IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 1, JANUARY/FEBRUARY 2007
97
Reactive-Power Compensation of Coal Mining
Excavators by Using a New-Generation STATCOM
Hazim Faruk Bilgin, Student Member, IEEE, Muammer Ermis, Member, IEEE, K. Nadir Kose, Alper Cetin,
Isik Cadirci, Member, IEEE, Adnan Acik, Turan Demirci, Alper Terciyanli, Student Member, IEEE,
Cetin Kocak, and Mustafa Yorukoglu
Abstract—This paper deals with the development and implementation of a current-source-converter-based static synchronous
compensator (CSC-STATCOM) applied to the volt-amperereactive (VAR) compensation problem of coal mining excavators.
It is composed of a ±750-kVAR full-bridge CSC with selective
harmonic elimination, a low-pass input filter tuned to 200 Hz,
and a ∆/Y -connected coupling transformer for connection to
medium-voltage load bus. Each power semiconductor switch is
composed of an asymmetrical integrated gate commutated thyristor (IGCT) connected in series with a reverse-blocking diode and
switched at 500 Hz to eliminate 5th, 7th, 11th, and 13th current
harmonics produced by the CSC. Operating principles, power
stage, design of dc link, and input filter are also described in
this paper. It has been verified by field tests that the developed
STATCOM follows rapid fluctuations in nearly symmetrical lagging and leading VAR consumption of electric excavators, resulting in nearly unity power factor on monthly basis, and the
harmonic current spectra in the lines of CSC-STATCOM at the
point of common coupling comply with the IEEE Std. 519-1992.
Index Terms—Current-source converter (CSC), harmonic
elimination, power quality, static VAR compensator (SVC).
I. I NTRODUCTION
E
LECTRIC excavators, i.e., power shovels and draglines,
are the key equipment in open-cast coal mining (Fig. 1).
In addition to their intermittent character as a load on the
network, they may be the sources of harmonics and consumers
Paper PID-06-05, presented at the 2005 Industry Applications Society Annual Meeting, Hong Kong, October 2–6, and approved for publication in the
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Mining Industry
Committee of the IEEE Industry Applications Society. Manuscript submitted
for review October 15, 2005 and released for publication August 25, 2006.
H. F. Bilgin, A. Cetin, T. Demirci, and A. Terciyanli are with the
TUBITAK Information Technologies and Electronics Research Institute, TR
06531 Ankara, Turkey, and also with the Electrical and Electronics Engineering
Department, Middle East Technical University, TR 06531 Ankara, Turkey
(e-mail: faruk.bilgin@bilten.metu.edu.tr; alper.cetin@bilten.metu.edu.tr; turan.
demirci@bilten.metu.edu.tr; alper.terciyanli@bilten.metu.edu.tr).
M. Ermis is with the Electrical and Electronics Engineering Department, Middle East Technical University, TR 06531 Ankara, Turkey (e-mail:
ermis@eee.metu.edu.tr).
K. N. Kose and A. Acik are with the TUBITAK Information Technologies and Electronics Research Institute, TR 06531 Ankara, Turkey (e-mail:
nadir.kose@bilten.metu.edu.tr; adnan.acik@bilten.metu.edu.tr).
I. Cadirci is with the TUBITAK Information Technologies and Electronics
Research Institute, TR 06531 Ankara, Turkey, and also with the Electrical and
Electronics Engineering Department, Hacettepe University, TR 06531 Ankara,
Turkey (e-mail: cadirci@bilten.metu.edu.tr).
C. Kocak is with Turkish Coal Enterprises, TR 06330, Ankara, Turkey.
M. Yorukoglu is with Enerjisa Corporation, TR 34330 Istanbul, Turkey
(e-mail: myorukoglu@enerjisa.com.tr).
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/TIA.2006.887308
Fig. 1. General view of the Southern Aegean open-cast lignite mining site
(top: power shovels; bottom: dragline).
of reactive power. The severity of power quality problems
arising from electric excavators largely depend on their electric motor drive technology: 1) Ward Leonard drives; 2) dc
motor drives based on phase-controlled thyristor rectifiers; and
3) variable-frequency ac motor drives based on dc-link converters. In newest excavators, device-commutated rectifiers have
been used on the supply side of the dc-link converter system,
thus providing theoretically harmonicless operation at nearly
unity power factor [1].
The usual practice is to solve the power quality problems on
each electric excavator individually. On the supply side, the use
of overexcited synchronous motors, induction motors compensated by permanently connected shunt capacitors, permanently
connected shunt filters, and thyristor-switched shunt filters are
the most common solutions. However, in some of the applications, the aforementioned solution techniques cannot provide an
adequate reactive-power compensation, resulting in a need for
group compensation of electric excavators [2]. In recent years,
reactive-power compensation problem of coal mining equipment existing in the stocks of Turkish Coal Enterprises (TKI)
has been solved by the use of unified relocatable static voltampere-reactive (VAR) compensators (SVC); each of which
is composed of thyristor-controlled reactors (TCR) and shunt
harmonic filters tuned to the fifth and seventh harmonics.
The power quality problems are not new, but customer
awareness of this problem has increased. In recent times,
power quality issues and custom solutions have generated a
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TABLE I
REACTIVE ENERGY PENALTY LIMITS RECENTLY IMPOSED BY THE
ENERGY MARKET REGULATORY AUTHORITY OF TURKEY
(IN NOVEMBER 2004)
tremendous amount of interest among power system authorities
and engineers. This led to more stringent regulations and limits
imposed by electricity authorities, although they differ from
one country to another in a limited extent. As an example,
the progress in reactive energy penalty limits for the near
future is recently imposed by the Energy Market Regulatory
Authority of Turkey, as summarized in Table I. More stringent
requirements for reactive-power compensation have made
the static synchronous compensator (STATCOM) topology
a viable solution technique in coal mining applications in
addition to conventional SVC topologies.
