Aalborg Universitet
Distributed Secondary Control for Islanded MicroGrids - A Novel Approach
Shafiee, Qobad; Guerrero, Josep M.; Vasquez, Juan Carlos
Published in:
I E E E Transactions on Power Electronics
DOI (link to publication from Publisher):
10.1109/TPEL.2013.2259506
Publication date:
2014
Document Version
Early version, also known as pre-print
Link to publication from Aalborg University
Citation for published version (APA):
Shafiee, Q., Guerrero, J. M., & Vasquez, J. C. (2014). Distributed Secondary Control for Islanded MicroGrids - A
Novel Approach. I E E E Transactions on Power Electronics, 29(2), 1018-1031.
https://doi.org/10.1109/TPEL.2013.2259506
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IEEE TRANSACTIONS ON POWER ELECTRONIC
1
Abstract— This paper presents a novel approach to conceive
the secondary control in droop-controlled MicroGrids. The
conventional approach is based on restoring the frequency and
amplitude deviations produced by the local droop controllers by
using a MicroGrid Central Controller (MGCC). A distributed
networked control system is used in order to implement a
distributed secondary control (DSC) thus avoiding its
implementation in MGCC. The proposed approach is not only
able to restore frequency and voltage of the MicroGrid but also
ensures reactive power sharing. The distributed secondary
control does not rely on a central control, so that the failure of a
single unit will not produce the fail down of the whole system.
Experimental results are presented to show the feasibility of the
DSC. The time latency and data drop-out limits of the
communication systems are studied as well.
Keywords – Secondary control, Distributed Control, Networked
Control Systems, Droop Control, Cooperative Control.
I. INTRODUCTION
icroGrids (MGs) are local grids comprise different
technologies such as power electronics converters,
distributed generations (DGs), energy storage systems, and
telecommunications which can operate connected to the
traditional centralized grid (macrogrid) but also could operate
autonomously in islanded mode.
Control structures are essential to proper control of MGs
providing stability and efficient operation. The important roles
that can be achieved using these control structures are
frequency and voltage regulation, active and reactive power
control between DG units and with the main grid,
synchronization of MG with the main grid, energy management
and economic optimization [1]-[13]. Recently, hierarchical
control for MGs has been proposed in order to standardize their
operation and functionalities [1]. In such a hierarchical
approach, three main control levels have been defined. The
primary control is the first level which is independent, dealing
with the local control loops of the DG units. This can be
performed by voltage and current loops, droop functions, and
virtual impedances. Conventionally, the active power–
Q. Shafiee, J. M. Guerrero, and J. C. Vasquez are with the Institute of
Energy Technology, Aalborg University, Aalborg East DK-9220, Denmark
(e-mails: qsh@et.aau.dk, joz@et.aau.dk, juq@et.aau.dk).
frequency droop control and the reactive power–voltage droop
are adopted as the decentralized control strategies in the power
electronic based MGs for the autonomous power sharing
operations. Although the primary level does not require for
communications, in order to achieve global controllability of
the MG, secondary control is often used.
The conventional secondary control approach relays on
using a MicroGrid Central Controller (MGCC), which includes
slow controls loops and low bandwidth communication
systems in order to measure some parameters in certain points
of the MG, and to send back the control output information to
each DG unit [1], [2]. On the other hand, this MGCC also can
include tertiary control, which is more related to economic
optimization, based on energy prices and electricity market [1].
Tertiary control exchanges information with the distribution
system operator (DSO) in order to make feasible and to
optimize the MG operation within the utility grid.
Secondary control is conceived to compensate frequency and
voltage deviations produced inside the MG by the virtual
inertias and output virtual impedances of primary control. This
concept was used in large utility power systems for decades in
order to control the frequency of a large area electrical network
[14], [15] and it has been applied to MGs to restore frequency
and voltage deviations [1], [2], [9]-[13]. Furthermore, global
objectives regarding voltage control and power quality of the
MG, such as voltage unbalance and harmonic compensation
have been proposed recently in additional secondary control
loops [16], [17]. In all of these literatures, a central secondary
control (CSC) has been used in order to manage the MG.
