CN104779635A - Controller device suitable for VSC-MTDC (voltage source converter-multi-terminal direct current) system - Google Patents

Abstract

The invention provides a controller device suitable for a VSC-MTDC (voltage source converter-multi-terminal direct current) system. The controller device comprises a rectifier-side local controller used for controlling a current transformer station with a rectification effect in an MTDC power transmission system, an inverter-side local controller used for controlling a current transformer station with an inversion effect in the MTDC power transmission system, and a coordination controller used for coordinating electric power physical parameters such as power, current and the like of each current transformer station in the MTDC power transmission system. The controller device effectively avoids PI (proportional integration) parameter setting and calculation of conventional double closed loop control and greatly lowers the complexity of control on rectification and inversion of current transformers without a linear controller and PWM (pulse width modulation); at the same time, a discrete mathematical model is fully utilized based on a P-DPC (predictive-direct power control) strategy, calculation is simple, and digitization is easy to realize.

Description

Be applicable to the control device of VSC-MTDC system

Technical field

The present invention relates to electrical engineering field, specifically one is applicable to the controller of VSC-MTDC (multi-terminal HVDC transmission based on voltage source converter) system, is especially applicable to the control device of VSC-MTDC system based on prediction-Direct Power (P-DPC).

Background technology

Along with the approach exhaustion of traditional energy, new forms of energy more and more receive the concern of people, and offshore wind farm then gains great popularity because of its plurality of advantages.At present, the long-distance transmissions of Large Scale Offshore Wind Farm adopts the system based on VSC-HVDC usually.On the basis of both-end high voltage direct current transmission, multi-terminal HVDC transmission MTDC technology is improved gradually and is applied.Multi-terminal direct current transmission system at least comprises the rotary substation of more than 3 or 3.Due to developing rapidly of electric power transfer, traditional two ends HVDC can not meet the demands gradually, and multi-terminal HVDC transmission is more and more subject to people and payes attention to.Compared with both-end high-voltage direct current, multi-terminal direct current transmission system is except needing to consider the control of each rotary substation self, also to consider the cooperation control between each rotary substation, adopt the stable operation of effective Controller gain variations to VSC-MTDC transmission system most important.Generally speaking, tradition has following shortcoming based on PI controller:

1, the local control design case of current transformer is complicated, comprises voltage, current controller;

2, have multiple PI controller, PI parameter regulates complicated;

3, cooperation control cannot carry out operational mode handoff functionality, and system reliability is low.

Summary of the invention

For defect of the prior art, the object of the invention is to overcome above proposed shortcoming and defect, propose a kind of control device being applicable to VSC-MTDC system.The present invention is directed to VSC-MTDC system, this locality of current transformer is controlled, devise rectification side local controller and inverter side local controller respectively; Devise the tuning controller between rotary substation simultaneously.

Rectification side local controller, for the control of rotary substation playing rectified action in multi-terminal direct current transmission system.

Inverter side local controller, for the control of rotary substation playing reversion reaction in multi-terminal direct current transmission system.

Tuning controller, for the controller coordinated electric power physical quantitys such as the power in multi-terminal direct current transmission system between each rotary substation, electric currents.

According to a kind of control device being applicable to VSC-MTDC system provided by the invention, comprise model apparatus for establishing;

Described model apparatus for establishing, for setting up three-phase VSC model, specific as follows:

Three-phase VSC comprises IGBT switch S 1, S2, S3, S4, S5, S6, also comprises DC power supply;

Be parallel with A phase brachium pontis, B phase brachium pontis, C phase brachium pontis between DC power supply, wherein, IGBT switch S 1, S4 form A phase brachium pontis, and IGBT switch S 2, S5 form B phase brachium pontis, and IGBT switch S 3, S6 form C phase brachium pontis;

The IGBT switch function of IGBT switch S 1, S2, S3, S4, S5, S6 is respectively S ₁, S ₂, S ₃, S ₄, S ₅, S ₆;

Brachium pontis on off state is defined as follows:

S _arepresent A phase brachium pontis switch function, S _brepresent B phase brachium pontis switch function, S _crepresent C phase brachium pontis switch function; Then the resultant vector S of A phase, B phase, C phase brachium pontis switch function is:

S=S _a+ α S _b+ α ²s _c(4) wherein, α=e ^{j2 π/3}, e is natural constant;

Calculate three-phase VSC output voltage vector U _i, calculating formula is:

U _i＝SU _dci＝0,1,2,…,7 (5)

U _irepresent i-th three-phase VSC output voltage vector, U _dcrepresent the direct voltage of DC power supply;

Obtain through three phase static abc coordinate system and two-phase static α β coordinate system transformation:

$\left\{\begin{array}{c}{U}_{\mathrm{\α}}=\sqrt{\frac{2}{3}}{U}_{\mathrm{dc}}[S_a-\frac{(S_b+S_c)}{2}]\\ U_{\mathrm{\β}}=\frac{\sqrt{2}}{2}{U}_{\mathrm{dc}}(S_b-S_c)\end{array}\right.---\left(6\right)$

U _αrepresent the three-phase VSC output voltage along α axle in two-phase static α β coordinate system, U _βrepresent the three-phase VSC output voltage along β axle in two-phase static α β coordinate system;

Following transient current equation is obtained by Kirchhoff's current law (KCL):

$\left\{\begin{array}{c}\frac{{\mathrm{di}}_a}{\mathrm{dt}}=\frac{(U_{\mathrm{aN}}-e_a)}{L}-\frac{R}{L}{i}_a\\ \frac{{\mathrm{di}}_b}{\mathrm{dt}}=\frac{(U_{\mathrm{bN}}-e_b)}{L}-\frac{R}{L}{i}_b\\ \frac{{\mathrm{di}}_c}{\mathrm{dt}}=\frac{(U_{\mathrm{cN}}-e_c)}{L}-\frac{R}{L}{i}_c\end{array}\right.---\left(7\right)$

I _a, i _b, i _crepresent A phase, B phase, C phase current respectively, t represents the time, U _aN, U _bN, U _cNrepresent the voltage of A phase, B phase, the relative neutral point N of C respectively, e _a, e _b, e _crepresent A phase, B phase, the C phase voltage of three-phase alternating current system respectively, R represents resistance, and L represents reactance;

Suppose three-phase grid balance, above-mentioned transient current equation obtains following equation through three phase static abc coordinate system and two-phase static α β coordinate system transformation:

$\left\{\begin{array}{c}L\frac{{\mathrm{di}}_{\mathrm{\α}}}{\mathrm{dt}}=U_{\mathrm{\α}}-e_{\mathrm{\α}}-{\mathrm{Ri}}_{\mathrm{\α}}\\ L\frac{{\mathrm{di}}_{\mathrm{\β}}}{\mathrm{dt}}=U_{\mathrm{\β}}-e_{\mathrm{\β}}-{\mathrm{Ri}}_{\mathrm{\β}}\end{array}\right.---\left(8\right)$

E _αrepresent the three-phase alternating current system voltage component along α axle in two-phase static α β coordinate system, e _βrepresent the three-phase alternating current system voltage component along β axle in two-phase static α β coordinate system, i _αrepresent the electric current along α axle in two-phase static α β coordinate system, i _βrepresent the electric current along β axle in two-phase static α β coordinate system.

