A method for improving the stability of the circulating current of an off-grid parallel system

Abstract

This invention provides a method for improving the circulating current stability of an off-grid parallel system. The method involves calculating the rotation angle of the converter output voltage using a phase-locked loop (PLL) and applying it to the dq transformation of the voltage-current loop and the average active power P. x and average reactive power Q x The calculation of the virtual inductive reactance and the negative-sequence d- and q-axis voltage drops is performed. When performing the d-q inverse transformation, the off-grid local oscillator angle of the converter is used as the rotation angle to distinguish it from the former, so that the control loop of the converter is no longer affected by the positive-sequence q-axis component of its output voltage, effectively suppressing the positive-sequence q-axis circulating current of the converter. At the same time, a virtual inductive reactance is added to the negative-sequence loop to suppress the negative-sequence circulating current of the converter, realizing the effective suppression of parallel circulating current, improving the circulating current stability of the off-grid parallel system, and eliminating the need for communication between converters, thus not affecting the converter's adaptability to reactive loads.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of new energy, in particular to a method for improving the stability of loop current of off-grid parallel operation system. BACKGROUND

[0002] To ensure the normal and stable operation of power load, when renewable energy is off-grid, it is often necessary to interconnect and expand multiple renewable energies, and in this process, multiple renewable energies are connected in parallel by means of converters, forming a multi-machine parallel environment. In off-grid mode, droop control is widely used in parallel operation of converters.

[0003] In a three-phase off-grid parallel operation system using droop control, the positive sequence components of the converter output voltage and current are used as the feedback of the droop control, and the feedback and the given value of the power of the converter generate the control quantity of the droop loop, which acts on the rotation angle and the given value of the positive sequence D-axis voltage of the converter respectively, but this scheme does not increase the output current control of the positive sequence q-axis control loop of the converter, which is easy to cause periodic oscillation of the parallel operation loop current.

[0004] In the prior art, the methods for solving the periodic oscillation of the parallel operation loop current mainly include current sharing control method and virtual impedance method, but the former needs to rely on real-time communication network, which increases the cost and complexity of the system and reduces the reliability of the system, which is not conducive to long-distance and large-scale off-grid parallel operation; the latter reduces the voltage control accuracy of the converter, and needs to cooperate with secondary regulation relying on parallel operation communication to compensate for voltage drop, which is complex, has low reliability and is not conducive to long-distance parallel operation. In addition, increasing virtual impedance in the positive sequence q-axis control loop reduces the adaptability of the converter to reactive load. SUMMARY

[0005] The purpose of the present application is to provide a method for improving the stability of loop current of off-grid parallel operation system, to solve the problems of low stability of loop current of existing off-grid parallel operation system and low adaptability to reactive load.

[0006] The present application provides a method for improving the stability of loop current of off-grid parallel operation system, which comprises:

[0007] S1, sampling the three-phase output voltage, three-phase bridge arm inductance current and three-phase output current of the converter x, wherein x represents the number of the converter, and the number of the converter is greater than or equal to two;

[0008] S2, performing abc / dq transformation and positive and negative sequence separation on the parameters sampled in S1 to obtain the positive and negative sequence d and q axis components of the control loop; wherein in the abc / dq transformation, the rotation angle θ used is PLL the rotation angle of the converter output voltage calculated by the phase-locked loop;

[0009] S3, obtain the input quantity of the voltage loop positive sequence control loop:

[0010] S3.1: Calculate the average active power P of converter x. x and average reactive power Q x ;

[0011] S3.2: Use a droop controller to control P respectively. x Q x The process is performed to obtain the off-grid local oscillation angle θ of converter x. droop Positive sequence d-axis output voltage reference value U refx_Pd The positive-sequence q-axis output voltage reference value U refx_Pq The given value is 0;

[0012] The formulas for the droop controller are shown in equations (8) and (9):

[0013] ω droop =ω n -m(P ref -P x (8);

[0014] U refx_Pd =U ref -n(Q ref -Q x (9);

[0015] Where, ω droop , m, n, ω n U ref P ref Q ref These are, respectively, the off-grid local oscillator angular frequency, droop active power coefficient, droop reactive power coefficient, off-grid angular frequency reference value, voltage amplitude reference value, active power reference value, and reactive power reference value; for the ω... droop Integrating, we obtain θ. droop ;

[0016] S3.3, according to the U refx_Pd U refx_Pq The input quantity ΔU of the positive sequence control loop of the voltage loop is obtained. x_Pd ΔU x_Pq ;

[0017] S4, obtain the negative sequence control loop input ΔU based on the negative sequence voltage drop generated by the virtual inductive reactance. x_Nd ΔU x_Nq ;

[0018] S5, according to the ΔU x_Pd ΔU x_Pq Obtain the reference values ​​I for the positive sequence d-axis and q-axis inductor currents. Lrefx_Pd ILrefx_Pq ;

[0019] S6, according to the I Lrefx_Pd I Lrefx_Pq Obtain the positive sequence voltage modulation signal;

[0020] S7, according to the ΔU x_Nd ΔU x_Nq Obtain the negative sequence voltage modulation signal;

[0021] S8, with θ droop The rotation angle is used to perform an inverse dq transform on the positive and negative sequence voltage modulation signals to obtain the three-phase voltage modulation signal;

[0022] S9 obtains the PWM control signal based on the three-phase voltage modulation signal.

