发电机类似的稳态和机电动态特性。但微网离网运行模式下，传统虚拟同步机控制存在频率偏差，属于有差调节；In the AC-DC hybrid microgrid, the microgrid bidirectional converter coordinates the power distribution of the AC sub-network and the DC sub-network, and is responsible for maintaining the frequency of the AC bus and the voltage of the DC bus. The traditional virtual synchronous machine control simulates the steady-state droop characteristics and transient inertia and damping of synchronous generators, and has similar steady-state and electromechanical dynamic characteristics to synchronous generators. However, in the off-grid operation mode of the micro-grid, there is a frequency deviation in the traditional virtual synchronous machine control, which is a differential adjustment;
当交直流混合微网中交流子网内负荷突增导致频率波动时，各分布式电源及微网双向换流器调整各自的出力维持子网内功率平衡。传统虚拟同步机控制中，有功频率环节在负荷出现变化时仅能实现一次调节，而频率的偏移会影响微网中部分负荷的正常运行。当负荷突增时，机械功率与电磁功率出现偏差，同步发电机转子运动方程可变形为：When the frequency fluctuates due to the sudden increase of the load in the AC sub-network in the AC-DC hybrid micro-grid, each distributed power source and the micro-grid bidirectional converter adjust their respective outputs to maintain the power balance in the sub-network. In the traditional virtual synchronous machine control, the active frequency link can only be adjusted once when the load changes, and the frequency offset will affect the normal operation of some loads in the microgrid. When the load suddenly increases, the mechanical power and the electromagnetic power deviate, and the equation of motion of the rotor of the synchronous generator can be deformed as:
对该一阶线性微分方程求解得：Solving this first-order linear differential equation yields:
式中，Ki为积分环节的积分系数，J为同步发电机的转动惯量，ω0为电网同步角速度，ω为电网实际的角速度，d为微分算子，t为时间，D为阻尼系数，s为拉普拉斯算子，ΔP为微网双向换流器输出的虚拟机械有功功率和有功功率的差值，C为常数，由状态的初始条件决定。In the formula, K i is the integral coefficient of the integral link, J is the moment of inertia of the synchronous generator, ω 0 is the synchronous angular velocity of the grid, ω is the actual angular velocity of the grid, d is the differential operator, t is the time, D is the damping coefficient, s is the Laplace operator, ΔP is the difference between the virtual mechanical active power and the active power output by the microgrid bidirectional converter, and C is a constant, determined by the initial conditions of the state.
由上式分析可得，负荷突增后的角频率由两部分组成，稳态分量和暂态分量暂态分量最终衰减至零，衰减时间常数为故而传统VSG控制下，角频率最终稳态值为属于有差调节。频率偏移量取决于功率偏差及阻尼系数，适当增大阻尼系数可以减小稳态频率偏移量。From the analysis of the above formula, it can be obtained that the angular frequency after a sudden load increase consists of two parts, the steady-state component and transient components The transient component eventually decays to zero with a decay time constant of Therefore, under the traditional VSG control, the final steady-state value of the angular frequency is It is a differential adjustment. The frequency offset depends on the power deviation and damping coefficient. Properly increasing the damping coefficient can reduce the steady-state frequency offset.
本发明的基于虚拟同步机的微网双向换流器控制方法，如图1所示，是在已有虚拟同步机控制中的转子运动方程阻尼环节上并联一个积分环节，所述的积分环节为其中，s为拉普拉斯算子，Ki为积分环节的积分系数，由确定取值范围，其中，ξ表示有功功率环闭环传递函数的阻尼比，D为阻尼系数，Xs是引入虚拟阻抗后的微网双向换流器总的输出电抗，E0表示稳态运行情况下微网双向换流器输出电势，Ue0为稳态运行情况下微网双向换流器输出端口电压，ω0为电网同步角速度，J为同步发电机的转动惯量。The microgrid bidirectional converter control method based on the virtual synchronous machine of the present invention, as shown in FIG. 1 , is to connect an integral link in parallel with the damping link of the rotor motion equation in the control of the existing virtual synchronous machine, and the integral link is: Among them, s is the Laplace operator, and K i is the integral coefficient of the integral link, which is given by Determine the value range, where ξ represents the damping ratio of the closed-loop transfer function of the active power loop, D is the damping coefficient, X s is the total output reactance of the microgrid bidirectional converter after introducing the virtual impedance, and E 0 represents the steady-state operation. The output potential of the lower microgrid bidirectional converter, U e0 is the output port voltage of the microgrid bidirectional converter under steady-state operation, ω 0 is the grid synchronous angular velocity, and J is the moment of inertia of the synchronous generator.
