Low voltage direct current distribution grounding protection system and method

Abstract

The embodiment of the present application provides a low-voltage direct-current power distribution grounding protection system and method, to solve the technical problem that the existing low-voltage direct-current power distribution grounding protection method requires the electrical equipment to quickly cut off the power supply, and the power supply continuity is poor. The system comprises: a medium-voltage power grid, a transformer, a line impedance, an active front-end converter, a filter, a grounding capacitor, a direct-current circuit breaker and a direct-current load; the medium-voltage power grid is stepped down by the transformer to obtain 380V / 50Hz three-phase alternating voltage, and is connected to the active front-end converter through the filter; the voltage of the direct-current bus is adjusted to the rated value by controlling the on-off of the semiconductor switch; the direct-current bus is constructed to supply power to the direct-current load; the fault type of the direct-current bus is determined, and corresponding protection is taken. The grounding fault current is small, the metal shell of the electrical equipment will not produce dangerous contact voltage, and the load can be continuously powered. In the case of uninterrupted power supply, the alarm is given, the fault is repaired in a planned manner, and the power supply continuity is improved.

Description

Technical Field

[0001] This application relates to the field of low-voltage DC power distribution technology, and in particular to a low-voltage DC power distribution grounding protection system and method. Background Technology

[0002] Currently, distributed renewable energy generation and energy storage are being widely integrated into the user side. The proportion of DC loads is showing a significant upward trend, such as data centers and LED lighting. Residential buildings, as an important component of the user side, have enormous potential for renewable energy production and consumption. Currently, the vast majority of residential buildings use AC power distribution systems, requiring DC-AC inverter stages to integrate photovoltaic and energy storage systems, and AC-DC rectification stages to power DC loads.

[0003] Compared to low-voltage AC power distribution, low-voltage DC power distribution systems offer significant advantages in several aspects. Specifically: distributed generation and consumption can be completed within the DC system, reducing the need for AC-DC power electronic converters and improving power conversion efficiency and system reliability. Considering the skin effect, low-voltage DC power distribution cables have higher power transmission capacity compared to AC systems. DC systems do not have the concepts of frequency and reactive power; equipment connection does not require synchronization, and there is no need to consider the negative-sequence and zero-sequence components generated by unbalanced loads in AC systems, significantly simplifying system control strategies. Based on these advantages of low-voltage DC power distribution, constructing DC buses in civil buildings to connect new energy sources, energy storage, and DC loads can significantly improve system efficiency, scalability, and stability. In power distribution systems, grounding design and fault protection are crucial for reliable power supply, protecting users, various equipment, converters, and lines, preventing fault propagation, and avoiding or reducing losses.

[0004] For equipment-level active front-end rectifiers, their DC buses are typically internal nodes within the equipment casing, resulting in a very low probability of ground faults. Generally, no special grounding scheme or protection measures are required. Conversely, the DC output of active front-end converters used in DC distribution systems needs to serve as a bus covering the power consumption area, supplying power to multiple devices, and must accommodate frequent connection and disconnection of load equipment. Therefore, the probability of short circuits between DC buses and short circuits to ground increases significantly, necessitating the design of reasonable grounding methods and protection strategies to ensure safe and reliable power supply. However, low-voltage DC power distribution technology in civil buildings is still in its early stages of development, and standards and specifications related to grounding design and fault protection are not yet fully developed. In the published invention CN106253319A, "A Grounding Circuit for a Low-Voltage DC Distribution Network Based on Flexible DC Networking Technology," a scheme is disclosed where the AC side filter of the active front-end converter is directly grounded, while the DC side is not grounded. This scheme belongs to the TT grounding system, and its disadvantages include large fault currents, requiring rapid power disconnection for electrical equipment, and poor power supply continuity. Summary of the Invention

[0005] This application provides a low-voltage DC power distribution grounding protection system and method to solve the technical problem that existing low-voltage DC power distribution grounding protection methods require electrical equipment to be quickly disconnected from the power supply, resulting in poor power supply continuity.

[0006] This application provides a low-voltage DC power distribution grounding protection system, the system comprising: a medium-voltage power grid, a transformer, a line impedance, an active front-end converter, a filter, a grounding capacitor, a DC circuit breaker, and a DC load; wherein, the first end of the transformer is connected to the medium-voltage power grid, and the second end of the transformer is connected to the line impedance; the first end of the filter is connected to the line impedance, and the second end of the filter is connected to the active front-end converter; the first end of the DC circuit breaker is connected to the active front-end converter, and the second end of the DC circuit breaker is connected to the DC load.

