Download A Novel Control Method for Unified Power Quality Conditioner

A Novel Control Method for Unified Power Quality
Conditioner (UPQC) Under Non-Ideal Mains
Voltage and Unbalanced Load Conditions
Metin Kesler
Engin Ozdemir
Kocaeli University Technical Education Faculty, 41380
Umuttepe Kocaeli Turkey
Kocaeli University Technical Education Faculty, 41380
Umuttepe Kocaeli Turkey
[email protected]
[email protected]
II.
Abstract--This paper presents a new control method to
compensate the power quality problems through a three-phase
unified power quality conditioner (UPQC) under non-ideal
mains voltage and unbalanced load conditions. The
performance of proposed control system was analyzed using
simulations with Matlab/Simulink program, and experimental
results with the hardware prototype. The proposed UPQC
system can improve the power quality at the point of common
coupling (PCC) on power distribution system under non-ideal
mains voltage and unbalanced load conditions.
I.
INTRODUCTION
Unified power quality control was widely studied by
many researchers as an eventual method to improve power
quality of electrical distribution system [1-3]. The function
of unified power quality conditioner is to compensate supply
voltage flicker/imbalance, reactive power, negative-sequence
current, and harmonics. In other words, the UPQC has the
capability of improving power quality at the point of
installation on power distribution systems or industrial
power systems. Therefore, the UPQC is expected to be one
of the most powerful solutions to large capacity loads
sensitive to supply voltage flicker/imbalance [2]. The UPQC
consisting of the combination of a series active power filter
(APF) and shunt APF can also compensate the voltage
interruption if it has some energy storage or battery in the dc
link [3].
vTa
vSa
+
vSabc
3∼
RS
Mains
Voltage
The shunt APF is usually connected across the loads to
compensate for all current-related problems such as the
reactive power compensation, power factor improvement,
current harmonic compensation, and load unbalance
compensation [1-2], whereas the series APF is connected in
a series with the line through series transformers. It acts as
controlled voltage source and can compensate all voltagerelated problems, such as voltage harmonics, voltage sag,
voltage swell, flicker, etc.
iTabc
vLa
=
iSabc
iCa
iSa
=
vTabc
iLabc
RL
RT
LT
Series
APF
CDC
Shunt
APF
+
vLabc
PCC
LS
iLa
Zload
LL
RC
LC
Nonlinear
Load
iCabc
Fig. 1. Unified power quality conditioner configuration.
A. Reference Voltage Signal Generation for Series APF
The function of the series APF is to compensate the
voltage disturbance in the source side, which is due to the
fault in the distribution line at the PCC. The series APF
control algorithm calculates the reference value to be
injected by the series APF transformers, comparing the
positive-sequence component with the load side line
voltages. The proposed series APF reference voltage signal
generation algorithm is shown in Fig. 3. In equation (1),
supply voltages vSabc are transformed to d-q-0 coordinates.
In this paper a new control algorithm for the UPQC
system is optimized without measuring transformer voltage,
load and filter current, so that system performance is
improved. The proposed control technique has been
evaluated and tested under non-ideal mains voltage and
unbalanced load conditions using Matlab/Simulink software.
The proposed method is also validated through experimental
study.
978-1-4244-4783-1/10/$25.00 ©2010 IEEE
UPQC CONTROL ALGORITHM
The UPQC consists of two voltage source inverters
connected back to back with each other sharing a common
dc link. One inverter is controlled as a variable voltage
source in the series APF, and the other as a variable current
source in the shunt APF. Fig. 1 shows a basic system
configuration of a general UPQC consisting of the
combination of a series APF and shunt APF. The main aim
of the series APF is harmonic isolation between load and
supply; it has the capability of voltage flicker/ imbalance
compensation as well as voltage regulation and harmonic
compensation at the utility-consumer PCC. The shunt APF is
used to absorb current harmonics, compensate for reactive
power and negative-sequence current, and regulate the dclink voltage between both APFs. The proposed UPQC
control algorithm block diagram in Matlab/Simulink
simulation software is shown in Fig. 2.
374
vSabc
bB
B
cC
cC
cC
C
iCabc
1
1
iSabc
1-Phase Seies Transformers
+ -i
Rdc3
-
iLn
vTabc
A
+
B
-
Cdc1
LLa1
Discrete,
Ts = 5e-006 s.
