Fast Frequency Regulation Research Articles

Analysis and Design of Inertia for Grid-Tied Electric Vehicle Chargers Operating as Virtual Synchronous Machines

Renewable energy resources (RES), e.g., solar photovoltaic (PV), wind energy, and electric vehicles (EVs) are extensively integrated into modern power electronics-based power systems due to their sustainable and environmentally friendly features. This has gradually reshaped the frequency regulation structure of power systems. Specifically, the duty of frequency regulation, previously supported by synchronous generators, will be taken over by grid-tied power converters (such as EV chargers), which can be controlled as virtual synchronous machines (VSMs) to mimic the inertial response of synchronous generators. However, being different from synchronous generators, VSMs enable fast frequency regulation. As a consequence, it is identified in this paper that the elimination of frequency oscillations is possible. Therefore, instead of simply increasing inertia for oscillations suppression, the desirable inertia coefficient can be different for various cases. In order to illustrate this point, relationships between several important performance indices, such as the rate-of-change-of-frequency, frequency nadir, overshoot, settling time, and inertia coefficient, are analyzed in detail. Furthermore, a straightforward method for designing the optimal inertia coefficient for VSMs is put forward. Finally, experiment results obtained from a 1-kW three-phase VSM prototype are demonstrated to validate the effectiveness of the theoretical analysis and proposed design method.

New hybrid scheme with local battery energy storages and electric vehicles for the power frequency service

This paper proposes a new hybrid scheme using the EV battery and the local battery as a unit, taking an active part in the grid services. Both electric vehicles and grid-scale battery energy storage have been growing fast in recent years. The active combination of these two kinds of energy sectors is challenging but will unlock extra flexibility at the distribution level. Therefore, the EV battery (EVB) and local battery (LB) are studied in a hybrid scheme for the first time. The scheme delivers an improved optimal power schedule for the fast frequency regulation (FFR) in the microgrid. A hybrid power management strategy based on an improved model predictive control (IMPC) in a microgrid is developed. The IMPC is advanced by adding the battery degradation prediction and EV capacity prediction in the loop and designed for optimal power-sharing with the minimum effect on battery lifetime. The EV battery status, which is critical for both the IMPC and the battery degradation quantification, is predicted by the deep learning approach. The proposed hybrid power management with IMPC is verified to be very effective with optimal power-sharing and battery anti-aging control in the microgrid application. • A new hybrid scheme with active combination of the EV and the local energy storage is developed. • An improved MPC is advanced by introducing the EV capacity prediction and battery degradation within the control loop. • The lifetime for both the local battery and the EV battery is proved to be extended under the hybrid scheme.

Grid-Following Mode Operation of Small-Scale Distributed Battery Energy Storages for Fast Frequency Regulation in a Mixed-Source Microgrid

As the share of power converter-based renewable energy sources (RESs) is high, a microgrid, in islanded mode, is more vulnerable to frequency instability due to (1) sudden power imbalance and (2) low inertia. One of the most common approaches to address this issue is to provide virtual inertia to the system by appropriately controlling the grid-side converter of the RESs. However, the primary frequency controller (PFC) presented in this paper focuses on the fast compensation of power imbalance without adding inertia to the system. The proposed method is based on estimating the real-time power imbalance caused by a disturbance and compensating it using multiple small-scale distributed battery energy storage systems (BESSs). The power imbalance is estimated by observing the initial rate of change of frequency (RoCoF) following a disturbance. Based on the estimated power imbalance and the rating of the BESSs, the reference power for the BESSs is determined. The BESSs are controlled in grid-following mode to compensate for the power imbalance. The performance of the proposed PFC is verified using a Typhoon real-time simulator for various scenarios and is compared with the conventional virtual synchronous generator (VSG) controller.

