Method for solving for converter valve states and valve currents based on valve-side current timing characteristics

Abstract

The present invention discloses a calculation method for a converter valve state and a valve current based on temporal features of a valve side current, the process is follows: collecting three-phase AC currents, DC currents on a valve side of a converter of a DC transmission system, and trigger pulses of converter valves; establishing a node current equation of the AC currents and valve currents; when detecting a rising edge of a trigger pulse of a converter valve, latching the number of the converter valve; according to the trigger pulses of the converter valves, and amplitude characteristics of the AC currents and characteristics of AC variations, to perform a conducting state and a blocking state judgment of valve states, and obtaining valve states; judging whether each phase has a bypass state; through summing the valve bypass states of the three phases to judge a number of bypass phases; supplementing bypass loop voltage equation; calculating the converter valve currents; when the calculated value of the valve currents is negative, the valve state is corrected to blocking state according to a one-way conductivity of the converter valves, otherwise do not correct; repeating the above steps again to calculate the valve current.

Claims

1. A method for determining a converter valve state and a valve current based on temporal features of a valve side current, for use in fault analysis and control and protection optimization of DC project, characterized in that, the calculation method comprises following steps: S1. collecting three-phase AC currents i.sub.a, i.sub.b, i.sub.c, and DC currents i.sub.dH, i.sub.dN on a converter valve side of a DC transmission system, and trigger pulses FP.sub.1, FP.sub.2, FP.sub.3, FP.sub.4, FP.sub.5, FP.sub.6 of converter valves; S2. according to a topology structure of the converter of the DC transmission system, establishing a node current equation Ai=y of the three-phase AC currents i.sub.a, i.sub.b, i.sub.c, the DC currents i.sub.dH, i.sub.dN and valve currents i.sub.VTm on the valve side; $\begin{array}{cc}\{\begin{array}{c}{i}_{\mathrm{VT}2}+i_{\mathrm{VT}4}+i_{\mathrm{VT}6}=i_{\mathrm{dH}}\\ i_{\mathrm{VT}1}+i_{\mathrm{VT}3}+i_{\mathrm{VT}5}=i_{\mathrm{dN}}\\ i_{\mathrm{VT}4}-i_{\mathrm{VT}1}=i_a\\ i_{\mathrm{VT}6}-i_{\mathrm{VT}3}=i_b\\ i_{\mathrm{VT}2}-i_{\mathrm{VT}5}=i_c\end{array}& \left(\mathrm{G1}\right)\end{array}$ $\mathrm{wherein}A=\left[\begin{array}{cccccc}0& 1& 0& 1& 0& 1\\ 1& 0& 1& 0& 1& 0\\ -1& 0& 0& 1& 0& 0\\ 0& 0& -1& 0& 0& 1\\ 0& 1& 0& 0& -1& 0\end{array}\right],i=\left[\begin{array}{c}{i}_{\mathrm{VT}1}\\ i_{VT2}\\ i_{VT3}\\ i_{VT4}\\ i_{VT5}\\ i_{\mathrm{VT}6}\end{array}\right],y=\left[\begin{array}{c}{i}_{\mathrm{dH}}\\ i_{\mathrm{dN}}\\ i_a\\ i_b\\ i_c\end{array}\right],$ is a valve current coefficient matrix, i is a valve current vector, y is a current vector at both ends of the valve side, i.sub.VTm are respective six converter valve currents, m is a number of six converter valves, m=1, 2, 3, 4, 5, 6; S3. according to the trigger pulses FP.sub.1, FP.sub.2, FP.sub.3, FP.sub.4, FP.sub.5, FP.sub.6 of the converter valves obtained in step S1, when detecting a rising edge of a trigger pulse of a converter valve, latching the number m of the converter valve into a register; S4. according to the trigger pulses FP.sub.1, FP.sub.2, FP.sub.2, FP.sub.4, FP.sub.5, FP.sub.6 of the converter valves obtained in step S1, and amplitude characteristics of the AC currents and characteristics of AC variations, to perform a conducting state and a blocking state judgment of valve states, and obtaining six valve states of the converter valves S.sub.VT1, S.sub.VT2, S.sub.VT3, S.sub.VT4, S.sub.VT5, S.sub.VT6; S5. according to the valve states obtained in step S4, constructing a valve state matrix S: $\begin{array}{cc}S=\left[\begin{array}{cccccc}{s}_{\mathrm{VT}1}& & & & & \\ & s_{\mathrm{VT}2}& & & & \\ & & s_{\mathrm{VT}3}& & & \\ & & & s_{\mathrm{VT}4}& & \\ & & & & s_{\mathrm{VT}5}& \\ & & & & & s_{\mathrm{VT}6}\end{array}\right];& \left(G2\right)\end{array}$ in the formula (G2): S.sub.VT1, S.sub.VT2, S.sub.VT3, S.sub.VT4, S.sub.VT5, S.sub.VT6 are the valve states of the six converter valves; S6. by calculating a product of an upper valve state and a lower valve state of each phase, judging whether each phase has a bypass state, valve bypass states S.sub.a, S.sub.b, S.sub.c of each phase are: $\begin{array}{cc}\{\begin{array}{c}{S}_a=s_{\mathrm{VT}1}{s}_{VT4}\\ S_b=s_{\mathrm{VT}3}{s}_{\mathrm{VT}6}\\ S_c=s_{\mathrm{VT}2}{s}_{\mathrm{VT}5}\end{array}& \left(\mathrm{G3}\right)\end{array}$ in the formula (G3): S.sub.a represents a valve bypass state of phase a, S.sub.b represents a valve bypass state of phase b, S.sub.c represents a valve bypass state of phase c, S.sub.k=1 represents a bypass, S.sub.k=0 represents no bypass, and k is serial numbers of a, b, and c phases; S7. summing the valve bypass states of the three phases to judge a number of bypass phases, where a number of bypass phases S.sub.total is expressed as follows:
S.sub.total=(S.sub.a+S.sub.b+S.sub.c)(G4); in the formula (G4): S.sub.total represents the number of bypass phases in the three phases abc; S8. according to the number of bypass phases and corresponding phases obtained in step S7, supplementing a bypass loop voltage equation C.sub.pi=D.sub.p; wherein C.sub.p is a coefficient matrix of the bypass loop voltage equation, i is a valve current vector, D.sub.p is a constant vector, and p is serial number of the bypass phases; S9. according to step S2 and step S8, and in conjunction with step S4, calculating the valve currents i.sub.VTm: S10. when the valve currents i.sub.VTm calculated in step S9<0, considering a one-way conductivity of the converter valves, correcting the valve states, correcting the valve states to blocking state S.sub.VTm=0, otherwise, do not correct; S11. repeating steps S5 to S10 until the valve currents i.sub.VTm0 is calculated; and S12. coordinating control of the converter valves, based on the corrected valve states and the calculated valve currents.

