CN113067357B - Direct-current voltage self-adaptive droop control method and system for alternating-current and direct-current hybrid power distribution network - Google Patents

Direct-current voltage self-adaptive droop control method and system for alternating-current and direct-current hybrid power distribution network Download PDF

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CN113067357B
CN113067357B CN202110289172.5A CN202110289172A CN113067357B CN 113067357 B CN113067357 B CN 113067357B CN 202110289172 A CN202110289172 A CN 202110289172A CN 113067357 B CN113067357 B CN 113067357B
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droop
direct
converter station
voltage
current
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CN113067357A (en
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裴雪军
张曼
杨敏
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Huazhong University of Science and Technology
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J1/00—Circuit arrangements for dc mains or dc distribution networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/04—Circuit arrangements for ac mains or ac distribution networks for connecting networks of the same frequency but supplied from different sources
    • H02J3/06—Controlling transfer of power between connected networks; Controlling sharing of load between connected networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

The invention discloses a direct-current voltage self-adaptive droop control method and system for an alternating-current and direct-current hybrid power distribution network, and belongs to the technical field of power electronics. The droop coefficient and the droop rated working point of droop control are adaptively improved and changed, the droop coefficient is adjusted in the first step, when the direct current voltage is out of limit, the droop coefficient is adaptively calculated and updated by taking the control voltage deviation in the limit range as a target according to the locally detected direct current bus voltage and output power information, and the direct current voltage is quickly controlled to return to the limit range; and secondly, adjusting a droop rated working point, stabilizing the voltage of the direct current bus at an upper limit or a lower limit, taking the output power of the corresponding converter station as the balance power of the system, constructing a new droop curve by taking the power as a new rated working point, and controlling the direct current voltage to return to the rated working point along the new droop curve, thereby solving the problem that the direct current voltage is out of limit caused by the trend change when the alternating current-direct current hybrid power distribution network adopts voltage droop control.

Description

Direct-current voltage self-adaptive droop control method and system for alternating-current and direct-current hybrid power distribution network
Technical Field
The invention belongs to the technical field of power electronics, and particularly relates to a direct-current voltage self-adaptive droop control method and system for an alternating-current and direct-current hybrid power distribution network.
Background
The running state of the alternating-current and direct-current hybrid power distribution network is changeable and the functional structure is complex, so that the alternating-current and direct-current hybrid power distribution network needs a system control strategy which is reasonable in design, and the system is guaranteed to run safely, stably and economically. The alternating current-direct current hybrid power distribution network can comprise a plurality of converter ports, and the plurality of converter ports need to be coordinated and controlled to realize the cooperative and stable operation of the alternating current-direct current hybrid power distribution network.
Currently, there are three main control methods for dc distribution voltage: a master-slave control mode, a voltage margin control mode and a voltage droop control mode. In a multi-terminal system, when a master-slave control mode is adopted, one terminal is used as a master converter station, and the other terminal is used as a slave converter station, so that the stable control of direct-current voltage is realized through high-speed reliable communication. The main converter station adopts constant direct current voltage control, the slave converter station adopts constant active reactive power control or constant alternating current voltage and frequency control, and when the power flow of the system changes, the main converter station changes the active output balance system power in real time to maintain the stability of the direct current voltage of the system. However, the master-slave control method is highly dependent on communication, and the accuracy of communication directly affects the control effect. When the voltage margin control mode is adopted, when the direct current voltage drops or rises to a set value, the control mode corresponding to the converter station is automatically switched to constant direct current voltage control, and communication between the converter stations is not needed. However, when a plurality of converter stations are controlled by the voltage margins, the complexity of the design of the parameters of the controller and the coordination among the voltage margins of the plurality of stages can be greatly increased along with the increase of the number of stages. The voltage droop control combines the characteristics of direct current voltage control and power control, and the direct current voltage stability can be maintained in real time while the output power of the current conversion station is controlled. Voltage droop control does not rely on communication, and expansibility is better simultaneously, does not need complicated control mode to switch over the link, more is suitable for the multiterminal direct current system, consequently current research and engineering adopt droop control mostly. However, since the voltage droop control cannot freely control the output power and the dc voltage also deviates from the rated value, it is necessary to improve the droop control.
