SUPPRESSION METHOD OF HIGH-FREQUENCY RESONANCE FOR VSC-HVDC SYSTEM BASED ON NONLINEAR FILTER IN VOLTAGE FEED-FORWARD CONTROL

Abstract

A suppression method of high-frequency resonance for VSC-HVDC (Voltage Source Converter-High Voltage Direct Current Transmission) system based on nonlinear filter in voltage feed-forward control. It includes that the nonlinear filter is used for the voltage feed-forward link in the current inner loop control. The specific method is to extract each sequence component of the AC (Alternating Current) voltage, calculate the average value of the sequence component at N sample points, compare the current average value with the output result of nonlinear filter in the previous period, and output the result of nonlinear filter in the current period. After corrected, the result is a fixed value varying according to gradient which acts on the voltage feed-forward link of the inner loop. The invention can effectively suppress the high frequency resonance caused by impedance matching between VSC-HVDC system and grid system without losing the rapidity, and at the same time, it can adjust with AC voltage when voltage disturbance such as AC fault occurs, so as to reduce the risk of over-current of the converter valve. At present, the invention only needs to be realized in the secondary control software link without adding a new equipment, which is also low cost, no occupancy of land and convenient for engineering practice.

Claims

1. A suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control, wherein said a nonlinear filter is used for the voltage feed-forward link in the current inner loop control of VSC-HVDC system, the specific method is to extract each sequence component of the AC voltage, calculate the average value of the sequence component at sample points, compare the current average value with the output result of nonlinear filter in the previous period, and correct the output result of nonlinear filter in the current period, after corrected, the result acts on the voltage feed-forward link of the inner loop.

2. A suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control, wherein the steps of the method are as follows: S1: extract the grid voltage and convert the per-unit value of three-phase instantaneous AC voltage into each sequence component according to Parker transformation; S2: calculate the average values of four voltage sequence components at N sampling points and obtain U.sub.dp_NAV, U.sub.qp_NAV, U.sub.dn_NAV, U.sub.qn_NAV; S3: compare the current average value of each voltage sequence component with the output of the nonlinear filter in the previous period separately and calculate the deviation D.sub.udp_y, D.sub.uqp_y, D.sub.udn_y, D.sub.uqn_y; S4: correct the output results of respective nonlinear filters in the current period according to the respective voltage deviation values and the fixed values V.sub.udp, V.sub.uqp, V.sub.udn, V.sub.uqn with gradient change are obtained; S5: the output results V.sub.udp, V.sub.uqp, V.sub.udn, V.sub.uqn of the nonlinear filters are fixed values which are varied with gradient and acting on the voltage feed-forward link of the inner loop.

3. The suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control according to claim 2, wherein the per-unit value of three-phase instantaneous AC voltage is converted into each sequence component at step S1, specifically: D-axis positive sequence voltage component U.sub.dp, Q-axis positive sequence voltage component U.sub.qp, D-axis negative sequence voltage component U.sub.dn and Q-axis negative sequence voltage component U.sub.dn.

4. The suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control according to claim 2, wherein the specific method to calculate the average value of each voltage sequence component at step S2 is as follows:
U.sub.dp_NAV(U.sub.dp+U.sub.dp_1+ . . . +U.sub.dp_N-2+U.sub.dp_N-1)/N
U.sub.qp_NAV(U.sub.qp+U.sub.qp_1+ . . . +U.sub.qp_N-2+U.sub.qp_N-1)/N
U.sub.dn_NAV(U.sub.dn+U.sub.dn_1+ . . . +U.sub.dn_N-2+U.sub.dn_N-1)/N
U.sub.qn_NAV(U.sub.qn+U.sub.qn_1+ . . . +U.sub.qn_N-2+U.sub.qn_N-1)/N wherein, U.sub.dp_n, U.sub.qp_n, U.sub.dn_n U.sub.qn_n are the values at N sampling points of the D-axis positive sequence voltage component U.sub.dp, the Q-axis positive sequence voltage component U.sub.qp, the D-axis negative sequence voltage component U.sub.dn and the Q-axis negative sequence voltage component U.sub.qn respectively, The value of n is from 0 to N1, a total of N sampling points, U.sub.dp_NAV, U.sub.qp_NAV, U.sub.dn_NAV, U.sub.qn_NAV are the average value of N sample points of D-axis positive sequence voltage component, the Q-axis positive sequence voltage component, the D-axis negative sequence voltage component and the Q-axis negative sequence voltage component respectively.

