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# IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO.

9, SEPTEMBER 2009

2111

## Evaluating Voltage Notch Problems Arising from

AC/DC Converter Operation
Reza Ghandehari, Student Member, IEEE, and Abbas Shoulaie

AbstractThis paper analyzes the voltage notches in ac/dc converters and the problems that they create. Voltage notch disturbs
the voltages waveform and excites the natural frequencies of the
system. In some systems, these excisions create considerable highfrequency oscillations in the voltages of the converter and the adjacent buses; this can damage capacitor banks, create parallel resonance, and generate radio disturbances. This paper analyzes the
effect of snubber circuits on the voltage oscillations that arise from
commutation. It also presents the theoretical foundation and analytical derivation that are used to calculate notch depth and the
frequency of oscillations. To achieve accurate and applicable results, various loads of the converter, the dc current ripple, and
the characteristic impedance of the system are considered. Several
experimental results are shown in order to illustrate the effects
of changes in the system and converter parameters on the voltage waveform of the converter. Computer simulations and experimental results indicate the accuracy of the theoretical relations.
The proposed equations make it possible to effectively study the
harmonic and power quality in networks that include high-power
converters.
Index TermsAC/DC converter, high-frequency oscillations,
snubbers, voltage notch.

I. INTRODUCTION
HE NUMBER and power ratings of ac/dc converters have
continuously increased in power systems, and have become more effective on power quality indexes. Although harmonics are known as the most critical disturbance of converters
and have been studied in many papers , voltage notches
can considerably affect the voltages of converters and adjacent
buses but have been considered less frequently .
Simultaneous conduction of switches in an ac/dc converter
during the commutation period causes a two-phase short circuit via switching elements. The voltage loss that results from
this short circuit in the converter voltages causes disturbances
in the voltage waveform, which is called the voltage notch.
These abrupt and regular changes in the voltage of the converter destroy the voltage sinusoidal waveform, which causes
the excitation of the natural frequencies of the electrical network. Excitation of the systems natural frequencies via the

Manuscript received January 21, 2009; revised March 23, 2009. Current version published August 21, 2009. Recommended for publication by Associate
Editor J. HR Enslin.
R. Ghandehari is with the Tehran Regional Electrical Company (TREC)
Tehran 16846-13114, Iran, and also with the Electrical Engineering Department, Iran University of Science and Technology (IUST), Tehran 16846-13114,
Iran (e-mail: r_ghandehari@iust.ac.ir).
A. Shoulaie is with the Electrical Engineering Department, Iran University of Science and Technology (IUST), Tehran, 16846-13114, Iran (e-mail:
shoulaie@iust.ac.ir).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPEL.2009.2021058

Fig. 1.

## voltage notch can generate noticeable oscillations in the voltage

and the current characteristics of the converter . These
oscillations can cause some problems, such as damaged capacitor banks, parallel resonance, radio disturbances, and electronic
device malfunction .
The frequency and magnitude of such oscillations depend on
the circuitry structure of the network, the size of the system
impedance as seen from the converter bus, and the size of the
capacitor banks . If the capacitor banks of the network are
neglected, then the capacitors of the snubber circuits will produce some high-frequency oscillations in the voltage and current
waveforms , .
The notch depth and area are proper indexes of the voltage
notch phenomenon, which is noticeable in the study and investigation of power quality status. The depth value of the voltage
notch and its area in the converter bus depends on the impedance
value between the converter and the point of common coupling
(PCC) (XT ), and on the system impedance, as seen from PCC
(XS ) (see Fig. 1). Higher impedance between converter and
PCC correlates to a lower value of voltage notch depth in that
feeder, which improves the power quality . Certain definitions and recommendations have been proposed regarding the
allowable limits of notch indexes , .
This paper presents an accurate study of the theoretical and
experimental aspects of the converters voltage notch and its
problems. First, the classical definitions and relations of the
voltage notch are described. Snubber circuits are considered as
one of the oscillations that create factors during the commutation
period; next, their effects on converter voltage and current are
examined. In the next step, these characteristics are obtained
regarding the ripple of the converters dc line current.
The effects of the networks capacitive elements, such as
the capacitor banks, filters, and cables on the converter voltage and the current oscillations during and after commutation
are explained. In each step, one of the conduction periods is

