CN104393745A - MMC (modular multilevel converter) harmonic circulating current restraining and direct-current power fluctuation restraining method - Google Patents

Abstract

The invention discloses an MMC (modular multilevel converter) harmonic circulating current restraining and direct-current power fluctuation restraining method. The method includes a modulation link based on submodule voltage estimation and a proportional control link based on direct-current fluctuation feedback. According to the modulation link, submodule voltage of each bridge arm is firstly estimated, and then the number of submodules expected to be invested for each bridge arm is obtained by the aid of a certain modulation technology. According to the method, MMC harmonic circulating current can be effectively restrained, the number of the submodules expected to be invested for each bridge arm is modified in the proportional control link, circulating current fluctuation caused by estimation errors can be further eliminated, the internal circulating current of an MMC can be restrained, the internal equivalent damping of the MMC can be effectively increased, and the dynamic response characteristic of direct-current is improved. The method is used for restraining the MMC harmonic circulating current and direct-current power fluctuation, and an alternating-current system can reliably work and is fine in robustness when being normal or faulted. Besides, the method does not need to sample current of each bridge arm.

Description

The harmonic circulating current of a kind of MMC suppresses and direct current power fluctuation suppressing method

Technical field

The present invention relates to power electronic technology application and electric power system power transmission and distribution field, the harmonic circulating current particularly relating to a kind of MMC (modularization multi-level converter) suppresses and direct current power fluctuation suppressing method.

Background technology

Owing to adopting full-control type power electronic power device (such as IGBT and IEGT) and pulse-width modulation strategy, therefore, be that the Technology of HVDC based Voltage Source Converter of core has active reactive and independently to control and trend is reversed and the advantage such as to be facilitated with voltage source converter.Compared with the customary DC technology of transmission of electricity based on thyristor, Technology of HVDC based Voltage Source Converter is easy to form multi-terminal HVDC transmission, to passive network system power supply (such as isolated island, offshore drilling platform), new energy power generation grid-connection (such as wind power generation, photovoltaic generation etc.) can be particularly suitable for.Compared with 2-level or 3-level voltage source converter, modularization multi-level converter (Modular Multilevel Converter, MMC) by the mutual concatenation technology of submodule that operational voltage level is lower, effectively can avoid the tandeming problems of electronic power switch device, solve dynamic, the static state voltage equipoise problem of Tandem devices with this.

Circulation is the specific question that MMC (modularization multi-level converter) topological structure brings, and adverse effect is brought to the normal operation of MMC: first, circulation increases the effective value of bridge arm current, increase the levels of current flowing through submodule power device, the active loss of aggravation power device, the final Power operation scope reducing inverter unit; Secondly, the amplitude of circulation aggravation submodule voltage fluctuation, under the condition of assigned voltage fluctuation range, circulation increases submodule capacitance, increases the cost investment of converter.Therefore, in Practical Project, need to consider loop current suppression problem.Should be used for from current, loop current suppression technology adopts the loop current suppression controller (CN101854061A) based on negative phase-sequence 2 frequency multiplication rotating coordinate transformation mostly, this controller utilizes three-phase MMC circulation based on the feature of 2 harmonics, utilize negative phase-sequence 2 frequency multiplication rotating coordinate transformation, the negative phase-sequence 2 frequency multiplication circulation of 3 phase MMC is converted to 2 DC component, and then proportion of utilization integral controller is suppressed it.It is simply direct that the major advantage of this controller is to control thinking, and it is convenient to realize; But this controller must be sampled to the bridge arm current of MMC, and only can be applied in 3 phase MMC, in the asymmetric situation of AC system, the control effects of this controller is bad.For this reason, the harmonic circulating current that the present invention proposes a kind of MMC suppresses and direct current power fluctuation suppressing method, not only can suppress the inside circulation of MMC, can also improve the dynamic response characteristic of direct current; No matter in or fault in ac transmission system situation normal in AC system, all energy reliably workings, robustness is good; In addition, the method does not need to sample to each bridge arm current.

Summary of the invention

The harmonic circulating current that the present invention aims to provide a kind of MMC suppresses and direct current power fluctuation suppressing method, and the present invention not only can suppress the inside circulation of MMC, can also improve the dynamic response characteristic of direct current; No matter in or fault in ac transmission system situation normal in AC system, all energy reliably workings, robustness is good.

The object of the invention is to be achieved through the following technical solutions: the harmonic circulating current of MMC of the present invention suppresses and direct current power fluctuates suppressing method, comprise following process:

1) based on the modulation link of submodule voltage estimate: the submodule voltage estimating each brachium pontis, modulator approach is utilized to obtain the submodule number of each brachium pontis expectation input;

2) based on the proportional control link of direct current fluctuation feedback: each brachium pontis of proportional control link correction expects the submodule number dropped into.

