Neutral after phase conversion

[rmitch13]

Hello, I am a new EIT and have much to learn - thank you for any help.

I am trying to work on feasibility of upgrading some EV chargers from 25 kW to 50 kW. The sites in question only have single phase 240 supply. The 50kW chargers require 480V and three phase w/ a neutral. So, first step up the voltage from 240->480, which will feed a phase converter to convert from single to three phase, but all phase converters I've looked at so far have no neutral. Is there an efficient way to add the needed neutral so the charger upgrade can work? So far the only thought is adding another transformer to create the neutral, but if you're upgrading 4 chargers, that's already 8 transformers for the circuit. Any advice would be appreciated.

 

[winnie]

My thought is that this would be a horrible kludge. I can think of several ways to do this, but in the end I would look for a charging system compatible with my existing supply.

Perhaps one of the 'buffered' fast charger suppliers has a system with a 240V input. These are systems with a built in battery, they charge their internal battery from the grid and than charge the car from their battery.

It sounds like you are considering standard phase converters which synthesize the 'third leg' to create three phases given the existing two leg single phase system.

Higher end static converters synthesize all three legs, and can be had with wye output. This is generally only done when desperately needed (eg for frequency conversion) because it is much more expensive.

In theory you could use a 240V phase converter followed by the step up to 480V, eliminating one transformer from your scheme.

Jon

 

[synchro]

My thought is that this would be a horrible kludge. I can think of several ways to do this, but in the end I would look for a charging system compatible with my existing supply.

I certainly agree. If the charger has a bridge-rectifier front end, then the current sharing between the phases would be highly dependent on the voltage balance of the 480V supply. And so there's a risk that the phase converter would not be contributing its share of input current. Or even worse, it would be taking the brunt of supplying the current.

 

[retirede]

My initial approach would be to contact the utility regarding the availability of a proper 3 phase service.
Compare the cost of a new service to the equipment required for your workaround, and don’t forget the efficiency losses of the workaround that will be incurred forever.

 

[rmitch13]

My thought is that this would be a horrible kludge. I can think of several ways to do this, but in the end I would look for a charging system compatible with my existing supply.

Perhaps one of the 'buffered' fast charger suppliers has a system with a 240V input. These are systems with a built in battery, they charge their internal battery from the grid and than charge the car from their battery.

It sounds like you are considering standard phase converters which synthesize the 'third leg' to create three phases given the existing two leg single phase system.

Higher end static converters synthesize all three legs, and can be had with wye output. This is generally only done when desperately needed (eg for frequency conversion) because it is much more expensive.

In theory you could use a 240V phase converter followed by the step up to 480V, eliminating one transformer from your scheme.

Jon

Thanks Jon, that was my initial thought as well - I was working off an existing system that was built with the trans-phase-trans series by necessity, and figured we could eliminate one of the transformers. It would definitely be ideal to upgrade the service or use 240V chargers, but the remoteness of the locations make that impossible at the moment. I'll move forward with some drawings going phase conversion -> step up. Thanks again for your thoughts!

 

[rmitch13]

My initial approach would be to contact the utility regarding the availability of a proper 3 phase service.
Compare the cost of a new service to the equipment required for your workaround, and don’t forget the efficiency losses of the workaround that will be incurred forever.

Yes, that will 100% be my 'ideal' scenario to this, however, many of the sites in question are remote - used in rest areas for long road trips, so convincing the provider/client to incur those costs for this is predictably a no-go.

 

[winnie]

Wait, you _already_ have a trans-phase-trans system in use? And now you are looking to upgrade for higher power chargers?

-Jon

 

[rmitch13]

Wait, you _already_ have a trans-phase-trans system in use? And now you are looking to upgrade for higher power chargers?

-Jon

No, sorry for the confusion. There is a site that initially did a trans-phase convert system but later found out the chargers do not work on that system due to the lack of neutral. To remedy that site, a possible additional transformer is being explored - though i have nothing to do with any of that and don't have information beyond that. I'm looking at different sites that require the 25->50kW upgrade (these sites only have the basic infrastructure)

 

[winnie]

At the site that has the trans-phase converter system, do you have details on the specific components used? The actual model/nameplates of the transformer and phase converters?

Basically with the kludge already in place you might consider using it as a test bench.

