A method for controlling active damping parameters of current source rectifier

Abstract

The present invention discloses a method for controlling active damping parameters of a current source rectifier. First, a small signal model of a current source rectifier is established based on a modulation strategy and power balance to obtain a transfer function from the inductor current on the DC side to the duty cycle; secondly, the cutoff frequency of the active damping high-pass filter is set according to the resonant frequency of the AC side filter; then, considering digital control and modulation delay, a current loop open-loop transfer function is established to analyze the influence of the active damping gain on the closed-loop poles; finally, the law of the damping ratio and modulus value of the closed-loop poles is analyzed to design the active damping gain. The present invention provides an active damping method for a current source rectifier, which can suppress the resonance of the AC side filter, improve the control performance, and optimize the active damping gain by scanning the active damping gain and drawing the current loop root trajectory, and has the advantages of good damping effect and strong loop stability.

Description

Technical Field

[0001] The invention relates to the technical field of rectifier control, and in particular to a method for controlling active damping parameters of a current source rectifier. Background Art

[0002] The rectifier circuit for converting AC to DC can be divided into voltage source rectifier and current source rectifier according to the characteristics of the energy storage elements on the DC side. Among them, the current source rectifier has a simple structure and has the advantages of step-down output, easy parallel operation, output short-circuit current suppression, high reliability, small input filter size, direct start, etc., which makes it have good application prospects in power factor correction, low-voltage DC distribution system, battery charging and other fields. With the development of magnetic materials and wide bandgap semiconductors, current source rectifiers have broad development space.

[0003] In the prior art, an LC filter is connected to the AC side of the current source rectifier to filter out the high-frequency switching ripple of the bridge arm output current, so that the grid current is a sine wave. However, the LC filter has series resonance, which makes it difficult to control the current source rectifier. The traditional passive damping method has the problem of additional loss and reduced filter performance. At present, there is no report on the active damping parameter design method for indirect current control of the current source rectifier. Summary of the invention

[0004] The problem to be solved by the present invention is to provide a method for controlling active damping parameters of a current source rectifier, by setting the cut-off frequency and active damping gain of a high-pass filter, suppressing the series resonance of the LC filter, improving the loop stability, and avoiding the oscillation of the AC side current.

[0005] The present invention adopts the following technical solution: a method for controlling active damping parameters of a current source rectifier, comprising the following steps:

[0006] Step 1: Based on the main circuit of the current source rectifier, an equivalent small signal model is established through the power balance and modulation strategy of the input and output of the current source rectifier to obtain the transfer function of the DC side inductor current to the duty cycle;

[0007] Step 2, calculate the resonant frequency of the AC side filter of the current source rectifier, and set the cut-off frequency of the active damping high-pass filter;

[0008] Step 3: Considering digital control and modulation delay, establish the current loop open-loop transfer function, analyze the law of closed-loop pole damping ratio and modulus value, and set the active damping gain.

[0009] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0010] 1. The method of the present invention is based on a current source rectifier capacitor voltage and a feedback active damping complete small signal model, and the parameter design is more accurate and reliable.

[0011] 2. The method of the present invention takes into account the influence of digital control and modulation delay, and can more effectively suppress the resonance of the LC filter and improve the control performance.

[0012] 3. The method of the present invention can optimize the active damping gain by scanning the active damping gain and drawing the current loop root trajectory. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is the main circuit and control block diagram of the active damping parameter control method of the current source rectifier of the present invention;

[0014] Figure 2 It is the post-active damping pole trajectory of the current source rectifier of the present invention;

[0015] Figure 3 is the damping rate of the pole trajectory after active damping of the current source rectifier of the present invention;

[0016] Figure 4 is the output voltage simulation result obtained by applying the control method of the present invention;

[0017] Figure 5 It is the output voltage simulation result obtained by applying the conventional control method. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the application is further elaborated in detail below in conjunction with the accompanying drawings. The described embodiments are only a part of the embodiments involved in the present invention. All non-innovative embodiments of other researchers in the field on this embodiment belong to the protection scope of the present invention. At the same time, for the step numbering in the embodiment of the present invention, it is only set for the convenience of explanation, and the order between the steps is not limited in any way. The execution order of each step in the embodiment can be adaptively adjusted according to the understanding of those skilled in the art.

