A centralized DC power dissipation device based on thyristors and its control method

Abstract

A centralized DC power dissipation device based on thyristors includes: N switching modules, reactors, and power dissipation resistors; the negative terminal of each switching module is connected to the positive terminal of an adjacent switching module; the output terminal of the Nth switching module is connected in series with a reactor and a power dissipation resistor; characterized in that: each switching module includes: a thyristor, a capacitor, an inductor, and a voltage equalization resistor; the input terminal of each switching module is connected to the anode of the thyristor, and its output terminal is connected to the cathode of the thyristor; one end of the capacitor is connected to the anode of the thyristor, and the other end is connected to the cathode of the thyristor through series connection with the inductor; the power dissipation resistor is connected in parallel across the two ends of the thyristor; N ≥ 2 natural numbers.

Description

Technical Field

[0001] This invention relates to the field of flexible DC power transmission technology, and specifically to a centralized DC power dissipation device based on thyristors. Background Technology

[0002] Flexible DC transmission technology has been widely used in large-scale offshore wind power transmission due to its advantages such as flexible control, low loss, and black start capability. However, in offshore wind power flexible DC transmission systems, when a low-voltage fault occurs on the AC side of the receiving-end converter station, the voltage drop in the AC grid leads to a decrease in the power transmission capacity of the receiving-end converter station. Meanwhile, the power output of the wind turbines remains unchanged in the short term, resulting in a power surplus that causes a voltage rise in the DC transmission line, jeopardizing the safety of core equipment such as the flexible DC converter valve.

[0003] In existing technologies, fault ride-through is achieved using DC power dissipation devices. Centralized DC power dissipation devices utilize fully controlled components and power-dissipating resistors. When the DC voltage is too high, the fully controlled components are activated, activating the power-dissipating resistors. The resistors absorb power, causing the DC voltage to drop. The power dissipation function ends by deactivating the fully controlled components, and the DC voltage rises again. This process is repeated during fault ride-through, thus stabilizing the DC voltage. The main problems with this method are: the DC power dissipation devices use a large number of fully controlled components, making them expensive; and because the turn-on and turn-off times of these components are on the order of μs, rapidly changing currents can cause significant impacts on the DC bus voltage, resulting in high DC voltage ripple.

[0004] To address the aforementioned shortcomings, current technologies generally employ distributed or semi-centralized DC power consumption device solutions. Detailed technical solutions can be found in the references below.

[0005] [1] J.Maneiro, S.Tennakoon, C.Barker and F.Hassan, "Energy divertingconverter topologies for HVDC transmission systems," 2013 15th European Conference on Power Electronics and Applications (EPE), Lille, France, 2013, pp.1-10.

[0006] [2] Xu Bin, Gao Chong, Zhang Jing. A novel DC energy dissipation device topology for AC fault ride-through on the main grid side of VSC-HVDC system applied to offshore wind power access [J]. Proceedings of the CSEE, 2021, 41(01): 88-97+400.

[0007] In the distributed approach, centralized resistors are distributed across each module. Gradual switching reduces the impact of current on the DC bus voltage. However, during fault ride-through, the resistors dissipate significant heat, necessitating a water-cooling system. The long-term operation of this system incurs additional losses and costs. The semi-centralized DC power dissipation device solution consists of sub-modules and centralized power dissipation resistors. Gradual switching of the sub-modules changes the current flowing through the power dissipation resistors. However, this solution's sub-modules include supporting capacitors and numerous control components, resulting in high costs. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention discloses a centralized DC power consumption device based on thyristors and its control method, the technical solution of which is as follows:

[0009] A centralized DC power dissipation device based on a thyristor includes: N switching modules, a reactor, and a power dissipation resistor; the negative terminal of each switching module is connected to the positive terminal of an adjacent switching module; the output terminal of the Nth switching module is connected in series with a reactor and a power dissipation resistor; characterized in that: each switching module includes: a thyristor, a capacitor, an inductor, and a voltage equalization resistor; the input terminal of each switching module is connected to the anode of the thyristor, and its output terminal is connected to the cathode of the thyristor; one end of the capacitor is connected to the anode of the thyristor, and the other end is connected to the cathode of the thyristor through series connection with the inductor; the power dissipation resistor is connected in parallel across the two ends of the thyristor; N ≥ 2 natural numbers.

