A high-frequency induction heating system feed from parallel connected cascaded non-inverting buck-boost converter

1. A high-frequency induction heating system
feed from parallel connected cascaded
non-inverting buck-boost converter
1
Report submitted to Dr. Ibrahim Abdelsalam
Done by: Mahmoud Abd Elfattah Mahmoud
ID number: 22223723

2. Abstract
➢ This presentation proposes a back to back converter for high-frequency induction heating system.
➢ The rectifier side operates under discontinuous conduction mode (DCM), and draw sinusoidal
current at a unity power factor from the supply.
➢ The rectifier side converter also controls the system output temperature by regulating the total input
power.
➢ The inverter side has a resonant operation utilizing a simplified phase lock loop scheme and can
achieve zero current resonant switching at a high power factor with operating frequency up to 100
kHz.
➢ The validity of the proposed high-frequency induction heating system is discussed and confirmed
utilizing PSCAD/EMTDC simulations considering different operating conditions.
2

3. INTRODUCTION
➢ Induction Heating (IH) technologies are one of the most efficient and well accepted
heating technology, because of its advantages in industrial, domestic and medical
applications.
➢ Induction Heating (IH) is the preferred choice over classical heating techniques such as
flame heating, resistance heating or traditional ovens or furnaces due to:
▪ Fast heating.
▪ Cleanness.
▪ Safety.
▪ Efficiency and accurate control.
▪ IH is a non-contact heating process that uses a power source to generate heat
by induction.
3

4. 4
Typical arrangement of an induction heating system: (a) general view and (b) top view.

5. How Induction Heating (IH) Work
5
➢ According to Faraday’s law of induction by a changing magnetic field in the conductor, eddy currents flow in closed loops
within conductors in planes perpendicular to the magnetic field, magnetic hysteresis creates additional heating in
ferromagnetic materials and Joule's first law which is known as Joule Effect expresses the relationship between heat
generated in a conductor and current flow, resistance, and time.
➢ AC power source is connected to back to back (BTB) converter to supply alternating voltage to the induction heating coil.
➢ The coil generates alternating magnetic field to induce of electrical current loops in the load to produce heat.
➢ Resonant inverters with output frequency (20 kHz to 100kHz) are widely used in IH system to reduce losses by soft
switching.
➢ High frequency operation reduces footprint and weight.
➢ the rectifier side of IH systems is diode rectifier or thyristor-based rectifier which results in drawing high total harmonic
distortion and large ac-filter in the rectifier side.

6. PROPOSED HIGH-FREQUENCY IH SYSTEM
6
➢ IH system consists of:
• Three single-phase transformers.
• Three ac/dc converter.
• One dc/ac inverter.
➢ The primary sides of the transformers are connected is star connection with the grid.
➢ The secondary side of each transformer is connected to individual single-phase ac-dc non inverting buck-boost
converter.
➢ The three ac/dc converter are connected in parallel to reduce the current stress on the power electronic devices.
➢ The ac-dc non inverting buck-boost converter operates under discontinuous conduction mode (DCM) to have a fast
dynamic response, low total harmonic distortion (THD) and unity power factor.
➢ The output of the non inverting ac/dc buck-boost converter is connected to a high-frequency inverter.
➢ The inverter output connected to a matching transformer with a turns ratio of 10/1 to match the converter
impedances and induction heating coil and to isolate the system.
➢ The transformer secondary side is connected to a compensating capacitor C connected in series with the heater coil L.

7. 7

8. IH SYSTEM THERMAL DYNAMICS
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▪ The working principle of IH system is similar to transformer in which the primary winding is the IH coil
and the secondary winding is the metallic objective that is heated or melted.
▪ The equivalent circuit of the IH system consist of resistance connected in series with the inductance and
capacitance to make resonance condition.

9. IH SYSTEM THERMAL DYNAMICS
9
▪ The equivalent resistance (Req) is the summation of the coil resistance (Rcoil) and the shunt resistance (Rsh):
Req = Rcoil + Rsh
▪ The coil resistance is calculated by the following equation:
Rcoil =
𝜌𝑐
𝑙𝑐
𝐴𝑐
Where 𝜌𝑐 is the coil resistivity, 𝑙c is the length of the coil and Ac is the cross sectional area of the coil.
▪ The shunt resistance which is the mutual inductance resistance is calculated as follow:
Rsh= 𝑁2RL
▪ The load resistance RL of metallic objective which is heated or melted is calculated as follow:
RL =
𝜌𝑚
𝑙𝑚
𝐴𝑚
Where 𝜌𝑚 is the material resistivity of iron (9.71 x 10-8 ohms.m), 𝑙𝑚 is the length of the tube, and the surface
cross sectional area of the metallic tube is:
𝐴𝑚 =2π rm 𝑙m + 2π 𝑟𝑚
2
▪ The equivalent inductance (Leq) is the total of self-inductance which is reduced by mutual inductance. It is
represented as:
Leq= 𝐿1 − 𝑁𝐿𝑀
Where L1 is the self-inductance and LM is the mutual inductance.

