Primary Frequency Control Research Articles

Stress-informed control of medium- and high-head hydropower plants to reduce penstock fatigue

The displacement of conventional generation in favor of stochastic renewable requires increasing regulation duties from the remaining dispatchable resources. In high- and medium-head hydropower plants (HPPs), providing regulation services to the grid and frequently changing the plant's set-point causes water hammer, which engenders pressure and stress transients within the pressurized conduits, especially the penstock, damaging it in the long run. This paper proposes a model predictive control (MPC) that explicitly models the hydraulic transients within the penstock. It achieves to reduce the mechanical loads on the penstock wall, and, consequently, fatigue effectively. Thanks to a suitable linearization of the plant model, the optimization problem underlying the MPC scheme is convex and can be solved with off-the-shelf optimization libraries. The performance of the proposed controller is tested with numerical simulations on a 230MW medium-head HPP with Francis turbine providing primary frequency control. Simulation results show substantially reduced penstock fatigue, existing approaches outperformed, and problem resolution times compatible with real-time control requirements.

Development and Performance Verification of Frequency Control Algorithm and Hardware Controller Using Real-Time Cyber Physical System Simulator

Frequency stability is a critical factor in maintaining the quality of the power grid system. A battery energy storage system (BESS) with quick response and flexibility has recently been used as a primary frequency control (PFC) resource, and many studies on its control algorithms have been conducted. The cyber physic system (CPS) simulator, which can perform virtual physical modelling and verification of many hardware systems connected to the network, is an optimal solution for the performance verification of control algorithms and hardware systems. This study introduces a large-scale real-time dynamic simulator that includes the national power system. This simulator comprises a power grid model, an energy management system (EMS) model, a BESS system model, and a communication model. It performs the control algorithm performance evaluation and the hardware controller’s response performance evaluation. The performance of the control algorithm was evaluated by tracking the power system’s characteristic trajectory in the transient state based on the physical response delay time between the output instruction of the frequency regulation controller (FRC), a hardware controller, and the output response of the BESS. Based on this, we examined the response performance evaluation results by linking them to the optimally designed actual FRC. As a result, we present an analysis of the BESS’s characteristic trajectories in the transient state, such as frequency, power system inertia, and power grid constant, and provide FRC response performance evaluation results at a level of 163 ms, by connecting the BESS installed at the actual site with the CPS simulator.

Reserve replacement from governor to energy storage system on conventional generator for operating-cost reduction

This paper proposes an operation strategy for a steam turbine generator (GEN) with a built-in energy storage system (ESS) that replaces the role of the governor. The proposed strategy cuts the operating cost of GEN by minimizing the throttling loss and the amount of fuel with the harmonious operation of ESS. For the conventional governor droop control, the valve cannot be fully open for emergency release; it causes losses in the throttle valve and fuel cost due to the reserve. Beyond these limitations, in the proposed coordinated operation between GEN and ESS, the GEN can operate in the natural sliding pressure mode, which fully opens the steam turbine valves, reduces the steam pressure, and improves the efficiency of the turbine by minimizing the throttling loss. In this method, while a generator produces constant power without the reserve for an emergency condition in the proposed coordinated operation, an ESS is in charge of fast primary frequency control. Subsequently, the GEN can achieve a replaced reserve operation by ESS, which removes the constant operational loss by reserved steam and provides economic advantages by reducing operation costs. We theoretically demonstrate that the proposed operation provides a quick and reliable frequency control performance through frequency- and time-domain analysis compared with the conventional operation. Using a power generation simulator practically installed at Korea Western Power Co., Ltd., we also proved that the proposed operation can yield the economic benefit based on the cost reduction by minimizing losses of GEN, and provide an improved frequency regulation performance with the coordinating operation.

Comparison of primary frequency control methods for wind turbines based on the doubly fed induction generator

The penetration of large levels of wind energy leads to technical problems affecting the frequency stability of the power system. Naturally, a wind power plant does not provide an inertial response nor does it regulate the system frequency in case of generation-demand imbalance due to the decoupling between the rotor speed and the grid frequency through the electronic converter (AC/DC/AC). This paper reviews, analysis and compares the performance of four control methods based on the modification of the electromagnetic torque set point and one of the mechanical torque. These methods allow that activation of the wind turbine inertia when a frequency deviation occurs, contributing to the regulation of the system frequency similarly to a conventional synchronous generator. However, wind turbines only change their operating point for a limited time and not permanently.

