CN109494697B - Cosine similarity-based new energy station multi-end pilot protection method - Google Patents
Cosine similarity-based new energy station multi-end pilot protection method Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/28—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for meshed systems
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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- Y02E10/56—Power conversion systems, e.g. maximum power point trackers
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Abstract
The invention discloses a cosine similarity-based new energy station multi-terminal pilot protection method, which aims at new energy stations which are connected with the grid through a power electronic converter and combines a common control strategy for inhibiting negative sequence current to obtain a transient current analytic formula of new energy station faults and a new energy station short-circuit current analytic formula; analyzing and obtaining the characteristic that the waveform of the short-circuit of the new energy station is superposed by sine waves and non-power frequency sine waves attenuated by non-power frequency according to the analysis formula of the short-circuit current of the new energy station, and constructing a multi-end pilot protection criterion of the new energy station based on cosine similarity; and setting the protection fixed value in the multi-terminal pilot protection criterion according to the angle error and the amplitude error when the system normally operates. The method is not influenced by the capacity and the operation mode of the new energy, has the characteristics of high reliability and sensitivity and good quick action, and improves the safety and the reliability of the operation of the large-scale new energy grid-connected system.
Description
Technical Field
The invention relates to the technical field of new energy power supply grid-connected protection, in particular to a method for multi-terminal pilot protection of a new energy station based on cosine similarity.
Background
In order to solve the increasingly severe problems of energy crisis and environmental pollution, new energy power sources such as photovoltaic power sources, wind power sources and the like are rapidly developed. The large-scale new energy station mostly adopts a centralized boosting grid-connected structure, and in order to save the investment of power equipment, T-connection type topologies of the new energy station in 110/220kV grid-connected lines are common. A plurality of new energy field stations T are connected to the same sending-out line, so that the influence range of the system caused by incorrect protection is larger, and in order to improve the safety of new energy power utilization, the faults need to be removed reliably and quickly through power grid relay protection, so that the research on the protection of the T-connection type sending-out line of the new energy field stations is of great significance.
The output of the new energy power supply has characteristics of waveform, randomness and the like, so that fault characteristic quantity is difficult to extract, and as the new energy station is generally integrated into a power grid by adopting a power electronic converter, the short-circuit current of the new energy station has unique characteristics of limited amplitude, frequency deviation, distortion, harmonic waves and the like, and is completely different from the fault characteristics of a synchronous generator. Due to the reasons, the traditional protection principle based on the fault characteristics of the synchronous generator has the adaptability problem of sensitivity reduction, and even has the risks of error and refusal, and the traditional protection is challenged, so that the characteristics of short-circuit current after the fault of the new energy station need to be researched, and a protection means suitable for a T-connection grid-connected system of the new energy station needs to be constructed.
Disclosure of Invention
The invention aims to provide a cosine similarity-based new energy station multi-terminal pilot protection method which is not influenced by new energy capacity and an operation mode, has the characteristics of high reliability and sensitivity and good quick action, and improves the operation safety and reliability of a large-scale new energy grid-connected system.
The purpose of the invention is realized by the following technical scheme:
a method for multi-terminal pilot protection of a new energy station based on cosine similarity, the method comprising:
step 2, analyzing and obtaining the characteristic that the waveform of a short circuit of the new energy station is a superposition of sine waves attenuated by non-power frequency and non-power frequency sine waves according to the analytic formula of the transient short circuit current of the new energy station, and constructing a multi-end pilot protection criterion of the new energy station based on cosine similarity;
and 3, setting a protection fixed value in the multi-terminal pilot protection criterion according to the angle error and the amplitude error when the system normally operates.
According to the technical scheme provided by the invention, the method is not influenced by the capacity and the operation mode of the new energy, has the characteristics of high reliability and sensitivity and good quick action, and improves the safety and reliability of the operation of the large-scale new energy grid-connected system.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.
Fig. 1 is a schematic flow chart of a method for multi-terminal pilot protection of a new energy station based on cosine similarity according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a T-connection type outgoing line topology and a fault location of a scaled new energy station in an example of the present invention;
fig. 3 is a schematic connection diagram of a relay protection device of a T-connection type outgoing line of a new energy station according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of the protection operation (intra-area fault) when an A-phase ground fault occurs at a T-junction in the area according to an exemplary embodiment of the present invention;
FIG. 5 is a schematic diagram of the protection operation (intra-area fault) when a BC two-phase short circuit fault occurs at a T-junction in the area according to an exemplary embodiment of the present invention;
FIG. 6 is a schematic diagram of the protection operation (intra-area fault) when a BC two-phase ground fault occurs at a T-junction in the area according to an exemplary embodiment of the present invention;
FIG. 7 is a schematic diagram of the protection operation (intra-area fault) when an ABC three-phase fault occurs at the T-junction in the area according to the embodiment 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 the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without making any creative effort, shall fall within the protection scope of the present invention.
