Variable Speed Induction Generator Power Converter Control
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Solution Overview
Problem
Renewable-energy turbines with variable voltage and frequency generated by wind, wave, or tidal flows face challenges in matching these outputs to a nominally constant power grid voltage and frequency, requiring efficient conversion methods to optimize energy capture and transmission.
Innovation Solution
A power transmission system utilizing induction generators connected to a power converter via transmission cables, with a control strategy that adjusts stator voltage and frequency based on turbine operating speed and fluid flow speed, eliminating the need for pitch control or stall regulation by maintaining optimal blade tip speed and efficiency across varying flow rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the speed of rotation of the turbine rotor is varied to optimize energy capture from fluid flows, then the energy capture efficiency is improved, but the voltage and frequency at the generator terminals become variable and incompatible with the fixed voltage and frequency requirements of the power network
Solution Approach 1:
A power converter is introduced as an intermediary device between the variable-speed generator and the fixed-frequency power network. The power converter includes a variable frequency drive that converts the variable frequency output from the generator to a fixed frequency suitable for grid connection, while maintaining optimal turbine speed for energy capture. This mediator resolves the contradiction by decoupling the speed requirements of the turbine from the frequency requirements of the power network.
Solution Approach 2:
The system dynamically changes electrical parameters (voltage and frequency) at the generator terminals to match optimal turbine operating conditions, then uses power electronic conversion to transform these variable parameters into fixed parameters suitable for power network connection. The controller adjusts stator voltage and frequency based on measured turbine speed and fluid flow conditions to maintain peak efficiency while ensuring grid compatibility.
2Adaptability or versatility
If pitch control or stall regulation mechanisms are added to the turbine to control operating speed, then the matching with power network is improved, but the device complexity and cost increase
Solution Approach 1:
The invention replaces mechanical pitch control systems or stall regulation mechanisms with an electrical control approach using power electronics. Instead of mechanically adjusting blade pitch or relying on aerodynamic stall, the system uses a variable frequency power converter to control generator speed and maintain optimal turbine operation. This substitution eliminates complex mechanical moving parts while achieving the same speed control objective, reducing device complexity and maintenance requirements.
3Reliability
If multiple transmission cables are used to connect each induction generator to the power converter, then the reliability of power transmission is improved, but the quantity of cables and installation complexity increase
Solution Approach 1:
Multiple induction generators are electrically connected in parallel to a single power converter through a combined transmission cable system. Rather than using separate cables for each generator, the invention merges the electrical connections into a unified cable assembly that carries combined power from multiple turbines to the power converter. This merging reduces the total quantity of cables required while maintaining reliable power transmission through the parallel electrical connection architecture.
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 system ensures efficient energy transmission to the power grid by maintaining optimal stator voltage and frequency, maximizing turbine efficiency and reducing the need for costly infrastructure, such as subsea equipment, while allowing for bi-directional power flow and grid fault ride-through capabilities.
Implementation Method 1
The ac frequency that is developed at the stator terminals of the generator (the 'stator voltage') is directly proportional to the speed of rotation of the rotor
Data Source
Figure 1
Figure 2
AI summary
A power transmission system may include a plurality of renewable-energy devices (4a, 4b, 4c) such as wind turbines or subsea turbines. The devices are connected together in parallel to a subsea cable (10) that carries an ac transmission voltage. Each device includes a turbine assembly (6) that is rotated by wind or water current flows, and a variable speed ac induction generator (G1, G2, G3). A power converter (2) is connected to the subsea cable (10) and is used to interface the generators (G1, G2, G3) to a supply network or power grid. The power transmission system is operated such that an indicated operating speed of one or more of the devices is used to control the power converter (e.g. the PWM strategy that is used to open and close the power semiconductor devices) to achieve desired stator electrical quantities at each generator (G1, G2, G3).