CN105743371A - Manufacturing method of MMC controller suitable for unbalanced voltage - Google Patents

Abstract

The invention provides a manufacturing method of an MMC controller suitable for an unbalanced voltage. The method comprises the following steps: (1) enabling an upper bridge arm voltage and a lower bridge arm voltage to tend to a target reference value through control of a current controller on the upper bridge arm voltage and the lower bridge arm voltage; and (2) uniformly controlling a positive-sequence component and a negative-sequence component of an AC component of an unbalanced current idiffj through an MMC circulation controller and independently controlling a zero-sequence component. The designed MMC controller is not complicated in principle and suitable for the conditions of balanced and unbalanced voltages, and the system stability is greatly improved.

Description

Controller manufacture method suitable in the MMC under unbalance voltage

Technical field

The present invention relates to electrical engineering, in particular it relates to a kind of controller manufacture method suitable in the MMC under unbalance voltage.

Background technology

Modular multi-level converter (Modularmultilevelconverter, MMC) has numerous advantage at high-voltage dc transmission electrical domain, important role in future transmission new forms of energy electric energy.The brachium pontis of MMC topology not adopts a large amount of switching device directly to connect, but adopts half-bridge sub module cascade form, is absent from the problems such as dynamic voltage balancing, is particularly well-suited to D.C. high voltage transmission occasion.Due to the structure that the three-phase brachium pontis DC side of MMC is in parallel with dc bus, determine MMC and operationally between three-phase, necessarily lead to circulation.Circulation is superimposed upon in bridge arm current, not only increases the rated current capacity of power device, adds system cost；Add switching loss simultaneously, make power device heating serious, affect device service life, thus be necessary circulation is suppressed.Tradition circulation controller adopts two frequency multiplication negative phase-sequence rotating coordinate transformations to be two DC component by the three-phase Circulation Decomposition of inverter inside, thus the Circulation Components eliminated in bridge arm current, but this circulation controller is only applicable to three-phase equilibrium AC system.

Through existing literature search is found, " IEEETransactionsonPowerDelivery " has delivered the article being entitled as " Predictivecontrolofamodularmultilevelconverterforaback-t o-backHVDCsystem (the modular multilevel PREDICTIVE CONTROL for back-to-back HVDC system) ", this article passes through discretization circulation mathematical model, a kind of circulation controller based on model prediction method is proposed, but the method is computationally intensive, when MMC only has N+1 level, on off state hasKind, control process loaded down with trivial details.

Summary of the invention

For defect of the prior art, it is an object of the invention to provide a kind of controller manufacture method suitable in the MMC under unbalance voltage.The present invention is directed to MMC and carry out detailed Derivation of Mathematical Model, made a concrete analysis of the situation of change of the exchange active power under unbalance voltage and circulation instantaneous power, devise the internal ring current controller controlled based on positive and negative sequence, eliminate active power two double-frequency fluctuation；Meanwhile, have also been devised a kind of circulation controller controlled based on positive and negative, zero sequence circulation.

According to the controller manufacture method suitable in the MMC under unbalance voltage provided by the invention, comprise the steps:

Step 1: make upper bridge arm voltage, lower bridge arm voltage trend towards target reference the control of upper bridge arm voltage, lower bridge arm voltage by current controller；

Step 2: by MMC circulation controller by out-of-balance current i_diffjThe positive-sequence component of AC compounent and negative sequence component be uniformly controlled, zero-sequence component individually controls.

Preferably, described step 1 comprises the steps:

Step 101: calculate in every phase brachium pontis brachium pontis, lower bridge arm voltage as follows:

$\left\{\begin{array}{c}{U}_{pj}={\mathrm{\Σ}}_{i=1}^n{v}_{dci}{s}_i\\ U_{nj}={\mathrm{\Σ}}_{i=1}^n{v}_{dci}{s}_i\end{array}\right.$

Wherein, i={1,2,3 ... n}, n are the quantity of submodule SM, j={a, b, c}, and a, b, c represent the three-phase of alternating current, U_pjBridge arm voltage in expression, U_njRepresent lower bridge arm voltage, v_dciFor i-th submodule capacitor voltage, s_iOn off state for i-th submodule；Upper brachium pontis, lower brachium pontis are in series by N number of submodule SM, constitute N+1 level current transformer；

Step 102: assuming that the submodule SM voltage constant in MMC, in MMC, each bridge arm voltage is equivalent to controlled voltage source, obtains one phase equivalent circuit:

The continuous mathematics model representation of three-phase of MMC is:

$u_{vj}=e_j-\frac{R_0}{2}{i}_{vj}-\frac{L_0}{2}\·\frac{{\mathrm{di}}_{vj}}{dt}----j=a,b,c$

$u_{diffj}=L_0\·\frac{{\mathrm{di}}_{diffj}}{dt}+R_0{i}_{diffj}=\frac{U_{dc}}{2}-\frac{u_{pj}+u_{nj}}{2}$

Wherein:

$e_j=\frac{u_{nj}-u_{pj}}{2}$

e_jIt is defined as internal emf, is worth the half of difference for upper and lower bridge arm voltage；

$i_{diffj}=\frac{i_{pj}+i_{nj}}{2}=\frac{i_{dc}}{3}+i_{zj}$

i_diffjFor internal out-of-balance current,Represent the DC component of internal out-of-balance current, i_dcFor DC current, i_zjRepresenting the AC compounent of internal out-of-balance current, this AC compounent is brachium pontis circulation；L₀Represent brachium pontis inductance, R₀Represent brachium pontis loss equivalent resistance；Controlled voltage source U_pjRepresent the upper bridge arm voltage of equivalence, U_njRepresent the lower bridge arm voltage of equivalence；i_pjBridge arm current in expression, i_njRepresent lower bridge arm current, i_diffjRepresent the inside out-of-balance current flowing through upper and lower brachium pontis；u_vj、i_vjRespectively the j phase voltage at level current transformer output point V place, electric current, u_diffjFor unbalance voltage；U_dcRepresent DC voltage；

