A constant voltage control method based on non-minimum phase system and DC power feedforward

Abstract

The present invention provides a constant voltage control method based on a non-minimum phase system and DC power feedforward, belonging to the field of converter control technology. The method comprises: calculating an error signal representing the square value of the actual DC bus voltage based on the square of a preset DC bus voltage control target value; inputting the error signal into a non-minimum phase system compensator and superimposing it with the DC power feedforward signal to obtain an active power target signal; inputting the active power target signal into an inner loop controller to obtain an AC side active power signal; and processing the AC side active power signal through a DC voltage transfer function to obtain the actual DC bus voltage square value. This method can solve the stability problem of DC bus voltage constant control when a three-phase converter operates in rectification mode and improve the dynamic performance of constant voltage control.

Description

Technical Field

[0001] The present invention belongs to the technical field of converter control, and in particular relates to a constant voltage control method based on a non-minimum phase system and direct current power feedforward. Background Art

[0002] Typically, three-phase converters require DC voltage as a control target in certain application scenarios. Maintaining a stable DC voltage is crucial when the transmission power varies. Depending on the direction of the power transmission, three-phase converters can operate in either rectification or inverter modes. In rectification mode, active power flows from the AC side to the DC side; in inverter mode, active power flows from the DC side to the AC side. In rectification mode, the bus voltage after rectification and filtering is the DC bus voltage. Since the input converter voltage can be unstable, the DC bus voltage can also increase or decrease. Excessively high DC bus voltage can damage the machine, while excessively low voltage can cause unreliable circuit breaker protection. Therefore, DC bus voltage regulation control is necessary.

[0003] The traditional method is a DC voltage control method based on a PI controller. The defect of this method is that it cannot maintain stability within the entire power range of the converter. Therefore, the control parameters need to be adjusted according to the power direction and size of the converter operation. The process is relatively complicated, and the traditional method is difficult to ensure good performance during transient processes of sudden increases and decreases in load, which affects the stability of the converter operation. Summary of the Invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a constant voltage control method based on a non-minimum phase system and DC power feedforward to solve the above-mentioned technical problems.

[0005] The present invention provides a constant voltage control method based on a non-minimum phase system and DC power feedforward, comprising:

[0006] Calculate the error signal of the actual DC bus voltage square value according to the square of the preset DC bus voltage control target value;

[0007] The error signal is input into the non-minimum phase system compensator and superimposed with the DC power feedforward signal to obtain the active power target signal;

[0008] Inputting the active power target signal into the inner loop controller to obtain an AC side active power signal;

[0009] The AC side active power signal is processed by a DC voltage transfer function to obtain the actual DC bus voltage square value.

[0010] Furthermore, the non-minimum phase system compensator is a leading phase compensator, and the transfer function of the non-minimum phase system compensator is:

[0011] In the above formula, k0 is the open-loop gain constant, z0 is the zero point introduced in the leading phase compensator, and p0 is the pole introduced in the leading phase compensator.

[0012] Furthermore, the parameter configuration of the zeros and poles in the transfer function of the non-minimum phase system compensator is determined by the response speed required by the control method and the phase margin required for system stability under the worst working conditions.

[0013] Furthermore, the transfer function of the inner loop controller is:

[0014] τ i is the closed-loop time constant of the inner-loop controller.

[0015] Furthermore, the output of the inner loop controller is used as the modulation wave signal of the converter to generate a PWM signal to drive the switching tube of the converter to be turned on and off.

[0016] Furthermore, the process of establishing the DC voltage transfer function is as follows:

[0017] According to the dynamic characteristics of the DC side capacitor voltage, the relationship between the power of each part of the converter system is determined, and the obtained nonlinear equation is as follows:

[0018]

[0019]

[0020] P ext is the DC side power, P loss is the sum of the loss of the open tube and the loss of the AC side filter inductor, V DC 2 is the square value of the actual DC bus voltage, P s is the active power signal at the AC side grid connection point, P t is the inverter output active power, P DC is the active power on the DC side; Q s is the reactive power on the AC side; L is the filter inductance, r on is the loss of the open tube, R is the loss of the AC side filter inductor; is the square of the AC voltage amplitude; is the AC current amplitude; The corresponding value is the square of the AC current amplitude; represents the AC current complex vector; represents the conjugate complex vector of alternating current; Indicates the instantaneous active power stored in the AC side filter inductor;

[0021] The above nonlinear equation is linearized by the small signal interference method, and V DC 2 and AC side active power P s The transfer function between is:

[0022] τ is the time constant of the DC voltage transfer function, and C is the DC side capacitance of the converter.

