Continuous Current Rating Research Articles

The Through-Flow Electrochemical Cell - a Breakthrough Technology

The paper describes a new approach to energy-storage electrochemical cells that is “chemistry-agnostic” and promises to solve several problems of existing batteries. These problems include: Charging rates are too slowIntrinsic failure-mechanisms compromise safetyEnergy Density is inadequate for mobile applicationsLifetime is too short for large-scale permanent installations The key idea of the “Through-Flow Cell” is that the electrolyte is pumped continuously in a closed loop through a porous anode, porous separator, and porous cathode, in a direction aiding the active ionic flow. Although the idea of moving a liquid electrolyte is not new, there have not been any successful commercial products until now. It should not be confused with the “Flow-Battery”, an entirely different concept where two different fluids flow separately within anode and cathode. Previous attempts to develop a reliable Through-Flow Cell foundered on the difficulty of immobilizing active materials on the anode and cathode collectors while fluid flows freely through surrounding pore-spaces. I describe a new active-material encapsulation method to achieve that crucial objective, and a new more convenient cell configuration.Applications will include large grid-scale installations and could include high-power transportation vehicles such as trains and buses. The basic cell is configured as a stack-able, de-mountable, and repairable module. It is a completely self-contained package incorporating an adaptive electrolyte pumping system with optional heating and/or cooling to optimize efficiency in a wide range of environmental conditions. In grid-scale systems the ability to easily remove a low-performing cell from a stack, repair, and replace it is a very desirable feature that extends the life of a high-CAPEX installation almost indefinitely.The new cell design has a conservative capacity rating of 3000 Amp.hr (using 300 mA.hr/gm active materials) and a continuous current rating of 2000 Amps, with the cell voltage being determined by the chemistry employed. For the Lithium-ion chemistry with a nominal voltage of 3.7v it has 10 KW.hr energy capacity @ 90% discharge. A linear stack of 100 cells makes a battery storing 1 MW.hr on a rack 9 meters in length; easily accommodated in an enclosure the size of a shipping container that could contain up to eight such racks.For some people the idea of a battery with moving parts runs counter to notions of simplicity found in the standard sealed cylindrical cells. In practice, however, large assemblies of such cells require complex external liquid cooling and possibly fire-suppression systems. When individual cells fail, they are usually disconnected by blowing fuses, resulting in a steady degradation that is not easily repairable.In contrast to sealed cells, some unique advantages of the electrolyte circulation technique can be summarized as follows: Effective Mobility of active-species ions is dramatically boosted by the flow velocity, translating into reduced series resistance between electrodes. Peak charging or discharging currents can be ten times higher without overheating or loss of charge capacity when the flow velocity is only one centimeter per second.Flow direction is reversed when switching from charging to discharging, which reverses dendrite growth that is a potential source of catastrophic failure.The interior temperature of the cell can be directly controlled and excess heat removed from the electrolyte by an external heat-exchanger, or added in cold climates.The Anode and Cathode, being 3-Dimensional porous structures, incorporate greatly increased volumes of active material compared to typical 2-Dimensional foil electrodes.Electrolyte flowing across the surface of active material inside porous electrodes efficiently sweeps away ion-deficient liquid from within the Slipping-Plane near the Solid-Electrolyte Interface (SEI), reducing the parasitic Zeta-Potential. Results from a proof-of-principle laboratory cell using the Nickel Metal-Hydride chemistry will be briefly described, and a full-scale commercial cell design presented in more detail. The very important active-material encapsulation technique is also described along with its unique ability to tolerate large changes in volume of the active material without degradation. Current cylindrical “jelly-roll” format cells cannot tolerate significant volume change caused by ion-intercalation inside active materials without physical destruction.The new technology may enable complete replacement of Graphite by Silicon in the Anodes of Lithium-ion cells. Silicon has a charge capacity almost four times greater than Graphite, but expands up to 400%. In another example, Cathodes in the Lithium-Sulfur chemistry may tolerate far higher Sulfur loading with corresponding increased energy density.In summary, this new focus on the mechanics of energy-storage cells shows promise for a step-change increase in power-density, reliability, and safety. As described, the idea can be applied to any new improvement in chemistry that uses a liquid electrolyte as soon as it achieves maturity. Figure 1

