Transmission Facilities Rating Methodology

Transcription

1 Document title Transmission Facilities Rating Methodology Document number EGR-TRMC Applies to: Transmission Engineering, Transmission System Operations, and Transmission Planning- Progress Energy Carolinas Keywords: engineering; transmission - engineering standards NERC Compliance Related This document is NERC Reliability Standard compliance related. Changes to this document could impact PE s compliance status to the following NERC Reliability Standard FAC-008. Proposed changes to this document must be reviewed as applicable by the following units: Standards, Transmission Planning, Line Engineering, Substation Engineering, P&C Engineering, and the Energy Control Center (Operations). 1.0 Purpose R3 The purpose of this document is to convey the criteria, or methodology, for determination of the ampacity rating of the transmission system facilities on the Progress Energy Carolinas (PEC) system. EGR-TRMC Rev. 8 (01/13) Page 1 of 16

2 2.0 References R IEEE Standard ANSI C ANSI CS2 2.4 IEEE Standard IEEE Standard C IEEE Standard C IEEE Standard C IEEE Standard C ICEA S AEIC CG NEMA Standard CP National Electrical Safety Code (NESC) 2.13 NERC Standard FAC NERC Glossary of Terms Used in Reliability Standards 3.0 General The facility rating shall equal the most limiting applicable equipment rating of the individual equipment that comprises that facility. R3.4.1 R3.1 R3.2 R3.2.2 R3.2.1 R3.2.3 The scope of equipment addressed in determining ratings of the bulk electric system shall include, but not be limited to transmission conductors, transformers, relay protective devices, terminal equipment, and series and shunt compensation devices. There are no fuses used in the BES. Consideration of the following will be utilized in the Facilities ratings methodology and are reflected in the various equipment ratings as shown in Attachment A: ratings provided by equipment manufacturers design criteria (industry practices, industry standards, etc), ambient temperature operating conditions other assumptions EGR-TRMC Rev. 8 (01/13) Page 2 of 16

3 R3.4.2 PEC normal and emergency ratings are equal on the majority of the transmission facilities. The exceptions are transmission lines that are listed in Table 4.1b that have emergency ratings (two hours). If a rating methodology other than what is specified in this document is desired and utilized, an engineering evaluation will document how the rating methodology was determined as well as the basis for the methodology. The documentation associated with the deviation will be retained by the ratings engineer. At generating plants, the generator owner is responsible for facility ratings from the high side of the GSUs to the point of interconnection with the transmission system. 3.1 Operating Conditions Operating conditions can affect facility ratings. These operational limitations can occur when a limiting element for a facility changes due to switching. For example, where a transmission line is terminated with two breakers in parallel, the Facility Rating may be reduced when one of the breakers is open and the remaining breaker or other associated equipment has an ampacity lower than the established Facility Rating when both breakers are closed. For operating limitations due to equipment failure or malfunction, temporary Facility Ratings can be established. Such situations are identified and handled in the PEC Energy Control Center with specific tags in the SCADA system. Protective device settings can be utilized to limit facility ratings when operational concerns exist that require the facility s rating to be lower than what is dictated by the equipment comprising the facility. In those cases, the protective device setting becomes the facility rating. When there are concerns associated with a rating determined by this methodology and the facility must remain in service due to grid reliability or customer load, an engineering evaluation shall be used to determine an interim facility rating until the issue is resolved. In these cases, the basis for the interim rating will be documented and maintained for the duration of the limitation by the ratings engineer. 4.0 Rating Criteria and Methodology 4.1 Transmission Line Conductors IEEE Std IEEE Standard for Calculating the Current-Temperature of Bare Overhead Conductors is referenced for the methodology used to establish overhead conductor ratings. Transmission overhead lines are designed to meet or exceed the requirements of the National Electrical Safety Code (NESC), federal regulation, state codes, and local codes. Transmission line conductors are designed to operate at temperatures up to but not exceeding the temperatures in Table 4.1a for Normal Ratings. The lines in Table 4.1b were designed to operate with Emergency ratings (2 hours) up to but not exceeding the corresponding maximum temperatures shown. EGR-TRMC Rev. 8 (01/13) Page 3 of 16

