A current-type virtual inertia control strategy for MMC-HVDC

Abstract

The present invention discloses a current-type virtual inertia control strategy for MMC-HVDC, including a basic control strategy for MMC and a virtual inertia control strategy, wherein the virtual inertia control strategy is specifically: determining the output of the virtual inertia control according to the AC grid frequency on the low-voltage side of the step-up transformer, as well as system constants, inertia coefficients, and disturbance variables; the basic control strategy includes inner-loop control and outer-loop control. The virtual inertia strategy of the present invention can improve the inertia of MMC-HVDC.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronics, and in particular relates to a current-type virtual inertia control strategy for MMC-HVDC. Background Art

[0002] With the rapid development of new energy technologies, there are a large number of new energy devices in modern power grids. At present, these new energy sources are connected to the power grid through modular multilevel converters (MMC); these MMCs are composed of power electronic switching devices, and the switching of these devices is fast, that is, the switching action of the device is completed instantly. Therefore, when the power grid is disturbed, such as frequency changes or voltage fluctuations, the change of MMC voltage or frequency is often very fast. This characteristic of MMC equipment is also called low inertia. At present, there is an urgent need for a control strategy to improve the inertia of MMC. Summary of the invention

[0003] The object of the present invention is to provide a current type virtual inertia control strategy to improve the inertia of MMC-HVDC.

[0004] The technical solution to achieve the purpose of the present invention is: a current-type virtual inertia control strategy for MMC-HVDC, wherein the MMC-HVDC is composed of MMC converter stations with exactly the same two ends, the MMC converter stations at both ends are connected through a DC bus, and the MMC converter station at each end includes three phases a, b and c with exactly the same structure, each phase is collected through an AC bus and then connected to an external AC power grid through a step-up transformer, and each phase has two upper and lower bridge arms, each bridge arm includes a bridge arm inductor and N sub-modules cascaded in sequence, and the strategy includes a basic control strategy of MMC and a virtual inertia control strategy, and the virtual inertia control strategy is specifically: the output of the virtual inertia control is determined according to the AC power grid frequency on the low-voltage side of the step-up transformer, as well as the system constant, inertia coefficient, and disturbance variable; the basic control strategy includes inner loop control and outer loop control, and the outer loop control is performed by subtracting the active power set value from the actual active power value output by the MMC, or by using the DC bus voltage and The actual value of the DC bus voltage is subtracted, and the difference is sent to the PI controller to obtain the d-axis current reference value; the reactive power given value is subtracted from the actual value of the reactive power output by the MMC, or the AC bus voltage is subtracted from the actual value of the AC bus voltage, and the difference is sent to the PI controller to obtain the q-axis current reference value; the inner loop control is performed by adding the d-axis current reference value to the output of the virtual inertia control and then subtracting the d-axis current actual value from the d-axis current reference value and sending it to the PI controller to obtain the d-axis voltage reference value, the q-axis current reference value is subtracted from the q-axis current actual value and the product of the bridge arm inductance value, and then the actual value of the d-axis voltage on the low-voltage side of the step-up transformer is added to obtain the d-axis voltage reference value on the low-voltage side of the step-up transformer, and the q-axis voltage reference value is subtracted from the q-axis voltage reference value to obtain the product of the d-axis current actual value and the bridge arm inductance value, and then the actual value of the q-axis voltage on the low-voltage side of the step-up transformer is added to obtain the q-axis voltage reference value on the low-voltage side of the step-up transformer.

[0005] Preferably, the specific method for determining the output of the virtual inertia control according to the AC grid frequency on the low-voltage side of the step-up transformer and the system constant, inertia coefficient and disturbance variable is as follows: multiplying the AC grid frequency by the system constant and inertia coefficient and then subtracting the disturbance variable to obtain the output of the virtual inertia control.

[0006] Compared with the prior art, the present invention has the following significant advantages: the present invention does not require the MMC to add additional equipment, and only requires changes to be made in the control structure of the MMC; the present invention directly acts on the inner loop current control of the MMC and has a faster response performance; the present invention increases the inertia of the MMC by controlling the output voltage of the MMC, and the inertia size of the MMC can be set as needed.

[0007] The present invention is further described in detail below in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 This is a schematic diagram of the power outer loop control, current inner loop control and virtual inertia control strategy of the MMC-HVDC system.

[0009] Figure 2 This is a schematic diagram of the MMC-HVDC system structure.

[0010] Figure 3 Output active power for MMC.

[0011] Figure 4 is the AC bus voltage on the inverter side of the MMC.

[0012] Figure 5 is the DC bus voltage on the inverter side of the MMC. DETAILED DESCRIPTION

[0013] In order to more clearly describe the ideas, technical solutions and advantages of the present invention, the specific implementation methods are illustrated by examples and drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than 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.

[0014] like Figure 2 As shown in the figure, the high voltage direct current transmission based on modular multilevel converter (MMC-HVDC) topology structure is composed of MMC converter stations with exactly the same structure at both ends, which are connected by a DC bus. The MMC converter station at each end includes three phases a, b and c with exactly the same structure. Each phase is collected by an AC bus and then connected to the external AC power grid through a step-up transformer. Each phase has two upper and lower bridge arms, and each bridge arm includes a bridge arm inductor Larm and N submodules (Submodule) cascaded in sequence; the submodule includes two IGBT integrations and submodule capacitors, and each IGBT integration is composed of an IGBT anti-parallel diode.

