Motor T.D. Undervoltage Restart

[nightfox1925]

We have one 13.8kV double ended switchgear with NO bus tie with two outgoing feeders feeding two 4.16kV MCCs via two 13.8-4.16kV step down transformers in each bus (total=4 MCCs). These MCCs supplies power to several 4kV induction motors. There are 5 nos 4kV motors in each MCCs which have time delayed auto-restart control scheme. Based from load flow and motor starting, it is proposed to group these motors with auto-restart into two (Group A and Group B). In the event of an open transition transfer scheme, group A motors will auto-re-start, run up to full load speed and then group B will automatically restart.

The existing auto restart control on each motor starter consists of two timers: ON-Delay TD2 which is set at 0.6 sec used to start the contactor and wired to bypass the seal-in contact. The OFF-Delay timer TD1 acts to delay breaking the continuity of the auto-start contact of TD1. During a temporary outage of control supply, TD1 opening is delayed by 6 seconds, if control power has returned within 6 seconds, ON-delay TD1 coil is energized and its contact closes within 0.6 seconds and energizes the main contactor via TD2 permissive contact. If control power resumption exceeds 6 seconds, the OFF-delay timer TD1 breaks and the auto-restart function is blocked.

It is proposed that the Group A motors existing timer settings will be retained. However, for group B motors, a 10 second additional timing should be added to both TD1 and TD2 to ensure that the highest rated motor in Group A has successfully reached full load speed. All motors are rated for auto-restart duty.

Are there any concerns or recommendations? We appreciate any valuable comments. Thank you.

 

[nightfox1925]

Anyone?

 

[jraef]

I see that you have a Multilin relay in the scheme, why not do all this in the Multilin? In a typical restart scheme you would stagger the motors, but is there something about the process that requires that 5 of them be started together?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[nightfox1925]

I do not have any information about the actual process requirements but the recent scheme is that all these motors were getting restarted at the same time.

A motor starting study was conducted and found that starting these babies resulted in unacceptable voltage drops on particular buses downstream. Because of this, it was recommended to group them into two and two sequential starts.

The highest rated motor on group A is a 4kV (323kW)Forced Draft Fan motor. I know this is a high inertia load and is it safe to assume a worse starting time for this motor at 15 seconds? or 20 seconds?

 

[Pikoplatt]

Does your 13.8kV Bus-Tie auto-close if you lose power on either the 'A' or 'B' incomer? If yes, the the closure of the Bus-Tie should be permitted by a 27R (Bus residual Undervoltage relay) located on the de-energized 13.8 kV Bus,to ensure that all motor back-emf has decayed before closure of tie breaker. Control power will be restored to all 4kV motor starters as soon as 13.8 kV Tie Breaker closes. The auto restart sequence can then begin from this point with the larger motor group first. Any reason why you cannot stagger each motor start within a group by 6-10 secs? Will your process allow it? This will be much easier on the 13.8/4kV Transformer, and result in less severe voltage dip as seen by other loads connected to the system, during the auto-restart sequence. If you have 4 motors in one group, within a space of 18 secs, you can have all started using a 6-sec gap, and remember to always start the largest motor first, unless the process requires a different sequence.