In order to solve the above problems, in a conventional solution, patent CN 112186793 a discloses a method for controlling droop in a dc power distribution network, in which when a dc voltage deviates from a rated value, a droop coefficient is adaptively adjusted to reduce a voltage deviation, but frequent changes in the droop coefficient may cause system instability, and a low voltage deviation cannot be completely eliminated. Patent CN 110957734 a discloses a voltage droop control method suitable for a multi-terminal flexible dc power transmission and distribution system, which adjusts the reference voltage of an operating working point in real time during the power flow disturbance process to ensure that the dc voltage does not exceed the limit during the adjustment process, but the invention cannot eliminate the voltage deviation. Patent CN111224420A discloses a droop control method and system for self-adaptation after large disturbance of a converter station, which adaptively adjusts a droop coefficient when a dc voltage exceeds a limit to prevent the dc voltage from exceeding the limit, but cannot finally eliminate a steady-state error of the dc voltage.
Disclosure of Invention
The invention provides a direct-current voltage self-adaptive droop control method and system for an alternating-current and direct-current hybrid power distribution network, aiming at the defect that the direct-current voltage is out of limit and the improvement requirement generated by a traditional droop strategy under the condition of frequent and large fluctuation of the tide in the alternating-current and direct-current hybrid power distribution network in the prior art.
In order to achieve the above object, according to an aspect of the present invention, a dc voltage adaptive droop control method for an ac/dc hybrid power distribution network is provided, where a dc power distribution network in the ac/dc hybrid power distribution network is "hand-in-hand", and the ac/dc hybrid power distribution network includes a plurality of converter stations operating in a droop control mode;
for each converter station operating in a droop control mode, the method comprises:
s1, acquiring local direct current bus voltage of the convertor station in real time;
s2, judging whether the obtained direct current bus voltage is out of limit, if so, entering step S3, otherwise, entering step S5;
s3, adaptively calculating and updating the droop coefficient of the converter station until the voltage of the direct-current bus is recovered to be not out of limit, and entering the step S4;
s4, acquiring the output power of the converter station at the moment as a new rated power point of the converter station, updating a droop rated working point of a droop curve to obtain an improved droop curve, and controlling the converter station;
and S5, not improving the currently used droop related parameters.
Preferably, in step S2, when | Δ Vdci|＞εVratediJudging that the voltage of the direct current bus is out of limit;
wherein ε represents the percentage of deviation, Δ VdciRepresenting real-time direct current bus voltage of converter station i and rated value V of direct current bus voltageratediThe difference of (a).
Preferably, in step S3, the calculation formula of the droop coefficient of the converter station is as follows:
k′droopi＝kdroopi(1+ΔVdcili)
Figure BDA0002981731910000031
Figure BDA0002981731910000032
wherein, k'droopiRepresents the updated droop coefficient, k, of the converter station idroopiRepresenting the droop coefficient, Δ V, before the converter station i is updateddciRepresenting real-time direct current bus voltage of converter station i and rated value V of direct current bus voltageratediDifference of (l)iRepresenting an intermediate parameter, Δ P, of the converter station iiRepresenting the difference, sign [ Δ V ], between the actual output power of the converter station i and the power ratingdci]Represents Δ VdciAnd epsilon represents the percentage of deviation.
Preferably, ε is 5%.
In order to achieve the above object, according to another aspect of the present invention, a dc voltage adaptive droop control system for an ac/dc hybrid power distribution network is provided, where a dc power distribution network in the ac/dc hybrid power distribution network is "hand-in-hand", and the ac/dc hybrid power distribution network includes a plurality of converter stations operating in a droop control mode;
for each converter station operating in a droop control mode, each converter station corresponding to a controller, the controller comprising:
the out-of-limit judging module is used for acquiring the local direct current bus voltage of the converter station in real time, judging whether the acquired direct current bus voltage is out of limit or not, if so, entering the droop curve updating module, and otherwise, not improving the currently used droop related parameters;
the droop curve updating module comprises a first updating module and a second updating module;
the first updating module is used for adaptively calculating and updating the droop coefficient of the converter station until the voltage of the direct-current bus is recovered to be not out of limit, and entering the second updating module;
and the second updating module is used for acquiring the output power of the converter station at the moment, taking the output power as a new rated power point of the converter station, updating a droop rated working point of the droop curve, and obtaining an improved droop curve so as to control the converter station.