Rdc1
pow ergui
VDC
RTabc
LTabc
CTabc
+
-
-
A
B
C
(Seri es Active Power Filter)
abc
sin_cos
dq0
CCabc
g
+
B
C
(Shunt Active Power Filter)
C2
em
p
vSabc
Vabc
iSabc
v Sabc PLL
abc
sin_cos
Valf a
Vbeta
Iabc
Vo
Vo
Io
Io
PWM
PWM
I'Salf a
I'Sbeta
I'0
Vdc
vDA
iSabc
iSabc
v Tabc
p
Valf a
Ebeta
dq0
v 'Tabc
vLabc
1-Phase
Non-Linear Load
RCabc
LCabc
C1
g
A
vSabc
Ldc3
+
3-Phase
Non-Linear Load
iTabc
Cc
Aa
iSn
LLabc
iLn
2
iSn
2
2
+ -i
bB
1
3-Phase Source
A
bB
Bb
C
aA
Cc
h5
B
g
aA
Bb
N
Pulse
aA
Aa
A
iLabc
vLabc
I'0
i'Sabc
i'Sabc
i'Sbeta
i'Salf a
Fig. 2. The proposed UPQC control algorithm block diagram in MATLAB Simulink.
1
⎡ 1
⎢ 2
⎡ vS0 ⎤
2
⎢
⎢ ⎥ 2
π
⎢ vSd ⎥ = 3 ⎢ sin(wt) sin(wt - 2 3 )
⎢
⎢v ⎥
π
⎢
⎣ Sq ⎦
cos(wt) cos(wt - 2 )
3
⎣⎢
1
⎤
⎥ v
2
⎡ Sa ⎤
π ⎥⎢ ⎥
sin(wt + 2 ) ⎥ ⎢ vSb ⎥
3 ⎥
π ⎢⎣ vSc ⎥⎦
cos(wt + 2 )⎥⎥
3 ⎦
(1)
The voltage in d axes ( v Sd ) given in (2) consists of
average and oscillating components of source voltages
( v Sd and ~
vSd ). The average voltage vSd is calculated by
using second order LPF (low pass filter).
vSd = vSd + ~
vSd
B. Reference Current Signal Generation for Shunt APF
The shunt APF described in this paper used to compensate
the current harmonics and reactive power generated by the
nonlinear load. The shunt APF reference current signal
generation block diagram is shown in Fig. 3. The
instantaneous reactive power (p-q) theory is used to control of
shunt APF in real time. In this theory, the instantaneous
three-phase currents and voltages are transformed to α-β-0
coordinates as shown in equation (4) and (5).
(2)
vLa
vLb
vLc
v S0
d-q
transform. vSd
The load side reference voltages v∗Labc are calculated as given
0
vSd
LPF
v Sq
in equation (3). The switching signals are assessed by
comparing reference voltages ( v∗Labc ) and the load voltages
vS0
0
v Sq
v∗
Lα
d-q
Inv.
transfor m
∗
v Lb
∗
v Lc
Series
APF
Sinusoidal
PWM
GAH
GAL
GB H
GB L
GCH
GC L
PLL
( v Labc ) and via sinusoidal PWM controller.
⎤
⎡
⎡ v ∗La ⎤
⎢ sin(wt)
cos(wt)
1⎥ ⎡ v ⎤
⎥ 2⎢
⎢
⎥ Sd
π
π
⎢ v ∗Lb ⎥ = ⎢ sin(wt - 2 ) cos(wt + 2 ) 1⎥ ⎢ 0 ⎥
3
3
⎥ 3⎢
⎢
⎥ ⎢⎢ 0 ⎥⎥
⎦
⎢⎣ v ∗Lc ⎥⎦
⎢sin(wt + 2 π ) cos(wt + 2 π ) 1⎥ ⎣
⎥⎦
3
3
⎣⎢
vSa
vSb
vSc
(3)
iSa
iSb
iSc
α-β
conv.
α-β
conv.
vo
vα
vβ
io
iα
iβ
p
LPF
Instantaneous
Power
calculate
q
p0
0
-1
V DC1 + Σ
+
VDC2
These produced three-phase load reference voltages are
compared with load line voltages and errors are then
processed by sinusoidal PWM controller to generate the
required switching signals for series APF IGBT switches.
375
p
q
∗
α-β
iSα
Reference ∗
current i Sβ
calc. i∗
S0
p0
*
VDC
α-β
Inv.
Trans.
∗
i Sa
i ∗Sb
i ∗Sc
Shunt
APF
Hysteresis
Band
PWM
ploss
PI
DC
Fig. 3. Series APF reference voltage and shunt APF reference current
signal generation block diagram.
