Inertia Time Constant Research Articles

Study on the stability and ultra-low frequency oscillation suppression method of pumped storage power plant with dual units sharing one pipeline

This paper aims to investigate the stability of the pumped storage power plant (PSPP) with dual units sharing one pipeline (DUSOP) and the method of suppressing the ultra-low frequency oscillation (ULFO) characteristics. Firstly, four nonlinear models of pumped storage regulation system (PSRS) with DUSOP are established under the design scenarios of no surge tank, upstream surge tank (UST), downstream surge tank, and upstream and downstream dual surge tanks. Then the stability of the PSRS under different surge tank arrangements (STAs) in the long diversion system design scenarios (LDSDS) is comparatively analyzed. Finally, based on the concept of stability margin, the influence mechanism of the operating points on the ULFO characteristics is investigated in the LDSDS with UST, and the influence of the governor parameters, hydrodynamic parameters, and mechanical parameters on the damping ratio and frequency of the system is also analyzed. The results show that among STAs, the design scheme of arranging the UST is more reasonable for PSPP with LDSDS from the comprehensive consideration of the economy of the project and the stability of the system. For PSPP with UST, on the line of equal stability margin, increasing the proportional gain, reducing the unit inertia time constant, reducing the water flow inertia time constant, and appropriately increasing the area of the surge tank can increase the frequency and reduce the amplitude, which is conducive to the suppression of ULFO. In the stability domain range, increasing the proportional gain increases the frequency and the system stability increases and then decreases. Increasing the integral gain decreases the frequency and then increases the system stability. At the same time, the frequency of the unit can be changed by adjusting the parameters of the system to avoid resonance between the frequency of the unit and the frequency of the power grid and to ensure the stability and safety of the unit operation.

Influence of system inertia time constant on transient stability level of AC power grid

AbstractPower system inertia is receiving increasing attention because the inclusion of renewable energy generation equipment weakens the inertia of the regional equivalent system and affects the transient stability of the system. In this paper, the influence of variation of inertia constant on the transient stability of the system is studied analytically by adding short‐circuit faults, Equal Area Criterion, and derivation of equations. On the basis of theoretical analysis, multi‐machine system is established to simulate and verify the correctness of the analysis results.

Stability and sensitivity characteristic analysis for the hydropower unit considering the sloping roof tailrace tunnel and coupling effect of the power grid

This paper focuses on the stability and dynamic characteristics of the coupled system of nonlinear hydraulic turbine regulating system (HTRS) and power grid (PG). By establishing a nonlinear mathematical model considering the downstream surge chamber and sloping roof tailrace tunnel, the coupling effect and influence mechanism between the hydropower station and power grid are revealed. First, with regard to the coupled system, HTRS considering downstream surge chamber and sloping roof tailrace tunnel and PG model is established. Then, dynamic performance of the coupled system is investigated based on the nonlinear mathematical model as well as Hopf bifurcation theory and validated by numerical simulation. Meanwhile, the impact mechanism of HTRS and PG is revealed by investigating dynamic characteristics. In addition, stability is studied by using eigenvalue method according to the Jacobian matrix of the coupled system. Finally, parameter sensitivity is investigated to quantify parameter effects on system performance. The experimental results indicate that bifurcation line divides the whole proportional–integral adjustment coefficient plane into two parts and the region at the bottom of bifurcation line is stability region. HTRS and PG possess a coupling effect on stable domain and dynamic properties of the coupled system. The variation of HTRS parameters is most significant for the coupled system, especially for the inertia time constant of the hydraulic turbine unit and penstock flow inertia time constant.

Inertia Evaluation of Wind Farm Under Overspeed Power Shedding Control

As the penetration rate of wind power generation gradually increases in most grids, it is necessary to evaluate wind farms’ inertia to provide inertia information for electric grids and boost grid stability. This paper firstly models the inertia control mode of overspeed power shedding, which can realize the inertia control under the operation of different blade tip speed ratios, so that the wind turbine has a high inertia support capability. Then the energy change process in the inertia control process is analyzed, and the virtual inertia time constant is derived to evaluate a single unit’s inertia. Finally, a simple wind condition and the wind farm’s inertia control requirements in a day are simulated using an equivalent model, and the inertia level of the wind farm is calculated. The proposed research approach can alleviate the sophistication of inertia evaluation which also effectively consider the influence of inertia control.

