Harmonic protection method of direct current voltage divider considering heat accumulation effect

Abstract

The invention discloses a DC voltage divider harmonic protection method considering a heat accumulation effect, and the method comprises the following steps: measuring the secondary side voltage of a DC voltage divider body through a secondary voltage dividing plate voltage transformer, and calculating the DC voltage of a flexible DC system according to the calibration transformation ratio N of the DC voltage divider body; performing fast Fourier transform on the DC voltage of the flexible DC system obtained by calculation to obtain the amplitude of each harmonic component of the DC voltage of the flexible DC system; calculating the amplitude of each harmonic voltage component of a high-voltage arm, a low-voltage arm and a secondary side voltage dividing plate according to the voltage transmission and transformation characteristics of the direct-current voltage divider; calculating harmonic loss of a high-voltage arm, a low-voltage arm and a secondary side voltage dividing plate by considering a resistance skin effect and capacitance loss; and comparing the harmonic heat accumulation values of the high-voltage arm, the low-voltage arm and the secondary side voltage dividing plate with the protection action setting values of the high-voltage arm, the low-voltage arm and the secondary side voltage dividing plate to obtain a protection scheme. According to the invention, the DC voltage divider can be protected from being affected by the harmonic heat accumulation effect and being burnt out.

Description

Technical Field

[0001] The invention belongs to the field of harmonic protection of a DC voltage divider, and in particular relates to a harmonic protection method for a DC voltage divider taking into account heat accumulation effect. Background Art

[0002] The flexible direct current transmission system (flexible direct current system) is a high-voltage direct current transmission system based on fully controlled power electronic devices. It has the characteristics of active and reactive power decoupling control and no risk of phase change failure. It is widely used in scenarios such as large-scale new energy grid connection and asynchronous interconnection of urban power grids. However, the power electronic equipment in the flexible direct current system is a nonlinear device, which leads to a large number of harmonics in the operation of the flexible direct current system, affecting the normal operation of electrical equipment.

[0003] In the prior art, the harmonic protection of flexible DC systems mainly includes harmonic overload protection of bridge arm reactors, valve-side harmonic current protection, grid-side voltage harmonic distortion rate protection, etc., but there is no harmonic protection for DC voltage dividers. DC voltage dividers are key devices for measuring DC voltage signals in flexible DC systems. When there are a large number of harmonics in the DC voltage, the DC voltage dividers may burn out due to the harmonic heat accumulation effect, causing the flexible DC system to shut down. Summary of the invention

[0004] The present invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a harmonic protection method for a DC voltage divider taking into account the heat accumulation effect.

[0005] The technical solution of the present invention is: a DC voltage divider harmonic protection method considering the heat accumulation effect, comprising the following steps:

[0006] A. Use the secondary voltage transformer to measure the secondary voltage U of the DC voltage divider body mes , calculate the DC voltage U of the flexible DC system according to the DC voltage divider body calibration ratio N dc ;

[0007] B. The calculated DC voltage U of the flexible DC system dc Perform fast Fourier transform to obtain the amplitude of each harmonic component of the DC voltage of the flexible DC system

[0008] C. Based on the fast Fourier transform results of DC voltage, the amplitude of each harmonic voltage component of the high voltage arm, low voltage arm and secondary side voltage divider is calculated according to the voltage transmission characteristics of the DC voltage divider.

[0009] D. Considering the skin effect of resistors and capacitor losses, calculate the harmonic losses P of the high-voltage arm, low-voltage arm and secondary voltage divider high , P low , P 2nd ;

[0010] E. The harmonic heat accumulation value P of the high voltage arm, low voltage arm and secondary side voltage divider high , P low , P 2nd Respectively with the high voltage arm, low voltage arm, secondary side voltage divider protection action setting value Compare and get the protection plan.

[0011] Furthermore, the protection scheme in step E is as follows:

[0012] If any harmonic heat loss is greater than the protection action setting value, the DC voltage divider harmonic protection will issue a DC voltage divider overheating warning and select the corresponding protection action output delay;

[0013] If any harmonic loss of the high voltage arm, low voltage arm and secondary side voltage divider board is always greater than the corresponding action setting value during the delay period, the DC voltage divider harmonic protection will issue a DC voltage divider overheat warning and trip, otherwise the DC voltage divider harmonic protection will not operate.

