Method for reducing influence of harmonic waves on measurement accuracy of resistance-capacitance direct current voltage divider

Abstract

A method for reducing the influence of harmonic waves on the measurement accuracy of a resistance-capacitance direct-current voltage divider comprises the following steps: establishing an impedance equivalent model of the resistance-capacitance direct-current voltage divider based on the structure of the resistance-capacitance direct-current voltage divider; analyzing the voltage dividing principle of the resistance-capacitance type direct current voltage divider to obtain the resistance-capacitance matching relation of the resistance-capacitance type direct current voltage divider; when harmonic waves and stray capacitance are not considered, the influence of the stray capacitance of the main voltage divider is analyzed; when harmonic waves and stray capacitance are considered, the resistance-capacitance matching relation of the resistance-capacitance type direct-current voltage divider is analyzed, so that the values of matching resistance and matching capacitance are calculated; and the stray capacitance is calculated, the matching capacitance under the harmonic waves is solved, and the influence of the harmonic waves on the measurement accuracy of the resistance-capacitance direct current voltage divider is reduced. According to the method, the matching capacitance can be adjusted to meet the resistance-capacitance matching relation through the calculated stray capacitance of the resistance-capacitance direct-current voltage divider, and the broadband voltage measurement accuracy of the resistance-capacitance direct-current voltage divider is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of direct current power transmission harmonic analysis, and in particular relates to a method for reducing the influence of harmonics on the measurement accuracy of a resistor-capacitor direct current voltage divider. Background Art

[0002] High-voltage direct current transmission has the advantages of long transmission distance, large transmission capacity and high stability. It is currently widely used in long-distance, large-capacity transmission and power system networking. The converter station of high-voltage direct current transmission will generate a large number of harmonics during its conversion process. A large number of harmonics injected into the AC and DC networks will bring a series of hazards to the equipment and loads of the power system itself: additional harmonic losses and heating of equipment such as DC transmission interlocks, rotating motors and capacitors; harmonic resonance overvoltage causes failure and damage to electrical components and equipment; harmonics will deteriorate the working conditions of the converter, causing inverter commutation failure or converter control instability; harmonic measurement errors; harmonics in the range of 200 to 35,000 Hz will cause relatively serious interference to communication equipment; protection devices malfunction.

[0003] Therefore, harmonic analysis and control have become basic issues in HVDC transmission. With the increase of HVDC transmission projects in China, especially the commissioning of UHVDC transmission, harmonic problems have become more and more serious, and have led to the shutdown of DC systems. Therefore, for DC systems, how to accurately measure and evaluate harmonic levels has become an important research topic.

[0004] In the power grid system, in order to measure the balanced voltage, the UHV converter station is usually equipped with a direct current voltage transformer (DCVT). The DC voltage transformer is used to measure the DC voltage and maintain the stability of the transmission line. Its function is the same as that of the AC voltage transformer. With the continuous development of the power system, new energy has gradually become the main power source for installed capacity, and the power grid has gradually transformed towards flexibility, intelligence, and digitalization, which has greatly promoted the development of UHV DC transmission. In the power grid system, the UHV converter station is usually equipped with a direct current voltage transformer (DCVT) to measure the DC voltage to maintain the safe and stable operation of the transmission line.

[0005] In the current power grid system, the DC voltage transformer measures the DC voltage of the pole bus and the neutral bus. Its working principle is that the resistance and capacitance voltage divider components of the DC voltage transformer measure the sampled value of the voltage signal, and connect the sampled value to the merging unit through a transmission cable or optical cable. The sampled value is processed in the merging unit, which can control and protect DC power transmission. It is an important part of the DC measurement, control and protection system.

