Anti-reflux ac input bus soft start circuit

Abstract

This invention discloses a simple, small-sized, low-cost AC input bus soft-start circuit with fault reporting capability, which is designed to prevent backflow. It includes a control system, a grid-side circuit breaker, a DC auxiliary power supply, a bus capacitor, first and second rectifier bridges, an isolation auxiliary power supply, a bus soft-start relay, a grid-side input relay, and a rectifier circuit. When connected to the grid, the bus soft-start relay and the grid-side input relay are disconnected, and the isolation auxiliary power supply supplies power to the control system via the DC auxiliary power supply. After the control system is powered on, it controls the bus soft-start relay to close and charges the bus capacitor. When the bus voltage is stable, the bus soft-start relay remains closed to drive the DC auxiliary power supply, determining whether the AC input bus soft-start circuit meets the soft-start conditions. If it does, the control system disconnects the bus soft-start relay and closes the grid-side input relay; otherwise, it reports an AC bus soft-start fault. When the control system is powered off, the isolation auxiliary power supply isolates the bus capacitor from the AC input circuit.

Description

Technical Field

[0001] This invention relates to the field of uninterruptible power supply technology, and in particular to a soft-start circuit for AC input bus that prevents backflow. Background Technology

[0002] Data centers host massive amounts of critical business data and numerous important applications, placing near-stringent demands on the continuity and stability of their power supply. Uninterruptible power supply (UPS) systems, due to their flexible expansion capabilities, ability to meet diverse needs, ease of maintenance, and high system stability, are widely used in data center power supply systems. The AC input circuits of UPS systems employ rectification and filtering circuits. At the moment the grid-side circuit breaker closes, the filter capacitors (usually electrolytic capacitors with relatively large capacities) are fully discharged, effectively at zero voltage. Based on the characteristic that capacitor voltage cannot change abruptly, the capacitors immediately absorb a large current to charge, generating a significant instantaneous closing current. This can damage components in the input circuit (such as fuses, semiconductor devices, PCBs, and bus capacitors). Therefore, a bus soft-start circuit is needed to reduce the instantaneous closing current.

[0003] There are three types of bus soft-start circuits in conventional uninterruptible power supplies (UPS). The first type uses an NTC (Negative Temperature Coefficient) resistor. Adding an NTC to the bus soft-start circuit suppresses the starting inrush current, utilizing its negative temperature characteristic to achieve the bus soft-start function. This solution is only suitable for scenarios with small bus capacitance. As UPS power increases, the capacitance of the bus also becomes relatively large, making the NTC method unusable. The second type uses a current-limiting resistor connected in parallel with a relay to achieve bus soft-start, such as... Figure 1 As shown, the current-limiting resistor and the normally open relay are connected in parallel and then in series in the bus charging circuit. The grid charges the bus through the rectifier bridge and the current-limiting resistor. After the bus capacitor soft-start is completed, the current-limiting resistor is short-circuited by closing the normally open relay. This scheme requires a high-power relay, which is costly, and the power consumption of the current-limiting resistor is large at the moment of closing, resulting in low reliability and short resistor life. Thirdly, a normally closed relay, a current-limiting resistor, and a rectifier bridge are used to achieve soft starting of the bus, such as... Figure 2As shown, when the grid circuit breaker is closed, the power grid charges the bus capacitor through the auxiliary power source normally closed relay via the grid-side circuit breaker and the rectifier bridge. When the bus voltage reaches the auxiliary power source's starting voltage, the DC auxiliary power source starts operating. The control system monitors the bus voltage and, when it reaches a set pre-threshold, closes the grid-side input relay and opens the auxiliary power source normally closed relay. The grid then charges the bus capacitor through the inverter circuit, completing the soft-start process of the AC bus. This scheme requires additional circuitry to complete the bus soft-start, increasing the complexity and cost of the control system. All the above bus soft-start circuits share a common drawback: North American and European safety certifications explicitly state that under specific power failure conditions, bus soft-start circuits using normally closed relays and current-limiting resistors (diodes are not considered safety devices in safety regulations) can cause high-voltage transfer to the AC side. If personnel are performing maintenance on the AC side, this could lead to dangerous situations endangering personal safety. Therefore, AC bus soft-start circuits must ensure the safety of maintenance personnel.

