A control system and method for improving the dynamic performance of a current-source inverter connected to a grid

Abstract

The application discloses a control system and method for improving grid-connected dynamic performance of a current source type inverter, comprising a grid current control loop, a capacitor voltage control loop and an active damping control loop. Wherein, the difference between a reference current and a feedback grid-connected current passes through a current loop controller to form the grid current control loop; a filter capacitor reference voltage passes through a proportional-proportional-delay controller to form the capacitor current control loop; the filter capacitor voltage is fed back to the output of the capacitor current control loop through a virtual resistance link to form the active damping control loop. According to the optimal damping ratio, the optimal feedback coefficient of the damping branch is designed, the problem of grid current oscillation caused by sudden change of the reference current can be solved, and when the parameters of the capacitor current controller are designed, the optimal feedback coefficient of the damping branch is combined, the problem of inner loop saturation caused by the feedback filter capacitor voltage of the damping branch can be solved, and the grid-connected dynamic performance of the current source type inverter can be improved.

Description

Technical Field

[0001] This invention relates to a control system and method for improving the grid-connected dynamic performance of a current-source inverter. It mainly includes suppressing the resonance of the current-source inverter, improving the grid-connected dynamic performance of the system, and solving the problem of inner-loop saturation that is easily caused by conventional control methods. It belongs to the technical field of grid-connected inverter control. Background Technology

[0002] As the interface between distributed energy and the power grid, the performance of the grid-connected inverter directly determines the quality of distributed energy power generation. Currently, most distributed power generation uses voltage source inverters, while research on current source inverters is relatively limited. Compared with voltage source inverters, current source inverters have the following advantages: (1) Photovoltaic arrays exhibit current source characteristics in the stable operating region, so current source inverters are more suitable for photovoltaic grid-connected power generation; (2) Current source inverters are boost converters, so compared with voltage source inverters, they can reduce the number of photovoltaic cells connected in series on the DC side for photovoltaic power generation and reduce the wind speed required for wind power generation, i.e., current source inverters can increase the power generation of photovoltaic and wind turbine units; (3) The large inductance on the DC side of the current source inverter can suppress DC side current harmonics and increase the inertia of the DC side current, which can slow down the rate of increase of the DC side current when the system is short-circuited, which is beneficial to increase the margin of protection action time. Moreover, compared with DC filter capacitors, DC side inductors have a longer lifespan and higher system reliability.

[0003] However, current-source inverters use CL-type filters, which can cause a resonant spike in the system, affecting its stability. The simplest and most effective method is to implement active damping through feedback of the filter capacitor voltage. However, since the amplitude of the filter capacitor voltage is relatively large compared to the DC-side current amplitude, directly implementing active damping through feedback of the filter capacitor voltage can easily lead to inner-loop saturation when the system achieves the optimal damping ratio, thus causing system instability. Furthermore, reducing the feedback coefficient of the damping branch to avoid loop saturation can result in insufficient system damping, which can easily cause continuous oscillations in the grid current and the AC-side filter capacitor voltage during sudden changes in the reference current.

[0004] Therefore, for current source inverters, if the problem of loop saturation caused by the active damping of the feedback filter capacitor voltage can be solved while achieving active damping, it will help improve the dynamic characteristics of the system. Summary of the Invention

[0005] The purpose of this invention is to provide a control system and method for improving the grid-connected dynamic performance of current source inverters, so as to solve the problems of poor system dynamic characteristics and grid-connected current quality when using conventional control methods for current source inverters.

