CN115498708A - Frequency response method-based interaction analysis method of grid-connected VSC and power grid - Google Patents
Frequency response method-based interaction analysis method of grid-connected VSC and power grid Download PDFInfo
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Abstract
A grid-connected VSC and power grid interaction analysis method based on a frequency response method belongs to the technical field of operation and maintenance of power systems. According to the invention, on the basis of determining a dynamic response model and a control strategy of the grid-connected converter, a linear small-signal model of active output and reactive output of the grid-connected converter is deduced under the condition of small disturbance on the power grid side, and then the response characteristics of the active power and the reactive power of the grid-connected converter under an electromechanical time scale and the interaction between the response characteristics and the alternating current power grid are analyzed by using a frequency response method, the influence of a permanent magnet wind turbine generator set based on the grid-connected converter and the power grid on the electromechanical oscillation characteristics of the power grid is theoretically analyzed, and the method has important significance for safe and stable operation of a power system with high-proportion access of new energy. Therefore, the invention has high practical application value.
Description
Technical Field
The invention belongs to the technical field of operation and maintenance of electric power systems, and particularly relates to a grid-connected VSC and power grid interaction analysis method based on a frequency response method.
Background
Climate change has become one of the major environmental and development challenges facing human society at present, and the power system will play a crucial leading and supporting role in the low-carbon transformation of energy. New energy power generation technologies represented by wind power and photovoltaic are developed at high speed in all countries in the world, and the new energy power generation proportion in a power grid is continuously improved. Different from a traditional synchronous unit, a new energy power supply needs to be connected into a power grid through a Voltage Source Converter (VSC), the controllability of the new energy power generation system is enhanced through the VSC, and meanwhile, the dynamic process of the power system after disturbance is abnormal and complicated through multi-time scale cascade control of the VSC. The dynamic characteristics of the grid-connected converter under the electromechanical time scale are deeply researched, the interaction between the grid-connected converter and an alternating current power grid in the electromechanical dynamic process is analyzed, and the method has important significance for ensuring the safe operation of a power system after large-scale new energy grid connection.
For the research on the electromechanical oscillation of a system after the large-scale new energy unit is connected to the grid, the method mostly focuses on the aspect of linear state space modeling of a new energy power generation system at present, and combines an established state space model with a small signal model of an alternating current system, and calculates the electromechanical characteristic parameters of the system by using a traditional small interference analysis method, so that the electromechanical characteristic analysis of the whole system after the new energy is connected to the grid is realized. However, the complex multi-time scale response characteristic of the grid-connected converter causes a significant difference with the dynamic process of the synchronous machine. In the electromechanical oscillation process under the small disturbance action of the network side, each synchronous unit can generate interaction through the coupling of an alternating current power grid while oscillating. For a new energy source unit connected to the grid, after the new energy source unit is connected to the power grid, power is interacted with a synchronous generator set in a system through a grid-connected converter, so that the research on the dynamic characteristics of the grid-connected converter and the interaction between the grid-connected converter and the dynamic characteristics in the electromechanical process is of great significance, however, related research is less at present.
In summary, there is a need in the prior art for a method capable of effectively analyzing dynamic interaction between a new energy grid-connected converter and an ac power grid in an electromechanical oscillation process, so as to solve the above problems.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the method is used for solving the technical problems that in the prior art, in order to analyze electromechanical oscillation of a new energy grid-connected power system, a small signal model is mainly constructed, characteristic values are adopted for analysis, and influence of dynamic characteristics of a grid-connected converter and interaction between the grid-connected converter and a power grid on electromechanical characteristics cannot be considered.
