A method and system for controlling inverter network construction with improved frequency support capability

Abstract

The present application provides a method and system for controlling a grid-type converter that improves the frequency support capability, and belongs to the field of new power system control technology. The method includes: based on the DC voltage control loop of the grid-type converter, collecting the DC bus voltage of the grid-type converter to obtain the phase angle of the virtual internal potential of the grid-type converter; at the same time, based on the reactive power control loop of the grid-type converter, obtaining the amplitude of the virtual internal potential of the grid-type converter; synthesizing the virtual internal potential phase angle and amplitude to obtain a virtual internal potential vector, and after passing through the voltage loop and the current loop as a PWM modulation signal, obtain the control signal of the switch tube of the grid-type converter to control the grid-type converter. The present application solves the problems of inertia reduction and insufficient frequency support caused by the access of a high proportion of new energy devices to the power grid, eliminates the lowest point of the power grid frequency, and ensures the safe and stable operation of the power grid frequency under the access of a high proportion of new energy devices.

Description

Technical Field

[0001] The present application belongs to the field of novel power system control technology, and more specifically, relates to a converter network control method and system for improving frequency support capability. Background Art

[0002] With the widespread application of renewable energy in power grids, the traditional power system dominated by synchronous generators has gradually transformed into a power system dominated by renewable energy. The renewable energy connected to the grid through converters has almost no inertia response capability, which leads to under-frequency load switching, SG under-frequency relay tripping, and even system power outages. Therefore, it is urgent to increase the inertia of the renewable energy-dominated power system, limit the lowest frequency point, and ensure the safe and stable operation of the power grid.

[0003] In order to improve the inertia of new energy, grid-following control introduces a frequency difference adjustment link in the active power outer loop of the converter, drawing on the operating experience of traditional power systems, so that the converter has external characteristics similar to those of a synchronous generator. The power command of the converter is directly modified or the power command is indirectly modified through the DC voltage, thereby changing the active output of new energy under grid frequency disturbance. However, grid-following control relies on a phase-locked loop to synchronize the converter with the grid. The interaction between the phase-locked loop and the grid can easily cause problems such as subsynchronous oscillation and harmonic oscillation, which deteriorates system stability.

[0004] As the stability problem of weak grids continues to become prominent, grid-forming control has begun to attract widespread attention. Grid-forming control does not require additional phase-locked links and can spontaneously provide active frequency support to the grid, which helps to improve the interactive stability of the converter and the weak grid. At present, the most representative grid-forming control is virtual synchronous control. By drawing on the swing equation of the synchronous generator, the grid-forming converter based on virtual synchronous control has inertia characteristics. In fact, actual new energy generally operates in maximum power point tracking mode to make full use of renewable energy, which requires the function of stabilizing the DC side voltage. However, virtual synchronous control achieves synchronization through active power-frequency control and cannot take into account the control of the DC side voltage. In order to achieve DC side voltage control and converter synchronization at the same time, a grid-forming control method using DC capacitors to dynamically synchronize the grid is proposed. According to the dynamic similarity between the converter and the synchronous generator, the DC capacitor voltage directly generates the grid frequency reference, and synchronization can be achieved without active power but only with DC voltage, which can ensure the maximum power point tracking control of new energy. The equivalence of the DC voltage synchronous grid-type converter and the synchronous generator in the mathematical model and the inertia size that can be provided are studied, revealing that the DC side capacitor size is equivalent to the inertia constant of the synchronous machine, and the DC voltage is equivalent to the rotor rotation speed of the synchronous machine. However, due to the linear mapping relationship between the DC bus voltage and the grid frequency, in the power system dominated by renewable energy, when the DC voltage drops to the lowest point, the grid frequency will also drop to the lowest point, affecting the system frequency safety. Summary of the invention

[0005] In view of the defects of the prior art, the purpose of this application is to provide a converter grid control method and system with improved frequency support capability, aiming to solve the problem that based on grid-based converters, due to the linear mapping relationship between the DC bus voltage and the grid frequency, in the power system dominated by renewable energy, when the DC voltage drops to the lowest point, the grid frequency will also drop to the lowest point, affecting the frequency safety of the grid system.

