IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 44, NO.4, JULY/AUGUST 2008 1027 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 tripping delay and the circuit breaker opening time. The contact parting time is fixed; however, the tripping delay can be increased. Although this method is not discussed in current technical 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 cost savings and process downtime. The problem occurred in a large paper mill distribution system, where the circuit breakers were applied well within their close and latch ratings but the interrupting duty exceeded by 6%. By introducing an additional tripping delay of one cycle, the interrupting duty is reduced and the existing breakers are retained in service. This paper demonstrates these calculations. Index Terms—Contact parting time, faults fed from 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 components of short-circuit current and total asymmetrical wave. I. I NTRODUCTION S 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, (1) Iint,total (rms, asym) = (acsym )2 + (dc)2 . 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 short-circuit current at t = 0. As the contact parting time is increased from C to C in Fig. 2, the following becomes evident. 1) The ac symmetrical component interrupted by the breaker decreases. 2) The total current interrupted by the breaker, 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 PID-07-10, presented at the 2007 IEEE Pulp and Paper Industry Conference, Williamsburg, VA, June 24–28, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS 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, GA 30084 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/TIA.2008.926235 Fig. 2. Decrease of ac symmetrical component and total asymmetrical interrupting current with increased contact parting time from C 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 interrupting duty is calculated at the contact parting 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. 3 shows the ac current interruption in current-zero circuit breakers. Short circuit occurs at t = 0, and at t = t2 , the contacts start parting. Interval t2 −t0 is the contact parting time. A tripping delay of 1/2 cycle is shown as t1 −t0 , and the 0093-9994/$25.00 © 2008 IEEE both interrupting and first cycle (close and latch). the interrupting rating is 42. the source must supply 63. a feeder breaker does not see the fault current contributed by the load connected to that feeder. and there is a reactive power loss in the reactor itself..2.and 2. It is not the intention to discuss these short-circuit current limiting techniques.. 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. 4) application of short-circuit current limiters [2].35-kA rms symmetrical. 37-kA rated interrupting current. Current limiting reactors are a popular method of reducing short-circuit duties. the current is interrupted. 4. 3. All circuit beakers on these buses are applied within their short-circuit rating.e. A generator reactor of 0. i. except that the interrupting duties on feeder circuit breakers at 13. AC current interruption by a current-zero circuit breaker. The circuit breakers of 50-kA interrupting rating. according to this reference. a fault at F2 in Fig. The short-circuit duty to which a breaker will be subjected is calculated by basing upon its maximum through fault current.0 per unit. VOL. When this breaker is applied at 13. S YSTEM C ONFIGURATION Fig.3.e. 3.1028 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS.31 kA or more is required. 4. In similar. II.4-kV bus interconnections of the distribution system. The generator breaker does not see the fault current contribution from the generator itself. Retrofitting and Replacement of Breakers The 2000 revision of [4] has made K factor (voltage range factor) equal to one for all indoor oil-less circuit breakers. 4. a reduction of 2. opening time is t2 –t1 . 4. i. The existing circuit breakers are rated 15 kV. (The interrupting rating of the breaker is a function of recovery voltage characteristics. A. These solutions resulted in adverse load flow during normal and contingency operations. and 3) adding additional reactors in the synchronizing bus tie circuits are considered. 2) redistribution of motor loads. five-cycle symmetrical interrupting. which are not of interest in this paper. 4 provide voltage support in addition to harmonic mitigation.) Figs. The variations in the results of . Provide Current Limiting Reactors Fig. 1–3 clearly show that the interrupting duty can be reduced by increasing the tripping delay. NO. the relative magnitude of shortcircuit current reduction decreases. 