CN115940101A - Pilot protection method for offshore wind power output line with simulated power and difference waveform comparison - Google Patents
Pilot protection method for offshore wind power output line with simulated power and difference waveform comparison Download PDFInfo
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Abstract
The invention belongs to the technical field of power system relay protection, and provides a pilot protection method for an offshore wind power transmission line with a comparison between a pseudo power waveform and a differential waveform, which comprises the steps of respectively installing relay protection devices at two ends of an alternating current transmission line, and acquiring voltage instantaneous values and current instantaneous values at two sides of the line; when the sudden change of continuous three current instantaneous values at any side meets the starting condition, taking a first sudden change point as a fault occurrence moment, selecting data of a cycle before the first current sudden change point as voltage operation data and current operation data under normal conditions, selecting data of a cycle after the first sudden change point as fault values of the voltage instantaneous values and the current instantaneous values, and calculating the difference between the quasi-power sum and the quasi-power sum by combining the quasi-power theorem; and measuring the difference degree between the pseudo power and the pseudo power difference waveform by adopting the Kendel correlation coefficient, and controlling the protection action by combining the setting value of the protection action. The method has the advantages of no influence of fault types and strong transient resistance.
Description
Technical Field
The invention belongs to the technical field of relay protection of power systems, and particularly relates to a pilot protection method for an offshore wind power transmission line with a pseudo-power sum-difference waveform comparison mode.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
With the proposal of the 'double-carbon' target, the new energy power generation technology is developed unprecedentedly, and offshore wind power has the remarkable advantages of no land resource occupation, high utilization hours and the like, and is widely concerned by researchers. In consideration of the characteristics of large offshore wind capacity and long-distance transmission, the flexible high voltage direct current transmission (VSC-HVDC) mode is usually adopted to be connected with a power grid. Meanwhile, a direct-drive permanent magnet synchronous motor (PMSG) is high in efficiency, small in size, wide in air volume adjusting range and good in low-voltage ride through performance, and is often selected as a main generator set of a large-scale offshore wind farm.
However, due to the fact that power electronic devices are adopted on two sides of the offshore wind power plant which is subjected to flexible direct grid connection, the two sides of the line have the characteristics of limited amplitude, phase difference of current phases on the two sides, frequency offset and the like, and therefore the traditional pilot protection cannot act correctly.
Meanwhile, with the increase of the capacity of the offshore wind farm, the fault ride-through capability is often required to be provided for a period of time after the fault occurs, and the different control strategies on the two sides of the line also enable the alternating current transmission line to present the fault characteristics different from those of the traditional power supply.
Disclosure of Invention
The invention provides a pilot protection method for an offshore wind power sending-out line, aiming at solving the problem that the traditional pilot protection cannot be applied to an offshore wind farm alternating current sending-out line, and the pilot protection method is characterized in that a basic pilot power theorem in a circuit is utilized, an expression meeting the pilot power theorem at two ends of the line before and after a fault is mathematically operated by collecting instantaneous values of voltage and current at the two ends of the line, and the difference degree between the pilot power and a pilot power difference waveform after operation is measured, so that the faults inside and outside the area are judged. The method has the advantages of no influence of control strategies on two sides of the circuit and the length of a data window, strong transition resistance, no need of circuit parameters and the like.
According to some embodiments, the invention adopts the following technical scheme:
a pilot protection method for an offshore wind power sending-out line based on a comparison of a pseudo-power waveform and a difference waveform comprises the following steps:
respectively installing relay protection devices at two ends of an alternating current transmission line, and acquiring voltage instantaneous values and current instantaneous values at two sides of the line;
when the sudden change of any one side of three continuous current instantaneous values meets the starting condition, taking a first sudden change point as a fault occurrence moment, selecting data of a cycle before the first current sudden change point as voltage operation data and current operation data under normal conditions, selecting data of a cycle after the first sudden change point as fault values of a voltage instantaneous value and a current instantaneous value, and calculating a pseudo-power sum and a pseudo-power difference by combining a pseudo-power theorem;
and measuring the difference degree between the pseudo power and the pseudo power difference waveform by adopting the Kendel correlation coefficient, and controlling the protection action by combining the setting value of the protection action.
