CN103618332A - Method for controlling controllable capacitor in CSCC-HVDC system - Google Patents

Abstract

The invention provides a method for controlling a controllable capacitor in a CSCC-HVDC system. The method includes the following steps that first, the critical equivalent impedance value Zcr of an alternating current system is calculated; second, the equivalent impedance value Zequ of the alternating current system and the voltage value Uinv of a conversion current bus are detected in real time, whether the Zequ is larger than or equal to the Zcr is judged, if yes, the third step is executed, and if not, the fourth step is executed; third, constant value impedance control over a commutated converter of the controllable capacitor is achieved; fourth, classification switching control over the commutated converter of the controllable capacitor is achieved. According to the method for controlling the controllable capacitor in the CSCC-HVDC system, the controllable capacitor of the CSCC is controlled with the equivalent impedance value of the alternating current system and the voltage value of the conversion current bus used as the judgment basis, the stability of an ac/dc interconnection system under different operation conditions is effectively improved, the effectiveness of the control method is verified, and meanwhile it has been shown that the CSCC-HVDC system has good fault ride-through and fault recovery characteristics.

Description

The control method of controlled capacitance in a kind of CSCC-HVDC system

Technical field

The present invention relates to a kind of control method, specifically relate to the control method of controlled capacitance in a kind of CSCC-HVDC system.

Background technology

Along with the propelling successively of extra-high voltage direct-current transmission engineering, direct current drop point City Regions grid structure is day by day complicated, causes exchanging receiving-end system under contrast and shows slightly weak, therefore, direct current transportation connecting system has been proposed to higher specification requirement.For adopting electrical network commutation converter (line commutated converter, LCC) traditional DC transmission system of technology, because its commutation voltage is directly provided by the AC network being attached thereto, therefore, fault in ac transmission system may cause direct current system commutation failure, during catastrophe failure, even may cause the chain reactions such as direct current locking.Adopt electric capacity commutation commutation technique can effectively improve AC/DC interconnected system operation stability, reduce the risk of direct current system commutation failure in AC fault situation.

In view of the characteristic of conventional DC transmission system, various countries scholar has recognized the superiority of electric capacity commutation commutation technique very early, and has carried out more theoretical research.This change of current topological structure is proposed in the 1950's by Buseman the earliest, and the people such as Reeve has carried out further analysis to the basic theories of electric capacity commutation commutation technique subsequently.Since phase late 1970s and the initial stage eighties, lot of domestic and foreign experts and scholars are studied electric capacity commutation change of current structure.But due to the restriction of the technical conditions such as valve rated value, this technology fails to be applied in Practical Project always.But since the nineties, fast development along with power electronic technology, the development of continuous adjustable filter apparatus and active filter, reactive power and high-performance harmonic can be independent control, the raising of valve rated value and the reduction of cost, converter need to move in more weak system, and the problem of electrical network commutation is more outstanding, the maturation of thyristor controlled series capacitor (controlled capacitance) technology, electric capacity commutation converter becomes study hotspot again.At present, international esbablished corporation has continued to increase the R&D intensity of electric capacity commutation HVDC Transmission Technology.Wherein, ABB AB has succeeded in developing the high voltage direct current transmission project of first employing electric capacity commutation technology in the world in 2000, and puts into commercial operation in Brazilian Caribbean; The 2003 Nian You U.S. have built up the Rapid City DC engineering that connects U.S. the western and eastern electrical network.

As Fig. 1, high voltage direct current transmission (high-voltage direct current, HVDC) controlled capacitance commutation converter (controlled series capacitor converter, CSCC) structure is controllable series capacitance to be placed on to the primary side of converter transformer, it is controlled series compensation (the thyristor controlled series capacitor that success is used, TCSC) technology and traditional electrical network commutation converter (line commutated converter, LCC) combination, by the dynamic adjustment to series electrical capacitance, can overcome contingent ferro resonance.

The economy of CSCC, technical operation advantage can be summarized as following 5 points:

(1) improve the power factor of converter;

(2) overvoltage while reducing load rejection;

(3) probability of inversion side generation commutation failure while effectively reducing fault;

(4) improve HVDC operation stability;

(5) without large capacity compensation device.

Summary of the invention

In order to overcome above-mentioned the deficiencies in the prior art, the invention provides the control method of controlled capacitance in a kind of CSCC-HVDC system, take AC system equivalent impedance and change of current bus voltage value carries out the control of CSCC controlled capacitance as criterion, effectively improved the stability of AC/DC interconnected system under different operating conditions, verified the validity of this control method, shown that CSCC-HVDC system has good fault traversing and failover characteristic simultaneously.