WEATHER-DEPENDENT CURRENT RATINGS OF OVERHEAD LINES IN THE PROBABILISTIC HIGH VOLTAGE DISTRIBUTION GRID PLANNING

This paper presents a comparison of the necessary grid reinforcements of a distribution grid considering the approaches for the static seasonal and the continuous weather-dependent dynamic current rating for overhead lines. The aim is to increase the transmission capacity by considering the environmental and weather influences. For this purpose, the required grid expansion will be estimated by applying the grid planning method, which will be presented in this paper, under application of the static seasonal and the dynamic current rating. In a second step, these results will be compared with the calculations obtained by the nominal current rating according to the state-of-the-art. This investigation is carried out in a probabilistic network planning environment. In addition, the paper shows the synergy effects between the different approaches for the estimation of the line current rating and the application of the dynamic curtailment of renewable power generation. Finally, an economic comparison of necessary grid reinforcements is carried out in order to determine the potential of the particular solutions.

Piezoelectrically Actuated Fast Mechanical Switch for MVDC Protection

Medium voltage direct current (MVDC) power systems are in their early phase of development, but the lack of efficient and reliable DC circuit breakers is currently one of the most significant bottlenecks for widespread adoption of MVDC applications. Compared to protection devices in high voltage direct current (HVDC) systems, MVDC circuit breakers are expected to interrupt fault currents with higher rates of rise in more power-dense and more controllable structures. A hybrid approach with a piezoelectrically actuated fast mechanical switch in parallel to a solid-state circuit breaker is expected to meet the above requirements. A prototype of a 15 kV-class piezoelectrically actuated fast mechanical switch is presented in this paper. The prototype demonstrates an actuator opening time of 1 ms, a breakdown voltage of over 20 kV, and a continuous current rating of 500 A. After integrating the fast mechanical switch into a hybrid circuit breaker testbed, the fast mechanical switch was able to open within 1.5 ms and the fault was cleared within 5 ms. Therefore, piezoelectrically actuated fast mechanical switches can enable fast, reliable and controllable protection for various MVDC systems like electric ships, electric aircraft and microgrids that integrate distributed energy resources.

Sheath Loss Factor for Cross-Bonded Cable Systems With Unknown Minor Section Lengths – Field Verification of the IEC Standard

The IEC Standard 60287-1-1 contains the formulae for the calculation of the sheath and armor loss factors of a 3-phase cable system. Sheath circulating current losses are computed for single-, multiple-, or cross-bonded systems. This paper deals with cross-bonded cable systems. For unequal minor section lengths, the standard gives a formula allowing calculation of the circulating current losses when the lengths of the minor sections are known. The origin of this formula is not documented; hence, this paper gives its derivation. When the lengths of the minor sections are not known, the standard suggests using a factor of 0.004, which multiplies the circulating current loss factor computed for a multi-point bonded system. In this paper, the cable sheath circulating loss factor was found using different methods with the aim of checking the applicability of the 0.004 multiplier to practical cable systems with differences in the length of minor sections and system voltage. Three different cable systems are studied; the first is a 132 kV circuit with known lengths of minor and major sections. The second circuit is a 220 kV system where the lengths of minor sections are unknown and the third is a 220 kV real system with known, highly irregular section lengths. The calculated values of sheath circulating current loss factor, applying the formulas from the IEC 60287-1-1, were compared to the values calculated based on practical measurements of sheath induced voltages and currents. The effect of taking the measured and IEC values on the continuous current rating (ampacity) is also presented for the first two circuits and the comparison of the unknown and known section lengths for the third circuit is offered.

Design and Testing of a Low Voltage Solid-State Circuit Breaker for a DC Distribution System

In this study, a low voltage solid-state circuit breaker (SSCB) was implemented for a DC distribution system using commercially available components. The design process of the high-side static switch was enabled through a voltage bias. Detailed functional testing of the current sensor, high-side switch, thermal ratings, analog to digital conversion (ADC) techniques, and response times of the SSCB was evaluated. The designed SSCB was capable of low-end lighting protection applications and tested at 50 V. A 15 A continuous current rating was obtained, and the minimum response time of the SSCB was nearly 290 times faster than that of conventional AC protection methods. The SSCB was implemented to fill the gap where traditional AC protection schemes have failed. DC distribution systems are capable of extreme faults that can destroy sensitive power electronic equipment. However, continued research and development of the SSCB is helping to revolutionize the power industry and change the current power distribution methods to better utilize clean renewable energy systems.

