A flexible DC power transmission system energy consumption device

Abstract

The present invention discloses an energy dissipation device for a flexible direct current transmission system, which is used to solve the problem of direct current overvoltage on the direct current side caused by the surplus direct current power when the alternating current system fails. The energy dissipation device is composed of a plurality of cascaded power submodules M1-Mn, a centralized energy dissipation resistor R1, a current limiting inductor L1-L2, and an isolating switch S1-S2; the power submodule topology is composed of a capacitor C, a fully controlled switch device T, an anti-parallel diode D, a voltage balancing resistor R, a through-current diode D1, a bypass switch BRK, and a thyristor VB. The energy dissipation device is used to consume the surplus power on the direct current side of the flexible direct current transmission system, cooperate to realize the AC fault crossing of the flexible direct current system, and also strive for action time for the fan when the fault cannot be cleared, so as to ensure the safety and reliability of the entire flexible direct current system.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible direct current (DC) transmission, and particularly relates to an energy-consuming device for a flexible DC transmission system. Background Art

[0002] Flexible DC transmission systems are very suitable for new energy grid connection, island power supply, grid interconnection and other fields with their significant technical advantages. Especially in the case of long-distance and large-capacity new energy, flexible DC transmission systems are basically the optimal solutions for new energy transmission at present.

[0003] At present, the demand for energy transformation and energy upgrading is becoming increasingly urgent, and the development pace of new energy has increased significantly. China has a vast territory, rich offshore and far-sea wind resources. Sending wind resources can well relieve the tense situation of electricity in China. Although flexible DC transmission is the optimal solution for new energy transmission at present, due to the large mechanical inertia and slow response time of wind turbines, when an AC fault occurs at an onshore flexible DC substation, the wind turbines cannot be cut off in time, resulting in blocked new energy transmission, energy accumulation in the converter valve, increased module voltage and DC voltage, endangering the entire system. To address the above problems, it is necessary to add an energy-consuming device to the flexible DC system to absorb surplus energy, gain action time for the wind turbines, and protect the system safety. Currently, the scheme of thyristor + energy-consuming resistor is usually adopted in engineering. The energy-consuming device is connected to the AC side of the flexible DC substation and needs to be switched in groups, but the DC current will flow through the converter valve, increasing the tolerance pressure of the converter valve. Summary of the Invention

[0004] The present invention provides an energy-consuming device for a flexible DC transmission system, which adopts a power sub-module series centralized energy-consuming scheme to limit the surplus energy from flowing to the receiving-end converter valve and causing the module voltage in the converter valve to rise.

[0005] To achieve the above object, an energy-consuming device for a flexible DC transmission system of the present invention includes n cascaded power sub-modules M1 to Mn, a centralized energy-consuming resistor R1, a current-limiting inductor L1, a current-limiting inductor L2, a disconnecting switch S1 and a disconnecting switch S2;

[0006] The positive terminal of the disconnecting switch S1 is connected to the positive pole of the DC bus, the negative terminal of the disconnecting switch S1 is connected to the positive terminal of the current-limiting inductor L1, the negative terminal of the current-limiting inductor L1 is connected to the positive terminal of the power sub-module M1, the power sub-modules M1 to Mn are cascaded, the negative terminal of the power sub-module Mn is connected to the positive terminal of the current-limiting inductor L2, the negative terminal of the current-limiting inductor L2 is connected to the positive terminal of the centralized energy-consuming resistor R1, the negative terminal of the centralized energy-consuming resistor R1 is connected to the positive terminal of the disconnecting switch S2, and the negative terminal of the disconnecting switch S2 is connected to the negative pole of the DC bus.

[0007] Furthermore, the power sub-module includes at least one basic unit. When the number of basic units is greater than 1, the second port of the basic unit is connected to the first port of the adjacent second basic unit.

[0008] Further, the basic unit includes a capacitor C, a fully-controlled switching device T, an antiparallel diode D, and a voltage-sharing resistor R. The positive terminal of the capacitor C, the collector of the fully-controlled switching device T, the cathode of the antiparallel diode D, and the positive terminal of the voltage-sharing resistor R are connected to the positive terminal of the power sub-module port. The negative terminal of the capacitor C, the negative terminal of the voltage-sharing resistor R, the emitter of the fully-controlled switching device T, and the cathode of the antiparallel diode D are connected to the negative terminal of the power sub-module.

