1. Advisor: Prof. Hong-Hee Lee
Presenter: Trinh Quoc Nam
Date: May, 14th, 2014
University of Ulsan, Ulsan, Korea
Industrial Network and Power Electronics Laboratory
Advanced Control Strategies for UPQC to
Improve Power Quality of Distribution
Power Systems
Thesis for Doctor of Philosophy in Electrical Engineering

2. 2
1 Necessity of the Research
3
4
5
6
Review of PQ improvement and control objective
Harmonics compensations using resonant controllers
Harmonics compensations using repetitive controller
Voltage sag and unbalance compensations
Conclusions and Future Works
Industrial Network and Power Electronics Laboratory
Presentation Outline
2 /72

3. 2
1 Necessity of the Research
3
4
5
6
Review of PQ improvement and control objectives
Harmonics compensations using resonant controllers
Harmonics compensations using repetitive controller
Voltage sag and unbalance compensations
Conclusions and Future works
Presentation Outline
Industrial Network and Power Electronics Laboratory 3 /72

4. Chapter 1
 Modern society relies heavily on electricity.
Household appliances
Office equipment
 Nowadays, electricity become an important
service. Quality of electricity directly affect to
quality of people life.
Manufacturing process
Importance of electricity
4 /72

5. Supply
voltage
Lv
NLi
Linear,
Sensitive
loads
Nonlinear
load
LC LR
sLsR
1LiSv
(Voltage sag)
(Line impedance)
(Voltage sag)
Current & voltage
harmonic
Voltage unbalance
Voltage sag
Chapter 1
 Power quality of electric system indicates both the quality of the voltage supplied to
loads and the quality of the current consumed by users.
 Good quality supply voltage should be a sinusoidal waveform with constant magnitude
and frequency.
 Good quality load current should be a sinusoidal waveform and in-phase with the
corresponding supply voltage.
PCC
Which characteristic define the power quality?
Si
5 /72

6. 1. Nowadays, most of electrical equipment are nonlinear loads, which
causes propagation of current and voltage harmonics in power
distribution systems.
2. More and more electronic-based equipment are used in power
system, which is very sensitive to voltage disturbances.
3. Poor power quality causes huge financial losses.
Due to these serious impacts of poor power quality, improvement
of power quality of electric systems becomes an mandatory
requirement.
Chapter 1 Why does power quality become important nowadays?
6 /72

7. 2
1 Necessity of the Research
3
4
5
6
Review of PQ improvement and control objective
Harmonics compensations using resonant controllers
Harmonics compensations using repetitive controller
Voltage sag and unbalance compensations
Conclusions and Future works
Presentation Outline
7 /72

8. Supply
voltage
1Li
Linear,
Sensitive
loads
Nonlinear
load
LC LR
sLsR
2Li
Sv
Shunt
Active
Power
Filter
Fi
Si
Chapter 2
Lv
 Shunt APF is a voltage source inverter (VSI), which is installed in power system to
compensate harmonic current caused by nonlinear loads.
h < 11 11≤h<17 17≤h<23 23≤h<35 35≤h THD
4 % 2% 1.5% 0.6% 0.3% 5%
Maximum permissible harmonic current distortion allowed by IEEE
519-1992 standard
Current harmonic compensation using shunt APF
8 /72

9. 1Li
Linear,
Sensitive
loads
Nonlinear
load
LC LR
2Li
LvDynamic
voltage
restorer
Srv
Chapter 2
Supply
voltage
sLsR
Sv
(Voltage sag)
(No voltage sag)
(Voltage sag)
 Dynamic voltage restorer (DVR) is a voltage source inverter (VSI), which is installed in
power system to protect load from voltage sag at supply side.
Voltage sag compensation using dynamic voltage restorer
9 /72

10. Chapter 2
Supply
voltage
1Li
Linear,
Sensitive
loads
Nonlinear
load
LC LR
sLsR
2Li
Sv
(Voltage sag)
Si
(No voltage sag)
Series
Active
Power
Filter
Shunt
Active
Power
Filter
Srv
Fi
UPQC
Lv
 Unified power quality conditioner (UPQC) is the combination of series and shunt APF,
which can deal with most of common power quality problems such as current and voltage
harmonic, voltage sag, voltage unbalance, etc.
UPQC for power quality improvement
10 /72

11. Chapter 3
Voltage
control
Current
control
,S abcv
,f abcv
,F abci
,S abci
Voltage sag/
harmonic
detector
,f abcv
Harmonic
detector
,L abci
,F abci
dcV
dcV-
+
+
+
Supply
Voltage
dcV
,S abci ,L abci
,L abcv,S abcv
fL
fC
shL
dcC
Linear,
Sensitive
loads
,Sr abcv
,F abci Nonlinear
load
LC LR
,f abcv Series
APF
Shunt
APF
UPQC control system
Conventional control method of UPQC:
1. Consist of many control
components.
2. Require a large number of sensors
3. Performance depends on:
Voltage sag detection, harmonic
detector, and voltage/ current controllers
PI
PWM PWM
Conventional control strategy for UPQC
11 /72

12. Common power
quality problems
Shunt
APF
Series APF
(DVR)
UPQC
1. Current harmonics YES NO YES
2. Voltage harmonics NO YES YES
3. Voltage sag/swell NO YES YES
4. Voltage unbalance NO YES YES
Chapter 2
 However, traditional control strategies of the UPQC is quite complex, requires
too many control components and large number of sensors, and furthermore,
most of control methods are unable to offer a satisfied control performance.
This thesis introduces advanced control strategies to improve performance
of UPQC.
Comparison of common power custom devices
12 /72

