Method and device for measuring residual magnetism of power transformer based on excitation voltage integration method

Abstract

The invention discloses a power transformer residual magnetism measuring method and device based on an excitation voltage integration method, and the device comprises a control module which controls an excitation power supply to generate an excitation voltage signal, and carries out the digital processing of collected voltage and current signals; the peripheral equipment comprises a touch screen, a USB (Universal Serial Bus) interface card and an embedded printer; the excitation power supply is used for outputting an excitation signal, and the output voltage signal is programmed by the control module; and the voltage and current acquisition module comprises a voltage transformer, a current transformer and a dual-channel analog-to-digital converter and is used for acquiring voltage at two ends of the transformer winding and current flowing through the transformer winding, and the acquired data is transmitted to the control module for analysis and calculation through the dual-channel analog-to-digital converter. According to the method, the residual magnetism of the iron core can be reflected by utilizing the percentage of the residual magnetic field intensity and the saturated magnetic field intensity of the transformer iron core, quantitative evaluation of the residual magnetism of the transformer iron core is realized, and a basis is provided for a subsequent demagnetization process and a demagnetization effect; and the test process is simple to operate.

Description

Technical Field

[0001] The invention relates to the field of power transformers, and in particular to a method and a device for measuring residual magnetism of a power transformer based on an excitation voltage integration method. Background Art

[0002] Power transformer is one of the important equipment for transmitting electric energy. The quality and reliability of the transformer are directly related to the safe and reliable operation of the power grid. Power transformers need to undergo preventive tests after commissioning and maintenance.

[0003] Due to the inherent hysteresis of ferromagnetic materials, residual magnetism will be generated in the transformer core after the DC test of the transformer. When the transformer is put into operation, the residual magnetism of the core will saturate the transformer core half-cycle, and generate a large number of harmonics in the excitation current, which not only increases the reactive power consumption of the transformer and shortens the life of the transformer, but also may cause the relay protection device to malfunction, resulting in serious equipment accidents and economic losses. Therefore, the transformer must be demagnetized after the DC test to ensure the safe and stable operation of the transformer.

[0004] At present, there are two main demagnetization methods used at home and abroad: DC and AC. However, the residual magnetism of the transformer is generally measured by indirect measurement. Even if some products use methods such as saturation time to measure the residual magnetism of the transformer, the excitation process of the transformer is simplified to a constant voltage process, and the measurement error is relatively large. Summary of the invention

[0005] The present invention aims to solve the above problems existing in the prior art and provide a method and device for measuring residual magnetism of a power transformer based on an excitation voltage integration method.

[0006] The technical solution of the present invention is: a method for measuring residual magnetism of a power transformer based on an excitation voltage integration method, the steps of which are:

[0007] Step 1: Equivalent the winding on one side of the power transformer to an RLC circuit, where RL is connected in series and then in parallel with C, where R is the resistance of the winding, L is the equivalent inductance of the winding, and C is the capacitance between the turns of the winding and the bushing. When working at a low frequency, the capacitance between the turns can be ignored;

[0008] Step 2: When the voltage at both ends of the power transformer is slowly increased, the effect of the capacitor C on the circuit can be ignored. Assuming that the magnetic field in the transformer core is uniform or the strength ratio of the magnetic field is stable, the relationship between the voltage U and the current I at both ends of the winding can be expressed as:

[0009]

[0010] Where:

[0011] N is the number of turns of the transformer winding;

[0012] S is the cross-sectional area of ​​the core;

[0013] From formula 1), it can be deduced that the relationship between the saturation magnetic field intensity Bs and the voltage and current is as follows:

[0014]

[0015] Where:

[0016] Br: initial magnetic field strength;

[0017] Formula 2) can be written as:

[0018]

[0019] Where:

[0020] Bs: saturation magnetic field intensity;

[0021] Step 3: When the current and voltage are applied positively from the zero voltage state without considering the direction of the current and voltage, the formula for magnetic field saturation is as follows:

[0022]

[0023] Ignoring the reverse voltage direction from zero voltage, the formula for magnetic field saturation is as follows:

[0024]

[0025] Step 4: Combining the above formulas, we can get:

[0026]

[0027] Step 5: If B% is used to represent the residual magnetic rate of the transformer, and the size is calculated by the ratio of Br to Bs, the calculation formula for the residual magnetic rate is as follows:

[0028]

[0029] Where:

[0030] B%: residual magnetic rate of transformer core;

[0031] R: resistance of transformer winding;

[0032] U - (t): voltage function when reverse voltage is injected;

[0033] U + (t): voltage function when the forward voltage is injected;

[0034] I -(t): Current function when reverse voltage is injected;

[0035] I + (t): Current function when the forward voltage is injected;

[0036] Considering that the DC resistance of the transformer winding is very small in practice, formula 8) can be simplified as follows:

[0037]

[0038] Where:

[0039] B%: residual magnetic rate of transformer core;

[0040] U - (t): voltage function when reverse voltage is injected;

[0041] U + (t): Voltage function when forward voltage is injected.

