Disclosure of Invention
The invention aims to solve the problems that the existing sliding mode control based on an observer of a three-level NPC converter has large buffeting and is very sensitive to measurement noise, and provides a sliding mode control method of the three-level NPC converter.
The invention discloses a sliding mode control method of a three-level NPC converter, which comprises the following steps:
the method adopts a direct-current voltage regulating ring and utilizes the actual value v of the direct-current side voltage of the three-level NPC converter
dcAnd a DC side voltage reference value
Obtaining an active power reference value p of a current sampling point moment direct current side through a self-adaptive sliding mode controller
*;
An instantaneous power tracking loop is adopted, and the active power reference value p of the current sampling point on the direct current side is utilized*An active power actual value p, a reactive power actual value q and a preset reactive power reference value q*Acquiring the average duty cycle number delta of the three-level NPC converter through a second-order sliding mode controllerαβ;
Average duty ratio delta for three-level NPC converterαβCarrying out alpha beta/abc coordinate transformation to obtain the average duty ratio delta of the three-level NPC converter under the abc coordinate systema'bc;
Using a voltage balancing loop to measure the actual value e of the DC-side unbalanced voltage
dcWith reference value of DC-side unbalanced voltage
Making difference, and performing PI regulation on the difference value of the two values to obtain a balance duty ratio delta
ba;
For balanced duty cycle deltabaAverage duty cycle delta of sum level NPC convertera'bcAnd after addition, a control signal of a switching tube of the three-level NPC converter is obtained through a pulse width modulator, so that the three-level NPC converter is controlled.
Further, in the invention, a direct-current voltage regulating ring is adopted, and the actual value v of the direct-current side voltage of the three-level NPC converter is utilized
dcAnd a DC side voltage reference value
Obtaining an active power reference value p of a load at the moment of a current sampling point through an Adaptive Sliding Mode Controller (ASMC) and a non-linear high-gain observer (NHGO)
*The specific method comprises the following steps:
step A1, utilizing the actual value v of the DC side voltage of the three-level NPC converter
dcAnd a DC side voltage reference value
Calculating the tracking error s of DC voltage regulation loop
v(ii) a Wherein,
step A2, tracking error s of DC voltage regulation loop through self-adaptive sliding mode controller ASMC
vCorrecting and outputting the corrected tracking error
Step A3, adopting a nonlinear high-gain observer NHGO to perform on x in step A1
1And the active power reference value p at the last sampling point moment
*Active power x to DC load
2Observing to obtain the estimated value of the load power at the DC side
Step A4, correcting the corrected tracking error
And estimated value of DC side load power
Adding to obtain the active power reference value p at the current sampling point moment
*。
Further, in the present invention, in step a2, the dynamic equation of the adaptive sliding mode controller ASMC is:
wherein, K
vAdaptive rate of adaptive sliding mode controller for gain of adaptive sliding mode controller ASMC, for a time variable
Comprises the following steps:
wherein, KlIs the gain change rate, K, of the adaptive sliding mode controller ASMCmIs a gain decision parameter of the adaptive sliding mode controller ASMC, baIs a gain gradient decision parameter of the adaptive sliding mode controller ASMC.
Further, in the present invention, in step a3, the dynamic equation of the nonlinear high-gain observer NHGO is:
wherein C is the capacitance of the DC side capacitor,
is a variable x
1Is determined by the estimated value of (c),
is a variable x
1An estimate of the derivative of (a) is,
active power x being a DC load
2Is estimated from the derivative of (a)
1Preceding stage gain parameter, alpha, of a non-linear high-gain observer NHGO
2Post-stage gain parameter, epsilon, of a non-linear high-gain observer NHGO
1And ε
2Two gain parameters of a non-linear high-gain observer NHGO, and epsilon
2Greater than epsilon
1,b
sAnd the gain judgment parameter of the non-linear high-gain observer NHGO is a saturation function sat (·).
