Large Frequency Deviations Research Articles

Generic System Frequency Response Model for Power Grids With Different Generations

With the increasing integration of renewable generation, many power grids have gradually formed AC-DC hybrid systems. Abnormal operations, such as DC blocking faults and generation trips, have led to several incidents of large frequency deviations. However, current simulation methods result in large errors when estimating the frequency regulation capacity of the system. This paper proposes a generic system frequency-response (SFR) model that can be used to estimate the dynamic frequency behavior of modern large-scale power systems. The limitations of the classical SFR model is first analyzed. Second, a generic SFR model with a more reasonable structure is presented, and the parameter-determination strategy is proposed using both the dynamic and steady-state data. Then, the generic SFR model is built and verified by a simulation case. Finally, a generic SFR model with satisfactory accuracy is established for the power grid in East China based on the measured disturbance data. The results show that the proposed model is promising for broad potential applications.

Long-Term Frequency Stability Assessment Based on Extended Frequency Response Model

Large frequency deviation degrades the operation performance of the boiler and its auxiliaries. It affects the output of thermal power units. A significant change in power generation leads to a great frequency deviation. An extended frequency response model for long-term frequency stability assessment is constructed to consider the effects of frequency deviations on boiler auxiliaries. The static power-frequency characteristic of the thermal power unit within an extensive frequency variation range is analyzed. It reveals that the active power output of the generating unit is not monotonically increasing with frequency dropping. An inflection point is observed on the static power-frequency characteristic curve when considering boiler auxiliary frequency characteristics. In the range of greater frequency deviation, the output power decreases rapidly with frequency declination, and an unstable equilibrium point (UEP) of frequency stability is identified. A quantitative index is proposed for frequency stability assessment based on UEP. The stability characteristic and the frequency stability quantitative index based on UEP are analyzed. The frequency dynamic behaviors of the extended model are demonstrated to analyze frequency stability characteristics within an extensive frequency variation range by a single machine system and the IEEE 39-bus system with considering boilers and its auxiliaries.

An adaptive filters based PMU algorithm for both steady-state and dynamic conditions in distribution networks

Although synchronized phasor measurement unit (PMU) technology is widely applied in transmission networks, conventional PMU algorithms suffer from new challenges (e.g. large frequency deviation, multiple interferences and dynamic behaviors, etc.) if directly applied to distribution networks. On the basis of predecessors' research, this paper presents a comprehensive PMU algorithm, using adaptive filters to simultaneously satisfy the high-precision and dynamic-response requirements of phasor estimation in distribution networks. In this algorithm, out-of-band (OOB) interferences that are difficult to be filtered out, are specifically considered in both steady-state and dynamic signal modelling processes. Based on the models, a finite impulse response (FIR) filter and a Taylor–Fourier transform (TFT) filter that both contains notches at the OOB interferences are elaborately designed and implemented in two parallel channels. With the support of a super-resolution frequency analysis method for signal components, the filters’ central frequency and notches’ position can be online adaptively adjusted. Finally, the outputs are switched between the two-channel estimations according to a designed transient recognition strategy. Test results in IEEE standard scenarios and some other realistic but severe scenarios demonstrate the effectiveness and reliability of this proposed algorithm.

Voltage and Frequency Regulation of Microgrid With Battery Energy Storage Systems

This paper presents a novel primary control strategy based on output regulation theory for voltage and frequency regulations in microgrid systems with fast-response battery energy storage systems (BESS). The proposed control strategy can accurately track voltage and frequency set points while mitigating system transients in the presence of disturbance events. Therefore, it overcomes the key weaknesses of droop-based control methods such as large steady-state voltage and frequency deviations and poor transient performance. Throughout this paper, four control schemes are derived with tradeoffs between communication requirement and system dynamic performance. Their effectiveness is validated through MATLAB Simulink simulation studies involving a medium-voltage microgrid with both synchronous generation resources and BESS. Although the proposed control schemes are centralized, practical implementation is possible with available communication links in microgrids and embedded hardware technologies.

