A grounding device for a flexible DC transmission system of offshore wind power and its control method

Abstract

The present invention discloses a grounding device for a flexible DC transmission system of offshore wind power and its control method. The grounding device includes a reactor bank, a disconnector bank, and a grounding resistor. The reactor bank is composed of three-phase iron-core large reactors connected in a star configuration, forming an artificial neutral point. The disconnector bank is composed of three-phase fast BPS connected in a star configuration, forming an artificial grounding point. The reactor bank and the disconnector bank form a double-star connection, and the two artificial neutral points are connected through a disconnector SN. The grounding resistor is composed of two groups of resistors, where resistor R1 and resistor R2 are respectively connected to the neutral points of the reactor and the disconnector, and the lightning arrester is connected in parallel with the grounding resistor R2. The device freely switches the value of the grounding resistor according to requirements such as system steady-state operation, modulation mode, startup process, transient process, and fault state to meet different operating conditions and modulation modes. Moreover, the star disconnector device adopted is used to protect the reactor bank in severe transient faults, ensuring the safety of the device.

Description

Technical Field

[0001] The present invention belongs to the field of new energy and electric power engineering technology, and specifically relates to a grounding device for an offshore wind power flexible direct current transmission system and a control method thereof. Background Art

[0002] With the continuous development of power technology, offshore wind power generation capacity is constantly expanding, especially the effective development and utilization of wind resources in offshore areas is getting farther and farther. The development of offshore wind farms in offshore areas is of great significance to solving the energy crisis. With the increasing aggravation of energy problems and the continuous commissioning of onshore flexible DC transmission and new energy grid-connected projects. Offshore wind power technology is about to usher in a comprehensive outbreak, and a large number of offshore wind power projects will enter the construction stage in the next few years. Grid-connected operation has become the most effective way to utilize wind energy on a large scale. The offshore wind power flexible DC access system based on modular multilevel converters (MMC) is more suitable for long-distance, large-scale offshore wind power access systems. Combined with the characteristics of offshore wind power DC transmission, symmetrical monopole MMC will be the best transmission method.

[0003] In recent years, with the continuous development and commissioning of DC transmission technology, MMC technology has gradually matured. However, due to the high maintenance cost, long cycle and difficulty of offshore converter stations, the safe and stable operation of offshore wind power flexible DC transmission systems has always been the key technology in the design of large-capacity offshore wind power transmission solutions, and there are still many problems. Since the offshore MMC converter station must adopt the design method of an offshore platform, the area is relatively limited. Therefore, the design process of the offshore flexible DC transmission system requires both intensiveness, simplification and high reliability. Especially for symmetrical monopole MMC systems, the design of the grounding method has a greater impact on the safe operation of offshore wind power. The grounding method and grounding device have a greater impact on the safety of the offshore wind power flexible DC converter valve, and the safety of the grounding device itself is a very critical design factor in the offshore wind power flexible DC transmission system.

[0004] At present, some flexible direct current projects have encountered problems such as grounding device failure or grounding method not matching the transient and steady-state processes of the system. However, there is very little research on the grounding method of offshore wind power, and the grounding method used is still based on the grounding of onshore flexible direct current system as the main reference. In the design process, it is impossible to simultaneously meet the requirements of steady-state operation, transient process, startup and fault conditions. In addition, the industry has not conducted research on the safety of the grounding device of the offshore wind power flexible direct current transmission system itself. Summary of the invention

[0005] The present invention provides a grounding device and a control method thereof for a flexible DC transmission system of offshore wind power. By adopting this device, the grounding requirements of various operating conditions of the flexible DC transmission system of offshore wind power can be balanced, and the design of the safety of the grounding method itself is increased. While meeting the functional requirements of the grounding device, its own characteristics can be taken into account, and it will not affect the grid-side power grid, so as to improve the reliability of the flexible DC transmission system of offshore wind power.

