bottom-up and recursive interconnection for multi-layer dc ... · bidirectional dc-dc converters...
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Bottom-up and Recursive Interconnection forMulti-layer DC Microgrids
Annette WerthThe University of Tokyo
Sony Computer Science Laboratories Inc.
Tokyo, Japan
Mario TokoroSony Computer Science Laboratories Inc.
Tokyo, Japan
Kenji TanakaThe University of Tokyo
Tokyo, Japan
Abstract—We propose a recursively scalable DC infrastructurestarting off from simple DC nanogrids (subsystem) that areinterconnected via a DC power bus line to form a cluster.
We have conceived a non-droop based procedure to exchangepower from one subsystem to another in order to balancedemand/generation fluctuations within the community withoutrequiring any central monitoring or control.
To scale the procedure to higher layers we further proposethree approaches that could be used for managing power ex-changes between clusters. The approaches are compared usinganalogies with the internet architecture i.e. circuit-switching,packet-switching and virtual switching.
Each approach is analyzed in its ability to provide decouplingbetween analog and digital-centric goals as well as betweeninfrastructures. Furthermore, we discuss whether the approachescould verify the four ground rules of the internet.
The 2-layer architecture and the procedure for DC powerexchange has been validated in practice on a full-scale 19DC nanogrids installed in inhabited houses in Okinawa. Theevaluation of the multi-layer exchange procedure is still ongoing.
Keywords—DC power distribution, Distributed power system,DC Power transmission, microgrid, smart grid
I. INTRODUCTION
Grid access capacity is now considered as the largest im-
pediment to the introduction of renewables [1]. Research focus
has shifted from power generation towards the underlying
power grid structure and its ability to handle the inherently
distributed and unstable power supplies. Indeed, the current
AC-dominated grid infrastructure is increasingly weakened
because the higher penetration of Distributed Generation (DG).
Fueled by AC-related issues such as frequency and voltage
stabilization, Low Voltage Direct Current (LVDC) solutions
such as DC microgrids have gained increasing research inter-
est. In general, nanogrids or microgrids provide a bottom-up
solution for integrating large amounts of micro-generation by
managing fluctuations locally and thus reducing the negative
impact to the utility network [2]. Nevertheless, usually they
still rely on the utility grid for avoiding energy shortcomings
and guaranteeing high reliability.
According to Farhangi [3], the future electricity grid will
require “plug-and-play integration of smart microgrids that
will need to be interconnected through dedicated highways for
command, data, and power exchange”. Pervasive monitoring
and control functions are distributed across all levels [3]
and interoperability will be key for integrating DER into the
grid [4], [5].
This bottom-up, interconnected evolution bears a clear
resemblance with what the internet has successfully gone
through which is why it has stimulated academic discus-
sions and comparisons [2]. Applying the internet concept
to the power grid could provide a solutions for networking
distributed resources in a bottom-up manner, instead of just
accessing them as the conventional grid is doing [6].
We contribute to this research by proposing a method for
exchanging DC power and approaches on how it can be scaled
in a bottom-up manner by exploiting internet concepts and
terminology.
In this paper, we first present the concept of a multi-layer
DC microgrid. A distributed, non-droop based procedure to
control the DC power exchange is explained. Then, we expand
the procedure over several layers using a circuit-switching,
packet-switching and virtual circuit-switching approach and
discuss each of them by applying basic rules of the internet
architecture. Finally, we show a full scale implementation of
the exchange procedure currently running on our platform in
Okinawa.
II. RELATED WORK
This section summarizes related works on interconnected
grids, power merging procedures and methods to scale using
internet concepts.
A. Architecture of Interconnected Grids
Though the concept of hierarchical control for AC power
system is well known (standards: IEEE 1547 for integrating
DER and IEEE2030 for smart grid interoperability), no overall
control studies for interconnecting energy systems such as
microgrids can be found, claimed Guerroro in 2011 in [7].
Since then, several research papers on interconnections of
microgrids have started to appear in literature [8], [9], [7], [10],
[11], [12], [13], [2], [14]. However, all of them only address
a subproblem without giving a complete solution including a
real implementation.
