Substation dc system ground capacitance multi-frequency detection method based on variable frequency bridge

Abstract

This invention discloses a multi-frequency detection method for the ground capacitance of a substation DC system based on a frequency converter bridge. According to the frequency characteristics of the ground capacitance of the substation DC system, a multi-frequency detection signal S containing n different frequency components is generated. nf ; using a frequency of f c carrier S c The above multi-frequency detection signal S containing n different frequency components nf Modulated into a pair of positive inverter bridge switch control signals S in complementary high and low level states + and the control signal S of the negative frequency converter bridge switch tube ‑ ; control signal S of the positive frequency conversion bridge switch tube + and negative frequency conversion bridge switch control signal S ‑ The positive switch S1 and the negative switch S2 are injected separately to achieve independent control of each switch; the capacitance to ground is calculated based on the acquired and detected signals; this invention achieves high-precision and reliable detection of the capacitance to ground of the DC system in substations, avoiding the problem of misjudgment of the capacitance to ground of long cables in the current DC system of substations during insulation testing.

Description

Technical Field

[0001] This invention relates to the field of insulation condition monitoring of DC systems in substations, and specifically to a multi-frequency detection method for ground capacitance of DC systems in substations based on a frequency conversion bridge. Background Technology

[0002] The DC system of a substation is the "lifeline" ensuring the reliable operation of circuit breakers and relay protection devices, and its insulation condition directly affects the safe and stable operation of the power grid. With the continuous development of power systems, substations are expanding in scale, and the length of cables laid within them is also increasing significantly. The capacitance to ground of long DC cables increases significantly with length (also known as the "capacitive rise effect of long DC cables"), thus generating a significant capacitive current during insulation testing, becoming a key factor affecting the accuracy of DC system insulation testing.

[0003] Specifically, in the insulation testing of current substation DC systems, traditional methods such as the bridge method or low-frequency signal injection are commonly used. However, the capacitive current of long cables can mask the true insulation leakage current, leading to inaccurate insulation resistance measurements and even missed or false detections. It is particularly important to note that the capacitive rise effect of long DC cables can also cause relay failures, delayed switching changes, and circuit breaker malfunctions, resulting in relay protection control function failures. Furthermore, during long-term operation, substation DC system cables are subjected to a combination of electrical, thermal, and environmental stresses. When moisture or early degradation occurs, their capacitance to ground changes significantly, serving as a health indicator for assessing early cable degradation. Therefore, accurate online measurement of the distributed capacitance to ground parameters of long substation DC system cables provides crucial information for accurately assessing the insulation status of the DC system and ensuring its safe and reliable operation.

[0004] For example, domestic invention patent application CN115524561A discloses an online monitoring method and device for the distributed capacitance to ground of a DC system. It adopts a "single-frequency resistor switching" detection logic, which calculates the capacitance by periodically switching on a negative detection resistor with a fixed resistance value and relying on the voltage fluctuation caused by the disturbance of this single resistor. However, this device relies on the single-frequency disturbance generated by switching on a single resistor. On the one hand, if monitoring is required for different types of cables such as long cables and short cables, different resistance values ​​need to be switched on repeatedly and the detection process needs to be started multiple times, which cannot achieve efficient adaptation. On the other hand, the single-frequency disturbance can only cover a narrow frequency range and cannot match the different capacitance characteristics of long cables and short cables. This directly leads to the problem that the capacitance measurement accuracy of long cables is difficult to meet the standard and that short cables are prone to missed capacitance detection in practical applications.

[0005] For example, the domestic utility model patent with authorization announcement number CN219417640U discloses an insulation monitoring device that also has a capacitance detection function. When detecting branch capacitance and total system capacitance, it is necessary to perform the detection in stages. It is not possible to inject multiple frequency band signals at one time to simultaneously complete the detection of the two types of capacitance. The two types of detection need to be carried out in stages: branch capacitance detection requires the use of a bridge circuit module and an AC current transformer to complete the detection, while total system capacitance detection requires the use of a bridge circuit module and an ultra-low frequency current transformer to complete the detection. Since the two have different detection frequencies, they must be started and the detection process must be executed independently in stages.

[0006] In summary, existing insulation testing methods for DC systems in substations all have significant limitations. In particular, the single-frequency resistance switching method requires repeated operation to adapt to different cables, and suffers from significant issues with measurement accuracy and missed detections. The multi-stage testing device cannot simultaneously complete capacitance monitoring, resulting in low efficiency. Traditional methods such as the low-frequency signal injection method are susceptible to interference and have long testing cycles, while the dielectric response method also suffers from weak anti-interference, high cost, and difficulty in real-time monitoring. Summary of the Invention

[0007] In view of this, the purpose of this invention is to provide a multi-frequency detection method for ground capacitance of substation DC system based on frequency conversion bridge, so as to achieve high-precision and reliable detection of ground capacitance of substation DC system, and avoid the problem of misjudgment of ground capacitance of long cable in substation DC system during insulation detection in the current technology.

