A Model Predictive Control Method for Grid-Connected Inverters with Current Limitation

Abstract

The present invention relates to a model predictive control method for a grid-connected inverter with current limitation, and constructs a main circuit of a three-phase three-level neutral-point clamped grid-connected inverter; Step S2: Collect grid signals at the current moment; Step S3: Detect the phase of the grid voltage and extract the positive and negative sequence components of the grid voltage in the double synchronous reference coordinate system; Step S4: Calculate the power reference value of the grid-connected inverter; Step S5: Calculate the grid-connected current reference value; Step S6: Calculate the positive and negative sequence current reference values in the two-phase stationary coordinate system; Step S7: Synthesize the reference current vector; Step S8: Predict the voltage value across the DC capacitor; Step S9: The model predictive controller selects the optimal operation and applies it to the grid-connected inverter, so that the grid-connected current can accurately track the reference current. The present invention can ensure that the grid-connected current of the inverter is always less than the maximum allowable value under grid unbalance faults.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected inverter control, and particularly relates to a model predictive control method for a grid-connected inverter with current limitation. Background Art

[0002] With the gradual increase of the proportion of new energy in China in the future, the construction and development of new energy power generation will enter an accelerated period. When the penetration rate of the new energy power generation system in the power grid reaches a certain level, it is required to have the fault ride-through ability when short-term faults occur, such as voltage dips, phase jumps, and frequency changes. That is, it is required that the grid-connected inverter still maintains normal operation without disconnecting from the grid when a short-term voltage drop occurs in the power grid. Only when the fault is very serious can it be disconnected from the power grid. Therefore, when new energy is incorporated into the power grid, a reasonable control strategy must be adopted to improve the output performance of the grid-connected inverter to ensure the safe and stable operation of the power grid and users.

[0003] At present, the control strategies of grid-connected inverters mainly include proportional-integral (PI) control, proportional-resonant (PR) control, hysteresis comparison control, repetitive control, predictive control, etc. PI control technology is widely used in grid-connected inverters due to its simple principle and mature theory. However, this method cannot achieve zero-static-error tracking of the grid-connected current, and the dynamic response speed is slow. Inappropriate control parameters are likely to cause the grid-connected current to exceed the maximum allowable range, affecting the safe and reliable operation of the system. PR control is essentially an improvement of PI control. Its basic idea is to introduce a resonant link in the proportional link, aiming to increase the open-loop gain of the system at the resonant frequency and improve the anti-interference ability of the control system. However, when the frequency of the power grid fluctuates, this method cannot effectively suppress the power grid harmonics. Hysteresis comparison control has the advantages of strong stability, simple structure, and fast dynamic response. However, since the hysteresis width remains unchanged during the control process, the switching frequency of the inverter is not restricted, resulting in difficulties in the design of the filter inductor and particularly prominent EMI problems. Repetitive control can well eliminate the steady-state error of the control system caused by repetitive disturbances. However, due to the existence of the delay link, the dynamic response of the system is slow. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a model predictive control method for a grid-connected inverter with current limitation, which improves the operation performance of the grid-connected system and ensures that the grid-connected current of the inverter is always less than the maximum allowable value under unbalanced grid faults.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions:

[0006] A model predictive control method for a grid-connected inverter with current limitation, comprising the following steps:

[0007] Step S1: Construct the main circuit of a three-phase three-level neutral-point clamped grid-connected inverter;

[0008] Step S2: Collect the grid-connected current i(k), grid voltage e(k), and DC-link capacitor voltages v c1 (k) and v c1 (k), and the DC-side input current i dc (k);

[0009] Step S3: Use a decoupled double synchronous reference frame PLL to detect the phase θ of the grid voltage and extract the positive and negative sequence components of the grid voltage in the double synchronous reference frame

[0010] Step S4: According to what is obtained in Step S3 and the maximum allowable value I of the grid-connected current max , calculate the active and reactive reference values P * and Q * of the grid-connected inverter in the balanced current mode, constant active power mode, and constant reactive power mode;

[0011] Step S5: According to the P * and Q * obtained in Step S4, calculate the grid-connected current reference values in the balanced current mode, constant active power mode, and constant reactive power mode

[0012] Step S6: Perform inverse Park transformation on what is obtained in Step S5 and and respectively to obtain the positive sequence current reference value in the αβ coordinate system and the negative sequence current reference value

[0013] Step S7: Add what is obtained in Step S6 to to obtain the current reference value in the αβ coordinate system and and synthesize the reference current vector i * ;

[0014] Step S8: Use Ohm's law to predict the voltage values across the DC capacitors and

[0015] Step S9: Feed the i * obtained in Step S7 into the model predictive controller, and the model predictive controller selects the optimal operation and applies it to the grid-connected inverter so that the grid-connected current can accurately track the reference current.

