Complex ship direct current micro-grid line protection method using characteristic impedance

Abstract

The invention relates to a complex ship DC micro-grid line protection method using characteristic impedance, and the method comprises the steps: collecting real-time voltage values and current values at the measurement positions of protection devices at the two sides of a medium-voltage DC line, carrying out the noise reduction, calculating a fault protection starting element, extracting a voltage signal component and a current signal component of a characteristic frequency band, and carrying out the calculation of a fault protection starting element; calculating the characteristic impedance; comparing the characteristic impedance with a corresponding threshold value to obtain a fault direction, sending a result of the fault direction to a protection device on the other side of the line, receiving the result of the fault direction of the protection device on the other side of the line, identifying a fault line area according to a fault direction criterion, and calculating a fault pole selection criterion. And finally, protecting the faulted line according to the result. According to the method, the electrical quantity coupling effect caused by the annular region architecture of the medium-voltage direct-current ship power system is overcome, the fault of the medium-voltage direct-current line can be accurately identified, the method has good anti-transition resistance capability, and the requirement for the sampling frequency is low.

Description

Technical Field

[0001] The present invention relates to the technical field of DC power system relay protection, and particularly to a method for protecting complex ship DC microgrid lines using characteristic impedance. Background Art

[0002] With the improvement of the energy utilization efficiency of ships, the integrated power system (IPS) has become a development trend. Combining electric propulsion technology, power electronics technology, and energy storage technology, the characteristics of high efficiency, flexibility, and environmental protection give IPS great advantages in terms of operating efficiency and cost. Compared with traditional AC systems, the medium-voltage DC (MVDC) power system is considered to have more potential in high-performance applications due to its higher power output capacity. Similar to land power grids, the shipboard power system (SPS) is regarded as an isolated microgrid except when connected to shore power. Therefore, reliability and survivability are particularly important, and MVDC SPS usually requires fault detection and isolation to be completed within 8 - 10 ms.

[0003] The protection of DC microgrids is more challenging than other DC power grids due to the influence of short lines and transition resistances. For DC power grids, common protection schemes include traveling wave protection, differential protection, overcurrent protection, injection protection, etc. However, traveling wave protection is considered not suitable for physically compact microgrids, and other traditional methods may require strict time synchronization, resulting in a decrease in reliability and an increase in cost. For MVDC SPS, the compact space leads to the coupling effect of electrical quantities. Especially for the ring area architecture, when a fault occurs in the MVDC lines forming the ring, the fault characteristics will quickly spread to the entire ring network, and the situation of high transition resistance is difficult to detect quickly. Therefore, there is an urgent need for a protection scheme for complex ship DC microgrids that does not require time synchronization, has a fast action speed, and has good resistance to transition resistance. Summary of the Invention

[0004] Based on the above problems, the present invention proposes a method for protecting complex ship DC microgrid lines using characteristic impedance. The present invention utilizes the topological characteristics of the ring area architecture of the medium-voltage DC ship power system, discriminates faults through the characteristic impedance obtained from both sides of the line, has a low requirement for the sampling frequency, does not require time synchronization for the required data communication, has a fast action speed, and has good resistance to transition resistance.

[0005] A method for protecting complex ship DC microgrid lines using characteristic impedance, the method comprising:

[0006] Collect the real-time voltage value and real-time current value at the measurement points of the protection devices on both sides of the medium-voltage DC line as voltage signals and current signals;

[0007] Perform real-time noise reduction processing on the voltage signals and current signals, and calculate the starting element of the fault protection in real time;

[0008] Use a filter to filter the voltage signals and current signals, and extract the voltage signal components and current signal components in the characteristic frequency band;

[0009] Calculate the characteristic impedance using the filtered voltage signal components and current signal components;

[0010] Compare the calculated characteristic impedance with the threshold set for the corresponding characteristic impedance to obtain the fault direction;

[0011] Send the result of the fault direction to the protection device on the other side of the medium-voltage DC line through communication, and at the same time receive the result of the fault direction of the protection device on the other side of the medium-voltage DC line;

[0012] Identify the fault line area according to the fault direction criterion;

[0013] Calculate the fault pole selection criterion using the filtered voltage signal components and current signal components, and the line protection for the fault operates.

[0014] Furthermore, the complex ship DC microgrid features a medium-voltage DC ship power system ring area architecture. The ring area architecture consists of a busbar and medium-voltage DC lines that are circularly connected to adjacent busbars. Among them, the lines are connected through a fault current limiter (FCL) and a DC current breaker (DCCB). The DC current breaker is then connected to the busbar, and the busbar has a large capacitor or DC filter as its impedance characteristic due to the connection of the converter.

