FACTS for dynamic load balancing and voltage support in rail traction

This paper presents application guidelines for a new generic plant controller model that can be used for renewable plant dynamic system analysis that may include the control of static compensators (STATCOMs), static var compensators (SVCs), and/or energy storage devices. The authors originally presented the need for such a model to the Western Electricity Coordinating Council (WECC) and contributed to various stages of the new model development led by WECC. As Independent System Operators (ISOs)/Regional Reliability Organizations (RTOs)/Transmission Service Providers (TSPs) are moving toward acceptance and use of generic models only the authors have for the first time applied this new model for an operating renewable plant. The new model was configured, parameterized, and tuned to control two STATCOMs and multiple switched capacitors/reactors to represent a master/auxiliary control strategy as implemented in the plant. Dynamic simulations show that the configured and parameterized model is performing properly as expected. This tuned model was submitted to the system operator for compliance with the interconnection requirements of the plant. This is the first submission of the new generic model for this type of control for a renewable project. The submitted model is appropriate for dynamic simulations, which are routinely performed by ISOs/RTOs/TSPs in system impact studies or transmission planning studies. This paper provides useful guidelines in applying this new model for similar projects.

Enhancing voltage stability against system disturbances is essential for the reliable operation of LCC-HVDC systems under weak grid conditions and high levels of renewable energy integration. LCC-HVDC systems inherently possess the capability to provide over 90% of their rated reactive power as dynamic voltage support. However, conventional control schemes lack the mechanism to coordinate this reactive power in response to voltage disturbances. This paper introduces a droop-based inverter control strategy that enables dynamic DC voltage adjustment, thereby unlocking the reactive power regulation potential of LCC-HVDC systems. A theoretical framework is established to quantify the relationship among voltage deviation, disturbance magnitude, and grid strength. Analytical and simulation results demonstrate that allocating reactive disturbances to stronger grids can reduce voltage deviation by up to 25% and expand the maximum tolerable disturbance (MTD) by nearly 2 times. The proposed strategy offers a control-oriented solution that extends the functional role of LCC-HVDC beyond power transmission, enabling it to serve as a distributed voltage support asset in modern power systems.

With increasing focus on economical as well as environmentally friendly mass transit, rail transport is gaining momentum in many countries. This means investing in novel rail infrastructure as well as upgrading and electrifying existing facilities. The TEN (Trans-European Network) programme within the European Union is but one current example of the importance attributed to more and better mass transit all over the world. Modern high-speed rail traction systems exert heavy and complex loadings on supply grids. Usually, power is taken between two phases, thereby causing substantial unbalance between phases in networks originally not at all built for this kind of operation. Unless remedied, the result will be deterioration of power quality, not only harmful to the traction system itself, but also prone to spreading through the supply grid, disturbing other users of power in the same grid. Consequently, with electrified rail traction, it is essential to ensure that Grid Codes set up by grid owners and operators are fulfilled at all times. The paper treats STATCOM as a means for power quality improvement in grids dominated by high speed rail traction, as well as highlights a recent installation for this same purpose in Europe.

Wind generation is required to provide dynamic voltage support (DVS) during fault ride-through (FRT) to enhance system voltage stability. However, DVS causes the FRT behavior of wind generation to be segmented, and its impact on voltage stability in the wind farm (WF) integrated linecommutated converter-based high-voltage direct current (LCCHVDC) sending-end system remains unclear. To achieve a quantitative analysis to this gap, a novel quasi-static piecewise model for the sending-end system is developed, in which the segmented FRT behavior of WF is fully incorporated. Beyond the well-known saddle-node bifurcation (SNB), a new FRT-induced nonsmooth bifurcation (FRTNB), is identified. The influence of WF and the LCC-HVDC system on both SNB-related and FRTNBrelated voltage stability is thoroughly analyzed. Based on the acquired insights, a control strategy is proposed for the LCCHVDC system to proactively utilize its DVS capability, which is adaptive to the segmented FRT behavior of WF and can effectively enhance system voltage stability without requiring knowledge of grid parameters. Dynamic simulations validate the theoretical analysis and the effectiveness of the proposed method under both voltage sag and swell conditions

Dynamic voltage support is a type of critical ancillary service in electric power networks. With the increasing penetration of inverter-based renewable energy resources, utilizing their idle capacity to provide dynamic voltage support is becoming essential. However, the performance of traditional control methods is ofter limited due to the frequently changing operating point and network parameters, which are tuned only in a specific condition and suffer performance deterioration in others. This article presents a model-free dynamic voltage control method for inverter-based energy resources to supply dynamic voltage support. This method can dynamically estimate the system with pseudo gradients, in which a continuous parameter function is proposed to adaptively approximate the optimal intermediate parameters. It guarantees the performance when the host system operating states vary without tuning the controller's parameters, while in comparison the PID controllers require the parameters being tuned repeatedly. Both numerical simulation and field tests demonstrate that the proposed method significantly outperforms the classic PI and adaptive PI controllers, especially in the case when the host system varies over time.

