Microgrids:

Non-exhaustive Review of
Technical Issues
Emmanuel De Jaeger
Mons - 30/03/2017

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Table of content

 Introduction – Microgrids general features

 Microgrids control
 Microgrids protection
 Microgrids power quality
 Architecture – AC vs DC concepts
 Conclusion

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 Definition of a microgrid
(in the context of electric power systems)

Group of interconnected loads and distributed energy

resources (DER) with defined electrical boundaries that
acts as a single controllable entity and is able to operate
in both grid-connected and island mode.

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 Use case scenarios for microgrids

 Microgrids that aim at improving reliability, and securing the energy supply
for all or part of their loads by islanding
 Distribution microgrid e.g. part of utility grid, campus, activity zone, etc.;
 Facility microgrid e.g. microgrids in a customer installation, a military
base, a hospital, etc.;

 Microgrids that aim at providing power to remote areas with lower cost, e.g.
isolated microgrids in rural electrification, oceanic islands, etc.;

 Microgrids that aim at reducing energy costs for microgrid users in the grid-
connected mode by optimizing the assets such as energy storage,
dispatchable loads and generators, providing ancillary services to the grid
etc.;

 Microgrids that aim at providing disaster-preparedness by optimizing the

assets such as energy storage, dispatchable loads and generators. This kind of
microgrids may be built in natural disaster prone areas, designed for the zone
where enhanced power supply is required for some critical loads, etc. 4
Microgrids: various types and operating modes

 Non-isolated microgrids
• Grid-connected mode (“on-grid” mode)
• Island mode
• Mode transfer ?

 Isolated microgrids

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 Non-isolated microgrid
Point of Connection (POC)
Transfer switch

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Isolated microgrid

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 Microgrids : general characteristics

 Distributed Energy Resources (Generator or Storage):

grid forming units vs grid following units

 Power Electronics Interface

 Provide the fixed local voltage
/ current regulation,

 Facilitate the DG unit to fast

track the load demand using
the energy storage devices,

 Incorporate the control

methods for load sharing
between the DG units.

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 Microgrids operation & control: main features
 DER work properly at predefined operating point or slightly different
from the predefined operating point but still satisfy the operating
limits;
 Market participation is optimized by acting on the production of local
DER and power exchanges with the utility;
 Active and reactive powers are transferred according to necessity of
the microgrid and/or the distribution system;
 Disconnection and reconnection processes are conducted
seamlessly;
 Sensitive loads, such as medical equipment and computer servers
are supplied uninterruptedly;
 In case of general failure, the microgrid is able to operate through
black-start;
 Energy storage systems can support the microgrid and increase the
system reliability and efficiency. 9
 Operation modes: non-isolated microgrids

 Control of the grid-connected mode

• DER and other components follow the same
requirements as in the utility grid (e.g. Synergrid Tech.
Spec. C10/11)
• Voltage and frequency response characteristics

 Control of the island mode

• Voltage: power quality ?
• Frequency:
o At least one (or one group of) controllable DER
to provide frequency reference
o Load tracking, load management, load shedding
o Transient stability
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 Mode transfer of non-isolated microgrid
 Mode transferring from grid-connected mode to island
mode can be divided into two types: intentional islanding
and unintentional islanding.
 The intentional islanding requires the microgrid to be
disconnected from the utility grid seamlessly. When there
is a fault in the utility grid which causes power quality to
deteriorate beyond the predefined limit at POC, the
microgrid is separated from the utility grid passively, and
this is called unintentional islanding.
 A microgrid may have black start capability, which is
needed if the mode transferring fails.
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 Control of microgrids