[0016] Further, the main circuit of the three-phase three-level neutral-point clamped grid-connected inverter includes an equivalent DC power supply output by a new energy distributed generation system, a DC-side capacitor, a three-phase three-level NPC grid-connected inverter, a filter inductor L, and a power grid e, which are connected in sequence.

[0017] Further, step S4 is specifically as follows:

[0018] Step S41: Express the grid voltage unbalance degree as:

[0019] In the formula, respectively represent the amplitudes of the positive and negative sequence components of the grid voltage;

[0020] Step S42: Judge the grid operation state; when ε ≤ 4%, the grid operates normally, and the active and reactive reference values P * and Q * of the grid-connected inverter are the active and reactive reference values calculated by the following formula:

[0021]

[0022] In the formula, P set and Q set are the preset output active power and reactive power of the grid-connected inverter respectively;

[0023] When ε > 4%, the grid has an asymmetric fault, and the active and reactive reference values P * and Q * of the grid-connected inverter are the active and reactive reference values calculated by the following formula:

[0024]

[0025] In the formula, σ is a unified coefficient; in the balanced current mode, σ = 0; in the constant active power mode, σ = +1; in the constant reactive power mode, σ = -1; k ∈ [0, 1] is a power adjustment coefficient.

[0026] Further, the grid-connected current reference value is the grid-connected current reference value calculated by the following formula according to different control objectives, specifically as follows:

[0027] 1) Control objective I: Balanced current mode

[0028] The expression of the grid-connected current reference value is:

[0029]

[0030] In the formula, Control objective I realizes the balance of the three-phase grid-connected current of the inverter;

[0031] 2) Control Objective II: Active Power Constant Mode

[0032] The expression of the grid-connected current reference value is as follows:

[0033]

[0034] In the formula, Control Objective II realizes the constant active power output of the inverter;

[0035] 3) Control Objective III: Reactive Power Constant Mode

[0036] The expression of the grid-connected current reference value is as follows:

[0037]

[0038] Control Objective III realizes the constant reactive power output of the inverter.

[0039] Furthermore, the voltage value across the DC capacitor and is the voltage value across the DC capacitor calculated by the following formula:

[0040]

[0041] In the formula, the current i c1 (k) and i c2 (k) depend on the switching states of each leg of the grid-connected inverter and the DC-side input current i k at time t dc (k); T s is the sampling period.

[0042] The present invention has the following beneficial effects compared with the prior art:

[0043] The present invention improves the operation performance of the grid-connected system and ensures that the grid-connected current of the inverter is always less than the maximum allowable value under the grid unbalance fault. Description of the Drawings

[0044] Figure 1 is the main circuit structure diagram of the grid-connected inverter of the present invention;

[0045] Figure 2 is the model predictive control block diagram of the grid-connected inverter of the present invention;

[0046] Figure 3 is the method flow chart of the present invention;

[0047] Figure 4 is the control result diagram of the grid-connected current in the balanced current mode in the embodiment of the present invention;

[0048] Figure 5This is the control result diagram of the grid-connected current in the constant active power mode in the embodiments of the present invention;

[0049] Figure 6 This is the control result diagram of the grid-connected current in the constant reactive power mode in the embodiments of the present invention. Specific embodiments

[0050] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0051] Please refer to Figure 3 , the present invention provides a model predictive control method for a grid-connected inverter with current limitation, which specifically includes the following steps:

[0052] Step S1: As Figure 1 shown, establish the main circuit of a three-phase three-level NPC grid-connected inverter and select the main circuit parameters, including the equivalent DC power supply V dc output by the new energy distributed generation system is 700V, the DC side capacitors C1 and C2 are 4400 μF, the filter inductor L is 5 mH, and the grid voltage amplitude e is

