Model Predictive and Zero-Sequence Voltage Balancing Control Method for Three-Phase Five-Level PWM Rectifier

Abstract

The present invention discloses a model prediction and zero-sequence voltage balance control method for a three-phase five-level PWM rectifier. The control method comprises adopting voltage outer loop PI control, voltage inner loop model prediction control, space vector modulation and switching sequence design, and a zero-sequence voltage injection method to control the three-phase five-level PWM rectifier, thereby achieving grid-side unity power factor operation of the three-phase five-level PWM rectifier, low grid-side current harmonics, and balanced and stable DC-side output voltage.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular to a three-phase five-level PWM rectifier model prediction and zero-sequence voltage balance control method. Background Art

[0002] Compared to traditional multi-level rectifier topologies, the new three-phase, five-level PWM rectifier offers advantages such as a reduced number of switches and the absence of capacitor voltage buildup, thus holding broad development prospects. Numerous mature control methods exist for PWM rectifier control strategies, including hysteresis current control, transient current control, proportional resonant control, and dq coordinate PI current decoupling control. Hysteresis current control offers simple implementation and fast system response, but its variable switching frequency results in a wide distribution of current harmonics, hindering filter design. Transient current control can maintain a fixed switching frequency, but due to the use of a PI controller in the inner loop, steady-state errors may occur. Proportional resonant control can effectively eliminate steady-state current errors, but the control effect is highly sensitive to grid-side voltage variations. dq coordinate PI current decoupling control utilizes a conventional PI controller, which is simple to implement but struggles to achieve ideal control results, resulting in poor system dynamic performance.

[0003] In recent years, model predictive control (MPC) has been increasingly applied to the power electronics field. As a digital control technology, MPC has broad application prospects due to its inherent advantages. It is easy to implement, flexible in control, and can replace traditional proportional-integral controllers while enabling multi-objective optimization control. However, when applied to three-phase, five-level rectifiers, traditional MPC suffers from drawbacks such as numerous switching vectors, complex selection, difficult control system design, and high computational complexity. Furthermore, in three-phase rectifiers without a neutral line, variations in line parameters, phase switch parameters, and similar load impedances can lead to interphase power flow, resulting in imbalanced DC voltage and grid current, the presence of negative-sequence fundamental components, and increased harmonic content, polluting the power grid. Summary of the Invention

[0004] To address the shortcomings of novel three-phase, five-level PWM rectifier control technologies, a model prediction and zero-sequence voltage balance control method for a three-phase, five-level PWM rectifier is proposed. This method uses a three-phase, five-level PWM rectifier as the control object. By employing a PI controller for the voltage outer loop and an improved model predictive control for the current inner loop, combined with space vector modulation and switching sequence design, and a zero-sequence voltage injection method, the method achieves unity power factor operation on the grid side of the three-phase, five-level PWM rectifier, low grid-side current harmonics, and balanced and stable DC output voltage.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A model prediction and zero-sequence voltage balance control method for a three-phase five-level PWM rectifier, the control method comprising adopting voltage outer loop PI control, voltage inner loop model prediction control, space vector modulation and switching sequence design, and zero-sequence voltage injection method to control the three-phase five-level PWM rectifier, thereby achieving grid-side unity power factor operation of the three-phase five-level PWM rectifier, low grid-side current harmonics, and stable and balanced DC-side output voltage. The control method specifically comprises the following steps:

[0007] Establishing a mathematical model of the main circuit topology of the three-phase five-level PWM rectifier in the abc coordinate system;

[0008] Performing dq synchronous rotating coordinate system conversion to establish a state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system;

[0009] The three-phase five-level PWM rectifier adopts voltage outer loop PI control, and the error between the DC side voltage of the three-phase five-level PWM rectifier and the given reference value is used through the voltage outer loop PI controller to obtain the inner loop current d-axis current reference signal in the dq synchronous rotating coordinate system. And the q-axis current reference signal Under unity power factor control, the preferred

[0010] The inner loop current d-axis current reference signal And the q-axis current reference signal As the input signal, according to the state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system, a current inner loop improved model predictive controller is designed, and the three-phase five-level PWM rectifier adopts the current inner loop improved model predictive control. The current inner loop improved model predictive control includes obtaining the modulation function m in the dq synchronous rotating coordinate system through discretization operation, equation transformation operation, and construction of evaluation function. d 、m q And the modulation function m in the abc coordinate system a 、m b 、m c ;

[0011] A zero-sequence voltage injection method is used to eliminate the influence of negative-sequence current and balance and stabilize the DC side output voltage. The zero-sequence voltage injection method includes: using the detected output voltage of each phase on the DC side of the three-phase five-level rectifier as the input control variable, establishing a mathematical model of the power of each phase of the three-phase five-level rectifier after zero-sequence voltage injection and a mathematical model of the power adjustment term, and obtaining the zero-sequence voltage amplitude and zero-sequence voltage phase injected into the three-phase five-level rectifier;

[0012] According to the zero-sequence voltage amplitude, a zero-sequence voltage time-domain mathematical model is obtained, and the zero-sequence voltage time-domain mathematical model is normalized to obtain a zero-sequence voltage normalization function, combined with the modulation function m in the abc coordinate system. a 、m b 、m c , sent to the modulation module to obtain the PWM modulation wave for controlling the switch tube of the three-phase five-level PWM rectifier, thereby completing the control of the three-phase five-level PWM rectifier.

