Fault ride-through energy consumption device topology suitable for current source converter offshore wind power sending-out system

Abstract

The invention provides a fault ride-through energy consumption device which is suitable for a current source converter (CSC) open sea wind power output system. In a CSC-based open-sea wind power output system, when an onshore alternating current system breaks down, direct current voltage on an inverter side is reduced, transmittable power is greatly reduced, direct current voltage on a sea side is basically kept unchanged, a voltage difference is generated in a direct current loop, and transient overcurrent occurs on a direct current line. The energy consumption device provided by the invention can solve the problem, the resistor in the energy consumption device consumes surplus power and limits the increase of direct current overcurrent, and meanwhile, by utilizing the characteristic that the inductor passes through low frequency and resists high frequency, the direct current is converted into high-frequency alternating current through the full-control switch, so that the purpose of limiting the direct current by the inductor is achieved. The energy consumption device provided by the invention can consume the fault surplus power, achieves the suppression of overcurrent, reduces the configuration number of the energy consumption device, and solves the cooling problem.

Description

Technical Field

[0001] The present invention relates to the fields of offshore DC power transmission and power electronics technology, and particularly to a topology of an energy-consuming device for fault ride-through of a far-sea wind power transmission system. Background Art

[0002] Driven by the global energy low-carbon transformation and carbon neutrality goals, far-sea wind power, with its advantages of stable wind energy resources, large single-unit capacity, and prominent potential for large-scale development, is becoming the core direction of the development of offshore wind power. However, far-sea wind farms are usually hundreds of kilometers away from the land, and power transmission faces severe challenges such as long distance, large capacity, and high loss. To break through the technical bottleneck, high-voltage DC (HVDC) or low-frequency AC (LFAC) and other efficient power transmission technologies are mostly adopted in far-sea wind power transmission systems.

[0003] However, when a short-circuit fault occurs on the onshore AC side, the strong coupling characteristics between the far-sea wind power transmission system and the onshore power grid will cause complex dynamic interaction problems: the flexible DC converter station is affected by the grid voltage drop, which may lead to blocked DC power transmission, overvoltage on the DC side, and overcurrent blocking of the converter; while for far-sea wind farms, due to the time-delay effect and lack of inertia of long-distance submarine cables, the power fluctuation during the fault may further increase the risk of grid transient instability.

[0004] In a far-sea wind power transmission system based on a current-source converter, when a fault occurs in the onshore converter station or the AC side, the voltage at the onshore outlet continuously drops, and the wind farm continuously outputs power, resulting in the active power input to the DC system being greater than the output active power. The voltage on the offshore side remains unchanged, causing a voltage difference in the DC loop, and the DC current in the line rapidly rises, thus resulting in a transient overcurrent. To ensure the stable operation of the system and achieve effective power transmission, it is urgent to study a fault ride-through energy-consuming device and its control strategy applicable to the far-sea wind power transmission system of the current-source converter. Summary of the Invention

[0005] To ensure that the offshore AC system can operate normally after a fault occurs and to solve the above transient overcurrent problem, the present invention proposes a fault ride-through energy-consuming device applicable to a CSC far-sea wind power transmission system, including an energy-consuming device topology and a control strategy. For the energy-consuming device topology, such as Figure 1As shown in the figure. It is characterized in that it is connected in series to a DC line, adopts an H-bridge topology, and there is a switching device on each of the four bridge arms, which is a reverse-blocking IGCT. Each bridge arm is connected through an inductor, and an IGCT device is connected in parallel to the H-bridge to control the switching of the energy-consuming device. Utilizing the triggering control characteristics of the IGCT, three working states are controlled to be generated. When a fault occurs in the onshore AC system, the resistor consumes the surplus power and restricts the increase of the DC overcurrent. At the same time, by utilizing the characteristic of the inductor to pass low-frequency current and block high-frequency current, the DC current is converted into a high-frequency alternating current through a fully-controlled switch, so as to achieve the purpose of the inductor restricting the DC current.