Compared with the prior art, the invention has the following beneficial effects:
(1) The method is not influenced by weak feed performance of an offshore wind field and current phase angle difference on two sides, and can solve the dilemma faced by a split-phase ratio braking protection method.
(2) The flexible direct grid-connected offshore wind field with both sides being power electronic devices has strong adaptability, is less influenced by distributed capacitance and control strategies, and has strong transition resistance.
(3) Compared with a protection mode utilizing phasor values, the method can effectively prevent inaccuracy and frequency leakage in Fourier transform, and has important significance on an offshore wind farm containing a large number of power electronic devices.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the invention and not to limit the invention.
Fig. 1 is a schematic flow diagram of a pilot protection method for an offshore wind power transmission line of a pseudo-power and difference waveform comparison type according to an embodiment of the present invention;
FIG. 2 is a single line schematic diagram of an equivalent system of an AC transmission line of an offshore wind farm connected through a flexible direct current network;
fig. 3 is a schematic diagram of a system structure and a fault location of an offshore wind farm connected through a flexible direct grid connection provided by an embodiment of the present invention.
The specific implementation mode is as follows:
the invention is further described with reference to the following figures and examples.
It is to be understood that the following detailed description is exemplary and is intended to provide further explanation of the invention as claimed. Unless defined otherwise, all 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.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the invention. As used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
As shown in fig. 1, the embodiment provides a flow chart of a pilot protection method for an offshore wind power transmission line with a pseudo-power and difference waveform comparison type, and the specific steps include:
the method comprises the following steps that firstly, the same relay protection devices are respectively installed on two sides of an alternating current transmission line M and two sides of a wind power plant W, the relay protection devices on the two sides independently collect instantaneous values of voltage and current on the two sides of the line, and the time of fault occurrence is judged according to the following formula:
in the formula:
is distinguished by the current>
A current bump representing the kth sample point, <' >>
Representing the kth current sample point and N representing the sample value of one cycle of current.
If when any side is continuously provided with three points
And then, starting to judge the fault.
Step two, after the starting condition is met, selecting the moment of occurrence of the first catastrophe point as a fault starting moment, selecting a cycle wave voltage and a current instantaneous value before the moment of occurrence of the first catastrophe point as numerical values in normal operation, selecting a cycle wave voltage and a current instantaneous value after the first catastrophe point as numerical values after the fault, and combining a graph 2 and a quasi-power theorem to write a circuit as shown in a formula (1):
in the formula, d represents the total number of branches, u m ,i m ,
Voltage and current transients on the soft-straight side and the soft-straight side after a fault, respectively, during normal operationValue u w ,i w ,/>
Instantaneous values u of wind field side voltage and current during normal operation and wind field side voltage and current after fault c ,i c ,/>
The instantaneous values u of the voltage and the current of each branch of the circuit after normal operation and fault are respectively f ,i f ,/>
The voltage and the current before the fault branch circuit fault and the voltage and the current after the fault are respectively adopted, under the general condition, when the system normally operates and has an out-of-area fault, the fault branch circuit in the circuit can be regarded as infinite impedance, and the current before the fault can be regarded as i f ＝0。
Step three, carrying out summation and difference operation on the column-written Taylor root pseudo-power theorem to obtain the pseudo-power sum and pseudo-power difference, and when the system normally operates, obtaining the fault branch current i f =0, and further can be obtained in formula (1)
The sum and difference of the pseudo power can be represented as equation (2) and equation (3), respectively:
step four, expressing the voltage instantaneous value by using the current instantaneous value, as shown in fig. 2, the voltage instantaneous value during normal operation and after fault of each branch in the ac transmission line equivalent model can be expressed as formula (4) and formula (5), respectively:
in the formula: r c ,C c ,L c Respectively, the equivalent impedance of each branch in FIG. 2 includes a resistor, a capacitor, and an inductor i c ,
The instantaneous values of the current before and after the fault of the c-th branch are respectively.