[0006] To achieve the above object, a grounding device for a flexible DC transmission system of offshore wind power according to the present invention includes a reactor bank, a disconnecting switch bank and a grounding resistor. The reactor bank is a three-phase reactor formed by star-connected reactors LU, LV and LW, and forms an artificial neutral point N1. The disconnecting switch bank is a three-phase disconnecting switch formed by star-connected disconnecting switches SU, SV and SW, and forms an artificial neutral point N2. The reactor bank and the disconnecting switch bank are connected in parallel to form a double-star wiring mode. The artificial neutral point N1 and the artificial neutral point N2 are connected through a disconnecting switch SN. The grounding resistor includes a resistor R1 and a resistor R2. The first end of the resistor R1 is connected to the artificial neutral point N1, and the second end is grounded. The first end of the resistor R2 is connected to the artificial neutral point N2, and the second end is grounded.

[0007] Further, a lightning arrester is connected in parallel with the resistor R2.

[0008] Further, the resistance value of the resistor R2 is greater than the resistance value of the resistor R1.

[0009] Further, the three-phase reactors constituting the reactor bank adopt iron-core reactors.

[0010] Further, during the steady-state operation of the grounding device, the three-phase disconnecting switches are in the open state, and the neutral point switch SN is in the closed state.

[0011] A control method based on the above grounding device

[0012] Before the start of the flexible DC transmission system of offshore wind power: first set the three-phase disconnecting switches to the open state, and then start the sequence control operation;

[0013] During the steady-state operation of the system, the drive signal QL = 0. The MMC valve side forms an artificial neutral point through the star connection of the three-phase reactors. The artificial neutral point is grounded through the grounding resistor. The three-phase disconnecting switches are in the off state, and the neutral point connection switch SN is in the closed state. The resistors R1 and R2 are in parallel state, and the grounding resistance value is

[0014] When a serious fault occurs in the system or the current stress level of the grounding device rises by more than 3 times, make the drive signal QL = 1; when the drive signal QL = 1, first close the three-phase disconnector, and then open the disconnector SN, and the system locks and shuts down; when the three-phase disconnector is closed and then the disconnector SN is opened, the parallel relationship between the resistor R1 and the resistor R2 is released, and the grounding resistance is R1.

[0015] Furthermore, when the system adjusts the operation model, the resistance value of the grounding resistance is switched through the disconnector SN.

[0016] Furthermore, after the system enters the steady state, the disconnector SN is placed in the closed state.

[0017] Compared with the prior art, the present invention has at least the following beneficial technical effects:

[0018] Combined with the characteristics of the flexible DC transmission of offshore wind power, the present invention can make the grounding system in a large-resistance or small-resistance grounding state according to the operating characteristics, and adjust the grounding resistance value by opening and closing the switch SN, which can meet the system requirements for various operating and debugging conditions;

[0019] During the steady-state operation process, the inductive reactance of the reactor in the grounding device is relatively large, the current stress level of the grounding device is very low, and the loss in the resistor is very small; in extreme fault conditions, through the control of the disconnector, the free switching of the grounding resistance value can be realized. When it is switched to the state with a large resistance value, the grounding device can suppress the fault current to ensure the equipment safety of the MMC; in severe fault conditions, the grounding device can effectively suppress its own current stress level, realizing the stability of the offshore wind power flexible DC transmission system and the safety of the equipment.

[0020] Furthermore, the reactor adopts an iron-core reactor, and the volume of the device is greatly reduced. The whole grounding device is small in volume and simple in wiring method. The device adopts conventional equipment such as reactors and disconnectors, with mature technology, high reliability in engineering, very low technical cost, and good economy.

[0021] Furthermore, a lightning arrester is connected in parallel on the resistor R2, which is mainly used to protect the resistor and prevent the resistor from being damaged due to too high voltage.

[0022] Furthermore, the resistance value of the resistor R2 is greater than the resistance value of the resistor R1. When a fault occurs, the resistance value of the grounding resistance is the resistance value of the resistor R2, and the heat dissipation and safety performance are better.