[0023] Furthermore, the positive and negative sequence d-axis components of the control loop in S2 include: the positive sequence d-axis component U of the output voltage. ox_Pd U ox_Pq The negative sequence d-axis and q-axis components of the output voltage U ox_Nd U ox_Nq Positive sequence d-axis and q-axis components of the bridge arm inductor current I Lx_Pd I Lx_Pq Negative sequence d-axis and q-axis components of the bridge arm inductor current I Lx_Nd I Lx_Nq The positive sequence d-axis and q-axis components of the output current I ox_Pd I ox_Pq and the negative sequence d-axis and q-axis components of the output current I ox_Nd I ox_Nq .

[0024] Furthermore, the calculation formula in S3.1 is shown in equation (7):

[0025]

[0026] (7).

[0027] Furthermore, the method in S3.3 is shown in equation (10); where the positive sequence d-axis and q-axis voltage controller input ΔU x_Pd ΔU x_Pq These are collectively referred to as the positive sequence control loop inputs of the voltage loop;

[0028] ΔU x_Pd =U refx_Pd -U ox_Pd

[0029] ΔU x_Pq =U refx_Pq -U ox_Pq (10).

[0030] Further, in S4, the method for obtaining the voltage loop negative sequence control loop input quantity is shown in formula (11), wherein the negative sequence d, q axis output voltage reference value U refx_Nd , U refx_Nq are both given values 0, the negative sequence d, q axis voltage controller input quantity ΔU x_Nd , ΔU x_Nq are collectively referred to as the voltage loop negative sequence control loop input quantity; the negative sequence d, q axis voltage drop U Lvx_Nd , U Lvx_Nq generated by the virtual inductance Lv is shown in formula (12);

[0031] ΔU x_Nd =U refx_Nd -U Lvx_Nd -U ox_Nd

[0032] ΔU x_Nq =U refx_Nq -U Lvx_Nq -U ox_Nq (11);

[0033] U Lvx_Nd =-ω PLL ×L v ×I ox_Nq

[0034] U Lvx_Nq =ω PLL ×L v ×I ox_Nd (12);

[0035] Further, S5 is specifically that the ΔU x_Pd , ΔU x_Pq respectively pass through the positive sequence d, q axis voltage controller to obtain the positive sequence d, q axis inductance current reference value I Lrefx_Pd , I Lrefx_Pq .

[0036] Further, S6 is specifically that the positive sequence d, q axis current controller input quantity ΔI x_Pd , ΔI x_Pq is obtained according to formula (13) and then respectively passes through the positive sequence d, q axis current controller to obtain the positive sequence d, q axis voltage modulation signal U rx_Pd , U rx_Pq ;

[0037] ΔI x_Pd =I Lrefx_Pd -I Lx_Pd

[0038] ΔI x_Pq =I Lrefx_Pq -I Lx_Pq(13).

[0039] Further, the S7 comprises:

[0040] S7.1: the ΔU x_Nd , ΔU x_Nq is obtained through negative sequence d, q axis voltage controller respectively to get negative sequence d, q axis inductance current reference value I Lrefx_Nd , I Lrefx_Nq ;

[0041] S7.2: the ΔI x_Nd , ΔI x_Nq is obtained according to formula (14) to get negative sequence d, q axis voltage modulation signal U rx_Nd , U rx_Nq after negative sequence d, q axis current controller respectively;

[0042] ΔI x_Nd = I Lrefx_Nd -I Lx_Nd

[0043] ΔI x_Nq = I Lrefx_Nq -I Lx_Nq (14).

[0044] Further, the S8 comprises: with the θ droop as the rotation angle, the positive sequence voltage modulation signal, negative sequence voltage modulation signal is carried out dq / abc coordinate inverse transformation to get three-phase voltage modulation signal

[0045] Further, the S9 comprises:

[0046] If the converter is three-phase four-wire system, the three-phase voltage modulation signal is carried out SPWM modulation after summing with zero sequence control loop output, and the PWM control signal is outputted;

[0047] If the converter is three-phase three-wire system, the three-phase voltage modulation signal is carried out SPWM modulation, and the PWM control signal is outputted.

[0048] Further, the voltage controller is proportional integral regulator.

[0049] Further, the current controller is proportional regulator.

[0050] The above method for improving the stability of the off-grid parallel system ring current uses phase-locked loop to calculate the rotation angle of the converter output voltage, and uses it for dq transformation of voltage and current loop, average active power P x and average reactive power Q xThe calculation of the virtual inductance and the calculation of the virtual inductive negative sequence d, q axis voltage drop are carried out, and when the dq inverse transformation is carried out, the off-grid local oscillator angle of the converter is used as the rotation angle, thereby being distinguished from the foregoing dq transformation and related calculation, so that the control loop of the converter is no longer affected by the positive sequence q axis component of the output voltage of the converter, and further, the positive sequence q axis circulating current of the converter is suppressed; meanwhile, the virtual inductance is added in the negative sequence loop, and the negative sequence circulating current of the converter is suppressed; the method improves the stability of the circulating current of the system, and does not need to add communication between the converters, and does not affect the adaptability of the converter to the reactive load. BRIEF DESCRIPTION OF DRAWINGS

[0051] Figure 1 The off-grid multi-converter parallel circuit structure relied on by the application is shown in the figure;

[0052] Figure 2 The droop control block diagram of the application is shown in the figure;

[0053] Figure 3 The overall control block diagram of the positive and negative sequence loops of the converter is shown in the figure. DETAILED DESCRIPTION

[0054] In order to facilitate the understanding of the application, the application will be described in more detail below with reference to the relevant drawings. The drawings show several embodiments of the application. However, the application can be realized in many different forms, and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the application more thorough and comprehensive.

