Network construction type control method and system for cascaded SVG (Static Var Generator)

Abstract

The invention relates to a network construction type control method and system for cascaded SVG, and belongs to the technical field of grid-connected control. The method comprises the following steps that a three-phase network construction current instruction value is input into a current control link to obtain a three-phase modulation wave used for controlling SVG sub-module switching, and the three-phase network construction current instruction value is obtained by inputting three-phase network construction voltage instructions into virtual impedance links of corresponding phases respectively; the three-phase networking voltage instruction is the product of the sinusoidal quantity of the three-phase networking phase instruction and the voltage amplitude instruction of the corresponding phase. According to the method, a network construction voltage phase, a network construction voltage instruction and a network construction current instruction are generated in a split-phase manner by adopting a method that each phase independently generates own reference voltage and reference phase, and then the generated three-phase independent network construction current instruction values are input into a current control link to obtain three-phase modulation waves for controlling switching of SVG sub-modules; the method can adapt to a power grid asymmetric fault of alternating current voltage phase abrupt change, and effectively improves the fault ride-through capability of the networking SVG.

Description

Technical Field

[0001] The invention relates to a network-building control method and system for cascaded SVGs, belonging to the technical field of grid-connected control. Background Art

[0002] The grid-type SVG consists of three bridge arms connected in a star or triangle topology and then connected in parallel to the grid. Figure 1 As shown in the figure, the existing mesh-type SVG topology adopts an angular connection mode, and each bridge arm is composed of a bridge arm reactor ( Figure 1 L in the middle g ) and multiple full-bridge sub-modules in series, each full-bridge sub-module ( Figure 1 The H-bridge submodule in the figure contains four fully controlled switching devices, a capacitor and other auxiliary circuits. Existing network control strategies, such as Figure 2 As shown in FIG. 1 , it includes a network voltage phase generation link, a network voltage command generation link, a virtual impedance link, and a current control link. Among them, the control block diagram of the network voltage phase generation link is as follows: Figure 3 As shown in the figure, the grid voltage phase generation link usually adopts a virtual synchronous control strategy based on the rotor motion equation, which is achieved by adjusting the average voltage U smavg Perform closed-loop control to generate power command P ref , and then the potential phase θ in the network SVG is obtained through the rotor motion equation; then, by adding 0°, -120°, and +120° to θ and performing sine calculation, the calculation results are compared with the network voltage amplitude U P Multiply to get the three-phase grid voltage command U refa , U refb , U refc ; Then the three-phase grid voltage command is calculated through the virtual impedance link to generate the current command value I refd with I refq ; Send the current command value to the current control link to generate a three-phase modulation wave, and after the modulation link, generate a pulse signal and send it to the SVG main circuit for sub-module switching control. Among them, SVG (Static Var Generator) is a traditional power electronic device used to quickly and effectively adjust the reactive power of the power grid to ensure voltage stability.

[0003] The phase relationship of the three-phase voltage instructions generated by the above control strategy is fixed, that is, the three-phase voltage instructions differ by 120°. At this time, the phase of the compensation current instruction generated by the virtual impedance link after the three-phase voltage instruction is subtracted from the grid voltage will be mainly determined by the grid voltage phase. However, when it comes to asymmetric grid faults involving sudden changes in the AC voltage phase, the phase relationship of the current instructions of different phases is no longer 120°, and the energy charging or discharging levels of the bridge arms are also different, and even one phase is charged and the other phase is discharged. At this time, relying solely on a unified sub-module average voltage controller obviously cannot meet the stable operation requirements of each of the three-phase modules, causing the module capacitors in different bridge arms to continuously charge or discharge, and eventually overvoltage or undervoltage occurs, resulting in the SVG fault lockout exit, seriously affecting the fault crossing capability of the networked SVG. Typical working conditions of asymmetric faults in power grids are, for example, short circuit faults between phases AB. After the fault, the voltages of the two phases are exactly the same, but the generated current commands are equal in magnitude and opposite in direction, and power cannot be allocated by zero-sequence current injection. This causes the A-phase bridge arm to discharge as a whole, and the voltage of the A-phase bridge arm submodule to continue to decrease. The B-phase bridge arm is charged as a whole, and the voltage of the B-phase bridge arm submodule continues to increase, eventually leading to fault shutdown. Figure 4 It can be seen from the phase-to-phase short-circuit fault waveform that when there is no obvious continuous overcurrent, the average capacitor voltage of the bridge arm of phase C drops to below 0.25 pu (per unit value) 100 ms (milliseconds) after the fault, while the average bridge arm voltage of phase A rises to more than 1.5 pu and continues to deteriorate over time. Summary of the invention

[0004] The object of the present invention is to provide a grid-forming control method and system for cascading SVGs, so as to solve the problem of low fault ride-through capability of the grid-forming SVGs when an asymmetric fault occurs in the power grid.

