Active Power Fluctuations Research Articles

Auto-Tracking Frequency Regulation Based Energy Saving Technology for Motor Systems With Dynamic and Potential Energy Load

Beam pumping motor system (BPMS) is a typical periodic potential energy load which is affected by factors such as down-hole load, mechanical transmission and geological structure. During a running cycle, the power fluctuates in a large range and multiple operating conditions where potential energy, electrical energy and kinetic energy are converted to one another and thus leads to lower system efficiency. For this type of load, a four-quadrant inverter is an effective method to improve the efficiency of the motor system. However, in practice, the two-quadrant variable frequency control is widely used due to its lower cost. The drawback of this control is the difficulty in realizing efficient conversion and utilization of electrical energy under the power generation mode, since the generated electrical energy is converted into thermal energy by the installed resistance brake inside the inverter, which causes additional energy consumption. To resolve this problem, this paper systematically studies the influence of frequency regulation on the load torque of BPMS, an automatic tracking frequency regulation method is then proposed. With this method, the controllable conversion between the kinetic and potential energy forms can be realized, and the fluctuation power under dynamic load conditions can also be smoothed. Meanwhile, energy consumption in power generation mode is also reduced effectively. The correctness and practicability of the proposed method are verified by the simulations and experiments of a standard oil well. The results show that, comparing to the traditional two-quadrant inverter, the proposed method can reduce the active power fluctuation of the system by about 20% under the premise of still using a two-quadrant inverter, and the power saving ratio is increased by more than 10%.

Enhanced Control Scheme for a Three-Phase Grid-Connected PV Inverter under Unbalanced Fault Conditions

This paper presents an improved control strategy to cancel the double grid frequency oscillations in the active power, reactive power, and DC-link voltage of a three-phase grid-connected photovoltaic (PV) system under unbalanced grid condition. To achieve these goals, an enhanced positive–negative-sequence control (PNSC) to remove oscillations of active power and an instantaneous active–reactive control (IARC) to mitigate the fluctuations of active and reactive power, simultaneously, are suggested. These methods are also effective to reduce the oscillations of the DC-link voltage. To track the desired unbalanced or harmonic reference currents, improved proportional resonant (PR) current controllers have been designed using the Bode frequency analysis. Simulation studies are carried out via Matlab/Simulink® software to verify the effectiveness of the suggested control strategies.

Quantifying robustness of Type 4 wind power plant as reactive power source

The quantification of reactive power reserves can be essential for transmission system operators for the secure operation of large power systems with high share of renewables. Reactive power support from renewable energy sources needs to be included in reactive power reserve estimation. The reactive power capability of variable renewable energy sources like wind power plants depends on active power production. The stochastic nature of wind speed, and thereby active power production, makes reactive power reserve estimation quite complex. This article proposes a novel methodology to estimate reactive power reserve of wind power plants, taking the uncertainties such as the active power fluctuation, the technical availability of wind turbines as well as the on-load tap changer position of wind power plant transformers into account. The methodology incorporates the definition and determination of robustness index for wind power plant as a reactive power source. The proposed methodology is evaluated on a test system, by assessing the impact of the robustness index on long-term voltage stability. The results show that the choice of an appropriate time period for assessment of wind power plant as a reactive power source is critical to have a high degree of confidence in reactive power reserve estimation. Furthermore, the investigation reveals the importance of considering the power fluctuation and availability in the estimation of the reactive power reserve. The presented case study shows that ignoring fluctuation and availability can lead to overestimation of maximum power transfer, resulting in wrong assessment of the power system security. The proposed methodology can be used by the transmission system operators to quantify the reactive power reserve in real-time and allow better integration of wind power plants in voltage stability assessment studies.

Operation Control for Improving Energy Efficiency of Shipboard Microgrid Including Bow Thrusters and Hybrid Energy Storages

The frequent start/stop operation of a bow thruster presents a high pulsating power demand in shipboard microgrids while reducing the fuel efficiency of the diesel engines, which may induce potential instabilities. In this article, a hybrid energy storage system, including batteries and ultracapacitors (UCs), is connected to the dc bus of the thruster driver, and a hierarchical controller is proposed for the bidirectional dc/dc converters that interface storages, thus providing the following functions: 1) the diesel generator (DG) sets provide the required average power to the bow thruster; 2) the batteries smooth the active power fluctuations; and 3) the UCs provide the pulsating active power due to the slow charging response of batteries. A $V - I$ droop control and high-/low-pass filters are used inside the primary controller to proper power sharing between storages in different time scales. The secondary controller is developed to ensure the power from the DG sets to the bow thruster being equal to the required average power. Simulation results shows that this control scheme can effectively reduce the capacity of DG sets and batteries and can improve the fuel consumption efficiency. When the thruster machine operates as a generator during fast breaking, the hybrid energy storage can absorb the braking energy.

