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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 44, NO. 4, JULY/AUGUST 2008
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Reducing Interrupting Duties of High-Voltage Circuit Breakers by Increasing Contact Parting Time J. C. Das, Fellow, IEEE
Abstract—The interrupting duty of high-voltage circuit breakers can be reduced by increasing the contact parting time, which is the sum of trippi tripping ng delay delay and the circuit circuit break breaker er openin opening g time. The contact contact parting time is ﬁxed; however however,, the tripping delay can be increased. Although this method is not discussed in current technical literature, literature, it is valid according to the ANSI/IEEE standards. Immediate replacement of the circuit breakers or other alternatives to reduce interrupting duties can be avoided, resulting in large large cost cost savin savings gs and proce process ss downti downtime me.. The probl problem em occurred in a large paper mill distribution system, where the circuit breakers were applied well within their close and latch ratings but the interrupti interrupting ng duty exceeded exceeded by 6%. By introduci introducing ng an additional additional tripping delay of one cycle, the interrupti interrupting ng duty is reduced and the existing breakers are retained in service. This paper demonstrates these calculations. Index Terms—Cont —Contact act partin parting g time, time, faults faults fed from from local local sources, faults fed from remote sources, no ac-decrement (NACD) ratio, tripping delay, weighted (interpolated) multiplying factors.
Fig. 1. The ac and dc decaying decaying components components of short-circ short-circuit uit current current and total asymmetrical wave.
I. I NTRODUCTION HORT-CIRCUIT currents are decaying transients. Fig. 1 shows the ac and dc decay of the short-circuit current, close to a generator. The presence of dc component results in an asymmetrical wave shape, and the rms time-current proﬁles of the ac component and total current can be calculated. Referring to Fig. 1,
S
I int int,total (rms, asym) =
(acsym)2 + (dc)2 .
(1)
Fig. 2 shows the rms time–current proﬁle of the ac component and total current with contact parting times (arbitrary) shown by vertical lines AA and BB from the initiation of the t = 0. As the contact parting time is inshort-circuit current at t = creased from C to to C in Fig. 2, the following becomes evident. 1) The ac symmetrical component component interrupted by the breaker decreases. 2) The total curren currentt interr interrupt upted ed by the break breaker er,, i.e., i.e., the asymmetrical duty, decreases. The asymmetrical duties were speciﬁed in [1] with respect to contact parting time and ratio-S, which is the required asymmetrical interrupting capability per unit of the symmetrical interrupting capability. In the year 1999 revision of this standard, Paper Paper PID-07-10 PID-07-10,, presented presented at the 2007 IEEE Pulp and Paper Industry Conference, Williamsburg, VA, June 24–28, and approved for publication in the IEEE TRANSACTIONS ON I NDUSTRY A PPLICATIONS by the Pulp and Paper Industry Committee of the IEEE Industry Applications Society. Manuscript submitted for review June 28, 2007 and released for publication November 29, 2007. Published July 23, 2008 (projected). The author is with Electrical Power Systems, AMEC E&C Services, Inc., Tucker, Tucker, GA 30084 USA (e-mail:

[email protected]).

[email protected]). Digital Object Identiﬁer 10.1109/TIA.2008.926235 10.1109/TIA.2008.926235
Fig. 2. Decrease of ac symmetrical component component and and total asymmetrical asymmetrical interinterrupting current with increased contact parting time from C to to C .
this ratio is replaced with the percentage of dc component that the breaker should be capable of interrupting, depending upon its contact parting time. The interr interrupt upting ing duty duty is calcul calculate ated d at the contac contactt partin parting g time. time. A ﬁve-cycle breaker has a contact opening time of 2.5 cycles (which is the lesser of the actual opening time of the breaker or 2.5 cycles), and one-half cycle is considered the tripping delay (relay operating time). Thus, the interrupting duty is calculated at three cycles. Fig. Fig. 3 shows shows the ac curren currentt interr interrupt uption ion in curren current-z t-zero ero = t 2 , circuit breakers. Short circuit occurs at t = 0, and at t = t the contacts start parting. Interval t 2 −t0 is the contact parting time. A tripping delay of 1/2 cycle is shown as t 1 −t0 , and the
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3) replacement or retroﬁtting overduty breakers; 4) application of short-circuit current limiters [2]; 5) use of duplex reactors [3]. It is not the intention to discuss these short-circuit current limiting techniques, except to brieﬂy underline some investigations relevant to the distribution system being discussed. A. Provide Current Limiting Reactors
Fig. 3. AC current interruption by a current-zero circuit breaker.
