Bipolar DC micro-grid fault diagnosis method based on voltage balancer

Abstract

According to the bipolar DC micro-grid fault diagnosis method based on the voltage balancer, the positive and negative electrode output currents of the voltage balancer are measured, the output current of a neutral line is calculated by using the theorem that the sum of the three-wire output currents is zero, and different faults are detected by constructing characteristic quantities based on current difference. The inter-electrode fault, the positive electrode grounding fault and the negative electrode grounding fault are classified through the relative relation of the three-wire current, and finally the line where the faults are located is determined through non-real-time communication between adjacent sources. According to the method, rapid fault diagnosis is realized under the conditions that the number of the sensors is small and no communication link exists between the nodes, the fault can be detected in the first sampling period after the fault occurs, and the high-resistance fault diagnosis has relatively high accuracy and relatively good reliability and economical efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of DC microgrid fault diagnosis, and in particular relates to a bipolar DC microgrid fault diagnosis method based on a voltage balancer. Background Art

[0002] The demand for energy in modern society is increasing. The development and utilization of renewable energy is an important measure to solve the energy crisis, reduce environmental pollution and promote sustainable development. DC microgrid is an emerging solution that can flexibly integrate various distributed power generation devices such as photovoltaic arrays, wind turbines and energy storage equipment. It has attracted widespread attention due to its efficient power transmission path and control structure, which is much simpler than AC microgrid, and has higher operating efficiency and power supply quality.

[0003] DC microgrids can be divided into three types: asymmetric unipolar structure, symmetric unipolar structure (pseudo bipolar structure) and true bipolar structure (three-wire bipolar structure). The true bipolar structure is similar to a three-phase AC system with three transmission lines, namely the positive line, the negative line and the neutral line. In the event of a fault, the load on the faulty line can be transferred to the healthy line, thereby improving the reliability of the bipolar microgrid. In the true bipolar structure, the power difference of the load or generator connected asymmetrically between the positive and negative poles causes current to flow on the neutral line and generates an unbalanced voltage. The unbalanced current may trigger overcurrent protection or cause harmonics, heating and other problems due to the grounding point of the transformer, while also increasing power losses. Therefore, a three-wire bipolar DC microgrid always requires a voltage balancing solution, and it is the most common practice to arrange a voltage balancer at the converter outlet or use a converter with this function integrated to control the unbalanced voltage. The voltage balancer can be flexibly installed on the load side or the source side to directly balance the node voltage.

[0004] However, a three-wire bipolar DC microgrid with unipolar DC sources, loads, and voltage balancers is a highly complex bipolar DC microgrid. Its low inertia, small impedance, and complex nonlinear coupling characteristics make it have a very fast transient response in the event of a fault, and the fast-rising fault surge current under a low-impedance fault can endanger the safety of power electronic equipment with low overcurrent capacity. Therefore, rapid detection and troubleshooting of faults is required.

[0005] Fault detection in DC microgrids can be divided into three categories. The first category is based on direct measurement, measuring parameters such as current, voltage and their derivatives and comparing them with predetermined thresholds to detect faults at high speed. The second category is based on signal processing techniques, including methods such as Fourier transform and wavelet transform. Although these methods have high accuracy, they are often limited by long computation time and complex implementation process. The third category includes emerging methods that integrate artificial intelligence, such as artificial neural networks and machine learning. These methods overcome the threshold selection problem associated with traditional diagnostic methods. However, these methods are characterized by high model complexity, long training time, and usually require a large number of labeled training samples. The quality of training data largely affects the performance of the diagnostic model. In the current methods for fault diagnosis of bipolar microgrids equipped with voltage balancers, line current or ground current is mainly used to detect faults, but the fault dynamics of three-wire lines have not been analyzed. The structure of a three-wire DC microgrid is complex, especially when equipped with a voltage balancer and connected to a unipolar power supply. Therefore, the fault characteristics are very different from those of a two-wire system. Therefore, the existing fault diagnosis methods for two-wire bipolar DC microgrids are not fully applicable, and it is necessary to design appropriate diagnostic methods for the fault characteristics unique to the three-wire structure.

