Frequency Stability Of Power System Research Articles

LFC enhancement concerning large wind power integration using new optimised PID controller and RFBs

To improve the frequency stability in an interconnected power system including renewable energy sources, the control actions need to be more robust and efficient. For this reason, this study proposes a new optimised proportional–integral–derivative (PID) controller coordinated with redox flow batteries (RFBs) for the enhancement of load frequency control (LFC) of power system concerning large penetration of wind power generation. The PID controller parameters were obtained using a recently developed meta-heuristic optimisation algorithm named Grey Wolf optimiser. To show the effectiveness of the proposed control strategy, the interconnected two-area IEE Japan East 107-bus-30-machine power system was investigated for the simulation. The system dynamic responses were obtained considering load change in area-1 and large wind farm integration in area-2. A comparative study of performance of the proposed strategy with some well-known optimisation techniques was performed. Furthermore, the sensitivity analysis of the proposed controller was also examined by varying the penetration range of the integrated wind farm. Dynamic responses obtained from the simulation satisfy the LFC requirements. In addition, the results reveal that the frequency control concept-based optimised PID controller coordinated with the RFB will enhance the frequency stability in terms of settling time, peak undershoot, and peak overshoot.

Maximum Penetration Level of Micro-Grids in Large-Scale Power Systems: Frequency Stability Viewpoint

Reduction of overall system inertia in response to increasing penetration level of Micro-Grids (MGs) makes frequency control more challenging. In this paper, a new analytical approach to mathematically investigate the impact of Micro-Grids penetration level on power system frequency stability is proposed. The method interprets the frequency dynamic behavior of the penetrated system based on the conventional system in terms of minimum instantaneous frequency, steady state deviation and a 15-second rolling window. Then, new indices are proposed to determine the maximum allowable MGs penetration level. These indices are derived by adapting the developed analytical approach to the required standards regarding the aforementioned criteria. Each index determines an upper bound on reduction of system inertia, which, in turn specifies the associated maximum penetration level. The maximum value of the inertia constant related to the developed indices (whichever is the larger) determines the system maximum allowable penetration level. The effectiveness of the proposed analytical approach and indices is demonstrated on a 16-machine 68-bus system and the IEEE 50-machine systems. Simulation results for large-scale power systems with high stiffness reveal that based on the operating condition, one of the minimum instantaneous frequency or 15-second rolling window impose an upper bound on penetration level.

Impact of Photovoltaics on Frequency Stability of Power System During Solar Eclipse

This paper analyses the influence of variable production of photovoltaic plants on the synchronous continental Europe interconnection during the solar eclipse of March 20, 2015. For countries with a high share of photovoltaics, the daily photovoltaics generation is estimated for the worst-case scenario – a clear sunny day. The presented results of long-term dynamic simulations focus on frequency stability (taking into account daily load diagram, secondary and primary frequency control, system inertia, settings of frequency protections and load frequency shedding).

Grid frequency control with electric vehicles by using of an optimized fuzzy controller

Nowadays, due to the increased price of fossil fuels and their decreasing resources on the one hand, and the importance of environmental pollution on the other, use of electric vehicles (EVs) has been increased. Charging of EVs has imposed new loads on power systems. These new and major loads along with the deregulation of power systems, which introduces new uncertainties to grid, have caused new challenges for the frequency control and stability of power systems. Use of EVs as moving batteries in deregulated power systems is one of the ways for dealing with this problem. In this method, charging EVs is controlled and, when necessary, EV battery is discharged in grid. This concept is called vehicle to grid (V2G), which was employed in this study for the control of the frequency of a smart deregulated grid. For this purpose, an optimized fuzzy controller was used to control EVs. Using the proposed method, charging or discharging of batteries was carried out with respect to grid frequency and battery state of charge. To investigate the proposed approach, a 35-bus system as a deregulated system was assumed. Then, this system was converted into a three area system in order for the frequency analysis. Simulations were performed in MATLAB/SIMULINK environment and the results illustrated the good performance of the proposed method in terms of frequency control of deregulated system. Moreover effect of proposed method on under frequency load shedding was verified. It was illustrated that by using of proposed method under frequency load shedding is postponed.

