CN105720844A - Novel three-phase serial modular multilevel converter HVDC converter - Google Patents

Abstract

The invention discloses a novel three-phase serial modular multilevel converter HVDC converter, comprising a DC output end, a three-phase AC input end, a first transformer, a second transformer, a third transformer, a first single-phase converter, a second single-phase converter and a third single-phase converter. The novel three-phase serial modular multilevel converter HVDC converter disclosed by the invention has a low cost and can block the DC short circuit current.

Description

A kind of Three phase serial module structureization many level HVDC inverter

Technical field

The invention belongs to high voltage, high-power power conversion device topological structure and control strategy field thereof, relate to a kind of Three phase serial module structureization many level HVDC inverter.

Background technology

Modularization multi-level converter (multilevelmodularconverter, MMC) it is voltage source converter (voltagesourceconverter, VSC) a kind of topological structure, proposed at IEEEPowerTechConference in 2002 by Marquardt and Lesnicar, be the newest fruits of high voltage dc transmission technology development.Relative to two level and three-level converter topological structure, MMC topological structure has plurality of advantages: modularized design, can realize the flexible change of voltage and power grade by adjusting the series connection number of submodule；Switching frequency is low, and loss declines；Output voltage waveforms smooths very much and close to ideal sinusoidal waveform, harmonic content is few, need not Large Copacity alternating current filter in net side；Fault ride-through capacity is strong.Based on These characteristics, MMC is applied to direct current transportation and can significantly improve reliability and the adaptability of DC transmission system.But, in order to export the DC voltage of same electric pressure, use the MMC system of half-bridge or full-bridge to need the derailing switch number of packages used to be the twice of traditional two level VSC topologys or four times, thus cost too high be a major defect of current MMC system.Meanwhile, dc-side short-circuit fault be also current MMC faced by subject matter.Practical Project adopts half-bridge submodule, when DC side breaks down, owing to anti-paralleled diode remains to provide path, system approximation generation three-phase shortcircuit into fault current, and disengagement failure electric current, the safe and stable operation of serious harm system cannot be come by locking inverter.And owing to DC current is absent from zero crossing, blow-out difficulty, the manufacturing process of high-voltage large-capacity dc circuit breaker is still immature, at present still in the experimental stage, rarely has application in engineering.

Summary of the invention

It is an object of the invention to the shortcoming overcoming above-mentioned prior art, it is provided that a kind of Three phase serial module structureization many level HVDC inverter, the cost of this inverter is low, can block direct-current short circuit electric current simultaneously.

For reaching above-mentioned purpose, Three phase serial module structureization many level HVDC inverter of the present invention includes DC output end, three-phase alternating current input, first transformator, second transformator, 3rd transformator, first single-phase converter, second single-phase converter and the 3rd single-phase converter, wherein, three outfans of three-phase alternating current input respectively with one end of primary coil in the first transformator, in second transformator primary coil one end and in the 3rd transformator one end of primary coil be connected, the other end of primary coil in first transformator, in second transformator primary coil the other end and in the 3rd transformator the other end of primary coil be connected, one end of secondary coil in first transformator, in second transformator secondary coil one end and in the 3rd transformator one end of secondary coil respectively with in the first single-phase converter first auxiliary brachium pontis, the first auxiliary brachium pontis in second single-phase converter and the first auxiliary brachium pontis in the 3rd single-phase converter are connected；

The second auxiliary brachium pontis in first single-phase converter is connected with the positive pole of DC output end, main brachium pontis in first single-phase converter and in the first transformator the second auxiliary brachium pontis in the other end of secondary coil and the second single-phase converter be connected, main brachium pontis in second single-phase converter and in the second transformator the 3rd auxiliary brachium pontis in the other end of secondary coil and the 3rd single-phase converter be connected, main brachium pontis in the 3rd single-phase converter and in the 3rd transformator the other end of secondary coil and the negative pole of DC output end be connected；

The second auxiliary brachium pontis in the second auxiliary brachium pontis, the second single-phase converter and the second auxiliary brachium pontis in the 3rd single-phase converter in first single-phase converter are sequentially connected in series form by n/2 full-bridge submodule, the first inductance and the first resistance, and n is the even number be more than or equal to 2.

The secondary coil of the first transformator, the first auxiliary brachium pontis and main brachium pontis in first single-phase converter form the first AC loop, the secondary coil of the second transformator, the first auxiliary brachium pontis and main brachium pontis in second single-phase converter form the second AC loop, the secondary coil of the 3rd transformator, the first auxiliary brachium pontis and main brachium pontis in 3rd single-phase converter form the 3rd AC loop, the wherein interior circular current in the first AC loop, the interior circular current in the second AC loop and the interior circular current in the 3rd AC loop all adopt AC to gain merit, reactive current feed forward decoupling control method is controlled, main brachium pontis in first AC loop, main brachium pontis in second AC loop and the main brachium pontis in the 3rd AC loop all adopt to be determined active power and determines alternating voltage control method to be controlled, the first auxiliary brachium pontis in first AC loop, the first auxiliary brachium pontis in second AC loop and the first auxiliary brachium pontis in the 3rd AC loop all adopt the constant DC voltage control method of current oriention to be controlled.

Introduce reactive power partition coefficient k, distribution between main brachium pontis and the first auxiliary brachium pontis and distribution between main brachium pontis and the first auxiliary brachium pontis in the 3rd AC loop by regulating reactive power partition coefficient k and realizing reactive power distribution between main brachium pontis and the first auxiliary brachium pontis in the first AC loop, in the second AC loop.

The second auxiliary brachium pontis and main brachium pontis in first monophase current device form the first DC circuit, the second auxiliary brachium pontis and main brachium pontis in second monophase current device form the second DC circuit, the second auxiliary brachium pontis and main brachium pontis in 3rd monophase current device form the 3rd DC circuit, wherein, interior circular current in first DC circuit, interior circular current in second monophase current device and the interior circular current in the 3rd monophase current device are controlled each through the feedforward closed loop control method of DC current, main brachium pontis in first DC circuit, main brachium pontis in second monophase current device and the main brachium pontis in the 3rd monophase current device are controlled by determining DC voltage, the second auxiliary brachium pontis in first DC circuit, the second auxiliary brachium pontis in second monophase current device and the second auxiliary brachium pontis in the 3rd monophase current device pass through constant DC voltage control.