[0006] To achieve the above objectives, on the one hand, the present application provides a method for controlling a converter network with improved frequency support capability, comprising the following steps:

[0007] S1: According to the DC voltage control loop of the grid-type converter, the DC bus voltage of the grid-type converter is collected to obtain the phase angle of the virtual internal potential of the grid-type converter; at the same time, according to the reactive power control loop of the grid-type converter, the amplitude of the virtual internal potential of the grid-type converter is obtained;

[0008] Among them, a hysteresis compensator link is set in the DC voltage control loop; a damping compensation link of the DC voltage control loop is set in the reactive power control loop;

[0009] S2: A virtual internal potential vector is obtained by synthesizing the virtual internal potential phase angle obtained by the DC voltage control loop and the virtual internal potential amplitude obtained by the reactive power control loop. After passing through the voltage loop and the current loop, it is used as a PWM modulation signal. The control signal of the grid-type converter switch tube is obtained through the PWM modulator to control the main circuit of the grid-type converter.

[0010] Further preferably, the DC voltage control loop of the grid-connected converter is expressed as:

[0011]

[0012] Where, ω represents the DC bus voltage loop angular frequency of the grid-type converter; u dc is the DC bus voltage of the grid-type converter; u dcn is the given value of the DC bus voltage of the grid-type converter; K0 is the coupling coefficient between the DC bus voltage and the angular frequency; a and b are the compensator coefficients of the hysteresis compensator; ω n is the given value of the DC bus voltage angular frequency of the grid-type converter; s is the Laplace operator; θ is the phase angle of the virtual internal potential of the grid-type converter.

[0013] Further preferably, the method for obtaining the amplitude of the virtual internal potential of the grid-connected converter based on the reactive power control loop is:

[0014] Collect the grid-connected point voltage and current of the grid-connected converter, calculate the grid-connected point reactive power, compare the grid-connected point reactive power with the reactive power reference value, and obtain the reactive power control error signal;

[0015] The error signal of reactive power control is subjected to droop control and is subtracted from the damping compensation link of the DC voltage control loop to obtain the amplitude of the virtual internal potential of the grid-connected converter.

[0016] Further preferably, the reactive power control loop of the grid-connected converter is expressed as:

[0017] E=-(QQ ref )·K q +E n -(u dc -u dcn )·M

[0018] Where, E represents the amplitude of the virtual internal potential of the grid-connected converter, K q represents the reactive power-voltage droop coefficient, Q represents the reactive power of the grid-connected point of the grid-connected converter, Q ref Indicates the reactive power given value, E n represents the given value of AC voltage, and M represents the damping coefficient;

[0019] Further preferably, the value range of the lag compensator parameter is obtained according to the frequency minimum point elimination principle, and the frequency minimum point elimination principle satisfies the following conditions:

[0020]

[0021] or p1+p2≤2z1;

[0022] Wherein, z1 represents the zero point of the system frequency response model considering the frequency modulation effect of the grid-type converter; p1 and p2 represent the two poles of the system frequency response model considering the frequency modulation effect of the grid-type converter;

[0023] The system frequency response model is based on the change rate of the power grid load ΔP L To the grid angular frequency change Δω g The system transfer function G PL_ωg (s) is:

[0024]

[0025] Where, M3 represents the coefficient of the system transfer function;

[0026] Grid angular frequency change Δω g The rate of change of the converter output power ΔP e The system transfer function G ωg_Pe (s) is:

[0027]

[0028] Among them, T J Represents the DC voltage coefficient, G eq Indicates the power factor, K dc Denotes the energy storage coefficient, D N represents the equivalent damping coefficient; K0 is the coupling coefficient between the DC bus voltage and the angular frequency; a and b are the compensator coefficients of the hysteresis compensator; ω n is the given value of the DC bus voltage angular frequency of the grid-connected converter; s is the Laplace operator.

[0029] On the other hand, the present application provides a converter network control system for improving frequency support capability, including:

[0030] A DC voltage control circuit is used to provide a DC voltage control loop, collect the DC bus voltage of the grid-forming converter, and obtain the phase angle of the virtual internal potential of the grid-forming converter;

[0031] A reactive power control circuit is used to provide a reactive power control loop to obtain the amplitude of the virtual internal potential of the grid-forming converter;

[0032] Among them, a hysteresis compensator is provided in the DC voltage control circuit; a damping compensator is provided in the reactive power control circuit;

[0033] A virtual internal potential vector synthesis circuit is used to synthesize a virtual internal potential vector according to a virtual internal potential phase angle obtained by a DC voltage control loop and a virtual internal potential amplitude obtained by a reactive power control loop;

[0034] The voltage-current loop circuit is used to convert the virtual internal potential vector into a PWM modulation signal after passing through the voltage loop and the current loop;

[0035] The PWM modulator is used to receive the PWM signal and obtain the control signal of the switch tube of the grid-type converter so as to control the main circuit of the grid-type converter.