[0008] The technical solution adopted in this invention is as follows:

[0009] A multi-frequency detection method for ground capacitance of a substation DC system based on a frequency converter bridge, wherein the frequency converter bridge includes a positive switch S1 and a negative switch S2 connected sequentially between the DC positive bus and the DC negative bus; the multi-frequency detection method for ground capacitance of the substation DC system includes:

[0010] Based on the frequency characteristics of the DC system's capacitance to ground in the substation, a multi-frequency detection signal S containing n different frequency components is generated. nf ;

[0011] Using a frequency of f c carrier S c The above multi-frequency detection signal S containing n different frequency components nf Modulated into a pair of positive inverter bridge switch control signals S in complementary high and low level states + and the control signal S of the negative frequency converter bridge switch tube - ;

[0012] The control signal S of the positive frequency conversion bridge switch transistor + and negative frequency conversion bridge switch control signal S -The positive switch S1 and the negative switch S2 are injected separately to achieve independent control of each switch.

[0013] At least the current signal i flowing into the detection branch is collected and detected through the detection branch of the capacitance to ground. x+ (t) and the current signal i flowing out of the detection branch. x- (t); Calculate the capacitance to ground based on the acquired and detected signals.

[0014] Preferably, the multi-frequency detection signal S containing n different frequency components is calculated based on the acquired and detected signal. nf The corresponding positive terminal to ground capacitance C x+ and negative terminal to ground capacitance C x- The positive electrode to ground capacitor C x+ This refers to the capacitance to ground of the DC positive bus cable connected to the detection branch of the capacitance to ground, wherein the negative capacitance to ground C x- This refers to the capacitance to ground of the DC negative bus cable connected to the detection branch for capacitance to ground.

[0015] Preferably, the connection branch between the positive switch S1 and the negative switch S2 is grounded; the DC positive bus and the DC negative bus are respectively connected to the detection branch of the capacitance to ground; the frequency converter bridge includes a positive filter resistor R. F+ Positive filter capacitor C F+ Negative filter resistor R F- Negative filter capacitor C F- ;in,

[0016] The positive filter resistor R F+ It is connected between the DC positive bus and the positive switch S1;

[0017] The negative filter resistor R F- It is connected between the DC negative bus and the negative switch S2;

[0018] The positive filter capacitor C F+ and negative filter capacitor C F- It is connected sequentially between the DC positive bus and the DC negative bus;

[0019] The injected switching control signal is filtered by various filter resistors and capacitors to remove high-frequency noise introduced by the operation of each switching transistor.

[0020] Preferably, the frequency characteristics of the substation DC system's ground capacitance include the ground capacitance and resistance parameters of the substation DC system's positive and negative bus cables under normal and fault conditions, determining the multi-frequency detection signal S to be injected. nf The frequency range.

[0021] Preferably, the multi-frequency detection signal S nf The frequency range is set to AC signals with an impedance angle of 15° to 75°. It should be specifically noted that the impedance angle referred to in this application refers to the corresponding sinusoidal multi-frequency detection signal S. nf After the variable frequency bridge is injected, the impedance Z is obtained by calculating the ratio of voltage to current in the variable frequency bridge circuit. The impedance Z is a complex number (containing a real part and an imaginary part). The angle between the imaginary part and the real part of the impedance Z is the impedance angle described in the entire application. The acquisition and detection of the impedance angle of the sinusoidal signal is common knowledge to those skilled in the art and is not considered as the innovative content of this application. Therefore, it will not be described in detail.

[0022] Preferably, when the positive frequency conversion bridge switch control signal S + When the signal is high, the positive switch S1 is turned on, forming a signal injection channel on the DC positive bus side. At this time, the control signal S of the negative inverter bridge switch is... - When the signal is low, the negative switch S2 is off; when the negative frequency converter bridge switch control signal S... - When the signal is high, the negative switch S2 is turned on, forming a signal injection channel on the DC negative bus side. At this time, the control signal S of the positive inverter bridge switch is... + When the voltage is low, the positive switch S1 is turned off.

[0023] Preferably, before calculating the capacitance to ground, the acquired and detected signal is reconstructed using a digital lock-in amplifier to suppress interference signals at non-target frequencies; wherein the calculation process of the reconstruction process includes the following:

[0024] ;

[0025] in: The signal to be reconstructed is T; the integration time is T; and the current time is t. It is the reference signal, which forms the multi-frequency detection signal S. nf A sinusoidal signal; It is the frequency of the reference signal; It is the initial phase of the reference signal; X out The signal after reconstruction processing.

[0026] Preferably, the detection branch for the capacitance to ground includes a first current sensor CT1 installed at the outlet of the branch being detected and a second current sensor CT2 installed at the inlet of the branch being detected; wherein, the outlet of the branch being detected is connected between the DC negative bus and the detection branch for the capacitance to ground; the inlet of the branch being detected is connected between the DC positive bus and the detection branch for the capacitance to ground; and the current signal i flowing out of the detection branch is detected by the first current sensor CT1.x- (t), the current signal i flowing into the detection branch is obtained by the second current sensor CT2. x+ (t).

