Micro grids: different structures for various applications

The aforementioned definition of a micro grid is a term that covers many possible implementations. This article attempts to give a concise overview of the different structures one encounters with micro grids. The choice of a certain structure is usually tied into its place in the grid, where the micro grid is to be connected, who the grid’s owner is and its principal aim.

Figure 1:  Residential micro grid in Bronsbergen, Nederland [1]

At present, as in the past, the largest part of electricity production occurs in large central power stations linked with the transmission grid. The distribution grid was a passive grid, without sources, linked to and supplied by the active transmission grid. Little or no distinction was made between the different types of consumers connected to the grid. In the case of demand exceeding supply, loads are disconnected to maintain the grid frequency and parts of the grid can be suffer blackouts. Clients requiring a highly reliable energy supply must install a UPS system, which shares several basic principles with the micro grid philosophy: local energy (a battery or a diesel generator) is used, which can work both linked in a grid as well as in an island state. These days, there is a growing trend for active distribution grids, where distributed sources account for an important part of the electrical production. These sources are linked with the distribution grid (at medium or low voltage). In the case of network problems, it is still standard to de-energise certain loads and disconnect distributed sources. The sources should, however, be able to support the grid. This is important if a large amount of the production occurs in the distributed way. For the future, the possibility of micro grids continuing to work in island mode in the case of a fault on the electricity grid is being considered. Microgrids could even offer grid support. Demand Side Management allows the option of shutting off particular loads (e.g. certain household machines) in the case of demand exceeding supply. Also, consumers could be classified into different classes, with different levels of power quality (e.g. critical load, which sets high requirements on the availability of the mains voltage and the pollution of it, against non-critical loads) and controllability.

The first type of micro grid is a so-called utility micro grid. This is (part of) a feeder for a distribution grid, with local energy sources and consumers. This type of micro grid can facilitate a large-scale introduction of distributed sources and can locally receive the growth of the user’s power (either completely or partially), so that congestion problems can be avoided or reduced. A utility micro grid can also deliver ancillary services to the grid, for example the local delivery or absorption of reactive power and the guarantee of a very good power quality for (some of) the local users. The principal objectives for the implementation of this structure are the reduction in impact of grid faults on local users (as the micro grid can, if necessary, work in island mode, independent of the rest of the grid) and the simplification of connecting distributed sources. This can be applied both in urban as well as rural areas.

Figure 2: Micro grid in San Lorenzo, Ecuador [2], an example of a remote micro grid

Industrial or commercial micro grids form a second class of micro grid structures. These are typically a collection of critical and/or sensitive loads requiring high power quality and reliability. Typical examples are a data centre or a university campus, but also a shopping centre, a factory, an industrial installation or even a residential neighbourhood. The principal aims of this variety are an increase of the power quality, better reliability and also frequently energy efficiency compared to the electricity grid. Potentially, different loads can be further sub-divided into groups within the micro grid according to the required grade of power quality and reliability. The micro grid can switch over to island operation in the event of a grid fault, during maintenance, periods of poor power quality, or when grid energy prices are high.

The third and final type of micro grid is the so-called remote micro grid. For the provision of energy in remote areas, developing countries and (geographic) islands, locally available energy sources, often renewable, are usually chosen. Combined heat and power (CHP) may be used. An autonomous micro grid is a good network structure for such cases, where it can also be possible to connect this micro grid to the electricity grid in the future. There are often problems with extending the electricity grid to these remote areas, and it is necessary to work with a pure island grid. It is thus very important that the local generator is adequate and its energy production is sufficiently reliable to ensure that the local consumers receive the highest possible availability in the energy provision. Failing this, it is possible that certain loads would have to be disconnected on an irregular basis to guarantee the stability and correct operation of the net. The application of energy storage can help the spread of this type of micro grid to a large extent.

Of course, the emergence of such micro grids brings many changes. In a vertically integrated market, where generation, transmission and distribution assets are in the hands of a single entity, optimum decisions over the placement and connection of distributed sources can be sought relatively easily. When the distribution operator is no longer owner of the distributed sources, as can be the case in a free electricity market, conflicts can occur between the wishes of the grid operator and those of the owner. The owner is primarily interested in the profit of his installation, determined by the aggregated energy production, whereas the grid operator makes his plans with regard to the operation and expansion of the grid and the resulting necessary investments, principally from the perspective of the (maximum) energy. Measures must thus be taken to ensure that the distributed sources do not look only at the profits, but also contribute to attaining an optimal operation of the entire distribution grid.

The first micro grid in The Netherlands: Bronsbergen

The first micro grid in the Netherlands is located in the Bronsbergen holiday park, near Zutphen (about 100km to the west of Amsterdam). This park consists of 208 holiday homes, of which 108 have been fitted with a solar PV installation on the roof. Peak generation capacity is 315 kW compared with a peak load of 150 kW. Furthermore, two battery banks have been installed as energy storage. It is, after all, not out of the question that when the power demand is high, the PV installations will not be able to produce the necessary power. To allow for this, the energy from the batteries can be recovered, and these can then be recharged later on (when the PV installations are producing more than the demand).

