A method for reducing the influence of harmonics on the measurement accuracy of a resistance-capacitance DC voltage divider

Abstract

A method for reducing the influence of harmonics on the measurement accuracy of a resistance-capacitance DC voltage divider includes: establishing an impedance equivalent model of the resistance-capacitance DC voltage divider based on the structure of the resistance-capacitance DC voltage divider; analyzing the voltage division principle of the resistance-capacitance DC voltage divider to obtain a resistance-capacitance matching relationship of the resistance-capacitance DC voltage divider; analyzing the influence of stray capacitance of a main voltage division body without considering harmonics and stray capacitance; analyzing the resistance-capacitance matching relationship of the resistance-capacitance DC voltage divider considering harmonics and stray capacitance to calculate the values of a matching resistance and a matching capacitance; calculating the stray capacitance and solving the matching capacitance under harmonics to reduce the influence of harmonics on the measurement accuracy of the resistance-capacitance DC voltage divider. The method can adjust the matching capacitance to meet the resistance-capacitance matching relationship through the calculated stray capacitance of the resistance-capacitance DC voltage divider, and improve the wide-frequency voltage measurement accuracy of the resistance-capacitance DC voltage divider.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of harmonic analysis of direct current transmission, and particularly relates to a method for reducing the influence of harmonics on the measurement accuracy of a resistance-capacitance type direct current voltage divider. BACKGROUND

[0002] High-voltage direct current transmission has the advantages of long transmission distance, large transmission capacity and high stability, and is currently widely used in long-distance and large-capacity power transmission and power system networking. A large amount of harmonics is generated in the commutation process of the converter station of high-voltage direct current transmission. The injection of a large amount of harmonics into the AC / DC network will cause a series of hazards to the equipment and loads of the power system itself: direct current transmission lockout, additional harmonic loss and heating of devices such as rotating machines and capacitors; harmonic resonance overvoltage causing failure and damage of electrical components and devices; harmonics will worsen the working conditions of the converter, causing inverter commutation failure or unstable converter control; harmonic metering error; harmonics in the range of 200-35000 Hz will cause serious interference to communication equipment; and protection device misoperation.

[0003] Therefore, harmonic analysis and control have become a basic problem of high-voltage direct current transmission. With the increasing number of high-voltage direct current transmission projects in China, especially the operation of ultra-high-voltage direct current transmission, the harmonic problem is becoming more and more serious, which has caused the shutdown of the direct current system. Therefore, for the direct current system, how to accurately measure and evaluate the harmonic level has become an important research topic.

[0004] In the power grid system, in order to measure the balanced voltage, the ultra-high-voltage converter station is usually equipped with a direct current voltage transformer (DCVT). The direct current voltage transformer is used to measure the direct current voltage, maintain the stability of the transmission line, and has the same effect as the alternating current voltage transformer. With the continuous development of the power system, new energy has gradually become the main power source, and the power grid has gradually transformed into flexibility, intelligence and digitization, which has greatly promoted the development of ultra-high-voltage direct current transmission. In the power grid system, the ultra-high-voltage converter station is usually equipped with a direct current voltage transformer (DCVT) to measure the direct current voltage, so as to maintain the safe and stable operation of the transmission line.

[0005] In the current power grid system, the measurement object of the direct current voltage transformer is the direct current voltage of the pole bus and the neutral bus. The working principle is that the sampling value of the voltage signal is measured by the resistance-capacitance voltage division component of the direct current voltage transformer, the sampling value is connected to the merging unit through a transmission cable or an optical cable, and the sampling value is processed in the merging unit. The direct current transmission can be controlled and protected, and it is an important part of the direct current measurement control and protection system.

[0006] With the development of new power system, new energy gradually becomes the main power source of installation, and the power grid gradually transforms to flexibility, intelligence and digitization, which promotes the development of ultra-high voltage direct current transmission. The DC voltage divider is an important device in the power transmission system, and the voltage changes rapidly when a fault occurs. The DC voltage divider is used to measure the DC voltage, maintain the stability of the transmission line, and has the same effect as the AC voltage transformer. In the current power grid system, the measurement object of the DC voltage divider is the DC voltage of the pole bus and the neutral bus, and the sampling data is transmitted to the converter station and the protection system. The DC control and protection system needs the DC voltage divider to provide real and reliable primary voltage information. Therefore, the reliability of the operation of the DC voltage divider is an important guarantee for the reliable monitoring of high voltage DC voltage, and is directly related to the safe and stable operation of the DC transmission system.

[0007] The DC voltage can be measured using a capacitive voltage divider, a resistive voltage divider, an inductive voltage transformer, and a resistance-capacitance voltage divider. In current DC projects, high-voltage DC voltage dividers are mainly based on resistance-capacitance voltage division principles.

[0008] For the influence of the frequency characteristics of the transformer, the resistance-capacitance voltage divider (also known as "resistance-capacitance DC voltage divider") is dominant, and the resistance or capacitance plays a decisive role under certain conditions. When the resistance-capacitance matching relationship is met, the resistance-capacitance DC voltage divider is not affected by the frequency. When designing the parameters of the resistance-capacitance DC voltage divider, the resistance-capacitance matching relationship is met, but the existence of stray capacitance in the actual working condition will destroy the resistance-capacitance matching relationship. Moreover, the capacitance value of the stray capacitance will change with the frequency. When measuring the harmonic voltage, the division ratio of each frequency is different, the harmonic distortion of the output signal is synthesized, and the resistance-capacitance DC voltage divider cannot achieve the function of measuring and monitoring the voltage of the DC transmission line.

