Renewable Energy Systems
The Next Step Forward
Governmental institutions have implemented rigorous rules aimed at reducing carbon dioxide emissions. Since most carbon dioxide is produced by coal-based power plants, there is a demand to increase the production of renewable energy – especially from wind and sun sources – and to reduce the number of coal-based power stations. The costs involved in a system that uses a greater proportion of renewable energy sources concern not only direct investment in renewable energy generation but also realising radical changes in energy system infrastructure, to maintain the requisite high levels of reliability and security in energy supply. The turn of the century and beginning of the new millennium will be remembered for the commitments made to replacing large coal and nuclear power plants with renewable resources, especially wind and solar energy. These commitments implicitly emphasised an increased use of ICT to control, monitor and protect future power systems; the final goal being to provide cheap and clean energy through a power system that operates with high levels of reliability and security of supply. This is reflected in what has become a generally accepted definition of a smart grid.
Wind and sun
Renewable energy resources (predominantly solar and wind parks) provide a potentially unlimited amount of clean electrical energy, that in the future – we hope – will pay back the investment made in them and provide cheap and clean electricity for everyone. For the electrical power engineer, however, wind and solar energy generation involve low system inertia, bi-directional current flow, and a completely different composition of loads and electricity generation. These are concerns related to security of supply and other unpredictable phenomena such as power quality, transients, and low fault currents. All of these factors raise the overarching question of how the systems of the future will be monitored, protected and controlled. Future power systems need to meet the requirements of being secure and reliable with high levels of sustainability.
Phasor measurement units (PMUs)
Currently, the Supervisory Control and Data Acquisition (SCADA) system is used to provide the topology and system state of transmission and distribution systems. SCADA, however, generates data every few seconds. Is this enough? Not according to Dr. Marjan Popov of the Electrical Sustainable Energy department at the TU Delft.
“In the future, we need to learn to deal with low system inertia – so faster measurements will be needed. Phasor Measurement Units (PMUs) are seen as a solution [to this], providing data to run smart algorithms. These algorithms will not only be used for state estimation but also for adaptive protection, fault location, situation awareness, controlled islanding and many other applications. PMUs will be very helpful providing the system with enough units to make the system observable. However, we still need to prevent severe disturbances. The utility of PMUs has already been demonstrated in weak systems with long lines – by General Electric (GE) on the Icelandic power system – which is very remote from the powerful European system to provide frequency and voltage stability. However, I still do not believe that this is enough for the future. The reason is this is that the systems of the future will be vulnerable to far more disturbances than classical power systems: more transient phenomena, more switching, more dispersed microgrids (to produce renewable generation on low voltage level) and new types of distribution all raise the major question of if we are still safe when a power system is affected by natural faults.”
New adaptive protection schemes
“When a fault occurs, the power system protection needs to function quickly to isolate the fault. Currently, through the Horizon 2020 Migrate project, the performance of existing protection algorithms are being investigated when a network is nearing 100 % penetration. Weak points must be identified and new algorithms are being developed. In order to observe their performance in a non-existing network, real-time simulations are being run with sophisticated models developed specifically for this project.”
Can faults be avoided?
Faults are sometimes natural phenomena. Lightning mostly strikes high objects like transmission towers and results in single-phase currents. Furthermore, the Dutch distribution system is predominantly a cable network, with most cables being several decades old. When a cable is nearing the end of its lifetime, the insulation becomes weaker which may result in a fault.
“In the newly formed TU Delft Power System Protection Centre (PSPC), we obtained NWO funding to investigate different phenomena by monitoring faster transient oscillations. How fast it will be necessary and how fast it is possible? This is the question that will be answered in the years to come!”
Marjan Popov is an associate professor in the Intelligent Electrical Power Systems Group (IEPG), of the Electrical Sustainable Energy department at TU Delft. His expertise is in power system protection, large-scale power system transients, CB interruption phenomena, high-frequency analysis and modeling and wide area monitoring and protection.
Text: Susan Penninks and Marjan Popov I Photo: Thierry Schut | August 2018