Aalborg Universitet
Power control flexibilities for grid-connected multi-functional photovoltaic inverters
Yang, Yongheng; Blaabjerg, Frede; Wang, Huai; Simoes, Marcelo
Published in:
Proceedings of the 4th International Workshop on Integration of Solar Power into Power Systems
Publication date:
2014
Document Version
Early version, also known as pre-print
Link to publication from Aalborg University
Citation for published version (APA):
Yang, Y., Blaabjerg, F., Wang, H., & Simoes, M. (2014). Power control flexibilities for grid-connected multi-
functional photovoltaic inverters. In Proceedings of the 4th International Workshop on Integration of Solar Power
into Power Systems (pp. 233-239). Energynautics GmbH.
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Power Control Flexibilities for Grid-Connected
Multi-Functional Photovoltaic Inverters
Yongheng Yang
†
, Frede Blaabjerg
†
, Huai Wang
†
, and Marcelo Godoy Sim
˜
oes
‡
†
Department of Energy Technology
‡
Electrical Engineering and Computer Science
Aalborg University Colorado School of Mines
Aalborg DK-9220, Denmark Golden, CO 80401, United States
yoy@et.aau.dk; fbl@et.aau.dk; hwa@et.aau.dk mgsimoes@ieee.org
Abstract— This paper explores the next-generation Photo-
Voltaic (PV) system integration issues at a high penetration level,
where the grid is becoming more decentralized and vulnerable.
Therefore, the PV systems are expected to be more controllable
with higher efficiency and higher reliability. Provision of ancillary
and intelligent services, like Low Voltage Ride-Through (LVRT),
reactive power compensation, and reliability-oriented thermal
management/control by PV systems is a key to attain higher
utilization of solar energy. Those essential functionalities for the
future PV inverters can contribute to reduced cost of energy.
To implement the advanced features, a flexible power controller
is developed in this paper, which can be configured in the PV
inverter and flexibly be changed from one to another. This power
control strategy is based on the single-phase PQ theory, and
it offers the possibilities to generate appropriate references for
the inner current control loop. The references depend on the
system conditions and also specific demands from both system
operators and customers. Besides, this power control strategy can
be implemented in a commercial PV inverter as standardized
functions, and also it can be achieved online in a predesigned
PV inverter. Case studies with simulations and experiments are
provided to verify the effectiveness and flexibilities of the power
control strategy.
I. INTRODUCTION
Another spectacular growth of grid-connected photovoltaic
(PV) systems has been witnessed in the year of 2013 [1].
The penetration level of PV systems will be further increased
in the future [2], since it is an effective solution to carbon-
dioxide reduction and also an essential part of “smart” grid.
However, the increase installations of PV systems into the
grid also bring side effects on the entire distributed network
due to the intermittent nature of solar PV energy (e.g. solar
irradiance variations and temperature fluctuations), which will
as a consequence affect the availability, the reliability, and
the quality of the distributed grid. Even for residential PV
systems of a few kWs, the impact can not be ignored today [3].
Possibly, the controllability of the whole power system will
be weakened, especially when a very high penetration level
is reached. In order to facilitate a reliable and efficient power
generation from solar PV energy, grid integration guidance
associated with critical customer demands is continuously
being updated [3], [4], which imposes more challenges for
the interfaced PV inverters. As a result, it calls for advanced
and intelligent control strategies for the next-generation multi-
functional PV inverters to be of much flexibility in order to
achieve those goals.
Hence, it is expected for the future PV systems to be of
much controllability by providing ancillary and intelligent ser-
vices, e.g. Low Voltage Ride-Through (LVRT) [5]–[9], reactive
power compensation (Var Comp.) [10]–[12], power qual-
ity enhancement [13]–[16], frequency control through active
power curtailment (Freq.-Watt function) [7], [17]–[19], and
reliability-oriented thermal management/control [20]–[23]. To-
gether with higher efficiency and higher reliability demands,
those functionalities for the future PV inverters are t he key to
reduce the total cost of PV energy. To implement those ad-
vanced features, a flexible power controller thus is developed
in this paper, which can be configured in the PV inverter to
fulfill the above services and flexibly be changed from one
to another according to the grid requirements and/or the end-
customer demands. This power control strategy is based on
the single-phase PQ theory [24], and it offers the possibilities
to generate appropriate power references, which are dependent
on the system conditions and also specific demands from both
system operators and customers. Besides, this power control
strategy can be implemented in a commercial PV inverter as
standardized functions, and also it can be achieved online in
a predesigned PV inverter in accordance to the PV system
operation conditions.
