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 a VSC-HVDC AC-DC hybrid power grid, considering constraints such as VSC-HVDC capacity limitations, a voltage-stabilized AC-DC method is established. Hybrid system VSC‑HVDC active power and reactive power optimization calculation model. By solving the Lagrangian function, the optimal active power transmission capacity, reactive power output and maximum power supply load of the AC-DC hybrid system can be obtained for VSC‑HVDC. The present invention provides a feasible method for improving voltage stability of an AC-DC hybrid system by optimizing the VSC-HVDC active power and reactive power output.

Description

A VSC-HVDC Power Control Method for Improving Voltage Stability

technical field

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

Background technique

The traditional VSC-HVDC is a high-voltage direct current transmission technology developed since the 1990s. It has the advantages of dynamic reactive power compensation for the AC grid and voltage support for the receiving end system. Therefore, VSC-HVDC becomes a more potential scheme to improve the voltage stability of AC system.

However, in the DC hybrid system, both the active power and reactive power delivered by VSC-HVDC to the AC system have an impact on voltage stability. Generally speaking, under the premise of constant reactive power, the greater the active power transmitted, the better the voltage stability; under the premise of constant active power, the greater the reactive power transmitted, the better the voltage stability. However, limited by capacity, the active power and reactive power delivered by VSC-HVDC to the AC system are mutually restricted. Therefore, in the process of voltage drop, the current VSC-HVDC active power and reactive power control methods can be divided into the following three types: One is to obtain greater reactive output at the cost of reducing active output; the other is to obtain greater active output at the cost of reducing reactive output; third is to reduce active power and reactive power in the same proportion. What kind of control method should be adopted is the most beneficial to improve the system voltage stability, and how to calculate the optimal control value of VSC-HVDC power based on improving voltage stability based on the parameters of the AC and DC grid itself, the existing technology has not fully revealed. In view of the above problems, the present invention proposes a VSC-HVDC power control method aiming at improving the voltage stability level, that is, considering constraints such as VSC-HVDC capacity limitation, and establishing an AC-DC hybrid system VSC-HVDC active power based on voltage stability Power and reactive power optimization calculation model, by solving the Lagrangian function, the optimal active power transmission capacity of VSC-HVDC, reactive power output and the maximum power supply load of the AC-DC hybrid system are obtained, which are used to improve the voltage of the AC-DC hybrid system stability.

Contents of the invention

The object of the present invention is 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 above object, the technical solution of the present invention is: a VSC-HVDC power control method for improving voltage stability, the flow chart of which is shown in Figure 1, including the following steps:

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

Step S2: Calculate the Thevenin equivalent model parameters of the AC system;

Step S3: Calculate the VSC-HVDC active and reactive operating range;

Step S4: Establishing an active voltage relational expression including VSC-HVDC;

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

Step S6: Perform VSC-HVDC power control.

Further, in the step S1, the AC power grid parameter data includes: the head end and end node numbers of the transmission line, transformer ratio, impedance, series resistance, reactance, parallel conductance, and susceptance; the DC network parameters Including: VSC-HVDC bridge arm reactor impedance, converter variable capacity, impedance, converter modulation ratio and maximum allowable current I _lim .

Further, in the step S2, the AC system and the VSC-HVDC equivalent circuit are obtained, as shown in Figure 2, the AC bus connected to the VSC-HVDC converter station is the i-th node of the AC grid, and its voltage Phasor is The fundamental wave voltage phasor output by the converter is The equivalent connection impedance between the converter and the AC bus i is Z ₁ ∠θ ₁ =R ₁ +jX ₁ , and the system equivalent impedance Z ₂ ∠θ ₂ =R ₂ +jX ₂ determined according to the i-th node , Thevenin equivalent impedance R ₂ and X ₂ can be obtained by PSD-BPA calculation software for AC systems.

