Harmonic and ultra-harmonic measuring system and measuring method based on CVT capacitive current principle

Abstract

The invention discloses a harmonic and super-harmonic measuring system and measuring method based on the CVT capacitive current principle, and belongs to the technical field of harmonic and super-harmonic measurement. According to the method, capacitive current analog signals on a voltage dividing capacitor C1 and a voltage dividing capacitor C2 are collected, the capacitive current analog signals are transmitted to a harmonic and super-harmonic measuring device, and the harmonic and super-harmonic measuring device measures the harmonic and super-harmonic. And the harmonic and super-harmonic measuring device is used for restoring a primary side total voltage waveform based on the capacitance current analog signal, the equivalent capacitance of the voltage dividing capacitor C1 and the equivalent capacitance of the voltage dividing capacitor C2. Influences of internal inductance and capacitance equivalent resistance of the capacitive voltage divider are considered, a primary side voltage reduction formula of the capacitive current method is corrected, and the corrected capacitive current method is suitable for ultra-harmonic measurement.

Description

Technical Field

[0001] The invention belongs to the technical field of harmonic and superharmonic measurement, and in particular relates to a harmonic and superharmonic measurement system and a measurement method based on a CVT capacitance current principle. Background Art

[0002] Electricity is the most important pillar of economic development, social production and people's daily life. Its clean, economical, easy to convert and control characteristics make it an indispensable energy source in modern society. However, with the increasing penetration rate of new clean energy such as wind power and photovoltaic power, a large number of nonlinear and impact loads are connected to the power grid, which greatly increases the instability of the power system operation. The increasingly complex power grid environment and the increase in power grid disturbances will lead to power quality problems such as power grid harmonics and voltage fluctuations in the power system, which seriously threatens the efficient, safe and stable operation of the power system. Grid harmonics are a factor that deserves special attention in power quality issues. They not only cause distortion of grid signal waveforms, but also accelerate equipment aging, reduce system capacity, and affect the accuracy of power metering and the safety of power supply and use. With the rapid development of power electronics technology, power electronic devices such as IGBT are widely used in electrical equipment, and their switching frequencies have increased from several thousand hertz to hundreds of kilohertz or even higher. The disturbances generated have gradually shifted from low-frequency bands to high-frequency bands. The high-frequency characteristics of ultra-harmonics may seriously interfere with the normal operation of precision equipment in the power system, causing high-frequency resonance in the power supply circuit, and even causing fires in electrical equipment, endangering the safety of life and property. Therefore, accurate monitoring of harmonics and ultra-high frequency harmonics has become an urgent need for the power system.

[0003] At present, the capacitor voltage transformer (CVT) is a commonly used power quality monitoring device in the high-voltage grid of the power system. However, due to the influence of the transmission characteristics of the intermediate electromagnetic unit, CVT cannot accurately measure harmonics. To solve this problem, the industry has proposed a CVT capacitor current harmonic measurement method, the principle of which is to measure the current in the CVT voltage divider capacitor and then restore the primary voltage waveform through integration. Since this method avoids the influence of the electromagnetic unit, it can more accurately restore the actual CVT primary side low-frequency harmonics, but because the CVT is affected by parasitic parameters, under ultra-high frequency harmonics, the internal inductance of the capacitor voltage divider cannot be ignored, and the capacitor in the capacitor voltage divider does not meet the ideal capacitance conditions during actual operation. Its actual model is a series-parallel mode of capacitors and resistors. This method does not consider the influence of internal inductance. Therefore, this method cannot currently measure ultra-high frequency harmonics of 9k-150kHz and is not suitable for ultra-high frequency harmonics. In addition, the existing CVT capacitor current method for monitoring harmonics requires that the restored primary voltage signal be converted into a sampling value message in accordance with the IEC61850 specification, and then sent to the next measurement device to calculate the harmonic voltage. If this method is applied to super-harmonic monitoring that requires coordination of various IEC standards, it will cause massive data processing. Therefore, this system is not suitable for super-harmonic monitoring.