Main brachium pontis in main brachium pontis in first single-phase converter, the main brachium pontis in the second single-phase converter and the 3rd single-phase converter is sequentially connected in series by n half-bridge submodule and forms.

The first auxiliary brachium pontis in the first auxiliary brachium pontis, the second single-phase converter and the first auxiliary brachium pontis in the 3rd single-phase converter in first single-phase converter are sequentially connected in series form by the second resistance, the second inductance and n/2 half-bridge submodules.

The method have the advantages that

Three phase serial module structureization many level HVDC inverter of the present invention includes the first single-phase converter, second single-phase converter and the 3rd single-phase converter, wherein the first single-phase converter, second single-phase converter and the 3rd single-phase converter are connected in series, when using same switch device count, realize the output of more High Level DC Voltage, thus effectively reducing cost and the volume of system, the reliability of raising system, it is absent from alternate circulation simultaneously, without carrying out the control measure such as loop current suppression, additionally, the second auxiliary brachium pontis in first single-phase converter, the second auxiliary brachium pontis in second single-phase converter and the second auxiliary brachium pontis in the 3rd single-phase converter are sequentially connected in series by n/2 full-bridge submodule and form, when there is short trouble in DC side, the quickly block that can pass through the second auxiliary brachium pontis IGBT triggering pulse blocks the generation of direct-current short circuit electric current, from without adopting AC circuit breaker to cut off DC Line Fault.

Accompanying drawing explanation

Fig. 1 is the three-phase equivalent circuit of the present invention；

Fig. 2 is an equivalent circuit of the present invention；

Fig. 3 is the AC one equivalent circuit of the present invention；

Fig. 4 is the DC side one equivalent circuit of the present invention；

Fig. 5 is the control strategy block diagram being regulated the Feedforward Decoupling closed loop control realizing watt current and reactive current in the present invention by PI；

Fig. 6 is that outer-loop control strategy of the present invention adopts power and voltage-controlled control strategy block diagram；

Fig. 7 is the control strategy block diagram being regulated the closed loop control realizing average anode current in the present invention by PI；

Fig. 8 is that the pi regulator adopting voltage close loop in the present invention controls the control strategy block diagram that in main brachium pontis, the DC voltage of each submodule is constant；

DC side is assisted bridge arm voltage V in the present invention by Fig. 9_a1、V_b2、V_c3In containing DC voltage component V_bloThe control strategy block diagram being controlled；

Figure 10 is brachium pontis modulating wave schematic diagram during Three phase serial module structureization many level HVDC inverter steady-state operation；

The oscillogram of AC voltage x current in present invention when Figure 11 is k=1；

The oscillogram of DC voltage electric current in present invention when Figure 12 is k=1；

The oscillogram of transmitting active power and reactive power in present invention when Figure 13 is k=1；

The oscillogram of main brachium pontis and auxiliary brachium pontis DC capacitor voltage in present invention when Figure 14 is k=1；

The spectrum analysis figure of ac-side current in present invention when Figure 15 is k=1；

The spectrum analysis figure of DC side electric current in present invention when Figure 16 is k=1；

The oscillogram of AC auxiliary brachium pontis modulating wave in present invention when Figure 17 is k=1；

The oscillogram of DC side auxiliary brachium pontis modulating wave in present invention when Figure 18 is k=1；

The oscillogram of main brachium pontis modulating wave in present invention when Figure 19 is k=1；

The oscillogram of AC voltage x current in present invention when Figure 20 is k=0.6；

The oscillogram of DC voltage electric current in present invention when Figure 21 is k=0.6；

The oscillogram of transmitting active power and reactive power in present invention when Figure 22 is k=0.6；

The oscillogram of main brachium pontis and auxiliary brachium pontis DC capacitor voltage in present invention when Figure 23 is k=0.6；

The spectrum analysis figure of ac-side current in present invention when Figure 24 is k=0.6；

The spectrum analysis figure of DC side electric current in present invention when Figure 25 is k=0.6；

The oscillogram of AC auxiliary brachium pontis modulating wave in present invention when Figure 26 is k=0.6；

The oscillogram of DC side auxiliary brachium pontis modulating wave in present invention when Figure 27 is k=0.6；

The oscillogram of main brachium pontis modulating wave in present invention when Figure 28 is k=0.6；

Figure 29 is the circuit diagram of the present invention.

Detailed description of the invention

Below in conjunction with accompanying drawing, the present invention is described in further detail:

With reference to Figure 29, Three phase serial module structureization many level HVDC inverter of the present invention includes DC output end, three-phase alternating current input, first transformator T1, second transformator T2, 3rd transformator T3, first single-phase converter, second single-phase converter and the 3rd single-phase converter, wherein, three outfans of three-phase alternating current input respectively with one end of primary coil in the first transformator T1, in second transformator T2 primary coil one end and in the 3rd transformator T3 one end of primary coil be connected, the other end of primary coil in first transformator T1, in second transformator T2 primary coil the other end and in the 3rd transformator T3 the other end of primary coil be connected, one end of secondary coil in first transformator T1, in second transformator T2 secondary coil one end and in the 3rd transformator T3 one end of secondary coil respectively with in the first single-phase converter first auxiliary brachium pontis u1, the first auxiliary brachium pontis v2 in second single-phase converter and the first auxiliary brachium pontis w3 in the 3rd single-phase converter is connected；The second auxiliary brachium pontis a1 in first single-phase converter is connected with the positive pole of DC output end, in main brachium pontis 1b in first single-phase converter and the first transformator T1, the second auxiliary brachium pontis b2 in the other end of secondary coil and the second single-phase converter is connected, in main brachium pontis 2c in second single-phase converter and the second transformator T2, the 3rd auxiliary brachium pontis c3 in the other end of secondary coil and the 3rd single-phase converter is connected, and in main brachium pontis 3d and the three transformator T3 in the 3rd single-phase converter, the other end of secondary coil and the negative pole of DC output end are connected；The second auxiliary brachium pontis b2 in the second auxiliary brachium pontis a1, the second single-phase converter and the second auxiliary brachium pontis c3 in the 3rd single-phase converter in first single-phase converter are sequentially connected in series form by n/2 full-bridge submodule, the first inductance and the first resistance, and n is the even number be more than or equal to 2.

