Dedicated 24VDC Supply for shunt trip breakers and protective relays 3

[kleppik]

I am working on grid connecting a synchronous generator. The control signal to trip the generator circuit breaker is 24VDC. 24VDC is also used to power the generator protective relay and intertie protective relay.

This 24VDC (a battery bank) is also used for cranking the prime mover. Is there a UL or CSA or IEEE standard that requires a dedicated supply for the shunt trip signal or protective relays?

 

[davidbeach]

I wouldn't share. What happens when you have a failed charger and just barely get the engine started before the batteries are down too far in voltage to be of any use? How are you going to trip for a fault now?

 

[dpc]

I don't know about an IEEE standard, but I agree with David Beach - leave the cranking battery as a stand-alone system and put the relaying and breaker control power on a quality station battery.

 

[kleppik]

Your advice is appreciated. I would have more leverage with the generator supplier if I could refer to an industry standard.

 

[rbulsara]

There is no code or standard preventing this as far as I know. You can do this, but you will have a system with very poor reliability.

A good design will have a separate battery source for the protective relays and the gen start batteries used as back up via proper blocking diodes.

Standards are not design manuals and not every aspect of a system design is covered by a code or a standard.


Rafiq Bulsara

http://www.srengineersct.com

 

[alehman]

Rafiq's suggestion is common practice in my experience.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[catserveng]

Rafiq describes how we do it, when the engine cranks the starting battery voltage can drop as low as 16 VDC on a 24 VDC system, also when the pinion solenoid kicks out it throws a pretty substantial spike, hard on the smart pieces of control equipment.

On smaller systems like rental modules where a second battery system is a tough sell we use a buffer power supply, like the Phoenix Quint or the 1606 series from Allen Bradley.

I have also used a 120VAC to 24VDC power supply for the controls and protections and backed it up with starting batteries and blocking diodes as mentioned above, but frankly, the seperate systems seem to give the best service life on the control and protection components.

Just recently added a second battery system on a smaller (1.5 MW) cogen unit that used to be setup as you're looking at, only the starting batteries providing all control power, after the third inter-tie relay failure. Comparison, each inter-tie relay repair cost $2500 plus our labor, the second battery system, a SENS charger, two motorcycle batteries, wiring and install was $1250 and I'll likely never go back for that problem. Maybe my boss won't be too happy to lose the ongoing revenue, but hopefully the customer gets the idea, hope you do too.

Hope that helps.

 

[busbar]

Another problem is that motive-power batteries are almost always grounded, where protective-relay systems are floated, increasing reliability. [Don’t put all your eggs in one basket.]

 

[racobb]

I have also seen instances when the voltage dip on engine starting would cause electronic microprocessor equipment to restart with the associated grief.

Alan

 

[rhatcher]

Check the schematics to be certain where the power source is. I will bet frothy beers on the house for anyone in my area that the relay and circuit breaker source is already a separate supply from the generator output through a transformer and rectifier.

If it is connected to the generator's alternator and starting batteries, that is not necessarily a bad thing. The generator must start and come to speed first, must have excitation applied, and must have the circuit breaker closed before there is any output that may activate a protective device. At ths point, the engine alternator will be the power source, not the batteries.

That is my experience and opinion. What do you guys think?

 

[rhatcher]

I will add that if the generator output is the source of the power to the protective relays, then loss of power to the protective relays also equals loss of power to supply a fault.

 

[rbulsara]

rhatcher:

What you are describing can be true for a shut trip of stand-alone generator breaker where there are no other discrete protective relays and generator breaker relies on its built-in trip unit for the overcurrent protection. There shut trip is not part of the primary protection but a back up or emergency stop. (Usually small low voltage units).

The systems we described above are for more extensive set up where generator(s) are also paralleled with the utility and/or other gens for some extended period. There many discrete or multifunction relays are employed, for not only the gen breaker but also for the utility breakers and other system breakers (ties and feeders). In those cases, a clean and reliable relay control power is required all the time, even when the gens are not running. There also is a continuous draw of power for those relays. You do not want them to drain the engine batteries during normal conditions.

