Re-configuring a Victron Energy isolation transformer
This is a companion article to a
YouTube video covering the same topic.
What are they and why do I need one?
Victron Energy isolation transformers are often used on boats that connect to shore power in order to provide a means of providing isolation between the shore power and the AC power that exists on the boat. The transformer sits in between those two worlds and isolates that two power supplies through magnetic flux and an air-gap.
The reason this is needed on a marine vessel is because without it, small leakage currents can flow between the earth ground for shore power and the "earth ground" connection on the boat, which is usually connected to water through a propeller shaft, out-drive, sail-drive or sacrificial zincs. That creates a loop and that small leakage current greatly accelerates any galvanic corrosion that naturally happens. Current flows through the water because of the salt content, or even in fresh-water by any dissolved particles which can aid in conductance. Because the shore power connection's ground and the boat's ground are not directly connected through a physical wire with the transformer in place, a ground current "loop" is not formed and that method of corrosion is stopped.
An isolation transformer is one means to prevent this connection. It should be noted there are others, such as galvanic isolators, however an isolation transformer is the ideal solution. (Galvanic isolators are typically low voltage Schottky diodes which just reduce the amount of voltage potential and therefore limit the amount of current that can flow).
Some of Victron Energy's isolation transformers also provide voltage doubling or voltage halving, so that if, for example, you have a 230V boat (like Mira) and you want to connect to 120V shore power, you still can! (The AC frequency can NOT be changed in this manner, which presents some other challenges, but we'll save that for another time.)
Why would I need to re-configure it?
By default, Victron Energy isolation transformers are designed to boost the input voltage by about 4%. It is believed that this is done because, often times, the marina's shore power cables that run from the utility company out to the docks can be very long. When that happens, the resistance in these wires causes a voltage drop, so what started out at 120V from the power company's connection to the marina, may drop to something like 115V by the time it gets to the power pedestals on the docks.
So, there's a transformer as part of our isolation solution, why not kill two birds with one stone and boost the voltage while we are at it? A noble objective to be sure and one that basically costs nothing to implement in their product However, that is all great until you don't need that boost, and that boost actually causes problems.
How could boosting be bad?
Some marinas (especially those in western Mexico) are known to have already high voltages at their power pedestals. So instead of 120V, it is something like 130V, then it gets boosted by 4-5% and ends up at 136V after the transformer. Now the voltage is too high and other Victron Energy products, like their inverter/chargers, which are usually right after the Victron Energy isolation transformers see this as an out of spec voltage and don't ever connect to shore power (as they shouldn't!).
Another possible problem, one which we experienced on Mira, is seen when connecting a small, portable, 2000W gasoline generator to our shore power inlet. By the way, we do this when we have extended time without sun, yet don't want to run the (much more expensive to maintain) diesel engines which have high-output alternators to re-charge the batteries.
We have found that when they are being run without a load, our small generator puts out a voltage of about 127V, then when under any nominal load, drops down to 120V.
Since Mira is a 230V boat, our transformer also doubles the incoming voltage. So 127V becomes 254V, and then we add about 4% to that and it's now at about 265V, when it should be 230V! That is 15% too high and well outside of the acceptable voltage range.
Yup, that's bad, how do we fix it?
First, we have to understand how transformers work. Fortunately, transformers are very simple electrical devices. They consist of one or more primary windings, to which power is fed in, and one or more secondary windings, from which power is then extracted. These windings are wrapped around what is fundamentally a shared iron core. Read more on Wikipedia if you want, but basically, the alternating current coming in induces a magnetic flux into the iron core (think of this as transferring potential energy), then when that alternating current switches polarity, that flux that is stored in the iron core with what we will call a "positive" flux needs to be forced out and replaced by an opposing flux, which we'll call negative. They are really clockwise and counter-clockwise, but again, Wikipedia if you care.
When that energy gets forced out of the core, it gets pushed onto the secondary windings and a voltage is created on those wires. The neat thing about transformers and why they are ultimately simple, is because the input voltage is proportionate to the output voltage in the same ratio as the number of windings between the primary winding(s) and the secondary winding(s). All transformers are notated with their ratio in the form of x:y, where x and y indicate the ratio between primary and secondary.
You may have guessed that the ratio of the transformer used in Victron Energy's isolation transformers is 1:1.04. In other words, for every volt applied to the input side, 1.04V will appear on the output side.
Okay, now can you tell me how to fix it?
With that knowledge in hand, there are three possible solutions to remove this boost.
