Wireless EV Charging

# Wireless Electric Vehicle Charging 
### IPT technology, Current status & Recent Developments

.TitleAuthor[Duleepa J Thrimawithana]

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.logo-slide[]
.footer[[Duleepa J Thrimawithana](https://www.linkedin.com/in/duleepajt), Department of Electrical, Computer and Software Engineering (2023)]

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# The University of Auckland

- Highest ranked New Zealand university and 68th in the QS World University Ranking
- Over 5,000 staff members and 40,000 students
- Nine faculties including Medical & Health Sciences, Engineering, Business & Economics and Science

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# Dept. of Electrical, Computer & Software Eng.

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- One of the 5 departments in the Faculty of Engineering
- Offers 3 undergraduate degree programs
 - Electrical & Electronics, Computer Systems and Software
 - Project based teaching
- 35+ full-time academic staff members and 15+ post-doctoral research fellows
- 150+ postgraduate students and 700+ undergraduate students
- Regular visiting research scholars and research students
- Research groups include Power Electronics, Power Systems, Signal Processing, Robotics, Embedded Systems, Parallel Computing, Telecommunications and Control Systems
]

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# Dept. of Electrical, Computer & Software Eng.

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# Power Electronics Research Group

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# Part I - Wireless Power Transfer
### A Review of Current Status

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.logo-slide[]
.footer[[Duleepa J Thrimawithana](https://www.linkedin.com/in/duleepajt), Department of Electrical, Computer and Software Engineering (2023)]

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# Wireless Power Transfer

.left-column[
- Can be divided in to two broad categories
- Radiative transmission through far-field principles
 - Examples include power transfer through radio-frequency, microwave, optical and ultrasonic technologies
 - At an experimental stage of development
 - AirFuel Alliance is developing standards
- Non-radiative transmission through near-field principles
 - Inductive power transfer (IPT) and capacitive power transfer (CPT) technologies are utilised
 - IPT is the most mature technology 
 - WPC, SAE, etc. standards are being developed
]

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# Early Developments

- 1890s to 1920s: Tesla (US), Hutin (FR) and LeBlanc (FR) demonstrated wireless power transfer 
- 1960s: Shuder (US) and his team worked on high power IPT systems for biomedical applications
- 1970s: Otto (NZ) and Bolger (US) proposed roadway power
- 1980s: Santa Barbara electric bus project (US) 
- Early 1990s: Boys (NZ), Green (NZ) and Covic (NZ) developed IPT systems for dynamic industrial systems & stationary people movers which were adopted and commercialized by Difuku and Wamplfer in materials handling applications
- Late 1990s: Meins (DE) developed multiphase IPT systems for industrial applications

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# Recent Developments

- Early 2000s: Boys (NZ) and Covic (NZ) developed IPT systems with Conductix-Wamplfer and Daifuku for automated guided vehicles and busses
- Mid 2000s: Hui (HK), Hu (NZ) and Soljacic (US) developed low power IPT systems for powering/charging biomedical implants, sensors and consumer devices
- Late 2000s: Hu's (NZ) work lead to forming Power by Proxi, Soljacic's (US) work lead to forming WiTricity  
- Early 2010s: Boys's (NZ) and Covic's (NZ) work lead to forming HaloIPT, which lead to Qualcomm Halo and Qi standard was released 
- Late 2010s: WiTricity holds the largest patent portfolio as they acquired Qualcomm Halo, while Apple acquires Power by Proxi, and number of new research groups as well as companies starts to emerge
- Early 2020s: Standards for EV stationary charging completed

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# WI-Charge Technology

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# Energous WattUp Technology

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# CPT Technologies

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# IPT Technologies

- Power ratings of tens of kWs with uni or bi-directional power flow
 - Magnetic designs include circular coil, solenoidal coil, polarised coil, multi coil structures as well as track based systems 
 - Ferrites and/or reflection coils are often used to shape the magnetic fields generated
 - Transmission range of over 300 mm with over ± 200 mm XY tolerance easily achievable 
 - Efficiency typically over 85% but can be as high as 97%
 - Operating frequency typically ranges from tens of kHz to tens of MHz