[0010] This invention discloses a centralized DC power consumption method based on thyristors. This method is based on the aforementioned centralized DC power consumption device based on thyristors, and its features are as follows:

[0011] Step 1: When the offshore wind power system is operating normally, the energy-consuming devices are not working and are in the off state;

[0012] Step 2: When a low-voltage fault occurs on the receiving end side, the system has surplus power and the DC voltage exceeds the set value. The energy dissipation device adjusts the operating frequency of the thyristor and triggers the thyristor with a pulse signal of fixed pulse width to make it conduct. Current flows through the energy dissipation resistor and energy dissipation begins. At the same time, the LC circuit starts to oscillate, and the oscillating current also flows through the thyristor. When the current flowing through the thyristor is 0, the thyristor turns off.

[0013] Step 3: When the DC voltage drops to the set value, a stop is triggered, and the energy-consuming device exits.

[0014] This invention discloses a control method for the fault ride-through process of a thyristor-based centralized DC energy dissipation device in an offshore wind power VSC-HVDC system; the method is based on the aforementioned thyristor-based centralized DC energy dissipation device, and its features are as follows:

[0015] Before time t0, when the offshore wind power system is operating normally, the DC energy consumption device monitors the DC voltage in real time. When the detected value is lower than or exceeds the set value, the energy consumption device does not work.

[0016] At time t1, a low-voltage fault occurs on the receiving end of the wind power system, resulting in surplus power and a continuous rise in DC voltage. The DC energy dissipation device monitors the DC voltage in real time; once it detects that the DC voltage exceeds a set value, it controls the DC energy dissipation device to enter energy dissipation mode. In the initial stage of thyristor activation, current is limited by the series inductor L to reduce the rate of current change. At this time, the current flowing through the resistor can be expressed as:

[0017]

[0018] In the formula, i R U represents the current flowing through the energy-consuming resistor R. dc This indicates the DC side voltage of the offshore wind power system.

[0019] The current then gradually increases until it reaches the rated value. At this point, the current flowing through the energy-consuming resistor is constant, and can be approximated as:

[0020]

[0021] In the formula, R is the resistance value of the energy-consuming resistor.

[0022] When the thyristor is turned on, the LC circuit also begins to oscillate, and the oscillation current i at this time... O It can be expressed as (3), the oscillation frequency f can be expressed as (4), and correspondingly, the current i flowing through the thyristor s This can be represented as (5), which shows that the current flowing through the thyristor first increases and then gradually decreases.

[0023]

[0024]

[0025] i s =i R -i o (5)

[0026] In the formula, Co represents the capacitance of the oscillating capacitor, Lo represents the inductance of the oscillating inductor, N is the number of sub-modules, and U... dc This refers to the DC voltage of a flexible DC system.

[0027] At time t2, the current flowing through the thyristor gradually decreases to 0, and the thyristor turns off. At this time, the DC voltage of the flexible DC transmission system charges the oscillating capacitor. During this process, the current still flows through the energy-consuming resistor until it reaches the rated voltage. The current flowing through the energy-consuming resistor drops to 0, and at this time the DC voltage rises.

[0028] The above process is repeated after time t3, and the DC voltage remains stable.

[0029] At time t4, the DC power consumption device monitors the DC voltage in real time. Once the DC voltage drops to the set value, the device locks all sub-modules and exits.

[0030] Beneficial effects

[0031] The technical solution provided by this invention uses a semi-controlled thyristor in conjunction with an LC oscillation circuit to replace the expensive fully controlled devices used in traditional DC power-consuming devices. Compared with existing solutions, this technology is more mature, more reliable, and lower in cost.

[0032] The technical solution provided by this invention uses an inductor of several hundred mH to reduce the rate of change of current when the thyristor is turned on, and the current charges the oscillation capacitor when the thyristor is turned off, thereby reducing the rate of change of current when the thyristor is turned off, significantly reducing DC voltage ripple, and improving system stability. Attached Figure Description

[0033] Figure 1 This is an electrical topology diagram of a DC power-consuming device in the existing technology.

[0034] Figure 2 This is an electrical topology diagram of a centralized DC power consumption device based on thyristors provided by the present invention.

[0035] Figure 3 This is a structural diagram of a centralized DC power dissipation device based on thyristors installed on an offshore wind power system, provided by the present invention.

[0036] Figure 4 This invention provides a control diagram for the fault ride-through process of a centralized DC power consumption device based on thyristors.

