CN110231545B - Island detection method and reference current extreme value calculation method for alternating current-direct current series-parallel power grid - Google Patents
Island detection method and reference current extreme value calculation method for alternating current-direct current series-parallel power grid Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R19/00—Arrangements for measuring currents or voltages or for indicating presence or sign thereof
- G01R19/04—Measuring peak values or amplitude or envelope of ac or of pulses
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/08—Locating faults in cables, transmission lines, or networks
- G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
- G01R31/086—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution networks, i.e. with interconnected conductors
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/08—Locating faults in cables, transmission lines, or networks
- G01R31/088—Aspects of digital computing
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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
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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
- 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]
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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
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/388—Islanding, i.e. disconnection of local power supply from the network
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Abstract
The invention relates to the technical field of direct current transmission, and discloses an island detection method and a reference current extreme value calculation method for an alternating current-direct current hybrid power grid, wherein the island detection method for the alternating current-direct current hybrid power grid is characterized in that a soft direct system local electric quantity d-axis reference current is detected according to the change of the soft direct system local electric quantity d-axis reference current after an island is formed. The invention has the beneficial effects that: the system island response speed is high, and the system island can be judged quickly; the possibility of erroneous judgment under large interference can be avoided by setting a reasonable threshold, so that the island and non-island conditions can be accurately and reliably distinguished; the realization is convenient, and a complex control link is not required to be introduced.
Description
Technical Field
The invention relates to the technical field of direct current transmission, in particular to an island detection method and a reference current extreme value calculation method for an alternating current-direct current series-parallel power grid.
Background
The VSC-HVDC technology based on the Voltage Source Converter is proposed in 1990 by Boon-Teck Ooi of McGill university, Canada, and is a novel power Transmission technology based on the Voltage Source Converter, a self-turn-off device and a Pulse Width Modulation (PWM) technology. As a new generation of dc transmission technology, flexible dc transmission is still composed of a converter station and a dc transmission line (usually a dc cable), similar in structure to high voltage dc transmission. Unlike current source converter type high voltage direct current transmission based on a phase control commutation technology, a converter in flexible direct current transmission is a Voltage Source Converter (VSC), and the converter is mainly characterized by adopting a turn-off device (usually, an IGBT) and a high-frequency modulation technology. By adjusting the amplitude of the outlet voltage of the converter and the power angle difference between the outlet voltage of the converter and the system voltage, the output active power and reactive power can be independently controlled. Therefore, the mutual transmission of active power between two alternating current networks can be realized by controlling the converter stations at the two ends, and simultaneously, the converter stations at the two ends can independently adjust the reactive power absorbed or emitted by the converter stations respectively, so that the connected alternating current system is supported in a reactive mode.
The islanding phenomenon refers to a distributed grid-connected power generation system of each user side when the power supply of a power grid is tripped due to a fault accident or power failure maintenance, such as: photovoltaic power generation, wind power generation, fuel cell power generation, etc.) fail to detect the power failure state immediately and disconnect itself from the utility grid, forming a self-powered island composed of a distributed power plant grid-connected power generation system and surrounding loads. Aiming at the island condition, the island condition needs to be quickly and accurately detected and correspondingly controlled and switched so as to realize stable power supply. The existing island detection technology of the flexible direct current transmission system has the following defects: the traditional island detection technology of the flexible direct current transmission system is based on phase and frequency detection, although the frequency and the phase are sensitive to power and can quickly reflect the island condition, larger fluctuation can also occur during load switching and alternating current system faults. In order to avoid the converter station being switched by mistake in the above situation, a large threshold value and a large duration threshold value are usually set for the offset phase and the frequency, but the setting of the threshold value may lengthen the process of the system recovering to a steady state, thereby causing frequency collapse and the like. Therefore, a method is needed to be found, which can not misjudge in the interference such as load disturbance, fault transient process and the like, and can quickly respond when an island condition occurs, and shorten the process of switching the networking control to the island control, so that the process can be smoothly and seamlessly transited.
Disclosure of Invention
Aiming at the problems, the invention provides an island detection method and a reference current extreme value calculation method for an alternating current-direct current series-parallel power grid; the method is characterized in that d-axis reference current output by an outer ring controller of the flexible direct system is used as electrical detection quantity, reasonable PI parameters and d-axis reference current threshold values are set, rapid response can be carried out on the d-axis reference current when an island is formed, meanwhile, the d-axis reference current is opposite to the change trend after the island is formed under the condition of large interference such as alternating current fault, the problem of misjudgment under the condition of large interference can be effectively solved, and compared with the traditional frequency-based and phase island detection technology, the method has faster response speed and more concise detection flow.