The STATCOM or STATCON or SSC is the static counterpart of the rotating synchronous condenser, but it has the capability of generating and/or absorbing reactive power at a faster
rate [3]–[9]. The STATCOM is a shunt-connected reactivepower compensation device, which has been built so far around
a voltage-source converter (VSC) [10]–[13]. The VSC produces
three-phase output voltages, each nearly in phase with and
coupled to the corresponding ac supply voltage through a
relatively small reactance [3]–[14]. During the operation of
the converter, the dc voltage is usually provided by an energy
storage capacitor. The converter consists of self-commutated
power semiconductors such as gate turn-off thyristor (GTO)
or GCT switches, which are turned on and off through gate
drive circuits at supply frequency. Large STATCOM systems
have been applied to high-voltage transmission systems in order
to increase transmission capacity and damp out oscillations in
system voltage and power. Harmonics produced by the VSC are
eliminated by complex coupling-transformer connections and
multiphase converter topologies [3]–[12].
Nowadays, with the advents in high-voltage insulated gate
bipolar transistor (IGBT) technology (up to 6.5 kV, 600 A),
medium-size STATCOMs are an attractive solution to power
quality problems of medium-voltage distribution systems
[13]. In such applications, the VSC fed from the dclink capacitor should have some additional capabilities such
as harmonic filtering and load balancing in addition to
reactive-power compensation. These are known as distribution
STATCOM (D-STATCOM) in the literature. These features of
D-STATCOM make necessary generation of harmonic components superimposed on the fundamental voltage component
at the ac side of VSC by turning on and off the IGBT
switches at frequencies much higher than the supply frequency,
overlapping with switching frequencies, operation, and control
strategies of low-voltage shunt active power filters [5], [7], [12].
A current-source converter (CSC) fed from a dc-link reactor,
which is the dual of the VSC fed from a dc-link capacitor, can
also be used in the transmission and D-STATCOMs. Only a
few research works have been reported in the literature, investigating various aspects of CSC-based STATCOMs [15]–[18].
This paper describes the first industry application of a newgeneration CSC-STATCOM. It is identified as a new-generation
scheme because a single full-bridge CSC with a selective
harmonic elimination technique and the simplest couplingtransformer topology for connection to a medium-voltage bus
can serve as a full substitute for complicated transformer connections and/or multiphase VSCs in conventional STATCOMs.
The CSC-based STATCOM has been designed for group compensation of electric excavators containing traditional motor
drives. Power consumptions of a typical dragline and a power
shovel are given in the Appendix for a few operation cycles. The
developed CSC-based STATCOM can also provide harmonic
filtering of the load if necessary by redesigning its low-pass
input filter and/or adding tuned second-order harmonic filters
to the ac side of the present system. Unified and relocatable
features, operating voltages, and most of the technical specifications of the developed STATCOM are compatible with those
of TCR-based SVCs recently being used in TKI mines [2].
II. S YSTEM D ESCRIPTION
Block diagram of the switchgear equipment for the overall
system, including load, CSC-STATCOM, electricity meters
used for billing, and medium-voltage switchgear equipment,
is given in Fig. 2. Fig. 3 shows the schematic diagram and
all protection facilities for CSC-based STATCOM. Some of
these circuit elements can be matched with those shown in
Figs. 4–6. Custom design mounting platform, including guard
fence, has been used in order to eliminate electromagnetic
interference problems, which may arise from possible closed
loops (Fig. 6).
The implemented CSC-based STATCOMs were installed by
the end of 2004 at two different sites of TKI, as shown in
Fig. 4(a) and (b). Each STATCOM is a +250-kVAR low-pass
input filter and a −750/+500-kVAR integrated gate commutated thyristor (IGCT)-based CSC. Both STATCOMs are connected to a 31.5-kV 50-Hz distribution bus through an 800-kVA
31.5/1 kV ∆/Y -connected semicustom coupling transformer.
CSC has been designed at a relatively low voltage level of
1 kV ac at the expense of a poor utilization of semiconductor
voltage blocking capability because: 1) 1 kV is a standard
low voltage level [19], thus permitting the use of standard
components and switchgear devices; and 2) this choice makes
STATCOM specifications compatible with our earlier TCRbased SVC design for TKI [2].
In CSC-STATCOM, a three-phase full-bridge converter is
employed for the reactive-power compensation of rapidly
changing balanced three-phase loads. Reverse voltage blocking
capability of CSC is provided by fast recovery diodes (hard
driven diodes, EUPEC-D911SH4500V) connected in series
with asymmetric IGCTs (ABB-5SHY35L4510). Natural aircooling of power semiconductors has been preferred for higher
reliability. The use of “asymmetric IGCT + reverse-blocking
diode” combination meets this objective better than symmetric
IGCT alternative. This is because power dissipation (switching
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BILGIN et al.: REACTIVE-POWER COMPENSATION OF COAL MINING EXCAVATORS USING STATCOM
Fig. 2.
Block diagram of the switchgear equipment (air-insulated metal-enclosed switchgear with SF6 circuit breakers and disconnectors).
Fig. 3.
Schematic diagram including all protection facilities of the implemented CSC-STATCOM.
and conduction losses) of the reverse-blocking switch (asymmetric IGCT + reverse-blocking diode) will be transferred to
the heat sinks through a conduction area nearly two times
bigger than that of symmetric IGCT having a reverse-blocking
capability, and the forward voltage drop across the switch
(4.5 V in our design) is lower than that of symmetric IGCT
having the same virtual wafer size. Indeed, symmetric IGCTs
having the desired current ratings are not yet available in the
market [21], [22].
