Coordinated control method for minimizing active and reactive power comprehensive oscillation quantity under current constraint

Abstract

The application discloses a current constraint condition active and reactive comprehensive oscillation quantity minimization collaborative control method, analyzes an inverter side converter in a power grid voltage unbalanced state, quantifies double frequency fluctuation quantities of active power and reactive power caused by a negative sequence component; adopts a third harmonic voltage compensation method to suppress the negative sequence current, dynamically adjusts third harmonic voltage compensation quantities of the converter, regulates and controls three-phase current balance; based on active and reactive power fluctuation suppression of the negative sequence current compensation method, utilizes controllable negative sequence current compensation quantities to eliminate double frequency fluctuation quantities of the active and reactive power, constructs an optimization function with active and reactive comprehensive fluctuation quantity minimization as a target, and adopts a particle swarm optimization algorithm to solve an optimal negative sequence current compensation scheme in real time, realizes collaborative suppression of three-phase current balance and power oscillation, and significantly improves operation stability and power transmission capacity of the inverter side converter under an asymmetric fault under current limiting conditions.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new energy grid-connected cooperative control technology, and particularly relates to a method for minimizing active and reactive power comprehensive oscillation under current constraint condition. BACKGROUND

[0002] Modular multilevel converter on the inverter side is widely used in new energy grid-connected power transmission due to its high output waveform quality and strong scalability. However, when the voltage is unbalanced due to asymmetric fault of the power grid, negative sequence current will appear on the alternating current side, which will further cause the double-frequency oscillation of active power and reactive power, and even endanger the safety of the equipment. The existing technology uses third harmonic voltage compensation method or negative sequence current compensation method to suppress negative sequence current or power fluctuation, but both are single target control, which is difficult to balance the contradiction among current balance, active power fluctuation and reactive power fluctuation, and still has the risk of system instability.

[0003] Therefore, there is an urgent need for a cooperative control method that can minimize the active and reactive power comprehensive oscillation under the premise of current amplitude limitation to solve the above problems. SUMMARY

[0004] The purpose of the present application is to solve the problem of insufficient system stability caused by single target control in the prior art when suppressing negative sequence current or power fluctuation, and to balance the contradiction among current balance, active power fluctuation and reactive power fluctuation. The method for minimizing active and reactive power comprehensive oscillation under current constraint condition is proposed to improve the operating stability and power transmission capacity of the converter on the inverter side under asymmetric fault.

[0005] In order to achieve the above purpose, the present application adopts the following technical scheme:

[0006] A method for minimizing active and reactive power comprehensive oscillation under current constraint condition, which comprises the following contents:

[0007] The inverter side converter under unbalanced grid voltage condition is analyzed, and the double-frequency fluctuation of active power and reactive power caused by negative sequence component is quantified;

[0008] The third harmonic voltage compensation method is used to suppress negative sequence current, and the third harmonic voltage compensation amount of the converter is dynamically adjusted to control the balance of three-phase current;

[0009] Based on the active power and reactive power fluctuation suppression of negative sequence current compensation method, the controllable negative sequence current compensation amount is used to eliminate the double-frequency fluctuation of active power and reactive power;

[0010] Taking current amplitude limitation as the constraint condition, a multi-objective optimization model is constructed to minimize the active and reactive power comprehensive fluctuation, and the particle swarm optimization algorithm is used to solve the optimal negative sequence current compensation scheme in real time.

[0011] As a further optimized scheme of the application, the analysis of the inverter side converter in the grid voltage unbalanced state, quantifying the active power and reactive power caused by the double frequency fluctuation of the negative sequence component, includes the following contents:

[0012] In the grid voltage unbalanced state, the poor running state of the inverter side converter is divided into: the existence of negative sequence current component in the grid side destroys the balance of three-phase current, the system active power appears double frequency fluctuation and the system reactive power appears double frequency fluctuation.

[0013] Using Park transformation, the AC side voltage and current in the grid voltage unbalanced state are obtained in the rotating coordinate system;

[0014] According to the instantaneous reactive power theory, combined with the AC side voltage and current in the rotating coordinate system, the instantaneous active power and reactive power of the system in the grid voltage unbalanced state are obtained;

[0015] Combined with the average value of the instantaneous active power and reactive power in the grid voltage balanced state, the negative sequence component existing in the AC side voltage and current in the grid voltage unbalanced state is obtained, which causes the double frequency fluctuation of the instantaneous active power and reactive power of the system.

[0016] As a further optimized scheme of the application, the three harmonic voltage compensation method is used to suppress the negative sequence current, and the three harmonic voltage compensation amount of the converter is dynamically adjusted to control the balance of three-phase current, including:

[0017] In the grid voltage unbalanced state, the outlet voltage of the converter is composed of positive sequence component and negative sequence component;

[0018] When the grid voltage is unbalanced, a three harmonic voltage compensation amount is injected into the converter sub-module to keep the voltage on the three-phase impedance balanced;

[0019] A free parameter , , is introduced to represent the ratio of the amplitude of the three harmonic voltage compensation amount in the three-phase to the amplitude of the positive sequence component of the outlet voltage of the converter;

[0020] After injecting the three harmonic voltage compensation amount into the converter sub-module, the voltage adjustment amplitude on the impedance is obtained by using the phase angle of the three harmonic voltage compensation amount in the two-phase;

[0021] Based on the size and phase of the three-phase voltage when the voltage is symmetrical, the relationship between the free parameter , , and the unbalance degree is derived;

[0022] When the grid voltage unbalance state occurs and the target is to suppress the negative sequence current, the free parameter is determined according to the unbalance degree 、 、 and the phase angle of the third harmonic voltage compensation amount, and the compensation amount of the third harmonic voltage is obtained according to the ratio of the amplitude of the third harmonic voltage compensation amount to the amplitude of the positive sequence voltage, and then the reference value of the third harmonic voltage compensation amount of each sub-module, the sub-module capacitor voltage reference value and the adjustment amount of the capacitor voltage equalization link are obtained to regulate the three-phase current balance.

