CN104993510A - Flexible DC power transmission system based on modularized multi-level converter - Google Patents

Abstract

The invention discloses a flexible DC power transmission system based on a modularized multi-level converter, comprising a modularized multi-level converter MMC circuit. The MMC circuit comprises a current switching arm constituting by a plurality of slave modules and inductors which are connected in series. The flexible DC power transmission system based on the modularized multi-level converter can further reduce the complexity of the control system, can improve the reliability and can reduce the content of the harmonic wave in the output voltage.

Description

Flexible DC Transmission System Based on Modular Multilevel Converter

technical field

The invention relates to flexible direct current transmission technology, in particular to a flexible direct current transmission system based on a modular multilevel converter.

Background technique

Flexible HVDC transmission is a new type of HVDC transmission technology based on voltage source converter (VSC), controllable turn-off device and pulse width modulation (PWM) technology. The DC transmission technology can instantaneously realize independent decoupling control of active power and reactive power, can supply power to passive networks, does not require communication between converter stations, and is easy to form a multi-terminal DC system. In addition, this power transmission technology can provide emergency support of active power and reactive power to the system at the same time, and has obvious advantages in improving system stability and power transmission capacity.

The flexible DC transmission technology based on voltage source converter (VSC), compared with the traditional high-voltage DC transmission technology, has the characteristics of flexible control and operation mode, and is widely used in power supply for cities and remote isolated loads, renewable energy and offshore Power generation and grid connection, etc.

However, the existing flexible DC transmission technology still has problems such as large system loss (that is, large switching loss) and failure to control the fault current when the DC side fails. The IGBT device is used in the flexible DC transmission system, and the PWM technology is used in the trigger control, which can achieve a high switching frequency, and the harmonic content of the output voltage of the converter station is also small. Compared with the traditional DC transmission system, the flexible DC transmission The capacity of filtering devices installed in converter stations has been greatly reduced. However, there is still room for further improvement in the above-mentioned flexible DC transmission system in terms of trigger control technology and further reducing the harmonic content of the output voltage.

Contents of the invention

In view of this, the main purpose of the present invention is to provide a flexible direct current transmission system based on a modular multilevel converter, so as to further reduce the complexity of its control system, improve reliability and reduce the harmonic content of the output voltage.

In order to achieve the above object, technical solution of the present invention is achieved in that way:

A flexible direct current transmission system based on a modular multilevel converter includes a modular multilevel converter MMC circuit; the MMC circuit includes a commutation arm composed of multiple sub-modules and inductors combined in series.

Wherein: said sub-module further includes an IGBT half-bridge and a DC capacitor.

The inductors are used to control the phase-to-phase current and provide protection for the converter in the event of a fault.

Each phase topology of the MMC includes 2N sub-modules and can output N+1 levels.

A kind of MMC control method based on carrier phase-shift PWM modulation, comprises the steps:

A. For N power units, use the triangular carrier wave to sequentially phase-shift the carrier period T _c = 1/N of 2π, and then compare it with the same sinusoidal modulation wave to generate N groups of PWM modulation waves to drive the N power units;

B. The output voltages of each power unit are superimposed on each other to generate an equivalent PWM output voltage waveform of the multilevel converter.

The flexible direct current transmission system based on the modular multilevel converter provided by the present invention has the following advantages:

The modular multilevel converter MMC circuit can reduce the complexity of the control system, improve the reliability and reduce the harmonic content of the output voltage. Using the MMC control method, only the carrier wave of each unit needs to be phase-shifted, while the modulation wave remains unchanged, so it is easy to implement. And adopting this control method will not increase the harmonic component of the output voltage. The four-level MMC simulation experiment proves that the modulation method is correct.

