3.1 Overview of ATCO Electric s URD System Design

Transcription

1 Page: URD DESIGN GUIDELINES 3.1 Overview of ATCO Electric s URD System Design ATCO Electric s design for distribution power supply to URD areas is a primary looped supply system. This is generically illustrated in Figure 3.1. The supply voltage is 25 kv. Figure Looped Primary URD System The power supply to all single lot underground residential services is through front lot service. Single phase transformers are connected to secondary pedestals, which in turn provide the connection to the residential service. The secondary supply design provides an interconnection or looping of the secondary conductors between the service pedestals. This allows for more efficient and responsive restoration of service to customers in the event of a transformer failure. Figure 3.2 illustrates a typical front lot service with the subdivision primary and secondary pedestal cable and secondary service cable connections. The supply is at 120/240 volts.

2 Page: 3-2 Figure 3.2A - Single Family Dwelling Lot Service with Looped Secondary Cables With the exception of commercial customers fed from a separate transformer, all secondary services are served from a secondary pedestal box connected to a transformer.

3 Page: 3-3 Figure 3.2B Multiple Family Dwelling Lot Service Bare Land Development

4 Page: URD Design This section outlines the responsibilities and standards for design of an underground electrical distribution development within a neighborhood or major subdivision. It is intended to assist the Developer and/or its agent(s) in preliminary design discussions with ATCO Electric s Engineering Representative Design Responsibilities ATCO Electric is responsible for planning and designing the express and source primary feeders which supply an URD. In addition, ATCO Electric is responsible for the planning, design, construction, installation and termination of three phase switch cubicle facilities. ATCO Electric s Engineering Representative will at the request of the Developer and/or its agent(s), provide: a) information on the switch cubicle requirements (dimensions, schedule and locations); and b) normally open/closed points at transformers for the specification of lightning arrestor locations. Figure 3.3 is a schematic representation of the facilities ATCO Electric and the Developer are each responsible to provide and install. Facilities shown in blue are ATCO Electric s responsibility, and facilities shown in black are the responsibility of the Developer and/or its agent(s).

5 Page: 3-5 Figure URD System Design Responsibilities

6 Page: Design Standards All underground electrical distribution facilities must be designed in accordance with the Alberta Electrical Utility Code (AEUC) and any amendments thereto under the Alberta Safety Codes Act; and, where applicable, the Canadian Electrical Code (CEC) Part III. The minimum acceptable design standards to be met by the Developer and/or its agent(s) will be the higher of the ATCO Electric standards established in these specifications or that of the governing authority. Adherence to the planning and design standards ensures ATCO Electric is able to maintain adequate service levels. Safety (grounding), voltage quality, reliability and equipment life are key factors Ultimate Design The design proposed by the Developer and/or its agent(s) must consider future stage development (i.e., the "ultimate" design). Ultimately, all subdivision primary cables and transformers must have a looped source or an alternate source of supply; that is, a minimum of two primary feed-in points (refer to Figure 3.1). The commissioning or use of the alternate feed in normal system operations or in emergency situations must require only the switching of normally open or closed switches at the switch cubicles supplying a subdivision area. All secondary services are to be fed directly from pedestals and looped (refer to Figure 3.3). Supply to three phase transformers (i.e., commercial feeders) must not be used or integrated from power cables designed for supplying single phase residential services. Where three phase loads and services are required, the three phase system must be planned and designed so that all three phase transformers are isolated from the

7 Page: 3-7 single phase residential feeders by the use of switch cubicles and/or separate feeders. Figure 3.3 illustrates this concept Initial Design Subdivision development may proceed in several phases and time frames. As such, temporary (in this context, for less than 1 year) arrangements for primary power cable and equipment are required; that is, provision for temporary looping and/or extension to future phases. The Developer and/or its agent(s) are responsible to coordinate these arrangements with ATCO Electric s Engineering Representative prior to the commencement of any detailed engineering or construction of the underground distribution system. ATCO Electric s Engineering Representative will make the final decisions regarding the system configuration. Options to be considered include: a) creation of temporary phase changes/loops by parking primary power cables in the padmount transformers nearest the stage limits; b) cap and coil 30m of primary power cables at the end of the stage limits; c) installation of ducts to the development boundary; and d) placement of padmount transformers at the development boundaries. Note: ATCO Electric will not accept splices in the design of underground distribution facilities for an URD (see Section 5.3.1). Each unnecessary splice introduces another element which can fail and degrade the performance of the system.

