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Abstract Protection of synchronous generators (SGs) against internal faults, such as stator earth fault (SEF) and turn‐to‐turn fault (TTF), is crucial for ensuring the stability and security of the power system. This paper presents a phase domain model for simulating SEF and TTF in SGs, requiring only nameplate data and avoiding the need for complex geometric data or lengthy simulations typical of FEM models. The stator winding is divided into three sections, allowing for the calculation of magnetic field distribution in both healthy and faulty conditions. The model is capable of simulating short‐circuit turns at various locations within the stator winding with high accuracy and speed. The dynamic response of the generator is also incorporated into the model. The model's accuracy is validated through comparison with results from multiphysics simulation software. Furthermore, this study addresses the limitations of conventional protection methods in detecting TTF and proposes a novel, simple, fast, and accurate protection logic that can be implemented in digital protection relays and is effective across a wide range of TTF scenarios.

Abstract In order to enhance the dynamic performance of aircraft high‐voltage direct current wound rotor synchronous generator (WRSG) system under load variations, a dual internal model control (DIMC) method utilising improved load current feedback (ILCF) is proposed. It employs internal model controllers in both voltage and current loops, thereby significantly augmenting the system's disturbance rejection capabilities. Improved load current feedback is incorporated between the DC voltage loop and the field current loop. Additionally, a method for designing a feedback fitting function is established, enhancing the system's adaptability to changes in operational speed and load. The authors outline parameter design methods for voltage and current loops in DIMC, based on the internal model control principle. Adjustments to these parameters can further enhance the system's disturbance rejection capabilities. Finally, comparative experiments between the PI method with ILCF and the DIMC method with ILCF are conducted on a 15 kW WRSG experimental platform. These experiments demonstrate that the proposed DIMC method with ILCF optimises dynamic performance across the entire speed range, enhancing the competitiveness of WRSG systems in aircraft applications.

Abstract With the increasing application of renewable energy sources (RESs), more and more grid‐connected voltage‐source converters (VSCs) in the grid need to operate in a “grid‐forming” manner. However, when large disturbances occur in the grid, grid‐connected converters may lose synchronization and cause the current to exceed set limits. To ensure the stable and safe operation of VSC during large disturbances and to improve the power supply capacity during voltage dips of RES, this paper proposes an emergency control method that takes into account the virtual synchronous generator (VSG) current limit and transient stability based on a large‐signal mathematical model of the VSG during fault and normal states. The method divides the operation area of the VSG into non‐emergency area and emergency area. If the operating point is in the non‐emergency region after a fault, only the active power reference value is changed to avoid current exceeding the limit and transient instability; if the operating point is in the emergency region, the current exceeding the limit and transient instability are avoided by changing the active power reference value and adding in the virtual impedance. Finally, the effectiveness of the proposed control strategy is verified by simulation.

Abstract In autonomous alternating current microgrids, the grid‐forming virtual synchronous generators can cooperate with the conventional synchronous generators to improve system inertia and frequency regulation capability. However, undesired active power oscillations between the synchronous generators and grid‐forming virtual synchronous generators may trigger their overcurrent protection and even result in a blackout. To explicitly reveal the oscillatory modes over all frequency bands, a high‐fidelity full‐order state‐space model is first developed. A potentially destabilising sub‐synchronous oscillation mode resulting from the interaction between grid‐forming virtual synchronous generators voltage controller and synchronous generators q‐axis damper winding is identified. Other modes reflecting the low‐frequency oscillation and frequency restoration dynamics are also assessed. Subsequently, to make a reasonable trade‐off between the accuracy and simplicity of system modelling, an enhanced quasi‐stationary model dedicated to low‐frequency oscillation evaluation is simplified from the full‐order type. The enhanced quasi‐stationary model features simplicity and low‐order benefits, which makes it more practical for multi‐generator system analysis. Moreover, by considering the dynamics of synchronous generators field winding and excitation system, the enhanced quasi‐stationary model significantly improves the low‐frequency oscillation characterisation accuracy compared with the existing quasi‐stationary model. The two developed models are comprehensively compared with the existing small‐signal models. Real‐time simulations based on RT‐LAB are conducted to verify the correctness of the theoretical analysis and the accuracy of the proposed small‐signal models.

Abstract One of the common methods to detect Loss of Excitation (LOE) is the impedance‐based technique. But it is too slow, and may show incorrect performance when faced with power system disturbances like stable power swings (SPS). To improve the aforementioned problems, a new index is introduced based on the change of transient states of the synchronous generator which are estimated by Extended Kalman filter (EKF). The principal idea of the proposed method is that the angle between the transient voltage and q‐axis internal voltage phasor will be increasing during LOE events. Therefore, this angle and its derivative are used. Estimation of the internal variables of the generator is used to achieve the angle between transient voltage and internal voltage phasor. To evaluate the efficiency of this algorithm, numerous simulations are performed on various networks and in different operating conditions. The simulation results of the method demonstrate that this approach both reduces the detection time in most cases and improves its security in the encounter of the other disturbances. Experimentally, the performance of the proposed method is evaluated by using a 5 kVA laboratory generator. Experimental studies show that the presented algorithm performed correctly detects LOE and SPS conditions.