Above-mentioned steps 1) in, estimate that the submodule voltage of each brachium pontis is independently carry out in each facies unit of modularization multi-level converter, concrete methods of realizing is as follows:

In formula (1) ~ (2): u _cpxand u _cnxbe respectively the submodule voltage estimate value of the upper and lower brachium pontis of MMC xth phase, U _creffor the specified runtime value of submodule capacitor voltage, ω is fundamental frequency angular frequency, ε _1xand ε _2xbe respectively the submodule voltage fundamental frequency fluctuation of MMC xth phase and the amplitude of 2 double-frequency fluctuation, with be respectively its initial phase angle, ε _1x, ε _2x, with respectively by following various calculating:

${\mathrm{\ϵ}}_{2x}=\frac{{\mathrm{NS}}_x}{4{\mathrm{\ωCU}}_{\mathrm{dc}}^2}---\left(4\right)$

In formula (3) ~ (9): N is the brachium pontis submodule number of MMC, and C is submodule capacitance, U _dcfor direct voltage, m _xfor the modulation ratio of MMC xth phase, S _xfor the apparent capacity of MMC xth phase, with be respectively the internal virtual electromotive force of MMC xth phase and the initial phase angle of output current.

After obtaining the submodule voltage estimate value of the upper and lower brachium pontis of MMC xth phase according to formula (1) ~ (2) estimation, recycle certain modulator approach and obtain the submodule number N that the mutually upper and lower brachium pontis of MMC xth expects input _px1and N _nx1:

$N_{\mathrm{px}1}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}-e_{\mathrm{refx}}}{u_{\mathrm{cpx}}}\right)---\left(10\right)$

$N_{\mathrm{nx}1}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}+e_{\mathrm{refx}}}{u_{\mathrm{cnx}}}\right)---\left(11\right)$

In above formula: N _px1and N _nx1the upper and lower brachium pontis being respectively MMC xth phase expects the submodule number of input; Mod (x) represents modulation system, in concrete implementation procedure, adopts nearest level to approach modulation, phase-shifting carrier wave modulation, the stacked modulation of carrier wave; e _refxfor the reference voltage of the internal virtual electromotive force of MMC xth phase, the power for MMC controls, and this control adopts ripe control method.

Above-mentioned steps 2) in, the main purpose of proportional control link revises the brachium pontis reference voltage of MMC, and concrete methods of realizing is as follows: first will measure the DC current values i obtained _dccarry out filtering through high pass filter HPF (s), obtain the wave component of direct current; Then the wave component of direct current is inputed to proportional control link; The output of proportional control link is the correction value Δ u of brachium pontis reference voltage _ref;

Obtain the correction value Δ u of brachium pontis reference voltage _refafterwards, on the basis of formula (9) ~ (10), the submodule number that the mutually upper and lower brachium pontis of MMC xth needs to drop into can be revised further.The mutually upper and lower brachium pontis of revised MMC xth needs the submodule number N dropped into _pxand N _nxdetermination mode be:

$N_{\mathrm{px}}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}-e_{\mathrm{refx}}-\mathrm{\Δ}{u}_{\mathrm{ref}}}{u_{\mathrm{cpx}}}\right)---\left(12\right)$

$N_{\mathrm{nx}}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}+e_{\mathrm{refx}}-\mathrm{\Δ}{u}_{\mathrm{ref}}}{u_{\mathrm{cnx}}}\right)---\left(13\right)$

Beneficial effect of the present invention is: the present invention, for suppressing harmonic circulating current and the direct current power fluctuation of MMC, not only can suppress the inside circulation of MMC, can also improve the dynamic response characteristic of direct current; No matter in or fault in ac transmission system situation normal in AC system, all energy reliably workings, robustness is good; In addition, the method does not need to sample to each bridge arm current.Be specially adapted to the fields such as power electronics drive technology, flexible AC transmission technology and Technology of HVDC based Voltage Source Converter.

Accompanying drawing explanation

Fig. 1 is the schematic diagram of proportional control link implementation method of the present invention;

Fig. 2 is the comparison diagram between the estimation result of submodule voltage algorithm for estimating of the present invention and submodule voltage actual value;

Fig. 3 is in simulation model of the present invention, does not consider circulation oscillogram during loop current suppression

Fig. 4 is in the present invention when the proportionality coefficient of proportional control link gets different numerical value, loop current suppression effect contrast figure

Fig. 5 is when AC system is asymmetric, the control effects schematic diagram of method proposed by the invention.

Embodiment

Below in conjunction with embodiment, the present invention is described further.

