Current transformer residual magnetism elimination circuit and method

Abstract

The invention discloses a residual magnetism elimination circuit and method for a current transformer, and the method employs a fluxgate principle, applies sine excitation voltage to a secondary winding of the current transformer, extracts a fluxgate signal through a response current, and achieves the detection of the residual magnetism of an iron core of the current transformer. And according to the detected residual magnetism flux linkage information, accurately calculating a voltage parameter required by demagnetization, and generating a degaussing voltage with a specific waveform. And finally, applying the degaussing voltage obtained by calculation to the secondary side of the current transformer, controlling the whole degaussing time to continue at the moment at which the fundamental wave angular frequency can be rotated to + / -90 degrees, and accurately eliminating residual magnetism by utilizing sine waves of a quarter period. According to the method, rapid demagnetization of the iron core of the current transformer can be realized, the reliability and safety of a power system are improved, and demagnetization can be carried out in the running process of the current transformer without offline operation or change of an original transformer structure. The residual magnetism of the iron core is not changed in the detection process, and the normal operation of the current transformer is ensured.

Description

Technical Field

[0001] The present invention relates to the field of current transformer demagnetization, and in particular to a circuit and method for eliminating residual magnetism of a current transformer. Background Art

[0002] As a key secondary device in the power system, the stability and reliability of the current transformer are crucial to the safe operation of the power system. The magnetization curve and hysteresis loop of the ferromagnetic material of the current transformer are important characteristics of the material. The residual magnetic induction intensity of the iron core of the current transformer has many effects on the current transformer. The current transformer is often in a biased state during operation, resulting in distortion of the secondary side transmission current. Due to the inherent hysteresis phenomenon of ferromagnetic materials, residual magnetism will also remain in the iron core after the voltage ratio and DC resistance measurement of the power current transformer. The residual magnetism saturates the iron core of the current transformer and generates a large number of harmonics in the excitation current, which not only increases the reactive power consumption of the current transformer, but also may cause the relay protection device to malfunction.

[0003] This study conducts an in-depth analysis of the rapid elimination method of residual magnetism / bias magnetism of protective current transformers, and proposes a demagnetization method based on flux information. The effectiveness of this method is verified through simulation analysis. This method can achieve rapid demagnetization of the core of the current transformer, improving the reliability and safety of the power system.

[0004] At present, the demagnetization methods used at home and abroad mainly include DC demagnetization, AC demagnetization, switch resistor demagnetization, voltage parallel negative feedback, additional material demagnetization, RL inductance energy storage, etc. Conventional AC demagnetization equipment demagnetizes thoroughly, but requires larger test equipment and takes a long time to demagnetize; in addition, the switch resistor demagnetization method and voltage parallel negative feedback method can only be used online, and the RL inductance energy storage method can only be used offline, both of which have certain limitations. Summary of the invention

[0005] The purpose of the present invention is to provide a current transformer demagnetization method based on flux linkage information. The method utilizes the fluxgate principle, applies an excitation voltage to the secondary winding of the current transformer, extracts the fluxgate signal by responding to the current, and can extract the fluxgate signal for magnetic detection without changing the residual magnetism of the current transformer core, thereby quickly demagnetizing the current transformer core and improving the reliability and safety of the power system.

[0006] To achieve the above object, the present invention adopts the following technical solution: The present invention provides a current transformer residual magnetism elimination circuit, the circuit includes a current transformer, a DC power supply V dc , single phase inverter, load switch SW 1. Residual magnetism detection circuit switch SW2. Current measurement auxiliary CT, bandpass filter, harmonic analysis module and load resistor R L ; Characterized in that the current measurement auxiliary CT is connected to the current transformer, the bandpass filter is connected to the current measurement auxiliary CT, and the residual magnetism detection circuit switch SW 2 is connected to the current transformer, the unidirectional inverter and the DC power supply V dc The current transformer is connected to the harmonic analysis module; The DC power supply V dc and a single-phase inverter to generate a sinusoidal excitation voltage; load switches SW 1 and residual magnetism detection circuit switch SW 2 Used to switch in and out single-phase inverters and load resistors R L ; Current measurement auxiliary CT is used for secondary current measurement; Bandpass filter filters the secondary current output and normal secondary current under power frequency i 2out = i 2L , maintain the normal operation of the devices carried by the current transformer; the harmonic analysis module is used to extract the harmonic current used for residual magnetism detection i 2nd .