Further, in the step S3, the 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:

Where: P _dc and Q _dc are the active and reactive power injected into node i by VSC-HVDC respectively, and δ _ik =δ _i -δ _k =δ-δ _k is the voltage phase angle difference between node i and node k; M is the modulation ratio of the converter, U _d is the DC side voltage of the converter, μ is the utilization rate of the PWM DC voltage;

From the above two formulas, it can be deduced that:

in, When VSC outputs reactive power to the AC system, Q _dc is positive;

Maximum allowable current limit during VSC-HVDC operation:

Among them, I _lim is the maximum allowable current of VSC-HVDC.

Further, in the step S4, a relationship equation between P and P _dc , Q _dc , U is established based on the power flow equation. According to the AC branch in the equivalent circuit of the AC system and the VSC-HVDC system, calculate the AC side power:

Among them, P _ac and Q _ac are the active and reactive power injected into node i by the AC branch respectively, with are the voltages at both ends of the AC branch, δ _ij = δ _i - δ _j = δ;

Then you can get:

in,

Then according to P _ac +P _dc =P, Q _ac +Q _dc =Q, Z _D =R _D +jX _D , After derivation, the relational equations of P and P _dc , Q _dc , U, R _D , X _D can be obtained (calculation formula of load power of AC/DC hybrid system):

In the formula: P and Q are the active power and reactive power of the load supplied by the AC-DC hybrid grid respectively; Z _D , R _D and X _D are the equivalent impedance, equivalent resistance and equivalent value of the load supplied by the AC-DC hybrid grid Reactance.

Normally, the load power factor Little change in a short period of time, it can be assumed that is a constant, after derivation, the relationship equation between P and P _dc , Q _dc , U can be obtained:

Further, in the step S5, P is a function about P _dc , Q _dc , U, R _D , X _D , namely

Write down the Lagrangian function L(P _dc , Q _dc , U, R _D , X _D ) as:

In the formula, as a constraint;

Taking the partial derivative of the above formula, we can get the system of equations

Solving the above 6 equations can get 6 unknowns P _dc , Q _dc , U, RD , X _D and λ, and substituting them into the equation P=f(P _dc , Q _dc , U, R _D , X _{D )} can be solved in The maximum load power P under the constraint conditions, and P _dc and Q _dc are the optimal power settings of VSC-HVDC based on improving the voltage stability level.

Further, in the step S6, the 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 _dc and constant reactive power Q _dc control.

Under normal operation, it is assumed that the VSC-HVDC sets the active power reference value P _ref and the reactive power reference value Q _ref as P _ref0 and Q _ref0 respectively according to the system requirements, namely:

P _ref = P _ref0 , Q _ref = Q _ref0 ;

When the AC system is in the process of voltage instability, that is, the voltage amplitude U of the AC bus i connected to the VSC-HVDC is less than the limit value U _lim and the duration is longer than t _lim (t _lim is set according to the time to avoid the fault, etc.), The control system at the receiving end of the VSC-HVDC switches into a VSC-HVDC power control mode that improves voltage stability.

In this mode, first, according to the network equivalent parameters calculated in step S1 and step S2, and the calculation method in step S5, the optimal power settings P _dc and Q _dc of VSC-HVDC under the load power at the moment are calculated;

Secondly, in order to reduce the impact of VSC-HVDC active power and reactive power adjustment on the system, the VSC-HVDC active power reference value P _ref and reactive power reference value Q _ref are respectively changed from the original set values P _ref0 and Q _ref0 , transition to its optimal power setting P _dc , Q _dc in a step-by-step manner, so that the power reference value is finally set to:

P _ref = P _dc , Q _ref = Q _dc ;

Finally, the d and q axis current reference values i _dref and i _qref are generated by the output of the active and reactive power outer loop PI controllers respectively, and passed into the current inner loop control; after modulation and triggering, the VSC-HVDC output power will be The stepping method gradually transitions to P _dc and Q _dc , which is the optimal power of VSC-HVDC to improve the voltage stability level.

Compared with the prior art, the present invention has the following beneficial effects: A VSC-HVDC power control method for improving voltage stability proposed by the present invention optimizes VSC-HVDC active power and reactive output to improve the AC-DC hybrid system Voltage stabilization offers one possible approach. It lays the foundation for the voltage stability improvement and operation control of the AC-DC hybrid system containing VSC-HVDC.