[0013] Furthermore, according to the voltage-current law, the mathematical model of the main circuit topology of the three-phase five-level PWM rectifier in the abc coordinate system is shown in formula (1):

[0014]

[0015] In formula (1), u xin (x=a, b, c) represents the AC side input voltage of the three-phase five-level PWM rectifier, u a 、u b 、u c Indicates the three-phase grid-side voltage of the three-phase five-level PWM rectifier, i a 、i b 、i c represents the three-phase grid-side current of the three-phase five-level PWM rectifier, L represents the three-phase grid-side transmission line inductance of the three-phase five-level PWM rectifier, and R represents the three-phase grid-side equivalent resistance of the three-phase five-level PWM rectifier.

[0016] Furthermore, the state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system is shown in formula (2):

[0017]

[0018] In formula (2), u d 、u q They represent the d-axis component and q-axis component of the grid-side voltage of the three-phase five-level PWM rectifier, respectively. d 、i q They represent the d-axis component and q-axis component of the grid-side current of the three-phase five-level PWM rectifier, respectively. din 、u qin They represent the d-axis component and q-axis component of the input voltage of the three-phase five-level PWM rectifier respectively, and ω represents the angular velocity of the grid-side voltage.

[0019] Furthermore, a current inner loop improved model predictive controller is designed, and the specific steps of adopting the current inner loop improved model predictive control on the three-phase five-level PWM rectifier include:

[0020] S1: Discretize the state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system. The state equation after discretization is shown in formula (3):

[0021]

[0022] The sampling period of the discretization operation is T s ;

[0023] In formula (3), i d (k), i q (k) represents the value of the grid-side current in the kth cycle in the dq synchronous rotating coordinate system, i d (k+1), i q (k+1) represents the value of the grid-side current in the k+1th period in the dq synchronous rotating coordinate system, u din (k),u qin (k) is the value of the AC side input voltage in the kth cycle in the dq synchronous rotating coordinate system;

[0024] where u din (k),u qin (k) can also be expressed by formula (4) as follows:

[0025]

[0026] In formula (4), Indicates the given DC side voltage reference value when the voltage outer loop PI controller is controlling;

[0027] S2: Combining equations (3) and (4), the predicted grid-side current value of the k+1th cycle in the dq synchronous rotating coordinate system is obtained through equation transformation, as shown in equation (5):

[0028]

[0029] S3: Introduce weight parameters, construct an evaluation function, and obtain the modulation function in the dq synchronous rotating coordinate system. There are two ways to construct the evaluation function, including calculating the square of the current difference and the absolute value of the difference;

[0030] The evaluation function J is shown in formula (6):

[0031]

[0032] In formula (6), λ is a weight parameter. Under unity power factor control, λ = 1 is preferred.

[0033] Let the evaluation function J be used to evaluate the modulation function m in the dq synchronous rotating coordinate system. d 、m qThe result of partial derivative is 0, as shown in formula (7):

[0034]

[0035] Solve equations (5), (6), and (7) together, as shown in equation (8), to obtain the modulation function m in the dq synchronous rotating coordinate system: d 、m q :

[0036]

[0037] S4: Perform an inverse transformation on the modulation function in the dq synchronous rotating coordinate system, as shown in formula (9), to obtain the three-phase modulation function m in the abc coordinate system a 、m b 、m c :

[0038]

[0039] Furthermore, the specific steps of obtaining the zero-sequence voltage amplitude and zero-sequence voltage phase injected into the three-phase five-level rectifier include:

[0040] S1: Assume the zero-sequence voltage to be injected. Assume that the injected zero-sequence voltage is as shown in equation (10):

[0041] u z =U z sin(ωt+θ z ) (10)

[0042] In formula (10), U z represents the zero-sequence voltage amplitude, ω represents the zero-sequence voltage angular velocity, θ z Indicates the zero-sequence voltage phase;

[0043] S2: Establish a mathematical model of the power of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected. The power of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected is p a 、p b 、p c As shown in formula (11):

[0044]

[0045] In formula (11), I p 、U p are the effective values of the positive sequence components of current and voltage, is the grid-side current phase, θ p is the grid-side voltage phase.