Preferably, the out-of-limit judging module judges whether the obtained dc bus voltage is out of limit in the following manner:
when | Δ Vdci|＞εVratediJudging that the voltage of the direct current bus is out of limit;
wherein ε represents the percentage of deviation, Δ VdciRepresenting real-time direct current bus voltage of converter station i and rated value V of direct current bus voltageratediThe difference of (a).
Preferably, the first updating module adaptively calculates the droop coefficient by the converter station by:
k′droopi＝kdroopi(1+ΔVdcili)
Figure BDA0002981731910000041
Figure BDA0002981731910000042
wherein, k'droopiRepresents the updated droop coefficient, k, of the converter station idroopiRepresenting the droop coefficient, Δ V, before the converter station i is updateddciRepresenting real-time direct current bus voltage of converter station i and rated value V of direct current bus voltageratediDifference of (l)iRepresenting an intermediate parameter, Δ P, of the converter station iiRepresenting the difference, sign [ Δ V ], between the actual output power of the converter station i and the power ratingdci]Represents Δ VdciAnd epsilon represents the percentage of deviation.
Preferably, ε is 5%.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
the invention adaptively improves and changes the droop coefficient and the droop rated working point of the droop control on the basis of the traditional droop control. Firstly, the method is based on droop control, only local information is needed, communication reliability is not needed among converter stations, and communication is not relied; secondly, when the direct current voltage exceeds the limit, the direct current voltage can be quickly regulated back to the limit range, and the system instability caused by voltage exceeding the limit is avoided; in addition, through regulation, the direct-current voltage can be stabilized to a rated value, and compared with the traditional droop control, the provided control strategy eliminates the steady-state deviation of the direct-current voltage; finally, the capacity margin of the converter station adopting the self-adaptive improved droop control is greatly improved, and the capability of the converter station subjected to tidal current fluctuation is enhanced.
Drawings
Fig. 1 is a flow chart of a dc voltage adaptive droop control method for an ac/dc hybrid power distribution network according to the present invention;
FIG. 2 is a typical structure diagram of an AC/DC hybrid power distribution network;
FIG. 3 is a block diagram of converter station control based on adaptive droop;
FIG. 4 is a block diagram of the bottom control structure of the converter, wherein (a) is an outer loop control block diagram; (b) is an inner loop control block diagram;
FIG. 5 is a block diagram of a process for adaptive droop control;
FIG. 6 is a graph of a droop curve variation process for adaptive droop control;
FIG. 7 is a simulation model of an AC/DC hybrid power distribution network;
fig. 8 is a waveform diagram of a system using a conventional fixed droop control, in which (a) is a dc bus voltage variation graph; (b) a graph of the output power change of the three-terminal converter station is shown;
fig. 9 is a waveform diagram of a system using adaptive droop control, wherein (a) is a dc bus voltage variation graph; (b) a graph of the output power change of the three-terminal converter station is shown; (c) a droop coefficient change curve chart of the two-end droop converter station is shown; (d) a graph of the rated power point change of the two-end droop converter station is shown.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
The invention concept of the invention is as follows: the invention adaptively improves and changes the droop coefficient and the droop rated working point of the droop control on the basis of the traditional droop control. The method comprises the following steps that firstly, a droop coefficient is adjusted, when the direct current voltage is out of limit, each end converter station calculates and updates the droop coefficient in a self-adaptive manner by taking a control voltage deviation as a target in a limit range according to locally detected direct current voltage and output power information, and the direct current voltage can be quickly controlled back to the limit range; and the second step is to adjust a droop rated working point, the direct current voltage is stabilized at the upper limit or the lower limit when the previous step is finished, the output power of the corresponding converter station is the balance power of the system at the moment, a new droop curve is constructed by taking the power as a new rated working point, the direct current voltage is controlled to return to the rated working point along the new droop curve, namely the rated voltage, and the adjustment of the droop rated working point realizes the elimination of voltage deviation.
As shown in fig. 1, the invention provides a direct-current voltage adaptive droop control method for an alternating-current and direct-current hybrid power distribution network, wherein the direct-current power distribution network in the alternating-current and direct-current hybrid power distribution network is in a hand-in-hand type, and the alternating-current and direct-current hybrid power distribution network comprises a plurality of converter stations working in a droop control mode; for each converter station operating in a droop control mode, the method comprises:
and S1, acquiring the local direct current bus voltage of the convertor station in real time.
