The document describes a seven-level cascaded multilevel inverter used as a shunt active power filter. It extends the capacitor voltage control technique used for two-level inverters to the seven-level filter. A predictive current controller is applied based on the supply current as the reference. Phase-shifted space vector modulation is used as the PWM technique. Simulation results validate the seven-level shunt active power filter, showing the load current, supply current, capacitor voltages, and other outputs meet expectations.

1. 2004 11th International Conference on Harmonics and Quality of Power
Seven-Level Shunt Active Power Filter
A. M. Massoud, S. J. Finney, and B.W. Williams
Abstract-. In this paper, the seven-level cascaded type For the second solution there are two aspects. For the first
inverter is used as a shunt active power filter to exploit part, series operation of semiconductor switches faces
multilevel inverter advantages. The capacitor voltage control problems of unbalanced static and dynamic voltage sharing
technique used as a harmonic current extraction method for the due to the spread of device characteristics and/or mismatch of
twiMevel inverter is extended to the seven-level shunt active
drive circuits. For the second part, multilevel conveners
power filter. A predictive current controller based on the supply
current (not the active filter current) is applied. Phase-shifted achieve high voltage switching by means of a series of
space vector modulation for the multilevel inverter is used as a accumulated voltage steps, each of which lies within the rating
PWM technique. The proposed seven-level shunt active power of the individual power devices. Also lower total harmonic
filter is validated by simulation. distortion can k achieved. There are three basic types of
e
Index Terms- Active filter, multilevel inverter, power multilevel converters, viz., cascaded multilevel, neutral point
quality. clamped, and flying capacitor conveners. In this paper, the
cascaded type, seven-level inverter is used as a shunt active
I. INTRODUCTION power filter. The multilevel inverter as a shunt active power
armontc nutigation in power systems is a goal which has filter is used in [31,[41 for neutral point clamping, in [5],[6] as
Hoccupied a g r i t deal of research since the early 60s. a cascaded type, and in [71,[8] as a flying capacitor for three
Harmonics are a by-product of modern electronics. They are level inverter. Static VAR compensation is introduced in [9]
associated with a large numbers of personal computers (single for the eleven-level cascaded type inverter but for only a single
phase loads), uninterruptible power supplies (WSs), variable pulse per half cycle for each cell, hence the controller
frequency drives (AC and DC), and any electronic device bandwidth is low. But for levels higher than three there is the
using solid state power switching supplies to convert incoming problem of maintaining capacitor balance while maintaining
AC to DC. Problems caused by harmonics are numerous viz., good current tracking performance. Fig. 1 shows the seven-
elevated RMS current, circuit breaker trips, nuisance fuse level inverter used as'a shunt APF.
operation, reduced equipment life (transformers, conductors,
breakers, switches), equipment malfunctions, high-frequency
current flow, reduced effective power factor, and
computersltelephones may experience interference or failures.
