Active damping control method for grid-connected converter

Abstract

The invention discloses an active damping control method for a grid-connected converter, and belongs to the field of converter control. According to the control method, a first-order unstable digital filter is connected in series to a capacitor voltage positive feedback path, reasonable parameter design is used, and phase compensation characteristics of the filter are fully utilized to carry out phase compensation on phase lag introduced by digital control. The filter can guarantee the stability of a system under the conditions of non-linear change of the impedance inductance value of a power grid and a weak power grid, improves the robustness of the system, and is few in design parameters and simple in design process. The converter adopting the control method is simple in loop structure, convenient to design, large in system stability margin and suitable for three-phase LCL filtering network construction / network following type converter occasions.

Description

Technical Field

[0001] The invention belongs to the technical field of grid-connected converters, and in particular relates to an active damping control method for a grid-connected converter. Background Art

[0002] In recent years, the penetration rate of renewable energy distributed generation in the power grid has increased year by year. The grid-connected converter is an interface device that connects distributed power sources with the public power grid, and plays a vital role in the distributed power generation system. The filters of the grid-connected converter can be divided into two types: L-type filters and LCL-type filters. Compared with the L-type filter, the LCL-type filter has better high-frequency harmonic attenuation capability, and under the same filtering effect, the total inductance required by the LCL-type filter is significantly smaller than that of the single-inductor filter, which is conducive to reducing the volume and weight of the converter and reducing costs. However, the LCL-type filter has a resonance problem. Its amplitude-frequency response has a resonance peak at the resonant frequency, and the phase will jump by -180°. The grid-connected system is prone to instability, so it is necessary to damp the resonance peak to stabilize the system.

[0003] One of the most direct methods to suppress resonant spikes is to add resistors to the LCL filter, also known as passive damping. This strategy is simple to operate and has high reliability, but it also brings about a lot of power loss. To overcome the above defects, an active damping strategy has been developed to achieve the effect of virtual resistance through appropriate control methods. Compared with the passive damping strategy, the active damping strategy has higher flexibility and higher system efficiency.

[0004] In addition, with the widespread application of digital control in the field of grid-connected converters, control delays are introduced into the control loop, including calculation delays and pulse width modulation (PWM) delays, which will lead to a reduction in the bandwidth and stability of the control loop. Phase advance compensation filters are an effective control strategy to mitigate the negative impact of digital control delays, but existing phase advance compensation filters are difficult to adapt to a wider range of system resonant frequency changes. Under weak grid conditions or when the filter inductance changes, the stability of the grid-connected system will still be greatly challenged. Summary of the invention

[0005] In order to solve the technical problems mentioned in the above background technology, in the field of active damping control methods of grid-connected converters, it is necessary to study a phase compensation filter that can effectively suppress the resonance peak of the system within a wide resonant frequency range and has a convenient design and implementation process, which provides damping for the system while taking into account stability and grid-side current quality.

[0006] To achieve the above object, the technical solution of the present invention is:

[0007] The present invention relates to an active damping control method for a grid-connected converter. The grid-connected converter comprises a converter module and a filter module. The output end of the converter module is connected in series with the filter module. The filter module comprises an A-phase first inductor, a B-phase first inductor, a C-phase first inductor, an A-phase second inductor, a B-phase second inductor, a C-phase second inductor, an A-phase capacitor, a B-phase capacitor, and a C-phase capacitor. An X-phase first inductor, an X-phase second inductor, and an X-phase capacitor (X=A, B, C) are connected together to form a three-phase LCL filter. The active damping control method comprises: inserting a first-order unstable digital filter in series on a path of output voltage feedback of the grid-connected converter, sampling the X-phase capacitor voltage v of the grid-connected converter, and cx (X=A,B,C;x=a,b,c), the sampled X-phase capacitor voltage v cx As the input signal of the first-order unstable digital filter, the input signal of the first-order unstable digital filter is passed through the first-order unstable digital filter to generate a first-order unstable digital filter output signal; sampling the current i of the first inductor current of the X phase Lx1 The reference value i of the first inductor current of phase X Lx1 * The output signal after difference is used as the current controller G i (s) input signal; the output signal of the first-order unstable digital filter and the current controller G i The output signals of (s) are summed to form the modulation wave u m '; The first-order unstable digital filter G ad (s) is:

[0008]

[0009] In the formula, ω α To adjust the angular frequency, α is the compensation coefficient and s is the Laplace operator.

[0010] In the above-mentioned active damping control method for grid-connected converter, the modulation wave u m ' as the input signal of the digital delay link; wherein the digital control delay G in the digital delay link d (s) includes 1 times the sampling period and PWM delay, and the output signal of the digital delay link is u m ; Output signal u of the digital delay link m As the input signal of the grid-connected converter; and compensating the phase angle lag caused by the digital control delay by connecting the first-order unstable digital filter in series on the output voltage feedback path.

[0011] In the above-mentioned active damping control method for grid-connected converter, the first-order unstable digital filter is an unstable link in the s domain. The first-order unstable digital filter is discretized using the backward difference discretization method, and some unstable poles in the s domain are mapped to the unit circle in the z domain. The expression G after the first-order unstable digital filter is discretized is ad (z) is:

[0012]

[0013] In the formula, z is e sTs , A1=T s ω α +1, A0 = -1, B1 = aT s ω α -1, B0=1, T s is the sampling period, ω α To adjust the angular frequency, α is the compensation coefficient.