Furthermore, in the invention, an instantaneous power tracking loop is adopted, and the active power reference value p of the current sampling point moment direct current side is utilized*An active power actual value p, a reactive power actual value q and a preset reactive power reference value q*Obtaining the average duty ratio delta of the three-level NPC converter through a second-order sliding mode controllerαβThe specific method comprises the following steps:
step B1, obtaining the active power reference value p at the current sampling point moment*Comparing with the actual value p of the active power to obtain the tracking error s of the active powerpSimultaneously, the real reactive power value q at the current sampling point moment and a preset reactive power reference value q are obtained*Comparing to obtain the tracking error s of reactive powerq;
Step B2, tracking error s of active power through second-order sliding mode controller SOSM
pAnd reactive power tracking error s
qCorrecting to obtain the corrected active power tracking error
And the corrected reactive power tracking error
Step B3, tracking error s according to active power
pReactive power tracking error s
qLast sampling point moment active power correction u
pAnd the reactive power correction u at the moment of the last sampling point
qUsing a high gain observer NHGO to correct the internal disturbance l caused by the uncertain parameters of the system
pAnd l
qObserving to obtain the estimated value of internal disturbance
And
wherein the correction amount u of active power
pAnd a reactive power correction u
qIs 0;
step B4, according to the corrected tracking error
And
estimation of internal disturbances
And
updating the active power correction u at the current sampling point moment
pAnd a reactive power correction u
q;
Step B5, the real value p of the active power and the real value q of the reactive power are differentiated to obtain the derivative of the active power respectively
And reactive power derivative
Order to
Obtaining an average duty cycle of the equivalent point
Step B6, correcting quantity u according to active power
pAnd a reactive power correction amount u
qAnd average duty cycle of equivalent point
Obtaining the average duty cycle delta
αβ。
Further, in the present invention, in step B2, the dynamic equation of the second-order sliding mode controller SOSM is:
wherein k isi1And ki2Is the gain of the second order sliding mode controller SOSM, t is time.
Further, in the present invention, in step B3, an estimated value of the internal disturbance is obtained
And
by the formula:
and
a computational implementation in which, among other things,
wherein v is
αAnd v
βIs alpha component and beta component of converter AC side voltage in alpha beta coordinate system, L is AC side line inductance, alpha
3Is a preceding-stage gain parameter, alpha, of a high-gain observer of an active power loop
4Is a back-stage gain parameter, alpha, of the active power loop high-gain observer
5Is the preceding-stage gain parameter, alpha, of the high-gain observer of the reactive power loop
6Is a post-stage gain parameter, epsilon, of a reactive power loop high-gain observer
pIs the gain, ε, of a high-gain observer of an active power loop
qIs the gain of the reactive power loop high-gain observer.
Further, in the present invention, in step B4, the current sampling point time active power correction upAnd a reactive power correction uqComprises the following steps:
further, in the present invention, in step B5, the average duty ratio of the equivalent point
Comprises the following steps:
wherein v is
αβThe voltage of the AC side of the converter under an alpha beta coordinate system; j is a matrix, and
omega is the angular frequency of the grid voltage; l is an alternating current side wire inductor.
Furthermore, in the invention, a voltage balance ring is adopted to compare the actual value e of the unbalanced voltage on the direct current side
dcWith reference value of DC-side unbalanced voltage
Making difference, and performing PI regulation on the difference value of the two values to obtain a balance duty ratio delta
baComprises the following steps:
wherein k ispbThe proportional link gain of the PI controller is obtained; k is a radical ofibThe integral link gain of the PI controller is obtained; t is time.