[0006] With the development of new power systems, new energy sources have gradually become the main power source for installed capacity, and the power grid has gradually transformed towards flexibility, intelligence, and digitalization, all of which have promoted the development of UHV DC transmission. The DC voltage divider is an important device in the transmission system, and the voltage changes rapidly when a fault occurs. The DC voltage divider is used to measure the DC voltage and maintain the stability of the transmission line. Its function is the same as that of the AC voltage transformer. In the current power grid system, the DC voltage divider measures the DC voltage of the pole bus and the neutral bus. The sampled data is transmitted to the converter station and the protection system. The DC control and protection system requires 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 the high-voltage DC voltage, which is directly related to the safe and stable operation of the DC transmission system.

[0007] DC voltage can be measured using capacitive voltage dividers, resistive voltage dividers, inductive voltage transformers, and resistor-capacitor voltage dividers. In current DC projects, high-voltage DC voltage dividers are mainly based on the resistor-capacitor voltage divider principle.

[0008] Regarding the influence of transformer frequency characteristics, the RC DC voltage transformer (also called "RC DC voltage divider") is dominant, and under certain conditions, the resistor or capacitor plays a decisive role. When the RC matching relationship is met, the voltage division ratio of the RC DC voltage transformer is not affected by the frequency. When the parameters of the RC DC voltage transformer are designed, the RC matching relationship is met, but the presence of stray capacitance in actual working conditions will destroy the RC matching relationship. In addition, the capacitance of the stray capacitance will change with the frequency. When measuring voltage containing harmonics, the voltage division ratios of each frequency are different, and the harmonic distortion of the synthesized output signal cannot achieve the purpose of measuring and monitoring the voltage of the DC transmission line by the RC DC voltage transformer.

[0009] Domestic and foreign research on stray capacitance is mostly focused on DCVT, lightning arresters, converter transformers, etc. The presence 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 voltage divider ratio of DCVT; for lightning arresters, they form a multi-potential conductor system with surrounding live equipment, generating stray capacitance and causing current interference, thus affecting the existing online monitoring technology; the high-frequency resonance quality factor of the converter transformer will change due to the presence of stray capacitance, and the resistance value of the branch will change. It can be seen that stray capacitance will affect the performance of the conductor itself, so the calculation of stray capacitance is crucial for the design and effect of power equipment.

[0010] In theory, there will be capacitive coupling between the charged body around the test object and the tested equipment, which will produce different electric field interference. To prevent this influence, relevant measures should be taken. However, it is impossible to completely eliminate the influence of stray capacitance. Summary of the invention

[0011] In order to solve the above technical problems, the present invention proposes a method for reducing the influence of harmonics on the measurement accuracy of a RC DC voltage divider. The method can adjust the matching capacitor to satisfy the RC matching relationship by calculating the stray capacitance of the RC DC voltage divider, thereby improving the broadband voltage measurement accuracy of the RC DC voltage divider.

[0012] The technical solution adopted by the present invention is:

[0013] A method for reducing the influence of harmonics on the measurement accuracy of a resistor-capacitor DC voltage divider comprises the following steps:

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

[0015] Step 2: Analyze the voltage division principle of the RC DC voltage divider to obtain the RC matching relationship of the RC DC voltage divider;

[0016] Step 3: When considering stray capacitance without considering harmonics, analyze the influence of stray capacitance of the main voltage divider; when considering harmonics and stray capacitance, analyze the resistance-capacitance matching relationship of the RC DC voltage divider, and calculate the value of the matching resistor;

[0017] 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.

[0018] In step 1, the RC DC voltage divider includes: a main voltage divider, a RC matching unit, a secondary voltage divider plate, and a signal acquisition device; the main voltage divider is connected to the RC matching unit, the RC matching unit is connected to the secondary voltage divider plate, and the secondary voltage divider plate is connected to the signal acquisition device;

[0019] The RC matching unit includes a matching resistor and a matching capacitor. By adjusting the matching resistor and the matching capacitor, the RC DC voltage divider can maintain a stable voltage division ratio. At the same time, the RC matching unit is used to reduce the measurement error caused by frequency and stray capacitance.

