# Mastering Differential Protection

Hi,

What you’ll learn

• Basic Definition of differential relay.
• Working principle of differential relay.
• CT polarity, why it is important.
• Current notation methodology, dot & cross.
• CT compared to the current source.
• Differential relay in the zone and out of zone fault.
• Through fault.
• Single Source feed differential protection scheme.
• Double Source feed differential protection scheme.
• Considering  internal and external fault on double feed system.
• Pick Up value of a differential relay.
• Max stabilizing current.
• Stabilizing factor.
• Through fault stability.
• The actual behavior of CT.
• Ratio and phase error of CT.
• Biased Differential protection.
• Circulating current & spill current.
• Slope of differential Relay.
• Restaining and operating coils.
• Restaining and operating current.
• Comparison of stability between simple and biased differential relay.
• Variable pickup value for biased differential protection.
• Calculation of slope for internal fault.
• Calculation of slope for external fault.
• The operating point of the relay.
• Single Phase Earth Leakage Relay Three Phase Earth Leakage Relay.
• Three Phase Earth Leakage Relay.

Course Content

• Introduction –> 1 lecture • 18min.
• CT Polarity Dot Notation –> 1 lecture • 16min.
• Differential protection working principle –> 1 lecture • 11min.
• Simplified Differential Protection In Zone and Out of Zone faults –> 1 lecture • 5min.
• Differential Protection feed from two ends –> 1 lecture • 5min.
• Actual behavior of CT –> 1 lecture • 12min.
• Through Fault stability and stability ratio –> 1 lecture • 13min.
• Percentage or Biased Differential Protection –> 1 lecture • 35min.
• Single Phase Earth Leakage Differential Protection –> 1 lecture • 6min.
• Three Phase Earth Leakage Protection –> 1 lecture • 6min.

Requirements

• Basics of electrical engineering.

Hi,

The relays used in power system protection are of different types. Among them, differential relay is a very commonly used relay for protecting transformers and generators from localized faults.

The differential relay is one that operates when there is a difference between two or more similar electrical quantities exceeds a predetermined value. In the differential relay scheme circuit, there are two currents come from two parts of an electrical power circuit. These two currents meet at a junction point where a relay coil is connected. According to Kirchhoff Current Law, the resultant current flowing through the relay coil is nothing but the summation of two currents, coming from two different parts of the electrical power circuit. If the polarity and amplitude of both the currents are so adjusted that the phasor sum of these two currents, is zero at normal operating condition. Thereby there will be no current flowing through the relay coil at normal operating conditions. But due to any abnormality in the power circuit, if this balance is broken, that means the phasor sum of these two currents no longer remains zero and there will be non-zero current flowing through the relay coil thereby relay being operated.

I have covered the following topics in this course,

• Basic Definition of differential relay
• Working principle of differential relay
• CT polarity, why it is important
• Current notation methodology, dot & cross
• CT compared to the current source
• Differential relay in the zone and out of zone fault
• Through fault
• Single Source feed differential protection scheme
• Double Source feed differential protection scheme
• Considering  internal and external fault on the double feed system
• Pick Up  value of a differential relay
• Max stabilizing current
• Stabilizing factor
• Through fault stability
• The actual behavior of CT
• Ratio and phase error of CT
• Biased Differential protection
• Circulating current & spill current
• Slope of differential Relay
• Restaining and operating coils
• Restaining and operating current
• Comparison of stability between simple and biased differential relay
• Variable pickup value for biased differential protection
• Calculation of slope for internal fault
• Calculation of slope for external fault
• The operating point of the relay
• Single Phase Earth Leakage Relay
• Three Phase Earth Leakage Relay

Thanks