Active inertia filter

Abstract

The invention discloses an active inertia filter, which belongs to the technical field of active filtering compensators, is a network-forming inverter with a super capacitor on a direct current side, and comprises an inverter super capacitor module, a DC-DC (Direct Current-Direct Current) module and a DC-AC (Direct Current-Alternating Current) module. The control of the inverter comprises two parts, one part is basic voltage and current control which has rapid dynamic response and realizes automatic rapid response to power supply intermittency and power grid frequency change, and the other part realizes slow power control to simulate inertia, so that a power filter and frequency inertia intersection are realized. According to the coupling scheme that the new energy is connected to the alternating-current power grid through the active inertia filter, the reliability of the system is greatly improved, and the transformation cost of the system is reduced. Moreover, the system has a quick power response function, can achieve the inertial power supply and power filtering functions, and improves the power tracking performance.

Description

Technical Field

[0001] The present invention relates to the technical field of power system operation control, and in particular to an active inertia filter. Background Art

[0002] Frequency control is very important for the stability of AC systems because the grid frequency reflects the balance between generators and demand. In frequency events, primary frequency regulation is always required to prevent a rapid decline in frequency on the time scale of seconds or minutes. The function of primary frequency control is usually achieved through inertia support and frequency droop. For synchronous generators (SGs), the kinetic energy stored in their rotors serves as inertial energy against frequency deviation, thus providing sufficient time for the effectiveness of frequency decline control. For grid-connected voltage-controlled inverters, their inertia should include two aspects. The first is the inherent frequency inertia. This requires that when the output power of the voltage-controlled inverter changes, its frequency should change slowly. This aspect can be achieved through the well-known virtual synchronous generator (VSG) control, in which the swing equation is simulated. The second inertial function is the inertial support of the grid. In this case, the inertia support requires a very fast but short-duration active power injection / absorption into the grid. For this inertia support, two functions should be performed: first, when the grid frequency changes, the inverter should supply / absorb active power to resist the frequency change; second, if pulsating power is generated due to intermittent photovoltaic power generation, the inverter should filter the pulsating power to prevent the pulsating power from entering or leaving the grid. Frequency droop is an active support function that remains unchanged for a long time and is used to restore the grid frequency with a static error, and the energy required is greater than the inertial support. Based on a well-tuned inner voltage control loop, primary frequency regulation can be achieved through an outer-loop power control. Summary of the Invention

[0003] The object of the present invention is to provide an active inertia filter that can transform a new energy access system from a grid-following system to a grid-forming system, effectively improving the reliability of the system. The technical solution adopted by the present invention is as follows.

[0004] In a first aspect, the present invention provides an active inertia filter, including a supercapacitor module, a DC-DC module, a DC-AC module, and a control module connected in sequence. A DC bus capacitor is connected in parallel on the DC bus between the cascaded DC-DC module and the DC-AC module. The AC side bus of the DC-AC module is connected to the grid connection point of the new energy system through an LC filter circuit unit;

[0005] The control module includes:

[0006] A DC current and voltage control unit for controlling a DC-DC module according to the DC bus voltage, DC bus capacitance, DC bus reference voltage, active power at the grid connection point, resistance on the supercapacitor side, inductance on the supercapacitor side, and the output value of the current inner loop in which the supercapacitor module is located;

[0007] An AC voltage and current control unit for controlling a DC-AC module according to the phase reference value of the inverter output voltage, the capacitor voltage and reference phase, reference capacitor voltage in the LC filter circuit unit, and the inductor current on the inverter side;

[0008] A power control unit for tracking the active power deviation at the grid connection point to obtain the reference phase of the capacitor voltage, obtaining the reference phase of the capacitor voltage, and for tracking the reactive power deviation at the grid connection point to obtain the reference capacitor voltage.

[0009] Optionally, the DC current and voltage control unit includes a DC current control inner loop and a DC voltage control outer loop;

[0010] The DC current control inner loop is used to determine the control pulse output to the DC-DC module according to the current inner loop reference value and the current inner loop output value;

[0011] The DC voltage control outer loop is used to obtain the power state of the current inner loop based on the DC bus voltage power model according to the DC bus voltage, DC bus capacitance, DC bus reference voltage, and active power at the grid connection point, and to determine the current inner loop reference value according to the power state of the current inner loop and the voltage of the supercapacitor module.