Description of drawings

FIG. 1 is a flowchart of a VSC-HVDC power control method for improving voltage stability in the present invention.

Fig. 2 is an equivalent circuit of the VSC-HVDC AC/DC hybrid system in the present invention.

Fig. 3 is a typical PV curve in the present invention.

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

Fig. 5 is a schematic diagram of an AC/DC hybrid system including a VSC-HVDC in an embodiment of the present invention.

detailed description

The technical solution of the present invention will be specifically described below in conjunction with the accompanying drawings.

The present invention provides a VSC-HVDC power control method for improving voltage stability, as shown in Figure 1, comprising the following steps:

(1) Input AC and DC grid parameters to form a node admittance matrix. Among them, the AC grid parameter data includes: the head end and terminal node number of the transmission line, the transformer ratio, impedance, series resistance, reactance, parallel conductance, and susceptance; the DC network parameters include: VSC-HVDC bridge arm reactor impedance, commutation current Variable capacity, impedance, converter modulation ratio and maximum allowable current I _lim .

(2) Calculate the Thevenin equivalent model parameters of the AC system. Obtain the equivalent circuit of the AC system and VSC-HVDC, as shown in Figure 2, note that the AC bus connected to the VSC-HVDC converter station is the i-th node of the AC grid, and its voltage phasor is The fundamental wave voltage phasor output by the converter is The equivalent connection impedance between the converter and the AC bus i is Z ₁ ∠θ ₁ =R ₁ +jX ₁ , and the system equivalent impedance Z ₂ ∠θ ₂ =R ₂ +jX ₂ determined according to the i-th node , in this embodiment, the Thevenin equivalent impedance R ₂ and X ₂ are obtained by the PSD-BPA calculation software for the AC system.

(3) Calculate the active and reactive operating range of VSC-HVDC

According to Figure 2, the DC side power equation can be obtained:

In the formula: P _dc and Q _dc are the active and reactive power injected into node i by VSC-HVDC respectively, and δ _ik =δ _i -δ _k =δ-δ _k is the voltage phase angle difference between node i and node k; M is the modulation ratio of the converter, U _d is the DC side voltage of the converter, μ is the utilization rate of the PWM DC voltage;

From formulas (1) and (2), it can be deduced that

in, When VSC outputs reactive power to the AC system, Q _dc is positive.

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

In the formula, I _lim is the maximum allowable current of VSC-HVDC.

Furthermore, in general, in the operation constraints of VSC-HVDC, especially in the process of voltage drop, the constraint of formula (4) will play a key role.

(4) Establish the relational expression of active power (P) and voltage (U) including VSC-HVDC.

exist In the case of a constant, the active (P) voltage (U) relationship including VSC-HVDC can be described as the following form:

P=f(P _dc ,Q _dc ,U) (5)

In the formula: P is the load active power. For the circuit in Figure 2, the PV relationship can be deduced through the power equation, as follows.

For the AC branch in Figure 2, the AC side power equation can be listed:

Among them, P _ac and Q _ac are the active and reactive power injected into node i by the AC branch respectively, with are the voltages at both ends of the AC branch, δ _ij = δ _i - δ _j = δ;.

According to formulas (6) and (7), we can get:

in,

In this example, considering the load power factor Little change in a short period of time, that is, assuming is a constant, and according to P _ac +P _dc ＝P, Q _ac +Q _dc ＝Q, after derivation, the relationship equation between P and P _dc , Q _dc , U can be obtained:

In the formula: P and Q are the active power and reactive power of the load supplied by the AC-DC hybrid grid, respectively.

According to formula (9), the typical PV curve of the AC/DC hybrid power grid can be obtained, as shown in Figure 3.

(5) Calculation of optimal power value of VSC-HVDC based on voltage stability

P is a function about P _dc , Q _dc , U, R _D , X _D , namely

Define the Lagrange function L(P _dc ,Q _dc ,U,R _D ,X _D ) as

In the formula, as constraints.

Taking the partial derivative of formula (11), we can get the equation system

Solving the above 6 equations can get 6 unknowns P _dc , Q _dc , U, RD , X _D and λ, and substituting them into the equation P=f(P _dc , Q _dc , U, R _D , X _{D )} can be solved in The maximum load power P under the constraint conditions, and P _dc and Q _dc are the optimal power settings of VSC-HVDC based on improving the voltage stability level.

