CN104659802A - Coordination control method of VSC-HVDC (voltage source converter based high voltage direct current) alternating-current voltage-frequency for improving transient stability of alternating-current system - Google Patents

Abstract

The invention discloses a coordination control method of VSC-HVDC (voltage source converter based high voltage direct current) alternating-current voltage-frequency for improving the transient stability of an alternating-current system. An alternating-current voltage-frequency coordination control target is increased in a transient state control target of a VSC station; the control strategy can be involved in transient state control of voltage and frequency of the alternating-current system by utilizing reactive and active rapid adjustment capability of VSC-HVDC respectively on the premise of guaranteeing the running safety of VSC-HVDC equipment. Based on the strategy, the invention designs a VSC controller structure capable of controlling the target priority ranking seriously, so that the transient state stability of voltage and frequency of a weak alternating-current power grid linked to the VSC can be improved effectively.

Description

A kind of VSC-HVDC alternating voltage-frequency coordination control method improving AC system transient stability

Technical field

The invention belongs to technical field of electric power system control, be specifically related to a kind of VSC-HVDC alternating voltage-frequency coordination control method improving AC system transient stability.

Background technology

Voltage source converter type DC transmission system (voltage source converter based high voltagedirect current, VSC-HVDC) adopting can from the power electronic device turned off, do not need additional commutation voltage to realize commutation, therefore can be used for connecting weak AC network.

In the electric network composition of current China, regional partial electric grid is connected with major network mainly through alternating current circuit, when regional partial electric grid and major network exchange that contact is more weak, passage trend is heavier time, easily there is transient stability and the problem such as small disturbed stability is poor in system.Along with the maturation gradually of VSC-HVDC technology, a kind of selection is newly become with the regional partial electric grid of the asynchronous connection of VSC-HVDC and major network, the outstanding advantage of this connected mode is to eradicate low-frequency oscillation problem, Asynchronous Interconnection structure can either avoid regional partial electric grid unit to participate in the low frequency oscillation mode of major network, in addition, even if the problem such as low-frequency oscillation or power forced oscillation appears in regional partial electric grid inside, fault also can not conduct to major network.This technical advantage is specially adapted to the low-frequency oscillation problem that small power station of solution Southwestern China portion City Regions is caused by AC synchronous networking mode.

Although VSC-HVDC Asynchronous Interconnection scheme can solve the low frequency oscillations problem that area power grid causes, but when area power grid is more weak, this weak AC system adopts during VSC-HVDC Asynchronous Interconnection also can run into new problem, as problems such as Voltage Instability, frequency unstabilitys.The area power grid comprising a large amount of small hydropower system lacks large-scale power supply, and when small hydropower system electrical network is by VSC-HVDC Asynchronous Interconnection, small hydropower system electrical network belongs to typical weak AC system, is characterized in: (1) VSC current conversion station short circuit ratio is low; (2) electrical network mechanical inertia is little; (3) AC network shelf structure is weak.Because the grid structure of small hydropower system electrical network is weak, part N-1 AC fault will cause grid disconnection or power plant's off-the-line, thus cause the unbalanced power of small hydropower system electrical network and VSC-HVDC short circuit ratio to reduce, and then may cause voltage or frequency unstability.

When VSC-HVDC is used for above-mentioned small hydropower system electrical network Asynchronous Interconnection scene, need expand the control objectives of VSC-HVDC, to improve small hydropower system line voltage and frequency stability.

Current VSC (voltage source converter) extensively adopts Current Vector Control technology, and this technology makes VSC current conversion station have the control dimension of d, q axle two decoupling zeros, can realize respectively gaining merit, idle control.Existing document is studied control strategy during VSC-HVDC connection weak AC system at present.ZhangLidong, Harnefors Lennart, Nee Hans-Peter is Interconnection oftwo veryweak AC systems by VSC-HVDC links using power-synchronization control (IEEETransactions on Power Systems at title, 2011, 26 (1): 344-355) a kind of power synchronous control strategy is proposed in document, this strategy enables VSC automatically control the phase place of output AC voltage, alternating voltage reference phase place is provided without the need to extraneous AC system, therefore the scene that VSC is coupled to weak AC system is applicable to.Zhu Ruike, Wang Yuhong, Li Xingyuan etc. are the additional frequency control strategy (Automation of Electric Systems for VSC-HVDC interconnected systems at title, 2014,38 (16): 81-87) determine in Active Power Controller, to introduce frequency-active power and direct voltage-active power slope characteristics to VSC in document, to pull-in frequency-direct voltage slope characteristics in VSC constant DC voltage control device, this control method can improve the frequency stability of the two ends AC system that VSC-HVDC connects.

But, current VSC control strategy do not consider each control objectives in transient process (1, VSC-HVDC direct voltage keeps stable; 2, its valve side fault current can be controlled during VSC AC fault; 3, VSC ac bus voltage control is in safe range; 4, VSC AC system frequency controls in safe range) situation that may clash, therefore may be unfavorable for the transient stability of the AC system that VSC-HVDC connects.

