Secondary Frequency Regulation Research Articles

A novel two-stage energy management of hybrid AC/DC microgrid considering frequency security constraints

The increasing penetration of renewable energy in the hybrid AC/DC microgrid (MG) reduces the inertia and further threatens the frequency security. Furthermore, the stochastic and intermittent characteristics of the renewable energy challenge the optimal scheduling of MG. To deal with these issues, this paper proposes a novel two-stage energy management strategy to minimize the operating cost of the hybrid AC/DC MG while ensuring frequency security by considering the uncertainties in both the renewable energy resources (RES) and load demand. Firstly, the relation between the maximal frequency deviation and the power reserve is derived by analyzing the dynamic frequency response model. Secondly, scenario generation and scenario reduction based on Monte Carlo simulation (MCS) are carried out to simulate the unbalanced system power. Thirdly, the day-ahead scheduling stage determines the optimal power reservation of the distributed generators (DGs) for primary and secondary frequency regulation. Then, in the intraday scheduling stage, the day-ahead power reservation plan is adjusted to meet the frequency security constraints by using the real-time updated forecasting information. Finally, the proposed energy management system (EMS) is formulated and solved. The simulation results demonstrate that the proposed method can significantly reduce the operating cost, the maximal frequency deviation, and load shedding.

A control strategy for a grid-connected virtual synchronous generator with virtual impedance

Virtual synchronous generator (VSG) can simulate synchronous generator turning and damping characteristics, which can further improve the penetration rate of renewable energy in the grid. However, the output impedance of virtual synchronous generator in medium and low voltage microgrids is dominated by resistance, which leads to the coupling of active/reactive power and affects the grid integration control effect, and the phase of VSG and grid voltage will be biased in the absence of a pre-synchronization link. For this purpose, a strategy of grid-connected control of VSG with virtual impedance is proposed. Firstly, the VSG mathematical model is established and virtual impedance is introduced into the VSG electrical portion to improve the grid-connected inverter output characteristics. Secondly, a secondary frequency and voltage regulation strategy to improve VSG control is proposed to restore the frequency and voltage of the VSG, and the SOGI phase-locked loop collects the grid voltage phase to complete the grid-connected pre-synchronization design. Finally, the effectiveness and correctness of the proposed strategy were verified by the simulation results based on MATLAB/ Simulink.

Optimal Secondary Frequency Regulation With ON-OFF Loads in Power Networks

Load-side participation can provide support to the power network by appropriately adapting the demand when required. In addition, it enables an economically improved power allocation. In this study, we consider the problem of providing an optimal power allocation among generation and ON-OFF loads within the secondary frequency control timeframe. In particular, we consider a mixed-integer optimization problem, which ensures that the secondary frequency control objectives (i.e., generation–demand balance and the frequency attaining its nominal value at steady state) are satisfied. We present analytic conditions on the generation and ON-OFF load profiles such that an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\epsilon $ </tex-math></inline-formula> -optimality interpretation of the steady-state power allocation is obtained, providing a nonconservative value for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\epsilon $ </tex-math></inline-formula> . Moreover, we develop a hierarchical control scheme that provides ON-OFF load values that satisfy the proposed conditions. We study the interaction of the proposed control scheme with the physical dynamics of the power network and provide analytic stability guarantees. Our results are verified with numerical simulations on the Western System Coordinating Council (WSCC) 9-bus system and the Northeast Power Coordinating Council (NPCC) 140-bus system, where it is demonstrated that the proposed algorithm yields a close to optimal power allocation.

Power management optimization of electric vehicles for grid frequency regulation: Comparative study

Electric vehicles (EVs) have shown promise in providing ancillary services, e.g., frequency regulation. This is mainly due to their capacities and fast response. On the contrary, the rapid integration of EVs in the grid poses challenges, such as frequency and voltage stability. In order to mitigate the above-mentioned issues, several dispatching strategies have been introduced in the recent literature to optimize the charging/discharging rates of EVs. In this paper, a comparative study of power management strategies for secondary frequency regulation (SFR) employing a fleet of EVs is presented. A hierarchical control scheme is employed to compare two cases, namely control at the charging station (CS) level and novel control at the EVs level. Under both cases, a multi-objective optimization approach is utilized to define the optimal charging and discharging rates of EVs using a pattern search algorithm. Furthermore, the performance of the two models is experimented under contingency cases, a notable contribution of this study. Finally, simulations are carried out using OPAL-RT real time simulator to validate the performance of the two models based on real-time traces obtained from Pennsylvania, New Jersey, and Maryland (PJM) interconnection and California independent system operator (CAISO). To further validate the proposed model, a comparison with a mixed-integer linear programming (MILP) based model is presented.