Two-Stage Grid-Connected Frequency Regulation Control Strategy Based on Photovoltaic Power Prediction

The large number of photovoltaics connected to the distribution network via power electronic converters squeezes the functional space of traditional synchronous generators in the power system and reduces the inertia of the network itself. However, due to the random and fluctuating nature of PV power generation, different types of meteorological conditions can also affect the inertia support capability of PV output power. Therefore, this paper proposes a frequency regulation control strategy based on the dynamic characteristics of the grid-side DC capacitor. Firstly, the control strategy of the grid-side inverter is improved and the mechanism of the frequency dynamic response model under PV penetration is analysed. Secondly, data from different weather types are correlated and analysed to predict PV power, and wireless sensor technology is used to introduce the data signals into the control link. Finally, a simulation model of a two-stage PV power generation system is developed and the fast-response capability of the system under different control parameters when the load is increased or decreased is analysed. The results show that under the proposed frequency regulation strategy, the larger the regulation coefficient and virtual inertia time constant, the greater the virtual inertia provided by the PV power generation, which improves the stability of the distribution network.

Research on multi-energy cooperative participation of grid frequency inertia response control strategy for energy storage type doubly-fed wind turbine considering wind speed disturbance

With the proposal of carbon peaking and carbon neutralization, the penetration rate of wind power generation continues to increase. This paper focuses on the problem that doubly fed induction wind turbines are vulnerable to input “source” disturbances and have weak frequency modulation ability, which reduces the stability of the power grid. Based on the structural model of energy storage system embedded in doubly fed wind power generation system, it is compared the ability of super capacitor energy storage and releasing rotor kinetic energy to provide inertia response power and energy, and the feasibility of multi-energy coordinated inertia response is analyzed. Based on the inertia time constant of conventional synchronous generator set, the inertia time constant and actual inertia constant of energy storage doubly fed wind power generation system under variable wind speed are defined. An extended state observer is used to estimate the change of captured mechanical power caused by the change of wind speed, and a control strategy for doubly fed induction generator with super capacitor to participate in power grid frequency regulation is designed. Finally, considering the aggregation of wind power and the difference of the state of charge during the operation of distributed energy storage, the 3*3*3 wind farm model is established using Matlab/Simulink simulation software. The feasibility and advantages of the frequency modulation control strategy proposed in this paper are verified by building a power grid frequency modulation simulation involving wind farms and traditional generators.

Study on dynamic model and dynamic characteristics of Delingha 50 MW trough solar field

China General Nuclear Power Group (CGNPC) Delingha 50 MW parabolic trough solar thermal power plant is the first commercial trough solar plant in China, and its solar field consists of 190 parallel heat collecting loops. For large-scale trough solar plant, balancing the flow rate of heat absorbing medium in each loop and stabilizing the outlet temperature are the key technologies and difficult problems. Regarding the Delingha 50 MW solar field as the research object, this paper mainly focusing on the dynamic characteristics of flow and heat transfer in the solar field. The hydrodynamic calculation model and the heat transfer dynamic model are established on the real-time dynamic simulation platform STAR-90. With the actual operation data, the built solar field model is validated by comparing the simulation results. On this basis, the disturbance simulations of direct normal irradiance (DNI), heat transfer oil’s mass flow and heat transfer oil’s inlet temperature are carried out. The dynamic response curves of disturbance and the thermal inertia time constant of the loops are obtained. The conclusions lay a theoretical foundation for the formulation of outlet medium’s temperature control strategy in the solar field of large-scale trough solar thermal power generation system.

A Novel Probabilistic Method for Under Frequency Load Shedding Setting Considering Wind Turbine Response

The Under Frequency Load Shedding (UFLS) is the last automated action to restore power system frequency stability when a severe contingency causes a significant drop in the frequency of interconnection or islanded areas. In this paper, a novel probabilistic algorithm that considers the wind turbine frequency response is proposed to design and optimize the UFLS scheme. The uncertainties of power systems such as wind speed, hourly load, inertia time constant, and generation losses are taken into account. The Monte-Carlo Simulation (MCS) method is used to model the power system uncertainties. Additionally, a novel method is developed to calculate the power system frequency response and accelerate the multistage UFLS scheme optimization processes. In this method, a time-independent formulation for power system frequency response is utilized to reduce the computational complexity. More than Twenty different UFLS plans are designed using the proposed probabilistic algorithm. Besides, some time-domain simulations are carried on the modified IEEE 39-bus test system to investigate the performance of the designed UFLS schemes. Results indicate that designed UFLS schemes provide stable frequency response and improve the power system frequency stability.

