Motor Protection Scheme In Question

[nightfox1925]

One of the projects I am looking on involves some 3.3kV motor starters involving Fuse-Contactor motor starter arrangement. Multi-function Motor protection relays (MPR) are also installed in these starters connected to phase and zero-sequence CTs. I observed on our contractor drawings that all the protection elements including the 50/51, 50G element of the MPR are set to open the contactor.

Is this a good design since I always been eyeing on using the 50/51, 50G to trip a breaker and not a contactor? The 415Vac system is solidly grounded and hence the contactor might not be rated to trip on a phase or ground fault.

Further, all there are 415Vac MCP+Contactor starters with MPR relays and are wired in the same manner. I am proposing that the 50/51 and 50G wired to directly trip the MCP via shunt trip and not the contactor.

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[AllanUK]

The following is applicable to LV systems - not sure whether it also applies to 3.3kV but I don't see why not...

Fuse-contactor arrangement is ok provided that the fuses and contactor are selected to co-ordinate with each other (type 2 co-ordination - IEC 60947-4-1). Check whether the switchgear is designed for type 2 co-ordination.

"Type ‘2’ co-ordination requires that, under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and shall be suitable for further use".

Under short circuit conditions (with high fault currents) the fuses should operate before the contactor can act to interrupt the fault. Hence the breaking capacity of the contactor is not exceeded.

 

[dpc]

You do have to take into account that the contactor cannot interrupt short-circuit current and make sure your system gives the fuse sufficient time to clear these faults.

Some medium-voltage motor starters (at least in the NEMA world) have built-in time delay on contactor drop-out to make sure this happens.

But fused contactors for medium-voltage are used all the time. Much less expensive than a circuit breaker and actually rated for many more open/close operations.

 

[Electrifier]

Further to good comments as mentioned above, NEC Article 430-52 and Table 430-152 dictate phase and ground fault protection for low voltage motor circuits but the type of protective device is a designer’s choice. There are five types, each having different benefits depending on the size of the motor, cost of protection. The five types commonly used for low voltage motor protection are:
• Non-time delay fuses (non-current limiting).
• Time delay fuses (current limiting).
• Inverse time circuit breaker.
• Magnetic only circuit breaker.
• Motor circuit protector (MCP).

Medium voltage motors are fault protected by NEMA Type R current limiting fuses or Circuit Breakers operated by Instantaneous O/C relays and/or differential relays.
Ground fault protection is provided by a residual scheme, but zero sequence protection is the preferred scheme. (For Low resistance grounded systems Ground fault shall be isolated by either fused contactor or circuit breaker but for solid grounded systems ground fault shall be isolated either by fuses or Circuit breakers)

Hope this help

 

[Electrifier]

MANDATORY PROTECTION REQUIREMENTS FOR LV AND MV STARTERS PROTECTED BY MULTIFUNCTION RELAYS

• Thermal O/L Protection (Ansi Device 49): Overload (O/L) protection is always applied to motors to protect them from overheating.(Sends trip signal to contactor or Circuit Breaker)
• Instantaneous Phase Over-current (Ansi Device 50): This function provides short circuit protection while allowing the motor to start. (This feature shall be disabled for fused contactors since contactor are usually not rated to trip high fault current. The protection is provided by the fuse. Per NEC guidelines, the fuse size selection varies based on type of motor. Table 430-152 provides sufficient details for more specific sizing. Fuse provides short circuit protection while allowing the motor to start. The fuse used for motors is usually current limiting type (i.e. it clears the high current faults within a quarter of a cycle to limit the peak current that the contactor or the breaker may experience.)
• Instantaneous Ground Over-current (Ansi Device 50G): This function provides ground fault protection. (It shall be disabled for fused contactors used in solidly grounded system.)

There are some other features as well in Multifunction relays and used as per system requirement, motor operation and cost of equipment. These functions are normally time delayed i.e. under voltage, acceleration timer, mechanical jam, RTD, unbalance current etc. These functions are commonly assigned to trip contactor for fused contactors.