[0015] Furthermore, the fault current limiter has a large inductor as its impedance characteristic and has the function of suppressing the rising rate of the fault current.

[0016] Furthermore, the DC current breaker is a hybrid breaker, including a two-port or multi-port DC current breaker.

[0017] Furthermore, the protection device is located between the DC current breaker connected to the busbar and the fault current limiter connected to the line.

[0018] Furthermore, the noise reduction is to filter out the noise through wavelet transform, which has strong anti-noise interference ability and retains the original characteristics of the signal.

[0019] Furthermore, the starting element is the derivative of the voltage signal and the derivative of the current signal, so as to quickly determine the occurrence of a fault.

[0020] Further, the filter is a digital band-pass filter, and the voltage signal and current signal in the characteristic frequency band are extracted.

[0021] Further, the characteristic frequency band is 500Hz–700Hz, and the characteristic frequency band is obtained by analyzing the parameters of the rectifier, inverter, DC / DC converter, and fault current limiter in the loop area architecture of the medium-voltage DC ship power system, where the rectifier is characterized by a DC filter, and the inverter and DC / DC converter are characterized by large capacitors as impedance.

[0022] Further, the calculation formula for the characteristic impedance is:

[0023]

[0024] In the formula, is the characteristic impedance, nx is the node where the protection device is located, and the characteristic frequency f within the characteristic frequency band ci is 600Hz (i = 1, 2) for both, and k s is the sampling sequence number of the collected data, is the root mean square value of the voltage signal component of node nx at the characteristic frequency f ci and is the root mean square value of the current signal component of node nx at the characteristic frequency f ci ; X represents U or I, is the voltage signal component of node nx at the characteristic frequency f ci or the current signal component N is the number of sampling points; the sampling frequency f is 10kHz, and the sliding time window T s is 2ms. w

[0025] Further, the fault direction is:

[0026]

[0027] In the formula, is the threshold set for the corresponding characteristic impedance.

[0028] Further, the communication transmits signals through optical fibers or other means and needs to have as low a delay as possible.

[0029] Further, the fault direction criterion for determining whether it is an internal fault or an external fault according to the result of the bilateral fault direction criterion is:

[0030]

[0031] ​Among them, nx and ny are the nodes where the protection devices on both sides of the same line are located, i, j = 1, 2 and i ≠ j. That is, when the fault directions on both sides of the fault location are judged as forward faults, the fault location is an in-zone fault for the protection devices on both sides.

[0032] Furthermore, the fault pole selection criterion is to obtain the sum B of the ratios of the positive and negative components of the voltage and current in the characteristic frequency band respectively, and then compare it with the threshold set by the fault pole selection criterion. Falling within different ranges can determine positive pole faults, negative pole faults, and bipolar faults.

[0033] Furthermore, the preset overall delay time is about 5 ms.

[0034] Since the requirements for fault detection and isolation do not exceed 8 ms, the present invention can quickly and accurately identify the fault line while reserving a margin, and has good anti-transition resistance ability. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a flowchart of the method for protecting complex ship DC microgrid lines using characteristic impedance in the present invention;

[0036] Figure 2 is a schematic diagram of the ring area structure of the ±15 kV medium voltage DC ship power system;

[0037] Figure 3 is the amplitude-frequency curve of the impedance of the DC filter and capacitor;

[0038] Figure 4 is the waveform diagram of the simulation result of the positive pole to ground fault at point F12. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0040] Such as Figure 1 、 Figure 2As shown, in an embodiment of a ring - shaped regional architecture of a ±15 kV medium - voltage DC ship power system, it includes a 12 - pulse uncontrolled rectifier, a voltage source converter (VSC), a dual active bridge (DAB) type DC / DC converter, a multi - terminal DC circuit breaker, a fault current limiter, as well as generators and loads. A complex ship DC micro - grid line protection method using characteristic impedance includes the following steps:

[0041] Step S110, collect voltage signals and current signals at the measurement points of each protection device in the DC ship power system in real time. The measurement points of the protection devices are set at Figure 2 nx_a and nx_b in, where x = 1, 2,..., 7 (such as n1_a, n1_b, n2_a, etc.).

[0042] Step S120, perform real - time noise reduction processing on the voltage signals and current signals through wavelet transform. The sampling frequency is selected as 10 kHz, and the wavelet "db2" is used to obtain the noise - reduced signals after four - level wavelet decomposition. At the same time, calculate the starting element of the fault protection in real time. When the voltage derivative and current derivative reach the fault threshold, it is determined that a fault has occurred.

