Why are control circuits designed to require power to trip?

[Girl Engineer]

Most switchgear has battery backup or capacitive trip units for control power. It seems like it would be more effective to use electrically held trip circuits, where the springs charge when power is applied, and breakers trip on a loss of power? What is the advantage of having power-to-trip controls?

 

[zog]

Most switchgear has battery backup

Usually the battery system is not a back up, it is your CP source

or capacitive trip units for control power. It seems like it would be more effective to use electrically held trip circuits, where the springs charge when power is applied,

They usually do, typiclaly when the breaker is closed (For an EO breaker) the charging motor will charge the close springs after the breaker closes. So in case you need to close it without CP available, the springs will nto have to be manually charged.

and breakers trip on a loss of power?

That is used for a UV feature.

What is the advantage of having power-to-trip controls?

Not sure how else you would do it? Trip springs are charged when the breaker is closed. Protective relay sends trip signal to breaker to trip, you need a power source to energize the trip coil.

Maybe I am just not following your question.

 

[Girl Engineer]

Thanks zog, you definately understood my question.
We are actually looking into starters that have a trip coil that opens the contacts to the trip circuit when it loses power. We also have an 27 for undervoltage, but we were wanting to add another level of fail-safe. I am trying to understand how this will work, because it is not a common configuration.
So with the trip circuit as I described above, it seems like the only component that needs to be powered would be the charging motor for the close spring. What exactly does the close spring do, and what is its purpose? Just hold the contacts closed? What holds the contacts closed immediately after a trip while the close spring starting to charge?

 

[markstg]

Most switchgear has battery backup or capacitive trip units for control power. It seems like it would be more effective to use electrically held trip circuits, where the springs charge when power is applied, and breakers trip on a loss of power? What is the advantage of having power-to-trip controls?

I think that is a great question. Below is an excerpt from a google search, on Fail-safe design:


"One interesting application of fail-safe design is in the power generation and distribution industry, where large circuit breakers need to be opened and closed by electrical control signals from protective relays. If a 50/51 relay (instantaneous and time overcurrent) is going to command a circuit breaker to trip (open) in the event of excessive current, should we design it so that the relay closes a switch contact to send a "trip" signal to the breaker, or opens a switch contact to interrupt a regularly "on" signal to initiate a breaker trip? We know that an open connection will be the most likely to occur, but what is the safest state of the system: breaker open or breaker closed?

At first, it would seem that it would be safer to have a large circuit breaker trip (open up and shut off power) in the event of an open fault in the protective relay control circuit, just like we had the fire alarm system default to an alarm state with any switch or wiring failure. However, things are not so simple in the world of high power. To have a large circuit breaker indiscriminately trip open is no small matter, especially when customers are depending on the continued supply of electric power to supply hospitals, telecommunications systems, water treatment systems, and other important infrastructures. For this reason, power system engineers have generally agreed to design protective relay circuits to output a closed contact signal (power applied) to open large circuit breakers, meaning that any open failure in the control wiring will go unnoticed, simply leaving the breaker in the status quo position.

Is this an ideal situation? Of course not. If a protective relay detects an overcurrent condition while the control wiring is failed open, it will not be able to trip open the circuit breaker. Like the first fire alarm system design, the "silent" failure will be evident only when the system is needed. However, to engineer the control circuitry the other way -- so that any open failure would immediately shut the circuit breaker off, potentially blacking out large potions of the power grid -- really isn't a better alternative."

 

[zog]

Good explanation Mark, I agree 100% with all of that. Pick your poison I suppose. The key here is testing, the trip system should be tested frequently to ensure it operates properly. Also, it is very common to have a loss of control power alarm in the system, such as a low batteyr voltage. Perhaps the most common failure I have seen is the battery charger breaker tripping or being left open, which causes the battery to drain don over time to a level that may be too low to operate the trip coils.

 

[zog]

Thanks zog, you definately understood my question.
We are actually looking into starters that have a trip coil that opens the contacts to the trip circuit when it loses power. We also have an 27 for undervoltage, but we were wanting to add another level of fail-safe. I am trying to understand how this will work, because it is not a common configuration.
So with the trip circuit as I described above, it seems like the only component that needs to be powered would be the charging motor for the close spring. What exactly does the close spring do, and what is its purpose? Just hold the contacts closed? What holds the contacts closed immediately after a trip while the close spring starting to charge?

Are you asking about starters or breakers? Different animals.

 

[markstg]

Good explanation Mark, I agree 100% with all of that. Pick your poison I suppose. The key here is testing, the trip system should be tested frequently to ensure it operates properly. Also, it is very common to have a loss of control power alarm in the system, such as a low batteyr voltage. Perhaps the most common failure I have seen is the battery charger breaker tripping or being left open, which causes the battery to drain don over time to a level that may be too low to operate the trip coils.

On line indication of TC available in the old days, and still done today, is to wire a pilot light/resistor in series with the TC. If the Pilot Light is on, there is control voltage and TC is in tact. This has its drawback that there isn't a remote alarm. The new Solid State Relays usually have TC monitoring that can be wired into the circuit, and alarm contacts for remote alarming (which can be configured Fail Safe) of TC fail and Control Power fail. Redundant TC can be provided where reliability is esssential.

Put another way, put your eggs in one basket and Watch That Basket.

 

[zog]

On line indication of TC available in the old days, and still done today, is to wire a pilot light/resistor in series with the TC. If the Pilot Light is on, there is control voltage and TC is in tact. This has its drawback that there isn't a remote alarm. The new Solid State Relays usually have TC monitoring that can be wired into the circuit, and alarm contacts for remote alarming (which can be configured Fail Safe) of TC fail and Control Power fail. Redundant TC can be provided where reliability is esssential.