Adaptive load shedding scheme for frequency stability enhancement in microgrids

The imbalance between the generated power and the load demand is the major factor that is usually responsible for frequency instability in power systems, most especially islanded microgrids. To determine the size of the loads that should be shed and their appropriate locations in the power system, to maintain the system frequency within the permissible limits, this paper presents an effective adaptive control scheme. In the proposed controller, a stepwise load-shedding approach is designed in the islanded MGs to regulate the grid frequency while providing the amount of power shortage. To this achieve, it locally measures the system parameters most especially voltage and frequency signals. Thereafter, a stepwise load-shedding will take place in locations where the highest voltage drop and frequency variation are experienced. The load-shedding step changes according to certain factors such as shedding speed, location and value, and the rate of frequency change. The proposed approach eliminates the adjustable loads to return the frequency back to the desired value. Simulation results of the proposed method under different practical scenarios, when compared with the conventional PID controller, provide considerable enhancement in the power system frequency stability.

A new smart charging method for EVs for frequency control of smart grid

Nowadays, due to the shortage of fossil fuels on the one hand and their high prices on the other hand, using electric vehicles (EVs) has been increased. Charging of EVs has imposed new loads on power systems. These new and major loads have faced the frequency control and stability of power systems with new challenges. One way to deal with this new challenge is smart charging of EVs. In this method, grid condition is a key parameter that affects the charging of EV. In other words, in smart charging method, charging is performed with respect to power system parameters such as frequency. In this paper, a smart charging method based on fuzzy controller is proposed, in which charging process is performed with respect to the frequency deviation of grid and state of charge (SOC) of EV battery. To evaluate the performance of the proposed controller in control of grid frequency, IEEE 39-bus system in the presence of renewable energy sources is considered as test system. In order to the frequency analysis, this system is converted into a three-area system and, for each area, several EV categories with different numbers of EVs, battery capacity, start time of charging, and initial SOC are supposed. Moreover performance of proposed method is compared with an optimized PI controller in terms of frequency control. To investigate performance of proposed method in charging of EVs, a two area system is assumed and charging of EVs is verified by applying step loads to both areas. Simulations are carried out in MATLAB/SIMULINK environment. Results of the simulations reveal the good performance of the proposed controller in terms of frequency control of grid and charging of EVs.

Robust H∞ load frequency control of delayed multi-area power system with stochastic disturbances

In this paper, by taking into account stochastic disturbances induced by the integration of large number of renewable energy resources into power grid, the robust H∞ load frequency control problem for multi-area power system with time delay is discussed in detail. Considering time delay induced by signal transmission between different control areas, two different controller designing strategies, namely the state feedback controller and the delayed state feedback controller designing strategies are proposed, respectively. Based on Lyapunov stability theory, several less conservative stability conditions are developed in the form of linear matrix inequalities to ensure the frequency stability of multi-area power system, where the optimal attenuation level can also be obtained by solving the constrained optimization problem. Finally, simulation results based on a two-area interconnected power system are provided to demonstrate the usefulness and effectiveness of the developed results.

Dynamic equivalencing of an active distribution network for large‐scale power system frequency stability studies

This study presents an approach for developing the dynamic equivalent model of an active distribution network (ADN), consisting of several micro-grids, for frequency stability studies. The proposed grey-box equivalent model relies on Prony analysis to establish stop time and load damping as the required modelling parameters. Support vector clustering (SVC) and grouping procedure are employed for aggregation and order-reduction of ADN. This significantly decreases the sensitivity of the estimated parameters to operating point changes which, in turn, guarantees the model robustness. This is done through representing the SVC output, that is, clusters, by cluster substitutes. The final ADN dynamic equivalent model is represented by several groups, in which their mutual interactions are taken into account by a new developed mathematical-based criterion. Simulation results reveal that the proposed model is robust which could successfully take into account the continuous and discontinuous uncertainties.

Optimal Reserve Allocation to Maximize Kinetic Energy in a Wind Power Plant

Modern wind generators (WGs) are forced or encouraged to participate in frequency control in the form of inertial and/or primary control to improve the frequency stability of power systems. To participate in primary control, WGs should perform deloaded operation that maintains reserve power using speed and/or pitch-angle control. This paper proposes an optimization formulation that allocates the required reserve to WGs to maximize the kinetic energy (KE) stored in a wind power plant (WPP). The proposed optimization formulation considers the rotor speed margin of each WG to the maximum speed limit, which is different from each other because of the wake effects in a WPP. As a result, the proposed formulation allows a WG with a lower rotor speed to retain more KE in the WPP. The performance of the proposed formulation was investigated in a 100-MW WPP consisting of 20 units of 5-MW permanent magnet synchronous generators using an EMTP-RV simulator. The results show that the proposed formulation retains the maximum amount of KE with the same reserve and successfully increases the frequency nadir in a power system by releasing the stored KE in a WPP in the case of a disturbance.

