Method for improving circulating current stability of off-grid parallel operation system

Abstract

The invention provides a method for improving circulating current stability of an off-grid parallel operation system. According to the method for improving the circulating current stability of the off-grid parallel operation system, the phase-locked loop is used for calculating the rotation angle of the output voltage of the converter, and the rotation angle is used for dq conversion of a voltage and current loop, calculation of the average active power Px and the average reactive power Qx and calculation of virtual inductive reactance negative sequence d-axis voltage drop and q-axis voltage drop; an off-grid local oscillation angle of the converter is used as a rotation angle, so that the off-grid local oscillation angle is distinguished from the rotation angle, a control loop of the converter is not influenced by an output voltage positive-sequence q-axis component any more, and positive-sequence q-axis circulating current of the converter is effectively restrained; meanwhile, virtual inductive reactance is added in a negative sequence loop, the negative sequence circulating current of the converter is suppressed, effective suppression of parallel operation circulating current is achieved, the circulating current stability of an off-grid parallel operation system is improved, communication does not need to be added between the converters, and the adaptability of the converters to reactive loads is not affected.

Description

Technical Field

[0001] The present invention relates to the field of new energy technology, and in particular to a method for improving the circulation stability of an off-grid parallel system. Background Art

[0002] In order to ensure the normal and stable operation of the power load, when renewable energy is operated off-grid, it is often necessary to interconnect and expand the capacity of multiple renewable energy sources. In this process, multiple renewable energy sources rely on converters to operate in parallel, forming a multi-machine parallel environment. In the off-grid operation mode, droop control is widely used in converter parallel operation.

[0003] In a three-phase off-grid parallel system using droop control, the positive-sequence components of the converter output voltage and current are used as feedback for droop control. The feedback and the converter power set value generate the control quantity of the droop loop, which acts on the converter rotation angle and the positive-sequence D-axis voltage setting respectively. However, this scheme does not add output current control to the converter positive-sequence q-axis control loop, which can easily lead to periodic oscillation of the parallel circulating current.

[0004] In the prior art, the methods for solving the periodic oscillation of the parallel machine circulation current mainly include the current sharing control method and the virtual impedance method. However, the former needs to rely on the real-time communication network, which increases the cost and complexity of the system, reduces the reliability of the system, and is not conducive to long-distance, large-scale off-grid paralleling; the latter reduces the voltage control accuracy of the converter, and needs to cooperate with the secondary regulation that relies on parallel machine communication to compensate for the voltage drop. The method is complex, the reliability is not high, and it is not conducive to long-distance paralleling. In addition, adding virtual impedance to the positive sequence q-axis control loop reduces the converter's adaptability to reactive loads. Summary of the invention

[0005] The purpose of the present invention is to provide a method for improving the circulation stability of an off-grid parallel system, so as to solve the problems of low circulation stability and low adaptability of reactive load in the existing off-grid parallel system.

[0006] The present invention provides a method for improving the circulation stability of an off-grid parallel system, the method comprising: S1, sampling the three-phase output voltage, three-phase bridge arm inductance current and three-phase output current of converter x, where x represents the converter number, and the number of converters is greater than or equal to two; S2, performs 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 PLL is the rotation angle of the converter output voltage calculated by the phase-locked loop; S3, obtain the voltage loop positive sequence control loop input: S3.1: Calculate the average active power P of converter x x and average reactive power Q x ; S3.2: Use the droop controller to control the P x , Q x Processing is performed to obtain the off-grid local oscillator angle θ of the 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 is a given value of 0; The formulas of the droop controller are shown in equations (8) and (9): ω droop =ω n -m(P ref -P x ) (8); U refx_Pd =U ref -n(Q ref -Q x ) (9); Among them, ω droop , m, n, ω n , U ref , P ref , Q ref are respectively the off-grid local oscillator angular frequency, the droop active coefficient, the droop reactive coefficient, the off-grid angular frequency reference value, the voltage amplitude reference value, the active power reference value, and the reactive power reference value; for the said ω droop Integrate to obtain the θ droop ; S3.3, according to the U refx_Pd , U refx_Pq Get the voltage loop positive sequence control loop input ΔU x_Pd , ΔU x_Pq ; S4, obtain the voltage loop negative sequence control loop input ΔU according to the negative sequence voltage drop generated by the virtual inductive reactance x_Nd , ΔU x_Nq ; S5, according to the ΔU x_Pd , ΔU x_Pq Get the positive sequence d and q axis inductor 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, according to the ΔU x_Nd , ΔU x_Nq Obtaining a negative sequence voltage modulation signal; S8, with θdroop is the rotation angle, and the positive and negative sequence voltage modulation signals are subjected to dq inverse transformation to obtain the three-phase voltage modulation signal; S9, obtaining a PWM control signal according to the three-phase voltage modulation signal.