[0023] A control method for the grounding device of a flexible DC transmission system for offshore wind power. For different working conditions, the resistance value of the grounding resistor is adjusted by opening or closing the disconnector SN to meet the system requirements for various operation and commissioning conditions. When a fault occurs in the system, the three-phase fast disconnector is closed, so that the fault current flows through the three-phase fast disconnector, avoiding the thermal stability and dynamic stability damage of the coils in the reactor.

[0024] Further, after the system enters the steady-state operation, in order to ensure that the overvoltage level during the system operation will not increase, and in order to detect the operation state and some characteristics of the MMC, the disconnector SN is placed in the closed state. Brief Description of the Drawings

[0025] Figure 1 Shown is the typical topology of the flexible DC transmission system for offshore wind power;

[0026] Figure 2 Shown is the high-reliability grounding device of the flexible DC transmission system for offshore wind power;

[0027] Figure 3 Shown is the steady-state operation mode of the grounding device of the flexible DC transmission system for offshore wind power;

[0028] Figure 4 Shown is the transient operation mode of the grounding device of the flexible DC transmission system for offshore wind power;

[0029] Figure 5 Shown is the control flow of the grounding device of the flexible DC transmission system for offshore wind power. Detailed Description of the Invention

[0030] In order to make the objectives and technical solutions of the present invention clearer and easier to understand, the following further details the present invention with reference to the drawings and embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0031] Figure 1 Shown is the typical topology of the flexible DC transmission system for offshore wind power. A typical flexible DC transmission system for offshore wind power consists of an offshore wind farm, an AC collection system, an AC booster station, an offshore converter station, a DC transmission system, and an onshore converter station. Among them, equipment such as the converter valves of the offshore converter station are all located on the offshore platform. Each wind farm collects power through the offshore wind power collection system, and the collection system generally adopts the AC collection method.

[0032] Due to problems such as economic costs, offshore environmental requirements, and difficulties in selecting the location of the DC grounding electrode, the flexible DC transmission system for offshore wind power generally adopts a symmetrical monopolar connection method. During the operation of the symmetrical monopolar system, a reference potential needs to be provided on the valve side, so a grounding point needs to be set on the valve side.

[0033] The grounding device in the HVDC flexible transmission system for offshore wind power first needs to meet the requirements for the reference potential during system operation. At the same time, due to the high safety requirements for primary equipment such as converter valves and transformers in the HVDC flexible transmission system for offshore wind power and the high difficulty of equipment maintenance work, faults such as loss of ground, overvoltage breakdown, and overcurrent burnout are not allowed to occur during the operation of the grounding device. Therefore, a highly reliable grounding device is very crucial for the HVDC flexible transmission system for offshore wind power.

[0034] Figure 2 It is a highly reliable grounding device for the HVDC flexible transmission system for offshore wind power.

[0035] The ultimate goal of the HVDC flexible transmission system for offshore wind power is to send the electric energy of the offshore wind farm to the onshore area and transmit the electric energy to the local large onshore power grid. In the HVDC flexible transmission system for offshore wind power, the fault types are relatively complex and the severity of the faults is relatively high. However, the maintenance cost of the offshore station is relatively high. Therefore, the impact of the grounding device on the fault circuit should be considered emphatically in the system design.

[0036] The grounding device applicable to the HVDC flexible transmission system for offshore wind power consists of four parts, namely, a reactor bank, a disconnector bank, a grounding resistor, and a lightning arrester. The reactor bank is composed of three-phase large-reactance star-connected reactors LU, LV, and LW, and an artificial neutral point N1 is formed. The large reactance in this application refers to a reactor with a reactance in the range of dozens of henries to thousands of henries.

[0037] In the grounding device, in order to reduce the volume of the equipment, a core reactor is used for the large-reactance reactor, and the volume of the device is greatly reduced; since the current stress level inside the reactor is relatively low under the rated operating condition, the wire diameter of the reactor winding is relatively small, which further reduces the volume of the equipment.

