CN109861262B - Method and system for controlling stability of flexible direct current and weak alternating current power grid - Google Patents
Method and system for controlling stability of flexible direct current and weak alternating current power grid Download PDFInfo
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Abstract
The invention discloses a method and a system for controlling the stability of a flexible direct current and weak alternating current power grid, belonging to a power systemThe field of systems. The method comprises the following steps: determining the power transmission characteristic curve U of an AC power grid at an operating points‑PsAnd flexible DC power characteristic curve Us‑PcThe position of (a); determining the location of the operating point, including the operating point being located at Us‑PcContinuously changing interval, operating point at Us‑PcForced recovery areas or no operating point and other different conditions; when the voltage U of the unstable operation point or the operation point of the flexible direct current and weak alternating current power grid series-parallel system is not stablesIs lower than a set threshold value UsthThen reset FRTS parameter IcdF(ii) a Monitoring the running state of the direct current and weak alternating current power grid series-parallel system after fault disturbance in real time, and stopping i when the dynamic instability is caused by current recoverycdAnd icqIncreasing; if U issIs still lower than UsthAnd the duration exceeds TsThen the DC power DeltaP is decreasedcTo lift UsIn which F isDTo avoid frequent activation of the flag bit by current perturbation control. The invention can restore the stable operation of the hybrid system.
Description
Technical Field
The present invention relates to the field of power systems, and more particularly, to a method and system for controlling the stability of a flexible dc and weak ac power grid.
Background
A Voltage Source Converter (VSC) is a core component in a flexible direct current transmission (VSC-HVDC) system, and is widely applied in different scenarios, so that the VSC becomes an important form for promoting power electronic development of a power system. The response characteristic of the disturbed VSC is obviously applied to the multi-time scale dynamic behavior of the alternating current-direct current hybrid system. For weak alternating current power grids, the effect is more prominent, and under the influence, the system stability characteristic is more complicated. For this reason, research around the operation characteristics and stability control of the flexible dc and weak ac grid hybrid system has become a hot spot and a focus of current power system research.
The operation characteristic of the VSC-HVDC and weak alternating current power grid series-parallel system relates to 3 main aspects of steady state operation characteristic, small disturbance stability and large disturbance stability. Compared with the small disturbance stability research, the large disturbance stability characteristic research aiming at the VSC-HVDC and weak alternating current power grid series-parallel system is relatively lagged. In the research on the large-disturbance stability characteristics, the frequency stability and related control problems of the hybrid system caused by the active imbalance are mostly focused.
Under the impact of large disturbance, control protection links such as current amplitude limiting and fault ride-through strategies can obviously influence the disturbed response characteristics of the VSC, so that the safety and stability of the alternating current-direct current hybrid system are more complex, and the stable control meets new requirements.
Disclosure of Invention
In order to solve the above problems, the present invention provides a method for controlling the stability of a flexible dc and weak ac power grid, the method comprising:
scanning expected faults of the flexible direct current and weak alternating current power grid series-parallel connection system, determining faults which obviously weaken the strength of the direct current and weak alternating current power grid series-parallel connection system, and obtaining a power grid power transmission characteristic curve U of the flexible direct current and weak alternating current power grid series-parallel connection system after large disturbances-PsAnd flexible DC power characteristic curve Us-PcAnd determining the operating point on the power transmission characteristic curve U of the AC power grids-PsAnd flexible DC power characteristic curve Us-PcThe position of (2):
determining location of operating point, including Us-PcContinuously changing interval, operating point at Us-PcForcing to recover the area or no operating point and the like under different conditions, when the flexible direct current and weak alternating current power grid series-parallel system has no steady state operating point or operating point voltage UsIs lower than a set threshold value UsthThen reset FRTS parameter IcdF；
Monitoring the running state of the direct current and weak alternating current power grid series-parallel system after fault disturbance in real time, and stopping i when the dynamic instability is caused by current recoverycdAnd icqIncreasing;
if U issIs lower than a set threshold value UsthAnd the time exceeds TDThen, a superimposed active current pickup amount Δ i is appliedcdrefIf U is presentsHovering over UsL2And UsHThe unstable balance point between the two can ensure that the running point of the series-parallel system automatically slides to the U under the action of the VSC controllers-PsStable equilibrium point of the upper half of the curve, if UsIs still lower than UsthAnd the duration exceeds TSThen the DC power DeltaP is decreasedcTo lift UsIn which F isDTo avoid frequent activation of the flag bit by current perturbation control.