[0007] Furthermore, the positive and negative sequence d and q axis components of the control loop in S2 include: the output voltage positive sequence d and q axis components U ox_Pd , U ox_Pq , the output voltage negative sequence d, q axis component U ox_Nd , U ox_Nq , the positive sequence d-axis and q-axis components of the bridge arm inductor current I Lx_Pd ,I Lx_Pq , the negative sequence d-axis and q-axis components of the bridge arm inductor current I Lx_Nd ,I Lx_Nq , the output current positive sequence d, q axis components I ox_Pd ,I ox_Pq And the output current negative sequence d, q axis components I ox_Nd ,I ox_Nq .

[0008] Furthermore, the calculation formula in S3.1 is shown in formula (7): (7).

[0009] Furthermore, the method in S3.3 is shown in equation (10); where the positive sequence d and q axis voltage controller input ΔU x_Pd , ΔU x_Pq Collectively referred to as voltage loop positive sequence control loop input; ΔU x_Pd =U refx_Pd -U ox_Pd ΔU x_Pq =U refx_Pq -U ox_Pq (10).

[0010] Furthermore, in S4, the method for obtaining the input quantity of the voltage loop negative sequence control loop is shown in equation (11), where the negative sequence d and q axis output voltage reference values ​​U refx_Nd , U refx_Nq The given value is 0, and the negative sequence d and q axis voltage controller input ΔU x_Nd , ΔU x_Nq Collectively referred to as the voltage loop negative sequence control loop input; the negative sequence d and q axis voltage drop U generated by the virtual inductive reactance Lv Lvx_Nd , U Lvx_Nq The calculation formula is shown in formula (12); ΔU x_Nd =Urefx_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);

[0011] Further, S5 is specifically, the ΔU x_Pd , ΔU x_Pq After the positive sequence d and q axis voltage controllers respectively, the positive sequence d and q axis inductor current reference values ​​I are obtained. Lrefx_Pd ,I Lrefx_Pq .

[0012] Furthermore, S6 is specifically as follows: According to formula (13), the positive sequence d and q axis current controller input ΔI is obtained: x_Pd , ΔI x_Pq After that, the positive sequence d and q axis current controllers are used to obtain the positive sequence d and q axis voltage modulation signals 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).

[0013] Further, the S7 includes: S7.1: ΔU x_Nd , ΔU x_Nq The negative-sequence d-axis and q-axis inductor current reference values ​​I are obtained through the negative-sequence d-axis and q-axis voltage controllers respectively. Lrefx_Nd ,I Lrefx_Nq ; S7.2: According to equation (14), the negative sequence d and q axis current controller input ΔI is obtained x_Nd , ΔI x_Nq After that, the negative-sequence d-axis and q-axis current controllers are used to obtain the negative-sequence d-axis and q-axis voltage modulation signals 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).

[0014] Further, the S8 includes: using the θ droop is the rotation angle, and the positive-sequence voltage modulation signal and the negative-sequence voltage modulation signal are subjected to dq / abc coordinate inverse transformation to obtain a three-phase voltage modulation signal Further, the S9 includes: If the converter is a three-phase four-wire system, the three-phase voltage modulation signal is summed with the zero-sequence control loop output and then subjected to SPWM modulation to output the PWM control signal; If the converter is a three-phase three-wire system, the three-phase voltage modulation signal is modulated by SPWM to output the PWM control signal.

[0015] Furthermore, the voltage controller is a proportional-integral regulator.

[0016] Furthermore, the current controller is a proportional regulator.

[0017] The above method for improving the circulation stability of the off-grid parallel system uses a phase-locked loop to calculate the rotation angle of the converter output voltage and uses it for the dq transformation of the voltage and current loop, the average active power P x and average reactive power Q x The calculation of the negative-sequence d and q axis voltage drops of the virtual inductive reactance is performed. When performing the dq inverse transformation, the off-grid local oscillator angle of the converter is used as the rotation angle, thereby distinguishing it from the aforementioned dq transformation and related calculations, 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; 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; this method improves the circulating current stability of the system, and does not require additional communication between converters, and does not affect the adaptability of the converter to reactive loads. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A schematic diagram of the circuit structure of multiple off-grid converters in parallel on which the present invention relies; Figure 2 This is a droop control block diagram of the present invention; Figure 3 This is the overall control block diagram of the positive and negative sequence loops of the converter described in the present invention. DETAILED DESCRIPTION

[0019] In order to facilitate understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. Several embodiments of the present invention are provided in the drawings. However, the present invention can be implemented 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 present invention more thorough and comprehensive.

[0020] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly on the other element or there may be a central element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be a central element at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0022] S1: Sampling the three-phase output voltage u of converter x ox_a 、u ox_b 、u ox_c , three-phase bridge arm inductor current i Lx_a 、i Lx_b 、i Lx_c And the three-phase output current i ox_a 、i ox_b 、i ox_c , where x represents the converter number, and x∈[1,Y], Y represents the number of the converter, and Y≥2.