The method for analyzing interaction between grid-connected VSC and power grid based on frequency response method comprises the following steps, and the following steps are carried out in sequence:
step one, establishing a topological structure and a corresponding control system for connection of a grid-connected converter VSC and an alternating current power grid, and obtaining a system formed by connection of the grid-connected converter VSC and the alternating current power grid:
(1) the method comprises the following steps that direct current power rectified by a rectifier of the wind turbine generator is stabilized through a direct current capacitor, and then fed into an alternating current power grid through a grid-connected converter VSC and a filter reactor, so that a topological structure of the grid-connected converter VSC connected with the alternating current power grid is obtained;
(2) establishing a control system corresponding to the topological structure, wherein the topological structure and the corresponding control system form a system formed by connecting a grid-connected converter VSC and an alternating current power grid, the control system adopts a d-q decoupling double-closed-loop control structure combining a voltage outer loop and a current inner loop to the VSC on the alternating current power grid side to realize the directional vector control of the voltage of the power grid, wherein d-axis current is active current, q-axis current is reactive current, the active power transmitted to the alternating current power grid is controlled by using the d-axis current, and the reactive power transmitted to the alternating current power grid is controlled by using the q-axis current;
the output of the d-axis current is a reference value of the d-axis current and a deviation value of the d-axis actual current; the outer ring of the q-axis current is controlled by alternating voltage, and the output of the q-axis current is a reference value of the q-axis current and a deviation value of the q-axis actual current;
step two, obtaining a small signal model after linearization of active output and reactive output of the grid-connected converter VSC under small disturbance of the AC power grid side:
the analysis process of a system formed by connecting the grid-connected converter VSC and the alternating current power grid is simplified through three conditions of a, b and c,
a. the power of the direct current side is kept constant;
b. neglecting the internal loss of the VSC of the grid-connected converter;
c. the voltage of a public connection point of a grid-connected converter VSC and an alternating current grid is always coincided with a d axis in the electromechanical process;
according to the instantaneous power theory, under the condition that the q-axis voltage is 0, the output active power and reactive power of the VSC of the grid-connected converter meet the following requirements:
in the formula, P is active power at the VSC outlet of the grid-connected converter; q is reactive power at the VSC outlet of the grid-connected converter; v. of pcc The alternating voltage is the alternating voltage at the public connection point of the grid-connected converter VSC and the alternating current power grid; i.e. i d Is the d-axis current; i.e. i q Is the q-axis current;
under the condition of small disturbance at the side of the alternating current network, the linearization of the formula (1) at the steady-state operation point of the system is as follows:
in the formula, Δ P is the variation of the active power P; Δ Q is the amount of change in reactive power Q; Δ v pcc For grid-connected point AC voltage v pcc The amount of change of (c); v pcc0 For grid-connected point AC voltage v pcc A steady state value of; i is d0 Is the steady state component of the d-axis current; i is q0 Is the steady state component of the q-axis current; Δ i d Is the amount of change in d-axis current; Δ i q Is the amount of change in the q-axis current;
under the control system effect, the interaction exists in the voltage of grid-connected converter VSC and alternating current electric network public connecting point department to utilize this characteristic to simplify formula (2), satisfy between each controller inner loop current in the control system and its corresponding reference value:
in the formula, G id (s) a PI controller for a d-axis current inner loop; g iq (s) a PI controller for q-axis current inner loop; delta i dref Is Δ i d A reference value of (d); Δ i qref Is Δ i q A reference value of (d); k is a radical of p_id Is the proportional gain of the d-axis current loop; k is a radical of i_id Is the integral gain of the d-axis current loop; k is a radical of formula p_iq Proportional gain for the q-axis current loop; k is a radical of i_iq Is the integral gain of the q-axis current loop; l is a Laplace transform operator; s is the complex frequency;
further, the relation between the variable quantities of the current reference values of the d axis and the q axis and the corresponding control quantity of the outer ring meets the following conditions:
in the formula, G udc Is a transfer function of a direct current voltage outer loop PI controller; g upcc Is the transfer function of the AC voltage outer loop PI controller; Δ u dcref Is a variable Δ u of DC voltage dc A reference value of (d); Δ v pccref Is the variation delta v of the AC voltage of the point of connection pcc A reference value of (a); k is a radical of p_udc Proportional gain of the direct current voltage control loop; k is a radical of i_udc Is the integral gain of the dc voltage control loop; k is a radical of formula p_vpcc Proportional gain for the ac voltage control loop; k is a radical of i_vpcc Integral gain of the ac voltage control loop;
d.c. voltage variation delta u dc AC voltage variation delta v of sum point grid pcc All the external loop voltage variation quantities are external loop voltage variation quantities, and subsequent expansion analysis is inconvenient, so that in order to visually analyze the dynamic response characteristics of the grid-connected converter VSC under small disturbance of the alternating current grid side, the dynamic characteristics of the direct current capacitor given by the formula (5) are further combined, and the direct current voltage component, namely the direct current voltage variation delta u in the input component is eliminated dc ：
-ΔP＝U dc0 CsΔu dc (5)
In the formula, Δ P is the voltage variation between two ends of the dc capacitor; u shape dc0 Is a voltage steady state voltage value; c is the capacitance value of the direct current capacitor;
