Direct-current converter station based on bipolar structure

Abstract

The invention discloses a direct-current converter station based on a bipolar structure. The direct-current converter station consists of two convertor units connected in series, the serial connection point of the two convertor units is grounded by a neutral bus, and each convertor unit consists of a plurality of MMC (Modular Multilevel Converter) which are connected in series and in parallel. With the structure, when direct current fails, only the fault pole is affected, and a perfect pole is hardly affected, so the system reliability is improved; meanwhile, according to the direct-current converter station, the neutral bus is led out to be grounded, the stage construction and capacity-increasing construction of a system can be conducted easily, and a single pole is firstly put into operation, then double poles are put into operation, so that investment benefits can be obtained early; and simultaneously, the neutral bus is grounded by a grounding pole guide line to provide a current backflow access to a system in a single-pole ground loop operation mode, and when the system is in balance operation, the grounding current is extremely low.

Description

technical field [0001] The invention belongs to the technical field of electric power system commutation, and in particular relates to a DC converter station based on a bipolar structure. Background technique [0002] Modular Multilevel Converter (MMC) adopts the form of sub-module cascading, which has low requirements for consistent triggering of devices and dynamic voltage equalization, good scalability, high-quality output voltage waveform, low switching frequency, and low operating loss. Low and many other advantages, it is the leader of the new generation of voltage source converter topology. The MMC-based high-voltage direct current transmission system (MMC-HVDC) is better than the traditional thyristor-based direct current in the grid connection of new energy sources such as photovoltaics, wind power and tidal currents, ultra-large-scale urban power transmission and distribution, remote islands, isolated loads and passive network power transmission. The power transmi...

AI Technical Summary

Problems solved by technology

[0004] First, the design and installation of the grounding method are difficult

There are mainly two existing technologies. One is to install a three-phase star-connected reactance branch on the MMC valve side to provide a reference potential for the converter station. The DC transmission system using this technology is as follows: figure 1 As shown in (a), it is difficult to select the reactance parameters of this technology, and it greatly affects the operating range of the reactive power of the converter itself

The other is the large resistance clamp grounding method on the DC side. The DC transmission system using this grounding method is as follows: figure 1 As shown in (b), but this grounding method is closely related to the selection of resistance parameters. When the resistance is too small, the steady-state operation loss will be large. When the resistance is too large, the entire system will not be grounded, and the requirements for system insulation coordination are high.

[0005] Second, the reliability of the system operation is low

None of the existing technologies clearly pointed out that smoothing reactors should be installed on the DC side, so it is necessary to increase the parameters of the bridge arm reactors to suppress the rising rate of the fault current on the DC side; and it can only operate in bipolar operation composed of a single voltage source converter. mode, once the DC line fails, it is very prone to bipolar outage, that is, the entire system is out of service, and the system reliability is low

[0006] Third, the delivery capacity is limited

Although MMC can theoretically realize infinite cascading of sub-modules, the transmission capacity will be limited to the capacity of the converter transformer. In the existing technology, it is difficult to achieve high-voltage large-capacity power similar to traditional DC transmission in the mode of using a single MMC to form a bipolar power transmission system. delivery

[0007] Fourth, the degree of standardization of system design, manufacture and installation is still low

When the number of cascaded MMC per bridge arm increases, the harmonics of the output voltage and current waveforms will be smaller and the quality will be higher. However, when the number reaches a certain level, increasing the number of sub-modules will have little effect on improving the harmonic characteristics of the system.

In addition, when the number of sub-modules is too large, the processing speed of capacitor voltage sorting and measurement will be greatly affected.

And there is no complete concept of standard design. For example, in engineering applications with different system capacities and voltage levels, almost all related content (such as controller parameter setting, measurement and sensing systems, cooling systems, etc.) need to be redesigned. System design, Both parameter setting and equipment manufacturing have brought great inconvenience, which is not conducive to the realization of equipment standardized manufacturing and production, and at the same time brought design, manufacturing, assembly, commissioning, operation and maintenance cycle

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