Load capability estimation of dry-type transformers used in PV-systems by employing field measurements
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ORIGINAL PAPER
Load capability estimation of dry-type transformers used in PV-systems by employing field measurements David L. Alvarez1
· Jorge Restrepo1 · F. Faria da Silva2 · Javier Rosero1
Received: 18 February 2020 / Accepted: 30 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Transformer insulation aging is a critical issue for both reliable and economic operations, and for planning of electrical systems. As insulation aging depends on the hottest-spot temperature, transformer management can be improved with a suitable model for temperature estimation and prediction. However, the temperature inside transformers varies dynamically because of changes in both the cooling conditions and the load cycles. Hence, this paper presents an algorithm to estimate and predict the hottest-spot in dry-type distribution transformers, so that their capability and insulation life can be assessed. This procedure is focused on transformers used to directly connect PV-inverters to the grid in order to consider the uncontrolled power generation of distribution PV-systems. To implement the algorithm, it is assumed that records of ambient temperature, PV-system power generation cycle and winding temperature are available. With these data, the parameters of an equivalent thermal circuit are fitted in order to dynamically model the transformer hottest-spot. The method was validated using twelveday records of a 70 kWp PV-generation system connected to a 75 kVA dry-type transformer. Results show that an enhancement in the hot-spot estimation is reached, and an assessment of the performance in real-time monitoring of the transformer capacity is achieved employing the proposed algorithm. Keywords Dry-type transformer · Dynamic thermal rating · Extended Kalman Filter · State estimation · Thermal transient
1 Introduction Photovoltaic (PV) generation at the distribution level has increased in recent years. These generation systems are usually located on the rooftops of houses or other buildings. When the output voltage of the PV-system inverter is different from grid voltage, it is necessary to use transformers. Due to space restrictions in buildings, as well as security requirements, dry-type transformers are frequently used for distributed PV-generation. One characteristic of PV-generation is its intermittent power injection. To reduce this impact, energy storage can
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David L. Alvarez [email protected] F. Faria da Silva [email protected]
1
Department of Electrical and Electronic Engineering, Universidad Nacional de Colombia, Cra. 30 No 45-03, Edificio 453-208, Bogotá, Colombia
2
Department of Energy Technology, Aalborg University, Aalborg, Denmark
be used. However, this technology still faces economic challenges for implementation at the distribution level, and so PV-systems are often connected directly to the grid through a transformer. In those transformers, the load varies continuously and so does the hottest-spot temperature, as a consequence of the uncontrolled power injection. This tempe
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