The method improves the dynamic and steady-state performance and the anti-interference capability of the three-phase NPC converter. The sliding mode control method of the three-level NPC converter is realized based on a direct-current voltage adjusting ring, an instantaneous power tracking ring and a voltage balancing ring. The actual value v of the DC side voltage is adjusted by a DC voltage adjusting ring
dcControl is performed so that the sum v of the DC-side capacitor voltages
dcAdjusted to corresponding desired values
The real value p of the active power and the real value q of the reactive power are controlled by an instantaneous power tracking loop, so that the active power p and the reactive power q accurately track respective reference values p
*And q is
*And the actual value e of the unbalanced voltage on the DC side is calculated by the voltage balance ring
dcControl is carried out to ensure that the unbalanced voltage of two capacitors on the direct current side is close to 0 and the combined action of three ringsThe control signal is generated to control the three-level NPC converter, the control process is simple, the direct-current bus voltage is regulated through the direct-current voltage regulating ring, so that the rapid dynamic response of a voltage step stage is ensured, and the fluctuation of the direct-current side voltage caused when an uncertain interfered direct-current side load is connected into the circuit can be effectively inhibited. The voltage balance of the direct-current side capacitor is realized, and the control stability is improved.
In the direct-current voltage regulating loop, a self-Adaptive Sliding Mode Controller (ASMC) is adopted to quickly regulate the direct-current bus voltage, so that buffeting is reduced, and quick dynamic response of a voltage step stage is ensured. Meanwhile, due to the existence of external uncertain disturbance, a nonlinear high-gain observer (NHGO) is added on the basis of an Adaptive Sliding Mode Controller (ASMC) (direct current voltage regulating loop) to inhibit the external uncertain disturbance (the existence of the external uncertain disturbance), and the observer is insensitive to noise and has strong disturbance rejection capability; in the instantaneous power tracking loop, a simple and effective Direct Power Control (DPC) strategy is adopted to realize the aim of power tracking, thereby simplifying the control process of an inner loop. In addition, in order to obtain alternating current with low harmonic distortion and robustness to system parameter perturbation, a second-order sliding mode controller (SOSM) based on a High Gain Observer (HGO) is adopted to ensure that active power and reactive power can rapidly converge to a stable state; finally, in the voltage balancing loop, a PI regulator is used to ensure voltage balancing of the dc link capacitor. Through experimental tests, the NPC power converter control strategy provided by the invention is compared with other control schemes, and the effectiveness and superiority of the scheme are proved.
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.
It should be noted that the embodiments and features of the embodiments may be combined with each other without conflict.
The first embodiment is as follows: the present embodiment is described below with reference to fig. 1, and the sliding mode control method of the three-level NPC converter according to the present embodiment includes:
the method adopts a direct-current voltage regulating ring and utilizes the actual value v of the direct-current side voltage of the three-level NPC converter
dcAnd a DC side voltage reference value
Obtaining an active power reference value p of a current sampling point moment direct current side through a self-adaptive sliding mode controller
*;
An instantaneous power tracking loop is adopted, and the active power reference value p of the current sampling point on the direct current side is utilized*An active power actual value p, a reactive power actual value q and a preset reactive power reference value q*Acquiring the average duty cycle number delta of the three-level NPC converter through a second-order sliding mode controllerαβ;
Average duty ratio delta for three-level NPC converterαβCarrying out alpha beta/abc coordinate transformation to obtain the average duty ratio delta of the three-level NPC converter under the abc coordinate systema'bc;
Using voltage balancingLoop, versus actual value e of DC side unbalance voltage
dcWith reference value of DC-side unbalanced voltage
Making difference, and performing PI regulation on the difference value of the two values to obtain a balance duty ratio delta
ba;
For balanced duty cycle deltabaAverage duty cycle delta of sum level NPC convertera'bcAnd after addition, a control signal of a switching tube of the three-level NPC converter is obtained through a pulse width modulator, so that the three-level NPC converter is controlled.
Before the method is applied specifically, a state space average model of the three-level NPC converter is established according to the operation principle of the three-level NPC converter; determining a control target of the three-level NPC converter according to a state space average model of the three-level NPC converter; the control targets include: make the sum v of the voltages of two capacitors on the DC side
dcAdjusting to desired value of DC side voltage reference value
Making active power p and reactive power q always track respective reference value p
*And q is
*And ensuring that the unbalanced voltage of the two capacitors on the direct current side approaches 0 to generate a corresponding control signal to control the three-level NPC converter.