[0014] Furthermore, the DC voltage of the flexible DC system in step A is expressed as:

[0015]

[0016] Furthermore, in step C, the high voltage arm voltage U high , low voltage arm voltage U low And the secondary side voltage divider voltage U 2nd With DC voltage U dc The transfer function G high (s), G low (s), G 2nd (s), specifically expressed as follows:

[0017]

[0018] In the formula, R high , R low , R 2nd They are the equivalent resistances of the high voltage arm, low voltage arm and secondary voltage divider plate respectively; C high , C low , C 2nd They are the equivalent capacitances of the high voltage arm, low voltage arm and secondary voltage divider plate respectively.

[0019] Furthermore, based on the high voltage arm voltage U high , low voltage arm voltage U low And the secondary side voltage divider voltage U 2nd With DC voltage U dc The transfer function between the high-voltage arm, the low-voltage arm and the secondary side voltage divider is calculated to calculate the amplitude of each harmonic voltage component The details are as follows:

[0020]

[0021] In the formula, ω 0 is the fundamental angular frequency.

[0022] Furthermore, step D considers the skin effect of resistors and capacitor losses to calculate the harmonic loss P of the high-voltage arm. high , the specific process is as follows:

[0023] First, considering the skin effect, the resistance under the hth harmonic can be expressed as:

[0024]

[0025] In the formula, R 0 is the fundamental resistance value;

[0026] Then, considering the skin effect, the loss caused by the hth harmonic voltage on the resistor can be expressed as:

[0027]

[0028] Then, the capacitance loss caused by the hth harmonic voltage can be expressed as:

[0029] P C_h =U h 2 h 2 ω 0 C tanδ 0

[0030] Where, tanδ 0 is the tangent of the capacitor dielectric loss.

[0031] Finally, considering the skin effect of the resistor and the capacitor loss, the harmonic loss of the high-voltage arm P high It can be calculated by the following formula:

[0032]

[0033] In the formula, Considering the skin effect of the resistor and the loss of the capacitor, tanδ high is the dielectric loss tangent of the high voltage arm capacitor.

[0034] Furthermore, step D considers the skin effect of resistors and capacitor losses to calculate the harmonic loss P of the low-voltage arm. low , the specific process is as follows:

[0035] When considering the skin effect of resistors and capacitor losses, the harmonic loss of the low-voltage arm P low It can be calculated by the following formula:

[0036]

[0037] In the formula, Considering the skin effect of the resistor and the capacitance loss, tanδ low is the dielectric loss tangent of the low voltage arm capacitor.

[0038] Furthermore, step D considers the skin effect of resistors and capacitor losses to calculate the harmonic loss P of the secondary voltage divider. 2nd , the specific process is as follows:

[0039] When considering the skin effect of the resistor and the capacitor loss, the harmonic loss P of the secondary voltage divider is 2nd It can be calculated by the following formula:

[0040]

[0041] In the formula, Considering the skin effect of the resistor and the capacitance loss, tanδ is the loss coefficient of the secondary voltage divider hth harmonic. 2nd is the dielectric loss tangent of the secondary voltage divider capacitor.

[0042] Furthermore, in step E, the high voltage arm protection action setting value The calculation expression is as follows:

[0043]

[0044] Where U high_B is the power frequency withstand voltage of the single high-voltage arm capacitor, n high is the number of high-pressure nodes, k high It is the long-term overload multiple of the high-voltage arm capacitor.

[0045] Furthermore, in step E, the low voltage arm protection action setting value The calculation expression is as follows:

[0046]

[0047] Where U low_B is the power frequency withstand voltage of a single low voltage arm capacitor, n low is the number of capacitors in series with the low voltage arm, k low It is the long-term overload multiple of the low-voltage arm capacitor.