[0042] A residual magnetism measuring device for a power transformer based on an excitation voltage integration method, comprising:

[0043] The control module is used to control the entire test process, including communicating with peripheral equipment, controlling the excitation power supply to generate an excitation voltage signal, and digitally processing the collected voltage and current signals;

[0044] Peripheral devices, including a touch screen for information output and input, a USB interface card for data export, and an embedded printer for report printing;

[0045] Excitation power supply, used to output excitation signal, the output voltage signal is programmed by the control module;

[0046] The voltage and current acquisition module includes a voltage transformer, a current transformer and a dual-channel analog-to-digital converter, which is used to collect the voltage at both ends of the transformer winding and the current flowing through the transformer winding. The collected data is sent to the control module for analysis and calculation through the dual-channel analog-to-digital converter;

[0047] Furthermore, the control module adopts a single-chip microcomputer of the 8051 series, and the dual-channel analog-to-digital converter adopts an ADS7862 analog-to-digital converter.

[0048] Furthermore, the maximum sampling rate of the voltage and current acquisition module is 500KS / s.

[0049] The present invention can use the percentage of the residual magnetic field strength of the transformer core to the saturation magnetic field strength to reflect the residual magnetism of the core, realize quantitative evaluation of the residual magnetism of the transformer core, and provide a basis for the subsequent demagnetization process and demagnetization effect; and the test process is simple to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is an equivalent structure diagram of the winding on one side of the transformer in the present invention;

[0051] Figure 2 The transformer winding is equivalent to an RLC circuit structure diagram in the present invention;

[0052] Figure 3 It is a circuit block diagram of a residual magnetism measuring device for a power transformer based on an excitation voltage integration method of the present invention;

[0053] Figure 4 The present invention is a test principle diagram of a residual magnetism measuring device for a power transformer based on an excitation voltage integration method. DETAILED DESCRIPTION

[0054] A method for measuring residual magnetism of a power transformer based on an excitation voltage integration method, the steps of which are:

[0055] Step 1: The power transformer is generally a three-phase integrated structure. Taking the YN winding with a neutral point as an example, the equivalent structure of one side of the transformer winding is as follows: Figure 1 As shown; in order to facilitate analysis and eliminate interference from other windings, taking a single winding as an example, the transformer winding is equivalent to an RLC circuit, as shown Figure 2 As shown, the connection method is RL connected in series and then in parallel with C, where R is the resistance of the winding, L is the equivalent inductance of the winding, and C is the capacitance between the turns of the winding and the bushing. When working at low frequency, the inter-turn capacitance can be ignored; where R is the DC resistance of the winding, L is the inductance of the winding, and C is the inter-turn capacitance and the bushing capacitance.

[0056] Step 2: When the voltage at both ends of the power transformer is slowly increased, the effect of the capacitor C on the circuit can be ignored. Assuming that the magnetic field in the transformer core is uniform or the strength ratio of the magnetic field is stable, the relationship between the voltage U and the current I at both ends of the winding can be expressed as:

[0057]

[0058] Where:

[0059] N is the number of turns of the transformer winding;

[0060] S is the cross-sectional area of ​​the core;

[0061] From formula 1), it can be deduced that the relationship between the saturation magnetic field intensity Bs and the voltage and current is as follows:

[0062]

[0063] Where:

[0064] Br: initial magnetic field strength;

[0065] Formula 2) can be written as:

[0066]

[0067] Where:

[0068] Bs: saturation magnetic field intensity;

[0069] Step 3: When the current and voltage are applied positively from the zero voltage state without considering the direction of the current and voltage, the formula for magnetic field saturation is as follows:

[0070]

[0071] Ignoring the reverse voltage direction from zero voltage, the formula for magnetic field saturation is as follows:

[0072]

[0073] Step 4: Combining the above formulas, we can get:

[0074]

[0075] Step 5: If B% is used to represent the residual magnetic rate of the transformer, and the size is calculated by the ratio of Br to Bs, the calculation formula for the residual magnetic rate is as follows:

[0076]

[0077] Where:

[0078] B%: residual magnetic rate of transformer core;

[0079] R: resistance of transformer winding;

[0080] U - (t): voltage function when reverse voltage is injected;

[0081] U + (t): voltage function when the forward voltage is injected;

[0082] I - (t): Current function when reverse voltage is injected;

[0083] I + (t): Current function when the forward voltage is injected;

[0084] Considering that the DC resistance of the transformer winding is very small in practice, formula 8) can be simplified as follows:

[0085]

[0086] Where:

[0087] B%: residual magnetic rate of transformer core;

[0088] U - (t): voltage function when reverse voltage is injected;

[0089] U + (t): Voltage function when forward voltage is injected.