Abstract In this article, the stability of a synchronous motor generator pair (SMGP) used for improving the inertia of grid‐connected renewable energy systems is investigated. The useful operational regime for different sets of system parameters is identified, such as electromagnetic torque, damping co‐efficient, and inertia by employing bifurcation analysis to detect stability boundaries. For the first time, the existence of bi‐stable regimes for the SMGP with non‐linear stability analysis is revealed. The authors' analysis unravels the possibility of the system getting transited to unsafe operation even when the system is in the linearly stable region. The existence of the bistable regime indicates the possibility of the system becoming unstable even when the eigenvalues are in the left half plane. The authors also identify globally stable and globally unstable regimes in the parameter space. The safe operating range of inertia and damping co‐efficient values helps in the design of a suitable MGP set that is robust to frequency deviations, even with a low inertia source. With the recommended values of electromagnetic torque, the authors' analysis provides a safe operational regime for power generation from renewable energy sources.

Abstract Here, a dynamic analytical model is provided for salient pole synchronous generators (SPSGs) under different eccentricity faults. A useful technique is developed first to describe the rotor position‐dependent non‐uniform air gap distribution by a proper function in the healthy state, which includes the stator slotting effect. Then, the exact air gap function is used along with the modified winding function theory to extract non‐integral equations for computing the SPSG various inductances that are playing critical role during analytical modelling and simulation of the machine. Afterword, the air gap function and the inductance equations are modified to include all types of the eccentricity fault. A 3 kW SPSG is simulated and then tested in laboratory under the healthy and the mixed eccentricity (ME) conditions. There are good accordance between the simulation and corresponding experimental results; they show that some harmonic components are introduced in the rotor and the stator currents due to the ME fault whose frequencies are integer multiples of the rotor speed frequency and whose amplitudes increase by increasing the fault severity. The load level change also affects the amplitudes of the harmonics.

Abstract Incidents of sub‐synchronous oscillations (SSOs) reported in the Hami power network of western China indicate that the torsional dynamics of a synchronous generator (SG) can be excited by a grid‐connected wind farm and trigger torsional SSOs. The concept of open‐loop modal resonance (OLMR) reveals the excitation mechanism from the standpoint of system modal condition. This paper proposes a novel damping torque analysis (DTA) for detecting and assessing the risk of OLMR exciting torsional SSOs in a complex power system with multiple grid‐connected wind farms. The proposed analysis is modal‐computational free and can also accurately identify the grid‐connected wind farm responsible for the excitation of the torsional SSOs. Moreover, the proposed DTA provides a physical insight into the OLMR to facilitate an understanding of the essential reasons that the OLMR may excite torsional SSOs. The effectiveness of the proposed DTA is demonstrated and evaluated in an example power system with multiple grid‐connected wind farms.

Abstract A three‐phase to single‐phase direct AC power generation system based on the open‐winding permanent magnet synchronous generator (PMSG) with the dual‐inverter is proposed in this study for the standalone power system. The negative DC terminals of the dual‐inverter are commonly connected, and the positive DC terminals are reconstructed as the output port of the single‐phase AC power; the three‐phase AC power generated by PMSG could be converted to be the high power quality single‐phase AC power directly without any LC filters. Different from the traditional OW‐PMSG system, zero‐sequence current is employed to achieve adjustable frequency and voltage of the single‐phase AC power, and the corresponding dual voltage‐loop control strategy with the separate positive‐sequence and zero‐sequence currents regulation of PMSG is given to achieve high output performance. The effectiveness of the proposed control strategy and the output characteristics have been verified by simulation and the experiment of the prototype, and the results show that the high‐quality single‐phase AC power can be obtained at a wide speed range and at different load operating conditions.

Abstract Synchronous generator (SG) plays a vital and critical role in the power system by supplying electric power to consumers. Various faults in SGs can cause some catastrophic events such as power disruption or blackout. These faults can be classified into two electrical and mechanical faults. Short circuit in stator windings and field winding are electrical fault while bearing, static/dynamic eccentricity, and broken damper bars faults are mechanical one. Unlike the induction machines, there are no much researches in SGs condition monitoring owing to its complex behaviour against the faults. Herein, the SG modelling approaches are presented briefly to elaborate shortcoming and challenging issues in the modelling, and then a comprehensive review of various electrical and mechanical fault detection methods is presented.

The first swing rotor angle stability is still an important stability challenge for modern power systems integrated with a large number of renewable energy‐based sources, such as wind farms and solar‐photovoltaic farms. Therefore, innovative strategies must be developed to exploit the full potential of doubly fed induction generators (DFIGs) to improve the first swing rotor angle stability in the system. This study presents a new active power logic (APL) controller for DFIGs which can reduce the active power during the fault and slowly recover it after the fault to allow synchronous generators to increase the electrical power during and after the fault; thus, enabling synchronous generators to improve the first swing rotor angle stability. The feasibility of the proposed control scheme is investigated via theoretical analysis and simulation studies. The reliability and voltage stability test system is used to demonstrate the effectiveness of the proposed scheme for local and remote faults and increased DFIG penetration conditions. The comparative results show that the proposed APL controller of the DFIG improves the first swing rotor angle stability, specifically when the DFIGs are located near the synchronous generators.

