阅读说明：本技术 一种基于pwm激励和定频分析的ct二次回路检测方法 (CT secondary circuit detection method based on PWM excitation and fixed frequency analysis ) 是由 葛玉磊 苏超 曹笑凡 孟海彦 刘腾昇 李斌 于 2021-01-21 设计创作，主要内容包括：本发明公开了一种基于PWM激励和定频分析的CT二次回路检测方法。该检测方法主要用于智能用电管理类终端上,检测系统包括与外部被检电流互感器串接的内部电流互感器、开关激励单元、信号采集单元、微型控制芯片；检测方法通过开关激励单元对检测系统内部的电流互感器注入一固定频率的PWM电压波,以反馈到信号采集单元的电压波形作为分析判断依据,控制芯片采用AD采样、快速傅里叶变换和定频分析技术,准确提取到所需频率的电压幅值,并与预定的标准电压值进行比较,以判断外部电流互感器二次回路的所处状态,快速发现外部电流互感器二次回路的短路、开路异常情况。(The invention discloses a CT secondary circuit detection method based on PWM excitation and fixed frequency analysis. The detection method is mainly used for intelligent power management terminals, and the detection system comprises an internal current transformer, a switch excitation unit, a signal acquisition unit and a micro control chip, wherein the internal current transformer, the switch excitation unit, the signal acquisition unit and the micro control chip are connected with an external current transformer to be detected in series; the detection method comprises the steps that a PWM voltage wave with fixed frequency is injected into a current transformer in a detection system through a switch excitation unit, the voltage wave fed back to a signal acquisition unit is used as an analysis and judgment basis, a control chip accurately extracts a voltage amplitude with required frequency by adopting AD sampling, fast Fourier transform and fixed frequency analysis technologies and compares the voltage amplitude with a preset standard voltage value to judge the state of a secondary circuit of the external current transformer, and the abnormal conditions of short circuit and open circuit of the secondary circuit of the external current transformer are quickly found.)

1. A CT secondary loop detection method based on PWM excitation and fixed frequency analysis is characterized by comprising the following steps:

step one, a switch excitation unit injects a PWM voltage wave with fixed frequency f into an internal current transformer in a mode of periodically switching on and off a switch tube, generates induced current on a secondary loop of an external current transformer and further induces and feeds back the induced current to a secondary side of another magnetic ring coil of the internal current transformer;

step two, the signal sampling unit detects a voltage signal fed back on the secondary side of the internal current transformer in real time and generates a time domain voltage amplitude waveform;

thirdly, the control chip converts the time domain voltage amplitude waveform into a frequency domain voltage amplitude waveform by using a Fast Fourier Transform (FFT) technology;

step four, the control chip continuously takes a plurality of voltage values (wherein L is a lag frequency value and is generally obtained by multiple times of experimental accumulation) which are integer times of the f + L frequency of the injected PWM wave from the frequency domain voltage amplitude waveform, and calculates the arithmetic average value of the voltage values;

and step five, judging the state of the secondary circuit of the external current transformer by comparing the calculated relation between the average voltage and the preset standard voltage.

2. The method as claimed in claim 1, wherein in the step one, a PWM voltage waveform with frequency f, duty ratio d and amplitude a is injected into a current transformer inside the intelligent power management terminal through a MOS switch tube, and the injected excitation generates an induced current on a secondary loop of an external current transformer and further induces and feeds back the induced current to a secondary side of another magnetic loop coil of the internal current transformer.

3. The CT secondary circuit detection method based on PWM excitation and fixed frequency analysis as claimed in claim 1 or 2, wherein in the second step, the signal sampling unit collects the voltage signal fed back on the secondary side of the internal current transformer at the frequency multiple of the injected PWM wave under the control of the MCU, and generates the time domain voltage amplitude waveform W_t。

4. The CT secondary loop detection method based on PWM excitation and fixed frequency analysis as claimed in claim 1, 2 or 3, wherein in step three, the micro control chip MCU utilizes fast Fourier transform FFT technique to convert the time domain voltage amplitude waveform W_tConverting into frequency domain voltage amplitude waveform W_f；

The calculation formula for converting the time domain voltage amplitude into the frequency domain voltage amplitude by the Fast Fourier Transform (FFT) algorithm is as follows:

wherein W (f) represents frequency domain voltage amplitude, w (t) represents time domain voltage amplitude, f represents frequency of injected PWM wave, t₀To represent the start sample time, t represents the end sample time, and dt represents the sample time interval.