[0006] The patents that have been applied for are as follows:

[0007] Patent number: CN202110783338.9, patent name A fault component identification method suitable for a fault-clearing DC distribution network. A fault component identification method is proposed for a DC distribution mode of a fault-clearing converter combined with a fast disconnector. According to the fault characteristics of each bus at different fault locations, a bus virtual voltage characteristic quantity is constructed to identify the fault line; according to the switching process of some sub-modules of the hybrid converter, an auxiliary identification criterion is constructed based on the capacitor charge transfer amount to complete the identification of the fault bus. The inventive method can realize the accurate identification of fault components and accurate fault ranging, has a strong ability to withstand transition resistance, and reduces the investment in line sensors.

[0008] It constructs bus virtual voltage characteristic quantities based on the fault characteristics of each bus at different fault locations in the DC distribution network to identify the fault line. The calculation of this characteristic quantity requires real-time communication to obtain the voltage information of multiple buses and the output current information of multiple converters. In addition, the fault bus identification criterion based on the capacitor charge transfer quantity proposed in the invention method aims to distinguish between bus faults and bus-related line end faults, and requires the switching of sub-modules in the converter and the opening and closing of all bus-related line disconnectors to cooperate. The fault identification time is relatively long, which has a greater impact on the power supply reliability of the system.

[0009] The fault characteristic quantity constructed by the present invention only needs to obtain the output voltage and current of the local converter to complete the calculation, and the fault detection does not require communication. The fault classification method based on the error of the converter output voltage estimation value proposed by the present invention realizes the distinction between inter-pole faults, positive grounding faults and negative grounding faults, and there is no need to operate the isolating switch or circuit breaker before the fault is removed.

[0010] Patent number: CN 201811362494.2, patent name: Ring DC distribution network fault detection method based on voltage prediction, belongs to the field of safe operation of DC distribution network, and protection devices are installed at both ends of the DC distribution network, and the protection devices include circuit breakers, sensors, relays, and reactors; it is characterized in that: the predicted voltage value of the line side at the next moment is predicted according to the sampled voltage values ​​at any three sampling time points on the line side, and the difference between the predicted voltage value and the measured voltage value on the line side at the moment of the fault is used as the action basis of the protection. The present invention solves the problems of low speed of traditional DC distribution network protection, excessive dependence on communication and the damping state of the circuit. The present invention only relies on the voltage value on the line side, and effectively avoids the influence of excessive dependence on the damping state of the circuit at the moment of the fault when setting protection using elements such as current, first-order derivative and second-order derivative of current, and first-order derivative of voltage.

[0011] It identifies faults by the difference between the predicted voltage and the measured voltage value on the line side at the moment of fault. Its voltage prediction method is designed based on the second-order derivative of voltage. The premise that this fault detection method can quickly and accurately identify faults is that there is a very large voltage drop at the moment of fault. When the fault impedance is large, the voltage drops slowly. At this time, it is difficult to ensure the speed and accuracy of fault detection using this method.

[0012] The fault detection method proposed in the present invention is achieved by measuring the positive and negative output currents of the voltage balancer, calculating the neutral line output current, calculating the difference of the three-line output currents and comparing them with the preset threshold value. The fault can be detected within the first sampling cycle after the fault occurs, and different types of faults can be quickly and accurately detected for high-resistance faults. The fault classification method of the present invention is based on the dynamic analysis of the faults of the three-line bipolar microgrid. Different types of faults are distinguished by comparing the difference of the three-line output currents, which is significantly different from the two-line microgrid targeted by the above invention. Summary of the invention

[0013] In order to solve the above problems, the present invention proposes a three-wire bipolar DC microgrid fault diagnosis method based on a voltage balancer. The method realizes rapid fault diagnosis when there is a small number of sensors and no communication link between nodes. It has a high accuracy in diagnosing high-resistance faults and can detect and classify any two-pole faults, positive pole grounding faults and negative pole grounding faults in a three-wire bipolar DC microgrid without installing additional sensors.

