SECTION DVD01: VOLTAGE DROP/VOLTAGE FLICKER AND TRANSFORMER SIZING GUIDELINES

Transcription

1 Issued DESIGN Section DVD00 INFORMATION SECTION Page 1 of 1 INDEX SECTION DVD01: VOLTAGE DROP/VOLTAGE FLICKER AND TRANSFORMER INTRODUCTION... Page 2 LOAD ESTIMATING AND TRANSFORMER SIZING GUIDELINES...Page 3 TOOLS FOR ESTIMATING TRANSFORMER LOAD AND CALCULATING VOLTAGE DROP... Page 7 VOLTAGE DROP AND TRANSFORMER SIZING PROGRAM... Page 7 VOLTAGE DROP CHARTS...Page 12 SECTION DVD02: VOLTAGE FLICKER DEFINITION... Page 1 INTRODUCTION... Page 1 PERMISSIBLE FLICKER LIMITS... Page 1 VOLTAGE FLUCTUATION CAUSES... Page 1

2 Issued DESIGN Section DVD01 INFORMATION SECTION Page 1 of 21 ALLIANT ENERGY VOLTAGE DROP / VOLTAGE FLICKER AND TRANSFORMER September, 1998 Voltage Drop Team: Mark Peterson Mike Warntjes Tony Mak Ron Johnson Revised January 2005: Mark Peterson

3 Section DVD01 DESIGN Issued Page 2 of 21 INFORMATION SECTION 1. INTRODUCTION Voltage Drop: The difference between the voltage magnitudes at the source and the receiving (load) ends of a feeder, secondary, or service. This drop is based on the length, the resistance and the reactance of the conductor and the amount of load being carried. Impedance: The characteristic of electrical conductors (lines or transformers) that tends to oppose or resist the flow of alternating current (AC) electricity. Impedance is composed of two parts: resistance and reactance. The impedance of a conductor decreases as the length of the conductor decreases or the size of the conductor selected increases. Nominal Voltage: A term used to describe a class of voltage levels, such as 120/240 V, 120/208 V, 277/480 V, 7.2/12.47 kv, etc. Service Voltage: The voltage at the customer s meter or point of interconnection between the utility s system and the customer s system. Steady State Voltage: A voltage level that exists for an extended period of time, usually five minutes or longer. Voltage Fluctuations: Sharp changes in voltage level which persist for only a few seconds or cycles. The objective of system voltage control is to economically provide to each customer the voltage that conforms to the voltage design limitations of electrical equipment. Voltage drop exists in each part of the power system from the source to the customer s service entrance and is proportional to the magnitude and phase angle (power factor) of the load current flowing through the entire power system. This means that a customer electrically close to the source would receive a higher voltage than a customer more remote from the source. Each customer operates practically the same electrical equipment, which makes it necessary to provide each with almost equal supply voltage. A compromise has been required between the allowable deviation from nameplate voltage that is supplied and the deviation above and below the nameplate voltage at which satisfactory performance can be obtained. If voltage limits maintained by the power company were too broad, the cost of appliances would be high because they would have to be designed to operate at any voltage within the limits. If voltage limits were too narrow, the cost of providing power would become too high. Alliant Energy has several nominal voltages that it offers to its customers in the various operating companies electric tariffs. Generally, for retail service the variations of voltage will be no more than five percent above or below the nominal voltage.