Bridge arm current i in j phase_pj, lower bridge arm current i_njFor:

$\left\{\begin{array}{c}{i}_{pj}=i_{diffj}+\frac{i_{vj}}{2}\\ i_{nj}=i_{diffj}-\frac{i_{vj}}{2}\end{array}\right.$

Obtain bridge arm voltage U in j phase_pj, lower bridge arm voltage U_nj:

$\left\{\begin{array}{c}{U}_{pj}=\frac{U_{dc}}{2}-u_{diffj}-e_j\\ U_{nj}=\frac{U_{dc}}{2}-u_{diffj}+e_j\end{array}\right.$

Step 103: obtain target reference according to following formula, specific as follows:

$\left\{\begin{array}{c}{U}_{pj\_ref}=\frac{U_{dc}}{2}-u_{diffj\_ref}-e_{j\_ref}\\ U_{nj\_ref}=\frac{U_{dc}}{2}-u_{diffj\_ref}+e_{j\_ref}\end{array}\right.$

U_{pj_ref}In expression, bridge arm voltage shows reference value, U_{nj_ref}Represent that lower bridge arm voltage shows reference value, u_{diffj_ref}For unbalance voltage reference value, e_{j_ref}For the reference value of internal emf, subscript _ ref represents reference value；

Step 104: when under unbalance voltage, positive-sequence component and negative sequence component to voltage independently control, specifically, obtain positive-sequence component and the negative sequence component expression formula of voltage, are designated as expression formula A:

$\left\{\begin{array}{c}{u}_{vj}^{+}=e_j^{+}-\frac{R_0}{2}{i}_j^{+}-\frac{L_0}{2}\·\frac{{\mathrm{di}}_j^{+}}{dt}\\ u_{vj}^{-}=e_j^{-}-\frac{R_0}{2}{i}_j^{-}-\frac{L_0}{2}\·\frac{{\mathrm{di}}_j^{-}}{dt}\end{array}\right.$

Wherein,For the positive-sequence component of the j phase voltage at level current transformer output point V place,For the positive-sequence component of internal emf,For the positive-sequence component of j phase current,For the negative sequence component of the j phase voltage at level current transformer output point V place,For the negative sequence component of internal emf,Negative sequence component for j phase current；T is the time；

Expression formula A being converted into d, q rotational coordinates, by decoupling, independently controls d axle and q axle component, the expression formula B under rotational coordinates is as follows:

Wherein,For d, q axle positive-sequence component of electric current,For d, q axle positive-sequence component of voltage of level current transformer output,For d, q axle positive-sequence component of internal emf,For d, q axle negative sequence component of electric current,For d, q axle negative sequence component of voltage of level current transformer output,D, q axle negative sequence component for internal emf；

Obtain the current controller of correspondence according to expression formula B, adopt PI controller, obtain internal emf reference value e_{j_ref}The positive and negative sequence reference value of d, q axle componentThat is:

Wherein, ω is electrical network angular frequency, and L is line reactance value, and R is line resistance, and PI () is pi controller.

Preferably, also comprise the steps:

-when under unbalance voltage, due to the existence of voltage x current negative sequence component, the active power of level current transformer net side and reactive power will produce 2 times of fundamental frequency fluctuations；

Level current transformer net side active power and reactive power all create positive sequence and negative sequence component, specific as follows:

$\left[\begin{array}{c}{P}_{g0}\\ Q_{g0}\\ P_{g\sin 2}\\ P_{g\cos 2}\end{array}\right]=\frac{3}{2}\left[\begin{array}{cccc}{V}_{gd}^{+}& V_{gq}^{+}& V_{gd}^{+}& V_{gq}^{-}\\ V_{gq}^{+}& -V_{gd}^{+}& V_{gq}^{-}& -V_{gd}^{-}\\ V_{gq}^{-}& -V_{gd}^{-}& -V_{gq}^{+}& V_{gd}^{+}\\ V_{gd}^{-}& V_{gq}^{-}& V_{gd}^{+}& V_{gq}^{+}\end{array}\right]\left[\begin{array}{c}{i}_d^{+}\\ i_q^{+}\\ i_d^{-}\\ i_q^{-}\end{array}\right]$

P_g=P_g0+P_gsin2sin2ω_gt+P_gcos2cos2ω_gt

Wherein, P_g0Represent active power, Q_g0Represent reactive power,Represent the q axle component of current on line side positive-sequence component,Represent the q axle component of current on line side positive-sequence component,Represent the d axle component of current on line side negative sequence component,Represent the q axle component of current on line side negative sequence component,Represent the d axle component of voltage on line side net side component positive-sequence component,Represent the d axle component of voltage on line side net side component negative sequence component,For the q axle component of voltage on line side net side component positive-sequence component,Q axle component for voltage on line side net side component negative sequence component, i represents current on line side, V represents voltage on line side, subscript "+", "-" represent positive-sequence component, negative sequence component respectively, subscript d, q represent the d axle component under rotational coordinates, q axle component respectively, subscript g represents net side component, and subscript 0 represents fundamental component；The sinusoidal 2 times of fundamental components of subscript sin2 and cos2 represent 2 times of fundamental frequency wave components of cosine；P_gFor total active power, ω is electrical network angular frequency；P_g0Fundamental component for total active power；P_gsin2For the sinusoidal 2 times of fundamental frequency wave components of total active power and P_gcos22 times of fundamental frequency wave components of cosine for total active power；