The technical scheme adopted by the invention is as follows:
an island detection method for an AC/DC hybrid power grid comprises the following steps:
to take into account avoidance of fault interferencedrefThe d-axis reference current threshold value setting process of the extreme value determines the minimum value of the converter station output active power under the most serious three-phase alternating current fault through the amplitude limiting value set by the converter station outer ring controller, and finally determines I through a slope input modeldrefAnd (4) an extreme value.
Further, the method for determining the output active power extreme value under the outlet three-phase alternating current fault of the receiving end converter station comprises the following steps:
and the converter station outer ring controller respectively sets dq axis reference current limiting values, and can limit fault current injected to a fault point by the converter station under the condition of alternating current fault. Under the three-phase alternating current fault, the converter station can be regarded as a current source model, and the fault current injected into the fault point of the converter station can be calculated through the dq axis reference current limiting value.
Further, under the three-phase alternating current fault, the controller loses steady-state control, the minimum value of the fault current output by the converter station is calculated according to the amplitude limiting value of the reference current output by the controller, and the corresponding power factor can be obtained according to the relation between the dq axis currents
Further, after the power factor is obtained, the residual voltage amplitude under the three-phase alternating current fault at the position of the bus at the outlet of the converter station is obtained according to ohm's law. And voltage and current do not have negative sequence components under the three-phase alternating current fault, and the minimum value of the output active power of the converter station can be obtained according to an instantaneous power method.
Further, after the fault is eliminated, for the active power recovery process of the converter station, the input of the active power outer loop controller is (P)ref-P), analyzing the active power outer loop controller transfer function, applying a suitable mathematical model to its input (P) during power recoveryref-P) output response is analyzed.
Further, the method for determining the d-axis reference current extreme value output by the converter station active power outer loop controller during the power recovery period specifically comprises the following steps:
for the power recovery period (P)ref-P) as a ramp input, which is subjected to a Laplace transform, the transformed (P)ref-P) is multiplied by the transfer function to obtain the output response of the complex frequency domain, and then the output response of the complex frequency domain is subjected to inverse Laplace transform, so that the corresponding output response of the time domain during the power recovery period can be obtained.
Further, the output response of the time domain is subjected to mathematical analysis, and the I of the zero state during the power recovery period can be obtaineddrefAnd obtaining the d-axis reference current setting threshold value by adding the d-axis reference current amplitude limit value during the three-phase alternating current fault period and considering the reliability coefficient.
In conclusion, the beneficial effects of the invention are as follows:
1. the invention uses d-axis real-time current IdrefThe method has the advantages that the method is an electrical quantity basis for judging whether the system is in an island state, the situation that the AC line is judged to be in the island state by mistake due to the fact that the AC system breaks down is reduced, and whether the AC line is in the island state is detected quickly and accurately;
2. the invention can reduce the fluctuation of the alternating voltage amplitude value at the outlet bus of the receiving end converter station in the engineering that the alternating current circuit has the island effect and needs to control and switch urgently, has the effect of shortening the recovery time of the system and can realize smooth and seamless control and switch.
Drawings
FIG. 1 is a schematic flow diagram of the present invention;
fig. 2 is a model schematic diagram of a 12-node flexible direct-current parallel operation system built in the embodiment of the invention;
fig. 3 is a schematic diagram of parameters of a model of a 12-node flexible direct-current parallel operation system and parameters of a receiving-end converter station controller, which are built in the embodiment of the present invention;
fig. 4 is a schematic diagram for showing the variation of the active power output by the receiving end converter station in embodiment 1 of the present invention;
fig. 5 is a schematic diagram for showing a variation of the receiving end converter station (Pref-P) in embodiment 1 of the present invention;
fig. 6 is a dialectical diagram for showing d-axis reference current of the receiving end converter station in embodiment 1 of the present invention;
fig. 7 is a schematic diagram used in embodiment 2 of the present invention to show a change in an effective voltage value at an outlet bus of a receiving-end converter station when the island detection method proposed by the present invention is used;
fig. 8 is a schematic diagram of embodiment 2 of the present invention for showing a frequency change of a receiving-end island system when the island detection method proposed by the present invention is adopted;
fig. 9 is a schematic diagram for showing the variation of the real-time reference current of the d-axis of the receiving end converter station in embodiment 2 of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention are clearly and completely described below with reference to fig. 1 to 9 of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 1, an island detection method for an alternating current-direct current hybrid power grid is shown, wherein I is shown after an island is formeddrefRapidly rises to reach the amplitude limit value,however, in order to avoid misjudgment of the ac system when a fault or other disturbance occurs, a threshold needs to be set so as to avoid the maximum value of the d-axis reference current under the large disturbance of the system.