A novel solid state transformer based control topology for interconnected MV and LV hybrid microgrids

Hybrid microgrid continues to be an attraction for researchers as it brings the advantages of both ac and dc systems under one roof. The interlinking converter (ILC) of this system regulates the ac frequency and dc voltage while maintaining bidirectional power. This paper proposes a novel solid-state transformer (SST) based topology for interconnected medium voltage (MV) and low voltage (LV) hybrid microgrids. Based on the topology, various control modes are identified and correspondingly, an autonomous power sharing scheme is presented with dc voltage and ac frequency regulation. A linear voltage-frequency control is discussed for ac/dc and dc/ac converters of SST which provides a secondary control mechanism. The dual active bridge (DAB) of the SST operates at voltage control mode while providing electrical isolation and stable system operation. The proposed topology and control scheme is significant in the sense that it allows integration of ac and dc grids with different voltage and frequency levels. In addition, the secondary dc voltage or ac frequency regulation control at any hybrid microgrid automatically regulates the ac frequency and dc voltage in the system which reduces system cost. Various simulated scenarios in PSCAD with load changes highlight the optimal performance of the proposed scheme.

Impact of grid-scale energy storage systems on energy and frequency regulation Colombian markets

This article proposes a co-optimization model that allows evaluating the simultaneous participation of energy storage systems (ESS) in arbitrage applications, secondary frequency regulation, and transmission investment deferral. The model considers the ESS degradation through an upper-piecewise linear approximation; constraints related to primary frequency energy reserve are also included in the model. The formulation is evaluated using historical data from the Colombian wholesale energy market. Eight case studies corresponding to the possible combinations of ESS participation in the three applications are analyzed. Subsequently, a financial evaluation is carried out for each case study, both from the system and the investor's point of view. The results show that the exclusive use of ESS to provide arbitrage is not economically viable in Colombia, while simultaneously providing secondary frequency regulation and restriction relief services is profitable, both for the system and for an investor agent.

Emergency power control strategy of HVDC FLC based on modified SFR model in islanded HVDC sending system

In the current islanded High Voltage Direct Current (HVDC) sending system for new energy base, with the increase of wind power penetration, the pressure to maintain the frequency stability of the system is increasing. There is a need to use HVDC Frequency Limit Control (FLC) for inter-regional emergency power support. Now, the dead zone (DZ) of FLC is always set to be larger than that of primary and secondary frequency regulation (FR) and maintain a certain margin, which weakens the supporting effect of HVDC on the FR in emergency. Based on the modified System Frequency Response (SFR) model, different disturbance types and sizes can be distinguished in advance by measuring the rate of change of frequency (RoCoF) of system, then a control strategy of HVDC FLC with small dead zone is proposed to deal with disturbances that would lead to large frequency deviation by conventional primary and secondary FR alone. And it will not increase the burden of the HVDC under normal wind power fluctuation. The PSCAD/EMTDC simulation results show that the proposed strategy can effectively improve the frequency stability of the sending system while the drawbacks of traditional FLC is avoided.

Flexible selection framework for secondary frequency regulation units based on learning optimisation method

Grid connections of new energy sources have increased system frequency fluctuations, which requires secondary frequency regulation(SFR) to ensure the safe and stable operation of the system. SFR balances the power deviation between generation and consumption in real time by changing the setpoints of the units. In this paper, we propose a dynamic area control error (ACE) threshold value-based method to rationalise unit selection for SFR tasks. Each conventional unit or virtual generation unit has an independent threshold to aid dispatchers in determining the response priority of each unit to SFR tasks relative to scheduled tasks. In this manner, power generation units can switch their control mode flexibly and track different setpoints using automatic generation control(AGC) and load control technology. We designed a threshold solver based on a sequential decision framework using the deep reinforcement learning method. The solver can dynamically update thresholds based on the real-time system operational state such that the selected units can better meet the current dispatching needs. In the experiment, we tested the performance of the solver in several typical scenarios; the proposed method was found to be effective in maintaining the frequency stability of the system, with lower operation costs.