[0048] The beneficial effects of the present invention are as follows:

[0049] The present invention can protect the DC voltage divider from being burned due to the harmonic heat accumulation effect. Since the present invention calculates the harmonic voltages of the high-voltage arm, the low-voltage arm, and the secondary voltage divider based on the voltage transmission characteristics of the DC voltage divider and the voltage signal measured by the voltage transformer of the secondary voltage divider, and further calculates the harmonic heat accumulation values ​​of the high-voltage arm, the low-voltage arm, and the secondary voltage divider based on the harmonic voltage calculation results, there is no need to configure an additional electrical signal measurement device. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 is a flow chart of the method of the present invention;

[0051] Figure 2 A schematic diagram of the structure of a DC voltage divider used in the present invention;

[0052] Figure 3 A schematic diagram of a DC voltage divider harmonic protection module considering the heat accumulation effect of the present invention;

[0053] Figure 4 This is a schematic diagram of the action logic of the harmonic protection of the DC voltage divider considering the heat accumulation effect of the present invention;

[0054] Figure 5 is the FFT result of the harmonic voltage at the DC voltage divider port in a specific embodiment;

[0055] Figure 6 It is the calculation result of the temperature change of the low-voltage arm and the secondary-side voltage divider plate of the DC voltage divider before the protection trip in the specific embodiment. DETAILED DESCRIPTION

[0056] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings and embodiments:

[0057] like Figures 1 to 6 As shown, a DC voltage divider harmonic protection method considering heat accumulation effect includes the following steps:

[0058] A. Use the secondary voltage transformer to measure the secondary voltage U of the DC voltage divider body mes , calculate the DC voltage U of the flexible DC system according to the DC voltage divider body calibration ratio N dc ;

[0059] B. The calculated DC voltage U of the flexible DC system dc Perform fast Fourier transform to obtain the amplitude of each harmonic component of the DC voltage of the flexible DC system

[0060] C. Based on the fast Fourier transform results of DC voltage, the amplitude of each harmonic voltage component of the high voltage arm, low voltage arm and secondary side voltage divider is calculated according to the voltage transmission characteristics of the DC voltage divider.

[0061] D. Considering the skin effect of resistors and capacitor losses, calculate the harmonic losses P of the high-voltage arm, low-voltage arm and secondary voltage divider high , P low , P 2nd ;

[0062] E. The harmonic heat accumulation value P of the high voltage arm, low voltage arm and secondary side voltage divider high , P low , P 2nd Respectively with the high voltage arm, low voltage arm, secondary side voltage divider protection action setting value Compare and get the protection plan.

[0063] The protection scheme in step E is as follows:

[0064] If any harmonic heat loss is greater than the protection action setting value, the DC voltage divider harmonic protection will issue a DC voltage divider overheating warning and select the corresponding protection action output delay;

[0065] If any harmonic loss of the high voltage arm, low voltage arm and secondary side voltage divider board is always greater than the corresponding action setting value during the delay period, the DC voltage divider harmonic protection will issue a DC voltage divider overheat warning and trip, otherwise the DC voltage divider harmonic protection will not operate.

[0066] The DC voltage of the flexible DC system in step A is expressed as:

[0067]

[0068] In step C, the high voltage arm voltage U high , low voltage arm voltage U low And the secondary side voltage divider voltage U 2nd With DC voltage U dc The transfer function G high (s), G low (s), G 2nd (s), specifically expressed as follows:

[0069]

[0070] In the formula, R high , R low , R 2nd They are the equivalent resistances of the high voltage arm, low voltage arm and secondary voltage divider plate respectively; C high , C low , C 2nd They are the equivalent capacitances of the high voltage arm, low voltage arm and secondary voltage divider plate respectively.

[0071] Based on the high voltage arm voltage U high , low voltage arm voltage U low And the secondary side voltage divider voltage U2nd With DC voltage U dc The transfer function between the high-voltage arm, the low-voltage arm and the secondary side voltage divider is calculated to calculate the amplitude of each harmonic voltage component The details are as follows:

[0072]

[0073] In the formula, ω 0 is the fundamental angular frequency.

