System for measuring earth capacitance and method

Abstract

The application discloses a kind of system ground capacitance measurement method and system, sampling the secondary side opening delta voltage of voltage transformer of set length voltage as first voltage sequence, sampling after t1 time when direct current power supply access voltage transformer secondary side opening delta end, t2 time when direct current power supply exits voltage transformer secondary side opening delta end, the t2 time is after t1 time, the set length is equal to t1+t2;First voltage sequence is delayed t2 time and obtains second voltage sequence, the difference of first voltage sequence and second voltage sequence is calculated as third voltage sequence;According to the oscillation frequency of third voltage sequence, the system ground capacitance value is calculated.It is less dangerous to the throw of direct current power supply in voltage transformer secondary side, and the hardware implementation effect is relatively easy, the accuracy of measured ground capacitance value is high, the error is small, is not influenced by power grid symmetry, application occasion is wider, adapts to the power grid of multiple neutral point grounding mode.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power grid detection, in particular to a system-to-ground capacitance measurement method and system. BACKGROUND

[0002] The medium voltage distribution network is mainly small current grounding, and the neutral point grounding modes mainly include arc suppression coil grounding, high resistance grounding and non-grounding mode. For a power grid system, the specific selection of which mode needs to accurately measure the size of the system-to-ground capacitance current.

[0003] The current measurement of the power grid capacitance current can be divided into direct measurement method and indirect measurement method, and the direct measurement method is divided into neutral point non-grounded single-phase metallic grounding method, neutral point arc suppression coil grounding single-phase metallic grounding method. The direct measurement method directly measures the artificially set single-phase grounding current with an instrument, and the principle is simple, and the accurate capacitance current value can be obtained without complex calculation; the indirect measurement method mainly includes neutral point external capacitance method, neutral point different frequency signal injection method and neutral point displacement voltage method, which does not change the operation mode of the power grid, the operation environment is safe, and the measurement precision is also high. It is the current mainstream capacitance current measurement method.

[0004] However, the direct measurement method needs to artificially construct a single-phase metallic grounding, so that the personnel electric shock risk is very large when operating the device, and the power grid enters an abnormal state during the measurement process, which has a great adverse effect on the stable operation of the power grid.

[0005] Although the indirect measurement method is relatively safe in measurement environment, and has no safety threat to the person and equipment, the calculation is complicated, the hardware implementation is difficult, and the measurement result accuracy is easily affected by the signal processing loop hardware. In addition, such methods are often only applicable to a certain neutral point grounding mode, such as the neutral point external capacitance method which is only applicable to the neutral point non-grounding system, and the neutral point displacement voltage method which can only be used for the neutral point arc suppression coil grounding system. At present, there is no indirect measurement method of capacitance current which can be applied to various neutral point grounding modes. SUMMARY

[0006] The technical problem to be solved by the present application is to provide a system-to-ground capacitance measurement method and system which can adapt to power grids with various neutral point grounding modes and realize the measurement of the power grid-to-ground capacitance.

[0007] In order to solve the above technical problems, the technical scheme adopted by the present application is:

[0008] A system-to-ground capacitance measurement method, comprising the steps of:

[0009] S1, sampling the voltage of the secondary side open triangle of the voltage transformer for a set time length as a first voltage sequence, connecting the DC power supply to the secondary side open triangle of the voltage transformer at t1 after the sampling starts, disconnecting the DC power supply from the secondary side open triangle of the voltage transformer at t2, wherein t2 is after t1, and the set time length is equal to t1+t2;

[0010] S2, delaying the first voltage sequence by t2 to obtain a second voltage sequence, and calculating the difference between the first voltage sequence and the second voltage sequence as a third voltage sequence;

[0011] S3, calculating the system-to-ground capacitance value according to the oscillation frequency of the third voltage sequence.

