Distance Relay Problem CramForThePE

[Cram For The PE]

A distance relay is located at Bus A that looks outwards toward Bus C as shown in the diagram below.





Zone 1 of the distance relay covers transmission line AB and Zone 2 covers just beyond Bus C.
A fault occurs on Bus C. When Generator B is online and breaker 52 is closed, which of the following statements is true?




a)      Generator B connected to the system has no effect on how the impedance relay sees a fault at Bus C.

b)      With Generator B connected, the impedance seen by the relay appears to be greater.

c)       The more current Generator B provides to Bus B, the more likely the relay will trip due to Zone 1.

d)      None of the above.

http://cramforthepe.com/index.php/2020/02/08/distance-relay-problem-2/

 

[lturner]

B? Gen B will support Voltage in line BC in turn raising impedance.

 

[Cram For The PE]

B? Gen B will support Voltage in line BC in turn raising impedance.

You are on the right track.

 

[JayD]

Whats the right answer?

B?

 

[BebeshKing PE]

 



A distance relay is located at Bus A that looks outwards toward Bus C as shown in the diagram below.



View attachment 16397

Zone 1 of the distance relay covers transmission line AB and Zone 2 covers just beyond Bus C.
A fault occurs on Bus C. When Generator B is online and breaker 52 is closed, which of the following statements is true?




a)      Generator B connected to the system has no effect on how the impedance relay sees a fault at Bus C.

b)      With Generator B connected, the impedance seen by the relay appears to be greater.

c)       The more current Generator B provides to Bus B, the more likely the relay will trip due to Zone 1.

d)      None of the above.

http://cramforthepe.com/index.php/2020/02/08/distance-relay-problem-2/

@Cram For The PE, was this problem in one of your books? I can't seem to find it though.   

Also the electronics (bridge diode) that you posted previously....

 

[Orchid PE]

Let's imagine the breaker for Generator B is open. During a fault at Bus C, the distance relay at Bus A will see the fault (essentially it sees the impedance of lines AB and BC). Generator A is contributing 100% of the fault current.

Now, with Generator B connected to the system, it helps supply fault current to the fault at Bus C. In the original scenario, the current through line BC was supplied entirely by Generator A, but now the current is supplied by both Gen 1 and Gen 2. This means Generator A is supplying less fault current than the first scenario.

Distance relays detect impedance by Z = V / I_F.

Let's make up some numbers for the first scenario I mentioned. V = 1000 and I_f = 1000. For this scenario, the impedance is 1000 / 1000 = 1, and Generator A is supplying all of I_f.

Now, for the second scenario, let's say the current in line BC is split by the generators. Now, Generator A is only supplying 500 amps to the fault. The distance relay then sees an impedance of 1000 / 500 = 2.

With Generator B supplying fault current, the impedance seen by the distance relay is greater than if Generator B was not connected.

 

[Cram For The PE]

@Cram For The PE, was this problem in one of your books? I can't seem to find it though.   

Also the electronics (bridge diode) that you posted previously....

This is volume IV. I am in the process of developing it now. It wont be done for another few months.

 

[Cram For The PE]

Let's imagine the breaker for Generator B is open. During a fault at Bus C, the distance relay at Bus A will see the fault (essentially it sees the impedance of lines AB and BC). Generator A is contributing 100% of the fault current.

Now, with Generator B connected to the system, it helps supply fault current to the fault at Bus C. In the original scenario, the current through line BC was supplied entirely by Generator A, but now the current is supplied by both Gen 1 and Gen 2. This means Generator A is supplying less fault current than the first scenario.

Distance relays detect impedance by Z = V / I_F.

Let's make up some numbers for the first scenario I mentioned. V = 1000 and I_f = 1000. For this scenario, the impedance is 1000 / 1000 = 1, and Generator A is supplying all of I_f.

Now, for the second scenario, let's say the current in line BC is split by the generators. Now, Generator A is only supplying 500 amps to the fault. The distance relay then sees an impedance of 1000 / 500 = 2.