4 Table 4.1a Overhead Conductor Maximum Operating Temperatures CONDUCTOR CONDUCTOR TEMPERATURES Normal C ( F) ACSR 100 (212) ACSR TW 100 (212) ACAR 100 (212) AACSR 93.3 (200) CU 100 (212) CW CU 100 (212) ACSS/TW 185 (365) Table 4.1b Transmission Lines with Emergency Ratings CONDUCTOR TRANSMISSION LINES WITH ACSR TEMPERATURES CONDUCTORS C ( F) CRAGGY - VANDERBILT 115KV LINE (336 SECTION) 124 (255) FAYETTEVILLE - FAYETTEVILLE EAST 230KV LINE 148 (298) PERSON VP HALIFAX 230KV LINE 148 (298) WEATHERSPOON PLANT LAURINBURG 230KV LINE 125 (256) IEEE Standard is the reference document used in our in- house program to establish individual overhead line conductor ratings. Ratings for two-bundled and three-bundled conductor are considered to be two and three times, respectively, the rating provided for a single conductor of the same size-type. Conductor manufacturers do not provide conductor ratings but they provide data used in performing the rating calculations. Ampacity rating criteria for two winter and two summer conditions in each of three regions of the PEC system are provided (Coastal, Piedmont, and Mountain; see Attachment B). The two summer ratings defined by the criteria for each region are: 1) a hot (peak) summer rating which can be used at any time during the defined summer period, and 2) a warm summer rating that that can be used during the summer period when system ambient temperatures are below 78ºF. The 78ºF ambient was chosen since 7/8 of the time during a typical year, the average temperature is below 78ºF. The summer months are defined as March 1 through November 30. EGR-TRMC Rev. 8 (01/13) Page 4 of 16

5 The two winter ratings for each region are: 1) a warm winter rating that can be used at any time during the winter period, and 2) a cold winter rating that can be used when system ambient temperatures are below 32ºF. The winter months are defined as December 1 through February 28. The hot summer and warm winter ratings for the Mountain region are based on lower ambient temperatures than the corresponding ratings in the Piedmont and Coastal regions. The Coastal ampacity ratings are based on a three ft./sec. wind speed normal to the conductor while the Piedmont and Mountain region ratings are based on a two ft./sec. wind speed (see below). Coastal (50 Miles Inland) Hot Summer Rating 95ºF 3 ft/sec Wind 100% Solar Warm Summer Rating 78ºF 3 ft/sec Wind 50% Solar Warm Winter Rating 70ºF 3 ft/sec Wind 50% Solar Cold Winter Rating 32ºF 3 ft/sec Wind 50% Solar Piedmont Hot Summer Rating 95ºF 2 ft/sec Wind 100% Solar Warm Summer Rating 78ºF 2 ft/sec Wind 50% Solar Warm Winter Rating 70ºF 2 ft/sec Wind 50% Solar Cold Winter Rating 32ºF 2 ft/sec Wind 50% Solar Mountains Hot Summer Rating 87ºF 2 ft/sec Wind 100% Solar Warm Summer Rating 78ºF 2 ft/sec Wind 50% Solar Warm Winter Rating 65ºF 2 ft/sec Wind 50% Solar Cold Winter Rating 32ºF 2 ft/sec Wind 50% Solar EGR-TRMC Rev. 8 (01/13) Page 5 of 16

6 The following is applicable to all regions: Table 4.1c Overhead Conductor Rating Criteria Description Unit Criteria Wind Angle to Conductor emissivity, Al (Cu) absorptivity, Al (Cu) degree (0.85) 0.5 (0.85) With Sun Yes Line Direction N-S or E-W E-W Line Latitude N 35 Time of Day Noon Atmosphere clear or industrial clear Elevation (from MSL) ft Transmission Underground Line Conductors PEC has one underground transmission conductor on its system. The design is a 2500 kcmil segmented CU conductor high pressure fluid filled pipe type cable. It is rated in accordance with the Association of Edison Illuminating Companies (AEIC) and is designed for a maximum conductor temperature of 85 C for normal rating. 4.3 Equipment Rating (Substations and Transmission Lines) Terminal equipment at substations and equipment within line sections are specified, designed, and applied for the full range of system voltage conditions and ranges to which they will be subjected. Ambient temperature, appropriate to the region, is specified in the equipment purchase specifications. The equipment is rated per the manufacturer s nameplate rating in accordance with industry standards, as follows, or as specified in subsections of this section. The emergency ratings are applicable to the lines that are identified in Table 4.1b. Equipment Industry Standard Circuit Breakers, Circuit Switchers and IEEE Std C37 Disconnect Switches Transformers, Reactors, Current IEEE Std C57 Transformers (CT s) Capacitors IEEE Std 18/NEMA CP1 Line Switches IEEE Std C37 Line Traps IEEE Std C57 Protective Relays IEEE std C37.90 EGR-TRMC Rev. 8 (01/13) Page 6 of 16

