阅读说明：本技术 一种适用于新能源mmc-mtdc系统的改进下垂控制方法 (Improved droop control method suitable for new energy MMC-MTDC system ) 是由 宋汶秦 杨婷婷 柴文博 刘永成 王海亮 王著秀 赵玲霞 赵春娟 杨茜 于 2021-08-16 设计创作，主要内容包括：本发明公开了一种适用于新能源MMC-MTDC系统的改进下垂控制方法,属于柔性直流输电技术领域,对传统下垂控制进行改进,提出直流电压裕度跟踪下垂控制。该方法以直流电压裕度跟踪为基础,在下垂系数中加入直流电压裕度校正因子,利用直流电压偏差值实时调整换流站运行工作点的直流电压参考值,并利用该参考值实现换流站的直流电压裕度跟踪,实现了换流站之间的协同控制,保证系统在受到功率扰动或换流站故障时直流电压稳定分布及有功功率合理分配。该方法结构简单,抗干扰能力强,不依赖通讯,可防止系统在扰动工况下的直流电压越限,保证系统稳定运行。(The invention discloses an improved droop control method suitable for a new energy MMC-MTDC system, belongs to the technical field of flexible direct current transmission, improves the traditional droop control, and provides direct current voltage margin tracking droop control. The method is based on direct-current voltage margin tracking, a direct-current voltage margin correction factor is added into a droop coefficient, a direct-current voltage reference value of a converter station operation working point is adjusted in real time by using a direct-current voltage deviation value, direct-current voltage margin tracking of the converter station is achieved by using the reference value, cooperative control among the converter stations is achieved, and stable direct-current voltage distribution and reasonable active power distribution of a system when the system is disturbed by power or faults of the converter station are guaranteed. The method has the advantages of simple structure and strong anti-interference capability, does not depend on communication, can prevent the direct current voltage of the system from exceeding the limit under the disturbance working condition, and ensures the stable operation of the system.)

1. An improved droop control method suitable for a new energy MMC-MTDC system is characterized in that n converter stations are arranged, wherein m converter stations adopting droop control are provided, and the rest n-m converter stations adopt constant active power control, wherein m is less than n;

in case of normal operation of the MMC-MTDC system, the m converter stations with droop control are at their reference point (P)_ref，I，U_ref，I) Operation, P_ref，IAnd U_ref，IRespectively an active power reference value and a direct current voltage reference value;

when the MMC-MTDC system is disturbed, the converter station adopting droop control bears the active power redundancy and the direct-current voltage deviation according to the proportion of the droop control coefficient and automatically moves to a new working point (P)_x，i，U_x，i)，P_x，iAnd U_x，iRespectively controlling real-time active power and real-time direct current voltage of the converter station by adopting droop;

the method comprises the following steps:

s1, when the system is in normal operation, determining the direct current voltage reference value U of the converter station adopting droop control_ref，IAnd an active power reference value P_ref，ITo obtain the droop coefficient k_iAnd a DC voltage reference value U_ref，IAnd an active power reference value P_ref，IThe relational expression of (1);

s2, when the system is in a disturbance condition, calculating the direct current voltage margin M of the converter station adopting droop control_i；

S3, when the system is in a disturbance working condition, calculating a direct current voltage reference value U of the converter station adopting droop control_re，I；

S4, solving the obtained direct current voltage margin M according to S2 and S3_iAnd a DC voltage reference value U_re，IFurther solving to obtain a direct current voltage margin correction factor zeta_i；

S5, solving the direct-current voltage margin correction factor zeta according to S4_iFor fixed droop coefficient k_iCorrected to obtain improved droop coefficient k_i′；

S6, utilizing the improved droop coefficient k_i' update the DC voltage margin M_iTracking droop control by DC voltage margin M_iReal-time tracking system DC voltage U_x，i。

