DYNAMIC STABILITY ANALYSIS AND CONTROL METHOD FOR VOLTAGE SOURCED CONVERTER BASED HIGH VOLTAGE DIRECT CURRENT TRANSMISSION SYSTEM

Abstract

A dynamic stability analysis and control method for a voltage sourced converter based high voltage direct current (VSC-HVDC) transmission system. The method includes the following steps: unlocking a converter station of the VSC-HVDC transmission system to make the VSC-HVDC transmission system run in a non-island control mode; extracting corresponding parameters of the VSC-HVDC transmission system, wherein the parameters include an effective voltage value U.sub.t0 of an AC system, an outgoing reactive power Q.sub.vsc0 of the VSC-HVDC transmission system, a gain k.sub.p of a phase-locked loop (PLL), and a proportional integral time constant k.sub.i of the PLL; calculating a short-circuit ratio (SCR), an unit value of U.sub.t0 and an unit value of Q.sub.vsc0; calculating a key stable component; checking the sign of the key stable component to determine the stability of the VSC-HVDC transmission system.

Claims

1. A method for determining a dynamic stability of a VSC-HVDC transmission system, comprising the following steps: step 1: unlocking a converter station of the VSC-HVDC transmission system to make the VSC-HVDC transmission system run in a non-island control mode; step 2: extracting a plurality of parameters corresponding to the VSC-HVDC transmission system, wherein the plurality of parameters comprise an effective voltage value U.sub.t0 of an AC system, an outgoing reactive power Q.sub.vsc0 of the VSC-HVDC transmission system, a gain k.sub.p of a PLL, and a proportional integral time constant k.sub.i of the PLL; step 3: taking a rated capacity of the converter station and a voltage of the AC system as reference values, and then calculating a SCR, an unit value of the U.sub.t0 and an unit value of the Q.sub.vsc0; step 4: calculating a key stable component $\left(\mathrm{SCR}{U}_{t.Math..Math.0}-\frac{Q_{\mathrm{VSC}.Math..Math.0}}{U_{t.Math..Math.0}}\right).Math.\frac{\left[\left(U_{t.Math..Math.0}.Math.k_p^2-k_i\right).Math.{}^2+U_{t.Math..Math.0}.Math.k_i^2\right]}{\left(k_i^2+k_p^2.Math.{}^2\right)},$ wherein, is an angular frequency; step 5: checking a sign of the key stable component $\left(\mathrm{SCR}{U}_{t.Math..Math.0}-\frac{Q_{\mathrm{VSC}.Math..Math.0}}{U_{t.Math..Math.0}}\right).Math.\frac{\left[\left(U_{t.Math..Math.0}.Math.k_p^2-k_i\right).Math.{}^2+U_{t.Math..Math.0}.Math.k_i^2\right]}{\left(k_i^2+k_p^2.Math.{}^2\right)}$ to determine the dynamic stability of the VSC-HVDC transmission system; wherein when the key stable component is positive and greater than 0.5, the VSC-HVDC transmission system is stable, and at this time, a VSC-HVDC is put into operation; when the key stable component is negative, the VSC-HVDC transmission system is unstable; when the key stable component is greater than or equal to 0 and less than or equal to 0.5, the VSC-HVDC transmission system is critically stable; when the VSC-HVDC transmission system is unstable or critically stable, the VSC-HVDC is not put into operation.