[0012] Optionally, in the DC current control inner loop, the difference between the current inner loop reference value and the current inner loop output value is respectively passed through a first proportional-integral controller and then subjected to PWM modulation to obtain the control pulse signal of the DC-DC module.

[0013] Optionally, the closed-loop transfer function of the DC bus current control of the DC current control unit is:

[0014]

[0015] where is the current inner loop reference value, is the current inner loop output value, is the inductance on the supercapacitor side, is the resistance on the supercapacitor side, is the proportional coefficient of the first proportional controller, is the integral coefficient of the first integral controller. The current inner loop output value is the current on the supercapacitor side.

[0016] The above DC current control adopts the average model of the switching frequency. Since the capacitance value is large and the disturbance of the supercapacitor voltage changes slowly, it is negligible.

[0017] Optionally, the DC bus voltage power model is expressed as:

[0018]

[0019] The closed-loop transfer function of the outer loop of the DC voltage control is:

[0020]

[0021] In the formula, is the DC bus voltage, is the DC bus reference voltage, 、 are the proportional coefficient and integral coefficient of the second proportional-integral controller, is the DC bus side capacitor.

[0022] From the closed-loop transfer function of the outer loop of the above DC voltage control and the DC bus voltage power model, the reference value of the current inner loop can be obtained.

[0023] Optionally, the loop bandwidth of the outer loop of the DC voltage control is less than one-fifth of the loop bandwidth of the inner loop of the DC current control, so that the inner and outer loops are decoupled.

[0024] In the above DC current and voltage control, the current and voltage control can both select the parameters of the PI controller through pole placement.

[0025] Optionally, the AC voltage and current control unit controls the DC-AC module according to the capacitor voltage and the reference capacitor voltage in the LC filter circuit unit, and the inductor current on the inverter side, including:

[0026] Obtain the state space model of the three-phase inverter with an LC filter as:

[0027]

[0028] In the formula, is the capacitor voltage, is the inductor current on the inverter side, is the inductor current on the grid side, is the output voltage of the inverter bridge arm;

[0029] Through the Park transformation, the three-phase stationary abc coordinate system is transformed into the two-phase synchronous rotating dq coordinate system to obtain:

[0030]

[0031] In the formula, is the capacitor voltage after Park transformation, is the inductor current on the inverter side after Park transformation, is the inductor current on the grid side after Park transformation, is the output voltage of the inverter bridge arm after Park transformation;

[0032] Applying the Laplace transform to the inverter model in the two-phase rotating coordinate system and using a proportional-integral controller for control, the control equation for the output voltage of the inverter bridge arm is obtained as:

[0033]

[0034] where, 、 are the reference inductor currents on the d-axis of the inverter side after Park transformation, are the inductor currents on the q-axis of the inverter side after Park transformation, 、 are the capacitor voltages on the dq-axis after Park transformation, are the output voltages of the inverter bridge arm on the q-axis after Park transformation, 、 are the proportional coefficient and integral coefficient of the third proportional-integral controller; 、 The control equation for the inverter bridge arm current is: are the reference capacitor voltages on the q-axis after Park transformation, and are the inductor currents on the q-axis of the inverter side after Park transformation,

[0035] where,

[0036]

[0037] are the reference capacitor voltages on the q-axis after Park transformation, 、 are the capacitor voltages on the q-axis after Park transformation, are the inductor currents on the q-axis of the inverter side after Park transformation, 、 are the inductor currents on the q-axis of the grid side after Park transformation, are the proportional coefficient and integral coefficient of the fourth proportional-integral controller. 、 are the inductor currents on the q-axis of the inverter side after Park transformation, are the inductor currents on the q-axis of the grid side after Park transformation, 、 are the inductor currents on the q-axis of the grid side after Park transformation, are the proportional coefficient and integral coefficient of the fourth proportional-integral controller. and are the proportional coefficient and integral coefficient of the fourth proportional-integral controller.

[0038] In the above AC voltage and current control, a capacitor reference voltage is introduced as a feed-forward compensation, which can achieve dq-axis decoupling control.