Fast Frequency Response Capability of Photovoltaic Power Plants: The Necessity of New Grid Requirements and Definitions

In recent years, only a small number of publications have been presented addressing power system stability with the increased use of large-scale photovoltaic (PV) generation around the world. The focus of these publications was on classical stability problems, such as transient and small signal stability, without considering frequency stability. Nevertheless, with increased PV generation, its effects on system frequency response during contingencies can no longer be ignored, especially in the case of weakly interconnected networks or isolated power systems. This paper addresses the impacts of large scale PV generation on the frequency stability of power systems. The positive effects of deloaded PV power plants (PV-PPs) able to support system frequency recovery during the initial seconds after major contingencies are also examined. Because this type of frequency support is not covered by current definitions, a new terminology is proposed that includes the frequency response of inertia-less generation units immediately after major power imbalances. We refer to this type of frequency support as fast frequency response (FFR). Finally, a discussion is also presented regarding the applicability and pertinence of frequency-related grid requirements for PV-PPs in the case of real power systems. The investigation is based on the isolated power system of northern Chile. The obtained results indicate that in the case of major power imbalances, no significant effects arise on the system frequency response until PV penetration levels exceed approximately 20%. From a system security perspective, the problems arise for PV penetration levels of approximately 50%, in which case, the frequency response capability in PV-PPs would be justified during certain hours of the year.

Utilisation of alkaline electrolysers to improve power system frequency stability with a high penetration of wind power

Controlling the frequency of power systems with high wind power penetration is more difficult due to the high variability of the wind power. One possible mainstream energy carrier in the future, particularly for the transportation sector, is Hydrogen, and water electrolysis is one of the most attractive ways to produce it. In this work, a detailed model of a steam turbine generator has been produced in MATLAB Simulink and used to investigate a scenario in which there is a 25% penetration of wind power. To improve the frequency stability of the power system, large scale alkaline electrolysers used in future Hydrogen filling stations could adjust their load with respect to the frequency deviation from nominal and can significantly reduce fluctuations in system frequency. For the case examined, five times less spinning reserve is required in order to maintain the power system frequency within operational limits when electrolysers are utilized as a form of demand side management (DSM), compared to the base case where no electrolyser DSM plant is available. Actual operational data from a pressurised alkaline electrolyser is used to evidence the fast load changing capability of such electrolysers.

Capacity Configuration of BESS as an Alternative to Coal-Fired Power Units for Frequency Control

Coal-fired power units show defects in meeting the demands of maintaining power system frequency stability. This is mainly due to their inherent characteristics and a continuous increase in renewable generation. Battery Energy Storage System (BESS) has such advantages as fast response and precise tracking, thus may be a new frequency regulation method. On basis of the principle of energy storage for regulation, capacity configuration of energy storage is made as an alternative to coal-fired power unit accounting for both primary and secondary frequency regulation. Example calculations and simulation in MATLAB/SIMULINK are conducted to verify the feasibility of the proposed capacity configuration methods and the reliability of the energy storage system for regulation.

Study on Emergency Load Shedding Based on Frequency and Voltage Stability

Under frequency load shedding is widely used under emergency situations in the power system. It significantly ensures frequency stability of power system by dropping excessive loads at certain places. In order to improve the traditional UFLS schemes, the rate of frequency change is taken into consideration when calculating the quantity of active power imbalance in the power system. The measure factors of frequency change rate are analyzed and a new function is created to calculate the active power imbalance based on frequency and voltage. Moreover, a new method which utilizes frequency and voltage sensitivity to determine the location and corresponding quantity of load shedding is proposed. Voltage stability should be maintained at the time of recovering system frequency stability. The simulation results based on IEEE39 buses system show that the new self-adaptive scheme of load shedding is more reasonable and superior.

Development of a simulation platform for studying on primary frequency regulation characteristics of nuclear units

With the rapid development of nuclear units, the primary frequency regulation (PFR) characteristics of nuclear units have a significant affect on the frequency stability of power system. Power system will suffer great challenge if nuclear units do not participate in PFR, so study on the PFR characteristics of nuclear units has becoming a pressing issue. A detailed, nonlinear, time-varying dynamic mathematical model of a whole pressurized water reactor (PWR) nuclear power plant has been established in the present study. The simulation platform for studying on the PFR characteristics of nuclear units has been developed according to the model established. Using the simulation platform, the operation and control mode of PWR nuclear units participating in PFR of power system is simulated and studied. The simulations results show that PWR nuclear units are feasible in participating in PFR from safety and economy by adopting the operation and control mode presented in the study, which can contribute to the practical operation of power system.

