A method for suppressing fault oscillations in a low-voltage DC Buck-type active current limiter

Abstract

This invention relates to a method for suppressing fault oscillations in a low-voltage DC Buck-type active current limiter. The low-voltage DC Buck-type active current limiter includes a current limiter output capacitor. The key feature is that a power dissipation branch is connected in parallel with the current limiter output capacitor. This power dissipation branch includes an anti-parallel power diode and a load-dissipating resistor connected in series. This invention considers the load-dissipating circuit for inductor energy at the fault point, effectively limiting the current peak of the current limiter and almost eliminating periodic oscillations on the current-limiting inductor, thus better ensuring equipment safety.

Description

Technical Field

[0001] This invention relates to an improved method for Buck-type active current limiters in the field of low-voltage DC. Background Technology

[0002] In recent years, with the continuous development of distributed generation and DC energy storage devices, DC distribution networks have received widespread attention from the academic community due to their advantages such as easy access to clean energy, strong controllability, and low line loss. [1][2] .

[0003] For low-voltage DC distribution systems, bus voltage is primarily controlled by converters connected to the bus. Due to the complex system topology and high integration of power electronics, the probability of DC short-circuit faults is correspondingly increased. Furthermore, compared to AC distribution systems, DC faults are characterized by rapid current rise, high amplitude, and the lack of a zero-crossing point in the fault current, making arc extinguishing difficult. These features pose significant challenges to the system's protection and isolation. [3][4] .

[0004] Currently, the main solutions for handling DC short-circuit faults in low-voltage DC systems connected to flexible substations include:

[0005] 1) Fault clearance is achieved by relying on DC circuit breakers. This scheme requires the installation of DC circuit breakers on the DC side. At present, the research focus of fault clearance technology based on DC circuit breakers is mainly on the type of DC circuit breaker used. Currently, DC circuit breakers are mainly classified into molded case circuit breakers, solid-state circuit breakers and hybrid circuit breakers. References [5][6] provide a good summary of this.

[0006] 2) Fault current limiting is achieved by relying on converter control. Reference [7] uses a solid-state switch composed of a controllable thyristor connected in series on the AC side of the converter. When a DC side fault occurs, the development of DC side current is limited by disconnecting the AC power grid, which has a good fault current limiting effect. After the fault point is cleared, the system can be restored to normal operation by restarting the converter. However, Reference [8] points out that when a short circuit fault occurs in the system, the module capacitor will discharge rapidly and the capacitor voltage will drop deeply. It is necessary to restart through a complex timing sequence such as slow start and charging. The equipment has poor fault ride-through capability.

[0007] 3) Relying on fault current limiting devices to limit the development of fault current. Reference [9] proposes a new type of flexible current limiter. When a fault occurs, the bypass current limiter is connected in series with the DC line through the natural switching of the bridge circuit. The DC bias power supply provides a variable clamping voltage for the fault circuit, thereby realizing flexible control of the voltage drop of the DC / AC or DC / DC output capacitor to limit the development of fault current.

[0008] Existing research focuses primarily on resistive short-circuit faults, while most electrical equipment, aside from some resistive loads, consists of inductive loads. For low-voltage DC Buck-type active current limiters, when the fault characteristics exhibit resistive-inductive or purely inductive properties, the fault current of the current limiter will show characteristics of large peak values ​​and periodic oscillations. This invention addresses this characteristic by improving the structure of the Buck-type active current limiter.

[0009] References

[0010] [1] Li Xialin, Guo Li, Wang Chengshan, et al. A review of key technologies for DC microgrids [J]. Proceedings of the CSEE, 2016, 36(1): 2-17.

[0011] [2] Wu Yuebin, Qi Tianxing, Ma Tao, et al. Fault current limiting characteristics analysis of DC microgrids [J]. Power Grid and Clean Energy, 2021, 37(6): 18-34.

[0012] [3]HE Jinghan, CHEN Kero, LI Meng, et al. Review ofprotection and faulthandling for a flexible DC grid[J]. Protection and Control of Modern PowerSystems, 2020, 5(2): 151-165.

[0013] [4] Nian Heng, Kong Liang. A review of research on fault protection technology for DC microgrids [J]. High Voltage Engineering, 2020, 46(7): 2241-2254.

[0014] [5] Xue Shimin, Qi Jinlong, Liu Chong. A review of DC microgrid protection [J]. Proceedings of the CSEE, 2021, 37(6): 18-34.

[0015] [6]Jackson JJ, Mwasilu F, Lee J, et al.AC-microgrids versus DC-microgrids with distributed energy resources: Areview[J].Renewable andSustainable Energy Review, 2013, 24: 387-405.

[0016] [7]Nayak G,Nath S.Effect of power electronic protections of inverterson protection of micro-grids[C].2016IEEE 6th Int.Conf.on Power Systems(ICPS),2016:1-6.

