CN104716656A - Cascading static synchronous reactive compensator topology with energy exchange unit and control method thereof - Google Patents

Abstract

The invention discloses a cascading static synchronous reactive compensator topology with an energy exchange unit and a control method thereof, and relates to the power unit capacitor voltage balance control technology of a cascaded reactive compensation device. The cascading static synchronous reactive compensator topology with the energy exchange unit and the control method thereof aim at solving the problems existing in the method adopted by an existing star connection cascade converter for the capacitor voltage unbalance among phases that the output current of the converter changes and breaks the control reliability and extracting of positive and negative sequence components has lowpass filtering and influences the dynamic performance of the cascading static synchronous compensator. The cascading static synchronous reactive compensator topology comprises N-1 power modules which adopt the star connection, and the N is a positive integer; the N power module adopts the energy exchange unit EEU, the energy exchange unit EEU is connected with a star point, the energy exchange unit EEU comprises a three-phase bridge and two common capacitors, and the three-phase bridge and the common capacitors adopt the star connection. The cascading static synchronous reactive compensator topology with the energy exchange unit and the control method thereof are used in the cascaded reactive compensation device based on the power unit.

Description

There is cascade Static Synchronous reactive-load compensator topology and the control method thereof of energy conversion unit

Technical field

The present invention relates to the power cell capacitor voltage balance control technology of cascade connection type reactive power compensator.

Background technology

Based on the reactive power compensator of cascade of power units due to without the need to step-up transformer, device volume and power consumption can be reduced, in middle pressure (common 6kV, 10kV) power quality controlling, occupy more and more important position.In practical application, cascade reactive power compensator often selects star connection, as shown in Figure 1.Compare with delta connection, the submodule number required for same nominal voltage devices is less, and converter output current harmonics is less.

The subject matter that star connection cascade converter exists is that three alternate capacitor voltage balance problems are comparatively difficult to resolve certainly.When each power model of cascade STATCOM is in suspended state, when carrying out reactive power compensation, a power frequency period internal power module charge-discharge energy is identical, and module capacitance voltage can keep stable.But under non-ideal conditions, such as current break, when module device parameter is inconsistent, module capacitance voltage is by imbalance, if do not take responsible measures, this imbalance causes convertor controls failure by expanding further.

Module capacitance Voltage unbalance comprises two parts: the capacitance voltage between single-phase internal module is uneven, and capacitance voltage imbalance between phase and phase.For the former, there are many solutions at present, wherein most popular solution comprises sequence, voltage modulated ripple compensates and the direct method of adjustment of module conducting duty ratio, its overall thought is carries out trickle adjustment to the charge and discharge process of each module, the holding capacitor balance of voltage, converter output voltage can not be changed simultaneously, substantially solve this problem by these methods.

But uneven for capacitance voltage between phase and phase, current solution is also few.From existing document, main solution is by Japanese scholars Akagi put forward the methods, the i.e. every mutually total output voltage of adjustment, by the voltage that superposition is consistent with converter output current phase place, in the real component that three alternate generations are different, to change the stored energy between three-phase, the capacitance voltage between balance three-phase.But the voltage of the method superposition can cause the change of converter output current, and converter three-phase output voltage also no longer balances, and its neutral potential differs and is decided to be zero, this destroys the reliability of control simultaneously.The method that the domestic scholar of having is separated by positive-negative sequence controls unsymmetrical current voltage, but the extraction of positive-negative sequence component exists low-pass filtering link, can have a strong impact on the dynamic property of cascade STATCOM.

Summary of the invention

The present invention seeks to, in order to solve existing star connection cascade converter, the change of converter output current, destruct limit reliability are existed for the method that capacitance voltage imbalance between phase and phase adopts, there is low-pass filtering, affect the problem of cascade STATCOM dynamic property in the extraction of positive-negative sequence component, provides a kind of cascade Static Synchronous reactive-load compensator topology and the control method thereof with energy conversion unit.