Recently, transformerless (TL) inverters are being extensively used in small-scale photovoltaic (PV) systems due to their compact-size, lighter-weight, lower-cost and higher-efficiency compared to their counterpart with transformer. However, the growing penetration of these small-scale based PV systems and low-inertia induction motor (IM) loads in low-voltage distribution networks (DNs) make the grid more vulnerable to short-term voltage stability (STVS). Hence, this paper thoroughly examines the STVS of DN with high-penetration of TL-PV units, and provides countermeasures by TL-PV systems to mitigate any short-term voltage instability. The detailed dynamic model of the TL inverter is developed first. Next, three control strategies: (1) constant peak current (CPC), (2) constant active current (CAC), and (3) constant active power (CAP) with low voltage ride through (LVRT) and dynamic voltage support (DVS) capabilities are proposed to improve STVS. The impacts of different level of PV penetrations and LVRT capability of TL-PV inverters on the STVS are explored. Moreover, countermeasures, such as LVRT with CAC and CAP controls and DVS by the TL-PV systems are designed and implemented. Several case studies are carried out on an IEEE 4 bus system first, and later extended to IEEE 13 node test feeder. The results show that DVS can further improve STVS in residential DNs. In addition, CAP and CAC control strategies can speed-up the postdisturbance voltage recovery compared with the conventional CPC control.

Now-a-days renewable energy sources are playing a vital role in global energy systems. Wind energy conversion method is cheaper compared to other conversions such as solar and tidal etc. There are many power quality issues in wind farms such as voltage sag, swell and harmonics arise due to grid, nonlinear load and three phase faults. These voltage fluctuations can be eliminated with the help of advanced reactive power compensation devices such as SVC (Static Var Compensator), STATCOM (Static Synchronous Compensator) and DVR (Dynamic Voltage Restorer). To reduce such power quality issues, it is essential to design, model and analyse the FACTS (Flexible AC Transmission line) device. In this project, STATCOM is used as a compensating device. STATCOM is a static synchronous generator operated as a parallel connection static reactive compensator. It is capable of generating or absorbing real and reactive power. Also, the compensator performance is analysed with the help of Proportional Resonant (PR) controller device. The STATCOM is connected parallel with the transmission line. When fault occurs, it will inject or absorb the reactive power to the line through the transformer according to the voltage requirement. The whole system is simulated through MATLAB/SIMULINK application. The simulation gives the effective influence of STATCOM on the improvement of voltage profile as well as reduction of Harmonics in grid connected Wind Energy Conversion System.

Voltage stability is very important in a power system operation. This paper presents a detailed study on voltage stability and line losses of a radial distribution system incorporating Renewable Distributed Generation (RDG) using a two method approach of Index Based planning (IBP) and Genetic Algorithm (GA). RDGs have been largely adopted in most power systems due to their ability to meet increased electrical load faster and cheaply without expanding the transmission network, they have minimal environmental effects and they cannot be depleted. Formulation and solution of distribution voltage stability problem of a distribution network with RDG is very critical in the modern power system operation and control. Researchers have previously solved the voltage stability problem by obtaining optimal placement of Flexible AC Transmission Systems (FACTS), Static VAR Compensators (SVC) and static capacitors to provide reactive power to the system and improve the voltage profile. Other researchers have used Under Frequency Load Shedding (UFLS) and others have used optimal sizing and siting of DG units to improve the distribution network's voltage profile. This paper outlines a detailed research of the effect of interconnecting RDGs on the voltage stability and line loss of the IEEE 14 bus radial distribution system using Index Based planning and Genetic Algorithm. The results indicate the total voltage deviation and the total line loss for Photovoltaic DG, Wind DG and Static compensators (STATCOMS). In addition it gives a detailed formulation of the problem. The results obtained shows that appropriate sizing and location of RDGs improves the voltage profile and reduces the line losses of the radial distribution system.

Design and installation of a ±250 kVAr D-STATCOM for a distribution substation

Inverter-based resources (IBRs) are required to provide dynamic voltage support (DVS) during voltage dips to enhance the low-voltage ride-through capability. In this paper, a model-free control method is developed to achieve the optimal DVS (ODVS) without relying on the knowledge of grid parameters. Delving into the optimum trajectory of the ODVS problem, it is found that the current constraint and the maximum active power constraint of IBRs are both binding, or one of them is binding. This inspires us to search for the optimum in a closed-loop way by a perturb-and-observe (P&O)-based optimum seeking (OS) controller with either the power factor angle or the reactive current being the manipulated (perturbed) variable. The system is guaranteed to converge asymptotically to the optimum provided the stepsize sequence is diminishing and non-summable. The proposed model-free optimal control is finally implemented within a single-stage photovoltaic (PV) system, where dynamic simulations demonstrate the optimal and fast DVS performance. Moreover, the implementation strategy for other types of IBRs that are not self-protected by nature is also discussed.

