A Residual Remanence Elimination Circuit and Method for Current Transformers

Abstract

The present invention discloses a residual remanence elimination circuit and method for a current transformer. The method utilizes the fluxgate principle to apply a sinusoidal excitation voltage to the secondary winding of the current transformer, extracts the fluxgate signal through the response current, and realizes the detection of the remanence of the iron core of the current transformer. Then, according to the detected remanence flux linkage information, the voltage parameters required for demagnetization are accurately calculated, and a demagnetization voltage with a specific waveform is generated. Finally, the calculated demagnetization voltage is applied to the secondary side of the current transformer, and the entire demagnetization time is controlled to last until the fundamental angular frequency rotates to ±90°, and the remanence is accurately eliminated using a quarter-cycle sine wave. This method can achieve rapid demagnetization of the iron core of the current transformer, improve the reliability and safety of the power system, and can demagnetize during the operation of the current transformer without offline operation or changing the original transformer structure. During the detection process, the remanence of the iron core can be kept unchanged to ensure the normal operation of the current transformer.

Description

Technical Field

[0001] The present invention relates to the field of current transformer degaussing, and particularly to a residual remanence elimination circuit and method for a current transformer. Background Art

[0002] As a key secondary device in the power system, the stability and reliability of a current transformer are crucial for 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, and the residual magnetic induction intensity of the iron core of the current transformer has various effects on the current transformer. The current transformer is often in a biased magnetic state during operation, resulting in distortion of the transmitted current on the secondary side. Due to the inherent hysteresis phenomenon of ferromagnetic materials, residual remanence will also remain in the iron core after operations such as voltage ratio measurement and DC resistance measurement of power current transformers. The remanence saturates the iron core of the current transformer, generating a large number of harmonics in the exciting current, which not only increases the reactive power consumption of the current transformer but also may cause misoperation of the relay protection device.

[0003] This research deeply analyzes a fast elimination method for remanence / biased magnetism of protective current transformers and proposes a degaussing method based on magnetic flux linkage information. The effectiveness of this method is verified through simulation analysis. This method can achieve fast degaussing of the iron core of the current transformer and improve the reliability and safety of the power system.

[0004] Currently, the degaussing methods used at home and abroad mainly include DC degaussing method, AC degaussing method, switching resistance degaussing method, voltage shunt negative feedback method, additional material demagnetization method, RL inductor energy storage method, etc. Conventional AC degaussing equipment can degauss thoroughly, but it requires large test equipment and takes a long time for degaussing; in addition, the switching resistance degaussing method and the voltage shunt negative feedback method can only be used online, while the RL inductor energy storage method can only be used offline, all of which have certain limitations. Summary of the Invention

[0005] The purpose of the present invention is to provide a current transformer degaussing method based on magnetic flux linkage information. This method utilizes the fluxgate principle, applies an exciting voltage to the secondary winding of the current transformer, extracts the fluxgate signal through the response current, and can extract the fluxgate signal for magnetic inspection without changing the remanence of the iron core of the current transformer, thereby quickly degaussing the iron core of the current transformer and improving the reliability and safety of the power system.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions: The present invention provides a residual remanence elimination circuit for a current transformer, which circuit includes a current transformer, a DC power supply V dc , a single-phase inverter, a load switch SW 1 , a remanence detection circuit switch SW2 Current measurement auxiliary CT, band-pass filter, harmonic analysis module and load resistor R L ; characterized in that the current measurement auxiliary CT is connected to the current transformer, the band-pass 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 single-phase inverter is connected to the DC power supply V dc is connected, and the current transformer is connected to the harmonic analysis module;

[0007] The DC power supply V dc and the single-phase inverter are used to generate a sinusoidal excitation voltage; the load switch SW 1 and the residual magnetism detection circuit switch SW 2 are used to cut in and cut out the single-phase inverter and the load resistor R L ; the current measurement auxiliary CT is used for secondary side current measurement; the band-pass filter filters the secondary side current and outputs the normal secondary side current at power frequency i 2out = i 2L , maintaining the normal operation of the device carried by the current transformer; the harmonic analysis module is used to extract the harmonic current for residual magnetism detection i 2nd .

