Chromatic Assessment of High-Voltage Circuit Breaker Gases

C. R. Jones, J. W. Spencer, L M. Shpanin and J. D. Yan

Introduction to Current Interruption

Switching electric currents in high-voltage circuits involves mechanically separating two contacts to form an electric arc plasma which is then quenched. This operation is conducted in a tank containing a vacuum or one type of gas which has good arc-quenching properties. The quenching process may be enhanced by blowing the gas through the plasma orelectromagnetically driving the arc through the gas.

Monitoring such circuit breakers is important to ensure that there are no power supply disruptions. The monitoring involves addressing many parameters such as the current flowing, various voltages, gas pressure, movement of the switch contacts and so on. Interpreting the monitored results can be a complex process in which chromatic processing can assist. Examples are given comparing data from a small group of medium-voltage air circuit breakers with electromagnetic arc control and non-B field interrupters with several different arc-quenching gases both experimentally and theoretically.

Chromatic Interpretation of Conventionally Measured Circuit Breaker Parameters

Introduction to Current Interruption Parameters

As a result of controlling and quenching an arc plasma during the interruption of high alternating currents, there are complicated variations in electrical parameters of the switch such as the high

Parameters evaluated from the tests

FIGURE 21.1 Parameters evaluated from the tests.

current phase arc voltage peak (Vpkc), arc extinction voltage peak (Vec), peak recovery voltage [V(rcc)] and peak fault current (Ipkc) (Figure 21.1) (Jones et al„ 2008; Shpanin et ah, 2009). The current interruption process will also be influenced by additional features such as the pressure of the gas inside the switch (pc), the gap length between the contacts (gc) and so on (Jones et ah, 2008; Shpanin et ah, 2009). In one form of switch, the electric arc may also be controlled by a separate magnetic field (Be) (Shpanin, 2006; Jones et ah, 2008; Shpanin et ah, 2009). There is therefore a need for an approach to extract relevant information from such a complexity of data in a manner which is traceable and capable of identifying emerging conditions. This may be achieved through the use of chromatic processing of the data.

Chromatic Analysis Method

Applying the chromatic approach to the current interruption situation involves first ordering and classifying the operational parameters of the circuit breaker in a suitably normalised form into three groups (Shpanin. 2006; Shpanin et ah, 2009) (Appendix 21 A).

Group I includes a priori-determined control parameters (gas pressure, contact gap, arc controlling magnetic field [B]), group II features the proposed interruption conditions (B field, fault current [Ipk], arc voltage [Vpk]) and group III provides the interrupter responses (Vpk, extinction peak [Vext], recovery voltage [Vrc]). The В field belongs to both groups I and II, while the arc voltage belongs to both groups II and III. The normalisation is arranged so that an increase in a parameter value represents a reduction in interrupter performance. Three overlapping chromatic processors (R, G, B), corresponding to each group (I, II, III) are superimposed upon the ordered set of parameters (Figure 21.2a). The outputs from each processor (R. G, B) are fed into chromatic algorithms (Figure 21.2b) to yield three chromatic parameters FI, S, L (Chapter 1). Polar chromatic maps of H versus L and H versus S may then be produced to indicate the switching condition (Figure 21.2c).

Values of the FI parameter indicate the dominance of each of the seven interruption parameters (gc through to Vrec). Values of L indicate the severity of conditions, and values of S indicate the spread of the influence among various parameters.

Examples of Chromatic Analysis Application

Figure 21.3a shows the normalised values of the operational parameters (gc, pc. Be, Ipkc, Vpkc, Vec, Vrc) for an air-filled, magnetic field-driven arc interrupter (Ipk = 13.3 kA. Bpeak = 345.3 mT, g = 0.10 6 m) with R. G. В processors superimposed. An H-L polar diagram is shown in Figure 21.3b with results for this interrupter at different peak currents lying in the range 0 < H < 56. This implies that the interrupter’s operational conditions are dominated by gap length and gas pressure.

Figure 21.3b shows results for other interrupters - non-rotary arcs in nitrogen air and SF(), air interrupters with В field producing coils. These results show

Chromatic analysis of current interrupter data,

FIGURE 21.2 Chromatic analysis of current interrupter data, (a) Non-orthogonal processors R. G. В superimposed on the three groups monitoring data; (b) chromatic transformation R. G. B>H:L, H:S; (c) H:L; H:S polar diagrams indicating location of components of the three monitoring groups.