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2112

## IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 9, SEPTEMBER 2009

TABLE I
TWELVE CONDUCTION PERIODS OF THE CONVERTER

discussed. The obtained results can be generalized to other conduction periods. Lastly, the experimental and simulation results
are compared with the achieved analyses at different conditions.
II. CLASSICAL ANALYSIS OF VOLTAGE NOTCH
In general, for a six-pulse ac/dc converter analysis, 12 conduction periods are considered; six periods for a two-switch
operation (complete conductance) and six periods for a threeswitch operation (see Table I).
If the voltages of the converter bus are considered symmetrical, then the related time equations of the main bus voltage,
based on the parameters presented in Fig. 1, are given by
ea (t) = Vm Sin(t)


2
eb (t) = Vm Sin t
3


2
.
ec (t) = Vm Sin t +
3

Vdep(b)

(4)
.
t= + /2

Voltage notch with fire angle variations in the third conductance period.

TABLE II
SYSTEM PARAMETERS FOR AC/DC CONVERTER

(1)

## To analyze the voltage notch, it is sufficient to select one of

the commutation periods in Table I, write the system equations,
and then generalize the obtained relations to other periods. Considering that the unbalanced voltage in the process of analysis
is simple, in order to avoid the complexity of relations, however, the three-phase voltages of system are considered to be
balanced. In the first step, the dc current of the converter is
supposed to be smooth, and the snubber circuits are neglected.
According to Fig. 1, for example, in the third conduction period,
the current equation of the commutation circuit is , 
 t



ib
i d c i b
3Vm Sin t dt.
Lc ib |i b =i d c Lc ic |i b =0 =
2
( + /2)
(2)
Based on this equation, the current and voltage of phase b
(ib and vb ) and the voltage notch depth (Vdep(b) ) are obtained
as follows:

3Vm
[Cos() Sin(t)] + idc
ib (t) =
(3)
2LC

LS
vb (t) = Vm Sin(t 2/3)
3Vm Cos(t)
2(LT + LS )



LS

=
3Vm Cos(t)

2(LS + LT )

Fig. 2.

(5)

## Voltage notch depth depends on three factors: firing angle

(), system impedance as seen from PCC (XS ), and impedance
between the PCC and the converter (XT ). In the third conductance period, with respect to (5), increasing the firing angle of
the converter from 0 to 90 , as indicated in Fig. 2, increases
the voltage notch depth. According to (5), the higher value of
XT /XS causes the lower value of the notch depth in PCC voltages. To certify these equations, a converter with RL load and
without capacitor banks at the systems buses is considered. The
converter is simulated using the specifications given in Table II,
while snubber circuits are neglected.
The results obtained from (4) and system simulation are compared in Fig. 3. The compatibility of the achieved waveforms
with good precision indicates the accuracy of the obtained results regarding relative assumptions.
III. VOLTAGE NOTCH ANALYSIS USING SNUBBER CIRCUITS
In the analysis of voltage notches, particularly in systems with
low short-circuit levels, it is impossible to neglect the effects of
snubber circuits on the voltages and current characteristics of
converters.
Fig. 4 shows an ac/dc converter during the first conduction
period, where two phases a and c are in commutation at the
upper switches of the converter and where phase b is in the
complete conduction state at the lower switches. In this status, it

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GHANDEHARI AND SHOULAIE: EVALUATING VOLTAGE NOTCH PROBLEMS ARISING FROM AC/DC CONVERTER OPERATION

Fig. 5.

Fig. 3.

