Analysis and Control of Bipolar LVDC Grid With DC Symmetrical Component Method

In this paper, the simulating dynamic behavior of a grid-linked wind farm power system in Taiwan is investigated. Some models including wind model, aerodynamic model, machine model, electromechanical model are employed to cater for the dynamic behavior of the system. Simulation results show that the system operates satisfactorily under normal and abnormal conditions.

The short circuit fault and the unbalanced three-phase loads are the common reasons causing unbalanced voltage distortion (UVD) in the distribution system. In the past, the decline in electric motor performance and life cycle was a well-known fact caused by UVD. However, the UVD deserves more attention in recent years due to the rapid growth of the wind power penetration in the distribution system. The wind power system operated under the UVD suffers not only the pulsation torque but also penetrates unbalanced current and consequently deteriorates the UVD of the distribution system. This paper presents an approach to dynamically resolve the symmetrical components from the three-phase instantaneous voltages in order to clarify the degree of the UVD for compensation. The vector representation for the symmetrical components involves the information of angular frequency into the steady-state phasor, which can push much of the complexity down on the controller design for compensating the UVD. A dynamic voltage restorer (DVR) is employed as a voltage compensator in the series path of the feeder for refining the percent voltage unbalance of the UVD. The impact of the UVD reduction on a grid-tied 3-kW squirrel cage induction generator (SCIG) shows the effectiveness of the proposed approach.

Micro-grids are becoming increasingly popular to consumers and as such in the future, a great number of them will be installed at consumer's sites. In this way, conventional distribution grid structures that accept distributed generation connections may face serious problems when their grid control and protection functions become more complicated. These problems have led to a new trend of generation power locally at distribution voltage level by using non-conventional sources like wind power, fuel cells, micro-turbines and biogas and their integration into the utility distribution system. Micro-grids can cause several fundamental problems in its system and grid control when operate as autonomous mode. This paper is a review of fundamental key issues on micro-grids with respect to voltage and frequency flow grid control and protection of micro-grids. Moreover, in the paper the symmetrical current components are analyzed on a larger system of distribution network. All of the studies are simulated using DIgSILENT PowerFactory to establish claims in this paper.

Impact of FACTS devices on the voltage stability is usually determined with the power-flow calculations. Results, gained in this way, is sometimes difficult, if not impossible, to explain in qualitative way and to draw the general conclusions. Therefore analytical approach using simple basic models may submit more insight into basic features of tested device behavior in an electric power system (EPS) than numerical calculations at real-like system model. The paper presents such an approach for Unified Power-Flow Controller (UPFC), which may be considered a synonym for multi-parameter controlled FACTS device. The basic goal is to acquire impact of an UPFC on voltage stability enhancement and voltage control respectively. Therefore it is positioned into the standard model, widely used for voltage stability representation purposes. In the paper the mathematical solution of the problem is explained and the impact of the UPFC on voltage stability is shown.

This paper proposes a new method for determining the optimal placement and sizing of voltage balancers (VBs) in bipolar DC distribution systems. In bipolar DC systems, asymmetric structures, such as distributed generation, loads, and line parameters, can cause voltage imbalances with respect to the poles. Such imbalances can be mitigated by using VBs that balance local positive and negative voltages in a decentralized manner, similar to voltage compensators in conventional AC systems. In this study, a deterministic optimization model is formulated to minimize the installation cost of VBs while maintaining the voltage imbalance within the allowable range. The model is proposed with mixed-integer linear programming using the big-M method and current-injection power flow equations. Subsequently, the uncertainty boundaries of renewable energy sources and loads are incorporated to formulate a robust optimization model as a tri-level optimization problem, which is solved using a column and constraint generation framework. The case study validates the effectiveness of the proposed method for mitigating voltage imbalances with minimum investment cost in bipolar DC distribution systems.