[0006] To achieve the above objectives, this invention extends the traditional single-loop control of current-source inverters to multi-loop control, and the inner loop employs an open-loop controller called proportional-proportional-delay controller, specifically including the following steps:

[0007] A control system and method for improving the grid-connected dynamic performance of a current-source inverter includes a first subtractor, a current loop controller, an inner loop proportional controller 1, an inner loop proportional controller 2, an inner loop internal delay module, a grid current feedforward module, a second subtractor module, a damping branch module, a third subtractor module, and a PWM module. The first subtractor and the current loop controller constitute the outer current control loop, the inner loop proportional controller 1, the inner loop proportional controller 2, the inner loop internal delay module, the grid current feedforward module, and the second subtractor module constitute the inner loop control loop, and the damping branch module and the third subtractor module constitute the damping control loop. The outer current control loop is used to control the grid-connected current to track the reference current, the inner current control loop is used to control the filter capacitor current, and the damping control loop is used to suppress system resonance.

[0008] In some disclosures, in the power circuit, the input terminal of the inverter bridge is connected to a constant current source, and the output terminal is connected in parallel with a filter capacitor, which is connected to a filter inductor, which is connected to the power grid; the control circuit is connected as follows:

[0009] 1) The reference current and grid current are connected to the input of the subtractor, and the output of the subtractor is connected to the input of the current loop controller;

[0010] 2) The output of the current loop controller is connected to the input terminals of the inner loop proportional controller 1 and proportional controller 2 respectively. The output terminal of proportional controller 2 is connected to the input terminal of the internal delay module of the inner loop controller. The output terminals of proportional controller 1 and the delay module are both connected to the input terminal of the second subtractor to obtain the current of the current filter capacitor.

[0011] 3) The feedforward grid current is connected to the input of the second subtractor, and together with the filter capacitor current calculated by the inner loop controller, they form the inverter current, which is then connected to the input of the third subtractor.

[0012] 4) The damping branch is connected to the input of the third subtractor and together with the inverter current forms a modulation wave, which is connected to the input of the PWM module. The PWM module outputs a drive signal to control the power device to switch on and off and generate inverter current.

[0013] In some disclosures, the system includes multiple loops, including a mains current control loop, a filter capacitor current control loop, and an active damping loop.

[0014] In some disclosures, the virtual resistor branch is used to suppress resonance in a current-source inverter, and the value of the virtual resistor is obtained based on the optimal damping ratio of the system.

[0015] In some disclosures, the inner loop proportional controller 1 and the inner loop proportional controller 2 are proportional-proportional-delay controllers.

[0016] In some disclosures, the inner loop proportional controller 1 and inner loop proportional controller 2 use the filter capacitor current as the control target, which can solve the problem that conventional controllers, which use the filter capacitor voltage as the control target, are prone to inner loop saturation and system instability.

[0017] A control method for improving the grid-connected dynamic performance of a current-source inverter includes the following steps:

[0018] 1) The grid current control loop uses a conventional PI controller. The input is the reference grid current, the feedback is the sampled grid current, and the output is the reference voltage of the filter capacitor voltage.

[0019] 2): The capacitor current control loop adopts a proportional-proportional-delay controller. The input is the reference voltage of the filter capacitor voltage, and there is no need to sample and feedback the filter capacitor voltage. The output is the current flowing through the filter capacitor.

[0020] 3): Based on the optimal damping ratio, the optimal feedback coefficient is designed, the voltage of the filter capacitor is sampled, and the voltage is fed back to the inner loop output through the optimal damping coefficient to form a damping branch, which is used to suppress system resonance;

[0021] 4): The output of the capacitor current control loop, the output of the damping branch, and the feedforward grid current constitute a modulation wave, which is connected to the PWM module to generate a drive signal for controlling the on / off state of the power devices.

[0022] In step 1), the grid current is directly controlled. By reasonably designing the controller parameters, the system can be ensured to have appropriate bandwidth and stability margin.

[0023] In step 2), the capacitor current control loop adopts a proportional-proportional-delay controller, which does not require feedback filter capacitor voltage and has the advantages of simple parameter design and low calculation amount.

[0024] In step 3), the feedback coefficient of the damping branch is obtained based on the optimal damping ratio, which can effectively suppress the resonance peak of the system and ensure the stability of the system.