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# IPT Applications

- Factory automation and materials handling
 - Includes hoists, AGVs and clean room systems  
- Consumer electronics and appliances
 - Charging phones, laptops, etc. as well as powering appliances 
- Transportation electrification 
 - Charging stationary and in-motion electric vehicles, charging electric ferries, etc.
- Biomedical
 - Charging and powering medical devices as well as implants
- Lighting  
 - Wireless power and communication to tunnel lighting, stage lighting and mines 
- Industrial applications 
 - Industrial machinery, robots, electric fense energizers, etc.

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# Part II - IPT Fundamentals
### Modelling the Coils and Power Transfer

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.logo-slide[]
.footer[[Duleepa J Thrimawithana](https://www.linkedin.com/in/duleepajt), Department of Electrical, Computer and Software Engineering (2023)]

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# Components of an IPT System

- Primary consists of a DC-AC switched-mode converter, a compensation network and a transmitter coil(s) 
 - A lumped transmitter coil is often referred to as a primary pad/coil/coupler 
 - An elongated coil is often called a primary track
 - Can be directly fed by a DC source or through a grid-connected AC-DC converter
- Secondary/pick-up consists of a receiver coil(s), a compensation network and an AC-DC converter
 - The reciever coil is is often referred to as a secondary/pick-up pad/coil/coupler
- Primary and pick-up coils are magnetically coupled but the coupling coefficient is typically less than 40%
- The compensation networks help improve efficiency by minimizing the VA requirements of converters

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# Modelling the Primary and Pick-up Coils

- The behaviour of the coils can be modelled using a T-equivelent transformer model
 - `\(L_{pt}\)` and `\(L_{st}\)` are the self-inductances of primary and pick-up coils, respectively
 - `\(\phi_{m}\)` represents flux linkage while `\(\phi_{lk_p}\)` and `\(\phi_{lk_s}\)` represent leakage flux
 - The coupling coefficient, k, between the coils is less than 0.4 in a typical IPT system
- The mutual inductance, M, between the two coils is given by 
\\[M = k\sqrt{L\_{pt}L\_{st}}\\]
- If the mean-turn lengths of the coils do not change, then `\(n = \frac{N_{pt}}{N_{st}} = \sqrt{\frac{L_{pt}}{L_{st}}}\)`, where `\(n\)` is the turns ratio

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# Coupled Inductor Model

- Although a T equivalent transformer model can be used to model the coils, the coupled inductor model is widely used when analysing an IPT system in the phasor-domain
- In the coupled inductor model, `\(L_{pt}\)` and `\(L_{st}\)` represent the self-inductances of primary and pick-up coils
- `\(V_{sr}\)` represents voltage induced across `\(L_{st}\)` due to current `\(I_{pt}\angle 0\)` flowing through `\(L_{pt}\)` and is given by
\\[V\_{sr} = \omega MI\_{pt}e^{j\pi/2}\\]
- Similarly, `\(V_{pr}\)` represents voltage induced across `\(L_{pt}\)` due to `\(I_{st}\angle\theta\)` and is given by
\\[V\_{pr} = \omega MI\_{st}e^{j(\theta+\pi/2)}\\]

???

If the current through primary coil is `\(I_{pt} \sin (\omega t)\)` then,
\\[\phi\_{m} \propto I\_{pt} \sin (\omega t) \\]
The voltage induced on the pick-up coil is therefore,
\\[v\_{sr} \propto \frac{\mathrm{d} \phi\_{m} }{\mathrm{d} t} \\]
\\[v\_{sr} \propto \omega I\_{pt} \cos (\omega t) \\]
Since `\(M\)` represent the coupling between the two coils,
\\[v\_{sr} = \omega M I\_{pt} \cos (\omega t) \\]
In the phasor-domain,
\\[V\_{sr} = \omega M I\_{pt} e^{j\pi/2} \\]

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# Open-Circuit Voltage & Short-Circuit Current

