Method for evaluating anti-saturation performance of P-level current transformer
By calculating the transient area coefficient of the current transformer at the maximum short-circuit current and combining the detection function of the protection device, its anti-saturation performance is divided into four levels, which solves the problem of insufficient evaluation of transient anti-saturation performance in the prior art and improves the safety of power grid operation.
Patent Information
- Application Number
- CN202311590793.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2023-11-27
- Publication Date
- 2025-05-27
AI Technical Summary
When evaluating the anti-saturation performance of current transformers, the prior art lacks clear requirements for the transient anti-saturation performance, resulting in protection errors caused by transient saturation and large-scale power outages.
By taking into account the C-O working cycle and the residual magnetic coefficient of CT under the maximum short-circuit current of full offset, the transient area coefficient is calculated, and combined with the protection detection function of the protection device, the anti-saturation performance of the current transformer is divided into four levels.
The problem of lack of evaluation standards for the transient anti-saturation performance of P-class current transformers was solved, and the transient anti-saturation performance evaluation was quantitatively given, which reduced the risk of transient saturation and improved the safety of the power grid operation.
Smart Images
Figure CN120044460A_ABST
Abstract
Description
Technical Field
[0001] The invention belongs to the technical field of current transformers, and in particular relates to a method for evaluating the anti-saturation performance of a P-level current transformer. Background Art
[0002] The current transformer is an instrument that converts the large current on the primary side into the small current on the secondary side for measurement based on the principle of electromagnetic induction. The current transformer is composed of a closed iron core and a winding. Its primary winding has very few turns and is connected in series in the circuit of the current to be measured. Therefore, it often has the entire current of the circuit flowing through it. The secondary winding has more turns and is connected in series in the measuring instrument and the protection circuit. When the current transformer is working, its secondary circuit is always closed, so the impedance of the series coil of the measuring instrument and the protection circuit is very small, and the working state of the current transformer is close to short circuit. The existing technical method of using the 5% / 10% error curve of the current transformer to evaluate its anti-saturation performance is not comprehensive enough, because it only considers the steady-state anti-saturation performance and does not make clear requirements for the transient anti-saturation performance. However, in the actual operation process, accidents such as large-scale power outages caused by false protection due to transient saturation of the current transformer have indeed occurred. Summary of the invention
[0003] In view of the technical problem that the above-mentioned current transformer transient saturation causes the accident of large-scale power outage due to protection malfunction, the present invention provides a method for evaluating the anti-saturation performance of a P-class current transformer. The anti-saturation performance of the current transformer is divided into four levels by using the results of the steady-state and transient anti-saturation performance evaluation of the current transformer and combining the protection detection function of the protection device, thereby solving the problem that the 5% / 10% error curve evaluation method of the current transformer lacks clear requirements for the transient anti-saturation performance of the current transformer.
[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is:
[0005] A method for evaluating the anti-saturation performance of a P-level current transformer comprises the following steps:
[0006] S1. Evaluate the steady-state anti-saturation performance of the current transformer using the error curve and the measured value of the secondary load impedance;
[0007] S2. Under the maximum short-circuit current of full offset, considering the CO working cycle and the residual magnetism coefficient of CT, the transient area coefficient is calculated to evaluate the transient anti-saturation performance of the current transformer;
[0008] S3. Based on the calculation results of the steady-state and transient anti-saturation performance of the current transformer and the saturation detection function of the protection device, the anti-saturation performance of the current transformer is divided into four levels, and the current transformer to be evaluated is classified into one of the levels.
