An over-voltage protection scheme with very fast
response for application in a CSI-fed induction
heating prototype
Molay Roy∗ and Mainak Sengupta†
∗ Department
of Electrical and Electronics Engineering,
National Institute of Technology, Sikkim
Email: molay.roy@gmail.com, molay.roy@nitsikkim.ac.in
† Department of Electrical Engineering,
Indian Institute of Engineering Science and Technology, Shibpur,
Email: mainak.sengupta@gmail.com, msg@ee.becs.ac.in
Abstract—Over-voltage protection is one of the key issues that
needs to be considered in the operation of a current source
inverter (CSI). Initially a brief study, on over-voltage protection
methods for CSIs used in induction heating application, is
presented in this paper. The CSIs are made using thyristors or
IGBTs with the series diodes which act as unidirectional current
switches with bidirectional voltage blocking capability. A large
inductor is connected in the DC-link of the CSI. An inadvertent
open-circuiting of switching devices of the CSI or in other series
path connections leads to dangerous over-voltage in the DC-link.
This important feature has been taken into account to design and
fabricate CSI-fed induction heating apparatus. Accordingly, an
appropriate over-voltage protection strategy has been planned,
implemented and demonstrated to operate successfully in the
practical set-up. An over-voltage protection scheme with a very
fast response has been used to track any such unwarranted open
circuiting. Such a scheme may even be considered to improve
the reliability in industrial applications.
I. I NTRODUCTION
In induction heating applications, two inverter topologies
might be used: voltage sourced inverters (VSI) and current
sourced inverters (CSI). However, the CSI configuration lends
itself naturally for suitable use in induction heating applications. At the output terminals of the CSI is connected a parallel
resonant circuit where the heating coil is shunt-connected
with a compensating capacitor [1]–[5] that effectively supplies
the reactive current requirement of the coil. The working
frequency of the inverter is close to resonant frequency, which
depends on the value of the capacitor and the coil (equivalent
R-L) parameters.
Fig. 1 shows the overall circuit diagram of induction heating
unit where the CSI is configured with switching devices made
out of IGBTs and series diodes. The aim of this paper is (i) to
present a review of existing over-voltage protection methods,
(ii) to suggest a novel modification over previously used
methods and finally, (iii) to demonstrate through an experiment
(using laboratory fabricated prototype) the verification of the
correctness of the proposed modification. This over-voltage
protection strategy is essential for such converters for preventing failures that may arise out of unwarranted/accidental
open-circuiting (of any CSI, for that matter) when carrying a
large current through the DC-link. This is all the more critical
for CSIs used in induction melting furnaces since such an
application can lead to a catastrophe due to presence of molten
metal. First, starting with operating principle of the CSI and
study on over-voltage protection strategies are presented to
prevent over-voltage in the circuit. This is followed by an
analysis and simulation of the protection scheme. Finally, the
circuit fabrication and experimental results are presented for
the adopted scheme. The experiments demonstrate very fast
response of the protection scheme.
II. O PERATING P RINCIPLE OF AN IGBT- BASED CSI
A parallel resonant circuit fed from an ideal IGBT-based
CSI is shown in Fig. 1. The figure shows parasitic inductance
(Lp ) in series with the load resonant circuit. This inductance
arises due to the stray inductances of connecting wires and
leads, device ESL etc. in the series path from the inverter
terminals to the heating coil. H-bridge inverters are mostly
used in the industry because among all the converter configurations, it has the ability to deliver maximum active power to the
load. Hence an H-bridge inverter has been considered where
ideally the inverter switches T1 & T2 and T3 & T4 commutate
alternatively with a duty-cycle of 50% (or just above that
considering the small mandatory overlap that is provided). The
induction coil-workpiece is modeled by a constant resistance
and inductance in series. An IGBT-based CSI may be used
to get almost unity power factor operation. In case of IGBTbased CSI, series diodes are added to block the reverse voltage
across the switch, as shown in Fig. 1.
The switching strategy of the IGBTs for a reliable operation
of the CSI considering the parasitic inductance in the switching
model and with an appropriate control strategy, must include
a method to keep the current under tight control even during
all the switching transitions, thus preventing undesirable overvoltages. The optimum switching process can be defined when
there are no sudden changes in current flow. This shall occur
when the total overlap time, during which all the switches
are on, is co-terminus in time with the current change from
−Id to +Id , where Id is the magnitude of the idealised level
(a)
Fig. 1. Circuit diagram of load resonant current source inverter for induction
heating application.
current in the DC-link. This duration is the optimum overlap
time because it minimises the turn-on switching losses in the
IGBTs and no over-voltage is produced. Moreover, if the turnoff matches with the zero crossing of the load voltage (vL ) the
turn-off losses are reduced to minimum (ZCS).