[0043] Step S130, when a fault occurs, obtain the voltage signal component and current signal component in the characteristic frequency band (500 Hz - 700 Hz) through a Chebyshev digital band - pass filter, and calculate the characteristic impedance through the voltage signal component and current signal component. The time window is set to 2 ms. At the same time, obtain the fault direction after comparing with the threshold set for the corresponding characteristic impedance.

[0044] Step S140, through optical fiber communication, send the result of the fault direction to the protection device on the other side of the line (such as n1_b and n2_a are on the other side of line 12), and at the same time receive the result of the fault direction of the protection device on the other side of the line.

[0045] Step S150, if it is determined as an in - zone fault according to the result of the bilateral fault - direction criterion, calculate the fault - pole - selection criterion through the voltage signal component and current signal component, and compare it with the threshold.

[0046] Step S160, actuate the corresponding line protection according to the result of the fault - pole - selection criterion, that is, send an opening instruction to the corresponding DC circuit breaker, and finally complete the fault detection and isolation within 3 - 4 ms after the fault occurs.

[0047] The principle part of the above - mentioned fault - direction criterion is explained as follows:

[0048] All nodes where the protection devices are located can be divided into two categories: rectifier nodes with a DC filter as the impedance characteristic (such asFigure 2 n1, n3, n4, and n6 therein), with a characteristic frequency of f c1 ; Inverters and DC / DC converter nodes characterized by large capacitors (such as Figure 2 n2, n5, n7 therein), with a characteristic frequency of f c2 . The amplitude-frequency curves of the DC filter and capacitor impedance are as Figure 3 shown. The DC filter on the 12-pulse uncontrolled rectifier side is a 12k-th harmonic filter. At the power frequency of 50 Hz for the AC generator, it can filter out the frequency component of 600 Hz. At this frequency, the impedance of the capacitor is also relatively low, while the impedance of the fault current limiter is relatively high. Therefore, the characteristic frequencies with significant impedance differences are selected, that is, f c1 and f c2 are both set to 600 Hz. To improve robustness and anti-interference ability, the adjacent characteristic frequency band of 500 Hz - 700 Hz is selected as the target impedance. According to the Shannon sampling theorem, the sampling frequency only needs to be greater than 1400 Hz, so the requirement for the sampling frequency is relatively low. According to the superposition theorem, the voltage and current in the power system fault state are the superposition of the steady-state component and the transient component caused by the fault. Therefore, through the fault additional network composed only of the excitation source at the fault point and other passive impedances, applying the series and parallel characteristics of the impedance, it can be deduced that the characteristic impedances in the case of in-zone faults and out-of-zone faults have significant magnitude differences and are related to the parameters of the capacitors and inductors used in the system.

[0049] The formula for selecting the threshold corresponding to the above characteristic impedance setting is:

[0050]

[0051] where |Z DCF | is the impedance amplitude of the DC filter, |Z FCL | is the impedance amplitude of the fault current limiter, |Z C(min) | = min(Z C(VSC) |, Z C(DAB) |) is the minimum value of the capacitor impedance amplitude, where |Z C(VSC) | is the capacitor impedance amplitude on the DC side of the voltage source inverter, and |Z C(DAB) | is the capacitor impedance amplitude of the DC / DC converter. and are both set to 1.9 Ω.

[0052] Taking the positive pole-to-ground fault of the fault point F12 located in Figure 2 as an example, the moment when this fault occurs is taken as t = 0. The simulation results of the positive pole voltage, positive pole current, and positive pole calculated characteristic impedance are as Figure 4As shown in the figure. It can be seen that at t = 0.1 ms, the calculated characteristic impedance has dropped below the threshold. Take the value at t = 2 ms as the input of the fault direction. The nodes (n1_b, n2_a) on both sides of the faulty line are determined to be in the forward fault (step S130). After data communication, according to the fault direction criterion, it can be determined that this fault is an in-zone fault (step S140). At this time, by calculating the fault pole selection criterion, the specific faulty line can be obtained according to the result (step S150). Then, by sending an opening instruction to the corresponding DC circuit breaker, the line protection can be accurately and quickly actuated (step S160).