The difference between positive and negative pole dc loads in a bipolar dc distribution network causes unbalanced current in the neutral line. This unbalanced current increases the power losses in the network and leads to unbalanced voltage in each node. An automatic commutation switch (ACS)-based method is developed in this study to suppress unbalanced current and voltage. Expressions of the unbalanced voltage and current of a single node and multiple nodes are derived. The equations indicate that the unbalanced power between positive and negative poles can be reduced by adjusting the power supply polarity (PSP) of the dc load. The PSP of the dc load and the corresponding switching command is determined by comparing the amplitude and polarity of the neutral line current. Afterwards, the switching command is implemented through ACS. The topology of ACS and the implementation of the switching command are presented. In addition, a simulation platform of a radial bipolar dc distribution network is built in MATLAB/Simulink. The platform verifies the effectiveness of the unbalanced voltage analysis method and the proposed suppression strategy. The proposed method is also validated using a scaled-down hardware setup in the laboratory.

Because of increased DG penetration level, maintaining a good voltage profile in distribution systems is challenging. In addition, because of slow expansion of distribution systems and fast growth of load, voltage stability becomes an important issue. Voltage and reactive power sensitivity, dV/dQ, is one of key important factors in maintaining a good voltage profile and analyzing voltage stability problem. In this work, we investigate some fundamental properties and identify several patterns of dV/dQ based on the IEEE 13-node test feeder. The results have important implications on the application of voltage control and on the validity of modal analysis for voltage stability analysis for three-phase unbalanced distribution systems.

The increasing penetration of inverter-based renewable energy sources has fundamentally altered the dynamic behavior of modern power systems by displacing conventional synchronous generation. Reduced system inertia, higher rates of change of frequency, modified fault responses, and complex voltage regulation have emerged as critical stability challenges in low-inertia grids. This paper presents a comprehensive study of power system stability under high renewable energy penetration through a structured literature review, a unified methodological framework, and a realistic transmission-level case study. The proposed framework integrates grid characterization, component and control modeling, multi-domain stability assessment, contingency analysis, and coordinated mitigation strategies using both phasor-domain and electromagnetic transient simulations. A three-zone network with varying levels of renewable integration is analyzed to quantify impacts on frequency, voltage, and small-signal stability following severe contingencies. Results demonstrate that stability degradation increases nonlinearly with renewable share in baseline grid-following configurations. The deployment of grid-forming inverter controls, synthetic inertia, and fast frequency response from battery energy storage significantly improves system performance by reducing RoCoF, restoring frequency nadir, and enhancing voltage recovery. Eigenvalue analysis further confirms improved damping of critical interaction modes. The findings highlight the necessity of hybrid control architectures and coordinated mitigation strategies to ensure reliable operation of future power systems with high renewable penetration.

This paper presents a new method for voltage stability analysis of unbalanced distribution systems connected with wind turbine energy systems (WTGSs). The developed continuation power flow uses secant predictor technique. The predicted solution is corrected using a radial power flow solver. The advantage radial load flow solver is interesting as it contains no matrices and hence it avoids the singularity found in Jacobian matrix. The WTGSs has been modeled based on sequence components. The developed model accounts for distribution system unbalance with the slip as independent variable which has been calculated during the power-flow solution process. A 33-node balanced feeder and The IEEE 13-node and 123-node distribution feeders with/without wind penetration are used to test out the developed method. Different case studies are used to test out the effect of WTGSs on both voltage profile and stability of unbalanced distribution systems.

Using VSC-MTDC for wind farm integration is considered to be a promising solution. In order to provide a stable ac voltage control and avoid active power control conflicts, the amplitude-phase control strategy, rather than the active/reactive power decoupled control strategy, is generally applied for the VSC connected to wind farms. However, using such a control strategy, the flexibility of power flow control will be partly lost. As the positive and negative pole dc networks in the bipolar VSC-MTDC can operate independently, a novel hybrid control strategy is proposed in this paper. In the proposed hybrid control strategy, the amplitude-phase control and active/reactive power decoupled control can be applied to the positive and negative poles, respectively. Hence, the hybrid strategy combines the advantages of the amplitude-phase control and active/reactive power decoupled control and can be adapted to bipolar VSC-MTDC for large-scale wind farm integration. Simulation results using PSCAD show that the hybrid control strategy can provide the stable ac voltage control and active/reactive power flow control for the integration of wind farms; moreover, such a hybrid control strategy is helpful to avoid the overload of dc lines, stabilize the dc voltage, and enhance the power transfer capability.

