Power angle tracking-based hybrid multi-machine transient stability control method for network tracking / constructing type converter

Abstract

The invention provides a power angle tracking-based hybrid multi-machine transient stability improvement control method for a network tracking / constructing type converter, and relates to the field of converter operation control. According to the method, a hybrid multi-machine mathematical model containing a grid following / grid constructing type converter is established based on the dynamic output characteristics of the converter, so that a system balance equation containing power angle interaction is obtained, a balance curve is drawn by adopting an equal-area rule for stability analysis, the relation between the power angle difference value and the stability margin of the hybrid converter is researched, and the stability of the hybrid converter is improved. Therefore, a power angle tracking strategy is provided, the system frequency fluctuation under a power grid fault is effectively inhibited, and the hybrid multi-machine transient stability of the follow-up construction grid type converter is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of converter operation control, and in particular to a method for improving transient stability of a hybrid multi-machine grid-type converter based on power angle tracking. Background Art

[0002] Power electronic converters can be divided into grid-following converters and grid-forming converters according to different control methods. The former has the external characteristics of a current source and can achieve precise power output, while the latter has the external characteristics of a voltage source and can achieve voltage and frequency support. However, the coexistence of grid-following and grid-forming converters in the power grid has made hybrid multi-machine system scenarios increasingly common in new power systems, bringing new transient stability issues.

[0003] Traditional methods for improving converter transient stability usually focus on the control characteristics analysis of single-machine converters, and are mainly applicable to systems with only a single grid-following or grid-forming control. However, the direct electrical connection between different converters in a hybrid multi-machine system causes their control state quantities to be coupled with each other. This coupling effect affects the dynamic response between converters, further exacerbating the transient stability problem, making it impossible for traditional single-machine transient stability improvement methods to be directly applied to hybrid multi-machine systems. Summary of the invention

[0004] 1. Technical issues to be resolved

[0005] In view of the deficiencies and gaps in the prior art, the present invention provides a hybrid multi-machine transient stability improvement control method for a grid-type converter based on power angle tracking to solve the problems existing in the above-mentioned background technology.

[0006] (II) Technical solution

[0007] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0008] On the one hand, a hybrid multi-machine transient stability improvement control method for a grid-connected converter based on power angle tracking is provided, comprising:

[0009] Step A: Considering the dynamic output characteristics of the grid-following type and the grid-forming type respectively, a mathematical model of a hybrid multi-machine system including the grid-following / grid-forming converters is established, which not only retains the line admittance characteristics between the nodes of the multi-machine system, but also realizes the analysis of the interaction impact between the hybrid converters;

[0010] Step B: Based on the established mathematical model, the power angle equation of the phase-locked loop of the grid-following converter is introduced to derive the balance equation containing the characteristic power angle quantity of the multi-machine converter;

[0011] Step C: Use the equal area rule to perform stability analysis on the system balance equation and study the relationship between the power angle difference and the stability margin of the hybrid converter;

[0012] Step D: By controlling the grid-type power angle to actively track the grid-type power angle, a multi-machine converter controller based on power angle tracking is designed to effectively suppress system frequency fluctuations under power grid faults and improve the transient stability of the hybrid multi-machine grid-type converter.

[0013] Furthermore, in the step A, establishing a mathematical model of a multi-machine system based on a hybrid of a grid-following / grid-forming converter includes steps A01 to A02:

[0014] Step A01: The grid-following converter adopts phase-locked loop synchronous vector current control to achieve accurate active and reactive output, that is,

[0015]

[0016] In the formula, is the q-axis component of the output voltage of the grid-following converter in the system; K p_PLL and K i_PLL are the proportional and integral coefficients of the phase-locked loop respectively; ω gn is the output frequency reference value; θ PLL It is the output voltage phase angle of the grid-type converter measured by the phase-locked loop.

[0017] The grid-type converter adopts droop control with a low-pass filter to achieve stable voltage and frequency support, that is,

[0018]

[0019] In the formula, ω GFM and V GFM are the output angular frequency and voltage of the grid-connected converter in the system; ω0 and V0 are the rated values ​​of the output frequency and voltage respectively; m p and n q are the droop coefficients of active and reactive power respectively; ω p and ω q are the cut-off frequencies of the active and reactive power droop low-pass filters respectively; P GFM and Q GFM Represent the output active power and reactive power respectively.

