Direct power prediction control method without grid voltage sensor based on MRAS

Abstract

The present invention discloses a direct power prediction control method without a grid voltage sensor based on MRAS, which adopts model reference adaptation (MRAS) to identify the grid-side inductance in real time, reduces the volume and cost of the system, and makes up for the excessive dependence of traditional model predictive control (MPC) on system parameters, so that the control is more stable and reliable, has stronger anti-interference ability to AC voltage, and has good harmonic characteristics; at the same time, the control method without a grid voltage sensor eliminates the grid-side voltage sensor, reduces the system volume, and saves system cost.

Description

Technical Field

[0001] The present invention relates to the technical field of power prediction of off-grid voltage sensors, and in particular to a direct power prediction control method of off-grid voltage sensors based on MRAS. Background Art

[0002] In recent years, three-phase voltage source rectifiers (VSRs) have been increasingly widely used in various industrial technology fields, such as high-voltage direct current transmission, uninterruptible power supplies, and new energy vehicles, due to their advantages such as high power factor, bidirectional energy flow, and controllable DC side current. Model predictive control (MPC), as an emerging control algorithm, has been widely used in many power electronic devices due to its simplicity, flexibility, and multi-input and multi-output characteristics. Among them, the finite control set model prediction (FCS-MPC) uses the mathematical model of the system to record all switch states, predict all possible values ​​at the next sampling time, and minimize the cost function to obtain the optimal switch state. Due to its simplicity and accuracy, this predictive control method is the most widely used in MPC.

[0003] The invention of patent number CN112994482A proposes a model prediction power dynamic compensation control method for a dual PWM frequency conversion system. Model prediction control is used on both sides of the rectifier-inverter. The input voltage vector and input current vector at the current moment are used to calculate the power prediction value at the next moment. Combined with the predicted current under rolling optimization and the ESO optimization estimation principle, the current value at the moment k+2 is predicted by rolling prediction to obtain the predicted value of the bus voltage at the moment k+2. On the basis of bus voltage tracking estimation, according to the principle of instantaneous energy balance on both sides, an energy flow balance equation is established to calculate the power compensation amount predicted in advance on the rectifier side and perform feedforward correction. This invention can suppress the bus voltage fluctuation problem caused by load mutation and system disturbance, and achieve coordinated improvement of the control performance on both sides. The invention with patent number CN112550024A proposes a three-phase PWM rectifier new energy vehicle charging method based on model prediction. Aiming at different charging requirements of users for new energy vehicles, including charging speed, charging economy, and charging battery damage, the optimal voltage curve is set by selecting the optimal index through a multi-objective algorithm. The on-board lithium battery and the new energy battery are charged at the same time, and the two batteries are monitored in real time to ensure the charging safety of the battery and meet the special charging needs of users. The obtained optimal voltage value is set as the DC side reference value, and adjusted according to the real-time feedback of the battery capacity. The optimal voltage value is obtained by the outer loop sliding mode control to obtain the rated active power value of the system. In order to reduce the loss caused by the number of changes in the grid-side switch, a switch coefficient table is designed according to the rated active power value to reduce the amount of calculation and thus improve the response speed. At the same time, the frequent changes of the switch are reduced to extend the life of the equipment.

[0004] However, since the mathematical model of the system is required for prediction, MPC is overly dependent on the accuracy of the system parameters. Inaccurate system parameters will lead to inaccurate prediction values. Therefore, it is very important to find a way to get the accurate values ​​of the system parameters. Grid-free voltage sensor rectification has many advantages over traditional VSR, such as reducing system size and cost. Voltage sensors may bring noise and interference, and the reliability of the entire system will be affected due to possible erroneous values. Summary of the invention

[0005] In view of the deficiencies in the prior art, the present invention provides a direct power prediction control method without grid voltage sensor based on MRAS, which makes up for the excessive dependence of traditional model predictive control (MPC) on system parameters, making the control more stable and reliable, with stronger anti-interference ability to AC voltage, and good harmonic characteristics. At the same time, the grid-side voltage sensor is omitted, the system volume is reduced, and the system cost is saved.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The embodiment of the present invention proposes a direct power prediction control method without a grid voltage sensor based on MRAS, and the control method comprises the following steps:

[0008] S1, collects the grid-side current i at the current sampling moment s (t), according to the rectifier output voltage V at the current sampling moment afe (t) and the estimated line voltage V s (t), adaptively estimate the grid-side inductance through model reference;

[0009] S2, collect the DC side voltage V at the current sampling time dc (t), according to the grid-side current i collected in step S1 s (t) and the switch state S at the current sampling time a (t), S b (t) and S c (t), calculate the grid line voltage at the current sampling moment;

[0010] S3, based on the collected grid-side current i s , the rectifier mathematical model is used to perform model prediction and obtain the value of the grid-side current at the next sampling time i s (t+1);

[0011] S4, through the predicted value i of the grid-side current in step S3 s (t+1) and the grid line voltage estimated in step S2, calculate the active power P(t+1) and reactive power Q(t+1) at the next sampling moment;

[0012] S5, given DC side reference voltage V * dc , and the actual DC side voltage V dc (t) is compared, and its error is multiplied by the DC side voltage after passing through the PI regulator to obtain the reference value of active power P * , and at the same time give the reference value of reactive power Q * ; Set the active power reference value P * , actual value of active power P(t), reference value of reactive power Q * The actual value of reactive power Q(t) is brought into the cost function, and all switch states are recorded to obtain the optimal switch state S that minimizes the cost function. a (t+1), S b (t+1) and S c (t+1), acting on the rectifier.

[0013] Furthermore, in step S1, the process of adaptively estimating the grid-side inductance by model reference includes the following sub-steps:

[0014] S11, collects grid-side current i s (t), and perform Clark transformation on it to obtain the network measured current i in the stationary two-phase αβ coordinate system sα (t) and i sβ (t), and the estimated grid-side voltage v s (t) and the rectifier output voltage v afe (t) Carry out clark change to obtain the grid-side voltage v in the static two-phase αβ coordinate system sα (t), v sβ (t) and the rectifier output voltage v afeα (t), v afeβ (t), calculate the estimated grid-side current:

[0015]

[0016] In the formula, represents the estimated value of the network current at time t+1 in the two-phase αβ coordinate system, represents the estimated value of the network current at time t in the two-phase αβ coordinate system; v sαβ (t) represents the grid-side voltage value at time t in the two-phase αβ coordinate system, v afeαβ (t) represents the rectifier output voltage value at time t in the two-phase αβ coordinate system, which is related to the switch state; R s represents the resistance value of the three-phase rectifier, Represents the estimated value of grid-side inductance; T s Represents the sampling time.

[0017] S12, subtract the estimated value of the grid-side current from the actual value of the grid-side current to obtain the difference e iα (t) and e iβ (t); the error e(t) between the actual model and the estimated model of the inductor is calculated according to the following formula:

[0018]

[0019] In the formula, and Represents the estimated grid current at the current moment;

[0020] S13, the error between the actual model of the inductor and the estimated model is passed through the PI regulator to obtain the estimated value of the grid-side inductance

[0021]

[0022] In the formula, K p and K I are the proportional and integral parameters of the PI regulator.

[0023] Furthermore, in step S2, the process of calculating the grid line voltage at this time includes the following sub-steps:

[0024] S21, collects the three-phase current i on the grid side s (t), and perform Clark transformation on it to obtain the network measured current i in the stationary two-phase coordinate system sα (t) and i sβ (t), collect the DC side voltage V dc (t), and the switch state S at the current sampling time a (t), S b (t), S c (t) and the estimated grid-side inductance The system's active power P(t) and reactive power Q(t) are calculated according to the following formula:

[0025]

[0026]

[0027] In the formula, R s Represents the resistance value of the three-phase rectifier, L s is the filter inductance of the three-phase rectifier, where S α (t), S β (t) is S a (t), S b (t), S c (t) is transformed to the switching state in the stationary two-phase coordinate system. It is calculated by the following formula:

[0028]

[0029] S22, according to the instantaneous power theory, the grid-side line voltage is calculated using the following formula:

[0030]

[0031] Further, in step S3, the value i of the grid-side current at the next sampling time is calculated using the following formula: s (t+1):

[0032]

[0033] In the formula, R s Represents the resistance value of the three-phase rectifier, L s is the filter inductance of the three-phase rectifier, v afe (t) represents the rectifier output voltage value at the current sampling moment, v s (t) represents the grid voltage value at the current sampling time, T s Represents the sampling time.

