WO2013004067A1 - Parallel structure of three-phase multi-level pwm converters - Google Patents

Abstract

The present invention relates to a parallel structure of three-phase multi-level PWM converters which are suitable for distributed power supplies, and systems for grid-connected power generation with renewable energies, charging and discharging and energy storage therefor and the like. Then present invention includes more than two three-phase multi-level PWM converters and control circuit units thereof, a digital signal processor and a three-phase alternating current power supply circuit; and further includes a common unified voltage regulator, the unified voltage regulator consisting of a voltage sampling circuit, a voltage sensor, a third adder and an external loop voltage PI regulator, the voltage sampling circuit being connected in parallel to a load R_L. The advantages of the present invention are as follows: the energy flow direction consistency problem of each three-phase multi-level PWM converter when operating in parallel is solved, the generation of a loop current is avoided, and at the same time the problem of equal current when each three-phase multi-level PWM converter operates in parallel is solved. The present invention is suitable for scenarios in which high power and scaled power electronics conversion is applied.

Description

 Parallel structure of three-phase multilevel PWM converter

 Field of the Invention This invention relates to the field of electrical power, and more particularly to a parallel configuration of a three-phase multi-level PWM converter.

Background technique

 In recent years, power electronics technology has been continuously developed and is widely used in various fields requiring power conversion. In the field of low-voltage and low-power electricity, power electronics technology has matured, and in the field of high-voltage and high-power applications, multi-level power conversion technology has gradually become the core and hot issue of research.

 Multi-level conversion technology is a new type of converter that realizes high-voltage and high-power output by improving the topology of the converter itself. It does not require a step-up transformer and a voltage-dividing transformer circuit. As the number of output voltage levels increases, the output waveform has a better harmonic spectrum, and the voltage stress experienced by each switching device is reduced. Multi-level conversion technology has become a new research field in power electronics technology, with high-voltage and high-power conversion as the research object. The reason why the multilevel converter has become a research hotspot of high voltage and high power conversion is because it has the following characteristics:

 (1) Each power device is only subjected to a bus voltage of 1/(n-1) (n is the number of levels);

 (2) The increase in the number of levels improves the output voltage waveform and reduces the output voltage waveform distortion (THD);

 (3) The same output voltage waveform as the two-level converter at high switching frequency can be obtained at a lower switching frequency, thereby reducing switching loss;

 (4) No output transformer is needed, which greatly reduces the volume and loss of the system.

The multi-level PWM converter can effectively suppress the higher harmonics caused by the PWM control. The PWM control can reduce the low-order harmonics contained in the stepped voltage output from the multi-level PWM converter, so the combination of the two can be optimal. Spectral characteristics. With the development of technologies such as distributed power, renewable energy generation, grid charging, charging and discharging, and energy storage, the application of multi-level PWM converters has been further deepened. High-power, large-scale engineering applications are imperative. However, the method of multi-level PWM converters using parallel building blocks to expand capacity has encountered problems. Since the multi-level PWM converter generally adopts a double closed-loop control strategy, the outer loop controls the DC output voltage U _{dc of the} multi-level PWM converter, the inner loop controls the AC current of the converter network side; and the constant-value control DC voltage U _{dc is} satisfied. Under the target, the energy is automatically bidirectionally transformed, ie: when the DC side voltage is higher than the given value, the energy is automatically flowing from the DC side to the converter network side while the regulator is acting; when the DC side voltage is lower than the given value, At the same time as the regulator acts, the energy automatically flows from the converter network side to the DC side. Due to the given parameters of each converter and the dispersion of the adjustment parameters, it may cause small differences in the given parameters and the inconsistency of the adjustment parameters. When two or more multi-level PWM converters are connected in parallel, at the same time, it is possible Part of the converter works in the rectified state, and some converters work in the inverter state. Since the internal resistance of each multi-level PWM converter is extremely small, this partial rectification and partial reversal may form a large circulating current between parallel multi-level PWM converters, which may affect the normal operation of the converter. , affecting the stability of the entire system, reducing the performance of the system; seriously harm, and even damage the parallel structure of the three-phase multi-level PWM converter. Therefore, multilevel PWM converters are generally not allowed to work in parallel.