obtaining a transfer function corresponding to the active control path as the variation delta v of the grid-connected point alternating voltage pcc The active power control method comprises the steps of taking active power variation injected into an alternating current power grid as input, and taking the active power variation as output, wherein the active power variation corresponds to a small signal model; obtaining a transfer function corresponding to the reactive power control path as a variable quantity delta v of the AC voltage of the grid-connected point pcc A small signal model corresponding to a reactive power control path which takes the reactive power variation as output is used as input;
the transfer function corresponding to the active control path and the transfer function corresponding to the reactive control path are respectively as follows:
in the formula: g p (s) is a transfer function corresponding to the active control path; g q (s) is a transfer function corresponding to the reactive power control path;
analyzing the active output and reactive output characteristics of the grid-connected converter VSC under the electromechanical time scale and the dynamic interaction between the grid-connected converter VSC and an alternating current power grid by using a frequency response method:
utilizing the small signal model and the none corresponding to the active control path obtained in the step two under the electromechanical time scaleSelecting parameters of a VSC (voltage source converter) designated controller of the grid-connected converter, namely parameters k of a direct-current voltage control outer ring PI (proportional integral) controller, according to a small-signal model corresponding to a power control path p_udc ,k i_udc Parameter k of outer ring PI controller controlled by AC voltage p_vpcc ,k i_vpcc And d-axis and q-axis PI controller parameters k of the current control inner loop p_id 、k p_iq ，k i_id 、k i_iq And a DC capacitance value C, a filter reactance X f And system reactance X s And substituting the equation into the equation (6), and qualitatively analyzing an active control path transfer function G between the VSC of the grid-connected converter and the AC power grid under the action of small disturbance of the AC power grid by using a frequency response method p (s) and reactive control path transfer function G q (s) logarithmic amplitude-frequency characteristics under the electromechanical time scale, qualitatively analyzing dynamic characteristics of active output and reactive output of the grid-connected converter VSC injected into the alternating current power grid according to amplitude values in the logarithmic amplitude-frequency characteristics, analyzing and obtaining power interaction characteristics under the electromechanical time scale between the grid-connected converter VSC and the alternating current power grid under the small disturbance action of the alternating current power grid according to dynamic characteristic analysis results of the active output and the reactive output, and completing interaction characteristic analysis of the grid-connected converter VSC and the alternating current power grid based on a frequency response method.
Through the design scheme, the invention can bring the following beneficial effects:
according to the invention, on the basis of determining a dynamic response model and a control strategy of the grid-connected converter, a linear small-signal model of active output and reactive output of the grid-connected converter is deduced under the condition of small disturbance on the power grid side, and then the response characteristics of the active power and the reactive power of the grid-connected converter under an electromechanical time scale and the interaction between the response characteristics and the alternating current power grid are analyzed by using a frequency response method, the influence of a permanent magnet wind turbine generator set based on the grid-connected converter and the power grid on the electromechanical oscillation characteristics of the power grid is theoretically analyzed, and the method has important significance for safe and stable operation of a power system with high-proportion access of new energy. Therefore, the invention has high practical application value.
Compared with the prior art, the invention has the following characteristics:
according to the method, on the basis of analyzing a dynamic response model and a control strategy of a grid-connected converter VSC, under the condition that small disturbance occurs on a power grid side, linear small signal models of active and reactive outputs of the grid-connected converter VSC are deduced, namely small signal models corresponding to an active control path and small signal models corresponding to a reactive control path, the active characteristics and reactive characteristics of the grid-connected converter VSC in the electromechanical oscillation process are analyzed by using a frequency response method, and the interaction characteristics between the active and reactive outputs and the power grid of the grid-connected converter VSC are analyzed; compared with the previous research for analyzing the influence of the new energy machine group to the electromechanical dynamic behavior of the power grid, the invention can take the interaction between the new energy machine group and the alternating current power grid into consideration, and effectively analyzes the influence of the system dynamic response characteristic of the interactive coupling of the power electronic equipment and the power grid on the electromechanical oscillation characteristic of the system, thereby having higher practical application value.
Drawings
The invention is further described in the following detailed description in conjunction with the drawings in which:
fig. 1 is a structural diagram of a topology of a grid-connected converter and a control system thereof in an embodiment of a grid-connected VSC and grid interaction analysis method based on a frequency response method.
Fig. 2 is a small-signal model diagram of a grid-connected converter VSC in an embodiment of a frequency response method-based grid-connected VSC and grid interaction analysis method of the present invention, where a common connection point voltage and a direct current voltage are used as inputs.
Fig. 3 is a first small-signal model diagram of the grid-connected converter VSC using the voltage at the common connection point as a single input in the embodiment of the grid-connected VSC and grid interaction analysis method based on the frequency response method.
Fig. 4 is a second small-signal model diagram of the grid-connected converter VSC in the embodiment of the method for analyzing interaction between the grid-connected VSC and the grid based on the frequency response method, where the common connection point voltage is used as a single input.
Fig. 5 is a logarithmic amplitude-frequency characteristic diagram of Gp(s) and Gq(s) in an embodiment of a grid-connected VSC and grid interaction analysis method based on a frequency response method.