[0012] To solve the above technical problems, another technical solution adopted by the present application is:

[0013] A system-to-ground capacitance measurement system, comprising a voltage transformer, an electric switch, a voltmeter and a system-to-ground capacitance measurement terminal, the system-to-ground capacitance measurement terminal is electrically connected to the electric switch and the voltmeter, the primary side of the voltage transformer is connected to three phases of a power grid respectively, the DC power supply and the electric switch are connected in series and then connected across two ends of the secondary side open triangle of the voltage transformer, and the voltmeter is connected to the two ends of the secondary side open triangle of the voltage transformer.

[0014] The system-to-ground capacitance measurement terminal comprises a memory, a processor and a computer program stored in the memory and executable on the processor, and the processor implements the following steps when executing the computer program:

[0015] S1, sampling the voltage of the secondary side open triangle of the voltage transformer for a set time length as a first voltage sequence, connecting the DC power supply to the secondary side open triangle of the voltage transformer at t1 after the sampling starts, disconnecting the DC power supply from the secondary side open triangle of the voltage transformer at t2, wherein t2 is after t1, and the set time length is equal to t1+t2;

[0016] S2, delaying the first voltage sequence by t2 to obtain a second voltage sequence, and calculating the difference between the first voltage sequence and the second voltage sequence as a third voltage sequence;

[0017] S3, calculating the system-to-ground capacitance value according to the oscillation frequency of the third voltage sequence.

[0018] The beneficial effect of the present application is that a system ground capacitance measurement method and system, the on-off of the direct current power supply is carried out at the secondary side of the voltage transformer, the operation process has no unsafe factors to the equipment and the human body, the danger is small, the sampling data processing algorithm is simple, the hardware implementation effect is relatively easy, the measured ground capacitance value is high in accuracy, small in error, is not affected by the symmetry of the power grid, is widely used, and is suitable for various neutral point grounding mode power grids. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 A system ground capacitance measurement system structure schematic diagram of an embodiment of the present application;

[0020] Figure 2 A system ground capacitance measurement method flowchart of an embodiment of the present application;

[0021] Figure 3 A switch S control signal timing diagram related to an embodiment of the present application;

[0022] Figure 4 A PT secondary side open delta voltage signal processing mode structure schematic diagram related to an embodiment of the present application;

[0023] Figure 5 A voltage transformer open delta winding equivalent structure schematic diagram related to an embodiment of the present application;

[0024] Figure 6 A different line commissioning two neutral point grounding mode under the decay component waveform diagram related to an embodiment of the present application;

[0025] Figure 7 A system ground capacitance measurement terminal structure schematic diagram of an embodiment of the present application.

[0026] Label explanation:

[0027] 1, a system ground capacitance measurement terminal; 2, a processor; 3, a memory. DETAILED DESCRIPTION

[0028] In order to explain the technical content, the purpose and the effect of the present application in detail, the following will be described in conjunction with the drawings.

[0029] Please refer to Figures 1 to 6 A system ground capacitance measurement method, comprising the steps of:

[0030] S1, the secondary side opening delta voltage of the voltage transformer is sampled as a first voltage sequence, the DC power supply is connected to the secondary side opening delta end of the voltage transformer at t1 moment after sampling starts, the DC power supply exits the secondary side opening delta end of the voltage transformer at t2 moment, the t2 moment is after the t1 moment, and the set time length is equal to t1+t2.

[0031] S2, the first voltage sequence is delayed by t2 to obtain a second voltage sequence, and the difference between the first voltage sequence and the second voltage sequence is calculated as a third voltage sequence.

[0032] S3, the oscillation frequency of the third voltage sequence is calculated to obtain the system ground capacitance value.

[0033] As described above, the system ground capacitance measurement method and system have the advantages that the DC power supply is connected and disconnected at the secondary side of the voltage transformer, the operation process is safe for equipment and personnel, the danger is small, the sampling data processing algorithm is simple, the hardware implementation effect is relatively easy, the measured ground capacitance value is accurate, the error is small, the application occasion is wide, and the system is suitable for various neutral point grounding mode power grids.

[0034] Further, the system is a power frequency system, and the t2 is a non-zero integer multiple of a power frequency period.

[0035] As described above, the measurement of the ground capacitance applied to the power frequency system is realized.