[0027] Preferably, the calculation process for the capacitance to ground includes the following:

[0028] After the switching transistor signal is injected, the voltage to ground of the DC positive bus is measured by the first voltage sensor and the second voltage sensor respectively after the corresponding switching transistor. and the ground voltage of the DC negative bus ;

[0029] Based on the acquired voltage and current signals, and the calculated phase difference between the voltage and current signals, the multi-frequency detection signal S with different frequency components is calculated. nf The corresponding susceptance B i ;

[0030] Using the capacitance to ground C and susceptance B i The following linear relationship exists: ;

[0031] Where, ω i For the multi-frequency detection signal S with different frequency components nf The corresponding angular frequency; through n sets of susceptance B i - angular frequency ω i The data were linearly fitted, and the positive electrode capacitance to ground C was calculated separately. x+ and negative terminal to ground capacitance C x- .

[0032] Preferably, the susceptance B i The calculation process is as follows:

[0033] ; ;in: In mathematics, it refers to taking the imaginary part, that is, when a complex number is written as a + bi, taking b. The amplitude of the voltage signal. The amplitude of the current signal. Current phase Phase with voltage The difference;

[0034] The positive electrode to ground capacitor C x+ and negative terminal to ground capacitance C x- The calculation process is as follows:

[0035] ;

[0036] C in the formula x C represents the positive terminal capacitance to ground. x+ Or the negative terminal capacitance to ground C x- ; k is the proportionality coefficient to be solved; i is the index of different frequency components, from 1 to n.

[0037] Preferably, the calculated positive-to-ground capacitance C is... x+ The insulation status of the DC positive bus cable is assessed by comparing it with the corresponding standard range and selectively incorporating changes in the positive-to-ground resistance; the calculated negative-to-ground capacitance C is then used to... x- The insulation status of the DC negative bus cable is assessed by comparing it with the corresponding standard range and selectively incorporating changes in the negative pole-to-ground resistance.

[0038] The main benefits of this application are:

[0039] Multi-band single-injection: Overcoming the limitations of traditional single-frequency injection, this method utilizes the frequency conversion bridge deployed within the substation's DC system to inject a multi-frequency signal containing n frequency bands (i.e., a multi-frequency detection signal S containing n different frequency components) in a single injection. nf This enables "one-time injection, multi-band synchronous monitoring"; compared with the inefficient mode of traditional methods that require multiple injections, it significantly shortens the detection cycle, while covering a wide frequency range and adapting to the distributed capacitance characteristics of DC cables of different lengths.

[0040] Complementary modulation anti-disturbance: The switching transistor is driven by a complementary modulation multi-frequency signal, so that the DC positive and negative bus voltages to ground maintain independent and complementary variation characteristics; when injecting multi-frequency signals, the disturbance to the original DC bus voltage is reduced, which solves the problem of the traditional bridge method affecting the system stability due to voltage fluctuations, ensures the normal operation of the substation DC system during the detection process, and prevents device malfunction.

[0041] Multi-band detection is adopted: multi-band data acquisition and detection are performed, and the capacitance to ground is solved by fitting the multi-band data of the acquired and detected signals. This significantly reduces the dependence on accurate measurement of impedance angle and effectively avoids the problems of false high measurement and missed detection caused by capacitor current masking leakage current in traditional methods. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the circuit connection structure of the DC system (including the frequency converter bridge) of the substation under the specific implementation of this application;

[0043] Figure 2 The control signal S of the positive pole frequency converter bridge switch in the specific embodiment of this application is... + and the control signal S of the negative frequency converter bridge switch tube - The modulation generation process ( Figure 2A schematic diagram labeled "Multi-frequency Modulation Carrier Control Signal Generation Module";

[0044] Figure 3 This is a flowchart illustrating the steps of a multi-frequency detection method for ground capacitance of a substation DC system based on a frequency conversion bridge, according to a specific embodiment of this application.

[0045] Figure 4 This is an experimental circuit diagram built on the Simulink platform in a specific embodiment of this application;

[0046] Figure 5 The waveform diagrams of the selected multi-frequency detection signal and the pair of complementary control signals S+ and S- obtained after modulation are shown in a specific embodiment of this application.

[0047] Figure 6 The waveforms of the positive busbar to ground voltage and the negative busbar to ground voltage are shown in a specific embodiment of this application.

[0048] Figure 7 This is a waveform diagram of the DC bus voltage after injecting multi-frequency detection signals in a specific embodiment of this application;

[0049] Figure 8 The voltage and current waveforms in a specific embodiment of this application are after reconstruction by a digital lock-in amplifier.

[0050] Figure 9 The image shows the AC component spectra of the reconstructed voltage and current obtained by performing Fourier analysis on the reconstructed voltage and current signals in a specific embodiment of this application.

