Frequency Regulation Performance Research Articles

Coordinated Control of Electric Vehicles and Renewable Energy Sources for Frequency Regulation in Microgrids

A Control technique of electric vehicles (EVs) cooperating with ac microgrids is considered as an important role with integration of renewable energy sources (RES), i.e. wind and solar farms. As known, the intermittent power generations of these RESs can provide significant changes of the frequency in microgrids. Consequently, outputs of these generations are regarded as continuous disturbances. Previously, the ability to permit frequency stabilizing effect was usually neglected in microgrid design; thereupon, the performance of controller may be ineffective to regulate the frequency in such a microgrid. To address this problem, a new coordination of EV, wind farm (WF), and photovoltaic (PV) for microgrid frequency regulation is proposed in this article. In the control design, the proposed adaptive PI controller is developed by using practical proportional integral (PI) controllers. An effect of a small delay is also considered in input-output pairs of the adaptive PI controllers. Simulation model is developed for validating the proposed controller. Simulation results demonstrate that the proposed coordinated control technique of EVs, WF, and PV power generation provides a better frequency regulation performance than a fixed PI controller under various uncertainties such as wind and solar power fluctuations, ${N}$ -1 outages, disconnection of RESs, load variations, and the number of EVs.

Improved virtual synchronous generator control for frequency regulation with a coordinated self-adaptive method

Power inverter adopting virtual synchronous generator (VSG) control can provide inertia support for distributed generation systems. However, it cannot take into account the dynamic regulation characteristics of frequency. Thus, when the system encounters a sudden change in load or disturbance, the dynamic process of frequency regulation will be greatly influenced. In view of this issue, an improved VSG control strategy based on a coordinated self-adaptive (CSA) method is proposed. The time domain analysis method is used to study the influences of virtual inertia and damping parameter perturbation on the system steady and dynamic performances. Further, in order to make the control strategy suitable for large load changes and suppress frequency variation beyond the limit, the secondary frequency modulation is introduced into the control loop. Through the coordinated adaptive control of virtual inertia, virtual damping and frequency modulation, the dynamic performance of VSG frequency regulation can be improved obviously. Simulation and experiment results have verified the effectiveness of the proposed CSA control strategy.

Advanced Frequency Support Strategy of Double-Stage Grid-Connected PV Generation

With an increasing penetration of PV generation in the electrical grid, the increasing replacement of large conventional synchronous generators by PV generation will result in deteriorated frequency regulation performance due to the reduced system inertia response. It is urgent for PV generation to take part in frequency regulation. In this paper, three virtual inertial control strategies are proposed for double-stage grid-connected PV generation: virtual inertial control based on the dynamics of phase-locked loop (PLL), virtual inertial control based on the dynamics of low voltage DC capacitor, virtual inertial control based on the dynamics of high voltage DC capacitor. The influence of control parameters on virtual inertial control strategies is also analyzed. Besides, in order to get an even better frequency behavior, a coordinated control strategy between PV generation and conventional synchronous generators (SGs) is also proposed. Finally, the theoretical analysis and control strategies are verified by simulation results.

Enabling and Evaluation of Inertial Control for PMSG-WTG Using Synchronverter With Multiple Virtual Rotating Masses in Microgrid

With the ever-increasing penetration level of renewable energy generation units and the replacement of traditional synchronous generators, the issues associated with the deteriorated performance of power system frequency regulation and potential grid frequency instability become more significant. In this paper, the inertial control algorithm using a synchronverter is enabled for variable-speed wind turbines. Besides, the type-4 permanent magnet synchronous generator wind turbine generator using synchronverter inertial control is modeled and the comprehensive analysis and comparison are conducted between frequency-based inertial control and synchronverter inertial controls. Aiming to address the compromised ability of a synchronverter with desirable inertial response to deal with fast changing wind condition, a synchronverter with multiple virtual rotating masses is proposed in order to improve the active power tracking performance as well as to boost inertial control of the synchronverter. The performance of inertial controls is verified in the modified 10MVA IEEE 14 bus microgrid system. The assessment of the simulation results provides a deep insight into different inertial control methods while demonstrating the applicability of synchronverter on wind power generation systems.