[0023] As a further optimized scheme of the application, the active power and reactive power fluctuation suppression based on the negative sequence current compensation method uses the controllable negative sequence current compensation amount to eliminate the double frequency fluctuation of the active power and reactive power fluctuation, and the process is as follows:

[0024] To suppress the active power fluctuation, the negative sequence current compensation amount is introduced into the current, and the free parameter is introduced to determine the amplitude and phase of the compensated negative sequence current 、 、 , which represents the ratio of the amplitude of the compensated negative sequence current to the amplitude of the original positive sequence current, and the current on the AC side is obtained after the negative sequence current compensation, and the negative sequence current compensation amount is obtained after Park transformation , the negative sequence current compensation amount in the rotating coordinate system, according to the instantaneous power theory, the active power fluctuation component caused by the negative sequence current compensation and the active power fluctuation component are obtained after the negative sequence current compensation;

[0025] When the grid voltage is unbalanced, the target is to suppress the active power fluctuation, and the negative sequence current compensation amount and the free parameter 、 、 are used to obtain the negative sequence current compensated to the system, and the current reference value obtained by the voltage outer loop controller and the current value in the system are used to complete the current inner loop control link.

[0026] As a further optimized scheme of the application, when the target is to suppress the double frequency fluctuation of the reactive power, the unbalanced state , the AC side voltage, current and negative sequence current compensation amount in the rotating coordinate system are combined to obtain the reactive power of the system after the negative sequence current compensation and the reactive power fluctuation component caused by the negative sequence current compensation;

[0027] When the grid voltage is unbalanced, the free parameter is obtained based on the double frequency fluctuation of the instantaneous reactive power and the active power fluctuation component caused by the negative sequence current compensation 、 、 , and the negative sequence current compensation amount and the free parameter are used 、 、 The negative sequence current needing compensation to the system is obtained, and the current reference value obtained by the voltage outer loop controller and the current value in the system are used to complete the current inner loop control link.

[0028] As a further optimized scheme of the application, the multi-objective optimization model with the minimum active power and reactive power comprehensive fluctuation as the target is constructed under the current amplitude limitation constraint condition, comprising:

[0029] Based on the active power and reactive power of the system after the negative sequence current compensation, variables and are introduced to represent the active power and reactive power fluctuation respectively:

[0030]

[0031] In the formula, , are the maximum and minimum values of the active power respectively, , are the maximum and minimum values of the reactive power respectively, is the amplitude of the positive sequence voltage, is the amplitude of the positive sequence current;

[0032] The fluctuation of the active power and the fluctuation of the reactive power are linearly weighted, and the multi-objective optimization model with the minimum active power and reactive power comprehensive fluctuation as the target is:

[0033]

[0034] In the formula, , are the weighting coefficients of the active power and reactive power fluctuation respectively;

[0035] The maximum value of the three-phase current in a cycle is:

[0036]

[0037] The constraint condition of the optimization equation is:

[0038]

[0039] In the formula, is the phase current amplitude limitation, , , represent the three-phase current on the AC side.

[0040] As a further optimization of this invention, based on a multi-objective optimization model aiming to minimize the combined fluctuation of active and reactive power, a particle swarm optimization algorithm is used to solve for the optimal negative sequence current compensation scheme in real time. The process is as follows:

[0041] In the particle swarm optimization algorithm, each particle represents a potential negative-order current compensation scheme, and its position is determined by the following parameters:

[0042]

[0043] In the formula, For the current particle Location, That is, the current particle The negative sequence current compensation parameters represented;

[0044] The fitness function is used to evaluate the quality of negative sequence current compensation parameters. The fitness function is:

[0045]

[0046] In the formula, and Using the current particle position At that time, the fluctuation of active and reactive power in the system;

[0047] The maximum three-phase current of each particle satisfies the phase current limiting constraint determined by system safety:

[0048]

[0049] During the iterative process of the particle swarm optimization algorithm, for any particle that does not satisfy the position constraint... Set its fitness value to a penalty value;

[0050] Initialization: Randomly initialize a group of particles within a preset parameter range. Each particle contains a set of randomly generated particles. And randomly initialize the speed;

[0051] Iterative updates, for each particle in each generation, based on the current position The fluctuation of active power is calculated based on a multi-objective optimization model. Fluctuation in reactive power and the maximum value of three-phase current ;

[0052] Determine feasibility based on constraints and calculate the fitness value for each particle. ;

[0053] The search yields the active power fluctuation at the individual optimal position of each particle. and reactive power fluctuation amount and the global optimal position of the whole population , update the speed and position of each particle;

[0054] a termination condition, when the preset maximum iteration number is reached or the change of the global optimal fitness value is less than the set threshold, the iteration is stopped, and the global optimal position at this time is output , which is the optimal negative sequence current compensation scheme under the current system state.

