Interview Questions on Transformer Protection

• Q. What parameters or technical particulars are important to be considered while designing protection scheme of the transformer? Answer:

1. Network Diagram showing the position of the transformer in the system
   
2. kVA or MVA rating of the transformer
3. Fault Level at the transformer
4. Voltage Ratio
5. Winding Connections
6. Per Unit Impedance
7. Neutral Point Earthing Resistance
8. Value of the System Earthing Resistance
9. Whether Indoor or Outdoor
10. Dry or Oil Filled
11. Length and area of cross section of the connecting leads between CTs and Relay Panel

• Q. What can be the reasons for the failure of the transformer? Answer: Transformer is one of the most expensive equipments in the Power System. Failure of the transformer can cause outage of power supply for even days. However, multiple things can go wrong with the transformer: There can be two types of failure - Internal Faults and External Faults.
• Internal Faults (the faults within the transformer):
  

1. Core Failure
2. Winding Failure
3. Lamination Failure
4. Bushing Failure
   
5. Terminal Board Failure
6. Tap Changer Failure (OLTC failures)
   
7. Transformer Oil
8. Oil level low
9. Moisture absorption
10. Transformer cooling system
11. Failure of insulation between Lamination and Core Bolt
12. Badly made joints and connections.
13. Frequent exposure to Lightning strokes

• External Faults (the faults outside the transformer):

1. OverLoad ... increases copper losses and consequence temperature rise.
   
2. Through Faults or System Faults ... not detected by differential protection of transformer, however, if through faults persist for longer duration, transformer gets damaged by thermal and electromagnetic mechanical stress ... detected by overcurrent relays.
   
3. Short Circuits
   
4. Open Circuits
5. Earth Faults
6. Over Voltage
7. Reduced system frequency
   

• Q. How Transformer Protection provided for internal faults? Answer:

1. Bucholz Relay (1st Stage Alarm and 2nd Stage Trip)
2. Sudden Pressure Relay (1st Stage Alarm and 2nd Stage Trip)
3. Pressure Relief Valve (1st Stage Alarm and 2nd Stage Trip)
4. Pressure Switch (1st Stage Alarm and 2nd Stage Trip)
5. Oil level Gauge Indicator (Alarm)
   
6. Vacuum Level Indicator (Alarm)
7. Oil Temperature Indicator (Alarm)
   
8. Gas Temperature Indicator (Alarm)
   
9. Silicon Breather (Passive - no alarm)
   
10. Smoke Detector (Alarm)
    

• Q. How Transformer Protection provided for external faults? Answer:

1. Differential Protection
2. Restriced EarthFault Protection
3. Over Current Protection
4. Over Fluxing Protection
5. Over current relays with under voltage blocking
6. Zero sequence protection.
7. Negative phase sequence protection
   

• Q. What are the tests done to minimize internal faults? Answer:

1. Dissolved Gas Analysis (DGA): Faults due to arcing, high current breakdown of oil, localized sparking, partial discharge of oil, localized overheating or hot spots, etc. causes thermal degradation of oil and used for analysis.
2. Thermography of Transformer: Hot Spots on the transformer bushing, circulation of oil in radiator tubes, distribution of heat on transformer body, etc.
3. Capacitance Tan Delta: to detect oil degradation.
4. Oil Sample Test: to check the oil quality.

• Q. What are the reasons for Oil degradation? Answer:
  

1. Degradation
2. Oxidation
3. Contamination

4. a) Dielectric breakdown voltage indicates the presence of electrically conductive contaminants in oil.

   b) Interfacial tension and acid number (sometimes called neutralization number or acidity) are affected by oxidation and contamination.

   c) Water content is temperature dependent.

   d) Power factor is also temperature dependent.

• Q. What protection are provided to transformer depending on their kVA or MVA ratings?

• Protection	Transformer Rated below 500 kVA	Transformer Rated between 500 kVA - 5MVA	Transformer Rated above 500 kVA
  Differential	No	No	Yes
  Restricted Earth Fault	No	No	Yes
  Back Up Over Current	No	Yes	Yes
  Back Up Earth Fault	No	Yes	Yes
  Over Fluxing	No	No	Yes
  Fuses	Yes	No	No
  Bucholz, PRV, OSR	No	Yes	Yes
  Oil and Winding Temperature Indicator	No	Yes	Yes
  Oil level monitoring equipment	No	Yes	Yes

• References:

1. Basler: Transformer Protection Application Guide http://www.basler.com/html/html/transformer.htm
2. IEEE Guide for Protective Relay Applications to Power Transformers (IEEE Std C37.91-2000)
3. Dissolved Gas Analysis Wiki http://en.wikipedia.org/wiki/Dissolved_gas_analysis
4. Thermography of transformer. http://us.fluke.com/usen/Solutions/TI/Thermal-Imaging-Electrical-Systems.htm
5. Implementation of Thermography in Power utilities www.ndt.net/article/afndt2008/papers/tahir.pdf (good material on what is thermography and its basic concept).
6. Capacitance Tan Delta test http://cr4.globalspec.com/thread/32684#comment341158
   

More commands on -AVD to create, delete, move, update...

