Current identification method and system
By linearly unfolding the instantaneous short-circuit current and combining iterative calculations, the problem of short-circuit current peak and zero-crossing point prediction is solved, and the short-circuit current is accurately predicted, which improves the interruption life of the fault current limiter and the grid safety.
Patent Information
- Application Number
- CN202510463734.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-14
- Publication Date
- 2025-08-08
AI Technical Summary
There is a lack of effective methods in the prior art to predict short-circuit current peaks and zero crossings, resulting in the number of interruptions and current magnitude of fast switching fault current limiters that cannot be accurately controlled, affecting equipment life and grid safety.
By linearizing the DC component of the instantaneous short-circuit current, using matrix representation, combining iterative calculations and real-time data updates, predicting the short-circuit current peak and zero crossing point, data is collected using current transformers and voltage transformers, and calculations are performed through the data processing system.
Accurate prediction of short-circuit current peak and zero crossing point is achieved, the calculation amount is reduced, and the breaking life and grid safety of fast switching fault current limiters are improved.
Smart Images
Figure CN120446566A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates specifically to the field of current identification methods, and in particular to a current identification method and system. Background Art
[0002] Economic development has placed higher demands on power supply reliability. With increasingly complex grid structures and an increasing number and capacity of power sources, excessive short-circuit currents have become a new challenge for power systems. Excessive short-circuit currents can damage power equipment such as transformers and busbars due to dynamic and thermal instability. Furthermore, circuit breakers are unable to interrupt the fault current, causing the fault to spread and seriously threatening grid operation safety. Inserting a fault current limiter (FCL) in series close to the power source is one effective way to address this issue. During a short circuit, the FCL acts as an impedance; during normal operation, it acts as a conductor. During a short circuit, the FCL activates before the circuit breaker, limiting the fault current and ensuring reliable circuit breaker operation.
[0003] Fast-switch fault current limiters are a common current limiter solution used in engineering applications. Their topology features an inductive parallel fast switch. During normal operation, the fast switch is closed. During a short-circuit fault, the fast switch opens, subjecting the fast switch contacts to the arcing current of the fault current. A fast switch has a breaking life of only a few dozen times at its rated fault current, but the lower the breaking current, the greater the number of times it can break. Therefore, it is necessary to reduce the number of fault currents it can break and the current magnitude at the time of breaking. During a short-circuit fault, the peak short-circuit current is predicted. If the peak short-circuit current does not exceed the specified value, the fast switch is locked and does not operate. It only opens when the peak short-circuit current exceeds the specified value, effectively reducing the number of times the fast switch breaks and improving its breaking life. The fast switch opens before the first zero-crossing of the short-circuit current. If the time of the first zero-crossing of the short-circuit current is predicted, the fast switch begins shortly before this zero-crossing, subjecting the fast switch to a very small arcing current, effectively extending its breaking life.
[0004] Currently, there is a lack of practical methods for predicting short-circuit current peak value and zero-crossing point. How to provide a current identification method and system to solve the short-circuit current peak value and zero-crossing point is a technical problem that needs to be solved urgently. Summary of the Invention
[0005] The object of the present invention is to provide a current identification method and system to solve the problems raised in the above background technology.