In order to realize foregoing invention object, the present invention takes following technical scheme:

The invention provides the control method of controlled capacitance in a kind of CSCC-HVDC system, described HVDC (High Voltage Direct Current) transmission system and AC transmission system form ac and dc systems, described HVDC (High Voltage Direct Current) transmission system comprises inversion side system, rectification side system and DC line between the two, described inversion side system comprises controlled capacitance commutation converter, between the change of current bus and ac bus of described controlled capacitance in inversion side system; Said method comprising the steps of:

Step 1: calculate the critical equivalent impedance Z of AC system _cr;

Step 2: detect in real time AC system equivalent impedance Z _equwith change of current bus voltage value U _inv, and judge whether to exist Z _equ>=Z _crif, there is execution step 3, if do not exist, do not perform step 4:

Step 3: controlled capacitance commutation converter is carried out to definite value impedance Control;

Step 4: controlled capacitance commutation converter is carried out to the control of classification switching.

In described step 1, HVDC (High Voltage Direct Current) transmission system is described as with equation:

$\left\{\begin{array}{c}{P}_d={{\mathrm{CU}}_{\mathrm{inv}}}^2[\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="HDA0000423994170000022.png,HDA0000423994170000031.png"\; state="\{\{state\}\}">cos</figure-callout>}2\mathrm{\&gamma;}-\cos (2\mathrm{\&gamma;}+2\mathrm{\&mu;})]\\ Q_d={{\mathrm{CU}}_{\mathrm{inv}}}^2[2\mathrm{\&mu;}+\sin 2\mathrm{\&gamma;}-\sin (2\mathrm{\&gamma;}+2\mathrm{\&mu;})]\\ I_d={\mathrm{KU}}_{\mathrm{inv}}[\cos \mathrm{\&gamma;}-\cos (\mathrm{\&gamma;}+\mathrm{\&mu;})]\\ U_d=P_d/I_d\end{array}\right.---\left(1\right)$

Wherein, P _dand Q _dbe respectively DC side active power and reactive power, I _dand U _dfor DC side electric current and voltage, C and the K constant for being determined by HVDC (High Voltage Direct Current) transmission system, μ and γ are respectively the angle of overlap of converter valve in HVDC (High Voltage Direct Current) transmission system and close the angle of rupture;

AC transmission system is described as with equation:

$\left\{\begin{array}{c}{P}_{\mathrm{ac}}=P_d=\frac{1}{\left|Z_s\right|}[{\mathrm{<figure-callout\; id="2"\; label="U\; inv"\; filenames="HDA0000423994170000022.png,HDA0000423994170000031.png"\; state="\{\{state\}\}">U</figure-callout>}}_{\mathrm{inv}}^{}\cos \left({\mathrm{\&theta;}}^{\&prime;}\right)-{\mathrm{EU}}_{\mathrm{inv}}\cos ({\mathrm{\&theta;}}^{\&prime;}+\mathrm{\&delta;})]\\ Q_{\mathrm{ac}}=\frac{1}{\left|Z_s\right|}[{\mathrm{<figure-callout\; id="2"\; label="U\; inv"\; filenames="HDA0000423994170000022.png,HDA0000423994170000031.png"\; state="\{\{state\}\}">U</figure-callout>}}_{\mathrm{inv}}^{}\sin \left({\mathrm{\&theta;}}^{\&prime;}\right)-{\mathrm{EU}}_{\mathrm{inv}}\sin ({\mathrm{\&theta;}}^{\&prime;}+\mathrm{\&delta;})]\\ Q_c=B_{\mathrm{<figure-callout\; id="2"\; label="c\; U\; inv"\; filenames="HDA0000423994170000022.png,HDA0000423994170000031.png"\; state="\{\{state\}\}">c</figure-callout>}}{U}_{\mathrm{inv}}^{}{}_2^{}=Q_d+Q_{\mathrm{ac}}\end{array}\right.---\left(2\right)$

Wherein, P _acand Q _acbe respectively AC active power and reactive power, B _cfor the equivalent admittance of alternating current filter and reactive compensation capacitor, Q _cfor equivalent admittance B _cthe reactive power of sending, Z _sfor seeing into that from the change of current bus equiva lent impedance of AC transmission system, the equivalent electromotive force that E is AC transmission system, θ ' are equiva lent impedance Z _svectorial angle, δ is change of current busbar voltage vectorial angle;

The equiva lent impedance Z of controlled capacitance commutation converter _cSCCbe expressed as:

$Z_{\mathrm{CSCC}}=Z_C-\frac{Z_C^2}{(Z_C-Z_L)}\frac{2\mathrm{\&beta;}+\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="HDA0000423994170000022.png,HDA0000423994170000031.png"\; state="\{\{state\}\}">sin</figure-callout>}2\mathrm{\&beta;}}{\mathrm{\&pi;}}+\frac{4Z_C^2}{(Z_C-Z_L)}\frac{{\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="HDA0000423994170000022.png,HDA0000423994170000031.png"\; state="\{\{state\}\}">cos</figure-callout>}}^2\mathrm{\&beta;}}{(k^2-1)}\frac{(k\tan \mathrm{k\&beta;}-\mathrm{tna\&beta;})}{\mathrm{\&pi;}}---\left(3\right)$

Wherein, Z _cfor controlled capacitance branch road capacitor value, Z _lfor inductive branch induction reactance value, the gating advance angle that β is anti-parallel thyristor, k and β are expressed as:

$k=\sqrt{\frac{Z_c}{Z_L}}---\left(4\right)$

β＝π-α （5）

Wherein, the Trigger Angle of α anti-parallel thyristor;

The short circuit ratio SCR of described ac and dc systems is the capacity of short circuit S of ac bus _acwith DC side active power rated value P _dNratio, that is:

$\mathrm{SCR}=\frac{S_{\mathrm{ac}}}{P_{\mathrm{dN}}}=\frac{U_N^2}{P_{\mathrm{dN}}}\&times;\frac{1}{\left|Z_r\right|}---\left(6\right)$

Wherein, U _nfor ac bus rated voltage, Z _rfor see into the equiva lent impedance of AC transmission system from ac bus;

When the short circuit ratio SCR of ac and dc systems gets short circuit ratio critical value CSCR, have:

$\mathrm{CSCR}=\mathrm{SCR}=\frac{U_N^2}{P_{\mathrm{dN}}}\&times;\frac{1}{\left|Z_{\mathrm{cr}}\right|}---\left(7\right)$

The critical equivalent impedance Z of known AC system _crbe expressed as:

$Z_{\mathrm{cr}}=\frac{U_N^2}{P_{\mathrm{dN}}}\&times;\frac{1}{\left|\mathrm{CSCR}\right|}---\left(8\right).$

Described step 3 comprises the following steps:

Step 3-1: calculate Z _equwith Z _crdifference Z _err, have Z _err=Z _equ-Z _cr;

Step 3-2: setting the target impedance value of adjusting is Z _ref, have Z _ref=Z _err, complete controlled capacitance commutation converter definite value impedance Control.

In described step 4, according to change of current bus voltage value U _inv, controlled capacitance commutation converter is carried out to the control of classification switching, minute following three kinds of situations:

A) if U _inv>=0.8pu, the controlled capacitance place branch road in controlled capacitance commutation converter, by short circuit, is equivalent to electrical network commutation converter;

B) if 0.6pu≤U _inv< 0.8pu, the controlled capacitance capacitor value putting into operation is set as AC system equivalent impedance Z _equ20%;

C) if U _inv< 0.6pu, the controlled capacitance capacitor value putting into operation is set as AC system equivalent impedance Z _equ50%.

Compared with prior art, beneficial effect of the present invention is:

1. the present invention be take AC system equivalent impedance and change of current bus voltage value and is selected corresponding controlled capacitance control measure as criterion, has effectively improved the stability of AC/DC interconnected system under different operating conditions;

2. verified the validity of this control method, shown that CSCC-HVDC system has good fault traversing and failover characteristic simultaneously;

3. make full use of the technical advantage of TCSC dynamic adjustment performance, with the control effect of realizing ideal.

Accompanying drawing explanation

Fig. 1 is CSCC structural representation;

Fig. 2 is ac and dc systems model schematic diagram;

Fig. 3 is that in controlled capacitance commutation converter CSCC, controlled capacitance is controlled model schematic diagram;

Fig. 4 is embodiment 1 model simplification topological diagram;

Fig. 5 is LCC and CSCC direct current power and time relationship comparison diagram under N-2 fault in embodiment 1;

Fig. 6 is LCC and CSCC direct voltage and time relationship comparison diagram under N-2 fault in embodiment 1;

Fig. 7 is LCC and CSCC direct current and time relationship comparison diagram under N-2 fault in embodiment 1;

Fig. 8 is controlled capacitance impedance Control response curve in CSCC in embodiment 1;