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. As the contacts start parting. Considering that the source voltage is maintained at 1.3 Ω is required to limit the shortcircuit currents at bus 4 and retain the existing circuit breakers. 44. This is not practical and disrupts the process integrity. Redistribute Motor Loads The shifting of rotating motor loads to reduce interrupting duty requires the removal and reconnection of at least 12 000 hp of motors to other buses.e. It is estimated that the reactance of UR1 in the utility tie circuit should be doubled. To supply 50 Mvar of load through a reactor of 0.2 kV.63 Ω. 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 large 14. first cycle duty calculations.4-Mvar capacitor filters shown in Fig. and excessive voltage drops occur. 5) use of duplex reactors [3]. The recovery voltage profile after current interruption shows transient recovery voltage and power frequency component. and the voltage across the parting contacts increases. JULY/AUGUST 2008 3) replacement or retrofitting overduty breakers. and t3 –t1 is the interrupting time. an arc is drawn. However. As the size of the reactor is increased. C. This is the arc voltage drop. B. The reactive power flow in a mainly reactive tie circuit requires ∆V at the sending and receiving ends. IV. Fig.2 Mvar of reactive power. To retain existing breakers in service.04-kA rms symmetrical. 1) Adding a generator reactor. shown exaggeratedly in Fig. and K = 1. The calculated interrupting duty for a bus fault is 44. Consider a fault at F1 in Fig. this will not have any impact on the close and latch capability. 13. When the dielectric strength builds up and a current zero occurs.. Interval t3 –t2 is the arcing time. the load side voltage will dip by 21%. The current limiting reactors have nonlinear characteristics with respect to short-circuit current limitation. 2) increasing the reactor size UR1 in Fig.2-kV bus 4 exceed their ratings by approximately 6%. except to briefly underline some investigations relevant to the distribution system being discussed. i.2 Mvar is lost in the reactor. 4 shows that there are large reactors in the distribution system. can be provided in the existing breaker cubicles at a considerable cost and process downtime. III. 4 shows the main 13. where the resistance component can be high.DAS: REDUCING INTERRUPTING DUTIES OF CIRCUIT BREAKERS BY INCREASING CONTACT PARTING TIME Fig.and 2. In Table I. using alternate techniques. E/Z calculation should be used for accurate results. The multiplying factors for short-circuit current contributions from the local sources consider ac and dc decay. however. When short-circuit current is predominantly fed through no more than one transformation or per unit reactance external to the generator. V. Figs. 5 and 6 show these multiplying factors for five and eight cycle breakers [1]. depending upon whether the fault is fed from the local or remote sources. whereas the multiplying factors from the remote sources consider dc decay only. obvious that ANSI/IEEE calculations are generally more conservative when compared with dynamic simulation or ElectroMagnetic Transients Program (EMTP).5 times the generator per unit reactance on the same megavoltampere base. which is less than 1. 4. all impedances and their X/R ratios should be correctly modeled. However. have been studied by various authors [5]–[7] and are summarized in Table I. ANSI/IEC/EMTP. in some cases. Table I shows that. AND DYNAMIC SIMULATION METHODS ANSI/IEEE standards permit E/X or E/Z calculation. Utility contributions are considered as remote sources. Postinterrupting duty multiplying factors. for low-voltage systems. unless rigorous alternate techniques are explored and acceptable to the owners of the plant. otherwise. These figures show the multiplying . it is. are applied to E/Z calculation. 1029 Single-line diagram of the interconnections of main 13.2. TABLE I COMPARISON OF SHORT-CIRCUIT CALCULATIONS. ANSI calculations are 30% higher than the corresponding results obtained with dynamic simulation. These calculations show considerable differences. a question arises whether the calculated overduty by a certain percentage can be ignored? This will not be a prudent approach. E/X will be more conservative. For accurate results.4-kV buses in the distribution system. In view of the large variations of the short-circuit currents shown in Table I with various techniques. A CCOUNTING FOR AC AND DC D ECAYS the short-circuit calculations. it is a remote source. it is considered a local source. Note that the total short-circuit current at t = 0 (time of short circuit) is approximately 15 times the motor full-load current. the E/Z multiplier for breaker selection can be reduced to account for the fault current decay during the additional two cycles. the decay depending upon the motor rating. a breaker of three-cycle parting time. Rm = 100. full-load current = 202 A. shown for five. From the initiation of . howsoever remote. VOL. JULY/AUGUST 2008 factors not only for the minimum contact parting time of the breaker (shown in a rectangle) but also for longer contact parting times. 