System Protection Technology

The objective of this module is to present the concepts on which fault calculations are made. Both balanced and non-balanced faults are discussed, including phase-to-phase and phase-to-ground faults. The use of “percent impedance” is shown and the use of symmetrical components is discussed.

After completion of this videotape and associated workbook, participants will understand the following concepts and be able to apply them to their day-to-day activities. They will also be able to answer related questions on these subjects.

• Fault characteristics (introduced in module SPT 4)
• Phasor diagrams (introduced in module SPT 3)
• Classification of faults
• Characteristics of the 3-phase balanced fault
• Impedance between source and fault
• Calculation of fault current (single phase)
• Per unit (percent) impedance, base MVA
• Conversion of per unit values to a different base MVA
• Calculation of fault MVA at different locations
• Calculation of fault current
• Conversion between OHMIC and PERCENT impedance
• Vectorial addition of impedances; the “j” operator
• Equivalent (substitute) circuits to aid analysis
• Symmetrical components; positive, negative and zero sequence quantities
• System phase rotation
• Characteristics of positive, negative and zero sequence components
• Phase-to-phase faults; study of sequence components
• Addition of sequence components to find fault current
• Single line-to-ground faults, study of sequence components
• Magnitude of sequence components
• Vectorial addition of sequence components; operator “a”
• Positive, negative and zero sequence impedances
• Examples of fault calculations using sequence impedances
• Relay settings, significance of relay location
• CT ratio, VT ratio
• Measuring zero sequence current