Four Multiscalar Control Methods | Gdańsk University of Technology

doctoral dissertation, entitled “Control Structures of Doubly-Fed Induction Generator Connected to Current Source Converter,” was supervised by Prof. Marcin Morawiec (Gdańsk Tech) and the reviewers were DSc Eng. Grzegorz Iwański (Warsaw University of Technology), DSc Eng. Andrzej Popenda (Częstochowa University of Technology) and Prof. PhD. DSc Eng. Andriy Lozynskyy (Lviv Polytechnic National University).         

The current trends in power generation show a shift from large-scale fossil fuel-based power plants, such as coal and crude oil, to renewable energy systems, such as solar, hydro and wind power. The continuing dwindling of fossil fuel resources has led to rising fuel prices. Therefore, scientists’ interest in the possibility of using renewable energy sources has increased.

Paweł Kroplewski studied wound rotor induction machines, also known as doubly-fed induction generators (DFIGs), which are commonly used in wind turbines. In these machines, there are two sets of windings: one connected to the AC grid, typically the stator winding, and the other connected to a power electronic converter, usually the rotor winding. This configuration provides better control of the generator’s angular speed and output voltage.

‘Since the 1980s, systems based on doubly-fed induction machines have been the subject of extensive research and development. Their high efficiency, low cost and ease of control have resulted in their widespread use in wind energy conversion systems. The main reasons for using these in wind power systems are: reduced noise, low strain on mechanical structures, and independent control of active and reactive power,’ said the scientist. ‘In my thesis, I examined the control of a two-level, three-phase Current Source Converter connected to a Doubly-Fed Induction Generator. I proposed four control methods to provide independent control of the variables, thus reducing steady-state and transient oscillations and providing resilience to grid failures.’

During the study, simulations and laboratory experiments were performed to assess the response of the analysed system to various changes that can occur during normal operation in wind power systems, including spikes: changes in active and reactive power, changes in rotor speed and voltage dips of varying depth and duration.
Five different power control methods have been tested for the doubly-fed induction machine, including the most commonly used Field-Oriented Control method, and four non-linear control methods that have been developed based on the dependencies of the multiscalar models of the doubly-fed induction generator. Simulation software was also developed to test these structures.

‘Three of the four proposed multiscalar control methods were implemented on a laboratory test stand to assess their dynamic characteristics, thus allowing for comparison and analysis,’ explains Paweł Kroplewski. ‘The last of the proposed multiscalar control methods, due to its computational complexity, was only analysed as a simulation.’

The most important conclusions drawn from the simulation and laboratory studies:

• the designed control structures enable independent regulation of active and reactive power of a doubly-fed induction generator under normal operating conditions;
• the proposed power control systems for a doubly-fed induction generator with non-linear multiscalar control have good dynamic properties, such as a reaction time to changes in active and reactive power under 75 ms;
• in cases of shallow voltage dips, the proposed power control systems allow for the control of rotor currents, yet their effectiveness depends on the depth of the dip;
• in the event of a deep voltage dip, additional protection circuits are required for the Current Source Converter on the rotor side; an example is an additional energy storage system connected to the DC-link via a bridge converter and switched on for the duration of the dip. This system would operate as a temporary power supply.

‘The proposed structures do not fully explore the potential for the synthesis of new control systems using multiscalar models of a doubly-fed induction machine. Thus, the control system configurations presented in my thesis provide a basis for future research on multiscalar systems with doubly-fed induction machines, which is currently underway at Gdańsk University of Technology,’ adds the scientist.

Paweł Kroplewski graduated with a Master's degree from the Faculty of Electrical and Control Engineering, studying Electrical Engineering, followed by a four-year Doctoral School. He is currently waiting for his doctoral thesis defence date. He links his career to working in the industry and developing further innovative technical solutions that can be implemented in businesses.