[0036] Further preferably, the DC voltage control loop in the DC voltage control circuit is expressed as:

[0037]

[0038] Where, ω represents the DC bus voltage loop angular frequency of the grid-type converter; u dc is the DC bus voltage of the grid-type converter; u dcn is the given value of the DC bus voltage of the grid-type converter; K0 is the coupling coefficient between the DC bus voltage and the angular frequency; a and b are the compensator coefficients of the hysteresis compensator; ω n is the given value of the DC bus voltage angular frequency of the grid-type converter; s is the Laplace operator; θ is the phase angle of the virtual internal potential of the grid-type converter.

[0039] Further preferably, the reactive power control circuit comprises: a grid connection point reactive power calculation unit, a first adding unit, a droop control unit and a second adding unit;

[0040] The grid-connected point reactive power calculation unit is used to collect the grid-connected point voltage and current of the grid-connected converter and calculate the grid-connected point reactive power;

[0041] The first adding unit is used to compare the reactive power of the grid connection point with the reactive power reference value to obtain an error signal for reactive power control;

[0042] The droop control unit is used to perform droop control on the error signal of reactive power control;

[0043] The second adding unit is used for subtracting the result output by the droop control unit from the damping compensation link of the DC voltage control loop to obtain the amplitude of the virtual internal potential of the grid-type converter.

[0044] Further preferably, the reactive power control loop in the reactive power control circuit is expressed as:

[0045] E=-(QQ ref )·K q +E n -(u dc -u dcn )·M

[0046] Where, E represents the amplitude of the virtual internal potential of the grid-connected converter, K q represents the reactive power-voltage droop coefficient, Q represents the reactive power of the grid-connected point of the grid-connected converter, Q ref Indicates the reactive power given value, E n represents the given value of AC voltage, and M represents the damping coefficient.

[0047] Further preferably, the DC voltage control circuit obtains the value range of the lag compensator parameter according to the frequency minimum point elimination principle, and the frequency minimum point elimination principle satisfies the following conditions:

[0048]

[0049] or p1+p2≤2z1;

[0050] Wherein, z1 represents the zero point of the system frequency response model considering the frequency modulation effect of the grid-type converter; p1 and p2 represent the two poles of the system frequency response model considering the frequency modulation effect of the grid-type converter;

[0051] The system frequency response model is based on the change rate of the power grid load ΔP L To the grid angular frequency change Δω g The system transfer function G PL_ωg (s) is:

[0052]

[0053] Where, M3 represents the coefficient of the system transfer function;

[0054] Grid angular frequency change Δω g The rate of change of the converter output power ΔP e The system transfer function G ωg_Pe (s) is:

[0055]

[0056] Among them, T J Represents the DC voltage coefficient, G eq Indicates the power factor, K dc Denotes the energy storage coefficient, D N represents the equivalent damping coefficient; K0 is the coupling coefficient between the DC bus voltage and the angular frequency; a and b are the compensator coefficients of the hysteresis compensator; ω nis the given value of the DC bus voltage angular frequency of the grid-connected converter; s is the Laplace operator.

[0057] In general, the above technical solutions conceived by this application have the following beneficial effects compared with the prior art:

[0058] Focusing on the new power system with a high proportion of new energy penetration, the DC bus voltage and the grid frequency are linearly mapped in the grid-forming control method based on the DC bus voltage dynamics. When the DC voltage drops to the lowest point, the grid frequency will also drop to the lowest point, affecting the system frequency safety. Based on this, the present application provides a converter grid-forming control method that improves the frequency support capability, reshapes the system frequency dynamics (wherein the system frequency is proportional to the angular frequency of the DC bus voltage loop of the grid-forming converter), so that the system frequency curve no longer linearly tracks the shape of the DC bus voltage curve (wherein the DC voltage control loop in the DC voltage control circuit is expressed as: u dc is the DC bus voltage of the grid-type converter; ω represents the DC bus voltage loop angular frequency of the grid-type converter), thereby eliminating the lowest frequency point and ensuring the safe and stable operation of the new power system. This solves the problems of inertia and damping drop, insufficient voltage and frequency support caused by the access of a high proportion of new energy devices to the grid, and helps the construction and development of new power systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 is a flow chart of an analysis method of a converter network control method provided in an embodiment of the present application;

[0060] Figure 2 is a topological diagram of a grid-type converter provided in an embodiment of the present application;

[0061] Figure 3 is a schematic diagram of similarity between a synchronous machine and a converter provided in an embodiment of the present application;

[0062] Figure 4 It is a structural diagram of a DC voltage control loop and a reactive power control loop of a grid-type converter provided in an embodiment of the present application;