[0004] In summary, the commonly used capacitance-current method currently ignores the effects of the internal inductance of the capacitor divider, and is not suitable for ultra-high frequency harmonic measurement. Moreover, the harmonic measurement system established by this method cannot solve the problem of massive data caused by ultra-high frequency harmonics. Therefore, it is of great significance to propose an optimization method for the calculation formula of the capacitance-current method and a measurement system solution suitable for ultra-harmonics for power quality monitoring of power systems. Summary of the invention

[0005] The present invention provides a harmonic and superharmonic measurement system and a measurement method based on the CVT capacitor current principle, so as to realize the measurement of superharmonics (9k-150kHz).

[0006] In order to achieve the above object, the present invention adopts the following technical solution: A harmonic and superharmonic measurement system based on the CVT capacitor current principle includes two wide-band current transformers, which are respectively used to measure capacitor current analog signals on a voltage-dividing capacitor C1 and a voltage-dividing capacitor C2, and transmit the capacitor current analog signals to a harmonic and superharmonic measurement device, which is used to restore a primary-side total voltage waveform based on the capacitor current analog signals, an equivalent capacitance of the voltage-dividing capacitor C1, and an equivalent capacitance of the voltage-dividing capacitor C2, wherein the equivalent capacitance of the voltage-dividing capacitor C1 and the equivalent capacitance of the voltage-dividing capacitor C2 are equivalent capacitances that take into account their respective internal inductance and capacitance.

[0007] Furthermore, the harmonic and superharmonic measuring device includes an ADC, an MCU and an optical fiber LAN. The high-speed ADC is used to sample and cache the capacitor current analog signal at a speed of 10MHz-50MHz, complete AD conversion, and output a capacitor current digital signal; the MCU is used to sample and process the capacitor current digital signal according to the IEC standard definition, and restore the primary side total voltage waveform, and obtain the harmonic or superharmonic voltage by Fourier transforming the primary voltage waveform.

[0008] Furthermore, the output end of the harmonic and superharmonic measuring device is connected to the input end of the IEC protocol converter. Furthermore, the output end of the IEC protocol converter is connected to the input end of the digital power quality online monitoring terminal.

[0009] Furthermore, a capacitor C3 is arranged between the voltage-dividing capacitor C2 and the grounding point, both ends of the capacitor C3 are connected to the input end of the rectifier circuit, the output end of the rectifier circuit is connected to the input end of the DC / DC energy extraction circuit, and the DC / DC energy extraction circuit is used to power the harmonic and superharmonic measuring device.

[0010] Furthermore, the capacitor C3 is connected in parallel with a protection circuit, which includes a triac V1, a resistor R1 and a reverse series zener diode D1, wherein a main electrode T1 of the triac is connected to a first end of the resistor R1 and a first end of the capacitor C3, a main electrode T2 is connected to a second end of the capacitor C3, a control electrode G is connected to a second end of the reverse series zener diode D1, and a first end of the reverse series zener diode D1 is connected to a second end of the resistor R1.

[0011] A harmonic and superharmonic measurement method based on CVT capacitance and current principle comprises the following steps: Step 1: The first wideband current transformer and the second wideband current transformer respectively measure the capacitive current analog signals on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2; Step 2, sampling and caching the capacitor current analog signals on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2, and performing AD conversion to obtain the capacitor current digital signals on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2; Step 3, based on the capacitive current digital signal on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2, the equivalent capacitance of the voltage-dividing capacitor C1 and the equivalent capacitance of the voltage-dividing capacitor C2, restore the primary voltage waveform, and perform Fourier transform on the primary voltage to obtain the waveform, effective value and content rate of the harmonic or superharmonic; the equivalent capacitance of the voltage-dividing capacitor C1 and the equivalent capacitance of the voltage-dividing capacitor C2 are equivalent capacitances that take into account their respective internal inductance and capacitance.

[0012] Furthermore, in step 2, the capacitor currents on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2 are sampled and cached at a speed of 10-50 MHz.