2. The calculation method for a converter valve state and a valve current based on temporal features of a valve side current according to claim 1, characterized in that, processes of step S4 are as follows: S41. when it is detected in step S1 that the trigger pulses of the converter valves are at a high electric level and a conducting state criterion of the converter valves is satisfied, setting the valve states S.sub.VTm=1, wherein the conducting state criterion of the converter valves is: an amplitude |i.sub.k| of an AC current of the phase where the converter valve numbered m is located is greater than an AC current fixed value, namely:
|i|>I.sub.set3(G5) in the formula (G5): i.sub.k is a, b, c three-phase AC currents, I.sub.set3 is the AC current fixed value; S42. when the converter valve numbered m is in a conducting state S.sub.VTm=1, if the AC current meets a blocking state criterion, then the converter valve numbered m is in a blocking state S.sub.VTm=0; otherwise, the converter valve numbered m continues to maintain the conducting state S.sub.VTm=1, wherein the blocking state criterion of the converter valve is: the amplitude |i.sub.k| of the AC current of the phase where the converter valve numbered m is located is less than a blocking state current threshold fixed value, and a variation rate of the AC current is less than a variation threshold fixed value, namely: $\begin{array}{cc}.Math.i_k.Math.<I_{\mathrm{set}4}\mathrm{and}\frac{.Math.i_{k\left(t\right)}-i_{k\left(t-t\right)}.Math.}{t}<I_{\mathrm{set}5}& \left(\mathrm{G6}\right)\end{array}$ in the formula (G6): I.sub.set4 is the blocking state current threshold fixed value, I.sub.set5 is the variation threshold fixed value; S43. when only an upper valve and a lower valve of a single-phase are conducting, the converter valves operate as a single-phase bypass pair, and the two valves of the bypass pair are judged to be in the conducting state, a single-phase bypass pair judgment condition is: a maximum value of the DC currents minus a maximum value of absolute values of the AC currents is greater than a commutation failure threshold fixed value, and the maximum value of the absolute values of the AC currents is less than a single-phase bypass pair threshold fixed value, namely: $\begin{array}{cc}\{\begin{array}{c}\max \left(i_{\mathrm{dH}},i_{\mathrm{dN}}\right)-\max \left(.Math.i_a.Math.,.Math.i_b.Math.,.Math.i_c.Math.\right)>I_{\mathrm{set}1}\\ \max \left(.Math.i_a.Math.,.Math.i_b.Math.,.Math.i_c.Math.\right)<I_{set2}\end{array}& \left(\mathrm{G7}\right)\end{array}$ in the formula (G7): I.sub.set1 is the commutation failure threshold fixed value, I.sub.set2 is the single-phase bypass pair threshold fixed value; S44. when the AC and DC currents satisfy formula (G7) of step S43, the converter valves operate as a single-phase bypass pair, wherein a valve state judgment condition of the single-phase bypass pair is: 1) When bypass state flags of all valves S.sub.bypass_m=0, if a register signal in step S3 is equal to m, a phase where the converter valve numbered m is located is a bypass phase, setting the bypass state flags of the two valves in the bypass phase equal to 1, keeping the two valves of the bypass phase in the conducting state, namely: S.sub.bypass_m=1 and S.sub.bypass_(mod(m+3,6))=1, S.sub.VTm=1 and S.sub.VT(mod(m+3,6))=1; 2) When the bypass status flags of the phase where the converter valve numbered m is located is S.sub.bypass_m=1 or S.sub.bypass_(mod(m+3,6))=1, keeping the two valves of the phase where the converter valve numbered m is located in the conducting state.

