A research proposal to improve the stability, efficiency, and reliability problems of low voltage DC microgrids from a communication control strategy point of view.

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INVESTIGATION OF THE CHALLENGES IN ESTABLISHING PLUG AND PLAY LOW
VOLTAGE DC MICROGRID WITH A VIEW TO INCREASING SYSTEM STABILITY,
EFFICIENCY, AND RELIABILITY.
Promise Beshel, PhD Research Aspirant, Sept. 2018, promise.beshel@hotmail.com
INTRODUCTION
The wake of industrialization came with its merits and demerits. Its development owes
huge credit to the growth of electricity [0]. At least with respect to climate change, cost,
and energy waste, the demand for more refined industrial processes and domestic
applications means that electricity has to be better generated, better transmitted, better
distributed, and better utilized, in a way and manner that offers the best of safety, stability,
efficiency, and reliability. Thoughts in this direction are responsible for the present-day
deviation from traditional methods of electricity generation, transmission and distribution,
towards Microgrid (MG) systems which importantly, offer the advantage of lower carbon
footprints.
The MG concept was originally proposed in 2002 [1]. It is considered the building block
of future low voltage distribution systems [1][3]. The Consortium for Electric Reliability
Technology Solutions (CERTS) defines Microgrid as an aggregation of micro sources and
loads operating as a single system, each one of them based on power electronics to
provide flexibility and controlled operation [2]. Other than this, a Microgrid is a system
comprising Micro generation sources, Distributed Energy Resources (DER), Energy
Storage Systems (ESS), and power loads. The terms DERs and micro generation
sources are loosely aligned; they make up local renewable or non-renewable generation
in small scale, connected to the AC or DC or Hybrid grid. The sources vary from solar
Photo Voltaic (PV) systems to wind turbines, natural gas turbines, biomass generators,
microbial fuel cells, micro hydro, ground source heat pumps, etc. The loads in a DC
Microgrid can be Direct Current (DC) or Alternating Current (AC), connected through a
Power Electronic Device [PED], offering an improvement in the system. However, this
improvement due to the coupling of the loads through PEDs is not without the risk of
introducing system instability due to the nonlinear behavior associated to the Constant
Power Load (CPL) [3].
Whereas nearness to electricity consumption domains from points of generation is a perk
that favors reduction of line losses in the Microgrid concept, its bi-directional nature of
electricity flow creates room for system imbalances that must be quickly responded to
when grid elements are connected and disconnected. To keep the output power constant,
a CPL such as a switching regulator varies its impedance in response to change in input
voltage. Therefore, the whole idea of a Distributed Generation (DG) can only make most
sense if the plug and play devices necessary for the implementation of the MG have least
negative impact on system stability, efficiency, reliability, and power quality. This proposal

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focuses on Low Voltage DC MG with a view to investigating the challenges unique to the
coupling of converters to the DC Microgrid.
Research works [1] [2][8…] have been done in a bid to investigate and improve on DC
Microgrids, and the challenges (please see the problem statement section of this
proposal) have generally ranged from safety considerations, to cost, reliability, efficiency,
and stability. With respect to stability, researchers have mostly adopted a generic method
of extracting a linear model of the system and applying such analytical methods as eigen
values against different factors to investigate the stability of the power converters in
relation to DC Microgrids.
PROBLEM STATEMENT
DC load profile is increasingly growing and DC Microgrid has comparatively proven
advantageous in meeting the load demands [7] [15], as well as complementing the power
demand of AC loads, but not without its pronounced challenges. DC Microgrids hold
extraordinary promise for a wide variety of situations, but there are still a few barriers to
deploy this technology, such as the initial cost of the system, lack of standards and code
of practice, etc.[15]
3
DC ESSDC Loads
PV Array Wind
Turbine
Utility Grid
DC/AC
Converter
DC Bus
DC/DC
Converter
Plug in EV
AC
Transformer
DC/AC
Converter
DC/DC
Converter
DC/DC
Converter
1
Research
focus
2
DC/DC
Converter
AC ESS
DC/AC
Converter
4
Where;
PV = Photo Voltaic
EV = electric Vehicle

