Help Understanding what role mechanically latched contactors play for MV motors 1

[Wfg42438]

Hello,

I have some basic questions regarding contractors when used for medium voltage motors. Any help on this would be much appreciated, my goal is to broaden my knowledge of this equipments application.

A colleague of mine was working on a system study where the facility had a 7.2 kV Mitsubishi switchgear had mechanically latched contactors in series with power fuses for each motor connected to the switchgear (see image "PF_Snap"). The image "PF" element refers to the power fuse and "VCS" element refers to the vacuum contactor.

In addition to the fuse and contactor the loads had relays with 51/51G capabilities. I grew curious about what the purpose of the contactor might be in the event of a bolted fault.


The contactor model number is VZ4-PL-E rated for 400A continuous current and breaking current rating of 4kA.

What I can't seem to understand is the purpose of the relay if there is a fused contactor connected per load.


I assumed the motor would be protected solely by the fuse but not I am not 100% if that's a valid assumption.

Does this mean that the contactor can be placed in an open state if a fault occurs by the motor relay with 51/51G/49 functions?

I read a catalog I found of the contactor online but I can't seem to find a clear indication of how long it would take for the contactor to open if it indeed could be operated by a relay in the event of a fault.


Thanks in Advance!



PF Snap Image :

Catalog Images:

Catalog Link:

https://www.mitsubishielectric.com.au/assets/IASW/VZ-E-Product_Catalogue-Sept2011.pdf

 

[che12345]

Dear Mr.Wfg42438
Q1. "...I have some basic questions regarding (contactor) contractors when used for medium voltage motors. ... is to broaden my knowledge of this equipments application......
... the facility had a 7.2 kV ... switchgear had mechanically latched contactors in series with power fuses for each motor connected to the switchgear (see image "PF_Snap")...".
A1. In some cases, in MV but also in LV, " mechanically latched contactors " are used. The mechanical latch device* latches (usually) the standard contactor in the (close position) after [energisation]. The contactor coil supply can be/is cut off, but the contactor remains close. To open the contactor, a momentary impulse is issued to the mechanical latching device*.
This feature is useful in preventing the contactor from damage by chattering when (due to poor closing signal), momentarily voltage dip or power failures and returns in a vey short interval etc....
Q2. "...In addition to the fuse and contactor the loads had relays with 50/51G capabilities. I grew curious about what the purpose of the contactor might be in the event of a bolted fault..".
A2. All MV and LV contactors have very low short-circuit kA rating. In the event of a (bolted high kA fault), the [fuses take over in breaking the high kA]. The contactor shall remain close. Only on 51/51G which are of (much lower kA), the [contactor opens], while the fuse shall remained intact.
Q3. "... The contactor ... rated for 400A continuous current and breaking current rating of 4kA.
A3. You see that the contactor is rating of 4kA; which is very very low compared to the (system short-circuit level) which is usually in 20-50kA or higher. See above A2.
Q4. "...What I can't ... understand is the purpose of the relay if there is a fused contactor connected per load.
A4. See above A2.
Q5. " ...I assumed the motor would be protected solely by the fuse but not(now) I am not 100% if that's a valid assumption...."
A5. The motor is protected by 50/51G (low kA) by opening the contactor. The fuses take over on bolted-fault of high kA. See above A2.
Q6. "...Does this mean that the contactor can be placed in an open state (air) if a fault occurs by the motor relay with 51/51G/49 functions?..."
A6. Yes, you see that the vacuum bottle (which is the switching device) is [without any arc-chute]. There is NO open arc (out side the vacuum bottle) when [it breaks the fault current irrespective of the kA level].
Q6. "... I can't seem to find a clear indication of how long it would take for the contactor to open if it indeed could be operated by a relay in the event of a fault.
A6. In the LV, contactor are classed AC1...AC4 utility categories. From this chart you can read the details, but I don't see this chart in your attachment.
Che Kuan Yau (Singapore)

 

[waross]

Power circuit breakers are often used as motor starters for medium voltage motors.
They are typically operated by stored energy that is released by a small solenoid.
One solenoid is used to close the breaker, a second solenoid is used to open the breaker.
The open solenoid is available as reverse acting.
That is, it is held by power and trips on loss of power.
The choice is a design consideration.
Do you want your breaker to trip open or to ride through a voltage dip?

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

 

[Kiribanda]

Is it possible for you to provide the "Prospective 3-ph short circuit current" at the 7.2kV swgr bus?
Most of the answres to your questions can be found in that kA value.

 

[LionelHutz]

The mechanical latching is likely a site desire.

The contactor should co-ordinate with a fuse up to a certain size, be it latching or not. Even not latched, the contactor takes some time to open and by the time it starts to open the fault is already cleared by the fuse. We've done testing multiple times to show 50kA co-ordination with a similar non-latching contactor using E and R rated fuses.

The relay is added to provide better protection than fuses alone would provide. Your installation codes likely require at least a motor overload and the fuses won't provide this overload function. In your case, the relay will be set to trip the contactor on lower current <4kA faults but to not trip it on any higher fault currents.

 

[RRaghunath]

Contactor is not designed for opening heavy currents beyond 8xtimes its rating, safely. There is a maximum fuse rating and type mentioned on the contactor label that can ensure its safety.
The motor protection relay has prolonged start protection, stall protection, unbalance current protection, earth fault protection, overload protection, no. of starts limit protection etc. that will not involve currents beyond the contactor rating. These protections can be allowed to open the contactor. Motor protection relay also has short circuit protection but is always kept disabled when the motor is being switched with Contactor.
The above applies whether the contactor is latched or unlatched. Only additional thing with latched contactor is Under Voltage protection for motor. This is also to be enabled and set in the motor protection relay. Unlatched contactor drops-off automatically when there is voltage dip in (or loss of) power supply and hence, no separate Under Voltage protection is provided.

 

[thermionic1]

I have seen a few installations where the digital relay has an instantaneous element set to block the operation of a relay based trip, thus preventing any damaging operation of the contactor trip and letting the fuses taken it.

 

[oldfieldguy]

The latching feature gives the motor a 'ride-through' on momentary power blips. Choosing a setting for the protective relay's undervoltage element allows the engineer to decide how long that time might be. Even unlatched contactors can take a cycler or two before dropping out and some manufacturers provide that feature as one of the capabilities of the contactor closing circuitry.

As far as opening on a short circuit, the fuses will win the race between their clearing time and the clearing time of the protective relay, preventing the contactor from trying to clear short circuit current.

Most relay applications I've seen for motor contactors do not use the 50 (instantaneous overcurrent) element because we don't want the contactor trying to interrupt high currents. The 49/51 elements have enough delay, even at high currents, to allow properly selected fuses to clear the fault.

old field guy