Preferably, inverter side controller is also comprised;

Described inverter side controller, for realizing following function:

By formula (8) discretization, obtain following equation:

$\left\{\begin{array}{c}{i}_{\mathrm{\α}}(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i_{\mathrm{\α}}\left(k\right)\frac{T_s}{L}(U_{\mathrm{\α}}\left(k\right)-e_{\mathrm{\α}}\left(k\right))\\ i_{\mathrm{\β}}(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i_{\mathrm{\β}}\left(k\right)+\frac{T_s}{L}(U_{\mathrm{\β}}\left(k\right)-e_{\mathrm{\β}}\left(k\right))\end{array}\right.---\left(9\right)$

K represents current time, and k+1 represents subsequent time, i _α(k+1) electric current of subsequent time along α axle is represented, i _β(k+1) electric current of subsequent time along β axle is represented, T _srepresent the sampling time, i _αk () represents the electric current of current time along α axle, i _βk () represents the electric current of current time along β axle, U _αk () represents the output voltage of current time along the three-phase VSC of α axle, U _βk () represents the output voltage of current time along the three-phase VSC of β axle, e _αk () represents the three-phase alternating current system voltage component of current time along α axle, e _βk () represents the three-phase alternating current system voltage component of current time along β axle;

The equation under dq two-phase rotating coordinate system can be obtained through rotation transformation to formula (9):

$\left\{\begin{array}{c}{i}_d(k+1)=i_{\mathrm{\α}}(k+1)\cos \mathrm{\θ}+i_{\mathrm{\β}}(k+1)\sin \mathrm{\θ}\\ i_q(k+1)=-i_{\mathrm{\α}}(k+1)\sin \mathrm{\θ}+i_{\mathrm{\β}}(k+1)\cos \mathrm{\θ}\end{array}\right.---\left(10\right)$

I _d(k+1) the three-phase grid-connected inverter output current d axle component that the k moment dopes is represented, i _q(k+1) represent the three-phase grid-connected inverter output current q axle component that the k moment dopes, θ represents the space angle of electrical network;

Active power and the reactive power in (k+1) moment is drawn by following prediction equation:

$\left\{\begin{array}{c}P(k+1)=e_d{i}_d(k+1)+e_q{i}_q(k+1)\\ Q(k+1)=e_q{i}_d(k+1)-e_d{i}_q(k+1)\end{array}\right.---\left(11\right)$

P (k+1) represents the active power in (k+1) moment, and Q (k+1) represents the reactive power in (k+1) moment, e _drepresent the three-phase alternating current system voltage component of d axle, e _qrepresent the three-phase alternating current system voltage component of q axle;

Calculate the first cost function g, calculating formula is:

g＝|(Q ^*-Q(k+1)|+|(P ^*-P(k+1)| (12)

P ^*for given active power reference value, Q ^*for given reactive power reference qref;

Calculate respectively at 8 three-phase VSC output voltage vector U _i8 the first cost function g under state, obtain the minimum value g of the first cost function g _min; Obtain the minimum value g corresponding to the first cost function g _minclosest to expecting power, thus obtain corresponding U _α(k), U _βk the size of (), by U _α(k), U _βk () substitutes into the U in formula (6) respectively _α, U _β, obtain the switch function that this k moment expects, and the switch function of this expectation passed to the brachium pontis on off state of subsequent time, and then control inverter side current transformer; Wherein, described closest to expecting that power is the minimum performance number Q (k+1) of the value of instruction first cost function g and P (k+1), i=0,1,2 ... 7.

Preferably, rectification side controller is also comprised;

Described rectification side controller, for realizing following function:

Set up rectification side three-phase VSC model:

$\left\{\begin{array}{c}L\frac{{\mathrm{di}}_{\mathrm{\α}}}{\mathrm{dt}}=e_{\mathrm{\α}}-U_{\mathrm{\α}}-{\mathrm{Ri}}_{\mathrm{\α}}\\ L\frac{{\mathrm{di}}_{\mathrm{\β}}}{\mathrm{dt}}=e_{\mathrm{\β}}-U_{\mathrm{\β}}-{\mathrm{Ri}}_{\mathrm{\β}}\end{array}\right.---\left(13\right)$

Discretization formula (13) obtains predicted current function:

$\left\{\begin{array}{c}{i}_{\mathrm{\α}}(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i_{\mathrm{\α}}\left(k\right)+\frac{T_s}{L}(e_{\mathrm{\α}}\left(k\right)-U_{\mathrm{\α}}\left(k\right))\\ i_{\mathrm{\β}}(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i_{\mathrm{\β}}\left(k\right)+\frac{T_s}{L}(e_{\mathrm{\β}}\left(k\right)-U_{\mathrm{\β}}\left(k\right))\end{array}\right.---\left(14\right)$

K represents current time, and k+1 represents subsequent time, i _α(k+1) electric current of subsequent time along α axle is represented, i _β(k+1) electric current of subsequent time along β axle is represented, T _srepresent the sampling time, i _αk () represents the electric current of current time along α axle, i _βk () represents the electric current of current time along β axle, U _αk () represents the output voltage of current time along the three-phase VSC of α axle, U _βk () represents the output voltage of current time along the three-phase VSC of β axle, e _αk () represents the three-phase alternating current system voltage component of current time along α axle, e _βk () represents the three-phase alternating current system voltage component of current time along β axle;

The equation under dq two-phase rotating coordinate system can be obtained through rotation transformation to formula (14):

$\left\{\begin{array}{c}{i}_d(k+1)=i_{\mathrm{\α}}(k+1)\cos \mathrm{\θ}+i_{\mathrm{\β}}(k+1)\sin \mathrm{\θ}\\ i_q(k+1)=-i_{\mathrm{\α}}(k+1)\sin \mathrm{\θ}+i_{\mathrm{\β}}(k+1)\cos \mathrm{\θ}\end{array}\right.---\left(10\right)$

Active power and the reactive power in (k+1) moment is drawn by following prediction equation:

$\left\{\begin{array}{c}P(k+1)=e_d{i}_d(k+1)+e_q{i}_q(k+1)\\ Q(k+1)=e_q{i}_d(k+1)-e_d{i}_q(k+1)\end{array}\right.---\left(11\right)$

P (k+1) represents the active power in (k+1) moment, and Q (k+1) represents the reactive power in (k+1) moment, e _drepresent the three-phase alternating current system voltage component of d axle, e _qrepresent the three-phase alternating current system voltage component of q axle; i _d(k+1) the three-phase grid-connected inverter output current d axle component that the k moment dopes is represented, i _q(k+1) the three-phase grid-connected inverter output current q axle component that the k moment dopes is represented; θ represents the space angle of electrical network;

Calculate the second cost function g, calculating formula is:

g＝|(Q ^*-Q(k+1)|+|(P _dc+P _i-P(k+1))| (15)

Q ^*for given reactive power reference qref, P _dcrepresent the direct current power after regulating, P _irepresent the active power that i-th other rotary substation consumes except rectification side current transformer in multi-terminal direct current transmission system, i>0;

Calculate respectively at the active-power P that i other rotary substation consumes _ii under state the second cost function g, obtains the minimum value g of the second cost function g _min; Obtain the minimum value g corresponding to the second cost function g _minclosest to expecting power, thus obtain corresponding U _α(k), U _βk the size of (), by U _α(k), U _βk () substitutes into the U in formula (6) respectively _α, U _β, obtain the switch function that this k moment expects, and the switch function of this expectation passed to the brachium pontis on off state of subsequent time, and then control rectification side current transformer; Wherein, described closest to expecting that power is the minimum performance number Q (k+1) of the value of instruction second cost function g and P (k+1), i>0.

Preferably, also tuning controller is comprised;

Described tuning controller, for controlling main convertor and there is the current transformer of power adjustments, specific as follows:

Work as P ₁(k+1) allow in adjustable range at power, i.e. P _1min<P ₁(k+1) <P _1max, then under making VSC2, VSC3 be operated in power mode; When VSC1 power adjustments is not enough or out of service, P ₁(k+1) adjustable range [P is not allowed at power _1min, P _1max] in, then make the operational mode of VSC2 switch to direct voltage pattern to bear power shortage, under VSC3 is operated in power mode by power mode;

Wherein: VSC1 represents main convertor, VSC2 is the current transformer with power adjustments, and VSC3 is the current transformer without regulating power; P _1minrepresent that rotary substation is exerted oneself lower limit, i.e. minimum load, P ₁(k+1) value of current predictive power is represented, P _1maxrepresent that rotary substation is exerted oneself the upper limit, i.e. maximum output.

Compared with prior art, the present invention has following beneficial effect:

(1) local controller method for designing is simple, and principle is also uncomplicated, without the need to voltage and current controller, does not also need PWM.

(2) there is no PI controller in local controller, without the need to PI calculation of parameter and the adjustment of complexity, greatly improve control efficiency.

(3) tuning controller designed by has the function switching current transformer mode of operation, greatly improves the stability of a system.

(4) take full advantage of discrete mathematical model, calculate simple, easy Digital Realization.

Accompanying drawing explanation

By reading the detailed description done non-limiting example with reference to the following drawings, other features, objects and advantages of the present invention will become more obvious:

Fig. 1 is three-phase VSC combining inverter.

Fig. 2 is the design of inverter side local controller.

Fig. 3 is the design of rectification side local controller.

Fig. 4 is the tuning controller based on prediction-direct Power Control.

Fig. 5 is VSC-MTDC total system simulation model.

Fig. 6 is normal operating conditions DC voltage waveform.

Fig. 7 is each current transformer active power waveform under normal operating conditions.

Fig. 8 is the DC voltage waveform under active power sudden change exceeds VSC1 adjustable range.

Fig. 9 is each current transformer active power waveform under active power sudden change exceeds VSC1 adjustable range.

Figure 10 is the DC voltage waveform of VSC3 rotary substation fault when exiting in short-term.

Figure 11 is each current transformer active power waveform of VSC3 rotary substation fault when exiting in short-term.

Embodiment

Below in conjunction with specific embodiment, the present invention is described in detail.Following examples will contribute to those skilled in the art and understand the present invention further, but not limit the present invention in any form.It should be pointed out that to those skilled in the art, without departing from the inventive concept of the premise, some distortion and improvement can also be made.These all belong to protection scope of the present invention.

The control device being applicable to VSC-MTDC system provided by the invention, comprising: model apparatus for establishing, rectification side local controller, inverter side local controller, tuning controller;

Described model apparatus for establishing, for setting up three-phase VSC Mathematical Modeling, as shown in Figure 1, comprises 6 IGBT switches.Wherein, have three brachium pontis, upper and lower each 1 the IGBT switch of each brachium pontis.

Three-phase VSC comprises IGBT switch S 1, S2, S3, S4, S5, S6, also comprises DC power supply;

Be parallel with A phase brachium pontis, B phase brachium pontis, C phase brachium pontis between DC power supply, wherein, IGBT switch S 1, S4 form A phase brachium pontis, and IGBT switch S 2, S5 form B phase brachium pontis, and IGBT switch S 3, S6 form C phase brachium pontis;

The IGBT switch function of IGBT switch S 1, S2, S3, S4, S5, S6 is respectively S ₁, S ₂, S ₃, S ₄, S ₅, S ₆;

In Fig. 1, U _dcrepresent direct voltage, a represents A phase, and b represents B phase, and c represents C phase, i _arepresent A phase current, i _brepresent B phase current, i _crepresent C phase current, e _a, e _b, e _crepresent each phase voltage of three-phase alternating current system, N represents neutral point;

Brachium pontis on off state is defined as follows:

S _arepresent A phase brachium pontis switch function, S _brepresent B phase brachium pontis switch function, S _crepresent C phase brachium pontis switch function;

Then the resultant vector S of above-mentioned switch function is:

S=S _a+ α S _b+ α ²s _c(4) wherein, α=e ^{j2 π/3}, e is constant, and be a nonterminating and non-recurring decimal, its value approximates 2.718281828459.