Full-bridge converters, like the one described in this paper,
would produce significant 5th, 7th, 11th, 13th, and higher order
odd current harmonic components when their semiconductor
switches are turned on and then off at supply frequency in pro-
99
ducing quasi-square ac current waveforms at supply frequency.
However, the CSC-STATCOM should comply with IEEE Std.
519.1992 when it is taken as an independent electrical installation. This is achieved by controlling STATCOM IGCTs according to precalculated selective harmonic elimination strategy
instead of harmonic elimination by transformer connections,
as in the case of large transmission STATCOMs. In [20],
selective harmonic elimination method has been described in
a generalized manner for CSCs.
In this paper, in order to eliminate the 5th, 7th, 11th, and 13th
current harmonics that will be produced by CSC, five chopping
angles and, hence, six independent pulses are determined by the
optimization tool of MATLAB. This corresponds to a switching
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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 1, JANUARY/FEBRUARY 2007
Fig. 4. CSC-based STATCOM installed at (a) Tinaz transformer substation and (b) Sekkoy transformer substation of TKI.
Fig. 5. Power stage of CSC.
of switching frequency harmonic component and its sidebands,
as in the case of pulse-width-modulation (PWM) control strategy, but increases considerably harmonics higher than 13th, in
comparison with those of quasi-square waveform. In order to
filter out these higher order harmonics successfully, a low-pass
filter has been designed and connected to the input of CSC
(Fig. 3). Its corner frequency is adjusted to 197 Hz by making
a compromise between the filtering performance and voltage
regulation and filter size.
In the design and implementation of first CSC-STATCOM
prototypes, all filter and dc-link reactors are chosen to be
air-cored reactors, thus eliminating entirely possible problems
arising from magnetic nonlinearity. Custom design iron-core
reactor choice would make a reduction in I 2 R losses and could
lead to a more compact system footprint, thus permitting all
system components to be located in larger containers.
III. O PERATING P RINCIPLES
A. Selective Harmonic Elimination
Fig. 6. Footprint of CSC-based STATCOM.
frequency of 500 Hz, which is lower than the practical upper
limit of 1 kHz for IGCTs commercially available at the present
time. This switching strategy does not lead to the generation
Several modulation techniques can be applied to CSCs in
producing desired current waveforms, as discussed in [23]. In
low- and medium-power applications where IGBTs or power
MOSFETs have been used as the main switching element at
relatively high switching frequencies, the PWM has become
the most commonly adopted technique for voltage and current
control and/or harmonic minimization. However, in mediumand high-power applications where IGCTs and GTOs are
employed, since the switching frequency is much lower than
those of power MOSFET and IGBT-based converters, the use
of PWM technique is not suitable for harmonic-free voltage
and/or current waveform generation. As will be shown in
this paper, selective harmonic elimination technique provides
elimination of low-order harmonics at moderate switching
frequencies. The relationship between the switching frequency
and the number of harmonics to be eliminated is given in [20]
as in the following:
(2K + 2)f1 , if K
2 is even
fs =
(1)
K
(2K + 3)f1 , if 2 is odd
where fs is the switching frequency, f1 is the supply frequency
and, hence, fundamental frequency, and K is the number of
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BILGIN et al.: REACTIVE-POWER COMPENSATION OF COAL MINING EXCAVATORS USING STATCOM
Fig. 7.
101
Circuit diagram of CSC.
Fig. 9. Definition of the five independent chopping angles for 5th, 7th, 11th,
and 13th harmonic elimination.
Fig. 8. Switching patterns of S1 and S2 and the corresponding theoretical
converter input current iR (t).
harmonics to be eliminated. Furthermore, E, given in (2), is
the number of chopping angles, as defined in [20]
E = K + 1.
(2)
In this paper, the number of current harmonics to be eliminated is four (5th, 7th, 11th, and 13th) resulting in five independent chopping angles, which can be determined by a proper
optimization algorithm.
Circuit diagram of the CSC and switching patterns of the
first leg (S1 and S2 ) for the elimination of 5th, 7th, 11th, and
13th current harmonics in the lines of converter input [iR (t)]
are given in Figs. 7 and 8, respectively. The switching patterns
of the second and third legs are to be shifted with respect to first
leg, respectively, by 2π/3 and 4π/3 rads.
In Fig. 8, it is assumed that the fundamental component
of iR1 (t) leads line-to-line voltage vST (t) by θ, resulting in
finite active (P ) and reactive (Q) power flow from the supply
to the converter. The amount of reactive power produced by
the CSC-STATCOM is controlled by varying angle θ (phase
lag/lead angle between iR and vST ) with fixed IGCT switching
patterns and pulse durations over the entire control range. The
fundamental frequency is to be synchronized with the supply
frequency (50 Hz). As θ is varied for control purposes, only
the mean value of unidirectional dc-link current (Idc ) changes
and, hence, the amplitude of line current pulses in Fig. 8, by the
same amount.
The Fourier series expansion of a nonsinusoidal periodic
waveform at a predetermined fixed fundamental frequency
(f1 = 50 Hz) is well known [24]. Since the current waveform
is an odd quarter wave, only the coefficients of sinusoidal terms
(bn ) are to be evaluated over the quarter of a period. The
five independent angles (α1 , α2 , . . . , α5 ) for the elimination
of harmonic current components (5th, 7th, 11th, and 13th)
over the quarter wave are shown in Fig. 9. The Fourier series
coefficient bn of converter input current iR is expressed in terms
of five independent angles (α1 , α2 , . . . , α5 ), Idc , and harmonic
number n, as in the following:
4Idc
bn =
nπ


2 )+cos(nα
π
4 )−cos(nα
π 5 ) cos(nα
π 1 )−cos(nα
×+cosn 3 −α4 −cosn 3 −α3 +cosn 3 −α1  .