An optimal control and sizing strategy for a coordinated WTG-ES system to provide frequency support

In power grids with large wind power integration, long-term wind turbine generator (WTG) de-loading to provide frequency reserves will greatly constrain the economic benefits of a wind farm (WF). Although energy storage (ES) often cooperates with the WF as a combined system to improve the frequency regulation performance, the coordination control inside the combined system and the trade-offs between the expensive investment costs of the ES, the long-term de-loading costs of the WF and the frequency regulation requirements should be further studied. Therefore, in this paper, a fuzzy-based coordinated control and sizing strategy is proposed to achieve dynamic cooperation inside the combined WTG-ES system and optimize the economic performance of the combined system for providing frequency support power. The fuzzy controller is designed to allocate the frequency regulation demand with considering the operation state and the frequency support capability of the WTG-ES system and the type of frequency incidents. Under the proposed control strategy, the WTG can operate at a rate closer to the maximum power point instead of having to maintain a large power reserve level for frequency support; SOC of the ES is separated into several sections with different features and operational rules to make full use of the ES energy capacity and constrain the SOC limit violation problems effectively. Subsequently, based on the proposed coordinated control strategy, an optimization model is presented that determines the optimal WTG de-loading level in different time intervals, ES rating powers and energy capacity, with the objective of minimizing the operation and frequency regulation costs of the combined WTG-ES system. The optimization model is solved by an improved PSO algorithm. Finally, case studies using actual wind power generation and frequency data were conducted to validate the performance of the proposed control and sizing strategy.

A decentralized multivariable controller for hydrostatic wind turbine drivetrain

AbstractA hydrostatic drivetrain transmits wind turbine energy to a generator. One hydrostatic transmission system (HTS) configuration utilizes a fixed displacement pump and a variable displacement motor. The system dynamics are captured in a nonlinear multi‐input multi‐output mathematical model. This paper introduces a decentralized control configuration based on this model to achieve two desired objectives: maximizing the harvested energy without direct measurement of wind and regulating the frequency of the generator without using power electronic converters. To accomplish these objectives, suitable pairing of control actuators and system responses are identified through nonlinear relative gain arrays (RGA) analysis. The pairing also provides a strong decoupling of control loops. So maximum power point tracking (MPPT) is achieved independently while the generator speed is regulated to maintain the frequency of generated power at 60 Hz. Simulation results demonstrate robust performance of MPPT and frequency regulation in the presence of uncertainties in the turbine and HTS model. We also demonstrate that the RGA paired input‐out control configuration offers superior performance over other possible input–output paired control configurations.

Performance Evaluation of BESS for Korean Power System Using Equivalent Power System Circuit Model

Abstract This paper describes a method of modeling power system elements involved in frequency-active power control using frequency data of south Korea power system. A particle swarm optimization method is used for estimating the parameters of the transfer function which illustrates the relationship between frequency and active power deviation. To obtain the optimal parameter estimation, we use the mean absolute percentage error (MAPE) index for evaluating the accuracy of estimation compared to real data. Real frequency and power data from KPX were used for comparison studies. From the proposed equivalent power system circuit model, we achieved 0.0052% MAPE compared to real data. To evaluate the BESS frequency regulation performance, we considered the BESS control algorithm which is based on the data of Korea electric power corporation (KEPCO). From this case study, we confirmed that the BESS which is operated in south Korea can improve frequency regulation in case of frequency event by increasing nadir frequency up to 0.0335Hz.