[0020] The main voltage divider includes a plurality of single-stage voltage divider modules, each of which uses the same resistor and capacitor elements.

[0021] The structure of each 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.

[0022] In the main voltage divider, the level at the bottom of the main voltage divider is a low voltage arm, and the resistance and capacitance of the low voltage arm are R L , C L ;

[0023] Several voltage divider modules above the low voltage arm constitute 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 ;

[0024] The resistors of each voltage divider module in the high-voltage arm 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 .

[0025] 1) When there is only DC voltage without harmonics on the DC bus, the equivalent impedance of the high-voltage arm and the low-voltage arm of the main voltage divider 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 is dc It 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, ω is the measured voltage frequency; when the DC voltage does not contain harmonics, ω=0, and the voltage divider ratio K of the main voltage divider of the RC DC voltage divider dc for:

[0028]

[0029] In formula (3): R H is 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;

[0030] 2) When the DC voltage contains harmonics, the voltage divider ratio K of the main voltage divider of the RC DC voltage divider ac for:

[0031]

[0032] 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 equations (3) and (4), we can see that the resistance and capacitance parameters should satisfy the following equations:

[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 the 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, due to the stray capacitance C of the high-voltage arm of the main voltage divider 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 RC 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 in frequency will cause the change in 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 division ratio of each harmonic frequency during transmission will vary with the frequency.

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

[0040]

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

[0042] R L ′=R L ||R0||R i (8);

[0043] Among them, R0 is the matching resistance of the RC matching unit, R i is the equivalent resistance of the secondary voltage divider.

[0044]

[0045] 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.

[0046] 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:

[0047]

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

[0049]

[0050] Among them, since the DC voltage divider voltage ratio K can be adjusted by matching resistor R0 dc ′ Calibrated to the design value K dc , so

[0051]

[0052] That is, we get:

[0053]

[0054] Substituting the calculated value of the matching resistor R0 into formula (11), the value of the matching capacitor C0 can be obtained.

[0055] In step 4, the stray capacitance is the undesired mutual capacitance generated between two charged bodies. Assuming that n conductors carry charges Q1, Q2, ..., Qn, the potentials of these conductors are:

[0056]

[0057] Among them, P ij is the potential coefficient, i, j = 1, 2, ..., n; solving this linear equation system, we can get the charge as:

[0058]

[0059] Among them, C ii is the capacitance of the ith conductor, C ij is the inductance, i≠j; assuming the potential of the ith conductor is 1V and the potential of other conductors is 0V, then for this system, the current carrying capacity of the ith conductor is equal to its capacitance, that is:

[0060]

[0061] Among them, Q j is the charge on the conductor, j = 1, 2, ..., n; V i is the electric potential of the above conductor, i=1,2,…,n.

[0062] It can be seen from this that the capacitance matrix of the entire system is:

[0063]

[0064] Helmholtz and Thomson showed that these coefficients are symmetric, that is,

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

[0066] Among them, C ji It is the inductance of the potential of conductor i to the charge of conductor j, i≠j.

[0067] In field theory, the Maxwell capacitance matrix is ​​used to describe the charge of the i-th conductor (Q1, Q2, ..., Q n ) and the voltages V1, V2, ..., V of all conductors in the system n The relationship between:

[0068]

[0069] 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;

[0070] It can be seen that the Maxwell capacitance matrix and the mutual capacitance matrix have a direct conversion relationship. By finding the Maxwell capacitance matrix, 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 desired or undesired capacitance (charge accumulation) generated between two charged bodies.

[0071] Among them, the Maxwell capacitance matrix is:

[0072]

[0073] The mutual capacitance matrix C is:

[0074]

[0075] Assume that there are only three conductors in space with charges Q1, Q2, and Q3, respectively. The distances between Q1 and Q2, Q1 and Q3, and Q2 and Q3 are r respectively. 12 、r 13 、r 23 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] Where C is the capacitance and U is the voltage difference between the conductors.

