A Method for Ground Fault Location of Switching Power Supplies in a DC Microgrid System

Abstract

The present invention discloses a method for locating the grounding fault of a switching power supply in a DC microgrid system. This method utilizes the power information composite modulation technology of the switching power supply to modulate digital information onto the energy flowing into and out of the switching power supply and simultaneously perform power conversion, that is, injecting characteristic signals for grounding fault detection into the microgrid system without adding additional circuits and cables, which simplifies the circuit structure of the fault location system. In addition, the present invention adopts the code division multiplexing technology to assign a unique address code sequence to each switching power supply in the system, so that the characteristic signals of different switching power supplies will not interfere with each other; the bus fault detection unit detects the characteristic signals injected into the system by each switching power supply in the system through non-invasive detection methods such as magnetic ring detection, without modifying the system structure, and only needs to detect the characteristic signals of the bus. This detection method is easy to install and disassemble, and does not require detecting each switching power supply branch, saving the detection cost.

Description

Technical Field

[0001] The present invention belongs to the technical field of grounding fault detection in DC microgrid systems, and particularly relates to a method for locating grounding faults of switching power supplies in DC microgrid systems. Background Art

[0002] The DC microgrid system is an important part of the power system for power transmission and transformation, and provides DC power supply for various electrical equipment in the system. In order to ensure the stable and reliable operation of the DC microgrid system, eliminate potential safety hazards in the DC microgrid system, and thus avoid the occurrence of major safety accidents, it is necessary to monitor the grounding faults of the DC microgrid system and locate the faults immediately. In the DC microgrid system, there are many parallel branches of switching power supplies. The switching power supplies interact with the DC microgrid system for electrical energy, and their safety and reliability will directly or indirectly affect the safety of the DC microgrid system. When there is a single-point grounding fault in the switching power supply, since the grounding current is small, the switching power supply can continue to work, but this potential problem must be detected and located early to avoid serious accidents caused by multi-point grounding; when there are grounding faults on both the positive and negative poles of the switching power supply, it will greatly increase the instability of the switching power supply operation, and even cause a short-circuit problem, affecting the entire DC microgrid system.

[0003] In order to locate the grounding faults in the DC microgrid system in a timely and effective manner, the bridge method and the signal injection method are generally used.

[0004] For the bridge method, as recorded in documents [Fei Wanmin, Zhang Yanli, Wu Zhaolin. New principle for detecting DC grounding resistance in power systems [J]. Automation of Electric Power Systems, 2001(06): 54-56], [Huang Yuwei, Qin Jinfei, Liu Nini, Huang Haihong. Detection method for differential current of unbalanced bridge of DC insulation double bridge arms [J]. Electrical & Energy Efficiency Management Technology, 2019(11): 57-61. DOI: 10.16628 / j.cnki.2095-8188.2019.11.011] and [Liu Nini. Research on insulation detection and calibration of substation DC power supply system [D]. Hefei University of Technology, 2019], the bridge method uses the change of the equivalent resistance of the positive and negative buses to the ground in combination with a bridge to locate the grounding faults in the DC microgrid system. However, it can only detect whether there is a grounding fault in the entire DC microgrid system. If it is necessary to determine the branch with the grounding fault, the current of each branch needs to be collected, and the detection accuracy is greatly affected by the sensor accuracy and the detection algorithm, and false actions are likely to occur.

[0005] The signal injection method, as recorded in the literature [Liu Bije. Research on Insulation Monitoring and Fault Line Searching Device for DC System [D]. Northeast Electric Power University, 2019] and the literature [Zhou Jun, Liu Bije, Li Shuguang. DC System Insulation Detection Technology Based on Improved Variable Frequency Secondary Injection Method [J]. Electrical Measurement & Instrumentation, 2019, 56(16): 129-133. DOI: 10.19753 / j.issn1001-1390.2019.016.021], uses an additional signal injection device to inject current signals into the DC system, and determines the location of the grounding fault according to the characteristics of the current signals. It is vulnerable to the interference of the system distributed capacitance, and the injected signals will affect the power quality of the switch power supply output, which may have an impact on the safe and stable operation of the DC microgrid system. Summary of the Invention

[0006] In view of the above, the present invention provides a method for locating the grounding fault of the switch power supply in a DC microgrid system, which detects the positive and negative bus currents of the DC microgrid system through a non-invasive detection method, and locates the switch power supply with a grounding fault in the DC microgrid system without affecting the operation of the DC microgrid system.

