CN106712072A - Voltage class optimization design method for flexible direct current transmission system - Google Patents

Abstract

The invention discloses a voltage class optimization design method for a flexible direct current transmission system. The method comprises the following steps: converting bridge arm current determining conduction loss and switching loss of an MMC converter valve into an expression containing a voltage class variable Udc only; obtaining a transient expression of the conduction loss according to the bridge arm current, and combining a switch action sequence of the bridge arm current to perform subsection integral on the transient expression of the conduction loss so as to obtain a conduction loss calculation formula; summating switching losses of all the modules in a fundamental frequency cycle so as to obtain a switching loss expression; and finally, deriving the total loss, thereby obtaining the transmission pressure at the lowest loss, wherein the voltage refers to a voltage class after optimization design. According to the method disclosed by the invention, the voltage class at the lowest loss is determined, the system loss can be reduced, and a reference standard is provided for determining the voltage class of the power grid.

Description

A kind of flexible direct current power transmission system voltage class Optimization Design

Technical field

Patent of the present invention belongs to flexible direct-current transmission field, and more particularly to a kind of flexible direct current power transmission system voltage class is excellent Change method for designing.

Background technology

With the development of Power Electronic Technique, modularization multi-level converter (MMC) is greatly promoted high-voltage dc transmission The development of power technology.After 2001 are suggested first, MMC is damaged by the output waveform and relatively low power of its high-quality Consumption, the interest of researcher is widely caused in academia and industrial quarters, its topological structure, mathematical modeling, coordination control, failure The aspects such as protection have been studied more thorough.As one kind of voltage source converter (VSC), MMC have concurrently VSC own While advantage, low dynamic voltage balancing requirement, favorable expandability are also unanimously triggered with device, switching frequency is low and running wastage is low Deng many advantages.At present, based on modularization multi-level converter D.C. high voltage transmission (MMC-HVDC) system extensively should For the occasion of the new-energy grid-connecteds such as wind-powered electricity generation, solar energy, existing Shanghai Nanhui direct current transportation demonstration project, Zhoushan Of Zhejiang Province multiterminal are soft Property direct current transportation demonstration project, Nanao, Guangdong Province Multi-end flexible direct current transmission demonstration project etc. put into operation or build. MMC-HVDC can also be applied to improve the occasion of city power distribution, such as be located at Transbay Cable engineerings, the Liaoning in San Francisco Dalian great science and technology demonstration project of flexible DC power transmission over strait etc..For the particular applications such as island power supply, MMC-HVDC There is its unique advantage.It is contemplated that, in the composition of Future Power System, it is essential that MMC-HVDC will turn into its Composition.

Different types of loss is different with the variation tendency of output voltage grade in modularization multi-level converter (MMC) , it is therefore desirable to determine a kind of output voltage grade for causing system total losses minimum.

The content of the invention

Technical problem solved by the invention is, it is proposed that a kind of flexible DC power transmission voltage class Optimization Design, The analytical expression of the various losses of converter valve is derived, by converter valve total losses expression formula derivation, having drawn converter valve damage Voltage class when consuming minimum.

To achieve the above object, the technical solution used in the present invention is：

A kind of flexible direct current power transmission system voltage class Optimization Design, described flexible direct current power transmission system is used MMC converter valves, MMC converter valves use the bridge arm topological structure of three-phase six, and per upper and lower two bridge arms are mutually included, each bridge arm is by N number of SM submodules SM₁~SM_NIt is in series with an inductance L, the tie point of upper and lower bridge arm draws phase line；Three phase line access is public Power network；

Flexible direct current power transmission system voltage class optimization method is：First, MMC converter valves on-state loss and switch will be determined The bridge arm current of loss is converted into containing only voltage class variable U_dcExpression formula；Secondly, on-state loss is drawn according to bridge arm current Transient expression formula, in conjunction with the switching sequence of movement of bridge arm current, the transient expression formula to on-state loss carries out subsection integral, Draw on-state loss；Then, the switching loss of all modules is sued for peace in a power frequency period, is drawn switching loss； Finally, the total losses of converter valve are calculated by on-state loss and switching loss, to total losses derivation, transmission of electricity when showing that loss is minimum Voltage, the voltage is the voltage class after optimization design.