Description of drawings

Figure 1 is a schematic diagram of the single-line principle of the existing flexible direct current transmission system;

Fig. 2 is a schematic diagram of the topology of a circuit composed of a modular multilevel converter (MMC) used in an embodiment of the present invention;

3 is a schematic diagram of the principle of a rectifier control system according to an embodiment of the present invention;

FIG. 4 is a schematic diagram of the principle of an inverter control system according to an embodiment of the present invention;

Figure 5 shows the line voltage (AC, per unit value) on the inverter side;

Figure 6 shows the phase voltage (AC, per unit value) on the inverter side;

Figure 7 is the harmonic spectrum of the line voltage at the inverter side;

Fig. 8 is the phase voltage harmonic spectrum of the inverter side;

Fig. 9 is the phase voltage (AC, per unit value) of the inverter side;

Figure 10 shows the line voltage on the inverter side (AC, per unit).

Detailed ways

The flexible direct current transmission system based on the modular multilevel converter of the present invention will be further described in detail below in conjunction with the accompanying drawings and the embodiments of the present invention.

Figure 1 is a schematic diagram of the single-line principle of the existing flexible direct current transmission system. Among them, the converter stations at both ends are composed of a voltage source converter (VSC) structure, a converter transformer, a converter reactor, a DC capacitor and a transmission cable.

Fig. 2 is a schematic diagram of the topology structure of the circuit composed of the modular multilevel converter (MMC) used in the embodiment of the present invention. The voltage source converter adopts the modular multilevel converter (MMC), because it has many advantages such as simple control system, high reliability and low harmonic content, so it has a wide application prospect.

It can be known from the topology of the component circuit of the modular multilevel converter (MMC) shown in Figure 2 that the topology uses a series combination of multiple sub-modules (SM, Sub-Module) to form each bridge arm of the converter , that is, the converter arm. As far as the MMC module (or circuit) is concerned, at any time, the state of IGBT1, whether it is turned on or off, is opposite to that of IGBT2, that is, when IGBT1 is turned on, IGBT2 is turned off; when IGBT1 is turned off, IGBT2 is turned on. Only in this way can the DC capacitor output a stable voltage.

In the modular multilevel converter of this embodiment, there are 6 commutation arms, and each commutation arm is composed of several sub-modules and inductors connected in series. Each sub-module includes: an IGBT half-bridge and a DC capacitor. Inductors are mainly used to control phase-to-phase commutation and provide protection for the converter in case of faults. DC capacitors are mainly used to support the voltage on the DC side.

For the topology of the three-phase MMC (that is, when the number of MMC circuits in the modular multilevel converter is 3), the DC side voltage is divided by the DC capacitor and the inductor, that is

${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; j</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}\mathrm{<span\; class="notranslate">\; u</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; l</span>}\frac{\mathrm{<span\; class="notranslate">\; d</span>}}{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; )</span>$

Where: U _d is the DC side voltage, j=1, 2,..., N is the N sub-modules of phase u, v _ju is the output voltage of the jth group module of phase u; l is the inductance; i _oa and i _Na are the upper and lower arm currents, respectively.

From the topology of each phase of MMC, it can be known that each phase is composed of 2N sub-modules, which can output N+1 levels, and the capacitor is charged by the current of the main circuit.

Fig. 3 is a schematic diagram of the principle of the rectifier control system according to the embodiment of the present invention. The control principle of the MMC-based rectifier is as follows: On the one hand, the actual value of reactive power is compared with the reference value of reactive power, the difference is passed through the PI (proportional integral) link, and the modulation ratio M value is obtained, and after a delay and limit The amplitude link is sent to PWM; on the other hand, the DC power is calculated according to the DC voltage and DC current, and compared with the DC power reference value, and after a delay, the difference is sent to the power flow controller to calculate the firing angle. And send the command of trigger angle to PWM. According to the M value instruction and firing angle instruction, PWM generates pulses to control IGBT, thereby controlling active power and reactive power.