[0063] Figure 5 It is a system frequency response model diagram based on the network control of the present application provided in an embodiment of the present application;

[0064] Figure 6 It is the change trajectory of the poles p1 and p2 of the system frequency response model based on the network control of the present application along with the lag compensator parameters a and b provided by the embodiment of the present application;

[0065] Figure 7It is the change trajectory of the zero point z1 of the system frequency response model based on the network control of the present application along with the lag compensator parameters a and b provided by the embodiment of the present application;

[0066] Figure 8 It is a feasible domain for eliminating the lowest frequency point based on the network control of the present application provided in the embodiment of the present application;

[0067] Fig. 9 It is a schematic diagram of the principles of the network control method of the present application and the traditional method provided in the embodiments of the present application;

[0068] Fig.10 It is a three-machine nine-node experimental topology diagram provided in an embodiment of the present application;

[0069] Fig.11 are active power curves obtained by using the grid-building control method provided by the present application and the traditional control method respectively;

[0070] Fig.12 are DC bus voltage curves obtained by using the grid-forming control method provided by the present application and the traditional control method respectively;

[0071] Fig.13 These are the system frequency response curves obtained by using the network control method provided by the present application and the traditional control method respectively. DETAILED DESCRIPTION

[0072] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0073] The terms “first”, “second” and the like in the specification and claims herein are used to distinguish different objects rather than to describe a specific order of the objects.

[0074] In the embodiments of the present application, words such as "exemplary" or "for example" are used to indicate examples, illustrations or descriptions. Any embodiment or design described as "exemplary" or "for example" in the embodiments of the present application should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of words such as "exemplary" or "for example" is intended to present related concepts in a specific way.

[0075] In the description of the embodiments of the present application, unless otherwise specified, “plurality” means two or more than two.

[0076] The present application provides a method for controlling a converter network to improve frequency support capability, comprising the following steps:

[0077] Step 1: According to the DC voltage control loop of the grid-type converter, the DC bus voltage of the grid-type converter is collected to obtain the phase angle of the virtual internal potential of the grid-type converter; wherein a hysteresis compensator link is provided in the DC voltage control loop; at the same time, according to the reactive power control loop of the grid-type converter plus the damping compensation link of the DC voltage control loop, the amplitude of the virtual internal potential of the grid-type converter is obtained;

[0078] Specifically, the DC voltage control loop of the grid-connected converter is expressed as:

[0079]

[0080] Where, ω represents the DC bus voltage loop angular frequency of the grid-type converter; u dc is the DC bus voltage of the grid-type converter; u dcn is the given value of the DC bus voltage of the grid-type converter; K0 is the coupling coefficient between the DC bus voltage and the angular frequency; a and b are the compensator coefficients of the hysteresis compensator; ω n is the given value of the DC bus voltage angular frequency of the grid-type converter; s is the Laplace operator; θ is the phase angle of the virtual internal potential of the grid-type converter;

[0081] More specifically, the design method of the reactive power control loop is as follows: collecting the grid connection point voltage and current of the grid-connected converter, calculating the grid connection point reactive power, comparing the grid connection point reactive power with the reactive power reference value, and obtaining the reactive power control error signal; performing droop control on the reactive power control error signal, and subtracting it from the damping compensation link of the DC voltage control loop to obtain the amplitude of the virtual internal potential of the grid-connected converter;

[0082] Further preferably, the reactive power control loop of the grid-connected converter is expressed as:

[0083] E=-(QQ ref )·K q +E n -(u dc -u dcn )·M

[0084] Where, E represents the amplitude of the virtual internal potential of the grid-connected converter, K q represents the reactive power-voltage droop coefficient, Q represents the reactive power of the grid-connected point of the grid-connected converter, Q ref Indicates the reactive power given value, E n represents the given value of AC voltage, and M represents the damping coefficient;

[0085] More specifically, considering the dynamics of the center of inertia in the power system, the entire power grid system can be modeled as an equivalent synchronous machine; e.g. Figure 5The system frequency response model of the single-region power grid system is shown, which can be equivalent to a first-order inertial system. It should be noted that it is only applicable to small signal analysis. In the system frequency response model, the influence of the optimized grid control is Figure 5 It is marked in the figure, that is, the grid angular frequency change Δω g The rate of change of the converter output power ΔP e The system transfer function is:

[0086]

[0087] Among them, T J Represents the DC voltage coefficient, G eq Indicates the power factor, K dc Denotes the energy storage coefficient, D N represents the equivalent damping coefficient;

