Multi-Infeed HVDC Interaction Studies Using Small-Signal Stability Assessment

This paper presents an analysis of multi-in-feed HVDC interactions using small signal analysis techniques. The modeling details necessary to adequately represent the dynamics of the HVDC converters and the AC network are investigated and the models are validated against an Electromagnetic Transient Simulation program. The paper shows that AC network dynamics must be modeled in order to obtain meaningful results from the small signal stability study. A small test system with two HVDC in-feeds is then used to demonstrate the presence of interactions in that system. The case studies presented in the paper indicate that it is possible to have interactions between the HVDC terminals in an AC system. The paper recommends that a small signal interaction study similar to what is presented in the paper should be performed to identify these interactions.

Faster calculation methods are desired for electromagnetic transient (EMT) simulation programs, since circuits simulated are becoming increasingly larger and more complex. If there are time-varying and non-linear elements in the circuit to be solved, the solution process requires repeated factorization of the coefficient matrix of the circuit equations. The repeated factorization, which is often called refactorization, significantly increases the computation time. Up to now, the bottom right arranging (BRA) method has been applied for refactorization in EMT simulation programs in order to reduce the computation load. In this paper, the author proposes the use of the varying elements tracing (VET) method, which is one of the refactorization methods proposed for power flow calculations, for an offline EMT simulation program. Using some test circuits, the computation time by the BRA method and that by the VET method are examined, and the following facts are found. In most test circuits, both methods greatly improve refactorization speed, but in some test circuits, the BRA method shows degraded refactorization speed due to increased fill-ins. As for the entire EMT simulation time, the VET method improves the refactorization speed by a factor of 1.6 times at most. On the other hand, the BRA method improves the refactorization speed by a factor of 1.5 times in the best case but degrades it in some test circuits.

Considering electrical network as a continuum has become popular for electromechanical wave analysis. This paper reviews the concept of electromechanical wave propagation. Analysis of large number of generator ring system will be an easy way to illustrate wave propagation. The property of traveling waves is that the maximum and minimum values do not occur at the same time instants and hence the difference between these time delays can be easily calculated. The homogeneous, isotropic 10 generator ring system is modeled using electromagnetic transient simulation programs. The purpose of this study is to investigate the time delays and wave velocities using Power System Computer Aided Design (PSCAD)/Electromagnetic Transient Program (EMTP). The disturbances considered here are generator disconnections and line trips.

To address the limited speed and scale of the simulation of modular multilevel converter (MMC) in electromagnetic transient simulation program (EMTP), this paper put forward a MMC fast electromagnetic transient model based on rotation transformation. The crucial advantage of the rotation transformation is to reduce the fundamental frequency of the original signal via the rotation transformation of coordinate system. With proposed method implemented in the EMTP of MMC, the simulation step size can be relatively increased, and the fast electromagnetic transient simulation can be achieved. In this paper, the equivalent model of MMC switch function model expressed by the state equation is derived. Based on this, the novel MMC model based on rotation transformation is further derived combined with the concept of rotation transformation. Then, the state equation of proposed novel MMC model is calculated in Matlab, and the simulation results and simulation time-consuming are compared with which of 51-level topological MMC electromagnetic transient model established in PSCAD/EMTDC. The results of comparative analysis show that the simulation duration of the MMC model based on rotation transformation is far less than the traditional MMC electromagnetic transient model without sacrificing accuracy.

Conventional fault analysis algorithms may not be suitable for the AC/DC hybrid systems. To solve this problem, a mathematical model for HVDC (High Voltage Direct Current) system, which is suitable for analyzing faults in AC system, is presented in this paper. Firstly, based on the switching functions method and sequence components method, an equivalent model of HVDC converter under the asymmetric AC system is put forward. The unique advantage of the converter model lies in that it has taken harmonic characteristics into account and can improve the calculation accuracy significantly. Secondly, response performances of HVDC control system in cases of AC faults are analyzed and expressions for the control response characteristics are discussed under 3 modes. Finally, a general algorithm is developed to incorporate the proposed HVDC model into the conventional AC short-circuit calculation algorithm. The effectiveness of the proposed HVDC model and algorithm has been verified by simulation studies and by comparing the results with those obtained from the EMTDC simulations.

Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) technology is a very attractive power transmission technology. The VSC based HVDC transmission uses IGBT (Insulated Gate Bipolar Transistors) switches having turn-on and turn-off capability and having special characteristics compared to Current Source Converter (CSC) based conventional HVDC technology. Among various control methodologies used for VSC based HVDC, vector control, nonlinear control and independent P-Q controller based on analytic expressions have been developed and analyzed in this work for sending end control station 1. Similarly, in the receiving end station 2, DC and AC voltage controllers are realized with traditional PI controllers have been analyzed. The performance of the proposed controller has been investigated under different operating conditions including post fault recovery. The effectiveness of the proposed various controllers are demonstrated through digital simulation studies on VSC-HVDC transmission system, using PSCAD/EMTDC software package under short time Line-Ground fault in both AC systems in two terminal VSC - HVDC systems.

SUMMARY This paper presents details on the implementation and application of power hardware in-the-loop simulation for several physical high voltage direct current (HVDC) simulators using transmission line modeling (TLM) method. The implementation of the TLM method to the decoupled simulation of HVDC links is presented. A small test system is then used to demonstrate the feasibility of the TLM decoupling method. A real-time interface method relying on the propagation delay of transmission lines is developed and verified. Case studies presented in this paper indicate that it is possible to have interactions between the HVDC terminals in an AC system. A PHIL simulation similar to what is presented in this paper will provide an alternative to study the muti-infeed power grid. Copyright © 2012 John Wiley & Sons, Ltd.

Arcing horns interrupting fault current is a new type of arcing horns installed on transmission-line towers, which act to interrupt fault current independently within one cycle of commercial frequency. In this paper, we have developed an Electromagnetic Transients Program (EMTP) simulation model of the arcing horns interrupting fault current to ascertain the application effects and devise an installation strategy. The EMTP simulation model can consecutively simulate the entire process, from fault occurrence to fault clearing, by integrating two different kinds of macroscopic arc models, and is capable of dealing with the entire short-circuit current range by taking into account the characteristics of the arc parameters. Finally, we have verified the validity of the EMTP simulation model by comparing simulation and experimental waveforms, and have shown the simulation of current and voltage waveforms in the event of a system fault by using the Japanese Power System Model as an application example of the EMTP simulation model.

Summary form only given. Small signal stability analysis in a power system is typically concerned with electromechanical oscillations. For this purpose, it is adequate to model the transmission system using a constant admittance matrix. For torsional oscillations and HVDC interactions, the frequency of interest is much higher and the constant admittance representation is not sufficient. This paper proposes a hybrid model, which allows the parts of the transmission network in the vicinity of HVDC converters or any other dynamic devices to be modeled with their dynamics and the remaining parts to be modeled as constant admittances. The proposed hybrid methodology for small signal stability assessment is the main contribution of this paper. The proposed methodology is validated against an electromagnetic transient simulation program (PSCAD/EMTDC) using time responses. The proposed model and two other small signal models are compared against each other in the frequency domain using modal analysis.

(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.