[0055] Compared with the prior art, the beneficial effects of the present application are: compared with the existing single target control, the present application constructs a multi-objective optimization model with the minimum of the comprehensive fluctuation amount of active power and reactive power as the target under the constraint condition of current limiting, and adopts the particle swarm optimization algorithm to solve the optimal negative sequence current compensation scheme in real time, realizes the collaborative control of three-phase current balance and power oscillation, and significantly improves the operation stability and power transmission capacity of the inverter on the inverter side under asymmetric fault under the condition of current limiting. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 is the control schematic diagram of the third harmonic voltage compensation method in the embodiment of the present application;

[0057] Figure 2 is the equivalent circuit schematic diagram of the a-phase on the alternating current side in the embodiment of the present application;

[0058] Figure 3 is the control logic schematic diagram of the third harmonic voltage compensation method in the embodiment of the present application;

[0059] Figure 4 is the converter control schematic diagram based on the negative sequence current compensation method in the embodiment of the present application;

[0060] Figure 5 is the control schematic diagram of the negative sequence current compensation method for suppressing active power fluctuation in the embodiment of the present application;

[0061] Figure 6 is the control schematic diagram of the negative sequence current compensation method for suppressing reactive power fluctuation in the embodiment of the present application;

[0062] Figure 7 is the flowchart of the particle swarm optimization algorithm for obtaining the optimal negative sequence current compensation scheme in the embodiment of the present application. DETAILED DESCRIPTION

[0063] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments.

[0064] The embodiment proposes a current constraint active and reactive power comprehensive oscillation quantity minimization collaborative control method, which comprises the following contents:

[0065] The inverter side converter in the grid voltage unbalanced state is analyzed, and the active power and reactive power double frequency fluctuation caused by the negative sequence component is quantified.

[0066] The negative sequence current compensation method is used to suppress the negative sequence current, and the third harmonic voltage compensation of the converter is dynamically adjusted to regulate the three-phase current balance.

[0067] Based on the active power and reactive power fluctuation suppression of the negative sequence current compensation method, the controllable negative sequence current compensation is used to eliminate the double frequency fluctuation of active power and reactive power respectively.

[0068] Taking current limiting as the constraint condition, a multi-objective optimization model with the minimum active and reactive power comprehensive fluctuation as the target is constructed, and the particle swarm optimization algorithm is used to solve the optimal negative sequence current compensation scheme in real time, realizing the collaborative control of three-phase current balance and power oscillation.

[0069] The inverter side converter in the grid voltage unbalanced state is analyzed, and the active power and reactive power double frequency fluctuation caused by the negative sequence component is quantified.

[0070] The inverter side converter runs in normal condition, with high output waveform quality and good running condition. However, in actual operation process, the power system cannot always be in ideal state, and is often affected by some factors, resulting in some fault conditions, and the probability of grid voltage imbalance caused by asymmetric fault is the highest. When designing the converter, it is generally expected that even if it runs in fault condition, it can also work well.

[0071] Under the condition of grid voltage imbalance, there are mainly three kinds of bad running states of inverter side converter: the existence of negative sequence current component in grid side destroys the balance of three-phase current, the active power of system appears double frequency fluctuation and the reactive power appears double frequency fluctuation. If these three conditions are not improved in time, it will threaten the safe operation of the system.

[0072] When the system runs normally, only positive sequence components are contained in three-phase voltage and current, and three-phase voltage and current remain balanced. When the grid voltage imbalance occurs, the AC side voltage and current of the converter (MMC) generally consist of three parts, positive sequence, negative sequence and zero sequence components, and the balance of three-phase voltage and current is destroyed. At the same time, since the converter in the system generally adopts Y / ∆ connection form, it can block the flow of zero sequence component, so the voltage and current on the AC side only need to consider the positive sequence and negative sequence, which can be respectively expressed as:

[0073] (1)

[0074] (2);

[0075] In the formula, , , This represents the three-phase voltage on the AC side. , , This indicates the three-phase current on the AC side; , These represent the amplitudes of the positive-sequence and negative-sequence voltages. , These represent the amplitudes of the positive-sequence current and the negative-sequence current. Represents angular frequency. Represents a time variable. The impedance angle of the system is denoted as .

[0076] Assume that the magnitudes of the positive and negative sequence components of voltage and current satisfy the following relationships:

[0077] (3);

[0078] In the formula , The variables are the imbalance of voltage and current on the grid side.

[0079] As can be seen from equation (2), the presence of the negative sequence component in the current causes an imbalance in the three-phase current on the AC side of the converter. If the imbalance of the three-phase current in the system is large, it will threaten the safe operation of the converter.