Even with a single generator system, where discrete relays are employed, a separate control power source is advisable.

Rafiq Bulsara

http://www.srengineersct.com

 

[rhatcher]

rbulsara:

I stand by my previous post, although I will add that it would be better if the separate source (generator-transformer-rectifier) included a battery. Sometimes that is the case and sometimes not. I also still believe that for small systems that a source derived from the starting batteries and engine alternator is not necessarily a recipe for disaster. It is not the best thing, but not a disaster. Finally, I do not claim to be an expert on anything.

That being said, your post seemed to indicate that my response was not correct for the question at hand. I have read the thread a few times and I am not sure what I am missing. The OP states that he has a grid connected generator with two protective relays. You state that:

"The systems we described above are for more extensive set up where generator(s) are also paralleled with the utility and/or other gens for some extended period. There many discrete or multifunction relays are employed, for not only the gen breaker but also for the utility breakers and other system breakers (ties and feeders)."

What did I miss?

 

[rbulsara]

The point.

Rafiq Bulsara

http://www.srengineersct.com

 

[catserveng]

If the generator is the only power source, then control and protection from it's starting batteries is likely ok because if something happens to the engine, it will likely go down and power will be lost. But maybe the breaker won't trip and on restart that may be a problem.

When paralleled, to either other units or the grid, the generator can be either closed into dead (inadvertent energinzation-very bad) or if the prime mover fails the generator can motorize, and can cause damage. Once a unit becomes part of a networked system, it control and protection are no longer the concern of only that unit, but the entire connected system.

On newer systems with electronic controls, if power supply voltage starts to fail, unexpected things happen, the controls usually don't just turn off, they get erratic, and can make things interesting.

Hope that helps.

 

[busbar]

If the relay load is not too great, you might consider one of these.

http://www.selinc.com/sel-ups

 

[racobb]

Please see my and catserveng previous post.

Alan

 

[rcw retired EE]

The Ontario Electric Safety Code (OESC) which applies in Ontario, Canada specifically requires that power for protection and tripping not supply anything else.

Subrule 14-308 "Battery Control Power for Circuit Breakers (1) When power for operating a circuit breaker is dervied from a battery, the battery shall not supply any load other than the circuit breaker and its associated control circuits and the battery voltage shall be continuously monitored."


This section deviates from the Canadian Electrical Code (CEC) which only requires monitoring. Both codes require either tripping on loss of control voltage or a continuous alarm when voltage is too low to operate the breaker.

Go with a seperate battery system.

 

[FreddyNurk]

rhatcher (Electrical)
14 Oct 09 20:38
I will add that if the generator output is the source of the power to the protective relays, then loss of power to the protective relays also equals loss of power to supply a fault.

If in the case (as highlighted in the first post regarding grid connect) if you can't trip the breaker then you're faced with the situation that the breaker may be closed but the engine is off, which isn't particularly good for it.

If in the case of losing the grid supply, you're then faced with that situation when the grid does come back, which again, isn't particularly pleasant.

That said, my preference is a separate station battery system feeding all the controls. You can get away with 240V (I'm in IEC land) spring charging motors, which has the effect as rhatcher stated that you can't close the breaker unless the generator is running (providing you get the power for it from the generator rather than the bus), but its still better to provide power from somewhere else for the controls.

We've got away with the generator fed power supply to run the controls, but in those cases getting the thing to start off the engine batteries in the first place isn't nice. If you're running a rental that is set up with one set of batteries, expect to have fun getting the thing started at least once during the hiring period.

 

[kleppik]

Rcwilson, thank-you so much. And to the other posters who have made it quite clear that it is better to have a separate battery (linked to cranking batteries via diode and a common battery charger) to power the protective relays and shunt trip signal to the generator circuit breaker.