[0036] Further, the system is a neutral point non-grounding system, and step S3 is specifically for calculating the system ground capacitance value according to the following formula:

[0037]

[0038] In the formula, f is the oscillation frequency of the third voltage sequence, L PT is the equivalent inductance of the voltage transformer opening delta single-phase primary and secondary windings.

[0039] As described above, the measurement of the ground capacitance applied to the neutral point non-grounding system is realized.

[0040] Further, the system is a neutral point grounding system through an arc suppression coil, and step S3 is specifically for calculating the system ground capacitance value according to the following formula:

[0041]

[0042] In the formula, L is the central point ground inductance value, f is the oscillation frequency of the third voltage sequence, L PT is the equivalent inductance of the voltage transformer opening delta single-phase primary and secondary windings.

[0043] From the above description, it is realized that the application is applied to the neutral point arc suppression coil grounding system to realize the measurement of the ground capacitance.

[0044] Further, the equivalent inductance L of the open delta single-phase primary and secondary winding of the voltage transformer PT The system ground capacitance value is calculated according to the following formula:

[0045]

[0046] In the formula, L m The voltage transformer single-phase excitation inductance value (three phases are considered to be consistent).

[0047] From the above description, it is realized that the calculation of the equivalent inductance of the open delta single-phase primary and secondary winding of the voltage transformer is realized.

[0048] A system ground capacitance measurement system comprises a voltage transformer, a motor switch, a voltmeter and a system ground capacitance measurement terminal, the system ground capacitance measurement terminal is electrically connected with the motor switch and the voltmeter, the primary side of the voltage transformer is connected with three phases of a power grid respectively, the direct current power supply and the motor switch are connected in series and are connected across two ends of the secondary side of the voltage transformer, and the voltmeter is connected with the two ends of the secondary side of the voltage transformer.

[0049] The system ground capacitance measurement terminal comprises a memory, a processor and a computer program stored in the memory and capable of running on the processor, and the processor realizes the following steps when executing the computer program:

[0050] S1, the secondary side open delta voltage of the voltage transformer for a set time length is sampled as a first voltage sequence, the direct current power supply is connected to the secondary side open delta end of the voltage transformer at t1 after the sampling starts, the direct current power supply is disconnected from the secondary side open delta end of the voltage transformer at t2, the t2 is after the t1, and the set time length is equal to t1+t2.

[0051] S2, the first voltage sequence is delayed for t2 to obtain a second voltage sequence, and the difference between the first voltage sequence and the second voltage sequence is calculated as a third voltage sequence.

[0052] S3, the system ground capacitance value is calculated according to the oscillation frequency of the third voltage sequence.

[0053] From the above description, the beneficial effects of the present application are that a system ground capacitance measurement method and system, the on-off of the direct current power supply is carried out at the secondary side of the voltage transformer, there is no unsafe factor for the equipment and the human body in the operation process, the danger is small, the sampling data processing algorithm is simple, the hardware implementation effect is relatively easy, the measured ground capacitance value is high in accuracy, small in error, is not affected by the symmetry degree of the power grid, is widely used, and is suitable for various neutral point grounding mode power grids.

[0054] Further, the system is a power frequency system, and the t2 is a non-zero integer multiple of a power frequency period.

[0055] From the above description, it is known that the measurement of the ground capacitance applied to the power frequency system is realized.

[0056] Further, the system is a neutral point ungrounded system, and the step S3 is specifically to calculate the ground capacitance value of the system according to the following formula:

[0057]

[0058] In the formula, f is the oscillation frequency of the third voltage sequence, L PT is the equivalent inductance of the open delta single-phase primary and secondary winding of the voltage transformer.

[0059] From the above description, it is known that the measurement of the ground capacitance applied to the neutral point ungrounded system is realized.

[0060] Further, the system is a neutral point arc-extinguishing coil grounded system, and the step S3 is specifically to calculate the ground capacitance value of the system according to the following formula:

[0061]

[0062] In the formula, L is the center point ground inductance value, f is the oscillation frequency of the third voltage sequence, L PT is the equivalent inductance of the open delta single-phase primary and secondary winding of the voltage transformer.