Because switch combination number has 8 kinds, the output voltage vector of three-phase VSC can calculate:

U _i＝SU _dci＝0,1,2,…,7 (5)

U _irepresent i-th three-phase VSC output voltage vector, U _dcrepresent direct voltage;

Calculate the output voltage of three-phase VSC, obtain through abc/ α β coordinate transform:

$\left\{\begin{array}{c}{U}_{\mathrm{\&alpha;}}=\sqrt{\frac{2}{3}}{U}_{\mathrm{dc}}[S_a-\frac{(S_b+S_c)}{2}]\\ U_{\mathrm{\&beta;}}=\frac{\sqrt{2}}{2}{U}_{\mathrm{dc}}(S_b-S_c)\end{array}\right.---\left(6\right)$

U _αrepresent the output voltage along the VSC of α axle, U _βrepresent the output voltage along the VSC of β axle;

Following transient current equation is obtained by Kirchhoff's current law (KCL):

$\left\{\begin{array}{c}\frac{{\mathrm{di}}_a}{\mathrm{dt}}=\frac{(U_{\mathrm{aN}}-e_a)}{L}-\frac{R}{L}{i}_a\\ \frac{{\mathrm{di}}_b}{\mathrm{dt}}=\frac{(U_{\mathrm{bN}}-e_b)}{L}-\frac{R}{L}{i}_b\\ \frac{{\mathrm{di}}_c}{\mathrm{dt}}=\frac{(U_{\mathrm{cN}}-e_c)}{L}-\frac{R}{L}{i}_c\end{array}\right.---\left(7\right)$

I _a, i _b, i _crepresent three-phase current, t represents the time, U _aN, U _bN, U _cNrepresent the voltage of three-phase centering point N, e _a, e _b, e _crepresent each phase voltage of three-phase alternating current system, R represents resistance, and L represents reactance;

Suppose three-phase grid balance, above-mentioned transient current equation obtains following equation through coordinate transform:

$\left\{\begin{array}{c}L\frac{{\mathrm{di}}_{\mathrm{\&alpha;}}}{\mathrm{dt}}=U_{\mathrm{\&alpha;}}-e_{\mathrm{\&alpha;}}-{\mathrm{Ri}}_{\mathrm{\&alpha;}}\\ L\frac{{\mathrm{di}}_{\mathrm{\&beta;}}}{\mathrm{dt}}=U_{\mathrm{\&beta;}}-e_{\mathrm{\&beta;}}-{\mathrm{Ri}}_{\mathrm{\&beta;}}\end{array}\right.---\left(8\right)$

E _αrepresent the AC system component of voltage along α axle, e _βrepresent the AC system component of voltage along β axle, i _αrepresent the electric current along α axle, i _βrepresent the electric current along β axle;

Described inverter side controller, for realizing following function:

By discretization three-phase VSC Mathematical Modeling, introduce prediction-Direct Power (P-DPC) control strategy.By formula (8) discretization, obtain following equation:

$\left\{\begin{array}{c}{i}_{\mathrm{\&alpha;}}(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i_{\mathrm{\&alpha;}}\left(k\right)\frac{T_s}{L}(U_{\mathrm{\&alpha;}}\left(k\right)-e_{\mathrm{\&alpha;}}\left(k\right))\\ i_{\mathrm{\&beta;}}(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i_{\mathrm{\&beta;}}\left(k\right)+\frac{T_s}{L}(U_{\mathrm{\&beta;}}\left(k\right)-e_{\mathrm{\&beta;}}\left(k\right))\end{array}\right.---\left(9\right)$

K represents current time, i _α(k+1) electric current of subsequent time along α axle is represented, i _β(k+1) electric current of subsequent time β axle is represented, T _srepresent the sampling time, i _αk () represents the electric current of current time along α axle, i _βk () represents the electric current of current time β axle, U _αk () represents the output voltage of the VSC of current time α axle, U _βk () represents the output voltage of the VSC of current time β axle, e _αk () represents the AC system component of voltage of current time α axle, e _βk () represents the AC system component of voltage of current time β axle;

From formula (9), after discretization inverter Mathematical Modeling, contacting between the electric current of subsequent time and a upper moment electric current can be obtained, namely can go out the electric current in (k+1) moment from the current forecasting in k moment.

Equation under rotation transformation can obtain d, q rotational coordinates:

$\left\{\begin{array}{c}{i}_d(k+1)=i_{\mathrm{\&alpha;}}(k+1)\cos \mathrm{\&theta;}+i_{\mathrm{\&beta;}}(k+1)\sin \mathrm{\&theta;}\\ i_q(k+1)=-i_{\mathrm{\&alpha;}}(k+1)\sin \mathrm{\&theta;}+i_{\mathrm{\&beta;}}(k+1)\cos \mathrm{\&theta;}\end{array}\right.---\left(10\right)$

I _d(k+1) the three-phase grid-connected inverter output current d axle component that the k moment dopes is represented, i _q(k+1) represent the three-phase grid-connected inverter output current q axle component that the k moment dopes, θ represents the space angle of electrical network;

Suppose the stabilization of power grids, then three-phase voltage can be regarded as substantially constant.Active power and the reactive power in (k+1) moment so just can be drawn by following prediction equation.

$\left\{\begin{array}{c}P(k+1)=e_d{i}_d(k+1)+e_q{i}_q(k+1)\\ Q(k+1)=e_q{i}_d(k+1)-e_d{i}_q(k+1)\end{array}\right.---\left(11\right)$

P (k+1) represents the active power in (k+1) moment, and Q (k+1) represents the reactive power in (k+1) moment, e _drepresent the AC system component of voltage of d axle, e _qrepresent the AC system component of voltage of q axle.

Next is selected cost function.Valency value function minimum principle and power-balance, adopt the first cost function g:

g＝|(Q ^*-Q(k+1)|+|(P ^*-P(k+1)| (12)

Wherein P ^*with Q ^*for given active power and reactive power reference qref.Utilize the first cost function formula (12), pass through global optimizing, instead can release the size closest to expecting power, closest to expecting that power is the minimum performance number Q (k+1) of instruction type (12) value and P (k+1), thus the voltage U under this state can be obtained _α(k), U _β(k) size, more most suitable on off state can be obtained by formula (6).

Be specially, calculate respectively at 8 three-phase VSC output voltage vector U _i8 the first cost function g under state, obtain the minimum value g of the first cost function g _min; Obtain the minimum value g corresponding to the first cost function g _minclosest to expecting power, thus obtain corresponding U _α(k), U _βthe size of (k), then through type (6) provides the brachium pontis on off state of subsequent time, and then control inverter side current transformer; Wherein, described closest to expecting that power is the minimum performance number Q (k+1) of the value of instruction first cost function g and P (k+1), i=0,1,2 ... 7.

Whole process is and provides switching vector selector by prediction-direct Power Control.Whole controller architecture design as shown in Figure 2.