Passive filters are used for harmonic mitigation due to their
advantages of simplicity, low cost, and easy maintenance. But
the disadvantages [1],[2] that these filters introduce are
numerous as; the filter can be overloaded, parallel resonance
between the power system and the passive filter, series
resonance with the power system, de-tuning of the harmonic
frequency with aging of passive components, and the filtering
characteristics are dependent on the source impedance. Due to
these disadvantages, the attention of researchers has been
drawn to the active power filter (APF). There are two main
types of active power filters, namely series and shunt active
power filters. The shunt active power filter can be used for
harmonic and reactive power compensation. It acts as a
harmonic current source which injects an anti-phase but equal
magnitude to the harmonic and reactive load current to
eliminate the harmonic and reactive components of the supply
current. There are two ways to apply the shunt APF in the
medium voltage range, either by using a transformer or by
extending the semiconductor switches rating (via multilevel 0
converters or series connection of semiconductor devices). For Fig. I . The even-level inmncr u x d as a shunt active power film
the transformer case, the problem lies in the high cost for
medium voltage transformers. Also, as the voltage decreases at II. PROPOSED METHOD
the secondary side, the current increases which means parallel
operated converters may be needed. A. Inrroduction
There are two important controlling parts to the shunt
The aulhon am with Dcpanmcnt of Cornpuling and Elecuical Enpinering
HaiolbWarl Universily. %canon Campus, Edinburgh, UK. EH14 4AS. active power filter design. The first is the harmonic extraction
0-7803-8746-5/04/$20.00 02004 IEEE. 136
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2. technique and the second is the current conuol technique. The performance as the reference supply current is sinusoidal
capacitor voltage control harmonic extraction technique [lo]- while the active filter current has significant harmonic
[I31 is extended to the seven-level APF. Among the most components. The computational burden is reduced. Using a
common three current controllers (hysteresis [ 141, ramp seven-level inverter reduces the ratings of the semiconductor
comparison 1141, and predictive[14],[15]), the predictive switches and improves total harmonic distortion. Using seven-
current controller is the most commonly used as it is suitable level PS-SVM as a PWM technique enables the inverter to
for DSP-implementation, provides more precise current compensate for the load harmonics and reactive current at a
control, has minimum distortion, and provides g o d static and lower capacitor voltage because SVM gives an increase of
dynamic performance. Therefore, current control is herein 15% in gain, as do third harmonic injection and dead banding,
employed using innovative predictive current control. Seven- but has the lowest THD [16]. Decreasing the capacitor voltage
level phase shifted space vector modulation (PS-SVM) is used decreases the switching losses and also reduces semiconductor
as a pulse width modulation (PWM) technique. Besides the ratings. SVM gives constant switching frequency operation.
advantages of using predictive current control, the proposed B. Capacitor voltage control
method has advantages. The number of sensors used is
minimized (the measured quantities are the supply currents, Harmonic extraction is performed using capacitor voltage
voltage of point of common coupling, and capacitor voltages). control as in [10]-[13] but extended to seven levels. This
The reference currents are the supply currents instead of the technique is based on power balance where the supply real
active filter currents which gives good current tracking power must be equal to the load real power plus inverter
big. 2. Thc s c ~ c n - l c ~shunt active power film hlock diazram
el
137
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3. losses. To maintain the power balance, the capacitor must m.PHASE-SHIFTED SVM I'OR MW,TLEVEL INVERTERS
instantaneously compensate for the difference between the In phase shifted SVM, superposition theory is applied to the
supply and the load power. Controlling three capacitor. multilevel inverter semiconductor switches. Conventional two-
voltages (vdcalr vdcbl, and vdcJ using three proportional- level space vector modulation is applied to each three
integral (PI) controllers results in outputs which are semiconductor switch groups of the three-phases and their
proportional to the instantaneous changes in power balance. complementary groups in the multilevel converter. A pre-
Multiplying each of these outputs hy the corresponding per calculated shift in time is introduced into the updown
unit voltage of the common coupling point results in the counters of each switch group, then the output is summed and
corresponding reference supply currents. To maintain the the required output voltage is obtained. For the seven-level
balance of the nine capacitor voltages, an exchange of the inverter, six up-down counters are used to provide a phase
digital output signals to the semiconductor devices between shift of T,J6 between these counters as shown in Fig.3. T,,,, is
each cell of the three cells in each phase is done and the switching cycle. The main advantage of using PS-SVM in
completed in three cycles in a similar manner to that in 191. the seven-level shunt APF is that capacitor symmetrical
The process continuously is repeated. Symmetrical loading of loading can be achieved as all the switches are turned on and
off once in each switching cycle. The penalty of using this
capacitors will he achieved by seven-level phase shifted SVM
PWM technique is that the switching loss is increased
which will be discussed later. Fig. 2 shows the proposed block
compared to conventional multilevel SVM.
-
diamam.
C. Predictive currrnf control secondgroup ............
The reference, measured supply currents, and voltages of
the common coupling point are used to predict the reference
< third e r ~ w
tourlhwup
----
output voltage of the inverter, required to make the measured fifth group -. -
current reach its reference at the next sampling instant. In Fig.