And S2, judging whether the obtained direct current bus voltage is out of limit, if so, entering the step S3, otherwise, entering the step S5.
In an alternating current-direct current hybrid power distribution network, after a tidal current is greatly changed, the voltage deviation of a direct current bus may exceed an upper limit or a lower limit. When the dc bus voltage is out of limit, each end converter station first bases on locally detected dc voltage and output power information.
Preferably, in step S2, when | Δ Vdci|＞εVratediJudging that the voltage of the direct current bus is out of limit; wherein ε represents the percentage of deviation, Δ VdciRepresenting real-time DC bus voltage and DC bus voltage nominal value VratediThe difference of (a). Preferably, ε is 5%.
And S3, adaptively calculating and updating the droop coefficient of the converter station until the direct current bus voltage is recovered to be not out of limit, and entering the step S4.
And adaptively calculating and updating the droop coefficient by taking the control voltage deviation as a target within a limit range, updating a droop control curve in real time by the converter station according to the calculated droop coefficient, and gradually controlling the direct-current voltage to return to be within the limit range.
Preferably, in step S3, the calculation formula of the droop coefficient of the converter station is as follows:
k′droopi＝kdroopi(1+ΔVdcili)
Figure BDA0002981731910000071
Figure BDA0002981731910000072
wherein, k'droopiRepresents the updated droop coefficient, kdroopiRepresenting the sag factor before update, Δ VdciRepresenting real-time DC bus voltage and DC bus voltage nominal value VratediDifference of (l)iDenotes the intermediate parameter, Δ PiRepresenting the difference between the actual output power of the converter station and the power rating, sign [ Δ V ]dci]Represents Δ VdciAnd epsilon represents the percentage of deviation.
And S4, acquiring the output power of the converter station at the moment as a new rated power point of the converter station, updating a droop rated working point of the droop curve to obtain an improved droop curve, and controlling the converter station.
When the direct-current voltage returns to the limit range through short-time regulation, the output power of the corresponding converter station is the new balance power of the system. And taking the balance power obtained in the previous step as a new rated power point, updating a droop rated working point of the droop curve to obtain an improved droop curve, and controlling the flow switching station. Since the rated power at this time is the balance power of the system, the converter station will return to the rated operating point along the new droop curve, i.e. the dc voltage will also return to the dc voltage rated value.
And S5, not improving the currently used droop related parameters.
If the direct current voltage is not out of limit, the currently used droop related parameters are not improved.
Examples
A typical configuration of an ac/dc hybrid power distribution network is shown in fig. 2, where a plurality of ac networks are interconnected with a dc network via converter stations. The alternating current network is connected with units such as a local load and new energy power generation, and the direct current network is connected with units such as a direct current load, new energy power generation and an energy storage system.
The overall architecture diagram of the direct-current voltage control strategy based on the adaptive variable droop is shown in fig. 3, wherein the control structure is a double-loop control structure, and the double loops work under a dq coordinate system. The outer ring can work in a droop control mode, a constant active power and a constant reactive power control mode according to different working modes. The control block diagram of the outer loop is shown in fig. 4 (a). The output of the outer loop is a reference value of the current inner loop, the current inner loop adjusts the current injected into the feeder line by the current converter according to the reference value, and a control block diagram of the inner loop is shown in (b) of fig. 4. And the output of the inner ring controller generates a switching tube trigger signal in a pulse width modulation mode after coordinate transformation so as to realize regulation and control of the converter. And when the direct-current voltage is out of limit, adaptively changing the droop coefficient and the droop rated operating point of droop control according to the proposed strategy.
For an n-end alternating current-direct current hybrid system, the m end works in a droop control mode to cooperatively control direct current voltage stability and system power balance, and the other ends work in a constant active power and constant reactive power control mode. Initially, the droop curve of the converter station i operating in the droop control mode is shown as a solid line in fig. 6, and the slope of the droop curve is the droop coefficient. The relationship between the dc voltage and the output power of the converter station i is:
Vdci-Vratedi＝kdroopi(Pi-Pratedi) (1)
in the formula, VratediAnd PratediRated direct current voltage and rated output power of the i converter station are respectively; vdciAnd PiRespectively detecting the direct current voltage and the output power of the i converter station in real time; k is a radical ofdroopiIs the droop coefficient of the i converter station.