[0051] Figure 10 The continuous angular frequency w-susceptance B obtained by fitting in a specific embodiment of this application i The graph shows the angular frequency ω on the horizontal axis (in rad / s) and the susceptance B on the vertical axis. i ×10 -6 . Detailed Implementation

[0052] Please refer to the above. Figure 1 , Figure 2 and Figure 3 As shown, this embodiment proposes a multi-frequency detection method for the ground capacitance of a substation DC system based on a frequency converter bridge. The frequency converter bridge includes components sequentially connected to the DC positive bus (…). Figure 1 Marked as "P") and DC negative bus ( Figure 1 The positive switch S1 and the negative switch S2 are connected by a branch grounded (marked "N"). Figure 1 Marked as "G"); the DC positive bus and DC negative bus are respectively connected to the detection branch of the capacitance to ground ( Figure 1(labeled "branch x", belonging to a known circuit structure); preferably, in this embodiment, the frequency converter bridge includes a positive filter resistor R. F+ Positive filter capacitor C F+ Negative filter resistor R F- Negative filter capacitor C F- ;in,

[0053] Positive filter resistor R F+ It is connected between the DC positive bus and the positive switch S1;

[0054] Negative filter resistor R F- It is connected between the DC negative bus and the negative switch S2;

[0055] Positive filter capacitor C F+ and negative filter capacitor C F- It is connected sequentially between the DC positive bus and the DC negative bus;

[0056] The injected switching control signal is filtered by each filter resistor and each filter capacitor (as a low-pass filter, with low-pass filter parameters set) to remove high-frequency noise introduced by the operation of each switching transistor.

[0057] Preferably, in this embodiment, the detection branch for the capacitance to ground includes a first current sensor CT1 installed at the outlet of the branch being detected and a second current sensor CT2 installed at the inlet of the branch being detected; wherein, the outlet of the branch being detected is connected between the DC negative bus and the detection branch for the capacitance to ground; the inlet of the branch being detected is connected between the DC positive bus and the detection branch for the capacitance to ground; and the current signal i flowing out of the detection branch is detected by the first current sensor CT1. x- (t), the current signal i flowing into this detection branch is obtained by the second current sensor CT2. x+ (t);

[0058] In this embodiment, the multi-frequency detection method for ground capacitance of a substation DC system includes:

[0059] Based on the frequency characteristics of the DC system's capacitance to ground in the substation, a multi-frequency detection signal S containing n different frequency components can be generated using a DSP (Digital Signal Processor). nf (i.e., corresponding) Figure 3 The step shown is "Based on the frequency characteristics of the DC system's capacitance to ground, generate a multi-frequency detection signal S containing n frequency components". nf() Preferably, in this embodiment, the frequency characteristics of the substation DC system's ground capacitance include the ground capacitance and resistance parameters of the substation DC system's positive and negative bus cables under normal and fault conditions, to determine the multi-frequency detection signal S to be injected. nf The frequency range.

[0060] Preferably, in this embodiment, the multi-frequency detection signal S nf The frequency range is set to AC signals with an impedance angle of 15° to 75°; by expanding the impedance angle range, the dependence on voltage and current signal phase angle detection is reduced, and the 50Hz power grid interference is avoided; generating signals containing n different frequencies. Components (corresponding angular frequencies) The multi-frequency detection signal S nf ( Figure 2 Labeled as "Multi-frequency modulation signal S" nf "; In the implementation of this application, the mixing method includes, but is not limited to, linear superposition, pulse modulation, frequency modulation signal, etc.; Compared with single-frequency signal, multi-frequency detection signal can simultaneously acquire system impedance information at multiple frequencies, and reduce the detection error at a single frequency and the requirements for detection sensitivity through information in multiple frequency dimensions."

[0061] Using a frequency of f c carrier S c The above multi-frequency detection signal S containing n different frequency components nf Modulated into a pair of positive inverter bridge switch control signals S in complementary high and low level states + and the control signal S of the negative frequency converter bridge switch tube - (i.e., corresponding) Figure 3 The steps shown "using a frequency of f" c carrier S c S nf Modulated into a pair of complementary frequency converter bridge switching transistor control signals S + S - (”);

[0062] The control signal S of the positive frequency conversion bridge switch transistor + and negative frequency conversion bridge switch control signal S - The positive switch S1 and the negative switch S2 are injected separately to achieve independent control of each switch.

[0063] Preferably, in this embodiment, when the positive frequency conversion bridge switch control signal S... + When the signal is high, the positive switch S1 is turned on, forming a signal injection channel on the DC positive bus side. At this time, the control signal S of the negative inverter bridge switch is... -When the signal is low, the negative switch S2 is off; when the negative frequency converter bridge switch control signal S... - When the signal is high, the negative switch S2 is turned on, forming a signal injection channel on the DC negative bus side. At this time, the control signal S of the positive inverter bridge switch is... + When the signal is low, the positive switch S1 is cut off; this independent control method ensures that the multi-frequency detection signal S... nf It is injected evenly and stably into the DC positive and negative busbars;