Tidal turbine support in microgrid frequency regulation through novel cascade Fuzzy-FOPID droop in de-loaded region

The goal of this study is to introduce an effective load frequency control scheme with the integration of tidal turbines in a standalone microgrid (μG) system. As standalone μG experiences lower inertia and lacks primary frequency control, the use of variable tidal turbines in the de-loaded region may be accepted as one of the feasible solutions for managing frequency regulation issues. In this condition, the de-load region alludes to an area where tidal turbines liberate their accumulated kinetic energy in rotational parts pursuing frequency fluctuations. An effectual cascade fractional order fuzzy PID-integral double derivative (CFOFPID-IDD) controller suggested for efficient utilization of tidal turbines, whose design variables are tuned through a recently appeared Jaya algorithm. An investigation is made between the acquired outcomes of the studied CFOFPID-IDD droop controller with fractional order fuzzy PID droop control to analyze the proposed strategy performance in various load conditions, with different physical constraints like time delay, dead zone, and generation rate constraints. Moreover, the sensitivity test reveals that the Jaya-optimized CFOFPID-IDD controller can undergo ± (10–25) % variation in various coefficients without retuning the design variable values. The simulation outcomes validate the effectiveness and adequacy of the proposed regulator.

Inertial and Primary Frequency Response of PLL Synchronized VSC Interfaced Energy Resources

Fast frequency response (FFR) is an option in maintaining system security under low inertia conditions. Power electronics interfaced energy resources such as wind and photovoltaics have the capability to deliver FFR. At present, almost all installed power electronics interfaced energy resources are connected to the grid by the phase locked loop (PLL)-synchronized voltage source converter (VSC) and they usually implement inertial response by measuring the rate of change of frequency (RoCoF). However, RoCoF measurements are highly susceptible to disturbances in the grid. In this paper, a novel inertia controller is developed for PLL-synchronized VSC interfaced energy resources to implement natural inertial response without measuring RoCoF. A thorough stability analysis is performed to study the impact of proposed inertia controller on the VSC in weak grid. The proposed inertia controller makes the deviation of DC voltage proportional to the deviation of system frequency. Further, a method of primary frequency control is proposed by measuring DC voltage rather than grid frequency. The proposed FFR scheme of inertia and primary frequency control is applied to a direct-drive windfarm connected to a grid, and its correctness and advantages are verified by simulation.

Battery Energy Storage System Sizing, Lifetime and Techno-Economic Evaluation for Primary Frequency Control: A Data-driven Case Study for Turkey

The share of renewable energy sources (RES) in power systems has been increasing in recent years. Future power systems will have lower inertia and difficult controllability, especially due to intermittent and variable renewable energy that is not dispatchable easily due to its fluctuating nature. Thus, it is necessary to increase the grid’s flexibility to ensure system stability. For this need, new technologies such as battery energy storage systems (BESS) are widely discussed. It is thought to be very useful to create a fast and accurate response in frequency control services with BESSs, especially in low inertia grid conditions. The sizing, charge-discharge control, and lifetime of a BESS providing frequency control service depend heavily on the changes that may occur in the power systems. So, it is a very complex issue to decide on during the investment phase. In this study, the optimum sizing, lifetime, and techno-economic evaluations of BESS providing primary frequency control (PFC) service have been made by grid's frequency data-driven. For this purpose, firstly; the BESS design providing PFC is created for Turkey’s electricity system. Secondly, with the developed algorithm, the number of charge-discharge cycles of the BESS is calculated and the lifetime and capacity fading of the BESS are determined according to the frequency deviation. Finally, economic evaluations have been made for BESS considering the investment- operating costs and PFC market prices.

