Increasing amounts of solar power connected to the low-voltage network will adversely affect the performance of the network. The two impacts that will most often set the limit are overvoltage with the customers and overloading the distribution transformer. In this work, alternative methods have been studied for determining when a transformer is overloaded, to allow more solar power to be connected to the low-voltage network, i.e., increasing the hosting capacity for solar power.A limit-based method on the highest temperature inside the transformer (the hotspot temperature) and a method based on the loss-of-life of the transformer insulation due to hotspot temperatures above the design temperature are those alternative methods in this study. These methods are known as "dynamic transformer rating", a technology proposed in the literature but with very little practical experience in distribution networks.Two models were developed and implemented in MATLAB: a thermal model of the transformer calculating the hotspot temperature for a given time series of loading and ambient temperature; and a model for the loss-of-life of the winding insulation for given time series of the hotspot temperature. These models have been applied to existing distribution networks: measured consumption patterns with high time resolution (10-minute time step) for nine different distribution transformers for 1.5 years (network operator); measured ambient temperature (SMHI); and solar-power production calculated from satellite measurements (Renewables Ninja).For these nine distribution transformers, the time series of the hotspot temperature and the loss-of-life over the 1.5 years have been calculated for different values of the solar power installed capacity on the low-voltage side of the distribution transformer. The resulting time series are used to estimate the hosting capacity for solar power of a 200 kVA transformer. Using the existing design methods, the hosting capacity is 200 kW. Once that value is reached, the further connection of solar power should be stopped until a larger transformer is available. According to IEC design methods, the hosting capacity is about 270 kW using a limit to the hotspot temperature. This value somewhat depends on the loading patterns of the transformer before the connection of solar power. Once that value is reached, the further connection should again be stopped. Even for installed capacity exceeding 270 kW, the loss of life of the transformer insulation is still small and acceptable. This allows for further connection of PV without the immediate need to replace the transformer. Even values up to 350 or 400 kW may be acceptable, but a limit based on loss-of-life will require a detailed risk analysis as the pre-solar loading of the transformer is shown to play an important role.This work has shown that dynamic transformer rating allows more solar power to be connected to a distribution network than using classical rating methods without unacceptable risk for transformer loss-of-life.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:du-38914 |
| Date | January 2021 |
| Creators | Khatun, Amena |
| Publisher | Högskolan Dalarna, Institutionen för information och teknik |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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