[0008] The present invention also provides a method for eliminating the residual magnetism of a current transformer residual magnetism elimination circuit, and the method includes the following steps:

[0009] (1) Before demagnetization, 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, according to the residual magnetism information, a specific waveform is applied to the secondary side of the protection current transformer for demagnetization, and the value of the magnetic flux linkage during the demagnetization process is calculated; Calculate the value of the magnetic flux linkage during the demagnetization process

[0010] , and its expression is as follows: ; where is the demagnetization voltage amplitude; is the angular frequency of the demagnetization voltage; is the starting phase angle of the demagnetization voltage; is the remaining magnetic flux of the iron core; N 1 , N 2 are the number of turns of the primary side and secondary side of the transformer respectively; is the angular frequency of system operation; is the pure resistive load on the secondary side; is the degaussing start time;

[0011] (2) Calculate the voltage parameters required for degaussing based on the residual magnetic flux linkage information detected in the current transformer, and set the sinusoidal degaussing voltage , and its expression is as follows:

[0012] ;

[0013] (3) For the transient component due to the non-mutation of magnetic flux linkage, control the entire degaussing time to last until the fundamental angular frequency rotates to ±90°, that is, control the starting phase angle of the degaussing voltage to be 90° and the starting time t 1 satisfy ;

[0014] (4) Control , and use a quarter-cycle sine wave to eliminate it specifically.

[0015] Further, in the on-line degaussing state, the frequency of the degaussing power supply used should satisfy that the system fundamental frequency is an even multiple of the degaussing frequency; while in the off-line degaussing state, the frequency of the degaussing voltage can be arbitrarily selected.

[0016] Specifically, during the off-line degaussing, a voltage of 2.5V - 10V needs to be added to the original degaussing voltage to eliminate the influence of coercivity.

[0017] Specifically, after eliminating the coercivity during the off-line degaussing, the final magnetic flux linkage is controlled within ±0.2%.

[0018] Further, the on-line degaussing can also eliminate the bias magnetism generated during the operation of the current transformer.

[0019] Specifically, the degaussing voltage can effectively work with a degaussing voltage of 10 - 35Hz.

[0020] Further, the on-line degaussing is applicable to degaussing when the iron core is at any different residual magnetic levels.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] The present invention can achieve on-line rapid degaussing of the iron core of the current transformer without off-line operation or changing the original structure of the transformer; the residual magnetism of the iron core is not changed during the detection process, ensuring the normal operation of the current transformer; the detection accuracy is improved by increasing the magnetic flux linkage change amount and the magnetic field saturation depth during the detection process. It is applicable to current transformers under various operating conditions, improving the stability and reliability of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1Circuit diagram for detecting residual magnetism of current transformer for protection of the present invention;

[0024] Figure 2 Magnetic flux waveform diagram of the offline demagnetization process of 50% initial residual magnetism of the present invention;

[0025] Figure 3 Waveform diagram (load resistance freewheeling) of the offline demagnetization process of 50% initial residual magnetism of the present invention;

[0026] Figure 4 Schematic diagram of the principle of the DC rapid demagnetization method of the present invention;

[0027] Figure 5 Waveform diagram (control coercive force) of the offline demagnetization process of 50% initial residual magnetism of the present invention;

[0028] Figure 6 Magnetic hysteresis curve diagram of offline demagnetization with different initial residual magnetisms of the present invention;

[0029] Figure 7 Waveform diagram (primary side current 2000 A) of the online demagnetization process of 50% initial residual magnetism of the present invention;

[0030] Figure 8 Demagnetization result diagram of the current transformer under different primary side currents of the present invention. Detailed implementation manners

[0031] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a 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 those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0032] As Figure 1 shown, the residual magnetism detection circuit of the current transformer for protection mainly includes a DC power supply V dc , a single-phase inverter, a load switch SW 1 , a residual magnetism detection circuit switch SW 2 , a current measurement auxiliary CT, a band-pass filter, a harmonic analysis module, a load resistance R L and so on. It is characterized in that the auxiliary CT is connected to the current transformer, the band-pass filter is connected to the auxiliary CT, the residual magnetism detection circuit switch SW 2 is connected to the current transformer, the single-phase inverter is connected to the DC power supply V dc , and the harmonic analysis module is connected to the current transformer.