On the other hand, the reactive power sharing of the Q–V
droop control is hard to achieve, since the voltage is not
constant along the MG power line, as opposed to the frequency
[18]. Consequently, reactive power sharing can be achieved by
implementing an external loop in the secondary level [19].
Significant efforts have been done in order to improve the
primary control method for power sharing in the recent years.
In [20], a power controller was proposed, which contains a
virtual inductor loop for both active and reactive power
decoupling, and an accurate reactive power sharing algorithm
with an online impedance voltage drop effect estimation
considering different location of the different local loads in a
MG. This strategy, which is an improvement of the
conventional droop method, operates in the primary control
level therefore it does not need physical communications
among DG units. Alternatively, a reactive power-sharing
Distributed Secondary Control for Islanded
MicroGrids - A Novel Approach
Qobad Shafiee, Student Member, IEEE, Josep M. Guerrero, Senior Member, IEEE, and Juan C. Vasquez,
Member, IEEE
M
IEEE TRANSACTIONS ON POWER ELECTRONIC
2
scheme has been presented in [21], which introduces an integral
control of the load bus voltage, combined with a reference that
is drooped against reactive power output. Further, active power
sharing has improved by computing and setting the phase angle
of the DGs instead of its frequency in conventional frequency
droop control. In [22], a control strategy which increases the
droop gain to improve the accuracy of reactive power sharing is
proposed by making a feedback reactive power injection loop
around the conventional droop loop of each DG, while
maintaining the system stability. Additionally, secondary
control loops implemented in the MGCC has proposed to share
reactive power between DG units and also to restore the voltage
deviations in [19]. In all those techniques, reactive power
sharing cannot be achieved completely since voltage is a local
variable, as a contrary of frequency.
Moreover, primary and tertiary controls are decentralized
and centralized control levels respectively, since while one is
taking care of the DG units, the other concerns about the MG
global optimization. However, although secondary control
systems conventionally have been implemented in the MGCC,
in this paper we propose to implement it in a distributed way
along the local control with communication systems. In this
sense, a local secondary control is determined for each DG to
generate set-points of the droop control to restore of the
deviations produced by the primary control.
This kind of distributed control strategies, which are also
named networked control systems (NCS), have been reported
recently in some literatures [9], [23]-[24]. In [9], technical
aspects of providing frequency control reserves (FCRs) and the
potential economic profitability of participating in FCR
markets for both decentralized and centralized coordination
approach based on a setup of multiple MGs are investigated. In
[23], a pseudo-decentralized control strategy has been
presented for distributed generation networks which operate in
distributed manner using a Global Supervisory Controller
(GSC) and local controllers with some intelligence. In the other
hand, a master-slave control by using networked control
strategy for the parallel operation of inverters has been
introduced in [24]. The method is employed to achieve the
superior load-sharing accuracy compared to conventional
droop scheme with low-bandwidth communication. Further,
the system robustness has been considered in the case of
communication failure as well. Distributed control strategies
have been used in all these literatures, however, the application
of these control strategies to secondary control of MGs still has
not been proposed.
In this paper, a distributed secondary control strategy is
proposed for power electronics-based MGs, including
frequency, voltage and reactive power sharing controllers. This
way, every DG has its own local secondary control which can
produce appropriate control signal for the primary control level
by using the measurements of other DGs in each sample time.
In order to investigate the impact of communication on this new
control strategy, the communication latency is considered when
sending/receiving information to/from other DG units and the
results are compared with the conventional MGCC.
The paper is organized as follows. In section II, the structure
of the primary control in MGs is described. Then, details of
centralized secondary control for MGs are discussed in Section
III. Section IV is dedicated to the proposed secondary control
strategy, which includes frequency control, voltage control and
reactive power sharing. Experimental results and discussion are
presented in Section V. Furthermore, the proposed secondary
control is applied on a two paralleled 2.2kW-inverter system as
a case study. Finally, the paper is concluded in Section VI.