[0074] Step D: Consider the skin effect of resistors and capacitor losses to calculate the harmonic loss P of the high-voltage arm. high , the specific process is as follows:

[0075] First, considering the skin effect, the resistance under the hth harmonic can be expressed as:

[0076]

[0077] In the formula, R 0 is the fundamental resistance value;

[0078] Then, considering the skin effect, the loss caused by the hth harmonic voltage on the resistor can be expressed as:

[0079]

[0080] Then, the capacitance loss caused by the hth harmonic voltage can be expressed as:

[0081] P C_h =U h 2 h 2 ω 0 C tanδ 0

[0082] Where, tanδ 0 is the tangent of the capacitor dielectric loss.

[0083] Finally, considering the skin effect of the resistor and the capacitor loss, the harmonic loss of the high-voltage arm P high It can be calculated by the following formula:

[0084]

[0085] In the formula, Considering the skin effect of the resistor and the loss of the capacitor, tanδ high is the dielectric loss tangent of the high voltage arm capacitor.

[0086] Step D: Consider the skin effect of resistors and capacitor losses to calculate the harmonic loss P of the low voltage arm. low , the specific process is as follows:

[0087] When considering the skin effect of resistors and capacitor losses, the harmonic loss of the low-voltage arm P low It can be calculated by the following formula:

[0088]

[0089] In the formula, Considering the skin effect of the resistor and the capacitance loss, tanδ low is the dielectric loss tangent of the low voltage arm capacitor.

[0090] Step D Consider the skin effect of resistors and capacitor losses to calculate the harmonic loss P of the secondary voltage divider. 2nd , the specific process is as follows:

[0091] When considering the skin effect of the resistor and the capacitor loss, the harmonic loss P of the secondary voltage divider is 2nd It can be calculated by the following formula:

[0092]

[0093] In the formula, Considering the skin effect of the resistor and the capacitance loss, tanδ is the loss coefficient of the secondary voltage divider hth harmonic. 2nd is the dielectric loss tangent of the secondary voltage divider capacitor.

[0094] In step E, the high voltage arm protection action setting value The calculation expression is as follows:

[0095]

[0096] Where U high_B is the power frequency withstand voltage of the single high-voltage arm capacitor, n high is the number of high-pressure nodes, k high It is the long-term overload multiple of the high-voltage arm capacitor.

[0097] In step E, the low voltage arm protection action setting value The calculation expression is as follows:

[0098]

[0099] Where U low_B is the power frequency withstand voltage of a single low voltage arm capacitor, n low is the number of capacitors in series with the low voltage arm, k low It is the long-term overload multiple of the low-voltage arm capacitor.

[0100] Specifically, the secondary side voltage divider protection action setting The calculation expression is as follows:

[0101]

[0102] Where U 2nd_B is the power frequency withstand voltage of a single low voltage arm capacitor, n 2nd is the number of capacitors in series with the low voltage arm, k 2nd It is the long-term overload multiple of the low-voltage arm capacitor.

[0103] Specifically, in step E, the high voltage arm protection delay time is calculated and expressed as follows:

[0104]

[0105] in, C high is the heat capacity of the high-voltage arm equipment, is the heat dissipation coefficient of the high-voltage arm equipment.

[0106] Specifically, the calculation expression of the low-voltage arm protection delay time is as follows:

[0107]

[0108] in, C low is the heat capacity of the low voltage arm equipment, is the heat dissipation coefficient of the low-voltage arm equipment.

[0109] Specifically, the calculation expression of the secondary side voltage divider protection delay time is as follows:

[0110]

[0111] in, C 2nd is the heat capacity of the secondary side voltage divider equipment, is the heat dissipation coefficient of the secondary side voltage divider device.