[0039] Optionally, the power control unit includes an active power control section;

[0040] In the active power control section, the grid power is described as:

[0041]

[0042] where represents the grid connection point power reference value, represents the change in grid frequency, represents the power interference caused by new energy connected to the grid, is the power tracking function, is the frequency inertia function, is the filtering performance function;

[0043] Let , then there is:

[0044]

[0045] where and are the integral coefficients of the integral controller with the active power deviation as the input respectively, is the proportional coefficient of the proportional controller with the active power reference value as the input;

[0046] Let , , then the filtering performance function is expressed as:

[0047]

[0048] Among them, the relationship between the control parameters and the zero point and the pole points , is:

[0049] .

[0050] As can be seen above, in the characteristic polynomial replaces the droop coefficient , as the coefficient of the first-order term s, which means that is introduced as the damping of the system, making the system stable. An adjustable zero point is introduced, and this zero point can be used to improve the power tracking performance. Based on this, by designing p1 and p2, the maximum power and transient time during the dynamic response when the grid frequency changes can be adjusted, as well as the filter time constant for compensating the intermittent output such as photovoltaic and wind energy at the access point for filtering performance.

[0051] Optionally, the power control unit further includes a reactive power control part, and the corresponding control form is expressed as:

[0052]

[0053] In the formula, is the voltage output by the reactive - voltage link, is the reactive power reference value, is the reactive power detection value, is the grid voltage, is the droop coefficient, is the inertia coefficient.

[0054] Beneficial effects

[0055] Without changing the inverter itself, the present invention effectively transforms the existing distributed power access system from a grid - following system to a grid - forming system. This AC - coupling scheme greatly improves the reliability of the system and reduces the transformation cost of the system. The two - stage converter topology is used for the supercapacitor inverter. The DC voltage control and AC voltage control constitute the fast power response function of the grid - forming system. At the same time, the power control method proposed by the present invention realizes the inertial power supply and power filtering functions, and improves the power tracking performance. The frequency inertia has a certain hindering effect on the grid frequency offset based on df / dt power control. When the grid frequency decreases, the grid - connected inverter injects active power to prevent the frequency from decreasing, and vice versa. When the parallel photovoltaic intermittent power generation generates pulsating power, the grid - connected inverter can act as a power filter to automatically filter the pulsating power, thereby reducing the impact of the pulsating power on the grid. Description of the drawings

[0056] Figure 1 shows the schematic diagram of the active inertia filter control system architecture;

[0057] Figure 2 shows the schematic diagram of the active inertia filter control system principle;

[0058] Figure 3 shows the schematic diagram of the DC - DC current control principle;

[0059] Figure 4 shows the schematic diagram of the DC - DC voltage control principle;

[0060] Figure 5The figure shows a schematic diagram of the active power control principle. Specific implementation manners

[0061] The following further describes in conjunction with the accompanying drawings and specific embodiments.

[0062] The active inertia filter proposed by the present invention is an active filter used to filter the power access of distributed energy such as photovoltaic and wind energy and provide inertia support. It is a grid-forming inverter with a supercapacitor on the DC side. The inverter can be divided into two parts. The first part is for basic voltage and current control, enabling the system to have fast dynamic response performance and realizing automatic and rapid response to power source intermittency and grid frequency changes, as Figure 1 shown in the red part; the second part realizes slow power control to simulate inertia, thereby realizing power filtering and frequency inertia, as Figure 1 shown in the blue part. The specific control principle block diagram is as Figure 2 shown.

[0063] As Figure 1 , the active inertia filter includes a supercapacitor SC, i.e., a supercapacitor module, a DC converter, i.e., a DC-DC module, a DC-AC inverter, i.e., a DC-AC module, and a control module; a DC bus capacitor is connected in parallel on the DC bus between the cascaded DC-DC module and the DC-AC module. The AC side bus of the DC-AC module is connected to the grid connection point of the new energy system through an LC filter circuit unit to form a traditional grid-connected AC coupling inverter; the supercapacitor SC is connected in parallel on the DC side of the traditional grid-connected inverter to form a grid-forming inverter.