In fact, it is necessary to adopt an efficient control method. The method can ensure that active power and reactive power are kept near an equivalent point when a system reaches a stable state, and ensures higher current quality. The three-level NPC converter can not only realize different control targets, but also improve the dynamic and steady-state performance and the anti-interference capability of the three-level NPC converter.
In fig. 1, a three-phase ac power source and an inductor are connected to the ac side of a three-level NPC converter to provide power transfer. On the dc side, a three-level NPC converter connects two capacitors to store energy and stabilize the dc voltage. Here, the dc side may be regarded as a dc microgrid, which is mainly composed of a dc load, other converters, various renewable energy sources, and the like.
The method of the invention improves the dynamic and steady-state performance and the anti-interference capability of the three-phase NPC converter. The sliding mode control method of the three-level NPC converter is realized based on a direct-current voltage adjusting ring, an instantaneous power tracking ring and a voltage balancing ring, and the actual value v of the direct-current side voltage is adjusted by the direct-current voltage adjusting ring
dcControl is performed so that the sum v of the DC-side capacitor voltages
dcAdjusted to corresponding desired values
Tracking the actual value p of the active power and the actual value q of the reactive power through an instantaneous power tracking loop, so that the active power p and the reactive power q accurately track respective reference values p
*And q is
*And the actual value e of the unbalanced voltage on the DC side is calculated by the voltage balance ring
dcThe control is carried out, unbalanced voltages of two capacitors on the direct current side are enabled to be close to 0, a control signal is generated by combining the combined action of three rings to control the three-level NPC converter, the control process is simple, the direct current bus voltage is adjusted through the direct current voltage adjusting ring, the rapid dynamic response of the voltage step stage is ensured, and the fluctuation of the direct current side voltage caused when an uncertain interfered direct current side load is connected into the circuit can be effectively inhibited. The voltage balance of the direct-current side capacitor is realized, and the control stability is improved.
Further, in the present embodiment, the first and second substrates,
the method adopts a direct-current voltage regulating ring and utilizes the actual value v of the direct-current side voltage of the three-level NPC converter
dcAnd a DC side voltage reference value
Obtaining an active power reference value p of a direct current side at the current sampling point moment through an Adaptive Sliding Mode Controller (ASMC) and a non-linear high-gain observer (NHGO)
*The specific method comprises the following steps:
step A1, utilizing the actual value v of the DC side voltage of the three-level NPC converter
dcAnd a DC side voltage reference value
Calculating the tracking error s of DC voltage regulation loop
v(ii) a Wherein,
step A2, tracking error s of DC voltage regulation loop through self-adaptive sliding mode controller ASMC
vCorrecting and outputting the corrected tracking error
Step A3, adopting a nonlinear high-gain observer NHGO to perform on x in step A1
1And the active power reference value p at the last sampling point moment
*Active power x to DC load
2Observing to obtain the estimated value of the load power at the DC side
Step A4, correcting the corrected tracking error
And estimated value of DC side load power
Adding to obtain the active power reference value p at the current sampling point moment
*。
Further, in the present embodiment, the first and second substrates,
in step a2, the dynamic equation of the adaptive sliding mode controller ASMC is:
wherein, K
vAdaptive rate of adaptive sliding mode controller for gain of adaptive sliding mode controller ASMC, for a time variable
Comprises the following steps:
wherein, KlIs the gain change rate, K, of the adaptive sliding mode controller ASMCmIs a gain decision parameter of the adaptive sliding mode controller ASMC, baIs a gain gradient decision parameter of the adaptive sliding mode controller ASMC. Further, in the present invention, the first and second substrates,
in step a3, the dynamic equation of the nonlinear high-gain observer NHGO is:
wherein C is the capacitance of the DC side capacitor,
is a variable x
1Is determined by the estimated value of (c),
is a variable x
1An estimate of the derivative of (a) is,
active power x being a DC load
2Is estimated from the derivative of (a)
1Preceding stage gain parameter, alpha, of a non-linear high-gain observer NHGO
2Post-stage gain parameter, epsilon, of a non-linear high-gain observer NHGO
1And ε
2Two gain parameters of a non-linear high-gain observer NHGO, and epsilon
2Greater than epsilon
1,b
sAnd the gain judgment parameter of the non-linear high-gain observer NHGO is a saturation function sat (·).