[0112] Embodiment 1

[0113] The present invention uses a resistive-capacitive DC voltage divider, the structure of which is as follows: Figure 2 As shown. The RC DC voltage divider consists of a primary body and a secondary voltage divider. The primary body of the DC voltage divider is connected to the high voltage arm resistor and capacitor R high , C high And the low voltage arm resistor and capacitor R low , C low For DC voltage U dc The secondary voltage divider uses the same structure to perform the secondary voltage divider, and the output voltage signal U mes The DC voltage divider calculates the DC voltage signal according to the body calibration ratio N, that is:

[0114]

[0115] The equivalent resistance and equivalent capacitance of the secondary voltage divider can be expressed as:

[0116]

[0117] In this embodiment, the equivalent resistance and capacitance parameters of the high voltage arm are R high =71.75MΩ, The equivalent resistance and capacitance parameters of the low voltage arm are R low =35.8kΩ, C low =3.29μF, the equivalent resistance and capacitance parameters of the secondary voltage divider are R 2nd =141kΩ, C 2nd =0.99μF, the DC voltage divider body calibration ratio is N=1:50000.

[0118] A schematic diagram of a DC voltage divider harmonic protection module considering the heat accumulation effect of the present invention is shown in FIG. Figure 3 As shown, it includes a data acquisition unit, a data processing unit, a logic judgment unit, and a protection export unit.

[0119] Data acquisition unit, realizes DC voltage signal U dc Collection function;

[0120] The data processing unit realizes the calculation of the harmonic content of the DC voltage signal, the calculation of the harmonic loss of the high-voltage arm, the low-voltage arm, and the secondary-side voltage divider, and the calculation of the protection action delay of the high-voltage arm, the low-voltage arm, and the secondary-side voltage divider, etc.

[0121] Logic judgment unit, realizing the logic judgment function of DC voltage divider harmonic protection considering the heat accumulation effect;

[0122] The protection output unit realizes the output of the DC voltage divider harmonic protection action signal considering the heat accumulation effect. When the DC voltage divider harmonic protection criterion considering the heat accumulation effect is met, the protection issues a DC voltage divider overheat warning and counts according to the delay time calculated by the data processing unit. If the criterion is continuously met during the timing, the protection is activated, the DC voltage divider overheat warning is issued and the DC voltage divider is disconnected; otherwise, the protection does not act.

[0123] The above module adopts the following DC voltage divider harmonic protection method considering the heat accumulation effect to realize the DC voltage divider harmonic protection. The specific flow chart is as follows: Figure 1 As shown, the following steps are included:

[0124] A. Use the secondary voltage transformer to measure the secondary voltage U of the DC voltage divider body mes, calculate the DC voltage U of the flexible DC system according to the DC voltage divider body calibration ratio N dc ;

[0125] B. The calculated DC voltage U of the flexible DC system dc Perform Fast Fourier Transform (FFT) to obtain the amplitude of each harmonic component of the DC voltage of the flexible DC system

[0126] C. Based on the DC voltage FFT results, calculate the amplitude of each harmonic voltage component of the high voltage arm, low voltage arm and secondary side voltage divider according to the voltage transmission characteristics of the DC voltage divider

[0127] First, calculate the transfer function G between the high-voltage arm voltage, low-voltage arm voltage, secondary side voltage divider voltage and DC voltage of the DC voltage divider. high (s), G low (s), G 2nd (s), which is specifically expressed as follows:

[0128]

[0129] In the formula, R high , R low , R 2nd are the equivalent resistances of the high voltage arm, low voltage arm and secondary voltage divider plate, respectively. high , C low , C 2nd They are the equivalent capacitances of the high voltage arm, low voltage arm and secondary voltage divider plate respectively.

[0130] Then, according to the transfer function G between the high-voltage arm voltage, the low-voltage arm voltage, the secondary side voltage divider voltage and the DC voltage high (s), G low (s), G 2nd (s), the amplitude of each harmonic voltage component of the high voltage arm, low voltage arm and secondary side voltage divider It can be calculated by the following formula:

[0131]

[0132] In the formula,

[0133]

[0134] Among them, ω 0 =2π*50 is the fundamental angular frequency.

[0135] D. Consider the skin effect of resistors and capacitor losses to calculate the harmonic losses P of the high-voltage arm, low-voltage arm and secondary voltage divider high , Plow P 2nd .

[0136] When the skin effect is considered, the resistance under the hth harmonic can be expressed as:

[0137]

[0138] In the formula, R 0 is the fundamental resistance value.