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From Fig.1, it is clear to see that the Power Electronic Devices (PED) referred to in the
introductory part of this proposal are the power converters, coupled to the DC Bus at
nodes 1,2, 3, and 4, in this arbitrary model. However, Points of Common Coupling (PCC)
do not yet exist for isolated systems (such as data centres, telecommunications base
stations, etc.), either due to technical reasons or due to cost.
• Shuffling Between Modes
An LVDC MG either operates in Islanded (or Stand-alone) mode wherein it is
disconnected from the main utility grid either passively (say, due to fault) or actively
(perhaps for routine maintenance), or in Parallel mode in which case it runs concurrently
with the Main utility grid. A desirable feature is the ability to work in both grid-connected
and stand-alone modes of operation, including a smooth transition between them.
Different control strategies might be defined for each mode of operation and, therefore, a
high-speed islanding detection algorithm is very important in order to adjust the control
strategy accordingly [4]. With the utilization of DC distribution, telecommunication
stations, data centers, and other remote areas where power quality and reliability are
paramount are well supplied, nearly exclusively. The Particularly interesting concept is to
resolve the power supply of these kinds of systems only using Renewable Energy
Sources (RES). However, the variable nature of RES then imposes a power balancing
challenge in case of isolated operation [5].
• Harmonics
The main effects of harmonics in power systems are heating, overloading, and ageing of
equipment, and increased losses. Besides the increase in the number of harmonic
sources, the number of devices sensitive to the harmonic distortion has also increased.
Harmonics may lead to malfunctioning of power system components and electronic
devices. The harmonic distortion is one of the indices of power quality of a power system.
The estimation of harmonics is of high importance for efficiency of the power system
network. Harmonic measurements are also important for grid companies and end users
to characterize the performance of their networks and develop solutions to harmonic
problems. As utilities and customers change their connected loads, system impedance
also changes, resulting into resonances and harmonic problems which were initially
none existent [9].
• Lack of Standards
Integration of several renewable technologies which are intermittent may compromise
reliability which is of prime importance for utilities. The control systems required for any
distributed generation technology or storage technology to plug into the utility
Fig.1 Block diagram of an arbitrary small scale Low Voltage DC Microgrid.

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infrastructure are quite complex. Also there are questions around forming standards
around technologies to be allowed to plug in, and in what way [6].
According to Associated Power Technologies, while there is no national standard
dictating Total Harmonic Distortion limits on systems; there are only
suggested/recommended values for acceptable levels of harmonic distortion [10]. The
IEC 1000-2-4 (1994), IEC 1000-1-1 (1992), IEC 1000-2-2 (1990), and IEEE 519-1992
standards are merely recommendations, hence are not enforced.
• Losses
With the challenges of frequency and synchronization eliminated, and line losses reduced
reasonably well in the LVDC MG as opposed to the AC MG’s, there are however the
challenges of conversion losses, switching losses, and grid impedance fluctuations due
to converters.
The driving factor in efficiency for many of today’s devices is the conversion from AC
power to DC power. It is not possible to get a 100% efficient conversion from one form to
the other. The energy is lost to either heat, magnetism, or shunted to ground depending
on the conversion technique. On the other hand, as loads go more and more to DC the
amount of these losses is increasing at the same rate. These conversions are
happening all over buildings and they are often not considered when designing a system.
By the same standard, many of these losses are amplified in conditioned spaces [7].
• Reliability and Performance
From a Control point of view, there are three main strategies; The Independent Control
strategy which although reliable, offers a shortfall in performance due to the lack of
operational information between converters; The Centralized control strategy which due
to a central controller, offers a single point of failure through external communication links;
and the Distributed Control strategy which, although the best strategy amongst the first
two due to the elimination of single point of failure and utilization of DC bus signaling
distributed control methods, offers the limitation of Bit Error Rate (BER) at 2kbps, and
limited bandwidth for information exchange between converters[15].
LITERATURE REVIEW
A lot of work has been done in the area of control strategies aimed at the stability of
Microgrids, generally. As regards the LVDC MG, however, the concerns are usually bent
on harmonics suppression, switching and conversion losses. Impedance-based stability
analysis, droop control, power line signaling, and distributed bus signaling are methods
amongst others that have been researched and applied. Plug and Play functionality has
been viewed in different respects but this proposal dwells on the parameters unique to
connection and disconnection of converters from the LVDC MG.