2. According to claim 1The improved droop control method suitable for the new energy MMC-MTDC system is characterized in that in the step S1, the droop coefficient k is_iThe relation between the direct-current voltage reference value and the active power reference value of the converter station is as follows:

U_x，i＝-k_i(P_x，i-P_ref，I)+U_ref，I； (1)。

3. the improved droop control method suitable for the new energy MMC-MTDC system according to claim 1, wherein in S2, the DC voltage margin M_iComprises the following steps:

M_i＝ψ_i-(U_x，i-U_dcn，i)； (2)

wherein psi_iFor voltage margin clipping factor, #_i5% -10% of rated voltage of the converter station; u shape_dcn，iIs the rated voltage of the converter station; u shape_x，i-U_dcn，iIndicating the dc voltage deviation of the converter station.

4. The improved droop control method suitable for the new energy MMC-MTDC system according to claim 3, wherein in S3, when the system is in a disturbance condition, the DC voltage reference value U is obtained_re,IComprises the following steps:

U_re,I＝U_dcn,i-(M_i/λ_i)； （3）

the DC voltage reference value U_re,IWith real-time DC voltage U_x,iConstantly changing, wherein λ_iFor evaluating the DC voltage margin M_iParameter of (a)_iThe value ranges are as follows: lambda is more than or equal to 0.05_i≤0.1；

From equation (3), it can be derived that the real-time dc voltage U of the converter station_x,iNear rated voltage U_dcn,iTime, DC voltage margin M_iContinuously increasing the reference value U of the DC voltage_re,IReduced to prevent voltage from exceeding rated voltage U_dcn,i(ii) a When the real-time DC voltage U_x,iEqual to rated voltage U_dcn,iBy adjusting lambda_iMake the DC voltage reference value U_re,IEqual to rated voltage U_dcn,iI.e. U_re,I=U_dcn,i。

5. The improved droop control method suitable for the new energy MMC-MTDC system according to claim 4, wherein in S4, the DC voltage margin correction factor ζ is_iComprises the following steps:

wherein, 0 is not less than ζ_i≤1；The maximum value of the converter station direct-current voltage margin is the upper limit value of the converter station direct-current voltage margin, the maximum value of the converter station direct-current voltage is represented, and the positive and negative fluctuation values of the voltage do not exceed 10% of the rated voltage under the normal condition;

from equation (4), the system uses the new DC voltage reference value U obtained by calculation_re,IAnd the upper limit value of the DC voltage marginThe direct-current voltage of the converter station is tracked in real time, so that direct-current voltage margin tracking control is realized.

6. The method for controlling droop control in the MMC-MTDC system as claimed in claim 5, wherein in S5, the droop coefficient k is modified_i' is:

from the equation (5), when the DC voltage U is real-time_x,iThe closer to the upper limit value of the DC voltage margin of the converter stationTime, voltage margin correction factor ζ_iGradually increasing sag factor k_i' with it, the unbalanced power borne by the converter station increases; when in useTime, voltage margin correction factor ζ_iIncrease to 1, sag factor k_i' to the minimum, the converter station is equivalently in a constant active power control mode, and system active power redundancy during disturbance can be reasonably distributed.

7. The improved droop control method suitable for the new energy MMC-MTDC system according to claim 6, wherein in S6, formula (1) is replaced by formula (5) and formula (3), resulting in:

the above formula shows that m convertor station real-time direct-current voltages U adopting improved droop control_x,iAnd real-time active power P_x,iThe relationship between them is quadratic and no longer linear; according to the formula, the voltage margin correction factor is increased along with the increase of the real-time direct-current voltage, so that the droop coefficient is converted into a variable from a fixed constant, and the tracking control of the direct-current voltage margin of the converter station is realized.

Technical Field

The invention relates to the technical field of flexible direct current transmission, in particular to an improved droop control method suitable for a new energy MMC-MTDC system.

Background

With the prominence of the global energy crisis and the continuous worsening of environmental problems, countries around the world have begun to transition from traditional fossil energy to green renewable energy. The development of renewable energy has become an important strategic approach for many countries to actively cope with the core content of climate change and to promote energy transformation.