Put another way, put your eggs in one basket and Watch That Basket.

Yep, like this one. Problem is they often burn out and don't ever get checked.

 

[mxslick]

<snip>.......meaning that any open failure in the control wiring will go unnoticed, simply leaving the breaker in the status quo position.

Is this an ideal situation? Of course not. If a protective relay detects an overcurrent condition while the control wiring is failed open, it will not be able to trip open the circuit breaker. Like the first fire alarm system design, the "silent" failure will be evident only when the system is needed.....

A prime and well-known example of this drawback can be found in THIS VIDEO of the Ives Dairy substation in Florida. (This is not the original post of the video, where a detailed explanation of what happened was in the description.)

The problem was caused by a defective fuseholder in the control system which prevented detection of a short circuit in a capacitor bank. The fault escalated and eventually destroyed the substation, including the transformer in the video, which, after venting and igniting the cooling oil, blew an expulsion fuse shutting down the station. (The flash and bang at the end of the video.)

That simple open fuseholder caused millions of dollars in damage and a lot of downtime. Cost of the fuseholder? $10.00

Can we say OOPS?

They had since re-designed the substation and upgraded the SCADA system to report any and all blown fuses, or loss of control power to any relays.

However, that didn't stop the station from nearly being totally destroyed-again-when a cat got into one of the main busses and caused multiple faults.

I won't post the link here since the pictures are graphic.

 

[dbuckley]

The way i look at it is that if there is a safety-of-life issue, I insist (and many jurisdictions in Europe require) that the cutout system is fail-safe, so electrically latched series contactors are the norm. So a machine that is about to chop a person in two: I want the most reliable cutout system possible. On the other hand, these systems are less reliable as the control circuits do "let go" from time to time resulting in unnecessary outages.

If its not that safety important (and in many cases, frankly unliklely to get used), then a power to trip system is the way to go.

Ok, so the lack of a $10 fuse caused a substation to get trashed, but to the poco, the improved security of supply would make the math come down heavily in favour of the more reliable when on system, rather than the more reliable when needed to switch off. But not having SCADA (or even a regular site inspection) not noticing the fuse had failed was amateur hour.

 

[Girl Engineer]

I agree that in a distribution system, reliability is the important thing. I am looking at starters and breakers feeding starters, so reliability is important for environmental and production reasons, but safety is a big issue too. We want an operator to be able to trip that starter no matter what.

I am still interested in zogs comment about the close spring. In a situation where we want the breaker to open, we don't really care about the close spring charging again until the breaker gets its control power back, right? We don't want to start that motor until the protection is in place. Are there other issues I am not seeing?

 

[Girl Engineer]

Woops, I keep on saying breaker. I mean starter.

 

[dbuckley]

What Zog is on about is shunt trip breakers, I think, where the spring that releases the contacts is charged by the act of closing the contacts, and a latch holds the contacts clsoed. A solenoid operates the release mechanism of the breaker, and the spring then opens it.

A mechanically latcehd electrically released contactor works the same way.

For safety-of-life I want to work the other way; the thing opens when a safety circuit loses power. With a simple DOL starter, that can be as easy as interrupting the circuit to the contacter coil when you want stuff to stop. Thats how normal DOL starter 'stop' buttons work; they just interrupt power to the contactor.

But, that wont necessarily hold the contactor off; press the green button and the contacator will engage again.

For a safety application an external electrically contactor in the supply is usually used, and that usually has a key reset.

 

[SG-1]

So with the trip circuit as I described above, it seems like the only component that needs to be powered would be the charging motor for the close spring. What exactly does the close spring do, and what is its purpose? Just hold the contacts closed? What holds the contacts closed immediately after a trip while the close spring starting to charge?

For a breaker.
1. The motor charges the closing spring.
2. The breaker is held open by a mechanical latch. ( think of cocking a pistol, the hammer is back waiting for you to pull the trigger.)
3. The operator sends an electrical signal to energize the close coil.
4. The close coil assembly mechanically "pulls the trigger". ( rotates a shaft )
5. The breaker closes. The closing springs force is divided to close the breaker & charge (mechanically ) the opening spring(s).
6. If the trip latch is in position ( no trip signal present ) the breaker latches closed. ( mechanically held )
7. The motor recharges the closing spring. ( This does not have to be complete before 8 can function. In other words the breaker can trip with the motor runing. )
8. An electrical trip signal is generated.
9. The trip coil energizes.
10. The trip coil assembly mechanically rotates the trip shaft releasing the energy in the opening spring(s)
11. The breaker opens.
12. Loop back up to 3.

There is a little more to it then this, but I think you have your mechanical answer.

 

[markstg]

Thanks zog, you definately understood my question.
We are actually looking into starters that have a trip coil that opens the contacts to the trip circuit when it loses power. We also have an 27 for undervoltage, but we were wanting to add another level of fail-safe. I am trying to understand how this will work, because it is not a common configuration.
So with the trip circuit as I described above, it seems like the only component that needs to be powered would be the charging motor for the close spring. What exactly does the close spring do, and what is its purpose? Just hold the contacts closed? What holds the contacts closed immediately after a trip while the close spring starting to charge?

I'm confused. Is this a 13.8KV Motor? 5KV motor starters are deenergize to trip or at least can be. Circuit Breakers at these voltage levels are energize to trip.

 

[Girl Engineer]

They are 13.8kV wound rotor motors.
Why is the trip circuit different between voltage levels?

 

[Tarbaby]

Have both prefer the breaker

Have both prefer the breaker

I have two medium voltage(4kv) motors at our plant. One on a breaker one on a starter. I prefer the breaker because small voltage dips(tree branch contacting the utilitily line) don't trip the unit.