Novel cascading scheme of VSC‐HVDC with DC voltage synchronisation control for system frequency support

This paper introduces a novel cascading frequency regulation scheme (CFRS) for VSC-HVDC to provide strong support for the connected weak AC grid without phase-locked loop and remote-communication considering the cost-effective property. The proposed CFRS utilises a DC voltage synchronisation control to enable the self-synchronisation of VSC-HVDC without phase-locked loop, while the corresponding inertia response can be exerted via utilising the stored HVDC capacitor energy. Meanwhile, the DC voltage acts as the reflection of system frequency, enabling the coordination between two-end VSC stations without communication. More importantly, to fully utilise the VSC-HVDC potentials, the CFRS that sequentially activates HVDC capacitor and primary frequency control is implemented so that the energy loss and the detrimental control impacts on the sending-end systems can be minimised while making sure the requirement of system frequency support. Analytical derivations have been done to evaluate the contribution of the proposed CFRS to system frequency regulation. Furthermore, the effectiveness of the proposed scheme has been well validated in PSCAD/EMTDC under several power system contingencies by fully comparing with existing control schemes. The proposed CFRS stands out by the reduced complexity of the control structure, the robustness for connecting the very weak AC grid and the fast frequency regulation ability with consideration of cost-effective property.

Exploring the gaps in renewable energy integration to grid

The continued integration of diverse renewable energy sources into the power grid has led to a significant decrease in power system inertia, thus resulting in several challenges in the electric power system such as frequency instability and sharp rise in rate of change of frequency (rocof). In order to combat these challenges, several technologies such as the use of energy storage systems, hybrid energy storages system and hybrid systems have been used to provide fast frequency response and regulation. This paper gives a comprehensive review of these technologies thus highlighting their applications, merits and demerits for frequency regulation in renewable energy sourced grid. Furthermore, the research reveals that hybridized system provides better characteristics than singly sourced renewable energy system, therefore the best suited hybrid system can be formed based on the desired power grid characteristics and specific power requirement. Finally, it was revealed that FESS, SMES and SCES have similar characteristics, nonetheless, their characteristics is complimentary to that of BESS, CAES and PHES technology, and so a hybrid combination comprising of BESS and SCES could be best suited for fast frequency response in renewable energy sourced grid.

Study and Simulation of a Wind Hydro Isolated Microgrid

Isolated microgrids are microgrids which operate autonomously. This paper presents an isolated microgrid which combines a Hydraulic Turbine Generator (HTG) with a Wind Turbine Generator (WTG) to supply consumers forming a Wind Hydro Isolated Microgrid (WHIM). The WHIM includes a Dump Load (DL) to dissipate the active power excess. The WHIM has been modeled and its operation has been simulated in two modes: Wind-Hydro (WH), where both HTG and WTG supply power, and Wind-Only (WO) mode, where the WTG is the active power supplier and the HTG keeps connected to the grid with null power to generate the grid voltage. In WO, a fast frequency regulation is achieved by means of a controller which commands the DL to consume the WTG power excess. Additionally, the simulation of the mode transition from WO to WH, which is triggered by a system active power deficit in WO mode, is shown. A kick starting system designed to speed up the HTG power production improves the transient from WO to WH mode change. Finally, the simulations in WH mode show the interaction between the HTG and WTG. The two controls proposed have been proved effective and the simulations show a good WHIM dynamic performance.

Fast frequency regulation and synthetic inertia in a power system with high penetration of renewable energy sources: Optimal design of the required quantities

Abstract Fast primary regulation and synthetic inertia supply are two novel ancillary services for power system frequency control. Although general agreement about their definition is still missing, these services have started to be recognized and inserted into the grid codes of some transmission system operators, especially to handle isolated systems with large penetration of non-programmable renewable energy sources. The main goal here is to propose a methodology to quantify the capacity needed for each service, in order to guarantee system stability and security when the regulation margins of the conventional generation for standard primary frequency control have already been exploited. In terms of control theory, identifying the required service amounts means computing the values of the gains associated to the related controllers. These are found by solving a non-linear optimization problem: the functional to be minimized is related to the maximum value of the power exchanged for each service after an imbalance and to the relative costs of such maximum power exchanges, while constraints are defined to limit the amplitude and the gradient of the response of grid frequency to the imbalance. The optimization procedure has been tested on an annual scenario for the power system of the Sardinia island: more precisely, with reference to predictions for 2030 of the hourly operating conditions of the system, control parameters have been optimized for each hour, by assuming the loss of the maximum generation or of the maximum absorption as the imbalance source. The most critical situation found in the simulations is for the loss of the maximum absorption: a high gain for the synthetic inertia controller and an even higher gain for the fast primary controller are needed to avoid both the curtailment of renewables and load shedding.