Suppress to be zero by 2 times of fundamental frequency wave components of active power, namely make P by control_gsin2=0, P_gcos2=0；P represents active power, and Q represents reactive power；Specifically, be initial condition analysis when taking the vertical q axle of voltage on line side V, thenAnd then electric current negative sequence component expression formula:

$\left\{\begin{array}{c}{i}_d^{-}=\frac{V_{gd}^{-}{i}_q^{+}}{V_{gd}^{+}}-\frac{V_{gq}^{-}{i}_d^{+}}{V_{gd}^{+}}\\ i_q^{-}=-\frac{V_{gd}^{-}{i}_d^{+}}{V_{gd}^{+}}-\frac{V_{gq}^{-}{i}_q^{+}}{V_{gd}^{+}}\end{array}\right.$

In formula,WithCalculated by value and power reference and voltage on line side and obtain, specifically,

$\left\{\begin{array}{c}{i}_d^{+}=\frac{2P}{3V_{gd}^{+}}\\ i_q^{+}=\frac{2Q}{3V_{gd}^{+}}\end{array}\right..$

Preferably, described step 2 comprises the steps:

Step 201: when under unbalance voltage, the single-phase instantaneous power P of a phase_puaFor,

$\begin{array}{c}{P}_{pua}=\frac{U_{dc}{I}_{dc}}{6}\[k^{-}{m}^{+}\cos \left(2{\mathrm{\ω}}_{0}t+\mathrm{\α}\_+{\mathrm{\γ}}_{+}\right)+k^{+}{m}^{+}\cos \left(2{\mathrm{\ω}}_{0}t+{\mathrm{\α}}_{+}+{\mathrm{\γ}}_{+}\right)-\\ 2l^{-}\sin \left(2{\mathrm{\ω}}_{0}t+\mathrm{\β}\_\right)-k^{-}{m}^{+}\cos \left(\mathrm{\α}\_-{\mathrm{\γ}}_{+}\right)\]\end{array}$

In formula, α₊、α_-The respectively phase angle of the positive sequence of internal emf, negative sequence component；k⁺、k^-Respectively internal emf positive sequence voltage, negative sequence voltage modulation are compared；l^-Represent circulation bucking voltageRatio with DC voltage；m⁺Ratio, γ is modulated for forward-order current₊Represent current transformer current on line side phase angle；U_dcRepresent DC voltage；ω₀For electrical network initial angular frequency；I_dcFor DC current；β_-It is 2 times of initial phase angles of fundamental frequency negative sequence component；

$k^{\±}=\frac{E_a^{\±}}{\frac{u_{dc}}{2}}$

$l^{-}=\frac{u_{diff}^{-}}{\frac{u_{dc}}{2}}$

Wherein,Represent the positive-sequence component of internal emf and the amplitude of negative sequence component；

Step 202: out-of-balance current i_diffjPositive and negative, 03 sequence expression formulas, as follows,

$i_{diffj}=\frac{I_{dc}}{3}+i_{zj}=\frac{I_{dc}}{3}+i_{zj}^{+}+i_{zj}^{-}+i_{zj}^0$

Wherein,For the positive-sequence component of the AC compounent of internal out-of-balance current,For the negative sequence component of the AC compounent of internal out-of-balance current,For the zero-sequence component of the AC compounent of internal out-of-balance current, I_dcRepresent DC current.

Step 203: by out-of-balance current i_diffjAC compounent, namely brachium pontis loop current suppression is zero；Specifically, out-of-balance current i_diffjThe positive-sequence component of AC compounent and negative sequence component be uniformly controlled, zero-sequence component individually controls, thus MMC circulation controller is:

$u_{diff}^{{\±}^{*}}=PI\[\left(\frac{I_{dc}}{3}\right)-i_{diffj}\]$

$u_{diff}^{0^{*}}=PI\[i_{dc}^{*}-I_{dc}\]$

$i_{dc}^{*}=\frac{P_g}{V_{dc}}$

$u_{diff}^{*}=u_{diff}^{{\±}^{*}}+u_{diff}^{0^{*}}$

Represent DC current reference value, I_dcRepresenting DC current, PI () represents PI controller,For the positive-sequence component of unbalance voltage reference value, negative sequence component,Represent unbalance voltage reference value,Expression unbalance voltage reference value, subscript+,-representing positive-sequence component, negative sequence component respectively, upper table 0 represents zero-sequence component, P_g、V_dcRepresent net side active power, DC voltage respectively.

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

1, the present invention is directed to MMC changed power under unbalance voltage, devise the internal ring current controller controlled based on positive and negative sequence, eliminate active power two double-frequency fluctuation.

2, the present invention is based on instantaneous power theory analysis, it is proposed to and devise that a kind of circulation is positive and negative, the circulation controller of 03 order components based on directly controlling.

3, MMC controller principle designed by the present invention uncomplicated, it is adaptable to the situation under balance and unbalance voltage, greatly improves system stability.

Accompanying drawing explanation

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

Fig. 1 is MMC Basic Topological in the present invention；

Fig. 2 is the one phase equivalent circuit of three-phase MMC in the present invention；

Fig. 3 is MMC electric current control block diagram in the present invention；

Fig. 4 is MMC circulation control block diagram in the present invention；

Fig. 5 is the complete control block diagram of MMC in the present invention；

Fig. 6 is dc current waveform under symmetrical alternating current electrical network in the present invention；

Fig. 7 is brachium pontis circulation waveform under symmetrical alternating current electrical network in the present invention；

Fig. 8 is the active power waveform in the present invention under unbalance voltage；

Fig. 9 is the dc current waveform in the present invention under unbalance voltage；

Figure 10 is the brachium pontis circulation waveform in the present invention under unbalance voltage；

Figure 11 is the structural representation of MMC basic structure Neutron module SM in the present invention.

