Leakage Voltage at Neutral

dear , kindly check your nearest Eearthing Pit .If it is ok you will get 3v w.r.t ground .

<...cannot earth the neutral point.because of the unbalanced earth condition of ship....>

This doesn't make sense. If the neutral point were to be earthed then there would be no voltage between neutral and earth at the transformer.

Please check out the Wikipedia article on earthing and advise further.

Since the equipment has qualified the tests there is no need to worry.

You may use Restricted Earth Fault Relay (64) to initiate an alarm above the threshold to circumvent the anamoly if that is your consideration.

thank u 4 ur valid response . really I am not full filled with ur reply.further I have recnnected the primary as y connection and earthed the primary Y point to a good earth.still I couldnot attain the customer demand of less than 3 volt .As I mentioned earlier the secondary neutral cannot be earthed due to Ship is not having valid earth point or in built established nuetral points. Only when the ship is fed with shore suply, the nuetral may be available and hull of ship cannot be cosidered as a good earth.I have introduced screen earthing after primary winding and after secondary winding and connected to the body of the transformer.Still ,the problem exists. as u suggested introduction of EFR is not reccomended by the OEM of the system. which is installed by BEL bangalore.

any way thank u for the response and expecting more.

joseph

You are correct a Delta connection has no neutral.

On a boat it is common to use the star point of a Y generator system to "earth" the boat and the isolation transformer should have compatable wiring.

Galvanic isolators are also often used on boats to control the floating sacrificial anode and the inclusion of one of these items will lower the voltage between "earth" and neutral by a small amount ( I think they are just a few diodes in an expensive box.)

And yes the question does not explain enough.

thank u mountk2

(I mean delta winding should not have Neutral, any tertiary winding you did not mention? and also how did the primary got good earth while on the same ship?).

my response:-I have rearranged the primary connection to Y for testing purpose and connected the Y to earth .All these tests are done on shop floor .Only after the Qc checked is passed we could install to on board ship.

(To my assessment voltage gradient between tank and neutral could have emerged due to leakage reactance cused by some careless workmanship or quality of the material used in one of the phase windings.)

my response:-this wiil be rechecked

If you can measure primary voltage with respect to earth, measure for all three phases and see if it is equal in all the three phases.

The problem of this shifting may be reflected from primary.

I think you are seeing very low current leakages, try measuring it with an old fashioned analogue meter, I'll bet it goes away.

Your point floating not to earth When connect to ships point it not or a shift the problem away. You seem be worried about something that is directly related to the way you are testing. Try connecting it to earth & then measure the volts again.

During manufacturing/ rewinding work, a thin copper foil ( covering 3/4th. perifery of LV winding with insulation) is wound over LV winding and is connected to transformer body to compensate the effect of voltage generation ( between Tr. neutral to body)due to leakage flux. In this particular case please check whether the same has been done.

Please carry out Polarization Index Test (PI) using megger.

(PI is defined as a ratio of 10 minute insulation resistance to 1 minute insulation resistance. For class A insulation value of 1.5 or more and for class B insulation minimum 2.0 is satisfactory)

Please also carry out Dummy load test at different loading conditions to observe the voltage values at neutral.

I will try to Google out any code or standard reference favoring you against the client's set limitation.

I want to add:

When isolation transformer is employed, to create isolation between ship and shore power supply, it is done by establishing a new ground on board your vessel. The shore ground is terminated at the shield between the primary and secondary windings of the isolation transformer.

At Google, information at following sites might be of your interest:

http//shop.pkys.com/isolation transformers.aspx

I agree with manindra post # 10

I want to add more on this issue. I am working in Port sector for long time as Electrical Engineer and from my working experience I have seen sometimes voltage between neutral point to transformer body increased beyond tolerable limit which is detrimental for operation of sophisticated electronic instruments/ equipments. In most of the cases it is seen that earthing connection from shield to transformer body snapped due corrosion( saline weather); and if the connection is remade the problem is solved.

PI test with HV break down tester (0 - 10 KV) has been conducted giving 3KV for 1 minute between primary sheild (disconnected from body) and primary winding read 0.003micro A,between primary winding and body is 0.005micro A and primary and srcodary is 0.005micro A. We are using class 'H' insulation( Nomex) for the inter layer separation (12mill X 2 layer) 3 layer between sheild and winding

I am not aware of PI test to have been done with an 'HV break down tester'. I had given requirement of megger.

Refering to one part in your orignal post I mean the toppest one, "Can any body suggest or explain the reason.we cannot earth the neutral point.because of the unbalanced earth condition of ship" Portugal philip's post # 7 has indicated one of the possible reasons.

Since the transformer has not left the workshop, please consider jayesh100375 post #1