In recent years, the contribution of power generation from renewable energy sources (RES) has increased immensely, as might be inferred from stringent environmental protection rules, the reduced accessibility of fossil fuels, and the need to satisfy a raised global power demand [1]. For example, China has set a goal to produce 35% of power from RES by 2030 [2]. India has set an ambitious target to generate 175 GW of power from RES by 2020 [3]. The European Union and the United States have additional targets for RES [4]. In 2017, few countries effectively integrated larger shares of RES into the main grid. Countries leading the way in RES penetration include Denmark (nearly 53%), Uruguay (28%), and Germany (26%). India, Ireland, Portugal, and Spain also have RES penetration levels above 20%. Fig. 1 shows the 2017 RES power generation shares of several countries and their targets for the year 2050.
The volatility and uncertainty of RES like solar and wind energy can be a significant problem for the operation of the power system [7]. The restoration of a conventional synchronous generator (SG) by a wide number of power electronic inverters increases efficiency, stability, quality, and flexibility [8]. However, power management among these sources leads to an immense challenge in system design and monitoring [9,10]. Further, large-scale integration of RES into the grid leads to frequency stability issues [11]. Generally, RES has low or no inertial response. For instance, photovoltaic (PV) arrays require power electronic dc–ac inverters to integrate with the grid and do not offer an inertial response to a grid, and wind turbines need variable frequency ac – dc – ac converters, which decouple the wind turbine inertia from the grid. Consequently, the inertia of the power system decreases as the penetration of RES increases [12]. The reduced inertia in the power system leads to an increase in the rate of change of frequency (ROCOF) and frequency deviations in a very short time, under power imbalances that substantially affect the frequency stability of the system [13].
A grid operator, i.e., the electric reliability council of Texas, has identified a persistent decrease in the inertia of its system [14]. The nominal system frequency of the electric reliability council of Texas system is 60 Hz. In the electric reliability council of Texas system, the power generation from RES has gradually increased from the year 2010–2017. Simultaneously, the frequency of the power system has been decreasing after the tripping of a 2750 MW generation unit in different years with an increasing share of RES from 2010 to 2017. After the contingency event (an unexpected loss of generation or load), the frequency nadir (the lowest point of frequency) rises with less amount of inertia. Fig. 2 proves that after a disturbance, the frequency nadir is increasing as the power generation from RES increases from the year 2010–2017.
SG regulates the frequency stability of the power system when the contribution of power generation from RES is small. As the RES penetration level increases each year, the frequency stability issues and power oscillations of the power system increase under disturbances [16,17]. The critical RES penetration limit is the instantaneous penetration value of RES, above which the frequency can fall below the allowable range after a contingency event. There is no unique critical RES penetration value, as system dynamics change from one moment to another [18]. The critical RES penetration limits are depending on the frequency droop controller, voltage droop controller, and transients [19].
Traditionally, the inertial response from the SG is an inherent characteristic, and it is not treated as an ancillary service. However, with the increase in penetration of RES, the grid operators in various countries have identified inertia as an ancillary service. From a grid operator view, the reduced inertia has two consequences on the frequency stability of the system. First, a high ROCOF leads to a trip of relays, and second, a high-frequency nadir results in unintentional load shedding. Hence, to overcome the frequency stability issues characterized by low inertia, different control techniques are required at the power electronic converters to allow RES to participate in frequency regulation, and different technologies need to be installed to enhance the inertia of the power system.
This review mainly focuses on inertia issues in power systems with a high penetration level of RES. This manuscript provides an overview of grid requirements and various measures followed by different countries to operate stably. Furthermore, this study reviews existing inertia emulation algorithms applied to inverters, wind turbines, PV systems, and paves a pathway to selecting an appropriate algorithm. A discussion on different control techniques explores the research gap and provides future direction to researchers working in this area. This study also analyzes the possibility of installing various appropriate technologies for the low-inertia power system.
The remainder of this paper is organized as follows. Section 2 examines the inertia of a traditional power system and the impact of RES on inertia. Section 3 discusses the various measures taken by different grid operators for a low-inertia power system. Section 4 explains various inertia emulation control techniques applied to inverters, wind turbines, PV systems to offset low inertia of a power system. Section 5 provides discussions on appropriate technologies used to enhance the inertia. Section 6 explores future directions, and section 7 provides a conclusion.