GAH
GAL
GBH
GBL
GCH
GCL
⎡v0 ⎤
⎢ ⎥
⎢vα ⎥ =
⎢v ⎥
⎣ β⎦
⎡i 0 ⎤
⎢ ⎥
⎢i α ⎥ =
⎢i ⎥
⎣ β⎦
⎡1/ 2
2
⎢ 1
3⎢ 0
⎣
2
3
⎡1/ 2
⎢ 1
⎢ 0
⎣
1/ 2
- 1/2
3/2
1/ 2
- 1/2
3/2
⎤ ⎡ vSa ⎤
- 1/2 ⎥ ⎢ vSb ⎥
⎥⎢ ⎥
- 3/2 v
⎦ ⎣ Sc ⎦
(4)
⎤ ⎡i Sa ⎤
- 1/2 ⎥ ⎢i Sb ⎥
⎥⎢ ⎥
- 3/2 i
⎦ ⎣ Sc ⎦
(5)
1/ 2
1/ 2
unbalanced load current conditions. The simulated UPQC
system parameters are given in Table I. In simulation studies,
the results are specified before and after UPQC system are
operated. In addition, when the UPQC system is operated, the
load has changed and dynamic response of the system is
tested. The proposed control method has been examined
under non-ideal mains voltage and unbalanced load current
conditions. Before harmonic compensation, the THD of the
supply current is 26.23%. The obtained results show that the
proposed control technique allows the 3.4% mitigation of all
harmonic components.
TABLE I.
The source side instantaneous real and imaginary power
components are calculated by using source currents and
phase-neutral voltages as given in (6). The instantaneous real
and imaginary powers include both oscillating and average
components as shown in (7). Average components of p and q
consist of positive sequence components ( p and q ) of source
current. The oscillating components ( ~
p and ~
q ) of p and q
Parameters
Source
Load
include harmonic and negative sequence components of
source currents [4]. In order to reduce neutral current, p 0 is
calculated by using average and oscillating components of
imaginary power and oscillating component of the real
power; as given in (8) if both harmonic and reactive power
compensation is required. isα* , isβ* and is0* are the reference
currents of shunt APF in α-β-0 coordinates. These currents
are transformed to three-phase system as shown in (9).
β
p=p+~
p
p0 = v0 ∗ i0
;
⎡i∗Sα ⎤
1
⎢∗ ⎥= 2
⎢⎣iSβ ⎥⎦ v α + vβ2
⎡ v α -vβ ⎤ ⎡ p + p 0 + ploss ⎤
⎢v
⎥⎢
⎥
0
⎦
⎢⎣ β v α ⎥⎦ ⎣
⎡1/ 2
2⎢
⎢1/ 2
3⎢
1/ 2
⎣
0 ⎤ ⎡i *S0 ⎤
⎥⎢
⎥
- 1/2
3/2 ⎥ ⎢i *Sα ⎥
- 1/2 - 3/2⎥ ⎢⎣ i *Sβ ⎥⎦
⎦
Series
APF
(6)
(7)
(8)
1
(9)
III.
Value
vSabc
380 Vrms
50 Hz
Frequency
f
3-Phase ac Line Inductance
LLabc
2 mH
1-Phase ac Line Inductance
LLa1
1 mHΩ
3-Phase dc Inductance
Ldc3
10 mH
3-Phase dc Resistor
Rdc3
30 Ω
1-Phase dc Resistor
Rdc1
87,5 Ω
1-Phase dc Capacitor
Cdc1
Voltage
VDC
240 μF
700 V
Capacitor 1/2
C1 C 2
ac Line Inductance
LCabc
2200 μF
3.5 mH
Filter Resistor
RCabc
5Ω
Filter Capacitor
CCabc
10 μF
Switching Frequency
fpwm
~15 kHz
ac Line Inductance
LTabc
1.5 mH
Filter Resistor
RTabc
5Ω
Filter Capacitor
CTabc
Switching Frequency
fpwm
20 μF
12 kHz
200
0
-200
0.15
The reference currents are calculated in order to
compensate neutral, harmonic and reactive currents in the
load. These reference source current signals are then
compared with sensed three-phase source currents, and the
errors are processed by hysteresis band PWM controller to
generate the required switching signals for the shunt APF
switches [6].