[0004] Chinese invention patent (CN117811338A) discloses a power-on buffer automatic bypass circuit, which enables soft starting of the busbar and also has anti-reverse current function to solve the safety risks caused by the aforementioned reverse current. Please refer to [link / reference]. Figure 3 The circuit uses a buffer resistor R1 to limit the inrush current at power-on, preventing damage to the bus capacitor EC1 due to high-current charging. Simultaneously, the starting capacitor CT and power device T1, along with voltage divider and current-limiting resistors RH1, RH2, and RH3, allow the bus voltage to rise slowly, achieving a smooth bus start-up. After the bus starts, relay K1 bypasses soft-start components such as the buffer resistor R1, reducing power consumption during normal operation. If the control system loses power, the circuit loses its control signal, but the reverse-current protection diode DT remains in reverse cutoff, directly cutting off the path for reverse current from the bus to the input side. Furthermore, the parallel structure of the bus capacitor and the series circuit slowly dissipates the residual bus voltage, further reducing the risk of reverse current leakage, eliminating the need for additional residual voltage discharge control logic. However, the reverse-current protection diode, buffer resistor, and other components in this circuit are limited by rated power and withstand voltage. In high-power bus systems, high-specification components must be selected, leading to increased component size and cost. When abnormal situations such as reverse-current protection diode breakdown or relay failure occur, potential circuit hazards may be difficult to detect, potentially causing the fault to escalate and even affecting the normal operation of the entire bus system. Summary of the Invention

[0005] Therefore, it is necessary to provide a simple, small, low-cost AC input bus soft-start circuit that can report faults and prevent backflow, addressing the above shortcomings.

[0006] A soft-start circuit for preventing backflow in AC input busbars includes a control system, a grid-side circuit breaker for grid connection, and a control circuit. The control circuit includes a DC auxiliary power supply, a busbar capacitor, and a first circuit, a second circuit, and a third circuit connected in parallel. The first circuit includes a first rectifier bridge connected to the grid-side circuit breaker and an isolated auxiliary power supply connected in series with the first rectifier bridge. The isolated auxiliary power supply is connected to the DC auxiliary power supply, which is connected to the control system. The second circuit includes a busbar soft-start relay connected to the grid-side circuit breaker and a second rectifier bridge connected in series with the busbar soft-start relay. The third circuit includes a grid-side input relay connected to the grid-side circuit breaker and a rectifier circuit connected in series with the grid-side input relay. One end of the busbar capacitor is connected to the second rectifier bridge and the rectifier circuit, and the other end of the busbar capacitor is connected to the connection line between the isolated auxiliary power supply and the DC auxiliary power supply. When the grid-side circuit breaker is closed, the grid is connected, the bus soft-start relay and the grid-side input relay are disconnected, and the control system is powered through the isolation auxiliary power supply and the DC auxiliary power supply. After the control system is powered on, it controls the bus soft-start relay to close and charges the bus capacitor. When the bus voltage is stable, the control system keeps the bus soft-start relay closed to drive the DC auxiliary power supply and determines whether the AC input bus soft-start circuit meets the soft-start conditions. If it does, the control system disconnects the bus soft-start relay and closes the grid-side input relay; otherwise, it reports an AC bus soft-start fault. When the control system is powered off, the isolation auxiliary power supply isolates the bus capacitor from the AC input circuit.

[0007] In one embodiment, the first circuit further includes an AC auxiliary power relay disposed between the grid-side circuit breaker and the first rectifier bridge; when the grid is connected, the AC auxiliary power relay is closed; when the bus voltage is stable, the control system disconnects the AC auxiliary power relay and keeps the bus soft start relay closed; when the AC input bus soft start circuit meets the soft start conditions, the control system disconnects the AC auxiliary power relay and the bus soft start relay, and closes the grid-side input relay.

[0008] In one embodiment, the AC auxiliary power relay is a normally closed relay, and the bus soft start relay and the grid-side input relay are normally open relays.

[0009] In one embodiment, when the bus capacitor drives the DC auxiliary power supply and the AC input bus soft start circuit meets the soft start conditions, the control system keeps the AC auxiliary power relay open. After the bus soft start relay is opened, the grid-side input relay is closed after a delay. When an AC bus soft start fault is reported, the AC auxiliary power relay is opened. If the AC input bus soft start circuit does not meet the soft start conditions, the bus soft start relay remains closed for a preset time and then reports the AC bus soft start fault to the control system.

[0010] In one embodiment, the AC input bus soft-start circuit further includes a current-limiting resistor connected in series between the AC auxiliary power relay and the first rectifier bridge.

[0011] In one embodiment, the soft-start conditions are that none of the devices are over-temperature, none of the devices are overloaded, none of the relays are faulty, and the peak voltage difference between the bus voltage and the grid voltage is within the limit range.

[0012] In one embodiment, a first diode is provided between the connection point of the bus capacitor on the connection line between the isolated auxiliary power supply and the DC auxiliary power supply and the isolated auxiliary power supply. The anode of the first diode is connected to the isolated auxiliary power supply, and the cathode of the first diode is connected to the DC auxiliary power supply.

[0013] In one embodiment, a second diode is connected in series between the bus capacitor and the access point, with the anode of the second diode connected to the positive terminal of the bus capacitor and the cathode of the second diode connected to the access point.

[0014] In one embodiment, a soft-start resistor is connected in series between the second rectifier bridge and the positive terminal of the bus capacitor.

[0015] In one embodiment, the isolation auxiliary power supply is a forward power supply or a flyback power supply.