从而将转子运动方程由一阶方程变为二阶方程，所述的转子运动二阶方程为：Thus, the rotor motion equation is changed from a first-order equation to a second-order equation, and the second-order rotor motion equation is:
其中，Ki为积分环节的积分系数，J为同步发电机的转动惯量，ω0为电网同步角速度，ω为电网实际的角速度，d为微分算子，t为时间，Pm表示微网双向换流器输出的虚拟机械有功功率，Pe表示微网双向换流器输出的有功功率，D为阻尼系数，s为拉普拉斯算子。Among them, K i is the integral coefficient of the integral link, J is the moment of inertia of the synchronous generator, ω 0 is the synchronous angular velocity of the grid, ω is the actual angular velocity of the grid, d is the differential operator, t is the time, and P m is the bidirectional microgrid The virtual mechanical active power output by the converter, Pe represents the active power output by the microgrid bidirectional converter, D is the damping coefficient, and s is the Laplace operator.
将上式整理为二阶常系数微分方程标准形式：The above formula is organized into the standard form of the second-order constant coefficient differential equation:
为使系统保持稳定运行，需设定参数，使得该转子运动方程标准形式的解满足以下形式：In order to keep the system running stably, parameters need to be set so that the solution of the standard form of the rotor equation of motion satisfies the following form:
其中：t为时间，C1和C2表示暂态情况下角频率分量1和2的初始值，T1和T2分别表示暂态情况下角频率分量1和2的衰减时间常数，ω*(t)表示暂态情况下角频率分量衰减后的稳态值。负荷突增时，频率发生突变并趋于稳定。求解式(5)可得稳态情况下频率表达式为：where: t is time, C 1 and C 2 represent the initial values of angular frequency components 1 and 2 under transient conditions, T 1 and T 2 represent the decay time constants of angular frequency components 1 and 2 under transient conditions, respectively, ω * (t ) represents the steady-state value after the angular frequency component is attenuated under transient conditions. When the load suddenly increases, the frequency changes abruptly and tends to be stable. Solving equation (5), the frequency expression in steady state can be obtained as:
ω＝ω*(t)＝ω0 (6)ω = ω * (t) = ω 0 (6)
可见，微网离网运行模式下，通过对暂态情况下二阶同步发电机转子运动方程推导，证明其控制属于无差调节，实现微网离网模式下频率的无偏差控制。It can be seen that in the off-grid operation mode of the micro-grid, by deriving the motion equation of the second-order synchronous generator rotor under transient conditions, it is proved that its control belongs to the non-difference regulation, and the frequency-free control of the micro-grid off-grid mode is realized.
考虑到微网双向换流器的双向传输特性，改变虚拟机械有功功率参考值给定方式，实现交流频率和直流母线电压控制目标的实时切换。直流微电网中直流电压控制策略一般采用有功/电压下垂控制，但存在电压偏差问题。图1所示的有功控制器，引入频率偏差用以计算虚拟机械有功功率，通过调整角频率从而控制有功功率输出。类比于交流子网中频率的无偏差控制，当负荷波动发生在直流子网中时，控制量变为直流母线电压，将电压偏差送入PI调节器计算得到虚拟机械有功功率，实现直流母线电压的稳定控制。本发明设定，微网双向换流器功率从直流子网流向交流子网为正。Considering the bidirectional transmission characteristics of the microgrid bidirectional converter, the given method of the virtual mechanical active power reference value is changed to realize the real-time switching of the control target of the AC frequency and the DC bus voltage. The DC voltage control strategy in DC microgrid generally adopts active power/voltage droop control, but there is a problem of voltage deviation. The active controller shown in Figure 1 introduces a frequency deviation to calculate the virtual mechanical active power, and controls the active power output by adjusting the angular frequency. Analogous to the no-bias control of the frequency in the AC sub-network, when the load fluctuation occurs in the DC sub-network, the control variable becomes the DC bus voltage, and the voltage deviation is sent to the PI regulator to calculate the virtual mechanical active power to realize the DC bus voltage. Stable control. The present invention sets that the power of the microgrid bidirectional inverter flows from the DC sub-network to the AC sub-network as positive.
