Virtual Inertia Response Research Articles

Integrated Assessment of the Reliability and Frequency Deviation Risks in Power Systems Considering the Frequency Regulation of DFIG-Based Wind Turbines

Recently, frequency regulation strategies have been widely adopted in the operation and control of doubly fed induction generator-based wind turbines (DFIG-WTs). Thus, wind power, which is becoming an increasingly important energy source, is expected to play a significant role in both power generation and frequency regulation in modern power systems. Under such circumstances, maintaining the reliability and frequency of power systems at a designated level may be more challenging due to inherent uncertainties in wind power generation. In this paper, the integrated assessment of the reliability and frequency deviation risks of power systems with a high penetration level of wind power is investigated. A multi-time scale analytical framework is proposed to calculate the integrated reliability and frequency deviation indices. The coupling between the reliability and frequency deviation is further addressed by developing a novel frequency-sensitive reliability model of the electric generator. Frequency deviations under supply/demand fluctuations and device failures are analyzed, and the power system frequency regulation process is modeled with the fuzzy adaptive virtual inertial response of DFIG-WTs and energy storage system (ESS). Furthermore, IEEE-RTS79 is used to verify the validity of the proposed model and solution method.

Virtual Inertia Response and Frequency Control Ancillary Services From Hydrogen Electrolyzers

This paper presents the modeling foundations to study the capabilities of hydrogen electrolyzers (HEs) to provide frequency control ancillary services (FCAS), including virtual inertia and primary and secondary frequency response. To do so, we propose a general, unified HE dynamic model for the electrolyzer stack circuit, power-electronics interface (PEI), and relevant converter-level control loops. The equivalent circuit of the stack is derived from its current transfer function, with poles and zero obtained from its step response characteristics (e.g., rise/settling time). The stack model also considers relevant physical nonlinearities and downstream hydrogen buffer/process operational constraints. The PEI control loops account for stack model parameters, hydrogen production operational constraints, stack temperature dynamics, and active power reference generation strategy for contingency and regulation frequency support services. Further, we propose a virtual synchronous machine (VSM) control approach to study the VSM HE capabilities to also provide virtual inertia response. We apply the modeling to both alkaline and proton exchange membrane (PEM) technologies, including design of appropriate control schemes. Finally, we assess the FCAS performance of alkaline and PEM HEs via dynamic simulation of the Australian south-east interconnection in a 50%-renewable scenario, also discussing comparison and cooperation with battery energy storage systems.

Decoupling active power control scheme of doubly-fed induction generator for providing virtual inertial response

Virtual inertial control (VIC) is implemented in the controller of doubly-fed induction generators (DFIGs) to compensate for the reduction in capability of the frequency regulation. However, the conventional VIC might give negative impacts on improve the frequency nadir due to the coupling power dynamics between the variation of the power references of the maximum power tracking operation (MPTO) and VIC. In addition, when regaining the rotor speed, a double frequency drop (DFD) trends to produce due to the sudden output drop. This paper suggests a decoupling active power control scheme of a wind power plant to improve the frequency nadir and minimize the DFD. To improve the frequency nadir, the proposed VIC suggests various operating power tracking operations to improve the frequency nadir. The variable-coefficient-PI-control based rotor speed recovery strategy (RSRS) is suggested to minimize the DFD and automatically switches to the MPTO. Test results demonstrate that the suggested decoupling active power control scheme with proposed RSRS and VIC strategy can minimize the DFD and reduce mechanical stresses on the wind turbine while enhancing the frequency nadir.

