Mesh quality is very important and is not subject to quality control before use. The mains voltage supplied by the energy supply companies for households, commerce and industry is ideally a uniform sinusoidal voltage with constant amplitude and frequency. However, this ideal case can no longer be found in public networks today. This is partly due to loads that draw a non-sinusoidal load current from the mains or have a non-linear characteristic curve, such as PCs, televisions, switching power supplies, energy-saving lamps or frequency converters.
Mains perturbations and the effects on connected devices
All the loads mentioned above draw a non-sinusoidal load current. Today, the increasing proportion of these non-linear loads is generating ever greater deviations from the pure sine wave that should be present in the supply networks. This ideal sinusoidal shape is no longer found in today’s supply networks, which is unavoidable due to the non-linear loads and is also permissible within certain limits.
For the assessment of power quality, the harmonics up to 2.5 kHz, corresponding to the 50th harmonic, are typically considered for the low-frequency system perturbations. The 3rd, 5th and 7th harmonics, i.e. the frequencies of 150, 250 and 350 Hz, have the strongest effects.
If the harmonic content is too high, sensitive electronic control systems, computers and control devices, for example, may no longer function properly or even individual loads may fail prematurely, seemingly without any logical explanation. Other possible effects are
- Increased load up to the destruction of power factor correction systems
- Oversizing of grid components such as transformers or cables required
- Limited functional reliability and service life of grid components and connected loads
The harmonic load on the grid therefore has considerable disadvantages for the system and consumers.
The transmission grid, transformers integrated into it or compensation systems in industry are calculated and designed for the nominal frequency of the grid. Additional loads due to grid perturbations are rarely taken into account when planning the systems. They result in a higher reactive power load on transmission media such as cables and transformers; an “overload” due to harmonics can lead to damage to individual consumers.
Measures to safeguard network quality
As there are a large number of consumer groups and different limits for interference emissions, it is important to objectively assess the mains voltage quality with regard to the existing standards. There are now methods that ensure trouble-free operation of the power grids and avoid feedback effects.
Passive harmonic filters, for example, which consist of an LC circuit, can be used universally. They have a high degree of efficiency. This is typically 98.5 percent. The technology is also very robust and maintenance-free except for any cooling air fans.
The following must be observed with passive filters:
- If they are operated in idle mode, they act as a capacitive reactive power source due to filter-related circulating currents.
- Depending on the application, grouping the filters and selectively switching them on and off may be useful.
A new approach based on optimized semiconductors and modern microprocessor technology is the use of active electronic filter systems. These permanently measure the grid quality and feed into the grid in a targeted manner with the help of an active power source. The result is then a sinusoidal current. The construction of this new generation of filters is complex and costly compared to the previously mentioned filter measures, as high-resolution and fast data acquisition and high computer performance are required. The most cost-effective method of reducing grid perturbations is the additional installation of chokes, either in the DC link or at the input of frequency converters. The use of a mains choke in the frequency inverter extends the current flow for charging the DC link capacitors and reduces the current intensity. This significantly reduces the distortion of the mains voltage and there is less mains feedback. In addition, the mains choke also increases the service life of the DC link capacitors, as they charge more gently by capping the current peaks. In addition, mains chokes improve the dielectric strength of the frequency inverters during mains transients. Due to the low input current, the cable cross-sections and mains fuses are also smaller.