[0005] To achieve the above object, the solution of the present invention includes:

[0006] A mesh control method for cascading SVGs of the present invention comprises the following steps:

[0007] The three-phase grid current command value is input into the current control link to obtain a three-phase modulation wave used to control the switching of the SVG submodule. The three-phase grid current command value is obtained by inputting the three-phase grid voltage command into the virtual impedance link of the corresponding phase respectively. The three-phase grid voltage command is the product of the sinusoidal quantity of the three-phase grid phase command and the voltage amplitude command of the corresponding phase respectively.

[0008] The three-phase grid phase command is obtained in the following way:

[0009] The difference between the square of the submodule voltage rating and the square of the submodule voltage average value of the SVG three-phase bridge arm is used to generate the three-phase frequency deviation through the inertia simulation link. The three-phase frequency deviation is added to the rated frequency to generate the three-phase frequency command. The three-phase frequency command is integrated to obtain the three-phase network phase command.

[0010] Furthermore, the expression of the three-phase grid voltage command is:

[0011] Uref i =Up i *sin(θ i )

[0012] θ i =∫(Δf i +f rated )

[0013] Where Uref i is the grid voltage instruction of the i-th phase in the three-phase system, Up i is the voltage amplitude command of the i-th phase, θ i is the network phase instruction of the i-th phase, Δf i is the frequency deviation of the i-th phase, f rated is the rated frequency.

[0014] Furthermore, the frequency deviation output by the inertia simulation link is limited, and the frequency deviations of the three phases after the limit are added to the rated frequency to generate the three-phase frequency command.

[0015] Furthermore, the expression of the three-phase grid current command value is:

[0016]

[0017] Where, Iref i is the grid current command of the i-th phase among the three phases, Uref i is the grid voltage command of the i-th phase, U i is the actual voltage at the common connection point of the i-th phase, R V is the virtual resistance, L V is the virtual reactance, and s is the frequency domain transformation parameter.

[0018] Furthermore, the current control link is used to convert the three-phase grid-forming current command value into a grid-forming current command in a rotating coordinate system, and perform closed-loop control on the grid-forming current command in the rotating coordinate system through a proportional-integral controller.

[0019] Furthermore, the current control link performs closed-loop control on the three-phase grid current command value through a proportional resonant controller.

[0020] A meshed control system for cascading SVGs of the present invention includes a processor, and the processor is used to execute a computer program to implement the steps of the meshed control method for cascading SVGs as described above.

[0021] Beneficial effects of the present invention:

[0022] The present invention is a pioneering invention, which provides a network control method for cascading SVG. The method adopts a method in which each phase independently generates its own reference voltage and reference phase to generate a network voltage phase, a network voltage instruction and a network current instruction in phase, and then the generated three-phase independent network current instruction values ​​are input into the current control link to obtain a three-phase modulation wave for controlling the switching of SVG submodules. The method can adapt to asymmetric faults of the power grid with sudden changes in the phase of the AC voltage, solves the problem that when an asymmetric fault involving a sudden change in the phase occurs in the power grid, the charging and discharging directions and sizes of the three-phase currents are inconsistent, resulting in overvoltage or undervoltage of the capacitor voltage of the SVG submodule, thereby causing SVG fault lockout, and effectively improves the fault ride-through capability of the networked SVG. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is an existing network-type SVG topology diagram in the background technology;

[0024] Figure 2 It is a control block diagram of an existing mesh-type SVG control strategy in the background technology;

[0025] Figure 3 It is a control block diagram of a network voltage phase generation link in the background technology;

[0026] Figure 4 is a waveform diagram of a phase-to-phase short circuit fault under an existing control strategy in the background technology;

[0027] Figure 5 It is a control block diagram of a mesh-type control method for cascading SVGs;

[0028] Figure 6 It is a control block diagram for the three-phase grid voltage command generation link in the grid control method for cascaded SVG;

