A three-phase four-leg converter VSG control method based on resonant sequence control

Abstract

The present invention relates to a three-phase four-bridge arm converter VSG control method based on resonant sequence control, which belongs to the field of new energy grid converter control technology. It is implemented by a coordinate transformation module, a positive and negative sequence separation module, an improved power calculation module, an improved VSG control module, and a voltage and current controller; a voltage and current controller is designed under an αβγ reference system, and an improved VSG control module is proposed. The instantaneous active and reactive power calculated by the positive sequence current is used as control feedback, and a positive sequence virtual impedance is introduced to improve system performance. Compared with the traditional method, this method simplifies the control structure of the system, reduces the frequency ripple under load imbalance conditions, and improves the negative sequence imbalance rate suppression rate by 89%.

Description

Technical Field

[0001] The invention relates to a three-phase four-bridge arm converter VSG control method based on resonance sequence control, belonging to the technical field of new energy grid converter control. Background Art

[0002] The development of renewable energy and power electronics technology has promoted the formation of microgrids. However, due to the intermittent and uncertain nature of renewable energy generation and the lack of inertia and damping in power electronic converters, the stability of isolated microgrids faces challenges. Virtual synchronous generators (VSGs) can simulate the characteristics of synchronous generators and help improve the stability of isolated microgrids.

[0003] In the actual power supply system, there are some single-phase loads and three-phase unbalanced loads, which easily lead to three-phase voltage imbalance. In recent years, the three-phase four-bridge arm topology is a converter structure that realizes effective power supply of unbalanced loads. It not only provides a neutral line loop, but also has the advantages of high DC voltage utilization, flexible control, small size and weight. In view of the above problems, the use of three-phase four-bridge arm topology for VSG control strategy is considered to be an ideal solution. However, the existing methods have defects, such as the control framework is only applicable to balanced load conditions and the number of controllers is large, and the converter output voltage is highly unbalanced; the double frequency component in the converter output power is ignored, resulting in the output voltage frequency containing double frequency components; the role of virtual impedance in current limiting, transient oscillation damping and small signal stability improvement is ignored. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a three-phase four-bridge-arm converter VSG control method based on resonant sequence control, which can improve the power quality of the output voltage, including frequency and three-phase imbalance.

[0005] Terminology explanation:

[0006] 1. abc reference system and αβγ reference system: Figure 7 is the abc reference system, where u a 、u b 、u c is the three-phase voltage. After Clarke transformation, the voltage value in the αβγ(αβ0) reference frame can be obtained, as Figure 8 As shown, the Clarke transformation matrix is

[0007]

[0008] 2. αβγ / abc module: It can realize the conversion from abc reference system to αβγ reference system. The formula is:

[0009]

[0010] 3. abc / αβγ module: It can realize the conversion from αβγ reference system to abc reference system. The formula is:

[0011]

[0012] The technical solution of the present invention is as follows:

[0013] A three-phase four-bridge-arm converter VSG control method based on resonant sequence control is implemented by a coordinate transformation module, a positive and negative sequence separation module, an improved power calculation module, an improved VSG control module, and a voltage and current controller;

[0014] The coordinate transformation module collects the abc three-phase load voltage u of the common connection point (PCC) of the three-phase four-bridge arm converter in real time. an ,u bn ,u cn And the three-phase load current i abc ,u an ,u bn ,u cn After being transformed into the voltage u in the αβγ reference frame by the αβγ / abc module αn ,u βn ,u γn ;i abc After being transformed into the current i in the αβγ reference frame by the αβγ / abc module αβγ ; The transformation formula can be expressed as:

[0015]

[0016] The positive and negative separation module is based on the current i in the αβγ reference frame αβγ The positive sequence current i in the αβγ reference frame is obtained αβ + , i.e. i α + and i β + , and then transformed by the abc / αβγ module to obtain the positive sequence three-phase current i abc + ;

[0017] The improved power calculation module is based on the collected abc three-phase load voltage u an ,u bn ,u cn The positive sequence three-phase current i obtained by transformation abc + Calculate the active power containing only the DC component and reactive power