I can see one theoretical way to derive a neutral and wye system from a rotary phase converter, but it depends on how the motor being used internally is wired.

-Jon

 

[retirede]

Yes, that will 100% be my 'ideal' scenario to this, however, many of the sites in question are remote - used in rest areas for long road trips, so convincing the provider/client to incur those costs for this is predictably a no-go.

Depending on how many chargers are at a given location, doubling the capacity of a single phase service could also be a problem.

 

[zbang]

This sounds like an exercise in how much loss can be added to the system.

 

[synchro]

I would recommend contacting the maker of the 50 kW charger that you intend to use, and ask them what their requirements are for voltage balance between the three phases of 480V. Also ask what the maximum load current on the neutral would be.

 

[rmitch13]

At the site that has the trans-phase converter system, do you have details on the specific components used? The actual model/nameplates of the transformer and phase converters?

Basically with the kludge already in place you might consider using it as a test bench.

I can see one theoretical way to derive a neutral and wye system from a rotary phase converter, but it depends on how the motor being used internally is wired.

-Jon

I know a 240/480, 75 kVA transformer is used for a step up, and a 480V, single->three phase 75kVA phase converter is used. I believe phase perfect was used for that converter? But can't be sure 100%.

 

[rmitch13]

This sounds like an exercise in how much loss can be added to the system.

yes, it's a headache for sure - probably why it was pawned onto the new guy

 

[electrofelon]

but in the end I would look for a charging system compatible with my existing supply.

That seems like an infinitely better option. But perhaps there are no such products available? I am thinking perhaps this is analogous to PV string inverters, where there are just are not any large single phase or even 240/208 3 phase product offerings.....

 

[Little Bill]

From my (limited) experience with phase converters, the one's I have done said not to use the manufactured leg with any 120V line to neutral connections. Its the same as trying to use the high leg on a delta system, where the high leg is 208V, and you have to avoid that leg for line to neutral loads. So the OP would need to design/install the system with the converter so that the mfg leg is not used for line to neutral loads.

 

[synchro]

... So, first step up the voltage from 240->480, which will feed a phase converter to convert from single to three phase, but all phase converters I've looked at so far have no neutral. Is there an efficient way to add the needed neutral so the charger upgrade can work? So far the only thought is adding another transformer to create the neutral, but if you're upgrading 4 chargers, that's already 8 transformers for the circuit. Any advice would be appreciated.

Using a wye-delta as a grounding transformer to derive a neutral would avoid most of the copper losses of a delta-wye feeding a balanced 3-phase load. But it still doesn't avoid the need for an extra transformer to provide a neutral.

 

[winnie]

The existing system (that doesn't work) starts at 120/240V single phase. A transformer boosts this to 480V (grounding as yet unspecified), which then feeds a phase converter.

The phase converter may be a 'Phase Perfect' brand. According to their website they only make 'digital solid state converters'. So the 480V single phase power is being rectified to DC and then passed through power electronics to synthesize AC again. Presumably only 1/3 of the power follows this path to create the 'third leg'.

The now 3 phase 480V goes to an EV charger where it gets rectified to DC for further processing/ control, then this DC goes to the EV.

1) I wonder if the lack of neutral is a red herring. I would double check if the output of the single phase step up transformer is grounded, and if it is center point or corner grounded.

My _guess_ is that the EV charger doesn't need the neutral, but is expecting a grounded wye supply. An ungrounded or corner grounded system might be causing the issues.

2) I wonder if the PWM synthesized 3 phase is not playing nice with the charger input rectifier. Adding a transformer might not change this.

3) I don't know anything about the charger topology. The chargers might use a controlled input rectifier to vary the output DC, rather than a dumb rectifier followed by DC control electronics. But we know it is AC input DC output. And it probably has power factor correction on the input.

Jon

 

[wwhitney]

Perhaps one of the 'buffered' fast charger suppliers has a system with a 240V input. These are systems with a built in battery, they charge their internal battery from the grid and than charge the car from their battery.

The first google hit I found on that idea (incorporates 160 kWh of battery!), which does accept 120/240V for battery charging:

FreeWire Pro Series

Boost Power Pro is a fully-integrated ultrafast EV charging technology that can be deployed anywhere and everywhere.

freewiretech.com

No idea if it exists or is vaporware.

Cheers, Wayne