5. The CT secondary circuit detection method based on PWM excitation and fixed frequency analysis as claimed in claim 1, 2, 3 or 4, wherein in step four, the micro control chip MCU applies the frequency domain voltage amplitude waveform W_fContinuously taking n integral multiples of the voltage value U at the frequency of f + L of the injected PWM wave₁、U₂、U₃、……、U_n(where L is the lag frequency value, typically accumulated over multiple experiments) and calculating the arithmetic mean U_a。

6. The CT secondary loop detection method based on PWM excitation and fixed frequency analysis as claimed in claim 1, 2, 3, 4 or 5, wherein in step five, the average voltage U obtained by comparing step four is compared_aWith a predetermined reference value voltage U_sTo determine the state of the secondary loop of the external current transformer; by comparison, calculating the average value U_aIs more than 10 times higher than the standard value U_sIf the external current transformer secondary loop is in a short circuit state; if the standard value U is_sIs more than 10 times higher than the arithmetic mean value U_aIf the external current transformer secondary loop is in an open circuit state; if the two conditions are not met, the secondary circuit of the external current transformer is in a normal connection state;

wherein the standard value voltage U_sThe typical value of sampling voltage of an external current transformer commonly used in a transformer area in the field of distribution network automation in a normal connection state of a secondary circuit is taken.

Technical Field

The invention belongs to the field of distribution network automation, and relates to a method for identifying the state of a secondary circuit of an external current transformer by using the characteristics of a voltage frequency domain generated by the work of power grid equipment, in particular to a CT secondary circuit detection method based on PWM excitation and fixed frequency analysis.

Background

The total quantity of the transformers in the power system transformer area mainly comprises a current transformer, a secondary circuit, an intelligent power utilization management terminal and the like, and the situations of short circuit and open circuit of the secondary circuit of the current transformer caused by objective misconnection or artificial subjective electricity stealing reasons occur occasionally, so that the problem that the measured electric quantity is smaller than the actual power utilization is finally caused, and huge economic losses are caused to the state and a power grid company. The manual inspection mode for the state of the secondary circuit of the current transformer is low in efficiency, time and labor are consumed greatly, and the actual inspection is inconvenient; therefore, the function of automatically detecting the state of the secondary circuit of the external current transformer is increased by the existing intelligent power utilization management terminal in the transformer area.

Therefore, a detection method which can be applied to an intelligent power management terminal and can accurately identify three states of short circuit, open circuit and normal state of the secondary circuit of the external current transformer needs to be found.

Disclosure of Invention

In order to solve the problems, the invention provides a CT secondary circuit detection method based on PWM excitation and fixed frequency analysis, which comprises the steps of firstly injecting a PWM voltage wave with fixed frequency onto a current transformer through a switch excitation circuit, secondly sampling the excitation voltage on the secondary side of the current transformer, accurately extracting the voltage component of an injected frequency point through an FFT (fast Fourier transform) technology and a fixed frequency analysis method, and finally performing main control calculation to judge the state of a secondary circuit of an external current transformer by comparing the calculated voltage value with a preset standard voltage value.

In order to realize the purpose, the invention adopts the following technical scheme:

step one, a switch excitation unit injects a PWM voltage wave with fixed frequency f into an internal current transformer in a mode of periodically switching on and off a switch tube, generates induced current on a secondary loop of an external current transformer and further induces and feeds back the induced current to a secondary side of another magnetic ring coil of the internal current transformer;

step two, the signal sampling unit detects a voltage signal fed back on the secondary side of the internal current transformer in real time and generates a time domain voltage amplitude waveform;

thirdly, the control chip converts the time domain voltage amplitude waveform into a frequency domain voltage amplitude waveform by using a Fast Fourier Transform (FFT) technology;

step four, the control chip continuously takes a plurality of voltage values (wherein L is a lag frequency value and is generally obtained by multiple times of experimental accumulation) which are integer times of the f + L frequency of the injected PWM wave from the frequency domain voltage amplitude waveform, and calculates the arithmetic average value of the voltage values;

and step five, judging the state of the secondary circuit of the external current transformer by comparing the calculated relation between the average voltage and the preset standard voltage.