Coordinated Active Power Control Strategy for Deloaded Wind Turbines to Improve Regulation Performance in AGC

The large scale integration of wind energy generation adds a new burden to frequency regulation because of the volatility of wind power. Incorporating wind turbines (WTs) into automatic generation control (AGC) is a new approach for maintaining a satisfactory frequency regulation performance, which requires the active power control (APC) of WTs. Conventionally, the APC of WTs is achieved by pitch angle control (PAC) or rotor speed control (RSC), which respectively show a slow response and a narrow regulating range. To improve the regulation performance of variable speed WTs, a coordinated APC strategy that includes the simultaneous activation of PAC and RSC and takes full advantage of rotor kinetic energy to absorb or release extra mechanical energy is proposed in this paper. The simulations focusing on both the WT and power system levels have shown the effectiveness of proposed control strategy.

Retaining of Frequency in Micro-grid with Wind Turbine and Diesel Generator

This paper aims to compare the performance of frequency regulation with two control modes of controller including power control scheme and rotor speed control scheme. The frequency control in this research is based on the frequency droop control method but fuzzy logic is used to define the frequency droop coefficient. To compare the performance of these control modes, a simulation of a micro-grid with the existence of a group of doubly fed induction generator wind turbine system and a diesel generator is fulfilled in Matlab/Simulink. Simulation results indicated that the frequency in the micro-grid with two control schemes always remains in the operation range. With the power control scheme, the frequency in the micro-grid is smoother than that with the rotor speed control. Additionally, DFIG wind turbine with the power control scheme has a better performance in terms of electrical energy when compared to the rotor speed control scheme, and hence the cost of fuel used by diesel is less costly.

Switched distributed load-side frequency control of power systems

This paper presents a frequency control scheme for transmission networks with multiple control areas using load-side controllers in cooperation with automatic generation control (AGC) to regulate the system frequency and the tie-line powers between control areas. In transmission networks, a switched consensus-based distributed controller is proposed for each load bus. Once the frequency violates a pre-defined threshold, load-side controllers will start to work in a frequency regulation mode (FRM) in which each controller communicates with neighboring controllers to discover the power imbalance of the corresponding control area. The control centre in each control area will send the tie-line power information to all the buses in the area, which together with the power imbalance of the area determines the control output of each load-side controller. After the frequency goes back to a satisfactory range, load-side controllers will switch to a load recovery mode (LRM) and shift their duties to the generators. Case studies involving a contingency of load increase and wind farm power fluctuation in the IEEE 118-bus multi-area system show that the performance of frequency and tie-line power regulation is improved significantly by the proposed control scheme compared with AGC and with minimized impacts on end-users.

Optimization of Governor Parameters to Prevent Frequency Oscillations in Power Systems

Negative damping of prime movers is a major cause of sustained frequency oscillations in power systems. Increasing the damping torque of prime movers by adjusting governor parameters is an effective measure to prevent frequency oscillations. The oscillation frequency is volatile and turbine parameters change under loading conditions. To guarantee robustness and adaptability, the governor parameters should be adjusted to ensure sufficient damping torque over the entire range of oscillation frequency and under different turbine parameters. An optimization model of governor parameters is proposed. A constraint of the model is that the damping torque of the prime mover over the entire oscillation frequency range, and under different turbine parameters, must be greater than the threshold. The composite dynamic performance of primary frequency regulation under different loading conditions is the optimization objective. Test results validate the proposed method. The stability of the frequency oscillation mode under different conditions can be guaranteed.

A New Mileage Payment for EV Aggregators With Varying Delays in Frequency Regulation Service

Electric vehicle (EV) aggregators participating in frequency regulation services with a large number of EVs may have a delayed response problem due to their underlying communication infrastructure and scheduling. In this paper, we investigate the effect of delays in EV aggregators’ domain on their frequency regulation payments, especially mileage payment. Since the delays affect frequency regulation performance, in order to consider the delay effect on regulation performance, we enhance the conventional load frequency control model by including multiple EV aggregators with varying delays. Under this model, we evaluate the effect of delays in EV aggregators’ domain on their profits under two ISOs’ mileage payments such as PJM and NYISO. Through simulations, it is shown that the profit of EV aggregators increases as the delays increases. The conventional mileage payments did not motivate EV aggregators to reduce the delay in order to improve regulation performance. To solve the problem, we propose a new mileage payment to reduce the delay. We finally show that the proposed payment can provide an incentive to reduce the delay.

