CN113315104B

CN113315104B - Method for reducing differential protection misoperation

Info

Publication number: CN113315104B
Application number: CN202110792380.7A
Authority: CN
Inventors: 薛占钰; 邢进春; 杨贤; 丁同同
Original assignee: Baoding Yuxin Electrical Technology Co ltd
Current assignee: Baoding Yuxin Electrical Technology Co ltd
Priority date: 2021-07-14
Filing date: 2021-07-14
Publication date: 2022-05-17
Anticipated expiration: 2041-07-14
Also published as: CN113315104A

Abstract

The invention discloses a method for reducing differential protection misoperation, which is characterized in that a current transformer is arranged at each port of an electric network, the current quantity is converted into pulses through a voltage-frequency conversion circuit, the generation sum of the number of the pulses is monitored to judge whether line faults occur, and in order to reduce the misoperation, the pulse number transmitted by each line is monitored and compared with a preset value within preset time to judge whether line disconnection or positive saturation or negative saturation of the current transformer occurs. The method can avoid the false operation caused by the disconnection of the optical fiber or the saturation of the current transformer, and improve the accuracy and the safety of the operation of the differential protection.

Description

Method for reducing differential protection misoperation

Technical Field

The invention relates to the field of power line protection, in particular to a method for reducing differential protection misoperation.

Background

The invention discloses a protection method of a power line, which comprises a trunk line and branch lines, wherein an electric energy inlet switch A and a switch B adjacent to the switch A are arranged on the trunk line, a plurality of branch lines are arranged between the switch A and the switch B, a switch F is arranged on the branch lines, the current quantities passing through the switch A, the switch B and the switch F are respectively converted into pulse quantities with corresponding relations, the pulse quantities are directly transmitted to a conversion calculation unit point to point through a communication medium, the conversion calculation unit converts and calculates the pulse quantities, whether the sum of the current quantities passing through the switch A and the current quantities passing through the switch B and the switch F is equal or not is judged by calculating the pulse quantities, and the switch A is cut off if the sum of the current quantities is not equal and exceeds a threshold value, so that the protection of the line is realized. However, in the operation process of the power system, a communication medium such as an optical fiber for transmitting pulses is sometimes disconnected, or a current transformer for acquiring the current amount is saturated, so that when a fault such as an interphase short circuit does not occur in a line, the sum of the current amounts is still unequal to cut off the switch a, and at this time, a malfunction occurs, which affects the operation reliability of the power system.

Disclosure of Invention

The invention aims to provide a method for reducing differential protection misoperation, which can avoid misoperation caused by fiber disconnection or current transformer saturation and improve the accuracy and safety of differential protection operation.

In order to achieve the purpose, the invention adopts the following technical scheme:

a method for reducing differential protection misoperation is characterized in that a current transformer is used for collecting the current quantity of each port of an electric network, the current quantity is converted into pulses through a voltage-frequency conversion circuit and is transmitted to a comparison unit, the comparison unit takes the algebraic sum of the number of the pulses, the number of the pulses is positive when the installation direction of the current transformer is the same as the direction of the port pointing to the electric network, the number of the pulses is negative when the installation direction of the current transformer is opposite to the direction of the port pointing to the electric network, or the number of the pulses is negative when the installation direction of the current transformer is the same as the direction of the port pointing to the electric network, the number of the pulses is positive when the installation direction of the current transformer is opposite to the direction of the port pointing to the electric network, and a first alarm signal is sent when the algebraic sum exceeds a threshold value, the fault detection mechanism detects the pulse number transmitted to the comparison unit by each pulse transmission line, when the pulse number is smaller than a first preset value in a first preset time period, the pulse transmission line is judged to be disconnected, when the pulse number is larger than a second preset value in a second preset time period, the current transformer is judged to reach positive saturation, when the pulse number is smaller than a third preset value in a third preset time period, the current transformer is judged to reach negative saturation, and the fault detection mechanism judges that a pulse line is disconnected or the current transformer reaches the positive saturation or the negative saturation and then sends a second alarm signal.

Preferably, the fault detection mechanism is implemented by a detection module, and the detection module receives the pulses transmitted by the pulse transmission line, calculates the number of pulses, and compares the number of pulses with the first preset value, the second preset value, and the third preset value.