Summary of the invention

 The technical problem to be solved by the present invention is to provide a three-phase multi-electricity capable of solving the consistency problem of the energy flow direction of the parallel multi-level PWM converter, avoiding the generation of the circulating current, and solving the parallel current sharing problem of the multi-level PWM converter. The parallel structure of the flat PWM converter.

 The technical solution adopted by the present invention to solve the technical problem thereof:

 A parallel structure of a three-phase multi-level PWM converter including two or more three-phase multi-levels

a PWM converter and its control circuit unit, a digital signal processor and a three-phase AC power supply circuit; it further includes a common unified voltage regulator; inputs of the three-phase multi-level PWM converters and their control circuit units The parallel connection is followed by the three-phase AC power supply circuit, and the output ends thereof are connected in parallel and connected to the same load R _L ; the unified voltage regulator is composed of a voltage sampling circuit, a voltage sensor, a third adder and an outer loop voltage PI regulator; The voltage sampling circuit and the load! ^Parallel, the voltage sampling circuit The output end is connected to the input end of the outer loop voltage PI regulator via the voltage sensor and the third adder, and the other input end of the third adder is connected to the direct current of each three-phase multilevel PWM converter The output voltage U _dc * is output, and the output terminals of the outer loop voltage PI regulator are respectively connected to respective input terminals of the control circuits in the three-phase multi-level PWM converters and their control circuit units.

The three-phase AC power supply circuit is composed of a phase A, a B phase, a C phase, a resistor R ₃ - R ₅ , and an inductor L _r L ₃ of a three-phase AC power supply; a first bridge of the three-phase multilevel PWM converter The point a of the upper and lower arm of the arm is sequentially passed through the resistor R ₃ and the inductor L ^ A phase, and the b point of the upper arm of the second bridge arm is sequentially connected to the B phase through the resistor R ₄ and the inductor L ₂ , and the third bridge arm The c-point of the upper and lower arm connection is connected to the C phase through the resistor R ₅ and the inductor L ₃ , and the A phase, the B phase, and the C phase are connected to the center point N.

The voltage sampling circuit is a voltage dividing circuit composed of a resistor and a resistor R _{2 in} series. The resistor is connected in series with R _{2 in} parallel with the load, and the node of the resistor and the resistor R ₂ is connected to the input end of the voltage sensor.

 The beneficial effects of the present invention are as follows:

 (1) Separate the outer loop voltage regulators of each parallel three-phase multilevel PWM converter to form a common unified voltage regulator, which solves the energy flow of each three-phase multilevel PWM converter in parallel operation. The consistency problem avoids the generation of circulation.

 (2) The output voltage of the unified voltage regulator is used as the active component of each parallel three-phase multi-level PWM converter, and the power factor or grid-side reactive power is converted into the reactive current reference component. The multi-level PWM converter adopts closed-loop current control to realize parallel current sharing control, and solves the current sharing problem when each three-phase multi-level PWM converter is operated in parallel.

 (3) The present invention is applicable to high power, large scale power electronic converter applications.

DRAWINGS

 Figure 1 is a schematic block diagram of the present invention;

 2 is a topological structural diagram of a main circuit of a typical three-phase three-level PWM converter;

3 is a schematic block diagram of a control circuit of a three-phase multi-level PWM converter; 4 is a block diagram of a specific implementation of a multi-cell parallel connection of a three-phase multi-level PWM converter.

detailed description

 This embodiment is a parallel structure of a three-phase multilevel PWM converter (three-phase three-level PWM converter) (see Figs. 1-4).

As shown in FIG. 1, the embodiment includes two or more three-phase multi-level PWM converters, a control circuit unit thereof and a digital signal processor; and a common unified voltage regulator; The input terminal of the level PWM converter and its control circuit unit is connected in parallel to the same three-phase AC power supply circuit, and the output ends thereof are connected in parallel to the same load R _L ; the unified voltage regulator is composed of a voltage sampling circuit 1 and a voltage sensor 2 The third adder 3 and the outer loop voltage PI regulator 4 are configured; the voltage sampling circuit 1 is connected in parallel with the load R _L , and the output end of the voltage sampling circuit 1 is sequentially subjected to the voltage sensor 2 and the third addition The third input of the third adder 3 is connected to the DC output given voltage U _dc * of the three-phase multi-level PWM converter, The output terminals of the outer loop voltage PI regulator 4 are respectively connected to respective input terminals of the control circuits in the three-phase multi-level PWM converters and their control circuit units.