2. A method for controlling the dynamic stability of the VSC-HVDC transmission system according to a plurality of determination results obtained by the method of claim 1, comprising the following steps: when the VSC-HVDC transmission system is unstable, firstly, calculating a right half part of the key stable component, to determine a sign of a formula $\frac{\left[\left(U_{t.Math..Math.0}.Math.k_p^2-k_i\right).Math.{}^2+U_{t.Math..Math.0}.Math.k_i^2\right]}{\left(k_i^2+k_p^2.Math.{}^2\right)};$ when the formula $\frac{\left[\left(U_{t.Math..Math.0}.Math.k_p^2-k_i\right).Math.{}^2+U_{t.Math..Math.0}.Math.k_i^2\right]}{\left(k_i^2+k_p^2.Math.{}^2\right)}$ is negative, adjusting the gain k.sub.p of the PLL and the proportional integral time constant k.sub.i of the PLL to make the formula $\frac{\left[\left(U_{t.Math..Math.0}.Math.k_p^2-k_i\right).Math.{}^2+U_{t.Math..Math.0}.Math.k_i^2\right]}{\left(k_i^2+k_p^2.Math.{}^2\right)}$ become positive and greater than 0.5, thereby ensuring that the VSC-HVDC transmission system is stable and the VSC-HVDC is put into operation; when the formula $\frac{\left[\left(U_{t.Math..Math.0}.Math.k_p^2-k_i\right).Math.{}^2+U_{t.Math..Math.0}.Math.k_i^2\right]}{\left(k_i^2+k_p^2.Math.{}^2\right)}$ is positive, the key stable component being negative is caused by a left half part of the key stable component, and a formula $\left(\mathrm{SCR}{U}_{t.Math..Math.0}-\frac{Q_{\mathrm{VSC}.Math..Math.0}}{U_{t.Math..Math.0}}\right)$ is negative, increasing the SCR of the VSC-HVDC transmission system or adding a reactive power compensation equipment to reduce the Q.sub.VSC0 to make the formula $\left(\mathrm{SCR}{U}_{t.Math..Math.0}-\frac{Q_{\mathrm{VSC}.Math..Math.0}}{U_{t.Math..Math.0}}\right)$ become positive and greater than 0.5, thereby ensuring that the VSC-HVDC transmission system is stable and the VSC-HVDC is put into operation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a vector control-based VSC control system connected to an AC system.

[0022] FIG. 2 is a control schematic of PLL.

[0023] FIG. 3 shows a two-end VSC-HVDC transmission system.

[0024] FIG. 4 shows the simulation verification result when the VSC-HVDC transmission system operates in the first kind of parameters.

[0025] FIG. 5 shows the simulation verification result when the VSC-HVDC transmission system operates in the first kind of parameters after adjusting the control parameters of the PLL.

[0026] FIG. 6 shows the simulation verification result when the VSC-HVDC transmission system operates in the first kind of parameters after adjusting the control parameters of the PLL again.

[0027] FIG. 7 shows the simulation verification result when the VSC-HVDC transmission system operates in the second kind of parameters.

[0028] FIG. 8 shows the simulation verification result when the VSC-HVDC transmission system operates in the second kind of parameters after adjusting the control parameters of the PLL.

[0029] FIG. 9 shows the simulation verification result when the VSC-HVDC transmission system operates in the second kind of parameters after adjusting the control parameters of the PLL again.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The present disclosure is further described in detail below with reference to the drawings and the specific embodiments. A method for determining the dynamic stability of a VSC-HVDC transmission system in the present disclosure includes the following steps.

[0031] Step 1: The converter station of the VSC-HVDC transmission system is unlocked to make the VSC-HVDC transmission system run in a non-island control mode.

[0032] Step 2: The corresponding parameters of the VSC-HVDC transmission system are extracted, wherein the parameters include the effective voltage value U.sub.t0 of the AC system, the outgoing reactive power Q.sub.vsc0 of the VSC-HVDC, the gain k.sub.p of the PLL, and the proportional integral time constant k.sub.i of the PLL.

[0033] Step 3: The rated capacity of the converter station and the voltage of the AC system are taken as the reference values, and then the SCR, the unit value of U.sub.t0 and the unit value of Q.sub.vsc0 are calculated.

[0034] Step 4: The key stable component

[00009]$\left(\mathrm{SCR}{U}_{t.Math..Math.0}-\frac{Q_{\mathrm{VSC}.Math..Math.0}}{U_{t.Math..Math.0}}\right).Math.\frac{\left[\left(U_{t.Math..Math.0}.Math.k_p^2-k_i\right).Math.{}^2+U_{t.Math..Math.0}.Math.k_i^2\right]}{\left(k_i^2+k_p^2.Math.{}^2\right)}$

is calculated, where, is the angular frequency.

