CN104407195A - Asymmetric fault short circuit current detection method for AC-DC hybrid system - Google Patents

Abstract

The invention discloses an asymmetric fault short circuit current detection method for an AC-DC hybrid system. The method comprises steps that, each phase voltage of an AC side and electrical operation parameters of a DC side of the system are monitored, and a commutation angle and a trigger deviation angle of every two phases of a converter valve are calculated when an asymmetric fault occurs in the system; each phase voltage and each phase current of the AC side are represented by employing symmetric components; a switch function model is established, and association relations between the voltage of the DC side and each phase voltage of the AC side and association relations between the current of the DC side and the current of the AC side are acquired; under the control of VDCOL, function relations between each phase current of the AC side and each phase voltage of the AC side are acquired; each phase voltage of the AC side is timely monitored, and a short circuit current injecting from the DC side to the AC side is acquired according to the function relations between each phase current of the AC side and each phase voltage of the AC side. The method can accurately detect the short circuit current injecting from the DC system to the AC system when the asymmetric fault occurs in the system.

Description

A kind of unbalanced fault short-circuit current detection method of alternating current-direct current combined hybrid system

Technical field

The present invention relates to Power System and its Automation technical field, particularly relate to a kind of unbalanced fault short-circuit current detection method of alternating current-direct current combined hybrid system.

Background technology

Along with developing rapidly of D.C. high voltage transmission in recent years (High-Voltage Direct Current being called for short HVDC), China just progressively forms the interconnected network containing Multi-infeed HVDC transmission system maximum in the world.But alternating current-direct current combined hybrid system interacts more and more obvious on the impact of power grid security steady-state operation, brings many technical difficulties to Electric Power Network Planning and operation.And when AC system generation unbalanced fault, each the characteristic sum uncharacteristic harmonics that alternating current-direct current combined hybrid system produces is by the safe and stable operation of likely influential system.

Although existing scholar proposes when system generation symmetric fault at present, straight-flow system injects the detection method of the short-circuit current of AC system, but still lacking description when system generation unbalanced fault, straight-flow system injects the detection method of the short-circuit current of AC system.

Summary of the invention

Based on this, the invention provides a kind of unbalanced fault short-circuit current detection method of alternating current-direct current combined hybrid system, can when alternating current-direct current combined hybrid system generation unbalanced fault, detection computations goes out the short-circuit current that straight-flow system injects AC system exactly.

A unbalanced fault short-circuit current detection method for alternating current-direct current combined hybrid system, comprises the steps:

The each phase voltage of AC of monitoring alternating current-direct current combined hybrid system and the electrical operation parameter of DC side, ask for angle of overlap and the trigger offset angle of every two-phase of converter valve during alternating current-direct current combined hybrid system generation unbalanced fault;

Adopt symmetrical component method that the asymmetrical voltage phasor of described AC and electric current phasor are decomposed into three-phase symmetrical phasor respectively;

Set up switch function model, in conjunction with the angle of overlap of described every two-phase, trigger offset angle and three-phase symmetrical phasor, obtain the incidence relation between DC voltage and each phase voltage of AC, and the incidence relation between DC side electric current and ac-side current;

Under the control of current limit instruction depending on voltage, utilize the incidence relation between described DC voltage and each phase voltage of AC and the incidence relation between described DC side electric current and each phase current of AC, obtain the funtcional relationship between each phase current of AC and each phase voltage of AC;

Monitor each phase voltage of described AC, according to the funtcional relationship between each phase current of described AC and each phase voltage of AC, when detecting described alternating current-direct current combined hybrid system generation unbalanced fault, DC side injects the short-circuit current of AC.

The unbalanced fault short-circuit current detection method of above-mentioned alternating current-direct current combined hybrid system, first each phase voltage of AC of alternating current-direct current combined hybrid system and the electrical operation parameter of DC side is monitored, strictly based on the system change of current and control characteristic, according to the incidence relation between DC voltage and each phase voltage of AC and the incidence relation between DC side electric current and each phase current of AC, acquisition can reflect the funtcional relationship between each phase current of the AC of ac and dc systems interaction mechanism and each phase voltage of AC intuitively, solve the straight-flow system when there is unbalanced fault and inject the computational problem of the electric current of AC system, fill up the blank of the computing method injecting the short-circuit current size of AC system under system unbalanced fault from straight-flow system, overcome the shortcoming that the variable of existing description AC system and straight-flow system mutual relationship is various, the interactional technical method of ac and dc systems is studied for Electric Power Network Planning and operation provide one.The unbalanced fault short-circuit current detection method of alternating current-direct current combined hybrid system of the present invention is research AC system and the interactional core methed of straight-flow system, can be applicable to alternating current-direct current Electric Power Network Planning and run control.

Accompanying drawing explanation

Fig. 1 is the schematic flow sheet of unbalanced fault short-circuit current detection method in an embodiment of alternating current-direct current combined hybrid system of the present invention.

Fig. 2 is the structural representation of alternating current-direct current combined hybrid system in an embodiment.

Fig. 3 is the angle of overlap of every two-phase and the schematic flow sheet at trigger offset angle of converter valve when obtaining alternating current-direct current combined hybrid system generation unbalanced fault in Fig. 1.