[0006] To achieve the above object, the present invention provides the following technical solutions:
[0007] A current identification method comprises the following steps:
[0008] S101. Linearize the DC component of the instantaneous short-circuit current:
[0009]
[0010] Where: I dc e -t / τ is the DC component of the instantaneous short-circuit current, τ is the time constant;
[0011] S102. Use matrix to represent instantaneous short-circuit current i t :
[0012] i t =H t X;
[0013] H t =[sinωt cosωt 1-t];
[0014] X=[I ac *cosφI ac *sinφI dc I dc / τ] T ;
[0015] S103, calculating the estimated data of X at time t=0;
[0016] S104. Based on the estimated data of X at time t-1, predict the data of X at time t:
[0017] X t|t-1 =X t-1|t-1 ;
[0018] P t|t-1 =P t-1|t-1 +Q t ;
[0019] Where: X t|t-1 is the state prediction of X at time t, X t-1|t-1 is the state estimate of X at time t-1; P t|t-1 is the variance matrix of the predicted state at time t, P t-1|t-1 is the variance matrix of the estimated state at time t-1, Q t is the variance matrix of the predicted state error;
[0020] S105, using instantaneous short-circuit current i t The measured data of time t is used to update the predicted data of X to the estimated data of X;
[0021]
[0022] X t|t =X t|t-1 +K t (i t +v t-H t X t|t-1 );
[0023] P t|t =(IK t H t )P t|t-1 ;
[0024] Where: R t is the instantaneous short-circuit current i t Variance matrix of measurement error; v t is the instantaneous short-circuit current i t The measurement error, whose variance is R t ;
[0025] S106, calculating the variance D of the estimated values of X at at least two adjacent moments. If the variance D is less than a preset required value, then the calculation of the estimated value of X is terminated; if the variance D is greater than or equal to the preset required value, then executing step S104;
[0026] S107. Calculate the first peak value of the fault current and the first zero-crossing point of the fault current according to the estimated value of X.
[0027] As a further solution of the present invention: in step S101, the instantaneous short-circuit current is:
[0028] i t =I ac *sinωt*cosφ+I ac *cosωt*sinφ+I dc e -t / τ ;
[0029] i t is the short-circuit current value at time t, I ac is the amplitude of the AC component of the short-circuit current, ω is the angular frequency, φ is the current phase angle at the initial moment of the fault, I dc is the DC component value of the short-circuit current at the initial moment of the fault.
[0030] As a further solution of the present invention: in step S103, X ac The solution method includes the following steps:
[0031] S201, calculate the instantaneous short-circuit current i t The derivative i′ t ,
[0032]
[0033] S202, calculate the instantaneous short-circuit current i t The second-order derivative i″ t ,
[0034]
[0035] S203, Solution I ac :
[0036]
[0037] As a further solution of the present invention: in step S103, the method for solving φ in X includes the following steps:
[0038] S301. Before the fault occurs, record the current peak time as t peak ;
[0039] S302: The time when the fault current overcurrent is detected is t over ;
[0040] S303, according to the current peak time t peak and the moment when the fault current flows over t over Calculated The calculation formula is as follows:
[0041]
[0042] Among them, φ peak The time t is the peak current moment peak Phase angle at time t, positive peak time t peak The corresponding φ peak for Negative peak time t peak The corresponding φ peak for
[0043] As a further solution of the present invention: In step S103, the solution method for τ in X is: before the fault occurs, record the voltage peak time as t Upeak and time t Upeak The current peak time in the next cycle is t peak , according to the voltage peak time t Upeak and the current peak time t peak Calculate τ using the following formula:
[0044]
[0045] As a further solution of the present invention: In step S103, the I in the matrix X dc The solution is: according to the instantaneous measurement value i0 of the short-circuit current at t = 0 and I ac ,φ,τ to get I dc , the calculation formula is as follows:
[0046]
[0047] As a further solution of the present invention: In step S107, the method for solving the first peak value of the fault current is: for the instantaneous short-circuit current i t Take the derivative and let the instantaneous short-circuit current derivative i t '=0, find the t value, the minimum t value is the peak moment, substitute the minimum t value into the instantaneous short-circuit current i t The peak value of the fault current is calculated using the calculation formula.
[0048] As a further solution of the present invention: In step S107, the method for solving the first zero-crossing point of the fault current is: let the instantaneous short-circuit current i t =0, find the t value, the smallest t value greater than zero is the first zero crossing point.