[0064] Referring to Figure 2 , the control module includes:

[0065] A DC current and voltage control unit for controlling the DC-DC module according to the DC bus voltage, DC bus capacitor, DC bus reference voltage, grid connection point active power, supercapacitor side resistance, supercapacitor side inductance, and the output value of the current inner loop where the supercapacitor module is located;

[0066] An AC voltage and current control unit for controlling the DC-AC module according to the phase reference value of the inverter output voltage, the capacitor voltage and reference phase in the LC filter circuit unit, the reference capacitor voltage, and the inductor current on the inverter side;

[0067] A power control unit for tracking the active power deviation of the grid connection point to obtain the reference phase of the capacitor voltage, obtaining the reference phase of the capacitor voltage, and for tracking the reactive power deviation of the grid connection point to obtain the reference capacitor voltage.

[0068] The working principle of the present invention is as follows:

[0069] When the power generation power or frequency of distributed energy connected to the grid suddenly changes, they need to quickly inject / absorb active power into the grid in a short time. For this purpose, the response of the grid-forming inverter should be as fast as possible to avoid the sudden power demand of the grid. Grid power depends on the voltages on both sides of the inductor : one is the grid voltage and the other is the inverter voltage.

[0070] Taking photovoltaic as an example, when the photovoltaic generates pulsating power, since the frequency and phase of the inverter remain unchanged at this time, this pulsating power must first flow into the grid-connected inverter. Since the voltage across the inductor remains unchanged, the grid power must remain unchanged. This physical mechanism is an inherent characteristic of the voltage source, which ensures that the pulsating power automatically flows into the supercapacitor inverter side, thus avoiding negative impacts on the grid. Subsequently, the power loop slowly adjusts the inverter output power to the steady-state value. In the present invention, since only supercapacitors are used, the steady-state value is zero (only providing inertia). This mechanism can also be explained from the perspective of impedance. The power distribution between the two voltage sources depends on the ratio of the impedances. The approximate zero impedance of the inverter ensures that Zi(s) ~ Zg(s) near the fundamental frequency. Therefore, the fluctuating power must be provided by the supercapacitor inverter until the power loop adjusts the phase and frequency of the supercapacitor inverter.

[0071] Specifically, the implementation of the present invention includes the following parts.

[0072] I. DC-DC control

[0073] Combined with Figures 1 to 3 as shown, this part of the control mainly controls the DC-DC module according to the DC bus voltage, DC bus capacitor, DC bus reference voltage, active power at the grid connection point, resistance on the supercapacitor side, inductance on the supercapacitor side, and the output value of the current inner loop in which the supercapacitor module is located.

[0074] The content of the DC-DC control includes a DC current control inner loop and a DC voltage control outer loop. The DC current control inner loop is used to determine the control pulse output to the DC-DC module according to the current inner loop reference value and the current inner loop output value: the difference between the current inner loop reference value and the current inner loop output value is respectively passed through the first proportional-integral controller and then subjected to PWM modulation to obtain the control pulse signal of the DC-DC module. The DC voltage control outer loop is used to obtain the power state of the current inner loop based on the DC bus voltage power model according to the DC bus voltage, DC bus capacitor, DC bus reference voltage, and active power at the grid connection point, and to determine the current inner loop reference value according to the power state of the current inner loop and the voltage of the supercapacitor module.

[0075] (1.1) Current control

[0076] The current control block diagram is as follows Figure 3 As shown, the present invention adopts the average switching frequency model. Since the capacitance value is large, the disturbance change of the supercapacitor voltage is slow and can be ignored. At this time, the closed-loop transfer function of the current loop is as follows:

[0077]

[0078] In the formula, is the reference value of the inner current loop, is the output value of the inner current loop, is the inductor on the supercapacitor side, is the resistor on the supercapacitor side, is the proportional coefficient of the first proportional controller, is the integral coefficient of the first integral controller. The output value of the inner current loop is the current on the supercapacitor side.

[0079] (1.2) Voltage control

[0080] The DC bus voltage control proposed by the present invention is based on the following power model of the DC bus voltage:

[0081]

[0082] Figure 4 is the DC voltage control block diagram based on the above power model. To decouple the inner and outer loops, the bandwidth of the voltage outer loop should be designed to be less than one-fifth of the current loop bandwidth. The inverter output voltage is used as feedforward to improve the dynamic response of the system. Then the closed-loop transfer function of the DC bus voltage control is as follows:

[0083]

[0084] In the formula, is the DC bus voltage, is the DC bus reference voltage, , are the proportional coefficient and integral coefficient of the second proportional-integral controller, is the capacitor on the DC bus side.

