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Asymmetric operation control method for DC side single-pole ground fault in mmc‑hvdc system

An MMC-HVDC, single-pole grounding fault technology, applied in power transmission AC network, emergency treatment AC circuit layout, etc., can solve the problems of high construction cost, increase the difficulty of manufacturing transformers and related equipment in the connection area, etc., and achieve low construction cost. , The effect of improving active defense capability and fast system recovery

Active Publication Date: 2017-10-31
HUAZHONG UNIV OF SCI & TECH
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Problems solved by technology

However, compared with the MMC-HVDC system with unipolar symmetrical wiring, this wiring method requires the AC side of each pole to bear the DC bias of half the rated DC voltage during normal operation, thus improving the relationship between the transformer and the connection area. The equipment is difficult to manufacture and the construction cost is high. At present, the only flexible direct current transmission project in the world that adopts this connection method is the Caprivi project connecting Namibia and Zambia

Method used

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  • Asymmetric operation control method for DC side single-pole ground fault in mmc‑hvdc system

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Experimental program
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Effect test

Embodiment 1

[0048] For the MMC-HVDC system with parallel reactance and resistance grounding on the AC side, after a unipolar ground fault occurs on the negative line, each bridge arm is equivalent to a controllable voltage source, then the AC side grounding electrode, sub-module capacitor and DC side fault are connected The location forms a fault circuit, such as Picture 10 Shown by the dotted line.

[0049] According to Kirchhoff's voltage law, the following relationship can be derived:

[0050]

[0051] According to Kirchhoff's current law, the following relationship can be derived:

[0052] I fault = I faultA +I faultB +I faultC

[0053] From the above formula, we can finally get:

[0054]

[0055] When the fault occurs to the steady state, the positive and negative bus voltages are:

[0056]

[0057] At this time, the three-phase outlet voltage of the AC side of the converter V a , V b , V c Expressed as:

[0058]

[0059] Where I fault Is the fault current at the fault point, I faultA , ...

Embodiment 2

[0066] For the MMC-HVDC system where the Y winding of the AC side transformer is grounded with a resistance, after a unipolar ground fault occurs, the fault path is as Figure 14 Shown by the dotted line, and Picture 10 The grounding method of the external resistance of the parallel reactor on the AC side is similar. Also according to Kirchhoff's voltage and current law, it can be deduced that after a negative bus unipolar ground fault occurs, the positive and negative DC bus voltages at steady state are:

[0067]

[0068] Similarly, at this time, the three-phase outlet voltage V of the AC side of the converter a , V b , V c Expressed as:

[0069]

[0070] The relationship satisfied between the DC side positive and negative bus voltage and the AC side outlet voltage in the steady state is the same as that of the AC side shunt reactor in the first embodiment after the unipolar grounding fault occurs.

[0071] Suppose that at 1.0S, it is detected that a ground fault occurs on the nega...

Embodiment 3

[0077] For the MMC-HVDC system with parallel clamping and large resistance grounding on the DC side, when a single-pole ground fault occurs, the equivalent circuit is simplified such as Figure 17 As shown, the grounding point changes from position ① to position ②. Because the resistance of the parallel resistor on the DC side is extremely large, which is almost open, each module capacitor does not have a discharge path with the fault grounding point, the capacitor voltage remains stable, and the capacitor current component remains the same as before the fault. Also due to the change of the grounding point, change The voltage on the AC side of the converter is equivalent to the voltage of the lower bridge arm, and a DC bias of half the rated DC voltage appears. The non-faulty pole DC line bears the entire DC voltage at this time, which is twice as high as before the fault. It can be seen that a unipolar ground fault on the DC side poses a serious insulation threat to the AC side...

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Abstract

The invention discloses an asymmetric operational control method of the direct current side monopolar grounding fault of an MMC-HVDC system. For the MMC-HVDC system based on monopolar symmetric wiring, after a direct current side monopolar grounding fault occurs, over voltage and fault current at AC and DC sides can be rapidly eliminated by setting the DC component of the output voltage of a bridge arm of a fault pole without locking a current converter, and further the insulation threat to AC / DC systems can be eliminated. Through adjusting phase angles of alternating components of different bridge arm voltages, the system still can continue transmitting half of a rated active power and provide reactive power support for the AC system while isolating the direct current side monopolar grounding fault, which has positive significance for the stability of the connected AC / DC systems; during the fault period, the current converter does not need to exit from operating, the restore speed of the system is high, the current converter is in a controlled state in the whole process, and therefore the active defense capability of the MMC-HVDC system based on monopolar symmetric wiring against the direct current side monopolar grounding fault can be improved.

Description

Technical field [0001] The invention belongs to the technical field of multilevel power electronic converters, and more specifically, relates to an asymmetric operation control method for a unipolar ground fault on the DC side of an MMC-HVDC system. Background technique [0002] The DC grid technology based on flexible DC transmission is considered to be the most effective technical solution in terms of large-scale distributed renewable energy access, ocean archipelago power supply, offshore wind farm cluster transmission, and new urban power grid construction. Research hotspots in the field of international power. The technology and construction of the DC transmission network has become an important development direction and part of the future power grid. The Modular Multilevel Converter Based High Voltage Direct Current (MMC-HVDC) system based on modular multilevel converters has been used in flexible DC transmission due to its advantages in system loss, capacity improvement, ...

Claims

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Application Information

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Patent Type & Authority Patents(China)
IPC IPC(8): H02J3/36
CPCH02J3/001H02J3/36Y02E60/60
Inventor 胡家兵徐克成万敏
Owner HUAZHONG UNIV OF SCI & TECH
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