The flexible direct current transmission technology (VSC-HVDC) has strong controllability, rapid power flow reversal, no distance limitation and no need of providing commutation current at an alternating current side. The decoupling of the sending end and the receiving end can be realized during long-distance transmission, and the influence of new energy grid connection on a power transmission system is reduced. Therefore, the flexible direct-current transmission technology becomes an ideal transmission mode for accessing new energy into a power grid, wherein the flexible direct-current transmission technology based on the MMC is more widely applied.

The concept of conventional voltage droop control is similar to the P-f droop control characteristic of ac systems. When the converter station adopts a droop control strategy, the relation between the direct-current voltage and the active power can be simply represented by a straight line, and the voltage and the power are automatically balanced and adjusted by utilizing the linear relation, so that the direct-current voltage of the MMC-MTDC system is kept stable. The proportion of the fixed direct-current voltage control and the fixed active power control in the converter station control can be determined through the droop coefficient, when the grid-connected power is in continuous and violent fluctuation, the converter station detects that the direct-current voltage of the system fluctuates, and the output power of the converter station is adjusted according to a certain rule to enable the system to reach new balance. However, the traditional droop control strategy has limitations, that is, no matter how large the residual direct current voltage margin of the converter station is, the converter station can only bear unbalanced power or direct current voltage deviation according to the initial preset proportion, so that part of the converter stations are close to the limit operation of the direct current voltage, and the regulation capability of the converter station on the fluctuation of new energy is lost. And the single droop coefficient is difficult to adapt to complex power fluctuation working conditions, and cannot flexibly respond to tidal current changes, so that the direct-current voltage is seriously deviated. If the converter station works in a state of deviating from the rated direct-current voltage, the safety and the reliability of converter station equipment are adversely affected, and the operation stability of the MMC-MTDC system is further affected. The existing methods for improving droop control mainly focus on two aspects, including improved design of the converter station active power-voltage curve and improved droop factor from a power margin perspective. The improved droop control proposed according to both aspects above achieves stable operation of the system when disturbed, but ignores the important role of dc voltage deviation in the system.

Disclosure of Invention

Aiming at the problems in the prior art, the invention provides an improved droop control method suitable for a new energy MMC (modular multilevel converter) -MTDC system (multi-terminal flexible direct current transmission system). A direct current voltage margin correction factor is added into a traditional droop coefficient, a direct current voltage reference value of a converter station operation working point is adjusted in real time through a direct current voltage deviation value, and direct current voltage margin tracking of the converter station is realized by utilizing the reference value.

In order to achieve the purpose, the invention adopts the following technical scheme:

an improved droop control method suitable for a new energy MMC-MTDC system is characterized in that n converter stations are arranged, wherein m converter stations adopting droop control are provided, and the rest n-m converter stations adopt constant active power control, wherein m is less than n;

in case of normal operation of the MMC-MTDC system, the m converter stations with droop control are at their reference point (P)_ref,_I,U_ref,_I) Operation, P_ref,_IAnd U_ref,_IRespectively an active power reference value and a direct current voltage reference value;

when the MMC-MTDC system is disturbed, the converter station adopting droop control bears the active power redundancy and the direct-current voltage deviation according to the proportion of the droop control coefficient and automatically moves to a new working point (P)_x,i，U_x,i)，P_x,iAnd U_x,iRespectively controlling real-time active power and real-time direct current voltage of the converter station by adopting droop;

the method comprises the following steps:

s1, when the system is in normal operation, determining the direct current voltage reference value U of the converter station adopting droop control_ref,_IAnd an active power reference value P_ref,ITo obtain the droop coefficient k_iAnd a DC voltage reference value U_ref,IAnd an active power reference value P_ref,IThe relational expression of (1);

s2, when the system is in a disturbance condition, calculating the direct current voltage margin M of the converter station adopting droop control_i；