[0007] The present invention also provides a method for eliminating residual magnetism of a current transformer, the method comprising the following steps: (1) Before demagnetization, the single-phase inverter is required to output a periodic sinusoidal excitation voltage, and the harmonic analysis module calculates the amplitude and phase of the second harmonic; then, based on the residual magnetism information, a specific waveform is applied to the secondary side of the protective current transformer for demagnetization, and the value of the magnetic flux during the demagnetization process is calculated; the value of the magnetic flux during the demagnetization process is calculated , which is expressed as follows: ;in, is the demagnetization voltage amplitude; is the angular frequency of the demagnetizing voltage; is the starting phase angle of the demagnetization voltage; is the residual magnetic flux of the core; N 1. N 2 are the turns of the primary and secondary sides of the transformer respectively; is the angular frequency of the system operation; It is a purely resistive load on the secondary side; is the demagnetization start time; (2) According to the residual magnetic flux information detected in the current transformer, calculate the voltage parameters required for demagnetization and set the sinusoidal demagnetization voltage , which is expressed as follows: ; (3) For the transient component where the flux linkage cannot change suddenly, the entire demagnetization time is controlled to last at a moment when the fundamental angular frequency can rotate to ±90°, that is, the starting phase angle of the demagnetization voltage is controlled. is 90° and the starting time t 1 Satisfaction ; (4) Control , using a quarter-cycle sine wave to eliminate it specifically.

[0008] Furthermore, in the online demagnetization state, the frequency of the demagnetization power supply used should satisfy that the system fundamental frequency is an even multiple of the demagnetization frequency; and in the offline demagnetization state, the frequency of the demagnetization voltage can be arbitrarily selected.

[0009] Specifically, during the offline demagnetization, a voltage of 2.5V-10V needs to be added to the original demagnetization voltage to eliminate the influence of coercive force.

[0010] Specifically, the final magnetic flux linkage of the offline demagnetization is controlled within ±0.2% after eliminating the coercive force.

[0011] Furthermore, the online demagnetization can eliminate the bias magnetism generated by the current transformer during operation.

[0012] Specifically, the demagnetization voltage can work effectively using a demagnetization voltage of 10-35 Hz.

[0013] Furthermore, the online demagnetization is applicable to demagnetization when the core is at any different residual magnetization level.

[0014] Compared with the prior art, the present invention has the following beneficial effects: The present invention can realize online rapid demagnetization of the core of the current transformer without offline operation or changing the original transformer structure; the core residual magnetism is not changed during the detection process, ensuring the normal operation of the current transformer; by increasing the flux change and the magnetic field saturation depth during the detection process, the detection accuracy is improved. It is suitable for current transformers under various operating conditions and improves the stability and reliability of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The present invention is a circuit diagram of residual magnetism detection of a protective current transformer; Figure 2 It is the magnetic flux waveform diagram of the 50% initial residual magnetism offline demagnetization process of the present invention; Figure 3 This is a waveform diagram (load resistance continuous current) of the 50% initial residual magnetism offline demagnetization process of the present invention; Figure 4 It is a schematic diagram of the principle of the DC rapid demagnetization method of the present invention; Figure 5 This is a waveform diagram (controlling coercive force) of the 50% initial residual magnetism offline demagnetization process of the present invention; Figure 6 hysteresis curve diagram of offline demagnetization with different initial residual magnetism of the present invention; Figure 7 This is a waveform diagram of the 50% initial residual magnetism online demagnetization process (primary side current 2000 A) of the present invention; Figure 8 This is a diagram of the demagnetization results of the current transformer of the present invention under different primary side currents. DETAILED DESCRIPTION

[0016] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0017] like Figure 1 As shown in the figure, the residual magnetism detection circuit of the current transformer for protection mainly includes a DC power supply V dc , single phase inverter, load switch SW 1. Residual magnetism detection circuit switch SW 2. Current measurement auxiliary CT, bandpass filter, harmonic analysis module, load resistor R L The invention is characterized in that the auxiliary CT is connected to the current transformer, the bandpass filter is connected to the auxiliary CT, and the residual magnetism detection circuit switch SW 2 is connected to the current transformer, the unidirectional inverter and the DC power supply V dc The harmonic analysis module is connected to the current transformer.

[0018] DC power supply V dc and a single-phase inverter to generate a sinusoidal excitation voltage; load switches SW 1 and residual magnetism detection circuit switch SW 2 Used to switch in and out single-phase inverters and load resistors R L ; Current measurement auxiliary CT is used for secondary current measurement; Bandpass filter filters the secondary current output Normal secondary current under power frequency (50Hz) i 2out = i 2L , maintain the normal operation of the devices carried by the current transformer; the harmonic analysis module is used to extract the harmonic current used for residual magnetism detection i 2nd .

[0019] Before demagnetization begins, the single-phase inverter needs to output a periodic sinusoidal excitation voltage, and the harmonic analysis module calculates the amplitude and phase of the second harmonic; then, based on the residual magnetism information, a specific waveform is applied to the secondary side of the protective current transformer for demagnetization.

