CN104659815B - Dynamic equivalence method of grid-connected inverter system - Google Patents

Abstract

The invention belongs to the field of power electronics, and relates to a dynamic equivalence method for a multi-grid-connected inverter system, including two steps of grouping and parameter aggregation: defining the dissimilarity to determine the difference between the dynamic admittances of each inverter, and Inverters with small dynamic admittance differences are divided into the same group; the parameter aggregation of inverters in the same group includes two parts: structural parameter aggregation and control parameter aggregation. In the present invention, multiple inverters with similar dynamic characteristics are equivalent to one, and while retaining the dynamic characteristics of the inverters, it effectively reduces the complexity of power system analysis with a large number of inverters and saves computing resources , has important engineering application value.

Description

A dynamic equivalence method for multi-grid-connected inverter systems

technical field

The invention belongs to the field of power electronics and relates to a dynamic equivalent method for a multi-grid-connected inverter system.

Background technique

With the vigorous development of new energy, more and more photovoltaic and wind power are integrated into the grid through inverters. The connection of a large number of inverters makes the grid have new features. In the dynamic analysis of the power grid, the influence of the inverter must be considered. If a detailed model is used for calculation, a large number of inverters will undoubtedly bring a huge amount of calculation and dimension disaster. The dynamic research on the power grid is generally only interested in a certain area, and for the area far away from this area, only its influence on the research area is considered in the study, and its interior does not need to be described in detail, and it can be reduced and analyzed. simplify. By performing equivalent value on the non-research area, it can not only save manpower and material resources greatly, but also ensure the research accuracy required by the project.

There are relatively mature studies on the dynamic equivalent of generators, loads and networks in power systems, among which the more practical dynamic equivalent methods can be divided into three categories: (1) coherent equivalent method; (2) modal equivalent ; (3) Gray box or black box equivalent based on parameter identification. However, there are few researches on the dynamic equivalence of the inverter, and it is simply equivalent to a current source or a negative load according to its output characteristics.

The invention groups the inverters based on the dynamic admittance of the inverters, divides the inverters with similar dynamic characteristics into the same group, and aggregates the inverters in the same group. While simplifying the system, it retains the dynamic characteristics of the inverter and ensures the accuracy of system analysis.

Contents of the invention

In view of the above problems, the present invention proposes a dynamic equivalence method for multiple grid-connected inverters. The present invention performs equivalence for grid-connected inverters connected in parallel to the same busbar, as shown in FIG. 1 . Multiple inverters with similar dynamic characteristics are equivalent to one inverter. The technical solution adopted is divided into two steps:

A step of grouping the grid-connected inverters: divide the M inverters in the grid into N groups, and each group contains W inverters; the specific method is: the inverter Based on the dynamic admittance of the inverter, the dynamic admittance dissimilarity is used as the grouping index of the inverter; the formula for calculating the dynamic admittance dissimilarity between the i-th inverter and the j-th inverter is:

Among them, G _i (jω) and G _j (jω) represent the frequency-domain expressions of the dynamic admittance of the i-th inverter and the j-th inverter after the unit respectively; n is the number of frequency points used in the calculation; ω ₁ and ω _n represent the start frequency point and end frequency point used in the calculation respectively; when the grouping index S _ij is less than a certain small positive real number ε, inverter i and inverter j are in the same group, The size of ε is selected according to the actual precision;

A step of parameter aggregation of inverters connected to the grid: parameter aggregation of W inverters in the same group, which is equivalent to one inverter; the parameter aggregation steps include:

Aggregation 1: Aggregation steps of structural parameters: including the aggregation of DC side capacitance and AC side filter parameters; the DC side capacitance of the equivalent inverter is equal to the sum of the DC side capacitances of each inverter in the group before the equivalent; the equivalent inverter The filter parameters on the AC side of the inverter are equal to the parallel values of the filter parameters in the group before the equivalence; the calculation formula is as follows

Among them, the subscript eq represents the parameters of the equivalent inverter, m refers to the number of inverters in the group, C represents the DC side capacitance, L and R represent the AC side filter inductance and equivalent resistance, respectively;

Aggregation 2: Aggregation steps of control parameters: In order to ensure that the dynamic admittance of the inverter before and after the equivalent value is similar, on the premise of determining the aggregation of structural parameters, the control parameters should satisfy:

Among them, G _Σ (jω) represents the sum of the dynamic admittances of the previous inverters at the equivalent value, G _eq (jω) represents the dynamic admittance of the equivalent inverter, and the expression of G _eq (jω) is:

Among them, Δi _d and Δe _d represent the output port current and voltage disturbance of the equivalent inverter, respectively, i _d0 and e _d0 represent the steady-state value of the output port current and voltage of the equivalent inverter, respectively, and U _dc0 represents the DC side voltage stability value, _is0 is the steady-state value of the DC side input current, k _p and _ki represent the outer loop control parameters of the equivalent inverter respectively.