[0025] In steps 2) and 3), by incorporating the damping branch feedback coefficient when designing the capacitor current controller parameters, the fundamental component of the damping branch feedback filter capacitor voltage can be filtered out without affecting the resonant voltage. This effectively solves the problem of inner-loop saturation caused by the damping branch feedback filter capacitor voltage in conventional control methods. Furthermore, since the damping branch feedback coefficient ensures the system achieves the optimal damping ratio, it can quickly and effectively suppress the oscillation of the grid current and filter capacitor voltage during sudden changes in the reference current.

[0026] Beneficial effects:

[0027] (1) A method for designing the optimal damping branch feedback coefficient is provided, and it can be decoupled from the grid current controller and the capacitor current controller, which has the advantage of simple design.

[0028] (2) When designing the parameters of the capacitor current controller, combined with the optimal feedback coefficient of the damping branch, the capacitor current control loop can be approximately equivalent to a high-pass filter to filter out the fundamental wave in the feedback filter capacitor voltage of the damping branch. This can solve the problem that the inner loop is prone to saturation when the feedback filter capacitor voltage of the conventional control method achieves active damping. It can also solve the problem that the grid current and filter capacitor voltage continue to oscillate due to the sudden change of the reference current. Moreover, compared with the method of connecting a high-pass filter in series in the damping branch, there is no problem of parameter coupling. It has the advantages of simple parameter design and small loop calculation.

[0029] Therefore, the present invention can effectively improve the stability and dynamic characteristics of current source inverters, thereby improving the practicality of current source inverters. Attached Figure Description

[0030] The invention will now be further described with reference to the accompanying drawings.

[0031] Figure 1 This is an overall block diagram of the system when the present invention is used.

[0032] Figure 2 This is a system block diagram showing the conversion of inverter current to grid-connected current.

[0033] Figure 3 This is a Bode plot of the inverter current to grid current transfer function without any additional measures.

[0034] Figure 4 It is a passive damping method.

[0035] Figure 5 This is the equivalent of active damping for passive damping.

[0036] Figure 6 Equivalent transformation of system block diagram.

[0037] Figure 7 The characteristics of the system after the damping branch feedback coefficient is designed.

[0038] Figure 8 System overall control block diagram.

[0039] Figure 9 Simplification of the overall system block diagram.

[0040] Figure 10The open-loop transfer function characteristics of the system with and without compensation network.

[0041] Figure 11 Validation of the effectiveness of the active damping method.

[0042] Figure 12 The steady-state characteristics of the system when using this invention.

[0043] Figure 13 The dynamic characteristics of the system when conventional control methods are used;

[0044] Figure 14 The dynamic characteristics of the system when using this invention. Detailed Implementation

[0045] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] This invention extends the traditional single-loop control of current-source inverters to multi-loop control, and the inner loop uses an open-loop controller called proportional-proportional-delay controller, specifically including the following steps:

[0047] A control system and method for improving the grid-connected dynamic performance of a current-source inverter includes a first subtractor, a current loop controller, an inner loop proportional controller 1, an inner loop proportional controller 2, an inner loop internal delay module, a grid current feedforward module, a second subtractor module, a damping branch module, a third subtractor module, and a PWM module. The first subtractor and the current loop controller constitute the outer current control loop, the inner loop proportional controller 1, the inner loop proportional controller 2, the inner loop internal delay module, the grid current feedforward module, and the second subtractor module constitute the inner loop control loop, and the damping branch module and the third subtractor module constitute the damping control loop. The outer current control loop is used to control the grid-connected current to track the reference current, the inner current control loop is used to control the filter capacitor current, and the damping control loop is used to suppress system resonance.

