Dumb Switchgear Question Part Deux

[iwire]

I was looking at this thread Dumb Switchgear Question and it got me to thinking.

With MV breakers and the relays that control them, are they wired in a 'fail open' way or do we need to count on the circuit working probably when needed?

 

[jim dungar]

With MV breakers and the relays that control them, are they wired in a 'fail open' way or do we need to count on the circuit working probably when needed?

They are not fail-safe.
No control power - no protection.

 

[iwire]

Is there anything that monitors this?

Is the circuit itself supervised for continuity?

Just seems odd when you have never really thought about it before.

We wire E-stops on machines as fail safe, fire alarm systems are supervised.

 

[SG-1]

The breakers can be fitted with Undervoltage Trip Units that will open the breaker if control power fails.

Many customers do monitor their control circuits for undervoltage. The majority of control power is still Battery supplied DC. For AC control a capacitor stores a charge sufficient for one trip. Many overcurrent relays are still powered by the CTs, some others are Dual powered.

Some relays can be configured to close their trip contacts if the relay itself fails & protection is lost. A more common approach is to monitor the healthy relay contact. I think this is called mode 2 operation or fail safe.

I think SEL has a voltmeter built in some models for monitoring the control voltage.

 

[mxslick]

Heck even POCO substations have that same problem, but they are monitored and alarmed.

However, if the alarms aren't set up correctly, you can get into big trouble:

NERC DOCUMENT

Details
The system operator received a low-voltage alarm at 109 volts DC from a 230/69kV substation and dispatched field personnel to investigate the alarm. When field personnel arrived they determined the battery charger was not producing proper voltage. Before proper action could be taken the DC voltage continued to decay and fell below the 85 volt DC operating threshold and all protective relays began to alarm and shutdown. Additionally, control circuits became non-functional and SCADA measurements and status information became erroneous. At this point it was decided the substation and surrounding BPS was at risk if a fault occurred in the unprotected relay protection system zones; so orders were given to shed load and remove the substation from service.

(Bold added by me.)

Sometimes even the most foolproof system can be tripped up by a fool.

Another example I just read about is the "blowout preventer" on that Gulf Coast oil well. It was discovered to have 260(!) failure modes and it didn't work in this case due to modifications, faulty wiring and a dead battery!

Oooppsss...

 

[zog]

They are not fail-safe.
No control power - no protection.

Iwire, while this statement is true it is rarely the case. Most MV subs either use DC control power fed from a battery bank that is kept on a float charge or if they use AC they use a Cap Trip device for the trip coil. Either way a complete loss of power to a facility will still permit the breakers trip coils to operate.

Loss of a control power source usually triggers an alarm, sometimes just a stupid horn int he sub on older stuff but most lineups installed in the last 20 years have SCADA systems to warn you of such an issue via an intranet to your computer or cell phone.

 

[jim dungar]

Loss of a control power source usually triggers an alarm, sometimes just a stupid horn int he sub on older stuff but most lineups installed in the last 20 years have SCADA systems to warn you of such an issue via an intranet to your computer or cell phone.

I have come across about 5 locations, just this year, that did not have monitoring on their control power. Two of them did not have any active power at all, someone had turned off the DC breakers. One location had AC with a CAP trip device that discharged due to not having under voltage protection and so was not available when it was finally needed.

This seems to be most problematic with facilities that do not have a large number of protective relays and do not understand their operation or maintenance requirements.

 

[zog]

I have come across about 5 locations, just this year, that did not have monitoring on their control power. Two of them did not have any active power at all, someone had turned off the DC breakers. One location had AC with a CAP trip device that discharged due to not having under voltage protection and so was not available when it was finally needed.

This seems to be most problematic with facilities that do not have a large number of protective relays and do not understand their operation or maintenance requirements.

Yep, that is why I said "usually". When I come across a situation like that, say for example the battery charger breaker tripped and the batteries discharged for example I get all the key guys together and explain the situation, thier faces usually turn white when they realize they have no protection.

 

[jimmac49]

Another level of protection that we use is to have trip coil monitors on our circuit breakers as well as the coils of 86 LOR devices. They all report to our SCADA system as alarms. They will indicate open trip coil , loss of voltage, or breaker trip failure all from one device.

Jim

 

[jim dungar]

Yep, that is why I said "usually".

I guess my disagreement is with your phrase "usually triggers an alarm".

I just got back from a new data center construction site. Had to have customer provide a temp source of 120VAC control power so we could close the 34.5KV station breaker. Customer appeared to be pleased to find out that by removing the control power he would be able to 'prevent' nuisance trips during commissioning, but he promised to turn it back on before the job was finished.

 

[gadfly56]

I guess my disagreement is with your phrase "usually triggers an alarm".

I just got back from a new data center construction site. Had to have customer provide a temp source of 120VAC control power so we could close the 34.5KV station breaker. Customer appeared to be pleased to find out that by removing the control power he would be able to 'prevent' nuisance trips during commissioning, but he promised to turn it back on before the job was finished.

"We've got trouble, trouble, trouble,
Right here in River City,
That starts with a "T"
And rhymes with "P"
And that stands for PINHEAD"

Customer, that is.

 

[jim dungar]

"We've got trouble, trouble, trouble,
Right here in River City,
That starts with a "T"
And rhymes with "P"
And that stands for PINHEAD"

Customer, that is.

kkkkkkkkkkkkk

 

[zog]

I guess my disagreement is with your phrase "usually triggers an alarm".

Well it does, I am in a different facility nearly every day for the last 15 years and more often than not there is some sort of alarm, mayne not anything through a SCADA system but at least something local.

I just got back from a new data center construction site. Had to have customer provide a temp source of 120VAC control power so we could close the 34.5KV station breaker. Customer appeared to be pleased to find out that by removing the control power he would be able to 'prevent' nuisance trips during commissioning, but he promised to turn it back on before the job was finished.

Yeah, sure he will. "Prevent nuisance trips", how do these people get these postitions?

I ran into a organization at a conference last week that has certification programs for Maintenence Manager, Plant Managers, Facility Managers, etc... Testing and (real) certification. I am so happy to see that and hope it becomes the norm because I have had to work with some people in those positions that have no business having that title.

 

[iwire]

Well it does, I am in a different facility nearly every day for the last 15 years and more often than not there is some sort of alarm, mayne not anything through a SCADA system but at least something local.

It strikes me that the facilities you spend most of your time in have a common theme, they are interested in doing the right thing that is why you are there.

The few times I have run into MV gear has been at universities or hospitals and they did not appear to do any up keep. I asked one facility electrician to kill the feed to a 4000 amp 480 volt panelboard they wanted me to hot swap a breaker in. He explained to me that would have required unracking a switch that they where having trouble with and did not want to touch. The equipment looked to me to be from the 50s.

BTW I refused to do the swap and stood about 50' away while the two in house guys did it in street clothes.

 

[zog]

It strikes me that the facilities you spend most of your time in have a common theme, they are interested in doing the right thing that is why you are there.

You know, I never thought of it that way and I suppose it makes sense. Most of the stuff we do is preventative and predictive maintenence testing.

It is the emergency calls, the blow ups at 3AM on Sunday, that have all the scary looking stuff.