[0006] The working principle and working state diagram of the energy-consuming device proposed by the present invention are as Figure 2 shown. The energy-consuming device has a total of 3 working states: when there are no trigger pulses for both T1 and T2, the energy-consuming device is short-circuited, which is working state 1; when there is a trigger pulse for T1, it is working state 2; when there is a trigger pulse for T2, it is working state 3. Among them: T1 is the general term for T11 and T12; T2 is the general term for T21 and T22. As can be seen from Figure 2 it, when the system is operating normally, the series-type energy-consuming device is always in working state 1. All the fully-controlled switches in the device are turned off, and the fast mechanical switch S is closed, and the DC current flows through the switch S. When it is detected that a fault occurs in the AC system, the natural commutation technology is adopted to quickly disconnect the switch S and trigger the conduction of the fully-controlled switch T. The current transfers from the switch S to the fully-controlled switch T, and then T is turned off and the fully-controlled switches T11 and T12 are triggered to conduct, forming Figure 2 the T11-L-T12 current flow path shown in working state 2 in Figure 2 . In a short time, the inductor restricts the increase of the DC current, and the energy-consuming resistor R consumes the surplus power of the DC system. The function of the switch T is to realize the reliable transfer of the current from S to the inductor loop. Then T11 and T12 are turned off and T21 and T22 are triggered to conduct, forming

[0007] the T21-L-T22 current flow path shown in working state 3 in Figure 3 . In this loop, the inductor also plays a role in restricting the increase of the DC current in a short time. Once the current is stable, the inductor will be equivalent to a wire. By utilizing the triggering control of the IGCT, the current flow path is switched between working states 2 and 3 at a certain frequency during the fault, converting the DC current into an alternating current, and the energy-consuming resistor will also consume more surplus power due to the skin effect, thereby realizing the suppression of the overcurrent.

[0007] The topological equivalent circuit model diagram of the series-type DC energy-consuming device installed at the DC outlet of the DC transmission inverter station is as Figure 3 shown, where E is the equivalent power supply voltage, R dcLet \(R_0\) be the DC line resistance, \(R\) be the energy-consuming resistor, \(L_0\) be the line smoothing reactor, and \(L\) be the inductor of the energy-consuming device. Before the fault occurs, the energy-consuming device operates in state 1, switch \(S_0\) is closed, \(S_1\) and \(S_2\) are open, and the DC current is the rated value \(I_0\). When the fault occurs, the energy-consuming device acts, switch \(S_0\) opens, \(S_1\) closes. Since a power electronic switch is used, the action process can be ignored. According to the flux linkage conservation equation, we have:

[0008] \(LI_1(0\) + ) + \(L_0I_0(0\) + ) = \(LI_1(0\) - ) + \(L_0I_0(0\) - ) (1)

[0009] When the fault occurs, the moment when switch \(S_0\) opens and \(S_1\) closes is recorded as time \(t = 0\). The initial conditions are \(I_1(0\) - ) = 0, \(I_0(0\) - ) = \(E / R\) dc， Substituting into equation (1), we get:

[0010]

[0011] According to Kirchhoff's voltage and current laws and combining with the initial conditions, it is solved that in the loop where \(S_1\) is closed:

[0012]

[0013] Suppose the frequency of the energy-consuming device switching from working state 2 to working state 3 is \(f\), and the period is \(T\). Then the moment from state 2 to state 3 is \(T / 2\). At the moment \(T / 2\), the switch switches, and the energy-consuming device switches from working state 2 to working state 3. Re-recording this moment as time \(t = 0\), then:

[0014]

[0015] According to the law of conservation of magnetic flux linkage, at the moment \(T / 2\), the initial value of the current in working state 3 is:

[0016]

[0017] It can be seen from equations (2) and (5) that the mutual switching between working states 2 and 3 reduces the initial value of the DC current, thus limiting the growth rate and amplitude of the DC current.

[0018] The beneficial effect of the present invention is that compared with the conventional energy-consuming device, the ability to suppress DC transient overcurrent is improved; compared with the control method of the traditional energy-consuming device, the fault ride-through effect can be improved while reducing the number of energy-consuming device configurations and the cooling problem, which has great application value for practical engineering. Description of the Drawings

[0019] Figure 1 It is the topological structure diagram of the energy-consuming device provided by the present invention;

[0020] Figure 2 It is the working principle and working state diagram provided by the present invention;

[0021] Figure 3 It is the topological equivalent circuit model diagram provided by the present invention;

[0022] Figure 4 It is the self-regulating energy-consuming resistor control diagram provided by the present invention;

[0023] Figure 5 It is the main network AC fault ride-through strategy diagram provided by the present invention. Detailed implementation manners

[0024] The preferred embodiments will be described in detail below with reference to the accompanying drawings. It should be emphasized that the following description is merely exemplary and not intended to limit the scope of the present invention and its applications.