Voltage
Simulation results for the load and source voltages under
unbalanced-distorted mains voltage conditions are shown in
Fig. 4. Load current compensation simulation results under
non-ideal (unbalanced-distorted) mains voltage conditions are
given in Fig. 5.
vSabc(V)
⎡i *Sa ⎤
⎢i * ⎥ =
⎢ Sb ⎥
⎢⎣i *Sc ⎥⎦
Shunt
APF
vTabc(V)
⎤ ⎡i ⎤
⎥ ⎢i α ⎥
v
α⎥
⎦ ⎢⎣ β ⎥⎦
v
dc-link
0.2
0.25
0.2
0.25
200
0
-200
0.15
vLabc(V)
v
⎡p ⎤ = ⎡⎢ α
⎢⎣q ⎥⎦ ⎢− v
⎣ β
UPQC SYSTEM PARAMETERS
Filter Voltages
0.3
t(s)
200
0
-200
0.15
Source Voltages
0.3
Load Voltages
0.2
0.25
0.3
Fig. 4. Simulation results for unbalanced and distorted mains voltage
condition.
SIMULATOIN RESULTS
In this study, a new control algorithm for the UPQC is
evaluated by using simulation results given in
Matlab/Simulink software under non-ideal mains voltage and
The neutral current compensation results are given in Fig.
6. The proposed UPQC control algorithm has ability to
compensate both harmonics and reactive power of the load
376
and neutral current is also eliminated. The proposed control
technique has been evaluated and tested under dynamical and
steady-state load conditions. Simulation results for under load
changing are shown in Fig. 7.
iLabc(A)
40
20
0
-20
-40
0.25
Load Currents
0.3
0.35
0.3
0.35
0.4
iCabc(A)
20
0
-20
Filter Currents
0.25
iSabc(A)
20
0
-20
0.4
t(s)
0.25
Source Currents
0.3
0.35
harmonics produced by a diode-bridge rectifier of 10 kVA,
but also to eliminate the voltage harmonics contained in the
receiving terminal voltage of the load. The UPQC consists of
two back to back connected voltage source inverters and three
DSP processors for controlling shunt and series APF’s and
computer communication for all system control functions.
The dc link of both APFs is connected to a common dc
capacitor of 1100 microfarad and 700 V dc. All of the circuit
parameters and experimental conditions are set up exactly the
same as the simulation conditions. Although the proposed
control scheme cannot be studied experimentally for
unbalanced mains voltage conditions, an optimal control can
be designed to eliminate this problem, which will have been
discussed as a future work.
0.4
iLn(A)
Fig. 5. Simulation results for unbalanced and non-linear load current
condition.
20
0
-20
Load Neutral Current
iCn(A)
0.25
0.35
0.3
0.35
0.3
0.35
0.4
20
0
-20
Filter Neutral Current
0.25
iSn(A)
0.3
20
0
-20
0.4
Source Neutral Current
0.25
0.4
Fig. 6. Simulation results for neutral current compensation.
Fig. 8. The photograph of the proposed UPQC system.
Before UPQC
After UPQC Operation
The source and load voltages are sensed using LEM LV
25P voltage sensors, whereas, all the currents are sensed
using LEM LA-55P Hall-Effect current sensors. The series
and shunt inverters are built using SEMIX 101GD128Ds sixpack IGBT switches from Semikron. CONCEPT 6SD106EI
and Semikron SKHI 61 IGBT drivers are used for series and
shunt APF respectively. The IGBT driver modules have short
circuit and over current protection functions for every IGBT
and provides electrical isolation for all PWM signals applied
to the digital signal processor (DSP). The proposed
experimental control system consists of three DSP cards from
TI (TMS320F28335). Three DSP cards are designed to
control shunt and series APF and one of them is responsible
for all system operation and power quality analysis. Both
inverters use the variable frequency hysteresis band
controller.
VDC(V)
iSn(A)
iSabc(A)
iLabc(V)
vLabc(V)
Load Variation (Step-up)
200
0
-200
Load Voltages
0.1
0.15
0.2
0.25
40
20
0
-20
-40
0.1
0.15
0.2
0.25
40
20
0
-20
-40
0.1
0.15
0.2
0.25
0.15
0.2
0.25
0.15
0.2
0.3
Load Currents
0.3
Source Currents
40
20
0
-20
-40
0.1
800
0.3
Source Neutral Current
0.3
700
600
0.1
t(s)
DC Link Voltage
0.25
0.3
Fig. 7. Simulation results for operational performance of the UPQC system.
IV.