III. OVER - VOLTAGE PROTECTION
(b)
Fig. 2. Load resonant current source inverter circuits with two different active
crowbar protection circuit (a) using resistive network only (b) using capacitive
and resitive network.
Cause of over-voltage
In operation of the CSI, it was assumed that the path
for current source Id is always closed. However, in practical
installations, sudden inadvertent turn-off of switching devices
may cause, an over-voltage (due to open-circuiting in the path
of Id causing a violation of KCL) that can of course damage
the converter. The over-voltage can also occur if the current is
interrupted on the load side, finally destroying the converter
devices.
Over-voltage protection scheme
To prevent this, an active crowbar circuit is generally used
in IH applications [6], [7]. An active crowbar circuit has been
shown in Fig. 2(a). In this circuit, a thyristor (Tcrowbar ) is
connected in a series with a resistor that can dissipate the
stored energy from the DC-link inductor. The DC-link voltage
is needed to be sensed and Tcrowbar is turned on when overvoltage occurs during open-circuit fault.
The protection scheme should essentially have very fast response for such systems. Therefore some additional capacitor
(Ccrowbar ) may be used to slow down the fast rising voltage
peak across the DC-link inductor during open circuit (Fig. 2(b)
[8]). These capacitors permit more time for the activation of
a second and more powerful active protection system. This
second protection system usually contains a resistor switchedon by a thyristor that can dissipate the energy stored in the DClink inductor of the CSI. Eventually the power supply has to
be disconnected, which will take more time because relays and
contactors have their usual longer mechanical time constants.
To make the Tcrowbar turn-on fast, it is important to sense
the fault condition. Very little discussion is found on this
issue in the available literature. However, sensing the DC-bus
voltage is one of the strategies used in most of the cases. In
this paper an alternative strategy has been proposed to achieve
faster protection. Here the fault condition has been monitored
by sensing the current in Ccrowbar path which helps to turn-on
the Tcrowbar at the instant the current exceeds a set threshold.
This gives a faster response than sensing the DC-bus voltage.
However, in both cases delay is present due to the inherent
delay in the sensing circuit and the finite turn-on time of the
thyristor (Tcrowbar ).
Presence of the crowbar capacitor extends the voltage buildup duration thus providing the thyristor ample turn-on time.
The maximum time for thyristor (Tcrowbar ) turn-on is
∆tmax =
Ccrowbar × ∆V
Id
(1)
where, Ccrowbar is the capacitance of the crowbar capacitor,
∆V is the difference between maximum voltage allowed in the
inverter and peak of the load voltage (vL ), Id is the DC-link
current fed to the CSI.
Alternatively, if the total delay time is known then Ccrowbar
can be chosen depending on the maximum voltage allowed in
the circuit. An interesting point may be however added here.
The resonant tank configuration of the load ensures that even
in case of any open-circuiting failure of the CSI, the load
will not be at all affected since the current, though large, will
circulate in the local coil and capacitor loop in the load. This
is all the more advantageous in such furnace applications since
it will not lead to any accident near the load site.
Simulation of over-voltage protection scheme
The performance of the protection scheme (Fig. 3) is simulated in SEQUEL [9]. In this proposed scheme, the current,
icrowbar is to be sensed & compared with set value of the DClink current. If any open circuit occurs in the inverter, the DClink current Id will try to flow through the diode (Dcrowbar )
and charge the capacitor (Ccrowbar ), which will cause a slow
rise of the voltage. When icrowbar is equal to Id a fault signal
(to shut down the system) is generated. This logic fault signal,
through appropriate gate driver circuit, turns on the thyristor
(Tcrowbar ) and simultaneously turns off the chopper switch
in the DC-link to cut off the power from the mains. Resistor
(Rcrowbar ) will dissipated the storage energy of the inductor
(Ld ). A small inductor (Lcworbar ) is connected in the path to
di
.
protect the thyristor from experiencing large dt
(a)
(b)
Fig. 4. Simulated waveform of an active crowbar protection scheme during
fault condition.
Fig. 3. Load resonant current source inverter with active crowbar protection
circuits.
In the simulation it has been considered that there is a
10µs delay between current sensing and thyristor turn-on. The
chopper gets turned off at the same time when thyristor turns
on. The simulation results are presented in Fig. 4. Fig. 4(a)
shows current icrowbar shooting up to the value of Id at the
instant when there is an open circuit at 0.04s and voltage
increase in the capacitor. At 0.04001s the thyristor turn-on and
capacitor start to discharged through thyristor. Fig. 4(b) shows
thyristor turn-on and current build up through the thyristor.
The simulation has been done at identical conditions as the
practical tests detailed below. The waveforms obtained from
simulation and the experimental waveforms obtained from
practical tests have been compared and found to be in excellent
agreement.