[0023] Understandably, ox_a 、u ox_b 、u ox_c are the output voltage of phase a, phase b and phase c of converter x respectively; i Lx_a 、i Lx_b 、i Lx_c are the inductor current of the a-phase bridge arm, the b-phase bridge arm, and the c-phase bridge arm of the converter x respectively; i ox_a 、i ox_b 、i ox_c They are the a-phase output current, b-phase output current and c-phase output current of converter x respectively.

[0024] The converter is a three-phase three-wire system or a three-phase four-wire system.

[0025] S2: Perform 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; in the abc / dq transformation, the rotation angle θ used is PLL is the rotation angle of the converter output voltage calculated by the phase-locked loop.

[0026] That is, the three-phase output voltage, three-phase bridge arm inductor current and three-phase output current in S1 are all subjected to abc / dq transformation and positive and negative sequence separation to obtain the positive and negative sequence d and q axis components of the control loop.

[0027] It can be understood that abc / dq conversion can be performed first and then positive and negative sequence separation can be performed, or positive and negative sequence separation can be performed first and then abc / dq conversion can be performed, which is collectively referred to as abc / dq conversion and positive and negative sequence separation.

[0028] Specifically, the three-phase output voltage u in S1 ox_a 、u ox_b 、u ox_c Perform abc / dq transformation and positive and negative sequence separation to obtain the output voltage positive sequence d-axis component U ox_Pd , output voltage positive sequence q-axis component U ox_Pq , and the output voltage negative sequence d-axis component U ox_Nd , output voltage negative sequence q-axis component U ox_Nq , as shown in formula (1) and formula (4) respectively.

[0029] The three-phase bridge arm inductor current i in S1 Lx_a 、i Lx_b 、i Lx_c Perform abc / dq transformation and positive and negative sequence separation to obtain the positive sequence d-axis component I of the bridge arm inductor current Lx_Pd , the positive sequence q-axis component of the bridge arm inductor current I Lx_Pq , and the negative sequence d-axis component of the bridge arm inductor current I Lx_Nd , bridge arm inductor current negative sequence q-axis component I Lx_Nq , as shown in formula (2) and formula (5) respectively.

[0030] The three-phase output current i in S1 ox_a 、i ox_b 、i ox_c Perform abc / dq transformation and positive and negative sequence separation to obtain the output current positive sequence d-axis component I ox_Pd , output current positive sequence q-axis component I ox_Pq , and the output current negative sequence d-axis component I ox_Nd , output current negative sequence q-axis component I ox_Nq , as shown in formula (3) and formula (6) respectively.

[0031] Among them, ωPLL is the output voltage rotation angular frequency of the converter calculated using the phase-locked loop, by PLL Integrate to get the output voltage rotation angle θ of the converter PLL , that is, ω PLL t.

[0032] It can be understood that in formulas (1)-(6), U ox_Z ,I Lx_Z ,I ox_Z They are respectively the zero-sequence component of the output voltage, the zero-sequence component of the bridge arm inductor current, and the zero-sequence component of the output current.

[0033] Among them, the output voltage positive sequence d and q axis components U ox_Pd , U ox_Pq is the 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, q axis components U ox_Nd , U ox_Nq is the output voltage negative sequence d-axis component U ox_Nd , output voltage negative sequence q-axis component U ox_Nq The output voltage positive sequence d, q axis components and the output voltage negative sequence d, q axis components are collectively referred to as the output voltage positive, negative sequence d, q axis components.

[0034] Among them, the positive sequence d-axis and q-axis components of the bridge arm inductor current I Lx_Pd ,I Lx_Pq is the positive sequence d-axis component of the bridge arm inductor current I Lx_Pd , the positive sequence q-axis component of the bridge arm inductor current I Lx_Pq ; Bridge arm inductor current negative sequence d, q axis component I Lx_Nd ,I Lx_Nq is the negative sequence d-axis component of the bridge arm inductor current I Lx_Nd , bridge arm inductor current negative sequence q-axis component I Lx_Nq The positive-sequence d-axis and q-axis components of the bridge arm inductor current and the negative-sequence d-axis and q-axis components of the bridge arm inductor current are collectively referred to as the positive-sequence d-axis and q-axis components of the bridge arm inductor current.

[0035] Among them, the output current positive sequence d and q axis components I ox_Pd ,I ox_Pq is the positive sequence d-axis component of the output current I ox_Pd , output current positive sequence q-axis component I ox_Pq ; Output current negative sequence d, q axis components I ox_Nd ,I ox_Nq is the negative sequence d-axis component of the output current I ox_Nd , output current negative sequence q-axis component I ox_NqThe output current positive sequence d, q axis components and the output current negative sequence d, q axis components are collectively referred to as the output current positive, negative sequence d, q axis components.

[0036] Among them, the positive and negative sequence d and q axis components of the output voltage, the positive and negative sequence d and q axis components of the bridge arm inductor current and the positive and negative sequence d and q axis components of the output current are collectively referred to as the positive and negative sequence d and q axis components of the control loop.