The following will further describe the embodiment of the present invention in detail with reference to the accompanying drawings, and as shown in fig. 1, a flow chart of a method for multi-terminal pilot protection of a new energy station based on cosine similarity according to the embodiment of the present invention is shown, where the method includes:
the specific process of the step is as follows:
firstly, a new energy station inverter grid-connected loop equation under a dq coordinate system is expressed as follows:
in the formula, L is equivalent inductance of an inverter grid-connected topology; r is equivalent resistance of the inverter grid-connected topology; i.e. i d D-axis current under dq coordinate system; i.e. i q Is q-axis current under dq coordinate system; u. of d Is the d-axis voltage at the inverter outlet; u. of q Is the q-axis voltage at the inverter outlet; e.g. of the type d Is the d-axis component of the electric network electromotive force; e.g. of the type q Is the q-axis component of the grid electromotive force; omega is angular frequency;
when the new energy station inverter adopts a voltage-current double closed-loop controller, under a control strategy of inhibiting negative sequence current, a control equation of the inverter can be written as follows:
in the formula, k ip 、k ii Respectively is a current control loop proportion and an integral time constant;
respectively representing a d-axis current reference value and a q-axis current reference value; i.e. i d ，i q Respectively representing d-axis current actual values and q-axis current actual values; l is equivalent inductance of the inverter grid-connected topology; e.g. of the type d Is the d-axis component of the electric network electromotive force; e.g. of the type q The grid electromotive force q-axis component.
The voltage outer ring is cut off when the inverter responds in a transient state, at the moment, the dq axis current is mainly influenced by the current control inner ring, the PI current ring only needs to track a given reference value, and the inverter transient response equation can be obtained by combining a loop equation of inverter grid connection and an inverter control equation:
in the formula, k ip 、k ii Respectively is a current control loop proportion and an integral time constant;
respectively representing a d-axis current reference value and a q-axis current reference value; i all right angle d ，i q Respectively representing d-axis current actual values and q-axis current actual values; and L is equivalent inductance of the inverter grid-connected topology.
Since the new energy station generally keeps the rated power factor operation in normal operation, the initial current i of the dq axis d0 ＝1p.u.,i q0 Solving the transient response equation of the inverter can obtain an analytic formula of the transient current under the dq axis as:
wherein i d ，i q Respectively representing d-axis current actual values and q-axis current actual values;
respectively representing a d-axis current reference value and a q-axis current reference value; xi is the damping ratio of the second order system, which has a value of
ω n Is the natural oscillation angular frequency of the second order system, and has a value of
ω d Representing a damped oscillation frequency of value
Beta is the damping angle, the value of which is
And further converting the transient state current analytic expression into a new energy station short circuit current analytic expression under an ABC coordinate system through coordinate transformation, wherein the analytic expression is specifically represented as follows:
wherein i φ Short-circuit current of any one of the three phases; theta φ Representing the initial phase angle after the three-phase fault; ω represents the angular frequency detected by the phase locked loop; t is t 0 Is the fault start time.
Step 2, analyzing and obtaining the characteristic that the waveform of the short-circuit of the new energy station is a superposition of sine wave attenuated by non-power frequency and non-power frequency sine wave according to the analytic formula of the short-circuit current of the new energy station, and constructing a multi-end pilot protection criterion of the new energy station based on cosine similarity;
in this step, the short-circuit current analytic expression of the conventional synchronous power supply is as follows:
in the formula, i is the short-circuit current of any one of three phases; e q|0| And x d The synchronous internal potential and the synchronous reactance are respectively; x ″) d And x' d 、T″ d And T' d The time constants of the synchronous machine reactance and the alternating current component attenuation in the sub-transient state and the transient state process are respectively; omega is the power frequency angular frequency; t is a Is the decay time constant of the dc component;
is the fault initiation angle.
