An adaptive adjustment method for ratio braking coefficient after CT saturation

Abstract

The present invention belongs to the technical field of current differential protection, and specifically relates to a method for adaptively adjusting the ratio braking coefficient after CT saturation. The single CT and dual CT access information are set in advance according to the configuration mode of the current transformer; whether the CT is saturated is determined by detecting the harmonic content in each CT; after CT saturation occurs, the ratio braking coefficient is adjusted to a reliable range through the current amplitude of each CT, the different positions where the CT saturation occurs, and the configuration mode of the current transformer. This method reduces the dependence of the fault identification criterion inside and outside the zone under CT saturation, is easier to implement, and can also improve the ability of differential protection to resist transient severe saturation of CT. The criterion itself has a higher degree of recognition and wider adaptability.

Description

Technical Field

[0001] The invention belongs to the technical field of current differential protection, and in particular relates to a method for adaptively adjusting a ratio braking coefficient after CT saturation. Background Art

[0002] In the power system, relay protection devices are crucial to ensure the safe and stable operation of the system. When a system fault occurs, relay protection is used to identify the fault area, cut off the faulty equipment, and ensure the safe operation of the system. Longitudinal current differential protection is an excellent protection function, but the saturation of current transformers (CTs) in extreme cases may pose a challenge to the correct operation of relay protection devices. CT saturation is a complex issue, especially when faults occur inside or outside the zone. How to ensure the reliability of differential protection is a topic worthy of in-depth study.

[0003] Affected by the inductive and capacitive elements of the line, a large number of high-frequency components will appear simultaneously when a short-circuit fault occurs. The time difference method, sampling point differential and other methods may be interfered and cannot accurately determine the CT saturation caused by the fault outside the zone. Although the saturation characteristics can be identified by the harmonic content detection method, it is difficult to clearly identify whether the CT saturation is caused by the fault inside the zone or the fault outside the zone, which may cause the differential protection to operate incorrectly.

[0004] In the saturation state, the excitation current of the current transformer increases, which will cause the measurement deviation to become larger and may affect the correct operation of the relay protection, especially the differential protection. In particular, when an out-of-zone fault occurs and causes CT saturation, how to ensure that the differential protection does not operate incorrectly is a problem that needs to be solved. At the same time, when the fault quickly develops from outside the zone to inside the zone, how to ensure that the differential protection can operate in time to avoid protection refusal to operate is also an issue that needs attention.

[0005] In the 3 / 2 wiring form, two sets of switches are installed at the outlet of the transmission line, among which the side switch is connected to the busbar, and the middle switch is shared with the adjacent line or transformer. Both sets of switches are equipped with CTs. At this time, the protection device needs to be connected to two sets of switch currents; while other wiring forms are all single-switch configurations, and the protection device only needs to be connected to one set of switch currents. The impact of CT saturation on differential protection in these two cases is also different. Case 1: In the 3 / 2 wiring mode, when the single CT is saturated due to an out-of-zone fault, the through current between the side switch and the middle switch will amplify the differential current and reduce the braking current, thereby increasing the risk of false protection. Case 2: In the single-side power supply mode, when the fault in the zone causes the CT on the power supply side to be severely saturated, the load side cannot provide sufficient short-circuit current, the differential current amplitude is weakened, and it is easy to cause protection failure.

[0006] In order to solve these problems, it is necessary to study the adjustment strategy of the ratio braking coefficient after CT saturation. Specifically, it is necessary to determine to what extent the braking coefficient should be raised to ensure the reliability of differential protection in different situations, such as single CT saturation caused by out-of-zone faults and severe saturation of CT on the power supply side caused by in-zone faults, etc. Summary of the invention

[0007] In order to overcome the problems in the prior art, the present invention proposes a method for adaptively adjusting the ratio braking coefficient after CT saturation.

[0008] The technical solution of the present invention to solve the above technical problems is as follows:

[0009] The present invention provides a method for adaptively adjusting a ratio braking coefficient after CT saturation, comprising the following steps:

[0010] Step 1. Set the single CT and double CT access information in advance according to the configuration mode of the current transformer;

[0011] Step 2. Check whether the relay protection device is activated. If so, calculate the startup time; detect the current and harmonic content of the CTs on both sides of the transmission line; based on the currents on both sides, determine whether the differential current exceeds the limit; if not, the ratio braking coefficient uses the preset default coefficient K0; if it exceeds the limit, determine whether the CT is saturated based on the content of the 2nd and 3rd harmonics in each CT;