[0049] A current identification system includes a current transformer, a voltage transformer, and a data processing system, wherein:
[0050] The current transformer and voltage transformer are used to collect the current and voltage values of the circuit respectively;
[0051] The data processing system includes a processor, a data acquisition unit, a data storage unit, a data calculation unit and a communication unit, wherein:
[0052] The data acquisition unit is used to collect the current and voltage data of the current transformer and the voltage transformer, and forward it to the processor, which then sends the current and voltage data to the data storage unit for storage;
[0053] The data storage unit is also used to store temporary data during the calculation process;
[0054] The data calculation unit calculates the required data and obtains it from the data storage unit through the processor; the calculation results of the short-circuit current peak value and the zero-crossing point are sent by the processor to the control and protection system through the communication unit.
[0055] Compared with the prior art, the present invention has the following beneficial effects:
[0056] 1. The present invention linearizes the DC component of the instantaneous short-circuit current, which is beneficial to reducing the amount of calculation;
[0057] 2. The present invention iterates in sequence according to the real-time measurement value of the instantaneous short-circuit current at each moment, calculates and obtains the amplitude of the short-circuit current AC component, the current phase angle at the initial moment of the fault, the short-circuit current DC component value and the time constant in the instantaneous short-circuit current formula, and thus obtains the short-circuit current peak value and zero-crossing point according to the instantaneous short-circuit current formula. This method has accurate prediction and small calculation amount and can be used in actual engineering applications.
[0058] 3. In the present invention, the estimated data of X at time t=0 is obtained by relatively accurate calculation rather than manual random assignment, which can effectively reduce the number of iterations and improve the efficiency of iterative calculation; the amplitude of the AC component of the short-circuit current I ac According to the instantaneous short-circuit current i t The derivative i′ t , derivative i′ t The derivative i″ t The calculation method is simple, and the minimum three sampling points can be used to calculate I ac , the calculation efficiency is high; the current phase angle φ and time constant τ at the initial moment of the fault can be obtained through only two times, and the calculation method is simple.
[0059] 4. In the present invention, when the variance matrix P_(0|0) of the estimated state at time t=0 is manually assigned a large value, it can speed up the iterative convergence. t The iterative process is adjusted in real time, taking the largest value first and then gradually decreasing it as the number of iterations increases, which is beneficial to improving the convergence efficiency and accuracy.
[0060] 5. The variance matrix P of the estimated state in the present invention t-1|t-1 , the variance matrix P of the predicted state t|t-1 , K t Offline calculations are performed in advance and stored in the data storage unit for use during short-circuit fault calculations, significantly reducing the amount of calculations. BRIEF DESCRIPTION OF THE DRAWINGS
[0061] Figure 1 Flowchart of the current identification method.
[0062] Figure 2 In the current identification method step S103, I in X ac Flowchart of the solution method.
[0063] Figure 3 This is the system topology diagram of the current identification system. DETAILED DESCRIPTION
[0064] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.
[0065] See also Figure 1-Figure 2 , the present invention proposes a current identification method, comprising the following steps:
[0066] S101, the instantaneous short-circuit current i t =Iac *sinωt*cosφ+I ac *cosωt*sinφ+I dc e -t / τ The DC component of is linearized:
[0067]
[0068] Where: I dc e -t / τ is the DC component of the instantaneous short-circuit current, τ is the time constant, i t is the short-circuit current value at time t, I ac is the amplitude of the AC component of the short-circuit current, ω is the angular frequency, φ is the current phase angle at the initial moment of the fault, I dc is the DC component value of the short-circuit current at the initial moment of the fault. This embodiment reduces the amount of calculation by linearizing the DC component of the instantaneous short-circuit current. It should be noted that the iteration time in this embodiment generally converges within a quarter cycle. Within the first quarter cycle of the short-circuit current, the DC component can be approximately equivalent to linear, which has little effect on the system calculation accuracy.
[0069] S102. Use matrix to represent instantaneous short-circuit current i t :
[0070] i t =H t X;
[0071] H t =[sinωt cosωt 1-t];
[0072] X=[I ac *cosφI ac *sinφI dc I dc / τ] T ;
[0073] S103, calculating the estimated data of X at time t=0. The estimated data of X at time t=0 is obtained through relatively accurate calculation rather than manual random assignment, which can effectively reduce the number of iterations and improve iterative calculation efficiency.