[0020] Step A02: According to the control characteristics of the grid-following / grid-forming converters, the grid-following converter operates as a current source, while the grid-forming converter operates as a voltage source. Considering the line admittance between each node, the parallel grid-forming-grid system is modeled as follows:

[0021]

[0022] In the formula, I GFL and I GFMare the branch currents of the grid-following and grid-building converters respectively; I g is the grid-connected current; Y GFL , Y GFM and Y g is the admittance of each branch of the multi-machine system; V GFL , V GFM They are the grid-following type and grid-building type voltages respectively; V PCC is the grid connection point voltage; V g is the voltage voltage.

[0023] Furthermore, in step B, the balance equation containing the characteristic power angle quantity of the multi-machine converter is derived according to the following steps B01 to B03:

[0024] Step B01: Based on the hybrid multi-machine mathematical model of grid-following / grid-forming converter, the grid-connection point voltage vector expression is as follows:

[0025]

[0026] Step B02: Based on the branch power flow relationship, the grid-type converter output voltage is constructed as follows:

[0027]

[0028] Where, Y0∠θ0, Y1∠θ1, Y2∠θ2, Y3∠θ3 are 1 / Y GFL , 1 / (Y GFM +Y g ), Y g / (Y GFM +Y g ), Y GFM / (Y GFM +Y g ) is a simplified definition of .

[0029] Step B03: Introduce the phase-locked loop power angle equation. According to the q-axis component of the grid-following converter output voltage, the balance equation containing the characteristic power angle of the multi-machine converter is as follows:

[0030]

[0031] In the formula, is the output power factor angle of the grid-following converter; δ GFL , δ GFM is the converter power angle based on the grid voltage.

[0032] Furthermore, in step C, the stability analysis of the system balance equation is performed using the equal area rule in the following steps to study the relationship between the power angle difference of the hybrid converter and the stability margin:

[0033] Step C01: During transient faults, the power angle ranges of the grid-following and grid-forming converters are as follows:

[0034]

[0035] δ GFM -δ GFL The range is [-π / 2,0], that is, sin(δ GFM -δ GFL ) is a negative value.

[0036] Step C02: Consider that the grid-following converter operates at unity power factor in steady state and outputs unity reactive power in transient state. The equivalent balance equation is as follows:

[0037]

[0038] In the formula, I n is the rated output current of the grid-following converter; A is defined as the key term Y in the balance equation GFM V GFM sin(δ GFM -δ GFL ).

[0039] Furthermore, in step D, a multi-machine converter controller based on power angle tracking is designed according to the following steps to effectively suppress system frequency fluctuations under power grid faults and improve the transient stability of hybrid multi-machine grid-type converters:

[0040] Step D01: Actively track the grid-following power angle by controlling the grid-forming power angle to achieve system frequency fluctuation suppression under grid faults. The power angle controller is designed as

[0041]

[0042] Wherein, switch = 1 represents that the system is in a steady-state operation period, and switch = 2 represents that the system is in a transient fault period.

[0043] Beneficial Effects

[0044] The present invention is a hybrid multi-machine transient stability improvement control method based on power angle tracking of a grid-type converter. By controlling the interaction of the converter power angle, the transient stability can be effectively improved. Based on the dynamic output characteristics of the converter, a mathematical model of a hybrid multi-machine system including a grid-type / grid-type converter is established, which not only retains the line admittance characteristics between the nodes of the multi-machine system, but also realizes the analysis of the interaction influence between the hybrid converters. The phase-locked loop power angle equation is introduced to derive the balance equation containing the characteristic power angle quantity of the multi-machine converter. On this basis, the equal area rule is used to analyze the stability of the system balance equation, and the relationship between the power angle difference of the hybrid converter and the stability margin is studied. By controlling the grid-type power angle to actively track the grid-type power angle, a multi-machine converter controller based on power angle tracking is designed. The present invention realizes the effective suppression of system frequency fluctuations under transient faults, and reduces the adverse effects of power grid operation faults and disturbances on the system. The present invention provides a new idea for improving the transient stability of a hybrid multi-machine system based on a power electronic converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a flow chart of the control method of the present invention;

[0046] Figure 2 It is a hybrid multi-machine system simulation system used in the embodiment of the present invention;

[0047] Figure 3 is a system frequency waveform diagram under a 30% symmetrical voltage drop in an embodiment of the present invention;

[0048] Figure 4 is a system frequency waveform diagram under 50% symmetrical voltage drop in an embodiment of the present invention;

