Virtual Inertia Control to Enhance Frequency Stability of Power Systems
By Aref Pouryekta, Vigna K. Ramachandaramurthy
A growing number of Voltage Source Converter (VSC) based generators in modern power systems results in a decrease of inertia and, consequently, to frequency instability. The reduced number of synchronous machines and the integration of wind turbines via VSCs into the power systems decouples the rotational components from the grid. Hence, reduction of inertia in the system threatens frequency stability. Virtual inertia is a solution in the described premises. Here, the VSC will be able to produce virtual inertia using appropriate control structure that enables it to behave as a synchronous generator and improve the frequency stability profile of the system.
Conventional power systems consist of large power plants with synchronous generators (SG) which provide considerable inertia in the system. However, in recent years, the number of renewable energy sources like photovoltaic generation plants and wind farms that are connected via VSCs into the electrical grid has increased significantly. For instance, Doubly Fed Generators (DFIG) are widely employed in wind farms using VSC interface devices as they provide opportunity to work in different frequencies and different wind speed. Consequently, decoupling the actual inertia of the blades of wind turbines leads to the risk of frequency instability during the disturbance in the system. Hence, despite the fast response time of VSCs compared to the synchronous generators, the lack of any spinning component leads to lower frequency stability and consequently a lesser resilient grid.
Frequency stability refers to the capability of a power system to retrieve its steady state frequency without violating the acceptable margins once the fault is removed. Typically, frequency instability occurs in power systems that lack sufficient active power generation, are heavily loaded, as well as power systems with high penetration of VSC-based generators. In power systems with high-density of VSC-interfaced generators, primary frequency support is more challenging, due to lack of spinning machines. Therefore, the available time to commence remedial actions are considerably reduced compared to power systems dominate by conventional generators. To overcome this problem, virtual inertia is introduced to ensure the short-term frequency stability of the grid. Generally, frequency control should be done in three stages:
- Inertial response (response to the rate of change of frequency)
- Primary frequency control
- Secondary frequency control
To achieve proper inertial response, the required virtual inertia to maintain the frequency stability must be determined. Subsequently, the control system should be able to adjust the output inertia from VSC-interfaced generators. It is vital to react quickly against frequency decline before under frequency relays start to trip the generators, which exacerbates the problem. In order to prevent frequency instability, the control method should be provided for VSC to increase the inertia of the grid in the presence of renewable energy resources. The control method should be able to emulate the mass behavior of conventional synchronous generators.
A conventional voltage source convertor can be changed into a synchronverter using a proper control strategy. Combining the advantages of a synchronous generator with a VSC provides an opportunity to increase the kinetic energy extracted from renewables. A control approach based on droop scheme shall be utilized by a synchronverter to produce higher inertia compared to the conventional VSC. In this method, the synchronverter would be able to mimic the synchronous generators output and improve the system performance and damp the oscillations in the power systems. Increasing the inertia of the system provides additional time to initiate self-healing scenarios while maintaining the robustness of system operation. Despite the synchronous machines that only contribute a fixed amount of inertia (due to the inherent characteristics of their rotating parts), a synchronverter can be designed to produce variable amounts of inertia as needed to maintain the frequency stability of the system (considering the physical rating limits of VSC). Additionally, albeit in the long term, synchronverters enable the utilities to decommission their conventional power plants and replace them with large storage systems without the need to re-adjust the control parameters on vicinity power plants as well as protection system.
To conclude, modern networks including microgrids and high penetration of VSC-interfaced power generation units suffer from lack of inertia that leads to frequency instability. In order to overcome the deficit of inertia in the system, a combination of conventional SG and inverters can be used to produce virtual inertia and improve the system stability.
Aref Pouryekta received his Ph.D. degree in electrical engineering from Tenaga National University, Malaysia, in 2017.He was awarded 2 years Postdoctoral Research Fellow with Tenaga National University. Currently he is a senior consultant engineer with DNV GL. His research interests include renewable energy integration, micro grid stability, and power system modeling.
Vigna K. Ramachandaramurthy completed his Bachelors Degree and PhD at the University of Manchester Institute of Science and Technology (UMIST), UK in 1998 and 2001 respectively. He has worked as an electrical engineer in a large electrical utility in Malaysia, with stints in the transmission and distribution subsidiaries and power plant. He is presently a Professor in the Institute of Power Engineering, Universiti Tenaga Nasional (UNITEN), Malaysia. Prof Vigna has received many awards for research and leadership such as the IET Mike Sargeant Award, Institution of Engineers Malaysia (IEM) Young Engineers Award and the IET Malaysia Engineers Award. He has served in the IET Council, IEM Council and the IEEE PES Excomm in Malaysia.
Prof Vigna advocates industry based research. He heads the Power Quality Research Group in UNITEN with 28 researchers under his supervision at the moment. The Group conducts numerous research and consultancy projects in South East Asia. His area of interest includes power systems related studies, smart grid, renewable energy, energy storage, power quality and electric vehicle. He is the consultant to many renewable energy projects and the RE industry in Malaysia and South East Asia. He has conducted the technical feasibility for more than 300 renewable energy plants in Malaysia, in the areas of hydro, solar PV, biomass, biogas, wind and energy storage. He was the principal consultant to the government and utility to formulate and develop the technical guidelines for grid-interconnection of solar PV and distributed generation in Malaysia.
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