The dynamic stability improvement of a longitudinal power system using a power system stabilizer (PSS) and a static VAr (reactive volt-ampere) compensator (SVC) is reported. An analytical approach is developed for the determination of PSS parameters. The effect of static VAr compensators installed at several different locations along 345 kV trunk lines on system responses is examined. Results from time-domain simulations indicate that the PSS and the SVC are very effective in damping system oscillations. >

To ensure a good quality of electrical energy at all points of the distribution network, the grid operators install several static compensators of reactive power (STATCOM) and many active power filters at some common coupling points PCC. This paper focuses on the operation of a static converter connected at PCC point performing the function of a static compensator (STATCOM) in the case of a voltage drop exceeding the standards and ensuring the function of an active power filter (APF) in the opposite case. This static converter ensures the regulation of the PCC voltage and eliminates the current harmonics produced by the non-linear loads connected to the grid. The control laws are designed to allow the converter to switch between the operation of a STATCOM and an active power filter. Furthermore, this research article represents an important aspect in scientific research concerning the control of FACT devices. A static reactive power compensator (STATCOM) for reactive power control and an active power filter (APF) performance are investigated by Improved Virtual flux based Direct Power Control (IVF_DPC). The association of a static converter with the (IVF_DPC) control allows to react in a very fast time to the grid perturbations. Simulation results of an improved control scheme (IVF-DPC) for STATOM / APF are affected by MATLAB / Simulink software to demonstrate the dynamic performance of the proposed control strategy. The adopted control scheme is implemented using the dSPACE 1104 board. Real time implementation results prove the efficiency and uniqueness of the adopted strategy

The transmission of electrical energy using AC started at the end of the 19th century and replaced smaller existing local DC distribution systems. By extending local supply areas and providing energy transfer over longer distances various problems regarding mainly voltage control and stability were observed caused mainly by reactive power unbalances in the systems. Switched reactive power compensation (shunt capacitors, shunt reactors) were primarily used to control the steady state system voltages. Dynamic reactive compensation were based on rotating machines, eg synchronous condensors. In the mid 60's of the 20 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> century first static compensation devices, ie DC controlled reactors (mercury arc bulbs) and thyristor controlled devices (thyristor switched capacitors-TSC, thyristor controlled reactors-TCR) came up. Fast response times, lower losses and less maintenance requirements of thyristor controlled devices resolved the limitations of rotating machines and DC controlled devices. Evaluation of operating losses resulted more and more in the use of Static Var Compensators (SVC) which were built up by combinations of TCR and TSC branches. These shunt devices together with series connected devices TCSC (thyristor controlled series capacitors) provided a base for FACTS (Flexible AC transmission systems). FACTS allows transmission systems to be more efficiently used with regard to improved dynamic system voltage control on one side and higher power transfer capabilities on the other side. In AC transmission systems SVCs are installed presently with a total size of more than 100000 MVA. New power electronic devices (GTOs, IGCT, IGBT) were introduced to the FACTS market and allowed the use of current and voltage source converters (VSC) for providing fast reactive power compensation. With further improvement of controls, development of semiconductors and new arrangements of VSC technology to-days reactive power compensation is a key method to establish a reliable AC power transmission. The paper gives an overview from the early days of reactive power compensation towards the present transmission system situation. It compares early solutions with to-days devices, provides decisive factors for the steps in development of arrangements and discusses advantages of the present devices.

A new approach for static reactive power compensation has been presented in this paper. Conventional thyristor controlled Static VAr Compensators (SVC) have inherent disadvantages like slow response times and poor harmonic performance as these FACTS devices are based on slow switching power electronics devices. Alternately, Pulse Width Modulated (PWM) dc-ac inverters and direct ac-ac converter structures offer higher bandwidth and push the spectral content to higher switching frequencies that are easier to filter. However, the application space of this approach is limited by the low voltage blocking capability of power devices employed in these converters. A center-point-clamped ac-ac direct power converter has been reported recently in literature which operates on the principle of neutral-point-clamped dc-ac inverter. By clamping the grid voltage to its mid-point, the center-point-clamped converter structure reduces voltage stress on the bi-directional switches by 50%. Compared to the conventional two-level and multilevel dc-ac inverters, the proposed compensator based on direct ac-ac conversion has a simpler structure and control. The operating principle as well as dynamic analysis for the proposed VAr compensation approach has been presented in the paper. A feedback controller has been designed for closed loop control. Simulation results presented in the paper verify that proposed converter offers better control of reactive power, retrofit capability, and reduced voltage stress on the bi-directional switches. Furthermore, it has been shown that leading and lagging reactive compensation can be accomplished with a smooth control of the reactance through duty cycle modulation.

As power electronic based controllers and loads become more prevalent in power systems, there is a growing concern about how the harmonics generated by these controllers and loads affect the power quality of the system. One widely used power electronic based load is the Variable Frequency Drive (VFDs) used to vary the speed of an induction motor; whereas a common example of a power electronic based controller used in power systems is the Static VAR Compensator (SVC) for improving a system’s power factor. In this thesis, the harmonic content and overall performance of a system including both a VFD and a SVC will be studied and analyzed. Specifically, the cases of no compensation, static capacitor compensation, and power electronic based static VAR compensation are examined. A small-scale model of a system for study was constructed in lab. Several cases were then performed and tested to simulate a system which contained both fixed and power electronic based harmonic generating loads. The performance of each case was determined by total harmonic current and voltage distortions, true power factor, and RMS current levels at different points in the system.