Stator Inter-Turn Fault - GE 869 Instruction Manual

CHAPTER 4: SETPOINTS

Stator Inter-turn Fault

869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
When the insulation of the stator windings deteriorates due to age and other factors, this
creates an inter-turn fault. This type of fault is local and can happen either on the same
phase or different phases. An inter-turn fault causes heating at the local level, but the heat
rapidly propagates, causing the fault to extend to other areas of the stator winding. If an
inter-turn fault can be detected in time, it provides warning of upcoming major damage to
the system.
The stator inter-turn fault element uses sequence components to detect stator turn failure
of the induction machine.
The operating condition can be defined as:
Where:
Z = positive sequence impedance
pp
Z = cross-coupled negative-to-positive sequence impedance
np
Z = learned unbalance base impedance
UBbase
Z /Z can be calculated from V1, V2, I2 and Z
np pp
Where:
V1 = positive sequence voltage calculated from the motor terminal voltages
V2 = negative sequence voltage calculated from the motor terminal voltages
I1 = positive sequence current calculated from the motor terminal currents
I2 = negative sequence current calculated from the motor terminal currents
Z = negative sequence impedance
nn
For an ideal symmetrical machine Z
sequence component circuit for the induction machine. However, in practice the situation
is not ideal and due to inherent asymmetry in the machine the Z
small non-zero quantities. When a turn fault occurs, the asymmetry in the system is
further aggravated which results in these cross-coupling terms increasing. The normalized
cross-coupled impedance, or ratio of Z
key operating signal that can effectively detect a stator inter-turn fault.
The inherent asymmetries in the machine at the time of commissioning and without a
stator inter-turn fault present are represented as:
Z = (Z / Z )
UBbase np pp at 0 inter-turn fault
The setpoint "Neg Seq Impedance" (Z
be set manually if "Neg Seq Imp Autoset" is set toManual. This value can be calculated
from the machine equivalent circuit parameters (i.e. winding inductance and resistance). It
can also be measured by deliberately applying the unbalance condition during
commissioning.
as follows:
nn
= Z = 0 i.e., it is a decoupled positive and negative
pn np
to Z as defined by the above equation, is the
np pp
) required for the implementation of the above can
nn
MONITORING
and Z values are
pn np
4–293