[0007] A method for locating the grounding fault of the switch power supply in a DC microgrid system, the DC microgrid system includes positive and negative DC buses and N multi-port switch power supplies. One port of the switch power supply is connected to the positive and negative DC buses, and each switch power supply has a unique address code sequence in the system. N is a natural number greater than 1; the switch power supply uses the power information composite modulation technology to modulate the respective address code sequence information onto the energy flowing into and out of the switch power supply, and at the same time regularly detects the current signals on the positive and negative DC buses, and then uses the corresponding positioning algorithm to realize the positioning of the faulty power supply when a grounding fault occurs.

[0008] Further, the address code sequence uses binary code.

[0009] Further, the power information composite modulation technology is to perform data modulation on the PWM carrier wave or PWM modulation wave that generates the PWM drive signal of the switch power supply.

[0010] Further, when performing data modulation on the PWM carrier wave, the amplitude of the PWM carrier wave is not changed, and the phase of the PWM carrier wave is used to represent the address code sequence information. The frequency of the PWM carrier wave is the working frequency of the switch power supply.

[0011] Further, when performing data modulation on the PWM modulation wave, the switch power supply generates a sine carrier wave with a frequency lower than its working frequency, the amplitude of the sine carrier wave is not changed, and the phase of the sine carrier wave is used to represent the address code sequence information, and the sine carrier wave is superimposed on the original PWM modulation wave as a new PWM modulation wave.

[0012] Furthermore, a non-invasive current sensor magnetic ring is used to detect the current signal i on the positive and negative DC bus respectively. + (t) and i - (t), and then the two sets of signals are conditioned and filtered and superimposed to obtain the current signal i(t). Finally, the positioning algorithm is applied to locate the switching power supply with a ground fault.

[0013] Furthermore, the specific implementation of the positioning algorithm is as follows:

[0014] (1) Generate a fixed amplitude, a frequency equal to the data carrier frequency, and a duration of T code A sinusoidal signal, where T code is the symbol period;

[0015] (2) For any switching power supply i, the address code sequence x i (m), replace "1" with "-1" and "0" with "1", and get the new address code sequence x' i (m); Combine the above sinusoidal signal with the new address code sequence x′ i Multiply each digit in (m) and concatenate them to get a duration of mT code signal y i (t), where i is the switch power supply number and m is the length of the address code sequence;

[0016] (3) Calculate signal y i (t) and the cross-correlation of the current signal i(t), and compare its peak value with the threshold. If it exceeds the threshold, it is determined that the switching power supply i has a ground fault;

[0017] (4) If no ground fault is detected in any switching power supply, steps (2) to (3) are repeated periodically.

[0018] Furthermore, after data modulation is performed on the PWM carrier, the symbol period T code =K·T s ; After data modulation of PWM modulated wave, the symbol period T code =K·M·T s , where T s is the duty cycle of the switching power supply, K and M are both positive integers and M≥2.

[0019] Furthermore, when data modulation is performed on the PWM carrier wave, the data carrier frequency is the operating frequency of the switching power supply; when data modulation is performed on the PWM modulated wave, the data carrier frequency is 1 / M of the operating frequency of the switching power supply.