In the conventional droop control, the droop curve of the converter station is always kept unchanged, the capacity margin of the converter station is limited, the capacity of the converter station cannot be fully utilized, and the direct-current voltage may exceed the limit after a large tidal current variation is experienced.
Compared with the traditional droop control, the invention introduces an influence factor l when the direct current voltage is out of limitiThe self-adaptation of the droop coefficient is improved as follows:
k′droopi＝kdroopi(1+ΔVdcili) (2)
of formula (II) k'droopiThe improved droop coefficient is obtained; Δ VdciIndicating the difference between the real time DC voltage and the DC voltage nominal value, i.e. Δ Vdci＝Vdci-VratediWhen | Δ Vdci|＞εVratediIf the dc voltage exceeds the limit, then, in general, ∈ is 5%.
Substituting formula (2) for formula (1):
Vdci-Vratedi＝kdroopi(1+ΔVdcili)(Pi-Pratedi) (3)
from formula (3):
Figure BDA0002981731910000091
in the formula,. DELTA.Pi＝Pi-PratediAnd represents the difference between the actual output power of the converter and the power rated value.
In order to control the DC voltage not to exceed the limit, | Delta Vdci|＝εVratediInstead of formula (4), one can obtain:
Figure BDA0002981731910000092
in the formula, sign [ Delta V ]dci]Is DeltaVdciCan be expressed as
Figure BDA0002981731910000093
When the rated power of the converter station is on the right half plane, a droop curve is constructed by taking the point M as a limit point, in this case, when the direct-current voltage is higher than the upper limit, through the improvement on the droop coefficient, the droop curve of the converter is shown in (a) in fig. 6, and when the converter station works at the point C in (a) in fig. 6, the power value at the point C is the balance power of the current system. Taking the power of the point C as a new rated power, a new rated working point B of the converter station can be obtained, and the droop curve after the secondary improvement is shown as (a) in fig. 6; when the direct current voltage is lower limit, the converter outputs fully. When the rated power of the converter station is in the left half plane, constructing a droop curve by taking a point N as a limit point, in this case, when the direct-current voltage is beyond the lower limit, the conversion process is similar to the above, and the conversion process is shown as (b) in fig. 6; when the direct current voltage is higher than the upper limit, the converter outputs fully. After the above modification, as shown in fig. 6, the converter station capacity margin Δ P 'after the droop control is modified'maxiCapacity margin Δ P of the converter station compared to conventional droop controlmaxiThe lifting is greatly improved, and the adjustability is realized. The transformation process is shown in fig. 5.
When the system undergoes a large tidal current change, droop control of the converter station is improved twice, finally, the output power of the converter station is the balance power of the current system, the direct-current voltage is stabilized at the rated direct-current voltage, and the steady-state error of the direct-current voltage is eliminated. In the conversion process, the direct current voltage can be regulated and controlled back to the limit range through a short and small out-of-limit process.
In order to verify the self-adaptive drooping direct-current voltage control capability, an alternating-current and direct-current hybrid power distribution network model shown in fig. 7 is constructed for simulation verification. In the simulation, the voltage of 3 alternating current buses is 10KV, and the voltage of a direct current bus is 20 KV. The capacity of the T1 end is 10MW, the capacity of the T2 end is 5MW, the capacity of the T3 end is 10MW, the T1 end and the T2 end work in a droop control mode, and the T3 end works in a constant active power and constant reactive power mode. No communication is needed between the terminals 3, the terminals T1 and T2 take local direct current voltage information as a criterion to adaptively improve the current droop control, and the terminal T3 receives an upper layer power command.
Assuming that all 3 ac networks can fully satisfy the active power requirement on the ac feeder, the present invention focuses on the control of the dc network voltage and active power by the cooperation of a plurality of converter stations. With the power flowing from the alternating current side to the direct current side as a positive direction, in an initial state, the T3 end outputs active power of-10 MW, the T1 end and the T2 end respectively output 7MW and 3MW, and the voltage of the direct current bus is stabilized at 20KV, namely the T1 end and the T2 end respectively work at a rated working point in the current state. At 2s, the output active power of the end T3 has a step of-4 MW, and the power flow has a large change. And 4s, the output active power step of the T3 end is 8MW, and the power flow is reversed.