[0009] Further, the positive terminal of the power sub-module port is connected to the positive terminal of the conduction bypass switch BRK, and the negative terminal of the conduction bypass switch BRK is connected to the negative terminal of the power sub-module.

[0010] Further, the positive terminal of the power sub-module port is connected to the anode of the thyristor VB, and the cathode of the thyristor VB is connected to the negative terminal of the power sub-module.

[0011] Further, the negative terminal of the capacitor C is connected to the anode of the current-carrying diode D1, and the cathode of the current-carrying diode D1 is connected to the negative terminal of the power sub-module.

[0012] Further, the centralized energy-consuming resistor R1 is a metal resistor.

[0013] The present invention has at least the following beneficial technical effects:

[0014] The energy-consuming device is connected across the positive and negative busbars on the DC side of the flexible DC system. When an AC grid fault occurs, the energy-consuming device discharges the surplus energy to the centralized energy-consuming resistor R1 by turning on the fully-controlled switching devices T of all power sub-modules, limiting the DC voltage rise, and restricting the surplus energy from flowing to the receiving converter valve, which causes the module voltage in the converter valve to rise, thus ensuring the safety of the flexible DC converter valve. After the flexible DC system is shut down, a large amount of electrical energy is stored in the DC cable. By utilizing the characteristic of the energy-consuming resistor in the energy-consuming device to consume energy, the rapid discharge of the DC cable can be realized, creating favorable conditions for the system to start again.

[0015] The centralized energy-consuming resistor R1 has a certain energy tolerance ability, which can provide favorable conditions for successfully crossing the AC fault. When the AC fault cannot be eliminated, it buys time for the fan to operate.

[0016] Further, affected by the voltage-sharing resistor R, when the voltage of the capacitor C of a certain power sub-module is relatively high, by triggering the fully-controlled switching device T of the power sub-module, the system stops charging the capacitor C. The capacitor C discharges to the voltage-sharing resistor R to reduce the voltage. When the voltage is lower than the set value, the fully-controlled switching device T of the power sub-module is turned off, and the charging of the capacitor C of the power sub-module in the system is restored, controlling the capacitor voltages of each power sub-module in the energy-consuming device within a certain range.

[0017] Furthermore, the energy-consuming device is beneficially affected by the capacitor C. The power sub-module takes power from the capacitor C, eliminating the need for an additional power supply.

[0018] Furthermore, the centralized energy-consuming resistor R1 can be a metal resistor with mature technology and low price. It uses natural cooling and can be placed outdoors to reduce the HVAC requirements of the valve hall, thereby reducing the cost of the energy-consuming device.

[0019] Furthermore, the energy-consuming device is beneficially affected by the bypass switch BRK. When a fault occurs in the power sub-module, it can be isolated.

[0020] Furthermore, the energy-consuming device is beneficially affected by the thyristor VB. When overvoltage occurs in the power sub-module and the bypass switch BRK fails to isolate the power sub-module, the overvoltage of the power sub-module causes the thyristor VB to break down and short-circuit, achieving the purpose of isolating the faulty sub-module.

[0021] Furthermore, the energy-consuming device is beneficially affected by the current-carrying diode D1. When the bypass switch BRK is closed, it prevents the capacitor C from discharging through the bypass switch BRK and causing damage to the bypass switch. Description of the Drawings

[0022] Figure 1 is the topology of the energy-consuming device for the flexible DC transmission system;

[0023] Figure 2 is the topology of the power sub-module in Embodiment 1;

[0024] Figure 3 is the topology of the second power sub-module in Embodiment 2;

[0025] Figure 1 wherein, 9 - the first basic unit, 10 - the second basic unit, 91 - the first port of the first basic unit, 92 - the second port of the first basic unit, 101 - the first port of the second basic unit, 102 - the second port of the second basic unit. Detailed Embodiments

[0026] In order to make the objectives and technical solutions of the present invention clearer and easier to understand, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0027] Embodiment 1

