IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 5, SEPTEMBER/OCTOBER

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1 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 5, SEPTEMBER/OCTOBER Circuit Breaker Transient Recovery Voltage Requirements for Medium-Voltage Systems With NRG Rasheek Rifaat, Senior Member, IEEE, Tarjit Singh Lally, and James Hong Abstract The industrial distribution power systems in Northern Alberta supplies electrical energy to satellite locations and mining areas. In some aspects, these systems differ from their counterparts in regular utility distribution cases. Accordingly, they require special attention when performing transient recovery voltage (TRV) studies and identifying ratings for new breakers to be added to the system. Meanwhile, North American (IEEE) and European (IEC) Standards are embarking on significant efforts to harmonize breaker specifications and testing requirements, including TRV tests. An electromagnetic transient program (EMTP) study has been performed to verify system TRV requirements under different conditions against Standard requirements and supplier s provided test data. Concerns, lessons learned, and some other findings associated with the study are documented in this paper for future references and for advancing robust usage of EMTP (alternative transient program) for the performance of such studies for subtransmission and distribution systems in different electrical systems. Index Terms Breaker ratings, electromagnetic transient program (EMTP), high-voltage (HV) circuit breakers, mediumvoltage circuit breaker, transient recovery voltage (TRV). I. INTRODUCTION THE deregulation of electrical generation in Alberta allowed large industrial system identities to own and/or operate their electrical generation, transmission and distribution equipment, lines, and systems within and in-between their facilities. Upon getting the required permits and after being granted the status of Industrial System Designate, industrial facilities could generate, transmit, and distribute their electrical energy within the designated area. As long as they adhered to the applicable requirements, real-time connections to the Alberta Integrated System are permitted at specific points of common coupling where they may export excess energy or import shortfall energy as the case may be. Other transmission or distribution facilities and systems are owned and operated by Manuscript received June 29, 2013; accepted November 5, Date of publication February 28, 2014; date of current version September 16, Paper 2013-PSPC-401, presented at the 2013 IEEE Industry Applications Society Annual Meeting, Orlando, FL, USA, October 6 11, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Power Systems Protection Committee of the IEEE Industry Applications Society. The authors are with Jacobs Canada Inc., Calgary, AB T2C 3E7, Canada ( rasheek.rifaat@jacobs.com; tarjitsingh.lally@jacobs.com; james.hong@ jacobs.com). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TIA an area designated utility as listed in the Electric Utility Act, the Alberta Utilities Commission, and the Alberta Electric System Operator. When initially developed, the subject area was remote with a sparse population, little generating capacity, and scarce voltage support. Since then, the oil recovery in Fort MacMurray considerably changed every aspect of the area s electrical system. Now, there are several generation and cogeneration facilities, a number of interconnecting lines, and far more complex system configurations. The system voltages in that area run with + 5% + 10% voltage above their midclass point. The area s 69-kV system is running at 72- to 74-kV continuous operating voltage with some areas having a maximum continuous operating voltage as high as 75.9 kv. Another aspect of the subject systems is associated with how the system neutral is connected to ground. The 69-kV systems in most North American utilities have solidly grounded neutral systems. In the subject case, the main source transformers are equipped with 200-A neutral resistance grounding arrangements to suit mining applications for the area. The Mining Codes and Standards require that ground potential rise be limited to 100 V. Such limitations would have been difficult with solidly neutral grounded systems, due to high-line to groundfault currents. The case in this paper is selected to highlight factors associated with transient recovery voltage (TRV) studies in industrial and mining areas where system voltages, neutral connections, and overall operating conditions may differ from typical distribution utility cases. It is assumed that the facility substation is an existing substation that, over two decades, encountered several changes. A new breaker was to be added to the existing substation in order to provide a looped circuit supplying satellite and mining areas. Inherited operating practices require the system to operate with a minimum bus voltage of 72.5 kv with a potential to increase maximum continuous operating voltage unless restricted by identified system needs. The breaker shall be specified to North American Standards which have been recently updated to harmonize with IEC Standards. Fig. 1 shows an overall single-line diagram for the case study. II. BREAKER TRVs A. What Is TRV? When a breaker trips, its three-phase poles mechanically travel apart introducing gaps between their stationary and moving parts. In simple arrangements, each side of the pole could IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