Fig. 9 is LCC under three-phase resistance earth fault in embodiment 2, improve LCC and CSCC inversion top-cross stream busbar voltage and time relationship comparison diagram;

Figure 10 is CSCC equivalent impedance under three-phase resistance earth fault in embodiment 2, improve in LCC fixedly series capacitance equivalent impedance and CSCC equivalent impedance reference value and time relationship comparison diagram;

Figure 11 is LCC under three-phase resistance earth fault in embodiment 2, improve LCC and CSCC direct current power and time relationship comparison diagram;

Figure 12 is LCC under three-phase phase fault in embodiment 2, improve LCC and CSCC inversion top-cross stream busbar voltage and time relationship comparison diagram;

Figure 13 is CSCC equivalent impedance under three-phase phase fault in embodiment 2, improve in LCC fixedly series capacitance equivalent impedance and CSCC equivalent impedance reference value and time relationship comparison diagram;

Figure 14 is LCC under three-phase phase fault in embodiment 2, improve LCC and CSCC direct current power and time relationship comparison diagram.

Embodiment

Below in conjunction with accompanying drawing, the present invention is described in further detail.

The invention provides the control method of controlled capacitance in a kind of CSCC-HVDC system, described HVDC (High Voltage Direct Current) transmission system and AC transmission system form ac and dc systems, described HVDC (High Voltage Direct Current) transmission system comprises inversion side system, rectification side system and DC line between the two, described inversion side system comprises controlled capacitance commutation converter, between the change of current bus and ac bus of described controlled capacitance in inversion side system.In order to improve AC/DC interconnected system operation stability, while reducing AC fault, the risk of direct current system commutation failure considers the factors such as performance driving economy simultaneously, and in the CSCC-HVDC system providing, the control method of controlled capacitance comprises the following steps:

Step 1: calculate the critical equivalent impedance Z of AC system _cr;

Step 2: detect in real time AC system equivalent impedance Z _equwith change of current bus voltage value U _inv, and judge whether to exist Z _equ>=Z _crif, there is execution step 3, if do not exist, do not perform step 4:

Step 3: controlled capacitance commutation converter is carried out to definite value impedance Control;

Step 4: controlled capacitance commutation converter is carried out to the control of classification switching.

As Fig. 2, in ac and dc systems, HVDC (High Voltage Direct Current) transmission system is described as with equation:

$\left\{\begin{array}{c}{P}_d={{\mathrm{CU}}_{\mathrm{inv}}}^2[\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="HDA0000423994170000022.png,HDA0000423994170000031.png"\; state="\{\{state\}\}">cos</figure-callout>}2\mathrm{\&gamma;}-\cos (2\mathrm{\&gamma;}+2\mathrm{\&mu;})]\\ Q_d={{\mathrm{CU}}_{\mathrm{inv}}}^2[2\mathrm{\&mu;}+\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="HDA0000423994170000022.png,HDA0000423994170000031.png"\; state="\{\{state\}\}">sin</figure-callout>}2\mathrm{\&gamma;}-\sin (2\mathrm{\&gamma;}+2\mathrm{\&mu;})]\\ I_d={\mathrm{KU}}_{\mathrm{inv}}[\cos \mathrm{\&gamma;}-\cos (\mathrm{\&gamma;}+\mathrm{\&mu;})]\\ U_d=P_d/I_d\end{array}\right.---\left(1\right)$

Wherein, P _dand Q _dbe respectively DC side active power and reactive power, I _dand U _dfor DC side electric current and voltage, C and the K constant for being determined by HVDC (High Voltage Direct Current) transmission system, μ and γ are respectively the angle of overlap of converter valve in HVDC (High Voltage Direct Current) transmission system and close the angle of rupture;

AC transmission system is described as with equation:

$\left\{\begin{array}{c}{P}_{\mathrm{ac}}=P_d=\frac{1}{\left|Z_s\right|}[U_{\mathrm{inv}}^2\cos \left({\mathrm{\&theta;}}^{\&prime;}\right)-{\mathrm{EU}}_{\mathrm{inv}}\cos ({\mathrm{\&theta;}}^{\&prime;}+\mathrm{\&delta;})]\\ Q_{\mathrm{ac}}=\frac{1}{\left|Z_s\right|}[U_{\mathrm{inv}}^2\sin \left({\mathrm{\&theta;}}^{\&prime;}\right)-{\mathrm{EU}}_{\mathrm{inv}}\sin ({\mathrm{\&theta;}}^{\&prime;}+\mathrm{\&delta;})]\\ Q_c=B_c{U}_{\mathrm{inv}}^2=Q_d+Q_{\mathrm{ac}}\end{array}\right.---\left(2\right)$