[0009] The method for evaluating the steady-state anti-saturation performance of the current transformer in S1 is:
[0010] S1.1. Obtain the following parameters from the current transformer nameplate: Rated primary current I pn , rated secondary current I sn , accuracy level, rated accuracy limit coefficient K alf , rated load Z bn From the short-circuit calculation sheet related to the current transformer installation location, the rated maximum three-phase short-circuit current steady-state effective value I is obtained. pcf , calculate the protection calibration coefficient K pcf ;
[0011] S1.2. Use the current transformer analyzer to measure the actual secondary load impedance value of the current transformer to be Z b ', Secondary circuit time constant T s , Remanence coefficient K r , error curve, find the horizontal coordinate Z on the error curve b ', read its ordinate as K alf ', this value is the actual accurate limit coefficient of the current transformer under actual load;
[0012] S1.3. Definition of transient margin factor K ts , transient margin factor K ts Indicates: In addition to meeting the basic steady-state requirements, the anti-saturation performance of the P-level current transformer also has a surplus of performance to resist the impact of transient saturation, the transient margin coefficient K ts The larger the value, the stronger the anti-saturation performance of the current transformer.
[0013] The protection check coefficient K is calculated in S1.1 pcf The method is:
[0014] K pcf =I pcf / I pn
[0015] Where: K pcf Indicates the protection calibration coefficient, I pcf Indicates the protection verification fault current, I pn Indicates the rated primary current of the current transformer.
[0016] The transient margin factor K in S1.3 ts The calculation method is:
[0017] K ts =K alf ' / K pcf
[0018] Where: K tsRepresents the transient margin factor, K alf ' represents the actual accurate limit coefficient, K pcf Indicates the protection calibration coefficient.
[0019] The method for calculating the transient area coefficient in S2 and then evaluating the transient anti-saturation performance of the current transformer is:
[0020] S2.1. From the instruction manual of the protection device connected to the current transformer to be evaluated, it is known that the saturation detection time of the protection device is t c Second;
[0021] S2.2. To ensure the absolute safety of the power system operation, all situations are considered according to the worst conditions, that is, the short-circuit current is the maximum value and is fully offset, and the residual magnetism of the current transformer is the maximum value in the same direction at this time, and then the transient area coefficient K at this time is calculated. td ;
[0022] S2.3, Transient area coefficient K of full offset short-circuit current after t seconds td , the current transformer is more likely to saturate in transient state than in steady state, and a larger core cross-sectional area is required to achieve the goal of transient unsaturation;
[0023] S2.4, take t = t c Calculate the transient area factor K required for the current transformer to be transiently unsaturated during the saturation detection period of the protection device td K td 1;
[0024] S2.5, take t = ∞ to calculate the transient area coefficient K required for the transient unsaturation of the current transformer during the entire short-circuit fault period td K td 2.
[0025] The transient area coefficient K is calculated in S2.2 td The method is:
[0026]
[0027] Where: T p Indicates the rated primary time constant, T p The value of is 0.1s, ω represents the special constant symbol of the power system, and the value of ω is 100π; the transient area coefficient K td It means: To achieve the goal of being unsaturated under transient conditions, the cross-sectional area of the current transformer core needs to be several times the steady-state unsaturated area.
[0028] The method of dividing the anti-saturation performance of the current transformer into four levels in S3 is: the anti-saturation performance of the P-level current transformer has been quantitatively expressed as the transient margin coefficient K ts, the following uses the parameters obtained by previous calculation to quantitatively evaluate the anti-saturation performance of the current transformer; due to the fact that T p ≈0.1s, T s Between 1s and 100s, K r Between 0.5 and 1, substitute the transient area coefficient K td Always satisfy 1<K td 1<K td 2, with 1, K td 1. K td 2 The three values are used as the basis to divide the grades and evaluate the anti-saturation performance of the current transformer, and the anti-saturation performance is divided into four levels from low to high.
[0029] The four levels of anti-saturation performance from low to high are:
[0030] Level 1: K ts <1, indicating that the current transformer has poor anti-saturation performance and does not meet the basic steady-state anti-saturation performance requirements. It cannot accurately measure the primary current value, which may easily cause the protection device to malfunction or refuse to operate. There is an extremely high risk and it is rated as unqualified;
[0031] Level 2: 1≤K ts <K td 1. This indicates that the current transformer has acceptable anti-saturation performance and can maintain an unsaturated state in a steady state. It also has a certain resistance to transient saturation. However, it cannot guarantee that it will maintain an unsaturated state during the entire cycle of the protection device performing saturation detection. This may result in measurement errors. Whether the protection device will erroneously operate depends on the device's algorithm. There is still a risk, so it is rated as qualified.