[0080] Substituting equation (3) into equations (1) and (2), and using the Park transformation, we obtain the unbalanced state. The AC side voltage and current in the rotating coordinate system are expressed as follows:

[0081] (4);

[0082] (5);

[0083] In the formula, and When indicating the state of voltage imbalance in the power grid AC side voltage in a rotating coordinate system and When indicating the state of voltage imbalance in the power grid AC current in a rotating coordinate system. Based on the instantaneous reactive power theory, and combining equations (4) and (5), the instantaneous active power of the system under grid voltage imbalance can be obtained. and reactive power , respectively, are the average values of the instantaneous active power and the instantaneous reactive power under the balanced state of the grid voltage,

[0084] (6);

[0085] in the formula, , , respectively, are the average values of the instantaneous active power and the instantaneous reactive power under the balanced state of the grid voltage, , , respectively, are the double-frequency fluctuation amounts of the instantaneous active power and the instantaneous reactive power, respectively, are:

[0086] (7);

[0087] (8);

[0088] As can be seen from the formula (4) to the formula (8), under the unbalanced state of the grid voltage, the negative sequence components existing in the alternating current side voltage and the alternating current will cause the active power and the reactive power of the system to appear double-frequency fluctuations, so that the converter loses the ability of stable operation and is difficult to bear the function of power transmission. When the unbalance degree is too large, the inverter device will even be burned out.

[0089] The third harmonic voltage compensation method is adopted to suppress the negative sequence current, and the third harmonic voltage compensation amount of the converter is dynamically adjusted to control the balance of the three-phase current. The process is as follows:

[0090] When the unbalanced state of the grid voltage occurs, the negative sequence current appears, and the balance of the three-phase current is destroyed. The unbalance of the three-phase current on the alternating current side can be reflected to the unbalance of the voltage on the three-phase impedance, and the purpose of the balance of the three-phase current can be achieved by controlling the voltage on the three-phase impedance. The voltage on the grid side impedance is related to the grid voltage and the outlet voltage of the converter. When the unbalanced state of the grid voltage occurs, adjusting the outlet voltage of the converter can make the three-phase current keep balance.

[0091] The third harmonic compensation method is introduced into the carrier phase shift modulation technology (CPS-SPWM). The modulation method not only can retain the advantages of the carrier phase shift technology, but also can improve the utilization rate of the direct current voltage and the quality of the output waveform. Under the unbalanced state of the grid voltage, the third harmonic voltage compensation method is adopted to suppress the negative sequence current, the mathematical relationship between the unbalance degree β of the voltage and the compensation amount of the third harmonic voltage is analyzed, and the negative sequence current component is suppressed. Figure 1 As shown in the figure, the inverter side converter control logic diagram based on the third harmonic voltage compensation method is shown. Through the voltage outer loop control, the current inner loop control and the capacitor voltage balance control, the third harmonic voltage compensation method control is added. Compared with the control method under the normal condition, a part of voltage component is added in the modulation link, so that the purpose of the balance of the three-phase current can be achieved.

[0092] Under grid voltage imbalance, the converter's output voltage consists of positive-sequence and negative-sequence components, and its expression can be given as:

[0093] (9);

[0094] In the formula, This represents the positive sequence component of the converter output voltage.

[0095] like Figure 2 For the communication side Equivalent circuit diagram, , For the equivalent resistance and inductance of the circuit, with Taking one phase as an example, the AC side current can be expressed as:

[0096] (10);

[0097] Because the three-phase impedance used in the system is symmetrical, the balance of the three-phase current is mainly related to the AC side output voltage. and converter output voltage related. Represents the imaginary unit. This represents the inductor component, which experiences phase lag in response to current. It is the difference between the grid-side voltage and the converter output voltage when the grid voltage is unbalanced. If three-phase symmetry cannot be maintained, the three-phase currents will also be unbalanced. To balance the three-phase currents, a suitable third harmonic voltage compensation is injected into the converter submodule to keep the voltages across the three-phase impedances balanced.

[0098] Therefore, this invention introduces three free parameters. , , , respectively represent The ratio of the amplitude of the third harmonic voltage compensation in the three phases to the amplitude of the positive sequence component of the converter output voltage:

[0099] (11);

[0100] In the formula, , , for Third harmonic voltage compensation in three phases.

[0101] After injecting the third harmonic voltage compensation, the converter output voltage is expressed as:

[0102] (12);

[0103] In the formula, for 、 The phase angle of the three harmonic voltage compensation amount in two phases, which is to correct the phase shift caused by unbalanced fault 、 , the expression is as follows:

[0104] (13);

[0105] The voltage adjustment amplitude on the impedance of the converter sub-module after injecting the three harmonic voltage compensation amount is:

[0106] (14);

[0107] (15);

[0108] (16);

[0109] To ensure the symmetry of the three-phase voltage, the size and phase of the voltage need to meet the following conditions:

[0110] 1) The amplitude of the three-phase voltage (voltage adjustment amplitude on the impedance) is equal; 2) The phase angle difference is , that is:

[0111] (17);

[0112] From formula (12) to formula (17), it can be deduced that when the three-phase current is balanced, the free parameter 、 、 and the unbalance degree have the following relationship:

[0113] (18);

[0114] When the voltage unbalance degree , 、 The phase voltage is reduced, and the voltage amplitude is lower than that of the phase voltage, and 、 , The phase voltage amplitude is equal. In order to maintain the balance of three-phase current, it is necessary to appropriately reduce the phase voltage and increase the phase voltage, so at this time The phase introduces the parameter 、 The phase introduces the parameter , The phase introduces the parameter , The phase introduces the parameter . .

