## 4 Results
### Off-Grid Operation--Dynamic Parameters during Load Power Changes
#### 4.3.3 Battery and DC Circuit Limit Tests  
Measurements were carried out in the off-grid mode to test the operation of the DC circuit. In the off-grid mode, the voltage of the DC circuit is stabilized by the PC3.1 and  
Figure 27: Off-grid operation with 5.5 kW load power changes and regulated generator power: (**a**) turning on a load; (**b**) turning off a load. (**b**)—load phase current; (**c**)—battery current; (**d**)—DC bus voltage; (**e**)—induction generator current. Source: author’s development.  
Figure 26: Turning off a 9.5 kW load in off-grid mode, with constant generator power: (**a**) current and voltage waveforms; (**b**) enlarged fragment of waveforms from Figure 26a during load switching. (**d**)—load phase current; (**e**)—battery current; (**f**)—DC bus voltage; (**f**)—battery voltage. Source: author’s development.  
PC3.2 converters. The PC3.1 converter is the source of energy for the DC circuit in order to maintain the set voltage of approx. 710 V. If the battery cannot receive the voltage of the surplus energy, the voltage of the DC circuit increases. With an excessive increase in the DC bus voltage (above 720 V), the PC3.2 converter is switched on, transferring the surplus energy to the resistor, R. When the load power increases, the battery is the source of energy for the DC bus, until a new operating point of the drive motor is established. The voltage and current waveforms in the DC circuit when the 8.5 kW load is switched on are shown in Figure 27. The AC output voltage of the converter system is shown in Figure 28. The load voltage is shown in Figure 28. Before switching on the load, the battery current was set to 0 A, the output power of the power supply system was also approx. 0 W (the generator power covered the needs of the power supply system).  
Figures 28 and 29 show that after about 200 ms the output voltage supplying the receivers was turned off. The battery current increased to approximately 160 A and the DC bus voltage decreased to the limit of 600 V. The PC2 grid converter turned off. The battery current waveform consists of two straight lines, inclined at different angles. The first angle depends on the inductance of the transformer circuit of the DAB converter. After approximately 120 ms, the battery current increase ramp was limited to a safe value for the battery circuit, to protect against exceeding the maximum current. After approx. 160 ms, the current has not yet reached its maximum value (165 A), but the voltage in the DC link has decreased to approx. 600 V and the PC2 grid converter has switched off. For the next 40 ms, the current was discharged the battery to rebuild the voltage on the DC bus. After exceeding the maximum value of the battery current, the converter was turned off. The rise time of the battery current is decisive for the maximum power obtained from the battery, because during this time the voltage in the DC link decreases. The second factor is the voltage of the battery. For a nominal voltage of 48 V (lead-acid battery), the discharge voltage may drop to 40 V. The final charging voltage is approx. 56 V.  
The waveforms of the voltages and currents when a load of approximately 6.5 kW was switched on are shown in Figure 30.  
A step change in the load power of approx. 6.5 kW, with an average battery voltage of approx. 42 V, did not cause the DC bus voltage to drop below 600 V. The energy drawn from the DC circuit capacitors was compensated by the battery current after approx. 180 ms. Then the DC bus voltage was restored to approx. 710 V.  
Figure 28: Off-grid operation, switching on a load with a power of 8.5 kW.  
–load phase current;  
–output AC voltage;  
–DC bus voltage. Source: author’s development.