Fig. 4 is the schematic flow sheet of the incidence relation realized in Fig. 1 between each phase voltage of DC voltage, electric current and AC, each phase current.

Fig. 5 is the family curve schematic diagram of DC voltage in Fig. 1 corresponding to VDCOL link and DC current.

Embodiment

Below in conjunction with embodiment and accompanying drawing, the present invention is described in further detail, but embodiments of the present invention are not limited thereto.

As shown in Figure 1, the unbalanced fault short-circuit current detection method being alternating current-direct current combined hybrid system of the present invention, at the schematic flow sheet of an embodiment, comprises the following steps:

Step S1: each phase voltage of AC of monitoring alternating current-direct current combined hybrid system and the electrical operation parameter of DC side, asks for the angle of overlap μ of every two-phase of converter valve during alternating current-direct current combined hybrid system generation unbalanced fault _mnwith trigger offset angle θ _mn;

Further, the actual Trigger Angle α of converter valve during alternating current-direct current combined hybrid system generation unbalanced fault is calculated _mn(m, n are any two-phase in a phase of three-phase circuit, b phase, c phase).During concrete enforcement, alternating current-direct current combined hybrid system comprises two input links, one of them input link is straight-flow system (i.e. the DC side of alternating current-direct current combined hybrid system) input end, and another input link is AC system (i.e. the AC of alternating current-direct current combined hybrid system).

Referring to Fig. 2, be that a kind of of the alternating current-direct current combined hybrid system of the present embodiment can the structural representation of implementation.Wherein, the DC side of alternating current-direct current combined hybrid system comprise connect successively sample holding unit 201, lock unit 202, collect (Min) unit 203; The AC of alternating current-direct current combined hybrid system comprises the smoothing processing unit 204, electric current and voltage limiting command (Voltage Dependent Current Order Limit the is called for short VDCOL) unit 205 that connect successively; Further, smoothing processing unit 204 comprises a variable transitions process: wherein, parameter s is an expression in complex field; Parameter T is time constant, can value be preferably 20ms (millisecond).The object of smoothing processing unit 204 is the DC current instruction smooth change enabling to calculate.DC current i _dmin unit 203 is arrived via sample holding unit 201, lock unit 202; AC signal u _d/ u _acmin unit 203 is arrived via after the process of smoothing processing unit 204 and VDCOL unit 205; The signal of Min unit 203 to two input ends integrates rear output combined hybrid system current signal i _ord.

During concrete enforcement, described step S1 can be realized by numerous embodiments.

Referring to Fig. 3, be the angle of overlap of every two-phase of converter valve and the schematic flow sheet of a kind of implementation at trigger offset angle when asking for alternating current-direct current combined hybrid system generation unbalanced fault in step S1.

Concrete, as shown in Figure 3, described step S1 comprises:

Step S11: adopt mode at equal intervals to carry out trigging control to each converter valve in described alternating current-direct current combined hybrid system;

Step S12: detect the commutation voltage of AC three-phase circuit, commutating reactance X _rand DC current average I _d, and calculate the commutation voltage U of any two-phase _mn(comprise a phase, b phase, c phase in three-phase circuit, m, n are any two-phase in three-phase circuit) and the phase differential with reference to phasor

In the present embodiment, introduce abc-α β coordinate transform, represent three-phase commutation voltage with rotating vector, the α component of commutation voltage can be obtained by following equation (1) with β component

$\left[\begin{array}{c}{\stackrel{\·}{U}}_{\mathrm{\α}}\\ {\stackrel{\·}{U}}_{\mathrm{\β}}\end{array}\right]=\frac{2}{3}\left[\begin{array}{ccc}1& -1/2& -1/2\\ 0& \sqrt{3}/2& -\sqrt{3}/2\end{array}\right]\left[\begin{array}{c}{\stackrel{\·}{U}}_{\mathrm{ca}}\\ {\stackrel{\·}{U}}_{\mathrm{ab}}\\ {\stackrel{\·}{U}}_{\mathrm{bc}}\end{array}\right]---\left(1\right)$

Choose phaselocked loop and export synchronizing voltage for reference phasor, under system is normally run, due to the regulating action of phaselocked loop, commutation voltage phase place is zero with reference phasor phase differential, then when system generation unbalanced fault, commutation voltage U _mnwith the phase differential with reference to phasor can be expressed as:

Wherein, U in above formula (2) _αand U _β2 components that three-phase commutation voltage is corresponding on α axle and β axle after coordinate transform (abc-α β), with be U after coordinate transform _αand U _βangle.

Step S13: according to described commutation voltage U _mnwith the phase differential with reference to phasor when can try to achieve AC system generation unbalanced fault, actual Trigger Angle α during each converter valve conducting _mn:

In above formula (3), m is any phase in three-phase circuit, and n is any phase in three-phase circuit; for the commutation voltage U of described any two-phase _mnwith the phase differential with reference to phasor; α ₀for the trigger delay angle under converter valve normal condition.

Step S14: according to actual Trigger Angle α during described converter valve conducting _mn, described commutation voltage U _mn, described commutating reactance X _rand described DC current average I _d, calculate angle of overlap μ when obtaining every two-phase commutation _mn.

