Contactor versus Breaker 8

[fieldserveng]

I am involved in an application where a synchronous generator is being paralleled to the utility across an electrically held contactor. A manual circuit breaker is being used upstream of this contactor to provide overcurrent protection and fault clearing. In all of my experience I have never run into this scenario. In other words the industry standard that I am used to has allways employed syncronizing across a circuit breaker.

I do not believe that this (using a contactor versus a breaker)is a robust design yet I cannot find any reference that discusses this subject.

As I am preparing for an FMEA of this design is anyone aware of a published standard or recommendation regarding the use of a contactor in such an application?

 

[alehman]

I've never seen this either. Electrically held contactors are a poor choice unless a reliable source of control power is available. If there is a momentary drop out of the control power source the contactor may open and close, causing loss of synchronization which could be disasterous. At least consider a mechanically held contactor. Circuit breakers are generally more rugged and reliable, and provide overcurrent protection so may be more economical.

 

[PWR]

I have seen small hydro units (under 1 MW) using latched vacuum contactors with fuses providing short circuit protection and conventional protective relays for other functions. I doubt if you'll find such a practice endorsed by a standard; even for small units.

 

[ScottyUK]

The OP doesn't state what size the generator is.

We installed a small generator and source transfer & synch control system delivered as a package by Cummins at our site last year. They proposed use of a contactor for connecting the generator (350kVA) to our internal 415V network. Their PCCP control system was set up to do this, and they said it couldn't be configured for a pulse-ON pulse-OFF control of a circuit breaker using separate closing and tripping coils, which I struggle to believe. We used a hard-wired solution to convert their relay output to a pair of signals interfacing to our circuit breaker. I didn't like the idea of using a contactor in place of a breaker any more than you guys, and for the same reasons, but it seems that it is not unheard of and may even be standard these days.




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

 

[fieldserveng]

Thank You all for your responses. I honestly don't expect to end up with a documented reference to using a circuit breaker over the contactor for I have done quite an exhaustive search on the subject.

I do welcome any opinions or experiences that you could share on the subject though as I intend to use these resposes in my presentation to the design engineers. Pro's and Con's alike.

Obviously the biggest pro of using a contactor is the cost reduction for the hardware. However the cons are the convoluted logic required to get the contactor to act like a breaker (from the controls perspective not the protective functions), and the extra real estate for the additional hardware.

The application is for a small DG (<500 kw). Also worth noting is that already in the development of this application several events have occurred where out of sync closures have happened and equipment damage has been involved. Unfortunately the fix so far has been to add more devices and protection in the contactor's control circuit.

As you can tell by the flavor of my comments I do not support the use of the contactor. Unfortunately, so far, I'm forced to go along with it. I'm hoping that I can find some real ammo to use in my argument for a circuit breaker.

Thanks...

 

[ScottyUK]

Another factor which I meant to include above is that contactors are great for making onto high currents such as the starting current for an induction motor, but have very little ability to break fault current. Contactors for motor control are always backed up by a breaker or HRC fuses for this exact reason, and the coordination between the thermal overload relay and the fault protection has to be such that the contactor does not try to open when fault current is flowing.

The upshot of that is: what is the fault level on your system at the point where your generator is connected? If the generator can be sync'd to the utility then the fault level could be quite large. You need to consider if the contactor could open under fault conditions, and then ask if it could safely break the fault current. Is there something else backing up the contactor such as HRC fuses?

Most generators can't deliver exceptionally high fault levels because the AVR will act to limit the set's output, but do you trust the AVR enough to rely on the built-in protection alone? I don't, and I deal with AVRs from the 350kVA set I mentioned up to those on the 365MVA turbo-alternators. On all these sets, dedicated short-circuit protection is provided external to the generator's control gear.




----------------------------------

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

 

[buzzp]

Scotty makes some good points. But the breaker should be the one clearing fault currents. Of course, any other protective relays will have to use the contactor if the breaker does not have a trip coil so the points about breaking fault current are valid.
I have also seen contactors "chatter". This is where the control power is somewhat weak or low. If the supply for the coil voltage is marginal, the contactor may attempt to close in but when the coil is energized, it pulls the voltage down a little bit but enough to drop the contactor out. Then the process repeats itself. Granted this is likely a rare event but nonetheless could be very detrimental to your generator (and anyone around it).