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- The voltage measured across an open-circuited pick-up coil, `\(V_{oc}\)`, is the same as `\(V_{sr}\)` and therefore
\\[V\_{oc} = V\_{sr} = \omega MI\_{pt}e^{j\pi/2}\\]
- The current flowing through a short-circuited pick-up coil, `\(I_{sc}\)`, is 
\\[I\_{sc} = \frac{V\_{sr}}{j\omega L\_{st}} = I\_{pt}\frac{M}{L\_{st}}\\]
- The uncompensated volt-ampere (VA) capacity of the pick-up coil can be given by
\\[S\_{u} = \left | V\_{oc} \right | \left | I\_{sc} \right | = I\_{pt}^2\frac{\omega M^2}{L\_{st}}\\]
]

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# Power Transferred Across Airgap

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- Assume that the `\(V_{sr}\)` induced by `\(I_{pt}\angle 0\)` causes a current `\(I_{st}\angle\theta\)` to flow through a load, `\(Z_{sc}\)`, attached across the pick-up coil 
- Under these conditions, power transferred is
\\[P\_{o} = \Re \left \\{ V\_{sr}I\_{st}e^{-j(\theta)} \right \\} = \left | V\_{sr} \right | \left | I\_{st} \right | \cos(\pi/2-\theta)\\]
- The VA rating of the coils required to achieve this is
\\[\mathit{VA}\_{pt} = \omega L\_{pt} \left | I\_{pt} \right |^2 \quad \text{and} \quad \mathit{VA}\_{st} = \omega L\_{st} \left | I\_{st} \right |^2 \\]
- Thus `\(P_{o}\)` is related to the VA in the coils as given by
\\[P\_{o} = k \sqrt{\mathit{VA}\_{pt}\mathit{VA}\_{st}} \sin(\theta) \\]
 - Typically, `\(\sin(\theta) \approx 1\)`, and thus `\( P_o \approx k \sqrt{\mathit{VA}_{pt}\mathit{VA}_{st}} \)`
]

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Note that 
\\[P\_{o} = \Re \left \\{ V\_{sr} \bar {I\_{st}} \right \\} \\]
Substituting `\( \left| V_{sr} \right | = \omega M \left| I_{pt} \right | \)`, 
\\[P\_{o} = \omega M \left | I\_{pt} \right | \left | I\_{st} \right | \cos(\pi/2-\theta)\\]
\\[P\_{o} = k\sqrt{ (\omega L\_{pt} \left | I\_{pt} \right |^2 ) (\omega L\_{st} \left | I\_{st} \right |^2) } \sin(\theta)\\]
\\[P\_{o} = k \sqrt{\mathit{VA}\_{pt}\mathit{VA}\_{st}} \sin(\theta) \\]

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# Coil Q, Losses & Maximum Efficiency

- Assume that the losses in `\(L_{pt}\)` and `\(L_{st}\)` are modelled using equivelent series resistances (ESRs) `\(r_{pt}\)` and `\(r_{st}\)`
- The quality factors, `\(Q_{pt}\)` and `\(Q_{st}\)`, of the primary and pick-up coils are
\\[Q\_{pt} =  {\omega L\_{pt}}/{r\_{pt}} \quad \text{and} \quad Q\_{st} = {\omega L\_{st}} / {r\_{st}} \\]
- The total losses in the two coils can be expressed as
\\[P\_{loss(c)} = \left | I\_{pt}^2 \right | r\_{pt} + \left | I\_{st}^2 \right | r\_{st} = \frac{\mathit{VA}\_{pt}}{Q\_{pt}} + \frac{\mathit{VA}\_{st}}{Q\_{st}}\\]
- The maximum efficiency is achieved when losses in the coils are matched (i.e. `\(\mathit{VA}_{pt}/Q_{pt} = \mathit{VA}_{st}/Q_{st}\)` )
\\[ \eta\_{c\_{max}} = \frac{ k \sin(\theta) -  \frac {1} { \sqrt {Q\_{pt} Q\_{st}} }} {k \sin(\theta) +  \frac {1} {\sqrt {Q\_{pt} Q\_{st}} }} \\]