[0055] It should be noted that when an element is referred to as being "fixed to" another element, it can be directly on the other element or there can be a middle element. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or there can be a middle element. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only.

[0056] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0057] S1: sampling the three-phase output voltage u of the converter x ox_a , u ox_b , u ox_c , the three-phase bridge arm inductor current i Lx_a , i Lx_b , i Lx_c , and the three-phase output current iox_a ox_b ox_c wherein x represents the number of the converter, and x∈[1,Y], Y represents the number of the converters, and Y≥2.

[0058] It can be understood that u ox_a u ox_b u ox_c are the a-phase output voltage, the b-phase output voltage, and the c-phase output voltage of the converter x respectively; i Lx_a i Lx_b i Lx_c are the a-phase bridge arm inductor current, the b-phase bridge arm inductor current, and the c-phase bridge arm inductor current of the converter x respectively; i ox_a i ox_b i ox_c are the a-phase output current, the b-phase output current, and the c-phase output current of the converter x respectively.

[0059] wherein the converter is a three-phase three-wire system or a three-phase four-wire system.

[0060] S2: performing abc / dq transformation and positive and negative sequence separation on the parameters obtained by sampling in S1 to obtain positive and negative sequence d and q axis components of the control loop; wherein in the abc / dq transformation, the rotation angle θ PLL is the rotation angle of the converter output voltage calculated through a phase-locked loop.

[0061] That is, performing abc / dq transformation and positive and negative sequence separation on the three-phase output voltage, the three-phase bridge arm inductor current, and the three-phase output current in S1 to obtain positive and negative sequence d and q axis components of the control loop.

[0062] It can be understood that the abc / dq transformation can be performed first, and then the positive and negative sequence separation can be performed, or the positive and negative sequence separation can be performed first, and then the abc / dq transformation can be performed, which are collectively referred to as performing abc / dq transformation and positive and negative sequence separation.

[0063] Specifically, performing abc / dq transformation and positive and negative sequence separation on the three-phase output voltage u ox_a u ox_b u ox_c in S1 to obtain output voltage positive sequence d axis component U ox_Pd , output voltage positive sequence q axis component U ox_Pq , output voltage negative sequence d axis component U ox_Nd , and output voltage negative sequence q axis component U ox_Nq , as shown in formula (1) and formula (4) respectively.

[0064] Performing abc / dq transformation and positive and negative sequence separation on the three-phase bridge arm inductor current i Lx_a i Lx_b i​​Lx_c The abc / dq transformation and positive and negative sequence separation are performed on the bridge arm inductor current to obtain a positive sequence d-axis component I Lx_Pd , a positive sequence q-axis component I Lx_Pq , and a negative sequence d-axis component I Lx_Nd , a negative sequence q-axis component I Lx_Nq of the bridge arm inductor current, as shown in formulas (2) and (5) respectively.

[0065] The abc / dq transformation and positive and negative sequence separation are performed on the three-phase output currents i ox_a , i ox_b , i ox_c in S1 to obtain a positive sequence d-axis component I ox_Pd , a positive sequence q-axis component I ox_Pq , and a negative sequence d-axis component I ox_Nd , a negative sequence q-axis component I ox_Nq of the output current, as shown in formulas (3) and (6) respectively.

[0066] Wherein, ω PLL is the rotational angular frequency of the output voltage of the converter calculated using a phase-locked loop, and the output voltage rotational angle θ PLL of the converter is obtained by integrating ω PLL , that is, ω PLL t.

[0067] It can be understood that in formulas (1)-(6), U ox_Z , I Lx_Z , I ox_Z are zero sequence components of the output voltage, the bridge arm inductor current, and the output current respectively.

[0068] Wherein, the positive sequence d-axis and q-axis components U ox_Pd , U ox_Pq of the output voltage are a positive sequence d-axis component U ox_Pd and a positive sequence q-axis component U ox_Pq of the output voltage; the negative sequence d-axis and q-axis components U ox_Nd , U ox_Nq of the output voltage are a negative sequence d-axis component U ox_Nd and a negative sequence q-axis component U ox_Nq of the output voltage. The positive sequence d-axis and q-axis components and the negative sequence d-axis and q-axis components of the output voltage are collectively referred to as the positive and negative sequence d-axis and q-axis components of the output voltage.

[0069] Wherein, the positive sequence d-axis and q-axis components I Lx_Pd , I Lx_Pq of the bridge arm inductor current are a positive sequence d-axis component I Lx_Pd and a positive sequence q-axis component ILx_Pq ; bridge arm inductor current negative sequence d, q axis component I Lx_Nd , I Lx_Nq is the bridge arm inductor current negative sequence d axis component I Lx_Nd , bridge arm inductor current negative sequence q axis component I Lx_Nq . The bridge arm inductor current positive sequence d, q axis component and the bridge arm inductor current negative sequence d, q axis component are collectively referred to as the bridge arm inductor current positive, negative sequence d, q axis component.

[0070] wherein, output current positive sequence d, q axis component I ox_Pd , I ox_Pq is the output current positive sequence d axis component I ox_Pd , output current positive sequence q axis component I ox_Pq ; output current negative sequence d, q axis component I ox_Nd , I ox_Nq is the output current negative sequence d axis component I ox_Nd , output current negative sequence q axis component I ox_Nq . The output current positive sequence d, q axis component and the output current negative sequence d, q axis component are collectively referred to as the output current positive, negative sequence d, q axis component.

[0071] wherein, output voltage positive, negative sequence d, q axis component, bridge arm inductor current positive, negative sequence d, q axis component and output current positive, negative sequence d, q axis component are collectively referred to as the positive, negative sequence d, q axis component of the control loop.