When the alternating current sending line has a fault, the current of each branch circuit also comprises a direct current component and a high frequency component besides a fundamental frequency component, so that the current after the fault can be written as shown in the formula (6):
in the formula:
represents the current after the fault of the c-th branch>
Represents a DC component, is selected and/or selected>
Represents the n-times high-frequency component amplitude of the c-th branch after the fault occurs, omega represents the angular frequency, and/or the value of the amplitude of the high-frequency component of the c-th branch after the fault occurs>
And (4) representing the initial phase angle of the nth harmonic of the c branch after the fault.
The normal operation branch current can be expressed as shown in equation (7):
in the formula: i.e. i c (t) represents the instantaneous value of the current during normal operation of the c-th branch, I c Representing the magnitude of the current.
In practical application, the contents of the fundamental frequency component and the second harmonic component in the current after the fault are greater than those of the other high frequency components, and the cable line resistance is much smaller than the line inductance, so for simplicity, only the fundamental frequency component and the second harmonic component in the fault current are considered in the formula derivation process, and the influence of the resistance in the line is ignored.
Combining the above formulas, substituting formulas (4) to (7) into formula (1) respectively to simplify to obtain the pseudo power, and formula (8):
the pseudo-power difference is shown as equation (9):
in formulae (8) and (9): r c ,C c ,L c Respectively the resistance, capacitance and inductance of each branch, I c Representing the magnitude of the current during normal operation,
respectively showing the amplitudes of a direct current component, a fundamental frequency component and a frequency doubling component in the fault current of the c branch. />
An initial phase angle representing the current in the c-th branch in normal operation>
Respectively representing the initial phase angles of the fundamental frequency component and the second harmonic frequency component after the fault.
The analysis of the formula (8) and the formula (9) showsWhen an out-of-range fault occurs, no fault branch exists in the line, and at the moment
in-AND branch capacitor C c In related components, the first term is completely the same, the third term and the fourth term are the same except that the amplitude is different, and in addition, the two terms also have the difference of a double frequency component and a direct current component.
And is connected with the inductor L c In the related part, the first terms in the two formulas are identical, and at this time, the two formulas have smaller difference in the terms and have higher similarity, and the two formulas and the inductor L have higher similarity c The third term in the related components has the relationship of opposite sign and 3 times difference in amplitude, while the fourth term still has the condition of opposite sign and unequal amplitude, and besides, the quasi-power sum and the quasi-power difference still have the relationship of one doubled frequency component and direct current component. Therefore, and the inductance L c The related components contain both the identical components and the components of opposite signs.
Meanwhile, for a large-scale offshore wind power alternating current transmission line, which is usually a submarine cable, the distributed capacitance is larger than that of a traditional overhead line, and according to the analysis, the components contained in the waveforms of the pseudo power and the pseudo power difference are different when an out-of-range fault occurs, but the content of the similar components contained in the waveforms is relatively high, and meanwhile, the similarity between the pseudo power and the double medium-frequency content of the pseudo power is also greatly influenced.
Thus, when an out-of-range fault occurs in the ac transmission line, the faultless branch within the line, i.e. the sum and difference between the pseudo-power
Is 0 and therefore->
And/or>
Completely reflect the shape and change characteristics of the waveform, and are based on the two characteristicsThe waveform of the two waveforms cannot be completely overlapped due to certain difference of the contained components, and certain difference is inevitably generated.
When an intra-area fault occurs, the method
u f Is the line voltage before the fault, is asserted>
For current flowing to the faulty branch after a fault, and->
Is far greater than or equal to>
And/or>
In the waveform of the sum and difference of the pseudo power
Both predominate, and it can be assumed that both equations contain only->
Ignoring the remaining components, the waveforms of the pseudo-power and the pseudo-power difference should be close to perfect agreement at this time.
Moreover, according to the derivation, the difference between the pseudo power and the pseudo power also comprises different direct current components, and in order to reduce the influence of the direct current components in the waveforms, the Kendell correlation coefficient which is not influenced by the waveform amplitude is selected to measure the difference degree of the two waveforms, so that the position of the fault is judged.