 Control of non-isolated microgrids

 Control of the grid-connected mode

 Control of the island mode

 Control of isolated microgrids

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 Control of microgrids

 Control of non-isolated microgrids

 Control of the grid-connected mode

• Microgrid response as a whole, as seen from the POC

• Control of the active power flow to the utility grid
• Control of the reactive power
- constant power factor λ;
- reactive power as a function of active power Q(P);
- fixed reactive power Qfix;
- reactive power as a function of voltage Q(U);
- reactive power as a function of active power and voltage
at the same time Q(P,U).
• Control of the internal resources in order to achieve
those objectives
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Control of microgrids
 Control of isolated microgrids: specific aspects
 The steady state control of the isolated microgrid might be different from
the island mode of the non-isolated microgrid.
 The ratio of electrical energy storage capacity to the total of other DER
capacity in the isolated microgrid could be much larger than that of the
island mode;
 The desired power quality of the isolated microgrid could be different from
that of the island mode depending on the load demand requirements;
 The isolated microgrid works currently in a self-sustained and independent
way based on the load requirements, while the non-isolated microgrid only
operates in island mode in limited time duration;
 From the control point of view such as voltage and frequency control,
strategies might be different even there are some common points;
 The frequency and voltage of the non-isolated microgrid in island mode
should be monitored all time in order to reconnect back to the utility grid;
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 Control strategies of microgrids

• centralized control: in this control scheme, a central

controller gives commands to the entire system
through a master-slave control configuration between
the central controller and the controllable distributed
devices;
• decentralized control: this control scheme is
accomplished through independent controls
communicating with each other. This strategy uses
intelligent devices that are strategically located to
detect the conditions and initiate the required
actions;
• hierarchical control: this control scheme combines the
central and decentralized control;
• autonomous control: this kind of control scheme is
accomplished through independent controls without
communication with other devices. 15
General configuration for the
control of a microgrid

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Microgrids control: general architecture (1)

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Microgrids control: general architecture

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Microgrids control: general architecture (2)

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Microgrids control: general architecture (3)

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Microgrids control: level 0 (inner control loop)

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Conventional (P/f and Q/V) droop control

𝑅 + 𝑗𝑋
𝐸1 𝐸2

𝐸1
𝑃= 2 2
𝑅 𝐸1 − 𝐸2 cos 𝛿 + 𝑋𝐸2 sin 𝛿
𝑅 +𝑋
𝐸1
𝑄= 2 2
−𝑅𝐸2 sin 𝛿 + 𝑋 𝐸1 − 𝐸2 cos 𝛿
𝑅 +𝑋
𝐸1 𝐸2
𝑃≅ 𝛿
𝑋
𝐸1
𝑄≅ 𝐸1 − 𝐸2
𝑋
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Conventional (P/f and Q/V) droop control

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Conventional (P/f) droop control: example

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Checking stability issues ?

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P/V and Q/f droop control in LV microgrids ?

𝑅 + 𝑗𝑋
𝐸1 𝐸2

𝐸1
𝑃= 2 2
𝑅 𝐸1 − 𝐸2 cos 𝛿 + 𝑋𝐸2 sin 𝛿
𝑅 +𝑋
𝐸1
𝑄= 2 2
−𝑅𝐸2 sin 𝛿 + 𝑋 𝐸1 − 𝐸2 cos 𝛿
𝑅 +𝑋
𝐸1
𝑃≅ 𝐸1 − 𝐸2
𝑅
𝐸1 𝐸2
𝑄≅ − 𝛿
𝑋
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Control strategies with full communication:
Master / Slave approach (from UPS technologies)

 Good voltage regulation and power sharing

 Communication
 Expensive
 Potentially vulnerable
 System reliability ?
 System expandability ?

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Control strategies with full communication
Master/slave control without central controller

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Control strategies with full communication
Master/slave control without central controller

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Control strategies with full communication
Master/slave control with central controller

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Microgrids Protection

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Microgrids Protection: generals
 Non-isolated and isolated microgrids shall have the
corresponding protective relaying functions to prevent
equipment from being damaged and to guarantee safe
operation.
 When a non-isolated microgrid transfers from grid-connected
mode to island mode, the configuration, power flow, neutral
earthing and short-circuit current values will change. Therefore,
the microgrid protection setting values shall be reconfigured
accordingly.
 In the non-isolated microgrid, there must be one set of
hardware protection and at least two sets of algorithm and
software.
 The microgrid protection shall also meet the requirements of
reliability, selectivity, sensitivity and speed.
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Principle for protection in a non-isolated microgrid
 Depending on the configuration, both the utility grid and
the DER in the microgrid contribute to the short-circuit
currents. The contribution to short-circuit currents from
the microgrid depends on the configuration of the
microgrid at any given time.
 In some microgrids, fiber-optic current differential
protection shall be used as the main protection, and
directional or non-directional overcurrent protection shall
be used as the backup protection.