In this study, virtual excitation emulation (VEE) is proposed along with the virtual inertia emulation (VIE) for a virtual synchronous generator (VSG) in a standalone microgrid to improve the AC voltage and frequency profile through the control of power electronic DC–AC converter in the microgrid. The VIE emulates the inertia and damping characteristics of a synchronous generator by mimicking the swing equation. The VEE is proposed to emulate the behaviour of the field circuit and exciter system of a synchronous generator. In addition, a restoration control is also added with the inertia and excitation emulation to restore the magnitude and frequency of the output voltage at the rated value and achieve isochronous control. A linearised small-signal model of the VSG with the proposed control technique is also presented to analyse the system stability and design the control parameters. The standalone microgrid is simulated in MATLAB/Simulink environment to show the effectiveness of the proposed control technique. From the simulation results, it is reflected that the proposed control technique improves the voltage and frequency response during transient and steady-state including the frequency nadir of the microgrid.

Considering the internal and external disturbances of the direct-driven wind energy conversion systems, i.e. multi-variable coupling, non-linear characteristic and modelling error, and the environmental disturbances, an adaptive sliding mode observer with active disturbance rejection control (ADRC) method for a direct-driven permanent magnet synchronous generator (PMSG) based wind power conversion system is proposed to track the maximum power point precisely. First, adaptive sliding mode observer based on phase-locked loop can effectively observe the rotor position information of PMSG, and weak the inherent chattering of traditional sliding mode. Second, ADRC can enhance the anti-disturbance ability when the system is under linear state feedback. Finally, the simulation results show that this method can estimate the rotor position exactly, and the system has the strong robustness and the optimum power tracking performance.

The synchronverter (SV) is currently one of the most promising grid-friendly inverters that mimic synchronous generators (SGs). Almost all the available SV use virtual current signal for synchronization, which require a complicated controller that lacks plug–play capability. This paper proposes a new generation of self-synchronised SV with a simpler controller, which provides plug–play capability with no need for switching between actual controller current and a virtual current for each ON–OFF process. The proposed SV (PSV) can also share the load with other SGs and SVs and also it is able to take part in regulating system frequency and voltage. A linearised small-signal model is derived for the controller to investigate its stability. The PSV is also validated through multiple simulation studies carried out in MATLAB/Simulink environment. The results show the capability of the PSV to achieve self-synchronisation and plug–play operation without a dedicated PLL or a virtual current. Compared to the current self-synchronised SV, the PSV guarantees a smoother grid connection and gives more accurate reactive power sharing. Moreover, the PSV is applied to a modified IEEE 14-bus test system considering three different contingencies to prove its impact on enhancing system dynamics and stability.

The variable hydrological conditions that are found in run-of-the river projects require operations over a wide range of water-level variations. This article develops a novel optimisation strategy for permanent magnet synchronous generator (PMSG) used in hydroelectric plant. The basis of this strategy is a simplified subdomain (SD) analytical model of the surface-mounted PM generator combined with an improved genetic algorithm, which searches the optimal generator parameters that maximise the operating efficiency of generators at different water-level conditions and minimise the material cost. The electromagnetic performances of the optimised generator are investigated and compared with conventional optimal design. It is found that the proposed method allows the emergence of higher performances in variable hydrological conditions.

Wind energy technology is developing more and more from recent decades in many countries. Therefore, addressing the latest wind turbine research issues, this study develops a dynamic model of a permanent magnet synchronous generator and an active rectifier—as part of a unified dynamic model of a wind turbine generator in MATLAB/Simulink. In addition, a novel technique for the verification of the open-loop and the closed-loop transfer functions in the frequency domain is presented. The technique develops by analysing the frequency responses of the transfer function at various frequencies. Furthermore, the cascaded control loops of the developed model are designed and the stability of the designed controllers is tested in response to step changes in reference values.

This article proposes a new control paradigm for the seamless operation of a permanent magnet synchronous generator (PMSG) based wind energy conversion system (WECS) in both islanded and grid-connected modes. Conventionally, grid-connected WECS are controlled to extract the maximum power from the wind, known as maximum power point tracking (MPPT). However, in islanded operation, the objective is to control the power generated from the WECS to match the load demand, which here is called load following power generation (LFPG). This article proposes a power management system (PMS) which integrates the MPPT and LFPG capabilities of the WECS into a single-control paradigm. This enables the WECS to seamlessly achieve MPPT in grid operation, LFPG in islanded mode and low-voltage ride-through (LVRT) during fault. The proposed scheme is validated using MATLAB/SIMULINK simulations.

