The aim of this study is to use combinations of a novel DCCB arrangement and additional inductances on the DC line to reduce losses and capital costs and to improve system reliability, while ensuring continuous operation of the healthy parts in a multi-terminal HVDC system during a DC fault. The paper is organized as follows. The novel DCCB delta- configuration is proposed in Section II and its advantages over a conventional star-configuration are presented in detail. In Section III, protection structures combining delta DCCB configurations with DC inductances are introduced to isolate the DC fault and delay fault propagation through the healthy branches. DC fault tolerant operation with the proposed DC fault protection structures are assessed in Section IV, by considering a pole-to-pole DC fault at the DC-link node in a radial three-terminal HVDC system. Finally Section V draws the conclusions.
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network will then be the core of such an interconnection system. MTDC can also open new power market opportunities and result in better utilization of transmission lines, in the study, J.G. Slootweg.. For such a Multi-Terminal offshore network, where large power would be transmitted over long distance, application of high-voltage alternating-current transmission (HVAC) technology may be difficult to implement due to large amount of reactive power compensation required. Thus, an alternative is to use high-voltage direct-current transmission (HVDC) technology. Moreover, since the offshore network may act as a power pool where power may be injected to and extracted from the network at different nodes, flexibility to control direction of power whilst maintaining voltage in the network is required. For such a situation, implementation of voltage sourced converter HVDC (VSC-HVDC) technology is favorable, in the study, Z. Lubosny and Lars Weimers .
High voltage direct current (HVDC) systems are becoming increasingly prevalent within power transmission systems as they offer very high capacity connections; the opportunity to connect asynchronous or otherwise independent networks; and to connect over large distances with particular relevance to connecting subsea, thereby enabling connection of offshore wind at scale. As power electronic technologies, such systems additionally allow power flows to be controlled which can provide further benefits to existing alternating current (AC) network infrastructure, as seen in back-to-back configurations. Their comparative size (fewer conductors and greater use of a conductor cross-section without having a skin effect) also allows existing AC connections to be restrung for direct current (DC) toward hybrid grid applications , and HVDC technologies can even be combined toward multi-terminal direct current (MTDC) networks. The increasing prevalence of HVDC is in part the reason for a recently proposed CIGRE working group with a focus on power system stability despite increasing expected harmonics and system resonance effects . Different topologies of HVDC each provide respective benefits and drawbacks. Conventional HVDC links use thyristor-based valves to maintain near constant DC current, but rely on AC waveforms to return through a zero-crossing to commutate. Thyristors consist of ‘pnpn’ semiconductor layers, and are fired by applying a gate voltage. As semiconductor technologies developed, the advent of water-cooled insulated gate bipolar transistors (IGBTs) enabled voltage source converter (VSC) technologies. Since IGBTs can both fire and block according to a control signal, VSC topologies allow greater controllability and consume less reactive power relative
Abstract— This paper implements and compares between the key concepts to enable wind power short-term frequency support from electrical and mechanical loads perspectives. Pitch de-loading, kinetic energy extraction and wind turbine (WTG) over-speeding are investigated, where each concept is integrated as a supplementary controller to the conventional controls of WTG. Different patterns of wind speed are examined, step-change and real intermittent of high resolution. The examined aggregated synchronous area has a relatively high wind penetration with frequency support. The overall dynamic inertia of the system is assessed to analyze the impact of the integrated support methods and their key parameters. The coordination between synchronous areas and wind farms, which are interconnected through a multi- terminal high voltage direct current network (MT-HVDC) is examined. A novel definition of the virtual inertia of MT-HVDC grid is proposed. Results show that pitch de-loading secures support reserve most of the time, and kinetic energy extraction provides sustainable support for a short interval, while accelerative de-loading could reach a compromise. The three methods are adaptable with the MT-HVDC holistic frequency support controller, with a slight advantage of kinetic energy extraction over the virtual inertia of the MT-HVDC. Matlab/Simulink® is the simulation environment.