Carbon nanotube wires with continuous current rating exceeding 20 Amperes

A process to fabricate carbon nanotube (CNT) wires with diameters greater than 1 cm and continuous current carrying capability exceeding 20 A is demonstrated. Wires larger than 5 mm are formed using a multi-step radial densification process that begins with a densified CNT wire core followed by successive wrapping of additional CNT material to increase the wire size. This process allows for a wide range of wire diameters to be fabricated, with and without potassium tetrabromoaurate (KAuBr4) chemical doping, and the resulting electrical and thermal properties to be characterized. Electrical measurements are performed with on/off current steps to obtain the maximum current before reaching a peak CNT wire temperature of 100 °C and before failure, yielding values of instantaneous currents in excess of 45 A for KAuBr4 doped CNT wires with a diameter of 6 mm achieved prior to failure. The peak temperature of the wires at failure (∼530 °C) is correlated with the primary decomposition peak observed in thermal gravimetric analysis of a wire sample confirming that oxidation is the primary failure mode of CNT wires operated in air. The in operando stability of doped CNT wires is confirmed by monitoring the resistance and temperature, which remain largely unaltered over 40 days and 1 day for wires with 1.5 mm and 11.2 mm diameters, respectively. The 100 °C continuous current rating, or ampacity, is measured for a range of doped CNT wire diameters and corresponding linear mass densities ρL. To describe the results, a new form of the fuse-law, where the critical current is defined as I∝ρL3/4, is developed and shows good agreement with the experimental data. Ultimately, CNT wires are shown to be stable electrical conductors, with failure current densities in excess of 50 A in the case of a convectively cooled 11.2 mm doped CNT wire, and amenable for use in applications that have long-term, high-current demands.

Cyclic Load Profiles for OffshoreWind Farm Cable Rating

By its nature, the power generated by offshore wind farms varies substantially with time. Despite this fact, the cable systems which connect such wind farms back to the onshore grid are typically sized based on a maximum, continuous current rating. In order to develop techniques by which the sizing of such cable connections might be optimized, this paper presents a method for deriving equivalent cyclic load curves from realistic, time-variant generation data. These cyclic load profiles may be used with the conventional IEC 60853–2 cable rating calculation, allowing the cable to be sized for a more realistic duty cycle. The example calculations presented show that this reduction in conservatism permits the use of smaller cable sizes, helping to drive down project costs. The implications of using such methods are discussed, highlighting the tradeoff between cost and conservatism.

High‐Ampacity Power Cables of Tightly‐Packed and Aligned Carbon Nanotubes

The current‐carrying capacity (CCC), or ampacity, of highly‐conductive, light, and strong carbon nanotube (CNT) fibers is characterized by measuring their failure current density (FCD) and continuous current rating (CCR) values. It is shown, both experimentally and theoretically, that the CCC of these fibers is determined by the balance between current‐induced Joule heating and heat exchange with the surroundings. The measured FCD values of the fibers range from 107 to 109 A m−2 and are generally higher than the previously reported values for aligned buckypapers, carbon fibers, and CNT fibers. To the authors’ knowledge, this is the first time the CCR for a CNT fiber has been reported. The specific CCC value (i.e., normalized by the linear mass density) of these CNT fibers are demonstrated to be higher than those of copper.

Switchgear in Unusual Environments: Application Guidelines for Assuring System Reliability

With an increased emphasis on reducing the size of installations, switchgear designers are faced with the diametrically opposed requirements of increasing shortcircuit withstand and continuous current ratings while reducing space for the air that provides both primary insulation and cooling for the energized parts. Using modern material science, compact integrated dielectric systems can be developed without compromising the essential features for achieving higher short-circuit withstand and continuous current ratings. IEEE standards define the usual service conditions for proper equipment installation and also offer application guidelines for these conditions. This article will discuss these guidelines and define the owners' and manufacturers' responsibilities to assure system reliability by examining a specific installation and issues experienced.