[0018] The improved VSG control module receives the calculated active power and reactive power The output no-load electromotive force E is obtained through the virtual synchronization control algorithm. m and the angle θ of the virtual no-load electromotive force; in order to reduce the three-phase imbalance of the output voltage, the reference voltage U of the VSG output in the abc reference system is obtained through the positive sequence virtual impedance drop refabc , and then the reference voltage U output by the converter in the αβγ reference frame is obtained through coordinate transformation refαβγ , and sent to the voltage and current controller;

[0019] The voltage controller in the voltage and current controller adopts proportional resonance control, the current controller adopts proportional control, and the voltage and current controller accepts the reference voltage U of the improved VSG control module. refαβγ , the modulation signal of the three-phase four-bridge-arm converter is obtained, and the trigger signal of each bridge arm is obtained through 3D-SVPWM modulation to achieve accurate tracking of the output voltage reference value of the three-phase four-bridge-arm converter.

[0020] Preferably, the positive and negative separation module is composed of the current i in the αβγ reference frame αβγ The positive sequence current i in the αβγ reference frame is obtained αβ + The process is based on the second-order generalized integrator (SOGI) and is expressed as:

[0021]

[0022] Among them, i α + is the positive sequence current of the α axis in the αβγ reference frame; i β + is the positive sequence current of β axis in αβγ reference frame; i α is the current of the α axis in the αβγ reference frame; i β is the current of the β axis in the αβγ reference frame; j is the rotation factor, j = e j90° ;

[0023] Then the positive sequence three-phase current i is obtained through the abc / αβγ module transformation abc + , i.e. i a + 、i b + and i c + , where the transformation matrix is ​​expressed as follows:

[0024]

[0025] i a + represents the current of phase a in the abc coordinate system; i b + represents the current of phase b in the abc coordinate system; i c + Represents the current of phase c in the abc coordinate system.

[0026] Preferably, in the improved power calculation module, based on the collected three-phase voltage u an ,u bn ,u cn And the transformed i abc + Calculate the active power containing only the DC component and reactive power The formula is:

[0027]

[0028] Preferably, the control algorithm of virtual synchronization in the improved VSG control module includes a VSG active power-frequency control algorithm and a VSG reactive power-voltage control algorithm;

[0029] The classic second-order model is used to model the active power-frequency control of VSG. The classic second-order model is the mathematical model expressed by formula (4). The difference is that P e The calculation formula is P e =u an i a +u bn i b +u cn i c , in order to avoid the double frequency component in the voltage frequency, compared with the traditional method, the input contains only the active power of the DC component, and the expression is as follows:

[0030]

[0031] Among them, P ref is the reference active power; J and D1 represent the virtual inertia constant and damping coefficient respectively; ω n and ω are the reference angular frequency and the angular frequency of the system output respectively; t represents time;

[0032] In order to simulate the excitation regulation system of the synchronous generator, a virtual excitation controller is constructed, and its expression is as follows:

[0033]

[0034] Among them, Q refis the reference value of reactive power; D2 and K represent the voltage regulation coefficient and integral regulation coefficient respectively (K represents the integral coefficient, which is obtained by experience adjustment); U ref and U m is the reference and measured value of the voltage RMS (U m for u an ,u bn ,u cn The average of the effective values); s represents a complex frequency domain variable, and the formula is an expression in the s domain.

[0035] Preferably, in order to simulate the output characteristics of the synchronous generator, a virtual synchronous generator output voltage equation is constructed. In order to make the output voltage a balanced three-phase voltage, compared with the traditional method, a virtual positive sequence impedance voltage drop is used to simulate the positive sequence impedance to ensure the output balanced three-phase voltage. The reference voltage U of the VSG output in the abc reference system is refabc The expression is as follows:

[0036] U refabc =E m -i abc + Z v + (6)

[0037] Among them, Z v + It is the virtual impedance of the positive sequence (generally, when multiple machines are connected in parallel, the value of the virtual impedance is related to the line impedance, but the present invention only considers a single inverter and can take a value close to the filter impedance).