Receding horizon control based secondary frequency regulation for power systems with wind energy integration

During the last decade, the integration of large-scale renewable energy resources like wind energy into the existing power girds has increased dramatically. In particular, the inherent intermittency and uncertainty of wind power poses new challenges for reliable and stable of power grids. Frequency control is one of the most significant challenges, especially in systems with few control reserves. To address this problem, we propose an optimization-based method for secondary frequency regulation that enables wind turbines to respond to frequency variations. Instead of relying on energy storage systems, the wind turbine’s kinetic energy storage and self-regulation capabilities are leveraged to optimize the trade off between providing frequency regulation and maximizing wind power yield. Moreover, the proposed method is amenable to online implementation and shows good performance in scenarios with time varying wind speeds. A detailed case study in a dedicated co-simulation environment is used to validate the proposed optimal control method and demonstrate its improved performance over standard approaches combining maximum power point tracking and conventional automatic generation control.

Control strategy for seamless switching of virtual synchronous generators based on secondary frequency and voltage regulation

In microgrid, virtual synchronous generators can enhance the system stability by simulating the operation mechanism of synchronous generators. However, a large impact current could be triggered during the grid-access of VSG inverters, resulting in switching failure. Aiming at this problem, based on real-time digital simulator (RTDS), this paper proposes a complete and simple pre-synchronization control strategy based on secondary frequency modulation and voltage regulation. Firstly, the grid-access structure of VSG was defined, and a mathematical model of VSG was constructed, followed by the design of power-frequency and excitation regulators. Next, secondary frequency and voltage regulation was performed on the microgrid in islanded mode to eliminate the frequency and voltage bias induced by load changes. On this basis, the grid phase was collected by phase-locked loop, the phases were synchronized through phase difference detection, and the grid-access synchronization was completed, realizing the seamless switching between islanded and grid-connected modes. Compared with the universality of applying matlab simulation software, real-time digital simulator (RTDS) has great advantages compared with physical simulation and general simulation software. Finally, the proposed strategy was proved effective with a self-designed RTDS simulation model.

Tracking-differentiator-based dynamic virtual inertial control of offshore wind power plant for frequency regulation

The increasing penetration of electronic-converter-interfaced wind generations remarkably declines the inertia of the power grid, which deteriorates the grid frequency stability. In this regard, various virtual inertial control (VIC) techniques have been developed. Among them, the proportional–derivative (PD) VIC is currently considered a standard solution. However, the problem with the PD-type VIC is that its structure amplifies the measurement noise and other high-frequency disturbances. This disadvantage greatly limits the overall quality of energy generation. Motivated by this practical problem, in this paper, a tracking-differentiator-based dynamic VIC is proposed for primary frequency regulation in offshore wind power plant. The idea is to derive the derivative of the frequency deviation, which canbe used to reject the adverse effect of noise amplification. This method can also help to avoid frequent adjustments of wind turbines, thus decreasing both damage and maintenance costs. In order to further enhance the control system anti-disturbance capability, a linear active disturbance rejection controller is designed to robustify the secondary frequency regulation. A single area modern power system including wind power and thermal power is simulated to verify the effectiveness of the proposed control strategy.

Research on Pre-Synchronization Control Strategy of Optical Storage VSG Off-Grid Switching

In the optical storage DC microgrid in island mode, in view of the large inrush current problem of the inverter controlled by the optical storage VSG when switching from off-grid to grid-connected operation mode, a pre-synchronization control strategy based on secondary frequency regulation and voltage regulation was proposed. This strategy can adjust the large frequency and voltage fluctuations caused by active and reactive load changes under the independent DC microgrid of the optical storage. At the same time, it can complete the effective tracking of the grid phase before connecting to the grid, and reduce the impulse current when the optical storage microgrid is isolated and connected to the grid. The real-time digital simulation platform (RTDS) is used to simulate and analyze the proposed control scheme, and the simulation results verify the feasibility of the proposed control strategy.