Optionally, the operating point is located at Us-PcAnd when the interval is changed continuously, applying a small shooting amount of the VSC reference current to enable the operating point to be transited to a stable balance point.
Optionally, the operating point is located at Us-PcWhen the forced recovery interval is in, the U is impacted by large disturbances<UsL1Then, in the process of the dynamic track rising of the hybrid system, Us>UsHWhen FRTS is exited, when U issLess than UsL2Then FRTS will force i againcdIs IcdFAccording to the formula:
or
Us≤Usth (2)
As icdAnd icqRestoring the growing stopping criterion and maintaining PcMaximum power transmission or UsTo set voltage Usth。
Optionally, Us-PsAnd Us-PcWhen there is no common operating point, if UsWith the solution, the corresponding system reference capacity and VSC reference capacity are respectively valued as SB、SVSCX of (2)sAnd IcdFSaid X issAnd IcdFThe following formula is satisfied:
Xs≤EsSB/IcdFSVSC (3)
IcdF≤EsSB/XsSVSC (4)
when U is turnedsReach the set threshold value UsthCorresponding to a predetermined XsAdjustable of IcdFSet up, then XsAnd IcdFThe following formula is satisfied:
the invention also provides a system for controlling the stability of the flexible direct current and weak alternating current power grid, which comprises:
the parameter acquisition module is used for scanning expected faults of the flexible direct current and weak alternating current power grid series-parallel connection system, determining faults which obviously weaken the strength of the direct current and weak alternating current power grid series-parallel connection system, and acquiring a power grid power transmission characteristic curve U of the flexible direct current and weak alternating current power grid series-parallel connection system after large disturbances-PsAnd flexible DC power characteristic curve Us-PcAnd determining the operating point on the power transmission characteristic curve U of the AC power grids-PsAnd flexible DC power characteristic curve Us-PcThe position of (2):
determining fault module, determining operating point position including Us-PcContinuously changing interval, operating point at Us-PcForced recovery areas or no operating point and other different conditions;
a first control module for controlling the voltage U of the unstable operation point or the operation point of the series-parallel system of the flexible DC and weak AC power networkssIs lower than a set threshold value UsthThen reset FRTS parameter IcdF；
The second control module monitors the running state of the direct current and weak alternating current power grid series-parallel system after fault disturbance in real time, and stops the operation of the i when the dynamic instability is caused by current recoverycdAnd icqIncreasing;
a third control module, if UsIs lower than a set threshold value UsthAnd the time exceeds TDThen, a superimposed active current pickup amount Δ i is appliedcdrefIf U is presentsHovering over UsL2And UsHThe unstable balance point between the two can ensure that the running point of the series-parallel system automatically slides to the U under the action of the VSC controllers-PsStable equilibrium point of the upper half of the curve, if UsIs still lower than UsthAnd the duration exceeds TSThen the DC power DeltaP is decreasedcTo lift UsIn which F isDTo avoid frequent activation of the flag bit by current perturbation control.
The VSC-HVDC and AC power grid series-parallel connection system has various unstable phenomena corresponding to different strengths of the AC power grid after large disturbance impact, including small disturbance instability, dynamic recovery instability, no stable working point and the like, and the stabilization mechanisms are different.
The invention adjusts and reduces the low-voltage current limiting value of the VSC in advance or operates the voltage U of the VSC according to the disturbed VSC after the faultsAnd power PcThe static and dynamic characteristics of the system are realized, the current reference value control is implemented, and the stable operation of the hybrid system can be restored.
Drawings
FIG. 1 is an equivalent model diagram of a VSC-HVDC and AC grid series-parallel system for controlling the stability of a flexible DC and weak AC grid;
FIG. 2 is a typical operating condition curve of a series-parallel system under different strengths of an AC power grid by a method for controlling the stability of a flexible DC and weak AC power grid;
FIG. 3 is a graph showing a small disturbance instability phenomenon and a current perturbation control effect of a method for controlling the stability of a flexible direct current and weak alternating current power grid;
FIG. 4 is a graph showing the dynamic instability phenomenon and the recovery current control effect of a method for controlling the stability of a flexible DC and weak AC power grid;
FIG. 5 is a diagram of a method for controlling the stability of a flexible DC and weak AC power grid without a steady-state operating point phenomenon and resetting IcdFControl effect curve chart
FIG. 6 is a flow chart of a method of controlling the stability of a flexible DC and weak AC power grid;
fig. 7 is a diagram of a system for controlling the stability of a flexible dc and ac grid.