[0064] It should be noted that after the switching transistor signal is injected, the DC bus voltage to ground U(t) specifically refers to the DC positive bus voltage to ground. Or the voltage to ground of the DC negative bus The following changes will occur accordingly:

[0065] n = 1, 2, 3… (that is, the sequence number of each frequency component signal);

[0066] in, It is the DC component of the voltage between the DC positive or negative bus and ground, and is the base voltage when the DC system is operating normally;

[0067] The amplitude of each frequency component of the sub-signal;

[0068] The initial phase of each frequency component signal;

[0069] The above process corresponds to Figure 3 The step shown is "the multi-frequency pulse modulation signal is filtered by a low-pass filter and injected into the DC system to make the DC bus voltage U(t)";

[0070] At least the current signal i flowing into the detection branch is collected and detected through the detection branch of the capacitance to ground. x+ (t) and the current signal i flowing out of the detection branch. x- (t);

[0071] Based on the acquired and detected signals, the multi-frequency detection signal S containing n different frequency components is calculated. nf The corresponding positive terminal to ground capacitance C x+ and negative terminal to ground capacitance C x- ; Positive terminal to ground capacitance C x+ This refers to the capacitance to ground of the DC positive bus cable connected to the detection branch for capacitance to ground, and the capacitance to ground of the negative terminal C. x- This refers to the capacitance to ground of the DC negative bus cable connected to the detection branch for capacitance to ground.

[0072] Preferably, in this embodiment, the calculation process for the capacitance to ground includes the following:

[0073] After the switching transistor signal is injected, the voltage to ground of the DC positive bus is measured by the first voltage sensor and the second voltage sensor respectively after the corresponding switching transistor. and the ground voltage of the DC negative bus The current signal i flowing into the detection branch is obtained by collecting and detecting the detection branch of the ground capacitance. x+ (t) and the current signal i flowing out of the detection branch. x- (t); that is, the corresponding Figure 3 The figure shows "Measurement of positive busbar to ground voltage". negative bus voltage to ground and each branch circuit i k (t)”;

[0074] More preferably, in this embodiment, before calculating the capacitance to ground, the acquired and detected signals (voltage signals, current signals) are pre-reconstructed using a digital lock-in amplifier to suppress interference signals at non-target frequencies; wherein the calculation process of the reconstruction process includes the following:

[0075] ;

[0076] in: The signal to be reconstructed is T; the integration time is T; and the current time is t. It is the reference signal, which forms the multi-frequency detection signal S. nf A sinusoidal signal; It is the frequency of the reference signal; It is the initial phase of the reference signal; X out To complete the reconstructed signal;

[0077] After the above reconstruction process, the reconstructed voltage and current signals are obtained, which correspond to... Figure 3 The step shown is "Reconstructing the voltage and current W using a digital lock-in amplifier". i "frequency band voltage and current signals";

[0078] Based on the reconstructed voltage and current signals and the calculated phase difference between them, the multi-frequency detection signal S for each frequency component is calculated. nf The corresponding susceptance B i The specific calculation process is as follows:

[0079] Using the capacitance to ground C and susceptance B i The following linear relationship exists: ;

[0080] Where, ω iFor the multi-frequency detection signal S with different frequency components nf The corresponding angular frequency; through n sets of susceptance B i - angular frequency ω i The data were linearly fitted, and the positive electrode capacitance to ground C was calculated separately. x+ and negative terminal to ground capacitance C x- .

[0081] Preferably, in this embodiment, the multi-frequency detection signal S with different frequency components nf The corresponding susceptance B i The calculation process is as follows:

[0082] ; ;in: The amplitude of the voltage signal. The amplitude of the current signal. Current phase Phase with voltage The difference, that is, the corresponding Figure 3 The steps shown are: "Calculate the susceptance B for each frequency band". i Fitting multi-band (ω) i B i Data acquisition: B=ωC relationship;

[0083] In this embodiment, the positive electrode to ground capacitance C x+ and negative terminal to ground capacitance C x- The calculation process is as follows:

[0084] ;

[0085] In the formula, C x It can refer to the positive terminal capacitance to ground C. x+ Or the negative terminal capacitance to ground C x- ; k is the scaling factor to be solved; i is the index of different frequency components, from 1 to n; that is, the corresponding Figure 3 The steps shown are: "Extract the fitting coefficient k and output the detection branch's capacitance to ground C". x "

[0086] It should be noted that, based on the calculation process of the above implementation scheme, it can be further extended to show that the capacitance to ground can be calculated simply by collecting the current signal from the detection branch without measuring the DC bus voltage to ground. This is because... constant term The relationship between current and frequency Where B is the susceptance, U is the voltage, I is the current, C is the capacitance to ground, R is the resistance to ground, and N is the number of points of the frequency signal, i.e., n. In this formula, k is i, representing the sequence number of the frequency signal. Based on these relationships, the capacitance to ground can be calculated without measuring the DC bus voltage to ground, simply by collecting and detecting the current signal from the detection branch. These calculation changes also fall within the scope of this application.