Further, the on-state loss P_condComputing formula be：

In above formula, N=round (U_dc/U_SM), U_dcIt is direct-current transmission voltage, U_SMIt is the rated voltage of SM submodules；T is work Frequency cycle, n is the number of the SM submodules that bridge arm needs conducting in t in a phases, i_paT () is the electricity of bridge arm in t a phases Stream, x₀, x₁And x₂It is i_paThe moment of (t) zero crossing, P_{T_con}(i_pa(t)) it is IGBT on-state loss, P_{D_con}(i_pa(t)) it is two poles Pipe on-state loss；Each calculation method of parameters is as follows：

In above formula, m is modulation ratio,It is the power factor angle of flexible direct current power transmission system transmitted power, ω is line voltage Fundamental wave angular frequency, S is flexible direct current power transmission system transmitted power rated value, U_F0It is diode threshold resistance, r_fFor diode is logical State resistance；U_CE0It is the column voltage of holding up of IGBT, r_CEIt is IGBT on state resistances；

Further, the switching loss includes (1) necessity switching loss P_sw1(2) additional switching losses P_addloss；

(1) necessary switching loss P_sw1Computing formula be：

Wherein, E_off(i_pa(t_x)) for IGBT in t_xThe turn-off power loss at moment, E_rec(i_pa(t_x)) for diode in t_xMoment Reverse recovery loss, E_on(i_pa(t_x)) for IGBT in t_xThe turn-on consumption at moment；E_off(i_pa(t'_x)) for IGBT in t'_xMoment Turn-off power loss, E_rec(i_pa(t'_x)) for diode in t'_xThe reverse recovery loss at moment, E_on(i_pa(t'_x)) for IGBT in t'_xWhen The turn-on consumption at quarter；Each calculation method of parameters is as follows：

In above formula, U_CEThe virtual voltage born during by IGBT and diode current flow or shut-off, U_SMIt is SM submodules electricity Hold rated voltage；U_{CE_ref}In the databook be given for IGBT manufacturers measure single switching loss when colelctor electrode- Emitter voltage value；a₁, b₁And c₁It is the fitting coefficient of IGBT turn-on consumptions；a₂, b₂And c₂It is the fitting system of IGBT turn-off power losses Number；a₃, b₃And c₃It is the fitting coefficient of diode reverse recovery losses；a₁、b₁、c₁、a₂、b₂、c₂、a₃、b₃And c₃Can be given birth to from IGBT Obtained in the databook for being given for producing producer；

(2) additional switching losses P_addlossComputing formula be：

In above formula, T_sIt is controlling cycle, k is from 1 to T/T_sBetween natural number, P_addlossK () meets following formula：

Wherein, E_on(i_pa(kT_s)) for IGBT in kT_sThe turn-on consumption at moment, E_off(i_pa(kT_s)) for IGBT in kT_sMoment Turn-off power loss, E_rec(i_pa(kT_s)) for diode in kT_sThe reverse recovery loss at moment；

L (k) is the submodule number that upper bridge arm should be turned in k-th controlling cycle, need to meet following formula：

Further, the total losses of the converter valve are：

To the total losses of converter valveDerivation, evenSolve U_dcValue, U now_dcValue is Voltage class after optimization design.

Further, a₁, b₁, c₁It is the fitting coefficient of IGBT turn-on consumptions, by databook " at 125 DEG C of junction temperature Typical collector current-turn-on consumption " curve is obtained by the way of conic fitting, a₁It is secondary in approximating method Term coefficient, b₁It is the Monomial coefficient in approximating method, c₁It is the constant term coefficient in approximating method；a₂, b₂, c₂It is IGBT shut-offs The fitting coefficient of loss, uses by " typical collector current-turn-off power loss at 125 DEG C of junction temperature " curve in databook The mode of conic fitting is obtained, a₂It is the secondary term coefficient in approximating method, b₂It is the Monomial coefficient in approximating method, c₂It is the constant term coefficient in approximating method；a₃, b₃, c₃It is the fitting coefficient of diode reverse recovery losses, by data hand " typical on state current-reverse recovery loss at 125 DEG C of junction temperature " curve is obtained by the way of conic fitting in volume, a₃ It is the secondary term coefficient in approximating method, b₃It is the Monomial coefficient in approximating method, c₃It is the constant term system in approximating method Number.