Wherein for the commutation voltage of m, n two-phase and the phase differential of corresponding reference phasor;

Preferably, angle of overlap μ when specifically calculating every two-phase commutation by following equation _mn:

μ _mn＝arccos(cosα _mn-2X _rI _d/u _mn)-α _mn(4)

In above formula (4), α _mnfor actual Trigger Angle during described converter valve conducting, m, n are any two-phase in three-phase circuit, u _mnfor the commutation voltage amplitude of m, n two-phase; X _rfor commutating reactance; I _dfor DC current mean value.

Step S15: according to the commutation voltage U of described any two-phase _mnwith the phase differential with reference to phasor the trigger offset angle θ of converter valve when calculating corresponding every two-phase commutation _mn.

Described step S15 specifically can be:

The trigger offset angle θ of converter valve when calculating every two-phase commutation by following equation _mn:

In above formula (5), m is any phase in three-phase circuit, and n is any phase in three-phase circuit; for the commutation voltage U of described any two-phase _mnwith the phase differential with reference to phasor; α ₀for the trigger delay angle under converter valve normal condition.

In the present embodiment, obtain alternating current-direct current combined hybrid system generation unbalanced fault time every two-phase commutation time converter valve angle of overlap μ _mnwith trigger offset angle θ _mnafter, can utilize in its step below and represent relevant parameter.

Step S2: adopt symmetrical component method that the asymmetrical voltage phasor of described AC and current component are decomposed into three-phase symmetrical phasor.

For asymmetrical voltage phasor, in the three-phase circuit of the AC of alternating current-direct current combined hybrid system, for any one group of asymmetric three-phase phasor (voltage or electric current), the phasor of three kinds of three-phase symmetricals can be decomposed into.Therefore asymmetric phase voltage can be expressed as with the symmetrical components of voltage:

$\left\{\begin{array}{c}{u}_a=u_a^0+u_a^1+u_a^2\\ u_b=u_a^0+e^2{u}_a^1+e{u}_a^2\\ u_c=u_a^0+e{u}_a^1+e^2{u}_a^2\end{array}\right.---\left(6\right)$

Wherein, u _athe a phase voltage of AC, u _bthe b phase voltage of AC, u _cit is the c phase voltage of AC; it is a phase zero sequence voltage component; it is a phase positive sequence voltage component; it is a phase negative sequence voltage components; Parameter e=1 ∠ 120 °, e ²=1 ∠ 240 °.Wherein, a phase voltage of AC, b phase voltage and c phase voltage are also referred to as AC change of current busbar voltage.

For a phase, each symmetrical components of a phase can be expressed as with asymmetric each phase voltage:

$\left\{\begin{array}{c}{u}_a^0=\frac{1}{3}(u_a+u_b+u_c)\\ u_a^1=\frac{1}{3}(u_a+e{u}_b+e^2{u}_c)\\ u_a^2=\frac{1}{3}(u_a+e^2{u}_b+e{u}_c)\end{array}\right.---\left(7\right)$

During concrete enforcement, under asymmetric case, because AC zero sequence voltage does not affect DC voltage, zero sequence voltage component can not considered.

Step S3: set up switch function model, in conjunction with the angle of overlap of described every two-phase, trigger offset angle and three-phase symmetrical phasor, obtain the incidence relation between DC voltage and each phase voltage of AC, and the incidence relation between DC side electric current and ac-side current.

During concrete enforcement, described switch function model is the process adopting switch function to represent each parameter in alternating current-direct current combined hybrid system.Such as, rectifier bridge and inverter bridge are typical switching device, can introduce unipolarity two-valued function switch function and describe on off state on rectifier bridge and inverter bridge brachium pontis:

S in above formula (8) _rqfor switch function expression formula, r represents system rectification side or inverter side, and q represents three-phase a, b, c in three-phase circuit.

Utilize the ultimate principle of above switch function, switch function model can be set up for alternating current-direct current combined hybrid system, thus obtain the incidence relation between DC voltage and each phase voltage of AC, and, the incidence relation between DC side electric current and ac-side current.

Referring to Fig. 4, be provided by the invention realize a kind of of incidence relation between each phase voltage of DC voltage, electric current and AC, each phase current can the flow chart of steps of implementation.

Particularly, described step S3 comprises:

Step S31: set up switch function model, and simulate according to the Interruption performance of described switch function model to converter valve;

Step S32: utilize described switch function model to modulate each phase voltage of AC, the incidence relation obtained between DC voltage and each phase voltage of AC is:

u _d＝u _as _ua+u _bs _ub+u _cs _uc(9)

Wherein, u _dfor DC voltage, u _afor AC a phase voltage, s _uafor the switch function of AC a phase voltage; u _bfor AC b phase voltage, s _ubfor the switch function of AC b phase voltage; u _cfor AC c phase voltage, s _ucfor the switch function of AC c phase voltage.

