Disclosure of Invention
In order to improve the dynamic performance of the direct-current output voltage of the three-phase PWM rectifier, the invention provides a direct power boundary control method based on a three-phase three-throw two-level rectifier, which can effectively improve the dynamic performance of the direct-current output voltage of the three-phase PWM rectifier. The control method is based on a mathematical model under a dq coordinate system, and deduces a natural track of a rectifying system when the current on the d-axis alternating current side is lowered and raised; combining with instantaneous power theory to obtain P, Q-variable direct power control model, and outputting S to direct power hysteresis comparatorqAnd a new rule is adopted, and the analyzed natural track is used as a switch surface, so that the effect of boundary control is achieved.
The technical scheme adopted by the invention is as follows:
the direct power boundary control method based on the three-phase three-opening two-level rectifier comprises the following steps of:
step 1: analyzing the working process of the three-phase three-switch two-level rectifier, and establishing a mathematical model of the rectifier under a synchronous rotation dq coordinate system by using coordinate transformation;
step 2: combining an instantaneous power theory, converting a mathematical model of the rectifier under the synchronous rotation dq coordinate system into a power model with P and Q as variables under the dq coordinate system;
and step 3: analyzing the boundary control conditions of the three-phase three-switch two-level rectifier, namely establishing a standard phase plane by taking the voltage at the direct current side as the horizontal axis of the phase plane and the current at the alternating current side as the vertical axis of the phase plane; in a standard phase plane, the rectifier has different natural tracks in different states, and the natural tracks of the rectifier when the current on the alternating current side is reduced and increased are analyzed;
and 4, step 4: based on the power quantity, firstly, a natural switch surface of boundary control is selected according to a natural track of the rectifier when the alternating current side current is reduced and increased, then the rule of the output of a power hysteresis comparator in direct power control is updated by using the natural switch surface, and finally the direct power boundary control method based on the three-phase three-switch two-level rectifier is obtained.
Step 1, analyzing the working process of the three-phase three-switch two-level rectifier, defining a switch function, and establishing a mathematical model of the three-phase three-switch two-level rectifier under a three-phase static coordinate system; because a mathematical model under a three-phase static coordinate system is complex, coordinate transformation needs to be introduced into a modeling process of the system, and the mathematical model of the three-phase three-switch two-level rectifier under a synchronous rotation dq coordinate system is obtained by utilizing the coordinate transformation.
In step 2, obtaining a calculation formula of instantaneous active power and instantaneous reactive power based on a dq coordinate system according to an instantaneous power theory; and (3) adopting the voltage orientation of the power grid, selecting the initial phase angle of the d axis to be equal to the initial phase angle of the a phase, and substituting the calculation formula into the mathematical model under the synchronous rotation dq coordinate system established in the step 1 to obtain a power model with P and Q as variables under the dq coordinate system.
And 3, establishing a phase plane by taking the voltage at the direct current side as the horizontal axis of the phase plane and the current at the alternating current side as the vertical axis of the phase plane, and obtaining the natural tracks of the rectifier when the current at the alternating current side is reduced and when the current is increased in the standardized phase plane through simple standardized calculation.
In step 4, in the direct power boundary control method, the reactive power hysteresis comparator outputs SQNumerical valueThe rule and the switching vector table are the same as the traditional direct power control method, and only the output S of the direct active power hysteresis comparatorPThe rule of the values is different from the conventional direct power control method.
The invention discloses a direct power boundary control method based on a three-phase three-opening two-level rectifier, which has the following technical effects:
1: direct power control is created by using boundary control conditions based on power quantities, using a new rule to control the power hysteresis comparator output SpThe numerical value of (c).
2: aiming at the problem of load sudden change in practical application, the dynamic performance of the direct-current side voltage of the three-phase three-throw two-level rectifier can be effectively improved.
Detailed Description
Step 1: analyzing the working process of the three-phase three-switch two-level rectifier, and establishing a mathematical model of the rectifier under a synchronous rotation dq coordinate system by using coordinate transformation;
step 2: combining an instantaneous power theory, converting a mathematical model of the rectifier under the synchronous rotation dq coordinate system into a power model with P and Q as variables under the dq coordinate system;
and step 3: analyzing the boundary control conditions of the three-phase three-switch two-level rectifier, namely establishing a standard phase plane by taking the voltage at the direct current side as the horizontal axis of the phase plane and the current at the alternating current side as the vertical axis of the phase plane; in a standard phase plane, the rectifier has different natural tracks in different states, and the natural tracks of the rectifier when the current on the alternating current side is reduced and increased are analyzed;
and 4, step 4: based on the power quantity, firstly, a natural switch surface of boundary control is selected according to a natural track of the rectifier when the alternating current side current is reduced and increased, then the rule of the output of a power hysteresis comparator in direct power control is updated by using the natural switch surface, and finally the direct power boundary control method based on the three-phase three-switch two-level rectifier is obtained.