[0090] A residual magnetism measuring device for a power transformer based on the excitation voltage integration method, such as Figure 3 As shown, including:

[0091] The control module is used to control the entire test process, including communicating with peripheral equipment, controlling the excitation power supply to generate an excitation voltage signal, and digitally processing the collected voltage and current signals;

[0092] Peripheral devices, including a touch screen for information output and input, a USB interface card for data export, and an embedded printer for report printing;

[0093] Excitation power supply, used to output excitation signal, the output voltage signal is programmed by the control module;

[0094] Voltage and current acquisition module, including voltage transformer, current transformer and dual-channel analog-to-digital converter, combined Figure 2 , which is used to collect the voltage across the transformer winding and the current flowing through the transformer winding. The collected data is sent to the control module for analysis and calculation through a dual-channel analog-to-digital converter.

[0095] Furthermore, the control module adopts a single-chip microcomputer of the 8051 series, and the development program adopts C++ language; the dual-channel analog-to-digital converter adopts an ADS7862 analog-to-digital converter.

[0096] Furthermore, the maximum sampling rate of the voltage and current acquisition module is 500KS / s.

[0097] Test results:

[0098] The demagnetization test was carried out using a power transformer. Since the transformer core has no residual magnetism, an excitation power supply was used to excite the transformer on the low-voltage side to generate an initial magnetic field in the transformer core. The magnetic field was then measured using the measuring device (tester) through the high-voltage side of the transformer. The sampling rate of this tester is 250KS / s. The measurement principle is as follows: Figure 4 shown.

[0099] After measurement, the relationship between the excitation current Is and the residual magnetism measured by the instrument is shown in the following table:

[0100]

[0101] It can be seen from the above table that the test results of the residual magnetism of the equipment have a relatively good positive correlation with the excitation current. In this way, the magnetic field strength of the transformer can be well evaluated, providing a basis for the subsequent demagnetization process and demagnetization effect.

[0102] The above are only specific embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for measuring residual magnetism of a power transformer based on an excitation voltage integration method, characterized in that: The following steps are involved: Step 1: Equivalent the winding on one side of the power transformer to an RLC circuit, where RL is connected in series and then in parallel with C, where R is the resistance of the winding, L is the equivalent inductance of the winding, and C is the capacitance between the turns of the winding and the bushing. When working at a low frequency, the capacitance between the turns can be ignored; Step 2: When the voltage at both ends of the power transformer is slowly increased, the effect of the capacitor C on the circuit can be ignored. Assuming that the magnetic field in the transformer core is uniform or the strength ratio of the magnetic field is stable, the relationship between the voltage U and the current I at both ends of the winding can be expressed as: Where: N is the number of turns of the transformer winding; S is the cross-sectional area of ​​the core; From formula 1), it can be deduced that the relationship between the saturation magnetic field intensity Bs and the voltage and current is as follows: Where: Br: initial magnetic field strength; Formula 2) can be written as: Where: Bs: saturation magnetic field intensity; Step 3: When the current and voltage are applied positively from the zero voltage state without considering the direction of the current and voltage, the formula for magnetic field saturation is as follows: Ignoring the reverse voltage direction from zero voltage, the formula for magnetic field saturation is as follows: Step 4: Combining the above formulas, we can get: Step 5: If B% is used to represent the residual magnetic rate of the transformer, and the size is calculated by the ratio of Br to Bs, the calculation formula for the residual magnetic rate is as follows: Where: B%: residual magnetic rate of transformer core; R: resistance of transformer winding; U - (t): voltage function when reverse voltage is injected; U + (t): voltage function when the forward voltage is injected; I - (t): Current function when reverse voltage is injected; I + (t): Current function when the forward voltage is injected; Considering that the DC resistance of the transformer winding is very small in practice, formula 8) can be simplified as follows: Where: B%: residual magnetic rate of transformer core; U - (t): voltage function when reverse voltage is injected; U + (t): Voltage function when forward voltage is injected.

2. A residual magnetism measuring device for a power transformer based on an excitation voltage integration method, characterized in that: include: The control module is used to control the entire test process, including communicating with peripheral equipment, controlling the excitation power supply to generate an excitation voltage signal, and digitally processing the collected voltage and current signals; Peripheral devices, including a touch screen for information output and input, a USB interface card for data export, and an embedded printer for report printing; Excitation power supply, used to output excitation signal, the output voltage signal is programmed by the control module; The voltage and current acquisition module, including a voltage transformer, a current transformer and a dual-channel analog-to-digital converter, is used to collect the voltage at both ends of the transformer winding and the current flowing through the transformer winding. The collected data is sent to the control module for analysis and calculation through the dual-channel analog-to-digital converter.

3. The residual magnetism measuring device for power transformer based on excitation voltage integration method according to claim 1, characterized in that: The control module adopts a single-chip microcomputer of the 8051 series, and the dual-channel analog-to-digital converter adopts an ADS7862 analog-to-digital converter.

4. The residual magnetism measuring device for power transformer based on excitation voltage integration method according to claim 1, characterized in that: The maximum sampling rate of the voltage and current acquisition module is 500KS / s.

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