[0072] (1);

[0073] (2);

[0074] (3);

[0075] (4);

[0076] (5);

[0077] (6).

[0078] S3: Obtain the voltage loop positive sequence control loop input quantity.

[0079] S3.1: Calculate the average active power P x and the average reactive power Q x of the converter x.

[0080] Specifically, the output voltage positive sequence d axis component U ox_Pd , the output voltage positive sequence q axis component U ox_Pq and the output current positive sequence d axis component Iox_Pd , output current positive sequence q-axis component I ox_Pq The average power calculation is performed to obtain the average active power P x and the average reactive power Q x of the converter x, and the calculation formula is shown in equation (7).

[0081]

[0082] (7).

[0083] S3.2: The average active power P x and the average reactive power Q x are processed by using the droop controller respectively, and the off-grid local oscillator angle θ droop and the positive sequence d-axis output voltage reference value U refx_Pd of the converter x are obtained; and the positive sequence q-axis output voltage reference value U refx_Pq is a given value 0.

[0084] The formula of the droop controller is shown in equations (8) and (9). The average active power P x is processed by equation (8) to obtain the off-grid local oscillator angle frequency ω droop , and the off-grid local oscillator angle frequency ω droop is integrated to obtain the off-grid local oscillator angle θ droop of the converter x; the average reactive power Q x is processed by equation (9) to obtain the positive sequence d-axis output voltage reference value U refx_Pd , and in addition, the positive sequence q-axis output voltage reference value U refx_Pq is a given value 0.

[0085] ω droop = ω n -m (P ref -P x ) (8);

[0086] U refx_Pd = U ref -n (Q ref -Q x ) (9);

[0087] Wherein, m is the droop active coefficient, n is the droop reactive coefficient, ω n is the off-grid angle frequency reference value of the converter, U ref is the voltage amplitude reference value (which is √2 times of the rated output voltage effective value, i.e. the peak value of the rated output voltage), P ref is the active power reference value, and Q ref is the reactive power reference value.

[0088] In one embodiment, m is in the range of (0, 10 Hz / Pn], where Pn is the rated active power of the converter; n is in the range of (0, 20%Un / Qn], where Un and Qn are the rated output voltage and the rated reactive power of the converter, respectively; ω n is in the range of [49 Hz, 61 Hz], for example, 50 Hz or 60 Hz, etc.; P ref is in the range of [-Pn, Pn]; Q ref is in the range of [-Qn, Qn].

[0089] In other embodiments, m, n, ω n , P ref , Q ref may also be selected according to actual needs.

[0090] S3.3: obtaining voltage loop positive sequence control loop input quantity ΔU refx_Pd , ΔU refx_Pq according to the U x_Pd , U x_Pq .

[0091] Subtracting the positive sequence d-axis output voltage reference value U refx_Pd obtained in S3.2 from the output voltage positive sequence d-axis component U ox_Pd , the positive sequence d-axis voltage controller input quantity ΔU x_Pd is obtained; similarly, subtracting the positive sequence q-axis output voltage reference value U refx_Pq of S3.2 from the output voltage positive sequence q-axis component U ox_Pq , the positive sequence q-axis voltage controller input quantity ΔU x_Pq is obtained, as shown in equation (10):

[0092] ΔU x_Pd =U refx_Pd -U ox_Pd

[0093] ΔU x_Pq =U refx_Pq -U ox_Pq (10).

[0094] Wherein, the positive sequence d-axis voltage controller input quantity ΔU x_Pd , the positive sequence q-axis voltage controller input quantity ΔU x_Pq , namely the positive sequence d-axis and q-axis voltage controller input quantities ΔU x_Pd , ΔU x_Pq , are collectively referred to as voltage loop positive sequence control loop input quantities.

[0095] S4: obtaining voltage loop negative sequence control loop input quantity ΔU x_Nd , ΔUx_Nq .

[0096] Negative sequence d-axis output voltage reference value U refx_Nd Given a value of 0, the negative sequence d-axis output voltage reference value U refx_Nd The negative-order d-axis voltage drop U generated by virtual inductive reactance Lvx_Nd The difference is then subtracted from the negative sequence d-axis component U of the output voltage. ox_Nd The negative sequence d-axis voltage controller input ΔU is obtained. x_Nd Similarly, the negative-sequence q-axis output voltage reference value U refx_Nq Given a value of 0, the negative sequence q-axis output voltage reference value U refx_Nq The negative-order q-axis voltage drop U generated by virtual inductive reactance Lvx_Nq The difference is then subtracted from the negative-sequence q-axis component U of the output voltage. ox_Nq The negative-sequence q-axis voltage controller input ΔU is obtained. x_Nq As shown in equation (11); where the negative-sequence d-axis voltage drop U generated by the virtual inductor Lvx_Nd The negative-sequence q-axis voltage drop U generated by the virtual inductor Lvx_Nq The calculation formula is shown in equation (12), where the virtual inductive reactance is represented by Lv. The negative-sequence d-axis and q-axis voltage drop U generated by the virtual inductive reactance Lvx_Nd U Lvx_Nq The negative-sequence d-axis voltage drop U generated by the virtual inductor Lvx_Nd The negative-sequence q-axis voltage drop U generated by the virtual inductor Lvx_Nq Negative sequence d-axis output voltage reference value U refx_Nd Negative sequence q-axis output voltage reference value U refx_Nq That is, the negative sequence d-axis and q-axis output voltage reference value U refx_Nd U refx_Nq .