The most serious bus three-phase alternating current short circuit fault at the outlet of the converter is considered, at the moment, the converter adopting constant active power control cannot realize power balance any more, and the current reaches the amplitude limit value. Taking the direction of the absorbed power of the current converter as the positive direction, at this time Idref＝Isdrefmin，Iqref＝Isqrefmin，IsdrefminAnd IsqrefminMinimum values set for the dq-axis reference currents, respectively.
The minimum value of the amplitude of the short-circuit current contributed by the converter during the short-circuit period is as follows:
and neglecting the grounding resistance and not containing a negative sequence component under the symmetrical fault so as to obtain the minimum value of the output power under the fault. The output power of the converter is minimum P when the fault is considered to be endedminAnd after a period of time, the power is restored to the set value. After the fault is eliminated (P)refP) gradually rising, fitting the controller output response of this power recovery process to a ramp response in order to simplify the tuning process. When the time of the fault ending is set as zero time, the following can be obtained:
kPr＝(Pmin-Pref)/tP (7)
in the formula, PrefOutputting an active power instruction value for the current converter at the receiving end, P being an actual active power value output by the current converter station at the receiving end, PminFor outputting the minimum value of active power, k, of the receiving end converter station during the three-phase AC fault of the outlet bus thereofPrDuring power recovery (P)ref-P) an approximate slope of the curve, approximated by a power recovery period (P)ref-P) average slope. At the power minimumThe most extreme, k, is taken into account in the value calculationPrIs approximately (P)min-Pref)/tp，trTime required for power to return to the set value, tpDuring power recovery (P)refP) time required to reach the peak.
For (P) during power recoveryref-P) applying a laplace transform to P (t) ═ Pref-P), available:
the response of the power recovery function is then:
subjecting it to inverse Laplace transform to define I'dref(t) is the inverse laplace transform of P(s) H(s) to obtain:
during power recovery, I'dref(t) maximum value at trIs obtained from (1)'dremaxThen the maximum value of the d-axis reference current during power recovery is:
Idrefmax＝Isdrefmin+I’drefmax (11)
taking k in consideration of reliability coefficientrSetting value of d-axis reference current threshold value Isd：
Isd＝krIdrefmax (12)
Under large disturbances such as AC faults, IdrefThe island condition is not judged by the detection method under the condition of large interference. After island formation, IdrefThe island can be quickly and accurately judged by taking the set threshold value as a criterion.
Referring to fig. 2, a model of a 12-node flexible direct-current parallel operation system is built in power system electromagnetic transient simulation software PSCAD/EMTDC, and both embodiments 1 and 2 are implemented based on the model. With reference to fig. 3 and 4, the rated dc voltage in the model of the 12-node flexible dc parallel operation system is ± 200 kV; rated voltage grades of the sending end alternating current system and the receiving end alternating current system are both 230 kV; the power transmission method comprises the following steps that 2 alternating current transmission lines are used for transmitting power to a set island system, and the rated transmission power of alternating current lines 1 and 2 is 70 MW; the rated transmission power of the flexible direct current transmission line is 120MW, and the direction of the absorbed power of the converter station is positive. Control strategy of the converter station: the sending end is controlled by constant direct current voltage and constant reactive power, and the receiving end is controlled by constant active power and constant alternating current voltage amplitude.
Referring to fig. 3, according to the parameters of the model and the parameters of the receiving end converter station controller, the minimum value I of the short circuit current amplitude value contributed by the converter is calculatedmin2.56kA, minimum value of AC voltage amplitude Umin2.62kV, power factor during short circuit
Is 0.78. Time t required for power to return to power set value PrIs 0.024s, (P)min-Pref) Is 112.15MW, calculated as I'drefmax2.69KA, then Idrefmax0.69 kA. In the system, a reliability coefficient K is setr1.2, calculating d-axis reference current setting value IsdIt was 0.828 kA.
Example 1
The method is used for detecting whether the situation that the AC line is in the island state due to fault of the AC system can be avoided.
And setting a three-phase short-circuit fault at the outlet bus of the receiving end converter station, wherein the fault occurs in 0.6s and disappears after lasting for 50 ms. With reference to fig. 4 and 5, the output active power of the converter station during the short circuit continuously decreases and reaches a minimum value of-10.6 MW at 0.653s, and the power difference is 109.4 MW. Thereafter, the active power starts to ramp back up to the set point. In connection with fig. 6, the maximum value during power ramp-back is 0.49kA with a 69% margin compared to the setting value of 0.828 kA.
Example 2
The embodiment is used for detecting whether the method can quickly detect whether the alternating current line is in an island state or not.
Referring to fig. 2, the ac line 1 is cut off at 0.6s, and the ac line 2 is cut off at 0.7s to form a receiving-end island system, and the island detection method provided by the invention is adopted for the system and the constant ac voltage control is adopted after switching.