[0013] Furthermore, in step 3, the voltage waveform is restored by equations (1)-(5): (1) (2) (3) (4) (5) in, is the capacitive current flowing through the voltage divider capacitor C2, is the capacitive current flowing through the voltage divider capacitor C1, is the voltage across the voltage divider capacitor C2, is the voltage on both sides of the voltage divider capacitor C1, is the total voltage on the primary side of the CVT, is the equivalent resistance of the voltage divider capacitor C1, is the equivalent resistance of the voltage divider capacitor C2, is the dielectric loss angle of the voltage divider capacitor C1, is the dielectric loss angle of the voltage divider capacitor C2, is the offset value of the voltage divider capacitor C1, is the offset value of the voltage divider capacitor C2, is the equivalent capacitance value of the voltage divider capacitor C1, is the equivalent capacitance value of the voltage divider capacitor C2, To measure frequency.

[0014] Furthermore, after step 3 is completed, the obtained waveform, effective value and content rate of the harmonic or superharmonic are converted into the IEC61850 message format and transmitted to the digital power quality online monitoring terminal through optical fiber.

[0015] Compared with the prior art, the present invention has at least the following beneficial technical effects: The harmonic and superharmonic measurement system based on the CVT capacitance current principle of the present invention is preset with an improved capacitance current method, and the influence of the inductance and capacitance equivalent resistance inside the capacitance voltage divider is considered. The capacitance current method is to calculate the voltage by taking the current flowing through the voltage-dividing capacitors C1 and C2 and combining the capacitance of the capacitors themselves. At present, the capacitance current method regards the voltage-dividing capacitors C1 and C2 as ideal capacitors, but its actual model is a capacitor and resistor series-parallel mode. In addition, the inherent inductance inside the capacitance voltage divider inside the CVT is uH level, which has almost no effect on the capacitance current method at low frequencies, but at high frequencies of superharmonics (9k-150kHz), the inductance of uH level cannot be ignored. Therefore, considering the influence of the inductance and capacitance equivalent resistance inside the capacitance voltage divider can accurately calculate the primary voltage at high frequencies of superharmonics (9k-150kHz), and expand the measurement range. The improved capacitance current method can be used to measure superharmonics (9k-150kHz), which can be coordinated to meet the levels defined by standards such as IEEE 519, IEC 61000-3-12, and IEC 61000-4-30, such as the first method in Part C.3 of GB / T 17626.30-2023, and can be applied to on-site power quality measurement.

[0016] Furthermore, it also includes an energy-taking circuit. By adding a capacitor C3 that has almost no effect on the voltage division to the CVT, the energy-taking circuit serves as the power supply for the harmonic and superharmonic measuring device, and a protection circuit is added to protect the device from damage due to overvoltage.

[0017] The capacitance-current method suitable for superharmonic measurement provided by the present invention improves the capacitance-current method by ignoring the influence of the internal inductance of the capacitance voltage divider and the equivalent resistance of the capacitance voltage divider, considers the influence of the internal inductance of the capacitance voltage divider and the equivalent resistance of the capacitance, and corrects the primary side voltage restoration formula of the capacitance-current method. The corrected capacitance-current method is suitable for superharmonic measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is the principle diagram of harmonic and superharmonic measurement system based on CVT capacitance and current principle; Figure 2 This is the equivalent circuit of the capacitance current method that can measure superharmonics. DETAILED DESCRIPTION

[0019] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] In order to enable those skilled in the art to better understand the technical solutions in the present invention, 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, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0021] It should be noted that when an element is referred to as being "disposed on" another element, it may be directly on the other element or there may be another element centered thereon. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be another element centered thereon at the same time. The terms "upper", "lower", "front", "back", "left", "right", "top", "bottom", "inside", "outside", etc. used in the present invention indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention.

[0022] Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used in the present invention in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used in the present invention includes any and all combinations of one or more of the related listed items.