[0038] The disconnector bank is composed of three-phase fast disconnectors SU, SV, and SW connected in parallel to the large reactance, and an artificial neutral point N2 is formed. The three-phase disconnectors and the three-phase large reactance form a parallel connection to form a double-star connection mode. The neutral point N1 and the neutral point N2 are connected through the disconnector SN.

[0039] The grounding resistor is composed of resistor R1 and resistor R2. The first end of resistor R1 is connected to the neutral point N1, and the second end is grounded; the first end of resistor R2 is connected to the neutral point N2, and the second end is grounded; according to different operating conditions, by controlling the three-phase disconnectors, the free switching of the grounding resistance value can be realized. When a small-resistance operating condition is required, the grounding resistance value is the parallel value of the two resistors. When resistor R2 and resistor R2 are in parallel, the grounding resistance R GThe resistance value is 4 kΩ to 6 kΩ; when a large resistance value is required, the grounding resistance value is the resistance value of the larger one of the resistors R1 and R2. The grounding terminals of the resistors R1 and R2 are connected, and the other ends of the resistors R1 and R2 are separated by the disconnect switch SN. The resistance values of the resistors R1 and R2 are 8 kΩ to 10 kΩ, and the resistance value of the resistor R2 is greater than that of the resistor R1.

[0040] Figure 2 The MOV in it is a lightning arrester, and the lightning arrester is connected in parallel with the grounding resistor R2 to protect the grounding resistor and prevent the resistor from being damaged due to excessive voltage.

[0041] Figure 3 The figure shows the steady-state operation mode of the grounding device of the offshore wind power flexible DC transmission system.

[0042] During the steady-state operation process, the three-phase star disconnect switch is in the open state, and the neutral point connection switch SN is in the closed state. The resistors R1 and R2 are in parallel, so the grounding resistance value R G is:

[0043]

[0044] During the steady-state operation process, an artificial neutral point is formed by connecting the iron core large reactance in star. In order to reduce the insulation level of the equipment, the grounding resistance value should not be too large. The parallel connection of the two resistors is realized through SN, and the grounding resistance value is R G .

[0045] The impedance values of the three-phase large reactance during the steady-state operation process are:

[0046] X 1U,V,W = ω0L = 2πfL = (100 / 120)πL (1-2)

[0047] In the above formula, X 1U,V,W is the reactance value of the three-phase large reactance, ω0 is the angular frequency of the fundamental voltage, L is the inductance value of the three-phase large reactance, f is the frequency of the fundamental voltage. The above formula shows the fundamental impedance of the large reactance when there is only the fundamental voltage (50HZ / 60HZ) in the valve-side phase voltage. When there is only the fundamental wave, the impedance value of the three-phase large reactance is relatively large, so the inductive current in the three-phase reactor is relatively small.

[0048] When there are harmonic voltages in the three-phase voltage, the harmonic impedance values of the three-phase large reactance are:

[0049] X 2U,V,W = ω h L = h2πfL = (100 / 120)hπL (1-3)

[0050] In the above formula, ω hω is the angular frequency corresponding to the harmonic voltage, and h is the harmonic order of the harmonic voltage. It can be seen from the above formula that for the harmonic voltage, the inductive reactance value of the three-phase large reactance increases by h times, and the harmonic current is effectively suppressed.

[0051] Under the steady-state operation condition, the current value flowing through the large reactance is relatively small, and the iron core in the reactor is in the linear working region, and the reactor is in the state of large reactance. Since the current stress level is relatively low, the current value in the grounding resistance R G is also relatively low. In addition, due to the suppression of the harmonic current by the reactor, the zero-sequence current in the resistance is very small, and the zero-sequence current value injected into the grid side is basically zero.