[0032] Level 3: K td 1≤K ts <K td 2. This indicates that the current transformer has good anti-saturation performance and can maintain an unsaturated state in a steady state. At the same time, it has a strong resistance to transient saturation. As long as the saturation detection function of the protection device works normally, no false operation or refusal to operate will occur. The risk is very low and it is rated as good.
[0033] Level 4: K td 2≤K ts This indicates that the current transformer has excellent anti-saturation performance and will not be saturated in either steady state or transient state during the entire short-circuit fault period. The saturation detection function of the protective device realizes double-layer protection without any risk and is rated as excellent.
[0034] Compared with the prior art, the present invention has the following beneficial effects:
[0035] The present invention considers the CO working cycle and the residual magnetism coefficient of the CT under the maximum short-circuit current of full offset, calculates the transient area coefficient and then evaluates the transient anti-saturation performance of the current transformer, solves the problem of lack of evaluation standards for the transient anti-saturation performance of the P-level current transformer, and quantitatively gives the transient anti-saturation performance evaluation. In addition, the present invention provides a four-level evaluation of the anti-saturation performance, solves the problem of uncertain transient saturation risk of the current transformer on site, provides scientific evaluation and suggestions for the saturation risk of the current transformer and the possible hazards, and improves the safety of power grid operation. BRIEF DESCRIPTION OF THE DRAWINGS
[0036] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. Obviously, the drawings in the following description are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.
[0037] The structures, proportions, sizes, etc. illustrated in this specification are only used to match the contents disclosed in the specification so as to facilitate understanding and reading by persons familiar with the technology. They are not used to limit the conditions under which the present invention can be implemented, and therefore have no substantial technical significance. Any structural modification, change in proportion or adjustment of size shall still fall within the scope of the technical contents disclosed in the present invention without affecting the effects and purposes that can be achieved by the present invention.
[0038] Figure 1 It is the error curve diagram of the present invention. DETAILED DESCRIPTION
[0039] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. These descriptions are only to further illustrate the features and advantages of the present invention, rather than to limit the claims of the present invention. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.
[0040] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.
[0041] In this embodiment, the anti-saturation performance evaluation method of the P-level current transformer includes the following steps:
[0042] Step 1: Use the error curve and the measured value of the secondary load impedance to evaluate the steady-state anti-saturation performance of the current transformer.
[0043] 1) Obtain the following parameters from the current transformer nameplate: Rated primary current I pn , rated secondary current I sn , accuracy level (5P or 10P, P stands for Percent, expressed as 5% or 10%), rated accuracy limit coefficient K alf , rated load Z bn From the short-circuit calculation sheet related to the current transformer installation location, the rated maximum three-phase short-circuit current steady-state effective value I is obtained. pcf , calculate the protection calibration coefficient K according to formula (1) pcf .
[0044] K pcf =I pcf / I pn (1)
[0045] Where: K pcf Indicates the protection calibration coefficient, I pcf Indicates the protection verification fault current, I pn Indicates the rated primary current of the current transformer.
[0046] 2) The actual secondary load impedance value of the current transformer measured by the current transformer analyzer is Z b ', Secondary circuit time constant T s , Remanence coefficient K r , error curve. The error curve is as follows Figure 1 As shown, the horizontal axis is the secondary load impedance value, and the vertical axis is the accurate limit coefficient.
[0047] Find the horizontal coordinate Z on the error curve b ', read its ordinate as K alf ', this value is the actual accurate limit coefficient of the current transformer under actual load.
[0048] 3) Define the transient margin coefficient Kts and calculate its value according to formula (2).
[0049] K ts =K alf ' / K pcf (2)
[0050] Where: K ts Represents the transient margin factor, K alf ' represents the actual accurate limit coefficient, K pcf Indicates the protection calibration coefficient.