Referring to fig. 7, a graph shows the variation of the effective voltage value U at the outlet bus of the receiving end converter station. With reference to fig. 8, a curve in the diagram shows a change situation of a receiving end island system frequency f when the system adopts the island detection method provided by the present invention, and with reference to fig. 9, when the 0.6s ac line 1 is cut off, the receiving end system frequency and voltage are not fluctuated, the d-axis reference current of the receiving end converter station is not changed, and the converter station output active power is not changed. After the alternating current line 2 is cut off for 0.7s, the d-axis reference current of the receiving end converter station rapidly rises and reaches a setting threshold value after 6 ms. And in the switching process, the system frequency has no obvious fluctuation, the amplitude fluctuation of the alternating voltage at the outlet bus of the receiving end converter station is small, and the voltage can be quickly recovered to a rated voltage value.
In the description of the present invention, it is to be understood that the terms "counterclockwise", "clockwise", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate orientations or positional relationships based on those shown in the drawings, and are used for convenience of description only, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be considered as limiting.
Claims (2)
1. An island detection method for an alternating current-direct current hybrid power grid is characterized by comprising the following steps:
s1: inputting the topology and related parameters of the direct current system, taking the direction of the absorbed power of the receiving end converter station as the positive direction, and solving the output of the converter station under the three-phase alternating current fault at the outlet bus of the receiving end converter stationMinimum value of powerP minAnd the power difference value (P ref-P) Wherein, in the step (A),P refoutputs an active power instruction value for the receiving end converter,Poutputting an actual value of active power for the receiving end converter station;
s2: establishing a mathematical model of the ramp input response for the power recovery process, wherein the mathematical model of the ramp input response can be expressed as:
in the formula (I), the compound is shown in the specification, P minfor the receiving end converter station to output the minimum value of active power during the three-phase ac fault of its outlet bus,k Prduring power recovery period (P ref-P) Approximate slope of the curve, approximated as a power recovery periodP ref-P) The average slope of the power, in the most extreme case considered in the calculation of the power minimum,k Pris approximately (P min-P ref)/t p，t rIn order to restore the power to the set value,t pduring power recovery period (P ref-P) The time required for reaching the peak value is subjected to mathematical analysis of a time domain and a complex frequency domain, and a d-axis reference current extreme value under zero-state response is obtained;
s3: according to the d-axis reference current amplitude limit value of the VSC-HVDC system controller, solving a d-axis reference current extreme value during power recovery, wherein the input power difference value of the external loop controller is (P ref-P) Modeling and fitting are carried out, Laplace transformation is carried out on input of the d-axis reference current extremum, response output in a zero state is obtained, then inverse Laplace transformation is carried out on the d-axis reference current extremum to obtain d-axis reference current extremum in a time domain, amplitude limiting values set by a controller are considered, namely d-axis reference current extremum in a full response state can be obtained, and reliability coefficient is considered, namely the d-axis reference current extremum in the full response state can be obtainedObtaining a setting threshold value of the device;
s4: and detecting whether the d-axis reference current reaches a setting threshold value, if not, judging that the flexible direct current transmission system is in an alternating current/direct current parallel operation mode, and if so, judging that the receiving end of the flexible direct current transmission system forms an island system.
2. A method for solving a reference current extreme value of an alternating current-direct current hybrid power grid is characterized by comprising the following steps of: difference in input power to outer loop controller (P ref-P) Modeling and fitting are carried out, Laplace transformation is carried out on input of the d-axis reference current extremum, response output under a zero state is obtained, then reverse Laplace transformation is carried out on the d-axis reference current extremum, d-axis reference current extremum under full response can be obtained by considering the amplitude limit value set by the controller, and d-axis reference current extremum output by the outer ring controller after the fault is ended is obtained: to pair
Performing a laplace transform yields:
multiplying the above formula by the transfer function of the outer ring controller to obtain complex frequency domain response in a zero state, performing inverse Laplace transform on the response to obtain a d-axis reference current extreme value in a time domain, and determining the d-axis reference current extreme value according to the following formula:
in the formula (I), the compound is shown in the specification,P minfor the receiving end converter station to output the minimum value of active power during the three-phase ac fault of its outlet bus,k Prduring power recovery period (P ref-P) The approximate slope of the curve is such that,t rin order to restore the power to the set value,t pduring power recovery period (P ref-P) The time required to reach the peak value is,I ’ drefmaxthe d-axis reference current extreme value under the zero-state response can be obtained by finally considering the d-axis reference current amplitude limiting value during the power recovery period, wherein,P refoutputs an active power instruction value for the receiving end converter,Pand outputting the actual value of the active power for the receiving end converter station.
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| CN111276997A (en) * | 2020-04-13 | 2020-06-12 | 国网福建省电力有限公司 | Alternating current fault current limiting method for flexible direct current transmission system |
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