Usage:

=====================================================
android [global options] action [action options]
=====================================================
Global options:
-h --help This help.
-s --silent Silent mode: only errors are printed out.
-v --verbose Verbose mode: errors, warnings and informational messages are printed. =====================================================
Valid actions are composed of a verb and an optional direct object:
================================================

- list : Lists existing targets or virtual devices.
Action "list ":
Lists existing targets or virtual devices.
Options:
No options

=====================================================
- list avd : Lists existing Android Virtual Devices.
Action "list avd":
Lists existing Android Virtual Devices.
Options:
No options

=====================================================
- list target : Lists existing targets.
Action "list target":
Lists existing targets.
Options:
No options

=====================================================
- create avd : Creates a new Android Virtual Device.
Action "create avd":
Creates a new Android Virtual Device.
Options:
-t --target Target id of the new AVD [required]
-c --sdcard Path to a shared SD card image, or size of a new sdcard for the
new AVD
-p --path Location path of the directory where the new AVD will be created

-n --name Name of the new AVD [required]
-f --force Force creation (override an existing AVD)
-s --skin Skin of the new AVD ====================================================
- move avd : Moves or renames an Android Virtual Device.

Action "move avd":
Moves or renames an Android Virtual Device.
Options:
-p --path New location path of the directory where to move the AVD
-n --name Name of the AVD to move or rename [required]
-r --rename New name of the AVD to rename
====================================================
- delete avd : Deletes an Android Virtual Device.
Action "delete avd":
Deletes an Android Virtual Device.
Options:
-n --name Name of the AVD to delete [required]

for eg.

D:\android-sdk_1.5_windows_16july\tools>android delete avd -n avd51
AVD 'avd51' deleted.

====================================================
- update avd : Updates an Android Virtual Device to match the folders of a new SDK.
Action "update avd":
Updates an Android Virtual Device to match the folders of a new SDK.
Options:
-n --name Name of the AVD to update [required] ====================================================
- create project: Creates a new Android Project.
Action "create project":
Creates a new Android Project.
Options:
-k --package Package name [required]
-n --name Project name
-a --activity Activity name [required]
-t --target Target id of the new project [required]
-p --path Location path of new project [required]
=====================================================
- update adb : Updates adb to support the USB devices declared in the SDK add -ons.
Action "update adb":
Updates adb to support the USB devices declared in the SDK add-ons.
Options:
No options

=====================================================

How to capture Screen of the Android Phone or emulator?

1. Run emulator either using cmd or eclipse.
2. If device i.e. Android Phone, connect it with the PC using specific cable use to load Apps.
3. Go to Eclipse.
4. Go to Devices as shown in A.
5. Check for Circle B, it is Screen Capture,it will get the current instance of the emulator or Android Phone (device).
6. Go to Save.
7. It will save the screen as a .png file as shown below.

How to Install Games/Applications on Your Android Phone or Emulator

1. This is how you can directly install any applications or Games and check the working of the Touch Screen on Android Phone.
2. Make sure that Phone is powered up and on. If using Emulator, create the Emulator Instance and it is running.
3. However, if emulator and Phone both are running, the command will run only on emulator. So, you have to quit the emulator instance, when loading the app on the Phone.
4. This shows that the Phone is being detected by the PC.
5. D:\android-sdk_1.5_windows\tools>adb devices
6. List of devices attached
   ? device
7. This shows how to load the games on the actual phone or emulator.
8. D:\android-sdk_1.5_windows\tools>adb install Puzzles.apk
   1300 KB/s (1352247 bytes in 1.015s)
9. D:\android-sdk_1.5_windows\tools>adb install Snake.apk
   244 KB/s (19545 bytes in 0.078s)
10. D:\android-sdk_1.5_windows\tools>adb install Android_Sudoku.apk
    601 KB/s (741288 bytes in 1.203s)
    
    You can download few Game Apps from the below site for free.
    http://www.androidfreeware.org/download

How to Create Two or multiple instances of Emulator

1. cmd1 ... D:\android-sdk_1.5_windows\tools>android create avd -n avd1 -t 1
   Android 1.5 is a basic Android platform. Do you wish to create a custom hardware profile [no] Created AVD 'avd1' based on Android 1.5
2. cmd1 ... D:\android-sdk_1.5_windows\tools>android create avd -n avd2 -t 1
   Android 1.5 is a basic Android platform. Do you wish to create a custom hardware profile [no] Created AVD 'avd2' based on Android 1.5
3. cmd1 ... D:\android-sdk_1.5_windows\tools>D:\android-sdk_1.5_windows\tools>emulator –avd adv1
4. cmd2 ... D:\android-sdk_1.5_windows\tools>emulator –avd adv2
5. Two emulator instances are created. One with Port 1 = 5554 and other with Port 2 = 5556.
6. 
7. Similarly multiple instances of the emulator can be created by creating multiple AVDs and running emulator deploying those AVDs.
8. AVD can be easily created using ECLIPSE too.
9. Go to Eclipse -> Run Configurations -> Target tab
10. 
11. You can see the list of all the existing AVDs.
12. Now go to AVD Manager (Android Virtual Devices Manager) and choose the option of Create AVD as in the next window.
13. 
14. You can also Delete AVD ... go back to AVD Manager and select the particular AVD.
15. 
16. You can create emulator instance directly from the ECLIPSE.
17. Eclipse -> Run Configurations -> Target -> Select a preferred AVD for deployment and then RUN.
18.