−cos n π3 +α2 +cos n π3 +α3 −cos n π3 +α5
(3)
The five independent angles (α1 , α2 , . . . , α5 ) can be determined from (3) by equating b5 , b7 , b11 , and b13 to zero, in
turn. One more equation is needed in determining these five
independent angles. The fifth equation relates peak value of the
fundamental current (b1 ) to dc-link current (Idc ) in terms of
the modulation index M , as in (4). This is because, according
to the universal definition of M , it is the ratio of the peak value
of the fundamental component IR1(peak) to the amplitude of the
pulsed iR waveform (Idc )
b1 = M Idc .
(4)
Chopping angles as a function of the modulation index are
then found as shown in Fig. 10.
Line current waveforms at the input of CSC [iR (t), iS (t), and
iT (t)] are generated by the corresponding switching patterns
which are composed of six independent pulses shown in Fig. 9.
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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 1, JANUARY/FEBRUARY 2007
Fig. 10. Independent chopping angles in degrees at f1 = 50 Hz for the
elimination of the 5th, 7th, 11th, and 13th current harmonics.
TABLE II
DURATION OF INDEPENDENT PULSES IN SWITCHING PATTERNS IN
DEGREES WITH RESPECT TO THE MODULATION INDEX AT f1 = 50 Hz
Fig. 11. Principle phasor diagram of CSC for (a) lossless and (b) practical
systems.
these, M = 0.8 is chosen for implementation by considering
the second and third constraints given above.
B. Control Strategy
Table II gives the duration of these pulses as a function of M ,
for f1 = 50 Hz. An optimum value for M should be chosen in
view of the following constraints.
1) For safe operation of IGCTs chosen (ABB5SHY35L4510), the duration of each pulse should
be at least 50 µs, as recommended by the manufacturer,
which results in a minimum pulse duration of Tmin = 1◦
at f1 = 50 Hz. In the design, a safer value of Tmin = 2◦
has been chosen.
2) For a prespecified value of generated reactive power by
CSC and hence input current IR1 , as the modulation index
is reduced, Idc increases. Since copper loss of the dc-link
reactor is proportional to the square of Idc , it is therefore
better to work at low Idc values.
3) From the performance view of the control system, full
control range of θ should be as large as possible. This
forces the use of low modulation index values.
The first constraint imposes an M value in the range from
0.8 to 0.9. This range is gray-shaded in Table II. Among
Reactive power produced by the CSC-STATCOM is controlled by applying phase angle control strategy to the switching
pattern with respect to the supply line-to-line voltage, i.e., by
varying θ between iR (t) and VST (t), as defined in Fig. 7. In
the implementation, fundamental line current phasor IR1 is
shifted by θ with respect to the line-to-line voltage phasor VST .
Voltage and current phasors are defined in Fig. 11, respectively,
for an ideal (lossless) and a practical system. The implemented
CSC is said to be operating as an inductive load and, hence,
consuming reactive power if IR1 leads VST by a small value
of θ(0 ≤ θ ≤ 5◦ ) in this application. On the other hand, if IR1
leads by a large θ value (175◦ ≤ θ ≤ 180◦ ), CSC is operating
as a capacitive load and, hence, delivering reactive power. If
the CSC under investigation was an ideal one, there will be
no power loss in the converter, dc link, and all the circuit
elements, and then, θ = 0 would correspond to the consumption
of reactive power, while θ = 180◦ to production.
In terms of the effective value of the line-to-line voltage
(V1 ) and fundamental line current (I1 ) just at the input of
the converter, reactive-power input Q to the converter can be
expressed as in the following:
√
Q = 3V1 I1 cos θ.
(5)
Note that cos θ appears in the reactive-power expression
because θ has been defined as the phase angle of IR1 with
respect to VST , and VST lags behind VR by 90◦ . S and T line
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BILGIN et al.: REACTIVE-POWER COMPENSATION OF COAL MINING EXCAVATORS USING STATCOM
103
Fig. 12. Implemented control system.
quantities are shifted, respectively, by 120◦ and 240◦ with respect to R. Equation (6) is obtained by substituting the effective
value of the peak line current given in (4) into (5)
3
Q=
M V1 Idc cos θ.
(6)
2
Similarly, active power P consumed by CSC can be
pressed as in the following:
√
P = 3V1 I1 sin θ.
√
Dividing both sides of (4) by 2 and substituting into
the following expression is obtained:
3
M V1 Idc sin θ.
P =
2
ex(7)
(7),
(8)
Active power P consumed by CSC can be expressed, as
in (9), in terms of the dc-link quantities by assuming ideal
switches and infinitely large dc-link inductance
2
.
P = Rdc Idc
(9)
Idc can then be obtained by combining (8) and (9) as in the
following:
3 M V1 sin θ
Idc =
.
(10)
2
Rdc
A substitution of (10) into (6) yields
Q=
3 M 2 V12
sin 2θ.
4 Rdc
(11)
The following conclusions can be drawn from (11).
1) The reactive power produced by CSC can be controlled
by varying θ.
2) For very small values of θ, sin 2θ can be approximated to
2θ in radians, resulting in a linear relationship.
3) The reactive power can also be controlled by varying
the modulation index M , but this method is not implemented for IGCT and GTO converters owing to minimum
pulsewidth restriction of these semiconductors.
4) V1 may vary significantly from inductive to capacitive
operating modes at full load depending on how large the
series inductance at the input of the converter is. Also,
there will be a difference between hot and cold dc-link
resistance, although it is very small. These will affect
the direct dependence of Q on θ to a certain extent in
a practical CSC.