Transient stability enhancement in multiple‐microgrid networks by cloud energy storage system alongside considering protection system limitations

AbstractThe gas turbine synchronous generators (GTSGs) are widely deployed as distributed generations (DGs) in countries with massive natural gas production owing to their low prices. However, due to the low inertia time constants of these synchronous‐based DGs, they are more susceptible to power grid faults which stands as a transient stability issue in networks with multiple microgrids (MGs). On the other hand, the cloud energy storage system (CESS) is a new concept that centralizes the individual distributed energy storage of one or more MGs. Here, the employment of CESS with synchronverter grid connections creates a suitable opportunity for improving the transient stability of the network by providing higher inertia. This is while the fault current contribution of the synchronverter‐based CESS imperils protection constraints in these networks. Therefore, a proper protection coordination index (PCI) is considered in the conducted study to identify the optimal size of the synchronverter‐based CESS through a two‐stage optimization algorithm that preserves the protection constraints among protective devices. Finally, the transient stability of the network with synchronverter‐based CESS is assayed by calculation of the critical clearing time (CCT) for faults. Numerical studies are carried out on the IEEE 33‐bus test system. Results are discussed in depth.

Nonlinear Modeling and Stability of a Doubly-Fed Variable Speed Pumped Storage Power Station with Surge Tank Considering Nonlinear Pump Turbine Characteristics

This paper investigates the nonlinear modeling and stability of a doubly-fed variable speed pumped storage power station (DFVSPSPS). Firstly, the mathematical model of DFVSPSPS with surge tank considering nonlinear pump turbine characteristics was derived and established. Then, Hopf bifurcation analysis of DFVSPSPS was performed. The stable region was identified and verified by example analysis. Moreover, the effect mechanism of nonlinear pump turbine characteristics on the stability of DFVSPSPS was explored. Finally, the influence of factors on the stability and dynamic response of DFVSPSPS was studied. The results indicate that the emerged Hopf bifurcation of DFVSPSPS is supercritical and the region on the low side of the bifurcation line is the stable region. Nonlinear head characteristics have a significant influence on the stability and dynamic response of DFVSPSPS. Nonlinear speed characteristics have an obvious effect on the stability and dynamic response of DFVSPSPS only under positive load disturbance and unstable surge tank. Nonlinear head characteristics are unfavorable for the stability of DFVSPSPS under positive load disturbance and favorable under negative load disturbance. A smaller flow inertia of penstock, a smaller head loss of penstock and a greater unit inertia time constant are favorable for the stability of DFVSPSPS. The stable region under the positive disturbance of active power is larger than that under the negative disturbance of active power. The time constant of the surge tank presents a saturation characteristic on the stability of DFVSPSPS.

Power system equivalent inertia evaluation algorithm based on intelligent optimization

To improve the accuracy of the evaluation of the equivalent inertia of a power system, a method for evaluating the equivalent inertia level of a power system based on an intelligent optimization algorithm is proposed. The proposed method constructs the objective function based on different degrees of system perturbation, ignores the frequency characteristics of the load, constructs the objective function with the generator swing equation, solves the inertia time constant of each generator using the intelligent optimization algorithm, and then evaluates the equivalent inertia of the whole system, eliminates the influence of inconsistent frequency change rate under perturbation on the system equivalent inertia evaluation, and thus obtains a more accurate system equivalent inertia evaluation, experimental result shows the evaluation accuracy can be improved by 10.79% and 3.48% respectively compared with other two comparison method.

Variable-Inertia Emulation Control Scheme for VSC-HVDC Transmission Systems

This paper proposes a novel variable-inertia emulation control (VIEC) scheme that enables voltage-source-converter based high-voltage DC (VSC-HVDC) transmission systems to flexibly support AC grid frequency stability like synchronous generators. The VIEC scheme allows us to extract the energy from the augmented DC capacitance for inertia emulation by controlling the DC voltage without affecting the stability of the AC system connected on the remote side. In particular, with the proposed VIEC scheme, the inertia time constant can be flexibly emulated and its value can be automatically adjusted according to the rate of change of grid frequency as well as grid frequency deviations. This leads to more effective inertial support across different timescales. Modal analysis is carried out to investigate the impacts of inertia and DC capacitance, and to obtain the optimal control parameters. The effectiveness and advantages of the proposed VIEC scheme are demonstrated on an IEEE benchmark system in the presence of faults and load changes.

Study on Dynamic Model and Dynamic Characteristics of Solar Field of 50 Mw Trough Solar Thermal Power Generation Plant

Solar thermal power generation solves the problem of fluctuation and intermittency through energy storage system, and can bear the basic power load and peak load regulation. It is an important technical way for China's energy structure transformation. As a complex thermal system with multivariable disturbance, large inertia and large delay, the operation control of the solar field is a difficult issue. In this paper, the solar field of the first 50 MW trough solar thermal demonstration power plant in China is taken as the research object. Based on the topological structure and working principle of the solar field and the basic laws of optics and thermodynamics, the hydrodynamic calculation model and the heat transfer dynamic model are developed. The correctness of the model is verified by comparing the simulation results with the actual operation data. On this basis, the disturbance simulations are carried out. In addition, the dynamic response curves of step disturbance and the inertia time constant of the heat collecting loops are obtained. The conclusions of this paper have important guiding significance for the operation control strategy formulation and system optimization design of trough solar thermal power generation system.