13. Control objective:
1. Develop advanced control strategies for UPQC to tackle various power
quality problems such as current and voltage harmonics, voltage sag,
voltage unbalance.
2. The final control target is to maintain the load voltage and supply current
sinusoidal and balanced despite the disturbances at the supply voltage
and the distorted nonlinear load current.
3. THD values of load voltage and supply current after compensation are
less than 5% to comply with IEEE 519-1992 standard.
Chapter 2 Control objective of the Thesis
13 /72

14. 2
1 Necessity of the Research
3
4
5
6
Review of PQ improvement and control objective
Harmonics compensations using resonant controllers
Harmonics compensations using repetitive controller
Voltage sag and unbalance compensations
Conclusions and Future works
Presentation Outline
Industrial Network and Power Electronics Laboratory 14 /72

15. Industrial Network and Power Electronics Laboratory
3.A. Current harmonics compensations
using shunt APF
15 /72

16. Shunt
APF
1
1
sin( ) sin( )L L Lh
h
i I t I h t 

  
,S abci
Supply
voltage
,S abcv
FL
,F abci
dcV
,L abci
1
sin( )F Lh
h
i I h t

 
1 sin( )S Li I t
The control target is to make the supply
current sinusoidal
Shunt APF must inject a harmonic current as
PWM
Nonlinear
load
acL
LC LR
Nonlinear load such as diode rectifier
consumes harmonic currents
Chapter 3 Operation principle of shunt active power filter (APF)
16 /72

17. Conventional control method of shunt APF:
1. Require load current measurement.
2. Performance depends on:
Harmonic detector and current control
Harmonic detector can be implemented by:
1. Instantaneous p-q power theory
2. HPF, LPF or BPF in synchronous (d-q)
reference frame.
Harmonic detector can make the whole control
scheme more complex or even imprecise!
Chapter 3
Supply
voltage
Nonlinear
load
dcV
FL
,S abci
,F abci
,L abci
acL,S abcv
Fhi
1Fi
dcV
PCC
LC LR
abcS
DC-link
voltage
control
Current
control
Harmonic
detector
PWM
signals
 Elimination of the harmonic detector can simplify the control system and improve
the shunt APF performance.
Conventional control system of shunt APF
17 /72

18. Shunt
APF
,S abci
Supply
voltage
,S abcv
FL
,F abci
dcC
,L abci
PWM
Nonlinear
load
acL
LC LRPower consumed by load:
Chapter 3
S SP jQ L LP jQ
APF APFP jQ
L LP jQ
Power generated by source: S SP jQ
Power compensated by APF: APF APFP jQ
Because DC-link side of shunt APF is
only a capacitor Cdc, PAPF ≈ 0.
 At steady-state condition:
1 1S L S d L dP P I I  
 Control of DC-link voltage is equivalent to power balancing between supply side and load side.
Control of
By control the DC-link voltage (Vdc) and supply current (iS,abc), the shunt APF can properly
operate without load current measurement and harmonic detector.
dcV
1 sin( )S Li I t
Reference current:
sin( )dc Sd S SdV I i I t  
  
Control shunt APF without harmonic detector
18 /72

19. dcV
1C
2C
a b c
+
-
Sdi
PI
controller
+dcV
-
Proposed
PI-VPI
current
controller
,F abv
sj
e 
,S dqi
0Sqi

,S abci
Supply
voltage
,S abcv
FL
,F abci
,L abci
Nonlinear
load
acL LC LR
Features of proposed control scheme:
1. Control scheme consists of DC-link voltage control and supply current control.
2. Load current measurement and harmonic detector are eliminated.
3. Supply current is controlled by proposed PI-VPI current controller.
4. System cost decreased by reducing current sensors and number of power switches.
PLL
s
Chapter 3 Proposed control strategy for shunt APF
19 /72

20.  In three-phase systems, (6n±1)th harmonics are
dominant components.
 In d-q frame, (6n±1)th harmonics become 6nth
 Proposed current controller is designed to
compensate 6nth (n=1..5) harmonic currents
5th
7th
11st
13rd17th
FFT analysis of three-phase
nonlinear load current
19th 23rd25th 29th31st
(6n±1) harmonics
6th
12th
18th
24th
30th
FFT analysis of load current
in d-q frame
6nth harmonics
-60
-40
-20
0
20
40
60
Magnitude(dB)
200 400 600 800 1000 1200 1400 1600 1800 2000
-90
-45
0
45
90
Phase(deg)
Bode Diagram
Frequency (Hz)
6th 12th
18th 24th
30th
2 2
1...5
2
( )
(6 )
rh
PR ph
n s
K s
G s K
s n
 


Transfer function of PR controller
Chapter 3
Li
Characteristics of harmonic currents
20 /72

21. Transfer function
(TF) of PI-R controller
Transfer function of
PI-VPI controller
Chapter 3
1
1 2 2
6...30
2
( )
i rh
PI R p
sh
K K s
G K
s s h


  


2
1
1 2 2
6...30
2
( )
ph rhi
PI VPI p
sh
K s K sK
G K
s s h



  


-100
-50
0
50
100
Magnitude(dB)
500 1000 1500 2000
-180
-90
0
90
Phase(deg)
Bode Diagram
Frequency (Hz)
PI-VPI controllerPI-R controller
1
FL
F F
G
sL R


1
F
F
PI R L
C
PI R L
G G
G
G G




Transfer function of LF filter
Closed loop TF of PI-R and
PI-VPI with LF filter
1
F
F
PI VPI L
C
PI VPI L
G G
G
G G




Proposed current controller
21 /72

22. Chapter 3
2
1
1 2 2
6...30
2
( )
ph rhi
PI VPI p
sh
K s K sK
G K
s s h



  