Detailed Description
The invention relates to a grid-connected VSC and power grid interaction analysis method based on a frequency response method, which is characterized in that a linear model of active and reactive power output of a grid-connected converter is pushed under the condition that small disturbance occurs on a conductive grid side, and then the active and reactive characteristics of the grid-connected converter and the interaction between the active and reactive characteristics and a power grid under an electromechanical time scale are analyzed by the frequency response method. The calculation process comprises the following steps:
step one, determining a topological structure of a power system containing a grid-connected converter VSC and a corresponding control system:
firstly, a topology structure diagram of the connection of the converter VSC and the alternating current power grid is given, namely, the direct current power of the wind turbine generator after passing through the machine side rectifier is fed into the alternating current power grid by the grid-connected converter VSC and the filter reactance after being stabilized by the direct current capacitor C.
The control system corresponding to the topological structure is described as follows: in order to ensure the stability of the direct-current bus voltage of the VSC of the machine side grid-connected converter and the voltage quality of a power grid, the vector control of the voltage orientation of the power grid is generally adopted for the VSC of the grid side, namely, the VSC of the grid side grid-connected converter generally adopts a d-q decoupling double closed-loop control structure with a voltage outer loop and a current inner loop, wherein the d-axis current and the q-axis current are respectively active current and reactive current, so that the active power transmitted to the power grid is controlled by controlling the d-axis current, and the reactive power flowing to the power grid is controlled by utilizing the q-axis current. The topological structure and a corresponding control system form a system formed by connecting a grid-connected converter VSC and an alternating current power grid, for ensuring the stable operation of the system and considering the requirement of a machine side converter on the reactive power support of the system, for d-axis current, the outer ring of the d-axis current is controlled by direct current voltage, and the output of the d-axis current is a reference value of the d-axis current; for q-axis current, its outer loop is controlled by an alternating voltage, and its output is a reference value for the q-axis current.
Step two, deducing a small signal model after linearization of active output and reactive output of the grid-connected converter VSC under small disturbance of the AC power grid side:
the dynamic characteristic of a system formed by connecting the grid-connected converter VSC and the alternating current power grid is mainly determined by a multi-scale cascade control system, and the dynamic characteristic is complex. To simplify the analysis process, the following three assumptions are made first:
a. the dc side power remains constant.
b. And neglecting the VSC internal loss of the grid-connected converter.
c. The voltage of a public connection point of the grid-connected converter VSC and the power grid is always coincident with the d axis in the electromechanical process.
According to the instantaneous power theory, under the condition that the q-axis voltage is 0, the output active power and reactive power of the VSC of the grid-connected converter meet the following requirements:
in the formula, P is active power at the VSC outlet of the grid-connected converter; q is reactive power at a VSC outlet of the grid-connected converter; v. of pcc The alternating voltage at the public connection point of the grid-connected converter VSC and the alternating current grid; i all right angle d Is the d-axis current; i all right angle q Is the q-axis current;
under small disturbance at the ac grid side, the linearization for equation (1) at the steady-state operating point of the system is as follows:
in the formula, Δ P is the variation of the active power P; Δ Q is the amount of change in reactive power Q; Δ v pcc For a grid point AC voltage v pcc The amount of change in (c); v pcc0 For a grid point AC voltage v pcc A steady state value of; i is d0 Is the steady state component of the d-axis current; I.C. A q0 Is the steady state component of the q-axis current; Δ i d Is the amount of change in d-axis current; Δ i q Is the amount of change in the q-axis current;
under the action of the control system, the interaction between the grid-connected current and the voltage at the public connection point of the power grid exists, so that the equation (2) can be simplified by utilizing the characteristic. The inner ring current of the controller and the reference value thereof meet the following conditions:
in the formula, G id (s) a PI controller for a d-axis current inner loop; g iq (s) a PI controller for a q-axis current inner loop; Δ i dref Is Δ i d A reference value of (a); Δ i qref Is Δ i q A reference value of (d); k is a radical of p_id Is the proportional gain of the d-axis current loop; k is a radical of i_id Is the integral gain of the d-axis current loop; k is a radical of p_iq Proportional gain for the q-axis current loop; k is a radical of formula i_iq Is the integral gain of the q-axis current loop; l is a Laplace transform operator; s is a complex frequency;
further, the relation between the variable quantity of the current reference values of the d axis and the q axis and the corresponding control quantity of the outer ring meets the following requirements:
in the formula, G udc An outer loop PI controller transfer function of the direct current voltage is obtained; g upcc An outer loop PI controller transfer function for the AC voltage; Δ u dcref Is a variable quantity delta u of direct current voltage dc A reference value of (d); Δ v pccref Is the variation delta v of the AC voltage of the point of connection pcc A reference value of (a); k is a radical of formula p_udc Proportional gain of the dc voltage control loop; k is a radical of formula i_udc Is the integral gain of the dc voltage control loop; k is a radical of formula p_vpcc Proportional gain for the ac voltage control loop; k is a radical of i_vpcc Integral gain of the ac voltage control loop;
