Segmented Voltage Initiation for Flexible DC Isolated Island Systems
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Solution Overview
Problem
Existing methods for initiating flexible DC transmission systems under isolated island conditions often cause disturbances and shocks, and fail to effectively manage high-frequency components and distortions during power loss of island AC grids.
Innovation Solution
A method involving zero-voltage initiation through open-loop control followed by dual closed-loop control, where the voltage reference wave is segmented with increasing rates, ensuring a steady-state voltage and minimizing disturbances, with specific constraints on initial values and rates to prevent excitation surge currents and protection malfunctions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional initiation methods are used under isolated island conditions, then the system can be initiated, but large disturbances and shocks occur during initiation
Solution Approach 1:
The voltage reference wave is divided into multiple segments with different increase rates. The first segment uses a rapid increase rate (α≥2.0 p.u/s) to quickly establish voltage, while subsequent segments use slower rates to smoothly reach rated voltage, thereby avoiding large disturbances and shocks during initiation.
Solution Approach 2:
The method performs zero-voltage initiation through open-loop control first to establish a stable baseline, then transitions to dual closed-loop control. This preliminary action prepares the system in advance to avoid sudden shocks when full control is engaged.
2Productivity
If voltage is rapidly increased during initiation, then initiation speed is improved, but high-frequency components and voltage/current distortions are generated
Solution Approach 1:
The voltage increase process is segmented into multiple stages. The first segment rapidly increases voltage from zero or fixed value Ux to Ua at rate α≥2.0 p.u/s to ensure fast initiation. Subsequent segments then slowly increase voltage at rates not greater than α to smoothly reach rated voltage UN, effectively suppressing high-frequency components and distortions.
Solution Approach 2:
The voltage increase rate is made dynamic rather than constant. The system adjusts the rate according to the initiation stage: high rate (α≥2.0 p.u/s) in the first segment for speed, then lower rates in subsequent segments for quality, achieving both fast initiation and clean waveforms.
Data Source
AI summary
During initiation under an isolated island condition, the amplitude of a voltage reference wave output generated by a flexible DC transmission control and protection system is in a segmented mode, wherein a first segment of the reference wave amplitude begins at zero or a fixed value Ux and rapidly increases to Ua kV at a rate of a kV/s, avoiding a low output voltage reference wave and high error and harmonic content of a sampled voltage and current. A second segment increases to Ub kV at a rate of β kV/s, a third segment increases to Uc kV at a rate of γ kV/s, and the Nth segment increases to a rated voltage of UN kV at a rate of ε kV/s, thereby achieving non-disturbance and shockless initiation of a flexible DC transmission system under an isolated island condition.