*
( )( )idc
Sd pdc dc dc
K
i K V V
s

  
Supply
voltage
dcV
FL
1C
2C
a b c
+
-
+
-
0Sqi

*'
Fqv
Sdi Sdi
Sqi
PI
+dcV
LPF
dcV
-
PI-VPI
*'
Fdv
Sqv
Sdv
+
+
+
+
*
,Fv 
*
,F abv
PWM
,S abci
abS
,F abci
Nonlinear
load
acL
LC LR

ab
,L abci
PI-VPI

dq
s
abc
,S dqi
dq
s
,Sv 
,S abcv
,S dqv
PLL
s
abc

BPF
s
dt
s,Sv 
sj
e 
PLL
abc,S abcv , 1Sv 
Sqv, 1Sv 
PI
s
PLLs
Detail of proposed control strategy implemented in DSP
22 /72

23. - Experiments are implemented using DSP
TMS320F28335 of Texas Instrument.
- Supply voltage is generated by an AC Programmable Power Source.
- A three-phase diode rectifier with RL or RLC load is used as a nonlinear load.
Parameters Value
Fundamental voltage 128 V (l-l RMS)
Frequency 60 Hz
Fifth order voltage 7%
Seventh order voltage 5%
Reference DC-link voltage
for six-switch APF
Vdc=220 V
Reference DC-link voltage
for four-switch APF
Vdc=440 V
DC-link capacitor of six-
switch APF
Cdc=1100 µF
DC-link capacitor of four-
switch APF
C1=C2=2200 µF
Filter inductance (LF) 2 mH
Filter resistance (RF) 0.1 Ω
Switching frequency 5 kHz
Nonlinear load
RL = 20 Ω
LL= 1 mH
CL = 2200 μF
Chapter 3 Experimental setup and parameters of shunt APF
23 /72

24. Traditional PI current controller Proposed PI-VPI current controller
Chapter 3
 The proposed PI-VPI current controller offers a good performance in harmonic current
compensation.
Performance of shunt APF with different controllers
24 /72

25. Dynamic response under load change
Chapter 3
Dynamic response when load on
 The proposed current controller offers robust operation and fast dynamic response
with load change.
Dynamic response of shunt APF
25 /72

26. Nonlinear RLC load
Chapter 3
Nonlinear RL load
 Good performance of shunt APF is maintained even under distorted supply voltage.
Performance of APF under distorted supply voltage
26 /72

27. Sai
Fai
Lai
SaFFT of i
1CV
2CV
(10A/di
(10A/d
(10A/d
(200V
(10ms/div)t
Sai
Fai
Lai
SaFFT of i
1CV
2CV
(10A/div)
(10A/div)
(10A/div)
(200V/div)
(10ms/div)t
Nonlinear RL load
Nonlinear RLC load
Chapter 3 Performance of three-phase four-switch shunt APF
27 /72

28. Load types
Load
current
THD
Traditional
PI current
controller
Proposed current controller
Six-switch shunt APF Four-switch shunt APF
Ideal
supply
Distorted
supply
Ideal
supply
Distorted
supply
RL 25.2% 11.3% 1.65% 1.84% 1.77% 1.89%
RLC 30.2% 12.7% 1.72% 1.93% 1.86% 1.97%
 PI current controller is unable to offer a good performance of harmonic
current compensation.
 Proposed current controller provides a good performance with six-
switch and four-switch shunt APF under either ideal or distorted supply
voltage conditions.
Comparison on performance of shunt APF with different
current controllers
28 /72

29.  In this section, an advanced current control scheme with PI-VPI controller is proposed to
effectively compensate the supply current to be sinusoidal despite the distorted load current.
 The absence of harmonic detector and load current measurement does not degrade the shunt
APF performance and help reduce the system cost.
 The supply current’s THD is reduced to less than 2%, which totally complies with IEEE 519-
1992 standard.
 However, shunt APF can only deal with harmonic current. The voltage harmonic in the
supply voltage cannot be fully compensated.
 To overcome this drawback, the series APF is installed together with shunt APF to be UPQC
for simultaneous current and voltage harmonic compensation.
Chapter 3 Conclusions
29 /72

30. Industrial Network and Power Electronics Laboratory
3.B. Current and voltage harmonics
compensations using UPQC
30 /72

31. Hysteresis
voltage
control
Hysteresis
current
control
,S abcv
,f abcv
,F abci
,S abci Harmonic
detector
,f abcv
Harmonic
detector
,L abci,F abci
dcV 
dcV-
+
+
+
Supply
Voltage
dcV
,S abci ,L abci
,L abcv
,S abcv
fL
fC
shL
dcC
Linear,
Sensitive
loads
,Sr abcv
,F abci Nonlinear
load
LC LR
,f abcv
Series APF
Shunt
APF
Major drawbacks of the conventional control strategies:
1. The control system requires harmonic detectors which limit the control performance if they are
not well designed.
2. It needs a large number of sensors. (9 current sensors and 7 voltage sensors)
3. The use of hysteresis controller cause large switching noises, control performance is not good.
Chapter 3
PI
Conventional control strategy of UPQC

32. Advantages of the proposed control strategy:
1. Does not require harmonic detectors, which simplify the control system.
2. Reduces the number of sensors. (3 current sensors and 5 voltage sensors)
3. Resonant controller can enhance control performance of UPQC.
Supply
Voltage
dcV
PI-R
voltage
controller
PI-3R
current
controller
dcV ,S dqi
,L abcv
,L dqv
,L dqv
,S abci
abc
dq
,S dqi
dcV-
+
-
+
-
+
,S abci ,L abci
,L abcv
,S abcv
fL
fC
shL
dcC
Linear,
Sensitive
loads
,Sr abcv
,F abci Nonlinear
load
LC LR
s
Series APF
Shunt
APF
abc
dq
abc
dq
s
s
ss
abc
dq
PI
PLL
Proposed control strategy for UPQC using resonant controllers
32 /72