如图2所示，本发明将虚拟同步机控制中的虚拟机械有功功率的给定方式由单向选择改变为双向选择，具体是，当交流子网出现功率缺额时，微网双向换流器传输功率为正，此时控制目标为交流子网频率；当直流子网出现功率缺额时，微网双向换流器传输功率为负，此时控制目标为直流母线电压。其中，As shown in Figure 2, the present invention changes the given mode of virtual mechanical active power in the virtual synchronous machine control from one-way selection to two-way selection. When the transmission power is positive, the control target is the frequency of the AC sub-network; when there is a power shortage in the DC sub-network, the transmission power of the microgrid bidirectional converter is negative, and the control target is the DC bus voltage. in,
所述的交流子网频率公式如下：The stated AC subnet frequency formula is as follows:
Pm＝Pref+kf(fref-f) (7)P m =P ref +k f (f ref -f) (7)
其中Pm表示微网双向换流器输出的虚拟机械有功功率，Pref和fref分别为微网双向换流器输出的有功功率参考值和系统频率参考值，kf为频率调节系数；where P m represents the virtual mechanical active power output by the microgrid bidirectional inverter, P ref and f ref are the active power reference value and the system frequency reference value output by the microgrid bidirectional inverter, respectively, and k f is the frequency adjustment coefficient;
所述的直流母线电压公式如下：The DC bus voltage formula is as follows:
其中Udc表示直流母线电压，Udc_ref表示直流母线电压参考值，kpu和kpi分别表示直流母线电压PI调节器的比例系数和积分系数。Wherein U dc represents the DC bus voltage, U dc_ref represents the DC bus voltage reference value, k pu and k pi respectively represent the proportional coefficient and integral coefficient of the DC bus voltage PI regulator.
本发明的基于虚拟同步机的微网双向换流器控制方法的稳定性分析，是通过建立微网双向换流器输出功率的小信号模型，对微网双向换流器控制方法进行稳定性分析，具体如下：The stability analysis of the microgrid bidirectional inverter control method based on the virtual synchronous machine of the present invention is to analyze the stability of the microgrid bidirectional inverter control method by establishing a small signal model of the output power of the microgrid bidirectional inverter. ,details as follows:
步骤1)step 1)
图3中，Re和Le为虚拟同步机的等效输出电阻和电抗，则虚拟同步机的等效输出阻抗Z为：In Figure 3, Re and Le are the equivalent output resistance and reactance of the virtual synchronous machine, then the equivalent output impedance Z of the virtual synchronous machine is:
Z＝Re+jωLe≈jX (9)Z=R e +jωL e ≈jX (9)
微网双向换流器桥臂中点电压相量表示为E∠δ，交流子网端口电压为Ue∠0，则微网双向换流器输出的有功功率和无功功率为：The voltage phasor at the midpoint of the bridge arm of the microgrid bidirectional converter is expressed as E∠δ, and the AC sub-network port voltage is U e ∠0, then the active power and reactive power output by the microgrid bidirectional converter are:
将转子运动二阶方程、微网双向换流器输出的有功功率Pe和无功功率Qe以及微网双向换流器输出电势方程中的变量表示为稳态量和扰动量之和，即The second-order equation of rotor motion, the active power Pe and reactive power Q e output by the microgrid bidirectional converter , and the output potential equation of the microgrid bidirectional converter are calculated as The variable in is expressed as the sum of the steady state quantity and the disturbance quantity, namely
其中，Kq表示微网双向换流器输出电势微分系数，E表示微网双向换流器输出电势，Qe和Qref分别为微网双向换流器输出无功功率和输出无功功率参考值，Ue和Uen分别为微网双向换流器输出端口电压和输出端口电压额定值，Dq为电压调节系数；E0表示稳态运行情况下微网双向换流器输出电势，表示微网双向换流器输出电势扰动量，δ为功角，δ0为稳态运行情况下功角，为功角扰动量，Pe0和Qe0分别为稳态运行情况下微网双向换流器输出的有功功率和无功功率，和分别为微网双向换流器输出的有功功率扰动量和无功功率扰动量，Pm表示微网双向换流器输出的虚拟机械有功功率，Pm0表示稳态运行情况下微网双向换流器输出的虚拟机械有功功率，表示微网双向换流器输出的虚拟机械有功功率扰动量，ω为电网实际的角速度，ω0为电网同步角速度，为电网实际的角速度的扰动量，Ue0为稳态运行情况下微网双向换流器输出端口电压，为微网双向换流器输出端口电压扰动量。