Autonomous Synchronizing and Frequency Response Control of Multi-terminal DC Systems With Wind Farm Integration

Recent analyses have shown that the grid-integration of offshore wind farms through MTDC systems has brought low inertia and small-signal stability issues, in which the dynamics of phase-locked-loop (PLL) play a crucial role. To address this issue, this article proposes a control strategy for the multi-terminal VSCs aiming at PLL-less synchronization and autonomous frequency response of the MTDC system. One of the significant features of the proposed control is that the deviation of the grid frequency can be instantaneously reflected on the deviation of the DC voltage without ancillary control. Based on this feature, a fast inertia response and primary frequency regulation among wind farms and AC systems interconnected by the MTDC system can be achieved. A small-signal model is established to evaluate the overall system stability using the proposed control. Finally, comparative studies of this proposed control with the conventional PLL-based vector control are conducted in PSCAD/EMTDC based on a practical MTDC system in China, the Zhangbei four-terminal HVDC transmission system. The analysis shows that the proposed control exhibits advantages in weak grid operation and autonomous frequency response.

Vehicle to Grid Frequency Regulation Capacity Optimal Scheduling for Battery Swapping Station Using Deep Q-Network

Battery swapping stations (BSSs) are ideal candidates for fast frequency regulation services (FFRS) due to their large battery stock capacity. In addition, BSSs can precharge batteries for customers and the batteries that are not in charging can provide a stable regulation capacity to the market. However, uncertainties, such as ACE signals and the EV per-hour visit counts, introduce stochastic nonlinear dynamics into the operation of a BSS-based FFRS. Currently, there is no quantification method to ensure its optimal economical operation. To close this gap, in this article, we propose a novel deep Q-learning-based FFRS capacity dynamic scheduling strategy. This method can autonomously schedule the hourly regulation capacity in real time to maximize the BSS's revenue for providing FFRS. Case studies using real-world data verify the efficacy of the proposed work.

Economic Assessment for Battery Swapping Station Based Frequency Regulation Service

Battery swapping stations (BSSs) have great potential in providing fast frequency regulation service (FFRS) owing to their large battery storage capacity. However, compared to a regular BSS, a BSS providing FFRS faces the following financial risks: first, higher infrastructure investment to support the vehicle-to-grid services; second, higher battery aging costs due to FFRS; third, FFRS causes uncertainties to batteries’ charging costs. Under such a context, in this article, we propose an economic risk assessment model for the BSS-based FFRS by comparing its economics with a regular BSS. In this model, the value at risk (VaR) of daily revenue and the long-term return on investment (ROI) of a regular BSS and a BSS providing FFRS are compared. The assessment results are obtained in three steps: first, we develop the operation and economic models for the BSS-based FFRS. Next, the mathematical models of the VAR and ROI analyses are formulated. Finally, the VaR of daily revenue is compared through a statistical analysis of a large number of scenarios. The ROI comparison is conducted by a policy gradient based reinforcement learning algorithm, which can handle the nonconvexity and stochastic dynamics brought by the electric vehicle visits. The practicality of the proposed framework is demonstrated by using the real ancillary market data from utilities and the traffic count data from onsite traffic sensors.