Described rectification side controller, for realizing following function:

Rectification side current transformer assume responsibility for stable DC voltage and provides the function of system power, and therefore, rectification side current transformer must adopt constant DC voltage control pattern, controls direct voltage and reactive power.Due to the structural similarity with inverter side current transformer, be not difficult to obtain rectification side VSC Mathematical Modeling:

$\left\{\begin{array}{c}L\frac{{\mathrm{di}}_{\mathrm{\&alpha;}}}{\mathrm{dt}}=e_{\mathrm{\&alpha;}}-U_{\mathrm{\&alpha;}}-{\mathrm{Ri}}_{\mathrm{\&alpha;}}\\ L\frac{{\mathrm{di}}_{\mathrm{\&beta;}}}{\mathrm{dt}}=e_{\mathrm{\&beta;}}-U_{\mathrm{\&beta;}}-{\mathrm{Ri}}_{\mathrm{\&beta;}}\end{array}\right.---\left(13\right)$

Owing to thinking DC voltage stability, U _α, U _βstill can be calculated by formula (6).Discretization formula (13) can obtain predicted current function:

$\left\{\begin{array}{c}{i}_{\mathrm{\&alpha;}}(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i_{\mathrm{\&alpha;}}\left(k\right)+\frac{T_s}{L}(e_{\mathrm{\&alpha;}}\left(k\right)-U_{\mathrm{\&alpha;}}\left(k\right))\\ i_{\mathrm{\&beta;}}(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i_{\mathrm{\&beta;}}\left(k\right)+\frac{T_s}{L}(e_{\mathrm{\&beta;}}\left(k\right)-U_{\mathrm{\&beta;}}\left(k\right))\end{array}\right.---\left(14\right)$

Again through d, q rotation transformation and mathematical computations, the active power and the reactive power that input VSC can be obtained, avatar still can be represented by formula (11), is specially: can to obtain the equation under dq two-phase rotating coordinate system through rotation transformation to formula (14):

$\left\{\begin{array}{c}{i}_d(k+1)=i_{\mathrm{\&alpha;}}(k+1)\cos \mathrm{\&theta;}+i_{\mathrm{\&beta;}}(k+1)\sin \mathrm{\&theta;}\\ i_q(k+1)=-i_{\mathrm{\&alpha;}}(k+1)\sin \mathrm{\&theta;}+i_{\mathrm{\&beta;}}(k+1)\cos \mathrm{\&theta;}\end{array}\right.---\left(10\right)$

Active power and the reactive power in (k+1) moment is drawn by following prediction equation:

$\left\{\begin{array}{c}P(k+1)=e_d{i}_d(k+1)+e_q{i}_q(k+1)\\ Q(k+1)=e_q{i}_d(k+1)-e_d{i}_q(k+1)\end{array}\right.---\left(11\right)$

Because rectification side current transformer must bear the function of regulating power, so direct current power governing loop need be added at controller.Choosing for cost function, whole system must could reliability service under power-balance, adopts following cost function:

g＝|(Q ^*-Q(k+1)|+|(P _dc+P _i-P(k+1))| (15)

In formula (15), P _dcfor the direct current power after adjustment, P _ifor the active power that other rotary substations except rectification side current transformer consume, i represents number, and the span of i is i>0.Each P _isize-dependent in communication system faster.Whole process is still the vector control based on prediction-Direct Power, and control design case block diagram as shown in Figure 3.

Particularly, calculate respectively at the active-power P that i other rotary substation consumes _ii under state the second cost function g, obtains the minimum value g of the second cost function g _min; Obtain the minimum value g corresponding to the second cost function g _minclosest to expecting power, thus obtain corresponding U _α(k), U _βthe size of (k), then through type (6) provides the brachium pontis on off state of subsequent time, and then control rectification side current transformer; Wherein, described closest to expecting that power is the minimum performance number Q (k+1) of the value of instruction second cost function g and P (k+1), i>0.

In Fig. 3,4, U ^* _dcrepresent direct voltage reference value, Pref represents active power reference value, P _minrepresent rotary substation power output lower limit, i.e. minimum load, P _maxrepresent the rotary substation power output upper limit, i.e. maximum output.

Described tuning controller, for controlling main convertor and there is the current transformer of power adjustments, specific as follows:

Work as P ₁(k+1) allow in adjustable range at power, i.e. P _1min<P ₁(k+1) <P _1max, then under making VSC2, VSC3 be operated in power mode; When VSC1 power adjustments is not enough or out of service, P ₁(k+1) adjustable range [P is not allowed at power _1min, P _1max] in, then make the operational mode of VSC2 switch to direct voltage pattern to bear power shortage, under VSC3 is operated in power mode by power mode;

Wherein: VSC1 represents main convertor, VSC2 is the current transformer with power adjustments, and VSC3 is the current transformer without regulating power; P _1minrepresent that rotary substation is exerted oneself lower limit, i.e. minimum load, P ₁(k+1) value of current predictive power is represented, P _1maxrepresent that rotary substation is exerted oneself the upper limit, i.e. maximum output.

Particularly, design at tuning controller, if only specify a current transformer to adopt constant DC voltage control in multi-terminal system, system operation reliability difference and active power can be caused accurately not to control, therefore need to adopt pattern switching controls to some current transformers, when main convertor cannot meet power adjustments time, the current transformer with power regulation functions can bear power shortage automatically.Be implemented as follows, the power that each current transformer is born is transferred to main convertor by communication system and has the current transformer of power adjustments, change main convertor by prediction-direct Power Control or there is the control signal of current transformer of power adjustments, the operational mode changing current transformer and then the power controlling current transformer are born, and ensure system power balance.As shown in Figure 3, be designed tuning controller, this controller be applied to main convertor and there is the current transformer of power adjustments.As shown in Figure 4, when being operated in normal condition, P _1min<P ₁(k+1) <P _1max, switch is in 1 state, under VSC2 is operated in power mode; When VSC1 power adjustments is not enough or out of service, P ₁(k+1) do not allow in adjustable range at power, switch is in 2 states, and VSC2 operational mode switches to direct voltage pattern.P _1minrepresent that rotary substation is exerted oneself lower limit, i.e. minimum load, P ₁(k+1) value of current predictive power is represented, P _1maxrepresent that rotary substation is exerted oneself the upper limit, i.e. maximum output; Wherein: VSC1 represents main convertor, VSC2 is the current transformer with power adjustments of specifying.

Tuning controller is operated in following three kinds of operating states:

(1) normal operating conditions, the predicted power of each current transformer is transferred to main convertor by communication system, utilize formula (6) and cost function formula (15), provide the control gate signal of main convertor under normal operating conditions, realize prediction-direct Power Control, ensure that each current transformer Direct Power balances; Now VSC1 is operated in and determines direct voltage pattern.VSC2, VSC3 are operated in power mode.Normal conditions, system works is in this state.VSC3 represents the current transformer without regulating power.