[0115] When the voltage unbalance degree , The phase voltage is reduced, and the voltage amplitude is lower than , The phase voltage amplitude. In order to maintain three-phase current balance, the phase voltage needs to be appropriately raised The phase voltage is reduced , The phase voltage, so at this time The phase introduction parameter , The phase introduction parameter , The phase introduction parameter

[0116] As Figure 3 is a schematic diagram of the third harmonic voltage compensation method control, when the grid voltage unbalance state occurs and the target is to suppress the negative sequence current, the sizes of , , and are determined according to formula (18) and formula (13) respectively, and the compensation amount of the third harmonic voltage is obtained according to formula (11), and finally the third harmonic voltage compensation signal of three phases is constructed according to formula (12) and (13) .

[0117] The third harmonic voltage compensation reference value of each sub-module is:

[0118] (19);

[0119] In the formula, The number of cascaded sub-modules.

[0120] The circulating current reference value is obtained by using the voltage outer loop control link, and then the output is obtained through the current inner loop control, and this is used as the adjustment amount of the capacitor voltage balance control. The average value of the sub-module capacitor voltage

[0121] (20);

[0122] In the formula, Indicates the number of sub-modules, Indicates the measured value of the capacitor voltage of each sub-module.

[0123] The capacitor voltage reference value of each sub-module is:

[0124] (21);

[0125] In the formula,​​​ is the DC side voltage.

[0126] (22);

[0127] In the formula, is the controller parameter in the voltage balancing control in the MMC, is the current flowing through the upper bridge arm of the phase, respectively.

[0128] Further, the sub-module capacitor voltage reference value and the regulation amount of the capacitor voltage balancing link , , the modulation of three-phase current balance is carried out, and the purpose of three-phase current balance is achieved.

[0129] Based on the active power and reactive power fluctuation suppression method of negative sequence current compensation, the double frequency fluctuation of active power and reactive power is eliminated by using controllable negative sequence current compensation amount, and the process is as follows:

[0130] According to the analysis of instantaneous power, under the condition of unbalanced grid voltage, the negative sequence component in voltage and current leads to the double frequency fluctuation of active power and reactive power in the system. If the negative sequence component in the system is compensated by a certain amount, the double frequency fluctuation of power will be affected.

[0131] When the unbalanced grid voltage state occurs, the negative sequence component in the voltage is determined by the state at the time of fault, and it is difficult to adjust. The present application proposes a control method for suppressing active power and reactive power based on the negative sequence current compensation method, and analyzes and analyzes the mathematical relationship between the voltage unbalance degree and the negative sequence current compensation amount.

[0132] As Figure 4 the converter control logic diagram based on the negative sequence current compensation method proposed by the present application, in the current inner loop control link, the negative sequence current compensation method is used for control, the negative sequence current compensation amount is used, and the voltage outer loop control is used to achieve the current inner loop control, so as to change the reference value of the current, and then cooperate with the capacitor voltage balancing control to form a modulation strategy, so as to achieve the purpose of suppressing the double frequency fluctuation of active power or reactive power.

[0133] In order to suppress the fluctuation of active power, the negative sequence current compensation amount is introduced into the current, and the amplitude and phase of the compensated negative sequence current need to be determined. Three free parameters , , respectively represent the ratio of the amplitude of the compensated negative sequence current to the amplitude of the original positive sequence current:

[0134] ​​(23);

[0135] wherein, , , is the amplitude of the three-phase negative sequence current compensation.

[0136] At this time, the compensation amount of the negative sequence current is:

[0137] (24);

[0138] wherein, , , are the phases of the three-phase negative sequence current compensation, respectively, and are selected .

[0139] Combining equation (2) and equation (24), the current on the AC side of the system after negative sequence current compensation is represented as:

[0140] (25);

[0141] After Park transformation of equation (24), the negative sequence current compensation in the coordinate system is:

[0142] (26);

[0143] Combining equation (4), equation (5) and equation (26), according to the instantaneous power theory, the active power of the system after negative sequence current compensation is obtained as:

[0144] (27);

[0145] wherein, is the active power fluctuation component caused by the negative sequence current compensation:

[0146] (28);

[0147] In order to suppress the two-frequency fluctuation of the active power, let , and accordingly the free parameters , , are respectively:

[0148] (29);

[0149] The control method based on the negative sequence current compensation method mainly realizes it by changing the current inner loop element, such as Figure 5The control schematic diagram for suppressing active power fluctuation by negative sequence current compensation is shown in Fig. 4. When the grid voltage is unbalanced, the amount of negative sequence current that needs to be compensated to the system is calculated by formula (24) and formula (29) with the target of suppressing the active power fluctuation , , and the current value in the system , together complete the current inner loop control link. ,

[0150] Similarly, when the target is to suppress the double frequency fluctuation of reactive power, the reactive power of the system after negative sequence current compensation is obtained by combining formula (4), formula (5) and formula (26):

[0151] (30);

[0152] In the formula, is the reactive power fluctuation component caused by the negative sequence current compensation amount:

[0153] (31);

[0154] In order to suppress the amount of double frequency fluctuation of reactive power, let , and the free parameters , , are respectively:

[0155] (32);

[0156] As Figure 6 is the control schematic diagram for suppressing reactive power fluctuation based on negative sequence current compensation method. When the grid voltage is unbalanced, the amount of negative sequence current that needs to be compensated to the system is calculated by formula (24) and formula (32) with the target of suppressing the reactive power fluctuation, and the current reference value obtained by the voltage outer loop controller PI, the current value in the system, together complete the current inner loop control link.

