The role of Distributed Energy Resources (DERs) in smart grid stability

Abstract

The power system grid is designed to be flexible and dynamic. Its flexibility is helping to re-adjust when the system operating conditions are changing. The conventional power systems are heavily dependent on fossil fuels such as coal as the energy source. Due to the global warming effect, traditional power generation through coal is becoming a significant concern, and policies have been put in place to mitigate air pollution. Most African countries have subscribed to those international policies, and South Africa is one of them. The only option to achieve an environmentally friendly power generation system is to integrate renewable energy resources. The integration of these energy resources can be done at the distribution level, with the benefit of minimizing transmission power losses. However, integrating the distributed energy resources into the conventional power system can result in a complex power grid. The energy management system will need to be improved. The other challenge about the distributed energy resources is that they quickly respond to system dynamic events. Lack of primary resources such as wind or sun can also be challenging. However, when the resources are available, these energy sources can be adequately utilized in various applications. The thesis explores utilizing the wind power generation system as an active power compensator following load demand increase in the power system to stabilize the system frequency. As the load demand increases, the frequency is declining. In the conventional generation system, the governing system is utilized as the primary control to release an additional active power to the grid flowing a load demand increase event. However, the governing system fails to restore the system frequency to its nominal value. A load reduction is the only viable solution for other control systems for the system frequency to recover and go back to its nominal state. The implementation of secondary control, which utilizes a decentralized automatic generation control, was developed and implemented to restore the system frequency when the governing system failed. When the load demand kept increasing, the wind power plant was integrated into the grid to improve frequency stability. Both decentralized automatic generation control and wind power active power compensator control loops were developed on DIgSILENT Power factory simulation software. The simulations were performed on the DIgSILENT software and were verified on the Real-Time Digital Simulator (RTDS) simulation platform. In the conventional system, Programmable Logic Controllers (PLC) are used for the control system, this requires a large number of separated control cards, and there is too much wiring involved. The complexity of the PLC control system makes troubleshooting to be difficult when there is a fault. The information to the control center is made through telemetry, and there is often a delay in obtaining the data due to signal transmission traffic. The utilization of the IEC 61850 standard has advanced communication and the mitigation of using too much wiring to achieve a working control system. The developed frequency control system was practically implemented using an IEC 61850 standard protocol. The control logic was developed using SEL-3555 Real-Time automatic Controller (RTAC) intelligent electronic device (IED).