Electric power systems basics

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Electric power systems basics

A basic power system consists of three components. The first component is the load, which demand a certain amount of power to be utilized for a designated purpose. The second component is the generator, the component that generates electric power. The third component is a branch conductor that connects the generator and load to each other, thus creating a basic electric power system. The electric power systems can either have an alternating current (AC) or direct current (DC), and the electric power transported in the system is calculated using equation 1 is the complex power, the voltage over the conductor and ∗ the conjugate of the current passing through the conductor. The components of a system would ideally transport electric power without any losses, yet in practice this is seldom the case. For instance, conductors have active power losses due to resistivity as seen in equation 2 is the active power losses and the resistance of the conductor. As can be seen in equation 2, the current is squared, which means it has a significant impact on the active power losses.
In order to reduce the power losses, it is desirable to minimize the current, especially during long transportation distances. However, lowering the current will according to equation 1 also reduce the delivered complex power. If the complex power is to remain the same the voltage must increase.
An increase in voltage is done by a fourth type of component called transformers. A transformer is usually placed close to power plants in order to increase the voltage, and thereby minimizing the active power losses. The electricity is transported along a high-voltage transmission grid until it reaches a high/medium-voltage transformer. There, the voltage is lowered and transported along the distribution grid. The voltage is usually lowered once more to 400 V when the power is close to the end loads, such as one or multiple household. The power flowing through these last-mentioned transformers are what the loads in the system model are representing, since every load can consist of multiple households. A diagram presenting how an electric power system with generators, conductors, loads and transformers are connected to one another is shown in Figure 1. The low-voltage transformer represented as loads in the system model have been marked in Figure 1 with a red circle.
Figure 1. An illustrative diagram of how generators, conductors, loads and transformers are connected to each other in an electric power system. In addition, the low-voltage transformer which is represented by a load in the system has been marked with a circle. The diagram is a modified version of a diagram made by EIA (U.S. Energy Information Administration) [16].

System investigated on Gotland

Gotland is a Swedish island located approximately 100 km (kilometres) from the mainland of Sweden. The island has an area of 3000 km2 (square kilometres). The distance between the northernmost and southernmost point of Gotland is approximately 176 km, and 50 km between its most eastern and western points. The total amount of private cars on Gotland at the end of 2017 was about 36 000 vehicles, which is approximately 600 cars for each 1000 citizen on Gotland [5].
The power system which was examined in the project was a rural area mainly consisting of household loads. All information regarding the system was obtained from GEAB, a daughter company of Vattenfall AB which owns the electric distribution grid on Gotland. The area was located in the eastern parts of Visby, the largest urban area of Gotland. The system consisted of a low voltage grid, 400 V, which was connected to the 11-kV distribution grid of Gotland through a transformer. A diagram of the system is presented in Appendix A. The system contained approximately 150 loads, yet most of these loads had a significantly larger average power consumption than a regular household has. A regular Swedish household has an annual electricity consumption of approximately 25 000 kWh [17,18], which corresponds to an average power consumption of approximately 3 kW. Most loads in the system had an average power consumption much larger than 3 kW. This indicated that multiple household were tied together into one load, meaning a load likely represented a local distribution feeder rather than an individual household. Based on mentioned average power consumption of a household, the number of households connected to the investigated system was estimated to approximately 600 households.

Definition of power quality in the electric power system.

In order to measure how well an electric power system performs it is important to first determine what defines a well-performing power system. The purpose of an electric distribution power system is to provide its consumers with the right amount of electric power, at the right time, without major losses and without compromising electrical components connected to the system. If a power system is performing perfectly, it will always balance the electric power supply and demand and keep the voltage level of the grid at nominal voltage. In practice this is a difficult task because the power system has undesirable behaviours and losses, for example the power losses explained in section 2.1. Therefore, regulation standards for different disturbances are used to ensure a sufficient level of grid stability. One of these regulations concerns voltage drops, and in Sweden any voltage levels in a the power grid should be between 90 and 110 percent of the nominal voltage [19].

Electric power consumption of a regular household in Sweden.

The electric consumption of a household can roughly be described by having a power peak in the morning after the residents wake up, and in the evening when they arrive home from work. During working hours most people are not at their homes, and the households will only utilize power for basic maintenance such as heating, ventilation and refrigerating. Even though the system is in a low power state during most of the day, the grid must be designed based on the peak load since the grid must be able to deliver the necessary peak power.
When examining the total power consumption of Gotland, it is observed that the daily power consumption trends are similar to a residential area. The load of the whole system increases during the morning and keep a nearly constant power consumption during work hours until late afternoon. The reason for this is because industries and offices with power demanding processes are mainly active during daytime, but then reduced or shut down when the workers end their work shifts.
A third perspective to look at the power consumption of Gotland is the annual trends; power loads based on weather and season. The power consumption of Gotland is at its peak during the winter season, since people utilize more electricity for heating and lighting, and at its lowest during the beginning of summer. In Figure 2, which presents the normalized values of Gotland power consumption during 2016 for each month, it may be observed that the power consumption is approximately one third less during the months of summer compared to consumption during winter.

