Battery Charger Load Causing High Voltage in Outlets and at Load Center

[msemp1]

Hello,

I am new here and work as a quality inspector for a mobile trailer/tower company. I am currently working alongside of one of our electricians, but we cannot seem to get to the bottom of our problem (forgive me if the answer is rather simple). I am trying to grow in knowledge on the electrical side.

We are running a (60A, 208V, 3-phase shore power cord (using only two lines) through a buck/boost step up transformer (https://www.larsonelectronics.com/p...primary-120-240v-secondary-46-88-amps-50-60hz) to split-phase 120/240V (you can see the electrical schematic at the link below). This shore power cord is powered by a generator. All voltage readings are measuring correctly, however, when the two battery chargers are plugged in (on the same outlet - two 12V batteries to each), we see a spike in voltage in all the outlets and in the load center.

Please also keep in mind that this schematic and wiring setup was driven by our customer's electrical engineers. Is more voltage entering the system to account for the battery chargers?

7b8cf9380d6a3d4be3abffbea47d7ab6.png hosted at IMGTR

Image 7b8cf9380d6a3d4be3abffbea47d7ab6.png hosted in IMGTR

imgtr.ee

 

[LarryFine]

Welcome to the forum.

It sounds like a classic open neutral (break in the white-wire pathway).

 

[msemp1]

Thank you, sir. That was our thought as well, but we are trying to determine where that is. We have six of these units that are wired the exact same and have tested three of them so far - all of which show the same issue. My thought is that it has something to do with the way we have wired the shore power cord and transformers.

 

[augie47]

By design, your 240v buck &, boost transformer will not provide a stable 120v output

 

[zbang]

we see a spike in voltage in all the outlets and in the load center.

This more suggests an issue with the generator's voltage regulator than a neutral problem.

 

[Joethemechanic]

120/208 from a wye source can't be made into 120/240 with a boost/buck. You end up with 139/240

 

[synchro]

The Larson buck-boost generates a neutral at its output having a voltage that is the common-mode voltage of its L1 and L2 inputs (which are each 120V_RMS and they are 120 degrees from each other in a 208V 3-phase system). The common mode voltage waveform is the sum of the L1 and L2 voltage waveforms (each being measured relative to the same reference) and then divided by 2. As a result, the voltage at the output neutral relative to the equipment ground (EGC) will be 60V_RMS due to that 120 degree angle. This is assuming that the 208V generator or shore power has its neutral bonded to the EGC. This 60V offset from ground is at 90 degrees from the L-N outputs of the buck boost And so the voltage on each line output of the buck boost measured relative to the EGC will be √ (60² + 120²) = 134.2V.

It's possible that the battery chargers draw a transient current through the equipment ground when they are plugged because the neutral output voltage from the buck boost is 60V_RMS relative to the equipment ground. And this might then cause a spike in the voltage that you're seeing. Are the battery chargers being run off of 120V or 240V?

 

[msemp1]

Thanks for everyones input. We have narrowed it down do a large charger that we have, which is charging 3 batteries. Two 24V batteries and a 12V battery for the generator on the unit (not the test generator). When that charger is unplugged, voltages are within tolerance. I am not sure why this is the case, though.

That makes sense synchro. The battery chargers are run off of 120V GFCI.

 

[msemp1]

Just to provide more information:

Voltages from the main breaker lines the charger unplugged are L1 to N: 112-114V and L2 to N: 132-135V.

Voltages from the main breaker with the charger plugged in are L1 to N: 120-135V and L2 to N: 135-154V (fluctuating wildly).

 

[msemp1]

The Larson buck-boost generates a neutral at its output having a voltage that is the common-mode voltage of its L1 and L2 inputs (which are each 120V_RMS and they are 120 degrees from each other in a 208V 3-phase system). The common mode voltage waveform is the sum of the L1 and L2 voltage waveforms (each being measured relative to the same reference) and then divided by 2. As a result, the voltage at the output neutral relative to the equipment ground (EGC) will be 60V_RMS due to that 120 degree angle. This is assuming that the 208V generator or shore power has its neutral bonded to the EGC. This 60V offset from ground is at 90 degrees from the L-N outputs of the buck boost And so the voltage on each line output of the buck boost measured relative to the EGC will be √ (60² + 120²) = 134.2V.

It's possible that the battery chargers draw a transient current through the equipment ground when they are plugged because the neutral output voltage from the buck boost is 60V_RMS relative to the equipment ground. And this might then cause a spike in the voltage that you're seeing. Are the battery chargers being run off of 120V or 240V?

120V GFCI that the charger is plugged into

 

[synchro]

A concern I have about generating an output neutral from the buck boost like in your schematic is that the output impedance it presents may not be sufficiently low for L-N loads, clearing one-pole breakers on short circuits, etc. That could be a bigger issue than the higher N-EGC voltage I mentioned above.
I would suggest plugging in a significant 120V load such as a milkhouse heater, and checking how much voltage drop it creates.
Actually, given the potential issues, it would be better to start first with a smaller load like an incandescent light bulb to see what it does.