 

[WoodrowJWeen]

I believe Generac is now using this arrangement (generator breaker plus contactor) for paralleling in their modular power system. Anyone had any experience with this system?
I'm curious though...the theme in this thread seems to be that breakers are better than contactors for switching high currents. If so, then why do most engineers prefer contactor type ATSs (ASCO, Russelectric, Zenith)instead of breaker type ATSs (Cutler-Hammer)?

 

[fieldserveng]

Speaking from experience the Russelectric ATS is NOT a contactor type device. It is a mechanically held switch and utilized a motor operator. If anything the Russ switch resembles a Breaker more than a contactor. Asco and Zenith both at one time tried modified motor contactors as did even Onan. I know these three mfgrs eventually gave up the practice of using contactors in an ATS application. Although I'm not really sure why. Nonetheless most ATS's built these days are truly switches using a mechanical means of maintaining the position of the load contacts not magnetically holding/maintaining these contacts.

 

[WoodrowJWeen]

I'll rephrase my question. Why do so many engineers (including the USACOE) reject circuit breaker type automatic transfer switches.

 

[davidbeach]

Why do so many engineers (including the USACOE) reject circuit breaker type automatic transfer switches.

Over time there have been a lot of bad designs for circuit breaker transfer switches. There are two general types of circuit breaker transfer switches; electrically operated and mechanically operated.

The electrically operated switches rely on the trip and close coils of insulated case or power circuit breakers, and these breakers also need a spring charging motor. There should not be any particular reliability concerns with this type of transfer switch.

The mechanically operated switches rely on some mechanism to move the breaker handles. In the older designs, this mechanism rotated on a point midway between the two breakers and applied a off-center torque to the breaker handles with the result that sometimes the handle broke. Newer designs apply the force to the breaker handle directly on its axis of motion and should be more reliable.

 

[alehman]

I suspect the COE requirement refers to designs involving molded case breakers with some sort of mechanism to operate the handles. MCCB's aren't intended for this type of duty. Most ATS mfg's have gotten away from this type of design.

Circuit breakers used for ATS applications should be electrically operated, insulated case or power circuit breakers.

 

[RRaghunath]

"Automatic transfer of power supplies" always demands fast action on the part of switching apparatus involved. This is where the contactors score over the circuit breakers (in terms of fast close / open operations) and hence preferred.

The circuit breakers are preferred only when the contactors are not available for the current ratings required, in my experience.

Trust the above is helpful.

 

[fieldserveng]

Just a friendly note to bring any further comments back on track with the original question. The application does not involve transfer switches. It is a small DG with a synchronous generator being paralleled to the utility using an Electrically held contactor. The genset manufacterer is sychronizing across this contactor. Personally I would prefer to use a Breaker yet I am trying to find support for my argument. The comments regarding chatter and control power problems are acknowledged and are quite viable. I also very much appreciate the recommendation of a mechanically latched device. If you have any experiences (good, bad or indifferent) with a similar combination of hardware that your willing to share I would certainly appreciate your comments.

Thanks

 

[stevenal]

IEEE 1547 doesn't specify that a breaker be used, but does have some parameters. Can the contactor and control meet these parameters consistently? Does the manufacturer of the contactor agree with the application? Does it meet the listing? And what happens during a fault. Will the fault induced voltage sag cause the contactor to release before the breaker can clear the fault?

 

[jraef]

fieldserveng,
I think that the application needs to consider the duty cycle involved. I did some low-head hyro generator control systems back in the early '80s where farmers were using irrigation ditch drops to recover electricity (let's not get into that tangent though). We used motorized MC circuit breakers in the arrangement based upon conventional designs, but quickly discovered that MCCBs are not designed for use as a contactor because the mechanical components wear out. When we confronted the manufacturer, Westinghouse, about it they denied responsibility. They stated that breakers are designed as protection devices with limited mechanical life, not control devices that would come on and off repeatedly because the mechanical life attributed to MCCB design is only 10,000 operations (I have never seen that corraborated though). They suggested the exact design you described in the beginning. Breakers for protection, contactors for switching. We did change, and used latching vacuum contactors. The mechanical wear out problems went away.

&quot;Venditori de oleum-vipera non vigere excordis populi&quot;

 

[RRaghunath]

I think latched contactors (with fuse back-up) are as good as breakers and in fact better when it comes to high number of switching operations and speed of operation.