[0085] The parameters of the PI controller for both DC-DC current and voltage control can be selected through pole placement.

[0086] II. DC-AC control

[0087] This part of the control is used to control the DC-AC module according to the phase reference value of the inverter output voltage, the capacitor voltage and reference phase, reference capacitor voltage in the LC filter circuit unit, and the inductor current on the inverter side. The DC-AC control part also includes the content of AC voltage and current control.

[0088] (2.1) Voltage control

[0089] The state - space model of a three - phase inverter with an LC filter is as follows:

[0090]

[0091] Where, is the capacitor voltage, is the inductor current on the inverter side, is the inductor current on the grid side, is the output voltage of the inverter bridge arm;

[0092] By applying the Park transformation to transform the three - phase stationary abc coordinate system to the two - phase synchronous rotating dq coordinate system, we get:

[0093]

[0094] Where, is the capacitor voltage after Park transformation, is the inductor current on the inverter side after Park transformation, is the inductor current on the grid side after Park transformation, is the output voltage of the inverter bridge arm after Park transformation;

[0095] Applying the Laplace transform to the inverter model in the two - phase rotating coordinate system and using a proportional - integral controller for control, the control equation for the output voltage of the inverter bridge arm is:

[0096]

[0097] Where, , are the reference inductor currents on the d - axis of the inverter side after Park transformation, axis inverter - side reference inductor current, , are the inductor currents on the q - axis of the inverter side after Park transformation, axis inverter - side inductor current, , are the dq - axis capacitor voltages after Park transformation, , are the output voltages of the inverter bridge arm on the axis after Park transformation, and are the proportional coefficient and integral coefficient of the third proportional - integral controller;

[0098] Similarly, using a proportional - integral controller (PI) for control, the control equation for the inverter bridge arm current can be obtained as:

[0099]

[0100] In the formula, and are the axis reference capacitor voltages after Park transformation, and are the axis capacitor voltages after Park transformation, and are the axis inverter-side inductor currents after Park transformation, and are the axis grid-side inductor currents after Park transformation, and are the proportional coefficient and integral coefficient of the fourth proportional-integral controller.

[0101] As Figure 2 shown, in the above AC voltage and current control, a capacitor reference voltage is introduced as feed-forward compensation, and axis decoupling control can be achieved.

[0102] III. Power Control

[0103] This part of the control includes an active power control part and a reactive power control part, which are respectively used to track the active power deviation of the grid connection point to obtain the reference phase of the capacitor voltage, obtain the reference phase of the capacitor voltage, and to track the reactive power deviation of the grid connection point to obtain the capacitor reference voltage.

[0104] (3.1) Active Power Control

[0105] The small-signal control block diagram of the active power is as Figure 5 shown, where is added to adjust the tracking performance, represents the grid frequency, represents the change in the grid frequency, represents the power interference from the photovoltaic or load, and and are all control parameters. As Figure 5 shown, the system has three inputs: power reference , change in the grid frequency and power interference . Then, the grid power can be described as:

[0106]

[0107] Let , then the closed-loop transfer function of the system is:

[0108]

[0109] As can be seen from the above formula, in the characteristic polynomial, Replacing the droop coefficient , as the coefficient of the first-order term s, this means that The introduction of makes the system stable by acting as the damping of the system.

[0110] Next, the pole placement method for active power control will be described.

[0111] First, rewrite as:

[0112]

[0113] The relationship between the control parameters and the zeros and poles is:

[0114]

[0115] This indicates that An adjustable zero is introduced. Using this zero can improve the power tracking performance. Let , then is:

[0116]

[0117] Since it is a first-order system, it has good dynamic performance, and its settling time can be adjusted through for adjustment.