[0029] Figure 7 It is a control block diagram of the virtual impedance link in the mesh control method for cascaded SVG;

[0030] Figure 8 It is a control block diagram of the current control link in the mesh-type control method for cascaded SVG. DETAILED DESCRIPTION

[0031] In order to solve the problems in the background technology, the present invention adopts a method in which each phase independently generates its own reference voltage and reference phase. While retaining the networking characteristics of SVG's self-generated phase and voltage, the flexibility of networking control is improved through phase control. This solves the problem that when an asymmetric fault involving a sudden phase change occurs in the power grid, the charging and discharging directions and sizes of the three-phase currents are inconsistent, resulting in overvoltage or undervoltage of the sub-module capacitor voltage, thereby causing SVG fault lockout, and effectively improves the fault ride-through capability of the networking SVG; it can also effectively improve the supporting capability of the networking SVG under power grid fault conditions without increasing the capacitance value and investment cost.

[0032] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0033] An embodiment of a mesh control method for cascading SVG:

[0034] A mesh control method for cascading SVGs comprises the following steps:

[0035] The difference between the square of the rated voltage of the submodule and the square of the average voltage of the submodule of the three-phase bridge arm of the SVG is respectively used to generate the frequency deviation of the three phases through the inertia simulation link, and the frequency deviation of the three phases is respectively added to the rated frequency to generate the three-phase frequency command, and the three-phase frequency command is integrated to obtain the three-phase network phase command; the product of the sinusoidal quantity of the three-phase network phase command and the voltage amplitude command of the corresponding phase is used as the three-phase network voltage command, and the three-phase network current command value is obtained by inputting the three-phase network voltage command into the virtual impedance link of the corresponding phase, and finally the three-phase network current command value is input into the current control link to obtain the three-phase modulation wave used to control the switching of the SVG submodule.

[0036] like Figure 5 As shown, the grid voltage command generation link is divided into three phases and runs independently to generate independent three-phase grid voltage commands Uref i , i = a, b, c, the virtual impedance link is also divided into three phases and operates independently to generate independent three-phase current instructions Iref i , i = a, b, c. Where:

[0037] 1) Phase-by-phase network voltage command generation link, such as Figure 6 As shown:

[0038] The square of the rated voltage of the submodule is compared with the square of the average voltage of the submodule of the i-th (i=A, B, C) phase bridge arm, and the comparison result is used to generate the frequency deviation value Δf through the inertia simulation link. i ;Frequency deviation value Δf i With rated frequency f ratedAfter adding, we get the frequency command f of the i-th phase i , the frequency command f of the i-th phase i After integration, the network phase instruction θ of the i-th phase is obtained i ; Finally, the voltage amplitude command Up of the i-th phase is i and the network phase instruction θ of the i-th phase i The sine value of the i-th phase is multiplied to obtain the grid voltage command Uref i , that is, Uref i =Up i *sin(θ i ).

[0039] Among them, the transfer function of the inertia simulation link is:

[0040]

[0041] In the formula, K J is the inertia simulation parameter, K T is the integration parameter, K D is the damping simulation parameter, and s is the frequency domain transformation parameter.

[0042] The frequency deviation output by the inertia simulation link is also limited, and the frequency deviations of the three phases after limiting are added to the rated frequency to generate the three-phase frequency command, that is, rated The added frequency deviation value Δf i is the frequency deviation value after limiting.

[0043] Among them, the expression of the three-phase grid voltage command is:

[0044] Uref i =Up i *sin(θ i )

[0045] θ i =∫(Δf i +f rated )

[0046] Where Uref i is the grid voltage instruction of the i-th phase in the three-phase system, Up i is the voltage amplitude command of the i-th phase, θ i is the network phase instruction of the i-th phase, Δf i is the frequency deviation of the i-th phase, f rated is the rated frequency.

[0047] 2) Phase split virtual impedance link, such as Figure 7 As shown:

[0048] The grid voltage command Uref of the i-th phase iThe actual voltage U at the common connection point (PCC point) of the i-th phase i Subtract and divide by virtual impedance R V +sL V Get the grid current command Iref of phase i i Among them, R V is the virtual resistance, L V is the virtual reactance, and s is the frequency domain transformation parameter.

