Direct-current bus voltage feedforward control method based on nonlinear disturbance observer

Abstract

The invention discloses a DC bus voltage feedforward control method based on a non-linear interference observer, and relates to the field of converter DC bus voltage control, and the method comprises the steps: inputting and outputting a differential equation of the DC bus capacitor voltage of a photovoltaic grid-connected system based on a photovoltaic grid-connected inverter, and observing the disturbance in the system based on the differential equation. Designing a non-linear interference observer, and generating a current inner ring reference value based on the output of the non-linear interference observer and the improved feedforward controlled voltage ring. The feed-forward function is added on a traditional voltage loop PI control link, the feed-forward function is a first-order differential link, and time delay caused by DC-AC current loop control response and observer convergence speed can be effectively compensated; the DC bus voltage feedforward control method based on the output result of the nonlinear disturbance observer can effectively suppress the impact of various transient working conditions on the DC bus voltage, and the dynamic performance of a DC bus voltage control system is remarkably improved.

Description

Technical Field

[0001] The invention relates to the field of converter DC bus voltage control, in particular to a DC bus voltage feedforward control method based on a nonlinear disturbance observer. Background Art

[0002] In recent years, the installed capacity of photovoltaic power generation systems has been continuously improved. As the interface device between renewable energy and the power grid, the control performance of the grid-connected inverter directly affects the output power quality and operating efficiency of the system. According to the number of energy conversion links in the main circuit, the photovoltaic grid-connected system can be divided into single-stage, two-stage and multi-stage types, among which the two-stage structure is the most widely used. For the two-stage photovoltaic grid-connected inverter, in addition to achieving maximum power point tracking (MPPT) and controlling the grid-connected current, the bus capacitor voltage connecting the front and rear stages must be controlled within a reasonable range. If the voltage is too high, the protection device will be triggered, and if it is too low, the power will flow from the grid side to the DC side. Generally speaking, the bus voltage of the two-stage photovoltaic grid-connected inverter is adjusted by the rear-stage inverter. At the same time, maintaining the power and energy balance between the front and rear stages can ensure the stability of the front-stage bus voltage. Therefore, the control of the rear-stage inverter is the core of the control of the entire photovoltaic inverter system. To improve the dynamics of DC bus voltage control, traditional methods are mainly based on load or source power feedforward, which requires additional voltage and current sensors and strict remote measurement or communication.

[0003] Traditional methods for suppressing transient disturbances in DC bus voltage are all feedforward control, which uses the mismatched current or power information of the photovoltaic power generation system as a disturbance signal to feed forward to the double closed-loop structure, directly tracking the disturbance current or power to reduce bus voltage fluctuations. However, the above feedforward control method generally requires additional sensors to measure the feedforward disturbance, which not only increases the cost but also reduces the reliability of operation. Summary of the invention

[0004] In view of the problems existing in the existing DC bus voltage feedforward control method based on nonlinear disturbance observer, the present invention is proposed.

[0005] Therefore, the problem to be solved by the present invention is the strong randomness and volatility of the output power of the photovoltaic grid-connected system, and the problem that the fluctuation of the photovoltaic power generation output power leads to a large fluctuation amplitude of the DC bus voltage.

[0006] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0007] In a first aspect, an embodiment of the present invention provides a DC bus voltage feedforward control method based on a nonlinear disturbance observer, which includes a differential equation of a DC bus capacitor voltage of a photovoltaic grid-connected system input and output based on a photovoltaic grid-connected inverter. Based on the differential equation, disturbances in the system are observed and a nonlinear disturbance observer is designed. Based on the output of the nonlinear disturbance observer, a current inner loop reference value is generated through an improved voltage loop of the feedforward control.

[0008] As a preferred solution of the DC bus voltage feedforward control method based on nonlinear disturbance observer described in the present invention, the photovoltaic grid-connected inverter circuit adopts a two-stage topology, the front stage is connected to the two ends of the bus capacitor via a boost circuit, and the PV unit is controlled by the MPPT algorithm to output maximum power; the rear stage inverter circuit is composed of a three-phase voltage source PWM inverter, a filter inductor, an equivalent resistor and a power grid.