[0015] like Figure 1 As shown, a current type virtual inertia control strategy of MMC-HVDC includes a basic control strategy of MMC and a virtual inertia control strategy.

[0016] The virtual inertia control strategy 1) uses the AC grid frequency on the low-voltage side of the step-up transformer as the input of the current-type virtual inertia control strategy; 2) multiplies the input AC grid frequency at the grid connection point by a system constant C and an inertia coefficient H; 3) subtracts the system disturbance variable K2 to obtain the output i of the virtual inertia controld_vic ;4) Change i d_vic Superimposed on the d-axis reference signal given by the current inner loop control.

[0017] The basic control strategy of the MMC includes inner loop control and outer loop control. The outer loop control is based on the active power given value P ref The difference between the actual active power value P output by the MMC and the actual active power value P is sent to the PI controller, or the DC bus voltage U dcref The actual value of the DC bus voltage U dc The difference is sent to the PI controller to obtain the d-axis current reference value i dref , and then through the reactive power given value Q ref The difference between the actual reactive power value Q output by the MMC and the actual reactive power value Q is sent to the PI controller, or the AC bus voltage U acref The actual value of the AC bus voltage U ac The difference is sent to the PI controller to obtain the q-axis current reference value i qref The inner loop control is performed by setting the d-axis current reference value i dref Add the output i of the virtual inertia control d_vic Then compare it with the actual value of d-axis current i d The difference is sent to the PI controller to obtain the d-axis voltage reference value v d , q-axis current reference value i qref The actual value of the q-axis current i q The difference is sent to the PI controller to obtain the q-axis voltage reference value v q , the d-axis voltage reference value v d Subtract the actual value of the q-axis current i q The product of the bridge arm inductance value plus the actual value of the d-axis voltage on the low-voltage side of the step-up transformer v sd Get the d-axis voltage reference value v on the low-voltage side of the step-up transformer cd , the q-axis voltage reference value v q Subtract the actual value of the d-axis current I d The product of the bridge arm inductance value plus the actual value of the q-axis voltage on the low-voltage side of the step-up transformer v sq Get the q-axis voltage reference value v on the low-voltage side of the step-up transformer cq , the q-axis voltage reference value v cq and d-axis voltage reference value v cd The submodules of the upper and lower bridge arms of the MMC are switched on and off through modulation, so that the desired three-phase voltage is obtained on the low-voltage side of the step-up transformer.

[0018] Example 1

[0019] The short-circuit ratio of the AC side of the receiving end of the MMC-HVDC is reduced to 2. A three-phase ground short-circuit fault with a duration of 0.075 s is triggered at 1.2 s at the point where the MMC is connected to the grid, without using virtual inertia control and with using virtual inertia control. The effect of virtual inertia control is illustrated by taking the output active power, output reactive power and DC bus voltage of the MMC receiving end as examples.

[0020] In the virtual inertia control strategy, the input of the virtual inertia control is the AC grid frequency at the grid connection point, and then the input AC grid frequency at the grid connection point is multiplied by a system constant C.

[0021] Preferably, the specific form of the system constant C is as follows:

[0022]

[0023] Where S MMC is the transmission capacity of the MMC-HVDC, f0 is the power frequency, which is 50HZ, and U s is the amplitude of the AC bus voltage, R eq is the equivalent resistance of MMC-HVDC, C eq The equivalent capacitance for MMC-HVDC can be calculated by the following formula.

[0024] Preferably, the equivalent capacitance C of MMC-HVDC eq The specific form is as follows:

[0025]

[0026] Wherein C is the capacitance value of the submodule, and N is the number of submodules in the MMC bridge arm.

[0027] Preferably, the equivalent resistance R of MMC-HVDC eq The specific form is as follows:

[0028]

[0029] Among them U dc is the DC bus voltage of MMC-HVDC.

[0030] In the virtual inertia control strategy, the grid frequency is multiplied by the system constant C and then multiplied by the inertia coefficient H, where H is selected as needed.

[0031] In the virtual inertia control strategy, the grid frequency is multiplied by the system constant C and the inertia coefficient H, and then the system disturbance variable K2 is subtracted to obtain the output of the virtual inertia control.

[0032] Preferably, the specific form of the disturbance variable K2 of MMC-HVDC is as follows:

[0033]

[0034] In the virtual inertia control strategy, after the grid frequency is multiplied by the system constant C and the inertia coefficient H, the system disturbance variable K2 is subtracted to obtain the output of the virtual inertia control. The output of the virtual inertia control is then superimposed on the d-axis reference signal given by the current inner loop control to realize the virtual inertia in the MMC-HVDC.