[0025] Figure 2 It is the working principle and working state diagram provided by the present invention. The energy-consuming device has a total of three working states: when there are no trigger pulses for both T1 and T2, the energy-consuming device is short-circuited, which is the working state 1, and this working state is the normal working state; when there is a trigger pulse for T1, it is the working state 2; when there is a trigger pulse for T2, it is the working state 3. Among them: T1 is the general term for T11 and T12; T2 is the general term for T21 and T22. The working states 2 and 3 indicate that the energy-consuming device is put into operation. The switching between the working states 2 and 3 changes the DC current into an alternating current, so that the energy-consuming resistor absorbs more surplus power due to the skin effect.

[0026] Figure 4 It is the self-regulating energy-consuming resistor control diagram provided by the present invention. According to the different degrees of faults and the different magnitudes of fault overcurrents at each moment, there will be different power differences, and n s (1 ≤ n s ≤ n) sub-modules will be correspondingly put into operation. After the sub-modules are put into operation, the sub-modules switch according to the working states 2 and 3 to absorb the surplus power. The number of sub-modules n s to be put into operation during a fault is as shown in Equation (6):

[0027] n s = n[(P in - P out ) / P dc (6)

[0028] In the formula: n is the number of sub-modules; P in is the input power of the system from DC to AC; P out is the output power of the grid-side converter station; P dcis the DC rated transmission power of the CSC HVDC transmission system.

[0029] Figure 5 is the main network AC fault ride-through strategy diagram provided by the present invention. When the power grid is operating normally, the DC current will not exceed the threshold I dc_h = 1.25I dc_N , the logical operation output value is S = 0. At this time, n s = 0, and the series DC energy dissipation device does not need to be put into operation. If a fault occurs on the AC side of the system and the DC current exceeds the threshold, the output value of the logic judgment module is S = 1. At this time, the number of sub-modules n that need to be put into operation can be calculated through the power input-output difference s . After the self-regulating energy dissipation device is put into operation, the surplus power is dissipated in the form of heat to achieve the power balance of the system and prevent the DC current from rising too high. Subsequently, before the main network fault is cleared, the number of sub-modules put into operation is adjusted according to the power deficit. After detecting the clearance of the main network fault, the main network AC fault ride-through control strategy is cut out and the steady-state control strategy is restored to achieve a smooth main network AC fault ride-through.

[0030] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Those of ordinary skill in the art can still modify or equivalently replace the specific implementation manners of the present invention with reference to the above embodiments. Any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention are within the scope of the claims of the present invention pending approval.

Claims

1. A fault ride-through energy consumption device topology suitable for CSC offshore wind power transmission system, characterized in that: It is connected in series at the DC outlet of the receiving end of the offshore wind power transmission system. When the energy consumption device is put into operation, the DC voltage is equivalently increased to suppress DC overcurrent.

2. The energy consumption device topology according to claim 1, characterized in that: It adopts H-bridge topology, with a switching device on each of the four bridge arms, which is a reverse-resistance IGCT. Each bridge arm is connected through an inductor, and an IGCT device is connected in parallel to the H bridge to control the switching of the energy-consuming device. There are three working states in total.

3. The energy consumption device topology according to claim 1 or 2, characterized in that: By utilizing the characteristics of inductance that passes DC and blocks AC, the increase of DC current can be limited in a short period of time. During a fault, the trigger control of IGCT is used to switch the current flow path between two different working conditions, turning the DC current into an alternating current. Due to the influence of the skin effect, the energy-consuming resistor will consume more surplus power.

4. According to the energy consumption device topology described in claims 1 and 2, a self-regulating energy consumption device control strategy is proposed, characterized in that: The entire energy consumption device is divided into multiple identical sub-modules, each of which can be switched on and off independently. By controlling the switching on and off of each sub-module, a smooth crossing of the system can be achieved.

5. The self-regulating energy consumption device control strategy according to claim 4, characterized in that: The difference between the system's DC to AC input power and the grid-side converter station's output power is used to determine the severity of the fault and invest in a corresponding number of sub-modules, greatly reducing costs.

Images