[0035] Step 5: The sign of the key stable component

[00010]$\left(\mathrm{SCR}{U}_{t.Math..Math.0}-\frac{Q_{\mathrm{VSC}.Math..Math.0}}{U_{t.Math..Math.0}}\right).Math.\frac{\left[\left(U_{t.Math..Math.0}.Math.k_p^2-k_i\right).Math.{}^2+U_{t.Math..Math.0}.Math.k_i^2\right]}{\left(k_i^2+k_p^2.Math.{}^2\right)}$

is checked to determine the stability of the VSC-HVDC transmission system. If the key stable component is positive and greater than 0.5, the VSC-HVDC transmission system is stable, and at this time, the VSC-HVDC can be put into operation. If the key stable component is negative, the VSC-HVDC transmission system is unstable. If the key stable component is greater than or equal to 0 and less than or equal to 0.5, the VSC-HVDC transmission system is critically stable. When the VSC-HVDC transmission system is unstable or critically stable, the VSC-HVDC cannot be put into operation.

[0036] Based on the determination results obtained by the above-mentioned method for determining the dynamic stability of the VSC-HVDC transmission system, if the VSC-HVDC transmission system is unstable, the control method for improving the stability of the system is as follow. Firstly, the right half part of the key stable component is calculated, that is, the sign of the formula

[00011]$\frac{\left[\left(U_{.Math.t.Math..Math.0}.Math..Math.k_{.Math.p}^{.Math.2}.Math.-.Math.k_{.Math.i}\right).Math..Math.{}^{.Math.2}+U_{.Math.t.Math..Math.0}.Math..Math.k_{.Math.i}^{.Math.2}\right]}{(.Math.k_{.Math.i}^{.Math.2}+k_{.Math.p}^{.Math.2}.Math..Math.{}^{.Math.2})}$

is determined. If the formula

[00012]$\frac{\left[\left(U_{.Math.t.Math..Math.0}.Math..Math.k_{.Math.p}^{.Math.2}.Math.-.Math.k_{.Math.i}\right).Math..Math.{}^{.Math.2}+U_{.Math.t.Math..Math.0}.Math..Math.k_{.Math.i}^{.Math.2}\right]}{(.Math.k_{.Math.i}^{.Math.2}+k_{.Math.p}^{.Math.2}.Math..Math.{}^{.Math.2})}$

is negative, then the gain k.sub.p of the PLL and the proportional integral time constant k.sub.i are adjusted to make the formula

[00013]$\frac{\left[\left(U_{.Math.t.Math..Math.0}.Math..Math.k_{.Math.p}^{.Math.2}.Math.-.Math.k_{.Math.i}\right).Math..Math.{}^{.Math.2}+U_{.Math.t.Math..Math.0}.Math..Math.k_{.Math.i}^{.Math.2}\right]}{(.Math.k_{.Math.i}^{.Math.2}+k_{.Math.p}^{.Math.2}.Math..Math.{}^{.Math.2})}$

become positive and greater than 0.5, thereby ensuring that the system is stable and the VSC-HVDC can be put into operation. If the formula

[00014]$\frac{\left[\left(U_{.Math.t.Math..Math.0}.Math..Math.k_{.Math.p}^{.Math.2}.Math.-.Math.k_{.Math.i}\right).Math..Math.{}^{.Math.2}+U_{.Math.t.Math..Math.0}.Math..Math.k_{.Math.i}^{.Math.2}\right]}{(.Math.k_{.Math.i}^{.Math.2}+k_{.Math.p}^{.Math.2}.Math..Math.{}^{.Math.2})}$

is positive, the key stable component is negative, which is caused by the left half part of the key stable component, that is,

[00015]$\left(\mathrm{SCR}{U}_{t.Math..Math.0}-\frac{Q_{\mathrm{VSC}.Math..Math.0}}{U_{t.Math..Math.0}}\right)$

is negative, then the SCR of the system is increased or the reactive power compensation equipment is added to reduce the Q.sub.VSC0 to make

[00016]$\left(\mathrm{SCR}{U}_{t.Math..Math.0}-\frac{Q_{\mathrm{VSC}.Math..Math.0}}{U_{t.Math..Math.0}}\right)$

become positive and greater than 0.5, thereby ensuring that the system is stable and the VSC-HVDC can be put into operation.

[0037] The two-end VSC-HVDC transmission system in FIG. 3 is taken as an example for verification.

