transient of power pulse and its sequence in

8/11/2019 Transient of Power Pulse and Its Sequence In

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352 ZHAO ZhengMing et al. Sci China Ser E-Tech Sci| June 2007 | vol. 50 | no. 3 | 351-360

teristics are often neglected. Hence many modulation techniques are not feasible for the safe op-

eration in the real application, especially in the high power converter whose dynamic process of

energy transformation is more rigorous. Hence the investigation on the transient process of power

pulse and its sequences has significant potential in the modern power electronics.The concepts of pulse are not uniform. Ref. [1] divided the pulse into three components: rising

edge, trailing edge and pulse width. Some additional physical indexes, including the pulse repeti-

tion frequency (prf) and pulse repetition rate (prr) are prescribed. This definition is very practical in

the real applications of power converters. For example, in the start-up process of a 380 V/160 kW

AC adjustable speed system with DC pre-excitation[2]

, the relevant output voltage and current are

illustrated in Figure 1.

Figure 1 The waveforms of output voltage and current in DC pre-excitation.

Here two distinct steps are illustrated. The first one is the current rising process, where current

does not reach the objective value and only a single vector is imposed on the motor terminals. At

the same time, the pulses are periodic with prf1= 1/(250 s) = 4 kHz. The second is the currentholding process, where the control algorithm generates different voltage vectors so as to hold the

current in a narrow hysteresis. The periodic characteristic of single pulse is replaced by that of

pulse sequence, withprf2=133 Hz andprr = 950 Hz. Hereprf1,prf2andprr are the functions of the

pulse duty, amplitude, load time constant, etc. In this example, we could see that the rising edge,

falling edge and pulse width of single pulse and the prf/prrof pulse sequence are all important

factors in the investigation on the power pulse and its sequence.

1 The transient in semiconductor switch, topology and control algorithmThe power electronic converter is made up of power semiconductors, commutation loop and con-

trol modules. A typical converter is shown in Figure 2.

In this system, the time constants of energy conversion in subsystems are different. For example,

the time constants of commutating loops by passive devices are milli-second, those of power

semiconductors and part of control modules are several micro-seconds, and those of some

soft-switching systems and digital controller are in the scales of n-second. If this converter is

connected with mechanical load like an induction machine, the time constant of electromechanical


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ZHAO ZhengMinget al. Sci China Ser E-Tech Sci| June 2007 | vol. 50 | no. 3 | 351-360 353

system is in the scale of second. The whole power electronic converter is made up of the above

subsystems with different time constants with the balance of the energy conversion, transmission

and storage as the key points. Most of the failures in the subsystems and their components occur in

the transients, i.e. from one steady state to another. In these dynamic processes especially in thecooperation of different subsystems, the energy unbalance often arises, which leads to the partial

convergence of energy flow[3]

.

Figure 2 A typical power electronic converter.

More specifically, the main forms of these transients in power electronic converters are as fol-

lows.

(1) Turn-off over voltage and turn-on over current of semiconductors (micro-second or

n-second), the reverse recovery process of freewheeling diode (ns), etc. In the macroscopic view,

they are the di/dt (more than 1000 A/s) and dv/dt (over 500 V/s) issues.The essence of power semiconductor switches lies on the correlation of PN junctions, whose

main carriers are electrons and holes. Diffusion and drift occur in the semiconductors following thediffusion theory and Ohm theory, respectively. With constant external restriction, these two

transportations keep dynamic balance. When operational conditions are not steady, this balance

will be broken. The abrupt change in the distribution and transportation of carriers gives rises to the

voltage spikes, over current, current convergence, latch-up effect, local overheating, etc.[4]

. This

would damage the components and systems if no timely energy leading is carried out. For example,

the IGCT in turn-on and turn-off processes behave as thyristor or transistor. This transformation

would finish within 1 micro-second and the physical characteristics are quite divergent. Some

experimental waveforms of the voltage and current on IGCT are illustrated in Figure 3 where UTis

the voltage across IGCT,Iloadis the load current and Gate is the triggering signal.