Hopf Bifurcation and Parameter Sensitivity Analysis of a Doubly-Fed Variable-Speed Pumped Storage Unit

The doubly-fed variable speed pumped storage unit is a storage system suitable for joint operation with renewable energy sources to smooth the imbalance between renewable energy supply and electricity demand. However, its working principle and operation control are more complex than those of constant speed pumped storage. In this study, a nonlinear model of doubly-fed variable speed pumped storage units (VSPSUs) considering nonlinear characteristics of the head loss is established. The study finds that a supercritical Hopf bifurcation occurs in the system, and the area enclosed by the lower side of the bifurcation line and the coordinate axis is the stability domain of the system. The active power step perturbation from −0.3 to 0.3 will gradually reduce the area of the stability domain and narrow the adjustable range of the control parameters. In addition, the sensitivity of the model full state variables and the primary and secondary relationships to the changes of subsystem parameters is analyzed systematically using the trajectory sensitivity. It is found that there is a large difference in the sensitivity of different state variables to the parameters. The state variables are much more sensitive to the transfer coefficient of hydraulic turbine torque to guide vane opening, the unit inertia time constant, and the controller proportional gain change than other parameters, which are defined as highly sensitive parameters. The receiver response time constant and the turbine flow-to-head transfer coefficient are the corresponding low-sensitivity parameters.

A start-up optimization strategy of a hydroelectric generating system: From a symmetrical structure to asymmetric structure on diversion pipes

This study focuses on the start-up optimization strategies for a Hydroelectric Generating System (HGS). A mathematical model of HGS to describe the complex hydraulic coupling relationship, between the water diversion pipes, the hydro-turbine, the governor and the generator, is proposed. This allows to accurately describe the start-up process and identifying the effect of successive start-up time interval (ΔT) and the pipe structure on the water head and the rotational speed of HGS. The result shows that the successive start-up strategy with ΔT = 9s maximally reduces the water head fluctuation of units #1 and #2 by 30.94% and 25.77% compared with the simultaneous start-up strategy when the branch pipe is symmetric. For the asymmetric branch pipe, the unit behind branch B# starts up first is beneficial to reduce the water head fluctuation of the unit behind branch #A when the flow inertia time constant of branch #A is larger. Finally, the parameters of the start opening as well the PID governor parameters are optimized by a multi-objective particle swarm optimization for the asymmetric pipe in order to minimize the unit speed overshoot and the speed rise time. These results in this work provide a theoretical guidance for the selection of optimal start-up strategy in the case of symmetric and asymmetric HGS branch pipe structures.

A New DC Microgrid Voltage Regulation Control Strategy for Photovoltaic Power Plant Based on Variable Power Reserve Level

With the gradual increase of photovoltaic power penetration, in order to ensure the voltage quality of DC micro-grid, photovoltaic power plants need to be able to participate in voltage regulation tasks to cope with the impact of photovoltaic power fluctuations and load fluctuations on voltage quality. Researches show photovoltaic power plants under load-reduction operation can participate in voltage regulation without using energy storage equipment. In this paper, a photovoltaic voltage regulation control strategy with variable load-reduction rate considering photovoltaic power fluctuation is proposed, which can change the load-reduction rate of photovoltaic plants according to the fluctuation of photovoltaic. Then, aiming at the problem of fast voltage change caused by small inertia time constant of DC micro-grid, the method of using photovoltaic backup capacity to provide inertia control for the system is adopted, so that photovoltaic power plants can cope with both photovoltaic fluctuation and load fluctuation at the same time. Finally, a simulation model is established in PSCAD/EMTDC. Simulation results verify that under the proposed control strategy, grid-connected converters for the PV can participate in the DC voltage regulation.

