# Distance Relay Problem CramForThePE



## Cram For The PE (Feb 8, 2020)

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/


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## lturner (Feb 16, 2020)

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


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## Cram For The PE (Feb 20, 2020)

lturner said:


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


You are on the right track.


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## JayD (Mar 5, 2020)

Whats the right answer?

B?


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## BebeshKing PE (Mar 7, 2020)

Cram For The PE said:


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


@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....


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## Orchid PE (Mar 8, 2020)

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.


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## Cram For The PE (Mar 8, 2020)

BebeshKing said:


> @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.


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## Cram For The PE (Mar 8, 2020)

Chattaneer PE said:


> 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.
> 
> ...


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


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## Dude99 (Mar 8, 2020)

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


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## Cram For The PE (Mar 8, 2020)

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?"


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## Dude99 (Mar 8, 2020)

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


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## Orchid PE (Mar 9, 2020)

Cram For The PE said:


> 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.


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## Cram For The PE (Mar 9, 2020)

Chattaneer PE said:


> 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?


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## Orchid PE (Mar 9, 2020)

Cram For The PE said:


> 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.


Only Gen A Connected

Bus A Voltage = 100kV

Current from Gen A = 1000 A

Z = 100


[*]Both Gens Online


Bus A Voltage = 105kV

Current from Gen A = 1000 A

Z = 105


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## Dude99 (Mar 9, 2020)

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


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## Dude99 (Mar 9, 2020)

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).


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## Dude99 (Mar 9, 2020)

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.


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## Orchid PE (Mar 11, 2020)

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


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## Cram For The PE (Mar 11, 2020)

Chattaneer PE said:


> @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.


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## Orchid PE (Mar 12, 2020)

Cram For The PE said:


> 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?


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## Cram For The PE (Mar 12, 2020)

Chattaneer PE said:


> 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?


I plan to keep the books in print. All I could really do is rely on feedback from people who take the test and tell me how it compares. This fourth volume will probably be my last though. 
All I know about is the FE transition. I never heard of any PE test yet going this way. My hunch is they will probably still let you come in with as many books as you want.


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## Orchid PE (Mar 12, 2020)

Cram For The PE said:


> I plan to keep the books in print. All I could really do is rely on feedback from people who take the test and tell me how it compares. This fourth volume will probably be my last though.
> All I know about is the FE transition. I never heard of any PE test yet going this way. My hunch is they will probably still let you come in with as many books as you want.


The mechanical PE exam has already transitioned, and this is what NCEES has on their website:




Here is the CBT schedule:

https://ncees.org/exams/cbt/


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## Cram For The PE (Mar 12, 2020)

Chattaneer PE said:


> The mechanical PE exam has already transitioned, and this is what NCEES has on their website:
> 
> View attachment 16745
> 
> ...


Interesting. I did not know about this. So they will probably follow the mechanical and only allow their book in the exam. I will change to my questions to Follow the format of the "alternative questions". It looks like there will still be 80 questions. I don't understand when they say you won't be penalized for wrong answers


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## Cram For The PE (Mar 14, 2020)

The answer to this question has been published here:

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


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## Orchid PE (Mar 15, 2020)

Well shoot, as soon as I Google "distance protection infeed," and then "distance protection of mutli terminal transmission lines" I get tons of resources. NERC even has a paper with this is exact same scenario. None of my protection books cover this topic. Though, I don't have Blackburn's book on me at the moment, so he may or may not cover it.


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## Orchid PE (Mar 15, 2020)

Cram For The PE said:


> I don't understand when they say you won't be penalized for wrong answers


I've never really understood why this still has to be pointed out. In my entire life I've never taken an exam where a question would count against you if you got it wrong. However, I think this used to be a thing back in the day. A long time ago my dad told me a story (that he thinks is hilarious) about a test he took in grade school. The teacher said they would only be graded on the questions they answered, and the ones they got wrong would count against them, but unanswered ones would not (which seems bizarre to me). So he, with the logical mindset he had that eventually led him into engineering, found a question he knew how to answer 100% and answered only that question! His reasoning was that they weren't graded on questions they didn't answer, and the teacher never specified how many questions they needed to answer. So if he got this one question right, he would get a 100 on the test.

He had to retake the test.


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## bdhlphcdh (Mar 15, 2020)

Blackburn does cover it in section 12.12.


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## Cram For The PE (Mar 15, 2020)

Chattaneer PE said:


> I've never really understood why this still has to be pointed out. In my entire life I've never taken an exam where a question would count against you if you got it wrong. However, I think this used to be a thing back in the day. A long time ago my dad told me a story (that he thinks is hilarious) about a test he took in grade school. The teacher said they would only be graded on the questions they answered, and the ones they got wrong would count against them, but unanswered ones would not (which seems bizarre to me). So he, with the logical mindset he had that eventually led him into engineering, found a question he knew how to answer 100% and answered only that question! His reasoning was that they weren't graded on questions they didn't answer, and the teacher never specified how many questions they needed to answer. So if he got this one question right, he would get a 100 on the test.
> 
> He had to retake the test.


Funny story! I do not believe I have ever taken a test that did not penalize you for getting the wrong answer. But I guess that is why I do not understand it. Just answer one question and get a 100%. They should explain further on this point. Anyway in real life and your career you have to figure out answers to questions and problems. You can not just skip it and not be penalized. When you make wrong decisions in life or engineering decisions there are consequences. The philosophy of a test set up like this is not something I personally would agree with.


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## Orchid PE (Mar 15, 2020)

Well, I take that back. The old SAT format used to count wrong answers against you as a "guess penalty."

I could see how it would be beneficial if the PE exam was that way. It would prevent people from passing if they had to guess on too many questions.

They would need to set a base passing score, and each question would count towards the your total (not percentage based, but still use weighted questions). So even if you only answered 1 question, your total score would not be enough. Then, if you answered wrong, it would count negative (based on weight).

This would probably significantly drop pass rates.


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## Cram For The PE (Mar 15, 2020)

Chattaneer PE said:


> Well shoot, as soon as I Google "distance protection infeed," and then "distance protection of mutli terminal transmission lines" I get tons of resources. NERC even has a paper with this is exact same scenario. None of my protection books cover this topic. Though, I don't have Blackburn's book on me at the moment, so he may or may not cover it.


Blackburn touches it in his book. However, I do not like the way he describes it. I think my explanation and pictures makes it easier to understand. But I will let you guys decide on that.


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## Orchid PE (Mar 16, 2020)

https://www.nerc.com/comm/PC/System Protection and Control Subcommittee SPCS DL/SPCTF-3TerminalLines091906.pdf

This is the paper I found by NERC.

Page of particular interest (though the entire paper is good):


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