CN106655199A - VSC-HVDC power control method for improving voltage stability - Google Patents

Abstract

The invention relates to a VSC-HVDC power control method for improving voltage stability. For the voltage stability problem of an alternating and direct current hybrid power network containing VSC-HVDC, through consideration of constraint conditions such as VSC-HVDC capacity limitation, an alternating and direct current parallel series system VSC-HVDC active power and reactive power optimal calculation model based on voltage stability is established. Through solution of a Lagrange function, the VSC-HVDC optimal active power transmission capacity and reactive output and the maximum power supply load of the alternating and direct current parallel series system can be obtained. According to the method, through optimization of the VSC-HVDC active power and reactive output, the feasible method for improving the voltage stability is provided for the alternating and direct current hybrid system.

Description

VSC-HVDC power control method for improving voltage stability

Technical Field

The invention relates to the technical field of analysis and control of an electric power system, in particular to a VSC-HVDC power control method for improving voltage stability.

Background

The traditional VSC-HVDC is a high-voltage direct-current transmission technology developed in the 90 s of the 20 th century, and has the advantages of dynamic reactive compensation of an alternating-current power grid and voltage support of a receiving end system. Therefore, VSC-HVDC has become a more promising solution for improving the voltage stability of ac systems.

However, in a dc-series-parallel system, both the active power and the reactive power delivered by the VSC-HVDC to the ac system have an effect on the voltage stabilization. Generally speaking, on the premise of constant reactive power, the larger the active power to be transmitted, the better the voltage stability; on the premise that the active power is unchanged, the larger the transmitted reactive power is, the better the voltage stability is. Due to the capacity limitation, the active power and the reactive power transmitted by the VSC-HVDC to the alternating current system are restricted with each other, so that in the voltage reduction process, the current VSC-HVDC active power and reactive power control modes can be divided into the following three types: firstly, obtaining larger reactive output by reducing active output; secondly, obtaining larger active output at the cost of reducing reactive power output; and thirdly, the active power and the reactive power are reduced in the same proportion. The prior art does not fully disclose what control method should be adopted to be most beneficial to improving the voltage stability of the system and how to calculate the optimal control value of the VSC-HVDC power based on the improvement of the voltage stability according to the parameters of the AC/DC power grid. Aiming at the problems, the invention provides a VSC-HVDC power control method aiming at improving the voltage stability level, namely considering constraint conditions such as VSC-HVDC capacity limit and the like, establishing a VSC-HVDC active power and reactive power optimization calculation model based on a voltage stabilization alternating current-direct current hybrid system, and obtaining the optimal active transmission capacity, reactive power and the maximum power supply load of the alternating current-direct current hybrid system and the like of the VSC-HVDC by solving a Lagrangian function, so as to improve the voltage stability of the alternating current-direct current hybrid system.

Disclosure of Invention

The invention aims to provide a VSC-HVDC power control method for improving voltage stability so as to overcome the defects in the prior art.

In order to achieve the purpose, the technical scheme of the invention is as follows: a VSC-HVDC power control method for improving voltage stability is shown in a flow chart of fig. 1 and comprises the following steps:

step S1: inputting AC and DC power grid parameter data to form a node admittance matrix;

step S2: calculating thevenin equivalent model parameters of the alternating current system;

step S3: calculating the active and reactive operating ranges of the VSC-HVDC;

step S4: establishing an active voltage relational expression containing VSC-HVDC;

step S5: calculating the optimal power value of the VSC-HVDC based on improving the voltage stability;

step S6: and performing VSC-HVDC power control.

Further, in the step S1, the ac grid parameter data includes: the serial numbers of the nodes at the head end and the tail end of the power transmission line, the transformation ratio, the impedance, the series resistance, the reactance, the parallel conductance and the susceptance of the transformer; the direct current network parameters include: VSC-HVDC bridge arm reactor impedance, commutation variable capacitance, impedance, converter modulation ratio and maximum allowable current I_lim。

Further, in step S2, an equivalent circuit of the ac system and the VSC-HVDC is obtained, and as shown in fig. 2, it is assumed that the ac bus accessed by the VSC-HVDC converter station is the i-th node of the ac power grid, and the voltage phasor thereof isThe fundamental voltage phasor output by the converter isThe equivalent connection impedance between the current converter and the alternating current bus i is Z₁∠θ₁＝R₁+jX₁And the system equivalent impedance Z is determined according to the ith node₂∠θ₂＝R₂+jX₂Equivalent resistance R of Thevenin₂And X₂The communication system can be obtained through PSD-BPA calculation software.