[0046] S3: Establish a mathematical model of the power regulation term of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected. The power regulation term Δp of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected is: a ,Δp b ,Δp c As shown in formula (12):

[0047]

[0048] The power deviation of each phase is calculated according to the law of conservation of energy and the DC side voltage deviation. The power adjustment term Δp of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected is: a ,Δp b ,Δp c It can also be shown as formula (13):

[0049]

[0050] In formula (13), U dc Indicates the DC voltage reference value, U dc1 Indicates the DC side voltage value of the a-phase PWM rectifier in the three-phase five-level rectifier, U dc2 Indicates the DC side voltage value of the b-phase PWM rectifier in the three-phase five-level rectifier, K p1 , K p2 Respectively represent the calculation of power adjustment term Δp a ,Δp b The adjustment coefficient when

[0051] S4: Solve equations (13) and (14) together, as shown in equation (14), to obtain the injected zero-sequence voltage amplitude U z and zero sequence voltage phase θ z :

[0052]

[0053] S5: Substitute equation (14) into equation (10) to obtain the time-domain mathematical model of the zero-sequence voltage.

[0054] Furthermore, the modulation module includes using space vector modulation to sectorize the modulation function in the abc coordinate system and using a five-segment symmetrical distribution to design a switch sequence.

[0055] Furthermore, the sequence vector action order of each switching cycle in the five-segment symmetrical distribution design switching sequence can be expressed by formula (15):

[0056] V a1 →V b →V a2 →V b →V a1(15)

[0057] In formula (15), V a1 、V a2 Indicates different sequence vectors that can reach the same input level in the same sector, used to distinguish redundant switch states, V b is a sequence vector without redundancy, V a1 、V a2 and V b The three sequence vectors represent different switching states in different sectors, thus forming four different switching sequences;

[0058] In formula (15), the action time corresponding to the sequence vector of each switching cycle in the switching sequence can be expressed by formula (16):

[0059] T a1 / 2→T b / 2→T a2 →T b / 2→T a1 / 2 (16)

[0060] In formula (16), T a1 、T a2 Respectively represent the sequence vector V a1 、V a2 The action time, T a Represents the sequence vector V a1 、V a2 The total action time, T a1 =T a2 =T a / 2, T b Represents the sequence vector V b action time.

[0061] Each phase of the three-phase five-level rectifier has five different input levels, corresponding to eight different switching states. Combined with space vector sector control, it can be divided into four sectors: I, II, III, and IV. Each sector can select a set of optimal switching sequences, so four switching sequences can be defined. Each switching sequence contains the three switching states contained in the corresponding sector, of which there is one redundant switching state.

[0062] Furthermore, the specific steps of sending the PWM modulation wave to the modulation module to obtain the PWM modulation wave for controlling the switch tube of the three-phase five-level PWM rectifier include:

[0063] S1: Sectorize the modulation function by space vector modulation, where the first sector contains U dc 、U dc / 2 two level states, from the input modulation function, according to the area equivalence principle, we can get:

[0064]

[0065] In formula (17), u xin (x=a,b,c) represents the abbreviation of the rectifier input side voltage, m x (x=a,b,c) represents the abbreviation of the modulation function in the abc coordinate system;

[0066] S2: Solve equation (15) to obtain the sequence vector V as shown in equation (18). a1 、V a2 and V b Required action time T a1 、T a2 、T b for:

[0067]

[0068] S3: Repeat S1 and S2 to calculate the action time of the remaining three sector sequence vectors;

[0069] S4: The switching sequences of different sectors represent different PWM wave states and orders. By using the above-set switching sequence and combining it with the action time calculation corresponding to the sequence vector in formula (18), PWM modulation wave generation can be completed to complete the control of the switch tube.

[0070] Compared with the existing technology, the present invention has the following beneficial effects:

[0071] 1. An improved model predictive control is used for the inner current loop of the control system, combined with space vector modulation and switching sequence design. This solves the problems of multiple switching states, difficult redundant vector selection, and complex control system design in the three-phase five-level PWM rectifier. It achieves unity power factor operation on the grid side of the three-phase five-level PWM rectifier and low grid-side current harmonics.

[0072] 2. The zero-sequence voltage injection method is used to regulate the inter-phase power. This solves the problems of inter-phase power flow and DC-side voltage imbalance caused by differences in switch tube parameters, line impedance parameters, equivalent impedance of similar loads, and the complexity of grid operation in the actual operation of three-phase five-level rectifiers. It eliminates the negative sequence current on the grid side and balances the grid-side current and DC-side voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 This is the overall control flow chart of the present invention;

[0074] Figure 2 This is the block diagram for the model predictive control;

[0075] Figure 3 This is the block diagram for zero-sequence voltage injection calculation;

[0076] Figure 4 Sector division diagram for modulation function;

[0077] Figure 5 This is the five-level waveform diagram of the AC side of the three-phase five-level PWM rectifier;

[0078] Figure 6 This is the grid-side unity power factor waveform of the three-phase five-level PWM rectifier;

[0079] Figure 7 Grid-side current waveform of three-phase five-level PWM rectifier under different modulation conditions

[0080] Figure 8 The harmonic analysis diagram of the proposed modulation method and SPWM modulation grid-side current;

[0081] Figure 9 This is the DC side voltage waveform when the load is unbalanced;

[0082] Figure 10 This is the grid-side current waveform when the load is unbalanced;

[0083] Figure 11 This is the DC side voltage waveform after zero-sequence voltage is injected;

[0084] Figure 12 This is the grid-side current waveform after zero-sequence voltage is injected. DETAILED DESCRIPTION

[0085] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts, any modifications, equivalent substitutions, improvements, etc., shall be included in the scope of protection of the present invention.