[0052] Figure 4 The grounding device of the offshore wind power flexible DC transmission system in the transient operation mode is shown as follows

[0053] When a fault occurs in the system, especially for faults at positions such as the DC side and the valve top, the fault current will form a loop of the fault current through the grounding device. After the fault occurs, through the detection and positioning of the substation control system, the disconnectors SU, SV, and SW in parallel for three phases in the grounding device are first closed.

[0054] Then, the disconnector SN between the two neutral points in the grounding device is disconnected. Since the opening speed of the disconnector is greater than the closing speed, after SN is opened, the parallel relationship of the two resistors is released. The grounding resistance becomes:

[0055] R G = R1 (1 - 4)

[0056] After the grounding resistance value becomes larger, the impedance value in the fault loop increases, which can effectively suppress the development of the fault current. Since the inductive reactance value of the reactor is relatively large before saturation, the suppression effect of this grounding device on the system fault current is very obvious in the initial stage of the fault. And it suppresses the current stress level of the reactor to ensure the equipment safety of the grounding device.

[0057] Subsequently, the disconnectors SU, SV, and SW in parallel for three phases in the grounding device are closed. During the closing process, the fault current continues to rise. However, the fault current flows through the disconnector, avoiding the thermal stability and dynamic stability damage of the coil in the reactor.

[0058] The transient process is mainly the cooperative action process of the three-phase disconnectors, and the resistance values of the three-phase reactances do not change.

[0059] Due to the particularity of the HVDC transmission system for offshore wind power, the failure rates at the valve top position and on the DC side are very high; additionally, due to special modulation strategies, operating modes, and other factors, significant losses are generated in the grounding system; based on these two factors, the grounding device is severely damaged. The highly reliable grounding device proposed in the present invention can meet the requirements of low losses and safe operation.

[0060] Figure 5 The control flow of the grounding device is shown as follows.

[0061] Before the start of the HVDC transmission system for offshore wind power, first set the three-phase disconnector to the open state, and then the station control system starts the sequence control operation to adjust the magnitude of the grounding resistance according to the system operating mode and commissioning mode. During the natural charging and controllable charging processes of the MMC, according to the operating characteristics, the grounding system is put into a large-resistance or small-resistance grounding state, and the grounding resistance value is adjusted by opening and closing the SN.

[0062] During the steady-state operation process, the valve side of the MMC forms an artificial neutral point through a star connection of three-phase large reactors, and the neutral point is grounded through R G The current level flowing through the grounding device during the steady-state operation process is very low, and the fault judgment function of the station control system is not activated, and the drive signal QL = 0.

[0063] When a severe fault occurs in the system, or when the current stress level of the grounding device rises by more than 3 times significantly, after the judgment of the station control system, the drive signal QL = 1. When the drive signal QL = 1, first close the parallel disconnector of the three-phase large reactor, and then open the neutral point disconnector SN. For the HVDC transmission system for offshore wind power, a symmetric monopole topology structure is generally adopted. And the DC transmission line is a submarine cable, and the AC side of the offshore substation also uses a submarine cable. Therefore, the faults in the system are generally permanent faults, and QL can only be triggered under severe permanent faults. When QL is triggered, the system needs to be blocked and shut down.

[0064] In addition, when the system is in the process of adjusting the operation model, for example, when the system needs to be in light load operation, heavy load operation, or economizer test, the requirements for the grounding resistance value may be inconsistent, and the resistance value is switched through the disconnector SN.

[0065] After the system enters the steady-state operation, in order to ensure that the overvoltage level during the system operation process does not increase. And in order to detect the operation state and some characteristics of the MMC, the grounding method needs to be set to a suitable resistance grounding. Therefore, after the system enters the steady state, the disconnector SN is placed in the closed state.