[0014] In the above grid-connected converter active damping control method, the compensation angle of the discretized first-order unstable digital filter in the discrete domain is:

[0015]

[0016] In the formula, A1 = T s ω α +1, A0 = -1, B1 = aT s ω α -1, B0=1, T s is the sampling period, and ω is the angular frequency.

[0017] In the above-mentioned active damping control method for grid-connected converter, the first-order unstable digital filter is at an angular frequency ω=ω s The compensation angle at / 2 is:

[0018]

[0019] In the formula, α is the compensation coefficient, ω α To adjust the angular frequency, T s is the sampling period.

[0020] In the above-mentioned active damping control method for the grid-connected converter, after using the first-order unstable digital filter, the equivalent admittance Y eq The phase is:

[0021]

[0022] In the formula, z is e sTs , A1=T s ω α +1, A0 = -1, B1 = aTs ω α -1, B0=1, T s is the sampling period, ω is the angular frequency; when the resonant frequency takes the maximum value ω=ω s / 2, the equivalent conductance G eq When the compensation coefficient α is between (0.52, 1), it is greater than 0.

[0023] In the above-mentioned active damping control method for the grid-connected converter, the parameter design of the first-order unstable digital filter comprises the following steps:

[0024] Step S1, select the compensation coefficient α of the first-order unstable digital filter, α>2 / (T s ω α ),ω s is the resonant angular frequency; when the angular frequency ω is adjusted α =ω s / π, the equivalent conductance G in the Nyquist band is satisfied eq >0, the compensation coefficient α is (0.52,1); when the angular frequency ω is adjusted α =ω s / 6, the equivalent conductance G in the Nyquist band is satisfied eq The compensation coefficient α of >0 is (1.77, 1.909);

[0025] Step S2, selecting the adjustment angular frequency ω of the first-order unstable digital filter α ,ω α ≤ω s / π; satisfies the equivalent conductance G within the Nyquist band eq Under the condition that the compensation coefficient a is greater than 0, the critical value 2 / (T s ω α ), select the adjustment angular frequency ω α =ω s / π, at this time the equivalent conductance G eq The amplitude is the largest, which can provide the maximum damping;

[0026] Step S3: the compensation coefficient α and the adjustment angular frequency ω α After the parameters of the first-order unstable digital filter are designed, the gain H ad Select; the gain H of the first-order unstable digital filter ad According to the stability margin of the Bode diagram of the inner loop open-loop transfer function of the capacitor voltage positive feedback active damping with a calculated delay of one beat after using the first-order unstable digital filter, the first-order unstable digital filter gain H ad It is necessary to ensure that the minimum amplitude margin of the stability margin of the inner open-loop transfer function Bode diagram is 3 dB and the minimum phase margin is 15°.

[0027] In the above-mentioned active damping control method for the grid-connected converter, the compensation coefficient α of the first-order unstable digital filter is selected in step S1, including, based on the high-pass filter, the value range of the compensation coefficient α is:

[0028]

[0029] In the formula, α is the compensation coefficient, ω s is the resonant angular frequency, ω α To adjust the angular frequency, T s is the sampling period.

[0030] In the above-mentioned active damping control method for grid-connected converter, the adjustment angular frequency ω of the first-order unstable digital filter selected in step S2 is α Including, after using the first-order unstable digital filter, the damping ratio ζ of the active damping equivalent to the capacitor voltage positive feedback:

[0031]

[0032] In the formula, G eq is the equivalent conductance, ω s is the resonant angular frequency, C x is the X-phase filter capacitor, X=A, B, C, x=a, b, c; under the same compensation coefficient α, adjust the angular frequency ω α =ω s / π equivalent conductance G eq The amplitude is larger; the damping ratio decreases with the equivalent conductance G eq increases with the increase of, adjusting the angular frequency ω α =ω s / π can provide the maximum damping, select the adjustment angular frequency ω α =ω s / π.

[0033] By inserting a first-order unstable digital filter in series on the output voltage feedback path, the phase compensation characteristics of the filter are fully utilized to compensate for the phase lag introduced by the digital control. The filter can ensure the stability of the system in the case of nonlinear changes in the inductance of the grid inductance and weak grid. At the same time, the design parameters are small and the design process is simple. The converter loop structure using this control method is simple, the design is convenient, and the system stability margin is large. It is suitable for three-phase LCL filter grid / grid-following converter occasions.

[0034] In order to make the above features and advantages of the invention more obvious and easy to understand, embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1The circuit topology of a three-phase LCL filter three-level grid-connected converter and its control circuit.

[0036] Figure 2 Schematic diagram of active damping control structure based on capacitor voltage positive feedback.

[0037] Figure 3 Schematic diagram of equivalent changes of active damping based on capacitor voltage positive feedback.

[0038] Figure 4 This is the equivalent admittance vector diagram of the capacitor voltage positive feedback active damping based on the calculation delay of one beat.

[0039] Figure 5 Bode diagram of the open-loop transfer function of the capacitor voltage positive feedback active damping inner loop.

[0040] Figure 6 To calculate the capacitor voltage positive feedback active damping with a delay of one beat, the equivalent conductance G is satisfied. eq >0 compensation coefficient α and adjustment angular frequency ω α Schematic diagram of the relationship between them.

[0041] Figure 7 When the compensation coefficient α is the same, adjust the angular frequency ω a The equivalent conductance G of the system at different times eq Schematic diagram.

