April 19, 2021

Why phase optimization matters

Baptiste Feron
Head of Energy Management

The ever increasing number of EVs is putting more and more pressure on the grid and driving demand for smart charging solutions. One thing, however, is often overlooked, despite its great potential: phase optimization.

Grids carry power on three phases

Continental electrical grids run on three phases. This means – without going into too much detail – that three conductors carry an alternating current of the same frequency and voltage amplitude. Each specific conductor (one per phase) is protected by a dedicated fuse that is triggered when there is an overcurrent. In practice, the dynamic load management solution has to protect each of these conductors by actively controlling the EV charging on it. Visualizing this (see below) not only looks pretty but also aids one’s understanding.

Illustration: Amplitude and frequency of three phase

Knowing the phase utilized by each EV can significantly increase the charging speed 

Some EVs only use a single (mostly plug-in hybrids or small-range EVs) or two grid phases to charge. Thus, if the charging phase is unknown, assumptions must be made. And to prevent overloads, the worst case scenario has to be assumed. Let’s look at an example to illustrate this:

Setup: GCP with a capacity of 32A per phase and three EVCS with a capacity of 16A per phase

We have a setup with three charging stations, each of which can charge with a maximum of 16A per phase, as well as a grid connection point (GCP) with a maximum capacity of 32A per phase. 

Without phase optimization we have to rely on assumptions

Without specific phase measurements, assumptions are required to prevent overloads. In practice, to protect the fuse one has to assume the worst case i.e. all EVs charge on the same phase. And while this prevents overloads, it may also lead to reduced charging speeds, as the following example demonstrates.

Scenario 1: three single-phase EVs charging (unknown phase utilization)

Consider three single-phase EVs, which can charge with a maximum of 16A. Without phase optimization, the EV charging phases are unknown. Therefore – to prevent overloads – one has to assume the worst case i.e. that all three EVs charge on the same phase. Thus, the total load is limited to the capacity of a single phase at the GCP (32A) and so the load per EV is capped to a third of 32A. The greyed out area represents the lost charging capacity.

With phase optimization the full capacity can be used

In Scenario 1 we were able to demonstrate that the assumption of the worst case may mean that EVs cannot charge with their maximum power.

Scenario 2: three single-phase EVs charging (known phase utilization)

Let's consider the same scenario but this time the EV charging phases are known. If the EVs – as assumed in scenario 1 – charge on the same phase, there is no benefit to knowing the phases. If, however, the EVs charge on different phases (scenario 2) they can be charged with their respective maximum. This means that the total charging load increases from 32A (scenario 1) to 48A (scenario 2) – an increase of 50%.

So what’s the challenge here?

Phase optimization is not that simple. Not all charging stations provide specific phase measurements and thus one additional meter per charging station may be required. But beyond the physical requirements there are further technical challenges.

  • Phase mapping: The reported phase measurement of the GCP is not necessarily equivalent to the measurements of the charging station i.e. phase 1 at the GCP might be physically connected to phase 2 or 3 at the charging station. And experience shows that the phases are often mapped incorrectly, leading to suboptimal phase usage. For this reason, an additional validity check and eventual remapping are required to mitigate the associated risks.
  • Characterizing the EV: Neither the EV nor the charging station directly provide the actual number of charging phases (single-, two- or three-phase). This information, crucial for optimization and fuse protection, has to be derived from the phase power measurements. To measure this, any new EV is assumed to charge on three phases and given a higher charging priority for a brief period of time to determine its actual charging characteristics.
  • Dealing with unexpected behavior: Experience shows that EVs do not always adhere to “standard” behavior when it comes to charging. For example, sometimes EVs reduce charging power without prior notice and for no apparent reason. Moreover, some EVs consume up to 30% reactive power. The load management must be alert and able to react to such behavior to ensure the best charging performance.

And then there is also some regulation

VDE-AR-N 4100 might sound like the name of Elon Musk’s next child but is in fact a norm governing the maximum permissible phase imbalance. This limit is set to 4.6 kVA and requires all operators of charging facilities for EVs to maintain a balance between the phases. A phase exact loadmanagement ensures compliance with this regulation.

Know your phases. Keep your balance.

In summary, knowing EV charging phases can improve the utilization of the available capacity significantly and thus speed up the charging process without any changes to the infrastructure. Additionally, it ensures compliance with regulation on phase imbalance. 

A practical tip: Lastly, single-phase EVs typically charge on the first phase of the charge point. Therefore, during the installation, it might be beneficial to rotate phases between the charging stations and the GCP to maximize the charging speed – as long as the load management solution supports phase optimization and can map the phases correctly. This can speed up the charging process by more than 50%, depending on the considered scenario.

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