[0051] Transient margin factor K tsThe meaning is: In addition to meeting the basic steady-state requirements, the P-level current transformer's anti-saturation performance has more surplus performance to resist the impact of transient saturation. The larger the value, the stronger the anti-saturation performance of the current transformer.
[0052] Step 2: Under the maximum short-circuit current of full offset, considering the CO working cycle and the residual magnetism coefficient of the CT, calculate the transient area coefficient and then evaluate the transient anti-saturation performance of the current transformer.
[0053] 1) From the instruction manual of the protection device connected to the current transformer to be evaluated, it is known that the saturation detection time of the protection device is t c Second.
[0054] 2) To ensure the absolute safety of the power system operation, all situations are considered according to the worst conditions, that is, the short-circuit current is the maximum value and is fully offset, and the residual magnetism of the current transformer is the maximum value in the same direction at this time, and then the transient area coefficient K at this time is calculated. td .
[0055] 3) Transient area coefficient K of full offset short-circuit current after t seconds td (Considering the CO working cycle and remanence coefficient), calculate according to formula (3):
[0056]
[0057] Where: T p Indicates the rated primary time constant, T p The value of is 0.1s, ω represents the special constant symbol of the power system, and the value of ω is 100π; the transient area coefficient K td It means: To achieve the goal of being unsaturated under transient conditions, the cross-sectional area of the current transformer core needs to be several times the steady-state unsaturated area.
[0058] 4) Take t = t c Calculate the transient area factor K required for the current transformer to be transiently unsaturated during the saturation detection period of the protection device td K td 1.
[0059] 5) Take t = ∞ to calculate the transient area coefficient K required for the transient unsaturation of the current transformer during the entire short-circuit fault period td K td 2.
[0060] Step 3: Based on the calculation results of the steady-state and transient anti-saturation performance of the current transformer and the saturation detection function of the protection device, the anti-saturation performance of the current transformer is divided into four levels, and the current transformer to be evaluated is classified into one of the levels.
[0061] After previous calculations, the anti-saturation performance of the P-class current transformer has been quantitatively expressed as the transient margin coefficient K ts , the following uses the parameters obtained by previous calculation to quantitatively evaluate the anti-saturation performance of the current transformer. p ≈0.1s, T s Between 1s and 100s, K r Between 0.5 and 1, substituting into formula (3) always satisfies 1<K td 1<K td 2, so 1, K td 1. K td 2 The three values are used as the basis to divide the grades and evaluate the anti-saturation performance of the current transformer, and the anti-saturation performance is divided into four levels from low to high.
[0062] The four levels of anti-saturation performance from low to high are:
[0063] Level 1: K ts <1, indicating that the current transformer has poor anti-saturation performance and does not meet the basic steady-state anti-saturation performance requirements. It cannot accurately measure the primary current value, which can easily cause the protection device to malfunction or refuse to operate. There is an extremely high risk and it is rated as unqualified.
[0064] Level 2: 1≤K ts <K td 1, indicating that the anti-saturation performance of the current transformer is acceptable. It can maintain an unsaturated state in a steady state and has a certain resistance to transient saturation. However, it cannot guarantee that it will maintain an unsaturated state during the entire cycle of the protection device performing saturation detection. Measurement errors may occur. Whether the protection device will erroneously operate depends on the algorithm of the device. There is still a risk, so it is rated as qualified.
[0065] Level 3: K td 1≤K ts <K td 2. This indicates that the current transformer has good anti-saturation performance and can maintain an unsaturated state under steady-state conditions. At the same time, it has a strong resistance to transient saturation. As long as the saturation detection function of the protection device works normally, no false operation or refusal to operate will occur. The risk is very low and is rated as good.
[0066] Level 4: K td 2≤K ts This indicates that the current transformer has excellent anti-saturation performance and will not be saturated in either steady state or transient state during the entire short-circuit fault period. The saturation detection function of the protective device realizes double-layer protection without any risk and is rated as excellent.