Summary of the invention

For the above-mentioned technical problem existing for prior art, the invention provides a kind of VSC-HVDC alternating voltage-frequency coordination control method improving AC system transient stability, strictly can realize the VSC controller architecture of control objectives prioritization, effectively improve voltage and the frequency transient stability of weak AC network.

Improve VSC-HVDC alternating voltage-frequency coordination control method of AC system transient stability, comprise the steps:

(1) for the VSC of weak AC system side in VSC-HVDC, detect and obtain its net the AC system frequency f of side, the effective value u of three-phase alternating voltage _s, exchange active-power P _s, exchange reactive power Q _sand the DC bus-bar voltage U of its DC side _dc;

(2) according to described interchange active-power P _s, AC system frequency f and DC bus-bar voltage U _dc, adopt based on band dead band meritorious-control algolithm that frequency droop characteristic adds the abundant volume control characteristic of direct voltage calculates this VSC valve side d shaft current reference quantity i _vdref;

(3) according to described interchange reactive power Q _swith three-phase alternating voltage effective value u _s, adopt based on band dead band idle-control algolithm of alternating voltage droop characteristic calculates this VSC valve side q shaft current reference quantity i _vqref;

(4) according to described d axle reference quantity i _vdrefwith q axle reference quantity i _vqrefadopt inner ring current Control Algorithm, calculate the three-phase modulations voltage signal generating and obtain this VSC, and then utilize three-phase modulations voltage signal to generate one group of switching signal to control the device for power switching in this VSC by modulation technique;

What in VSC-HVDC, the VSC employing of another AC system side was conventional determines direct voltage and determines the control mode of reactive power (or determining alternating voltage).

Adopt in described step (2) based on band dead band meritorious-control algolithm that frequency droop characteristic adds the abundant volume control characteristic of direct voltage calculates VSC valve side d shaft current reference quantity i _vdrefdetailed process as follows:

A1. the active power reference P preset is made _srefdeduct interchange active-power P _s, and then to subtract each other result carry out PI regulate obtain VSC valve side d shaft current reference quantity i _vdref1;

A2. active power reference P is made _srefdeduct interchange active-power P _s, and then carry out proportion adjustment obtain active power margin of error Δ P to subtracting each other result _s1; Make the dead band upper frequency limit reference quantity f preset _ref1deduct AC system frequency f, and then carry out proportion adjustment obtain AC system frequency margin of error Δ f to subtracting each other result _s1; Make active power margin of error Δ P _s1with AC system frequency margin of error Δ f _s1be added, so to addition result carry out PI regulate obtain VSC valve side d shaft current reference quantity i _vdref2;

A3. to VSC valve side d shaft current reference quantity i _vdref1with i _vdref2carry out size to compare, get smaller value as VSC valve side d shaft current reference quantity i _vdref3;

A4. active power reference P is made _srefdeduct interchange active-power P _s, and then carry out proportion adjustment obtain active power margin of error Δ P to subtracting each other result _s2; Make the dead band lower-frequency limit reference quantity f preset _ref2deduct AC system frequency f, and then carry out proportion adjustment obtain AC system frequency margin of error Δ f to subtracting each other result _s2; Make active power margin of error Δ P _s2with AC system frequency margin of error Δ f _s2be added, so to addition result carry out PI regulate obtain VSC valve side d shaft current reference quantity i _vdref4;

A5. to VSC valve side d shaft current reference quantity i _vdref3with i _vdref4carry out size to compare, get higher value as VSC valve side d shaft current reference quantity i _vdref5;

A6. the direct voltage upper limit reference amount U preset is made _dcref1deduct DC bus-bar voltage U _dc, and then to subtract each other result carry out PI regulate obtain VSC valve side d shaft current reference quantity i _vdref6;

A7. to VSC valve side d shaft current reference quantity i _vdref5with i _vdref6carry out size to compare, get smaller value as VSC valve side d shaft current reference quantity i _vdref7;

A8. the direct voltage lower limit reference amount U preset is made _dcref2deduct DC bus-bar voltage U _dc, and then to subtract each other result carry out PI regulate obtain VSC valve side d shaft current reference quantity i _vdref8;

A9. to VSC valve side d shaft current reference quantity i _vdref7with i _vdref8carrying out size to compare, getting higher value through being limited to ± i up and down _vdlimcurrent limit, obtain VSC valve side d shaft current reference quantity i _vdref; i _vdlimfor the d axle outer shroud current limit threshold value of VSC.

