STAR DELTA TRAFO

[quest]

Recently we purchased a star Delta trafo for battery charger.
when testing the trafo at site for ratio test,we gave 3ph and neutral to primary and measured the respective voltages and currents.
Surprisingly we got current in the neutral inspite o balanced voltages.
Then, we retested using only 3-ph,neutral current became zero.
but the reason for neutral current in the 1st case is a mystery.
help

 

[electricpete]

#1 - What's a Trafo? Is it a transformer?

#2 - Was the secondary of your transformer loaded while you performed the test? If so, then the neutral current could be explained by either:
A - Unbalanced load on the secondary
B - Harmonic load on the secondary. Harmonics of course behave differntly than fundamental. Well known is that 3rd harmonic currents from each of three will add in the neutral return wire. To the extent that you have in-phase 3rd harmonic currents circulating around the secondary delta, you will have proportional in-phase 3rd harmonics flowing in each of the primary wye legs. These in-phase 3rd harmonic currents do NOT add to zero, but rather add together to form a neutral 3rd harmonic which is 3 times as big as each of the phase third harmonics.

The part that may be a little harder to understand is why the circulating 3rd harmonic currents would appear in the secondary delta in presence of a harmonic load. I believe that is the case... but I'll have to think on it for awhile to come up with an explanation.

 

[electricpete]

I thought about this one some more. Here's the way I'm looking at it.

Let's say we model the load as three single phase nonlinear loads connected in delta. Each of these "wants" to draw a 3rd harmonic current. IF the transformer could supply these third harmonic currents, they would all be in phase with each other (they are 1/3 of a 60-cycle waveform apart which corresponds to 3/3 of a 180 cycle waveform... which is in-phase). However, the transformer secondary, being delta connected is unable to supply these third harmonic currents because of their zero-sequence nature. Therefore the load will create a large 3rd harmonic voltage at the secondary terminals of the transformer as the load "tries" unsuccessfuly to draw it's 3rd harmonic current. With each of the phase-to-phase voltage at the secondary terminals in phase, the total voltage around the delta of the transformer secondary is 3 times that imposed by each single-phase load. This induces a circulating current in the secondary delta. Think of this circulating current as the excitation current drawn in response to a (3rd-harmonic) voltage source hooked to the delta side of the transformer. It will create a 3rd harmonic voltage at the primary wye terminals of the transformer. To the extent that there are loads/sources connected to the transformer primary bus, then third harmonic currents will flow on the primary side (and add up in the primary winding neutral). However, if this transformer is supplied by yet another upstream transformer whose secondary winding is delta, and there are no other loads on the bus, then there would be no path for 3rd harmonic currents in the downstream transformer primary neutral.

So to repeat... my model of the system would be superposition of two voltage sources. The fundamental voltage source is applied at the trasnformer hi-side. The third-harmonic voltage source is applied at the transformer low side. Third harmonic currents flow in the transformer hi-side only if there is some device hooked to the hi-side bus which is capable of drawing zero-sequence (third harmonic) currents. If you have the luxury of lugging in a delta transformer to temorarily supply your transformer, and you can isolate other loads from the bus, I'll bet you neutral currents would disappear (although I'm not suggesting you do this without carefully considering the implications for personnel and equipment safety).

It's worth mentioning that the internal circulating excitation currents on the secondary side may be quite high if the core is going into saturation... affects transformer temperature and load capability.

If you can get a scope, it'd be interesting to look at the primary wye neutral current and the secondary voltage to see how distorted the waveform is.

One question may come to mind in discussing circulating currents within the delta winding... where is the return path for flux? If it is a 5 or 7 leg transformer core, then the flux flows in the unwound legs. In this case the reluctance of the path is low, so exciting inductance is high and excitation current (circulating current) will be fairly low.

However, if this were a 3-leg transformers cores, then the flux would have to travel outside the core to complete the return path. This means high reluctance path, low exciting inductance, high exciting (circulating current).

I believe that core form are generally 3-leg and shell form are 5 or 7 leg. But it's been awhile. Does anyone know?