Hope this help

 

[nightfox1925]

DPC, does any contactor have sufficient making current rating to withstand a phase or a ground fault current until the fuse totally interrupts the S.C. current? I have checked some contactor ratings and they do have published continuous maximum (peak) service current ratings which are unfortunately less to overcome a short circuit current.

Electrifier: to sum up, the fuse to take care of the Phase and Ground S.C. currents and the contactor to open against overcurrent and undervoltage at normal operating conditions...then 50/51/50G/51G functions of the motor protection relay will be disabled? I think I can use them to at least send an alarm...what do you think?

Anymore suggestions on where I may apply the 50/51/50G/51G MPR elements on a fused-contactor motor starter application?

Above all, thanks you in advance for all the suggestions and comments.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[Electrifier]

Your are right for disabling Instantaneous O/C trip functions of Multifunction relays. On a solidly grounded system 50G element should not trip contactor as well. 51 Element (Normally available as Mechanical Jam)is set to trip contactor. It can be set to initiate alarm as well, on a lower setting. 49 Element O/L Alarm(Normally set for few seconds delay at O/L Pickup current) is also used to initiate alarm. Ground Fault Alarm(with a lower P/U and few seconds delay) is also useed to initiate alarm.
As DPC said, you need to care about that contactor can not interrupt short circuit currents. Just draw time/current curve of any function you are looking to configure and make sure that it is in safe limit for contactor.
Hope this help

 

[dpc]

Normally the contactor withstand rating is adequate for maximum fault current. At high fault currents, the curren-limiting fuse will clear the fault within 1/2 cycle.

You should be able to get withstand ratings for the contactors and check against your fault current levels.

The contactor's weak spot is in the interrupting capability.

As just an example, a Cutler-Hammer 800 Amp 5kV starter has a 1/2 cycle withstand rating of 86,000 A, a 1 sec rating of 10,800 A and a 30 sec rating of 4320 A. Its maximum interrupting rating (3 times) per NEMA is 13,200 A. IEC Break capability is 6400 A.

 

[nightfox1925]

Thank you very much to all who contributed their views and I will consider all the homogenous suggestions I got.

Aloha!

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[kptx171]

What is the 3.3 kv system fault level?I suspect 3.3KV system is resistance grounded.U dont have a breaker to be tripped by 50/51/50G.If the motor is used for an application where in the system fault level is low and within contactor ratings then the application of MPR may be more meaningful.

 

[nightfox1925]

Hi there kptx171, yes it is resistance grounded as per our contractor's design and the ground fault will have limitations in terms of current flow. However, the MPR are attached to protect the motor in both mechanically (jam, bearing temp., winding temp. etc) and electrically (overload, unbalance etc.) I refer to the 50/51: phase S.C. protection and 50G:Ground fault (if current flow is high)and if the contactor breaking capacity will be sufficient for all maximum fault current values. As I have stated earlier, I do disable them for this specific application and rely on the fuse to blow and open the faulty circuit.

The contactor breaking capacity is not designed for these type of faults. If it possible, is it a reliable justification that the current limiting effects of the fuse will limit the current to an acceptable value for the fuse to break? However, for the limiting effect to take place, the fuse should melt within the specified high current region and opens the circuit before the contactor and hence make the idea of tripping the contactor via 50/51, 50G of the MPR seem useless on the other hand.


GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[nightfox1925]

Hi again, I would like to correct the phrase "...acceptable value for the fuse to break?"

Should be "...acceptable value for the CONTACTOR to break?

my apologies

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[mohanlal]

I am familiar with the protection system for 3.3kv motors. The basic control is VCU (vacuum contactor unit)with back up fuse protection. The following protections are normally provided in the ABB-make REF542+ relays.Similar protections are available in the case of GE -make relays too.
1.0 Thermal O/L 49
2.0 Zero seq 51G
3.0 Neg seq 46
4.0 Locked rotor
5.0 U/V 27
Short circuit ptot. is taken care of back up fuses.
subra