[0086] like Figure 1 As shown, the model prediction and zero-sequence voltage balance control method of the three-phase five-level PWM rectifier includes using voltage outer loop PI control, voltage inner loop model prediction control, space vector modulation and switching sequence design, and zero-sequence voltage injection method to control the three-phase five-level PWM rectifier to achieve the unity power factor operation of the PWM rectifier on the grid side, low harmonics of the grid side current, and balanced and stable DC side output voltage. The control method specifically includes the following steps:

[0087] Establishing a mathematical model of the main circuit topology of the three-phase five-level PWM rectifier in the abc coordinate system;

[0088] Performing dq synchronous rotating coordinate system conversion to establish a state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system;

[0089] The three-phase five-level PWM rectifier adopts voltage outer loop PI control, and the error between the DC side voltage of the three-phase five-level PWM rectifier and the given reference value is used through the voltage outer loop PI controller to obtain the inner loop current d-axis current reference signal in the dq synchronous rotating coordinate system. And the q-axis current reference signal Under unity power factor control, the preferred

[0090] The inner loop current d-axis current reference signal And the q-axis current reference signal As the input signal, according to the state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system, a current inner loop improved model predictive controller is designed, and the three-phase five-level PWM rectifier adopts the current inner loop improved model predictive control. The current inner loop improved model predictive control includes obtaining the modulation function m in the dq synchronous rotating coordinate system through discretization operation, equation transformation operation, and construction of evaluation function. d 、m q And the modulation function m in the abc coordinate system a 、m b 、m c ;

[0091] A zero-sequence voltage injection method is used to eliminate the influence of negative-sequence current and balance and stabilize the DC side output voltage. The zero-sequence voltage injection method includes: using the detected output voltage of each phase on the DC side of the three-phase five-level rectifier as an input control variable, obtaining the zero-sequence voltage amplitude and zero-sequence voltage phase injected into the three-phase five-level rectifier, and establishing a mathematical model of the power of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected and a mathematical model of the power regulation term;

[0092] According to the zero-sequence voltage amplitude, a zero-sequence voltage time-domain mathematical model is obtained, and the zero-sequence voltage time-domain mathematical model is normalized to obtain a zero-sequence voltage normalization function, combined with the modulation function m in the abc coordinate system. a 、m b 、m c , sent to the modulation module to obtain the PWM modulation wave for controlling the switch tube of the three-phase five-level PWM rectifier, thereby completing the control of the three-phase five-level PWM rectifier.

[0093] According to the voltage-current law, the mathematical model of the main circuit topology of the three-phase five-level PWM rectifier in the abc coordinate system is shown in formula (1):

[0094]

[0095] In formula (1), u xin (x=a, b, c) represents the AC side input voltage of the three-phase five-level PWM rectifier, u a 、u b 、u c Indicates the three-phase grid-side voltage of the three-phase five-level PWM rectifier, i a 、i b 、i c represents the three-phase grid-side current of the three-phase five-level PWM rectifier, L represents the three-phase grid-side transmission line inductance of the three-phase five-level PWM rectifier, and R represents the three-phase grid-side equivalent resistance of the three-phase five-level PWM rectifier.

[0096] The state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system is shown in formula (2):

[0097]

[0098] In formula (2), u d 、u q They represent the d-axis component and q-axis component of the grid-side voltage of the three-phase five-level PWM rectifier, respectively. d 、i q They represent the d-axis component and q-axis component of the grid-side current of the three-phase five-level PWM rectifier, respectively. din 、u qin They represent the d-axis component and q-axis component of the input voltage of the three-phase five-level PWM rectifier respectively, and ω represents the angular velocity of the grid-side voltage.

[0099] like Figure 2 As shown, the inner loop current d-axis current reference signal And the q-axis current reference signal As an input signal, according to the state equation (2) of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system, a current inner loop improved model predictive controller is designed, and the three-phase five-level PWM rectifier is controlled by the current inner loop improved model predictive control. The specific steps of controlling the three-phase five-level PWM rectifier by the current inner loop improved model predictive control include:

[0100] S1: Discretize the state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system. The state equation after discretization is shown in formula (3):

[0101]

[0102] The sampling period of the discretization operation is T s ;

[0103] In formula (3), i d (k), i q (k) represents the value of the grid-side current in the kth cycle in the dq synchronous rotating coordinate system, i d (k+1), i q (k+1) represents the value of the grid-side current in the k+1th period in the dq synchronous rotating coordinate system, u din (k),u qin (k) is the value of the AC side input voltage in the kth cycle in the dq synchronous rotating coordinate system;

[0104] where u din (k),u qin (k) can also be expressed by formula (4) as follows:

[0105]

[0106] In formula (4), Indicates the given DC side voltage reference value when the voltage outer loop PI controller is controlling;

[0107] S2: Combining equations (3) and (4), the predicted grid-side current value of the k+1th cycle in the dq synchronous rotating coordinate system is obtained through equation transformation, as shown in equation (5):

[0108]

[0109] S3: Introduce weight parameters, construct an evaluation function, and obtain the modulation function in the dq synchronous rotating coordinate system. There are two ways to construct the evaluation function, including calculating the square of the current difference and the absolute value of the difference;

[0110] The evaluation function J is shown in formula (6):

[0111]

[0112] In formula (6), λ is a weight parameter. Under unity power factor control, λ = 1 is preferred.