Study of Optimization of Operating Modes of Solar Power Plants in the Power System of the Republic of Crimea and Sevastopol with the Possibility of Frequency Control

The relevance of improving the methods of RES integration into power systems is increasing every year, and therefore there is a need to improve the management characteristics of large RES when working in the power system. One of the ways of RES participation is frequency regulation in the power system. With a large percentage of installed RES capacity, this issue is relevant, since RES is of great importance for the management of the power system and ensuring the reliability of power supply. Supported by the Government of the Russian Federation (CDA RES) the number of RES withing the structure of electric energy generation grows yearly therefore increasing its effect on power supply system. Positive trends in the development of renewable energy sources require the System Operator (hereinafter - JSC SO UES) to study the operating modes in advance and work out the technical requirements for them. The experience of operating renewable energy sources in Russia is still not large enough and the regulatory and technical base has not yet been fully formed. This article discusses the basic requirements for renewable energy sources, enshrined in the regulatory and technical base and the problems of meeting these requirements. The article presents the results of studies carried out at the Crimean solar power plant (SPP) in the village of Mityaevo. In the course of the research, the SPP was tested with a general primary frequency control by automatically reducing the output active power of the power plant with an increase in frequency. As a result of the experiments, the possibility of SES participation in the general primary frequency regulation was confirmed, which is an important technical parameter that affects the operation of the power system. The maneuverable characteristics of power plants based on renewable energy sources make it possible to increase the flexibility and resilience of the power system to various fluctuations. The experiments were carried out in order to solve the problems arising in power systems, taking into account the increasing influence of renewable energy sources (RES).

Distributed generation contribution to primary frequency control through virtual inertia and damping by reference conditioning

Penetration of converter-based distributed generation units (DGUs) rises quickly in the power systems. These generation units, which do not have any rotating masses, reduce the total system inertia, thus challenging both frequency control and system stability. In this work, to allow DGUs to contribute to primary frequency control, a sliding-mode reference conditioning loop (SMRC) is proposed. In this way, the DGUs aim to constrain both frequency deviation and rate of change of frequency within the boundaries established in the grid codes. The conditioning loop only acts if there is a risk of exceeding any of the established boundaries. Through variable structure system theory is shown that the conditioning loop behaves like a temporary contribution of virtual inertia and damping. Simulation results show that the conditioning loop allows DGUs to contribute to improving primary frequency control performance. Thus, exceeding the established boundaries is avoided with efficient use of energy.

A Novel Approach for the Implementation of Fast Frequency Control in Low-Inertia Power Systems Based on Local Measurements and Provision Costs

Transitioning towards carbon-free energy has brought severe difficulties related to reduced inertia in electric power systems. Regarding frequency stability, low-inertia systems are more sensitive to disturbance, and traditional frequency control is becoming insufficient to maintain frequency within acceptable limits. Consequently, there is a necessity for faster frequency support that can be activated before the primary frequency control and that can decelerate further frequency decay. This paper proposes a local control strategy for a multi-stage fast frequency response (FFR) provided as an ancillary service that considers the location of the disturbance and the distribution of system inertia. The novelty of the presented control strategy is the ranking of FFR resources by price, which takes the economic component into consideration. The proposed control is simple, based only on RoCoF measurements that trigger the activation of FFR resources. Its advantage over other methods is the ability to adapt the FFR resource response to the disturbance without complex calculations and the ability to ensure a bigger response closer to the disturbance, as well as in low-inertia parts of the system. In that way, there is a bigger activation of resources in the parts of the system that are more endangered by disturbances, which, as a result, minimizes the propagation of the disturbance’s impact on system stability. The applicability of the presented method is demonstrated in a simple 3-area power system and IEEE 68-bus system implemented in MATLAB/Simulink. The results show that the proposed control enables the largest response closer to the disturbance, thus mitigating the propagation of the disturbance. Furthermore, the results confirm that the proposed control enables lower provision costs and more support in low-inertia areas that are more vulnerable to disturbances.