d.c. voltage variation delta u dc AC voltage variation delta v of sum point grid pcc All the direct-current voltage components are external loop voltage variation and are inconvenient for subsequent development and analysis, so that the dynamic response characteristic of the grid-connected converter VSC under small disturbance of the alternating current network side is visually analyzed, the direct-current capacitance dynamic characteristic given by the formula (5) is further combined, and the direct-current voltage component, namely the direct-current voltage variation delta u in the input component is eliminated dc ：
-ΔP＝U dc0 CsΔu dc (5)
In the formula, Δ P is the voltage variation between two ends of the dc capacitor; u shape dc0 Is a voltage steady state voltage value; c is the capacitance value of the direct current capacitor;
further, it is derived only by Δ v pcc For input, a single-input small-signal model with the active power and reactive power variation injected into the power grid as output, namely transfer functions of an active control loop and a reactive control loop are respectively as follows:
in the formula, G p (s) is a transfer function corresponding to the active control path; g q (s) is a transfer function corresponding to the reactive power control path;
analyzing the active output and reactive output characteristics of the grid-connected converter VSC under the electromechanical time scale and the dynamic interaction between the grid-connected converter VSC and an alternating current power grid by using a frequency response method:
and (3) under the electromechanical time scale, selecting parameters of the VSC specified controller of the grid-connected converter, namely parameters k of the direct-current voltage control outer-loop PI controller by using the small-signal model corresponding to the active control path and the small-signal model corresponding to the reactive control path obtained in the step two p_udc ,k i_udc Parameter k of outer ring PI controller controlled by AC voltage p_vpcc ,k i_vpcc And d-axis and q-axis PI controller parameters k of the current control inner loop p_id 、k p_iq ，k i_id 、k i_iq And a DC capacitance value C, a filter reactance X f And system reactance X s And substituting the equation into the equation (6), and qualitatively analyzing an active control path transfer function G between the VSC of the grid-connected converter and the AC power grid under the action of small disturbance of the AC power grid by using a frequency response method p (s) and reactive control path transfer function G q (s) logarithmic amplitude-frequency characteristics under the electromechanical time scale, so that the dynamic characteristics of active output and reactive output injected into the alternating current power grid by the grid-connected converter VSC are qualitatively analyzed according to the amplitude in the logarithmic amplitude-frequency characteristics, and the on-line state between the grid-connected converter VSC and the alternating current power grid under the small disturbance action of the alternating current power grid is analyzed and obtained according to the analysis result of the dynamic characteristics of the active output and the reactive outputAnd the power interaction characteristic under the electrical time scale is obtained, so that the interaction characteristic analysis of the grid-connected converter VSC and the alternating current power grid based on the frequency response method is completed.
Examples
The method for analyzing interaction between grid-connected VSC and power grid based on frequency response method comprises the following steps:
step one, determining a topological structure and a corresponding control strategy of a power system containing a grid-connected converter VSC:
a topology structure diagram of the connection of the grid-connected converter VSC and the alternating current grid is determined, as shown in fig. 1, that is, the direct current power of the wind turbine generator after passing through the machine side rectifier is stabilized by the direct current capacitor C and then fed into the alternating current grid by the grid-connected converter VSC and the filter reactance. The PLL is a phase-locked loop and provides a reference phase for d-q conversion so as to ensure that a grid-connected converter VSC and an alternating current grid are kept synchronous; q vsc Injecting reactive power into a system for the grid-connected converter VSC; x f Is the filter reactance.
In addition, in order to guarantee the stability of the direct-current bus voltage of the machine side grid-connected converter VSC and the voltage quality of the power grid, the grid side grid-connected converter VSC is generally subjected to grid voltage directional vector control, namely, the grid side grid-connected converter VSC generally adopts a d-q decoupling double closed-loop control structure with a voltage outer loop and a current inner loop combined together, wherein d-axis current and q-axis current are respectively active current and reactive current, so that the active power transmitted to the power grid is controlled by controlling d-axis current, and the reactive power flowing to the power grid is controlled by using q-axis current. The topological structure and a corresponding control system form a system formed by connecting a grid-connected converter VSC and an alternating current power grid, for ensuring the stable operation of the system and considering the requirement of a machine side converter on the reactive power support of the system, for d-axis current, the outer ring of the d-axis current is controlled by direct current voltage, and the output of the d-axis current is a reference value of the d-axis current; for q-axis current, the outer loop is controlled by alternating voltage, and the output is a reference value of the q-axis current.
Step two, deducing a linear small-signal model of active output and reactive output of the grid-connected converter VSC under small disturbance of the power grid side:
the dynamic characteristic of a system formed by connecting the grid-connected converter VSC with the alternating current grid is mainly determined by a multi-scale cascade control system of the system, and the dynamic characteristic is complex. To simplify the analysis process, the following three assumptions are made first:
a. the dc side power is maintained constant.
b. And neglecting the VSC internal loss of the grid-connected converter.
c. The voltage of a public connection point of the grid-connected converter VSC and the power grid is always coincident with the d axis in the electromechanical process.
