SPD in MCC 1

[wroggent]

We recently had a surge in a plant 'due to one the utility's fuses blowing.' The surge destroyed 3 contactors in one of our MCCs. We don't currently use any SPDs in the plant with a possible exception to small point of use units, e.g., integrated in a power strip. I've done a bit of reading on SPDs and I understand that there can be a great variability in there effectiveness, depending greatly on where or how they're installed. For example, wall units that potentially make use of long leads are less effective than (panel) integrated units because of the inductive reactance introduced by the leads. So far I've only looked at GE and Eaton's offerings for SPDs. Eaton offers a unit that supposedly is designed for installation within an MCC. Do any of you have experience with these units? The intention of installing an SPD in the MCC would be to avoid damage to the MCC components (and also connected apparatus) due to a surge from occurring again. Would the SPD function effectively in this capacity? Would the placement of the SPD in the MCC be a significant consideration, i.e., are the buses in an MCC of sufficiently low impedance to allow free placement of the SPD? What approach would you suggest for avoiding damages due to surges (internally or externally created) in a plant? MCCs are 1600A 480Y/277V from various manufacturers.


Thanks

 

[waross]

Define "Surge" I spent some time in an area where "surges" were commonplace. The only thing, the issues, while real were actually under voltage conditions.
We recently had a surge in a plant 'due to one the utility's fuses blowing.' Did the surge blow the fuses or did the fuses blowing cause the surge?
In what way were the contactors destroyed?
It may be wise to identify the origin of the "surge" before trying to mitigate future issues.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

We recently had a surge in a plant 'due to one the utility's fuses blowing.' The surge destroyed 3 contactors in one of our MCCs.

I'm having trouble right away with this part of it. More likely you had a single phasing event by blowing only one fuse and you have nothing in your system to shut down motors and other loads in the case of a phase loss. The contactors were damaged because of the added load, and/or chattering, something like that. An SPD would not help with that, you would need a Phase Monitor Relay, or better overloads in your MCC that have it built in.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[wroggent]

I'm not sure what happened first. I was told that the plant personnel contacted the utility when they lost (partial?) power. Allegedly the utility explained that they had an issue with a residential load and one of their (the utility's) fuses had blown. My guess is there was a L-G fault somewhere on the residential feeder which caused the fuse to blow. I don't know if the plant is on the same feeder that the fuse blew on, i.e., we may have never lost a phase (apart from the brief period when one of the phases was grounded). - I guess I should verify that with the utility. The contactors we use have an overload relay attached - if it was set (dialed) correctly wouldn't it have tripped? Or, could the contactors have been destroyed before the fault was cleared, e.g., the damage is due to over voltage on the other two phases? I don't know in what way the contactors where destroyed; I just know they needed to be replaced. In short, it seems unrealistic to me that the contactors could have been destroyed due to TOV during the fault, or due to overloading from continuing to run on two phases after the fault cleared - hence me mentioning a surge. Sorry about the lack of information; I'm an EE intern (with little experience) in an office with only ChemEs (so there isn't anyone to ask). Thanks again for the assistance.

 

[waross]

There are many ways for a contactor to fail. Some designs may flash across from phase to phase if they open under a heavy load. A simultaneous over voltage condition may make this more likely. This does not apply to all designs of contactor.
An under voltage condition may destroy the coil. If the coil swells when if burns up, some electricians will replace the entire contactor.
A contactor should be able to interrupt reasonable overloads and over voltages without suffering damage.
I am leaning towards burned coils as a result of under voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hdp]

I have been involved in a couple of situations where a single-phasing event during a storm was interpreted as a "surge" issue. In both cases, the contractor control power was derived inside the customer facility from a 480V-208Y120V transformer. The damage was caused by the reduced voltage on the 120V control power circuit, due to the loss of voltage on one of the 480V phase conductors. The reduced voltage led to contractor chatter and the ultimate failure of the contractor coils.

 

[jraef]

Bingo on hdp's post. That's exactly where I was going with that.

If you have an industrial facility with a lot of 3 phase motors that are already running when a phase is lost, those 3 phase motors will act like a "spinning reserve" to regenerate a 3rd phase, just like a Rotary Phase Converter does. But also just like an RPC, the control power must come from the "real" phases, otherwise it can drop precipitously and cause contactor coils to chatter. It's a very common problem and a key issue with regards to having phase loss protection that will shut down motor controllers.