Further, in step S3, an equivalent circuit of the ac system and the VSC-HVDC system is obtained, and the dc-side power is calculated from the equivalent circuit of the ac system and the VSC-HVDC system:

wherein: p_dc、Q_dcActive and reactive power of node i are injected for VSC-HVDC respectively,_ik＝_i-_k＝-_kis the voltage phase angle difference between node i and node k;m is the modulation ratio of the current converter, U_dThe voltage of the direct current side of the converter is mu, and the utilization rate of the PWM direct current voltage is mu;

the following two formulas can be derived:

wherein,when VSC outputs reactive power to AC system, Q_dcIs positive;

maximum allowed current limit during VSC-HVDC operation:

wherein, I_limIs the maximum allowable current of VSC-HVDC.

Further, in the step S4, P and P are established based on the power flow equation_dc、Q_dcAnd U. According to an alternating current branch circuit in the equivalent circuit of the alternating current system and the VSC-HVDC system, calculating the power of an alternating current side:

wherein, P_ac、Q_acThe active and reactive power at node i are injected into the ac branch respectively,andrespectively the voltages at the two ends of the alternating current branch circuit, _ij＝_i-_j＝；

then it can be obtained:

wherein,

then according to P_ac+P_dc＝P，Q_ac+Q_dc＝Q，Z_D＝R_D+jX_D，By derivation, P and P can be obtained_dc、Q_dc、U、R_D、X_DIs onSystem equation (calculation formula of load power of AC/DC series-parallel system):

in the formula: p, Q load active power and reactive power for supplying power to the AC/DC series-parallel power grid respectively; z_D、R_DAnd X_DThe equivalent impedance, the equivalent resistance and the equivalent reactance of the power supply load of the alternating current-direct current series-parallel power grid are obtained.

Load power factor in normal conditionsThe change is not large in a short time, and can be assumedIs constant, and P can be obtained by derivation_dc、Q_dcThe relation equation of U:

further, in the step S5, P is about P_dc、Q_dc、U、R_D、X_DA function of, i.e.

Lagrange function L (P)_dc,Q_dc,U,R_D,X_D) Comprises the following steps:

in the formula,is a constraint condition;

the above formula is subjected to partial derivation to obtain an equation set

Solving the above 6 equations can yield 6 unknowns P_dc、Q_dc、U、R_D、X_DAnd λ, substituting the equation P ═ f (P)_dc,Q_dc,U,R_D,X_D) Can be solved atMaximum load power under constraint P, and P_dc、Q_dcI.e. the VSC-HVDC optimal power setting based on increasing the voltage stability level.

Further, in step S6, the VSC-HVDC employs a conventional power outer loop and dq decoupling current inner loop control manner; the inverter side of the receiving end adopts constant active power P_dcConstant and constant reactive power Q_dcAnd (5) controlling.

Under the normal operation condition, setting VSC-HVDC to reference the active power value P according to the system requirement_refAnd a reactive power reference value Q_refAre respectively set to P_ref0And Q_ref0Namely:

P_ref＝P_ref0,Q_ref＝Q_ref0；

when the alternating current system is in the voltage instability process, namely the voltage amplitude U of the alternating current bus i connected with the VSC-HVDC is smaller than the threshold value U_limAnd has a duration greater than t_lim(t_limSet according to the time for avoiding fault removal and the like), the control system of the VSC-HVDC receiving end is switched to enter a VSC-HVDC power control mode for improving the voltage stability.