[0033] DC power supply V dc and a single-phase inverter are used to generate a sinusoidal excitation voltage; a load switch SW 1 and a residual magnetism detection circuit switch SW 2 are used to switch in and out the single-phase inverter and the load resistor R L ; a current measurement auxiliary CT is used for secondary side current measurement; a band-pass filter filters the secondary side current to output the normal secondary side current at the power frequency (50 Hz) i 2out = i 2L to maintain the normal operation of the device carried by the current transformer; a harmonic analysis module is used to extract the harmonic current for residual magnetism detection i 2nd .

[0034] Before the demagnetization starts, a single-phase inverter needs to output a one-cycle sinusoidal excitation voltage, and the harmonic analysis module calculates the amplitude and phase of the second harmonic; subsequently, a specific waveform is applied to the secondary side of the protective current transformer according to the residual magnetism information for demagnetization.

[0035] The offline demagnetization process is as follows:

[0036] Taking the offline demagnetization with a 25 Hz sine wave as an example, when the initial residual magnetism of the current transformer is 50% of the saturation magnetic flux, as Figure 2 shown, the demagnetization operation is performed on the transformer during stage 2 (0.005 - 0.015 s). The demagnetization power supply frequency is 25 Hz, and a quarter-cycle waveform, that is, 10 ms, is applied. According to the core parameters, its saturation magnetic flux linkage is 0.11254 Wb, and when the initial residual magnetism is 50%, the required demagnetization voltage amplitude is:

[0037] ;

[0038] where, is the angular frequency of the demagnetization voltage; is the remaining magnetic flux of the core.

[0039] When the demagnetization stage ends, the single-phase inverter is removed, and the secondary side coil of the transformer continues to flow through the load resistor. After the demagnetization ends, the current will slowly decay, and the magnetic flux linkage will also change accordingly, as Figure 3 shown. When the demagnetization stage ends, Figure 3 in (a) shows that the excitation current decays according to an exponential trend, Figure 3 in (b) shows that the magnetic flux linkage increases slightly, from Figure 3As can be seen from (c) in [reference], the final stable magnetic flux linkage is 4.8%. This is called the "return point memory effect", which in this case means that for the hysteresis curves ending at the same "coercivity point", when the excitation current decays to zero, the final remanence is the same.

[0040] To reduce the influence of coercivity on the demagnetization result, in this method, when the magnetic flux linkage first becomes zero, the demagnetization does not end. Instead, the power supply amplitude is slowly reduced to reduce the hysteresis curve, and finally both the magnetic flux linkage and the excitation current are controlled to be zero. Therefore, it is necessary to increase the voltage on the original demagnetization voltage to eliminate the influence of coercivity. The magnetic flux during the demagnetization process is as shown in (a) in [reference], and the demagnetization voltage waveform is as shown in (a) in [reference]. Measured through experiments, Figure 4 as shown in (a) in [reference], Figure 4 as shown in (a) in [reference], Figure 4 If it is better to take the magnetic flux linkage at point D in (b) in [reference] as 1 / 16 of the saturation magnetic flux linkage, then the demagnetization voltage should be:

[0041] ; is the demagnetization voltage in the first quadrant; is the demagnetization voltage in the second, third, and fourth quadrants; is the saturation magnetic flux linkage.

[0042] Using this voltage, an off-line demagnetization is performed on a current transformer with an initial remanence of 50% of the saturation magnetic flux. The waveforms during the demagnetization are as shown in [reference]. Figure 5 as shown. Figure 5 In [reference], during stage 2 (0.01 - 0.04 s), a demagnetization operation is performed on the transformer. During 0.01 - 0.02 s, the remanence is eliminated to zero, but the excitation current is not zero; during 0.02 - 0.04 s, the excitation current is controlled. At 0.04 s, both the magnetic flux linkage and the excitation current are controlled to be close to zero. According to (a) in [reference] and (b) in [reference], after the magnetic flux linkage 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 influence on the remanence is relatively small. Figure 5 in (a) in [reference] and Figure 5 in (b) in [reference],

[0043] Under different initial remanences, using the demagnetization voltage shown above, a demagnetization operation is performed on the current transformer. The hysteresis curves during part of the demagnetization process are as shown in [reference]. Figure 6 as shown in the hysteresis curves of off-line demagnetization with different initial remanences. Figure 6 The voltage waveform on the secondary side of the current transformer during the whole process in (a) in [reference]. During stage 2, the voltage on the secondary side 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 The magnetic flux linkage waveform of the transformer core during the whole process in (b) in [reference]. During the demagnetization process in stage 2, the magnetic flux linkage of the core changes from 50% to 0. During the elimination process, the magnetic flux linkage of the transformer does not saturate, and the overall amplitude of the excitation current is maintained within a small range.