2113

## When idc is smooth, the current of phase b and its derivative

at to = + /6 are calculated as follows:


 

(t to )
2idc
(1)
(1)
ib (t) = VC (t) exp
VC (t) +
ci
3Lc Ceq

2Req (1)
1
Cosh (ci (t to )) +
V (t)
2ci 3Lc C


3Lc Ceq
4i b (to )

(7)
Sinh (ci (t to ))
3Lc Ceq
2

## Voltage notch on the voltage waveforms of converter bus.

where

3Vm
Vm
2idc
=
Cos(t) +
Sin(t)
2Lc Ceq
2Lc Ceq
3Lc Ceq

2
4Req
8Lc
1
3Lc

ci =
ci =

to = +
2Lc 3Ceq
9
Req
6

2Vm
3
dib (to )
i b (to ) =
=
Sin() 3 Cos() .
(8)
dt
3Lc
6

(1)
VC (t)

Fig. 4. Three-phase ac/dc converter during first conduction period with snubber circuits.

## is clear that the snubber circuits of the ON-switches (1, 5, and 6)

play no role in this analysis because of a short circuit in their two
ends. Only the snubber circuits of the OFF-switches (2, 3, and
4) should be considered. By simplifying the converter circuit in
Fig. 4, the equivalent circuit shown in Fig. 5 is achieved.
The differential equation of this figure can be simply achieved
as a function of phase b current, as follows:

2ib (t)
d2 ib (t) 2Req dib (t)


+
+
=

V
Cos
t
+
m
dt2
3Lc dt
3Lc Ceq
Lc
3

2idc
3Lc Ceq

(6)

2
If the inequality (4Req
/9 < 8Lc /3Ceq ) was true, then the
current of phase b would contain oscillations with angular
frequencies (ci ). The current oscillations in the feeder lines
of the converter create oscillations in the converter bus and the
PCC voltages in the commutation process. The damping timeconstant of current oscillations (ci ) depends on Lc and Req .
A higher value of XC or a lower value of Req increases the
damping time of oscillations, which arises from the voltage
notch.
Similarly, in order to analyze the equivalent circuit of the
converter after the commutation period, the frequency (co )
and the damping time-constant (co ) of the oscillations on the
voltage and current waveform are discovered by

36Lc
1
2Lc
2
co =
9Req
co =
.
(9)
6Lc
Ceq
Req

## The voltage notch depth in the first conduction period, with

the mentioned assumptions in this status, can be obtained as
follows:


3

XS
Vm
Cos t +
.
Vdep(b) =
3(XS + XT )
3
3
t= + /6

where Req = Rsn /3, Ceq = 3Csn , and XC = XS + XT . Additionally, Rsn and Csn are the resistance and the capacitor of
snubber circuits, respectively.

(10)
The voltage notch depth (Vdep ) is an instantaneous variation of the voltage magnitude at the beginning of the

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2114

## IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 9, SEPTEMBER 2009

TABLE III
SYSTEM SPECIFICATIONS FOR NOTCH ANALYSIS USING SNUBBER CIRCUITS

Fig. 7.

Fig. 6.

## commutation period. The positive and negative values of the

notch depth (Vdep ), calculated from (5) or (10), refer to overshoots and undershoots on the converter voltage waveform,
respectively.
In general, in all analytical states, the voltage notch depth
depends on XS /(XS + XT ). This means that a higher value of
XS /XT increases the notch depth in PCC voltages. Therefore,
in a weak system (with a lower short-circuit level or high system
impedance), the voltage notch becomes deeper. To verify the
validity of obtained relations, an ac/dc converter is simulated by
the parameters value that is noted in Table III.
The dc current ripple factor (Kid = (idc(m ax)
idc(m in) )/2idc(avg) ) suitably shows the smoothness of the
current. A sufficiently large inductor load value of the converter
is chosen so that the assumption of the smoothness of the
dc current can be obtained. In this case, Kid is calculated to
be 0.001. The obtained results for the current of phase b are
compared with those from (7) in Fig. 6. This figure reveals
that the results overlap with one another, which is proper.
Interestingly, the oscillation frequencies are equal in both
manners. Obviously, the frequency of these oscillations (fci )
by simulation is approximately 4300 Hz; by (8), it is calculated
to be 4291 Hz.
Increasing 8LC /3Ceq causes the converter current to shift to
oscillatory mode during commutation time, which affects the
converters ac voltage. Hence, the higher system impedance and
the lower snubber circuit capacitance push the converter toward

Fig. 8. Simplified equivalent circuit of the converter during the first conduction

## an oscillatory status in the commutation period. Equations (8)

(10) are true only when considering the smoothness of the dc
line current (idc ); whenever this current is not smooth, these
equations cannot be reliable. If the inductance value of the converter load decreases to 0.15 H (Kid = 0.013), then the dc line
current will not be smooth; consequently, analytical and simulation results are not compatible (see Fig. 7). This matter mentions
that this method is suitable for application in current-source converters, and that the ripple of dc current must be considered in
general ac/dc converters.