[0037] (1); (2); (3); (4); (5); (6).

[0038] S3: Obtain the voltage loop positive sequence control loop input.

[0039] S3.1: Calculate the average active power P of converter x x and average reactive power Q x .

[0040] Specifically, for the output voltage positive sequence d-axis component U in S2 ox_Pd , output voltage positive sequence q-axis component U ox_Pq The positive sequence d-axis component of the output current I ox_Pd , output current positive sequence q-axis component I ox_Pq Perform average power calculation to obtain the average active power P of converter x x and average reactive power Q x , the calculation formula is shown in formula (7): (7).

[0041] S3.2: Use droop controller to control the average active power P x and average reactive power Q x Processing is performed to obtain the off-grid local oscillator angle θ of the 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 is given a value of 0.

[0042] The formulas of the droop controller are shown in equations (8) and (9). The average active power P is calculated by equation (8). x After processing, the off-grid local oscillator angular frequency ω is obtained droop, for the off-grid local oscillator angular frequency ω droop Integrate to get the off-grid local oscillator angle θ of converter x droop ; The average reactive power Q is calculated by formula (9): x Processing is performed to obtain the positive sequence d-axis output voltage reference value U refx_Pd , in addition, the positive sequence q-axis output voltage reference value U refx_Pq is given a value of 0.

[0043] ω droop =ω n -m(P ref -P x ) (8); U refx_Pd =U ref -n(Q ref -Q x ) (9); Among them, m is the droop active coefficient, n is the droop reactive coefficient, ω n is the off-grid angular frequency reference value of the converter, U ref is the voltage amplitude reference value (the peak value of the rated output voltage, i.e. √2 times the effective value of the rated output voltage), P ref is the active power reference value, Q ref is the reactive power reference value.

[0044] In one embodiment, the value range of m is (0,10Hz / Pn], where Pn is the rated active power of the converter; the value range of n is (0,20%Un / Qn], where Un and Qn are the rated output voltage and rated reactive power of the converter respectively; ω n The value range is [49Hz, 61Hz], for example, it can be 50Hz or 60Hz, etc.; P ref The value range of Q is [-Pn, Pn]; ref The value range is [-Qn, Qn].

[0045] In other embodiments, m, n, ω n , P ref , Q ref The value of can also be selected according to actual needs.

[0046] S3.3: According to the U refx_Pd , U refx_Pq Get the voltage loop positive sequence control loop input ΔU x_Pd , ΔU x_Pq .

[0047] The positive sequence d-axis output voltage reference value U obtained by S3.2 refx_Pd The positive sequence d-axis component of the output voltage U ox_PdThe difference is obtained to obtain the positive sequence d-axis voltage controller input ΔU x_Pd Similarly, the positive sequence q-axis output voltage reference value U of S3.2 refx_Pq The positive sequence q-axis component of the output voltage U ox_Pq The difference is obtained to obtain the positive sequence q-axis voltage controller input ΔU x_Pq , as shown in formula (10): ΔU x_Pd =U refx_Pd -U ox_Pd ΔU x_Pq =U refx_Pq -U ox_Pq (10); Among them, the positive sequence d-axis voltage controller input ΔU x_Pd , positive sequence q-axis voltage controller input ΔU x_Pq That is, the positive sequence d and q axis voltage controller input ΔU x_Pd , ΔU x_Pq , collectively referred to as the voltage loop positive sequence control loop input.

[0048] S4: Obtain the voltage loop negative sequence control loop input ΔU based on the negative sequence voltage drop generated by the virtual inductive reactance x_Nd , ΔU x_Nq .

[0049] Negative sequence d-axis output voltage reference value U refx_Nd The negative sequence d-axis output voltage reference value U refx_Nd The negative-sequence d-axis voltage drop U generated by the virtual inductive reactance Lvx_Nd The difference is then subtracted from the output voltage negative sequence d-axis component U ox_Nd , and obtain the negative sequence d-axis voltage controller input ΔU x_Nd Similarly, the negative sequence q-axis output voltage reference value U refx_Nq Set the negative sequence q axis output voltage reference value U to 0. refx_Nq The negative-sequence q-axis voltage drop U generated by the virtual inductive reactance Lvx_Nq The difference is then subtracted from the output voltage negative sequence q-axis component U ox_Nq , and the negative sequence q-axis voltage controller input ΔU is obtained x_Nq , as shown in formula (11); where the negative-sequence d-axis voltage drop U generated by the virtual inductance Lvx_Nd , the negative sequence q-axis voltage drop U generated by the virtual inductance Lvx_Nq The calculation formula is shown in formula (12), where the virtual inductive reactance is represented by Lv. The negative sequence d- and q-axis voltage drop U generated by the virtual inductive reactance Lvx_Nd , U Lvx_Nq is the negative sequence d-axis voltage drop U generated by the virtual inductance Lvx_Nd , the negative sequence q-axis voltage drop U generated by the virtual inductance 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 and q axis output voltage reference value U refx_Nd , U refx_Nq .

