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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.

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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

Transfer switch

Point of Connection (POC)

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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.

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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.

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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

𝑃 =𝐸1

𝑅2 + 𝑋2 𝑅 𝐸1 − 𝐸2 cos 𝛿 + 𝑋𝐸2 sin 𝛿

𝑄 =𝐸1

𝑅2 + 𝑋2 −𝑅𝐸2 sin 𝛿 + 𝑋 𝐸1 − 𝐸2 cos 𝛿

𝑃 ≅ 𝐸1𝐸2

𝑋𝛿

𝑄 ≅𝐸1

𝑋 𝐸1 − 𝐸2

𝑅 + 𝑗𝑋 𝐸 1 𝐸 2

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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 ?

𝑄 ≅ −𝐸1𝐸2

𝑋𝛿

𝑃 ≅𝐸1

𝑅 𝐸1 − 𝐸2

𝑃 =𝐸1

𝑅2 + 𝑋2 𝑅 𝐸1 − 𝐸2 cos 𝛿 + 𝑋𝐸2 sin 𝛿

𝑄 =𝐸1

𝑅2 + 𝑋2 −𝑅𝐸2 sin 𝛿 + 𝑋 𝐸1 − 𝐸2 cos 𝛿

𝑅 + 𝑗𝑋 𝐸 1 𝐸 2

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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.

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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)

2

1

0,5

1 2 5 10 9

0,65

1,4

3

Frequency (kHz)

Co

mp

ati

bilit

y l

eve

l (

%)

132

120

5 10 30 50 9

129,5

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Frequency (kHz)

Co

mp

ati

bilit

y l

eve

l (

dB

(µV

))

126

122

124

128

130

Compatibility levels are still being defined in this frequency range !

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 distributing electricity 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)