CN111541382B - A control method for Vienna rectifier current distortion under heavy load - Google Patents

Abstract

The invention discloses a method for controlling the current distortion of a Vienna rectifier under the condition of heavy load, which obtains the modulation degree m of the Vienna rectifier according to the actual operation condition, and then obtains the modulation degree m of the Vienna rectifier according to m and theta_mTo obtain a corresponding maximum allowable lag angle theta_m(ii) a Calculating the actual lag angle theta and the maximum allowable lag angle theta_mComparing to determine whether to compensate for reactive power, if theta is less than or equal to theta_mNo compensation is required, if theta>θ_mReactive power needs to be compensated; and (5) completing the control of the Vienna rectifier by adopting a zero-sequence component injection method. The invention can not only improve the current distortion under the condition of large load, but also maximize the power factor angle.

Description

A control method for Vienna rectifier current distortion under heavy load

technical field

The invention relates to power electronic technology, in particular to a control method for Vienna rectifier current distortion under the condition of heavy load.

Background technique

For the Vienna rectifier, due to the included angle between the phase current and the reference voltage vector, the diode in the topology will force commutation when the three-phase current crosses the zero point, which will cause current distortion and lead to current deterioration. The traditional zero-sequence component injection method can solve the current distortion problem under certain circumstances, but this method is limited by the modulation degree, which leads to the limitation of the angle between the phase current and the reference voltage vector, so when the load current increases When it exceeds a certain limit, only the zero-sequence component injection method will no longer be applicable. The principle of the zero-sequence component injection method is described in detail below.

Figure 1 shows the topology of a traditional three-phase three-wire Vienna rectifier. The Vienna rectifier is a three-level converter. Figure 2 is a three-level space vector diagram. According to the three-phase voltage distribution, it is divided into 6 sectors, from sector I to sector I District VI. Due to the voltage drop on the inductor, a lag angle is generated between the reference voltage v _ref and the current is _s , as shown in sector I in Figure 2. Assuming that v _ref operates in the a region under a certain modulation degree, the basic vector combination is [100], [10-1], [00-1], [0-1-1]. But when entering the b region, due to the uncontrollable characteristics of the diode, it can only output [01-1] instead of [0-1-1]. Therefore, the current distortion can be avoided by adopting a five-stage SVPWM such as [000]→[100]→[10-1]→[100]→[000]. In other words, current distortion can be avoided as long as the switch is kept open when the current is commutated. Therefore, the analysis of the first sector is extended to the other sectors, and the analysis in SVPWM is equivalent to the calculation in SPWM to obtain the zero-sequence component injection method.

The traditional zero-sequence component method is limited by the modulation degree m, as shown in the first sector space vector diagram in Figure 3, when v _ref operates in regions 1 and 2, it can be replaced by a redundant vector, when entering region 3, only The large vector [1-1-1] can be used and there is no redundant vector to replace, and [1-1-1] itself cannot be output during current commutation, so once it enters region 3, if the modulation degree m exceeds a certain limit , the current must be distorted.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a control method for Vienna rectifier current distortion under the condition of heavy load.

The technical solution for realizing the object of the present invention is: a method for controlling the current distortion of Vienna rectifiers under a large load, comprising the following steps:

Step 1. Obtain the modulation degree m of the Vienna rectifier from the actual operating conditions, and then obtain the corresponding maximum allowable lag angle θ _{m according to the relationship between m and θ m ;}

Step 2: Calculate the actual lag angle θ and compare it with the maximum allowable lag angle _{θ m} to determine whether reactive power _compensation is required.

Step 3. Use the zero-sequence component injection method to complete the Vienna rectifier control.

Further, in step 1, the relationship between m and θ _m is

Further, in step 2, the actual lag angle calculation formula is

Among them, _{id and ed are the active current and grid voltage respectively, V out is the output voltage of the DC side, and i out is the output} current.