In this mode, the calculation results are first obtained in accordance with steps S1 and S2Calculating the optimal power setting P of VSC-HVDC under the load power at the moment by using the network equivalent parameters and the calculation method in the step S5_dc、Q_dc；

Secondly, in order to reduce the impact of the regulation of the VSC-HVDC active power and the reactive power on the system, the VSC-HVDC active power reference value P is used_refAnd a reactive power reference value Q_refRespectively by the original set value P_ref0And Q_ref0Transition to its optimum power setting P in a stepwise manner_dc、Q_dcThe power reference value is finally set to:

P_ref＝P_dc,Q_ref＝Q_dc；

finally, d-axis and q-axis current reference values i are generated through the output of an active power outer loop PI controller and a reactive power outer loop PI controller respectively_drefAnd i_qrefAnd the current is transmitted into the current inner loop control; after modulation and triggering, the VSC-HVDC output power will gradually transit to P in a stepping mode_dc、Q_dcI.e. the VSC-HVDC optimum power for increasing the voltage stability level.

Compared with the prior art, the invention has the following beneficial effects: the VSC-HVDC power control method for improving the voltage stability, provided by the invention, provides a feasible method for improving the voltage stability of an alternating current-direct current hybrid system through optimizing the active power and the reactive power output of the VSC-HVDC. The method lays a foundation for improving the voltage stability and controlling the operation of the VSC-HVDC-containing alternating current and direct current hybrid system.

Drawings

Fig. 1 is a flow chart of a VSC-HVDC power control method for improving voltage stability according to the present invention.

FIG. 2 is an equivalent circuit containing a VSC-HVDC alternating current-direct current parallel-serial system in the invention.

FIG. 3 is a typical PV curve of the present invention.

Fig. 4 is a flow chart of the VSC-HVDC optimal power control process based on voltage stability improvement in the present invention.

Fig. 5 is a schematic diagram of an ac/dc hybrid system including VSC-HVDC in accordance with an embodiment of the present invention.

Detailed Description

The technical scheme of the invention is specifically explained below with reference to the accompanying drawings.

The invention provides a VSC-HVDC power control method for improving voltage stability, which comprises the following steps as shown in figure 1:

(1) inputting AC/DC power grid parameters to form a node admittance matrix. Wherein the AC power grid parameter data includes: the serial numbers of the nodes at the head end and the tail end of the power transmission line, the transformation ratio, the impedance, the series resistance, the reactance, the parallel conductance and the susceptance of the transformer; the direct current network parameters include: VSC-HVDC bridge arm reactor impedance, commutation variable capacitance, impedance, converter modulation ratio and maximum allowable current I_lim。

(2) And calculating thevenin equivalent model parameters of the alternating current system. Obtaining the equivalent circuit of the AC system and the VSC-HVDC, as shown in FIG. 2, recording the I-th node of the AC power grid as the AC bus accessed by the VSC-HVDC converter station, and the voltage phasor of the I-th nodeThe fundamental voltage phasor output by the converter isThe equivalent connection impedance between the current converter and the alternating current bus i is Z₁∠θ₁＝R₁+jX₁And the system equivalent impedance Z is determined according to the ith node₂∠θ₂＝R₂+jX₂In the present embodiment, the Thevenin equivalent impedance R₂And X₂And acquiring the communication system through PSD-BPA calculation software.

(3) Calculating the active and reactive operating ranges of VSC-HVDC

From fig. 2, the dc side power equation can be derived:

in the formula: p_dc、Q_dcActive and reactive power of node i are injected for VSC-HVDC respectively,_ik＝_i-_k＝-_kis the voltage phase angle difference between node i and node k;m is the modulation ratio of the current converter, U_dThe voltage of the direct current side of the converter is mu, and the utilization rate of the PWM direct current voltage is mu;

can be derived from the formulas (1) and (2)

Wherein,when VSC outputs reactive power to AC system, Q_dcIs positive.

At the same time, the maximum allowable current limit during VSC-HVDC operation is considered, namely:

in the formula I_limIs the maximum allowable current of VSC-HVDC.

Further, in general, the constraint of equation (4) plays a key role in VSC-HVDC operation constraints, especially during voltage sag.

(4) And establishing an active power (P) voltage (U) relation containing VSC-HVDC.

In thatWith constants, the VSC-HVDC containing active (P) voltage (U) relation can be described in the form:

P＝f(P_dc,Q_dc,U) (5)

in the formula: and P is the load active power. The PV relationship can be derived from the power equation for the circuit of fig. 2, as follows.

