Unit 2 power quality esrmnotes.in, Lecture notes of Electrical and Electronics Engineering

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UNIT 2.VOLTAGE SAG

- Sources of sags

- Magnitude & duration of sag

- Effect of sag on computer and consumer

electronics

- Monitoring and mitigation of voltage sag.

S. No Disturbance Duration Cause 1 Voltage Sag Typically 0.5 to 30 cycles 1.Faults on power system 2.Starting of large loads (Motors) 2 Momentary interruptions Typically 2 to 5 s 1.Leads to complete loss of voltage as a result of action taken taken by utilities to clear transient faults on their systems. 2.Sustained interruptions of longer than 1 min are due to permanent faults.

Utility power System

• Voltage sag and interruption are caused by faults on utility system.
• If there is a fault on feeder- 1 , 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 cleared, interruption will be temporary.
• In fault on same or parallel feeder, the customer will experience a voltage sag during the period that the fault is actually on the system.
• As soon as breakers open to clear the fault, normal voltage will be restored at the customer.
• Any of these fault locations can cause equipment to misoperate

Momentary Interruption or Voltage sag

prior to fault clearing and the subsequent

two fast recloser operations.

Reclosing time 2s , 1 st^ operation tree branch not completely removed 2 nd^ operation system was restored

Estimating Voltage Sag Performance

It is important to understand the expected

supply system voltage sag performance is

to assure the optimum operation of

production facilities.

• Production facilities can be

• Design

• Equipment specifications

AREA OF VULNERABILITY (WEAKNESS, EXPOSURE)

• Find the probability of sensitivity equipment being subjected to voltage lower than its minimum voltage sag ride through capability.
• The minimum voltage magnitude a piece of equipment can withstand or tolerate without misoperation or failure. This is also know as equipment voltage sag immunity or susceptibility limit.
• The area of vulnerability describes all the fault locations that can cause equipment to misoperate.

AREA OF VULNERABILITY (WEAKNESS, EXPOSURE) - contd..,

• Area of vulnerability is developed to help Equipment voltage sag ride-through capability
• It is determined by the TOTAL CIRCUIT MILES OF EXPOSURE (contact) to faults that can cause voltage magnitudes at an end-user facility to drop below the equipment minimum voltage sag ride-through capability

• The loads will be subject to faults on both the transmission system and the distribution system.
• Number of voltage sags that a customer can expect is determined by combining the area of vulnerability with the expected fault performance. Equipment Sensitivity To Voltage Sags
• Equipment within an end-user facility may have different sensitivity to voltage sags. i.e

• Missing voltage
• Three-phase voltage unbalance during the sag event
• Point-in-the-wave at which the sag initiates and terminates 3.TYPES OF EQUIPMENT SENSITIVITY TO VOLTAGE SAGS Equipment sensitive to,
• Only the magnitude of voltage sag
• Both the magnitude and duration of voltage sag.
• Characteristics other than magnitude and duration.

Equipment sensitivity divided into three category 1 .Only the Magnitude of Voltage Sag

• Devices in this group are sensitive to the minimum voltage magnitude experienced during sag.
• Duration of the disturbance is secondary importance for these devices. - Under voltage relays - Process controls - Motor drive controls - Automated machines (e.g., semiconductor manufacturing equipment)

3. Equipment sensitive to characteristics other than magnitude and duration - Phase unbalance during the sag event - Point-in-the wave at which the sag is initiated - Any transient oscillations occurring during the disturbance

Equipment sensitivity contd..

• End user’s sensitive processes, voltage sag ride-through capability is the most important characteristics.
• Equipment susceptibility (vulnerability/weakness) to voltage sags measured by Magnitude- duration plot - Voltage sag magnitude that will cause equipment to misoperate as a function of the sag duration Magnitude-duration plot for Voltage sag