With Generator B supplying fault current, the impedance seen by the distance relay is greater than if Generator B was not connected.

Why do you assume the fault current supplied from generator A would be different with generator B connected?

 

[Dude99]

He is assuming the fault is a fixed Z. So the 'load' would be served by both G's.

 

[Cram For The PE]

What the relay "sees" due to component Zab will never change. Shown above is what it would look like if the breaker is open. To state the question differently:
  "After Zab is passed, does the slope change and if so how and why?"

 

[Dude99]

My take, won't be the first time I'm wrong (definitely not the last).  Using rough numbers And basic calcs for example

 

[Orchid PE]

Why do you assume the fault current supplied from generator A would be different with generator B connected?

Would it not be different? Even without a fault, it looks like load is served off Bus C. If Generator A is the only generator connected, it will be serving all the load. If Generator B was connected as well, both would be serving the load. With a fault at Bus C, both generators will be feeding the fault.

 

[Cram For The PE]

Would it not be different? Even without a fault, it looks like load is served off Bus C. If Generator A is the only generator connected, it will be serving all the load. If Generator B was connected as well, both would be serving the load. With a fault at Bus C, both generators will be feeding the fault.

If you look at what dude99 did, you can see Gen A supplies 10MVA no matter if Gen B is connected. Obviously, at the point of fault there is more MVA_SC but Gen A SC remains. Iturner is very close to the correct answer. I would just ask that he prove it a little more.

@Dude99  Think about this. If you were the relay at bus A, why would you see the current different between the scenarios? What about the voltage at Bus A?

 

[Orchid PE]

What the relay "sees" due to component Zab will never change. Shown above is what it would look like if the breaker is open. To state the question differently:
  "After Zab is passed, does the slope change and if so how and why?"

Well if the fault contribution doesn't change for Gen A, but Gen B provides more fault current, then it would appear as though the impedance of Zbc is less with both generators connected when compared to Gen A alone.

But if Gen B is connected, I imagine the voltage at Bus B would be a little higher than if it wasn't connected. That would lessen the drop in voltage at Bus A, and if we assume the current remains the same in both scenarios, the impedance would be greater with the higher voltage at Bus A.

Scenarios (made up values):

Typical Bus Voltage 115kV.

1. Only Gen A Connected
   
   Bus A Voltage = 100kV
   
2. Current from Gen A = 1000 A
   
3. Z = 100
   

[*]Both Gens Online

1. 
   Bus A Voltage = 105kV
   
2. Current from Gen A = 1000 A
   
3. Z = 105
   

 

[Dude99]

Gen has 10 mva 'available' but it only supplies a portion of the fault's 3.75, 1/3 of it.. The fault is no different than any other 'load' and will be supplied by both gens.

there may be a potential difference at  bus A but it will be equalized by i flow between the gens and load/i division

 

[Dude99]

This may help.  Gen B does increase total  fault S (and reduces Gen A fault S) but the relay trips at same point.  Gen B will proportionally reduce A's fault Vdrop and I but the ratio (Z) should be ~ the same.  (Numbers are from my previous example).

 

[Dude99]

I added the A and B individual contributions to A+B.  As you can see A (with B) is proportionally lowered from A alone. The Z 'seen' (sensed) is the same.

 

[Orchid PE]

@Cram For The PE What is the correct answer and what is your process of getting there?

 

[Cram For The PE]

@Cram For The PE What is the correct answer and what is your process of getting there?

I will post it at my website in a few days.

 

[Orchid PE]

I will post it at my website in a few days.

For this 4th sample exam you are developing, how do you think it will compare to a CBT Power exam? NCEES has the Power exam tentatively scheduled to transition in 2021.

Based on how other exams have transitioned, NCEES may provided the only approved reference book. Do you feel like the breadth of problems in the NCEES test bank will be reduced since it's difficult to reduce the standard 20+ references that examinees normally bring into a single reference? Do you still plan on keeping your reference book in print, even though it may not be allowed during the exam?