[0097] Among them, the negative sequence d-axis and q-axis voltage controller input ΔU x_Nd ΔU x_Nq That is, the negative sequence d-axis voltage controller input ΔU x_Nd Negative sequence q-axis voltage controller input ΔU x_Nq These are collectively referred to as the input quantities of the voltage loop negative sequence control loop.

[0098] The negative-sequence d-axis voltage controller and the negative-sequence q-axis voltage controller can be two independent voltage controllers or the same voltage controller, such as a two-port voltage controller.

[0099] ΔU x_Nd =U refx_Nd -U Lvx_Nd -U ox_Nd

[0100] ΔU x_Nq =Urefx_Nq -U Lvx_Nq -U ox_Nq (11);

[0101] U Lvx_Nd =-ω PLL ×L v ×I ox_Nq

[0102] U Lvx_Nq =ω PLL ×L v ×I ox_Nd (12);

[0103] It can be understood that the sequence of the foregoing S3 and S4 can be interchanged, or both are performed simultaneously.

[0104] S5: obtaining positive sequence d-axis inductance current reference value I x_Pd and positive sequence q-axis inductance current reference value I x_Pq according to the ΔU Lrefx_Pd and ΔU Lrefx_Pq .

[0105] The positive sequence d-axis voltage controller input quantity ΔU x_Pd and the positive sequence q-axis voltage controller input quantity ΔU x_Pq respectively pass through the positive sequence d-axis voltage controller and the positive sequence q-axis voltage controller to obtain the positive sequence d-axis inductance current reference value I Lrefx_Pd and the positive sequence q-axis inductance current reference value I Lrefx_Pq ; the positive sequence d-axis inductance current reference value I Lrefx_Pd and the positive sequence q-axis inductance current reference value I Lrefx_Pq , that is, the positive sequence d-axis inductance current reference value I Lrefx_Pd and the positive sequence q-axis inductance current reference value I Lrefx_Pq .

[0106] Wherein, the positive sequence d-axis voltage controller input quantity ΔU x_Pd and the positive sequence q-axis voltage controller input quantity ΔU x_Pq respectively pass through the positive sequence d-axis voltage controller and the positive sequence q-axis voltage controller to indicate the positive sequence d-axis voltage controller input quantity ΔU x_Pd passes through the positive sequence d-axis voltage controller, and the positive sequence q-axis voltage controller input quantity ΔU x_Pq passes through the positive sequence q-axis voltage controller.

[0107] Wherein, the positive sequence d-axis voltage controller and the positive sequence q-axis voltage controller are simply referred to as positive sequence d-axis and q-axis voltage controllers; the positive sequence d-axis voltage controller and the positive sequence q-axis voltage controller can be two independent voltage controllers, or can be the same voltage controller, for example, a dual-port voltage controller.

[0108] S6: obtaining I Lrefx_Pd and ILrefx_Pq The positive sequence voltage modulation signal is obtained.

[0109] The positive sequence d-axis inductor current reference value I Lrefx_Pd in S5 is subtracted from the positive sequence d-axis component I Lx_Pd of the bridge arm inductor current in S2 to obtain the positive sequence d-axis current controller input quantity ΔI x_Pd . Similarly, the positive sequence q-axis inductor current reference value I Lrefx_Pq in S5 is subtracted from the positive sequence q-axis component I Lx_Pq of the bridge arm inductor current in S2 to obtain the positive sequence q-axis current controller input quantity ΔI x_Pq ; the positive sequence d-axis current controller input quantity ΔI x_Pd and the positive sequence q-axis current controller input quantity ΔI x_Pq are respectively subjected to the positive sequence d-axis current controller and the positive sequence q-axis current controller to obtain the positive sequence d-axis voltage modulation signal U rx_Pd and the positive sequence q-axis voltage modulation signal U rx_Pq , as shown in equation (13). The positive sequence d-axis voltage modulation signal U rx_Pd and the positive sequence q-axis voltage modulation signal U rx_Pq are collectively referred to as the positive sequence voltage modulation signal. rx_Pd , U rx_Pq .

[0110] ΔI x_Pd =I Lrefx_Pd -I Lx_Pd

[0111] ΔI x_Pq =I Lrefx_Pq -I Lx_Pq (13);

[0112] wherein the positive sequence d-axis current controller input quantity ΔI x_Pd and the positive sequence q-axis current controller input quantity ΔI x_Pq are respectively subjected to the positive sequence d-axis current controller and the positive sequence q-axis current controller to indicate that the positive sequence d-axis current controller input quantity ΔI x_Pd is subjected to the positive sequence d-axis current controller and the positive sequence q-axis current controller input quantity ΔI x_Pq is subjected to the positive sequence q-axis current controller; the positive sequence d-axis current controller input quantity ΔI x_Pd and the positive sequence q-axis current controller input quantity ΔI x_Pq are collectively referred to as the positive sequence d-axis and q-axis current controller input quantities ΔI x_Pd , ΔI x_Pq .

[0113] Wherein, the positive sequence d-axis current controller and the positive sequence q-axis current controller are collectively referred to as a positive sequence d, q-axis current controller; the positive sequence d-axis current controller and the positive sequence q-axis current controller can be two independent current controllers; or can be the same current controller, for example, a dual-port current controller.

[0114] S7: According to the ΔU x_Nd , ΔU x_Nq Obtain the negative sequence voltage modulation signal.

[0115] S7.1: The ΔU x_Nd , ΔU x_Nq Respectively through the negative sequence d, q-axis voltage controller to obtain the negative sequence d, q-axis inductance current reference value I Lrefx_Nd , I Lrefx_Nq .