In an embodiment, the basic control objective of the dc voltage regulation loop is to regulate the dc bus voltage to a specified value. In order to obtain the performances of quick transient response, insensitivity to external interference and the like, an ASMC control strategy based on NHGO is proposed for a voltage regulation loop. ASMC is called Adaptive sliding mode control in English, and Adaptive sliding mode control; the English language of NHGO is known as Nonlinear high-gain observer.
The control strategy of the adaptive sliding mode controller ASMC overcomes the compromise problem of buffeting and dynamic performance existing in the traditional Sliding Mode Control (SMC), reserves the quick dynamic performance of the traditional sliding mode control, and simultaneously weakens the buffeting phenomenon, so that the ASMC is applied to the field of power electronics and can further improve the performance of a converter. On the other hand, although ASMC can improve the robustness of the system, its ability to achieve interference cancellation is not sufficient due to the lack of interference information, which means that interference cannot be compensated for immediately to the controller. As a technique for observing states and disturbances, observers are suitable to compensate for this disadvantage of systems, such as Sliding Mode Observers (SMO) and Linear Extended State Observers (LESO). However, the conventional observer has a disadvantage of being very sensitive to noise, and thus has limited performance in practical applications. The nonlinear high-gain observer (NHGO) is an improved version of the high-gain observer, is not sensitive to noise, has strong disturbance rejection capability, and is very suitable for being applied to the field of power electronics. Therefore, in the embodiment, the interference elimination is realized by adopting a non-linear high-gain observer NHGO.
Further, in this embodiment, an instantaneous power tracking loop is adopted, and the active power reference value p on the dc side at the current sampling point is used*An active power actual value p, a reactive power actual value q and a preset reactive power reference value q*Acquiring the average duty cycle number delta of the three-level NPC converter through a second-order sliding mode controllerαβThe specific method comprises the following steps:
step B1, obtaining the active power reference value p at the current sampling point moment*Comparing with the actual value p of the active power to obtain the tracking error s of the active powerpSimultaneously, the actual reactive power value q at the current sampling point moment and a preset reactive power reference are obtainedValue q*Comparing to obtain the tracking error s of reactive powerq;
Step B2, tracking error s of active power through second-order sliding mode controller SOSM
pAnd reactive power tracking error s
qCorrecting to obtain the corrected active power tracking error
And the corrected reactive power tracking error
Step B3, tracking error s according to active power
pReactive power tracking error s
qLast sampling point moment active power correction u
pAnd the reactive power correction u at the moment of the last sampling point
qUsing a high gain observer NHGO to correct the internal disturbance l caused by the uncertain parameters of the system
pAnd l
qObserving to obtain the estimated value of internal disturbance
And
wherein the correction amount u of active power
pAnd a reactive power correction u
qIs 0;
step B4, according to the corrected tracking error
And
estimation of internal disturbances
And
updating the active power correction u at the current sampling point moment
pAnd a reactive power correction u
q;
Step B5, the real value p of the active power and the real value q of the reactive power are differentiated to obtain the derivative of the active power respectively
And reactive power derivative
Order to
Obtaining an average duty cycle of the equivalent point
Step B6, correcting quantity u according to active power
pAnd a reactive power correction amount u
qAnd average duty cycle of equivalent point
Obtaining the average duty cycle delta
αβ。
Further, in the present embodiment, the first and second substrates,
in step B2, the dynamic equation of the second-order sliding mode controller SOSM is:
wherein k isi1And ki2Is the gain of the second order sliding mode controller SOSM, t is time.
Further, in the present embodiment, the first and second substrates,
in step B3, an estimate of the internal disturbance is obtained
And
the method is realized by the following formula:
and
wherein,
v
αand v
βIs alpha component and beta component of converter AC side voltage in alpha beta coordinate system, L is AC side line inductance, alpha
3Is a preceding-stage gain parameter, alpha, of a high-gain observer of an active power loop
4Is a back-stage gain parameter, alpha, of the active power loop high-gain observer
5Is the preceding-stage gain parameter, alpha, of the high-gain observer of the reactive power loop
6Is a post-stage gain parameter, epsilon, of a reactive power loop high-gain observer
pIs the gain, ε, of a high-gain observer of an active power loop
qIs the gain of the reactive power loop high-gain observer.
