Main-Tie-Main

Status
Not open for further replies.
J

JeffWalsh

Member
Main-Tie-Main
A customer has had what seems to be a recurring event which has resulted in a large chiller being damaged each of the past two summers. This spring my company sold six multi-use (PFC/TVSS) power conditioners to try to help. These devices contain capacitors, inductors and MOVs - all cross coupling the phases (patented design). This summer, one of our units blew up. The MOVs were all fried (voltage event?), and the cover was blown off. The capacitors and inductors were intact.

No damage occured to the chiller and our product will be replaced or rebuilt under warantee. We may have saved the customer hundreds of thousands of dollars, but they will still incurr installation costs, we will pay for shipping, and our manufacturer needs to fix or replace the unit. We need to try to find and fix the root cause.

The Chief Building Engineer (not an Electrical Engineer) explained that he believes the problem may lie with an automatic transfer switch and a Main-Tie-Main breaker. The tranfer switch was installed less that a year before the problems began; it sits on the customer's premesis, but is owned by the utility; it is is fed by circuits "A" and "B" that come from different substations; "A" is the primary. The Main-Tie-Main is fed on one side by this transfer switch, and on the other directly by circuit "B".

Is this a configuration that could result in a voltage surge or other potential hazard? Is there a possible simple fix such as adjusting the timings on the MTM?
 
Last edited:
JeffWalsh said:
Main-Tie-Main
A customer has had what seems to be a recurring event which has resulted in a large chiller being damaged each of the past two summers. This spring my company sold six multi-use (PFC/TVSS) power conditioners to try to help. These devices contain capacitors, inductors and MOVs - all cross coupling the phases (patented design). This summer, one of our units blew up. The MOVs were all fried (voltage event?), and the cover was blown off. The capacitors and inductors were intact.

No damage occured to the chiller and our product will be replaced or rebuilt under warantee. We may have saved the customer hundreds of thousands of dollars, but they will still incurr installation costs, we will pay for shipping, and our manufacturer needs to fix or replace the unit. We need to try to find and fix the root cause.

The Chief Building Engineer (not an Electrical Engineer) explained that he believes the problem may lie with an automatic transfer switch and a Main-Tie-Main breaker. The tranfer switch was installed less that a year before the problems began; it sits on the customer's premesis, but is owned by the utility; it is is fed by circuits "A" and "B" that come from different substations; "A" is the primary. The Main-Tie-Main is fed on one side by this transfer switch, and on the other directly by circuit "B".

Is this a configuration that could result in a voltage surge or other potential hazard? Is there a possible simple fix such as adjusting the timings on the MTM?
Click to expand...

Yes it can cause a voltage surge if it is a close transition scheme and there is a phase shift on either bus A or B.
 
First question: Does it (as Zog suggested) have a closed transition scheme that allows (1) the A Bus to power one side, and (2) the B Bus to power the other side, and (3) the two to be in parallel during the short transfer time from one to the other? If so, that could easily cause a major failure, and a downright explosion is not impossible.

Second question: Does the Main-Tie-Main setup include an interlock to prevent all three from being closed at the same time? If so, my suspicions would next fall upon the transfer switch. That leads to my . . .

. . . Third question: How quickly does the transfer switch operate? If the A Bus is powering a large motor, such as a chiller plant, and if that power source is lost, and if the transfer switch acts so quickly that the large motor is still spinning at nearly full speed when the B Bus power source is applied, then you will be essentially placing two power sources (i.e., the large motor, temporarily acting as a generator as it spins down, and the new power source from B Bus) in parallel. They are not likely to be in sync with each other, so a large current could easily result.
 
charlie b said:
First question: Does it (as Zog suggested) have a closed transition scheme that allows (1) the A Bus to power one side, and (2) the B Bus to power the other side, and (3) the two to be in parallel during the short transfer time from one to the other? If so, that could easily cause a major failure, and a downright explosion is not impossible.

Second question: Does the Main-Tie-Main setup include an interlock to prevent all three from being closed at the same time? If so, my suspicions would next fall upon the transfer switch. That leads to my . . .

. . . Third question: How quickly does the transfer switch operate? If the A Bus is powering a large motor, such as a chiller plant, and if that power source is lost, and if the transfer switch acts so quickly that the large motor is still spinning at nearly full speed when the B Bus power source is applied, then you will be essentially placing two power sources (i.e., the large motor, temporarily acting as a generator as it spins down, and the new power source from B Bus) in parallel. They are not likely to be in sync with each other, so a large current could easily result.
Click to expand...

Sounds to me that the M/T/M tie provided by the power company has some issues and sine they have control over it, they need to be called on the carpet. If they do switching as Charlie stipulates it then they are responsible for the incurred damages. When paralleling sources your short circuit duty increases and if they are out of phase - should had a 25 relay - there could be an unmitigated disaster. Even if it is in phase there could be equalizing current flowing between the sources which could be a problem.
 
weressl said:
(cut)Even if it is in phase there could be equalizing current flowing between the sources which could be a problem.
Click to expand...
I'm not familiar with the term "equalizing current" Where does it flow (from where to where)? What causes it to flow? How does it raise the voltage to the customer?

cf
 
Cold Fusion said:
I'm not familiar with the term "equalizing current" Where does it flow (from where to where)? What causes it to flow? How does it raise the voltage to the customer?

cf
Click to expand...