[0042] Figure 8 When the compensation coefficient α is 1.001, the equivalent conductance G eq >0 frequency range and each resonant frequency f r The first-order unstable digital filter gain H that satisfies the inner loop stability is ad The value range of .

[0043] Fig. 9 The Bode diagram of the inner open-loop transfer function of the capacitor voltage positive feedback active damping with a calculation delay of one beat under different grid impedances after using a first-order unstable digital filter in the specific implementation.

[0044] Fig.10 The Bode diagram of the outer loop open-loop transfer function of the capacitor voltage positive feedback active damping with a one-beat calculation delay after using a first-order unstable digital filter in the specific implementation is shown.

[0045] Fig.11 To calculate the X-phase grid impedance L of the capacitor voltage positive feedback active damping with a delay of one beat gx Schematic diagram of the change in position of the closed-loop poles of the system when (X=A,B,C; x=a,b,c) changes.

[0046] Fig.12 is the X-phase grid impedance L gx=0mH, schematic diagram of grid-connected current waveform using active damping scheme of grid-connected converter using first-order unstable digital filter.

[0047] Fig.13 The power grid is a weak power grid (short circuit ratio SCR = 2, X phase power grid impedance L gx =5.8mH, X=A, B, C; x=a, b, c) schematic diagram of the grid-connected current waveform of the active damping scheme of the grid-connected converter using the first-order unstable digital filter under the working conditions. DETAILED DESCRIPTION

[0048] In order to make the purpose and technical solution of the embodiment of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings of the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the described embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0049] The active damping control method of the grid-connected converter of the present invention is realized based on sampling delay and a first-order unstable digital filter. The grid-connected converter of the present invention is a three-phase LCL type filter three-level grid-connected converter. Figure 1 The figure is a circuit topology diagram of a three-phase LCL filter three-level grid-connected converter and its control circuit. Figure 1 As shown, the three-phase LCL type filter three-level grid-connected converter 1 includes a DC power supply module 11, a three-level grid-connected converter module 12, an LCL filter module 13, a common coupling point 14, a grid impedance module 15, and a grid equivalent voltage source module 16.

[0050] The DC power supply module 11 includes a DC power supply V dc , DC power supply V dc The two levels of the three-level grid-connected converter module 12 are connected to the three-level grid-connected converter module 12, and the output ends of the three-level grid-connected converter module 12 are the A-phase bridge arm voltage output point A inv , B phase bridge arm voltage output point B inv , C phase bridge arm voltage output point C inv The LCL filter module 13 includes a first inductor L1, a second inductor L2, and a filter capacitor C, wherein the first inductor L1 includes a first inductor L of the A phase. a1 , B phase first inductor L b1 , and the first inductor L of phase C c1 The second inductor L2 includes the second inductor L of the A phase a2 、B phase second inductor L b2 , and the second inductor L of phase C c2 ; Filter capacitor C includes A phase filter capacitor Ca 、B phase filter capacitor C b , and C phase filter capacitor C c Among them, through the first inductor L of phase A a1 The current on the A phase is the first inductor current i La1 , through the first inductor L of phase B b1 The current on the first inductor of phase B is i Lb1 , through the first inductor L of phase C c1 The current on the first inductor of phase C is i Lc1 ; Through the second inductor L of phase A a2 The current on the A phase second inductor current i La2 , through the second inductor L of phase B b2 The current on the second inductor of phase B is i Lb2 , through the second inductor L of phase C c2 The current on the C phase second inductor current i Lc2 ; Through the A phase filter capacitor C a The current on the filter capacitor is the A phase current i ca , through the B phase filter capacitor C b The current on the filter capacitor is the B phase current i cb , through the C phase filter capacitor C c The current on the filter capacitor is the C phase current i cc ; A phase filter capacitor C a The voltage between the first terminal voltage and the ground is the A phase filter capacitor voltage v ca , B phase filter capacitor C b The voltage between the first terminal voltage and the ground is the B phase filter capacitor voltage v cb , C phase filter capacitor C c The voltage between the first terminal voltage and the ground is the C phase filter capacitor voltage v cc ; The first inductor L of phase A a1 The first end and the A-phase bridge arm voltage output point A of the three-level grid-connected converter module 12 inv Connect the second end to the A phase filter capacitor C a The first end of the A phase second inductor L a2 The first end of the B phase first inductor L b1 The first end and the B-phase bridge arm voltage output point B of the three-level grid-connected converter module 12 inv Connect the second end to the B phase filter capacitor C b The first end of the B phase second inductor L b2 The first end of the C phase first inductor L c1 The first end and the C phase bridge arm voltage output point C of the three-level grid-connected converter module 12 inv Connect the second end to the C-phase filter capacitor Cc The first end of the C phase second inductor L c2 The first end of the A phase filter capacitor C a The second end of the B phase filter capacitor C b The second end of the C phase filter capacitor C c The second ends of the three-phase filter capacitors are connected together to form a star connection.