(6) Realization of power control

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 _dc and constant reactive power Q _dc control. Under normal circumstances, it is assumed that the VSC-HVDC sets the active power reference value P _ref and the reactive power reference value Q _ref as P _ref0 and Q _ref0 respectively according to the system requirements, namely:

P _ref = P _ref0 , Q _ref = Q _ref0 ;

As shown in Figure 4, when the AC system is in the process of voltage instability, that is, the voltage amplitude U of the AC bus i connected to the VSC-HVDC is less than the limit value U _lim and the duration is greater than t _lim (t _lim is the time to avoid the fault etc.), the control system at the VSC-HVDC receiving end switches into the VSC-HVDC power control mode to improve voltage stability.

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

Secondly, in order to reduce the impact of VSC-HVDC active power and reactive power adjustment on the system, the VSC-HVDC active power reference value P _ref and reactive power reference value Q _ref are respectively changed from the original set values P _ref0 and Q _ref0 Transition to its optimal power setting P _dc , Q _dc in a step-by-step manner, so that the power reference value is finally set to:

P _ref = P _dc , Q _ref = Q _dc ;

Finally, the d and q axis current reference values i _dref and i _qref are generated by the output of the active and reactive power outer loop PI controllers respectively, and passed into the current inner loop control; after modulation and triggering, the VSC-HVDC output power will be The stepping method gradually transitions to P _dc and Q _dc , which is the optimal power of VSC-HVDC to improve the voltage stability level. The optimal power control process of VSC-HVDC based on improving voltage stability is shown in Fig. 4.

Below in conjunction with example the present invention is described in detail.

Taking the AC/DC hybrid power transmission system containing VSC-HVDC shown in Figure 5 as an example for illustration, the voltage stability of the system is analyzed by using the method provided by the invention, which specifically includes the following steps:

1. Calculate the equivalent circuit parameters of AC and DC systems

In the calculation example in Figure 5, the main parameters of the flexible HVDC system are shown in Table 1. The sending end is controlled by constant DC voltage U _d and constant AC reactive power Q; the receiving end is controlled by constant active power P and AC reactive power Q. The maximum load at the receiving end is 1998MW, and the power factor In the maximum operation mode, the equivalent impedance of the AC grid at the receiving end is R ₂ =1.587Ω, X ₂ =5.766Ω, and the equivalent potential of the AC system E _s =1.1pu (reference voltage 230kV).

Table 1 Main parameters of VSC-HVDC system

According to the given data above, the AC/DC hybrid power grid containing VSC-HVDC shown in Figure 5 is simplified and equivalent according to Figure 2, and the parameters of the equivalent circuit are shown in Table 2. Table 2 gives the parameter name value and standard unit value. For convenience, the following calculations are carried out with standard unit value.

Table 2 Calculation results of circuit parameters

Note: The reference values are respectively, U _B ＝230kV, S _B ＝100MVA, Z _B ＝529Ω.

2. Calculation of optimal power value of VSC-HVDC based on voltage stability

Substitute the parameters (per unit value) in Table 1 and Table 2 into the following formula

In the formula:

(Pu)

Taking the partial derivative of L(P _dc ,Q _dc ,U), we can get the equation system

Solving the above equations, considering that P _dc , Q _dc , and U are real number solutions greater than 0, a set of meaningful real number solutions can be obtained

The equation system solution P _dc is expressed as 554.6MW with a famous value, which is the best active power value of VSC-HVDC to improve voltage stability.

Substituting the above solution into f(P _dc ,Q _dc ,U), the maximum transmission power P of the system can be solved as

P _max = f(P _dc , Q _dc , U) = 44.420

P _max is expressed as 44420MW with a well-known value, which is the maximum transmittable power of the AC-DC hybrid system.