Considering the growing wind power penetration in power systems, using the flexible control ability of these turbines to enhance power system dynamic behaviour has been investigated. One of the major challenges regarding the stability problem of power systems is the low-frequency oscillations (LFOs) of synchronous generators in a power system. Usually, power system stability units are used to damp these oscillations, but using wind turbine systems for this purpose has been suggested in recent studies. In this regard, mostly a linear supplementary control loop has been employed in the control system of wind turbines. Applying a linear structure for the supplementary control loop does not result in optimum performance since the power system operating conditions and the wind turbine power generation change continuously. In this study, adaptive control is used to design the supplementary loop. Doubly-fed induction generator (DFIG)-based wind turbine systems are used for this study. Since adaptive control parameters tend to optimum values, applying this controller will provide better damping for power system (LFOs at different wind speeds, as shown in this study.

The electromagnetic design of a six-phase permanent magnet synchronous generator (PMSG) for application in medium/high-speed wind energy conversion systems (WECS) is studied in this work. The dynamic model and the analytical design process of the six-phase PMSG are presented and some design aspects, such as the winding configuration and the geometry of the permanent magnets are discussed in order to improve the performance of the generator in both healthy and fault-tolerant conditions, i.e. when only one set of three-phase windings is connected to the grid. Finally, several simulation results obtained through finite element analysis are presented in order to validate the proposed design.

The fast growth of offshore all-DC wind farm will bring problems such as the weakening of grid frequency stability and the increase of equivalent grid impedance. To overcome this, a coordination control strategy for offshore all-DC wind farm is proposed in this study with two salient features: better performance under weak grid condition, and real-time frequency support from wind farm. This strategy consists of three parts: the inertia synchronising control of receiving end converter, the constant ratio control of DC transformer and the frequency response of wind farm. With the proposed strategy, the all-DC wind farm operates like a synchronous generator to onshore grid, which provides fast frequency support when the onshore grid frequency changes. The effectiveness of the proposed method is validated in PSCAD/EMTDC.

This paper proposes a coordinated frequency control method for variable speed wind turbines with permanent magnet synchronous generators (VSWT-PMSGs) based offshore wind farm (OWF), which is connected to the main onshore grid through voltage source converter (VSC) based high voltage DC (HVDC) transmission system. The purpose of the proposed system is to damp the frequency oscillations of the onshore grid due to the installation of large-scale fixed speed wind turbines with squirrel cage induction generators (FSWT-SCIGs) based wind farm (WF) and photovoltaic (PV) power station. A novel centralised droop controller with the dead band is designed for VSWT-PMSGs to decrease the frequency fluctuations of the onshore main power system. In the proposed system, primary frequency reserve is implemented by deloading operation of VSWT-PMSGs in the OWF. The effectiveness of the proposed centralised frequency controller is verified through simulation analysis on a modified IEEE nine-bus model system in PSCAD/EMTDC software.

In recent years, a new type SSO issue, which has occurred in direct-drive permanent magnetic synchronous generators (PMSGs) based wind farm, brings a new challenge to the stability of the power system. In this paper, the SSO issue in grid-connected PMSGs system is studied by the eigenvalue analysis. The linearised state-space model of this system is built firstly. The impacts on the SSO characteristics from short-circuit ratio (SCR), impedance-frequency characteristic, resonant frequency of AC network and control parameters of PMSGs are observed by the root loci analysis. After that, electromagnetic transient simulation is carried out to verify the eigenvalue analysis. Meanwhile, the participation factors of SSO mode are calculated to analyse the subsynchronous control interaction (SSCI) between AC network and PMSG. The result shows that there is a SSO mode which is greatly influenced by AC network and DC capacitor, grid side converter (GSC) control of PMSG. The characteristics of AC network will affect the SSCI between AC network and PMSG.

An energy storage system (ESS) sizing method is proposed to enable wind farm (WF) to be a black-start (BS) source. This method handles three challenges: firstly, ESS has enough power to help WF start up. Secondly, the WF together with ESS is modelled as a virtual synchronous generator (VSG), and its frequency regulation capability is considered when sizing ESS. Thirdly, the power and capacity of ESS are determined so that VSG can deliver the required power during black-start. Instead of using wind speed probability distribution, kernel density estimation is used in this paper to directly estimate the probability distribution characteristics of ESS sizing. Simulation results show that with minimum investment cost on ESS, WF is able to be BS source.

This study presents a detailed methodology for non-intrusive measurement of cooling air mass flow rate that enables an overall machine evaluation. This approach enables the simultaneous measurement of air mass flow with shaft torque at differing operating points while minimising the change in air flow introduced by the measurement system. The impact of geometric parameters in the designed system is investigated using a detailed 180° computational fluid dynamics (CFD) model. Special attention was paid to minimising their influence on pressure drop, the mass flow rate through the machine, and measurement uncertainty. Based on the results of this investigation, the system was designed and manufactured, and the experimentally measured data was used to validate the CFD predictions. For the as optimal identified configuration, the flow rate is predicted to decrease by 2.2% relative to unrestricted operation. The achieved measurement uncertainty is ± 2.6% at synchronous speed.