According to the formula, the waveform of the short-circuit current of the traditional synchronous power supply is expressed as a power frequency sine wave attenuated according to an exponential law; and the waveform of the short circuit of the new energy station presents the characteristic of superposition of non-power frequency attenuated sine waves and non-power frequency sine waves. The two characteristics have great difference, and the new energy station multi-end pilot protection criterion based on cosine similarity can be constructed by using the difference, specifically:
combining with the T-connection type sending-out line topology of the new energy station, correspondingly adding the current sampling values of all the T-connection new energy stations with clock synchronization within 10ms window length, and recording the operation result as a station side and current vector
In the formula, N represents that N new energy stations are accessed on a T-connection type sending-out line; i.e. i wk A current sampling value vector of clock synchronization within 10ms window length of the kth new energy station; n is the total number of all sampling points within a 10ms window length;
the current sampling value of clock synchronization within the window length of 10ms on the synchronous system side is taken as the current vector on the system side, and is expressed as: i.e. i s ＝(i s1 ,i s2 ,…,i sn )；
And substituting the two current vectors into the following cosine similarity formula for calculation:
in the formula, cos (i) w ,i s ) Calculating cosine similarity representing two current vectors;
when an out-of-range fault occurs, the transient current waveforms of the station side current and the system side current are opposite, and the similarity coefficient cos (i) of the two current vectors is obtained through calculation w ,i s ) -1; when an intra-area fault occurs, because the fault characteristics of the new energy power supply and the traditional synchronous power supply are different, the similarity of transient current waveforms on two sides is poor, and the similarity coefficient obtained through calculation is larger than-1; because the neutral point of a main transformer in the wind power station is directly grounded, when a grounding fault occurs in a region, the zero-sequence current in the system is large, so that the calculated value of the cosine similarity is near 1, and the multi-end pilot protection criterion of the new energy station based on the cosine similarity is constructed and expressed as follows:
cos(i wΣ ,i s )＞cos set
in the formula, the subscript phi is any phase in the ABC three phases, and the similarity coefficient is calculated by using the data of the same phase; cos (chemical oxygen demand) set For protecting constant value, the value range is [ -1,1 [ ]]In the meantime.
And 3, setting a protection fixed value in the multi-terminal pilot protection criterion according to the angle error and the amplitude error when the system normally operates.
Here, the formula for setting the protection constant value in the multi-terminal pilot protection criterion is as follows:
wherein K is a reliability coefficient; k ang Is an angle error coefficient; k amp Is an amplitude error coefficient; k is mar Is a margin coefficient; cos (chemical oxygen demand) set The values are fixed for protection.
For example, the angle error mainly comes from a pilot protection communication synchronization error and a CT angle error, a current reference direction is selected as a bus pointing line, and under the condition that the angle error is not considered, the waveforms of the station side current and the system side current are completely opposite, and the cosine similarity calculation value is-1. Considering that the maximum synchronization error is 0.5ms, and the corresponding power frequency angle is 9 degrees; the CT angle error is generally considered to be 7, so the angle error totals 16. When the angle error is 16 degrees, the corresponding cosine similarity calculation value is-0.960, and in order to avoid the angle error, a coefficient K related to the angle error is set ang 。
In addition to the angle error, the magnitude error also affects the cosine similarity calculation, which is mainly derived from the CT magnitude error. Since the CT accuracy is generally 5% or 10%, under the amplitude error of 10%, the cosine similarity calculation value is-0.993. In order to avoid amplitude errors, a coefficient K is set for the amplitude errors amp 。
In addition, since a certain margin is generally set for the protection constant value setting in order to ensure reliability, a coefficient K is set for the margin mar 。
For example, the angle error coefficient is 0.95, the amplitude error coefficient is 0.99, and the margin coefficient is 0.95, and the reliability coefficient K is 0.9, so that the setting value of the protection constant value is cos set ＝-1×0.9＝-0.9。
The process of the method is described in detail below by specific examples, as shown in fig. 2, which is a schematic diagram of a topology and a fault location of a T-connection type sending-out line of a large-scale new energy field station in an example of the present invention, where one power electronic unit in fig. 2 represents one new energy field station (generally rated capacity is 50MW or 100MW), one sending-out line can be T-connected with a plurality of new energy field stations, and then is merged into an external synchronization system through an overhead line, and two 100MW new energy field stations are connected to the T-connection type sending-out line with voltage level of 220kV in fig. 2, where #1 new energy field station is a permanent magnet wind farm, a single machine has rated capacity of 1.5MW, and there are 99 fans in total, and terminal voltage is 0.69 kV; the #2 new energy station is a double-fed wind power station, the rated capacity of a single machine is 3MW, 33 fans are used in total, and the voltage at the machine end is 0.69 kV; the rated capacity of the box transformer substation is 3.5MVA, the transformation ratio is 35kV/0.69kV, Dyn is connected with a wire, and the short-circuit impedance is 6.76%; equivalent collector resistance 0.11 Ω and inductance 409.5 μ H. The rated capacity of a main transformer is 120MVA, the transformation ratio is 220kV/35kV, YNd is connected, and the short circuit impedance is 6%. The total length of the transmission line is 40km, the positive sequence impedance and the negative sequence impedance are both 0.076+ j0.338 omega/km, the zero sequence impedance is 0.284+ j0.824 omega/km, and the T joint is positioned at the midpoint of the line. According to the topological structure in fig. 2 and the parameters, a new energy source T-connection grid-connected system electromagnetic transient model is built in a Real Time Digital Simulator (RTDS) to verify the method provided by the invention.