[0118] Secondly, for the frequency inertia, when the grid frequency changes, due to the limited energy storage of the supercapacitor, the steady-state output power of the inverter should be zero, that is, to provide dynamic power support. Observe , meets the requirements of the final value. In this case, two indicators in the dynamic process are crucial. One is the duration of the dynamic process; the other is the maximum power in the transient process. The duration depends on the energy required for inertia support and can be adjusted by and the positions of. The maximum power amplitude in the transient process is not only related to the rated power of the supercapacitor inverter but also affects the energy required for the dynamic support duration. Therefore, to clarify the design of the maximum power amplitude in the transient process when the grid frequency changes, can be rewritten as:

[0119]

[0120] By calculating the derivative of the above formula with respect to the frequency , can be determined The maximum amplitude is:

[0121]

[0122] When , this value is actually the maximum power during the dynamic response when the grid frequency changes. Correspondingly, the power amplitude is quantitatively determined by and . The smaller the designed poles, the longer the transient time and the greater the maximum power. Therefore, the placement of the poles should comprehensively consider these two aspects and the dynamic characteristics of power tracking control.

[0123] Finally, the fluctuating power generated by the photovoltaic or load can be regarded as a power disturbance . Let , , then the performance of the power filter can be described as:

[0124]

[0125] This is a low-pass filter, and its filtering ability can be designed by and . Therefore, by adopting the proposed active filter design for photovoltaic power filtering with inertia support, the intermittent output of the photovoltaic can be compensated by the supercapacitor inverter. The filter time constant depends on and .

[0126] (3.2) Reactive power control

[0127] The reactive power-voltage link mimics the synchronous excitation regulation to achieve the reactive power droop characteristic and the error control of reactive power with respect to voltage. When considering inertia support, the instantaneous reactive power control is a first-order system with a closed loop and has the following control form:

[0128]

[0129] Where: is the voltage output by the reactive power-voltage link, is the reference value of reactive power, is the detected value of reactive power, is the grid voltage, is the droop coefficient, is the inertia coefficient.

[0130] The parameter design of the above formula is convenient, and it is also the same as the active power-frequency link in the reactive power-voltage link, providing a certain amount of inertia support.

[0131] With the technical solutions of the above embodiments, the active inertia filter proposed by the present invention provides a certain amount of inertia support for the power grid. Assume that the inertia of the power grid is:

[0132]

[0133] In the formula, is the moment of inertia, (the positive direction of flowing into the power grid) is the power difference when there is a frequency deviation between the power grid and the load. For such a power grid, generates a frequency change rate . Then rewrite as:

[0134]

[0135] Under steady state ( ), the output power of the supercapacitor inverter is proportional to the frequency change rate , and the coefficient is .

[0136]

[0137]

[0138] Among them can be defined as the moment of inertia provided by the active inertia filter. Obviously, the power provided by the active inertia filter will compensate for the power difference , thereby reducing the frequency change rate, because the output power is the feedback of the frequency change rate: , so the active inertia filter can absorb to prevent the frequency from decreasing.

[0139] The design of the pole can be balanced between better filtering performance and greater inertia, and greater power and energy ratings. The smaller the pole, the better the filtering performance, the greater the inertia, and at the same time, the greater the power and energy ratings.

[0140] The above describes the embodiments of the present invention in conjunction with the accompanying drawings. However, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many forms without departing from the purpose of the present invention and the scope protected by the claims. These all fall within the protection scope of the present invention.

Claims

1. An active inertia filter, characterized in that: It includes a super capacitor module, a DC-DC module, a DC-AC module and a control module connected in sequence, a DC bus capacitor is connected in parallel to the DC bus between the cascaded DC-DC module and the DC-AC module, and the AC side bus of the DC-AC module is connected to the grid connection point of the new energy system after passing through an LC filter circuit unit; The control module comprises: A DC current and voltage control unit, used to control the DC-DC module according to the DC bus voltage, DC bus capacitance, DC bus reference voltage, grid-connected point active power, supercapacitor side resistance, supercapacitor side inductance and the current inner loop output value of the supercapacitor module; An AC voltage and current control unit, used to control the DC-AC module according to the phase reference value of the inverter output voltage, the capacitor voltage and reference phase in the LC filter circuit unit, the reference capacitor voltage, and the inverter side inductor current; A power control unit is used to track the active power deviation of the grid connection point to obtain a capacitor voltage reference phase, obtain the reference phase of the capacitor voltage, and to track the reactive power deviation of the grid connection point to obtain the capacitor reference voltage.