7 4.3.1 Power Transformers Transformer normal ratings are rated per the manufacturer s nameplate rating in accordance with industry standards such as IEEE C Transformers are required to be capable of carrying rated load continuously at five per cent (5%) above rated secondary voltage without exceeding an average winding temperature rise of 55ºC above a 40ºC maximum ambient and a 30ºC average ambient over a 24 hour period for normal ratings. For newer transformers that have a 55ºC/65ºC average winding temperature rise design rating, two (2) nameplate ratings are provided; a 55ºC winding temperature rise rating and a 65ºC winding temperature rise rating. By design, the 65ºC rise ratings are one hundred and twelve per cent (112%) of the 55ºC rise ratings and can be used for normal ratings. Bushings and load tap changers are considered a part of the transformer, and as such are subject to the same load rating criteria of the transformer. If emergency ratings are needed for transformers, they will be determined using one of two methods. Not all transformers will have an emergency rating. When emergency ratings are implemented, the emergency summer rating and emergency winter rating will be the same. The first method is for transformers purchased starting in Due to transformer design specification requirements implemented in 2005 which required the manufacturer to perform a 4 hour overload heat run at 125% of nameplate rating, a 2 hour emergency rating of 125% can be applied to these units pending a review of the factory test data. For transformers purchased prior to 2005, the PT-Load computer program developed by Electric Power Research Institute (EPRI) is to be used along with a typical 24 hour loading profile and 24 hour ambient temperature cycle for the rating period. This program utilizes the algorithms in the IEEE Std C57. The PT-Load program can be used in conjunction with condition assessments to determine emergency ratings Line Traps The design criteria for line traps are based on ANSI C , Requirements for Power-Line Carrier Traps. Rating Methodology The determination of adjusted normal continuous current ratings is based on the temperature rise proportional to an exponential value of the current. The exponential value of 1/2.0 (typically, between 1/1.6 to 1/2.0) is used per utility design practice. The emergency ratings are from ANSI C Table A1 and interpolated values from the table. A summer ambient temperature of 95 F (35 C) and a winter ambient temperature of 32 F (0 C) are used. EGR-TRMC Rev. 8 (01/13) Page 7 of 16

8 Normal continuous current rating The normal continuous current rating of line traps is per manufacturer s nameplate and based at 40 C ambient temperature. This current rating can be adjusted for specific ambient temperature without exceeding the normal allowable maximum temperature a line trap can withstand. Table shows the percentage values of the nameplate rating for the calculation of adjusted normal continuous current ratings at different ambient temperatures. Summer and winter ratings are noted in the table. Emergency overload current rating It should be realized that overload conditions will result in some loss of life on line traps. The emergency overload values should not be applied frequently. This accelerates aging of the unit resulting in shortening of life expectancy. If a frequent overload is to occur, it is recommended to upgrade the unit. Table shows the normal and emergency ratings in percentage values of continuous current (nameplate) ratings. Note: It is at PEC s discretion to utilize the most conservative loadability factors in either the nameplate ratings or the ratings in Table when establishing facility ratings. Table Line Trap Normal Continuous and Emergency Overload Current Ratings Timeframe Ambient Temp Normal (% of continuous current Emergency (% of continuous current C F rating) rating) SUMMER WINTER Disconnect Switches (including Line Switches) Progress Energy Carolinas utilized the industry standards referenced below as guidelines when creating the allowable disconnect switch normal and emergency load current ratings. IEEE C37.30 Standard Requirements for High Voltage Switches. IEEE C37.32 American National Standard for High Voltage Switches, Bus Supports, and Accessories Schedules of Preferred Ratings, Construction Guidelines, and Specifications. IEEE C37.37 Loading Guide for AC High-Voltage Air Switches (in Excess of 1000V) EGR-TRMC Rev. 8 (01/13) Page 8 of 16

