Current Control in AC Charging for EVs

Current Control in AC Charging for EVs

Read the articles OBC in EVs , Battery Charging Modes to undetstand this article better.

This article focuses solely on the current control aspect of AC charging and does not cover the entire charging sequence. Detailed charging sequences for various charging standards will be discussed in separate articles. Here, we aim to explain how current control operates during AC charging in EVs. For simplicity, we assume that all necessary steps for initiating the charging process have been completed.
The diagrams below illustrate the system setup for single-phase AC charging, ready to start.

System setup for single-phase AC charging

Abbreviations:
EV = Electric Vehicle
EVI = Electric Vehicle Charging Inlet
EVCC = Electric Vehicle Communication Controller
EVSE = Electric Vehicle Supply Equipment (Charging Station)
HVB = High Voltage Battery
HMI = Human Machine Interface (Display or Mobile)
OBC = On-Board Charger
SOC = State of Charge
PI Contrl = Proportional-Integral Control

Key Elements in Current Control

The main aspects of current control include determining when to start charging, when to stop charging, and the limits that must be adhered to for safe and efficient operation. Let’s explore these elements.

When to Start Charging?

In our assumed scenario, all necessary steps (establishing the communication with EVSE, HVB contactors closing etc.) for initiating charging are complete. and the vehicle is ready to charge.

When to Stop Charging?

Charging should be stopped under the following conditions:

  1. Customer-Selected SOC Reached:
    • The customer specifies the desired SOC via the HMI, such as the vehicle display or a mobile app.
    • Once this SOC is achieved, charging shall be stopped.
  2. Scheduled Charging Time Ended:
    • Customers can set specific charging schedules through the HMI, e.g., charging from 6 PM to 10 PM.
    • Charging shall stop at the end of this time frame, regardless of the SOC.
  3. Intentional Customer Request to Stop Charging:
    • Unlock Button Pressed: When the unlock button is pressed, the vehicle performs authentication (e.g., by detecting the proximity of the vehicle key). If the key is verified within a specified range, the vehicle will stop charging. Unauthorized attempts are ignored.
    • Shutdown Request via HMI: The customer can manually request to stop charging from the HMI.
  4. Fault Conditions:
    • Communication loss with the charging station.
    • Overheating of the OBC or similar critical faults.

Current Limits to Maintain

For safety and system reliability, multiple current limits must not be exceeded. These include:

  1. Customer-Selected AC Current Limit:
    • Customers can set a phase current limit through the HMI based on the fuse capacity at their residence. For instance, if the EV is capable of drawing high current but the home fuse has a lower limit, exceeding this limit may trip the fuse. Hence, this adjustable option is provided.
  2. Charging Cable AC Current Limit:
    • Charging cables have specific current ratings, indicated by their proximity resistance value as per standards (example as per IEC 61851-1 is below).
      • 1500Ω –> 13A
      • 680Ω –> 20A
      • 220Ω –> 32A
      • 100Ω –> 63A
    • Detachable charging cables, commonly used in Europe, might support lower current than the EV or EVSE. Drawing more current than the cable’s capacity could damage the cable.
  3. EVSE AC Current Limit:
    • This is the current limit of the EVSE, communicated via control pilot duty cycle or ISO 15118 protocols.
    • EVSE current limits may dynamically vary, especially in public charging settings, where load-sharing mechanisms adjust limits based on the number of EVs charging simultaneously.
  4. OBC DC Current Limit:
    • The OBC has a maximum current delivery limit on the DC side. This may vary dynamically due to temperature or module faults. OBC cannot deliver more current than this limit even if requested.
  5. HVB DC Charge Current Limit:
    • The HVB has a charge current limit that typically decreases as the SOC increases. Exceeding this limit can damage the battery or reduce its lifespan.
Information flow among the components during charging

Current Control Mechanism

Current control is managed by the EVCC. The EVCC gathers information from multiple systems, including the HVB, HMI, and EVSE. It then regulates the current by sending an appropriate DC current setpoint to the OBC. To ensure no limits are violated, the EVCC considers all the current limits in the calculation of DC current setpoint to the OBC. Importantly, AC current limits must be converted to their DC equivalents, as HVBs are charged using DC current. The conversion involves accounting for the OBC efficiency (as the AC to DC conversion is done by OBC):

Before delving into advanced control methods, it is essential to understand the concept of simple control and the issues associated with it.

Simple Current Control

Simple current control is a straightforward approach where the lowest current limit among all the applicable limits is used as the DC current setpoint. This ensures that none of the limits are exceeded. The OBC then regulates the current accordingly and delivers it to the HVB.

Simple current control in EVCC

Let us examine a few use cases to highlight the limitations of this method. For the purpose of illustration, assume the following:

  • HVB DC Charge Current Limit at Low SOC = 100 A
  • HVB DC Charge Current Limit at High SOC = 10 A
  • Minimum of All Other Current Limits = 20 A
  • Auxiliary Load Current (when active) = 5 A

1. Auxiliary Load Off:

  • At Low SOC:
    • DC Current Setpoint to OBC = min (HVB DC Charge Current Limit at Low SOC, Minimum of All Other Current Limits)
    • DC Current Setpoint to OBC = min (100 A, 20 A) = 20 A
    • The OBC delivers 20 A to the HVB.
    • Result: No issue arises in this case.
  • At High SOC:
    • DC Current Setpoint to OBC = min (HVB DC Charge Current Limit at High SOC, Minimum of All Other Current Limits)
    • DC Current Setpoint to OBC = min (10 A, 20 A) = 10 A
    • The OBC delivers 10 A to the HVB.
    • Result: Again, no issue arises in this case.