Fig. 4 is a schematic diagram of the principle of the inverter control system according to the embodiment of the present invention. The control principle of the MMC-based inverter is: on the one hand, the actual value of the DC voltage is compared with the reference value, the difference is passed through the proportional integral (PI) link, and the firing angle is calculated and sent to the PWM; on the other hand, the The actual value of the AC voltage is compared with the reference value, and the difference is passed through the proportional integral (PI) link to calculate the modulation ratio M and send it to the PWM. According to the M value command and the firing angle command, the PWM generates pulses to control the IGBT, thereby controlling the DC voltage and the bus voltage at the receiving end.

The present invention also proposes a MMC control method based on carrier phase-shift PWM modulation, that is, for N power units, the triangular carrier is sequentially phase-shifted and the carrier period T _c = 1/N of 2π, that is Then compare with the same sinusoidal modulation wave to generate N groups of PWM modulation waves to drive N power units. The output voltages of each power unit are superimposed on each other to generate an equivalent PWM output voltage waveform of the multilevel converter. The MMC control method has the following advantages: 1) The algorithm only needs to shift the phase of the carrier wave of each unit, while the modulation wave remains unchanged, which is easy to implement; 2) The algorithm does not increase the harmonic component of the output voltage.

The embodiment of the present invention is proved by the simulation experiment of the four-level MMC, and the result proves that the modulation method is correct, as shown in FIGS. 5 to 10 in detail.

In the present invention, a simulation experiment is carried out on a flexible direct current transmission system based on a voltage source converter, and analysis is carried out. Taking the capacitor voltage of each sub-module as a reference value, then the unit value of the capacitor voltage is U _C =1(pu), and the DC side voltage is 3(pu). The frequency is f ₀ =50Hz, and the inductance is L=0.004H.

When the flexible direct current transmission system is in steady state operation, the line voltage output of the inverter is as follows:

Figure 5 shows the line voltage (AC, per unit) on the inverter side; Figure 6 shows the phase voltage (AC, per unit) on the inverter side. Among them, Fig. 7 is the line voltage harmonic spectrum of the inverter side; Fig. 8 is the phase voltage harmonic spectrum of the inverter side.

In order to study the transient voltage stability of the flexible DC transmission system:

In the embodiment of the present invention, a three-phase short-circuit fault is set on the inverter side, and the fault clearing time is 0.5s. The waveforms of the AC voltage on the inverter side are as follows:

Figure 9 shows the phase voltage (AC, per unit value) on the inverter side; Figure 10 shows the line voltage (AC, per unit value) on the inverter side.

From the above simulation results, it can be seen that after the three-phase short-circuit fault is cleared, the AC system on the inverter side can maintain transient voltage stability.

Therefore, the following conclusions can be drawn:

1) The control method based on the carrier phase shift PWM algorithm can control the MMC very well.

2) The control method of the rectifier and inverter is correct and effective, which can ensure that the voltage harmonics on the inverter side are very small.

3) The flexible DC transmission system based on the voltage source converter has good transient voltage stability.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention.

Claims (5)

1. A flexible direct current transmission system based on a modular multilevel converter, characterized in that it includes a modular multilevel converter MMC circuit; the MMC circuit includes a series combination of multiple submodules and inductors The completed commutation arm. 2. The flexible direct current transmission system based on a modular multilevel converter according to claim 1, wherein said sub-module further comprises an IGBT half bridge and a direct current capacitor. 3. The flexible direct current transmission system based on a modular multilevel converter according to claim 1, wherein the inductor is used to control phase-to-phase current and provide protection for the converter in case of a fault. 4. The flexible direct current transmission system based on a modular multilevel converter according to claim 1, wherein each phase topology of the MMC includes 2N sub-modules and can output N+1 levels. 5. A kind of MMC control method based on carrier phase shift PWM modulation, it is characterized in that, comprises the steps: A. For N power units, use the triangular carrier wave to sequentially phase-shift the carrier period T _c = 1/N of 2π, and then compare it with the same sinusoidal modulation wave to generate N groups of PWM modulation waves to drive the N power units; B. The output voltages of each power unit are superimposed on each other to generate an equivalent PWM output voltage waveform of the multilevel converter.