3. The calculation method for a converter valve state and a valve current based on temporal features of a valve side current according to claim 1, characterized in that, processes of step S8 are as follows: S81. according to the number of bypass phases obtained in step S7, when the number of bypass phases S.sub.total1, the converter valves operate without a bypass pair or one phase bypass pair, at this time, there is no need to construct the bypass loop voltage equation, skip to step S91, otherwise, go to step S82; S82. when the number of bypass phases S.sub.total=2, it indicates that two phase bypass pairs of the converter valves operate, supplementing the bypass loop voltage equation C.sub.pi=D.sub.p, skip to step S92; otherwise, go to step S83; wherein, 1) If S.sub.a=S.sub.b, then a following relationship exists:
i.sub.VT1+i.sub.VT4=i.sub.VT3+i.sub.VT6(G8) supplementing the bypass loop voltage equation C.sub.abi=D.sub.ab, wherein C.sub.ab=[1 0 1 1 0 1], D.sub.ab=[0], p=ab, C.sub.ab is a coefficient matrix of the bypass loop voltage equation when the ab phases bypass, D.sub.ab is a constant vector of the bypass loop voltage equation when the ab phases bypass; 2) If S.sub.a=S.sub.c, then a following relationship exists:
i.sub.VT1+i.sub.VT4=i.sub.VT2+i.sub.VT5(G9) supplementing the bypass loop voltage equation C.sub.aci=D.sub.ac, wherein C.sub.ac=[1 1 0 1 1 0], D.sub.ac=[0], p=ac, C.sub.ac is a coefficient matrix of the bypass loop voltage equation when the ac phase bypass, and D.sub.ac is a constant vector of the bypass loop voltage equation when the ac phase bypass; 3) If S.sub.b=S.sub.c, then a following relationship exists:
i.sub.VT2+i.sub.VT5=i.sub.VT3+i.sub.VT6(G10) supplementing the bypass loop voltage equation C.sub.bci=D.sub.bc, C.sub.bc=[0 1 1 0 1 1], D.sub.bc=[0], p=bc, C.sub.bc is a coefficient matrix of the bypass loop voltage equation when the bc phase bypass, and D.sub.bc is a constant vector of the bypass loop voltage equation when the bc phase bypass; S83. when the number of bypass phases S.sub.total=3, it indicates that three phase bypass pairs of the converter valves operate, supplementing the bypass loop voltage equation C.sub.pi=D.sub.p, skip to step S92; at the same time, a following relationship exists: $\begin{array}{cc}\{\begin{array}{c}{i}_{\mathrm{VT}1}+i_{VT4}=i_{VT3}+i_{VT6}\\ i_{\mathrm{VT}1}+i_{VT4}=i_{VT2}+i_{VT5}\end{array}& \left(\mathrm{G11}\right)\end{array}$ supplementing the bypass loop voltage equation C.sub.abci=D.sub.abc, wherein C.sub.abc= $\left[\begin{array}{cccccc}1& -1& 0& 1& -1& 0\\ 1& 0& -1& 1& 0& -1\end{array}\right],$ D.sub.abc=[0 0]T, p=abc, C.sub.abc is a coefficient matrix of the bypass loop voltage equation when the abc phases bypass, D.sub.abc is a constant vector of the bypass loop voltage equation when the abc phases bypass; processes of step S9 are as follows: S91. constructing a state equation based on valve state characteristics ASi=y, inverting its coefficient matrix to calculate the valve current i.sub.VTm:
i=(AS).sup.1y(G12) in the formula (G12): i=[i.sub.VT1 i.sub.VT2 i.sub.VT3 i.sub.VT4 I.sub.VT5 i.sub.VT6].sup.T; S92. combining Ai=y and C.sub.pi=D.sub.p, combining the valve state characteristics S to construct a state equation ESi=z, inverting its coefficient matrix, and calculating the valve current i.sub.VTm:
i=(ES).sup.1x(G13) in the formula (G13): i=[i.sub.VT1 i.sub.VT2 i.sub.VT3 i.sub.VT4 i.sub.VT5 i.sub.VT6].sup.T, $E=\left[\begin{array}{c}A\\ C_p\end{array}\right],z=\left[\begin{array}{c}y\\ D_p\end{array}\right],$ p is a serial number of a bypass phase, E is a coefficient matrix formed by the coefficient matrix A and the coefficient matrix C.sub.p, z is a vector formed by the current vector y and the constant vector D.sub.p at both ends of the valve side.

4. The calculation method for a converter valve state and a valve current based on temporal features of a valve side current according to claim 1, characterized in that, in step S3, when a rising edge of a trigger pulse of a converter valve is detected FP.sub.m(t)FP.sub.m(tt)=1, latching the number m of the converter valve into the register; wherein FP.sub.m(t) is a trigger pulse signal of the converter valve numbered m at time t, FP.sub.m(tt) is a trigger pulse signal of the converter valve numbered m at time (tt), t is a time at a certain moment, t is a sampling time interval.