  • 1. For the non-B field interrupters, the 1 bar air-filled interrupter had L ~ 0.7, the 3 bar N2 interrupter had L ~ 0.65 and the SF6 interrupter had L ~ 0.55.
  • 2. For the В field interrupter with 1 bar air, L = 0.65 —» 0.75.

Figure 21.3c shows a more expanded H-S Cartesian chromatic map. This shows that for all of the interrupters and conditions investigated, no single parameter is outstandingly dominant, since in all cases, S < 0.34 (S = 0 - equal influence; S = 1 - single totally dominant feature). The variation with peak current is indicated by arrows.

For an air-filled, magnetic field-driven arc interrupter. Figure 21.3b and c show that increasing the contact length further equilibrates the effects of the various parameters (S reduced from 0.22 to

0.12) by promoting the influence of Be, pc(H —> 60) while reducing the stress level (L reduced from 0.725 to 0.64). (Further details may be found in Shpanin et al., 2009.)

Summary and Overview

The application of chromatic analysis enables chromatic maps to be produced which clearly and simply highlight the relative significance of various operational parameters for different types of current interrupters. The parameters involved are divided into three groups corresponding to control

Chromatic processing of data for current interrupters,

FIGURE 21.3 Chromatic processing of data for current interrupters, (a) Data set ordering with non-orthogonal processors superimposed (example data for В field air interrupter with I peak = 13.3 kA. В peak = 345.3 mT, gap = 0.106 m); (b) polar H:L diagram; (c) Cartesian H:S diagram (expanded region; arrow indicates increasing fault current). (Interrupters: No В field : “a" SF6, N2, (3Bar); Air (1 Bar), With В field : “0” “O”

Air (1 Bar) (Shpanin. 2006).

parameters, current interruption conditions and interrupter responses so that guidance is provided for possible design improvements.

Comparison of High-Voltage Arc-Quenching Gases: Test Data

Introduction to Arc-Quenching Test Data

A widely used gas for high-voltage circuit breakers is sulphur hexafluoride (SF6) (Glaubitz et al. 2014) However, SF6 also has a high global warming potential (GWP ~ 23500) over a long time horizon (100 years) (Stocker et al., 2013). Its use is therefore regulated and restricted. A search for alternative gases has been ongoing for about two decades (Kieffel et al.. 2014; Seeger et al., 2017). The complexity of comparing various alternatives to SF6 and the amount and variety of information available have contributed to the difficulty of assessments and to find a solution. Such difficulties may be alleviated using chromatic approaches.

Central to the consideration of the choice of gas for arc quenching for high-voltage alternating current (AC) interruption are three of the parameters considered in Section 21.2; see Figure 21.1.

  • 1. The highest peak AC (Ipk) which can be interrupted
  • 2. The dielectric strength (Vd) of the gas
  • 3. Gas pressure change due to decomposition caused by arcing (Dp)

A chromatic-based comparison of the previous parameters can provide a quantitative comparison of various gases.

Chromatic Analysis

Potential arc-quenching gases considered by several investigators are carbon dioxide (C02) and fluoroketone (FK) plus fluoronitrite (FN), each mixed with air or C02 (Uchii et al., 2002, 2004; Kieffel et ah, 2016; Seeger et ah, 2017). Values of Ipk, Vd and Dp for these gases may be normalised respectively with regard to their values for SF6, (FN + air) and SF6. The three normalised parameters are treated as the outputs from three chromatic processors, that is. R = (Ipk)'=Ipk/Ipk(SF6), G = (Vd)' = Vd/Vd(FN + air), В = (Dp)' = Dp/Dp(SF6).

These can be transformed into chromatic parameters L', X', Y'. Z' (Chapter 1)

A chromatic parameter (1 - L') may be used to provide an overall quantification of each arcquenching gas mixture, whilst the X', Y', Z' parameters indicate the relative significance of each component (Ipk)', (Vd)', (Dp)'. A graph of (1 - L') versus gas type is presented in Figure 21.4, whilst a chromatic map of X' versus Y' is given in Figure 21.5.