Wind-Storage Combined Virtual Inertial Control Based on Quantization and Regulation Decoupling of Active Power Increments

With the increasing proportion of wind turbines in power grids, they are required to have capabilities of active and efficient virtual inertial response to maintain grid frequency stability. However, the virtual inertial control methods currently used in doubly-fed induction generator (DFIG) units suffer from a secondary frequency drop (SFD) problem. Although the SFD can be inhibited by reducing the active power support strength of the DFIG units during inertia response, it will undoubtedly weaken the virtual inertia of the units. Therefore, how to eliminate the SFD while increasing the virtual inertia of the units is a worthy issue for studying. To solve this issue, a wind-storage combined virtual inertial control system based on quantization and regulation decoupling of active power increments is proposed in this paper. First, by setting the parameters of a proportional–differential (P-D) algorithm, the total active power increments required for virtual inertial response are quantified at the DFIG level. Secondly, a curve-shifting method based on the rate of change of frequency is adopted to adjust the active power output of the DFIG units. Finally, a battery energy storage system (BESS) is used to compensate for the power shortages of the units according to the quantized value of the active power increments. Simulations show that the control method can not only eliminate SFD but also effectively increase the system’s virtual inertia.

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.

Research on Coordinated Control Strategy of Energy Storage Participating in Primary Frequency Regulation Considering Frequency Deviation Change Rate

In order to further improve the performance of primary frequency modulation (PFM) by battery energy storage, a new control strategy is proposed. By analysing the characteristics of virtual inertia response and virtual droop control, the artificial dead zone of energy storage participating in frequency modulation is set based on virtual inertial control to make it respond quickly and improve the characteristics of energy storage to suppress frequency deviation change rate; In this paper, the remaining energy and load disturbance of the energy storage battery are taken as the double inputs, and the fuzzy control strategy is used to derive the function relationship between the virtual droop coefficient and the two inputs to ensure that the charging and discharging power of the energy storage battery is suitable for the actual needs. The load change rate is used to predict the load change at the next moment. Based on this, the coefficients of virtual inertia control and virtual droop control are determined, and the output coefficients of both are optimized to achieve the optimal frequency regulation effect. Three evaluation indexes are proposed to evaluate the control strategy in this paper. Finally, the simulation model is established in MATLAB/Simulink and the case study is carried out in a regional power grid. The proposed strategy can effectively improve the primary frequency regulation of power grid and has positive significance for maintaining energy storage battery.

Wind Inertial Response Based on the Center of Inertia Frequency of a Control Area

As a result of the increased integration of power converter-connected variable speed wind generators (VSWG), which do not provide rotational inertia, concerns about the frequency stability of interconnected power systems permanently arise. If the inertia of a power system is insufficient, wind power plants’ participation in the inertial response should be required. A trendy solution for the frequency stability improvement in low inertia systems is based on utilizing so-called “synthetic” or “virtual” inertia from modern VSWG. This paper presents a control scheme for the virtual inertia response of wind power plants based on the center of inertia (COI) frequency of a control area. The PSS/E user written wind inertial controller based on COI frequency is developed using FORTRAN. The efficiency of the controller is tested and applied to the real interconnected power system of Southeast Europe. The performed simulations show certain conceptual advantages of the proposed controller in comparison to traditional schemes that use the local frequency to trigger the wind inertial response. The frequency response metrics, COI frequency calculation and graphical plots are obtained using Python.

Stochastic frequency constrained unit commitment incorporating virtual inertial response from variable speed wind turbines

Increasing the penetration of converter-interfaced renewable energy sources like variable speed wind turbines (VSWTs) leads to drastic challenges in keeping the system frequency stability due to system inertia decrement. In this study, a novel variable virtual inertia control strategy for VSWTs is developed which considers their mechanical restriction. It also makes the implication of VSWTs virtual inertial response straightforward to be analytically considered in the system frequency dynamic constraints like rate of change of frequency and frequency nadir. Furthermore, these constraints are formulated and incorporated into a chance frequency constrained unit commitment in order to minimize system operation cost while co-optimizing both inertial response from VSWTs/conventional units and governor-based primary frequency response. The proposed stochastic methods can curtail the load and wind power to satisfy a predefined level of operation risk which can coordinate the unit commitment problem with wind turbines. The comparison analysis on a 10 units standard test system and 4 case studies illustrates that increasing the penetration level of VSWTs from 17% to 36% can provide important savings up to 22.5% while the frequency constraints are also met by VSWTs virtual inertia provision. This helps increasing the penetration of renewables from the view point of frequency stability.