[0113] Let the evaluation function J be used to evaluate the modulation function m in the dq synchronous rotating coordinate system. d 、m q The result of partial derivative is 0, as shown in formula (7):

[0114]

[0115] Solve equations (5), (6), and (7) together, as shown in equation (8), to obtain the modulation function m in the dq synchronous rotating coordinate system: d 、mq :

[0116]

[0117] S4: Perform an inverse transformation on the modulation function in the dq synchronous rotating coordinate system, as shown in formula (9), to obtain the three-phase modulation function m in the abc coordinate system a 、m b 、m c :

[0118]

[0119] When the three-phase grid-side current of the three-phase five-level PWM rectifier is unbalanced, the grid-side current can be decomposed into positive and negative sequences for analysis. The grid-side power of any phase of the corresponding three-phase five-level PWM rectifier can be expressed as: P = P p +P n , where P p It is expressed as the positive sequence of the grid side voltage and current when unbalanced, P n Indicates the power generated by the negative sequence of the grid-side voltage and current when it is unbalanced, P n This causes three-phase power deviation, resulting in power imbalance on the three-phase grid side. In the unbalanced situation, the present invention adopts the zero-sequence voltage injection method to perform phase-to-phase power control.

[0120] Use P z It represents the regulated power generated after the zero-sequence voltage is injected, compensating for the power imbalance of each phase caused by the unbalanced load. After the zero-sequence voltage is injected and the system stabilizes again, the grid-side power output only contains P p part,

[0121] P n =P z , thereby eliminating the influence of negative sequence current.

[0122] like Figure 3 As shown, the output voltage U of each phase of the three-phase five-level rectifier DC side is detected dc1 、U dc2 As an input control variable, the zero-sequence voltage injection method is used to balance the DC side output voltage, and the specific steps of obtaining the zero-sequence voltage amplitude and zero-sequence voltage phase injected into the three-phase five-level rectifier include:

[0123] S1: Assume the zero-sequence voltage to be injected. Assume that the injected zero-sequence voltage is as shown in equation (10):

[0124] u z =U z sin(ωt+θ z ) (10)

[0125] In formula (10), Uz represents the zero-sequence voltage amplitude, ω represents the zero-sequence voltage angular velocity, θ z Indicates the zero-sequence voltage phase;

[0126] S2: Establish a mathematical model of the power of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected. The power of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected is p a 、p b 、p c As shown in formula (11):

[0127]

[0128] In formula (11), I p 、U p are the effective values of the positive sequence components of current and voltage, is the grid-side current phase, θ p is the grid-side voltage phase.

[0129] S3: Establish a mathematical model of the power regulation term of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected. The power regulation term Δp of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected is: a ,Δp b ,Δp c As shown in formula (12):

[0130]

[0131] The power deviation of each phase is calculated according to the law of conservation of energy and the DC side voltage deviation. The power adjustment term Δp of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected is: a ,Δp b ,Δp c It can also be shown as formula (13):

[0132]

[0133] In formula (13), U dc Indicates the DC voltage reference value, U dc1 Indicates the DC side voltage value of the a-phase PWM rectifier in the three-phase five-level rectifier, U dc2 Indicates the DC side voltage value of the b-phase PWM rectifier in the three-phase five-level rectifier, K p1 , K p2 Respectively represent the calculation power adjustment term Δp a ,Δp b The adjustment coefficient when

[0134] S4: Solve equations (13) and (14) together, as shown in equation (14), to obtain the injected zero-sequence voltage amplitude U zand zero sequence voltage phase θ z :

[0135]

[0136] S5: Substitute equation (14) into equation (10) to obtain the time-domain mathematical model of the zero-sequence voltage.

[0137] like Figure 4 As shown, it is a diagram of sector division of the modulation function. The modulation module includes using space vector modulation to sectorize the modulation function in the abc coordinate system and using a five-segment symmetrical distribution to design a switch sequence.