Description

DESCRIPTION OF FIGURES

(1) FIG. 1 is a flow chart of a valve state judgment and a valve current calculation of the present invention, clockVT represents a symbol of a register, t_end represents a running end time;

(2) FIG. 2 is an illustrative diagram of a valve state and a valve current calculation model of the present invention, wherein i.sub.a, i.sub.b, i.sub.c, i.sub.dH, i.sub.dN are input ports for collecting current on a valve side of a converter, FP.sub.1, FP.sub.2, FP.sub.3, FP.sub.4, FP.sub.5, FP.sub.6 are collecting input ports for trigger pulses of converter valves, s.sub.VT1, s.sub.VT2, s.sub.VT3, s.sub.VT4, s.sub.VT5, S.sub.VT6 are output ports of valve status signals; i.sub.VT1 i.sub.VT2 i.sub.VT3 i.sub.VT4 i.sub.VT5 i.sub.VT6 are output ports of valve current calculations;

(3) FIG. 3 is an illustrative diagram of a converter valve for a high-voltage DC transmission of the present invention, wherein i.sub.a, i.sub.b, i.sub.c are the a phase, b phase, and c phase of a three-phase AC current, i.sub.dH is a high-voltage side DC current, i.sub.dN is a low-voltage side DC current, i.sub.VT1 i.sub.VT2 i.sub.VT3 i.sub.VT4 i.sub.VT5 i.sub.VT6 are valve currents, directions indicated by arrows in the figure is positive currents;

(4) FIG. 4 are waveforms of three-phase AC currents i.sub.a, i.sub.b, i.sub.c, and a DC current i.sub.d max on a valve side of a converter valve of the present invention, and the DC current i.sub.d max is the maximum value of the high-voltage side DC current i.sub.dH and the low-voltage side DC current i.sub.dN, that is i.sub.d max=max(i.sub.dH, i.sub.dN);

(5) FIG. 5 are trigger pulses of six converter valves of a HVDC power transmission of the present invention, FP.sub.1, FP.sub.2, FP.sub.3, FP.sub.4, FP.sub.5, FP.sub.6 are the trigger pulses of the six converter valves;

(6) FIG. 6 is a current loop diagram of a state judgment of bypass pair of the converter valve of the present invention;

(7) FIG. 7 is a flow chart of a valve state judgment of six pulses of the present invention;

(8) FIG. 8 is a valve state judging flow chart of the converter valve numbered 1 of the present invention;

(9) FIG. 9 is a bypass pair state judging flow chart of the present invention;

(10) FIG. 10 is a valve state waveform diagram based on the variation characteristics of the AC current of the present invention, in the figure, s.sub.VT1, s.sub.VT2, s.sub.VT3, s.sub.VT4, s.sub.VT5, S.sub.VT6 are valve state signals;

(11) FIG. 11 is a flow chart of supplementing bypass equation of the present invention;

(12) FIG. 12 is a valve current waveform diagram calculated based on valve states of the present invention, in the figure, i.sub.VT1 i.sub.VT2 i.sub.VT3 i.sub.VT4 i.sub.VT5 i.sub.VT6 are valve current signals;

(13) FIG. 13 is a valve state waveform diagram considering a one-way conductivity correction of a valve of the present invention, in the figure, s.sub.VT1, s.sub.VT2, s.sub.VT3, s.sub.VT4, s.sub.VT5, S.sub.VT6 are valve state signals;

(14) FIG. 14 is a valve current waveform diagram calculated based on corrected valve states of the present invention, in the figure, i.sub.VT1 i.sub.VT2 i.sub.VT3 i.sub.VT4 i.sub.VT5 i.sub.VT6 are valve current signals.

DETAILED DESCRIPTION

(15) In order to make the purposes, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are some, but not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

Embodiments

(16) The invention discloses a calculation method for a converter valve state and a valve current based on temporal features of a valve side current, and the method is used to judge the valve state and calculate the valve current in actual power grid project. The operating conditions of the converter valve after an AC fault change differently. In order to more comprehensively cover the operating conditions of the converter valve after the fault, this embodiment takes a three-phase fault of a six-pulse inverter side commutation bus as an example. The present invention will be further described in detail according to FIGS. 1 and 2.

(17) S1. first, the converter involved in the present invention is briefly described. The commutation topology of this embodiment is shown in FIG. 3. In the figure, six valve arms are numbered in the order in which they are normally opened. VT1 represents the converter valve numbered 1, VT2 represents the converter valve numbered 2, VT3 represents the converter valve numbered 3, VT4 represents the converter valve numbered 4, VT5 represents the converter valve numbered 5, VT6 represents the converter valve numbered 6. Valve VT4, valve VT6, valve VT2 form an upper bridge arm. Valve VT1, valve VT3, valve VT5 form a lower bridge arm. Collecting three-phase AC currents i.sub.a, i.sub.b, i.sub.c and DC currents i.sub.dH, idN on a valve side of a converter of FIG. 3 of a DC transmission system, and trigger pulses FP.sub.1, FP.sub.2, FP.sub.3, FP.sub.4, FP.sub.5, FP.sub.6 of converter valves. According to the collected signals, the current waveforms of the three-phase AC currents and DC currents on the valve side before and after the fault are obtained as shown in FIG. 4. The trigger pulse waveforms of the six converter valves are shown in FIG. 5;

(18) S2. according to a topology structure of the converter of the DC transmission system in step S1, according to Kirchhoff's law, establishing a node current equation Ai=y of the three-phase AC currents i.sub.a, i.sub.b, i.sub.c, DC currents i.sub.dH, i.sub.dN and valve currents i.sub.VTm on the valve side;

