Current interruption

Pelenc.Y. Review of the main current interruption techniques Voltas Limited, Switchgear Division, India. 

Information on defects can be obtained with good approximation from Eq. (3-5 la). The value of is all that is necessary for an overview. should be as high as possible to increase the sensitivity. In addition, to eliminate foreign voltages in the soil, it is necessary to switch the polarization current on and off with the help of a current interrupter periods of about 2 s off and 18 s on are convenient. Potential differences independent of the polarization current that are the result of foreign currents or electrode faults (see Section 3.2) are totally excluded by this method. On the other hand, the IR component of a compensation current can also be... 

Seals are required at entries by conduit or cable to explosion-proof enclosures containing arcing or high-temperature devices in Division 1 and Division 2 locations. It is not required to seal IM in. or smaller conduits into explosion-proof enclosures in Division 1 areas housing switches, circuit breakers, fuses, relays, etc., if their current-interrupting contacts are hermetically sealed or under oil (having a 2-in. minimum immersion for power contacts and 1-in. for control contacts). 

In addition to circuit breakers, there are other classes of automatic switches that can be controlled or operated remotely, but with current-interrupting capability. These include circuit switchers, reclosers, and sectionalizers. 

This protection technique shall be permitted for current-interrupting contacts in those Class I, Division 2, Class II, Division 2, and Class III locations for which the equipment is approved. 

This protection technique shall be permitted for current-interrupting contacts in Class I, Division 2 locations. 

The latter equals IRwc where RWc is the ohmic resistance between the working and counter electrode. Experimentally it is rather easy to measure the riohmic.wc term using the current interruption technique as shown in Figure 4.9. Upon current interruption the ohmic overpotential r 0i,mjCtwc vanishes within less than 1 ps and the remaining part of the overpotential which vanishes much slower is t w+T c (Eq. 4.9). 

Figure 4.9. Use of the current interruption technique to measure the ohmic overpotential, r ohmic,wc> between the working (W) and counter (C) electrode.

Thus, to a good approximation the t w determined via current interruption (Fig. 4.10) can be considered to be an activation overpotential. 

How can one explain such a huge Faradaic efficiency, A, value As we shall see there is one and only one viable explanation confirmed now by every surface science and electrochemical technique, which has been used to investigate this phenomenon. We will see this explanation immediately and then, in much more detail in Chapter 5, but first let us make a few more observations in Figure 4.13. It is worth noting that, at steady-state, the catalyst potential Uwr, has increased by 0.62 V. Second let us note that upon current interruption (Fig. 4.13), r and UWr return to their initial unpromoted values. This is due to the gradual consumption of Os by C2H4. 

How can we confirm this sacrificial promoter model By simply looking at the r vs t transient behaviour of Figure 4.13 or of any galvanostatic NEMCA experiment upon current interruption (1=0). 

Thus the average lifetime, x, of O on the catalyst surface (x=TOF ) equals 770 s or 13 min. This then should be the time needed for the rate, r, to decay to its unpromoted value upon current interruption. This is in excellent agreement with experiment (Fig. 4.13) and nicely confirms the sacrificial promoter concept of NEMCA The promoter (O5 ) is sacrificed by eventually reacting with the oxidizable species (C2H4). But before being sacrificed , i.e. consumed, it has caused on the average the reaction of A extra oxygen atoms with the oxidizable species. 

Upon positive current application the rate of C2H4 oxidation increases by a factor of 13 (p=14) with a A value of the order of 100. The important aspect of the figure is that upon current interruption neither the rate nor O return to their initial open- circuit values (Fig. 4.49). There is a permanent... 

As shown on Figs. 8.31 to 8.33 the rate and UWR (or 0) oscillations of CO oxidation can be started or stopped at will by imposition of appropriate currents.33 Thus on Fig. 8.31 the catalyst is initially at a stable steady state. Imposition of a negative current merely decreases the rate but imposition of a positive current of200 pA leads to an oscillatory state with a period of 80s. The effect is completely reversible and the catalyst returns to its initial steady state upon current interruption. 

The opposite effect is depicted on Fig. 8.32 where the catalyst under open-circuit conditions exhibits stable limit cycle behaviour with a period of 184 s. Imposition of a negative current of -400 pA leads to a steady state. Upon current interruption the catalyst returns to its initial oscillatory state. Application of positive currents leads to higher frequency oscillatory states. 

As shown in Fig. 9.25, upon current interruption rH2, ro and Urj,e return to their open circuit values, showing the reversibility of the effect. It is worth noting that the rate transient parallels, to a large extent, the catalyst potential. This shows the important role of catalyst potential in describing electrochemical promotion. 

A method based on the oseillographie observation of the response after current interruption or tuming-on suitable for its interpretation taking into account the effect of side reactions on potential-time curves has been deseribed [59Khe, 61Zin]. (Data obtained with this method are labelled OCS.)... 

Figure 9. Typical discharge (lithiation) voltage profile of the Li/11.7%Cu-graphite cell at 50 °C in 1 1 EC DEC (1 MLiPF6 LP-40). Inset is an expanded region showing the voltage relaxation change during current interruption at about 0.08 V of the Li/11,7%Cu-graphite...

There are no electrolyzers developed specifically for operation with wind turbines. However, the rapid response of electrochemical systems to power variations makes them suitable "loads" for wind turbines. Industrial electrolyzers are designed for continuous operation, mainly because their elevated investment cost requires high-capacity factors for reasonable payback times, but they are subject to a considerable number of current interruptions through their lifetime due to occasional power interruptions, accidental trips of safety systems, and planned stops for maintenance. Current interruptions are more frequent in specialty applications, where electrolyzers supply hydrogen "on demand." Therefore, the discontinuous use of the equipment is not new, and most commercial electrolyzers may be used in intermittent operation although a significant performance decrease is expected with time. In fact, it is not power variation, but current interruptions that may cause severe corrosion problems to the electrodes, if the latter are not protected by the application of a polarization current when idle. 

The voltage drop I R should be estimated and eliminated from the measured potential. It can be directly determined by fast current interruption measurements about 1 xs after shutting down the current, the potential is already decreased by all ohmic voltage drops while all other... 

CID current interrupter device activated by internal pressure. 

Other less prominent types of additives, also intended for overcharge protection, were termed shutdown additives in the battery industry based on their tendency at high potentials to release gas, which in turn would activate a current interrupter device (CID), or to polymerize and block the ion passage in the electrolyte. The former included such... 

Figure 10. Typical short-circuit behavior of a 18650 lithium-ion cell with shutdown separator and without PTC (positive temperature coefficient) and CID (current interrupt device). This test simulates an external short circuit of a cell.

Figure 5. Measurement and analysis of steady-state i— V characteristics, (a) Following subtraction of ohmic losses (determined from impedance or current-interrupt measurements), the electrode overpotential rj is plotted vs ln(i). For systems governed by classic electrochemical kinetics, the slope at high overpotential yields anodic and cathodic transfer coefficients (Ua and aj while the intercept yields the exchange current density (i o). These parameters can be used in an empirical rate expression for the kinetics (Butler—Volmer equation) or related to more specific parameters associated with individual reaction steps.(b) Example of Mn(IV) reduction to Mn(III) at a Pt electrode in 7.5 M H2SO4 solution at 25 Below limiting current the system obeys Tafel kinetics with Ua 1/4. Data are from ref 363. (Reprinted with permission from ref 362. Copyright 2001 John Wiley Sons.)...

---