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].
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 voltage-related problems, such as voltage harmonics, voltage sag, voltage swell, flicker, etc.
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.
II. 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 dc-link voltage between both APFs. The proposed UPQC control algorithm block diagram in Matlab/Simulink simulation software is shown in Fig. 2.
+ = = +
3∼
RT LT
Nonlinear Load
iLabc
RL LL
vTabc
Mains Voltage
vSabc
CDC iTabc
vLabciSabc
RC LC
RS LS
Zload
iCabc
ShuntAPF
SeriesAPF
vSa vTa vLa iSa iCa iLa
PCC
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.
Fig. 2. The proposed UPQC control algorithm block diagram in MATLAB Simulink.
vvv
vvv
Sc
Sb
Sa
Sq
Sd
S0
)3π2cos(wt)
3π2-cos(wtcos(wt)
)3π2sin(wt)
3π2-sin(wtsin(wt)
21
21
21
32
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢
⎣
⎡
=⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
+
+ (1)
The voltage in d axes ( Sdv ) given in (2) consists of
average and oscillating components of source voltages ( Sdv and Sdv~ ). The average voltage Sdv is calculated by using second order LPF (low pass filter).
SdSdSd v~vv += (2)
The load side reference voltages ∗
Labcv are calculated as given in equation (3). The switching signals are assessed by comparing reference voltages ( ∗
Labcv ) and the load voltages ( Labcv ) and via sinusoidal PWM controller.
00Sdv
1)3π
2cos(wt)3π
2sin(wt
1)3π
2cos(wt)3π
2-sin(wt
1cos(wt)sin(wt)
32
Lcv
Lbv
Lav
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢
⎣
⎡
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
++
+=∗
∗
∗ (3)
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.
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).
α-β Reference
current calc.
α-β Inv.
Trans.
α-β conv.
α-βconv.
vαvβ Instantaneous
Power calculate
iαiβ
LPF∗Sβi
∗Sαi
lossp
GAH
GAL
GBH
GBL
GCH
GCL
Shunt APF
HysteresisBand PWM
DC VDC1 PI
-
+
*DCV
Σ
p p
q
∗Sai
∗Sbi
∗Sciio
vo
VDC2
+
d-q Inv.
transform
PLL
d-qtransform.
LPF
GAH
GAL
GBH
GBL GCH
GCL
Series APF
Sinusoidal PWM
vLa vLb vLc
∗Lαv
∗Lbv
∗Lcv
iSa iSb iSc
vSa vSb vSc
vS0
vSq
vSd
vS0
vSq
Sdv0
0
∗S0i
q
p0
0
-1p0
Fig. 3. Series APF reference voltage and shunt APF reference current signal generation block diagram.
375
ScvSbvSav
/23-/2301/2-1/2-1
21/21/21/
3
2
βv
αv
0v
⎥⎥
⎦
⎤
⎢⎢
⎣
⎡
⎥⎥
⎦
⎤
⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
= (4)
SciSbiSai
/23-/2301/2-1/2-1
21/21/21/
3
2
βi
αi
0i
⎥⎥
⎦
⎤
⎢⎢
⎣
⎡
⎥⎥
⎦
⎤
⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
= (5)
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 include harmonic and negative sequence components of source currents [4]. In order to reduce neutral current, 0p 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).
⎥⎥⎦
⎤
⎢⎢⎣
⎡
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎥⎦⎤
⎢⎣⎡
−=
βiα
i
αv
βv
βv
αv
qp
(6)
000 ivp ∗= ; p~pp += (7)
⎥⎦⎤
⎢⎣⎡
⎥⎥⎦
⎤
⎢⎢⎣
⎡
⎥⎥⎦
⎤
⎢⎢⎣
⎡ ++
+=∗
∗
0lossppp
αvβvβ-vαv
2βv2
αv
1
SβiSαi 0 (8)
S*iS*iS0*i
Sc*iSb*iSa*i
/23-1/2-21//231/2-21/
0121/
32
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
=⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
β
α (9)
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].
III. 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
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. UPQC SYSTEM PARAMETERS
Parameters Value
Source Voltage vSabc 380 Vrms
Frequency f 50 Hz
Load
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 240 μF
dc-link Voltage VDC 700 V
Capacitor 1/2 C1 C2 2200 μF
Shunt APF
ac Line Inductance LCabc 3.5 mH Filter Resistor RCabc 5 Ω
Filter Capacitor CCabc 10 μF Switching Frequency fpwm ~15 kHz
Series APF
ac Line Inductance LTabc 1.5 mH Filter Resistor RTabc 5 Ω
Filter Capacitor CTabc 20 μF Switching Frequency fpwm 12 kHz
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.
0.15 0.2 0.25 0.3
-2000
200
vSab
c(V
)
0.15 0.2 0.25 0.3
-2000
200
vTab
c(V
)
0.15 0.2 0.25 0.3
-2000
200
vLab
c(V
)
Fig. 4. Simulation results for unbalanced and distorted mains voltage condition.
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
Source Voltages
t(s)
Filter Voltages
Load Voltages
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.
0.25 0.3 0.35 0.4-40-20
02040
iLab
c(A
)
0.25 0.3 0.35 0.4
-200
20
iCab
c(A
)
0.25 0.3 0.35 0.4
-200
20
iSab
c(A
)
Fig. 5. Simulation results for unbalanced and non-linear load current condition.
0.25 0.3 0.35 0.4
-200
20
iLn(
A)
0.25 0.3 0.35 0.4
-200
20
iCn(
A)
0.25 0.3 0.35 0.4
-200
20
iSn(
A)
Fig. 6. Simulation results for neutral current compensation.
0.1 0.15 0.2 0.25 0.3
-2000
200
vLab
c(V
)
0.1 0.15 0.2 0.25 0.3-40-20
02040
iLab
c(V
)
0.1 0.15 0.2 0.25 0.3-40-20
02040
iSab
c(A
)
0.1 0.15 0.2 0.25 0.3-40-20
02040
iSn(
A)
0.1 0.15 0.2 0.25 0.3600
700
800
VD
C(V
)
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
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.
Fig. 8. The photograph of the proposed UPQC system.
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 six-pack 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.
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.
Load Neutral Current
Filter Neutral Current
Source Neutral Current
t(s) Source Currents
Load Currents
Filter Currents
Load Currents
Load Voltages
Source Currents
Source Neutral Current
DC Link Voltage
Before UPQC After UPQC Operation
Load Variation (Step-up)
t(s)
377
Fig. 9. Experimental results for source voltage (vSa) and source current (iSa)
before filter operation.
Fig. 10. Experimental results for source voltage (vSa) and filter current (iCa)
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.
Fig. 12. Experimental results for source current (iSabc) before and after filter
operation.
Fig.13. Experimental results for dc link voltage and source current (iSa) before and after load variation (load step-up).
Fig.14. Experimental results for source current (iSabc) and neutral current iSn before and after filter operation
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 three-phase form before and after filter operation.
DC-link Voltage
iSa
iSn Source Neutral Current
iSabc Source Currents
iLn
iCn
iSn
iSa vSa
iSa harmonic spectrum
iCa harmonic spectrum
iSa harmonic spectrum
iCa vSa
iSa vSa
iSabc Source Currents
378
Fig. 16. Experimental results for mains and load voltages before filter
operation.
Fig. 17. Experimental results for load voltages in three-phase form before
and after filter operation.
V. 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.
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.
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).
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