A grid networking support system of LCC-HVDC drive variable-frequency variable-voltage transformer and a topological function design method thereof

Abstract

The application provides a grid networking support system of an LCC-HVDC driven variable-frequency transformer and a topological function design method thereof, first proposes a reconstruction scheme and a constituting scheme of a converter station of a conventional DC LCC-HVDC system driven variable-frequency transformer, and proposes an underground shallow-buried variable-frequency transformer system and a constituting design thought, further establishes a grid networking support function design method based on variable-frequency transformer rotor kinetic energy, a weak AC system fault defense and protection system design method, and an LCC-HVDC driven variable-frequency transformer capacity estimation and support local wind and light field station MPPT maximum power generation estimation method.

Description

Technical Field

[0001] This invention belongs to the field of active support and security of new power systems, specifically involving a power grid support system for an LCC-HVDC driven frequency converter and its topology functional design method. Background Technology

[0002] The primary utilization of renewable energy sources such as wind and solar power is power generation, and its power generation and grid connection methods differ significantly from traditional synchronous generator sets. For example, wind and solar power generation typically connects to the grid via power electronic devices, thus failing to provide inertial support to the grid. Currently, almost all wind and solar power generation units use power electronic converters to track the grid frequency, interacting with the grid as a current source. Against this backdrop, the random fluctuations in wind and solar power generation and the low inertia of power electronic devices pose serious challenges to the synchronization stability, anti-interference, and control and protection performance of the sending-end and receiving-end AC power systems of my country's high-proportion wind and solar power generation, threatening the safe and stable operation of the high-proportion renewable energy power system.

[0003] In this context, such as Figure 1 As shown, when a transient disturbance occurs in the weak receiving-end AC system, such as a momentary power loss, conventional DC will respond to the disturbance instantaneously within 10 milliseconds through rapid control methods such as firing angle, arc extinction angle, and current control. However, this response will have a cascading effect on the weak sending-end AC system, causing a loss of active power in the weak sending-end system, thereby affecting the safe and stable operation of the sending-end system. Summary of the Invention

[0004] To address the challenges and problems encountered in scenarios involving high-proportion wind and solar power generation and high-proportion LCC-HVDC DC transmission, such as weakened AC system performance in terms of low inertia, interference resistance, synchronization stability, and relay protection, this invention proposes a grid support system for an LCC-HVDC drive frequency converter and its topology functional design method. The LCC-HVDC drive frequency converter directly draws DC power from the HVDC DC system, eliminating the need for an additional rectification stage for the DC motor. Simultaneously, the rotor side of the frequency converter is connected to the AC side of the converter valve, and the voltage regulation function of the frequency converter will contribute to the stability of the AC side voltage of the converter valve, reducing the probability of commutation failure in the LCC-HVDC transmission.

[0005] The frequency converter of this invention integrates related technologies such as transformer, phase shifter, hydro generator, doubly fed motor, and DC drive control. Its core is a rotary transformer with three-phase windings installed on the stator and rotor sides, and a DC motor drives the rotor to adjust the phase shift of the rotor magnetic field relative to the stator magnetic field, thereby controlling the magnitude and direction of the active power transmitted by the frequency converter.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A grid construction support system for an LCC-HVDC driven frequency converter transformer and its topology function design method are characterized by: firstly, modifying the converter station of the conventional DC LCC-HVDC system driving the frequency converter transformer and setting up an underground shallow-buried frequency converter transformer system; further designing the grid construction support function based on the rotor kinetic energy of the frequency converter transformer, the fault prevention and protection system for the weak AC system, and estimating the capacity of the LCC-HVDC driven frequency converter transformer and the maximum power generation of the local wind and solar power plant MPPT.

[0008] Furthermore, the modification of the converter station driving the frequency converter transformer of the conventional DC LCC-HVDC system includes: on the basis of the original conventional DC, i.e. LCC-HVDC, connecting low-capacity semi-controllable thyristor converter valves in series at the highest DC voltage and lowest DC voltage of the converter valves on the rectifier side and inverter side respectively, adding the AC side of the converter valves to be connected to the rotor of the frequency converter transformer through slip rings, while the stator of the frequency converter transformer is connected to the external AC system and stepped up through a three-winding transformer;

[0009] The rotor of the frequency converter transformer is driven to rotate by a DC motor. At the same time, the DC system driving the DC motor draws power directly from the DC side of the LCC-HVDC and does not require an additional DC rectification stage.

