# Disturbances Generated by Lighting Systems with LED Lamps and the Reduction in Their Impacts
## 4 Two-Stage Power Supply (with a Double Conversion)  
In the case of the connection of power supply circuits to loads with strong non-linearities, which draw strongly distorted currents (with a high content of higher harmonics), it is necessary to apply circuits that reduce the occurrence of current and voltage higher harmonics. Recipients with a significant grouping of such loads are commercial facilities, where a lot of computer hardware (containing switching power supplies), electronic regulators of the consumed power (e.g., in ventilation systems, air-conditioning systems, industrial systems), or energy-saving lighting (discharge lamps or LED lamps) are in use. Besides the production sphere, other facilities may also include shopping centres, banks, office buildings, educational, medical and service facilities as well as other business units and residential buildings. One device that significantly limits the occurrence of current higher harmonics in the power grid is a two-stage power supply. The double conversion of electricity takes place in it: the sinusoidal voltage of the power supply system is rectified in the rectifier block at the power supply input and is then supplied to the inverter through the DC-DC bus where it is converted into sinusoidal voltage with much more advantageous parameters. In this circuit, an improvement in the quality of the supply voltage parameters (values, frequencies, shape) is obtained. The energy separation of loads and the network takes place, and at the same time, a significant limitation in the impact of the current higher harmonics of non-linear loads on the power mains is achieved. In practice, such power supplies are the main component of the uninterrupted power supply (UPS), operating in the VFI (Voltage Frequency Independent) topology, i.e., on-line. A block diagram of  
Figure 5: Spectral characteristic of current higher harmonics flowing through LED lamp 1 in reference to the permissible values according to [50]. Figure 6: Spectral characteristic of current higher harmonics flowing through LED lamp 2 in reference to the permissible values according to [50]. this system is presented in Figure 7. The use of a battery pack additionally allows for the uninterrupted supply of loads in the event of a power failure or improper parameters of the supply voltage (until the energy collected in energy storages is exhausted) [4; 5]. In view of the properties of the two-stage power supply and its common use in UPS VFI, this chapter of the article presents the results of testing of a three-phase, non-linearly loaded UPS for the purposes of quantitative and qualitative representation of this problem. The tests were performed using the UPS EVER Powerline Green 33/Lite series. Figure 8 includes oscillograms of the distorted current with which the power supply output was loaded and the voltage at the UPS output, generated by its inverter circuit. The waveforms were vertically shifted in relation to each other, in order to make them clearly legible. The results of the testing of the higher harmonics and the total harmonic distortions (THD) of these signals are depicted in Figure 9.  
For research purposes, the waveform of the load current is intentionally shaped in a non-linear and non-symmetric manner by the specialist electronic circuit. Its \(\mathit{THD_{I}}\) is 41.6%. In the frequency spectrum, there are mainly even harmonics, which is the consequence of the lack of symmetry of the load (the current is drawn in a single wave). Voltage generated at the UPS output (supplied to loads) is sinusoidal, with a small \(\mathit{THD_{U}}\), which is equal to 0.7%. Figure 8: Oscilloscope waveforms of load current and voltage at the UPS output. Figure 7: Block diagram of the two-stage power supply in the VFI two-stage power supply (UPS). The current drawn from the mains by the UPS, loaded non-linearly and non-symmetrically, has the \(THD_{I}\) at the level of 17%, and thus about 2.5 times lower than the THD of the power supply load current.