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

Based on the flexible direct current power transmission system of modularization multi-level converter

Technical field

The present invention relates to Technology of HVDC based Voltage Source Converter, particularly relate to a kind of flexible direct current power transmission system based on modularization multi-level converter.

Background technology

Flexible DC power transmission is a kind of VSC-HVDC technology based on voltage source converter (VSC), controlled shutoff device and pulse-width modulation (PWM) technology.This HVDC Transmission Technology can instantaneous realization gain merit and idle independent uneoupled control, can power to passive network, between current conversion station without the need to communication, be easy to form MTDC transmission system.In addition, this technology of transmission of electricity can provide the Emergency Assistance of active power and reactive power simultaneously to system, has clear superiority improving in the stability of system and ability to transmit electricity etc.

Based on the Technology of HVDC based Voltage Source Converter of voltage source converter (VSC), compare traditional high voltage dc transmission technology, there is control and operational mode feature flexibly, be widely used in for city and far-end island load power, regenerative resource and the occasion such as marine power generation is grid-connected.

But, still there is system loss large (namely switching loss is larger) in existing Technology of HVDC based Voltage Source Converter, the problems such as fault current when can not control DC side fault.IGBT device is have employed in flexible direct current power transmission system, trigging control adopts PWM technology, very high switching frequency can be realized, the harmonic wave of output voltage amount of current conversion station is also less, relative to traditional DC transmission system, the capacity of the filter that flexible direct current transmission converter station is installed greatly reduces.But above-mentioned flexible direct current power transmission system, in trigging control technology and further reduction harmonic wave of output voltage content, also has the space of improving further.

Summary of the invention

In view of this, main purpose of the present invention is to provide a kind of flexible direct current power transmission system based on modularization multi-level converter, to reduce the complexity of its control system further, to improve the harmonic content of reliability and reduction output voltage.

For achieving the above object, technical scheme of the present invention is achieved in that

Based on a flexible direct current power transmission system for modularization multi-level converter, comprise modularization multi-level converter MMC circuit; Described MMC circuit comprises the converter arm become through tandem compound with inductor by multiple submodule.

Wherein: described submodule comprises an IGBT half-bridge and a direct current capacitor further.

Described inductor for controlling three-phase current, and when fault for converter provides protection.

Every topology mutually of described MMC, comprises 2N submodule, can export N+1 level.

Based on a MMC control method for Carrier-shifted PWM modulation, comprise the steps:

A, to N number of power cell, utilize triangular carrier phase shift carrier cycle T successively _cn/mono-of=2 π, then compares with identical sinusoidal modulation wave, produces N group PWM ripple and carrys out a driving N power cell;

B, the output voltage of each power cell to be superposed mutually, create the equivalent PWM output voltage waveforms of multilevel converter.

Flexible direct current power transmission system based on modularization multi-level converter provided by the present invention, has the following advantages:

Adopt modularization multi-level converter MMC circuit, the harmonic content of the complexity of control system, raising reliability and reduction output voltage can be reduced.Adopt MMC control method, only need the carrier wave of phase shift unit, and modulating wave is constant, be therefore easy to realize.And adopt this control method can not increase the harmonic component of output voltage.Through four level MMC simulation results shows, result proves that this modulator approach is correct.

Accompanying drawing explanation

Fig. 1 is existing flexible direct current power transmission system single line principle schematic;

Fig. 2 is the topological structure schematic diagram of modularization multi-level converter (MMC) built-up circuit adopted in the embodiment of the present invention;

Fig. 3 is the principle schematic of the rectifier control system of the embodiment of the present invention;

Fig. 4 is the principle schematic of the inverter control system of the embodiment of the present invention;

Fig. 5 is the line voltage (interchange, perunit value) of inverter side;

Fig. 6 is the phase voltage (interchange, perunit value) of inverter side;

Fig. 7 is the line voltage harmonic frequency spectrum of inverter side;

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

Fig. 9 is the phase voltage (interchange, perunit value) of inverter side;

Figure 10 is the line voltage (interchange, perunit value) of inverter side.

Embodiment

Below in conjunction with accompanying drawing and embodiments of the invention, the flexible direct current power transmission system that the present invention is based on modularization multi-level converter is described in further detail.

Fig. 1 is existing flexible direct current power transmission system single line principle schematic.Wherein, the current conversion station at two ends all adopts the part compositions such as voltage source converter (VSC) structure, converter transformer, converter reactor, direct current capacitor and power transmission cable.

Fig. 2 is the topological structure schematic diagram of modularization multi-level converter (MMC) built-up circuit adopted in the embodiment of the present invention.Voltage source converter adopts modularization multi-level converter (MMC), because it has the multiple advantages such as control system is simple, reliability is high, harmonic content is low, is thus with a wide range of applications.

The topological structure of the built-up circuit of modularization multi-level converter (MMC) is as shown in Figure 2 known, and this topological structure utilizes the tandem compound of multiple submodule (SM, Sub-Module) to become each brachium pontis of converter, i.e. converter arm.With regard to MMC module (or circuit), at any time, the state of IGBT1, conducting or shutoff are all contrary with IGBT2, and namely when IGBT1 conducting time, IGBT2 turns off; When IGBT1 turns off time, IGBT2 conducting.Only in this way, the voltage of direct current capacitor ability stable output.

In the modularization multi-level converter of the present embodiment, have 6 converter arms, each converter arm is composed in series by some submodules and inductance.Each submodule comprises: an IGBT half-bridge and a direct current capacitor.Inductor is mainly used to control the alternate change of current, and when fault for converter provides protection.Direct current capacitor is mainly used to the voltage supporting DC side.

