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Ateliers 1 & 2
Gestion du Réseau de Distribution Rurale
Michel VANDENBERG
Dr. Ingénieur, ISET – Université de KASSEL – Allemagne
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HYBRID MINIGRIDS FOR RURAL ELECTRIFICATION
Michel Vandenbergh
Institut für Solare Energieversorgungstechnik (ISET) e. V.
Königstor 59, D-34119 Kassel
tel +49-561-7294103
[email protected]
ABSTRACT: A hybrid power system is defined as an electricity production and distribution system which supply consists
of a combination of two or more types of electricity generating sources (e.g. solar photovoltaic panels, wind turbine
generators, hydro plants, fuel gensets, fuel cells, ...). Hybrid systems may also include several forms of energy storage
(e.g. battery, pumped storage, ...). The main goal of the minigrid is to provide reliable power with a good quality and at the
best price possible to the electricity consumers. With a good energy management, it is also possible to maximize the
share of renewable energies and to reduce the contribution of fossil fuels to a minimum. In order to design a stable
minigrid, four main architectures have been identified: Single fixed master, Single switched master, Multi-master diesel
dominated and Multi-master inverter dominated. The additional possibility of connecting the minigrid to the national grid
offers many advantages for the rural electrification planners. Recent innovations allow to design minigrids offering smooth
transitions during the connection and disconnection phases to the main grid.
RÉSUMÉ: Dans un mini-réseau hybride, plusieurs sources d‘énergie ( par exemple diesel, solaire, éolien, hydro) sont
combinées afin de produire l‘électricité qui alimente un petit réseau de distribution autonome. Le but principal du miniréseau est d’alimenter les charges électriques sans interruption, avec une bonne qualité de tension et au moindre coût.
Un mini-réseau bien géré permet également de maximiser l’utilisation des énergies renouvelables et de minimiser la
consommation en combustible fossile.
La technologie actuelle permet de concevoir des mini-réseaux qui sont alimentés par des sources d‘énergie
renouvelables, tout en fournissant une électricité de qualité aux consommateurs ruraux.
Afin de garantir la stabilité du mini-réseau, différentes architectures sont possibles. Quatre catégories principales sont
présentées: “Single master” fixe, “Single master” changeant, “Multi-master” avec générateurs diesel et “Multi-master”
avec onduleurs.
La possibilité de connecter facilement le mini-réseau au réseau électrique national est souhaitable, ce qui simplifie la
planification de l’électrification rurale. Des innovations récentes autorisent une transition en douceur lors de la connexion
et de la déconnexion au réseau principal.
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INTRODUCTION
Village electrification represents a high potential market for hybrid power systems. Many stand-alone diesel units
are powering mini-grids all around the world and could be retrofitted with renewable power generators and battery
storage units. Small hybrid power systems have proven to be a cost-effective solution for powering single user
applications. New technologies are today available for powering robust multi-user mini-grids.
The additional possibility of connecting the minigrid to the national grid offers many advantages for the rural
electrification planners. Recent innovations allow to design minigrids offering smooth transitions during the
connection and disconnection phases to the main grid.
Microgrids comprise electrical distribution systems with distributed energy sources, storage devices and
controllable loads, operated connected to the main power network or islanded, in a controlled and coordinated
way. The operation of microgrids offers distinct advantages to customers and utilities, i.e. improved energy
efficiency, minimisation of overall energy consumption, reduced environmental impact, improvement of reliability
and resilience, network operational benefits and more cost efficient electricity infrastructure replacement.
ARCHITECTURES FOR MINI-GRIDS
Current PV hybrid mini-grid system architectures can be classified into four categories. The classification is based
on which ac power sources in the mini-grid perform the “grid forming” function to control the mini-grid frequency
and voltage.
The single fixed master mini-grid architecture (see Figure 1a) has only the battery inverter connected to the minigrid and therefore it does the grid forming.
The single switched master mini-grid architecture (see Figure 1b) has multiple ac sources connected to the minigrid (typically the battery inverter and a fossil fuel genset), but only one source at any time does the grid forming.
In single master architectures, several battery inverters could be paralleled in a master-slave mode.
The multi-master rotating machine dominated mini-grid (see Figure 1c) has multiple ac sources (fossil fuel gensets
and PV inverters) connected to the mini-grid and simultaneously supplying power. The gensets do the grid
forming and the PV inverters follow the mini-grid voltage and frequency.
The multi-master inverter dominated mini-grid (see Figure 1d) is characterized by the coupling on the AC-bus of
most power sources via inverters. The power sources can be distributed in the mini-grid. E g. PV generators can
be integrated in the roofs of different houses. Several grid forming units (like battery storage or rotating machines)
could operate in parallel in a multi-master mode.
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Fig. 1c Multi-master rotating machine dominated mini-grid.
