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Ateliers 1 & 2

Gestion du Réseau de Distribution Rurale

Michel VANDENBERGMichel VANDENBERGDr. Ingénieur, ISET – Université de KASSEL – Allemagne


HYBRID MINIGRIDS FOR RURAL ELECTRIFICATIONMichel 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 mini-réseau 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.

Michel VANDENBERGMichel VANDENBERG


• 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 mini-grid and therefore it does the grid forming.

• The single switched master mini-grid architecture (see Figure 1b) has multiple ac sources connected to the mini-grid (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.



Fig. 1a Single fixed master mini-gridFig. 1c Multi-master rotating machine dominated mini-grid.

Fig. 1b Single switched master mini-grid Fig. 1d Multi-master inverter dominated mini-gridMichel VANDENBERGMichel VANDENBERG


• 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 co-ordination 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• 3 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).



Figure 1: Communication infrastructure in the DeMoTec laboratory



DIESEL16 kW

10 kV

UCTE

Load 6 kW

WIND11 kW

Load 6 kW

High priorityLoad 0.5 kW

Manually operated relay

Relay controlled by Microgrid Superviser

Relay controlled by generator

BATTERYINVERTER 1

13 kW

PV2 kW

230 mOhms LV line


NormalLoad 9 kWBATTERY

INVERTER 213 kW

DIESEL16 kW

10 kV

UCTE

Load 6 kW

WIND11 kW

Load 6 kW


Manually operated relay

Relay controlled by Microgrid Superviser

Relay controlled by generator

BATTERYINVERTER 1

13 kW

PV2 kW

230 mOhms LV line


NormalLoad 9 kWBATTERY

INVERTER 213 kW

Figure 2: Layout of the microgrid installation in the DeMoTec



• 4 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.



• 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



• 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.



Figure 4: Islanding test with secondary control strategy 2. The microgrid frequency is controlled at 50 Hz. (screenshot of DeMoTec SCADA)



• 6 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.



• 7 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

GRID CONNECTED

ISLAND OPERATION ON GRID

Figure 6a: Voltage transient on phase 1 during reconnection to mains

ISLAND OPERATION

GRID CONNECTED

ISLAND OPERATION ON GRID CONNECTED

Figure 6b: Current transient on phase 1 during reconnection to mains



8 CONCLUSIONS• 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/island-grids/sunny-island/index.html

• [6] http://microgrids.power.ece.ntua.gr/: Microgrids project homepage

ISLAND OPERATION GRID CONNECTED ISLAND OPERATION ON GRID

Figure 7a: Voltage transient on phase L1 during unintentional islanding

ISLAND OPERATION GRID CONNECTED ISLAND OPERATION ON GRID

Figure 7b: Current transient on phase L1 during unintentional islanding


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