Further, in step 3, it is necessary to compensate the reactive power for

i _q ^* = i _d ^* tanΔθ

where Δθ is obtained by

θ=θ _v -Δθ

In the formula, θ _v is the angle between the reference vector _{v ref} and the grid voltage ed, _id is the active current, ω is the grid angular frequency, and L is the inductance value of the inductance; as the compensated reactive power increases, θ _v will also be slightly Therefore, it is difficult to directly determine Δθ, so that the actual lag angle θ is equal to the maximum lag angle θ _m allowed by the current modulation degree, and then θv and Δθ can be obtained by the above formula.

Compared with the prior art, the present invention has significant advantages in: analyzing the space vector diagram of the Vienna rectifier, determining the applicable range of the traditional zero-sequence component injection method, and reducing the current and reference voltage vector by compensating a certain reactive power The lag angle θ between the two and make it within the range of the zero-sequence component method, so that the zero-sequence component injection method can be used again, which can not only improve the current distortion under heavy load conditions, but also maximize the power factor angle.

Description of drawings

Figure 1 is a topology diagram of a three-phase three-wire Vienna rectifier.

Figure 2 is a three-level space vector diagram. .

FIG. 3 is a space vector diagram of the first sector.

FIG. 4 is a graph showing the relationship between the modulation degree m and the maximum allowable lag angle θ _m .

Figure 5 is an operational vector diagram of unity power factor.

Figure 6 is a schematic diagram of reactive power compensation.

FIG. 7 is a graph showing the relationship between θ _m and Δθ and θ _v and Δθ.

FIG. 8 is a current simulation result graph.

Detailed ways

The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and specific examples.

Step 1. Obtain the modulation degree m of the Vienna rectifier from the actual operating conditions, and then obtain the corresponding maximum allowable lag angle θ _{m according to the relationship between m and θ m .}

In this step, the relationship between the modulation degree m and the maximum allowable lag angle θ _m can be obtained by analyzing the first sector space vector diagram in FIG. 3 , as follows:

For the convenience of expression, normalization is used in the above formula, assuming that the length of the side of the triangle is 2, the length of the reference vector is ɑ, and the length of the other side is x.

Simplification can get the relationship between m and θ _m :

Curve 4 can be drawn according to the relational expression, from which it can be seen that when the modulation degree m increases, the maximum allowable lag angle θ _m decreases gradually. The maximum allowable lag angle can be obtained by selecting θ _m corresponding to the modulation degree m.

Step 2: Calculate the actual lag angle θ and compare it with the maximum allowable lag angle _{θ m} to determine whether reactive power _compensation is required.

The expression for the actual lag angle θ is as follows

Among them, _{id and ed are the active current and grid voltage respectively, V out is the output voltage of the DC side, and i out is the output} current.

Step 3. When θ>θ _m , the reactive power compensation needs to be i _q ^* = id ^* _tanΔθ , and then combined with the zero-sequence component injection method to solve the problem.

Step 3 will be described in detail below. Fig. 5 is the operation vector diagram of the rectifier when the power factor is unity, wherein the lag angle between the current and the reference vector reaches a maximum of θ _m . When the load increases, the output current i _out increases, and the _id also increases. When the _id increases to the value shown in Figure 6, the corresponding lag angle θ>θ _m , by compensating a certain reactive power to make the _id and the The lag angle θ of _vref decreases, and the compensated current is i as shown in Figure 6. It only needs to be compensated to use the zero-sequence component injection method when θ= _θm , which can ensure that the power factor is maximized, but it can be found from Figure 6 that after compensating for a certain amount of reactive power, the reference voltage vector v _ref and the grid The angle between the voltages _{ed will also increase slightly, denoted as θ v ,} which makes it difficult to determine the specific value of the compensation reactive power, but the relationship between the lag angle θ and the compensation angles Δθ and θ _v can be obtained from Figure 6:

θ=θ _v -Δθ (4)

From the cosine theorem and the Pythagorean theorem, we can get:

In the above formula, u _L is the inductor voltage, which can be simplified to the following formula:

From Figure 6, we can intuitively see the change of v _ref , that is, after compensating for a certain amount of reactive power, v _ref decreases, so that the modulation degree m also decreases, so the maximum allowable lag angle θ _m actually increases. In other words, by compensating for reactive power, the original θ>θ _m can be changed to θ≤θ _m . In order to maximize the power factor angle, it is only necessary to satisfy θ=θ _m , and then use formulas (4) and (6) Solve θ _v and Δθ at this time, Δθ is the reactive angle that should be compensated, and the reactive power command is

i _q ^* = _tanΔθ ·id ^* (7)

Example

In order to verify the effectiveness of the scheme of the present invention, the following simulation experiments are carried out.