For the topology of three-phase MMC (when namely modularization multi-level converter MMC circuit quantity is 3), DC voltage by direct current capacitor and inductor dividing potential drop, namely

$U_d=\underset{j=1}{\overset{N}{\Σ}}{v}_{ju}+l\frac{d}{dt}(i_{pa}+i_{Na})$

Wherein: U _dfor DC voltage, j=1,2 ..., N is N number of submodule of u phase, v _jufor the output voltage of a jth group module of u phase; L is inductance; i _oaand i _nabe respectively upper arm and underarm electric current.

From the topology of every phase of MMC, be often made up of 2N submodule, N+1 level can be exported, and capacitor be by the current charges of main circuit.

Fig. 3 is the principle schematic of the rectifier control system of the embodiment of the present invention.Control principle based on the rectifier of MMC is: on the one hand, the actual value of reactive power and the reference value of reactive power are compared, difference, through PI (proportional integral) link, obtains modulation ratio M value, postpone and amplitude limit link through one, be sent to PWM; On the other hand, calculate direct current power according to direct voltage and direct current, and compare with direct current power reference value, after one postpones, difference is sent to flow controller, to calculate Trigger Angle, and PWM is sent in the instruction of Trigger Angle.PWM, according to the instruction of M value and Trigger Angle instruction, generates the pulse of control IGBT, thus controls active power and reactive power.

Fig. 4 is the principle schematic of the inverter control system of the embodiment of the present invention.Control principle based on the inverter of MMC is: on the one hand, the actual value of direct voltage and reference value is compared, and difference, through proportional integral (PI) link, calculates Trigger Angle and is sent to PWM; On the other hand, the actual value of alternating voltage and reference value are compared, difference, through proportional integral (PI) link, calculates modulation ratio M value and is sent to PWM.PWM, according to the instruction of M value and Trigger Angle instruction, generates the pulse of control IGBT, thus controls direct voltage and receiving terminal busbar voltage.

The present invention also proposes a kind of MMC control method based on Carrier-shifted PWM modulation, and namely to N number of power cell, triangular carrier is phase shift carrier cycle T successively _cn/mono-of=2 π, namely then compare with identical sinusoidal modulation wave, carry out a driving N power cell to produce N group PWM ripple.The output voltage of each power cell superposes mutually, creates the equivalent PWM output voltage waveforms of multilevel converter.Adopt this MMC control method, there is following advantages: 1) this algorithm only needs the carrier wave of phase shift unit, and modulating wave is constant, be easy to realize; 2) this algorithm can not increase the harmonic component of output voltage.

Embodiments of the invention are by the simulation results show of four level MMC, and result proves that this modulator approach is correct, specifically as shown in Fig. 5 ~ Figure 10.

The present invention has just carried out emulation experiment based on the flexible direct current power transmission system of voltage source converter, and analyzes.The capacitance voltage getting each submodule is fiducial value, and so the perunit value of capacitance voltage is just U _c=1 (p.u.), DC voltage is just 3 (p.u.).Frequency is f ₀=50Hz, inductance is L=0.004H.

When described flexible direct current power transmission system is in steady operation, the line voltage of inverter exports as follows respectively:

Fig. 5 is the line voltage (interchange, perunit value) of inverter side; Fig. 6 is the phase voltage (interchange, perunit value) of inverter side.Wherein, Fig. 7 is the line voltage harmonic frequency spectrum of inverter side; Fig. 8 is the phase voltage harmonic spectrum of inverter side.

For studying the Enhancement of Transient Voltage Stability of flexible direct current power transmission system:

The embodiment of the present invention is provided with a three phase short circuit fault in inverter side, and the fault clearance time is 0.5s.The waveform of its inverter side alternating voltage is as follows respectively:

Fig. 9 is the phase voltage (interchange, perunit value) of inverter side; Figure 10 is the line voltage (interchange, perunit value) of inverter side.

From above simulation result, after three phase short circuit fault is removed, the AC system of inverter side can maintain Enhancement of Transient Voltage Stability.

Therefore can draw the following conclusions:

1) control method based on Carrier-shifted PWM algorithm can be good at control MMC.

2) control method of rectifier and inverter is correct and effectively, can guarantees that the voltage harmonic of inverter side is very little.

3) flexible direct current power transmission system based on voltage source converter has good Enhancement of Transient Voltage Stability.

The above, be only preferred embodiment of the present invention, be not intended to limit protection scope of the present invention.

Claims (5)

1. based on a flexible direct current power transmission system for modularization multi-level converter, it is characterized in that, comprise modularization multi-level converter MMC circuit; Described MMC circuit comprises the converter arm become through tandem compound with inductor by multiple submodule.

2. according to claim 1 based on the flexible direct current power transmission system of modularization multi-level converter, it is characterized in that, described submodule comprises an IGBT half-bridge and a direct current capacitor further.

3., according to claim 1 based on the flexible direct current power transmission system of modularization multi-level converter, it is characterized in that, described inductor for controlling three-phase current, and when fault for converter provides protection.

4. according to claim 1 based on the flexible direct current power transmission system of modularization multi-level converter, it is characterized in that, every topology mutually of described MMC, comprises 2N submodule, can export N+1 level.

5., based on a MMC control method for Carrier-shifted PWM modulation, it is characterized in that, comprise the steps:

A, to N number of power cell, utilize triangular carrier phase shift carrier cycle T successively _cn/mono-of=2 π, then compares with identical sinusoidal modulation wave, produces N group PWM ripple and carrys out a driving N power cell;

B, the output voltage of each power cell to be superposed mutually, create the equivalent PWM output voltage waveforms of multilevel converter.