33. Distorted voltage at supply side
1
1
sin( ) sin( )S S Sh
h
v v t v h t 

  
1
sin( )Sr Sh
h
v v h t

 
1 sin( )L Sv v t 
Chapter 3
Si
Supply
Voltage
fL
fC
dcV
Srv
Series
APF
PWM
,L abcv,S abcv
Nonlinear
load
LC LR
,L dqv
-
+ ,Sr dqv
,L dqv
i
p
K
K
s

6
2 2
2 (6 )
r c
c s
K s
s s

  
2
1
1f f f fL C s R C s +
+ +
,S dqv
+
,Sr dqv
Control of series APF
Compensating voltage injected by series APF
33 /72

34. 1
6 1
sin( ) sin( )L h
h n
i I t I h t 
 
  
A three-phase nonlinear load has odd harmonic
currents with orders (n = 1, 2, 3 …)6 1n 
6 1
sin( )F h
h n
i I h t
 
 
1 sin( )Si I t
,
6
sin( )F dq h
h n
i I h t

 
The control objective is to make the supply
current sinusoidal
Shunt APF must inject a harmonic current as
Or in d-q frame
Shunt
APF
,S abci
Supply
voltage
,S abcv
shL
,F abci
dcV
,L abci
PWM
Nonlinear
load
acL
LC LR
Chapter 3
,S dqi
-
+ ,sh dqv,L dqv
+
+
+
,S dqi1
sh shL s R
+
,L dqi
-
,F dqi
i
p
K
K
s
PI
+dcV
-
dcV
2 2
6,12,18 2 ( )
rh c
h c s
K s
s s h

   

Control of shunt APF
34 /72

35. ( ) ( )
1 ( ) ( )
PI R LC
C
PI R LC
G s G s
G
G s G s




6
2 2
2 (6 )
i r c
PI R p
c s
K K s
G K
s s s

 
   
 
2
1/ ( 1)LC f f f fG L C s R C s  
3
3
( ) ( )
1 ( ) ( )
PI R L
C
PI R L
G s G s
G
G s G s




1/ ( )L sh shG L s R 
3 2 2
6,12,18 2 ( )
i rh c
PI R p
c sh
K K s
G K
s s s h

 


  
 

Analysis of voltage and current controllers
0.5
0.6
0.7
0.8
0.9
1
Magnitude(abs)
200 400 600 800 1000 1200 1400
-90
-45
0
45
Phase(deg)
Bode Diagram
Frequency (Hz)
Resonant
frequency (360Hz)
wc=10
Kr6=500, wc=5
Kr6=2000
Resonant peak
of LC filter
-30
-20
-10
0
10
20
30
Magnitude(dB)
200 400 600 800 1000 1200 1400
-180
-135
-90
-45
0
Phase(deg)
Bode Diagram
Frequency (Hz)
1080Hz
wc=10
Krh=1000
360Hz 720Hz
wc=5
Krh=1000
wc=5
Krh=2000
Chapter 3
35 /72

36. - Simulations are performed by PSIM software.
- Experiments are implemented using DSP TMS 320F28335 of Texas Instrument.
- Supply voltage is generated by a Programmable Power Source.
- A three-phase diode rectifier with RL or RLC load is used as a nonlinear load.
Parameters Value
Fundamental voltage 190 V (l-l RMS)
Frequency 60 Hz
Fifth order voltage 7%
Seventh order voltage 5%
Reference DC-link voltage Vdc=350 V
DC-link capacitor Cdc=2200 µF
Filter inductance (Lsh) 2 mH
Filter resistance (Rsh) 0.1 Ω
LC filter inductance (Lf) 0.5 mH
LC filter capacitance (Cf) 12 µF
Damping resistance (Rf) 0.5 Ω
Switching frequency 5 kHz
Nonlinear load
RL = 30 Ω
LL= 1 mH
CL = 2200 μF
Chapter 3 Simulation results and experimental verifications
36 /72

37. Sav
Lav
Sai
Lai
[100 V/div]
[10 A/div] [10 ms/div]
Sav
Lav
Sai
Lai
[100 V/div]
[10 A/div] [10 ms/div]
PI controllerHysteresis controller
Chapter 3 Simulation results of UPQC with different controllers
37 /72

38. Load
types
Hysteresis
controller
PI controller Proposed control
scheme
νL iS νL iS νL iS
RL load 3.2% 11.3% 6% 14.8% 0.92% 1.61%
RLC load 3.6% 13.7% 6.5% 17.8% 0.98% 1.74%
Sav
Lav
Sai
Lai
[100 V/div]
[10 A/div] [10 ms/div]
Proposed control scheme
Chapter 3
THD of load voltage and supply current with different control methods
Simulation results of UPQC with proposed controller
38 /72

39. Sav
Lav
Sai
Lai
[100 V/div] [5 A/div] [10 ms/div]
Sav
Lav
Sai
Lai
[100 V/div] [5 A/div] [10 ms/div]
PI controller Proposed control scheme
Chapter 3
 The proposed control strategy of UPQC provides a good control performance in
harmonic voltage and current compensations.
Experimental results of UPQC with different controllers
39 /72

40. Sav
Lav
Sai
Lai
Full load50% load
[100 V/div] [5 A/div] [20 ms/div]
Chapter 3
 The proposed control strategy of UPQC offers fast dynamic response with load change.
Dynamic response of UPQC with proposed controller
40 /72