[0023] See also Figure 1 , Figure 1 The schematic diagram of the harmonic and superharmonic measurement system based on the CVT capacitor current principle is shown. The measurement system consists of two wide-band current transformers, an energy extraction circuit with an additional capacitor C3, a harmonic and superharmonic measurement device, an IEC protocol converter, and a digital power quality online monitoring terminal. The voltage-dividing capacitor C1 and the voltage-dividing capacitor C2 form a capacitive voltage divider. The compensating reactor L and the intermediate transformer T are located inside the CVT electromagnetic unit. When the CVT is in a steady state, the system voltage is first divided by the capacitor voltage divider, and then stepped down again by the intermediate transformer T to change the voltage value to a standard value. In essence, the CVT is a voltage divider, and the voltage standard value is obtained by dividing the voltage by the capacitor voltage divider and the intermediate transformer. However, due to the influence of the transmission characteristics of the electromagnetic unit, the CVT cannot accurately measure harmonics. Among them, the voltage-dividing capacitor C1 is a high-voltage capacitor, and the voltage-dividing capacitor C2 is a low-voltage capacitor.

[0024] Two wide-band current transformers measure analog signals of currents on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2 respectively, and transmit the analog signals to the harmonic and superharmonic measuring device; The harmonic and superharmonic measurement device consists of a high-speed analog-to-digital converter (ADC), a microcontroller unit (MCU) and a fiber-optic local area network (LAN). In order to measure superharmonics (9k-150kHz), the high-speed ADC is used to sample and cache the two-way capacitor current analog signals at a speed of 10-50MHz, complete AD conversion, and output the capacitor current digital signal to the MCU.

[0025] The MCU further samples and processes the digital signal of the capacitor current transmitted by the high-speed ADC according to various IEC standard definitions. Different IEC standards have different methods for selecting the frequency points of each harmonic. The IEC 61000-4-30 standard requires that the harmonic frequency and its adjacent frequency points be taken as the effective value of the harmonic voltage. Specific standards such as IEC 61000-3-12 require that a single 5Hz frequency component be taken as the effective value of the harmonic voltage. For example, for the fifth harmonic, IEC 61000-4-30 requires the effective value of the harmonic voltage at the frequency point of 245-255Hz, and IEC 61000-3-12 requires the effective value of the harmonic voltage at the frequency point of 250Hz. Therefore, the MCU sets the effective value of the harmonic voltage to be calculated subsequently according to the desired IEC standard, and then uses the improved capacitor current method of the present invention to restore the total voltage waveform of the CVT primary side and obtain the harmonic and superharmonic voltage waveforms, effective values ​​and content rates through Fourier transform. Finally, the waveforms, effective values ​​and content rates of each harmonic obtained are transmitted to the IEC protocol converter through the optical fiber LAN. The IEC protocol converter converts the waveforms, effective values ​​and content rates of each harmonic received into the IEC61850 message format, and transmits them to the digital power quality online monitoring terminal through the optical fiber. The digital power quality online monitoring terminal displays the waveforms, effective values ​​and content rates of each harmonic, so as to realize the monitoring of CVT harmonics and superharmonics.

[0026] In addition, the present invention innovatively introduces an energy-taking solution of an additional capacitor C3, adding a uF-level capacitor C3 between the voltage-dividing capacitor C2 and the grounding point to realize a self-energy power supply for the harmonic and superharmonic measuring device, which only requires a few watts of power to operate.

[0027] In the DC / DC energy harvesting circuit, a rectifier bridge composed of four diodes converts the alternating current (AC) power transmitted by C3 into direct current (DC) power. The DC / DC energy harvesting circuit converts the rectified high-voltage DC power into DC power with a lower amplitude to provide the harmonic and superharmonic measurement device with the required rated voltage, thereby realizing self-energy harvesting of the device.

[0028] The voltage-dividing capacitors C1 and C2 in CVT are at the pF level, so the introduction of capacitor C3 only produces a very small voltage-dividing effect. In view of the fact that CVT is a widely used power quality monitoring device in voltage levels of 35kV and above, the error caused by the voltage-dividing capacitor C3 is negligible, only one ten-thousandth, and this error can be ignored in practical applications. In order to protect the harmonic and superharmonic measuring device, the present invention designs a protection circuit for the energy-taking circuit with C3 attached. When overvoltage occurs, the protection circuit can protect the harmonic and superharmonic measuring device and the energy-taking circuit from being damaged.