[0009] The research on stray capacitance at home and abroad is mainly focused on DCVT, surge arresters, and converter transformers. The existence of stray capacitance will affect the transmission of DCVT harmonics and the accuracy of harmonic measurement, and on the other hand, it will affect the division ratio of DCVT; for surge arresters, a multi-potential conductor system is formed with the surrounding live equipment, and the stray capacitance brings current interference, thereby affecting the existing online monitoring technology; the high-frequency resonance quality factor of the converter transformer will change due to the existence of stray capacitance, and the resistance value of the branch will change. As can be seen, stray capacitance often affects the performance of the conductor itself, so the calculation of stray capacitance is crucial for the design and effect of power equipment.

[0010] The test device and the surrounding live equipment will theoretically have capacitive coupling, thereby producing different electric field interference. In order to prevent this effect, relevant measures should be taken, but it is impossible to completely eliminate the influence of stray capacitance, SUMMARY

[0011] To solve the above technical problems, the present application provides a method for reducing the influence of harmonics on the measurement accuracy of a resistance-capacitance DC voltage divider, which can calculate the stray capacitance of the resistance-capacitance DC voltage divider, adjust the matching capacitance to meet the resistance-capacitance matching relationship, and improve the wideband voltage measurement accuracy of the resistance-capacitance DC voltage divider.

[0012] The technical solutions adopted by the present application are as follows:

[0013] A method for reducing the influence of harmonics on the measurement accuracy of a resistance-capacitance DC voltage divider, comprising the following steps:

[0014] Step 1: Based on the structure of the resistance-capacitance DC voltage divider, an impedance equivalent model of the resistance-capacitance DC voltage divider is established;

[0015] Step 2: The principle of voltage division of the resistance-capacitance DC voltage divider is analyzed to obtain the resistance-capacitance matching relationship of the resistance-capacitance DC voltage divider;

[0016] Step 3: Without considering harmonics, the influence of the stray capacitance of the main voltage dividing body is analyzed; considering harmonics and stray capacitance, the resistance-capacitance matching relationship of the resistance-capacitance DC voltage divider is analyzed to calculate the value of the matching resistance;

[0017] Step 4: The stray capacitance is calculated, and the matching capacitance under harmonics is solved to reduce the influence of harmonics on the measurement accuracy of the resistance-capacitance DC voltage divider.

[0018] In the step 1, the resistance-capacitance DC voltage divider comprises a main voltage dividing body, a resistance-capacitance matching unit, a secondary voltage dividing plate, and a signal acquisition device; the main voltage dividing body is connected to the resistance-capacitance matching unit, the resistance-capacitance matching unit is connected to the secondary voltage dividing plate, and the secondary voltage dividing plate is connected to the signal acquisition device;

[0019] The resistance-capacitance matching unit comprises a matching resistance and a matching capacitance, and by adjusting the matching resistance and the matching capacitance, the resistance-capacitance DC voltage divider can maintain a stable voltage division ratio; at the same time, the resistance-capacitance matching unit can reduce the measurement error caused by frequency and stray capacitance.

[0020] The main voltage dividing body comprises a plurality of single-stage voltage dividing modules, each single-stage voltage dividing module uses the same resistance and capacitance elements,

[0021] The structure of each single-stage voltage dividing module is that a plurality of resistors are uniformly distributed around a capacitor in the circumferential direction, and the structure of the plurality of resistors is connected in parallel across the capacitor.

[0022] In the main voltage dividing body, the lowest stage of the main voltage dividing body is a low-voltage arm, and the resistance and capacitance of the low-voltage arm are R L and C L , respectively.

[0023] The several voltage division modules above the low-voltage arm constitute a high-voltage arm 1.2 of the main voltage division body 1, and the equivalent resistance and the equivalent capacitance of the high-voltage arm 1.2 are R H , C H , respectively.

[0024] The resistances of each voltage division module in the high-voltage arm are connected in series, i.e., R H =R1+R2+R3+…+R n , and the capacitances of each voltage division module are connected in parallel, i.e., C H =C1∥C2∥C3∥…∥C n .

[0025] 1) When only the DC voltage without harmonics exists on the DC bus, the equivalent impedances of the high-voltage arm and the low-voltage arm of the main voltage division body of the resistance-capacitance DC voltage divider are as shown in the following formula (1) and formula (2); the voltage division ratio K dc of the main voltage division body can be equivalent to formula (3).

[0026]

[0027] In the above formula, Z H is the equivalent impedance of the high-voltage arm, Z L is the impedance of the low-voltage arm, and ω is the frequency of the measured voltage; when the DC voltage does not contain harmonics, ω=0, and the voltage division ratio K dc of the main voltage division body of the resistance-capacitance DC voltage divider is as follows:

[0028]

[0029] In formula (3), R H is the equivalent resistance of the high-voltage arm of the main voltage division body, R L is the equivalent resistance of the low-voltage arm of the main voltage division body, C H is the capacitance of the high-voltage arm, C L is the capacitance of the low-voltage arm, Z H is the equivalent impedance of the high-voltage arm, Z L is the impedance of the low-voltage arm, u H (t) is the measured voltage connected to the high-voltage end of the resistance-capacitance voltage divider, and u L (t) is the output voltage of the low-voltage end.

[0030] 2) When the DC voltage contains harmonics, the voltage division ratio K ac of the main voltage division body of the resistance-capacitance DC voltage divider is as follows:

[0031]

[0032] In step 2, in order to ensure that the resistance-capacitance DC voltage divider has the same voltage division ratio when measuring the DC voltage and the voltage containing harmonics, K dc =K ac, from formula (3) and (4), we can know that the resistance and capacitance parameters should satisfy the following formula:

[0033] R H C H =R L C L (5);

[0034] When R H C H =R L C L When the voltage divider ratio K of the RC DC voltage divider under harmonic voltage is ac The voltage division ratio K under the DC voltage without harmonics dc Equal, the voltage divider ratio is not affected by frequency; R H C H =R L C L It is the resistance-capacitance matching relationship.