Nowadays, wind power is one of the most used renewable energy sources due to its economic possibilities. Consequently, penetration of wind turbines in electrical power systems has increased and they may influence overall power system behavior. It is therefore important to study the performance of wind turbines in electrical power systems and their interaction with other generation equipment and loads. In this paper, the behavior of a transmission system with high level of wind energy penetration using doubly fed induction generator (DFIG) based wind farms is analyzed. The technical factors analyzed are steady-state voltage profile, the dynamics during voltage sags and voltage stability.

SummaryHigh penetration of renewable energy in a power system may cause the problem of power dispatching and stability. The detailed dynamic behavior analysis for such a system is therefore important to the planning and operation of the power system. This study discussed the dynamic characteristics of an island power system with variable‐pitch wind turbines under high wind speed and high small hydraulic output power. The system primarily consisted of three diesel engine power generation systems, three constant‐speed variable‐pitch wind turbines, a small hydraulic induction generation system, and lumped static loads. The maximum penetration of renewable energy in this system could reach almost 60%. Detailed models based on MATLAB/Simulink were developed to cater for the dynamic behavior of the system. The results suggested that this island power system can operate stably in this operational mode with the help of variable‐pitch wind turbines. This study can serve as an important reference for planning, operation, and further expansion of island power systems. Copyright © 2014 John Wiley & Sons, Ltd.