[0080] In actual solution, the Maxwell capacitance matrix is ​​solved by the above energy method to obtain the stray capacitance value, that is, the mutual capacitance value.

[0081] In step 4, the calculated capacitance matrix can be used to substitute the calculated stray capacitance into the matching resistance R0 obtained by combining the formula (13) into the formula (11), so as to solve the matching capacitance of the RC matching unit under the main characteristic harmonics, thereby reducing the influence of harmonics on the measurement accuracy of the RC DC voltage divider.

[0082] The present invention provides a method for reducing the influence of harmonics on the measurement accuracy of a resistor-capacitor DC voltage divider, and the technical effects are as follows:

[0083] 1) The present invention analyzes the structural principle of the RC DC voltage divider and concludes that when the RC matching relationship is met, the voltage division ratio of the RC voltage divider can be kept constant and is not affected by the frequency.

[0084] 2) The present invention introduces a resistance-capacitance matching unit, and the matching resistor can calibrate the voltage divider ratio of the DC voltage divider to the designed value; the matching capacitor can achieve resistance-capacitance matching by changing the matching capacitor value through the calculated stray capacitance value, thereby improving the frequency response characteristics of the DC voltage transformer and realizing accurate measurement of wide-band DC voltage. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0086] Among them: 1-main voltage divider, 2-resistance-capacitance matching unit, 3-secondary voltage divider board, 4-signal acquisition device;

[0087] 1.1-outer wall, 1.2-high pressure arm, 1.3-low pressure arm;

[0088] Figure 2 is an equivalent circuit diagram of a single-stage voltage divider module in the present invention;

[0089] Figure 3 This is a circuit diagram of the main voltage divider of the RC DC voltage divider in the present invention.

[0090] Figure 4 The figure is an equivalent circuit diagram of the main voltage divider impedance of the RC DC voltage divider in the present invention.

[0091] Figure 5 The circuit diagram of the secondary voltage divider plate of the RC DC voltage divider in the present invention. DETAILED DESCRIPTION

[0092] A method for reducing the influence of harmonics on the measurement accuracy of a resistor-capacitor DC voltage divider comprises the following steps:

[0093] Step 1: Analyze the structure of the RC DC voltage divider, establish the impedance equivalent model of the RC DC voltage divider, analyze the voltage division principle of the RC DC voltage divider, and conclude that in order to ensure accurate measurement of the RC DC voltage divider, the RC matching relationship should be met.

[0094] Step 2: By analyzing the resistance-capacitance matching relationship of the RC DC voltage divider, it can be seen that when R H C H =R L C L When the voltage divider ratio K of the RC DC voltage divider under the harmonic voltage is ac The voltage division ratio K under the DC voltage without harmonics dc The voltage divider ratio is not affected by the frequency.

[0095] Step 3: Analyze the influence of stray capacitance of the main voltage divider when harmonics are not considered and when stray capacitance is considered, analyze the overall resistance-capacitance matching relationship of the RC DC voltage divider when harmonics and stray capacitance exist, and analyze and calculate the matching resistance and capacitance values ​​of the RC matching unit that can correct the voltage divider ratio and reduce the influence of harmonics.

[0096] Step 4: By obtaining the value of stray capacitance and combining it with the above method, the RC matching relationship can be satisfied by changing the matching capacitance value, thereby improving the broadband measurement accuracy of the RC DC voltage divider and reducing the influence of stray capacitance on the RC DC voltage divider under harmonic conditions.

