Bidirectional Wireless EV Charging and Smart Grid Integration

The rate of electrification of mobility in industries or transportation depends upon the deployment of the charging infrastructure. The existing solution, namely conductive charging, has several concerns in safety, robustness, and comfort due to the plugging-in of huge cables, especially for a higher power. Wireless charging is touted to provide a safe, clean, and autonomous solution.

 

What is Wireless and Inductive Power Transfer (IPT)?

The scientist Nikola Tesla coined the term “Wireless Power Transfer” (WPT) and presented a contactless system in 1893. The fundamental principles that govern this technology are Lenz’s law and Michael Faraday’s law of induction. There are many methods by which this can be employed. The most successfully commercialized (at low power levels) is “Inductive Power Transfer” (IPT). IPT uses near-field technology where the energy remains within a small region of the transmitter.

Finepower develops solutions for wireless (inductive) power transfer since several years. Now we are extending this technology to bidirectional operation in conjunction with high power, low voltage batteries in the research project BiLiA which is funded by the Bavarian Ministry of Economics and project execution organization VDI-VDE-I.

 

IPT Magnetic Coil System

The magnetic coupling stage is the most important part deciding the design of power electronics, efficiency, and transferable power. In a typical application as electric vehicle charging, the secondary side coil is attached on the bottom side of the vehicle. The primary coil side is put on the ground. This assembly is ensured to have flux in between those two coils by using ferrite and aluminum on the outer sides of each coil. Stacking or shaping of the ferrite blocks is also possible. The airgap between the coils can be quite large, depending upon the vehicle’s ground clearance. This leads to leakage inductance of similar dimensions as the mutual inductance. Each coil in the IPT system can have circular, rectangular, solenoid, DD, DDQ, bipolar, etc. shapes. The advantages of each coil system vary across interoperability, size, flux leakage, positional tolerances, and operational complexity. At higher powers, to reduce the ampere-turns (or Magnetomotive force) a bifilar winding is used. The efficiency of the entire IPT system is limited by the native quality factor of the coils. This can be increased by using a Litz wire carefully reducing the skin and proximity losses at both bundle and strand level.

 

Figure 1: Block diagram of a typical inductive charging station. Grid power is rectified and converted to a high-frequency signal using PFC and inverter, respectively. This high frequency current signal through the primary coil generates a flux. Thereby, inducing a voltage across the secondary. The signal is later rectified to deliver power to a DC battery load.

 

Figure 2: a) On the secondary side, a series capacitor Css is added. A properly chosen value can cancel the secondary inductance (ωLs) to improve the power transfer. The impedance seen by Voc is purely resistive at this frequency. This is typically used in constant voltage applications. b) A parallel capacitor replacing the series one is useful in constant current applications. c+d) Hybrid compensation type where the series and parallel capacitors can be adjusted are also possible.

 

Optimizing Resonant Circuit Maximizes Efficiency

The simplified model of a typical inductive charging system is shown in Figure 1. A high frequency (i.e., 80-90 kHz) inverter after a PFC converts the rectified grid voltage into an AC square wave which is necessary for effective power transfer.

This high frequency current through the primary coil generates a flux inducing a voltage across the secondary side. This voltage is called open-circuit voltage (Voc), given by Equation 1 where Ip is the primary coil current, M is the mutual inductance and, ω is the angular frequency.

 

V_oc = jωMIp

 

Voc, when connected to a load delivers power and is given by Equation 2 where Rac is the equivalent load resistance (The impedance presented by the active rectifier and load to the secondary side.) Ls, is the secondary inductance.

 

P_{out} = \frac{V_{2}^{oc}R_{ac}}{R_{2}^{ac}+(\omega^{2}L_{2}^{s})}

 

Using maximum power transfer theorem with Equation 2, the maximum output power is achieved at Rac = ωLs. Adding a series capacitor in the equation with 1/ωC2 to cancel the term ωLs can double the maximum transferrable power. But instead of a series, different other compensation topologies are also possible. They can be any T (or n) - network built using passive energy storage components. Some simplified tuning networks on the secondary side are shown in Figure 2. 

The output power of the circuit can also be written as in Equation 3, where Isc is the current of the secondary side under short circuit condition and Q2 is the secondary load quality factor.

 

P_{out} = V_{oc}I_{sc}Q_{2}=\frac{\omega M^{2}}{L_{s}}I_{2}^{p}Q_{2}

 

From the Equation 3, the primary coil cur-rent can be reduced by increasing Q2 and thus, reduces the losses. But the bandwidth of the system will be reduced, making the implementation of the control system more difficult. The required volt-ampere rating of the secondary coil also increases.