Dynamic Frequency Support from a DFIG-Based Wind Turbine Generator via Virtual Inertia Control

As the penetrated level of wind in power grids increases, the online system inertia becomes weak. Doubly-fed induction generator (DFIG)-based wind turbine generators (WTGs) are required to provide virtual inertia response to support system frequency. The present inertia control strategy with fixed control gain is not suitable and may cause stall of the DFIG-based WTG, as the virtual inertia response potential from the DFIG-based WTG is different with various wind speed conditions. This paper addresses a virtual inertia control method for the DFIG-based WTGs to improve the system frequency stability without causing stalling of the wind turbine for various wind speed conditions. The effectiveness of the proposed virtual inertia control method is investigated in a small system embedded with the DFIG-based WTG. Results demonstrate that the proposed virtual inertia strategy improves the frequency stability without causing the rotor speed security issue. Thus, the proposed control strategy can secure the dynamic system frequency security of power systems under the scenarios of full and partial loads, and, consequently, the proposed method provides a promising solution of ancillary services to power systems.

Combined Primary Frequency and Virtual Inertia Response Control Scheme of a Variable-Speed Dish-Stirling System

The potential of variable-speed dish-Stirling (VSDS) solar-thermal generating plant in providing grid frequency support is investigated. In the proposed VSDS frequency support control scheme, the reference speed of the Stirling engine is regulated to track a deloaded power curve which is governed by the solar insolation level. The gain of a supplementary speed-frequency droop controller is then set to meet the primary frequency control requirement. Further uniqueness of the VSDS control scheme pertains to the provision of virtual inertia response by regulating the kinetic energy in the rotating mass of the engine-generator and the thermal energy in the heat absorber/receivers. Small-signal analysis shows that the frequency support scheme is inherently stable, and it will provide higher degree of damping as the penetration level of the VSDS system and/or the solar insolation level increase. The efficacy of the proposed scheme is validated by computer simulation.

Model Predictive Control for Indirect Boost Matrix Converter Based on Virtual Synchronous Generator

Indirect boost matrix converter is potentially a great alternative to a back-to-back converter for permanent magnet synchronous generators based distributed generation since it can achieve a voltage-boost functionality without utilizing a bulky DC-link capacitor. Despite the success of the indirect boost matrix converter topology, there still exist some issues in the relevant control structure that must be resolved appropriately. First, the existing controls are grid-following controls, which is incapable of islanded operation. Secondly, the exiting controls generate a highly distorted current waveform, which needs to be suppressed by a passive damping resistor. Moreover, without an energy storage element, the distributed generations have no short-time power reserve unit for providing an inertial power to support the utility. In order to solve these issues, a novel approach based on a modified virtual synchronous control and a finite control set model predictive control scheme is proposed in this paper. The former is adopted to ensure proper operations in both grid-connected and islanded modes and to emulate the virtual inertial response by drawing inertial power from the input source. The latter utilizes multi-controls of real-time variables to avoid complicated coupling between the input side and the output side controls and to grant the indirect boost matrix converter with the capability of providing active filter resonance damping. Comparative studies between the proposed control and its existing counterpart are conducted with several simulations in PSCAD/EMTDC software to demonstrate the superior performances of the proposed strategy. Finally, the proposed control is verified in a scale-down experiment testbed.

Control Strategy of BESS for Providing Both Virtual Inertia and Primary Frequency Response in the Korean Power System

As the battery energy storage system (BESS) has been considered to be a solution to the diminished performance of frequency response in the Korean power system, in which renewable energy resources (RESs) are expected to increase rapidly, this paper proposes a control strategy for providing both the virtual inertia and primary frequency response considering the MW-scale BESS installed by the Korea Electricity Power Corporation (KEPCO). The benefit of such a fast and flexible BESS can be maximized by the proposed control strategy for making it provide both the inertia and primary frequency response, which would be deficit with the increased RES. In the proposed control strategy, the state of charge (SOC) is maintained in the specific range in which the life cycle is maximized, the interference of SOC recovery by frequency control is minimized, the responding capacity for providing the virtual inertia response is maximized during the transient period, and the performance requirements for frequency response are satisfied. The effectiveness of the proposed strategy is verified by both Korean power system model-based simulation and on-site operations.

