Wye-connected transformers are often used for power transfer over long distances. Solar activity may cause a potential difference between the neutral point of wye connected transformers. Generally referred to as Earth Surface Potential (ESP), the ESP may cause an induced flux in such power transformers which may produce very low frequency currents called Geomagnetically Induced Currents (GIC). In large power systems, transformer banks such as those formed from three single-phase transformer units may be used due to the large power requirements. In systems where these transformers are connected in a delta-wye configuration, GIC may flow into the transformers through the neutral to ground connection of the wye-connected secondary windings and split into the secondary winding of each single-phase transformer. This may result in a circuit where the ESP and the secondary windings of each transformer are in parallel such that the low frequency ESP may cause a dc shift in the flux and bring the transformer into half-cycle saturation.
Another concern in power transfer systems is harmonics. Harmonics are signals generated in the process of electrical conversion from ac to dc, dc to ac or ac from one frequency to another. Harmonics generated in this process are multiples of the fundamental power system frequency (60 Hz) such as 120 Hz (2nd harmonic), 180 Hz (3rd harmonic), etc. Among these, harmonics that have a frequency multiple of three are called triplen harmonics. Triplen harmonics are more common in buildings (due to fluorescent, power supplies, etc.) while other harmonics such as 5th, 7th, 11th, and 13th are more problematic in industrial applications using Variable Frequency Drives (VFD) for motor speed control. Harmonics cause a number of problems such as hindering the power factor, overheating the transformer, overloading the neutral cable, equipment failure, inefficient operation of motors, false trips (loss of revenue), non-compliance with standards, and flickering in parallel connected circuits. It is estimated that 70% of distribution loads involve electronics that generate some form of harmonics. As per IEEE Std. 519, the total harmonic distortion has to be limited to 5% of the fundamental current at point of interconnection.
Furthermore, a key concern in delta connected power systems is the lack of reference to ground for ground fault detection, safety and insulation concerns. Such delta connected power systems may, for example be used in wind farm applications where the wind turbine transformers are configured in a star/delta configuration, which may be collected through one or more transmission lines to a substation step up delta-wye transformer. Such power transfer systems may suffer from instability in the voltage transfer signal, insulation failure, and failure to detect ground faults.
USPTO Backgrounds
USPTO Backgrounds