[0016] The AC input bus soft-start circuit for backflow prevention of this invention, by adding an isolation auxiliary power supply and optimizing the circuit connection and control method, isolates the bus capacitor from the AC input side circuit when the control system loses power, thereby preventing high-voltage components from flowing back into the AC input side, providing strong protection for the safety of maintenance personnel. Backflow prevention can be achieved simply by adding an isolation auxiliary power supply, requiring fewer components, simplifying the circuit structure, and reducing the overall circuit size and cost. When a fault occurs, the control system promptly reports the fault for timely handling, preventing the fault from escalating and affecting the entire bus system, thus ensuring the normal operation of the bus system. Attached Figure Description

[0017] Figure 1 The circuit diagram for implementing bus soft start using a current-limiting resistor and a relay in parallel; Figure 2 The circuit diagram shows the implementation of bus soft start using normally closed relays, current-limiting resistors, and rectifier bridges. Figure 3 Circuit diagram of the automatic bypass circuit for power-on buffer; Figure 4 This is a circuit diagram of the AC input bus soft start circuit for preventing backflow in one embodiment of the present invention; Figure 5This is a schematic diagram of the current flow direction in the first stage of the AC input bus soft-start circuit for backflow prevention in one embodiment of the present invention. Figure 6 This is a schematic diagram of the current flow direction in the second stage of the AC input bus soft-start circuit for backflow prevention in one embodiment of the present invention. Figure 7 This is a schematic diagram of the current flow direction in the third stage of the AC input bus soft-start circuit for backflow prevention in one embodiment of the present invention. Figure 8 This is a schematic diagram of the current flow direction in the fourth stage of the AC input bus soft-start circuit for backflow prevention in one embodiment of the present invention. Figure 9 This is a logic judgment diagram of the AC input bus soft start circuit for preventing backflow in one embodiment of the present invention; Figure 10 This is a schematic diagram of the current flow direction of the AC input bus soft-start circuit for backflow prevention in one embodiment of the present invention when the control system is powered off. Detailed Implementation

[0018] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0019] In conventional bus soft-start circuits, there is a lack of effective electrical isolation between the bus capacitor and the AC input side. When the control circuit experiences a power outage due to unforeseen factors (such as a single point of failure), the high-voltage components are highly likely to flow backward to the AC input side through some electrical components in the bus soft-start circuit (such as normally closed relays and current-limiting resistors). This backflow poses a significant risk of electric shock to personnel performing system maintenance and repair, directly threatening their personal safety. While the power-on buffer automatic bypass circuit disclosed in invention patent CN117811338A can achieve bus soft-start and has anti-backflow functionality, this circuit requires the use of high-specification components in high-power bus systems, increasing component size and cost. Furthermore, it cannot send fault signals in abnormal situations, posing a significant safety hazard. To address this, the present invention sets up an isolated auxiliary power supply in the circuit to isolate the bus capacitor from the AC input side circuit in the event of a system power failure, thereby preventing high-voltage components from flowing back into the AC input side. This provides strong protection for the safety of maintenance personnel, simplifies the circuit structure, can be matched with high-power scenarios, and can also report bus faults in a timely manner to prevent the fault from escalating.

[0020] For details, please refer to Figure 4 The AC input bus soft-start circuit for backflow prevention in this embodiment includes a control system, a grid-side circuit breaker QF1 for grid connection, and a control circuit. The control circuit includes a DC auxiliary power supply, a bus capacitor, and a first circuit, a second circuit, and a third circuit connected in parallel. The first circuit includes a first rectifier bridge D1 connected to the grid-side circuit breaker QF1 and an isolation auxiliary power supply connected in series with the first rectifier bridge D1. The isolation auxiliary power supply is connected to the DC auxiliary power supply, and the DC auxiliary power supply is connected to the control system. The second circuit includes a bus soft-start relay RLY2 connected to the grid-side circuit breaker QF1 and a second rectifier bridge D2 connected in series with the bus soft-start relay RLY2. The third circuit includes a grid-side input relay RLY3 connected to the grid-side circuit breaker QF1 and a rectifier circuit connected in series with the grid-side input relay RLY3. One end of the bus capacitor is connected to the second rectifier bridge D2 and the rectifier circuit, and the other end of the bus capacitor is connected to the connection line between the isolation auxiliary power supply and the DC auxiliary power supply. In this embodiment, the control circuit is connected to the power grid through the grid-side circuit breaker QF1 on one hand, and to the control system through the DC auxiliary power supply on the other hand. The second and third circuits are connected in parallel and then connected to the bus capacitor to charge the bus capacitor so as to drive the DC auxiliary power supply through the bus capacitor and thus supply power to the control system.