3. Power control

In this embodiment, the initial values of VSC-HVDC active power reference value P _ref and reactive power reference value Q _ref are set to 800MW and 100Mvar respectively according to system requirements; The value is set to 1s. When the VSC-HVDC monitors that the voltage amplitude of the connected AC bus is less than 0.9pu and lasts longer than 1s, it starts power control based on improving voltage stability. According to the calculation results, it is based on the best active and reactive power The power values are 554.6MW and 518.0Mvar. VSC-HVDC automatically transitions _Pref and Q _ref to its optimal active and reactive power values of 554.6MW and 518.0Mvar in a step-by-step manner. After being controlled by the inner and outer loops of VSC-HVDC power, the output active power of VSC-HVDC will gradually transition to 554.6MW and 518.0Mvar in a stepwise manner until the system voltage recovers.

The above are the preferred embodiments of the present invention, and all changes made according to the technical solution of the present invention, when the functional effect produced does not exceed the scope of the technical solution of the present invention, all belong to the protection scope of the present invention.

Claims (7)

1. a VSC-HVDC power control method that improves voltage stability, is characterized in that, comprises the steps: Step S1: Input AC and DC power grid parameter data to form a node admittance matrix; Step S2: Calculate the Thevenin equivalent model parameters of the AC system; Step S3: Calculate the VSC-HVDC active and reactive operating range; Step S4: Establishing an active voltage relational expression including VSC-HVDC; Step S5: Calculate the optimal power value of VSC-HVDC based on improving voltage stability; Step S6: Perform VSC-HVDC power control. 2. A kind of VSC-HVDC power control method that improves voltage stability according to claim 1, is characterized in that, in described step S1, described AC grid parameter data comprises: the head end of transmission line, terminal node serial number, transformer ratio, impedance, series resistance, reactance and parallel conductance, susceptance; the DC network parameters include: VSC-HVDC bridge arm reactor impedance, converter variable capacity, impedance, converter modulation ratio and maximum allowable Current I _lim . 3. A kind of VSC-HVDC power control method that improves voltage stability according to claim 1, it is characterized in that, in described step S2, obtain AC system and VSC-HVDC equivalent circuit, denote VSC-HVDC exchange The AC bus connected to the flow station is the i-th node of the AC grid, and its voltage phasor is The fundamental wave voltage phasor output by the converter is The equivalent connection impedance between the converter and the AC bus i is Z ₁ ∠θ ₁ =R ₁ +jX ₁ , and the system equivalent impedance Z ₂ ∠θ ₂ =R ₂ +jX ₂ determined according to the i-th node , Thevenin equivalent impedance R ₂ and X ₂ can be obtained by PSD-BPA calculation software for AC systems. 4. A kind of VSC-HVDC power control method that improves voltage stability according to claim 1, is characterized in that, in described step S3, obtains AC system and VSC-HVDC system equivalent circuit, according to this AC system Calculate the DC side power with the equivalent circuit of the VSC-HVDC system: ${\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}\mathrm{<span\; class="notranslate">\; u</span>}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ;</span>$ ${\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{\mathrm{<span\; class="notranslate">\; sin\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}\mathrm{<span\; class="notranslate">\; u</span>}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{\mathrm{<span\; class="notranslate">\; sin\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ;</span>$ Where: P _dc and Q _dc are the active and reactive power injected into node i by VSC-HVDC respectively, and δ _ik =δ _i -δ _k =δ-δ _k is the voltage phase angle difference between node i and node k; M is the modulation ratio of the converter, U _d is the DC side voltage of the converter, μ is the utilization rate of the PWM DC voltage; From the above two formulas, it can be deduced that: ${<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; Y</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ;</span>$ in, When VSC outputs reactive power to the AC system, Q _dc is positive; Maximum allowable current limit during VSC-HVDC operation: ${\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&le;</span>{<span\; class="notranslate">\; (</span>\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; UI</span>}}_{\mathrm{<span\; class="notranslate">\; lim</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}$ Among them, I _lim is the maximum allowable current of VSC-HVDC. 5. A kind of VSC-HVDC power control method that improves voltage stability according to claim 1, is characterized in that, in described step S4, establishes the relational equation of P and _{Pdc, Qdc ,} U based on power flow equation . According to the AC branch in the equivalent circuit of the AC system and the VSC-HVDC system, calculate the AC side power: ${\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; j</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; UE</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ;</span>$ ${\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; j</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; sin\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; UE</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; sin\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ;</span>$ Among them, P _ac and Q _ac are the active and reactive power injected into node i by the AC branch respectively, with are the voltages at both ends of the AC branch, δ _ij = δ _i - δ _j = δ; Then you can get: ${<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; Y</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ;</span>$ in, Then according to P _ac +P _dc =P, Q _ac +Q _dc =Q, Z _D =R _D +jX _D , After derivation, the relational equations of P and P _dc , Q _dc , U, R _D , X _D can be obtained (calculation formula of load power of AC/DC hybrid system): $\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}<span\; class="notranslate">\; \&PlusMinus;</span>\sqrt{<span\; class="notranslate">\; -</span>{<span\; class="notranslate">\; \&lsqb;</span>{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&rsqb;</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; Y</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}$ In the formula: P and Q are the active power and reactive power of the load supplied by the AC-DC hybrid grid respectively; Z _D , R _D and X _D are the equivalent impedance, equivalent resistance and equivalent value of the load supplied by the AC-DC hybrid grid Reactance. due to load power factor Little change in a short period of time, remember is a constant, after derivation, the relationship equation between P and P _dc , Q _dc , U can be obtained: 6. A kind of VSC-HVDC power control method that improves voltage stability according to claim 1, is characterized in that, in described step S5, P is about P _dc , Q _dc , U, R _D , X _D function, that is $\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; dc</span>}}<span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; dc</span>}}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}<span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; dc</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; dc</span>}}\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}<span\; class="notranslate">\; \&PlusMinus;</span>\sqrt{<span\; class="notranslate">\; -</span>{<span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; dc</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; dc</span>}}\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; Y</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}$ Write down the Lagrangian function L(P _dc , Q _dc , U, R _D , X _D ) as: In the formula, as a constraint; Taking the partial derivative of the above formula, we can get the system of equations Solving the above 6 equations can get 6 unknowns P _dc , Q _dc , U, RD , X _D and λ, and substituting them into the equation P=f(P _dc , Q _dc , U, R _D , X _{D )} can be solved in The maximum load power P under the constraint conditions, and P _dc and Q _dc are the optimal power settings of VSC-HVDC based on improving the voltage stability level. 7. A kind of VSC-HVDC power control method that improves voltage stability according to claim 1, is characterized in that, in described step S6, VSC-HVDC adopts power outer loop, dq decoupling current inner loop control mode ; The inverter side of the receiving end is controlled by constant active power P _dc and constant reactive power Q _dc ; Under normal operation, it is assumed that the VSC-HVDC sets the active power reference value P _ref and the reactive power reference value Q _ref to P _ref0 and Q _ref0 respectively according to the system requirements, namely: P _ref = P _ref0 , Q _ref = Q _ref0 ; When the AC system is in the process of voltage instability, that is, the voltage amplitude U of the AC bus i connected to the VSC-HVDC is less than the limit value U _lim and the duration is longer than t _lim , the control system at the receiving end of the VSC-HVDC switches to improve voltage stability VSC-HVDC power control mode; In this mode, first calculate the network equivalent parameters according to step S1 and step S2, and use the calculation method of step S5 to calculate the optimal power settings P _dc and Q _dc of the VSC-HVDC under the load power at the moment; Secondly, in order to reduce the impact of VSC-HVDC active power and reactive power regulation on the system, the VSC-HVDC active power reference value P _ref and reactive power reference value Q _ref are respectively changed from the original set values P _ref0 and Q _ref0 , transition to its optimal power settings P _dc and Q _dc in a step-by-step manner, so that the power reference value is finally set as: P _ref =P _dc , Q _ref =Q _dc ; Finally, the d and q axis current reference values i _dref and i _qref are generated through the output of the active and reactive power outer loop PI controllers respectively, and passed into the current inner loop control; after modulation and triggering, the VSC-HVDC output power will be The stepping method gradually transitions to P _dc and Q _dc , which is the optimal power of VSC-HVDC to improve the voltage stability level.