Single-conductor AC / DC hybrid system DC side overvoltage suppression circuit and method based on band elimination filter

Abstract

The invention relates to a band elimination filter-based single-conductor AC / DC hybrid system DC side overvoltage suppression circuit, which comprises a distributed photovoltaic system PV, the output end of the distributed photovoltaic system PV is connected with the input end of a band elimination filter BSF, the output end of the band elimination filter BSF is connected with the neutral point of a first zigzag-zag transformer ZT1, the input end of the first zigzag-zag transformer ZT1 is connected with an AC current, and the output end of the first zigzag-zag transformer ZT1 is connected with the AC current. The output end of the first zigzag-zag transformer ZT1 is connected with the input end of the second zigzag-zag transformer ZT2 through a hybrid line, and the output end of the second zigzag-zag transformer ZT2 outputs a direct-current load and an alternating-current load. According to the invention, the overvoltage at the direct-current side during the fault is effectively suppressed, the safety of direct-current equipment is protected, an overvoltage generation path during the fault can be blocked in a targeted manner, and the overvoltage at the direct-current side is avoided from the source; normal operation and electric energy transmission of the system are not affected, and compatibility is high; implementation is easy, complex control is not needed, and practicability and economical efficiency are high; the structure is simple, no extra control burden exists, and the deployment difficulty is low.

Description

Technical Field

[0001] This invention relates to the field of power system technology, and in particular to a DC-side overvoltage suppression circuit and method for a single-conductor AC / DC hybrid system based on a band-stop filter. Background Technology

[0002] With the continuous development of my country's economy and the constant improvement of people's living standards, the demand and supply of electricity, as a crucial foundational industry for national economic and social development, are growing rapidly. Currently, the expansion space of existing transmission corridors is very limited, and laying new underground cable lines is not only difficult to construct but also extremely costly. Therefore, it is essential to enhance the transmission capacity of existing distribution lines and tap their potential transmission capacity. Simultaneously, with the continuous advancement of power system economics and technology, an increasing number of DC loads, such as electric vehicles and subways, are being connected to the power system. Furthermore, the widespread application of new energy sources has also brought new challenges to the power system. Users' increasingly higher demands for power quality pose a severe test to the power supply and distribution technologies of the power system.

[0003] Single-conductor AC / DC transmission technology features large transmission capacity, maximizing the utilization of transmission corridors and increasing the transmission capacity of existing lines. It can also improve the stability of hybrid systems and reduce system oscillations by rapidly controlling the output power of the DC system during faults. Compared to transmission systems, distribution systems are easier to couple and separate AC and DC power. Zig-zag transformers, due to their unique winding configuration, effectively achieve DC power injection and separation. Therefore, applying single-conductor AC / DC transmission technology to existing distribution systems increases the transmission capacity of existing AC lines, effectively addressing the problem of power shortages in traditional AC distribution networks.

[0004] Due to the unique structure of single-conductor AC / DC systems, their fault characteristics are more complex than those of traditional single-phase AC / DC systems. When an asymmetrical fault such as a single-phase grounding occurs in the mixed circuit of a single-conductor AC / DC system, the zero-sequence voltage generated by the mixed circuit can intrude into the DC system through the neutral point of the Zig-Zag transformer, generating overvoltages in the DC system and threatening the system insulation, requiring relevant protective measures. Because of its unique structure, the AC / DC coupling characteristics of a single-conductor AC / DC system differ from those of traditional systems, making its fault overvoltage characteristics more complex. Existing overvoltage protection methods are mainly applicable to single-phase AC / DC systems and may not be suitable for single-conductor AC / DC mixed systems. Summary of the Invention

[0005] To address the problem that in existing single-conductor AC / DC hybrid systems, when an asymmetrical fault occurs in the hybrid circuit, the DC side is prone to overvoltage and lacks effective suppression methods, the primary objective of this invention is to provide a DC-side overvoltage suppression circuit for single-conductor AC / DC hybrid systems based on a band-stop filter that can effectively suppress DC-side overvoltage during faults, protect the safety of DC equipment, and specifically block the overvoltage generation path during faults.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a DC-side overvoltage suppression circuit for a single-conductor AC / DC hybrid system based on a band-stop filter, comprising a distributed photovoltaic system (PV), a band-stop filter (BSF), a first zig-zag transformer (ZT1), a second zig-zag transformer (ZT2), and a hybrid circuit. The output terminal of the distributed photovoltaic system (PV) is connected to the input terminal of the band-stop filter (BSF), the output terminal of the band-stop filter (BSF) is connected to the neutral point of the first zig-zag transformer (ZT1), the input terminal of the first zig-zag transformer (ZT1) is connected to an AC current, the output terminal of the first zig-zag transformer (ZT1) is connected to the input terminal of the second zig-zag transformer (ZT2) through the hybrid circuit, and the output terminal of the second zig-zag transformer (ZT2) outputs both DC and AC loads.