41. Chapter 3
 A novel control strategy for the UPQC is proposed with the aid of resonant
controllers to deal with current and voltage harmonics.
 The proposed control strategy effectively compensate voltage and current
harmonics without requirement of the load current measurement and harmonic
detectors.
 In simulations and experiments, the THD values of the load voltage and the
supply current after compensation are greatly reduced to be less than 2% and 3%,
respectively, to comply with the IEEE 519-1992 standard.
Conclusions
41 /72

42. Chapter 3
 This chapter exhibits the feasibility of the resonant controllers applied in the shunt APF and
UPQC for harmonic compensation application.
 Thanks to the effectiveness of the resonant controllers, the load voltage and the supply current
are compensated to be sinusoidal with the THD values less than 2% and 3%, respectively, to
agree to the IEEE 519-1992 standard.
 Both proposed control algorithms of the shunt APF and UPQC are operated without load
current measurement and harmonic detector. Therefore, the complexity of the control system
and system cost are significantly reduced.
 However, drawbacks of resonant controller are that many resonant controllers must be used to
compensate a large number of harmonic components and digital implementation of the
resonant controllers consume long computation time due to trigonometric functions.
 In order to overcome this limitation, repetitive controller is considered to replace resonant
controller. The feasibility of repetitive controller will be investigated in the next chapter.
Conclusions of Chapter
42 /72

43. 2
1 Necessity of the Research
3
4
5
6
Review of PQ improvement and control objectives
Harmonics compensations using resonant controllers
Harmonics compensations using repetitive controller
Voltage sag and unbalance compensations
Conclusions and Future works
Presentation Outline
Industrial Network and Power Electronics Laboratory 43 /72

44. -10
0
10
20
30
40
50
Magnitude(dB)
200 400 600 800 1000 1200 1400 1600 1800
-90
-45
0
45
90
Phase(deg)
Bode Diagram
Frequency (Hz)
6th 12th 18th 24th
30th 36th
 Traditional RC provides high gain at
every harmonic frequency, so it can
compensate all harmonic components.
 Due to the long time delay of traditional
repetitive controller, it have very slow
dynamic response.
 it should be improved.
Chapter 4
 One single repetitive controller
with the time delay Tp/6 equal to a
bank of resonant controllers tuned
at 6nωs.
2
d p
s
T T


 
  
  ( )E s

rK( ) dsT
Q s e ( )pG s


( )R s ( )Y s
( )RCG s
( )RCU s
Characteristics of the traditional repetitive controller
-10
0
10
20
30
40
50
Magnitude(dB)
50 100 150 200 250 300 350 400 450
-90
-45
0
45
90
Phase(deg)
Bode Diagram
Frequency (Hz)
1st 3rd 5th 7th 9th8th6th4th2nd
( ) ( )
( )
( ) 1 ( )
d
d
sT
s
RC r
RC T
U s K Q s
G s
E s Q s
e
e


 

/6
/6
( ) ( )
( )
( ) 1 ( )
p
p
sT
sT
RC r
RC
U s K Q s e
e
G s
E s Q s


 


45. Main features of the proposed control strategy:
1. A single repetitive controller (RC) can replace a bank of resonant controllers, which can greatly
simplify the control system.
2. A frequency adaptive scheme is developed to maintain good performance of UPQC under
frequency deviations.
Supply
Voltage
dcV
RC
voltage
controller
PI-RC
current
controller
dcV ,S dqi
,L abcv
,L dqv
,L dqv
,S abci
abc
dq
,S dqi
dcV-
+
-
+
-
+
,S abci ,L abci
,L abcv
,S abcv
fL
fC
shL
dcC
,Sr abcv
,F abci Nonlinear
load
LC LR
s
Series APF
Shunt
APF
abc
dq
abc
dq
abc
dq
PI
PLL
s s
ss
Proposed control scheme of UPQC using repetitive controller
45 /72

46. Chapter 4
,S dqi
-
,sh dqv,L dqv
+
+
+
,S dqi1
sh shL s R
+
,L dqi
-
,F dqi
i
p
K
K
s
PI
+dcV
-
+
dcV /6
/6
( )
1 ( )
p
p
sT
r
sT
K Q s e
Q s e



Power stage
,L dqv
-
+,L dqv
2
1
1f f f fL C s R C s 
+
,S dqv
+
,Sr dqv
,Sr dqv
Power stage
+
/6
/6
( )
1 ( )
p
p
sT
r
sT
K Q s e
Q s e



+
Control scheme for shunt APF
Control scheme for series APF
Detail on control of shunt and series APFs
46 /72

47. If Tp changes (grid frequency varies), N can be a non-integer value.
We can not implement z-N if N is a non-integer number.
The delay function in RC is rearranged as:
1
2 ( )
1
2
fr s
fr s
N T s
fr s
N T
s
e C s
N T
s


 

1
1
(1 ) (1 )
( )
(1 ) (1 )
fr fr
fr fr
N N z
C z
N N z


  

  
Ni is integer part of the delay samples N, Nfr is fraction part of N
(for example N=60.3, Ni=60, Nfr=0.3)
i s iN T s N
e z 

/6 ( )p i fr s fr ss i s
sT N N T s N T sNT s N T s
e e e e e
    
  
6
p
i fr
s
T
N N N
T
  
/6
/6
( ) ( )
( )
( ) 1 ( )
p
p
sT
RC r
RC sT
U s K Q s e
G s
E s Q s e


 

RC in
s-domain
Chapter 4
6
p
s
T
N
T
 
 
 