[0017] [8] Zhang Zhongfeng, Xie Yeyuan, Yang Chen, et al. Method for bipolar short-circuit fault ride-through of DC transformers in distribution networks [J]. Electric Power Engineering Technology, 2019, 38(4): 2-9.

[0018] [9] Zhang Feng, Wu Jiang, Tang Shasha, Huang Wangchunzi, Zheng Yong. Fault characteristics analysis of DC distribution network based on novel flexible current limiter [J / OL]. Journal of Electric Power System and Automation: 1-11. [2021-10-15].

[0019] https: / / doi.org / 10.19635 / j.cnki.csu-epsa.000816. Summary of the Invention

[0020] In practical engineering applications, the presence of line inductance causes current limiters to exhibit large peak values ​​and periodic oscillations during fault current. This invention proposes a method to suppress fault oscillations in low-voltage DC Buck-type active current limiters operating under equivalent short-circuit resistance-inductive conditions. This invention considers a load-dissipating circuit for the inductor energy at the fault point, which limits the peak current in the current limiter inductor and reduces periodic oscillations in the current-limiting inductor. The technical solution is as follows:

[0021] A method for suppressing fault oscillations in a low-voltage DC Buck-type active current limiter, wherein the low-voltage DC Buck-type active current limiter includes a current limiter output capacitor, characterized in that a power dissipation branch is connected in parallel with the current limiter output capacitor, the power dissipation branch including an anti-parallel power diode and a load-dissipating resistor connected in series.

[0022] This invention considers the unloading circuit of inductor energy at the fault point, which can effectively limit the current peak of the current limiter and almost eliminate the periodic oscillation on the current limiting inductor, thus better ensuring the safety of the equipment. Attached Figure Description

[0023] Figure 1 Buck active current limiter structure, (a) and (b) are two-level Buck active current limiter structure and three-level Buck active current limiter structure, respectively.

[0024] Figure 2 Two-level Buck type active current limiter structure

[0025] Figure 3 Equivalent circuit after current limiter fault blocking

[0026] Figure 4 Simulation structure of a single Buck active current limiter system

[0027] Figure 5 Fault characteristics of the current limiter under resistive-inductive DC faults: (a) and (b) show the port voltage and current-limiting inductor current of the current limiter under resistive-inductive DC faults, respectively.

[0028] Figure 6 Improved Structure of Two-Level Buck Active Current Limiter

[0029] Figure 7 Improved structure of three-level Buck type active current limiter

[0030] Figure 8 Fault characteristics of an improved current limiter with an equivalent short-circuit impedance exhibiting resistivity-inductance. (a) and (b) show the port voltage and current-limiting inductor current of the improved current limiter with an equivalent short-circuit impedance exhibiting resistivity-inductance, respectively. Detailed Implementation

[0031] The present invention will now be described in conjunction with the accompanying drawings and embodiments.

[0032] 1. Problem Statement

[0033] A low-voltage DC Buck-type active current limiter for use in low-voltage DC applications, such as Figure 1 As shown, where Figure 1 (a) is a two-level structure. Figure 1 (b) is a three-level structure. Considering the presence of line inductance in actual engineering, the fault characteristics may exhibit inductive or purely inductive characteristics. In this case, the fault current characteristics of the current limiter differ significantly from those of a purely resistive fault. Figure 2 The oscillation process is illustrated using a two-level current limiter as an example.

[0034] In the initial state, i L (0)>0, diode D conducts, and at this time the diode can be regarded as a wire. Figure 2 It can be equivalent to a circuit. Figure 3 According to KVL and KCL, the system's operating state can be described as follows:

[0035]

[0036] The information obtained from i L The third-order homogeneous differential equation with positive coefficients:

[0037]

[0038] The characteristic equation corresponding to the homogeneous equation is:

[0039]

[0040] The characteristic equation corresponding to equation (3) is:

[0041] r 3 +ar 2 +br+c=0 (4)

[0042] in:

[0043]

[0044] Solving equation (4) yields three characteristic roots:

[0045]

[0046] in:

[0047]

[0048] Considering the oscillation phenomenon of the inductor current, the system is in an underdamped state. The characteristic root r1 of equation (4) is a negative real root, and r2 and r3 are a pair of conjugate complex roots.