5. The suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control according to claim 4, wherein the deviations D.sub.udp_y, D.sub.uqp_y, D.sub.udn_y, D.sub.uqn_y at step S3 are calculated as:
D.sub.udp_y=U.sub.dp_NAVV.sub.udp_1
D.sub.uqp_y=U.sub.qp_NAVV.sub.uqp_1
D.sub.udn_y=U.sub.dn_NAVV.sub.udn_1
D.sub.uqn_y=U.sub.qn_NAVV.sub.uqn_1 wherein, V.sub.udp_1, V.sub.uqp_1, V.sub.udn_1, V.sub.uqn_1 are the output values of the nonlinear filter of each sequence component in the previous period respectively; D.sub.udp_y, D.sub.uqp_y, D.sub.udn_y, D.sub.uqn_y are the deviation of D-axis positive sequence voltage component, the Q-axis positive sequence voltage component, the D-axis negative sequence voltage component and the Q-axis negative sequence voltage component respectively.

6. The suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control according to claim 2, wherein the output result of each nonlinear filter in the current period is corrected according to the voltage deviation at step S4, the specific correction method is as follows: the preset threshold X ranges from 0.05 to 0.25 and the preset nonlinear gradient Z ranges from 0.05 to 0.25, if the deviation of D-axis positive sequence voltage component D.sub.udp_y is greater than the preset threshold value X, the output of the nonlinear filter V.sub.udp is equal to the output value of the previous period V.sub.udp_1 plus the preset nonlinear gradient Z; if the deviation D.sub.udp_y is less than the preset threshold value-X, the output of the nonlinear filter V.sub.udp is equal to the output value of the previous period V.sub.udp_1 minus the preset nonlinear gradient Z; if the deviation D.sub.udp_y is between X and X, the output of the nonlinear filter V.sub.udp is equal to the output value of the previous period V.sub.udp_1, that is:
V.sub.udp=V.sub.udp_1+Z,(D.sub.udp_y>X)
V.sub.udp=V.sub.udp_1Z,(D.sub.udp_y<X)
V.sub.udp=V.sub.udp_1,(XD.sub.udp_yX), if the deviation of Q-axis positive sequence voltage component D.sub.uqp_y is greater than the preset threshold value X, the output of the nonlinear filter V.sub.uqp is equal to the output value of the previous period V.sub.uqp_1 plus the preset non-linear gradient Z; if the deviation D.sub.uqp_y is less than the preset threshold value-X, the output of the non-linear filter V.sub.uqp is equal to the output value of the previous period V.sub.uqp_1 minus the preset non-linear gradient Z; if the deviation D.sub.uqp_y is between X and X, the output of the nonlinear filter V.sub.uqp is equal to the output value of the previous period V.sub.uqp_1, that is:
V.sub.uqp=V.sub.uqp_1+Z,(D.sub.uqp_y>x)
V.sub.uqp=V.sub.uqp_1Z,(D.sub.uqp_y<x)
V.sub.uqp=V.sub.uqp_1,(XD.sub.uqp_yx), if the deviation of the D-axis negative sequence voltage component D.sub.udn_y is greater than the preset threshold value X, the output of the nonlinear filter V.sub.udn is equal to the output value of the previous period V.sub.udn_1 plus the preset nonlinear gradient Z; if the deviation D.sub.udn_y is less than the preset threshold value X, the output of the nonlinear filter V.sub.udn is equal to the output value of the previous period V.sub.udn_1 minus the preset nonlinear gradient Z; if the deviation D.sub.udn_y is between X and X, the output of the nonlinear filter V.sub.udn is equal to the output value of the previous period V.sub.udn_1, that is:
V.sub.udn=V.sub.udn_1+Z,(D.sub.udn_y>x)
V.sub.udn=V.sub.udn_1Z,(D.sub.udn_y<x)
V.sub.udn=V.sub.udn_1,(XD.sub.udn_yx). if the deviation of the Q-axis negative sequence voltage component D.sub.uqn_y is greater than the preset threshold value X, the output of the nonlinear filter V.sub.uqn is equal to the output value of the previous period V.sub.uqn_1 plus the preset nonlinear gradient Z; if the deviation D.sub.uqn_y is less than the preset threshold value X, the output of the nonlinear filter V.sub.uqn is equal to the output value of the previous period V.sub.uqn_1 minus the preset nonlinear gradient Z; if the deviation D.sub.uqn_y is between X and X, the output of the nonlinear filter V.sub.uqn is equal to the output value of the previous period V.sub.uqn_1, that is:
V.sub.uqn=V.sub.uqn_1+Z,(D.sub.uqn_y>X)
V.sub.uqn=V.sub.uqn_1Z,(D.sub.uqn_y<X)
V.sub.uqn=V.sub.uqn_1,(XD.sub.uqn_yX).

