Title:	Analysing the Voltage Stability of Photovoltaic Inverters Reactive Power Control in the Laboratory Including the Distribution GRID Transformer
Author(s):	F.P. Baumgartner, C. Messner, C. Seitl, G. Lauss, F. Carigiet, R. Bründlinger, T.I. Strasser
Keywords:	Cost Reduction, Qualification and Testing, PV Inverter, Grid Integration, Voltage Regulation
Topic:	PV Systems - Performance, Applications and Integration
Subtopic:	Grid and Energy System Integration
Event:	35th European Photovoltaic Solar Energy Conference and Exhibition
Session:	6EO.2.6
Pages:	1754 - 1759
ISBN:	3-936338-50-7
Paper DOI:	10.4229/35thEUPVSEC20182018-6EO.2.6
Price:	0,00 EUR
Document(s):	paper

Abstract/Summary:

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Photovoltaic (PV) inverters increase the line voltage in Distribution Grids (DG) by active power feed in. Today, modern PV inverters are also able to feed in reactive power to mitigate the above voltage rise. The favoured, cost effective implementation is the control of reactive power feed in according to the instantaneous measured line voltage. The stability of this decentralised Q(U) PV inverter closed-loop control is mandatory and analysed in this work. The DG operator must guarantee the voltage limits given in the regulatory framework. This is challenging due to fast changing solar irradiance, load flows and the interaction of an overlying automatic voltage control-loop of a connected substation. The performed tests in the AIT SmartEST laboratory resulted in very stable operation even at small Time Constants (TC) below 5 seconds of the Q(U) control parameter. As one test scenario, an abrupt rise of solar irradiance immediately followed by load drop is realized by use of the PV and load emulators. The PV inverter reduces the resulting voltage rise by increasing its reactive power, depending on the Q(U) control time constant. It was found that even at smaller Q(U) time constants than the typical applied values of 5 or 10 seconds no sign of instability arises. It is recommended to the DG operator to apply TC of 1 or 2 seconds of Q(U) control to minimise the duration of overvoltage condition during the transient voltage adjustment. Applying irradiance conditions of a typical cloudy day in the lab test yields 45% of the time the line voltage was above a given grid voltage limit, while applying Q(U) at TC of 1 second there was no occurrence of overvoltage. In detail at Q(U) TC setting of 20 seconds the overvoltage arises 3.4% of the total period and only 0.3% at TC 5 seconds. Only stable operation conditions were found including the automatic voltage control of the transformer sub-station at a typical setting of 10 seconds delay time of that sub-station control setting. Summarized, in combination with that delay time setting the smaller Q(U) time constant of the PV inverters below 5 seconds are beneficial due to the minimised overvoltage time.