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The offshore WPP contains the PMSG based number of wind turbines connected either in series and shunt configuration. The power transfer between offshore WPP and onshore grid is achieved through high voltage direct current (HVDC) transmission system. The proposed configuration has voltage source converter (VSC) based compensator units. It employs modern semiconductor switches such as IGBT/GTO which is compact in size compared to classic thyristor valve based converters. It is based on self- commutated pulse width modulation (PWM) technology. Also IGBT has the ability to turn ON and OFF with much higher frequency and does not requires any reactive power support . Hence this enables easy in changing the reactive power flow within the system. The different configurations of VSC-HVDC system is monopole, bipole, back-to-back or asymmetric, multi terminal . The figure 1 shows the system configuration of proposed multi-terminal VSC-HVDC system for wind power plant. The proposed configuration is called as UHVDC system which provides both series and shunt compensation. The WPP of proposed system has offshore and onshore VSC station. The Offshore station accommodate one converter and the onshore station contains two independent converters namely series and shunt converters. The onshore VSC station is connected with the electrical grid system through two shunt connected transformers (T r3 and T rn ) and this assures
Abstract: The AC voltage control of a DC/DC converter based on the modular multilevel converter (MMC) is considered under normal operation and during a local DC fault. By actively setting the AC voltage according to the two DC voltages of the DC/DC converter, the modulation index can be near unity, and the DC voltage is effectively utilized to output higher AC voltage. This significantly decreases submodule (SM) capacitance and conduction losses of the DC/DC converter, yielding reduced capital cost, volume, and higher efficiency. Additionally, the AC voltage is limited in the controllable range of both the MMCs in the DC/DC converter; thus, over-modulation and uncontrolled currents are actively avoided. The AC voltage control of the DC/DC converter during local DC faults, i.e., standby operation, is also proposed, where only the MMC connected on the faulty cable is blocked, while the other MMC remains operational with zero AC voltage output. Thus, the capacitor voltages can be regulated at the rated value and the decrease of the SM capacitor voltages after the blocking of the DC/DC converter is avoided. Moreover, the fault can still be isolated as quickly as the conventional approach, where both MMCs are blocked and the DC/DC converter is not exposed to the risk of overcurrent. The proposed AC voltage control strategy is assessed in a three-terminal high-voltage direct current (HVDC) system incorporating a DC/DC converter, and the simulation results confirm its feasibility.
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In1882first time Thomas Edison transfer hvdc power in transmission and distribution of electrical power in New York. After 1954 the establishment of 20 MW HVDC link between Swedish mainlands - Gotland Island. When invention of induction motor which is main part of industries and transformer’s widely use of voltage level to high level to low level as required for our uses. AC system are used for commercial purpose but some limitation like power transmission capacity and inefficiency for long transmission. However increase the demand of electric power so that increase the voltage drop of transmission line, this is create problem in transmission line. This problem minimize busing power electronics component, this combined system known as HVDC system. In this system all problem is minimize for transmission system. There are some advantages of HVDC system over HVAC
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This paper has successfully integrated and evaluated the performance of VSC based multi-terminal HVDC system with resistive SFCL. An effective model of resistive type of SFCL has applied in VSC based multi-terminal HVDC system, considering the DC breakers overcurrent withstand capability. Based on the electric field intensity and current density characteristics, the SFCL has been modelled in this paper and it is successfully integrated into a VSC based MTDC system for fault current limitation under AC/DC faults. The transient analysis is carried out for different perturbations and from the simulation results. Simulation results show that SFCL not only reduces the current transients for DC faults but also for AC faults. The method was validated by time domain simulations undertaken with the PSCAD/EMTDC, and the results showed that the fault current could be reduced up to 65%. The use of SFCL can effectively suppress the short circuit fault on DC line. Hence, the robustness of VSC based multi-terminal HVDC system can be improved against the DC fault to a certain extent. The DC resistive SFCL is proved to be a very promising technology in multi-terminal HVDC systems.
With extensive research and applications, VSC technology has gradually achieved a high degree of maturity, and there has been numerous projects on VSC- based HVDC applications, including the applications of MTDC and renewable energy integration in recent years. There has been a variety of topologies with the VSC development. Among them, one of these, the MMC, has salient features and shows its strong competitiveness, which has been well recognized by research and applications. Since there are a number of energy capacitors in the SMs of the MMCs, it is important to precharge these capacitors during the startup stage and the system startup control is essential. In, a startup control scheme for the MMC was proposed.
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Silicon carbide has been projected to have tremendous potential for high voltage solid-state power devices with very high voltage and current ratings because of its electrical and physical properties. The rapid development of the technology for producing high quality single crystal SiC wafers and thin films presents the opportunity to fabricate solid- state devices with power-temperature capability far greater than devices currently available. This capability is ideally suited to the applications of power conditioning in new more- electric or all- electric military and commercial vehicles.
The control principles for the rectifier and inverter converter depicted in Figure 3. The new control principle to be used for the operation of the inverter-side converter of the HVDC transmission is based on the ability of the CCC to control the exchange of reactive power with the AC network from the AC terminal of the converters, and by doing that controlling the alternating voltage of the converter bus. The new proposed control scheme for the inverter includes the following control loops: A main control loop that gives the inverter operation with constant alternating voltage (the controller uses the alternating voltage measured at the filter bus); Transiently, a direct- current controller can be selected (normally the rectifier controls the direct current, but during transients like disturbances in the connected AC network, the direct- current controller at the inverter can be selected to recover the HVDC transmission link from the fault); Transient operation of the inverter-side converter can also be made with constant commutation margin, when the calculated commutation margin is lower than a minimum reference, to avoid commutation failures;
Current harmonics: Current harmonics are caused by non-linear loads. When a non-linear load, such as a rectifier, is connected to the system, it draws a current that is not necessarily sinusoidal. The current waveform can become quite complex, depending on the type of load and its interaction with other components of the system. Regardless of how complex the current waveform becomes, as described through Fourier series analysis, it is possible to decompose it into a series of simple sinusoids, which start at the power system fundamental frequency and occur at integer multiples of the fundamental frequency.