[0007] This application embodiment also provides a low-voltage DC power distribution grounding protection method, the method comprising: stepping down the medium-voltage power grid through a transformer to obtain a 380V / 50Hz three-phase AC voltage, and connecting it to an active front-end converter through a filter; wherein the active front-end converter is used to adjust the voltage of the DC bus to a rated value by controlling the on / off state of semiconductor switches; constructing a DC bus to supply power to the DC load; determining the fault type of the DC bus and taking corresponding protection measures; wherein the fault type includes: DC bus short-circuit fault and DC bus positive and negative short-circuit to ground.

[0008] In one implementation of this application, when the fault type of the DC bus is a DC bus short circuit, the protection measures are as follows: determining the magnitude of the short-circuit resistance; when the short-circuit resistance is greater than a preset resistance threshold, the active front-end converter maintains the voltage of the DC bus at its rated value; when the short-circuit resistance is less than or equal to the preset resistance threshold, the DC circuit breaker trips to isolate the DC bus from the fault point.

[0009] In one implementation of this application, when the fault type of the DC bus is a short circuit between the positive and negative DC bus and ground, the protection measures are as follows: the DC circuit breaker trips to isolate the DC bus from the fault point; the voltage between the metal casing of the electrical equipment and ground is controlled within a preset safe voltage range for the human body, and a fault alarm is triggered.

[0010] The low-voltage DC power distribution grounding protection system and method provided in this application have the following advantages: different protection strategies are determined for different fault types; the grounding fault current is small; the metal casing of electrical equipment will not generate dangerous contact voltage; and power supply to the load can be continuous. An alarm is triggered without power interruption, allowing for planned fault repair and improving power supply continuity. Attached Figure Description

[0011] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0012] Figure 1 A diagram illustrating the composition of a low-voltage DC power distribution grounding protection system provided in this application embodiment;

[0013] Figure 2 A flowchart of a low-voltage DC power distribution grounding protection method provided in this application embodiment;

[0014] Figure 3 A schematic diagram of a DC bus short-circuit fault provided in an embodiment of this application;

[0015] Figure 4 A schematic diagram of a DC bus positive and negative short-circuit fault to ground provided in an embodiment of this application;

[0016] Figure 5 The DC-side equivalent circuit and current loop diagram under a short-circuit fault to ground are provided in the embodiments of this application;

[0017] Figure 6 The simulation waveform diagram of the key components under a short-circuit fault to ground at bus L+ provided in the embodiments of this application is shown. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] This application provides a low-voltage DC power distribution grounding protection system and method to solve the technical problem of poor power supply continuity caused by the requirement for rapid power disconnection of electrical equipment in existing low-voltage DC power distribution grounding protection methods. This application proposes a design scheme based on capacitor grounding at the DC bus outlet and fault protection technology, which can achieve DC bus-to-ground short-circuit protection and human electric shock protection, thus improving the system's power supply continuity.

[0020] The technical solutions proposed in the embodiments of this application will be described in detail below with reference to the accompanying drawings.

[0021] Figure 1 This is a diagram illustrating the composition of a low-voltage DC power distribution grounding protection system, provided as an embodiment of this application. Figure 1As shown, the system mainly includes: a medium-voltage power grid, a transformer, a line impedance, an active front-end converter, a filter, a grounding capacitor, a DC circuit breaker, and a DC load; wherein, the first end of the transformer is connected to the medium-voltage power grid, and the second end of the transformer is connected to the line impedance; the first end of the filter is connected to the line impedance, and the second end of the filter is connected to the active front-end converter; the first end of the DC circuit breaker is connected to the active front-end converter, and the second end of the DC circuit breaker is connected to the DC load.

[0022] Specifically, the neutral point on the low-voltage side of the AC distribution transformer is not grounded. The DC bus L+ and L- are grounded through Y capacitors Y1 and Y2 with equal capacitance. To limit the duration of leakage current during grounding faults, the capacitance of the grounding capacitors is typically selected in the μF range. After the DC bus output port and the grounding point, circuit breakers K1 and K2 with residual current monitoring / protection functions are configured. When the vector sum of the currents flowing through circuit breakers K1 and K2 exceeds the corresponding threshold value, the circuit breakers take corresponding protective actions. The metal casing on the load side is directly grounded. When the power supply system is in a fault-free state and enters a steady state, the voltages across grounding capacitors Y1 and Y2 are both half of their rated values.

