Some Sample Problems I Composed

[TNSparky]

Was running the other day and thought of these. Good things to think about.

1. A transmission line has an impedance of 10 ohms (ignore reactive and capacitive impedance). A distance relay is set to protect 120% of this line. The current transformer connected to the relay has a ratio of 600:1. The potential transformer connected to the relay has a ratio of 1000:1. The ohmic reach the relay is set to is most nearly:

A. 20 ohms, B. 12 ohms, C. 10 ohms, D. 7 ohms

2. A 3-phase, 2-pole, 60 Hz synchronous machine has a rating of 600MVA at 0.9 power factor. The machine is brought up to synchronous speed, and is left to sit in these conditions, not connected to a power system. What will happen if the exciter field current is raised in these conditions?

A. The machine will speed up and power factor will become unity. B. The machine's real power output will increase. C. The machine's reactive power output will increase. D. The machine's stator terminal voltage will increase.

3. A certain power system is protected with a non-time delay 67 element and a 51 element. The purpose of the 51 element is to:

A. Establish fault current directionality and initiate trip instantaneously. B. Wait a specified amount of time, then initiate a trip. C. Initiate trip after an unhealthy voltage is detected. D.  Initiate trip instantaneously, regardless of the direction of the fault.

4. A 3-phase transformer has a primary winding of 115kV, secondary winding of 69kV, and is rated at 10MVA with 9% impedance. The apparent power output on the secondary side is most nearly:

A. 17 MVA. B. 6 MVA C. 10 MVA D. 15  MVA

5. According to the 2014 NEC, the primary purpose of a ground-fault circuit interrupter (GFCI) is to:

A. Protect personnel from ground faults. B. Protect equipment from ground faults. C. Protect switchgear from ground faults. D. Protect wiring and feeders from ground faults.

6. During a single-line to ground fault, the following sequence currents are available:

A. Positive and negative ONLY. B. Positive and zero ONLY. C. Negative and Zero ONLY. D. Positive, negative and zero sequence.

I'll provide answers by Sunday (4/9). These did not come from a practice exam or an actual exam. These are original questions composed by me. I just wanted to share with you all because they are good things to think about.

 

[rg1]

Was running the other day and thought of these. Good things to think about.

1. A transmission line has an impedance of 10 ohms (ignore reactive and capacitive impedance). A distance relay is set to protect 120% of this line. The current transformer connected to the relay has a ratio of 600:1. The potential transformer connected to the relay has a ratio of 1000:1. The ohmic reach the relay is set to is most nearly:

A. 20 ohms, B. 12 ohms, C. 10 ohms, D. 7 ohms

Explanation- 120% of 10 is 12. Other info in the question are red herrings

2. A 3-phase, 2-pole, 60 Hz synchronous machine has a rating of 600MVA at 0.9 power factor. The machine is brought up to synchronous speed, and is left to sit in these conditions, not connected to a power system. What will happen if the exciter field current is raised in these conditions?

A. The machine will speed up and power factor will become unity. B. The machine's real power output will increase. C. The machine's reactive power output will increase. D. The machine's stator terminal voltage will increase. 

Explanation-The voltage will rise till saturation of field circuit and rise will follow B-H curve of field circuit. 

3. A certain power system is protected with a non-time delay 67 element and a 51 element. The purpose of the 51 element is to:

A. Establish fault current directionality and initiate trip instantaneously. B. Wait a specified amount of time, then initiate a trip. C. Initiate trip after an unhealthy voltage is detected. D.  Initiate trip instantaneously, regardless of the direction of the fault.

Explanation- 51 is IDMT element so none of the answers fit into it. However C may be a near answer.

4. A 3-phase transformer has a primary winding of 115kV, secondary winding of 69kV, and is rated at 10MVA with 9% impedance. The apparent power output on the secondary side is most nearly:

A. 17 MVA. B. 6 MVA C. 10 MVA D. 15  MVA

Explanation- The apparent does not change; it is same for Xmer 

5. According to the 2014 NEC, the primary purpose of a ground-fault circuit interrupter (GFCI) is to:

A. Protect personnel from ground faults. B. Protect equipment from ground faults. C. Protect switchgear from ground faults. D. Protect wiring and feeders from ground faults.

Explanation- I do not have the book but, but it protects personnel while touching a unhealthy leaking ( failed insulation) device primarily. Others B,C,D are also correct and everything gets protected in the end.

6. During a single-line to ground fault, the following sequence currents are available:

A. Positive and negative ONLY. B. Positive and zero ONLY. C. Negative and Zero ONLY. D. Positive, negative and zero sequence.

Explanation- Only positive - balanced faults; Positive and Negative- Unbalanced without involving ground.; All when- unbalanced with ground fault.

I'll provide answers by Sunday (4/9). These did not come from a practice exam or an actual exam. These are original questions composed by me. I just wanted to share with you all because they are good things to think about.

Hey TNSparky

I am sorry. I am always tempted to solve the question I see. Please comment on the answers. 