Mechanism research on active power fluctuation caused by servo valve jamming of turbo-generator

To solve forced power oscillations arising from a thermal power unit’s load fluctuation frequently, load control mechanisms and solutions were studied in combining with these actual accidents data. This paper comprehensively analyzes the load control principle and provides a basic accident handling procedure according to the unit’s actual condition. According to analysis, inspection and dynamic test results, load fluctuation causes locked-in-place jamming of servo valve. An actuator model is established to simulate the unit’s operation under different control parameters. The simulation results show that setting unreasonable control parameters intensifies load fluctuations into forced load oscillation, which is verified by dynamic testing of the unit and the effect of control optimization on operation. This analysis and the resolution of this issue are effective and accurate, providing a valuable reference in the event of a similar accident. It is determined that strict process control of overhaul and in operation can reduce the probability of load fluctuation, and control parameter optimization can avoid forced load oscillation.

RETRACTED: Performance enhancement of grid-interfaced inverter using intelligent controller

The high-performance grid-interfaced inverters are in demand as they are rapidly used in renewable energy systems. The main objective of grid-interfaced inverters is to inject high-quality active and reactive power with sinusoidal current. Many control schemes have been proposed earlier in the literature, but the operation under parametric uncertainties has not been given much attention. In this article, an adaptive network–based fuzzy inference control algorithm for a three-phase grid-interfaced inverter under parametric uncertainties is proposed. The main purpose of the proposed technique is to enhance the response time, decrease the steady-state oscillation in the injected active and reactive power and enhance the power quality even with parametric uncertainties. For assessment and evaluation reason, the conventional proportional–integral control is compared with the proposed controller. For a fair comparison, the gain setting for the proportional–integral control is obtained by Particle swarm optimization algorithm. The suggested system is developed and simulated in MATLAB/Simulink. Simulation results demonstrate that both the controllers work well to regulate the powers to required values, even with parametric variations. However, the proposed control demonstrates superiority in comparison to conventional proportional–integral control in terms of speedy response, decreased steady-state fluctuations, better power quality and increased robustness. The rise time and fluctuations in the per-unit active and reactive power are much less with the proposed control. Total harmonic distortion of the injected current and grid current are significantly better than the conventional proportional–integral control.

An asynchronized synchronous machine to control a flexible grid operation

In this paper, a system of two asynchronized synchronous machines is proposed to control a power-flow in a meshed grid of 220/500 kV and a control algorithm, including a system parameterization guide. The proposed system is supposed to qualitatively affect the parameters of steady-state modes and shirt-circuit-caused transient processes and to increase the transfer capacity of the grid. Wherein, this double-fed synchronous machines complex allows for a significant increase in the power quality and reliability within the grid’s normal operation. Long-term dynamics is considered. As a result, it is shown that the proposed system parametrization provides a significant decrease in active power fluctuations and power system swinging.

Model Predictive Control Method of hybrid Battery energy storage system for Smoothing Wind Power Fluctuation

In a large-scale wind power generation system, active power fluctuation caused by random wind speed will have a serious impact on the power grid. In order to limit the power fluctuation that wind farm transmits to the power grid and protect the energy storage battery, this paper has proposed a model predictive control method of hybrid energy storage by optimizing the objective function and constraint conditions. Firstly, the mathematical model of predictive control method has been established in a wind power system with hybrid energy storage. Then, with the goal of minimum energy storage output and maximum charging-discharging capacity of the super-capacitor, the predictive control process has been optimized. Meanwhile, the constraint on the output power of the battery has been dynamically changed to reduce its charging-discharging switching frequency, and the model predictive control strategy of the hybrid energy storage has been formed. Finally, compared with the model prediction control method of single energy storage, based on a wind farm data, the simulation results show that the proposed method can smooth wind power fluctuation, reduce the time that the power does not satisfy the fluctuation requirements, ensure the capability of the super-capacitor, and reduce the charging-discharging switching frequency of the energy storage battery.