Step S33: utilize described switch function model to modulate DC side electric current, obtains the incidence relation between DC side electric current and each phase current of AC:

$\left\{\begin{array}{c}{i}_a=i_d{s}_{\mathrm{ia}};\\ i_b=i_d{s}_{\mathrm{ib}};\\ i_c=i_d{s}_{\mathrm{ic}};\end{array}\right.---\left(10\right)$

In above formula (10), i _a, i _band i _cbe respectively each phase current of AC; i _dfor DC side electric current; s _iafor the switch function of AC a phase current; s _ibfor the switch function of AC b phase current; s _icfor the switch function of AC c phase current.

Further, described step S3 also comprises:

Step S34: according to angle of overlap and the trigger offset angle of every two-phase of converter valve during described alternating current-direct current combined hybrid system generation unbalanced fault, obtain the fundametal component of the switch function of each phase voltage of the AC in switch function model respectively, revise component and commutation component, and the fundametal component of the switch function of each phase current of AC, correction component and commutation component;

Step S35: the fundametal component of the switch function of each phase voltage of described AC, correction component and commutation component are superposed, as the switch function of each phase voltage of AC; The fundametal component of each phase current of described AC, correction component and commutation component are superposed, as the switch function of each phase current of AC.

Particularly, the present embodiment considers converter valve turn-on instant deviation angle and the uneven impact on switch function of angle of overlap simultaneously, each switch function can be regarded as fundametal component, revise the superposition of component and commutation component, wherein:

For voltage switch function, its voltage fundametal component s _unwith voltage correction component s _urbe preferably the square wave that amplitude is 1, width is respectively 2 π/3 and θ; Voltage commutation component s _{u μ}be preferably the square wave that amplitude is 0.5, width is angle of overlap μ;

For current switch function, its electric current fundametal component s _inwith electric current correction component s _irbe preferably the square wave that amplitude is 1, width is respectively 2 π/3 and θ; Current commutation component s _{i μ}be preferably the class sawtooth wave that width is μ.

During concrete enforcement, the switch function of each phase voltage and each phase current can be expanded into Fourier series, can obtain:

$s_{\mathrm{un}}\left(\mathrm{\ωt}\right)=\underset{k=-\∞}{\overset{\∞}{\mathrm{\Σ}}}\frac{1}{\mathrm{k\π}}[\sin \frac{\mathrm{k\π}}{3}+j(\cos \mathrm{k\π}+\cos \frac{2\mathrm{k\π}}{3})]e^{\mathrm{jk\ωt}}---\left(11\right)$

$s_{\mathrm{ur}}({\mathrm{\θ}}_{\mathrm{mn}},\mathrm{\ωt})=\underset{k=-\∞}{\overset{\∞}{\mathrm{\Σ}}}\frac{j}{2\mathrm{k\π}}\left[e^{-\mathrm{jk}{\mathrm{\θ}}_{\mathrm{mn}}}(e^{-\mathrm{jk\π}}-1)(e^{-\mathrm{jk}{\mathrm{\θ}}_{\mathrm{mn}}}-1)\right]e^{\mathrm{jk\ωt}}---\left(12\right)$

$s_{\mathrm{u\μ}}({\mathrm{\μ}}_{\mathrm{mn}},\mathrm{\ωt})=\underset{k=-\∞}{\overset{\∞}{\mathrm{\Σ}}}\frac{j}{4\mathrm{k\π}}\left[e^{-\mathrm{jk}{\mathrm{\μ}}_{\mathrm{mn}}}(e^{-\mathrm{jk\π}}-1)(e^{-\mathrm{jk}{\mathrm{\μ}}_{\mathrm{mn}}}-1)\right]e^{\mathrm{jk\ωt}}---\left(13\right)$

The switch function of each phase current can be expanded into Fourier series, wherein, electric current fundametal component s _inwith electric current correction component s _irwith voltage fundametal component s _unwith voltage correction component s _ursimilar, current commutation component s _{i μ}for:

$\left\{\begin{array}{c}{s}_{\mathrm{i\μ}}(u_{\mathrm{mn}},{\mathrm{\α}}_{\mathrm{mn}},{\mathrm{\μ}}_{\mathrm{mn}},\mathrm{\ωt})=\underset{k=-\∞}{\overset{\∞}{\mathrm{\Σ}}}{s}_{\mathrm{i\μ}\left(k\right)}{e}^{\mathrm{jk\ωt}}\\ s_{\mathrm{i\μ}\left(k\right)}=\frac{1-e^{-\mathrm{jk}{\mathrm{\μ}}_{\mathrm{mn}}}}{2\mathrm{\π}{X}_r}(u_{\mathrm{mn}}\cos {\mathrm{\α}}_{\mathrm{mn}}-j2X_r)\end{array}\right.---\left(14\right)$

Wherein, parameter μ _mnand θ _mnthe angle of overlap of converter valve and trigger offset angle during every two-phase commutation during alternating current-direct current combined hybrid system generation unbalanced fault; α _mnfor actual Trigger Angle during converter valve conducting; u _mnfor the commutation voltage amplitude of m, n two-phase; X _rfor commutating reactance.

So, according to equation (11) ~ (14), the switch function of each phase voltage that the fundametal component that can find application, the superposition revising component and commutation component represent and the switch function of each phase current.

