Key Aspects in Modelling Transformers for Real Time Simulation: Open Phase, DC Bias and Internal Faults

Abstract

Transformer open phase conditions, DC Bias (e.g. caused by Geomagnetically Induced Current) and internal faults are three important scenarios in protection and control testing of power systems. It is challenging to model these three scenarios in EMT simulation programs because precise modelling of the core structure and the saturation effects is required. The conventional three-phase transformer models used in EMT-Type simulation programs assume that a three-phase transformer is made up of three magnetically identical single-phase transformers. In reality, the saturation of the iron in three-phase transformers is not uniform in various parts of the core magnetic path. This can be observed in both 3-limb and 5-limb transformer cores, even under normal operating conditions. This paper reports our experiences in simulating the three scenarios described above with a UMEC (Unified Magnetic Equivalent Circuit Circuit) transformer model using RTDS® real time simulator. The paper first provides background knowledge of protection and control testing using Real Time Digital Simulators, and continues with an introduction to transformer open phase and DC Bias conditions in power system operation. Then the UMEC transformer model with the capability of modelling internal faults is proposed, followed by the validation and accuracy comparison. The paper demonstrates that transformer models based on UMEC theory can correctly represent the actual characteristics of 3-limb and 5-limb transformers in open phase conditions, DC Bias and internal faults for the purpose of protection and control HIL (Hardware-in-the-Loop) testing of power systems equipment.