The application of power electronics can introduce flexibility into power systems, but it can also cause harmonic problems. As a typical application of power electronics, high-voltage direct current (HVDC) transmission exhibits very prominent harmonic problems. In recent years, many HVDC protection maloperation events caused by harmonic problems have occurred in the field. The phenomena of harmonic transfer and amplification in HVDC transmission have an undeniable impact on the overall operation of a power system. Previous studies of these phenomena have relied mainly on detailed electromagnetic transient simulations. However, these simulations normally suffer from a large computational burden and low efficiency due to the complexity of the models and the small simulation time steps used. Therefore, an efficient analysis model of harmonics for various HVDC operating modes is presented in this paper. This model is based on port theory; every part of the HVDC transmission system is represented by an equivalent two-port component to construct a unified equivalent circuit. The proposed model is simple to calculate, and only the connections of the two-port components need to be changed to model different operation modes. The proposed two-port model is applied for the harmonic analysis of an HVDC transmission system, and the validity and accuracy of the model are confirmed by detailed electromagnetic simulations. Moreover, the model can be easily applied to HVDC systems for harmonic transfer and amplification, and the effects of the HVDC transmission power and smoothing reactor on harmonic transfer and amplification are analyzed.

Over long distances bulk power transfer can be carried out by a high voltage direct current (HVDC) connection cheaper than by a long distance AC transmission line. HVDC transmission can also be used where an AC transmission scheme could not (e.g. through very long cables or across borders where the two AC systems are not synchronised or operating at the same frequency). However, in order to achieve these long distance transmission links, power convertor equipment is required, which is a possible point of failure and any interruption in delivered power can be costly. It is therefore of critical importance to design a HVDC scheme for a given availability. (11 pages)

This paper deals with the analysis, design and control of a three-level 24-pulse Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) system. A three level VSC operating at fundamental frequency switching (FFS) is proposed with 24-pulse VSC structure to improve the power quality and reduce the converter switching losses for high power applications. The design of three-level VSC converter and system parameters such as ac inductor and dc capacitor is presented for the proposed VSC based HVDC system. It consists of two converter stations fed from two different ac systems. The active power is transferred between the stations either way. The reactive power is independently controlled in each converter station. The three-level VSC is operated at optimized dead angle (β). A coordinated control algorithm for both the rectifier and an inverter stations for bidirectional active power flow is developed based on FFS and local reactive power generation. This results in a substantial reduction in switching losses and avoiding the reactive power plant. Simulation is carried to verify the performance of the proposed control algorithm of the VSC based HVDC system for bidirectional active power flow and their independent reactive power control.

The interior overvoltage in the extra-high voltage (EHV) power system determines the equipment insulation and safe operation of the system. At first, the paper expounds the problems brought from the application of traditional fixed high voltage shunt reactor to limit the overvoltage in our national EHV power system. Therefore this paper brings forward a proposition to solve these problems by adopting magnetically controlled shunt reactor. This paper mainly introduces a novel type of controlled shunt reactor: Magnetically controlled shunt reactor (MCR). Then its structure and principle of magnetically controlled shunt reactor is presented. The harmonic distribution characteristics, the voltage-current characteristics, control characteristics and response time of MCR are analyzed successively. Afterwards, this paper takes a 500 kV EHV transmission line from Sanyou to Jingzhou for an example, the line model and MCR model are built up. Based on Electromagnetic Transient Program (EMTP), analysis and numerical simulation has been performed to calculate frequency overvoltage and switching overvoltage in the EHV power system with magnetically controlled reactor. (7 pages)

Large grid interconnection is an important form of Energy Internet. Ultra-high voltage direct current (UHVDC) plays an increasingly important role in the interconnection of large power grids, which helps to optimize the allocation of resources in a wide area. UHVDC hierarchical connection mode, in which the UHVDC inverter station connects to different voltage level AC system, provides an effective means to evacuate the high-capacity power of UHVDC access to the load center. DC terminal location selection is a multi-objective decision problem. Considering the characteristics and influence of UHVDC hierarchical connection to power grid, the comprehensive evaluation indexes are established in this paper, on the basis of which DC terminal location selection strategy by the use of Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) method. Finally, an actual project of UHVDC hierarchical connection to system is taken for an example and four different UHVDC hierarchical connection schemes are discussed in this paper. The result shows that effective decision support for UHVDC hierarchical terminal location planning can be provided with the proposed method.