The art of protecting electrical systems, part 16—software speeds electrical design

Part 16 in this continuing series looks at computer programs used to assist in electrical system design and protection.

02/20/2008


Editor’s Note: From 1965 through 1970, Consulting-Specifying Engineer’s predecessor, Actual Specifying Engineer, ran a series of articles on overcurrent protection. Due to the immense popularity of the 31 installments in the series, the authors, George Farrell and Frank Valvoda, PE, reprised the series in an updated version beginning in the Feb. 1989 issue of CSE. Over the years since the last installment ran in the late ’90s, we have received many requests to re-run this series. Mr. Valvoda passed away in Dec. 2001, and his long-time friend and editorial partner, George Farrell, passed away in early 2006.

The reader should keep in mind that the authors' software evaluation www.skm.com


In the past few years the art of protecting electrical systems has undergone a dramatic change. The ready availability of

Available software has simplified calculation of such formerly involved tasks as average lighting; point-by-point lighting; panel board arrangements; conduit sizing; sizing of feeders and transformers; voltage drop; load-flow and short-circuit studies; harmonic analysis; effect of motor starting; use of leading power-factor devices for power-factor correction; and preparation of coordination drawings to ensure selective overcurrent protection. The programs often tie into computer-aided design programs to make layouts of lighting, panel boards, and switchboards, motor control centers, single-line and riser diagrams, and impedance diagrams for voltage-drop and short-circuit studies.

This article begins the review of program availability with a detailed examination of one of the larger, more complex software programs on the market:ical system evaluation.

The program
The program being reviewed here provides a complete electrical distribution system analysis, with the capability of including on-site generation. It consists of the following interactive, related modules: design load analysis to establish and record loads on the system; sizing of feeders and transformers to meet load requirements; voltage-drop and load-flow studies to determine voltage, percent voltage drop and actual power flow at every significant point in the system; and short-circuit calculations (both three phase and unbalanced) to determine fault current.

This article (and the next article in our series) discuss each module using the small electrical system shown in Figure 1 as an example. The example system has been kept deliberately simple to meet space constraints.

This article establishes the electrical system and describes the data input used in all modules. The furnished libraries of feeders and transformers are illustrated, and use of the program’s Demand Load Library is described. Sample data screens, as they appear on the monitor, are shown.

The next article will describe data input required for the program’s Voltage Drop and Load Flow Module and for the Short Circuit Module. Excerpts of Design Load Analysis, Size Feeders & Transformers, Load Flow and Short Circuit reports will be presented, together with an evaluation of each. And, finally, the additional capabilities of the program will be discussed.

Example system
Figure 1 is a single-line diagram of the electrical system to be studied. It consists of a utility connection at 12.47 kilovolts (kV), with transformation down to 4.16 kV, 480 V, and 208Y/120 V. By convention, points where impedance changes (called nodes or buses) are identified by circled numbers. Lines between the buses are identified only with reference to the buses, i.e., the lines do not carry a specific identification.


In this example it is assumed for simplicity that protective devices have no impedance. In addition, certain connections between components are assumed to be short enough that ignoring their impedance will not contribute to significant errors. The effect of these omissions will be discussed later. Therefore, only the impedance for cable connections between buses 1 and 2, 3 and 4, 5 and 6, 5 and 7, and 8 and 9 are included. Transformers connect buses 2 and 3, 4 and 5, and 7 and 8. Protective devices in series with the transformers are assumed to have no impedance.

Motor loads on the system are connected directly to buses 3 and 6. The load on bus 3 is a 900-hp synchronous motor, while the load on bus 6 is grouped small motors totaling 800 hp, with the largest at 50 hp. The lighting and receptacle load is connected directly to bus 9.

Sources of fault energy are 500 megavolt-amps (MVA) at X/R = 20 from the utility and the 800- and 900-hp motor loads.

The 15-kV and 5-kV cable is assumed to be Type XLP, while 600-V cable is assumed to be Type THWN. Operating temperature for both voltage drop/load flow and short circuit is 75 C.

(Note: It is recommended that short-circuit studies be conducted with conductor operating temperature at 25 C, simulating a lightly loaded condition, because conductor resistance is less than at 75 C. Users can circumvent this problem by entering data for cable at the proper operating temperatures into the library.)

Main menu
Operations in the

Branch Records permits the defining of all feeder, transformer, and contribution information. Such data must be input before load data is input.

Libraries accesses the provided feeder and transformer libraries. As with all comprehensive programs of this type, these libraries may be changed or added to at will.

Bus Records allows inputting of load data on individual buses. Here we will consider only “End Use Loads,” i.e., the final load on a panel, switchboard, or motor control center. “Special bus loads” (impact loads such as motor starting and capacitors for power factor correction) may be input when the system warrants.

Execute Studies enables the user to perform any of the program study modules provided.

Link to CAD,

Inputting branch records
Data input screens are provided to allow systematic input of information about the branches. Figure 2 illustrates such a screen for the feeder from bus 8, MAINLTG to bus 9, LPC. In the right half of the screen, the voltage must be specified, along with the conductor material (C = copper), type duct (M = magnetic) and insulation (Code Type THWN). The length of the feeder (100 ft.), quantity/phase (one, as a first approximation), and circuit description (four wire plus full neutral) are entered as is the ambient temperature (30 C). When the program is used to size the feeder, the feeder size and neutral data fields are left blank as is the raceway size.