[0051] The common coupling point 14 includes the A phase common coupling point v PCCa 、B phase common coupling point v PCCb , and the C phase common coupling point v PCCc ; The grid impedance module 15 includes a phase A grid impedance L ga , Phase B grid impedance L gb , and the C phase grid impedance L gc The second inductor L of phase A a2 The second end is connected to the A phase grid impedance L ga The first end passes through the A phase common coupling point v PCCa Connect the second inductor L of phase B b2 The second end is connected to the B phase grid impedance L gb The first end passes through the B phase common coupling point v PCCb Connect the second inductor L of phase C c2 The second end is connected to the C phase grid impedance L gc The first end passes through the C phase common coupling point v PCCc The grid equivalent voltage source module 16 includes a phase A grid equivalent voltage source v ga 、B-phase grid equivalent voltage source v gb , and the equivalent voltage source v of the C-phase grid gc ; The A-phase grid equivalent voltage source v ga The first end is connected to the A phase grid impedance L ga The second end is connected to the B-phase grid equivalent voltage source v gb The first end is connected to the B phase grid impedance L gb The second end is connected to the C-phase grid equivalent voltage source v gc The first end is connected to the C phase grid impedance L gc The second end of the A-phase grid equivalent voltage source v ga The second end, B phase grid equivalent voltage source v gb The second end and the C-phase grid equivalent voltage source v gc The second ends are connected together to form a star connection.

[0052] The control circuit 2 is used to control the three-phase LCL filter three-level grid-connected converter 1, and the control circuit 2 includes a driving module 21, a capacitor voltage sampling module 22, a DSP control module 23, and an inductor current sampling module 24. The DSP control module 23 includes a modulation wave abc / dq coordinate converter 231, a capacitor voltage abc / dq coordinate converter 232, a first-order unstable digital filter 233, an inductor current abc / dq converter 234, a current controller 235, an adder 236, and a subtractor 237.

[0053] The three input terminals of the capacitor voltage sampling module 22 respectively sample the A-phase filter capacitor C a The voltage of the A-phase filter capacitor between the first terminal voltage and ground v ca 、B phase filter capacitor C b The voltage v between the first terminal voltage and the ground of the B-phase filter capacitor cb , and C phase filter capacitor C c The voltage between the first terminal voltage and the ground of the C-phase filter capacitor v cc The output end of the capacitor voltage sampling module 22 is connected to the input end of the capacitor voltage abc / dq coordinate converter 232; the two output ends of the capacitor voltage abc / dq coordinate converter 232 are connected to the two input ends of the first-order unstable digital filter 233; the three input ends of the inductor current sampling module 24 respectively sample the first inductor L of phase A a1 The first inductor current i of phase A on La1 , through the first inductor L of phase B b1 The first inductor current i of phase B on Lb1 , and through the first inductor L of phase C c1 The first inductor current i of phase C Lc1 The output end of the inductor current sampling module 24 is connected to the input end of the inductor current abc / dq converter 234; the two output ends of the inductor current abc / dq converter 234 are respectively connected to the subtractor 237 and respectively compared with the given value i d * and given value i q *Subtraction; the two output ends of the subtractor 237 are connected to the two input ends of the current regulator 235; the two output ends of the current controller 235 and the two output ends of the first-order unstable digital filter 233 are connected to the adder 236 and added respectively; the two output ends of the adder 236 are connected to the two input ends of the modulation wave abc / dq coordinate converter 231; the output end of the modulation wave abc / dq coordinate converter 231 is connected to the drive module 21, and the output of the drive module 21 is connected to the three-level grid-connected converter module 12.

[0054] Figure 2FIG. 1 is a schematic diagram of the active damping control structure based on capacitor voltage positive feedback of the present invention. Figure 2 As shown, 31 is the inductor current outer loop, 32 is the X-phase first inductor current reference value i Lx1 * , 33 is the current controller G i (s), 34 is the capacitor voltage active damping inner loop, 35 is the X-phase grid voltage, 36 is the system model, and 37 is the current flowing through the X-phase second inductor L x1 The X-phase second inductor current i Lx2 , the capacitor voltage active damping inner loop 34 includes, a first-order unstable digital filter G ad (s) and its gain H ad The product of 341, digital control delay G d (s) 342, X-phase bridge arm voltage 343, flows through the X-phase first inductor L x1 The X-phase first inductor current i Lx1 344, X-phase filter capacitor C x X-phase capacitor voltage v cx 345; The present invention adopts the first inductor L flowing through the X phase x1 The X-phase first inductor current i Lx1 344 feedback control strategy realizes overcurrent protection and power control (X = A, B, C; x = a, b, c), the inductor current outer loop 31 feeds back the sampled current flowing through the first inductor L of the X phase x1 The X-phase first inductor current i Lx1 344 and the X-phase first inductor current reference value i Lx1 * 32 compares the difference and passes through the current controller G i (s) 33 obtains the error signal; the capacitor voltage active damping inner loop 34 uses capacitor voltage positive feedback to achieve active damping, using a first-order unstable digital filter G ad (s) Perform phase compensation on the conductance equivalent to active damping to make the equivalent conductance positive in the entire Nyquist band; x X-phase capacitor voltage v cx 345 sampling (X = A, B, C; x = a, b, c), and using the sampled X-phase capacitor voltage v cx 345 as the first-order unstable digital filter G ad (s) and its gain H ad The input signal of the product 341 is passed through the first-order unstable digital filter G ad (s) and its gain H ad The product 341 generates the output signal of the first-order unstable digital filter. The first-order unstable digital filter and its gain H adThe product of the output signal of 341 and the current controller G i (s)33 The error signal obtained is summed as the modulation wave u' m ; The digital control delay G d (s) 342 includes 1 times the sampling period and PWM delay, where the calculation delay is 1 times the sampling period, also known as one-beat calculation delay; the modulation wave u' m As the input signal passes through the digitally controlled delay G d (s)342 After that, the output signal is the final modulation wave u m The input variable of the transfer function of the system model 36 is the X-phase bridge arm voltage v invx (s) 343, the output variable is the current flowing through the second inductor L of phase X x1 The X-phase inductor current i Lx2 (s)37, the disturbance variable is the equivalent voltage source v of the X-phase power grid gx (s)35.