[0049] Among them, the expression of the three-phase grid current command value is:

[0050]

[0051] Where, Iref i is the grid current command of the i-th phase among the three phases, Uref i is the grid voltage command of the i-th phase, U i is the actual voltage at the common connection point of the i-th phase, R V is the virtual resistance, L V is the virtual reactance, and s is the frequency domain transformation parameter.

[0052] 3) Current control link, such as Figure 8 As shown:

[0053] The current control link is mainly used to limit the SVG current during grid fault ride-through to avoid overcurrent lockout.

[0054] The SVG output current is controlled to the command current Iref through closed-loop control. This can be achieved in the following two ways: one is to control the three-phase current command Iref a 、Iref b 、Iref c Iref converted to the rotated coordinate system d 、Iref q The closed-loop control is performed through a proportional-integral controller, and the other is to directly use the three-phase current command Iref a 、Iref b 、Iref c Closed-loop control is performed through a proportional resonant controller. After calculation, the three-phase modulation wave m is finally obtained. a 、m b 、m c , completing the entire control process.

[0055] The present invention adopts a method in which each phase independently generates its own reference voltage and reference phase. While retaining the networking characteristics of SVG self-generated phase and voltage, the flexibility of networking control is improved through phase-splitting control. It can solve the problem that when an asymmetric fault involving phase mutation occurs in the power grid, the charging and discharging directions and sizes of the three-phase currents are inconsistent, resulting in overvoltage or undervoltage of the sub-module capacitor voltage, thereby causing SVG fault lockout. It effectively improves the fault ride-through capability of the networking SVG, and can also effectively improve the supporting capability of the networking SVG under power grid fault conditions without increasing the capacitance value and investment cost.

[0056] An embodiment of a mesh control system for cascading SVGs:

[0057] A mesh control system for cascading SVGs includes a processor, the processor is used to execute a computer program to implement the steps of a mesh control method for cascading SVGs. The mesh control method for cascading SVGs has been described in detail in an embodiment of a mesh control method for cascading SVGs, and will not be repeated here.

Claims

1. A mesh control method for cascading SVG, characterized in that: The steps include: The three-phase grid current command value is input into the current control link to obtain a three-phase modulation wave for controlling the switching of the SVG submodule. The three-phase grid current command value is obtained by inputting the three-phase grid voltage command into the virtual impedance link of the corresponding phase respectively. The three-phase grid voltage command is the product of the sinusoidal quantity of the three-phase grid phase command and the voltage amplitude command of the corresponding phase respectively. The three-phase grid phase instruction is obtained in the following way: The difference between the square of the submodule voltage rated value and the square of the submodule voltage average value of the SVG three-phase bridge arm is used to generate the three-phase frequency deviation through the inertia simulation link, and the three-phase frequency deviation is added to the rated frequency to generate the three-phase frequency command, and the three-phase frequency command is integrated to obtain the three-phase network phase command.

2. The mesh control method for cascaded SVG according to claim 1, characterized in that: The expression of the three-phase grid voltage instruction is: Uref i =Up i *sin(θ i ) θ i =∫(Δf i +f rated ) Where Uref i is the grid voltage instruction of the i-th phase in the three-phase system, Up i is the voltage amplitude command of the i-th phase, θ i is the network phase instruction of the i-th phase, Δf i is the frequency deviation of the i-th phase, f rated is the rated frequency.

3. The mesh control method for cascaded SVG according to claim 1, characterized in that: The frequency deviation output by the inertia simulation link is also limited, and the frequency deviations of the three phases after the limit are added to the rated frequency to generate the three-phase frequency command.

4. The mesh control method for cascaded SVG according to claim 1, characterized in that: The expression of the three-phase grid current command value is: Where, Iref i is the grid current command of the i-th phase among the three phases, Uref i is the grid voltage command of the i-th phase, U i is the actual voltage at the common connection point of the i-th phase, R V is the virtual resistance, L V is the virtual reactance, and s is the frequency domain transformation parameter.

5. The mesh control method for cascaded SVG according to claim 1, characterized in that: The current control link is used to convert the three-phase grid-forming current command value into a grid-forming current command in a rotating coordinate system, and to perform closed-loop control on the grid-forming current command in the rotating coordinate system through a proportional-integral controller.

6. The mesh control method for cascaded SVG according to claim 1, characterized in that: The current control link performs closed-loop control on the three-phase grid current command value through a proportional resonant controller.

7. A mesh control system for cascading SVGs, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of the mesh control method for cascaded SVG according to any one of claims 1 to 6.

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