According to the instantaneous power theory, under the condition that the q-axis voltage is 0, the output active power and reactive power of the VSC of the grid-connected converter meet the following requirements:
in the formula, P is active power at the VSC outlet of the grid-connected converter; q is reactive power at a VSC outlet of the grid-connected converter; v. of pcc The alternating voltage is the alternating voltage at the public connection point of the grid-connected converter VSC and the alternating current power grid; i.e. i d Is the d-axis current; i.e. i q Is the q-axis current.
Under small disturbance at the ac grid side, the linearization for equation (1) at the steady-state operating point of the system is as follows:
in the formula, Δ P is the variation of the active power P; Δ Q is the amount of change in reactive power Q; Δ v pcc For grid-connected point AC voltage v pcc The amount of change in (c); v pcc0 For grid-connected point AC voltage v pcc A steady state value of; i is d0 Is the steady state component of the d-axis current; i is q0 Is the steady state component of the q-axis current; delta i d Is the amount of change in d-axis current; Δ i q Is the amount of change in the q-axis current.
Under the action of the control system, the interaction exists between the grid-connected current and the voltage at the public connection point of the power grid, so that the expression (2) can be simplified by utilizing the characteristic. The inner ring current of the controller and the reference value thereof meet the following conditions:
in the formula, G id (s) a PI controller for a d-axis current inner loop; g iq (s) a PI controller for q-axis current inner loop; delta i dref Is Δ i d A reference value of (d); delta i qref Is Δ i q A reference value of (d); k is a radical of p_id Is the proportional gain of the d-axis current loop; k is a radical of i_id Is the integral gain of the d-axis current loop; k is a radical of p_iq Proportional gain for the q-axis current loop; k is a radical of i_iq Is the integral gain of the q-axis current loop; l is a Laplace transform operator; s is the complex frequency.
Further, the relation between the variable quantity of the current reference values of the d axis and the q axis and the corresponding control quantity of the outer ring meets the following requirements:
in the formula, G udc Is a transfer function of a direct current voltage outer loop PI controller; g upcc An outer loop PI controller transfer function for the AC voltage; Δ u dcref Is a variable quantity delta u of direct current voltage dc A reference value of (d); Δ v pccref Is the variation quantity delta v of the grid-connected point alternating voltage pcc A reference value of (d); k is a radical of p_udc Proportional gain of the dc voltage control loop; k is a radical of i_udc Integral gain of the dc voltage control loop; k is a radical of p_vpcc Proportional gain for the ac voltage control loop; k is a radical of i_vpcc Is the integral gain of the ac voltage control loop;
the outer-loop voltage variation (including the dc voltage variation Δ u) can be derived by combining equations (2) to (4) dc AC voltage variation delta v of sum point grid pcc ) For input, the active power and reactive power variation injected into the grid by the grid-connected converter VSC is a small-signal model of output, as shown in fig. 2. As is clear from fig. 2, the common node voltage v is determined with the outer loop reference value and the controller parameter pcc Reactive power acting on grid-connected converter outlet through two pathsPower: wherein one part changes its outlet reactive power through the current path and the other part influences the reactive power output dynamic process through the alternating voltage path. Different from reactive power, the active power at the outlet of the grid-connected converter is simultaneously subjected to direct-current voltage u dc And a common connection point voltage v pcc In common, i.e. a portion of the voltage v from the common connection point pcc Steady state current I through d-axis d0 The other part is influenced by the direct voltage and the direct voltage control loop.
However, FIG. 2 shows the voltage output from a DC voltage u dc And a common connection point voltage v pcc The formed multi-input model is difficult to visually analyze the dynamic response characteristic of the grid-connected converter under the system disturbance. For this purpose, the dc voltage component, i.e., the dc voltage variation Δ u, is removed by the dc capacitance dynamic characteristic model in equation (5) dc 。
-ΔP＝U dc0 CsΔu dc (5)
In the formula: delta P is the voltage variation at two ends of the capacitor; u shape dc0 Is a voltage steady state voltage value; c is the capacitance.