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1. Harmonics and resonance concerns of power electronics have been investigated widely
in distribution systems, and they are crucial issues to be addressed for the integration of
converter interfaced generators (CIGs). These are practical detection and mitigation
solutions to the existing harmonics and resonance problems, that is, harmonics from the
converters and their interaction with the harmonic impedance generated by passive grid
elements such as cables, transformers, and harmonic filters. These solutions have
focused on the steady-state harmonic interactions. However, wide-bandwidth controls
may cause system instability or oscillations owing to the lack of stability margins at
frequencies ranging from a few hundred hertz to several kilohertz [8].
2. According to Santiago Sanchez and Marta Molinas, the design of Microgrids at the level
of distribution systems requires a stable behavior for multiple operation states. The tools
used to study the stability of such systems require the estimation of the grid impedance.
By the use of the grid impedance estimation around an operation point, it is possible to
define a space variable-parameter to obtain a qualitative or quantitative measure from the
operation to the unstable boundary. Their study presents a comparison of the Kalman
filter and the recursive least squares method for the estimation of the grid impedance.
The grid impedance is estimated by the technique based on two neighbor operation
points. The results were validated by a hardware in the loop and an experimental setup.
Finally, the estimated values of the grid impedance of a Microgrid were used with a large
signal stability study of a dc constant power load [3].
3. Existing time-domain phasor-based and Electromagnetic Transient (EMT) simulation
tools lack the capability of capturing the fast dynamics of power electronic converters and
providing analytical insights. Motivated by this observation, an analytical approach is
developed in the current study for the impact analysis of a new CIG interconnection. First,
the output impedance of the new CIG is theoretically developed by incorporating control
dynamics and an interface filter, such as the LCL filter in this study. On the basis of this
converter output impedance model, the equivalent impedance of the power grid at the
PCC is subsequently derived. This impedance model represents the new as well as the
existing nearby converters and is used to evaluate the influence of the interactions among
converters under various planning options and credible operating conditions on the
system stability (e.g., points of interconnection (or sites), line distance (or impedance),
control parameters, generator tripping, etc.). Detailed EMT simulations have
demonstrated the accuracy and efficacy of the derived models and methods [8].
4. The application of power electronic devices in Microgrid, bring the problem of converter
cascade stability. As nonlinear switching circuits, the Impedance is time-varying and
nonlinear. When the structure of the power electronic devices is complicated, it is difficult
to predict the stability with conventional impedance ratio methods. Besides adjusting the
impedance ratio, adding intermediary filter and designing bus controller are effective
methods to improve the cascade stability. At the moment, the domestic study on cascade
stability is very limited. Abroad, only a few scholars are doing the relevant theoretical

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analysis and application research, such as F. C. Lee of VT, M. Ehasani of Texas A&M
University. In Microgrid, converters cascade stability still needs a lot of research work
[14].
5. Andre´ Pires et al investigate the stability of DC Microgrids with a control strategy adopted
for both stand-alone and parallel modes of operation. In the islanded operation mode,
droop control was the basic method for bus voltage stabilization without communication
among the sources. The paper goes on to show the consequences of droop
implementation on the voltage stability of dc power systems, whose loads are active and
nonlinear, e.g. constant power loads. The proposed equivalent model keeps the
qualitative behavior of the system while reducing the complexity of the stability analysis.
The set of parallel sources and their corresponding transmission lines were modeled by
an ideal voltage source in series with an equivalent resistance and inductance. The
approximate model allowed performing a nonlinear stability analysis to predict the system
qualitative behavior due to the reduced number of differential equations. Additionally, non-
linear analysis provides analytical stability conditions as a function of the model
parameters and it leads to a design guideline to build reliable Microgrid System (MGS)
based on safe operating regions [11].
6. The Bus-Signaling Method for the autonomous coordinated control of DC islanded
Microgrid is classified as ESS Master Control, RES Virtual Inertia Control and Demand
Side Control. These control strategies can be combined together in order to target at
different State of Charge (SoC) scenarios of ESS in a decentralized way. In terms of
coordinated performance, these control strategies based on droop method meet the
limitations: i) Droop Control is usually implemented on Voltage Control Mode (VCM)
converters, while most RES units embrace Current Control Mode (CCM) converters ii)
the conditions of SoC are not taken into account when developing decentralized power
control strategies. In this sense power line communication methods are proposed, which
inject a range of high frequency components over AC or DC power lines as
communication signals to achieve power management among converters. They attract
much attention since the coordinated signals (i.e. SoC of ESS, power generation of RES)
that can be exchanged depend on power lines instead of using external fast
communication links. However, these methods intensively introduce a series of high
frequency noise to the power cables. Another similar technique employs bus voltage
levels as communication signals. Based on these bus voltage signals, ESS and RES units
change output power or operation modes. However, this control law needs the mode
changing actions, which makes the parameters of each mode hard to be designed and
even may cause system instabilities during the dynamic switching process. In addition,
few of them discuss the full scenario considering both power generation and Demand
Side Management (DSM). In this paper, a novel coordinated control for islanded DC
Microgrids is proposed, which consists of two levels: a primary local control and
centralized secondary control. The primary control is based on bus-signaling method
(BSM), where the bus voltage is regulated as a function of SoC and acts as a coordination