EXPERIMENTAL TEST RESULTS
Fig. 8 shows an experimental system configuration
photograph of the proposed UPQC system. The aim of the
UPQC system is not only to compensate for the current
Fig. 9 shows source voltage and current waveforms before
filtering. After compensation, source current becomes
sinusoidal and in phase with the source voltage; hence, both
harmonics and reactive power are compensated
simultaneously. Before harmonic compensation, the THD of
the supply current is 29.13% and after the harmonic
compensation, it is reduced to 5.3% which complies with the
IEEE 519 harmonic standards. Fig. 10 and Fig. 11 show
experimental results for source voltage (vSa), filter current
(iCa) and source current (iSa) after filter operation respectively.
377
iSa
vSa
iSa
iSa harmonic spectrum
Fig. 9. Experimental results for source voltage (vSa) and source current (iSa)
before filter operation.
DC-link Voltage
Fig.13. Experimental results for dc link voltage and source current (iSa)
before and after load variation (load step-up).
iCa
vSa
iCa harmonic spectrum
iSabc Source Currents
Fig. 10. Experimental results for source voltage (vSa) and filter current (iCa)
after filter operation.
iSa
iSn Source Neutral Current
vSa
iSa harmonic spectrum
Fig.14. Experimental results for source current (iSabc)
and neutral current iSn before and after filter operation
Fig. 11. Experimental results for source voltage (vSa) and source current (iSa)
after filter operation.
Fig. 12 shows experimental results for three-phase source
currents (iSabc) before and after filter operation. Fig. 13 shows
experimental results for the dc link voltage and the source
current (iSa) before and after load variation (load step-up), the
shunt APF tested under dynamical and steady-state load
conditions under load changing. Fig. 14 shows the
experimental results for source currents (iSabc) and neutral
current (iSn) before and after filter operation. Fig. 15 shows
results for load neutral (iLn), filter neutral (iCn) and source
neutral current (iSn) before and after filter operation.
iSabc Source Currents
Fig. 12. Experimental results for source current (iSabc) before and after filter
operation.
iLn
iCn
iSn
Fig.15. Experimental results for load neutral (iLn),
filter neutral (iCn) and source neutral current (iSn).
These experimental results given above shows that the
harmonic compensation features of the UPQC, by appropriate
control of the shunt APF can be done effectively. The shunt
APF with reduced current measurement based control method
can be compensating neutral, harmonic and reactive currents
effectively, in the unbalanced and distorted load conditions.
The series APF experimental results for mains and load
voltages before filter operation is shown in Fig. 16. Fig. 17
shows the experimental results for the load voltages in threephase form before and after filter operation.
378
ACKNOWLEDGEMENT
This study is supported financially by TUBITAK research
fund number 108E083 and Kocaeli University Scientific
Research Fund.
This work is also supported by Concept Inc. (Concept
IGBT driver), Semikron Inc. (IGBT and IGBT driver), LEM
Inc. (voltage and current sensor) and TI Inc. (F28335 eZdsp),
which is gratefully acknowledged. The authors gratefully
acknowledge the contributions of Halim Ozmen (from
Semikron Turkey) and Robert Owen (from TI).
vLa Load voltage
vSa Source voltage
Fig. 16. Experimental results for mains and load voltages before filter
operation.
REFERENCES
[1]
[2]
[3]
vLabc Load voltages
[4]
[5]
[6]
[7]
Fig. 17. Experimental results for load voltages in three-phase form before
and after filter operation.
V.
[8]
CONCLUSION
This paper describes a new control strategy used in the
UPQC system, which mainly compensate reactive power and
voltage and current harmonics in the load under non-ideal
mains voltage and unbalanced load current conditions. The
proposed control strategy use only loads and mains voltage
measurements for series APF based on the synchronous
reference frame theory. The instantaneous reactive power
theory is used for shunt APF control algorithm by measuring
mains voltage and currents. The conventional methods
require measurements of the load, source and filter voltages
and currents.
The simulation results show that, when unbalanced and
nonlinear load current or unbalanced and distorted mains
voltage conditions, the above control algorithms eliminate the
impact of distortion and unbalance of load current on the
power line, making the power factor unity. Meanwhile, the
series APF isolates the loads voltages and source voltage, the
shunt APF provides three-phase balanced and rated currents
for the loads.
[9]
[10]
[11]
[12]
The experimental results obtained from a laboratory
model of 10 kVA, along with a theoretical analysis, are
shown to verify the viability and effectiveness of the
proposed UPQC control method.
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