Article Effect of Large Load Loss on Voltage Stability in California Power Systems

Study Case Effect of Large Load Loss on Voltage Stability in California Power Systems Michael Kurniawan (2018123456) University of Cincinnati, 2600 Clifton Ave, Cincinnati, OH 45221, United States michaelkurniawan2019@gmail.com Abstract The electric power system is an interconnected network that plays a role in delivering electricity from the generator to the user. This system consists of several main components, namely power plants, transmission lines, and distribution lines, which work together to meet the electrical power needs of consumers. Because this system operates continuously, the possibility of disturbances in the electricity distribution process cannot be avoided. One of the disturbances that can occur is sudden load shedding, which can result in frequency and voltage instability in the California power system. To maintain voltage stability, effective mitigation measures are required. One solution that can be applied is the installation of an Automatic Voltage Regulator (AVR), which functions to control the rotation speed of the synchronous generator so that the voltage remains stable during a disturbance. This study aims to analyze voltage stability when load shedding occurs in the California power system. System modeling is done using DIgSILENT PowerFactory software with the help of RMS/EMT Simulation tools. Voltage stability simulations were performed under three different conditions, namely day peak load, night peak load, and low load. AVRs are installed on some of the main generators to ensure voltage stability is maintained when disturbances occur. Keywords: Voltage Stability, Load Shedding, Automatic Voltage Regulator (AVR), DIgSILENT PowerFactory. INTRODUCTION One of the most economical, efficient, and safe methods of delivering energy is through electrical energy. In power plants, primary energy sources such as fossil fuels (oil, natural gas, and coal), hydropower, geothermal, and nuclear are converted into electrical energy. Synchronous generators play a role in converting mechanical energy from the turbine shaft into three-phase electricity. Furthermore, the electrical energy is increased in voltage through a transformer before being sent through a high-voltage transmission network to the load MICHAEL KURNIAWAN 1 Study Case centers. The increase in voltage aims to reduce the current flowing in the transmission network, so as to reduce power losses due to heat (heat loss). When the transmission network reaches the load centers, the voltage is again lowered through transformers before being distributed to users. At this stage, electrical energy is converted into various forms of energy needed, such as mechanical energy for motors, lighting, heating, and cooling. Today, the issue of electrical energy is a major concern in the sustainability of future energy supply. California's electric power system still relies on fossil fuels as the main source of generation. Although renewable energy such as hydropower has begun to be utilized to meet electricity demand, its capacity is still insufficient to meet the growing demand. As a result, the government and private sector still rely on fossil fuel-based power generation to make up for the energy supply deficit. The high dependence on fossil fuels has caused negative environmental and economic impacts. Therefore, innovative solutions are needed to overcome this challenge. One strategic step that can be implemented is to accelerate the transition towards renewable energy that is more environmentally friendly. Wind energy, for example, has great potential as an alternative energy source that can help reduce dependence on fossil fuels in California. Wind energy potential in California has been identified in areas with wind speeds that support electricity generation. Some strategic locations known to have high wind resources include Tehachapi, Altamont Pass, and San Gorgonio Pass, which have been the centers of wind energy development in the state. A number of developers have proposed wind farms in other locations with similar potential, but further studies are needed to confirm the feasibility of the projects. One important aspect in the integration of wind farms into the power system is the stability of the system in receiving the plants. Since wind farms rely on intermittent energy sources, the amount of electrical energy produced can fluctuate. Therefore, each region with different power grid characteristics requires special analysis to ensure the reliability and operational efficiency of wind farm projects, especially on a large scale. Wind energy utilization in California has grown rapidly with the construction of various wind farms that use wind turbines as the main drive for electricity generators. Areas such as the Tehachapi Wind Resource Area and the Altamont Pass Wind Farm exemplify the successful implementation of wind energy to support the transition to a more environmentally friendly power system. In developed countries such as Germany, Australia and the Netherlands, wind power generation has become a major part of the energy mix, and California continues to develop similar infrastructure to reduce dependence on fossil fuels. MICHAEL KURNIAWAN 2 Study Case RESEARCH METHOD Figure 1: Research flowchart Based on Figure 1, the steps taken in this final project research include collecting data related to the California interconnection system, modeling the single line diagram of the interconnection system, analyzing the power flow in the system, conducting simulation tests, evaluating simulation results, and drawing conclusions based on the findings obtained. Simulation Planning The simulation planning conducted in this study aims to analyze the voltage stability of the power system in California due to the loss of large loads. The voltage stability analysis is performed through power flow studies to obtain information about the voltage profile at each bus in the system. The simulation is designed to evaluate the voltage changes when some loads in the system are suddenly removed. Some of the simulation scenarios include disturbances in the form of load