The wind farm side converter collects energy from wind farm and then transmit it to the power grid via the dc transmission and grid side converter. For the normal op- eration of a VSC transmission system, its dc link voltage must be maintained at a constant value under all condi- tions. A constant dc voltage indicates balanced active power flow between the two sides. Abnormal dc link voltage can cause the system to trip and disrupt its nor- mal operation. Furthermore, to achieve this balance, the grid side converter is assigned to control the dc voltage, to ensure the energy collected by the wind farm side converter is transmitted to the grid network. The control system of the grid side converter is shown as Figure 5.
Mr. R.RAMESH BABU was born in India in the year of 1979. He received B.Tech degree in Electrical and Electronics Engineering in the year of 2004 & M.Tech degree in Electrical Power System in the year of 2015 from JNTUA, Anantapuram. He was expert in electrical machines, Power system, Measurements and Instrumentation Subjects. He is currently working as An Lecturer in EEE Department in G.M.R.Polytechnic, Srisailam. Andhra Pradesh State ,India.
being buffered in each hardware repeater. It gets even worse when PWM signal synchronization are required to be done. Due to harmonics considerations, each PWM signal in any controller should be synchronized to a reference time stamp and the resolution of time difference should be in the order of sub-micro second. The added latency of signal propagation in each stage will cause problems in the controller synchronization. Beside all these cons, the major benefit of this architecture is its ability to be implemented by fiber optic based decoupled hardware. Since data line is point to point, fiber optic communication is possible and this will give a high degree of robustness to the entire design.
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There are no piece of electrical equipment that does not depends on electrical insulation in one form or an other to maintain the flow of electric current in desired path of the circuit. If due to some reasons the current deviates from the desired path, the potential will drop. In real insulation systems with solid insulation, partial discharges usually occur far below the breakdown voltage. In the long run, these lead to the destruction of nearly all solid insulating materials.
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The occurrence of substantial AC frequency events, brought about by an imbalance between the generated power and the power demand within an AC network can destabilise the system and potentially risk de-synchronisation. Primary frequency support can be provided by the inherent inertia of conventional rotating machines thus automatically offsetting frequency deviations within the overall system. However, when proposed large multi-terminal HVDC (MTDC) systems, incorporating a significant number of interconnected wind farms are considered, the decoupling of both the large rotating mass of the modern, variable speed wind turbines, via their power electronic converters and the further decoupling of the different power networks through interconnected HVDC systems, could result in a system with a large generating capacity but with its overall inertia being substantially reduced making the system frequency more susceptible to changes in load and generation .
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ABSTRACT:A high-voltage direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power over a long distance and our basic requirement of transmitting large amount of power over a long distance with minimum losses. The converter transformer is an integral part of an HVDC system. The major loss in the converter transformer is a harmonic loss. This paper presents review in literature of converter transformer based on HVDC system. Traditional converter transformer produces more harmonic current. So to overcome the existing problem of the traditional converter transformer, new converter transformer and an inductive filtering method are presented in this paper.
The equivalent line resistance is approximately reduced by 36.6%. The droop gain design is carried out for both radial parallel and meshed parallel MTDC systems. This paper presents a methodology to design a voltage–current droop controller for an MTDC system with a three-wire bipolar HVDC transmission line. It has been shown that switching from two-wire mode to a three-wire mode of the three-wire bipolar line affects the equivalent line resistance. In case of radial MTDC systems, the line currents, mainly depend on the line resistance (in power-sharing mode), corresponding droop gains of the VSCs should be updated to ensure distributing of current with the desired sharing ratio. However, their values can be obtained offline. On the other hand, a meshed MTDC system necessitates an online algorithm to update the droop gain constants with any variation in input/output current magnitudes and/or line resistance variation.
Power system oscillation damping has always been a major concern for the reliable operation of PS. To increase damping, several approaches have been proposed. The most common ones being excitation control through PSS and supplementary damping control of HVDC, SVC, TCSC and other FACTS devices. In this thesis, I’m focusing to design PSS using Differential Evolution (DE) to damp the oscillation with HVDC. There are variety method to design PSS and the previous PSS design using Pole Placement , H∞ robust control technique , Linear Matrix Inequalities (LMI) robust control technique , μ-Synthesis (or singular value decomposition) a robust control technique .
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