Further, a₁Value is 6.558 × 10^-4, b₁Value is 3.659, c₁Value is 684.4, a₂Value is 6.071 ×10^-5, b₂Value is 4.025, c₂Value is 378.2, a₃Value is 7.984 × 10^-4, b₃Value is 3.103, c₃Value is 644.2。

Further, T values are 0.02s, T_sValue is 0.5ms,Value is 0, m values for 0.95, S values are 1000MW, ω value are the model Infineon-FZ1200R45HL, U of 100 π rad/s, IGBT pipes_SMValue is 3000V, U_CE Value is 3000V, U_{CE_ref}Value is 2800V, U_CE0Value is 1.342V, r_CEValue is 0.00126 Ω, U_F0Value is 1.079V, r_fValue is 0.001109 Ω.

The invention discloses a kind of flexible direct current power transmission system voltage class Optimization Design, will determine that MMC is changed first The bridge arm current of stream valve on-state loss and switching loss is converted into containing only voltage class variable U_dcExpression formula；Secondly according to bridge arm Electric current draws the transient expression formula of on-state loss, in conjunction with the switching sequence of movement of bridge arm current, to the instantaneous table of on-state loss Subsection integral is carried out up to formula, on-state loss computing formula is drawn；Then by the switching loss of all modules in a power frequency period Inside sued for peace, drawn switching loss expression formula；Finally to total losses derivation, transmission voltage when showing that loss is minimum, the electricity Pressure is the voltage class after optimization design.Present invention determine that voltage class when loss is minimum, it is possible to reduce system loss, For the determination of line voltage grade provides normative reference.

Beneficial effect：

This flexible DC power transmission voltage class Optimization Design disclosed by the invention, analyzes all kinds of losses of converter valve With the relation of transmission voltage grade, it is certain in capacity, under the particular condition that submodule voltage is fixed, by total losses are expressed into Row derivation, solved loss it is minimum when voltage class.The present invention is 1) for the setting of voltage class provides the reference of science； 2) system power loss is reduced.

Brief description of the drawings

Fig. 1 three-phase MMC system construction drawings；

Bridge arm current puts into number graph of a relation with submodule in Fig. 2 a phases；

Fig. 3 bridge arm currents and submodule switching sequence；

Fig. 4 additional switching losses calculation flow charts；

The relation of Fig. 5 on-state loss and voltage class；

The relation of Fig. 6 switching losses and voltage class；

The relation of Fig. 7 total losses and voltage class；

Specific embodiment

In order that technical problem solved by the invention, technical scheme and beneficial effect become more apparent, below in conjunction with Accompanying drawing, the present invention will be described in further detail.It should be appreciated that instantiation described herein is only used to explain this hair It is bright, it is not intended to limit the present invention.

Fig. 1 is three-phase MMC systems, and three-phase MMC systems are made up of three phase bridge arms, and each phase bridge arm is by upper half bridge arm with Half bridge arm two parts are constituted, and each half bridge arm (is designated as SM successively by N number of SM submodules respectively₁, SM₂..., SM_N) and a bridge arm Inductance is sequentially connected in series and forms, and the output end of each phase bridge arm is drawn from two tie points of bridge arm inductance, and three exits are respectively v_ao、v_bo、v_co；Each SM submodule is a half-bridge current transformer, by two IGBT pipes T1 and T2, two diode D1 and D2 and One electric capacity C is constituted；Wherein, the emitter stage of IGBT pipes T1 is connected with the colelctor electrode of IGBT pipes T2 and constitutes the anode of SM, IGBT The colelctor electrode of pipe T1 is connected with the positive pole of electric capacity C, and the emitter stage of IGBT pipes T2 is connected with the negative pole of electric capacity and constitutes the negative terminal of SM； D1 and T1 reverse parallel connections, D2 and T2 reverse parallel connections；The gate pole of IGBT pipes T1 and T2 receives control wave；The volume of submodule Voltage is determined for U_SM, DC voltage is U_dc。

Below in conjunction with accompanying drawing to the on-state loss analytical expression of MMC converter valves submodule, switching loss solution in the present invention The derivation for analysing expression formula is illustrated.