 

YES,TRAFO MEANS TRANSFORMER.
ALSO,Note that the above tests are done at no-load.

 

[rhatcher]

When you perform an open secondary test on a transformer you can determine the excitation current and the turns ratio. I have never performed this type of test except as an exercise, so I cannot tell you if the results you saw are to be expected. We perform insulation resistance and TTR testing on transformers.

Ideally, the excitation current should balance out so that no neutral current exists. However, on small 3 phase transformers the construction of the core is not symmetrical in a magnetic sense. Most of these I have seen have a core shaped like an "H" with the ends closed laying on it's side. I would think that the center phases of this type of arrangement would have a different magnetizing susceptance (B_M in the equivalent circuit) than the outer phases due to the relationship between B_M and core saturation.

If so, this may account for the neutral component of the magnetizing current measured during the open secondary test.

 

[peterb]

Some questions -
What was the value of neutral current measured? How about phase currents?
What is the transformer rating?
What are the neutral grounding (earthing) conditions of the supply system and the transformer in question?

 

[electricpete]

One point of clarification... when you say unloaded, do you mean there is no battery charger connected to transformer or that there's no load on the battery charger?

I'll assume tere is nothing whatsoever connected to the transformer. As rhatcher said, the only currents expected in that situation are excitation currents. When Doble testing high-voltage transformers by applying 10kv to primary winding of one phase, the single-phase excitation currents are limited to the order of 300 milliamps (that's where the test set trips). And you see a pattern of two highs and one low. The low current corresponds to the phase wrapped on the center leg, which has a lower reluctance due to two parallel flux return paths, therefore higher exciting reactance and lower exciting curent. I think that lower voltage transformers would tend to have higher exciting currents because #1 - they tend to have fewer turns and #2 - lower core cross section and higher reluctance.

Now comes the million dollar question... how would a three-phase excitation test compare to a single-phase excitation test? Anyone know? There was a Doble article on the subject. I'll see if I can dig it up.

Actually, I did perform measurements of excitation currents for each of three phases of an unloaded generator stepup transformer a few years back. We had had a pilot-wire relay trip and were just being extra-careful as we brought the plant back up to make sure we didn't have another trip. I honestly can't remember the results, it seems like there may have been some imbalance.

Before we leave the subject of third harmonics altogether, it's worth mentioning that IF there is any third harmonic present on your primary voltage (perhaps due to other nonlinear loads hooked of of transformers fed by the same bus?), then you would once again draw third harmonic currents in each of the three phases of the primary wye, which would add in the neutral. (also you'd see circulating currents in the secondary delta). But compared to the directly loaded case discussed above, magnitude of currents would be less.

 

[quest]

Actually the transformer was tested outside the battery charger.It is a spare transformer and was to be tested before using it in the charger.
specs. of transformer:

95KVA,Y/DELTA,415/140V
CASE 1:GIVING 3-PH AND NEUTRAL SUPPLY TO PRIMARY:
AT NO-LOAD:
PRIMARY VOLTS PRIMARY CURRENT
RY=401.7 RPH=5.1A
YB=406.8 YPH=6.11A
RB=405.1 BPH=5.97A
N=8.21A
SEC VOLTS:
ry=144.4,yb=145.2,rb=145.9
SEC CURRENTS
ry=4.73A
yb=8.66A
rb=4.5A
CASE 2:GIVING 3-PH TO PRIMARY W/O NEUTRAL
PRIMARY VOLTS PRIMARY CURRENT
RY=401.9 RPH=5.15A
YB=410.0 YPH=3.25A
RB=408.0 BPH=5.4A
N=8.21A
SEC VOLTS:
ry=144.1,yb=145.1,rb=145.7
SEC CURRENTS
ry=1.15A
yb=1.15A
rb=1.10A

 

[quest]

The supply is from 2MVA Transformer which is a common distribution transformer for various loads.
It is solidly ground,2MVA 6.6KV/415V,DELTA/STAR.
The transfomrer in question has neutral floating as per the drawing of battery charger.

