Conventional Synchronous Generators Research Articles

Improved Virtual Synchronous Generator Control of DFIG to Ride-Through Symmetrical Voltage Fault

This article presents an improved virtual synchronous generator (VSG) control strategy of doubly-fed induction generator (DFIG) during symmetrical voltage fault. Based on VSG control, DFIG can behave like a conventional synchronous generator (SG), which can support frequency and voltage in grid-connected mode. Moreover, when microgrid disconnects from grid, DFIG can provide voltage and power for local loads. However, during symmetrical voltage fault, the transient components in DFIG cannot be controlled properly by VSG. To solve this problem, the air-gap flux feedback is designed in VSG control to accelerate the decay of transient components, and rotor current limiting method is also introduced in the article. In the proposed method, there is no need to switch the control structure or control targets. DFIG always works in the VSG control. Therefore, DFIG can operate in the standalone mode when the connection to grid is cut off. The decay time constant, the voltage control, current limiting control and parameter variation of the improved VSG control are analyzed in detail. Finally, the simulation and experimental results in both normal and fault grid condition are given to validate the effectiveness of proposed VSG control strategy.

A Novel VSG-Based Accurate Voltage Control and Reactive Power Sharing Method for Islanded Microgrids

Islanded microgrids (IMGs) are more likely to be perturbed by renewable generation and load demand fluctuation, thus leading to system instability. The virtual synchronous generator (VSG) control has become a promising method in the microgrids stability control area for its inertia-support capability. However, the improper power sharing and inaccurate voltage control problems of the distributed generations (DGs) in microgrids still has not been solved with a unified method. This paper proposes a novel VSG equivalent control method named Imitation Excitation Control (IEC). In this method, a multi-objective control strategy for voltage and reactive power in a low voltage grid that considers a non-negligible resistance to reactance ratio (R/X) is proposed. With the IEC method, the voltage drop across feeders is compensated, thus the terminal voltage of each inverter will be regulated, which will effectively stabilize the PCC (point of common coupling) voltage and inhibit the circular current. Meanwhile, this method can realize accurate reactive power tracking the reference value, making it accessible for reactive power scheduling. What is more, the reasonability of the IEC model, namely the equivalent mechanical characteristic and transient process inertia support between VSGs and conventional synchronous generators (SG), is illustrated in this paper. Moreover, steady-state stability is proved by the small-signal modeling method, and the energy required by inertia support is given. Finally, the simulation result validates the effectiveness of the proposed method, and it is also demonstrated that the proposed method outperforms the conventional droop control method.

Virtual Synchronous Generator Based on Hybrid Energy Storage System for PV Power Fluctuation Mitigation

The application of renewable energy is stimulating since the environmental pollution and the increase in demand for global energy consumption have become the main concerns of humanity. However, the intermittent nature of renewable sources could seriously affect the frequency stability of the system which needs to be solved. In this paper, the Virtual Synchronous Generator (VSG) based on battery/supercapacitor Hybrid Energy Storage System (HESS) is proposed to handle the stochastic power output of Photovoltaic (PV). First, the power allocation methods for HESS and its comparison are illustrated. Second, the comparison of the frequency deviation suppression strategies is presented. Moreover, as the adjustable parameters of VSG (J, D) is a key superior to the conventional synchronous generator; hence, a part of this paper will be introduced a new evolutionary algorithm called Backtracking Search Optimize Algorithm (BSA) to tune the parameters of the VSG in real time. To investigate the control performance, the standalone microgrid is modeled in the MATLAB/Simulink environment. Several case studies are conducted, and the results prove the improvement of the system frequency by attenuating the maximum overshoot of frequency deviation from 50.18 Hz to 50.03 Hz.

Transient stability improvement of wind farm integrated power system using STATCOM

The need for Interconnected power system is increasing day by day because of continuous growth of Electrical energy demand and to transmit Electric power to remote places at minimum cost and minimum losses. With the operation of power system in interconnected manner, maintaining the system security is difficult task i.e. whenever a disturbance occurs, the system undergoes stability problems. Even though Conventional energy sources are available, Electrical Engineers prefer Renewable energy sources integration because of Energy crisis and pollution problems related to the former, one such Renewable energy source isWind power.Wind energy has major share in Renewable energy sources because of its abundant availability in the nature. Whenever Wind generators coupled to the power system, the system exhibits drooping voltage characteristics and this situation becomes worse during faults. This condition can be neutralised with FACTS (Flexible AC transmission system) devices, one such FACTS device is STATCOM (static synchronous compensator). STATCOMsupports reactive and real power exchange and also improves Transient stability of the system because of its superior characteristics and quick response. In this paper a 9 bus Wind farm integrated test power system is taken and stability studies are done. Since, Wind farm is integrated with the system whenever a fault occurs, overall system stability is reduced i.e. the conventional synchronous generators can withstand it, whereas the Wind generators can’t. So to enhance the Transient stability of the system, a STATCOM is installed and the system behaviour is observed.

