The Art of Protecting Electrical Systems, Part 17: Computer Software and Electrical Design
Part 16 of this continuing series looked at computer programs used to assist in electrical system design and protection. Part 17 continues a look at how one computer program assists engineers in electrical system design.
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.
A wide variety of computer programs has become available in recent years for enhancing the art of protecting electrical systems. Although long available for mainframe computers, many programs now are available for more readily accessible microcomputers. These programs cover the design of power distribution systems.
This series examines one program as an example of the genre. The program facilitates the complete design of an electrical system from loading, sizing of feeders, and transformers, and voltage drop and load flow to short circuit calculations.
[Editor’s Note: What follows is a review of a software program as it existed in 1992. For a description of current capabilities of the DAPPER system, go to www.skm.com .
This discussion is not meant as a specific endorsement of the program over others. The program was selected for presentation because it is representative of the type available and because it permits a complete evaluation of an electrical system.
Figure 1 is a single-line diagram of the electrical system being studied. It was deliberately kept simple. It consists of a utility connection at 12.47 kV with transformation to 4.16 kV, 480 V, and 208Y/120 V. By convention, points where impedance changes (called nodes or buses) are identified by encircled numbers. Lines between the buses are identified only with reference to the buses, i.e., the lines do not carry a special identification.
A complete description of the sample is contained in Part 16 of this series. In that article, we examined the program’s main menu and data input screens for branches and buses, together with excerpts from the feeder, transformer, and end-use-load libraries.
Fault duty branch records
Figure 2 is a fault duty branch record input screen. Data entered is the three-phase fault duty of the utility: 500 mega-volt-amperes (MVA) at a resistance-to-reactance ratio (X/R) of 20 at 12,470 V. Single-line-to-ground duty may be entered if known.
Figure 3 is a representation of the fault duty branch record input for the synchronous motor on bus 3, 5KVSWGR. The standard input data fields for synchronous motors include the motor’s kilovolt-ampere (kVA) base and X/R ratio, plus subtransient reactance. Default values suit most conditions, and the user may make desired changes.
Figure 4 shows the fault duty branch record input for the induction motors on bus 6, MCC1. Input fields include kVA or horsepower, subtransient reactance and X/R ratio.
The menu brings up a screen with eight study choices. The following studies were applied to reach results illustrated in this article: demand-load analysis, size feeders and transformers, load flow and voltage drop, and three-phase and unbalanced faults.
The load center summary permits panel boards and switchboards to be specified by circuit numbers. It is fully integrated with the load analyses. However, it is beyond the scope of this article. The three-phase fault study is a subset of the three-phase and unbalanced fault study. Because it only calculates three-phase faults, it is not discussed here.
The transient motor starting study is used in studying motor starting requirements, torque requirements, accelerating time studies, stall studies, load-change studies, controller applications, and voltage-dip studies. This time-simulation motor starting analysis also is beyond the scope of this article and is not discussed.
The network topology diagrammer presents a method of creating simplified one-line diagrams from branch records. It, too, is beyond the scope of this article.
The program’s demand-load analysis assembles loads from various end-use-load screens and, according to demand levels and other attributes of the demand-load library, presents system-load information. Figure 5 shows a portion of the study’s report.
The first entry is the load schedule for bus 2, T1PRI. The load is a “branch load” only; no “end-use loads” are connected to the bus. The connected load is 1,857.1 kVA and the demand load is 1,849.9 kVA, indicating certain demand factors were applied to the connected load. The design load is 2,108.3 kVA, indicating a continuous load factor (for lighting and/or the largest motor on the branch) was applied. Net power factor of the load is 81.7% lagging.
The next entry is the load schedule for bus 3, 5KV SWGR. The 900-hp synchronous motor, which is connected directly to the bus, is an end-use load. The load coming from bus 4, TR2PRI, is a branch load. The program carries through these different categories so demand and design factors may be applied appropriately.
The last entry shown is the load schedule for bus 4, 808V SWGR. In all schedules, load amperes for various load types (connected, demand, and design) are given at the bus voltage.