44. and 3) the total short-circuit current. a part of short-circuit current contribution will be termed “local” and a part will be termed “remote. magnetizing reactance Xm = 3. rotor resistance r2 = 0. Three-phase and line-to-ground fault multiplying factors that include the effects of dc decrement only.3-kV four-pole motor. it implies that motors.165 pu.0 pu. rotor reactance X2 = 0. The motor data are as follows: full-load efficiency = 94. The ANSI/IEEE contribution to interrupting duty current is shown as a straight line. For short-circuit current contributions from the motors irrespective of their type and location in the system. the short-circuit currents contributed by induction motors decay and approach zero value in a couple of cycles. starting from 1. the ac decay is built into the premultiplying impedance factors. if so relayed that it actually parts contacts in five cycles after fault initiation (Fig. This implies that. which decays to zero in three cycles.0295 s.0984 pu. and lockedrotor current = six times the full-load current. Three-phase fault multiplying factors that include ac and dc decrement. 4. ac short-circuit time constant T = 0. 2) dc component of the short-circuit current. stator reactance X1 = 0. full-load power factor = 88%.00 pu. motor transient reactance X = 0. 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..5 cycles. Figs. NACD source currents . Fig. will continue feeding a nondecaying current into the fault until it is cleared by the circuit breaker. given by NACD ratio = Fig. Practically.0656 pu. for five. NO.7%. the multiplying factor shown for five-cycle contact parting time can be used.e. which decays to almost zero in five cycles. 6. Consider a 900-hp 2.1030 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS. and dc short-circuit time constant Tdc = 0.” The fraction of the interrupting current that is contributed by remote sources is identified as no ac-decrement (NACD) ratio.0142 s. W EIGHTED (I NTERPOLATED ) I NTERRUPTING D UTY M ULTIPLYING F ACTORS In an industrial system with generation.and eight-cycle breakers [1]. Fig. 7 shows the calculated: 1) ac symmetrical component of the current. VI. 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. 3). 5. The calculated parameters of this motor are as follows: stator resistance r1 = 0.015 pu. This means that all the branch current contributions to the fault point have to be traced throughout the system and labeled local or remote.031 pu. i. Thus.and eight-cycle breakers [1]. The calculation is facilitated by the digital computers. For proper application of a circuit breaker VII. the calculations will give remote and local contributions. (4) Motor contribution = 10. also see [9]. Irrespective of the contact parting time. AND THREE-PHASE FAULT CURRENT CONTRIBUTIONS TO BUS 4 IN FIG.000368 + j0. Step 2) Verify computer calculations.sym.56 kA rms sym. rms S YMMETRICAL TABLE III CALCULATIONS OF REMOTE/LOCAL COMPONENTS OF SHORT-CIRCUIT CURRENT AND NACD Fig. which consider the decay from motor loads. ANSI/IEEE METHOD. X/R = 42.124. and for each specific calculation time ts after the short-circuit starts. C ALCULATION P ROCEDURE The following procedure can be adopted to consider the effect of increased contact parting time on the interrupting duty calculations. we get the following. Td is an X/R ratio time constant in radians at the same frequency. the ANSI first cycle current can be used.242.56 kA rms at an X/R of 23. fault to approximately 1. . E/Z = 39. NACD = 9. Tables II and III show these results.45.sym Rated > Iint. Analytical calculations of 900-hp-motor short-circuit current versus ANSI/IEEE interrupting duty contributions. it is nondecaying. Total remote contribution = 9. Moreover. the contribution is constant. Step 1) Run computer-based calculations which will give all branch flows. This contrasts with the IEC calculations. Local = 19.56/39.DAS: REDUCING INTERRUPTING DUTIES OF CIRCUIT BREAKERS BY INCREASING CONTACT PARTING TIME 1031 TABLE II COMPUTER-BASED CALCULATIONS. (3) No postinterrupting duty multiplying factors are applied to motor contributions.5 cycles. Section 6. Reference [10] directly gives the weighted (interpolated) multiplying factors based upon NACD. which is the “Large induction motors with prolonged contributions.11082 pu (100-MVA BASE). Then.sym = MFremote (Iremote )+MFlocal (Ilocal )+Imotor . Reference [8.447 = 0.447 kA rms sym. The total E/Z current is now broken into the following constituents.31 kA rms sym at an X/R of approximately 72. calculated. Note that motor contributions are neither remote nor local but are a part of E/Z calculations. Sec.447 kA AT −88.” does qualify the ANSI calculation methods and states that. 7.10◦ . and again. the interrupting duty is calculated as follows: Iint. for each large motor with a significant short-circuit contribution. Remote = 9.577 kA rms at X/R = 15. Z1 = 0. 