Bias Current - ABB Relion Technical Manual

Section 6
Differential protection
6.1.3.4
6.1.3.5
98

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.
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 wye-delta type, for example
YNd1. Transformers of this type do not transform the zero sequence quantities, but zero
sequence currents can flow in the grounded wye connected winding. In such cases, an
external ground-fault on the wye-side causes zero sequence current to flow on the wye-
side of the power transformer, but not on the delta side. This results in false differential
currents - consisting exclusively of the zero sequence currents. If high enough, these false
differential currents can cause an unwanted disconnection of the healthy power
transformer. They must therefore be subtracted from the fundamental frequency
differential currents if an unwanted trip is to be avoided.
For delta windings this feature shall be enabled only if a grounding transformer exist
within the differential zone on the delta side of the protected power transformer.
1MRK 502 048-UUS A
Technical manual