2 is a topology diagram of a main circuit of a three-phase three-level PWM converter. The power switch tube V ₁₂ is an IGBT including an anti-parallel diode, and ¥ and ¥ ₄ constitute a first upper arm. with. Forming the first lower arm, V ₂ and V ₅ constitute the second upper arm, V ₈ and V _u constitute the second lower arm, V ₃ and V ₆ constitute the third upper arm, and V ₉ and V ₁₂ constitute the third lower arm, first The upper arm and the first lower arm are connected in series to form a first bridge arm, the second upper arm and the second lower arm are connected in series to form a second bridge arm, the third upper arm and the third lower arm are connected in series to form a third bridge arm, and a DC capacitor is connected to the DC side. , C ₂ , the first three-phase AC power line A through the resistor R ₃ , the linear inductor is connected to the upper arm of the first bridge arm at point a, the second three-phase AC power line B through the resistor R ₄ , the linear inductor L ₂ Connect to the b point of the upper and lower arm connection of the second bridge arm. The third three-phase AC power line C is connected to the upper and lower arm connection point c of the third bridge arm via the resistor R ₅ and the linear inductor L ₃ ; e _a , e _b , e _c , three three-phase AC power supplies are connected to the center point N.

The parallel structure of the three-phase multilevel PWM converter is performed by N three-phase multilevel PWM conversion The device and its control circuit unit are connected in parallel (see Figure 3-4). The characteristics are as follows: The AC side of each converter circuit is taken from the same AC power circuit, and the DC side is connected in parallel to form a DC bus, which shares the DC load R.

 Figure 3 shows the block diagram of the control circuit of a three-phase multilevel PWM converter. The control method is based on the vector control technology of grid voltage orientation. It adopts double closed loop control, the outer loop is the voltage control loop, and the inner loop is the grid side current control loop. The details are as follows:

The outer loop uses the DC output voltage signal as the voltage feedback amount, and is obtained by the voltage dividing circuit and the voltage sensor ₂ composed of R ₂ , and the DC voltage is given a constant voltage U _dc * as a constant value target, in the third adder 3 For comparison, the output of the third adder 3 is subjected to proportional-integral processing by the outer loop voltage PI regulator 4, and the output control current i _d * is output; the inner loop is divided into a d-axis PID regulator 6 and a q-axis PID regulator 5, The process is to first mathematically transform the three-phase instantaneous alternating current i _a , i _b , i _{c to} obtain a direct current component i _{d in the} same direction as the voltage composite vector and a direct current component iq perpendicular to the voltage composite vector; id reactive power and voltage vectors synthesizing the same direction, and therefore a current id called active component, the active power converter can be adjusted to control the id, iq and reactive current component referred to, the control of the converter can be adjusted i _q; e _a to For example, the e _a phase voltage is connected to the phase locked loop 8 and the space vector phase angle calculation link 9, and the space vector phase angle calculation link 9 outputs the sine amount (sin e , sin( 6 -120. ), sin( 0 + 12O°)), cosine amount (cos Θ , cos( θ -120. ) , cos ( θ +120. )) to dq /abc converter 12, dq /abc converter 12 is connected to the main circuit of the three-phase multilevel PWM converter via the SVPWM signal generator 13; in the three-phase linear inductor L1, L2 The phase fire line 7 connected to the main circuit of the PWM converter is connected to the current sensor 10 and the abc/dq converter 11, respectively, and the abc/dq converter 11 has two output signals, wherein the iq signal passes through the first adder 14, q. The axis PID regulator 5 outputs a u _q *, i _d signal through the second adder 15, the d-axis PID regulator 6 outputs u _d *; a voltage divider circuit composed of R1 and R2 in parallel between the DC positive and negative bus bars, The output of the voltage dividing circuit is connected via a voltage sensor 2, a third adder 3, an outer loop voltage PI regulator 4, and a second adder 15. The voltage sensor 2 uses a Hall voltage sensor of the type SKIT_V25V6. The third adder 3 and the outer loop voltage PI regulator 4 are each implemented by software, and the software is installed in the digital signal processor, and the digital signal processor is model number 2812. The specific operation process is described in detail as follows:

1. First select the grid three-phase composite voltage vector as the d-axis vector orientation reference, and detect the phase of the A-phase electromotive force e _a of the grid in real time through the phase-locked loop (PLL) circuit 8. Determine the voltage orientation vector through the space vector phase angle calculation link 9. Position angle θ, find the sine amount (sin6, sin(e-120.), sin( Θ +120.)), cosine amount (cosS, cos(6 -120.), cos (θ+120° )) and Output it to the abc/dq converter 11 and the dq/abc converter 12;

2. The AC currents i _a , i _b , i _c extracted from the phase fire lines 7 are the current feedback amount, and the current sensor

After 10, the three-phase stationary coordinate system is transformed into the two-phase synchronous rotating coordinate system by the abc/dq converter 11, and the three-phase currents i _a , i _b , i _c with phase differences of 120° are transformed into phase mutual differences of 90. The two-phase current i _d , i .

3. The abc/dq converter 11 is based on the input sine quantity ( sin Θ , sin( Θ -120° ), sin( θ +120 ) ) ), cosine amount (cosS, cos(6 -120. ), cos (Θ +120.)), realizes the transformation of abc three-phase stationary coordinate system to dq synchronous rotation stop coordinate system, and finally transforms into DC component i _d , iq in synchronous rotating coordinate system.

4. Output i _d * of the outer loop voltage PI regulator 4 as a given parameter of the d-axis PID regulator 6, the DC component i _d obtained by the decoupling of the alternating current as the feedback of the d-axis PID regulator 6, the outer loop voltage PI The output i _d * of the regulator 4 and the DC component i _d decoupled from the alternating current first pass through the second adder 15 and then through the d-axis PID regulator 6 proportional-integral-derivative operation output control voltage u _d *;

5. The reactive current component iq* converted by reactive power or power factor is used as a given parameter of the q-axis PID regulator 5, and the DC component i _q obtained by decoupling the alternating current is used as feedback of the q-axis PID regulator 5, The DC component iq obtained by decoupling the work current component iq* from the AC current first passes through the first adder 14, and then passes through the q-axis PID regulator 5 proportional-integral-derivative operation output control voltage Uq*;

6. The dq /abc converter 12 transforms the sine according to the input ( sin Θ , sin( θ -120. ), sin( θ +120 ) ), cosine (cos0, cos(0-12O° ), cos ( θ+120° )), realize the transformation of the d _q synchronous rotating coordinate system to the abc three-phase stationary coordinate system, and output the d-axis PID regulator 6 in the synchronous rotating coordinate system. The control voltage u _d *, the control voltage Uq* output by the q-axis PID regulator 5 is transformed into the sinusoidal components U _a *, U _b *, U _c * in the three-phase stationary coordinate system;

 7. After the SVPWM signal generator 13 pulse width modulation, the control signal of the six-way PWM inverter bridge power tube is output.

 After decoupling, the active power of the three-phase multilevel PWM converter is proportional to the d-axis current component, and the reactive power is proportional to the q-axis current component. The law satisfies the following relationship (1), where UG is the grid. Phase voltage rms value.

 Therefore, controlling the d-axis current component can adjust the active power, that is, the DC bus voltage, and control the q-axis current component to adjust the reactive power or power factor, realize independent control of the DC voltage of the PWM converter and the reactive power of the grid side, and make the system Has good static and dynamic performance.

 4 is a block diagram of a specific implementation of a multi-cell parallel connection of a three-phase multi-level PWM converter. The parallel structure of the multi-cell three-phase three-phase multi-level PWM converter is composed of N three-phase multi-level PWM converters and their control circuit units connected in parallel. The AC side of each three-phase multi-level PWM converter is connected in parallel to the same AC power supply circuit; the DC output of each three-phase multi-level PWM converter is connected in parallel to the DC bus. The key technologies for parallel operation are:

1. Separate the outer loop voltage regulators of each parallel three-phase multilevel PWM converter to form a common unified voltage regulator, specifically the DC output voltage signal as the voltage feedback amount, and the partial voltage of the sum The circuit voltage divider, the voltage sensor 2, the third adder 3, and the outer loop voltage PI regulator 4 obtain the voltage feedback amount, and the DC voltage output output voltage U _dc * is a constant value target, and is performed by the outer loop voltage PI regulator 4 After the proportional-integral processing, the output control current i _d * is output as each parallel three-phase multi-level The closed loop current of the PWM converter controls a given signal of the d-axis PID regulator;