Autonomous vehicle-to-grid design for provision of frequency control ancillary service and distribution voltage regulation

We develop a system-level design for the provision of Ancillary Service (AS) for control of electric power grids by in-vehicle batteries, suitably applied to Electric Vehicles (EVs) operated in a sharing service. An architecture for cooperation between transportation and energy management systems is introduced that enables us to design an autonomous Vehicle-to-Grid (V2G) for the provision of multi-objective AS: primary frequency control in a transmission grid and voltage amplitude regulation in a distribution grid connected to EVs. The design is based on the ordinary differential equation model of distribution voltage, which has been recently introduced as a new physics-based model, and is utilized in this paper for assessing and regulating the impact of spatiotemporal charging/charging of a large population of EVs to a distribution grid. Effectiveness of the autonomous V2G design is evaluated with numerical simulations of realistic models for transmission and distribution grids with synthetic operation data on EVs in a sharing service. In addition, we present a hardware-in-the-loop test for evaluating its feasibility in a situation where inevitable latency is involved due to power, control, and communication equipments.

Frequency control strategy for sending-end of hybrid cascaded UHVDC system with large-scale wind power access

For the frequency overrun problem caused by the high proportion of wind power in the sending grid of hybrid UHVDC transmission system, the coordinated frequency control strategy of wind power and hybrid UHVDC system in the sending system is proposed. Firstly, the additional frequency control strategy considering primary frequency control and inertia characteristics is introduced in the wind turbines and the line commuted converter (LCC) station respectively, and the virtual inertia control strategy is introduced in the modular multilevel converter (MMC) station. Secondly, the action rules and control strategy of wind power and hybrid UHVDC system coordination involved in the power grid are designed. At the same time, the mechanism of frequency control strategy is analyzed​ based on the frequency response model of wind power and UHVDC integrated system. Finally, the simulation results indicate that the proposed frequency control strategy can enhance the inertia and primary frequency capability of the AC system at the sending end, and effectively improve the frequency stability problem of the AC system under different operating conditions.

An approach to arranging primary and secondary control of the operating parameters in microgrids featuring inverter-connected generator sets

Various low-power generator sets (GS) are mainly integrated into microgrids by means of inverters coupled with energy storage systems (ESS). ESSs are connected either to the DC busbars of GSs or to the AC busbars of the microgrid, in this case they are equipped with individual inverters. Use of ESS helps balance power in cases of load surge / loadshedding when the microgrid is islanded. This paper overviews approaches to arranging primary and secondary control of the operating parameters of microgrids. It discusses the technical challenges of, and possible solutions for, implementing primary and secondary control in low- and medium-voltage distribution grids featuring inverter-connected GSs. Calculations of electromechanical transients show that inverter-connected GSs have advantages over conventional GSs when used microgrids that are weakly connected to the power system (the external grid) or islanded. In a microgrid featuring inverter-connected GSs, active and reactive power need to be controlled separately as in power systems featuring conventional GSs; implementation of such controls must be adjusted for the parametric and topological characteristics of low- and medium-voltage distribution grids. To address the issues of linked control, this paper proposes a comprehensive approach that adjusts microgrid design for the specifics of inverter-connected GSs, improves inductance, and incorporates algorithmic solutions. It details upon the virtual impedance algorithm, which extends the common principle of droop speed control. Its implementation helps load the GSs and the ESS with adjustments for their installed capacity; it can also simulate the mechanical constant of GS inertia. Primary control has a low margin that mainly comes from the ESS; thus, backups available to secondary control must be used to compensate for the deviations in operating parameters; this is a key challenge of secondary frequency and voltage control in microgrids. When primary and secondary voltage and frequency controls have been duly implemented in a microgrid, its GSs, the ESS, and electricity delivery to the loads within the microgrid can all operate reliably in a variety of operating situations.

Frequency Modulation Control of Hydraulic Wind Turbines Based on Ocean Used Wind Turbines and Energy Storage

Based on the energy storage type of hydraulic wind turbines (HWTs) and in view of the unit frequency drop problem under high wind power proportion conditions, this paper proposes a method of primary frequency control under maximum power point tracking (MPPT). HWT power output is affected by wind speed randomness and volatility. In addition, traditional wind turbines do not have inertial adjustment ability, leading to a decrease in the frequency stability of the power system caused by the increase in wind power permeability. In the paper, a hydraulic energy storage system and synchronous generator are combined to carry out primary frequency modulation, and a mathematical model of the hydraulic energy storage system, the hydraulic main transmission system, and the generator active power regulation system after grid connection is established. By analyzing the load changing rules of power systems and frequency fluctuation caused by the power system load after the wind turbine is grid-connected, the variable parameter frequency modulation compensation control strategy of combined turbine-energy storage systems is established, and simulation verification under different load fluctuations is carried out, verifying the effectiveness of the frequency modulation control strategy, which achieves a good control effect for improving the frequency modulation ability of hydraulic wind turbines.