2. Auxiliary Load On:

  • At Low SOC:
    • The DC Current Setpoint to OBC remains the same as calculated above:
      • DC Current Setpoint to OBC = 20 A
    • Current to HVB = DC Current Setpoint to OBC – Auxiliary Load
    • Current to HVB = 20 A – 5 A = 15 A
    • While the OBC delivers 20 A, 5 A is consumed by the auxiliary load. Since the setpoint is dominated by the Minimum of All Other Current Limits (20 A), there is no way to increase the HVB charging current. So, no issue arises in this case.
  • At High SOC:
    • The DC Current Setpoint to OBC remains the same as calculated above:
      • DC Current Setpoint to OBC = 10 A
    • Current to HVB = DC Current Setpoint to OBC – Auxiliary Load
    • Current to HVB = 10 A – 5 A = 5 A
    • Problem: The OBC delivers only 10 A, with 5 A consumed by the auxiliary load. Although the Minimum of All Other Current Limits is 20 A, allowing the OBC to deliver up to 15 A (5 A for the auxiliary load and 10 A for the HVB), the simple current control method cannot adjust the setpoint beyond the lowest current limit.

Simple current control, while straightforward, fails to account for scenarios where auxiliary loads impact the effective charging current delivered to the HVB. This limitation arises from its rigid adherence to the lowest current limit without dynamic adjustments. Advanced current control techniques can address these issues by dynamically adjusting the setpoint to optimize current delivery based on real-time conditions.

One might wonder why the auxiliary load current is not included as an input in this control strategy. In any design, it is generally preferable to minimize the use of additional hardware components unless absolutely necessary. As a result, it is possible that the system does not include current measurement sensors for all auxiliary loads.

Advanced PI Current Control: Addressing the Challenges of Simple Current Control

Building on the limitations of simple control, we explore an advanced PI control strategy. This approach not only resolves the issues highlighted earlier but also addresses additional challenges such as sensor inaccuracies.

Why PI Control?

PI control dynamically adjusts the output to ensure that the feedback (measured current) matches the reference (desired current) while not exceeding the limits. This adaptability makes it highly effective for optimizing current delivery in EV charging systems. Beyond resolving the issues of auxiliary loads, it corrects discrepancies caused by sensor tolerances. For instance:

  • Scenario: The EVCC requests 20 A to charge the battery. While the OBC delivers this current based on its own measurement, inaccuracies can occur. If the OBC measures 20 A but the HVB interprets it as 17 A, the battery receives less current than its limit, prolonging the charging time.
  • Solution: PI control adjusts the setpoint to compensate for such differences, ensuring the HVB receives the current as per its limits.

Principles of PI Current Control:

In the context of EV charging:

  • Reference: HVB DC Charge Current Limit (the desired charging current for the HVB)
  • Feedback: Measured HVB DC Charge Current
  • Output: DC Current Setpoint to the OBC
  • Limits: Minimum of All Other Current Limits

The PI controller continuously regulates its output to minimize the error between the reference and feedback, ensuring the HVB charges optimally.

Note: The internal workings of PI control are not explained here. Instead, I have focused on explaining the core concept of how a PI controller is utilized.

Basic PI Control Representation
PI current control in EVCC

Revisiting the Use Cases:

Using the same parameters as in the simple control example, we evaluate how PI control resolves the challenges (Sensor inaccuracies are not considered here):

1. Auxiliary Load Off

  • At Low SOC:
    • Reference = 100 A
    • Since the other limits constrain the setpoint to 20 A, the PI controller saturates at 20 A, maintaining safe operation.
    • Result: No change in behavior compared to simple control.
  • At High SOC:
    • Reference = 10 A
    • The PI controller limits the setpoint to 10 A, aligning with the HVB’s requirements.
    • Result: No change in behavior compared to simple control.

2. Auxiliary Load On

  • At Low SOC:
    • Reference = 100 A
    • The PI controller recognizes the limitation imposed by the other limits (20 A) and saturates at this value. The auxiliary load consumes 5 A, leaving 15 A for the HVB.
    • Result: No change in behavior compared to simple control.
  • At High SOC:
    • Reference = 10 A
    • The PI controller dynamically adjusts the setpoint to 15 A (10 A for the HVB and 5 A for the auxiliary load), ensuring both receive the required current.
    • Result: Unlike simple control, the PI controller optimizes current delivery to meet the demands of both the HVB and auxiliary loads.

Handling Sensor Inaccuracies:

To illustrate the effectiveness of PI control in compensating for sensor inaccuracies, consider the following:

  • HVB DC Charge Current Limit = 10 A
  • Minimum of All Other Current Limits = 20 A

The OBC and HVB measure current independently, often with slight deviations. For example:

  • If the HVB measures 10 A and the OBC measures the same current as 12 A, the PI controller adjusts the setpoint to 12 A. This ensures the OBC delivers the correct 10 A from the HVB’s perspective, compensating for measurement differences.

PI current control offers a robust solution to the challenges posed by simple control. By dynamically adjusting the setpoint based on real-time feedback, it ensures optimal current delivery to the HVB while addressing issues such as auxiliary load consumption and sensor inaccuracies. This advanced control strategy is essential for efficient and reliable EV charging systems.

When any of the specified charging stop conditions are met (as outlined in the article), the charging current is gradually decreased to 0 A and then the charging process is fully terminated.

This strategy is also applicable to three-phase charging, where the current limits of all three phases must be considered when checking for current limits.

Amar Reddy Avatar
Amar Reddy, a seasoned professional in EV Charging with more than a decade of expertise in the Electric Vehicles domain. With a Master’s degree in Electrical Engineering, he is eager to share valuable insights into EV charging, catering to enthusiasts and fostering a deeper understanding of the field.

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