Step 1, analyzing the working process of the three-phase three-switch two-level rectifier, defining a switch function, and establishing a mathematical model of the three-phase three-switch two-level rectifier under a three-phase static coordinate system; because a mathematical model under a three-phase static coordinate system is complex, coordinate transformation needs to be introduced into a modeling process of the system, and the mathematical model of the three-phase three-switch two-level rectifier under a synchronous rotation dq coordinate system is obtained by utilizing the coordinate transformation.
In step 2, obtaining a calculation formula of instantaneous active power and instantaneous reactive power based on a dq coordinate system according to an instantaneous power theory; and (3) adopting the voltage orientation of the power grid, selecting the initial phase angle of the d axis to be equal to the initial phase angle of the a phase, and substituting the calculation formula into the mathematical model under the synchronous rotation dq coordinate system established in the step 1 to obtain a power model with P and Q as variables under the dq coordinate system.
And 3, establishing a phase plane by taking the voltage at the direct current side as the horizontal axis of the phase plane and the current at the alternating current side as the vertical axis of the phase plane, and obtaining the natural tracks of the rectifier when the current at the alternating current side is reduced and when the current is increased in the standardized phase plane through simple standardized calculation.
In step 4, in the direct power boundary control method, the reactive power hysteresis comparator outputs SQThe rule of the numerical value and the switching vector table are the same as the traditional direct power control method, and only the direct active power hysteresis comparator outputsGoes out of SPThe rule of the values is different from the conventional direct power control method.
The method comprises the following specific steps:
step 1: as can be seen from the analysis of the topology structure of the three-phase three-switch two-level rectifier shown in fig. 1, the three-phase three-switch two-level rectifier is a PWM rectifier, the rectifying circuit includes three switching tubes, and the output voltage at the dc side is only vdcAnd-vdcTwo types of three-phase three-switch two-level rectifiers have the characteristics of simplicity, strong robustness, easy acquisition of power modules and auxiliary devices and the like.
ea、eb、ecThe three-phase static coordinate system is the power grid electromotive force; i.e. ia、ib、icThree-phase current at the alternating current side under a three-phase static coordinate system; l isA、LB、LCIs an AC side inductor, LA=LB=LCL; r is an equivalent resistance of an alternating current side circuit; c is a direct current side capacitor; rLIs a direct current side load; v. ofdcIs a direct current side voltage; e.g. of the typed、eq、id、iqThe voltage and current of the AC side under the two-phase synchronous rotating coordinate system.
In order to establish a general mathematical model of a three-phase three-opening two-level rectifier, the following assumptions are made: 1) the electromotive force of the power grid is an ideal three-phase sine wave; 2) the alternating current filter inductor is linear and is not saturated; 3) the power switch tube ignores dead time and is an ideal switch.
Defining a switching function:
a. b and c represent a three-phase stationary coordinate system.
Establishing a general mathematical model of a three-phase three-switch two-level rectifier:
Sa、Sb、Screpresenting abc three-phase switching functions, respectively.
In addition, kirchhoff's current law is applied to the positive node of the direct current measurement capacitor, and the following results are obtained:
c represents a DC capacitor, RLRepresenting a dc load.
And (3) adopting coordinate transformation to the equations (2) and (3), transforming the three-phase stationary coordinate system (a, b, c) to the two-phase synchronous rotating coordinate system (d, q), and obtaining a mathematical model of the three-phase three-switch two-level rectifier as follows:
in the formula: w is angular velocity, Sd、SqIs a switching function transformed into dq coordinate system.
Step 2: and (3) balancing the three-phase power grid, and obtaining the calculation formula of instantaneous active power and instantaneous reactive power under the dq coordinate system by combining an instantaneous power theory:
P=edid+eqiq,Q=eqid-ediq (5)
by using grid voltage orientation, e is obtainedqWhen the above formula is substituted by formula (5), 0 can be obtained:
P=edid,Q=-ediq (6)
multiplying both sides of the formula (4) by edAnd obtaining a power model with P and Q as variables under the dq coordinate system as follows:
and step 3: firstly, two axes of a phase plane are selected, and according to the existence and uniqueness theory of differential equation solutions, under the condition that any initial data condition is selected, a phase track is required to correspond to the phase plane on the phase plane. For a rectifying system, any of its states corresponds to a point in the phase plane, so that system motion can be represented in the phase plane. The movement of points on the phase plane corresponds to the change of the system state with time, and these movement trajectories are the phase trajectories. The switch surface used by the boundary control of the invention is a natural switch surface, so that the alternating current side current is selected as the longitudinal axis of the phase plane, the direct current side voltage is selected as the transverse axis of the phase plane, the phase plane is established, and the natural track is selected as the phase track.
To obtain the specific power factor, i is controlledqWhen the inductance equivalent resistance R is negligibly small, i.e., R is 0, equation (4) can be simplified as:
v in formula (8)
Sd=v
dcS
d,
According to the operating principle of three-phase three-switch two-level rectifiers v
SdThe maximum value of (d) is:
when the AC side current decreases, v
SdShould be positive, i.e.
Order to
Substituting the compound in the formula (8) to obtain:
by using the following trigonometric identity:
equation (10) can be transformed into the following form:
wherein k is and ide、vdcInitial value-dependent constants.
