# A Real Model of a Micro-Grid to Improve Network Stability
## 4 Microgrid Voltage Stability Control
### Microgrid Voltage Level Stability Control: Control Strategy of the CC  
The control framework is centrally implemented in LabView. The algorithm recognizes two control levels, a higher and a lower one (Figure 8). The lower layer is responsible for distributing the energy as needed or according to the higher level strategy. The control commands are transmitted at a very high speed, in the order of milliseconds. The safety of the system and the basic operations are ensured mainly at the lower level. In terms of security, the primary problem lies in monitoring the battery charge to prevent power off take at a low voltage or, conversely, to eliminate any charging if the voltage is too high. These protective elements are contained twice in the model: first, they are installed as hardware in the CB1 and remain effective during the energy transfer between the main network and the other batteries; second, the system comprises software protection, which duplicates its hardware counterpart, and also controls the transfer of energy from the power stations directly into the battery. Specifically, the elements are used in simulating the remote sources, another part of the lower layer. The data from these sources are processed, enabling the influx of a corresponding amount of energy to the grid through two 12 V batteries (denoted as BAT2 in Figure 1). Yet another item affiliated with this level consists of the energy transfer control, materialized on the basis of the actual voltage values in each battery; here, the central idea is invariably to secure enough energy in the main network, from which the appliances draw the required power. Figure 1 introduces four different types of storage, \(A\) to \(D\). For better orientation in the network management, we will describe their function within the grid. The first part, \(A\), is directly connected  
Figure 8: The structure of the SEM control and the control levels. to the main smart grid network, with the energy stored in two 12 V batteries connected in series; the main line is thus 24 V. As already mentioned, all appliances are connected to this line. To eliminate any unnecessary energy transfer, the largest source of energy is directly connected to the 24 V storage tool, and therefore directly to the main network. In this case, the grid comprises also the two photovoltaic panels. The remaining parts of the sequence \(A\) to \(D\), starting with portion \(B\), are used predominantly for temporary energy storage or to store the energy supplied by the other sources. In block \(B\), the simulated energy from the remote sources is stored in two 12 V batteries connected in parallel and can be transferred to the main line, if necessary. Simultaneously, however, the batteries are used as temporary storage for the main line, especially when the remote sources do not generate sufficient energy. Part \(C\) is only used to store electricity temporarily and finds application mainly in situations when the solar panels and remote sources provide too much power and the consumption or storage size cannot cover the entire incoming volume. The last part, \(D\), is hydrogen storage. The overall control is set to a three-day cycle. In this cycle, the normal operating condition occurs if the power plants supply enough energy during one day but provide only a reduced amount over the two days that follow. For such operation, it is sufficient to regulate the energy transfer only at the lower level of control. In any case when the sources ensure only a low volume of energy during all the three days, the public power grid must be used to compensate for the shortage. Otherwise, with surplus energy, the higher level is involved in the control process. In this context, one of the aims of the higher layer is to use all the energy from the sources available at a given moment. If there are more days with surplus supply, the storage of energy is disabled by the high voltage of the batteries, and the source powers must be reduced.