[0023] The beneficial effect of the present invention is that the present invention is based on a non-minimum phase system compensator and a DC power feedforward link. The non-minimum phase system compensator link is mainly used to solve the problem of instability caused by insufficient phase margin when the three-phase converter operates in rectification mode because the entire system exhibits the characteristics of a non-minimum phase system. The addition of a DC power feedforward link can improve the dynamic response speed of the DC bus constant voltage control and reduce the ringing amplitude when the DC bus voltage command suddenly changes or the load changes. It can solve the stability problem of the DC bus voltage constant control when the three-phase converter operates in rectification mode and improve the dynamic performance of the constant voltage control. In addition, the design principle of the present invention is reliable, the structure is simple, and it has a very broad application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0025] Figure 1 is a schematic diagram of power flow of a converter in one embodiment of the present invention;

[0026] Figure 2 This is the controller composition and the block diagram of the entire closed-loop control system proposed by the present invention;

[0027] Figure 3 It is a waveform comparison diagram of the simulation results using the traditional PI controller and the controller proposed in this invention. DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0029] like Figure 1 The three-phase converter shown in the figure needs to use DC voltage as the control target in certain application scenarios. When the transmission power in the system changes, the DC voltage needs to be kept stable at all times. Depending on the direction of the transmission power, the three-phase converter can operate in two modes: rectification and inversion. When operating in rectification mode, active power flows from the AC side to the DC side; when operating in inversion mode, active power flows from the DC side to the AC side. The traditional DC voltage control method based on the PI controller cannot remain stable within the entire power range of the converter. Therefore, the control parameters need to be adjusted according to the power direction and size of the converter operation. In addition, it is difficult for the traditional method to ensure good performance during transient processes with sudden increases and decreases in load.

[0030] According to the dynamic characteristics of the DC side capacitor voltage, the relationship between the power of each part of the converter system is determined, and the obtained nonlinear equation is as follows:

[0031]

[0032] P ext is the DC side power, that is, the power input to the converter from the outside, P loss is the sum of the loss of the open tube and the loss of the AC side filter inductor. Figure 1 In the example, use the equivalent i loss Indicates; V DC is the square value of the actual DC bus voltage, P s is the active power signal at the AC side grid connection point, P t is the active power at the inverter output, P DC is the active power on the DC side; Q s is the reactive power on the AC side; L is the filter inductance, r on is the loss of the open tube, R is the loss of the AC side filter inductor; is the square of the AC voltage amplitude; is the AC current amplitude; The corresponding value is the square of the AC current amplitude; represents the AC current complex vector; represents the conjugate complex vector of alternating current; Indicates the instantaneous active power stored in the AC side filter inductor;

[0033] The above nonlinear equation is linearized by the small signal interference method, and V DC 2 and AC side active power P s The transfer function between is:

[0034] τ is the time constant of the DC voltage transfer function, and C is the DC side capacitance of the converter.

[0035] The expression of the time constant τ is further obtained as: P ext0 is the reactive power on the DC side at the steady-state voltage operating point selected when the small signal interference method is used for linearization, P s0 is the active power on the AC side at the steady-state voltage operating point.

[0036] From the expression of the time constant τ, it can be seen that when the AC side filter inductance and the AC grid voltage are determined, its value is related to the power P transmitted on the DC side at the steady-state voltage operating point. ext0 When P ext0 When the value of (τs+1) / s is very small, the term (τs+1) / s can be simplified to an integral link. The controller designed by simplifying the mathematical model will have potential risks. ext0 When τ is negative, τ is also negative. The zero point provided by the τs+1 term for the system will be located to the right of the imaginary axis in the polar coordinate plot, causing the phase-frequency characteristic curve to shift downward or increase its downward slope, which may lead to insufficient system phase margin. In fact, when τ is negative, the entire control system exhibits the characteristics of a non-minimum phase system, so a non-minimum phase system compensator is required to ensure that the control system stability margin meets the requirements.