[0021] When the grid-side circuit breaker closes, the grid is connected, and the bus soft-start relay RLY2 and the grid-side input relay RLY3 are disconnected. Power is supplied to the control system via an isolation auxiliary power supply and a DC auxiliary power supply. In other words, when the control system is not powered on, the initial state of the bus soft-start relay RLY2 and the grid-side input relay RLY3 is open. The first circuit remains connected throughout the entire process from grid connection to bus soft-start, so that the control system can be powered on through the first circuit when the grid is connected. After the control system is powered on, it controls the bus soft-start relay RLY2 to close and charge the bus capacitor until the voltage of the bus capacitor stabilizes. In this embodiment, bus voltage stability means that the peak voltage difference between the bus voltage and the grid voltage is within a limited range, and the fluctuation value of the bus capacitor within a continuous time period is within a preset range; for example, the change in the bus capacitor value within 10 seconds does not exceed 0.5V. When the bus voltage is stable, the control system keeps the bus soft-start relay RLY2 closed to drive the DC auxiliary power supply, thereby enabling the DC auxiliary power supply to power the control system. The system then checks whether the AC input bus soft-start circuit meets the soft-start conditions. If it does, the control system disconnects the bus soft-start relay RLY2 and closes the grid-side input relay RLY3 to achieve a soft start on the AC bus. Otherwise (i.e., when the soft-start conditions are not met), it reports an AC bus soft-start fault so that maintenance personnel can promptly troubleshoot the problem. When the control system is powered off, the isolation auxiliary power supply separates the bus capacitor from the AC input circuit to prevent the fault from escalating.

[0022] In this embodiment, the soft-start conditions are that none of the devices are overheating, none of the devices are overloaded, none of the relays are fault-free, and the peak voltage difference between the bus voltage and the grid voltage is within the limit range. Specifically, in this embodiment, each device (i.e., all devices in the AC input bus soft-start circuit except the control system) is equipped with a temperature sensor or temperature sensing element for collecting the device's temperature. Each device and its corresponding temperature sensor or temperature sensing element are connected to the control system signal. In this way, the control system can analyze whether each device has overheating, overload, or fault conditions based on the received temperature, voltage, and current information of each device, and calculate and analyze that the peak voltage difference between the bus voltage and the grid voltage is within the limit range. Before the AC bus is soft-started, it is necessary to ensure that the devices are free from overheating and overload, the relays are fault-free, and the bus voltage conditions are met simultaneously to ensure the safety of the circuit.

[0023] Furthermore, when the bus capacitor drives the DC auxiliary power supply and the AC input bus soft start circuit meets the soft start conditions, the control system keeps the AC auxiliary power relay RLY1 open. After opening the bus soft start relay RLY2, it delays closing the grid-side input relay RLY3. Because there is a slight delay between the relay's electrical disconnect command and the actual separation of the mechanical contacts, if the grid-side input relay RLY3 is closed immediately after opening the bus soft start relay RLY2, the contacts of the bus soft start relay RLY2 may not be fully separated while the grid-side input relay RLY3 is already closed. This could lead to a parallel short circuit between the second and third circuits, generating a large current that could burn out the relay contacts or surrounding components. By delaying the closure of the grid-side input relay RLY3, the control system ensures that the bus soft start relay contacts are fully open before closing the grid-side input relay RLY3. The direct power supply path eliminates short-circuit faults in terms of timing; it also avoids voltage surges caused by the inrush current generated by the instantaneous voltage difference when the grid-side input relay RLY3 closes. Furthermore, it allows the control system to recheck the differential voltage status after the bus soft-start relay RLY2 opens, confirming that it remains within limits before executing the closing action. This prevents erroneous closing due to dynamic voltage drift, ensures the accuracy of the soft-start switching logic, and prevents overcurrent and voltage instability problems caused by excessive differential voltage after closing. It also prevents arcing caused by the closing of the grid-side input relay RLY3 under differential voltage conditions. After the bus soft-start relay RLY2 is disconnected, the bus capacitor provides separate power to the DC auxiliary power source and the control system. A brief delay allows this independent power supply to stabilize before connecting to the grid directly, avoiding fluctuations in the DC auxiliary power source input voltage and instability in the control system power supply due to sudden changes in the power supply circuit during path switching. This prevents command errors and detection failures in the control system due to voltage disturbances, ensuring the continuity and stability of the circuit control logic.

[0024] Furthermore, when reporting an AC bus soft-start fault, the AC auxiliary power relay RLY1 disconnects. If the AC input bus soft-start circuit does not meet the soft-start conditions, the bus soft-start relay RLY2 remains closed for a preset time before reporting the AC bus soft-start fault to the control system. Since the peak voltage difference between the bus voltage and the grid-side voltage does not meet the limit, there is a high probability of temporary anomalies such as instantaneous grid instability and brief bus voltage drift. These anomalies may recover on their own. By uniformly ensuring that the bus soft-start relay RLY2 remains closed for a preset time before reporting the AC bus soft-start fault to the control system, a time threshold is essentially set for fault diagnosis. This filters out instantaneous anomalies, ensuring that only when the state of not meeting the soft-start conditions, such as voltage difference anomalies, persists, is it determined to be a genuine soft-start fault and reported. This avoids invalid fault alarms caused by brief fluctuations and improves the accuracy of fault diagnosis.