[0012] Step 3. If any CT is saturated, based on the configuration of the current transformer, determine whether both sides of the transmission line are connected in a single CT mode; if so, execute step 4; if not, if the dual CT access side is saturated, execute step 5, otherwise execute step 6;

[0013] Step 4. Both sides of the transmission line are connected in a single CT mode, and the ratio braking coefficient is adjusted to a first coefficient K1, wherein the first coefficient K1 is greater than a preset default coefficient K0;

[0014] Step 5. Determine whether the position where CT saturation occurs is at the dual CT access side, and determine the current amplitude of the dual CT; based on the saturation position and the current amplitude, adjust the ratio braking coefficient; the dual CT is a side switch CT and a middle switch CT;

[0015] Step 6. Determine whether the start time is within the preset time; based on the start time, adjust the ratio braking coefficient;

[0016] Step 7. If the CT saturation state is not detected on all sides, restore the ratio braking coefficient to the preset default coefficient K0.

[0017] Furthermore, the step 5 specifically includes:

[0018] If this side is a dual CT access side, determine whether the current amplitude of the side switch CT and the middle switch CT is less than the preset current threshold;

[0019] If the current amplitudes are all greater than the preset current threshold, the ratio braking coefficient is adjusted to the second coefficient K2;

[0020] If any CT current is less than the preset current threshold or any switch is in the open position, execute step 6;

[0021] If only one CT is connected to the local side, go to step 6.

[0022] Furthermore, the step 6 specifically includes:

[0023] Determine whether the start time is less than the preset time. If the start time is less than the preset time, adjust the ratio braking coefficient to the second coefficient K2; if the start time is greater than the preset time, adjust the ratio braking coefficient to the first coefficient K1;

[0024] The first coefficient K1 is greater than the preset default coefficient K0, and the second coefficient K2 is greater than the first coefficient K1.

[0025] Furthermore, the preset time is 40ms.

[0026] Furthermore, the first coefficient K1<1, and is 0.85-0.9.

[0027] Furthermore, the second coefficient K2>1, and is 1.1-1.5.

[0028] Furthermore, the preset default coefficient K0 has a value range of 0.5-0.7.

[0029] Compared with the prior art, the present invention has the following technical effects:

[0030] (1) In the present invention, when either side is in a dual CT access mode, the saturation of the CT is determined by the harmonic content in the current. When saturation occurs on the dual CT access side and the side and middle switch currents are both large, the ratio braking coefficient is raised to the second coefficient K2 (K2>1). By setting a braking coefficient greater than 1, the braking threshold is raised. When situation 1 occurs, the problem of misoperation of the differential protection due to out-of-zone faults can be solved.

[0031] (2) In the present invention, when dual CTs are connected on one side and single CTs are connected on the other side, if the saturated side is on the single CT access side, the ratio braking coefficients on both sides are the second coefficient K2 within 40ms, which can improve the ability of differential protection to resist severe transient CT saturation. After 40ms, the braking coefficients on both sides are adjusted to the first coefficient K1. When there is an intra-area fault in situation 2, the differential protection can be operated because the first coefficient K1<1;

[0032] (3) The present invention sets the differential protection braking coefficient to a reliable range according to different scenarios by adaptively adjusting the ratio braking coefficient after CT saturation, so that the differential protection will not be locked, and the protection refusal to operate when an out-of-zone fault develops into an in-zone fault can be effectively avoided;

[0033] (4) After CT saturation occurs, the present invention reduces the dependence of the fault identification criteria inside and outside the zone, making it easier to implement. The criteria themselves have a higher degree of recognition and wider adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the drawings required for use in the embodiments or the prior art descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0035] Figure 1 It is a schematic diagram of the process of the present invention. DETAILED DESCRIPTION

[0036] In order to further explain the technical means and effects taken by the present invention to achieve the predetermined invention purpose, the specific implementation methods, structures, features and effects of the technical solutions proposed by the present invention are described in detail below in conjunction with the accompanying drawings and preferred embodiments. The specific features, structures or characteristics in one or more embodiments may be combined in any suitable form. Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as those commonly understood by technicians in the technical field of the present invention.

[0037] It is worth noting that the criteria for longitudinal current differential protection are:

[0038]

[0039] In the above formula, I cd is the differential current, I zd is the braking current, K is the ratio braking coefficient, is the current phasor on both sides of the line, I set It is the differential current action setting value; in the 3 / 2 connection form, the single-side current phasor is the phasor sum of the two CT groups of the side switch and the middle switch.