The secondary coil of the first transformator T1, the first auxiliary brachium pontis u1 and main brachium pontis 1b in first single-phase converter forms the first AC loop, the secondary coil of the second transformator T2, the first auxiliary brachium pontis v2 and main brachium pontis 2c in second single-phase converter forms the second AC loop, the secondary coil of the 3rd transformator T3, the first auxiliary brachium pontis w3 and main brachium pontis 3d in 3rd single-phase converter forms the 3rd AC loop, the wherein interior circular current in the first AC loop, the interior circular current in the second AC loop and the interior circular current in the 3rd AC loop all adopt AC to gain merit, reactive current feed forward decoupling control method is controlled, main brachium pontis 1b in first AC loop, main brachium pontis 2c in second AC loop and the main brachium pontis 3d in the 3rd AC loop all adopts and determines active power and determine alternating voltage control method to be controlled, the first auxiliary brachium pontis u1 in first AC loop, the first auxiliary brachium pontis v2 in second AC loop and the first auxiliary brachium pontis w3 in the 3rd AC loop all adopts the constant DC voltage control method of current oriention to be controlled.

Introduce reactive power partition coefficient k, distribution between main brachium pontis 2c and the first auxiliary brachium pontis v2 and distribution between main brachium pontis 3d and the first auxiliary brachium pontis w3 in the 3rd AC loop by regulating reactive power partition coefficient k and realizing reactive power distribution between main brachium pontis 1b and the first auxiliary brachium pontis u1 in the first AC loop, in the second AC loop.

The second auxiliary brachium pontis a1 and main brachium pontis 1b in first monophase current device forms the first DC circuit, the second auxiliary brachium pontis b2 and main brachium pontis 2c in second monophase current device forms the second DC circuit, the second auxiliary brachium pontis c3 and main brachium pontis 3d in 3rd monophase current device forms the 3rd DC circuit, wherein, interior circular current in first DC circuit, interior circular current in second monophase current device and the interior circular current in the 3rd monophase current device are controlled each through the feedforward closed loop control method of DC current, main brachium pontis 1b in first DC circuit, main brachium pontis 2c in second monophase current device and the main brachium pontis 3d in the 3rd monophase current device is controlled by determining DC voltage, the second auxiliary brachium pontis a1 in first DC circuit, the second auxiliary brachium pontis b2 in second monophase current device and the second auxiliary brachium pontis c3 in the 3rd monophase current device passes through constant DC voltage control.

Main brachium pontis 3d in main brachium pontis 1b in first single-phase converter, the main brachium pontis 2c in the second single-phase converter and the 3rd single-phase converter is sequentially connected in series by n half-bridge submodule and forms；The first auxiliary brachium pontis v2 in the first auxiliary brachium pontis u1, the second single-phase converter and the first auxiliary brachium pontis w3 in the 3rd single-phase converter in first single-phase converter are sequentially connected in series form by the second resistance, the second inductance and n/2 half-bridge submodules.

Requirement according to input and output voltage amplitude and system transmission power, the present invention can select number and the parameter of brachium pontis submodule flexibly, by the submodule of each brachium pontis is modulated when properly functioning, make brachium pontis output relevant voltage to realize the control target of AC and DC side.Mathematical modeling for convenience of the present invention, n/2 full-bridge submodule in n half-bridge submodule in main brachium pontis, n/2 half-bridge submodule in AC auxiliary brachium pontis and AC auxiliary brachium pontis all can be equivalent to controlled voltage source, thus obtains the equivalent-circuit model of the present invention as shown in Figure 1.

According to kirchhoff Circuit Theorem, Fig. 1 obtain the loop-voltage equation of the present invention and node current equation be as follows:

$\left\{\begin{array}{c}{V}_u={\mathrm{Ri}}_u+L\frac{{\mathrm{di}}_u}{dt}+V_{u1}+V_{1b}\\ V_v={\mathrm{Ri}}_v+L\frac{{\mathrm{di}}_v}{dt}+V_{v2}+V_{2c}\\ V_w={\mathrm{Ri}}_w+L\frac{{\mathrm{di}}_w}{dt}+V_{w3}+V_{3d}\end{array}\right.---\left(1\right)$

$\left\{\begin{array}{c}{V}_a=-{\mathrm{Ri}}_a-L\frac{{\mathrm{di}}_a}{dt}+V_{a1}+V_{1b}\\ V_b=-{\mathrm{Ri}}_b-L\frac{{\mathrm{di}}_b}{dt}+V_{b2}+V_{2c}\\ V_c=-{\mathrm{Ri}}_c-L\frac{{\mathrm{di}}_c}{dt}+V_{c3}+V_{3d}\end{array}\right.---\left(2\right)$

$\left\{\begin{array}{c}{i}_u=i_{1b}+i_a\\ i_v=i_{2c}+i_b\\ i_w=i_{3d}+i_c\\ i_a=i_b=i_c=i_{dc}\end{array}\right.---\left(3\right)$

The transformation matrix of β o is C from abc to α to adopt constant power conversion_abc/αβo, wherein,

$C_{abc/\mathrm{\α}\mathrm{\β}o}=\sqrt{\frac{2}{3}}\left[\begin{array}{ccc}1& -1/2& -1/2\\ 0& \sqrt{3}/2& -\sqrt{3}/2\\ 1/\sqrt{2}& 1/\sqrt{2}& 1/\sqrt{2}\end{array}\right]---\left(4\right)$

It is C by the transformation matrix of α β o to abc_αβo/abcFor:

$C_{\mathrm{\α}\mathrm{\β}o/abc}=\sqrt{\frac{2}{3}}\left[\begin{array}{ccc}1& 0& 1/\sqrt{2}\\ -1/2& \sqrt{3}/2& 1/\sqrt{2}\\ -1/2& -\sqrt{3}/2& 1/\sqrt{2}\end{array}\right]---\left(5\right)$