[0048] In some disclosures, in the power circuit, the input terminal of the inverter bridge is connected to a constant current source, and the output terminal is connected in parallel with a filter capacitor, which is connected to a filter inductor, which is connected to the power grid; the control circuit is connected as follows:

[0049] 1) The reference current and grid current are connected to the input of the subtractor, and the output of the subtractor is connected to the input of the current loop controller;

[0050] 2) The output of the current loop controller is connected to the input terminals of the inner loop proportional controller 1 and proportional controller 2 respectively. The output terminal of proportional controller 2 is connected to the input terminal of the internal delay module of the inner loop controller. The output terminals of proportional controller 1 and the delay module are both connected to the input terminal of the second subtractor to obtain the current of the current filter capacitor.

[0051] 3) The feedforward grid current is connected to the input of the second subtractor, and together with the filter capacitor current calculated by the inner loop controller, they form the inverter current, which is then connected to the input of the third subtractor.

[0052] 4) The damping branch is connected to the input of the third subtractor and together with the inverter current forms a modulation wave, which is connected to the input of the PWM module. The PWM module outputs a drive signal to control the power device to switch on and off and generate inverter current.

[0053] In some disclosures, the system includes multiple loops, including a mains current control loop, a filter capacitor current control loop, and an active damping loop.

[0054] In some disclosures, the virtual resistor branch is used to suppress resonance in a current-source inverter, and the value of the virtual resistor is obtained based on the optimal damping ratio of the system.

[0055] In some disclosures, the inner loop proportional controller 1 and the inner loop proportional controller 2 are proportional-proportional-delay controllers.

[0056] In some disclosures, the inner loop proportional controller 1 and inner loop proportional controller 2 use the filter capacitor current as the control target, which can solve the problem that conventional controllers, which use the filter capacitor voltage as the control target, are prone to inner loop saturation and system instability.

[0057] A control method for improving the grid-connected dynamic performance of a current-source inverter includes the following steps:

[0058] 1) The grid current control loop uses a conventional PI controller. The input is the reference grid current, the feedback is the sampled grid current, and the output is the reference voltage of the filter capacitor voltage.

[0059] 2): The capacitor current control loop adopts a proportional-proportional-delay controller. The input is the reference voltage of the filter capacitor voltage, and there is no need to sample and feedback the filter capacitor voltage. The output is the current flowing through the filter capacitor.

[0060] 3): Based on the optimal damping ratio, the optimal feedback coefficient is designed, the voltage of the filter capacitor is sampled, and the voltage is fed back to the inner loop output through the optimal damping coefficient to form a damping branch, which is used to suppress system resonance;

[0061] 4): The output of the capacitor current control loop, the output of the damping branch, and the feedforward grid current constitute a modulation wave, which is connected to the PWM module to generate a drive signal for controlling the on / off state of the power devices.

[0062] In step 1), the grid current is directly controlled. By reasonably designing the controller parameters, the system can be ensured to have appropriate bandwidth and stability margin.

[0063] In step 2), the capacitor current control loop adopts a proportional-proportional-delay controller, which does not require feedback filter capacitor voltage and has the advantages of simple parameter design and low calculation amount.

[0064] In step 3), the feedback coefficient of the damping branch is obtained based on the optimal damping ratio, which can effectively suppress the resonance peak of the system and ensure the stability of the system.

[0065] In steps 2) and 3), by incorporating the damping branch feedback coefficient when designing the capacitor current controller parameters, the fundamental component of the damping branch feedback filter capacitor voltage can be filtered out without affecting the resonant voltage. This effectively solves the problem of inner-loop saturation caused by the damping branch feedback filter capacitor voltage in conventional control methods. Furthermore, since the damping branch feedback coefficient ensures the system achieves the optimal damping ratio, it can quickly and effectively suppress the oscillation of the grid current and filter capacitor voltage during sudden changes in the reference current.

[0066] by Figure 1 Taking the single-phase current source grid-connected inverter shown as an example, a control system and method for improving the grid-connected dynamic performance of the current source inverter are disclosed. The control system consists of a current outer loop main control circuit, a capacitor current control circuit, and an active damping control circuit.