[0050] Among them, the negative sequence d, 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 , collectively referred to as the voltage loop negative sequence control loop input quantity.

[0051] The negative-sequence d-axis voltage controller and the negative-sequence q-axis voltage controller may be two independent voltage controllers, or may be the same voltage controller, such as a dual-port voltage controller.

[0052] 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); It can be understood that the order of the aforementioned S3 and S4 can be interchanged, or both can be performed simultaneously.

[0053] S5: According to the ΔU x_Pd , ΔU x_Pq Get the positive sequence d and q axis inductor current reference value I Lrefx_Pd ,I Lrefx_Pq .

[0054] Positive sequence d-axis voltage controller input ΔU x_Pd , positive sequence q-axis voltage controller input ΔU x_Pq The positive-sequence d-axis inductor current reference value I is obtained by the positive-sequence d-axis voltage controller and the positive-sequence q-axis voltage controller respectively. Lrefx_Pd , positive sequence q-axis inductor current reference value I Lrefx_Pq ; Positive sequence d-axis inductor current reference value I Lrefx_Pd , positive sequence q-axis inductor current reference value ILrefx_Pq That is, the positive sequence d and q axis inductor current reference value I Lrefx_Pd ,I Lrefx_Pq .

[0055] Among them, the positive sequence d-axis voltage controller input ΔU x_Pd , positive sequence q-axis voltage controller input ΔU x_Pq The positive sequence d-axis voltage controller and the positive sequence q-axis voltage controller refer to the positive sequence d-axis voltage controller input ΔU x_Pd After the positive sequence d-axis voltage controller, the positive sequence q-axis voltage controller input ΔU x_Pq After the positive sequence q-axis voltage controller.

[0056] Among them, the positive-sequence d-axis voltage controller and the positive-sequence q-axis voltage controller are referred to as positive-sequence d-axis and q-axis voltage controllers for short; the positive-sequence d-axis voltage controller and the positive-sequence q-axis voltage controller can be two independent voltage controllers or the same voltage controller, such as a dual-port voltage controller.

[0057] S6: According to the I Lrefx_Pd ,I Lrefx_Pq A positive sequence voltage modulation signal is obtained.

[0058] Set the positive sequence d-axis inductor current reference value I in S5 Lrefx_Pd The positive sequence d-axis component I of the bridge arm inductor current in S2 Lx_Pd The difference is obtained to obtain the positive sequence d-axis current controller input ΔI x_Pd Similarly, the positive sequence q-axis inductor current reference value I in S5 is Lrefx_Pq The positive sequence q-axis component I of the bridge arm inductor current in S2 Lx_Pq The difference is obtained to obtain the positive sequence q axis current controller input ΔI x_Pq ; Positive sequence d-axis current controller input ΔI x_Pd , positive sequence q axis current controller input ΔI x_Pq The positive-sequence d-axis voltage modulation signal U is obtained by the positive-sequence d-axis current controller and the positive-sequence q-axis current controller respectively. rx_Pd , positive sequence q-axis voltage modulation signal U rx_Pq , as shown in equation (13). Positive sequence d-axis voltage modulation signal U rx_Pd , positive sequence q-axis voltage modulation signal U rx_Pq That is, the positive sequence d, q axis voltage modulation signal U rx_Pd , U rx_Pq , collectively referred to as positive sequence voltage modulation signal.

[0059] ΔI x_Pd =I Lrefx_Pd -I Lx_Pd ΔI x_Pq =ILrefx_Pq -I Lx_Pq (13); Among them, the positive sequence d-axis current controller input ΔI x_Pd , positive sequence q axis current controller input ΔI x_Pq The positive sequence d-axis current controller and the positive sequence q-axis current controller refer to the positive sequence d-axis current controller input ΔI x_Pd After the positive-sequence d-axis current controller, the positive-sequence q-axis current controller input ΔI x_Pq After the positive sequence q axis current controller; the positive sequence d axis current controller input ΔI x_Pd , positive sequence q axis current controller input ΔI x_Pq Referred to as positive sequence d, q axis current controller input ΔI x_Pd , ΔI x_Pq .

[0060] Among them, the positive-sequence d-axis current controller and the positive-sequence q-axis current controller are referred to as positive-sequence d-axis and q-axis current controllers for short; the positive-sequence d-axis current controller and the positive-sequence q-axis current controller can be two independent current controllers; or they can be the same current controller, such as a dual-port current controller.

[0061] S7: According to the ΔU x_Nd , ΔU x_Nq A negative sequence voltage modulation signal is obtained.

[0062] S7.1: ΔU x_Nd , ΔU x_Nq The negative-sequence d-axis and q-axis inductor current reference values ​​I are obtained through the negative-sequence d-axis and q-axis voltage controllers respectively. Lrefx_Nd ,I Lrefx_Nq .