opening time is t2 –t1 . As the contacts start parting, an arc is drawn, and the voltage across the parting contacts increases. This is the arc voltage drop, shown exaggeratedly in Fig. 3. When the dielectric strength builds up and a current zero occurs, the current is interrupted. Interval t 3 –t2 is the arcing time, and t3 –t1 is the interrupting time. The recovery voltage proﬁle after current interruption shows transient recovery voltage and power frequency component, which are not of interest in this paper. (The interrupting rating of the breaker is a function of recovery voltage characteristics.) Figs. 1–3 clearly show that the interrupting duty can be reduced by increasing the tripping delay. However, this will not have any impact on the close and latch capability, i.e., ﬁrst cycle duty calculations. II. S YSTEM C ONFIGURATION
Current limiting reactors are a popular method of reducing short-circuit duties, both interrupting and ﬁrst cycle (close and latch). The current limiting reactors have nonlinear characteristics with respect to short-circuit current limitation. As the size of the reactor is increased, the relative magnitude of shortcircuit current reduction decreases. Fig. 4 shows that there are large reactors in the distribution system. 1) Adding a generator reactor, 2) increasing the reactor size UR1 in Fig. 4, and 3) adding additional reactors in the synchronizing bus tie circuits are considered. It is estimated that the reactance of UR1 in the utility tie circuit should be doubled. Doubling the reactors in the tie circuits or sectionalizing the synchronous tie bus has little impact on short-circuit currents at bus 4 in Fig. 4. A generator reactor of 0.3 Ω is required to limit the shortcircuit currents at bus 4 and retain the existing circuit breakers. These solutions resulted in adverse load ﬂow during normal and contingency operations, and excessive voltage drops occur. The reactive power ﬂow in a mainly reactive tie circuit requires ∆V at the sending and receiving ends, and there is a reactive power loss in the reactor itself. To supply 50 Mvar of load through a reactor of 0.63 Ω, the source must supply 63.2 Mvar of reactive power, i.e., 13.2 Mvar is lost in the reactor. Considering that the source voltage is maintained at 1.0 per unit, the load side voltage will dip by 21%. The large 14.4-Mvar capacitor ﬁlters shown in Fig. 4 provide voltage support in addition to harmonic mitigation.
Fig. 4 shows the main 13.2- and 2.4-kV bus interconnections of the distribution system. All circuit beakers on these buses are applied within their short-circuit rating, except that the interrupting duties on feeder circuit breakers at 13.2-kV bus 4 exceed their ratings by approximately 6%. The existing circuit breakers are rated 15 kV, ﬁve-cycle B. Redistribute Motor Loads symmetrical interrupting, 37-kA rated interrupting current, and The shifting of rotating motor loads to reduce interrupting K = 1.3. When this breaker is applied at 13.2 kV, the interduty requires the removal and reconnection of at least 12 000 hp rupting rating is 42.04-kA rms symmetrical. The calculated of motors to other buses. This is not practical and disrupts the interrupting duty for a bus fault is 44.35-kA rms symmetrical. process integrity. To retain existing breakers in service, a reduction of 2.31 kA or more is required. The short-circuit duty to which a breaker will be subjected is C. Retroﬁtting and Replacement of Breakers calculated by basing upon its maximum through fault current. The 2000 revision of [4] has made K factor (voltage range Consider a fault at F1 in Fig. 4. The generator breaker does not see the fault current contribution from the generator itself. factor) equal to one for all indoor oil-less circuit breakers. The In similar, a feeder breaker does not see the fault current circuit breakers of 50-kA interrupting rating, according to this contributed by the load connected to that feeder, i.e., a fault reference, can be provided in the existing breaker cubicles at a considerable cost and process downtime. at F2 in Fig. 4. III. L IMITING THE S HORT -C IRCUIT C URRENTS The short-circuit currents can be limited by one or more of the following methods: 1) adding current limiting reactors; 2) redistribution of motor loads;
IV. V ARIATIONS IN S HORT -C IRCUIT C ALCULATIONS BY A LTERNATE T ECHNIQUES ANSI/IEEE standards recognize that the short-circuit calculations can be performed by using other acceptable techniques which give reliable results. The variations in the results of
DAS: REDUCING INTERRUPTING DUTIES OF CIRCUIT BREAKERS BY INCREASING CONTACT PARTING TIME
Fig. 4.