[0020] The offline degaussing process is as follows: Take the 25 Hz sine wave demagnetization as an example. When the initial residual magnetism of the current transformer is 50% saturation flux, Figure 2 As shown in Figure 2, the transformer was demagnetized in stage 2 (0.005-0.015s). The demagnetization power supply frequency is 25Hz, and a quarter-cycle waveform is applied, i.e., 10 ms. According to the core parameters, its saturation flux is 0.11254 Wb, and when the initial remanence is 50%, the required demagnetization voltage amplitude is: ; in, is the angular frequency of the demagnetizing voltage; is the residual magnetic flux of the core.

[0021] When the demagnetization phase is over, the single-phase inverter is removed and the secondary coil of the transformer continues to flow through the load resistor. After the demagnetization is completed, the current will slowly decay and the flux will also change accordingly, such as Figure 3 When the demagnetization phase is completed, Figure 3 (a) shows that the excitation current decays exponentially. Figure 3 (b) shows a slight increase in magnetic flux, due to Figure 3 From (c) in the figure, we can see that the final stable flux linkage is 4.8%. This is called the "return point memory effect", which is manifested in this case as the hysteresis curve ending at the same "coercivity point" and the final remanence is the same when the excitation current decays to zero.

[0022] In order to reduce the influence of coercive force on the demagnetization result, this method does not end the demagnetization when the flux becomes zero for the first time, but slowly reduces the power supply amplitude, thereby reducing the hysteresis curve, and finally controlling the flux and excitation current to be zero. Therefore, it is necessary to increase the voltage on the original demagnetization voltage to eliminate the influence of coercive force. The magnetic flux in the demagnetization process is as follows: Figure 4 As shown in (a) in the figure, the demagnetization voltage waveform is as follows Figure 4 As shown in (a) in the figure, it is experimentally measured that Figure 4 In (b), the magnetic flux at point D is 1 / 16 of the saturation magnetic flux, so the demagnetization voltage should be: ; is the first quadrant demagnetization voltage; is the demagnetization voltage of the second, third and fourth quadrants; is the saturation flux.

[0023] Using this voltage, the current transformer with an initial residual magnetism of 50% saturation flux is demagnetized offline. The waveform during demagnetization is as follows: Figure 5 shown. Figure 5 In the process, the transformer was demagnetized in stage 2 (0.01-0.04 s), in which the residual magnetism was eliminated to zero in 0.01-0.02 s, but the excitation current was not zero; the excitation current was controlled in 0.02-0.04 s, and at 0.04 s, both the flux linkage and the excitation current were controlled to be close to zero. Figure 5 (a) and Figure 5 In (b), after the magnetic flux becomes zero at 0.04 s, the final excitation current is less than one tenth of the excitation current at 0.02 s, and its subsequent effect on residual magnetism is relatively small.

[0024] Under different initial remanence conditions, the demagnetization voltage shown in the above formula is used to demagnetize the current transformer. The hysteresis curve during the partial demagnetization process is shown in the figure below: Figure 6 The offline demagnetization hysteresis curves with different initial remanence are shown. Figure 6 (a) is the voltage waveform on the secondary side of the current transformer during the whole process. In stage 2, the secondary side voltage of the current transformer is a PWM square wave generated by the inverter, and its equivalent voltage is the reference voltage corresponding to the red line in the figure. Figure 6 (b) in the figure is the flux waveform of the transformer core during the whole process. During the demagnetization process in stage 2, the core flux changes from 50% to 0. During the demagnetization process, the transformer flux is not saturated, and the overall amplitude of the excitation current is maintained in a small range.

[0025] The process of online demagnetization is as follows: Online demagnetization requires the ability to eliminate the bias magnetization of the current transformer during online operation. When the effective value of the primary current of the current transformer is 2000 A and the initial operation bias magnetization is 50%, the waveforms during the demagnetization process are as follows Figure 7 shown.

[0026] exist Figure 7 In the figure, 0-0.03 s is stage 1, 0.03-0.06 s is stage 2, and after 0.06 s is stage 3. During stage 2, the inverter is put into operation and the current transformer is demagnetized; the current transformers in stages 1 and 3 are in normal operation. Figure 7 (a) is the voltage waveform on the secondary side of the current transformer during the whole process. In stage 2, the secondary side voltage of the current transformer is a PWM square wave generated by the inverter, and its equivalent voltage is the reference voltage corresponding to the red line in the figure. Figure 7 (b) is the flux waveform of the transformer core during the whole process. In the demagnetization process of stage 2, the core flux changes from 50% to 0. During the demagnetization process, the transformer flux is not saturated, and the overall amplitude of the excitation current is maintained in a small range, such as Figure 7 As shown in (c) in . Figure 7 In (d), the hysteresis curve fluctuates within the range of 40%-50% in stage 1, the hysteresis curve decreases from 50% to 0 in stage 2, and the hysteresis curve fluctuates within the range of ±10% in stage 3.