8 Page: Street Light Design The location of street light facilities is a critical consideration in the design of an URD subdivision, to ensure required lighting levels are achieved in all areas. Space is limited within subdivision URWs and/or easements, and the location of street lights will impact the available locations for padmount switching facilities and transformers. Further, it is critical to ensure that no more than one distribution facility is installed in close proximity to a single lot (e.g., transformer/pedestal and street light). These facilities should be reasonably and equitably located on lots throughout the subdivision area. 3.3 Performance Requirements Grounding The proper design and installation of the grounding system is fundamental to the safe operation of an URD system. Protective systems (i.e., fuses, circuit breakers) must be in place and operate to rapidly disconnect a faulted piece of equipment (e.g., a cable, transformer, etc.). In addition, the grounding system must limit the rise in step-and-touch voltage to a safe level for persons in proximity to the equipment when it faults. The Consulting Engineer is responsible to ensure all the requirements of the AEUC and Part III of the Canadian Electrical Code are met for the grounding of all equipment Short Circuit Ratings The Consulting Engineer is responsible to ensure all equipment to be included within an URD has a sufficient short circuit rating. The Developer and/or its agent(s) are responsible to contact and obtain the required information from ATCO Electric s Engineering Representative.

9 Page: Voltage Drop The nominal system voltage and voltage limits at the service entrance are as specified in the Canadian Electrical Association (CEA) Reference Standard for Canadian Practice Preferred Voltage Levels For AC Systems Volts, and CSA Standard CAN 3 C235 - B3, Table 3. For circuits up to 1,000 volts, the recommended voltage variation limits at the service entrance are summarized in Table 3.1. Table Limits of Operating Voltage EXTREME OPERATING CONDITIONS NORMAL OPERATING CONDITIONS SINGLE PHASE 120/ / / / / THREE PHASE 4 CONDUCTOR 120/208Y 110/ / / / /600Y 306/ / / /635 THREE PHASE CONDUCTOR ATCO Electric will try to ensure that the minimum voltage under normal operating conditions, at the primary source points, is not less than 13.6 kv line-to-neutral. This is equivalent to a maximum 12.8 V drop on a 240 V base. The maximum voltage drop for the subdivision primary and secondary to the customer service entry point for any customer is 7.2 V, or 3.0% on a 240 V base.

10 Page: 3-10 For design purposes, the load for residential services and street lights should be modeled as a constant kva load at a 90% power factor. This is a conservative design which, in a contingency situation, should allow customers to be supplied with a voltage within emergency operating limits. The voltage drop at the customer s service entry point on the loss of a single phase padmount transformer is to be calculated with the load modeled at 50% constant kva and 50% constant impedance. This reflects a mix of service motor, lighting and resistive loads. See Table 3.2 for the cable impedances to be utilized for the calculation of voltage drop for subdivision single phase primary cables. As the return path for the current is through the neutral conductor and the ground (the cables are grounded at both ends), associated effects must be considered when calculating the effective cable impedance. Table Loop Impedances of Single Phase Concentric Neutral Aluminum Cable 1 Conductor Size Concentric Neutral (size and number R X of strands of wire ) 2 10 # # /0 16 # /0 13 # MCM 18 # Reference: Short, T.A., Electric Power Distribution Handbook, CRC Press, 2004, p. 21, Table Impedances are in Ohms/km. Conductor temperature = 90 C, Neutral Temperature = 80 C, 15-kV class 220-mil insulation, soil resistivity = 100 Ohms/metre. 2. Values interpolated between 250 MCM and 350 MCM in the referenced table.

