A new bi-directional power electronic transformer (PET) for induction heating .... VS1 and N2/N1 are effective main component of secondary voltage and winding ...
BI-DIRECTIONAL POWER ELECTRONIC TRANSFORMER FOR INDUCTION HEATING SYSTEMS S.H. Hosseini1, M.B. Sharifian2, M. Sabahi3, A. Yazdanpanah4, G.H. Gharehpetian5 1,2,3,4 5
Faculty of Electrical & Computer Engineering, University of Tabriz, Tabriz, Iran Electrical Engineering Department, Amirkabir University of Technology, Tehran, Iran ABSTRACT
A new bi-directional power electronic transformer (PET) for induction heating applications is described. Proposed PET contains primary bi-directional cycloconverter switches, high frequency isolation transformer, matching coil and a parallel resonant heating system. Switching routine is applied in such a way that the output voltage regulation is achieved in the full load variations, even at asymmetrical utility conditions. Simulations are obtained for the proposed PET which consists of 380 V / 3φ/ 50 Hz input voltage and 210 V / 1φ /10 kHz output voltage. The mathematical analysis are presented and verified by simulation results. Index Terms — Power electronic transformer, High frequency switching, Induction heating, Parallel resonance.
providing bi-directional operation utilizing of PETs could be a proper solution. In this paper a three phase input single phase output direct PET (without energy storage element) induction heating device is presented. Proposed PET-IHD is designed to have equal power consumption between phases with unity power factor, so a resistive symmetric load from the utility point of view can be considered. Galvanic isolation and voltage level conversion are other features of proposed PET-IHD. Additionally, this new topology provides higher power rating with respect to utility voltage at the load side. Cycloconverter switches are controlled under a specific pattern to obtain three level high frequency voltage source to supply parallel resonance tank through a matching coil. A circuit model for 380 V / 3φ/ 50 Hz to 210 V / 1φ /10 kHz converter has been simulated with PSCAD/EMTDC [9] software. The simulation results show the good performance of proposed circuit and verify the mathematical analysis.
2. OPERATION OF THE PROPOSED INVERTER 1. INTRODUCTION Medium power high frequency converter now is widely used in several applications such as automobile and aircraft converters, computer and electronic devices, switching power supplies and uninterruptible power supplies. Meanwhile, in some applications which inductive loads should be supplied the concept of bidirectional power flow figures out as a serious issue. On the other hand, in order to minimize the size of isolation transformer and output filter components high frequency switching technique should be used [1]. Both high frequency switching and bidirectional power flow are achieved by power electronic transformers (PET). In addition, compact size, high power flow rate, multi function operation, voltage or frequency regulation and galvanic isolation aspect could be obtained. Therefore, many researches have recently studied several types of the PETs [1-4], proposed to be a suitable replacement of conventional converters. Most of researches have been studied the theory of PET and its usage in power systems. Meanwhile the special applications of PET could also be interesting, such as motor drive applications and excitation control [5]. Also solid state frequency converters have been widely used as induction heating and melting systems , which several literatures discussed about their structures and control manners [6-8]. Because induction heating devices (IHD) are classified as unusual or special loads [8], input rectifier and energy storage elements are used to obtain power factor correction. But these elements, increase losses and reduce efficiency. Thus, in order to eliminate input rectifier and storage elements as well as
Fig. 1 illustrates the power circuit of proposed IHD that employs six bi-directional switches at primary side of isolating transformer to establish some parts of three phase utility voltages on the primary winding. Possible switching states are illustrated in Table 1.
SA
SB
LC
SC TR
VA VB VC
N1: N2
+ Vp _
Input Filter
S'A
S'B
+ V_s
Cp
+ V _o Lp
S'C
Fig.1. Power electronic transformer based induction heating device.
978-1-4244-1643-1/08/$25 © 2008 IEEE 000347
States 0,1 and 2 create zero voltage on the primary side so RMS value of the cycloconverter output can be controlled by adjusting zero intervals. Positive and negative intervals are selected according to the maximum and the minimum input utility voltages respectively, defined by (1):
v max (t ) = max{v AB (t ), v BC (t ), vCA (t )}
v min (t ) = min{− v AB (t ),−v BC (t ),−vCA (t )}
(1)
therefore (5) can be simplified as (6):
1 − jω 0 C P − 2 ω L ω 0 LP (6) Z T ( jω 0 ) ≈ jω 0 L F + 2 1 2 ω 0 C P − 2 ω 0 LP RO 2 0
Imaginary part of (6), taken equal with zero, is solved to obtain f0 as the following equation:
Duration of none zero intervals, Ton, is given by (2):
TS 2 Vref
t ON = D (kTS )
(2)
D (kTS ) =
(3)
v max (kTS )
ω0 =
where TS , D and Vref are switching period, duty cycle and reference adjusting voltage respectively. Secondary side of isolation transformer is coupled to parallel resonance tank circuit through matching (or filter) coil LC. Table 1: Possible states of switching State Active Switches 0 SA, S’A 1 SB, S’B 2 SC, S’C 3 SA, S’B 4 SA, S’C 5 SB, S’A 6 SB, S’C 7 SC, S’A 8 SC, S’B
2
(8)
Ll2
Ll1
LC
Ideal + Vs
+
Vp _
+
Io
N1: N2
LM
RM
LP Vo
CP Ro
Zo
_
(a) LF Ideal +
Vp _
LM
RM
LP Vo
CP
ZT
Zo
Ro
_
(b) Fig. 2. a) Equivalent circuit of figure 1; b) simplified circuit.