Fig. 8 shows a dc bus voltage variation curve and a three-terminal converter station output power variation curve when droop control is performed at T1 and T2 ends by using a conventional fixed droop coefficient. In fig. 8, (a) shows a dc bus voltage variation curve, and it can be seen from the graph that, when the power flow makes the first step, the dc voltage exceeds the upper limit 21KV already because the capacity margin of the converter station is fixed and small. When the power flow is subjected to the second step, namely, the power flow is reversed, the direct-current voltage continues to rise and deviates far from the upper limit, which greatly harms the stable operation of the actual system. Fig. 8 (b) shows a variation curve of output power of the three-terminal converter station, and when the power at the T3 terminal is changed in steps twice, the terminals T1 and T2 can adjust the power according to the droop characteristics so that the system power is balanced.
Fig. 9 shows a system response curve of the T1 and T2 terminals by using droop control based on adaptive droop. Fig. 9 (a) shows a dc bus voltage variation curve, and it can be known from the graph that, in the process of going through two steps, the dc bus voltage is regulated and controlled to return to the limit after a short small-amplitude out-of-limit, and finally returns to the rated value, and the steady-state error of the dc bus is 0. Compared with the traditional droop control with fixed droop coefficients, the droop control based on the self-adaptive variable droop is more beneficial to the safe operation of the system. Fig. 9 (b) shows a variation curve of output power of the three-terminal converter station, and when the power at the T3 terminal has a step change, the terminals T1 and T2 can adjust the power according to their respective droop characteristics so as to balance the system power. Fig. 9 (c) shows a droop coefficient adaptive variation curve in the system control process, and fig. 9 (d) shows a droop power point variation curve in the system control process. As can be seen from a comparison of (a), (c) and (d) in fig. 9, after two tidal current steps, when the dc voltage is out of limit, the droop coefficients at the T1 and T2 ends start adaptive adjustment; after the direct current voltage is regulated back to be within the limit range, the rated power points of the ends T1 and T2 start to step, and then the ends T1 and T2 regulate the direct current voltage back to be the rated voltage according to a new droop curve. Referring to fig. 9 (a), (c), and (d), after the first step, when T is 2.15s, the dc voltage gets higher, at which time the droop coefficients k1 and k2 at the ends of T1 and T2 start to increase adaptively, and reach a temporary steady state when T is 2.2s, at which time the dc bus voltage has been adjusted to fall back into the limit range due to the change of the droop coefficient. At 2.3s, the rated power points of the ends T1 and T2 start to step, the rated power of the ends T1 and T2 after the step is 3.5MW and 0.5MW respectively, and the sum of the rated power and the rated power is the balance power of the system at the moment, namely the power of the output power of the end T3. After the step of the rated power point, the droop rated working points at the ends T1 and T2 jump, and the droop coefficients at the ends T1 and T2 also jump. At this time, the voltage corresponding to the rated operating point of the T1 and T2 droop curves is the rated direct-current voltage, the power is the power for balancing the current system, the voltage of the direct-current bus gradually returns to the rated voltage from the upper voltage limit along with the new droop curve, and the elimination of the steady-state error of the direct-current voltage is realized. The second step is a power flow reversal, after the second step, when T is 4.1s, the direct current voltage is over the upper limit, at this time, the droop coefficients at the ends of T1 and T2 are adaptively changed again, and a temporary stable state is reached when T is 4.15s, at this time, the direct current bus voltage is regulated to fall back to the limit range. At 4.25s, the rated power points of the ends T1 and T2 are stepped again, the rated powers of the two ends T1 and T2 after the step are respectively-3.5 MW and-4.5 MW, and the sum of the rated powers is the balance power of the system at the moment, namely the power of the output power of the end T3. After the step of the rated power point, the droop rated working points at the ends T1 and T2 jump, and the droop coefficients at the ends T1 and T2 also jump. At this time, the voltage corresponding to the rated operating point of the T1 and T2 droop curves is the rated direct-current voltage, the power is the power for balancing the current system, the voltage of the direct-current bus gradually returns to the rated voltage from the upper voltage limit along with the new droop curve again, and the steady-state error of the direct-current voltage is eliminated. The invention can maintain the direct current voltage not to exceed the limit and eliminate the steady-state error of the direct current voltage under the conditions of the power flow step and the power flow reversal.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (6)