Detailed Description
The exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, however, the present invention may be embodied in many different forms and is not limited to the embodiments described herein, which are provided for complete and complete disclosure of the present invention and to fully convey the scope of the present invention to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, the same units/elements are denoted by the same reference numerals.
Unless otherwise defined, terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Further, it will be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense.
The invention provides a method for controlling the stability of a flexible direct current and weak alternating current power grid, as shown in fig. 6, comprising the following steps:
forecasting flexible direct current and weak alternating current power grid series-parallel connection systemFault scanning, namely determining a fault which obviously weakens the strength of the direct current and weak alternating current power grid series-parallel connection system, and acquiring a power grid power transmission characteristic curve U after large disturbance of the flexible direct current and weak alternating current power grid series-parallel connection systems-PsAnd flexible DC power characteristic curve Us-PcAnd determining the operating point on the power transmission characteristic curve U of the AC power grids-PsAnd flexible DC power characteristic curve Us-PcThe position of (2):
as shown in fig. 1, in the figure, Uc、PcdVoltage and active power at the VSC outlet side are respectively; pc、QcAnd Ps、QsRespectively active power and reactive power output by the VSC and injected into an alternating current power grid; u shapesIs the commutation bus voltage; u. ofd、id、IcVoltage, current and alternating current side current on the direct current side of the VSC are respectively; rc、LcAnd CdThe resistance, the inductance and the direct-current side capacitance of the commutation reactor are respectively; xsiAnd XseThevenin equivalent reactance of a receiving end power grid and an external alternating current power grid are respectively; esThe equivalent potential is the Thevenin equivalent potential of the alternating current power grid.
If large disturbance faults such as disconnection and the like occur in part of external interconnected alternating current channels, the strength of the receiving-end equivalent alternating current power grid is weakened, and the operation condition of the series-parallel system is changed. On one hand, whether the parallel-serial system is stable depends on whether the VSC power characteristic curve after disturbance and the AC power grid power characteristic curve have intersection points, namely whether the intersection points have balance points; on the other hand, depending on the VSC control system dynamic tuning, the series-parallel system can be transitioned to a stable equilibrium point.
The VSC adopts a control mode of fixed active power and fixed reactive power, and considers an amplitude limiting link and a fault ride-through strategy, so that the VSC corresponds to the nose-shaped curve cluster U of the power transmission characteristic of the alternating current power grid with different intensities after large disturbances-PsAnd a VSC large disturbance power characteristic curve Us-PcAs shown in fig. 2.
When the operating point is determined to be located at Us-PcContinuously changing interval, operating point at Us-PcForced recovery zones or no-run pointsWhen the voltage U of the unstable operating point or the operating point of the flexible direct current and weak alternating current power grid series-parallel system is not stablesIs lower than a set threshold value UsthThen reset FRTS parameter IcdF；
After a fault, the power transmission characteristic curve U of the AC power grids-PsAnd flexible DC power characteristic curve Us-PcPoint of intersection OβIn the latter continuous variation interval of a' b, due to OβIn Us-PsThe low-voltage and high-current running state of the lower half part of the nose-shaped curve is output by VSC (voltage source converter)cdIs limited by a Fault Ride Through Strategy (FRTS). In this state, if the series-parallel system is disturbed slightly, the U is causedsIncreasing, then at a certain PcUnder the action of a regulator, icdWill correspondingly decrease, i decreasescdActing on the ac grid will further increase UsAnd then continuously decrease icd. Corresponding to the process, the operating point of the hybrid system is to be determined by OβAutomatically slide to the same PcCorresponding high voltage low current state, i.e. O'βDots, as shown in fig. 2. Thus, OβIs a small disturbance unstable equilibrium point, O'βThe point of stable equilibrium after the fault.
Monitoring the running state of the direct current and weak alternating current power grid series-parallel system after fault disturbance in real time, and stopping i when the dynamic instability is caused by current recoverycdAnd icqIncreasing;
subjected to large disturbance impact to make Us<UsL1Then, in the process that the dynamic track of the hybrid system rises along o' c, the point c is crossed, namely Us>UsHTime, FRTS quit, icdAnd icqWill be under the action of the recovery strategy at a set rate IcuplimAnd (4) linear growth. i.e. icdAnd icqAnd its application to the AC mains XsFormed UsJointly determining the VSC output power Pc，PcDynamic trajectory along AC mains Us-PsAnd (4) changing.