Power systems are frequently subjected to variations of the operating conditions due to the occurrence of contingencies such as faults, load or generator tripping. These disturbances might enforce the system to enter the instability region and thus deteriorate its overall performance. This paper proposes a systematic approach for transient stability assessment using generator-based threshold. An individual stability limit is designed for each generator instead of using only one pre-defined threshold for all generators. The considered contingencies are characterized by a fault type, location, loading condition and critical clearing time to consider wide range of operating conditions. Two fault locations are only considered to reduce the search space for stability limits; the generator terminals and mid of transmission lines. The critical clearing time is used to find the maximum relative rotor angle (MRRA) of individual generators attained during the entire time domain simulation. The MRRA of each generator represents the maximum angle which can be reached without losing synchronism at a given contingency. Conservative stability limits are drawn around the minimum stable drift of each generator which is chosen from the set of MRRAs of all contingencies. The effectiveness of the proposed approach is tested using the modified New-England, 39-bus system.

The frequency response test on synchronous generators has been increasing in the last decades, but the high cost of equipment used for conducting the test is still a stumbling block for both manufacturers and end consumers. This study aims to propose a methodology for obtaining parameters, through the use of neural networks. This study treats the results designed through frequency tests, in which the proposal was the use of low-cost equipment to perform them. In addition, a process of optimisation of the neural network was developed during the development of this study.

Maintaining the frequency within an allowable range is the necessity to ensure the stable operation of the grid. When a large-scale frequency disturbance occurs in the grid, the synchronous generators (SGs) will temporarily release the kinetic energy to make the generation and demand balanced. This is well known as the inertial response of the SG. However, the traditional variable-speed wind turbines (VSWTs) only generate a negligible inertial response when a frequency disturbance happens. The traditional VSWT is designed to maximise the wind energy injected into the grid. Thus, when a frequency disturbance is detected in the grid, the traditional VSWT cannot change the active power output to support the grid. To solve this problem, the concepts of the inertial control and inertial support capability for VSWT were proposed. The inertial control measures the frequency of the grid and adjusts the active power output of VSWT. Thus, during the frequency disturbance, the VSWT has the inertial support capability. The purpose of this study is to provide a comprehensive review of the inertial control for the wind power plant (WPP) consisting of VSWTs. With the inertial control, the integration of the WPPs can enhance the frequency stability of the grid.

In a large-scale power system, electromagnetic transient simulation is facing the contradiction between simulation speed and simulation accuracy, so the electromagnetic transient models of the components in the power system have been continually being improved. The synchronous generator is an important part of the power system, and the Voltage Behind Reactance (VBR) model of the synchronous machine is deduced and improved on the basis of the classical model. The method of Single-phase rotation transformation is based on the idea of a rotating coordinate system, and the frequency of the signal can be reduced by the rotation transformation. Moreover, the simulation step length of the electromagnetic transient is related to the frequency of the change of the electric quantity. The lower the change frequency is, the larger the simulation step can be. Here, according to the theory of single-phase rotation transformation, the VBR model of synchronous generator based on time scale transform is deduced, which can simulate in the case of large steps without the loss of the precision, and thereby which the speed of electromagnetic transient simulation of power system will be effectively improved.

Recently, the virtual synchronous generator (VSG) has drawn considerable attention for its function of simulating the electromechanical transient of SG to make converters equip with the characters of inertia, damping, voltage regulating, and frequency regulating. Energy storage systems (ESSs), which are connected to the converter by a DC/DC circuit, are the basis of VSG. The output active power of VSG is restricted by the capacity margin of the grid-side converter and DC/DC circuit, and the output energy is bounded by the energy margin of ESS, which means that VSG is restrained by treble constraints: DC side power, DC-side energy, and AC-side power. A small signal model of VSG is built, and the method of getting the treble constraints is proposed. Through the analysis of the dynamic response characteristics of the VSG, the operating zone of VSG under constraints is confirmed. Simulations in power system computer aided design (PSCAD)/electromagnetic transient including DC (EMTDC) verified the theoretical analysis and the transient characteristics of VSG when the constraints are broken and simulated.

Large-scale wind and solar power generations have got rapid development in recent years in China and abroad. They always connect to load centres through high-voltage direct current (HVDC) transmission or ultra HVDC (UHVDC) transmission lines. Such systems are so large and it is necessary to make appropriate equivalence according to the purpose of different research. At present, the analysis model of this kind of real system is few, and simplified single-machine infinite-bus system or IEEE example system is adopted most of the time, which may affect the accuracy of analysis results. Here, the UHVDC project from a practical renewable energy base to load centres in China is chosen as an example, the actual structure and parameters are used to establish a typical example system for AC/DC hybrid power grid with high-proportion renewable energy. The new-generation synchronous condenser with large capacity and the supporting synchronous generators are all considered. First, the installed power generation, power grid structure, and load configuration of example system are introduced. Then the load flow distribution and transient stability characteristics are analysed by using DIgSILENT PowerFactory software. Finally, the feasibility of the proposed example system is shown by simulation results.