[0014] To achieve the above object, the technical solution adopted by the present invention is:

[0015] A bipolar DC microgrid fault diagnosis method based on a voltage balancer, characterized in that it comprises the following steps:

[0016] Step 1) Measure the positive output current I of the voltage balancer P , Negative output current I N , and calculate the output current I of the neutral line Z ;

[0017] The output current of the neutral line is calculated as follows:

[0018] I P +I Z +I N =0 (1)

[0019] Step 2) Calculate the fault characteristic quantity - the difference ΔI of the three-line current P , ΔI N and ΔI Z , compared with the preset threshold ε1. If any ΔI exceeds the threshold ε1, it is considered that the line is faulty;

[0020] The calculation steps of the fault characteristic quantity ΔI are as follows:

[0021] ΔI[n]=I[n]-I[n-1] (2)

[0022] Where I[n] is the sampled value or calculated output current of the voltage balancer at time n;

[0023] Step 3) Based on the differential ΔI of the three-wire current P , ΔI N and ΔI Z Fault classification is performed based on the relative relationship between

[0024] Further, step 3) includes the following specific steps:

[0025] First, ΔI P , ΔI N and ΔI Z Compare with the preset threshold ε2.

[0026] A) If no ΔI exceeds the threshold ε2, proceed to step B. If any ΔI exceeds the threshold ε2, determine whether ΔI P , ΔI N and ΔI Z Whether |ΔI is satisfied N |>|ΔI P |>|ΔI Z |or ΔI P, ΔI N Approximately equal; if the condition is met, it is considered that an inter-pole fault has occurred between the positive line and the negative line; if the condition is not met, go to step B;

[0027] ΔI P , ΔI N Approximate equality is determined using δ PN %, and the calculation formula is as follows:

[0028]

[0029] If δ PN % is less than 1%, then it is considered that ΔI P , ΔI N approximately equal;

[0030] B.1) Determine |ΔI Z |Is it greater than |ΔI N |or|ΔI P |. If the conditions are met, determine ΔI Z Is it greater than 0? If ΔI Z >0, it is considered that an inter-pole fault has occurred between the negative line and the neutral line; if ΔI Z <0, it is considered that an inter-pole fault has occurred between the positive line and the neutral line;

[0031] B.2) Determine |ΔI N Is |greater than |ΔI P | and ΔI P , ΔI Z Approximately equal. If the conditions are met, it is considered that a ground fault has occurred in the negative line;

[0032] ΔI P , ΔI Z Approximate equality is determined using δ PZ %, and the calculation formula is as follows:

[0033]

[0034] If δ PN % is less than 1%, then it is considered that ΔI P , ΔI N approximately equal;

[0035] B.3) Determine |ΔI Z Is | less than |ΔI P | and |ΔI N |or ΔI N , ΔI Z Approximately equal. If the conditions are met, it is considered that a ground fault has occurred in the positive line;

[0036] ΔIN , ΔI Z Approximate equality is determined using δ NZ %, and the calculation formula is as follows:

[0037]

[0038] If δ NZ % is less than 1%, then it is considered that ΔI N , ΔI Z Approximately equal.

[0039] Beneficial effects:

[0040] The bipolar DC microgrid fault diagnosis method based on a voltage balancer proposed in the present invention can realize fast and accurate detection of high-resistance faults, and can classify any two-pole faults, positive pole grounding faults and negative pole grounding faults. In addition, the method does not require communication between nodes and has advantages in reliability and economy. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a flow chart of the present invention;

[0042] Figure 2 It is a schematic diagram of the DC microgrid structure of the present invention;

[0043] Figure 3 It is a schematic diagram of the discharge circuit of a two-line bipolar microgrid under a positive grounding fault;

[0044] Figure 4 Schematic diagram of the discharge circuit of a three-wire bipolar microgrid under a positive grounding fault;

[0045] Figure 5 It is a schematic diagram of current differential fault characteristic quantity detection of positive pole grounding fault under symmetrical wiring of the present invention;

[0046] Figure 6 It is a schematic diagram of detecting the current differential fault characteristic quantity of the positive-negative inter-pole fault under symmetrical wiring of the present invention;

[0047] Figure 7 It is a schematic diagram of detecting the current differential fault characteristic quantity of the positive-negative inter-pole fault under the asymmetric wiring of the present invention;

[0048] Figure 8 It is a schematic diagram of detecting the current differential fault characteristic quantity of the negative pole-neutral line inter-pole fault under asymmetric wiring of the present invention;

[0049] Fig. 9 It is a schematic diagram of current differential fault characteristic quantity detection of positive pole grounding fault under asymmetric wiring of the present invention;

