Power Quality Control in Hybrid AC/DC
Microgrid Systems
Yunwei (Ryan) Li 李运帷
PhD, P.Eng, Professor
Department of Electrical and Computer Engineering
University of Alberta
Email: yunwei.li@ualberta.ca
Web: www.ece.ualberta.ca/~yunwei1
1
Introduction
University of Alberta
Location:
Edmonton
Alberta, Canada
Founded in 1908
18 Faculties
400 programs
U of A
ECE
research
building
Donadeo Innovation Centre of Engineering
Faculty of Engineering:
4000 undergraduate
1600 graduate students
2
Introduction
Topics
1. Hybrid AC/DC Microgrids
- Hybrid AC/DC microgrids structure
- Power quality in hybrid AC/DC microgrid
- Interfacing converters and control
2. Power Quality Control Through Interfacing Converters
- AC and DC grids support
- Unbalance compensation
- Harmonics control
3
1.1 Hybrid AC/DC Microgrid
Future Power Systems
Source: www.clean-coalition.org
4
1.1 Hybrid AC/DC Microgrid
AC Coupled Microgrid
Hybrid Microgrid
AC Bus
DC
Distributed
Generation
DC/AC
AC
Distributed
Generation
AC/DC/AC
Storage
Element (if
necessary)
Bidirectional
Converter
Utility
Grid
AC/DC
DC Load
AC Load
.
.
.
 AC-coupled microgrid is the dominant structure now due to its
simple structure and simple control and power management
scheme.
 High frequency AC bus/link is possible.
5
1.1 Hybrid AC/DC Microgrid
DC Coupled Microgrid
Hybrid Microgrid
DC Bus
DC
Distributed
Generation
DC/DC
AC
Distributed
Generation
AC/DC
Storage
Element (if
necessary)
Bidirectional
Converter
.
.
.
Interlinking
Converters
(ILCs)
AC Bus
Utility
Grid
DC/AC
DC/AC
.
.
.
DC/AC
DC Load
AC Load
AC Load
 Interlinking converters (ILCs) are used to link the DC and AC buses.
 Variable frequency AC load can be connected to DC bus.
 The DC-coupled microgrid features simple structure and does not need
any synchronization when integrating different DGs.
 Some SEs can be connected to DC bus directly without converters
6
1.1 Hybrid AC/DC Microgrid
Hybrid AC/DC Coupled Microgrid
Hybrid Microgrid
DC Bus
DC
Distributed
Generation
Interlinking
Converters
(ILCs)
Utility
Grid
AC/DC/AC
AC
Distributed
Generation
AC/DC
DC
Distributed
Generation
Bidirectional
Converter
Storage
Element (if
necessary)
DC/DC
DC/AC
DC/AC
AC
Distributed
Generation
AC/DC
.
.
.
Storage
Element (if
necessary)
Bidirectional
Converter
DC/AC
.
.
.
AC Bus
DC Load
.
.
.
AC Load
AC Load
 Both DC and AC buses have DGs and SEs, and these buses are linked
by interlinking converters (ILC).
 Probably the most promising microgrid structures in the near future.
 Requires more coordination between the DC and AC subsystems.
7
1.2 Power Quality
General residential distribution feeder in North America
Distribution system loads:
 Distributed loads
 Single phase loads
 Nonlinear loads – energy efficient loads.
8
1.2 Power Quality
50
Harmonic Load Current (%)
40
Appliance
1st
3rd
5th
7th
9th
11th
13th
CFL
LED
100
100
87
73
60
26
47
27
47
58
40
15
26
16
PC
100
78
49
19
12
14
14
30
20
10
0
2010 2012 2014 2016 2018 2020 2022
Household CFLs growth prediction
Data: Lawrence Berkeley National Lab