[0063] Negative sequence d-axis voltage controller input ΔU x_Nd , negative sequence q-axis voltage controller input ΔU x_Nq The negative-sequence d-axis inductor current reference value I is obtained by the negative-sequence d-axis voltage controller and the negative-sequence q-axis voltage controller respectively. Lrefx_Nd , negative sequence q-axis inductor current reference value I Lrefx_Nq .

[0064] Among them, the negative sequence d-axis voltage controller input ΔU x_Nd , negative sequence q-axis voltage controller input ΔU x_Nq The negative sequence d axis voltage controller and the negative sequence q axis voltage controller refer to the negative sequence d axis voltage controller input ΔU respectively. x_Nd After the negative sequence d-axis voltage controller and the negative sequence q-axis voltage controller input ΔU x_Nq After the negative sequence q-axis voltage controller.

[0065] Among them, the negative sequence d-axis inductor current reference value I Lrefx_Nd , negative sequence q-axis inductor current reference value I Lrefx_Nq Referred to as negative sequence d, q axis inductor 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 referred to as negative-sequence d-axis and q-axis voltage controllers in short.

[0066] S7.2: According to equation (14), the negative sequence d and q axis current controller input ΔI is obtained x_Nd , ΔI x_Nq After that, the negative-sequence d-axis and q-axis current controllers are used to obtain the negative-sequence d-axis and q-axis voltage modulation signals U rx_Nd , U rx_Nq .

[0067] Set the negative sequence d-axis inductor current reference value I in S7.1 Lrefx_Nd The negative sequence d-axis component I of the bridge arm inductor current in S2 Lx_Nd The difference is obtained to obtain the negative sequence d-axis current controller input ΔI x_Nd Similarly, the negative sequence q-axis inductor current reference value I in S7.1 is Lrefx_Nq The negative sequence q-axis component I of the bridge arm inductor current in S2 Lx_Nq The negative sequence q axis current controller input ΔI is obtained by subtracting x_Nq , as shown in formula (14); the negative sequence d-axis current controller input ΔI x_Nd , negative sequence q axis current controller input ΔI x_Nq The negative-sequence d-axis voltage modulation signal U is obtained by the negative-sequence d-axis current controller and the negative-sequence q-axis current controller respectively. rx_Nd , negative sequence q-axis voltage modulation signal U rx_Nq ; Negative sequence d-axis voltage modulation signal U rx_Nd , negative sequence q-axis voltage modulation signal U rx_Nq Collectively referred to as negative sequence voltage modulation signal.

[0068] Among them, the negative sequence d-axis current controller input ΔI x_Nd , negative sequence q axis current controller input ΔI x_Nq Referred to as negative sequence d, q axis current controller input ΔI x_Nd , ΔI x_Nq ; Negative sequence d, q axis voltage modulation signal U rx_Nd , U rx_Nq Referred to as negative sequence d, q axis voltage modulation signal U rx_Nd , U rx_Nq .

[0069] Among them, the negative sequence d-axis current controller input ΔI x_Nd , negative sequence q axis current controller input ΔI x_NqThe negative sequence d axis current controller and the negative sequence q axis current controller refer to the negative sequence d axis current controller input ΔI respectively. x_Nd After the negative sequence d-axis current controller and the negative sequence q-axis current controller input ΔI x_Nq After the negative sequence q-axis current controller.

[0070] ΔI x_Nd =I Lrefx_Nd -I Lx_Nd ΔI x_Nq =I Lrefx_Nq -I Lx_Nq (14).

[0071] Among them, the negative-sequence d and q-axis current controllers; the negative-sequence d-axis voltage controller and the negative-sequence q-axis voltage controller are referred to as negative-sequence d and q-axis voltage controllers for short; 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 dual-port voltage controller.

[0072] It can be understood that the order of the aforementioned S6 and S7 can be interchanged, or both can be performed simultaneously.

[0073] S8, with θ droop is the rotation angle, and the positive and negative sequence voltage modulation signals are subjected to dq inverse transformation to obtain the three-phase voltage modulation signal.

[0074] Among them, the positive-sequence voltage modulation signal and the negative-sequence voltage modulation signal are referred to as positive-sequence voltage modulation signals and negative-sequence voltage modulation signals for short.

[0075] Specifically, the positive-sequence voltage modulation signal and the negative-sequence voltage modulation signal are subjected to dq / abc coordinate inverse transformation (i.e., dq inverse transformation) to obtain a three-phase voltage modulation signal U rx_a , U rx_b , U rx_c , that is, the a-phase voltage modulation signal U rx_a , b-phase voltage modulation signal U rx_b and c-phase voltage modulation signal U rx_c .

[0076] Among them, the rotation angle used in the inverse transformation of dq / abc coordinates is the off-grid local oscillator angle θ droop , that is, the off-grid local oscillator angular frequency ω of the converter droop The integral of the off-grid angular frequency reference value ω n The integral of the difference between the active power droop frequency loop output and the active power droop frequency loop output, where the active power droop frequency loop output is m(P ref -P x ).