Let v
n=v
dc,
e
dn=e
d,
And substituting the natural path lambda of the rectifying system when the alternating current side current is reduced in an expression (11)
downComprises the following steps:
in the normal phase plane, λ is shown in FIG. 2
downIs one to
A circle with a radius of l as the center of the circle.
When the AC side current rises, v
SdShould be negative, i.e.
The natural track lambda of the rectifying system is obtained when the current of the alternating current side rises by adopting the same derivation method when the current of the alternating current side falls
upComprises the following steps:
in the normal phase plane, λ, as shown in FIG. 3
upIs one to
A circle with m as the radius as the center of the circle.
And 4, step 4: since only the active power has an effect on the DC output voltage, and inCan be represented by a change in the instantaneous active power P, and a change in the state of the switch will cause a change in P. Therefore, the system operation track can be controlled by the method.
In order to obtain the direct power boundary control rule of the three-phase three-switch two-level rectifier, the following definitions are firstly made:
i in formulae (14) and (15)n_T、vn_TAre the current and voltage at the operating target point on the phase plane.
By natural trajectory λ when the alternating side current is reduceddownAnd a natural locus lambda of when the alternating-current side current risesupThe selected boundary condition natural switch surfaces are:
RT_downindicating a falling boundary resistance
ednRepresenting the voltage to ground on the DC side, RT_upRepresenting the rising boundary resistance.
The α β plane is divided into 12 vector sectors, each of which is 30 degrees in the α β plane, and the phase angle range thereof can be represented by equation (18).
When the space where the grid voltage vector is located needs to be determined, the phase angle of the grid voltage vector E is calculated firstly
e
α、e
βThe interval of the grid voltage vector E is determined by using an equation (18) for the alternating-current side voltage in the alpha beta coordinate system.
Reactive power hysteresis comparator output S in direct power boundary control algorithm of three-phase three-throw two-level rectifierQThe rules for the values are the same as the conventional direct power control rules:
in the formula (19), q is an instantaneous reactive power estimated value, qrFor instantaneous reactive power reference value, HqThe hysteresis width of the reactive power hysteresis comparator. Generally, the hysteresis width of the hysteresis comparator is affected by parameters in the main circuit, such as the inductance L on the ac side and the voltage v on the dc sidedc. If the hysteresis width is too small, the switching frequency is too high, and as a result, the loss of the system switch is increased, and the aging of the switch is accelerated; if the hysteresis width is too large, the power tracking is too slow to satisfy real-time control. Therefore, when designing a system, the size of the hysteresis loop width should be selected to meet the system requirement and meet the practical requirement.
Active power hysteresis in direct power boundary control algorithm of three-phase three-switch two-level rectifierOutput S of loop comparatorPThe value adopts a new rule:
i: if v isdc<vn_TOnly when σdown<At 0, S P1, otherwise SP=0;
II: if v isdc>vn_TOnly when σup>At 0, S P0, otherwise SP=1。
Firstly, a sector where a power grid voltage vector is located is judged by adopting a formula (18), and then S is output according to a power hysteresis comparatorP、SQThe sector of the grid voltage vector and the S outputP、SQAnd sending the switching vector into a switching table to select the switching vector.
The switch table used in the direct power boundary control algorithm of the three-phase three-switch two-level rectifier is the same as the switch table used in the traditional direct power control:
TABLE 1 direct power boundary control algorithm switching table for three-phase three-switch two-level rectifier
Setting line parameters of a three-phase three-switch two-level rectifier: the effective value of the three-phase circuit is 220V/50Hz, and the input inductance L at the alternating current sideA=LB=LCThe line equivalent resistance R is 0.1 Ω, the dc-side filter capacitance C is 3300 μ F, the load R is 30 Ω, the switching frequency is 20kHz, and at 0.15s, the load is doubled on the dc side.
When the load is rated, the waveform diagram of the A-phase voltage current at the AC side of the three-phase three-switch two-level rectifier is shown in figure 5, and the current at the network side of the three-phase three-switch two-level rectifier adopting a direct power boundary control algorithm can be obtained from the waveform diagram, so that the sine is realized, the voltage current is basically in the same phase, and the power factor is close to 1.
When the load is suddenly changed to 2 times of load from the rated load, the A-phase current waveform diagram of the AC side of the three-phase three-switch two-level rectifier is shown in fig. 6, and the current of the network side of the three-phase three-switch two-level rectifier adopting the direct power boundary control algorithm can quickly reach a stable state when the load fluctuates.
When the load suddenly changes from the rated load to 2 times of the load, the voltage and current waveform diagram of the direct current side of the three-phase three-switch two-level rectifier is shown in fig. 7, and the direct current side voltage and current of the three-phase three-switch two-level rectifier adopting the direct power boundary control algorithm can be obtained from the diagram and reach the stable state again in a short time when the load fluctuates, so that the three-phase three-switch two-level rectifier has good dynamic performance.
From the analysis of the oscillogram, the direct power boundary control algorithm based on the three-phase three-switch two-level rectifier can effectively improve the dynamic performance of the direct current output current and the direct current output voltage of the three-phase three-switch two-level rectifier, achieves the expected experimental result and has certain practical value.