The scope of work concerned a HV/LV substation supplied by transmission overhead lines and feeding 20MW motors.
The objective was to analyze the transformer energization of the power transformer and verify if the breakers were correctly sized. In particular, a Transient Recovery Voltage analysis of the breakers was requested.
Powersys used for this study the software EMTP and its capabilities to simulate accurately the switching transients.
A model of the substation and the incoming transmission lines is built in EMTP. The upstream network is represented as an ideal voltage source with short-circuit impedances and a constant parameter is used for the overhead lines. Power transformer is represented using the T model and a particular attention is given to the modelling of the saturation, which is the origin of the inrush currents. Saturation data is derived from the B-H curve and the air core reactance is modelled using the CIGRE C4.307 Brochure 568 (“Transformer Energization in Power Systems: A Study Guide”). Each model of component has been validated from unit tests.
Different cases are simulated in EMTP:
Since the voltage drop and the inrush current are function of the time of the switching, a statistical study has been performed for each case: the switching time is varied on a period and the highest inrush currents and deepest voltage drops are noted.
For the TRV analysis, the model of the substation has to be valid in the high frequency range. Small capacitance of all equipment are added in compliance with the recommendations of the book “Transients in Electrical systems” (J. C. Das). Busbar, CT and CVT, breakers and transformers are concerned.
High-Frequency model of the breaker (EMTP)
Then, the TRV study has been performed on the HV breaker. Different scenarios were simulated following:
• The fault location (HV side or LV side),
• The type of fault (Line-Line-Line, Line-Line-Line to ground, Line-Line, Line-Line to ground and Line to Ground).
The simulated TRV are compared with IEC 62271-100 recommendations.
The study reveals that the RMS voltage drop due to transformer energization on the HV bus of the substation never exceeds 8% and the worst inrush current stays below 10p.u.
For the TRV study, the maximum TRV of the circuit breaker is around 300 kV which is within the limits given by the IEC standard 62271 – 100.
Simulated overvoltage (kV) at the breaker (red) and standard TRV (blue)