[0077] S9, obtaining a PWM control signal according to the three-phase voltage modulation signal.

[0078] 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 SPWM modulation, the PWM control signal of the converter switch tube (referred to as PWM control signal for short) is output.

[0079] If the converter is a three-phase three-wire system, the three-phase voltage modulation signal does not need to be consistent with the zero-sequence control loop output U 0x_0 The three-phase voltage modulation signal U in S8 is directly rx_a , U rx_b , U rx_c After SPWM modulation, the PWM control signal of the converter switch tube (abbreviated as PWM control signal) is output.

[0080] Among them, the zero-sequence control loop is a dual closed-loop control of voltage and current, PWM refers to pulse width modulation, and SPWM refers to sinusoidal pulse width modulation.

[0081] like Figure 3 As shown, in one embodiment, the zero-sequence control loop is specifically: the zero-sequence voltage loop setting (0) and the zero-sequence component U of the output voltage ox_Z After the difference is made, it passes through the zero-sequence voltage controller and the zero-sequence component I Lx_Z After the difference is made and passed through the zero-sequence current controller, the output is U 0x_0 , U 0x_0 That is the output of the zero-sequence control loop.

[0082] It can be understood that the aforementioned corresponding reference values ​​can be selected according to actual needs.

[0083] In one embodiment, the voltage controller is a proportional-integral regulator, and the current controller is a proportional regulator.

[0084] It should be noted that, in the embodiments of the present invention: Droop control: The basic principle of droop control is to achieve load current distribution and stabilize system frequency by simulating the inductive droop characteristics between active power, reactive power, frequency and voltage.

[0085] Off-grid local oscillator angle: In the off-grid power generation mode, the converter acts as a voltage source, and the instantaneous phase angle of the sinusoidal signal with a specific oscillation frequency generated by the control chip's own oscillation.

[0086] Phase-locked loop: A control system that can keep the frequency and phase of a controlled oscillator in a certain relationship with the input signal. It is generally composed of components such as a phase detector, a loop filter, and a voltage-controlled oscillator.

[0087] Off-grid parallel operation: refers to an off-grid power system where multiple devices are connected in parallel to provide power. This system does not rely on the public power grid, but stores electricity through batteries to form a self-sufficient power system.

[0088] Virtual inductive reactance: In microgrids, virtual inductive reactance is used to improve the stability and power distribution of microgrids. Virtual inductive reactance is added to the output of the converter through the control algorithm. The impact of line impedance imbalance on system stability is reduced, achieving the purpose of optimizing power distribution and improving system stability.

[0089] dq transformation: also known as abc / dq transformation, is used to convert three-phase voltage and current into components in a two-phase orthogonal coordinate system. This transformation is based on a rotating dq reference frame, where the d axis is usually aligned with the magnetic field direction of the motor, while the q axis leads by 90 degrees.

[0090] The above method for improving the circulation stability of the off-grid parallel system uses a phase-locked loop to calculate the rotation angle of the converter output voltage and uses it for the dq transformation of the voltage and current loop, the average active power P x and average reactive power Q x The calculation of the droop power and the calculation of the negative sequence d and q axis voltage drops of the virtual inductive reactance are performed. When performing the dq inverse transformation, the off-grid local oscillator angular frequency ω of the converter is used. droop , thus distinguishing 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, thereby suppressing the positive-sequence q-axis circulating current of the converter, and at the same time adding virtual inductive reactance to the negative-sequence loop to suppress the negative-sequence circulating current of the converter, thereby improving the circulating current stability of the system; in addition, this method is based on inductive droop control, does not require communication between parallel converters, and does not affect the adaptability of the converter to reactive loads.

[0091] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for improving the circulation stability of an off-grid parallel system, characterized in that: The method comprises: S1, sampling the three-phase output voltage, three-phase bridge arm inductance current and three-phase output current of converter x, where x represents the converter number, and the number of converters is greater than or equal to two; S2, performs 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 PLL is the rotation angle of the converter output voltage calculated by the phase-locked loop; S3, obtain the voltage loop positive sequence control loop input: S3.1: Calculate the average active power P of converter x x and average reactive power Q x ; S3.2: Use the droop controller to control the P x , Q x Processing is performed to obtain the off-grid local oscillator angle θ of the 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 is a given value of 0; The formulas of the droop controller are shown in equations (8) and (9): oh droop =ω n -m(P ref -P x ) (8); U refx_Pd =U ref -n(Q ref -Q x ) (9); Among them, ω droop , m, n, ω n , U ref , P ref , Q ref are respectively the off-grid local oscillator angular frequency, the droop active coefficient, the droop reactive coefficient, the off-grid angular frequency reference value, the voltage amplitude reference value, the active power reference value, and the reactive power reference value; for the said ω droop Integrate to obtain the θ droop ; S3.3, according to the U refx_Pd , U refx_Pq Get the voltage loop positive sequence control loop input ΔU x_Pd , ΔU x_Pq ; S4, obtain the voltage loop negative sequence control loop input ΔU according to the negative sequence voltage drop generated by the virtual inductive reactance x_Nd , ΔU x_Nq ; S5, according to the ΔU x_Pd , ΔU x_Pq Get the positive sequence d and q axis inductor 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, according to the ΔU x_Nd , ΔU x_Nq Obtaining a negative sequence voltage modulation signal; S8, with the θ droop is the rotation angle, and the positive and negative sequence voltage modulation signals are subjected to dq inverse transformation to obtain the three-phase voltage modulation signal; S9, obtaining a PWM control signal according to the three-phase voltage modulation signal.

