Large AC Power Research Articles

Predicting Failure Cascades in Large Scale Power Systems via the Influence Model Framework

Large blackouts in power grids are often the consequence of uncontrolled failure cascades. The ability to predict the failure cascade process in an efficient and accurate manner is important for power system contingency analysis. In this paper, we propose to apply the influence model for the prediction and screening of failure cascades in large scale DC and AC power networks. Then, the trained influence model is applied to some large power grids with thousands of buses and transmission lines. The prediction performance is evaluated in four different aspects, from the global perspective of the overall failure size to the individual information regarding the link failure time. The results show that under limited training samples, the proposed framework is capable of predicting the failure cascade size with a 7% error rate, the final state of links with a 10% error rate, and the failure time within 1 time unit for both the DC and AC model. One major advantage of the proposed method is that it can reduce the computational time of the failure cascade prediction by a few orders of magnitude with limited compromise in accuracy, as compared to the power flow based contingency analysis. Another important feature of the proposed method is that the trained influence parameters can reveal the critical initial contingencies. This information is very helpful for identifying the worst contingency scenarios for system operators.

Hilbert‐Huangtransform based fault identification and classification technique forACpower transmission line protection

In the modern power system, the fault identification and classification is one of the challenging tasks during high fault impedance due to low change in the magnitude of fault current from steady-state to fault inception. In this paper, a high impedance fault detection technique is proposed based on Hilbert-Huang transform (HHT). The energy for each phase is computed from the intrinsic mode function (IMF) of three-phase voltage/current signals. An average relay energy index (AREI) is proposed to detect the transmission line fault in the large interconnected power system. Furthermore, a fault classification technique is also proposed based on relay energy index (REI) of each phase of the three-phase voltage/current signals. The proposed HHT based fault detection technique is validated on standard WSCC 9 bus and IEEE 14 bus test power systems using PSCAD and MATLAB software. The simulation results reveal that the proposed Hilbert-Huang transform based fault identification and classification technique can be best suitable for protection of transmission lines during high impedance faults in complex and large AC power transmission system.

Guest Editorial Special Section on Asynchronous Interconnect and Generation

With the rapid growth of modern power systems and the wide application of high-capacity HVDC systems, the stability, security, and reliability of AC-DC synchronous system may become compromised by various complex faults in the system including the possibility of multi-HVDC blocking faults. Interconnecting two or more large AC power subsystems or transmitting large amounts of energy from power plants directly to load centers through HVDC links is deemed as an effective pathway for expanding the grid and enabling new solutions paradigms. However, a wider adaptation of this new technology also brings many technical challenges, including lack of standardized equipment, the design of versatile control to ensure stability, mismatch of generation and load, interactions between various equipment and their controllers, harmonics, resonances, sub- and super-synchronous oscillations in weak AC systems, etc. Research and development efforts are currently underway in many countries to address these challenges at the system and equipment levels. The areas of focus include the planning, design, construction, modeling, analysis, operation, control, protection, and stability of asynchronous interconnection of modern and future power systems. New high-efficiency topologies and effective measurements are also among the key nontrivial issues when developing practical asynchronous interconnected power systems.

Experimental study of subcooled flow boiling heat transfer of water in a circular channel under one-side heating conditions

Heat transfer experiments on subcooled water flow boiling were carried out in a vertical upward circular channel, which was set off-center in a rectangular nickel block. Particular attention was paid to the heat transfer of water under high and heat fluxes and high mass fluxes under one-side heating conditions, with respect to the heat removal technologies for the divertor in the International Thermonuclear Experimental Reactor (ITER). The test section was electrically heated by a large AC power supply with an effective heated length of 360 mm. The test parameters covered pressures of 3–5 MPa, mass fluxes of 3000–8000 kg·m−2·s−1, inlet bulk temperatures of 40–220 °C, and equivalent heat fluxes up to 8 MW·m−2. The effects of the parameters on the heat transfer coefficients have been discussed in detail, from single-phase forced convention to fully developed nucleate boiling. A series of heat transfer correlations were evaluated using the experimental data, and most of the correlations did not adequately fit the experimental results. A modified Liu-Winterton correlation and modified Jens-Lottes correlation were used to predict the heat transfer coefficients in the partially boiling region and fully developed boiling region, respectively. The average errors (AEs) of the two correlations were −0.28% and 0.6%, and the root mean square errors (RMSEs) of them were 7.93% and 2.89%.

