TCSC Damping Control for Sub-Synchronous Resonance Mitigation
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Solution Overview
Problem
Existing control systems for damping subsynchronous resonances (SSR) in electric power systems rely on mechanical damping, which is difficult to determine and guarantee, making it challenging to effectively mitigate SSR without assuming mechanical damping values.
Innovation Solution
A thyristor-controlled series capacitor (TCSC) system that provides positive damping in a narrow band around a discrete frequency, selected based on natural oscillation behavior, using a control apparatus with a damping controller, firing circuit, and phase-locked loop to independently control electrical damping, reducing dependence on mechanical damping.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If TCSC control relies on mechanical damping to mitigate SSR, then damping effect is achieved, but the system becomes dependent on difficult-to-determine mechanical damping values
Solution Approach 1:
The patent replaces dependence on mechanical damping with electrical damping provided by the TCSC. The control system generates a damping signal that directly modulates the TCSC's impedance to provide positive damping in the subsynchronous frequency range, eliminating the need to rely on mechanical damping values from the turbine-generator shaft system.
Solution Approach 2:
The patent implements a feedback control mechanism where the damping controller receives signals representing subsynchronous oscillations and generates appropriate control signals to the firing circuit. This closed-loop feedback system actively suppresses SSR by continuously adjusting the TCSC's impedance based on detected oscillation conditions.
2Adaptability or versatility
If TCSC provides broad frequency range compensation, then power flow control is improved, but subsynchronous resonance risk increases
Solution Approach 1:
The patent applies local quality by providing different impedance characteristics at different frequency ranges. The TCSC provides capacitive compensation at power frequency for improved power flow control, while simultaneously providing inductive impedance in the subsynchronous frequency range to prevent SSR. This frequency-dependent local quality resolution allows both benefits to coexist.
Solution Approach 2:
The patent uses dynamic impedance control where the TCSC's effective impedance changes dynamically based on the frequency of the current. The thyristor firing control adjusts the conduction angle to provide different reactance values at different frequencies, enabling the same device to provide both power flow control and SSR protection.
3Speed
If TCSC response time is reduced for SSR damping, then subsynchronous resonance control is improved, but power system control capability is reduced
Solution Approach 1:
The patent segments the control function into different time scales and frequency ranges. The firing control provides fast response (less than 10 ms) for subsynchronous frequency components to damp SSR, while the boost controller provides slower response for power frequency and electromechanical oscillation damping. This segmentation allows the TCSC to operate effectively at multiple time scales simultaneously.
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 TCSC system effectively dampens SSR by providing positive electrical damping, independent of mechanical damping, thereby enhancing the stability and transfer capability of the power system without requiring pre-defined mechanical damping values.
Implementation Method 1
In a TCSC, the whole capacitor bank, or alternatively a section of the capacitor bank, is provided with a parallel thyristor controlled inductor which circulates current pulses that add in phase with the line current
Implementation Method 2
The TCSC acts to eliminate this risk for coinciding resonance frequencies by making the series capacitors act inductive in the subsynchronous frequency band
Data Source
Figure 1~2
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Figure 5
AI summary
An apparatus (17) for controlling a thyristor controlled series capacitor means connected to a power transmission line (12), the control apparatus comprising a thyristor firing control (18) comprising means for effectuating a desired capacitor voltage zero crossing in dependence of the line current and the capacitor voltage in response to a command signal, and a command control (19) for providing the command signal to the thyristor firing control. The command control comprises a damping control (24) comprising means for providing damping at a least one discrete frequency.