Doubly-Fed Electric Machine Control Near Synchronous Speed
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Solution Overview
Problem
Doubly-fed electric machines face thermal limitations and component overload when operating around synchronous speed, leading to reduced power output and increased risk of overheating due to asymmetrical rotor currents, which existing control strategies attempt to mitigate by avoiding synchronous speed or reducing power output.
Innovation Solution
The method involves controlling the AC-to-DC converter to inject stator reactive power at a frequency different from the rated frequency, with the DC-to-AC converter compensating this power to reduce duty cycle asymmetry and avoid external grid perturbations, thereby allowing operation within predetermined speed ranges while minimizing thermal stress on power conversion system components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the DFIG operates around synchronous speed, then the aerodynamic performance is optimized, but thermal limitations and component overload occur due to asymmetrical rotor currents
Solution Approach 1:
The patent applies parameter changes by injecting reactive power at a different frequency than the rated frequency of the machine. This frequency parameter modification creates a harmonic that alters the rotor current characteristics, transforming the asymmetrical current pattern into a more balanced one, thereby reducing thermal stress while maintaining operation around synchronous speed
Solution Approach 2:
The patent introduces reactive power injection as an intermediary mechanism between the power conversion system and the rotor currents. The AC-to-DC converter forces injection of stator reactive power, which acts as a mediator to modify the rotor current distribution and reduce asymmetry without directly controlling the rotor currents themselves
2Reliability
If the AC-to-DC converter forces injection of stator reactive power to create a harmonic, then duty cycle asymmetry is reduced, but the DC-to-AC converter must compensate for the created reactive power and harmonic
Solution Approach 1:
The patent merges the functions of the AC-to-DC converter and DC-to-AC converter into a coordinated control system. The AC-to-DC converter creates the harmonic by injecting reactive power, while the DC-to-AC converter simultaneously compensates for it, combining their actions to achieve duty cycle symmetry without requiring external grid perturbations
Solution Approach 2:
The patent implements feedback control where the DC-to-AC converter detects the reactive power and harmonic created by the AC-to-DC converter and adjusts its operation to compensate for these effects. This closed-loop feedback mechanism ensures that the harmonic injection achieves the desired duty cycle balancing without causing grid issues
3Reliability
If existing control strategies avoid synchronous speed operation, then thermal stress is reduced, but aerodynamic performance and productivity are compromised
Solution Approach 1:
The patent inverts the conventional approach by not avoiding synchronous speed operation, but rather enabling it through reactive power injection. Instead of restricting operation away from synchronous speed to prevent thermal issues, the invention uses harmonic injection to allow safe operation at synchronous speed, thereby maintaining both reliability and productivity
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 approach optimizes aerodynamic performance, reduces thermal damage, and extends the operational limits of the power conversion system, preventing unexpected turbine stops and enhancing efficiency by avoiding speed exclusion zones around synchronous speed.
Implementation Method 1
the AC-to-DC converter of the power conversion system is controlled to force injection of a stator reactive power to create a harmonic at a frequency different than a rated frequency of the machine
Implementation Method 2
the DC-to-AC converter is controlled to compensate the created stator reactive power and the harmonic
Implementation Method 3
rotor windings of a rotor are connected to the electrical grid via a power conversion system, comprising an AC-to-DC converter and a DC-to-AC converter and being adapted to control a rotor current
Data Source
AI summary
A method for computer-implemented controlling a doubly-fed electric machine, where stator windings are directly connected to an electrical grid and where rotor windings of a rotor are connected to the electrical grid via a power conversion system, includes an AC-to-DC converter and a DC-to-AC converter and being adapted to control a rotor current, the method including obtaining a rotational speed of the machine; determining, whether the obtained rotational speed is within a predetermined operational speed range around the synchronous speed; and if it is determined that the obtained rotational speed is within the predetermined operational speed range, controlling the AC-to-DC converter of the power conversion system to force injection of a stator reactive power to create a harmonic at a frequency different than a rated frequency of the machine; and controlling the DC-to-AC converter of the power conversion system to compensate the created stator reactive power and the harmonic.