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Single-line diagram of the interconnections of main 13.2- and 2.4-kV buses in the distribution system.
TABLE I COMPARISON OF S HORT-CIRCUIT C ALCULATIONS , ANSI/IEC/EMTP, A ND DYNAMIC S IMULATION M ETHODS
ANSI/IEEE standards permit E/X or E/Z calculation. E/X will be more conservative. However, for low-voltage systems, where the resistance component can be high, E/Z calculation should be used for accurate results. In view of the large variations of the short-circuit currents shown in Table I with various techniques, a question arises whether the calculated overduty by a certain percentage can be ignored? This will not be a prudent approach, unless rigorous alternate techniques are explored and acceptable to the owners of the plant. For accurate results, all impedances and their X/R ratios should be correctly modeled. V. A CCOUNTING FOR AC A ND D C D ECAYS
the short-circuit calculations, using alternate techniques, have been studied by various authors [5]–[7] and are summarized in Table I. These calculations show considerable differences. In Table I, it is, however, obvious that ANSI/IEEE calculations are generally more conservative when compared with dynamic simulation or ElectroMagnetic Transients Program (EMTP). Table I shows that, in some cases, ANSI calculations are 30% higher than the corresponding results obtained with dynamic simulation.
Postinterrupting duty multiplying factors, depending upon whether the fault is fed from the local or remote sources, are applied to E/Z calculation. When short-circuit current is predominantly fed through no more than one transformation or per unit reactance external to the generator, which is less than 1.5 times the generator per unit reactance on the same megavoltampere base, it is considered a local source; otherwise, it is a remote source. Utility contributions are considered as remote sources. The multiplying factors for short-circuit current contributions from the local sources consider ac and dc decay, whereas the multiplying factors from the remote sources consider dc decay only. Figs. 5 and 6 show these multiplying factors for ﬁve and eight cycle breakers [1]. These ﬁgures show the multiplying
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factors not only for the minimum contact parting time of the breaker (shown in a rectangle) but also for longer contact parting times. This implies that, a breaker of three-cycle parting time, if so relayed that it actually parts contacts in ﬁve cycles after fault initiation (Fig. 3), the E/Z multiplier for breaker selection can be reduced to account for the fault current decay during the additional two cycles, i.e., the multiplying factor shown for ﬁve-cycle contact parting time can be used. Figs. 5 and 6 show that this multiplying factor is lower compared with the minimum contact parting time multiplying factor for the same X/R ratio. VI. W EIGHTED ( I NTERPOLATED) I NTERRUPTING D UT Y M ULTIPLYING F ACTORS In an industrial system with generation, a part of short-circuit current contribution will be termed “local” and a part will be termed “remote.” The fraction of the interrupting current that is contributed by remote sources is identiﬁed as no ac-decrement (NACD) ratio, given by NACD ratio =
Fig. 5. Three-phase fault multiplying factors that include ac and dc decrement, shown for ﬁve- and eight-cycle breakers [1].
Fig. 6. Three-phase and line-to-ground fault multiplying factors that include the effects of dc decrement only, for ﬁve- and eight-cycle breakers [1].