9 Rating Methodology In developing the normal and emergency ratings for air disconnect switches; Progress Energy Carolinas utilizes the guidelines provided in clauses 5 & 6 of IEEE Std C These guidelines utilize the ACCC (Allowable Continuous Current Class) code designation as defined in sub clause 5.3 of IEEE Std C The ACCC designation identifies the composite curve relating the loadability factor of the switch to the ambient temperature, as determined by the limiting switch part class designations. The IEEE Std C37.37 has established curves or an Emergency Load Current Formula to determine switch normal and emergency current ratings. As stated in Annex A background information of IEEE Std 37.34: This standard recognizes that air switches can be operated at allowable continuous currents in excess of rated continuous currents when reduced ambient temperatures prevail. Indoor and outdoor air switches built prior to 1971 in accordance with standards were allowed a 30 C temperature rise over a maximum ambient of 40 C. In this way, they were allowed to operate at a maximum temperature of 70 C. With a 25 C ambient, these switches could operate at 1.22 times rated current without exceeding the 70 C maximum temperature. To determine the normal and emergency rating, the loadability factor is multiplied by the rated continuous current of the air disconnect switch (nameplate value). Application Considerations NOTE: If a disconnect switch (or line switch) is equipped with a load-break device or interrupter, it may not successfully interrupt currents above the nameplate rating of the interrupter. Switches carrying loads and being subjected to outdoor environmental conditions for several years rely upon adequate maintenance for satisfactory performance. A switch not properly aligned, with poor or dirty contact condition, or without proper contact pressure will not carry rated current without excessive temperatures or resistance. Per IEEE C , the guidelines for emergency ratings assume that the allowable maximum temperatures of the switch parts were not exceeded during the 2 hours prior to an emergency. The limits of total temperature for switches stated in IEEE C will be exceeded under emergency overload currents, and these higher temperatures may cause a reduction in the operating life of the switch. EGR-TRMC Rev. 8 (01/13) Page 9 of 16

10 Allowable Continuous Current Capability Ratings (Normal) Allowable continuous current capability ratings for switches under different ambient temperatures are not provided by the manufacturer. The loadability of each switch part class at various temperatures is determined by utilizing the formula in of IEEE Std C and table 1 of C Per IEEE C , section 5.1 Allowable continuous current, The allowable continuous current is a function of maximum allowable total temperature of the switch parts and the ambient temperature. Normal allowable continuous current loadability factors are determined from figure 1 of IEEE Std C See table below. Emergency Ratings Emergency Ratings for switches are not provided by the manufacturer. They are calculated based on the methodology defined in IEEE Std C Progress Energy Carolinas emergency ampacity ratings are approved for ambient temperatures of 32 F/0 C (Winter Ratings) and 95 F/35 C (Summer Ratings). Emergency ratings are limited to 2 hour durations and determined from figure 3 of IEEE Std C Note: It is at PEC s discretion to utilize the most conservative loadability factors in either the nameplate ratings or the ratings in Table when establishing facility ratings. Table Loadability factor for normal continuous and emergency overload current capability of switches. Air Disconnect Switches Default ACCC designation Summer 95 F (35 C) Ambient Temp Continuous Current Loadability Factor Emergency Current Loadability Factor Winter 32 F (0 C) Ambient Temp Continuous Current Loadability Factor Emergency Current Loadability Factor AO1 (switch manufactured before 1971) DO6 (switch manufactured after 1971) EGR-TRMC Rev. 8 (01/13) Page 10 of 16