For a phase, utilize each component of a phase voltage, a phase voltage switch function s can be obtained _uafor:

s _ua＝s _uan+s _uar+s _uaμ(15)

According to the waveform of equation (11) ~ (13) and each component of voltage, can obtain:

$\left\{\begin{array}{c}{s}_{\mathrm{uan}}=s_{\mathrm{un}}\left(\mathrm{\ωt}\right)\\ s_{\mathrm{uar}}=s_{\mathrm{ur}}({\mathrm{\θ}}_{\mathrm{ab}},\mathrm{\ωt}-\mathrm{\π}/3)-s_{\mathrm{ur}}({\mathrm{\θ}}_{\mathrm{ca}},\mathrm{\ωt}+\mathrm{\π}/3)\\ s_{\mathrm{ua\μ}}=s_{\mathrm{u\μ}}({\mathrm{\μ}}_{\mathrm{ab}},\mathrm{\ωt}-\mathrm{\π}/3-{\mathrm{\θ}}_{\mathrm{ab}})-s_{\mathrm{u\μ}}({\mathrm{\μ}}_{\mathrm{ca}},\mathrm{\ωt}+\mathrm{\π}/3+{\mathrm{\θ}}_{\mathrm{ca}})\end{array}\right.---\left(16\right)$

Wherein, s _uana phase voltage switch function s _uafundametal component, s _uara phase voltage switch function s _uacorrection component and s _{ua μ}a phase voltage switch function s _uacommutation component.

Utilize each component of a phase current, a phase current switch function s can be obtained _iafor:

s _ia＝s _ian+s _iar+s _iaμ(17)

According to the waveform of equation (11), equation (12), equation (14) and each current component, can obtain:

$\left\{\begin{array}{c}{s}_{\mathrm{ian}}=s_{\mathrm{in}}\left(\mathrm{\ωt}\right)\\ s_{\mathrm{iar}}=s_{\mathrm{ir}}({\mathrm{\θ}}_{\mathrm{ab}},\mathrm{\ωt}-\mathrm{\π}/3)-s_{\mathrm{ir}}({\mathrm{\θ}}_{\mathrm{ca}},\mathrm{\ωt}+\mathrm{\π}/3)\\ s_{\mathrm{ia\μ}}=s_{\mathrm{i\μ}}(u_{\mathrm{ab}},{\mathrm{\α}}_{\mathrm{ab}},{\mathrm{\μ}}_{\mathrm{ab}},\mathrm{\ωt}-\mathrm{\π}/3-{\mathrm{\θ}}_{\mathrm{ab}})-s_{\mathrm{i\μ}}(u_{\mathrm{ca}},{\mathrm{\α}}_{\mathrm{ca}},{\mathrm{\μ}}_{\mathrm{ca}},\mathrm{\ωt}+\mathrm{\π}/3+{\mathrm{\θ}}_{\mathrm{ca}})\end{array}\right.---\left(18\right)$

Wherein, s _iana phase current switch function s _iafundametal component, s _iara phase current switch function s _iacorrection component and s _{ia μ}a phase current switch function s _iacommutation component.

In like manner, the voltage switch function s of b phase can be obtained _ubwith current switch function s _ibbe respectively:

s _ub＝s _ubn+s _ubr+s _ubμ(19)

s _ib＝s _ibn+s _ibr+s _ibμ(20)

Wherein, s _ubnb phase voltage switch function s _ubfundametal component, s _uarb phase voltage switch function s _ubcorrection component and s _{ub μ}b phase voltage switch function s _ubcommutation component; s _ibnb phase current switch function s _ibfundametal component, s _ibrb phase current switch function s _ibcorrection component and s _{ib μ}b phase current switch function s _ibcommutation component.

In like manner, the voltage switch function s of c phase can be obtained _ucwith current switch function s _icbe respectively:

s _uc＝s _ucn+s _ucr+s _ucμ(21)

s _ic＝s _icn+s _icr+s _icμ(22)

Wherein, s _ucnc phase voltage switch function s _ucfundametal component, s _ucrc phase voltage switch function s _uccorrection component and s _{uc μ}c phase voltage switch function s _uccommutation component; s _icnc phase current switch function s _icfundametal component, s _icrc phase current switch function s _iccorrection component and s _{ic μ}c phase current switch function s _iccommutation component.

So, in the present embodiment, more than acquisition after each switch function, by in conjunction with above equation (6), equation (9) and equation (10), incidence relation between the DC voltage that can be represented by each component of voltage and each phase voltage of AC, incidence relation between the DC side electric current represented by each current component and each phase current of AC, and, the funtcional relationship between each phase current of AC and each phase voltage of AC.

Step S4: depending on current limit instruction (the Voltage Dependent Current OrderLimit of voltage, be called for short VDCOL) control under, utilize the incidence relation between described DC voltage and each phase voltage of AC and the incidence relation between described DC side electric current and each phase current of AC, obtain the funtcional relationship between each phase current of AC and each phase voltage of AC.

In order to prevent continuous commutation failure, current-order (VDCOL) limiting unit depending on voltage is introduced in engineering practice, to be controlled DC current instruction during low-voltage, reduction straight-flow system to the reactive requirement of AC system, improves the recovery characteristics of DC transmission system during fault recovery.

