A synchronverter is an inverter that mimics synchronous generators, which offers a mechanism for power systems to control grid-connected renewable energy and facilitates smart grid integration. Similar to other grid-connected inverters, it needs a dedicated synchronization unit, e.g., a phase-locked loop (PLL), to provide the phase, frequency, and amplitude of the grid voltage as references. In this paper, a radical step is taken to improve the synchronverter as a self-synchronized synchronverter by removing the dedicated synchronization unit. It can automatically synchronize itself with the grid before connection and track the grid frequency after connection. This considerably improves the performance, reduces the complexity, and computational burden of the controller. All the functions of the original synchronverter, such as frequency and voltage regulation, real power, and reactive power control, are maintained. Both simulation and experimental results are presented to validate the control strategy. Experimental results have shown that the proposed control strategy can improve the performance of frequency tracking by more than 65%, the performance of real power control by 83%, and the performance of reactive power control by about 70%.

https://www.sheffield.ac.uk/polopoly_fs/1.323811!/file/IEEEPE_SelfSynch.pdf

Self-Synchronized Synchronverters: Inverters Without a Dedicated Synchronization Unit

This paper proposes a low-voltage ride-through scheme for the permanent magnet synchronous generator (PMSG) wind power system at the grid voltage sag. The dc-link voltage is controlled by the generator-side converter instead of the grid-side converter (GSC). Considering the nonlinear relationship between the generator speed ωm and the dc-link voltage Vdc , a dc-link voltage controller is designed using a feedback linearization theory. The GSC controls the grid active power for a maximum power point tracking. The validity of this control algorithm has been verified by simulation and experimental results for a reduced-scale PMSG wind turbine simulator.

LVRT Scheme of PMSG Wind Power Systems Based on Feedback Linearization

The growing trends in wind energy technology are motivating the researchers to work in this area with the aim towards the optimization of the energy extraction from the wind and the injection of the quality power into the grid. Over the last few years, wind generators based on permanent magnet synchronous machines (PMSMs) are becoming the most popular solution for the modern wind energy conversion systems (WECSs). This paper presents a concise review of the grid-integrated WECSs employing permanent magnet synchronous generators (PMSGs). It reviews the trends in converter topologies, control methodologies, and methods for maximum energy extraction in PMSG based WECSs, which have been reported in various research literatures primarily in reputed research journals and transactions during last few years. It also presents an overview to the grid interconnection issues related to output power smoothing and reactive power control in addition to fault-ride-through (FRT) and grid support capabilities of PMSG based WECSs. This review article will serve the researchers working in the area of grid-integrated PMSG based WECSs in the exploration of trends, developments and challenges in the past research works and in finding out the relevant references for their research work.

Grid-integrated permanent magnet synchronous generator based wind energy conversion systems: A technology review

With the wide application of voltage source converters (VSCs) in power system, dc-bus voltage control instabilities increasingly occurred in practical conditions, especially in weak ac grid, which poses challenges on stability and security of power converters applications. This paper aims to give physical insights into the stability of dc-bus voltage control affected by ac-bus voltage control in VSC connected to weak grid. The concepts of damping and restoring components are developed for dc-bus voltage to describe the stability of dc-bus voltage control. The impact of ac-bus voltage control on dc-bus voltage control stability can be revealed by investigating the impact of ac-bus voltage control on damping and restoring components essentially. Furthermore, the detailed analysis for the impact of ac-bus voltage control on damping and restoring components is presented considering varied ac system strengths, operating points, and ac-bus voltage control parameters. The simulation results from 1.5-MW full-capacity wind power generation system are demonstrated which conform well to the analysis. Finally, the experimental results validate the analysis.

DC-Bus Voltage Control Stability Affected by AC-Bus Voltage Control in VSCs Connected to Weak AC Grids

Static synchronous compensators have been broadly employed for the provision of electrical ac network services, which include voltage regulation, network balance, and stability improvement. Several studies of such compensators have also been conducted to improve the ac network operation during unbalanced voltage sags. This paper presents a complete control scheme intended for synchronous compensators operating under these abnormal network conditions. In particular, this control scheme introduces two contributions: a novel reactive current reference generator and a new voltage support control loop. The current reference generator has as a main feature the capacity to supply the required reactive current even when the voltage drops in amplitude during the voltage sag. Thus, a safe system operation is easily guaranteed by fixing the limit required current to the maximum rated current. The voltage control loop is able to implement several control strategies by setting two voltage set points. In this paper, three voltage support control strategies are proposed, and their advantages and limitations are discussed in detail. The two theoretical contributions of this paper have been validated by experimental results. Certainly, the topic of voltage support is open for further research, and the control scheme proposed in this paper can be viewed as an interesting configuration to devise other control strategies in future works.