1. A direct-current voltage self-adaptive droop control method for an alternating-current and direct-current hybrid power distribution network is characterized in that the direct-current power distribution network in the alternating-current and direct-current hybrid power distribution network is in a hand-in-hand type, and the alternating-current and direct-current hybrid power distribution network comprises a plurality of converter stations working in a droop control mode;
for each converter station operating in a droop control mode, the method comprises:
s1, acquiring local direct current bus voltage of the convertor station in real time;
s2, judging whether the obtained direct current bus voltage is out of limit, if so, entering step S3, otherwise, entering step S5;
s3, with the aim of controlling the voltage deviation within the limit range, adaptively calculating and updating the droop coefficient of the converter station until the direct-current bus voltage is recovered to be not out of limit, and entering the step S4;
s4, acquiring the output power of the converter station at the moment as a new rated power point of the converter station, updating a droop rated working point of a droop curve to obtain an improved droop curve, and controlling the converter station;
s5, the currently used droop related parameters are not improved;
in step S3, the calculation formula of the droop coefficient of the converter station is as follows:
k′droopi＝kdroopi(1+ΔVdcili)
Figure FDA0003508032480000011
Figure FDA0003508032480000012
wherein, k'droopiRepresents the updated droop coefficient, k, of the converter station idroopiRepresenting the droop coefficient, Δ V, before the converter station i is updateddciRepresenting real-time direct current bus voltage of converter station i and rated value V of direct current bus voltageratediDifference of (l)iRepresenting an intermediate parameter, Δ P, of the converter station iiRepresenting the difference, sign [ Δ V ], between the actual output power of the converter station i and the power ratingdci]Represents Δ VdciAnd epsilon represents the percentage of deviation.
2. The method of claim 1, wherein in step S2, when | Δ Vdci|>εVratediJudging that the voltage of the direct current bus is out of limit;
wherein ε represents the percentage of deviation, Δ VdciIndicating commutationReal-time direct current bus voltage of station i and direct current bus voltage rated value VratediThe difference of (a).
3. A method as claimed in claim 1 or 2, wherein ∈ 5%.
4. A direct-current voltage self-adaptive droop control system of an alternating-current and direct-current hybrid power distribution network is characterized in that the direct-current power distribution network in the alternating-current and direct-current hybrid power distribution network is in a hand-in-hand type, and the alternating-current and direct-current hybrid power distribution network comprises a plurality of converter stations working in a droop control mode;
for each converter station operating in a droop control mode, each converter station corresponding to a controller, the controller comprising: the out-of-limit judging module is used for acquiring the local direct current bus voltage of the converter station in real time, judging whether the acquired direct current bus voltage is out of limit or not, if so, entering the droop curve updating module, and otherwise, not improving the currently used droop related parameters;
the droop curve updating module comprises a first updating module and a second updating module;
the first updating module is used for adaptively calculating and updating the droop coefficient of the converter station by taking the control voltage deviation as a target within a limit range until the direct-current bus voltage is recovered to be not out of limit, and entering a second updating module;
the second updating module is used for acquiring the output power of the converter station at the moment, taking the output power as a new rated power point of the converter station, updating a droop rated working point of the droop curve to obtain an improved droop curve, and controlling the converter station;
the first updating module adaptively calculates the droop coefficient by the converter station in the following way:
k′droopi＝kdroopi(1+ΔVdcili)
Figure FDA0003508032480000021
Figure FDA0003508032480000022
wherein, k'droopiRepresents the updated droop coefficient, k, of the converter station idroopiRepresenting the droop coefficient, Δ V, before the converter station i is updateddciRepresenting real-time direct current bus voltage of converter station i and rated value V of direct current bus voltageratediDifference of (l)iRepresenting an intermediate parameter, Δ P, of the converter station iiRepresenting the difference, sign [ Δ V ], between the actual output power of the converter station i and the power ratingdci]Represents Δ VdciAnd epsilon represents the percentage of deviation.
5. The system of claim 4, wherein the out-of-limit determining module determines whether the obtained DC bus voltage is out-of-limit by:
when | Δ Vdci|>εVratediJudging that the voltage of the direct current bus is out of limit;
wherein ε represents the percentage of deviation, Δ VdciRepresenting real-time direct current bus voltage of converter station i and rated value V of direct current bus voltageratediThe difference of (a).
6. A system as claimed in claim 4 or 5, wherein ε is 5%.
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