Control of Photovoltaic Plants Interconnected via VSC to Improve Power Oscillations in a Power System

This paper presents an integrated methodology applied to photovoltaic (PV) plants for improving the dynamic performance of electric power systems. The proposed methodology is based on primary frequency control, which adds an ancillary signal to the voltage reference of the DC-link for the voltage source converter (VSC) in order to reduce power oscillations. This ancillary signal is computed by relating the energy stored in the VSC of the DC-link and the energy stored in the synchronous machine’s shaft. In addition, the methodology considers the operating limits of the VSC, which prioritizes active power over reactive power. Furthermore, the VSC control is assessed with interconnection and damping assignment passivity-based control (IDA-PBC), as well as compared to conventional PI control. IDA-PBC is employed to design a Lyapunov asymptotically stable controller using the Hamiltonian structural properties of the open-loop model of the VSC. A 12-bus test system that considers PV plants is employed to compare the proposed IDA-PBC control with a classical proportional-integral control approach. The impact of the proposed methodology is analyzed in four scenarios with different PV penetration levels (10%, 30%, 50%, and 80%) and four large disturbances in the test power system. In addition, a decrease in the inertia of the synchronous machines from 100 to 25% is analyzed. The time-domain simulation results show that the frequency oscillations are reduced by 16.8%, 38.43%, 37.53%, and 76.94% in comparison with the case where the proposed methodology was not implemented. The simulations were conducted using the SimPowerSystems toolbox of the MATLAB/Simulink software.

Plug-in Electric Vehicles Participation in Primary Frequency Control Considering Charging Demand

ABSTRACT Electric Vehicle is now a global term, affecting the transportation sector, automobile industry, and electric grid scenario. As Plug-in Electric Vehicles (PEVs) penetration level is escalating, PEVs will create new challenges and opportunities for transportation and the electric grid in the near future. The distributed stored energy of PEVs with high active power controllability turns PEVs into new distributed energy sources (DES) for the grid. With a high penetration level, PEVs can also provide ancillary services to the grid, e.g., primary frequency control (PFC). Although grid support provided by PEVs is limited because of additional operational limitations and various constraints of PEVs and the grid. A new charging model for PEVs has been proposed in this paper to investigate the potential of PEVs for primary frequency control. The proposed model incorporates several PEV characteristics such as charger topology, drive power limitation, and operational modes with proper consideration of PEV’s charging demand profile. Furthermore, a control scheme is proposed to decide the availability of PEVs for ancillary services and compute the participation level of PEVs without affecting the charging demands. In the end, several scenarios have been used to evaluate and show the potential of PEVs for the PFC in the power system. The maximum deviation of frequency is reduced by 17.39% in case 1 of Scenario A, 26.08% in case 2 of Scenario A and 21.73% in both cases of Scenario B with proper consideration of charging demands.

Adaptive Under-Frequency Load Shedding Control Strategy of Power Systems With Wind Turbines and UHVDC Participating in Frequency Regulation

Represented by wind turbines and ultra high-voltage DC (UHVDC), the power-electronic interfaced power sources participate in fast frequency control, which has a significant impact on the power system frequency. However, the conventional under-frequency load shedding (UFLS) scheme doesn’t take into account the impact above, resulting in unreasonable load shedding after a large loss of generation. To this end, this article proposes an adaptive UFLS control strategy of power systems with wind turbines and UHVDC participating in frequency regulation. Firstly, based on the virtual inertia control model and the primary frequency control model of wind turbines and UHVDC, the study establishes the simplified frequency response model of the power system considering the participation of wind turbines and UHVDC in frequency regulation. Furthermore, the impact of the active power response characteristics of wind turbines and UHVDC participating in frequency regulation on the magnitude of the active power deficiency is comprehensively analyzed. Thus the precise estimation of the magnitude of the power deficiency can be achieved, which provides technical guidance for multi-stage UFLS. Finally, simulation results demonstrate that the proposed UFLS strategy is capable of reflecting the power system frequency more objectively after a large loss of generation event. In addition, the proposed UFLS strategy outperforms the conventional UFLS strategy in terms of shedding less amount of load when the same desired effect of frequency recovery is achieved.