[0038] Preferably, the implementation process of the voltage and current controller is:

[0039] (1) Mathematical model of three-phase four-bridge arms in the αβγ reference frame

[0040] The three bridge arms a, b, and c of the converter are connected to the three-phase AC load through an LC filter. The filter inductance is L f , the capacitance is C f , the fourth bridge arm passes through the neutral inductor L n Connected to the neutral point, the DC voltage source U dc Considered as a constant value, according to Kirchhoff's law:

[0041]

[0042] i La +i Lb +i Lc +i n =0 (9)

[0043] Among them, u ag ,ubg ,u cg are the bridge arm voltages of a, b, and c bridge arms relative to the fourth bridge arm, i La ,i Lb ,i Lc is the current flowing through the filter inductor L f The three-phase current of n Represents the current flowing through the neutral inductor L n The current;

[0044] After substituting formula (7), (8), (9), we can get:

[0045]

[0046]

[0047] Due to L n Due to the effect of , there is coupling between the three phases abc. In order to eliminate the coupling, Clarke transformation is used to realize the transformation from abc to αβγ. The transformation matrix is ​​expressed as follows:

[0048]

[0049] After Clarke transformation, the mathematical model of the three-phase four-bridge arm in the αβγ reference frame can be obtained from formulas (10) and (11):

[0050]

[0051] Among them, [u αg ,u βg ,u γg ] T is the bridge arm voltage of converter abc relative to the fourth bridge arm (g) in the αβγ reference frame; [u αn ,u βn ,u γn ] T is the load voltage in the αβγ reference frame; [i Lα ,i Lβ ,i Lγ ] T and[i α ,i β ,i γ ] T They represent the inductor current and load current in the αβγ reference frame respectively;

[0052] From (13) and (14), it can be seen that there is no coupling between α, β and γ, that is, the parameter of α is only related to the parameter with the subscript α, but not to the parameters of β and γ. Therefore, the controllers under the α, β and γ components can be designed independently;

[0053] (2) Construction of voltage and current control structure in αβγ reference frame

[0054] From formulas (13) and (14), it can be obtained that the mathematical model of the three-phase four-bridge arm in the αβγ reference system is equivalent to the mathematical model of the three-phase three-bridge arm in the abc coordinate system. Therefore, the voltage and current control structure in the αβγ reference system can be established by using the dual method. The voltage and current control loop structure of the α axis is the same as that of the β axis, but due to the existence of the neutral point inductance, the equivalent filter inductance on the γ axis is L f +3L n ; Control delay of digital system G d (s) and zero-order holder G ZOH (s) is expressed as:

[0055]

[0056] Among them, T s is the sampling period;

[0057] The voltage controller adopts proportional resonant control, which accepts the reference voltage U generated by VSG. refαβγ , so that the coordinate transformation u αn ,u βn ,u γn Tracking the voltage reference value generated by VSG to achieve voltage control at a given AC side common connection point (PCC point), the proportional controller is expressed as:

[0058]

[0059] Among them, k v is the proportionality coefficient, k i is the resonance coefficient;

[0060] The voltage controller generates a reference value I for current control through a proportional resonant controller. refαβγ , reference value I refαβγ Compared with the capacitor current in the αβγ reference frame, the modulation signal of the three-phase four-bridge arm is obtained through the proportional controller, and then the trigger signal of each bridge arm is obtained through 3D-SVPWM modulation, which can finally realize the accurate control of the AC side voltage under unbalanced load.

[0061] For any details not provided in the present invention, please refer to the prior art.

[0062] The beneficial effects of the present invention are:

[0063] The present invention discloses a three-phase four-bridge-arm converter VSG control method based on resonant sequence control, which uses positive sequence current and positive sequence virtual impedance to improve the power calculation method and VSG control method. Based on the mathematical model of the three-phase four-bridge-arm αβγ reference system, a voltage and current controller in the αβγ reference system is designed. The voltage controller adopts proportional resonance control, and the current controller adopts proportional control. Compared with the traditional method, the three-phase four-bridge-arm converter VSG control method based on resonant sequence control of the present invention greatly reduces the voltage frequency ripple under load unbalanced conditions, and at the same time, the three-phase voltage imbalance rate is suppressed to 0.26%, which is reduced by 89% compared with the traditional method, and the quality of the output voltage is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] The drawings in the specification, which constitute a part of the present application, are used to provide further understanding of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute improper limitations on the present application.

