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Microgrids

Microgrid architectures



Bertrand Cornélusse
[email protected]

Content of this lecture

In this lecture we review microgrids architectures, that is which components form a microgrid and how to interconnect them.

In the next lectures we will focus on the components themselves, on features that are important for operation, both from a technical point of view and from an economical point of view.

AC grids

An alternating current (AC) microgrid is a microgrid where components are coupled through one AC bus (if there is only one voltage level).

  • Most microgrids are AC
  • Typically, AC microgrids where the demand > 5kW are three-phase!
  • Required if you want to connect to the public grid (in Belgium)
  • Equipment in general require less components per unit of power transferred
  • Easy to generate a rotating field for motors
  • (Three-phase power transfer is a constant expression, if the phases are balanced)

Grid topologies

Most common: radial architecture

  • Subject to availability issues (one single path to a load)

Alternatives:

  1. provide a redundant path to each load (more robust than radial)
  2. provide spatially diverse paths (more robust than 1)
  3. ring-type distribution (Can isolate a fault and still feed all but problematic zone)
  4. ladder type distribution (yet more connection possibilities)

Note: a more complex system also needs more complex protection schemes.

See chapter 7 of [1] for more information.

AC coupling example

Let's take the example of a house or a small company that is running at low-voltage (230V or 400V) and has a grid connection plus a backup diesel generator, some PV panels, a battery, and some appliances.

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Automatic transfer switches

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Automatic transfer switch principle

Power electronics interfaces

Power electronic circuits are interfaces

  • between devices (DERs and loads) and the power distribution grid
  • between the microgrid and the distribution grid (PCC)

Purpose: enable a controllable (bidirectional) flow between devices

*DER: sources of electric power that are not directly connected to a bulk power transmission system. Distributed energy resources include both generators and energy storage technologies. (T.Ackermann, G.Andersson, and L.Söder, “Distributed Generation: A Definition,” Electric Power Systems Research, vol. 57, issue 3, April 2001, pp. 195–204.)

Types of power electronics interfaces (from [2]) $\rightarrow$ More in next lecture.

Grid-following converters (Fig (b)): can be represented as an ideal current source setting the active and reactive power injected into / withdrawn from the grid.

Grid-forming converters (Fig (a)): can be represented as an ideal AC voltage source setting the voltage amplitue and frequency of the local electrical grid.

Grid-supporting converters (Fig (c)): "inbetween the two others", implementing functions to support the grid, e.g. droop control.

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All these functions are achieved using several nested control loops.

Example: solar inverter

.grid[ .kol-1-2[.width-100[]] .kol-1-2[.width-100[]] ] Here it is a three-phase inverter from SMA. Source: website of SMA

Requires a network signal to work!

Example: Vehicle to grid

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Example: Automatic transfer switch, grid forming inverter & battery charger

.grid[ .kol-1-2[.width-100[]] .kol-1-2[.width-100[]] ] Source: website of Victron.

Characterizing power distribution architectures based on how power conversion is performed

  • Centralized: power conversion is performed at a single power electronic interface. Example: .grid[ .kol-1-2[.width-80[]] .kol-1-2[.width-80[]] ]
  • Distributed: power conversion functions are spread among converters
  • may lead to parallel or cascade structures

.footnote[Source:https://www.alma-solarshop.com/solax-power-inverter/982-hybrid-solax-inverter-x1-50t-hv.html]

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Rules for connecting DER to the grid (Belgian case, C10/11)

Synergrid

Synergrid is the federation of electricity and gas network operators in Belgium. Synergrid establishes prescriptions for a series of topics related to distribution systems.

In the "Technical Prescription C10/11 of Synergrid, edition 2.1 (01.09.2019) (English translation)", you can find the rules that apply to a new installation.

"This document C10/11 lays down the technical requirements relating to the connection of power generating plants capable to operate in parallel to the distribution network. The objectives of this document are the following:

  • ensuring proper operation of the distribution networks;
  • improving the safety of staff active in these networks;
  • protecting the distribution network’s infrastructure;
  • and contributing to the general system stability. "

Application domain of C10/11

Applies to:

  • Plants < 25MW connected to the distribution grid
  • Any energy source
  • Without limitation regarding the possibility of actually injecting energy to the distribution network; this means, for example, that it is also applicable to power-generating plants equipped with a zero export relay. (...)
  • ...