[0034] Furthermore, in step S4, the following formula is used to calculate the active power P(t+1) and reactive power Q(t+1) at the next sampling moment:

[0035] P(t+1)=v sα (t)i sα (t)+v sβ (t)i sβ (t)

[0036] Q(t+1)=v sβ (t)i sα (t)-v sα (t)i sβ (t).

[0037] Furthermore, the cost function is:

[0038] g=|P * -P(t+1)|+|Q * -Q(t+1)|;

[0039] Where P * is the active power given value, Q * is the reactive power given value.

[0040] The beneficial effects of the present invention are:

[0041] The MRAS-based direct power predictive control method without grid voltage sensor proposed in the present invention can be used in three-phase two-level voltage type rectifiers, which makes up for the excessive dependence of traditional model predictive control (MPC) on system parameters, making the control more stable and reliable, with stronger anti-interference ability to AC voltage and good harmonic characteristics. At the same time, the grid-side voltage sensor is omitted, the system volume is reduced, and the system cost is saved. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is the main circuit topology of three-phase voltage type rectifier VSR.

[0043] Figure 2 Schematic diagram of grid-side inductance estimation for model reference adaptive (MRAS).

[0044] Figure 3 It is the three-phase voltage source rectifier VSR sector and output voltage vector diagram.

[0045] Figure 4 It is a schematic diagram of a direct power prediction control method based on MRAS without grid voltage sensor according to an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The present invention will now be described in further detail with reference to the accompanying drawings.

[0047] It should be noted that the terms such as "upper", "lower", "left", "right", "front", "back", etc. cited in the invention are only for the convenience of description and are not used to limit the scope of implementation of the present invention. Changes or adjustments to their relative relationships should be regarded as the scope of implementation of the present invention without substantially changing the technical content.

[0048] In order to facilitate understanding of the technical solution, the symbols and numbers of this embodiment are first explained: s , R s - The filter inductance and resistance of the three-phase rectifier; v sk (k=a,b,c)-three-phase stationary coordinate system grid voltage; i s (i sa ,i sb ,i sc )-grid-side current of three-phase stationary coordinate system; i sα 、i sβ -Grid-side current of two-phase stationary coordinate system; R-DC-side load resistance; C-DC-side filter capacitor; V dc - Actual value of DC side voltage; V * dc -DC side voltage given value; I dc -DC side current; P * - Active power set value; Q *-Reactive power given value; S k (k=a,b,c)-switch state; v afe (v afeα , v afeβ )-rectifier output voltage value; -Estimated three-phase voltage on the grid side; - estimated value of grid-side inductance; i s (t+1)(i a ,i b ,i c )-three-phase grid-side current prediction value; P(t+1)-active power prediction value; Q(t+1)-reactive power prediction value; V a ,V b ,V c ,V' a ,V' b ,V' c -IGBT name; VD a ,VD b ,VD c ,VD' a ,VD' b ,VD' c - diode name; e iα ,e iβ - The error between the actual current and the estimated current on the grid side in the two-phase stationary coordinate system; the error between the actual model and the estimated model of the e-inductor; i sβ -Estimated grid-side current in two-phase stationary coordinate system; - estimated value of grid-side inductance; v 0 , v 1 ,v 2 ,v 3 ,v 4 ,v 5 ,v 6 ,v 7 -8 switching vectors on the three-phase two-level rectifier side.

[0049] See also Figure 4 , specifically follow the steps below:

[0050] Step 1: Collect the grid-side current i s , according to the rectifier output voltage V at this moment afe , Line voltage estimated value V s , the grid-side inductance is adaptively estimated through model reference.

[0051] Step 2: Collect the DC voltage V dc , according to the grid-side current i collected in step 1 s And the switch state S at the current sampling time (time t) a,S b ,S c Calculate the grid line voltage at this time.

[0052] Step 3: Based on the collected grid-side current i s , the rectifier mathematical model is used to perform model prediction and obtain the value i of the grid-side current at the next sampling moment (the t+1 moment) s (t+1).

[0053] Step 4: Predicted value of grid-side current i s (t+1) and the estimated line voltage, the active power P(t+1) and reactive power Q(t+1) at the next sampling moment can be calculated.