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# Impact of k and Coil Q on Performance

- Typically, k is in the range 0.1 to 0.3 and coil Q is in the range 400 to 600 for EV charging
- Design of a 10 kW IPT system 
 - Assume both `\(Q_{pt}\)` and `\(Q_{st}\)` are 400
 - Note that `\( P_o \approx k \sqrt{\mathit{VA}_{pt}\mathit{VA}_{st}} \)` needs to be always met

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# Part III - Coil Structures
### Field Patterns and Coil Parameters

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.logo-slide[]
.footer[[Duleepa J Thrimawithana](https://www.linkedin.com/in/duleepajt), Department of Electrical, Computer and Software Engineering (2023)]

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# Types of Coil Structures

- Can be broadly divided into ditributed track type coil structures and lumped coil structures
- Typically, ferrites are used to help shape the field generated 
 - Maximize coupling, minimize leakage and improve coil Q
- As an alternative for ferrites, energised or passive field shapping coils can be used
- Lumped coil structures can be further divided as non-polarised, polarised and multi-coil structures
- Multi-coil structures typically employ multiple decoupled coils 
 - Each coil can be driven seperately to create the desired field pattern
- Repeater coils can be used to extend the range

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# Construction of a Lumped Coil

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- Contructed as planar spiral winding and take the shape of a circle/square/rectangle/etc.
 - Multifilar windings can be used to increase the current rating, but special attention is required to ensure current sharing 
- High-frequency Litz wire is used to improve coil Q
 - Need to ensure that the windings are appropriately isolated 
- Ferrite plates/disks/bars are used to create a single sided flux pattern 
- The housing that provides electrical isolation holds the Litz wire and the ferrites 
 - Typically the windings and ferrites are potted 
- Aluminum shielding is used to reduce leakage flux
]

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# Non-Polarised Coils

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- The single sided magnetic field distribution is from centre to edge of the coil
- Typically used to transfer power over a gap that is roughly equivalent to one quarter of the coil diameter
- Changes in the displacement between the primary and pick-up coils causes
 - Changes to `\(k\)` between the coils 
 - Changes to `\(L_{pt}\)` and `\(L_{st}\)` of the coils
- With lateral/longitudinal displacement `\(k\)` will drop to 0
 - The point at which `\(k\)` drops to 0 is referred to as the decoupled position of the coils
 - A negative `\(k\)` after the decoupled position indicates a `\(180^0\)` phase-shift in induced voltage
]

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# DD Type Polarised Coils

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- The single sided magnetic field distribution is from one end to the other end of the coil
- Typically used to transfer power over a gap that is roughly equivalent to one half of the coil length
- Flux-pipe formed by the two coils at the center of the pad should be as large as possible
- Changes in the displacement between the primary and pick-up coils causes
 - Changes to `\(k\)` between the coils 
 - Changes to `\(L_{pt}\)` and `\(L_{st}\)` of the coils
- With longitudinal (x) displacement `\(k\)` will drop to 0
 - At the point at which `\(k\)` drops to 0, primary and pick-up DD coils are decoupled
]

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# Multi Coil Structures

- DDQ structure is one of the most common multi-coil structures
 - The quadrature (Q) winding is decoupled from the DD winding
 - Thus, both windings can be used independently to transfer power
 - A DD primary and DDQ secondary provides wide spatial tolerance since the Q winding helps transferring power when `\(k\)` between the DD windings drop
- Bi-polar structure employs two overlapping windings that are decoupled from each other
 - When used as a transmitter, each coil can be driven at a desired magnitude and a phase
- Tri-polar and other multi-coil structures typically employ decoupled overlapping windings

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# Demo: Fields Generated by a DD Coil

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# Part IV - Compensating the Coils
### Behavior of the Compensated Coils

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.logo-slide[]
.footer[[Duleepa J Thrimawithana](https://www.linkedin.com/in/duleepajt), Department of Electrical, Computer and Software Engineering (2023)]