An independently controlled energy storage to support short term frequency fluctuations in weak electrical grids

This paper proposes the use of Energy Storage (ES) to support frequency fluctuations in weak grids powered by low-power synchronous generators (SGs). Traditionally, large synchronous generators rely on high inertia and governor control to guarantee the frequency stability of the network. The scenario changes in networks with a large penetration of small distributed generators (DGs) with smaller inertia, where load variations can lead to large frequency deviations, violating the over/under frequency protection limits. The proposed energy storage control injects active power during load transients so that the frequency is constrained within predefined thresholds. The system frequency is detected by a Double Second-Order Generalised Integrator Frequency Locked Loop (DSOGI-FLL), eliminating the need for communication between energy storage and synchronous generator and providing “plug-and-play” capability. The proposed method combined with the continuous speed governing supports the frequency well within the primary frequency transient requirements. The solution has been validated using an 8kW laboratory system.

Power-Imbalance Allocation Control of Power Systems-Secondary Frequency Control

The traditional secondary frequency control of power systems restores nominal frequency by steering Area Control Errors (ACEs) to zero. Existing methods are a form of integral control with the characteristic that large control gain coefficients introduce an overshoot and small ones result in a slow convergence to a steady state. In order to deal with the large frequency deviation problem, which is the main concern of the power system integrated with a large number of renewable energy, a faster convergence is critical. In this paper, we propose a secondary frequency control method named Power-Imbalance Allocation Control (PIAC) to restore the nominal frequency with a minimized control cost, in which a coordinator estimates the power imbalance and dispatches the control inputs to the controllers after solving an economic power dispatch problem. The power imbalance estimation converges exponentially in PIAC, both overshoots and large frequency deviations are avoided. In addition, when PIAC is implemented in a multi-area controlled network, the controllers of an area are independent of the disturbance of the neighbor areas, which allows an asynchronous control in the multi-area network. A Lyapunov stability analysis shows that PIAC is locally asymptotically stable and simulation results illustrate that it effectively eliminates the drawback of the traditional integral control based methods.

Distributed load shedding for microgrid with compensation support via wireless network

Due to the limited generation and finite inertia, microgrid suffers from the large frequency and voltage deviation which can lead to system collapse. Thus, reliable load shedding to keep frequency stable is required. Wireless network, benefiting from the high flexibility and low deployment cost, is considered as a promising technology for fine-grained management. In this paper, for balancing the supply-demand and reducing the load-shedding amount, a distributed load shedding solution via wireless network is proposed. Firstly, active power coordination of different priority loads is formulated as an optimisation problem. To solve it, a distributed load shedding algorithm based on subgradient method (DLSS) is developed for gradually shedding loads. Using this method, power compensation can be utilised and has more time to lower the power deficit so as to reduce the load-shedding amount. Secondly, to increase the response rate and enhance the reliability of our method, a multicast metropolis schedule based on TDMA (MMST) is developed. In this protocol, time slots are dedicatedly allocated and a checking and retransmission mechanism is utilised. Finally, the proposed solution is evaluated by NS3-Matlab co-simulator. The numerical results demonstrate the feasibility and effectiveness of our solution.

A Modified Differential Coherent Bit Synchronization Algorithm for BeiDou Weak Signals with Large Frequency Deviation

BeiDou system navigation messages are modulated with a secondary NH (Neumann-Hoffman) code of 1 kbps, where frequent bit transitions limit the coherent integration time to 1 millisecond. Therefore, a bit synchronization algorithm is necessary to obtain bit edges and NH code phases. In order to realize bit synchronization for BeiDou weak signals with large frequency deviation, a bit synchronization algorithm based on differential coherent and maximum likelihood is proposed. Firstly, a differential coherent approach is used to remove the effect of frequency deviation, and the differential delay time is set to be a multiple of bit cycle to remove the influence of NH code. Secondly, the maximum likelihood function detection is used to improve the detection probability of weak signals. Finally, Monte Carlo simulations are conducted to analyze the detection performance of the proposed algorithm compared with a traditional algorithm under the CN0s of 20~40 dB-Hz and different frequency deviations. The results show that the proposed algorithm outperforms the traditional method with a frequency deviation of 50 Hz. This algorithm can remove the effect of BeiDou NH code effectively and weaken the influence of frequency deviation. To confirm the feasibility of the proposed algorithm, real data tests are conducted. The proposed algorithm is suitable for BeiDou weak signal bit synchronization with large frequency deviation.