Detailed description of the invention

Below in conjunction with specific embodiment, the present invention is described in detail.Following example will assist in those skilled in the art and are further appreciated by the present invention, but do 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, it is also possible to make some deformation and improvement.These broadly fall into protection scope of the present invention.

Step 1: set up the continuous mathematical model of MMC three-phase；Wherein MMC basic structure is made up of three-phase six brachium pontis, and each brachium pontis is in series by N number of submodule SM (SubModule, SM), constitutes N+1 level current transformer, as shown in Figure 1.

Step 101: calculate upper brachium pontis in every phase brachium pontis, lower bridge arm voltage by DC capacitor voltage, switch function as follows:

$\left\{\begin{array}{c}{U}_{pj}={\Σ}_{i=1}^n{v}_{dci}{s}_i\\ U_{nj}={\Σ}_{i=1}^n{v}_{dci}{s}_i\end{array}\right.---\left(1\right)$

Wherein, i={1,2,3 ... n}, n are the quantity of submodule SM, j={a, b, c}, and a, b, c represent the three-phase of alternating current, in Fig. 1, SM represents submodule, Ua, Ub, and Uc represents three-phase alternating voltage, U_pjBridge arm voltage in expression, U_njRepresent lower bridge arm voltage, U_dcRepresenting DC voltage, C represents DC capacitor.v_dciFor i-th submodule capacitor voltage, s_iFor i-th submodule on off state.

Step 102: if supposing SM voltage constant, in MMC, each bridge arm voltage can be equivalent to controlled voltage source, then can obtain one phase equivalent circuit；

As in figure 2 it is shown, L₀Represent brachium pontis inductance, R₀Represent brachium pontis loss equivalent resistance；Controlled voltage source U_pjRepresent the upper bridge arm voltage of equivalence, U_njRepresent the lower bridge arm voltage of equivalence；i_pjBridge arm current in expression, i_njRepresent lower bridge arm current, i_diffjRepresent the inside out-of-balance current flowing through upper and lower brachium pontis；

The j phase voltage at level current transformer output point V place and electric current respectively u_vjAnd i_vj.According to Kirchhoff's law, the continuous mathematical model of three-phase of MMC can be expressed as:

$u_{vj}=e_j-\frac{R_0}{2}{i}_{vj}-\frac{L_0}{2}\·\frac{{\mathrm{di}}_{vj}}{dt},\left(j=a,b,c\right)---\left(2\right)$

$u_{diffj}=L_0\·\frac{{\mathrm{di}}_{diffj}}{dt}+R_0{i}_{diffj}=\frac{U_{dc}}{2}-\frac{u_{pj}+u_{nj}}{2}---\left(3\right)$

Wherein:

$e_j=\frac{u_{nj}-u_{pj}}{2}---\left(4\right)$

e_jBeing defined as internal emf, its value is the half of the difference of upper and lower bridge arm voltage；

$i_{diffj}=\frac{i_{pj}+i_{nj}}{2}=\frac{i_{dc}}{3}+i_{zj}---\left(5\right)$

i_diffjFor internal out-of-balance current, it is made up of two parts,Represent DC component, i_zjRepresenting AC compounent, this AC compounent is brachium pontis circulation, u_diffjFor unbalance voltage；

Meanwhile, as shown in Figure 2, bridge arm current i in j phase_pj, lower bridge arm current i_njIt is represented by:

$\left\{\begin{array}{c}{i}_{pj}=i_{diffj}+\frac{i_{vj}}{2}\\ i_{nj}=i_{diffj}-\frac{i_{vj}}{2}\end{array}\right.---\left(6\right)$

In like manner, it is possible to obtain bridge arm voltage U in j phase_pj, lower bridge arm voltage U_nj:

$\left\{\begin{array}{c}{U}_{pj}=\frac{U_{dc}}{2}-u_{diffj}-e_j\\ U_{nj}=\frac{U_{dc}}{2}-u_{diffj}+e_j\end{array}\right.---\left(7\right)$

Step 2: MMC carries out power analysis, current-order calculates；

In unbalance voltage situation, due to the existence of voltage x current negative sequence component, the active power of level current transformer net side and reactive power non-constant, but 2 times of fundamental frequencies fluctuations can be produced；

Level current transformer net side active power and reactive power all create positive sequence and negative sequence component, specific as follows:

$\left[\begin{array}{c}{P}_{g0}\\ Q_{g0}\\ P_{g\sin 2}\\ P_{g\cos 2}\end{array}\right]=\frac{3}{2}\left[\begin{array}{cccc}{V}_{gd}^{+}& V_{gq}^{+}& V_{gd}^{+}& V_{gq}^{-}\\ V_{gq}^{+}& -V_{gd}^{+}& V_{gq}^{-}& -V_{gd}^{-}\\ V_{gq}^{-}& -V_{gd}^{-}& -V_{gq}^{+}& V_{gd}^{+}\\ V_{gd}^{-}& V_{gq}^{-}& V_{gd}^{+}& V_{gq}^{+}\end{array}\right]\left[\begin{array}{c}{i}_d^{+}\\ i_q^{+}\\ i_d^{-}\\ i_q^{-}\end{array}\right]---\left(8\right)$

P_g=P_g0+P_gsin2sin2ω_gt+P_gcos2cos2ω_gt(9)

Wherein, P_g0Represent active power, Q_g0Represent reactive power,Represent the q axle component of current on line side positive-sequence component,Represent the q axle component of current on line side positive-sequence component,Represent the d axle component of current on line side negative sequence component,Represent the q axle component of current on line side negative sequence component,Represent the q axle component of voltage on line side net side component positive-sequence component, V represents voltage on line side, subscript "+", "-" represent d axle component, the q axle component that positive-sequence component, negative sequence component, subscript d, q represent under rotational coordinates respectively respectively, subscript g represents net side component, and subscript 0 represents fundamental component；Subscript sin2 and cos2 represents 2 times of fundamental frequency wave components；P_gFor total active power, ω is electrical network angular frequency；P_g0Fundamental component for total active power；P_gsin2And P_gcos22 times of fundamental frequency wave components of total active power；, it can be seen that total active power is formed by stacking by active power fundamental component and 2 times of fundamental frequency wave components of active power.Owing to active power fluctuation can cause DC bus-bar voltage corresponding 2 times of fundamental frequencies fluctuation occur, thus affecting the quality of power supply.