Harmonics of some nonlinear loads
(typically THD > 100%)
Salles et al, “Assessing the Collective Harmonic
Impact of Modern Residential Loads”, IEEE Trans.
Power Delivr. vol. 27, no. 4, pp. 1937-1946, Oct.
2012.
Nonlinear loads are rapidly increasing in distribution systems
 More efficient loads (CFL, LED, ASD fridge, LED, high efficiency washer,
etc.) are increasingly adopted
 Many of them produce high harmonics
 Residential system voltage THD in North America will reach 5% soon
9
1.2 Power Quality
The power quality issues are becoming urgent for future
distribution systems:
 DC subgrid voltage variations and harmonics
 AC subgrid voltage variations:
 Frequency deviation
 Voltage magnitude change (particularly with DG)
 AC subgrid voltage unbalance:
 Utilities in Canada already experienced tripping of loads due to severe
unbalance
 AC subgrid voltage harmonics.
 Distribution system voltage THD is deteriorating
 PFC capacitors further complicate the situation
 Traditional centralized power quality compensation does not
work well
10
1.3 Interfacing Converters
DC/AC interfacing converters
DG/SE
(PV,
Fell cell,
battery)
DC
AC
AC/AC interfacing converters
AC
subsystem
or AC grid
Micro- or
wind
turbine
Single stage DC-AC
DG/SE
(PV,
Fell cell,
battery)
DC
DC
AC
DC
DC
AC
AC
subsystem
or AC grid
Two stage (AC-DC-AC)
AC
subsystem
or AC grid
DC
AC
Double stage (DC-DC and DC-AC)
DC/AC
Distributed
Generation
DC/AC
Distributed
Generation
.
.
Micro- or
wind
turbine
AC
DC
DC
DC
DC
AC
AC
subsystem
or AC grid
Multiple stage conversion
Multi-port converters
Multiple-Port
Power
Converter
AC or DC
subsystems
&
loads
11
1.3 Interfacing Converters
Control of Interfacing Converters
- Manly for Real and Reactive Power Flow
The DC-AC interfacing converters (including DC-AC bus
interlinking converter) can work on:
 Bi-directional power control mode:
 Current control method (CCM) – mostly used for grid-tied
converters
 Voltage control method (VCM) – droop control, virtual
synchronous generator.
 DC link voltage control mode (for DC-AC bus interlinking
converter): balancing the power generation and consumption
on DC bus.
 AC link voltage control mode: mainly for stand-alone microgrid
operation.
12
Topic 2
Power Quality Control using
Interfacing Converters
 Renewable energy based DG
systems, energy storage
system, and DC-AC
interlinking converter usually
have available rating for
ancillary services such as:
 Grid support
 Unbalance compensation
 Harmonics compensation
Generator
Distribution
transformer
Substation
transformer
Primary feeder
Distribution
line
PV
panel
Residential loads
 Ancillary services for DG/SE and microgrid systems are becoming an
important issue that can further improve the system cost effectiveness
 Distributed compensation is more effective than centralized compensation
due to the distributed loads
13
2.1 Grid Support
AC Grid Support Functions
The grid-supporting power converter can be controlled as a current source
(CCM) or voltage source (VCM).
*
P*
*
E
*
Q
+-
Kp
++
*
Pinv
PCC