Investigation of Power Networks and their Patterns by Mathematical methods in order to Voltage Stability Enhancement

Authors in the research paper has considered IEEE 14-bus and 34-bus test network for creation generation and load patterns. This paper aims at analysing the potential impacts that distributed generation (DG) might have on voltage stability of electrical power networks. It is expected that increasing amounts of new generation technologies will be connected to electrical power systems in the near future. Most of these technologies are considerably smaller scale than conventional synchronous generators. In this paper, the impact of DG pattern (size and location of DG) on the voltage stability of two test system is investigated. The simulation methodology is discussed as well. It is found that the effects of DG the voltage stability of a power system strongly depend on the size and location of the distributed generators. The results show that install of DG on the weak bus, improve loadability limit and voltage stability and decrease loss of system. By investigating and reviewing the results of similar patterns in these two systems, the different generation and load patterns impact mechanism of DG units over the behaviour of the systems in terms loadability limit and voltage stability is achieved.

Modular Multilevel Converter Synthetic Inertia-Based Frequency Support for Medium-Voltage Microgrids

High penetration of renewable energies through fast-response power converters results in a considerable displacement of conventional synchronous generators and losing of system inertia for frequency control. Modular multilevel converters (MMCs) can be employed serving as an interface between the large-scale renewable generation and power grids. In a microgrid with high shares of renewables integrating through MMCs, submodule (SM) capacitors can be used as energy storage to provide a degree of synthetic inertia for system frequency support. This paper quantitatively exploits the MMC synthetic inertia in a microgrid with flexible renewable penetration levels. An MMC inertia coefficient concept is proposed. It is mainly affected by the penetration ratio, the SM capacitance, and the modulation index of an MMC, with the system operation constraints. According to the analysis, a substantial portion of system inertia can be provided by properly designed MMCs. Detailed MMC frequency control loops are presented. The capacitor average voltage is proportionally linked to the frequency deviation, in order to flexibly adjust the capacitor energy during frequency events. The MMC output active power is deliberately and simultaneously regulated according to the capacitor energy change rate. By controlling the MMC capacitor voltage, dc side power, and output active power, an amount of energy can be released or absorbed by SM capacitors to improve the system frequency response during the frequency event transients, while the renewable generation is scarcely influenced. The proposed concept is experimentally verified and the results show that, with proper system parameters and control loops, the line frequency deviation and the rate of change of frequency can be significantly reduced. Good agreements of the system frequency characteristics have been achieved between the theoretical calculation and experimental results.

Eigenvalue-Oriented Dynamic Stability Examination to Enhance Designing a Microgrid Hosting Clusters of Inertial and Non-Inertial Distributed Generators

Self-governing microgrids may consist of several conventional synchronous generators, converter-based distributed resources, and energy storages. In this paper, an eigenvalue-oriented stability assessment method is established to study the small signal stability of such systems that have complex dynamic characteristics. A new index referred to as the microgrid's marginal frequency stability index is established. This index is then used to determine the influence and correlation between sizes, quantities and internal parameters of the resources and loads on the microgrid's stability. Likewise, using this index, the impact of the isolation and coupling impedance amongst resources and loads are studied on the microgrid's stability. The findings of this study can be employed when designing microgrids to realize more stable systems.

Sunny with a Chance of Curtailment: Operating the US Grid with Very High Levels of Solar Photovoltaics.

SummaryWith rapid declines in solar photovoltaic (PV) and energy storage costs, futures with PV penetrations approaching or exceeding 50% of total annual US generation are becoming conceivable. The operational merits of such a national-scale system have not been evaluated sufficiently. Here, we analyze in detail the operational impacts of a future US power system with very high annual levels of PV (>50%) with storage. We show that load and operating reserve requirements can be met for all hours while considering key generator operational constraints. Storage plays an active role in maintaining the balance of supply and demand during sunset hours. Under the highest PV penetration scenario, hours with >90% PV penetration are relatively common, which require rapid transitions between predominately conventional synchronous generation and mostly inverter-based generation. We observe hours with almost 400 GW (over 40%) of economic curtailment and frequent (up to 36%) hours with very low energy prices.