By constant power transformation matrix C_abc/αβoPremultiplication formula (1)～formula (3), obtains system voltage equation under α β coordinate system and current equation is:

$\left\{\begin{array}{c}{V}_{s\mathrm{\α}}={\mathrm{Ri}}_{s\mathrm{\α}}+L\frac{{\mathrm{di}}_{s\mathrm{\α}}}{dt}+V_{bs\mathrm{\α}}+V_{bm\mathrm{\α}}\\ V_{s\mathrm{\β}}={\mathrm{Ri}}_{s\mathrm{\β}}+L\frac{{\mathrm{di}}_{s\mathrm{\β}}}{dt}+V_{bs\mathrm{\β}}+V_{bm\mathrm{\β}}\\ V_{so}={\mathrm{Ri}}_{so}+L\frac{{\mathrm{di}}_{so}}{dt}+V_{bso}+V_{bmo}\end{array}\right.---\left(6\right)$

$\left\{\begin{array}{c}{V}_{l\mathrm{\α}}=-{\mathrm{Ri}}_{l\mathrm{\α}}-L\frac{{\mathrm{di}}_{l\mathrm{\α}}}{dt}+V_{bl\mathrm{\α}}+V_{bm\mathrm{\α}}\\ V_{l\mathrm{\β}}=-{\mathrm{Ri}}_{l\mathrm{\β}}-L\frac{{\mathrm{di}}_{l\mathrm{\β}}}{dt}+V_{bl\mathrm{\β}}+V_{bm\mathrm{\β}}\\ V_{lo}=-{\mathrm{Ri}}_{lo}-L\frac{{\mathrm{di}}_{lo}}{dt}+V_{blo}+V_{bmo}\end{array}\right.---\left(7\right)$

$\left\{\begin{array}{c}{i}_{s\mathrm{\α}}=i_{bm\mathrm{\α}}+i_{l\mathrm{\α}}\\ i_{s\mathrm{\β}}=i_{bm\mathrm{\β}}+i_{l\mathrm{\β}}\\ i_{so}=i_{bmo}+i_{lo}\end{array}\right.---\left(8\right)$

Wherein, V_sα、V_sβ、V_soAnd i_sα、i_sβ、i_soα β o component for AC voltage and current；V_lα、V_lβ、V_loAnd i_lα、i_lβ、i_loα β o component for three voltage and currents of DC side；V_bsα、V_bsβ、V_bsoThe α β o component of bridge arm voltage is assisted for AC；V_blα、V_blβ、V_bloThe α β o component of bridge arm voltage is assisted for DC side；V_bcα、V_bcβ、V_bcoAnd i_bcα、i_bcβ、i_bcoIt it is the α β o component of main bridge arm voltage and electric current.

If synchronous rotating angle matrix is:

$C_{\mathrm{\α}\mathrm{\β}o/dqo}=\left[\begin{array}{ccc}cos\mathrm{\ω}t& sin\mathrm{\ω}t& 0\\ -sin\mathrm{\ω}t& cos\mathrm{\ω}t& 0\\ 0& 0& 1\end{array}\right]---\left(9\right)$

$C_{dqo/\mathrm{\α}\mathrm{\β}o}=\left[\begin{array}{ccc}cos\mathrm{\ω}t& -sin\mathrm{\ω}t& 0\\ sin\mathrm{\ω}t& \cos \mathrm{\ω}t& 0\\ 0& 0& 1\end{array}\right]---\left(10\right)$

Formula (6) formula (8) is transformed to the voltage equation under dqo coordinate system for employing formula (9) and formula (10) and current equation is as follows:

$\left\{\begin{array}{c}{V}_{sd}={\mathrm{Ri}}_{sd}+L\frac{{\mathrm{di}}_{sd}}{dt}-{\mathrm{\ω}}_s{\mathrm{Li}}_{sq}+V_{bsd}+V_{bmd}\\ V_{sq}={\mathrm{Ri}}_{sq}+L\frac{{\mathrm{di}}_{sq}}{dt}+{\mathrm{\ω}}_s{\mathrm{Li}}_{sd}+V_{bsq}+V_{bmq}\\ V_{so}={\mathrm{Ri}}_{so}+L\frac{{\mathrm{di}}_{so}}{dt}+V_{bso}+V_{bmo}\end{array}\right.---\left(11\right)$

$\left\{\begin{array}{c}{V}_{ld}=-{\mathrm{Ri}}_{ld}-L\frac{{\mathrm{di}}_{ld}}{dt}+{\mathrm{\ω}}_s{\mathrm{Li}}_{lq}+V_{bld}+V_{bmd}\\ V_{lq}=-{\mathrm{Ri}}_{lq}-L\frac{{\mathrm{di}}_{lq}}{dt}-{\mathrm{\ω}}_s{\mathrm{Li}}_{ld}+V_{blq}+V_{bmq}\\ V_{lo}=-{\mathrm{Ri}}_{lo}-L\frac{{\mathrm{di}}_{lo}}{dt}+V_{blo}+V_{bmo}\end{array}\right.---\left(12\right)$

$\left\{\begin{array}{c}{i}_{sd}=i_{bmd}+i_{ld}\\ i_{sq}=i_{bmq}+i_{lq}\\ i_{so}=i_{bmo}+i_{lo}\end{array}\right.---\left(13\right)$

If AC input voltage is three-phase symmetrical, i.e. V_so=0, further, since i_a=i_b=i_c, it is known that i_lα=i_lβ=0, i_ld=i_lq=0；Then formula (11)-Shi (13) is converted to:

$\left\{\begin{array}{c}{V}_{sd}={\mathrm{Ri}}_{sd}+L\frac{{\mathrm{di}}_{sd}}{dt}-{\mathrm{\ω}}_s{\mathrm{Li}}_{sq}+V_{bsd}+V_{bmd}\\ V_{sq}={\mathrm{Ri}}_{sq}+L\frac{{\mathrm{di}}_{sq}}{dt}+{\mathrm{\ω}}_s{\mathrm{Li}}_{sd}+V_{bsq}+V_{bmq}\\ 0={\mathrm{Ri}}_{so}+L\frac{{\mathrm{di}}_{so}}{dt}+V_{bso}+V_{bmo}\end{array}\right.---\left(14\right)$

$\left\{\begin{array}{c}{V}_{ld}=V_{bld}+V_{bmd}\\ V_{lq}=V_{blq}+V_{bmq}\\ V_{lo}=-{\mathrm{Ri}}_{lo}-L\frac{{\mathrm{di}}_{lo}}{dt}+V_{blo}+V_{bmo}\end{array}\right.---\left(15\right)$

$\left\{\begin{array}{c}{i}_{sd}=i_{bmd}\\ i_{sq}=i_{bmq}\\ i_{so}=i_{bmo}+i_{lo}\end{array}\right.---\left(16\right)$

Wherein, formula (14)-Shi (16) is present invention mathematical model under dqo coordinate system, and this mathematical model is 4 order mode types.