Therefore, for ensureing that active power is constant, it is necessary to suppress to be zero by 2 times of fundamental frequency wave components of active power, namely make P by control_gsin2=0, P_gcos2=0.Utilize mathematical reverse footwork thought, the current-order in this situation of retrodicting.It is initial condition analysis when taking the vertical q axle of voltage on line side, thenElectric current negative sequence component expression formula can be obtained by formula (8):

$\left\{\begin{array}{c}{i}_d^{-}=\frac{V_{gd}^{-}{i}_q^{+}}{V_{gd}^{+}}-\frac{V_{gq}^{-}{i}_d^{+}}{V_{gd}^{+}}\\ i_q^{-}=-\frac{V_{gd}^{-}{i}_d^{+}}{V_{gd}^{+}}-\frac{V_{gq}^{-}{i}_q^{+}}{V_{gd}^{+}}\end{array}\right.---\left(10\right)$

In formula,WithCalculated by value and power reference and voltage on line side and obtain:

$\left\{\begin{array}{c}{i}_d^{+}=\frac{2P}{3V_{gd}^{+}}\\ i_q^{+}=\frac{2Q}{3V_{gd}^{+}}\end{array}\right.---\left(11\right)$

Step 3: current controller designs, to MMC control, can be understood as the suitable gate electrode drive signals of searching to go to control system variable x (t) so that it is as far as possible close to desired reference variable x* (t), i.e. the control to upper bridge arm voltage, lower bridge arm voltage.Therefore, control target reference and derived by formula (7) and go out, specific as follows:

$\left\{\begin{array}{c}{U}_{pj\_ref}=\frac{U_{dc}}{2}-u_{diffj\_ref}-e_{j\_ref}\\ U_{nj\_ref}=\frac{U_{dc}}{2}-u_{diffj\_ref}+e_{j\_ref}\end{array}\right.---\left(12\right)$

Under unbalance voltage, positive-sequence component and negative sequence component must independent control, formula (2) can obtain its positive sequence and negative phase-sequence expression formula:

$\left\{\begin{array}{c}{u}_{vj}^{+}=e_j^{+}-\frac{R_0}{2}{i}_j^{+}-\frac{L_0}{2}\·\frac{{\mathrm{di}}_j^{+}}{dt}\\ u_{vj}^{-}=e_j^{-}-\frac{R_0}{2}{i}_j^{-}-\frac{L_0}{2}\·\frac{{\mathrm{di}}_j^{-}}{dt}\end{array}\right.---\left(13\right)$

Formula (13) being converted into d, q rotational coordinates, by decoupling, independently controls d axle and q axle component, the expression formula under rotational coordinates is as follows:

Be can be designed that the current controller of correspondence by formula (14), adopt PI controller, obtain the e of internal emf_{j_ref}The positive-negative sequence reference value of d, q axle componentWithThat is:

Fig. 3 is designed MMC electric current control block diagram.

In Fig. 3, MMC is modular multi-level converter, and C represents that DC capacitor, R represent that line resistance, L represent line reactance, P^*Represent active power reference value, Q^*Represent reactive power reference qref.Subscript * represents that reference value, θ represent that grid voltage phase-angle, ω represent electrical network angular frequency, and PI represents PI controller.

Step 4:MMC circulation controller designs, under unbalance voltage, shown in single-phase instantaneous power such as formula (16), for A phase:

$\begin{array}{c}{P}_{pua}=\frac{U_{dc}{I}_{dc}}{6}\[k^{-}{m}^{+}\cos \left(2{\mathrm{\ω}}_{0}t+\mathrm{\α}\_+{\mathrm{\γ}}_{+}\right)+k^{+}{m}^{+}\cos \left(2{\mathrm{\ω}}_{0}t+{\mathrm{\α}}_{+}+{\mathrm{\γ}}_{+}\right)-\\ 2l^{-}-sin(2{\mathrm{\ω}}_{0}t+\mathrm{\β}\_)-k^{-}{m}^{+}cos(\mathrm{\α}\_-{\mathrm{\γ}}_{+})\]\end{array}---\left(16\right)$

In formula, α₊、α_-Represent the phase angle of the positive sequence of internal emf, negative sequence component；k⁺、k^-Represent internal emf positive sequence voltage, negative sequence voltage modulation ratio, specifically as shown in formula (17)；l^-Represent circulation bucking voltageWith the ratio of DC voltage, specifically calculate as shown in formula (18)；m⁺Represent forward-order current modulation ratio, γ₊Represent current transformer current on line side phase angle.It can be seen that under unbalance voltage, single-phase instantaneous power contains twice fundamental frequency zero-sequence component (Section 1), twice fundamental frequency positive and negative sequence component (middle two) and DC component (last).Zero-sequence component can cause that DC voltage and DC current produce fluctuation, and twice fundamental frequency positive and negative sequence component is directly related with MMC circulation, and DC component is due to mutual deviation 120 ° in each phase, thus automatically eliminates.

