A Shunt Active Power Filter for a Medium-Voltage 12-Pulse Current Source Converter Using Open Loop Control Compensation

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5840 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 11, NOVEMBER 2014
A Shunt Active Power Filter for a Medium-Voltage
12-Pulse Current Source Converter Using
Open Loop Control Compensation
Mostafa S. Hamad, Mahmoud I. Masoud, Senior Member, IEEE, Khaled H. Ahmed, Senior Member, IEEE,
and Barry W. Williams
Abstract—AC-side compensation for current source converters
using a shunt active power filter (SAPF) is affected by delays
introduced into the reference signals and/or the actual injected
current. The delays should be identified and compensated to
eliminate distortion from the supply current extracted from the
point of common coupling, such that the total harmonic dis-
tortion complies with standards. An improved distortion factor
is achieved if the delay for each harmonic order is considered
individually. This paper introduces an open-loop control strategy
for the SAPF, which is capable of mitigating specific and prede-
termined harmonic orders and consequently achieves low total
harmonic distortion. The delays are minimized or eliminated when
extracting the reference current and controlling the filter current.
The control technique is applied on a medium-voltage asymmetri-
cally controlled 12-pulse ac to dc current source converter, which
follows a specific power locus with the SAPF connected via taps on
the star connected secondary winding of the front end transformer,
to compensate the mains current dominant harmonics (5th, 7th,
11th, and 13th). Medium voltage simulation results are verified
experimentally using a scaled prototype.
Index Terms—AC-side compensation, active power filters
(APF), harmonic compensation, medium voltage (MV), open-loop
control, power quality, shunt active power filter (SAPF), 12-pulse
converters.
I. INTRODUCTION
CONTROLLED rectifiers such as 12-pulse converters are
commonly used in high power applications, especially
at medium voltage (MV) levels due to high reliability, ro-
bustness, low complexity, and low power losses. As well as
variable power factor, the main drawback is the harmonics gen-
erated cause a power quality problem at the converter ac-side
Manuscript received September 23, 2013; revised December 30, 2013;
accepted February 14, 2014. Date of publication March 12, 2014; date of
current version June 6, 2014.
M. S. Hamad is with the Arab Academy for Science, Technology, and
Maritime Transport, Alexandria 21532, Egypt (e-mail: eng_mostafa99@
yahoo.com).
M. I. Masoud is with the Electrical and Computer Engineering Department,
College of Engineering, Sultan Qaboos University, 123 Muscat, Oman (e-mail:
m.masoud@ieee.org).
K. H. Ahmed is with the School of Engineering, Kings College, University of
Aberdeen, Aberdeen, AB24 3FX, U.K., on leave from the Department of Elec-
trical Engineering, Faculty of Engineering, Alexandria University, Alexandria
21544, Egypt (e-mail: khaled_ahmed@ieee.org).
B. W. Williams is with the University of Strathclyde, Glasgow, G1 1XQ,
U.K. (e-mail: barry.williams@eee.strath.ac.uk).
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/TIE.2014.2311388
[1]–[3]. Compensation techniques such as passive or active
power filtering (APF) are used to improve the line side power
quality to comply with harmonic guideline standards such as
IEEE 519-1992 [4]. Power quality problems can be solved
with passive filters, however, passive filters have disadvantages;
such as dependency on the source impedances, parallel/series
resonance, aging of passive components, uncontrollable filter
currents and reactive power could be produced [5]. Due to semi-
conductor device development, the APF becomes a promising
compensator solution [6]–[9].
The APF alleviates passive filter drawbacks, moreover it is
robust and reliable [12], [13]. However, APF performance is
challenging in MV and high voltage systems due to switch
voltage rating and switching frequency limitations [1]. Reduc-
ing the filter-side voltage affects system size, cost, and allows
higher switching frequency operation [3], [5].
The compensating signal is generated by the contribution
of three control stages, namely; reference extraction, current
control, and the PWM [3], [6], [11]–[13]. The delay intro-
duced in the reference signals and/or the actual injected current
delays affect APF compensation quality, especially when the
switching frequency is low. The measuring devices, DSP and
the current control response times introduce a delay in the
compensation process. Consequently, the compensated mains
current THD can be higher than allowed standards.
Some solutions have been proposed to solve such a prob-
lem. Several conventional APF techniques used low-pass filters
(LPF) to extract the current harmonics and classical controllers
for the current control such as PI or hysteresis [14]. These
results in harmonic mis-cancellation due to phase shifts and
reference tracking errors, consequently degrade filtering perfor-
mance. A better solution, suitable for slow-varying loads has
been proposed. A Fourier series has been used to determine
individual harmonics [15]. A repetitive controller is used to im-
prove periodic current reference tracking but is only beneficial
if the current phase lag with respect to its reference is less than
60◦
. Moreover, a two-layer current control structure is required
[10]. In [16], the amplitude of the nth harmonic is extracted by
using a LPF in d − qn coordinates and the time delay is elimi-
nated by adding correcting phase shifts when transforming from
the d − qn coordinates to the d − q coordinate system. The
delay is determined using the experimental frequency response
of the current controller of the voltage source converter, and the
system requires a compensation function to reduce the effect
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