In HVDC system is applied, the control strategy of converting plant is different from the control strategy of Inverter Station.Controlling system with traditional LC C-HVDC similar, converting plant adopts constant dc power control, and Inverter Station adopts constant DC voltage control, will be controlled the research of strategy for converting plant below.

Now consider an equivalent circuit of Three phase serial module structureization many level HVDC inverter, as shown in Figure 2.

Application principle of stacking, AC one equivalent circuit is as shown in Figure 3.

By present invention mathematical modulo pattern (14) under dqo coordinate system it can be seen that for making AC input current not include DC component, make V_bso=-V_bmo；When converting plant straight-flow system voltage is DC voltage V_dcTime,For direct current, for making DC side output electric current not include AC compounent, V can be made_blo+V_bmoIn do not comprise AC compounent；Meanwhile, for realizing each single-phase inverters output DC voltage, DC side auxiliary bridge arm voltage V is made_bld=-V_bmd, V_blq=-V_bmq。

When systematic steady state runs, three main brachium pontis of the present invention are used for realizing alternating current-direct current power conversion, therefore main bridge arm voltage V_1b、V_2c、V_3dWith electric current i_1b、i_2c、i_3dInclude AC-frequency component and DC component.But owing to AC and DC side assist the DC side of brachium pontis not have burden with power, therefore when being left out active loss, it all can not absorb or send active power.Owing to AC assists bridge arm current i_u1、i_v2、i_w3For simple sinusoidal alternating current, do not contain DC component, therefore AC auxiliary bridge arm voltage V_u1、V_v2、V_w3In should contain only DC voltage component, and AC-frequency component can not be contained, i.e. V_bsd=V_bsq=0,Owing to DC side assists bridge arm current i_a1、i_b2、i_c3For DC current, do not contain AC compounent, therefore DC side auxiliary bridge arm voltage V_a1、V_b2、V_c3In should contain only AC-frequency component, i.e. V_blo=0.

When systematic steady state runs, front two formulas of formula (14) have:

$\left\{\begin{array}{c}{V}_{sd}={\mathrm{Ri}}_{sd}-{\mathrm{\ωLi}}_{sq}+V_{bd}\\ V_{sq}={\mathrm{Ri}}_{sq}+{\mathrm{\ωLi}}_{sd}+V_{bq}\end{array}\right.---\left(17\right)$

The active power of auxiliary brachium pontis and the total coabsorption of main brachium pontis and reactive power are respectively

$\left\{\begin{array}{c}{P}_{bs}=V_{bd}{i}_{sd}+V_{bq}{i}_{sq}\\ Q_{bs}=V_{bq}{i}_{sd}-V_{bd}{i}_{sq}\end{array}\right.---\left(18\right)$

It is zero for ensureing the active power that auxiliary brachium pontis absorbs, then has:

V_bsdi_sd+V_bsqi_sq=0 (19)

For realizing the dynamic distribution of absorbing reactive power ratio between auxiliary brachium pontis and main brachium pontis, introduce reactive power partition coefficient k so that main brachium pontis absorbing reactive power meets:

kQ_bs=V_bmqi_sd-V_bmdi_sq(20)

Namely, as k=1, reactive power is all absorbed by main brachium pontis.

Simultaneous formula (18)-Shi (20), then have:

$\begin{array}{c}{P}_{bs}=V_{bmd}{i}_{sd}+V_{bmq}{i}_{sq}\\ 0=V_{bsd}{i}_{sd}+V_{bsq}{i}_{sq}\\ {\mathrm{kQ}}_{bs}=V_{bmq}{i}_{sd}-V_{bmd}{i}_{sq}\\ (1-k)Q_{bs}=V_{bsq}{i}_{sd}-V_{bsd}{i}_{sq}\end{array}---\left(21\right)$

Solve formula (21),

$\left\{\begin{array}{c}{V}_{bsd}=-\frac{i_{sq}}{{i_{sd}}^2+{i_{sq}}^2}(1-k)Q_{bs}\\ V_{bsq}=\frac{i_{sd}}{{i_{sd}}^2+{i_{sq}}^2}(1-k)Q_{bs}\end{array}\right.---\left(22\right)$

$\left\{\begin{array}{c}{V}_{bmd}=V_{bd}-V_{bsd}\\ =V_{bd}+\frac{i_{sq}}{{i_{sd}}^2+{i_{sq}}^2}(1-k)Q_{bs}\\ V_{bmq}=V_{bq}-V_{bsq}\\ =V_{bq}-\frac{i_{sd}}{{i_{sd}}^2+{i_{sq}}^2}(1-k)Q_{bs}\end{array}\right.---\left(23\right)$

In practical operation, the active power that auxiliary brachium pontis absorbs is not 0, and it needs to absorb a small amount of active power to compensate the active loss in brachium pontis, it is to avoid the DC capacitor continuous discharge in submodule, thus ensureing that DC voltage is constant.If the active power that auxiliary brachium pontis absorbs is P_{bs_loss}, therefore constraint equation (19) is rewritten as

V_bsdi_sd+V_bsqi_sq=P_{bs_loss}(24)

Association type (18), formula (20) and formula (24), then have:

$\begin{array}{c}{P}_{bs}-P_{bs\_loss}=V_{bmd}{i}_{sd}+V_{bmq}{i}_{sq}\\ P_{bs\_loss}=V_{bsd}{i}_{sd}+V_{bsq}{i}_{sq}\\ {\mathrm{kQ}}_{bs}=V_{bmq}{i}_{sd}-V_{bmd}{i}_{sq}\\ (1-k)Q_{bs}=V_{bsq}{i}_{sd}-V_{bsd}{i}_{sq}\end{array}---\left(25\right)$

Solve formula (25),

$\left\{\begin{array}{c}{V}_{bsd}=-\frac{i_{sq}}{{i_{sd}}^2+{i_{sq}}^2}(1-k)Q_{bs}+\frac{i_{sd}}{{i_{sd}}^2+{i_{sq}}^2}{P}_{bs\_loss}\\ V_{bsq}=\frac{i_{sd}}{{i_{sd}}^2+{i_{sq}}^2}(1-k)Q_{bs}+\frac{i_{sq}}{{i_{sd}}^2+{i_{sq}}^2}{P}_{bs\_loss}\end{array}\right.---\left(26\right)$

$\left\{\begin{array}{c}{V}_{bmd}=V_{bd}-V_{bsd}\\ =V_{bd}+\frac{i_{sq}}{{i_{sd}}^2+{i_{sq}}^2}(1-k)Q_{bs}-\frac{i_{sd}}{{i_{sd}}^2+{i_{sq}}^2}{P}_{bs\_loss}\\ V_{bmq}=V_{bq}-V_{bsq}\\ =V_{bq}-\frac{i_{sd}}{{i_{sd}}^2+{i_{sq}}^2}(1-k)Q_{bs}-\frac{i_{sq}}{{i_{sd}}^2+{i_{sq}}^2}{P}_{bs\_loss}\end{array}\right.---\left(27\right)$

Control strategy shown in formula (26) and formula (27) is represented by following control structure figure, wherein, and the active-power P that AC auxiliary brachium pontis absorbs_{bs_loss}Produced by auxiliary brachium pontis DC voltage outer shroud.

Control strategy

(1) AC control strategy

The mathematical model of the present invention is similar to the mathematical model of traditional grid-connected inverter, adopts as follows based on the feed forward decoupling control strategy of grid voltage orientation:

In front two formulas of formula (14), order

$\left[\begin{array}{c}{V}_{bd}\\ V_{bq}\end{array}\right]=\left[\begin{array}{c}{V}_{bsd}\\ V_{bsq}\end{array}\right]+\left[\begin{array}{c}{V}_{bmd}\\ V_{bmq}\end{array}\right]---\left(28\right)$

$\left\{\begin{array}{c}{V_{bd}}^{\′}={\mathrm{Ri}}_{sd}+{\mathrm{Lsi}}_{sd}\\ {V_{bq}}^{\′}={\mathrm{Ri}}_{sq}+{\mathrm{Lsi}}_{sq}\end{array}\right.---\left(29\right)$

The Feedforward Decoupling closed loop control realizing watt current and reactive current is regulated by PI, wherein, V_bd' and V_bq' obtained by the pi regulator of current closed-loop, its control principle is as shown in Figure 5.

Outer shroud control strategy for the present invention adopts power and Control of Voltage, namely adopts for AC system and determines active power and determine alternating voltage to control, as shown in Figure 6.

(2) DC side control strategy

Constant for the DC voltage of each submodule in the main brachium pontis of the control present invention, by controlling V_blo+V_bmoRealize.When converting plant straight-flow system voltage is DC voltage V_dcTime,By the 3rd formula of present invention mathematical modulo pattern (15) under dqo coordinate system, the close-loop control scheme of design voltage outer shroud and current inner loop is as follows.

Obtained by the 3rd formula of formula (15)

$-{\mathrm{Ri}}_{lo}-L\frac{{\mathrm{di}}_{lo}}{dt}+V_{blo}+V_{bmo}=\frac{V_{dc}}{\sqrt{3}}---\left(30\right)$

Order

$\begin{array}{c}{V}_{bo}=V_{blo}+V_{bmo}\\ {V_{bo}}^{\′}={\mathrm{Ri}}_{lo}+{\mathrm{Lsi}}_{lo}\end{array}---\left(31\right)$

Then can pass through PI and regulate the closed loop control realizing average anode current.Wherein, V_bo' obtained by the pi regulator of current closed-loop, its control principle is as shown in Figure 7.

Current setting value i_lo ^*The setting value of electric current is exported for system dc side, constant for the DC voltage of each submodule in the main brachium pontis of the control present invention, adopt the pi regulator of voltage close loop, such as Fig. 8.

Actually being constantly present active loss in DC side auxiliary brachium pontis, the DC voltage in order to ensure DC side auxiliary brachium pontis is constant, in addition it is also necessary to DC side is assisted bridge arm voltage V_a1、V_b2、V_c3In containing DC voltage component V_bloIt is controlled.And V_bloThe DC voltage closed loop of each submodule of brachium pontis can be assisted to produce by DC side.And then V can be obtained by formula (31)_bmo, control structure can represent as shown in Figure 9.

Systematic steady state value calculates

If AC system is three-phase symmetrical system.Without loss of generality, it is assumed that during steady operation of the present invention, the voltage and current of input-output system is:

$\left\{\begin{array}{c}{V}_u=V_{sm}cos\left(\mathrm{\ω}t\right)\\ V_v=V_{sm}cos(\mathrm{\ω}t-2\mathrm{\π}/3)\\ V_w=V_{sm}cos(\mathrm{\ω}t+2\mathrm{\π}/3)\end{array}\right.---\left(32\right)$

$V_a=V_b=V_c=\frac{V_{dc}}{3}---\left(33\right)$

The steady state solution that previously described constant power transformation matrix and synchronous rotating angle matrix obtain the present invention is adopted to be:

$\begin{array}{c}\left[\begin{array}{c}{V}_{sd}\\ V_{sq}\end{array}\right]=\sqrt{\frac{3}{2}}\left[\begin{array}{c}{V}_{sm}\\ 0\end{array}\right]\\ \left[\begin{array}{c}{i}_{sd}\\ i_{sq}\end{array}\right]=\sqrt{\frac{3}{2}}{I}_{sm}\left[\begin{array}{c}cos\mathrm{\θ}\\ -sin\mathrm{\θ}\end{array}\right]\end{array}---\left(34\right)$