[0049] Let the characteristic roots r1, r2, and r3 be:

[0050]

[0051] The expression for inductor current is:

[0052] i L (t)=c1e γt +e αt (c2cosβt+c3sinβt) (9)

[0053] Let the initial current of the current-limiting inductor be I. L0 The initial voltage of the capacitor is U. C0 The initial current of the faulty inductor is I. f0 The undetermined coefficients can be determined as follows:

[0054]

[0055] To verify the correctness of the theoretical analysis of the oscillation process of the equivalent short-circuit impedance exhibiting a lower current-limiting inductor characteristic, a three-level DC resistive-inductive fault simulation structure was built in Matlab / Simulink for simulation. The simulation process was verified when line inductance was present. The simulation system structure diagram is shown below. Figure 4 As shown in Table 1. The current limiter is connected to a 750V DC power source on the high-voltage side and a 20kW DC load on the low-voltage side. The system parameters are shown in Table 1, and the fault resistance R... f The fault inductance is 0.1Ω, and the fault inductance L is... f It is 20uH.

[0056] Table 1 Current Limiter Parameters

[0057]

[0058] Assuming a DC resistive-inductive short circuit occurs at t=1s on the output side of the current limiter, the simulation results are as follows: Figure 5 As shown in the figure. From the fault characteristics under the inductive DC fault at this time, it can be found that under the action of the capacitor reverse voltage, the peak value of the current-limiting inductor under the inductive DC fault reaches 130A, and there is a large oscillation process, which threatens the safety of the equipment.

[0059] 2. Methods for suppressing fault oscillations in current limiters with equivalent short-circuit impedance exhibiting resistivity-inductance characteristics.

[0060] As the above analysis shows, when the equivalent short-circuit impedance is resistive-inductive, due to the presence of inductance at the fault location, the capacitor voltage will become negative at the moment of the fault due to the freewheeling current of the inductor. After the current limiter is locked out, the negative capacitor voltage will continue to rise under the action of the diode circuit, threatening equipment safety. In order to limit the voltage on the inductor L after the current limiter is locked out, it is necessary to limit the magnitude of the negative voltage on the output capacitor C, that is, to limit the freewheeling current of the inductor at the fault location.

[0061] In practical engineering applications, the presence of line inductance causes the fault current of the current limiter to exhibit large peak values ​​and periodic oscillation characteristics. This invention proposes a method for suppressing fault oscillations in a low-voltage DC Buck-type active current limiter. This invention employs an anti-parallel power diode D... d With a load-relief resistor R d The circuit is connected in series to form a power-dissipating branch, which is then connected in parallel to the current limiter's output capacitor C. The improved Bcuk-type active current limiter structure is as follows: Figure 6 and Figure 7 As shown, the dashed box represents the added energy-consuming branch.

[0062] For the improved active current limiter, when it is operating normally, the voltage u on the output capacitor C is... o When the current is positive, it is opposite to the conduction direction of the diode, and the diode is not conducting. When a fault occurs and the current limiter is blocked, the unloading resistor R... d Faster consumption of inductor L at the fault location f The energy in the limiting capacitor C is used to limit the negative voltage on the output capacitor C and the current in the current-limiting inductor L to continue to increase and oscillate.

[0063] 3. Simulation Verification of Fault Oscillation Suppression Method under Equivalent Short-Circuit Impedance of Resistive-Inductive Nature

[0064] Assuming a severe metallic short circuit occurs at the current limiter outlet side at t=1s, the short circuit point parameter L... f =20uH, R f=0.1Ω; unloading circuit parameter R d =0.2Ω, other simulation parameters are the same as in Table 1; fault blocking detection current is 50A, and the current limiter output capacitor voltage and current limiting inductor current before and after the improvement are as follows: Figure 8 (a) and Figure 8 As shown in (b).

[0065] Depend on Figure 8 It can be seen that after the fault occurred, the actual current reached 60A, triggering the current limiter to lock out. It can also be seen that the peak negative voltage of the improved current limiter's output capacitor is approximately 75A, compared to... Figure 5 The peak current of 130A is greatly reduced. After the current limiter fails, the current can be reduced rapidly after the lockout and the drop process is faster. The oscillation process almost disappears.

[0066] The above are merely embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structures or processes made using the contents of the present invention specification and drawings are similarly included within the patent protection scope of the present invention.

Claims

1. A method for suppressing fault oscillations in a low-voltage DC Buck-type active current limiter, wherein the low-voltage DC Buck-type active current limiter includes a current limiter output capacitor, characterized in that, A power dissipation branch is connected in parallel with the current limiter's output capacitor. The power dissipation branch includes an anti-parallel power diode and a load-relief resistor connected in series. Specifically, an anti-parallel power diode Dd and a load-relief resistor Rd are connected in series to form a power dissipation branch, which is then connected in parallel with the current limiter's output capacitor C to obtain an improved active current limiter. When operating normally, the voltage on the output capacitor C is positive, which is opposite to the conduction direction of the diode. At this time, the diode is not conducting. When a fault occurs and the current limiter is locked out, the energy in the inductor at the fault location is consumed through the unloading resistor Rd, thereby limiting the negative voltage on the output capacitor C and the process of the current in the current-limiting inductor L continuing to increase and oscillate.

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