[0037] Among them, 1 is the switch module, 2 is the capacitor, 3 is the inductor, 4 is the thyristor, 5 is the surge arrester, 6 is the reactor, and 7 is the energy-consuming resistor. Detailed Implementation

[0038] Please refer to Figure 2This invention provides a centralized DC power dissipation device based on thyristors, comprising: N switching modules, reactors, and power dissipation resistors; the negative terminal of each switching module is connected to the positive terminal of the adjacent switching module; the output terminal of the Nth switching module is connected in series with a reactor and a power dissipation resistor; characterized in that: each switching module includes: a thyristor, a capacitor, an inductor, and a voltage equalization resistor; the input terminal of each switching module is connected to the anode of the thyristor, and its output terminal is connected to the cathode of the thyristor; one end of the capacitor is connected to the anode of the thyristor, and the other end is connected to the cathode of the thyristor through series connection with the inductor; the power dissipation resistor is connected in parallel across the two ends of the thyristor; N ≥ 2. The series reactor in this invention is a smoothing reactor, which avoids a large rate of change in DC current during the switching process, significantly reduces DC voltage ripple, and improves system stability. The addition of the voltage equalization resistor ensures that the voltage of the switching modules remains balanced when they are not in operation.

[0039] To further illustrate the present invention, its working process is now described in detail as follows:

[0040] When an offshore wind power system is operating normally, the energy-consuming devices are not working and are in an off state.

[0041] After a low-voltage fault occurs on the receiving end, the system has surplus power and the DC voltage exceeds the set value. The energy-consuming device adjusts the operating frequency of the thyristor and triggers the thyristor with a pulse signal of fixed pulse width to make it conduct. Current flows through the energy-consuming resistor and energy consumption begins. At the same time, the LC circuit starts to oscillate, and the oscillating current also flows through the thyristor. When the current flowing through the thyristor is 0, the thyristor turns off.

[0042] When the DC voltage drops to the set value, a stop is triggered, and the energy-consuming device shuts down.

[0043] Figure 4 This invention provides a control diagram of the fault ride-through process of a centralized DC power consumption device based on thyristors in an offshore wind power VSC-HVDC system.

[0044] Please refer to Figure 4 The control process is now described in detail according to the time sequence:

[0045] Before time t0, when the offshore wind power system is operating normally, the DC energy consumption device monitors the DC voltage in real time. When the detected value is lower than or exceeds the set value, the energy consumption device stops working.

[0046] At time t1, a low-voltage fault occurs on the receiving end of the wind power system, resulting in surplus power and a continuous rise in DC voltage. The DC energy dissipation device monitors the DC voltage in real time; if it detects that the DC voltage exceeds a set value, it controls the device to enter energy dissipation mode. The DC energy dissipation device uses a fixed pulse width frequency conversion control method, where 100Hz corresponds to 100% surplus power and 10Hz corresponds to 10% surplus power. Specifically, at time t1, the thyristor conducts, the inductor acts as a current buffer, and the current flowing through the energy dissipation resistor gradually increases to its rated value, causing the DC voltage to decrease. Simultaneously, the LC current begins to oscillate. When the oscillating current reverses, the current flowing through the thyristor gradually decreases. This process is as follows: Figure 4 As shown in (a).

[0047] At time t2, the current flowing through the thyristor gradually decreases to 0, and the thyristor turns off. At this time, the system charges the oscillation capacitor. During this process, the current still flows through the energy-consuming resistor until it reaches the rated voltage. The current flowing through the energy-consuming resistor drops to 0, and the DC voltage rises.

[0048] The above process is repeated after time t3, and the DC voltage remains stable.

[0049] At time t4, the DC power consumption device monitors the DC voltage in real time. Once the DC voltage drops to the set value, the device locks all sub-modules and exits.

[0050] When a low-voltage fault occurs on the receiving end of an offshore wind power system, the AC grid voltage drop leads to a decrease in the power transmission capacity of the receiving-end converter station. The resulting active power surplus causes a rise in the DC transmission line voltage. This patent uses thyristors to control the absorption of surplus power, thus reducing the DC voltage. Existing solutions use fully controlled devices directly connected in series with energy-consuming resistors. These solutions employ numerous fully controlled devices, and the rapidly changing current has a significant impact on the DC voltage, resulting in substantial ripple during fault periods. Compared to similar existing devices, this invention has lower costs, lower system DC voltage ripple, and higher reliability.