As shown in FIG. 2, as the VSC injection current increases, when the dynamic trajectory is at Us-PsCurve 3 upper halfIn part, PcIncrease UsDecrease, after crossing the inflection point, PcReduce UsThen it will continue to decrease as UsLess than UsL2Then FRTS will force i againcdIs IcdF. Corresponding to the process, PcThe dynamic trajectory will be in Us-PcAnd Us-PsU of (1)sL2And UsHAnd the interval is periodically switched alternately, so that the parallel-serial system cannot establish a steady-state operating point. For this purpose, formula (1) or formula (2) can be used as icdAnd icqRestoring growing stopping criterion to maintain PcMaximum power transmission or UsTo set voltage Usth。
Us≤Usth (2)
If U issIs lower than a set threshold value UsthAnd the time exceeds TDThen, a superimposed active current pickup amount Δ i is appliedcdrefIf U is presentsHovering over UsL2And UsHThe unstable balance point between the two can ensure that the running point of the series-parallel system automatically slides to the U under the action of the VSC controllers-PsStable equilibrium point of the upper half of the curve, if UsIs still lower than UsthAnd the duration exceeds TSThen the DC power DeltaP is decreasedcTo lift UsIn which F isDTo avoid frequent activation of the flag bit by current perturbation control.
After a large disturbance impact, if Us-PsAnd Us-PcAnd if no public operating point exists, the corresponding series-parallel system will not have a steady operating point. If U issWith the solution, the corresponding system reference capacity and VSC reference capacity are respectively valued as SB、SVSCX of (2)sAnd IcdFThe relationships expressed by the formulas (3) and (4) should be satisfied, respectively.
Xs≤EsSB/IcdFSVSC (3)
IcdF≤EsSB/XsSVSC (4)
Corresponding UsReach the set threshold value UsthAnd above, then XsAnd IcdFThe formula (5) and the formula (6) should be satisfied, respectively.
After a fault, if the AC power grid X is availablesIncrease the U remarkablys-PsAnd Us-PcWithout intersection points, as shown by the curve (r) in fig. 2, the parallel-serial system will not operate stably. For recovering system stability, corresponding to a predetermined XsAdjustable of IcdFSetting to satisfy the formula (5) or (6).
As shown in FIG. 1, consider that the AC delivery path is disconnected resulting in XsAnd the stability of various corresponding working conditions and the effectiveness of corresponding control strategies are improved to different degrees. Wherein, VSC adopts the control mode of fixed active power and fixed reactive power, PcAnd Qc1250MW and 200MVar respectively, and the main parameters of FRTS are set as UsL1＝0.8pu、UsL2＝0.5pu、UsH＝0.9pu、IcdF0.4 pu. Reference capacity S of AC grid and VSCB、SVSCThe values are 100MVA and 1250MVA respectively. In addition, UsthThe value is considered to be different situations of 0.8pu, 0.85pu, 0.9pu and the like, TDAnd TS2.0s and 3.0s, respectively.
(1) Small disturbance instability phenomenon and stable control effect
For the system shown in FIG. 1, after a fault disturbance, if XsJump to 0.075pu, Us-PsAnd Us-PcThe intersection point of the two-phase parallel-serial system is located at the lower half part of the former, and as shown by a curve II in figure 2, the parallel-serial system is located at a small-disturbance unstable balance point of low-voltage and large-current operation.
Due to disturbed Us0.59pu suspended in UsthFor this purpose, a current Δ i is appliedcdrefAnd (4) perturbation control. As shown in FIG. 3, after perturbation control, the hybrid system can quickly transit to the same PcStable balance point, U, of corresponding high voltage and low currentsIt was 0.97 pu.
(2) Dynamic instability phenomenon and stable control effect
For the system shown in FIG. 1, after a fault disturbance, if XsJump to 0.08pu, Us-PsWill intersect at Us-PcThe dynamic recovery area is shown as a curve (c) in fig. 2. Under this condition, the transient response of the disturbed series-parallel system is shown in fig. 4, and it can be seen that the recovery period icdLinear lifting, corresponding to PcAlong Us-PsGradually decreases after increasing to the maximum inflection point, UsThen monotonically decreases and satisfies Us<UsL2Time icdReduced to I again by FRTS restrictioncdF。icdReduce make UsRapidly rises and enters a dynamic recovery area again, so that each electrical quantity of the series-parallel system forms periodic oscillation and cannot stably run. When the maximum transmission power criterion shown in the formula (1) or the voltage threshold criterion shown in the formula (2) is met, the output current of the VSC is stopped from increasing, and the system can be recovered to be stable, as shown in FIG. 4, wherein U is shown in the figuresth0.8pu and 0.9pu, respectively.