NACD source currents
. E/Z for interrupting network
(2)
The computation of NACD ratio requires additional calculations of remote and local currents contributed at the fault point from various sources. This means that all the branch current contributions to the fault point have to be traced throughout the system and labeled local or remote. The calculation is facilitated by the digital computers. For short-circuit current contributions from the motors irrespective of their type and location in the system, the ac decay is built into the premultiplying impedance factors. Thus, it implies that motors, howsoever remote, will continue feeding a nondecaying current into the fault until it is cleared by the circuit breaker. Practically, the short-circuit currents contributed by induction motors decay and approach zero value in a couple of cycles, the decay depending upon the motor rating. Consider a 900-hp 2.3-kV four-pole motor. The motor data are as follows: full-load efﬁciency = 94.7%; full-load power factor = 88%; full-load current = 202 A; and lockedrotor current = six times the full-load current. The calculated parameters of this motor are as follows: stator resistance r 1 = 0.031 pu; rotor resistance r2 = 0.015 pu; magnetizing reactance X m = 3.00 pu; Rm = 100.0 pu; stator reactance X 1 = 0.0656 pu; rotor reactance X 2 = 0.0984 pu; motor transient reactance X = 0.165 pu; ac short-circuit time constant T = 0.0295 s; and dc short-circuit time constant T dc = 0.0142 s. Fig. 7 shows the calculated: 1) ac symmetrical component of the current, which decays to almost zero in ﬁve cycles; 2) dc component of the short-circuit current, which decays to zero in three cycles; and 3) the total short-circuit current. Note that the total short-circuit current at t = 0 (time of short circuit) is approximately 15 times the motor full-load current. The ANSI/IEEE contribution to interrupting duty current is shown as a straight line, starting from 1.5 cycles. From the initiation of
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TABLE II COMPUTER-BASED C ALCULATIONS , ANSI/IEEE METHOD, A ND THREE-PHASE F AULT C URRENT C ONTRIBUTIONS TO B US 4 I N F IG . 4. E/Z = 39.447 kA AT −88.10◦ , X/R = 42.45, Z 1 = 0.000368 + j0.11082 pu (100-MVA B AS E), I NTERRUPTING D UT Y MF F OR F IV E-CYCLE S YMMETRICAL R ATED C IRCUIT B REAKER= 1 .124, AN D M AXIMUM I NTERRUPTING D UTY = 44.35 kA, rms S YMMETRICAL
TABLE III CALCULATIONS OF R EMOTE /LOCAL C OMPONENTS OF SHORT-CIRCUIT C URRENT AND NACD
Fig. 7. Analytical calculations of 900-hp-motor short-circuit current versus ANSI/IEEE interrupting duty contributions.
fault to approximately 1.5 cycles, the ANSI ﬁrst cycle current can be used, and again, it is nondecaying. Irrespective of the contact parting time, the contribution is constant. This contrasts with the IEC calculations, which consider the decay from motor loads. Section 6.6 of the new IEEE standard 551 [8], which is the “Large induction motors with prolonged contributions,” does qualify the ANSI calculation methods and states that, for each large motor with a signiﬁcant short-circuit contribution, and for each speciﬁc calculation time t s after the short-circuit starts, a reactance factor of e t /T be substituted for the standard multiplying factor. T d is an X/R ratio time constant in radians at the same frequency. Reference [8, Sec. 12] addresses the IEC calculation methods and compares these with the ANSI standards; also see [9]. Short-circuit current at a fault location can be divided into local and remote components and the current contributed by motor loads. Then, the interrupting duty is calculated as follows: s
d
I int,sym = MFremote(I remote)+ MFlocal (I local )+ I motor.
(3)
No postinterrupting duty multiplying factors are applied to motor contributions. For proper application of a circuit breaker
VII. C ALCULATION P ROCEDURE The following procedure can be adopted to consider the effect of increased contact parting time on the interrupting duty calculations. Step 1) Run computer-based calculations which will give all branch ﬂows. Moreover, the calculations will give remote and local contributions. Tables II and III show these results. From these tables, we get the following. Total remote contribution = 9.56 kA rms sym.
E/Z = 39.447 kA rms sym. NACD = 9.56/39.447 = 0.242. Note that motor contributions are neither remote nor local but are a part of E /Z calculations. Reference [10] directly gives the weighted (interpolated) multiplying factors based upon NACD. Step 2) Verify computer calculations. The total E/Z current is now broken into the following constituents. Local = 19.31 kA rms sym at an X/R of approximately 72. Remote = 9.56 kA rms at an X/R of 23.
Breaker I int,sym Rated > I int,sym. calculated.
(4)
Motor contribution = 10.577 kA rms at X/R = 15.