(19) $\begin{array}{cc}\{\begin{array}{c}{i}_{\mathrm{VT}2}+i_{\mathrm{VT}4}+i_{\mathrm{VT}6}=i_{\mathrm{dH}}\\ i_{\mathrm{VT}1}+i_{\mathrm{VT}3}+i_{\mathrm{VT}5}=i_{\mathrm{dN}}\\ i_{\mathrm{VT}4}-i_{\mathrm{VT}1}=i_a\\ i_{\mathrm{VT}6}-i_{\mathrm{VT}3}=i_b\\ i_{\mathrm{VT}2}-i_{\mathrm{VT}5}=i_c\end{array}& \left(1\right)\end{array}$$\mathrm{wherein}A=\left[\begin{array}{cccccc}0& 1& 0& 1& 0& 1\\ 1& 0& 1& 0& 1& 0\\ -1& 0& 0& 1& 0& 0\\ 0& 0& -1& 0& 0& 1\\ 0& 1& 0& 0& -1& 0\end{array}\right],i=\left[\begin{array}{c}{i}_{VT1}\\ i_{VT2}\\ i_{VT3}\\ i_{VT4}\\ i_{VT5}\\ i_{VT6}\end{array}\right],y=\left[\begin{array}{c}{i}_{\mathrm{dH}}\\ i_{\mathrm{dN}}\\ i_a\\ i_b\\ i_c\end{array}\right],$
A is a valve current coefficient matrix, i is a valve current vector, y is a current vector at both ends of the valve side, i.sub.VTm are respective six converter valve currents, m is a number of six converter valves, m=1, 2, 3, 4, 5, 6;

(20) S3. according to the trigger pulses FP.sub.1, FP.sub.2, FP.sub.3, FP.sub.4, FP.sub.5, FP.sub.6 of the converter valves collected in step S1, when a rising edge of a trigger pulse of a converter valve is detected FP.sub.m(t)FP.sub.m(tt)=1, latching the number m of the converter valve into the register; wherein FP.sub.m(t) is a trigger pulse signal of the converter valve numbered m at time t, FP.sub.m(tt) is a trigger pulse signal of the converter valve numbered m at time (tt), t is a time at a certain moment, t is a sampling time interval;

(21) For example: when the rising edge of the trigger pulse of the converter valve numbered 1 is detected, the number 1 is stored in the register;

(22) S4. combining steps S1 and S3, according to trigger pulses of the converter valves, the amplitude characteristics of the AC currents and the variation characteristics of the AC currents, performing a conducting state and a blocking state judgment of valve states, obtaining the valve states s.sub.VT1, s.sub.VT2, s.sub.VT3, s.sub.VT4, s.sub.VT5, S.sub.VT6. The valve state judgment of this method mainly uses the amplitude characteristics of the AC current. However, when the converter valves only have same-phase upper and lower arms conducting to form a single-phase bypass pair, as shown in FIG. 6, at this time, the three-phase AC current is 0, the AC and DC currents are completely isolated, and the AC currents cannot be used to judge the valve state. Therefore, it is necessary to judge the valve state of the single-phase bypass pair separately, wherein the flow chart of the complete valve state judgment is shown in FIG. 7. The figure includes a sub-module for judging the valve states of the six converter valves and a sub-module for judging the state of the single-phase bypass pair. The flow chart of the sub-module for judging the valve state of the six converter valves, taking valve VT1 as an example, is shown in FIG. 8. The sub-module for judging the state of the single-phase bypass pair is shown in FIG. 9. According to the flow charts of FIGS. 7, 8, and 9, the waveforms of the six valve states obtained based on the variation characteristics of the AC current are shown in FIG. 10. s.sub.VT1, s.sub.VT2, s.sub.VT3, s.sub.VT4, s.sub.VT5, S.sub.VT6 represent the valve states of the six converter valves respectively;

(23) In this embodiment, with reference to the flow chart of the valve state judgment in FIG. 7, step S4 specifically comprises the following steps:

(24) S41. according to step S1, taking the valve VT1 as an example, the valve state judgment flow chart of the valve VT1 is shown in FIG. 8. When it is detected that the trigger pulse of the VT1 converter valve is at a high electric level and a conducting state criterion of the converter valve is satisfied, setting the valve state s.sub.VT1=1. Wherein the conducting state criterion of the converter valve is: the amplitude |i.sub.a| of the AC current of the phase a where the valve VT1 is located is greater than the AC current fixed value. That is:
|i.sub.k|>I.sub.set3(2)
in the formula: i.sub.a is the a phase AC current; I.sub.set3 is an AC current fixed value.

(25) S42. according to step S41, when the valve VT1 is in the conducting state (s.sub.VT1=1), if the AC current satisfies the blocking state criterion, then the valve VT1 is in the blocking state (s.sub.vT1=0). Otherwise, valve VT1 continues to remain in the conducting state (s.sub.VT1=1). Wherein the blocking state criterion of the converter valve is: the amplitude |i.sub.a| of the AC current of phase a where the valve VT1 is located is less than a blocking state current threshold fixed value, and the variation rate of the AC current is less than a variation threshold fixed value. That is:

(26) 0$\begin{array}{cc}.Math.i_k.Math.<I_{\mathrm{set}4}\mathrm{and}\frac{.Math.i_{k\left(t\right)}-i_{k\left(t-t\right)}.Math.}{t}<I_{\mathrm{set}5}& \left(3\right)\end{array}$ in the formula: I.sub.set4 is the blocking state current threshold fixed value, I.sub.set5 is the variation threshold fixed value; S43. according to step S1, step S41 and step S42, when only an upper valve and a lower valve of a single-phase are conducting, the converter valves operate as a single-phase bypass pair, and the two valves of the bypass pair are judged to be in the conducting state. A maximum value of the DC currents minus a maximum value of absolute values of the AC currents is greater than a commutation failure threshold fixed value, and the maximum value of the absolute values of the AC currents is less than a single-phase bypass pair threshold fixed value. That is:

(27) $\begin{array}{cc}\{\begin{array}{c}\max \left(i_{\mathrm{dH}},i_{\mathrm{dN}}\right)-\max \left(.Math.i_a.Math.,.Math.i_b.Math.,.Math.i_c.Math.\right)>I_{\mathrm{set}1}\\ \max \left(.Math.i_a.Math.,.Math.i_b.Math.,.Math.i_c.Math.\right)<I_{set2}\end{array},& \left(4\right)\end{array}$ in the formula: I.sub.set1 is the commutation failure threshold fixed value, I.sub.set2 is the single-phase bypass pair threshold fixed value. S44. according to S1, step S3 and step S43, when the AC and DC currents satisfy the formula (4) of step S43, the converter valve operates as a single-phase bypass pair and the flow chart of state judgment under the bypass pair condition is shown in FIG. 9. The valve state of the single-phase bypass pair is judged as: 1) When the bypass state flags of all valves are s.sub.bypass_m=0, if the register signal in step S3 is equal to 1 or 4, phase a bypasses. Then the bypass state flags s.sub.bypass_1=1 and s.sub.bypass_4=1, the valve state s.sub.VT1=1 and s.sub.VT4=1; Otherwise: if the register signal in step S3 is equal to 3 or 6, phase b bypasses. Then s.sub.bypass_3=1 and s.sub.bypass_6=1, s.sub.VT3=1 and s.sub.VT6=1; Otherwise: if the register signal in step S3 is equal to 3 or 6, phase c bypasses. Then s.sub.bypass_2=1 and s.sub.bypass_5=1, s.sub.VT2=1 and s.sub.VT5=1; 2) When the bypass status flag is s.sub.bypass_1=1 or s.sub.bypass_4=1, maintaining the two valves of phase a continue to be in the conducting state; Otherwise: when the bypass status flag is s.sub.bypass_3=1 or s.sub.bypass_6=1, maintaining the two valves of phase b continue to be in the conducting state; Otherwise: when the bypass status flag is s.sub.bypass_2=1 or s.sub.bypass_5=1, maintaining the two valves of phase c continue to be in the conducting state; S5. according to the valve states obtained in step S4, constructing a valve state matrix S:

(28) $\begin{array}{cc}S=\left[\begin{array}{cccccc}{s}_{\mathrm{VT}1}& & & & & \\ & s_{\mathrm{VT}2}& & & & \\ & & s_{\mathrm{VT}3}& & & \\ & & & s_{\mathrm{VT}4}& & \\ & & & & s_{\mathrm{VT}5}& \\ & & & & & s_{\mathrm{VT}6}\end{array}\right]& \left(5\right)\end{array}$ in the formula: s.sub.VT1, s.sub.VT2, s.sub.VT3, s.sub.VT4, s.sub.VT5, S.sub.VT6 are the valve states of the six converter valves; S6. according to step S5, by calculating a product of an upper valve state and a lower valve state of each phase, judging whether each phase has a bypass state, valve bypass states S.sub.a, S.sub.b, S.sub.c of each phase are:

(29) $\begin{array}{cc}\{\begin{array}{c}{S}_a=s_{\mathrm{VT}1}{s}_{VT4}\\ S_b=s_{VT3}{s}_{VT6}\\ S_c=s_{VT2}{s}_{VT5}\end{array}& \left(6\right)\end{array}$ in the formula: S.sub.a represents a valve bypass state of phase a, S.sub.b represents a valve bypass state of phase b, S.sub.c represents a valve bypass state of phase c, S.sub.k=1 represents a bypass, S.sub.k=0 represents no bypass, and k represents three phases of a, b, and c; S7. according to step S6, summing the valve bypass states of the three phases to judge a number of bypass phases. A number of bypass phases S.sub.total is expressed as follows:
S.sub.total=(S.sub.a+S.sub.b+S.sub.c)(7); in the formula: S.sub.total represents the number of bypass phases in the three phases abc; S8. according to the number of bypass phases and the corresponding phases obtained in step S7, when S.sub.total>1, there are two or more bypass pairs in the converter valves, the valve current cannot be calculated directly according to the node current equation of the converter valves according to formula (1), or the equation solution is not unique, so it is necessary to supplement the bypass loop voltage equation C.sub.pi=D.sub.p. The flow chart of the supplementing judgment is shown in FIG. 11, wherein C.sub.p is a coefficient matrix of the bypass loop voltage equation, i is a valve current vector, D.sub.p is a constant vector, and p is serial number of the bypass phases;

(30) In this embodiment, the flow chart of supplementing the bypass loop voltage equation is shown in FIG. 11. Step S8 specifically comprises the following steps: S81. according to the number of bypass phases obtained in step S7, when the number of bypass phases S.sub.total1, there is no need to construct the bypass loop voltage equation, skip to step S91; otherwise, go to step S82; S82. according to step S7 and step S81, when the number of bypass phases S.sub.total=2, it indicates that two phase bypass pairs of the converter valves operate, skip to step S92; otherwise, go to step S83.