Among them, K q represents the differential coefficient of the output potential of the microgrid bidirectional inverter, E represents the output potential of the microgrid bidirectional inverter, and Q e and Qref are the output reactive power and output reactive power reference of the microgrid bidirectional inverter, respectively. U e and U en are the output port voltage of the microgrid bidirectional converter and the rated value of the output port voltage respectively, D q is the voltage regulation coefficient; E 0 represents the output potential of the microgrid bidirectional converter under steady-state operation, represents the output potential disturbance of the microgrid bidirectional converter, δ is the power angle, δ 0 is the power angle under steady-state operation, is the power angle disturbance, P e0 and Q e0 are the active power and reactive power output by the microgrid bidirectional converter under steady-state operation, respectively, and are the active power disturbance and reactive power disturbance output by the microgrid bidirectional inverter, respectively, P m represents the virtual mechanical active power output by the microgrid bidirectional inverter, and P m0 represents the microgrid bidirectional commutation under steady-state operation. The virtual mechanical active power output by the device, represents the virtual mechanical active power disturbance output by the microgrid bidirectional converter, ω is the actual angular velocity of the grid, ω 0 is the synchronous angular velocity of the grid, is the disturbance of the actual angular velocity of the power grid, U e0 is the output port voltage of the microgrid bidirectional converter under steady-state operation, is the voltage disturbance at the output port of the microgrid bidirectional converter.
消除扰动量二次分量和稳态量，得到微网双向换流器输出功率的小信号模型：By eliminating the secondary component of the disturbance quantity and the steady-state quantity, the small-signal model of the output power of the microgrid bidirectional converter is obtained:
式中：X为微网双向换流器等效输出电抗，D为阻尼系数，J为同步发电机的转动惯量，Ki为积分环节的积分系数；where X is the equivalent output reactance of the microgrid bidirectional converter, D is the damping coefficient, J is the moment of inertia of the synchronous generator, and K i is the integral coefficient of the integral link;
在计算微网双向换流器输出功率的小信号模型时，设定：When calculating the small-signal model of the output power of the microgrid bidirectional converter, set:
Ue＝E，sinδ0＝δ0，cosδ0＝1；U e =E, sinδ 0 =δ 0 , cosδ 0 =1;
对线性化后的方程进行拉普拉斯变换，可以得到有功环与无功环小信号模型传递框图，如图4所示。Laplace transform is performed on the linearized equation, and the block diagram of the small-signal model transfer between the active loop and the reactive loop can be obtained, as shown in Figure 4.
由图4可知，有功环和无功环存在耦合，但耦合支路增益均含有δ0项。微网正常运行情况下，功角数值很小。本发明通过引入虚拟阻抗控制算法，实现有功环和无功环的近似解耦控制。同步发电机电势E、输出定子电流I和端电压Ue关系如下所示：It can be seen from Figure 4 that there is coupling between the active loop and the reactive power loop, but the coupling branch gains all contain a δ 0 term. Under the normal operation of the microgrid, the power angle value is very small. The invention realizes the approximate decoupling control of the active loop and the reactive loop by introducing a virtual impedance control algorithm. The relationship between the synchronous generator potential E, the output stator current I and the terminal voltage U e is as follows:
式中，Lv为虚拟电抗对应的虚拟电感，Xs是引入虚拟阻抗后的换流器输出电抗。In the formula, L v is the virtual inductance corresponding to the virtual reactance, and X s is the output reactance of the converter after introducing the virtual impedance.
引入虚拟阻抗等效为与换流器输出端串联一个虚拟电感，增大了换流器的输出阻抗，从而使耦合支路增益大大减小，实现了有功环和无功环路近似解耦。The introduction of virtual impedance is equivalent to a virtual inductance in series with the output end of the converter, which increases the output impedance of the converter, thereby greatly reducing the gain of the coupling branch and realizing the approximate decoupling of the active loop and the reactive loop.