Fig. 2 is that bridge arm current turns on graph of a relation with submodule in a phases, as t ∈ (x₀,x₁) when, there is i_pa(t)>0, and i_pa(t) Flow through n D1 and neutralize (N-n) individual T2；As t ∈ (x₁,x₂) when, there is i_pa(t)<0, and i_paFlow through n T1 and neutralize (N-n) individual D2, institute With on-state loss P_condFor：

In above formula, T values are 0.02s, and n is the number of the submodule that bridge arm needs conducting in t in a phases, i_paT () is Bridge arm current, x in t a phases₀, x₁, x₂It is i_paThe moment of zero crossing, P_{T_con}(i_pa(t)) it is IGBT on-state loss, P_{D_con} (i_pa(t)) it is diode on-state loss.

In above formula, m values are 0.95,Value is that 0, ω values are 100 π, and S values are 1000MW, U_F0Value is 1.079, r_fValue is 0.0011109 Ω；U_CE0Value is 0.342V, r_CEValue is 0.00126 Ω.

Fig. 3 is bridge arm current and submodule action sequence, the moment t of each staircase voltage change_xCan be by following formula It is determined that：

Wherein x is the submodule number that bridge arm is putting into operation in a phases.

When t ∈ (0, t '₁) when, i_pa>0, therefore, in t_xMoment, it is necessary to which certain submodule is switched into input by excision state State, wherein t_xFor：

WhenWhen, i_pa<0, therefore, in t_xMoment, it is necessary to switch to certain submodule by input state to cut off shape State, wherein t_xFor：

WhenWhen, i_pa<0, therefore, in t_xMoment, it is necessary to switch to certain submodule from input state to cut off shape State, wherein t_xFor：

WhenWhen, i_pa>0, therefore, in t_xMoment, it is necessary to switch to certain submodule from input state to cut off shape State, wherein t_xFor：

Necessary switching loss P as shown in Figure 3_sw1Expression formula is：

Wherein, E_off(i_pa(t_x)) for IGBT in t_xThe turn-off power loss at moment, E_rec(i_pa(t_x)) for diode in t_xMoment Reverse recovery loss, E_on(i_pa(t_x)) for IGBT in t_xThe turn-on consumption at moment, E_off(i_pa(t'_x)) for IGBT in t'_xMoment Turn-off power loss, E_rec(i_pa(t'_x)) for diode in t'_xThe reverse recovery loss at moment, E_on(i_pa(t'_x)) for IGBT in t'_xWhen The turn-on consumption at quarter.

U in above formula_CEValue is 3000, U_{CE_ref}Value is 2800, a₁Value is 6.558 × 10^-4, b₁Value is 3.659, c₁Value is 684.4, a₂Value is 6.071 × 10^-5, b₂Value is 4.025, c₂Value is 378.2, a₃Value is 7.984 × 10^-4, b₃Value is 3.103, c₃Value is 644.2.

Fig. 4 is additional switching losses calculation flow chart, T_sIt is controlling cycle, during k-th controlling cycle, upper bridge arm should be turned on Submodule number be：

As l (k-1)≤N-l (k), if approached using nearest level, in k controlling cycle, it is necessary to kth -1 The submodule of controlling cycle conducting is all replaced with the submodule not turned on, and now additional switching losses are：

P_addloss(k)=l (k-1) (E_on(i_pa(kT_s))+E_off(i_pa(kT_s))+E_rec(i_pa(kT_s)))

As l (k-1)>During N-l (k), then in k-th controlling cycle, it is necessary to -1 submodule of controlling cycle conducting of kth In the excision of (N-l (k)) individual submodule, additional switching losses now are：

P_addloss(k)=(N-l (k)) (E_on(i_pa(kT_s))+E_off(i_pa(kT_s))+E_rec(i_pa(kT_s))))

So total additional switching losses P_addlossFor：

T in above formula_sValue is 0.5ms.