[0040] In one embodiment of the present invention, referring to Figure 1, provides an adaptive adjustment method for the ratio braking coefficient after CT saturation, comprising the following steps: step 1. pre-set the single CT and double CT access information according to the configuration mode of the current transformer; step 2. check whether the relay protection device is started, and if so, calculate the start time; detect the current and harmonic content of the switches on both sides of the transmission line; based on the current on both sides, determine whether the differential current exceeds the limit; if not, the ratio braking coefficient adopts the preset default coefficient K0; if it exceeds the limit, determine whether the CT is saturated based on the content of the 2nd and 3rd harmonics in each CT; step 3. If any CT is saturated, based on the current According to the configuration mode of the current transformer, determine whether both sides of the transmission line are connected in a single CT mode; if so, execute step 4; if not, if saturation occurs on the dual CT access side, execute step 5, otherwise execute step 6; Step 5. Determine whether the position where CT saturation occurs is on the dual CT access side, and determine the current amplitude of the dual CT; based on the saturation position and current amplitude, adjust the ratio braking coefficient; Step 6. Determine whether the start-up time is within the preset time; based on the start-up time, adjust the ratio braking coefficient; Step 7. If no saturation is detected on all sides, restore the ratio braking coefficient to the preset default coefficient K0.

[0041] The following is a detailed explanation of each of the above steps:

[0042] Step 1. Set the single CT and double CT access information in advance according to the configuration mode of the current transformer.

[0043] In the 3 / 2 wiring form, the currents of the side switch and the middle switch CT need to be connected to the protection device respectively, and two sets of current CTs are configured accordingly. In other wiring forms, only one set of current CT is required. The selection of the two corresponding protection devices is different. The device can determine the configuration information of the current transformer through its own selection, which is represented by 1 bit, where 0 represents single CT access and 1 represents double CT access. The state is transmitted through the optical fiber channel to identify the configuration method on both sides of the transmission line.

[0044] Step 2. Check whether the relay protection device is activated. If so, calculate the startup time; detect the current and harmonic content of the switches on both sides of the transmission line; based on the current on both sides, determine whether the differential current exceeds the limit; if not, the ratio braking coefficient uses the preset default coefficient K0; if it exceeds the limit, determine whether the CT is saturated based on the content of the 2nd and 3rd harmonics in each CT.

[0045] By checking the change of current sampling value, whether the relay protection device is started due to system disturbance or other reasons, and recording the start time of the start, the start time is calculated in real time.

[0046] Use current transformers (CTs) to detect the current on both sides of the transmission line in real time; calculate the differential current based on the current of the switches on both sides, and use I cd >I setDetermine whether the differential current exceeds the limit:

[0047] If the differential current exceeds the limit, the harmonic content is analyzed to determine whether the CT is saturated. When the CT is saturated, the output current waveform is obviously distorted and the harmonic content increases. Among them, the method of determining whether the CT is saturated can be implemented using existing technologies, such as harmonic method, time difference method, angle difference method, waveform recognition method, etc. This method determines the saturation state by detecting whether the amplitude of the 2nd and 3rd harmonics in each CT phase current is greater than a preset ratio of the fundamental amplitude. The preset ratio is 0.2 to 0.5.

[0048] If the differential current does not exceed the limit, the differential protection will not operate and the ratio braking coefficient will be the preset default coefficient K0.

[0049] Step 3. If any CT is saturated, determine whether both sides of the transmission line are connected in a single CT mode based on the configuration of the current transformer; if so, execute step 4; if not, if saturation occurs on the dual CT access side, execute step 5, otherwise execute step 6.

[0050] Step 4. Both sides of the transmission line are connected with a single CT, and the ratio braking coefficient is adjusted to the first coefficient K1, where the first coefficient K1 is greater than the preset default coefficient K0.

[0051] Step 5. Determine whether the position where CT saturation occurs is on the dual CT side, and determine the current amplitude of the dual CT; based on the saturation position and the current amplitude, adjust the ratio braking coefficient.

[0052] If this side is a dual CT access side, determine whether the CT currents of the side switch and the middle switch are greater than the preset current threshold;

[0053] If both are greater than the preset current threshold, the ratio braking coefficient is adjusted to the second coefficient K2;

[0054] If any CT current is less than the preset current threshold or any switch is in the open position, execute step 6.

[0055] If only one CT is connected to the local side, go to step 6.

[0056] The current preset threshold is set as follows: 1.5 to 2 times the differential current value during normal operation, with a minimum of no less than 0.1 In. The preset time can be 40 ms to 60 ms.

[0057] Step 6: Determine whether the start time is within the preset time; adjust the ratio braking coefficient based on the start time.

[0058] Specifically, it includes: judging whether the start time is less than the preset time, if the start time is less than the preset time, adjusting the ratio braking coefficient to the second coefficient K2; if the start time is greater than the preset time, adjusting the ratio braking coefficient to the first coefficient K1.