[0009] As a preferred solution of the DC bus voltage feedforward control method based on nonlinear disturbance observer of the present invention, the dynamic differential equation of the DC bus capacitor voltage of the photovoltaic grid-connected system is:

[0010]

[0011] Where, C is the DC bus capacitance; u dc 、i pv 、i dc They are the DC bus voltage, the output DC current of the photovoltaic unit, and the input DC current of the DC-AC converter; the grid voltage u in the dq coordinate i (i=d,q), grid-side current i i (i=d,q) and inverter output voltage u ci The relationship between (i=d,q) is expressed as,

[0012]

[0013] Active power p transmitted between photovoltaic power generation and AC grid g It is expressed as,

[0014] P g =u dc i dc =1.5u d i d (3)

[0015] As a preferred solution of the DC bus voltage feedforward control method based on nonlinear disturbance observer of the present invention, the equation of the DC bus voltage control system is obtained by combining equations (1) to (3), which is expressed as:

[0016]

[0017] definition:

[0018]

[0019] Where x1 and x2 are state variables, u is the control input, d(t) is the disturbance, and y is the output; then the equation of the DC bus voltage control system of equation (4) can be re-expressed as:

[0020]

[0021] in,

[0022] As a preferred solution of the DC bus voltage feedforward control method based on nonlinear disturbance observer of the present invention, wherein: the disturbance is observed and expressed as:

[0023]

[0024] Among them, z is the intermediate state quantity of NDO; is the estimated value of the disturbance; p(x) is the designed observation function, expressed as,

[0025] p(x)=l1x1+l2x2 (8)

[0026] The observer gain is obtained through l(x), which is expressed as,

[0027]

[0028] As a preferred solution of the DC bus voltage feedforward control method based on nonlinear disturbance observer of the present invention, wherein: the nonlinear disturbance observer is expressed as:

[0029]

[0030] in, is the estimated value of the disturbance.

[0031] As a preferred solution of the DC bus voltage feedforward control method based on nonlinear disturbance observer of the present invention, wherein: the current inner loop reference value is generated based on the voltage loop of improved feedforward control, that is, the input of the disturbance observer is input to the voltage loop input after passing through the feedforward function to generate the current inner loop reference value, and the feedforward function G ch (s) is expressed as,

[0032]

[0033] Among them, k ipd is the current inner loop proportional coefficient, and k is the converter gain.

[0034] In a second aspect, an embodiment of the present invention provides a DC bus voltage feedforward control system based on a nonlinear disturbance observer, which includes: a construction module, which establishes a photovoltaic grid-connected inverter output differential equation for characterizing a nonlinear system model; a design module, which designs a nonlinear disturbance observer according to the input and output differential equations; and an output module, which is used to feedforward the input of the disturbance observer into the voltage loop after passing through a feedforward function.

[0035] In a third aspect, an embodiment of the present invention provides a computer device, comprising a memory and a processor, wherein the memory stores a computer program, wherein: when the processor executes the computer program, any step of the above-mentioned DC bus voltage feedforward control method based on nonlinear disturbance observer is implemented.

[0036] In a fourth aspect, an embodiment of the present invention provides a computer-readable storage medium having a computer program stored thereon, wherein: when the computer program is executed by a processor, any step of the above-mentioned DC bus voltage feedforward control method based on nonlinear disturbance observer is implemented.

[0037] The beneficial effect of the present invention is that it can realize rapid tracking of photovoltaic panel output power disturbance without adding additional voltage / current sensors, providing a reference for the feedforward control of the voltage loop. In addition, the present invention adds a feedforward function to the traditional voltage loop PI control link, and the feedforward function is a first-order differential link, which can effectively compensate for the delay caused by the DC-AC current loop control response and the observer convergence speed; since no additional voltage / current sensors are required, it is conducive to the expansion and plug-and-play of DC-side distributed power supplies and loads; the DC bus voltage feedforward control method based on the output result of the nonlinear disturbance observer can effectively suppress the impact of various transient conditions on the DC bus voltage, and significantly improve the dynamic performance of the DC bus voltage control system. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. Among them:

[0039] Figure 1 is a flow chart of the present invention;

[0040] Figure 2 The photovoltaic grid-connected topology diagram of the present invention;

[0041] Figure 3 It is a control block diagram based on NDO of the present invention;

[0042] Figure 4It is a simulation comparison diagram of the DC side bus voltage output power and current curve of the present invention;

[0043] Figure 5 It is a comparison diagram of the DC side bus voltage waveform of the ordinary dual-loop control and the simulation waveform of the control strategy of the present invention. DETAILED DESCRIPTION

[0044] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in the art without creative work should fall within the scope of protection of the present invention.