S3, when the system is in a disturbance working condition, calculating a direct current voltage reference value U of the converter station adopting droop control_re,I；

S4, solving the obtained direct current voltage margin M according to S2 and S3_iAnd a DC voltage reference value U_re,IFurther solving to obtain a direct current voltage margin correction factor zeta_i；

S5, solving the direct-current voltage margin correction factor zeta according to S4_iFor fixed droop coefficient k_iCorrected to obtain improved droop coefficient k_i′；

S6, utilizing the improved droop coefficient k_i' update the DC voltage margin M_iTracking droop control by DC voltage margin M_iReal-time tracking system DC voltage U_x,i。

Further, in S1, the droop coefficient k_iThe relation between the direct-current voltage reference value and the active power reference value of the converter station is as follows:

U_x,i＝-k_i(P_x,i-P_ref,I)+U_ref,I； (1)

further, in S2, the dc voltage margin M_iComprises the following steps:

M_i＝ψ_i-(U_x，i-U_dcn，i)； (2)

wherein psi_iFor voltage margin clipping factor, #_i5% -10% of rated voltage of the converter station; u shape_dcn,iIs the rated voltage of the converter station; u shape_x,i-U_dcn,iIndicating the dc voltage deviation of the converter station.

Further, in S3, when the system is in the disturbance condition, the dc voltage reference value U_re,IComprises the following steps:

U_re,I＝U_dcn,i-(M_i/λ_i)； (3)

reference value U of DC voltage_re,IWith real-time DC voltage U_x,iConstantly changing, wherein λ_iFor evaluating the DC voltage margin M_iParameter of (a)_iThe value ranges are as follows: lambda is more than or equal to 0.05_i≤0.1；

From the equations (2) and (3), it can be derived that the real-time dc voltage U at the converter station_x,iNear rated voltage U_dcn,iTime, DC voltage margin M_iContinuously increasing the reference value U of the DC voltage_re,IReduced to prevent voltage from exceeding rated voltage U_dcn,i(ii) a When the real-time DC voltage U_x,iEqual to rated voltage U_dcn,iBy adjusting lambda_iMake the DC voltage reference value U_re,IEqual to rated voltage U_dcn,iI.e. U_re,I＝U_dcn,i。

Further, in S4, the dc voltage margin correction factor ζ_iComprises the following steps:

wherein, 0 is not less than ζ_i≤1；The maximum value of the converter station direct-current voltage margin is the upper limit value of the converter station direct-current voltage margin, the maximum value of the converter station direct-current voltage is represented, and the positive and negative fluctuation values of the voltage do not exceed 10% of the rated voltage under the normal condition;

from equation (4), the system uses the new DC voltage reference value U obtained by calculation_re,IAnd the upper limit value of the DC voltage marginThe direct-current voltage of the converter station is tracked in real time, so that direct-current voltage margin tracking control is realized.

Further, in S5, the modified droop coefficient k_i' is:

from the equation (5), when the DC voltage U is real-time_x,iThe closer to the upper limit value of the DC voltage margin of the converter stationTime, voltage margin correction factor ζ_iGradually increasing sag factor k_i' with it, the unbalanced power borne by the converter station increases; when in useTime, voltage margin correction factor ζ_iIncrease to 1, sag factor k_i' to the minimum, the converter station is equivalently in a constant active power control mode, and system active power redundancy during disturbance can be reasonably distributed.

Further, in S6, formula (5) and formula (3) are substituted for formula (1) to obtain:

the above formula shows that m convertor station real-time direct-current voltages U adopting improved droop control_x,iAnd real-time active power P_x,iBetweenThe relationship (a) is quadratic and no longer linear; according to the formula, the voltage margin correction factor is increased along with the increase of the real-time direct-current voltage, so that the droop coefficient is converted into a variable from a fixed constant, and the tracking control of the direct-current voltage margin of the converter station is realized.