So far, the control strategy has been described only for
operation in the steady state. Active power flow is always
unidirectional in the steady state, i.e., from the supply to the
dc-link reactor. However, in transient state, while bringing Idc
from one value to another lower or higher value, in order to
vary the amount of reactive power generated, power flow in
the dc link should be bidirectional, thus allowing charging or
discharging of the dc-link reactor. This is achieved by varying
θ between +90◦ and −90◦ in the inductive region and between
90◦ and 270◦ in the capacitive region. Any restriction imposed
on θ variation range makes the response of the system slower.
A wide variation range for θ causes a rapid rise or decay of
dc-link current.
Block diagram of the control system is shown in Fig. 12.
It is implemented by the use of a TMS320LF2407A DSP
microcontroller-based platform. The implemented control system consists of an outer reactive-power loop and an inner dclink current loop in order to compensate reactive-power demand
of the load. Since the CSC-based STATCOM is designed for
unmanned operation and therefore equipped with all necessary
protection relays (overload, overcurrent, over/undervoltage, and
residual voltage) with settings being made according to associated standards (except undervoltage relay whose setting is
lower than usual experience), frequent system trips are avoided
by the use of the control system in Fig. 12. This is achieved by
equipping the control system primarily by the dc-link current
limiter circuit and secondarily by θ-limiter circuit.
DC-link current limiter has been used in order to prevent
the dc-link reactor from over heating. θ limiter is an extra
protection facility against failure in dc-link current feedback
loop. After reaching a fail-proof design, it is going to be deleted
from the control system because it makes transient response of
the system slower.
C. Input Filter Design
In order to filter out successfully higher order harmonics produced by CSC, such as 17th, 19th, 23rd, etc., a low-pass filter
has been designed and connected to the input of the converter
(Fig. 3). The filter parameters and the corner frequency are
chosen in view of the following constraints.
1) For better filtering performance, corner frequency should
be set to a value as low as possible. This makes necessary
a larger shunt capacitor for a reasonable series reactor
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Fig. 13. Filter characteristics as seen from the converter side for different L and C pairs.
size. This may cause asymmetrical VAR characteristics
for the resulting STATCOM because of fixed capacitive
VAR generation of the filter capacitor. This may be
undesirable for a small CSC-based STATCOM which
compensates a load with nearly symmetrical VAR
demand. In large STATCOMs, this effect is not as
significant as in the small ones.
2) In order to avoid the drawback of large shunt capacitor
mentioned above, a higher series inductance can be chosen, resulting in an effective reduction in shunt capacitance while keeping the corner frequency constant. This
may lead to an unacceptable rise in line-to-line voltage
across the CSC input terminals from full-inductive to fullcapacitive VAR generation. Therefore, the inductance of
the series filter reactor should be kept at a reasonable size.
3) It is also not recommended to adjust the corner frequency
at a higher value, e.g., somewhere between the fifth
and seventh harmonic frequencies, because of ineffective
filtering of higher order harmonic components. This can
make necessary the use of seven chopping angles instead
of five, at the expense of higher IGCT switching frequency in complying with harmonic current standards.
In view of these considerations, the corner frequency of the
low-pass input filter is adjusted to 197 Hz, and a compromise
is made between the shunt capacitor (C = 240 µF/phase − ∆)
and series inductor (L = 895 µH/phase), as shown in Fig. 13.
In order to damp out the filter characteristics at the corner frequency, a 10-Ω resistor is connected in parallel with a 700-µH
external reactor part of the filter inductor. If the low-pass filter
were not equipped with these damping resistors, sustained
oscillations would be observed in supply current waveform
arising from transient changes.
The implemented low-pass filter generates 250-kVAR
reactive power in the capacitive region at 1 kV when CSC is
blocked. This does not remain constant during the operation
of CSC-STATCOM because the input voltage of CSC varies
considerably, depending upon the operation mode (lagging or
leading).
The performance of the implemented low-pass input filter
has also been examined from the viewpoint of voltage harmonics in the utility bus. No significant interaction has been
observed in simulation and field test results.
The load demands ±500 kVAR in this particular application.
However, the implemented CSC (without filter) can produce
±750 kVAR at constant a 1-kV CSC input voltage by considering current ratings of all CSC components. Theoretically,
this corresponds to +1000-kVAR/−500-kVAR reactive-power
generation capability for the implemented CSC-STATCOM.
D. Design of DC-Link Reactor
DC-link reactor is chosen in view of the CSC transient
response, total demand distortion (TDD) of the CSC input
current waveforms, and reactor cost. In the transient state, since
the reactive power produced by CSC is proportional to the
mean dc-link current, transition from one operating point to
another needs large amount of energy to be injected or extracted
2
2
from the dc link, i.e., (1/2)Ldc [Idc(final)
− Idc(initial)
]. From
this viewpoint, a small Ldc is to be chosen.
Since the dc-link reactor is subject to short duration voltage
pulses, during each pulse, the dc-link reactor is considered to be
operating in subtransient state. Therefore, the voltage–current
equation of dc-link reactor given in (12) can be approximated
to the first-order linear differential equation in (13)
d
dt
idc (t) = Vdc
(12)
Ldc
d
idc (t) ∼
= Vdc
dt
(13)
Rdc + Ldc
where Vdc is the applied step voltage. The solution of the
differential equation in (13) is
idc (t) = (Vdc /Ldc )t + I0
(14)
where I0 is the initial value of the dc-link current at t = 0, by
assuming that a step voltage is applied at that instant. Therefore,
the dc-link current rises and decays approximately on a straight
line in the steady state. Vdc /Ldc dictates the rate of rise of the
current. It is much smaller than the maximum permissible value
of IGCT’s di/dt rating (1 A/µs 1000 A/µs). In view of this
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BILGIN et al.: REACTIVE-POWER COMPENSATION OF COAL MINING EXCAVATORS USING STATCOM
105
design constraint, a much smaller value of dc-link inductance
could be used in the design.