[0010] Furthermore, the underground shallow-buried frequency converter system is shallowly buried in the underground location where the converter valve or conventional transformer of the converter station is located. This is used to reduce the floor space occupied by the newly installed frequency converter in the converter station, isolate the noise caused by the rotation of the frequency converter rotor, and facilitate the natural flow of water in the water cooling system, thereby quickly cooling the frictional heat generated by the high-speed operation of the slip ring.

[0011] Furthermore, by rapidly releasing or storing the rotor kinetic energy of the frequency converter transformer, it provides grid support functions to the power grid, including: 1) frequency adjustment function for weak AC systems; 2) voltage support and stabilization function for weak AC systems; 3) providing emergency power support and spinning reserve function.

[0012] Furthermore, when a weak AC system suffers severe disturbances such as a three-phase ground fault, the fault defense and protection system of the LCC-HVDC converter valve changes with or without a frequency converter transformer. Without a frequency converter transformer, the AC side current of the LCC-HVDC converter valve drops to zero instantaneously. However, this invention proposes installing a frequency converter transformer. Since the frequency converter transformer is an electromagnetic conversion device, its AC side short-circuit current exhibits a sine wave with a large amplitude, which will be beneficial to the design of the relay protection system.

[0013] Furthermore, the capacity estimation of the LCC-HVDC drive frequency converter and the estimation of the maximum power generation capacity (MPPT) supporting the local wind and solar power station include:

[0014] The change in rotor kinetic energy of the frequency converter transformer is derived as follows:

[0015]

[0016] In the above formula, ω1 is the current rotational speed of the frequency converter transformer rotor, ω(t) is the rotational speed that the frequency converter transformer rotor can reach, and J VFT Given the inertia of the frequency converter transformer rotor, differentiate equation (3):

[0017]

[0018] From equation (4), it can be deduced that by using the rotor rapid change rate of the frequency converter transformer, the capacity of the asynchronous machine, i.e., the inertia-free inverter, that the frequency converter transformer can support can be estimated, i.e., P. inverter RoCoF max The maximum frequency change rate of a weak AC system is much smaller than the rotor rapid change rate of a frequency converter transformer;

[0019] Based on equation (4), we can infer the maximum capacity of wind and solar power inverters or the maximum LCC-HVDC input power that can be installed in a weak AC system.

[0020] The LCC-HVDC drive frequency converter proposed in this invention eliminates the need for the DC-side rectifier in traditional frequency converters, instead directly drawing DC drive power from the LCC-HVDC transmission. Notably, the frequency converter possesses a certain inertia; by releasing its kinetic energy, it provides emergency power support to weak AC systems. It also facilitates the adjustment of voltage changes at the AC side port of the LCC-HVDC, thereby avoiding significant DC current fluctuations caused by AC voltage volatility and preserving the converter's firing angle operating range. Simultaneously, the LCC-HVDC drive frequency converter reduces the reactive power compensation capacity in the AC system and also constructs an AC short-circuit current protection mechanism, facilitating the protection device's safeguarding of the converter's AC side operation. Attached Figure Description

[0021] Figure 1 A schematic diagram of a conventional high-voltage direct current transmission method for connecting the sending and receiving ends of a weak AC system;

[0022] Figure 2 A schematic diagram showing the connection of a frequency converter transformer and its modification to a conventional DC LCC-HVDC system.

[0023] Figure 3 A schematic diagram showing the location of a shallowly buried underground frequency converter transformer;

[0024] Figure 4 The AC current curves of the frequency converter transformer and LCC converter valve after an AC system fault are shown.