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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 44, NO. 4, JULY/AUGUST 2008
The remote multiplying factor can be read from the curves or it is analytically calculable from the following:
MFremote = (1/S ) 1 + 2e−4πC/ (X/R)
1/2
(5)
where C is the contact parting time in cycles. In (5), the remote MF = 1.069. In Fig. 6, also a similar MF can be read, although not to three decimal places. Local multiplying factor cannot be calculated from a mathematical expression and must be read from the curve. The relevant portion of the curve can be enlarged for easy reading. In Fig. 5, Local MF = 1.21. Therefore, the calculated interrupting duty is as follows:
19.31(1.21) < < < <
−89.20◦ + 9.56(1.069) −87.51◦ + 10.577 −86.19◦ = 44.18 kA −88.27◦ .
This calculated result of 44.18 kA is fairly close to the computer calculations of 44.35 kA, as shown in Table II. Step 3) Calculate the interrupting duty at increased fourcycle contact parting time. The E/Z calculation does not change only the MF’s change. Remote MF = 1. Local MF = 1.1. Calculated interrupting duty
19.31(1.1) < −89.20◦ + 9.56 < −87.51◦ + 10.577 < −86.19◦ = |41.38| kA . This gives a reduction of 2.8 kA by increasing the contact parting time by one cycle. Circuit breaker rating at the operating voltage = 42.045 kA rms sym. Thus, the breakers are applied slightly below their ratings with one cycle increased contact parting time. VIII. I MPLEMENTATION
logic, avoiding the use of another discrete auxiliary device. Although the failure rate on an auxiliary trip device will be low, its elimination improves the reliability. IX. F URTHER C ONSIDERATIONS A. Transient Stability
In many situations, even a one-cycle delay in clearing a severe fault can make a difference between unstable and stable conditions. Consider a fault close to the bus on a feeder, at F2 in Fig. 4. In general, the stability of a generator is hard to ensure for a three-phase bolted bus fault close to it, and an increase in tripping delay of one cycle worsens this situation. For other unsymmetrical faults, the transient stability will be slightly compromised. There are eight-cycle breakers rated on TOT current basis (prior to 1964 ratings) on some of the buses in Fig. 4. B. Relay Coordination
Current limiting fuses are extensively applied in industrial systems for substation transformers’ primary protection. In addition, the feeder breakers serving these transformers have instantaneous overcurrent settings to protect the cable circuits feeding the transformers. Introducing one-cycle delay improves the coordination with downstream transformer fuses in a typical radial or loop fed distribution system—for a fault location downstream of the fuse. C. Arc-Flash Considerations
The incident energy release will generally be higher. The short-circuit current has somewhat decayed during an additional one cycle, whereas the fault clearance time (for incident energy release) increases by one cycle. D. Fault Energy Release
As the fault clearing time is increased by one cycle, the energy released into the fault will increase. However, it will not be in the ratio 6/5 due to the decaying nature of the fault current, in Fig. 1. For comparison, an integral should be taken
E =
i2 dt
(6)
or Simpson’s rule of averages can be applied. The calculation shows that the increased energy release will be approximately 7%.