Further, it is derived by Δ v alone pcc Single input small signal model with the active power variation injected into the grid as output for input (as shown in fig. 3), and Δ ν only pcc As an input, a single-input small-signal model (as shown in fig. 4) with the reactive power variation injected into the power grid as an output is used, and the two small-signal models are respectively a transfer function corresponding to an active control path and a transfer function corresponding to a reactive control path:
step three: the method comprises the following steps of analyzing the active output and reactive output characteristics of the grid-connected converter VSC under the electromechanical time scale and the dynamic interaction between the grid-connected converter VSC and a power grid by using a frequency response method:
and (3) selecting parameters of a grid-connected VSC typical controller by using the small signal model corresponding to the active control path and the small signal model corresponding to the reactive control path obtained in the step two under the electromechanical time scale, substituting the parameters into a formula (6) as shown in Table 1, and qualitatively analyzing dynamic characteristics of active output and reactive output of the grid-connected converter under the action of small disturbance of the power grid, so that the power interaction characteristics between the grid-connected converter and the alternating current power grid in the small disturbance electromechanical process are analyzed. Fig. 5 shows a logarithmic amplitude-frequency characteristic diagram of Gp(s) and Gq(s) obtained by the frequency response method. As can be seen from fig. 5, in the range of the electromechanical frequency band (the frequency range is 0.1 to 2.5 Hz), the amplitude of the transfer function Gp(s) corresponding to the active control path is small, and even at the upper limit of the electromechanical frequency band of 2.5Hz, the frequency response amplitude is still small and is smaller than-10 dB. Therefore, in the electromechanical oscillation process, the interaction between the grid-connected converter and the active power of the alternating current power grid is weak. Meanwhile, the amplitude of a transfer function Gq(s) corresponding to the reactive power control path is constantly larger than zero in an electromechanical oscillation frequency band, which indicates that the reactive power interaction between the permanent magnet direct-drive wind turbine generator and the power grid is larger than the active power in the electromechanical oscillation process, and the reactive power response is the main interaction form between the grid-connected converter and the alternating current power grid.
TABLE 1 parameters of control link of grid-connected converter
The invention provides an effective method for analyzing the dynamic interaction of the grid-connected VSC and the alternating current power grid under the electromechanical time scale, can qualitatively analyze the dynamic interaction of the permanent magnet synchronous wind turbine generator set based on the converter and the alternating current power grid in the electromechanical oscillation process, and provides an effective analysis method for the electromechanical dynamic behavior analysis of new energy high-proportion access to the power grid.
Claims (1)
1. A grid-connected converter VSC and power grid interaction analysis method based on a frequency response method is characterized by comprising the following steps: comprises the following steps which are sequentially carried out,
step one, establishing a topological structure and a corresponding control system for connection of a grid-connected converter VSC and an alternating current power grid, and obtaining a system formed by connection of the grid-connected converter VSC and the alternating current power grid:
(1) the method comprises the following steps that direct current power rectified by a rectifier of the wind turbine generator is stabilized through a direct current capacitor, and then fed into an alternating current power grid through a grid-connected converter VSC and a filter reactor, so that a topological structure of the grid-connected converter VSC connected with the alternating current power grid is obtained;
(2) establishing a control system corresponding to the topological structure, wherein the topological structure and the corresponding control system form a system formed by connecting a grid-connected converter VSC and an alternating current power grid, the control system adopts a d-q decoupling double-closed-loop control structure combining a voltage outer loop and a current inner loop to the VSC on the alternating current power grid side to realize the directional vector control of the voltage of the power grid, wherein d-axis current is active current, q-axis current is reactive current, the active power transmitted to the alternating current power grid is controlled by using the d-axis current, and the reactive power transmitted to the alternating current power grid is controlled by using the q-axis current;
the output of the d-axis current is a reference value of the d-axis current and a deviation value of the d-axis actual current; the outer ring of the q-axis current is controlled by alternating voltage, and the output of the q-axis current is a reference value of the q-axis current and a deviation value of the q-axis actual current;
step two, obtaining a small signal model after linearization of active output and reactive output of the grid-connected converter VSC under small disturbance of the AC power grid side:
the analysis process of a system formed by connecting the grid-connected converter VSC and the alternating current grid is simplified through three conditions of a, b and c,
a. the power of the direct current side is kept constant;
b. neglecting the internal loss of the VSC of the grid-connected converter;
c. the voltage of a public connection point of a grid-connected converter VSC and an alternating current grid is always coincided with a d axis in the electromechanical process;
according to the instantaneous power theory, under the condition that the q-axis voltage is 0, the output active power and reactive power of the VSC of the grid-connected converter meet the following requirements:
in the formula, PActive power at the VSC outlet of the grid-connected converter; q is reactive power at a VSC outlet of the grid-connected converter; v. of pcc The alternating voltage at the public connection point of the grid-connected converter VSC and the alternating current grid; i all right angle d Is the d-axis current; i all right angle q Is the q-axis current;