[0035] In step 3, when the harmonic effect is not considered and only the stray capacitance is considered, the stray capacitance C Z1 The order of magnitude is the same as the high voltage arm equivalent capacitance C H The stray capacitance C of the low voltage arm is equivalent to Z2 And the low voltage arm voltage divider capacitor C L is much smaller than that, so R H C H ′>R L C L ′, the resistance-capacitance matching relationship is destroyed. At this time, C H ′ is the total equivalent capacitance of the high voltage arm of the main voltage divider of the RC DC voltage divider, C L ′ is the total equivalent capacitance of the low-voltage arm of the main voltage divider of the RC DC voltage divider;

[0036] When considering the influence of harmonics, the change of frequency will cause the change of capacitance value, which will cause R H C H ′≠R L C L '; When a RC DC voltage divider is used to measure a voltage containing harmonics, the voltage divider ratio when each harmonic frequency is transmitted will vary with the frequency.

[0037] In step 3, the entire RC DC voltage divider with the RC matching unit added thereto is subjected to circuit equivalence. When the RC DC voltage divider satisfies the RC matching relationship shown in the following equation, the voltage divider ratio remains constant.

[0038] R H C H ′=R L 'C L ′ (6);

[0039] In formula (6), C H is the total equivalent capacitance of the high-voltage arm of the main voltage-dividing body of the resistance-capacitance DC voltage divider, and R L is the total equivalent resistance of the low-voltage arm port of the main voltage-dividing body of the resistance-capacitance DC voltage divider. L is the total equivalent capacitance of the low-voltage arm port of the main voltage-dividing body of the resistance-capacitance DC voltage divider.

[0040]

[0041] In formula (7), C Z1 is the total equivalent stray capacitance of the high-voltage arm of the main voltage-dividing body of the resistance-capacitance DC voltage divider.

[0042] R L ′ = R L || R0|| R i (8);

[0043] wherein R0 is the matching resistance of the resistance-capacitance matching unit, and R i is the equivalent resistance of the secondary voltage-dividing plate.

[0044]

[0045] wherein C Z2 is the total equivalent stray capacitance of the low-voltage arm of the main voltage-dividing body of the resistance-capacitance DC voltage divider, C0 is the matching capacitance of the resistance-capacitance matching unit, and C i is the equivalent capacitance of the secondary voltage-dividing plate.

[0046] It can be obtained from the above formula that the resistance-capacitance matching relationship of the resistance-capacitance DC voltage divider considering the existence of harmonics and stray capacitance is:

[0047]

[0048] In the formula, C Z1 and C Z2 are stray capacitances, the stray capacitance values are obtained by field calculation, and the matching resistance value R0 and the matching capacitance value C0 of the resistance-capacitance matching unit are analyzed and solved in combination with these parameter values.

[0049]

[0050] Since the DC voltage divider dividing ratio K dc ′ can be calibrated to the design value K dc by the matching resistance R0,

[0051]

[0052] That is, it is obtained:

[0053]

[0054] The value of the matching resistance R0 calculated is brought into formula (11), and the value of the matching capacitance C0 can be obtained.

[0055] In step 4, the stray capacitance is an undesired mutual capacitance between two charge bodies. Assuming that n conductors have charge amounts Q1, Q2, …, Qn, the potentials of the conductors are:

[0056]

[0057] Where P ij is a potential coefficient, i, j = 1, 2, …, n; solving this linear equation set can obtain the charge amount:

[0058]

[0059] Where C ii is the capacitance of the ith conductor, C ij is an induction coefficient, i≠j; assuming that the potential of the ith conductor is 1V and the potentials of the other conductors are 0V, the charge amount of the ith conductor is equal to its capacitance for this system, that is:

[0060]

[0061] Where Q j is the charge amount of the conductor, j = 1, 2, …, n; V i is the potential of the above-mentioned conductor, i = 1, 2, …, n.

[0062] Therefore, the capacitance matrix of the entire system is:

[0063]

[0064] Helmholtz and Thomson proved that these coefficients have symmetry, that is:

[0065] C ij = C ji (18);

[0066] Where C ji is the induction coefficient of the potential of the ith conductor to the charge of the jth conductor, i≠j.

[0067] In field theory, the Maxwell capacitance matrix is used to describe the relationship between the charge (Q1, Q2, …, Q n ) of the ith conductor and the voltage V1, V2, …, V n of all conductors in the system:

[0068]

[0069] Wherein, Q1, Q2, …, Q n Respectively, the total charge of the conductor i, i=1, 2, …, n; C ji Is the inductive coefficient of the potential of the conductor i to the charge of the conductor j, i, j=1, 2, …, n;

[0070] It can be seen that the Maxwell capacitance matrix and the mutual capacitance matrix have a direct conversion relationship, and the Maxwell capacitance matrix is obtained, and then the mutual capacitance matrix C of the system can be obtained through linear transformation. Mutual capacitance, also known as parasitic capacitance or stray capacitance, is the expected or unexpected capacitance (charge accumulation) generated between two charge bodies.

[0071] Wherein, the Maxwell capacitance matrix is:

[0072]

[0073] The mutual capacitance matrix C is:

[0074]

[0075] Suppose that there are only three conductors in the space with Q1, Q2 and Q3 charges, and the distances between Q1 and Q2, Q1 and Q3, and Q2 and Q3 are r 12 , r 13 , r 23 , respectively. The electrostatic energy W can be calculated according to formula (22).

[0076]

[0077] At the same time, the electrostatic energy W can be expressed by formula (23):

[0078]

[0079] Wherein, C is the capacitance value, and U is the voltage difference between the conductors.

[0080] In actual solving, the Maxwell capacitance matrix is solved by the above energy method, and then the stray capacitance value, i.e. the mutual capacitance value, is obtained.