Adopt in described step (3) based on band dead band idle-control algolithm of alternating voltage droop characteristic calculates VSC valve side q shaft current reference quantity i _vqrefdetailed process as follows:

B1. the reactive power reference Q preset is made _srefdeduct interchange reactive power Q _s, and then to subtract each other result carry out PI regulate obtain VSC valve side q shaft current reference quantity i _vqref1;

B2. reactive power reference Q is made _srefdeduct interchange reactive power Q _s, and then carry out proportion adjustment obtain reactive power error amount Δ Q to subtracting each other result _s1; Make the dead zone voltage upper limit reference amount u preset _sref1deduct three-phase alternating voltage effective value u _s, and then carry out proportion adjustment obtain alternating voltage margin of error Δ u to subtracting each other result _s1; Make reactive power error amount Δ Q _s1with alternating voltage margin of error Δ u _s1be added, so to addition result carry out PI regulate obtain VSC valve side q shaft current reference quantity i _vqref2;

B3. to VSC valve side q shaft current reference quantity i _vqref1with i _vqref2carry out size to compare, get smaller value as VSC valve side q shaft current reference quantity i _vqref3;

B4. reactive power reference Q is made _srefdeduct interchange reactive power Q _s, and then carry out proportion adjustment obtain reactive power error amount Δ Q to subtracting each other result _s2; Make the dead zone voltage lower limit reference amount u preset _sref2deduct three-phase alternating voltage effective value u _s, and then carry out proportion adjustment obtain alternating voltage margin of error Δ u to subtracting each other result _s2; Make reactive power error amount Δ Q _s2with alternating voltage margin of error Δ u _s2be added, so to addition result carry out PI regulate obtain VSC valve side q shaft current reference quantity i _vqref4;

B5. to VSC valve side q shaft current reference quantity i _vqref3with i _vqref4carrying out size to compare, getting higher value through being limited to ± i up and down _vqlimcurrent limit, obtain VSC valve side q shaft current reference quantity i _vqref; i _vqlimfor the q axle outer shroud current limit threshold value of VSC.

Described d axle outer shroud current limit threshold value i _vdlimwith q axle outer shroud current limit threshold value i _vqlimadopt following dynamic slice logic:

If U _dcref2<U _dc<U _dcref1, then i _vqlim=i _vlim, otherwise, i _vdlim=i _vlim, wherein, i _vlimfor the maximum through-current capacity of VSC preset.

VSC alternating voltage-frequency coordination control method of the present invention, alternating voltage-frequency coordination control objectives is added in VSC current conversion station transient state control objectives, the method can, under the prerequisite ensureing VSC-HVDC equipment safety operation, utilize the idle and meritorious rapid adjustability of VSC-HVDC to participate in voltage and the control of frequency transient state of AC system respectively.

Weak AC network is in fault transient process, the voltage of electrical network and frequency can fluctuate even unstability, but generally comparatively mains frequency fluctuation is more violent for AC voltage fluctuations, the more important thing is, the height of alternating voltage will directly affect meritorious, idle output (or absorption) ability of VSC current conversion station.Therefore, in fault in ac transmission system transient process, the inventive method is paid the utmost attention to and is controlled in safe range by alternating voltage, if VSC still has regulating power, then continues to control AC system frequency.

To sum up, four transient state control objectives of VSC alternating voltage-frequency coordination control strategy of the present invention and priority orders thereof are set as follows:

A () VSC-HVDC direct voltage keeps stable;

Can control valve side fault current during (b) AC fault;

C () VSC ac bus voltage control is in safe range;

D () VSC AC FREQUENCY CONTROL is in safe range.

Wherein, control objectives (a), (b) are the safety in order to protect VSC-HVDC equipment, and both are limit priority; Control objectives (c), (d) are to improve AC system stability, and target (c) is the second priority, and target (d) is the 3rd priority.Control method of the present invention effectively can improve voltage and the frequency transient stability of the weak AC network that VSC connects.

Accompanying drawing explanation

Fig. 1 is the control block diagram of weak AC system side VSC in VSC-HVDC.

-frequency droop characteristic the schematic diagram of gaining merit in the d axle band dead band that Fig. 2 (a) is small hydropower system grid side VSC.

The q axle control characteristic schematic diagram that Fig. 2 (b) is small hydropower system grid side VSC.

Fig. 3 is the d axle direct voltage abundant volume control characteristic schematic diagram of small hydropower system grid side VSC.

Fig. 4 (a) is the cooperation control block diagram of VSC alternating voltage-frequency d axle of the present invention.

Fig. 4 (b) is the cooperation control block diagram of VSC alternating voltage-frequency q axle of the present invention.

Fig. 5 is that the VSC-HVDC of A area power grid access Yunnan major network connects schematic diagram.

Fig. 6 (a) is under cooperation control of the present invention, and the alternating voltage waveform figure in rear A area occurs fault.

Fig. 6 (b) is under cooperation control of the present invention, and the frequency departure oscillogram in rear A area occurs fault.

Fig. 6 (c) is under cooperation control of the present invention, and the alternating voltage waveform figure of rear lonely net and major network side VSC occurs fault.

Fig. 6 (d) is under cooperation control of the present invention, and the active power oscillogram of rear lonely net and major network side VSC occurs fault.