[0029] The protection circuit includes a triac V1, a resistor R1 and an inverse series voltage regulator D1, wherein the main electrode T1 of the triac is connected to the first end of the resistor R1 and the first end of the capacitor C3, the main electrode T2 is connected to the second end of the capacitor C3, the control electrode G is connected to the second end of the inverse series voltage regulator D1, and the first end of the inverse series voltage regulator D1 is connected to the second end of the resistor R1. The inverse series voltage regulator D1 is composed of two Zener diodes in inverse series connection.

[0030] See also Figure 2 , Figure 2 The equivalent circuit used in the capacitor current method for measuring superharmonics is shown in the improved capacitor current method of the present invention. The influence of the internal inductance and the equivalent resistance of the capacitor voltage divider is taken into account. For the equivalent resistance of the voltage divider capacitor C1 and the capacitor C2, the equivalent resistance Rc1 and Rc2 of the capacitors in series can be calculated by the dielectric loss angle of the capacitors in the capacitor voltage divider. The internal inductance of the capacitor voltage divider cannot be ignored under superharmonics. Since the inductance and the capacitance cancel each other out, the inductance Lc1 and Lc2 can be regarded as canceling out part of the capacitance value of the voltage divider capacitors C1 and C2, thereby correcting the capacitance value in the capacitor current method, as shown in equations (2) and (3). The corrected results are shown in equations (1)-(5): (1) (2) (3) (4) (5) in, is the capacitive current flowing through the voltage divider capacitor C2, is the capacitive current flowing through the voltage divider capacitor C1, is the voltage across the voltage divider capacitor C2, is the voltage on both sides of the voltage divider capacitor C1, is the total voltage on the primary side of the CVT, is the equivalent resistance of the voltage divider capacitor C1, is the equivalent resistance of the voltage divider capacitor C2, is the dielectric loss angle of the voltage divider capacitor C1, is the dielectric loss angle of the voltage divider capacitor C2, is the offset value of the voltage divider capacitor C1, is the offset value of the voltage divider capacitor C2, is the equivalent capacitance value of the voltage divider capacitor C1, is the equivalent capacitance value of the voltage divider capacitor C2, To measure frequency.

[0031] The following are specific embodiments. It should be noted that these embodiments are preferred examples of the present invention and are used for those skilled in the art to understand the present invention, but the present invention is not limited to these embodiments.

[0032] Example 1 Reference Figure 1 This embodiment provides a harmonic and superharmonic measurement system based on the CVT capacitor current principle, including a first wide-band current transformer, a second wide-band current transformer, an energy extraction circuit, a harmonic and superharmonic measurement device, an IEC protocol converter and a digital power quality online monitoring terminal.

[0033] The output ends of the first wideband current transformer and the second wideband current transformer are connected to the input end of the harmonic and superharmonic measuring device, the output end of the harmonic and superharmonic measuring device is connected to the input end of the IEC protocol converter, and the output end of the IEC protocol converter is connected to the input end of the digital power quality online monitoring terminal. The energy extraction circuit is electrically connected to the harmonic and superharmonic measuring device.

[0034] A first wideband current transformer, used for measuring the capacitive current of the voltage dividing capacitor C1 and transmitting it to the harmonic and superharmonic measuring device; A second wide-band current transformer is used to measure the capacitive current of the voltage-dividing capacitor C2 and transmit it to the harmonic and superharmonic measuring device; The harmonic and superharmonic measuring device comprises a high-speed ADC, an MCU and an optical fiber LAN which are connected in sequence. The high-speed ADC is used to sample and cache two-way capacitor currents at a speed of 10-50MHz to complete AD conversion; the MCU is used to further sample and process the capacitor current digital signal transmitted by the high-speed ADC according to various IEC standard definitions, and then use the improved capacitor current method of the present invention to restore the primary voltage waveform, obtain the harmonic or superharmonic voltage through Fourier transformation of the primary voltage waveform, and finally transmit the obtained voltage to the IEC protocol converter through the optical fiber LAN.

[0035] The IEC protocol converter is used to convert the received voltage data into the IEC61850 message format and transmit it to the digital power quality online monitoring terminal via optical fiber.