The invention generates a current signal with specific frequency in the secondary circuit of the current transformer by a PWM excitation mode, and determines the state of the secondary circuit of the external current transformer by a fixed frequency analysis method.

Further, in the first step, a PWM voltage waveform with the frequency f, the duty ratio d and the amplitude a is injected into a current transformer inside the intelligent power management terminal through an MOS switching tube, and the injected excitation generates an induced current on a secondary loop of an external current transformer and further induces and feeds back the induced current to a secondary side of another magnetic ring coil of the internal current transformer;

further, in the second step, the signal sampling unit collects a voltage signal fed back on the secondary side of the internal current transformer under the control of the microcontroller MCU at a frequency multiple of the injected PWM wave frequency and generates a time domain voltage amplitude waveform W_t；

Further, in the third step, the micro control chip MCU utilizes fast fourier transform FFT technique to apply the time domain voltage amplitude waveform W_tConverting into frequency domain voltage amplitude waveform W_f；

The calculation formula for converting the time domain voltage amplitude into the frequency domain voltage amplitude by the Fast Fourier Transform (FFT) algorithm is as follows:

wherein W (f) represents frequency domain voltage amplitude, w (t) represents time domain voltage amplitude, f represents frequency of injected PWM wave, t₀To represent a start sampling time, t represents an end sampling time, and dt represents a sampling interval;

further, in the fourth step, the micro control chip MCU processes the frequency domain voltage amplitude waveform W_fContinuously taking n integral multiples of the voltage value U at the frequency of f + L of the injected PWM wave₁、U₂、U₃、……、U_n(where L is the lag frequency value, typically accumulated over multiple experiments) and calculating the arithmetic mean U_a；

Further, in step five, the average voltage U obtained in step four is compared_aWith a predetermined reference value voltage U_sTo determine the state of the secondary loop of the external current transformer; by comparison, calculating the average value U_aIs more than 10 times higher than the standard value U_sIf the external current transformer secondary loop is in a short circuit state; if the standard value U is_sIs more than 10 times higher than the arithmetic mean value U_aIf the external current transformer secondary loop is in an open circuit state; if the two conditions are not met, the secondary circuit of the external current transformer is in a normal connection state;

wherein the standard value voltage U_sThe typical value of sampling voltage of an external current transformer commonly used in a transformer area in the field of distribution network automation in a normal connection state of a secondary circuit is taken.

The invention relates to a CT secondary circuit detection method based on PWM excitation and fixed frequency analysis. The method is simple to realize, accurate in judgment and friendly to a power grid, and the state of the secondary loop of the current transformer can be judged through fixed frequency analysis only by injecting the PWM voltage wave with stable frequency, so that user equipment is not influenced; meanwhile, the data can be subjected to edge calculation, and only the result is uploaded to the master station.

Drawings

FIG. 1 is a flow chart of a CT secondary circuit detection method based on PWM excitation and fixed frequency analysis according to the present invention;

FIG. 2 is a structural diagram of a CT secondary circuit detection method based on PWM excitation and fixed frequency analysis according to the present invention;

FIG. 3 is a voltage amplitude spectrum diagram of the sampled voltage after fast Fourier transform by the control chip MCU of the invention (the secondary circuit of the external current transformer is in a normal connection state);

fig. 4 is a voltage amplitude frequency spectrum diagram of the sampling voltage after the sampling voltage is subjected to fast fourier transform by the control chip MCU (the external current transformer secondary circuit is in a short-circuit state).

Detailed Description

The technical solutions in the embodiments of the present invention will be further described below by describing exemplary embodiments of the present invention with reference to the drawings, and the described examples are only a part of implementation cases of the present invention, but not all implementation cases. The following description is only for clearly illustrating the technical solutions of the present invention, and the protection scope of the present invention is not limited thereby.

