Distribution System Training

The objective of this module is to demonstrate the principle of differential protection, and examine its application to protection of such equipment as generators, transformers, buses, and power lines.

On completion of this module, the participant should be able to understand the following concepts and apply them in day-to-day operation.

• The differential principle as applied to simple bus protection
• Significance of CT (Current Transformer) ratios
• Circulation of CT secondary current, including flow through the differential relay operating coil
• Differential operation under external fault conditions
• Differential operation under internal fault conditions
• Generator differential protection
• Transformer differential protection
• The flow of error current due to tap changing and discrepancies in CT secondary equipment
• Misoperation during through fault conditions due to error current
• The function of restraint coils in counteracting misoperation
• Differential relay misoperation due to transformer inrush current
• The function of the harmonic restraint unit
• Line differential protection principle
• Circulating current pilot wire differential protection
• Opposed voltage pilot protection
• The effect of pilot wire damage (open circuit or short circuit)
• Monitoring the integrity of the pilot wire circuit
• Pilot wire induced voltage resulting from power line fault current flow
• The effect of induced voltage in the pilot wires
• Discharge of induced voltage through mutual drainage reactors
• Station ground-mat potential rise under power line ground fault conditions
• Affect of potential difference between the substation ground mat and the local grounding of drainage reactors and pilot cable shield
• The function of neutralizing transformers