 

Bidirectional Power Flow Reduces Grid Costs

To reduce greenhouse-gas emissions, there is a strong push towards renewable ener-gies. Most prominently solar and wind power. But sunlight and wind flows are intermittent and such fluctuations can destabilize the grid. Also, in the pursuit of energy inde-pendence, many industries are installing their own systems. This is due to increas-ingly ease of access to renewable energy technologies. For instance, car fleet owners, driven to electrify their vehicles would benefit from the (cheaper) generation of their own power and therefore install grid systems or charging points. On the other hand, this could lead to an increasing need of large land space to cover peak power demands. Smart grid storage systems, however, can reduce the required peak power. By storing energy during peak availability and supplying it when needed, the power flow can be managed and external demand from the grid can be stabilized.

Due to relatively large capacities, electric vehicle batteries can be viewed as ideal energy storage elements for grid stabilization. Therefore, battery chargers including wireless systems, should be enhanced to provide bidirectional operation. 

A modified model of the IPT system with bidirectional functionality is shown in Figure 3 In the forward mode, power flows from the grid to a battery load. The block after the PFC acts as an inverter exciting the primary coil. A rectifier is required to convert the AC power from the secondary coil to a battery. The respective functions of these blocks will be interchanged, while in reverse mode. The choice of the compensation type and its values depend upon many criteria. Some of them are discussed below:

Controllability: The common control method is the primary control. This method controls the high frequency (HF) inverter output voltage as the primary coil input voltage. According to the Equation 4 either voltage control or phase control is possible. Where Vdc is the PFC output voltage and α is the phase angle.

 

V_{in,rms}=\frac{2\sqrt{2}}{\pi }V_{dc}cos\frac{\alpha }{2}

 

Typical voltage transfer functions of a S(erial)-S(erial) and LC-LC compensation Fig. 1.4. The coil position is fixed delivering 1 kW and 5 kW. The plots show the both forward and reverse modes. The load quality factor should not be too high, required operating range can exceed the specification of the inverter. On the other hand, a low quality factor will not fully utilize the available operating range. As can be seen in LC-LC compensated gain response, the variation of the gain is low. And in S-S forward response, pole splitting happens at higher powers. This complicates the design of the control system.

 

 

Figure 3: The system is line in Figure 1, but with dual operation possible for both DC-AC and AC-DC converters. This allows bidirectional power transfer.

 

 

Figure 4: AC can gain response in both the power directions are plotted for S-S and LC-LC compensations. Both are turned to operate at 85 kHz.

 

Figure 5: Input phase response to check for ZVS operation possibility in both the power modes

 

The corresponding input phase responses are plotted in Figure 5. The phase response starts flattening (across the operating range) in the S-S forward, limiting the available ZVS range. Whereas in reverse mode, the sharp changes due to high quality factors demands a lot of reactive power. The same trend can be observed in LC-LC, but with interchanged responses.

Simplicity: By extending LC to the partialseries topology LCC, the quality factor can be improved which suits the primary control. But it increases the cost and complexity due to the added components. Also using the same type of compensation on both the sides maintain symmetricity and can reduce the design efforts.

Reflected impedance: The reactive component of the reflected impedance effects resonance. In partial-parallel compensation, there is always some reactive component. Whereas both the S-S and LCC-LCC compensation would have zero reflected reactance when operated below resonance (except if there is an offset between the coils). If care is not taken, this can limit the soft turn-on of the transistors in some scenarios, thus reducing the operational efficiency. Adaptive tuning can help in alleviating this issue. With proper design techniques, an optimal choice of tuning can be obtained to ensure ZVS operation in both modes across positions.

To sum it up, it is necessary to consider all the constraints for both forward and backward operating modes from the design start of a bidirectional wireless charging system. The performance will degrade if the coil parameters and tuning system are selected in the same way as for a unidirectional design. Therefore, a bottom-up approach is needed to compile all requirements and constraints from the beginning to optimize the magnetic system while considering the cost and limitations on the power electronics.

Finepower is continuously extending the technical limitations of wireless charging and supports customers to efficiently implement this technology into their respective applications.

 

About the authors

Main author:

Sri Vijay Vangapandu received master’s degree at the Technical University of Munich in 2019. He works in the field of inductive charging for industrial and automotive applications at Finepower GmbH in Ismaning/Munich.

Co-Author:

As Field Application Engineer and Marketing Manager, Dipl.-Ing Tobias Herrmann is responsible for both the technical support of customers, as well as the technical and visual marketing activities of Finepower.

 

This article originally appeared in Bodo's Power Systems Magazine.