[0045] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0046] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0047] Example 1

[0048] Reference Figure 1 and Figure 2 , which is the first embodiment of the present invention, and provides a DC bus voltage feedforward control method based on a nonlinear disturbance observer, comprising:

[0049] S1: The differential equation of the DC bus capacitor voltage of the photovoltaic grid-connected system based on the input and output of the photovoltaic grid-connected inverter.

[0050] like Figure 2 As shown in the figure, the photovoltaic grid-connected inverter circuit adopts a two-stage topology. The front stage is connected to the two ends of the bus capacitor via a boost circuit, and the PV unit is controlled by the MPPT algorithm to output maximum power; the rear stage inverter circuit consists of a three-phase voltage source PWM inverter, a filter inductor, an equivalent resistor and a power grid. Its function is to maintain the power balance between the front and rear stages, stabilize the bus voltage, and generate AC current that meets the grid-connected conditions.

[0051] In order to improve the dynamics of DC bus voltage control, a DC bus voltage control method based on nonlinear disturbance observer (NDO) is designed. Nonlinear disturbance observer (NDO). NDO is used to track the power disturbance in the DC subgrid and provide a reference for the feedforward control of the voltage loop. The voltage outer control loop uses a PI regulator in parallel with the improved power disturbance feedforward control to compensate for the inherent delay in the current loop response and the observer convergence rate.

[0052] By using NDO, it is possible to quickly track the power disturbance of photovoltaic panels without adding additional voltage / current sensors, which is conducive to the expansion and plug-and-play of distributed power sources and loads on the DC side.

[0053] The dynamic differential equation of the DC bus capacitor voltage of the photovoltaic grid-connected system is:

[0054]

[0055] Where, C is the DC bus capacitance; u dc 、i pv 、i dc They are the DC bus voltage, the output DC current of the photovoltaic unit, and the input DC current of the DC-AC converter;

[0056] Grid voltage u in dq coordinates i (i=d,q), grid-side current i i (i=d,q) and inverter output voltage u ci The relationship between (i=d,q) is expressed as,

[0057]

[0058] Ignoring the energy loss of the converter, the active power p transferred between the photovoltaic power generation and the AC grid is g It is expressed as,

[0059] P g =u dc i dc =1.5u d i d (3)

[0060] Combining equations (1) to (3), the second-order system shown in equation (4) (considering only the active current dynamic equation) describes the DC bus voltage control system and obtains the equation of the DC bus voltage control system, which is expressed as:

[0061]

[0062] definition,

[0063]

[0064] Where x1 and x2 are state variables, u is the control input, d(t) is the disturbance, and y is the output;

[0065] Then the equation of the DC bus voltage control system in equation (4) is re-expressed as:

[0066]

[0067] in,

[0068] S2: Observe the disturbances in the system based on differential equations and design a nonlinear disturbance observer.

[0069] The disturbance is observed and expressed as,

[0070]

[0071] Among them, z is the intermediate state quantity of NDO; is the estimated value of the disturbance;

[0072] p(x) is the designed observation function, expressed as,

[0073] p(x)=l1x1+l2x2 (8)

[0074] The observer gain is obtained through l(x), which is expressed as,

[0075]

[0076] In order to simplify the observer design of system (6) and eliminate the influence of other uncertain factors on the observed value, the observer gain can be selected to be l1>0, l2=0, then the nonlinear disturbance observer is expressed as,

[0077]

[0078] in, is the estimated value of the disturbance.

[0079] S3: Based on the output of the nonlinear disturbance observer, the voltage loop of the improved feedforward control is used to generate the current inner loop reference value.

[0080] In order to reduce interference current DC bus voltage u dc Design an improved feedforward control voltage loop. The input of the disturbance observer is passed through the feedforward function G ch (s) is then input to the voltage loop input to generate the current inner loop reference value. ch (s) can be expressed as follows:

[0081]

[0082] Among them, k ipd is the current inner loop proportional coefficient, k is the converter gain, and is preferably 0.5 to 1.5.