[0157] With current limiting as the constraint condition, a multi-objective optimization model with the target of minimizing the comprehensive fluctuation of active power and reactive power is constructed, and the particle swarm optimization algorithm is used to solve the optimal negative sequence current compensation scheme in real time, realizing the collaborative control of three-phase current balance and power oscillation.

[0158] ​When three-phase current balance is achieved, the active power and the reactive power of the system still have a large fluctuation under the unbalanced state of the grid voltage. When the double-frequency fluctuation of the active power is eliminated, the fluctuation of the reactive power is more severe, and the unbalance degree of the three-phase current is increased. When the double-frequency fluctuation of the reactive power is eliminated, the fluctuation of the active power is more severe, and the unbalance degree of the three-phase current is increased. Although the compensation method can improve part of the operating characteristics of the system during the fault, it will also cause other adverse effects, which may threaten the safety of the system. Therefore, based on the control method of the negative sequence current compensation method, the comprehensive adjustment of the three targets of current, active power and reactive power is considered, and multi-objective control is achieved to ensure that the overall control effect is optimal.

[0159] As can be seen from the above, when the fluctuation of the active power or the reactive power is suppressed, the unbalance degree of the three-phase current becomes more serious, which may threaten the operation of the system. The peak value change of the current can reflect the unbalance degree of the current, and therefore, in the present embodiment, the amplitude limiting of the current is taken as a constraint condition, and the comprehensive fluctuation amount of the active power and the reactive power is taken as a target function, so that the comprehensive fluctuation amount of the two is minimized, and the operating state of the system under fault is improved.

[0160] According to the above analysis, after the negative sequence current compensation, the active power and the reactive power of the system are respectively , . In order to facilitate analysis, two variables and are introduced to represent the fluctuation amount of the active power and the reactive power respectively:

[0161] (33);

[0162] In the formula, , are the maximum and minimum values of the active power respectively, , are the maximum and minimum values of the reactive power respectively.

[0163] In the control process, the free parameters , , are adjusted, so that the double-frequency fluctuation amount of the active power and the reactive power can be adjusted. The fluctuation amount of the active power and the fluctuation amount of the reactive power are linearly weighted, and the comprehensive fluctuation amount of the active power and the reactive power is taken as a target function, and a multi-objective optimization model with the minimum active power and reactive power comprehensive fluctuation amount as the target is constructed, and the expression is as follows:

[0164] (34);

[0165] wherein, , are the weighting coefficients of active power and reactive power fluctuation respectively, satisfying .

[0166] The maximum value of three-phase current in a cycle is:

[0167] (35);

[0168] The constraint condition of the optimization equation is:

[0169] (36);

[0170] wherein, is the phase current limit, , , represents the three-phase current on the AC side.

[0171] Under the condition of grid voltage imbalance, there is a negative sequence current component in the grid side, which destroys the balance of three-phase current, and the active power of the system appears two-frequency fluctuation, and the reactive power appears two-frequency fluctuation. When the particle swarm optimization algorithm is used to solve the optimal negative sequence current compensation scheme in real time, the influence of grid voltage imbalance can be minimized, and the three-phase current balance and power collaborative control can be realized.

[0172] The basic principle of particle swarm optimization (PSO) is as follows:

[0173] PSO is initialized as a group of random particles (random solutions), and then the optimal solution is found through iteration. In each iteration, the particle updates itself by tracking two extreme values; the first is the optimal solution found by the particle itself, which is called individual extreme value; the other extreme value is the optimal solution found by the whole population at present, which is the global extreme value. In addition, instead of using the whole population, only a part of it can be used as the neighbor of the particle, and the extreme value in all neighbors is the local extreme value.

[0174] Suppose there is a target search space with dimension , which is composed of particles in a colony, wherein the particle is a dimensional vector, represented as:

[0175] (37);

[0176] The flight speed of the particle is also a dimensional vector, represented as:

[0177] (38);

[0178] The optimal value searched by the particle, i.e., individual extremum, is expressed as:

[0179] (39);

[0180] The optimal value searched, i.e., global extremum, is expressed as:

[0181] (40);

[0182] After searching for the individual extremum and the global extremum, the position and the speed of the particle need to be updated, and the updating method is:

[0183] (41);

[0184] In the formula, is the iteration number, , is a learning factor, , are two pseudo-random numbers independent of each other and obeying a uniform distribution between 0 and 1, indicates the current speed of the i-th particle, indicates the updated speed of the i-th particle, indicates the individual best position of the i-th particle, indicates the current position of the i-th particle, indicates the global best position, indicates the updated position of the i-th particle. For the optimal negative sequence current compensation scheme problem of the application, a particle swarm algorithm is adopted to solve it, the steps of the PSO algorithm are as shown in , and the process is as follows: 1. Optimization variable (particle position)

[0185] Each particle in the PSO algorithm represents a potential negative sequence current compensation scheme, and its position is composed of three free parameters: Figure 7

[0186] (42);

[0187] In the formula, is the position of the current particle,

[0188]

[0189] In the formula, is the position of the current particle, ​​​​​The represented negative sequence current compensation parameter.