During concrete enforcement, described step S4 realizes in the following manner, comprising:

Step S41: when described alternating current-direct current combined hybrid system generation unbalanced fault, described alternating current-direct current combined hybrid system DC voltage u _ddrop to when to a certain degree (presetting a definite value) and the described current limit instruction (VDCOL) depending on voltage is activated;

Step S42: described depend on electric current and voltage restriction instruction (VDCOL) control under, by DC side current i _doutput is:

$i_d=\left\{\begin{array}{cc}{i}_{d\min },& u_d<u_{d\min }\\ \frac{i_{d\max }-i_{d\min }}{u_{d\max }-u_{d\min }}(u_d-u_{d\min })+i_{d\min },& u_{d\max }<u_d<u_{d\min }\\ i_{d\max },& u_d>u_{d\max }\end{array}\right.---\left(23\right)$

In above formula (23), u _dfor the DC voltage of described alternating current-direct current combined hybrid system, u _{d max}for the maximal value of DC voltage, u _{d min}for the minimum value of DC voltage, i _{d min}for the maximal value of DC side electric current, i _{d max}for the minimum value of DC side electric current.

Referring to Fig. 5, which show DC voltage corresponding to VDCOL link and DC current (u _d-i _d) family curve.

In some unbalanced fault situation of generation, when direct current or alternating voltage occur declining and lower than setting value after, VDCOL link in DC control system will be activated, this link is according to certain DC voltage and DC current corresponding relation, follow DC voltage change trend adjustment DC current setting valve, and then change direct current transportation power, now straight-flow system operates in CC (Constant Current, the constant current) control model under VDCOL effect; Until AC and DC voltage resume to after to a certain degree, VDCOL link exits, straight-flow system returns to again conventional CP (Constant Power, constant power) or CC control model, namely DC current control objectives becomes power controller output valve or DC current controller input setting valve.

Step S43: according to described DC voltage u _dphase voltage (u each with AC _a, u _band u _c) between incidence relation, i.e. equation (9), and, described DC side current i _dphase current (i each with AC _a, i _band i _c) between incidence relation, i.e. equation (10), calculates the funtcional relationship f (u obtained between each phase current of AC and each phase voltage of AC _a, u _b, u _c).

During concrete enforcement, sequence component analysis method can be adopted to the analysis of electric system unbalanced fault.First the order components model of transverter is set up, as the equation (6) in above and equation (7), and, under asymmetric case, due to AC zero sequence voltage dC voltage is not affected, can zero sequence voltage component do not considered then, equation (9) can be expressed as:

$u_d=(u_a^1+u_a^2)s_{\mathrm{ua}}+(u_b^1+u_b^2)s_{\mathrm{ub}}+(u_c^1+u_c^2)s_{\mathrm{uc}}---\left(24\right)$

Wherein, it is a phase positive sequence voltage component; it is a phase negative sequence voltage components; it is b phase positive sequence voltage component; it is b phase negative sequence voltage components; it is c phase positive sequence voltage component; it is c phase negative sequence voltage components.

Order:

$f_a(u_a,u_b,u_c,i_d)=(u_a^1+u_a^2)s_{\mathrm{ua}}---\left(25\right)$

$f_b(u_a,u_b,u_c,i_d)=(u_b^1+u_b^2)s_{\mathrm{ub}}---\left(26\right)$

$f_c(u_a,u_b,u_c,i_d)=(u_c^1+u_c^2)s_{\mathrm{uc}}---\left(27\right)$

Equation (7) is substituted into equation (25), can funtcional relationship be obtained:

$f_a(u_a,u_b,u_c,i_d)=[\frac{2}{3}{u}_a+\frac{1}{3}(e+e^2)(u_b+u_c)]s_{\mathrm{ua}}---\left(28\right)$

In like manner can in the hope of f _b(u _a, u _b, u _c, i _d), f _c(u _a, u _b, u _c, i _d).

Then according to equation (24), DC voltage i can be obtained _dwith exchange change of current busbar voltage (the i.e. each phase voltage u of AC _a, u _b, u _c) between funtcional relationship:

u _d＝f _a(u _a,u _b,u _c,i _d)+f _b(u _a,u _b,u _c,i _d)+f _c(u _a,u _b,u _c,i _d) (29)

According to equation (10), equation (17), equation (20) and equation (22), the funtcional relationship that can obtain between ac-side current and DC side electric current is:

$\left\{\begin{array}{c}{i}_a=i_d{s}_{\mathrm{ia}}=i_d(s_{\mathrm{ian}}+s_{\mathrm{iar}}+s_{\mathrm{ia\&mu;}})\\ i_b=i_d{s}_{\mathrm{ib}}=i_d(s_{\mathrm{ibn}}+s_{\mathrm{ibr}}+s_{\mathrm{ib\&mu;}})\\ i_c=i_d{s}_{\mathrm{ic}}=i_d(s_{\mathrm{icn}}+s_{\mathrm{icr}}+s_{\mathrm{ic\&mu;}})\end{array}\right.---\left(30\right)$

Association type (23), (29) and (30), then can solve the funtcional relationship between each cross streams electric current and each cross streams (asymmetric) voltage:

$\left\{\begin{array}{c}{i}_a=f_1(u_a,u_b,u_c)\\ i_b=f_2(u_a,u_b,u_c)\\ i_c=f_3(u_a,u_b,u_c)\end{array}\right.---\left(31\right)$

Step S5: each phase voltage of what meaning AC of Real-Time Monitoring, according to the funtcional relationship between each phase current of described AC and each phase voltage of AC, i.e. equation (31), when detecting described alternating current-direct current combined hybrid system generation unbalanced fault, the measured value of each phase voltage (i.e. change of current busbar voltage) of AC is substituted into equation (31), the short-circuit current that DC side injects AC can be obtained.