The invention has the following beneficial effects:

(1) the invention adds a direct current voltage margin correction factor on the basis of the traditional droop control, utilizes the direct current voltage deviation value to adjust the direct current voltage reference value of the operating working point of the converter in real time, and then utilizes the reference value to realize the tracking of the direct current voltage, thereby realizing the direct current voltage margin tracking of the converter station and automatically reducing the direct current voltage of the converter station when the direct current voltage approaches the upper limit of the voltage margin. Meanwhile, the improved droop coefficient enables the converter station adopting droop control to effectively bear active power redundancy under system disturbance, and cooperative control among the converter stations of the system can be realized according to the actually available direct-current voltage margin, so that stable operation of the system is guaranteed.

(2) When the direct-current voltage deviation is too large and the active power is redundant due to power fluctuation or converter station faults of the system, self-adaptive adjustment is carried out between the converter stations under the action of improved droop control, a new steady-state operation point of the system is reached, and the stable direct-current voltage distribution and the reasonable active power distribution of the converter stations under the condition of power disturbance or converter station faults are ensured.

Drawings

FIG. 1 is a diagram of a four-terminal new energy MMC-MTDC system according to an embodiment of the present invention;

FIG. 2 is a block diagram of a DC voltage margin tracking droop controller according to one embodiment of the present invention;

FIG. 3 is a flow chart of droop control based on DC voltage margin tracking according to one embodiment of the present invention;

FIG. 4 is a graph of DC voltage margin tracking droop control operation according to one embodiment of the present invention;

Detailed Description

The following description of the embodiments of the present invention will be made with reference to the accompanying drawings:

as shown in the figure1, the MTDC system is a four-terminal new energy MTDC system, 4 converter stations are provided in total, 2 converter stations adopting droop control are provided, and the rest 2 converter stations adopt constant active power control. The system mainly comprises: wind farm and alternating current system voltage u_snWherein n ═ 1, 2, 3, 4 denotes the end count of the converter station; MMC exchanges side inductance L_n(ii) a Voltage V at the inlet of the converter station_n(ii) a The resistance and inductance of the DC line are R_dln、L_dln(ii) a Current I at AC side of converter station_sn(ii) a Equivalent resistance R on system AC side_n(ii) a The method for coordinating and controlling the MTDC system converter stations mainly comprises master-slave control, direct-current voltage margin control and droop control; the invention improves the traditional droop control by utilizing a direct-current voltage margin tracking method from the droop control.

As shown in fig. 2, the control principle of the present invention is: firstly, determining a direct current voltage reference value U when a system stably operates_ref,ITo rated value U_dcn,iAnd an active power reference value P_ref,I(ii) a Then calculating the DC voltage margin M of the system_iAnd passing through the DC voltage margin M_iRated voltage U of system_dcn,iObtaining a direct current voltage reference value U under the system disturbance working condition_re,I(ii) a The obtained DC voltage margin M is reused_iWith its reference value U_re,IAnd adding a DC voltage margin correction factor zeta_iThe droop coefficient is corrected to obtain an improved droop control coefficient k_i'. Finally, using the improved droop coefficient k_iAnd updating the direct current voltage margin tracking droop control, and tracking and controlling the direct current voltage of the real-time tracking system through the direct current voltage margin. The converter station measures the real-time direct-current voltage, adjusts the instruction value of the power based on the droop characteristic, and simultaneously performs power distribution, wherein the adjusted power reacts on the direct-current voltage to maintain the voltage stability; the method is characterized in that a direct current voltage margin correction factor is added into a droop coefficient, direct current voltage influence factors such as new energy output fluctuation and converter station faults are integrated, and a direct current voltage reference value of a converter station operation working point is adjusted in real time by utilizing a direct current voltage deviation value in combination with system direct current voltage deviation, so that the direct current voltage of a system is close to a voltage marginWhen the voltage is limited, the voltage is automatically reduced to prevent the voltage from exceeding the limit; k in FIG. 2_p+k_iS denotes a PI controller, k_pIs a proportionality coefficient, k_iIs an integral coefficient; i.e. i_dmaxAnd i_dminRespectively representing the upper limit value and the lower limit value of the current amplitude limiting link; the input value of the improved droop controller is a reference value and an actual value of direct-current voltage and active power, and the output value of the improved droop controller is a d-axis current reference value of a current converter inner ring current controller; by the active current reference value i_drefThe control of the converter station is realized, and the purpose of ensuring the stable operation of the system is achieved.