[0007] The distributed photovoltaic system PV includes photovoltaic panels and a DC-DC converter. The DC-DC converter consists of a photovoltaic capacitor Cpv, a filter inductor Lb, a MOSFET Q, a reverse diode D, a diode Db, and a DC-side capacitor Cb. The positive terminal of the photovoltaic panel is connected to one end of the filter inductor Lb and the positive terminal of the photovoltaic capacitor Cpv. The other end of the filter inductor Lb is connected to the anode of the diode Db and the collector of the MOSFET Q. The reverse diode D is connected across the collector and emitter of the MOSFET Q. The cathode of the diode Db is connected to the positive terminal of the DC-side capacitor Cb and the band-stop filter BSF. The negative terminals of the photovoltaic panel, the photovoltaic capacitor Cpv, the MOSFET Q, and the DC-side capacitor Cb are all grounded.

[0008] The band-stop filter consists of a capacitor C1 and an inductor L1 connected in parallel.

[0009] Another object of the present invention is to provide a method for suppressing overvoltage on the DC side of a single-wire AC / DC hybrid system based on a band-stop filter, the method comprising the following sequential steps:

[0010] (1) When the system is running normally, the band-stop filter does not impede the normal operating current of the DC system;

[0011] (2) When an asymmetrical fault occurs in the hybrid line of a single-conductor AC / DC system, the zero-sequence voltage generated by the hybrid line will invade the DC system. The band-stop filter will prevent the zero-sequence voltage from charging the inter-electrode capacitor of the DC system, thereby suppressing the overvoltage generated by the DC system.

[0012] As can be seen from the above technical solution, the beneficial effects of the present invention are as follows: First, the present invention effectively suppresses DC-side overvoltage during faults, protects the safety of DC equipment, and can specifically block the overvoltage generation path during faults. When an asymmetrical fault such as a single-phase grounding occurs in the hybrid line, the band-stop filter (BSF) will directly prevent the zero-sequence voltage generated by the line from charging the inter-electrode capacitor of the DC system, thus avoiding overvoltage on the DC side from the source. Second, it does not affect the normal operation of the system and power transmission, and has strong compatibility: the design of the band-stop filter (BSF) realizes "suppression during faults and no disturbance during normal operation". During normal operation, the DC current can be smoothly transmitted through the inductive element of the band-stop filter (BSF), without hindering the injection of DC power into the hybrid line or interfering with the function of the hybrid line to transmit AC and DC power simultaneously. After the fault is eliminated, the band-stop filter (BSF) will also return to the unobstructed state of DC current. Third, it is simple to implement, requires no complex control, and has high practicality and economy: the structure is simple and there is no additional control burden. It only needs to connect the band-stop filter (BSF) in series on the DC line between the DC system and the neutral point of the Zig-Zag transformer. There is no need to add complex control algorithms or additional control modules, and the deployment difficulty is low. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of a single-conductor AC / DC power distribution system in this invention;

[0014] Figure 2 This is a schematic diagram of the band-stop filter configuration in this invention. Detailed Implementation

[0015] like Figure 1 As shown, a DC-side overvoltage suppression circuit for a single-conductor AC / DC hybrid system based on a band-stop filter includes a distributed photovoltaic (PV) system, a band-stop filter (BSF), a first Zig-zag transformer (ZT1), a second Zig-zag transformer (ZT2), and a hybrid circuit. The output terminal of the PV system is connected to the input terminal of the BSF, and the output terminal of the BSF is connected to the neutral point of the first Zig-zag transformer (ZT1). The input terminal of the first Zig-zag transformer (ZT1) is connected to an AC current. The output terminal of the first Zig-zag transformer (ZT1) is connected to the input terminal of the second Zig-zag transformer (ZT2) through the hybrid circuit. The output terminal of the second Zig-zag transformer (ZT2) outputs both DC and AC loads. The hybrid circuit is... Figure 1 Hybrid line in the middle.