Frequency Regulation From Electrified Railway

With the improvement of traffic electrification, the coupling between power and transportation systems becomes more and more intensified. In this paper, electrified railway is firstly introduced into power system to support frequency regulation under high penetration of renewable energy sources. A novel frequency regulation framework consisting of day-ahead capacity estimation, frequency control parameters dispatch, and real-time response is constructed for electrified railway to provide primary frequency control support. Motion equations of electric trains under regulation are developed in detail. To fulfill the frequency regulation requirement and ensure punctual arrival, a multi-train joint response strategy is proposed to coordinate different electric trains for primary frequency control. The frequency regulation capacity estimation problem and frequency control power dispatch problem are established based on the multi-train joint response strategy. A novel sequential cutting plane algorithm is developed to linearly relax the nonlinear capacity estimation problem and power dispatch problem with ordinary differential equation constraints and iteratively tighten the relaxation area to improve solution optimality with desirable efficiency. Comprehensive numerical studies verify the effectiveness of the proposed primary frequency control framework, indicating the high potential of electrified railway to support frequency regulation of modern power systems.

Contingency Analysis of a Grid Connected EV's for Primary Frequency Control of an Industrial Microgrid Using Efficient Control Scheme

After over a century of internal combustion engines ruling the transport sector, electric vehicles appear to be on the verge of gaining traction due to a slew of advantages, including lower operating costs and lower CO2 emissions. By using the Vehicle-to-Grid (or Grid-to-Vehicle if Electric vehicles (EVs) are utilized as load) approach, EVs can operate as both a load and a source. Primary frequency regulation and congestion management are two essential characteristics of this technology that are added to an industrial microgrid. Industrial Microgrids are made up of different energy sources such as wind farms and PV farms, storage systems, and loads. EVs have gained a lot of interest as a technique for frequency management because of their ability to regulate quickly. Grid reliability depends on this quick reaction. Different contingency, state of charge of the electric vehicles, and a varying number of EVs in an EV fleet are considered in this work, and a proposed control scheme for frequency management is presented. This control scheme enables bidirectional power flow, allowing for primary frequency regulation during the various scenarios that an industrial microgrid may encounter over the course of a 24-h period. The presented controller will provide dependable frequency regulation support to the industrial microgrid during contingencies, as will be demonstrated by simulation results, achieving a more reliable system. However, simulation results will show that by increasing a number of the EVs in a fleet for the Vehicle-to-Grid approach, an industrial microgrid’s frequency can be enhanced even further.

Primary Frequency Control in an Ancillary Services Market in Real Time and its Relationship with Solar-Wind Generation

In Chile, the operation and market of renewable solar-wind generation and conventional gas-coal-diesel-hydraulic generation is made up of two electric generation systems that are managed in a complementary manner and currently interconnected under the name of Sistema Electrico Nacional, counting on the greatest potential and solar development in South America. However, the real-time operation is carried out by the Dispatch and Control Center, which treats them as two independent electrical generation systems in terms of supervision and control. The system faces the challenge associated with the massive potential and development of solar-wind generation concentrated in the northern area of Chile. This creates difficulties in its technical operation, especially in frequency control and the complexity of modeling in real-time the changing market for complementary services. Therefore, the primary frequency control is the complementary service that acts as the first action and line of defense to allow the system to integrate large blocks of renewable energy. It is necessary to deliver a minimum of dynamic reserves in times of ten seconds and five minutes to avoid the operation of stages of load disconnection / generation due to low / over frequency in front of large power disconnections of gas-fired plants (520 MW), hydraulic of reservoir (700 MW) and coal (380 MW) or loss of demand from large mining processes (300, 500 and 600 MW) for systemic frequency variations that should not exceed 0.7 Hz.