(English) Dynamic behavior of modern power system is increasingly influenced by the dynamics of power converters. At the generation level, a high volume of wind and solar energy with their large-scale power converters are replacing conventional synchronous generators. At the transmission level, many High Voltage Direct Current (HVDC) projects based on Modular Multilevel Converter (MMC) technology are commissioned to transfer offshore wind to the load centers, interconnect two non-synchronous AC grids, or two different nodes of the same AC grid (as an embedded link) to enhance the security of supply. The consumption level also faces such high integration of power converters since the modern flexible loads, such as electric vehicles, industrial motor drives, and recent hydrogen electrolyzers, are connected to the power system via power converters. Such transformation in generation, transmission, and consumption levels leads to the power dynamics that have never been experienced before in the conventional power systems. The high penetration of power converters may cause several challenges: reduction in the system total inertia and short-circuit current, excitation of undesirable interactions among control loops, reduction in stability margin, and loss of devices in the context of fault-ride through (FRT). Such challenges can be addressed by appropriate adjustments in the control systems of power converters. At the transmission level, this thesis focuses on improving the dynamic behavior of HVDC systems. A methodology is suggested to study the small-signal stability and evaluate potential interactions that may happen among control loops of two HVDC converter stations. Next, a multivariable controller based on H-inf design practice is developed to optimize the dynamic behavior of the system. As a follow-up study, mu-synthesis control approach is adopted to make HVDC control resilient against AC grid impedance variation. In the context of FRT performance, the fault operation of the HVDC system is improved by appropriate modification of the MMC control loops. Finally, a brief study on the DC voltage droop control design is conducted on a multi-terminal HVDC system. At the generation level, this thesis investigates the dynamic behavior of grid-forming control during fault events in a low-strength power system. It investigates the complication related to the existing grid codes and suggests two FRT controls for AC voltage support during fault recovery. As the last step, this thesis evaluates the feasibility of modern flexible loads to provide grid-forming services to the low-strength power grid. A hydrogen electrolyzer is used as an example and the operational constraints related to the hydrogen production are linked to the grid-forming functionalities. (Català) El comportament dinàmic del sistema de potència modern està cada vegada més influenciat per la dinàmica dels convertidors de potència. A nivell de generació, un gran volum d'energia eòlica i solar amb els seus convertidors de potència a gran escala està substituint els generadors síncrons convencionals. A nivell de transmissió, molts projectes de corrent continu d'alta tensió (HVDC) basats en la tecnologia Modular Multilevel Converter (MMC) s'encarreguen de transferir l'eòlica marina als centres de càrrega o interconnectar dues xarxes de CA per millorar la seguretat del subministrament. El nivell de consum també s'enfronta a una integració tan alta dels convertidors de potència, ja que les càrregues flexibles modernes, com ara vehicles elèctrics, accionaments de motors industrials i electrolitzadors d'hidrogen recents, es connecten al sistema elèctric mitjançant convertidors de potència. Aquesta transformació en els nivells de generació, transmissió i consum condueix a una dinàmica de potència que mai s'havia experimentat abans en els sistemes de potència convencionals. L'elevada penetració dels convertidors de potència pot provocar diversos reptes: reducció de la inèrcia total del sistema i de la potència de curtcircuit, excitació d'interaccions indesitjables entre llaços de control, reducció del marge d'estabilitat i pèrdua de dispositius en el context de fallada (FRT). Aquests reptes es poden abordar mitjançant ajustos adequats en els sistemes de control dels convertidors de potència. A nivell de transmissió, aquesta tesi se centra en la millora del comportament dinàmic dels sistemes HVDC. Es proposa una metodologia per estudiar l'estabilitat del senyal petit i avaluar les interaccions potencials que poden passar entre els bucles de control de dues estacions convertidores HVDC. A continuació, es desenvolupa un controlador multivariable basat en la pràctica de disseny H-inf per optimitzar el comportament dinàmic del sistema. Com a estudi de seguiment, s'adopta un enfocament de control de sintetització-mu per fer que el control HVDC sigui resistent a les variacions en la impedància de la xarxa de CA. En el context del rendiment FRT, el funcionament de falla del sistema HVDC es millora mitjançant les modificacions adequades als bucles de control MMC. Finalment, es realitza un breu estudi sobre el disseny de control de caiguda de tensió de CC en un sistema HVDC multiterminal. A nivell de generació, aquesta tesi investiga el comportament dinàmic del control de formació de xarxa durant esdeveniments de falla en un sistema d'energia de baixa inèrcia. Investiga la complicació relacionada amb els codis de xarxa existents i suggereix dos controls FRT per al suport de la tensió de CA durant la recuperació de fallades. Com a darrer pas, aquesta tesi avalua la viabilitat de càrregues flexibles modernes per proporcionar serveis de formació de xarxa a la xarxa elèctrica de baixa resistència. Com a exemple s'utilitza un electrolitzador d'hidrogen i les limitacions operatives relacionades amb la producció d'hidrogen estan vinculades a les funcionalitats de formació de la xarxa. (Español) El comportamiento dinámico de los sistemas eléctricos modernos está cada vez más influido por la dinámica de los convertidores de potencia. A nivel de generación, un gran volumen de energía eólica y solar con sus convertidores de potencia a gran escala están sustituyendo a los generadores síncronos convencionales. A nivel de transmisión, se están encargando muchos proyectos de corriente continua de alta tensión (HVDC) basados en la tecnología de convertidores multinivel modulares (MMC) para transferir la energía eólica marina a los centros de carga, interconectar dos redes de CA no síncronas o dos nodos diferentes de la misma red de CA (como enlace integrado) para mejorar la seguridad del suministro. El nivel de consumo también se enfrenta a una integración tan elevada de los convertidores de potencia, ya que las cargas flexibles modernas, como los vehículos eléctricos, los accionamientos de motores industriales y los recientes electrolizadores de hidrógeno, se conectan al sistema eléctrico a través de convertidores de potencia. Esta transformación en los niveles de generación, transmisión y consumo da lugar a una dinámica de potencia nunca antes experimentada en los sistemas eléctricos convencionales. La alta penetración de los convertidores de potencia puede causar varios retos: reducción de la inercia total del sistema y de la corriente de cortocircuito, excitación de interacciones indeseables entre los bucles de control, reducción del margen de estabilidad y pérdida de dispositivos en el contexto de la conmutación por fallo (FRT). Estos retos pueden abordarse mediante ajustes adecuados en los sistemas de control de los convertidores de potencia. A nivel de transmisión, esta tesis se centra en mejorar el comportamiento dinámico de los sistemas HVDC. Se propone una metodología para estudiar la estabilidad de pequeña señal y evaluar las posibles interacciones que pueden producirse entre los lazos de control de dos estaciones convertidoras HVDC. A continuación, se desarrolla un controlador multivariable basado en la práctica de diseño H-inf para optimizar el comportamiento dinámico del sistema. Como estudio complementario, se adopta un enfoque de control mu-síntesis para que el control HVDC sea resistente a la variación de la impedancia de la red de CA. En el contexto del rendimiento FRT, se mejora el funcionamiento en fallo del sistema HVDC mediante la modificación adecuada de los bucles de control MMC. Por último, se realiza un breve estudio sobre el diseño del control de droop de tensión de CC en un sistema HVDC multiterminal. A nivel de generación, esta tesis investiga el comportamiento dinámico del control de formación de red durante eventos de fallo en un sistema eléctrico de baja tensión. Investiga la complicación relacionada con los códigos de red existentes y sugiere dos controles FRT para el soporte de tensión de CA durante la recuperación de fallos. Como último paso, esta tesis evalúa la viabilidad de las cargas flexibles modernas para proporcionar servicios de formación de red a la red eléctrica de baja potencia. Se utiliza como ejemplo un electrolizador de hidrógeno y se relacionan las restricciones operativas relacionadas con la producción de hidrógeno con las funcionalidades de formación de red.