[0035] like Figure 3 As shown in the figure, an MMC-HVDC receiving end is connected to a weak grid with a short-circuit ratio of 2. The MMC output active power before and after a three-phase grounding short-circuit fault with a duration of 0.075s occurred at the MMC access grid at 1.2s without using virtual inertia control and with using virtual inertia control. When MMC-HVDC does not use virtual inertia control, after a three-phase grounding fault occurs, the MMC output active power will oscillate periodically, while when MMC-HVDC uses virtual inertia control, after a three-phase grounding fault occurs, the MMC output active power will quickly stabilize.

[0036] like Figure 4 As shown in the figure, an MMC-HVDC receiving end is connected to a weak grid with a short-circuit ratio of 2. The AC bus voltage on the MMC inverter side before and after a three-phase grounding short-circuit fault with a duration of 0.075s occurred at the MMC access grid at 1.2s, without using virtual inertia control and with using virtual inertia control. When MMC-HVDC does not use virtual inertia control, after a three-phase grounding fault occurs, the AC bus voltage on the MMC inverter side will oscillate periodically, while when MMC-HVDC uses virtual inertia control, after a three-phase grounding fault occurs, the AC bus voltage on the MMC inverter side will quickly stabilize.

[0037] like Figure 5 As shown in the figure, an MMC-HVDC receiving end is connected to a weak grid with a short-circuit ratio of 2. The DC bus voltage on the MMC inverter side before and after a three-phase grounding short-circuit fault with a duration of 0.075s occurred at the MMC access grid at 1.2s, without using virtual inertia control and with using virtual inertia control. When MMC-HVDC does not use virtual inertia control, after a three-phase grounding fault occurs, the DC bus voltage on the MMC inverter side will oscillate periodically, while when MMC-HVDC uses virtual inertia control, after a three-phase grounding fault occurs, the DC bus voltage on the MMC inverter side will quickly stabilize.

Claims

1. A current-type virtual inertia control strategy for MMC-HVDC, wherein the MMC-HVDC is composed of MMC converter stations with identical ends, the MMC converter stations at both ends are connected through a DC bus, and each MMC converter station includes three phases a, b and c with identical structures, each phase is collected through an AC bus and then connected to an external AC power grid through a step-up transformer, each phase has two upper and lower bridge arms, each bridge arm includes a bridge arm inductor and N sub-modules cascaded in sequence, characterized in that: The strategy includes the basic control strategy of MMC and the virtual inertia control strategy. The virtual inertia control strategy is specifically as follows: the output of the virtual inertia control is determined according to the AC grid frequency on the low-voltage side of the step-up transformer, as well as the system constant, inertia coefficient, and disturbance variable; the basic control strategy includes inner-loop control and outer-loop control. The outer-loop control makes a difference between the active power given value and the actual value of the active power output by the MMC, or makes a difference between the DC bus voltage and the actual value of the DC bus voltage, and sends the difference to the PI controller to obtain the d-axis current reference value; makes a difference between the reactive power given value and the actual value of the reactive power output by the MMC, or makes a difference between the AC bus voltage and the actual value of the AC bus voltage, and sends the difference to the PI controller to obtain the d-axis current reference value. The PI controller obtains a q-axis current reference value; the inner loop control obtains a d-axis voltage reference value by adding the output of the virtual inertia control to the d-axis current reference value and then subtracting the difference from the actual d-axis current value and sending the difference to the PI controller to obtain a q-axis voltage reference value; the d-axis current reference value is subtracted from the actual q-axis current value and the bridge arm inductance value and then adding the actual value of the d-axis voltage on the low-voltage side of the step-up transformer to obtain the d-axis voltage reference value on the low-voltage side of the step-up transformer; the q-axis voltage reference value is subtracted from the actual d-axis current value and the bridge arm inductance value and then adding the actual value of the q-axis voltage on the low-voltage side of the step-up transformer to obtain the q-axis voltage reference value on the low-voltage side of the step-up transformer; The specific method for determining the output of the virtual inertia control according to the AC grid frequency on the low-voltage side of the step-up transformer, the system constant, the inertia coefficient, and the disturbance variable is as follows: the output of the virtual inertia control is obtained by multiplying the AC grid frequency by the system constant and the inertia coefficient and then subtracting the disturbance variable; The specific form of the system constant C is: Among them, S MMC is the transmission capacity of MMC-HVDC, f0 is the power frequency, U s is the amplitude of the AC bus voltage, R eq is the equivalent resistance of MMC-HVDC, C eq is the equivalent capacitance of MMC-HVDC; The specific form of the disturbance variable K2 is as follows: S MMC is the transmission capacity of MMC-HVDC, H is the inertia coefficient, U s is the amplitude of the AC bus voltage, R eq is the equivalent resistance of MMC-HVDC, C eq is the equivalent capacitance of MMC-HVDC.

2. The current type virtual inertia control strategy of MMC-HVDC according to claim 1 is characterized in that: Equivalent capacitance C of MMC-HVDC eq The specific form is as follows: Wherein, C1 is the capacitance value of the submodule, and N is the number of submodules in the MMC bridge arm.

3. The current type virtual inertia control strategy of MMC-HVDC according to claim 1 is characterized in that: Equivalent resistance R of MMC-HVDC eq The specific form is as follows: Among them, U dc is the DC bus voltage of MMC-HVDC.

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