Charging 5000KVA Transformer Through 750KVA DG Set

What have the Electrical Engineers connected with this project got to say on the matter?

Talked to the consultant, who told that inrush current of transformer should be withstand by the DG set. But DG set supplier is not sure for that. That is why i am searching for the solution.

Assuming all the testing is complete and the circuit protective devices are set-up properly, just throw the switch. Either it will hold-up, or it will trip off. Either way, there's the answer.

Does this mean that you have no electrical engineers on staff?

A better place to search for a solution is to hire a competent electrical engineer, and not rely on strangers.

Take PWSlack's advice and throw the switch, from far, far away.

Ooooooooh! Got it! Get the consultant to throw the switch.

I like it! I like it!

Get the consultant to put his health where his mouth is. If he starts to back-pedal, then he is full of it.

You are being paid to do this, the consultant is being paid to do this, and you want free advice from strangers unfamiliar with your situation?

Here it is then, first strap the consultant to the transformer, then be ethical and quit your job and find one that you're trained for. I can imagine the look on your boss's face as you say, "The strangers on CR4 said it was ok to try..." while the smoke clears.

Another free hint: your setup may or may not work, it depends heavily on the order of operations and the engineering parameters of the protective devices, DG, transformers, VFD, cabling, and motor. btw- you didn't even consider that your weak link may be the VFD which may not like all the surges, sags, reactive current, harmonics, transients, and off nominal operating points that it will be subject to.

There are ways to do this all properly and safely, finding them out will be money well spent.

"Then close all the circuit breakers and switches and run the diesel generator output up to rated voltage."

As far as energizing the transformer goes this would most likely work but I would make sure the motor breaker/contactor is open.

Applying less than rated voltage to any motor will cause a locked rotor current condition to be present and could cause severe motor damage and/or lead to a catastrophic electrical failure of the system.

If "applying less than rated voltage to a motor creates a locked rotor condition",how do you justify "reduced voltage starters"?.

Verify from transformer manufacturer.

Commenting on your last paragraph "Other way is to install ...".

-I am not sure that the excitation will build up slowly! The apparent shortcircuit on the output of the alternator might not allow any build up of the magnetic flux on the Genset Alternator and subsequently the voltage buildup. It will depend on the type of excitation installed. Usually, Alternators are started and allowed to build up voltage before loading them.

Please feel free to correct me if I am wrong.

If we study further how the magnetising current on sudden switching on of a power transformer rises to extent as if a short circuit is applied across the terminals of the 3 phase generator - here are the causes in brief:

1. The inductive current lags the applied voltage by 90 deg electrical - hence depending upon the point of closing of contact of the switch relative to position of sine wave the current at closing of CB is minimum or maximum.

2. Depending upon the polarity of residual magnetism when the transformer was last de-energised, the current can go to any limit. For example if the polarity of magnetism of hysteresis magnetising curve and residual magnetism is same and at maximum value-on switching on of Transformer, the total sum of residual and exciting MMF can lead to saturation of core - which is equivalent of air core -resulting in zero inductance of charging coil and very high current (high to the extent that resistance is the only part of total impedance of winding left to limit the current).

Hence from above you will find when we build up the voltage after closing the switch the question of initial short circuit does not arise. There is no inherent short circuit. It turns out to a short circuit only on sudden switching.

I have tried both of above methods to limit the inrush currents and have experienced success.

I have cut and pasted a write up I had prepared in 2004 on this subject. The brand name of alternators I was dealing at time has been removed or replaced with ..... also my address is not mentioned. Hope this will be able to through further light on this subject.

Guidelines for Energisation of Transformer on DG set.:

Whenever a transformer is switched on, it draws very heavy inrush magnetising currents from the source of supply.

Magnitude of Inrush Current:

The magnitude of inrush depends upon following factors.

1) Value of instantaneous voltage of sinusoidal wave at which the contact of breaker closes (Inrush being maximum for breaker closing at zero instantaneous voltage).

2) Short circuit impedance of source.

3) Inherent design of transformer.

4) Magnetic remenance in the core at the time of switching off of transformer.

When transformer is energised from an infinite source, maximum inrush current varies from 7 to 14 times its full load current.

Factors 1& 2 above contribute towards limiting the maximum value.

Effect on 3 phase Alternators.

1) Depending upon the value of inrush currents, the energisation of transformer can be seen as a severe short circuit fault.

A short circuit on a generator has following effects.

a) High electromechanical forces between adjacent coils of stator winding in over hang portion.

b) Sudden overload of excitation systems as it is switched into field forcing mode to maintain output of magnetic circuit.

All alternators designed for commercial use have required capability to face short circuits faults. The design is in line with that of other equipment such as indoor switchgear system etc. That means standard design is for probability of foreseeing limited number of such faults in life cycle of the machine.

In case of captive power plant where there can be almost daily switching of transformers such switching means daily subjecting the alternator to condition of high voltage fault close to its terminals. This is severe for long life expectancy of the alternator. Such a duty of daily feeding magnetic inrush currents of transformers is even more severe than that of a dedicated 3 phase alternators used in Short Circuit Testing Laboratories which are of special design.

