Disclosure of Invention
Technical problem to be solved
Aiming at the defects of the prior art, the invention provides an ANPC three-level inverter and a model prediction control method thereof, which solve the problem of neutral point potential imbalance of an Active Neutral Point Clamped (ANPC) three-level inverter.
(II) technical scheme
In order to achieve the purpose, the invention is realized by the following technical scheme: the ANPC three-level inverter comprises three-phase bridge arms, wherein each phase bridge arm comprises six IGBTs, four IGBTs connected in series are connected with the positive electrode and the negative electrode of a direct-current bus, the two IGBTs connected in series realize a neutral-point clamping function, the input end of each bridge arm is connected with a direct-current voltage source, the neutral point is connected to the neutral point O of a direct-current side voltage-dividing capacitor, and each IGBT is controlled to be switched on and switched off by a driving circuit.
Preferably, the input direct-current power supply is connected with two voltage division capacitors, and the middle points of the two capacitors are connected with the middle point of the ANPC three-level inverter.
Preferably, the filter is a three-phase LC filter circuit.
Preferably, the control circuit comprises a DSP + CPLD main controller, a driving circuit and a signal sampling and conditioning circuit, the DSP chip is responsible for sampling voltage and current signals and calculating vector action time, and the CPLD is used for distributing PWM signals and transmitting the PWM signals to the driving circuit to drive the ANPC three-level inverter.
Preferably, the voltage of the direct current side voltage-dividing capacitor, the three-phase output current of the inverter and the three-phase output voltage are sampled and input into the AD conversion module of the DSP controller after passing through the signal conditioning circuit.
The invention also discloses a model prediction control method of the ANPC three-level inverter, which specifically comprises the following steps:
step one, establishing an equivalent relation of an inverter in an abc coordinate system according to a model of the inverter;
secondly, obtaining an equivalent relation under an alpha beta coordinate system under the condition of unbalanced inverter power grid by using clarke transformation, sending the given current and the transformed current into a cost function, deriving a voltage vector of the cost function to obtain an optimal voltage vector, and sending the optimal vector into space vector modulation;
comparing the difference value of the upper capacitor voltage and the lower capacitor voltage with 0 according to the upper capacitor voltage and the lower capacitor voltage on the direct current side, comparing and outputting the difference value to a repetitive controller, and then adjusting the state transition time of each phase of bridge arm to realize the neutral point potential balance control;
and step four, converting the action time of each vector into a duty ratio, modulating the duty ratio by the in-phase carrier to generate a PWM signal, and controlling the on or off of the IGBT by a driving circuit.
Preferably, in the first step, the grid voltage and the grid-connected current are detected, and a model expression of the ANPC three-level inverter system is obtained according to kirchhoff's law.
Preferably, in the second step, for simple calculation, clarke transformation is performed on the voltage and current of the abc axis to obtain an equivalent relation of an α β coordinate system of the ANPC three-level inverter under the condition of unbalanced power grid, then current expressions of the α axis and the β axis are obtained, the given currents of the α axis and the β axis and the obtained currents of the α axis and the β axis are sent to a cost function, derivation is performed on a voltage vector of the cost function to obtain an optimal voltage vector, and the optimal voltage vector is sent to space vector modulation to perform midpoint potential balance control.
Preferably, in the third step, when the upper capacitor voltage on the dc side is higher than the lower capacitor voltage, the output of the repetitive controller increases the P-type small vector action time, so as to reduce the upper capacitor voltage on the dc side and realize the midpoint potential balance control; when the voltage of the capacitor on the DC side is lower than that of the capacitor on the lower side, the output of the repetitive controller increases the action time of the N-type small vector, thereby reducing the voltage of the capacitor on the lower side of the DC side.
Preferably, in the fourth step, the PWM signal of the IGBT is determined by the switching state, and when the x (x ═ a, b, c) phase outputs the P state, Sx1, Sx2, Sx6 are turned on; when the x phase outputs the O state, Sx2, Sx3, Sx5 and Sx6 are switched on; when the x phase outputs the N state, Sx3, Sx4 and Sx5 are turned on.