[0049] Figure 5 is a system frequency waveform diagram under 80% symmetrical voltage drop in an embodiment of the present invention; DETAILED DESCRIPTION

[0050] The following will be combined with the accompanying drawings of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0051] like Figure 1 As shown, the present invention provides a hybrid multi-machine transient stability improvement control method for grid-type converters based on power angle tracking. In practical applications, the method specifically includes the following steps:

[0052] Step A: Based on the dynamic output characteristics of the converter, a mathematical model of a hybrid multi-machine system including a grid-following / grid-forming converter is established, which not only retains the line admittance characteristics between nodes of the multi-machine system, but also realizes the analysis of the interaction impact between the hybrid converters, and then proceeds to step B;

[0053] Step B: introduce the phase-locked loop power angle equation, derive the balance equation containing the characteristic power angle quantity of the multi-machine converter, and then enter step C;

[0054] Step C: Use the equal area rule to perform stability analysis on the system balance equation, study the relationship between the power angle difference of the hybrid converter and the stability margin, and then proceed to step D;

[0055] Step D: By controlling the grid-type power angle to actively track the grid-type power angle, a multi-machine converter controller based on power angle tracking is designed to effectively suppress system frequency fluctuations under power grid faults and improve the transient stability of the hybrid multi-machine grid-type converter.

[0056] In the above step A, from the following steps A01 to A02, a mathematical model of a multi-machine system based on a hybrid grid / grid-type converter is provided:

[0057] Step A01: The grid-following converter adopts phase-locked loop synchronous vector current control to achieve accurate active and reactive output, that is,

[0058]

[0059] In the formula, is the q-axis component of the output voltage of the grid-following converter in the system; K p_PLL and K i_PLL are the proportional and integral coefficients of the phase-locked loop respectively; ω gn is the output frequency reference value; θ PLL It is the output voltage phase angle of the grid-type converter measured by the phase-locked loop.

[0060] The grid-type converter adopts droop control with a low-pass filter to achieve stable voltage and frequency support, that is,

[0061]

[0062] In the formula, ω GFM and V GFM are the output angular frequency and voltage of the grid-connected converter in the system; ω0 and V0 are the rated values ​​of the output frequency and voltage respectively; m p and n q are the droop coefficients of active and reactive power respectively; ω p and ω q are the cut-off frequencies of the active and reactive power droop low-pass filters respectively; P GFM and Q GFMRepresent the output active power and reactive power respectively.

[0063] Step A02: According to the control characteristics of the grid-following / grid-forming converters, the grid-following converter operates as a current source, while the grid-forming converter operates as a voltage source. Considering the line admittance between each node, the parallel grid-forming-grid system is modeled as follows:

[0064]

[0065] In the formula, I GFL and I GFM are the branch currents of the grid-following and grid-building converters respectively; I g is the grid-connected current; Y GFL , Y GFM and Y g is the admittance of each branch of the multi-machine system; V GFL , V GFM They are the grid-following type and grid-building type voltages respectively; V PCC is the grid connection point voltage; V g is the voltage voltage.

[0066] In the above step B, the balance equation containing the characteristic power angle quantity of the multi-machine converter is derived from steps B01 to B03:

[0067] Step B01: Based on the hybrid multi-machine mathematical model of grid-following / grid-forming converter, the grid-connection point voltage vector expression is as follows:

[0068]

[0069] Step B02: Based on the branch power flow relationship, the grid-type converter output voltage is constructed as follows:

[0070]

[0071] Where, Y0∠θ0, Y1∠θ1, Y2∠θ2, Y3∠θ3 are 1 / Y GFL , 1 / (Y GFM +Y g ), Y g / (Y GFM +Y g ), Y GFM / (Y GFM +Y g ) is a simplified definition of .

[0072] Step B03: Introduce the phase-locked loop power angle equation. According to the q-axis component of the grid-following converter output voltage, the balance equation containing the characteristic power angle of the multi-machine converter is as follows:

[0073]

[0074] In the formula, is the output power factor angle of the grid-following converter; δ GFL , δ GFM is the converter power angle based on the grid voltage.

[0075] In the above step C, the equal area rule is used to perform stability analysis on the system balance equation from step C01 to step C03, and the relationship between the power angle difference of the hybrid converter and the stability margin is studied:

[0076] Step C01: During transient faults, the power angle ranges of the grid-following and grid-forming converters are as follows:

[0077]

[0078] δ GFM -δ GFL The range is [-π / 2,0], that is, sin(δ GFM -δ GFL ) is a negative value.