[0020] Furthermore, the signal y is calculated by the following formula i (t) is the cross-correlation with the current signal i(t);

[0021]

[0022] In the method of the present invention, the switching power supply in the system adopts the power information composite modulation technology, modulates digital information onto the energy flowing in and out of the switching power supply and performs power conversion simultaneously, that is, injects a characteristic signal for grounding fault detection into the microgrid system without adding additional circuits and cables, simplifying the circuit structure of the fault location system. In addition, the method of the present invention adopts the code division multiplexing technology, assigns a unique address code sequence to each switching power supply in the system, so that the characteristic signals of different switching power supplies will not interfere with each other; the bus fault detection unit detects the characteristic signals injected into the system by each switching power supply in the system through non-intrusive detection methods such as magnetic ring detection, does not need to modify the system structure, and only needs to detect the characteristic signals of the bus. This detection method is easy to install and disassemble, does not need to detect each switching power supply branch, and saves the detection cost.

[0023] Compared with the prior art, the present invention has the following beneficial technical effects:

[0024] 1. The present invention uses the switching power supply inside the DC microgrid system to inject a characteristic signal for grounding fault detection into the system, without the need to add external equipment to inject detection signals, simplifying the circuit structure of the fault location system.

[0025] 2. The intensity of the signal injected by the switching power supply of the present invention can be adjusted and will not interfere with the stable operation of the DC microgrid system.

[0026] 3. When the present invention performs grounding fault detection, it does not need to detect each parallel branch, but only detects the DC positive and negative buses, saving the detection cost. Description of the Drawings

[0027] Figure 1 It is a schematic structural diagram of a DC microgrid system.

[0028] Figure 2 It is a schematic diagram of the principle of phase shift keying modulation.

[0029] Figure 3 It is a schematic diagram of the principle of modulating PWM carrier data.

[0030] Figure 4 It is a schematic diagram of the principle of modulating PWM modulation wave data.

[0031] Figure 5 It is a schematic diagram of the principle of multiplying the address code sequence by the sine signal.

[0032] Figure 6 It is a schematic diagram of the simulation result of the cross-correlation degree between the signal y1(t) and i(t). Detailed Embodiments

[0033] To describe the present invention more specifically, the technical solutions of the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0034] The method for locating the grounding fault of the switching power supply in the DC microgrid system of the present invention is applicable to the DC microgrid system, such as Figure 1 shown, the system includes DC positive and negative buses and N multi-port switching power supplies. One port of each switching power supply is connected to the DC positive and negative buses. Only the ports connected to the DC positive and negative buses are drawn for the switching power supply in the figure.

[0035] Each switching power supply in the system has a unique address code sequence x i (m) (i = 1, 2,..., N), where m is the sequence length. The address code sequence of the switching power supply is composed of binary digits "0" and "1". The cross-correlation coefficient expression of the address code sequences of different switching power supplies is S is the number of corresponding code elements that are the same in x i and x j , D is the number of corresponding code elements that are different in x i and x j (C is a parameter set according to the on-site environment, etc.), and the cyclic shift sequence of any address code sequence in the system does not repeat with other sequences. When the value of C is 0, the address code sequences of the switching power supplies in the system are pairwise orthogonal; a possible address code sequence group x i (31) (i = 1, 2, 3) is shown in Table 1.

[0036] Table 1

[0037] Serial number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 <![CDATA[x1(31)]]> 1 0 0 0 1 1 0 1 1 1 0 1 0 1 0 0 <![CDATA[x2(31)]]> 1 0 0 0 1 0 0 1 0 1 0 1 1 0 0 0 <![CDATA[x3(31)]]> 1 0 0 0 1 0 1 0 1 1 0 1 0 0 0 0 Serial number 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 <![CDATA[x1(31)]]> 0 0 1 0 0 1 0 1 1 0 0 1 1 1 1 <![CDATA[x2(31)]]> 0 1 1 1 0 0 1 1 0 1 1 1 1 1 0 <![CDATA[x3(31)]]> 1 1 0 0 1 0 0 1 1 1 1 1 0 1 1

[0038] The switching power supply adopts the power information composite modulation technology, specifically, phase shift keying digital modulation is performed on the PWM carrier wave or PWM modulation wave that generates the PWM drive signal of the switching power supply. The principle of phase shift keying modulation is as Figure 2 shown. The amplitude and frequency of the carrier wave are fixed. The phase and of the carrier wave are used to represent "0" and "1" respectively. The duration of transmitting one code element is T code , and one code element contains an integer number of carrier wave cycles.