[0016] like Figure 2 As shown, the distributed photovoltaic system PV includes a photovoltaic panel 1 and a DC-DC converter. The DC-DC converter consists of a photovoltaic capacitor Cpv, a filter inductor Lb, a MOSFET Q, a reverse diode D, a diode Db, and a DC-side capacitor Cb. The positive terminal of the photovoltaic panel 1 is connected to one end of the filter inductor Lb and the positive terminal of the photovoltaic capacitor Cpv. The other end of the filter inductor Lb is connected to the anode of the diode Db and the collector of the MOSFET Q. The reverse diode D is connected across the collector and emitter of the MOSFET Q. The cathode of the diode Db is connected to the positive terminal of the DC-side capacitor Cb and the band-stop filter BSF. The negative terminals of the photovoltaic panel 1, the photovoltaic capacitor Cpv, the MOSFET Q, and the DC-side capacitor Cb are all grounded.

[0017] The band-stop filter consists of a capacitor C1 and an inductor L1 connected in parallel.

[0018] This method includes the following steps in sequence:

[0019] (1) When the system is running normally, the band-stop filter does not impede the normal operating current of the DC system;

[0020] (2) When an asymmetrical fault occurs in the hybrid line of a single-conductor AC / DC system, the zero-sequence voltage generated by the hybrid line will invade the DC system. The band-stop filter will prevent the zero-sequence voltage from charging the inter-electrode capacitor of the DC system, thereby suppressing the overvoltage generated by the DC system.

[0021] The following combination Figure 1 , Figure 2 The present invention will be further described below.

[0022] In this invention, the structure of the single-conductor AC / DC power distribution system is as follows: Figure 1 As shown, the AC component of a single-conductor AC / DC hybrid transmission line is composed of... Figure 1 The power generated by the generator or infinite power system on the left is transmitted to the busbar of the medium-voltage substation via the first zig-zag transformer ZT1, and then to the high-voltage side Z-shaped winding of the second zig-zag transformer ZT2 in the low-voltage substation via distribution lines. From there, it is transmitted to AC users via feeders and the substation. The DC portion of the system is mainly generated by the distributed photovoltaic system (PV) transmitting DC power to the substation busbar through the neutral point of the zig-zag winding of the first zig-zag transformer ZT1 in the medium-voltage substation. The DC power is then transmitted via transmission lines to the zig-zag winding of the second zig-zag transformer ZT2 in the low-voltage substation, and finally to DC users via the neutral point. The three-phase transmission lines simultaneously carry both AC and DC currents. Because the DC current is superimposed on all three phases, it is divided into three equal parts; therefore, each conductor carries one-third of the AC current and the total DC current.

[0023] Due to its inherent characteristics, a band-stop filter (BSF) impedes voltage and current at specific frequencies. During normal system operation, the BSF does not impede the normal operating current of the DC system. When an asymmetrical fault such as a single-phase grounding occurs in a hybrid AC / DC system, the zero-sequence voltage generated by the hybrid line will invade the DC system. The BSF will prevent the zero-sequence voltage from charging the inter-electrode capacitor of the DC system, thereby suppressing the overvoltage generated in the DC system. This suppression method does not require complex control procedures, greatly reduces the overvoltage in the DC system, and provides a guarantee for the stable operation of the single-conductor AC / DC system.

[0024] Under normal operating conditions, the band-stop filter does not impede the normal operating current of the DC system in the single-conductor AC / DC system. DC power is injected into the hybrid line through the neutral point of the first zig-zag transformer ZT1, and the hybrid line transmits both AC and DC power simultaneously.

[0025] When a single-phase ground fault occurs in a hybrid AC / DC distribution system with a single conductor, there are four initial stages. After the fault occurs, the DC inter-electrode capacitor discharges rapidly, and the MOSFET Q is blocked within microseconds. In the initial stage of the fault, because the photovoltaic system voltage is lower than the DC system output voltage, the diode Db does not conduct. At this time, the DC system fault current is mainly generated by the combined effect of the capacitor and the zero-sequence voltage. When the DC inter-electrode capacitor voltage drops to a certain value, the diode Db conducts. The DC capacitor, photovoltaic power supply, and zero-sequence voltage work together to provide the fault current. When the DC inter-electrode capacitor voltage drops to 0, the MOSFET Q's reverse freewheeling diode D conducts. For a fault circuit consisting of a photovoltaic power supply, DC capacitor, and band-stop filter Db, as the zero-sequence voltage changes, it will exceed the DC system inter-electrode voltage. The zero-sequence voltage will charge the DC system inter-electrode capacitor, thus causing an overvoltage in the DC system.