[0038] Solution 1: The parameters of the VSC-HVDC transmission system are set as follows (all values in the table are unit values):

TABLE-US-00001 calculated value of key stable k.sub.i k.sub.p SCR Q.sub.VSC0 U.sub.t0 component 100 1 2.55 0.17 1.0 0.0264

[0039] The calculated value of the key stable component is 0.0264, determining that the system is unstable and the VSC-HVDC cannot be put into operation. According to the simulation verification shown in FIG. 4, the system oscillates and is divergent unstable, which is consistent with the theoretical analysis.

[0040] In order to improve the stability of the system, the different parts of the key stable component need to be calculated. Firstly, the value of the right half part of the key stable component is calculated as 0.011085, which is negative, indicating that the negative sign of the key stable component is caused by the right half part of the key stable component, then the control parameter k.sub.p of the PLL is gradually adjusted to make the key stable component become positive. When k.sub.p is increased to 1.5, the system parameters and the calculated value of the key stable component are as follows.

TABLE-US-00002 calculated value of key stable k.sub.i k.sub.p SCR Q.sub.VSC0 U.sub.t0 component 100 1.5 2.55 0.17 1.0 0.0817

[0041] Now the calculated value of the key stable component is 0.0817, which is between 0 and 0.5, determining that the system is critically stable and the VSC-HVDC cannot be put into operation. According to the simulation verification shown in FIG. 5, the system undergoes a persistent oscillation and is indeed critically stable, which is consistent with the theoretical analysis.

[0042] Then the k.sub.p is increased to 10, and the system parameters and the key stable component value are as follows.

TABLE-US-00003 calculated value of key stable k.sub.i k.sub.p SCR Q.sub.VSC0 U.sub.t0 component 100 10 2.55 0.17 1.0 1.1855

[0043] The calculated value of the key stable component is 1.1855, which is positive, determining that the system is stable and the VSC-HVDC can be put into operation. According to the simulation verification shown in FIG. 6, the system converges, which is consistent with the theoretical analysis.

[0044] Solution 2: The parameters of the system are set as follows (all values in the table are unit values).

TABLE-US-00004 calculated value of key stable k.sub.i k.sub.p SCR Q.sub.VSC0 U.sub.t0 component 21000 100 2.55 0.4 0.75 0.1845

[0045] The calculated value of the key stable component is 0.1845, which is between 0 and 0.5, determining that the system is critically stable and the VSC-HVDC cannot be put into operation. According to the simulation verification shown in FIG. 7, the system undergoes a persistent oscillation and is indeed critically stable, which is consistent with the theoretical analysis.

[0046] In order to improve the stability of the system, the control parameter k.sub.i of the PLL is gradually adjusted to ultimately expectantly make the key stable component be a large positive value. Firstly, k.sub.i is reduced to 20000, then the system parameters and the calculated value of the key stable component are as follows.

TABLE-US-00005 calculated value of key stable k.sub.i k.sub.p SCR Q.sub.VSC0 U.sub.t0 component 20000 100 2.55 0.4 0.75 0.2019

[0047] The calculated value of the key stable component is 0.2019, which is not much improved and is between 0 and 0.5, determining that the system is still critically stable and the VSC-HVDC cannot be put into operation. According to the simulation verification shown in FIG. 8, the system undergoes a persistent oscillation and is indeed critically stable, which is consistent with the theoretical analysis.

[0048] In order to further improve the stability of the system, the control parameter k.sub.i of the PLL is adjusted to 1, and meanwhile the reactive power equipment is added, so as to improve the stability of the voltage, making U.sub.t0=1, and reducing Q.sub.vsc0. The system parameters and the calculated value of the key stable component are as follows.

TABLE-US-00006 calculated value of key stable k.sub.i k.sub.p SCR Q.sub.VSC0 U.sub.t0 component 1 100 2.55 0.17 1.0 2.3834

[0049] The calculated value of the key stable component is 2.3834, which is positive, determining that the system is stable and the VSC-HVDC can be put into operation. According to the simulation verification shown in FIG. 9, the system converges, which is consistent with the theoretical analysis.

[0050] In sum, it is feasible to determine the dynamic stability of DC transmission by adopting the key stable component, and the method of improving the stability of the system is effective.