Optimization strategy of secondary frequency modulation based on dynamic loss model of the energy storage unit

When the Energy Storage System (ESS) participates in the secondary frequency regulation, the traditional control strategy generally adopts the simplified first-order inertia model, and the power allocated to each energy storage unit follows the principle of equal distribution. Therefore, it is impossible to consider the inconsistency of each internal unit for a long time, increasing capacity loss of the energy storage system, and the difficulty in improving the frequency regulation effect. This paper proposes an optimal control strategy based on the ESS unit loss and dynamic model. Firstly, we established the dynamic variable-parameter model of lithium batteries and gave the capacity loss of the Li-cell model. And then on this basis, we deduced the capacity loss of each unit to apply in the power system. Secondly, the requirements of Automatic Generation Control (AGC) signals to the differences between generator unit and ESS units are analyzed, and the output constraints of generator units and ESS are given, and the weight coefficients are determined by using the fuzzy control theory. Finally, a double-level control strategy considering frequency deviation rate and the capacity loss rate is formed, and the implementation process is given. The capacity loss of each ESS unit is 16.19% lower than that of the traditional method, and the frequency deviation of the regional power grid is effectively improved by more than 23.07%. The control strategy of this paper is of great significance for the design of a large-scale ESS control system.

Hierarchical frequency control strategy of doubly-fed induction diesel generator under emergency condition

Natural disasters lead to the uncertain topological structure of the urban distribution network. It is difficult for the communication system to recover quickly, which brings great difficulties to restoring the power supply. Therefore, the deployment of an emergency power supply is one of the critical measures to ensure the stable operation of isolated network systems and reduce power loss. Compared with the traditional synchronous diesel generator (SDG) as an emergency power supply, a doubly-fed induction diesel generator (DFIDG) has the advantages of flexible and controllable, rapid response and high operation efficiency. This paper presents a layered frequency control strategy for a doubly-fed induction diesel generator in an emergency scenario. Firstly, the models of doubly-fed induction diesel generator and its control system, prime mover and its speed regulation system are established. Secondly, the hierarchical frequency control strategy is proposed, including primary integrated frequency control (including droop control and virtual inertia control) and secondary frequency regulation control based on translation method. The virtual inertia control can make the doubly-fed machine quickly release the rotor kinetic energy when the frequency changes rapidly, so as to improve the frequency supportability. According to the overall shortage of the system and the maximum output power of the doubly-fed induction diesel generator, the strategy calculates the secondary frequency regulation power adjustment as the DFIDG power setting value, and then can realize automatic secondary frequency regulation without a communication system. Finally, simulation results verify the effectiveness of the proposed strategy.

Scheduled Power Control and Autonomous Energy Control of Grid-Connected Energy Storage System (ESS) With Virtual Synchronous Generator and Primary Frequency Regulation Capabilities

This paper presents a combined control scheme for the grid-connected energy storage system (ESS). There are two control modes: the power control mode for the charging or discharging condition and the energy control mode for the stand-by condition. In the power control, a derivative element is added in the inertial power-frequency droop as a simple power system stabilizer to enhance the system damping. The energy control is developed from the power control by considering the energy storage dynamics. During system disturbances, both control modes are able to provide autonomous grid support. The small-signal model and the large-signal transient model of the ESS with the proposed control scheme are derived. As a result, the output power and&#x002F;or the stored energy of ESS is in proportional to the frequency deviation and will be restored to the pre-disturbance condition. By selecting appropriate control parameters, the stored energy is controlled within a safe range constrained by the frequency limits without manual interventions. The primary frequency regulation with both power-frequency and energy-frequency proportional relationships is realized by the energy control, which coordinates with the grid secondary frequency regulation. The control performance and the derived energy model are validated by detailed time-domain simulation studies.

A multi-agent deep reinforcement learning-based “Octopus” cooperative load frequency control for an interconnected grid with various renewable units

With the aim of improving the frequency regulation performance of load frequency control (LFC) in an interconnected power system as well as reducing wastage of frequency regulation resources, an “octopus” cooperative load frequency control (OC-LFC) method is proposed. This method addresses the secondary frequency regulation coordination problem affecting different areas, as well as the coordination problem arising between the controller and power distributor within the same area. In addition, for this framework a tracking-explorer-based distributed multi-agent deep deterministic policy gradient (TED-MADDPG) algorithm is proposed. This algorithm imitates the edge-cloud cooperation framework, akin to an octopus. In the performance-based frequency regulation market, the controller and power distributor in each area are regarded as independent agents, which output the total generated power command and the generated power commands of different units. In the proposed method, centralized training and distributed implementation are employed so that a global optimal decision with higher robustness can be obtained. Furthermore, this algorithm also employs a tracking-explorer strategy which incorporate multiple learning techniques and in turn enhance the robustness of the proposed method. The effectiveness of the proposed method is demonstrated using a China Southern Grid (CSG) four-area load frequency control model.

