CN110994660A

CN110994660A - MMC power operation interval optimization method based on energy flow law

Info

Publication number: CN110994660A
Application number: CN201911069790.8A
Authority: CN
Inventors: 夏向阳; 石超; 刘远
Original assignee: Changsha University of Science and Technology
Current assignee: Changsha University of Science and Technology
Priority date: 2019-11-07
Filing date: 2019-11-07
Publication date: 2020-04-10

Abstract

The invention discloses an MMC power operation interval optimization method based on an energy flow rule, which can realize maximum power transmission under the condition of an alternating current side fault of a flexible direct current transmission (MMC-HVDC) system. Firstly, analyzing the fault characteristics of an alternating current side, understanding the energy distribution condition and the fluctuation characteristics of instantaneous energy of the alternating current side under different fault types, constructing the coupling relation between the dynamic process of the energy and each electrical parameter of the system, finding out the energy conversion rule of the flexible direct current power transmission system, optimizing the MMC operation power interval on the basis of the energy conversion rule, constructing the analytical expression of the maximum active power transmission capability of the MMC under different fault conditions, and determining the optimal energy distribution of the MMC-HVDC system.

Description

MMC power operation interval optimization method based on energy flow law

Technical Field

The invention relates to maximum power transmission under the condition of a fault at an alternating current side of a flexible direct current transmission (MMC-HVDC) system, in particular to an MMC power operation interval optimization method based on an energy flow rule when the fault at the alternating current side of the system occurs, and belongs to the technical field of electric power.

Background

The high-voltage flexible direct-current transmission technology based on a Modular Multilevel Converter (MMC) is a new generation direct-current transmission technology taking a voltage source Converter as a core, and is a control high point of the application of power electronic technology in the world at present.

The fault of the flexible direct current transmission system comprises an internal fault of the converter, a fault of a direct current side and a fault of an alternating current side. The alternating current side fault generation mechanism is multifaceted, when an alternating current network is asymmetric, the alternating current side current of the MMC converter is unbalanced, the sum of three-phase instantaneous power at the alternating current side is time-varying fluctuation quantity, power oscillation is generated, meanwhile, the direct current side has the problems of larger 2-frequency doubling current, voltage fluctuation and the like, the safe and stable operation of the system is endangered, and particularly when the alternating current network has an asymmetric grounding fault, the problem is more serious. An electric power system adopting an overhead line is easy to have ground faults due to factors such as lightning stroke, dirt on an insulator or salt fog, and the like, and the asymmetric ground fault is the most common in all alternating-current faults and accounts for more than 70%. Therefore, it is important to prevent the power oscillation caused by the asymmetric fault of the alternating current network from being transmitted to the direct current system and ensure the continuous power transmission and good operation performance of the MMC-HVDC system.

So far, the optimal control of a converter station for realizing the fault ride-through of an alternating current side of an MMC-HVDC system under the condition of the fault of the alternating current side is relatively less, the existing methods comprise dead-beat direct current control, phase-locked loop control, sequence component methods and the like, but still some problems need to be solved, such as the dynamic flow process of system energy, the analysis of an internal energy mechanism of a converter, the optimization of a system operation power interval and the like in the absence of the fault of the alternating current side. Therefore, if the mutual relation between the occurrence of the fault at the AC side and the inside of the converter can be analyzed, the energy conversion rule of the flexible DC power transmission system is found; on the basis, an MMC power operation interval optimization method based on an energy flow law under an alternating-current side fault is provided in a multi-target safe operation domain under different fault working conditions, and the effect that system indexes approach the optimal output is achieved.

Disclosure of Invention

The invention aims to improve the MMC power transmission limit capacity of a system during fault ride-through, explores the energy conversion rule of a flexible direct-current transmission system, and has the innovation points that: the method has the advantages that the coupling relation between the internal and external electrical parameters of the system and energy distribution under different fault types of the AC side of the MMC-HVDC system is found, the dynamic mechanism of the system energy under different working conditions of the AC side is disclosed, the flow rule of the converter energy during the fault of the AC side is obtained, and the MMC power operation interval optimization method based on the energy flow rule is provided, so that the optimal energy distribution of the MMC-HVDC system is determined, and the maximum power transmission under the fault of the AC side is realized.

The invention provides an MMC power operation interval optimization method based on an energy flow rule, which comprises the following steps:

step S1: analyzing fault characteristics of an alternating current side of an MMC-HVDC system, comprehensively considering the problems of submodule capacitor voltage fluctuation, direct current bus overvoltage, overcurrent, double frequency fluctuation of active power, rated capacity limitation and the like faced by the MMC, and establishing a power operation interval boundary criterion of the MMC;

step S2: on the basis of the criterion of the power operation interval, analyzing the energy flow condition of the alternating current side of the system under different working conditions and the fluctuation characteristic changing along with the fault, constructing the coupling relation between the energy dynamic process and each electrical parameter of the system, and summarizing the MMC energy conversion rule based on different fault types and different voltage drop degrees;

step S3: comprehensively considering the multi-factor constraints, and constructing an analytical expression of the maximum active power transmission capability of the MMC under different working conditions on the AC side of the system;

step S4: and analyzing the influence of the redistribution of the fault energy on each electrical parameter of the system according to the analytical formula, so as to research the optimization method of the MMC power operation interval and determine the optimal energy distribution of the MMC-HVDC system.