[0054] Step 5: Given the DC side reference voltage V * dc , and the actual DC side voltage V dc For comparison, the error passes through the PI regulator and then multiplied by the DC side voltage to obtain the reference value P of active power. * , and at the same time give the reference value of reactive power Q * . Set the active power reference value P * , actual value of active power P, reference value of reactive power Q * , the actual value of reactive power Q is brought into the cost function, and all switch states are recorded to obtain the optimal switch state S a ,S b ,S c , and acts on the rectifier.

[0055] Figure 1 It is the main circuit topology of three-phase voltage source rectifier VSR. Figure 1 , step 1 is as follows:

[0056] Collect grid-side current i s , and perform Clark transformation on it to obtain the network measured current i in the static two-phase αβ coordinate system α ,i β , and the estimated grid-side voltage v s and the rectifier output voltage v afe Perform clark transformation to get v sα ,v sβ ,v afeα ,v afeβ , then the estimated grid-side current can be calculated:

[0057]

[0058] In the formula, '-' represents an estimated value. Rectifier output voltage value v afeαβ As shown in Table 1.

[0059] Table 1 Switching state and v afeαβ Value relationship table

[0060]

[0061] See also Figure 2 , subtract the estimated grid-side current from the actual grid-side current to get e iα and e iβ From the application of Lyapunov's theorem to the stability of the system, it can be seen that the error between the actual model and the estimated model of the inductor is:

[0062]

[0063] In the formula, and Represents the estimated grid current.

[0064] The error between the actual model of the inductor and the estimated model is converted into the estimated value of the grid-side inductor through the PI regulator, which is:

[0065]

[0066] In the formula, K p and K I are the proportional and integral parameters of the PI regulator.

[0067] Step 2 is as follows:

[0068] Collect the three-phase current i on the grid side s , perform Clark transformation on it, and obtain the network measured current i in the stationary two-phase coordinate system α ,i β , collect the DC side voltage V dc , and the switch state S at this moment a ,S b ,S c and the estimated grid-side inductance Calculate the active power P and reactive power Q of the system at this time:

[0069]

[0070]

[0071] According to the instantaneous power theory, the grid-side line voltage is:

[0072]

[0073] Step 3 is as follows:

[0074] According to the collected grid-side three-phase current i sThe mathematical model of the system can predict the grid-side current forecast value at the next sampling moment:

[0075]

[0076] Step 4 is as follows:

[0077] Based on the estimated grid-side voltage and the predicted grid-side current of the next sampling period, the active power P(t+1) and reactive power Q(t+1) of the system at the next moment can be calculated:

[0078] P(t+1)=v sα i sα +v sβ i sβ

[0079] Q(t+1)=v sβ i sα -v sα i sβ .

[0080] Step 5 is as follows:

[0081] The reference value of the given DC side voltage is compared with the actual value of the DC side voltage. The difference passes through the PI regulator and is multiplied by the actual value of the DC side to obtain the active power given value P. * , while given reactive power Q * . Set the active power given value P * , reactive power given value Q * The active power P(t+1) and reactive power Q(t+1) of the system at the next moment are brought into the cost function:

[0082] g=|P * -P(t+1)|+|Q * -Q(t+1)|.

[0083] Go through each switch state (as shown in Table 1) and find the switch state S that minimizes the cost function. a ,S b ,S c , and acts on the rectifier.

[0084] For a three-phase two-level rectifier, there are 8 switching states, and its vector diagram is as follows: Figure 3 As shown. Go through each switch state and find the switch state S that minimizes the cost function. a ,S b ,S c , and acts on the rectifier.

[0085] The above are only preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should be regarded as the protection scope of the present invention.