Characterization of a Fast Battery Energy Storage System for Primary Frequency Response

In response to increasing integration of renewable energy sources on electric grid systems, battery energy storage systems (BESSs) are being deployed world-wide to provide grid services, including fast frequency regulation. Without mitigating technologies, such as BESSs, highly variable renewables can cause operational and reliability problems on isolated grids. Prior to the deployment of a BESS, an electric utility company will typically perform modeling to estimate cost benefits and determine grid impacts. While there may be a comparison of grid operations before and after BESS installation, passive monitoring typically does not provide information needed to tune the BESS such that the desired services are maintained, while also minimizing the cycling of the BESS. This paper presents the results of testing from a live grid using a method that systematically characterizes the performance of a BESS. The method is sensitive enough to discern how changes in tuning parameters effect both grid service and the cycling of the BESS. This paper discusses the application of this methodology to a 1 MW BESS regulating the entire island of Hawaii (180 MW peak load) in-situ. Significant mitigation of renewable volatility was demonstrated while minimizing BESS cycling.

Fully-distributed Load Frequency Control Strategy in an Islanded Microgrid Considering Plug-In Electric Vehicles

With large-scale integration of electric vehicles, this paper investigates the load frequency control problem in an islanded microgrid with plug-in electric vehicles (PEVs), which can be regarded as mobile battery energy storages to provide a valuable contribution to frequency regulation. A novel fully-distributed control strategy is proposed to achieve fast frequency regulation of islanded microgrids and effective coordination control of distributed energy sources. Firstly, distributed control based on an improved linear active disturbance rejection algorithm is realized through a multi-agent system, and it greatly enhances the anti-disturbance capability of the microgrid. Then, in order to guarantee the effectiveness of PEVs in frequency regulation, PEVs are controlled following the controllable power rate (CPR) calculated from the consensus-based multi-agent system. Furthermore, the system control construction in this paper is well designed to avoid the negative effects caused by system communication time delay. Finally, numerical simulations under different disturbances are carried out to demonstrate the effectiveness of the proposed control strategy in comparison with other previous control strategies.

Control strategy of DFIG in hybrid micro‐grid using sliding mode frequency controller and observer

A doubly fed-induction generator (DFIG) has a significant advantage of fast dynamic frequency regulation. However, conventional virtual inertia control (VIC) design only takes frequency into account and may face secondary frequency drop during rotor speed restoration. To solve these problems, the novel decentralised control strategy is proposed for a hybrid micro-grid. Firstly, the load disturbance is considered to enhance control system accuracy, which is estimated by using the state observer based on the dynamic mathematical model. Then, the sliding mode VIC controller is designed by taking advantage of the observed disturbance value and frequency deviation for DFIG output dynamic adjustment according to the conventional frequency control (VIC) model. Furthermore, during DFIG rotor speed restoration, pitch angle control based on the mathematical model is applied for reserve capacity, and a new triggering condition according to the conventional condition is designed, so as to release reserve power at an appropriate time. Finally, the proposed strategy is tested in a typically isolated micro-grid with diesel and DFIG through a simulation and experiment.

Decentralised frequency‐based control of a population of heterogeneous ACs without power oscillations

In power system, the control performance deteriorates due to the penetration of the fluctuating renewable generation if no advanced control strategy is implemented. In this study, the authors investigate a class of thermostatically controlled loads – air conditioners (ACs) – to provide fast frequency regulation service in system contingencies by switching ON/OFF state based on the system frequency deviation in a decentralised way. First, an estimation of the steady-state aggregate power of a population of heterogeneous ACs is derived corresponding to the ambient temperature, and the power overshoot caused by a safe protocol (SP) is discussed analytically, both of which are verified by simulations. Then, a decentralised frequency-based control strategy is designed and a multi-stage frequency responsive model is implemented implicitly. An SP with the timer is introduced in the strategy in order to eliminate the power oscillations during the load pick-up period. Finally, the proposed strategy is incorporated into the primary control of the power system to provide fast frequency regulation service in power contingencies. Comparing with other decentralised strategies, the proposed method can obtain a lower-frequency deviation and the smoothest demand trajectory without long-term power oscillations when a power contingency occurs. Meanwhile, the comfort of the consumers is guaranteed during the demand response.