Feeder and transformer sizing
The next step is to size feeders and transformers by selecting from the studies menu. From the resulting presentation screen, the user selects the allowable percent voltage drop for each feeder. A default value of 2.5% is provided, or another value may be entered. Feeders for transformers may be sized for any percentage desired, depending on the National Electrical Code criteria or the design concept in question. The default value is 125%.
Options presented on this screen allow flexibility in the study. For example, if recalculating for any reason (load change, perhaps), the study may simply re-evaluate the results without changing them or updating the files.
Figure 6 shows a portion of the study report: the feeder schedule. Using values obtained by the design-load analysis program in conjunction with feeder and transformer criteria previously entered, the program calculates feeder sizes. All system feeders are shown on this schedule.
For example, from bus number 5, 480V SWGR two feeders to other buses exist; one to bus 6, MCC1, and one to bus 7, LTGDIST. The former consists of three sets of three single-conductor 400-kcmil, copper THWN-insulated conductors in three-in. metallic, rigid-steel conduit.
Figure 7 shows another portion of the sizing feeder and transformer report: the feeder design-load analysis. This section analyzes feeder selection so the user can judge the appropriateness of selected feeder size. Bearing in mind that feeder size is chosen to meet both ampacity and voltage-drop criteria, this report shows that, for example, the feeder from bus 5, 480V SWGR, to bus 6, MCC1, has a total design load of 977 amps while the feeder capacity is 1.005 amps. Should the user feel that other factors (future load growth, need for standardization on conductor sizes, spare capacity, etc.) make a change in feeder size desirable, he can now go back to the branch record input screens and change them. The final studies are then made with feeder sizes incorporating all known factors and necessary assumptions to fine tune the design.
Voltage drop and load flow
Figure 8 shows a load-flow criteria screen. This screen sets the stage for the load-flow criteria study. The program uses an iterative approach, so the final system analysis is a balanced solution—that is, the effect of a voltage drop in one portion of the system is balanced against the effects of voltage drop in all other portions. The user may select, however, an approximate solution where this balancing is not carried through. Except in very large systems, time saved by using the approximate method is not justified.
If the effect on the system of starting motors is required for the calculation, starting kVA may be input as a special bus load. The default study is without this modeling. If taps are specified on transformers and if voltage drop through transformers is of interest, the user may select their modeling. It sometimes is of interest to model the exact utility equivalent impedance. If “no” is selected for this option, a default impedance is used in calculations.
Solution criteria may be set up: per unit driving voltage (0.5 to 1.5) and voltages at bus and branch for which the load-flow report is flagged showing a discrepancy.
Figure 9 is an excerpt from the voltage-drop and load-flow study for the example system. Note that at load bus 5, 480V SWGR, design voltage is 480 V, while actual expected voltage is 442 V, a drop of 7.9%. The 7.9 is flagged with a “$” indicating that it does not meet the criteria of 5.0% requested in Figure 8. The user can compensate for this design flaw by changing feeder sizes or using transformer taps.
Actual power flow from bus 4, TR2PRI, is 730 kW, 541 kilovars, and 960 kVA at a power factor of 83% lagging. Losses through transformer T2 are 62.5 kVA. The voltage drop through the transformer is 21 V at a voltage drop of 4.28%.
This exceeds the criteria for branch voltage drops (4.0% requested in Figure 8). The discrepancy may be compensated for by selecting one 2.5% or one 5.0% primary transformer tap, keeping in mind the effect of no load or light load. Figure 10 shows the condition at bus 5 when the transformer is used.
The short-circuit currents program performs three-phase fault or unbalanced fault studies. Figure 11 is an excerpt from the unbalanced fault report.
This report shows that at bus 5, 480V SWGR, the three-phase fault duty is 18,775 amps at an X/R of seven, single-line-to-ground duty is 19,440 amps at an X/R of six, line-to-line fault duty is 16,260 amps and line-to-line-to-ground fault duty is 19,250 amps. Symmetrical component impedances of Z1, Z2, and Z0 are given, enabling calculation of other fault types.
Other information given is 25,554 momentary amps, 19,485 amps at three cycles, 19,441 amps at five cycles, and 19,440 amps at eight cycles. These latter values are of little interest at the 480-V bus voltage. They may, however, be of interest at buses 3, 5KV SWGR, and 4, TR2PRI, where the 5-kV protective equipment must be considered.