EV charger power distribution in an apartment complex

Question

I am on the executive committee for an apartment complex. We have had a number of requests from residents to add EV chargers to the basement carpark. It's obvious that the number of these requests will increase in the future, so we're trying to cater for the end goal - allowing everyone to install their own chargers. Of course, this requires that those chargers will be controlled by a central controller to prevent them from overloading the complex's electrical substation. My question is, what are the technologies available for distribution of power?

First, let me describe the physical infrastructure:

• We have a 500 kVA substation for the entire complex of 90 apartments.
• The average peak load is 175 kVA; the minimum (overnight) load is 55 kVA.
• The basement carpark is not a large square. Instead, think of a really stretched out 'S' shape, with the top and bottom curves quite tight - in other words, a long line of around 300m (1,000 ft). The substation and main switchboard are in the middle of the elongated 'S', so two 150m (500 ft) lengths.
• The basement carpark has a concrete ceiling with numerous existing services already installed: fire sprinkler system; sewerage and storm water piping; electrical cable trays; data cable trays; etc. It's quite "busy" up there - but there's nowhere else to install stuff.

Of the 90 units, to date there are only 5 applications for EV chargers. This will obviously increase, but the plan is for a phased installation to accommodate people as they put in their requests. The starting point is to install the master controller that all chargers will be controlled by, and a master utility electricity meter that will measure the total cost of electricity consumed. The master controller will log which chargers used how much power to allow us to bill the tenants for the power consumed. We're assuming that each space will only have a 7 kVA charger installed.

Our plan is to effectively distribute the whole complex's power to all the chargers. While this is overboard, it seems appropriate given that the overnight rate is only 10% of available power, leaving 90% available for the EV chargers. That means that SIGNIFICANT power will be distributed throughout the carpark.

My understanding of the technologies available suggest the following approaches:

1. The traditional "tree" structure: feed multiple distribution boards (DBs), which in turn feed individual chargers as necessary. For example, for 90 apartments install 6 DBs of 15 apartments each, and then slowly add the individual cable runs from each car space to its nearest DB.
2. Use a busway / busduct system to provide a "backbone" of power that individual car spaces can connect to. Extend the bus as more distant residents apply to be connected.

For 1), the problem is that even a single DB could conceivably require a LARGE amount of power at a time. 15 car spaces of 7 kVA each is over 100 kVA, requiring THICK cables for the feed. And since they each come from the central area to various places along the long line, the multiple THICK cable runs are parallel, which is expensively redundant!

The advantage of 2) is that there'd be a single "run" for the length of the carpark for the full power draw of the system. The problem is that busway / busduct is not very flexible. Dodging and weaving through the existing pipework infrastructure isn't easy, or perhaps even possible.

What I'm looking for is a hybrid of the two: a single, flexible backbone (in the twisty turny sense) for the length of the system, allowing for individual "tap off" points along the length. The problem is that a cable run of 500 kVA would require either absurdly thick cables, or so many parallel cables that the expense would be exorbitant.

Suggestions?

Answer 1

Well, I can see why you are confused. This is NOT your father's battery charger. And the people who designed this stuff - Tesla and SAE - are way ahead of you.

So let's red-pill you on the underlying tech, and then we can talk about how to approach this big installation. (which may be easy - and cheap too.)

Those aren't chargers. They're Energy Management Systems.

That thing on the wall, the EVSE or "charging point" is not a DC charger. It's a simple gateway with a relay and a small computer. That's it. It has One Job: tell the car the safe AC current which may be drawn right now. (well it's also an RCD).

Technology Connections has a wonderful video that says exactly how they work. The video says it doesn't apply to European 3-phase but it actually does!! in all the ways we're about to discuss. So please watch it.

Anyway, I'll never call it a charger. Its name is "EVSE".

The EVSE's actual job is to tell the car the safe current. This creates many options for energy management.

• The simplest units are simply hard-set based on the circuit size they are on (which is based on the spare ampacity of the residential service).
• Power Sharing allows several EVSE's in a group to share a single current allocation dynamically across multiple EVs - in this video, 15kW has provisioned for the four EVSEs to share. The Tesla Wall Connector is also very good at this.
• Limiting draw from the grid/service - in this case a CT (Current Transformer) is added to the home's service, and EV charging is dynamically adjusted to avoid exceeding the set maximum. Examples of such systems are the Emporia VUE or the Myenergi Zappi (which buries this feature deep in the docs as a "Grid Limit").

Getting the feel of this tech?

The max possible speed is not "needed" at all.

Don't make 'perfect' the enemy of the 'good'.

7kW charge rate sounds cool, but for at-home overnight charging, it's overkill.