II. PRIMARY CONTROL FOR MICROGRIDS
Power electronics based MG consists of a number of elements
that can operate in parallel either in islanded mode or connected
to the main grid. Fig. 1 shows a general structure of MG, which
composes n DG units. The MG is connected to the utility
system through a static transfer switch (STS) at the point of
common coupling (PCC). As depicted in Fig.1, each DG
system comprises a renewable energy source (RES), an energy
storage system (ESS), and a power electronic interface, which
normally consist of a dc-ac inverter. Each DG can be connected
to a predefined load or to the AC common bus directly in order
to supply power.
The dc/ac inverters are classified as voltage source inverters
(VSIs) and current source inverters (CSIs) which the former is
commonly used to inject current in grid connected modes and
the latter to keep the frequency and voltage stable in
autonomous operation. Both can operate in parallel in a MG.
However, VSIs are convenient since they can enhance power
quality and ride-through capability for DGs in a MG [1], [25].
The primary control of VSIs based MG includes voltage and
current control loops, virtual impedance loop and droop control
strategy as shown in Fig. 3. Linear and nonlinear control
strategies are designed and performed in order to regulate the
output voltage and to control the current while maintaining the
system stable. Normally, inner control loops include
proportional-resonant (PR) controller when they use stationary
framework (αβ), and proportional-integral (PI) controller when
they use the dq framework. The reference of the voltage control
loop will be generated, together with the droop controller and a
virtual impedance loop.
Droop control is responsible for adjusting the frequency and
the amplitude of the voltage reference according to the active
and reactive powers (P and Q), by using the well-known P/Q
droop method [1], [25]- [29]. Furthermore, a virtual impedance
loop is also added to the voltage reference in order to fix the
output impedance of the VSI which will determine the P/Q
power angle/amplitude relationships based on the droop
method control law. In contrast with physical impedance, this
virtual output impedance has no power losses, and it is possible
to implement resistance without efficiency losses [13]. More
details about the primary control can be found in [1], [13],
being out of scope of this paper.
IEEE TRANSACTIONS ON POWER ELECTRONIC
3
Renewable
Energy source
Energy Storage
System
Load
1
DG
. . .
Islanded MicroGrid
Grid
Load
2
DG
Load
n
DG
Renewable
Energy source
Energy Storage
System
Renewable
Energy source
Energy Storage
System
Fig. 1. General structure of MG.
III. CENTRALIZED SECONDARY CONTROL FOR MICROGRIDS
Since the primary control is local and does not have
intercommunications with other DG units, in order to achieve
global controllability of the MicroGrid, secondary control is
often used. Conventional centralized secondary control loop is
implemented in MGCC [2]. Fig. 2 shows MG secondary
control architecture consists of a number of DG units locally
controlled by a primary control and a secondary control, which
measures from a remote sensing block a number of parameters
to be sent back to the controller by means of a low bandwidth
communication system. Hence, those variables are compared
with the references in order to be compensated by the
secondary control, which will send the output signal through
the communications channel to each DG unit primary control.
The advantage of this architecture is that the communication
system is not too busy, since only unidirectional messages are
sent in only one direction (from the remote sensing platform to
the MGCC and from the MGCC to each DG unit). The
drawback is that the MGCC is not highly reliable since a failure
of this controller is enough to stop the secondary control action.
A. Frequency control
Traditionally, secondary controllers for large power systems
are based on frequency restoration, since the frequency of the
generator-dominated grids is highly dependent on the active
power. This fact is an advantage since frequency is a control
variable that provides information related to the
consumption/generation balance of the grid. This central
controller, named Load Frequency Control (LFC) in Europe or
Automatic Generation Control (AGC) in USA, is based on a
slow PI control with a dead band that restores the frequency of
the grid when the error is higher than a certain value, e.g. +/-50
mHz in the north of Europe.
