## 1 Introduction
### The Aim and the Scope of This Research Work  
Power electronic converters are commonly used to control the parameters of electricity from a source connected to the grid [44]. The developed solution allows for uninterrupted generation of energy in on/off-grid modes, with the possibility of regulating the generated active power at the same time by changing the generator rotational speed and the fuel dose (torque of the driving engine), adjusting the operating point of the set to the lowest fuel consumption.  
The battery in the off-grid mode provides the energy needed to excite the generator and regulate the voltage, and also provides energy during load power changes (it provides the equivalent of the power system inertia for frequency control). The time to take energy from the battery or give energy to the battery is relatively short, several seconds at maximum power (see Section 4). Therefore, the battery capacity is relatively small. In addition, such a solution allows for the introduction of a series of power types of cogeneration systems connected to the grid using one power converter system, in particular a converter connected to the power grid. This is important in the context of the NC RFG network code applicable in the European Union and the requirements for energy sources connected to the power grid. For practical reasons, for low-power sources, up to 50 kW, it is more cost-effective and less complicated to use a universal power converter system. It is also easier to fulfill the technical requirements, in accordance with the NC RFG, in the field of active and reactive power control in response to values of voltage and frequency in the network.  
The main goal of this research work was to develop and build a power electronic system that would allow the cooperation of an induction generator driven by an internal combustion engine with the low-voltage (LV) power grid (on-grid mode) and the island network (off-grid mode), with a power of up to 50 kW. For the construction of the power supply system, typical solutions of individual converters were used [45; 46; 47; 48], but in a new topology, with the use of a small battery as a source of excitation of the induction generator and power balancing. In addition, the system should ensure almost uninterrupted switching between operating modes. For practical reasons, it was assumed that the switching time should not be longer than 40 ms. During this time, the voltage and frequency values must be restored. Uninterruptible switching between operating modes is necessary in the case of powering a farm that requires continuity of its energy supply.  
It was assumed that the power supply system with an induction generator must fulfill the following technical requirements:  
* in on-grid and off-grid modes, mCHP active power is controlled by changing the rotational speed and torque (e.g., fuel dose) of the internal combustion engine;
* in the off-grid mode, ensuring the quality parameters of electricity in accordance with the PN-EN 50160:2010E standard [49];
* switching time between modes < 40 ms.  
Content of the article:
* the concept of the designed power supply system, its technical and functional requirements, as well as the topologies of the built power converters are presented in Section 2;
* a description of the tests carried out, as well as a description of the test stand and the devices used is presented in Section 3;
* the results of the experimental tests for the most important operating states are presented in Section 4.