[0023] The above describes a low-voltage DC power distribution grounding protection system provided by embodiments of this application. Based on the same inventive concept, embodiments of this application also provide a low-voltage DC power distribution grounding protection method. Figure 2 A low-voltage DC power distribution grounding protection method provided in the embodiments of this application, such as Figure 2 As shown, the method mainly includes the following steps:

[0024] Step 201: After the medium-voltage power grid is stepped down by a transformer, a 380V / 50Hz three-phase AC voltage is obtained, which is then connected to an active front-end converter via a filter.

[0025] The active front-end converter is used to regulate the voltage of the DC bus to a rated value by controlling the on / off state of the semiconductor switch.

[0026] Step 202: Construct a DC bus to supply power to the DC load.

[0027] Step 203: Determine the type of fault that occurred on the DC bus and take corresponding protection measures.

[0028] The fault types include: DC bus short circuit fault and DC bus positive and negative short circuit to ground.

[0029] In this embodiment of the application, when the fault type is a DC bus short circuit, such as Figure 3As shown, the protection measures are as follows: The short-circuit resistance is determined. When the short-circuit resistance is greater than a preset resistance threshold, the active front-end converter maintains the DC bus voltage at its rated value. That is, when the short-circuit current plus the load current is less than the maximum output current of the active front-end converter, the active front-end converter maintains the DC bus voltage at its rated value, providing the system with fault ride-through capability and ensuring the continuity of power supply. When the short-circuit resistance is less than or equal to the preset resistance threshold, that is, when the short-circuit current plus the load current is greater than the operating threshold of the DC circuit breaker, the DC circuit breaker trips, isolating the DC bus from the fault point.

[0030] In this embodiment of the application, when the fault type is a DC bus positive and negative short circuit to ground, such as Figure 4 As shown, this specifically includes short-circuiting the DC bus L+ to the metal casing of the electrical equipment and short-circuiting the DC bus L- to the metal casing of the electrical equipment. The protection measures are as follows: the DC circuit breaker trips, isolating the DC bus from the fault point to prevent electric shock; the voltage of the electrical equipment's metal casing to ground is controlled within a preset safe voltage range for humans, and a fault alarm is triggered. At this time, the short-circuit current is extremely small and does not affect the normal operation of the system.

[0031] Specifically Figure 5 The equivalent circuit on the DC side and the current loop under a short-circuit fault to ground are described. Figure 4 Further explanation: When a short-circuit fault to ground occurs at bus L+, the short-circuit current i k1 Discharge is applied along busbar L+, the short-circuit resistance to ground, and the ground-to-ground capacitance Y1. The discharge time and peak current are mainly determined by the short-circuit resistance and the capacitance of the grounding capacitor. Similarly, the short-circuit current i k2 The DC power supply, bus L+, short-circuit resistance to ground, and grounding capacitor Y2 are charged. The charging time and peak current are mainly determined by the short-circuit resistance and the capacitance of the grounding capacitor. This charging and discharging process causes the grounding capacitor Y1 to discharge rapidly, eventually reducing its voltage to zero. Correspondingly, the voltage of bus L+ relative to ground drops to zero, and the voltage of the metal casing of the electrical equipment relative to ground also becomes zero, falling within the safe voltage range for humans and posing no risk of electric shock. Similarly, after the grounding capacitor Y2 reaches a steady state, its voltage relative to ground is the negative rated voltage. During the charging and discharging process of the grounding capacitor, due to the leakage current i... k1 and i k2 The existence of , the vector sum of the currents flowing through DC circuit breakers K1 and K2 is not zero, that is;

[0032] I Δ =i L +i k1 -i L +i k2 =i k1 +i k2≠0;

[0033] Among them, I Δ For the residual current of the DC power distribution system, i L Let i be the load current. k1 and i k2 This is the leakage current flowing through the ground. The residual current I can be utilized. Δ This serves as a fault alarm trigger signal. On the other hand, after the grounding capacitor Y1 has discharged, the bus L+ is clamped to ground potential, and no short-circuit current is generated. Therefore, the power supply system can operate normally, ensuring the continuity of power supply. This is in conjunction with the residual current I... Δ The triggered fault alarm signal enables planned maintenance of the power distribution system.