PO =
(5)
To obtain proper switching angular frequency, ω0, imaginary part of (5) should be taken equal with zero. Due to low power factor of the induction coil, it is possible to assume RO2 > L P Also startup current must be limited by LF; the maximum possible startup primary current variation, neglecting transformer inrush current, can be approximated by (11): Startup
VO
2V m
Parameter Utility Vref LP RO Fmax(PLL)
(11)
Vm is the peak voltage of the utility. Maximum allowable switching devices current is found out from manufacture given datasheet which must be so greater than of maximum primary startup current. Thus LF can be determined by considering (10) and (11).
5. AUTO TUNING CONTROL Generally parallel resonance tank and load condition are not clear enough to achieve precision value of switching frequency. On the other hand the value of RO changes during warm-up procedure or output power adjusting. To design proper controller, output voltage is calculated as follows:
VO = I O Z O ( jω0 ) ≈ − j
LF ( LP + LF ) IO CP LP
(12)
Meanwhile main component of secondary voltage has the following relation with Io:
V S ,1
L = I O Z T ( jω 0 ) ≈ F LP
2
I O R O
90
PI
VCO
fS
o
Table 2: circuit parameters
2
TS 2
+_
Fig. 3. Simplified controller block diagram. Value 380V/50Hz 300V 10 µH 50 mΩ 11000 Hz
Parameter N1/N2 LF CP Fmin(PLL) LM
Value 1 200 µH 30 µF 8000Hz 20 mH
7. CONCLUSION In this paper an induction heating device with power electronic transformer structure has been described. Proper operation of proposed circuit is carried out both by mathematical analyzes and simulation results. Flexibility aspects of power electronic transformer leads to several advantageous such as unity power factor without using any storage elements, symmetric loading from utility point of view, isolation of working coil, compact dimensions and almost uniform sinusoidal output. Additionally, this topology can provide voltage level conversion ability; leads to obtaining desired power even at input low voltage level. Furthermore, proposed topology provides maximum output power and low output THD utilizing comparatively smaller size matching filter coil which work suitably with auto tuning of switching frequency controller.
(13) 600
Equations (12) and (13) show phase relationship between main component of secondary voltage and output voltage which is equal to 90o at f0. Thus a known type of phase locked loop controller can be used to adjust proper switching frequency to deliver maximum power to RO. A simple PLL controller is shown in Fig. 3.
0 -300 -600 400
6. SIMULATION RESULTS
000349
Vo
200 [V]
Sample circuit parameters are given by Table 2. The switching frequency is calculated about 9415.73 Hz by replacing given values in (7). Consequently, from (5) we have ZT ( jω o ) = 1.969∠ − 10o gives a suitable power factor close to 0.985. Fig 4 shows auto tuning procedure; waveforms of primary side voltage and output voltage on the induction coil. Secondary side of isolation transformer, output voltage, output current and induction coil current are shown in Fig. 5 respectively. Simulation summaries are given in Table 3. Fig. 6 shows one phase voltage and current. Fig. 7 shows startup conditions for asymmetric three phase utility input which verifies good output voltage regulation.
Vp
300 [ V]
max{∆I primary }
1 N2 ≈ L F N 1
Phase Comparator
VS
0 -200 -400 0.0000
0.0020
0.0040
0.0060
0.0080
0.0100
Fig. 4. Startup conditions of VP and VO (Time 1msec/div).
.
Vs
600
Vp
600
300
300
[ V]
[ V]
0 -300 -600
[V]
400
-300 Vo
-600
200
400
0
200 [V]
-200 -400 75
0
-400
25
0.0000
0.0020
0.0040
0.0060
0.0080
0.0100
.
[A]
0 -25
Fig. 7. Startup conditions of VP and VO for asymmetric input (Time 1msec/div).
-50 -75 600
ILp
8. REFERENCES
300 [ A]
0 -300 -600
[ Sec.]
0.0997
0.0998
0.0999
0.1000
.
Fig. 5. Secondary voltage, output voltage, output current and induction coil current respectively. 600
Va
400 200 [ V]
Vo
-200
Io
50
0 -200 -400 -600 40
Ia
20 [A]
0
0 -20 -40 0.0000
0.0050
0.0100
0.0150
0.0200
0.0250
0.0300
.
Fig. 6. One phase input voltage and current using a current filter (Time 5msec/div). Table 3: Simulation summaries Parameter
Value
Parameter
Value
VO rms ILp rms FS
212V 353A 9423Hz
IO rms THD(VO) POUT
43 A 0.7 % 6230W
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