[0097] (1). Structure of RC DC voltage transformer:

[0098] The RC DC voltage divider is composed of a main voltage divider 1, a secondary voltage divider board 3, and a signal acquisition device 4. The main voltage divider 1 uses RC voltage division to convert high voltage into low voltage. Since the voltage after the main voltage divider is still relatively high (generally 70V), the secondary voltage divider board 3 also uses the RC voltage division principle. On the one hand, it can convert the low-voltage signal output by the low-voltage arm of the main voltage divider into a multi-channel signal (generally 5V) for the signal acquisition module to perform signal acquisition work and send it to the control protection system. On the other hand, it can avoid the problem that all secondary measurement results of the voltage divider are abnormal due to a secondary circuit load failure.

[0099] The present invention adds a resistor-capacitor voltage divider unit 2 between the secondary voltage divider plate 3 and the main voltage divider 1 on the basis of the existing common resistor-capacitor DC voltage divider structure. By adjusting its matching resistor and matching capacitor, the resistor-capacitor DC voltage divider can maintain a stable voltage divider ratio. At the same time, the resistor-capacitor voltage divider unit 2 can reduce the measurement error caused by frequency and stray capacitance. The improved DC voltage transformer (DCVT) structure is as follows: Figure 1 shown.

[0100] Since DCVT is mostly used for voltage measurement of high-voltage direct current transmission, its operating voltage level is high, and cascade assembly is often required. For example, the ±800KV DCVT overall resistor-capacitor voltage divider module is composed of 16 single-stage voltage divider modules, and each stage of voltage divider is 50kV. At the same time, in order to reduce the measurement error caused by different resistor-capacitor temperature coefficients when the temperature rise reaches thermal equilibrium, each stage of voltage divider module uses the same resistor and capacitor elements. Each two-stage voltage divider module is connected in series with a rigid flange with bolts as the carrier. The equivalent circuit of a single-stage resistor-capacitor voltage divider module is as follows: Figure 2 shown.

[0101] Each level of voltage divider module structure is that the resistors are evenly distributed around the capacitor along the circumferential direction with the capacitor as the axis to prevent corona from being generated on the resistor surface. The capacitor is connected in parallel at both ends of the resistor. When the equipment is impacted by transient overvoltages such as lightning, the capacitor can flatten the wavefront steepness and protect the equipment. In order to optimize the electric field distribution inside the equipment, the single-stage resistor-capacitor voltage divider is often designed with a voltage equalizing ring.

[0102] The number of stages that make up the RC voltage divider module is related to the voltage level of the DCVT, such as Figure 3 The voltage dividing principle of the DCVT main voltage divider is shown. The first level at the bottom of the main voltage divider 1 is the low voltage arm 1.3. The resistance and capacitance of the low voltage arm are R L , C L , the several stages of RC voltage dividers above the low voltage arm constitute the high voltage arm of the main voltage divider, and the single-stage RC voltage divider is composed of several smaller RC elements connected in series and parallel. The equivalent resistance and equivalent capacitance of the high voltage arm of the main voltage divider are R H , C H The resistors of each voltage divider in the high-voltage arm are connected in series, that is, R H =R1+R2+R3+…+R n , the capacitors of each voltage divider are connected in parallel, that is, C H =C1∥C2∥C3∥…∥C n .

[0103] In order to meet the insulation level of resistors and capacitors and other electrical and mechanical performance requirements, the main resistor and capacitor voltage divider of DCVT is usually placed inside a hollow insulator, which is filled with insulating oil, SF6 or N2 and other insulating media. The upper part is encapsulated with a flange and the lower part is encapsulated and supported by a base.

[0104] RC DC voltage divider plays an important role in high-voltage DC transmission system. Its measurement principle is simple and the technology is mature. With the development of new power system, it is required that the ratio difference of RC DC voltage divider under harmonic conditions within 50Hz-3kHz is not more than 3%. The measurement bandwidth of RC DC voltage divider is determined by the matching degree of high and low voltage arm voltage divider resistor and voltage divider capacitor. In theory, when the RC matching relationship is met, the voltage divider ratio of RC DC voltage divider can be kept constant. In fact, RC parameters are affected by the operating environment, stray parameters, etc., and it is difficult to achieve RC matching. Harmonics will affect the size of stray parameters of RC DC voltage divider. Therefore, reducing the influence of harmonics on RC DC voltage divider is of great significance to achieve accurate measurement of broadband DC voltage.