[0063] From the above description, it is known that the measurement of the ground capacitance applied to the neutral point arc-extinguishing coil grounded system is realized.

[0064] Further, the equivalent inductance L PT of the open delta single-phase primary and secondary winding of the voltage transformer is calculated according to the following formula:

[0065]

[0066] In the formula, L m is the single-phase excitation inductance value of the voltage transformer (three phases are considered to be consistent).

[0067] From the above description, it is known that the calculation of the equivalent inductance of the open delta single-phase primary and secondary winding of the voltage transformer is realized.

[0068] The application is used for measuring the ground capacitance of a system and is suitable for power grid systems with various neutral point grounding modes.

[0069] Please refer to Figures 1 to 6 , the first embodiment of the application is:

[0070] A system for measuring the ground capacitance of a neutral point ungrounded system, a neutral point high resistance grounded system and a neutral point arc suppression coil grounded system.

[0071] Please refer to Figure 1 The principle of the specific scheme is that a direct current power supply is connected in parallel with the open delta voltage at the secondary side of a potential transformer (PT), the primary and secondary windings of the open delta voltage of the PT are equivalent to an inductance of a certain value (the inductance value can be calculated according to the parameters of the PT nameplate), when the direct current power supply is turned off, the open delta voltage at the secondary side of the PT is measured and sampled and is processed, the power frequency voltage component caused by the unbalanced voltage in the open delta voltage at the secondary side of the PT is filtered out, and only the decay oscillation component caused by the turning off of the direct current power supply is obtained. The decay oscillation component is frequency detected, and the size of the ground capacitance is obtained by using the relationship between the ground capacitance and the oscillation frequency.

[0072] The key points of the method are the control of the voltage change caused by the direct current power supply connected in parallel with the open delta voltage end at the secondary side of the PT, the filtering out of the power frequency component caused by the unbalanced voltage in the open delta voltage signal at the secondary side of the PT / the zero sequence voltage component when single-phase grounding, and the control of the turning on and turning off time of the direct current power supply. When the system is normally operated, the existence of the unbalanced voltage makes the open delta voltage at the secondary side of the PT not 0, and the voltage is a power frequency voltage. After the direct current power supply is turned on, the open delta voltage at the secondary side of the PT changes. After the direct current power supply is turned off, the open delta voltage at the secondary side of the PT should return to the original power frequency voltage state. In this transient process, the open delta voltage at the secondary side of the PT after filtering out the power frequency component presents a decay oscillation waveform related to the ground capacitance value, and the size of the ground capacitance can be obtained by using the relationship between the capacitance value and the decay oscillation frequency.

[0073] When the method is used to measure the ground capacitance of a neutral point ungrounded system or a neutral point high resistance grounded system, the inductance value resonating with the ground capacitance is the equivalent inductance size of the primary and secondary windings of the open delta voltage of the PT; when the method is used to measure the ground capacitance of a neutral point arc suppression coil grounded system, the inductance value resonating with the ground capacitance is the equivalent inductance size of the parallel connection of the equivalent inductance of the primary and secondary windings of the open delta voltage of the PT and the inductance of the arc suppression coil.

[0074] Specifically, please refer to Figure 2 The method comprises the following steps:

[0075] S1. The open delta voltage of the secondary side of the voltage transformer is sampled for a set duration as the first voltage sequence. After sampling starts, at time t1, the DC power supply is connected to the open delta terminal of the secondary side of the voltage transformer. At time t2, the DC power supply is disconnected from the open delta terminal of the secondary side of the voltage transformer. Time t2 is after time t1. The set duration is equal to t1 + t2.

[0076] Specifically, sampling of the open delta voltage on the secondary side of the PT begins at time 0. At time t1, switch S is closed, i.e., the DC power supply is activated. At time t2, switch S is opened, i.e., the DC power supply is deactivated. The control signal for switch S is as follows: Figure 3 As shown (low level indicates disconnect signal, high level indicates close signal).

[0077] S2. Delay the first voltage sequence by time t2 to obtain the second voltage sequence, and calculate the difference between the first voltage sequence and the second voltage sequence as the third voltage sequence.