[0074] See also Figure 2 In step S103 of the embodiment of the present application, X ac The solution method includes the following steps:
[0075] S201, calculate the instantaneous short-circuit current i t The derivative i′ t ,
[0076]
[0077] S202, calculate the instantaneous short-circuit current i t The second-order derivative i″ t ,
[0078]
[0079] S203, i′ t andi″ t The AC component is much larger than the DC component, so the DC component is ignored because the DC component is the AC component I ac *10%-40% of sin(ωt+φ), τ is generally 0.2-0.5, for power frequency system, ω is 314, so i′ t andi″ t The AC component is much larger than the DC component, and the solution is I ac , the amplitude of the AC component of the short-circuit current I ac According to the instantaneous short-circuit current i t The derivative i′ t , derivative i′ t The derivative i″ t The calculation method is simple, and the minimum three sampling points can be used to calculate I ac , with high computational efficiency.
[0080]
[0081] Furthermore, in step S103 of the embodiment of the present application, the method for solving φ in X includes the following steps:
[0082] S301. Before the fault occurs, record the current peak time as t peak , after each new current peak moment time record is made, the previous current peak moment time record is cleared;
[0083] S302: The time when the fault current overcurrent is detected is t over ;
[0084] S303, according to the current peak time t peak and the moment when the fault current flows over t over Calculate φ using the following formula:
[0085]
[0086] where φ peak The time t is the peak current moment peak Phase angle at time t, positive peak time t peak The corresponding φ peak for Negative peak time tpeak The corresponding φ peak for
[0087] In step S103 of the embodiment of the present application, the method for solving τ in X is: before the fault occurs, record the voltage peak time as t Upeak and time t Upeak The current peak time in the next cycle is t peak , according to the voltage peak time t Upeak and the current peak time t peak Calculate τ using the following formula:
[0088]
[0089] In step S103 of the embodiment of the present application, I in the matrix X dc The solution is:
[0090] According to the instantaneous measurement value i0 of the short-circuit current at t=0 and the preliminary calculated I ac ,φ,τ to get I dc The calculation formula is as follows:
[0091]
[0092] S104. Based on the estimated data of X at time t-1, predict the data of X at time t:
[0093] X t|t-1 =X t-1|t-1 ;
[0094] P t|t-1 =P t-1|t-1 +Q t ;
[0095] Where: X t|t-1 is the state prediction of X at time t, X t-1|t-1 is the state estimate of X at time t-1; P t|t-1 is the variance matrix of the predicted state at time t, P t-1|t-1 is the variance matrix of the estimated state at time t-1, Q t is the variance matrix of the predicted state error; the variance matrix P of the estimated state at time t = 0 0|0 The values are assigned manually. In this embodiment, the current phase angle φ and the time constant τ at the initial moment of the fault can be obtained through only two times, and the calculation method is simple.
[0096] S105, using instantaneous short-circuit current i t The measured data of time t is used to update the predicted data of X to the estimated data of X;
[0097]
[0098] X t|t =X t|t-1 +K t (i t +v t -H t X t|t-1 );
[0099] P t|t =(IK t H t )P t|t-1 ;
[0100] Where: R t is the instantaneous short-circuit current i t Variance matrix of measurement error; v t is the instantaneous short-circuit current i t The measurement error, whose variance is R t .
