Self-Commutated Converter Control for Weak AC Network Stability
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Solution Overview
Problem
Existing methods for controlling self-commutated converters in weak high-voltage AC networks, such as those connecting offshore wind farms to onshore power supplies, face challenges in stabilizing voltage and frequency due to non-linear impedance and lack of a separate energy source, leading to inefficient power transmission and network stability issues.
Innovation Solution
A method that determines active and reactive power differences at a common coupling point, using orthogonal and parallel controllers to minimize reactive power exchange and a frequency controller to stabilize network frequency, eliminating the need for a direct current controller and allowing for stable operation without a dedicated energy source.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a self-commutated converter is connected to a weak high-voltage AC network without a separate energy source, then the network can operate with reduced infrastructure, but the voltage and frequency stability deteriorates due to non-linear impedance
Solution Approach 1:
The converter controls its own operating point by autonomously regulating active and reactive power exchange with the network, without requiring external control systems or separate energy sources. The converter independently adjusts its power output to maintain stability in the weak AC network
Solution Approach 2:
The converter continuously monitors the actual active and reactive power exchange with the network and compares these values against reference values. Based on the deviations detected, the control system automatically adjusts the converter's operating parameters to maintain voltage and frequency stability
2Device complexity
If conventional control methods are used in weak AC networks with non-linear impedance, then the control system structure is simple, but the dynamic behavior and precision of power control deteriorates
Solution Approach 1:
The control task is divided into two independent segments: active power control and reactive power control. Each segment has its own reference value and control loop, allowing precise independent regulation of real and reactive power exchange with the network
Solution Approach 2:
The converter dynamically adjusts its operating point in real-time based on actual power measurements and reference values. The control system continuously modifies the converter's output to respond to changing network conditions and maintain optimal performance
3Reliability
If the converter autonomously controls its operating point through active and reactive power regulation, then the network stability improves, but the control algorithm complexity increases
Solution Approach 1:
The converter performs multiple functions simultaneously: it generates active power, regulates reactive power, maintains voltage stability, and ensures frequency stability. A single control system handles all these tasks by coordinating active and reactive power exchange with the network
Solution Approach 2:
The converter independently determines and maintains its operating point by self-regulating its active and reactive power output. The system uses internal measurements and control algorithms to autonomously ensure network stability without external intervention
Data Source
AI summary
A self-commutated converter is connected to further self-commutated converters by its AC voltage connection via an inductive component using a coupling point, which is common to all the converters, in an AC voltage network. An active power P and a frequency fN are determined from a network voltage at the coupling point and a converter current flowing via the inductive component. An active power difference value ΔP is supplied to an orthogonal controller and to a parallel controller. The output value from the parallel controller is used to minimize the reactive power exchanged between converter and coupling point. The frequency difference value Δf is supplied to a frequency controller and the output value from the frequency controller is logically combined with the output value from the orthogonal controller and the output value from the parallel controller, the frequency difference value Δf being simultaneously minimized.