The harmonic circulating current of a kind of MMC of the present invention suppresses and direct current power fluctuation suppressing method, comprise based on the modulation link of submodule voltage estimate and the proportional control link two parts based on direct current fluctuation feedback, first modulation link estimates the submodule voltage of each brachium pontis, then utilizes certain modulation technique to obtain the submodule number of each brachium pontis expectation input.The each brachium pontis of proportional control link correction expects the submodule number dropped into.

Method of the present invention effectively can suppress the harmonic circulating current of MMC.The each brachium pontis of proportional control link correction expects the submodule number dropped into, and not only can eliminate the circulation fluctuation brought because of estimation error further, effectively can also increase the inside equivalent damping of MMC, improve the dynamic response characteristic of direct current

Submodule voltage algorithm for estimating of the present invention is based on single-phase MMC in implementation process, and that is, algorithm for estimating is independently carry out in each facies unit of MMC.What for the xth of MMC algorithm for estimating is described mutually below realizes principle, specific as follows:

In formula (1) ~ (2): u _cpxand u _cnxbe respectively the submodule voltage estimate value of the upper and lower brachium pontis of MMC xth phase, U _creffor the specified runtime value of submodule capacitor voltage, ω is fundamental frequency angular frequency.ε _1xand ε _2xbe respectively the submodule voltage fundamental frequency fluctuation of MMC xth phase and the amplitude of 2 double-frequency fluctuation, with be respectively its initial phase angle.ε _1x, ε _2x, with respectively by following various calculating:

${\mathrm{\ϵ}}_{2x}=\frac{{\mathrm{NS}}_x}{4{\mathrm{\ωCU}}_{\mathrm{dc}}^2}---\left(4\right)$

In formula (3) ~ (9): N is the brachium pontis submodule number of MMC, and C is submodule capacitance, U _dcfor direct voltage, m _xfor the modulation ratio of MMC xth phase, S _xfor the apparent capacity of MMC xth phase, with be respectively the internal virtual electromotive force of MMC xth phase and the initial phase angle of output current.

After obtaining the submodule voltage estimate value of the upper and lower brachium pontis of MMC xth phase according to formula (1) ~ (2) estimation, recycle certain modulator approach and obtain the submodule number N that the mutually upper and lower brachium pontis of MMC xth expects input _px1and N _nx1:

$N_{\mathrm{px}1}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}-e_{\mathrm{refx}}}{u_{\mathrm{cpx}}}\right)---\left(10\right)$

$N_{\mathrm{nx}1}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}+e_{\mathrm{refx}}}{u_{\mathrm{cnx}}}\right)---\left(11\right)$

In above formula: N _px1and N _nx1the upper and lower brachium pontis being respectively MMC xth phase expects the submodule number of input; Mod (x) represents modulation system, in concrete implementation procedure, adopts nearest level to approach modulation, phase-shifting carrier wave modulation, the stacked modulation of carrier wave etc.; e _refxfor the reference voltage of the internal virtual electromotive force of MMC xth phase, the power for MMC controls, and can adopt ripe control method.

The main purpose of proportional control link revises the brachium pontis reference voltage of MMC, concrete methods of realizing is as shown in Figure 1: first carrying out filtering by measuring the DC current values idc obtained through high pass filter HPF (s), obtaining the wave component of direct current; Then the wave component of direct current is inputed to proportional control link (proportionality coefficient is K); The output of proportional control link is the correction value Δ u of brachium pontis reference voltage _ref.

Obtain the correction value Δ u of brachium pontis reference voltage _refafterwards, on the basis of formula (9) ~ (10), the submodule number that the mutually upper and lower brachium pontis of MMC xth needs to drop into can be revised further.The mutually upper and lower brachium pontis of revised MMC xth needs the submodule number N dropped into _pxand N _nxdetermination mode be:

$N_{\mathrm{px}}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}-e_{\mathrm{refx}}-\mathrm{\Δ}{u}_{\mathrm{ref}}}{u_{\mathrm{cpx}}}\right)---\left(12\right)$

$N_{\mathrm{nx}}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}+e_{\mathrm{refx}}-\mathrm{\Δ}{u}_{\mathrm{ref}}}{u_{\mathrm{cnx}}}\right)---\left(13\right)$

Now for concrete Digital Simulation example, action effect of the present invention is described, simulation example adopts three-phase (point A phase, B phase, C phase) modular multilevel converter type inverter.As seen from Figure 2, closely, error is less for the actual value of the voltage estimate value that submodule voltage estimate algorithm proposed by the invention obtains and submodule voltage.Figure 3 shows that A phase circulation oscillogram when not considering loop current suppression, Figure 4 shows that A phase circulation oscillogram when adopting the present invention to suppress circulation.Comparison diagram 3 and Fig. 4 can find out, the present invention can the inside circulation of remarkable suppression module multilevel converter.In Fig. 4, during time t≤1s, Proportional coefficient K=0; During 1s < t≤1.1s, K=10; During 1.1s < t≤1.2s, K=20; During t > 1.2s, K=30; Can find out, proportionality coefficient is larger, and loop current suppression effect is better.As seen from Figure 5, in the asymmetric situation of AC system, the present invention still can the circulation of effective suppression module multilevel converter, and direct current also can be suppressed to fluctuate.