Load profiles of household loads on Gotland

Three load curves of the investigated system, where data were provided from GEAB, is shown in Figure 3. The data is also presented in Appendix B. The curves represent the hourly mean power consumption for three different days during the year 2016. The different days were chosen to capture as much of the data variance as possible. For instance, the load curve of January represents a very cold day, while the day in May represents a warm day where the residents don’t use much electric power. The day in October represents a mean day. The y-axis on Figure 3 is normalized based on the mean load in October.

Electric vehicles (EVs)

Electric Vehicles (EVs) are vehicles which propel by using an electric powertrain. EVs are in turn classified into three types; hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs). The major differences between the three types are how they propel themselves. HEVs consists of both an electric power engine and an internal combustion engine (ICE) [20]. These engines can either be connected in a parallel or serial drivetrain. In a serial powertrain, the ICE propels a generator, providing power to the electric engine which propels the vehicle. In a parallel powertrain, the engines are coupled and utilized at different driving conditions [20]. PHEVs also have an electrical engine and a combustion engine which are coupled, such as a hybrid vehicle. However, the electric engine in a PHEV is powered by a small battery bank in the vehicle [20]. In 2018, the battery banks installed in PHEVs commonly had a size between 10 and 20 kWh [21–24]. It is possible to charge these batteries externally by connecting the PHEVs to an electric power grid [20]. The last type is called battery electric vehicles (BEVs). BEVs are vehicles which only propels using an electric power engine. The engine is powered by a battery bank which is recharged by connecting the BEV to an external power source, such as an electric power grid. The battery bank in BEVs are usually much larger than in PHEVs. In 2018, the storage in the battery bank of conventional BEVs ranged between 40 kWh and 70 kWh [25–27]. A summary of how the different types of EVs differ from each other is presented in Table 1.
The vehicles used for the V2G services in this study were all BEVs, since they had potential to store larger amounts of electrical power compared to the other types of EVs. The reason for only choosing one type of EV was due to simplicity, as it made it easier to set up a standardized battery capacity of the EVs in the system. PHEVs could also have been chosen instead for BEVs since they also interact with an electric power grid. However, BEVs had a higher battery capacity compared to PHEVs and thus allowed more V2G utilization in the system. Therefore, BEVs were regarded as more interesting for this study compared to PHEVs.

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V2G and BEV charging

The definition of V2G is simply that the charging of EVs has a bidirectional charging station, the electric car also has the possibility to feed power back to the grid. This differs from conventional electrical vehicle supply equipment (EVSE) which most commonly are unidirectional; they can only charge the EV. The concept of V2G is quite simple; when the grid is unstable and in need of extra power, the batteries in the EVs discharge to a threshold limit and provide power at locations where it’s required locally. The EVSE would become bidirectional instead of unidirectional. However, doing this practically is complex and demands a fast response controlling unit at each charge station. Just imagine how a system with a few large generators in a couple of years adds thousands of small generators distributed around the whole system. An illustrative diagram of how V2G works is presented in Figure 4.
Figure 4. Diagram of how the BEV and power grid interacts with each other using a bi-directional EVSE [28].
There are additional challenges and issues which concerns the implementation of V2G. One of these challenges is the need for a bidirectional communicational electric power grid, which is one characteristic of a smart grid [29]. The concept of a smart grid can generally be described as an electrical power system which has a number of characteristics which the traditional grid is lacking [29]. A few examples of these characteristics are digital devices, condition monitoring, self-healing, two-way communication and distributed power generation [29]. An increase in information rate and bidirectional communication between components in the system will allow much faster regulations of power, making it possible for the grid to handle more complex power systems.
An increase in the grid’s ability to handle distributed power generation is important since V2G would provide local auxiliary services, which requires fine regulation in order to operate. The power grid of Gotland does not presently have the characteristics of a smart grid, however for the sake of this study it is assumed to be developed for V2G to function properly.

Why electro-mobility?

As mentioned in the introduction of the report, society must change in order to achieve the goals for sustainability and climate. The transportation sector is no exception from these changes. The possibility of transporting goods and services is crucial for the modern society to function. At the same time, transportations were estimated to contribute approximately 20 percent of all annual global CO2-emissions during 2014 [30]. BEVs are an alternative that releases less CO2-emissions compared to fossil fuelled vehicles [31]. In a study from 2012 it was estimated that light-duty BEVs, and a European energy mix, had 10 to 24 percent less global warming potential (GWP) compared to their fossil fuelled counterparts [31].
In addition to lowering CO2- emissions from transportations, EVs have a few more benefits compared to fossil fuelled vehicles. These benefits are reduced noise and better air quality, since EVs are quiet and emit no tailpipe emissions [32]. These benefits are especially beneficial at locations with high traffic density, such as city centres.