12] addresses the IEC calculation methods and compares these with the ANSI standards. INTERRUPTING DUTY MF FOR FIVE-CYCLE SYMMETRICAL RATED CIRCUIT BREAKER= 1. AND M AXIMUM I NTERRUPTING D UTY = 44. a reactance factor of ets /Td be substituted for the standard multiplying factor.6 of the new IEEE standard 551 [8]. 4. From these tables.35 kA. Short-circuit current at a fault location can be divided into local and remote components and the current contributed by motor loads. E/Z = 39. Breaker Iint. Transient Stability In many situations. 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. the calculated interrupting duty is as follows: 19. at F2 in Fig.56(1.) This 94-trip relay can be a nondraw-out molded case device with adequate current interrupting and carrying capability similar to a lockout relay. Step 3) Calculate the interrupting duty at increased fourcycle contact parting time. and an increase in tripping delay of one cycle worsens this situation. Relay Coordination Current limiting fuses are extensively applied in industrial systems for substation transformers’ primary protection.18 kA is fairly close to the computer calculations of 44.51◦ + 10.045 kA rms sym. The short-circuit current has somewhat decayed during an additional one cycle. in Fig. The calculation shows that the increased energy release will be approximately 7%. Remote MF = 1. Calculated interrupting duty 19. For comparison.069) −87. the trip delay of one cycle can be programmed into the relay logic. In Fig. 4. which has an inherent pickup time delay of one cycle. The E/Z calculation does not change only the MF’s change.56 < −87.1032 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS. the breakers are applied slightly below their ratings with one cycle increased contact parting time. Therefore. Thus. avoiding the use of another discrete auxiliary device. although not to three decimal places. 4.19◦ = 44. IX. Fault Energy Release As the fault clearing time is increased by one cycle.8 kA by increasing the contact parting time by one cycle. There are eight-cycle breakers rated on TOT current basis (prior to 1964 ratings) on some of the buses in Fig. Indoor Oil-Less Circuit Breakers With K = 1 The calculation procedure is also applicable to breakers with K = 1. occur through an auxiliary trip relay 94.577 −86. In general.1. the energy released into the fault will increase. In an electromagnetic device.27◦ . JULY/AUGUST 2008 The remote multiplying factor can be read from the curves or it is analytically calculable from the following: 1/2 MFremote = (1/S) 1 + 2e−4πC/(X/R) (5) where C is the contact parting time in cycles. the transient stability will be slightly compromised. even a one-cycle delay in clearing a severe fault can make a difference between unstable and stable conditions.20◦ + 9. This gives a reduction of 2.19◦ = |41.21) < < < < −89. E. 6. whereas the fault clearance time (for incident energy release) increases by one cycle.069.31(1. C. F. Thus. an integral should be taken E = i2 dt (6) or Simpson’s rule of averages can be applied. Circuit breaker rating at the operating voltage = 42. whether manual or through protective relays.35 kA. F URTHER C ONSIDERATIONS A. In Fig.51◦ + 10. it will not be in the ratio 6/5 due to the decaying nature of the fault current. This calculated result of 44.577 < −86. the reduction that can . VIII. (Lockout relays of device 86 generally have a pickup time delay of 50–60 ms. its elimination improves the reliability. VOL.1) < −89. Although the failure rate on an auxiliary trip device will be low.38| kA. However.21. Local multiplying factor cannot be calculated from a mathematical expression and must be read from the curve. For other unsymmetrical faults. 5.20◦ + 9. This is acceptable. D. the pickup time will slightly vary. I MPLEMENTATION All trips. With multifunction microprocessor-based protective relays. In addition. 