2)不考虑有功环和无功环的耦合，通过微网双向换流器输出功率的小信号模型求出有功功率环的闭环传递函数公式如下：2) Without considering the coupling of the active loop and the reactive power loop, the closed-loop transfer function formula of the active power loop is obtained through the small signal model of the output power of the microgrid bidirectional converter as follows:
式中，G(s)为有功功率环闭环传递函数，和分别为复频域下微网双向换流器输出的有功功率扰动量和输出的虚拟机械有功功率扰动量，Xs是引入虚拟阻抗后的微网双向换流器总的输出电抗，s表示拉普拉斯算子；K为比例常数，ωn和ξ分别表示有功功率环闭环传递函数的自然振荡角频率和阻尼比，由下式求得：In the formula, G(s) is the closed-loop transfer function of the active power loop, and are the output active power disturbance of the microgrid bidirectional converter and the output virtual mechanical active power disturbance in the complex frequency domain, respectively, X s is the total output reactance of the microgrid bidirectional converter after introducing the virtual impedance, s represents the pull Plath operator; K is the proportionality constant, ω n and ξ represent the natural oscillation angular frequency and damping ratio of the closed-loop transfer function of the active power loop, respectively, and are obtained from the following equations:
3)根据有功功率环的闭环传递函数、交流子网频率公式，可得图5所示的频率控制模式下控制框图，并求得频率控制模式下开环传递函数为：3) According to the closed-loop transfer function of the active power loop and the frequency formula of the AC sub-network, the control block diagram in the frequency control mode shown in Figure 5 can be obtained, and the open-loop transfer function in the frequency control mode is obtained as:
正常情况下，直流子网侧按照上层优化功率稳定运行。暂态情况下，交流子网通过AC/DC双向换流器向直流子网注入或吸收有功功率。为简化设计，直流子网可等效为直流母线上的恒定电阻元件Req，电阻的阻值取决于注入直流子网侧有功功率。Under normal circumstances, the DC sub-grid side operates stably according to the optimized power of the upper layer. Under transient conditions, the AC sub-network injects or absorbs active power into the DC sub-network through the AC/DC bidirectional converter. To simplify the design, the DC sub-network can be equivalent to a constant resistance element Re eq on the DC bus, and the resistance value of the resistor depends on the active power injected into the DC sub-network side.
由图7可知，忽略换流器内部有功功率损耗时，换流器传输的有功功率为It can be seen from Figure 7 that when the internal active power loss of the converter is ignored, the active power transmitted by the converter is
式中，Req为直流子网等效电阻，Udc为直流母线电压，Pe1为微网双向换流器传输功率为负时输出的有功功率，C为直流侧电容，d为微分算子。考虑小信号干扰下，有功功率/直流电压传递函数如下所示：In the formula, Req is the equivalent resistance of the DC sub-network, U dc is the DC bus voltage, P e1 is the active power output when the transmission power of the microgrid bidirectional converter is negative, C is the DC side capacitance, and d is the differential operator . Considering the small signal interference, the active power/DC voltage transfer function is as follows:
式中，GP-U(s)为微网双向换流器传输功率为负时的有功功率/直流电压传递函数，和分别为稳态运行情况下直流母线电压扰动量和微网双向换流器传输功率为负时输出的有功功率扰动量。根据有功功率环的闭环传递函数、直流母线电压公式和微网双向换流器传输功率为负时的有功功率/直流电压传递函数，，可得图8所示的频率控制模式下控制框图，并求得直流母线电压控制模式下开环传递函数为：In the formula, G PU (s) is the active power/DC voltage transfer function when the transmission power of the microgrid bidirectional converter is negative, and are the DC bus voltage disturbance under steady-state operation and the active power disturbance output when the transmission power of the microgrid bidirectional converter is negative, respectively. According to the closed-loop transfer function of the active power loop, the DC bus voltage formula and the active power/DC voltage transfer function when the transmission power of the microgrid bidirectional converter is negative, the control block diagram in the frequency control mode shown in Figure 8 can be obtained, and The open-loop transfer function in the DC bus voltage control mode is obtained as:
其中：GU(s)为微网双向换流器控制目标为直流母线电压下的开环传递函数，kpu和kpi分别表示直流母线电压PI调节器的比例系数和积分系数。Among them: G U (s) is the open-loop transfer function of the microgrid bidirectional converter control target under the DC bus voltage, k pu and k pi represent the proportional coefficient and integral coefficient of the DC bus voltage PI regulator, respectively.