1, the equation relating the active filter current, inverter
output voltage, and the common coupling point voltage is:
where s represents the phase a, b or c, L, is the interfacing Fig. 3.Tb updawn wuntenusd for seven-lcvcl convmm
inductance, v, is the active filter output voltage for the x-
A. Seven level phase-shifted SVM (PS-SVM)
phase, e, is the s-phase voltage of the common coupling point,
and jfx is the active filter current. Inductor For the seven-level cascaded multilevel converter shown in
is neglected. Equation (1) can be represented in a discrete Fig. 4, the 36 switches are divided into 6-groups (each group
form as: represents a leg of one cell for the three phases). These groups
can be defined as follows: (Sal, SDI, Sc,) and their
v : ( n + I ) = t,[ii,(n+ : , - i f l ( r i ) ] + e, (4 (2) complementary, (Sa,, Sb4, Sc4) and their complementary, (Sa5,
Sh5, S,S) and their complementary, (SrB, Sh8, Sea) and their
where gn+and
I) + are the x.phase active filter complemenory, (sa9% SO) and their complementary. and
Sb12,SCl2) their complementary.
and
current and the predicted output voltage references Ph"C(*) Phsllbl Fkbrir,q
respectively, at sampling instant (n+lj. 7,is the sampling
time. Using Kirchhoff s current law at the common coupling
point:
i,r (ti) = i , ( n ) - i ( n )
, (3)
where ; A n ) and i,Jnn) are thex-phase load and supply currents
respectively at the sampling instant (n). Since the sampling
instant ( n + 1 ) is not available, i J r ( n + l )is replaced by i;,(n).
This introduces one sampling time delay which becomes less
significant as the sampling frequency increases. The active
filter reference current can be expressed as:
i;x(n) = i h ( ~ z ) - i : r ( t ~ ) (4)
Substituting (3) and (4) into (2) gives:
v:(n + I ) = L~ [ i,r(n)-C.(n)
T, )+e1(.) (5) 0
Equation ( 5 ) represents the predicted inverter output voltage swilchen (in p
'
a
y su; ~ ~ o u p I
expressed in terms of the reference and actual supply currents.
138
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4. Two-level SVM is used for each group but the up-down where A is defined as
counters used are shifted in time as in the phase-shift carrier
based PWM control technique in [16]. This shift in time (tp.) (14)
depends on the number of levels, m, and the switching time, A=[:::
T,
,. The submatrices A l l , Ail, A,,, and are defmed in the
T_ appendix. X, B, and U are defined as:
1 , =-
m-l (6)
With this method, the switching losses are increased by (m-1) -I
- 0 0
times that of conventional multilevel SVM. But the harmonics
5"
are shifted towards (m-lfs,,, instead off,$* as with normal -I
multilevel SVM. Also the complexity of the states in normal 0 - 0
multilevel SVM is avoided with PS-SVM. Lb
o n -- I
IV. SIMULATION X= B= L?
0 0 0
A. Seven-level shunt active powerfilter state-space model 0 0 0
The state-space model for the seven-level shunt active power 0 0 0
filter is used for MatlaWSimulink simulation. The seven-level o n o
inverter used as a shunt active power filter is shown in Fig. 1 n o 0
and the inverter output phase voltages are expressed as:
3
0 0 0
0 0 0
'TO (') = C'd<.x;
i=l ('.~,di-3) - 'r<4;-2) ) (7)
0 0 0
where x denotes phases a, b, and c. The switching states are
0 0 0
'1' for the on-state and '0' for the off-state. The switches in
B. Sirnulation results
The state-space model of the active filter is simulated using
MatlabISimulink. Seven-level PS-SVM is programmed using
six n-files with s-function blocks in Simulink. The parameters
used in simulation are shown in Table I . The nonlinear load is
a three-phase uncontrolled bridge rectifier feeding a 70 Ohm
resistive load. Parts a to f of Fig. 5 show the load current,
supply current, active filter current, capacitor voltages, and
inverter output line voltage and its power SpectNm,
respectively. As can be seen in Fig. 5e, the fust harmonic
appears at 72 kHz, six times the switching frequency which
improves the effectiveness of the interfacing inductance,
enabling lower inductance.