Unless otherwise defined, terms used herein have their commonly understood meanings to those skilled in the art.

Referring to fig. 1, a CT secondary loop detection method based on PWM excitation and fixed frequency analysis includes the following steps:

(1) the on-off of an N-MOS switching tube is controlled by adopting a Ruisar 5 series micro control chip, a PWM voltage wave with fixed frequency f being 1KHz, duty ratio being 18% and amplitude being 3.3V is injected on a secondary side coil of a current transformer in an intelligent power management terminal, a TVS diode can be incorporated into two sides of the MOS tube for considering the protection of the MOS tube, and the MOS tube is prevented from being broken down by large voltage generated on the current transformer. The injected excitation generates an induced current on a secondary loop of the external current transformer and further inductively feeds back to the secondary side of another magnetic loop coil of the internal current transformer.

(2) The dual-diode signal sampling unit shown in fig. 2 collects the voltage fed back on the secondary side of the internal current transformer and generates a time-domain voltage amplitude waveform W under the control of the microcontroller MCU with the frequency 5 times that of the injected PWM wave, i.e., 5f is 5KHz_t(ii) a The sampling resistor in the signal sampling unit is 300 omega, and the diode connected with the sampling resistor in parallel plays a role in voltage limiting, so that the waveform of the acquired voltage is complete as much as possible.

(3) The micro control chip MCU utilizes the fast Fourier transform FFT technology to convert the time domain voltage amplitude waveform W_tConverting into frequency domain voltage amplitude waveform W_fFor example, fig. 3 is a voltage amplitude spectrogram after the control chip MCU performs the fast fourier transform on the sampled voltage (the secondary circuit of the external current transformer is in a normal connection state), fig. 4 is a voltage amplitude spectrogram after the control chip MCU performs the fast fourier transform on the sampled voltage (the secondary circuit of the external current transformer is in a short circuit state), and it can be seen from the two diagrams that the voltage amplitudes at the positions of (1000+50) Hz and (2000+50) Hz in the short circuit state of the secondary circuit of the external current transformer are obviously more than 10 times higher than the voltage amplitudes at the positions of (1000+50) Hz and (2000+50) Hz in the normal connection state of the secondary circuit of the external current transformer; when the external current transformer secondary circuit is in an open state, the voltage amplitudes at (1000+50) Hz and (2000+50) Hz are almost 0;

the calculation formula for converting the time domain voltage amplitude into the frequency domain voltage amplitude by the Fast Fourier Transform (FFT) algorithm is as follows:

wherein W (f) represents frequency domain voltage amplitude, w (t) represents time domain voltage amplitude, f represents frequency of injected PWM wave, t₀To represent the start sample time, t represents the end sample time, and dt represents the sample time interval.

(4) The micro control chip MCU controls the frequency domain voltage amplitude waveform W_fContinuously taking a voltage value U which is 100 integral times of the frequency f + L of the injected PWM wave₁、U₂、U₃、……、U₁₀₀(where L is the hysteresis frequency value, typically found by accumulating from multiple experiments, where L is 50Hz), and calculating the arithmetic mean U_a。

(5) Average value voltage U obtained by comparing (4)_aWith a predetermined reference value voltage U_sTo determine the state of the secondary loop of the external current transformer; by comparison, calculating the average value U_aIs more than 10 times higher than the standard value U_sIf the external current transformer secondary loop is in a short circuit state; if the standard value U is_sIs more than 10 times higher than the arithmetic mean value U_aIf the external current transformer secondary loop is in an open circuit state; if the two conditions are not met, the secondary circuit of the external current transformer is in a normal connection state;

wherein the standard value voltage U_sThe typical value of sampling voltage of an external current transformer commonly used in a transformer area in the field of distribution network automation in a normal connection state of a secondary circuit is taken.

The foregoing illustrates and describes the principles, general features, and advantages of the present invention. The above embodiments are only for illustrating the technical solution of the present invention and not for limiting the same, and the above embodiments and descriptions are only for illustrating the principle of the present invention, and the present invention may have various changes and modifications without departing from the spirit and scope of the present invention, and these changes and modifications fall within the scope of the claimed invention. The scope of the invention is defined by the appended claims and equivalents thereof.