For the ac branch in fig. 2, the ac side power equation can be listed:

wherein, P_ac、Q_acThe active and reactive power at node i are injected into the ac branch respectively,andrespectively the voltages at the two ends of the alternating current branch circuit, _ij＝_i-_j＝；。

according to the formulae (6) and (7):

wherein,

in this embodiment, the load power factor is consideredWith little change in short time, i.e. assumingIs constant and is according to P_ac+P_dc＝P，Q_ac+Q_dcDerived from Q, P and P_dc、Q_dcThe relation equation of U:

in the formula: p, Q are the load active power and reactive power of the AC-DC hybrid power grid.

According to equation (9), a typical PV curve of the ac/dc hybrid grid can be obtained, as shown in fig. 3.

(5) Voltage stabilization-based VSC-HVDC optimal power value calculation

P is about P_dc、Q_dc、U、R_D、X_DA function of, i.e.

Defining a Lagrangian function L (P)_dc,Q_dc,U,R_D,X_D) Is composed of

In the formula,are constraints.

The partial derivatives of the formula (11) are calculated to obtain an equation set

Solving the above 6 equations can yield 6 unknowns P_dc、Q_dc、U、R_D、X_DAnd λ, substituting the equation P ═ f (P)_dc,Q_dc,U,R_D,X_D) Can be solved atMaximum load power under constraint P, and P_dc、Q_dcI.e. the VSC-HVDC optimal power setting based on increasing the voltage stability level.

(6) Power control implementation

VSC-HVDC adopts the traditional power outer loop and dq decoupling current inner loop control mode; the inverter side of the receiving end adopts constant active power P_dcConstant and constant reactive power Q_dcAnd (5) controlling. Under normal conditions, setting VSC-HVDC to reference the active power value P according to the system requirement_refAnd a reactive power reference value Q_refAre respectively set to P_ref0And Q_ref0Namely:

P_ref＝P_ref0,Q_ref＝Q_ref0；

as shown in FIG. 4, when the AC system is in voltage instability, i.e. the voltage amplitude U of the AC bus i connected to VSC-HVDC is smaller than the threshold value U_limAnd has a duration greater than t_lim(t_limSet according to the time for avoiding fault removal and the like), the control system of the VSC-HVDC receiving end is switched to enter a VSC-HVDC power control mode for improving the voltage stability.

In the mode, firstly, according to the network equivalent parameters calculated in the steps (1) and (2) and the calculation method in the step (5), the optimal power setting P of VSC-HVDC under the load power at the moment is calculated_dc、Q_dc；

Secondly, in order to reduce the impact of the regulation of the VSC-HVDC active power and the reactive power on the system, the VSC-HVDC active power reference value P is used_refAnd a reactive power reference value Q_refRespectively by the original set value P_ref0And Q_ref0Transition to its optimum power setting P in a stepwise manner_dc、Q_dcThe power reference value is finally set to:

P_ref＝P_dc,Q_ref＝Q_dc；

finally, d-axis and q-axis current reference values i are generated through the output of an active power outer loop PI controller and a reactive power outer loop PI controller respectively_drefAnd i_qrefAnd the current is transmitted into the current inner loop control; after modulation and triggering, the VSC-HVDC output power will gradually transit to P in a stepping mode_dc、Q_dcI.e. the VSC-HVDC optimum power for increasing the voltage stability level. The VSC-HVDC optimal power control procedure based on improved voltage stability is shown in fig. 4.

The present invention will be described in detail with reference to examples.

Taking the VSC-HVDC-containing alternating current and direct current hybrid power transmission system shown in fig. 5 as an example for explanation, the method provided by the present invention is used to analyze the voltage stability of the system, and specifically includes the following steps:

1. calculating equivalent circuit parameters of AC/DC system

In the example shown in fig. 5, the main parameters of the flexible dc power transmission system are shown in table 1, and the sending end of the flexible dc power transmission system adopts a constant dc voltage U_dConstant AC reactive powerQ control; the receiving end is controlled by constant active power P and alternating current reactive power Q. Maximum load at receiving end of 1998MW, power factorEquivalent impedance R of receiving-end alternating current power grid in maximum operation mode₂＝1.587Ω，X₂5.766 omega, equivalent potential E of alternating current system_s1.1pu (reference voltage 230 kV).