[0065] Figure 1 It is a schematic diagram of the implementation process of the three-phase four-bridge-leg converter VSG control method based on resonant sequence control;

[0066] Figure 2 It is the topological structure diagram of three-phase four-bridge arm;

[0067] Figure 3 It is the control structure diagram of voltage and current device;

[0068] Figure 4 This is a hardware prototype diagram of a three-phase four-bridge-arm converter;

[0069] Figure 5 It is the frequency waveform diagram under unbalanced load;

[0070] Figure 6 Figure 6 is a voltage and current waveform diagram of the steady state under unbalanced load, where Figure 6(a) is a voltage and current waveform diagram of the three-phase four-bridge arm VSG control method, and Figure 6(b) is a voltage and current waveform diagram of the three-phase four-bridge arm converter VSG control method based on resonant sequence control;

[0071] Figure 7 is the abc reference system;

[0072] Figure 8 It is the αβγ reference system. DETAILED DESCRIPTION

[0073] In order to enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the drawings in the implementation of this specification, but are not limited to this. Anything not fully described in the present invention shall be based on the conventional technology in the art.

[0074] Example 1

[0075] A three-phase four-leg converter VSG control method based on resonant sequence control, such as Figure 1 As shown, it is realized by a coordinate transformation module, a positive and negative sequence separation module, an improved power calculation module, an improved VSG control module, and a voltage and current controller;

[0076] The coordinate transformation module collects the abc three-phase load voltage u of the common connection point (PCC) of the three-phase four-bridge arm converter in real time. an ,u bn ,u cn And the three-phase load current i abc ,u an ,u bn ,u cn After being transformed into the voltage u in the αβγ reference frame by the αβγ / abc module αn ,u βn ,u γn ;i abc After being transformed into the current i in the αβγ reference frame by the αβγ / abc module αβγ ; The transformation formula can be expressed as:

[0077]

[0078] The positive and negative separation module is based on the current i in the αβγ reference frame αβγ The positive sequence current i in the αβγ reference frame is obtained αβ + , i.e. i α + and i β + , and then transformed by the abc / αβγ module to obtain the positive sequence three-phase current i abc + ;

[0079] The improved power calculation module is based on the collected abc three-phase load voltage u an ,u bn ,u cn The positive sequence three-phase current i obtained by transformation abc + Calculate the active power containing only the DC component and reactive power

[0080] The improved VSG control module receives the calculated active power and reactive power The output no-load electromotive force E is obtained through the virtual synchronization control algorithm. mand the angle θ of the virtual no-load electromotive force; in order to reduce the three-phase imbalance of the output voltage, the reference voltage U of the VSG output in the abc reference system is obtained through the positive sequence virtual impedance drop refabc , and then the reference voltage U output by the converter in the αβγ reference frame is obtained through coordinate transformation refαβγ , and sent to the voltage and current controller;

[0081] The voltage controller in the voltage and current controller adopts proportional resonance control, the current controller adopts proportional control, and the voltage and current controller accepts the reference voltage U of the improved VSG control module. refαβγ , the modulation signal of the three-phase four-bridge-arm converter is obtained, and the trigger signal of each bridge arm is obtained through 3D-SVPWM modulation to achieve accurate tracking of the output voltage reference value of the three-phase four-bridge-arm converter.