[0025] Figure 5 A schematic diagram of the power grid topology for a weak AC system supported by the instantaneous voltage of a frequency converter transformer. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0027] Step 1: Upgrading the frequency converter transformer in a conventional DC LCC-HVDC system

[0028] This invention improves upon the existing conventional DC, i.e., LCC-HVDC. Conventional DC, i.e., LCC-HVDC, includes a rectifier side, an inverter side with a large-capacity semi-controllable thyristor converter valve, a DC line, filtering and reactive power compensation devices, such as... Figure 1 As shown, when a conventional DC power supply is connected to a weak AC system, it is impossible to actively support the weak AC system and the access of new energy sources. This invention proposes to connect low-capacity semi-controllable thyristor (GTO) converter valves in series at the highest and lowest DC voltage points of the converter valves on the conventional DC rectifier side and inverter side, respectively. Figure 2 As shown. Assuming the DC side voltage of the LCC-HVDC is 500kV, then in the following... Figure 2 Additional converter valves are added at four locations: the highest DC voltage at the positive terminal of the LCC-HVDC rectifier, the lowest DC voltage at the negative terminal, the highest DC voltage at the positive terminal of the inverter, and the lowest DC voltage at the negative terminal. The DC side voltage of the added converter valves is designed to be 25kV, and the AC side voltage is designed to be 15kV.

[0029] The AC side of the added converter valve is connected to the rotor of the variable frequency transformer (VFT) via slip rings. Simultaneously, the stator side of the VFT is connected to the external AC system and stepped up to 110kV and 220kV AC systems via a three-winding transformer (15 / 110 / 220kV). The rotor of the VFT is driven to rotate by a DC motor, making the frequency of the voltage and current in the rotor windings time-varying, while the stator winding frequency is 50Hz. Correspondingly, the AC side frequency of the newly added converter valve in the LCC-HVDC transmission system has time-varying characteristics; however, considering the rotor speed of the VFT, the operating frequency of the AC side of the converter valve is typically less than 50Hz. When the rotor speed of the VFT is 50Hz, this time-varying frequency is 0Hz. In this invention, the DC system driving the DC motor draws power directly from the DC side of the LCC-HVDC system, eliminating the need for a DC rectification stage.

[0030] Step 2: Design and composition of underground shallow-buried frequency conversion transformer system

[0031] This invention is achieved through, as follows Figure 3 The illustrated shallow-buried frequency converter transformer unit is designed to reduce the footprint of the converter station, isolate noise from the rotating rotor of the frequency converter transformer, and facilitate the natural flow of water in the water-cooling system. This frequency converter transformer unit is typically shallowly buried underground at the location of the converter valve or conventional transformer in the converter station.

[0032] The newly added AC outlet three-phase line of the LCC-HVDC converter valve is connected to the three-phase rotor winding of the frequency converter transformer. Since the three-phase windings on the frequency converter transformer rotor move with the rotor, carbon brushes are needed as the interface between the rotor windings and the AC line of the converter valve. In this invention, the rotor of the frequency converter transformer will rotate at a speed less than 50Hz (the power frequency). This causes the rotor joints to run at high speed in the slip rings, generating some friction. However, the current frequency in the rotor is low, meaning the corresponding current stress is relatively small. Under these conditions, the frequency converter transformer operates equivalent to a synchronous condenser.

[0033] Step 3: Design the power grid support function based on the rotor kinetic energy of the frequency converter transformer

[0034] The rotor of the frequency converter transformer is at ω r The rotational speed at which it rotates corresponds to the kinetic energy it stores, which can be expressed as:

[0035]

[0036] In the above formula, J represents the inertia of the rotor of the frequency converter transformer during rotation, and f r This refers to the frequency at which the rotor of the frequency converter rotates.

[0037] Variable frequency transformers primarily rely on speed and power regulation. By controlling the rotor speed, they can synchronize the magnetic field generated by the rotor winding current with the magnetic field generated by the stator winding current. Furthermore, they can release or absorb rotor kinetic energy to cope with disturbances or abnormal operating conditions in the power grid. Specifically, this includes the following active power grid support functions:

[0038] 1) Frequency adjustment function: When the frequency of the AC system changes beyond the rated value, the power input of the frequency converter to the AC system is changed by combining the control of the LCC-HVDC converter valve and the drive of the DC motor of the frequency converter transformer, thereby adjusting the frequency of the AC system so that its fluctuation is within the normal range.

[0039] 2) Powering weak systems: Traditional LCC-HVDC converters lack the synchronization and stabilization function for weak power grids, requiring a robust AC system to support their operation. In this invention, the frequency converter transformer has a power grid self-synchronization function, serving as a stable voltage source to support... Figure 1The normal operation of the LCC-HVDC commutation valve is crucial to prevent commutation failure and to serve as a voltage source to support the stable operation of the weak AC system.