This paper serves to explore the next-generation PV sys-
tem integration features, develop a power control solution to
achieve those advanced features, and initiate further research
perspectives. In § II, a summary of key features for next-
generation PV systems is given by reviewing the currently
active grid standards/codes, followed by the power control
strategy for multi-functional PV inverters. Case studies on
the LVRT, reactive power injection, and temperature man-
agement using the power control strategy are conducted on
a single-phase PV system. The results presented in § IV have
demonstrated the power control flexibilities for grid-connected
PV inverters of multiple functionalities. It can enable a more
controllable and more manageable integration of PV systems.
II. ADVANCED PV INVERTER FEATURES
Seen from the thriving trend of PV systems, it can be
predicted that the grid, where more PV systems are going to be
connected to, will become even decentralized and vulnerable.
This will result in complicated control systems but reduced
carbon-dioxide emission. However, it should be noted that PV
TABLE I
MULTIPLE FUNCTIONS FOR FUTURE PV INVERTERS.
Features
Remarks
Volt.-Var control
Reactive power control to maintain the grid
voltage level [27], [28].
Freq.-Watt control
Based on the droop characteristic between
grid frequency and active power production to
achieve a constant grid frequency [17], [18],
[29], [30].
P constraints
Active power constraints [23], [25], e.g. delta
power production, power ramp control, and
peak power limiting control.
Dynamic grid support
In response to voltage faults, PV systems
should stay connected to the grid with reac-
tive power injection, especially at a high PV
penetration level [5]–[7].
Lower downtime &
higher efficiency
To further reduce the cost of energy [31], [32].
Harmonic comp.
Take an active role in power quality control,
e.g. as an active power filter [33], [34].
Smart operation
Var injection/compensation at nights, when
there is no solar irradiance.
systems are not just about decarbonisation, and they can be
beneficial in different ways to both grid operators and the
customers beyond the basic electricity generation [25].
In order to reach a goal of wider scale adoption of PV
systems and also to expand the benefits, a smoother transition
has to be initiated by the grid system operators and the PV
generators by means of revising the integration regulations and
developing advanced control strategies, respectively. Nonethe-
less, most of the currently active grid standards/codes seem
largely to require the grid-connected PV inverters to cease
energizing once a grid disturbance is confirmed [25], [26],
which is against the transition. To make the most of solar PV
energy in a cost-effective way, a common control strategy has
to be developed, and it should be simple but flexible for future
advanced PV inverters with the features listed in Table I.
Notably, the “Volt.-Var control” and “Freq.-Watt control”
are based on the droop relationship between grid voltage and
reactive power injection, and the relationship between grid
frequency and active power production , respectively [35],
[36]. Due to the very low X/R ratio of single-phase feeders,
grid voltage and frequency control through reactive power and
active power control is not very effective in a single-phase PV
system. While this could be an option for the PV systems at
a high penetration level by appropriately managing the active
power production and properly using the reactive power. In this
paper, a part of the advanced features shown in Table I for grid-
friendly PV systems have been demonstrated. Additionally,
to implement these functions, forecasting, monitoring, and
communication technologies have to be advanced.
III. FLEXIBLE POWER CONTROL STRATEGY
A. System Description
Fig. 1 exemplifies a grid-connected single-phase PV sys-
tem, which is a commonly used configuration for residential
applications of lower power ratings (e.g. up to 5 kW). As
TABLE II
SPECIFICATIONS OF THE SIN GLE-PHASE PV SYSTE M.
Parameter Symbol Value
PV nominal maximum power P
MP P
1∼3 kW
PV nominal voltage v
MP P
400 V
Grid voltage (RMS) V
g
230 V
Grid frequency ω
0
2π×50 rad/s
DC-link capacitor C
dc
2200 µF
LCL filter
L
1
C
f
L
2
3.6 mH
2.35 µF
708 µH
Sampling and switching freq. f
s
, f
sw
10 kHz
i
pv
v
pv
v
g
i
g
Inverter
LCL-filter
PWM
inv
Grid
Z
g
C
Load
Inverter
Control
C
dc
PV Panels
L
1
L
2
C
f
MPPT
Q
*
P
*
PCC
Fig. 1. A typical single-stage single-phase grid-connected PV system
(transformerless) with an LCL filter.
achieving high efficiency is always of interest for the inverter
manufacturers and also the PV users, normally, the isolation
transformer is removed, being the popular transformerless
PV inverters. To flexibly maximize the output PV energy
with extended operational hours, a DC-DC converter can be
adopted between the PV panels and the PV inverter, where
the Maximum Power Point Tracking (MPPT) is implemented
[23]. In that case, the DC-link control is aimed at power
injection by controlling the PV inverter. A current controller
with harmonic compensation is normally implemented in the
inverter control unit as shown in Fig. 1, since the current
controller is responsible for the power quality of injected
current, being synchronized with the grid voltage usually by
means of a Phase Locked Loop (PLL) in the normal operation
mode [37], [38]. Table II shows the system specifications.