[0082] Example 2

[0083] A control method for a three-phase four-bridge-arm converter VSG based on resonant sequence control, as described in Example 1, except that the positive and negative separation module is composed of a current i in the αβγ reference frame αβγ The positive sequence current i in the αβγ reference frame is obtained αβ + The process is based on the second-order generalized integrator (SOGI) and is expressed as:

[0084]

[0085] Among them, i α + is the positive sequence current of the α axis in the αβγ reference frame; i β + is the positive sequence current of β axis in αβγ reference frame; i α is the current of the α axis in the αβγ reference frame; i β is the current of the β axis in the αβγ reference frame; j is the rotation factor, j = e j90° ;

[0086] Then the positive sequence three-phase current i is obtained through the abc / αβγ module transformation abc + , i.e. i a + 、i b + and i c + , where the transformation matrix is ​​expressed as follows:

[0087]

[0088] i a + represents the current of phase a in the abc coordinate system; ib + represents the current of phase b in the abc coordinate system; i c + Represents the current of phase c in the abc coordinate system.

[0089] Example 3

[0090] A three-phase four-bridge-arm converter VSG control method based on resonant sequence control, as described in Example 2, except that in the improved power calculation module, based on the collected three-phase voltage u an ,u bn ,u cn And the transformed i abc + Calculate the active power containing only the DC component and reactive power The formula is:

[0091]

[0092] Example 4

[0093] A three-phase four-bridge-arm converter VSG control method based on resonant sequence control, as described in Example 3, except that the virtual synchronization control algorithm in the improved VSG control module includes a VSG active power-frequency control algorithm and a VSG reactive power-voltage control algorithm;

[0094] The classic second-order model is used to model the active power-frequency control of VSG. The classic second-order model is the mathematical model expressed by formula (4). The difference is that P e The calculation formula is P e =u an i a +u bn i b +u cn i c , in order to avoid the double frequency component in the voltage frequency, compared with the traditional method, the input contains only the active power of the DC component, and the expression is as follows:

[0095]

[0096] Among them, P ref is the reference active power; J and D1 represent the virtual inertia constant and damping coefficient respectively; ω n and ω are the reference angular frequency and the angular frequency of the system output respectively; t represents time;

[0097] In order to simulate the excitation regulation system of the synchronous generator, a virtual excitation controller is constructed, and its expression is as follows:

[0098]

[0099] Among them, Q ref is the reference value of reactive power; D2 and K represent the voltage regulation coefficient and integral regulation coefficient respectively (K represents the integral coefficient, which is obtained by experience adjustment); U ref and U m is the reference and measured value of the voltage RMS (U m for u an ,u bn ,u cn The average of the effective values); s represents a complex frequency domain variable, and the formula is an expression in the s domain.

[0100] In order to simulate the output characteristics of the synchronous generator, a virtual synchronous generator output voltage equation is constructed. In order to make the output voltage a balanced three-phase voltage, compared with the traditional method, a virtual positive sequence impedance drop is used to simulate the positive sequence impedance to ensure the output balanced three-phase voltage. The reference voltage U of the VSG output in the abc reference system is refabc The expression is as follows:

[0101] U refabc =E m -i abc + Z v + (6)

[0102] Among them, Z v + It is the virtual impedance of the positive sequence (generally, when multiple machines are connected in parallel, the value of the virtual impedance is related to the line impedance, but the present invention only considers a single inverter and can take a value close to the filter impedance).

[0103] Example 5

[0104] A control method for a three-phase four-bridge-arm converter VSG based on resonant sequence control is as described in Example 4, except that the implementation process of the voltage and current controller is as follows:

[0105] (1) Mathematical model of three-phase four-bridge arms in the αβγ reference frame

[0106] The topological structure of three-phase four-bridge arm is as follows Figure 2 As shown, the three bridge arms a, b, and c of the converter are connected to the three-phase AC load through an LC filter, and the filter inductance is L f , the capacitance is C f , the fourth bridge arm passes through the neutral inductor L n Connected to the neutral point, U dc is a DC voltage source, usually from energy storage or distributed generation. Ignoring the constraints on the DC side of the converter, g represents the position of the midpoint of the two IGBTs in the fourth bridge arm.dc Considered as a constant value, according to Kirchhoff's law:

[0107]

[0108] i La +i Lb +i Lc +i n =0 (9)