[0053] Step S2: Collect the grid-connected current i(k), grid voltage e(k), and DC side capacitor voltages v c1 (k) and v c1 (k), and the DC side input current i dc (k) of the grid-connected inverter at the current moment;

[0054] Step S3: Use the decoupled double synchronous reference frame phase-locked loop (DDSRF-PLL) to detect the phase θ of the grid voltage and extract the positive and negative sequence components of the grid voltage in the double synchronous reference frame

[0055] Step S4: According to the obtained in step S3 and the maximum allowable value I max of the grid-connected current, calculate the active and reactive reference values P * and Q * of the grid-connected inverter in the balanced current mode, constant active power mode, and constant reactive power mode;

[0056] Step S5: According to the P * and Q * obtained in step S4, calculate the grid-connected current reference values in the balanced current mode, constant active power mode, and constant reactive power mode

[0057] Step S6: Combine the and and Perform the inverse Park transformation respectively to obtain the positive-sequence current reference value in the αβ coordinate system and the negative-sequence current reference value

[0058] Step S7: Add the obtained in step S6 to to obtain the current reference value in the αβ coordinate system and and synthesize the reference current vector i * ;

[0059] Step S8: Predict the voltage values at both ends of the DC capacitor and

[0060] Step S9: Feed the i * obtained in step S7 into the model predictive controller. The model predictive controller selects the optimal operation and applies it to the grid-connected inverter, so that the grid-connected current can accurately track the reference current.

[0061] In this embodiment, step S4 specifically includes the following steps:

[0062] Step S41: Express the grid voltage unbalance degree as:

[0063] wherein respectively represent the amplitudes of the positive and negative sequence components of the grid voltage;

[0064] Step S42: Judge the grid operation state; when ε ≤ 4%, the grid operates normally, and the active and reactive power reference values P * and Q * of the grid-connected inverter are the active and reactive power reference values calculated by the following formula:

[0065]

[0066] wherein P set and Q set are the preset output active power and reactive power of the grid-connected inverter respectively;

[0067] When ε > 4%, the grid has an asymmetric fault, and the active and reactive power reference values P * and Q * of the grid-connected inverter are the active and reactive power reference values calculated by the following formula:

[0068]

[0069] Wherein, σ is the unity coefficient; in the balanced current mode, σ = 0; in the constant active power mode, σ = +1; in the constant reactive power mode, σ = -1; k ∈ [0, 1] is the power regulation coefficient.

[0070] In this embodiment, the grid-connected current reference value described in step S5 is the grid-connected current reference value calculated by the following formula according to different control objectives:

[0071] 1) Control objective I: Balanced current mode

[0072] The expression of the grid-connected current reference value is:

[0073]

[0074] Wherein, Control objective I can achieve the balance of the three-phase grid-connected current of the inverter;

[0075] 2) Control objective II: Constant active power mode

[0076] The expression of the grid-connected current reference value is:

[0077]

[0078] Wherein, Control objective II can achieve the constancy of the active power output by the inverter;

[0079] 3) Control objective III: Constant reactive power mode

[0080] The expression of the grid-connected current reference value is:

[0081]

[0082] Control objective III can achieve the constancy of the reactive power output by the inverter.

[0083] In this embodiment, the voltage value across the DC capacitor described in step S5 and is the voltage value across the DC capacitor calculated by the following formula:

[0084]

[0085] Wherein, the current i c1 (k) and i c2 (k) depend on the switching states of each bridge arm of the grid-connected inverter at time t k and the DC-side input current i dc (k); T s is the sampling period.

[0086] In this embodiment, in step S9, the model predictive controller applies the model predictive control strategy based on the phase angle compensation method to the control of the grid-connected inverter.

[0087] In this embodiment, the output active power P of the grid-connected inverter is set set = 10 kW and the reactive power Q set = 0, and the maximum allowable value I of the grid-connected current max = 25 A, the power regulation coefficient k = 1, and the system sampling frequency f s is 40 kHz; it is set that during t = 0.2 s to t = 0.6 s, an asymmetrical voltage sag occurs in the power grid, and the unbalance degree ε = 40%. According to the three modes described in step S4, three simulations are respectively performed, and the results are as Figure 4 、 Figure 5 and Figure 6 . It can be seen that under the asymmetrical grid fault, the method of the present invention can always maintain the peak value of the grid-connected current within the safe threshold range, improving the reliability of the system operation.