[0116] The negative sequence d-axis voltage controller input quantity ΔU x_Nd , the negative sequence q-axis voltage controller input quantity ΔU x_Nq Respectively through the negative sequence d-axis voltage controller, the negative sequence q-axis voltage controller to obtain the negative sequence d-axis inductance current reference value I Lrefx_Nd , the negative sequence q-axis inductance current reference value I Lrefx_Nq .

[0117] Wherein, the negative sequence d-axis voltage controller input quantity ΔU x_Nd , the negative sequence q-axis voltage controller input quantity ΔU x_Nq Respectively through the negative sequence d-axis voltage controller, the negative sequence q-axis voltage controller to the negative sequence d-axis voltage controller input quantity ΔU x_Nd Through the negative sequence d-axis voltage controller, the negative sequence q-axis voltage controller input quantity ΔU x_Nq Through the negative sequence q-axis voltage controller.

[0118] Wherein, the negative sequence d-axis inductance current reference value I Lrefx_Nd , the negative sequence q-axis inductance current reference value I Lrefx_Nq Are collectively referred to as negative sequence d, q-axis inductance current reference value I Lrefx_Nd , I Lrefx_Nq ; the negative sequence d-axis voltage controller and the negative sequence q-axis voltage controller are collectively referred to as a negative sequence d, q-axis voltage controller.

[0119] S7.2: According to formula (14) to obtain the negative sequence d, q-axis current controller input quantity ΔI x_Nd , ΔI x_Nq Respectively through the negative sequence d, q-axis current controller to obtain the negative sequence d, q-axis voltage modulation signal U rx_Nd , U rx_Nq .

[0120] In S7.1, the negative sequence d-axis inductance current reference value I Lrefx_NdThe negative-sequence d-axis component I Lx_Nd Subtracting the negative-sequence d-axis component I x_Nd , the negative-sequence q-axis component I Lrefx_Nq Subtracting the negative-sequence q-axis component I Lx_Nq , the negative-sequence q-axis component I x_Nq , the negative-sequence q-axis component I x_Nd , the negative-sequence q-axis component I x_Nq The negative-sequence d-axis voltage modulation signal U rx_Nd , the negative-sequence q-axis voltage modulation signal U rx_Nq The negative-sequence d-axis voltage modulation signal U rx_Nd , the negative-sequence q-axis voltage modulation signal U rx_Nq are collectively referred to as negative-sequence voltage modulation signals.

[0121] The negative-sequence d-axis current controller input ΔI x_Nd , the negative-sequence q-axis current controller input ΔI x_Nq are collectively referred to as negative-sequence d, q-axis current controller inputs ΔI x_Nd , ΔI x_Nq The negative-sequence d-axis voltage modulation signal U rx_Nd , the negative-sequence q-axis voltage modulation signal U rx_Nq are collectively referred to as negative-sequence d, q-axis voltage modulation signals U rx_Nd , U rx_Nq .

[0122] The negative-sequence d-axis current controller input ΔI x_Nd , the negative-sequence q-axis current controller input ΔI x_Nq The negative-sequence d-axis current controller input ΔI x_Nd The negative-sequence d-axis current controller input ΔI x_Nq The negative-sequence q-axis current controller input ΔI

[0123] ΔI x_Nd = I Lrefx_Nd - I Lx_Nd

[0124] ΔI x_Nq = I Lrefx_Nq - I Lx_Nq (14).

[0125] Among them, the negative sequence d-axis and q-axis current controller; the negative sequence d-axis voltage controller and the negative sequence q-axis voltage controller are simply referred to as the negative sequence d-axis and q-axis voltage controller; the negative sequence d-axis voltage controller and the negative sequence q-axis voltage controller can be two independent voltage controllers; or they can be the same voltage controller, such as a two-port voltage controller.

[0126] Understandably, the order of S6 and S7 can be interchanged, or both can be performed simultaneously.

[0127] S8, with θ droop The rotation angle is used to perform an inverse dq transformation on the positive and negative sequence voltage modulation signals to obtain the three-phase voltage modulation signal.

[0128] Among them, positive sequence voltage modulation signal and negative sequence voltage modulation signal are simply referred to as positive and negative sequence voltage modulation signals.

[0129] Specifically, the aforementioned positive-sequence voltage modulation signal and negative-sequence voltage modulation signal are subjected to an inverse dq / abc coordinate transformation (i.e., inverse dq transformation) to obtain the three-phase voltage modulation signal U. rx_a U rx_b U rx_c That is, the voltage modulation signal U of phase a. rx_a b-phase voltage modulation signal U rx_b and c-phase voltage modulation signal U rx_c .

[0130] The rotation angle used in the inverse transformation of dq / abc coordinates is the off-grid local oscillation angle θ. droop That is, the off-grid local oscillator frequency ω of the converter. droop The integral; or in other words, the off-grid angular frequency reference value ω of the converter. n The integral of the difference between the active droop frequency loop output and the active droop frequency loop output, where the active droop frequency loop output is m(P) ref -P x ).

[0131] S9 obtains the PWM control signal based on the three-phase voltage modulation signal.

[0132] If the converter is a three-phase four-wire system, the three-phase voltage modulation signal U in S8 rx_a U rx_b U rx_c With zero-sequence control loop output U 0x_0 After being processed and modulated by SPWM, the output converter switching transistors receive PWM control signals (referred to as PWM control signals).