[0109] Among them, u ag ,u bg ,u cg are the bridge arm voltages of a, b, and c bridge arms relative to the fourth bridge arm, i La ,i Lb ,i Lc is the current flowing through the filter inductor L f The three-phase current of n Represents the current flowing through the neutral inductor L n The current;

[0110] After substituting formula (7), (8), (9), we can get:

[0111]

[0112] Due to L n Due to the effect of , there is coupling between the three phases abc. In order to eliminate the coupling, Clarke transformation is used to realize the transformation from abc to αβγ. The transformation matrix is ​​expressed as follows:

[0113]

[0114] After Clarke transformation, the mathematical model of the three-phase four-bridge arm in the αβγ reference frame can be obtained from formulas (10) and (11):

[0115]

[0116] Among them, [u αg ,u βg ,u γg ] T is the bridge arm voltage of converter abc relative to the fourth bridge arm (g) in the αβγ reference frame; [u αn ,u βn ,u γn ] T is the load voltage in the αβγ reference frame; [i Lα ,i Lβ ,i Lγ ] T and[i α ,i β ,iγ ] T They represent the inductor current and load current in the αβγ reference frame respectively;

[0117] From (13) and (14), it can be seen that there is no coupling between α, β and γ, that is, the parameter of α is only related to the parameter with the subscript α, but not to the parameters of β and γ. Therefore, the controllers under the α, β and γ components can be designed independently;

[0118] (2) Construction of voltage and current control structure in αβγ reference frame

[0119] From formulas (13) and (14), it can be obtained that the mathematical model of three-phase four-bridge arms in the αβγ reference frame is equivalent to the mathematical model of three-phase three-bridge arms in the abc coordinate system. Therefore, the voltage and current control structure in the αβγ reference frame can be established by using the dual method, as follows: Figure 3 As shown, the voltage and current control loop structure of the α-axis is the same as that of the β-axis, but due to the existence of the neutral point inductance, the equivalent filter inductance on the γ-axis is L f +3L n ; Control delay of digital system G d (s) and zero-order holder G ZOH (s) is expressed as:

[0120]

[0121] Among them, T s is the sampling period;

[0122] The voltage controller adopts proportional resonant control, which accepts the reference voltage U generated by VSG. refαβγ , so that the coordinate transformation u αn ,u βn ,u γn Tracking the voltage reference value generated by VSG to achieve voltage control at a given AC side common connection point (PCC point), the proportional controller is expressed as:

[0123]

[0124] Among them, k v is the proportionality coefficient, k i is the resonance coefficient;

[0125] The voltage controller generates a reference value I for current control through a proportional resonant controller. refαβγ , reference value I refαβγ Compared with the capacitor current in the αβγ reference frame, the modulation signal of the three-phase four-bridge arm is obtained through the proportional controller, and then the trigger signal of each bridge arm is obtained through 3D-SVPWM modulation, which can finally realize the accurate control of the AC side voltage under unbalanced load.

[0126] In order to verify the proposed three-phase four-bridge-arm converter VSG control method based on resonant sequence control, a hardware prototype was built for experimental verification. Figure 4 The main circuit parameters of the prototype are shown in Table 1.

[0127] Table 1: Main circuit parameters of three-phase four-bridge-arm converter prototype

[0128]

[0129] In the experimental verification, the virtual inertia J is 0.5. The damping coefficient D1 is 30, and the virtual impedance Z v The voltage modulation D2 is 321.

[0130] The three-phase four-bridge-arm virtual synchronous control method and the three-phase four-bridge-arm converter VSG control method based on resonant sequence control were experimentally tested. The output voltage quality of the three-phase four-bridge-arm converter VSG control method based on resonant sequence control of the present invention was compared and analyzed through two experimental cases.

[0131] Case I is the three-phase four-bridge output voltage frequency test under unbalanced load, such as Figure 5 As shown. The loads of phases a, b, and c are set to 7.5Ω, 15Ω, and 30Ω respectively. The three-phase four-bridge-arm converter adopts two strategies. When the system starts, the voltage-frequency waveform is as follows: Figure 5 Compared with the traditional control strategy, the three-phase four-leg converter VSG control method based on resonant sequence control can significantly suppress the voltage frequency ripple.