(2) VSC3 chugging, the power output of VSC1 current transformer exceeds its freely regulated allowed band [P _1min, P _1max], exceed power section and bear by current transformer VSC2 with power adjustments ability all in direct current system.Now, VSC1 keeps maximum power output or minimum power, is operated in constant DC voltage control pattern.Based on P-DPC cooperation control, VSC2 is switched to direct voltage pattern by power mode, automatically bears power shortage, makes whole system again keep power-balance.VSC3 is still operated in power mode.

(3) VSC1 main convertor is out of service, and VSC2 bears power shortage completely, and be switched to direct voltage pattern by VSC2 by power mode, VSC3 is still operated in power mode.Wherein: VSC1 represents main convertor, VSC2 is the current transformer with power adjustments of specifying, and VSC3 is the current transformer without power regulation functions.

Further, as shown in Figure 2, net side three-phase voltage and electric current is gathered in the k moment, through type (6), (9), (10), (11) calculate the meritorious of (k+1) moment and reactive power, compare with reactive power reference qref with given active power, by cost function formula (12) and formula (6), global optimizing, select the switching signal vector of the most applicable subsequent time, and then control inverter side current transformer.Wherein, with active power predicted value and the absolute value of difference of active power reference value and the absolute value sum of the difference of reactive power predicted value and reactive power reference qref for being worth reference index, its concrete form is as follows:

g＝|(Q ^*-Q(k+1)|+|(P ^*-P(k+1)|

As shown in Figure 3, rectification side local controller designs, net side three-phase voltage and electric current is gathered in the k moment, through type (6), (14), (10), (11) calculate the meritorious of (K+1) moment and reactive power, with by communication system transmission come each current transformer power sum and modulation direct current power compared with, by cost function formula (15) and formula (6), global optimizing, select the switching signal vector of the most applicable subsequent time, and then control rectification side current transformer.Wherein, with active power predicted value and the absolute value of difference of all current transformer active power and the direct current power summation after modulating and the absolute value sum of the difference of reactive power predicted value and system reactive power reference value for being worth reference index, concrete form is as follows:

g＝|(Q ^*-Q(k+1)|+|(P _dc+P _i-P(k+1))|

Wherein, P _dcfor the direct current power after adjustment, P _ifor the active power that other rotary substations except rectification side current transformer consume.Each P _isize-dependent in communication system faster.

As shown in Figure 4, tuning controller designs, and is designed tuning controller, this controller is applied to main convertor and has the current transformer of power adjustments.When being operated in normal condition, P _1min<P ₁(k+1) <P _1max, switch is in 1 state, under VSC2 is operated in power mode; When VSC1 power adjustments is not enough or out of service, P ₁(k+1) do not allow in adjustable range at power, switch is in 2 states, and VSC2 operational mode switches to direct voltage pattern.Wherein: VSC1 represents main convertor, VSC2 is the current transformer with power adjustments of specifying.

Further, the application of controller provided by the invention in multi-terminal direct current transmission system.Fig. 5 is three end DC transmission system simulation models.Wherein VSC1 is main convertor, adopts constant DC voltage control; VSC2 introduces P-DPC cooperation control, and operating state can switch between power mode and direct voltage pattern, namely has power adjustments ability; VSC3 is operated in power mode, not changeable.Concrete simulation parameter is as following table:

Table 1 simulation parameter

The present embodiment will verify the validity of designed controller from following three kinds of operating states.

(1) normal operating conditions, VSC3 rotary substation side active power is suddenlyd change, and VSC1 power output is within adjustable range;

(2) VSC3 rotary substation side active power sudden change, VSC1 power output exceeds adjustable range;

(3) VSC1 rotary substation because of fault out of service in short-term.

In Fig. 5: Pulse1 ~ 3 represent control impuls; V-I 1 ~ 4 represents voltage and current measurement module; RL1 ~ 3 represent line impedance.

System responses is as shown in Figure of description.

Wherein, Fig. 6, Fig. 7 are the system responses under (1) state, can find out when being in normal operating conditions, and be that power or direct voltage are all very stable, wherein, direct voltage is the same with preset value, remains on 750V.When 0.1s ~ 0.2s, current transformer VSC3 power produces fluctuation, and when changing to-6KW by 6KW, the automatic regulating power balance of main variable flow station VSC1, system run all right, illustrates that the control strategy based on P-DPC is effective.In the case, VSC1 is operated in and determines direct voltage pattern, VSC2 and VSC3 is operated in power mode.

Fig. 8, system responses under Fig. 9 correspondence (2) state, can find out, when 0.1s ~ 0.2s, the active power sudden change of VSC3, changes to-1.3MW by original 6kW, cause direct current system unbalanced power, the power injecting DC network is less than its power output, and direct voltage declines, and system works point changes.For checking the validity of controller, VSC2 initial power is set to 2MW here, and now power stage has exceeded the adjustable range [-3MW, 3MW] of main convertor VSC1.VSC1 keeps maximum power 3MW to export, and VSC2 mode of operation automatically switches to direct voltage pattern by power mode, regulating system power-balance.

System responses under Figure 10, Figure 11 correspondence (3) state, 0.1s ~ 0.2s, VSC1 rotary substation out of service in short-term, cause direct current system unbalanced power, direct voltage fluctuates.VSC2 switches to direct voltage pattern by power mode, regulating system power-balance, stable DC voltage.After 0.2s, circuit breaker reclosing, system is recovered normal and is run in 0.05.

The present invention devises current transformer local controller based on P-DPC and tuning controller, effectively prevent PI parameter tuning and the calculating of traditional double closed-loop control, without the need to linear controller and PWM, greatly reduces the control complexity of current transformer rectification and inversion.Meanwhile, the control strategy based on P-DPC takes full advantage of discrete mathematical model, calculates simple, easy Digital Realization.

Above specific embodiments of the invention are described.It is to be appreciated that the present invention is not limited to above-mentioned particular implementation, those skilled in the art can make various distortion or amendment within the scope of the claims, and this does not affect flesh and blood of the present invention.