[0019] In one embodiment of the present invention, the current source rectifier main circuit, such as Figure 1 Shown: three-phase voltage e a 、e b 、e c After the LC filter is connected to the D x and Q x (x=1, 2, 3, 4, 5, 6) Six-switch current source rectifier, where the AC side inductor L a =L b =L c =L, AC side filter capacitor C a=C b =C c =C, D0 is the freewheeling diode, DC side, L dc is the DC side inductance, C dc is the DC side capacitance, R L is the load equivalent resistance.

[0020] The control link is mainly: the DC voltage reference value and DC voltage V dc The difference is sent to the voltage loop proportional integral (PI) controller. The output of the PI controller is the inductor current reference value.

[0021] Will With the inductor current i L The difference is sent to the current controller k p The output of the current controller is the modulation ratio m′, and the active damping modulation ratio m is superimposed. damp The modulation ratio m of pulse width modulation (PWM) is formed, and finally the PWM link generates the switch tube drive signal.

[0022] Based on the above current source rectifier main circuit, the active damping parameters of the current source rectifier are controlled:

[0023] Firstly, a small signal model of current source rectifier is established based on modulation strategy and power balance, and the transfer function from DC side inductor current to duty cycle is obtained.

[0024] Secondly, the cutoff frequency of the active damping high-pass filter is set according to the resonant frequency of the AC side filter.

[0025] Then, considering digital control and modulation delay, the open-loop transfer function of the current loop is established, and the influence of active damping gain on the closed-loop poles is analyzed.

[0026] Finally, the rules of closed-loop pole damping ratio and modulus value are analyzed to design active damping gain.

[0027] In this embodiment, the control method is as follows:

[0028] Step 1: Based on the rectifier input and output power balance, establish the current source rectifier equivalent small signal model to obtain the transfer function from the DC side inductor current to the duty cycle. The specific steps are as follows:

[0029] (1.1) Divide one cycle of the AC side voltage into 12 sectors. Taking the first sector as an example, the grid voltage angle θ∈[0,π / 6] is calculated, and the average DC voltage of the output equivalent small signal model is calculated.

[0030] The driving signal of the current source rectifier in the first sector is as follows: Figure 2 As shown, the line voltages that form the DC side voltage are: u ac =u a -u c and u ab =u a -u b , assuming that the three-phase voltage expression is as follows:

[0031]

[0032] Among them, V m is the phase voltage peak value, θ=ω g t is the grid voltage angle, ω g Grid voltage frequency.

[0033] Calculate the average DC voltage as:

[0034]

[0035] The calculations for other sectors are similar and the results are the same.

[0036] (1.2) Based on the principle of conservation of energy, calculate the equivalent inductance and equivalent capacitance values ​​of the equivalent small signal model.

[0037] Assume that the AC side current expression is as follows:

[0038]

[0039] Among them, I N is the phase current peak value, then the current stored in the AC side inductor L a , L b , L c The energy is:

[0040]

[0041] Among them, L a =L b =L c =L.

[0042] Stored in the equivalent inductance L eq The energy is:

[0043]

[0044] Among them, i eq is the current flowing through the equivalent inductor and i eq =I N , based on the law of conservation of energy, then:

[0045]

[0046] Combine i eq =I N , we can get:

[0047]

[0048] Similarly, the AC side capacitor C a , C b and C c The energy is:

[0049]

[0050] Among them, C a =C b =C c =C.

[0051] Stored in equivalent capacitance C eq The energy is:

[0052]

[0053] Among them, u eq is the voltage applied to the equivalent capacitor and u eq =V m , based on the law of conservation of energy, then:

[0054]

[0055] Combine u eq =V m , we can get:

[0056]

[0057] (1.3) to obtain the small signal circuit of the current source rectifier, such as Figure 3 As shown, where M max is the maximum modulation ratio, I L is the inductor current at full load, V C is the capacitance C eq Steady-state value, v c is the disturbance component.