RC in
z-domain
( )( )
( )
( ) 1 ( )
N k
r
RC N
K Q z z zU z
G z
E z Q z z


 

Where:
Q(z) is a filter,
zk is a phase lead term,
and Kr. is controller gain.
Repetitive controller with frequency adaptive scheme
(4.13)
47 /72

48. ( ) ( ) ( )
( )
( ) 1 ( ) ( )
i
i
N k
RC r
RC N
U z K Q z C z z z
G z
E z Q z C z z


 

Chapter 4
s
PLL
,S abcv 2
s


pT
6
p
s
T
N
T

iN
frN
Eq. (4.13)
( )C z
( )E z ( )U z
( )P
G z


( )R z ( )Y z
( )D z


( )RCG z


k
rz K( ) ( )iN
Q z z C z
1
i
p
K z
K
z

 

1
1
(1 ) (1 )
( )
(1 ) (1 )
fr fr
fr fr
N N z
C z
N N z


  

  
(4.13)
Repetitive controller with frequency adaptive scheme
48 /72

49. PI controller PI-3R controller
Lai
Lav
Sai
[10 A/div]
[100 V/div]
[10 A/div]
Sav [100 V/div]
t [10ms/div]
Lai
Lav
Sai
[10 A/div]
[100 V/div]
[10 A/div]
Sav [100 V/div]
t [10ms/div]
Chapter 4 Performance of UPQC with different controllers
49 /72

50. PI
controller
PI-3R
controller
Proposed PI-
RC controller
THD of νL 6.37% 1.27% 0.64%
THD of iS 12.7% 3.65% 1.57%
Calculation time 41μs 95μs 51μs
Proposed PI-RC controller
Lai
Lav
Sai
[10 A/div]
[100 V/div]
[10 A/div]
Sav [100 V/div]
t [10ms/div]
Chapter 4 Performance of UPQC with proposed controller
50 /72

51. Grid frequency = 49.5 Hz Grid frequency = 50.5 Hz
 Without frequency adaptive scheme, the UPQC is unable to provide a good performance
under grid frequency different from 50 Hz.
Performance of the UPQC without frequency
adaptive scheme
Chapter 4
51 /72
Lai
Lav
Sai [5 A/div]
[50 V/div]
[5 A/div]
Sav [50 V/div]
t [20ms/div]
Lai
Lav
Sai [5 A/div]
[50 V/div]
[5 A/div]
Sav [50 V/div]
t
[20ms/div
]

52. Grid frequency = 49.5 Hz Grid frequency = 50.5 Hz
 With frequency adaptive scheme, the UPQC provides a good performance at all test cases.
Performance of the UPQC with frequency adaptive
scheme
Chapter 4
52 /72
Lai
Lav
Sai [5 A/div]
[50 V/div]
[5 A/div]
Sav [50 V/div]
t [20ms/div]
Lai
Lav
Sai [5 A/div]
[50 V/div]
[5 A/div]
Sav [50 V/div]
t [20ms/div]

53. Without frequency-adaptive
scheme
With frequency-adaptive scheme
Frequency 49.5 Hz 50 Hz 50.5 Hz 49.5 Hz 50 Hz 50.5 Hz
THD of νL 5.49 % 0.64 % 4.35 % 0.88 % 0.64 % 0.77 %
THD of iS 16.65 % 1.57 % 12.45 % 1.75 % 1.57 % 1.63 %
Chapter 4
 Without frequency adaptive scheme, the UPQC provides a good performance only
at 50 Hz.
 With frequency adaptive scheme, the UPQC provides a good performance at all test
cases.
Performance of UPQC under different grid frequencies
53 /72

54. Chapter 4
 In this chapter, a simplified and effective solution for harmonic compensations was
proposed by using the modified RC which has reduced time delay and the frequency
adaptive function.
 The UPQC with the proposed RC is able to effectively compensate the load voltage
and the supply current to be sinusoidal irrespective of the distortions of the supply
voltage and the load current.
 The excellent compensation performance of the UPQC is maintained despite the grid
frequency deviations thanks to the frequency-adaptive scheme.
 The effectiveness of the proposed control method was verified through experiments:
the load voltage and supply current are compensated to be sinusoidal with THD
values less than 1% and 2%, respectively, in all test cases.
Conclusions of Chapter
54 /72

55. 2
1 Necessity of the Research
3
4
5
6
Review of PQ improvement and control objectives
Harmonics compensations using resonant controllers
Harmonics compensations using repetitive controller
Voltage sag and unbalance compensations
Conclusions and Future works
Industrial Network and Power Electronics Laboratory 55 /72

56. ( )L S pre sagV V 
1SV
1SrV
2SrV
2SV
LocusLV

1.0pu
s
( )s 
1.0pu
Chapter 5
Balanced voltage sag
Unbalanced voltage sag
Voltage sag could be a balanced or a unbalanced sag, or even with or without a phase jump.
In any situation, the UPQC must compensate voltage sag to maintain the load voltage
balanced and sinusoidal at nominal amplitude.
Operation of UPQC for voltage sag compensation
56 /72

57. Chapter 5
Main features of the proposed control strategy:
1. With PR controller at the series APF control, the UPQC is able to compensate both balanced and
unbalanced voltage sag.
2. The proposed control strategy can effectively deal with various power quality problems as voltage
sag, unbalance, and distortions as well as current harmonics at the load side.
Proposed control strategy for UPQC
Supply
Voltage
dcV
PR-R
voltage
controller
PI-VPI
current
controller
dcV ,S dqi
,L abcv
,L dqv
,L dqv
,S abci
abc
dq
,S dqi
dcV-
+
-
+
-
+
,S abci ,L abci
,L abcv
,S abcv
fL
fC
shL
dcC
Linear,
Sensitive
loads
,Sr abcv
,F abci Nonlinear
load
LC LR
s
Series APF
Shunt
APF
abc
dq
abc
dq
s
s
ss
abc
dq
PI
SOGI
PLL