Wherein, P _acand Q _acbe respectively AC active power and reactive power, B _cfor the equivalent admittance of alternating current filter and reactive compensation capacitor, Q _cfor equivalent admittance B _cthe reactive power of sending, Z _sfor seeing into that from the change of current bus equiva lent impedance of AC transmission system, the equivalent electromotive force that E is AC transmission system, θ ' are equiva lent impedance Z _svectorial angle, δ is change of current busbar voltage vectorial angle;

The equiva lent impedance Z of controlled capacitance commutation converter _cSCCbe expressed as:

$Z_{\mathrm{CSCC}}=Z_C-\frac{Z_C^2}{(Z_C-Z_L)}\frac{2\mathrm{\&beta;}+\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="HDA0000423994170000022.png,HDA0000423994170000031.png"\; state="\{\{state\}\}">sin</figure-callout>}2\mathrm{\&beta;}}{\mathrm{\&pi;}}+\frac{4Z_C^2}{(Z_C-Z_L)}\frac{{\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="HDA0000423994170000022.png,HDA0000423994170000031.png"\; state="\{\{state\}\}">cos</figure-callout>}}^2\mathrm{\&beta;}}{(k^2-1)}\frac{(k\tan \mathrm{k\&beta;}-\mathrm{tna\&beta;})}{\mathrm{\&pi;}}---\left(3\right)$

Wherein, Z _cfor controlled capacitance branch road capacitor value, Z _lfor inductive branch induction reactance value, the gating advance angle that β is anti-parallel thyristor, k and β are expressed as:

$k=\sqrt{\frac{Z_c}{Z_L}}---\left(4\right)$

β＝π-α （5）

Wherein, the Trigger Angle of α anti-parallel thyristor;

The short circuit ratio SCR of described ac and dc systems is the capacity of short circuit S of ac bus _acwith DC side active power rated value P _dNratio, that is:

$\mathrm{SCR}=\frac{S_{\mathrm{ac}}}{P_{\mathrm{dN}}}=\frac{U_N^2}{P_{\mathrm{dN}}}\&times;\frac{1}{\left|Z_r\right|}---\left(6\right)$

Wherein, U _nfor ac bus rated voltage, Z _rfor see into the equiva lent impedance of AC transmission system from ac bus;

When the short circuit ratio SCR of ac and dc systems gets short circuit ratio critical value CSCR, have:

$\mathrm{CSCR}=\mathrm{SCR}=\frac{U_N^2}{P_{\mathrm{dN}}}\&times;\frac{1}{\left|Z_{\mathrm{cr}}\right|}---\left(7\right)$

The critical equivalent impedance Z of known AC system _crbe expressed as:

$Z_{\mathrm{cr}}=\frac{U_N^2}{P_{\mathrm{dN}}}\&times;\frac{1}{\left|\mathrm{CSCR}\right|}---\left(8\right).$

Clearly, when interconnected systems transmits the power of certain capacity, if Zr>Z _cr, system is difficult to stable operation.CSCC controlled capacitance can effectively reduce the equivalent impedance of AC system, suitably increases SCR, thereby improves the operation stability of system.

When receiving end AC system breaks down, the voltage of change of current bus falls and easily causes commutation failure.Under normal operation, Inverter Station ac bus magnitude of voltage remains near 1pu; When fault occurs, busbar voltage is instantaneous to be fallen, because the adjustment of converter transformer voltage ratio is slower, now can think that no-load voltage ratio is constant, therefore converter transformer valve-side voltage equal proportion is fallen, the amplitude of falling depends on electrical distance between fault origination point and change of current bus and the operational mode of system at that time.It has been generally acknowledged that the instantaneous 0.8pu of dropping to of inversion side change of current busbar voltage easily causes commutation failure when following.CSCC controlled capacitance can improve voltage under failure condition and fall value, is conducive to system and recovers rapidly stable operation.

Described step 3 comprises the following steps:

Step 3-1: calculate Z _equwith Z _crdifference Z _err, have Z _err=Z _equ-Z _cr;

Step 3-2: setting the target impedance value of adjusting is Z _ref, have Z _ref=Z _err, complete controlled capacitance commutation converter definite value impedance Control.