[0028] Figure 1In the flexible DC transmission system energy consumption device topology, the device consists of multiple cascaded power sub-modules M1 to Mn, a centralized energy consumption resistor R1, a current-limiting inductor L1, a current-limiting inductor L2, a disconnecting switch S1, and a disconnecting switch S2. Assume that the positive end of each device in the energy consumption device is the positive terminal close to the positive pole of the DC bus, and the negative end is the negative terminal close to the negative pole of the DC bus. The positive end of the disconnecting switch S1 of the device is connected to the positive pole of the DC bus, the negative end of the disconnecting switch S1 is connected to the positive end of the current-limiting inductor L1, the negative end of the current-limiting inductor L1 is connected to the positive end of the power sub-module M1, and the power sub-modules M1 to Mn are cascaded, where n≥2; the negative end of the power sub-module Mn is connected to the positive end of the current-limiting inductor L2, the negative end of the current-limiting inductor L2 is connected to the positive end of the centralized energy consumption resistor R1, the negative end of the centralized energy consumption resistor R1 is connected to the positive end of the disconnecting switch S2, and the negative end of the disconnecting switch S2 is connected to the negative pole of the DC bus.

[0029] Figure 2 It is the power sub-module topology of the flexible DC transmission system energy consumption device. The power sub-module topology includes a capacitor C, a fully controlled switch device T, an antiparallel diode D, a voltage-sharing resistor R, a current-carrying diode D1, a bypass switch BRK, and a thyristor VB; the positive end of the capacitor C, the collector of the fully controlled switch device T, the cathode of the antiparallel diode D, the positive end of the voltage-sharing resistor R, the positive end of the bypass switch BRK, and the anode of the thyristor VB are all connected to the positive end of the power sub-module port. The negative end of the capacitor C and the negative end of the voltage-sharing resistor R are connected to the anode of the current-carrying diode D1. The emitter of the fully controlled switch device T, the cathode of the antiparallel diode D, the negative end of the bypass switch BRK, the cathode of the thyristor VB, and the cathode of the current-carrying diode D1 are connected to the negative end of the power sub-module.

[0030] When the DC bus voltage is higher than the set value Uset1, the fully controlled switch device T of all power sub-modules conducts, and the impedance of the energy-consuming device is mainly the energy-consuming resistor R1. Part or all of the DC-side current flows through the centralized energy-consuming resistor R1, and the DC bus voltage drops; when the DC bus voltage is lower than the set value Uset2, the fully controlled switch device T of all power sub-modules turns off. Since the sum of the capacitor voltages of all power sub-modules is approximately equal to the set value Uset1, and Uset1 > Uset2, due to the reverse cut-off effect of the current-carrying diode D1 of the power sub-module, no current of the energy-consuming device flows to the DC side, and all the DC-side current flows to the receiving-end converter valve, causing the DC bus voltage to rise. When it reaches Uset1 again, the energy-consuming device conducts to discharge energy, and the cycle repeats. At the moment when the fully controlled switch device T of all power sub-modules in the energy-consuming device conducts, the current-limiting inductors L1 and L2 limit the rate of change of the current on the energy-consuming device R1, providing a certain buffer for the current flowing through the energy-consuming device. When the DC-side voltage is lower than Uset1 for a long time, the energy-consuming device operates in a steady-state voltage equalization state. When the voltage of the capacitor C of a certain power sub-module is relatively high, by triggering the fully controlled switch device T of this power sub-module, the system stops charging the capacitor C, and the capacitor C discharges through the voltage equalization resistor R to reduce the voltage. When the voltage is lower than the set value, the fully controlled switch device T of this power sub-module is turned off, and the system resumes charging the capacitor C of this power sub-module, controlling the capacitor voltages of each power sub-module in the energy-consuming device within the set range, which is generally within ±10% of the rated voltage. When a power sub-module fails, the bypass switch BRK can be triggered to isolate the fault. When the bypass switch BRK fails to isolate the power sub-module, the voltage of the power sub-module further rises. When it reaches the breakdown voltage of the thyristor VB, the thyristor VB breaks down and shorts, achieving the purpose of isolating the faulty power sub-module. When the system is shut down, there is a large amount of electrical energy stored in the DC cable, and the DC voltage drops slowly. The set values Uset1 and Uset2 can be reduced, and the electrical energy in the cable can be quickly released through the centralized energy-consuming resistor R1, creating favorable conditions for the system to restart. When the energy-consuming device needs to be repaired, the disconnecting switches S1 and S2 are opened, ensuring the safety of the repair. This device designs the energy tolerance of the centralized energy-consuming resistor based on the rated power discharge time of the flexible DC system, and strives for a certain operation time for the fan when the AC fault cannot be smoothly crossed, ensuring the safety of the system.