[0139] Therefore, when the skin effect is considered, the loss caused by the hth harmonic voltage on the resistor can be expressed as:

[0140]

[0141] The capacitance loss caused by the h-th harmonic voltage can be expressed as:

[0142] P C_h =U h 2 h 2 ω 0 C tanδ 0

[0143] Where, tanδ 0 is the tangent of the capacitor dielectric loss.

[0144] Therefore, when considering the skin effect of the resistor and the capacitor loss, the harmonic loss of the high-voltage arm P high It can be calculated by the following formula:

[0145]

[0146] In the formula, Considering the skin effect of the resistor and the loss of the capacitor, tanδ high is the dielectric loss tangent of the high-voltage arm capacitor. In this embodiment, tanδ high =0.00067.

[0147] When considering the skin effect of resistors and capacitor losses, the harmonic loss of the low-voltage arm P low It can be calculated by the following formula:

[0148]

[0149] In the formula, Considering the skin effect of the resistor and the capacitance loss, tanδ low is the dielectric loss tangent of the low-voltage arm capacitor. In this embodiment, tanδ low =0.0028.

[0150] When considering the skin effect of the resistor and the loss of the capacitor, the harmonic heat accumulation value P of the secondary voltage divider is 2nd It can be calculated by the following formula:

[0151]

[0152] In the formula, Considering the skin effect of the resistor and the capacitance loss, tanδ is the loss coefficient of the secondary voltage divider hth harmonic. 2nd is the dielectric loss tangent of the secondary voltage divider capacitor. In this embodiment, tanδ 2nd =0.00198;

[0153] E. The harmonic loss P of the high voltage arm, low voltage arm and secondary side voltage divider high , P low , P 2nd Respectively with the high voltage arm, low voltage arm, secondary side voltage divider protection action setting value By comparison, if any harmonic loss is greater than the protection action setting value, the DC voltage divider harmonic protection will issue a DC voltage divider overheat warning and select the corresponding protection action output delay. If during the delay period, any harmonic loss of the high-voltage arm, low-voltage arm and secondary side voltage divider board is always greater than the corresponding action setting value, the DC voltage divider harmonic protection will issue a DC voltage divider overheat warning and trip, otherwise the DC voltage divider harmonic protection will not operate.

[0154] High voltage arm protection action setting The calculation method is:

[0155]

[0156] Where U high_B is the power frequency withstand voltage of the single high-voltage arm capacitor, n high is the number of high-pressure nodes, k high is the long-term overload multiple of the high-voltage arm capacitor. In the embodiment of the present invention, n high =7, k high =1.3, U high_B =25kV, calculated

[0157] Low voltage arm protection action setting The calculation method is:

[0158]

[0159] Where U low_B k is the power frequency withstand voltage of a single low voltage arm capacitor, low is the long-term overload multiple of the low-voltage arm capacitor. In the embodiment of the present invention, k low =1.3, Ulow_B =100V, calculated

[0160] Secondary side voltage divider protection action setting The calculation method is:

[0161]

[0162] Where U 2nd_B is the power frequency withstand voltage of a single low voltage arm capacitor, n 2nd is the number of capacitors in series with the low voltage arm, k 2nd is the long-term overload multiple of the low-voltage arm capacitor. In the embodiment of the present invention, n 2nd =5, k 2nd =1.3, U 2nd_B =20V, calculated

[0163] In terms of the selection of protection action outlet delay, the calculation method of high voltage arm protection delay time is:

[0164]

[0165] in, C high is the heat capacity of the high-voltage arm equipment, is the heat dissipation coefficient of the high-voltage arm equipment. In this embodiment, C high =0.11kJ / K, Therefore, τ high =1.1s

[0166] The calculation method of low voltage arm protection delay time is:

[0167]

[0168] in, C low is the heat capacity of the low voltage arm equipment, is the heat dissipation coefficient of the low-voltage arm equipment. In this embodiment, C low =0.22J / K, Therefore τ low =1.1s

[0169] The calculation method of the secondary side voltage divider protection delay time is:

[0170]

[0171] in, C 2nd is the heat capacity of the secondary side voltage divider equipment, is the heat dissipation coefficient of the secondary side voltage divider device. In this embodiment, C 2nd=0.12J / K, Therefore τ low =1.33s.

