Notes

Fault Calculations - Typical Equipment Parameters 

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A frequent problem in fault calculations is the obtaining of equipment parameters. While it is always preferable to use the actual parameters of the equipment, sometimes these are not available. In this instance it is necessary to resort to the use of typical parameters.

This note is intended to become a collection of typical parameters. If there is something missing or needs adding, please leave a comment in the discussion.

HV Network Impedance

Impedance of HV network referred to LV side of transformer

Psc Uo (V) Ra (mΩ) Xa (mΩ)
250 MVA 420 0.106 0.71
500 MVA 420 0.053 0.353

Low Voltage Power Transformers

Transformer Short Circuit Impedance

Typical values short circuit impedance, USC for different kVA ratings (HV winding < 20 kV)

  Type of Transformer
transformer rating oil-immersed cast-resin
50 to 630 4% 6%
800 to 2500 6% 6%

Transformer Impedance (400 V)

Resistance, reactance and impedance for typical transformers (HV winding <20 kV)

transformer rated power (kVA) 50 100 160 250 315 400 500 630 800 1000 1250 1600 2000 2500
oil-immersed Usc % 4 4 4 4 4 4 4 4 6 6 6 6 6 6
  Rtr mΩ 95.3 37.9 16.2 9.2 6.9 5.1 3.9 2.9 2.9 2.3 1.8 1.4 1.1 0.9
  Xtr mΩ 104.1 59.5 41.0 26.7 21.3 16.9 13.6 10.8 12.9 10.3 8.3 6.5 5.2 4.1
  Ztr mΩ 141.1 70.5 44.1 28.2 22.4 17.7 14.1 11.2 13.2 10.6 8.5 6.6 5.3 4.2
cast-resin Usc % 6 6 6 6 6 6 6 6 6 6 6 6 6 6
  Rtr mΩ   33.5 18.6 10.7 8.2 6.1 4.6 3.5 2.6 1.9 1.5 1.1 0.8 0.6
  Xtr mΩ   100.4 63.5 41.0 32.6 25.8 20.7 16.4 13.0 10.4 8.3 6.5 5.2 4.2
  Ztr mΩ   105.8 66.2 42.4 33.6 26.5 21.2 16.8 13.3 10.6 8.4 6.6 5.3 4.2

 

Synchronous Machines

Performance Under Fault Conditions

Sub-transient emf:  myElectrical Equation

Transient emf:  myElectrical Equation 

Synchronous emf:  myElectrical Equation
Fault current: myElectrical Equation

Typical Machine Reactances

Typical Machine Reactances (& Time Constants

  Cylindrical Rotor
Turbine Generators
Salient Pole Generators
  Salient Pole
Synchronous
Condensers
Conventional 		Direct
Cooled
4 Pole
Multi-Pole
Synchronous
Direct axis Xd pu 1.6-2.0 0.8-1.0 2.0-2.3 2.1-2.4 1.3-2.1 1.3-1.5
Quadrature axis Xq pu 1.0-1.25 0.5-0.65 1.95-2.1 1.95-2.25 0.6-1.2 0.8-1.0
Transient
Reactance  Xd' pu 0.3-0.5 0.2-0.35 0.18-0.25 0.27-0.30 0.15-0.35 0.4-0.5
Short circuit τd' s 1.5-2.5 1.0-2.0 0.75-1.0 0.75-1.0 0.8-1.2 0.8-1.2
Open circuit τdo' s 5-10 3-7 4-8 6-9.5 4-8 3-7
Sub-transient
Direct axis Xd'' pu 0.2-0.4 0.12-0.25 0.11-0.13 0.19-0.23 0.1-0.25 0.2-0.35
Quadrature axis Xq'' pu 0.25-0.6 0.15-0.45 0.11-0.13 0.19-0.23 0.14-0.35 0.2-0.35
Short circuit - direct axis τd' s 0.04-0.09 0.05-0.10 0.015-0.025 0.02-0.03 0.02-0.04 0.02-0.07
Open circuit - direct axis τd'' s 0.07-0.11 0.08-0.25 0.015-0.025 0.02-0.03 0.04-0.07 0.02-0.07
Short circuit - quadrature axis τq'' s 0.04-0.6 0.05-0.6 0.015-0.025 0.02-0.03 0.10-0.15 0.02-0.07
Open circuit - quadrature axis τq0'' s 0.1-1.2 0.2-0.9 0.015-0.025 0.02-0.03 0.3-0.7 0.1-0.2
Other Reactances
Negative sequence X-ve pu 0.25-0.5 0.14-0.35 0.11-0.13 0.19-0.23 0.12-0.3 0.2-0.35
Zero sequence  Xo pu 0.12-0.16 0.06-0.10 0.05-0.075 0.11-0.16 0.03-0.10 0.1-0.2

Cables

myElectrical Equation
myElectrical Equation

where:

d = distance between centre
r = geometric mean radius
a = cross sectional area (mm)

TypicalEquipmentCables

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Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

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