Dynamic characteristics of virtual inertial response provision by DFIG-based wind turbines

Power converter technology partially or fully electrically decouples the wind energy source from the grid which results in the decrease of system inertia. However, when those units participate in virtual inertial response their electromechanical dynamics become coupled to the grid electromechanical modes. To date, there were no comprehensive studies on how do different elements and parameters of a wind energy conversion system (WECS) impact its virtual inertial response provision. This is important from the standpoint of understanding the expected wind farm response during frequency containment process as well as from the standpoint of developing better inertial response controllers. In this paper we have investigated how do operating point, line-side and machine-side converter, phase-locked loop and pitch angle control impact the inertial response of the total power controlled type III WECS (DFIG) which is one of the most common wind turbine topologies used today. We show that the operating point, pitch angle control and outer loop of the machine-side converter have a visible impact on strength of the inertial response, while other elements do not and some can even be neglected in inertial response studies.

Delivering combined droop and inertial response from wind plant for power system frequency stability

As wind penetration level into power systems increases, provision of frequency support from wind plant is anticipated to become a requirement in grid codes. In addition to the virtual inertial response, two different approaches to delivering droop response from wind plant are presented and compared in this paper. The combination of droop and inertial response from wind plant has been assessed for a frequency time series. Simulation results show that the proposed approaches to delivering combined droop and inertial response can provide superior performance to either inertial or droop response alone.

Comprehensive inertia control for hybrid AC/DC distribution system

In this study, a comprehensive inertia control strategy is proposed for the distribution system consisting of AC/DC sub-grids. The coupling relationship of the AC and DC systems is analysed, and the comprehensive inertia control method of hybrid AC/DC system is proposed. More specifically, the operational characteristics of the wind turbine generator (WTG) are utilised to design the virtual inertial response strategy, which enables the WTGs in distribution network to provide flexible voltage and frequency support. Further, since the distributed energy storage (DES) is taken into account, the virtual inertial response of WTGs can be more controllable, which fully utilises the capacity of DES without the risk of halt of the WTGs. In addition, the mutual power support is designed based on the coupling relationship of the hybrid system, which can enhance the overall inertia of the distribution network. Finally, the effectiveness of the proposed strategy is verified by the simulation studies, which shows that the strategy can enhance the power quality and dynamics of the hybrid AC/DC system and thus improves the robustness of the distribution network.

Virtual inertia control of PV systems for dynamic performance and damping enhancement of DC microgrids with constant power loads

DC bus voltage in islanded DC microgrids (MGs) is prone to power fluctuations of sources and loads. This is due to the lack of generational inertia, which is caused by high penetration of converter-based renewable energy sources (RESs). With the growing penetration of RESs, especially photovoltaic arrays (PVAs), they are required to provide grid support services such as inertial response in a similar way to conventional synchronous generators. Here, a virtual inertia control (VIC) is proposed for PVAs to enhance the inertia of a hybrid PVA-battery DC MG. The proposed VIC employs active power control of PVAs to provide virtual inertial response (VIR) without using any high-power energy storage system such as supercapacitors. An adaptive virtual inertia gain is introduced to achieve dynamic power sharing between the PVAs that provide VIR. Impedance-base stability analysis is utilised to study the impact of the virtual inertia gain on the system stability and to show the impact of the proposed VIC on improving the stability margin of the DC MG in the presence of destabilising constant power loads (CPLs). Finally, simulation results are presented to verify the effectiveness of the proposed method in dynamic performance and damping enhancement of the DC MG and reducing the high-current stress on the battery.