[0051] The technical solution provided by this invention uses thyristors and an LC oscillation circuit to control the energy consumption of the resistor. Compared with fully controlled devices, thyristors are lower in cost, more reliable, and have stronger withstand voltage and current surge capabilities. Compared with traditional DC energy dissipation devices, the cost is greatly reduced and the reliability is improved.

[0052] The technical solution provided by this invention uses a large inductor to reduce the rate of change of current when the thyristor is turned on, and the current charges the oscillation capacitor when the thyristor is turned off, further reducing the rate of change of current during turn-off and thus reducing the voltage ripple of the DC system. Furthermore, by employing a control strategy that changes the frequency with a fixed pulse width, surplus power can be absorbed from 0% to 100%, improving system stability.

[0053] 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 VSC-HVDC system fault ride-through control method based on offshore wind power, which is based on a thyristor-based centralized DC energy dissipation device, the device comprising N switching modules, an electric reactor and an energy dissipation resistor; the negative pole of a switching module is connected to the positive pole of the adjacent switching module; the output end of the Nth switching module is sequentially connected to the electric reactor and the energy dissipation resistor; characterized in that: The switch module comprises a thyristor, a capacitor, an inductor and a lightning arrester; an input end of the switch module is connected with an anode of the thyristor, and an output end thereof is connected with a cathode of the thyristor; one end of the capacitor is connected with the anode of the thyristor, and the other end thereof is connected with the cathode of the thyristor through the inductor connected in series; the lightning arrester is connected in parallel across the thyristor; N is a natural number greater than or equal to 2; and the switch module is characterized in that: Before t0, when the offshore wind power system is in normal operation, the DC energy consumption device detects the DC voltage in real time, and when the detected value is lower than a set value, the energy consumption device does not work; At t1, a low-voltage fault occurs at the receiving end of the wind power system, the system has surplus power, the DC voltage continues to rise, the DC energy consumption device detects the DC voltage in real time, and when the detected DC voltage exceeds the set value, the DC energy consumption device is controlled to enter the energy consumption state; in the initial stage of the thyristor being put into operation, the inductor L connected in series limits the current and reduces the current change rate, and the current flowing through the resistor is represented as: In the formula, i R represents the current flowing through the energy dissipation resistor R, U dc represents the DC side voltage of the offshore wind power system; Thereafter, the current gradually rises to the rated value, and the current flowing through the energy consumption resistor is constant and is approximately represented as: In the formula, R is the resistance value of the energy consumption resistor. When the thyristor is turned on, the LC circuit also starts to oscillate, and the oscillation current i O The oscillation frequency f is represented by formula (4), and the current i flowing through the thyristor is represented by formula (5). s The current flowing through the thyristor is obtained first increases and then gradually decreases, In the formula, Co represents an oscillation capacitor, Lo represents an oscillation inductor, and N is the number of sub-modules; At t2, the current flowing through the thyristor gradually decreases to 0, the thyristor is turned off, the DC voltage of the flexible DC power transmission system charges the oscillation capacitor, the current still flows through the energy consumption resistor during the process, until the energy consumption resistor reaches the rated voltage, and the current flowing through the energy consumption resistor decreases to 0, at which time the DC voltage rises; After t3, the above process is repeated, and the DC voltage is maintained stable; At t4, the DC energy consumption device detects the DC voltage in real time, and when the detected DC voltage drops to the set value, the DC energy consumption device is controlled to block all sub-modules, and the DC energy consumption device exits.

2. The offshore wind farm VSC-HVDC system fault ride-through control method according to claim 1, characterized in that: At t1, the thyristor is turned on, the inductor functions as a current buffer, the current flowing through the energy consumption resistor gradually increases to the rated value, and the DC voltage decreases; at the same time, the LC current starts to oscillate, and when the oscillation current is reversed, the current flowing through the thyristor gradually decreases.

3. A non-volatile storage medium, characterized by, The non-volatile storage medium comprises a stored program, wherein the program controls the device in which the non-volatile storage medium is located to execute the method of claim 1 when the program is run.

4. An electronic device, comprising: The device comprises a processor and a memory; the memory stores computer readable instructions, and the processor is configured to run the computer readable instructions, wherein the computer readable instructions execute the method of claim 1 when the computer readable instructions are run.

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