As shown in fig. 3, the specific implementation steps are as follows:

an improved droop control method suitable for a new energy MMC-MTDC system is characterized in that a total of 4 converter stations are arranged, wherein 2 converter stations adopting droop control are provided, and the rest 2 converter stations adopt constant active power control;

in case of a normal operation of the MMC-MTDC system, 2 converter stations with droop control are at their reference point (P)_ref,_I,U_ref,_I) Operation, P_ref,_I、U_ref,_IRespectively an active power reference value and a direct current voltage reference value;

when the MMC-MTDC system is disturbed, the converter station adopting droop control bears the active power redundancy and the direct-current voltage deviation according to the proportion of the droop control coefficient and automatically moves to a new working point (P)_x,i，U_x,i)，P_x,i、U_x,iRespectively controlling real-time active power and real-time direct current voltage of the converter station by adopting droop;

s1, when the system is in normal operation, determining the direct current voltage reference value U of the converter station adopting droop control_ref,_IAnd an active power reference value P_ref,ITo obtain the droop coefficient k_iAnd a DC voltage reference value U_ref,IAnd an active power reference value P_ref,IThe relation of (1):

U_x,i＝-k_i(P_x,i-P_ref,I)+U_ref,I； (1)

s2, seriesWhen the system is in a disturbance working condition, calculating the direct-current voltage margin M of the converter station adopting droop control_i；

The direct-current voltage deviation is an important index for evaluating the performance of the multi-terminal MTDC system, and the direct-current voltage margin M is usually_iThe voltage margin amplitude limiting coefficient and the real-time direct-current voltage and direct-current voltage rated value of the converter station are used for obtaining:

M_i＝ψ_i-(U_x，i-U_dcn，i)； (2)

wherein psi_iFor voltage margin clipping factor, #_i5% -10% of rated voltage of the converter station; u shape_dcn,iIs the rated voltage of the converter station; u shape_x,i-U_dcn,iRepresenting a dc voltage deviation of the converter station;

s3, when the system is in a disturbance working condition, calculating a direct current voltage reference value U of the converter station adopting droop control_re,I；

In order to realize that the voltage does not exceed the limit, the direct-current voltage reference value U of the converter station_re,IRated value U of system DC voltage_dcn,iWith a DC voltage margin M_iDetermining:

U_re,I＝U_dcn,i-(M_i/λ_i)； (3)

reference value U of DC voltage_re,IWith real-time DC voltage U_x,iConstantly changing, wherein λ_iFor evaluating the DC voltage margin M_iParameter of (a)_iThe value ranges are as follows: lambda is more than or equal to 0.05_i≤0.1；

From the equations (2) and (3), it can be derived that the real-time dc voltage U at the converter station_x,iNear rated voltage U_dcn,iTime, DC voltage margin M_iContinuously increasing the reference value U of the DC voltage_re,IReduced to prevent voltage from exceeding rated voltage U_dcn,i(ii) a When the real-time DC voltage U_x,iEqual to rated voltage U_dcn,iBy adjusting lambda_iMake the DC voltage reference value U_re,IEqual to rated voltage U_dcn,iI.e. U_re,I＝U_dcn,i；

S4, solved according to S2 and S3DC voltage margin M_iAnd a DC voltage reference value U_re,IFurther solving to obtain a direct current voltage margin correction factor zeta_i；