The significant over-current would deduce a large surface pressure, large switching off loss and

current convergence, thus damaging the semiconductors. Figure 4 shows the surface of one IGCTdestructed with the partial burn and surface rupture. Here the power pulse is not only a time func-

tion but also a space one. Different failure mechanisms lead to different destructive positions. In the

research of failure mechanisms of GTOs, Januszewski[5]

found that if the failure mechanism is over

turn-off power, the destructive position is in the middle of silicon cells; if it is due to high di/dtin

the turn-on process, the destructive position lies on the edge; if it is due to the long term over

current, the silicon cells burn out in large areas. Similar phenomena are also found in IGCTs

(Figure 4(a)(d)).


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Figure 3 The turn-off voltage/current waveforms.

Figure 4 The section of destructed IGCT in high power inverter. (a) Trace 1 (rupture in large area); (b) trace 2 (in the middle of

silicon cell); (c) trace 3 (at the edge of silicon cell); (d) trace 4 (across several silicon cells).

The above failures traces are quite similar to those proposed in ref. [5]. However, there are

differences between IGCT and GTO in their respective structures. Hence the failure mechanisms

are not uniform. Only on the basis of commutating characteristics and inner structure of IGCT,

could the in-dept research be carried out. If there are problems in the peripheral circuit design, such


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as inability to limit di/dtor mismatch of load and devices, power electronic engineers should select

the IGCT and its relevant peripheral components especially the snubber parameters, set appropriate

protection thresholds, configure the corresponding sensors, calculate the various design parameters,

and implement the proper protecting action so as to guarantee the converter be operating in its ownSOA (safe operational area).

(2) The topology of a power converter consists of the commutating loop, heat sink and carrier

loop, which are different tunnels of power flowing with the same energy carrier. From the aspect of

energy equivalence, they could all be described with different topologies. The time constant of heat

disperse is minutes or hours while that of commutating loop is at microsecond level and semi-

conductor is at ns or even ps level. They interact with each other and maintain the energy balance in

the steady work. Since they have different time constants, different velocities of energy flow make

the transient quite complicated. What is more, the stray and parasitic parameters in various to-

pologies make it more complex[6]

. Three main parts in the three-level high power inverters are

shown in Figure 5.

Figure 5 Three main parts in the three-level high power inverter. (a) Heat sink (minutes or hours); (b) DC bus (s); (c) carrierloop in semiconductors (nsps).

Although the time constants of different loops vary significantly, their energy flows are ex-

pressed in similar equations. The principle of heat flow is[7]

2 2

12 2.

T T Ta

x t t

= +

(1)

And the carrier transportations in the power semiconductors are[8]

2 2

22 2p

p p pD

x t t

= +

and

2 2

22 2n

n n nD

x t t

= +

. (2)

The dynamic equations of voltage and current in the commutating loops are[9]


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2 2

32 2

u u uk

x t t

= +

and

2 2

32 2

i i ik

x t t

= +

. (3)

The variables of T, p, n, uand iare temperature, hole distribution, electron distribution, voltage

and current, respectively.Dp, Dnand k are diffusion coefficients. 1, 2and 3are the time constants.

By eqs. (1)(3), the heat, carrier and electric fields have similar transportation discipline except

the different coefficients and their own time constants, which are related with materials, structures,

shapes and even the operational conditions. Take eq. (3) as an example, whose analytical solution is

expressed as

3

/

2

0, / ,

( , )

(0, / )e ( , ), / .

x c

t x c

u x t

u t x c U x t t x c

(4)

Here 3ck

= , and

3

22

1 23 2

23 / 2

2

1

2( , ) e (0, )

2

t

x c

xI

cxU x t u t d

k x

c

=

.

1I is the one-order imagine Bessel function. ( , )i x t is similar to eq. (4). Except the speed, the

above three wave propagations have the same transmission principles. These three functions co-

exist in the same converter and correlate with each other. The unbalance of energy distribution in

electrical process would lead to non-uniform heat distribution in the space, and the partial over heat

would degrade and even invalidate the power semiconductors. Improper assemblies in mechanical

design, like the long interconnects, large stray inductance in DC bus and long distance betweensnubber circuit and power semiconductors would induct the voltage spikes on the switches. The

coefficients in partial eqs. (1)(3) possess the strong non-linearity in most cases, such as the dif-

ferent characteristics of silicon devices under high and low votage[10]

. Any negelect of these

propagations would bring about risks to the power electronic converter.