$k^{\±}=\frac{E_a^{\±}}{\frac{u_{dc}}{2}}---\left(17\right)$

$l^{-}=\frac{u_{diff}^{-}}{\frac{u_{dc}}{2}}---\left(18\right)$

In formula (17),Represent the positive-sequence component of internal emf and the amplitude of negative sequence component.Rewriting formula (5), it is possible to obtain positive and negative, the 03 sequence expression formulas of out-of-balance current, as follows:

$i_{diffj}=\frac{i_{dc}}{3}+i_{zj}=\frac{i_{dc}}{3}+i_{zj}^{+}+i_{zj}^{-}+i_{zj}^0---\left(19\right)$

The AC compounent of out-of-balance current, i.e. brachium pontis circulation, it is necessary to be suppressed to zero, due to existence positive and negative, zero sequence currents, if each component individually adopts PI controller, then also need to notch filter.Considering in three-phase alternating current system, the positive-sequence component of circulation and negative sequence component sum are zero, therefore, can be uniformly controlled for positive sequence and negative sequence component, namely circulation directly be controlled；Zero-sequence component affects DC current fluctuation, and it is individually controlled, and designed controller is as follows:

$u_{diff}^{{\±}^{*}}=PI\[\left(\frac{i_{dc}}{3}\right)-i_{diffj}\]---\left(20\right)$

$u_{diff}^{0^{*}}=PI\[i_{dc}^{*}-i_{dc}\]---\left(21\right)$

$i_{dc}^{*}=\frac{P_g}{V_{dc}}---\left(22\right)$

$u_{diff}^{*}=u_{diff}^{{\±}^{*}}+u_{diff}^{0^{*}}---\left(23\right)$

Corresponding controller block diagram is as shown in Figure 4.In Fig. 4,Represent DC current reference value, i_dcRepresenting DC current, PI represents PI controller, i_pjBridge arm current in expression, i_njRepresent lower bridge arm current,Expression unbalance voltage reference value, subscript+,-represent positive-sequence component and negative sequence component respectively, 0 represents zero-sequence component.By formula (13) it can be seen that current controller controls e_{j_ref}, circulation controller controls u_{diffj_ref}, and then controlling upper and lower bridge arm voltage, whole MMC control block diagram is as figure 5 illustrates.

In Fig. 5, i_pjBridge arm current in expression, i_njRepresent lower bridge arm current,Represent DC current reference value, i_dcRepresent DC current, P_g、V_dcRepresent net side active power and DC voltage, P^*Represent active power reference value, Q^*Represent reactive power reference qref.Subscript+,-representing positive-sequence component and negative sequence component respectively, subscript d, q represent the d axle component under rotational coordinates and q axle component respectively, and subscript g represents net side component, and subscript * represents reference value.

In the present embodiment, 21 level MMC systems are carried out Research of digital simulation by utilization simulation software MATLAB/Simulink, and the effectiveness of checking model and control strategy, simulation parameter, in Table 1.

Table 1 simulation parameter

Fig. 6, Fig. 7 give system response under balanced voltage.When 0.3s, accessing proposed circulation controller, it can be seen that under this control strategy, output current of converter, voltage stabilization, loop current suppression effect is obvious, and DC current fluctuation substantially diminishes.Simulation result shows that proposed control strategy is suitable under balanced voltage.

Fig. 8, Fig. 9, Figure 10 give system response under unbalance voltage, and contrast with tradition loop current suppression device (CCSC).When 0.3s, system generation singlephase earth fault, it is in unbalance voltage environment.Simulation result shows, the control under tradition loop current suppression device (CCSC) inapplicable unbalance voltage environment.Control method in this paper overcomes this shortcoming, and it is notable that circulation controls inhibition, simultaneously effective inhibits the fluctuation of system active power.

Above specific embodiments of the invention are described.It is to be appreciated that the invention is not limited in above-mentioned particular implementation, those skilled in the art can make various deformation or amendment within the scope of the claims, and this has no effect on the flesh and blood of the present invention.

Claims (4)

1. the controller manufacture method of the MMC being applicable under unbalance voltage, it is characterised in that comprise the steps:

Step 1: make upper bridge arm voltage, lower bridge arm voltage trend towards target reference the control of upper bridge arm voltage, lower bridge arm voltage by current controller；

Step 2: by MMC circulation controller by out-of-balance current i_diffjThe positive-sequence component of AC compounent and negative sequence component be uniformly controlled, zero-sequence component individually controls.

2. the controller manufacture method suitable in the MMC under unbalance voltage according to claim 1, it is characterised in that described step 1 comprises the steps:

Step 101: calculate in every phase brachium pontis brachium pontis, lower bridge arm voltage as follows:

$\left\{\begin{array}{c}{U}_{pj}={\mathrm{\Σ}}_{i=1}^n{v}_{dci}{s}_i\\ U_{nj}={\mathrm{\Σ}}_{i=1}^n{v}_{dci}{s}_i\end{array}\right.$

Wherein, i={1,2,3 ... n}, n are the quantity of submodule SM, j={a, b, c}, and a, b, c represent the three-phase of alternating current, U_pjBridge arm voltage in expression, U_njRepresent lower bridge arm voltage, v_dciFor i-th submodule capacitor voltage, s_iOn off state for i-th submodule；Upper brachium pontis, lower brachium pontis are in series by N number of submodule SM, constitute N+1 level current transformer；

Step 102: assuming that the submodule SM voltage constant in MMC, in MMC, each bridge arm voltage is equivalent to controlled voltage source, obtains one phase equivalent circuit:

The continuous mathematics model representation of three-phase of MMC is:

$u_{vj}=e_j-\frac{R_0}{2}{i}_{vj}-\frac{L_0}{2}\·\frac{{\mathrm{di}}_{vj}}{dt},j=a,b,c$

$u_{diffj}=L_0\·\frac{{\mathrm{di}}_{diffj}}{dt}+R_0{i}_{diffj}=\frac{U_{dc}}{2}-\frac{u_{pj}+u_{nj}}{2}$

Wherein:

$e_j=\frac{u_{nj}-u_{pj}}{2}$

e_jIt is defined as internal emf, is worth the half of difference for upper and lower bridge arm voltage；

$i_{diffj}=\frac{i_{pj}+i_{nj}}{2}=\frac{i_{dc}}{3}+i_{zj}$

i_diffjFor internal out-of-balance current,Represent the DC component of internal out-of-balance current, i_dcFor DC current, i_zjRepresenting the AC compounent of internal out-of-balance current, this AC compounent is brachium pontis circulation；L₀Represent brachium pontis inductance, R₀Represent brachium pontis loss equivalent resistance；Controlled voltage source U_pjRepresent the upper bridge arm voltage of equivalence, U_njRepresent the lower bridge arm voltage of equivalence；i_pjBridge arm current in expression, i_njRepresent lower bridge arm current, i_diffjRepresent the inside out-of-balance current flowing through upper and lower brachium pontis；u_vj、i_vjRespectively the j phase voltage at level current transformer output point V place, electric current, u_diffjFor unbalance voltage；U_dcRepresent DC voltage；

Bridge arm current i in j phase_pj, lower bridge arm current i_njFor:

$\left\{\begin{array}{c}{i}_{pj}=i_{diffj}+\frac{i_{vj}}{2}\\ i_{nj}=i_{diffj}-\frac{i_{vj}}{2}\end{array}\right.$

Obtain bridge arm voltage U in j phase_pj, lower bridge arm voltage U_nj:

$\left\{\begin{array}{c}{U}_{pj}=\frac{U_{dc}}{2}-u_{diffj}-e_j\\ U_{nj}=\frac{U_{dc}}{2}-u_{diffj}+e_j\end{array}\right.$

Step 103: obtain target reference according to following formula, specific as follows:

$\left\{\begin{array}{c}{U}_{pj\_ref}=\frac{U_{dc}}{2}-u_{diffj\_ref}-e_{j\_ref}\\ U_{nj\_ref}=\frac{U_{dc}}{2}-u_{diffj\_ref}+e_{j\_ref}\end{array}\right.$

U_{pj_ref}In expression, bridge arm voltage shows reference value, U_{nj_ref}Represent that lower bridge arm voltage shows reference value, u_{diffj_ref}For unbalance voltage reference value, e_{j_ref}For the reference value of internal emf, subscript _ ref represents reference value；

Step 104: when under unbalance voltage, positive-sequence component and negative sequence component to voltage independently control, specifically, obtain positive-sequence component and the negative sequence component expression formula of voltage, are designated as expression formula A:

$\left\{\begin{array}{c}{u}_{vj}^{+}=e_j^{+}-\frac{R_0}{2}{i}_j^{+}-\frac{L_0}{2}\·\frac{{\mathrm{di}}_j^{+}}{dt}\\ u_{vj}^{-}=e_j^{-}-\frac{R_0}{2}{i}_j^{-}-\frac{L_0}{2}\·\frac{{\mathrm{di}}_j^{-}}{dt}\end{array}\right.$

Wherein,For the positive-sequence component of the j phase voltage at level current transformer output point V place,For the positive-sequence component of internal emf,For the positive-sequence component of j phase current,For the negative sequence component of the j phase voltage at level current transformer output point V place,For the negative sequence component of internal emf,Negative sequence component for j phase current；T is the time；

Expression formula A being converted into d, q rotational coordinates, by decoupling, independently controls d axle and q axle component, the expression formula B under rotational coordinates is as follows:

Wherein,For d, q axle positive-sequence component of electric current,For d, q axle positive-sequence component of voltage of level current transformer output,For d, q axle positive-sequence component of internal emf,For d, q axle negative sequence component of electric current,For d, q axle negative sequence component of voltage of level current transformer output,D, q axle negative sequence component for internal emf；

Obtain the current controller of correspondence according to expression formula B, adopt PI controller, obtain internal emf reference value e_{j_ref}The positive and negative sequence reference value of d, q axle componentThat is:

Wherein, ω is electrical network angular frequency, and L is line reactance value, and R is line resistance, and PI () is pi controller.

3. the controller manufacture method suitable in the MMC under unbalance voltage according to claim 2, it is characterised in that also comprise the steps:

-when under unbalance voltage, due to the existence of voltage x current negative sequence component, the active power of level current transformer net side and reactive power will produce 2 times of fundamental frequency fluctuations；

Level current transformer net side active power and reactive power all create positive sequence and negative sequence component, specific as follows:

$\left[\begin{array}{c}{P}_{g0}\\ Q_{g0}\\ P_{g\sin 2}\\ P_{g\cos 2}\end{array}\right]=\frac{3}{2}\left[\begin{array}{cccc}{V}_{gd}^{+}& V_{gq}^{+}& V_{gd}^{-}& V_{gq}^{-}\\ V_{gq}^{+}& -V_{gd}^{+}& V_{gq}^{-}& -V_{gd}^{-}\\ V_{gq}^{-}& -V_{gd}^{-}& -V_{gq}^{+}& V_{gd}^{+}\\ V_{gd}^{-}& V_{gq}^{-}& V_{gd}^{+}& V_{gq}^{+}\end{array}\right]\left[\begin{array}{c}{i}_d^{+}\\ i_q^{+}\\ i_d^{-}\\ i_q^{-}\end{array}\right]$