[0088] The above are only the preferred embodiments of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the present invention.

Claims

1. A model predictive control method for grid-connected inverters with current limitation, characterized in that Including the following steps: Step S1: Construct the main circuit of a three-phase three-level neutral-point clamped grid-connected inverter; Step S2: Collect the grid-connected current \(i(k)\), grid voltage \(e(k)\), and DC-side capacitor voltages \(v\) c1 (k) and \(v\) c1 (k), the DC-side input current \(i\) dc (k) of the grid-connected inverter; Step S3: Detect the phase θ of the grid voltage using a decoupled double synchronous reference frame phase-locked loop, and extract the positive and negative sequence components of the grid voltage in the double synchronous reference frame Step S4: According to the and the maximum allowable value I of the grid-connected current max , calculate the active and reactive reference values P * and Q * of the grid-connected inverter in the balanced current mode, constant active power mode, and constant reactive power mode; Step S5: Based on P obtained in Step S4 * and Q * , calculate the grid-connected current reference values in the balanced current mode, constant active power mode, and constant reactive power mode Step S6: Perform the inverse Park transformation on the and and respectively to obtain the positive-sequence current reference value and the negative-sequence current reference value Step S7: Add the result obtained in Step S6 to to obtain the current reference value in the αβ coordinate system and and synthesize the reference current vector i * ; Step S8: Predict the voltage value across the DC capacitor using Ohm's law and Step S9: Feed the i obtained in Step S7 * into the model predictive controller, and the model predictive controller selects the optimal operation and applies it to the grid-connected inverter, so that the grid-connected current can accurately track the reference current.

2. The model predictive control method for a grid-connected inverter with current limitation according to claim 1, wherein, The main circuit of the three-phase three-level neutral-point clamped grid-connected inverter includes an equivalent DC power supply output by a new energy distributed generation system, a DC-side capacitor, a three-phase three-level NPC grid-connected inverter, a filter inductor L, and a power grid e, which are connected in sequence.

3. A model predictive control method for a grid-connected inverter with current limitation according to claim 1, characterized in that, The specific content of step S4 is as follows: Step S41: Represent the grid voltage unbalance degree as: wherein, respectively represent the amplitudes of the positive and negative sequence components of the grid voltage; Step S42: Determine the grid operation status; when ε ≤ 4%, the grid operates normally, and the active and reactive power reference values P * and Q * are the active and reactive power reference values calculated by the following formula: Where P set and Q set are respectively the preset active power and reactive power of the grid-connected inverter output; When ε > 4%, an asymmetrical fault occurs in the power grid, and the active and reactive reference values P * and Q * of the grid-connected inverter are the active and reactive reference values calculated by the following formula: Where σ is a unified coefficient; in the balanced current mode, σ = 0; in the constant active power mode, σ = +1; in the constant reactive power mode, σ = -1; k ∈ [0, 1] is a power regulation coefficient.

4. A model predictive control method for a grid-connected inverter with current limitation according to claim 1, characterized in that, The grid-connected current reference value is the grid-connected current reference value calculated by the following formula according to different control objectives, specifically as follows: 1) Control objective I: Balanced current mode The expression for the grid-connected current reference value is: In the formula, The control target I realizes the balance of the three-phase grid-connected current of the inverter; 2) Control objective II: Constant active power mode The expression for the grid-connected current reference value is: In the formula, Control objective II achieves a constant active power output of the inverter; 3) Control objective III: Constant reactive power mode The expression for the grid-connected current reference value is: Control objective III realizes the constant reactive power output of the inverter.

5. A model predictive control method for a grid-connected inverter with current limit according to claim 1, characterized in that, The voltage value across the DC capacitor and is the voltage value across the DC capacitor calculated by the following formula: where the current is i c1 (k) and i c2 (k) depend on t k At the moment, the switching states of each bridge arm of the grid-connected inverter and the DC-side input current i dc (k); T s is the sampling period.

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