Robust Frequency and Voltage Stability Control Strategy for Standalone AC/DC Hybrid Microgrid

The microgrid (MG) concept is attracting considerable attention as a solution to energy deficiencies, especially in remote areas, but the intermittent nature of renewable sources and varying loads cause many control problems and thereby affect the quality of power within a microgrid operating in standalone mode. This might cause large frequency and voltage deviations in the system due to unpredictable output power fluctuations. Furthermore, without any main grid support, it is more complex to control and manage the system. In past, droop control and various other coordination control strategies have been presented to stabilize the microgrid frequency and voltages, but in order to utilize the available resources up to their maximum capacity in a positive way, new and robust control mechanisms are required. In this paper, a standalone microgrid is presented, which integrates renewable energy-based distributed generations and local loads. A fuzzy logic-based intelligent control technique is proposed to maintain the frequency and DC (direct current)-link voltage stability for sudden changes in load or generation power. Also from a frequency control perspective, a battery energy storage system (BESS) is suggested as a replacement for a synchronous generator to stabilize the nominal system frequency as a synchronous generator is unable to operate at its maximum efficiency while being controlled for stabilization purposes. Likewise, a super capacitor (SC) and BESS is used to stabilize DC bus voltages even though maximum possible energy is being extracted from renewable generated sources using maximum power point tracking. This newly proposed control method proves to be effective by reducing transient time, minimizing the frequency deviations, maintaining voltages even though maximum power point tracking is working and preventing generators from exceeding their power ratings during disturbances. However, due to the BESS limited capacity, load switching (load shedding scheme) as last option is also introduced in this paper. Simulation results prove the effectiveness of the proposed control strategy from both frequency and voltage perspectives.

Optimal System Frequency Response Model and UFLS Schemes for a Small Receiving-End Power System after Islanding

Large frequency deviations after islanding are exceedingly critical in small receiving-end power systems. The under-frequency load shedding (UFLS) scheme is an efficient protection step for preventing system black outs. It is very important to get an exact model to design the UFLS schemes. In this paper, an optimization model to achieve the system frequency response (SFR) model either from the full-scale power system or from test records was proposed. The optimized SFR model took into account the response of governors-prime movers and the dynamic characteristics of loads developed in the modern power system. Then the UFLS schemes were designed via the optimized SFR model and particle swarm optimization (PSO) method. The time-domain simulation with the actual small receiving-end power system was presented to investigate the validity of the presented model and the developed technique.

A Modified Dynamic Synchrophasor Estimation Algorithm Considering Frequency Deviation

When the power system is suffered from disturbances, such as unbalance, load variation, and fault occurrence, not only the phase and magnitude of the power signal will change but also the fundamental frequency can deviate away from the nominal one up to 5 Hz as per IEEE standard C37.118. As large frequency deviation will lead to great errors on Taylor approximation, the accuracy can be improved by expanding the dynamic characteristics in form of Taylor series near real fundamental frequency of supplied signals instead of nominal frequency (50/60 Hz). The corresponding coefficient against the different frequency estimations is applied to the dynamic phasor estimator to yield accurate synchrophasor estimation with the consideration of large frequency deviation. First, raw phasors attained by short time Fourier transform, are employed to provide the frequency estimation. Then, according to the frequency estimation, the coefficient matrix calculated off-line in form of lookup table is selected to calculate phasor estimation at the center of data window. Finally, a shift process is taken to give phasor estimation at report time. Simulated signals, PSCAD-generated signal, RTDS-generated data, and field data are employed to evaluate the performance of the proposed algorithm. The results show that the proposed algorithm can get more accurate phasor estimations than our previous work at most of the time with the cost of minor increase of computational power.

State of Charge-Based Active Power Sharing Method in a Standalone Microgrid with High Penetration Level of Renewable Energy Sources

Standalone microgrids, which are mainly constructed on island areas have low system inertia, may result large frequency deviations even for small load change. Moreover, increasing penetration level of renewable energy sources (RESs) into standalone microgrids makes the frequency stability problem even worse. To overcome this problem, this paper proposes an active power sharing method with zero frequency deviations. To this end, a battery energy storage system (BESS) is operated as constant frequency (CF) control mode, whereas the other distributed generations (DGs) are operated as an active and reactive power (PQ) control mode. As a result, a state of charge (SOC) of the BESS is changed as the system load varies. Based on the SOC deviation, DGs share the load change. The SOC data is assumed to be sent via communication system, hence the communication time delay is considered. To enhance reliability, controllers of DGs are designed to take account of the failure of communication system. The simulation results show that active power can be shared among DGs according to desired ratio without frequency deviations even for large variation of output power of RESs.

