DC side voltage control method of flexible multi-state switch when grid voltage is unbalanced

Abstract

The invention discloses a direct-current side voltage control method for a flexible multi-state switch at an unbalanced power grid voltage; the method comprises the following steps that when multipleports of the flexible multi-state switch suffer from the unbalanced power grid voltage at the same time, a direct current side voltage active power fluctuation mathematical model is established; positive-sequence and negative-sequence synchronous rotation coordinates are established under the traditional UdcQ control, wherein in the positive sequence, the traditional PI control voltage outer ringis adopted to inhibit transient voltage fluctuation; meanwhile, the two-frequency-frequency fluctuation amount of the direct-current side voltage is transformed into a negative-sequence current instruction after passing through a resonance controller, a resonance closed loop is formed to directly restrain the two-frequency ripples of the direct-current side voltage; and finally, floating control is realized by adopting positive and negative sequence current inner ring PI control. According to the method, the two-frequency voltage fluctuation caused by unbalanced power grid voltage can be effectively inhibited, and the service life of the capacitor is prolonged; and meanwhile, generation of harmonic current is avoided, and the method is suitable for single-port and multi-port unbalanced conditions.

Description

technical field [0001] The invention relates to a method for controlling the DC side voltage of a flexible multi-state switch, in particular to a method for controlling the DC side voltage when there are multiple ports and the grid voltage is unbalanced at the same time. Background technique [0002] With the continuous increase in the penetration rate of distributed new energy such as wind energy and solar energy, the operating environment of the power grid is becoming increasingly complex, and the traditional primary equipment of the distribution network is seriously insufficient in controlling the power grid. In recent years, the flexible multi-state switch (FMSS) has attracted the deep attention and research of many scholars because of its flexible ability to control the distribution network. The flexible multi-state soft switch is a power electronic device installed at the traditional tie switch, which can quickly and accurately control its own power flow, change the po...

AI Technical Summary

Problems solved by technology

The three-phase unbalance of the FMSS grid voltage or the asymmetrical load connected to the feeder will cause the exchange power of the converter to appear double-frequency AC fluctuation components, so that the DC side voltage will appear periodic double-frequency fluctuations under steady state

[0003] Most of the existing methods are only suitable for single-port networks, and are used to suppress power fluctuations generated when the grid voltage is unbalanced, and cannot be applied to multi-port converters.

There are also some literatures that propose resonant control of the DC side voltage, but a large amount of harmonic current will be generated

[0004] The double frequency fluctuation of the DC side voltage will in turn cause the AC side current to generate negative sequence and triple frequency harmonic currents

What's more serious is that the fluctuation of the DC side voltage will not only affect the current quality of the AC side output, but also increase the stress on the capacitor and reduce the service life of the capacitor

At the same time, the stability of the DC side voltage is the basis for realizing the seamless switching of other port operation modes of the FMSS. When the fluctuation is severe, the system will oscillate and cannot work normally. Therefore, an effective control method is needed to realize the DC side voltage of the FMSS under the unbalanced grid voltage. no fluctuation control

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