[0067] Several examples of calculating the anti-saturation performance of P-class current transformers are given in table form, as shown in Table 1. Due to the large number of values in the error curve, the specific values are not included in the table, and the results read on the coordinate axis are directly filled in the table. Because the three-phase values are close, only the A-phase data is taken for each current transformer.
[0068] Table 1 P-level current transformer anti-saturation performance evaluation table
[0069]
[0070]
[0071] In this embodiment, due to cost considerations, not too much anti-saturation performance margin is reserved when designing and selecting the current transformer. Therefore, through analysis of previous measured data and after calculation and evaluation according to this embodiment, the anti-saturation performance of general current transformers is mainly concentrated in the qualified and good levels, with occasional unqualified and no excellent rating.
[0072] This embodiment, under the maximum short-circuit current of full offset, considers the CO working cycle and the residual magnetism coefficient of the CT, calculates the transient area coefficient and then evaluates the transient anti-saturation performance of the current transformer, solves the problem of lack of evaluation standards for the transient anti-saturation performance of the P-level current transformer, and quantitatively gives the transient anti-saturation performance evaluation. In addition, this embodiment provides a four-level evaluation of the anti-saturation performance, solves the problem of uncertain transient saturation risk of the current transformer on site, and provides scientific evaluation and suggestions for the saturation risk of the current transformer and the possible hazards, thereby improving the safety of power grid operation.
[0073] Only the preferred embodiments of the present invention are described in detail above, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of the present invention, and various changes should be included in the protection scope of the present invention.
Claims
1. A method for evaluating the anti - saturation performance of a P - class current transformer, characterized in that: it includes the following steps: S1. Evaluate the steady - state anti - saturation performance of the current transformer by using the error curve and the measured value of the secondary load impedance; S2. Under the maximum short - circuit current of full offset, considering the C - O operating cycle and the remanence coefficient of the CT, calculate the transient area coefficient and then evaluate the transient anti - saturation performance of the current transformer; S3. Based on the results of calculating the steady - state and transient anti - saturation performances of the current transformer, combined with the saturation detection function of the protection device, divide the anti - saturation performance of the current transformer into four levels and classify the current transformer to be evaluated into one of the levels.
2. The method for evaluating the anti - saturation performance of a P - class current transformer according to claim 1, characterized in that: the method for evaluating the steady - state anti - saturation performance of the current transformer in S1 is: S1.
1. Obtain the following parameters from the nameplate of the current transformer: rated primary current I pn , rated secondary current I sn , accuracy class, rated accuracy limit factor K alf , rated load Z bn Obtain the steady-state effective value I of the rated primary maximum three-phase short-circuit current from the short-circuit calculation book related to the installation location of the current transformer pcf , calculate the protection verification coefficient K pcf ; S1.
2. Measure the actual secondary load impedance value Z of the current transformer using a current transformer analyzer b ’, the secondary circuit time constant T s , the remanence coefficient K r , the error curve. Locate the point on the error curve with abscissa Z b ’, and read its ordinate as K alf ’. This value is the actual accurate limit factor of the current transformer under the actual load; S1.
3. Define the transient margin coefficient K ts , the transient margin coefficient K ts indicates that in addition to meeting the basic steady-state requirements, how much surplus performance of the class P current transformer can be used to resist the influence of transient saturation. The larger the value of the transient margin coefficient K ts , the stronger the anti-saturation performance of the current transformer.
3. The method for evaluating the anti - saturation performance of a P - class current transformer according to claim 2, characterized in that: The method for calculating the protection verification coefficient K in S1.1 pcf is as follows: K pcf = I pcf / I pn Where: K pcf represents the protection verification coefficient, I pcf represents the protection verification fault current, I pn represents the rated primary current of the current transformer.
4. The method for evaluating the anti - saturation performance of a P - class current transformer according to claim 2, characterized in that: The transient margin coefficient K in S1.3 ts is calculated as follows: K ts = K alf ’ / K pcf Where: K ts represents the transient margin coefficient, K alf ’ represents the actual accurate limit factor, K pcf represents the protection verification coefficient.