[0105] (2). Resistor-capacitor matching relationship of RC DC voltage divider:

[0106] The impedance equivalent circuit diagram of the main voltage divider of the RC DC voltage divider is as follows: Figure 3When there is only DC voltage without harmonics on the DC bus, the equivalent impedance of the high-voltage arm and the low-voltage arm of the DC voltage transformer's resistor-capacitor voltage divider is as follows: dc It can be equivalent to formula (3).

[0107]

[0108] Where ω is the measured voltage frequency. When the DC voltage does not contain harmonics, ω = 0, and the voltage divider ratio K of the main voltage divider of the RC DC voltage divider is dc for:

[0109]

[0110] In the formula, R H is 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 section of the RC voltage divider, u L (t) is the output voltage at the low voltage end.

[0111] When the DC voltage contains harmonics, the voltage division ratio K of the main voltage divider of the RC DC voltage divider ac for:

[0112]

[0113] To ensure that the DCVT has the same voltage division ratio when measuring DC voltage and harmonic voltage, K dc =K ac , from equations (3) and (4), we can see that the resistance and capacitance parameters should satisfy the following equations:

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

[0115] When R H C H =R L C L When the voltage divider is in harmonic voltage, the voltage divider ratio Kac of the RC DC voltage divider is equal to the voltage divider ratio Kdc of the DC voltage without harmonics. The voltage divider ratio is the same at different frequencies. H C H =R L C L It 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 resistor-capacitor DC voltage divider in the voltage divider body. The situation of the secondary voltage divider is consistent with that of the main voltage divider. Here, the analysis is only 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, it can avoid distortion in the measurement, thus ensuring the measurement accuracy of the DC voltage divider. Under actual working conditions, the harmonics contained in the measured DC voltage and the existence 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, due to the stray capacitance C of the high voltage arm of the main voltage divider 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 RC 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 make 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 in frequency will cause the change in capacitance value, which will cause R H C H ′≠R L C LWhen a RC DC voltage divider measures a voltage containing harmonics, the voltage divider ratio of each harmonic frequency during transmission will vary with the frequency. Thus, the harmonic voltage synthesized by 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, harmonic frequency and 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. The addition of the RC matching unit can calibrate the voltage division ratio of the RC DC voltage divider to the design value through the matching resistor R0. At the same time, the capacitance of the high and low voltage arm stray capacitance can be analyzed and calculated, and the capacitance of C0 can be adjusted to reduce the influence of harmonics on the measurement accuracy of the RC DC voltage divider.

[0123] The RC DC voltage divider with the RC matching unit added is circuit equivalent as a whole. When the RC DC voltage divider satisfies the RC matching relationship shown in the following equation, the voltage division ratio of the RC DC voltage 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 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.

[0126] By analyzing the circuit equivalent of the RC DC voltage divider, we can know that:

[0127]

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

[0129] R L ′=R L ||R0||R i (8);

[0130] Among them, R0 is the matching resistance of the RC matching unit, R i is the equivalent resistance of the secondary voltage divider.

[0131]

[0132] Among them, C Z2is 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.

[0133] 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:

[0134]

[0135] In the formula, C H , C L , C i , R H , R L , R i The resistor and capacitor components are fixed when they leave the factory, and the present invention considers them unchanged; C Z1 , C Z2 The stray capacitance can be obtained by field calculation, and the matching resistance value R0 and the matching capacitance value C0 of the RC matching unit are analyzed and solved by combining these parameter values.

[0136]

[0137] Among them, since the voltage division ratio K of the RC DC voltage divider can be changed by matching the resistor R0 dc ′ Calibrated to the design value K dc , so

[0138]

[0139] You can get:

[0140]

[0141] Substituting the calculated value of the matching resistor R0 into formula (11), the value of the matching capacitor C0 can be obtained.