Virtual Inertia Control-Based Model Predictive Control for Microgrid Frequency Stabilization Considering High Renewable Energy Integration

Renewable energy sources (RESs), such as wind and solar generations, equip inverters to connect to the microgrids. These inverters do not have any rotating mass, thus lowering the overall system inertia. This low system inertia issue could affect the microgrid stability and resiliency in the situation of uncertainties. Today’s microgrids will become unstable if the capacity of RESs become larger and larger, leading to the weakening of microgrid stability and resilience. This paper addresses a new concept of a microgrid control incorporating a virtual inertia system based on the model predictive control (MPC) to emulate virtual inertia into the microgrid control loop, thus stabilizing microgrid frequency during high penetration of RESs. The additional controller of virtual inertia is applied to the microgrid, employing MPC with virtual inertia response. System modeling and simulations are carried out using MATLAB/Simulink® software. The simulation results confirm the superior robustness and frequency stabilization effect of the proposed MPC-based virtual inertia control in comparison to the fuzzy logic system and conventional virtual inertia control in a system with high integration of RESs. The proposed MPC-based virtual inertia control is able to improve the robustness and frequency stabilization of the microgrid effectively.

Integrated wind turbine controller with virtual inertia and primary frequency responses for grid dynamic frequency support

An integrated controller of wind turbines with both inertial response and primary frequency regulation (PFR) to provide complete dynamic frequency support for the grid with high wind power penetration is investigated. The wind turbine control governor contains two cross-coupled controllers: pitch controller and maximum power point tracking (MPPT) controller. First, as a precondition for the PFR, a de-loading pitch control scheme is proposed to reserve capacity required for frequency regulation. Then, by optimizing the MPPT scheme, the rapid virtual inertia response is achieved even under de-loading operation condition. Based on the analysis of the steady-state characteristics of wind turbines with frequency droop control, the primary frequency control strategy, which enables the adjustment of frequency droop coefficient, is further proposed through pitch angle changes. Thus, the PFR and inertial response can be both achieved by the proposed de-loading pitch controller and optimized MPPT controller. A three-machine prototype system containing two synchronous generators and a Doubly Fed Induction Generator (DFIG)-based wind turbine with 30% of wind penetration is implemented to validate the proposed integrated control strategies on providing inertial response and subsequent load sharing in the event of frequency change.

A novel method for obtaining virtual inertial response of DFIG‐based wind turbines

AbstractThis paper investigates virtual inertia control of doubly fed induction generator (DFIG)‐based wind turbines to provide dynamic frequency support in the event of sudden power change. The relationships among DFIGs' virtual inertia, rotor speed and network frequency variation are analysed, and a novel virtual inertia control strategy is proposed. The proposed control strategy shifts the maximum power point tracking (MPPT) curve to the virtual inertia control curves according to the frequency deviation so as to release the ‘hidden’ kinetic energy and provide dynamic frequency support to the grid. The calculation of the virtual inertia and its control curves are also presented. Compared with a PD regulator‐based inertial controller, the proposed virtual inertia control scheme not only provides fast inertial response in the event of sudden power change but also achieves a smoother recovery to the MPPT operation. A four‐machine system with 30% of wind penetration is simulated to validate the proposed control strategy. Simulation results show that DFIG‐based wind farms can provide rapid response to the frequency deviation using the proposed control strategy. Therefore, the dynamic frequency response of the power grid with high wind power penetration can be significantly improved. Copyright © 2015 John Wiley & Sons, Ltd.

Coordinated frequency regulation by offshore wind farms and VSC-HVDC transmission

With the increasing wind penetration, large-scale offshore wind farms exert significant impact on power system security and operation, and thus are required to contribute to system frequency regulation. This paper develops a coordinated control strategy for offshore wind farms with voltage source converter-based HVDC (VSC-HVDC) transmission system to participate in power system frequency regulation. The strategy explores the frequency support capability of offshore wind farms and VSC-HVDC. By implementing the proposed coordinated control, the VSCHVDC link is able to provide quick virtual inertial response to onshore grid frequency drops. Shortly afterwards, the offshore wind farm detects the frequency changes and start to participate in inertial response and primary frequency control. The offshore wind farms, in this study, consist of full-converter wind turbines (FCWTs). Simulation results are presented to demonstrate the frequency regulation capability of VSC-HVDC and validate the effectiveness of the proposed coordinated control strategy.