Virtual Inertia: Current Trends and Future Directions

The modern power system is progressing from a synchronous machine-based system towards an inverter-dominated system, with large-scale penetration of renewable energy sources (RESs) like wind and photovoltaics. RES units today represent a major share of the generation, and the traditional approach of integrating them as grid following units can lead to frequency instability. Many researchers have pointed towards using inverters with virtual inertia control algorithms so that they appear as synchronous generators to the grid, maintaining and enhancing system stability. This paper presents a literature review of the current state-of-the-art of virtual inertia implementation techniques, and explores potential research directions and challenges. The major virtual inertia topologies are compared and classified. Through literature review and simulations of some selected topologies it has been shown that similar inertial response can be achieved by relating the parameters of these topologies through time constants and inertia constants, although the exact frequency dynamics may vary slightly. The suitability of a topology depends on system control architecture and desired level of detail in replication of the dynamics of synchronous generators. A discussion on the challenges and research directions points out several research needs, especially for systems level integration of virtual inertia systems

Control of Voltage-Source Converters Considering Virtual Inertia Dynamics

Controlling power-electronic converters in power systems has significantly gained more attention due to the rapid penetration of alternative energy sources. This growth in the depth of penetration also poses a threat to the frequency stability of modern power systems. Photovoltaic and wind power systems utilizing power-electronic converters without physical rotating masses, unlike traditional power generations, provide low inertia, resulting in frequency instability. Different research has developed the control aspects of power-electronic converters, offering many control strategies for different operation modes and enhancing the inertia of converter-based systems. The precise control algorithm that can improve the inertial response of converter-based systems in the power grid is called virtual inertia. This thesis employs a control methodology that mimics synchronous generators characteristics based on the swing equation of rotor dynamics to create virtual inertia. The models are also built under different cases, including grid-connected and islanded situations, using the swing equation with inner current and voltage outer loops. Analysis of the simulation results in MATLAB/Simulink demonstrates that active and reactive power are independently controlled under the grid-imposed mode, voltage and frequency are controlled under the islanded mode, and frequency stability of the system is enhanced by the virtual inertia emulation using swing equation. On this basis, it is recommended that the swing equation-based approach is incorporated with the current and voltage control loops to achieve better protection under over-current conditions. Further works are required to discover other factors that could improve the effectiveness of the models

Virtual Inertia Emulation to Improve Dynamic Frequency Stability of Low Inertia Microgrids

Due to low inertia and the intermittent nature of photovoltaic systems, dynamic frequency stability issues arise in microgrids with large photovoltaic systems. This limits the maximum amount of photovoltaic systems that can be penetrated in the microgrid. In order to increase the penetration of photovoltaic systems, the dynamic frequency controller, that is faster than the primary frequency controller (governor control) needs to be added in the microgrid system. For dynamic frequency control, inertial response can be provided from the energy storage system (such as battery, ultra-capacitor, photovoltaic system, etc.), which is termed as virtual inertia. A virtual inertia can be defined as the combination of an energy storage system, a power electronics converter and a proper control algorithm that improves the dynamic frequency stability of the microgrid. A virtual inertia supplies or absorbs the active power to and from the energy storage system to improve the dynamic frequency stability. This thesis presents the design and implementation of a hardware prototype of 1 kW virtual inertia in a microgrid with a real diesel generator and a load. For a step change in load, the virtual inertia improved the frequency response of the system from 57.39 Hz to 58.03 Hz. This improvement in frequency response proves the concept of existing proportional derivative based virtual inertia experimentally. With the addition of virtual inertia, the frequency of the system returns to the nominal frequency slower. Once the primary controller (governor control) of the system takes the action to regulate the frequency, virtual inertia no longer needs to add inertia to the system. So the dynamics of the VI needs to be improved so that the frequency returns to nominal frequency faster. This thesis also proposes an online learning controller based virtual inertia using adaptive dynamic programming that learns online and improves the dynamics of the controller of existing VI. The output of this controller supplements the output of the existing proportional derivative controller of virtual inertia. The supplementary controller is trained to increase the dynamics of the outer controller and to bring the system frequency to nominal frequency faster. Due to faster dynamics, the net energy delivered by the VI can be reduced significantly and improve the total possible discharge cycles from the battery. For performance evaluation, the proposed controller was implemented in a microgrid with a photovoltaic system, a diesel generator and a variable load. With the proposed controller, the frequency of the system returned to nominal frequency faster. The net energy delivered by the proposed controller in a photovoltaic diesel generator microgrid was 46.14% of the net energy delivered by the existing virtual inertia. Due to the decrement in the total energy delivered, the total number of possible battery discharge cycles with ADP based VI was 2.17 times of the total number of possible battery discharge cycles from VI