[0087] Furthermore, in specific implementation and application, the capacitance to ground C can be determined based on... x (Referring to the positive terminal capacitance to ground C) x+ Or the negative terminal capacitance to ground C x- The insulation performance of DC bus cables is evaluated based on changes in the following:

[0088] The ground capacitance C calculated in this embodiment x By comparing with the reference range, the insulation condition of the DC cable can be preliminarily determined directly:

[0089] If the capacitance to ground C x If the capacitance to ground is within the standard range and the total capacitance of the entire system does not exceed the system-level upper limit, it indicates that the cable insulation performance meets the operational requirements; if the capacitance to ground C x If the value exceeds the normal range, an insulation abnormality warning will be triggered directly. Furthermore, the capacitance to ground C can be periodically detected and stored using the method provided in this embodiment. x This forms a "time-to-ground capacitance" change curve. If the curve shows a slow upward trend, even if the current ground capacitance C... x If the curve is still within the normal range, it can be determined as early insulation deterioration, and early intervention is required; if the curve shows a sudden upward trend, it is determined as "rapid insulation deterioration", and the machine must be shut down for maintenance immediately to avoid the fault from spreading.

[0090] More preferably, in other embodiments of this application, the capacitance to ground C can also be acquired simultaneously during detection. x The resistance to ground (corresponding to the capacitance to ground) also includes the resistance to ground at the negative terminal of the branch and the resistance to ground at the negative terminal of the branch, which is the branch. Figure 1 The "branch x" parameter shown can be combined with other parameters to avoid misjudgments caused by a single parameter, forming a more comprehensive insulation assessment system as follows:

[0091] 1. If the capacitance to ground C x If the resistance to ground is normal, then the insulation condition is considered good.

[0092] 2. If the capacitance to ground C x Normal; if the resistance to ground decreases, it is determined to be a local leakage in the insulation layer.

[0093] 3. If the capacitance to ground C xIf the resistance to ground exceeds the standard, it is determined that the insulation is severely deteriorated and requires emergency treatment.

[0094] 4. If the capacitance to ground C x If the resistance to ground is normal but exceeds the standard, it is determined that the dielectric properties of the insulation layer have deteriorated, and monitoring needs to be strengthened.

[0095] It should be noted that since this application calculates the capacitance to ground and has obtained the reconstructed voltage and current, the resistance to ground can be obtained by calculating the real part of the impedance Z. The calculation process of the resistance to ground is a conventional technical means that can be made by those skilled in the art based on the content described in this application. In order to save space, this embodiment will not be described in detail.

[0096] To enable those skilled in the art to better understand the technical solutions of this invention, based on the above implementation schemes, the following specific embodiments will be proposed in conjunction with the accompanying drawings of the embodiments of this invention. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0097] In the following embodiment, according to the "QCSG1203003 Technical Specification for DC Power Supply Systems in Substations," the normal range of the ground capacitance of the DC bus in a substation is generally 0-300μF. Considering that a substation's electrical cabinet has over 50 connection points, the ground capacitance for each branch of the DC bus is selected as 0.02μF. However, under adverse conditions such as rain or humidity, the ground capacitance can increase significantly, sometimes reaching more than twice the normal value. Therefore, a ground capacitance range of 0.02μF-0.18μF is chosen, covering both normal operating conditions and extreme degradation scenarios, which aligns with actual operating characteristics. In summary, the ground resistance range is selected as 10kΩ-100kΩ, and the ground capacitance range is selected as 0.02μF-0.18μF.

[0098] To simultaneously adapt to different DC cable parameters, a multi-frequency signal S containing five frequency components (10Hz, 25Hz, 45Hz, 55Hz, and 70Hz) is selected. nf The selection of these five frequency components fully considers the differences in the response of the capacitance to ground at different frequencies, and can capture the characteristic information of the capacitance more comprehensively.

[0099] This experiment uses the Simulink platform to build the experimental circuit. Please refer to the diagram for its structure. Figure 4 As shown; the steps described in the above embodiments of this application (see also [reference]) Figure 3 As shown in the figure, the calculation of the capacitance to ground is finally completed. Figure 4 The relevant module parameters in the experimental circuit shown are shown in Table 1 below:

[0100] Table 1

[0101]

[0102] in, Figure 4 In the experimental circuit shown, “≥” is the comparison module, which outputs 1 when greater than or equal to and 0 when less than; “NOT” is the inverting module, which is used to perform inversion processing.

[0103] The multi-frequency detection signal S nf ( Figure 5 (labeled as "multi-frequency signal") and carrier signal S with a frequency of 15kHz c Modulation is performed to generate a pair of square wave positive frequency conversion bridge switch control signals S, which are in complementary high and low level states. + ( Figure 5 Labeled as "Control Signal S" + (Waveform) and negative frequency converter bridge switch control signal S - ( Figure 5 Labeled as "Control Signal S" - Waveform (complementary)", such as Figure 5 As shown; these complementary square wave signals are used to control the positive switch S1 and the negative switch S2, respectively, and then pass through the positive filter resistor R. F+ Positive filter capacitor C F+ Negative filter resistor R F- Negative filter capacitor C F- After processing by the low-pass filter, the DC positive bus voltage to ground is... ( Figure 6 Marked as "positive busbar to ground voltage U" p ) and DC negative bus voltage to ground ( Figure 6 Marked as "Negative busbar to ground voltage U" n All of these will follow the multi-frequency detection signal S nf It changes with the changes, such as Figure 6 As shown; however, the DC bus voltage between the DC positive bus and the DC negative bus does not fluctuate significantly, as... Figure 7 As shown.