A total of 6 failure positions provided in the outgoing line W 1 、W 2 The S-side outer outlet and the inner proximal end are designated as K11, K12, K13, K14, K15, and K16, respectively. The fault types include all 4 types, including single-phase earth fault, two-phase short-circuit fault, and three-phase short-circuit fault, which take a-phase earth, BC two-phase short-circuit, and ABC three-phase short-circuit as examples, and the above 4 fault types are abbreviated as AG, BCG, BC, ABC, respectively.
Fig. 3 is a schematic diagram illustrating a connection of a relay protection device of a T-connection type outgoing line of a new energy station according to an embodiment of the present invention. As shown in fig. 3: delivery line W 1 Side, W 2 A relay protection device W is respectively arranged on the side and the side S 1 、W 2 And S, each set of protection device independently measures the three-phase current of the side and acquires sampling values of other two sides through a communication channel. According to the multi-terminal pilot protection criterion of the new energy station based on the cosine similarity, the cosine similarity is calculated by using current sampling values of the same-name phase station side and the current and system side, the fault position is judged, and a trip signal is sent to the corresponding breaker.
Fig. 4 shows the protection operation condition (i.e. the in-zone fault) when the a-phase ground fault occurs at the T-junction in the area according to the exemplary embodiment of the present invention; fig. 5 shows a protection operation condition (i.e., an in-region fault) when a BC two-phase short circuit fault occurs at a T junction in an exemplary embodiment of the present invention; fig. 6 shows a protection operation condition (i.e., an intra-area fault) when a BC two-phase ground fault occurs at a T junction in an area according to an exemplary embodiment of the present invention; fig. 7 shows the protection operation (i.e., the in-zone fault) when an ABC three-phase fault occurs at the point T in the zone in the example of the present invention. Wherein the solid line is the calculated value of the cosine similarity of phase A, the dotted line is the calculated value of the cosine similarity of phase B, and the dash-dot line is the calculated value of the cosine similarity of phase C; in order to clearly show the protection performance, the protection constant value is indicated by a two-dot chain line. In the figure, 0ms is the moment when a fault occurs, the cosine similarity coefficient calculated value of a fault phase exceeds a protection fixed value of-0.9 within 5ms after the fault occurs in a zone, the cosine similarity coefficient calculated value tends to be stable within 10-15 ms, the protection can reliably and quickly identify the faults inside and outside the zone, and the action is taken to trip the circuit breaker.
In order to further verify the effectiveness of the method provided by the invention, a large number of simulation studies are carried out in a real-time digital simulator (RTDS) aiming at the conditions of different fault positions, different fault types and the like shown in FIG. 2, all simulation results are given in the following Table 1, and the cosine similarity coefficient variation range in the Table 1 refers to the variation range of the cosine similarity coefficient from the starting moment of the fault to within 20ms after the fault; table 2 shows the behavior of the protection in the case of a short circuit at the T-junction via a transition resistance. The "cosine similarity calculated value" in table 2 is the cosine similarity calculated using all sample values within one data window length after the fault; table 3 shows the calculated cosine similarity of the fault at the T-junction when the T-junction of the new energy station with different capacity is sent out. The "cosine similarity calculated value" in table 3 is the cosine similarity calculated using all sample values within one data window length after the fault.