Preferably, when the pulse transmission line is an optical fiber line, the current amount is first converted into an electrical pulse through a voltage-to-frequency conversion circuit, the electrical pulse is converted into an optical pulse through a first photoelectric conversion device, then the optical pulse is transmitted to a second photoelectric conversion device through the optical fiber line, and the second photoelectric conversion device converts the optical pulse into an electrical pulse again and transmits the electrical pulse to the comparison unit and the detection module; or the current magnitude is converted into electric pulses through the voltage-frequency conversion circuit and directly transmitted to the comparison unit and the detection module.

Preferably, the second preset timeSegment is Deltat₂N detection modules are arranged aiming at any one pulse transmission line, and the N detection modules are sequentially spaced at a delta t interval₂The method comprises the following steps of performing delta t on pulses transmitted by the pulse transmission line₂Counting time, namely judging that the current transformer reaches positive saturation when the number of pulses counted by any one of the N detection modules exceeds the second preset value; the third preset time period is Deltat₃M detection modules are arranged aiming at any pulse transmission line, and the M detection modules are sequentially spaced at a delta t interval₃The M carries out delta t on the pulse transmitted by the pulse transmission line₃And counting time, namely judging that the current transformer reaches negative saturation when the pulse number calculated by any one of the M detection modules is smaller than the third preset value, wherein N, M are integers larger than 1.

Preferably, said Δ t₂=△t₃And N = M, only one group of N detection modules is set, and when the number of pulses counted by any one of the detection modules exceeds the second preset value, it is determined that the current transformer reaches positive saturation, and when the number of pulses counted by any one of the detection modules is smaller than the third preset value, it is determined that the current transformer reaches negative saturation.

Preferably, twice of the second preset time period is less than or equal to the time for positive saturation of the current transformer, and twice of the third preset time period is less than or equal to the time for negative saturation of the current transformer.

The invention has the advantages that the disconnection of the pulse transmission line and the saturation of the current transformer are detected by additionally arranging the fault detection mechanism, and a specific algorithm of the fault detection mechanism is given, so that the algebraic sum of pulses of each port can exceed the threshold value due to the disconnection of the pulse transmission line and the saturation of the current transformer, but the algebraic sum of pulses of each port can be distinguished from the algebraic sum of pulses of each port exceeding the threshold value due to interphase short-circuit faults and the like through the second alarm signal, thereby effectively reducing the occurrence of misoperation and improving the safety of system operation and the accuracy of protection. Meanwhile, the scheme provides the relation between the installation direction of the current transformer and the positive and negative of the pulse number, can be better suitable for a multi-electric-energy inlet or random power flow electric network, and simplifies the calculation relation of the pulse number. The invention mainly collects the pulse quantity signal and utilizes the characteristics of the pulse quantity signal to identify the hardware fault in the whole protection component in time, thereby improving the accuracy and reliability of the protection operation. By arranging a plurality of detection modules on each line and performing detection and calculation within a preset time period, the judgment sensitivity can be improved.

Drawings

FIG. 1 is a schematic diagram of an embodiment of the method of the present invention;

FIG. 2 is a diagram illustrating the relationship between a plurality of detection modules and a comparison unit.

Detailed Description

The invention will be further illustrated by the following specific embodiments in conjunction with the accompanying drawings:

in an embodiment of the invention, the electrical network has four ports, port a, port B, port F1 and port F2. The current quantity passing through the port A is converted into weak current voltage signals at the port A through the current transformer 1, then the weak current voltage signals are directly converted into electric pulses with frequencies in direct proportion to the input voltage quantity through the first voltage-frequency conversion circuit VFC1, and then the electric pulses are directly transmitted to the detection module 5 and the comparison unit 12 in real time through a cable or a circuit on a circuit board. Similarly, the current quantities at the port B, the port F1 and the port F2 are converted into weak current voltage quantities through the current transformer 2, the current transformer 3 and the current transformer 4, and then are output to the second voltage-to-frequency conversion circuit VFC2, the third voltage-to-frequency conversion circuit VFC3 and the fourth voltage-to-frequency conversion circuit VFC4, respectively, the corresponding current quantities are first converted into electric pulses with frequencies in a proportional correspondence relationship, then the electric pulses are converted into optical pulses through the first photoelectric conversion device, and are directly transmitted to the second photoelectric conversion device at one side of the comparison unit 12 point to point through the optical fiber line, and then the electric pulses are converted into corresponding electric pulses by the second photoelectric conversion device, and are transmitted to the

respective detection modules

6, 7, 8 and the comparison unit 12. The comparing unit 12 is provided with corresponding counters for receiving the pulses corresponding to the port a, the port B, the port F1 and the port F2, and converting the pulses into digital quantities representing the number of the pulses. The digital values are sent to an adder for operation, the algebraic sum of the pulse numbers of the port A, the port B, the port F1 and the port F2 is calculated, and if the algebraic sum exceeds a threshold value, a first alarm signal is sent out. The algebraic sum is calculated as follows: it is specified that the installation direction of the