I'm a Yankee with a Yankee commute of 80 miles / 130km. You know what I actually need for that? 3kW giving 75-110 miles (120-180km) charge in 10 hours. I can install that with our cheapest 14 AWG twin-and-earth. The 12 AWG we use for sockets would give 4kW.

But don't take my word on it, Technology Connections addresses this here.

7kW means 180-270 miles (300-450km) in 10 hours. If you can do it great, but don't hang up or delay EV charging thinking nothing less will do.

Now that you understand the technology basics, let's have a conversation.

Most of the time, a dwelling takes very little power

Our infamously audacious American homes have enormous 5.5kW dryers, 4.5kW water heaters, 12-16kW hobs/ovens, and of course our mighty air conditioners. And yet, an American home's average consumption is - ready for this? 1.3 kW. Really.

Thus it is perfectly obvious that the large energy users are very intermittent, and most of the time, the house is quiescent - at 0.2 kW or even less. It's very important to understand this. The same applies to a British home. Almost all the time, there is headroom for productive EV charging. All you need is a "Load Limiting" feature on the EVSE, which backs down EV charge rate when the house is using a lot of power.

Wait. Aren't the apartment directly above the parking spaces?

I gather you would very much prefer that the tenant pay their EV electric use, yes? And the tenants presently have meters on the electric service in their flats, yes?

Well, aren't those flats directly above the parking spaces you have (or could) assign them?

So maybe you don't need a monster project of distributing 500kW all around the complex, or dealing with metering individual units. This has already been done.

All you need to do is get power from their individual consumer unit (CU; American service panel) in the flat, down to the parking spot. And you need some sort of energy management system to assure you don't set their CU's on fire. (I am assuming the builder sized the unit services and CUs closely to the requirements of the flat, and there is not a lot of headroom available, yes?)

So use Grid Limiting (aka "digital fuses")

This is what Grid Limiting is for (such as the Myenergi Zappi or Emporia Vue have implemented). A current transformer on the supply tells the EV to throttle back EV charge rate when other loads are drawing a lot, which isn't very often.

The difficulty here is that the Emporia system uses a wireless signal between the Vue energy monitor and the EVSE. However, the Zappi seems to have forseen this issue, offering several ways to wire the Current Transformer to the EVSE. So you run whichever size of cable you deem appropriate (16A for 3.5kW or 32A for 7kw) and a cat3 wire for the Current Transformer.

The owner of each individual flat could decide (and pay for) the size of wire they want. The wire size would be configured as a current limit in the EVSE. (same as you normally do).

What I don't know is whether your country's codes will require you to chase lines all the way to the flat's consumer unit, or whether you can somehow tap at their meter. The current transformer can work in either location.

So my approach would be to lay the conduit routes needed to route cabling from the parking spots to viable places to tap the consumer's side of the meter - be it the meter area, risers, or consumer units. Lay down the infrastructure for all so this is easy in the future, instead of having to cost $2500 as you mentioned in a comment. Then when a resident orders it, have your electricians pull the wires into the conduits and install a grid-limiting EVSE.

Alternately, use Power Sharing to limit charging groups.

When a group of EVs uses Power Sharing to share a single allotment of power, a funny thing happens. Not all cars need a heavy charge every night. In fact, only a few do. The Law of Averages applies.

Suppose you have 16 cars in a Power Sharing group. How many cars had a very busy day and really need 70 kWH (7 kW at 10 hours) by morning? Only two. The others drove between 10 and 80 km and as such, all of them together only need 100 kWH. So we need to restore 240 kWH to all cars.

But it gets better - with Power Sharing, charging begins when the first car arrives, and finishes when the last car leaves. So we don't need to assume 10 hours, we can call it 16 hours. How much power do we need, then? 240 kWH divided by 16 hours = 15 kW. For 16 cars.

I would pad that by running 20kW... but that's less than 1/5 the power you were thinking that you had to provision. 20kW is only about 80 amps @ 240V - Tesla's Wall Connector v3.0 will do exactly what I just said - 20kW for 16 cars, with a 11.5kW maximum for any one car.

Given 3-phase distribution I would put one group of sixteen on each phase, so just like that - 60kW and 80A wire, and we have provided 48 charging spots.

This greatly reduces wire sizes needed, too, in to the realm of the reasonable.

If you had to distribute 500kW over such distances, I wouldn't do it at 230/400V, you're talking almost 400A in each direction (750 kcmil/400mm² aluminum wire x 4?) I would step it up to higher voltage (or simply take it as it comes from the power company, which I presume is in the 2000 volt range?) and distribute 230V transformers near the parking spaces.