[0034] In the embodiments of this application, Figure 6 Simulation waveforms of key components under a short-circuit fault to ground at bus L+ are presented. From top to bottom, they represent the bus voltage, grounding capacitor voltage, current flowing through the DC circuit breaker, and the ground short-circuit alarm signal, described in detail below. At 0.35s, a short-circuit fault to ground occurs at bus L+. Grounding capacitor Y1 discharges rapidly, while grounding capacitor Y2 charges rapidly. Upon reaching steady state, the voltage across capacitor Y1 is zero, and the voltage across capacitor Y2 is the negative rated voltage (-800V). During charging and discharging, the current flowing through DC circuit breaker K1 is not equal to the current flowing through DC circuit breaker K2, meaning the residual current is not zero. This residual current protection detection device identifies the current and triggers the ground short-circuit alarm signal. Simulation results show that the output voltage remains at the rated value (800V), providing normal power to the load. In the simulation, the short-circuit resistance is set to 1kΩ (simulating human body resistance), and the grounding capacitors Y1 = Y2 = 0.55μF. The simulation waveform shows that the short-circuit current is pulsed, with a peak value of approximately 400mA and a duration of approximately 2.7ms. According to standard IEC 60479-1, "Effects of current on human beings and livestock," the aforementioned current envelope falls within region 2, generally not causing harmful physiological effects on the human body. The simulation results demonstrate that the low-voltage DC power distribution grounding system and protection method disclosed in this application exhibits a small ground fault current, prevents dangerous contact voltages on the metal casing of electrical equipment, and allows for continuous power supply to the load, thus improving the system's power supply continuity.

[0035] The various embodiments in this application are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the device embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions of the method embodiments.

[0036] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0037] The above description is merely an embodiment of this application and is not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

Claims

1. A method for grounding protection of low-voltage DC power distribution, employing a low-voltage DC power distribution grounding protection system, the system comprising: The system comprises a medium-voltage power grid, a transformer, a line impedance, an active front-end converter, a filter, a grounding capacitor, DC circuit breakers K1 and K2, and a DC load; wherein, the first end of the transformer is connected to the medium-voltage power grid, and the second end of the transformer is connected to the line impedance; the first end of the filter is connected to the line impedance, and the second end of the filter is connected to the active front-end converter; the first ends of DC circuit breakers K1 and K2 are connected to the active front-end converter, and the second ends of DC circuit breakers K1 and K2 are connected to the DC load; the low-voltage side neutral point of the AC distribution transformer is not grounded, and the metal casing on the load side is directly grounded; the method is characterized by comprising: The medium-voltage power grid is stepped down by a transformer to obtain a 380V / 50Hz three-phase AC voltage, which is then connected to an active front-end converter via a filter. The active front-end converter is used to regulate the DC bus voltage to the rated value by controlling the on / off state of semiconductor switches. A DC bus is constructed to supply power to the DC load. The DC bus L+ and DC bus L- are grounded through grounding capacitors Y1 and Y2 with equal capacitance values, respectively. The capacitance values ​​of the grounding capacitors are in the μF range. DC circuit breakers K1 and K2 with residual current monitoring / protection functions are configured after the DC bus output port and the grounding point, respectively. Determine the type of fault occurring on the DC bus and take corresponding protection measures; wherein, the fault type includes: DC bus short circuit fault and DC bus positive and negative short circuit to ground; When a short-circuit fault to ground occurs on the DC bus L+, the residual current vector sum flowing through the DC circuit breakers K1 and K2 is detected based on the charging and discharging process of the grounding capacitors Y1 and Y2. When the residual current vector sum is not zero, a fault alarm is triggered. After the grounding capacitor Y1 has discharged, the bus L+ is clamped to ground potential, and no short-circuit current is generated, so the power supply system can operate normally to ensure the continuity of power supply. After the fault alarm, maintenance is arranged according to the plan.

2. The low-voltage DC power distribution grounding protection method according to claim 1, characterized in that, When the fault type of the DC bus is a DC bus short circuit, the protection measures taken are as follows: The magnitude of the short-circuit resistance is determined, and when the short-circuit resistance is greater than a preset resistance threshold, the active front-end converter maintains the voltage of the DC bus at the rated value. When the short-circuit resistance is less than or equal to a preset resistance threshold, DC circuit breakers K1 and K2 trip, isolating the DC bus from the fault point.

3. The low-voltage DC power distribution grounding protection method according to claim 1, characterized in that, When the fault type of the DC bus is a short circuit to ground between the positive and negative terminals of the DC bus, the protection measures taken are as follows: DC circuit breakers K1 and K2 tripped, isolating the DC bus from the fault point; The voltage to ground of the metal casing of the DC load is controlled within the preset safe voltage range for the human body, and a fault alarm is triggered.

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