[0138] The sequence vector action order of each switching cycle in the five-segment symmetrical distribution design switching sequence can be expressed by formula (15):

[0139] V a1 →V b →V a2 →V b →V a1 (15)

[0140] In formula (15), V a1 、V a2 Indicates different sequence vectors that can reach the same input level in the same sector, used to distinguish redundant switch states, V b is a sequence vector without redundancy, V a1 、V a2 and V b The three sequence vectors represent different switching states in different sectors, thus forming four different switching sequences;

[0141] In formula (15), the action time corresponding to the sequence vector of each switching cycle in the switching sequence can be expressed by formula (16):

[0142] T a1 / 2→T b / 2→T a2 →T b / 2→T a1 / 2 (16)

[0143] In formula (16), T a1 、T a2 Respectively represent the sequence vector V a1 、V a2 The action time, T a Represents the sequence vector V a1 、V a2 The total action time, T a1 =T a2 =T a / 2, Tb Represents the sequence vector V b action time.

[0144] Each phase of the three-phase five-level rectifier has five different input levels, corresponding to eight different switching states. Combined with space vector sector control, it can be divided into four sectors: I, II, III, and IV. Each sector can select a set of optimal switching sequences, so four switching sequences can be defined. Each switching sequence contains the three switching states contained in the corresponding sector, of which there is one redundant switching state.

[0145] The specific steps of sending the PWM modulation wave to the modulation module to obtain the PWM modulation wave for controlling the switch tube of the three-phase five-level PWM rectifier include:

[0146] S1: Sectorize the modulation function by space vector modulation, where the first sector contains U dc 、U dc / 2 two level states, from the input modulation function, according to the area equivalence principle, we can get:

[0147]

[0148] In formula (15), u xin (x=a,b,c) represents the abbreviation of the rectifier input side voltage, m x (x=a,b,c) represents the abbreviation of the modulation function in the abc coordinate system;

[0149] S2: Solve equation (17) to obtain the sequence vector V as shown in equation (18). a1 、V a2 and V b Required action time T a1 、T a2 、T b for:

[0150]

[0151] S3: Repeat S1 and S2 to calculate the action time of the remaining three sector sequence vectors;

[0152] S4: The switching sequences of different sectors represent different PWM wave states and orders. By using the above-set switching sequence and combining it with the action time calculation corresponding to the sequence vector in formula (18), PWM modulation wave generation can be completed to complete the control of the switch tube.

[0153] In order to verify the correctness of the theoretical analysis and the feasibility of the control system, this paper built an experimental platform and conducted relevant experiments to verify the feasibility of the control algorithm.

[0154] like Figure 5As shown in FIG, the five-level waveform on the input side of the three-phase five-level PWM rectifier of the present invention is consistent with the theoretical analysis. Figure 6 As shown in FIG, the voltage and current waveforms of phases a and b of the three-phase five-level PWM rectifier of the present invention are shown. It is obvious that both phases a and b can achieve a rectification effect close to unity power factor. Figure 7 As shown, Figure 7 (a) and Figure 7 (b) respectively describes the waveforms of the three-phase grid-side current when SPWM modulation and the switching sequence modulation of the present invention are used. It is obvious that the current waveform when the switching sequence modulation is used is closer to the sine wave and has smaller fluctuations than that when the SPWM modulation is used. Figure 8 As shown, Figure 8 (a) and Figure 8 (b) The harmonic content of the grid-side current of the three-phase five-level PWM rectifier of the present invention is analyzed under two modulation strategies, SPWM modulation and switching sequence modulation of the present invention. It can be seen that the switching sequence modulation of the present invention can effectively reduce the harmonics of the grid-side current. The harmonics of the three-phase grid-side current are all controlled at about 4%, which is within the allowable range of the power grid.

[0155] This experiment simulates the differences in line impedance, switch tube parameters, and similar load impedance by using different load values. Figure 9 As shown in Figure 1, the DC side current waveform is described when the load is unbalanced without zero-sequence voltage injection. The experiment switches the load to three unequal resistance values at 1s. It can be seen that under the three resistance values, the DC side voltage fluctuates greatly, with fluctuations of about ±50V. This fluctuation will seriously affect the operation of the equipment. Figure 10 As shown in FIG, the grid-side current when the load is unbalanced without zero-sequence voltage injection is described. According to the reference straight line, it can be seen that the amplitudes of the three-phase grid-side currents are not equal and contain negative-sequence current components.

[0156] like Figure 11 and Figure 12 As shown in Figure 2, the experimental waveforms of DC side voltage and grid side current are described respectively when the inter-phase power balance control is realized after zero-sequence voltage injection. Figure 11 As shown in Figure 1, the DC side voltage waveform when zero-sequence voltage control is injected 0.5s after the load is unbalanced, it can be seen that after about 40ms, the DC side voltage reaches a balanced state again. Figure 12 As shown, the three-phase grid-side current and Figure 7 (b) Approximately, in a balanced state with no negative sequence current. This experiment was set up to inject zero sequence voltage 0.5s after the load becomes unbalanced, which can effectively balance and stabilize the DC side voltage.

[0157] The present invention designs an improved model predictive control method for a three-phase five-level PWM rectifier containing coupled inductors, which can effectively avoid a large number of optimization calculations and reduce the system's computational workload. In addition, compared with traditional SPWM modulation, the modulation method combining space vector modulation with switching sequence design can reduce the grid-side current distortion rate, THD content, and switching frequency.