2) Almost all application in power generation is for 3 phase alternators. 3 phase voltages being 120° (Electrical) apart, even if at the time of switching, assuming instantaneous voltage of one phase at peak (particularly zero inrush condition of transformer switching) the instantaneous voltage of other two phases will be passing through negative 50% of peak. Therefore energization of 3 phase transformer is always equivalent of heavy asymmetrical fault on the 3 phase alternator.

Of the two designs of rotors (cylindrical and salient poles) alternators with 4 pole (1500 rpm for 50 Hz) and higher poles (for lower speeds) are of salient pole design. The salient pole design has inherent asymmetrical reluctance of air gap of Alernator.

A symmetrical 3 phase short fault current produces only 2^nd harmonic in stator current of 3 phase salient pole alternator which is dampened by damper cage winding in main field.

But in case of asymmetrical short circuit (switching on 3 phase transformer), the wave form of stator current becomes complex containing both even and odd harmonics. Attempt of alternator designer while designing damper cage is to achieve Xd" = Xq" so that this unevenness of air gap reluctance is minimised. For example, In ....... 11 KV, 4 pole machines values of Xq" & Xd" are practically within 10% of each other up to 6000 kVA rating. Thus this design feature of .... Alternators helps in minimising the effect of additional harmonics produced by transformer switching, still we can not ignore the effect of sudden heating generated by harmonics of this asymmetrical inrush currents.

General Guidelines:

In light of basics discussed above, the general guidelines for small 3 phase synchronous machines are

1) The transformer to be energised shall not be of rating greater than that of 3-phase alternator.

2) Value of maximum inrush currents and their decay time shall be obtained from transformer manufacture and informed to DG supplier at initial stage to verify its suitability for switching on to DG set and check requirements of any special design from the quotation stage. Special design in case of Transformer switching can be

i) Strengthening of over hangs

ii) Design of rotating of Diodes to meet inrush and modify surges on diode due to inrush currents.

iii) Designing of special damper cage winding if necessary.

3) Based on decay of transformer inrush & expected harmonics in field poles, generator manufacturer can advise pause time ( cooling time of rotor) between switching of two transformers ( in case more than one transformer has to be energised.)

For further clarification please feel free to contact us at following address

Thanks for the details on the transformers behaviours during switching on etc. Very good.

The point I was addressing is to do with the build-up of voltage at the alternator: If there is a short circuit like condition before any build-up has taken place, and if the excitation type depends solely on the remanent magnetism of the rotor/stator irons in the Alternator, then you might not be able to build-up the voltage at the alternator. If the excitation system relies on a Permanent magnet for the initial magnetising (as for special designs to garanty short circuit currents ...), then your proposed transformer energising process will work.

There was another thread here about comparing to Soft starting motors: Whenthat is done, a low voltage is applied and then increased gradually to full voltage. In this case, the voltage is maintained at each stage to allow a steady current to flow. There is no short circuit condition and the magnetising current will be present ...

LAA please note that under open circuit condition of transformer (Load not connected), the winding has very high impedance (Inrush current only 1.5 to 2% of full load current and angle almost 90 deg lagging behind applied voltage). This is not a short circuit condition.

Hence if you are slowly building up the voltage by either of the two methods described - the voltage will build up as usual, because the impedance connected across generator is large (as good as No Load). This will simultaneously charge the transformer.

The short circuit LIKE behaviour appears when we apply suddenly rated voltage to de-energised transformer - That is a transient condition as flux density in the core at closing of switch is not the desired to be corresponding to instantaneous voltage of sinusoidal wave form.

RAM can also note above.

I had also graphically analysed the inrush current when I wrote the subject note - I have lost the pages as I did not have the scanner then - otherwise it would have understood very clearly.

Thanks for the info. I will refresh on it.

LAA is correct, the type of excitation system will have a profound effect on the results. For example if it is a compound(ing) type with CTs that provide "extra" boost under short circuit or overload conditions, then the results could be quite different than others' scenarios suggest.

While I'm not saying that this can't be done by trained experienced personnel working from a professionally prepared plan; the thought of blindly charging all these magnetics, "fooling" all the related protective circuits, bypassing control circuitry, modifying operational sequences, the amount of technical analysis required, and the apparent lack of technical expertise at the site, are some of the reasons why I was so adamant about not giving out enough information in post #7 ↑.

The "instructions" provided by others contain general information on "what to do" without providing any information on "how to do it" that is specific to exactly what equipment the OP has at his site. They are just as useful (and dangerous) as providing information on open heart surgery such as, "Anesthetize patient, crack open chest, remove heart, replace heart, close patient up, remove anesthesia, visit in recovery room".

The "Reader's Digest Condensed Version" does not cut it when lives, time, and money are on the line. Sometimes we're b*st*rds because we've "been there, done that" and see the dangers that others don't, or worse have seen the results of such short-sighted but well intentioned actions.

I have to trust that our more "sensitive" members understand this.