 

[electricpete]

What's the difference between Case 1 and 2. Case 2 says W/O Neutral but there is a neutral current of 8.21A reported. Where was that measured if there was no neutral? I'll assume the 8.21A of case 2 is a typo.

How did you measure a secondary current with no load connected? Were you able to measure the internal circulating current of the delta? i.e. you were able to access internal cable? Or perhaps this device has 6 secondary terminals with external jumpers to close the corners of the delta? I'll assume you were able to measure internal current of the delta.

The question at this point appears to be whether these represent normal excitation currents or some type of abnormal current. I haven't got my hands on that Doble article yet but I'd expect it'd shed some light.

Off-hand, I'd say that I would not expect there to be any circulating current in the delta under no-load. (but take it with a grain of salt). That seems to suggest there is some kind of zero-sequence excitation. Triplen harmonics seem to be that comes to mind... from one of two sources#A - you have harmonics on your primary voltae due to other plant loads as discussed above; #B - the transformer is going into saturation producing non-linear excitation current. The third harmonic component results in the excitation current circulates in the secondary. Transformer going into saturation as discussed in item #B in this case would not be due to the voltage applied (you were a little bit below nameplate), but possibly just a part of the design.

Bottom line - at this point it seems this might be normal 3rd harmonic excitation curren due to designed core saturation, or it might be influenced by harmonic component of the voltage on your 415v bus. Either way it doesn't seem be a tremendously large current... probably not a big concern unless you plan on loading the tranformer full up.

The above is just a lot of guesses. I'd be interested to hear what others have to say. I'm going to think about it a little more. Of course a real-world measurement on neutral current in a few other similar transformers would be worth more than all the guessing in the world to determine what is normal.

 

[peterb]

I agree with electricpete - we need some further explanation of the measurements. Note that the voltage supply is not in fact balanced, there is clearly some 3rd harmonic present, as evidenced by the neutral current. It may be helpful if there were readings taken phase-neutral as well as phase-phase, as this would allow derivation of the sequence components. The current values sound high for no-load excitation current (3.9-4.6% of full load current for Case 1, 2.5-4.1% for Case 2 - typical value expected in the area of 0.5%)
I am also curious about the secondary voltage rating of the transformer, which is stated as 140 V phase-phase. This doesn't sound to be a standard voltage and, along with the 95 kVA rating, leads me to wonder whether this is a specialty transformer. If this is the case, then the manufacturer should be contacted for details of the factory tests, for comparison purposes.

 

[jbartos]

Suggestion:
In many instances, the upstream connection is grounded star. Therefore, by connecting star with neutral to the transformer, the zero sequence current will flow through the neutral because of possible asymmetry of the overall load from the source (the star connection with grounded neutral-assumed). Obviously, if the neutral is not connected the zero sequence current path is open; therefore, the zero sequence current is negligible, i.e. zero for all practical purposes).

 

[electricpete]

Good comments from all. My previous comments addressed only triplen harmonics as possible source of the zero sequence currents. I'd have to agree with jbartos that it could also be simple voltage unbalance on 415v system (due to unbalanced single phase load at some point in the system or unbalanced supply from utility)which causes zero-sequence excitation of the transformer and creates primary neutral current and circulating secondary currents.

 

[NormGA]

If you have access to a phase angle meter it would be most helpful to read the magnitude and phase angle of the voltages and currents, including the neutral. Should make it a lot easier to calculate what is going on.

Wye-Delta banks on an overhead distribution system are normally operated with a floating primary neutral. The reason being you would not know that one of the single phase transformers had failed, it would just operate like an open delta. With the neutral floating you find out immediately. That could happen with your battery charger transformer too. So why not float the neutral of the battery charger transformer?

 

[jbartos]

Suggestion to the previous posting marked by ///\\\:
Wye-Delta banks on an overhead distribution system are normally operated with a floating primary neutral.
///The y-connections on transformers primary are more frequently used in medium and high voltage power distribution systems where y-connection with a suitable neutral grounded scheme upstream transformer and no grounded neutral at y-connection downstream transformer.\\\