[0190] 2. Fitness function design

[0191] The fitness function is used to evaluate the merit of each particle (i.e. each set of negative sequence current compensation parameter). The objective of the present application is to minimize the total power fluctuation while satisfying the current peak constraint, so the fitness function is designed as:

[0192] (43);

[0193] Where, and are the active power fluctuation and reactive power fluctuation respectively calculated from the system model (equations (4)-(8), (27)-(32)) using the current particle position is the weighting factor, satisfying , which can be adjusted according to the system sensitivity to active / reactive power fluctuation.

[0194] 3. Constraint handling

[0195] Each particle's three-phase current maximum also needs to satisfy the phase current clipping constraint determined by system security:

[0196] (44);

[0197] In the process of particle swarm optimization (PSO) iteration, for any particle that does not satisfy the position constraint , its fitness value is set to a very large penalty value (for example ), so as to ensure that the search process is automatically concentrated in the feasible solution region.

[0198] 4. PSO initialization and iteration process

[0199] (1) Initialization: Randomly initialize a group of particles (population size usually set to 20-50) within the preset parameter range, each particle contains a set of randomly generated , and its speed is randomly initialized.

[0200] (2) Iterative update: for each particle in each generation, according to its current position , calculate the corresponding active power fluctuation , reactive power fluctuation and three-phase current maximum based on the multi-objective optimization model;

[0201] ​​(3)Judge the feasibility according to the constraint condition, and calculate the fitness value of each particle ;

[0202] (4)Search the active power fluctuation and the reactive power fluctuation of the individual optimal position of each particle and the global optimal position of the whole population ; ;

[0203] (5)Update the speed and position of each particle according to the standard PSO speed and position update formula (41).

[0204] (6)Termination condition: when the preset maximum iteration number (such as 50-100 times) is reached or the change of the global optimal fitness value is less than the set threshold, the iteration stops. The global optimal position at this time is output , which is the optimal negative sequence current compensation scheme under the current system state.

[0205] 5. Real-time integration scheme

[0206] The above PSO optimization process is embedded in the digital control system (such as DSP or FPGA) of the converter, and runs in an online manner. The system monitors the grid voltage unbalance degree , current unbalance degree and the current power fluctuation state in real time, periodically (such as every 100ms) calls the PSO algorithm to solve the optimal compensation parameters, and dynamically adjusts the negative sequence current injection amount, realizes the adaptive cooperative suppression control.

[0207] 6. Algorithm acceleration and real-time guarantee

[0208] In order to meet the stringent requirements of real-time control on calculation speed, the following strategies are adopted to improve the PSO calculation efficiency:

[0209] (1) Use the optimization result of the last control period as the center point of the PSO initialization of this period, and generate part of the particles in its neighborhood, so as to greatly reduce the number of iterations required for convergence.

[0210] (2) Strictly limit the maximum number of iterations of PSO each round, and ensure that the time consumption of single optimization calculation is much less than the control period.

[0211] It should be noted that the part of the present application not involved is the same as or can be realized by the prior art. The above description is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can make equivalent replacement or change according to the technical scheme and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered in the protection scope of the present application.

Claims

1. A collaborative control method for minimizing the combined active and reactive power oscillations under current constraints, characterized in that, Includes the following: The inverter-side converter under grid voltage imbalance is analyzed to quantify the second harmonic fluctuation of active and reactive power caused by negative sequence components. The third harmonic voltage compensation method is used to suppress negative sequence current, dynamically adjust the third harmonic voltage compensation amount of the converter, and regulate the three-phase current balance, including: Under grid voltage imbalance, the converter's output voltage consists of positive-sequence and negative-sequence components. When grid voltage imbalance occurs, third harmonic voltage compensation is injected into the converter submodule to maintain voltage balance across the three-phase impedances, introducing free parameters. , , This represents the ratio of the amplitude of the third harmonic voltage compensation in the three phases to the amplitude of the positive sequence component of the converter outlet voltage. After the third harmonic voltage compensation is injected into the converter submodule, the phase angle of the third harmonic voltage compensation is used to obtain the voltage adjustment amplitude on the impedance. Based on the magnitude and phase of the voltage when the three-phase voltages are symmetrical, the free parameters are derived. , , Relationship with unbalance degree; when a grid voltage imbalance occurs and the goal is to suppress negative sequence current, the free parameters are determined based on the unbalance degree. , , The phase angle of the third harmonic voltage compensation amount is obtained, and the compensation amount of the third harmonic voltage is obtained according to the ratio of the amplitude of the third harmonic voltage compensation amount to the amplitude of the positive sequence voltage. Then, the reference value of the third harmonic voltage compensation amount of each sub-module, the reference value of the capacitor voltage of the sub-module, and the adjustment amount of the capacitor voltage balancing link are obtained to regulate the three-phase current balance. Active and reactive power fluctuation suppression based on negative sequence current compensation method: By utilizing a controllable negative sequence current compensation amount, the second harmonic fluctuations of active and reactive power are eliminated. The process is as follows: To suppress active power fluctuations, a negative sequence current compensation is introduced into the current. To determine the amplitude and phase of the compensated negative sequence current, free parameters are introduced. , , The ratio of the compensated negative-sequence current amplitude to the original positive-sequence current amplitude is used to obtain the AC current after negative-sequence current compensation. The compensated negative-sequence current is then transformed by Park to obtain... The negative sequence current compensation in the rotating coordinate system, based on instantaneous power theory, yields the system's active power after negative sequence current compensation and the active power fluctuation component caused by the negative sequence current compensation. When the grid voltage is unbalanced, the goal is to suppress active power fluctuations by utilizing the negative sequence current compensation and free parameters. , , It obtains the negative sequence current to compensate the system, and together with the current reference value obtained by the voltage outer loop controller and the current value in the system, completes the current inner loop control loop. Using current limiting as a constraint, a multi-objective optimization model is constructed with the goal of minimizing the combined fluctuation of active and reactive power. The optimal negative sequence current compensation scheme is solved in real time using the particle swarm optimization algorithm.