## IV. IMPACT OF DC CURRENT RIPPLE ON THE NOTCH

For an applicable analysis of the voltage notch, it is necessary
to consider the nonsmoothness of the dc line current. The ripple
size of the dc current depends on the inductance value of the
include a wide range of ac/dc converter loads; therefore, in this
analysis, the RLE load is chosen in order to study the voltage
notch, considering the nonsmoothness of the dc line current.
Because of the load, the simplified equivalent circuit of the
converter during the first conduction period is shown in Fig. 8.

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GHANDEHARI AND SHOULAIE: EVALUATING VOLTAGE NOTCH PROBLEMS ARISING FROM AC/DC CONVERTER OPERATION

Fig. 10.

Fig. 9.

## Solving differential equations of the voltages and currents for

this equivalent circuit requires long and complex equations for
converter ac currents. In this case, to achieve the converter ac
currents, the frequency domain equations are used in the manner
as follows (11), shown at the bottom of the page, where

s 3
3Vm
+ Lc idc (to )
F (s) =
2 (s2 + 2 )
ZL = RL + sLL

s + 3
+ Lc idc (to )
G(s) =
3Vm
2 (s2 + 2 )
Zeq = Req +

K(s) =

1
sCeq

3Vm E
2
+ (2Lc + LL ) idc (to ) . (12)
s + 2
s

## This complicated relation in the frequency domain involves

longer and more sophisticated relations in the time domain or
else its presentation will not be useful. Numerical solutions can
help solve this problem. With this method, the results of the
aforementioned equations can be compared with the characteristics obtained from the simulation depicted in Fig. 9. Precise
compatibility of these two characteristics indicates the accuracy
of the analysis process at the commutation time by considering
the dc current ripple (Kid = 0.024).
In this case, it is necessary to know the dependency of the oscillations frequency and their existence condition as a relation.
This is possible when the natural frequencies of the network are

2115

## Dead equivalent circuit seen from two ends of the switch T 1 .

known. Fig. 10 shows the dead equivalent circuit of the converter at the first conduction period with its system impedances
from the viewpoint of the switch that turns ON at the beginning
of this conduction period. This equivalent circuit indicates the
natural frequencies of the system, which are excited by voltage
notches. For example, the transfer function of the phase b
current, which is excited by the voltage notch (Vdep ), has the
following form:
H(s) =

## Csn LL s2 + (Rsn Csn Lc + Rsn Csn )s + 1

Ib (s)
=
Vdep (s)
a3 s3 + a2 s2 + a1 s + a0

a0 = 2RL

## a2 = 9RL Csn Lc + 3Rsn Csn Lc + 2Rsn Csn LL

a3 = 9Csn Lc LL .

(13)

## The natural frequencies of this equivalent circuit are the poles

of transfer function (or the roots of characteristic equation) of
(13), which are easily obtainable. In fact, if some of these poles
were imaginary, then an oscillatory response during commutation would be expected, and the frequency of the oscillations would be determined from the imaginary portion of such
poles. The aforementioned characteristic equation for a converter with the specifications detailed in Table III is concluded:
s3 + 7445.7s2 + 7.44 108 s + 3.7 109 = 0.
The roots of this equation are s1 = 4.97, s2,3 =
3720.36 27020.77j, and the frequency of oscillations for
the current of the converter is: fci = 27020.77/2 = 4300.5.
This frequency is identical to the previous value that was obtained from the simulation and (8).
V. EFFECTS OF SYSTEM CAPACITIVE ELEMENTS ON THE
VOLTAGE NOTCH
In the process of notch analysis, the power factor correction
capacitors are considered, in addition to the snubber circuits and

 

2ZL G(s) + (Req ZL )F (s) 2Zeq K(s) + 1 sCeq F 2 (s)
Ib (s) =
(3Lc Zeq + 3Lc ZL )s + 2Zeq ZL

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(11)

2116

## IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 9, SEPTEMBER 2009

Fig. 11. Dead equivalent circuit seen from two ends of the switch T 1 with
system impedance Z s (s).
Fig. 13.

