Unit 02 – Voltage Sag
Syllabus: Sources of Voltage Sag: Motor starting, ARC Furnace, Fault Clearing, etc.
Establishing voltage sag performance and principle of its protection.
Solution at End user level, Isolation Transformer, Voltage Regulator, Static UPS, Rotary UPS, Active Series Compensator.
Voltage Sag and its sources:
- Voltage sag is the decrease in the rms voltage between 0.1 to 0.9pu at the power frequency for duration from 0.5cycle to 1minute.
- They are generally caused by system faults, energizing of heavy load, starting of large motor, etc.
- 46% of the voltage sag occurs due to fault on parallel circuit.
- 31% of the voltage sag occurs due to fault on transmission system.
- 23% of the voltage sag occurs due to fault on owner circuit.
- Consider a customer that is supplied from the feeder supplied by circuit breaker 1 on the diagram:
- If there is a fault on the same feeder, the customer will experience a voltage sag during the fault followed by an interruption when the breaker opens to clear the fault.
- If the fault is temporary in nature, a reclosing operation on the breaker should be successful and the interruption will only be temporary.
- Breaker usually requires about 5 to 6 cycle to operate and during this time, voltage sag occurs.
- Sensitivity equipment will surely trip during the interruption.
- During a fault on a parallel feeder or a fault on transmission system, the customer will experience voltage sag during the period that the fault is actually on the system.
- As soon as breaker open to clear the fault or close after clearing fault, normal voltage will be restored at customer.
Equipment Sensitive to Voltage Sag:
a. Equipment sensitive to only the magnitude of the voltage sag:
- These devices which are sensitive to the minimum voltage magnitude during sag.
- Ex: under-voltage relay, process controls, motor drive control, etc.
b. Equipment sensitive to both the magnitude and duration of voltage sag:
- This group includes all equipment that uses electronic power supplies.
- Such equipment miss-operates or fails when the power supply output voltage drops below specified values.
c. Equipment sensitive to characteristics other than magnitude and duration:
- Some devices are affected by other sag characteristics such as the phase unbalance during the sag event, the point-in-the wave at which the sag is initiated, or any transient oscillations occurring during the disturbance.
Motor Starting Sag:
- Motor has undesirable effect of drawing several times their full load current while starting.
- This large current will flow through system impedances causes a voltage sag.
- This voltage sag may dim lights, cause damage to controllers and disturb sensitive equipment.
- This condition is made worse, followed by an extremely poor starting, displacement factor usually in range of 15 to 30%.
- The time required for the motor to accelerate to rated speed increases with the magnitude of sag.
- Excessive sag may prevent the motor from starting successfully.
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