[0101] S106, calculating the variance D of the estimated values of X at at least two adjacent moments. If the variance D is less than a preset required value, then the calculation of the estimated value of X is terminated; if the variance D is greater than or equal to the preset required value, then executing step S104;
[0102] S107. Calculate the first peak value of the fault current and the first zero-crossing point of the fault current according to the estimated value of X. The method for calculating the first peak value of the fault current is:
[0103] For instantaneous short-circuit current i t Take the derivative and let the instantaneous short-circuit current derivative i t '=0, find the t value, the minimum t value is the peak moment, substitute the minimum t value into the instantaneous short-circuit current i t Calculation formula, calculate the fault current peak value;
[0104] In this embodiment, the instantaneous short-circuit current is i t =I ac *sinωt*cosφ+I ac *cosωt*sinφ+I dc e -t / τ ,i t is the short-circuit current value at time t, I ac is the amplitude of the AC component of the short-circuit current, ω is the angular frequency, φ is the current phase angle at the initial moment of the fault, I dc is the DC component of the short-circuit current at the initial moment of the fault; let the instantaneous short-circuit current derivative i t '=0, we get:
[0105]
[0106] Furthermore, in step S107 of this embodiment, the method for solving the first zero-crossing point of the fault current is: let the instantaneous short-circuit current i t =0, find the t value, the smallest t value greater than zero is the first zero crossing point,
[0107]
[0108] It should be noted that, in this embodiment, the variance matrix P of the estimated state t-1|t-1 , the variance matrix Q of the state error t , instantaneous short-circuit current i t The variance matrix R of the measurement error t is a diagonal matrix; the variance matrix P_(0|0) of the estimated state at time t=0 is artificially assigned a large value to speed up the iterative convergence; Q t The iterative process is adjusted in real time, taking the largest value first and then gradually decreasing it as the number of iterations increases; R t and the instantaneous short-circuit current i t The measurement error v t The value is determined by the accuracy of the current transformer.
[0109] like Figure 3 As shown, the present invention also proposes a system corresponding to a current identification method, comprising a current transformer (1), a voltage transformer (2) and a data processing system (3), wherein:
[0110] The current transformer (1) and the voltage transformer (2) respectively collect the current and voltage values of the circuit;
[0111] The data processing system (3) includes a processor (31), a data acquisition unit (32), a data storage unit (33), a data calculation unit (34), and a communication unit (35), wherein:
[0112] The data acquisition unit (32) is used to collect current and voltage data of the current transformer (1) and the voltage transformer (2), and forward the data to the processor (31), and then the processor (31) sends the current and voltage data to the data storage unit (33) for storage;
[0113] The data storage unit (33) is also used to store temporary data during the calculation process. The temporary data is temporary data obtained during the calculation process of the data calculation unit (34) and is sent to the data storage unit (33) for storage through the processor (31); the data required for the calculation by the data calculation unit (34) is obtained from the data storage unit (33) through the processor (31); and the calculation results of the short-circuit current peak value and the zero-crossing point are sent by the processor (31) to the control and protection system through the communication unit (35).
[0114] It should be noted that, in the embodiment of the present application, the variance matrix P of the estimated state t-1|t-1 , the variance matrix P of the predicted state t|t-1 , K t Offline calculations are performed in advance and stored in a data storage unit (33) for calculation and recall, thereby reducing the amount of calculation when a short circuit fault occurs.
[0115] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.
[0116] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.
Claims
1. A current identification method, characterized in that: The following steps are involved: S101. Linearize the DC component of the instantaneous short-circuit current: Where: I dc e -t / τ is the DC component of the instantaneous short-circuit current, τ is the time constant; S102. Use matrix to represent instantaneous short-circuit current i t : i t =H t X; H t =[sinωt cosωt 1-t]; X=[I ac *cosφ I ac *sinφ I dc I dc / τ] T ; S103, calculating the estimated data of X at time t=0; S104. Based on the estimated data of X at time t-1, predict the data of X at time t: X t|t-1 =X t-1|t-1 ; P t|t-1 =P t-1|t-1 +Q t ; Where: X t|t-1 is the state prediction of X at time t, X t-1|t-1 is the state estimate of X at time t-1; P t|t-1 is the variance matrix of the predicted state at time t, P t-1|t-1 is the variance matrix of the estimated state at time t-1, Q t is the variance matrix of the predicted state error; S105, using instantaneous short-circuit current i t The measured data of time t is used to update the predicted data of X to the estimated data of X; X t|t =X t|t-1 +K t (i t +v t -H t X t|t-1 ); P t|t =(I-K t H t )P t|t-1 ; Where: R t is the instantaneous short-circuit current i t Variance matrix of measurement error; v t is the instantaneous short-circuit current i t The measurement error, whose variance is R t ; S106, calculating the variance D of the estimated values of X at at least two adjacent moments. If the variance D is less than a preset required value, then ending the calculation of the estimated value of X; If the variance D is greater than or equal to the preset required value, step S104 is executed; S107. Calculate the first peak value of the fault current and the first zero-crossing point of the fault current according to the estimated value of X.