7. The suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control according to claim 6, wherein the preset threshold X is 0.15 and the preset nonlinear gradient Z is 0.2.

8. The suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control according to claim 2, wherein the output results V.sub.udp, V.sub.uqp, V.sub.udn, V.sub.uqn of the nonlinear filter at step S5 are fixed values with gradient variation, which acts on the voltage feed-forward link of four inner current loops, namely, positive sequence D-axis, positive sequence Q-axis, negative sequence D-axis and negative sequence Q-axis.

9. The suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control according to claim 5, wherein the output result of each nonlinear filter in the current period is corrected according to the voltage deviation at step S4, the specific correction method is as follows: the preset threshold X ranges from 0.05 to 0.25 and the preset nonlinear gradient Z ranges from 0.05 to 0.25, if the deviation of D-axis positive sequence voltage component D.sub.udp_y is greater than the preset threshold value X, the output of the nonlinear filter V.sub.udp is equal to the output value of the previous period V.sub.udp_1 plus the preset nonlinear gradient Z; if the deviation D.sub.udp_y is less than the preset threshold value X, the output of the nonlinear filter V.sub.udp is equal to the output value of the previous period V.sub.udp_1 minus the preset nonlinear gradient Z; if the deviation D.sub.udp_y is between X and X, the output of the nonlinear filter V.sub.udp is equal to the output value of the previous period V.sub.udp_1, that is:
V.sub.udp=V.sub.udp_1+Z,(D.sub.udp_y>X)
V.sub.udp=V.sub.udp_1Z,(D.sub.udp_y<X)
V.sub.udp=V.sub.udp_1,(XD.sub.udp_yX), if the deviation of Q-axis positive sequence voltage component D.sub.uqp_y is greater than the preset threshold value X, the output of the nonlinear filter V.sub.uqp is equal to the output value of the previous period V.sub.uqp_1 plus the preset non-linear gradient Z; if the deviation D.sub.uqp_y is less than the preset threshold value-X, the output of the non-linear filter V.sub.uqp is equal to the output value of the previous period V.sub.uqp_1 minus the preset non-linear gradient Z; if the deviation D.sub.uqp_y is between X and X, the output of the nonlinear filter V.sub.uqp is equal to the output value of the previous period V.sub.uqp_1, that is:
V.sub.uqp=V.sub.uqp_1+Z,(D.sub.uqp_y>X)
V.sub.uqp=V.sub.uqp_1Z,(D.sub.uqp_y<X)
V.sub.uqp=V.sub.uqp_1,(XD.sub.uqp_yX), if the deviation of the D-axis negative sequence voltage component D.sub.udn_y is greater than the preset threshold value X, the output of the nonlinear filter V.sub.udn is equal to the output value of the previous period V.sub.udn_1 plus the preset nonlinear gradient Z; if the deviation D.sub.udn_y is less than the preset threshold value X, the output of the nonlinear filter V.sub.udn is equal to the output value of the previous period V.sub.udn_1 minus the preset nonlinear gradient Z; if the deviation D.sub.udn_y is between X and X, the output of the nonlinear filter V.sub.udn is equal to the output value of the previous period V.sub.udn_1, that is:
V.sub.udn=V.sub.udn_1+Z(D.sub.udn_y>X)
V.sub.udn=V.sub.udn_1Z(D.sub.udn_y<X)
V.sub.udn=V.sub.udn_1(XD.sub.udn_yX), if the deviation of the Q-axis negative sequence voltage component D.sub.uqn_y is greater than the preset threshold value X, the output of the nonlinear filter V.sub.uqn is equal to the output value of the previous period V.sub.uqn_1 plus the preset nonlinear gradient Z; if the deviation D.sub.uqn_y is less than the preset threshold value X, the output of the nonlinear filter V.sub.uqn is equal to the output value of the previous period V.sub.uqn_1 minus the preset nonlinear gradient Z; if the deviation D.sub.uqn_y is between X and X, the output of the nonlinear filter V.sub.uqn is equal to the output value of the previous period V.sub.uqn_1, that is:
V.sub.uqn=V.sub.uqn_1+Z,(D.sub.uqn>X)
V.sub.uqn=V.sub.uqn_1Z,(D.sub.uqn<X)
V.sub.uqn=V.sub.uqn_1,(XD.sub.uqnX).