From those studies, we infer that for making an intercon-
nected grid practical and scalable, two conditions are required:
978-1-5090-2320-2/16/$31.00 c©2016 IEEE
1) not one but two independent, decoupled infrastruc-
tures: power infrastructure and information infrastruc-
ture [15]. The information infrastructure provides the
control structure (networking) on which high level con-
trol logic can be implemented.
2) a clear decoupling between analog-centric control goals
(related to power stability) and digital-centric control
goals (related to system automation) [16]. While it is
clear that both goals must be achieved by the future
grid, they should not be interrelated.
Further more, only loose coupling between software and
hardware layers can provide enough freedom for developing
universal solutions.
B. Power merging
A bottom-up grid requires methods to merge power from
distributed resources. In this paper, we focus on direct current
(DC) because DC present certain advantages over AC [17]:
• Power merging and system analysis is easier (no fre-
quency, phase or waveform control, thus no synchroniza-
tion) [18]
• More efficient transmission lines and improved system
stability because of the absence of reactance and external
disturbances [18]
• Raw electricity output from renewable sources, batteries
and 80% of today’s consumer equipments already use
DC [19]
• DC conversion efficiencies above 90% have made it prac-
tical to convert voltage sources to current sources [20]
• New protection strategies such as electronic fuses [21]
allow safe interruption and suppressing arcs which has
been a main issue in DC because of the absence of natural
zero crossing point
• Digital control developed for DC provides enhanced
flexibility for smart grid development
Two main methods allow merging power in DC [18]:1) Droop-based methods: DC voltage is added by relying
on the internal resistance of the DC voltage sources and the
line resistance. This method is widely used for the DC electric
railway system as it does not require a communication net-
work, but disadvantages include “poor transient performance
or instability issues, inability for black-start-up after system
collapse and inability to provide accurate power sharing among
DER units due to output impedance uncertainties” [22].2) Non-droop-based methods: A single DC voltage source
(constant voltage - CV) stabilizes the voltage of the power
line to which DC current sources (constant current - CC) can
then be connected. As it requires one fixed voltage source for
stabilizing the bus voltage (master), it could be considered
as master/slave approach. For this reason, the number of CC
nodes and the geographic expansion are limited. Yet, this
method also presents the important advantage of being con-
sidered “flexible with respect to connection and disconnection
of DER units” [22].
The latter system is analytically consistent to boost the
current of the voltage power source by keeping the voltage sta-
bilized without needing any critical control system. However, a
single voltage source (master) constitutes a serious bottleneck
in case of failure of that unit. Fortunately, today’s DC-DC
converter can work both in CV and CC mode [20], meaning
that any converter could be dynamically take on the master
role for controlling the bus voltage.
C. Internet analogies for multi-layered grids
In both merging methods, adding units using a flat ex-
pansion of the topology leads to an overall degradation in
efficiency and resilience. Internet analogies could help to scale
power grids by clustering or layering power grids from the bot-
tom up. Indeed, a main difference between the internet and the
power grid is that the latter still lacks a comparable layering
of the infrastructure that is simple, clear and effective [2].
Increased capacity and decreased delays in the internet
infrastructure have made a lot of the original research efforts
on Quality of Service obsolete [2]. Similarly, advances in
storage technologies, may also mitigate efforts on how to
integrate fluctuations caused by renewables [2].
An often picked up notion is the one of routing electricity
by using so called "power packet" [23] or "packetized en-
ergy" [24], [25] as an analogy to an IP packet. However, while
routing data makes sense because all packets are different,
electrons are all the same, only timing, quantity and location
matters [2]. Moving power is associated with an efficiency
cost [2], meaning that the transfers should be minimized. Also,
routing energy seems to infringe the two conditions stated in
II-A as it makes it hard to decouple infrastructure and goals.