Modular Motor Driver with Torque Control for Gripping Mechanism

This paper presents the algorithms for controlling the torque of a DC motor via a motor driver. Controlling the torque of the DC motor is achieved via PWM variations. The core of the project lies in the capability of the algorithm to regulate PWM to the motor by sensing the average current flow. This algorithm, when implemented on a DC motor that is coupled to a gripping actuator will enable the actuator to have torque control to grasp an object. Torque control algorithm in a gripping actuator will allow sufficient force to be exerted to the object grasped without damaging the object. Apart from that, this algorithm will also protect the hardware of the actuator to ensure it is operating within the continuous current rating of the motor and driver. This is possible by taking in the magnitude of the current drawn by the load as the feedback to the system and set appropriate action to protect the system from damage when a certain setpoint of current is reached. In implementing the project, several algorithms were tested to achieve the torque control algorithm requirement including on-off algorithm, P algorithm, PI algorithm, PD algorithm and PID algorithm. Using the same hardware configuration, the different algorithms produced different outcomes on the output of the controller. The output of the controller is monitored on a computer via serial communication. The serial communication over the USB port on the computer is done by utilizing the Processing programming language and integrated development environment (IDE). This software will enable the output of the controller to be mapped on the computer monitor to provide a real-time graphical view of the output values to the user. These graphical outputs of the controller was evaluated based on the response time, degree of overshooting, and steady-state error to determine the most optimal method of control to be implemented in torque control scheme. The advantage of this method is the minimal cost incurred and the system built does not involve complex feedback system. The application of this system for now is limited to controlling motors connected directly to the load (gripper) without passing through any gear conversions.

Field test results for a multi-shot 12.47 kV fault current limiter

During ten months of 1989, a prototype multishot 12.47 kV fault current limiter (FCL) was installed in a field test mode at a simulation in Lincoln, Nebraska. The authors present the principles of operation, computer simulations, and the results of the field test. Nineteen fault occurrences were successfully handled by the FCL. Examples of actual faults include currents of 3900 A reduced to 2140 A and bus voltage sags improved from 0.33 to 0.9 per unit. The FCL features tested and proven in this field test were the dynamic insertion of the resistive rail to limit the first one half cycle of fault current without causing voltage or current transients, limiting the remaining cycles of fault current with the limiting resistor, automatic resetting of the device during current zero, and the large continuous current rating with no additional losses.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A New Fault-Interrupting Device for Improved Medium-Voltage System and Equipment Protection

A newly developed fault-interrupting device employs radical departures in overcurrent sensing, response, and interruption techniques. It offers a large selection of time-current characteristic curves plus high continuous current ratings combined with efficient current-limiting action in a compact, self-contained package not requiring external sensing or control power. Improved protection and coordination can be provided in a wide variety of medium-voltage circuit and equipment applications, some of which cannot be properly addressed with previously available devices. Sensing and response functions of the electronic control module and operation of the interrupting module are described. Examples are presented illustrating use of the flexible response characteristics in the solution of difficult protection and coordination applications.

Vacuum switchgear for fusion experiments

Fusion experiments frequently have demanding switching requirements that cannot be satisfied by conventional switchgear. Two frequent requirements and specialized vacuum switches designed to meet them are presented. Experimental results from testing of prototype devices are discussed. A frequently encountered switch application is the interruption of a high level, pulsed, dc current followed by a fast rising recovery voltage. A 7-in. vacuum interrupter with an axial magnetic field has proven to be an effective switch for currents up to 25 kA. Beyond 25 kA, a parallel configuration becomes necessary. The influence of the axial magnetic field on the behavior of parallel vacuum interrupters and test results up to 50 kA are discussed. Also, test results on experimental devices with novel electrode geometries such as the rod-array vacuum interrupter and the Amsler contact interrupter are presented. Interrupters which can carry large dc currents on a continuous basis are frequently required in fusion applications. This requirement occurs in superconducting coil protection circuits and generally involves recovery voltages of 5 kV or less. A specially designed vacuum interrupter with water-cooled electrodes has been used to extend the continuous current rating of a commercial interrupter from 1200 A to 13 kA. Two second generation devices with special electrode materials are currently being developed for use with a special actuator and are intended to extend the continuous rating to 25 kA.