[0036] Example 2 Reference Figure 2 This embodiment provides a method for measuring superharmonics, comprising the following steps: Step 1: The first broadband current transformer and the second broadband current transformer respectively measure the capacitive current analog signals on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2 and transmit them to the harmonic measurement device; Step 2: The harmonic and superharmonic measuring device samples and caches the two-way capacitor current analog signals at a speed of 10-50MHz, completes AD conversion, and outputs capacitor current digital signals to the MCU; the MCU further samples and processes the current digital signals transmitted by the high-speed ADC according to various IEC standard definitions; Step 3, restore the primary voltage waveform, and obtain the harmonic or superharmonic voltage through Fourier transform; The total voltage on the primary side of the CVT can be restored by the following formula (1) (2) (3) (4) (5) in, is the capacitive current flowing through the voltage divider capacitor C2, is the capacitive current flowing through the voltage divider capacitor C1, is the voltage across the voltage divider capacitor C2, is the voltage on both sides of the voltage divider capacitor C1, is the total voltage on the primary side of the CVT, is the equivalent resistance of the voltage divider capacitor C1, is the equivalent resistance of the voltage divider capacitor C2, is the dielectric loss angle of the voltage divider capacitor C1, is the dielectric loss angle of the voltage divider capacitor C2, is the offset value of the voltage divider capacitor C1, is the offset value of the voltage divider capacitor C2, is the equivalent capacitance value of the voltage divider capacitor C1, is the equivalent capacitance value of the voltage divider capacitor C2, To measure frequency.

[0037] Step 4: The obtained voltage information is transmitted to the IEC protocol converter via the optical fiber LAN. The IEC protocol converter converts the received data into the IEC61850 message format and transmits it to the digital power quality online monitoring terminal via the optical fiber.

[0038] The term "consisting of" describing a combination shall include the identified elements, ingredients, parts or steps and other elements, ingredients, parts or steps that do not substantially affect the basic novel characteristics of the combination. The use of the terms "comprising" or "including" to describe a combination of elements, ingredients, parts or steps herein also contemplates embodiments that consist essentially of these elements, ingredients, parts or steps. By using the term "may", it is intended to illustrate that any of the attributes described that "may" be included are optional.

[0039] Multiple elements, ingredients, parts or steps can be provided by a single integrated element, ingredient, part or step. Alternatively, a single integrated element, ingredient, part or step can be divided into separate multiple elements, ingredients, parts or steps. The disclosure "one" or "an" used to describe an element, ingredient, part or step is not intended to exclude other elements, ingredients, parts or steps.

[0040] It should be understood that the above description is for illustration and not for limitation. Many embodiments and many applications beyond the examples provided will be apparent to those skilled in the art upon reading the above description. Therefore, the scope of the present teachings should not be determined with reference to the above description, but rather with reference to the foregoing claims and the full scope of equivalents to which such claims are entitled. For the purpose of comprehensiveness, all articles and references, including disclosures of patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended to be a waiver of such subject matter, nor should it be considered that the applicant has not considered such subject matter to be part of the disclosed inventive subject matter.

Claims

1. Harmonic and superharmonic measurement system based on CVT capacitance and current principle, characterized by: The invention comprises two wide-band current transformers, which are respectively used to measure the capacitor current analog signals on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2, and transmit the capacitor current analog signals to the harmonic and superharmonic measuring device, and the harmonic and superharmonic measuring device is used to restore the primary-side total voltage waveform based on the capacitor current analog signals, the equivalent capacitance of the voltage-dividing capacitor C1 and the equivalent capacitance of the voltage-dividing capacitor C2, and the equivalent capacitance of the voltage-dividing capacitor C1 and the equivalent capacitance of the voltage-dividing capacitor C2 are equivalent capacitances that take into account their respective internal inductance and capacitance.

2. The harmonic and superharmonic measurement system based on the CVT capacitance and current principle according to claim 1 is characterized in that: The harmonic and superharmonic measuring device includes an ADC, an MCU and an optical fiber LAN. The high-speed ADC is used to sample and cache the capacitor current analog signal at a speed of 10MHz-50MHz, complete AD conversion, and output a capacitor current digital signal; the MCU is used to sample and process the capacitor current digital signal according to the IEC standard definition, and restore the primary side total voltage waveform, and obtain the harmonic or superharmonic voltage by Fourier transforming the primary voltage waveform.