In this paper, we will take a higher level perspective when
comparing to the internet: instead of implementing internet
concepts we rather borrow terms and concepts for better
describing our approach with the well-established ones in the
ICT sector. In particular, we claim that the four ground rules
of the internet by R. Kahn [26] should also hold for the future
electricity grid:
• Each network (here: grid cluster) would stand on its own.
• Communication (here: power transfer) would be best
effort basis.
• Black boxes like routers/gateways would be used to
connect networks.
• No global control at operations level.
III. MULTI-LAYER DC MICROGRID
In this section, we expand our original proposition of
interconnected DC nanogrids [27], [28], [29] for scaling
it over multiple layers by respecting the two main conditions
stated in II-A. The four ground rules of the internet are
satisfied. After a brief overview of the overall architecture, we
propose two procedures: a non-droop method for DC power
exchange and three approaches for scaling the power exchange
over several layers.
A. Architecture
On the lowest layer, DC nanogrids (i.e. DC subsystems)
including generation, storage and load are used as building
blocks. As shown in Fig. 1 they are interconnected using
bidirectional DC-DC converters that control the power flow to
form a DC microgrid (i.e. cluster) according to the procedure
described in subsection III-B).
Fig. 1. Basic architecture for two layers. Top: Connection between DCsubsystems. Bottom: Concept illustration for autonomous power exchangebased on battery SoC as decision variable
For interconnecting DC subsystems two components are
required (see Fig. 1, top):
1) A DC-DC converter for regulating power flow over the
DC power bus line.
2) A controller for interfacing internal modules and exter-
nal over a dedicated communication line.
This architecture can enable bottom-up, peer-to-peer power
exchanges between subsystems (see Fig. 1, bottom). We pro-
posed a decentralized implementation of such an autonomous
exchanges software in which subsystems negotiate powerdeals with their neighbors [27], but the negotiation logic is
out of scope of this paper.
The advantage of such a modular design that separates
the subsystems from the interconnecting modules is that,
in principle, any kind of subsystem that fulfills the basic
requirements can be used as long as the interface between
nanogrid controller and network controller is adapted so that
a minimum set of monitoring/control functionalities can be
accessible over the network. This allows the internal modules
to be very heterogenous, for instance to use solar energy or
wind or even a generator as power source.
DC clusters can only cover a small community on a geo-
graphically limited area. To scale, the procedure of connecting
DC subsystems can be recursively expanded to interconnect
entire clusters via higher voltage lines. Such a multi-layered,bottom-up DC grid enables loose coupling between layers
by making abstraction of their internal components and func-
tioning.
Fig. 2. DC-based multi-layer infrastructure made of interconnected DCsubsystems
B. Procedure for DC power exchange
In this subsection we describe a fully distributed, non-droop
based method that allows to exchange power to decouple the
analog and digital-centric goals (see II-A).
Every subsystem is connected to the DC bus line via a
bidirectional DC-DC converter that can work in 3 modes:
standby (off), constant voltage (CV) or constant current (CC).
Once a power deal is agreed, one of the participating converter
is dynamically chosen to work in CV mode (master) and thus
ensure a constant bus voltage. All other DC-DC converters are
either on stand-by or on CC mode. The master converter acts
either as load or as source depending on the sum of all other
units currents. In this way, voltage remains constant during
power exchanges. When no exchange is ongoing, the master
is released and the bus voltage drops to 0 V. We refer to this
strategy as intermittent control because when no exchange is
ongoing, all DC-DC converters are set standby which reduces
losses to a minimum.
Fig. 3. Decentralized procedure for exchanging DC power from n to n units
The example in Fig. 3 shows 5 interconnected subsystems
that are represented by their DC-DC converters which are
connected to the shared power bus line (nominal bus voltage
during power exchanges Vg = 350V ). In the example, we
use the current Ig to express the amount of power exchanged
(Pexchanged = Ig ∗ Vg).
We observe that the agreed energy deals may not correspond
to the actual power flow. As long as the maximum capacities of
the converters are respected, the power flow will be balanced
independently of the physical location of the subsystem on the
power bus line. No central control or monitoring is required
because the internal feedback loops of the controller will
handle the analog control and manage transient phenomena
locally. Hence, time and location constraints are taken care
of at the lowest level, allowing a clear decoupling of analog-
centric and digital-centric goals.