B. Flexible Power Control Strategy
With the help of Clarke transformation (abc → αβ), the
instantaneous power theory proposed by Akagi has been
widely used in the three-phase systems [39]. Although this
theory is not appropriately applicable to single-phase systems
due to a limited number of control variables (i.e. the grid
voltage v
g
and the grid current i
g
), its attractiveness of direct
and intuitive active power and reactive power control remains
in single-phase systems. Therefore, efforts have been devoted
to create an imaginary system [24], [37] in order to adopt the
instantaneous power theory as it is shown in Fig. 2, being the
single-phase PQ theory.
According to the single-phase PQ theory [24], the active
power and the reactive power in the αβ stationary reference
frame can be expressed as,
P =
1
2
(v
α
i
α
+ v
β
i
β
)
Q =
1
2
(v
β
i
α
− v
α
i
β
)
(1)
i
g
v
g
v
β
i
β
i
α
v
α
v
β
i
β
+
à
Real system Virtual system
αβ components
ωt
(a)
Quadrature
Signal Generator
i
α
v
α
i
β
v
β
i
g
v
g
(b)
Fig. 2. (a) graphic representations of the single-phase PQ theory and (b)
in-quadrature system generation.
i
g
G
c
(s)
v
inv
*
*
i
g
Plant
v
α
v
β
Current
controller
T
j
v
g
f
g
i
g
i
pv
v
pv
MPPT
LVRT
Temp. Control
P Constraints
Q Comp.
etc.
P
Q
*
*
v
gm
2
2
v
g
Multi-functions
Fig. 3. Closed-loop control block diagram of the single-phase PV system
based on the single-phase PQ theory.
with v
αβ
, i
αβ
being the grid voltage and grid current in the
αβ reference frame, and P , Q being the active power and
the reactive power, respectively. Referring to Fig. 2, the grid
current can be derived from (1) and it can be given by,
i
∗
g
= i
∗
α
=
2
v
2
α
+ v
2
β
v
α
v
β
P
∗
Q
∗
(2)
with
v
gm
=
v
2
α
+ v
2
β
(3)
where “*” denotes the reference signals and v
gm
is the grid
voltage amplitude. Subsequently, the entire flexible power
control diagram based on the single-phase PQ theory can be
illustrated in Fig. 3. The single-phase PQ theory also enables
the use of popular Proportional Integrator (PI) controllers in
the dq rotating reference frame for the grid current control
where a Park transformation is required. Moreover, it is also
possible to use PI controllers to control the active power and
the reactive power injected to the grid [5].
It can be observed in Fig. 3 and (2), the power control
solution does not require a PLL system to synchronize the
grid current with the grid voltage. Instead, the dynamics
of the power control system are highly dependent on the
performance of the built-up αβ system, i.e. v
α
and v
β
, where
the synchronization actually is also achieved. Consequently, as
it is shown in Fig. 2, the implementation of the power control
strategy is shifted to create a quadrature signal generator based
on like Hilbert transformation, inverse Park transformation,
and the Second Order Generalized Integrator (SOGI) [24],
[37], [40]. Due to its good harmonic rejection ability, the SOGI
based in-quadrature signal generation system has been adopted
in this power control strategy.
It should be pointed out that, in such a power control
system, all the current controllers in the αβ stationary ref-
erence frame like the Proportional Resonant (PR) and the
repetitive controller can directly be adopted to regulate the
injected grid current. In contrast to the PI-controlled system,
there is no need for Park and inverse Park transformations
(αβ → dq) for the current controllers acting in dq reference
frame. The “Multi-Functions” unit (i.e. power reference unit)
is an objective-determined reference generator for the power
control strategy. As a consequence, the power control of multi-
functional PV inverters can be achieved by flexibly setting
appropriate power references, in spite of its performance-
dependency on the in-quadrature system, as it is shown in
Fig. 3. In addition, in the flexible power control strategy, only
current controllers have to be designed (i.e. control parameter
tuning in the current controllers).
IV. APPLICATION EXAMPLES
A. Low Voltage Ride Through
LVRT requirements were firstly introduced to the renewable
systems of high power ratings (e.g. several megawatts) con-
nected to medium- or high-voltage grids, e.g. wind turbine sys-
tems and utility-scale PV power plants. As the PV penetration
level is continuously growing at a rapid rate and also the power
rating of an individual PV system is going higher, similar
requirements have been extended to and imposed on other PV
systems. A shift of those requirements towards next-generation
PV systems, covering a wide range of applications from single-
phase PV systems of lower power ratings to three-phase higher
power PV plants, has been initiated in some countries [3], [6],
[8], [9], e.g. Italy, Germany, and Japan, where PV systems
have a large share of the electricity generation.