The unbalanced fault short-circuit current detection method of the alternating current-direct current combined hybrid system that the embodiment of the present invention provides, the each phase voltage of AC of monitoring alternating current-direct current combined hybrid system and the electrical operation parameter of DC side, strictly based on the system change of current and control characteristic, according to the incidence relation between DC voltage and each phase voltage of AC and the incidence relation between DC side electric current and each phase current of AC, acquisition can reflect the funtcional relationship between each phase current of the AC of ac and dc systems interaction mechanism and each phase voltage of AC intuitively, solve the straight-flow system when there is unbalanced fault and inject the computational problem of the electric current of AC system, fill up the blank of the computing method injecting the short-circuit current size of AC system under system unbalanced fault from straight-flow system, overcome the shortcoming that the variable of existing description AC system and straight-flow system mutual relationship is various.The interactional technical method of ac and dc systems is studied for Electric Power Network Planning and operation provide one.The unbalanced fault short-circuit current detection method of the alternating current-direct current combined hybrid system that the embodiment of the present invention provides is research AC system and the interactional core methed of straight-flow system, can be applicable to alternating current-direct current Electric Power Network Planning and run control.

The above embodiment only have expressed several embodiment of the present invention, and it describes comparatively concrete and detailed, but therefore can not be interpreted as the restriction to the scope of the claims of the present invention.It should be pointed out that for the person of ordinary skill of the art, without departing from the inventive concept of the premise, can also make some distortion and improvement, these all belong to protection scope of the present invention.Therefore, the protection domain of patent of the present invention should be as the criterion with claims.

Claims (7)

1. a unbalanced fault short-circuit current detection method for alternating current-direct current combined hybrid system, is characterized in that, comprise the steps:

The each phase voltage of AC of monitoring alternating current-direct current combined hybrid system and the electrical operation parameter of DC side, ask for angle of overlap and the trigger offset angle of every two-phase of converter valve during alternating current-direct current combined hybrid system generation unbalanced fault;

Adopt symmetrical component method that the asymmetrical voltage phasor of described AC and electric current phasor are decomposed into three-phase symmetrical phasor respectively;

Set up switch function model, in conjunction with the angle of overlap of described every two-phase, trigger offset angle and three-phase symmetrical phasor, obtain the incidence relation between DC voltage and each phase voltage of AC, and the incidence relation between DC side electric current and ac-side current;

Under the control of current limit instruction depending on voltage, utilize the incidence relation between described DC voltage and each phase voltage of AC and the incidence relation between described DC side electric current and each phase current of AC, obtain the funtcional relationship between each phase current of AC and each phase voltage of AC;

Monitor each phase voltage of described AC, according to the funtcional relationship between each phase current of described AC and each phase voltage of AC, when detecting described alternating current-direct current combined hybrid system generation unbalanced fault, DC side injects the short-circuit current of AC.

2. the unbalanced fault short-circuit current detection method of alternating current-direct current combined hybrid system as claimed in claim 1, it is characterized in that, the each phase voltage of AC of described monitoring alternating current-direct current combined hybrid system and the electrical operation parameter of DC side, the step of the angle of overlap and trigger offset angle of asking for every two-phase of converter valve during alternating current-direct current combined hybrid system generation unbalanced fault comprises:

Mode is at equal intervals adopted to carry out trigging control to each converter valve in described alternating current-direct current combined hybrid system;

Detect the commutation voltage of AC three-phase circuit, commutating reactance and DC current average, and calculate the commutation voltage of any two-phase and the phase differential with reference to phasor;

According to described commutation voltage and described phase differential, when obtaining alternating current-direct current combined hybrid system generation unbalanced fault, actual Trigger Angle during each converter valve conducting;

According to actual Trigger Angle during described converter valve conducting, described commutation voltage, described commutating reactance and described DC current average, calculate angle of overlap during every two-phase commutation;

According to the commutation voltage of described any two-phase and the phase differential with reference to phasor, the trigger offset angle of converter valve when calculating corresponding every two-phase commutation.

3. the unbalanced fault short-circuit current detection method of alternating current-direct current combined hybrid system as claimed in claim 2, it is characterized in that, described according to actual Trigger Angle during described converter valve conducting, described commutation voltage, described commutating reactance and described DC current average, the step of angle of overlap when calculating every two-phase commutation comprises:

Angle of overlap μ when calculating every two-phase commutation by following equation _mn:

μ _mn＝arccos(cosα _mn-2X _rI _d/u _mn)-α _mn

Wherein, μ _mnfor angle of overlap during described every two-phase commutation; α _mnfor actual Trigger Angle during described converter valve conducting, m, n are any two-phase in three-phase circuit, u _mnfor the commutation voltage amplitude of m, n two-phase; X _rfor commutating reactance; I _dfor DC current mean value.

