Describe Common Types of Protective Relays Used In HV Systems

Question 1

What is the purpose of a protective relay?

Answer 1

To monitor circuit or equipment conditions and to initiate an alarm/trip if the conditions warrant it

Question 2

What is the most common HV relay?

Answer 2

The inverse-time overcurrent relay

Question 3

What are four common electrical quantities monitored by protective relays?

Answer 3

• Current
• Voltage
• Power
• Frequency

Question 4

What is the most common method of achieving time delay in an electromechanical relay?

Answer 4

Using a shaded-pole/induction-disk relay

Question 5

What are three non-electrical quantities that frequently need monitoring in electrical HV systems?

Answer 5

• Temperature
• Pressure
• Vibration

Question 6

What type of relay is becoming more common in protective systems?

Answer 6

Solid-state relay

Question 7

What is meant by the term “inverse time” with respect to the overcurrent relay?

Answer 7

Trip time decreases as current increases

Question 8

What causes the aluminium disk to turn and tend to close the contacts in the induction-type overcurrent relay? What causes the restraining torque?

Answer 8

Current flow in the coil of the relay coming from the associated CT. A spring causes the restraining torque.

Question 9

The time taken for the timed contacts to close depends on what three factors?

Answer 9

• CT current
• Travel distance to the disk
• Relay coil turns

Question 10

What is the purpose of using differential protection?

Answer 10

To protect equipment against ground faults and inter-phase faults

Question 11

Identify some positive features of solid-state relays

Answer 11

• Compactness
• Accuracy
• Memory
• Reliability

Question 12

What devices do the ANSI designations 50, 51 and 52 identify?

Answer 12

50 - Instantaneous overcurrent relay
51 - AC time-trip overcurrent relay
52 - AC circuit breaker