Voltage Support Control Strategies for Static Synchronous Compensators Under Unbalanced Voltage Sags

Fast acting static synchronous compensator (STATCOM), a representative of FACTS family, is a promising technology being extensively used as the state-of-the-art dynamic shunt compensator for reactive power control in transmission and distribution system. Over the last couple of decades, researchers and engineers have made path-breaking research on this technology and by virtue of which, many STATCOM controllers based on the self-commutating solid-state voltage-source converter (VSC) have been developed and commercially put in operation to control system dynamics under stressed conditions. Because of its many attributes, STATCOM has emerged as a qualitatively superior controller relative to the line commutating static VAR compensator (SVC). This controller is called with different terminologies as STATic COMpensator advanced static VAR compensator, advanced static VAR generator or static VAR generator, STATic CONdenser, synchronous solid-state VAR compensator, VSC-based SVC or self-commutated SVC or static synchronous compensator (SSC or S2C). The development of STATCOM controller employing various solid-state converter topologies, magnetics configurations, control algorithms, switching techniques and so on, has been well reported in literature with its versatile applications in power system. A review on the state-of-the-art STATCOM technology and further research potential are presented classifying more than 300 research publications.

Static synchronous compensators (STATCOM): a review

This paper is focused on design and modeling of a new fundamental frequency switching based 24-pulse 2-level plusmn100 MVAR STATCOM in Matlab platform for high power applications Only four 6-pulse elementary gate turn off voltage source converters (GTO-VSC) along with single stage magnetics meeting dual objectives of magnetic summing circuit and coupling transformer at PCC, and PI-controllers employing principle of phase angle control, are modeled to achieve an improved performance to regulate load voltage in an AC network with THD values within limits

A New 24-Pulse STATCOM for Voltage Regulation

A multi-pulse GTO based voltage source converter (VSC) topology together with a fundamental frequency switching mode of gate control is a mature technology being widely used in static synchronous compensators (STATCOMs). The present practice in utility/industry is to employ a high number of pulses in the STATCOM, preferably a 48-pulse along with matching components of magnetics for dynamic reactive power compensation, voltage regulation, etc. in electrical networks. With an increase in the pulse order, need of power electronic devices and inter-facing magnetic apparatus increases multi-fold to achieve a desired operating performance. In this paper, a competitive topology with a fewer number of devices and reduced magnetics is evolved to develop an 18-pulse, 2-level + 100MVAR STATCOM in which a GTO-VSC device is operated at fundamental frequency switching gate control. The inter-facing magnetics topology is conceptualized in two stages and with this harmonics distortion in the network is minimized to permissible IEEE-519 standard limits. This compensator is modeled, designed and simulated by a SimPowerSystems tool box in MATLAB platform and is tested for voltage regulation and power factor correction in power systems. The operating characteristics corresponding to steady state and dynamic operating conditions show an acceptable performance.

Modeling of 18-Pulse STATCOM for Power System Applications

Multi-pulse topology of converters using elementary six-pulse GTO - VSC (gate turn off based voltage source converter) operated under fundamental frequency switching (FFS) control is widely adopted in high power rating static synchronous compensators (STATCOM). Practically, a 48-pulse (6×8 pulse) configuration is used with the phase angle control algorithm employing proportional and integral (PI) control methodology. These kinds of controllers, for example the ±80MVAR compensator at Inuyama switching station, KEPCO, Japan, employs two stages of magnetics viz. intermediate transformers (as many as VSCs) and a main coupling transformer to minimize harmonics distortion in the line and to achieve a desired operational efficiency. The magnetic circuit needs altogether nine transformers of which eight are phase shifting transformers (PST) used in the intermediate stage, each rating equal to or more than one eighth of the compensator rating, and the other one is the main coupling transformer having a power rating equal to that of the compensator. In this paper, a two-level 48-pulse ±100MYAR STATCOM is proposed where eight, six-pulse GTO-YSC are employed and magnetics is simplified to single-stage using four transformers of which three are PSTs and the other is a normal transformer. Thus, it reduces the magnetics to half of the value needed in the commercially available compensator. By adopting the simple PI-controllers, the model is simulated in a MATLAB environment by SimPowerSystems toolbox for voltage regulation in the transmission system. The simulation results show that the THD levels in line voltage and current are well below the limiting values specified in the IEEE Std 519-1992 for harmonic control in electrical power systems. The controller performance is observed reasonably well during capacitive and inductive modes of operation.

Analysis of a Harmonics Neutralized 48-Pulse STATCOM with GTO Based Voltage Source Converters

A modified three-level 48-pulse ±100 MVAr static synchronous compensator (STATCOM) employing four pairs of elementary six-pulse neutral-point diode-clamped gate turn-off thyristor-based voltage source converters with fundamental frequency switching modulation is realised using angle control methodology and pre-calculated dead angle (the duration in which converter-terminal voltage is clamped to zero) in d–q synchronous rotating frame. With a magnetics configuration designed for summing up of output voltages of four converter pairs and simultaneously stepping up of its voltage to the transmission level, and employing proportional and integral controllers in the control, the operating performance equivalent to 96-pulse STATCOM is achieved through optimising harmonics distortion. This compensator is modelled and its performance is simulated in MATLAB using SimPowerSystem toolbox for voltage regulation in transmission system. The obtained results show reasonably good characteristics of the compensator to control system dynamics under steady-state and dynamic system conditions.

R. Saha

Papers

Static synchronous compensators (STATCOM): a review

A New 24-Pulse STATCOM for Voltage Regulation

Modeling of 18-Pulse STATCOM for Power System Applications

Analysis of a Harmonics Neutralized 48-Pulse STATCOM with GTO Based Voltage Source Converters

Improved 48-pulse static synchronous compensator for high-voltage applications