[0133] If the converter is a three-phase three-wire system, then the three-phase voltage modulation signal does not need to be coupled with the zero-sequence control loop output U. 0x_0 The three-phase voltage modulation signal U in S8 is directly converted into the sum. rx_a Urx_b , U rx_c After SPWM modulation, the PWM control signal of the output converter switch tube (referred to as PWM control signal) is output.

[0134] Wherein, the zero sequence control loop is a voltage and current double closed loop control, PWM refers to pulse width modulation, and SPWM refers to sine pulse width modulation.

[0135] As shown in Figure 3 , in one embodiment, the zero sequence control loop is specifically: the zero sequence voltage loop is given (0) and the zero sequence component U ox_Z of the output voltage is subtracted, and then the zero sequence component I Lx_Z of the bridge arm inductance current is subtracted through the zero sequence voltage controller, and then the output U 0x_0 , U 0x_0 , that is, the output of the zero sequence control loop.

[0136] It can be understood that the aforementioned corresponding reference value can be selected according to actual needs.

[0137] In one embodiment, the aforementioned voltage controller is a proportional integral regulator, and the aforementioned current controller is a proportional regulator.

[0138] It should be noted that in the embodiments of the present application:

[0139] Droop control: the basic principle of droop control is to realize load current distribution and stabilize system frequency by simulating the inductive droop characteristic between active power, reactive power and frequency, voltage.

[0140] Off-grid local angle: in off-grid power generation mode, the converter acts as a voltage source, and the instantaneous phase angle of the sine signal with a specific oscillation frequency generated by the oscillation of the control chip itself.

[0141] Phase-locked loop: a control system that can keep the frequency and phase of the controlled oscillator in a certain relationship with the input signal, generally composed of a phase detector, a loop filter, a voltage-controlled oscillator and other components.

[0142] Off-grid parallel machine: refers to the parallel operation of multiple devices in an off-grid power system to provide power. Such a system does not rely on the public power grid, but stores power in a battery to form a self-sufficient power system.

[0143] Virtual inductive reactance: in a microgrid, virtual inductive reactance is used to improve the stability and power distribution of the microgrid. By adding a virtual inductive reactance at the output end of the converter through a control algorithm, the impact of line impedance imbalance on system stability is reduced, achieving the purpose of optimizing power distribution and improving system stability.

[0144] DQ transformation: also known as abc / dq transformation, used to convert three-phase voltage and current into components in two-phase orthogonal coordinate system. This transformation is based on a rotating dq reference frame, in which the d-axis is usually aligned with the magnetic field direction of the motor, and the q-axis leads by 90 degrees.

[0145] The method for improving the stability of the loop current of the off-grid parallel system, by using the phase-locked loop to calculate the rotation angle of the converter output voltage, and using it for the dq transformation of the voltage and current loop, the calculation of the average active power P x and the average reactive power Q x (namely, the droop power calculation), and the calculation of the virtual inductive negative sequence d, q-axis voltage drop, and when performing dq inverse transformation, the off-grid local oscillator angle frequency ω droop of the converter is used to distinguish from the former, so that the control loop of the converter is no longer affected by the positive sequence q-axis component of its output voltage, thereby suppressing the positive sequence q-axis loop current of the converter, and at the same time increasing the virtual inductance in the negative sequence loop to suppress the negative sequence loop current of the converter, thereby improving the stability of the loop current of the system; in addition, this method is based on inductive droop control, and does not require communication between parallel converters, and does not affect the adaptability of the converter to reactive loads.