Claims (3)

1. the harmonic circulating current of MMC suppresses and a direct current power fluctuation suppressing method, it is characterized in that comprising following process:

1) based on the modulation link of submodule voltage estimate: the submodule voltage estimating each brachium pontis, modulator approach is utilized to obtain the submodule number of each brachium pontis expectation input;

2) based on the proportional control link of direct current fluctuation feedback: each brachium pontis of proportional control link correction expects the submodule number dropped into.

2. the harmonic circulating current of MMC according to claim 1 suppresses and direct current power fluctuation suppressing method, it is characterized in that above-mentioned steps 1) in, estimate that the submodule voltage of each brachium pontis is independently carry out in each facies unit of MMC, concrete methods of realizing is as follows:

In formula (1) ~ (2): u _cpxand u _cnxbe respectively the submodule voltage estimate value of the upper and lower brachium pontis of MMC xth phase, U _creffor the specified runtime value of submodule capacitor voltage, ω is fundamental frequency angular frequency, ε _1xand ε _2xbe respectively the submodule voltage fundamental frequency fluctuation of MMC xth phase and the amplitude of 2 double-frequency fluctuation, with be respectively its initial phase angle, ε _1x, ε _2x, with respectively by following various calculating:

${\mathrm{\&epsiv;}}_{2x}=\frac{{\mathrm{NS}}_x}{4\mathrm{\&omega;}{\mathrm{CU}}_{\mathrm{dc}}^2}---\left(4\right)$

In formula (3) ~ (9): N is the brachium pontis submodule number of MMC, and C is submodule capacitance, U _dcfor direct voltage, m _xfor the modulation ratio of MMC xth phase, S _xfor the apparent capacity of MMC xth phase, with be respectively the internal virtual electromotive force of MMC xth phase and the initial phase angle of output current;

After obtaining the submodule voltage estimate value of the upper and lower brachium pontis of MMC xth phase according to formula (1) ~ (2) estimation, recycle certain modulator approach and obtain the submodule number N that the mutually upper and lower brachium pontis of MMC xth expects input _px1and N _nx1:

$N_{\mathrm{px}1}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}-e_{\mathrm{refx}}}{u_{\mathrm{cpx}}}\right)---\left(10\right)$

$N_{\mathrm{nx}1}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}-e_{\mathrm{refx}}}{u_{\mathrm{cnx}}}\right)---\left(11\right)$

In above formula: N _px1and N _nx1the upper and lower brachium pontis being respectively MMC xth phase expects the submodule number of input; Mod (x) represents modulation system, in concrete implementation procedure, adopts nearest level to approach modulation, phase-shifting carrier wave modulation, the stacked modulation of carrier wave; e _refxfor the reference voltage of the internal virtual electromotive force of MMC xth phase, the power for MMC controls, and this control adopts ripe control method.

3. the harmonic circulating current of MMC according to claim 1 suppresses and direct current power fluctuation suppressing method, it is characterized in that above-mentioned steps 2) implementation method of proportional control link is as follows: first will the DC current values i that obtains of measurement _dccarry out filtering through high pass filter HPF (s), obtain the wave component of direct current; Then the wave component of direct current is inputed to proportional control link; The output of proportional control link is the correction value Δ u of brachium pontis reference voltage _ref;

Obtain the correction value Δ u of brachium pontis reference voltage _refafterwards, can revise the submodule number that the mutually upper and lower brachium pontis of MMC xth needs to drop on the basis of formula (9) ~ (10) further, the mutually upper and lower brachium pontis of revised MMC xth needs the submodule number N dropped into _pxand N _nxdetermination mode be:

$N_{\mathrm{px}}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}-e_{\mathrm{refx}}-{\mathrm{\&Delta;u}}_{\mathrm{ref}}}{u_{\mathrm{cpx}}}\right)---\left(12\right)$

$N_{\mathrm{nx}}=\mathrm{Mod}\left(\frac{{0.5U}_{\mathrm{dc}}-e_{\mathrm{refx}}-{\mathrm{\&Delta;u}}_{\mathrm{ref}}}{u_{\mathrm{cnx}}}\right).---\left(13\right)$