All trips, whether manual or through protective relays, occur through an auxiliary trip relay 94, which has an inherent pickup E. Indoor Oil-Less Circuit Breakers With K = 1 time delay of one cycle. (Lockout relays of device 86 generally The calculation procedure is also applicable to breakers with have a pickup time delay of 50–60 ms.) This 94-trip relay can be a nondraw-out molded case device with adequate current K = 1 . interrupting and carrying capability similar to a lockout relay. In an electromagnetic device, the pickup time will slightly vary. F. Expected Reduction in Interrupting Duty This is acceptable. With multifunction microprocessor-based protective relays, The maximum E/Z multiplying factor for local contributhe trip delay of one cycle can be programmed into the relay tions (ac and dc decay) is 1.25. Thus, the reduction that can
DAS: REDUCING INTERRUPTING DUTIES OF CIRCUIT BREAKERS BY INCREASING CONTACT PARTING TIME
be obtained in the interrupting duty is a function of the X/R ratio and increase in contact parting time. G. Conclusion
This paper shows that the interrupting duty can be reduced by increasing the contact parting time. Although ANSI/IEEE standards permit this method to avoid the immediate replacement of breakers, a detailed methodology, which is similar to the one presented in this paper, has not been presented in the current literature. While implementing this method, the related aspects discussed in this paper should be considered. For the system under study, this method saved much expense and downtime, as the conventional means of addressing this problem, discussed in this paper, had technoeconomical limitations. R EFERENCES [1] Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis, ANSI/IEEE Std. C37-010, 1979. (Revised 1999 and 2005). [2] J. C. Das, “Limitation of fault-current limiters for expansion of electrical distribution systems,” IEEE Trans. Ind. Appl., vol. 33, no. 4, pp. 1073– 1082, Jul./Aug. 1997. [3] J. C. Das, F. Robertson, and J. Twiss, “Duplex reactor for a large co-generation distribution system—An old concept revisited,” in Proc. TAPPI Eng. Conf., Nashville, TN, 1991, pp. 637–648. [4] AC High Voltage Circuit Breakers Rated on a Symmetrical Current Basis—Preferred Ratings and Related Capabilities , ANSI/IEEE Std. C37.06, 2000. [5] J. R. Dunki-Jacobs, B. P. Lam, and R. P. Strafford, “A comparison of ANSI-based and dynamically rigorous short-circuit current calculation procedures,” IEEE Trans. Ind. Appl., vol. 24, no. 6, pp. 1180–1194, Nov./Dec. 1988. [6] A. Berizzi, S. Massucco, A. Silvestri, and D. Zaninelli, “Short-circuit current calculations—A comparison between methods of IEC and ANSI standards using dynamic simulation as reference,” IEEE Trans. Ind. Appl., vol. 30, no. 4, pp. 1099–1106, Jul./Aug. 1994.
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[7] A. J. Rodolakis, “A comparison of North American (ANSI) and European (IEC) fault calculations guidelines,” IEEE Trans. Ind. Appl., vol. 29, no. 3, pp. 515–521, May/Jun. 1993. [8] IEEE Recommended Methods for Calculating AC Short-Circuit Currents in Industrial and Commercial Power Systems, Violet Book , IEEE Std. 551, 2006. [9] J. C. Das, “Short-circuit calculations—ANSI/IEEE and IEC methods, similarities and differences,” in Proc. 8th Int. Symp. Short-Circuit Calculations Power Syst., Brussels, Belgium, 1998, pp. 25–30. [10] W. C. Huening, Jr., “Interpretation of New American Standards for power circuit breaker applications,” IEEE Trans. Ind. Gen. Appl., vol. IGA-5, no. 5, pp. 121–143, Sep./Oct. 1969.
J. C. Das (SM’80–F’08) received the B.A. degree
in mathematics and the B.E.E. degree from Panjab University, Chandigarh, India, in 1953 and 1956, respectively, and the M.S.E.E. degree from Tulsa University, Tulsa, OK, in 1982. He is currently a Staff Consultant with Electrical Power Systems, AMEC E&C Services, Inc., Tucker, GA. He is responsible for power system studies, including short circuit, load ﬂow, harmonics, stability, arc-ﬂash hazard, grounding, switching transients, and, also, protective relaying. He conducts courses for continuing education in power systems, and he is the author or coauthor of about 50 technical publications. He is the author of Power System Analysis (Marcel Dekker, 2002). His interests include power system transients, harmonics, power quality, protection, and r elaying. Mr. Das is a member of the IEEE Industry Applications and IEEE Power Engineering Societies. He is a member of the Technical Association of the Pulp and Paper Industry and the International Council on Large Electric Systems, a Fellow of the Institution of Electrical Engineers (U.K.), a Life Fellow of the Institution of Engineers (India), and a member of the Federation of European Engineers (France). He is a Registered Professional Engineer in the States of Georgia and Oklahoma, a Chartered Engineer (C.Eng.) in the U.K., and a European Engineer (Eur.Ing.). He received the IEEE Pulp and Paper Industry CommitteeMeritorious Award in Engineering in 2005. He is active in thePower Distribution Subcommittee, IEEE Pulp and Paper Industry Committee of which he is a member.