(31) If: S.sub.a=S.sub.b, then a following relationship exists:
i.sub.VT1+i.sub.VT4=i.sub.VT3+i.sub.VT6(8) supplementing the bypass loop voltage equation C.sub.abi=D.sub.ab, wherein C.sub.ab=[1 0 1 1 0 1], D.sub.ab=[0], C.sub.ab is a coefficient matrix of the bypass loop voltage equation when the ab phases bypass, D.sub.ab is a constant vector of the bypass loop voltage equation when the ab phases bypass; when in the bypass condition of S.sub.a=S.sub.c and S.sub.b=S.sub.c, it is only needed to modify the corresponding bypass phases in the formula (8). S83. according to step S7, step S81 and step S82, when the number of bypass phases S.sub.total=3, it indicates that three phase bypass pairs of the converter valves operate, skip to step S92; at the same time. Then a following relationship exists:

(32) $\begin{array}{cc}\{\begin{array}{c}{i}_{\mathrm{VT}1}+i_{\mathrm{VT}4}=i_{VT3}+i_{\mathrm{VT}6}\\ i_{\mathrm{VT}1}+i_{\mathrm{VT}4}=i_{VT2}+i_{\mathrm{VT}5}\end{array}& \left(9\right)\end{array}$ in supplementing the bypass loop voltage equation C.sub.abci=D.sub.abc,

(33) $c_{\mathrm{abc}}=\left[\begin{array}{cccccc}1& -1& 0& 1& -1& 0\\ 1& 0& -1& 1& 0& -1\end{array}\right],$
D.sub.abc=[0 0].sup.T, C.sub.abc is a coefficient matrix of the bypass loop voltage equation when the abc phases bypass, D.sub.abc is a constant vector of the bypass loop voltage equation when the abc phases bypass; S9. according to step S2 and step S8, and in conjunction with step S4, calculating the converter valve currents i.sub.VTm, obtaining the valve currents i.sub.VT1 i.sub.VT2 i.sub.VT3 i.sub.VT4 i.sub.VT5 i.sub.VT6 calculated on the basis of the valve state obtained based on the variation characteristics of the AC currents, comparing the calculated valve current with the simulated value in DC project, The waveform is shown in FIG. 12. It is found that there is a significant difference between the calculated value and the simulated value, and there is a situation where the calculated value of the valve current has a negative value, and the calculation result needs to be corrected;

(34) In this embodiment, in conjunction with FIGS. 1 and 11, step S9 specifically comprises the following steps: S91. according to step S2, step S4 and step S81, constructing a state equation ASi=y based on valve state characteristics, and calculating valve current i.sub.VTm. Wherein:

(35) $\begin{array}{cc}ASi=\left[\begin{array}{cccccc}0& s_{\mathrm{VT}2}& 0& s_{\mathrm{VT}4}& 0& s_{\mathrm{VT}6}\\ s_{\mathrm{VT}1}& 0& s_{\mathrm{VT}3}& 0& s_{\mathrm{VT}5}& 0\\ -s_{\mathrm{VT}1}& 0& 0& s_{\mathrm{VT}4}& 0& 0\\ 0& 0& -s_{\mathrm{VT}3}& 0& 0& s_{\mathrm{VT}6}\\ 0& s_{\mathrm{VT}2}& 0& 0& -s_{\mathrm{VT}5}& 0\end{array}\right]\left[\begin{array}{c}{i}_{\mathrm{VT}1}\\ i_{\mathrm{VT}2}\\ i_{\mathrm{VT}3}\\ i_{\mathrm{VT}4}\\ i_{\mathrm{VT}5}\\ i_{\mathrm{VT}6}\end{array}\right]=\left[\begin{array}{c}{i}_{dH}\\ i_{dN}\\ i_a\\ i_b\\ i_c\end{array}\right]& \left(10\right)\end{array}$ in the formula,

(36) $AS=\left[\begin{array}{cccccc}0& s_{\mathrm{VT}2}& 0& s_{\mathrm{VT}4}& 0& s_{\mathrm{VT}6}\\ s_{\mathrm{VT}1}& 0& s_{\mathrm{VT}3}& 0& s_{\mathrm{VT}5}& 0\\ -s_{\mathrm{VT}1}& 0& 0& s_{\mathrm{VT}4}& 0& 0\\ 0& 0& -s_{\mathrm{VT}3}& 0& 0& s_{\mathrm{VT}6}\\ 0& s_{\mathrm{VT}2}& 0& 0& -s_{\mathrm{VT}5}& 0\end{array}\right],$
AS is the coefficient matrix. inverting the coefficient matrix AS. Calculating valve current i.sub.TVm:
i=(AS).sup.1y(11) in the formula: i=[i.sub.VT1 i.sub.VT2 i.sub.VT3 i.sub.VT4 i.sub.VT5 i.sub.VT6].sup.T. S92. according to step S2, step S4, step S81 and step S83, combining Ai=y and C.sub.pi=D.sub.p, combining the valve state characteristics S to construct a state equation ESi=z, inverting its coefficient matrix, and calculating the valve current i.sub.Vtm.