10. The suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control according to claim 9, wherein the preset threshold X is 0.15 and the preset nonlinear gradient Z is 0.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 is a flow chart of a suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control;

[0027] FIG. 2 is a schematic diagram of a VSC-HVDC system;

[0028] FIG. 3 is a schematic diagram showing the implementation position of voltage feed-forward nonlinear filter in the positive sequence current inner loop;

[0029] FIG. 4 is a schematic diagram showing the implementation position of voltage feed-forward nonlinear filter in the negative sequence current inner loop.

DETAILED DESCRIPTION

[0030] The following is further detailed description of the technical scheme of the application in conjunction with the drawings of the specification and specific embodiments.

[0031] Turning now to FIG. 1, the harmonic damping control method of the DC side of VSC-HVDC system needs to be configured in each converter terminal, that is, the converter terminal 1, the converter terminal 2 and the converter terminal N shown in FIG. 2. Following is an example of converter terminal 1. The specific implementation process of a suppression method of high-frequency resonance for VSC-HVDC system based on nonlinear filter in voltage feed-forward control is as follows.

[0032] Step S1: extract the grid voltage and convert the per-unit values of three-phase instantaneous AC voltage U.sub.a, U.sub.b, U.sub.c into sequence components according to Parker transformation, specifically: D-axis positive sequence voltage component U.sub.dp, Q-axis positive sequence voltage component U.sub.qp, D-axis negative sequence voltage component U.sub.dn, and Q-axis negative sequence voltage component U.sub.dn.

[0033] Step S2: calculate the average values of four voltage sequence components at N sampling points and obtain U.sub.dp_NAV, U.sub.qp_NAV, U.sub.dn_NAV, U.sub.qn_NAV. This embodiment is mainly for high-frequency resonance above 500 Hz, and the cut-off time for calculating the average value is 2 ms. The processing interruption of the known application device is 100 us, and N=20 is calculated, that is, the average value of 20 sampling points is calculated. The values are calculated as follows:

U.sub.dp_20AV=)U.sub.dp+U.sub.dp_1+ . . . U.sub.dp_18+U.sub.dp_19)/20

U.sub.qp_20AV=)U.sub.qp+U.sub.qp_1+ . . . U.sub.qp_18+U.sub.qp_19)/20

U.sub.dn_20AV=)U.sub.dn+U.sub.dn_1+ . . . U.sub.dn_18+U.sub.dn_19)/20

U.sub.qn_20AV=)U.sub.qn+U.sub.qn_1+ . . . U.sub.qn_18+U.sub.qn_19)/20,

herein, U.sub.dp_n, U.sub.qp_n, U.sub.qp_n, U.sub.qn_n are the values before N sampling cycles of the D-axis positive sequence voltage component, the Q-axis positive sequence voltage component, the D-axis negative sequence voltage component and the Q-axis negative sequence voltage component respectively.

[0034] Step S3: compare the current average value of each voltage sequence component with the output of the nonlinear filter in the previous period and calculate the deviations D.sub.udp_y, D.sub.uqp_y, D.sub.udn_y, D.sub.uqn_y as:

D.sub.udp_y=U.sub.dp_NAVV.sub.udp_1

D.sub.uqp_y=U.sub.qp_NAVV.sub.uqp_1

D.sub.udn_y=U.sub.dn_NAVV.sub.udn_1

D.sub.uqn_y=U.sub.qn_NAVV.sub.uqn_1,

herein, V.sub.udp_1, V.sub.uqp_1, V.sub.udn_1, V.sub.uqn_1 are the output values of the previous period of the nonlinear filter of each sequence component respectively.

[0035] Step S4: The output result of each nonlinear filter in the current period is corrected according to the voltage deviation. The specific correction method is as follows: the preset threshold X ranges from 0.05 to 0.25, the optimal value is 0.15, the preset nonlinear gradient Z ranges from 0.05 to 0.25 and the optimal value is 0.2.