 

[synchro]

By design, your 240v buck &, boost transformer will not provide a stable 120v output

I agree. It might be better to only use a buck boost for any loads that need 240V vs. 208V.

 

[gar]

230819-2342 EDT

This is a badly designed system. Shore power is 2 of 3 phases. On shore power voltage line to neutral is boosted to 120 *1.15 = 138 V to get 240 V line to line. This means nominal single phase is 15% too high. The battery chargers are designed for 120 V input.

But beyond this the two line to neutral voltages are not close in value.

You need to investigate your source voltage more closely.

None of this has anything to do with your voltage instability.

..

 

[winnie]

The transforme(s) shown in the schematic need to be replaced with a 208:120/240 single phase isolation transformer.

@LarryFine got it right, open neutral. But not because of a loose wire, but because of the transformer design.

The designer put two transformers in series, with the source neutral not connected, in an attempt to derive 120/240V split phase power.

Because any derived neutral would be at elevated voltage (60V to supply neutral) the designer instructed that supply neutral not be connected to derived neutral.

But because the two 'halves' of the system don't share a common transformer core, the derived neutral have very high impedance and L-N voltage unstable.

If a single larger buck boost transformer were used with a common core, then the derived L-N voltage would be stable. But there would still be the problem if the 60V to ground on the derived neutral.

If the supply neutral were connected then the existing transformers would provide stable L-N voltage, and the elevated N voltage would be eliminated. But the L-N voltage would be 139V, again not acceptable.

The correct transformer for the job is a 208V :120/240V isolation transformer, not a buck-boost autotransformer.

Or bring 120/208V from the supply, and only use the buck/boost for 240V loads.

But if you want everything exactly the same on generator or shore power, you need a different transformer.

Jon

 

[winnie]

For what it's worth, I just looked at the instructions on the Larson site, and they seem to describe the _incorrect_ use attempted here.

The Larson page linked in the OP explicitly states that the input is 208V and the output 120/240V. The instructions explicitly state that only the output neutral should be connected.

IMHO the Larson instructions are incorrect, but I'd appreciate one of the other theory folk checking because now I am second guessing myself.

Larson Electronics - 1-Phase Buck/Boost Step-Up Transformer - 208V Primary - 120/240V Secondary - 46.88 Amps - 50/60Hz

1-Phase Buck/Boost Step-Up Transformer - 208V Primary - 120/240V Secondary - 46.88 Amps - 50/60Hz

www.larsonelectronics.com

https://www.larsonelectronics.com/images/product/WiringDiagram/224054.PDF

 

[Joethemechanic]

There might be a N-G bond before this transformer. Maybe the generator is already bonded. If If N and G are to be kept at the same potential on the load side, that is where the only bond can be.

What if you avoid all this convolution and just connect the generator for 120/240?

 

[Joethemechanic]

I wonder if the battery chargers are creating some kind of N-G bond, maybe intermittent. Are the DC circuits of the batteries grounded in any way? Or bonded to anything?

 

[synchro]

For what it's worth, I just looked at the instructions on the Larson site, and they seem to describe the _incorrect_ use attempted here.

The Larson page linked in the OP explicitly states that the input is 208V and the output 120/240V. The instructions explicitly state that only the output neutral should be connected.

IMHO the Larson instructions are incorrect, but I'd appreciate one of the other theory folk checking because now I am second guessing myself.

Larson Electronics - 1-Phase Buck/Boost Step-Up Transformer - 208V Primary - 120/240V Secondary - 46.88 Amps - 50/60Hz

1-Phase Buck/Boost Step-Up Transformer - 208V Primary - 120/240V Secondary - 46.88 Amps - 50/60Hz

www.larsonelectronics.com

https://www.larsonelectronics.com/images/product/WiringDiagram/224054.PDF

That Larson page says is generates a neutral at it's output: "This step-up transformer has a split phase secondary voltage of 120/240 V that generates a neutral and provides up to 46.88 amps at 240 V or two legs of 46.88 amps at 120 V for a total capacity of 93.76 A at 120 V on the secondary side."

As you mentioned in a previous post, Larson shows two buck-boost transfomers that are each on their own separate core. And so it would be very poor in a neutral forming function because there's little to constrain the two transfomers primary voltages to be equal.
But even if they were on the same core, there is still the constraint that the primary winding current of the buck boost is 16/120 or 13.3% of the secondary winding current. And the load currents are supplied through the 16V secondary windnings. That alone puts a significant limitation on how much neutral current that the primary windings would need to supply when the neutral from the source of 208V is not connected.

 

[synchro]

What if you avoid all this convolution and just connect the generator for 120/240?

I think they are using the generator configured for 208/120V so that it matches what would be supplied from a shore power outlet in order to do some factory testing.

 

[gar]

230820-1454 EDT

I can not easily read the circuit diagram. But it appears there is a jumper between EGC and Neutral at the generator. Shore power has two hots and an EGC. So if it is assumed that at the shore power transformer the EGC and neutral are connected, then there should be little difference in voltage between these two lines at the boat. But there is the major voltage excess on the 120 lines.

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