Research on DFIG-ES System to Enhance the Fast-Frequency Response Capability of Wind Farms

With the increasing penetration of wind power generation, the frequency regulation burden on conventional synchronous generators has become heavier, as the rotor speed of doubly-fed induction generator (DFIG) is decoupled with the system frequency. As the frequency regulation capability of wind farms is an urgent appeal, the inertia control of DFIG has been studied by many researchers and the energy storage (ES) system has been installed in wind farms to respond to frequency deviation with doubly-fed induction generators (DFIGs). In view of the high allocation and maintenance cost of the ES system, the capacity allocation scheme of the ES system—especially for fast-frequency response—is proposed in this paper. The capacity allocation principle was to make the wind farm possess the same potential inertial energy as that of synchronous generators set with equal rated power. After the capacity of the ES system was defined, the coordinated control strategy of the DFIG-ES system with consideration of wind speed was proposed in order to improve the frequency nadir during fast-frequency response. The overall power reference of the DFIG-ES system was calculated on the basis of the frequency response characteristic of synchronous generators. In particular, once the power reference of DFIG was determined, a novel virtual inertia control method of DFIG was put forward to release rotational kinetic energy and produce power surge by means of continuously modifying the proportional coefficient of maximum power point tracking (MPPT) control. During the deceleration period, the power reference smoothly decreased with the rotor speed until it reached the MPPT curve, wherein the rotor speed could rapidly recover by virtue of wind power so that the secondary frequency drop could be avoided. Afterwards, a fuzzy logic controller (FLC) was designed to distribute output power between the DFIG and ES system according to the rotor speed of DFIG and S o C of ES; thus the scheme enabled the DFIG-ES system to respond to frequency deviation in most cases while preventing the secondary frequency drop and prolonging the service life of the DFIG-ES system. Finally, the test results, which were based on the simulation system on MATLAB/Simulink software, verified the effectiveness of the proposed control strategy by comparison with other control methods and verified the rationality of the designed fuzzy logic controller and proposed capacity allocation scheme of the ES system.

Micro-grid stabilizer design using sliding mode controller

Future of the network stability is endangered by increasing the number of Distributed Generation (DG) and Renewable Energy Source (RES) units. The idea of the Virtual Synchronous Machine (VSM) has been raised to control the power electronic-based DG/RES converters in order to have better integration with the grid. This paper introduces a new stabilizer design for VSM-based converters to guarantee the stability of the micro-grid (MG). In this regard, the Sliding Mode Control (SMC) theory, which is robust against the disturbances and uncertainties, is employed to cope with the intermittent and nonlinear nature of DGs. The mutual operation of the proposed inverter and MG stabilizer has the following advantages: (1) It provides a seamless and robust transition from the grid-connected to the islanded mode. (2) It is universal, sharing the real and reactive power during islanded mode and acting as a grid-supporting inverter in the grid-connected mode. (3) It mimics the behavior of the conventional synchronous generator resulting in better integration of DGs into the grid. (4) It can be used both in the voltage-controlled and the current-controlled Voltage Source Converters (VSC). (5) It obviates the need for the communication links, Phase Locked Loop (PLL) and islanding detection process. The mathematical model of the whole system has been investigated. The simulation results, conducted in the PSCAD/EMTDC environment, confirm the effectiveness of the proposed controller.

PERFORMANCE ANALYSIS OF AXIAL FIELD PERMANENT MAGNET GENERATOR

Axial Field Permanent Magnet (AFPM) synchronous generators are broadly connected for the little size power wind turbine this is ascribe to its powerful thickness per unit load than the conventional synchronous generator. It is a circle molded rotor and the stator structures. In this paper, examine the execution of both AFER-NS and AFIR-S generators having double side's slotted stator core is accomplished to improve the output generation characteristics. The electromagnetic plan examination was done to expand the power density. The enhancements design structure optimizations of the rotor pole-arc ratio of permanent magnet are proficient to increase output power and to decrease cogging torque. The output performances of AFER-NS and AFIR-S generators are associated with each other.