Interpretation of Chromatic Monitoring Results

An approximate indication of the overall behaviour of the various gases is provided by the (1 - L') chromatic parameter (Figure 21.4), whilst a more detailed comparison is provided by the X' : Y'

Effective current interruption performance (1 - L) of various gases (L = (R + G + B)/3)

FIGURE 21.4 Effective current interruption performance (1 - L) of various gases (L = (R + G + B)/3).

X : Y : Z chromatic map comparing relative magnitudes of current interruption effects (0 = SF,1 = (FK + CO,), 2 = (FK + air), 3 = (FN + CO,), 4 = (FN + air), 5 = (CO,)

FIGURE 21.5 X : Y : Z chromatic map comparing relative magnitudes of current interruption effects (0 = SF6,1 = (FK + CO,), 2 = (FK + air), 3 = (FN + CO,), 4 = (FN + air), 5 = (CO,).

chromatic map (Figure 21.5). The latter emphasises the advantages of SF6 in having a relatively high dielectric strength (Y' —> 1), no reduction in the maximum interrupted current (X' —> 0) and only a low increase in gas pressure (Z' —» 0).

CO, has no post-arc pressure increase (Z' —> 0), and its current interruption and dielectric strength are lower than SF6 (x —> 0.5, Y —> 0.5). Although the relative current interruption capabilities (X') of the FN and FK gas mixtures are poorer than SF6, they are better than CO,. However, the post-current interruption pressure increases (Z') are greater for all FN and FK gases than both SF6 and CO,. Also, the relative dielectric strength reductions (Y') are all lower than that of SF(>; two of them [(FN + CO,) and (FK + CO,)] have similar values to CO,.

Quantification of the post-arc pressure rise is an indication of the possible production of arc- induced undesirable environmental effects (Chapter 12).

The difference in the values of (1 - L') for CO,, FK-CO,0. FN-CO, may be ascribed to the complex combination of reduced dielectric strength, current interruption reduction and post-arc production of toxic compounds. Thus, if a slight reduction in dielectric strength is considered less significant for particular applications than a reduction in interrupted current, (FK + CO,) might be preferable to (FN + CO,). Consideration of such chromatic factors can assist in decision-making.

Summary and Overview

The gas examples treated provide an illustration of how the complex performance of various gases can in principle be conveniently quantified, compared and presented in an X', Y', Z' chromatic map. A calibrated effective performance parameter (1 - L') can be used for comparing the performance of a gas with a benchmark SF6. As new SF6 potential replacement gases become available, the approach can provide a basis for the quantification of performance.

Comparison of High-Voltage Arc-Quenching Gases: Theoretical Data

Introduction to Arc-Quenching Computations

Considerable computational modelling has been made historically of energy transfer during arc plasma quenching in gas flow, high-voltage circuit breakers (Zhang et al., 2016). Detailed interpretation of such results about the performance of various gases can prove difficult, particularly in identifying any significant property. However, chromatic analysis of such results can assist in the interpretation of such results.

The basis of such computational approaches is a thermodynamic consideration of the balance between the electrical energy input to the arc plasma (V.i); the energy stored in the arc plasma and the energy lost via convection, conduction and radiation. A chromatic examination of these processes for different gases can assist in identifying suitable gases for current interruption.

Chromaticity of Thermal Storage Temperature Variations

A comparison may be made of the thermal gas storage (mass density x specific heat) variation with temperature for various gases (Zhang et al., 2016; Guo et al., 2017). The variation of thermal storage with temperature in the range 0-104 К is shown in Figure 21.6 for SF6, CO, and air. The highly complex nature of the storage variations is apparent. The difficulty of interpreting such complex variations may be alleviated by addressing each variation chromatically with three triangular chromatic processors (Figure 21.6), with R covering the electrically nonconducting temperature range 0-5000 К and В the electrically conducting temperature range 5,000-10,000 K.

An X, Y, Z chromatic map for the thermal storage data is shown in Figure 21.7a, with X representing the relative amount of storage for the 0-5,000 К temperature range and Z representing the 5,000- 10,000 К range. These results show that at higher temperatures, air has the highest storage, whereas at the lower temperature range, SF6 has the highest storage. Thus, SF6 has the advantage of storing a relatively low amount of heat in the electrically conducting plasma range and a considerably higher amount at the non-electrically conducting temperature range.