Fig. 6 (e) is under cooperation control of the present invention, and the reactive power oscillogram of rear lonely net and major network side VSC occurs fault.

Fig. 6 (f) is under cooperation control of the present invention, and the DC voltage waveform figure of rear lonely net and major network side VSC occurs fault.

Embodiment

In order to more specifically describe the present invention, below in conjunction with the drawings and the specific embodiments, technical scheme of the present invention is described in detail.

For realizing four transient state control objectives and the priority orders of VSC alternating voltage-frequency coordination control strategy, present embodiment has redesigned VSC-HVDC control characteristic.

In VSC basic structure schematic diagram as shown in Figure 1, U _sfor VSC net side bus alternating voltage effective value, u _sjbe respectively VSC net side bus three-phase alternating voltage, u _sdand u _sqbe respectively d axle component and the q axle component of VSC net side bus three-phase alternating voltage, i _sjbe respectively VSC net side outlet place three-phase alternating current, i _vdand i _vqbe respectively d axle component and the q axle component of VSC valve side three-phase alternating current, R and L is respectively VSC AC equivalent resistance and inductance, and V is VSC valve top-cross stream voltage effective value, v _jrefbe respectively VSC valve side and export three-phase voltage reference value, v _drefand v _qrefbe respectively d axle component and q axle component that VSC valve side exports three-phase alternating voltage reference value.I _vdref, i _vqrefwith for control signal, wherein the transformation matrix of coordinates T used by VSC control system _abc/dqand T _dq/abcfixed phase.

Abc/dq conversion module expression formula is as follows:

$T_{\mathrm{abc}/\mathrm{dq}}=\frac{2}{3}.\left[\begin{array}{ccc}\cos \widehat{\mathrm{\&theta;}}& \cos (\widehat{\mathrm{\&theta;}}-\frac{2\mathrm{\&pi;}}{3})& \cos (\widehat{\mathrm{\&theta;}}+\frac{2\mathrm{\&pi;}}{3})\\ -\sin \widehat{\mathrm{\&theta;}}& -\sin (\widehat{\mathrm{\&theta;}}-\frac{2\mathrm{\&pi;}}{3})& -\sin (\widehat{\mathrm{\&theta;}}+\frac{2\mathrm{\&pi;}}{3})\end{array}\right]$

Dq/abc conversion module expression formula is as follows:

$T_{\mathrm{dq}/\mathrm{abc}}=\left[\begin{array}{cc}\cos \widehat{\mathrm{\&theta;}}& -\sin \widehat{\mathrm{\&theta;}}\\ \cos (\widehat{\mathrm{\&theta;}}-\frac{2\mathrm{\&pi;}}{3})& -\sin (\widehat{\mathrm{\&theta;}}-\frac{2\mathrm{\&pi;}}{3})\\ \cos (\widehat{\mathrm{\&theta;}}+\frac{2\mathrm{\&pi;}}{3})& -\sin (\widehat{\mathrm{\&theta;}}+\frac{2\mathrm{\&pi;}}{3})\end{array}\right]$

When Fig. 2 is the AC fault situation of small hydropower system electrical network, d, q axle control characteristic that small hydropower system grid side VSC adopts.Consider the AC fault situation of small hydropower system electrical network, the grid structure of small hydropower system electrical network is weak, and part N-1 AC fault will cause grid disconnection or power plant's off-the-line, thus causes the unbalanced power of small hydropower system electrical network and VSC short circuit ratio to reduce.For the voltage in raising small hydropower system electrical network transient process and frequency stability, the VSC of small hydropower system grid side should absorb more reactive power from electrical network when alternating voltage is too high, should absorb more active power from electrical network when mains frequency is too high, vice versa.Therefore, d, q axle control characteristic of small hydropower system grid side VSC can be designed to respectively: band dead band meritorious-frequency droop characteristic and band dead band idle-alternating voltage droop characteristic.

Fig. 3 is that when considering major network AC fault situation, small hydropower system grid side VSC adopts the abundant volume control characteristic of direct voltage.During there is AC fault in major network side, DC voltage control function need be born by small hydropower system grid side VSC, therefore small hydropower system grid side VSC can adopt the abundant volume control characteristic of direct voltage: when VSC-HVDC DC voltage fluctuation exceedes limited range, small hydropower system grid side VSC transfers constant DC voltage control pattern to.To sum up, the control characteristic of small hydropower system grid side VSC is as follows: d axle be band dead band meritorious-frequency droop characteristic adds the abundant volume control characteristic of direct voltage, q axle be band dead band idle-alternating voltage droop characteristic.