[0055] Depend on Figure 2 The available system model is:

[0056]

[0057] In the formula, X=A, B, C, x=a, b, c; G iL1 (s) is the X-phase bridge arm voltage v invx (s) to the first inductor L of phase X x1 The X-phase first inductor current i Lx1 The transfer function of (s), i Lx1 (s) is the transfer function G iL1 The output variable (s) flows through the first inductor L of the X phase x1 The first inductor current of phase X, v invx (s) is the transfer function G iL1 (s) Input variable X-phase bridge arm voltage, L T is the current flowing through the second inductor L of phase X x2 and X-phase grid impedance L gx The sum of C x is the X-phase filter capacitor, L x1 is the current flowing through the first inductor L of phase X x1 , s is the Laplace operator, ω r is the resonant angular frequency of the LCL filter, f r is the resonant frequency of the LCL filter; G vc (s) is the bridge arm voltage X phase v invx (s) to X phase capacitor voltage v cx The transfer function of (s), v cx (s) is the X-phase capacitor voltage; G ic(s) is the X-phase bridge arm voltage v invx (s) to the first inductor L of phase X x1 The X-phase first inductor current i Lx1 i Lx1 The transfer function of (s), i icx (s) is the current flowing through the X-phase filter capacitor C x The capacitor current.

[0058] Depend on Figure 2 It can be seen that the open-loop gain transfer function T of the capacitor voltage positive feedback control system is vc (s) can be expressed as:

[0059]

[0060] In the formula, G i (s) is the transfer function of the current controller, G d (s) is the digital control delay of the system, H ad is the first-order unstable digital filter gain, G ad (s) is a first-order unstable digital filter, C x is the X-phase filter capacitor of the LCL filter, and s is the Laplace operator.

[0061] Figure 3 FIG. 1 is a schematic diagram of equivalent changes of the active damping based on capacitor voltage positive feedback of the present invention. Figure 3 As shown, 41 is the inductor current outer loop, 411 is the capacitor voltage active damping inner loop, and 412 is the equivalent admittance Y eq , 42 is the virtual impedance Z eq ; Figure 3 The system described in the figure is a one-shot calculation delay system with capacitor voltage positive feedback active damping. The capacitor voltage positive feedback active damping is equivalent to the X-phase filter capacitor C x Parallel virtual impedance Z eq 42, the expression is:

[0062]

[0063] In the formula, H ad is the first-order unstable digital filter gain, R eq (s) is the equivalent virtual resistance, X eq (s) is the equivalent virtual reactance.

[0064] The virtual impedance Z eq 42 is converted to equivalent admittance Y eq The expression of 412 is as follows, the real part is the conductance and the imaginary part is the susceptance:

[0065]

[0066] In the formula, G eq (ω) is the equivalent conductance, j is the imaginary unit, B eq (ω) is the equivalent virtual susceptance, R eq (ω) is the equivalent virtual resistance, X eq (ω) is the equivalent virtual reactance, G ad (s) is a first-order unstable digital filter, T s is the sampling period, and ω is the angular frequency.

[0067] From formula (7), we can get the equivalent admittance Y of the system: eq Phase ∠Y eq (f) is:

[0068]

[0069] In the formula, f s is the sampling frequency of the system, and f is the independent variable frequency.

[0070] Figure 4 The equivalent admittance vector diagram of the capacitor voltage positive feedback active damping based on one-beat calculation delay. The equivalent admittance Y of the above system is eq The vector is Figure 4 As shown in the figure, it can be seen that the equivalent conductance G eq In the frequency range (0, f s / 3) is a positive number, and the equivalent admittance Y eq The phase is (-90°, 90°), which suppresses the resonance characteristics; the equivalent conductance G eq In the frequency range (f s / 3,2f s / 3) is a negative number, and the equivalent admittance Y eq The phase is (-90°, -270°). At this time, when the grid impedance changes and the system resonant frequency is greater than f s / 3, the active damping no longer damps the system. To deal with this situation, it is necessary to compensate the phase of the admittance.

[0071] The expression of the first-order unstable digital filter in the s domain is G ad (s) is:

[0072]

[0073] In the formula, ω α To adjust the angular frequency, α is the compensation coefficient, and the value of α should be greater than or equal to 1. This first-order unstable digital filter behaves as a high-pass filter and only feeds back the high-frequency component of the capacitor voltage.

[0074] The stability of the grid-connected converter can be judged by the Nyquist stability criterion, that is, Z = P-2 (N + -N- ), Z and P are the poles of the system in the closed loop and open loop right half plane of the s plane, respectively, N + N is the number of times the open-loop Bode diagram phase characteristic curve crosses -180° and the amplitude margin GM>0dB. - It is the number of times the open-loop Bode diagram phase characteristic curve negatively crosses -180° and the amplitude margin GM<0dB. Figure 5 The open-loop Bode diagram of the capacitor voltage positive feedback active damping inner loop for calculating the one-beat delay. Figure 5 It can be seen that the system is at the Nyquist frequency f N There is a gain "trap" at this point, which just offsets the high-frequency gain of the digital filter.