(III) advantageous effects
The invention provides an ANPC three-level inverter and a model prediction control method thereof.
The method has the following beneficial effects:
(1) compared with the traditional control method, the novel model prediction control considers the influence of unbalanced condition of the power grid, does not need complex positive and negative sequence separation and PI parameter adjustment, is simple to control, and has better grid-connected current waveform quality.
(2) The ANPC three-level inverter and the model prediction control method thereof send alpha and beta axis currents and given currents into a cost function, conduct on a voltage vector to obtain an optimal output vector, and send the voltage vector into space vector modulation.
(3) The ANPC three-level inverter and the model prediction control method thereof adopt the repetitive controller, do not need to adjust complex PI parameters, can realize the capability of actively controlling the neutral point potential balance, greatly reduce the neutral point potential fluctuation, and can effectively inhibit the neutral point potential deviation caused by abnormal conditions.
(4) The ANPC three-level inverter and the model prediction control method thereof can be applied to the field of renewable energy power generation such as photovoltaic power generation systems.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In order to solve the problems, the invention provides a model prediction control method which adopts a constant switching frequency method, calculates a current model under a static coordinate system according to a voltage model under the condition of power grid unbalance, realizes accurate current tracking, and compared with the traditional method, the method can eliminate double frequency fluctuation caused by power grid unbalance and does not need positive and negative sequence separation. And the midpoint potential control of the ANPC three-level inverter is realized through repeated control according to the capacitance difference value of the direct current side. The invention has the functions of midpoint potential control and grid-connected current tracking control, and can ensure that the ANPC works efficiently and stably.
The embodiment of the invention provides an ANPC three-level inverter and a model prediction control method thereof, wherein the ANPC three-level inverter comprises a three-phase bridge arm, the phase bridge arm comprises six IGBTs, four IGBTs connected in series are connected with the anode and the cathode of a direct-current bus, two IGBTs connected in series realize a midpoint clamping function, the input end of each bridge arm is connected with a direct-current voltage source, the midpoint is connected to the midpoint O of a direct-current side voltage-dividing capacitor, each IGBT is controlled by a driving circuit to perform switching action, a diode is connected in series with the connection side of an inductor and an input direct-current power supply. One ends of the two capacitors are connected to the connection position of the diode and the inductor, and the other ends of the two capacitors are connected to the connection position of the other inductor and the bridge arm of the ANPC three-level inverter.
Selecting the midpoint O of the DC-side capacitor1Taking A-phase bridge arm as an example as a reference point, when the power device Sa1、Sa2And Sa6When conducting, uAO1=Vdc_link/2, definition ofThe state is P; when power device Sa2、Sa3、Sa5And Sa6When conducting, u AO10, this state is defined as O; when power device Sa3、Sa4And Sa5When conducting, uAO1=-Vdc_linkAnd/2, defining the state as N.
Switching state and output voltage of each phase of bridge arm of Z-source ANPC three-level inverter (x ═ a, b, c)
In the invention, an input direct current power supply is connected with two voltage division capacitors, and the middle points of the two capacitors are connected with the middle point of an ANPC three-level inverter.
In the invention, the filter is a three-phase LC filter circuit.
In the invention, the control circuit comprises a DSP + CPLD main controller, a driving circuit and a signal sampling and conditioning circuit, wherein a DSP chip is responsible for sampling voltage and current signals and calculating vector action time, and the CPLD is used for distributing PWM signals and transmitting the PWM signals to the driving circuit to drive the ANPC three-level inverter.
In the invention, the voltage of a direct current side voltage-dividing capacitor, the three-phase output current of an inverter and the three-phase output voltage are sampled and input into an AD conversion module of a DSP controller after passing through a signal conditioning circuit.
