SBAA541 December   2022 AMC1202 , AMC1302 , AMC1306M05 , AMC22C11 , AMC22C12 , AMC23C10 , AMC23C11 , AMC23C12 , AMC23C14 , AMC23C15 , AMC3302 , AMC3306M05

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
    1. 1.1 DC Charging Station for Electric Vehicles
    2. 1.2 Current-Sensing Technology Selection and Equivalent Model
      1. 1.2.1 Sensing of the Current With Shunt-Based Solution
      2. 1.2.2 Equivalent Model of the Sensing Technology
  4. 2Current Sensing in AC/DC Converters
    1. 2.1 Basic Hardware and Control Description of AC/DC
      1. 2.1.1 AC Current Control Loops
      2. 2.1.2 DC Voltage Control Loop
    2. 2.2 Point A and B – AC/DC AC Phase-Current Sensing
      1. 2.2.1 Impact of Bandwidth
        1. 2.2.1.1 Steady State Analysis: Fundamental and Zero Crossing Currents
        2. 2.2.1.2 Transient Analysis: Step Power and Voltage Sag Response
      2. 2.2.2 Impact of Latency
        1. 2.2.2.1 Fault Analysis: Grid Short-Circuit
      3. 2.2.3 Impact of Gain Error
        1. 2.2.3.1 Power Disturbance in AC/DC Caused by Gain Error
        2. 2.2.3.2 AC/DC Response to Power Disturbance Caused by Gain Error
      4. 2.2.4 Impact of Offset
    3. 2.3 Point C and D – AC/DC DC Link Current Sensing
      1. 2.3.1 Impact of Bandwidth on Feedforward Performance
      2. 2.3.2 Impact of Latency on Power Switch Protection
      3. 2.3.3 Impact of Gain Error on Power Measurement
        1. 2.3.3.1 Transient Analysis: Feedforward in Point D
      4. 2.3.4 Impact of Offset
    4. 2.4 Summary of Positives and Negatives at Point A, B, C1/2 and D1/2 and Product Suggestions
  5. 3Current Sensing in DC/DC Converters
    1. 3.1 Basic Operation Principle of Isolated DC/DC Converter With Phase-Shift Control
    2. 3.2 Point E, F - DC/DC Current Sensing
      1. 3.2.1 Impact of Bandwidth
      2. 3.2.2 Impact of Gain Error
      3. 3.2.3 Impact of Offset Error
    3. 3.3 Point G - DC/DC Tank Current Sensing
    4. 3.4 Summary of Sensing Points E, F, and G and Product Suggestions
  6. 4Conclusion
  7. 5References

2.2.3.2 AC/DC Response to Power Disturbance Caused by Gain Error

Figure 2-14 shows a generic voltage controller and equivalent model of the power plant.

Figure 2-14 Simplified DC Bus Voltage Control Loop With Power Plant Model

As Figure 2-14 shows, the term derived analytically before (Equation 8 ) was introduced in the loop as a disturbance to check the voltage control performance. By observing the control loop topology, it can be demonstrated that, thanks to the presence of an integrative part of a PI controller, the steady-state error caused by the DC disturbance Equation 8 is completely rejected. Conversely, the AC component of the disturbance cannot be rejected completely, thus leading to a voltage ripple.

To evaluate the maximum acceptable gain error of a current sensor used in the AC side, simulations were run by applying the following hypothesis:

  • DC bus voltage working at the minimum rated voltage to maximized the ripple voltage (650 V)
  • Maximum power exchange between the AC and DC side, thus increasing the power disturbance (11 kW)
  • Gain error for the three phases applied to reach the worst-case scenario, as follows:
    ε1 = –ε2 = –ε3;
  • Current control loop bandwidth kept constant in all the simulations (3 kHz)
  • The AC filter is designed to keep the THD below 3% at the nominal output power when using prime current sensing
  • The power line frequency is 50 Hz

Figure 2-15 shows simulation results of an AC/DC converter working with sensors having different gain error.

Figure 2-15 DC Link Voltage Ripple Over Time With DC Link Bandwidth and Gain Error as Parameters

The following results are present in Figure 2-15:

  1. A 100-Hz ripple voltage on the DC-link. This is caused by the power ripple injected by the gain error of a current sensing stage.
  2. The mean value of the voltage in all the cases is still the same when steady state is reached thanks to the integrative part of the PI controller, as confirmed by the theory.
  3. The DC-link voltage ripple is correlated to the bandwidth of the DC-link voltage control loop. If the bandwidth of the voltage control loop is high enough, the controller tries to eliminate the ripple voltage by controlling very fast the current loops at the expense of grid THD.

In this example a 400-Hz bandwidth of the voltage control loop, paired with 3.7% gain error of the current sensor, leads to a THD of 3.3 % compared to a 3% THD with an ideal current sensor without gain error. Alternatively, a low bandwidth of the voltage control loop leads to low THD on the grid-side but the ripple voltage on the DC link can increase to an unacceptable level. Having a voltage ripple in the DC link can lead to power ripple on the battery which cannot be tolerated. Furthermore, low voltage control loop bandwidth leads to poor load-step response.

In conclusion, a current sensor located in the switching node with a gain error of 3.7% can lead to an increment of the grid current THD of more than 10%. To compensate for this increase, the input filter has to grow by more than 4% in volume to meet the design goal of < 3% THD at the grid-side of the converter.