Example:

You have two 10MVA distribution transformers with nearly identical impedance. One has a 2MVA load the other has 7MVA. When you parallel them there will be 2.5MVA flowing through the tie breaker toward the higher loaded side to equalize the load diffrential between the two transformer.
 
your customer should be putting pressure on the utility (as others stated) to resolve the issue, and you might help them prove to the utility that it is their problem. Anyone who is paying for that kind of service should be provided with pretty substantial help by the utility and treated well. IMSHO the utility should be providing them with conditioned power (as opposed to blowing their (Edit) up when one of the grids go down). I mean, the MTM is supposed to be helping them, not creating problems for them, right ? Of course, if the utility is a bunch of jerks, then you might have to help the customer document a few more blown up TVSS's (a data recorder would be nice if they currently don't have any logging) so you can prove with out a doubt who exactly is at fault.
 
Last edited by a moderator:
nakulak said:
your customer should be putting pressure on the utility (as others stated) to resolve the issue, and you might help them prove to the utility that it is their problem. Anyone who is paying for that kind of service should be provided with pretty substantial help by the utility and treated well. IMSHO the utility should be providing them with conditioned power (as opposed to blowing their (Edit) up when one of the grids go down). I mean, the MTM is supposed to be helping them, not creating problems for them, right ? Of course, if the utility is a bunch of jerks, then you might have to help the customer document a few more blown up TVSS's (a data recorder would be nice if they currently don't have any logging) so you can prove with out a doubt who exactly is at fault.
Click to expand...

Cool down Pilgrim.....:rolleyes:

We have NOT established that this was due to the Utility's fault. We suggested some avenues of investigations with strong suspicion about the M/T/M.
 
Last edited by a moderator:
I have stayed out of this discussion since I am not an engineer. However, discussion is what it should be and not flying off the handle with emotion and pointing fingers. :roll:

I do agree that the serving electric utility should be involved in the discussion. Additionally, I wonder how the switching scheme was originally decided upon since this seems to be a make before break setup? The reason I am inquiring is that we will provide the medium voltage feeds from different substations if the loading on the circuits permit this type of switching. Also, we never provide the switching so this is outside of my expertise. :)
 
Jeff,I don't see the physical connections in your circuit description. If ckts "A" & "B" are feeding the transfer switch and the transfer switch is feeding one side of the tie and as you stated that ckt "B" also feeds the other side of the tie as well as the transfer switch,,,that looks a little squirrelly> can you look see if thats what you meant?

dick
 
Again, I suspect the system is designed correctly but different type of loads are causing a phase shift on one of the busses so that when the tie closes (Assuming closed transition) the voltage differential from the phase shift is causing high circulating currents and a vlotage spike. Do phasing checks across the tie.
 
zog said:
Again, I suspect the system is designed correctly but different type of loads are causing a phase shift on one of the busses so that when the tie closes (Assuming closed transition) the voltage differential from the phase shift is causing high circulating currents and a vlotage spike. Do phasing checks across the tie.
Click to expand...

Voltage NEVER shifts due to load characteristics, current does.
 
You are right, i was thinking about a phase shift from different transformer configurations and got my thinking crossed, thanks for pointing out my error as you so much love to do.
 
But there may be a phase shift. If there is a big difference in distance from the common point (say a transmission substation), there will be a noticeable phase shift between the two points across the open switch. The two should come together, but it might be kind of exciting (as it would be when you open it again).

I'm thinking for a short distance to the common tie (say two feeders out of the same distribution substation), there will be some equalization through the switch because of the impedance difference, but I would not think you would see any huge transient voltages. I may be wrong but that would be my first thought.

I'm assuming everything is phased properly, of course, and that the circuits are tied at some point up the line.
 
Am I the only one having the problem of seeing these connections? Sketch this in a oneline,,,,,,,it sure doesn't look right on paper if drawn to the description

somebody straighten me out on this please

dick
 
I'm picturing something like with a make-before-break:

Main-Tie-Main.jpg
 
a sketch

a sketch

Here is a sketch I made from what I saw and what the Chief told me:
Main-Tie-Main.png

Chiller one is the one having the problems. The MTM was online for a few years before there were problems. The utility's transfer switch was only in place for a matter of months before the first incident. The one thing he was not sure about was the circuits the chillers were on. He thinks that is on circuit A, but in dicussing with a coleague we belive this is incorrect. It seems if power from substation B goes down, the utility's transfer switch does nothing. I don't have answers to a number of other questions posed. Thank you all for the spirited discussion and thoughtful responses.
 
Status
Not open for further replies.
Top