Description of drawings

FIG. 1 is a structural diagram of a multi-grid-connected inverter system in the present invention.

Fig. 2 is a control block diagram of the control strategy adopted by the inverter in the present invention.

Figure 3 is a comparison diagram of the d-axis current step response of the inverter before and after the equivalent.

detailed description

The present invention will be further elaborated below in conjunction with the accompanying drawings and specific implementation.

The main idea of the present invention is that when the power grid is disturbed, the inverter whose dynamic response is similar is equivalent to an inverter, and the change of the d-axis component of the grid-connected current under the grid-connected voltage disturbance is selected as its dynamic response describe. Explanation of symbols and labels in figures 1, 2, and 3: e _a , e _b , e _c —grid voltage, ia , _ib , _{ic —grid} current, L—grid-connected inverter filter inductance, C — grid-connected inverter DC side capacitance, U _dc — grid-connected inverter DC side voltage, U _dcref — grid-connected inverter DC voltage reference value, i _d grid-connected inverter d-axis current, i _dref — parallel grid inverter d-axis current reference value, i _q — grid-connected inverter q-axis current reference value, i _qref — grid-connected inverter q-axis current reference value, e _d — grid voltage d-axis component, e _q — Grid voltage q-axis component, G _v (s)—the outer loop controller of the grid-connected inverter, G _i (s)—the inner loop controller of the grid-connected inverter.

One, at first introduce the theoretical method of the present invention.

The present invention is divided into two steps:

Step 1: Grouping.

The output characteristics of an inverter are determined by its dynamic admittance, and inverters with similar dynamic admittances have similar dynamic characteristics. Based on the d-axis (active axis) dynamic admittance of the inverter, the dynamic admittance dissimilarity is used as the grouping index of the inverter. The smaller the dissimilarity, the closer the dynamic admittance between the two inverters. The formula for calculating the dynamic admittance dissimilarity between the i-th inverter and the j-th inverter is:

Among them, G _i (jω) and G _j (jω) represent the frequency-domain expressions of the dynamic admittance of the i-th inverter and the j-th inverter after the unit respectively. n is the number of frequency points used in the calculation. ω ₁ and ω _n represent the start frequency point and end frequency point used in the calculation, respectively. When the dissimilarity S _ij is less than a certain small positive real number ε, inverter i and inverter j are in the same group, and the size of ε is selected according to the actual equivalent accuracy.

Step 2: Parameter Aggregation

Perform parameter aggregation for inverters in the same group. Parameter aggregation includes two parts: the aggregation of structural parameters and the aggregation of control parameters.

The aggregation of structural parameters includes the aggregation of DC side capacitance and AC side filter parameters. The DC side capacitance of the equal inverters is equal to the sum of the DC side capacitances of the inverters in the pre-equivalent group; the AC side filter parameters of the equivalent inverters are equal to the parallel values of the filter parameters of the inverters in the pre-equivalent group. Calculated as follows:

Among them, the subscript eq represents the equivalent inverter, i in the following table represents the i-th inverter, m refers to the number of inverters in the group, C represents the DC side capacitor, R and L represent the AC side filter inductance and equal effective resistance.

Since the response speed of the inverter control current inner loop control is very fast, the dynamic process of the current inner loop can be ignored, and it is considered that the inner loop current can quickly track the change of the upper command value, so the aggregation of control parameters only needs to consider the voltage outer loop. In order to ensure that the dynamic admittance of the inverter before and after the equivalence is similar, on the premise that the structural parameters are aggregated and determined, the control parameters should satisfy:

Among them, G _Σ (jω) represents the sum of the dynamic admittances of the previous equivalent inverters, and G _eq (jω) represents the dynamic admittance of the equivalent inverters. According to Figure 2, the expression of G _eq (jω) can be obtained as:

Among them, Δi _d and Δe _d represent the output port current and voltage disturbance of the equivalent inverter, respectively, i _d0 and e _d0 represent the steady-state value of the output port current and voltage of the equivalent inverter, respectively, and U _dc0 represents the DC side voltage stability value, _is0 is the steady-state value of the DC side input current, k _p and _ki represent the outer loop control parameters of the equivalent inverter respectively.