P_g=P_g0+P_gsin2sin2ω_gt+P_gcos2cos2ω_gt

Wherein, P_g0Represent active power, Q_g0Represent reactive power,Represent the q axle component of current on line side positive-sequence component,Represent the q axle component of current on line side positive-sequence component,Represent the d axle component of current on line side negative sequence component,Represent the q axle component of current on line side negative sequence component,Represent the d axle component of voltage on line side net side component positive-sequence component,Represent the d axle component of voltage on line side net side component negative sequence component,For the q axle component of voltage on line side net side component positive-sequence component,Q axle component for voltage on line side net side component negative sequence component, i represents current on line side, V represents voltage on line side, subscript "+", "-" represent positive-sequence component, negative sequence component respectively, subscript d, q represent the d axle component under rotational coordinates, q axle component respectively, subscript g represents net side component, and subscript 0 represents fundamental component；The sinusoidal 2 times of fundamental components of subscript sin2 and cos2 represent 2 times of fundamental frequency wave components of cosine；P_gFor total active power, ω is electrical network angular frequency；P_g0Fundamental component for total active power；P_gsin2For the sinusoidal 2 times of fundamental frequency wave components of total active power and P_gcos22 times of fundamental frequency wave components of cosine for total active power；

Suppress to be zero by 2 times of fundamental frequency wave components of active power, namely make P by control_gsin2=0, P_gcos2=0；P represents active power, and Q represents reactive power；Specifically, be initial condition analysis when taking the vertical q axle of voltage on line side V, thenAnd then electric current negative sequence component expression formula:

$\left\{\begin{array}{c}{i}_d^{-}=\frac{V_{gd}^{-}{i}_q^{+}}{V_{gd}^{+}}-\frac{V_{gq}^{-}{i}_d^{+}}{V_{gd}^{+}}\\ i_q^{-}=-\frac{V_{gd}^{-}{i}_d^{+}}{V_{gd}^{+}}-\frac{V_{gq}^{-}{i}_q^{+}}{V_{gd}^{+}}\end{array}\right.$

In formula,WithCalculated by value and power reference and voltage on line side and obtain, specifically,

$\left\{\begin{array}{c}{i}_d^{+}=\frac{2P}{3V_{gd}^{+}}\\ i_q^{+}=\frac{2Q}{3V_{gd}^{+}}\end{array}\right..$

4. the controller manufacture method suitable in the MMC under unbalance voltage according to claim 2, it is characterised in that described step 2 comprises the steps:

Step 201: when under unbalance voltage, the single-phase instantaneous power P of a phase_puaFor,

$\begin{array}{c}{P}_{pua}=\frac{U_{dc}{I}_{dc}}{6}\[k^{-}{m}^{+}\cos (2{\mathrm{\ω}}_{0}t+{\mathrm{\α}}_{-}+{\mathrm{\γ}}_{+})+k^{+}{m}^{+}\cos (2{\mathrm{\ω}}_{0}t+{\mathrm{\α}}_{+}+{\mathrm{\γ}}_{+})-\\ 2l^{-}\sin (2{\mathrm{\ω}}_{0}t+{\mathrm{\β}}_{-})-k^{-}{m}^{+}\cos ({\mathrm{\α}}_{-}-{\mathrm{\γ}}_{+})\]\end{array}$

In formula, α₊、α_-The respectively phase angle of the positive sequence of internal emf, negative sequence component；k⁺、k^-Respectively internal emf positive sequence voltage, negative sequence voltage modulation are compared；l^-Represent circulation bucking voltageRatio with DC voltage；m⁺Ratio, γ is modulated for forward-order current₊Represent current transformer current on line side phase angle；U_dcRepresent DC voltage；ω₀For electrical network initial angular frequency；I_dcFor DC current；β_-It is 2 times of initial phase angles of fundamental frequency negative sequence component；

$k^{\±}=\frac{E_a^{\±}}{\frac{u_{dc}}{2}}$

$l^{-}=\frac{u_{diff}^{-}}{\frac{u_{dc}}{2}}$

Wherein,Represent the positive-sequence component of internal emf and the amplitude of negative sequence component；

Step 202: out-of-balance current i_diffjPositive and negative, 03 sequence expression formulas, as follows,

$i_{diffj}=\frac{I_{dc}}{3}+i_{zj}=\frac{I_{dc}}{3}+i_{zj}^{+}+i_{zj}^{-}+i_{zj}^0$

Wherein,For the positive-sequence component of the AC compounent of internal out-of-balance current,For the negative sequence component of the AC compounent of internal out-of-balance current,For the zero-sequence component of the AC compounent of internal out-of-balance current, I_dcRepresent DC current.

Step 203: by out-of-balance current i_diffjAC compounent, namely brachium pontis loop current suppression is zero；Specifically, out-of-balance current i_diffjThe positive-sequence component of AC compounent and negative sequence component be uniformly controlled, zero-sequence component individually controls, thus MMC circulation controller is:

$u_{diff}^{\±*}=PI\[\left(\frac{I_{dc}}{3}\right)-i_{diffj}\]$

$u_{diff}^{0*}=PI\[i_{dc}^{*}-I_{dc}\]$

$i_{dc}^{*}=\frac{P_g}{V_{dc}}$

$u_{diff}^{*}=u_{diff}^{\±*}+u_{diff}^{0*}$

Represent DC current reference value, I_dcRepresenting DC current, PI () represents PI controller,For the positive-sequence component of unbalance voltage reference value, negative sequence component,Represent unbalance voltage reference value,Expression unbalance voltage reference value, subscript+,-representing positive-sequence component, negative sequence component respectively, upper table 0 represents zero-sequence component, P_g、V_dcRepresent net side active power, DC voltage respectively.