Advantageous effects

The method has the advantages that the operation mechanism in the MMC-HVDC system is determined, understanding of the dynamic process of system energy flow and determination of the optimal distribution of system energy during the fault of the alternating current side are facilitated, active adjustment of fault energy and the distribution of the energy of each bridge arm in the converter are possible by controlling the current component of each bridge arm, and the effect that the index of the flexible direct current transmission system approaches to the optimal output effect in a multi-target safe operation domain can be achieved.

Drawings

FIG. 1 is a flow chart of fault signature analysis provided by the present invention;

FIG. 2 is a graph of the coupling of energy distribution to various electrical parameters of the system;

FIG. 3 is a power interval optimization diagram;

FIG. 4 is a flow diagram of a technique implementation.

Detailed Description

The invention is further described with reference to the accompanying drawings and the detailed description.

Referring to fig. 1, for analyzing the ac side fault characteristics of the MMC-HVDC system, first, an MMC-HVDC simulation model including an ac system interface, a dc line, two-end converter stations, a converter transformer, and a data conversion interface needs to be built in the ADPSS. Through interface variables, a control system built by adopting the model is associated with a built primary system to carry out simulation research on a direct-current power transmission system, various faults on an alternating-current side, such as two-phase short-circuit faults, disconnection faults, single-phase earth faults, two-phase earth faults, three-phase short-circuit faults, impedance imbalance and the like, are simulated to obtain a fault equivalent circuit model, and meanwhile, model parameters, boundary conditions, fault duration and the like are adjusted, so that various factors are fully considered. And extracting the voltage and current values of the bridge arm and the rectifying side of the converter and the fault time of the power transmission line of the inverter. And (2) introducing the signals into MATLAB, calculating extracted signal parameters into active and reactive energy values, analyzing energy distribution conditions at different positions in a time domain to solve energy flow conditions, deducing an analytical formula, converting the signals from the time domain to a frequency domain by using generalized S transformation, separating to obtain characteristic harmonics, non-characteristic harmonics and other signals, obtaining signal energy distribution conditions at different frequencies, and taking the signal energy distribution conditions as fault characteristics.

Referring to fig. 2, in order to find a coupling relationship between energy distribution and each electrical parameter of the system, the system energy operating characteristics under various alternating current side faults need to be analyzed, and by combining the energy conversion process under the steady-state operation of the MMC-HVDC system and the characteristic analysis of the change of each electrical parameter (each component of bridge arm current, sub-module voltage, alternating current three-phase voltage current and the like) of the system under the faults, the energy flow conditions of the system under different fault conditions are obtained according to the established MMC-HVDC system electromagnetic transient simulation model, and the MMC-HVDC system energy flow loop under the fault conditions is reconstructed.

Referring to fig. 3, the problems of overvoltage, overcurrent, power frequency doubling fluctuation, sub-module capacitance voltage fluctuation threshold, system capacity limitation and the like of a direct current bus which may be faced by the MMC when a system ac side fails are comprehensively considered, the influence of multiple factors on the safe operation area of the MMC is analyzed, and the safe operation area of the MMC under the constraint of the multiple factors is determined.

Referring to fig. 4, according to the safe operation area of the MMC, the transient energy conversion rules of the MMC with different fault types and different voltage drop degrees are analyzed, different alternating-current side faults are discussed in a classified manner, and an analytical expression that the maximum power transmission capability of the MMC changes along with the voltage drop degree (k) is constructed. Considering that the fluctuation characteristics of different internal and external electrical parameters can change the power limit of an MMC operable area, analyzing the influence of energy flow between lower bridge arms of different control targets on internal and external related electrical parameters, researching an MMC power operation interval optimization method, improving the maximum power transmission capability of the MMC during the fault ride-through period of an alternating current side, and solving the optimal energy distribution of the MMC-HVDC system under different fault working conditions.

The above description is only exemplary of the present invention and should not be taken as limiting the invention, as any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. An MMC power operation interval optimization method based on an energy flow law is characterized by comprising the following steps:

step S1: an MMC-HVDC simulation model is built in the ADPSS, fault characteristic analysis is carried out on the AC side of the MMC-HVDC system, and the problems of submodule capacitor voltage fluctuation, direct current bus overvoltage, overcurrent, active power double frequency fluctuation, rated capacity limitation and the like faced by the MMC are comprehensively considered;

step S2: describing energy distribution conditions of the system under different working conditions according to the analysis result of the system fault characteristics;

step S3: summarizing an MMC energy conversion rule based on different fault types and different voltage drop degrees, and analyzing the fluctuation characteristic of instantaneous energy;

step S4: and analyzing the influence of the redistribution of the fault energy on each electrical parameter of the system, and determining the optimization method of the MMC power operation interval.

2. The method of claim 1, wherein a power operating interval boundary criterion for the MMC is established.

3. The method of claim 1, wherein the energy dynamics process is coupled to the system electrical parameters.

4. The method of claim 1, wherein an analytical expression of the maximum active power transmission capability of the MMC is established for different operating conditions.

5. The method according to claim 1, characterized by determining an optimal distribution of the MMC-HVDC system energy, achieving maximum power transmission at fault.