10. IH SYSTEM THERMAL DYNAMICS
10
▪ Resonant circuit is connected in series with the capacitor bank, the total impedance is obtained by:
Zt = Req +2π f Leq +
1
2πf𝑐
▪ the secondary output voltage and current are shown in the two following equation.
VS = Zt Is
Is =
𝑉𝑠
𝑍𝑡
▪ IH system follow the equilibrium thermal energy principle is expressed by:
Input Heat = Output Heat + Heat Loss
▪ Input Heat is the IH BTB converter input power:
Input Heat =
1
2
𝐼𝑠
2𝑅𝑒𝑞
▪ Output heat is the total heat energy consumed by heat or melt metallic objective per unit time:
Output Heat =
𝑚𝑐 𝑇ℎ
−𝑇𝑎 +𝑚𝐿𝑝𝑡
𝑡

11. IH SYSTEM THERMAL DYNAMICS
11
▪ Mass energy and Heat Latent Energy are calculated by the two following equation respectively:
Qm = mc(Th - Ta)
QL = mLpt
Where m is the mass of charge (Kg), c is specific heat capacity of charge material (J/Kg. ◦K), Th is the melting
temperature of charge (◦K), Ta is the ambient temperature (◦K), and Lpt is the amount of heat to accomplish
phase transformation.
▪ Heat Loss is energy which is lost in IH coil is show in the following equation:
Heat Loss = 𝐼𝑠
2
𝑅𝑐𝑜𝑖𝑙
▪ From the previous equations:
𝐼𝑠
2
=
2𝑚(𝐶ϴ+Lpt )
𝑡(𝑅𝑠ℎ−𝑅𝑐𝑜𝑖𝑙)
So 𝐼𝑠
2 α θ , Where θ = Th−Ta .

12. CONTROL STRATEGY
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➢ IH system control system consist of two main sections:
▪ First section is applied to control the input power which is responsible of controlling the IH system output.
▪ Second section is utilized to insure that the inverter maintains resonant operation.

13. BTB converter Input power control loop
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➢ BTB converter input power control loop consists of two main control loops, inner control loop and outer control
loop. Based on the required temperate and the metallic objective equivalent resistance the user feeds the
controller with the required input reference power.
➢ The controller calculates the equivalent reference peak supply current (IP_ref) which is the input signal of the
outer control loop.
➢ In the outer control loop, the error between the (IP_ref) and the actual supply peak current (IP_act) passes through
a proportional integral (PI) controller to estimate the reference dc-side capacitor voltage. A voltage limiter is
added to protect the power electronics devices from over voltage.
➢ In the inner control loop the PI control utilized the error between the reference dc-side capacitor voltage and
the actual capacitor voltage to calculate the ac-dc buck-boost converter modulation index (m).

14. Inverter control loop
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➢ A simple phase lock loop (PLL) control algorithm is applied to maintain resonant operation over a wide range of
frequencies from 50 to 100 kHz. Where both capacitor voltage (vc) and inverter output voltage (vi) is compared
with zero, where result output of the comparison is used as an input to the XOR gate.
➢ Low pass filter (LPF) utilized to get the mean value from the XOR gate output, which proportional to the phase
difference between the inverter and capacitor voltages.
➢ The LPF output voltage is compared with a value corresponding to 90 degrees (0.5).
➢ The PI controller is utilized to adjust the inverter switching frequency to ensure that the difference is forced to
zero. When this condition is achieved, the capacitor and the inverter voltages are in quadrature, which ensures
that the inverter output voltage and current are in phase.

15. SIMULATION RESULTS
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➢ The proposed IH system is tested with three different reference input power 4kW, 8kW and 2kW
(equivalent IP_ref = 8.6A, 17.1A, and 4.3A respectively) to evaluate the dynamic performance of the proposed IH
system scheme, the ability of tracking the reference input power and inverter resonant operation at different
operating conditions.
➢ The overall system parameter are listed in table below:

16. SIMULATION RESULTS
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The following figure, shows that the controller succeeded to quickly track the reference supply peak current during
different operating conditions with minimum transient time.

17. SIMULATION RESULTS
17
The following figure, shows that the reference dc-side capacitor voltage increased and decreased to compensate
the change in the reference input power.

18. SIMULATION RESULTS
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The following figure represent the dc-link inductance current, where it can be observed that the proposed IH
system operates at the boundary between DCM and continuous conduction mode (CCM) at rated power to
decrease the current stress on the power electronics devices.

19. SIMULATION RESULTS
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The following figure shows phase ’a’ supply voltage and the three-phase supply current during the step change of
the reference power from 4kW to 8kW. It is observed that the proposed converter has a high quality sinusoidal
current in phase with the supply voltage at different operating conditions.

20. SIMULATION RESULTS
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The following figure show that the inverter output voltage and the resonant capacitor voltage are in quadrature
which means that the inverter is operating under resonant condition.

21. SIMULATION RESULTS
21
The following figure shows The inverter output voltage and output current are in phase.

22. 22
CONCLUSION
The induction heating system based on using three parallel connected ac-dc buck-boost converter to supply
adjustable dc input voltage for resonant high frequency inverter. The ac-dc converter operates under DCM and
parallel connected to reduce the current stress on the power electronics devices. The resonant high frequency
inverter has a simplified PLL that use single logic circuit and one low pass filter. Simulation results using
PSCAD/EMTDC verifies the operation of the proposed IH system under different operating conditions. The results
show that the proposed system has a good dynamic performance and sinusoidal unity PF supply current. Also the
results reveal that the PLL control scheme operates precisely as designed and the inverter output voltage and
current are kept in phase at different operating conditions.