[0037] On the other hand, in order to improve the dynamic performance of DC voltage control when the load changes, an additional DC power feedforward link is added, superimposed on the output of the DC voltage control compensator link, and jointly act on the inner loop controller.

[0038] Based on the above reasons, the control method of the present invention consists of two steps: a non-minimum phase system compensator and a DC power feedforward step, resulting in a closed-loop control system. On the one hand, the introduction of the non-minimum phase system compensator improves the system stability margin, ensuring stable operation of the three-phase converter over the full power range. On the other hand, the introduction of the DC power feedforward step improves the dynamic response of the three-phase converter to sudden increases and decreases in load, reducing the DC voltage ringing amplitude.

[0039] like Figure 2 As shown, the reference signal and feedback signal input in the present invention are different from those in the traditional control method. The reference signal input in this method is the square of the DC voltage control target value, and the feedback signal is the square of the measured DC voltage value.

[0040] The error signal e is obtained by subtracting the input reference signal from the feedback signal v First, it passes through a non-minimum phase system compensator, which can compensate for the phase of the system when the three-phase converter works in rectification mode, thereby meeting the requirements of voltage control stability.

[0041] The output of the non-minimum phase system compensator is superimposed on the DC power feedforward signal to obtain the active power target signal. The addition of the DC power feedforward link can improve the dynamic response speed of the DC bus constant voltage control and reduce the ringing amplitude when the DC bus voltage command changes suddenly or the load changes.

[0042] The active power target signal is input into the inner loop controller to obtain the AC side active power signal; the output of the inner loop controller is the modulation wave signal of the converter. By selecting a suitable modulation method, such as SPWM or SVPWM, and further generating a PWM signal, the converter switch tube can be driven to be turned on and off.

[0043] After the AC side active power signal passes through the DC voltage transfer function, the square of the actual DC bus voltage value is obtained, and then the square of the actual DC bus voltage value is used as the feedback signal to form a closed-loop control.

[0044] Optionally, as an embodiment of the present invention, the transfer function of the non-minimum phase system compensator is designed to be in the following form, which is essentially a leading phase compensator. The transfer function of the non-minimum phase system compensator is: In the above formula, k0 is the open-loop gain constant, z0 is the zero point introduced in the leading phase compensator, and p0 is the pole introduced in the leading phase compensator.

[0045] The design of the lead compensator will be based on the transmission power P ext0 A negative value represents the maximum power under rectification conditions, meaning that the design is based on the worst-case scenario of a non-minimum phase system when τ is negative. Therefore, the parameter configuration of the zeros and poles in the transfer function of the non-minimum phase system compensator is determined by the required response speed of the control method and the phase margin required for transformer stability under the worst-case operating conditions.

[0046] Optionally, as an embodiment of the present invention, the transfer function of the inner loop controller is: τ i is the closed-loop time constant of the inner-loop controller. The design principle of the closed-loop transfer function of the inner-loop controller has been mentioned in many papers related to three-phase converter control and will not be repeated here.

[0047] Optionally, as an embodiment of the present invention, the DC voltage transfer function is:

[0048] In summary, the open-loop transfer function of the entire closed-loop control system can be obtained as

[0049] The cutoff frequency ω of the control system cIt is usually selected to be 0.1 to 0.5 times the bandwidth of the current inner loop. The leading phase compensator needs to correct the phase-frequency characteristics of the system so that the converter system still has sufficient phase margin when operating in the rectification mode. It is generally selected to be 30 degrees to 60 degrees to ensure that the system has good dynamic characteristics. The selection of parameter k0 needs to ensure |l(jω c )|=1.