High voltage direct current (HVDC) transmission offers several operation advantages over high voltage alternating current (HVAC) transmission systems. HVDC transmission lines have no frequency and hence minimal problems of harmonics, oscillations or transients; they actually damp out transients with effective controllers. Precise and fast power control in either direction is easier to achieve in HVDC than HVAC systems as dc power is minimally affected by reactance. This curtails system instability, cascading effect or voltage collapse. The ease of power flow control in HVDC lines makes it easy to achieve the maximum power and thermal capacity of transmission lines. It is also simple to control active power transfer at a predetermined level or even to modulate this to improve system damping. For weak AC grids, voltage source converter (VSC) based HVDC systems offer better controllability and grid flexibility for integrating intermittent renewable energy sources such as wind and solar-power. HVDC systems offer technical advantages: all angular stability problems disappear and even connection of systems at different frequency is feasible. The problems of cable charging current are eliminated. Thus cables can readily be used for underwater crossings. HVDC is the established technology of choice for long distance transmission and subsea electrical power transmission systems. In this paper, an investigation of voltage stability margin on Eskom high voltage eastern grid is carried out. Using the Newton Raphson iterative method, the impact of HVDC link controller and transformer tap changers on voltage stability were evaluated on Athene substation of Eskom eastern grid, an area of rapid load growth. Results obtained show that by introducing HVDC line, transmission capacity is enhanced, with lower line losses and improved voltage profile. Simulations were carried out using DigSILENT PowerFactory.

The distribution system is the final link between bulk power system and consumer end. A distinctive load flow solution method is used for analysis of the load flow of radial and weakly meshed network based on Kirchhoff’s Current Law (KCL) and KVL. This method has excellent convergence characteristics for both radial as well as weakly meshed structure and is based on bus injection to branch current and branch-current to bus-voltage matrix. The main contribution of the paper is: (i) an analysis has been carried out for a weekly mesh network considering number of loops addition and its impact on the losses, kW and kVAr requirements from a system, and voltage profile, (ii) different load models, realistic ZIP load model and load growth impact on losses, voltage profile, kVA and kVAr requirements, (iii) impact of addition of loops on losses, voltage profile, kVA and kVAr requirements from substation, and (iv) comparison of system performance with radial distribution system. Voltage stability is a major concern in planning and operation of power systems. This paper also includes identifying the closeness critical bus which is the most sensitive to the voltage collapse in radial distribution networks. Node having minimum value of voltage stability index is the most sensitive node. Voltage stability index values are computed for meshed network with number of loops added in the system. The results have been obtained for IEEE 33 and 69 bus test system. The results have also been obtained for radial distribution system for comparison.