A Droop Frequency Control for Maintaining Different Frequency Qualities in a Stand-Alone Multimicrogrid System

Multiple microgrid (MG) systems can exist in a wide geographical area. They can interconnect and operate on different frequency qualities to enhance the penetration of renewable energy resources. To operate multiple MGs on different frequency qualities, a framework of the stand-alone multimicrogrid (MMG) system is proposed in this study. In the proposed MMG system, each MG connects to the common dc line through an ac/dc interlinking converter. A droop frequency control is proposed for the interlinking converter to improve frequency control performance and achieve autonomous power sharing. The sensitivity analysis with respect to the proposed control is performed to evaluate the stability of the converter system. The proposed droop frequency controller is evaluated in the MMG system with three MGs. The feasibility of the proposed controller is verified by the hardware-in-the-loop simulation using OPAL-RT technologies. The droop frequency controller is executed in digital signal processor TMS-320F-28335 while the proposed MMG system is modeled in a real-time simulator (OP5600). A comparison study on the proposed droop frequency control and conventional P/f control is presented. By using the proposed frequency control, the energy reserves in the adjacent microgrids can be shared effectively to achieve better performance of frequency regulation in the stand-alone MMG system.

Risk Assessment of Power System Considering Frequency Dynamics and Cascading Process

Frequency security is vital to the safety of power systems and has been scrutinized for many years. The conventional frequency security analysis only checks whether the frequency after anticipated initial failures can remain in the normal range based on some aggregated models, but the influence of potential cascading failures has not been considered yet. This is not enough, especially when the modern power system suffers the increasing threat of cascading failures. Therefore, this paper proposes a novel frequency simulation model considering the influence of cascading failure to reveal the security level of power systems comprehensively. The proposed model is based on a platform on which the frequency dynamics and the power flow distributions can be calculated jointly. Moreover, simulation models of protection devices and some supervisory operation-control schemes are also taken into account. Case studies validate the effectiveness of the proposed model on the IEEE 39-bus system. Moreover, the results of some further probabilistic simulations under different operation parameters are obtained, which show the great significance of improving the frequency regulation performance to cope with challenges of blackouts.

Modeling and dynamic response control for primary frequency regulation of hydro-turbine governing system with surge tank

Abstract This paper aims to study the modeling and dynamic response control for primary frequency regulation of hydro-turbine governing system with surge tank. Firstly, the index of dynamic response control for primary frequency regulation is selected and illustrated. Then, the sine wave assumption of water level oscillation in surge tank is proposed, and a novel simplified model for hydro-turbine governing system is obtained. Using the simplified model, the analytical expression for dynamic response of power output is derived. Finally, the concept for the domain of primary frequency regulation is proposed. The effects of influence factors on response performance of primary frequency regulation are analyzed. The results indicate that water level oscillation in surge tank can be assumed as a sine wave. The assumed sine oscillation describes the characteristics of the unsteady flow in headrace tunnel and surge tank. The analytical solution for dynamic response of power output obtained from the sine wave assumption is reasonable and has good accuracy. The dynamic response of power output is superposed by four independent subwaves. The response performance of primary frequency regulation can be quantitatively evaluated by domain of primary frequency regulation. The larger the domain of primary frequency regulation, the better the response performance.

Hydro turbine governor’s power control of hydroelectric unit with sloping ceiling tailrace tunnel

The primary frequency regulation and load regulation transient process when the hydro turbine governor is under the power mode of hydropower unit with sloping ceiling tailrace are analysed by field test and numerical simulation in this paper. A simulation method based on “three-zone model” to simulate small fluctuation transient process of the sloping ceiling tailrace is proposed. The simulation model of hydraulic turbine governor power mode is established by governor’s PLC program identification and parameter measurement, and the simulation model is verified by the test. The slow-fast-slow “three-stage regulation” method which can improve the dynamic quality of hydro turbine governor power mode is proposed. The power regulation strategy and parameters are optimized by numerical simulation, the performance of primary frequency regulation and load regulation transient process when the hydro turbine governor is under power mode are improved significantly.

