IMPEDANCE PROTECTION FOR POWER TRANSFORMERS

The distance protection principle, when applied to over-head lines and cables, is well known, widely applied and well proven. Also on large generating blocks it is often applied as one of the back-up protection functions. When it comes to typical power transformers, the conventional distance protection however has some limitations which have limited its application in this context. Due to the increase in decentralized generation the risk of not clearing transformer or high voltage faults from the low voltage side has increased. The inherent remote back-up functionality in the impedance principle presents itself as an ideal measure for addressing this risk. The paper will show what is possible with an impedance based protection function in terms of detecting faults in and "through" a transformer. In particular the ability to apply the impedance protection function at the low voltage side of the transformer will be addressed. The focus will be on the need for zone specific impedance measuring principles for different tasks. Compared with conventional distance protection where it is only possible to apply a single method for all zones. It will be shown why the conventional distance protection will often fail to produce the desired results due to the inability to measure with the optimum elements (loops). The manner in which faults are "mapped" onto the measuring loops via the transformer winding arrangement and vector group are keys to understanding the impedance protection in this context. Some simulation results will be used to confirm the theoretical findings. An application example will be used to shown the implementation with a focus on setting coordination.