4)根据微网双向换流器控制目标为交流子网频率和直流母线电压下开环传递函数进行稳定性分析。所述的分析包括：4) According to the control objective of the microgrid bidirectional converter, the stability analysis is carried out for the open-loop transfer function under the frequency of the AC sub-grid and the voltage of the DC bus. The analysis described includes:
根据微网双向换流器控制目标为交流子网频率和直流母线电压的开环传递函数，利用根轨迹法绘制参数J、D、Ki变化时，闭环传递函数极点在s平面上的变化轨迹，分析可得：According to the open-loop transfer function of the AC sub-network frequency and the DC bus voltage as the control target of the microgrid bidirectional converter, the root locus method is used to draw the change trajectory of the pole of the closed-loop transfer function on the s-plane when the parameters J, D, and K i change. , the analysis can be obtained:
由图6(a)可知，转动惯量J越大，闭环传递函数极点越靠近虚轴，系统越不稳定，转动惯量过大会引起超调量、调节时间的增大，从而造成功率振荡，系统越不容易稳定；由图6(b)(c)可知，随着阻尼系数的增大，系统稳定性增强；随着积分环节的积分系数的增大，闭环极点远离逐渐实轴，超调增大，但始终保持在虚轴左侧，系统稳定性不变。It can be seen from Figure 6(a) that the larger the moment of inertia J is, the closer the pole of the closed-loop transfer function is to the imaginary axis, and the more unstable the system is. If the moment of inertia is too large, the overshoot and adjustment time will increase, resulting in power oscillation, and the more the system is. It is not easy to stabilize; it can be seen from Figure 6(b)(c) that with the increase of the damping coefficient, the system stability is enhanced; with the increase of the integral coefficient of the integral link, the closed-loop pole is far away from the gradual real axis, and the overshoot increases. , but always remains on the left side of the imaginary axis, and the system stability remains unchanged.
当微网双向换流器控制目标为直流母线电压时，开环传递函数由比例环节、一阶微分环节、积分环节、二阶振荡环节和一阶惯性环节构成。为使系统保持稳定且具有较好的动态特性，截止频率fc位于中频段，且满足kpi/kpu＜fc＜ωn。When the control target of the microgrid bidirectional converter is the DC bus voltage, the open-loop transfer function consists of a proportional element, a first-order differential element, an integral element, a second-order oscillation element, and a first-order inertia element. In order to keep the system stable and have better dynamic characteristics, the cut-off frequency f c is located in the middle frequency band and satisfies k pi /k pu <f c <ω n .
下面给出实例：Examples are given below:
本发明的基于虚拟同步机的交直流混合微网双向换流器控制方法，应用在交直流混合微网中，依靠微网双向换流器实现交流频率的无偏差控制和直流母线电压的稳定控制。正常情况下，交直流混合微网稳定运行，微网双向换流器传输功率为零。当交流侧负荷发生突增时，频率下降，控制系统检测到频率偏差，经过运算生成虚拟机械有功功率信号，送入惯性阻尼环节，经过积分环节反馈补偿实现频率的无偏差调节。当直流侧发生负荷突增时，直流母线电压下降，系统切换控制模式，通过直流电压偏差信号生成虚拟机械有功功率信号，维持直流母线电压的稳定。The control method of the AC-DC hybrid micro-grid bidirectional inverter based on the virtual synchronous machine of the present invention is applied in the AC-DC hybrid micro-grid, and the non-bias control of the AC frequency and the stable control of the DC bus voltage are realized by means of the micro-grid bidirectional inverter. . Under normal circumstances, the AC-DC hybrid microgrid operates stably, and the transmission power of the microgrid bidirectional converter is zero. When the load on the AC side increases suddenly, the frequency decreases, and the control system detects the frequency deviation, generates a virtual mechanical active power signal through calculation, and sends it to the inertia damping link. When the load suddenly increases on the DC side, the DC bus voltage drops, the system switches the control mode, and generates a virtual mechanical active power signal through the DC voltage deviation signal to maintain the stability of the DC bus voltage.
为了验证本发明的有效性，建立交直流混合微网，其中交流母线电压等级为380V(线电压)，直流母线电压为800V，微网双向换流器滤波装置选择LC滤波器，其中电路参数设计为电感L＝0.6H，电容C＝20μF，控制电路参数设计为转动惯量J＝2kg.m2，阻尼系数D＝50，积分系数Ki＝1000。In order to verify the effectiveness of the present invention, an AC-DC hybrid microgrid is established, wherein the voltage level of the AC bus is 380V (line voltage), the voltage of the DC bus is 800V, and the filter device of the bidirectional converter of the microgrid selects an LC filter, wherein the circuit parameters are designed For the inductance L=0.6H, the capacitance C=20μF, the parameters of the control circuit are designed as the moment of inertia J=2kg.m 2 , the damping coefficient D=50, and the integral coefficient K i =1000.