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# An Uncompensated Pick-up

- Assume a load resistor, `\(R_{L}\)`, is connected directly across a pick-up coil, which is coupled to a primary coil 
- The primary coil is driven by the primary converter generating a current `\(I_{pt}\angle 0\)` at an angular frequency `\(\omega = 2\pi f\)`
- The current flowing through pick-up coil as well as the load will be
\\[ I\_{st} = \frac {-V\_{sr}} {R\_{L} + j\omega L\_{st}} = \frac {-\omega MI\_{pt}e^{j\pi/2}} {R\_{L} + j\omega L\_{st}}  \\]
- The impedance of `\(L_{st}\)` limits the ability to supply current to `\(R_{L}\)`

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# Power from an Uncompensated Pick-up

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- The power delivered to the load is then
\\[ P\_{o} =  \left | I\_{st} \right |^2 R\_{L} = \left ( \omega MI\_{pt} \right )^2  \frac {R\_{L}} {R\_{L}^2 + \omega^2 L\_{st}^2 } = k^2 \mathit{VA}\_{pt} \frac {\omega L\_{st} / R\_{L}} { 1 + \omega^2 L\_{st}^2 / R\_{L}^2 } \\]
- At a load `\(R_L = \omega L_{st}\)`, maximum power is delivered 
\\[ P\_{o\_{(max)}} =  \frac {\left ( \omega MI\_{pt} \right )^2} {2 \omega L\_{st} } = 0.5 V\_{oc}I\_{sc}  = 0.5k^2\mathit{VA}\_{pt} \\]
- As an example, when `\(k = 0.1\)`, to deliver 5 mW to the load, the primary coil should be driven at least at 1 VA
 - The impedance of `\(L_{st}\)` limits the power transfer
]

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# What Should an 'Ideal' Pick-up Look Like?

- The load `\(R_{L}\)` 'ideally' needs to be able to take any amount of power from pick-up
- This could be achieved, for example if the pick-up coil is made to to look like an ideal voltage source
 - The impedance of `\(L_{st}\)` need to be made zero 
 - In this case we would have an ideal voltage source, `\(V_{sr}\)`, supplying the load, `\(R_{L}\)` 
- Alternatively, the pick-up coil can be made to look like an ideal current source, `\(I_{sc}\)`
- Resonance can be used to achieve this 'ideal voltage source' or 'ideal current source' type behavior
 - A technique discovered by Hertz in the 1880's

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# Primary Compensation Networks

- Directly driving the highly inductive load consisting of `\(L_{pt}\)` and `\(Z_{pr}\)` with a power converter can be very inefficient and costly
 - For example the VA rating of the converter, which is required to deliver `\(1W\)` to the pick-up load, can be in excess of `\(1/(k^2Q_{ss/sp_{(op)}})\)`
 - If `\(k = 0.1\)` and `\(Q_{ss/sp_{(op)}}=5\)`, which are typical for an IPT system, the primary converter will need to be supply 20VA to deliver 1W to the load
- Resonance is also used to reduce the VA demand of the primary converter
 - Many different resonant networks have been developed to-date to cater for different applications

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# Compensation Methods

- The two most common resonant/compensation networks are the parallel and series types
 - Parallel resonance is not common on primary in modern systems as it needs to be driven by a current sourced inverter
- Primary side commonly employ LCL compensation networks 
- More complex compensation networks include
 - LCCL and LCLC
 - Hybrid compensation
 - Impedance compression networks

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# Compensation Networks - Summary

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# Research Advances
### Highlights & Updates

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.logo-slide[]
.footer[Duleepa J Thrimawithana, Department of Electrical, Computer and Software Engineering (2023)]

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# The Team

- Members from Power Electronics, Centre for Advanced Composites Materials, Transportation, Engineering Science,  Business School, GNS Science, Victoria University, Mahi Maioro and AUT
 - Collaborations with industry, ASPIRE, Utah State University and Cambridge University

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# Capabilities

- The power electronics team work with the other teams to develop converter, magnetic and controller designs and extensively test them

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# Active Rectification of Pick-up Output