[0025] In this embodiment, the grid-side circuit breaker QF1 serves as the main switch for the entire AC input bus soft start circuit. It is the first control node for the grid to connect to the circuit. Through manual or automatic closing, it enables the grid to connect with subsequent circuits, providing initial AC input power for the entire soft start circuit and the isolation auxiliary power supply. It is the basic control device for powering on the circuit.

[0026] The first rectifier bridge D1 converts the AC power input from the grid into DC power, providing a suitable DC input power for the isolation auxiliary power supply. This ensures the normal startup and operation of the isolation auxiliary power supply, serving as its power adapter. The isolation auxiliary power supply is crucial for achieving both backflow prevention and soft-start functionality, possessing both power supply and isolation protection functions. Specifically, during the initial grid connection phase, the isolation auxiliary power supply converts the power input from the first rectifier bridge D1 into suitable high-voltage power, which is then used as the input to the DC auxiliary power supply after being "overridden" with the bus voltage. This provides advance power to the control system, ensuring that the control system can be powered on and operated even when the bus capacitor has no energy, enabling logic control of subsequent relays. When the control system loses power, the isolation auxiliary power supply utilizes its electrical isolation characteristics to electrically isolate the high-voltage power on the bus side from the AC input side. Even when the first circuit is closed, it can prevent high-voltage power from backflowing to the AC side through the first circuit, fundamentally eliminating the risk of electric shock for maintenance personnel. In this way, by adding only this one device, the two core functions of powering the control system and providing backflow prevention isolation are simultaneously achieved, simplifying circuit design and reducing hardware costs. Preferably, the isolation auxiliary power supply is a forward power supply or a flyback power supply. Of course, the isolation auxiliary power supply can also be other commercially available power supplies that can achieve isolation between the input side and the output side.

[0027] In one embodiment, the first circuit further includes an AC auxiliary power relay RLY1 disposed between the grid-side circuit breaker QF1 and the first rectifier bridge D1, i.e., the AC auxiliary power relay RLY1, the bus soft start relay RLY2, and the grid-side input relay RLY3 are connected in parallel. When the grid is connected, the AC auxiliary power relay RLY1 is closed; when the bus voltage is stable, the control system opens the AC auxiliary power relay RLY1 while keeping the bus soft start relay RLY2 closed; when the AC input bus soft start circuit meets the soft start conditions, the control system opens the AC auxiliary power relay RLY1 and the bus soft start relay RLY2, and closes the grid-side input relay RLY3. Preferably, the AC auxiliary power relay RLY1 is a normally closed relay, and the bus soft start relay RLY2 and the grid-side input relay RLY3 are normally open relays. In this embodiment, the AC auxiliary power relay RLY1 is closed in the initial stage of grid connection, and the grid provides an AC input path for the isolated auxiliary power supply through the closed AC auxiliary power relay RLY1, ensuring that the isolated auxiliary power supply is powered on and supplies power to the control system. Once the bus voltage stabilizes, the AC auxiliary power relay RLY1 disconnects, switching the power supply circuit and allowing the bus capacitor to replace the grid as the DC auxiliary power source. When the control system loses power, the AC auxiliary power relay RLY1 automatically resets and closes, working with the isolation auxiliary power supply to isolate the high-voltage side from the AC side. This provides protection without additional control signals, making it one of the fundamental hardware components of power-down protection.

[0028] The AC input bus soft-start circuit also includes a current-limiting resistor R1 connected in series between the AC auxiliary power relay RLY1 and the first rectifier bridge. Before charging, the bus capacitor has no stored energy and is initially nearly short-circuited. If directly powered from the grid through the rectifier circuit, it will generate a very large instantaneous inrush current (surge current). The core function of the current-limiting resistor R2 is to limit the amplitude of this inrush current, keeping the charging current within the circuit's tolerance range. This is crucial for achieving soft-start charging of the bus and is the core manifestation of the soft-start characteristic in the soft-start circuit. Secondly, the current-limiting resistor R2 effectively shares the current load of the second rectifier bridge D2 and the bus soft-start relay RLY2 in the charging path, preventing these devices from burning out, sticking contacts, or breaking down due to the large instantaneous current. This significantly improves the lifespan and reliability of the charging path devices, meeting the circuit's safety design requirements. Furthermore, the current-limiting resistor R2, by limiting the charging current, allows the energy of the bus capacitor to accumulate slowly, enabling the bus voltage to rise steadily from 0V to a stable value, rather than a sudden surge. This provides a stable voltage foundation for disconnecting the AC auxiliary power relay RLY1 after the bus voltage stabilizes, allowing the bus capacitor to supply power to the DC auxiliary power source. It also ensures precise matching of the voltage difference between the bus voltage and the grid-side voltage peak, providing an accurate voltage reference for the voltage difference judgment conditions upon soft start, avoiding errors in voltage difference judgment and false alarms due to sudden voltage changes. The current-limiting resistor R2 only operates during the bus soft start charging phase. Once the bus voltage stabilizes and the soft start conditions are met, the control system disconnects the bus soft start relay RLY2, and the current-limiting resistor R2 is removed from the main circuit path. It is not connected in series in the direct power supply path of the subsequent grid-side input relay RLY3, avoiding energy loss caused by prolonged resistor operation and ensuring power supply efficiency during normal power supply phases. If a fault such as voltage difference or relay sticking occurs during the bus charging stage, the bus soft start relay RLY2 will remain energized. At this time, the current limiting resistor R2 will continuously limit the current in the circuit, preventing the charging current from being too large under fault conditions, preventing the circuit from entering a vicious overcurrent state, buying time for fault reporting and circuit protection, and helping to improve the circuit's abnormal operating condition protection capability.