current transformers

1, 2, 3, 4 is the same as the direction in which the respective ports point to the electrical network (i.e. the electrical network surrounded by port a, port B, port F1 and port F2), the pulse number is positive and the pulse number is negative if the direction is opposite (or that the installation direction of the current transformers is the same as the direction in which the respective ports point to the electrical network, the pulse number is negative and the pulse number is positive if the direction is opposite). Therefore, in any time period, the signs of the number of the pulses generated by the port where the electric energy flows in and the number of the pulses generated by the port where the electric energy flows out are opposite, the electric energy flowing in and the electric energy flowing out of the electric network are equal, the algebraic sum of the pulse numbers is zero, a threshold value can be set according to the required accuracy in consideration of the noise influence, the algebraic sum exceeds the threshold value, then faults such as interphase short circuit, single-phase grounding and the like occur in the electric network, at the moment, a first alarm signal is sent out, the electric power system can perform corresponding operation according to the first alarm signal, such as tripping and the like, at the moment, the normal processing is performed, and the false action is not performed.

However, if the cable transmitting the electrical pulse, or the optical fiber line transmitting the optical pulse is disconnected, or the current transformer is saturated (that is, the signal generated by the electromagnetic induction exceeds the positive range or the negative range of the current transformer), the algebraic sum of the pulse numbers corresponding to the port a, the port B, the port F1 and the port F2 is no longer zero, and if the algebraic sum is not recognized, the trip is still performed according to the first alarm signal, and a false operation occurs because the line does not actually have a fault at this time.

In order to avoid the above false actions, the

detection modules

5, 6, 7, 8 are used to detect the pulses sent by the respective pulse transmission lines, and compare the pulses with the preset values. If a certain line is disconnected, no pulse is transmitted on the line, the number of pulses detected by the detection module is always lower than a first preset value in a first preset time period (the first preset value is generated by noise, and a person skilled in the art can set the first preset time period and the first preset value according to actual needs, see the following example), and then the disconnection of the pulse transmission line is determined. If a certain current transformer is saturated, the detection module on the path will detect that the pulse number is maximum in a second preset time period, that is, greater than a second preset value (the second preset time period and the second preset value can be set by those skilled in the art according to actual situations, see the following example). If a certain current transformer is negatively saturated, the number of pulses in a third preset time period is smaller than a third preset value (the third preset time period and the third preset value can be set by those skilled in the art according to actual needs, see the following example). When the detection module detects the condition, the second alarm signal is sent to the power system, the power system comprehensively makes judgment, and at least the fault detection mechanism can not generate misoperation due to direct tripping caused by the existence of the first alarm signal, so that the occurrence of misoperation is reduced. When the above embodiment is implemented, a dc boost circuit is generally needed to boost ac voltage with positive and negative values obtained by the current transformer to ac voltage with voltage greater than 0, and then the ac voltage is sent to the voltage-to-frequency conversion circuit, for example, as follows:

a voltage-frequency converter with the model of VFC110 is adopted, a 4MHZ local oscillator is adopted, the whole measurement range is-5V to +5V, U = 0-10V is changed through a direct current lifting circuit, the range of the pulse number generated in 1 millisecond ranges from 0 to 4000 corresponding to 0-4 MHZ, the first preset value for evaluating whether a line is disconnected can be 50, if the first preset value can be set in a first preset time period, and if the pulse number in 5 millisecond (or 10 millisecond and the like) is smaller than 50, a pulse transmission line is disconnected. For positive saturation or negative saturation of the current transformer (assuming that the time of the positive saturation or the negative saturation in the specified scenario is 7 milliseconds), the second preset time period and the third preset time period may be both 3 milliseconds, then the second preset value is 4000 × 3-150=12000-150=11850, the third preset value may be set to 50 × 3=150, and 50 is assumed to be zero-shift data of 1 millisecond per unit time.

Further, to improve sensitivity:

setting the second preset time period as Deltat₂N detection modules are arranged aiming at any one pulse transmission line, and the N detection modules are sequentially spaced at a delta t interval₂The method comprises the following steps of performing delta t on pulses transmitted by the pulse transmission line₂Counting time when any of the N detection modules is countingWhen the pulse number exceeds the second preset value, the current transformer is judged to reach positive saturation; setting the third preset time period as Deltat₃M detection modules are arranged aiming at any pulse transmission line and are sequentially spaced at a distance delta t₃The M carries out delta t on the pulse transmitted by the pulse transmission line₃And counting time, namely judging that the current transformer reaches negative saturation when the pulse number calculated by any one of the M detection modules is smaller than the third preset value, wherein N, M are integers larger than 1.