[0081] In step 4, by calculating the capacitance matrix, the matching capacitance of the resistance-capacitance matching unit under the main characteristic harmonic can be solved by introducing the matching resistance R0 calculated by the stray capacitance into formula (11) according to formula (13), so as to reduce the influence of harmonics on the measurement accuracy of the resistance-capacitance DC voltage divider.

[0082] The method for reducing the influence of harmonics on the measurement accuracy of the resistance-capacitance DC voltage divider has the following technical effects:

[0083] 1) The application obtains that when the resistance-capacitance matching relationship is met, the resistance-capacitance divider can keep the division ratio constant and is not affected by the frequency through the analysis of the structural principle of the resistance-capacitance DC voltage divider.

[0084] 2) The application introduces the resistance-capacitance matching unit, the matching resistance can calibrate the division ratio of the DC voltage divider to the design value, and the matching capacitance can change the matching capacitance value to realize the resistance-capacitance matching through the calculated stray capacitance value, so as to improve the frequency response characteristic of the DC voltage transformer and realize the accurate measurement of the wide-band DC voltage. BRIEF DESCRIPTION OF DRAWINGS

[0085] Figure 1 It is a structural schematic diagram of the resistance-capacitance DC voltage divider in the application.

[0086] Among them: 1 is a main voltage dividing body, 2 is a resistance-capacitance matching unit, 3 is a secondary voltage dividing plate, and 4 is a signal acquisition device.

[0087] 1.1 is an outer wall, 1.2 is a high-voltage arm, and 1.3 is a low-voltage arm.

[0088] Figure 2 It is an equivalent circuit diagram of a single-stage voltage dividing module in the application.

[0089] Figure 3 It is a main voltage dividing body circuit diagram of the resistance-capacitance DC voltage divider in the application.

[0090] Figure 4 It is an impedance equivalent circuit diagram of the main voltage dividing body of the resistance-capacitance DC voltage divider in the application.

[0091] Figure 5 It is a secondary voltage dividing plate circuit diagram of the resistance-capacitance DC voltage divider in the application. DETAILED DESCRIPTION

[0092] A method for reducing the influence of harmonics on the measurement accuracy of a resistance-capacitance DC voltage divider, comprising the following steps:

[0093] Step one: analyzing the structure of the resistance-capacitance DC voltage divider, establishing the impedance equivalent model of the resistance-capacitance DC voltage divider, analyzing the voltage dividing principle of the resistance-capacitance DC voltage divider, and obtaining that the resistance-capacitance matching relationship should be met to ensure the accurate measurement of the resistance-capacitance DC voltage divider.

[0094] Step two: analyzing the resistance-capacitance matching relationship of the resistance-capacitance DC voltage divider, and analyzing that when R H C H =R L C L , the division ratio K ac of the resistance-capacitance DC voltage divider under the harmonic voltage is equal to the division ratio K dc under the DC voltage without harmonics, and the division ratio is not affected by the frequency.

[0095] Step three: the influence of the stray capacitance of the main voltage divider body is analyzed without considering the harmonic influence and considering the stray capacitance, the resistance-capacitance matching relationship of the resistance-capacitance type DC voltage divider is analyzed when the harmonic and stray capacitance exist, and the matching resistance and capacitance of the resistance-capacitance matching unit which can correct the voltage division ratio and reduce the harmonic influence are analyzed and calculated.

[0096] Step four: by obtaining the stray capacitance value, the resistance-capacitance matching relationship can be satisfied by changing the matching capacitance value, the wide frequency measurement accuracy of the resistance-capacitance type DC voltage divider is improved, and the influence of the stray capacitance on the resistance-capacitance type DC voltage divider under the harmonic condition is reduced.

[0097] (1). Resistance-capacitance type DC voltage transformer structure:

[0098] The resistance-capacitance type DC voltage divider is composed of a main voltage divider body 1, a secondary voltage divider plate 3 and a signal acquisition device 4. The main voltage divider body 1 adopts resistance-capacitance division, and high voltage is converted into low voltage by resistance division. Since the voltage after the division of the main voltage divider body is still high (generally 70V), the secondary voltage divider plate 3 also adopts resistance-capacitance division principle. On the one hand, it can convert the low voltage signal output by the low voltage arm of the main voltage divider body into a plurality of signals (generally 5V) for signal acquisition by the signal acquisition module and sent to the control protection system. On the other hand, it can avoid problems such as abnormality of all secondary measurement results of the voltage divider due to failure of a secondary circuit load.

[0099] The application adds a resistance-capacitance division unit 2 between the secondary voltage divider plate 3 and the main voltage divider body 1 on the basis of the original common resistance-capacitance type DC voltage divider structure. By adjusting the matching resistance and matching capacitance of the resistance-capacitance division unit 2, the resistance-capacitance type DC voltage divider can maintain the stability of the voltage division ratio. At the same time, the resistance-capacitance division unit 2 can reduce the measurement error caused by frequency and stray capacitance. The improved DC voltage transformer (DCVT) structure is shown in Figure 1 .

[0100] Since the DCVT is mainly used for voltage measurement of high voltage direct current transmission, its operating voltage level is high, and it often needs to be assembled in a cascade mode. For example, the overall resistance-capacitance division module of a ±800KV DCVT is composed of 16 single-stage division modules, each stage divides 50kV. In order to reduce the measurement error caused by different temperature coefficients of resistance and capacitance elements when the temperature rises to thermal equilibrium, the same resistance and capacitance elements are used in each stage division module. Each two stage division modules are connected in series by a steel flange with a bolt as the carrier. The equivalent circuit of a single-stage resistance-capacitance division module is shown in Figure 2 .