TABLE 1
TABLE 2
TABLE 3
The simulation result shows that the protection can reliably and quickly identify various types of internal and external faults, the calculated value of the protection cosine similarity after the internal fault is 10-15 ms greater than the protection fixed value of-0.9 and tends to be stable, and the protection speed is good; the protection is not influenced by the capacity and the operation mode of the new energy station, and higher sensitivity can be ensured under various conditions; under the condition of short circuit through the transition resistor, the protection still has reliability and can endure high-resistance faults.
It is noted that those skilled in the art will recognize that embodiments of the present invention are not described in detail herein.
In conclusion, the method of the embodiment of the invention avoids the situation that the sensitivity of the traditional protection is reduced or even the protection is refused to operate when the fault is in the area, and meanwhile, the protection can reliably operate when the fault is sent out from the line area, and the full-line quick operation can be realized; the method is not influenced by the capacity and the running mode of the new energy power supply and is not influenced by a control strategy.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (1)
1. A method for multi-terminal pilot protection of a new energy station based on cosine similarity is characterized by comprising the following steps:
step 1, aiming at a new energy station which is connected with the grid through a power electronic converter, combining a common control strategy for inhibiting negative sequence current to obtain a transient short-circuit current analytic expression of the fault of the new energy station;
in step 1, the obtained analytic formula of the transient current under dq axis is:
wherein i d ，i q Respectively representing d-axis current actual values and q-axis current actual values;
respectively representing a d-axis current reference value and a q-axis current reference value; xi is the damping ratio of the second order system, which has a value of
ω n Is the natural oscillation angular frequency of the second order system, and has a value of
k ip 、k ii Respectively is a current control loop proportion and an integral time constant; l is equivalent inductance of the inverter grid-connected topology; omega d Representing a damped oscillation frequency of value
Beta is the damping angle, the value of which is
And further converting the transient state current analytic expression into a new energy station short circuit current analytic expression under an ABC coordinate system through coordinate transformation, wherein the analytic expression is specifically represented as follows:
wherein i φ Short-circuit current of any one of the three phases; theta φ Representing the initial phase angle after the three-phase fault; ω represents the angular frequency detected by the phase locked loop; t is t 0 Is the starting moment of the fault;
step 2, analyzing and obtaining the characteristic that the waveform of a short circuit of the new energy station is a superposition of sine waves attenuated by non-power frequency and non-power frequency sine waves according to the analytic formula of the transient short circuit current of the new energy station, and constructing a multi-end pilot protection criterion of the new energy station based on cosine similarity;
in step 2, the process of constructing the new energy station multi-terminal pilot protection criterion based on the cosine similarity specifically includes:
combining with the T-connection type sending-out line topology of the new energy station, correspondingly adding the current sampling values of all the T-connection new energy stations with clock synchronization within 10ms window length, and recording the operation result as a station side and current vector
In the formula, N represents that N new energy stations are accessed on a T-connection type sending-out line; i.e. i wk A current sampling value vector of clock synchronization within 10ms window length of the kth new energy station; n is the total number of all sampling points within a 10ms window length;
the current sampling value of clock synchronization within the window length of 10ms on the synchronous system side is taken as the current vector on the system side, and is expressed as: i.e. i s ＝(i s1 ,i s2 ,…,i sn )；
And substituting the two current vectors into the following cosine similarity formula for calculation:
in the formula, cos (i) w ,i s ) Calculating cosine similarity representing two current vectors;
when an out-of-range fault occurs, the transient current waveforms of the station side current and the system side current are opposite, and the similarity coefficient cos (i) of the two current vectors is obtained through calculation w ,i s ) 1, is ═ 1; when an intra-area fault occurs, the calculated similarity coefficient is larger than-1; because the neutral point of a main transformer in the wind power station is directly grounded, when a grounding fault occurs in a region, the zero-sequence current in the system is large, so that the calculated value of the cosine similarity is near 1, and the multi-end pilot protection criterion of the new energy station based on the cosine similarity is constructed and expressed as follows:
cos(i wΣ ,i s )＞cos set
in the formula, the subscript phi is any phase in the ABC three phases, and the similarity coefficient is calculated by using the data of the same phase; cos (co) set For protection, the value range is [ -1,1 [ ]]To (c) to (d);
step 3, setting a protection constant value in the multi-terminal pilot protection criterion according to an angle error and an amplitude error when the system normally operates;
in step 3, a formula for setting the protection constant value in the multi-terminal pilot protection criterion is as follows:
wherein K is a reliability coefficient; k ang Is an angle error coefficient; k amp Is an amplitude error coefficient; k mar Is a margin coefficient; cos (chemical oxygen demand) set The values are fixed for protection.
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