 

[TNPE]

Hey TNSparky

I am sorry. I am always tempted to solve the question I see. Please comment on the answers. 

I believe you are right on all except #3.  A 51 element is a time over current device (does not look in both directions), therefore, I feel confident in B, not C, being the correct answer.  However, the 67 element is indicated to be a non-time delay (i.e. instantaneous) and is a device used for looking in both directions (therefore, you can eliminate a few of the given answers).

 

[rg1]

I believe you are right on all except #3.  A 51 element is a time over current device (does not look in both directions), therefore, I feel confident in B, not C, being the correct answer.  However, the 67 element is indicated to be a non-time delay (i.e. instantaneous) and is a device used for looking in both directions (therefore, you can eliminate a few of the given answers).

The meaning given by IEEE and ANSI for 51 and 67 is in attached file.

View attachment 9308

 

[TNSparky]

@rg1 and @TNPE. You both are mostly correct. Explanation below.

1. The question asked for relay ohms. This is different than the primary side impedance. Remember, a distance relay does nothing but measure current and voltage vectors and performs math to obtain a calculated impedance. If a phase fault occurs (important concept: distance protection, unless otherwise explicitly stated, refers to PHASE faults, not ground faults), the relay divides the measured voltage (off the PT) by the measured current (off the CT). In this case we need to set our reach to be 120% of 10 ohms or 12 ohms....PRIMARY. The relay sees the values reflected by instrument transformers. It can be proven that this reach can be translated into secondary (or relay) ohms by dividing the CT ratio by the PT ratio (dimensional analysis) and multiplying this by the primary reach. 12 ohms-primary * 600/1000 = 7.2 relay ohms....or most nearly 7 ohms (Answer D).

2. Correct. Answer is D.

3. Anytime you see a 67 element coupled with a 51 element, it should raise a red flag. It is implied that in this type of protection is a primary/backup setup. The 67 element acts instantaneously after it asserts a fault direction as either forward or backwards. If it is supervised to only operate upon forward faults, it will be restrained from tripping in the event of a reverse fault. Supervision occurs from usually a negative or zero sequence supervisor (vector math). The 51 element is a backup element that is non-directional and time delayed. Coordination is achieved with this element by setting the time dial such that the adjacent primary zone(s) of protection has/have a chance to operate, prior to backup protection clearing the fault. In this case, B is the correct answer.

4. Correct. Answer is C.

5. Correct. Answer is A. Remember, the question asked what is the PRIMARY purpose of GFCI's. Pay attention to question wording. The primary purpose of GFCI's is to protect personnel. That's it. There is inherent protection capability for the other items mentioned, but that is not the primary function of a GFCI device. 

6. Correct. Answer is D.

 

[TNSparky]

@rg1 and @TNPE. You both are mostly correct. Explanation below.

1. The question asked for relay ohms. This is different than the primary side impedance. Remember, a distance relay does nothing but measure current and voltage vectors and performs math to obtain a calculated impedance. If a phase fault occurs (important concept: distance protection, unless otherwise explicitly stated, refers to PHASE faults, not ground faults), the relay divides the measured voltage (off the PT) by the measured current (off the CT). In this case we need to set our reach to be 120% of 10 ohms or 12 ohms....PRIMARY. The relay sees the values reflected by instrument transformers. It can be proven that this reach can be translated into secondary (or relay) ohms by dividing the CT ratio by the PT ratio (dimensional analysis) and multiplying this by the primary reach. 12 ohms-primary * 600/1000 = 7.2 relay ohms....or most nearly 7 ohms (Answer D).

2. Correct. Answer is D.

3. Anytime you see a 67 element coupled with a 51 element, it should raise a red flag. It is implied that in this type of protection is a primary/backup setup. The 67 element acts instantaneously after it asserts a fault direction as either forward or backwards. If it is supervised to only operate upon forward faults, it will be restrained from tripping in the event of a reverse fault. Supervision occurs from usually a negative or zero sequence supervisor (vector math). The 51 element is a backup element that is non-directional and time delayed. Coordination is achieved with this element by setting the time dial such that the adjacent primary zone(s) of protection has/have a chance to operate, prior to backup protection clearing the fault. In this case, B is the correct answer.

4. Correct. Answer is C.

5. Correct. Answer is A. Remember, the question asked what is the PRIMARY purpose of GFCI's. Pay attention to question wording. The primary purpose of GFCI's is to protect personnel. That's it. There is inherent protection capability for the other items mentioned, but that is not the primary function of a GFCI device. 

6. Correct. Answer is D.

Correction to 3: A 51 element can be set to be directional. It depends on the relay and application. It is time delayed regardless.

 

[rg1]

Correction to 3: A 51 element can be set to be directional. It depends on the relay and application. It is time delayed regardless.

@TNSparky Thanks a lot. Yes questions are to be read more carefully.

 

[TNPE]

Correction to 3: A 51 element can be set to be directional. It depends on the relay and application. It is time delayed regardless.

Customarily, an electromechanical 51 element (reasonable assumption) is a time over current device, hence B is the appropriate answer for the given question/scenario.