Frequency and Voltage Control Strategy of Standalone Microgrids With High Penetration of Intermittent Renewable Generation Systems

In this paper, we propose a frequency and voltage control strategy for a standalone microgrid with high penetration of intermittent renewable generation systems, which might cause large frequency and voltage deviation in the system due to unpredictable output power fluctuations. To this end, a battery energy storage system (BESS) is suggested for generating the nominal system frequency instead of a synchronous generator, from a frequency control perspective. This makes the system frequency independent of the mechanical inertia of the synchronous generator. However, a BESS has a capacity limitation; a synchronous generator is used to maintain the state of charge (SOC) of the BESS at a certain value. For voltage control, we proposed that a reactive power/active power (Q/P) droop control be added to the conventional reactive power controller. By adding a Q/P droop control, renewable generation acquires a voltage-damping effect, which dramatically alleviates the voltage fluctuation induced by its own output power fluctuation. Simulation results prove the effectiveness of the proposed control strategy from both frequency and voltage control perspectives.

An Integrated Security-Constrained Model-Based Dynamic Power Management Approach for Isolated Microgrids in All-Electric Ships

Isolated microgrid power systems are prone to large frequency and voltage deviations, since they have limited generation and finite inertia and may include pulse power loads and large proportion of dynamic loads. Thus, they require effective power management methods to operate optimally, while satisfying operating and security constraints. In this paper, a novel integrated security-constrained model-based power management approach is proposed for isolated microgrids in all-electric ships during the normal/alert operating state. The new power management method was formulated as a multi-objective optimal control problem. A method based on the Nondominated Sorting Genetic Algorithm II was developed to solve the problem. The developed integrated security-constrained power management approach was applied to a notional all-electric ship computer model. The simulation results, conducted in PSCAD, showed the effectiveness of the new power management method.

Electric vehicles contribution for frequency control with inertial emulation

This work proposes a novel primary frequency control technique with electric vehicles (EV), the combination of inertial emulation and droop control, for isolated systems. Being EV dispersed along the grids, the impacts of possible delayed actions are assessed. Islanded systems have reduced inertia and so load/generation imbalance situations may lead to large frequency deviations. Therefore, this paper focuses essentially on the EV contribution for primary reserves provision, in order to allow a safe integration of further intermittent Renewable Energy Sources (RES). An avant-garde generation dispatch was adopted for the test system used in this work, fully reliant on RES, mainly conventional hydro units and some wind generation. The studied disturbances include a rapid shortfall on wind power production and a sequence of consecutive events caused by the variability of the wind resource in an ordinary situation.

Risk Assessment for Power System Operation Planning With High Wind Power Penetration

Integration of wind power generation (WPG) is increasing rapidly worldwide. The variability and uncertainty of wind energy may lead to significant load-generation imbalances resulting in large frequency deviations, hence increasing system operational risks, especially in small and isolated power systems with low inertia and limited capabilities of providing frequency responses. This raises the need for investigating alternatives to current power system operation planning approaches to cope with the uncertain nature of the intermittent generation. This paper presents a risk assessment approach to analyze power system security for operation planning under high penetration of wind power generation. The proposed approach deals with not only steady-state voltage and overload evaluations, but also frequency response adequacy. For fast identification of operational limit violations in the proposed risk assessment method, we develop an analytical procedure for approximating frequency response and assessing the consequences of limit violations without performing dynamic simulations. As a result, the frequency response adequacy assessment can be run simultaneously with the steady-state voltage and overload evaluations. The proposed risk assessment approach is illustrated via its application to a model of a power system with high wind power penetration.