In described step 4, according to change of current bus voltage value U _inv, controlled capacitance commutation converter is carried out to the control of classification switching, minute following three kinds of situations:

A) if U _inv>=0.8pu, the controlled capacitance place branch road in controlled capacitance commutation converter, by short circuit, is equivalent to electrical network commutation converter;

B) if 0.6pu≤U _inv< 0.8pu, the controlled capacitance capacitor value putting into operation is set as AC system equivalent impedance Z _equ20%;

C) if U _inv< 0.6pu, the controlled capacitance capacitor value putting into operation is set as AC system equivalent impedance Z _equ50%.

Because voltage under interchange catastrophe failure is minimum, fall value lower, so the secondary capacitor value of locating is greater than one-level capacitor value, to guarantee that interconnected systems can recover rapidly stable operation.Large and then cause control disorder for fear of the change of current busbar voltage fluctuating range detecting during fault recovery, in above-mentioned latter two situation, a period of time backed off after random operation of forcing to put into operation of CSCC controlled capacitance, the control selection effect of change of current bus voltage value was during this period lost efficacy, and system reverts to LCC structure afterwards.

In electromagnetic transient simulation program PSCAD/EMTDC, build following ac and dc systems: inversion side system adds CSCC, converter bridge part still adopts determines Current Control and surely closes the angle of rupture to control; Rectification side adopts LCC structure, determines Current Control; Inversion top-cross streaming system consists of the subsystem of three parallel runnings, and system parameters is as follows: system 1 equivalent impedance is 75 ° of Ω of 44.08 ∠, system 2,3 parameter homologous rays 1.Model simplification topological structure as shown in Figure 4.

3 system parallel runnings under normal circumstances, now whole AC system equivalence short circuit ratio is 3.6, belongs to strong system.Suppose that AC system, at 2.5s, N-2 fault occurs, now, whole AC system only surplus system 3, in operation, belongs to utmost point weak pattern system, known according to the calculating of AC system equivalent impedance detection criteria, need to access Z _ref=23.48 Ω.In this case, the system operation characteristic of contrast LCC structure and CSCC structure as shown in Figure 5-Figure 7.Clearly, under N-2 failure condition, LCC system generation commutation failure, and CSCC system can stable transfer power, shows that CSCC can improve the stability of system under fault.Figure 12-Figure 14 shows that CSCC controlled capacitance impedance Control response curve, wherein Z _cSCCrepresent the actual connecting system equivalent impedance of CSCC controlled capacitance, Z _refrepresent Ordering impedance value.When system runs on stable state, CSCC controlled capacitance is by short circuit making, and now connecting system equivalent impedance is 0; Occur after N-2 fault, calculate and knownly need to access Z _ref=23.48 Ω.Controller model design parameter sees attached list 1.

Table 1

Fig. 4 example model is slightly made an amendment, an inversion top-cross streaming system retention system 1 and 2.Respectively three-phase resistance earth fault and three-phase phase fault are studied below.

Suppose the inversion side change of current bus fault that is short-circuited when 2s, trouble duration 0.1s, for three-phase resistance earth fault, system AC system equivalent impedance Z now _eqube less than corresponding critical equivalent impedance Z _cr, but minimum the falling of change of current busbar voltage approaches 0.2pu; For three-phase phase fault, system AC system equivalent impedance Z now _equalso be less than corresponding critical equivalent impedance Z _cr, but minimum the falling of change of current busbar voltage approaches 0.5pu.Therefore inversion side all needs to put into operation under two kinds of failure conditions, (secondary is 11 to CSCC system secondary electric capacity herein Ω).Following three kinds of systems are carried out to simulation comparison:

1) traditional LC C system;

2) improved LCC system (inversion side current conversion station AC adds fixedly series capacitance);

3) CSCC system.

Wherein, in improvement LCC system, fixedly the equivalent capacitor value of series capacitance equates with the actual CSCC controllable capacitance value putting into operation; In this example, the CSCC controlled capacitance time of putting into operation is 3s, and controlled capacitance branch road is by short circuit afterwards.Simulation curve is to such as shown in Fig. 9-10 and Figure 12-14.

From Fig. 9 and Figure 12, add fixedly series capacitance to improving the minimum value effect that has some improvement of falling of voltage, but its magnitude of voltage fluctuating range is larger in failover procedure, this is because the fixed capacity accessing is easy and system impedance produces resonance, cause the vibration of voltage, be unfavorable for the stable operation of direct current system.Contrasting two figure can find, for phase-to phase fault situation, the castering action that CSCC system is fallen alternating voltage is more obvious, and its dominant mechanism is that CSCC controlled capacitance has reduced the equivalent impedance of system, can bear even more serious fault.In Fig. 9, CSCC system change of current busbar voltage returns to 1pu when 2.25s, tend towards stability rapidly afterwards, and now improved LCC system and traditional LC C system change of current bus recovery effects all poor; In Figure 12, CSCC system change of current busbar voltage returns to stationary value when 2.31s, and now traditional LC C system voltage value is still lower, and improved LCC system voltage curve fluctuation is larger.