This procedure assumes that the line resistance is within
some range so that the voltage drop becomes within the
controllable range of the DC-DC converter. Therefore, only
a limited number of subsystems on a geographically limited
area is possible, which has led us to develop a multi-layer
approach described in the next section.
C. Multi-layer DC exchange procedure
To be able to connect DC clusters, the DC exchange
procedure must be expanded to higher layers using higher
voltages in order to reduce transmission losses because we
expect longer inter-cluster distance. We use network theory
and terminology as an analogy and propose three methods that
could be described analogically to circuit-switching, packet-
switching as well as virtual circuit-switching applied to the
power grid.
1) Circuit-switching approach: In this approach, a single
DC-DC converter (gateway) is used to convert the voltage and
control the power flow to a higher layer bus (Fig. 4). Once
a power deal is agreed between DC subsystems belonging to
different clusters, all converters must be set one by one to
achieve a continuous power flow. However, since the gateway
converters do not dispose of batteries, they are powered off
when no exchange is happening (principle of intermittent
control). To start an exchange, one of the gateway converters
be powered on and then ramp up the higher layer bus line and
keep the voltage stable, always by being indirectly powered
by the DC subsystem that started the deal. In practice, this
approach is difficult not only because of the question on
who is powering the gateway converters (intermittent control
may not be possible) but also because the procedure requires
controlling the end-to-end deal as a "circuit". This implies
simultaneously managing three sub-deals with potentially dif-
ferent voltages and currents (I1,I2, I3), knowing that power is
reduced at each conversion step due conversion losses. Hence,
the control procedure is complex because it couples control of
both clusters and the gateway converters cannot be considered
as "black boxes" which would violate two rules of the internet
II-C.
2) Packet-switching approach: In this approach (Fig. 5)
interconnections include an energy buffer surrounded by a
gateway converter for each cluster. Power deals can only be
formed between units belonging to the same cluster (no ex-
plicit cross-cluster deals are possible). Gateway converters can
Fig. 4. Circuit-switching interconnections
use the same control procedures as all other DC subsystems
of the cluster and the only way they communicate with other
clusters is by measuring their buffer’s State of Charge (SoC)
and negotiate deals accordingly (Fig. 6). Indirectly, power may
still be transferred from subsystem N1.3 to N2.3, but the con-
trol and negotiation for each cluster is completely decoupled.
Furthermore, the buffers solve the issue of powering gateway
converters.
Fig. 5. Packet-switching interconnections
Fig. 6. Example of a control procedure inside gateway using packet-switchingapproach
3) Comparison: Tab. I shows a qualitative comparison of
the proposed approaches.
Table ICOMPARISON OF CIRCUIT-SWITCHING AND PACKET-SWITCHING APPLIED
TO POWER GRID
Circuit-switching Packet-switchingMinimal infrastructure: one converter
onlyTwo converters and energy storage
(Buffer) required
Low losses for inter-cluster transferHigher inter-cluster conversion losses
(2 conversion steps)
Guaranteed capacity No guarantees (best effort)
One complex procedure Multiple simple procedures
Wasted capacity if voltages aredifferent
More efficient usage of power lines
Communication between clusters No direct communication needed
Failures are propagated Failures are confined to one cluster
Thanks to the buffers, packet-switching approach allows a
complete separation of clusters which simplifies the control
procedures significantly and provides increased resilience,
which outweighs the additional hardware requirements. All
four ground rules of the internet II-C would be satisfied using
this approach.
However, in the packet-switching approach limits power
deal negotiations to be confined within one cluster. In future
applications, energy may be dealt similarly to stock exchange
where all participating subsystems can trade their spare en-
ergy even beyond their own cluster. For this, an inter-cluster
platform for power deal negotiation is required, but that also
conserves the simplicity and resilience provided by a packet-
switching approach for power transfer.