+-
Vd
Reference
Current
Generator
Kq
++
*
Qinv
I*
+-
Current
controller
PWM
I inv
Z
Inverter
Vd

PLL
Grid
VPCC
Grid voltage amplitude and frequency support control: CCM
 The frequency variation is controlled by the active power while the reactive power
controls the amplitude of voltage.
 The frequency and voltage controllers are proportional controllers (𝐾𝑝 and 𝐾𝑞 ) for
realizing the inverse 𝑃 − 𝑓 and 𝑄 − 𝑉 droop control.
14
2.1 Grid Support
AC Grid Support Functions
P*

*
Q*
E*
+-
Kp
+


PCC
Space
vector to
abc
Pm
+-

Kq
+
V
V*
+-
Voltage
controller
PWM
Inverter
Z
Grid
VPCC
Qm
Grid voltage amplitude and frequency support control: VCM
 When the power converter works as a controllable voltage source, a linking
impedance (physical or virtual) is necessary between the converter and grid.
 The active and reactive power controllers are proportional controllers (𝑘𝑃 and 𝑘𝑞 )
for realizing 𝑃 − 𝑓 and 𝑄 − 𝑉 droop control.
15
2.1 Grid Support
AC/DC Grid Support
Example on ILC control for AC/DC grid support
DC subsystem
droop
f P.U
VP.U
1
Loh et al, “Autonomous Operation of Hybrid
Microgrid With AC and DC Subgrids”, IEEE
Trans. Power Electron. vol. 28, no. 5, pp.
2214–2223, May. 2013
AC subsystem
droop
1
Pdc*
vP.U
Interlinking
Converter
Pdc
1
Pac*
f P.U
Equalizing the AC side
normalized frequency
and DC side normalized
voltage and determine
the ILC power reference
Pac
1
Controlling
v P .U  f P .U
Pdc _ max
𝑉𝑑𝑐, 𝑃.𝑈. =
Pac _ max
𝑉𝑑𝑐 − 0.5 × 𝑉𝑑𝑐,𝑚𝑎𝑥 + 𝑉𝑑𝑐,𝑚𝑖𝑛
𝑓𝑃.𝑈. =
0.5 × 𝑉𝑑𝑐,𝑚𝑎𝑥 − 𝑉𝑑𝑐,𝑚𝑖𝑛
Vdc
VPCC
Normalization

PLL
f
Normalization
Vdc, P.U
+-
fP.U
e
KP
𝑓 − 0.5 × 𝑓𝑚𝑎𝑥 + 𝑓𝑚𝑖𝑛
0.5 × 𝑓𝑚𝑎𝑥 − 𝑓𝑚𝑖𝑛
P*
Q
*
i*
Reference
current
*
Calculator i
Control block diagram of one ILC
A possible improvement is to
consider AC & DC grid stiffness
PCC

PWM
abc
Interlinking
Inverter
Z
Grid
ILC works in power control mode
16
2.2 Unbalance Compensation
 Unbalance voltage affects power equipment, introduces double-frequency
oscillations on DC bus, increase converter AC current peak/harmonics
 Negative sequence current can be generated through interfacing converter to
reduce negative sequence voltage at PCC
Equivalent circuit for unbalance compensation
i

R
Z 
v
2
2
P(1  k1 )