(3) Unstable operating point phenomenon and stable control effect
After fault disturbance, if XsWhen the jump is to 0.2pu and the corresponding ac grid is significantly weakened, U appears as shown by curve (r) in fig. 2s-PsAnd Us-PcAnd the hybrid system cannot operate under the working condition without a public operating point. When X is detectedsIf the formula (5) is not satisfied, the corresponding U can be calculated by using the formula (6)sthI of 0.8pu and 0.85pucdFThe maximum values are 0.24pu and 0.211pu, respectively. Transient response and operating point change of the series-parallel system are shown in FIG. 5, resetting IcdFValue taking is carried out, VSC power transmission power is limited, a series-parallel system can be kept stable, and voltage can be increased to Usth。
According to the time domain simulation result, after large disturbance impact is verified, the VSC-HVDC and weak alternating current power grid series-parallel connection system has instability phenomena in various forms, and the stability of the system can be maintained according to the running state of the series-parallel connection system.
The present invention further provides a system 200 for controlling the stability of a flexible dc and weak ac power grid, as shown in fig. 7, the system 200 includes:
the parameter obtaining module 201 is used for scanning expected faults of the flexible direct current and weak alternating current power grid series-parallel connection system, determining faults which obviously weaken the strength of the direct current and weak alternating current power grid series-parallel connection system, and obtaining a power grid power transmission characteristic curve U of the flexible direct current and weak alternating current power grid series-parallel connection system after large disturbances-PsAnd flexible DC power characteristic curve Us-PcAnd determining the operating point on the power transmission characteristic curve U of the AC power grids-PsAnd flexible DC power characteristic curve Us-PcThe position of (2):
determine Fault Module 202, determine operating Point location, including Us-PcContinuously changing interval, operating point at Us-PcForced recovery areas or no operating point and other different conditions;
the first control module 203 is used for controlling the voltage U of the unstable-state operation point or operation point of the flexible direct current and weak alternating current power grid series-parallel systemsIs lower than a set threshold value UsthThen reset FRTS parameter IcdF；
The second control module 204 monitors the running state of the direct current and weak alternating current power grid series-parallel system after fault disturbance in real time, and stops the operation when the dynamic instability is caused by current recoverycdAnd icqIncreasing;
third control Module 205, if UsIs lower than a set threshold value UsthAnd the time exceeds TDThen, a superimposed active current pickup amount Δ i is appliedcdrefIf U is presentsHovering over UsL2And UsHThe unstable balance point between the two can ensure that the running point of the series-parallel system automatically slides to the U under the action of the VSC controllers-PsStable equilibrium point of the upper half of the curve, if UsIs still lower than UsthAnd the duration exceeds TSThen the DC power DeltaP is decreasedcTo lift UsIn which F isDTo avoid frequent activation of the flag bit by current perturbation control.
The VSC-HVDC and AC power grid series-parallel connection system has various unstable phenomena corresponding to different strengths of the AC power grid after large disturbance impact, including small disturbance instability, dynamic recovery instability, no stable working point and the like, and the stabilization mechanisms are different.
The invention can reduce the VSC low-voltage current limiting value in advance, or implement current reference value control according to the static and dynamic characteristics of the VSC running voltage Us and the power Pc after fault disturbance, and can restore the stable running of the hybrid system.