An investigation into spatial and temporal aspects of transient stability in power systems with increasing renewable generation

This paper investigates the impact of generator dispatch (dictated by cost) and locational aspects related to renewable generation connection and consequent disconnection of synchronous generation, on transient stability. As conventional synchronous generation (SG) is displaced by power electronic converter interfaced generation (PEC), the transient behaviour of the power system may change significantly due both to the changed power flows resulting from the different locations of renewables compared with thermal plant and the different dynamic characteristics of PEC. This paper uses an AC Optimal Power Flow (OPF) to determine credible, minimum cost dispatches of generation in a test network with increasing penetration of PEC in different locations. It is found that the critical locations of the system can vary significantly with respect to economic dispatch, location of disconnection of SG and location of PEC, highlighting the increasing temporal and spatial change in system dynamic behavior.

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

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

Stability implications of bulk power networks with large scale PVs

With the shift in renewable portfolio standards, conventional fossil-fuel based generators are expected to be partially or fully replaced with renewable energy sources such as photovoltaics (PVs). However, replacing the conventional synchronous generators with PV generators may raise new stability concerns such as lack of inertia, insufficient reactive power and voltage fluctuations, which can jeopardize the reliability of the entire network. In this paper, various impacts of integrating utility-scale PVs and rooftop PVs and replacing the existing conventional generators on the stability of a power network are investigated. Detailed steady state, transient and small signal analyses are conducted on a large test system, namely Texas 2000-bus system for different PV penetration levels. System transients such as line faults, double-line faults, loss of generator, loss of PVs and cloud cover scenarios are simulated to study their impacts on the test system with different PV penetration levels. It is demonstrated from the impact analysis that the benefits of PVs are highly dependent on a spectrum of factors such as node criticality, type, location and penetration of PVs, and type of transients. Furthermore, the influence of battery energy storage system that are generally associated with PVs on the system stability is also discussed.

Centralized VSM control of an AC meshed microgrid for ancillary services provision

This paper proposes a centralized control strategy for distributed generation (DG) sources connected to an AC meshed microgrid (MG). Through the proposed strategy, power production of distributed generators is managed such that the injected active and reactive powers at the point of common coupling are equal to predefined set-points. Thus, the MG is able to provide the main ancillary services including frequency and voltage control. An extended load flow model is used to identify active and reactive power production of each DG unit such that the total power injected to the utility grid satisfies the required setpoints. Hence, from the point of view of the transmission system operator (TSO), the whole MG can be seen as a single entity that behaves as a «PQ» generator. This control is intended to facilitate the management of power systems including a large number of microgrids and DG sources in terms of stability analysis, optimal reactive power dispatch, optimal power flow, etc. Furthermore, since the centralized control allows to set «PQ» power production to any predefined value, it was proposed to operate the whole MG using the concept of Virtual Synchronous Machine (VSM). It has been shown that, through the proposed control, the MG can reproduce the behavior of conventional synchronous generators during normal conditions and during faults.

Research on Frequency Fuzzy Adaptive Additional Inertial Control Strategy for D-PMSG Wind Turbine

The traditional additional inertial control (T-AIC) strategy can provide frequency support for the directly-driven wind turbine with a permanent magnet synchronous generator (D-PMSG). However, due to the fixed control coefficients, the frequency modulation effect is poor under load and wind speed disturbances. In order to improve the frequency transient response of D-PMSG, a fuzzy adaptive additional inertial control strategy (FA-AIC) is proposed in this paper. A simplified D-PMSG model is established for the complexity and low calculation speed. A single-machine grid-connected system composed of a D-PMSG and an equivalent synchronous generator set (ESGS) is taken as the background and analysis of the principle of T-AIC. The proportional and derivative coefficient initial values in T-AIC are tuned by simulating the static characteristics and inertial response characteristics of the conventional synchronous generator set, and fuzzy control technology is introduced to adjust the proportional and derivative coefficients adaptively based on the frequency deviation and the frequency deviation change rate under load or wind speed disturbances. The simulation verification indicates that T-AIC, kinetic energy (KE)-based gain-AIC and FA-AIC all can utilize the D-PMSG additional inertial response to provide frequency support for grid-connected systems. Compared with T-AIC and KE-based gain-AIC, the proposed FA-AIC can not only provide more effective frequency support during load disturbances, but also suppress the frequency fluctuation caused by the wind speed variation and displays a better dynamic frequency regulation effect.

