Investigation of Inertia Response and Rate of Change of Frequency in Low Rotational Inertial Scenario of Synchronous Dominated System

The shift to a sustainable energy future is becoming more reliant on large-scale deployment of renewable and distributed energy resources raising concerns about frequency stability. Rate of Change of Frequency (RoCoF) is necessary as a system inertia metric in order for network operators to perform control steps to preserve system operation. This paper presents in a straightforward and illustrative way several relevant aspects of the inertia response and RoCoF calculation that could help to understand and explain the implementation and results of inertial response controllers on power converter-based technologies. Qualitative explanations based on illustrative numerical experiments are used to cover the effects on the system frequency response of reduced rotational inertia in synchronous dominated power systems. One main contribution of this paper is making evident the importance of the governor action to avoid the synchronous machine taking active power from the system during the recovering period of kinetic energy in an under frequency event.

Obtaining Robust Performance of a Current Fed Voltage Source Inverter for Virtual Inertia Response in a Low Short Circuit Ratio Condition

Low inertia levels are typical in island power systems due to the relatively small rotational generation. Displacing rotational generation units with static inertia-less PV power results in a significant increase in the frequency volatility. Virtual inertia provided by inverter-storage systems can resolve this issue. However, a low short circuit ratio (SCR) at the point of common coupling together with a fast phase locked loop (PLL) will compromise the response performance of the system. To address this issue, a robust PI controller (RPI) for the inner current-loop of a current fed grid-connected inverter is proposed. The PLL disturbance and grid impedance are incorporated into a single model and recast to a generalized representation of the system, thereby allowing easy tuning of the RPI by the mixed sensitivity H∞ method. The performance of the RPI is compared with that of a PI controller (PI) tuned by the regular loop-shaping method. The results show that when the SCR is above 10, the performance of both controllers is equivalent. However, lowering of the SCR compromises the performance of the system with PI and it becomes underdamped at SCR < 2. On the contrary, the system with the RPI is capable of maintaining the nominal performance throughout the same SCR decrease.

Virtual Inertia Coordinated Allocation Method Considering Inertia Demand and Wind Turbine Inertia Response Capability

Wind turbines can have inertia characteristics similar to synchronous generators through virtual inertia control, which helps to provide the inertia support for the system. However, there is the problem of how to coordinate the allocation of virtual inertia among wind turbines. In response to this problem, this paper first analyzes the inertia response capabilities of wind turbines and puts forward an evaluation index that quantifies the inertia response capability of wind turbines. The inertia response capability of a wind farm is evaluated at the entire system level. Based on the evaluation index, the virtual inertia coordinated allocation method considers the system inertia demand and the inertia response capabilities of the wind turbines. It is proposed to release the inertia response capability of each wind turbine while avoiding an excessive release of kinetic energy and bring a second impact by wind turbines’ exiting operation. Finally, the effectiveness of the proposed method is verified by a simulation case study.

Coordinated Sending-End Power System Frequency Regulation via UHVDC

The continuous improvement of new energy penetration reduces the inertia of the system, which leads to the frequency deviation and the rate of change of frequency (RoCoF) being easily exceeded. To improve the frequency stability of sending-end power systems with large-scale renewable energy access via ultra-high voltage direct current (UHVDC), the coordinated frequency control for UHVDC participating in system frequency regulation (FR) including primary FR and system inertial response is presented. Based on the simplified system model, the mechanism of UHVDC participation in system frequency support and its influence on receiving-end system frequency response characteristics are analyzed. Compared with the inertia response and primary FR of traditional synchronous generators, the parameter calculating method of UHVDC coordinated frequency response control is proposed. Based on the system root trajectory analysis, the influence of the frequency response control parameters on the sending-end system’s stability is analyzed, and the constraints of UHVDC participating in the system frequency response control are analyzed. Then, based on the RTDS verification platform containing the Lingshao ±800 kV UHVDC control and protection system, the system frequency response characteristics under different control strategies, operating conditions and control parameters are verified and analyzed. The experimental results show that the UHVDC frequency coordinated control can effectively increase the equivalent inertia of the sending-end system, restrain the RoCoF and the frequency deviation, and increase the FR capability of the UHVDC system.