2. The method for improving the circulation stability of the off-grid parallel system according to claim 1, characterized in that: The positive and negative sequence d and q axis components of the control loop in S2 include: the output voltage positive sequence d and q axis components U ox_Pd , U ox_Pq , the output voltage negative sequence d, q axis component U ox_Nd , U ox_Nq , the positive sequence d-axis and q-axis components of the bridge arm inductor current I Lx_Pd ,I Lx_Pq , the negative sequence d-axis and q-axis components of the bridge arm inductor current I Lx_Nd ,I Lx_Nq , the output current positive sequence d, q axis components I ox_Pd ,I ox_Pq And the output current negative sequence d, q axis components I ox_Nd ,I ox_Nq .

3. The method for improving the circulation stability of the off-grid parallel system according to claim 2, characterized in that: The calculation formula in S3.1 is shown in formula (7): （7）。 4. The method for improving the circulation stability of the off-grid parallel system according to claim 3, characterized in that: The method in S3.3 is shown in equation (10); where the positive sequence d and q axis voltage controller input ΔU x_Pd , ΔU x_Pq Collectively referred to as voltage loop positive sequence control loop input; ΔU x_Pd =U refx_Pd -U ox_Pd ΔU x_Pq =U refx_Pq -U ox_Pq (10) 5. The method for improving the circulation stability of the off-grid parallel system according to claim 3, characterized in that: In S4, the method for obtaining the input quantity of the voltage loop negative sequence control loop is shown in equation (11), where the negative sequence d and q axis output voltage reference value U refx_Nd , U refx_Nq The given value is 0, and the negative sequence d and q axis voltage controller input ΔU x_Nd , ΔU x_Nq Collectively referred to as the voltage loop negative sequence control loop input; the negative sequence d and q axis voltage drop U generated by the virtual inductive reactance Lv Lvx_Nd , U Lvx_Nq The calculation formula is shown in formula (12); 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).

6. The method for improving the circulation stability of the off-grid parallel system according to claim 4, characterized in that: S5 specifically refers to: x_Pd , ΔU x_Pq After the positive sequence d and q axis voltage controllers respectively, the positive sequence d and q axis inductor current reference values ​​I are obtained. Lrefx_Pd ,I Lrefx_Pq .

7. The method for improving the circulation stability of the off-grid parallel system according to claim 6, characterized in that: Specifically, S6 is: According to formula (13), the positive sequence d and q axis current controller input ΔI is obtained x_Pd , ΔI x_Pq After that, the positive sequence d and q axis current controllers are used to obtain the positive sequence d and q axis voltage modulation signals 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)。 8. The method for improving the circulation stability of the off-grid parallel system according to claim 7, characterized in that: The S7 includes: S7.1: ΔU x_Nd , ΔU x_Nq The negative-sequence d-axis and q-axis inductor current reference values ​​I are obtained through the negative-sequence d-axis and q-axis voltage controllers respectively. Lrefx_Nd ,I Lrefx_Nq ; S7.2: According to equation (14), the negative sequence d and q axis current controller input ΔI is obtained x_Nd , ΔI x_Nq After that, the negative-sequence d-axis and q-axis current controllers are used to obtain the negative-sequence d-axis and q-axis voltage modulation signals 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)。 9. The method for improving the circulation stability of the off-grid parallel system according to claim 8, characterized in that: The S8 comprises: taking the θ droop is the rotation angle, and the positive-sequence voltage modulation signal and the negative-sequence voltage modulation signal are subjected to dq / abc coordinate inverse transformation to obtain a three-phase voltage modulation signal.

10. The method for improving the circulation stability of the off-grid parallel system according to claim 9, characterized in that: The S9 includes: If the converter is a three-phase four-wire system, the three-phase voltage modulation signal is summed with the zero-sequence control loop output and then subjected to SPWM modulation to output the PWM control signal; If the converter is a three-phase three-wire system, the three-phase voltage modulation signal is modulated by SPWM to output the PWM control signal.

11. The method for improving the circulation stability of an off-grid parallel system according to any one of claims 4, 5, 6 and 8, characterized in that: The voltage controller is a proportional-integral regulator.

12. The method for improving the circulation stability of an off-grid parallel system according to claim 7 or 8, characterized in that: The current controller is a proportional regulator.

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