CHAPTER 4: SETPOINTS MONITORING869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–293Stator Inter-turn FaultWhen the insulation of the stator windings deteriorates due to age and other factors, thiscreates an inter-turn fault. This type of fault is local and can happen either on the samephase or different phases. An inter-turn fault causes heating at the local level, but the heatrapidly propagates, causing the fault to extend to other areas of the stator winding. If aninter-turn fault can be detected in time, it provides warning of upcoming major damage tothe system.The stator inter-turn fault element uses sequence components to detect stator turn failureof the induction machine.The operating condition can be defined as:Where:Zpp = positive sequence impedanceZnp = cross-coupled negative-to-positive sequence impedanceZUBbase = learned unbalance base impedanceZnp/Zpp can be calculated from V1, V2, I2 and Znn as follows:Where:V1 = positive sequence voltage calculated from the motor terminal voltagesV2 = negative sequence voltage calculated from the motor terminal voltagesI1 = positive sequence current calculated from the motor terminal currentsI2 = negative sequence current calculated from the motor terminal currentsZnn = negative sequence impedanceFor an ideal symmetrical machine Zpn = Znp = 0 i.e., it is a decoupled positive and negativesequence component circuit for the induction machine. However, in practice the situationis not ideal and due to inherent asymmetry in the machine the Zpn and Znp values aresmall non-zero quantities. When a turn fault occurs, the asymmetry in the system isfurther aggravated which results in these cross-coupling terms increasing. The normalizedcross-coupled impedance, or ratio of Znp to Zpp as defined by the above equation, is thekey operating signal that can effectively detect a stator inter-turn fault.The inherent asymmetries in the machine at the time of commissioning and without astator inter-turn fault present are represented as:ZUBbase = (Znp / Zpp) at 0 inter-turn faultThe setpoint “Neg Seq Impedance” (Znn) required for the implementation of the above canbe set manually if “Neg Seq Imp Autoset” is set toManual. This value can be calculatedfrom the machine equivalent circuit parameters (i.e. winding inductance and resistance). Itcan also be measured by deliberately applying the unbalance condition duringcommissioning.