Emulated in matlab, compared in different capabilities, the Dissipation change rule in the case of different voltage class.

Fig. 5 is the relation of on-state loss and voltage class, it can be seen that when voltage class is relatively low, with electricity The increase of grade is pressed, on-state loss declines rapidly.Such as when voltage class is 240kV, it is 10.33MW to be lost, when voltage etc. When level is increased to 600kV, loss quickly falls to 5.807MW；When voltage class is further improved, loss is slow to be declined, when Voltage class is 780kV, and loss drops to 5.111MW, and after voltage class is more than 800kV, loss is held essentially constant.

Fig. 6 is the relation of switching loss and voltage class, it can be seen that when voltage class is relatively low, due to son Number of modules is less, therefore switching loss is relatively low；When voltage class is raised, submodule number increase, switching loss also accordingly increases. Such as when voltage class is 240kV, switching loss is 7.755MW, when voltage class is increased to 780kV, switching loss liter Height arrives 9.612MW.

Different types of loss is different with the variation tendency of voltage class, it is therefore desirable to it is determined that a kind of cause that system is total Minimum voltage class is lost.

Fig. 7 is the relation of total losses and voltage class, it can be seen that with the progressively liter of transmission voltage grade Height, total losses have a minimum, appear in 720kV voltage class vicinity.This be due to when the capacity of converter valve determines, With the raising of voltage class, now on-state loss can gradually converge to fixed value, and at the same time switching loss but can gradually increase Plus.The transmission voltage of converter valve when can determine to be lost minimum by the inventive method, to determine line voltage grade and power network Capacity provides reference.

Claims (7)

1. a kind of flexible direct current power transmission system voltage class Optimization Design, described flexible direct current power transmission system uses MMC Converter valve, MMC converter valves use the bridge arm topological structure of three-phase six, and per upper and lower two bridge arms are mutually included, each bridge arm is by N number of SM Submodule and an inductance L are in series, and the tie point of upper and lower bridge arm draws phase line；Three phase line accesses public electric wire net；It is special Levy and be, flexible direct current power transmission system voltage class optimization method is：First, MMC converter valves on-state loss and switch will be determined The bridge arm current of loss is converted into containing only voltage class variable U_dcExpression formula；Secondly, on-state loss is drawn according to bridge arm current Transient expression formula, in conjunction with the switching sequence of movement of bridge arm current, the transient expression formula to on-state loss carries out subsection integral, Draw on-state loss；Then, the switching loss of all modules is sued for peace in a power frequency period, is drawn switching loss； Finally, the total losses of converter valve are calculated by on-state loss and switching loss, to total losses derivation, transmission of electricity when showing that loss is minimum Voltage, the voltage is the voltage class after optimization design.

2. flexible direct current power transmission system voltage class Optimization Design according to claim 1, the on-state loss meter Calculating formula is：

$P_{cond}=\frac{1}{T}\{{\&Integral;}_{x_0}^{x_1}\&lsqb;{\mathrm{nP}}_{D\_con}\left(i_{pa}\right(t\left)\right)+(N-n)P_{T-con}\left(i_{pa}\right(t\left)\right)\&rsqb;dt+{\&Integral;}_{x_1}^{x_2}\&lsqb;{\mathrm{nP}}_{T\_con}\left(i_{pa}\right(t\left)\right)+(N-n)P_{D\_con}\left(i_{pa}\right(t\left)\right)\&rsqb;dt\}$

In above formula, N=round (U_dc/U_SM), U_dcIt is direct-current transmission voltage, U_SMIt is the rated voltage of SM submodules；T is power frequency week Phase, n is the number of the SM submodules that bridge arm needs conducting in t in a phases, i_paT () is the electric current of bridge arm in t a phases, x₀, x₁And x₂It is i_paThe moment of (t) zero crossing, P_{T_con}(i_pa(t)) it is IGBT on-state loss, P_{D_con}(i_pa(t)) for diode it is logical State is lost；Each calculation method of parameters is as follows：