离网运行时，初始时刻，微网双向换流器工作在频率控制模式下，传输有功功率80kW，系统稳定运行。0.5s时交流子网投入负荷20kW，1s时切除负荷40kW。图9给出了传统VSG控制和本方法频率控制模式下的频率变化波形。换流器采用传统虚拟同步机控制时，初始频率稳定在49.92Hz，0.5s时频率下降并稳定在49.87Hz，1s时频率上升并经微小超调稳定至49.96Hz左右。采用本方法频率控制模式时，初始频率稳定在50Hz，0.5s和1s时频率经短暂变化能够迅速恢复并维持在额定频率。对比可知，两者均能减小频率的变化，起到为系统提供惯性支撑的作用，动态响应基本一致。但本方法频率控制模式作用下，可做到频率的无偏差调节。当频率偏差超过阈值时，采用本方法频率控制模式使得微网频率恢复至额定值，提高系统独立运行的频率稳定性。图10显示了本方法频率控制模式下交流母线电压幅值波形。0.5s负荷突增时，交流母线电压幅值经短暂振荡恢复稳定，1s负荷突降时，电压幅值略有降低，但一直保持稳定在311V左右。During off-grid operation, at the initial moment, the micro-grid bidirectional converter works in frequency control mode, transmits 80kW of active power, and the system runs stably. At 0.5s, the AC sub-network input load is 20kW, and at 1s, the load is cut off by 40kW. Fig. 9 shows the frequency change waveform under the traditional VSG control and the frequency control mode of this method. When the converter is controlled by a traditional virtual synchronous machine, the initial frequency is stable at 49.92Hz, the frequency decreases at 0.5s and stabilizes at 49.87Hz, and the frequency rises at 1s and stabilizes to about 49.96Hz after a slight overshoot. When the frequency control mode of this method is adopted, the initial frequency is stable at 50Hz, and the frequency can be quickly recovered and maintained at the rated frequency after a brief change at 0.5s and 1s. The comparison shows that both of them can reduce the change of frequency and play the role of providing inertial support for the system, and the dynamic response is basically the same. However, under the action of the frequency control mode of this method, the frequency can be adjusted without deviation. When the frequency deviation exceeds the threshold, the frequency control mode of this method is used to restore the frequency of the microgrid to the rated value, thereby improving the frequency stability of the independent operation of the system. Figure 10 shows the AC bus voltage amplitude waveform in the frequency control mode of this method. When the load suddenly increases for 0.5s, the voltage amplitude of the AC bus is stabilized after a brief oscillation. When the load suddenly drops for 1s, the voltage amplitude decreases slightly, but it remains stable at about 311V.
当微网双向换流器工作在直流母线电压控制模式时，传输180kW的有功功率，此时换流器的控制目的是保持直流母线电压稳定。假设在0.5s时，直流子网负荷突然增加50kW。图11所示为直流母线电压控制模式下直流母线电压波形，图12为直流母线电压控制模式下交流母线频率波形。初始时电压稳定在额定电压800V，并且在0.5s时直流母线电压突降至780V左右并且逐渐恢复到额定值附近。本方法直流母线电压控制模式能够保持直流母线电压的稳定。当负载突然增加时，交流母线频率略有增加，但频率值基本稳定在50Hz。When the microgrid bidirectional converter works in the DC bus voltage control mode, it transmits 180kW of active power. At this time, the control purpose of the converter is to keep the DC bus voltage stable. Assume that at 0.5s, the DC sub-network load suddenly increases by 50kW. Figure 11 shows the DC bus voltage waveform under the DC bus voltage control mode, and Figure 12 shows the AC bus frequency waveform under the DC bus voltage control mode. The voltage is stable at the rated voltage of 800V at the beginning, and the DC bus voltage suddenly drops to about 780V in 0.5s and gradually recovers to around the rated value. The DC bus voltage control mode of the method can keep the DC bus voltage stable. When the load suddenly increases, the frequency of the AC bus increases slightly, but the frequency value is basically stable at 50Hz.
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