2. The collaborative control method for minimizing the combined active and reactive power oscillations under current constraints according to claim 1, characterized in that, The analysis of the inverter-side converter under grid voltage imbalance conditions quantifies the second harmonic fluctuations of active and reactive power caused by negative sequence components, including the following: Under grid voltage imbalance, the poor operating conditions of the inverter-side converter are as follows: the presence of negative sequence current components on the grid side disrupting the balance of the three-phase currents, the occurrence of second harmonic fluctuations in the system's active power, and the occurrence of second harmonic fluctuations in the system's reactive power. Using the Park transform, the voltage imbalance state of the power grid is obtained. AC side voltage and current in a rotating coordinate system; Based on the theory of instantaneous reactive power, combined with By rotating the AC side voltage and current in the coordinate system, the instantaneous active and reactive power of the system under the grid voltage imbalance condition can be obtained. By combining the average values ​​of instantaneous active and reactive power under balanced grid voltage conditions, the negative sequence components in AC side voltage and current under unbalanced grid voltage conditions are obtained, causing the second harmonic fluctuation of instantaneous active and reactive power in the system.

3. The collaborative control method for minimizing the combined active and reactive power oscillations under current constraints according to claim 2, characterized in that, When the goal is to suppress the second harmonic fluctuation of reactive power, combined with the unbalanced state... The AC side voltage, current and negative sequence current compensation in the rotating coordinate system are used to obtain the reactive power of the system after negative sequence current compensation and the reactive power fluctuation component caused by the negative sequence current compensation. When the grid voltage is unbalanced, the free parameters are obtained based on the active power fluctuation component caused by the second harmonic fluctuation of instantaneous reactive power and the negative sequence current compensation. , , And utilize the compensation amount of negative sequence current and free parameters , , It obtains the negative sequence current that needs to be compensated to the system, and together with the current reference value obtained by the voltage outer loop controller and the current value in the system, completes the current inner loop control loop.

4. The collaborative control method for minimizing the combined active and reactive power oscillations under current constraints according to claim 1, characterized in that, The multi-objective optimization model, which uses current limiting as a constraint and aims to minimize the combined fluctuation of active and reactive power, includes: Based on the active and reactive power of the system after negative sequence current compensation, variables are introduced. and These represent the fluctuations in active power and reactive power, respectively. ； In the formula, , These are the maximum and minimum values ​​of active power, respectively. , These are the maximum and minimum values ​​of reactive power, respectively. This represents the amplitude of the positive sequence voltage. This represents the amplitude of the positive sequence current. Fluctuations in active power and reactive power fluctuations A linearly weighted multi-objective optimization model that aims to minimize the combined fluctuation of active and reactive power: ； In the formula, , These are the weighting coefficients for active power and reactive power fluctuations, respectively. The maximum value of the three-phase current in one cycle for: ； The constraints of the optimization equation are: ； In the formula, For phase current limiting, , , This indicates the three-phase current on the AC side.

5. The collaborative control method for minimizing the combined active and reactive power oscillations under current constraints according to claim 4, characterized in that, Based on a multi-objective optimization model aiming to minimize the combined fluctuation of active and reactive power, the optimal negative-sequence current compensation scheme is solved in real time using a particle swarm optimization algorithm, as follows: In the particle swarm optimization algorithm, each particle represents a potential negative-order current compensation scheme, and its position is determined by the following parameters: ； In the formula, For the current particle Location, That is, the current particle The negative sequence current compensation parameters represented; The fitness function is used to evaluate the quality of negative sequence current compensation parameters. The fitness function is: ； In the formula, and Using the current particle position At that time, the fluctuation of active and reactive power in the system; The maximum three-phase current of each particle satisfies the phase current limiting constraint determined by system safety: ； During the iteration of the particle swarm optimization algorithm, for any particle that does not satisfy the position constraint, its fitness value is set to a penalty value; Initialization: Randomly initialize a group of particles within a preset parameter range. Each particle contains a set of randomly generated particles. And randomly initialize the speed; Iterative updates, for each particle in each generation, based on the current position The fluctuation of active power is calculated based on a multi-objective optimization model. Fluctuation in reactive power and the maximum value of three-phase current ; Determine feasibility based on constraints and calculate the fitness value for each particle. ; The search yields the active power fluctuation at the individual optimal position of each particle. and reactive power fluctuation And the global optimal position of the entire population. Update the velocity and position of each particle; The termination condition is that iteration stops when the preset maximum number of iterations is reached or the change in the global optimal fitness value is less than a set threshold, and the global optimal position at this point is output. This is the optimal negative sequence current compensation scheme under the current system state.

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