[0029] A first diode is installed between the connection point of the bus capacitor on the connection line between the isolated auxiliary power supply and the DC auxiliary power supply. The anode of the first diode is connected to the isolated auxiliary power supply, and the cathode is connected to the DC auxiliary power supply. By setting the first diode, unidirectional conduction from the isolated auxiliary power supply to the DC auxiliary power supply can be achieved, enabling directional power supply. In the initial stage of grid connection, the bus capacitor has no energy and the voltage is 0V. The winding output of the isolated auxiliary power supply is relatively high voltage. At this time, the first diode conducts in the forward direction, providing a unique path for the transmission of electrical energy from the isolated auxiliary power supply to the DC auxiliary power supply. This ensures that the electrical energy from the isolated auxiliary power supply is successfully delivered to the DC auxiliary power supply, and after conversion, it powers the control system. This is a key path guarantee for the early power-on of the control system. After the bus voltage stabilizes, the AC auxiliary power relay RLY1 disconnects, and the DC auxiliary power supply is powered by the bus capacitor. At this time, the output voltage on the bus side is higher than the winding output voltage of the isolated auxiliary power supply. The first diode is reverse-biased and cut off, completely blocking the path of backflow of electrical energy from the bus side or the DC auxiliary power supply side to the isolated auxiliary power supply. This prevents the isolated auxiliary power supply from overvoltage breakdown, winding burnout, and other device failures due to reverse voltage or reverse charging, thus achieving core hardware protection for the isolated auxiliary power supply. In addition, the unidirectional conduction characteristic of the first diode eliminates the reverse interference of the bus voltage to the output of the isolated auxiliary power supply, allowing the high-voltage output of the isolated auxiliary power supply to form a stable "mutually opposed" state with the bus voltage. This provides a stable input voltage for the DC auxiliary power supply without reverse fluctuations, ensuring the stability of the operating voltage of the control system output by the DC auxiliary power supply and preventing logic errors such as relay on / off and voltage difference judgment caused by voltage fluctuations. Furthermore, when the circuit switches from power supply from the isolated auxiliary power supply to power supply from the bus capacitor, the reverse cutoff characteristic of the first diode allows the switching to be completed automatically without additional control signal intervention, simply by the voltage difference between the two sides.

[0030] The bus soft-start relay RLY2 is controlled by the control system. It engages after the control system is powered on, providing a dedicated path for charging the bus capacitors and serving as the core control switch during the bus charging phase. When the soft-start conditions are met, it disconnects under control system commands, ending the current-limiting charging phase of the bus. It automatically disconnects when the control system is powered off, cutting off the charging path between the bus capacitors and the AC input side, thus preventing high-voltage power from the bus side from flowing back to the AC side via the charging circuit. The second rectifier bridge D2, in conjunction with the bus soft-start relay RLY2, converts the AC power input from the grid into DC power, providing DC energy for charging the bus capacitors. It is adapted to the DC charging characteristics of the bus capacitors and serves as the power adapter during the bus charging phase.

[0031] In one embodiment, a soft-start resistor R2 is connected in series between the second rectifier bridge D2 and the positive terminal of the bus capacitor. The soft-start resistor R2 is used to suppress the surge current during the initial charging of the bus capacitor and can effectively share the current load of the second rectifier bridge D2 and the bus soft-start relay RLY2, preventing them from burning out, breaking down, or sticking contacts due to instantaneous high current, thus significantly improving the service life and operational reliability of the charging path hardware. Furthermore, by limiting the charging current through the soft-start resistor R2, the energy of the bus capacitor is allowed to accumulate slowly, causing the bus voltage to rise steadily to a stable value rather than a sudden surge. This ensures that once the bus voltage stabilizes, the power supply circuit switching from disconnecting the AC auxiliary power relay RLY1 to supplying power to the DC auxiliary power supply via the bus capacitor can be smoothly completed. It also keeps the voltage difference between the bus voltage and the grid-side voltage peak stable and detectable, providing an accurate voltage reference for the voltage difference judgment completed by the soft start (one of the core judgment conditions), avoiding errors in voltage difference judgment and false alarms of soft start faults due to sudden voltage changes. When the peak voltage difference between the bus voltage and the grid voltage does not meet the limit requirements, the bus soft start relay RLY2 remains energized. At this time, the soft start resistor R2 can continuously limit the current in the charging path (second circuit), avoiding excessive charging current under fault conditions that could cause circuit overcurrent damage. This provides a buffer time for the control system to report the AC bus soft start fault after a delay, preventing the circuit from entering a dangerous state of severe overcurrent and improving the protection capability of the circuit under abnormal operating conditions.