2 2990 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 5, SEPTEMBER/OCTOBER 2014 point that is different from other poles. Hence, one of the poles will be the first to clear. With basic symmetrical components analysis, Kpp =3X0/(X1+2X0). Accordingly, it will be a function of the system neutral grounding arrangement. With some assumption of that ratio, for effectively grounded systems, X0/X1 is 3, and the first pole to clear factor is 1.3. For a noneffectively grounded system or an ungrounded system is 1.5. The amplitude factor reflects the damping in the system. It will be affected by the lines connected at the subject breaker (overhead, cables, long lines, short lines). The Standards establish values for this factor based on worst case scenario for typical systems. If the system has a standard operating voltage and standard configurations, then a breaker that is designed and tested in accordance with the standards might be able to withstand the TRV. If the system operating voltage is nontypical and its configuration is special, then a TRV study might help in verifying if a selected breaker is capable to withstand the TRV. Fig kV distribution system in Industrial Northern Alberta. be a supply side and/or a load side of the breaker. The current continues to flow in ionized newly created air or a gas gap until it reaches a value of zero (zero crossing). In a typical ac system, the current will cross zero within 1/2 cycle (i.e., 8.3 ms in an 60-Hz system and 10 ms in an 50-Hz system) after contact departing. The zero crossing will occur at a different time for each phase of the three phases in the system. During interruption of ac current, the arc loses conductivity as the instantaneous current reaches zero. Within a few microseconds after current zero, the current stops flowing in the circuit. The electrical system s response to the sudden change in current differs on the two sides of the breaker. The difference in voltage between the two sides during the interruption process generates TRV. TRVs could be exponential (overdamped) or oscillatory (underdamped). The TRV is dependent on the electric circuit on the two sides of the breaker. Interruption will be fully successful if arcing does not reoccur due to TRV. The full definition of the circuit breaker ability to withstand the TRV and the power frequency recovery voltage is given in [3]. TRV is also affected by other system and breaker parameters such as prefault voltage, fault current, and system neutral grounding arrangement. Standards identified two categories of breakers those that are rated above 100 kv and those that rated below 100 kv. Circuit breakers with rated voltage less than 100 kv are classified into two classes, i.e., S1 and S2. A circuit breaker class S1 is intended to be used in a cable system. A circuit breaker class S2 is intended to be used in an overhead line system [6]. To simplify the required calculations needed to establish the Standards requirements, factors were used. Two important factors are used in IEEE Standards [3] and [5]; the first pole to clear factor Kpp and the amplitude factor Kaf. The definitions are in the relevant standards [3] and [5]. The first factor Kpp is due to each pole of the breaker having a zero-crossing B. Breaker Standards North American Standards (ANSI/IEEE) for high-voltage (HV) breakers have continuously been revised and updated. ANSI breaker standards are part of the IEEE C37 Series [3] [6]. IEEE Standard C [3] is the Application Guide for breaker TRV, and C [4] is the overall rating standard, which also include reference to TRV (main part and informative appendix). Recent changes in these standards have focused on the harmonization between IEC and ANSI/IEEE. In recent years, Standards [3] [5] were revised in the direction of harmonization. One key aspect of the harmonization is the supplier s ability to perform a single set of tests per breaker type that will address both North American and International markets. IEC also adopted harmonization policies as demonstrated in adding amendments such that Amendment 1 to IEC C. Applications of Standards to Case Study From Standard IEEE-C37.06, the following factors are addressed [4, Table 6]. Breaker class is S2; circuit breaker is rated below 100 kv for overhead lines. System is noneffectively grounded. Breaker rated maximum voltage is 72.5 kv. The rated maximum operating voltage for breakers was also defined in [5]. The definition stipulates that such rating is the upper limit for operation. Hence, for a typical 69-kV system with 5% margin, 72.5 kv is the proper rated maximum voltage. As explained in earlier sections of this paper, the 69-kV system in Northern Alberta is labeled as a 72.5-kV system. For this system, the rated maximum voltage is 72.5 kv +5% (76 kv), which is a nonstandard maximum voltage for breakers under the updated Standards. Nevertheless, selecting a suitable breaker for the subject system would be another part of the breaker selection exercise. The verifications for the TRV part might be done utilizing the updated Standards, supplier s data sheet,