[0044] The process of online degaussing is as follows:

[0045] Online degaussing is required to eliminate the bias magnetization of the current transformer during online operation. When the effective value of the primary-side current of the current transformer is 2000 A and the initial operating bias magnetization is 50%, the waveforms during the degaussing process are as Figure 7 shown.

[0046] In Figure 7 : 0 - 0.03 s is stage 1, 0.03 - 0.06 s is stage 2, and after 0.06 s is stage 3. During the time of stage 2, the inverter is put into operation to perform degaussing on the current transformer; the current transformer is in a normal operating state in both stage 1 and stage 3. Figure 7 In (a) of Figure 7 is the voltage waveform on the secondary side of the current transformer during the whole process. The voltage on the secondary side of the current transformer in stage 2 is the 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 In (b) of Figure 7 is the magnetic flux linkage waveform of the transformer core during the whole process. During the degaussing process in stage 2, the magnetic flux linkage of the core changes from 50% to 0. During the elimination process, the magnetic flux linkage of the transformer does not saturate, and the overall amplitude of the exciting current remains within a small range, as

[0047] In (c) of Figure 8 shown. Figure 8 In (d) of

[0048] Figure 8 : during stage 1, the hysteresis curve fluctuates in the range of 40% - 50%, during stage 2, the hysteresis curve decreases from 50% to 0, and during stage 3, the hysteresis curve fluctuates in the range of ±10%.

[0049] After considering the specification and the content disclosed herein, those skilled in the art will readily think of other embodiments of the present application. The present application aims to cover any variations, uses, or adaptations of the present application, and these variations, uses, or adaptations follow the general principles of the present application and include the common general knowledge or conventional technical means in the technical field not disclosed in the present application.

[0050] It should be understood that the present application is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope.

Claims

1. A method for eliminating residual magnetism of a current transformer, characterized in that: The circuit includes a current transformer, a DC power supply V dc , single-phase inverter, load switch SW1, 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, the residual magnetism detection circuit switch SW2 is connected to the current transformer, the unidirectional inverter is connected to the DC power supply V dc The current transformer is connected to the harmonic analysis module; The DC power supply V dc The single-phase inverter is used to generate a sinusoidal excitation voltage; the load switch SW1 and the residual magnetism detection circuit switch SW2 are used to switch in and out of the single-phase inverter and the load resistor R L ; Current measurement auxiliary CT is used for secondary current measurement; Bandpass filter filters the secondary current output Normal secondary current i under power frequency 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 i used for residual magnetism detection 2nd ; The elimination 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, a waveform is applied to the secondary side of the protective current transformer according to the residual magnetism information to perform demagnetization, and the value of the flux linkage during the demagnetization process is calculated; the value of the flux linkage during the demagnetization process ψ is calculated, and its expression is as follows: Among them, U dem is the demagnetization voltage amplitude; ω dem is the angular frequency of the demagnetization voltage; α dem is the starting phase angle of the demagnetization voltage; rem is the residual magnetic flux of the core; N1 and N2 are the turns of the primary and secondary sides of the transformer respectively; ω b is the angular frequency of the system operation; R L is a pure resistive load on the secondary side; t1 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 u dem , which is expressed as follows: in dem =U dem son[ω dem (t-t1)+α]; (3) For the transient component where the flux linkage cannot mutate 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 α of the demagnetization voltage is controlled. dem is 90° and the starting time t1 satisfies ω b t1=±90°; (4) Control U dem / ω dem =Ψ rem , using a quarter-cycle sine wave to eliminate it specifically.

2. The method according to claim 1, 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.

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

4. 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.

5. 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.

6. The method according to claim 5, characterized in that The final magnetic flux linkage of the offline demagnetization is controlled within ±0.2% after eliminating the coercive force.

7. The method according to claim 1, characterized in that The demagnetization voltage can work effectively when the demagnetization voltage is 10-35Hz.

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