11 4.3.4 Circuit Breakers Progress Energy Carolinas utilized the industry standards referenced below as guidelines when creating the allowable circuit breakers normal and emergency load current ratings. IEEE Std C , Application Guide for AC High- Voltage Breakers Rated on a Symmetrical Current Basis. IEEE Std C , Standard Rating Structure for AC High- Voltage Breakers. Rating Methodology The determination of adjusted normal continuous current ratings is based on the temperature rise proportional to an exponential value of the current. The exponential value of 1/1.8 (typically, between 1/1.6 to 1/2.0) is used per utility design practice and IEEE Std. C Emergency overload current ratings are based on IEEE Std C sub clause and interpolated calculations. Ratings are based upon pre-loading equal to the circuit breakers rated continuous current. Normal continuous current rating The normal continuous current rating of circuit breakers is per manufacturer s nameplate and based on a 40 C ambient temperature. This current rating can be adjusted for specific ambient temperature without exceeding the normal allowable maximum temperature a circuit breaker can withstand. Table shows the loadability factor of the nameplate rating for the calculation of adjusted normal continuous current ratings for different ambient temperatures. Emergency overload current rating Table specifies the maximum loadability factor that may be applied to circuit breaker nameplate ratings for emergency overload current applications. The limits of total temperature for the circuit breaker will be exceeded under the specified emergency load currents, and these higher temperatures may cause a reduction in operating life of the circuit breaker. Note: It is at PEC s discretion to utilize the most conservative loadability factors in either the nameplate ratings or the ratings in Table when establishing facility ratings. EGR-TRMC Rev. 8 (01/13) Page 11 of 16

12 Table Circuit breaker normal and emergency ratings Summer 35 C/95 F Ambient Winter 0 C/32 F Ambient Continuous Current Loadability Factor Emergency Current Loadability Factor Continuous Current Loadability Factor Emergency Current Loadability Factor Connectors Progress Energy Carolinas defines a connector as a device which joins two or more conductors for the purpose of providing a continuous electrical path. Progress Energy Carolinas considers the rating of any connector to be equal to, or greater than, the rating of the largest conductor(s) for which the connector was designed to connect. Reviews of NEMA Standard CC-1, ANSI/IEEE Standard C , various technical papers, manufacturer data, and interviews with other electric utilities were used to establish Progress Energy Carolinas s rating methodology for connectors Relay Protective Devices A relay protective device as defined in IEEE std 100 as: protective relay (power operations) - A device whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control action. Protective relays are rated in accordance with IEEE standard C Components within the secondary circuits of current transformers (CT) (transducers, meters, etc) shall have continuous ratings such that the current ratings of the devices in those circuits are not exceeded at the normal rating, or in cases where an emergency rating exists, at the emergency rating, of the facility. CT secondary wiring, terminal lugs, connectors, terminal blocks, etc are designed to accommodate the maximum current of the circuit. Adhering to this rating methodology will insure the protective devices are available to operate when called upon for overload or fault conditions. Protective device settings can be utilized to limit facility ratings when operational situations, equipment concerns, or interim conditions exist, that require the facility s rating to be lower than what is dictated by the equipment comprising the facility. In those cases, the protective device setting becomes the facility rating. EGR-TRMC Rev. 8 (01/13) Page 12 of 16

13 4.3.7 Potential Transformers (PTs), Coupling Capacitor Voltage Transformers (CCVTs) PTs and CCVTs are not thermal in line devices and as such do not affect Facility Ratings. Therefore, these devices are not included in specific details of the ratings for Transmission Line facilities, Transformer Facilities, or Series and Shunt Compensation Facilities Current Transformers (CTs) There are four types of construction used for current transformers (CTs): Wound, Bar-primary, Window, and Bushing. The Wound and Bar-primary types include the primary in the construction of the CT such that they are in line devices with thermal ratings. The Window and Bushing types have circular cores that contain the secondary windings and are designed in such a way that a primary conductor is inserted through the window of the CT. In the case of the Bushing type they are designed to fit on bushings. The Window and Bushing CTs have no primary and as such do not affect facility thermal ratings. The maximum current that a Current Transformer (CT) can carry depends on its connected ratio and the thermal rating factor k. A CT can be loaded up to its connected ratio rating times its k rating factor. For example, a 1000/5 multi ratio CT with a k rating factor of 1.5 and connected to the load at the 600/5 tap can carry up to 900A in the primary. If the K factor is unknown a value of 1 should be assumed Rigid Bus (Substation) Rigid bus conductors are rated in accordance with IEEE Guide for Bus Design in air Insulated substations, Annex B using the following assumptions: Description Ambient Temperature Latitude Bus Direction With Sun Wind Speed Solar Emissivity/Absorptivity 0.5 The maximum conductor temperatures are Value 40 C 40 N East- West Yes 0.6m/sec perpendicular to the conductor axis Normal C ( F) Emergency C ( F) Aluminum 90 (194) 100 (212) Copper 90 (194) 90 (194) Conductor manufacturers do not provide rigid bus conductor ratings but they provide data used in performing the rating calculations. EGR-TRMC Rev. 8 (01/13) Page 13 of 16