Figure 6 shows the line-line voltage on the motor terminals, which is far from the original con-

trol signal. Abundant ripples exist on the waveform, which are caused by the stray parameters on

the transmission line. These parameters on the loop of power flow cause the differences in state

variables.

The commutating process brings about the differences in current, voltage, electromagnetic field

and the temperature distribution at different time and space. This directly determines the ar-

rangement of key components in the converter. The definition and extraction of nonlinear pa-

rameters are important factors in analyzing energy transportation and transformation in this system.

Linear and lumped models are often adopted in the previous simulation and analysis in power

electronic techniques. Hence it is meaningless trying to analyze these dynamic processes with

uniform topology theory based on the ideal switches especially in high power inverter.

(3) The control modules are the signal-level subsystems. The transient is in micro-second or

nano-second. The digital control system in modern power electronics is mainly based on DSP and

other multi-CPU network, whose instruction executing time is micro-second or nano-second. The


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deformation and delay by parasitic parameters on the gate signal in the peripheral hardware drive

circuit are also from micro-second to nano-second. This transient significantly influences the drive

signal and non-ideal switching processes. Most of abnormal output pulses in the inverter are caused

by the transient. Figure 7 gives the abnormal pulses measured on the output terminals of athree-level 1.25 MW/6 kV inverter. All of them will lead to the distortion of output waveforms or

even commutating failure.

Figure 6 The line-line voltage on the motor terminals.

Additionally, because of the restriction by physical characteristics of semiconductor switches,

like the minimum pulse width, some macroscopic control strategies are not well implemented

especially in the low running speed in an adjustable speed system, where the duty cycle of switches

is very low. The minimum pulse width restricts its output and leads to the distortion and even

destruction[11]

. Figure 8 shows the distribution of the reference voltage vectors at 5 Hz operation in

three-level system with different minimum pulse widths. Obviously, the larger the minimum pulse

width, the more severe the waveform distortion.

Hence four main basic concepts on pulse and pulse sequence are presented.

(i) Electromagnetic pulse sequence is the foundation of realizing the effective energy trans-

formation in power electronic converter, and also is the fundamental of inverter operation.

(ii) Pulse sequence is equivalent to power sampling in power electronic converter, and it is the

digitalization of electromagnetic power as well.

(iii) The rising edge, trailing edge, width and amplitude are the fundamentals of power pulse.

(iv) Various modulation algorithms are the macroscopic control of power pulse sequence. They

are affected by the fundamentals of pulses. Only under this perspective could the pulse and its

sequence be controlled effectively.

2 The expected scientific innovation and conclusions

The research of transient of the power pulse sequence in power electronic converters is beyond the

modern power electronics, especially when the research perspective reaches the nanosecond

timescale. This transient is difficult to be explained by the theory of modern power electronics. A


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Figure 7 Abnormal pulses measured in the output terminals of the high power inverter. (a)Voltage pulse by asymmetric dead

band effect; (b) voltage pulse by snubber circuit; (c) voltage pulse by current zero-crossing in dead band; (d) voltage pulse by

insufficient pulse width.

common sense is that when the space dimension of materials reaches the nanometer scale, the

nanometer material brings up a revolution in material field. It is imaginable that the theory and

application of nanosecond process research in power electronics would make some expected

achievements as follows.(1) Theoretical innovation on the short-timescale power flow. The viewpoint would transfer

from the originalpower = voltagecurrentto the quantum physics and energy band theory so as to

get rid of the bondage of ideal and lumped model. Nonlinear and stray models should be estab-

lished such that the simultaneous equations can be based on the energy band. Schrodinger functions

can be utilized to precisely calculate the transient of pulse power phenomena. The operational and

failure mechanisms would be grasped and effective simulation could be achieved.

(2) Theoretical innovation on the traveling wave propagation. The theory of traveling wave

described by Einstein diffusion equation would be applied to the analysis of wave propagation in


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Figure 8 The voltage vector distribution under 5 Hz with different minimum pulse widths. (a) 40 s; (b) 55 s; (c) 70 s.

power electronic devices. We could comprehend the transportation theory of inner carriers in

semiconductors and free electrons in transmission lines. Thus the energy function is extended to

four-dimensional time and space functions. Then the energy flow and distribution at any time and

any place are reproduced precisely.