Analysis of Dynamic Voltage Fluctuation Mechanism in Interconnected Power Grid with Stochastic Power Disturbances

With the integration of large-scale renewable energy and various new types of loads, the stochastic fluctuation characteristic of the power has seriously affected the secure and stable operation of interconnected power grids. In addition to active power fluctuations of AC tie-lines that have been studied in the past, the problem of dynamic fluctuation of bus voltage is also increasingly prominent. Firstly, the typical power fluctuations of electric railway traction load, smelting load, and wind power generation in modern power systems are presented and the accompanying voltage fluctuations are analyzed. Secondly, the dynamic fluctuation mechanism and distribution characteristic of the system voltage under special forced oscillation and generalized forced oscillation are studied. It is found that when the periodic disturbance induces a special forced oscillation, the voltage fluctuation has the same mode information compared to the power angle fluctuation. And it also has obvious peak characteristics near the natural frequency. When the stochastic disturbance induces generalized forced oscillation, the voltage fluctuations can be decomposed into proportional and resonant components according to its frequency domain characteristic. Finally, the above conclusions are verified by the simulation results of an IEEE 4-generator 2-area system.

Enhanced Current Reference Calculation to Avoid Harmonic Active Power Oscillations

Energy storage systems play a key role in the rise of distributed power generation systems, hence there is great interest in extending their lifetimes, which are directly related to DC current ripple. One of the ripple sources is the low-frequency active power fluctuations under unbalanced and distorted grid voltage conditions. Therefore, this paper addresses a multifrequency control strategy where the harmonic reference currents are calculated to reduce harmonic active power oscillations. The stationary reference frame (StRF) approach taken here improves the precision and computational time of the current reference calculation method. Additionally, in order to ensure safe converter operation when a multifrequency reference current is provided, a computational efficient peak current saturator is applied while avoiding signal distortion every time step. If the injected current harmonic distortion is to be minimized, which is a feature included in this work, the peak current saturator is a necessary requirement. Active power ripple is reduced even with frequency variations in the grid voltage using a well-known frequency-adaptive scheme. The simulation and experimental results prove the optimized performance for the control objective: power ripple reduction with minimum current harmonic distortion.

INVESTIGATION OF THE MODE OF A THREE-PHASE ACTIVE POWER FILTER IN THE POWER CIRCUIT OF AN ARC FURNACE AT OSCILLATIONS OF ACTIVE POWER

Arc furnace is a common smelting furnace for metallurgical production. At the same time, there is a stochastic operating mode in the arc furnace, that widespread in the industry. The combustion arc in the arc furnace is accompanied by a generation to the power supply network of reactive power, higher harmonics and there is asymmetry of currents. This leads to fluctuations not only in reactive power, but also causes fluctuations in active power. Static compensating devices such as condenser batteries, power passive filters, synchronous compensators do not have time to react to the sharply variable operating mode of the arc furnace. The latest advances in the compensation of inactive components power are the use of threephase active power filters, which do not have the phenomenon of resonance of voltages and currents, as well as they react to change the load during their work. In the work the model of the three-phase active power filter with an arc furnace to the visual modeling environment is synthesized. The parameters of the arc furnace substitution scheme are brought to the low side of the electric transformer the furnace. Parameters of three-phase active power filter are calculated for this type of arc furnace. In the course of the study, obtained the data of the consumption of active and reactive power of the load and the network, as well as the coefficient of distortion of the sinusoidality of the current load curve and the network. The received oscillogram of network voltage, load current, current active power filter, network current and spectral diagram of higher harmonics of load current and network. All the results obtained were analyzed and perform conclusions for them.

Real-Time Identification of Power Fluctuations Based on LSTM Recurrent Neural Network: A Case Study on Singapore Power System

Fast and stochastic power fluctuations caused by renewable energy sources and flexible loads have significantly deteriorated the frequency performance of modern power systems. Power system frequency control aims to achieve real-time power balance between generations and loads. In practice, it is much more difficult to exactly acquire the values of unbalance power in both transmission and distribution systems, especially when there is a high penetration level of renewable energies. This paper explores a deep learning approach to identify active power fluctuations in real-time, which is based on a long short-term memory recurrent neural network. The developed method provides a more accurate and faster estimation of the value of power fluctuations from the real-time measured frequency signal. The identified power fluctuations can serve as control reference so that the system frequency can be better maintained by automatic generation control, as well as emerging frequency control elements, such as energy storage system. A detailed model of Singapore power system integrated with distributed energy storage systems is used to verify the proposed method and to compare with various classical methods. The simulation results clearly demonstrate the necessity for power fluctuation identification, and the advantages of the proposed method.