[0104] The measured voltage and current data are used to accurately extract the target multi-frequency detection signal S through a digital lock-in amplifier. nf The amplitude and phase information, suppression of background noise and residual interference, such as Figure 8 The reconstructed voltage and current waveforms shown provide high-quality time-domain signal data for subsequent frequency-domain calculations of the capacitance to ground.

[0105] By analyzing the reconstructed DC positive bus voltage to ground DC negative bus voltage to ground and current signal i x- (t), Current signal i x+ (t) Perform Fast Fourier Transform respectively, such as Figure 9 As shown: Distinct spectral peaks can be clearly seen at the injected frequencies of 10Hz, 25Hz, 45Hz, 55Hz, and 70Hz. These peaks directly reflect the AC components at the corresponding frequencies. It should be noted that... Figure 9 The “reconstructed voltage AC component spectrum” and “reconstructed current AC component spectrum” shown refer to the spectrum waveforms at the DC positive bus terminal. Meanwhile, the signal amplitudes at other frequencies in the spectrum are extremely low, indicating that the Fast Fourier Transform effectively extracted the target frequency components and the noise was well suppressed, further verifying the effectiveness of the previous signal reconstruction process.

[0106] based on Calculate each frequency ω i The susceptance B below i Multiple sets of frequency-susceptance data were acquired.

[0107] After acquiring multiple sets of frequency-susceptance data, in order to transform the discrete frequency point data into a continuous characteristic curve, the experiment performed fitting processing on these data to ensure that the fitted curve can accurately reflect the variation law between frequency and susceptance, and to minimize the error caused by discrete data. Figure 10 As shown.

[0108] After obtaining a continuous angular frequency-susceptance curve through fitting, the capacitance to ground C can be calculated by combining the mathematical relationship between susceptance and capacitance (C=B / ω). x The results are shown in Table 2 (the data in Table 2 are all simulation calculations for the positive bus end; the calculation process for the negative bus end is the same as that for the positive bus end, so it will not be explained again):

[0109] Table 2

[0110] Simulated capacitance to ground / μF Simulated resistance to ground / kΩ Calculate capacitance to ground / μF error 0.02 100 0.0195 2.5% 0.02 50 0.0198 1% 0.02 10 0.0199 0.5% 0.1 100 0.098 2% 0.1 50 0.099 1% 0.1 10 0.101 1% 0.18 100 0.1761 2.1% 0.18 50 0.1781 1.05% 0.18 10 0.1797 1%

[0111] As shown in Table 2 and the error analysis during the experiment, the overall error in estimating the capacitance to ground using the calculation method employed in this experiment is less than 2.5%, indicating that the method has high measurement accuracy. Furthermore, the experiment revealed that, for the same simulated capacitance, the error decreases as the resistance to ground decreases. This characteristic makes the method perform better in scenarios with low resistance to ground. In addition, the method is highly adaptable to different capacitance levels, achieving accurate measurement for both small and large capacitors, providing a reliable and efficient solution for measuring capacitance to ground in practical engineering applications.

[0112] It will be apparent to those skilled in the art that the present 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 its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0113] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for detecting the multi-frequency of the grounding capacitance of a variable frequency bridge based substation DC system, characterized in that, The variable-frequency bridge comprises a positive switch tube S1 and a negative switch tube S2 connected in sequence between a direct-current positive bus and a direct-current negative bus; the detection method comprises: According to the frequency characteristic of the substation DC system to ground capacitance, a multi-frequency detection signal S containing n different frequency components is generated nf ; a carrier S c with a frequency f c is modulated into a pair of positive and negative frequency bridge switch control signals S nf and S + in high and low complementary states - ; At least through the detection branch of the earth capacitance, the current signals i x+ (t) flowing into the detection branch and the current signals i x- (t) flowing out of the detection branch are collected respectively Based on the collected signals, the earth capacitance is calculated The calculation process of the ground capacitance comprises the following: After the switch tube signal injection, the corresponding switch tube is behind, through the first voltage sensor and the second voltage sensor respectively collect detection get measurement direct current positive bus voltage And the ground voltage of the direct current negative bus ; Based on the acquired voltage signal, current signal, and the calculated phase difference between the voltage signal and the current signal, a multi-frequency detection signal S of different frequency components is calculated nf The corresponding susceptance B i ; Utilizing the linear relationship of the earth capacitance value C and the earth susceptance B i as follows: ; where ω i is the angular frequency corresponding to each different frequency component of the multi-frequency detection signal S nf ; the n sets of susceptance B i - angular frequency ω i are linearly fitted with the data, respectively, to calculate the positive-to-ground capacitance C x+ and the negative-to-ground capacitance C x- , respectively.

