Cascade type static var generator control method based on virtual admittance method

Abstract

The invention relates to the technical field of power system and electric energy quality control, and discloses a cascade static var generator control method based on a virtual admittance method, which comprises the following steps of: measuring an instantaneous value of line voltage or phase voltage of a power grid in real time; extracting a power grid voltage unit rotation phasor through a software phase-locked loop; multiplying the instantaneous value of the power grid voltage by cos theta and sin theta, extracting a direct-current component through a digital low-pass filter, and generating a real part and an imaginary part of a complex phasor of the power grid voltage; and measuring the instantaneous value of the load phase current, and converting the instantaneous value into a line current or directly obtaining the phase current. By accurately matching load admittance and inverter equivalent admittance, error accumulation introduced by multi-step calculation is eliminated, so that the reactive compensation precision is improved, meanwhile, calculation delay is reduced through direct generation of an instantaneous current instruction, the system can quickly respond to load change and dynamically adjust compensation current, and the reactive compensation efficiency is improved. The problems of voltage fluctuation and three-phase imbalance are effectively suppressed, and the stability of a power system is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems and power quality control, and specifically to a control method for a cascaded static var generator based on the virtual admittance method. Background Art

[0002] In a power system, the fluctuation of reactive power and the imbalance of three-phase loads are the main reasons for the instability of the grid voltage, the increase in line losses, and the shortening of the equipment life. The traditional static var generator often uses the symmetrical decomposition method for compensation control. By decomposing the load current into positive-sequence, negative-sequence, and zero-sequence components, and then combining them to generate a compensation current command.

[0003] The traditional control method of the static var generator needs to generate a compensation command through the decomposition and coordinate transformation of positive-sequence, negative-sequence, and zero-sequence components. This process involves multi-step calculations, resulting in a high algorithm complexity and significant calculation delay. The decomposition of sequence components relies on complex matrix operations, introducing additional errors. The coordinate transformation needs to frequently update the rotation angle, increasing the calculation time consumption, resulting in limited compensation accuracy and slow dynamic response, and it is difficult to cope with voltage flicker and harmonic problems caused by load mutations. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present invention provides a control method for a cascaded static var generator based on the virtual admittance method, which solves the problems of high algorithm complexity, significant calculation delay, resulting in limited compensation accuracy, slow dynamic response, and difficulty in coping with voltage flicker and harmonics caused by load mutations.

[0005] To achieve the above objectives, the present invention is realized through the following technical solutions: A control method for a cascaded static var generator based on the virtual admittance method, including the following steps: Real-time measure the instantaneous value of the grid line voltage or phase voltage; Extract the unit rotating phasor of the grid voltage cosθ + jsinθ through a software phase-locked loop; Multiply the instantaneous value of the grid voltage by cosθ and sinθ, and extract the DC component through a digital low-pass filter to generate the real part and the imaginary part of the complex phasor of the grid voltage; Measure the instantaneous value of the load phase current, and convert it into a line current or directly obtain the phase current; Calculate the load equivalent admittance through the ratio of the load current to the complex phasor of the grid voltage; Determine the compensation admittance of the static var generator according to the load equivalent admittance; Generate the complex phasor of the compensation current command based on the compensation admittance and the complex phasor of the grid voltage; Convert the complex phasor of the compensation current into an instantaneous value command to control the cascaded H-bridge inverter to output the compensation current.

[0006] Preferably, the calculation formula for the compensation admittance for a three-phase three-wire system is: where BΔab, BΔbc, BΔca are the equivalent compensation susceptances of the static var generator, and B Lab , B Lbc , B Lca are the equivalent line susceptances of the load, and G Lca , G Lab , G Lbc are the equivalent line conductances of the load.

[0007] Preferably, the calculation of the compensation admittance for a three-phase four-wire system includes a star part and a delta part: The compensation susceptance of the star part is: The compensation susceptance of the delta part is: where B Ya , B Yb , B Yc are the equivalent compensation susceptances of the star part, B Δab , B Δbc , B Δca are the equivalent compensation susceptances of the delta part, B La , B Lb , B Lc are the equivalent phase susceptances of the load, and G La , G Lb , G Lc are the equivalent phase conductances of the load.

[0008] Preferably, the calculation formula for the complex phasor of the compensation current is: Three-phase three-wire system: Three-phase four-wire system: Among them, is the complex phasor of the compensation current, is the complex phasor of the grid voltage, and j is the imaginary unit.

[0009] Preferably, the calculation formula of the instantaneous value command of the compensation current is: Among them, Re and Im are the real part and the imaginary part of the complex number, and θ is the phase angle of the grid voltage.

[0010] Preferably, the cascaded H-bridge inverter is equivalent to a pure susceptance and does not contain a conductance component to maintain the stability of the capacitor voltage.

[0011] Preferably, the configuration method of the compensation susceptance is that when only reactive power needs to be compensated and three-phase unbalance is not compensated, the compensation susceptance values B Δab , B Δbc , B Δca of the triangular part are set to zero, and only the compensation susceptances B Ya , B Yb , B Yc of the star-connected part are enabled.