3 RIFAAT et al.: CIRCUIT BREAKER TRV REQUIREMENTS FOR MEDIUM-VOLTAGE SYSTEMS WITH NRG 2991 TABLE I T-VALUE VERSUS TRV PEAK FOR 72.5-kV BREAKERS breaker type tests, and the electromagnetic transient program (EMTP) model of the subject system. Breaker suppliers have been offering international breakers with higher voltage classes (i.e., 100 kv, 115 kv, etc.) and readjusting them for the area s specifications. Most of such voltages are not typical in North America. Hence, the supplier s approach would be to readapt the original breaker type tests as much as possible and supplement them if necessary by IEEE tests. Such readjustment will mean that TRV type tests could differ from breakers made entirely to North American Standards. In the case study, information was requested from the suppliers, and test results were received and used in the TRV Study. D. Breaker TRV and Test Requirements The Standards show what TRV parameters are to be expected from the supplier for each breaker class. The following shows the ratings for TRV in breakers less than 100-kV rating and class S2 [4, Table 7]. It might be observed that the Standard values are preferred values and not mandatory. It might be also observed that TRV parameters are shown at different values of circuit breaker short-circuit data, namely, T100 (100%), T60 (60%), T30 (30%), and T10 (10%). For 72.5-kV breakers, Table I of this paper shows a summary of TRV peak voltage values as extracted from Table 7 of the Standards to demonstrate how calculations are conducted on selected breaker ratings and for reference purposes. As an example, If the system s ultimate short-circuit current is between 24 and 40 ka, then the breaker T100 values shall be used. If the system s ultimate short-circuit current is between 12 and 24 ka, then T60 values of 146 ka can be used. Similar derivations could be used for T30 and T10 conditions. III. TRV STUDY USING EMTP The calculations to obtain the peak value at T10 for 72 kv breaker is given as an example in C [3]. Standard C37.04b [5] shows how to combine the envelope and the test values for a circuit breaker in order to devise testing procedure for breaker TRV capability. Standard C suggested for TRV studies to utilize an approach similar to the approach verifying that a breaker is tested for certain TRV requirements. TRV requirements shall be verified for peak value (TRV peak value) and the rate of rise of recovery voltage value (RRRV) (see Fig. 2). Some years ago, calculations of transient voltage were done using system equivalent circuits, initial conditions, and mathematical equations with some approximations and hand- Fig. 2. Simplified superimposition of TRV envelope over system TRV. held calculators. Similar exercises can be performed using EMTP alternative transient program (ATP) (see Fig. 3), by modeling the system fault condition on one side of the breaker, allowing fault clearing time, then switching the breaker off (switch circuit interruption at the zero crossing of each phase). In order to establish most representative modeling, care must be given to the following factors that affect the TRV calculations. Modeling of each side shall include the source with its most representative short-circuit characteristics (worst case scenario). Modeling shall include capacitances and reactances associated with the connected system. That shall include equipment and bus capacitances. Equipment may be lumped at each side; however, lumping of equipment must not affect the transient performance of each side. If the system includes a back-feed through a parallel path, it is recommended to isolate the parallel path as it could skew the results. The model configuration shall allow calculations to be done separately for: terminal TRV (TRV with a fault near breaker terminal); short-line TRV (Fault at the remote end of a short line); if required out-of-phase TRV shall be also calculated. If calculations are required for different source circuit ratings, the source modeling and configuration shall allow calculations to be done separately for: T100; T60; T30; T10. If calculations are required for different system maximum continuous operating voltages, as shown in the subsequent example. Consider the system shown in Fig. 1 as an example. Two breakers were to be examined for TRV, i.e., Breaker A and Breaker C. Breaker B shall be left open for the TRV calculation purposes.