[0142] (4) Calculation method of stray capacitance of RC DC voltage divider under actual working conditions:

[0143] Regarding the influence of the frequency characteristics of the transformer, the RC DC voltage divider is dominant, and under certain conditions, the resistor or capacitor plays a decisive role. In practical applications, each resistor has its own stray capacitance, and this nonlinear characteristic will cause the resistor to resonate at a certain frequency. When the DCVT works in the presence of harmonics, resonance may occur due to the broadband characteristics of the harmonics.

[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 cause 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 ith conductor is 1V and the potential of other conductors is 0V. For this system, the amount of charge carried by the ith conductor is equal to its capacitance, that is,

[0151]

[0152] It can be seen from this that the capacitance matrix of the entire system is

[0153]

[0154] Helmholtz and Thomson showed 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 ith 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 the Maxwell capacitance matrix and the mutual capacitance matrix have a direct conversion relationship. By finding the Maxwell capacitance matrix, the mutual capacitance matrix C of the system can be obtained through linear transformation, and then the stray capacitance value can be obtained.

[0159] The measurement error problem of the DC voltage divider is mainly caused by system frequency deviation, resistance value deviation and capacitance value deviation. Resistance and capacitance are not constant values. They are affected by many factors. The error values ​​generated during the production process will introduce measurement errors. The stray capacitance generated by the resistor itself in the environment will also affect the capacitance value, thereby affecting the voltage divider ratio of DCVT, resulting in DCVT measurement errors, affecting the accuracy and precision of the measurement. In addition, the stray capacitance will also be affected by the measured harmonic frequency, causing the voltage divider ratio of the RC DC voltage divider to change, reducing the measurement accuracy of the RC DC voltage divider for DC voltages containing harmonic signals.

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

Claims

1. A method for reducing the influence of harmonics on the measurement accuracy of a resistor-capacitor DC voltage divider, characterized in that The following steps are involved: Step 1: Based on the structure of the RC DC voltage divider, an impedance equivalent model of the RC DC voltage divider is established; Step 2: Analyze the voltage division principle of the RC DC voltage divider to obtain the RC matching relationship of the RC DC voltage divider; Step 3: When considering stray capacitance without considering harmonics, analyze the influence of stray capacitance of the main voltage divider; when considering harmonics and stray capacitance, analyze the resistance-capacitance matching relationship of the RC DC voltage divider, 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 RC matching unit (2) comprises a matching resistor and a matching capacitor. By adjusting the matching resistor and the matching capacitor, the RC DC voltage divider can maintain a stable voltage division ratio. At the same time, the RC matching unit (2) is used to reduce the measurement error 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 adopts 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 ; The plurality of 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 division ratio K of the main voltage divider (1) of the RC DC voltage divider dc for: In formula (3): R H is 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 division 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 equations (3) and (4), we can see that the resistance and capacitance parameters should satisfy the following equations: 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 the 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, due to the stray capacitance C of the high-voltage arm of the main voltage divider 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 RC 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 in frequency will cause the change in 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 division ratio of each harmonic frequency during transmission 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 RC DC voltage divider with the RC matching unit (2) added thereto is subjected to circuit equivalence as a whole. 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 It is the total equivalent stray capacitance of the high voltage arm of the main voltage divider of the RC DC voltage divider; R L ′=R L R0 R i (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: In the formula, 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 ′ Calibrated 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, the stray capacitance is the undesired mutual capacitance generated 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 ith conductor is 1V and the potential of other conductors is 0V, then for this system, the current carrying capacity of the ith 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; It can be seen from this that the capacitance matrix of the entire system is: Helmholtz and Thomson showed that these coefficients are symmetric, 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. The Maxwell capacitance matrix is ​​obtained, and then 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 formula (13) are substituted into formula (11), so that 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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