TABLE 1 VSC-HVDC System principal parameters

According to the given data, the VSC-HVDC-containing alternating current-direct current hybrid power grid shown in FIG. 5 is simplified and equalized according to FIG. 2, and all parameters of an equivalent circuit are shown in Table 2. The named and per-unit values of the parameters are given in table 2, and for convenience, the following calculations are performed using the per-unit values.

TABLE 2 Circuit parameter calculation results

Note: the reference values are respectively U_B＝230kV,S_B＝100MVA，Z_B＝529Ω。

2. Voltage stabilization-based VSC-HVDC optimal power value calculation

Substituting the parameters (per unit values) in tables 1 and 2 into the following equations

In the formula:

(per unit value)

To L (P)_dc,Q_dcU) calculating the partial derivative to obtain an equation set

Solving the above system of equations, taking into account P_dc、Q_dcU is a real number solution larger than 0, and a group of meaningful real number solutions can be obtained

Equation set solution P_dcThe value expressed by a famous value is 554.6MW, namely the optimal active power value of the VSC-HVDC for improving the voltage stability.

Substituting the solution into f (P)_dc,Q_dcU) can be solved to obtain the maximum transmission power P of the system as

P_max＝f(P_dc,Q_dc,U)＝44.420

P_maxThe value of the qualified value is 44420MW, namely the maximum power which can be transmitted by the AC-DC hybrid system.

3. Power control

In this embodiment, the VSC-HVDC active power reference value P_refAnd a reactive power reference value Q_refSetting initial values to be 800MW and 100Mvar respectively according to system requirements; the out-of-limit value of the alternating-current bus voltage is set to be 0.9p.u., and the out-of-limit value of the duration is set to be 1 s. When VSC-HVDC monitors that the amplitude of the connected alternating current bus voltage is less than 0.9p.u. and the duration is more than 1s, starting power based on improvement of voltage stabilityAnd controlling, according to the calculation result, the optimal active and reactive power values based on improving the voltage stability are 554.6MW and 518.0 Mvar. VSC-HVDC automatic P-mixing_ref、Q_refAnd the power is transited to the optimal active and reactive power values of 554.6MW and 518.0Mvar in a stepping mode. After the VSC-HVDC power inner and outer ring control, the VSC-HVDC output active power gradually transits to 554.6MW and 518.0Mvar in a stepping mode until the system voltage recovers.

The above are preferred embodiments of the present invention, and all changes made according to the technical scheme of the present invention that produce functional effects do not exceed the scope of the technical scheme of the present invention belong to the protection scope of the present invention.

Claims (7)

1. A VSC-HVDC power control method for improving voltage stability is characterized by comprising the following steps:

step S1: inputting AC and DC power grid parameter data to form a node admittance matrix;

step S2: calculating thevenin equivalent model parameters of the alternating current system;

step S3: calculating the active and reactive operating ranges of the VSC-HVDC;

step S4: establishing an active voltage relational expression containing VSC-HVDC;

step S5: calculating the optimal power value of the VSC-HVDC based on improving the voltage stability;

step S6: and performing VSC-HVDC power control.

2. A VSC-HVDC power control method for improving voltage stability according to claim 1, wherein in said step S1, said ac grid parameter data comprises: the serial numbers of the nodes at the head end and the tail end of the power transmission line, the transformation ratio, the impedance, the series resistance, the reactance, the parallel conductance and the susceptance of the transformer; the direct current network parameters include: VSC-HVDC bridge arm reactor impedance, commutation variable capacitance, impedance, converter modulation ratio and maximum allowable current I_lim。

3. The VSC-HVDC power control method for improving voltage stability according to claim 1, wherein in step S2, an equivalent circuit of the ac system and the VSC-HVDC is obtained, and it is assumed that an ac bus accessed by the VSC-HVDC converter station is an i-th node of the ac power grid, and a voltage phasor thereof isThe fundamental voltage phasor output by the converter isThe equivalent connection impedance between the current converter and the alternating current bus i is Z₁∠θ₁＝R₁+jX₁And the system equivalent impedance Z is determined according to the ith node₂∠θ₂＝R₂+jX₂Equivalent resistance R of Thevenin₂And X₂The communication system can be obtained through PSD-BPA calculation software.