[0088] according to Figure 5 The system frequency response model can be simplified to obtain the rate of change of the power grid load ΔP L To the grid angular frequency change Δω g The system transfer function is:

[0089]

[0090] Wherein, z1 represents the zero point of the system frequency response model considering the frequency modulation of the grid-type converter; p1 and p2 represent the two poles of the system frequency response model considering the frequency modulation of the grid-type converter; M3 represents the coefficient of the system transfer function;

[0091] More specifically, the root locus diagram of the system frequency response model is as follows: Figure 6 As shown in the figure, by drawing the parameter root locus, we can get the trajectory curves of p1 and p2 changing with the changes of the lag compensator parameters a and b. Similarly, we can get the trajectory curves of z1 changing with the changes of the lag compensator parameters a and b, as shown in the figure. Figure 7 shown.

[0092] More specifically, the lowest frequency point of the system can be eliminated only when p1, p2 and z1 first satisfy the condition that they are all on the real axis. Therefore, the zeros and poles in this application refer to the absolute values ​​of the coordinates on the real axis.

[0093] More specifically, according to the principle of eliminating the lowest frequency point, the constraints of p1, p2 and z1 can be obtained, and the value range of the lag compensator parameters can be further obtained. The basis theorem of the constraints of p1, p2 and z1 is:

[0094]

[0095] Or the theorem:

[0096] p1+p2≤2z1

[0097] According to the above theorem, the parameter range of the lag compensator that can eliminate the lowest frequency point can be drawn, such as Figure 8 As shown; when the parameters are selected within the feasible range within the shaded range in the figure, the lowest frequency point can be eliminated. Considering the parameter margin range, the lag compensator parameters can be selected as (a=16.7, b=1.2);

[0098] Step 2: The virtual internal potential vector is synthesized based on the virtual internal potential phase angle obtained by the DC voltage control loop and the virtual internal potential amplitude obtained by the reactive power control loop. After passing through the voltage loop and the current loop, it is used as the PWM modulation signal. The control signal of the grid-type converter switch tube is obtained through the PWM modulator to control the main circuit of the grid-type converter.

[0099] Example

[0100] This embodiment takes a three-machine nine-node circuit as an example. Figure 3 As shown, the circuit is composed of two synchronous machines and a grid-type converter, wherein the grid-type converter is composed of 200 500kVW sub-converters. The benchmark parameters of the grid-type converter considering the DC bus voltage are shown in Table 1.

[0101] Table 1

[0102]

[0103] like Figure 1 As shown, the present application provides a method for controlling a converter network with improved frequency support capability, and a preferred implementation scheme of the present application is described in detail below.

[0104] Figure 2 The main topology of the grid-connected system of the grid-connected converter is shown, including new energy, energy storage system, DC side capacitor C, converter, filter inductor L f and grid-side inductance L g , where E is the internal potential voltage; U t is the terminal voltage; U g is the grid-side voltage; U dc is the DC bus voltage; P R is the output power of new energy; P sto is the output power of the energy storage system; P in and P out are the active power input to the converter and the active power injected into the grid by the converter respectively; according to the energy conversion characteristics of the DC bus capacitor, we can get:

[0105]

[0106] Where C is the DC capacitance value; U dc0 is the initial value of the DC voltage of the grid-connected converter; U dc is the DC voltage value of the grid-type converter; P in is the input power; P e is the output power; and the rotor motion equation of the traditional synchronous machine is expressed as:

[0107]

[0108] Wherein, J represents the moment of inertia of the synchronous machine;

[0109] By comparing the above two equations, the dynamic equation of the converter DC bus voltage can be compared with the rotor motion equation of the synchronous machine, such as Figure 4 As shown, the coupling relationship between the DC bus voltage and angular velocity can be established, and it can be obtained:

[0110]

[0111] The above formula describes the relationship between the change in DC bus voltage and angular frequency of the grid-type converter; in order to reshape the frequency curve, a hysteresis compensator is added to the coupling coefficient of the DC voltage control loop, which can achieve the active balance between the DC side and the AC side of the grid-type converter through the charging and discharging of the DC side capacitor C, and achieve the effect of eliminating the lowest frequency point. Figure 4 As shown in the figure, the DC voltage control loop of the grid-connected converter is expressed as:

[0112]

[0113] Where, ω represents the DC bus voltage loop angular frequency of the grid-type converter; u dc is the DC bus voltage of the grid-type converter; u dcn is the given value of the DC bus voltage of the grid-type converter; K0 is the coupling coefficient between the DC bus voltage and the angular frequency; a and b are the compensator coefficients of the hysteresis compensator; ω n is the given value of the DC bus voltage angular frequency of the grid-type converter; s is the Laplace operator; θ is the phase angle of the virtual internal potential of the grid-type converter;