Further

$V_{bd}=V_{sd}-{\mathrm{Ri}}_{sd}+{\mathrm{\ωLi}}_{sq}=\sqrt{\frac{3}{2}}{V}_{sm}-\sqrt{\frac{3}{2}}{\mathrm{RI}}_{sm}cos\mathrm{\θ}-\sqrt{\frac{3}{2}}{\mathrm{\ωLI}}_{sm}sin\mathrm{\θ}---\left(35\right)$

$V_{bq}=V_{sq}-{\mathrm{Ri}}_{sq}-{\mathrm{\ωLi}}_{sd}=\sqrt{\frac{3}{2}}{\mathrm{RI}}_{sm}sin\mathrm{\θ}-\sqrt{\frac{3}{2}}{\mathrm{\ωLI}}_{sm}cos\mathrm{\θ}---\left(36\right)$

$Q_{bs}=V_{bqs}{i}_{sd}-V_{bd}{i}_{sq}=\frac{3}{2}{V}_{sm}{I}_{sm}sin\mathrm{\θ}-\frac{3}{2}{\mathrm{\ωLI}}_{sm}^2---\left(37\right)$

$V_b^2=V_{bd}^2+V_{bq}^2=\frac{3}{2}{V}_{sm}^2+\frac{3}{2}{R}^2{I}_{sm}^2+\frac{3}{2}{\mathrm{\ω}}^2{L}^2{I}_{sm}^2-3V_{sm}{\mathrm{RI}}_{sm}\cos \mathrm{\θ}-3V_{sm}{\mathrm{\ωLI}}_{sm}\sin \mathrm{\θ}---\left(38\right)$

$i_s=\sqrt{i_{sd}^2+i_{sq}^2}=\sqrt{\frac{3}{2}}{I}_{sm}---\left(39\right)$

As it is shown in fig. 7, wherein, the modulation degree of AC auxiliary brachium pontis is m to each brachium pontis modulation degree distribution schematic diagram during stable state₁-2m₃, the modulation degree of DC side auxiliary brachium pontis is-2m₂, the modulation degree of main brachium pontis is m₂+m₃。

When steady-state operation of the present invention, for making main brachium pontis and auxiliary brachium pontis only modulate, thus maximizing the meritorious of inverter and AC and DC side system exchange and reactive power, set optimization aim as:

$\begin{array}{c}\min \left\{\max \left(\right|m_1|+|2m_3|,|m_2|+|m_3|,|2m_2\left|\right)\right\}\\ \begin{array}{cc}s.t.& \left\{\begin{array}{c}\left|m_1\right|+\left|2m_3\right|\≤1,\\ \left|m_2\right|+\left|m_3\right|\≤1,\\ \left|2m_2\right|\≤1\end{array}\right.\end{array}\end{array}---\left(40\right)$

The power attenuation P of auxiliary brachium pontis can be ignored when systematic steady state_{bs_loss}, formula (22)～(23) obtain:

$m_1=\sqrt{\frac{2}{3}}\frac{V_{bs}}{\frac{{\mathrm{NV}}_{dcr}}{2}}=\sqrt{\frac{2}{3}}\frac{\sqrt{V_{bsd}^2+V_{bsq}^2}}{\frac{{\mathrm{NV}}_{dcr}}{2}}=2\sqrt{\frac{2}{3}}\frac{(1-k)Q_{bs}}{i_s{\mathrm{NV}}_{dcr}}---\left(41\right)$

$m_2=\frac{V_{bm}}{{\mathrm{NV}}_{dcr}}=\frac{\sqrt{V_{bmd}^2+V_{bmq}^2}}{{\mathrm{NV}}_{dcr}}==\frac{\sqrt{V_b^2+(k^2-1)\frac{Q_{bs}^2}{i_s^2}}}{{\mathrm{NV}}_{dcr}}---\left(42\right)$

$m_3=\frac{\frac{V_{bmo}}{\sqrt{3}}}{{\mathrm{NV}}_{dcr}}=\frac{\frac{V_{dc}}{3}}{{\mathrm{NV}}_{dcr}}---\left(43\right)$

Formula (22)～(23) are substituted in formula (41)～(43),

$m_1=\frac{V_{bs}}{\frac{{\mathrm{NV}}_{dcr}}{2}}=\frac{2\sqrt{\frac{3}{2}}(1-k)(V_{sm}sin\mathrm{\θ}-{\mathrm{\ωLI}}_{sm})}{{\mathrm{NV}}_{dcr}}---\left(44\right)$

$m_2=\frac{V_{bm}}{{\mathrm{NV}}_{dcr}}=\frac{\sqrt{k^2(\frac{3}{2}{V}_{sm}^2{\sin }^2\mathrm{\θ}-3V_{sm}{\mathrm{\ωLI}}_{sm}sin\mathrm{\θ}+\frac{3}{2}{\mathrm{\ω}}^2{L}^2{I}_{sm}^2)+\frac{3}{2}{V}_{sm}^2{\cos }^2\mathrm{\θ}-3V_{sm}{\mathrm{RI}}_{sm}cos\mathrm{\θ}+\frac{3}{2}{R}^2{I}_{sm}^2}}{{\mathrm{NV}}_{dcr}}---\left(45\right)$

$m_3=\frac{V_{bmo}}{{\mathrm{NV}}_{dcr}}=\frac{\frac{V_{dc}}{\sqrt{3}}}{{\mathrm{NV}}_{dcr}}---\left(46\right)$

By formula (44)～(46), main brachium pontis and the stable state modulation degree of auxiliary brachium pontis can be changed by regulating the value of k, and then calculate the optimal value of the k meeting optimization aim formula (40), thus the overall performance of system is optimized.

Simulation study

Building the system model of the present invention under MATLAB/SIMULINK platform, system major parameter is as shown in table 1.