And step five, selecting Kendell correlation coefficients capable of reflecting waveform similarity to measure the difference degree of the two waveforms, wherein when the sampling points of the two waveforms are simultaneously increased or simultaneously reduced along with time, the two waveforms are consistent, and when the sampling points of the two waveforms are changed in the opposite direction along with the time, the two waveforms are inconsistent, and the Kendel correlation coefficient formula is shown as a formula (10):
in the formula, τ 2 Representing the calculated kender correlation coefficient,
n is the number of sampling points, N c Representing the logarithm of the sample points satisfying consistency, N d Represents the logarithm of the inconsistent sample points, <' > or>
Wherein r represents the small number of sets of the same element composition on the wind field side, t z Indicating the number of elements contained in the z-th subset. Same principle N B This is also true with respect to the straight side.
The kender correlation coefficient is a statistical measure of the similarity, and the value of the kender correlation coefficient varies between [ -1,1], and when the kender correlation coefficient is close to 1, the compared two waveforms have a higher similarity, and when the kender correlation coefficient is close to-1, the compared two waveforms have a lower similarity, and the kender correlation coefficient has no relation with the amplitude of the comparison object.
When the system is in normal operation, the system does not meet the protection of the starting condition and is not started.
When an external fault occurs in an alternating current sending line, the components contained in the waveforms of the pseudo power and the pseudo power difference are obviously different, and the two waveforms are obviously different and have poor similarity according to a Kendel correlation coefficient calculation formula.
In the waveform of the sum and difference between the pseudo power when an in-zone fault occurs in the AC transmission line
The proportion of the two waveforms is far greater than that of the other components, the two waveforms can be considered to be completely the same, and according to a Kendell correlation coefficient calculation formula, the two waveforms are almost not different, and the similarity is strong and close to 1.
The result of Kendel coefficient calculation is located at [ -1,1]Meanwhile, since observation is not intuitive enough and is largely affected by uncertainty, the result of kendel calculation may be multiplied by a coefficient 10 to be amplified, and the amplified kendel correlation coefficient may be denoted by τ' 2 And (4) showing.
Step six, determining a protection action threshold value according to the protection principle, wherein the two waveforms of the simulated power and the simulated power difference obtained by the analysis contain components which are different, but the component with strong similarity is dominant, so that the waveforms have the characteristic of similarity to a certain extent, and therefore, according to experience, whether the amplified Kendall correlation coefficient acts or not can be considered to be the threshold tau' set The setting value is set to 6, a certain margin is reserved for ensuring, the result is multiplied by a margin coefficient of 1.4, and the setting value is 8.4.
Step seven, calculating tau 'according to each phase' 2 And judging the fault position and the fault type according to the relation between the set protection setting value and the set protection setting value.
In the concrete implementation process, the following conditions are available through comparison with the setting result:
if three phases A, B and C are provided in the AC transmission line of the offshore wind farm, there is one phase τ' 2 If the numerical value of the fault phase is greater than the setting value, the single-phase earth fault is generated, and the fault phase breaker is tripped.
If three phases A, B and C are sent out from the AC transmission line of the offshore wind farm, two phases or three phases tau 'exist' 2 If the numerical value of the fault is greater than the setting value, the fault is a two-phase short-circuit fault or a three-phase short-circuit fault, and at the moment, the relay protection device sends out a two-phase or three-phase tripping command.
If AC transmission lines A, B and C of offshore wind farm are three-phase, three-phase tau' 2 If the values of the two are all smaller than the setting value, the circuit is in an out-of-range fault, at the moment, the relay protection device does not send a tripping command, and the circuit breaker does not trip.
Examples
And (3) establishing a flexible direct grid-connected offshore wind farm grid-connected model through EMTP-RV simulation software, and performing simulation verification on the offshore wind farm output line pilot protection method based on the pseudo-power theorem waveform difference provided by the embodiment.