Preferably, in the above scheme, only one group of N detection modules may be provided, so as to make Δ t₂=△t₃And N = M, when any one of the calculated pulse numbers exceeds the second preset value, the current transformer is judged to reach positive saturation, and when the pulse number is smaller than the third preset value, the current transformer is judged to reach negative saturation.

In order to ensure that the second preset time period or the third preset time period of the detection module can be within the positive saturation time or the negative saturation time of the current transformer, two times of the second preset time period are required to be smaller than or equal to the positive saturation time of the current transformer, and two times of the third preset time period are required to be smaller than or equal to the negative saturation time of the current transformer. Because the detection module performs cyclic detection, the above arrangement can ensure that the detection module always has a complete detection period (i.e. the second preset time period or the third preset time period) falling within the saturation time of the current transformer.

For example: under a given working scene, the time of positive saturation or negative saturation of the current transformer is 6 milliseconds, then (6/2) × 18 degrees =54 degrees (the saturation is symmetrical on two sides of 90 degrees, so that 6/2 is used, alternating current changes by 18 degrees in 1 millisecond), 90-54=36 degrees, namely from 36 degrees to 144 degrees, and the angle interval cannot be linearly changed, so that the saturation is achieved. At this time, let Δ t₂=△t₃=3 ms, N = M =3, that is, 3 detection modules are arranged on each line (as shown in fig. 2, each line has a

detection module

51, 52, 53, a

detection module

61, 62, 63, a

detection module

71, 72, 73, and a

detection module

81, 82, 83), and detection is performed at intervals of 1 ms in sequence, then the second preset value is obtainedThe third preset value is 4000 × 3-150=12000-150=11850, and the third preset value can be set to 50 × 3=150, and 50 is assumed to be zero drift data of 1 millisecond per unit time. When the pulse number calculated by one of the three modules in 3 milliseconds is larger than 11850, the current transformer is judged to be positively saturated, or when the pulse number calculated in 3 milliseconds is smaller than 150, the current transformer is judged to be negatively saturated (a voltage-frequency converter with the model of VFC110, a 4MHZ local oscillator and the whole measurement range of-5V to +5V are still adopted, U =0 to 10V is changed through a direct current lifting circuit, and the pulse number generated in 1 millisecond ranges from 0 to 4000 corresponding to 0 to 4 MHZ).

The increased sensitivity is achieved by using a plurality of detection modules because if the current transformer is saturated in the 2 nd millisecond from the beginning of the detection period of the detection module 51, the detection module 51 does not reach the preset value after the detection of one period (3 milliseconds), because only 1 millisecond falls in the monitoring interval when the current transformer is saturated. If no other detection module exists, the detection module 51 is relied on only, the preset value can be reached only after the next 3 milliseconds detection period of the detection module 51 is finished, then the alarm is given, and theoretically, 4 milliseconds are needed for the alarm from the positive saturation of the current transformer. When the detection module 52 is arranged, the detection module 52 starts to detect after 1 millisecond from the detection module 51, theoretically, 5 milliseconds are needed from the occurrence of the positive saturation of the current transformer to the alarm; when the detection module 53 is arranged, the detection module 53 starts to detect 2 milliseconds after the detection module 51 starts to detect, and theoretically, it is enough that 3 milliseconds are needed for the alarm from the occurrence of the normal saturation of the current transformer.

In the above example, if positive saturation occurs in 2.5 ms during the detection process of the detection module 51, the alarm will be issued 3.5 ms after the current transformer saturation occurs by the detection module 51 alone. With the detection module 52 alone, the alarm will be 4.5 milliseconds after the current transformer is saturated. By means of the detection module 53 alone, the alarm will be 5.5 milliseconds after the current transformer is just saturated. If 6 detection modules (N = 6) are set, the time for starting detection will be 3 mm/6 =0.5 ms apart, the fastest alarm response time is 3 ms, the slowest alarm response time is 3.5 ms, and if 30 detection modules (N = 30) are set, the fastest alarm response time is 3 msThe slowest response time is 3.1 milliseconds. Namely, for positive saturation of the current transformer, the fastest alarm response time is delta t theoretically₂The slowest alarm response time is delta t₂+△t₂and/N, the more detection modules are arranged on a certain line, the easier the detection is carried out from the positive saturation period or the negative saturation period of the current transformer, and the alarm time is earlier.