11 Page: 3-11 The cable impedances for secondary USEB90 cables (where the neutral conductor surrounds both phase conductors) are not readily available in reference books. The impedances provided in Table 3.3 for the secondary conductors typically utilized in these specifications were obtained from the Engineering Design Group of the Alcan Cable Company. Table Loop Impedances of 2/c Single Phase USEB90 Concentric Neutral Aluminum Secondary Cables 1 Conductor Size Concentric Neutral (size and number R X of strands of wire ) 2 16 # # /0 16 # /0 32 # MCM 32 # Reference: Alcan Cable Company, Engineering Design Group. 1. Impedances are in Ohms/km. Conductor temperature = 90 C, Neutral Temperature = 80 C, 15-kV class 220-mil insulation, soil resistivity = 100 Ohms/metre. Detailed voltage drop calculations must be included in the information package provided to ATCO Electric for review (i.e., calculations completed on spreadsheets or by power analysis software such as, but not limited to ETAP or CYME). See Appendix C, Document C.1.6, for sample calculations completed on a spreadsheet. Note: The Consulting Engineer should utilize the impedance characteristics of the cable at the rated operating temperature to calculate the voltage drop. Refer back to Table 3.1. For the design to be accepted: a) the voltage must be maintained within the limits for Normal Operating Conditions at the customer meter;

12 Page: 3-12 b) on the loss of a single phase padmount transformer or subdivision primary feeder, the voltage must be maintained within the limits for Extreme Operating Conditions (i.e., emergency operating limits); and c) on the loss of a transformer, the site with the lowest voltage (i.e., the greatest voltage drop) fed off that transformer must be maintained within emergency operating limits Transformer Loading The number of customers who can be satisfactorily supplied from a transformer (in terms of the voltage quality and thermal limits of the equipment) depends on the diversity of the loads of the customers. Each customer will have different consumption patterns according to their routines, house comfort levels, and the efficiency of their electrical equipment and so on. The diversity factors provided in Table 3.4 are recommended for the design of single phase subdivision primary feeders and transformers. Table Diversity Factors No. of Lots Coincidence Factor Diversity Factor Reference: Willis, H. Lee, Power Distribution Planning Reference Book, CRC Press, 2004, p. 812, Figure

13 Page: 3-13 Tables 3.5, 3.6 and 3.7 identify the maximum number of 100 amp, 200 amp or a combination of 100/200 amp customers (services) that can normally be supplied from ATCO Electric s standard single phase 14.4kV 120/240V (nominal) rated primary and secondary loop-feed transformers to maintain the voltage within the limits identified in Section These tables are to be used as guidelines only. The number of services that can be supplied may be limited by size restrictions of the secondary compartments and/or the allowable secondary power cable voltage drop. ALL designs and calculations submitted must meet voltage and loading requirements. Table Maximum Number of 100 A Services Supplied by Padmount Transformers Transformer Size 100 A Services 50kVA 16 75kVA kVA 31 Table Maximum Number of 200 A Services Supplied by Padmount Transformers Transformer Size 200 A Services 50kVA 8 75kVA kVA 15 Table Maximum Number of a Combination of 100A and 200 A Services Supplied by Padmount Transformers Transformer Size 100 A Services 200 A Services 50kVA kVA kVA 22 8

14 Page: Cable Ampacity The maximum current (at a nominal 25 kv) that each conductor is allowed to carry is identified in Tables 3.8 and 3.9. Table Ampacities of Direct Buried Cable Cable Size Maximum Amps 1 #1 AL MCM AL MCM AL Assumes cables direct buried in trench with a minimum separation of one full cable diameter from another phase cable, phase max temperature 90 C, and ambient earth temperature 20 C. Table Ampacities of One Cable in One Duct Cable Size Maximum Amps 1 #1 AL MCM AL MCM AL Assumes phase max temperature 90 C. These ratings are in accordance with ATCO Electric s design standards. In developing the subdivision design, care must be taken to ensure the cables are not undersized for existing and future phase developments Reliability Considerations The secondary design is such that on the failure of a transformer, voltage is quickly restored to within emergency operating limits so the customer s essential equipment can continue to operate pending a full repair. A secondary loop system allows for quicker restoration of service than a radial secondary system following a transformer fault. Primary and secondary splices are not accepted or allowed in URD system design, in order to achieve an adequate reliability level (see Section 5.3.1).