The solution of (12) is given in the following:
idc (t) = (Vdc /Rdc )(1 − e−t/τdc )
(15)
where
τdc = Ldc /Rdc .
For the implemented system, τdc = 0.15 s, and hence theoretically in 5τdc time period, dc-link current rises to Vdc /Rdc =
75 kA, under loss of control. During the charging period of
the dc-link reactor, current rises to a rated value, theoretically,
in a very short time period (ts = 2 ms), by assuming that
Idc(rated) = 1000 A. However, for a practical system, such as
the one described in this paper, neither the steady-state value
of the dc-link current can reach to very high values, such as
75 kA, nor the dc-link reactor can be fully charged or discharged fast, such as in 2-ms time, but both of them are limited
by other circuit elements. Simulation studies have shown that
the final steady-state value of the dc-link current would reach
3.5 kA under loss of control and the dc-link reactor could be
charged and discharged in 4.5 ms.
Internal resistance Rdc of the dc-link reactor should be kept
at an implementable minimum value, thus reducing the dc
reactor losses. Operating the dc-link reactor at a higher voltage
for the same system size could make a more significant contribution to reactor loss reduction problem than Rdc reduction.
Superconducting reactor could only make a contribution to loss
reduction problem at the expense of much higher reactor costs.
From the viewpoint of distortion of line current waveforms
at the input of CSC, large dc-link inductors are more desirable.
This choice yields to better approximation of dc-link current to
a level current, thus improving the TDD of the input current
waveform. A large dc-link reactor means a huge and expensive
system component; therefore, it is not recommended.
In view of the above considerations and constraints, a compromise is therefore needed. For the 1-kV ac voltage level
and currently available harmonic standards, Ldc = 3 mH and
Rdc = 20 mΩ are found to be adequate.
Fig. 14. Typical IGCT current waveforms. (a) 500-kVAR inductive mode.
(b) 500-kVAR capacitive mode.
TABLE III
HARMONIC CONTENT OF CSC INPUT CURRENT iR AND SUPPLY CURRENT
iSR ON THE 1-kV SIDE
IV. F IELD T ESTS
Current waveforms of IGCT switches have been recorded by
the use of Rogowski coils (see the Appendix) in the field. Two
sample records, one for the 500-kVAR inductive and the other
for the 500-kVAR capacitive, are given in Fig. 14 for two supply
cycles (40 ms). Since the power semiconductors employed
in CSC are self-commutated devices, switching patterns and
semiconductor current waveforms need to be the same. A
comparison between waveforms in Figs. 8 and 14 shows that
the implemented CSC operates successfully.
The performance of harmonic elimination technique is
proven by field tests. These experimental results are compared
with the simulation results in Table III for 500-kVAR reactivepower generation in the inductive mode. Since the chopping
angles are determined by optimization, the harmonic elimi-
nation technique used in this research work does not yield
an absolute elimination of all low-order harmonics, e.g., 7th,
11th, and 13th harmonics in the second column of Table III
are not absolutely zero. On the other hand, the implemented
measuring, control, and actuating subsystems are not ideal. The
variations in supply voltage and frequency from one cycle to
another, computational delays of a 16-bit DSP, and nonideal
switching characteristics of IGCTs are the major nonidealities
affecting the success of harmonic elimination. To quantify the
effects of these nonidealities in the implemented system on
CSC input line current, measured IR values given in the fourth
column of Table III can be compared with those in the second
column.
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106
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 1, JANUARY/FEBRUARY 2007
TABLE IV
HARMONIC DISTORTION IN LINE CURRENTS OF CSC-BASED STATCOM
AT THE C OMMON C OUPLING P OINT (31.5-kV BUS )
Fig. 16. Variations in TDD of the CSC-based STATCOM line current on the
31.5-kV-side.
Fig. 15. 31.5-kV-side line current waveforms of CSC-based STATCOM
and their harmonic spectra for: (a) 500-kVAR inductive and (b) 500-kVAR
capacitive.
Further than these, low-pass input filter in Fig. 13 causes
magnification of third, and fifth harmonic current components.
Third and fifth harmonics appearing in the line current waveform of CSC will then be amplified theoretically by the factors
of 2.3 and 1.6, respectively (Fig. 13). In practice however,
owing to the component tolerances of low-pass input filter,
these are magnified by 2.75 and 2.1, respectively, as can be
determined from experimental values in the last two columns
of Table III. In summary, TDD value of the implemented
CSC-based STATCOM can be 20% higher than theoretically
expected result.
Some sample line current waveforms of CSC-based
STATCOM on the 31.5-kV side and their harmonic spectra are
shown in Fig. 15. For the implemented system, total harmonic
distortion TDD as well as the amplitudes of individual current
harmonic components of line current waveforms at the common
coupling point (31.5-kV medium-voltage distribution bus) is
compared with the limit values given in IEEE Std. 519-1992.
This comparison is summarized in Table IV for a 500-kVAR
reactive-power generation in the inductive region.
The variations in the TDD of the line current waveforms on
the medium-voltage side against the reactive power produced
by the CSC-based STATCOM are given in Fig. 16.