As shown in Fig. 4 (a), in VSC alternating voltage-frequency coordination control strategy of the present invention, calculate VSC valve side d shaft current reference quantity i _vdrefperforming step:

(1) P is made _srefdeduct interchange active-power P _s, obtain weak AC system side VSC net top-cross stream active power error amount, carry out signal transacting to interchange active power error amount, treated signal obtains VSC by an amplitude limit link and nets side alternating current d axle reference value i _vdref1;

Wherein: P _sreffor the weak AC system side VSC net top-cross stream active power reference preset;

Above-mentioned signal transacting can be realized by prior art, as: by VSC net top-cross stream active power error amount input proportional and integral controller, because the output of proportional and integral controller is made up of two parts addition, Part I obtains after producing an error amplification signal according to the error amount of input, Part II is obtained after time integral by the error amount of input, and the signal therefore produced after proportional and integral controller process exports the addition for first and second part exports.

(2) P is made _srefdeduct interchange active-power P _s, obtain weak AC system side VSC net top-cross stream active power error amount, make f _ref1deduct weak AC system side VSC net top-cross streaming system frequency f, obtain AC system frequency error amount, by VSC net top-cross stream active power error amount and Proportional coefficient K _p1product add AC system frequency error amount and Proportional coefficient K _f1product, obtain Δ i _vd2, to Δ i _vd2carry out signal transacting, treated signal obtains VSC by an amplitude limit link and nets side alternating current d axle reference value i _vdref2.

Δi _vd2＝K _p1(P _sref-P _s)+K _f1(f _ref1-f)

Wherein: f _ref1for the band dead band of presetting meritorious-frequency droop characteristic curve in dead band upper frequency limit reference quantity;

Above-mentioned signal transacting can be realized by prior art, as: by Δ i _vd2input proportional and integral controller, because the output of proportional and integral controller is made up of two parts addition, Part I obtains after producing an error amplification signal according to the error amount of input, Part II is obtained after time integral by the error amount of input, and the signal therefore produced after proportional and integral controller process exports the addition for first and second part exports.

(3) by VSC valve side d shaft current reference quantity i _vdref1with i _vdref2carry out size to compare, smaller value is exported and obtains VSC valve side d shaft current reference quantity i _vdref3;

(4) P is made _srefdeduct interchange active-power P _s, obtain weak AC system side VSC net top-cross stream active power error amount, make f _ref2deduct weak AC system side VSC net top-cross streaming system frequency f, obtain AC system frequency error amount, by VSC net top-cross stream active power error amount and Proportional coefficient K _p2product add AC system frequency error amount and Proportional coefficient K _f2product, obtain Δ i _vd4, to Δ i _vd4carry out signal transacting, treated signal obtains VSC by an amplitude limit link and nets side alternating current d axle reference value i _vdref4.

Δi _vd4＝K _p2(P _sref-P _s)+K _f2(f _ref2-f)

Wherein: f _ref2for the band dead band of presetting meritorious-frequency droop characteristic curve in dead band lower-frequency limit reference quantity;

Above-mentioned signal transacting can be realized by prior art, as: by Δ i _vd4input proportional and integral controller, because the output of proportional and integral controller is made up of two parts addition, Part I obtains after producing an error amplification signal according to the error amount of input, Part II is obtained after time integral by the error amount of input, and the signal therefore produced after proportional and integral controller process exports the addition for first and second part exports.

(5) by VSC valve side d shaft current reference quantity i _vdref3with i _vdref4carry out size to compare, higher value is exported and obtains VSC valve side d shaft current reference quantity i _vdref5;

(6) U is made _dcref1deduct direct voltage U _dc, obtain VSC direct current voltage error value, carry out signal transacting to direct current voltage error value, treated signal obtains VSC by an amplitude limit link and nets side alternating current d axle reference value i _vdref6;

Wherein: U _dcref1for the upper voltage limit reference quantity of default direct voltage abundant volume control characteristic curve;

Above-mentioned signal transacting can be realized by prior art, as: by VSC direct current voltage error value input proportional and integral controller, because the output of proportional and integral controller is made up of two parts addition, Part I obtains after producing an error amplification signal according to the error amount of input, Part II is obtained after time integral by the error amount of input, and the signal therefore produced after proportional and integral controller process exports the addition for first and second part exports.

(7) by VSC valve side d shaft current reference quantity i _vdref5with i _vdref6carry out size to compare, smaller value is exported and obtains VSC valve side d shaft current reference quantity i _vdref7;

(8) U is made _dcref2deduct direct voltage U _dc, obtain VSC direct current voltage error value, carry out signal transacting to direct current voltage error value, treated signal obtains VSC by an amplitude limit link and nets side alternating current d axle reference value i _vdref8;

Wherein: U _dcref2for the lower voltage limit reference quantity of default direct voltage abundant volume control characteristic curve;

Above-mentioned signal transacting can be realized by prior art, as: by VSC direct current voltage error value input proportional and integral controller, because the output of proportional and integral controller is made up of two parts addition, Part I obtains after producing an error amplification signal according to the error amount of input, Part II is obtained after time integral by the error amount of input, and the signal therefore produced after proportional and integral controller process exports the addition for first and second part exports.

