One of the advantages of mini-grids against other decentralized solutions, like stand-alone systems or Solar Home Systems, is the possibility of implementing energy management strategies in order to optimize the efficiency of the global system. If we ask to the users within a small community powered by SHS, for instance, we would probably find out that some of them do not use all the daily energy delivered by their systems whilst there are another ones that are not fully satisfied, since they would like to have more energy every day. These individual energy needs can also vary depending on the daytime, the day of the week or even the season. And this gap between production and consumption may become more dramatic when it affects to the amount of energy consumed (sold) in rural communities, where the monthly revenues from energy sales have to ensure the long-term operation of the plant.
So, if the target is to share energy among the users of individual systems to increase the global efficiency, how we could upgrade a cluster of stand-alone systems to a mini-grid with the minimum cost?
During the last PV-Hybrid and Mini-Grid Conference held in Chambéry (France), the Swiss company Studer-Innotec presented the case of a small community in the Swiss Alps. That community is composed of 32 chalets and the majority is powered by individual solar systems. Not all houses are occupied at the same time and, looking globally to the system, it was observed that there was a lot of solar energy production lost and therefore, a very low efficiency. In this context, the presentation by Studer described a method to use together many inverters, some acting as current sources and some as voltage sources. That is, a mini-grid of many individual Solar Home Systems (SHS) or also called a Distributed Mini-Grid (fig.1)
As introduced above, the goal is to share advantages but not problems and with that purpose, a special arrengement is implemented in each individual inverter to decide when it shares energy with the mini-grid or not.
How does it work?
A frequency control strategy has been implemented for this small community, what means that no information is carried by the voltage. Why frequency control? Because is a very robust information carrier since is not influenced by the quality/lenght of the LV line. Thus, there is a standard 230 V line but with a frequency varying from 48 to 54 Hz. And what is more important, no other communication between the individual systems is required.
To implement this strateggy, a central inverter (VSI) is added in order to provide the the voltage and the frequency to the island grid. The VSI sets a frequency to the grid and slightly modifies it around 50 Hz, as described above. Around this common point, the existing distributed inverters-chargers in all different houses are interconnected through a LV single phase line and act as current sources (CSI), being able to measure the value of frequency set by the VSI. Accordingly, the CSIs sychronize on the LV line and push or pull current on it without trying to modify the voltage. This is function of the frequency measured on the line and the status of their own batteries: from the measured frequency, they can deduce the battery voltage of the central inverter (VSI) and compare it ot their own state of charge (see fig. 2) This comparison allows knowing if one distributed inverter has more or less energy than the central one and then each one can follow predetermined energy managament rules.
Energy Sharing Rules
Under this strategy, decisions are decentralized (every CSI applies the rules for itself without knowing what the others are doing) but the rules were chosen so that the global system works in a coherent way to meet the goals of system optimization and losses minimization. In this regard, the logic implemented in each distributed inverter follows some parameters (see fig.3) Some of the most interesting are:
User energy limit: there is a quota of energy allocated for each user. Once it is gone, that individual system is disconnected to the mini-grid and obliged to work in standalone mode unless there is enough energy available in the global system (f > 51 Hz)
User power limit: every user is allowed to take a quota of power from the LV line. Above this value, the CS inverter has to add power from its own battery.
Excess energy of a CSI: when the battery voltage is higher than the battery setpoint, then the distributed inverter feeds the mini-grid with the excess power. This is done by setting the inverter battery voltage setpoint a little bit under the setting of the solar charge controller.
P.O. Moix, N. Zuchuat (Studer-Innotec); "A mini-grid of individual Solar Home Systems": A Distributed Mini-Grid. Concept and test site in Switzerland"; 6th European Conference on PV-Hybrids and Mini-Grids.