[0058] Since the output voltage V dc There is a large capacitance C dc Support, the DC voltage response speed is much lower than the current loop, so when analyzing the current loop, it can be considered that the output voltage V dc remains unchanged, that is, the output voltage disturbance υ dc =0.

[0059] Depend on Figure 3 You can get:

[0060]

[0061] Depend on Figure 1 It can be seen that:

[0062] m=m′+m damp =m′+υ C D(s) (14)

[0063] Combining equations (12), (13) and (14), we can obtain:

[0064]

[0065] Where D(s) = k d ·G HPF (s) is active damping, k d is the active damping gain, G HPF (s) = s / (s + ω c ) is a high-pass filter, ω c is the cut-off frequency.

[0066] Step 2: Calculate the resonant frequency of the rectifier AC side filter and set the cutoff frequency of the active damping high-pass filter. The specific steps are as follows:

[0067] (2.1) Calculate the resonant frequency of the rectifier AC side filter:

[0068]

[0069] (2.2) Set the active damping high-pass filter cutoff frequency ω c Satisfaction: 10ω g ≤ω c ≤πf res , where ω g is the grid frequency.

[0070] Step 3: Considering digital control and modulation delay, establish the current loop open-loop transfer function, analyze the law of closed-loop pole damping ratio and modulus value, and design active damping gain. The specific steps are as follows:

[0071] (3.1) The current loop control block diagram is as follows Figure 4 As shown, the open-loop transfer function is:

[0072] G OL (s) = k p G d (s)G iL-m (s) (17)

[0073] Among them, k p is the current loop proportional controller, The delay phase is The delay time is T d =1.5T s , T s is the sampling period.

[0074] (3.2) Set the active damping gain k d Starting from 0 and gradually increasing, the current loop with active damping gain k d The root locus curve of the change, such as Figure 5 shown.

[0075] (3.3) It can be seen that the current loop is composed of three root loci. After the conjugate poles p1 and 2 coincide at the junction, they move to both sides of the real axis, and the pole p3 is always located on the real axis. At the junction, the poles p1, 2, and 3 have a unit damping ratio, and the modulus of the poles p4 and 5 is much smaller than that of the poles p1, 2, and 3. Therefore, the excitation generated by p4 and 5 is rapidly attenuated, and the influence on the current loop can be ignored. The poles p1, 2, and 3 are the dominant poles.

[0076] From the above analysis, we can see that setting the dominant pole at the junction of the real axis is the optimal active damping gain.

[0077] In order to verify the superiority of the control method provided by the present invention, simulation analysis was carried out in MATLAB / simulink, and the system parameters are shown in Table 1 below.

[0078] Table 1: System parameters

[0079] parameter Value L 0.24mH C 6.8uF <![CDATA[L dc ]]> 2mH <![CDATA[C dc ]]> 750uF <![CDATA[M max ]]> 0.95 Vdc 370V Ts 0.0001s <![CDATA[I L ]]> 8A <![CDATA[ω g ]]> 100πrad / s <![CDATA[V m ]]> 311V <![CDATA[k p ]]> 11.7 <![CDATA[k opt ]]> 0.283

[0080] The simulation results are as follows Figure 6 As shown in Figure 2, the actual output voltage quickly tracks the reference voltage, and the regulation time is about 1.5ms, which is much shorter than Figure 5 The result obtained for conventional control is shown as 12ms.