[0053] When a positive pole-to-ground fault and a bipolar fault occur at the fault point F12, the simulation results are shown in Table 1, and the obtained results are all transient values at the 2nd ms after the start of the fault. According to the fault pole selection criterion, it can be known that Line 12 is identified as an in-zone fault, while other lines are identified as out-of-zone faults. For different fault types, the fault pole selection criterion can accurately identify. Therefore, the line protection method proposed by the present invention can accurately and quickly act for different line fault types. When a fault occurs in the area outside the line (such as the AC side of the rectifier and the AC side of the inverter), all line protections only identify out-of-zone faults, so no misoperation occurs. When dealing with faults with different transition resistances, as shown in Table 1, when the transition resistance rises to 500 ohms, the results of the fault direction criterion and the fault pole selection are both correct, and the calculated characteristic impedance A still has a large margin from the threshold. Therefore, the line protection method proposed by the present invention has good resistance to transition resistance.

[0054] Table 1 Simulation results of Line 12 protection under different faults

[0055]

[0056] Therefore, the line protection method proposed by the present invention is fast, accurate, and reliable, and has the following characteristics: 1) It overcomes the electrical quantity coupling effect brought by the loop area architecture of the medium-voltage DC ship power system and can accurately identify faults in medium-voltage DC lines; 2) Since the fault point based on the superposition theorem is regarded as the excitation source, it is also sensitive to faults with high transition resistance; 3) Due to the obvious difference in the components of the characteristic impedance in a specific frequency band, its requirement for the sampling frequency is low. 4) Since the communication only needs to transmit the criterion result signal and does not need to transmit the sampling signal, its communication does not require time synchronization, so the requirement for the communication system is low.

[0057] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent for the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the patent for the present invention shall be subject to the appended claims.

Claims

1. A method, characterized in that: The following steps are involved: Collect the real-time voltage and current values ​​at the measuring points of the protection devices on both sides of the medium voltage DC line as voltage signals and current signals; Perform real-time noise reduction on voltage and current signals, and calculate the start-up elements of fault protection in real time; Filter the voltage signal and the current signal using a filter to extract the voltage signal component and the current signal component of the characteristic frequency band; calculating a characteristic impedance using the filtered voltage signal component and the current signal component; The calculated characteristic impedance is compared with the threshold value set for the corresponding characteristic impedance to obtain the fault direction; Sending the result of the fault direction to the protection device on the other side of the medium voltage DC line through communication, and receiving the result of the fault direction of the protection device on the other side of the medium voltage DC line; According to the fault direction criterion, the fault line area can be identified; The filtered voltage signal component and current signal component are used to calculate the fault pole selection criterion and to perform protection actions on the faulty line.

2. The complex ship DC microgrid line protection method using characteristic impedance according to claim 1 is characterized in that: The complex ship DC microgrid is a ring-shaped regional architecture of a medium-voltage DC ship power system, including a busbar, a medium-voltage DC line connected to the adjacent busbar in a ring, wherein the medium-voltage DC line is connected to a DC circuit breaker through a fault current limiter, and the DC circuit breaker is connected to the busbar; wherein the busbar is connected to the converter, with a large capacitor or DC filter as the impedance characteristic, and the fault current limiter has a large inductance as the impedance characteristic, which has the function of suppressing the fault current rising rate.

3. The complex ship DC microgrid line protection method using characteristic impedance according to claim 2 is characterized in that: The characteristic frequency band is 500Hz-700Hz, which is obtained by parameter analysis of the rectifier, inverter, DC / DC converter and fault current limiter of the ring area architecture, wherein the rectifier is characterized by a DC filter, the inverter and DC / DC converter are characterized by large capacitance as impedance, and the fault current limiter is characterized by large inductance as impedance.

4. The complex ship DC microgrid line protection method using characteristic impedance according to claim 3 is characterized in that: The calculation formula of the characteristic impedance is: N=f s ×T w , In the formula, is the characteristic impedance, nx is the node where the protection device is located, and the characteristic frequency f in the characteristic frequency band ci All are 600Hz (i=1,2), k s is the sampling sequence number of the collected data, For node nx at characteristic frequency f ci The RMS value of the voltage signal component, For node nx at characteristic frequency f ci The root mean square value of the current signal component; X refers to U or I, For node nx at characteristic frequency f ci The voltage signal component Or current signal component N is the number of sampling points; sampling frequency f s is 10kHz, sliding time window T w is 2ms.

5. The complex ship DC microgrid line protection method using characteristic impedance according to claim 4 is characterized in that: The fault direction is: In the formula, The threshold value set for the corresponding characteristic impedance.

6. The complex ship DC microgrid line protection method using characteristic impedance according to claim 5 is characterized in that: The fault direction criterion is: Among them, nx and ny are the nodes where the protection devices on both sides of the same line are located, i,j=1,2 and i≠j, that is, when the fault directions on both sides of the fault location are judged to be forward faults, the fault location is an intra-zone fault for the protection devices on both sides.

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