4 Issued DESIGN Section DVD01 INFORMATION SECTION Page 3 of 21 Typically, the voltage level at the distribution substation is set at 125 Volts. The control on voltage regulators or load tap changer (LTC) will typically maintain this level within +/- 1 Volt of the voltage setting. This would be a 2 Volt bandwidth. Using 120 V as a base, +/- 5% gives a range of V as the allowable voltage level. The table below shows Alliant s voltage drop guidelines. MAXIMUM DESIGN DISTRIBUTION SYSTEM VOLTAGE DROPS Feeder Component Residential Feeder Rural Feeder Primary Feeder 4.35 % 5.6 % Distribution Transformer 2.0 % 2.0 % Secondary 2.0 % * Service 0.75 % 1.5 % TOTAL: 9.1% 9.1% * For rural applications, it is assumed the majority of applications will have no secondary. The maximum design voltage drops shown above are intended to be a guide for those responsible for the planning, design and engineering of the distribution system. The voltage drop for each component is intended to minimize the cost of the distribution system while maintaining an acceptable voltage to all Alliant Energy customers. The table is based on a typical Alliant Energy distribution system and will not be applicable to all situations. 2. LOAD ESTIMATING AND TRANSFORMER In 2005, new guidelines for estimating transformer loads and sizing transformers were developed by the Distribution Transformer Sizing Six Sigma Team. The team s recommendations for loading distribution transformers, along with a list of the team s members, are shown on the following page. Flow charts that were developed by the team for estimating load and sizing transformers are also included. The team also revised the equations used by the GIS transformer load management (TLM) program so the results provided will reflect the most resent company load data.

5 Section DVD01 DESIGN Issued Page 4 of 21 INFORMATION SECTION Lean Six Sigma Transformer Utilization Team Distribution Transformer Loading Recommendations 03/02/05 The following guidelines should be considered when installing distribution transformers. The guidelines are based on the loss of insulation life characteristics (of our present standard transformers), the average load profiles of each class of customers and average annual temperatures across the Alliant Energy territory. Residential For new developments or rebuilds, distribution transformers should be typically loaded at 110% at peak. This will allow 40 years of load growth at.75% per year, which has been the average load growth per transformer in the past. The goal is to ultimately load residential transformers to a target maximum of 140%. Transformers will be flagged in TLM when the load exceeds 180%. Residential load estimating tools Load on existing transformers can be estimated using the TLM program in ArcFM. Load for new homes can be estimated using the transformer sizing spreadsheet or the residential diversity curves. The diversity curves were developed using load research data and are a better representation of smaller existing homes. Tools will be added to the Design Voltage Drop (DVD) section of the Electric Construction Standards (ECS) manual. Small Commercial For new small commercial installations or rebuilds, or transformers serving a combination of residential and commercial customers, distribution transformers should typically be loaded to 110% of rating at peak if expected load growth is unknown. The goal is to ultimately load transformers to a target maximum of 140%. If enough information is available to estimated expected load growth, the transformer should be sized such that the initial load plus expected load growth does not exceed 140%. Voltage drop must be considered as it may be the most limiting factor. Large Commercial New or replacement transformer installations for large commercial customers (with load factors greater than 60%) should be loaded to a maximum of 105%. If excessive harmonic loads are suspected, the maximum loading limit may need to be reduced. Consult with distribution engineer. Industrial The typical load factor for industrial customers is about 80%. At this level, large pad mount transformers will not loose significant insulation life if loaded below 105% at peak. However, due to the high harmonic content of many of the more common industrial loads such as variable speed drives, florescent lighting, arc welders, arc furnaces, electronic power supplies and battery chargers, the maximum loading may need to be reduced significantly. Consult with distribution engineer when sizing transformers for industrial loads. Commercial and Industrial load estimating tools - Load on existing transformers can be estimated using the TLM program in ArcFM. Individual demand metered loads are available in CSS or CIS. Load for new customers can be estimated using information from the customer, load from similar customers or section D3PUD-3 (and appendix A) of the ECS manual. Team members: John Helbling, Black Belt Steve Carlson, Special Projects Manager Mark Peterson, Lead Distribution Engineer Centerville, IA Bruce Bielema, Manager Customer Service Clinton, IA Dan Holzman, Engineering Technician Berlin, WI Nate Pollock, GIS Business Analyst