[0172] A logic flow chart of a DC voltage divider harmonic protection considering the heat accumulation effect of the present invention is as follows: Figure 4 As shown. The DC voltage divider protection action criterion is jointly determined by the high-voltage arm protection action criterion, the low-voltage arm protection action criterion, and the secondary side voltage divider protection action criterion. When any of the protection action criterion of the high-voltage arm, the low-voltage arm, and the secondary side voltage divider is met, the DC voltage divider sends out a warning signal and starts timing. The delay time criterion setting value is the minimum value of the delay time of the high-voltage arm, the low-voltage arm, and the secondary side voltage divider, that is, T = min {T high ,T low ,T 2nd}. When the DC voltage divider time criterion and action criterion are met at the same time, the DC voltage divider trips.

[0173] Input the DC voltage divider port as Figure 5 The harmonic voltage shown, the high voltage arm loss calculation result is P high =5.95×10 2 If the value is lower than the action setting value, the high-voltage arm protection will not work. The loss of the low-voltage arm and the loss of the secondary side voltage divider are P low =0.766, P 2nd =0.163 are all higher than the corresponding protection action setting value and meet the action criterion, so the DC voltage divider protection issues an overheating warning and starts timing. The protection delay time of the high voltage arm, low voltage arm, and secondary side voltage divider board are T high =∞、T low =1.255s, T 2nd =2.144s, the DC voltage divider protection takes the minimum value as the delay time, that is, T = T low =1.255s. After applying the harmonic voltage for 1.255s, the DC voltage divider protection action criterion and time criterion are met at the same time, and the DC voltage divider protection trips to prevent the DC voltage divider protection from overheating and burning. According to the temperature rise formula Calculate and plot the temperature changes of the low voltage arm and the secondary voltage divider plate before the DC voltage divider protection trips. Figure 6 As shown, set the initial temperature of the device T 0 =20℃. Figure 6 As shown, the heat accumulation in the low-voltage arm is obvious, and the temperature rises to 59°C before tripping, which is slightly lower than the maximum temperature threshold of 60°C that the equipment can withstand.

[0174] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

Claims

1. A DC voltage divider harmonic protection method considering heat accumulation effect, characterized in that: The following steps are involved: A. Use the secondary voltage transformer to measure the secondary voltage U of the DC voltage divider body mes , calculate the DC voltage U of the flexible DC system according to the DC voltage divider body calibration ratio N dc ; B. The calculated DC voltage U of the flexible DC system dc Perform fast Fourier transform to obtain the amplitude of each harmonic component of the DC voltage of the flexible DC system C. Based on the fast Fourier transform results of DC voltage, the amplitude of each harmonic voltage component of the high voltage arm, low voltage arm and secondary side voltage divider is calculated according to the voltage transmission characteristics of the DC voltage divider. D. Considering the skin effect of resistors and capacitor losses, calculate the harmonic losses P of the high-voltage arm, low-voltage arm and secondary voltage divider high , P low , P 2nd ; E. The harmonic heat accumulation value P of the high voltage arm, low voltage arm and secondary side voltage divider high , P low , P 2nd Respectively with the high voltage arm, low voltage arm, secondary side voltage divider protection action setting value Compare and get the protection plan.

2. The method for harmonic protection of a DC voltage divider considering heat accumulation effect according to claim 1, characterized in that: The protection scheme in step E is as follows: If any harmonic heat loss is greater than the protection action setting value, the DC voltage divider harmonic protection will issue a DC voltage divider overheating warning and select the corresponding protection action output delay; If any harmonic loss of the high voltage arm, low voltage arm and secondary side voltage divider board is always greater than the corresponding action setting value during the delay period, the DC voltage divider harmonic protection will issue a DC voltage divider overheat warning and trip, otherwise the DC voltage divider harmonic protection will not operate.