[0059] Step 7. If the CT saturation state is not detected on all sides, that is, the harmonic content is less than the preset harmonic content threshold, the ratio braking coefficient is restored to the preset default coefficient K0.

[0060] When saturation occurs on the single CT access side, the braking coefficients on both sides are the first coefficient K1 after the preset time. When there is an intra-area fault in situation 2, the protection can be operated because the braking coefficient is less than 1. In addition, within 40ms, the ratio braking coefficients on both sides are the second coefficient K2, which can improve the ability of differential protection to resist severe transient saturation of CT.

[0061] When saturation occurs on the dual CT access side, the braking threshold is raised by setting a braking coefficient K2 greater than 1. When situation 1 occurs, the problem of false operation due to out-of-zone faults can be solved. The influence of severe transient saturation of CT caused by non-periodic components can also be avoided.

[0062] When the current on either side of the dual CT access is small or a single switch is in operation, the influence of the through current between the two switches on the differential protection under the 3 / 2 wiring method can be eliminated and it can be handled according to a ratio braking coefficient not greater than 1. When the zone is saturated in situation 2, the problem of refusal of fault protection in the zone can be avoided.

[0063] The present invention sets the differential protection braking coefficient to a reliable range according to different scenarios by adaptively adjusting the ratio braking coefficient after CT saturation, so that the differential protection will not be locked, and can effectively avoid the situation where the protection refuses to operate when the fault outside the zone develops into the zone. After CT saturation occurs, the dependence of the fault identification criterion inside and outside the zone is reduced, which is easier to implement, and the criterion itself has a higher recognition degree and wider adaptability.

[0064] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention.

Claims

1. A method for adaptively adjusting the ratio braking coefficient after CT saturation, characterized in that: The following steps are involved: Step 1. Set the single CT and double CT access information in advance according to the configuration mode of the current transformer; Step 2. Check whether the relay protection device is activated. If so, calculate the startup time; detect the current and harmonic content of the CTs on both sides of the transmission line; based on the currents on both sides, determine whether the differential current exceeds the limit; if not, the ratio braking coefficient uses the preset default coefficient K0; if it exceeds the limit, determine whether the CT is saturated based on the content of the 2nd and 3rd harmonics in each CT; Step 3. If any CT is saturated, based on the configuration of the current transformer, determine whether both sides of the transmission line are connected in a single CT mode; if so, execute step 4; if not, if the dual CT access side is saturated, execute step 5, otherwise execute step 6; Step 4. Both sides of the transmission line are connected with a single CT, and the ratio braking coefficient is adjusted to a first coefficient K1, wherein the first coefficient K1 is greater than a preset default coefficient K0; Step 5. Determine whether the location where CT saturation occurs is at the dual CT access side, and determine the current amplitude of the dual CT; Based on the saturation position and the current amplitude, the ratio braking coefficient is adjusted; the dual CT is a side switch CT and a middle switch CT; Step 6. Determine whether the start time is within the preset time; based on the start time, adjust the ratio braking coefficient; Step 7. If no CT saturation is detected on all sides, restore the ratio braking coefficient to the preset default coefficient K0; The step 5 specifically includes: If this side is a dual CT access side, determine whether the current amplitude of the side switch CT and the middle switch CT is less than the preset current threshold; If the current amplitudes are all greater than the preset current threshold, the ratio braking coefficient is adjusted to the second coefficient K2; If any CT current is less than the preset current threshold or any switch is in the open position, execute step 6; If only one CT is connected to the local side, go to step 6. The step 6 specifically includes: Determine whether the start time is less than the preset time. If the start time is less than the preset time, adjust the ratio braking coefficient to the second coefficient K2; if the start time is greater than the preset time, adjust the ratio braking coefficient to the first coefficient K1; The first coefficient K1 is greater than the preset default coefficient K0, and the second coefficient K2 is greater than the first coefficient K1.

2. The method for adaptively adjusting the ratio braking coefficient after CT saturation according to claim 1 is characterized in that: The preset time is 40ms.

3. The method for adaptively adjusting the ratio braking coefficient after CT saturation according to claim 1 is characterized in that: The first coefficient K1 has a value range of 0.85-0.

9.

4. The method for adaptively adjusting the ratio braking coefficient after CT saturation according to claim 1 is characterized in that: The second coefficient K2 has a value range of 1.1-1.

5.

5. The method for adaptively adjusting the ratio braking coefficient after CT saturation according to claim 1 is characterized in that: The preset default coefficient K0 has a value range of 0.5-0.7.

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