[0050] Figure 3 This paper simulates and compares the control effects of DC voltage using the traditional PI control method and the constant voltage control method based on non-minimum phase system compensator and DC power feedforward described in the present invention. In the simulation model, the DC voltage control target value is set to 1300V, the AC rated voltage is 690V, and the rated power is 1.725MW. The DC current at full power is 1327A. It should be noted that a positive value of the DC current indicates a rectifier condition, and a negative value indicates an inverter condition. At 0.3 seconds, the DC current increases from 0A to 1327A within one cycle of 20ms. At 0.6 seconds, the DC current jumps from 1327A to -1327A within two cycles of 40ms. At 0.75 seconds, the DC current suddenly drops to 0A. The simulation results show that when using the traditional PI controller method, the DC voltage becomes unstable and deviates significantly from the control target value when the DC current, i.e., the transmission power, changes suddenly. However, when using the constant voltage control method based on a non-minimum phase system and DC power feedforward described in the present invention, the DC voltage recovers to the control target value after a brief fluctuation when the DC current changes suddenly. To verify the impact of the DC power feedforward link on the dynamic performance of the control system, the simulation results before and after the feedforward link was added were compared. It can be seen that after the feedforward link was added, the voltage ringing during the transient process was significantly reduced, and the DC voltage recovery time was shortened. These simulation results demonstrate that the controller described in the present invention is effective and significantly improves the dynamic performance of DC voltage control.

[0051] The constant voltage control method based on a non-minimum phase system and DC power feedforward, described in this invention, was simulated and demonstrated using actual measurements on a converter device. The converter device parameters remained consistent with those used in the simulation. Whether operating in rectification or inversion mode, when the AC current was shelved from an effective value of 1400A to 0A (i.e., when the power dropped from full power to zero power), the DC voltage recovered to the control target value after a brief fluctuation. The voltage ringing during the transient process was minimal, demonstrating the effectiveness of the controller described in this invention and its excellent dynamic performance.

[0052] Although the present invention has been described in detail with reference to the accompanying drawings and in conjunction with preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, a person of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and such modifications or substitutions shall be within the scope of the present invention. Any changes or substitutions that can be easily conceived by a person of ordinary skill in the art within the technical scope disclosed in the present invention shall be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A constant voltage control method based on a non-minimum phase system and DC power feedforward, characterized in that: include: Calculate the error signal of the actual DC bus voltage square value according to the square of the preset DC bus voltage control target value; The error signal is input into the non-minimum phase system compensator and superimposed with the DC power feedforward signal to obtain the active power target signal; Inputting the active power target signal into the inner loop controller to obtain an AC side active power signal; The AC side active power signal is processed by a DC voltage transfer function to obtain the actual DC bus voltage square value; The process of establishing the DC voltage transfer function is as follows: According to the dynamic characteristics of the DC side capacitor voltage, the relationship between the power of each part of the converter system is determined, and the obtained nonlinear equation is as follows: P ext is the DC side power, P loss is the sum of the loss of the open tube and the loss of the AC side filter inductor, V DC 2 is the square value of the actual DC bus voltage, P s is the active power signal at the AC side grid connection point, P t is the active power at the inverter output, P DC is the active power on the DC side; Q s is the reactive power on the AC side; L is the filter inductance, r on is the loss of the open tube, R is the loss of the AC side filter inductor; is the square of the AC voltage amplitude; is the AC current amplitude; The corresponding value is the square of the AC current amplitude; represents the AC current complex vector; i * represents the conjugate complex vector of alternating current; Indicates the instantaneous active power stored in the AC side filter inductor; The above nonlinear equation is linearized by the small signal interference method, and V DC 2 and AC side active power P s The transfer function between is: τ is the time constant of the DC voltage transfer function, and C is the DC side capacitance of the converter.

2. The method according to claim 1, characterized in that The non-minimum phase system compensator is a leading phase compensator, and the transfer function of the non-minimum phase system compensator is: In the above formula, k0 is the open-loop gain constant, z0 is the zero point introduced in the leading phase compensator, and p0 is the pole introduced in the leading phase compensator.

3. The method according to claim 2, characterized in that The parameter configuration of the zeros and poles in the transfer function of the non-minimum phase system compensator is determined by the response speed required by the control method and the phase margin required for system stability under the worst working conditions.

4. The method according to claim 1, wherein The transfer function of the inner loop controller is: τ i is the closed-loop time constant of the inner-loop controller.

5. The method according to claim 1, wherein Also includes: The output of the inner loop controller is used as the modulation wave signal of the converter to generate a PWM signal to drive the switching tube of the converter to turn on and off.

Images