58. dq

,Lv 
-
+ ,Srv 

,L dqv
1
2 2
2
r c
p
c s
K s
K
s s

 

 
6
2 2
2 (6 )
r c
c s
K s
s s

  
2
1
1f f f fL C s R C s 
+
+
+
,Sv 
+
dq

,L dqv
dq

,Srv 
Resonant controller for harmonic
voltage compensation
PR controller for voltage sag and
unbalance compensation
Chapter 5
0
0.5
1
1.5
Magnitude(abs)
50 100 150 200 250 300 350 400 450
-135
-90
-45
0
45
90
Phase(deg)
Bode Diagram
Frequency (Hz)
Selected resonant
frequency (60Hz)
Selected resonant
frequency (360Hz)
C=Cf
L=Lf
C=Cf*2
L=Lf*2
L=Lf/2
C=Cf/2
Control scheme on series APF
58 /72

59. VPI controller for harmonic
current compensation
Chapter 5
0
0.2
0.4
0.6
0.8
1
Magnitude(abs)
200 400 600 800 1000 1200 1400
-90
-45
0
45
90
Phase(deg)
Bode Diagram
Frequency (Hz)
Kph=5
Kph=0.5
Kph=2
Resonant frequency
,S dqi
-
+ ,F dqv,L dqv
+
+
+
,S dqi1
sh shL s R
+
,L dqi
-
,F dqi
i
p
K
K
s

2
2 2
6,12,18 ( )
ph rh
h s
K s K s
s h



Control scheme on shunt APF
59 /72

60. ,S abcv
,Sr abcv
,L abcv
(50V/div)
(50V/div)
(50V/div)
(50ms/div)t
Voltage sag period
,S abcv
,Sr abcv
,L abcv
(50V/div)
(50V/div)
(50V/div) (50ms/div)t
Voltage sag period
Chapter 5 Performance of UPQC under unbalanced voltage sag
Conventional PI voltage controller Proposed PR-R voltage controller
60 /72

61. ,S abcv
,Sr abcv
,L abcv
[50V/div]
[50 ms/div]
Voltage sag period
[50V/div]
[50V/div]
,S abcv
,Sr abcv
,L abcv
[50V/div]
[50 ms/div]
Voltage sag period
[50V/div]
[50V/div]
Chapter 5 Voltage sag compensation under distorted supply
61 /72

62. Sav
Srav
Lai
[50V/div]
[10A/div]
Fai
Lav
Sai
Voltage sag period
[50V/div]
[50V/div]
[10A/div]
[10A/div]
Voltage sag, distortion, and current harmonic compensation
62 /72

63. Chapter 5
 In this chapter, an enhanced control strategy is proposed to deal with voltage sag and
unbalance problems.
 The proposed control method effectively tackle balanced and unbalanced voltage sag with
phase jump so that the load voltage is maintained balanced and sinusoidal at rated value.
 The proposed control strategy also simultaneously deal with voltage sag, unbalance, and
current distortions to maintain the load voltage and supply current balanced and sinusoidal.
 The feasibility of the proposed control strategy was verified through various experimental
tests.
Conclusions of Chapter
63 /72

64. 2
1 Necessity of the Research
3
4
5
6
Review of PQ improvement and control objectives
Harmonics compensations using resonant controllers
Harmonics compensations using repetitive controller
Voltage sag and unbalance compensations
Conclusions and Future works
Industrial Network and Power Electronics Laboratory 64 /72

65. Chapter 6
From the results achieved by the proposed control strategies, the main conclusions of this thesis
are summarized as below.
 Resonant controller is a suitable solution to deal with the current and voltage harmonics. The
proposed control algorithm developed for shunt APF and UPQC using resonant controller
effectively compensates the load voltage and the supply current to be almost sinusoidal with a
low THD less than 2% and 3%, respectively, which sufficiently agree to the IEEE 519-1992
standard.
 In addition to resonant controller, repetitive controller is also an effective solution for the
current and voltage harmonic compensation. One repetitive controller can replace a bank of
resonant controller, hence the control system of the UPQC is significantly simplified. The
proposed control strategy with repetitive controller compensate current and voltage harmonics
to make the load voltage and the supply current sinusoidal with a very low THD values about
1% and 2%, respectively, which completely satisfy the IEEE 519-1992 standard.
Conclusions of Thesis
65 /72

66. Chapter 6
 Apart from harmonic compensation, this thesis also take into account the voltage sag and
voltage unbalance issues. An enhanced control strategy is proposed to simultaneously tackle
voltage sags, unbalance, and distortions on the supply side as well as current harmonics on the
load side so that the load voltage is balanced and sinusoidal at rated value regardless of the
abnormal condition of supply voltage. Therefore, various power quality problems are
effectively compensated with a unified control algorithm.
 All suggested control strategies are developed and properly operated without demand of load
current measurement and harmonic detector. Therefore, they are more effective and simpler
control methods compared to conventional ones.
Conclusions of Thesis (cont.)
66 /72

67. Chapter 6
Although various power quality problems have been successfully tackled by the proposed control
methods for the UPQC in this thesis, there are some possible problems that need to be considered
and solved in the future research. They can be suggested as:
1. Developing intelligent and adaptive control for UPQC to optimize control objectives under
different conditions of supply voltage and loads.
2. Investigating the operation of UPQC for the power quality enhancement in the micro-grid
system.
3. Development of UPQC topologies that can offer low cost and high efficiency features for
high power applications.
Future Works
67 /72