Similar concept has been implemented in MGCC in order to
restore the frequency of P–f droop controlled MG [4]. The
frequency restoration compensator can be derived as follows.
Pf MG MG if MG MG
f k f kf f f dt
(1)
being kpf and kif the control parameters of the secondary control
PI compensator. The frequency levels in the MG (
) are
measured and compared to the references (
) and the errors
processed through the compensators (δf ) are sent to all the DG
units in order to restore the frequency of MG.
B. Voltage control
The voltage also can be controlled by using similar
procedure as the frequency secondary control [1]. When the
voltage in the MG is out from a certain range of nominal rms
values, a slow PI control that compensates the voltage
amplitude in the MG, pass the error through a dead band, and
send the voltage information by using low bandwidth
communications to each DG unit. Thus, it can be implemented
together with the frequency restoration control loop at the
MGCC. The voltage restoration control loop can be expressed
as follows:
PE MG MG iE MG MG
E k E kE E E dt
(2)
being k
PE
and k
iE
the PI controller parameters of the voltage
secondary control. The control signal ( ) is sent to the
primary control level of each DG in order to remove the steady
state errors produced by droop control.
Fig. 2. Centralized secondary control.
IEEE TRANSACTIONS ON POWER ELECTRONIC
4
Central Secondary Control
Virtual
Impedance Loop
Power
Calculation
o
v
L
i
o
i
Voltage Reference
Generator
sin( )Et
Voltage
Control Loop
E
f
Q
P
Droop Control
ref
v
L
o
L
C
Current
Control Loop
dc
V
DC Link
PWM
Frequency Control (Eq. 1)
MG
f
MG
E
i
P
kE
kE
s
Voltage Control (Eq. 2)
Communication Link
i
P
kf
kf
s
f
E
MG
f
MG
E
f
E
Primary Control
DGK
MicroGrid bus
Fig. 3. Scheme of the central secondary control for a DG unit in a MG.
This approach can be also extended to more resistive MGs by
using P–V droops in the primary control, and restoring the
voltage of the MG by sending the voltage correction
information to adjust the voltage reference. Thus, voltage and
frequency restoration controllers can be used in any R/X
condition by means of the park transformation in the primary
control. Consequently, the secondary control is transparent to
the R/X nature of the power lines, as opposed to the primary
control.
Fig. 3 depicts details of centralized secondary control
structure for an individual DG unit (DG
k
) in an islanded MG
based on equations (1) and (2). As seen, The frequency and
voltage levels in the MG are measured and compared to the
their references, then errors processed through the
compensators are sent to primary control level of all DG units
in order to restore the deviations in the MG.
IV. PROPOSED DISTRIBUTED SECONDARY CONTROL
The problem of using the MGCC for implementing
secondary control is that a failure can result in a bad function of
the whole system. In order to avoid a single centralized
controller, a distributed control system approach is proposed in
this paper. However, even with this new control strategy there
is need of MGCC for coordination of units during black start
process and among other management functionalities of MG.
The initial idea is to implement primary and secondary
controllers together as a local controller. Fig. 4 shows the
diagram of a fully distributed control system. Primary and
secondary controls are implemented in each DG unit. The
secondary control is placed between the communication system
and the primary control. Frequency control, voltage control,
and reactive power sharing will also be reviewed by using this
control approach. However, this control strategy can be used to
share active power in high R/X MGs as well.
In this case, secondary control in each DG collects all the
measurements (frequency, voltage amplitude, and reactive
power) of other DG units by using the communication system,
average them and produce appropriate control signal to send to
the primary level removing the steady state errors.
Fig. 5 illustrates details of the proposed distributed secondary
control for an individual DG (DG
k
) in a MG.
A. Frequency control
Taking the idea from large electrical power systems, in order
to compensate the frequency deviation produced by the local
P- droop controllers, secondary frequency controllers have
been proposed [26]. However, the approach needs
communications in order to avoid instability in the MG system
caused probably by different stories of each local inverter.