IV. H ARDWARE IMPLEMENTATION
The circuit diagram for the induction heating converter as
shown in Fig. 3 has been fabricated in the laboratory and a
photograph of the laboratory prototype is shown in Fig. 5. The
converter consists of 3-phase rectifier, which is connected to
the DC-link capacitor. This is followed by a buck chopper (to
control the power and it is used to get continuous current
at input of the CSI). Finally, a CSI using IGBT devices
with series diodes are connected to the load resonant circuit
(induction heating coil parallel with capacitor bank). Fig. 5(a)
shows IGBT of buck chopper, crowbar diode, crowbar thyristor
(a)
(b)
(c)
(d)
Fig. 5. Photographs of 2kVA, 10kHz laboratory prototype consisting of (a)
Buck chopper (Block-B of Fig. 3), crowbar diode, crowbar thyristor with their
respective gate driver circuits (b) Crowbar capacitor and crowbar resistance
(Block-C of Fig. 3) (c) IGBT based converter (Block-D of Fig. 3), its driver
circuit and load resonant circuit (Block-E of Fig. 3) (d) IGBT module place
in the heat-sink.
with their gate driver circuits, while Fig. 5(b) shows the photo
of the crowbar capacitor and the crowbar resistance. Fig. 5(c)
shows the IGBT based converter, its driver circuit and load
resonant circuit (capacitor bank and induction coil). Fig. 5(d)
shows the IGBT module placed on the heat-sink. The details
of design, fabrication and operation of the entire set-up may
be found in [10]. Some pertinent details are given in Table I.
TABLE I
S ET- UP DETAILS .
Sl.
no.
1
2
3
4
5
6
7
8
9
Elements
DC link inductor(Ld )
Coil inductance(L)
Coil resistance(R)
Capacitor across coil(C)
IGBT with series diode module
IGBT module
Thyristor module
Diode module
Overlap-time
Value or Rating
20 mH, 45A
15.4 µH
54 mΩ
20µF , 1200V
SKM300GBD12T4
BSM75GB120DN2
SKKT 58 B16 E
MEE 75-12DA
1 µs
To test the correctness of the proposed scheme, one has to
scale-down the levels of voltages and currents for the sake of
preventing irreversible damage of the set-up hardware. Thus,
(a) CH1: Current icrowbar (10A/div); CH2: Fault signal (10V/div)
maximum of 200V instantaneous. It has been observed that
when open-circuit occurs in converter, the current goes high
in the crowbar capacitor and a over-voltage fault signal is
generated (Fig. 6(a)). Fig. 6(b) shows increased voltage in
the crowbar capacitor and thyristor voltage waveforms when
fault occur. In this figure, it is clearly visible that with in
few micro-second the crowbar thyristor turn-on and voltage
increase in the capacitor is just 12 volt. In other words, if the
DC-link voltage increases by a small amount equal to 12 volts,
at once the thyristor turns on within 2.5 µs (Ch2 waveform
in Fig. 6(b)). This clearly establishes the effectiveness of the
suggested scheme and precision of the laboratory developed
circuitry. The scheme thus may be thus tried at higher ratings
also.
V. C ONCLUSIONS
The application of active crowbar circuit for reliable CSI operation is presented in this paper. The over-voltage protection
scheme may be well-suited to industrial applications too. The
fastness of response of the proposed protection scheme has
been experimentally verified at reduced voltage/current levels
on a 2kW, 10kHz laboratory fabricated prototype.
Interestingly, the resonant tank configuration of the load
ensures that even in case of any open-circuiting failure of the
CSI, the load will not be at all affected since the current,
though large, will circulate in the local coil and capacitor
loop in the load. This is very advantageous in such furnace
applications .
Another important point has been established here with
regard to the gate drivers used for the IGBTs when the same
is for a CSI application. Since most commercial converters in
medium power applications are now-a-days voltage-fed, most
driver incorporate their corresponding protection systems.
Thus, most medium power commercial driver incorporate
protection systems (like inherent dead-time between logic of
complementary devices of a module) in which the driver turns
off the device in case of short-circuit. Using these drivers in
a CSI can cause undesired open-circuiting of the current path
leading to the destruction of devices. These available drivers
cannot be directly used, therefore. Alternatively, a modified
logic with a small overlap time between devices of the same
leg (module) needs to be in-built into the gate drivers.
R EFERENCES
(b) CH1: Voltage across Ccrowbar (100V/div); CH2: Voltage
across thyristor Tcrowbar (100V/div)
Fig. 6. Experimental waveform of over-voltage protection during fault
condition of an induction heating converter.
this test was both simulated and experimented for 2A in the
DC-link (this corresponds to an instantaneous voltage of 50 V
max across the DC-link). The actual conditions where this
scheme is expected to work successfully is for a current
of 20 A and an average DC-link voltage of 100 V and a
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