Secondary frequency regulation and stabilization method of islanded droop inverters based on integral leading compensator

The microgrid multi-parallel inverters system with droop control strategy will produce microgrid frequency static error due to the reference power mismatch. In order to eliminate the frequency static error caused by droop control as well as enhance the stability of multi droop inverters system, a secondary frequency regulation and stabilization method for multi-parallel droop inverters system based on island operation of the frequency static error integral lead compensator is proposed. The proposed method includes no communication link among inverters. Through the state space modeling of the multi-parallel drooping inverter microgrid system based on the component connection method (CCM), the state space model of multi parallel inverters is established. Parameter design and analysis are carried out accordingly. The state space model of the load is also established with transfer function-state space mapping method as a comparison. The method proposed in this paper can effectively enhance the small signal stability of the system while realizing the frequency error-free operation of multiple parallel droop inverters. To verify the method proposed in this paper, MATLAB/Simulink simulation is conducted to compare the proposed method with the method based on communication consensus algorithm and proves the effectiveness of the proposed method and at the same time.

Battery Energy Storage Participation in Automatic Generation Control of Island Systems, Coordinated with State of Charge Regulation

Efficient storage participation in the secondary frequency regulation of island systems is a prerequisite towards their complete decarbonization. However, energy reserve limitations of storage resources pose challenges to their integration in centralized automatic generation control (AGC). This paper presents a frequency control method, in which battery energy storage systems (BESSs) participate in automatic frequency restoration reserve (aFRR) provision, through their integration in the AGC of an island system. A local state of charge (SOC) controller ensures safe operation of the BESS in case of disturbances, without jeopardizing system security when available energy reserves are diminishing. The aFRR participation factors of regulating units are altered when the storage systems approach their SOC limits, re-allocating their reserves to other load-following units. Restoration of BESS energy reserves is achieved by integrating SOC regulation in the real-time economic dispatch of the system, formulated as a mixed-integer linear programming problem and solved every few minutes to determine the base points of the AGC units. A small autonomous power system, comprising conventional units, renewable energy sources and a BESS, is used as a study case to evaluate the performance of the proposed method, which is compared with alternative approaches to secondary regulation with BESS participation.

Voltage/Frequency Regulation With Optimal Load Dispatch in Microgrids Using SMC Based Distributed Cooperative Control

Majority of the contemporary hierarchical control strategies for microgrids are either centralized or distributed, relying on leader-follower (or master-slave) consensus for secondary frequency and/or voltage regulation. Thus, in either case, these strategies are susceptible to single-point-failure (SPF). This potential research gap motivated the authors to propose a distributed three-layered hierarchical control strategy applicable to droop-controlled islanded AC microgrids. The proposed strategy can simultaneously ensure (i) frequency and voltage regulation of DGs within the prescribed frequency and voltage deviation limits as per IEC 60034-1 standard (i.e., &#x00B1;2% and &#x00B1;5%, respectively) without relying on leader-follower consensus at the secondary level, (ii) distributed economic dispatch of active power with minimum error, and (iii) distributed reactive power dispatch with plausible error. The proposed technique is fully distributed and shares the computation and communication burden among the neighboring nodes using a sparse communication network, thus, it is insusceptible to SPF. The feasibility of the proposed technique is guaranteed through various time-domain based numerical simulations executed in Matlab/Simulink under different loading conditions and microgrid expansion.

A Criterion and Stochastic Unit Commitment Towards Frequency Resilience of Power Systems

High-capacity long-distance transmission lines, such as high-voltage direct-current (HVDC) lines, are vulnerable to extreme weather. Their failures may cause significant power loss and frequency drop in the receiving-end power systems, which should be considered in the power system scheduling. In this paper, the failure rates of transmission lines subject to extreme weather conditions are firstly modeled to obtain the probability of failure events. Then, a novel criterion defined by the integration of abnormal frequency during the primary and secondary frequency regulation process is proposed to measure frequency security. This criterion is capable to reflect the cumulative frequency deviation effect and distinguish the contributions of different frequency regulation reserve providers, e.g., generators and flexible loads. Moreover, the linearity of the criterion makes it easy to be incorporated into optimization problems. Finally, a two-stage stochastic frequency constrained unit commitment (FCUC) model is developed to optimally schedule generators and flexible loads while satisfying the frequency security constraints under transmission line failure events. The proposed FCUC model is efficiently solved by a regularized L-shape algorithm. The proposed model and techniques are validated based on the modified IEEE 118 and 300 bus systems with realistic meteorological data.