Wye / Delta step up

[delhi81]

I am having some issues with a 208Y 4w / 4160D system. we are stepping up from 1200 amp 208 to a 300 kva 120/208 Y to 4160D transformer to another building and stepping back down to 120/208 3ph 4w system. the problem when we transfer from generator power to normal with inphase monitor on the transfer switch, we see a high in rush, enough to trip the 1200 amp main in most occasions. we are pulling about 150 kw at the time. the main is not ground faulted,we do have a full netural to the transformer. we feel this may be causing the problem. any thoughts?

 

[rbulsara]

some basic questions first:

Is this a new problem or has been there since initial installtion?

Have you checked that the in-phase monitor is in fact working correctly? Have you verified it with a voltmeter or some other method?

How many phases does this 'monitor' monitors?

 

[Rodmcm]

Where is the genset connected, first or second building,, where are the loads?

 

[delhi81]

rbulsara this is a new problem since we installed the step up transformer. we did use a volt meter with factory tech. and it monitors all three phases.. we have an allen bradley power monitor on line and it records a max current each time from 1.4ka to 1.6ka...

 

[delhi81]

rodmcm the gen-set is in the first building conneccted with main service... and load is connected to transfer switch.

 

[jghrist]

Is the transfer back to utility source open- or closed-transition? Does the 1200A main trip if you energize from a dead condition or only when transitioning from the generator?

The inrush on the 300 kVA transformer could be high enough to trip the main if there is an intantaneous overcurrent trip. If there is secondary protection on the tranformer, the primary protection can be 250% of full load. This may solve the inrush problem. Another way is to use an overcurrent device with an inverse characteristic that allows more time for the inrush to die down. Inrush could be as high as 25x full load current for 0.01 sec, 12x for 0.1 sec, 6x for 1 sec, 3x for 10 sec and 2x for 100 sec.

 

[delhi81]

jghrist

it is an open transistion... we have looked at an updated micrologic type breaker. this would allow for some "tweeking" of set points. rightnow it is a simple thermal / mag. breaker. the consederation of possible ferroressonence is my main consern on a delta wye config.

any thoughts on that?

 

[rbulsara]

Oh..if its open transition then....it is definitely the inrush...even if you check the phasing it does not matter as there is at least 5 -6 cycles of gap...minimum time a breaker would take to operate...

In fact in trying to close supply on a transformer before it is completley de-enegized (all magnetization decayed...), very heavy in rush will be created...

If anything waiting for a few seconds before reapplying the power will result in lesser inrush (more often than you are getting now)..Forget the in-phase monitoring..its only useful if you are doing closed transition.

A 300 kVA transformer can have inrush up to 12 times its rating for 0.1 sec. It appears that you are now energizing two transforemers in series, compounding the inrush issue..

You need to upgrade your breaker to 1600A and with static trip uit in which you can set (or eliminate) instantaneous setting to 10X pick up or more. (which equals 1.6kA)

(This may appear violating NEC requirement of no more than 125% OCPD, but Code does not preclude properly engineered installation if required to accomodate inrush)

Also you can replace the breakers with dual-element time delay fuses.. you need to check fuse time curves to see that 1.6kA at 0.1 seconds lie below the curve..

 

[busbar]

See §7 Ferroresonance in IEEE C57.105-1978 …Transformer Connections in Three Phase Distribution Systems. Delta/grounded-wye is cited as a transformer connection susceptible to ferroresonance, but also states: “…experience in the utility industry has shown that ferroresonance is not as likely at the lower voltages (4160Y and 8320Y) as at the higher voltages.” A single-pole condition {fuses} is needed to initiate ferroresonance.