[0040] 3) Spinning standby: The rotor of the frequency converter transformer in this invention is equivalent to a flywheel energy storage device. When a large disturbance occurs in the external system, the rotor kinetic energy is released quickly to provide emergency power support and spinning standby.

[0041] Step 4: Design a fault defense and protection system for the weak AC system of the LCC-HVDC drive frequency converter transformer.

[0042] When a weak AC system experiences severe disturbances such as a three-phase ground fault, the LCC-HVDC converter valve typically locks out, and its AC side short-circuit current drops directly to near zero, without exhibiting a sinusoidal wave. Figure 4 As shown, when an AC side of the LCC-HVDC converter is connected to a frequency converter transformer, since the latter is an electromagnetic conversion device, its AC side short-circuit current exhibits a large-amplitude sine wave. On the other hand, without considering the frequency converter transformer, severe voltage fluctuations occur on the AC side of the LCC-HVDC converter valve. These voltage fluctuations will affect the normal commutation process of the converter valve, causing it to lock up. The AC and DC side voltages of the converter valve will drop, causing the converter valve current to drop instantaneously to zero, thus causing the converter valve to stop working.

[0043] The instantaneous voltage support function of the LCC-HVDC drive frequency converter proposed in this invention enables the LCC converter valve to quickly recover to the normal voltage level after a voltage dip under severe external AC system faults, thereby preventing the converter valve from entering a locked-in or commutation failure state or stopping operation.

[0044] like Figure 5 As shown, when a three-phase ground fault occurs near the 220kV busbar of the external AC system, the first intelligent switch 1 and the second intelligent switch 2 will quickly open, while the third intelligent switch 3 will remain closed. At this time, the frequency converter transformer adjusts its port voltage to maintain the AC side voltage of the converter valve; simultaneously, the DC side voltage of the converter valve returns to normal after a brief voltage drop. The LCC-HVDC continues to supply power to the frequency converter transformer, and the rotor of the frequency converter transformer stores the energy transmitted by the LCC-HVDC. As the ground fault in the external AC system is cleared, the AC side voltage of the LCC-HVDC approaches its normal voltage level. Therefore, the design of the LCC-HVDC driving the frequency converter transformer facilitates the re-stable connection of the LCC-HVDC with the external AC system, while further releasing the remaining energy stored in the frequency converter transformer to ensure the normal transmission of active power by the LCC-HVDC.

[0045] Step 5: Estimating the capacity of the LCC-HVDC drive frequency converter and estimating the maximum power generation capacity (MPPT) of the local wind and solar power station.

[0046] Assuming the DC-side voltage was ±250kV and the maximum rated DC-side current was 4kA before the LCC-HVDC converter station upgrade, the maximum transmission capacity could reach 2000MW; on the other hand, the single frequency converter transformer used after the upgrade is 100MW, meaning the capacity of the two frequency converter transformers driven by the LCC-HVDC accounts for a proportion of the total HVDC transmission capacity.

[0047]

[0048] When considering that the kinetic energy of the frequency converter transformer rotor can be captured by the weak AC system, the installed capacity of the frequency converter transformer or the amount of kinetic energy that can be released can be used to estimate the asynchronous machine installed capacity of the weak AC system (including LCC-HVDC injected power, maximum MPPT power generation of local wind and solar power plants, etc.); conversely, the required installed capacity and inertia of the frequency converter transformer can be estimated by using the asynchronous machine installed capacity of the weak AC system.

[0049] The change in the rotor kinetic energy of the frequency converter transformer is:

[0050]

[0051] In the above formula, ω1 is the current rotational speed of the frequency converter transformer rotor, ω(t) is the rotational speed that the frequency converter transformer rotor can reach, and J VFT Let be the inertia of the frequency converter transformer rotor. Differentiate equation (3):

[0052]

[0053] From equation (4), it can be deduced that by using the rotor rapid change rate of the frequency converter transformer, the capacity of the asynchronous machine, i.e., the inertia-free inverter, that the frequency converter transformer can support can be estimated, i.e., P. inverter RoCoF max The maximum frequency change rate of a weak AC system is typically much smaller than the rotor rapid change rate of a frequency converter. In power systems with a rated frequency of 50Hz, the protection limit of RoCoF is usually between 0.1Hz / s and 1.0Hz / s, while in European countries such as Ireland, the RoCoF protection limit is 1.0Hz / s, in Denmark it is 2.5Hz / s, and in Spain it is 2Hz / s.