Three phase serial module structureization many level HVDC Converter DC-side voltage and current waveform during Figure 11-Figure 19 respectively k=1, AC voltage and current waveform, the active power of transmission and reactive power oscillogram, main brachium pontis and auxiliary brachium pontis DC capacitor voltage oscillogram, ac-side current spectrum analysis figure, DC side current spectrum analysis chart, AC auxiliary brachium pontis modulating wave oscillogram, DC side auxiliary brachium pontis modulating wave oscillogram, main brachium pontis modulating wave oscillogram.

Three phase serial module structureization many level HVDC inverter AC voltage and current waveform during Figure 20-Figure 28 respectively k=0.6, DC voltage current waveform figure, transmit active power and reactive power oscillogram, main brachium pontis and auxiliary brachium pontis DC capacitor voltage oscillogram, ac-side current spectrum analysis figure, DC side current spectrum analysis chart, AC auxiliary brachium pontis modulating wave oscillogram, DC side auxiliary brachium pontis modulating wave oscillogram, main brachium pontis modulating wave oscillogram.

Claims (6)

1. Three phase serial module structureization many level HVDC inverter, it is characterized in that, including DC output end, three-phase alternating current input, first transformator (T1), second transformator (T2), 3rd transformator (T3), first single-phase converter, second single-phase converter and the 3rd single-phase converter, wherein, three outfans of three-phase alternating current input respectively with one end of primary coil in the first transformator (T1), in second transformator (T2) primary coil one end and in the 3rd transformator (T3) one end of primary coil be connected, the other end of primary coil in first transformator (T1), in second transformator (T2) primary coil the other end and in the 3rd transformator (T3) other end of primary coil be connected, one end of secondary coil in first transformator (T1), in second transformator (T2) secondary coil one end and in the 3rd transformator (T3) one end of secondary coil respectively with in the first single-phase converter first auxiliary brachium pontis (u1), the first auxiliary brachium pontis (v2) in second single-phase converter and the first auxiliary brachium pontis (w3) in the 3rd single-phase converter are connected；

(a1's the second auxiliary brachium pontis in first single-phase converter is connected with the positive pole of DC output end, main brachium pontis (1b) in first single-phase converter is connected with the other end of secondary coil in the first transformator (T1) and auxiliary brachium pontis of second in the second single-phase converter (b2), main brachium pontis (2c) in second single-phase converter is connected with the other end of secondary coil in the second transformator (T2) and auxiliary brachium pontis of the 3rd in the 3rd single-phase converter (c3), main brachium pontis (3d) in 3rd single-phase converter is connected with the other end of secondary coil in the 3rd transformator (T3) and the negative pole of DC output end；

The second auxiliary brachium pontis (c3) in the second auxiliary brachium pontis (a1) in first single-phase converter, the second auxiliary brachium pontis (b2) in the second single-phase converter and the 3rd single-phase converter is sequentially connected in series forms by n/2 full-bridge submodule, the first inductance and the first resistance, and n is the even number be more than or equal to 2.

2. Three phase serial module structureization many level HVDC inverter according to claim 1, it is characterized in that, the secondary coil of the first transformator (T1), the first auxiliary brachium pontis (u1) and main brachium pontis (1b) in first single-phase converter form the first AC loop, the secondary coil of the second transformator (T2), the first auxiliary brachium pontis (v2) and main brachium pontis (2c) in second single-phase converter form the second AC loop, the secondary coil of the 3rd transformator (T3), the first auxiliary brachium pontis (w3) and main brachium pontis (3d) in 3rd single-phase converter form the 3rd AC loop, the wherein interior circular current in the first AC loop, the interior circular current in the second AC loop and the interior circular current in the 3rd AC loop all adopt AC to gain merit, reactive current feed forward decoupling control method is controlled, main brachium pontis (1b) in first AC loop, main brachium pontis (2c) in second AC loop and the main brachium pontis (3d) in the 3rd AC loop all adopt to be determined active power and determines alternating voltage control method to be controlled, the first auxiliary brachium pontis (u1) in first AC loop, the first auxiliary brachium pontis (v2) in second AC loop and the first auxiliary brachium pontis (w3) in the 3rd AC loop all adopt the constant DC voltage control method of current oriention to be controlled.

3. Three phase serial module structureization many level HVDC inverter according to claim 1, it is characterized in that, introduce reactive power partition coefficient k, by regulate reactive power partition coefficient k realize main brachium pontis (2c) in reactive power distribution between main brachium pontis (1b) with the first auxiliary brachium pontis (u1) in the first AC loop, the second AC loop assist with first distribution between brachium pontis (v2) and in the 3rd AC loop main brachium pontis (3d) and first assist the distribution between brachium pontis (w3).

4. Three phase serial module structureization many level HVDC inverter according to claim 1, it is characterized in that, the second auxiliary brachium pontis (a1) and main brachium pontis (1b) in first monophase current device form the first DC circuit, the second auxiliary brachium pontis (b2) and main brachium pontis (2c) in second monophase current device form the second DC circuit, the second auxiliary brachium pontis (c3) and main brachium pontis (3d) in 3rd monophase current device form the 3rd DC circuit, wherein, interior circular current in first DC circuit, interior circular current in second monophase current device and the interior circular current in the 3rd monophase current device are controlled each through the feedforward closed loop control method of DC current, main brachium pontis (1b) in first DC circuit, main brachium pontis (2c) in second monophase current device and the main brachium pontis (3d) in the 3rd monophase current device are controlled by determining DC voltage, the second auxiliary brachium pontis (a1) in first DC circuit, the second auxiliary brachium pontis (b2) in second monophase current device and the second auxiliary brachium pontis (c3) in the 3rd monophase current device pass through constant DC voltage control.

5. Three phase serial module structureization many level HVDC inverter according to claim 1, it is characterized in that, the main brachium pontis (3d) in the main brachium pontis (1b) in the first single-phase converter, the main brachium pontis (2c) in the second single-phase converter and the 3rd single-phase converter is sequentially connected in series by n half-bridge submodule and forms.

6. Three phase serial module structureization many level HVDC inverter according to claim 1, it is characterized in that, the first auxiliary brachium pontis (w3) in the first auxiliary brachium pontis (u1) in the first single-phase converter, the first auxiliary brachium pontis (v2) in the second single-phase converter and the 3rd single-phase converter is sequentially connected in series forms by the second resistance, the second inductance and n/2 half-bridge submodule.