(9) by VSC valve side d shaft current reference quantity i _vdref7with i _vdref8carry out size to compare, by higher value through amplitude be positive and negative i _vdlimcurrent limit link export obtain VSC valve side d shaft current reference quantity i _vdref;

As shown in Fig. 4 (b), calculate VSC valve side q shaft current reference quantity i _vdrefperforming step:

(1) Q is made _srefdeduct interchange reactive power Q _s, obtain weak AC system side VSC net top-cross stream reactive power error value, carry out signal transacting to interchange reactive power error value, treated signal obtains VSC by an amplitude limit link and nets side alternating current q axle reference value i _vqref1;

Wherein: Q _sreffor the weak AC system side VSC net top-cross stream reactive power reference preset;

Above-mentioned signal transacting can be realized by prior art, as: by VSC net top-cross stream reactive power error value input proportional and integral controller, because the output of proportional and integral controller is made up of two parts addition, Part I obtains after producing an error amplification signal according to the error amount of input, Part II is obtained after time integral by the error amount of input, and the signal therefore produced after proportional and integral controller process exports the addition for first and second part exports.

(2) Q is made _srefdeduct interchange reactive power Q _s, obtain weak AC system side VSC net top-cross stream reactive power error value, make u _sref1deduct three-phase alternating voltage effective value u _s, obtain alternating voltage error amount, by VSC net top-cross stream reactive power error value and Proportional coefficient K _q1product add alternating voltage error amount and Proportional coefficient K _u1product, obtain Δ i _vq2, to Δ i _vq2carry out signal transacting, treated signal obtains VSC by an amplitude limit link and nets side alternating current q axle reference value i _vqref2.

Δi _vq2＝K _Q1(Q _sref-Q _s)+K _u1(u _ref1-u _s)

Wherein: u _sref1for preset band dead band idle-voltage droop characteristic in dead zone voltage upper limit reference amount;

Above-mentioned signal transacting can be realized by prior art, as: by Δ i _vq2input proportional and integral controller, because the output of proportional and integral controller is made up of two parts addition, Part I obtains after producing an error amplification signal according to the error amount of input, Part II is obtained after time integral by the error amount of input, and the signal therefore produced after proportional and integral controller process exports the addition for first and second part exports.

(3) by VSC valve side q shaft current reference quantity i _vqref1with i _vqref2carry out size to compare, smaller value is exported and obtains VSC valve side q shaft current reference quantity i _vqref3;

(4) Q is made _srefdeduct interchange reactive power Q _s, obtain weak AC system side VSC net top-cross stream reactive power error value, make u _sref2deduct three-phase alternating voltage effective value u _s, obtain alternating voltage error amount, by VSC net top-cross stream reactive power error value and Proportional coefficient K _q2product add alternating voltage error amount and Proportional coefficient K _u2product, obtain Δ i _vq4, to Δ i _vq4carry out signal transacting, treated signal obtains VSC by an amplitude limit link and nets side alternating current q axle reference value i _vqref4.

Δi _vq4＝K _Q2(Q _sref-Q _s)+K _u2(u _ref2-u _s)

Wherein: u _sref2for preset band dead band idle-voltage droop characteristic in dead zone voltage lower limit reference amount;

Above-mentioned signal transacting can be realized by prior art, as: by Δ i _vq4input proportional and integral controller, because the output of proportional and integral controller is made up of two parts addition, Part I obtains after producing an error amplification signal according to the error amount of input, Part II is obtained after time integral by the error amount of input, and the signal therefore produced after proportional and integral controller process exports the addition for first and second part exports.

(5) by VSC valve side q shaft current reference quantity i _vdref3with i _vdref4carry out size to compare, by higher value through amplitude be positive and negative i _vqlimcurrent limit link export obtain VSC valve side q shaft current reference quantity i _vqref;

Finally, to d, q axle outer shroud current limit link (i in VSC alternating voltage-frequency coordination controller architecture _vdlim, i _vqlim) adopt following dynamic slice logic:

Work as U _dcref2<U _dc<U _dcref1time, i _vqlim=i _vlim,

Otherwise, i _vdlim=i _vlim,

Wherein: i _vlimfor the VSC maximum current capacity preset.