Also as to the motor thermal overloads tripping on phase loss: common misconception. Basic thermal overloads will NOT trip on a phase loss. They only trip on INCREASED CURRENT. But if your motor is only lightly loaded, the current increase may not be enough to trip the overload relay quickly enough to avoid damage to components in the system. If you had Solid State Overloads that included Phase Loss protection (most now do), then that is fine. But if not, you cannot rely upon a bi-metal or melting alloy OLR to protect your system from damage due to phase loss. This is I know anathema to a lot of the sales literature for IEC style bi-metal OLRs, but it is true nonetheless. IEC overloads really only offer phase loss "sensitivity" in that the trip curve is biased to trip earlier on a phase loss, but it is not full protection. That might protect the contactors I'll admit, but the motor can still be damaged by the extra heating from the severe current imbalance that occurs. But if you are in North America and you had NEMA style overload relays in an MCC (as is usually the case), most of them don't have that feature anyway and you can definitely over heat the contactors on phase loss.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[hdp]

Pardon my spell check for incorrectly providing "contractor" for contactor!

 

[jraef]

They all do that until you train them.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[wroggent]

I'm a bit confused about how the chatter occurs.

1.) Is the reduction of control voltage while the contactor is still closed due to the motor slowing down?

2.) For an RPC, if single phase 480V is supplied to two terminals, what should the expected voltage be on the third terminal when the RPC is lightly loaded, heavily loaded?

3.) If the phase that isn't used to derive the control power is lost (opened), would the chatter likely still occur? Example: The L-L voltage is normally 480V. Phase 1 and 2 power a 480/120V transformer, the 120V is used for control. If phase 3 is opened, wouldn't V_12 stay near 480V?

4.) Once the contactor opens, why would it reclose? Inductance of the control transformer? (v=Ldi/dt)

 

[waross]

Most of your questions fall in the "It depends" category.
But;
Phase loss;
The motors act as induction generators and attempt to support the missing phase. They are usually unable to fully support the non-motor loads on the lost phase and as a result the voltage drops on that phase.
If a contactor had a simple core it would tend to start to open every time that the sine wave passed zero. To avoid this there is a single turn winding across part of the core face called a shading coil. This causes a phase shift of the magnetizing force across part of the core face. This holds the armature firmly seated during the zero crossings of the supply voltage.
As the voltage drops, the shaded part of the core may not develop enough force to prevent the contactor from opening slightly only to be slammed closed by the next rising half cycle. Hence chatter with low voltage.
If the voltage drops so low that the contactor drops out completely, a large air gap is introduced into the magnetic circuit. As a result the inductive reactance drops considerably. This also drops the impedance and the current rises. If the voltage is not adequate to pull in the contactor, the coil may overheat and burn up with much less than rated voltage.
Remember that a phase loss is an abnormal event. Currents may rise to abnormal levels and voltage drops may increase to abnormal values.
Motors will seldom slow down more than a few RPM but they will draw more current. This will add heat but also the rotor heating may be disproportionally greater as the rotor transfers energy from the healthy phases to the faulted phase in it's role as an induction generator. Even though the motor current may remain below rated current for a motor with less than 100% loading, the motor may be overheated due to the extra rotor heating.
What's an RPC?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Rotary Phase Converter.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[wroggent]

If the voltage drops so low that the contactor drops out completely, a large air gap is introduced into the magnetic circuit. As a result the inductive reactance drops considerably. This also drops the impedance and the current rises. If the voltage is not adequate to pull in the contactor, the coil may overheat and burn up with much less than rated voltage

We use a push button to supply power to the contactor relay; when the button is pressed one of the relay contacts closes, the contact is wired in parallel with the push button and allows current to continue to flow into the relay after the push button is released. It seems to me that if the contactor drops out, power will be no longer be supplied to the relay and thus the relay contacts can't change state until the start button is pressed again. We don't use DC, but the following image is basically what I'm talking about.

 

[waross]

I am still wondering about the "failure" of the contactors.
That circuit can still chatter.
Many plants use remote control and switching of motor contactors and many pump and refrigeration applications use pressure, level or temperature controlled contactors all of which are at risk of low voltage failure without added protection schemes.
But we are all just guessing until we have more information concerning the details of your failures. I am sure that it seems obvious to you what has failed but not to us.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Bill has got the right of it. The contactor doesn't need to drop out fully, it just needs to release its magnetic grip just enough to make it chatter. A 3 wire circuit protects against a full control power loss for sure but under the right low voltage conditions, contactors can, and do, chatter and destroy themselves.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376