[0158] In addition, the use of zero-sequence voltage injection to control phase-to-phase power can effectively avoid grid-side current imbalance and DC-side voltage imbalance caused by differences in line impedance, switch tube parameters, and similar load impedance, and is practical.

Claims

1. A three-phase five-level PWM rectifier model prediction and zero-sequence voltage balance control method, characterized in that: The control method includes using voltage outer loop PI control, current inner loop model predictive control, space vector modulation and switching sequence design, and zero-sequence voltage injection method to control the three-phase five-level PWM rectifier, so as to achieve the three-phase five-level PWM rectifier grid-side unity power factor operation, low grid-side current harmonics, and balanced and stable DC side output voltage. The control method specifically includes the following steps: Establishing a mathematical model of the main circuit topology of the three-phase five-level PWM rectifier in the abc coordinate system; Performing dq synchronous rotating coordinate system conversion to establish a state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system; The three-phase five-level PWM rectifier is controlled by a voltage outer loop PI control to obtain an inner loop current d-axis current reference signal and a q-axis current reference signal in a dq synchronous rotating coordinate system; Taking the inner loop current d-axis current reference signal and the q-axis current reference signal as input signals, and based on the state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system, design a current inner loop improved model predictive controller, and adopt the current inner loop improved model predictive control for the three-phase five-level PWM rectifier. The current inner loop improved model predictive control includes obtaining the modulation function in the dq synchronous rotating coordinate system and the modulation function in the abc coordinate system through discretization operation, equation transformation operation, and construction of evaluation function; A zero-sequence voltage injection method is used to eliminate the influence of negative-sequence current and balance and stabilize the DC side output voltage. The zero-sequence voltage injection method includes: using the detected output voltage of each phase on the DC side of the three-phase five-level PWM rectifier as the input control variable, establishing a mathematical model of the power of each phase of the three-phase five-level rectifier after zero-sequence voltage injection and a mathematical model of the power adjustment term, and obtaining the zero-sequence voltage amplitude and zero-sequence voltage phase injected into the three-phase five-level rectifier; A zero-sequence voltage time-domain mathematical model is obtained based on the zero-sequence voltage amplitude. The zero-sequence voltage time-domain mathematical model is normalized to obtain a zero-sequence voltage normalization function. The zero-sequence voltage normalization function is combined with the modulation function in the abc coordinate system and sent to a modulation module to obtain a PWM modulation wave for controlling the switching tube of the three-phase five-level PWM rectifier, thereby completing the control of the three-phase five-level PWM rectifier.

2. The three-phase five-level PWM rectifier model prediction and zero-sequence voltage balance control method according to claim 1, characterized in that: According to the voltage-current law, the mathematical model of the main circuit topology of the three-phase five-level PWM rectifier in the abc coordinate system is shown in formula (1): In formula (1), u ain 、u bin 、u cin Indicates the input voltage of phase a, phase b, and phase c on the AC side of the three-phase five-level PWM rectifier, u a 、u b 、u c Indicates the three-phase grid-side voltage of the three-phase five-level PWM rectifier, i a 、i b 、i c represents the three-phase grid-side current of the three-phase five-level PWM rectifier, L represents the three-phase grid-side transmission line inductance of the three-phase five-level PWM rectifier, and R represents the three-phase grid-side equivalent resistance of the three-phase five-level PWM rectifier.

3. The three-phase five-level PWM rectifier model prediction and zero-sequence voltage balance control method according to claim 2, characterized in that: The state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system is shown in formula (2): In formula (2), u d 、u q They represent the d-axis component and q-axis component of the grid-side voltage of the three-phase five-level PWM rectifier, respectively. d 、i q They represent the d-axis component and q-axis component of the grid-side current of the three-phase five-level PWM rectifier, respectively. din 、u qin They represent the d-axis component and q-axis component of the input voltage of the three-phase five-level PWM rectifier respectively, and ω represents the angular velocity of the grid-side voltage.

4. The three-phase five-level PWM rectifier model prediction and zero-sequence voltage balance control method according to claim 3, characterized in that: The specific steps of designing a current inner loop improved model predictive controller and adopting the current inner loop improved model predictive control for the three-phase five-level PWM rectifier include: S1: Discretize the state equation of the three-phase five-level PWM rectifier in the dq synchronous rotating coordinate system. The state equation after discretization is shown in formula (3): The sampling period of the discretization operation is T s ; In formula (3), i d (k), i q (k) represents the value of the grid-side current in the kth cycle in the dq synchronous rotating coordinate system, i d (k+1), i q (k+1) represents the value of the grid-side current in the k+1th period in the dq synchronous rotating coordinate system, u din (k),u qin (k) is the value of the AC side input voltage in the kth cycle in the dq synchronous rotating coordinate system; where u din (k),u qin (k) is expressed as follows by formula (4): In formula (4), Indicates the given DC side voltage reference value when the voltage outer loop PI controller is controlling; S2: Combining equations (3) and (4), the predicted grid-side current value of the k+1th cycle in the dq synchronous rotating coordinate system is obtained through equation transformation, as shown in equation (5): S3: Introduce weight parameters, construct evaluation function, and obtain modulation function in dq synchronous rotating coordinate system; The evaluation function J is shown in formula (6): In formula (6), λ is the weight parameter; The partial derivative of the evaluation function on the modulation function in the dq synchronous rotating coordinate system is set to 0, as shown in formula (7): Solve equations (5), (6), and (7) together, as shown in equation (8), to obtain the modulation function m in the dq synchronous rotating coordinate system: d 、m q : S4: Perform an inverse transformation on the modulation function in the dq synchronous rotating coordinate system, as shown in formula (9), to obtain the three-phase modulation function m in the abc coordinate system a 、m b 、m c :