It is observed that all field test results comply with IEEE
Std. 519-1992, over the entire operating range of CSC-based
STATCOM. In the last column of Table IV, the limit values
for the worst case are also given. The worst case corresponds
to weakest supplies, which are not usual at medium-voltage
buses. The CSC-based STATCOM can comply even with the
worst case limit values by six harmonic eliminations, and/or a
more careful filter design. iR (t), iSR (t), iCR (t), and vST (t),
which are already defined in Fig. 7, are given in Fig. 17 for
two different cases. These waveforms reveal that the CSC-based
STATCOM operates as expected.
Fig. 18 shows the dc-link voltage and current waveforms.
The voltage waveform has been recorded by a high-voltage
differential probe and the peak-to-peak ripple current by a
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BILGIN et al.: REACTIVE-POWER COMPENSATION OF COAL MINING EXCAVATORS USING STATCOM
107
Fig. 18. DC-link waveforms for (a) 500-kVAR capacitive and (b) 500-kVAR
inductive.
Rogowski coil; the mean value of the dc-link current has been
deduced from the hall-effect device measurements (Appendix).
The following conclusions can be drawn from these
waveforms.
1) Against step changes in dc-link voltage, dc-link current
varies nearly on a straight line; this proves our subtransient behavior approximation.
2) For voltage pulses with longer duration, since the
dc-link voltage follows the supply voltage, dc-link current
variations have exponential rise or decays.
3) Peak-to-peak ripple is low in comparison with the
mean dc-link current as expected, i.e., an 80-A peakto-peak ripple with respect to a 700-A mean current for
500-kVAR inductive reactive power.
Fig. 17. 1-kV-side waveforms of CSC-based STATCOM for (a) 550-kVAR
inductive and (b) 625-kVAR capacitive.
Total CSC-based STATCOM losses, including coupling
transformer, cables, bus bars, semiconductors, dc-link reactor,
and all others, will vary as a function of the reactive power
generated by the overall system, as shown in Fig. 19.
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108
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 1, JANUARY/FEBRUARY 2007
Fig. 19. Total CSC-based STATCOM losses against the reactive power.
Fig. 20. Reactive power consumed by the load and generated by STATCOM
(1-s averaged data).
The total power loss of the implemented CSC-based
STATCOM can be separated as 20% in the coupling transformer, 25% in the power stage, 25% in the dc-link reactor, 25%
in the low-pass input filter, and 5% in other system elements
at full load. The efficacy of the CSC-based STATCOM (power
dissipation/reactive power generated) is found to vary in the
range from 2% to 7% over the entire operating range.
Two CSC-based STATCOMs have been operating in the
field since December 2004. The variations in the reactivepower consumption of load (dragline + power shovel) and the
reactive power generated by CSC-based STATCOM are shown
in Fig. 20 for nearly 2-min operating period, which corresponds
nearly to two operating cycles of dragline. The implemented
STATCOM (black-colored curve in Fig. 20) follows closely the
rapid variations in the reactive-power demand of the load bus
(gray-colored curve). The 24-h records of reactive and active
powers are given in Fig. 21, both for the load bus and the
STATCOM. Although the restricted ±500 kVAR capability of
STATCOM does not provide full compensation (Fig. 21), this
capacity can meet the reactive-power compensation needs of
the load bus even in the future (Table I), resulting in nearly unity
power factor (PF) (1% inductive and 3% capacitive reactive
energy of active energy demand of the load).
Fig. 21. Daily power variations. (a) Reactive and (b) active power variations
(1-s averaged data).
In Figs. 20 and 21, 1-s averaged data are plotted. Positive
kVAR consumption denotes lagging PF load, and positive
reactive-power generation for STATCOM denotes capacitive
operation. Curve portion A in Fig. 21(a) corresponds to consecutive starting of the dragline and power shovel.
Dragline demands nearly 4.5 MVAR during a starting period
of 30 s, while the power shovel is 2 MVAR for a period of 20 s.
On the curve portion B in Fig. 21(a), power shovel is excavating
coal, while the dragline is in the standby mode. In Fig. 21(b),
negative active power drawn from the supply corresponds to
regeneration of electric excavators.
During the regenerative braking period, electric excavator
supplies the STATCOM losses. Since buy and sell tariff does
not apply to TKI buses, only part of the STATCOM losses will
be reflected in the electricity bills.
V. C ONCLUSION
The first industry application of a new-generation CSCSTATCOM designed for group compensation of electric
excavators is presented in this paper. The developed system is composed of a single full-bridge CSC with selective
harmonic elimination and the simplest coupling-transformer
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BILGIN et al.: REACTIVE-POWER COMPENSATION OF COAL MINING EXCAVATORS USING STATCOM
109
4) LEM LT 1005 S Hall-effect closed-loop current
transducer;
5) Powertek, Rogowski current waveform transducers;
a) CWT 15B, 2 mV/A, and CWT 6 B, 5 mV/A.
6) National Instruments Data Acquisition System
a) DAQCard-6062E data acquisition card
b) SC-2040 sample-and-hold card.
7) Fluke 80i-110S ac/dc current probe.
R EFERENCES
Fig. 22. Reactive-power consumption of (a) power shovel and (b) dragline.
topology to serve as a full substitute for conventional VSCbased STATCOMs with complicated transformer connections
and/or multiphase converters for harmonic elimination. The
5th, 7th, 11th, and 13th current harmonics produced by CSC
are shown to be eliminated by switching semiconductors at a
moderate frequency of 500 Hz. The series combination of an
asymmetrical IGCT and a reverse-blocking diode meets the objective of natural air-cooling for high reliability and unmanned
operation. The line currents of CSC-based STATCOM at the
medium-voltage common coupling bus comply with IEEE Std.