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8 Issued DESIGN Section DVD01 INFORMATION SECTION Page 7 of 21 AVAILABLE TOOLS FOR ESTIMATING TRANSFORMER LOAD AND CALCULATING VOLTAGE DROP Several tools are available to estimate transformer load and to calculate voltage drop on the distribution transformer, secondary and service. A transformer sizing and voltage drop spreadsheet program is available as well as a set of load estimating and voltage drop charts. The spreadsheet is intended as a tool for designing the distribution systems for new residential subdivisions whereas the voltage drop charts may be used to calculate the voltage drop on any section of conductor. The transformer sizing spreadsheet is also provided in a hard copy table and graph. A table and graph is provided for estimating load for existing homes as well. Primary voltage drop and motor start calculations should be referred to the area Distribution Engineer. 1. VOLTAGE DROP AND TRANSFORMER SIZING PROGRAM An Excel spreadsheet program has been developed which will size the distribution transformer and calculate the voltage drop on the transformer, secondary and service (overhead or underground) to residential customers. There are four customer types to choose from and up to four secondaries or pedestals may be modeled. The program will calculate the load in kva on each secondary and on the distribution transformer. A suggested minimum transformer size is calculated based on a given loading factor. The advantage of this program is that the customer diversity load curves are built into the program. This means that the user does not have to adjust for the fact that most customers do not peak at the same time. As more customers are modeled on a transformer, the program automatically reduces the load per customer according to the diversity curves within the program. Detailed step by step instructions for using the spreadsheet and a hard copy of the program as it appears on the computer screen are included in the following pages. A graph of the output of the transformer sizing spreadsheet is also available for summer loads with air conditioning (type A and C) which may be easier to use. However, for the results to be accurate all of the customers served from the transformer must be of the same type. A table and graph is provided for estimating load for existing homes as well. Copies of the voltage drop and transformer sizing program as well as the graphs mentioned above are available on the IPL Field Engineering Team West web site. The methodology to calculate the transformer loading is based on the project team of Dick Wight of Energy Market Solutions, Diane Cunningham of Applied Energy Group, Dr. Russ Heikes and Dr. Bill Hines of Georgia Institute of Technology. Please refer to WP&L Residential Load Modeling 1995 report prepared by Energy Market Solutions.

9 Section DVD01 DESIGN Issued Page 8 of 21 INFORMATION SECTION VOLTAGE DROP AND TRANSFORMER SIZING PROGRAM INSTRUCTIONS Description Any questions call Mark Peterson (641) This spreadsheet contains three worksheets, a "Transformer Sizing" worksheet and two "Secondary / Service Voltage Drop" worksheets. The Transformer Sizing worksheet contains an algorithm to determine the peak kva demand for a group of "new" customers based on the number and type of each customer. The customers can be grouped by up to four load distribution points "nodes" which can be overhead or underground. The outputs include the non-coincident peak for each load point (Node 1 through Node 4) and the peak transformer load. The transformer load includes diversity between the load points. An additional table is included called Inputs to: "Coincident Peak kva" which displays the coincident load at each node (their sum equals the calculated transformer load). These loads can then be entered into either "Secondary / Service Voltage Drop" worksheet to calculate the voltage drop at any point on a secondary system. The voltage drop worksheets are identical except "Secondary-Service Volt Drop (%)" will calculate voltage drop in percent and "Secondary-Service Volt Drop (V)" will calculate the voltage on a 120 V base. Refer to section DVD-1 of the Electric Construction Standards manual for the maximum design distribution system voltage drops. Instructions Transformer Sizing Spreadsheet 1. Fill in "Location", "Pole or Pedestal No.", and "Tx No." if needed to identify file or hard copy. 2. Determine appliance codes from "Electric Appliance Combo Table". 3. Enter number of customers on a white box - always start with "Node 1". The "Noncoincident Peak kva" table will display the calculated peak load for each node (load point) and the transformer, as well as the minimum transformer size (load + loading factor) and the number of customers. 4. The "Coincident Peak kva" table will display the calculated transformer load, distributed proportionally to each pedestal. These outputs are also displayed in the "Secondary-Service Volt Drop" sheets. Secondary-Service Voltage Drop 1. Click the "Clear Sheet" button to zero the conductor lengths and kva loads on the pedestals and meters. 2. Enter the inputs as instructed for each input variable. The outputs from the "Transformer Sizing" worksheet are available in the table in the upper right portion of each "Secondary-Service Volt Drop". 3. Transformer voltage drop and load will be shown in the table representing the transformer. 4. The voltage drop for each pedestal and meter will be shown at each location modeled. Modeling More Than Four Load Points If it is desired to model more than four load points utilizing the Transformer Sizing worksheet, all of the customers served by the transformer of each type should be entered into a single node (1) location. The load should then be distributed to each node in the volt drop worksheet by the proportion of customers served at the node.