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Principle for protection in an islanded non-isolated
microgrid and in an isolated microgrid
 The contribution to short-circuit currents from the
microgrid depends on the configuration of the microgrid at
any given time. Moreover, due to the solid state inverters
or converters, small fault current values cannot guarantee
the accurate action of the traditional protective relays, and
specific algorithms sometimes are needed.
 In some cases, fiber-optic current differential protection
should be used as the main protection, and directional or
non-directional overcurrent protections should be used as
the backup protection. 35
 Microgrids Power Quality & EMC
Emission ?
 Grid-connected vs Islanded: different impedances at stake
 Low frequency harmonics (up to 2 kHz)
 High frequency harmonics (2 to 150 kHz)
 Rapid voltage fluctuations and flicker
 Voltage unbalance
 Transient overvoltages

Immunity ?
 Disturbance immunity levels higher for microgrids devices ?
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Microgrids Power Quality

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 High Frequency disturbances
(“Supra-harmonics” in the range 2-150 kHz)

HF harmonic components measurement results (one day) near a LV

connected photovoltaic installation
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 High Frequency disturbances
(“Supra-harmonics” in the range 2-150 kHz)
Compatibility levels are still being defined in this frequency range !
2 132

Compatibility level (dB(µV))

130
Compatibility level (%)

1,4 129,5
128

1 126

124

0,65
122

0,5 120
1 2 3 5 9 10 5 9 10 20 30 50
Frequency (kHz) Frequency (kHz)

Note: EMC in this frequency range

can definitely be a serious issue if
PLC is used
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Electricity Distribution in DC ?

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 LVDC network topologies

Bipolar Homopolar

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Microgrids : LVAC vs LVDC ?

 Why distributingelectricity in DC ?
 LVAC vs LVDC micro-grids:
 Controlling the frequency ?
 Switching from on-grid mode to islanded mode and
vice-versa
 Issues: stability, reactive power, power quality ?
 Losses

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Control strategies without communication :
Conventional droop control for DC microgrids

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 Conclusions

 Importance of the technical aspects of microgrids

(Reliability, Quality, Efficiency)
• Control
• Protection
• Power Quality
• Communication / Cyber-security

 Economic, reliable and safe operation of microgrids

implies additional investments, beside the cost of
electricity generation and storage devices
 We are still at the beginning of the story: interesting
technical challenges must be investigated and need
RD&D efforts (Research, Development & Demonstration)

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 Sources

 Engie Lab (Laborelec) internal documentation

 CIGRE-CIRED JWG C4.24 Power Quality & EMC of Smart Grids
 International Conference on Electricity Distribution, CIRED Special Reports
2011, 2013 & 2015
 E. Barklund et al., Energy Management in Autonomous Microgrid Using
Stability-Constrained Droop Control of Inverters (IEEE Trans. On Power
Electronics, 2008)
 T. Vandoorn et al., Review of primary control strategies for islanded microgrids
with power-electronic interfaces (Renewable and Sustainable Energy Reviews,
2013)
 J.J. Justo et al., AC-Microgrids versus DC-Microgrids with Distributed Energy
Resources: a Review (Renewable and Sustainable Energy Reviews, 2013)
 X. Wang et al., A Review of Power Electronics Based Microgrids (Aalborg
University) http://vbn.aau.dk/files/170816843/JPE_11076_1.pdf
 B. Grégoire, Etudes des Micro-Réseaux d’Energie Electrique sous l’Angle de la
Qualité de la Tension (MSc thesis, UCL, 2011)
 IEC/TS 62898-2 Ed. 1 (working document IEC 8/1437/CD)
 IEC /IS 61000-2-2 (under revision; IEC 77A WG 8 working documents)

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