(37) For example: when S.sub.a=S.sub.b, combining equations (1) and (8) to construct the state equation E.sub.abSi=z.sub.ab:

(38) $\begin{array}{cc}{E}_{ab}\mathrm{Si}=\left[\begin{array}{cccccc}0& s_{\mathrm{VT}2}& 0& s_{\mathrm{VT}4}& 0& s_{\mathrm{VT}6}\\ s_{\mathrm{VT}1}& 0& s_{\mathrm{VT}3}& 0& s_{\mathrm{VT}5}& 0\\ -s_{\mathrm{VT}1}& 0& 0& s_{\mathrm{VT}4}& 0& 0\\ 0& 0& -s_{\mathrm{VT}3}& 0& 0& s_{\mathrm{VT}6}\\ 0& s_{\mathrm{VT}2}& 0& 0& -s_{\mathrm{VT}5}& 0\\ s_{\mathrm{VT}1}& 0& -s_{\mathrm{VT}3}& s_{\mathrm{VT}4}& 0& -s_{\mathrm{VT}6}\end{array}\right]\left[\begin{array}{c}{i}_{\mathrm{VT}1}\\ i_{\mathrm{VT}2}\\ i_{\mathrm{VT}3}\\ i_{\mathrm{VT}4}\\ i_{\mathrm{VT}5}\\ i_{\mathrm{VT}6}\end{array}\right]=\left[\begin{array}{c}{i}_{\mathrm{dH}}\\ i_{\mathrm{dN}}\\ i_a\\ i_b\\ i_c\\ 0\end{array}\right]& \left(12\right)\end{array}$ in the formula,

(39) $E_{ab}S=\left[\begin{array}{cccccc}0& s_{\mathrm{VT}2}& 0& s_{\mathrm{VT}4}& 0& s_{\mathrm{VT}6}\\ s_{\mathrm{VT}1}& 0& s_{\mathrm{VT}3}& 0& s_{\mathrm{VT}5}& 0\\ -s_{\mathrm{VT}1}& 0& 0& s_{\mathrm{VT}4}& 0& 0\\ 0& 0& -s_{\mathrm{VT}3}& 0& 0& s_{\mathrm{VT}6}\\ 0& s_{\mathrm{VT}2}& 0& 0& -s_{\mathrm{VT}5}& 0\\ s_{\mathrm{VT}1}& -s_{\mathrm{VT}2}& 0& s_{\mathrm{VT}4}& -s_{\mathrm{VT}5}& 0\end{array}\right],$$i=\left[\begin{array}{c}{i}_{\mathrm{VT}1}\\ i_{\mathrm{VT}2}\\ i_{\mathrm{VT}3}\\ i_{\mathrm{VT}4}\\ i_{\mathrm{VT}5}\\ i_{\mathrm{VT}6}\end{array}\right],z_{ab}=\left[\begin{array}{c}{i}_{\mathrm{dH}}\\ i_{\mathrm{dN}}\\ i_a\\ i_b\\ i_c\\ 0\end{array}\right].$

(40) Inverting the coefficient matrix E.sub.abS to calculate the valve current i.sub.VTm:
i=(E.sub.abS).sup.1z.sub.ab(13) in the formula: i=[i.sub.VT1 i.sub.VT2 i.sub.VT3 i.sub.VT4 i.sub.VT5 i.sub.VT6].sup.T,

(41) 0$E_{ab}=\left[\begin{array}{c}A\\ C_{ab}\end{array}\right],z_{ab}=\left[\begin{array}{c}y\\ D_{ab}\end{array}\right],$
p=ab, E.sub.ab is a coefficient matrix formed by the coefficient matrix A and the coefficient matrix C.sub.ab, z.sub.ab is a vector formed by the current vector y and the constant vector D.sub.ab at both ends of the valve.

(42) The similar situations occur for other bypass conditions. S10. according to step S9, when the valve currents i.sub.VTm (m=1, 2, 3, 4, 5, 6) calculated in step S9<0, considering a one-way conductivity of the converter valves, in a specified positive direction, the current of the converter valves can only be a positive value, correcting the valve states at this time, and correcting the valve state to blocking state s.sub.VTm=0, otherwise, do not correct; S11. repeating steps S5 to S10 until the valve currents i.sub.VTm0 is calculated. The corrected valve state considering the unidirectional conductivity of the valve is shown in FIG. 13, s.sub.VT1, s.sub.VT2, s.sub.VT3, s.sub.VT4, s.sub.VT5, S.sub.VT6 are the corrected valve states of the six converter valves; recalculating the valve currents i.sub.VT1, i.sub.VT2, i.sub.VT3, i.sub.VT4, i.sub.VT5, i.sub.VT6 based on the corrected valve states, comparing the calculated valve currents with the simulated values after the valve states are corrected, and the waveform is shown in FIG. 14. It is found that the calculated values are consistent with the simulated values, indicating the effectiveness of this method.

(43) To sum up, a calculation method for a converter valve state and a valve current based on temporal features of a valve side current proposed in this embodiment uses trigger pulse signals of the converter valves and the variation characteristics of the AC current amplitude to judge the valve states; based on the valve states, the valve currents are calculated by combining a topological relationship between the AC and DC currents and the valve currents. Considering the one-way conductivity of the valves to correct the valve states, and the valve currents are recalculated so as to obtain the valve states and valve currents of the entire operation process. According to FIG. 14, the calculated value of the valve currents are consistent with the simulated values in actual DC power grid, which verifies the effectiveness of the method of the present invention. This method may be applied to practical project, and is very important for project fault analysis and control coordination.

(44) The above-mentioned embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the above-mentioned embodiments, and any other changes, modifications, substitutions, combinations, simplifications should be equivalent replacement manners, which are all included in the protection scope of the present invention.