[0050] Fig.10 It is a schematic diagram of the detection result of the current differential fault characteristic quantity under the line load switching superimposed noise of the present invention. DETAILED DESCRIPTION

[0051] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments, wherein the flow chart is as follows: Figure 1 As shown:

[0052] Embodiment 1:

[0053] This embodiment and the following embodiments are all based on Figure 2 The DC microgrid shown is used as an object for explanation. Figure 2 The DC microgrid shown is a ring bipolar DC microgrid with a voltage level of 760V (±380V), including 1 AC grid, 3 distributed generation units, 2 DC load units and 2 voltage balancers. Each distributed power source and load is connected to the bus through a DC / DC converter. The voltage balancer is equipped with load measurement, and the energy storage unit uses a droop control strategy to adjust the system voltage. The switching frequency of the converter in the system is set to 20kHz, and the current sampling frequency is set to 20kHz.

[0054] when Figure 2 When a positive pole ground fault occurs in line 1 of the DC microgrid shown in the figure, the positive pole output current of the voltage balancer adjacent to the line will rise rapidly, and its derivative will also increase rapidly. However, unlike the two-line bipolar microgrid (such as Figure 3 shown), Figure 4 The discharge loop of a three-wire two-terminal line with only one end grounded under a positive ground fault is shown. At this time, the capacitor on the ungrounded side will charge the negative electrode on the grounded side. Due to the creation of a new loop, loop 4, the current of the negative line tends to flow in the discharge direction. Therefore, the current of the negative electrode will also change significantly. That is, in the case of a positive fault, the current of the faulty positive electrode and the current of the healthy negative electrode will rise rapidly, and their derivatives will be relatively large. The same is true for negative faults.

[0055] against Figure 2 For the system shown in the figure, considering its fault dynamic characteristics, protection time requirements and sampling frequency, the detection threshold ε1 is set to 0.9. Figure 5 The fault detection results of the three-wire current differential when a positive ground fault occurs in line 1 are shown, and the fault resistance is 1Ω. As can be seen from the figure, the differential of the three-wire current reaches the maximum value at the first sampling point (0.05ms) and is much larger than ε1, and the fault is detected.

[0056] when Figure 2 When an inter-pole fault occurs between any two lines of line 1 of the DC microgrid shown, the corresponding line output current of the adjacent voltage balancer of the line will rise rapidly, and its derivative will also increase rapidly. Figure 6The fault detection results of the three-wire current differential when an inter-pole fault occurs between the positive line and the negative line of line 1 are shown, and the fault resistance is 1Ω. As can be seen from the figure, the difference between the positive current and the negative current reaches the maximum value at the first sampling point (0.05ms) and is much larger than ε1, and the fault is detected.

[0057] Embodiment 2:

[0058] This embodiment describes the fault classification method. Figure 1 For lines 1 and 5 in the figure, if any type of fault occurs, the current difference between two lines of the voltage balancer will be approximately equal at the first few sampling points. The fault type can be determined based on this:

[0059]

[0060] Figure 5 Positive ground fault and Figure 6 The positive-negative inter-pole fault exhibited strictly complies with the above classification conditions.

[0061] For the lines with asymmetrical wiring at both ends, Figure 1 For example, line 2 in Figure 1 is connected to the positive source on one side. Under fault conditions, |I N | will become larger, so |ΔI N | also becomes larger, so that the approximately equal relationship in the positive-negative pole fault, negative-neutral pole fault and positive pole grounding fault in equation 1 no longer holds. The new fault type judgment criterion is as follows:

[0062]

[0063] Similarly, the fault type criteria for connecting to the negative source are as follows:

[0064]

[0065] Figure 7 The fault detection results of line 2 using three-wire current differential when an inter-pole fault occurs between the positive line and the negative line are shown, and the fault resistance is 1Ω. As can be seen from the figure, the difference between the positive current and the negative current reaches the maximum value at the first sampling point (0.05ms) and is much larger than ε1, and satisfies |ΔI N |>|ΔI P |>|ΔI Z |, a positive-negative inter-pole fault is detected.