[0101] The structure of each stage voltage dividing module is resistance, which is evenly distributed around the capacitor in the circumferential direction with the capacitor as the axis, to prevent the surface of the resistance from generating corona. The capacitor is connected in parallel across the resistance. When the equipment is subjected to the impact of transient overvoltage such as lightning, the capacitor can play a role in flattening the wave front steepness to protect the equipment. In order to optimize the electric field distribution inside the equipment, the single-stage resistance-capacitance voltage dividing body often adopts voltage equalizing ring voltage equalization in design.

[0102] The number of stages of the resistance-capacitance voltage dividing module is related to the voltage level of the DCVT, such as Figure 3 The voltage dividing principle of the main voltage dividing body of the DCVT is shown, wherein the first stage at the lowermost end of the main voltage dividing body 1 is a low-voltage arm 1.3, and the resistance and the capacitor of the low-voltage arm are R L , C L , respectively. A plurality of stages of resistance-capacitance voltage dividing bodies above the low-voltage arm constitute a high-voltage arm of the main voltage dividing body, and each single-stage resistance-capacitance voltage dividing body is composed of a plurality of smaller resistance-capacitance elements connected in series and parallel. The equivalent resistance and the equivalent capacitance of the high-voltage arm of the main voltage dividing body are R H , C H , respectively. The resistances of each stage of the voltage dividing body are connected in series, i.e. R H = R1+ R2+ R3+…+ R n , and the capacitances of each stage of the voltage dividing body are connected in parallel, i.e. C H = C1∥C2∥C3∥…∥C n .

[0103] In order to meet the insulation level and other electrical and mechanical performance requirements of the resistance and the capacitor, the resistance-capacitance main voltage dividing body of the DCVT is usually arranged inside a hollow insulator, which is filled with insulating oil, SF6 or N2 and the like, and the upper part is packaged by a flange, and the lower part is packaged and supported by a base.

[0104] The resistance-capacitance DC voltage divider plays an important role in the high-voltage direct current transmission system, and its measurement principle is simple and mature. With the development of new power systems, it is required that the ratio error of the resistance-capacitance DC voltage divider under the condition of harmonics within 50Hz-3kHz is not greater than 3%. The measurement bandwidth of the resistance-capacitance DC voltage divider is determined by the matching degree of the high-voltage and low-voltage arm dividing resistors and capacitors. In theory, when the resistance-capacitance matching relationship is met, the dividing ratio of the resistance-capacitance DC voltage divider can remain constant. In fact, the resistance-capacitance parameters are affected by the operating environment and stray parameters, and it is difficult to achieve resistance-capacitance matching. Harmonics can affect the size of the stray parameters of the resistance-capacitance DC voltage divider, and therefore, reducing the influence of harmonics on the resistance-capacitance DC voltage divider is of great significance to realize accurate measurement of wide-band DC voltage.

[0105] (2). Resistance-capacitance matching relationship of the resistance-capacitance DC voltage divider:

[0106] The impedance equivalent circuit diagram of the main voltage dividing body of the resistance-capacitance DC voltage divider is as follows Figure 3The equivalent impedance of the high-voltage arm and the low-voltage arm of the resistance-capacitance voltage divider main body of the DC voltage transformer is shown as follows when only the DC voltage without harmonics is on the DC bus. The voltage division ratio K of the resistance-capacitance DC voltage divider main body dc The equivalent impedance of the high-voltage arm and the low-voltage arm of the resistance-capacitance voltage divider main body of the DC voltage transformer is shown as follows when only the DC voltage without harmonics is on the DC bus. The voltage division ratio K of the resistance-capacitance DC voltage divider main body

[0107]

[0108] In the formula, ω is the frequency of the measured voltage. When the DC voltage does not contain harmonics, ω = 0, and the voltage division ratio K of the resistance-capacitance DC voltage divider main body dc is:

[0109]

[0110] In the formula, R H is the equivalent resistance of the high-voltage arm of the main voltage divider body, R L is the equivalent resistance of the low-voltage arm of the main voltage divider body, C H is the capacitance of the high-voltage arm, C L is the capacitance of the low-voltage arm, Z H is the equivalent impedance of the high-voltage arm, Z L is the impedance of the low-voltage arm, u H (t) is the measured voltage connected to the high-voltage section of the resistance-capacitance voltage divider, u L (t) is the output voltage of the low-voltage end.

[0111] When the DC voltage contains harmonics, the voltage division ratio K of the resistance-capacitance DC voltage divider main body ac is:

[0112]

[0113] To ensure that the DC voltage transformer has the same voltage division ratio when measuring DC voltage and harmonic voltage, K dc = K ac , from formulas (3) and (4), the resistance-capacitance parameters should satisfy the following formula:

[0114] R H C H = R L C L (5);

[0115] When R H C H = R L C L , the voltage division ratio Kac of the resistance-capacitance DC voltage divider under harmonic voltage is equal to the voltage division ratio Kdc under DC voltage without harmonics, and the voltage division ratio is the same at different frequencies, R H C H = R L C L is the resistance-capacitance matching relationship.

[0116] (3) Analysis of transfer characteristics of RC DC voltage divider under actual working conditions:

[0117] In actual working conditions, there is stray capacitance between the series units from the high-voltage end to the low-voltage end of the RC DC voltage divider. The situation of the secondary voltage divider is the same as that of the main voltage divider. Here, the analysis is based on the main voltage divider.

[0118] The main voltage divider of the RC DC voltage divider satisfies the RC matching relationship R H C H =R L C L When measuring harmonic frequency voltage, distortion can be avoided, ensuring the measurement accuracy of the DC voltage divider. Under actual working conditions, the harmonics contained in the measured DC voltage and the presence of stray capacitance will affect the voltage division ratio of the RC DC voltage divider, thereby affecting the measurement accuracy of the RC DC voltage divider.