[0054] Therefore, based on equation (4), it can be inferred that the maximum capacity of wind and solar power inverters or the maximum input power of LCC-HVDC can be installed in a weak AC system interconnected with LCC-HVDC drive frequency converter transformers.

[0055] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A power grid support system for an LCC-HVDC drive frequency converter transformer and its topology functional design method, characterized in that: First, the converter station driving the frequency converter transformer of the conventional DC LCC-HVDC system is modified, and an underground shallow-buried frequency converter transformer system is set up. Then, the grid construction support function based on the rotor kinetic energy of the frequency converter transformer, the fault prevention and protection system of the weak AC system, the capacity estimation of the LCC-HVDC driving frequency converter transformer and the estimation of the maximum power generation of the local wind and solar power station MPPT are carried out. The modification of the converter station driving the frequency converter transformer of the conventional DC LCC-HVDC system includes: on the basis of the original conventional DC, i.e. LCC-HVDC, connecting low-capacity semi-controllable thyristor converter valves in series at the highest DC voltage and lowest DC voltage of the converter valves on the rectifier side and inverter side respectively; adding the AC side of the converter valves to be connected to the rotor of the frequency converter transformer through slip rings; at the same time, the stator of the frequency converter transformer is connected to the external AC system and stepped up through a three-winding transformer. The rotor of the frequency converter transformer is driven to rotate by a DC motor. At the same time, the DC system driving the DC motor draws power directly from the DC side of the LCC-HVDC and does not require an additional DC rectification stage.

2. The power grid support system for an LCC-HVDC drive frequency converter and its topology functional design method as described in claim 1, characterized in that: The underground shallow-buried frequency converter system is shallowly buried in the underground location where the converter valve or conventional transformer of the converter station is located. It is used to reduce the footprint of newly installed frequency converters in the converter station, isolate the noise caused by the rotation of the frequency converter rotor, and facilitate the natural flow of water in the water cooling system, thereby quickly cooling the frictional heat generated by the high-speed operation of the slip ring.

3. The power grid support system for an LCC-HVDC drive frequency converter transformer and its topology functional design method as described in claim 2, characterized in that: The frequency is adjusted by controlling the power input to the AC system through the frequency converter transformer; emergency power support and rotational reserve are provided by rapidly releasing rotor kinetic energy, including: 1) frequency adjustment function for weak AC system; 2) voltage support and stabilization function for weak AC system; 3) providing emergency power support and rotational reserve function.

4. The power grid support system for an LCC-HVDC drive frequency converter transformer and its topology functional design method as described in claim 3, characterized in that: When a weak AC system is subjected to severe disturbances such as a three-phase ground fault, the AC side short-circuit current of the frequency converter transformer, which is an electromagnetic conversion device, will exhibit a sine wave, which is beneficial to the relay protection system.

5. The power grid support system for an LCC-HVDC drive frequency converter transformer and its topology functional design method as described in claim 4, characterized in that: The capacity estimation of the LCC-HVDC drive frequency converter and the estimation of the maximum power generation of the local wind and solar power station MPPT include: The change in rotor kinetic energy of the frequency converter transformer is derived as follows: （3） In the above formula, This represents the current rotational speed of the frequency converter transformer rotor. This refers to the maximum speed that the rotor of the frequency converter transformer can achieve. Given the inertia of the frequency converter transformer rotor, differentiate equation (3): （4） From equation (4), it can be deduced that by using the rotor rapid change rate of the frequency converter transformer, the capacity of the asynchronous machine, i.e., the inertia-free inverter, that the frequency converter transformer can support can be estimated. , The maximum frequency change rate of the weak AC system is less than the rotor rapid change rate of the frequency converter transformer; Based on equation (4), we can infer the maximum capacity of wind and solar power inverters or the maximum LCC-HVDC input power that can be installed in a weak AC system.

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