The primary goals of compensation in wireless power transfer (WPT) for electric vehicles (EVs) are to improve power transfer efficiency, reduce reactive power, and ensure stable operation under varying conditions, such as load changes and coil misalignment.
Compensation in WPT systems for EVs refers to using reactive components, such as capacitors and inductors, to manage and optimize the power transfer process.
Refer to the previous FAQ, which discusses the simple and basic compensation topologies used in the EV WPT system. This article explains why compensation topology is required for the WPT system with a focus on EVs.
Making the WPT system tolerant to bifurcation
Bifurcation in WPT systems refers to splitting the resonant frequency into multiple frequencies under certain operating conditions. This phenomenon can occur in capacitive wireless power transfer (CWPT) and inductive wireless power transfer (IWPT) systems.
In CWPT systems, frequency bifurcation can occur when the distance between the transmitter and receivers varies. This can reduce the efficiency or power transfer of a fixed load system.
Applying the bifurcation phenomenon allows the system to operate in a coupling-independent regime, resulting in a nearly constant efficiency or power transfer despite fluctuating coupling.
In IWPT systems, bifurcation occurs under certain conditions, such as misalignment (coupling coefficient changes) or load changes. This is an important characteristic of traditional IWPT systems and adversely affects their efficient and stable operation.
Frequency bifurcation in IWPT systems can be eliminated by controlling the fractional order of the system components. This can expand the working range of the conventional IWPT system by reducing the boundary of critical distance and critical load.
Series-series compensation (Figure 1) is a technique used to improve the efficiency and stability of WPT systems. It involves adding a series of resonant components to the primary and secondary sides of the system.
Figure 1. An equivalent circuit diagram of a series-series compensated system. (Image: Elsevier)
This correction method lowers frequency bifurcation by changing the fractional order of the system’s parts. This makes the system’s working range bigger and boosts output power and transfer efficiency.
Reducing the apparent power input in WPT
Reducing apparent power input helps improve the efficiency of the WPT system. This is because the input power supply can operate at a lower VA (volt-ampere) rating, reducing energy losses and increasing the system’s overall efficiency.
Lowering apparent power input can lead to cost savings for the power supply. Minimizing the VA rating allows the power supply to be designed with smaller and less expensive components, reducing costs, as deduced from Figure 2.
Figure 2. A power triangle showing the relationship between real, reactive, and apparent power. (Image: Rakesh Kumar, Ph.D.)
WPT systems can be designed to operate over a broader range of applications, from low-power devices to high-power systems, by minimizing input apparent power. This flexibility is important for the widespread adoption of wireless power transfer technology.
Compensation topologies in WPT systems can reduce apparent power input by minimizing the power supply’s VA rating. This is achieved by resonating the primary and/or secondary inductances with compensation capacitors to provide the reactive power required for the inductances to generate an adequate magnetic field.
Regulating constant voltage and constant current in WPT
Different compensation topologies (Figure 3) can regulate the voltage and current at the output, ensuring stable and efficient power transfer.
Figure 3. A review of the different basic compensation topologies in the WPT system. (Image: Rakesh Kumar, Ph.D.)
For example, Series-Series (SS) compensation is known for maintaining a constant voltage gain regardless of load variations, making it suitable for applications requiring stable voltage output.
Similarly, Series-Parallel (SP) and Parallel-Series (PS) topologies can be optimized to provide constant current or voltage outputs, respectively, depending on the application requirements.
Summary
Compensation networks are mandatory in an EV’s WPT system. The most commonly used compensation topologies, SS, SP, PS, and PP, meet different needs for this system.
SS compensation topology provides stable voltage gain and is less sensitive to load and coupling variations, making it suitable for dynamic charging applications. The SP compensation topology Offers good voltage regulation and is effective for systems with strong magnetic coupling.
The PS compensation topology suits applications requiring constant current output and high power factor at weak coupling. The PP compensation topology is less commonly used due to its lower power factor but can be effective in specific scenarios.
References
- Frequency bifurcation conditions for capacitive wireless power transfer with multiple receivers, IEEE
- Frequency bifurcation in a series-series compensated fractional-order inductive power transfer system, Elsevier
- A comprehensive review of the recent development of wireless power transfer technologies for electric vehicle charging systems, IEEE
- Compensation topologies of high-power wireless power transfer systems, IEEE
- Wireless power transfer: systems, circuits, standards, and use cases, MDPI
Images
- Figure 1, Journal of Advanced Research, Elsevier, Page 11, Figure 21
- Figure 2-3, Rakesh Kumar, Ph.D.
Filed Under: FAQs, Wireless charging