Coordinated‐control strategy of scalable superconducting magnetic energy storage under an unbalanced voltage condition

Modular multilevel converters (MMCs) have the advantages of high-power density and small-harmonic distortion because of their modularity and flexibility, thus providing a new avenue for research into scalable superconducting magnetic energy storage (SMES) in renewable energy generation. This study presents coordinated control for a three-phase four-wire (3P4W) MMC-based SMES system under unbalanced voltage applications. First, the positive- and negative-sequence mathematical models and the port-controlled Hamiltonian with dissipation model of the 3P4W MMC-SMES system are established by introducing an additional path for zero-sequence current. Second, a multi-objective passivity-based control strategy that can effectively improve the power quality and system robustness and eliminate both double-frequency active and reactive power fluctuations or double-frequency active power fluctuation and negative-sequence current is proposed. The simulation results based on MATLAB/Simulink demonstrate the effectiveness of the proposed topology of the SMES and its control strategy.

A Fault-Tolerant Operation Approach for Grid-Tied Five-Phase Current-Source Converters With One-Phase Supplying Wire Broken

In this paper, an enhanced fault-tolerant operation approach is developed to maintain continued operation of five-phase grid-tied current-source converters (CSCs) during one-phase supplying wire broken. First, the instantaneous power flow of this system is analyzed based on the sequence decomposition method. It is found that through proper arrangement of positive-, negative-, and pseudozero-sequence components of line current reference and compensation of filter CL current offset, the CSC system can achieve reduction of instantaneous active power fluctuation and minimization of line current magnitude at the same time. Accordingly, multiphase CSCs are able to operate with smaller dc rail current ripples and to obtain more active power transfer capability during fault tolerant operation. In addition, this paper also briefly discusses the control of multiphase systems in the case of complex grid faults with simultaneous wire broken and imbalanced grid voltage dips. It is found that an intentional constant reactive power injection can help to reduce the peak current of the five-phase grid-tied system. Furthermore, it should be noted that the proposed method is realized in a direct current modulation manner. Thus, a large number of line current acquisition devices for conventional multiphase grid-tied converters are removed. Finally, this paper also discusses the modulation method of five-phase CSC with one-phase wire broken. A modified “ampere-second balance” principle is applied to direct obtain the gating signals that can satisfy the constraints of CSCs.

Facilitating static firm frequency response with aggregated networks of commercial food refrigeration systems

Aggregated electrical loads from massive numbers of distributed retail refrigeration systems could have a significant role in frequency balancing services. To date, no study has realised effective engineering applications of static firm frequency response to these aggregated networks. Here, the authors present a novel and validated approach that enables large scale control of distributed retail refrigeration assets. The authors show a validated model that simulates the operation of retail refrigerators comprising centralised compressor packs feeding multiple in-store display cases. The model was used to determine an optimal control strategy that both minimised the engineering risk to the pack during shut down and potential impacts to food safety. The authors show that following a load shedding frequency response trigger the pack should be allowed to maintain operation but with increased suction pressure set-point. This reduces compressor load whilst enabling a continuous flow of refrigerant to food cases. In addition, the authors simulated an aggregated response of up to three hundred compressor packs (over 2 MW capacity), with refrigeration cases on hysteresis and modulation control. Hysteresis control, compared to modulation, led to undesired load oscillations when the system recovers after a frequency balancing event. Transient responses of the system during the event showed significant fluctuations of active power when compressor network responds to both primary and secondary parts of a frequency balancing event. Enabling frequency response within this system is demonstrated by linking the aggregated refrigeration loads with a simplified power grid model that simulates a power loss incident.

Decoupled Power Control for a Modular-Multilevel-Converter-Based Hybrid AC–DC Grid Integrated With Hybrid Energy Storage

This paper presents a decoupled power control strategy for a modular multilevel converter (MMC)-based hybrid ac–dc grid integrated with a hybrid energy storage system. This system can mitigate the active power fluctuations caused by intermittent renewable generation and also realize reactive power compensation as required by voltage regulation. The proposed hybrid ac–dc system has a novel configuration in which batteries and supercapacitors are distributed into the upper arm and the lower arm of the MMC, respectively. Therefore, it is possible to design fully decoupled power control and simultaneously optimize the performances of batteries and supercapacitors. First, the system modeling and operational principles of the MMC are briefly introduced in this paper. On the basis of these analyses, the power decoupling between the upper arm and the lower arm is presented to realize different control objectives. Moreover, battery state-of-charge balancing control and supercapacitor voltage control are also presented, which are crucial for ensuring the normal operation of these energy storage components and the proposed MMC-based hybrid ac–dc grid. Finally, the proposed system and its control strategies are verified by laboratory experimental results.