The topology of the micro grid in Bronsbergen is shown in Figure 3. It consists of four parallel low voltage branches that the houses are connected to.

Figure 3: Topology of the micro grid in Bronsbergen

Each branch is protected with a safety fuse rated at 200 A and equipped with measuring equipment which determines the active and reactive power over the line. These measuring appliances are linked to the master, which measures the total amount of active and reactive power exchanged between the micro grid and the mid-voltage grid. The micro grid is linked to the mid-voltage grid by a 400 kVA transformer (400 V – 10 kV). The master can exchange data via GSM communication with a computer (e.g. in a dispatch centre). The battery banks are connected to the grid connection on the load side of the 630 A fuse.

The principal objectives of this micro grid are to guarantee the electrical energy’s quality and assure the supply. Moreover, it is used in research for the integration of micro grids, amongst other things to look at the practical problems that can be expected with such grid structures.

At the moment, several problems have been concretely identified with regard to harmonisation (among which there is sometimes an important third harmonic in the neutral conductor – see Figure 4), voltage imbalance resulting from an unequal division of the load over the three phases and maintenance of the voltage amplitude within the standard.

Figure 4: Neutral Conductor

Regarding research objectives, tests have already been completed successfully where the grid has been allowed to function as an autonomous island (disconnected from the mid-voltage grid) for a period of 24 hours. The opening and closing of the coupling circuit breaker with the 10 kV grid can also occur automatically. In the coming months, subsequent issues will be investigated further:

• Maintain the correct operation of the micro grid due faults on the medium-voltage grid or in one of the branches of the micro grid itself.
• Reduction of the harmonic pollution and buffer any potential resonance phenomena.
• The development of an optimal energy management system to maximise the battery banks’ life-span.

Although in this micro grid only PV installations and batteries are used, Continuon – the grid manager involved in the operation of the micro grid – also works on comparable projects concerning cogeneration units, another important small-scale energy source for the future.

Figure 5: Installation of the battery banks

The residential micro grid of Am Steinweg in Stutensee, Germany

One of the first pilot projects on micro grids with renewable energy in a residential neighbourhood was carried out in the neighbourhood Am Steinweg in Stutensee, a German village located about fifteen minutes north of the Karlsruhe.

This is a three-phase low voltage grid with a neutral conductor, which is linked in one place by a 400 kVA transformer to the medium-voltage grid and has a circular structure. The energy sources in this micro grid are a CHP (with an optional electrical power of 28 kW), different PV installations (with a nominal power of 35kWp) and a lead-free battery bank (880 Ah) with a bi-directional inverter. This inverter is designed for a power exchange of 1000kW. The batteries can deliver this power for half an hour. In total, 101 apartments are linked to this grid. The maximum active power through the transformer is determined as 150kW.

Figure 6: Structure of the residential micro grid in Stutensee

Figure 6 shows the structure of the micro grid, with the division of loads and sources.

The presence of a storage element prevents the over-voltage that would otherwise result when the PV installation produces more than the instantaneous demand and allows that peak shaving to be achieved. (During periods of high demand, the batteries deliver part of the power, reducing the amount drawn from the net.) Sometimes, it is also possible to match the local demand perfectly with the local production, so that no power runs through the transformer, and thus the micro grid works like a kind of virtual island, independent of the grid. Storage is important in grids with a significant proportion of renewable energy sources, considering the variable power output of these sources. Without storage, it is a nearly impossible task to ensure that the local demand can be fulfilled at any one time by the local generation.

Figure 7: Solar panels and battery storage in Stutensee

An energy management system regulates the energy current in the network so that the micro grid is used as efficiently as possible. This can lead to a reduction of the maintenance costs and an increase of the lifespan of the CHP as well as the pursuit of the lowest possible electricity price for consumers. It also tested whether it is possible to reconcile the residents’ use with local generation in any way. An example of this is the “Washing with the Sun” initiative. Residents received a text message if the PV installations produced too much energy, in which they were informed in what period the solar panels generated a lot. If they adjusted their peak consumption (including washing machines, hence the name of the initiative) with these periods of optimal photovoltaic production in mind, they received a financial bonus. This initiative seemed to be a success, considering the great response from the residents.

Figure 8 : Text message saying “Wash now!”

Apart from electrical storage, a thermal storage unit was also placed with the CHP, in which the electricity production of the WKK could be regulated more or less independently of local demand for heat.

Figure 9: Maximum power via the MS/LS transformer with (blue) and without (purple) energy management system

Figure 9 shows the maximum power travelling via the transformer with and without energy management system. It is clear that this system reduces the peak load of the transformer to an important extent and can save a lot of money in consequence.