[0067] The aforementioned multi-loop control circuit and method for improving the grid-connected dynamic performance of a current-source inverter are specifically implemented as follows: a subtractor and a current loop controller constitute an outer loop control circuit for the grid current, used to control the grid current to track the reference current; an inner loop proportional controller 1, an inner loop proportional controller 2, and an inner loop internal delay module are used to control the current flowing through the capacitor, and together with the grid current feedforward module, they form the inverter current through a second subtractor; the filter capacitor voltage passes through a damping branch module, and together with the inverter current, passes through a third subtractor to form a modulation wave, which passes through a PWM module to generate a drive signal.

[0068] Example: Combining Figure 1 This paper presents a design example for improving the grid-connected dynamic characteristics of current source inverters. Figure 2 Provide a control block diagram for the inverter current to the grid-connected current. Based on... Figure 2 The transfer function from inverter current to grid current can be derived:

[0069]

[0070] As can be seen from equation (1), the system is a second-order system and does not contain a damping term. Therefore, the system must be severely underdamped. The Bode plot of equation (1) is shown below. Figure 3 As shown.

[0071] Figure 3 The system exhibits resonance; in the frequency band where the amplitude gain is greater than zero, the phase gain jumps by -180°, indicating that the system is unstable from a control perspective. Increasing system damping, particularly passive damping, can be achieved in four forms, such as... Figure 4 As shown.

[0072] Using methods 1 and 3, the damping will affect the CL filter's suppression of higher harmonics. Consider using method 2, which is a passive damper. This resistor can be used as 1 / R corresponding to K1 in the proportional-proportional-delay controller parameter of the current source inverter. This can solve the problem that the insulation resistance of the filter capacitor is too large, resulting in an excessively small 1 / R.

[0073] Considering that passive resistors consume power and reduce system efficiency, the resistor in method 2 is replaced with a virtual resistor. Figure 4 It can be seen that adding this passive resistor is equivalent to adding a resistive branch to C, reducing the current flowing into C, and the magnitude of this current is u. C / R, therefore, if the resistor in method 2 is replaced with a virtual resistor, the control block diagram of the inverter current to the grid current can be transformed into... Figure 5 .

[0074] According to equation (1), the resonant frequency expression of the system can be calculated:

[0075]

[0076] Substituting the parameters, calculations show that the system resonant frequency is much higher than the fundamental frequency. Since the resonant frequency is much higher than the fundamental frequency, and the filter capacitor voltage mainly contains the fundamental component of the grid voltage compared to the grid-side inductor voltage, from the perspective of achieving active damping, proportional feedback of the filter capacitor voltage is equivalent to proportional feedback of the grid-side inductor voltage. Therefore, the filter capacitor voltage feedback can be transformed into grid-side inductor voltage feedback. Furthermore, by moving the damping branch feedback node backward, a first-order differential feedback of the grid current is obtained. Figure 5 Become Figure 6 .

[0077] according to Figure 6 The transfer function from inverter current to grid current can be derived:

[0078]

[0079] It can be seen that the transmission shown in equation (3) is not a typical second-order system. By transforming equation (3), we can obtain:

[0080]

[0081] It can be seen that the system shown in equation (4) is a typical second-order system, and the damping ratio of the system can be expressed as:

[0082]

[0083] In order to ensure that the system has good characteristics and can effectively suppress the system's resonance peak, let ζ = 0.707, then the value of the virtual resistance R of the damping branch can be obtained according to equation (5). Figure 7 The Bode plots of the inverter current to the grid current before and after adding the active damping branch are given. It can be seen that when the virtual resistor branch is added and the virtual resistor is designed according to the optimal damping ratio, the resonance spike is well suppressed.