2. The closed-loop current control of the parallel unit 1 is divided into a d-axis PID regulator 6 and a q-axis regulator 5 to uniformly control the output of the voltage regulator to control the current i _d * as a given signal of the d-axis PID regulator 6, alternating current The DC component i _d obtained by decoupling is used as the feedback of the d-axis PID regulator 6. After the proportional-integral-differential operation of the d-axis PID regulator 6, the control voltage u _d * is output; the reactive power is converted by reactive power or power factor. The current component i _q * is a given signal of the q-axis PID regulator 5, and the DC component iq obtained by decoupling the alternating current is used as feedback of the q-axis PID regulator 5, after the proportional-integral-derivative operation of the q-axis PID regulator 5 The output control voltage Uq*, the current vector control process of the parallel unit 1 is specifically the same as the inner loop current vector control of the three-phase multilevel PWM converter of FIG. 3 described above. The operation principle of the parallel unit 2 to the parallel unit N is the same as that of the parallel unit 1.

 In this way, on the one hand, the given parameters of the parallel three-phase multi-level PWM converter and the dispersion of the adjustment parameters are overcome, and some converters are operated in the rectification state during the parallel operation, and some converters work in the active inverter. The state eliminates the factor of forming a circulating current between the parallel three-phase multi-level PWM converters; on the other hand, the control strategy of each parallel three-phase multi-level PWM converter is reduced to the grid-side AC current closed-loop control The present invention substantially realizes the current sharing control between the parallel converters. Therefore, the present invention solves the problem of the consistency of the energy flow direction of the three-phase multi-level PWM converters in parallel operation, thereby avoiding the generation of the circulating current; The current sharing problem of the three-phase multi-level PWM converter in parallel provides technical support for multi-unit parallel connection of three-phase multi-level PWM converters, achieving high power, building blocks and large-scale applications.

 The above description is only the preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Any modifications, equivalents, improvements, etc. made within the spirit and scope of the invention are intended to be included within the scope of the invention.

Claims

Rights request

A parallel structure of a three-phase multilevel PWM converter comprising two or more three-phase multilevel PWM converters and a control circuit unit thereof, a digital signal processor and a three-phase AC power supply circuit; The utility model further comprises a common unified voltage regulator; the input terminals of the three-phase multi-level PWM converters and the control circuit unit thereof are connected in parallel to the three-phase AC power supply circuit, and the output ends thereof are connected in parallel and then connected to the same load. R _L ; The unified voltage regulator is composed of a voltage sampling circuit (1), a voltage sensor (2), a third adder (3) and an outer loop voltage PI regulator (4); the voltage sampling circuit (1) In parallel with the load R _L , the output of the voltage sampling circuit ( 1 ) is sequentially connected to the input of the outer loop voltage PI regulator (4 ) via the voltage sensor ( 2 ) and the third adder ( 3 ). The other input end of the third adder (3) is connected to a DC output given voltage U _dc * of each three-phase multi-level PWM converter, and the output of the outer loop voltage PI regulator (4) Separating the three-phase multi-level PWM converters and their control circuits The corresponding input of the control circuit in the unit.

2 . The parallel structure of a three-phase multi-level PWM converter according to claim 1 , wherein the three-phase AC power supply circuit comprises a phase A, a B phase, a C phase, and a resistor R ₃ − of the three-phase AC power source. R ₅ , the inductor L _r L _{3 is} composed; the upper arm of the first bridge arm of the three-phase multi-level PWM converter has a point a through the resistor R ₃ , the inductor L ^ ^ A phase, and the second bridge arm The b point of the upper and lower arm connection is connected to the B phase through the resistor R ₄ and the inductor L ₂ in turn, and the c point of the upper arm connection of the third bridge arm is sequentially connected to the C phase through the resistor R ₅ and the inductor L ₃ , the A phase and the B phase. The C phase is connected to the center point N.

The three-phase to the claim 2 parallel structure multilevel PWM converter, wherein said voltage sampling circuit (1) by the resistor R ₂ in series with a resistor divider circuit consisting of resistors

Ri is connected in series with R _{2 in} parallel with the load RL, and the node of the resistor and resistor R ₂ is connected to the input of the voltage sensor (2).

 A parallel structure of a three-phase multilevel PWM converter according to claim 3, characterized in that said voltage sensor (2) employs a Hall voltage sensor of the type SKIT_V25V6.