[0132] Case II is the three-phase four-bridge arm output voltage imbalance test under unbalanced load. Figure 6 As shown. Set the three-phase loads of a, b, and c to 7.5Ω, 15Ω, and 30Ω respectively. Voltage unbalance is usually measured by negative sequence voltage unbalance. According to the national standard, the calculation formula for negative sequence voltage unbalance is:

[0133]

[0134] Among them, U1 is the RMS value of the positive sequence component of the three-phase voltage, and U2 is the RMS value of the negative sequence component of the three-phase voltage.

[0135] Figure 6 (a) is the waveform of the traditional three-phase four-leg converter VSG control. The calculated negative sequence unbalance rate is 2.4%. However, the national standard requires that the negative sequence voltage unbalance should be less than 2%, so it cannot meet the conditions for normal system operation. Figure 6 (b) is the voltage waveform of the improved VSG control module of the present invention. The negative sequence unbalance rate is 0.26%, and the unbalance degree of the output voltage is greatly suppressed.

[0136] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A three-phase four-bridge-arm converter VSG control method based on resonant sequence control, characterized in that: It is realized through coordinate transformation module, positive and negative sequence separation module, improved power calculation module, improved VSG control module and voltage and current controller; The coordinate transformation module collects the abc three-phase load voltage u of the common connection point of the three-phase four-bridge arm converter in real time an ,u bn ,u cn And the three-phase load current i abc ,u an ,u bn ,u cn After being transformed into the voltage u in the αβγ reference frame by the αβγ / abc module αn ,u βn ,u γn ;i abc After being transformed into the current i in the αβγ reference frame by the αβγ / abc module αβγ ; The positive and negative sequence separation module is based on the current i in the αβγ reference frame. αβγ The positive sequence current i in the αβγ reference frame is obtained α + and i β + , and then transformed by the abc / αβγ module to obtain the positive sequence three-phase current i a + 、i b + and i c + ; The improved power calculation module is based on the collected abc three-phase load voltage u an ,u bn ,u cn The positive sequence three-phase current i obtained by transformation a + 、i b + and i c + Calculate the active power containing only the DC component and reactive power The improved VSG control module receives the calculated active power and reactive power The output no-load electromotive force E is obtained through the virtual synchronization control algorithm. m The angle θ of the virtual no-load electromotive force is obtained by the positive sequence virtual impedance voltage drop to obtain the reference voltage U output by the VSG in the abc reference system. refabc , and then the reference voltage U output by the converter in the αβγ reference frame is obtained through coordinate transformation refαβγ , and sent to the voltage and current controller; The voltage controller in the voltage and current controller adopts proportional resonance control, the current controller adopts proportional control, and the voltage and current controller accepts the reference voltage U of the improved VSG control module. refαβγ , the modulation signal of the three-phase four-bridge-arm converter is obtained, and the trigger signal of each bridge arm is obtained through 3D-SVPWM modulation, so as to realize the accurate tracking of the output voltage reference value of the three-phase four-bridge-arm converter; The positive and negative sequence separation module is composed of the current i in the αβγ reference frame αβγ The process of obtaining the positive sequence current in the αβγ reference frame is implemented based on a second-order generalized integrator and is expressed as: Among them, i α + is the positive sequence current of the α axis in the αβγ reference frame; i β + is the positive sequence current of β axis in αβγ reference frame; i α is the current of the α axis in the αβγ reference frame; i β is the current of β axis in αβγ reference frame; j is the rotation factor; Then the positive sequence three-phase current i is obtained through the abc / αβγ module transformation a + 、i b + and i c + , where the transformation matrix is ​​expressed as follows: i a + represents the current of phase a in the abc coordinate system; i b + represents the current of phase b in the abc coordinate system; i c + Represents the current of phase c in the abc coordinate system.