Study on hydraulic transient process of K hydropower station with low head, large flow and complex long tail water system

K hydropower station is a hydraulic engineering with the main task of power generation, which is also a typical hydropower station with low head, large discharge and long tail water conveyance system. The hydraulic characteristics of these-type hydropower stations are of high water inertia, large amplitude of surge chamber water level fluctuation and poor stability of units. Therefore, due to the control requirements of the stability, the area of the horizontal section of the surge chamber is usually large, which leads to the long period of water level fluctuation of the surge chamber and the relatively long time of unit output oscillation in the transition process, and the regulating quality of the unit is often difficult to meet the control requirements due to the large inertia time constant of the water flow in the water conveyance system and the unique characteristic curve of unit. It is important to design appropriately to ensure the safe and stable operation of the hydropower station. In this paper, it is shown the process of hydraulic design of these-type stations. According to the design of K hydropower station, the simple surge chamber and restricted orifice surge chamber are compared in terms of hydraulic characteristics including the head loss of steady operation, the water level fluctuation of transition process and the stability of operation. Based on the optimized restricted orifice surge chamber, the overall calculation and analysis of the hydraulic transition process and the stability analysis of K hydropower station are carried out, it is concluded that the unit stability quality of the K power station is fine and the maximum pressure of pipe is in control, which provide strong support for the safe and stable operation of K station. The comprehensive and thorough analysis in this paper can provide reference for the design of the similar projects.

Stability Analysis of Different Regulation Modes of Hydropower Units

The dynamic characteristics of hydropower unit governing systems considerably influence the stability of hydropower units and the connected power system. The dynamic performances of hydropower units with power regulation mode (PRM) and opening regulation mode (ORM) are different. This paper establishes a detailed linear model of a hydropower unit based on the Phillips–Heffron model. The damping characteristic and stability of two regulation modes with different water inertia time constants TW were analyzed. ORM tended to provide negative damping, while PRM often provided positive damping in the major parts of the frequency range within the normal frequency oscillations when TW was large. Eigenvalue analysis illustrated that PRM has better stability than ORM. To validate the analysis, a simulation under two typical faults WAS conducted based on a nonlinear model of a hydropower unit. The simulation results illustrated that the responses of units with PRM are more stable in terms of important operating parameters, such as output power, rotor speed, and power angles. For hydropower units facing challenges in stable operation, PRM is recommended to obtain good dynamic stability.

Compensation Design of Coordinated Control System for Supercritical Once-Through CHP Plants Based on Energy Analysis

To increase the flexibility of power generation, the supercritical once-through boiler combined heat and power (CHP) plant should improve its coordinated control system (CCS). The difficulty of CCS for supercritical once-through boiler unit is to determine the water-fuel ratio in the dynamic process. First, a CHP unit model is established and simplified into a three-input three-output object under pure condensing working conditions. Second, dynamic relative matrix gain method and simulation is used to analysis several once-through boiler coordinated control schemes commonly used in engineering in which the compensation link is designed on the basis of water-fuel ratio. Simulation results show that, the water following CCS scheme has better intermediate point temperature performance and worse throttle pressure performance than the fuel following CCS scheme. To improve pressure performance, the closed-loop characteristics of pressure response to fuel and feed water flow are analyzed and compared through wavelet analysis. Finally, a method for determining the inertia time constant in the dynamic compensation link by comparing the wavelet coefficients is proposed. Wavelet analysis show that the imbalance in the speed of energy changes to fuel and feed water flow is the essential cause of large fluctuations in intermediate point temperature and throttle pressure before turbine of the supercritical once-through boiler unit. Simulation results show that using the proposed dynamic compensation in water following CCS can effectively reduce fluctuations of the intermediate point temperature and fluctuations of throttle pressure before turbine when power load command changes.

Research on frequency control method for micro-grid with a hybrid approach of FFR-OPPT and pitch angle of wind turbine

Micro-grid is the main application scenario of the renewable energy. When wind power serves as the dominant source of the micro-grid, the uncertainty of the wind energy caused by the climate makes the maintenance of the frequency stability of the micro-grid more complicated. Provided in this paper is a novel method to control the electrical output of WTG to maintain the frequency of the micro-grid close to its target. Firstly, a fast frequency response optimized power point tracking method (FFR-OPPT) is designed. The double excitation from the first-order and second-order derivative of the grid frequency can improve the speed of kinetic energy extraction of the wind turbine due to the increase of the virtual inertia time constant. Secondly, in order to enhance the service life of wind turbine, a hybrid control strategy of FFR-OPPT and pitch angle is proposed based on boundary analysis of the wind turbine. The control strategy can reduce the adjustment frequency and range of the pitch angle of wind turbine significantly. The proposed method is tested in a micro-grid combining by wind turbines and small thermal generations. The load frequency control in micro-grid is dominated by wind turbine and is assisted by thermal generation. The simulation results illustrate that the control strategy can control the frequency of micro-grid close to its schedule value effectively, accompanied by decrease of the frequency and range of pitch angle adjustment during control process.