1. Replace the transformer in the isolation transformer unit with one having a turns ratio that matches our needs. Actually, replacing it with a transformer with multiple output taps that produce very similar ratios would be ideal, as you could fine tune the output voltage up or down, depending on the then-current needs.
2. Remove some windings from the transformer's secondary to lower the turns ratio. (Don't even think about this!)
3. Swap the primary winding connections with the secondary winding connections so we end up with a buck transformer with the inverse turns ratio, namely 1:0.96! It turns out this is very quick and easy to accomplish and will work for quite a few scenarios. It is also easy to switch back if needed.
For our scenario above it changes to the following: 127V comes in, which then gets doubled to 254V, but also reduced by 4%, which is 244V, which keeps the inverter/charger happy. Once an actual load is added and the generator output drops to 120V, the output is even better: 120V * 2 * 0.96 = 230.4V; just about perfect!
Now, this solution may cause problems for us if we connect to 230V pedestal, however we hardly ever do this and if we do, it is quick and easy to undo the changes.
That didn't tell me how to fix it. Now can you tell me?
Okay, so the only reasonable solution is to swap that primary winding connections with the secondary winding connections.
Before I continue, and more importantly, before you continue, you should not do this if you are not qualified. There are Victron Energy technicians and electricians that can make this swap for you. Doing this wrong could result in an overload, a boat fire or electrocution.
The following instructions are meant for them to make these changes for you. The author assumes no responsibility for any issues encountered in performing these changes, proceed at your own risk!!
- Make sure there is no AC connection to the boat. It's not a bad idea to de-power your inverter, but if it is wired correctly and working correctly, there should be no power coming "backwards" from the inverter.
- Remove the cover from the Victron isolation transformer. there are 4 screws that attach the large upper panel.
- Measure the incoming wires with a voltmeter set to AC Volts to make absolutely sure that no power is coming into the transformer.
- Inside, you will see that there are 8 wires coming from the top of the unit, from the toroidal transformer. There are four connectors on the left side and four on the right side. Take a picture of this starting point, both so you know how to go back to normal, and so it can be used as a reference when double-checking your work.
- These two sets of 4 wires need to be swapped, in order. Meaning, the first wire in the left set must become the first wire in the right set, and vice-versa. The second wire in the let set must become the second wire in the right set, and vice-versa. The same for wires 3 and 4.
When doing this, don't "braid" the connections, as there is barely enough length to make these connections. It is better to remove all connections first, then re-connect them one by one, but notes should be used to make sure the order is maintained. The connectors should each fully seat. - Print out this file and tape it on the the inside of the cover plate as a reference.
- Add a sticker to the outside of the cover plate indicating if the isolation transformer is wired for the (standard) boost configuration or the (modified) buck configuration.
- Compare the before picture taken to what it looks like when complete and double-check the work.
- Re-attach the cover plate to the unit with the 4 screws.
- Reattach AC power to the shore power input of the boat and verify that the voltage seen by the inverter or on AC outlets is within the range expected.
For the 3600W Auto-adjusting Isolation Transformer, here is what the connections looked like before changes:
And after changes:
Just curious, how is the transformer able to handle 115V and 230V inputs and always output the voltage I need?
The ability of this isolating transformer to be able to automatically adapt to 115V and 230V inputs is because both the primary coil and secondary coils are broken in half. Depending on how those primary halves are connected on the input side and how the secondary halves are connected to the output side allows that. If the primary coils are wired to be one long string, it is configured for 230V input. If the primary coils are wired to be two shorter strings in parallel, it is configured for 115V. The same is true for the output and the secondary coil, however, since the voltage at which your boat operated doesn't change, that connection can be done one time with the provided jumpers, and does not have to adapt.
Here is a basic block diagram of how this isolation transformer is built:
The connections on the input (left) side are done by a voltage sensing circuit that is driving relays. On the output (right) side, the connections are achieved by jumpers configured once during installation.
When the boat is connected to 230V, the configuration relays are set to achieve this:
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Configured for 230V input: one long winding on the primary side.
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When the boat is connected to 115V, the configuration relays are set to achieve this:
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| Configured for 115V input: two short windings in parallel on the primary side. |
So what changes in the block diagram to make this to a buck transformer?
We swapped the primary wining connections with the secondary winding connections to change this transformer from a boost to a buck. That changes the block diagram to this, with the changes in orange. You will see that the ratio is swapped and what was primary is now secondary and vice-versa:
Wow, that was a lot!
If you got here, thanks a lot for reading! Sorry I get so technical. Please leave any comments or questions below!