2. The three-phase four-bridge-arm converter VSG control method based on resonant sequence control according to claim 1 is characterized in that: Improved power calculation module to calculate active power containing only DC component and reactive power The formula is:

3. The three-phase four-bridge-arm converter VSG control method based on resonant sequence control according to claim 2 is characterized in that: The control algorithm of virtual synchronization in the improved VSG control module includes a VSG active power-frequency control algorithm and a VSG reactive power-voltage control algorithm. In the VSG active power-frequency control algorithm, the calculation formula of θ is as follows: Among them, P ref is the reference active power; J and D1 represent the virtual inertia constant and damping coefficient respectively; ω n and ω are the reference angular frequency and the angular frequency of the system output respectively; t represents time; In the VSG reactive power-voltage control algorithm, E m The calculation formula is as follows: Among them, Q ref is the reference value of reactive power; D2 and K represent the voltage regulation coefficient and integral regulation coefficient respectively; U ref and U m is the reference and measured value of the voltage RMS; s represents the complex frequency domain variable.

4. The three-phase four-bridge-arm converter VSG control method based on resonant sequence control according to claim 3 is characterized in that: The reference voltage U output by VSG in the abc reference frame refabc The expression is as follows: U refabc =E m -i abc + Z v + (6) Among them, Z v + is the virtual impedance of the positive sequence.

5. The three-phase four-bridge-arm converter VSG control method based on resonant sequence control according to claim 4 is characterized in that: The implementation process of the voltage and current controller is: (1) Mathematical model of three-phase four-bridge arms in the αβγ reference frame The three bridge arms a, b, and c of the converter are connected to the three-phase AC load through an LC filter. The filter inductance is L f , the capacitance is C f , the fourth bridge arm passes through the neutral inductor L n Connected to the neutral point, the DC voltage source U dc Considered as a constant value, according to Kirchhoff's law: i La +i Lb +i Lc +i n =0 (9) Among them, u ag ,u bg ,u cg are the bridge arm voltages of a, b, and c bridge arms relative to the fourth bridge arm, i La ,i Lb ,i Lc is the current flowing through the filter inductor L f The three-phase current of n Represents the current flowing through the neutral inductor L n The current; After substituting formula (7), (8), (9), we can get: Due to L n The abc three-phase is coupled. In order to eliminate the coupling, Clarke transformation is used to realize the transformation from abc to αβγ. The transformation matrix is ​​expressed as follows: After Clarke transformation, the mathematical model of the three-phase four-bridge arm in the αβγ reference frame can be obtained from formulas (10) and (11): Among them, [u αg ,u βg ,u γg ] T is the bridge arm voltage of converter abc relative to the fourth bridge arm (g) in the αβγ reference frame; [u αn ,u βn ,u γn ] T is the load voltage in the αβγ reference frame; [i Lα ,i Lβ ,i Lγ ] T and[i α ,i β ,i γ ] T They represent the inductor current and load current in the αβγ reference frame respectively; (2) Construction of voltage and current control structure in αβγ reference frame According to formulas (13) and (14), the mathematical model of the three-phase four-bridge arm in the αβγ reference frame is equivalent to the mathematical model of the three-phase three-bridge arm in the abc coordinate system. Therefore, the voltage and current control structure in the αβγ reference frame is established by the dual method. The voltage and current control loop structure of the α axis is the same as that of the β axis. However, due to the existence of the neutral point inductance, the equivalent filter inductance on the γ axis is L f +3L n ; Control delay of digital system G d (s) and zero-order holder G ZOH (s) is expressed as: Among them, T s is the sampling period; The voltage controller adopts proportional resonant control, which accepts the reference voltage U generated by VSG. refαβγ , so that the coordinate transformation u αn ,u βn ,u γn Tracking the voltage reference value generated by VSG to achieve voltage control of a given AC side common connection point, the proportional controller is expressed as: Among them, k v is the proportionality coefficient, k i is the resonance coefficient; The voltage controller generates a reference value I for current control through a proportional resonant controller. refαβγ , reference value I refαβγ Compared with the capacitor current in the αβγ reference frame, the modulation signal of the three-phase four-bridge arm is obtained through the proportional controller, and then the trigger signal of each bridge arm is obtained through 3D-SVPWM modulation.

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