[0079] Step C02: Consider that the grid-following converter operates at unity power factor in steady state and outputs unity reactive power in transient state. The equivalent balance equation is as follows:

[0080]

[0081] In the formula, I n is the rated output current of the grid-following converter; A is defined as the key term Y in the balance equation GFM V GFM sin(δ GFM -δ GFL ).

[0082] In the above step D, a multi-machine converter controller based on power angle tracking is designed by the following steps D01 to D02, which effectively suppresses the system frequency fluctuation under the power grid fault and improves the transient stability of the hybrid multi-machine grid-type converter:

[0083] Step D01: Actively track the grid-following power angle by controlling the grid-forming power angle to achieve system frequency fluctuation suppression under grid faults. The power angle controller is designed as

[0084]

[0085] Wherein, switch = 1 represents that the system is in a steady-state operation period, and switch = 2 represents that the system is in a transient fault period.

[0086] Applying the above-mentioned control scheme to practice, the hybrid multi-machine system model based on MATLAB / Simulink simulation platform is as follows Figure 2As shown, the system includes a grid-following converter and a grid-forming converter, which are interconnected and grid-connected through impedance branches. The system operates in a stable state at the initial moment. A system controller is constructed according to the hybrid multi-machine transient stability improvement control method of grid-forming converter based on power angle tracking proposed in the present invention to verify the effectiveness and superiority of the proposed method.

[0087] Corresponds to Figure 2 ,The operating conditions of the hybrid multi-machine system are set as follows: at the initial moment, the system is in steady-state operation. At 3 seconds, a transient grid fault occurs, which is manifested as a three-phase symmetrical grid voltage drop. At this time, the power angle tracking controller switch=2. Figure 3 This is the simulation waveform of the hybrid multi-machine system. Figure 3 This is the system frequency waveform under 30% symmetrical voltage drop. The horizontal axis represents time, unit: second, and the vertical axis represents frequency, unit: Hz. Figure 4 This is the system frequency waveform under 50% symmetrical voltage drop. The horizontal axis represents time, unit: seconds, and the vertical axis represents frequency, unit: Hz. Figure 5 The system frequency waveform diagram under 80% symmetrical voltage drop, the horizontal axis represents time, unit: seconds, the vertical axis represents frequency, unit: Hz. Figures 3 to 5 It can be seen that the multi-machine converter controller based on power angle tracking can effectively suppress the system frequency fluctuation under power grid fault and improve the transient stability of the hybrid multi-machine grid-type converter.

[0088] The present invention proposes a control method for improving transient stability of hybrid multi-machines of grid-type converters based on power angle tracking. First, a mathematical model of a hybrid multi-machine system including grid-type / grid-type converters is established based on the dynamic output characteristics of the converter, which not only retains the line admittance characteristics between nodes of the multi-machine system, but also realizes the analysis of the interaction between the hybrid converters. Then, the phase-locked loop power angle equation is introduced to derive the balance equation containing the characteristic power angle of the multi-machine converter. Then, the equal area rule is used to analyze the stability of the system balance equation, and the relationship between the power angle difference of the hybrid converter and the stability margin is studied. On this basis, the multi-machine converter controller based on power angle tracking is designed by controlling the grid-type power angle to actively track the grid-type power angle. This method can effectively suppress the system frequency fluctuation under transient faults and reduce the adverse effects of power grid operation faults and disturbances on the system. The present invention provides a feasible and effective implementation plan for improving the transient stability of hybrid multi-machine systems based on power electronic converters.

[0089] In the description of this specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0090] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the present invention to be protected.

Claims

1. A hybrid multi-machine transient stability control method for grid-connected / grid-connected converters based on power angle tracking, characterized in that: include: Step A: Considering the dynamic output characteristics of the grid-following type and the grid-forming type respectively, a mathematical model of a hybrid multi-machine system including the grid-following / grid-forming converters is established, which not only retains the line admittance characteristics between the nodes of the multi-machine system, but also realizes the analysis of the interaction impact between the hybrid converters; Step B: Based on the established mathematical model, the power angle equation of the phase-locked loop of the grid-following converter is introduced to derive the balance equation containing the characteristic power angle quantity of the multi-machine converter; Step C: Use the equal area rule to perform stability analysis on the system balance equation and study the relationship between the power angle difference and the stability margin of the hybrid converter; Step D: By controlling the grid-type power angle to actively track the grid-type power angle, a multi-machine converter controller based on power angle tracking is designed to effectively suppress system frequency fluctuations under power grid faults and improve the transient stability of the hybrid multi-machine grid-type converter.