The left half of the screen lists all the branches as they are input, so the status of the system may be fully described at all times. Letters at the left of the branch listings are indicative of type of branch when not a conductor (T = transformer, C = contribution of fault current. Note that no “from” bus identification is given for contributions, as the contribution is from the zero-impedance, infinite bus. See Part 9

Figure 3 illustrates the branch record for bus 4, TR2PRI to bus 5, 480-V SWGR, which is a transformer. The primary, a delta connection (D), is at 4,160 V. The secondary, a wye-grounded (YG) connection, is at 480 V. (The phase-to-neutral voltage of 277 V does not appear in this tabulation.) When studying the system from the beginning, it is not necessary to show nominal kilovolt-amps (kVA) or full-load kVA, as the Feeder and Transformer Sizing module will accomplish this task based on actual loads in the system. Full-load kVA will include provision for cooling fans.

The transformer has been specified as dry type (DT); transformer resistance and reactance will be obtained from the program’s transformer library. “Design” loading, when specified (contrasted with “demand” loading), applies selected design multiplying factors to the loads—such as 1.25 for long, continuous load types. Taps may be specified on the primary connection to raise or lower the secondary voltage to suit voltage-drop considerations.

If the library standard impedance does not apply, the user may enter specific values for the transformer under consideration. Ground impedance also may be specified.

Libraries
These programs provide libraries of data that may be used in calculation. The program includes four libraries: feeders, transformers, demand-load table, and motor control center devices.

Figure 4 is an excerpt from the feeder library for THWN conductors. Information given (and used by the Calculation modules) includes ampacity, reduced neutral size that may be used, ground wire size, permission to parallel, and positive and zero-sequence resistance and reactance per thousand feet. Users may add or revise these data as required.

Figure 5 is an excerpt from the dry-type transformer impedance library. Percent resistance and reactance are listed for various ratings of standard transformers. Other transformer types for which such data are furnished are dry-type with forced air, oil-cooled, and oil-cooled with forced air.

Figure 6 shows a page from the demand-load library. (An additional 10 categories of load may be handled via the library’s second page.) The loads shown are “end-use loads,” i.e., the load on the item of equipment last in the series arrangement of equipment. Demand percentages apply to various categories of load, based on the National Electrical Code.


For example, for category 3, receptacles, the first 10 kVA of load is to be calculated at 100%, while the balance of the load is to be calculated at 50%. The long continuous load (LCL) factor for the type of load appears in the last column. Loads are entered into the data input screens by these categories. The study modules automatically use the demand, LCL factors, and percent power factor in calculating individual and total loads on the system. All available competing programs have a comparable system for accumulating system loads.

Inputting bus records
The Demand Load Library is reflected in the input screens for bus loading. Figure 7 shows the loads to be input on bus 6, MCC1, for the example system. For this bus, only two categories of load are present: a grouped load of small motors totaling 750 hp (category 9) and one large motor (category 10) sized at 50 hp. The distinction between category 9 and 10 is made to permit the study modules to account for the code requirement of 125% ampacity for the largest motor on the equipment (in this case, the motor control center).


Grouped induction motor load is input on the basis of 1 hp equals 1 kVA. Other units that may be used are volt-amps (VA) and amps. The calculated data at the lower right portion of the screen does not appear until the studies have been completed. For this bus, the demand-load analysis study calculated a connected load of 962 amps and a design load (including the 25% additional for the largest motor) of 977 amps. The unbalanced fault study calculated a fault duty of 17,789 symmetrical three-phase amps and 17,351 single-line-to-ground amps.

The next part in this series will complete the study of the example system, including output reports of all study modules. It is important to again emphasize that this discussion of the DAPPER program in no way implies an endorsement of the software.

Related Stories:

The Art of Protecting Electrical Systems, Part 1: Introduction and Scope

The Art of Protecting Electrical Systems, Part 2: System Analysis

The Art of Protecting Electrical Systems, Part 3: System Analysis

The Art of Protecting Electrical Systems, Part 4: System Analysis

The Art of Protecting Electrical Systems, Part 5

The Art of Protecting Electrical Systems, Part 6

The Art of Protecting Electrical Systems, Part 7: Equipment Short Circuit Ratings

The Art of Protecting Electrical Systems, Part 8: Short-Circuit Calculations

The Art of Protecting Electrical Systems, Part 9: Assigning Impedance Values

The Art of Protecting Electrical Systems, Part 10: Assigning Impedance Values

The Art of Protecting Electrical Systems, Part 11: Impedance in Systems with Rotating Machinery

The Art of Protecting Electrical Systems, Part 12: Approximating Short-Circuit Calculations for Conductors


The art of protecting electrical systems, part 13:

The art of protecting electrical systems, part 14: single-phase short circuit calculations, a step-by-step guide

The art of protection electrical systems, part 15: calculating fault currents





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