In above formula, m is modulation ratio,It is the power factor angle of flexible direct current power transmission system transmitted power, ω is line voltage fundamental wave Angular frequency, S is flexible direct current power transmission system transmitted power rated value, U_F0It is diode threshold resistance, r_fIt is diode on-state electricity Resistance；U_CE0It is the column voltage of holding up of IGBT, r_CEIt is IGBT on state resistances.

3. flexible direct current power transmission system voltage class Optimization Design according to claim 2, the switching loss bag Include (1) necessity switching loss P_sw1(2) additional switching losses P_addloss；

(1) necessary switching loss P_sw1Computing formula be：

$\begin{array}{c}{P}_{sw1}=\frac{1}{T}\{\underset{x=1}{\overset{q}{\&Sigma;}}{E}_{off}\left(i_{pa}\right(t_x\left)\right)+\underset{x=q}{\overset{N}{\&Sigma;}}{E}_{on}\left(i_{pa}\right(t_x\left)\right)+\underset{x=q}{\overset{N}{\&Sigma;}}{E}_{rec}\left(i_{pa}\right(t_x\left)\right)+\underset{x=p}{\overset{N}{\&Sigma;}}{E}_{off}\left(i_{pa}\right(t_x^{\&prime;}\left)\right)+\\ \underset{x=1}{\overset{p}{\&Sigma;}}{E}_{on}\left(i_{pa}\right(t_x^{\&prime;}\left)\right)+\underset{x=1}{\overset{p}{\&Sigma;}}{E}_{rec}\left(i_{pa}\right(t_x^{\&prime;}\left)\right)\}\end{array}$

Wherein, E_off(i_pa(t_x)) for IGBT in t_xThe turn-off power loss at moment, E_rec(i_pa(t_x)) for diode in t_xMoment it is reverse Recover loss, E_on(i_pa(t_x)) for IGBT in t_xThe turn-on consumption at moment；E_off(i_pa(t'_x)) for IGBT in t'_xThe shut-off at moment Loss, E_rec(i_pa(t'_x)) for diode in t'_xThe reverse recovery loss at moment, E_on(i_pa(t'_x)) for IGBT in t'_xMoment Turn-on consumption；Each calculation method of parameters is as follows：

$\left\{\begin{array}{c}{E}_{on}\left(i_{pa}\right(t\left)\right)=\frac{U_{CE}}{U_{CE\_ref}}\left(a_1{i}_{pa}^2\right(t)+b_1|i_{pa}\left(t\right)|+c_1)\\ E_{off}\left(i_{pa}\right(t\left)\right)=\frac{U_{CE}}{U_{CE\_ref}}\left(a_2{i}_{pa}^2\right(t)+b_2|i_{pa}\left(t\right)|+c_2)\\ E_{rec}\left(i_{pa}\right(t\left)\right)=\frac{U_{CE}}{U_{CE\_ref}}\left(a_3{i}_{pa}^2\right(t)+b_3|i_{pa}\left(t\right)|+c_3)\end{array}\right.$

In above formula, U_CEThe virtual voltage born during by IGBT and diode current flow or shut-off, U_SMIt is SM submodule electric capacity volumes Determine voltage；U_{CE_ref}Colelctor electrode-transmitting in the databook be given for IGBT manufacturers when single switching loss is measured Pole tension value；a₁, b₁And c₁It is the fitting coefficient of IGBT turn-on consumptions；a₂, b₂And c₂It is the fitting coefficient of IGBT turn-off power losses； a₃, b₃And c₃It is the fitting coefficient of diode reverse recovery losses；a₁、b₁、c₁、a₂、b₂、c₂、a₃、b₃And c₃Can be from IGBT factories Obtained in the databook for being given of family；

(2) additional switching losses P_addlossComputing formula be：

$P_{addloss}=\frac{1}{T}\underset{k=1}{\overset{T/T_s}{\&Sigma;}}{P}_{addloss}\left(k\right)$