Fig. 1a Single fixed master mini-grid
Fig. 1b Single switched master mini-grid
Fig. 1d Multi-master inverter dominated mini-grid
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ISET has developed techniques for the multi-master inverter dominated architecture [3,4,5]. In the
innovative concept developed by ISET, reactive power/voltage and active power/frequency droops
are used for the power control of the inverters. The droops are similar to those in utility grids. The
supervisory control just provides parameter settings for each component. This way expensive
control bus systems are replaced by using the grid quantities voltage and frequency for coordination of the components. Such structure results in the following features: (1) simple
expansion of the system, (2) increased redundancy, as the system does not rely on a single grid
forming unit, (3) a supervisory control or a grid code is required.
Fig. 1c Multi-master rotating machine dominated mini-grid.
Fig. 1d Multi-master inverter dominated mini-grid
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INTERCONNECTION ISSUE
In order to validate the different functions of a low voltage microgrid, a specific test configuration
has been set up in the Design Centre for Modular Systems Technology (“DeMoTec”) of the Institut
für Solare Energieversorgungstechnik (ISET). In this highly innovative microgrid configuration, the
grid control is distributed among three distant inverters which are not linked by any fast
communication link. For primary control purposes, the sharing of power between these different
grid-forming inverters is made possible using the selfsyncTM algorithm.
Concerning the secondary control of the inverters, the implementation of a microgrid supervisory
controller was a crucial task for the demonstration of the microgrid. An adapted communication
environment based on internet and XML-RPC has been set up in order to allow the microgrid
supervisory controller to send control set points to the local generator controllers (Remote
Terminal Units) of the different power units (Figure 1).
Several critical situations have been studied, which included the transition from interconnected to
island operation after a fault on the main grid or the transition from island to interconnected
operation (Re-connection to mains after fault, microgrid black start).
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Figure 1: Communication infrastructure in the DeMoTec laboratory
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BATTERY
INVERTER 1
13 kW
PV
2 kW
High priority
Load 0.5 kW
BATTERY
INVERTER 2
13 kW
230 mOhms LV line
Manually operated relay
WIND
11 kW
Relay controlled by
Microgrid Superviser
Normal
Load 9 kW
High priority
Load 0.5 kW
Load 6 kW
DIESEL
16 kW
Load 6 kW
Relay controlled by
generator
UCTE
10 kV
Figure 2: Layout of the microgrid installation in the DeMoTec
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MICROGRID TEST CONFIGURATION
The three-phase microgrid under test includes the following components:
3 grid forming units (2 battery units and 1 diesel generator set)
2 renewable energy generators: PV (inverter) and wind (asynchronous
generator)
several loads with different priority levels
several automatic switches for sectionalizing the microgrid into up to 3
island grids, in order to increase the reliability.
supervisory control for a fully automatic operation of the microgrid
(disconnection, re-connection, black-start, optimal dispatch)
Connection to main medium voltage grid via a 100 kVA transformer
A purely resistive line simulator with a resistance of 230 mOhms,
representing about 400 meters of a weak low voltage line (NAYY 4*50 SE)
has been inserted between the microgrid bus and the Battery Inverter 1.
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5 TESTING THE ISLAND MODE
The island mode of operation has been validated on the DeMoTec
microgrid. The test has demonstrated that two battery inverters with
active power/frequency droops can share their active power in island
mode even if the distance from the main load to each battery unit is
very different. As is shown on Figure 2, a simulated low voltage line
has been inserted between the battery unit 1 and the microgrid bus.
The battery unit 2 is directly connected to the microgrid bus. Two
strategies for the secondary control in island mode have been tested:
The microgrid supervisor always starts the diesel generator and
regulates its power to keep the average power output of battery units
close to zero. Microgrid frequency is not regulated.
The microgrid supervisor starts the diesel generator only if a battery
state of charge is too low or if the power output of the storage units is
too high. New droop setpoints are sent to the batteries for regulating
the frequency at 50 Hz.
Figure 3a and Figure 3b present the results of the islanding test with
secondary control strategy 1. The actions of the supervisory controller
are also visible. The high impedance fault on the main grid with the
consequent islanding of the microgrid happens at time t1=2083 s. At
time t2=2400 s, after the diesel start, the supervisory controller, sends a
new setpoint to the diesel in order to reduce the power of the battery
units to zero. The remaining power fluctuations are due to the variations
of both the load profile and the wind power production. A new event
was an increase of the microgrid load by 6 kW (2 kW per phase). We
can see the immediate response of the battery inverters which take
each half of the new load. After a while, the new set point of the diesel
is sent by the supervisory controller and battery powers are close to
zero. The last event at time t3=2500 s is the reduction of the microgrid
load by 6 kW (2 kW per phase). Again, instant response of the battery
inverters, which take each half of the extra energy till the supervisory
controller sends a new power setpoint to the diesel unit.
Figure 3a: Inverters sharing power on
phase L1 before and after microgrid
islanding
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Figure 3b: Frequencies of mains (f_Mains) and microgrid (f_MG) before and after
microgrid islanding (no secondary control of frequency).