[0119] When the harmonic effect is not considered and only the stray capacitance is considered, the stray capacitance C Z1 The order of magnitude is the same as the high voltage arm equivalent capacitance C H The stray capacitance C of the low voltage arm is equivalent to Z2 And the low voltage arm voltage divider capacitor C L is much smaller than that, so R H C H ′>R L C L ′, the resistance-capacitance matching relationship is destroyed. At this time, C H ′ is the total equivalent capacitance of the high voltage arm of the main voltage divider of the RC DC voltage divider, C L ′ is the total equivalent capacitance of the low-voltage arm of the main voltage divider of the RC DC voltage divider.

[0120] In order to eliminate the influence of stray capacitance on the RC matching relationship, when selecting the voltage divider capacitor of the RC DC voltage divider, C H Lower than the theoretical design value or C L However, since the design value is ideal and the actual resistance value is the nominal value, if R H The resistance is lower than the design value or R L When the resistance value is higher than the design value, the voltage divider ratio of the main voltage divider will deviate from the original design value.

[0121] When considering the influence of harmonics, the change of frequency will cause the change of capacitance value, which will cause R H C H ′≠R L C LWhen the harmonic voltage is measured by the RC divider, the voltage ratio of each harmonic frequency transmission will change due to the different frequencies, so the harmonic voltage obtained by synthesizing the voltage output of each frequency will be distorted, and the function of measuring and monitoring the voltage of the DC transmission line cannot be achieved.

[0122] In summary, the harmonic frequency and the stray capacitance will affect the voltage division ratio of the RC divider, thereby affecting the measurement accuracy of the RC divider. The addition of the RC matching unit can calibrate the voltage division ratio of the RC divider to the designed value through the matching resistance R0. At the same time, the influence of the harmonic on the measurement accuracy of the RC divider can be reduced by adjusting the capacitance of C0 through the analysis and calculation of the capacitance of the high and low voltage arm stray capacitance.

[0123] The RC divider with the RC matching unit is equivalent to the circuit, and when the RC divider satisfies the RC matching relationship shown in the following formula, the voltage division ratio of the RC divider remains constant.

[0124] R H C H ′=R L ′C L ′ (6);

[0125] In the formula, C H ′ is the total equivalent capacitance of the high voltage arm of the main voltage dividing body of the RC divider, R L ′ is the total equivalent resistance of the low voltage arm port of the main voltage dividing body of the RC divider, and C L ′ is the total equivalent capacitance of the low voltage arm port of the main voltage dividing body of the RC divider.

[0126] The circuit equivalent analysis of the RC divider shows that:

[0127]

[0128] In the formula, C Z1 is the total equivalent stray capacitance of the high voltage arm of the main voltage dividing body of the RC divider.

[0129] R L ′=R L ||R0||R i (8);

[0130] In the formula, R0 is the matching resistance of the RC matching unit, and R i is the equivalent resistance of the secondary voltage dividing plate.

[0131]

[0132] In the formula, C Z2C0 is the matching capacitance of the resistance-capacitance matching unit, C i is the equivalent capacitance of the secondary voltage dividing plate.

[0133] From the above formula, it can be concluded that the resistance-capacitance matching relationship of the whole resistance-capacitance DC voltage divider considering the existence of harmonics and stray capacitance is:

[0134]

[0135] In the formula, C H , C L , C i , R H , R L , R i are fixed by the resistance-capacitance components when they are factory, and the present application considers that they are constant; C Z1 , C Z2 is the stray capacitance, which can be calculated by field calculation, and the matching resistance value R0 and the matching capacitance value C0 of the resistance-capacitance matching unit are analyzed and solved in combination with these parameter values.

[0136]

[0137] Since the voltage dividing ratio K dc of the resistance-capacitance DC voltage divider can be calibrated to the design value K dc by the matching resistance R0, therefore

[0138]

[0139] That is, the following can be obtained:

[0140]

[0141] The value of the matching resistance R0 calculated is brought into the formula (11), and the value of the matching capacitance C0 can be obtained.

[0142] (4) Stray capacitance calculation method of resistance-capacitance DC voltage divider under actual working conditions:

[0143] For the influence of the frequency characteristics of the mutual inductor, the resistance-capacitance DC voltage divider is dominant, and the resistance or capacitance plays a decisive role under certain conditions. In actual application, each resistance itself has stray capacitance, and this nonlinear characteristic will cause the resistance to resonate at a certain frequency. When the DCVT works in the presence of harmonics, due to the wide frequency characteristics of harmonics, resonance may occur.

[0144] In addition, the stray parameters will affect the equivalent capacitance values ​​of the high and low voltage arms, thereby affecting the resistance-capacitance matching relationship of the RC DC voltage divider. Different voltage divider ratios at different frequencies will lead to synthetic harmonic distortion, affecting the economy and safety of equipment operation.

[0145] To this end, the present invention establishes a stray capacitance calculation model to analyze the impact of stray capacitance.

[0146] Stray capacitance is the undesired mutual capacitance between two charged bodies. The definition of capacitance (C = Q / U) cannot be used directly for calculation. To solve this problem, Maxwell proposed the potential coefficient and inductance coefficient. Assuming that n conductors carry charges Q1, Q2, ..., Qn, the potential of these conductors is:

[0147]

[0148] Among them, P ij is the potential coefficient, i, j = 1, 2, ..., n. Solving this linear equation system, we can get the charge as

[0149]

[0150] Among them, C ii is the capacitance of the ith conductor, C ij is the inductance (i≠j). Assume that the potential of the i-th conductor is 1V and the potential of other conductors is 0V. Then, for this system, the amount of charge carried by the i-th conductor is equal to its capacitance, that is,

[0151]

[0152] From this we can see that the capacitance matrix of the entire system is

[0153]

[0154] Helmholtz and Thomson proved that these coefficients have symmetry, that is,

[0155] C ij =C ji (18);

[0156] In field theory, the Maxwell capacitance matrix is ​​often used to describe the charge of the i-th conductor (Q1, Q2, ..., Q n ) and the voltages of all conductors in the system (V1, V2, ..., V n ) between them.