[0032] The grid-side input relay RLY3 is the core switch for completing the AC bus soft start, enabling direct power supply from the grid to the bus and completing the final switching of the soft start process. It automatically disconnects when the control system loses power, cutting off the direct path between the bus capacitor and the grid input port. Together with the bus soft start relay RLY2, it forms a double-circuit protection, completely blocking the direct conduction path from the bus side to the AC side. The rectifier circuit serves as the power conversion element after the bus soft start, adapting to the DC operating characteristics of the bus, i.e., converting AC to DC to avoid faults such as abnormal charging and discharging of the bus capacitor and burnout of the DC module caused by direct AC power connection to the bus.

[0033] The DC auxiliary power supply, acting as a power adapter between the isolated auxiliary power supply and the control system, converts the high-voltage electrical energy output by the isolated auxiliary power supply into an operating voltage that the control system can adapt to, providing a stable DC power supply for the control system. After the bus voltage stabilizes, it receives the electrical energy from the bus capacitor and continuously supplies power to the control system, ensuring the logic control function of the control system throughout the entire process.

[0034] The bus capacitor, as a core energy storage device, stores DC power. After voltage stabilization, it replaces the grid to supply power to the DC auxiliary power supply, maintaining the operation of the control system. It is the core carrier of energy storage and subsequent power supply switching in the circuit, and its voltage stability is an important criterion for soft start completion. Furthermore, a second diode is connected in series between the bus capacitor and the connection point. The anode of the second diode is connected to the positive terminal of the bus capacitor, and the cathode is connected to the connection point. Initially, when the grid is connected, the isolated auxiliary power supply converts the energy input from the first rectifier bridge D1 into suitable high-voltage energy. At this time, the bus soft start relay RLY2 is not activated, and the bus capacitor is not charged; the bus voltage is 0V. After the bus soft start relay RLY2 closes, the bus capacitor is charged. As the bus voltage increases, when it exceeds the output voltage of the isolated auxiliary power supply, the circuit switches to the mode where the bus capacitor supplies power to the control system. By setting the second diode, the bus voltage and the isolated auxiliary power supply can be mutually offset, allowing the isolated auxiliary power supply to act as the input of the DC auxiliary power supply after offsetting with the bus voltage, thus powering the control system.

[0035] The following combination Figure 5-10 This section explains the working process of the AC input bus soft start circuit.

[0036] Specifically, when the grid is connected, the grid-side circuit breaker QF1 is closed. The grid supplies power to the isolation auxiliary power supply through the AC auxiliary power relay RLY1 and the first rectifier bridge D1. One winding of the isolation auxiliary power supply outputs a relatively high voltage, which "interlocks" with the bus voltage, serving as the input to the DC auxiliary power supply. The output of the DC auxiliary power supply powers the control system. At this time, the bus capacitor has no energy, but the control system already has power (e.g., Figure 5 (As shown).

[0037] When the control system is powered on, it issues a command to engage the bus soft-start relay RLY2. The power grid charges the bus through the bus soft-start relay RLY2, the second rectifier bridge D2, and the soft-start resistor R2 (e.g., Figure 6 (As shown). After the bus voltage stabilizes, disconnect the AC auxiliary power relay RLY1, and the DC auxiliary power supply will continue to operate by the bus capacitor (as shown). Figure 7 (As shown). If the judgment conditions are met, i.e., no overtemperature, no overload, no relay fault, and the peak voltage difference between the bus voltage and the grid-side voltage is within the limit range, disconnect the bus soft start relay RLY2, and after a 6ms interval, engage the grid-side input relay RLY3 to complete the AC bus soft start (e.g. Figure 8 (As shown). If the soft start conditions such as differential pressure are not met, the bus soft start relay RLY2 will remain energized for 20 seconds and report an AC bus soft start fault. The judgment process is as follows: Figure 9 As shown.

[0038] If the control system loses power and the circuit loses its control signal, the bus soft start relay RLY2 and the grid-side input relay RLY3 will disconnect the bus capacitor from the grid input port. The AC auxiliary power relay RLY1 will close, but the presence of the AC auxiliary power will isolate the high-voltage and AC sides, ensuring the safety of maintenance personnel (e.g., Figure 10 (As shown).