X
Z 
v e
j

2
Q(1  k2 )
ILC negative sequence
virtual impedance
Z  

iGrid
PCC

v



Load
i
Z

X Grid
Z 
 Grid
2
e
j

2
Z
X Grid

RGrid
Z 
 Grid
2
RGrid
Grid negative sequence
impedance
Z 

Grid
17
2.2 Unbalance Compensation
Unbalance Voltage Compensation Control – CCM based
P*
*
Q
Reference
Current
Generator
I*
+-
PCC
αβ
PWM
*
I
Current
controller
+-
Current
controller
Iβ
Vβ+ Vβ-
Sequence
Extractor
Z
Grid
abc
Iα
Vα+ Vα-
Inverter
Vα
αβ
abc
Iinv
αβ
abc
VPCC
Vβ
 Reference power and positive and negative sequence voltages generate the
reference currents of inverter:
𝐼𝛼∗ = 𝑃∗
k1 + (1 − k1 ) −
𝑉 +
𝑉 + 𝑄∗
v+ 2 𝛼
v− 2 𝛼
k 2 + (1 − k 2 ) −
𝑉 +
𝑉
v+ 2 𝛽
v− 2 𝛽
𝐼𝛽∗ = 𝑃∗
k1 + (1 − k1 ) −
𝑉 +
𝑉 − 𝑄∗
v+ 2 𝛽
v− 2 𝛽
k 2 + (1 − k 2 ) −
𝑉 +
𝑉
v+ 2 𝛼
v− 2 𝛼
 k1 and k 2 provide flexible control of positive and negative sequences while
adjusting power oscillation, max current, etc..
18
2.2 Unbalance Compensation
Unbalance Voltage Compensation Control
Control scheme switched from positive-sequence current injection to
unbalance compensation (𝑘1 and 𝑘2 adjusted to reduce real power oscillation)
Switching the
control strategy
Switching the
control strategy
V   4.7V
V   3.9V
p  30W
Positive-sequence
current injection
p  0W
Unbalance
compensation
19
2.2 Unbalance Compensation
Parallel Interfacing Converters under Unbalanced Voltage:
Parallel Interfacing Converters
ILC1
i1
Output
Filter
i2
Output
Filter
in
Output
Filter
PCC
iIFCs
ILC2
DC
Subsystem/Bus
v
AC
Subsystem/Bus
...
ILCn
Parallel interfacing converters with
common DC and AC links
 The active-reactive powers oscillations of 𝑖 𝑡ℎ -ILC can be controlled using
scalar coefficients 𝑘𝑝𝑖 and 𝑘𝑞𝑖 :
∗
∗
𝑖𝑖∗ = 𝑖𝑝𝑖
+ 𝑖𝑞𝑖
=
𝑣+ 2
𝑃𝑖 𝑘𝑝𝑖
𝑃𝑖
𝑣+ + + 2
−
2
+ 𝑘𝑝𝑖 𝑣
𝑣
+ 𝑘𝑝𝑖 𝑣 −
2
𝑣− +
𝑣+ 2
𝑄𝑖 𝑘𝑞𝑖
𝑄𝑖
𝑣⊥+ + + 2
−
2
+ 𝑘𝑞𝑖 𝑣
𝑣
+ 𝑘𝑞𝑖 𝑣 −
2
𝑣⊥−
 For n-parallel ILCs, the power (DC bus) oscillations caused by the
converters can be properly utilized to cancel each other
 A redundant ILC to cancel the effects of other ILCs
 All ILCs participate according to their available ratings
20
2.2 Unbalance Compensation
Parallel Interfacing Converters under Unbalanced Voltage
Three ILCs with rated power: S1=9kVA, S2=4kVA, S3=10kVA
From positive-seq current injection to unbalance comp with peak current sharing
ILC-1 output active power
ILC-2 output active power
ILC-3 output active power
ILC-3
ILC-1
ILC-2
ILCs collective active power
Peak currents of ILCs
PCC negative-seq voltage
21
2.3 Harmonics Compensation
Harmonics Compensation
 Nonlinear loads cause PCC voltage distortions, especially in a weak grid.
 Interlinking converter (or DG interfacing converter) can absorbed the nonlinear
load current and improve the PCC voltage quality.
 The converter side impedance should be controlled
Equivalent Circuit
VG
ZDG (s)
ZGRID (s)
ZLoad (s)
ILC
VGrid (s)
ILoad (s)
PCC Load
Grid
22
2.3 Harmonics Compensation
Harmonics Compensation - CCM based control
PRef
QRef
PCC
IDG_ref
P and Q IDG_f_ref
++
+Controllers
IDG_h_ref
Pfb
Qfb
1/Rh
Current
controller
PWM
Interfacing
inverter
Z
Grid
Ifb
VPCC_h
For
Synchronization
VPCC_f
Fundamental
and Harmonics
detection
VPCC
 Harmonic compensation achieved by controlling the converter as a shunt active
power filter (APF)
 Fundamental current reference is generated by the output power control
 Reference harmonic current produced by grid side voltage harmonics (VPCC_h)
and a virtual resistance Rh .
 The DG acts as a small resistance at the harmonic frequency (R-APF).
 Rh can be adaptively controlled according to the available converter rating (to