519-1992. Especially, TDD of STATCOM line current can be
reduced to an absolute minimum by applying six harmonic
component elimination strategy at the expense of higher switching frequency and designing the low-pass input filter more
carefully. The resulting system follows the rapid changes in
reactive-power demand of electric excavators, thus improving
the power factor of the overall system to nearly unity. The
proposed CSC-STATCOM may constitute a viable alternative
to conventional SVC and VSC-STATCOMs, offering a new
technology solution to reactive-power compensation problem
of rapidly fluctuating excavator loads.
A PPENDIX
A. Power Consumption of Electric Excavators
See Fig. 22.
B. Measuring Apparatus
1) Tektronix TDS5054 digital phosphor oscilloscope;
2) Tektronix P5210 high-voltage differential probe;
3) Tektronix P5050 voltage probe;
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Hazim Faruk Bilgin (S’98) received the B.Sc. and
M.Sc. degrees in electrical and electronics engineering from Middle East Technical University, Ankara,
Turkey, in 1998 and 2000, respectively, where he
is currently working toward the Ph.D. degree in the
field of electrical engineering on the current-sourceconverter-based static synchronous compensator.
He is a Chief Research Engineer with the
TUBITAK Information Technologies and Electronics Research Institute, Ankara. His areas of research
are power converters for power quality and electric
motor drive applications.
Muammer Ermis (M’99) received the B.Sc. degree
in electrical engineering from Middle East Technical
University, Ankara, Turkey, in 1972, the M.B.A.
degree in production management from Ankara
Academy of Commercial and Economic Sciences,
Ankara, in 1974, and the M.Sc. and Ph.D. degrees
in electrical engineering from Middle East Technical
University, in 1976 and 1982, respectively.
Currently, he is a Professor of electrical engineering with the Middle East Technical University.
His current research interests are power quality and
utility applications of power electronics.
Dr. Ermis received “The Overseas Premium” paper award from the Institution of Electrical Engineers, U.K., in 1992, the “2000 Committee Prize Paper
Award” from the Power Systems Engineering Committee and the “Meritorious
Paper Award 2003” from the Metal Industry Committee of the IEEE Industry
Applications Society, and the “IEEE Power Engineering Society Chapter
Outstanding Engineer Award 2003.”
K. Nadir Kose received the B.S. degree in electrical and electronics engineering from Middle East
Technical University, Ankara, Turkey, in 1993, and
the M.S. degree from Hacettepe University, Ankara,
in 2001.
He is a Chief Research Engineer with the
TUBITAK Information Technologies and Electronics Research Institute, Ankara. His areas of research
are active power filters, boost rectifiers, IGBT-based
high-power dc motor drives, synchronous motor field
exciters, static VAR compensators, electrostatic precipitators, and microcontroller-based control systems.
Alper Cetin received the B.Sc. and M.Sc. degrees
in electrical and electronics engineering from Middle
East Technical University, Ankara, Turkey, in 1995
and 2000, respectively, where he is currently working
toward the Ph.D. degree.
He is a Chief Research Engineer with the
TUBITAK Information Technologies and Electronics Research Institute, Ankara. His areas of research
are power quality and switch-mode power supplies.
Isik Cadirci (M’98) received the B.Sc., M.Sc., and
Ph.D. degrees in electrical and electronics engineering from Middle East Technical University, Ankara,
Turkey, in 1987, 1988, and 1994, respectively.
Currently, she is a Professor of electrical engineering with Hacettepe University, Ankara, and also
a Power Electronics Group Coordinator with the
TUBITAK Information Technologies and Electronics Research Institute, Ankara. Her research interests
include power quality, electric motor drives, and
switch-mode power supplies.
Dr. Cadirci received the “2000 Committee Prize Paper Award” from the
Power Systems Engineering Committee and the “Meritorious Paper Award
2003” from the Metal Industry Committee of the IEEE Industry Applications
Society.
Adnan Acik received the B.Sc. and M.Sc. degrees
in electrical and electronics engineering from Middle
East Technical University, Ankara, Turkey, in 1995
and 1998, respectively.
He is a Chief Research Engineer with the
TUBITAK Information Technologies and Electronics Research Institute, Ankara. His areas of research
are power quality and switch-mode power supplies.
Turan Demirci received the B.Sc. and M.Sc. degrees
in electrical and electronics engineering from Middle
East Technical University, Ankara, Turkey, in 2000
and 2002, respectively, where he is currently working
toward the Ph.D. degree.
He is a Senior Research Engineer with the
TUBITAK Information Technologies and Electronics Research Institute, Ankara. His research interests
include embedded systems and computer networks.
Alper Terciyanli (S’03) received the B.Sc. and
M.Sc. degrees in electrical and electronics engineering from Middle East Technical University, Ankara,
Turkey, in 2001 and 2004, respectively, where he is
currently working toward the Ph.D. degree.
He is a Senior Research Engineer with the
TUBITAK Information Technologies and Electronics Research Institute, Ankara. His areas of research are active power filters and power quality
monitoring.
Cetin Kocak received the B.Sc. degree in geophysical engineering from Istanbul University, Istanbul,
Turkey, in 1970.
He has been with Turkish Coal Enterprises since
1970. He was a Deputy Chairman of the Research,
Project and Establishment Department, Turkish Coal
Enterprises, during the STATCOM project and is
now a Counselor for the General Director of Turkish
Coal Enterprises.
Mustafa Yorukoglu received the B.Sc. and M.Sc.
degrees in mining engineering from Middle East
Technical University, Ankara, Turkey, in 1978 and
1990, respectively.
He was with Turkish Coal Enterprises from 1978
to 2004. He was the Chairman of the Research,
Project and Establishment Department, Turkish Coal
Enterprises, during the STATCOM project. He has
been a Project Leader of the mining group of Enerjisa
Corporation, Istanbul, Turkey.
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