4. The VSC-HVDC power control method for improving voltage stability according to claim 1, wherein in step S3, an equivalent circuit of the ac system and the VSC-HVDC system is obtained, and the dc side power is calculated according to the equivalent circuit of the ac system and the VSC-HVDC system:

$P_{dc}=\frac{U_k{U}_i}{Z_1}cos({\mathrm{\θ}}_1+{\mathrm{\δ}}_{ik})-\frac{U_i^2}{Z_1}{\mathrm{cos\θ}}_1=\frac{U_{k}U}{Z_1}cos({\mathrm{\θ}}_1+{\mathrm{\δ}}_{ik})-\frac{U^2}{Z_1}{\mathrm{cos\θ}}_1;$

$Q_{dc}=\frac{U_k{U}_i}{Z_1}sin({\mathrm{\θ}}_1+{\mathrm{\δ}}_{ik})-\frac{U_i^2}{Z_1}{\mathrm{sin\θ}}_1=\frac{U_{k}U}{Z_1}sin({\mathrm{\θ}}_1+{\mathrm{\δ}}_{ik})-\frac{U^2}{Z_1}{\mathrm{sin\θ}}_1;$

wherein: p_dc、Q_dcActive and reactive power of node i are injected for VSC-HVDC respectively,_ik＝_i-_k＝-_kis the voltage phase angle difference between node i and node k;m is the modulation ratio of the current converter, U_dThe voltage of the direct current side of the converter is mu, and the utilization rate of the PWM direct current voltage is mu;

the following two formulas can be derived:

${(P_{dc}+U^2{G}_1)}^2+{(Q_{dc}-U^2{B}_1)}^2=Y_1^2{U}_k^2{U}^2;$

wherein,when VSC outputs reactive power to AC system, Q_dcIs positive;

maximum allowed current limit during VSC-HVDC operation:

$P_{dc}^2+Q_{dc}^2\≤{\left(\sqrt{3}{\mathrm{UI}}_{\lim }\right)}^2$

wherein, I_limIs the maximum allowable current of VSC-HVDC.

5. The VSC-HVDC power control method for improving voltage stability of claim 1, wherein in step S4, P and P are established based on power flow equation_dc、Q_dcAnd U. According to an alternating current branch circuit in the equivalent circuit of the alternating current system and the VSC-HVDC system, calculating the power of an alternating current side:

$P_{ac}=\frac{U_i{U}_j}{Z_2}cos({\mathrm{\θ}}_2+{\mathrm{\δ}}_{ij})-\frac{U_i^2}{Z_2}{\mathrm{cos\θ}}_2=\frac{{\mathrm{UE}}_s}{Z_2}cos({\mathrm{\θ}}_2+\mathrm{\δ})-\frac{U^2}{Z_2}{\mathrm{cos\θ}}_2;$

$Q_{ac}=\frac{U_i{U}_j}{Z_2}sin({\mathrm{\θ}}_2+{\mathrm{\δ}}_{ij})-\frac{U_i^2}{Z_2}{\mathrm{sin\θ}}_2=\frac{{\mathrm{UE}}_s}{Z_2}sin({\mathrm{\θ}}_2+\mathrm{\δ})-\frac{U^2}{Z_2}{\mathrm{sin\θ}}_2;$

wherein, P_ac、Q_acThe active and reactive power at node i are injected into the ac branch respectively,andrespectively the voltages at the two ends of the alternating current branch circuit, _ij＝_i-_j＝；

then it can be obtained:

${(P_{ac}+U^2{G}_2)}^2+{(Q_{ac}-U^2{B}_2)}^2=U^2{E}_s^2{Y}_2^2;$

wherein,

then according to P_ac+P_dc＝P，Q_ac+Q_dc＝Q，Z_D＝R_D+jX_D，By derivation, P and P can be obtained_dc、Q_dc、U、R_D、X_DThe relation equation (load power calculation formula of the AC-DC hybrid system):