[0146] The above-described embodiments only express several embodiments of the present application, and the description is more specific and detailed, but it should not be understood as limiting the scope of the patent of the present application. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are all within the scope of protection of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method for improving the stability of the circulating current of an off-grid parallel system, characterized in that, The method comprises: S1, sampling three-phase output voltage, three-phase bridge arm inductor current and three-phase output current of a current transformer x, wherein x represents the number of the current transformer, and the number of the current transformer is greater than or equal to two; S2, performing abc / dq transformation and positive and negative sequence separation on the parameters obtained by sampling in S1 to obtain positive and negative sequence d and q axis components of the control loop; wherein, in the abc / dq transformation, the rotation angle θ used is the rotation angle of the converter output voltage calculated by the phase-locked loop PLL the rotation angle of the converter output voltage calculated by the phase-locked loop The positive and negative sequence d, q axis components of the control loop in S2 include: output voltage positive sequence d, q axis components U ox_Pd , ox_Pq , output voltage negative sequence d, q axis components U ox_Nd , ox_Nq , bridge arm inductance current positive sequence d, q axis components I Lx_Pd , Lx_Pq , bridge arm inductance current negative sequence d, q axis components I Lx_Nd , Lx_Nq , output current positive sequence d, q axis components I ox_Pd , ox_Pq , and output current negative sequence d, q axis components I ox_Nd , ox_Nq ; S3, obtaining voltage ring positive sequence control loop input quantity: S3.1: Calculate the average active power P obtained by the converter x x and the average reactive power Q x ; The calculation formula in S3.1 is shown in formula (7): （7）； S3.2: using a droop controller to respectively process the P x , Q x , to obtain the off-grid local oscillator angle θ droop of the converter x refx_Pd ; and the positive sequence d-axis output voltage reference value U refx_Pq is a given value 0; The formula of the droop controller is shown in formula (8) and (9): ω droop =ω n -m(P ref -P x ) (8); U refx_Pd =U ref -n(Q ref -Q x ) (9); Wherein, ω droop , m, n, ω n , U ref , P ref , Q ref are off-grid local oscillator frequency, droop active coefficient, droop reactive coefficient, off-grid angular frequency reference value, voltage amplitude reference value, active power reference value, reactive power reference value respectively; the integral of ω droop is carried out to obtain θ droop ; S3.3, according to the U refx_Pd , U refx_Pq get the voltage ring positive sequence control loop input quantity ΔU x_Pd , ΔU x_Pq ; The method in S3.3 is shown in equation (10); where the positive sequence d-axis and q-axis voltage controller input ΔU x_Pd , ΔU x_Pq These are collectively referred to as the positive sequence control loop inputs of the voltage loop; ΔU x_Pd =U refx_Pd -U ox_Pd ΔU x_Pq =U refx_Pq -U ox_Pq (10); S4, obtaining the input quantity ΔU of the voltage loop negative sequence control loop according to the negative sequence voltage drop generated by the virtual inductive reactance x_Nd , ΔU x_Nq ; In S4, the method for obtaining the voltage loop negative sequence control loop input quantity is shown in formula (11), wherein the negative sequence d, q axis output voltage reference value U refx_Nd , U refx_Nq are both given values 0, the negative sequence d, q axis voltage controller input quantity ΔU x_Nd , ΔU x_Nq are collectively referred to as the voltage loop negative sequence control loop input quantity; the negative sequence d, q axis voltage drop U Lvx_Nd , U Lvx_Nq generated by the virtual inductance Lv is shown in formula (12), wherein ω PLL is the rotation angle frequency of the converter output voltage calculated using a phase-locked loop. ΔU x_Nd =U refx_Nd -U Lvx_Nd -U ox_Nd ΔU x_Nq =U refx_Nq -U Lvx_Nq -U ox_Nq (11); U Lvx_Nd = -ω PLL × L v × I ox_Nq U Lvx_Nq =ω PLL ×L v ×I ox_Nd (12); S5, obtaining the positive-sequence d, q-axis inductance current reference value I x_Pd , ΔU x_Pq obtaining positive-sequence d, q-axis inductance current reference value I Lrefx_Pd , I Lrefx_Pq ; S6, according to the I Lrefx_Pd , I Lrefx_Pq obtaining a positive sequence voltage modulation signal; S7, obtaining the ΔU according to the ΔU x_Nd , ΔU x_Nq obtaining a negative sequence voltage modulation signal; S8, with the θ droop The rotation angle is used to perform an inverse dq transform on the positive and negative sequence voltage modulation signals to obtain the three-phase voltage modulation signal; S9, obtaining a PWM control signal according to the three-phase voltage modulation signal; The method calculates the rotation angle θ of the converter output voltage by using a phase-locked loop PLL and uses it for the dq transformation of the voltage and current loop, the calculation of the average active power P x and the average reactive power Q x , and the calculation of the virtual inductive negative sequence d, q axis voltage drop, and when performing the dq inverse transformation, uses the off-grid local oscillator angle θ of the converter droop as the rotation angle, thereby distinguishing from θ PLL , so that the control loop of the converter is no longer affected by the positive sequence q axis component of its output voltage, thereby suppressing the positive sequence q axis circulating current of the converter.

2. The method for improving the stability of the circulating flow of the off-grid parallel system according to claim 1, characterized in that, S5 is specifically, the ΔU x_Pd , ΔU x_Pq respectively through the positive sequence d, q-axis voltage controller, get positive sequence d, q-axis inductance current reference value I Lrefx_Pd , I Lrefx_Pq .

3. The method for improving the stability of the circulating flow of the off-grid parallel system according to claim 2, characterized in that, The S6 is specifically, according to formula (13) to obtain positive sequence d, q-axis current controller input ΔI x_Pd , ΔI x_Pq After respectively through positive sequence d, q-axis current controller, positive sequence d, q-axis voltage modulation signal U rx_Pd , U rx_Pq ; ΔI x_Pd =I Lrefx_Pd -I Lx_Pd ΔI x_Pq =I Lrefx_Pq -I Lx_Pq (13).

4. The method for improving the stability of the circulating flow of the off-grid parallel system according to claim 3, characterized in that, The S7 comprises: S7.1: the ΔU x_Nd , ΔU x_Nq , respectively, through the negative sequence d, q-axis voltage controller to obtain the negative sequence d, q-axis inductance current reference value I Lrefx_Nd , I Lrefx_Nq ; S7.2: Obtain negative sequence d, q axis current controller input ΔI according to formula (14) x_Nd , ΔI x_Nq After passing through negative sequence d, q axis current controller respectively, obtain negative sequence d, q axis voltage modulation signal U rx_Nd , U rx_Nq ; ΔI x_Nd =I Lrefx_Nd -I Lx_Nd ΔI x_Nq =I Lrefx_Nq -I Lx_Nq (14).

5. The method for improving the stability of the circulating flow of the off-grid parallel system according to claim 4, characterized in that, The S8 includes: with the θ droop as a rotation angle, the positive sequence voltage modulation signal, the negative sequence voltage modulation signal are dq / abc coordinate inverse transformed to obtain a three-phase voltage modulation signal.

6. The method for improving the stability of the circulating flow of the off-grid parallel system according to claim 5, characterized in that, The S9 comprises: If the current transformer is a three-phase four-wire system, the three-phase voltage modulation signal is added to the zero sequence control loop output, and then the PWM control signal is output after SPWM modulation; If the current transformer is a three-phase three-wire system, the three-phase voltage modulation signal is output after SPWM modulation.

7. The method of improving the stability of the circulating flow of the off-grid parallel system according to claim 1, characterized in that, The voltage controller is a proportional integral regulator.

8. The method for improving the stability of the circulating flow of the off-grid parallel system according to claim 3, characterized in that, The current controller is a proportional regulator.

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