[0036] The preset threshold value is X=0.15p.u., the initial output value of the nonlinear filter output is 0, and the preset nonlinear step is Z=0.2p.u. If the deviation D.sub.udp_y is greater than the preset threshold value 0.15, the output of the nonlinear filter V.sub.udp is equal to the output value of the previous period V.sub.udp_1 plus the preset nonlinear gradient 0.2; if the deviation D.sub.udp_y is less than the preset threshold value 0.15, the output of the nonlinear filter V.sub.udp is equal to the output value of the previous period V.sub.udp_1 minus the preset nonlinear gradient 0.2; if the deviation D.sub.udp_y is between 0.15 and 0.15, the output of the nonlinear filter V.sub.udp is equal to the output value of the previous period V.sub.udp_1, that is:

V.sub.udp=V.sub.udp_1+0.2,(D.sub.uqn_y>0.15)

V.sub.udp=V.sub.udp_1+0.2,(D.sub.uqn_y<0.15)

V.sub.udp=V.sub.udp_1+0.2,(0.15D.sub.uqn_y0.15).

[0037] If the deviation D.sub.uqp_y is greater than the preset threshold value 0.15, the output of the nonlinear filter V.sub.uqp is equal to the output value of the previous period V.sub.uqp_1 plus the preset nonlinear gradient 0.2; if the deviation D.sub.uqp_y is less than the preset threshold value 0.15, the output of the nonlinear filter V.sub.uqp is equal to the output value of the previous period V.sub.uqp_1 minus the preset nonlinear gradient 0.2; if the deviation D.sub.uqp_1 is between 0.15 and 0.15, the output of the nonlinear filter V.sub.uqp is equal to the output value of the previous period V.sub.uqp_1 that is:

V.sub.uqp=V.sub.uqp_1+0.2,(D.sub.uqp_y>0.15)

V.sub.uqp=V.sub.uqp_10.2,(D.sub.uqp_y<0.15)

V.sub.udp=V.sub.udp_1+0.2,(0.15D.sub.uqn_y0.15).

[0038] If the deviation D.sub.udn_y is greater than the preset threshold value 0.15, the output of the nonlinear filter V.sub.udn is equal to the output value of the previous period V.sub.udn_1 plus the preset nonlinear gradient 0.2; if the deviation D.sub.udn_y is less than the preset threshold value 0.15, the output of the nonlinear filter V.sub.udn is equal to the output value of the previous period V.sub.udn_1 minus the preset nonlinear gradient 0.2; if the deviation D.sub.udn_y is between 0.15 and 0.15, the output of the nonlinear filter V.sub.udn is equal to the output value of the previous period V.sub.udn_1, that is:

V.sub.udn=V.sub.udn_1+0.2,(D.sub.udn_y>0.15)

V.sub.udn=V.sub.udn_10.2,(D.sub.udn_y<0.15)

V.sub.udn=V.sub.udn_1+0.2,(0.15D.sub.udn_y0.15).

[0039] If the deviation D.sub.uqn_y is greater than the preset threshold value 0.15, the output of the nonlinear filter V.sub.uqn is equal to the output value of the previous period V.sub.uqn_1 plus the preset nonlinear gradient 0.2; if the deviation D.sub.uqn_y is less than the preset threshold value 0.15, the output of the nonlinear filter V.sub.uqn is equal to the output value of the previous period V.sub.uqn_1 minus the preset nonlinear gradient 0.2; if the deviation D.sub.uqn_y is between 0.15 and 0.15, the output of the nonlinear filter V.sub.uqn_y is equal to the output value of the previous period V.sub.uqn_1, that is:

V.sub.uqn=V.sub.uqn_1+0.2,(D.sub.uqn_y>0.15)

V.sub.uqn=V.sub.uqn_10.2,(D.sub.uqn_y<0.15)

V.sub.uqn=V.sub.uqn_1,(0.15D.sub.uqn_y0.15).