[0039] The principle of data modulation on the PWM carrier wave is as Figure 3As shown, the carrier generator performs phase - shift keying modulation according to the baseband data to generate a modulated PWM carrier containing the baseband data information. The frequency of the PWM carrier is the operating frequency of the switching power supply. After comparing the modulated PWM carrier with the PWM modulation wave, a PWM drive signal is generated; the symbol period T code after data - modulating the PWM carrier code satisfies T s = K·T + (K ∈ N s ), where T

[0040] is the operating period of the switching power supply.

[0040] The principle of data - modulating the PWM modulation wave is as Figure 4 [[ID=!5]]shown. The carrier generator performs phase - shift keying modulation according to the baseband data to generate a carrier containing the baseband data information. The frequency of the carrier is divisible by the operating frequency of the switching power supply. After adding the carrier and the PWM modulation wave, the sum is used as the modulated PWM modulation wave. After comparing it with the PWM carrier, a PWM drive signal is generated. The symbol period T code after data - modulating the PWM carrier code satisfies T s = K·M·T + (M ≥ 2, K, M ∈ N s ), where M·T

[0041] Figure 1 is the carrier period generated by the carrier generator.

[0041] Figure 1 The fault - detection unit in + detects the currents i

[0042] (t) and i-(t) of the positive and negative DC busbars, then uses a signal conditioning and filtering circuit to process the received signals to obtain the sum of the processed currents i(t), and finally applies a positioning algorithm to locate the switching power supply with a grounding fault. The specific implementation method is as follows: (1) The fault - detection unit generates a sine signal with a fixed amplitude, a frequency equal to the data - carrier frequency, and a duration of T code . Replace the "1" in the address - code sequence x i C (m)(i = 1, 2, …, N) with "-1" and the "0" with "1" to obtain x′ i (m)(i = 1, 2, …, N), and then multiply the N new address - code sequences x′ [[ID=4!]] i (m)(i = 1, 2, …, N) by the sine signal respectively to obtain N signals y code (t)(i = 1, 2, …, N) with a duration of mT i . The sequence - code group x i (31)(i = 1, 2, 3) shown in Table 1 is replaced by x′ i(31) (where \(i = 1, 2, 3\)), the results are shown in Table 2. Multiply the first address code \(x′1(31)\) in the code group by the sine signal generated by the fault detection unit to obtain the signal \(y1(t)\) as Figure 5 shown. For the remaining address codes, following the same rule can obtain \(y\) i (t) (\(i = 1, 2, 3\)).

[0043] Table 2

[0044] Serial number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 <![CDATA[x′1(31)]]> -1 1 1 1 -1 -1 1 -1 -1 -1 1 -1 1 -1 1 1 <![CDATA[x′2(31)]]> -1 1 1 1 -1 1 1 -1 1 -1 1 -1 -1 1 1 1 <![CDATA[x′3(31)]]> -1 1 1 1 -1 1 -1 1 -1 -1 1 -1 1 1 1 1 Serial number 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 <![CDATA[x′1(31)]]> 1 1 -1 1 1 -1 1 -1 -1 1 1 -1 -1 -1 -1 <![CDATA[x′2(31)]]> 1 -1 -1 -1 1 1 -1 -1 1 -1 -1 -1 -1 -1 1 <![CDATA[x′3(31)]]> -1 -1 1 1 -1 1 1 -1 -1 -1 -1 -1 1 -1 -1

[0045] (2) The DC positive and negative bus signal receivers sequentially calculate the cross - correlation values of \(y\) i (t) (\(i = 1, 2, …, N\)) and \(i(t)\), compare the operation results with a preset threshold value, and determine that the switching power supply corresponding to the address code exceeding the threshold has a grounding fault. When the sequence information of \(x1(31)\) is contained in \(i(t)\), a possible cross - correlation simulation result of \(y1(t)\) and \(i(t)\) is as Figure 6 shown, and its peak value is 77.5. If the preset threshold is 50 and this peak value exceeds the preset threshold, it is determined that the switching power supply corresponding to the \(x1(31)\) sequence has a grounding fault.