14 Strain Bus (Substation) Substation strain bus conductors are rated in accordance with IEEE Standard Standard for Calculating the Current-Temperature of Bare overhead Conductors using the assumptions the table below. Description Value Ambient Temperature 40 C Latitude 35 N Time of Day noon Bus Direction East- West Wind Speed 2 feet/sec perpendicular to the conductor axis Solar Absorptivity 0.5 Solar Emissivity 0.5 Atmosphere Clear The maximum conductor temperatures are Normal C ( F) Emergency C ( F) Aluminum 90 (194) 100 (212) Copper 90 (194) 90 (194) Conductor manufacturers do not provide strain bus conductor ratings but they provide data used in performing the rating calculations. 5.0 Joint Facilities Coordination Coordination of facility ratings data with neighboring systems is done through the SERC Long Term Study Group (LTSG) Data Bank where ratings are coordinated, maintained and updated by the PEC representative and the representatives of the neighboring systems. 6.0 Communication of Facility Ratings Methodology R4 R5 6.1 Transmission shall make its Facility Ratings methodology available for inspection and technical review by those Reliability Coordinators, Transmission Operators, Transmission Planners, and Planning Coordinators that have responsibility for the area in which the associated Facilities are located, within 21 calendar days of receipt of a request. 6.2 If a Reliability Coordinator, Transmission Operator, Transmission Planner, or Planning Coordinator provides written comments on its technical review of the Facility Ratings methodology, Transmission shall provide a written response to that commenting entity within 45 calendar days of receipt of those comments. The response shall indicate whether a change will be made to the Facility Ratings methodology and, if no change will be made to that Facility Ratings methodology, the reason why. 6.3 Documentation associated with 6.1 or 6.2 above shall be retained in PDF format. EGR-TRMC Rev. 8 (01/13) Page 14 of 16

15 R3.1 R Attachment A Rating Methodology Considerations Equipment Transmission Overhead Line Conductors Transmission Underground Line Conductors Substation Conductors Manufacturer Rating or Data Design/Industry Standards Ambient Temperature Operating conditions Other Assumptions Yes 1 IEEE Std 738 Section 4.1 Section 3.1 Wind, Sun, Line Sag Yes 1 Association of Edison Illuminating Companies (AEIC) Yes 1 IEEE Std 605 IEEE Std 738 Yes Section 3.1 Soil Thermal Properties, # of Circuits, Load Factor Section Section 3.1 Wind, Solar Section Circuit Breakers Yes IEEE Std C37 Yes 2 Section 3.1 Temperature rise, pre-contingency loading, thermal time constant 3 Circuit Switchers Air Disconnect Switches, Line Switches Yes IEEE Std C37 Yes 4 Section 3.1 Wind velocity, Altitude 4 Line Traps Yes IEEE Std C57 Yes 2 Section 3.1 Altitude 3 Power Yes IEEE Std C57 Section Section 3.1 Wind Loading, Altitude 3 Transformers Current Yes IEEE Std C57 Yes 2 Section 3.1 Wind Loading, Altitude 3 Transformers Reactors Yes IEEE Std C57 Yes 2 Section 3.1 Average Ambient, Altitude 3 Capacitors Yes IEEE Std 18/NEMA Yes 2 Section 3.1 Average Ambient, Altitude 3 CP1 Relay Protective Devices Yes IEEE Std C37.90 Yes 2 Section 3.1 Altitude, Relative Humidity 3 Notes: 1 Manufacturer provided data for calculation of rating 2 Ambient temperature appropriate to the region as specified in the equipment specification 3 Specified in the equipment purchase specifications 4 Ambient, wind velocity, and altitude are specified in IEEE Std C37 EGR-TRMC Rev. 8 (01/13) Page 15 of 16

16 8.0 Attachment B PEC Region Map EGR-TRMC Rev. 8 (01/13) Page 16 of 16