1. the feeder breakers serving these transformers have instantaneous overcurrent settings to protect the cable circuits feeding the transformers. B. 44. Expected Reduction in Interrupting Duty The maximum E/Z multiplying factor for local contributions (ac and dc decay) is 1. the remote MF = 1.25.18 kA −88. NO. Consider a fault close to the bus on a feeder. The relevant portion of the curve can be enlarged for easy reading. as shown in Table II. 4. Arc-Flash Considerations The incident energy release will generally be higher. In (5). Local MF = 1. Local MF = 1.31(1. the stability of a generator is hard to ensure for a three-phase bolted bus fault close to it. also a similar MF can be read. 2006. B.) in the U. respectively. degree from Panjab University. 1991. pp. and relaying. Appl.” IEEE Trans. TAPPI Eng. Ind. Sep.). “Short-circuit calculations—ANSI/IEEE and IEC methods. Das (SM’80–F’08) received the B. 1180–1194. 637–648. vol.. and a member of the Federation of European Engineers (France). Das is a member of the IEEE Industry Applications and IEEE Power Engineering Societies. R EFERENCES [1] Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis.. pp. J. Appl.K. a Life Fellow of the Institution of Engineers (India). [4] AC High Voltage Circuit Breakers Rated on a Symmetrical Current Basis—Preferred Ratings and Related Capabilities. [3] J. the related aspects discussed in this paper should be considered. 4. grounding. “Short-circuit current calculations—A comparison between methods of IEC and ANSI standards using dynamic simulation as reference. 1993. [10] W. Appl.. Strafford./Oct. 1073– 1082. . He is a member of the Technical Association of the Pulp and Paper Industry and the International Council on Large Electric Systems. C. S. He received the IEEE Pulp and Paper Industry Committee Meritorious Award in Engineering in 2005. TN. Ind. P.” IEEE Trans. load flow. vol.. degree from Tulsa University. has not been presented in the current literature. 5. Symp. 121–143. Silvestri. Jr. discussed in this paper. in 1953 and 1956. He conducts courses for continuing education in power systems. degree in mathematics and the B. Conf. ANSI/IEEE Std. pp. 24. Berizzi. AMEC E&C Services.” IEEE Trans. and the M. “Limitation of fault-current limiters for expansion of electrical distribution systems. [2] J. J. harmonics.. OK. vol.E. Twiss. had technoeconomical limitations.06.E. He is the author of Power System Analysis (Marcel Dekker. 2002). GA. harmonics. (Revised 1999 and 2005). A. [6] A.. 1099–1106.E. protection. 1994. stability. 6. C. Rodolakis. Appl. His interests include power system transients. arc-flash hazard. switching transients.S./Aug. Belgium. Huening. a Fellow of the Institution of Electrical Engineers (U. Das. Massucco. power quality. vol. C37. He is a Registered Professional Engineer in the States of Georgia and Oklahoma. He is active in the Power Distribution Subcommittee. 3./Dec. a detailed methodology.E. pp.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. While implementing this method.” in Proc. 1979. a Chartered Engineer (C. India. ANSI/IEEE Std. Inc. P. Tucker. no. He is currently a Staff Consultant with Electrical Power Systems. IEEE Std..A. 1969. Nov. and D. 29. 8th Int. no.. 551. Tulsa. “Interpretation of New American Standards for power circuit breaker applications. 1998. “A comparison of ANSI-based and dynamically rigorous short-circuit current calculation procedures. as the conventional means of addressing this problem. similarities and differences. in 1982. C37-010.. Violet Book. 4. Das. F. pp. Lam. Mr. He is responsible for power system studies.). and J. Robertson. also. and a European Engineer (Eur. [8] IEEE Recommended Methods for Calculating AC Short-Circuit Currents in Industrial and Commercial Power Systems.Ing. no./Aug. C. Brussels. C. 1988. Although ANSI/IEEE standards permit this method to avoid the immediate replacement of breakers. May/Jun. Ind. Jul. “A comparison of North American (ANSI) and European (IEC) fault calculations guidelines. which is similar to the one presented in this paper. pp. 30. 1033 [7] A. this method saved much expense and downtime. Gen. no. Dunki-Jacobs. Chandigarh. R. and. [9] J. For the system under study.” IEEE Trans.Eng. 2000. “Duplex reactor for a large co-generation distribution system—An old concept revisited. Short-Circuit Calculations Power Syst. Ind. IGA-5.. IEEE Pulp and Paper Industry Committee of which he is a member. C. Das.” in Proc. vol. [5] J.” IEEE Trans. Ind. G. protective relaying. Conclusion This paper shows that the interrupting duty can be reduced by increasing the contact parting time. pp. Zaninelli. and he is the author or coauthor of about 50 technical publications. Appl. Jul. Nashville. no. 1997. 33. 515–521. including short circuit.K. and R. 25–30.