New model of multi-parallel distributed generator units based on virtual synchronous generator control strategy

Virtual synchronous generator (VSG) control technique presents a promising alternative for controlling the inverter-based distributed generator (DG), due to its capability to improve the microgrid stability when the renewable energy has a high penetration level into the grid. The analyses and parameters design of the active and reactive power loop of DG are often based on a small-signal model, where a linear relationship between active and reactive powers versus control variables is obtained based on quasi-static method, in which the currents dynamic is neglected. This model gives qualitative results only in case of large conventional synchronous generators. However, for a small DG based on VSG controller with a small virtual inertia, fast power electronics devices and with coupling between active and reactive powers loops, this model does not give satisfactory results. To overcome the aforementioned problem, this paper proposes a new model of DG systems based on VSG operating in island microgrids. The proposed model incorporates the line impedance dynamic of the microgrid into the DG model. The accuracy of the proposed model has been investigated and compared with the conventional one using MATLAB/Simulink environment. Simulation obtained results prove that the new model is more precise compared to the conventional one in transit state reflecting by the way better the dynamics of the system and allowing as a consequence of a good analysis of the stability and behaviour of the global system.

Active Power Sharing and Frequency Restoration in an Autonomous Networked Microgrid

Microgrid (MG) concept is considered as the best solution for future power systems, which are expected to receive a considerable amount of power through renewable energy resources and distributed generation units. Droop control systems are widely adopted in conventional power systems and recently in MGs for power sharing among generation units. However, droop control causes frequency fluctuations, which leads to poor power quality. This paper deals with frequency fluctuation and stability concerns of f-P droop control loop in MGs. Inspired from conventional synchronous generators, virtual damping is proposed to diminish frequency fluctuation in MGs. Then, it is demonstrated that the conventional frequency restoration method inserts an offset to the phase angle, which is in conflict with accurate power sharing. To this end, a novel control method, based on phase angle feedback, is proposed for frequency restoration in conjunction with a novel method for adaptively tuning the feedback gains to preserve precise active power sharing. Nonlinear stability analysis is conducted by drawing the phase variations of the nonlinear second-order equation of the δ-P droop loop and it is proved that the proposed method improves the stability margin of f-P control loop. Simulation results demonstrate the effectiveness of the proposed method.

A Generalized Droop Control for Grid-Supporting Inverter Based on Comparison Between Traditional Droop Control and Virtual Synchronous Generator Control

In this paper, a generalized droop control (GDC) is proposed for a grid-supporting inverter based on a comparison between traditional droop control and virtual synchronous generator (VSG) control. Both the traditional droop control and VSG control have their own advantages, but neither traditional droop control nor VSG control can meet the demand for different dynamic characteristics in grid-connected (GC) and stand-alone (SA) modes at the same time. Rather than using a proportional controller with a low-pass filter, as in a traditional droop control, or fully mimicking the conventional synchronous generator parameters in a VSG control, the active power control loop of the GDC can be designed flexibly to adapt to different requirements. With a well-designed controller, the GDC can achieve satisfactory control performance; unlike a traditional droop control, it can provide virtual inertia and damping properties in SA mode; unlike a VSG control, the output active power of an inverter with GDC can follow changing references quickly and accurately, without large overshoot or oscillation in the GC mode. Moreover, given specific controller parameters, the GDC can function as both a traditional droop control and a VSG control. The GDC's controller parameter design is more intuitive and flexible, and this paper provides a distinct design process. Finally, the effectiveness of the proposed control method is validated by the simulation and experimental results.

Damping Low-Frequency Oscillations Through VSC-HVdc Stations Operated as Virtual Synchronous Machines

With proper control methods, grid-connected power converters can mimic the dynamics of conventional synchronous generators (SGs), i.e., acting as virtual synchronous machines (VSMs), for improving the grid frequency stability and ensuring a smooth transition toward converter-dominated power systems. However, the integration of VSMs will inevitably influence the low-frequency oscillations (LFOs) in power grids which are usually caused by the interactions among SGs. This influence should be of concern especially when the VSM's capacity is similar to that of SG, e.g., operating voltage-source converter (VSC)-HVdc stations as VSMs. This paper investigates how the VSM affects LFOs in power systems by analyzing its equivalent damping torque. We show that with proper control design, VSMs can provide considerable positive damping torque to effectively damp LFOs in power grids. Particularly, using the damping torque analysis, the influences of virtual impedance and grid frequency detector (i.e., phase-locked loop) are elaborately studied so as to provide design guidelines for better damping LFOs. Finally, case studies of a two-area test system with integration of a VSM-based VSC-HVdc station verify the validity of the analysis.