The cascade Static Synchronous reactive-load compensator topology with energy conversion unit of the present invention, this cascade Static Synchronous reactive-load compensator topology comprises N-1 power model, and N is positive integer, and N-1 power model adopts Y-connection; N number of power model of this cascade Static Synchronous reactive-load compensator topology adopts energy exchange unit EEU, and energy exchange unit EEU is connected with star point, and energy exchange unit EEU comprises a three-phase bridge and two shared electric capacity C _n1, C _n2, three-phase bridge and two shared electric capacity C _n1, C _n2adopt Y-connection, the Y-connection point O of energy exchange unit EEU is zero-potential point.

The control method with the cascade Static Synchronous reactive-load compensator topology of energy conversion unit of the present invention, the detailed process of this control method is:

Step 1, detected energy crosspoint EEU DC voltage U _dCN, by U _dCNwith DC side reference voltage U _dCref ^*subtract each other, difference regulates the output voltage adjustment angle θ obtaining energy exchange unit EEU through PI ₀;

Step 2, detection A, B, C three phase capacitance voltage, obtain three phase capacitance average voltage U _avgx, by U _avgxsubtract each other with three phase capacitance voltage reference value, difference regulates through PI and obtains each phase output voltage adjustment angle θ _x;

Step 3, detection three-phase current i _a, i _b, i _c, obtain i through 3/2 conversion _dand i _q, and then obtain the electrical degree θ of output current _i;

Step 4, according to the output voltage of energy exchange unit EEU adjustment angle θ ₀with each phase output voltage adjustment angle θ _xdifference be superimposed with the electrical degree θ of output current _i, then deduct pi/2, obtain energy exchange unit EEU output voltage angle θ ^*;

Step 5, basis obtain energy exchange unit EEU output voltage angle θ ^*square wave is adopted to control to energy exchange unit EEU;

Step 6, adopt PSPWM to modulate to the capacitance voltage of every phase front N-1 power model, the balance completing the cascade Static Synchronous reactive-load compensator topology with energy conversion unit controls.

Advantage of the present invention: the present invention discloses a kind of New Topological and modulation thereof of cascade STATCOM (STATCOM) and balances control strategy.Different from traditional topology, which increase an energy exchange unit be made up of three-phase bridge and common capacitor (EEU) at contiguous star tie point place.Wherein, energy exchange unit adopts square-wave frequency modulation, and all the other modules adopt Carrier-shifted PWM (PSPWM) modulation, and by changing the modulating wave electromagnetic point of energy exchange unit, realize the converter capacitive coupling balance of voltage.Topology proposed by the invention and control method are conducive to the capacitor voltage balance of cascade reactive-load compensator, by the improvement of topological structure, initiatively increase the Energy Transfer that an energy exchange unit is used between three-phase, and by special modulation and the control realization capacitive coupling balance of voltage.Relative to traditional capacitance voltage balancing control method, the method, by adding sub-fraction circuit, makes whole system more reliable and more stable.

Accompanying drawing explanation

Fig. 1 is the structured flowchart of typical cascade Static Synchronous reactive-load compensator topology;

Fig. 2 is the structured flowchart with the cascade Static Synchronous reactive-load compensator topology of energy conversion unit of the present invention;

Fig. 3 is energy exchange unit EEU and other unit output voltages, the overall output voltage of converter and output current relation schematic diagram;

Fig. 4 is the FB(flow block) of the control method of the cascade Static Synchronous reactive-load compensator topology with energy conversion unit;

Fig. 5 is a phase output voltage schematic diagram of energy exchange unit EEU and converter;

Fig. 6 is load current and offset current schematic diagram;

Fig. 7 is average capacitor voltage schematic diagram;

Fig. 8 is the capacitance voltage of three capacitance voltages and energy exchange unit EEU in a phase.

Embodiment

Embodiment one: present embodiment is described below in conjunction with Fig. 2, there is described in present embodiment the cascade Static Synchronous reactive-load compensator topology of energy conversion unit, this cascade Static Synchronous reactive-load compensator topology comprises N-1 power model, N is positive integer, and N-1 power model adopts Y-connection; N number of power model of this cascade Static Synchronous reactive-load compensator topology adopts energy exchange unit EEU, and energy exchange unit EEU is connected with star point, and energy exchange unit EEU comprises a three-phase bridge and two shared electric capacity C _n1, C _n2, three-phase bridge and two shared electric capacity C _n1, C _n2adopt Y-connection, the Y-connection point O of energy exchange unit EEU is zero-potential point.