[0078] Please refer to Figure 4 The first voltage sequence u0(t) is delayed by time t2 to obtain the second voltage sequence u. 01 (t), let u0(t) and u 01 (t) minus the result to get u 02 (t).

[0079] In a power frequency system, the signal delay time t2 should be a non-zero integer multiple of the power frequency period (0.02s), that is, it should satisfy the following formula:

[0080] 50t² = k;

[0081] In the formula, k is a non-zero integer.

[0082] S3. Calculate the system-to-ground capacitance value based on the oscillation frequency of the third voltage sequence.

[0083] Specifically, when the system is a neutral point ungrounded system (i.e.) Figure 1 (S1 switch is open), the equivalent inductance of the single-phase primary and secondary windings of the PT open delta is L. PT This value satisfies the following numerically:

[0084]

[0085] In the formula, L m The value of the magnetizing inductance of each phase of the PT is given.

[0086] A voltage transformer can be approximated as a transformer with an open secondary side; therefore, its primary and secondary windings (open delta terminals) can be equivalent to... Figure 5 As shown, Figure 5 In the diagram, L1 represents the primary side leakage inductance of each phase PT, and L'2 represents the secondary side leakage inductance of each phase PT converted to the primary side value.m This represents the magnetizing inductance value of each phase PT.

[0087] When switch S is open, the equivalent inductance of each phase can be expressed as L1+L. m Therefore, in the zero-sequence loop, the equivalent inductance L of the voltage transformer is... PT It can be represented as:

[0088]

[0089] Because of L m >> L1, therefore the above equation can be transformed into the following equation:

[0090]

[0091] The calculated value of the ground capacitance of an ungrounded neutral system can be obtained using the following formula:

[0092]

[0093] In the formula, f represents the third voltage sequence u 02 The oscillation frequency of (t).

[0094] When the system is a neutral point grounded system via an arc suppression coil (i.e.) Figure 1 With switch S1 closed, the calculated capacitance to ground is:

[0095]

[0096] In the formula, L is the inductance of the center point to ground.

[0097] The present invention conducted simulation tests on the above method. Specific simulation parameters were as follows: power line voltage of 10kV, arc suppression coil inductance of 0.4H, PT single-phase primary excitation inductance of 1H, DC power supply of 100V connected in parallel across the open delta of the PT, voltage divider resistor of 50Ω, leakage resistance of each phase of the three feeders of 12000Ω, and the three lines named P line, Q line, and M line respectively. The three-phase capacitances to ground of line P were 11μF, 10μF, and 13μF, the three-phase capacitances to ground of line Q were 5μF, 5μF, and 7μF, and the three-phase capacitances to ground of line M were 10μF, 10μF, and 10μF. The DC power supply was switched on at t1 of 0.4s and switched off at t2 of 0.6s, with a total simulation time of 1s.

[0098] The oscillation decay waveforms during the commissioning of different lines in the simulation are as follows: Figure 6 As shown in Table 1, the simulation results are as follows.

[0099] Table 1 Simulation Results

[0100]

[0101] Please refer to Figure 1 and 7 Embodiment two of the present application is:

[0102] Please refer to Figure 1 A system-to-ground capacitance measurement system includes a voltage transformer, an electric switch, a voltmeter and Figure 7 A system-to-ground capacitance measurement terminal shown in the figure, the system-to-ground capacitance measurement terminal is electrically connected with the electric switch and the voltmeter, the primary side of the voltage transformer is connected with three-phase power grid respectively, the DC power supply and the electric switch are connected in series and cross the two ends of the secondary side of the voltage transformer, and the voltmeter is connected with the two ends of the secondary side of the voltage transformer.

[0103] The system-to-ground capacitance measurement terminal 1 includes a memory 3, a processor 2 and a computer program stored in the memory 3 and capable of running on the processor 2, and the processor 2 realizes the steps of the above-mentioned embodiment one when executing the computer program.