[0075] The present invention adopts a compensation strategy of connecting a first-order unstable digital filter in series on the capacitor voltage positive feedback path. After using the present invention, the phase ∠G of the system ad (ω) is:

[0076]

[0077] In the formula, ω α To adjust the angular frequency, α is the compensation coefficient and ω is the angular frequency.

[0078] As shown in formula (10), the phase of the compensation link is given by ∠G α (ω) and ∠G β (ω) consists of two parts, when ω=ω α When ∠G α (ω) provides a 45° phase. When α is large, the phase in the low frequency band is mainly determined by ∠G α (ω) provided.

[0079] In the s domain, the first-order unstable digital filter is an unstable link. In order to improve the stability range of the system, the filter is discretized using the backward difference discretization method. The backward difference discretization can map some s-domain unstable poles to the z-domain unit circle. The discretized first-order unstable digital filter G ad (z) is:

[0080]

[0081] In the formula, z is e sTs , A1=T s ω α +1, A0 = -1, B1 = aT s ω α -1, B0=1, T s is the sampling period.

[0082] Therefore, the phase that can be compensated by the first-order unstable digital filter in the z domain is:

[0083]

[0084] In the formula, A1 = T s ω α +1, A0 = -1, B1 = aT s ω α -1, B0=1, T s is the sampling period, and ω is the angular frequency.

[0085] The first-order unstable digital filter has an angular frequency of ω=ω s The compensation angle at / 2 is:

[0086]

[0087] Where, T s is the sampling period, ω α To adjust the angular frequency, α is the compensation coefficient and ω is the angular frequency.

[0088] After using the first-order unstable digital filter, the damping ratio ζ of the active damping equivalent to capacitor voltage feedback is:

[0089]

[0090] In the formula, G eq is the equivalent conductance, ω s is the resonant angular frequency, C x It is the X-phase filter capacitor.

[0091] After using the first-order unstable digital filter, the active damping equivalent admittance phase of the system is:

[0092]

[0093] In the formula, z is e sTs , A1=T s ω α +1, A0 = -1, B1 = aT s ω α -1, B0=1, T s is the sampling period, and ω is the angular frequency. At this time, when the resonant frequency takes the maximum value, ω=ω s / 2, the equivalent conductance G eq When the compensation coefficient α is taken as an appropriate value, it is still greater than 0.

[0094] Specifically, in implementation, the parameters of the first-order unstable digital filter used in the invention need to be designed to achieve the optimal effect. The main design steps are as follows:

[0095] Step S1, select the compensation coefficient α of the first-order unstable digital filter. Because when α<2 / (T s ω α ), the first-order unstable digital filter has poles outside the unit circle in the z domain, and the system is unstable at this time. Therefore, the selection of the compensation coefficient α must satisfy α>2 / (T s ω α ). According to the definition of high-pass filter, the compensation coefficient α must be greater than or equal to 1, so the angular frequency ω is adjusted α and the resonant angular frequency ω s Need to meet ω α <ω s / π. Figure 6 To calculate the capacitor voltage positive feedback active damping with a delay of one beat, the equivalent conductance G is satisfied. eq >0 compensation coefficient α and adjustment angular frequency ω a Schematic diagram of the relationship between them. Figure 6 As shown, ω α =ω s / π, the equivalent conductance G in the Nyquist band is satisfied eq The compensation coefficient α > 0 is (0.52, 1); ω α =ω s / 6, the equivalent conductance G in the Nyquist band is satisfied eq >0 is (1.77, 1.909); and as the compensation coefficient α increases, the equivalent conductance G eq >0, the frequency interval is reduced, so the compensation coefficient α should be as close as possible to the minimum value α of the compensation coefficient min Considering the calculation accuracy of the digital controller variable, the compensation coefficient α cannot be infinitely close to the minimum value of the compensation coefficient α min ,observe Figure 6 The actual compensation coefficient α is selected as 1.001. Figure 8 When the compensation coefficient α is 1.001, the equivalent conductance G eq >0 frequency range and each resonant frequency f r The first-order unstable digital filter gain H that satisfies the inner loop stability is ad The value range is from Figure 8 It can be seen that the equivalent conductance G eq The frequency range for positive resistance is (0,0.491f s ).

[0096] Step S2, selecting the adjustment angular frequency ω of the first-order unstable digital filter α . Figure 7 When the compensation coefficient α is the same, adjust the angular frequency ω α Schematic diagram of equivalent conductance of the system at different times. Figure 7As shown, the equivalent conductance G is satisfied within the Nyquist band. eq When the compensation coefficient a is greater than 0, the critical value 2 / (T s ω α ), comparing different adjustment angular frequencies ω α The equivalent conductance G under the parameters eq size. It can be seen that in ω α =ω s / π parameter equivalent conductance G eq A larger amplitude can provide greater damping. Therefore, in the implementation process, a larger adjustment angular frequency ω is generally selected. α , here we adjust the angular frequency ω α Under the constraints of Figure 6 Select an appropriate compensation coefficient a.

