Full Converter Damping Subsynchronous Oscillations
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Solution Overview
Problem
Series-compensated transmission lines in wind turbines are prone to subsynchronous oscillations (SSOs) that can cause damage due to resonance interactions, leading to shaft fatigue and failure, and existing solutions like FACTS controllers are expensive and difficult to operate.
Innovation Solution
A full converter system with closed-loop current control within the line side converter is used to actively damp SSR oscillations, preventing excitation and isolation of wind turbine components from SSOs, employing vector control and pulse width modulation to maintain stability at synchronous frequencies while reducing subsynchronous frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If series capacitors are installed to compensate voltage drop and increase power transfer capacity, then power transfer capacity is improved, but subsynchronous oscillations are excited causing shaft fatigue and damage
Solution Approach 1:
The full converter system acts as an intermediary between the series-compensated transmission line and the wind turbine generator. It actively dampens subsynchronous oscillations before they can excite the turbine shaft, thereby protecting the shaft from fatigue and damage while allowing the series capacitor to remain in service for power transfer enhancement.
Solution Approach 2:
The control system dynamically adjusts the converter's impedance parameters to achieve high gain at synchronous frequencies (maintaining power transfer) and low gain at subsynchronous frequencies (damping oscillations). This frequency-dependent parameter adjustment resolves the contradiction by selectively treating different frequency components.
2Reliability
If FACTS controllers are used to control subsynchronous oscillations, then shaft protection is improved, but system cost and operational complexity increase
Solution Approach 1:
The full converter system performs multiple functions: it enables variable speed operation for maximum power point tracking, controls active and reactive power output, and actively damps subsynchronous oscillations. By integrating these functions into a single system, the patent avoids the need for separate FACTS controllers, thereby reducing overall system complexity while maintaining shaft protection.
3Reliability
If full converter system with closed-loop current control is used to damp SSR oscillations, then shaft protection is improved, but converter complexity increases
Solution Approach 1:
The closed-loop current control system continuously monitors the converter's output current and adjusts the switching commands to maintain the desired current waveform. This feedback mechanism enables the converter to actively damp subsynchronous oscillations by injecting counter-phase currents, providing shaft protection through a relatively simple control structure compared to traditional FACTS devices.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The full converter system effectively prevents the excitation of subsynchronous torsional and control interactions, ensuring the stability and safety of wind turbine components connected to series-compensated transmission lines by maintaining high gain at synchronous frequencies and low gain at subsynchronous frequencies, thus avoiding damage from SSR oscillations.
Implementation Method 1
A full converter system with closed-loop current control within the line side converter is used to actively damp SSR oscillations
Implementation Method 2
employing vector control and pulse width modulation to maintain stability at synchronous frequencies while reducing subsynchronous frequencies
Implementation Method 3
series-compensated transmission lines having a capacitor connected in series with the transmission line. The capacitor compensates the voltage drop across the primarily-inductive transmission line
Implementation Method 4
The series capacitor resonates with the electrical inductances of the machine and of the transmission line and interconnecting transformers
Implementation Method 5
SSR oscillations can also interact with a wind turbine control system and thereby potentially excite torsional oscillations in the wind turbine generator shaft
Implementation Method 6
When it involves a torsional natural frequency of a turbine-generator shaft, the SSR is commonly called an SSTI
Data Source
AI summary
A full converter connected to a series compensated transmission line. The full converter comprises a generator side converter, a DC link connected across an output of the generator side converter, a line side converter connected across an output of the DC link, and a converter controller configured to control the full converter to mitigate effects of subsynchronous oscillations present on the series compensated transmission line.