Synchronous generation is being displaced by renewable energy sources and high-voltage direct current (HVDC) interconnectors. These factors will lead to a diminishing provision of ancillary services. Conventional services tendered to provide network support rely on the availability of synchronous generation and because of this, future power system operators will have to develop new ways to obtain the ancillary services required. A technology rapidly gaining in popularity is voltage source converter (VSC)-based HVDC and further advances in the technology have led to the modular multilevel converter (MMC). Due to the design of an MMC there is intrinsic electrostatic energy that could potentially be utilised to provide ancillary services. This study determines the electrostatic energy available from a typical grid-scale MMC VSC for network support during disturbances and for power oscillation damping. Supporting simulations are provided to ascertain the impact that this electrostatic energy could have on network performance if utilised. This study displays that although an MMC possesses intrinsic energy, utilising it for frequency response would provide negligible system benefits. However, for POD, the available intrinsic MMC energy did improve system stability which leads to the potential of using multiple MMCs to increase the energy and further improve system stability.

High temperature superconductors (HTS) enable very compact electric machines with high power density. Here, an HTS power generator based on authors’ previous double claw pole design is studied and improved. The original design of the double claw pole machine features a stationary field core to provide support and access to the superconducting field winding and its cryostat. It does not serve any other purpose. However, a homopolar field passes through it. The improvement was made to replace the field core by iron-cored copper coils, creating an inner stator within the generator to increase the electric loading. Through this step, the power output of the generator was increased from 10 to 11.6 MW, increasing the power density from 54.28 to 63.54 W/kg while maintaining the same outer diameter.

With a higher and higher penetration of renewable energy in the grid, the principle of the virtual synchronous generator (VSG) is proposed as an attractive solution to controlling the grid-connected inverters. In common, active power-frequency and reactive power-voltage controllers play major roles in VSG control system. However, the line impedance and local loads are the critical case effecting on the control result of the two controllers. Compared with reactive power-voltage controller, active power-frequency controller is more easier to product the load sharing among the VSGs accurately. This paper presents a self-adaptable reactive power-voltage controller to deal with the sharing problem of reactive power in the parallel VSGs system, using the reactive power difference to adjust reactive power-voltage control coefficient. Then, a linearised small-signal model is established for stability analysis of the reactive power-voltage control coefficient to the parallel VSGs system. Finally, an experiment of two parallel VSGs system based on the self-adaptable reactive power-voltage controllers is performed, with different line impedance. Results obtained from the experiment verify the effectiveness of the proposed self-adaptable reactive power-voltage controllers in the parallel VSGs system.

The increasing penetration of renewable generation has led to the decrease of power systems’ overall inertia, which introduces significant challenges to frequency stability. In this paper, the potential of using Fast Frequency Response (FFR) to enhance frequency control in power systems with low inertia is investigated in detail. A Generic System Frequency Response (GSFR) model taking into account of the penetration level of Non-Synchronous Generation (NSG) and FFR has been developed and used to investigate the impact of reduced inertia on frequency control and demonstrate that the amount of reserve power to be scheduled can be significantly reduced with the deployment of FFR. The impact of the different FFR resources' characteristics (e.g. response delay, ramp rate, etc.) on the effectiveness of frequency control is also investigated, based on which the desirable specifications for FFR schemes are summarised. These desirable properties of FFR schemes are taken into account in the design of a wide-area monitoring and control system termed ‘Enhanced Frequency Control Capability (EFCC)’, which is proposed for the delivery of FFR in the future Great Britain transmission system. The design and operation of the EFCC scheme are presented, along with a case study demonstrating its effectiveness in enhancing the frequency control.

Complex harmonic analysis (CHA) provides a computationally efficient alternative to finite element analysis (FEA) for modelling multi-phase wound field synchronous machines. This paper presents three methods that could improve the accuracy of the CHA: by improving the equivalent air gap, by including a saturation factor, and by using image methods for computing end-winding inductance. The comparison against a finite element method and experiment shows the improved complex harmonic method could predict waveforms reliably, where a dynamic finite element method would be time-consuming. This paper shows the method is valid for a five-phase generator.

A modelling and stability analysis method of a virtual synchronous machine (VSM) is proposed in this study. VSM technology is used to solve the problems caused by the high permeability of distributed energy. Though VSM has inertia and damping characteristics, it is more prone to oscillate than a synchronous generator because of the characteristic and unexpected harmonics generated by power electronic elements. The traditional linear time invariant based model is difficult to analyse a time-variant system and cannot analyse harmonic coupling mechanism accurately. This study proposes a new model of a grid-connected VSM by using the harmonic state-space modelling approach, which is a typical linear time-varying periodic model and considers different harmonics of the system. The stability of the system is analysed by eigenvalues of system matrix, which is based on the harmonic state-space model. The theoretical modelling method and stability analysis are verified by direct time-domain simulation results.

Unlike the synchronous generators, the short-circuit current provided by the photovoltaic (PV) sources is limited by the grid-connected inverters and closely related to the normal conditions, which impacts the protection of the power systems. This study deals with the protection of the power lines (distribution feeders) that connect the PV power plants (PVPP) to the grid; the first part of this study analyses the impact of the grid-connected PV (GCPV) system on the conventional distance protection. A specific coordination between the over-current protection of the grid side and the distance protection of the PVPP side was used to eliminate this impact. This coordination requires delaying the distance protection and keeping the inverter connected to the grid, which was achieved by fault ride-through (FRT) feature accompanied with the inverter control during the fault condition. The second part studies the possibility of using the double-end-impedance-based fault location on the PVPP power lines, and some FRT strategies were suggested to reduce the effect of the inverter control loops on obtaining the accurate fault location. The proposed work was verified by GCPV system model using MATLAB/Simulink software.