[0083] In summary, the present invention can realize rapid tracking of photovoltaic panel output power disturbances without adding additional voltage / current sensors, providing a reference for the feedforward control of the voltage loop. In addition, the present invention adds a feedforward function to the traditional voltage loop PI control link, and the feedforward function is a first-order differential link, which can effectively compensate for the delay caused by the DC-AC current loop control response and the observer convergence speed; since no additional voltage / current sensors are required, it is conducive to the expansion and plug-and-play of distributed power supplies and loads on the DC side; the DC bus voltage feedforward control method based on the output results of the nonlinear disturbance observer can effectively suppress the impact of various transient conditions on the DC bus voltage, and significantly improve the dynamic performance of the DC bus voltage control system.

[0084] Example 2

[0085] Reference Figure 3 Based on the first embodiment, this embodiment further provides a DC bus voltage feedforward control system based on a nonlinear disturbance observer, including:

[0086] Building modules, establishing the output differential equation of photovoltaic grid-connected inverter to characterize the nonlinear system model;

[0087] Design module, design nonlinear disturbance observer according to input and output differential equations;

[0088] The output module is used to feedforward the input of the disturbance observer to the voltage loop after passing through the feedforward function.

[0089] like Figure 3 As shown in Figure 1, the control system includes a nonlinear disturbance observer, a DC bus voltage outer loop and a current inner loop control:

[0090] Nonlinear disturbance observer output It can quickly track the output power disturbance of the photovoltaic panel and provide a reference for the feedforward control of the voltage loop.

[0091] The voltage loop includes PI control and disturbance current feedforward, where the disturbance current Obtained by the nonlinear disturbance observer, the feedforward function is a first-order differential link, which can effectively compensate for the delay caused by the DC-AC current loop control response and the observer convergence speed.

[0092] The current loop adopts PI controller to achieve fast output current tracking.

[0093] This embodiment also provides a computer device, which is suitable for the case of a DC bus voltage feedforward control method based on a nonlinear disturbance observer, and includes a memory and a processor; the memory is used to store computer executable instructions, and the processor is used to execute the computer executable instructions to implement the DC bus voltage feedforward control method based on a nonlinear disturbance observer as proposed in the above embodiment.

[0094] The computer device may be a terminal, and the computer device includes a processor, a memory, a communication interface, a display screen and an input device connected via a system bus. The processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and a computer program. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The communication interface of the computer device is used to communicate with an external terminal in a wired or wireless manner, and the wireless manner can be achieved through WIFI, an operator network, NFC (near field communication) or other technologies. The display screen of the computer device may be a liquid crystal display screen or an electronic ink display screen, and the input device of the computer device may be a touch layer covering the display screen, or a key, trackball or touchpad provided on the housing of the computer device, or an external keyboard, touchpad or mouse, etc.

[0095] This embodiment further provides a storage medium on which a computer program is stored. When the program is executed by a processor, the DC bus voltage feedforward control method based on a nonlinear disturbance observer as proposed in the above embodiment is implemented.

[0096] The storage medium proposed in this embodiment and the data storage method proposed in the above embodiment belong to the same inventive concept. The technical details not fully described in this embodiment can be found in the above embodiment, and this embodiment has the same beneficial effects as the above embodiment.

[0097] Example 3

[0098] Reference Figure 4 and Figure 5 This is the third embodiment of the present invention. Based on the previous two embodiments, this embodiment provides a DC bus voltage feedforward control method based on a nonlinear disturbance observer. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through economic benefit calculation and simulation experiments.

[0099] Matlab / Simulink software was used to build a simulation model of a photovoltaic grid-connected inverter, and the proposed method was simulated and compared with the traditional double closed-loop control. Figure 1 Given, the control system structure is as follows Figure 3 shown.

[0100] At 0.2s, the light intensity changes, and the output power of the photovoltaic panel changes from 100kW to 150kW. The output power and current curves are as follows: Figure 4 The DC bus voltage waveform of the common dual-loop control is as follows: Figure 5 Under the same conditions, the simulation waveform of the control strategy proposed by the present invention is as shown in Figure 5 shown.

