Next Generation Protection of Transformers

The objective of the project has been to provide improved capabilities for determining the overvoltages by simulation in software programs, thereby enabling best use of protection against overvoltages. PROGRESS per 30/11-2021: The project has been finalized with all budget spendt. WP1 : - No activity. WP2 : Terminalmodeling of transformers based on measurements. - Finalized and implemented new method for inluding neutral points in transformer model. The methos has been poublished in an IEEE journal paper. WP3 : Modeleling of grounding systems - further improved and completed software for analysis of transient voltages in grounding structures. WP4 : Model interface with circuit simulation programs - No activity. WP5 : Overvoltage protection and analysis - Investigating possibility of replacing overhead ground wires in front of substation by surge arresters. The activity has been finalized and documented in a project memo. - Written a project report giving an overview of the results gained throuhout the project. WP6 : Lightning activity - Activity is finished. WP 7: Participation in CIGRE JWG A2/C4.52 - Finalizing writing of CIGRE technical reports (ongoing).

Blant de mange konkrete resultater nevnes: - Spenningssoppsving kan oppstå i HS viklingens nøytralpunkt for Y-Y koblede fordelingstransformatorer, pga.lynnedslag på luftledning. Bruk av overspenningsavledere på LS-siden er essensielt for å unngå at overspenningen i nøytralpunktet overstiger isolasjonsnivået. - Beregninger viser at konvensjonelt topplinevern i innføringssonen gir generelt bedre beskyttelse av 132 kV hovedtransformator enn bruk av overspenningsavledere plassert ute på master. - Resonante spenningsoppsving kan oppstå internt i viklinger pga. oscillerende overspenninger på transformators eksterne terminaler. For hovedtransformatorer kan slike oppsving beregnes gjennom transiente analyser, forutsatt at en hensiktsmessig modell er tilgjengelig. De oppnådde resultater og innsikt vil tilflyte nettselskapene gjennom REN-blader.

Transformers are vulnerabe to overvoltages. Lightning alone causes about 1% annual failure for distribution transformers in some areas in Norway. With a population of tens of thousands transformers in the grid, the potential for cost savings is substantial. The overvoltages (from lightning, switching, faults) are mitigated by use of protection devices in combination with the grounding system. Existing guidelines for protection of transformers are based on simulation studies using primitive models of the transformer and the grounding system. The proposed project shall revisit the procedures for protection using advanced models of the transformers and grounding structures, giving a more correct assessment of the overvoltage stresses, forming a basis for better protective solutions. Tasks: 1. Winding model. The detailed winding model used by manufacturers will be made available and used for calculating internal voltage stresses in power transformers. The known inaccuracies in damping of will be adressed by use of experimental damping factors. 2. Terminal model. A Measurement-based transformer model developed at SINTEF will be extended to allow more vector groups and grounding conditions. 3. Grounding system model. Frequency-dependent models will be developed based on EM field solver calculations. 4. Model interfacing. Capabilities will be developed for interfacing the models with at least one circuit simulator (EMTP-RV) to permit simulation of complete power systems. 5. Overvoltage protection. The models will be used in simulation studies to clarify the best choice of protection (surge arresters, snubber circuits, grounding design,...) for alternative networks. 6. The effect of climatic changes on lightning activity will be predicted (flash density and current stroke parameters) The work will be coordinated with activities in CIGRE WG A2/C4.52 on transformer high-frequency modeling. The results will be available to all utilities through REN-blad.