Distributed Optimization Decomposition for Joint Economic Dispatch and Frequency Regulation

Economic dispatch and frequency regulation are typically viewed as fundamentally different problems in power systems and, hence, are typically studied separately. In this paper, we frame and study a joint problem that co-optimizes both slow timescale economic dispatch resources and fast timescale frequency regulation resources. We show how the joint problem can be decomposed without loss of optimality into slow and fast timescale subproblems that have appealing interpretations as the economic dispatch and frequency regulation problems, respectively. We solve the fast timescale subproblem using a distributed frequency control algorithm that preserves network stability during transients. We solve the slow timescale subproblem using an efficient market mechanism that coordinates with the fast timescale subproblem. We investigate the performance of our approach on the IEEE 24-bus reliability test system.

A hybrid dynamic demand control strategy for power system frequency regulation

The rapid increase in renewable energy integration brings with it a series of uncertainty to the transmission and distribution systems. In general, large-scale wind and solar power integration always cause short-term mismatch between generation and load demand because of their intermittent nature. The traditional way of dealing with this problem is to increase the spinning reserve, which is quite costly. In recent years, it has been proposed that part of the load can be controlled dynamically for frequency regulation with little impact on customers' living comfort. This paper proposes a hybrid dynamic demand control (DDC) strategy for the primary and secondary frequency regulation. In particular, the loads can not only arrest the sudden frequency drop, but also bring the frequency closer to the nominal value. With the proposed control strategy, the demand side can provide a fast and smooth frequency regulation service, thereby replacing some generation reserve to achieve a lower expense.

Frequency Control in Isolated/Islanded Microgrids Through Voltage Regulation

This paper presents a frequency control mechanism for an isolated/islanded microgrid through voltage regulation. The proposed scheme makes use of the load voltage sensitivity to operating voltages and can be easily adopted for various types of isolated microgrids. The proposed controller offers various advantages, such as allowing the integration of significant levels of intermittent renewable resources in isolated/islanded microgrids without the need for large energy storage systems, providing fast and smooth frequency regulation with no steady-state error, regardless of the generator control mechanism. The controller requires no extra communication infrastructure, and only local voltage and frequency is used as feedback. The performance of the controller is evaluated and validated through various simulation studies in the PSCAD/EMTDC software environment based on a realistic microgrid test system, using small-perturbation stability analysis to demonstrate the positive effect of the proposed controller in system damping.

Distributed Frequency Control Strategy for Islanded Microgrid with Consideration of Transmission Congestion

To guarantee the security and stability of islanded microgrid, transmission congestion problem is considered during the frequency control so as to efficiently make use of power available without violating system constraints. A novel strategy was proposed, which combines distributed frequency control and congestion management and makes the distributive frequency control reasonable at the request of line congestion. It effectively solve multi-objective problem with multi-linear equality and inequality constraints. To testify the proposed strategy, an islanded microgrid with five-machines is provided in Matlab/Simulink. The results demonstrate that fast frequency regulation and effective settlement of transmission congestion are obtained by this approach.

Adaptive Roles of Islanded Microgrid Components for Voltage and Frequency Transient Responses Enhancement

This paper introduces a novel framework of coordinated voltage and frequency control strategy for islanded microgrid (MG) operation. The proposed control schemes rely on local measurements as communication-free control approach. Therefore, the distributed controllers of the MG components have been deployed based on their slow, medium, and fast dynamic responses to maintain the voltage and frequency in adherence to IEEE Standards 1547 and 929. The various voltage and frequency control responses associated with reactive power management scheme are efficiently utilized based on well-defined states of operation and transient management scheme. In each state, the roles of each device for voltage and frequency regulations are defined with its regulation capability, and response time based on its local measurements. Consequently, the fast reactive power compensation and rapid frequency regulation are ensured based on the inverter-based devices at challenging operating conditions. As a result, the proposed control strategy improves the voltage and frequency regulation, transient response, and MG stability. A comprehensive simulation study has verified the superior performance of the communication-free approach during steady state and in response to severe disturbances.