[0081] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for controlling active damping parameters of a current source rectifier, characterized in that: The steps include: Step 1: Based on the main circuit of the current source rectifier, an equivalent small signal model is established through the power balance and modulation strategy of the input and output of the current source rectifier to obtain the transfer function of the DC side inductor current to the duty cycle, including the following sub-steps: Step 1.1, divide the AC side voltage into 12 sectors in one cycle. Taking the first sector as an example, the grid voltage angle θ∈[0,π / 6], calculate the average DC voltage V of the output equivalent small signal model eq : Step 1.2: Based on the power balance principle, calculate the equivalent small signal model inductance as L eq =3 / 2L, the equivalent small signal model capacitance is C eq , C eq =2 / 3C, L is the inductance on the AC side, C is the capacitance on the AC side; Step 1.3: Establish an equivalent small signal model of the current source rectifier and obtain the transfer function from the DC side inductor current to the duty cycle: G iL-m (s)=(V C ·A-sI L L eq ·B) / (sL dc ·A+sL eq M max ·B) Among them, V C is the capacitance C eq The steady-state voltage, L dc is the DC side inductance, s is the Laplace operator, I L is the inductor current at full load, M max is the maximum modulation ratio, A=s 2 L eq C eq +sL eq I L D(s)+1,B=M max +V C D(s); D(s)=k d ·G HPF (s) is active damping, k d is the active damping gain, G HPF (s) = s / (s + w c ) is a high-pass filter, w c is the cut-off frequency; Step 2: Calculate the resonant frequency f of the AC side filter of the current source rectifier res , set the active damping high-pass filter cutoff frequency w c , w c satisfy: 10w g ≤w c ≤πf res Among them, ω g is the grid frequency; Step 3: Establish the current loop open-loop transfer function, analyze the law of closed-loop pole damping ratio and modulus value, and set the active damping gain, including the following sub-steps: Step 3.1: Considering digital control and modulation delay, establish the current loop open-loop transfer function G OL (s); Step 3.2: Set the active damping gain k d Starting from 0 and gradually increasing, the current loop with active damping gain k d Changing root locus curve; Step 3.3: Analyze the root locus curve pole damping and modulus, and set the point where the dominant poles meet on the real axis as the optimal active damping gain.

2. The current source rectifier active damping parameter control method according to claim 1, characterized in that: In step 1, the current source rectifier main circuit converts the three-phase voltage e a 、e b 、e c After passing through the LC filter, it is connected to the six-switch current source rectifier, and the freewheeling diode D0 is connected, respectively through the load equivalent resistance R L and DC link capacitor C dc ; Set the DC voltage reference value and DC voltage V dc The difference is sent to the PI controller to output the inductor current reference value Will Through the DC side inductor L dc The inductor current i L The difference is sent to the current controller k p , output modulation ratio m′; superimposed active damping modulation ratio m damp The PWM modulation ratio m is formed, and the switch tube driving signal is generated by the PWM link; Wherein, the six-switch current source rectifier is composed of D x and Q x Composition: x=1, 2, 3, 4, 5, 6.

3. The current source rectifier active damping parameter control method according to claim 1, characterized in that: In step 1.1, the grid voltage angle θ is expressed as follows: θ=ω g t Calculate the average DC voltage V of the output equivalent small signal model eq : In eq =9 / πV m Among them, w g The grid voltage frequency, t represents time; V m is the peak value of the phase voltage of the power grid.

4. The current source rectifier active damping parameter control method according to claim 1, characterized in that: In step 2, calculate the resonant frequency f of the AC side filter of the current source rectifier res , the formula is as follows:

5. The method for controlling active damping parameters of a current source rectifier according to claim 1, characterized in that: In step 3.1, establish the current loop open-loop transfer function: G oL (s)=k p G d (s)G iL-m (s) Among them, k p is the current loop proportional controller, and the delay link is T d For the delay time.

6. The method for controlling active damping parameters of a current source rectifier according to claim 5, characterized in that: The current loop is composed of three root locus curves p1, p2, and p3, and the conjugate pole p 1，2 After the confluence points coincide, they move to both sides of the real axis, and p3 is located on the real axis. At the confluence point of p1, p2, and p3, the pole p 1，2，3 The dominant pole is set as the optimal active damping gain at the point where the dominant pole meets the real axis.

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