[0084] For the inner voltage loop, the reference input is the output of the outer loop, which is also the reference voltage of the filter capacitor. The main function of the inner loop is to control the current of the filter capacitor. The relationship between the filter capacitor reference voltage and the filter capacitor current is expressed as follows:

[0085]

[0086] In the formula Z C To determine the capacitive reactance of the filter capacitor, we use backward differential pair equation (6) for discretization, and obtain:

[0087]

[0088] In equation (7) i C (k) represents the current of the current filtering capacitor, u C (k) represents the current reference voltage of the filter capacitor, u C (k-1) represents the reference voltage of the filter capacitor at the previous switching moment, T s This indicates the sampling calculation period. At this time, the expressions for the first and second proportional controllers of the voltage inner loop are respectively:

[0089]

[0090] Consider directly feeding back the filter capacitor voltage to achieve active damping. Since the amplitude of the filter capacitor voltage is relatively large compared to the DC side current, it can easily lead to inner loop saturation. The purpose of feeding back the filter capacitor voltage to achieve active damping is to feed back the resonant voltage contained in the filter capacitor voltage. Moreover, the most important component of the filter capacitor voltage is the fundamental voltage. Therefore, if the fundamental voltage can be filtered out, the problem of easy saturation of the inner loop can be solved.

[0091] High-pass filters can remove the fundamental frequency component without affecting the resonant voltage. However, the parameters of the high-pass filter are coupled with the feedback coefficient of the damping branch, increasing the design complexity. Furthermore, high-pass filters also increase the computational load. Considering that the fundamental frequency component of the filter capacitor voltage passes through a virtual resistor and becomes a fundamental frequency current, and the reference input of the inner loop is the reference value of the filter capacitor voltage, which is approximately equal to the actual filter capacitor voltage, but does not contain harmonic voltage information given that the system contains a damping branch, then if the reference voltage of the filter capacitor passes through the same virtual resistor, the fundamental frequency component of the current generated by the actual capacitor voltage passing through the virtual resistor can be canceled, thus solving the inner loop saturation problem. Placing this virtual resistor on the proportional controller 1 of the inner loop controller, the parameters of the proportional controller 1 can then be:

[0092]

[0093] At this point, all parameters of the inner loop control circuit, including the damping branch, have been designed, and the overall control block diagram of the system is as follows: Figure 8 As shown.

[0094] Figure 8 G represents i (s) represents the outer loop current loop controller, G d1 (s) represents the current sampling delay, G d2 (s) represents the sampling delay of the filter capacitor voltage and the PWM modulation delay. When using conventional sampling and PWM modulation methods, the sampling delay is 1T. s PWM modulation can be approximated as 0.5T s .

[0095] Because the delay in the inner loop affects not only the performance of the inner loop controller but also the performance of the damping branch, compensation is needed to address the inner loop control delay. Considering that Newton interpolation prediction is simple to implement and theoretically can completely eliminate the impact of control delay, it is used here to compensate for the control delay. The specific expression for Newton interpolation prediction is as follows:

[0096] u c_f (k+n)=u c (k)+n(u c (k)-u c (k-1)) (10)

[0097] In the formula, u c_F (k+n) represents the predicted capacitor voltage, u c (k) represents the capacitor voltage sampled during the current switching cycle, n represents the prediction step number, and u c (k-1) represents the capacitor voltage sampled in the previous switching cycle.

[0098] After predicting the control delay, it can be seen that the input of the inner loop is the reference voltage of the filter capacitor voltage, and the output is the filter capacitor voltage. Therefore, the inner loop can be regarded as a proportional element with a gain of 1. Figure 8 It can be simplified to Figure 9 .

[0099] Considering the simplicity of the outer loop control and the differential nature of the interpolation prediction, predicting the grid-connected current would easily amplify its harmonic components. Therefore, the sampling delay of the grid-connected current is not predicted here. A first-order inertial element is used to approximate the control delay, G. d1 (s) can be expressed as:

[0100]

[0101] Outer loop current loop controller G i (s) Using a conventional PI controller, it can be expressed as:

[0102]

[0103] No current loop controller G added i Before (s), the open-loop transfer function of the system is:

[0104]

[0105] From (13), it can be seen that the system has two open-loop poles, namely:

[0106]

[0107]

[0108] Substituting the system parameters, we can determine that s2 is closer to the imaginary axis, therefore s2 is the dominant pole of the system. Adding G... i After (s), the open-loop transfer function of the system is:

[0109]

[0110] The system's crossover frequency is set to f c At this point, we can obtain:

[0111]

[0112] By designing the zero point of the PI controller at the dominant pole of the open-loop transfer function before compensation, we have:

[0113]

[0114] By combining equations (17) and (18) and substituting the system parameters, the parameters of the outer loop controller can be calculated. A Bode plot is then drawn for the system with and without a PI controller, as shown below. Figure 10 As shown.

