ABB ret650, Technical Manual

Page: 99 / 670

6.1.4.3 Differential current alarm
The fundamental frequency differential current level is monitored at all times within
the differential function. As soon as all three fundamental frequency differential
currents are set above the set alarm level (
IDiffAlarm ), the pickup timer is started.
When the pre-set time, defined by setting parameter tAlarmDelay , has expired a
differential current alarm is generated and the output signal IDALARM is set to
logical value one.
6.1.4.4 Bias current
The bias current is calculated as the highest current amongst all individual winding
current contributions, compensated for eventual power transformer phase shift and
transferred to the power transformer reference side. All individual winding current
contributions are already referred to the power transformer winding one side (power
transformer HV winding) and therefore they can be compared regarding their
magnitudes. There are six (or nine in the case of a three-winding transformer)
contributions to the total fundamental differential currents, which are the candidates
for the common bias current. The highest individual current contribution is taken as a
common bias (restrain) current for all three phases. This "maximum principle" makes
the differential protection more secure, with less risk to operate for external faults and
in the same time brings more meaning to the breakpoint settings of the operate -
restrain characteristic.
The magnitudes of the common bias (restrain) current expressed in reference side
amperes can be read as service value from the function. At the same time it is available
as an output IBIAS from the differential protection function block. It can be connected
to the disturbance recorder and automatically recorded during any external or internal
fault condition.
6.1.4.5 Elimination of zero sequence currents
The zero sequence currents can be eliminated from the differential bias current on a
per winding basis via a parameter.
Elimination of the zero sequence current component is necessary whenever:
• the protected power transformer cannot transform the zero sequence currents to
the other side.
• the zero sequence currents can only flow on one side of the protected power
transformer.
In most cases, power transformers do not properly transform the zero sequence current
to the other side. A typical example is a power transformer of the star-delta type, for
example YNd1. Transformers of this type do not transform the zero sequence
quantities, but zero sequence currents can flow in the earthed star- connected winding.
In such cases, an external earth-fault on the star-side causes zero sequence current to
flow on the star-side of the power transformer, but not on the delta side. This results
1MRK 504 135-UEN A
Section 6
Differential protection
93
Technical manual