Continuous current rating for external and surface cooled cable systems

A method for computing continuous current rating of forced cooled cable systems is developed. It can be used for any number of heat sources, and represents the temperatures along the line as a sum of exponentials. Two different applications are considered. These are for cable systems externally cooled by water running in four pipes, and for surface cooled cables, each cable in a pipe. A complete numerical example is given for both cases

High voltage vacuum circuit breakers

A new line of high voltage vacuum power circuit breakers is being developed for 121 kV and above and will use the new vacuum interrupter module presented in a companion paper <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . The first ratings in production will be 242 kV with interrupting capabilities of 40,000 amperes, followed by breakers rated 50,000 amperes; both of these will have continuous current ratings of 2000 and 3000 amperes. This paper departs from normal practice by submitting the new vacuum circuit breaker design concepts prior to manufacture. A future paper on the breaker testing program and test results will be submitted to the Institute. The breakers will be dead tank design and use SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> gas only for insulation purposes. The interrupter contacts are to be mechanically driven by charged-spring type operating mechanisms. The breakers will be supplied for conventional outdoor switchyards and for high voltage gas-insulated station equipments.

The Service Current Rating of Power Conversion Equipment

The conventional method which has been used for specifying the load current/time capability of power conversion equipment is based on the provision of continuous and short-time current ratings under specified conditions of service. This method is not suitable for establishing the capability of the equipment for supplying arbitrary load current/time demands on a repetitive duty cycle basis. This paper develops the basis for a method of rating which can be used for this purpose. It is a method that is application rather than equipment oriented. It involves the construction of a service current profile which is actually a function rather than a value. The method has been incorporated in standards which have been proposed for thyristor power conversion equipment for dc motor drives by the appropriate committees within NEMA, IEEE, and the IEC. The concept, in fact, was developed through the deliberations of those committees. It should be remarked that the method can also be applied to other types of power equipment.

Fast Acting Fuses for the Protection of Semiconductors

Co-ordination of semiconductors and high speed fuses with respect to surge ratings has been a relatively simple matter because of ample safety factors present in the published data. This has resulted in considerable difficulty in obtaining full semiconductor utilization, because the fuses so chosen tend to limit the maximum continuous semiconductor current. Since the semiconductor industry trend is towards higher continuous current ratings with the same published surge ratings, a closer evaluation of all aspects relating to fuse and semiconductor surge ratings is necessary to permit full use of the semiconductor's maximum continuous current.

Late Model 230-kV Oil Circuit Breaker

Recent reports on the performance of oil breakers give the impression that they are not suitable for use on modem transmission systems. They imply that oil circuit breakers have reached a design maturity and that they cannot be made adequate for today's use. The record should truthfully say that these comments are applicable only to older oil circuit breakers and not to those of modem design. A 230-kV oil circuit breaker that was developed within the past year and designed to meet today's requirements is discussed. This involves meeting the requirements of ASA Standards C37.03 to C37.010 (Symmetrical Basis of Rating). It has a 20 000-MVA interrupting rating and a 2000-ampere continuous current rating. A description of the breaker and a full complement of test results are included.

Metal-Clad Switchgear for 23 and 34.5 kV

The extension of metal-clad switchgear with oilless power circuit breakers to the higher voltages of 23 kV and 34.5 kV has awaited the development of a suitable circuit breaker unit. Prompted by the need of Duquesne Light Company for such 23-kV equipment for their new Oakland substation, the breakers described in this article were designed and developed. As a result, indoor and outdoor metal-clad switchgear, with all of their well- known advantages, are now available for both voltage ratings up to a 1500-MVA interrupting rating and for continuous current ratings of 1200 and 3000 amperes.

Metal-Clad 4,160-Volt 350-Mva Switchgear with Symmetrical Ratings

Air break power circuit breakers of the magnetic type have been used for 2,300-and 4,160-volt service for almost the last 20 years. These breakers have been built in interrupting ratings from 50 to 250 mva (megavolt-amperes). Within the last 2 years a need has been established for breakers of higher interrupting and continuous current-carrying capacity. The need is attributable to the larger sized transformers used for distribution substations and for auxiliary power for modern generating stations. These require continuous current ratings up to 3,000 amperes and interrupting ratings up to 350 mva. The breakers and switchgear described in this paper were developed to meet this requirement. Because of industry interest in the basis of rating power circuit breakers, design tests were made to demonstrate that these breakers would meet the 350-mva interrupting rating on a symmetrical basis and with a 5-cycle interrupting time rating.