Embodiment two: present embodiment is described below in conjunction with Fig. 4, have the control method of the cascade Static Synchronous reactive-load compensator topology of energy conversion unit described in present embodiment, the detailed process of this control method is:

Step 1, detected energy crosspoint EEU DC voltage U _dCN, by U _dCNwith DC side reference voltage U _dCref ^*subtract each other, difference regulates the output voltage adjustment angle θ obtaining energy exchange unit EEU through PI ₀;

Step 2, detection A, B, C three phase capacitance voltage, obtain three phase capacitance average voltage U _avgx, by U _avgxsubtract each other with three phase capacitance voltage reference value, difference regulates through PI and obtains each phase output voltage adjustment angle θ _x;

Step 3, detection three-phase current i _a, i _b, i _c, obtain i through 3/2 conversion _dand i _q, and then obtain the electrical degree θ of output current _i;

Step 4, according to the output voltage of energy exchange unit EEU adjustment angle θ ₀with each phase output voltage adjustment angle θ _xdifference be superimposed with the electrical degree θ of output current _i, then deduct pi/2, obtain energy exchange unit EEU output voltage angle θ ^*;

Step 5, basis obtain energy exchange unit EEU output voltage angle θ ^*square wave is adopted to control to energy exchange unit EEU;

Step 6, adopt PSPWM to modulate to the capacitance voltage of every phase front N-1 power model, the balance completing the cascade Static Synchronous reactive-load compensator topology with energy conversion unit controls.

Embodiment three: present embodiment is described further embodiment two, the output voltage adjustment angle θ of adjusting energy crosspoint EEU described in step 1 ₀the capacitance voltage that can maintain energy exchange unit EEU is stablized.

Embodiment four: present embodiment is described further embodiment two, regulates each phase output voltage to adjust angle θ described in step 2 _xthe capacitive coupling balance of voltage can be realized.

Embodiment five: present embodiment is described further embodiment two, obtains energy exchange unit EEU output voltage angle θ described in step 4 ^*the exchange of energy exchange unit EEU power component energy can be controlled, thus change the stored energy of each phase electric capacity.

Embodiment six: present embodiment is described further embodiment two, adopts PSPWM to carry out modulating the capacitor voltage balance can guaranteeing every phase front N-1 power model to the capacitance voltage of every phase front N-1 power model described in step 6.

Embodiment seven: present embodiment is described further embodiment two, described in step 5 to the modulation system that energy exchange unit EEU carries out square wave control is:

$\left\{\begin{array}{c}{S}_N=\left\{\begin{array}{cc}1& 0\&le;{\mathrm{\&theta;}}^{*}<\mathrm{\&pi;}\\ -1& \mathrm{\&pi;}\&le;{\mathrm{\&theta;}}^{*}<2\mathrm{\&pi;}\end{array}\right.\\ M_x=\left(\frac{v_{\mathrm{refA}}-S_N\frac{V_{\mathrm{DCN}}}{2}}{\underset{i=1}{\overset{N-1}{\mathrm{\&Sigma;}}}{V}_{\mathrm{DCi}}}\right),x=\mathrm{1,2},...,N-1\end{array}\right.$

Wherein, S _nthe switch function of energy exchange unit EEU, M _xthe modulating wave of N-1 power model, v _refAfor A phase output voltage amplitude, V _dCNthe DC voltage of N-1 power model, V _dCirepresent the capacitance voltage in i-th power model.

The present invention proposes a kind of cascade STATCOM topological structure with energy exchange unit, as Fig. 2.Different with traditional topology (Fig. 1), at contiguous star tie point place, which increase an energy exchange unit EEU be made up of three-phase bridge and common capacitor.Namely the size of capacitance voltage represents the energy stored in electric capacity, in New Topological, by can exchange the energy of capacitance stores in three-phase to the control of EEU, thus makes capacitance voltage adjusted.