15 Page: Cable Trench Alignments/Depths Standard alignments for underground power and other joint-use facilities are generally defined by the Municipal Authority It is the responsibility of the Developer and/or its agent(s) to obtain the most current version from the Municipal Authority., at a minimum the alignment must conform with the drawings in Appendix D, specifically Drawings D.2.1, D.2.2, D.2.3, D.7.1A, D.7.1B, D.7.2, D.7.3, D.7.4, and D Express Feeders Any express feeders identified by ATCO at the Preliminary Design stage will be designed by ATCO Electric. Information will be provided to the Developer and/or it agent(s) at the design review. The express feeder will be installed in 102mm or 150mm ducts (if required, and dependent on cable size) in a separate/common trench located in a URW and/or an easement on a boulevard. The centre line of trench alignment will be determined by the ATCO Electric Engineering Representative. While ATCO Electric will design any express feeders within a subdivision, the Developer and/or its agent(s) may be required to provide and install the civil works and install ducts and omni balls wherever road crossings are necessary (see Section 2.2 and Table 2.1, Note 1). The road crossings will be outlined by ATCO Electric s Engineering Representative Subdivision Primary Feeders After submission of the development plans to ATCO Electric but prior to starting the design, the Developer and/or its agent(s) are responsible to obtain the distribution system planning requirements, which includes the source points, from the ATCO Electric Engineering Representative.

16 Page: Four Party Trenching Four party trenching involves the installation of subdivision primary distribution cables, secondary cables, street light cables, communications cable, cable TV and gas pipes in one (1) common trench, as per the typical front lot alignments and standard trench details shown in Appendix D, Drawing D.1.7. Any deviations to the standard vertical or horizontal alignments must be accepted in advance by ATCO Electric s Engineering Representative. The Developer must also obtain the approval of the other shallow utilities Three Party Trenching Three party trenching involves the installation of subdivision primary distribution cables, secondary cables, street light cables, communications cable and cable TV in one (1) common trench, as per the typical front lot alignments and standard trench details shown in Appendix D, Drawings D.1.5 and D.1.6. Any deviations to the standard vertical or horizontal alignments must be accepted in advance by ATCO Electric s Engineering Representative. The Developer must also obtain the approval of the other shallow utilities Two Party Trenching Two party trenching involves the installation of subdivision primary distribution cables, secondary cables, street light cables and communications cable or cable TV in one (1) common trench, as per the typical front lot alignments and standard trench details shown in Appendix D, Drawings D.1.5 and D.1.6.

17 Page: 3-17 Any deviations to the standard vertical or horizontal alignments must be accepted in advance by ATCO Electric s Engineering Representative. The Developer must also obtain the approval of the other shallow utilities One Party Trenching Single party trenching involves the installation of primary distribution cables, secondary cables and street light cables in one (1) common trench, as per the typical front lot alignments and standard trench details shown in Appendix D, Drawings D.1.1, D.1.2A and D.1.2B. Any deviations to the standard vertical or horizontal alignments must be accepted in advance by ATCO Electric s Engineering Representative Service Entry Trench to Lot For Single Family Dwelling service box and stake are to be installed 0.5m from the easement and 1.5m from the lot line and NOT under a driveway. See Appendix D, Drawings D.3.1A, D.3.2, D.5.1 and D.5.2. For Multiple Family Dwelling service box is typically not required as the building foundation is in place prior to construction. Service trench shall be minimum 1.5m from the building. See Appendix D, drawings D.3.1B 3.5 Primary and Secondary Cables Primary Cables The following cables are to be utilized for subdivision primary service:

18 Page: /C #1 AWG TRXLPE compact aluminum strand 100% Cu concentric neutral 1/C 350 MCM TRXLPE compact aluminum strand 100% Cu concentric neutral For detailed primary cable specifications, see Appendix E, Document E.1 and Specifications and Secondary and Service Drop Cables The following cables are to be utilized for secondary service: 2/C - #2 600V aluminum conductor jacketed type USEB90 reduced Cu concentric neutral 2/C 4/0 600V aluminum conductor jacketed type USEB90 reduced Cu concentric neutral 2/C 300 MCM 600V aluminum conductor type USEB90 100% Cu concentric neutral For detailed secondary cable specifications, see Appendix E, Document E.2 and Specifications and For Single Family Dwelling services, the design for the service drop must show a minimum length of 30 metres of secondary cable from the edge of a service box entrance to the customer s meter. The voltage drop at the end of the service drop cable must be within the limits identified in Section For Multiple Family Dwelling Services within Bare Land Residential Development the service conductor shall be 2/C MCM aluminium conductors. Developers shall be responsible for trench and installation of service conductors up the multi-meter junction box. See Appendix D, drawing D.3.1B and D.2.5.

19 Page: Switch Cubicles ATCO Electric's Engineering Representative will provide the Developer and/or its agent(s) with information related to the switch cubicles required to supply a subdivision area (i.e., preliminary location, schedule, size and configuration). The Developer and/or its agent(s) are responsible to propose a final location for the approval of ATCO Electric s Engineering Representative. The switch cubicle must be located a minimum distance of 3.0m from the curb, and a 6.0m x 6.0m easement is required. If necessary, the Developer and/or its agent(s) will provide ATCO Electric with the easement (see Section 4.2.2). Note: The distance from the curb must take into consideration the size and location of the base ground grid, which extends 0.5m to 1.0m beyond the boundaries of a switch cubicle base (see Appendix D, Drawing D.4.5). 3.7 Padmount Transformers and Secondary Pedestals Installation For the purposes of these specifications, padmount transformers will be single-phase, 14.4kV 120/240V (nominal) rated primary and secondary, loop-feed transformers, and three-phase, 25kV 120/208V or 347/600V (nominal) rated primary and secondary, loop-feed transformers. For Single Family Dwelling, no services will be fed directly from the transformer; a secondary pedestal is always required. A street light may be fed from either a transformer or a pedestal. For Multiple Family Dwelling, services will be fed directly from the transformer; a secondary pedestal is not required. A Street light maybe fed from either a transformer or a pedestal.

20 Page: 3-20 Padmount transformers are to be installed as follows: a) The transformers must be mounted on precast bases located at intersecting lot lines, and coordinated with other facilities located on the street so as not to encumber both lot lines of any residential family zoned lot. b) The precast base, where possible, should be located 0.5m from easement on a boulevard, or 0.5m from the edge of the trench, but only if the required ground grid will not be under existing or proposed sidewalks, curbs, roadways, and other known structures or on private property. lf such a conflict exists; the precast base must be shifted to allow for the ground grid installation. The Developer and/or its agent(s) are responsible to provide the required alignments. c) c) If the Consulting Engineer wishes to install a transformer on the opposite property line of a street light, the Developer and/or its agent(s) must include an approved driveway plan for all lots within the subdivision with the first submission of the detailed engineering plans. In addition, the Developer must provide a letter accepting responsibility for all costs of relocating the equipment due to changes in the location of driveways. This option is available only if all other conditions regarding transformer location are met. d) Transformer bases must NOT be: i. offset into pie-shape properties with frontages of less than 15.0m; ii. set closer to a corner cut than 6.0m; iii. located with less than a 1.0m side clearance to all other facilities, and less than a 2.0m clearance to bus stops and curb cuts; iv. set in a sidewalk; or v. placed on the driveway side of a lot. e) Furthermore:

21 Page: 3-21 i. No pedestal or street light should be placed on the driveway side of a lot. (If this cannot be avoided, ATCO Electric s CSR should be consulted). ii. The transformer is to be positioned so its access lid/door faces the roadway. iii. The street light is to be positioned as follows: Single Davit the access door is to be positioned so that the employee is facing oncoming traffic when working in the light standard where possible; Double Davit installed on an island the door is to be installed parallel to the road so that the employee has visibility of approaching traffic from either direction when working in the standard. iv. The pedestal is to be positioned as follows: When the pedestal is parallel to the road the pedestal door faces the house including pedestal/transformer combinations and street light/pedestal combinations; When the pedestal is placed perpendicular to the road surface the door shall be positioned so that when accessing the pedestal the worker is facing oncoming traffic.. See Appendix D, Drawings D.3.1, D.3.1B, D.3.2, D.4.15 D.7.1A, D.7.1B, D.7.2, D.7.4, D.7.6C, D.7.7, D.7.8 and D.7.9 for transformer and pedestal alignment, orientation and clearance details Equipment Identification The circuit and equipment identification and tagging requirements on the engineering drawings must be accurate and match the identification and tags on the cables and equipment installed. Appendix D, drawings D.5.6, D.5.7, D.5.8, D.5.9, D.5.10 and D Grounding

22 Page: 3-22 The Developer and/or its agent(s) are responsible, as part of the overall installation of the URD system, to complete the design, installation and interconnection of ground facilities at each padmount transformer location as per Appendix D, Drawings D.4.5 and D.4.6. The grounding materials must meet the requirements of Appendix E.5. All conductive above-ground facilities (e.g., metal guard posts or bollards, stand-alone cable TV and communication pedestals, metering pedestals, etc.) within 3.0m of a transformer pedestal must be tied into the pedestal s ground grid. 3.8 Primary Fusing ATCO Electric will complete the primary fuse coordination at the switch cubicle and supply the required on-site fuse units. 3.9 Street Lighting Typically, the Municipal Authority will dictate the roadway lighting requirements, including lighting levels, lighting types, festoon electric plugs, locations and ownership or responsibility for the facilities within the development area. Refer to the Municipal Authority s most recent guidelines. Festoon electric plugs must be installed on all light standards on roadways with four (4) or more lanes. Required illumination levels for a roadway are determined by the classification and use of the roadway. The Developer and/or its agent(s), unless directed otherwise by the governing authority, will utilize the American National Standard Practice for Roadway Lighting, publication RP-8 of the Illuminating Engineering Society of North America (IESNA), as the minimum standard for the design of safe and efficient roadway lighting in urban residential and commercial areas. The Developer and/or its agent(s) must check the roadway classifications with the appropriate roadway authority before proceeding with any detailed engineering or construction of street lights and the underground electrical distribution system.

23 Page: 3-23 Street lighting design calculations and documentation must be submitted for ATCO Electric s acceptance; the minimum requirements are described in Section 4.0. A sample of inputs and calculations utilizing a computer program is provided in Appendix C, Document C.1.7. All designs must be based on a uniform tilt of zero degrees (0 o ). For single lot, All street light circuits must be fed from secondary pedestals, in accordance with the requirements of Section The individual street light davits are to be controlled by a photoelectric cell mounted on each davit. In residential areas, street light standards should be placed on lot lines, where practical. In subdivisions with curved roads and irregular lot sizes, another location may have to be determined to meet lighting requirements. When installing pedestals, it is recommended to pair pedestals with street lights to minimize facility locations and the visual impact to lot owners. See Appendix D, Drawings D.7.1A, D.7.1B and D.7.2 for street light alignment details Landscaping Adjacent to Underground Electrical Equipment Before commencing the design of an URD system, the Developer and/or its agent(s) must confirm landscaping plans with the Municipal Authority, including all vegetation for the area. See Appendix D, Drawings D.7.6A, D.7.6B, and D.7.6C for details.