Associated with the fault ride-through, which is defined as
the stay-connected time for the system in response to voltage
faults as shown in Fig. 4(a), the reactive current injection
has also to be enabled to support the voltage recovery. Fig.
4(b) shows an example of the minimum reactive current for
medium- and high-voltage systems in response to a voltage
fault [3], [7]. Consequently, the PV system has to meet two
requirements in the case of voltage faults: (a) remain connected
to the grid during the transient and (b) provide reactive current
to support the voltage recovery. Both can be implemented in
the flexible power control strategy shown in Fig. 3. Notably,
the power injection is limited by the PV inverter rating, as it
can be given by
P
2
+ Q
2
≤ S
max
(4)
in which P , Q are the injected active power and reactive
power according to the voltage level, and S
max
is the inverter
maximum apparent power.
Considering the above operation constraints under grid
faults, a 1 kW single-phase grid-connected system has been
tested in LVRT operation mode referring to Fig. 3. Other pa-
rameters of the system has been given in Table II. A PR current
(b)
0 0.5
1.1
0.9
v
gm
(p.u.)
100
20
Dead Band
I
q
/I
N
(%)
LVRT- Support voltage
k = 2 p.u.
2 p.u.
1-
= ³
q N
gm
I I
k
v
I
q
: reactive current,
I
N
: rated current,
v
gm
: grid voltage.
High Voltage Ride- Through
Notes:
(a)
Time (s)
v
gm
(p.u.)
0
1.0
t
1
t
3
t
2
Stay Connected
v
2
v
1
0
Normal Operation
Fig. 4. Fault ride-through requirement: (a) stay-connected time under grid
faults and (b) reactive current profile in response to voltage faults [7].
controller with paralleled resonant Harmonic Compensators
(HC) has been adopted for the grid current control [38]. The
current controller G
c
(s) is given as (5) where only the 3
rd
,
5
th
, and 7
th
order harmonics are compensated.
G
c
(s) =
P R
k
p
+
k
r
s
s
2
+ ω
2
0
+
HC
h=3,5,7
k
ih
s
s
2
+ (hω
0
)
2
(5)
in which k
p
= 20, k
r
= 2000, and k
ih
= 5000 are the control
gains of the current controller.
Fig. 5 shows the performance of the single-phase system
under grid faults with the flexible power control strategy.
As it can be observed in Fig. 5, once a voltage fault is
confirmed, the system with the power control is able to inject
appropriate reactive power, which is dependent on the grid
voltage level. In the consideration of the inverter rating, the
injection current amplitude is maintained constant during this
short-term event. Besides, when the grid fault is cleared,
the power control solution can quickly change back to unity
power factor operation mode with the maximum active power
injection. The test results have demonstrated that the single-
phase PV inverter is capable of riding through voltage faults
enabled by the flexible power control strategy.
B. VAR Operation at Nights
Although double-stage PV inverters can extend the operat-
ing hours during a day, there is still a gap at nights where
the solar irradiance level is almost 0 kW/m
2
. Consequently,
no active power is available in that period, while the reactive
power is not like that case. The PV systems can provide
(a)
(b)
i
g
v
g
t
1
t
2
Sag Duration
Q
P
t
1
t
2
Sag Duration
Fig. 5. Low voltage ride-through operation of a single-phase PV system
(0.43 p.u. voltage sag): (a) grid voltage v
g
[100 V/div] and grid current i
g
[5 A/div] and (b) active power P [500 W/div] and reactive power Q
[500 var/div], time [40 ms/div].
P
Q
0
P
ins
Q
max
Q
max
S
max
S
ins
φ
Q
ins
Q
Fig. 6. Reactive power capability of a PV inverter.
reactive power which can be used to secure the entire grid
since it affects the grid voltage throughout the system. In
addition to reactive power operation at nights, there is a room
for most PV inverters to provide reactive power compensation
even in day-time operations [41]–[43], as it is shown in Fig.
6. Thus, according to (4) and Fig. 6, the maximum available
reactive power |Q
max
| can be determined by
|Q
max
| =
S
2
max
− P
2
ins
(6)
where P
ins
is the instantaneous active power and S
max
has
been defined previously.
Considering the reactive power constraint shown in (6), the
flexible power control strategy can enable the VAR operation
mode of PV system at nights. However, the key to implemen-