Coordinated control among PSS, WTG and BESS for improving Small-Signal Stability

The goal towards attaining a sustainable future has led to the rapid increase in the integration of converter control based generators (CCBGs). The low inertia response characteristics of CCBGs and the weak tie lines in interconnected systems pose a huge threat to Small-Signal Stability (SSS). Adequate damping of low-frequency oscillations (LFO) is pivotal in ensuring the maximum power transfer through the critical transmission corridors. These operational issues become more serious with the significant reduction in system inertia as a result of the high penetration of CCBGs. Therefore, appropriate control techniques are an absolute requirement for preventing LFOs from limiting the penetration of CCBGs in interconnected networks. This may also eventually lead to revisions in grid codes mandating CCBGs to provide auxiliary damping control. But, the progressive addition of multiple damping controllers for specific target modes can lead to the drifting of eigenvalues (EVs) associated with other electromechanical modes (EMs) in the system. This is due to the adverse interactions between multiple damping controllers in the uncoordinated control approach and may result in deteriorating SSS. Therefore, this paper proposes a simultaneous coordinated control among Battery Energy Storage System (BESS), Wind Turbine Generators (WTG) and Power System Stabilizer (PSS) for enhancing SSS in networks with high wind penetration by considering both inter-area (IA) and local modes. The performance of the proposed coordinated control is corroborated using IEEE 68 bus system for multiple operating scenarios for which the critical modes in the system have the lowest damping index (DI). The effectiveness of modulating the active power, reactive power and simultaneous modulation of both active and reactive power injected by BESS along with a dual-channel Optimized WTG Damping Controller (DOWDC) and PSS is evaluated. The impact of the different coordinated control strategies on voltage dynamics is also investigated. The simulation results validate the better performance of the proposed coordinated control over uncoordinated control approaches.

A Spectral Model of Grid Frequency for Assessing the Impact of Inertia Response on Wind Turbine Dynamics

The recent developments in renewable energy have led to a higher proportion of converter-connected power generation sources in the grid. Operating a high renewable energy penetration power system and ensuring the frequency stability could be challenging due to the reduced system inertia, which is usually provided by the conventional synchronous generators. Previous studies have shown the potential of wind turbines to provide an inertia response to the grid based on the measured rate of change of the grid frequency. This is achieved by controlling the kinetic energy extraction from the rotating parts by its converters. In this paper, we derive a spectral-based model of the grid frequency by analyzing historical measurements. The spectral model is then used to generate realistic, generic, and stochastic signals of the grid frequency for typical aero-elastic simulations of wind turbines. The spectral model enables the direct assessment of the additional impact of the inertia response control on wind turbines: the spectra of wind turbine output signals such as generator speed, tower base bending moment, and shaft torsional moment are calculated directly from the developed spectral model of the grid frequency and a commonly used spectral model of the turbulent wind. The calculation of output spectra is verified with non-linear time-domain simulations and spectral estimation. Based on this analysis, a notch filter is designed to significantly alleviate the negative impact on wind turbine’s structural loads due to the inertia response with only a small reduction on the grid support.

Novel Coordinated Control Strategy of BESS and PMSG-WTG for Fast Frequency Response

An increase in inverter-based resources (IBRs) can lower the inertia of a power system, which may adversely affect the power system by causing changes such as a frequency nadir reduction or an increased initial rate of change of frequency (RoCoF). To prevent this, an ancillary service called fast frequency response (FFR) helps the inertia response by using IBRs. The main resources used in FFR are variable-speed wind turbine generators (VSWTGs) or energy storage systems (ESSs), which can respond quickly through converter control. The control is applied to the frequency regulation service faster than the primary frequency response, so the second frequency nadir may fall below the first frequency nadir. This study proposed a novel coordinated control strategy to efficiently utilize energy to improve the frequency nadir through coordinated control of wind turbines based on permanent magnetic synchronous generators (PMSGs) and battery energy storage systems (BESSs). The simulation results confirmed that the two-bus test system was composed of PSCAD/EMTDC, and the frequency nadir increased by utilizing the same amount of energy as in traditional control systems.

Frequency and inertia response effects, capabilities, and possibilities in renewable energy sources

Background & Objectives: The global power system is in a state of continuous evolution, incorporating more and more renewable energy systems. The converter-based systems are void of inherent inertia control behavior and are unable to curb minor frequency deviations. The traditional power system, on the other hand, is made up majorly of synchronous generators that have their inertia and governor response for frequency control. For improved inertial and primary frequency response, the existing frequency control methods need to be modified and an additional power reserve is to be maintained mandatorily for this purpose. Energy self-sufficient renewable distributed generator systems can be made possible through optimum active power control techniques. Also, when major global blackouts were analyzed for causes, solutions, and precautions, load shedding techniques were found to be a useful tool to prevent frequency collapse due to power imbalances. The pre-existing load shedding techniques were designed for traditional power systems and were tuned to eliminate low inertia generators as the first step to system stability restoration. To incorporate emerging energy possibilities, the changes in the mixed power system must be addressed and new frequency control capabilities of these systems must be researched. Discussion: In this paper, the power reserve control schemes that enable frequency regulation in the widely incorporated solar photovoltaic and wind turbine generating systems are discussed. Techniques for Under Frequency Load Shedding (UFLS) that can be effectively implemented in renewable energy enabled micro-grid environment for frequency regulation are also briefly discussed. The paper intends to study frequency control schemes and technologies that promote the development of self- sustaining micro-grids. Conclusion: The area of renewable energy research is fast emerging with immense scope for future developments. The comprehensive literature study confirms the possibilities of frequency and inertia response enhancement through optimum energy conservation and control of distributed energy systems.