[0012] Preferably, the unit rotating phasor of the grid voltage is synchronously extracted from the line voltage or the fundamental component of the phase voltage through a phase-locked loop.

[0013] Preferably, the cut-off frequency of the digital low-pass filter is lower than

[0014] Preferably, the calculation formula of the load equivalent susceptance is: Three-phase three-wire system: Three-phase three-wire system: Among them, is the load equivalent susceptance, is the complex phasor of the load current, is the complex phasor of the grid voltage.

[0015] The present invention provides a control method for a cascaded static var generator based on the virtual susceptance method. It has the following beneficial effects: 1. In the present invention, the load is directly equivalent to admittance through the virtual admittance method, and the compensation admittance is calculated based on the real-time grid voltage and load current, avoiding the complex conversion process of positive-sequence, negative-sequence, and zero-sequence decomposition in the traditional symmetrical decomposition method. By precisely matching the load admittance and the inverter equivalent admittance, the error accumulation introduced by multi-step calculations is eliminated, thereby improving the accuracy of reactive power compensation. At the same time, the direct generation of instantaneous current commands reduces the calculation delay, enabling the system to quickly respond to load changes, dynamically adjust the compensation current, effectively suppress voltage fluctuations and three-phase unbalance problems, and enhance the stability of the power system.

[0016] 2. In the present invention, independent compensation admittance calculation logics are designed for three-phase three-wire systems and three-phase four-wire systems respectively. By flexibly configuring the compensation strategies for the star and delta connection parts, seamless adaptation to different grid structures is achieved. For three-phase three-wire systems, the compensation admittance only needs to be calculated based on line voltage and line current. For three-phase four-wire systems, step-by-step compensation is carried out in combination with phase voltage and phase current. The modular design does not require changing the core algorithm architecture, reducing the equipment transformation cost and the complexity of on-site commissioning, and is applicable to the reactive power compensation requirements of power systems in multiple scenarios and voltage levels.

[0017] 3. In the present invention, by equivalenting the cascaded H-bridge inverter to a pure susceptance and strictly restricting that there is no conductance component in its equivalent admittance, the problem of capacitor voltage fluctuation caused by active power flow is avoided, ensuring the long-term stability of the capacitor voltage on the DC side of the inverter, reducing the risk of capacitor overvoltage or undervoltage, and extending the service life of the equipment. In addition, based on a digital low-pass filter, the fundamental components of voltage and current are extracted, effectively filtering out high-frequency noise interference, further improving the purity of the control signal, reducing the switching loss of power devices, and enhancing the energy efficiency and reliability of the overall system. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic flow chart of a control method for a cascaded static var generator based on the virtual admittance method of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] Next, the technical solutions of the present invention will be clearly and completely described in conjunction with the accompanying drawings of the specification. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0020] Embodiment: Please refer to the attached Figure 1 , the embodiment of the present invention provides a control method for a cascaded static var generator based on the virtual admittance method, including the following steps: A control method for a cascaded static var generator based on the virtual admittance method, comprising the following steps: Measure the instantaneous value of the grid line voltage or phase voltage in real time; Extract the unit rotating phasor cosθ + j sinθ of the grid voltage through a software phase-locked loop; Multiply the instantaneous value of the grid voltage by cosθ and sinθ, and extract the DC component through a digital low-pass filter to generate the real part and the imaginary part of the complex phasor of the grid voltage; Measure the instantaneous value of the load phase current, and convert it into a line current or directly obtain the phase current; Calculate the load equivalent admittance through the ratio of the load current to the complex phasor of the grid voltage; Determine the compensation admittance of the static var generator according to the load equivalent admittance; Generate the complex phasor of the compensation current command based on the compensation admittance and the complex phasor of the grid voltage; Convert the complex phasor of the compensation current into an instantaneous value command to control the cascaded H-bridge inverter to output the compensation current.

[0021] Example 1: Overall control process of a cascaded static var generator based on the virtual admittance method Measurement of grid voltage and load current: Use a voltage sensor to measure the three-phase grid line voltage in real time, and collect the instantaneous value of the load phase current through a current sensor

[0022] Phase-locked loop synchronization and rotating phasor extraction: Use a software phase-locked loop to synchronize the fundamental component of the grid voltage to generate the unit rotating phasor cosθ + j sinθ, where θ is the phase angle of the grid voltage.

[0023] Generation of complex phasor of grid voltage: Multiply the instantaneous value of the grid voltage by cosθ and sinθ respectively, and extract the DC component through a digital low-pass filter to obtain the complex phasor of the grid voltage: Three-phase three-wire system: Three-phase four-wire system:

[0024] Calculation of load equivalent admittance: For a three-phase three-wire system, according to formulas (4)-(6): For a three-phase four-wire system, according to formulas (16)-(18): Compensation admittance calculation and current command generation: Three-phase three-wire system: Calculate the compensation susceptance B according to formulas (7)-(9) Δab , B Δbc , B Δca , and generate the complex-phase quantity formula (10-12) of the compensation current: Three-phase four-wire system: Calculate the star and delta compensation susceptances respectively according to formulas (19)-(24), and generate the star-part current (Formulas 25-27) and the delta-part current (Formulas 28-30).

