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Voltage drop calculations

Voltage drop calculations

(OP)
Hello everybody, i'm trying to estimate voltage at terminals of several motors based on estimations of voltage drop from motor control center. what assumptions can be made about conductor parameters? (e.g. when inductance and capacitance can be neglected) In some instances there are several circuits in a duct. Should mutual effects between them be considered?

More info:

Nominal voltage of feeders are 480 V and 4,16 kV, some feeders have a length of 200 meters or more (for Americans 1ft = 0,3048 m). Type of conductors are THHN, THW and XLPE. By the way, can i find a good catalogue on conductors parameters in the web?(i'm not happy with those included in some handbooks)

Thanks for your comments

RE: Voltage drop calculations


A good review is §3.11 in the IEEE Red Book.  Motor-starting and –running voltage drops are usually both considered.  Manufacturer’s impedance data for various configurations is available for the asking.
  

RE: Voltage drop calculations

Suggestion: Motor voltage drop during locked-rotor condition to evaluate whether the motor will start or stall, is included in:
IEEE Std 399-1997 IEEE Recommended Practice for Industrial and Commercial Power System Analysis,
Section 9.6.1 The Mathematical Relationships
on page 242
Basically, there are three methods that can be used to solve bus voltages, namely:
1. Impedance Method
2. Current Method
3. Load Flow Solution Method

RE: Voltage drop calculations

(OP)
Thanks jbartos, there is good practical information in the web site you recommended. I'm sure it will be helpful for me.

Busbar
Thanks for your comment, could you please explain the statement "Manufacturer’s impedance data for various configurations is available for the asking"?

IEEE Standars you recommended (busbar and jbartos) are not readily available for me, but i'll try tyo get them.

RE: Voltage drop calculations


IEEE standards are easily available—get out your credit card.
  

RE: Voltage drop calculations

The Cutler-Hammer Consulting Application Catalog has a simple voltage drop table that can be used for an approximation.  I believe this entire catalog is available in .pdf form at the Cutler-Hammer website.  

RE: Voltage drop calculations

(OP)
Up to know all approaches i've seen don't take into account capacitive effects, maybe because of voltage level. Can anyone explain if capacitive effects should be included(or not)in calculations of voltage drop in 4,16 kV feeders? I think the thickness of insulation at this voltage level can have an impact.

RE: Voltage drop calculations

1 - Cable capacitance
2 - Motor Capacitance
3 - Surge Capacitor Capacitance
4 - Power factor capacitance

1- Cable capacitance can be found from physics of cylinder within cylinder.
Substituting in unit conversions gives the following formula from Okonite:
C= 7 * 35*S1C / LOG(D/d)
Where
C = Cap in picoFarads per foot
S1C = dielectric constant from 2 to 8 (use higher value for conservatism).  PVC: 3.5-8.0  EP - 2.8-3.5 Polyethelene - 2.3 XLPE - 2.3-6.0
D = Outer Diameter in inches
d = inner diameter in inches(usually conductor diameter).

Our 4kv cables range from 0.15 to 0.122 microfarads per 1000 foot per phase.

2 - Motor capacitance to ground can be determined during Doble test.  Westinghouse provided us some typical values ranging from 0.037 microfarads for 400hp 2-pole to 0.149 microfarads for 1500hp 6-pole motor (4kv motors).

3 - Surge cap typically 1 microfarad or less for 13.8kv and below motors.

4 - Power factor correction caps may be more.

Evaluate how this affects your system.

Consider effect of one microfarad to ground on 4kv system (2.4kv to ground).
Xc =1 / (2PifC) = 1 / (2*Pi*60*C) = 1 / 377C
VARS = V^2 / Xc= V^2 * 377*C
2400^2*377*1E-6 ~ 2kvar.

I think you will find #1, 2, 3 are usually not significant for typical plant power systems but you should check for yourself.


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RE: Voltage drop calculations

Needless to say, the 3-phase kvar is 3x the 1-phase kvar calculated above.

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RE: Voltage drop calculations

(OP)
Pete, thanks for your contributions. With respect to your comments i have two questions:

Is the value of capacitance calculated by the above formula per phase capacitance (taking into account the effects of other phases) or single conductor capacitance?

It seems that the expresion doesn't take into account ground effects. Usually a major portion of total feeder length is underground.This condition inevitably increase capacitive effects, what's your opinion?  

RE: Voltage drop calculations

(OP)
jbartos:

The web site you suggested
http://www.okonite.com/engineering/voltage-regulation.html
shows a calculation for a 4/0 AWG cable. They take OD as 0,81 in.(I assume OD = outside diameter 0,810 inches or 20,574 mm)
I have seen in many catalogs and NEC tables a column that specify conductor approximate or nominal outer diameter for 4/0 AWG in the order of 0,68 in. or 17,3 mm. The question is, Does this diameter that appears in NEC tables and catalogs takes into account insulation and jacket thickness?
A diameter of 0,81(OD) seems to be equal to 0,68(Diameter from tables for THW) + 0,08 (insulation) + 0,045(jacket).

RE: Voltage drop calculations

Comment on the previous posting: I have an older issue of NEC, NFPA 70-1996. Table 5 Dimensions of Insulated Conductors and Fixture Wires on page 70-884 indicates an approximate diameter of 4/0 to be .688 in inches and approximate square inch area of .3718. It clearly states that the conductor is insulated. However, there is no reference to the conductor jacket.
Also, the conductor Class Stranding, e.g. B, C, etc. needs to be considered as well as conductor having the concentric stranding, compact round or compressed diameter. Namely, 4/0 dimensions:
.475 inch   Compact Diameter
.512 inch   Compressed Diameter
.528 inch   Class B Stranding (19 conductors, 105.5mil)
.529 inch   Class C Stranding (37 conductors, 75.6mil)
Reference: NEMA WC 7-1982   
 

RE: Voltage drop calculations

For voltage drop calculations the NEC or NFPA 70 is not concerned with insulation type or thickness, if you refer to chapter nine, table eight in the back of 2002 NEC it will list conductor properties for voltage drop calculations
and ohms per one thousand feet of resistance for each conductor, 4/0 copper is listed as .528 in. diameter and 211600 circular mills, a simple formula for voltage drop is 22 x wire length in feet x current in amps devided by circular mills of conductor, remember to double the wire length on single phase circuits, much more exact formulars are out there but off the top of my head I do not remember them. Tarzan

RE: Voltage drop calculations

Suggestions to the previous posting marked ///\\\
tarzan56007 (Electrical) May 16, 2004
For voltage drop calculations the NEC or NFPA 70 is not concerned with insulation type or thickness, if you refer to chapter nine, table eight in the back of 2002 NEC it will list conductor properties for voltage drop calculations
and ohms per one thousand feet of resistance for each conductor, 4/0 copper is listed as .528 in. diameter
///The .528 inch Class B Stranding (19 conductors, 105.5mil), copper.\\\
 and 211600 circular mills, a simple formula for voltage drop is 22 x wire length in feet x current in amps devided by circular mills of conductor,
///This formula is for DC resistance and associated voltage drop only. If AC is considered, the voltage drop across Zcable=Rcable+jXcable should be applied.
X=2xpixfxL. The cable Xc=1/(2xpixfxC) should be also considered for high voltage cables.\\\
The cable or conductor inductance depends on a type of conduit, number of conductors, power factor, etc.\\\
 remember to double the wire length on single phase circuits, much more exact formulars are out there but off the top of my head I do not remember them. Tarzan
///E.g. see Reference: IEEE Std 141-1993 Red Book.\\\

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