2. The method of claim 1, wherein the method further comprises: The connection branch between the positive switch tube S1 and the negative switch tube S2 is grounded; the direct current positive bus and the direct current negative bus are respectively connected with the detection branch of the ground capacitor; the frequency conversion bridge comprises a positive filter resistor R F+ , a positive filter capacitor C F+ , a negative filter resistor R F- , a negative filter capacitor C F- ; wherein, The positive electrode filter resistor R F+ Connected between the direct current positive bus and the positive switch tube S1; The negative filter resistor R F- is connected between the DC negative bus and the negative switch tube S2; The positive filter capacitor C F+ and the negative filter capacitor C F- are connected in sequence between the DC positive bus and the DC negative bus. The injected switch tube control signals are filtered by the filter resistors and the filter capacitors, and high-frequency noise introduced by the switch tube actions is filtered out.

3. The method of claim 1, wherein the method further comprises: The frequency characteristics of the substation DC system ground capacitance include the ground capacitance and resistance parameters of the substation DC system DC positive bus cable and negative bus cable in normal and fault states, determine the frequency range of the multi-frequency detection signal S to be injected nf ; based on the acquired detected signal, calculate the multi-frequency detection signal S containing n different frequency components nf corresponding to the positive ground capacitance C x+ and the negative ground capacitance C x- ; the positive ground capacitance C x+ refers to the ground capacitance of the detection branch connected to the DC positive bus cable, and the negative ground capacitance C x- refers to the ground capacitance of the detection branch connected to the DC negative bus cable.

4. The method according to claim 1 or 3, characterized in that, The multi-frequency detection signal S nf The frequency range of the alternating signal is set to an impedance angle of 15° to 75°.

5. The method of claim 1, wherein the method further comprises: When the positive frequency conversion bridge switch tube control signal S + is high level, the positive switch tube S1 is turned on, and the signal injection channel is formed at the DC positive bus side. At this time, the negative frequency conversion bridge switch tube control signal S - is low level, and the negative switch tube S2 is cut off. - When the negative frequency conversion bridge switch tube control signal S + is high level, the negative switch tube S2 is turned on, and the signal injection channel is formed at the DC negative bus side. At this time, the positive frequency conversion bridge switch tube control signal S is low level, and the positive switch tube S1 is cut off.

6. The method of claim 1, wherein the method further comprises: Before the ground capacitance is calculated, the collected and detected signals are reconstructed by a digital lock-in amplifier for suppressing interference signals of non-target frequencies; wherein, the calculation process of the reconstruction comprises the following: The injected switch tube control signals are filtered by the filter resistors and the filter capacitors, and high-frequency noise introduced by the switch tube actions is filtered out. Before the ground capacitance is calculated, the collected and detected signals are reconstructed by a digital lock-in amplifier for suppressing interference signals of non-target frequencies; wherein, the calculation process of the reconstruction comprises the following: ； wherein: is the signal to be reconstructed; T is the integration time; t is the current time; is the reference signal, is the sinusoidal signal forming the multi-frequency detection signal S nf ; is the frequency of the reference signal; is the initial phase of the reference signal; X out is the signal after completion of the reconstruction.

7. The method of claim 1, wherein the method further comprises: The detection branch of the earth capacitance comprises a first current sensor CT1 installed at an outgoing line of the detection branch and a second current sensor CT2 installed at an incoming line of the detection branch; wherein the outgoing line of the detection branch is connected between a DC negative bus and the detection branch of the earth capacitance; the incoming line of the detection branch is connected between a DC positive bus and the detection branch of the earth capacitance; a current signal i x- (t) flowing out of the detection branch is detected by the first current sensor CT1, and a current signal i x+ (t) flowing into the detection branch is detected by the second current sensor CT2.

8. The method of claim 1, wherein the method further comprises: The admittance B i The calculation process is as follows: ; ; wherein: is the amplitude of the voltage signal, is the amplitude of the current signal, is the difference between the current phase and the voltage phase . The positive electrode-to-ground capacitance C x+ and the negative electrode-to-ground capacitance C x- The calculation processes are as follows: ； C in the formula x represents the positive electrode-to-ground capacitance C x+ or the negative electrode-to-ground capacitance C x- k is a proportional coefficient to be solved; i is the serial number of different frequency components, from 1 to n.

9. The method of claim 1, wherein the method further comprises: The calculated positive electrode-to-ground capacitance C x+ is compared with a corresponding standard range set for it, and the change in the positive electrode-to-ground resistance is selectively combined to achieve the insulation state evaluation of the DC positive bus cable; the calculated negative electrode-to-ground capacitance C x- is compared with a corresponding standard range set for it, and the change in the negative electrode-to-ground resistance is selectively combined to achieve the insulation state evaluation of the DC negative bus cable.

Images