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11 Section DVD01 DESIGN Issued Page 10 of 21 INFORMATION SECTION Single Phase Secondary / Service Voltage Drop Calculator

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13 Section DVD01 DESIGN Issued Page 12 of 21 INFORMATION SECTION Load Estimating Graphs for New and Existing Homes 2. VOLTAGE DROP CHARTS The voltage drop charts on the following pages are designed to show the percent voltage drop between the source and load ends of a length of secondary or a service. The charts show percent voltage drop per 100 feet. Follow these steps to use the charts: 1. Determine the proper chart to use by determining whether the load is single- or three-phase and what the nominal voltage is. 2. Determine the AWG size and length of the secondary/service conductor. 3. Determine the load current in amperes. For parallel runs (i.e. More than one conductor per phase), divide the total load in amps by the number of runs. Use this number in the charts. 4. Locate the value of amps on the X (horizontal) axis. 5. Read the percent voltage drop from the Y (vertical) axis corresponding to the conductor being used. 6. For conductor lengths other than 100 feet, adjust this number accordingly. Example: for 200 feet, multiply the number by two. For 50 feet, multiply the number by 0.5

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23 Issued DESIGN Section DVD02 INFORMATION SECTION Page 1 of 6 VOLTAGE FLICKER 1. DEFINITION VOLTAGE FLICKER: A phenomenon caused by sudden changes of brief duration in the voltage supplied to a customer. 2. INTRODUCTION Electric Utility delivery systems provide relatively constant voltage. Power system voltage fluctuations are generally caused by large changes with customers loads. Large current changes are causing the voltage to fluctuate and another customer has seen lamp flicke, experienced problems with a process, or otherwise been disturbed. There are two forms of power system voltage fluctuations: (1) voltage fluctuations caused by utilization equipment resulting in lamp flicker, and (2) voltage fluctuations caused by transient system faults resulting in utilization equipment downtime. Human sensitivity and tolerance varies considerably and depends upon the magnitude and frequency of the voltage flicker. It is, therefore, difficult to establish flicker limits that are economically justified and satisfactory to all customers. Historically, voltage fluctuation limit guidelines have been based on customer and equipment tolerance curves. More recently, flicker limits and system design guidelines have been established by regulatory bodies through input from consumer organizations and utilities. 3. PERMISSIBLE FLICKER LIMITS The maximum amount of acceptable flicker voltage cannot be stated concisely for several reasons. Flicker that one individual may feel is objectionable may not be perceptible to another. The type of lighting fixture used is also important. Incandescent and fluorescent lamps are affected differently by a change in supply voltage. Cyclic or rapidly recurring voltage changes are generally more objectionable than non-cyclic. For non-cyclic changes, the annoyance due to the flicker is affected by the rate of change, duration of change, and frequency of occurrence of the flicker. 4. VOLTAGE FLUCTUATION CAUSES Most of the voltage fluctuations on distribution systems are due to the operation of customers utilization equipment. Following are some of the more common types of equipment known to cause flicker. Several more typical solutions for correcting the voltage fluctuation to within permissible limits are also provided. a. Motor Starting 1) Probably the most common cause of voltage flicker problems is the starting of motors. For reasons of cost, efficiency, and reliability, commercial general purpose motors require a momentary starting current several times their full load running current in order to produce sufficient starting torque. The magnitude of starting current may cause a dip in the reference voltage and result in objectionable flicker.