3. The method for harmonic protection of a DC voltage divider considering heat accumulation effect according to claim 1, characterized in that: The DC voltage of the flexible DC system in step A is expressed as:

4. The method for harmonic protection of a DC voltage divider considering heat accumulation effect according to claim 1, characterized in that: In step C, the high voltage arm voltage U high , low voltage arm voltage U low And the secondary side voltage divider voltage U 2nd With DC voltage U dc The transfer function G high (s), G low (s), G 2nd (s), specifically expressed as follows: In the formula, R high , R low , R 2nd They are the equivalent resistances of the high voltage arm, low voltage arm and secondary voltage divider plate respectively; C high , C low , C 2nd They are the equivalent capacitances of the high voltage arm, low voltage arm and secondary voltage divider plate respectively.

5. The method for harmonic protection of a DC voltage divider considering heat accumulation effect according to claim 4, characterized in that: Based on the high voltage arm voltage U high , low voltage arm voltage U low And the secondary side voltage divider voltage U 2nd With DC voltage U dc The transfer function between the high-voltage arm, the low-voltage arm and the secondary side voltage divider is calculated to calculate the amplitude of each harmonic voltage component The details are as follows: Where ω0 is the fundamental angular frequency.

6. The method for harmonic protection of a DC voltage divider considering heat accumulation effect according to claim 1, characterized in that: Step D Consider the skin effect of resistors and capacitor losses to calculate the harmonic loss P of the high-voltage arm. high , the specific process is as follows: First, considering the skin effect, the resistance under the hth harmonic can be expressed as: In the formula, R0 is the fundamental resistance value; Then, considering the skin effect, the loss caused by the hth harmonic voltage on the resistor can be expressed as: Then, the capacitance loss caused by the hth harmonic voltage can be expressed as: P C_h =U h 2 h 2 ω0Ctanδ0 Where tanδ0 is the tangent loss angle of the capacitor dielectric. Finally, considering the skin effect of the resistor and the capacitor loss, the harmonic loss of the high-voltage arm P high It can be calculated by the following formula: In the formula, Considering the skin effect of the resistor and the loss of the capacitor, tanδ high is the dielectric loss tangent of the high voltage arm capacitor.

7. The method for harmonic protection of a DC voltage divider considering heat accumulation effect according to claim 6, characterized in that: Step D Consider the skin effect of resistors and capacitor losses to calculate the harmonic loss P of the low voltage arm. low , the specific process is as follows: When considering the skin effect of resistors and capacitor losses, the harmonic loss of the low-voltage arm P low It can be calculated by the following formula: In the formula, Considering the skin effect of the resistor and the capacitance loss, tanδ low is the dielectric loss tangent of the low voltage arm capacitor.

8. The method for harmonic protection of a DC voltage divider considering heat accumulation effect according to claim 7, characterized in that: Step D Consider the skin effect of resistors and capacitor losses to calculate the harmonic loss P of the secondary voltage divider. 2nd , the specific process is as follows: When considering the skin effect of the resistor and the capacitor loss, the harmonic loss P of the secondary voltage divider is 2nd It can be calculated by the following formula: In the formula, Considering the skin effect of the resistor and the capacitance loss, tanδ is the loss coefficient of the secondary voltage divider hth harmonic. 2nd is the dielectric loss tangent of the secondary voltage divider capacitor.

9. The method for harmonic protection of a DC voltage divider considering heat accumulation effect according to claim 1, characterized in that: In step E, the high voltage arm protection action setting value The calculation expression is as follows: Where U high_B is the power frequency withstand voltage of a single high-voltage arm capacitor, n high is the number of high-pressure nodes, k high It is the long-term overload multiple of the high-voltage arm capacitor.

10. The method for harmonic protection of a DC voltage divider considering heat accumulation effect according to claim 1, characterized in that: In step E, the low voltage arm protection action setting value The calculation expression is as follows: Where U low_B is the power frequency withstand voltage of a single low voltage arm capacitor, n low is the number of capacitors in series with the low voltage arm, k low It is the long-term overload multiple of the low-voltage arm capacitor.

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