[0027] Demagnetization is performed on the current transformer at different primary currents and when the core is at different residual magnetism levels. The final operating bias magnetization results are as follows: Figure 8 shown. Figure 8 The detection effects when the primary side current is 0A, 500A, 1kA and 2kA are considered. The primary side current of the current transformer is 0, which means that the current transformer is in an offline state. The rated current of this current transformer is 2kA.

[0028] Figure 8 In the figure, although the demagnetization results are slightly different under different primary currents, the curve change trends are basically the same. In addition, overall, under different primary currents, the final residual magnetism after demagnetization is within ±0.2%. This solution can achieve good online demagnetization.

[0029] Those skilled in the art will readily appreciate other embodiments of the present application after considering the specification and practicing the contents disclosed herein. The present application is intended to cover any variations, uses or adaptations of the present application, which follow the general principles of the present application and include common knowledge or customary technical means in the art that are not disclosed in the present application.

[0030] It should be understood that the present application is not limited to the exact construction that has been described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof.

Claims

1. A current transformer residual magnetism elimination circuit, characterized in that: The circuit includes a current transformer, a DC power supply V dc , single phase inverter, load switch SW 1. Residual magnetism detection circuit switch SW 2. Current measurement auxiliary CT, bandpass filter, harmonic analysis module and load resistor R L ; Characterized in that the current measurement auxiliary CT is connected to the current transformer, the bandpass filter is connected to the current measurement auxiliary CT, and the residual magnetism detection circuit switch SW 2 is connected to the current transformer, the unidirectional inverter and the DC power supply V dc The current transformer is connected to the harmonic analysis module; The DC power supply V dc and a single-phase inverter to generate a sinusoidal excitation voltage; load switches SW 1 and residual magnetism detection circuit switch SW 2 Used to switch in and out single-phase inverters and load resistors R L ; Current measurement auxiliary CT is used for secondary current measurement; Bandpass filter filters the secondary current output and normal secondary current under power frequency i 2out = i 2L , maintain the normal operation of the devices carried by the current transformer; the harmonic analysis module is used to extract the harmonic current used for residual magnetism detection i 2nd .

2. A method for eliminating residual magnetism of a current transformer according to claim 1, characterized in that: The method comprises the following steps: (1) Before demagnetization, the single-phase inverter is required to output a periodic sinusoidal excitation voltage, and the harmonic analysis module calculates the amplitude and phase of the second harmonic; then, based on the residual magnetism information, a specific waveform is applied to the secondary side of the protective current transformer for demagnetization, and the value of the magnetic flux during the demagnetization process is calculated; the value of the magnetic flux during the demagnetization process is calculated , which is expressed as follows: ;in, is the demagnetization voltage amplitude; is the angular frequency of the demagnetizing voltage; is the starting phase angle of the demagnetization voltage; is the residual magnetic flux of the core; N 1. N 2 are the turns of the primary and secondary sides of the transformer respectively; is the angular frequency of the system operation; It is a purely resistive load on the secondary side; is the demagnetization start time; (2) According to the residual magnetic flux information detected in the current transformer, calculate the voltage parameters required for demagnetization and set the sinusoidal demagnetization voltage , which is expressed as follows: ； (3) For the transient component where the flux linkage cannot change suddenly, the entire demagnetization time is controlled to last at a moment when the fundamental angular frequency can rotate to ±90°, that is, the starting phase angle of the demagnetization voltage is controlled. is 90° and the starting time t 1 Satisfaction ; (4) Control , using a quarter-cycle sine wave to eliminate it specifically.

3. The method according to claim 2, characterized in that In the online demagnetization state, the frequency of the demagnetization power supply used should satisfy the system fundamental frequency which is an even multiple of the demagnetization frequency; while in the offline demagnetization state, the frequency of the demagnetization voltage can be selected arbitrarily.

4. The method according to claim 2, characterized in that: During the offline demagnetization, a voltage of 2.5V to 10V needs to be added to the original demagnetization voltage to eliminate the influence of coercive force.

5. The method according to claim 4, characterized in that The offline demagnetization controls the final flux linkage within ±0.2% after eliminating the coercive force.

6. The method according to claim 2, characterized in that The online demagnetization can eliminate the bias magnetism generated by the current transformer during operation.

7. The method according to claim 2, characterized in that The demagnetization voltage can work effectively when the demagnetization voltage is 10-35Hz.

8. The method according to claim 2, characterized in that: The online demagnetization is applicable to demagnetization when the core is at any different residual magnetism levels.

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