Claims (4)

1. be applicable to a control device for VSC-MTDC system, it is characterized in that, comprise model apparatus for establishing;

Described model apparatus for establishing, for setting up three-phase VSC model, specific as follows:

Three-phase VSC comprises IGBT switch S 1, S2, S3, S4, S5, S6, also comprises DC power supply;

Be parallel with A phase brachium pontis, B phase brachium pontis, C phase brachium pontis between DC power supply, wherein, IGBT switch S 1, S4 form A phase brachium pontis, and IGBT switch S 2, S5 form B phase brachium pontis, and IGBT switch S 3, S6 form C phase brachium pontis;

The IGBT switch function of IGBT switch S 1, S2, S3, S4, S5, S6 is respectively S ₁, S ₂, S ₃, S ₄, S ₅, S ₆;

Brachium pontis on off state is defined as follows:

S _arepresent A phase brachium pontis switch function, S _brepresent B phase brachium pontis switch function, S _crepresent C phase brachium pontis switch function; Then the resultant vector S of A phase, B phase, C phase brachium pontis switch function is:

S＝S _a+αS _b+α ²S _c(4)

Wherein, α=e ^{j2 π/3}, e is natural constant;

Calculate three-phase VSC output voltage vector U _i, calculating formula is:

U _i＝SU _dci＝0,1,2,…,7 (5)

U _irepresent i-th three-phase VSC output voltage vector, U _dcrepresent the direct voltage of DC power supply;

Obtain through three phase static abc coordinate system and two-phase static α β coordinate system transformation:

$\left\{\begin{array}{c}{U}_{\mathrm{\&alpha;}}=\sqrt{\frac{2}{3}}{U}_{\mathrm{dc}}[S_a-\frac{(S_b+S_c)}{2}]\\ U_{\mathrm{\&beta;}}=\frac{\sqrt{2}}{2}{U}_{\mathrm{dc}}(S_b-S_c)\end{array}\right.---\left(6\right)$

U _αrepresent the three-phase VSC output voltage along α axle in two-phase static α β coordinate system, U _βrepresent the three-phase VSC output voltage along β axle in two-phase static α β coordinate system;

Following transient current equation is obtained by Kirchhoff's current law (KCL):

$\left\{\begin{array}{c}\frac{{\mathrm{di}}_a}{\mathrm{dt}}=\frac{(U_{\mathrm{aN}}-e_a)}{L}-\frac{R}{L}{i}_a\\ \frac{{\mathrm{di}}_b}{\mathrm{dt}}=\frac{(U_{\mathrm{bN}}-e_b)}{L}-\frac{R}{L}{i}_b\\ \frac{{\mathrm{di}}_c}{\mathrm{dt}}=\frac{(U_{\mathrm{cN}}-e_c)}{L}-\frac{R}{L}{i}_c\end{array}\right.---\left(7\right)$

I _a, i _b, i _crepresent A phase, B phase, C phase current respectively, t represents the time, U _aN, U _bN, U _cNrepresent the voltage of A phase, B phase, the relative neutral point N of C respectively, e _a, e _b, e _crepresent A phase, B phase, the C phase voltage of three-phase alternating current system respectively, R represents resistance, and L represents reactance;

Suppose three-phase grid balance, above-mentioned transient current equation obtains following equation through three phase static abc coordinate system and two-phase static α β coordinate system transformation:

$\left\{\begin{array}{c}L\frac{{\mathrm{di}}_{\mathrm{\&alpha;}}}{\mathrm{dt}}=U_{\mathrm{\&alpha;}}-e_{\mathrm{\&alpha;}}-{\mathrm{Ri}}_{\mathrm{\&alpha;}}\\ L\frac{{\mathrm{di}}_{\mathrm{\&beta;}}}{\mathrm{dt}}=U_{\mathrm{\&beta;}}-e_{\mathrm{\&beta;}}-{\mathrm{Ri}}_{\mathrm{\&beta;}}\end{array}\right.---\left(8\right)$

E _αrepresent the three-phase alternating current system voltage component along α axle in two-phase static α β coordinate system, e _βrepresent the three-phase alternating current system voltage component along β axle in two-phase static α β coordinate system, i _αrepresent the electric current along α axle in two-phase static α β coordinate system, i _βrepresent the electric current along β axle in two-phase static α β coordinate system.

2. the control device being applicable to VSC-MTDC system according to claim 1, is characterized in that, also comprises inverter side controller;

Described inverter side controller, for realizing following function:

By formula (8) discretization, obtain following equation:

$\left\{\begin{array}{c}{i}_{\mathrm{\&alpha;}}(k+1)=(1-\frac{{\mathrm{RT}}_S}{L})i_{\mathrm{\&alpha;}}\left(k\right)+\frac{T_S}{L}(U_{\mathrm{\&alpha;}}\left(k\right)-e_{\mathrm{\&alpha;}}\left(k\right))\\ i_{\mathrm{\&beta;}}(k+1)=(1-\frac{{\mathrm{RT}}_S}{L})i_{\mathrm{\&beta;}}\left(k\right)+\frac{T_S}{L}(U_{\mathrm{\&beta;}}\left(k\right)-e_{\mathrm{\&beta;}}\left(k\right))\end{array}\right.---\left(9\right)$

K represents current time, and k+1 represents subsequent time, i _α(k+1) electric current of subsequent time along α axle is represented, i _β(k+1) electric current of subsequent time along β axle is represented, T _srepresent the sampling time, i _αk () represents the electric current of current time along α axle, i _βk () represents the electric current of current time along β axle, U _αk () represents the output voltage of current time along the three-phase VSC of α axle, U _βk () represents the output voltage of current time along the three-phase VSC of β axle, e _αk () represents the three-phase alternating current system voltage component of current time along α axle, e _βk () represents the three-phase alternating current system voltage component of current time along β axle;

The equation under dq two-phase rotating coordinate system can be obtained through rotation transformation to formula (9):

$\left\{\begin{array}{c}{i}_d(k+1)=i_{\mathrm{\&alpha;}}(k+1)\cos \mathrm{\&theta;}+i_{\mathrm{\&beta;}}(k+1)\sin \mathrm{\&theta;}\\ i_q(k+1)=-i_{\mathrm{\&alpha;}}(k+1)\sin \mathrm{\&theta;}+i_{\mathrm{\&beta;}}(k+1)\cos \mathrm{\&theta;}\end{array}\right.---\left(10\right)$

I _d(k+1) the three-phase grid-connected inverter output current d axle component that the k moment dopes is represented, i _q(k+1) represent the three-phase grid-connected inverter output current q axle component that the k moment dopes, θ represents the space angle of electrical network;

Active power and the reactive power in (k+1) moment is drawn by following prediction equation:

$\left\{\begin{array}{c}P(k+1)=e_d{i}_d(k+1)+e_q{i}_q(k+1)\\ Q(k+1)=e_q{i}_d(k+1)-e_d{i}_q(k+1)\end{array}\right.---\left(11\right)$

