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Electric arc furnaces, SAG-Mills, and many other loads cause dynamically fluctuating disturbances to the feeding network. Penalties or shut downs by the utility can be the result. Filters, SVCs and SVC PLUS help and even boost plant productivity.
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Rapidly changing loads like big drives for ore mills, compressors or pumps, or electric arc furnaces lead to undesired voltage fluctuation. Flicker in lighting – critical for human wellbeing, and degradation of other connected devices’ performance can result.
Varying load current – according to Ohm’s law – will result in a varying voltage at the load. For quick changes this is referred to as flicker. This is evidently noticed by humans in form of flicker in lighting. Especially in the range of 4 to 15 Hz (as typically caused by electric arc furnaces) this has sensitive impact on human well being.
As for industrial and public consumers, voltage fluctuation affects efficiency and performance of components. Therefore the vast majority of grid providers set stringent limits to the permitted flicker level imposed by industrial plants.
Nonlinear loads from converters or arc furnaces are a source of unwanted harmonics that could disturb other consumers connected to the network. Harmonics also decrease the efficiency of transmission networks – since they generate losses in all network elements, without contribution to driving a machine, melting steel or other tasks of industrial production.
Harmonic currents generated by nonlinear loads generate harmonic voltage distortion at the connecting point of the load. Utility companies accept a certain margin of harmonic voltage distortion, but distortion exceeding the limits is to be avoided.
To achieve that, harmonic currents should be “kept away” from the feeding grid and from other consumers. Instead they should be “directed” to flow into harmonic filtering devices, or currents must be generated with a waveform neutralizing the harmonic component of the load current.
Significant inductive loads like motors are present in all fields of industry. In the metallurgical industry arc furnaces constitute the largest inductive loads – operating at an extremely bad power factor. The reactive power required by such loads causes degrading of power factor (or cos φ). As also reactive power needs to be transported through the network, this leads to bad utilization of all transmission components such as cables, transformers, etc.
Different loads have different characteristic power factors. Some loads have a clearly defined and nearly constant power factors, for others it can vary rapidly and within a wide range. Extreme behavior comes with electric arc furnaces. Big drives for ore mill applications, rolling mill drives and pumps/compressors show fluctuating power factors due to variation in their operating point.
The main challenge is to measure the load behavior very fast to achieve according control of the compensation system: dynamic loads require dynamic compensation.
Evenly distribute power
Unsymmetrical loads are not – or at least not completely – balanced at all times. Single-phase loads or arc furnaces can cause uneven phase currents in the three-phase supply system and uneven phase voltages.
Generators are three-phase sources of energy that is transported through the grid up to the consumers. The whole system works best when all three phases are evenly loaded. But certain loads are only single phase, and certain three-phase loads can become extremely unsymmetrical in special operating conditions, e.g. an electric arc furnace for melting steel can be anything between balanced up to only two-phase operation at worst.
In order to limit unwanted effects of such unbalanced conditions in the grid, utilities allow only a limited degree of unbalance, and the owner of such consumers has to take measures to evenly distribute the power consumption of his plant to the three phases at his connection to the utility grid.
Industries worldwide rely on solutions from Siemens to optimize their power supply for smooth operation and compliance to utility regulations. Here you can find a selection of our references.
Replacing an old arc furnace with a new, twice as powerful one required dual flicker reduction
SVC PLUS with ±100 Mvar for the VSC and 60 Mvar for the filter circuits
Flicker reduction factor of 6 to 8 was achieved
Stable voltage on the mill bus allows trouble-free and stable operation of the mill drives
High power factor close to 1 ensures best utilization of the electrical supply
Reduction of harmonic currents ensures compliance with utility limitations
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