## Voltage harmonic spectrum related to the converter feeder supply.

TABLE IV
PARAMETERS OF THE EXPERIMENTAL SYSTEM

## Of course, by neglecting the effects of essential parameters

of the converter (the load characteristic and snubber circuits),
the other oscillatory frequencies of the voltage and the current
waveforms can be achieved easily from the system frequency
characteristic (Zs (S)).
Fig. 12.

## VI. EXPERIMENTAL RESULTS

the ripple of the dc current. The voltage notch in the network,
which contains power factor correction capacitors or other capacitors like filters and the capacitive impedance of the cables,
excites the natural frequencies of the network.
Because of the availability of the network structure, the characteristic impedance (ZS (s)) that is seen from the converter bus
can be drawn. Then, the frequencies that cause resonance can
be clarified . The number and value of oscillation frequencies, after the beginning of the commutation, depend on the
imaginary poles of the transfer function of the system equivalent
circuit. Fig. 11 illustrates this structure for the first conduction
period with system impedance function Zs (s).
Fig. 12 shows the phase b voltage of an ac/dc converter,
which is investigated in a system similar to a typical network
that was used in . The effects of the snubber circuits are
also considered. Fig. 13 depicts the harmonic spectrum of such
voltage shape; it is clearly observed that the existing oscillations
in this characteristic originate from several frequencies. The
61th-order harmonic was noted in the aforementioned reference,
and the 221th-order harmonic has a lesser amplitude.

For experimental investigation of the voltage notch at different conditions of the converter, several system parameters (such
as system impedance, firing angle, and converter smoothing reactor) must be changed so that the analytical equations can be
compared with the practical results. Therefore, a converter similar to the simulated system was considered, with the difference
that the experimental system impedances were obtained from
the system identification process , . The parameters of
the experimental system are presented in Table IV. The capacitor banks in the feeders of this system were so far away from
the converter bus that the related effects on the voltage notch
characteristic could be neglected.
Fig. 14 shows the voltage characteristic of phase a of this
converter at a firing angle of 15 . The relatively low impedance
of the system causes the notch width to narrow, and causes
the oscillatory behavior of the voltage to rapidly damp. In this
experiment, the system impedance is not great enough to allow
observation of the performed analysis. Therefore, by adding
impedance equal to Zadd = 0.122 + j0.945 in each phase,
the experiments were repeated. The voltage waveform of the
converter is shown in Fig. 15 at the firing angle of 30 .

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GHANDEHARI AND SHOULAIE: EVALUATING VOLTAGE NOTCH PROBLEMS ARISING FROM AC/DC CONVERTER OPERATION

Fig. 14.

## Fig. 16. Experimental voltage waveform with additional system impedances

(by sampler circuit).

Fig. 17.
Fig. 15. Experimental voltage waveform with additional system impedances
(by oscilloscope).

## To facilitate the comparison between the experimental and the

theoretical results, a sampler circuit with a sampling frequency
of 50 kHz was used in each channel in order to measure and
store the voltage and the current characteristics of the converter.
The results were transmitted to the MATLAB and plotted, as
shown in Fig. 16. In this figure, one of the voltage notches is
compared with the characteristic obtained from (11) and (12).
The coincidence of these two characteristics confirms the effectiveness and accuracy of the presented theoretical relations. The
frequencies of voltage oscillations in this practical waveform
during and after the commutation period are nearly 4100 and
5000 Hz, and the corresponding values are calculated to be 4057
and 4961 Hz from (8) and (9), respectively.
Fig. 17 shows the experimental characteristics of voltage
notch on the converter bus voltage with various firing angles
by identical scale. This figure demonstrates notch depth dependency on the firing angle and the oscillations frequency