In above formula, T_sIt is controlling cycle, k is from 1 to T/T_sBetween natural number, P_addlossK () meets following formula：

$\left\{\begin{array}{cc}{P}_{addloss}\left(k\right)=l(k-1)\&CenterDot;\left(E_{on}\right(i_{pa}\left({\mathrm{kT}}_s\right))+E_{off}(i_{pa}\left({\mathrm{kT}}_s\right))+E_{rec}(i_{pa}\left({\mathrm{kT}}_s\right)\left)\right)& l(k-1)\&le;N-l\left(k\right)\\ P_{addloss}\left(k\right)=(N-l(l\left)\right)\&CenterDot;\left(E_{on}\right(i_{pa}\left({\mathrm{kT}}_s\right))+E_{off}(i_{pa}\left({\mathrm{kT}}_s\right))+E_{rec}(i_{pa}\left({\mathrm{kT}}_s\right)\left)\right)& l(k-1)>N-l\left(k\right)\end{array}\right.$

Wherein, E_on(i_pa(kT_s)) for IGBT in kT_sThe turn-on consumption at moment, E_off(i_pa(kT_s)) for IGBT in kT_sThe pass at moment Breakdown consumes, E_rec(i_pa(kT_s)) for diode in kT_sThe reverse recovery loss at moment；

L (k) is the submodule number that upper bridge arm should be turned in k-th controlling cycle, need to meet following formula：

$l\left(k\right)=\frac{U_{dc}}{2U_{SM}}(1-{\mathrm{mcosk\&omega;T}}_s).$

4. flexible direct current power transmission system voltage class Optimization Design according to claim 3, the converter valve it is total It is lost and is：

To the total losses of converter valveDerivation, evenSolve U_dcValue, U now_dcValue is and optimizes Voltage class after design.

5. flexible direct current power transmission system voltage class Optimization Design according to claim 4, it is characterised in that a₁, b₁, c₁It is the fitting coefficient of IGBT turn-on consumptions, by " typical collector current at 125 DEG C of junction temperature-open-minded in databook Loss " curve is obtained by the way of conic fitting, a₁It is the secondary term coefficient in approximating method, b₁In being approximating method Monomial coefficient, c₁It is the constant term coefficient in approximating method；a₂, b₂, c₂It is the fitting coefficient of IGBT turn-off power losses, passes through " typical collector current-turn-off power loss at 125 DEG C of junction temperature " curve in databook is obtained by the way of conic fitting , a₂It is the secondary term coefficient in approximating method, b₂It is the Monomial coefficient in approximating method, c₂It is the constant in approximating method Term coefficient；a₃, b₃, c₃It is the fitting coefficient of diode reverse recovery losses, by " typical at 125 DEG C of junction temperature in databook On state current-reverse recovery loss " curve is obtained by the way of conic fitting, a₃It is the quadratic term in approximating method Coefficient, b₃It is the Monomial coefficient in approximating method, c₃It is the constant term coefficient in approximating method.

6. flexible direct current power transmission system voltage class Optimization Design according to claim 5, it is characterised in that a₁Take Be worth is 6.558 × 10^-4, b₁Value is 3.659, c₁Value is 684.4, a₂Value is 6.071 × 10^-5, b₂Value is 4.025, c₂ Value is 378.2, a₃Value is 7.984 × 10^-4, b₃Value is 3.103, c₃Value is 644.2.

7. the flexible direct current power transmission system voltage class Optimization Design according to any one of claim 2~6, it is special Levy and be, T values are 0.02s, T_sValue is 0.5ms,Value is that 0, m values are that 0.95, S values are 1000MW, and ω values are The model Infineon-FZ1200R45HL, U of 100 π rad/s, IGBT pipes_SMValue is 3000V, U_CEValue is 3000V, U_{CE_ref}Value is 2800V, U_CE0Value is 1.342V, r_CEValue is 0.00126 Ω, U_F0Value is 1.079V, r_fValue is 0.001109Ω。