Global Voltage Unbalance in MV Networks Due to Line Asymmetries

The International Electrotechnical Commission (IEC) has recently released the Technical Report IEC/TR 61000-3-13 for the assessment of voltage unbalance emission limits to individual customer installations connected to medium-voltage (MV), high-voltage, and extra-high-voltage power systems. As in the counterpart IEC technical reports for harmonics (IEC 61000-3-6) and flicker (IEC 61000-3-7) allocation, IEC/TR 61000-3-13 also apportions the global emission allowance to customers in proportion to their agreed apparent power. However, noting that voltage unbalance at a busbar can arise due to load and system (essentially lines) asymmetries, IEC/TR 61000-3-13 applies a scaling factor Kue to the apportioned allowance. This factor Kue represents the fraction of the global emission allowance that can be allocated to installations. Conversely, the factor K´ue = 1 - Kue accounts for the emission arising as a result of system inherent asymmetries. This paper addresses the global voltage unbalance emission in MV power systems arising as a result of line asymmetries on which a systematic evaluation method is not given in IEC/TR 61000-3-13. First, a detailed study with the view to investigate the influence of line asymmetries on the global emission and its dependency on various load types is carried out by employing a simple radial network. Second, a matrix-based methodology, covering radial and interconnected networks, for the assessment of this emission at the nodal level taking line, system and load characteristics, system operating conditions, and downstream load composition into account are proposed. The methodology is verified by using unbalance load-flow analysis.