Transcript

Introduction to Bidirectional Vehicle Charging

Good afternoon, everyone, and welcome to today’s white paper webinar. Today, we’re going to be talking about bidirectional vehicle charging. We have Andrew with us, who wrote this paper, and this is an interesting paper about the latest trend in electric vehicles.

Not only are these vehicles charging, so they’re a nonlinear load when they are connected to the grid and charging the battery, but sometimes they’re supplying power. They’re backfeeding back into the system, so they are not only power supplies or sinks, but sources. This is an interesting new type of load that utilities are facing, a bidirectional electric vehicle. And I’ll turn this over to Andrew to walk us through this paper.

EVs and Power Quality Impacts

Hi, I’m Andrew. I’m a software engineer here at PMI. Let’s get started. So, we’re all familiar with EVs and their power quality impacts. We talk about harmonic current injection, feeder loading, and some diversity effects. But with vehicle-to-grid charging, or V2G, they’re not nonlinear loads, as Chris mentioned, that they’re mobile, inverter-based, distributed energy resources, or DERs.

V2Gs are really good for frequency regulation, peak shaving, renewable balancing, and distribution support, but from a power quality standpoint, they introduce a lot of new challenges.

Charging Versus Discharging

You can see the difference between charging and discharging. In charging, the vehicle is acting as a nonlinear load, and in discharging, it’s acting as an inverter-based DER. In terms of charging, the grid dictates the voltage and frequency and draws shaped current, and also the harmonic load behavior is dominating, whereas in discharging, the inverter has to synchronize to the grid and it injects controlled current and the EV has to be a participant in the grid dynamics.

It’s an interesting situation. It’s a very tiny scale of a BESS, a battery energy storage system, that utilities are using on a distribution or even a transmission-level scale, where you have giant batteries that charge during the day and generally from solar and then supply power at night. But this is on a micro scale, so no individual car at a residential location is going to be more than a few kilowatts of charging or discharging, which you think is too small to matter. But on a small, say, 25 kVA transformer, especially if something is shared, a couple of vehicles can actually be a significant issue.

Potential Interactions and Problems

It’s still early to see what kind of problems we’re gonna have, but you may see things like you see in distribution with multiple solar sites on the same feeder. In that situation, let’s say you have your link feeder with three or four multi-megawatt PV sites, they can interact with each other. One will make an adjustment and see the other device and you’ll have sub-synchronous or low-frequency oscillations in the RMS voltage.

You might have anti-islanding pulses that triggers another one into some sort of instability. And in general, it’s hard to predict the type of misoperations or interactions you’ll see when you have multiple power supplies tied together that are active nonlinear devices. That’s a problem that you see in distribution, but you don’t typically see that on a secondary of a transformer in this small scale.

But that’s more likely to happen as you have more and more of these vehicles that have power flow in both directions. If you have a shared transformer in three or four or five houses on that secondary, and each one of those houses has one or two of these cars plugged in, I would predict that we’re gonna see some interesting interactions when that happens, as one starts discharging into the grid and another one sees that and they interact. And that’s a weak grid on that secondary.

Power Quality Measurement Recommendations

So, if you have a customer that’s seeing this or that has one of these cars, it’d be worth taking some power quality measurements. Measure the transformer current. Here you especially want to be aware of where you place your recorder, that determines what current you’re getting. If you put the recorder at the meter base, you’re capturing that customer’s load, including their car, but not anyone else on the transformer secondary.

That’s a situation where you might want more than one recorder at the same time, one on the transformer secondary capturing all of the customers on that secondary, and then at least one on a house that has this sort of a vehicle so you can look at power flow on a per-customer basis, or per this determinant that’s interacting.

You may want to go one step further. If that car’s amount of power injection is such that it’s still a net load on the house, that they’re not exporting power, then you may actually want to put a recorder even further downstream to capture the circuit breaker or circuit that that vehicle is on so you can measure their actual generation and tie that to what’s happening on the secondary of the transformer.

So there, you’d not only want multiple recorders, but also time synchronization, ideally even GPS time synchronization, so you could line up waveforms and determine if current pulses from the vehicle injecting power are interacting with other chargers in the system. That seems like a lot of elaborate recording for a residential situation, but you really do have almost a mini-distribution network on the transformer secondary when you have multiple houses and multiple cars that each can be these power sources.

Comparison to Other Residential DERs

In theory, you can have this with houses that have their own solar if you have three or four houses that each have inverters, but this is a unique kind of load because it’s a load and a supply. You may have houses that have some types of loads like this. If they have battery walls or other sort of large batteries, like the Tesla battery wall in our house, which is effectively an electric vehicle that doesn’t go anywhere. It’s a giant battery that charges and then discharges. But those aren’t as common yet as these sort of vehicles.

These vehicles are gonna present some interesting power quality challenges at the residential level on transformer secondaries. Where you might see this on a larger system would be like a high speed charger system, where you have lots of cars together. But there, they may be operating in a more synchronous fashion.

It’s definitely a very interesting load, where the car can determine whether it is a harmonic load, where it’s charging the battery. And most of these electric vehicle chargers are actually fairly well behaved for harmonics, but then discharging. And here, again, the challenge is not only harmonics, but also that synchronization and having them interact with each other as they are synchronized into the utility voltage, which on a secondary of a transformer is a weak system because you’re limited by the transformer impedance and having to interact with each other.

Closing Remarks

If you have any questions about this, this is really a brand new topic. These aren’t common out in the wild yet, and we’re rarely ahead of having any measurements at this point. This is just to give you a heads-up of what’s coming next and what you might be seeing in terms of residential complaints that might look like flicker or other types of traditional power quality problems. Then when you dig into it, you realize you’ve got something a lot more complicated when you are looking at the actual current flows.

Especially if anyone has seen this already or already have some horror stories that relate, feel free to chime in. If you have any questions, feel free to type them here in the questions box now, and we look forward to helping utilities try to deal with this. If you have a recording like this or dealing with this kind of situation, let us know and we’d be happy to help however we can.

We don’t have questions now. Again, if you have a question later or when you first run into this, if you are responding to a flicker complaint and you dig a little deeper and realize you’ve got some of these fancy cars involved with your customers, let us know and we’d be happy to take a look at the data with you. Thanks for attending, everyone, and everyone have a great day.