2117

## independency on the firing angle. As seen in this figure, the

oscillations magnitude, which is known as voltage overshoot,
increases concurrently with the increase of the fire angle.
The type of converter load and the size of dc current ripples affect the characteristics of the voltage notches, and the
load inductance (LL ) plays the most important role. Therefore,
this inductance was removed from the converter load, and the
experiments were repeated by resistive load. Fig. 18 compares
two wave shapes that are related to the converter voltage with
R and RL loads under similar conditions and at firing angles of
30 . This figure illustrates that decreasing load inductance has
a noticeable impact on the more rapid damping of the voltage
oscillations.
According to (8) and (9), the damping rates of voltage oscillations at and after the commutation periods depend on the snubber
circuit resistance values in such a way that when this resistance
increases, the generated oscillations are damped at higher rates.
The obtained experimental results, shown in Fig. 19, confirm
this fact.

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2118

## tions are highly accurate and effective. The influences of various

kinds of parameters on notch depth, frequency oscillations, and
related damping have been investigated in the laboratory. This
research will be useful in diagnosing power quality problems
arising from ac/dc converter applications and in accurate harmonic analysis.
Subsequent efforts will endeavor to analyze the effects of the
voltage notch on power networks, and to study the disturbances
that arise from the voltage notch in buses that are near the
converter bus.
REFERENCES

Fig. 18.

## Fig. 19. Damping of the voltage oscillations by increasing snubber circuit

resistance in the laboratory.

VII. CONCLUSION
This paper has analyzed the voltage notch problems that arise
from the operation of the ac/dc converter, which is one of the
most important disturbance sources in power systems. The analysis concentrated on the effects of the system and converter
parameters on the voltage notch. This matter concerns large
industrial consumers that have low short-circuit levels.
This paper investigated the effects of system impedances,
capacitor banks, snubber circuits, firing angles, and types of
converter loads on voltage notch characteristics. The proposed
relations accurately predict the frequency of oscillations in the
voltage and current waveforms of the converter bus that originate from voltage notch. In relation to the voltage notch, this
study found that an increase in the system impedances causes
the appearance of oscillations in the voltage of the converter bus,
both during and after the commutation periods. Additionally, a
decrease in the converter load inductance and an increase in the
resistance of snubber circuits provide a more rapid damping rate
for the oscillations. The simulation and the experimental results
demonstrate the theoretical foundation, and the proposed equa-

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 G. Fusco, A. Losi, and M. Russo, Constrained least squares methods
for parameter tracking of power system steady-state equivalent circuits,
IEEE Trans. Power Del., vol. 15, no. 3, pp. 10731080, Jul. 2000.
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GHANDEHARI AND SHOULAIE: EVALUATING VOLTAGE NOTCH PROBLEMS ARISING FROM AC/DC CONVERTER OPERATION

## Reza Ghandehari (S07) was born in Isfahan, Iran,

in 1971. He received the B.Sc. and M.Sc. degrees in
1995 and 2002, respectively, from Iran University of
Science and Technology (IUST), Tehran, Iran, where
he is currently working toward the Ph.D. degree in
electrical engineering.
He is with Tehran Regional Electrical Company
(TREC). His current research interests include power
electronics, power quality, electrical machine, and
HVDC.

2119

## Abbas Shoulaie was born in Isfahan, Iran, in 1949.

He received the B.Sc. degree from Iran University
of Science and Technology (IUST), Tehran, Iran, in
1973, and the M.Sc. and Ph.D. degrees in electrical
engineering from the Universite des Sciences et Technologies de Lille 1 (U.S.T.L.), Montpellier, France,
in 1981 and 1984, respectively.
He is currently a Professor of electrical engineering at IUST. His current research interests include
power electronics, magnetic systems and linear motors, flexible alternating current transmission systems
(FACTS) devices, and HVDC.

Authorized licensed use limited to: Iran Univ of Science and Tech. Downloaded on September 2, 2009 at 13:41 from IEEE Xplore. Restrictions apply.
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