Claims

1. A direct power prediction control method without grid voltage sensor based on MRAS, characterized in that: The control method comprises the following steps: S1, collects the grid-side current i at the current sampling moment s (t), according to the rectifier output voltage V at the current sampling moment afe (t) and the estimated line voltage V s (t), adaptively estimate the grid-side inductance through model reference; S2, collect the DC side voltage V at the current sampling time dc (t), according to the grid-side current i collected in step S1 s (t) and the switch state S at the current sampling time a (t), S b (t) and S c (t), calculate the grid line voltage at the current sampling moment; S3, based on the collected grid-side current i s (t), the rectifier mathematical model is used to perform model prediction to obtain the value of the grid-side current at the next sampling time i s (t+1); S4, through the predicted value i of the grid-side current in step S3 s (t+1) and the grid line voltage estimated in step S2, calculate the active power P(t+1) and reactive power Q(t+1) at the next sampling moment; S5, given DC side reference voltage V * dc , and the actual DC side voltage V dc (t), and its error is multiplied by the DC side voltage after passing through the PI regulator to obtain the reference value P* of active power, and the reference value Q* of reactive power is given at the same time; the active power reference value P*, the active power P(t+1) at the next sampling moment, the reactive power reference value Q* and the reactive power Q(t+1) at the next sampling moment are brought into the cost function, and all the switching states are recorded to obtain the optimal switching state S that minimizes the cost function. a (t+1), S b (t+1) and S c (t+1), acting on the rectifier; In step S1, the process of adaptively estimating the grid-side inductance by model reference includes the following sub-steps: S11, collects grid-side current i s (t), and perform Clark transformation on it to obtain the network measured current i in the stationary two-phase αβ coordinate system sα (t) and i sβ (t), and the estimated grid-side voltage v s (t) and the rectifier output voltage v afe (t) Carry out clark change to obtain the grid-side voltage v in the static two-phase αβ coordinate system sα (t), v sβ (t) and the rectifier output voltage v afeα (t), v afeβ (t), calculate the estimated grid-side current: In the formula, represents the estimated value of the network current at time t+1 in the two-phase αβ coordinate system, represents the estimated value of the network current at time t in the two-phase αβ coordinate system; v sαβ (t) represents the grid-side voltage value at time t in the two-phase αβ coordinate system, v afeαβ (t) represents the rectifier output voltage value at time t in the two-phase αβ coordinate system, which is related to the switch state; R s represents the resistance value of the three-phase rectifier, Represents the estimated value of grid-side inductance; T s represents the sampling time; S12, subtract the estimated value of the grid-side current from the actual value of the grid-side current to obtain the difference e iα (t) and e iβ (t); the error e(t) between the actual model and the estimated model of the inductor is calculated according to the following formula: In the formula, and Represents the estimated grid current at the current moment; S13, the error between the actual model of the inductor and the estimated model is passed through the PI regulator to obtain the estimated value of the grid-side inductance In the formula, K p and K I are the proportional and integral parameters of the PI regulator.

2. The MRAS-based direct power prediction control method without grid voltage sensor according to claim 1, characterized in that: In step S2, the process of calculating the grid line voltage at this time includes the following sub-steps: S21, collects the three-phase current i on the grid side s (t), and perform Clark transformation on it to obtain the network measured current i in the stationary two-phase coordinate system sα (t) and i sβ (t), collect the DC side voltage V dc (t), and the switch state S at the current sampling time a (t), S b (t), S c (t) and the estimated grid-side inductance The system's active power P(t) and reactive power Q(t) are calculated according to the following formula: In the formula, R s Represents the resistance value of the three-phase rectifier, L s is the filter inductance of the three-phase rectifier, where S α (t), S β (t) is S a (t), S b (t), S c (t) is transformed to the switching state in the stationary two-phase coordinate system; it is calculated by the following formula: S22, according to the instantaneous power theory, the grid-side line voltage is calculated using the following formula:

3. The MRAS-based direct power prediction control method without grid voltage sensor according to claim 1, characterized in that: In step S3, the value i of the grid-side current at the next sampling time is calculated using the following formula: s (t+1): In the formula, R s Represents the resistance value of the three-phase rectifier, L s is the filter inductance of the three-phase rectifier, v afe (t) represents the rectifier output voltage value at the current sampling moment, v s (t) represents the grid voltage value at the current sampling time, T s Represents the sampling time.

4. The MRAS-based direct power prediction control method without grid voltage sensor according to claim 1, characterized in that: In step S4, the following formula is used to calculate the active power P(t+1) and reactive power Q(t+1) at the next sampling moment: P(t+1)=v sα (t)i sα (t+1)+v sβ (t)i sβ (t+1) Q(t+1)=v sβ (t)i sα (t+1)-v sα (t)i sβ (t+1)。 5. The MRAS-based direct power prediction control method without grid voltage sensor according to claim 1, characterized in that: The cost function is: g=|P * -P(t+1)|+|Q * -Q(t+1)|; Where P * is the active power given value, Q * is the reactive power given value.

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