[0031] Embodiment 2

[0032] The difference between this embodiment and Embodiment 1 lies in the different topologies of the power sub-modules. In this embodiment, the topology of the power sub-modules is as Figure 3As shown, the power sub-module topology of the energy-consuming device in the flexible DC transmission system. This power sub-module topology is composed of the cascading of two identical basic units. The basic unit is the power sub-module topology in Embodiment 1. The second end 92 of the first basic unit 9 of the power sub-module is connected to the first port 101 of the second basic unit 10. The first port 91 of the first basic unit of the power sub-module is connected to the second port of the second basic unit of the previous power sub-module. The second port 102 of the second basic unit of the power sub-module is connected to the first port of the first basic unit of the next power sub-module. In this embodiment, the control method and function of the power sub-module topology are the same as those of the power sub-module topology in Embodiment 1. However, the structural components (encapsulated plates, some internal structural parts of the module) used in the power sub-module topology in this embodiment are fewer than those in the power sub-module topology 1, which can reduce the cost of the energy-consuming device to a certain extent.

[0033] Finally, it should be noted that: the technical solutions of the present invention are only described in combination with the above embodiments and are not limited thereto. Those of ordinary skill in the art should understand that: those skilled in the art can modify the specific implementation manners of the present invention or make equivalent replacements, but these modifications or changes are all within the scope of protection of the claims pending for approval.

Claims

1. A flexible DC power transmission system energy consumption device, characterized in that, It includes n cascaded power sub-modules M1 to Mn, a centralized energy-consuming resistor R1, a current-limiting inductor L1, a current-limiting inductor L2, a disconnector S1 and a disconnector S2; The positive terminal of the disconnector S1 is connected to the positive pole of the DC bus, the negative terminal of the disconnector S1 is connected to the positive terminal of the current-limiting inductor L1, the negative terminal of the current-limiting inductor L1 is connected to the positive terminal of the power sub-module M1, the power sub-modules M1 to Mn are cascaded, the negative terminal of the power sub-module Mn is connected to the positive terminal of the current-limiting inductor L2, the negative terminal of the current-limiting inductor L2 is connected to the positive terminal of the centralized energy-consuming resistor R1, the negative terminal of the centralized energy-consuming resistor R1 is connected to the positive terminal of the disconnector S2, and the negative terminal of the disconnector S2 is connected to the negative pole of the DC bus; The power sub-module includes at least one basic unit. When the number of basic units is greater than 1, the second port of the basic unit is connected to the first port of the adjacent second basic unit; The basic unit includes a capacitor C, a fully controlled switching device T, an antiparallel diode D and a voltage-sharing resistor R. The positive terminal of the capacitor C, the collector of the fully controlled switching device T, the cathode of the antiparallel diode D and the positive terminal of the voltage-sharing resistor R are connected to the positive terminal of the power sub-module port. The negative terminal of the capacitor C, the negative terminal of the voltage-sharing resistor R, the emitter of the fully controlled switching device T and the cathode of the antiparallel diode D are connected to the negative terminal of the power sub-module; The negative terminal of the capacitor C is connected to the anode of the current-carrying diode D1, and the cathode of the current-carrying diode D1 is connected to the negative terminal of the power sub-module; The positive terminal of the power sub-module port is connected to the anode of the thyristor VB, and the cathode of the thyristor VB is connected to the negative terminal of the power sub-module.

2. The energy-consuming device of a flexible DC transmission system according to claim 1, wherein The positive terminal of the power sub-module port is connected to the positive terminal of the conducting bypass switch BRK, and the negative terminal of the conducting bypass switch BRK is connected to the negative terminal of the power sub-module.

3. The energy-consuming device of a flexible DC power transmission system according to claim 1, wherein The centralized energy-consuming resistor R1 is a metal resistor.

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