Research on Wind Power Fluctuation and its Impacts on Power System Frequency

The output power of wind farms has significant randomness and variability, which results in adverse impacts on power system frequency stability. This paper extracts wind power fluctuation feature with the HHT (Hilbert-Huang Transform) method. Firstly, the original wind power data was decomposed into several IMFs (Intrinsic Mode Functions) and a tendency component by using the EMD (Empirical Mode Decomposition) method. Secondly, the instantaneous frequency of each IMF was calculated. On this basis, taking a WSCC 9-bus power system as benchmark, the impact on power system frequency caused by wind power fluctuation was simulated in a real-time simulation platform, and the key component which results in the frequency deviation was found. The simulation results validate the wind power fluctuation impacts on frequency deviation, underlying the following study on power system frequency stability under the situation of large-scale intermittent generation access into the grid.

Improvement of power system frequency stability using alkaline electrolysis plants

Hydrogen could become an important energy carrier, in particular used as an input to fuel cell electric vehicles. Alkaline electrolysers are an attractive technology to produce carbon-free hydrogen from renewable generated electricity. Large-scale alkaline electrolysers used in future hydrogen-filling stations could also be utilised to improve the frequency stability of the electricity power system. The electrolyser load can be controlled to respond to power system frequency variations, and in the case of a sudden loss of generation, these electrolysers could rapidly decrease their load on the system to maintain the power balance. In this study, the potential of alkaline electrolysers to dynamically stabilise the frequency of the power system is assessed. A model of steam turbine generation unit has been developed in MATLAB SIMULINK environment, and a scenario in which there is a sudden loss of generation in the system is examined. It is demonstrated that alkaline electrolysers could prevent unacceptable frequency drop, i.e. below the statutory limit, following by an abrupt loss of generation, even with no spinning reserve on the system. In this article for the first time, the ramping rate of an alkaline electrolyser is shown through experimental data.

The Study on Isolated Power Control System Based on Coordinated Control

This paper analysis the formation conditions of isolated power system, its influencing factors, revealing the operation mode, automatic safety device, generator protection regulation characteristics and these factors to stable operation of isolated power system. The dynamic models for isolated power system with generating unit of medium and small capacity are proposed in the paper to meet the need of the applicable ones to study their dynamic characteristics. The differences between isolated and the interconnected power system in the frequency characteristics are proposed and consider into low-frequency load shedding scheme and optimization methods that can meet both isolated and the interconnected power system frequency stability requirements. Not only more proper control strategy of units but their evaluation indexes are suggested to improve performance and achieve stability in islanded power system.

Coordinated frequency regulation by doubly fed induction generator-based wind power plants

The increasing penetration of wind power impacts the frequency stability of power systems. A doubly fed induction generator (DFIG)-based wind power plant naturally does not provide frequency response because of the decoupling between the output power and the grid frequency. DFIGs also lack power reserve margin because of the maximum power point tracking (MPPT) operation. Therefore this study presents a novel frequency regulation by DFIG-based wind turbines to coordinate inertial control, rotor speed control and pitch angle control, under low, medium or high wind speed mode. Inertial control emulates the inertia of wind generators and supports frequency control during transient. The gain of inertial control is calculated from a creative viewpoint of protecting the wind turbine from stalling. Rotor speed control and pitch angle control enable DFIGs to reserve sufficient active power for a steady-state frequency adjustment. The numerical simulations demonstrate that the coordinated control enhances the frequency regulation capability and damps the frequency oscillations effectively.

Determination of frequency stability border of power system to set the thresholds of under frequency load shedding relays

Frequency stability is a crucial consideration in power system operation and planning, particularly as a consequence of recent increase in load demand. There is an urgent need in each power system to consider load–generation balance to prevent system from breaking up into islands. In this paper, a system frequency response model has been presented and a new method has been offered to analytically calculate the border of frequency stability of power system. It can be realized, whether system is stable from frequency viewpoint or not. Moreover, the threshold of UFLS (under frequency load shedding) relays can more effectively be determined, compared to the previous works in the area. The proposed method is examined on a four unit test system. Obtained results confirm the validity of the developed approach.

Research on H2 speed governor for diesel engine of marine power station

The frequency stability of a marine power system is determined by the dynamic characteristic of the diesel engine speed regulation system in a marine power station. In order to reduce the effect of load disturbances and improve the dynamic precision of a diesel engine speed governor, a controller was designed for a diesel engine speed regulation system using H2 control theory. This transforms the specifications of the system into a standard H2 control problem. Firstly, the mathematical model of a diesel engine speed regulation system using an H2 speed governor is presented. To counter external disturbances and model uncertainty, the design of an H2 speed governor rests on the problem of mixed sensitivity. Computer simulation verified that the H2 speed governor improves the dynamic precision of a system and the ability to adapt to load disturbances, thus enhancing the frequency stability of marine power systems.