Fig. 5 is the VSC-HVDC networking blueprint of small hydropower system City Regions, Yunnan.Somewhere, the northwestward, Yunnan Province electrical network (hereinafter claiming A area) comprises a large amount of small power stations group of planes, and A area connects weakness with exchanging of outskirt electrical network, and this grid structure easily causes underdamping low-frequency oscillation problem, causes A area ability of supplying electric power to be obstructed.Dominating the inter-area oscillation mode of participation for eliminating an A area group of planes, devising the VSC-HVDC networking blueprint of small hydropower system City Regions for A area.Under PSS/E emulation platform, example comprises 11459 ac bus, 1640 generators, and wherein A area is containing small power station's unit (access 220kV and following) 71.The capacity of short circuit of side, A area VSC ac bus is 1827MVA, and short circuit ratio is 2.03.When system cloud gray model is to 1.0s, the TC-SSH 220kV circuit generation three-phase metallic earthing fault in A area, after fault continues 0.12s, circuit tripping and fault clearance, this N-1 fault causes SSH power plant three machines (55MW × 3) cut.A area side VSC adopts present embodiment alternating voltage-frequency coordination control strategy, and major network side VSC is for determine direct voltage and idle control.The controling parameters of A area side VSC is as follows: f _ref1=50.1Hz, f _ref2=49.9Hz, u _sref1=1.02pu, u _sref2=0.98pu, U _dcref1=1.06pu, U _dcref2=0.94pu, i _vlim=1.10pu (conveyance capacity of VSC is 1.1 times of rated condition).Active power and reactive power are to inject VSC for positive direction.

During VSC-HVDC employing present embodiment that Fig. 6 is shown, the transient response process of system.Between age at failure (about 1.00s to 1.12s): the alternating voltage in A area significantly falls, A area side VSC enters idle-voltage droop control pattern, and the outer circular current dynamic slice link of dq axle of A area side VSC is in idle mode of priority: A area side VSC send in a large number idle (about 950Mvar) to support its ac bus voltage, as can be seen from Fig. 6 (a), side, A area VSC ac bus voltage can maintain high value (about 1.00pu), and the global voltage drop-off range in A area also obviously alleviates; Simultaneously, the active power of A area side VSC drops to about 100MW in the effect of the outer circular current dynamic slice link of dq axle, as shown in Fig. 6 (d), this is conducive to the decline alleviating A area alternating voltage, but A area generating unit speed also can be caused to rise (mains frequency rising).After fault clearance, (about 1.12s to 1.50s): A area alternating voltage rises at once, and what A area side VSC sent is idlely down to normal level; A area power grid frequency starts to decline but still proceeds to meritorious-frequency droop control model higher than 50.0Hz, A area side VSC, increases the meritorious to reduce frequency of transmission.After fault clearance a period of time (about 1.50s to 4.00s): because SSH power plant is cut, there is meritorious vacancy in A area, frequency starts to be down to below 50.0Hz, and A area side VSC reduces the meritorious with control frequency of transmission.System enters new stable state about 4.00s, A area side VSC controlled by meritorious-frequency droop and idle-voltage droop control Automatic-searching to new stable operating point.Now VSC-HVDC transmitting active power is down to about 780MW, the alternating current circuit trend in A area alleviates, thus be conducive to A area alternating voltage and remain on higher level, therefore A area side VSC sends less idle (250Mvar) and just alternating voltage can be controlled in safe range.In whole transient process, major network side VSC by VSC-HVDC DC voltage control in safe range.Therefore after VSC-HVDC employing present embodiment alternating voltage-frequency coordination control strategy, line voltage and the frequency fluctuation of A area AC fault process meet service requirement; In whole transient process, VSC alternating voltage-frequency coordination control strategy priority orders as requested can realize four transient state control objectives.

Claims (4)

1. improve VSC-HVDC alternating voltage-frequency coordination control method of AC system transient stability, comprise the steps:

(1) for the VSC of weak AC system side in VSC-HVDC, detect and obtain its net the AC system frequency f of side, the effective value u of three-phase alternating voltage _s, exchange active-power P _s, exchange reactive power Q _sand the DC bus-bar voltage U of its DC side _dc;

(2) according to described interchange active-power P _s, AC system frequency f and DC bus-bar voltage U _dc, adopt based on band dead band meritorious-control algolithm that frequency droop characteristic adds the abundant volume control characteristic of direct voltage calculates this VSC valve side d shaft current reference quantity i _vdref;

(3) according to described interchange reactive power Q _swith three-phase alternating voltage effective value u _s, adopt based on band dead band idle-control algolithm of alternating voltage droop characteristic calculates this VSC valve side q shaft current reference quantity i _vqref;

(4) according to described d shaft current reference quantity i _vdrefwith q shaft current reference quantity i _vqrefadopt inner ring current Control Algorithm, calculate the three-phase modulations voltage signal generating and obtain this VSC, and then utilize three-phase modulations voltage signal to generate one group of switching signal to control the device for power switching in this VSC by modulation technique.