Chromatically addressed temperature variation of thermal storage of SF, CO,, air. (a) Thermal storage versus temperature; (b) chromatic processors (R, G. B) superimposed upon thermal storage

FIGURE 21.6 Chromatically addressed temperature variation of thermal storage of SF6, CO,, air. (a) Thermal storage versus temperature; (b) chromatic processors (R, G. B) superimposed upon thermal storage: temperature (CZC paper).

Figure 2l.7b shows a graph of the effective magnitude of the heat stored (Ls) versus the spread of the heat stored. SF6 has the highest stored heat capability with little spread, whereas air has the lowest stored heat with high spread.

These results provide one clear indication as to why SF6 is a better arc-quenching medium than air or CO,. It stores only little heat at the electrically conducting temperatures (>5,000 K) but has the highest capability of storing heat at the lower, non-electrically conducting region. Thus, a quest for alternative gases to SF6 needs to ensure that the relative heat storage between the electrically conducting temperatures is very much less than for the non-electrically conducting temperatures.

Chromatic characteristics for thermal storage of electrically conducting and non-conducting temperature ranges for SF, CO,, air

FIGURE 21.7 Chromatic characteristics for thermal storage of electrically conducting and non-conducting temperature ranges for SF6, CO,, air. (a) X, Y. Z chromatic map (Xs stored heat, < 5 x 10’K; Zs stored heat, > 5 x 10'K). (b) Effective magnitude (Ls) versus spread (Ss) of stored heat.

Chromatic characteristics of radiation, radial conduction and convection of air and SF6 (full symbols - air, open symbols - SF6

FIGURE 21.8 Chromatic characteristics of radiation, radial conduction and convection of air and SF6 (full symbols - air, open symbols - SF6: circle - 500A. triangle - 50A. square - 25A). (a) X,Y, Z chromatic map (relative values of conduction (Xr), convection (Yr). radiation (Zr)). (b) Effective magnitude (Lr) versus spread (Sr).

Chromatic Comparison of Different Forms of Thermal Losses

A chromatic comparison may be made of the thermal losses from an arc plasma carrying different currents (25-500 A) due to radiation, thermal conduction and convection. In this case, the chromatic processors can be applied so that R=thermal conduction, G=thermal convection, B=radiation. The resulting X. Y, Z chromatic map is shown in Figure 21.8a for air and SF6 at various currents (25- 500 A). At 25 A, this shows little radiation loss compared with conduction and convection for both air and SF(), whereas at 500 A. SF() has a relatively high radial conduction which is greater than air.

A chromatic map of effective strength (Ls) versus spread (Sv) (Figure 21.8b) shows that although SF6 and air have similar strengths (0.3-0.4) at the higher current (0.6-0.3), SF6 has a higher strength than air at 25 A.

Summary and Overview

Chromatic processing of detailed computational analysis of electric arc plasma behaviour (e.g., Guo et al., 2017) has highlighted features which warrant consideration when seeking possible replacement gases for SF6. The need for high thermal storage at non-electrically conducting temperatures (<5,000 K) has been indicated. A high thermal power transfer to this area is also highlighted, with both thermal conduction and radial convection having important roles.

Appendix 21A: Circuit Breaker Operational Parameters

  • • I Gas pressure (p) (p/pn = pc; pn = 1 bar);
  • • I Contact gap length (g) (1 - 1/gn = gc; gn = 1 m);
  • • II, I Magnetic field (В) (1 - B/Bn = Be; Bn = 1 T);
  • • II Fault current (Ipk) (Ipk/Ipkn = Ipkc; Ipkn = 20 к A);
  • • II, III Arc voltage (Vpk) (1 - Vpk/Vpkn = Vpkc; Vpkn = 1 kV);
  • • III Extinction peak (Vextn) (1 - Vext/Vextn = Vec; Vextn = 1 kV);
  • • III Recovery voltage (Vrc) (1 - Vrc/Vrcn = Vrcc; Vrcn = 1 kV).

n=normalising parameter value; c=normalised parameter.


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