4 2992 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 5, SEPTEMBER/OCTOBER 2014 Fig. 3. EMTP model for the example shown in Fig. 1. The following comments address the results of several runs and are shown in Figs Fig. 4(a): While source system voltage is at 79 kv, a breaker terminal fault occurs. TRV peak values are acceptable for T10 and T30, marginal for T60, and unacceptable for T100. This means that if the system ultimate short circuit would be 12 ka or less, the proposed breaker will be acceptable; otherwise, an alternative breaker shall be considered. RRRV, which is the initial slope of the voltage curve, meets the requirements for all test ratings. Fig. 4(b): While system voltage is at kv, a breaker terminal fault occurs. TRV peak values are acceptable for T10, T30, and T60 and unacceptable for T100. This means that if the system s ultimate short circuit is equal to 24 ka or less, the proposed breaker will be acceptable; otherwise, an alternative breaker shall be considered. RRRV, which is the initial slope of the voltage curve, meets the requirements for all test ratings. Fig. 4(c): While system voltage is at kv, a breaker terminal fault occurs. TRV peak values are acceptable for T10 and T30, and T60 and marginal for T100. This means that if the system s ultimate short circuit would be less than 40 ka, the proposed breaker will be acceptable. RRRV, which is the initial slope of the voltage curve, meets the requirements for all test ratings. Fig. 4(d): While system voltage is at 72.5 kv, a breaker terminal fault occurs. TRV peak values are acceptable for all T classes: T10, T30, T60, and T100. RRRV, which is the initial slope of the voltage curve, meets the requirements for all test ratings. To demonstrate the sensitivity of TRV to effective neutral grounding, additional model runs were performed. Fig. 5 shows the reduction in system TRV requirements when solidly grounded system. Short-line TRV analysis was also required. Since the line connected to the breaker is a short line, a fault was simulated at remote end of the line. Results show TRVs are within the breaker capability. Breaker C is in a remote substation, and it was requested to verify its capability. To simulate the case for Breaker C,

5 RIFAAT et al.: CIRCUIT BREAKER TRV REQUIREMENTS FOR MEDIUM-VOLTAGE SYSTEMS WITH NRG 2993 Fig. 4. (a) (Top) Breaker A, terminal fault, system voltage 79 kv. (b) Breaker A, terminal fault, system voltage kv. (c) Breaker A, terminal fault, system voltage 74 kv. (d) (Bottom) Breaker A, terminal fault, system voltage 72 kv.

6 2994 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 5, SEPTEMBER/OCTOBER 2014 IV. CONCLUSION If industrial system operating voltages are out of typical range, TRV analysis is critical for breakers performance verifications. In industrial and mining supply systems, where supply transformer neutral might not be solidly grounded such verification become more critical. Use of the updated IEEE Standards in combination with the appropriate EMTP (ATP) system models and runs will allow completion of the analysis and appropriate selection of breaker. An example of a 72.5-kV distribution system was discussed and demonstrated to assist preparation of TRV study cases for similar systems. Fig. 5. Hypothetical system with solidly grounded neutral, i.e., TRV at two different voltages. REFERENCES [1] C. L. Wagner, D. Dufournet, and G. F. Montilllet, Revision of the application guide for transient recovery voltage for AC high-voltage circuit breakers of IEEE C37.011: A working group paper of the high voltage circuit breaker subcommittee, IEEE Trans. Power Del., vol. 22, no. 1, pp , Jan [2] R. W. Alexander and D. Dufournet, Transient Recovery Voltage (TRV) for High-Voltage Circuit Breakers. St. Pete Beach, FL, USA: Tutorial IEEE Switchgear Committee, May 2003, ch. 7, Alstom T&D Tutorial. [3] Application Guide for Transient Recovery Voltage for AC High-Voltage Circuit Breakers, IEEE Std. C , [4] IEEE Standard for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis Preferred Ratings and Related Required Capabilities for Voltages above 1000 V, IEEE Std. C , [5] IEEE Standard Rating Structure for AC High-Voltage Circuit Breakers, IEEE Std. C (R2006), [6] IEEE Standard Rating Structure for AC High Voltage Circuit Breakers Rated on Symmetrical Current Basis, Amendment 2: To Change the Description of Transient Recovery Voltage for Harmonization with IEC , IEEE Std. C37.04, [7] High-Voltage Alternating Current Circuit Breakers, IEC Pub (Ed 4.0b), IEC, Geneva, Switzerland, [8] High Voltage Switchgear and Control Gear Part 100, Alternating Current Circuit Breakers, IEC Pub (Ed. 2), IEC, Geneva, Switzerland, [9] ATP Rule Book, Alternative Transient Program Rule Book, Canadian/American EMTP User Group, West Linn, OR, USA, [10] R. Rudenberg and G. McKay, Transient Performance of Electric Power Systems Phenomena in Lumped Networks, 1st ed. New York, NY, USA: McGraw-Hill, [11] J. A. Martinez-Velasco, Ed., Power System Transients Parameter Determination Chapter 7. Boca Raton, FL, USA: CRC Press, Fig. 6. Short-line TRV results are within the envelope. Breaker A was closed. The fault branch in the EMTP model was relocated to the load side of Breaker C. The fault was simulated and breaker C was similarly tripped as breaker A did in earlier cases. The TRV peak value is less than all test ratings of circuit breakers. Additionally, the RRRV is inside of the initial slope for all test ratings (see Fig. 6). Hence, the worst case scenario was confirmed to be at the source substation as anticipated. Rasheek Rifaat (M 76 SM 93) received the B.Sc. degree from Cairo University, Giza, Egypt, in 1972 and the M.Eng. degree from McGill University, Montreal, QC, Canada, in 1979, both in electrical engineering. Between 1975 and 1981, he was with Union Carbide Canada Ltd., Beauharnois, QC. In 1981, he joined Monenco Consultants, Calgary, AB, Canada, and Saskmont Engineering, Regina, SK, Canada, where he was involved in thermal power generating plant projects. Since 1991, he has been with Jacobs Canada Inc., Calgary, working on large- and medium-size power cogeneration projects, and oil and gas projects. He has published a number of papers on power system protection and industrial and cogeneration power system modeling and analysis. Mr. Rifaat is a Registered Professional Engineer in three Canadian Provinces: Alberta, Saskatchewan, and Ontario.

7 RIFAAT et al.: CIRCUIT BREAKER TRV REQUIREMENTS FOR MEDIUM-VOLTAGE SYSTEMS WITH NRG 2995 Tarjit Singh Lally received the B.Sc. degree in electrical engineering from Panjab University, Chandigarh, India, in He was with the Department of Atomic Energy, Mumbai, India, and then the State Electric Utility Company Board, Punjab, India. In 1995, he moved to Canada where he worked as an Electrical Engineer with Magna Projects, Calgary, AB, Canada, until Since 2002, he has been an Electrical Engineer with Jacobs Canada Inc., Calgary. His technical interest area includes system studies, protection coordination, and high-voltage substations equipment and systems. He enjoys coaching and providing technical support to future electrical system and highvoltage engineers. Mr. Lally is a Registered Professional Engineer in in two Canadian Provinces: Alberta and British Columbia and two US States: Georgia and Vermont. James (Jinsoo) Hong received the B.Sc. degree in electrical engineering from the University of Alberta, Edmonton, AB, Canada, in Since May 2012, he has been an Electrical Engineer (in Training EIT) with Jacobs Canada Inc., Calgary, AB. While working at Jacobs, his work has included electrical power systems modeling and analysis. He has prepared various industrial system models utilizing several commercial software packages, as well as the publicly available electromagnetic transient program (EMTP), and the alternative transient version (EMTP/alternative transient program). He has been also involved in studies associated with system modeling, including power system analysis, power system transients, harmonics, and protection studies.