avoid interference with primary function of real power generation)
23
2.3 Harmonics Compensation
Harmonics Compensation - CCM based control
Without compensation
With compensation
(a)
0
0.22
(b)
0.2
0.22
Mag (% of Fundamental)
(c)
0.18
0.2
0.22
0.24
Time
(s) phase voltage,
(a)
PCC
10
PCC voltage harmonics
5
0
0
5
10
15
20
Harmonic order
25
30
0.14
0.16
0.18
0.2
0.22
0.24
0.14
0.16
0.18
0.2
0.22
0.24
0.14
0.16
0.18
0.2
Time (s)
ILC current.
0.22
0.24
5
0
-5
0.24
A
0.18
0
-100
0.24
A
0.2
A
0.18
A
-100
0.16
5
(b)
0
-5
0.16
5
(c)
0
-5
0.16
V
100
5
0
-5
(b) Grid current, (c)
Mag (% of Fundamental)
(a)
V
100
10
PCC voltage harmonics
5
0
0
5
10
15
20
Harmonic order
25
30
24
2.3 Harmonics Compensation
Harmonics Compensation - VCM based control
PRef
+-
P
Controller
ω
1
s
Pfb
θ
PCC
Space
vector
to abc
Vf_ref
++
VDG_ref
+-
Voltage
controller
PWM
Interfacing
inverter
Z
Grid
Vh_ref
Vfb
QRef
+-
Qfb
Q
Controller
Vmag
G
VPCC_h
For
Synchronization
VPCC_f
Fundamental
and Harmonics
detection
VPCC
 Converter harmonic voltage is controlled as Vh _ ref  G  VPCC _ h
 Then the equivalent harmonic impedance at converter side: Z ILC _ eq  Z ILC / (1  G )
 Harmonic impedance at converter side can be controlled substantially lower than
that at grid side – nonlinear load currents flow to converter
.
25
2.3 Harmonics Compensation
Harmonics Compensation - VCM based control
Harmonic compensation (G>0)
0.72
0.74
0.76
0.78
(b)
0
-100
0.7
0.72
0.74
0.76
0.78
0.8
2.34
2.36
2.38
0
(b)
2.32
2.34
2.36
2.38
0.72
0.74
0.76
0.78
0
(c)
A
(d)
2.32
2.34
2.36
2.38
0.7
0.72
0.74
0.76
0.78
1.6
1.52
1.54
1.56
1.58
1.6
V
1.5
1.52
1.54
1.56
1.58
1.6
1.52
1.54
1.56
1.58
1.6
5
0
(d)
0
-5
2.3
0.8
1.58
0
2.4
-5
-5
1.56
-5
5
0
1.54
5
2.3
0.8
1.52
0
-100
1.5
2.4
-5
0.7
0
-100
1.5
100
2.4
A
A
A
(c)
0
5
A
2.32
5
-5
(d)
(a)
-100
2.3
5
(c)
100
0
-100
2.3
100
0.8
Harmonic rejection (G=-1)
A
(b)
V
-100
0.7
100
V
V
(a)
(a)
0
V
100
V
Uncontrolled (G=0)
100
2.32
2.34
2.36
2.38
2.4
1.5
Time (S)
Time (S)
Time (S)
(a) PCC phase voltage, (b) ILC phase voltage, (c) Grid current, (d) ILC current.
6
4
2
0
0
5
10
15
Harmonic order
20
25
30
10
10
Mag (% of Fundamental)
PCC voltage harmonics
8
Mag (% of Fundamental)
Mag (% of Fundamental)
10
PCC voltage harmonics
8
6
4
2
0
0
5
10
15
20
Harmonic order
25
30
PCC voltage harmonics
8
6
4
2
0
0
5
10
15
20
Harmonic order
25
26
30
Conclusions
 With more controllability and flexibility in a microgrid system, valuable ancillary
functions can be provided for better grid operation and better power quality.
 Voltage support is the most widely implemented ancillary functions now.
 Unbalance and harmonics compensation is becoming more important with the
increasing single phase and nonlinear loads - virtual impedance control can
facilitate the unbalance and harmonics compensation.
 Coordinated virtual impedance control is important in multiple converters for
optimal task sharing (considering PFC and harmonics resonance) and void
circulation current.
 To encourage more ancillary functions, relevant grid codes, polices and
markets are required.
27
Conclusions
Future research direction on hybrid AC/DC microgrid power quality
control:
 Parallel operation of DC-AC interlinking converters (ILCs) between DC and AC
subgrids.
 Harmonics compensation and control with low switching frequency – new
converter topologies, new PWM techniques.
 Multiple converter interactions – resonances, impedance variations, damping.
 System level coordination through supervisory control (SCADA).
28
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29