The above is the case of a detection period of 3 ms, but of course, this period may be set to 2 ms, or even 1 ms.

For undetected saturation, for example, a detection period of 2 milliseconds and only 1 millisecond, which is undetected, it is necessary to properly set the threshold for the differential alarm without causing differential malfunction and rejection.

According to the principle and the example, the method can select appropriate parameters according to a preset working scene, and can also design the response accuracy and sensitivity in advance to plan the hardware configuration of the power system, such as planning the positive and negative saturation time of the current transformer or planning the number of detection modules to be set.

The above embodiments are only a few illustrations of the inventive concept and implementation, not limitations thereof, and the technical solutions without substantial changes are still within the scope of protection under the inventive concept.

Claims

1. A method for reducing differential protection misoperation is characterized in that a current transformer is used for collecting the current quantity of each port of an electric network, the current quantity is converted into pulses through a voltage-frequency conversion circuit and is transmitted to a comparison unit, the comparison unit takes the algebraic sum of the number of the pulses, the number of the pulses is positive when the installation direction of the current transformer is the same as the direction of the port pointing to the electric network, the number of the pulses is negative when the installation direction of the current transformer is opposite to the direction of the port pointing to the electric network, or the number of the pulses is negative when the installation direction of the current transformer is the same as the direction of the port pointing to the electric network, the number of the pulses is positive when the installation direction of the current transformer is opposite to the direction of the port pointing to the electric network, and a first alarm signal is sent out when the algebraic sum exceeds a threshold value, the fault detection mechanism detects the pulse number transmitted to the comparison unit by each pulse transmission line, when the pulse number is smaller than a first preset value in a first preset time period, the pulse transmission line is judged to be disconnected, when the pulse number is larger than a second preset value in a second preset time period, the current transformer is judged to reach positive saturation, when the pulse number is smaller than a third preset value in a third preset time period, the current transformer is judged to reach negative saturation, and the fault detection mechanism judges that a pulse line is disconnected or the current transformer reaches the positive saturation or the negative saturation and then sends a second alarm signal.

2. The method of reducing differential protection glitches of claim 1 in which the fault detection mechanism is implemented by a detection module that receives pulses transmitted by the pulse transmission line and calculates a number of pulses, and compares the number of pulses to the first, second and third preset values.

3. The method for reducing differential protection unwanted operation according to claim 2, wherein when the pulse transmission line is an optical fiber line, the current amount is first converted into an electrical pulse by a voltage-to-frequency conversion circuit, the electrical pulse is converted into an optical pulse by a first photoelectric conversion device, and then the optical pulse is transmitted to a second photoelectric conversion device by the optical fiber line, and the second photoelectric conversion device converts the optical pulse into an electrical pulse again and transmits the electrical pulse to the comparison unit and the detection module; or the current magnitude is converted into electric pulses through the voltage-frequency conversion circuit and directly transmitted to the comparison unit and the detection module.

4. The method of reducing differential protection glitches of claim 1 in which the second predetermined time period is at₂N detection modules are arranged aiming at any one pulse transmission line, and the N detection modules are sequentially spaced at a delta t interval₂/N processing the pulses transmitted by the pulse transmission linet₂Counting time, namely judging that the current transformer reaches positive saturation when the number of pulses counted by any one of the N detection modules exceeds the second preset value; the third preset time period is Deltat₃M detection modules are arranged aiming at any pulse transmission line, and the M detection modules are sequentially spaced at a delta t interval₃The M carries out delta t on the pulse transmitted by the pulse transmission line₃And counting time, namely judging that the current transformer reaches negative saturation when the pulse number calculated by any one of the M detection modules is smaller than the third preset value, wherein N, M are integers larger than 1.

5. The method for reducing differential protection glitches of claim 4 in which said Δ t₂=△t₃And N = M, only one group of N detection modules is set, and when the number of pulses counted by any one of the detection modules exceeds the second preset value, it is determined that the current transformer reaches positive saturation, and when the number of pulses counted by any one of the detection modules is smaller than the third preset value, it is determined that the current transformer reaches negative saturation.

6. The method of reducing differential protection glitches of claim 1 in which twice the second predetermined period of time is less than or equal to the time for the current transformer to be positively saturated and twice the third predetermined period of time is less than or equal to the time for the current transformer to be negatively saturated.