1) Model building
The offshore wind power plant grid-connected model through flexible direct grid connection is shown in figure 3 and comprises 4 wind fields, each wind field is 100MW, the output rated voltage of a direct-drive wind generating set in each wind field is 0.69kV, the direct-drive wind generating set is changed to 35kV through a box, the direct-drive wind generating set is connected through a 35kV medium-voltage collecting wire, the direct-drive wind generating set is changed to 220kV through a main transformer, the direct-drive wind generating set is sent to a flexible direct end through a 220kV alternating-current sending circuit, the generated electric energy is sent to a land power grid through an MMC-HVDC, the rated voltage of the MMC-HVDC is +/-400 kV, the length of the 220kV alternating-current sending circuit of the wind power plant is 10km, the positive sequence resistance and the inductance of the circuit are 0.0529 omega/km and 0.45mH/km respectively, the zero sequence resistance and the inductance of the circuit are 0.0530 omega/km and 0.45mH/km respectively, and the positive sequence capacitance and the zero sequence capacitance of the circuit are c respectively 1 =0.155 μ F/km and c 0 =0.155 muf/km, a data window length of 20ms and a sampling frequency of 4kHz were used for the simulation.
2) Simulation analysis
In order to verify the feasibility of the pilot protection method for the offshore wind power transmission line based on the pseudo-power and difference waveform comparison, provided by the embodiment, faults under different conditions are set at points K2 and K3 respectively, and the results are recorded in tables 1, 2 and 3. Table 1 shows the results corresponding to different types of failure when a metallic failure occurs. Table 2 shows the results corresponding to different types of faults when short-circuit faults occur in the zones via different transition resistances. Table 3 shows the results corresponding to different types of faults when an out-of-range short circuit fault occurs through different transition resistances.
TABLE 1 Effect of different Fault types on protection
TABLE 2 Effect of different transition resistances on protection at K2 Point Fault
TABLE 3 Effect of different transition resistances on protection at K3 Point Fault
As can be seen from table 1, under different fault types, the marine wind power transmission line pilot protection method based on the pseudo power and difference waveform comparison provided by this embodiment can correctly determine the fault position. It can be seen from tables 2 and 3 that τ 'gradually increases with the transition resistance' 2 The value of the protection parameter is still far away from the protection setting value, the intra-area fault protection can reliably act, and the protection is reliable and does not act when the extra-area fault occurs.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A pilot protection method for an offshore wind power output line with a pseudo power and difference waveform comparison mode is characterized by comprising the following steps:
respectively installing relay protection devices at two ends of an alternating current transmission line, and acquiring voltage instantaneous values and current instantaneous values at two sides of the line;
when the sudden change of continuous three current instantaneous values at any side meets the starting condition, taking a first sudden change point as a fault occurrence moment, selecting data of a cycle before the first current sudden change point as voltage operation data and current operation data under normal conditions, selecting data of a cycle after the first sudden change point as fault values of the voltage instantaneous values and the current instantaneous values, and calculating the difference between the quasi-power sum and the quasi-power sum by combining the quasi-power theorem;
and measuring the difference degree between the pseudo power and the pseudo power difference waveform by adopting the Kendel correlation coefficient, and controlling the protection action by combining the setting value of the protection action.
2. The pilot protection method for the offshore wind power sending-out line with the pseudo-power and difference waveform comparison according to claim 1, wherein the starting conditions are as follows:
wherein the content of the first and second substances,
indicates the phase of the current, and>
a current bump representing the kth sample point, <' >>
Representing the kth current sample point, N representing the sample value of one period of the current, I set Starting a setting value for protection.
3. The pilot protection method for the offshore wind power transmission line with the quasi-power sum-difference waveform comparison according to claim 1, wherein before calculating the quasi-power sum-quasi-power difference, quasi-power theorem expressions at two ends of the line are obtained:
in the formula, d represents the total number of branches, u m ,i m ,
Voltage and current instantaneous values u of the soft and straight sides during normal operation and after fault w ,i w ,/>
Instantaneous values u of wind field side voltage and current during normal operation and wind field side voltage and current after fault c ,i c ,/>
The instantaneous values u of the voltage and the current of each branch of the circuit after normal operation and fault are respectively f ,i f ,/>
The voltage and current instantaneous values before the fault of the line fault branch circuit and the voltage and current instantaneous values after the fault are respectively.