Fast Electromagnetic Transient Simulations of Power Systems Utilizing Single‐Phase Phasor‐Based Modeling of Remote Systems

SUMMARYElectromagnetic transient (EMT) simulations of relatively large power systems have become quite common, for instance, in the case where simulations of HVDC converters are carried out with large ac power systems connected to the converters. Thus, the increase in computation time is a serious concern. To reduce computation time, this paper proposes a method to reduce computational demand of a remote power system which is located far from the source of a transient event to be simulated. In the proposed method, the remote power system, which is supposed to be represented by a three‐phase EMT‐based model, is reduced to a single‐phase phasor‐based model, and the size of the circuit to be simulated is thus reduced and the dynamics calculations of inductors and capacitors included in the remote power system are neglected. The calculation algorithm of generator models included in the remote power system is also simplified. The proposed method has been applied to EMT simulations of the WEST 10 benchmark power system prepared by the IEEJ, and it has been shown that the computation time is remarkably reduced without significant loss of accuracy if the portion assumed to be the remote power system is sufficiently far from the source of a transient event.

Critical heat flux of highly subcooled water flow boiling in circular tubes with and without internal twisted tapes under high mass fluxes

This study focuses on the critical heat flux (CHF) of highly subcooled water flow boiling under high mass fluxes, particularly with respect to the thermal–hydraulics analysis in fusion reactor components. The experiments were performed in vertical circular tubes those were uniformly heated by a large AC power supply. Twisted tape was employed to induce swirl flow in order to further enhance the subcooled CHF. This study obtained several high CHF data points in both smooth tubes and twisted-tape tubes, which can enhance relevant high-CHF databases. The influences of thermodynamic quality, mass flux, pressure, and twisted tape on subcooled CHF were discussed. The results show that twisted tapes can allow significant increase in subcooled CHF under high mass fluxes. The CHF enhancement effect is more obvious when using twisted tapes with smaller twist ratios. Available existing subcooled CHF correlations were compared with the experimental data from the present and others’ studies for both smooth tubes and twisted-tape tubes. For smooth tubes, the CHF look-up table has satisfactory prediction accuracy. For twisted-tape tubes, a modified correlation of Arment et al. was proposed to predict the swirl-flow subcooled CHF.

Subcooled flow boiling heat transfer of water in circular tubes with twisted-tape inserts under high heat fluxes

This paper presents the heat transfer of subcooled water in swirl flow under high heat fluxes with respect to the heat removal technologies for the divertors in the International Thermonuclear Experimental Reactor (ITER). The experiments were conducted in a vertical circular tube that was heated uniformly with a large AC power supply. A twisted tape with a twist ratio of 2 or 4 was inserted into the circular tube to induce the swirl flow. The test parameters were as follows: heat flux q=5–19.5MW/m2; system pressure P=3, 4.2, 5MPa; mass flux G=6000, 8000, 10,000kg/(m2s); and inlet bulk temperature Tb,i=50–200°C. The heat transfer coefficients and boiling curves are obtained from single-phase forced convection to fully developed nucleate boiling. The influences of mass flux, heat flux, system pressure, thermodynamic quality, and inlet subcooling on heat transfer are discussed in detail. Special attention is paid to the effect of the twisted tape on subcooled heat transfer. More importantly, a wide set of heat transfer correlations are evaluated on the basis of our experimental data, and relevant recommendations for engineering applications are made according to their prediction accuracies.

Online monitoring of the distributed lateral displacement in large AC power generatorsusing a high spatial resolution Brillouin optical fiber sensor

We report for the first time, to the best of our knowledge, online monitoring of thedistributed lateral displacement in large AC power generators using high spatial resolutiondifferential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA). Toperform the measurement of distributed lateral displacements with periods of only a fewcm in large AC power generators, a 2 cm spatial resolution strain measurement is realizedusing the differential pulse pair of 8/8.2 ns in DPP-BOTDA, and then the lateraldisplacements are reconstructed according to the strain–displacement relation with theassumption of a sine shape function. Using different fiberglass ripple springs, twotypes of lateral displacement with periods of 3 and 3.25 cm are demonstrated,obtaining a maximum displacement of 0.43 mm with a measurement accuracy of ∼ 40 µm. This provides the information on the stator coil tightness through online monitoring ofthe distributed lateral displacement caused by the fiberglass ripple springs, and ensuressafe operating conditions for large AC power generators. In addition, the largenumber of sensing points associated with distributed optical fiber sensors make iteconomically and technically practical to monitor large numbers of key components in agenerator without any interference from the large magnetic and electrical fields.