Regarding PMSG-based wind turbine generation system, this study proposes a super-capacitor energy storage unit (SCECU) connected parallel to the DC-link of the back-to-back converter to enhance its high voltage ride through performance. The analysis of the operation and control for the grid-side converter and SCECU are conducted. Based on real-time digital simulators (RTDS), a model of PMSG-based wind turbine with SCECU is developed, and the simulation results show that the SCECU absorbs the imbalanced energy and the grid-side converter absorbs inductive reactive power during the period of voltage swell. The simulation results based on RTDS verify the correctness and feasibility of the high voltage ride through control strategy.

Considering the negative resistance effect of power electronic equipment, unstable resonance problems may occur between wind farms and converter stations. This paper focuses on the resonance stability problem of a wind power base coupling with a multi-level converter−high-voltage direct-current transmission (MMC–HVDC) system. Firstly, the s-domain impedance models of the doubly-fed induction wind generator, the permanent magnet synchronous wind generator, and the modular MMC are built, through the theoretical derivation and simulation test. Secondly, based on the s-domain nodal admittance matrix method, the resonance structure of the wind power base coupling with an MMC–HVDC system is analysed, including the resonance mode frequencies, the resonance mode damping factors, the nodal participation factors, and the nodal voltage mode shapes. Thirdly, the main factors influencing the resonance structure are studied, including the topology of the wind power base, the capacity of the wind turbines, and the operation mode of the MMC station. Finally, a brief conclusion ends the paper.

Energy is an important driving force for social development. The shortage of natural resources urgently promotes the research of energy and the development of related technologies. Meanwhile, the distributed generation is one of the most important approaches. Virtual synchronous generator (VSG) technology, as a kind of control strategy that can participate actively in grid power regulation, has drawn much attention. Traditional VSG technology uses the two-level inverter as the main circuit. Based on the theory of VSG and the matrix converter (MC) as the main circuit of the VSG, the control system can imitate synchronous generator's characteristics such as inertial characteristics, frequency response characteristics and frequency modulation. The method provides a feasible solution for the power conversion of the distributed generation, which is the AC output, omitting the procedure of AC–DC–AC. Through the simulation, it is verified that the VSG technology can be applied to the MC, which provides a new idea for the exchange transformation in distributed generation.

As China's AC transmission system with series capacitor and HVDC transmission technology are widely used in long-distance and large-scale power transmission network, sub-synchronous resonance (SSR) problem becomes more prominent. This paper studied induction generator effect (IGE) and torsional interaction (TI) of synchronous generator by focusing on amplitude and frequency modulation of synchronous generator's flux linkage. IGE involves only electric system dynamics, which will only cause amplitude modulation of flux linkage, resulting in one or two frequency component added in stator current of non-salient pole synchronous generator (NPSG) or salient pole synchronous generator (SPSG) respectively. TI involves both electric and mechanical system dynamics, which will cause both amplitude and frequency modulation of flux linkage, resulting in multiple sub and super-synchronous frequency components added in stator current by means of Bessel function. This result will provide a new reference to understand the SSR phenomena containing multiple frequency components in practical power system. The IEEE first benchmark model is used to demonstrate the accuracy of the analysis results.

In recent years, the low-frequency oscillation (LFO) problem has become increasingly significant with rapid increase in the size of AC/DC interconnected power system containing large-scale wind power. This article aims to conduct the LFO analysis when an offshore wind farm is integrated into the AC power system through module multilevel converter (MMC)-based high-voltage direct current (HVDC). First, the basic mathematical models including the MMC with detailed control strategy and the permanent magnet synchronous generator (PMSG) are described. The eigenvalue analysis is then applied to conduct the LFO study. An interconnected AC/DC test system with a wind farm connected though the MMC-based HVDC transmission line is designed as benchmark. To validate the analysis results, the detailed simulation models built under the PSCAD/EMTDC^TM environment together with the Prony analysis are carried out. Different simulation scenarios involving the changes of point of common coupling (PCC), wind speed, length of transmission line, and wind penetration level are elaborately studied. Some meaningful conclusions are drawn to provide basic foundation for the future design of damping controller to mitigate LFO.

Currently, virtual synchronous generator (VSG) technology is proposed to be a solution to stability controlling of micro-source inverter. However, the deviation from phase locked loop (PLL) influences the reference frequency calculating which is important to the VSG control system. This paper focuses on the virtual inertia controller in the VSG control system, an enhanced controller is achieved for avoiding the effect of PLL on the VSG dynamics. For analysing dynamic characteristics of the enhanced VSG control system, the linearised small-signal model of VSG in isolated operation is well established, including electrical system, enhanced virtual inertia controller, voltage and current controller, virtual impedance, and droop controller. By small-signal stability analysis, the influence rules of the main parameters (load, line, virtual inertia and proportional-Integral gain) on VSG system impacts are provided, which helps identifying possible feedback signals for controller design to improve the system stability. Results obtained from time-domain small-signal model are compared with those from the electrical simulation to verify its accuracy.