3. The harmonic and superharmonic measurement system based on the CVT capacitance and current principle according to claim 1 is characterized in that: The output end of the harmonic and superharmonic measuring device is connected to the input end of the IEC protocol converter.

4. The harmonic and superharmonic measurement system based on the CVT capacitance and current principle according to claim 3 is characterized in that: The output end of the IEC protocol converter is connected to the input end of the digital power quality online monitoring terminal.

5. The harmonic and superharmonic measurement system based on the CVT capacitance and current principle according to claim 1 is characterized in that: A capacitor C3 is provided between the voltage-dividing capacitor C2 and the grounding point, both ends of the capacitor C3 are connected to the input end of the rectifier circuit, the output end of the rectifier circuit is connected to the input end of the DC / DC energy extraction circuit, and the DC / DC energy extraction circuit is used to power the harmonic and superharmonic measurement device.

6. The harmonic and superharmonic measurement system based on the CVT capacitance and current principle according to claim 5 is characterized in that: The capacitor C3 is connected in parallel with the protection circuit, which includes a three-terminal bidirectional thyristor V1, a resistor R1 and an inverse series zener diode D1. The main electrode T1 of the three-terminal bidirectional thyristor is connected to the first end of the resistor R1 and the first end of the capacitor C3, the main electrode T2 is connected to the second end of the capacitor C3, the control electrode G is connected to the second end of the inverse series zener diode D1, and the first end of the inverse series zener diode D1 is connected to the second end of the resistor R1.

7. A harmonic and superharmonic measurement method based on CVT capacitance and current principle, characterized in that: The following steps are involved: Step 1: The first wideband current transformer and the second wideband current transformer respectively measure the capacitive current analog signals on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2; Step 2: sampling and caching the analog signals of the capacitor current on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2, and performing AD conversion to obtain digital signals of the capacitor current on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2; Step 3, based on the capacitive current digital signal on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2, the equivalent capacitance of the voltage-dividing capacitor C1 and the equivalent capacitance of the voltage-dividing capacitor C2, restore the primary voltage waveform, and perform Fourier transform on the primary voltage to obtain the waveform, effective value and content rate of the harmonic or superharmonic; the equivalent capacitance of the voltage-dividing capacitor C1 and the equivalent capacitance of the voltage-dividing capacitor C2 are equivalent capacitances that take into account their respective internal inductance and capacitance.

8. The harmonic and superharmonic measurement method based on the CVT capacitance and current principle according to claim 7 is characterized in that: In the step 2, the capacitor currents on the voltage-dividing capacitor C1 and the voltage-dividing capacitor C2 are sampled and cached at a speed of 10-50 MHz.

9. The harmonic and superharmonic measurement method based on the CVT capacitance and current principle according to claim 7, characterized in that: In step 3, the primary voltage waveform is restored by equations (1)-(5): （1） （2） （3） （4） （5） in, is the capacitive current flowing through the voltage divider capacitor C2, is the capacitive current flowing through the voltage divider capacitor C1, is the voltage across the voltage divider capacitor C2, is the voltage on both sides of the voltage divider capacitor C1, is the total voltage on the primary side of the CVT, is the equivalent resistance of the voltage divider capacitor C1, is the equivalent resistance of the voltage divider capacitor C2, is the dielectric loss angle of the voltage divider capacitor C1, is the dielectric loss angle of the voltage divider capacitor C2, is the offset value of the voltage divider capacitor C1, is the offset value of the voltage divider capacitor C2, is the equivalent capacitance value of the voltage divider capacitor C1, is the equivalent capacitance value of the voltage divider capacitor C2, To measure frequency.

10. The harmonic and superharmonic measurement method based on the CVT capacitance and current principle according to claim 7, characterized in that: After step 3 is completed, the obtained waveform, effective value and content rate of the harmonic or superharmonic are converted into IEC61850 message format and transmitted to the digital power quality online monitoring terminal through optical fiber.

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