[0114] Considering the dynamics of the center of inertia in the power system, the entire power grid system can be modeled as an equivalent synchronous machine; Figure 5 The system frequency response model of a single-area power system is shown, which can be equivalent to a first-order inertial system; it should be noted that this is only applicable to small signal analysis; in the system frequency response model, the impact of the optimized grid control is marked in the figure;

[0115] According to the system frequency response model, the change rate of the grid load ΔP can be obtained.L To the grid angular frequency change Δω g The system transfer function is:

[0116]

[0117] Wherein, z1 represents the zero point of the system frequency response model considering the frequency modulation of the grid-type converter; p1 and p2 represent the two poles of the system frequency response model considering the frequency modulation of the grid-type converter; M3 represents the coefficient of the system transfer function;

[0118] Further preferably, the root locus diagram of the system frequency response model is as follows: Figure 6 As shown in the figure, by drawing the parameter root locus, we can get the trajectory curves of p1 and p2 changing with the changes of the lag compensator parameters a and b. Similarly, we can get the trajectory curves of z1 changing with the changes of the lag compensator parameters a and b, as shown in the figure. Figure 7 shown.

[0119] Further preferably, the lowest frequency point of the system can be eliminated only when p1, p2 and z1 first satisfy the condition that they are all on the real axis. Therefore, the zeros and poles in this application refer to the absolute values ​​of the coordinates on the real axis.

[0120] Further preferably, according to the principle of eliminating the lowest frequency point, the constraints of p1, p2 and z1 can be obtained, and the value range of the lag compensator parameters can be further obtained. The basis theorem of the constraints of p1, p2 and z1 is:

[0121]

[0122] Or the theorem:

[0123] p1+p2≤2z1

[0124] According to the above theorem, the parameter range of the lag compensator that can eliminate the lowest frequency point can be drawn, such as Figure 8 When the parameters are selected within the feasible range within the shaded range in the figure, the lowest frequency point can be eliminated. Considering the parameter margin range, the lag compensator parameters can be selected as (a=16.7, b=1.2);

[0125] based on Figure 2 ,like Figure 4 As shown, the reactive power control of the grid-connected converter adopts droop control, the reactive power of the grid-connected point is compared with the reactive power reference value, the error signal of the reactive power control is subjected to droop control, and the amplitude of the virtual internal potential of the grid-connected converter is obtained by subtracting it from the damping compensation link of the DC voltage control loop;

[0126] E=-(QQ ref )·Kq +E n -(u dc -u dcn )·M

[0127] Where, E represents the amplitude of the virtual internal potential of the grid-connected converter, K q represents the reactive power-voltage droop coefficient, Q represents the reactive power of the grid-connected point of the grid-connected converter, Q ref Indicates the reactive power given value, E n represents the given value of AC voltage, and M represents the damping coefficient;

[0128] Finally, the control circuit of the grid-type converter is obtained as follows: Figure 4 As shown, the process of controlling the main circuit of the grid-type converter by using the control circuit is as follows:

[0129] S1: Collect the DC bus voltage u of the grid-type converter dc , according to the relationship between the DC voltage control loop angular frequency and the DC bus voltage of the reshaped grid-type converter, the output angular frequency is obtained:

[0130]

[0131] Then the phase angle of the virtual internal potential of the grid-type converter is obtained;

[0132]

[0133] Specifically, the controller parameters and operating parameters of the grid-connected converter are shown in Table 1;

[0134] At the same time, the grid-connected point voltage and current of the grid-connected converter are collected, the grid-connected point reactive power is calculated and compared with the reactive power reference value, the obtained reactive power control error signal is subjected to droop control, and the amplitude of the virtual internal potential of the grid-connected converter is obtained by subtracting it from the damping compensation link of the DC voltage control loop;

[0135] E=-(QQ ref )·K q +E n -(u dc -u dcn )·M

[0136] S2: The phase and amplitude of the virtual internal potential are finally synthesized into a virtual internal potential; the virtual internal potential is used as a PWM modulation signal after passing through the voltage and current loop, and the control signal of the switch tube of the grid-type converter is obtained through the PWM modulator to control the main circuit of the grid-type converter; due to the added hysteresis compensator playing a filtering role, the grid frequency directly eliminates the lowest point, such as Fig. 9 shown.