[0040] Step S5: the output results V.sub.udp, V.sub.uqp, V.sub.udn V.sub.uqn of the nonlinear filter is fixed values varied by gradient, and are applied to the voltage feed-forward links of the four current inner loops of the positive sequence D-axis, the positive sequence Q-axis, the negative sequence D-axis, and the negative sequence Q-axis. As shown in FIG. 3 and FIG. 4, in FIG. 3, U.sub.dj, and U.sub.qp are the D-axis positive sequence voltage component and the Q-axis positive sequence voltage component of the grid voltage respectively. V.sub.udp and V.sub.uqp are the output results of the nonlinear filter. The result v.sub.d.sup.+ of the D-axis current reference value i.sub.dref.sup.+ and the actual value is output through PI controller are added with the output V.sub.udp of the nonlinear filter and the decoupling term i.sub.q.sup.+ multiplied by L to obtain the final modulation voltage reference value M of the positive sequence D-axis u.sub.cdref.sup.+. The result v.sub.q.sup.+ of the Q-axis current reference value i.sub.qref.sup.+ and the actual value i.sub.q.sup.+ output through PI controller are added with the output V.sub.uqp of the nonlinear filter and the decoupling term i.sub.q.sup.+ multiplied by L to obtain the final modulation voltage reference value M of the positive sequence Q-axis u.sub.cqref.sup.+. u.sub.cdref.sup.+ and u.sub.cqref.sup.+ are inversely transformed from DQ to ABC to form three-phase modulation voltage u.sub.ref_abc.sup.+.

[0041] In FIG. 4, U.sub.dn and U.sub.qn are the D-axis positive sequence voltage component and the Q-axis positive sequence voltage component of the grid voltage respectively. V.sub.udn and V.sub.nqn are the output results of the nonlinear filter. The result v.sub.d.sup. of the D-axis current reference value i.sub.dref.sup. and the actual value i.sub.d.sup. output through PI controller are added with the output V.sub.udp of the nonlinear filter and the decoupling term i.sub.q.sup. multiplied by L to obtain the final modulation voltage reference value M of the positive sequence D-axis u.sub.cdref.sup.. The result v.sub.q.sup. of the Q-axis current reference value i.sub.qref.sup. and the actual value i.sub.q.sup. output through PI controller are added with the output V.sub.uqp of the nonlinear filter and the decoupling term i.sub.q.sup. multiplied by L to obtain the final modulation voltage reference value M of the positive sequence Q-axis U.sub.cqref.sup.. u.sub.cdref.sup. and U.sub.cqref.sup. are inversely transformed from DQ to ABC to form three-phase modulation voltage u.sub.ref_abc.sup..

[0042] The embodiment is as follows: assuming that the positive sequence voltage component of D-axis U.sub.dp after Parker transform is 1.01 p.u. Taking the sampling values of this period together with the first 19 cycles, a total of 20 points are averaged and filtered. Assuming that the average value D.sub.udp_y still 1.01p.u. and the initial value of the filter at the first pass of the nonlinear filter is 0. The deviation between the average value of the current voltage sequence components and the output of the previous period nonlinear filter is calculated as 1.01. Since the deviation of 1.01 is greater than the preset threshold value of 0.15, the output V.sub.udp of the nonlinear filter in this period is equal to the output value of the previous period plus the preset nonlinear gradient of 0.2, which acts on the voltage feed-forward link of the inner current loop of the D-axis positive sequence. The current execution period is processed and the next period is calculated the same. In the fifth execution period, the output of the nonlinear filter is 1.0 p.u., which is close to the D-axis positive-sequence voltage component value of 1.01. At the same time, in order to ensure that the output result of the nonlinear filter is reasonable, in this embodiment, the minimum value of the output result of the preset nonlinear filter is 0.4 p.u., and the maximum value of the output result is 1.0 p.u. According to the preset nonlinear gradient of 0.2p.u., the output of the nonlinear filter may only be the following fixed values: 0.4p.u., 0.2p.u., 0 p.u., 0.2p.u., 0.4p.u., 0.6p.u., 0.8p.u., 1.0p.u.

[0043] Since the above technical solution is adopted, this present invention has the following advantages:

(1) the present invention can effectively improve the impedance characteristics in high frequency band, thereby effectively suppressing the high-frequency resonance caused by the impedance matching between the VSC-HVDC system and AC system;
(2) it's able to adjust the AC voltage when the voltage disturbance of AC fault occurs taking into account the rapidity so as to reduce the risk of overcurrent of the converter valve.
(3) it only needs to be realized in the secondary control software link without adding a new equipment, which is also low cost, no occupancy of land and convenient for engineering practice.

[0044] To some extent, other advantages, objects and features of the invention will be set forth in the description which follows, and based on the following investigation and study, it will be obvious to those skilled in the field, or can be taught from the practice of the present invention.

[0045] Finally, it should be noted that the above embodiments are used only to illustrate the technical scheme of the invention, not to limit it. Although the invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the specific embodiments of the invention can still be modified or replaced equally without any modification or modification that departs from the spirit and scope of the present invention. All of them shall be covered by the protection of the rights of the invention.