Claims (5)
1. A method of controlling the stability of a flexible direct current and weak alternating current power grid, the method comprising:
scanning expected faults of the flexible direct current and weak alternating current power grid series-parallel connection system, determining faults which obviously weaken the strength of the flexible direct current and weak alternating current power grid series-parallel connection system, and obtaining an alternating current power grid power transmission characteristic curve U after large disturbance of the flexible direct current and weak alternating current power grid series-parallel connection systems-PsAnd flexible DC power characteristic curve Us-PcAnd determining the operating point on the power transmission characteristic curve U of the AC power grids-PsAnd flexible DC power characteristic curve Us-PcIn which U issVoltage P of flexible DC and AC network interface bussFor injecting active power, P, into an AC networkcOutputting active power for flexible direct current:
determining location of operating point, including Us-PcContinuously changing interval, operating point at Us-PcDifferent conditions of forced recovery zones or no steady state operating points;
when the voltage U of the unstable operation point or the operation point of the flexible direct current and weak alternating current power grid series-parallel system is not stablesIs lower than a set threshold value UsthResetting the active current amplitude limiting parameter I in the fault ride-through strategy FRTScdF；
Monitoring the running state of the direct current and weak alternating current power grid series-parallel system after fault disturbance in real time, and stopping the active current i of the direct current and weak alternating current power grid series-parallel system when the dynamic instability is caused by current recoverycdAnd a reactive current icqIncreasing;
if U issIs lower than a set threshold value UsthAnd the time exceeds the set time delay TDThen, a superimposed active current pickup amount Δ i is appliedcdrefIf U is presentsHovering over a set value UsL2And a set value UsHThe unstable balance point between the two can ensure that the running point of the series-parallel system automatically slides to the U under the action of the VSC controllers-PsStable equilibrium point of the upper half of the curve, if UsIs still lower than UsthAnd the duration exceeds the set delay TsThen the DC power DeltaP is decreasedcTo lift Us。
2. The method of claim 1, wherein the operating point is located at Us-PcAnd when the interval is changed continuously, inputting a small shooting amount of the VSC reference current to enable the operating point to be transited to a stable balance point.
3. The method of claim 1, wherein the operating point is located at Us-PcWhen the forced recovery interval is in, the U is impacted by large disturbances<UsL1Then, UsL1In the process of ascending of the dynamic track of the hybrid system, U is a set values>UsHWhen FRTS is exited, when U issLess than UsL2Then FRTS will force i againcdIs IcdFAccording to the formula:
or
Us≤Usth (2)
As icdAnd icqRecovery increaseLong stopping criterion, maintenance of PcMaximum power transmission or UsTo set voltage Usth。
4. The method of claim 1, wherein the number of U is greater after the large disturbance shocks-PsAnd Us-PcNo steady state operating point, if UsWith solution, corresponding to the system reference capacity SBValue of (A) XsAnd VSC reference capacity SVSCValue of (1)cdFSaid X issAnd IcdFThe following formula is satisfied:
Xs≤EsSB/IcdFSVSC (3)
IcdF≤EsSB/XsSVSC (4)
when U is turnedsReach the set threshold value UsthCorresponding to a predetermined XsAdjustable of IcdFSet up, then XsAnd IcdFThe following formula is satisfied:
Esthe equivalent potential is the Thevenin equivalent potential of the alternating current power grid.
5. A system for controlling the stabilization of a flexible direct current and weak alternating current power grid, the system comprising:
the parameter acquisition module is used for scanning expected faults of the flexible direct current and weak alternating current power grid series-parallel connection system, determining faults which obviously weaken the strength of the flexible direct current and weak alternating current power grid series-parallel connection system, and acquiring an alternating current power grid power transmission characteristic curve U after large disturbance of the flexible direct current and weak alternating current power grid series-parallel connection systems-PsAnd flexible DC power characteristic curve Us-PcAnd determining the operating point on the power transmission characteristic curve U of the AC power grids-PsAnd flexible DC power characteristic curve Us-PcThe position of (2):
determining fault module including operating point at Us-PcContinuously changing interval, operating point at Us-PcDifferent conditions of forced recovery zones or no steady state operating points;
a first control module for controlling the voltage U of the unstable operation point or the operation point of the series-parallel system of the flexible DC and weak AC power networkssIs lower than a set threshold value UsthThen reset FRTS parameter IcdF；
The second control module monitors the running state of the direct current and weak alternating current power grid series-parallel system after fault disturbance in real time, and stops the active current i of the direct current and weak alternating current power grid series-parallel system when dynamic instability is caused by current recoverycdReactive current i of series-parallel system of direct current and weak alternating current power gridcqIncreasing;
a third control module, if UsIs lower than a set threshold value UsthAnd the time exceeds the set time delay TDThen, a superimposed active current pickup amount Δ i is appliedcdrefIf U is presentsHovering over a set value UsL2And a set value UsHThe unstable balance point between the two can ensure that the running point of the series-parallel system automatically slides to the U under the action of the VSC controllers-PsStable equilibrium point of the upper half of the curve, if UsIs still lower than UsthAnd the duration exceeds the set delay TSThen the DC power DeltaP is decreasedcTo lift Us。
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