Utilizing distributed energy resources to support frequency regulation services

Increasing penetration of small-scale intermittent distributed energy resources (DER) such as solar/wind in the power system poses frequency regulation problems due to the reduced system inertia. This paper proposes a new entity, namely, renewable energy aggregators (REA), which enables several small-scale renewable energy generators (SREG) and energy storage systems (ESS) to enhance the frequency stability in low-inertia systems. REA participates in the electricity market and provides frequency regulation services by employing dynamic schedule and control strategies (DSCS). The proposed DSCS consists of forecasting and frequency regulation blocks to schedule appropriate amount of renewable energy in real-time. It utilizes individual weather conditions such as solar insolation and wind speeds for SREG and determines variable de-loaded coefficients to energy spilling or wastage. The efficacy of DSCS based REA under both open-loop and closed-loop simulation studies indicate improved frequency regulation performance under varying weather conditions and load fluctuations.

State-of-the-art review on frequency response of wind power plants in power systems

With an increasing penetration of wind power in the modern electrical grid, the increasing replacement of large conventional synchronous generators by wind power plants will potentially result in deteriorated frequency regulation performance due to the reduced system inertia and primary frequency response. A series of challenging issues arise from the aspects of power system planning, operation, control and protection. Therefore, it is valuable to develop variable speed wind turbines (VSWTs) equipped with frequency regulation capabilities that allow them to effectively participate in addressing severe frequency contingencies. This paper provides a comprehensive survey on frequency regulation methods for VSWTs. It fully describes the concepts, principles and control strategies of prevailing frequency controls of VSWTs, including future development trends. It concludes with a performance comparison of frequency regulation by the four main types of wind power plants.

Assessment of the Effectiveness of Energy Storage Resources in the Frequency Regulation of a Single-Area Power System

An energy storage resource (ESR) has outstanding ramping capability, but its limited energy disables the provision of regulation service around the clock. As a comparison, a conventional generator (CG) is not restricted by the released energy, but the ramp rate is limited. In this paper, a method is proposed to evaluate the effectiveness of ESRs providing frequency regulation service in a single-area system. We measure the performance of frequency regulation by the standard deviation of system frequency excursions, and define the regulation requirement of an isolated power system as the minimum regulation capacity which satisfies the desired regulation performance. By analyzing the regulation requirements under different combinations of regulation resources, we can quantitatively compare the effectiveness of CGs and ESRs. Case studies show that ESRs can reduce regulation requirements, indicating that they are more effective than CGs in frequency regulation. However, they become less effective and even outperformed by CGs when they constitute a larger portion of the system regulation capacity.

Grid Frequency Estimation Using Multiple Model with Harmonic Regressor: Robustness Enhancement with Stepwise Splitting Method

Reduction of inertia in electricity networks due to high penetration level of renewable energy sources will require wind turbines to participate in frequency regulation via Active Power Control. The performance of frequency regulation and protection system depends strongly on the performance of network frequency estimation. Fast frequency variations and uncertainties associated with unknown harmonics and measurement noise in the network signals are the main obstacles to performance improvement of frequency estimation with classical zero crossing method, which is widely used in industry. The same uncertainties introduce challenges in model based frequency estimation. These challenges are addressed in this paper within the framework of multiple model with harmonic regressor.Additional challenges associated with computational complexity of matrix inversion algorithms and accuracy of inversion of ill-conditioned matrices in the multiple model are also discussed in the paper. New high order algorithms with reduced computational complexity are presented. Instability mechanism is discovered in Newton-Schulz and Neumann matrix inversion techniques in finite precision implementation environment. A new stepwise splitting method is proposed for elimination of instability and for performance improvement of matrix inversion algorithms in the multiple model. All the results are confirmed by simulations.