Droop Control of Microgrids based on Distributed Optimization
*
Peng Yi
1
, Feng Liu
2
, Yiguang Hong
1
1. Key Lab of Systems and Control, Academy of Mathematics and Systems Science
Chinese Academy of Sciences, Beijing 100190, P. R. China,
E-mail: yipeng@amss.ac.cn; yghong@iss.ac.cn
2. Electrical Engineering Department, Tsinghua University
E-mail: lfeng@mail.tsinghua.edu.cn
Abstract: In this paper, we discuss the optimal power balance and frequency stability in microgrids by imposing distributed
optimization scheme on droop control. Assign each agent for each power source to take the responsibility of tuning the generation
level in real time. Considering each agent only has its local information, we propose a novel distributed optimization algorithm
for each agent to get the optimal generation decision. The parameter of each droop controller is tuned according to the optimal
solution provided by the distributed algorithm. With the droop controller, the control structure achieves both the frequency
stabilization and economical load sharing. Moreover, we show some cases to demonstrate the effectiveness of our methodology.
Key Words: Microgrids; Droop control; Distributed optimization; Load sharing control; Multi-agent system; Economic dis-
patch
1 Introduction
In recent years, microgrid has been drawing more and
more attention for its capability to manage renewable en-
ergy locally and efficiently, and to enhance the reliability of
existing power girds [1] [2]. As a matter of fact, the power
sources of microgrids, which may consist of many elements
such as wind-turbines and photovoltaic panels, are funda-
mentally different from the traditional dispatchable power
sources such as thermal generators. These heterogeneous
power sources have different and time-varying capacities,
ramping characteristics, and cost functions, and may be un-
certain and intermittent. Therefore, it has become a big chal-
lenge to coordinate these heterogeneous power sources to
meet the load demand efficiently and securely [2].
Power balance is also closely related to frequency stability
problem in power grids, which is usually handled using the
hierarchical control framework [3]. Droop controller stabi-
lizes the frequency dynamics, which usually results in steady
state frequency deviation. Then automatic generator control
(AGC) and economic dispatch (ED) are performed for zero
frequency deviation and efficiency of load sharing. With the
hierarchical control framework, AGC and ED are performed
in a centralized manner, because all the relevant data must
be collected to one center [3]. Some recent works were done
to apply the hierarchical framework to microgrid control [4].
The most recent literature indicates that distributed con-
trol structure can be more appealing than the centralized one
in microgrid [2], because the diversified distributed genera-
tors (DGs) are usually owned by different utilities. Recent-
ly, distributed algorithms and controllers for the load shar-
ing and frequency stabilization in inverter-based microgrids
have been proposed in [7]- [14]. In [5, 6], the authors dis-
cussed how to control the inverters to mimic the synchronous
generator (SG). Under this framework, [8] and [10] analyzed
the frequency stability of the droop controlled microgrid.
Consensus-based distributed controller is proposed to per-
*
This work is supported partially by NNSF of China under Grants
61174071 and 61333001, and partially by 973 Program under Grant
2014CB845301/2/3. This work is also partly supported by the Special grand
from EPRI of China (XTB51201303968)
form the AGC in [11]. [12] and [13] investigated how to de-
sign decentralized control law to achieve the optimal load
sharing, while [14] designed a distributed control law by the
Internal Model principle to achieve dynamical power bal-
ance.
In practice, the capacity and ramping constraints of the
power sources must be taken into consideration. Since the
microgrid has relatively weak transmission lines and low
voltage working points, its network structure and transmis-
sion lines capacities may also limit the load sharing. How-
ever, most studies only considered the overall power balance
except very few results ( [11,14]).
In this paper, different from the existing hierarchical con-
trol framework, we propose a scheme to combine the droop
controller with distributed optimization algorithm for the fre-
quency stability control and optimal load sharing in a micro-
grid. We consider both the generation and transmission con-
straints in the optimization model for ensuring the security
of load sharing. In fact, we assign an agent to each power
generator to take charge of its power generation. Each agent
only knows its own cost function and generation capacity, as
well as the flow capacities of the transmission lines that the
power source is directly connected to. This multi-agent sys-
tem can solve the cost-minimization power balancing prob-
lem with both generation and transmission constraints in a
distributed way. The obtained optimal generation decision
is then used for tuning each power source’s droop controller,
achieving optimal droop-controlled power balancing and fre-
quency stability retaining. We believe this may provide a
promising way to change the current “master-salve” control
framework of microgrids into a “peer-to-peer” one, making
it possible to achieve the “plug-and-play” control for the fu-
ture microgrids.
The reminder of this paper is organized as follows: in
Section 2, we give the generic model of load sharing and
frequency stability control problem in microgrids. Then we
propose the optimal droop controller design in section 3. In
section 4, we propose a distributed algorithm to solve the op-
timization problem. We show some case studies in section
5. Finally, we give the concluding remarks in section 6.
Proceedings of the 34th Chinese Control Conference
Jul
28-30, 2015, Han
zhou, China
9002