[0026] By configuring a series band-stop filter, when the zero-sequence voltage charges the DC system inter-electrode capacitor, the band-stop filter BSF is in the cutoff state, preventing the zero-sequence voltage from charging the DC inter-electrode capacitor, thereby suppressing the overvoltage generated by the DC system and ensuring the safe and stable operation of the DC system equipment.

[0027] After a single-phase ground fault occurs in a hybrid AC / DC system and is cleared, the zero-sequence voltage becomes 0, the band-stop filter (BSF) resumes its conducting state and conducts DC current, and the system returns to normal operation.

[0028] The series band-stop filter (BSF) blocks the charging process of the DC inter-electrode capacitor by the zero-sequence voltage during a fault, keeping the overvoltage in the DC system within the insulation tolerance level, thus preventing damage to the equipment in the DC system.

[0029] In summary, this invention effectively suppresses DC-side overvoltage during faults, protecting the safety of DC equipment. It can specifically block the overvoltage generation path during faults. When an asymmetrical fault such as a single-phase grounding occurs in the hybrid line, the band-stop filter (BSF) directly prevents the zero-sequence voltage generated by the line from charging the inter-electrode capacitor of the DC system, thus avoiding overvoltage on the DC side from the source. It does not affect the normal operation of the system and power transmission, and has strong compatibility: the design of the band-stop filter (BSF) achieves "suppression during faults and no disturbance during normal operation." During normal operation, DC current can be smoothly transmitted through the inductive element of the band-stop filter (BSF), without hindering the injection of DC power into the hybrid line or interfering with the function of the hybrid line to transmit AC and DC power simultaneously. After the fault is cleared, the band-stop filter (BSF) will also return to an unobstructed state for DC current. It is simple to implement, requires no complex control, and is highly practical and economical: the structure is simple and has no additional control burden. It only needs to connect the band-stop filter (BSF) in series on the DC line between the DC system and the neutral point of the Zig-Zag transformer. There is no need to add complex control algorithms or additional control modules, and the deployment difficulty is low.

[0030] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. A DC-side overvoltage suppression circuit for a single-wire AC / DC hybrid system based on a band-stop filter, characterized in that: The system includes a distributed photovoltaic system (PV), a band-stop filter (BSF), a first zig-zag transformer (ZT1), a second zig-zag transformer (ZT2), and a hybrid circuit. The output of the distributed photovoltaic system (PV) is connected to the input of the band-stop filter (BSF). The output of the band-stop filter (BSF) is connected to the neutral point of the first zig-zag transformer (ZT1). The input of the first zig-zag transformer (ZT1) is connected to an alternating current. The output of the first zig-zag transformer (ZT1) is connected to the input of the second zig-zag transformer (ZT2) through the hybrid circuit. The output of the second zig-zag transformer (ZT2) outputs both DC and AC loads.

2. The DC-side overvoltage suppression circuit for a single-wire AC / DC hybrid system based on a band-stop filter according to claim 1, characterized in that: The distributed photovoltaic system PV includes photovoltaic panels and a DC-DC converter. The DC-DC converter consists of a photovoltaic capacitor Cpv, a filter inductor Lb, a MOSFET Q, a reverse diode D, a diode Db, and a DC-side capacitor Cb. The positive terminal of the photovoltaic panel is connected to one end of the filter inductor Lb and the positive terminal of the photovoltaic capacitor Cpv. The other end of the filter inductor Lb is connected to the anode of the diode Db and the collector of the MOSFET Q. The reverse diode D is connected across the collector and emitter of the MOSFET Q. The cathode of the diode Db is connected to the positive terminal of the DC-side capacitor Cb and the band-stop filter BSF. The negative terminals of the photovoltaic panel, the photovoltaic capacitor Cpv, the MOSFET Q, and the DC-side capacitor Cb are all grounded.

3. The DC-side overvoltage suppression circuit for a single-wire AC / DC hybrid system based on a band-stop filter according to claim 1, characterized in that: The band-stop filter consists of a capacitor C1 and an inductor L1 connected in parallel.

4. The overvoltage suppression method for the DC side of a single-conductor AC / DC hybrid system based on a band-stop filter according to any one of claims 1 to 3, characterized in that: The method includes the following steps in sequence: (1) When the system is running normally, the band-stop filter does not impede the normal operating current of the DC system; (2) When an asymmetrical fault occurs in the hybrid line of a single-conductor AC / DC system, the zero-sequence voltage generated by the hybrid line will invade the DC system. The band-stop filter will prevent the zero-sequence voltage from charging the inter-electrode capacitor of the DC system, thereby suppressing the overvoltage generated by the DC system.

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