Illustrate below in conjunction with Fig. 2-Fig. 8, the output voltage of the every phase of cascade converter be all mutually in submodule output voltage and, each submodule output voltage controls by modulating.The output voltage of converter can be represented by the capacitance voltage of each module and the index of modulation:

U _j=(V _dC1m ₁+ V _dC2m ₂+ ... + V _{dC (N-1)}m _n-1)+V _dCNm _n, j=A, B or C (1)

A front N-1 module is all H bridge type topology, and its three-phase output voltage is modulated by PWM method such as phase-shifting PWM (PSPWM).EEU also can adopt PWM, but it only has 2 level to export (+V _dCN/ 2 and-V _dCN/ 2), this is the place different with other modules, so it is difficult to participate in the modulation (such as PSPWM) of other modules.As can be seen from above formula also, if do not offset switch harmonic by other modules, N number of module work needs relatively high switching frequency when PWM mode.In order to reduce switching frequency and harmonic wave, the present invention adopts square-wave frequency modulation to EEU module, and other modules adopt PSPWM modulation.

Suppose that A phase output voltage amplitude is v _refA, during employing square-wave frequency modulation, its modulation system is such as formula shown in (2), θ in formula ^*it is EEU module modulating wave electromagnetic point.S _nthe switch function of N number of module, M _xbe the modulating wave of other modules, being used for same phase shift triangular carrier compares to obtain corresponding switching pulse.

Adjustment θ ^*angle can realize capacitor voltage balance, and principle as shown in Figure 3.For A phase, when reactive power compensation, suppose that A phase output current is i _a, output voltage is u _a, then i _awith u _aelectromagnetic point difference 90 degree.Suppose that EEU unit output voltage differs with current angle as θ, known when θ is pi/2,

$W={\&Integral;}_0^{2\mathrm{\&pi;}}{S}_N{I}_A\sin \left(\mathrm{\&omega;t}\right)d\left(\mathrm{\&omega;t}\right)=0---\left(3\right)$

And when 0< θ < pi/2, W>0; As-pi/2 < θ <0, W<0.Therefore, by controlling θ, energy exchange unit can be made to charge or electric discharge.Meanwhile, because all the other submodule modulating waves are such as formula shown in (2), namely the total output voltage of A phase remains unchanged, and during pure reactive power compensation, it differs 90 degree with current angle, and in a power frequency period, total meritorious of A phase is exchanged for 0.All the other submodule output voltages are that reference voltage deducts energy exchange unit voltage, therefore, if the energy exchange of a power frequency period self-energy crosspoint is greater than 0, then the energy exchange of all the other submodules will be less than 0, like this by the output voltage angle θ of adjusting energy crosspoint, the object of all the other module capacitance voltages of adjustment A phase can be reached.

Meanwhile, in order to keep energy exchange unit (EEU) its voltage to stablize, also needing to control the energy exchange of this unit, equally also being realized by its reference voltage of control and output current angle.

Finally, the capacitor voltage balance scheme of this converter as shown in Figure 4.First, closed-loop control is carried out to the voltage of energy exchange unit (EEU), obtain EEU output voltage adjustment angle θ ₀, then PI closed-loop control is carried out to three phase capacitance average voltage, obtains the output voltage adjustment angle θ of each phase EEU _x, finally, the angle θ of each phase current on this adjustment angular stack _i, and deduct pi/2, namely obtain EEU output voltage angle θ ^*.

${\mathrm{\&theta;}}^{*}={\mathrm{\&theta;}}_0-{\mathrm{\&theta;}}_x+{\mathrm{\&theta;}}_i-\frac{\mathrm{\&pi;}}{2}---\left(4\right)$

Afterwards, carry out modulating according to shown in formula (2).Finally, in order to the capacitance voltage of other power cells in equilibrium phase, to adopt in other documents conventional method, namely the modulating wave of each power cell is adjusted further, the component that the difference of superposition and this cell voltage and this phase average cell voltage is proportional.