[0137] In order to further demonstrate the implementation effect of this application, the network optimization control method proposed in this application is simulated with three machines and nine nodes, as shown in the following figure: Fig.10 As shown;

[0138] Fig.11 The figure shows the power output waveforms of the grid-based optimization control method proposed in the present application and the traditional method under the frequency event of load input at node 6; it can be seen that compared with the traditional method, the method proposed in the present application can output a larger output active power peak and a larger energy capacity;

[0139] Fig.12 The figure shows the DC bus voltage waveforms under the grid-type optimization control method proposed in the present application and the traditional method when the frequency event of load input occurs at node 6; it can be seen that compared with the traditional method, the DC bus voltage fluctuation amplitude of the method proposed in the present application is larger, and more energy is output to the system;

[0140] Fig.13 The figure shows the system frequency waveforms under the grid-based optimization control method proposed in this application and the traditional method when a load is put into the frequency event at node 6; it can be seen that, compared with the traditional method, the method proposed in this application can completely eliminate the lowest frequency point and greatly improve the frequency change rate.

[0141] In summary, compared with the prior art, the present application has the following advantages:

[0142] Focusing on the new power system with a high proportion of new energy penetration, the linear mapping relationship between the DC bus voltage and the grid frequency in the grid-forming control method based on the dynamics of the DC bus voltage is characterized; when the DC voltage drops to the lowest point, the grid frequency will also drop to the lowest point, affecting the system frequency safety; based on this, this application proposes a converter grid-forming control method that improves the frequency support capability, reshapes the system frequency dynamics (the system frequency is proportional to the DC bus voltage loop angle frequency of the grid-forming converter), so that the system frequency curve shape no longer linearly tracks the DC bus voltage curve shape, thereby eliminating the lowest frequency point and ensuring the safe and stable operation of the new power system. This solves the problems of inertia and damping reduction, insufficient voltage and frequency support caused by the access of a high proportion of new energy devices to the grid, and helps the construction and development of new power systems.

[0143] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A method for controlling a converter network to improve frequency support capability, characterized in that: The following steps are involved: S1: According to the DC voltage control loop of the grid-type converter, the DC bus voltage of the grid-type converter is collected to obtain the phase angle of the virtual internal potential of the grid-type converter; at the same time, according to the reactive power control loop of the grid-type converter, the amplitude of the virtual internal potential of the grid-type converter is obtained; Among them, a hysteresis compensator link is set in the DC voltage control loop; a damping compensation link of the DC voltage control loop is set in the reactive power control loop; S2: A virtual internal potential vector is synthesized based on the virtual internal potential phase angle obtained by the DC voltage control loop and the virtual internal potential amplitude obtained by the reactive power control loop, and is used as a PWM modulation signal after passing through the voltage loop and the current loop. The control signal of the switch tube of the grid-type converter is obtained through the PWM modulator to control the main circuit of the grid-type converter; Among them, the value range of the lag compensator parameters is obtained according to the frequency minimum point elimination principle, and the frequency minimum point elimination principle satisfies the following conditions: or ; in, It represents the zero point of the system frequency response model considering the frequency modulation effect of the grid-connected converter; p 1 and p 2 represents the two poles of the system frequency response model considering the frequency modulation effect of the grid-connected converter; The system frequency response model is based on the change rate of the power grid load Δ P L To the grid angular frequency change Δ The system transfer function for: in, represents the coefficients of the system transfer function; Grid angular frequency change Δ The rate of change of the output power of the converter Δ P e The system transfer function for: in, T J represents the DC voltage coefficient, G eq represents the power factor, K dc represents the energy storage coefficient, D N represents the equivalent damping coefficient; is the coupling coefficient between the DC bus voltage and the angular frequency; and is the compensator coefficient of the hysteresis compensator; is the given value of the DC bus voltage angular frequency of the grid-type converter; is the Laplace operator.

2. The method for controlling the inverter network construction according to claim 1, characterized in that: The DC voltage control loop of the grid-connected converter is expressed as: in, It represents the DC bus voltage loop angular frequency of the grid-type converter; is the DC bus voltage of the grid-type converter; is the DC bus voltage given value of the grid-type converter; is the coupling coefficient between the DC bus voltage and the angular frequency; and is the compensator coefficient of the hysteresis compensator; is the given value of the DC bus voltage angular frequency of the grid-type converter; is the Laplace operator; is the phase angle of the virtual internal potential of the grid-type converter.