Control of high frequency microactuators using active structures

A fluidically driven microactuator that generates supersonic, pulsed microjets has been implemented with smart materials to actively and precisely control the frequency of the microjets in a closed-loop manner. Since this actuator relies on a number of microscale flow and acoustic phenomena to produce the pulsed microjets, its resonant frequency is determined by its geometry and other flow parameters. The design discussed in this paper integrates piezoelectric stacks by connecting them to movable sidewalls within the actuator such that the microactuatorʼs internal geometry can be controlled by varying the voltage across the piezo-stacks. An open-loop control scheme demonstrates the frequency modulation capabilities that are enabled with this design: very large frequency deviations (up to ) around the actuator design frequency are attained at very high rates (up to 1 kHz). Closed-loop control of the microactuatorʼs frequency was also demonstrated, and the results indicate that (combined with appropriate sensors) this actuator could be used effectively for active, feedback control in high-speed, resonance-dominated flowfields. This proof of concept study clearly illustrates the ability of this robust and compact actuator to produce perturbations that can be modulated and controlled based on the desired control objective.

Control of cyclic fluctuations in an independent microgrid by an SOFC triple combined cycle inertia system

Abstract Technology was investigated to control cyclic fluctuations in an independent microgrid powered with unstable renewable energy by use of a solid oxide fuel cell (SOFC, 1 MW) in a triple combined cycle (SOFC-TCC) that included a gas turbine (G/T, 0.8 MW) and a steam turbine (S/T, 0.2 MW). A large-scale solar power system (0.8 MW) and a wind farm (0.8 MW) were interconnected with the electrical power network through an inverter. The cyclic fluctuations ingredient of the network was controlled by a suitably designed inertia system and by governor-free control of the G/T and S/T. The SOFC-TCC’s control block diagram was submitted to MATLAB/Simulink R 2013a, and the deviation of electrical power and frequency in the independent microgrid caused by the SOFC-TCC and renewable energy interconnection was clarified. As a result, a range of suitable inertial constants for G/T and S/T and the electrical output characteristics were determined. Selecting a small inertial constant for the simulation resulted in a large frequency deviation of G/T and S/T, with frequency stabilized for a short time. On the other hand, selecting a large inertial constant resulted in a controlled frequency deviation, although the unstable frequency of the power grid continued for a long time.

English

3 ABSTRACT: This paper present the analysis of tuning the parameters of PID controller applied to load frequency control problem. In case of an interconnected power system, any small sudden load change in any of the areas causes the fluctuation of the frequencies of each and every area and also there is fluctuation of power in tie line. For satisfactory operation of a power system, frequency should remain nearly constant. Frequency deviations can directly impact on a power system operation, system stability, reliability and efficiency. Large frequency deviations can damage equipments, degrade load performance, overload transmission lines and affect the performance of system protection schemes. These large-frequency deviation events can ultimately lead to a system collapse. A Load Frequency Control (LFC) scheme basically incorporates an appropriate control system for an interconnected power system, which is heaving the capability to bring the frequencies of each area and the tie line powers back to original set point values or very nearer to set point values effectively after the sudden load change. This can be achieved by the use of conventional and modern controllers. Modern controllers have a lot of advantages over conventional integral controller. They are much faster than integral controllers and also they give better stability response than integral controllers. In this proposed research work PID controller has been applied for LFC of two area power systems. The parameters of the PID controller are tuned by two different methods names as (1) Ziegler-Nichols (Z-N) Method and (2) Simplex Search Method for better results. It is seen that the results obtained are far better than the conventional controller.

Simultaneous Estimation of the Time of Disturbance and Inertia in Power Systems

Offnominal power system frequency regimes play a key role in the propagation of disturbed system conditions and blackouts. The evolution of power systems will make the availability of frequency-control services more uncertain, increasing the risk of large frequency deviations. Furthermore, the reduction in the inherent inertia of power systems and their increased variability will be a critical threat to the quality of frequency control. This paper proposes an online algorithm for estimating the time of disturbance and inertia after a disturbance. This is in contrast to previous inertia estimation methods that operated offline. The algorithm processes wide-area measurements of frequency and active power and could provide support for adaptive frequency control. Simulations, laboratory tests, and real transmission system measurements have been used to demonstrate the algorithm's ability to correctly detect a disturbance, accurately estimate the time of the disturbance and the inertia, and prevent false disturbance detections.