[0101] Depend on Figure 5 It can be seen that for the DC side bus voltage, both the traditional dual-loop control and the NDO-based control strategy proposed in the present invention have good steady-state characteristics, but their transient characteristics are different. The traditional dual closed-loop control has an overshoot of about 100V, and the adjustment time of the system under this control strategy is 0.08s; using the method proposed in the present invention, the overshoot of the system is 60V, and it can reach a stable state after only about 0.05s. The traditional dual closed-loop control and the method proposed in the present invention have similar dynamic qualities. From the perspective of the overshoot and adjustment time of the system, the former is slightly inferior to the latter.

[0102] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A DC bus voltage feedforward control method based on a nonlinear disturbance observer, characterized in that: include, The differential equation of the DC bus capacitor voltage of the photovoltaic grid-connected system based on the input and output of the photovoltaic grid-connected inverter; Observe the disturbance in the system based on differential equations and design a nonlinear disturbance observer; Based on the output of the nonlinear disturbance observer, the voltage loop of the feedforward control is improved to generate the current inner loop reference value.

2. The DC bus voltage feedforward control method based on nonlinear disturbance observer according to claim 1, characterized in that: The circuit of the photovoltaic grid-connected inverter adopts a two-stage topology. The front stage is connected to the two ends of the bus capacitor via a boost circuit, and the PV unit is controlled by an MPPT algorithm to output maximum power; the rear stage inverter circuit consists of a three-phase voltage source PWM inverter, a filter inductor, an equivalent resistor and a power grid.

3. The DC bus voltage feedforward control method based on nonlinear disturbance observer according to claim 2, characterized in that: The dynamic differential equation of the DC bus capacitor voltage of the photovoltaic grid-connected system is: Where, C is the DC bus capacitance; u dc 、i pv 、i dc They are the DC bus voltage, the output DC current of the photovoltaic unit, and the input DC current of the DC-AC converter; Grid voltage u in dq coordinates i (i=d,q), grid-side current i i (i=d,q) and inverter output voltage u ci The relationship between (i=d,q) is expressed as, Active power p transmitted between photovoltaic power generation and AC grid g It is expressed as, P g =u dc i dc =1.5u d i d 。 (3)。 4. The DC bus voltage feedforward control method based on nonlinear disturbance observer according to claim 3, characterized in that: Combining equations (1) to (3), the equation of the DC bus voltage control system is expressed as: definition, Where x1 and x2 are state variables, u is the control input, d(t) is the disturbance, and y is the output; Then the equation of the DC bus voltage control system in equation (4) is re-expressed as: in, 5. The DC bus voltage feedforward control method based on nonlinear disturbance observer according to claim 4, characterized in that: The disturbance is observed and expressed as, Among them, z is the intermediate state quantity of NDO; is the estimated value of the disturbance; p(x) is the designed observation function, expressed as, p(x)=l1x1+l2x2 (8) The observer gain is obtained through l(x), which is expressed as, 6. The DC bus voltage feedforward control method based on nonlinear disturbance observer according to claim 5, characterized in that: Then the nonlinear disturbance observer is expressed as, in, is the estimated value of the disturbance.

7. The DC bus voltage feedforward control method based on nonlinear disturbance observer according to claim 6, characterized in that: The current inner loop reference value is generated based on the voltage loop of the improved feedforward control, that is, the input of the disturbance observer is input to the voltage loop input after the feedforward function to generate the current inner loop reference value. The feedforward function G ch (s) is expressed as, Among them, k ipd is the current inner loop proportional coefficient, and k is the converter gain.

8. A DC bus voltage feedforward control system based on a nonlinear disturbance observer, based on the DC bus voltage feedforward control method based on a nonlinear disturbance observer according to any one of claims 1 to 7, characterized in that: include, Building modules, establishing the output differential equation of photovoltaic grid-connected inverter to characterize the nonlinear system model; Design module, design nonlinear disturbance observer according to input and output differential equations; The output module is used to feedforward the input of the disturbance observer to the voltage loop after passing through the feedforward function.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the DC bus voltage feedforward control method based on nonlinear disturbance observer according to any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the DC bus voltage feedforward control method based on a nonlinear disturbance observer according to any one of claims 1 to 7 are implemented.

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