[0104] In summary, the present application provides a system-to-ground capacitance measurement method and system, the on-off of the DC power supply is carried out at the secondary side of the voltage transformer, there is no unsafe factor for the equipment and the human body in the operation process, the danger is small, the sampling data processing algorithm is simple, the hardware implementation effect is relatively easy, the measured ground capacitance value is high in accuracy, small in error, is not affected by the symmetry degree of the power grid, is widely used in various neutral point grounding mode power grids.

[0105] The above-mentioned is only the embodiment of the present application, and does not limit the patent range of the present application, and any equivalent transformation, direct or indirect application in the related technical field by using the content of the specification and the drawings of the present application is also included in the patent protection range of the present application.

Claims

1. A method of measuring a system-to-ground capacitance, characterized by, The method comprises the steps of: S1, sampling the secondary side open delta voltage of the voltage transformer as a first voltage sequence, after sampling starts the moment the DC power supply accesses the secondary side open delta terminal of the voltage transformer, the moment the DC power supply exits the secondary side open delta terminal of the voltage transformer, the moment after the moment, the set duration is equal to ; S2, delaying the first voltage sequence a second voltage sequence is obtained, and a difference between the first voltage sequence and the second voltage sequence is calculated as a third voltage sequence; S3, calculating the system-to-ground capacitance value according to the oscillation frequency of the third voltage sequence; The system is a power frequency system, and the is a non-zero integer multiple of the power frequency period; The system is a neutral point ungrounded system, and step S3 specifically calculates the system-to-ground capacitance value according to the following formula: ； In the formula, is the oscillation frequency of the third voltage sequence, is the equivalent inductance of the open delta single-phase secondary winding of the voltage transformer. The system is a neutral point arc suppression coil grounding system, and step S3 specifically calculates the system-to-ground capacitance value according to the following formula: ； In the formula, L is the central point-to-ground inductance value, is the oscillation frequency of the third voltage sequence, is the equivalent inductance of the open-delta single-phase primary and secondary winding of the voltage transformer The voltage transformer open delta single-phase primary and secondary winding equivalent inductance The calculation is made according to the following formula: ； In the formula, is the magnetizing inductance of each phase of the voltage transformer.

2. A system-to-ground capacitance measuring system comprising a voltage transformer, an electric switch, a voltmeter and a system-to-ground capacitance measuring terminal, the system-to-ground capacitance measuring terminal being electrically connected to the electric switch and the voltmeter, the primary side of the voltage transformer being connected to three phases of a power grid respectively, a DC power supply and the electric switch being connected in series and then being connected across the two ends of the secondary side of the voltage transformer, and the voltmeter being connected to the two ends of the secondary side of the voltage transformer; The system for measuring earth capacitance comprises a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, The processor implements the following steps when executing the computer program: S1, sampling the secondary side open delta voltage of the voltage transformer as the first voltage sequence, after the sampling starts the moment the DC power supply accesses the secondary side open delta terminal of the voltage transformer, the moment the DC power supply exits the secondary side open delta terminal of the voltage transformer, the moment after the moment, the set duration is equal to ; S2, delaying the first voltage sequence a time to obtain a second voltage sequence, and calculating a difference between the first voltage sequence and the second voltage sequence as a third voltage sequence; S3, calculating the system-to-ground capacitance value according to the oscillation frequency of the third voltage sequence; The system is a power frequency system, and the is a non-zero integer multiple of the power frequency period; The system is a neutral point ungrounded system, and step S3 specifically calculates the system-to-ground capacitance value according to the following formula: ； In the formula, is the oscillation frequency of the third voltage sequence, is the equivalent inductance of the open delta single-phase secondary winding of the voltage transformer. The system is a neutral point arc suppression coil grounding system, and step S3 specifically calculates the system-to-ground capacitance value according to the following formula: ； In the formula, L is the central point-to-ground inductance value, is the oscillation frequency of the third voltage sequence, is the equivalent inductance of the open-delta single-phase primary and secondary winding of the voltage transformer The voltage transformer open delta single-phase primary and secondary winding equivalent inductance The calculation is made according to the following formula: ； In the formula, is the magnetizing inductance of each phase of the voltage transformer.

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