under small disturbance at the ac grid side, the linearization for equation (1) at the steady-state operating point of the system is as follows:
in the formula, Δ P is the variation of the active power P; Δ Q is the amount of change in reactive power Q; Δ v pcc For grid-connected point AC voltage v pcc The amount of change of (c); v pcc0 For a grid point AC voltage v pcc A steady state value of; i is d0 Is the steady state component of the d-axis current; i is q0 Is the steady state component of the q-axis current; Δ i d Is the amount of change in d-axis current; Δ i q Is the amount of change in the q-axis current;
under the control system effect, the interaction exists in the voltage of grid-connected converter VSC and alternating current electric network public connecting point department to utilize this characteristic to simplify formula (2), satisfy between each controller inner loop current in the control system and its corresponding reference value:
in the formula, G id (s) a PI controller for a d-axis current inner loop; g iq (s) a PI controller for q-axis current inner loop; Δ i dref Is Δ i d A reference value of (d); Δ i qref Is Δ i q A reference value of (d); k is a radical of p_id Proportional gain of d-axis current loop; k is a radical of i_id Is the integral gain of the d-axis current loop; k is a radical of p_iq Proportional gain for the q-axis current loop; k is a radical of i_iq Is the integral gain of the q-axis current loop; l is a Laplace transform operator; s is the complex frequency;
further, the relation between the variable quantities of the current reference values of the d axis and the q axis and the corresponding control quantity of the outer ring meets the following conditions:
in the formula, G udc Is a transfer function of a direct current voltage outer loop PI controller; g upcc Is the transfer function of the AC voltage outer loop PI controller; Δ u dcref Is a variable quantity delta u of direct current voltage dc A reference value of (a); Δ v pccref Is the variation quantity delta v of the grid-connected point alternating voltage pcc A reference value of (d); k is a radical of p_udc Proportional gain of the direct current voltage control loop; k is a radical of i_udc Is the integral gain of the dc voltage control loop; k is a radical of p_vpcc Proportional gain for the ac voltage control loop; k is a radical of i_vpcc Is the integral gain of the ac voltage control loop;
d.c. voltage variation delta u dc AC voltage variation delta v of sum point grid pcc All the external loop voltage variation quantities are external loop voltage variation quantities, and subsequent expansion analysis is inconvenient, so that in order to visually analyze the dynamic response characteristics of the grid-connected converter VSC under small disturbance of the alternating current grid side, the dynamic characteristics of the direct current capacitor given by the formula (5) are further combined, and the direct current voltage component, namely the direct current voltage variation delta u in the input component is eliminated dc ：
-ΔP＝U dc0 CsΔu dc (5)
In the formula, Δ P is the voltage variation between two ends of the dc capacitor; u shape dc0 Is a voltage steady state voltage value; c is the capacitance value of the direct current capacitor;
obtaining a transfer function corresponding to the active control path as the variation delta v of the grid-connected point alternating voltage pcc The active power control method comprises the steps of taking active power variation injected into an alternating current power grid as input, and taking the active power variation as output, wherein the active power variation corresponds to a small signal model; obtaining a transfer function corresponding to the reactive power control path as the variation delta v of the AC voltage of the grid-connected point pcc The small signal model is corresponding to the reactive power control path which takes the reactive power variation as output for input;
the transfer function corresponding to the active control path and the transfer function corresponding to the reactive control path are respectively as follows:
in the formula, G p (s) is a transfer function corresponding to the active control path; g q (s) is a transfer function corresponding to the reactive power control path;
analyzing the active output and reactive output characteristics of the grid-connected converter VSC under the electromechanical time scale and the dynamic interaction between the grid-connected converter VSC and an alternating current power grid by using a frequency response method:
and (3) selecting parameters of a VSC (voltage source converter) designated controller of the grid-connected converter, namely parameters k of a direct-current voltage control outer ring PI (proportional integral) controller, by using the small-signal model corresponding to the active control path and the small-signal model corresponding to the reactive control path obtained in the step two under the electromechanical time scale p_udc ,k i_udc Parameter k of outer ring PI controller controlled by AC voltage p_vpcc ,k i_vpcc And d-axis and q-axis PI controller parameters k for the current control inner loop p_id 、k p_iq ，k i_id 、k i_iq And a DC capacitance value C, a filter reactance X f And system reactance X s Substituting the data into the formula (6), and qualitatively analyzing an active control path transfer function G between the grid-connected converter VSC and the alternating current power grid under the action of small disturbance of the alternating current power grid by using a frequency response method p (s) and reactive control path transfer function G q (s) logarithmic amplitude-frequency characteristics under the electromechanical time scale, so that the dynamic characteristics of active output and reactive output injected into the alternating current power grid by the grid-connected converter VSC are analyzed qualitatively according to the amplitude in the logarithmic amplitude-frequency characteristics, and the power interaction characteristics under the electromechanical time scale between the grid-connected converter VSC and the alternating current power grid under the small disturbance action of the alternating current power grid are analyzed and obtained according to the analysis results of the dynamic characteristics of the active output and the reactive output, so that the interaction characteristic analysis of the grid-connected converter VSC and the alternating current power grid based on a frequency response method is completed.
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