Aiming for integration of a large-scale of distributed generators (DGs) such as photovoltaic into the microgrids, the virtual synchronous generator (VSG) control concept has been developed. By this concept, DGs based on inverters can exhibit the characteristics of synchronous generators (SGs) such as the inertial, damping, and droop functions and can participate in the microgrid control and stability. Here, a detailed VSG model is presented and analysed in order to investigate that, VSGs have really the similar structure of SGs. Then, the paper studies the microgrid with both VSG and ordinary SG. Then, parallel arrangements of VSG-VSG are set up in order to compare the dynamic behaviour of the VSG with the standard SG in an isolated microgrid. The dynamic responses are compared under equal sharing and proportional sharing of the load demand. Simulation results are presented using matlab/simulink.

Modular multilevel converter-battery energy storage system (MMC-BESS) has a good engineering application. When MMC-BESS is connected to the grid, the real-time phase angle of grid is an important parameter. When MMC-BESS is connected to the grid, a simulation model based on virtual synchronous generator (VSG) is built in MATLAB. The results show the control algorithm based on VSG can improve the dynamic response of the system and stably detect the phase angle of the grid.

Studies demonstrate that the power converter has one of the highest failure rates in a wind turbine, with a key failure driver being the power module junction temperature (T(j)). This paper details an experimental setup for simplified emulation of wind turbine conditions on a power converter with infra-red sensing of IGBT T(j). Results are compared to previous simulation work for a PMSG wind turbine, with the same trend of increasing mean T(j) with wind speed found, and the need to use an equivalent generator reactance in highlighted. A commercial-scale prototype for more accurate wind turbine converter emulation is also detailed.

This study proposes a new back-to-back (BTB) current source converter (CSC) configuration and presents its application for variable speed wind energy conversion system. The proposed configuration consists of three cascaded ac−dc non-inverting buck-boost converter. The ac side of each converter is connected to an open-end winding of the three-phase permanent magnet synchronous generator and the dc sides are connected in parallel to a current source inverter (CSI) without the need for a dc-side capacitor. Ac−dc converters operate under discontinuous conduction mode, and they draw sinusoidal input currents at the unity power factor from fixed or variable magnitude and frequency supply. In addition, the proposed BTB CSC addresses one of the main drawbacks of the conventional CSI which is the over-voltage experienced by switches during commutation. Moreover, the switching losses of the proposed CSI are eliminated. The ElectroMagnetic Transients including DC (EMTDC)/Power System Computer Aided Design (PSCAD) software is used to assess the feasibility of the proposed topology during normal operating conditions and during low voltage ride through.

Conventional automatic voltage regulator (AVR) schemes use thyristor-based rectifiers in which the generator field voltage is varied by changing the thyristor's firing angle. The conventional AVR has been known to cause voltage sags and extended under-voltages, especially under fault conditions. Several control schemes such as digital proportional–integral–derivative (PID) controllers, adaptive control schemes, and intelligent control schemes have been proposed in the literature. However, these schemes increase the complexity of the system. This work proposes a DC−DC converter-based AVR as an alternative to the rectifier-controlled AVR. The DC−DC converter is arranged in step-down (buck-type) configuration. The DC−DC converter uses metal-oxide-semiconductor field-effect transistor/insulated-gate bipolar transistors and has a faster response than the thyristors, thereby resulting in faster and precise control of the generator field voltage. The DC−DC converter is connected to the field circuit of the generator, and the terminal voltage of the generator is regulated by a PI controller by varying the converter duty cycle to control the generator field voltage. Simulation and experimental investigation has shown that the proposed scheme exhibits a better transient response and is robust under DC input voltage variation and generator load changes.

The interaction between renewable energy generation and high-voltage direct current (HVDC) transmission system becomes an essential cause of the subsynchronous oscillation (SSO) problem. In this study, the permanent magnet synchronous generator-based wind farm connected to HVDC transmission system is studied. The susceptivity analysis is carried out to find the main influence factors of the SSO problem. Besides, a parameter design method for subsynchronous damping controller is proposed. The configuration and the performance indicator are discussed, and then the optimisation algorithm is developed based on hybrid genetic algorithm and electromagnetic transient simulation. Finally, the proposed design method is validated by simulation in PSCAD/EMTDC.

The high-voltage DC transmission (HVDC) technology has become more and more popular for grid integration of large and remote wind farms. There was the new form of SSO appeared in Xinjing Grid which was the PMSG integration through an HVDC system, so the SSO was studied. The detailed model of the wind power transmission system through the two-terminal HVDC system was established. With the small signal method in case of the SSO fed into the system, due to the characteristics of the PMSG and HVDC topology, the expressions of the output signal of SSO were derived. Based on the ideal model，a control scheme for SSO suppression was proposed from the perspective of the PMSG controller. Finally, verified the rationality and effectiveness of the method based on the simplified Xinjiang grid model.