5. The three-phase five-level PWM rectifier model prediction and zero-sequence voltage balance control method according to claim 4, characterized in that: The specific steps of obtaining the zero-sequence voltage amplitude and zero-sequence voltage phase injected into the three-phase five-level rectifier include: S1: Assume the zero-sequence voltage to be injected. Assume that the injected zero-sequence voltage is as shown in equation (10): you z =U z sin(ωt+θ z ) (10) In formula (10), U z represents the zero-sequence voltage amplitude, ω represents the zero-sequence voltage angular velocity, θ z Indicates the zero-sequence voltage phase; S2: Establish a mathematical model of the power of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected. The power of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected is p a 、p b 、p c As shown in formula (11): In formula (11), I p 、U p are the effective values of the positive sequence components of current and voltage, is the grid-side current phase, θ p is the grid-side voltage phase; S3: Establish a mathematical model of the power regulation term of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected. The power regulation term Δp of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected is: a ,Δp b ,Δp c As shown in formula (12): The power deviation of each phase is calculated according to the law of conservation of energy and the DC side voltage deviation. The power adjustment term Δp of each phase of the three-phase five-level rectifier after the zero-sequence voltage is injected is: a ,Δp b ,Δp c It can also be shown as formula (13): In formula (13), U dc Indicates the DC voltage reference value, U dc1 Indicates the DC side voltage value of the a-phase PWM rectifier in the three-phase five-level rectifier, U dc2 Indicates the DC side voltage value of the b-phase PWM rectifier in the three-phase five-level rectifier, K p1 , K p2 Respectively represent the calculation of power adjustment term Δp a ,Δp b The adjustment coefficient when S4: Solve equations (12) and (13) together, as shown in equation (14), to obtain the injected zero-sequence voltage amplitude U z and zero sequence voltage phase θ z : S5: Substitute equation (14) into equation (10) to obtain the time-domain mathematical model of the zero-sequence voltage.

6. The three-phase five-level PWM rectifier model prediction and zero-sequence voltage balance control method according to claim 5, characterized in that: The modulation module includes using space vector modulation to sectorize the modulation function in the abc coordinate system and using five-segment symmetrical distribution to design a switch sequence.

7. The three-phase five-level PWM rectifier model prediction and zero-sequence voltage balance control method according to claim 6, characterized in that: The sequence vector action order of each switching cycle in the five-segment symmetrical distribution design switching sequence can be expressed by formula (15): In a1 →V b →V a2 →V b →V a1 (15) In formula (15), V a1 、V a2 Indicates different sequence vectors that can reach the same input level in the same sector, used to distinguish redundant switch states, V b is a sequence vector without redundancy, V a1 、V a2 and V b The three sequence vectors represent different switching states in different sectors, thus forming four different switching sequences; In formula (15), the action time corresponding to the sequence vector of each switching cycle in the switching sequence can be expressed by formula (16): T a1 / 2→T b / 2→T a2 →T b / 2→T a1 / 2 (16) In formula (16), T a1 、T a2 Respectively represent the sequence vector V a1 、V a2 The action time, T a Represents the sequence vector V a1 、V a2 The total action time, T a1 =T a2 =T a / 2, T b Represents the sequence vector V b action time.

8. The three-phase five-level PWM rectifier model prediction and zero-sequence voltage balance control method according to claim 7, characterized in that: The specific steps of sending the PWM modulation wave to the modulation module to obtain the PWM modulation wave for controlling the switch tube of the three-phase five-level PWM rectifier include: S1: Sectorize the modulation function by space vector modulation, where the first sector contains U dc 、U dc / 2 two level states, from the input modulation function, according to the area equivalence principle, we can get: In formula (17), the three-phase modulation method is the same, u bin =m b ×U dc ,u cin =m c ×U dc , m a 、m b 、m c They represent the modulation functions of phase a, phase b, and phase c in the abc coordinate system respectively; S2: Solve equation (17) as shown in equation (18) to obtain the sequence vector V a1 、V a2 and V b Action time T a1 、T a2 、T b for: S3: Repeat S1 and S2 to calculate the action time of the remaining three sector sequence vectors; S4: By designing the switch sequence with the five-segment symmetrical distribution and combining the action time of the sequence vector in formula (18), the PWM modulation wave can be calculated to complete the control of the switch tube.

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