(3) Achievements on arbitrary waveform transformation. The high-level ambit of power elec-

tronics is to realize the arbitrary waveform transformation. The power pulse and its sequence is the

precondition. Because of the rapid development in computer science and technology, the flexible

virtual reality enters the human life. However, the simulation by computer is at signal level. If we

want to turn it to the electromagnetic force to drive the real world, these signal-level parameters

must be transformed into the power-level ones with the same waveforms. This is the so-called

power electronic amplifier. A transformer has realized the large-scale and complex power system.

A high power and arbitrary wave transforming electronic amplifier will dominate the future elec-

trical world.

(4) Based on the theory of power pulse phenomena, the traditional control strategies, component

selection, device design, cooling calculation, etc. could be translated into the mathematical optimal

problems and physical energy-level issues. The design of power electronic converter will be for-

mulized and visualized and therefore will be more scientific.

(5) The research on power pulse in high power inverters is a new developing aspect. The tradi-


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tional power pulse in high voltage courses is mainly the single pulse[12]

. Also this pulse sequence is

not maneuverable and generated by large capacitors and their combination. Since the pulse se-

quence in power electronics is sequential, repetitive, controllable and generated by semiconductors

and corresponding topology, it would completely replace the former one and find wider applica-tions as soon as the capacities of power switches make a breakthrough.

Research on the power pulse phenomena and power electronic device is beneficial to achieving

the arbitrary waveform transformation with high effectiveness and high reliability, processing the

precise analysis and effective simulation on the failure mechanisms of power switches and esti-

mating the switching characteristics of the whole systems, including calculation of the switching

loss and heat generation, direction of arrangement of power commutating loop and its peripheral

circuits, debasement of EMI and possibility of error conduction and error protection. It has a very

important theoretical meaning and application value.

Certainly, the scientific meaning and the real applications could not be separated. This research

helps to establish the nonlinear model of power electronic switches and reflect the device status

under different operational conditions. Through studying the electromagnetic energy distribution[13]in high power inverter and the solution procedure of optimized energy function, we might be able

to optimize the system framework, design snubber circuit, depress dv/dtand di/dtand control the

transient of electromagnetic energy. The PWM algorithms and switching processes should be dealt

with in a combined way so as to attain the harmonious combination of optimal switching frequency,

minimum pulse width, dead band and modulation index in the analysis of power pulse phenomena.

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2006, 26(3): 159163

3 Cai X S, Qian Z M, Wang Z Y. A survey of the developing strategy of power electronics. Trans China Electrotech Soc, 1999,

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204 Zhao Z M, Zhang H T, Yuan L Q, et al. The failure mechanism and protection strategy of high voltage three-level inverter

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5 Januszewski S. The failure analysis of semiconductor under running condition of power inverter. Commut Tech Elect Power,

2000, 2: 2734

6 Ferreira J A, Van Wyk J D. Electromagnetic energy propagation in power electronic converters: Toward future electro-

magnetic integration. Proc IEEE, 2001, 89(6): 876889

7 Jiang R Q. Impulse Effect in Thermal Propagation, Diffusion and Momentum Transfer (in Chinese). Beijing: Science Press,

1996

8 Allen R H. Analytical modeling of device-circuit interactions for the power insulated gate bipolar transistor. IEEE Trans

Indust Appl, 1990, 26(6): 9951005

9 Giovanni Miano, Antonio Maffucci. Transmission lines and lumped circuits. San Diego: Academic Press, 2001

10 Lundstrom M. Fundamentals of Carrier Transport. New York: Addison-Wesley Publishing Company, 1990

11 Wei D, Francis J, Lee F C, et al. Maximum pulse width space vector modulation for soft-switching inverters.Power Electron

Special Conf, 2001, 2: 11531158

12 Astrov Y A. Semiconductor-gas discharge electronic devices: Stability, patterns and control. Int Conference Phys Control,

2003, 3: 883894

13 Wang D Y, Zhang L C, Bai T Y. Computation of transient magnetic energy density of the commuting process for MOSFET

converter circuits. J North Jiaotong Univ (in Chinese), 2000, 23(2): 97101
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