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signal to control power generation/consumption from RES/distributed loads. In addition,
a higher secondary level is presented to restore bus voltage for the applications that
require strict bus voltage regulation [12].
7. Zhengyu Lin et al proposed a novel hybrid control strategy whose methodology can be
very resourceful to this research work’s objective, albeit that it is formulated around the
AC/DC Microgrid model. The strategy leverages on the conventional state-space
averaging model for the DC side and the dynamic phasor model for the AC side, on a
Fourier Transform base. Small signal stability Analysis is conducted with the help of the
developed hybrid model to investigate the effect of the droop gains on the hybrid AC/DC
MG. Conclusions of stability analysis are verified through time domain simulation in
Matlab/Simulink [13].
8. Zhengyu Lin et al, in Novel Communication Method between Power Converters for DC
Microgrid Applications, address the challenge of implementing DC Microgrid from a
communication perspective, with focus on the Physical layer. The method their research
employed utilizes switching frequency detection method to provide cost-effective low
bandwidth communication links for low power rating DC Microgrid applications to develop
various distributed control algorithms.[15]
RESEARCH QUESTIONS
1. From a Distributed Control point of view, what parameter can be used to detect the
switching frequency other than the switching noise?
2. How can the bit error rate of the model be reduced without using lower order
modulation?
3. What can be done to increase the communication speed?
4. What can be done to bypass the IEC/IEEE lack of enforcement on design
parameters that bring the harmonic distortions to barest minimum?
RESEARCH OBJECTIVES
1. To understudy and understand control strategies of relevant past research works.
2. To develop a solution that harmonizes converter parameter variations of different
manufacturers, with an aim to proposing and developing a new LVDC MG topology
that suppresses losses more efficiently.
3. To develop, test, and analyse a novel control strategy that combines at least
communication and hardware strategies.

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PROPOSED SOLUTION
Preamble: There is often a trade-off between cost, bit error rate (BER), bandwidth, and
signal-to–noise ratio [16][18]. BER decreases with the increase in the Eb/No value.
Increasing the Eb/No value means increasing the Signal power (Eb) with respect to Noise
energy (No)[17]. The traditional way of measuring noise immunity is measuring/calculating
the probability of a bit error given Eb/No. This is a good way of making everything fair.
Frequency Shift Keying (FSK) signals have good noise rejection for their data rates, they
are easy to decode, and have good sensitivity, with lower probability of error (Pe) and
high Signal to Noise Ratio (SNR). However, they are not bandwidth efficient [22]. FSK is
a great choice, but comes with more noise in its side bands and therefore you will not be
able to place lots of systems close to each other in space or frequency[23]. If the medium
between the transmitter and receiver is good and the signal to noise ratio is high, then the
bit error rate will be very small - possibly insignificant and having no noticeable effect on
the overall system. However if noise can be detected, then there is a chance that the bit
error rate will need to be considered [18]. On the other hand, higher order FSK modulation
offers more bandwidth promise.
The Walsh-Hadamard transform (WHT) is the best known of the non-sinusoidal
orthogonal transforms. It has gained widespread use in digital signal processing since its
application is easy and it shortens processing time. WHT shares several characteristics
with Discrete Fourier Transform (DFT), however, Fourier Analysis is time-consuming
whereas WHT comparatively offers time saving to the tone of 48% because it requires
only elementary algebraic operations. These special aspects of WHT suggest its further
applications in various digital signal processing applications and multi-harmonic analysis.
Consequently its hardware implementation is also simpler [19][20].
From the foregoing, a proposed solution would be an important deviation from [15], which
should be comparatively more efficient while maintaining the advantages of lower cost,
flexibility, and reliability. The idea is to introduce two changes to [15];
i. Develop a variant of TI’s latest low cost Piccolo Microcontroller cited in [15], which
will use a WHT algorithm instead of the DFT algorithm, for direct analysis of the
harmonic spectrum of resulting outputs from the power converters synchronized
to the DC bus voltage. This should require lesser cost to implement than with DFT,
for earlier stated reasons.
ii. Adopt a communication method between power converters that utilizes higher
order FSK along with Pulse Width Modulation (PWM). For flexibility, ease of
implementation, and cost efficiency of such a solution, CML Microcircuit’s
CMX7164 Integrated Circuit (IC) can be employed.
The CMX7164 IC combines the benefits of non-linear Power Amplifiers and higher data
rates. It is a modem that includes full 2FSK, 4FSK, 8FSK (coding 2, 3 and 4 bits per

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symbol, respectively) and 16FSK, supporting symbol rates of 2k to 10k symbols per
second. Additionally, it is built to handle the modulation and demodulation digitally, with
Digital Signal Processing (DSP). The chip includes full data filtering before modulation,
using a root raised cosine filter with an alpha (α) of 0.2, and also offers raised cosine or
Gaussian filtering for the data prior to modulation, for a few more bits per Hz of bandwidth.
The interface to the system microcontroller is by way of a serial C-BUS [23]. The only
challenge with this is that the CMX7164 is still just a modem hence the need to add
external transmitter and receiver circuits.
METHODOLOGY
This research begins with analyzing the broad divisions of existing control strategies,
analyzing their results, and measuring them against desired outcomes. Mathematical
models will be developed for combinations of the most widely accepted methods and
analysis of the resulting hybrid model done using simulations with MATLAB/Simulink. The
results will then form a basis for modelling a universal grid topology that accommodates
the parameter variations in converter designs by manufacturers.
RESEARCH TIMELINE
FROM TO ACTION

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