What is the key difference between a directional and a non-directional relay?
A
Directional relays operate only on voltage
B
Non-directional relays require phase angle measurement
C
Directional relays consider the direction of current flow
D
Non-directional relays detect faults based on power factor
Analysis & Theory
Directional relays operate based on the direction of power flow or fault current, making them suitable for complex networks.
Which type of relay is suitable for radial distribution systems?
Analysis & Theory
In radial systems, power flows in one direction, so non-directional relays are sufficient and economical.
In a directional relay, which two quantities are compared to determine the direction of fault current?
D
Impedance and power factor
Analysis & Theory
Directional relays compare the phase angle between voltage and current to detect the direction of the power flow or fault.
Where are directional relays mainly used?
C
Ring and interconnected systems
Analysis & Theory
In ring and mesh systems, the power flow direction can change, so directional relays are essential to isolate the correct faulted section.
Which relay does NOT respond to the direction of power flow?
A
Directional overcurrent relay
C
Non-directional overcurrent relay
D
Directional impedance relay
Analysis & Theory
Non-directional relays respond only to the magnitude of the current, not its direction.
In directional relays, the torque is produced based on the angle between:
C
Voltage and current vectors
Analysis & Theory
The torque in directional relays depends on the phase angle between voltage and current vectors.
Which relay typically includes a polarizing voltage coil?
Analysis & Theory
Directional relays often use a polarizing voltage coil to determine the correct fault direction.
Directional overcurrent relays are particularly important in:
C
Interconnected power systems with bidirectional flow
D
Low-voltage systems only
Analysis & Theory
In interconnected systems, fault current may flow from multiple sources, requiring directional relays to locate the exact fault direction.
Which of the following is true about non-directional overcurrent relays?
A
They are sensitive to the direction of fault current
B
They need a voltage input to operate
C
They operate when current exceeds a set value regardless of direction
D
They compare power factor angle
Analysis & Theory
Non-directional relays trip if the current exceeds a threshold, without checking the direction of flow.
Why are directional relays not commonly used in simple radial feeders?
A
They are less sensitive
B
They require high voltage
C
They are more complex and not needed where current flows in one direction only
D
They are slower than non-directional relays
Analysis & Theory
Radial feeders have unidirectional current flow, making simple, cost-effective non-directional relays sufficient.