24 Section DVD02 DESIGN Issued Page 2 of 6 INFORMATION SECTION VOLTAGE FLICKER There are three general classes of motor installations: Single-phase fractional horsepower motors commonly used in homes and small stores. Small multi-horsepower single and polyphase motors operated from secondary distribution circuits, such as in small shops, large stores and buildings, and recently in an increasing number of homes for air conditioning. Large horsepower three-phase motors operated from primary lines, mostly for industrial applications. 2) The magnitude of voltage dip caused by a motor load during starting depends on: a) The system impedance (size and length of transmission line, size of substation transformer, and size and length of distribution line). b) The size and impedance of the (distribution) transformer feeding the load. c) The length and size of the secondary conductor between the motor and transformer. d) The starting current (locked rotor amps) requirements of the motor. 3) Flicker can be improved by reducing the system impedance by: a) Increasing the size of the substation and/or distribution transformers. b) Adding distribution transformers or moving existing transformers closer to the load to reduce the length of the secondary conductor. c) Increasing the size or number (parallel runs) of the secondary conductor. d) Reducing the starting current requirements of the motor through reduced voltage starting equipment (auto transformer and reactor type reduced voltage starter, wye-delta starting, or part winding starting). * Note this is the preferred method, however the effects of reduced voltage starting equipment on the starting torque characteristics of the motor must be analyzed. Consult your distribution engineer for assistance. Motor nameplate information, as well as specifics concerning the use and type of application, will be required. b. Electric Welders Due to their relatively small size and widespread use, welders can create voltage fluctuations on many segments of the distribution and delivery systems. Most welders have a smaller on time than off time, and consequently, the total energy consumed is small compared with the instantaneous demand. Fortunately, most welders are located in industrial applications with other processes, which may require a larger amount of power. Supply facilities typically are of larger capacity to reduce the amount of flicker. In isolated cases,

25 Issued DESIGN Section DVD02 INFORMATION SECTION Page 3 of 6 VOLTAGE FLICKER but nonetheless important, the welder may be the major load in the area, and serious flicker may be imposed on distribution systems that are otherwise adequate for serving ordinary loads. Practically all welders in service are single phase, and the most commonly used welder is the resistance welder. Various types of electric welders are: 1) Flash welders 2) Pressure butt welders 3) Projection welders 4) Resistance welders a. Spot b. Seam Basically, the source of voltage, usually 240, 480, or 2,300 volts, is stepped down to a few volts to send high currents through the parts to be welded. Although bridge rectifier circuits are used in resistance welders, some saturation of the welding transformer usually will exist. This effect, together with the short successive pulses of high current, creates the objectionable voltage dip. Corrective measures for voltage dips caused by welders range from motor-generator sets installed in the customer s plant to supply circuit changes by the utility. Supply circuit changes may be accomplished by increasing the feeder size or the substation capacity. Separate distribution feeder lines to the welding load also are possible economic solutions. As with all fluctuating loads, the object is to decrease the system impedance between the source and objectionable load. Consult your distribution engineer for assistance. Welder nameplate information, as well as specifics concerning the use and type of application, will be required. c. Electric Furnaces There are three types of electric furnaces resistance, induction, and arc. Depending on the size of the furnace, each type could cause a voltage fluctuation problem. When an electric furnace load is proposed for our system, distribution engineering and/or distribution planning must perform a detailed study of possible voltage problems. Induction furnaces are single-phase devices that are typically connected phase-tophase. When configured with capacitors and inductors, induction furnaces can simulate a three-phase load and operate relatively balanced with a power factor close to unity. Induction furnaces connected phase-to-phase cause significant voltage fluctuations due to delivery system load imbalance. One phase will see a significant voltage drop while the other phase incurs a slight voltage rise during the operation of the furnace. Induction furnaces,depending upon the size and operated to simulate a three-phase load create minimal, if any, voltage concerns.