P (k+1) represents the active power in (k+1) moment, and Q (k+1) represents the reactive power in (k+1) moment, e _drepresent the three-phase alternating current system voltage component of d axle, e _qrepresent the three-phase alternating current system voltage component of q axle;

Calculate the first cost function g, calculating formula is:

g＝|(Q ^*-Q(k+1)|+|(P ^*-P(k+1)| (12)

P ^*for given active power reference value, Q ^*for given reactive power reference qref;

Calculate respectively at 8 three-phase VSC output voltage vector U _i8 the first cost function g under state, obtain the minimum value g of the first cost function g _min; Obtain the minimum value g corresponding to the first cost function g _minclosest to expecting power, thus obtain corresponding U _α(k), U _βk the size of (), by U _α(k), U _βk () substitutes into the U in formula (6) respectively _α, U _β, obtain the switch function that this k moment expects, and the switch function of this expectation passed to the brachium pontis on off state of subsequent time, and then control inverter side current transformer; Wherein, described closest to expecting that power is the minimum performance number Q (k+1) of the value of instruction first cost function g and P (k+1), i=0,1,2 ... 7.

3. the control device being applicable to VSC-MTDC system according to claim 1, is characterized in that, also comprises rectification side controller;

Described rectification side controller, for realizing following function:

Set up rectification side three-phase VSC model:

$\left\{\begin{array}{c}L\frac{{\mathrm{di}}_{\mathrm{\&alpha;}}}{\mathrm{dt}}=e_{\mathrm{\&alpha;}}-U_{\mathrm{\&alpha;}}-{\mathrm{Ri}}_{\mathrm{\&alpha;}}\\ L\frac{{\mathrm{di}}_{\mathrm{\&beta;}}}{\mathrm{dt}}=e_{\mathrm{\&beta;}}-U_{\mathrm{\&beta;}}-{\mathrm{Ri}}_{\mathrm{\&beta;}}\end{array}\right.---\left(13\right)$

Discretization formula (13) obtains predicted current function:

$\left\{\begin{array}{c}{i}_{\mathrm{\&alpha;}}(k+1)=(1-\frac{{\mathrm{RT}}_S}{L})i_{\mathrm{\&alpha;}}\left(k\right)+\frac{T_S}{L}(e_{\mathrm{\&alpha;}}\left(k\right)-U_{\mathrm{\&alpha;}}\left(k\right))\\ i_{\mathrm{\&beta;}}(k+1)=(1-\frac{{\mathrm{RT}}_S}{L})i_{\mathrm{\&beta;}}\left(k\right)+\frac{T_S}{L}(e_{\mathrm{\&beta;}}\left(k\right)-U_{\mathrm{\&beta;}}\left(k\right))\end{array}\right.---\left(14\right)$

K represents current time, and k+1 represents subsequent time, i _α(k+1) electric current of subsequent time along α axle is represented, i _β(k+1) electric current of subsequent time along β axle is represented, T _srepresent the sampling time, i _αk () represents the electric current of current time along α axle, i _βk () represents the electric current of current time along β axle, U _αk () represents the output voltage of current time along the three-phase VSC of α axle, U _βk () represents the output voltage of current time along the three-phase VSC of β axle, e _αk () represents the three-phase alternating current system voltage component of current time along α axle, e _βk () represents the three-phase alternating current system voltage component of current time along β axle;

The equation under dq two-phase rotating coordinate system can be obtained through rotation transformation to formula (14):

$\left\{\begin{array}{c}{i}_d(k+1)=i_{\mathrm{\&alpha;}}(k+1)\cos \mathrm{\&theta;}+i_{\mathrm{\&beta;}}(k+1)\sin \mathrm{\&theta;}\\ i_q(k+1)=-i_{\mathrm{\&alpha;}}(k+1)\sin \mathrm{\&theta;}+i_{\mathrm{\&beta;}}(k+1)\cos \mathrm{\&theta;}\end{array}\right.---\left(10\right)$

Active power and the reactive power in (k+1) moment is drawn by following prediction equation:

$\left\{\begin{array}{c}P(k+1)=e_d{i}_d(k+1)+e_q{i}_q(k+1)\\ Q(k+1)=e_q{i}_d(k+1)-e_d{i}_q(k+1)\end{array}\right.---\left(11\right)$

P (k+1) represents the active power in (k+1) moment, and Q (k+1) represents the reactive power in (k+1) moment, e _drepresent the three-phase alternating current system voltage component of d axle, e _qrepresent the three-phase alternating current system voltage component of q axle; i _d(k+1) the three-phase grid-connected inverter output current d axle component that the k moment dopes is represented, i _q(k+1) the three-phase grid-connected inverter output current q axle component that the k moment dopes is represented; θ represents the space angle of electrical network;

Calculate the second cost function g, calculating formula is:

g＝|(Q ^*-Q(k+1)|+|(P _dc+P _i-P(k+1))| (15)

Q ^*for given reactive power reference qref, P _dcrepresent the direct current power after regulating, P _irepresent the active power that i-th other rotary substation consumes except rectification side current transformer in multi-terminal direct current transmission system, i>0;

Calculate respectively at the active-power P that i other rotary substation consumes _ii under state the second cost function g, obtains the minimum value g of the second cost function g _min; Obtain the minimum value g corresponding to the second cost function g _minclosest to expecting power, thus obtain corresponding U _α(k), U _βk the size of (), by U _α(k), U _βk () substitutes into the U in formula (6) respectively _α, U _β, obtain the switch function that this k moment expects, and the switch function of this expectation passed to the brachium pontis on off state of subsequent time, and then control rectification side current transformer; Wherein, described closest to expecting that power is the minimum performance number Q (k+1) of the value of instruction second cost function g and P (k+1), i>0.

4. the control device being applicable to VSC-MTDC system according to claim 1, is characterized in that, also comprise tuning controller;

Described tuning controller, for controlling main convertor and there is the current transformer of power adjustments, specific as follows:

Work as P ₁(k+1) allow in adjustable range at power, i.e. P _1min<P ₁(k+1) <P _1max, then under making VSC2, VSC3 be operated in power mode; When VSC1 power adjustments is not enough or out of service, P ₁(k+1) adjustable range [P is not allowed at power _1min, P _1max] in, then make the operational mode of VSC2 switch to direct voltage pattern to bear power shortage, under VSC3 is operated in power mode by power mode;

Wherein: VSC1 represents main convertor, VSC2 is the current transformer with power adjustments, and VSC3 is the current transformer without regulating power; P _1minrepresent that rotary substation is exerted oneself lower limit, i.e. minimum load, P ₁(k+1) value of current predictive power is represented, P _1maxrepresent that rotary substation is exerted oneself the upper limit, i.e. maximum output.