[0025] Instantaneous current command conversion and control output: Convert the complex-phase quantity of the compensation current into an instantaneous value command, and control the cascaded H-bridge inverter to output the compensation current through PWM modulation to achieve reactive power and unbalance compensation.

[0026] Example 2: Reactive power and unbalance compensation of a three-phase three-wire system Line current reconstruction: Convert the load phase current into line current according to formulas (1)-(3): iLab = (iLa - iLb) / 3; (1) iLbc = (iLb - iLc) / 3; (2) iLca = (iLc - iLa) / 3; (3) Compensation admittance calculation: The compensation susceptance is also calculated according to formulas (7)-(9): where

[0027] Embodiment 3: Step-by-step compensation strategy for a three-phase four-wire system Division of labor between star and delta compensation: Star part: Compensate the phase susceptance and part of the unbalanced conductance (formulas 19-21); Delta part: Compensate the remaining unbalanced conductance (formulas 22-24).

[0028] Current command synthesis: Star part current is generated by formulas (31)-(33); Delta part current is generated by formulas (34)-(36).

[0029] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A control method for a cascaded static var generator based on the virtual admittance method, characterized in that It includes the following steps: Measure the instantaneous value of the grid line voltage or phase voltage in real time; Extract the unit rotating phasor of the grid voltage cosθ + j sinθ through a software phase-locked loop; Multiply the instantaneous value of the grid voltage by cosθ and sinθ, and extract the DC component through a digital low-pass filter to generate the real and imaginary parts of the complex phasor of the grid voltage; Measure the instantaneous value of the load phase current and convert it to the line current or directly obtain the phase current; Calculate the load equivalent admittance through the ratio of the load current to the complex phasor of the grid voltage; Determine the compensation admittance of the static var generator according to the load equivalent admittance; Generate the complex phasor of the compensation current command based on the compensation admittance and the complex phasor of the grid voltage; Convert the complex phasor of the compensation current into an instantaneous value command to control the cascaded H-bridge inverter to output the compensation current.

2. The control method of the cascaded static var generator based on the virtual admittance method according to claim 1, characterized in that: The calculation formula of the compensation admittance for a three-phase three-wire system is: Among them, BΔab, BΔbc, and BΔca are the equivalent compensation susceptances of the static var generator, and B Lab , B Lbc , B Lca are the equivalent line susceptances of the load, and G Lca , G Lab , G Lbc are the equivalent line conductances of the load.

3. A control method for a cascaded static var generator based on the virtual admittance method according to claim 1, characterized in that: The calculation of the compensation admittance for a three-phase four-wire system includes a star part and a delta part: The compensation susceptance of the star part is: The compensation susceptance of the delta part is: Among which B Ya 、B Yb 、B Yc are the equivalent compensating susceptances of the star part, B Δab 、B Δbc 、B Δca are the equivalent compensating susceptances of the delta part, B La 、B Lb 、B Lc are the equivalent phase susceptances of the load, G La 、G Lb 、G Lc are the equivalent phase conductances of the load.

4. A control method for a cascaded static var generator based on the virtual admittance method according to claim 1, characterized in that: The calculation formula of the complex phasor of the compensation current is: Three-phase three-wire system: Three-phase four-wire system: Among them, is the complex phasor of the compensation current, is the complex phasor of the grid voltage, and j is the imaginary unit.

5. A control method for a cascaded static var generator based on the virtual admittance method according to claim 1, characterized in that: The calculation formula of the instantaneous value command of the compensation current is: Where Re and Im are the real and imaginary parts of the complex number, and θ is the phase angle of the grid voltage.

6. A control method for a cascaded static var generator based on the virtual admittance method according to claim 1, characterized in that: The cascaded H-bridge inverter is equivalent to a pure susceptance and does not contain a conductance component to maintain the stability of the capacitor voltage.

7. A control method for a cascaded static var generator based on the virtual admittance method according to claim 3, characterized in that: The configuration method of the compensation admittance is that when only reactive power needs to be compensated and three-phase unbalance does not need to be compensated, the compensation susceptance values B Δab 、B Δbc 、B Δca in the triangular part are set to zero, and only the compensation susceptance B Ya 、B Yb 、B Yc in the star-connected part is enabled.

8. A control method for a cascaded static var generator based on the virtual admittance method according to claim 1, characterized in that: The unit rotating phasor of the grid voltage is synchronously extracted through a phase-locked loop for the fundamental component of the line voltage or phase voltage.

9. A control method for a cascaded static var generator based on the virtual admittance method according to claim 1, characterized in that: The cut-off frequency of the digital low-pass filter is lower than 10. A control method for a cascaded static var generator based on the virtual admittance method according to claim 1, characterized in that: The calculation formula of the load equivalent admittance is: Three-phase three-wire system: Three-phase three-wire system: Among them, is the load equivalent admittance, is the complex phasor of the load current, is the complex phasor of the grid voltage.

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