DFIG DC-Chopper Control for Fast Power Reduction and Rotor Overspeed
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Solution Overview
Problem
The integration of wind turbines into power grids poses challenges due to their intermittent and fluctuating power outputs, leading to rotor over-speed issues during fast active power reduction (FPR) events, which can damage mechanical components and require continuous operation of energy dissipation devices like DC choppers, increasing operational costs.
Innovation Solution
A FPR system for doubly-fed induction generators (DFIG) that includes a pitch servo system, a DC-link, rotor-side converter (RSC), and grid-side converter (GSC) connected in parallel, along with a DC chopper circuit featuring a dump resistor and a fully-controlled power switching device, driven by a power switching device driver and dual closed-loop vector control units to coordinate pitch angle and power switching for timely energy dissipation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If FPR is achieved by controlling the VSCs inside the WTGs, then the power output can be rapidly reduced under grid emergency conditions, but the excessive power is accumulated in the form of rotor kinetic energy causing rotor over-speed that potentially damages the WTG mechanical components
Solution Approach 1:
The patent extracts the excessive kinetic energy from the rotor system by introducing a DC chopper circuit connected to the DC-link. The chopper circuit provides a separate energy dissipation path that diverts excess energy away from the rotor, preventing rotor over-speed while enabling rapid power reduction through VSC control.
Solution Approach 2:
The DC chopper circuit acts as an intermediary energy dissipation device between the VSCs and the rotor. It mediates the energy transfer by absorbing excess kinetic energy during FPR events, converting it to heat through the dump resistor, and preventing direct transmission of over-speed conditions to the rotor mechanical components.
2Object-affected harmful factors
If the pitch angle is adjusted to decrease the amount of mechanical power input, then the rotor over-speed can be mitigated, but the effect is inconspicuous due to the slow action of pitch angle
Solution Approach 1:
The patent replaces the slow mechanical pitch angle adjustment system with an electrical control system comprising VSCs and a DC chopper circuit. This substitution enables rapid energy management through electrical switching actions, achieving fast response times without the mechanical inertia delays inherent in pitch angle adjustment.
Solution Approach 2:
The DC chopper circuit is pre-configured and ready to operate, with the fully-controlled power switching device and dump resistor already in place. When FPR is commanded, the chopper can immediately begin dissipating excess energy without waiting for mechanical pitch adjustments to take effect, providing preliminary protective action against rotor over-speed.
3Device complexity
If conventional MPPT mode is maintained under the existing schemes introduced with DC chopper, then the system structure is simple, but the unbalanced energy continuously exists so that the issue of excessive energy after FPR is not fundamentally resolved and the DC chopper needs to operate throughout the execution of FPR
Solution Approach 1:
The patent implements dynamic control mode switching that adapts the system behavior based on operational conditions. During FPR events, the system transitions from conventional MPPT mode to a coordinated control mode where the DC chopper actively dissipates excess energy. After FPR completion, the system dynamically returns to MPPT mode, eliminating the need for continuous chopper operation and fundamentally resolving the excessive energy issue.
Solution Approach 2:
The system incorporates feedback mechanisms that monitor power output, rotor speed, and energy balance conditions. Based on this feedback, the control system intelligently switches between MPPT and coordinated control modes, ensuring the DC chopper operates only when necessary to dissipate unbalanced energy, thereby eliminating continuous operation while maintaining simple overall system structure.
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
This system effectively restrains rotor over-speed, eliminates unbalanced power, and minimizes the operational cost by coordinating DC chopper and pitch angle control, ensuring secure rotor operation and reducing the need for continuous DC chopper engagement.
Implementation Method 1
a DC chopper circuit, which includes a dump resistor and a fully-controlled power switching device
Implementation Method 2
the output end of the first inverting adder is connected to an input end of the first PI controller; an output end of the first PI controller is connected to an input end of the PWM modem
Implementation Method 3
the output end of the first PI controller is connected to an input end of the PWM modem; the PWM modem outputs the pulse signal to the control end of the fully-controlled power switching device
Data Source
AI summary
The present invention discloses a FPR system of DFIG, comprising a DC chopper circuit made up of a fully-controlled power switching device and a dump resistor first connected in series and then connected to the positive and negative poles of the DC-link; the fully-controlled power switching device is driven by a power switching device driver; the power switching device driver comprises a first inverting adder, a first PI controller and a PWM modem; the positive and negative input ends of the first inverting adder receive the real-time DC-link voltage signal and its threshold value respectively, and the output end of the first inverting adder is connected to the input end of the first PI controller; the output end of the first PI controller is connected to the input end of the PWM modem; the PWM modem outputs the pulse signal to the control end of the fully-controlled power switching device.