[0066] Figure 8The fault detection results of line 2 using three-line current differential when an inter-pole fault occurs between the negative line and the neutral line are shown, and the fault resistance is 1Ω. As can be seen from the figure, the difference between the negative current and the neutral line current reaches the maximum value at the first sampling point (0.05ms) and is much larger than ε1, and satisfies |ΔI Z |>|ΔI P | and |ΔI Z |>0, a negative-neutral line fault is detected.

[0067] Fig. 9 The fault detection results of line 2 using three-wire current differential when a ground fault occurs in the positive line are shown, and the fault resistance is 1Ω. As can be seen from the figure, the difference between the positive current and the negative current reaches the maximum value at the first sampling point (0.05ms) and is much larger than ε1, and satisfies |ΔI P |>|ΔI N |>|ΔI Z |, a positive ground fault is detected.

[0068] This embodiment illustrates that both ends of the line can accurately classify different types of faults.

[0069] Embodiment 3:

[0070] This embodiment describes the detection method under normal load switching and noise. Noise and load switching should not be diagnosed as faults. This embodiment verifies that this method will not falsely report under normal system operation. The power of DC load #1 increases by 50%, and the measured noise is 50db. Fig.10 The results of the three-wire current differential of line 2 at this time are shown. Until the system stabilizes again, the three-wire current differential is less than ε1. No fault is detected.

[0071] The above description is only a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any modification or equivalent change made based on the technical essence of the present invention still falls within the scope of protection required by the present invention.

Claims

1. A bipolar DC microgrid fault diagnosis method based on voltage balancer, characterized in that: The following steps are involved: Step 1) Measure the positive output current I of the voltage balancer P , Negative output current I N , and calculate the output current I of the neutral line Z ; The output current of the neutral line is calculated as follows: I P +I Z +I N =0 (1) Step 2) Calculate the fault characteristic quantity - the difference ΔI of the three-line current P , ΔI N and ΔI Z , compared with the preset threshold ε1. If any ΔI exceeds the threshold ε1, it is considered that the line is faulty; The calculation steps of the fault characteristic quantity ΔI are as follows: ΔI[n]=I[n]-I[n-1] (2) Where I[n] is the sampled value or calculated output current of the voltage balancer at time n; Step 3) Based on the differential ΔI of the three-wire current P , ΔI N and ΔI Z Fault classification is performed based on the relative relationship between the 2. A voltage-based DC microgrid fault diagnosis method according to claim 1, characterized in that: Step 3) The specific steps are as follows: First, identify the properties of the adjacent sources of the voltage balancer. If connected to the negative source, swap ΔI P and ΔI N The value of ΔI P , ΔI N and ΔI Z Compare with the preset threshold ε2; A) If no ΔI exceeds the threshold ε2, go to step B. If any ΔI exceeds the threshold ε2, determine whether ΔI P , ΔI N and ΔI Z Whether |ΔI is satisfied N |>|ΔI P |>|ΔI Z |or ΔI P , ΔI N Approximately equal; if the condition is met, it is considered that an inter-pole fault has occurred between the positive line and the negative line; if the condition is not met, go to step B; ΔI P , ΔI N Approximate equality is determined using δ PN %, and the calculation formula is as follows: If δ PN % is less than 1%, then it is considered that ΔI P , ΔI N approximately equal; B.1) Determine |ΔI Z |Is it greater than |ΔI N |or|ΔI P |, if the conditions are met, judge ΔI Z Is it greater than 0? If ΔI Z >0, it is considered that an inter-pole fault has occurred between the negative line and the neutral line; if ΔI Z <0, it is considered that an inter-pole fault has occurred between the positive line and the neutral line; B.2) Determine |ΔI N |Is it greater than |ΔI P | and ΔI P , ΔI Z Approximately equal, if the condition is met, it is considered that a ground fault has occurred in the negative line; ΔI P , ΔI Z Approximate equality is determined using δ PZ %, and the calculation formula is as follows: If δ PN % is less than 1%, then it is considered that ΔI P , ΔI N approximately equal; B.3) Determine |ΔI Z Is | less than |ΔI P | and |ΔI N |or ΔI N , ΔI Z Approximately equal, if the condition is met, it is considered that a ground fault has occurred in the positive line; ΔI N , ΔI Z Approximate equality is determined using δ NZ %, and the calculation formula is as follows: If δ NZ % is less than 1%, then it is considered that ΔI N , ΔI Z Approximately equal.

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