[0157]

[0158] It can be seen that Maxwell capacitance matrix and mutual capacitance matrix have a direct conversion relationship, and the mutual capacitance matrix C of the system can be obtained by linear transformation after the Maxwell capacitance matrix is obtained, and then the stray capacitance value can be obtained.

[0159] The measurement error of the DC voltage divider is mainly caused by the system frequency deviation, resistance value deviation and capacitance value deviation. The resistance and capacitance are not constant values and are affected by many factors. The error value generated in the production process will introduce measurement error. The stray capacitance generated by the resistance itself in the environment also affects the size of the capacitance value, thereby affecting the voltage division ratio of the DCVT, causing the measurement error of the DCVT, affecting the accuracy and precision of the measurement. And the stray capacitance will also be affected by the harmonic frequency of the measurement, causing the voltage division ratio of the resistance-capacitance DC voltage divider to change, reducing the measurement accuracy of the resistance-capacitance DC voltage divider for DC voltage containing harmonic signals.

[0160] By calculating the capacitance matrix, the matching capacitance of the resistance-capacitance matching unit under the main characteristic harmonic can be solved according to the calculated stray capacitance, thereby reducing the influence of harmonics on the measurement accuracy of the resistance-capacitance DC voltage divider.

Claims

1. A method for reducing the influence of harmonics on the measurement accuracy of a RC DC voltage divider, characterized in that The following steps are involved: Step 1: Based on the RC DC voltage divider structure, establish an impedance equivalent model of the RC DC voltage divider; Step 2: Analyze the voltage division principle of the RC DC voltage divider and obtain the resistance-capacitance matching relationship of the RC DC voltage divider; Step 3: Analyze the influence of the stray capacitance of the main voltage divider when considering stray capacitance but not harmonics. Analyze the resistance-capacitance matching relationship of the RC DC voltage divider when considering both harmonics and stray capacitance, and calculate the value of the matching resistor. Step 4: Calculate the stray capacitance and solve for the matching capacitance under harmonics to reduce the impact of harmonics on the measurement accuracy of the RC DC voltage divider.

2. The method for reducing the influence of harmonics on the measurement accuracy of a RC DC voltage divider according to claim 1, characterized in that: In step 1, the RC DC voltage divider comprises: a main voltage divider (1), a RC matching unit (2), a secondary voltage divider plate (3), and a signal acquisition device (4); the main voltage divider (1) is connected to the RC matching unit (2), the RC matching unit (2) is connected to the secondary voltage divider plate (3), and the secondary voltage divider plate (3) is connected to the signal acquisition device (4).

3. The method for reducing the influence of harmonics on the measurement accuracy of a RC DC voltage divider according to claim 2, characterized in that: The resistance-capacitance matching unit (2) comprises a matching resistor and a matching capacitor. By adjusting the matching resistor and the matching capacitor, the resistance-capacitance DC voltage divider can maintain a stable voltage division ratio. At the same time, the resistance-capacitance matching unit (2) is used to reduce measurement errors caused by frequency and stray capacitance.

4. The method for reducing the influence of harmonics on the measurement accuracy of a RC DC voltage divider according to claim 2, characterized in that: The main voltage divider (1) comprises a plurality of single-stage voltage divider modules, each stage of the voltage divider module uses the same resistor and capacitor elements, and the structure of each stage of the voltage divider module is that a plurality of resistors are evenly distributed around the capacitor along the circumferential direction with the capacitor as the axis, and the structure composed of the plurality of resistors is connected in parallel at both ends of the capacitor; In the main voltage divider (1), the first level located at the bottom of the main voltage divider (1) is a low-voltage arm (1.3), and the resistance and capacitance of the low-voltage arm (1.3) are R L 、C L ; Several voltage divider modules above the low voltage arm (1.3) form the high voltage arm (1.2) of the main voltage divider (1). The equivalent resistance and equivalent capacitance of the high voltage arm (1.2) are R H 、C H ; The resistors of each voltage divider module in the high voltage arm (1.2) are connected in series, that is, R H =R1+R2+R3+…+R n , the capacitors of each voltage divider module are connected in parallel, that is, C H =C1∥C2∥C3∥…∥C n .

5. The method for reducing the influence of harmonics on the measurement accuracy of a RC DC voltage divider according to claim 4, characterized in that: When there is only DC voltage without harmonics on the DC bus, the equivalent impedance of the high voltage arm (1.2) and the low voltage arm (1.3) of the main voltage divider (1) of the RC DC voltage divider is as shown in the following equations (1) and (2); the voltage divider ratio K of the main voltage divider (1) is dc It can be equivalent to formula (3); In the above formula, Z H is the equivalent impedance of the high voltage arm, Z L is the impedance of the low-voltage arm, ω is the measured voltage frequency; when the DC voltage does not contain harmonics, ω=0, the voltage divider ratio K of the main voltage divider (1) of the RC DC voltage divider is dc for: In formula (3): R H The equivalent resistance of the high voltage arm of the main voltage divider, R L The equivalent resistance of the low-voltage arm of the main voltage divider, C H is the high voltage arm capacitance, C L is the low voltage arm capacitance, Z H is the equivalent impedance of the high voltage arm, Z L is the impedance of the low voltage arm, u H (t) is the measured voltage connected to the high voltage end of the RC voltage divider, u L (t) is the output voltage of the low voltage end; When the DC voltage contains harmonics, the voltage divider ratio K of the main voltage divider (1) of the RC DC voltage divider ac for:

6. The method for reducing the influence of harmonics on the measurement accuracy of a RC DC voltage divider according to claim 5, characterized in that: In step 2, in order to ensure that the RC DC voltage divider has the same voltage division ratio when measuring DC voltage and harmonic voltage, it should be ensured that K dc =K ac , from formula (3) and (4), we can know that the resistance and capacitance parameters should satisfy the following formula: R H C H =R L C L (5); When R H C H =R L C L When the voltage divider ratio K of the RC DC voltage divider under harmonic voltage is ac The voltage division ratio K under the DC voltage without harmonics dc Equal, the voltage divider ratio is not affected by frequency; R H C H =R L C L It is the resistance-capacitance matching relationship.

