A hybrid diverter design for distribution level on-load tap changers

The on-load tap changer (OLTC) is used for voltage regulation on distribution networks. The traditional OLTC is a mechanical system limited by the number of tap changes it can perform before requiring maintenance and by the speed at which it can perform successive tap changes. An experimental validation of a new primary side hybrid diverter design for a 2 MVA 11 kV distribution transformer that addresses both longevity and speed limitations of the traditional OLTC is presented. An experimental apparatus is discussed that is used to simulate the inter-tap voltage and load currents appropriate for the target transformer, allowing long-term, continuous and low-cost testing of candidate diverter designs. The wear experienced by identical sets of contacts operating under the new `active' diverter is compared to that under a `passive' diverter scheme. The contact damage caused by the heavy arcing in a traditional OLTC diverter is also shown. The proposed diverter design effectively eliminates arcing and produces extremely high contact lifetimes in excess of 10
⁷
operations, compared to the passive scheme which may be expected to provide around 10
⁵
operations with contacts of a similar size. Fast tap change operations performed in under three half-cycles of the mains waveform are also demonstrated.