[0115] from Figure 10 It can be seen that without the PI controller, the system's crossover frequency is only 35Hz and the phase margin is close to 90°, which indicates that the system's bandwidth is low, the phase margin is too large, and the system's dynamic characteristics are very poor. After adding the PI controller, the system's crossover frequency is close to 600Hz, the phase margin is close to 60°, and it has sufficient amplitude stability margin, which indicates that the PI controller's parameter settings are more reasonable.

[0116] Figure 11 The effectiveness of the active damping method is verified by enabling the damping branch at the peak of the reference current. It can be seen that when the damping branch is not enabled, there are significant oscillations in both the grid current and the filter capacitor voltage. However, when the damping branch is enabled, the oscillations in both the grid current and the filter capacitor voltage are significantly suppressed.

[0117] Figure 12 The steady-state grid-connected current waveform and THD of the grid-connected current when the proposed method is used are given. It can be seen that the quality of the grid-connected current meets the requirements.

[0118] Figure 13 The dynamic waveforms of the system are presented when the damping branch feedback coefficient is too small and the damping coefficient is increased using conventional control methods. The experimental waveforms are also presented when the damping coefficient is set to the optimal feedback coefficient of the damping branch based on the optimal damping ratio at the peak. Figure 13 (a) It can be seen that although the damping branch feedback coefficient can keep the system stable in steady state, due to insufficient system damping, the capacitor voltage and grid current oscillate severely when the reference current changes abruptly, leading to damage to the power devices. From Figure 13 (b) It can be seen that although increasing the feedback coefficient of the damping branch did not result in device damage, the grid current and filter capacitor voltage still exhibited significant oscillations when the reference current suddenly decreased due to insufficient damping coefficient. Furthermore, changing the feedback coefficient of the damping branch to the optimal damping branch feedback coefficient at the peak of the wave would lead to saturation of the inner loop of the system and system instability.

[0119] Figure 14 The dynamic waveform of the system when using the method proposed in this invention is given. It can be seen that, by using the method proposed in this invention, since the influence of the fundamental frequency of the filter capacitor voltage can be eliminated, the feedback coefficient of the damping branch can be set according to the optimal damping ratio, and the influence of grid current and filter capacitor voltage oscillation can be quickly suppressed when the reference current changes abruptly.

[0120] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0121] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A control system for improving the grid-connected dynamic performance of a current-source inverter, characterized in that: Includes power circuits and control circuits; The power circuit includes a DC-side constant current source, a current-source inverter bridge, filter capacitors, filter inductors, and the power grid. The control circuit includes a first subtractor, a current loop controller, inner loop proportional controller 1, inner loop proportional controller 2, inner loop internal delay module, grid current feedforward module, a second subtractor module, a damping branch module, a third subtractor module, and a PWM module. In the control circuit, the first subtractor and the current loop controller constitute the outer current control loop, the inner loop proportional controller 1, inner loop proportional controller 2, inner loop internal delay module, grid current feedforward module, and second subtractor module constitute the inner loop control loop, and the damping branch module and the third subtractor module constitute the damping control loop. The outer current control loop is used to control the grid current to track the reference current; the inner current control loop is used to control the filter capacitor current; and the damping control loop is used to suppress system resonance. In the power circuit, the input terminal of the inverter bridge is connected to a constant current source, and the output terminal is connected in parallel with a filter capacitor. The filter capacitor is connected to a filter inductor, and the filter inductor is connected to the power grid. The control circuit is connected as follows: 1) The reference current and grid current are connected to the input of the subtractor, and the output of the subtractor is connected to the input of the current loop controller; 2) The output of the current loop controller is connected to the input terminals of the inner loop proportional controller 1 and proportional controller 2 respectively. The output terminal of proportional controller 2 is connected to the input terminal of the internal delay module of the inner loop controller. The output terminals of proportional controller 1 and the delay module are both connected to the input terminal of the second subtractor to obtain the current of the current filter capacitor. 3) The feedforward grid current is connected to the input of the second subtractor, and together with the filter capacitor current calculated by the inner loop controller, they form the inverter current, which is then connected to the input of the third subtractor. 4) The damping branch is connected to the input of the third subtractor and together with the inverter current forms a modulation wave, which is connected to the input of the PWM module. The PWM module outputs a drive signal to control the power device to switch on and off and generate inverter current.