Adjustable loads control and stochastic stability analysis for multi-energy generation system based on Markov model

This paper mainly addresses the issue of the power generation fluctuations due to the stochastic characteristics of the renewable energies, which threaten the stability of the power grid. First, the multi-energy generation system with integration of wind and photovoltaic power is modeled in the framework of Markov model to describe the dynamic changes. Second, to weaken the active power fluctuation of renewable energies generation and improve the utilization of renewable energies, the flexible loads are added for local consumption and an adjustable load control strategy is proposed to guarantee the output power continuous stability of the renewable energies generation. Then, the stochastic stability is analyzed by using Markov stability theory. Finally, the simulation results and analysis are provided to illustrate the effectiveness of the proposed schemes.

LS-Solar-PV System Impact on Line Protection

Large-scale photovoltaic power station access to the grid will profoundly change the fault current characteristics of the power station’s outgoing lines. This change results in adaptive problems in traditional protection phase selection components, which may cause incorrect actions in reclosing, protection ranging, and distance protection. Based on the fault current characteristics of the large-scale photovoltaic power station transmission line, this paper analyzes the adaptability of the phase current difference mutation and the sequence component phase selection component in protecting the Photovoltaic (PV) power plant side of the transmission line. Based on the fault current analytical formula, the phase relationship between the phase current difference and the current sequence component under different control targets, such as suppressing negative sequence current, suppressing the active power fluctuation, and suppressing the reactive power fluctuation, is derived. The operational performances of the phase–phase current difference of the abrupt phase selection component and the sequence component phase selection component of the power station side are degraded, which may cause incorrect operation of the phase selection component. Based on the actual engineering parameters of a PV power plant, a simulation model was built in Power System Computer Aided Design (PSCAD) to verify the correctness of the theoretical analysis.

Enhancing FRT performance and smoothing output power of DFIG wind farm equipped by SFCL and SMES in a fuzzy framework

Wind farms equipped by Doubly-Fed Induction Generators (DFIG) should have two capabilities so that system operators can efficiently utilizes them. These capabilities include Fault-Ride-Through (FRT) and smoothing the output active power fluctuations, especially when these generators provide significant electrical power. The Superconducting Fault Current Limiter (SFCL) and also the Superconducting Magnetic Energy Storage (SMES) are supplementary devices which are used to enhance these capabilities and reduce electrical power swings. In the present work, improvement of FRT capability and smoothing the output power of wind farms are formulated as a multi-objective problem in a fuzzy framework. The variables of optimization are the Proportional-Integral (PI) gains of DFIG and SMES controllers and also SMES and SFCL parameters. Minimization of the initial energy stored in the SMES unit, the energy losses of SFCL, deviations of the DC-link voltage of DFIG, deviations of output active power of DFIG, deviations of output voltage of DFIG, and DFIG speed deviations are six objective functions of the problem. These objective functions are scaled by a fuzzy operator, then the scaled objective functions are aggregated by the “max-geometric mean” operator to obtain the multi-objective function. For optimizing this multi-objective function, the Hybrid Big Bang Big Crunch (HBB-BC) as a meta-heuristic optimization algorithm is used. The proposed algorithm is implemented on a case study to numerically evaluate its efficiency. Simulation results show that the proposed algorithm is more effective to enhance FRT capability and smooth output power of wind farm in comparison with the other algorithms.

Improved Reference Generation of Active and Reactive Power for Matrix Converter With Model Predictive Control Under Input Disturbances

The existing model predictive control schemes for the matrix converter perform well under ideal input conditions, but are not optimal when source voltages are unbalanced and distorted. The analysis shows that, under input disturbances, the input filter capacitors cause active power fluctuation and further output power quality degradation, which has been ignored in literature. Therefore, a method is proposed to extract the fluctuation with a digital filter and compensate it in the reference generation of source active power, so as to eliminate the output low-frequency harmonics. In the case of input voltage unbalance, a method is proposed to generate the reference source reactive power which could achieve sinusoidal source currents, without the need of separating sequence components. Moreover, effects of source voltage distortions, which degrade the input power quality significantly and are ignored in literature, are suppressed with the proposed generation method of reference reactive power. The extensive experimental results in nine cases have verified the satisfactory steady-state and dynamic performance of the proposed methods.