[0097] Step S3: After the parameters of the first-order unstable digital filter are designed, in order to ensure the stability of the inner loop, the first-order unstable digital filter gain H ad The gain H of the first-order unstable digital filter is ad According to the stability margin of the Bode diagram of the inner loop open-loop transfer function of the capacitor voltage positive feedback active damping with a calculated delay of one beat after using the first-order unstable digital filter, the first-order unstable digital filter gain H ad The minimum amplitude margin must be 3dB and the minimum phase margin must be 15°. Figure 8 When the compensation coefficient a=1.001, the equivalent conductance G eq >0 frequency range and each resonant frequency f r The first-order unstable digital filter gain H that satisfies the inner loop stability is ad The value range of Figure 8 As shown in Figure 2, when the compensation coefficient α = 1.001, the inner loop is almost stable at the Nyquist frequency, K ad The smaller the value, the larger the inner ring stability range, where K ad =L T H ad / (L1+L T ). Table 1 is a system parameter table, giving specific experimental parameters. When the X-phase grid impedance L gx =5.8mH, short circuit ratio SCR = 2, the resonant frequency decreases to f r =0.22f s ; When the first inductor L of phase X x1 When the inductance fluctuation drops to 50%, the resonant frequency increases to f r =0.41f s .

[0098] Table 1 System parameters

[0099]

[0100] According to the possible working conditions in the above experiments, the first-order unstable digital filter gain H is designed ad ,observe Figure 6 It can be seen that the compensation coefficient α=1.001 and ω α =ω s / π can satisfy the equivalent conductance G eq In the frequency band (0,0.491f s ) within the positive resistance.

[0101] The present invention is further verified below in conjunction with specific implementation methods:

[0102] Fig. 9 The Bode diagram of the inner loop open-loop transfer function of the capacitor voltage positive feedback active damping with a delay of one beat under different grid impedances after using a first-order unstable digital filter in a specific implementation is shown in FIG. Fig. 9 As shown, when the first inductor L of the X phase x1 Inductance value fluctuates by 50%, resonant frequency f r Move to the high frequency range, at this time the amplitude margin GM3 and phase margin PM3 are minimum. xg When it increases, the resonant frequency f r Move to the low frequency range, at which time the phase margin PM1 is the smallest. Therefore, the resonant frequency f r The designed amplitude margin GM3 and phase margin PM3 at the maximum resonant frequency f r The phase margin PM1 is designed to be minimum. In order to make the system have sufficient stability margin, the minimum gain margin should be greater than 3dB and the minimum phase margin should be greater than 15°. ad When =0.046, the phase margin PM1=57°, the phase margin PM3=15°, and the amplitude margin GM3=3dB, which meets the stability requirements.

[0103] Fig.10 The Bode diagram of the outer loop open-loop gain of the capacitor voltage positive feedback active damping with a calculation delay of one beat after using a first-order unstable digital filter in a specific implementation is shown in FIG. Fig.10 As shown, the grid-connected converter active damping scheme of the first-order unstable digital filter can meet the X-phase grid impedance L gx Stability under the conditions of (0mH, 5.8mH) weak power grid and X-phase power grid impedance dropping by 50%. Fig.11 To calculate the X-phase grid impedance L of the capacitor voltage positive feedback active damping with a delay of one beat gx Schematic diagram of the position change of the closed-loop poles of the system when it changes. Fig.11 Where 111 is the position change trajectory of the closed-loop pole of the system, from Fig.11It can be seen that the system has good dynamic performance and robustness.

[0104] A prototype was built to further experiment with the proposed control strategy. Fig.12 is the X-phase grid impedance L gx = 0mH, the grid-connected current waveform diagram of the grid-connected converter active damping scheme using a first-order unstable digital filter, at which time the three-phase LCL filter resonant frequency f r =8.4kHz, the grid current has good steady-state and dynamic performance. In time period 121, the grid current undergoes a sudden change to half load. In time period 122, the active damping control strategy of the present invention is removed. After the present invention is removed, the system damping is insufficient and high-frequency resonance 123 occurs in the grid current. Fig.13 For weak grid (short circuit ratio SCR = 2, X phase grid impedance L gx =5.8mH) working condition, the grid-connected current waveform diagram of the active damping scheme of the grid-connected converter using the first-order unstable digital filter, at this time the three-phase LCL filter resonant frequency f r =5.2kHz, the dominant pole of the system moves to the low frequency band, the dynamic response of the grid current slows down, and in time period 131, the grid current undergoes a sudden change to half load. At time 132, the active damping control strategy of the present invention is removed. In time period 133, after the present invention is removed, the grid current oscillates and triggers the hardware overcurrent protection.

[0105] In summary, the present invention is used in the three-phase LCL filter grid / grid-following converter, which solves the system stability problem of the weak grid under the wide range of inductance changes. Without adding additional sensors, the parameter design is simple, with a high stability range, and is effective for the weak grid working condition and the first inductance L of the X phase. x1 The sensor is highly robust to fluctuations in inductance and is easy to use in engineering.

[0106] The above embodiments are only for illustrating the technical idea of ​​the present invention, and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for controlling active damping of a grid-connected converter, wherein the grid-connected converter comprises a converter module and a filter module, wherein an output end of the converter module is connected in series with the filter module; The filter module includes: a first inductor of phase A, a first inductor of phase B, a first inductor of phase C, a second inductor of phase A, a second inductor of phase B, a second inductor of phase C, a capacitor of phase A, a capacitor of phase B, and a capacitor of phase C; a first inductor of phase X, a second inductor of phase X, and a capacitor of phase X (X=A, B, C) are connected together to form a three-phase LCL filter; It is characterized in that The active damping control method includes: inserting a first-order unstable digital filter in series on the output voltage feedback path of the grid-connected converter, sampling the X-phase capacitor voltage v of the grid-connected converter cx (X=A,B,C;x=a,b,c), the sampled X-phase capacitor voltage v cx As the input signal of the first-order unstable digital filter, the input signal of the first-order unstable digital filter is passed through the first-order unstable digital filter to generate a first-order unstable digital filter output signal; sampling the current i of the first inductor current of the X phase Lx1 The reference value i of the first inductor current of phase X Lx1 * The output signal after difference is used as the current controller G i (s) input signal; the output signal of the first-order unstable digital filter and the current controller G i The output signals of (s) are summed to form the modulation wave u m '; The first-order unstable digital filter G ad (s) is: In the formula, ω α To adjust the angular frequency, α is the compensation coefficient and s is the Laplace operator.