But not to:

  • Off-grid systems
  • Backup systems (not actually able to feed in the grid)
  • ... (elevators)

Special cases

A power backup system (as specified in § 4.1.9) will only operate in parallel with the distribution network for a short time in the following sporadic cases:

  • During tests
  • In case of islanding / reconnection after a network faults ("make-before-break")
  • ...

There are maximum durations depending on the cases.

--

In case of infringment, either: .grid[ .kol-1-2[

  • all rules of C10/11 apply to the backup system
  • or parallel operation made impossible ] .kol-1-2[<iframe src="https://giphy.com/embed/x6RunS9u1L3AA" width="240" height="135" frameBorder="0" class="giphy-embed" allowFullScreen></iframe>

    ] ]

Maximum power limits for a small power-generating plant

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  • Each power-generating unit must be equipped with an automatic separation system
  • If the power-generating plant includes an energy storage system,
    • an EnFluRi sensor must be provided to control the power injected on the distribution network.
    • the EnFluRi sensor is a directional power sensor having a communication link with the energy storage system.
    • the power injected into the distribution network is limited to the maximum power of the other means of power-generation.

Settings of the automatic separation system (Annex C)

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Syncrocheck

The power-generating units which synchronize with the voltage on the distribution network (such as synchronous machines, island equipment ...), have to be equipped with a synchrocheck relay (equipped with a synchroscope) of a type homologated by Synergrid.

The synchrocheck is set as follows unless determined otherwise by the DSO:

  • Voltage difference < 5 %
  • Phase difference < 5°
  • Observation time = 0,5 seconds

Technical basic requirements regarding the power generating units (Annex D)

E.g. Specific for a small power-generating plant (D.7.1.1)

By default, the power generation unit must operate according to the following rules:

  • When the voltage $\leq 105 % U_n$ : $\cos \phi = 1 (Q=0)$
  • When the voltage $ > 105 % U_n $ : free operation with $1 \geq \cos \phi &gt; 0.9$ under-excited. (no overexcited operation allowed)

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DC microgrids

DC microgrids

  • The distribution system is DC
  • Requires DC to DC converters to adapt voltage to devices
  • DC to AC to power AC loads, or to inject in the public grid
  • AC to DC to convert AC generation to DC (e.g. from public grid to microgrid)
  • DC microgrids are not widespread but gain some interest

DC microgrid example

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DC vs AC: pros

.grid[ .kol-1-2[.center[:)]

  • DC systems enable a simpler integration of distributed energy resources (DERs*), since many of them are either DC by nature or require a DC interface anyway
  • Parallel distributed architectures are simpler to realize in DC:
  • unnecessary frequency control and phase synchronization
  • Frequency control is not necessary in DC systems
  • unwanted harmonic content may by easier to filter too] .kol-1-2[.center[:(]
  • Autonomous distributed control harder in DC than in AC because no information carried through the signal (frequency, phase)
  • There are stability issues due to DC-DC conversion stages
  • It is more difficult to clear fault currents: the signal “does not go through zero”. Hence protections are more costly and harder to set up.] ]

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A first microgrid demonstration

Lab visit

Assignment

By teams of 2, write a little report and draw an electrical diagram of the demonstration board:

  • wiring diagram
  • protections
  • equipment ratings (voltage, current, power)
  • types of controllers and regulations
  • cable sections
  • try to get some datasheets to understand how components work, can do and cannot do

References

[1] Kwasinski, Alexis, Wayne Weaver, and Robert S. Balog. Microgrids and other local area power and energy systems. Cambridge University Press, 2016.

[2] Rocabert, J., Luna, A., Blaabjerg, F., & Rodríguez, P. (2012). Control of power converters in AC microgrids. IEEE Transactions on Power Electronics, 27(11), 4734–4749. https://doi.org/10.1109/TPEL.2012.2199334

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The end.