2. The hybrid multi-machine transient stability control method of grid-connected / grid-connected converter based on power angle tracking according to claim 1 is characterized in that: In the step A, a mathematical model of a multi-machine system based on a hybrid of grid-following / grid-forming converters is established through the following steps A01 to A02: Step A01: The grid-following converter adopts phase-locked loop synchronized vector current control to achieve accurate active and reactive output, namely: Where V GFLq is the q-axis component of the output voltage of the grid-following converter in the system; K p_PLL and K i_PLL are the proportional and integral coefficients of the phase-locked loop respectively; ω gn is the output frequency reference value; θ PLL The phase angle of the grid-type converter output voltage measured by the phase-locked loop; The grid-type converter adopts droop control with a low-pass filter to achieve stable voltage and frequency support, namely: In the formula, ω GFM and V GFM are the output angular frequency and voltage of the grid-connected converter in the system; ω0 and V0 are the rated values ​​of the output frequency and voltage respectively; m p and n q are the droop coefficients of active and reactive power respectively; ω p and ω q are the cut-off frequencies of the active and reactive power droop low-pass filters respectively; P GFM and Q GFM Respectively represent the output active power and reactive power; Step A02: According to the control characteristics of the grid-following / grid-forming converters, the grid-following converter operates as a current source, while the grid-forming converter operates as a voltage source. Considering the line admittance between each node, the parallel grid-forming-grid system is modeled as follows: In the formula, I GFL and I GFM are the branch currents of the grid-following and grid-building converters respectively; I g is the grid-connected current; Y GFL , Y GFM and Y g is the admittance of each branch of the multi-machine system; V GFL , V GFM They are the grid-following type and grid-building type voltages respectively; V PCC is the grid connection point voltage; V g is the voltage voltage.

3. The method for controlling transient stability of hybrid multi-machines of grid-connected / grid-connected converters based on power angle tracking according to claim 2 is characterized in that: In step B, a balance equation containing characteristic power angle quantities of the multi-machine converter is derived through the following steps B01 to B03: Step B01: Based on the hybrid multi-machine mathematical model of the grid-following / grid-building converter, the grid-connection point voltage vector expression is as follows: Step B02: Based on the branch power flow relationship, the grid-type converter output voltage is constructed as follows: Where, Y0∠θ0, Y1∠θ1, Y2∠θ2, Y3∠θ3 are 1 / Y GFL , 1 / (Y GFM +Y g ), Y g / (Y GFM +Y g ), Y GFM / (Y GFM +Y g ) Step B03: Introduce the phase-locked loop power angle equation. According to the q-axis component of the grid-following converter output voltage, the balance equation containing the characteristic power angle of the multi-machine converter is as follows: In the formula, is the output power factor angle of the grid-following converter; δ GFL , δ GFM is the converter power angle based on the grid voltage.

4. The hybrid multi-machine transient stability control method of grid-connected / grid-connected converter based on power angle tracking according to claim 3 is characterized by: In step C, the equal area rule is used to perform stability analysis on the system balance equation, and the relationship between the power angle difference of the hybrid converter and the stability margin is studied, which specifically includes the following steps: Step C01: During transient faults, the power angle ranges of the grid-following and grid-forming converters are as follows: δ GFM -δ GFL The range is [-π / 2,0], that is, sin(δ GFM -δ GFL ) is a negative value; Step C02: Considering that the grid-following converter operates at unity power factor in steady state and outputs unity reactive power in transient state, the equivalent balance equation is as follows: In the formula, I n is the rated output current of the grid-following converter; A is defined as the key term Y in the balance equation GFM V GFM sin(δ GFM -δ GFL ).

5. The hybrid multi-machine transient stability control method of grid-connected / grid-connected converter based on power angle tracking according to claim 4 is characterized in that: In the step D, a multi-machine converter controller based on power angle tracking is designed to effectively suppress the system frequency fluctuation under power grid faults and improve the transient stability of the hybrid multi-machine grid-type converter, which specifically includes the following steps: Step D01: Actively track the grid-following power angle by controlling the grid-forming power angle to achieve system frequency fluctuation suppression under grid faults. The power angle controller is designed as follows: Wherein, switch = 1 represents that the system is in a steady-state operation period, and switch = 2 represents that the system is in a transient fault period.

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