Overcurrent relays are the most common form of protection used to operate only under fault conditions. They should not be installed purely as a means of protecting systems against overloads. The relay settings that are selected are often a compromise in order to cope with both overload and overcurrent conditions. Overcurrent relays can be classified as definite current, definite time, and inverse time as shown in Figure 9.1(a-c). The time delay units can work in conjunction with the instantaneous units as shown in Figure 9.1(d).

Chapter Contents:

• 9.1 Fundamentals of overcurrent protection
• 9.1.1 Protection coordination principles
• 9.1.2 Criteria for setting instantaneous units
• 9.1.3 Setting time-delay relays
• 9.1.4 Setting overcurrent relays using software techniques
• 9.2 Coordination across Dy transformers
• 9.3 Protection equipment installed along the feeders
• 9.3.1 Reclosers
• 9.3.2 Sectionalizers
• 9.3.3 Fuses
• 9.4 Setting criteria
• 9.4.1 Fuse-fuse coordination
• 9.4.2 Recloser-fuse coordination
• 9.4.3 Recloser-sectionalizer coordination
• 9.4.4 Recloser-sectionalizer-fuse coordination
• 9.4.5 Recloser-recloser coordination
• 9.4.6 Recloser-relay coordination
• 9.5 Protection considerations when distributed generation is available
• 9.5.1 Short circuit levels
• 9.5.2 Synchronization
• 9.5.3 Overcurrent protection
• 9.5.4 Adaptive protection
• Proposed exercise

Inspec keywords: fault diagnosis; relay protection; overcurrent protection; power distribution protection

Other keywords: instantaneous units; overload conditions; time delay units; definite time; relay settings; fault conditions; modern protection; overcurrent relays; inverse time; distribution systems; definite current

Subjects: Power system protection; Distribution networks