Second, the following is a specific case of using the above method.

Five grid-connected inverters are connected in parallel on the same bus. The specific structure is shown in Figure 1, where n=5 in the figure. The control block diagram of the grid-connected inverter is shown in Figure 2. Neglecting the dynamic process of the inner loop, it is considered that i _d = _{id ref} . The structure and control parameters of each inverter are shown in Table 1. The equivalent inverter has the same structure and control algorithm as the single inverter before the equivalent. Then the admittance of the equivalent value inverter can be expressed as (4) type.

Table 1 inverter parameters

Based on the above parameters, the dynamic admittance of each inverter is obtained. Using formula (1), the calculation results of the dissimilarity between the above five inverters are shown in Table 2.

Table 2 Calculation results of dissimilarity

Since the actual calculated dissimilarity is very small, the dissimilarity in Table 2 is the data obtained by multiplying the actual calculated result by 10 ⁶ . Take ε=8, according to Table 2, inverters 1 and 2 can be divided into one group, which is marked as group A; inverters 3, 4, and 5 can be divided into another group, which is marked as group B.

Perform parameter aggregation for the inverters in group A and group B. According to the aggregation of the structural parameters according to the formula (2), the aggregation of the control parameters is carried out according to the formula (3). The obtained equivalent inverter parameters are shown in Table 3.

Table 3 Equivalent inverter parameters

Comparing the equivalent inverter with the detailed model before equivalent, the step response is shown in Figure 3.

The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which the present invention belongs can make various modifications or supplements to the described specific embodiments or adopt similar methods to replace them, but they will not deviate from the spirit of the present invention or go beyond the definition of the appended claims range.

Claims (1)

1. a kind of many grid-connected inverter systems Dynamic Equivalence it is characterised in that：

One will be grid-connected in the inverter step that carries out point group：M inverter in will be grid-connected is divided into N number of group, in each group Include W inverter；Concrete grammar is：Based on the motional admittance of inverter, using motional admittance distinctiveness ratio as inversion Point group's index of device；The computing formula of i-th inverter and jth platform inverter motional admittance distinctiveness ratio is：

Wherein, G_i(j ω) and G_j(j ω) represents the frequency of i-th inverter and jth platform inverter motional admittance after perunit respectively Domain expression formula；The frequency points that n uses when being and calculating；ω₁And ω_nRepresent the initial frequency point used when calculating and end respectively Frequency point；Divide group's index S_ijDuring less than some little arithmetic number ε, inverter i and inverter j divides in same group, the size of ε Need to choose according to available accuracy；

One will be grid-connected in the inverter step that carries out parameter aggregation：W inverter in same group is entered with line parameter gather Close, be equivalent to an inverter；The step of this parameter aggregation includes：

Polymerization one：The polymerization procedure of structural parameters：Polymerization including DC bus capacitor and wave filter on AC side parameter；Equivalent inversion Device DC bus capacitor is equal to each inverter direct-flow side electric capacity sum in equivalent pre-group；Equivalent inverter ac side filter parameter etc. The parallel value of each filter parameter in equivalent pre-group；Computing formula is as follows

Wherein, subscript eq represents the parameter of equivalent inverter, and m refers to the number of units of inverter in group, and C represents DC bus capacitor, L and R Represent the equivalent resistance of AC filter inductance and AC respectively；

Polymerization two：The polymerization procedure of control parameter：Close for inverter motional admittance before and after ensureing equivalence, in structural parameters polymerization On the premise of determination, control parameter should meet：

Wherein, G_Σ(j ω) represents equivalent previous inverter motional admittance sum, G_eq(j ω) represents the dynamic of equivalent inverter Admittance, G_eqThe expression formula of (j ω) is：

Wherein, Δ i_dWith Δ e_dRepresent equivalent inverter output end mouth electric current and voltage disturbance amount, i respectively_d0And e_d0Represent respectively Equivalent inverter output end mouth electric current and voltage steady-state value, U_dc0Represent DC-side Voltage Stabilization value, i_s0It is DC side input current Steady-state value, k_pAnd k_iRepresent equivalent inverter outer shroud control parameter respectively.