2. The control system for improving the grid-connected dynamic performance of a current source inverter according to claim 1, characterized in that: The system includes multiple loops, including a grid current control loop, a filter capacitor current control loop, and an active damping loop.

3. The control system for improving the grid-connected dynamic performance of a current source inverter according to claim 1, characterized in that: The inner loop proportional controller 1 and the inner loop proportional controller 2 are proportional-proportional-delay controllers.

4. The control system for improving the grid-connected dynamic performance of a current source inverter according to claim 3, characterized in that: The inner loop proportional controller 1 and inner loop proportional controller 2 use the filter capacitor current as the control target, which can solve the problem that conventional controllers, which use the filter capacitor voltage as the control target, are prone to inner loop saturation and system instability.

5. The control system and method for improving the grid-connected dynamic performance of a current source inverter according to claim 4, characterized in that: The inner loop proportional controller 1 and inner loop proportional controller 2 take the filter capacitor current as the control target. When designing the parameters of the inner loop controller, the optimal feedback coefficient of the damping branch is combined to solve the problem of inner loop saturation caused by the feedback filter capacitor voltage to achieve active damping in conventional control strategies. This can effectively improve the dynamic characteristics of the current source inverter.

6. A control method for improving the grid-connected dynamic performance of a current-source inverter, characterized in that, Includes the following steps: 1) The grid current control loop uses a conventional PI controller. The input is the reference grid current, the feedback is the sampled grid current, and the output is the reference voltage of the filter capacitor voltage. 2): The capacitor current control loop adopts a proportional-proportional-delay controller. The input is the reference voltage of the filter capacitor voltage, and there is no need to sample and feedback the filter capacitor voltage. The output is the current flowing through the filter capacitor. 3): Based on the optimal damping ratio, the optimal feedback coefficient is designed, the voltage of the filter capacitor is sampled, and the voltage is fed back to the inner loop output through the optimal damping coefficient to form a damping branch, which is used to suppress system resonance; 4): The output of the capacitor current control loop, the output of the damping branch, and the feedforward grid current constitute a modulation wave, which is connected to the PWM module to generate a drive signal for controlling the on / off state of the power devices.

7. The control method for improving the grid-connected dynamic performance of a current source inverter according to claim 6, characterized in that: In step 1), the grid current is directly controlled. By rationally designing the controller parameters, it is possible to ensure that the system has appropriate bandwidth and stability margin. In step 2), the capacitor current control loop adopts a proportional-proportional-delay controller, which eliminates the need for feedback filter capacitor voltage and has the advantages of simple parameter design and low computational load. In step 3), the feedback coefficient of the damping branch is obtained based on the optimal damping ratio, which can effectively suppress the resonance peak of the system and ensure the stability of the system. In steps 2) and 3), by incorporating the damping branch feedback coefficient when designing the capacitor current controller parameters, the fundamental component of the feedback filter capacitor voltage of the damping branch can be filtered out without affecting the resonant voltage.

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