4. The pilot protection method for the quasi-power sum-difference waveform comparative offshore wind power transmission line according to claim 1, wherein the quasi-power sum is:
in the formula, d represents the total number of branches, u m ,i m ,
Voltage and current instantaneous values u of the soft and straight sides during normal operation and after fault w ,i w ,/>
Instantaneous values u of wind field side voltage and current during normal operation and wind field side voltage and current after fault c ,i c ,/>
The instantaneous values of the voltage and the current of each branch circuit of the circuit after normal operation and fault respectively f ,i f ,/>
The voltage and current instantaneous values before the fault of the line fault branch circuit and the voltage and current instantaneous values after the fault are respectively.
5. The pilot protection method for the quasi-power and difference waveform comparison type offshore wind power transmission line according to claim 1, wherein the quasi-power difference is as follows:
in the formula, d represents the total number of branches, u m ,i m ,
Voltage and current instantaneous values u of the soft and straight sides during normal operation and after fault w ,i w ,/>
Instantaneous values u of wind field side voltage and current during normal operation and wind field side voltage and current after fault c ,i c ,/>
The instantaneous values u of the voltage and the current of each branch of the circuit after normal operation and fault are respectively f ,i f ,/>
The voltage and current instantaneous values before the fault of the line fault branch circuit and the voltage and current instantaneous values after the fault are respectively.
6. The method for pilot protection of a quasi power sum and difference waveform comparative offshore wind power takeoff line according to claim 1, wherein the two sides of the line comprise a soft and straight side and a wind field side.
7. The pilot protection method for the offshore wind power transmission line with the pseudo-power and difference waveform comparison according to claim 1, wherein before the Kendell correlation coefficient is adopted, the method comprises the following steps: and expressing the instantaneous value of the voltage by using the instantaneous value of the current, and analyzing the expanded pseudo power and the difference degree of the pseudo power and the meaning component in the pseudo power difference expression.
8. The pilot protection method for the quasi-power sum-difference waveform comparative offshore wind power transmission line according to claim 1, wherein the Kendel correlation coefficient is an amplified Kendel correlation coefficient, and the amplified Kendel correlation coefficient is:
in formula (II) is τ' 2 Shows the Kendel correlation coefficient after 10 times of amplification,
n is the number of sampling points, N c Representing the logarithm of the sample points satisfying consistency, N d Represents the logarithm of the sample point satisfying the inconsistency, < >>
Wherein r represents the small collection number formed by the same numerical points at the wind field side, t z Represents the number of elements contained in the z-th sub-set, and N is the same B The same is true on the line-compliant side.
9. The pilot protection method for the quasi-power sum-difference waveform comparative offshore wind power transmission line according to claim 1, wherein the setting value of the protection action is determined according to the characteristic that the quasi-power sum-difference waveform comparative offshore wind power transmission line contains components when an internal fault and an external fault of a region occur and by combining the amplified Kendall correlation coefficient.
10. The pilot protection method for the quasi-power sum-difference waveform comparative offshore wind power transmission line according to claim 1, wherein the step of judging the position of the fault and the type of the fault according to the relation between the Kendell correlation coefficient and the set protection setting value calculated by each phase comprises the following steps:
if the AC transmission lines of the offshore wind power plant are three-phase, namely A, B and C, and one-phase Kendel correlation coefficient is greater than a setting value, the occurrence of a single-phase earth fault is indicated, and the fault phase breaker is tripped at the moment;
if the AC of the offshore wind farm sends out three phases A, B and C, and two phases or three phases of Kendall correlation coefficients are larger than a setting value, the occurrence of two-phase short-circuit fault or three-phase short-circuit fault is indicated, and at the moment, the relay protection device sends out a two-phase or three-phase tripping command;
if the AC of the offshore wind farm sends out three phases of A, B and C, and the Kendell correlation coefficients of the three phases are all smaller than the setting value, the circuit is in an out-of-area fault, at the moment, the relay protection device does not send out a tripping command, and the breaker does not trip.
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