MI v Sampling frqae,,c) 24 k H z
the supply phase voltages. CO, ifi and ifc are the active filter G€
139
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5. ,.
.....
...
Time i s ) Time ( s )
... .. ....... .. ...
. . ..... .. .......
.. . . . . . . . . . . .
. . . . . . .. . . . . .
.+ ...
...
-400; 0.01 I
0.02 0.03 0.04
Time i s ) Fmqucncy (kHz)
V. CONCLUSION -0 0 0 0 0
C,,
In this paper, a seven-level shunt active power filter with 0 0 0 0 .
novel predictive current control has been presented. Capacitor Cb,
, and A,, = [O],,
voltage control is extended to the seven-level inverter as a 0 0 0
C3
b
harmonic current extraction technique. Predictive current *n =
control 1s used as the current control technique. Seven-level
0 0 L C< 3 J O O 0
k I
SVM is used as the PWM technique. The proposed method
. . 0 0 M o o 0
enables the possibility of using active filtering in the medium
voltage range. C,,
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6. 2002. ISIE 2002. h e d i n g s of dit 2W2 IEEE lntmational Symposium l l S l Ghoudjehhaklou, H.: Kargar, A.: "A new predictive contml matcgy for
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swiwhing high-voltage aciive pawn 6ker wt flying eapa6tors Cor urhan
ih 481484
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al.
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VIE. BIOGRAPHIES
191Pang Zheng Pcng: McKeevcr, J.W.: Adam, DJ.: "A power line condilionm Ahmed M.Messoud reccived an BSc. degra: (first class honours) and M.Sc.
using cavade multilcvel invenm for dswihtion systems", lndubry f o the Rculw of Eqinering, Alexandria Univmsity, Egypt in 1997 and 2000
rm
Applications. IEEE Transactions an , Vol. 34, Issuc: 6. Nov.~Dcc. pp:
1998, and is curmnllv mrsuinc a Ph.D. deme in Elsclrical Enoinccrine in die
pp: 8 6 9 0 leturer at Herid-Watt University. His areas of intcrcst arc wflwvitchiq
1121 t i ~ p t ~N.:, Machmoum M.: '"Simplified digital-analogical contml for
t techniques, power semiconductor paeciion, energy recovay snuhlxr circuits,
shunt active power liitcn under unbalanced eondilions", Power Elcnronics and lowdistonion railer lopologis.
and Variable S p e d I)river. 1998. Scvcnth Into~laliondConference on (IEE BarryW.Williams w i v e d an M.Eng.Sc. dqree from the Univniity of
Conl.PUhl.No.456),21~23Sep1998,pp: 11-16 Adelaide, Adclaidc. Ausualia. in 1978, and Ph.D. dcgec from Cambridge
1131 Shyh-Jier Huang: JinnChang Wu; "A control algorithm far thmc-phasc University, Cambridge, U.K. in 1980. After swen years as a Lccturcr at Imperial
thrre-wid active power fillers undn non~idcal w i n s vollagcs", Pown Collcgc, University of London, U.K., he W ~ appoinlcd Io the chair of dcctrieal
F
ElcdrOniCS Trans., IEEE , Vo1.14, Jul 1999, pp: 753 -760 engin-ng at Ilaia-Watt University, Edinburgh, U.K., in 1986. His m a r c h
1141 Brando, G.: Del Pima A.: I'acccnda, E.: "A compariron hawen Same activity includcs power rmimndunor modcling and protection, mnvcncr
lEEE l n t m l i o m l Caracas
control a l p i l h m ofparallel active filt~ring".4'~ toplogics and saltswitching iechniqucs, and the application of ASlCs and
Conlcrcncc on Devices, Circuits and Sysams, Aruhu, April 17-19,2002, pp: micmproccswrs to induslririal electronics.
1-6
141
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