2. The current identification method according to claim 1, characterized in that: In step S101, the instantaneous short-circuit current is: i t =I ac *sinωt*cosφ+I ac *cosωt*sinφ+I dc e -t / τ ; i t is the short-circuit current value at time t, I ac is the amplitude of the AC component of the short-circuit current, ω is the angular frequency, φ is the current phase angle at the initial moment of the fault, I dc is the DC component value of the short-circuit current at the initial moment of the fault.
3. The current identification method according to claim 1, characterized in that: In step S103, X in I ac The solution method includes the following steps: S201, calculate the instantaneous short-circuit current i t The derivative i′ t , S202, calculate the instantaneous short-circuit current i t The second-order derivative i″ t , S203, Solution I ac :
4. The current identification method according to claim 3, characterized in that: In step S103, the method for solving φ in X includes the following steps: S301. Before the fault occurs, record the current peak time as t peak ; S302: The time when the fault current overcurrent is detected is t over ; S303, according to the current peak time t peak and the moment when the fault current flows over t over Calculate φ using the following formula: Among them, φ peak The time t is the peak current moment peak Phase angle at time t, positive peak time t peak The corresponding φ peak for Negative peak time t peak The corresponding φ peak for 5. The current identification method according to claim 4, characterized in that: In step S103, the method for solving τ in X is: before the fault occurs, record the voltage peak time as t Upeak and time t Upeak The current peak time in the next cycle is t peak , according to the voltage peak time t Upeak and the current peak time t peak Calculate τ using the following formula:
6. The current identification method according to claim 5, characterized in that: In step S103, I in matrix X dc The solution is: according to the instantaneous measurement value i0 of the short-circuit current at t = 0 and I ac ,φ,τ to get I dc , the calculation formula is as follows:
7. The current identification method according to claim 1, characterized in that: In step S107, the method for solving the first peak value of the fault current is: t Take the derivative and let the instantaneous short-circuit current derivative i t '=0, find the t value, the minimum t value is the peak moment, substitute the minimum t value into the instantaneous short-circuit current i t The peak value of the fault current is calculated using the calculation formula.
8. The current identification method according to claim 1, characterized in that: In step S107, the method for solving the first zero-crossing point of the fault current is: let the instantaneous short-circuit current i t =0, find the t value, the smallest t value greater than zero is the first zero crossing point.
9. A current identification system, characterized in that: The invention comprises a current transformer (1), a voltage transformer (2) and a data processing system (3), wherein: The current transformer (1) and the voltage transformer (2) are used to collect the current and voltage values of the circuit respectively; The data processing system (3) includes a processor (31), a data acquisition unit (32), a data storage unit (33), a data calculation unit (34) and a communication unit (35), wherein: The data acquisition unit (32) is used to collect current and voltage data of the current transformer (1) and the voltage transformer (2), and forward the data to the processor (31), and then the processor (31) sends the current and voltage data to the data storage unit (33) for storage; The data storage unit (33) is also used to store temporary data during the calculation process; The data calculation unit (34) calculates the required data and obtains it from the data storage unit (33) through the processor (31); The calculation results of the short-circuit current peak value and the zero-crossing point are sent by the processor (31) to the control and protection system through the communication unit (35).