Claims (7)

1. have the cascade Static Synchronous reactive-load compensator topology of energy conversion unit, this cascade Static Synchronous reactive-load compensator topology comprises N-1 power model, and N is positive integer, and N-1 power model adopts Y-connection; It is characterized in that, N number of power model of this cascade Static Synchronous reactive-load compensator topology adopts energy exchange unit EEU, and energy exchange unit EEU is connected with star point, and energy exchange unit EEU comprises a three-phase bridge and two shared electric capacity C _n1, C _n2, three-phase bridge and two shared electric capacity C _n1, C _n2adopt Y-connection, the Y-connection point O of energy exchange unit EEU is zero-potential point.

2., based on the control method of cascade Static Synchronous reactive-load compensator topology described in claim 1 with energy conversion unit, it is characterized in that, the detailed process of this control method is:

Step 1, detected energy crosspoint EEU DC voltage U _dCN, by U _dCNwith DC side reference voltage U _dCref ^*subtract each other, difference regulates the output voltage adjustment angle θ obtaining energy exchange unit EEU through PI ₀;

Step 2, detection A, B, C three phase capacitance voltage, obtain three phase capacitance average voltage U _avgx, by U _avgxsubtract each other with three phase capacitance voltage reference value, difference regulates through PI and obtains each phase output voltage adjustment angle θ _x;

Step 3, detection three-phase current i _a, i _b, i _c, obtain i through 3/2 conversion _dand i _q, and then obtain the electrical degree θ of output current _i;

Step 4, according to the output voltage of energy exchange unit EEU adjustment angle θ ₀with each phase output voltage adjustment angle θ _xdifference be superimposed with the electrical degree θ of output current _i, then deduct pi/2, obtain energy exchange unit EEU output voltage angle θ ^*;

Step 5, basis obtain energy exchange unit EEU output voltage angle θ ^*square wave is adopted to control to energy exchange unit EEU;

Step 6, adopt PSPWM to modulate to the capacitance voltage of every phase front N-1 power model, the balance completing the cascade Static Synchronous reactive-load compensator topology with energy conversion unit controls.

3. the control method with the cascade Static Synchronous reactive-load compensator topology of energy conversion unit according to claim 2, is characterized in that, the output voltage adjustment angle θ of adjusting energy crosspoint EEU described in step 1 ₀the capacitance voltage that can maintain energy exchange unit EEU is stablized.

4. the control method with the cascade Static Synchronous reactive-load compensator topology of energy conversion unit according to claim 2, is characterized in that, regulates each phase output voltage to adjust angle θ described in step 2 _xthe capacitive coupling balance of voltage can be realized.

5. the control method with the cascade Static Synchronous reactive-load compensator topology of energy conversion unit according to claim 2, is characterized in that, obtain energy exchange unit EEU output voltage angle θ described in step 4 ^*the exchange of energy exchange unit EEU power component energy can be controlled, thus change the stored energy of each phase electric capacity.

6. the control method with the cascade Static Synchronous reactive-load compensator topology of energy conversion unit according to claim 2, it is characterized in that, described in step 6, adopt PSPWM to carry out modulating the capacitor voltage balance can guaranteeing every phase front N-1 power model to the capacitance voltage of every phase front N-1 power model.

7. the control method with the cascade Static Synchronous reactive-load compensator topology of energy conversion unit according to claim 2, is characterized in that, described in step 5 to the modulation system that energy exchange unit EEU carries out square wave control is:

$\left\{\begin{array}{c}{S}_N=\left\{\begin{array}{cc}1& 0\&le;{\mathrm{\&theta;}}^{*}<\mathrm{\&pi;}\\ -1& \mathrm{\&pi;}\&le;{\mathrm{\&theta;}}^{*}<2\mathrm{\&pi;}\end{array}\right.\\ M_x=\left(\frac{v_{\mathrm{refA}}-S_N\frac{V_{\mathrm{DCN}}}{2}}{\underset{i=1}{\overset{N-1}{\mathrm{\&Sigma;}}}{V}_{\mathrm{DCi}}}\right),x=\mathrm{1,2},...,N-1\end{array}\right.$

Wherein, S _nthe switch function of energy exchange unit EEU, M _xthe modulating wave of N-1 power model, v _refAfor A phase output voltage amplitude, V _dCNthe DC voltage of N-1 power model, V _dCirepresent the capacitance voltage in i-th power model.