4. the unbalanced fault short-circuit current detection method of alternating current-direct current combined hybrid system as claimed in claim 2, it is characterized in that, the described commutation voltage according to described any two-phase with reference to the phase differential of phasor, the step at the trigger offset angle of converter valve when calculating corresponding every two-phase commutation comprises:

The trigger offset angle θ of converter valve when calculating every two-phase commutation by following equation _mn:

Wherein, θ _mnfor the trigger offset angle of converter valve during described every two-phase commutation; M, n are any two-phase in three-phase circuit; commutation voltage for described any two-phase and the phase differential with reference to phasor; α ₀for the trigger delay angle under converter valve normal condition.

5. the unbalanced fault short-circuit current detection method of the alternating current-direct current combined hybrid system as described in any one of Claims 1 to 4, it is characterized in that, describedly set up switch function model, in conjunction with the angle of overlap of described every two-phase, trigger offset angle and three-phase symmetrical phasor, obtain the incidence relation between DC voltage and each phase voltage of AC, and the step of incidence relation between DC side electric current and ac-side current comprises:

Set up switch function model, and simulate according to the Interruption performance of described switch function model to converter valve;

Utilize described switch function model to modulate each phase voltage of AC, the incidence relation obtained between DC voltage and each phase voltage of AC is: u _d=u _as _ua+ u _bs _ub+ u _cs _uc; Wherein, u _dfor DC voltage, u _afor AC a phase voltage, s _uafor the switch function of AC a phase voltage; u _bfor AC b phase voltage, s _ubfor the switch function of AC b phase voltage; u _cfor AC c phase voltage, s _ucfor the switch function of AC c phase voltage;

Utilize described switch function model to modulate DC side electric current, obtain the incidence relation between DC side electric current and each phase current of AC: i _a=i _ds _ia; i _b=i _ds _ib; i _c=i _ds _ic; Wherein, i _a, i _band i _cbe respectively each phase current of AC; i _dfor DC side electric current; s _iafor the switch function of AC a phase current; s _ibfor the switch function of AC b phase current; s _icfor the switch function of AC c phase current.

6. the unbalanced fault short-circuit current detection method of alternating current-direct current combined hybrid system as claimed in claim 5, it is characterized in that, describedly set up switch function model, in conjunction with the angle of overlap of described every two-phase, trigger offset angle and three-phase symmetrical phasor, obtain the incidence relation between DC voltage and each phase voltage of AC, and incidence relation between DC side electric current and ac-side current step also comprise:

According to angle of overlap and the trigger offset angle of every two-phase of converter valve during described alternating current-direct current combined hybrid system generation unbalanced fault, obtain the fundametal component of the switch function of each phase voltage of the AC in switch function model respectively, revise component and commutation component, and the fundametal component of the switch function of each phase current of AC, correction component and commutation component;

The fundametal component of the switch function of each phase voltage of described AC, correction component and commutation component are superposed, as the switch function of each phase voltage of AC; The fundametal component of each phase current of described AC, correction component and commutation component are superposed, as the switch function of each phase current of AC.

7. the unbalanced fault short-circuit current detection method of alternating current-direct current combined hybrid system as claimed in claim 6, it is characterized in that, under the described control of current limit instruction depending on voltage, utilize the incidence relation between described DC voltage and each phase voltage of AC and the incidence relation between described DC side electric current and each phase current of AC, the step obtaining the funtcional relationship between each phase current of AC and each phase voltage of AC comprises:

When described alternating current-direct current combined hybrid system generation unbalanced fault, described alternating current-direct current combined hybrid system DC voltage u _dwhen dropping to pre-set value, the described current limit instruction depending on voltage is activated;

Described depend on electric current and voltage restriction instruction control under, by DC side electric current export be:

$i_d=\left\{\begin{array}{cc}{i}_{d\min },& u_d<u_{d\min }\\ \frac{i_{d\max }-i_{d\min }}{u_{d\max }-u_{d\min }}(u_d-u_{d\min })+i_{d\min },& u_{d\max }<u_d<u_{d\min }\\ i_{d\max },& u_d>u_{d\max }\\ & \end{array}\right.$

Wherein, i _dfor described DC side electric current, u _dthe DC voltage of described alternating current-direct current combined hybrid system, u _{d max}for the maximal value of DC voltage, u _{d min}for the minimum value of DC voltage, i _{d min}for the maximal value of DC side electric current, i _{d max}for the minimum value of DC side electric current;

Incidence relation according between described DC voltage and each phase voltage of AC: u _d=u _as _ua+ u _bs _ub+ u _cs _uc, and the incidence relation between described DC side electric current and each phase current of AC: i _a=i _ds _ia; i _b=i _ds _ib; i _c=i _ds _ic, calculate the funtcional relationship obtained between each phase current of AC and each phase voltage of AC.