2. VSC-HVDC alternating voltage-frequency coordination control method according to claim 1, is characterized in that: adopt in described step (2) based on band dead band meritorious-control algolithm that frequency droop characteristic adds the abundant volume control characteristic of direct voltage calculates VSC valve side d shaft current reference quantity i _vdrefdetailed process as follows:

A1. the active power reference P preset is made _srefdeduct interchange active-power P _s, and then to subtract each other result carry out PI regulate obtain VSC valve side d shaft current reference quantity i _vdref1;

A2. active power reference P is made _srefdeduct interchange active-power P _s, and then carry out proportion adjustment obtain active power margin of error Δ P to subtracting each other result _s1; Make the dead band upper frequency limit reference quantity f preset _ref1deduct AC system frequency f, and then carry out proportion adjustment obtain AC system frequency margin of error Δ f to subtracting each other result _s1; Make active power margin of error Δ P _s1with AC system frequency margin of error Δ f _s1be added, so to addition result carry out PI regulate obtain VSC valve side d shaft current reference quantity i _vdref2;

A3. to VSC valve side d shaft current reference quantity i _vdref1with i _vdref2carry out size to compare, get smaller value as VSC valve side d shaft current reference quantity i _vdref3;

A4. active power reference P is made _srefdeduct interchange active-power P _s, and then carry out proportion adjustment obtain active power margin of error Δ P to subtracting each other result _s2; Make the dead band lower-frequency limit reference quantity f preset _ref2deduct AC system frequency f, and then carry out proportion adjustment obtain AC system frequency margin of error Δ f to subtracting each other result _s2; Make active power margin of error Δ P _s2with AC system frequency margin of error Δ f _s2be added, so to addition result carry out PI regulate obtain VSC valve side d shaft current reference quantity i _vdref4;

A5. to VSC valve side d shaft current reference quantity i _vdref3with i _vdref4carry out size to compare, get higher value as VSC valve side d shaft current reference quantity i _vdref5;

A6. the direct voltage upper limit reference amount U preset is made _dcref1deduct DC bus-bar voltage U _dc, and then to subtract each other result carry out PI regulate obtain VSC valve side d shaft current reference quantity i _vdref6;

A7. to VSC valve side d shaft current reference quantity i _vdref5with i _vdref6carry out size to compare, get smaller value as VSC valve side d shaft current reference quantity i _vdref7;

A8. the direct voltage lower limit reference amount U preset is made _dcref2deduct DC bus-bar voltage U _dc, and then to subtract each other result carry out PI regulate obtain VSC valve side d shaft current reference quantity i _vdref8;

A9. to VSC valve side d shaft current reference quantity i _vdref7with i _vdref8carrying out size to compare, getting higher value through being limited to ± i up and down _vdlimcurrent limit, obtain VSC valve side d shaft current reference quantity i _vdref; i _vdlimfor the d axle outer shroud current limit threshold value of VSC.

3. VSC-HVDC alternating voltage-frequency coordination control method according to claim 2, is characterized in that: adopt in described step (3) based on band dead band idle-control algolithm of alternating voltage droop characteristic calculates VSC valve side q shaft current reference quantity i _vqrefdetailed process as follows:

B1. the reactive power reference Q preset is made _srefdeduct interchange reactive power Q _s, and then to subtract each other result carry out PI regulate obtain VSC valve side q shaft current reference quantity i _vqref1;

B2. reactive power reference Q is made _srefdeduct interchange reactive power Q _s, and then carry out proportion adjustment obtain reactive power error amount Δ Q to subtracting each other result _s1; Make the dead zone voltage upper limit reference amount u preset _sref1deduct three-phase alternating voltage effective value u _s, and then carry out proportion adjustment obtain alternating voltage margin of error Δ u to subtracting each other result _s1; Make reactive power error amount Δ Q _s1with alternating voltage margin of error Δ u _s1be added, so to addition result carry out PI regulate obtain VSC valve side q shaft current reference quantity i _vqref2;

B3. to VSC valve side q shaft current reference quantity i _vqref1with i _vqref2carry out size to compare, get smaller value as VSC valve side q shaft current reference quantity i _vqref3;

B4. reactive power reference Q is made _srefdeduct interchange reactive power Q _s, and then carry out proportion adjustment obtain reactive power error amount Δ Q to subtracting each other result _s2; Make the dead zone voltage lower limit reference amount u preset _sref2deduct three-phase alternating voltage effective value u _s, and then carry out proportion adjustment obtain alternating voltage margin of error Δ u to subtracting each other result _s2; Make reactive power error amount Δ Q _s2with alternating voltage margin of error Δ u _s2be added, so to addition result carry out PI regulate obtain VSC valve side q shaft current reference quantity i _vqref4;

B5. to VSC valve side q shaft current reference quantity i _vqref3with i _vqref4carrying out size to compare, getting higher value through being limited to ± i up and down _vqlimcurrent limit, obtain VSC valve side q shaft current reference quantity i _vqref; i _vqlimfor the q axle outer shroud current limit threshold value of VSC.

4. VSC-HVDC alternating voltage-frequency coordination control method according to claim 3, is characterized in that: described d axle outer shroud current limit threshold value i _vdlimwith q axle outer shroud current limit threshold value i _vqlimadopt following dynamic slice logic:

If U _dcref2<U _dc<U _dcref1, then i _vqlim=i _vlim, otherwise, i _vdlim=i _vlim, wherein, i _vlimfor the maximum through-current capacity of VSC preset.