Challenges brought by electro-mobility

Even though BEVs are considered to have less environmental impact compared to fossil fuelled cars, there are some downsides and problems with BEVs which need to be addressed. There are in general four major challenges BEVs must deal with, in order to compete with conventional cars.
Two of these challenges are cost and driving distance before recharging, which are factors that directly affect people’s general opinion about BEVs. The general driving distance of commercial BEVs are today around 270 to 400 km [25,26]. Range anxiety and cost are challenges which are handled on an individual level, where a person is deciding what alternative that mostly fulfils a specific need. In this case, a person chooses between having an EV or a purely fossil propelled vehicle. In 2017 the EVs were still more expensive than cars with a combustion engine, yet studies have found that EVs are expected to be cheaper than similar fossil fuelled cars by 2025 [33]. The main argument for the reduction of price is a continued decrease for cost of material used in the batteries of BEVs [33]. As for range anxiety, there are multiple projects undergoing to install large quantities of publicly available charge stations with short distances between them. One example of these projects are InCharge, a project performed by Vattenfall AB, which have worked with expanding the charge infrastructure in the Nordics, Germany and the Netherlands [34].
The third and fourth challenges are challenges on a scale much larger than the daily life of a person. The third challenge is to balance a large increase in local power consumption. The power output from an EVSE connected to a household, which is connected by one phase and limited to 16 amperes (A), is approximately 3.7 kW. In comparison, a regular household usually has a mean power consumption of about 2.8 kW if it has an annual power consumption of 25 000 kWh [18]. This means the load from a household may instantly more than double when the BEVs starts charging. This lead to large power peaks, especially if the cars are charging immediately when people get home in the evening, which in turn may result in lack of electric power due to bottle-necks in the distribution grid. If the EV penetration in the Swedish vehicle fleet will increase, this may become a serious issue in rural areas where the grid can’t handle these power quantities. This shows the importance of creating smart grid solutions and reinforcing a power system for an upcoming increase in power consumption.
Lastly, the fourth challenge is the lack of materials for creating the necessary batteries. Most of the common BEVs uses Li+ batteries. The use of large quantities of lithium itself isn’t major problem, however the batteries use other metals in their cathodes as well as lithium. One of these materials is cobalt, which is scarce [35]. This lack of cobalt could result in very high prices, which means there might not be enough cobalt to satisfy the whole world’s need of BEVs. The manufacturing of cobalt also has issues in terms of social sustainability. Today a majority of the cobalt used in batteries are affiliated with cases of child labour and poor working conditions [36]. This is of course if BEVs will use the exact same technology in the future as in the present. The battery industry and the research on batteries is a highly developing area and it is therefore likely that future batteries of BEVs will consist of a different material composition than present batteries.

Driving habits

In modern society the car plays an important role for rural households. It creates easy and flexible transportation of goods and services for a household family, such as driving to work, shopping for groceries, dropping off children for school or visiting friends in other towns. However, even though the car grants a lot of flexibility, it has a low level of daily utilization. The driving habits of a Swedish citizen is to make 0.7 main trips every day, drive about 25 km each day for approximately 44 minutes [37]. If an assumption is made that people generally drive their cars between 8:00 in the morning and 20:00 in the evening, the car will be parked about 94 percent of the time where cars usually are utilized. If one also considers that the car can be utilized for V2G or charging the whole day, the total time during a day in this case is estimated to 97 percent of each day in average.

Table of contents :

1.1. Purpose of the study
1.2. Discussion about source material
2. Background
2.1. Electric power systems basics
2.2. System investigated on Gotland
2.3. Definition of power quality in the electric power system.
2.4 Electric power consumption of a regular household in Sweden.
2.5. Load profiles of household loads on Gotland
2.6. Electric vehicles (EVs)
2.7. V2G and BEV charging
2.8. Why electro-mobility?
2.9. Challenges brought by electro-mobility
2.10. Driving habits
3. Method
3.1. Scope and assumptions
3.2 Setting up cases
3.3. Input data to PSS®E simulation model
3.4. Creating a complementary python-script.
3.5. Output data
3.6. Simulation models
4. Results and Discussion
4.1. Utilization of EVs during a day
4.2. Strain relative maximum capacity of the conductors
4.3. Impact on Voltage
4.4. How well does the model fit the true system?
4.5. Future potential projects and model improvements
5. Conclusions


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