[0046] y i (t) The calculation formula for the cross - correlation value of \(y(t)\) and \(i(t)\) is as follows:

[0047]

[0048] (3) If no grounding fault is detected, repeat steps (1) and (2).

[0049] The above description of the embodiments is for the convenience of those of ordinary skill in the art to understand and apply the present invention. Those who are familiar with the technology in this field can obviously make various modifications to the above embodiments easily, and apply the general principles described herein to other embodiments without creative labor. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A method for locating the grounding fault of a switching power supply in a DC microgrid system. The DC microgrid system includes positive and negative DC buses and N multi-port switching power supplies. One port of each switching power supply is connected to the positive and negative DC buses, and each switching power supply has a unique address code sequence in the system. N is a natural number greater than 1. The method is characterized in that: The switching power supply uses power information composite modulation technology to modulate the respective address code sequence information onto the energy flowing into and out of the switching power supply. Non-invasive current sensor magnetic rings are used to detect the current signals i + (t) and i - (t) on the positive and negative DC buses respectively. Then, after conditioning and filtering these two groups of signals, they are superimposed to obtain the current signal i(t). Finally, a positioning algorithm is applied to locate the switching power supply where the grounding fault occurs. The specific implementation method of the positioning algorithm is as follows: (1) Generate a sine signal with a fixed amplitude, a frequency equal to the data carrier frequency, and a duration of T code , where T code is the symbol period; (2) For the address code sequence x i (m) of any switching power supply i, replace the "1" with "-1" and the "0" with "1" therein to obtain a new address code sequence x' i (m); multiply each digit in the above sine signal by the new address code sequence x' i (m) and splice them to obtain a signal y code with a duration of mT i (t), where i is the serial number of the switching power supply and m is the length of the address code sequence; (3) Calculate the cross-correlation between the signal y i (t) and the current signal i(t), compare the peak value with the threshold, and if it exceeds the threshold, it is determined that the switching power supply i has a ground fault; (4) If no grounding fault is detected in any switching power supply, steps (2) to (3) are repeated at regular intervals.

2. The method for locating the grounding fault of the switching power supply in the DC microgrid system according to claim 1, wherein: The address code sequence uses binary code.

3. The method for locating the grounding fault of the switching power supply in the DC microgrid system according to claim 1, wherein: The power information composite modulation technology is to perform data modulation on the PWM carrier wave or PWM modulation wave that generates the PWM drive signal of the switching power supply.

4. The method for locating the grounding fault of the switching power supply in the DC microgrid system according to claim 3, characterized in that: When performing data modulation on the PWM carrier wave, the amplitude of the PWM carrier wave is not changed, and the phase of the PWM carrier wave is used to represent the address code sequence information. The frequency of the PWM carrier wave is the operating frequency of the switching power supply.

5. The method for locating the grounding fault of the switching power supply in the DC microgrid system according to claim 3, characterized in that: When performing data modulation on the PWM modulation wave, a sine carrier wave with a frequency lower than its operating frequency is generated by the switching power supply. The amplitude of the sine carrier wave is not changed, and the phase of the sine carrier wave is used to represent the address code sequence information. The sine carrier wave is superimposed on the original PWM modulation wave to form a new PWM modulation wave.

6. The method for locating the grounding fault of the switching power supply in the DC microgrid system according to claim 1, characterized in that: After data modulation of the PWM carrier, the symbol period T code = K·T s ; after data modulation of the PWM modulation wave, the symbol period T code = K·M·T s , where T s is the operating period of the switching power supply, and both K and M are positive integers and M≥2.

7. The method for locating the grounding fault of the switching power supply in the DC microgrid system according to claim 1, wherein: When performing data modulation on the PWM carrier wave, the data carrier wave frequency is the operating frequency of the switching power supply; when performing data modulation on the PWM modulation wave, the data carrier wave frequency is 1 / M of the operating frequency of the switching power supply, where M is a positive integer and M≥2.

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