The invention also discloses a model prediction control method of the ANPC three-level inverter, which specifically comprises the following steps:
step one, establishing an equivalent relation of an inverter in an abc coordinate system according to a model of the inverter;
secondly, obtaining an equivalent relation under an alpha beta coordinate system under the condition of unbalanced inverter power grid by using clarke transformation, sending the given current and the transformed current into a cost function, deriving a voltage vector of the cost function to obtain an optimal voltage vector, and sending the optimal vector into space vector modulation;
comparing the difference value of the upper capacitor voltage and the lower capacitor voltage with 0 according to the upper capacitor voltage and the lower capacitor voltage on the direct current side, comparing and outputting the difference value to a repetitive controller, and then adjusting the state transition time of each phase of bridge arm to realize the neutral point potential balance control;
and step four, converting the action time of each vector into a duty ratio, modulating the duty ratio by the in-phase carrier to generate a PWM signal, and controlling the on or off of the IGBT by a driving circuit.
In the first step, the voltage and the grid-connected current of a power grid are detected, and a model expression of the ANPC three-level inverter system is obtained according to kirchhoff's law.
In the second step, for simple calculation, clarke transformation is performed on the voltage and current of the abc axis to obtain an equivalent relation of an alpha-beta coordinate system of the ANPC three-level inverter under the condition of unbalanced power grid, then current expressions of the alpha axis and the beta axis are obtained, given currents of the alpha axis and the beta axis and the obtained currents of the alpha axis and the beta axis are sent to a cost function, derivation is performed on a voltage vector of the cost function to obtain an optimal voltage vector, and the optimal voltage vector is sent to space vector modulation to perform midpoint potential balance control.
In the third step, when the upper capacitor voltage on the direct current side is higher than the lower capacitor voltage, the output of the repetitive controller increases the action time of the P-type small vector, so that the upper capacitor voltage on the direct current side is reduced, and the midpoint potential balance control is realized; when the voltage of the capacitor on the DC side is lower than that of the capacitor on the lower side, the output of the repetitive controller increases the action time of the N-type small vector, thereby reducing the voltage of the capacitor on the lower side of the DC side.
In the fourth step, the PWM signal of the IGBT is determined by the switching state, and when the x (x ═ a, b, c) phase outputs the P state, Sx1, Sx2, Sx6 are turned on; when the x phase outputs the O state, Sx2, Sx3, Sx5 and Sx6 are switched on; when the x phase outputs the N state, Sx3, Sx4 and Sx5 are turned on.
Three-phase unbalanced grid voltages and currents may be represented as
Wherein
Positive, negative, and zero components of voltage and current, respectively. The voltage and current imbalances transformed by Clark and Park can be expressed as
Wherein
The components of the unbalanced voltage and current in the α β stationary frame and the dq rotating frame, respectively.
The complex power transmitted by the three-level ANPC inverter may be expressed as
Wherein i*For the conjugation of the current, the equation (2) is substituted into (3) to obtain
s=[po+ps2 sin(2ωt)+pc2 cos(2ωt)]+j[qo+qs2 sin(2ωt)+qc2 cos(2ωt)]
(4)
Wherein
In order to avoid the complexity of calculation brought by positive and negative sequence separation, phase locking and power calculation, the currents of the alpha axis and the beta axis are obtained as follows:
the current reference value and the actual value of equation (6) are then brought into the cost function as:
G=(iα,ref-iα)2+(iβ,ref-iβ)2
(7)
the voltage vector is derived as:
the optimal voltage vector is obtained in equation (8), and the voltage vector is fed into the space vector.
And comparing the difference value of the upper capacitor voltage and the lower capacitor voltage with 0 according to the upper capacitor voltage and the lower capacitor voltage on the direct current side, comparing and outputting the comparison result to a repetitive controller, and then adjusting the state transition time of each phase of bridge arm to realize the neutral point potential balance control.
The model prediction control method provided by the invention can realize the functions of grid connection, neutral point potential balance control and the like of the ANPC three-level inverter under the condition of unbalanced power grid. Compared with the traditional control direction, the method does not need positive and negative sequence separation and PI parameter adjustment. Compared with an NPC three-level inverter, the loss of the power device is relatively balanced, the improvement of the power level of the system is facilitated, and the power device has wide application prospect in the field of renewable energy power generation such as a high-capacity photovoltaic power generation system.
It is noted that, herein, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.