[0066] Adopt the high-reliability grounding device of the present invention. This device can freely switch the grounding resistance value according to requirements such as the steady-state operation of the system, modulation mode, startup process, transient process, and fault state to meet different operating conditions and modulation modes. It can effectively meet the requirements for the grounding device under various operating conditions of the offshore wind power flexible DC transmission system. When using this grounding device, the current stress level flowing through during the steady-state operation process is very low, and the zero-sequence current flowing into the ground can be ignored. During the transient operation process, it can not only suppress the fault current but also limit the current stress level of the grounding device itself, thereby effectively protecting the grounding device. In this solution, conventional iron-core reactors, lightning arresters, resistors, and disconnectors are adopted, with relatively high reliability and very low primary equipment costs. There are no additional requirements for the secondary system, and the economy is relatively good in engineering. Moreover, the star-shaped disconnector device adopted in this device is used to protect the reactor group in severe transient faults to ensure the safety of the device. The reactor and disconnector adopted in the present invention are both conventional and mature devices, with no additional requirements for the primary system and secondary system. In addition, the device has a small volume and good economy in the offshore wind power flexible DC transmission project.

[0067] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. After those skilled in the art read this application, any modifications or changes made to the present invention with reference to the above embodiments are within the scope of protection of the claims of this invention patent.

Claims

1. A control method for a grounding device of an HVDC transmission system for offshore wind power, characterized in that: Before the startup of the offshore wind power HVDC transmission system: first set the three-phase disconnector to the open state, and then start the sequence control operation; During the steady-state operation of the system, the drive signal QL = 0. The valve side of the MMC forms an artificial neutral point through the star connection of the three-phase reactor. The artificial neutral point is grounded through a grounding resistor. The three-phase disconnector is in the open state, and the neutral point connection switch SN is in the closed state. The resistor R1 and the resistor R2 are in parallel, and the value of the grounding resistor is When a serious fault occurs in the system or the current stress level of the grounding device rises by more than 3 times, make the drive signal QL = 1; when the drive signal QL = 1, first close the three-phase disconnector, and then open the disconnector SN, and the system locks up and shuts down; after closing the three-phase disconnector and then opening the disconnector SN, the parallel relationship between the resistor R1 and the resistor R2 is released, and the grounding resistance is R1; The grounding device includes a reactor bank, a disconnector bank and a grounding resistor. The reactor bank is a three-phase reactor composed of reactors LU, LV and LW connected in star, and forms an artificial neutral point N1; the disconnector bank is a three-phase disconnector composed of disconnectors SU, SV and SW connected in star, and forms an artificial neutral point N2. The reactor bank and the disconnector bank are connected in parallel to form a double-star connection mode; the artificial neutral point N1 and the artificial neutral point N2 are connected through the disconnector SN; The grounding resistor includes a resistor R1 and a resistor R2. The first end of the resistor R1 is connected to the artificial neutral point N1, and the second end is grounded; the first end of the resistor R2 is connected to the artificial neutral point N2, and the second end is grounded.

2. The control method of a grounding device for a flexible DC transmission system of offshore wind power according to claim 1, wherein, A lightning arrester is connected in parallel with the resistor R2.

3. The control method of a grounding device for a flexible DC transmission system of offshore wind power according to claim 2, characterized in that, The resistance value of the resistor R2 is greater than the resistance value of the resistor R1.

4. The control method of a grounding device for a flexible DC transmission system of offshore wind power according to claim 1, characterized in that, The three-phase reactors constituting the reactor bank adopt iron-core reactors.

5. The control method of a grounding device for a flexible HVDC transmission system of offshore wind power according to claim 1, characterized in that During the steady-state operation of the grounding device, the three-phase disconnector is in the open state, and the neutral point switch SN is in the closed state.

6. The control method of a grounding device for a flexible DC transmission system of offshore wind power according to claim 1, characterized in that When the system adjusts the operation model, the grounding resistance value is switched through the disconnector SN.

7. The control method of a grounding device for a flexible DC transmission system of offshore wind power according to claim 1, characterized in that, After the system enters the steady state, the disconnector SN is placed in the closed state.

Images