5. The method for evaluating the anti - saturation performance of a P - class current transformer according to claim 1, characterized in that: the method for calculating the transient area coefficient and then evaluating the transient anti - saturation performance of the current transformer in S2 is: S2.
1. Obtain from the instruction manual of the protection device connected to the current transformer to be evaluated that the saturation detection duration of the protection device is t c seconds; S2.
2. To ensure the absolute safety of the power system operation, all situations are considered under the worst conditions, that is, the short-circuit current is at its maximum value, and it is fully offset. Exactly at this time, the remanent magnetism of the current transformer is at its maximum value in the same direction, and then the transient area coefficient K is calculated at this time td ; S2.
3. Transient area coefficient K of the full-offset short-circuit current after t seconds td The current transformer is more likely to saturate transiently than in the steady state and requires a larger core cross-sectional area to achieve the goal of non-saturation during the transient state; S2.
4. Take t = t c Calculate the transient area coefficient K required for the current transformer to be transiently unsaturated during the saturation detection performed by the protection device td Denote it as K td 1; S2.
5. Calculate the transient area factor K required for the current transformer to remain transiently unsaturated during the entire short-circuit fault period with t = ∞ td Denoted as K td 2.
6. The method for evaluating the anti - saturation performance of a P - class current transformer according to claim 5, characterized in that: The method for calculating the transient area coefficient K in S2.2 td is as follows: Where: T p represents the rated primary time constant, and the value of T p is 0.1 s. ω represents the special constant symbol of the power system, and the value of ω is 100π; the transient area coefficient K td represents: to achieve the goal of remaining unsaturated under transient conditions, how many times the cross-sectional area of the current transformer core needs to reach the steady-state unsaturated area.
7. The method for evaluating the anti - saturation performance of a P - class current transformer according to claim 1, characterized in that: The method of dividing the anti-saturation performance of current transformers into four levels in S3 is as follows: The anti-saturation performance of P-class current transformers has been quantitatively expressed as the transient margin coefficient K ts , and the anti-saturation performance of the current transformer is quantitatively evaluated with the parameters obtained from the previous calculation; since T p ≈0.1s, T s is between 1s and 100s, K r is between 0.5 and 1, substituting the transient area coefficient K td always satisfies 1 < K td 1 < K td 2. Using the three values of 1, K td 1, and K td 2 as benchmarks to divide grades, the anti-saturation performance of the current transformer is classified and evaluated, and the anti-saturation performance is divided into four levels from low to high.
8. The method for evaluating the anti - saturation performance of a P - class current transformer according to claim 7, characterized in that: the four levels of anti - saturation performance from low to high are respectively: Level 1: K ts <1 indicates that the current transformer has poor anti-saturation performance and does not meet the basic steady-state anti-saturation performance requirements. It cannot accurately measure the primary current value, is prone to causing misoperation or refusal of the protection device, and poses a very high risk, so it is rated as unqualified; Level 2: 1 ≤ K ts <K td 1 indicates that the current transformer has acceptable anti-saturation performance and can remain unsaturated under steady-state conditions. At the same time, it has a certain ability to resist transient saturation. However, it cannot guarantee to remain unsaturated throughout the entire cycle when the protection device performs saturation detection, and measurement errors may occur. Whether the protection device malfunctions depends on the algorithm of the device, and there is still a risk, so it is rated as qualified; Level Three: K td 1 ≤ K ts <K td td 2 indicates that the current transformer has good anti-saturation performance, can maintain an unsaturated state under steady-state conditions, and has a rather strong resistance to transient saturation. As long as the saturation detection function of the protection device works properly, misoperation or refusal to operate will not occur, and the risk is very low, so it is rated as good; Level Four: K td 2 ≤ K ts , indicating that the current transformer has excellent anti-saturation performance and will not saturate during the entire short-circuit fault period, whether in the steady state or the transient state. The saturation detection function of the cooperation protection device realizes double-layer protection, without any risk, and is rated as excellent.