In this embodiment, the control target to be realized is to control the actual active power value p and the actual reactive power value q so that they respectively track their reference values. The goal is achieved here using the SOSM control strategy to ensure fast response and steady state performance of the system. In addition, in order to improve the robustness of the system to the parameter perturbation, an HGO is designed to inhibit the influence of uncertain parameters on the system. The SOSM is called a Second-order sliding mode in English; the HGO is generally known in english as a High-gain observer.
Further, in the present embodiment, the first and second substrates,
in step B4, the real power correction u at the current sampling point timepAnd a reactive power correction uqComprises the following steps:
further, in the present embodiment, the first and second substrates,
in step B5, average duty ratio of equivalent point
Comprises the following steps:
wherein v is
αβThe voltage of the AC side of the converter under an alpha beta coordinate system; j is a matrix, and
omega is the angular frequency of the grid voltage; l is an alternating current side wire inductor.
Further, in the present embodiment, the first and second substrates,
using a voltage balancing loop to measure the actual value e of the DC-side unbalanced voltage
dcWith reference value of DC-side unbalanced voltage
Making difference, and performing PI regulation on the difference value of the two values to obtain a balance duty ratio delta
baComprises the following steps:
wherein k ispbThe proportional link gain of the PI controller is obtained; k is a radical ofibThe integral link gain of the PI controller is obtained; t is time.
In order to verify the superiority of the control strategy provided by the application, the sliding mode control method of the three-level NPC converter is compared with the traditional PI control strategy through experiments, wherein the sliding mode control strategy (LESO-SMC) is based on an extended state observer, and the sliding mode control strategy (NHGO-SMC) is based on a nonlinear high-gain observer. The parameters of the three-level NPC converter are shown in table I.
Table I experimental platform parameters
First, a test of dynamic performance was performed. Fig. 3a, 3b, 3c and 3d are dynamic response graphs when a voltage command is changed from 750V to 690V by using a PI controller, a LESO-SMC controller, a NHGO-SMC controller and a NHGO-ASMC controller proposed in the present application (i.e., a sliding mode control method of a three-level NPC converter according to the present invention), and fig. 4a, 4b, 4c and 4d are dynamic response graphs when a voltage command is changed from 690V to 750V, respectively. The results of the two experimental comparisons show that the dynamic response time is short, and the overshoot voltage is small.
Next, a test for steady state performance was performed. Connecting a DC load R1And R2And when the NPC operates stably, observing the quality of the three-phase alternating current under different control strategies. Fig. 5a, 5b, 5c and 5d are graphs of current harmonic spectra using a PI controller, a LESO-SMC controller, a NHGO-SMC controller, and a NHGO-ASMC controller as proposed in the present application, respectively, with Total Harmonic Distortion (THD) of 2.1%, 2.2%, 2.1% and 2.0%, respectively. Obviously, the current THD obtained by the NHGO-ASMC control strategy provided by the application is lower, and the current quality is better.
Finally, the disturbance resistance performance test is carried out. At R1The transient response of the DC bus voltage and phase a current when switched into the circuit is shown in FIG. 6, FIGS. 6a, 6b, 6c andFIG. 6d is a transient response using a PI controller, a LESO-SMC controller, a NHGO-SMC controller, and the NHGO-ASMC controller proposed herein, respectively. It can be observed that the method proposed by the present application achieves smaller voltage fluctuations and recovery times than other controllers. Therefore, according to the experimental results, compared with other control strategies, the NHGO-ASMC control strategy provided by the application has better anti-interference capability.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that features described in different dependent claims and herein may be combined in ways different from those described in the original claims. It is also to be understood that features described in connection with individual embodiments may be used in other described embodiments.