UNIT – 2: VOLTAGE SAGS AND INTERRUPTIONS TWO MARKS

UNIT – 2: VOLTAGE SAGS AND INTERRUPTIONS

TWO MARKS

1.      What is voltage sag?

                  A sag or dip is a decrease in RMS voltage or current at the power frequency for durations from 0.5 cycles to 1 minute, reported as the remaining voltage. Typical values are between 0.1 pu and 0.9 pu.

2.      When sag leads to interruption.

                  Voltage sag is a reduction in voltage for a short time. The voltage reduction magnitude is between 10 % to 90%  of  the normal root mean square (RMS) voltage at 50 Hz. An interruption is a complete loss of voltage or a drop to less than 10 % of nominal voltage in one or more phases.

3.      What are the causes of sag?

·      Voltage sags are usually associated with voltage sag.

·      Equipment sensitive to both the magnitude and duration of voltage sag.

·      Equipment sensitive to have characteristics other than magnitude and duration.

4.         What are the three levels of possible solutions to voltage sag and momentary interruption problems?

·         Power System Design

·         Equipment design

·         Power conditioning equipment.

5.      List some industry standards associated with voltage sags.

                  *SEMI F47-0200        8CBEMA curve

6.      What are the sources of sags and interruption?

·            A sudden increase in load results in a corresponding sudden drop in voltage.

·            Any sudden increase in load, if large enough will cause a voltage sag in motors, faults, switching.

·               Recloser operation.

7.      Give some economic impacts due to sag.

·               Process outrages

·               Damaged products

·               Lost time for restarting.

8.      What is the importance of estimating sag performance?

                  It is important to understand the expected voltage sag performance of the supply system so that facilities can be designed and equipment specifications developed to assure the optimum operation of production facilities.

9.      What are the various factors affecting the sag magnitude due to faults at a certain point in the system.

·               Distance to the fault

·               Fault impedance

·               Type of fault

·               Pre-sag voltage level

·               System configuration

·               System impedance

·               Transformer connections.

10.  Name the different motor starting methods.

                  Resistance and reactance starters

                  Autotransformer starters

                  Star-Delta starters

11.  What are the causes for voltage sags due to transformer energizing?

·               Normal system operation, which includes manual energizing of a transformer.

·               Reclosing actions.

12.  How voltage sag can be mitigated.

                  Voltage sag can be mitigated by voltage and power injections into the distribution system using power electronics based devices which are also known as custom power devices.

13.  Name the three levels of possible solutions to voltage sag and momentary interruption problems.

·               Equipment Design                  * Power conditioning equipment

·               Power system design 

14.  Name any four types of sag mitigation devices.

·               Dynamic Voltage Restorer(DVR)

·               Active Series Compensators

·               Distribution Static Compensator(DSTATCOM)

·               Solid State Transfer Switches(SSTS)

15.  Define Dynamic Voltage Restorer (DVR).

                  A DVR is a solid state power electronics switching device consisting of either GTO or IGBT , a capacitor bank as an energy storage device and injection transformers. It is connected in series between a distributed system and a load.

16.  What is the important role of a DVR?

                  The basic idea of a DVR is to inject a controlled voltage generated generated by a forced commuted converter in series to the bus voltage by means of an injecting transformer.

17.  Define active series compensation devices.

                  A device that can boost the voltage by injecting a voltage in series with the remaining voltage during a voltage sag condition.

18.  What is the need of DSTATCOM?

                  It allows effective control of active and reactive power exchanges between the DSTATCOM  and the ac system.

19.  What is the main function of DSTATCOM?

·            Voltage regulation and compensation of reactive power

·            Correction of power factor

·            Elimination of current harmonics.

20.  What is the role of SSTS?

·               Can be used very effectively to protect sensitive loads against voltage sags, swells and other electrical disturbance.

·               It ensures continuous high quality power supply to sensitive loads by transferring , within a time of milliseconds , the load from a faulted bus to a healthy one.

PART – B

1.      Discuss the sources of sags and interruption.

2.      Discuss in detail about the sag performance evaluation indices.

3.      Explain the sag performance evaluation methods.

4.      Explain the various causes and effects of voltage sags.

5.      What are the different voltage sag mitigation techniques? Explain in detail.

6.      Explain the principle of DVR operation used for sag mitigation.

7.      Discuss in detail about the active series compensator.

8.      Explain the solid state transfer switch with the transfer operation.

9.      Explain the system adapted to estimate the severity of the sag occurred due to various sources.

10.  Mention the standards associated with the voltage sag.