7. The method for reducing the influence of harmonics on the measurement accuracy of a RC DC voltage divider according to claim 1, characterized in that: In step 3, when the harmonic effect is not considered and only the stray capacitance is considered, the stray capacitance C Z1 The order of magnitude is the same as the high voltage arm equivalent capacitance C H The stray capacitance C of the low voltage arm is equivalent to Z2 And the low voltage arm voltage divider capacitor C L is much smaller than that, so R H C H ′>R L C L ′, the resistance-capacitance matching relationship is destroyed; at this time, C H ′ is the total equivalent capacitance of the high voltage arm of the main voltage divider of the RC DC voltage divider, C L ′ is the total equivalent capacitance of the low-voltage arm of the main voltage divider of the RC DC voltage divider; When considering the influence of harmonics, the change of frequency will cause the change of capacitance value, which will cause R H C H ′≠R L C L '; When a RC DC voltage divider is used to measure a voltage containing harmonics, the voltage divider ratio when each harmonic frequency is transmitted will vary with the frequency.

8. The method for reducing the influence of harmonics on the measurement accuracy of a RC DC voltage divider according to claim 7, characterized in that: In step 3, the entire RC DC voltage divider with the RC matching unit (2) added thereto is subjected to circuit equivalence. When the RC DC voltage divider satisfies the RC matching relationship shown in the following equation, the voltage divider ratio remains constant. R H C H ′=R L ′C L ′ (6); In formula (6), C H ′ is the total equivalent capacitance of the high voltage arm of the main voltage divider of the RC DC voltage divider, R L ′ is the total equivalent resistance of the low-voltage arm port of the main voltage divider of the RC DC voltage divider, C L ′ is the total equivalent capacitance of the low-voltage arm port of the main voltage divider of the RC DC voltage divider. In formula (7), C Z1 is the total equivalent stray capacitance of the high-voltage arm of the main voltage divider of the RC DC voltage divider; (8); Among them, R0 is the matching resistance of the RC matching unit, R i is the equivalent resistance of the secondary voltage divider; Among them, C Z2 is the total equivalent stray capacitance of the low-voltage arm of the main voltage divider of the RC DC voltage divider, C0 is the matching capacitance of the RC matching unit, C i is the equivalent capacitance of the secondary voltage divider; From the above formula, it can be concluded that when harmonics and stray capacitance are considered, the overall resistance-capacitance matching relationship of the RC DC voltage divider is: Where C Z1 、C Z2 The stray capacitance is obtained by field calculation, and the matching resistance value R0 and matching capacitance value C0 of the RC matching unit are analyzed and solved by combining these parameter values; Among them, since the DC voltage divider voltage ratio K can be adjusted by matching resistor R0 dc 'Calibrate to the design value K dc , so That is, we get:

9. The method for reducing the influence of harmonics on the measurement accuracy of a RC DC voltage divider according to claim 8, characterized in that: In step 4, stray capacitance is the undesired mutual capacitance between two charged bodies. Assuming that n conductors carry charges Q1, Q2, ..., Qn, the potentials of these conductors are: Among them, P ij is the potential coefficient, i, j = 1, 2, ..., n; solving this linear equation system, we can get the charge as: Among them, C ii is the capacitance of the ith conductor, C ij is the inductance, i≠j; assuming the potential of the i-th conductor is 1V and the potential of other conductors is 0V, then for this system, the charge carried by the i-th conductor is equal to its capacitance, that is: Among them, Q j is the charge on the conductor, j = 1, 2, ..., n; V i is the potential of the above conductor, i = 1, 2, …, n; From this we can see that the capacitance matrix of the entire system is: Helmholtz and Thomson proved that these coefficients have symmetry, that is, C ij =C ji (18); Among them, C ji is the inductance of the potential of conductor i to the charge of conductor j, i≠j; In field theory, the Maxwell capacitance matrix is ​​used to describe the charges Q1, Q2, ..., Q of the i-th conductor. n The voltages V1, V2, ..., V of all conductors in the system n The relationship between: Among them, Q1, Q2, ..., Q n are the total charge of conductor i, i = 1, 2, ..., n; C ji is the inductance of the potential of conductor i to the charge of conductor j, i, j = 1, 2, ..., n; It can be seen that the Maxwell capacitance matrix and the mutual capacitance matrix have a direct conversion relationship. By calculating the Maxwell capacitance matrix, the mutual capacitance matrix C of the system can be obtained through linear transformation. Mutual capacitance is also called stray capacitance. In actual solution, the Maxwell capacitance matrix is ​​solved by the energy method to obtain the stray capacitance value, that is, the mutual capacitance value. Among them, the Maxwell capacitance matrix is: The mutual capacitance matrix C is:

10. The method for reducing the influence of harmonics on the measurement accuracy of a RC DC voltage divider according to claim 9, characterized in that: In step 4, the calculated capacitance matrix, the calculated stray capacitance, and the matching resistance R0 obtained by combining formula (13) are substituted into formula (11), and the matching capacitance of the RC matching unit under the characteristic harmonic can be solved, thereby reducing the influence of harmonics on the measurement accuracy of the RC DC voltage divider.

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