The droop coefficient can clearly determine the proportion of the direct current voltage control and the active power control in the system control, the lower the droop coefficient is, the weaker the control capability on the direct current voltage is, but the stronger the control capability on the power is, and the direct current voltage margin M obtained by S2 and S3 is utilized_iAnd a DC voltage reference value U_re,IFurther obtaining a DC voltage margin correction factor Zeta_i：

Wherein, 0 is not less than ζ_i≤1；The maximum value of the converter station direct-current voltage margin is the upper limit value of the converter station direct-current voltage margin, the maximum value of the converter station direct-current voltage is represented, and the positive and negative fluctuation values of the voltage do not exceed 10% of the rated voltage under the normal condition; when the converter station of the system quits operation and power fluctuation, the converter station injects excessive active power to cause the rise of direct-current voltage, so the invention only considers the situation of the increase of the direct-current voltage, and the maximum value which can be reached is the upper limit value of the direct-current voltage margin; from equation (4), the system uses the new DC voltage reference value U obtained by calculation_re,IAnd the upper limit value of the DC voltage marginThe direct-current voltage of the converter station is tracked in real time to realize direct-current voltage margin tracking control;

s5, solving the direct-current voltage margin correction factor zeta according to S4_iFor fixed droop coefficient k_iCorrected to obtain improved droop coefficient k_i′：

From the equation (5), when the DC voltage U is real-time_x,iThe closer to the upper limit value of the DC voltage margin of the converter stationTime, voltage margin correction factor ζ_iGradually increasing sag factor k_i' with it, the unbalanced power borne by the converter station increases; when in useTime, voltage margin correction factor ζ_iIncrease to 1, sag factor k_iThe minimum value is reached, the converter station is equivalently in a constant active power control mode, and the system active power redundancy during disturbance can be reasonably distributed;

s6, utilizing the improved droop coefficient k_i' update the DC voltage margin M_iTracking droop control by DC voltage margin M_iReal-time tracking system DC voltage U_x,i；

Sag factor k to be improved_i' and a DC voltage reference value U_re,ISubstituting into equation (1), the proposed DC voltage margin M_iTracking droop control is given by:

the structure of the improved droop controller is shown in fig. 3, and the droop coefficient k of the traditional droop controller is compared with that of the traditional droop controller_iWith improved sag factor k_iThe' updating is carried out, so that the droop coefficient is converted into a variable from a fixed constant, and the direct-current voltage margin M of the commutation station is realized_iThe tracking control of (2);

from the above formula, with the real-time DC voltage U_x,iIncrease of (1), correction factor ζ of DC voltage margin_iAlso increases when the actual DC voltage U_x,iApproaching the upper limit of the DC voltage marginTime, DC voltage margin correction factor ζ_iIncreasing the reference value U of the DC voltage_re,ISelf-regulation is carried out to prevent the direct current voltage from exceeding the limit and maintain the stability of the direct current voltage of the converter station; correction factor zeta with DC voltage margin_iGradually increase and improve the droop coefficient k_iThe unbalanced power borne by the converter station is increased along with the reduction, the converter station is equivalently in a constant active power control mode, and the redundant active power of the converter station is reasonably distributed; a graph of the improved voltage droop control operating characteristic is shown in figure 4,andrespectively representing the maximum value and the minimum value of the direct-current voltage and the active power of the converter station during system disturbance, and simultaneously showing m real-time direct-current voltages U of the converter station adopting improved droop control through S6 and a characteristic diagram_x,iAnd real-time active power P_x,iThe relationship between them is quadratic and no longer linear; real-time DC voltage U_x,iThe closer to the upper limit of the DC voltage margin of the systemAnd the converter station performs self-adaptive adjustment according to the actual direct current voltage margin of the converter station, so that cooperative control among the converter stations is realized.

It is to be understood that the above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and those skilled in the art may make modifications, alterations, additions or substitutions within the spirit and scope of the present invention.