[0039] The AC input bus soft-start circuit for backflow prevention of this invention, by adding an isolation auxiliary power supply and optimizing the circuit connection and control method, isolates the bus capacitor from the AC input side circuit when the control system loses power, thereby preventing high-voltage components from flowing back into the AC input side, providing strong protection for the safety of maintenance personnel. Backflow prevention can be achieved simply by adding an isolation auxiliary power supply, requiring fewer components, simplifying the circuit structure, and reducing the overall circuit size and cost. When a fault occurs, the control system promptly reports the fault for timely handling, preventing the fault from escalating and affecting the entire bus system, thus ensuring the normal operation of the bus system.

[0040] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0041] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A soft-start circuit for AC input busbar to prevent backflow, characterized in that, The system includes a control system, a grid-side circuit breaker for grid connection, and a control circuit. The control circuit includes a DC auxiliary power supply, a bus capacitor, and a first circuit, a second circuit, and a third circuit connected in parallel. The first circuit includes a first rectifier bridge connected to the grid-side circuit breaker and an isolated auxiliary power supply connected in series with the first rectifier bridge. The isolated auxiliary power supply is connected to the DC auxiliary power supply, which is connected to the control system. The second circuit includes a bus soft-start relay connected to the grid-side circuit breaker and a second rectifier bridge connected in series with the bus soft-start relay. The third circuit includes a grid-side input relay connected to the grid-side circuit breaker and a rectifier circuit connected in series with the grid-side input relay. One end of the bus capacitor is connected to the second rectifier bridge and the rectifier circuit, and the other end of the bus capacitor is connected to the connection line between the isolated auxiliary power supply and the DC auxiliary power supply. When the grid-side circuit breaker is closed, the grid is connected, the bus soft-start relay and the grid-side input relay are disconnected, and the control system is powered through the isolation auxiliary power supply and the DC auxiliary power supply. After the control system is powered on, it controls the bus soft-start relay to close and charges the bus capacitor. When the bus voltage is stable, the control system keeps the bus soft-start relay closed to drive the DC auxiliary power supply and determines whether the AC input bus soft-start circuit meets the soft-start conditions. If it does, the control system disconnects the bus soft-start relay and closes the grid-side input relay; otherwise, it reports an AC bus soft-start fault. When the control system is powered off, the isolation auxiliary power supply isolates the bus capacitor from the AC input circuit.

2. The AC input bus soft start circuit according to claim 1, characterized in that, The first circuit also includes an AC auxiliary power relay disposed between the grid-side circuit breaker and the first rectifier bridge; when the grid is connected, the AC auxiliary power relay is closed; when the bus voltage is stable, the control system disconnects the AC auxiliary power relay and keeps the bus soft start relay closed; when the AC input bus soft start circuit meets the soft start conditions, the control system disconnects the AC auxiliary power relay and the bus soft start relay, and closes the grid-side input relay.

3. The AC input bus soft start circuit according to claim 2, characterized in that, The AC auxiliary power relay is a normally closed relay, while the bus soft start relay and the grid-side input relay are normally open relays.

4. The AC input bus soft start circuit according to claim 2, characterized in that, When the bus capacitor drives the DC auxiliary power supply and the AC input bus soft start circuit meets the soft start conditions, the control system keeps the AC auxiliary power relay open. After the bus soft start relay is opened, the grid-side input relay is closed after a delay. When an AC bus soft start fault is reported, the AC auxiliary power relay is opened. If the AC input bus soft start circuit does not meet the soft start conditions, the bus soft start relay will remain closed for a preset time before reporting the AC bus soft start fault to the control system.

5. The AC input bus soft start circuit according to claim 2, characterized in that, The AC input bus soft start circuit also includes a current-limiting resistor connected in series between the AC auxiliary power relay and the first rectifier bridge.

6. The AC input bus soft start circuit according to claim 1, characterized in that, The soft-start conditions are that none of the devices are over-temperatured, none of the devices are overloaded, none of the relays are faulty, and the peak voltage difference between the bus voltage and the grid voltage is within the limit range.

7. The AC input bus soft start circuit according to claim 1, characterized in that, A first diode is provided between the connection point of the bus capacitor on the connection line between the isolated auxiliary power supply and the DC auxiliary power supply and the isolated auxiliary power supply. The anode of the first diode is connected to the isolated auxiliary power supply, and the cathode of the first diode is connected to the DC auxiliary power supply.

8. The AC input bus soft start circuit according to claim 7, characterized in that, A second diode is connected in series between the bus capacitor and the access point. The anode of the second diode is connected to the positive terminal of the bus capacitor, and the cathode of the second diode is connected to the access point.

9. The AC input bus soft start circuit according to claim 1, characterized in that, A soft-start resistor is connected in series between the second rectifier bridge and the positive terminal of the bus capacitor.

10. The AC input bus soft start circuit according to claim 1, characterized in that, The isolation auxiliary power supply is either a forward power supply or a flyback power supply.

Images