68. Journal Articles:
1. Q.-N. Trinh and H.-H. Lee, “An Enhanced Grid Current Compensator for Grid-Connected
Distributed Generation under Nonlinear Loads and Grid Voltage Distortions,” accepted for publication
in IEEE Transactions on Industrial Electronics.
2. Q.-N. Trinh and H.-H. Lee, “An Advanced Current Control Strategy for Three-Phase Shunt
Active Power Filters,” IEEE Transactions on Industrial Electronics, vol.60, no.12, pp.5400-5410,
Dec. 2013.
3. Q.-N. Trinh, H.-H. Lee, T.-W. Chun, “An Enhanced Harmonic Voltage Compensator for General
Loads in Stand-alone Distributed Generation Systems,” Journal of Power Electronics, vol.13, no.6,
pp.1070-1079, Nov. 2013.
4. Q.-N. Trinh and H.-H. Lee, “Advanced Repetitive Controller to Improve the Voltage Characteristics of
Distributed Generation with Nonlinear Loads,” Journal of Power Electronics, vol.13, no.3, pp.409-
418, May 2013.
5. Q.-N. Trinh and H.-H. Lee, “Novel Control Strategy for a UPQC under Distorted Source and
Nonlinear Load Conditions,” Journal of Power Electronics, Vol.13, No.1, pp.161-169, Jan. 2013.
(Best journal paper award of JPE in 2013)
6. Q.-N. Trinh and H.-H. Lee, “A New Z-Source Inverter Topology with High Voltage Boost
Ability,” Journal of Electrical Engineering & Technology, Vol. 7, No. 5, pp. 714-723, Sept. 2012.
Publications
68 /72

69. Journal Articles (cont.):
7. Q.-N. Trinh and H.-H. Lee, “Improvement of UPQC Performance with Enhanced Resonant Control
Strategy,” under second review on IET Generation, Transmission, and Distribution.
8. Q.-N. Trinh and H.-H. Lee, “Digital Implementation of Frequency Adaptive UPQC Control System
with Modified Repetitive Controller,” submitted to IEEE Transactions on Industrial Electronics.
9. Q.-N. Trinh and H.-H. Lee, “Implementation of Low Cost High Performance UPQC with Four-
Switch Three-Phase Inverters,” submitted to Journal of Power Electronics.
Conference Articles:
1. Q.-N. Trinh and H.-H. Lee, “Harmonic Currents and Reactive Power Compensation with
Novel Shunt Hybrid Power Filter,” accepted for publication in Proc. International Conference on
Electrical Engineering (ICEE) 2014, Jeju, Korea, Jun. 2014.
2. Q.-N. Trinh and H.-H. Lee, “Improvement of Grid Current Performance for Grid-Connected
DG under Distorted Grid Voltage and Nonlinear Local Loads,” accepted for publication in Proc.
International Symposium on Industrial Electronics (ISIE) 2014, Istanbul, Turkey, Jun. 2014.
3. Q.-N. Trinh and H.-H. Lee, “An Enhanced Current Control Strategy for Three-Phase Shunt Active
Power Filters with Repetitive Controllers,” in Proc. IEEE International Conference on Electrical
Machines and Systems (ICMES) 2013, pp.1543-1548, Oct. 2013.
Publications
69 /72

70. Conference Articles (cont.):
4. Q.-N. Trinh and H.-H. Lee, “A Repetitive Control Scheme to Improve Performance of UPQC under
Distorted Source and Nonlinear Load Conditions,” in Proc. IEEE International Power and Energy
Conference (IPEC) 2012, pp. 418-423, Dec. 2012.
5. Q.-N. Trinh and H.-H. Lee, “Improvement of power quality under distorted source and nonlinear load
conditions,” in Proc. IEEE 7th International Power Electronics and Motion Control Conference
(IPEMC), 2012, pp.546-551, Jun. 2012.
6. Q.-N. Trinh and H.-H. Lee, “Improvement of Current Performance for Grid Connected Converter
under Distorted Grid Condition,” in Proc. of The IET Renewable Power Generation Conference
2011, pp. 95-102, Sep. 2011.
7. Q.-N. Trinh and H.-H. Lee, “A new Z-source inverter topology to improve voltage boost ability”, in
Proc. IEEE 8th International Conference Power Electronics and ECCE Asia (ICPE & ECCE), 2011,
Jun. 2011.
8. Q.-N. Trinh and H.-H. Lee, “Z-source inverter based grid connected for PMSG wind power system,” in
Proc. IFOST 2010, pp. 145 – 150, Oct. 2010.
9. Q.-N. Trinh and H.-H. Lee, “Maximum Power Point Tracking in PMSG Using Fuzzy Logic
Algorithm”, in Proc. 6th International Conference on Intelligent Computing, ICIC 2010, Changsha,
China, pp. 543-553, Aug. 2010.
Publications
70 /72

71.  I want to thank my supervisor, Professor Hong-Hee Lee for his support, encouragement,
and helpful advices for me to proceed through the doctoral program and complete my PhD
thesis at University of Ulsan.
 Special thanks to all Professors in Committee Members:
Professor Tae-Won Chun
Professor Dong-Choon Lee
Professor Jin Hur
Professor Sung-Jin Choi
for your precious time, guidance and helpful suggestions on my PhD thesis.
 I also would like to thank all my lab-mate and great friends for helping me overcome
difficult time during this long journey.
Acknowledgement
71 /72

72. Industrial Network and Power Electronics Laboratory
Q&A
Thank you for your attention!
Q & A
72 /72