The effect of hybrid electric vehicle charging on distribution transformers

Master's Thesis, Department of Electrical Engineering "M.SC."

Trend: Power

Abstract

The use of electric vehicles such as hybrid electric vehicles in the transportation industry is increasing day by day. The battery of these cars can be charged through a home power socket or through public parking lots. Additional load caused by car battery charging can cause harmful consequences on the distribution network, among these consequences is reducing the life of power network equipment. In this thesis, by using the thermal model of the transformer and calculating the temperature of the hot spot of the transformer, the effects of the presence of different numbers of electric vehicles and their charging time intervals on the rate of loss of life of distribution transformers are investigated and evaluated. The desired method is implemented on a sample distribution transformer that supplies electrical power for household use. The results obtained from the graphs quantitatively show that the night charging period has the least effect on the transformer life loss rate. This is despite the fact that car charging in the evening and the beginning of the night, which coincides with the peak household consumption, will have the greatest effect on the rate of loss of life of the transformer.

Further calculations will show that the presence of 30% of electric vehicles in the evening charging period, compared to the absence of the car, will cause a 900% increase in the rate of loss of life of the transformer.

Key words: distribution transformer, life loss rate, hybrid electric vehicles, distribution network, battery charging.

Introduction:

The rapid development of technology in the transportation sector, along with environmental concerns, has led to an increase in the price of oil, which has led to efforts to eliminate dependence on fossil fuels in many countries of the world. One of the efforts made in this field is investing in the design and construction of electric cars. Based on this, many reputable car manufacturing companies have begun the commercial production of electric cars.

The commercial production of electric cars is carried out in two ways: completely battery-based cars and gasoline-electric hybrid cars. Fully battery-based vehicles get their power from rechargeable batteries installed on the vehicle. For the daily operation of these cars, the batteries in the cars must be charged at stations in the city. The use of these cars was not welcomed due to the long waiting time to charge the battery at the charging stations and also the lower performance in terms of speed and acceleration compared to gasoline cars. Although many reputable car manufacturing companies are focusing on improving the quality of fully battery-based cars and using fast charging to charge them, it is expected that these cars will also find their market in special applications in the near future. On the other hand, in the past few years, the production of gasoline-electric hybrid vehicles has been on the agenda of car manufacturing companies, and the supply of these products to the market is increasing. Due to the reduction in the price of the technology used in these cars, it is expected that customers' acceptance of this car will increase significantly in the near future. In gasoline-electric hybrid cars, the driving force of the car is produced from the combined operation of the gasoline combustion engine and the electric motor. In this combination, on the one hand, the amount of fuel consumption in these cars is reduced compared to gasoline cars, and on the other hand, their performance is improved compared to fully battery-based electric cars.

A significant point in increasing the use of hybrid cars is that the technology used in the batteries of these cars allows them to be charged from any external source. This external power source can be the power grid, and the charging of said cars can be done in any possible place, such as residential houses where access to the power grid is easily possible.

The possibility of easy charging of these cars makes the owners use the car during their daily trips within the city and when they return home, they prepare their car for the next day's trip by connecting the car to the home electrical socket. During the period of connecting these cars to the electric socket, the energy needed to recharge the hybrid cars' batteries is provided from the power grid. In this context, it is necessary to pay attention to the fact that the use of these cars, which have been developed to save fuel consumption, can cause adverse effects on the power grid. Because the energy required to charge these cars can be modeled as the energy required to feed an additional load for the power grid, which can be connected to the grid at any time. Meanwhile, the design and construction of many used power networks have been done without considering the possibility of connecting these equipments to the network. Since the charging of these equipments through power networks without knowledge can lead to many problems in the normal operation of networks, many researchers have studied the technical effects of connecting rechargeable electric vehicles on the performance of power networks.

Given that understanding the effects of charging electric vehicles on the power network and taking action to eliminate or reduce these effects requires planning the power network and implementing long-term preventive solutions, it is necessary to have sufficient knowledge in this field before widespread use. Get it from them. For this reason, in many references[1]-[5], researchers have studied the various effects caused by the connection of these cars in the fields of changing the load profile of consumers, increasing losses and voltage drop of the power network. However, no special attention has been paid to the effects of charging electrical outlets on the life of electrical equipment of the power network. This is despite the fact that by connecting electric vehicles to the power grid, the amount of power passing through the network equipment increases and this can lead to a reduction in their life. Due to the connection of electric vehicles to home sockets, it is expected that distribution networks will accept more influence from the charging of said vehicles than the production and transmission sectors. Also, distribution transformers are one of the most important distribution network equipments that have high costs and are abundantly found in distribution networks. For this reason, this thesis focuses on the study of the life of distribution transformers in the presence of electric vehicles.

In this regard, as shown in references]8 [-]6 [shows that distribution transformers are mainly affected by thermal stress and experience aging or loss of life. In this research, the life analysis of distribution transformers under the influence of thermal stress is performed based on the hot spot temperature. Several methods are presented in references [9] and [10] to calculate this quantity. The method presented in reference [9] is widely accepted and its accuracy has been confirmed in many references. Therefore, in this thesis, this method is used to analyze the life of distribution transformers. Based on this method, changes in air temperature and load passing through the transformer are the determining parameters in the aging process of transformers. In other words, any increase in the ambient temperature and increase in the load of the transformer causes the aging process of the transformer to accelerate. On the other hand, the connection of electric vehicles to the distribution network causes the load of the transformer to increase in certain periods of the day and night. Therefore, it can be expected that the aging process of distribution transformers will increase in the presence of electric vehicles. However, in the existing references, there is no sufficient knowledge and mathematical model to determine the numerical size of the effect of electric vehicle charging, especially the penetration coefficient and their charging time intervals on the aging process of distribution transformers. In this thesis, a method is presented by which the effects of hybrid electric vehicle charging on the life loss process of distribution transformers can be investigated from a numerical point of view. Based on this, first, a comprehensive stochastic model of the electric energy consumption pattern by these cars when connected to the distribution network is presented. The obtained model is used together with the load pattern of the consumers of the distribution network and the pattern of air temperature changes in certain days of the year to analyze the life of distribution transformers in the presence of electric vehicles. Next, by modeling different charging scenarios, the effects of the connection time of electric vehicles to the network on the rate of loss of life of distribution transformers are analyzed.