$P=\frac{P_{dc}-U^2{G}_2+Q_{dc}\frac{X_D}{R_D}+U^2{B}_2\frac{X_D}{R_D}\±\sqrt{-{\[Q_{dc}-P_{dc}\frac{X_D}{R_D}+U^2(B_2+G_2\frac{X_D}{R_D})\]}^2+Y_2^2{U}^2{E}_s^2(1+\frac{X_D^2}{R_D^2})}}{1+\frac{X_D^2}{R_D^2}}$

in the formula: p, Q load active power and reactive power for supplying power to the AC/DC series-parallel power grid respectively; z_D、R_DAnd X_DThe equivalent impedance, the equivalent resistance and the equivalent reactance of the power supply load of the alternating current-direct current series-parallel power grid are obtained.

Power factor due to loadWithin a short time, the change is not large, andis constant, and P can be obtained by derivation_dc、Q_dcThe relation equation of U:

6. the VSC-HVDC power control method for improving voltage stability of claim 1, wherein in step S5, P is related to P_dc、Q_dc、U、R_D、X_DA function of, i.e.

$P=f(P_{\mathrm{dc}},Q_{\mathrm{dc}},U,R_D,X_D)=\frac{P_{\mathrm{dc}}-U^2{G}_2+Q_{\mathrm{dc}}\frac{X_D}{R_D}+U^2{B}_2\frac{X_D}{R_D}\±\sqrt{-{[Q_{\mathrm{dc}}-P_{\mathrm{dc}}\frac{X_D}{R_D}+U^2(B_2+G_2\frac{X_D}{R_D})]}^2+Y_2^2{U}^2{E}_s^2(1+\frac{X_D^2}{R_D^2})}}{1+\frac{X_D^2}{R_D^2}}$

Lagrange function L (P)_dc,Q_dc,U,R_D,X_D) Comprises the following steps:

in the formula,is a constraint condition;

the above formula is subjected to partial derivation to obtain an equation set

Solving the above 6 equations can yield 6 unknowns P_dc、Q_dc、U、R_D、X_DAnd λ, substituting the equation P ═ f (P)_dc,Q_dc,U,R_D,X_D) Can be solved atMaximum load power under constraint P, and P_dc、Q_dcI.e. the VSC-HVDC optimal power setting based on increasing the voltage stability level.

7. A VSC-HVDC power control method for improving voltage stability according to claim 1, wherein in said step S6, VSC-HVDC adopts a power outer loop and dq decoupling current inner loop control manner; the inverter side of the receiving end adopts constant active power P_dcConstant and constant reactive power Q_dcControlling;

under the normal operation condition, setting VSC-HVDC to reference the active power value P according to the system requirement_refAnd a reactive power reference value Q_refAre respectively set to P_ref0And Q_ref0Namely:

P_ref＝P_ref0,Q_ref＝Q_ref0；

when the alternating current system is in the voltage instability process, namely the voltage amplitude U of the alternating current bus i connected with the VSC-HVDC is smaller than the threshold value U_limAnd a duration greater than t_limControl system switch of VSC-HVDC receiving endSwitching into a VSC-HVDC power control mode for improving voltage stability;

in the mode, firstly, the network equivalent parameters are calculated according to the steps S1 and S2, and the optimal power setting P of VSC-HVDC under the load power at the moment is calculated by using the calculation method of the step S5_dc、Q_dc；

Secondly, in order to reduce the impact of the regulation of the VSC-HVDC active power and the reactive power on the system, the VSC-HVDC active power reference value P is used_refAnd a reactive power reference value Q_refRespectively by the original set value P_ref0And Q_ref0Transition to its optimum power setting P in a stepwise manner_dc、Q_dcThe power reference value is finally set to: p_ref＝P_dc,Q_ref＝Q_dc；

Finally, d-axis and q-axis current reference values i are generated through the output of an active power outer loop PI controller and a reactive power outer loop PI controller respectively_drefAnd i_qrefAnd the current is transmitted into the current inner loop control; after modulation and triggering, the VSC-HVDC output power will gradually transit to P in a stepping mode_dc、Q_dcI.e. the VSC-HVDC optimum power for increasing the voltage stability level.