- Identical primary and pick-up circuits employ inductor-capacitor-inductor (LCL) T-resonant networks
 - Can use other types of compensation networks 
- Both full-bridge converters are operated at the same frequency to drive the couplers
 - Compensation networks are typically tuned to the fundamental switching frequency
- Magnitude of power flow is regulated using voltage cancellation to control the RMS of `\( V_{pi} \)` and `\( V_{si} \)` and/or controlling the phase difference between `\( V_{pi} \)` and `\( V_{si} \)`
- Direction of power flow is controlled using the phase difference, `\( \theta \)`, between `\( V_{pi} \)` and `\( V_{si} \)`

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# Hybrid Compensation Topologies

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- Complementary compensation networks employed to improve misalignment tolerance 
- Require less or no control 
- Nearly constant power transfer across the entire operating region
- Minimize increase in VA due to detuning caused by changes in coupler inductanes  
- For example the power throughput of a parallel hybrid system can be given by

\\[P\_{out} = \{8V\_{in}V\_{out}\over\pi^2} \left(\frac{1}{\omega\_TM\_s} + \frac{M\_p}{\omega\_TL\_{pi}L\_{si}}\right)\\]

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<img src="img/updates/Hybrid_Conv2.png" width="350px">
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# Boost Active Bridge (BAB)

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- Integrated post/pre regulation capabilities enable maximum efficiency tracking
- Wide ZVS range and reduced current stresses
- Eliminates DC-blocking capacitor
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<img src="img/updates/BAB_W2.png" width="310px">
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# WPT2/Z2 VA with Integrated BAB

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- Boost inductors integrated in to the DD pad
- Designed to comply with WPT2/Z2 specifications
- Controlled both the GA and VA to maintain a nearly constant efficiency under all operating conditions

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# Integrated Boost Multilevel Converter (IBMC)
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- Derives a higher voltage to drive the compensation network
 - Reduces current stress and may eliminate step-up transformer
- Inherently eliminates circulating currents
 - Series-blocking capacitor can be omitted
- Generate a low THD staircase modulated output voltage
 - Can generate wide range of voltages
 - Can use lower voltage higher performance devices
 
<img src="img/updates/IBMMC_Conv.gif" height="160px">

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# IBMC Performance

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- Maintains a constant 7.7 kW power transfer through primary control
- Operate in 5-level mode at low k and 3-level mode at high k
- Efficiency ranges between 91.5% and 92.4% across entire operating region
- A simple algorithm used to ensure module voltages are balanced
]

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# A Digitized Modulation Scheme

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- Traditional duty/phase modulation causes loss of ZVS
- Pulse density modulation introduces unwanted resonances
- New scheme operates a module only at 0%, 50% or 100% duty ensuring ZVS
- Works like a DAC as the number of modules in each of the 3 states determine voltage amplitude 
]

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# An MMC Based IPT System

- In some applications, boosting capability of the IBMC may not be needed
 - An MMC based system can be used in such applications 
- Arm inductors also functions as part of the resonant circuit
 - Performance is similar to an IBMC
]

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# Low Cost IPT Systems

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- For example push-pull converters employed in both VA and GA 
 - DC indcutor(s) and current splitting transformer can be integrted in with the D coupler
 - Generates a higher voltage to drive the couplers   
 - Relative phase between converters controls the power flow
 - Switches maintain ZVS across the entire operating region

]

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# Push-Pull Coupler Array (PPCA)

- Each coupler driven by a single switch and controlled independently
- Swtich stresses do not scale with the number of couplers
- Switches only conduct the DC current
- A damaged module only impacts the two neighbouring modules 
]

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# Optimizing the Coil Structures

- An experimental method called the stepped resonant excitation (SRE) to obtain the core and copper losses is developed
 - Using SRE method high fidelity FEM models that also capture thermal behavior are developed
- As an example, the characteristics of WPT2/Z2 couplers show significantly higher copper losses
]

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# References
#### These slides were derived from a presentation prepared by the author on 
#### "Fundametals of Inductive Power Transfer: A Workshop with Example Designs"
#### @wirelesspower.github.io/SeminarsWorkshops/FundamentalsofIPTDJT/presentation.html

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# Thank You