In Figure 4, a screenshot of DeMoTec SCADA presents the impact of implementing the
islanding test with the secondary control strategy 2. The microgrid frequency is controlled at
50 Hz, by sending new droop setpoints to the battery inverters. After the increase of the
load by 6 kW, the frequency drops very fast to 49.4 Hz due to the primary control and is
restored to 50 Hz by the secondary control. After disconnecting the 6 kW load, frequency
now increases to 50.6 Hz. Figure 4 presents the situation at the time when the secondary
control has again restored the 50 Hz.
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Figure 4: Islanding test with secondary control strategy 2. The microgrid frequency is controlled at 50
Hz. (screenshot of DeMoTec SCADA)
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VALIDATION OF EMERGENCY
FUNCTIONS
The supervisory controller’s emergency
functions have been validated on the microgrid
described in Figure 2. The black-start
procedure is well illustrated by the frequency
plot in Figure 5a. At the start, the microgrid is
splitted in three island systems. Each battery
inverter is powering its own grid and the main
microgrid bus is out of voltage. We assume
also a power failure on the main grid. The first
action of the microgrid supervisory controller is
to start the back-up diesel unit, which restores
the voltage on the microgrid bus (light blue
line). The two battery inverters then
automatically synchronize to the microgrid bus.
In order to do this reconnection smoothly, they
reduce their frequency. The batteries are then
charged by the diesel unit. After restoration of
the mains, the microgrid central controller
activates the synchronizer of the microgrid
switch and after a few seconds, the whole
microgrid is reconnected.
Power is provided to both inverters firstly by the
diesel unit (370 s < time < 485 s), and secondly
by the main grid. After restoration of mains and
before synchronization (455 s < time < 485 s),
the little power provided by the mains is due to
the losses in the 0.4/10 kV transformer.
•Figure 5a: Black-start test: frequency of phase L1
Figure 5b presents the active power on phase L1 during
black-start.
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POWER QUALITY ISSUES
During emergency states, power quality
issues have been investigated. No
significant voltage dips have been
recorded during these procedures. The
two plots in Figure 6a and Figure 6b
present the current and voltage wave
forms on phase L1 during the
synchronization procedure to the main
grid. The microgrid main switch is
closed at time tclose=484.93 s, without
any dip in the voltage. The transients
during an unintentional islanding event
are presented on Figure 7a and Figure
7b. The current wave form shows also
how the two battery inverters, which
are connected with very different LV
line length to the microgrid bus, are
sharing the active power.
ISLAND
OPERATION
ISLAND
OPERATION
ON GRID
GRID
CONNECTED
Figure 6a: Voltage transient on phase
1 during reconnection to mains
ISLAND OPERATION
ISLAND
OPERATION
ON GRID
GRID
CONNECTED
CONNECTED
Figure 6b: Current transient on phase 1
during reconnection to mains
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CONCLUSIONS
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State of the art concerning robust power system architectures for hybrid
mini-grid systems technology has been presented. Innovative concepts
for power system stability and energy management in a multi-master
inverter dominated architecture have been investigated.
Detailed results of testing a three phase microgrid with three grid forming
units (one diesel 16 kW and two battery units 12 kW each), a wind
generator 15 kVA and several photovoltaic generators, have been
studied. Normal and emergency microgrid functionalities (black-start,
island mode, islanding, reconnection, frequency control, power
sharing,...) have been successfully tested. During microgrid islanding or
reconnection phases, smooth voltage transitions were measured without
any power quality problem.
In all these situations, it was demonstrated that it is today possible to
combine a reliable uninterruptible power supply with the integration of a
high penetration level of renewable energy sources (wind and PV).
REFERENCES
[1]
Microgrid Laboratory Facilities - M. Barnes, A. Dimeas, A.
Engler, C. Fitzer, N. Hatziargyriou, C. Jones, S. Papathanassiou, M.
Vandenbergh - International Conference on Future Power Systems,
Amsterdam, 16.-18.11.2005
[2]
www.iset.uni-kassel.de/abt/FB-A/demotec/ground/plan.html:
Virtual Visit in DeMoTec (Microgrid Laboratory facility of ISET)
[3]
A. Engler, “Vorrichtung zum gleichberechtigten Parallelbetrieb
von ein- oder dreiphasigen Spannungsquellen”, (European patent: 02
018 526.26, US patent: 10/222,310, Japanese patent: 2002-240991)
[4]
A.Engler, “Applicability of droops in low voltage grids”,
International Journal of Distributed Energy Resources, Vol.1 No.1,
January-March 2005
[5]
http://www2.sma.de/en/solar-technology/products/islandgrids/sunny-island/index.html
[6]
http://microgrids.power.ece.ntua.gr/: Microgrids project
homepage
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ON GRID
ISLAND OPERATION
GRID CONNECTED ISLAND OPERATION
Figure 7a: Voltage transient on phase
L1 during unintentional islanding
ON
GRID
GRID
CONNECTEDISLAND
ISLAND OPERATION
OPERATION
Figure 7b: Current transient on phase L1
during unintentional islanding
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