The experimental simulation model is built in Matlab/Simulink, and the system parameters are shown in Table 1. When the load increases to make id =12A, the _vref at this time is calculated and the modulation degree m= _0.894 is obtained, the corresponding θ _m =4°, but the actual lag angle at this time is 5.08°, so simply use the zero sequence The component injection method still suffers from current distortion. Therefore, the method of compensating reactive power is used to draw the relationship between θ _m and Δθ and the relationship between θ _v and Δθ. As shown in Figure 7, the increment of θ _v is very small when it increases, and the corresponding Δθ is selected according to the calculation. The reactive current command is about 0.21A.

Table 1 Vienna rectifier circuit parameters

parameter Numerical value Grid voltage (phase) 60V(50Hz) DC side voltage 165V Filter inductor L 2mH DC side capacitor C 2.05mF On-off level 30kHz

The simulation is shown in Figure 8. Before 0.5s, it is the current situation after only the zero-sequence component injection method. The injected components are shown in Table 2. After 0.5s, after compensating for part of the reactive power, the current is improved.

Table 2 Zero-sequence component injection method

sector zero sequence component injection range I -V_b V_a-2V_b+V_c&gt;0,V_a-V<sub>b</ sub>&lt;1,V_a-V_c&gt;1 II -V_a 2V_a-V_b-V_c&gt;0,V_a-V<sub>c</ sub>&lt;1,V_b-V_c&gt;1 III -V_c V_a+V_b-2V_c&gt;0,V_a-V<sub>b</ sub>&lt;-1,V_b-V_c&lt;1 IV -V_b V_a-2V_b+V_c&lt;0,V_a-V<sub>b</ sub>&gt;-1,V_a-V_c&lt;-1 V -V_a 2V_a-V_b-V_c&lt;0,V_a-V<sub>c</ sub>&gt;-1,V_b-V_c&lt;-1 VI -V_c V_a+V_b-2V_c&lt;0,V_a-V<sub>b</ sub>&gt;1,V_b-V_c&gt;-1

Claims (3)

1. the control method of Vienna rectifier current distortion under a large load situation, is characterized in that, comprises the steps: Step 1. Obtain the modulation degree m of the Vienna rectifier from the actual operating conditions, and then obtain the corresponding maximum allowable lag angle θ _{m according to the relationship between m and θ m ;} Step 2: Calculate the actual lag angle θ and compare it with the maximum allowable lag angle _{θ m} to determine whether reactive power _compensation is required. Step 3. When compensating for reactive power, the zero-sequence component injection method is used to complete the Vienna rectifier control. The reactive power compensation needs to be i _q ^* = i _d ^* tanΔθ Among them, i _d ^* and i _q ^* are the reference values of active current and reactive current, respectively, and Δθ is obtained by the following formula θ=θ _v -Δθ

In the formula, _θv is the angle between the reference vector _vref and the grid voltage _ed , _id is the active current, ω is the grid voltage fundamental frequency, L is the input side filter inductance; let the actual lag angle θ equal to the current modulation system The maximum allowable lag angle θ _m , and then θv and Δθ can be obtained by solving the above formula. 2. the control method of Vienna rectifier current distortion under the large load situation as claimed in claim 1, is characterized in that: in step 1, the relational expression of m and θ _m is

3. the control method of Vienna rectifier current distortion under the heavy load situation as claimed in claim 1 is characterized in that: in step 2, the actual lag angle calculation formula is

Among them, _id and _ed are the active current and grid voltage respectively, V _out is the DC side output voltage, i _out is the output current, ω is the grid voltage fundamental frequency, and L is the input side filter inductance.

Images