phase angle setting for syn check 1

[heavyCurrent]

Let's say there are three systems - A, B & C. They are all interconnected in a ring form A-B-C-A. Our system is A. System C is very weak and has limited generation. So, A & B are delivering power to system C. Whenever the system C & A are disconnected, the transmission system becomes a long radial line. Whenever, we tried to reclose the breaker between C & A, we have a huge phase angle difference even though all three systems are tied together. The breaker is located at the generation station at system A.

I was told that the generators in system A only can withstand 10 deg phase difference during switching. So, I always set my relay sync check angle at 10 deg. For the above case, we can't close the breaker between C & A because of the sync check relay. Typically, our linemen don't use the sync check relay (bypass it) and manually close the breaker. So far, our generators are still running.

Anyone wants to chime in on this situation. How can I find the generators tolorence on phase angle differences during switching at the generating station.

 

[rcw retired EE]

I envision the scenario as the generators synchronzing to themselves through an impedance. The loads are already connected to the generation. Closing the breaker just redistributes the load current over another path. I don't visualize a large torque on the generator when the breaker closes. It is not picking up any more load.

Do you have any recordings that show large generator current excursions when the breaker closes? Does the floor jump? Can the plant operators even tell it is happening? If not then there probably is not a problem.

My idea may be wrong. I'll check some of my analysis books for an answer.

 

[heavyCurrent]

I don't hear any complaint from the plant operators yet. So, I would guess nothing happens to the generators yet. But, as you know, it may cause a big stress on the machines. Who know? I am not very familiar with the machine.

 

[waross]

I would look at the kilowatt load on the generator that is picking up the load, before and after closing. The difference in loading before and after is probably a block load.
On a small plant with 350 KW and 600 KW machines I have seen consistent closing at 30 deg phase angle. A little more and the breaker tripped. A little more than that and a coupling would shear a key. This happened once or twice a year. I found a hardwired phase angle error of 30 degrees in the synchroscope circuit. I am happy closing at 15 degrees error.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

I agree with rcwilson. Synchronising a machine into a larger system through a stiff connection can really throw the generator around and has a lot of potential to wreck the machine. A window of 10 degrees phase angle for a machine synchronising to a larger system would be alarming, but this sounds like a check-sync relay between systems - correct?

In your case the impedance of the interconnection line acts to soften the shock because it limits the amount of synchronising current (and thus torque) that system C can impose onto the generators of systems A and B. If system C has very little generation then you could possibly (and very crudely) model system C as a lumped load through a series impedance.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi.
We had a good topic on the issue.
Please see attached

http://www.eng-tips.com/viewthread.cfm?qid=259102

Good Luck
Slava

 

[stevenal]

I think the three systems may be confusing the issue. Since the three systems are tied together both before and after the breaker closure, lets call it one system. Consider a single generator with an open looped line long enough to extend a significant portion of a wavelength. At the open point the angular will be large, but are you going to damage a generator syncing to itself?

 

[m3ntosan]

If it’s a planned synch, have you tried to reduce the phase angle before closing the breaker? IF POSSIBLE instruct “A” system generation to step back and raise the generation level in “B” system if available, for switching time to reduce phase angle.

Assuming we’re talking about HV system, what happens when you have a trip and some automation is trying to reconnect?

In some utilities for heavy loaded 400 kV lines, the reclosing automation will check synch at 60 deg close to power plants.


May you grow up to be righteous, may you grow up to be true...

 

[heavyCurrent]

Thank you very much for posting your comments.

These three systems are relatively small compared to most of the systems. I should have said that they are interconnected with the subtransmission lines such as 230kV between A & B, 115kV between A & C and B & C. The load in system C is around 180 MW and they have only 40 MW of generations.

To answer ScottyUK's question, yes 10 deg is the sync check relay phase angle difference allowance.

To answer stevenal's question, yes you may consider as one big system. Please note that they are 4 or 5 gas generators in system A, 4 or 5 gas generators in system B and 2 small Hydro generators in system C. Are you saying, since they are in sync between those generators, the phase angle difference doesn't matter.

To answer m3ntosan's question, I was thinking exactly. Raise system B & C generations and close the phase angle gap between A & C and close the breaker. However, I need to know how much the pahse angle differences the machines in system A can take. I don't know where to find this information. Is it something that I need to get it from the manufacturers?

I drew up some diagrams below for your visual aid.

Before BKR close
B
/ \
/ \
/X ___\
A C

After BKR close
B
/ \
/ \
/_____\
A C

 

[m3ntosan]

In my opinion, it shouldn’t be a problem because you’re not synchronizing separate systems; you’re just closing an open ring. I can’t see why this might be a problem, it is usually the case to close breakers with angles between 35 – 60 deg if it’s the same system, if not it will revert to 10 deg . Synchronizing with wide angles should be a problem for transmission system as a probability exists to trip circuits by protection because o the power flow rush when closing the CB. So maybe you should be concerned on breaking you’re ring in some other part of the system without any control.
There’s a problem for generators when you’re synchronizing separate systems with difference in frequency and the shock in the system will trip units.


May you grow up to be righteous, may you grow up to be true...

 

[m3ntosan]

My understanding from your previous posts is that you’re already overriding synch relay when closing the loop; it is interesting to know at what angle are you closing?

May you grow up to be righteous, may you grow up to be true...

 

[jghrist]

I agree with m3ntoson. You aren't synchronizing systems, just closing a loop. The phase difference is because of power flow between the different parts of the system. Closing the loop is changing the power flow so that the generators in A supply more of the load. It's more like picking up load than synchronizing.

I don't see how you would be able to change generation to reduce the angle across the open breaker. As long as there is load flowing from A to B to C, there will be phase angle differences.

 

[m3ntosan]

jghrist,

Power angle is influenced by active power flow, impedance path and voltage magnitude as per P (?) relation/diagram.

May you grow up to be righteous, may you grow up to be true...

 

[heavyCurrent]

I found this article on the following website.

http://www.o-t-s.com/synchronizing.html

As you can see in the link, Scenario #2 is very similar to my situation. The article suggests that we need to make sure the phase angle and voltage magnitude differences are within acceptable limits. My guess is the generators' limits.

What do you guys think about it?

The angle of separation between A & C is 25 deg when the breaker is open.