shedding in the Los Angeles region during day and night peak loads, as well as load shedding in the San Francisco region under day and night peak load conditions. In addition, the simulations also included scenarios of load shedding in Los Angeles or San Francisco when the system was at its lowest load condition. MICHAEL KURNIAWAN 3 Study Case Single Line Diagram California's electric power system is an interconnected network of power plants spread throughout the state. Electrical energy is transmitted through transmission networks with voltages of 500 kV, 230 kV, and 115 kV, and then distributed to consumers through distribution networks with voltages of 33 kV, 20 kV, and 11 kV. This study uses data from power system operators in California, with data coverage limited to the electricity system in the region. The data obtained includes single line diagrams, transmission network data, power plant data, and load data on the California interconnection system. Based on this data, a representation of the system in the form of a single line diagram is made, as shown in Figure 2. Figure 2: Single Line Diagram of California Electric Power System Simulation of Load Release Disturbance in Los Angeles In this study, two main disturbance scenarios were analyzed, namely sudden load shedding on the California power system. One of the scenarios is a load shedding in the Los Angeles region, which can be observed in Figure 3, which shows the location of the disturbance on the system. Figure 3: Location of Load Discharge Disturbances in Los Angeles MICHAEL KURNIAWAN 4 Study Case Simulation of Load shedding Disturbance in San Francisco Another fault scenario was a load shedding in the San Francisco region, which was performed by disconnecting two major loads connected at the 20 kV busbar in San Francisco. A visual representation of this disturbance is shown in Figure 4, which shows the location of the disturbance in the San Francisco power system. Figure 4: Location of Load Discharge Disturbances in San Francisco RESULTS AND DISCUSSION Simulation of Voltage Stability Maintaining Method This simulation aims to analyze how the system can maintain voltage stability after a disturbance in the form of load shedding on the system. The simulated disturbances include load events and switch events on the Los Angeles load, where the disturbance occurs at the 5th second after the simulation starts, and the switch on the Los Angeles load will be released at the 5th.09th second. Based on the simulation results, some buses experience instability during peak day load, peak night load, and low load, so efforts need to be made to maintain voltage stability. The methods used to overcome voltage instability are: Installation of AVR (Automatic Voltage Regulator) in several power plants with a capacity above 50 MW and a high level of loading to control the voltage. Installation of SVS (Static VAR System) on the bus with the lowest voltage after simulation is done to improve system stability. The loading data of the plants installed with AVR can be seen in Table 1. MICHAEL KURNIAWAN 5 Study Case Table 1. Load Data in AVR Simulation of Load Release Disturbance in Los Angeles This simulation was conducted in three scenarios, namely during the day peak load, night peak load, and low load. The simulation results show that after the Los Angeles load is released, there is a voltage increase at several buses. To overcome this, an AVR is installed to keep the voltage stable. The simulation results during the afternoon peak load are shown in Table 2. Table 2. Daytime Peak Load MICHAEL KURNIAWAN 6 Study Case Before the installation of AVR, the average increase in bus voltage around the Los Angeles load reached 1.0813 pu. After the installation of AVR, the voltage stabilized and approached 1 pu. The simulation results during the evening peak load are shown in Table 3. Table 3. Night Peak Load Before the installation of AVR, the average voltage increase reached 1.1577 pu. After AVR was installed, the unstable bus voltage dropped back to near 1 pu. The simulation results during low load conditions are shown in Table 4. Table 4. Low Load MICHAEL KURNIAWAN 7 Study Case Before the installation of AVR, the average voltage increase reached 1.0748 pu, and after the installation of AVR, the voltage dropped to close to 1 pu. Simulation of Load Release Disturbance in San Francisco The second simulation was conducted with load shedding disturbances in San Francisco at various load conditions. The simulation results during the afternoon peak load are shown in Table 5. Table 5. Noon Peak Load When the San Francisco load was released at noon peak load conditions, the average voltage increase reached 1.0527 pu. After the installation of AVR, the voltage dropped to close to 1 pu. The simulation results during the evening peak load are shown in Table 6. Table 6. Evening Peak Load The voltage increase before AVR installation averaged 1.0509 pu. After the AVR was installed, the voltage stabilized and approached 1 pu. MICHAEL KURNIAWAN 8 Study Case CONCLUSION 1. Under daytime peak load conditions, the California system experienced instability when the Los Angeles load was shed, with an average voltage increase of 1.0813 pu, and when the San Francisco load was shed, the average voltage increase reached 1.0527 pu. Similarly, at evening peak load, the system experienced instability with an average voltage increase of 1.1577 pu when the Los Angeles load was disconnected, and 1.0509 pu when the San Francisco load was disconnected. 2. Under low load conditions, the system experienced instability when the Los Angeles load was released, with an average voltage increase of 1.0748 pu. However, for the San Francisco load shedding disturbance, the system is still in a stable condition. 3. After the Los Angeles load shedding disturbance, the California system stabilized after the installation of AVRs on several generators to overcome overvoltage around the Los Angeles bus. The same thing happened with the San Francisco load shedding, where the system stabilized again after the installation of AVRs in several major plants. MICHAEL KURNIAWAN 9 Study Case REFERENCE MICHAEL KURNIAWAN 10