Figure 10 and Figure 13 are that equiva lent impedance is measured curve, and CSCC equivalent impedance reference value is one-level impedance 11 Ω.Here it is pointed out that between age at failure, electric current oppositely causes CSCC equivalent impedance track reference value well, can produce certain overshoot; Meanwhile, fixedly series capacitance equivalent impedance also can produce vibration, because fixed capacity in phase-to phase fault situation discharges and recharges instantaneous energy imbalance, causes its equivalent impedance degree of fluctuation larger.

In Figure 11 and Figure 14, can find, CSCC system can obviously be accelerated direct current power and return to rapidly rated value: under three-phase resistance ground fault condition, CSCC system returns to rated value when 2.4s; In three-phase phase fault situation, it is faster that CSCC system power recovers, and reaches rated value during 2.32s; And two corresponding traditional LC C systems of the moment are all very low with improvement LCC system power value.For ground fault condition, improved LCC system has reduced the performance number of sending between age at failure, but not obvious to the rapid restitution of direct current power; For phase-to phase fault situation, improved LCC system has even worsened its recovery process.Controller model design parameter sees attached list 2.

Table 2

The above analysis is known: 1) CSCC can improve system short-circuit ratio, for N-1(or N-2) fault recovery improves significantly, and can effectively improve AC/DC interconnected system operation stability; 2) voltage that CSCC system can improve under change of current busbar short-circuit failure condition falls value, accelerates direct current power and recovers, and has the fault traversing and the failover characteristic that are better than LCC system.

Finally should be noted that: above embodiment is only in order to illustrate that technical scheme of the present invention is not intended to limit, although the present invention is had been described in detail with reference to above-described embodiment, those of ordinary skill in the field are to be understood that: still can modify or be equal to replacement the specific embodiment of the present invention, and do not depart from any modification of spirit and scope of the invention or be equal to replacement, it all should be encompassed in the middle of claim scope of the present invention.

Claims (4)

1. the control method of controlled capacitance in a CSCC-HVDC system, described HVDC (High Voltage Direct Current) transmission system and AC transmission system form ac and dc systems, described HVDC (High Voltage Direct Current) transmission system comprises inversion side system, rectification side system and DC line between the two, described inversion side system comprises controlled capacitance commutation converter, between the change of current bus and ac bus of described controlled capacitance in inversion side system; It is characterized in that: said method comprising the steps of:

Step 1: calculate the critical equivalent impedance Z of AC system _cr;

Step 2: detect in real time AC system equivalent impedance Z _equwith change of current bus voltage value U _inv, and judge whether to exist Z _equ>=Z _crif, there is execution step 3, if do not exist, do not perform step 4:

Step 3: controlled capacitance commutation converter is carried out to definite value impedance Control;

Step 4: controlled capacitance commutation converter is carried out to the control of classification switching.

2. the control method of controlled capacitance in CSCC-HVDC system according to claim 1, is characterized in that: in described step 1, HVDC (High Voltage Direct Current) transmission system is described as with equation:

$\left\{\begin{array}{c}{P}_d={{\mathrm{CU}}_{\mathrm{inv}}}^2[\cos 2\mathrm{\&gamma;}-\cos (2\mathrm{\&gamma;}+2\mathrm{\&mu;})]\\ Q_d={{\mathrm{CU}}_{\mathrm{inv}}}^2[2\mathrm{\&mu;}+\sin 2\mathrm{\&gamma;}-\sin (2\mathrm{\&gamma;}+2\mathrm{\&mu;})]\\ I_d={\mathrm{KU}}_{\mathrm{inv}}[\cos \mathrm{\&gamma;}-\cos (\mathrm{\&gamma;}+\mathrm{\&mu;})]\\ U_d=P_d/I_d\end{array}\right.---\left(1\right)$

Wherein, P _dand Q _dbe respectively DC side active power and reactive power, I _dand U _dfor DC side electric current and voltage, C and the K constant for being determined by HVDC (High Voltage Direct Current) transmission system, μ and γ are respectively the angle of overlap of converter valve in HVDC (High Voltage Direct Current) transmission system and close the angle of rupture;

AC transmission system is described as with equation:

$\left\{\begin{array}{c}{P}_{\mathrm{ac}}=P_d=\frac{1}{\left|Z_s\right|}[U_{\mathrm{inv}}^2\cos \left({\mathrm{\&theta;}}^{\&prime;}\right)-{\mathrm{EU}}_{\mathrm{inv}}\cos ({\mathrm{\&theta;}}^{\&prime;}+\mathrm{\&delta;})]\\ Q_{\mathrm{ac}}=\frac{1}{\left|Z_s\right|}[U_{\mathrm{inv}}^2\sin \left({\mathrm{\&theta;}}^{\&prime;}\right)-{\mathrm{EU}}_{\mathrm{inv}}\sin ({\mathrm{\&theta;}}^{\&prime;}+\mathrm{\&delta;})]\\ Q_c=B_c{U}_{\mathrm{inv}}^2=Q_d+Q_{\mathrm{ac}}\end{array}\right.---\left(2\right)$

Wherein, P _acand Q _acbe respectively AC active power and reactive power, B _cfor the equivalent admittance of alternating current filter and reactive compensation capacitor, Q _cfor equivalent admittance B _cthe reactive power of sending, Z _sfor seeing into that from the change of current bus equiva lent impedance of AC transmission system, the equivalent electromotive force that E is AC transmission system, θ ' are equiva lent impedance Z _svectorial angle, δ is change of current busbar voltage vectorial angle;

The equiva lent impedance Z of controlled capacitance commutation converter _cSCCbe expressed as:

$Z_{\mathrm{CSCC}}=Z_C-\frac{Z_C^2}{(Z_C-Z_L)}\frac{2\mathrm{\&beta;}+\sin 2\mathrm{\&beta;}}{\mathrm{\&pi;}}+\frac{4Z_C^2}{(Z_C-Z_L)}\frac{{\cos }^2\mathrm{\&beta;}}{(k^2-1)}\frac{(k\tan \mathrm{k\&beta;}-\mathrm{tna\&beta;})}{\mathrm{\&pi;}}---\left(3\right)$

Wherein, Z _cfor controlled capacitance branch road capacitor value, Z _lfor inductive branch induction reactance value, the gating advance angle that β is anti-parallel thyristor, k and β are expressed as:

$k=\sqrt{\frac{Z_c}{Z_L}}---\left(4\right)$

β＝π-α （5）

Wherein, the Trigger Angle of α anti-parallel thyristor;

The short circuit ratio SCR of described ac and dc systems is the capacity of short circuit S of ac bus _acwith DC side active power rated value P _dNratio, that is:

$\mathrm{SCR}=\frac{S_{\mathrm{ac}}}{P_{\mathrm{dN}}}=\frac{U_N^2}{P_{\mathrm{dN}}}\&times;\frac{1}{\left|Z_r\right|}---\left(6\right)$

Wherein, U _nfor ac bus rated voltage, Z _rfor see into the equiva lent impedance of AC transmission system from ac bus;

When the short circuit ratio SCR of ac and dc systems gets short circuit ratio critical value CSCR, have:

$\mathrm{CSCR}=\mathrm{SCR}=\frac{U_N^2}{P_{\mathrm{dN}}}\&times;\frac{1}{\left|Z_{\mathrm{cr}}\right|}---\left(7\right)$

The critical equivalent impedance Z of known AC system _crbe expressed as:

$Z_{\mathrm{cr}}=\frac{U_N^2}{P_{\mathrm{dN}}}\&times;\frac{1}{\left|\mathrm{CSCR}\right|}---\left(8\right).$

3. the control method of controlled capacitance in CSCC-HVDC system according to claim 1, is characterized in that: described step 3 comprises the following steps:

Step 3-1: calculate Z _equwith Z _crdifference Z _err, have Z _err=Z _equ-Z _cr;

Step 3-2: setting the target impedance value of adjusting is Z _ref, have Z _ref=Z _err, complete controlled capacitance commutation converter definite value impedance Control.

4. the control method of controlled capacitance in CSCC-HVDC system according to claim 1, is characterized in that: in described step 4, according to change of current bus voltage value U _inv, controlled capacitance commutation converter is carried out to the control of classification switching, minute following three kinds of situations:

A) if U _inv>=0.8pu, the controlled capacitance place branch road in controlled capacitance commutation converter, by short circuit, is equivalent to electrical network commutation converter;

B) if 0.6pu≤U _inv< 0.8pu, the controlled capacitance capacitor value putting into operation is set as AC system equivalent impedance Z _equ20%;

C) if U _inv< 0.6pu, the controlled capacitance capacitor value putting into operation is set as AC system equivalent impedance Z _equ50%.

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