26 Section DVD02 DESIGN Issued Page 4 of 6 INFORMATION SECTION VOLTAGE FLICKER Consult your distribution engineer for assistance. Furnace nameplate information, as well as specifics concerning the use and type of application of the equipment and system supply impedance, will be required. d. Miscellaneous Under this category comes special equipment such as electric shovels, strip mining equipment, stone crushers, heavy rolling mills, and similar installations. Most of these must be considered individually as to special features and the type of power supply required. Again, your Distribution Engineer should be consulted for assistance to assure that these devices can be operated with the least amount of system voltage fluctuation. e. Residential Voltage Flicker Tables On the next several pages are tables showing maximum secondary and service lengths for various transformer sizes. These tables are based on voltage flicker limits only. In most cases, if not all, these limits are more restrictive than voltage drop limits. In other words, taking voltage flicker into account tends to restrict the length of secondary and services more so than voltage drop due to steady state load. However, the voltage drop should also be checked against the guidelines to make sure limits are not exceeded. The tables are set up to limit flicker at points common to multiple customers (transformers, pedestals) to 3% and at the meter to 4%.

27 Issued DESIGN Section DVD02 INFORMATION SECTION Page 5 of 6 VOLTAGE FLICKER 10 kva Transformer, No Secondary* Maximum Service Length (Ft.) #2 (OH) #1/0 (OH/UG) #4/0 (OH/UG) 350 MCM (UG) *Do not serve multiple residential customers from a 10 kva transformer. Assuming 60 amp inrush at 0.65 pf, which creates 3.4% flicker through 10 kva transformer. Allowing meter, primary flicker not accounted for. 25 kva Transformer, Maximum Service Length (Ft.) No Secondary #2 (OH) #1/0 (OH/UG) #4/0 (OH/UG) 350 MCM (UG) kva Transformer, 1/0 Secondary Length of 1/0 Secondary Maximum Service Length Conductor (Ft.) #2 (OH) #1/0 (OH/UG) #4/0 (OH/UG) 350 MCM (UG) kva Transformer, 4/0 Secondary Length of 4/0 Secondary Maximum Service Length (Ft.) Conductor (Ft.) #2 (OH) #1/0 (OH/UG) #4/0 (OH/UG) 350 MCM (UG) kva Transformer, 350 Secondary Length of 350 Secondary Maximum Service Length (Ft.) Conductor (Ft.) #2 (OH) #1/0 (OH/UG) #4/0 (OH/UG) 350 MCM (UG) Assuming 100 amp inrush at 0.65 pf, which creates 2% flicker through 25 kva transformer. Allowing 3% pedestal, meter, primary flicker not accounted for.

28 Section DVD02 DESIGN Issued Page 6 of 6 INFORMATION SECTION VOLTAGE FLICKER 50 kva Transformer, No Secondary Maximum Service Length (Ft.) #2 (OH) #1/0 (OH/UG) #4/0 (OH/UG) 350 MCM (UG) kva Transformer, 1/0 Secondary 50 kva Transformer, 4/0 Secondary Length of 1/0 Secondary Con- Maximum Service Length (Ft.) ductor (Ft.) #2 (OH) #1/0 (OH/UG) #4/0 (OH/UG) 350 MCM (UG) Length of 4/0 Secondary Maximum Service Length (Ft.) Conductor (Ft.) #2 (OH) #1/0 (OH/UG) #4/0 (OH/UG) 350 MCM (UG) kva Transformer, 350 Secondary Length of 350 Secondary Conductor (Ft.) Maximum Service Length (Ft.) #2 (OH) #1/0 (OH/UG) #4/0 (OH/UG) 350 MCM (UG) Assuming 100 amp inrush at 0.65 pf, which creates 1% flicker through 50 kva transformer. Allowing 3% pedestal, meter, primary flicker not accounted for. (END)