This could be achieved using an approach similar to Virtualcircuit-switching:
4) Virtual circuit-switching: Just like for the internet net-
work, this approach is built on top of the packet-switching
approach (Fig. 7). The gateways will continue to use the
packet-switching approach to achieve power exchange, but
deal negotiation may be done cross-clusters: negotiation units
can "virtually" exchange power to other clusters, even if in
reality no actual power may flow between clusters (power
may be absorbed by the buffers only). For inter-cluster deals,
a certain fee corresponding to the transfer/conversion losses
shall be added depending on the "distance" of the clusters.
This approach has for advantage that the actual power transfer
procedure remains simple and decoupled between layers, but
the negotiation algorithm can be used across multiple clusters.
IV. IMPLEMENTATION AND VERIFICATION
The feasibility of the procedure for DC power merging has
been tested and implemented on at a full-scale platform in
Okinawa where 19 inhabited houses are equipped with DC
nanogrids (see Fig. 8). All are interconnected with a DC
power bus line . An autonomous energy exchange software
(see details in [27]) negotiates deals between houses that are
then executed according to the procedure explained in III-B.
By following this procedure, we could achieve decoupling of
negotiation and physical power and therfore free the higher
level software from managing analog-centric goals.
The procedure has been running autonomously since De-
cember 2014. We have shown intermittent control is feasible
Fig. 7. Virtual Circuit Switching interconnections
Fig. 8. Full scale platform to test a 2-layer DC microgrid
and improves efficiency in a best-effort manner. During the
6 recorded utility blackouts, exchange procedure continued
normal operation showing that cluster can operate as stand
alone. DC subsystems can be considered as black boxes and
no central control or supervision is needed. All four rules of
the internet are therefore satisfied.
The approaches to expand the procedure to higher layers
for being able to interconnect clusters is now being tested.
The circuit-switching procedure was realized on a laboratory
prototype made of 2 subsystems connected via 2 higher
layers with different voltages. We could successfully set the
cascading DC-DC converters to transfer power between the
two subsystems (see Fig. 4). However, the experiment was
limited as it uses only 2 subsystems and the nominal voltage
used for the highest layer bus (3rd layer) was within a similar
range as the one of the 2nd layer bus. Further tests with more
subunits connected via converters with more suitable voltage
ranges are required for a more realistic evaluation. A similar
set-up would be needed for testing and comparing the packet-
switching approaches.
V. CONCLUSION
Interconnection systems will be an important requirement
for integrating renewable distributed generation. It allows us-
ing local and distributed storage and enables power balancing
and managing demand-response between energy clusters. This
can be used not only for day-to-day balancing demand/gener-
ation mismatches but it could also serve for exchanging power
between communities which could be of vital important in the
case of emergency.
This paper proposes a bottom-up approach for exchanging
DC power. We first described the procedure for a flat topology
made of two layers (DC subsystems connected to form a
DC cluster). Three approaches to scale this procedure over
several layers are then discussed using analogies with circuit-
switching, packet-switching and virtual circuit-switching ap-
proaches borrowed from the ICT sector.
The two-layer procedure has been proven in the laboratory
as well as on a full-scale platform (19 interconnected DC
nanogrids) over an extended period of time. A basic imple-
mentation using the circuit-switching approach as control has
been tested on a laboratory prototype with three layers.
Future research is needed for comparing and evaluating
the three approaches proposed for scaling the DC exchange
procedure to higher layers. For this, simulations as well as
prototypes should be designed with a realistic number of
subsystems as well as DC-DC converters with adapted voltage
ranges.
ACKNOWLEDGMENT
This research is partially supported by the “Subtropical
and Island Energy Infrastructure Technology Research Sub-
sidy Program” of the Okinawa Prefectural Government and
carried out by the research consortium of Sony Computer
Science Laboratories, Inc., Okisoukou Co. Ltd., Sony Business
Operations Inc. and OIST. Particular thank goes to Tadashi
Morita, Daisuke Kawamoto and Shigeru Tajima who greatly
contributed in forming these ideas.
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