Important things to know about REF ( Restricted Earth Fault Relays)

Why REF required?????
REF protection applied on transformer in order to detect ground faults on a given winding more sensitively than overall differential protection able to do.

On which principle they based ?????
The principle of comparison of measured variables by comparing the residual current of the phase CT of the given winding with the current associated with ground star point.

Why residual current occur????? or why CT saturated???
Saturation Saturation is the sudden variation of material permeability from high value to low value at the point known as the saturation bend.

In case of saturation induction then increases only slowly and Ie ( excitation current ) deforms to form peak.

Why stabilizing resistor inserted in the differential path for REF relay ???
If any CT ( phase CT or neutral CT ) get saturation (due to DC component of current during trafo energization (Inrush current)) .
So CT 's high magnetizing inductance breaks down to low value determined by permeability of air.
Differential current or residual current caused by CT saturation does not flow completely through differential path (due to high Impedance in path) but flows through low magnetizing inductance of saturated CT.
Hence insertion of resistance has a restraining effect.
Voltage across stabilizing resistance at Max. through fault = Max. through fault current ( CT resistance + 2 * Cable resistance )

BTW we are never allow to saturate our CT's by selecting proper knee point voltage.

High and Low Impedance Relays

Both low impedance & high impedance are working in merz.-prize circulating current principle (i.e) under normal operating condition, incoming and out going current remains the same and hence no current flows through the relay operating coil.

Basic difference lies in how we are achieving the stability for external faults.

In low impedance relays, we have bias characteristics.Based on the external fault magnitude, bias increases.(in our
static relays, bias current is the average of incoming and outgoing currents) Operating current of the relay should exceed the setting current and the bias current.During internal fault condition all the current converges and hence operating current increases more than the bias and the relay operates.

In high impedance relays, stability is achieved by the external resistors connected in series with the relay operating coils and making it as voltage dependent.(this is why we call it as high impedance relays).Resistance should be calculated in such a way that the relay will not operate for the external fault (with one CT becoming completely saturated).

Electrical Interview Questions

1. Tell us about your background.
   
2. Your short term goals
3. Your long term goals
4. How can we trust if you stay with our organisation.
5. GIS: Was there separate LCC and relay panels.
6. What projects you have worked before?
7. Why you want to work for us?
8. What did you do in your particular job profile.
9. How were you placed in that job profile
10. Were you assisting or were you leading the team?
11. Which group you would like to join - Relay, Automation, Electromechanical or Civil. and Why?
    

Electrical Interview Questions for Relay Protection Engineering III

1. Have you also worked on Motor or generator protection or it is only transmission and distribution?
2. What different kind of relaying you have worked with?
3. Did you also work with Electromechanical relays?
4. 78% of relaying is still electromechanical.
5. Can you explain how do you order the relay from the Step 1 to commissioning?
6. What type of relay you have used for SEL?
7. What kind of protection is there for Line?
8. Ok, it is distance.
9. what is the distance protection philosophy?
10. How many zones are there?
11. Can you discuss the characteristic of the zone?
12. How can you plot it on the paper?
13. Where will be the protected line on the characteristic?
14. What angle you can see the protected line?
15. Can you discuss where will be the Remote line in this cureve?
16. Where can you see the other lines on the R-X characteristic?
17. Are you aware of line differential?
18. What is the principle for the line differential?
19. How does the differential takes place?
20. Are you aware of POTT or DUB?
21. Have you used them in your project?
22. Which software you have used for ABB, simens for relay setting.
23. What is REF?
24. Why is it necessary?
25. Why the differential relay cannot protect it?
26. What is the philosophy of REF?
27. Does it depend on the grounding?
28. Are you aware of symmetrical components?
29. Why are they used?
30. What is the purpose of using symmetrical components?
31. How you can measure fault using hte symmetrical components?
32. What is unbalanced and balanced fault?
33. Is 3Ph a balanced fault?
34. Then what can be the unbalanced fault?
35. How do you read the Voltage and current?
36. How can you say that they are balanced?
37. How can you make a graph of it?
38. How is a displaced wrt b and wrt c?
39. At what angle?
40. what shall be the order of the abc?
41. Can you give the example where you do phase chagne?
42. What about the motor?
43. How do you revrerse the direction?
44. What did you study in your undergrad?
45. What all things you learnt as a course work there?
46. What are your goals 5-10 years down the line?
47. can you explain the gap in your service?
48. Have you worked on Aspen, Mathcad, before?
49. We only work with SEl relays.
50. What all books you have read for the relays?
51. How did you learn the relay setting?
52. How did you cross check the relay setting.
53. Do you do hand calculation for hte relay setting?
54. How was you site experience?
55. Did you assist somebody or helped somebody for relay commissioning.
56. How exactly you have worked on site?
57. I am kind of surprised, how can you commission so many relays in 5 years.
58. What kind of projects do you have?
59. Have you read the papers from SEL ?
60. How good are you with making presentations?
61. Have you made presentation before?
62. How good are you writing technical papers?
63. How did you learn all this?
64. What motivates you to work for us?
65. What kind of load shedding knowledge do you have?
66. How much familiar are you with Mark VI and load shedding at generation end?
67. What application you have seen?
68. Mainly you can focus on doing relay setting from the HMI?
69. Can you explain your recent relay setting, how did you do it exactly?
70. Did you make all calculations by yourself?
71. Somebody else did for you?

Load Flow Models are based on Static Network Model.

Utilities use real-time on-line models in SCADA for the optimization of generation, VAR control, losses, and tie-line control.

A load-flow study is carried out to determine the steady-state bus voltages, active
and reactive power flows, transformer tap settings, component or circuit loading,
generator exciter regulator voltage set points, system performance under contingency or emergency operations, and system losses.

Load flow can also be used to determine voltage profile at the time of starting a large motor. The starting motor is modeled as a constant-impedance shunt with the X/R ratio based on a locked rotor or starting power factor. The load-flow case is run with the starting motor disconnected, and the voltage at the relevant buses is recorded. The starting-motor locked-rotor impedance
is connected as a shunt, and the new case is run. The difference in voltage at any bus
is the voltage drop at the instant of starting the motor.

Two algorithms, Gauss-Siedel and Newton-Raphson, are used to solve the loadflow
equations.

The Gauss-Siedel method gives a simple and stable solution and works well up to 100 buses. The
solution iterates one bus at a time, corrects that bus voltage to the specified value, and continues until an error is detected.

The solution may not converge for the following reasons:
1. Error in the input data
2. System is too weak to carry the load
3. Insufficient VAR in the system to support the voltage

In the Newton-Raphson method, the n quadratic equations are first linearized by forming a Jacobian matrix. The present value of the bus voltage is then calculated, and then n linear equations are solved in steps. The number of iterations is small, between five and ten.

Electrical Interview Questions for Relay Protection Engineering

1. There will be always a lot of questions based on your resume. You should know your resume and what all things you have done in your project in a nice manner.
2. There will be also a lot of cross questioning about the profile you are looking for.
   
3. But apart from that, few basic questions might be asked to you:
4. What factors decide the pick up value of the current, considering distributed system.
5. What are the time margin for the overcurrent relay.
6. How are CT's connected on Delta-Star trafo considering you are not using any phase correcting CTs or any interposing CTs.
7. What are the inputs needed for Breaker Failure relay.
8. What is Zone 1 setting?
9. Why it is set to that percentage?
10. Say, PTR = 1000/1 and CTR = 200/1; what shall be Secondary Impedance.
11. What sequence current flows through P-G faults?
12. How does they flow?