2. The active inertia filter according to claim 1, characterized in that: The DC current and voltage control unit comprises a DC current control inner loop and a DC voltage control outer loop; The DC current control inner loop is used to determine the control pulse output to the DC-DC module according to the current inner loop reference value and the current inner loop output value; The DC voltage control outer loop is used to obtain the current inner loop power state based on the DC bus voltage power model according to the DC bus voltage, DC bus capacitance, DC bus reference voltage and grid-connected point active power, and to determine the current inner loop reference value according to the current inner loop power state and the supercapacitor module voltage.

3. The active inertia filter according to claim 2, characterized in that: In the DC current control inner loop, the difference between the current inner loop reference value and the current inner loop output value is respectively passed through a first proportional integral controller and then subjected to PWM modulation to obtain a control pulse signal of the DC-DC module.

4. The active inertia filter according to claim 3, characterized in that: The DC bus current control closed-loop transfer function of the DC current control unit is: In the formula, is the current inner loop reference value, is the current inner loop output value, is the inductance on the supercapacitor side, is the supercapacitor side resistance, is the proportional coefficient of the first proportional controller, is the integral coefficient of the first integral controller. The output value of the current inner loop is the current on the supercapacitor side.

5. The active inertia filter according to claim 2, characterized in that: The DC bus voltage power model is expressed as: The closed-loop transfer function of the DC voltage control outer loop is: In the formula, is the DC bus voltage, is the DC bus reference voltage, , are the proportional coefficient and integral coefficient of the second proportional-integral controller, is the DC bus side capacitance.

6. The active inertia filter according to claim 2, characterized in that: The loop bandwidth of the DC voltage control outer loop is less than one fifth of the loop bandwidth of the DC current control inner loop, so that the inner and outer loops are decoupled.

7. The active inertia filter according to claim 1, characterized in that: The AC voltage and current control unit controls the DC-AC module according to the capacitor voltage and the reference capacitor voltage in the LC filter circuit unit and the inductor current on the inverter side, including: The state space model of the three-phase inverter with LC filter is obtained as: In the formula, is the capacitor voltage, is the inductor current on the inverter side, is the grid-side inductor current, is the inverter bridge arm output voltage; The three-phase stationary abc coordinate system is transformed into the two-phase synchronously rotating dq coordinate system through Park transformation, and the following is obtained: In the formula, is the capacitor voltage after Park transformation, is the inverter side inductor current after Park transformation, is the grid-side inductor current after Park transformation, is the inverter bridge arm output voltage after Park transformation; Applying Laplace transform to the inverter model in the two-phase rotating coordinate system and adopting proportional integral controller for control, the inverter bridge arm output voltage control equation is obtained as follows: In the formula, , is the Park transformed Axis inverter side reference inductor current, , is the Park transformed Axis inverter side inductor current, , is the dq axis capacitor voltage after Park transformation, , is the Park transformed Axis inverter bridge arm output voltage, and are the proportional coefficient and the integral coefficient of the third proportional-integral controller; The inverter arm current control equation is: In the formula, , is the Park transformed Axis reference capacitor voltage, , is the Park transformed Shaft capacitance voltage, , is the Park transformed Axis inverter side inductor current, , is the Park transformed Axis grid side inductor current, and are the proportional coefficient and integral coefficient of the fourth proportional-integral controller.

8. The active inertia filter according to claim 1, characterized in that: The power control unit includes an active power control part; In the active power control part, the grid power is described as: In the formula, Indicates the grid-connected point power reference value, Indicates the change of grid frequency. Indicates the power interference caused by the new energy connected to the power grid, is the power tracking function, is the frequency inertia function, is the filtering performance function; make , then: In the formula, and are the integral coefficients of the integral controller with active power deviation as input, is the proportional coefficient of the proportional controller with the active power reference value as input; make , , then the filtering performance function is expressed as: Among them, the control parameters and zero point and pole , The relationships are: 。 9. The active inertia filter according to claim 8, characterized in that: The power control unit also includes a reactive power control part, and the corresponding control form is expressed as: Where: is the voltage output by the reactive-voltage link, is the reactive power reference value, Reactive power detection value, is the grid voltage, is the droop coefficient, is the inertia coefficient.

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