Finnish Scientist Could Revolutionise Inverter Design
If this researcher at the Tampere University of Technology (TUT), Finland, is right, generally used design principles of power inverters used in photovoltaic applications are wrong. As they are attracting growing international attention, the potentially revolutionary understandings could be key to solving the problems solar photovoltaic power systems have with power quality and disruptions in the power grid.
Is a photovoltaic cell a non-ideal current source? “I had some doubts about the real nature of the converters used in PV applications, because the PV generator seemed to be a current source not a voltage source,” says Professor Teuvo Suntio from the Department of Electrical Engineering at TUT. He had been reading too many articles describing converters interfacing with PV generators as if they were conventional voltage-fed converters that behave as current sinks at their input terminals. “According to [German physicist Gustav R.] Kirchhoff, connecting two current sources in series does not work properly at all,” Suntio evokes. “This is the basis for the 'revolutionary' ideas.”
That in mind, in 2008, the Finnish scientist began to search for the truth about properties of PV converters. The power electronics expert wondered why many shortcomings and malfunctions of renewable energy power systems remained unresolved. Suntio’s findings could provide game-changing explanations. First, he has explained why a converter directly connected to the PV generator does not work properly if its input current is used as a feedback variable; he recommends the use of the input voltage instead. Second, using output voltage or output current as a feedback variable leads to instability when the MPP [maximum power point] is reached. “This means that the output-side feedback cannot be used without compromising the stability of the PV system,” Suntio explains. Third, the voltage-sourced inverter (VSI) contains right-half-plane (RHP) zero in its output dynamics when the inverter operates in the constant-current region of the PV generator. “This means that the grid-current loop is actually unstable,” Suntio says. “The RHP zero actually turns into a RHP zero in the DC-link feedback loop and, consequently, instability would take place if the control bandwidth of the loop is too low. Such instabilities at rather low frequencies were reported, but their origin was unknown.” Fourth, Suntio and his team noticed that “the inverter output impedance exhibits 'negative incremental resistive' behavior at the frequencies close to the grid frequency and its low harmonics.” They explained that behavior with the cascaded control and phase locked loop (PLL) synchronizing the inverter with grid frequency.
Suntio’s progressive understandings could help improve the reliability of future inverter devices and, especially, the reliability of renewable energy systems as a whole. “Correctly considering the PV generator as an input source would also reduce costs if the sizing of the components are carried out without unnecessary oversizing," the scientist adds.
He also reckons, however, that there is still a long way to go before solar researchers “really understand and, especially, accept the facts related to the power electronics converters applied in the renewable energy systems.” Suntio predicts the main paradigm shift will come when one day “everyone recognises that the same power stage can have properties that are quite different from each other, and that the determining factors are the input and output sources as well as the feedbacks applied.” He even suggests changing currently used terminologies so that other engineers could understand what their colleagues are talking about. “Real knowledge about the origin of different undesired phenomena in the behavior of the inverters will enable their solving in a most optimal way,” he says, adding that many of today’s solutions focus on making inverters more complex or oversizing the devices to compensate for shortcomings. “Even though we have worked hard at making these discoveries known within the research community, there still is too much resistance.”
In 2008, TUT began collaborating with the global manufacturer of power and automation technologies ABB. Together, they have joined forces to pinpoint the optimal structure of photovoltaic power plants and develop new PV inverters ABB has patented. “ABB was the only company visionary enough to recognize the potentials of the research activities at Tampere University of Technology,” Suntio says. “As a consequence of that, the full-scale research projects started in 2009, yielding the 'revolutionary' knowledge.”