2. The active damping control method for a grid-connected converter according to claim 1, It is characterized in that The modulation wave u m ' as the input signal of the digital delay link; wherein the digital control delay G in the digital delay link d (s) includes 1 times the sampling period and PWM delay, and the output signal of the digital delay link is u m ; Output signal u of the digital delay link m As the input signal of the grid-connected converter; and compensating the phase angle lag caused by the digital control delay by connecting the first-order unstable digital filter in series on the output voltage feedback path.

3. The active damping control method for a grid-connected converter according to claim 2, It is characterized in that The first-order unstable digital filter is an unstable link in the s domain. The first-order unstable digital filter is discretized using the backward difference discretization method, and some unstable poles in the s domain are mapped to the unit circle in the z domain. The expression G after the first-order unstable digital filter is discretized is ad (z) is: In the formula, z is e sTs , A 1 =T s ω α +1, A 0 =-1, B 1 =aT s ω α -1, B 0 =1,T s is the sampling period, ω α To adjust the angular frequency, α is the compensation coefficient.

4. The active damping control method for a grid-connected converter according to claim 3, It is characterized in that The compensation angle of the discretized first-order unstable digital filter in the discrete domain is: In the formula, A 1 =T s ω α +1, A 0 =-1, B 1 =aT s ω α -1, B 0 =1,T s is the sampling period, and ω is the angular frequency.

5. The active damping control method for a grid-connected converter according to claim 4, It is characterized in that The first-order unstable digital filter has an angular frequency of ω=ω s The compensation angle at / 2 is: In the formula, α is the compensation coefficient, ω α To adjust the angular frequency, T s is the sampling period.

6. The active damping control method for a grid-connected converter according to claim 1, It is characterized in that After using the first-order unstable digital filter, the equivalent admittance Y eq The phase is: In the formula, z is e sTs , A 1 =T s ω α +1, A 0 =-1, B 1 =aT s ω α -1, B 0 =1,T s is the sampling period, ω is the angular frequency; when the resonant frequency takes the maximum value ω=ω s / 2, the equivalent conductance G eq When the compensation coefficient α is between (0.52, 1), it is greater than 0.

7. The active damping control method for a grid-connected converter according to claim 1, It is characterized in that The parameter design of the first-order unstable digital filter comprises the following steps: Step S1, select the compensation coefficient α of the first-order unstable digital filter, α>2 / (T s ω α ),ω s is the resonant angular frequency; when the angular frequency ω is adjusted α =ω s / π, the equivalent conductance G in the Nyquist band is satisfied eq >0, the compensation coefficient α is (0.52,1); when the angular frequency ω is adjusted α =ω s / 6, the equivalent conductance G in the Nyquist band is satisfied eq The compensation coefficient α of >0 is (1.77, 1.909); Step S2, selecting the adjustment angular frequency ω of the first-order unstable digital filter α ,ω α ≤ω s / π; satisfies the equivalent conductance G within the Nyquist band eq Under the condition that the compensation coefficient a is greater than 0, the critical value 2 / (T s ω α ), select the adjustment angular frequency ω α =ω s / π, at this time the equivalent conductance G eq The amplitude is the largest, which can provide the maximum damping; Step S3: the compensation coefficient α and the adjustment angular frequency ω α After the parameters of the first-order unstable digital filter are designed, the gain H ad Select; the gain H of the first-order unstable digital filter ad According to the stability margin of the Bode diagram of the inner loop open-loop transfer function of the capacitor voltage positive feedback active damping with a calculated delay of one beat after using the first-order unstable digital filter, the first-order unstable digital filter gain H ad It is necessary to ensure that the minimum amplitude margin of the stability margin of the inner open-loop transfer function Bode diagram is 3 dB and the minimum phase margin is 15°.

8. The active damping control method for a grid-connected converter according to claim 7, It is characterized in that The compensation coefficient α of the first-order unstable digital filter selected in step S1 includes, based on the high-pass filter, the value range of the compensation coefficient α is: In the formula, α is the compensation coefficient, ω s is the resonant angular frequency, ω α To adjust the angular frequency, T s is the sampling period.

9. The active damping control method for a grid-connected converter according to claim 7, It is characterized in that The adjustment angular frequency ω of the first-order unstable digital filter selected in step S2 α Including, after using the first-order unstable digital filter, the damping ratio ζ of the active damping equivalent to the capacitor voltage positive feedback: In the formula, G eq is the equivalent conductance, ω s is the resonant angular frequency, C x is the X-phase filter capacitor, X=A,B,C, x=a,b,c; Under the same compensation coefficient α, adjust the angular frequency ω α =ω s / π equivalent conductance G eq The amplitude is larger; the damping ratio decreases with the equivalent conductance G eq increases with the increase of, adjusting the angular frequency ω α =ω s / π can provide the maximum damping, select the adjustment angular frequency ω α =ω s / π.

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