3. The converter network control method according to claim 1, characterized in that: The method for obtaining the amplitude of the virtual internal potential of the grid-connected converter based on the reactive power control loop is: Collect the grid-connected point voltage and current of the grid-connected converter, calculate the grid-connected point reactive power, compare the grid-connected point reactive power with the reactive power reference value, and obtain the reactive power control error signal; The error signal of reactive power control is subjected to droop control and is subtracted from the damping compensation link of the DC voltage control loop to obtain the amplitude of the virtual internal potential of the grid-connected converter.

4. The method for controlling the inverter network construction according to claim 3, characterized in that: The reactive power control loop of the grid-connected converter is expressed as: in, E represents the amplitude of the virtual internal potential of the grid-connected converter, K q Represents the reactive power-voltage droop coefficient, Q Indicates the reactive power of the grid-connected point of the grid-connected converter, Q ref Indicates the reactive power given value, E n Indicates the AC voltage given value, M represents the damping coefficient; is the DC bus voltage of the grid-type converter; It is the given value of DC bus voltage of grid-connected converter.

5. A converter network control system for improving frequency support capability, characterized in that: include: A DC voltage control circuit is used to provide a DC voltage control loop, collect the DC bus voltage of the grid-forming converter, and obtain the phase angle of the virtual internal potential of the grid-forming converter; A reactive power control circuit is used to provide a reactive power control loop to obtain the amplitude of the virtual internal potential of the grid-forming converter; Among them, a hysteresis compensator is provided in the DC voltage control circuit; a damping compensator is provided in the reactive power control circuit; A virtual internal potential vector synthesis circuit is used to synthesize a virtual internal potential vector according to a virtual internal potential phase angle obtained by a DC voltage control loop and a virtual internal potential amplitude obtained by a reactive power control loop; The voltage-current loop circuit is used to convert the virtual internal potential vector into a PWM modulation signal after passing through the voltage loop and the current loop; A PWM modulator is used to receive a PWM signal and obtain a control signal of a switch tube of a grid-type converter so as to control a main circuit of the grid-type converter; Among them, the DC voltage control circuit obtains the value range of the lag compensator parameter according to the frequency minimum point elimination principle, and the frequency minimum point elimination principle meets the following conditions: or ; in, It represents the zero point of the system frequency response model considering the frequency modulation effect of the grid-connected converter; p 1 and p 2 represents the two poles of the system frequency response model considering the frequency modulation effect of the grid-connected converter; The system frequency response model is based on the change rate of the power grid load Δ P L To the grid angular frequency change Δ The system transfer function for: in, represents the coefficients of the system transfer function; Grid angular frequency change Δ The rate of change of the output power of the converter Δ P e The system transfer function for: in, T J represents the DC voltage coefficient, G eq represents the power factor, K dc represents the energy storage coefficient, D N represents the equivalent damping coefficient; is the coupling coefficient between the DC bus voltage and the angular frequency; and is the compensator coefficient of the hysteresis compensator; is the given value of the DC bus voltage angular frequency of the grid-type converter; is the Laplace operator.

6. The converter network control system according to claim 5, characterized in that: The DC voltage control loop in the DC voltage control circuit is expressed as: in, It represents the DC bus voltage loop angular frequency of the grid-type converter; is the DC bus voltage of the grid-type converter; is the DC bus voltage given value of the grid-type converter; is the coupling coefficient between the DC bus voltage and the angular frequency; and is the compensator coefficient of the hysteresis compensator; is the given value of the DC bus voltage angular frequency of the grid-type converter; is the Laplace operator; is the phase angle of the virtual internal potential of the grid-type converter.

7. The converter network control system according to claim 5, characterized in that: The reactive power control circuit comprises: a grid-connected point reactive power calculation unit, a first adding unit, a droop control unit and a second adding unit; The grid-connected point reactive power calculation unit is used to collect the grid-connected point voltage and current of the grid-connected converter and calculate the grid-connected point reactive power; The first adding unit is used to compare the reactive power of the grid connection point with the reactive power reference value to obtain an error signal for reactive power control; The droop control unit is used to perform droop control on the error signal of reactive power control; The second adding unit is used for subtracting the result output by the droop control unit from the damping compensation link of the DC voltage control loop to obtain the amplitude of the virtual internal potential of the grid-type converter.

8. The converter network control system according to claim 7, characterized in that: The reactive power control loop in the reactive power control circuit is expressed as: in, E represents the amplitude of the virtual internal potential of the grid-connected converter, K q Represents the reactive power-voltage droop coefficient, Q Indicates the reactive power of the grid-connected point of the grid-connected converter, Q ref Indicates the reactive power given value, E n Indicates the AC voltage given value, M represents the damping coefficient; is the DC bus voltage of the grid-type converter; It is the given value of DC bus voltage of grid-connected converter.

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