Spectroscopy of Rb atoms in hollow-core fibers

Recent demonstrations of light-matter interactions with atoms and molecules confined to hollow waveguides offer great promise for ultralow-light-level applications. The use of waveguides allows for tight optical confinement over interaction lengths much greater than what could be achieved in bulk geometries. However, the combination of strong atom-photon interactions and nonuniformity of guided light modes gives rise to spectroscopic features that must be understood in order to take full advantage of the properties of such systems. We use light-induced atomic desorption to generate an optically dense Rb vapor at room temperature inside a hollow-core photonic band-gap fiber. Saturable-absorption spectroscopy and passive slow-light experiments reveal large ac Stark shifts, power broadening, and transit-time broadening, that are present in this system even at nanowatt powers.

Input Impedance Modeling and Analysis of Line-Commutated Rectifiers

This paper presents the modeling and analysis of small-signal input impedance for line-commutated rectifiers. The recently developed impedance mapping method, which has been applied to single-phase rectifiers with sinusoidal line inputs, is generalized to single-phase rectifiers with distorted lines as well as three-phase rectifiers. The modeling method assumes known impedance of the circuit on the dc side of the rectifier bridge (including its output filter and the load), and determines the corresponding ac input impedance by using a current and voltage mapping function that describes the operation of the rectifier bridge in the frequency domain. In this approach, closed-form analytical expressions are derived for the ac input impedance as functions of the dc circuit impedance. The resulting models are intended for stability analysis and design of large ac power systems involving significant number of rectification loads, such as more-electric aircraft power systems and microgrids. Detailed derivation of the models is presented in this paper. Numerical simulation and experimental measurement results are also presented to validate the developed models.

Magnetization Loss and Shield Effect in Multi-Stacked Tapes With Various Stacking Configurations

AC loss is one of the major topics in large AC power applications using high temperature superconductor such as power transformers, transmission cables and fault current limiters because it is closely related to operation efficiency. Multi-stacked tape conductors should be used to transport the large current in those power applications. A research of various arrangements of HTS tapes for multi-stacked tapes has been performed to increase the capacity of transport current in HTS power applications. In this paper, we studied magnetization loss and shield effect from several different arrangements of BSCCO tapes such as Face-to-Face type, regular matrix type (m <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$times$</tex> 2) and irregular matrix type. The results show that the magnetization loss of the Face-to-Face arrangement was lower than those of the other matrix types for the same stacking numbers. We think that the result was due to the shield effect by demagnetization of adjacent HTS tapes which were located as face to face.

Point-to-point HVdc systems using 36-pulse operation

HVdc systems are adopted in the case of long distance power transmission, interconnection of two large ac systems or power transmission by cables to overloaded districts or isolated islands. Moreover, a division of a huge ac system by dc systems can reduce its short-circuit capacity. HVdc systems, however, need both ac and dc filters, which attempt to prevent the HVdc converters from injecting all its harmonic output into both the ac and dc systems. Unfortunately, filters have a complex design and performance, and constitute a considerable part of the volume and cost of present dc terminal stations. Thus, any possible new solution is welcome. In this paper a 36-pulse configuration for point-to-point HVdc systems is proposed, whereby both ac and dc harmonics are diminished throughout the system, thus opening the possibility of reducing both the ac and dc filtering, together with the associated drawbacks. Prospective advantages regarding reactive power requirements and reliability for the proposed configuration are also evaluated.

Low crosstalk packaging design for Josephson logic circuits

Theoretical and experimental studies are accomplished for inductive crosstalk noise reductions at Josephson chip-to-card connectors. This noise is induced by large AC power and high switching speed signal currents. The crosstalk mechanism was analyzed using a Partial Element Equivalent Circuits Model. Ground inductance causes not only crosstalk noise between connectors but also ground fluctuation noise inside the chip. This ground noise is large enough to cause false logic operations. Test chips and cards with improved connectors were produced for an experimental evaluation. Power crosstalk noise was measured using Josephson sampling circuits fabricated on the chip. The crosstalk noise - signal level ratio was less than 2.5%, when 250 MHz, 50 mA power currents were supplied. Crosstalk noise between neighboring signal connectors was also reduced to negligible level, including the worst case. These results favorably agree with calculations. This low crosstalk packaging design can be applied to high speed Josephson logic systems.