ABB Relion REG670, Техническое руководство

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(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.
It shall be noted that if the zero-sequence currents are subtracted from the separate
contributions to the total differential current, then the zero-sequence component is
automatically eliminated from the bias current as well. This ensures that for secondary
injection from just one power transformer side the bias current is always equal to the
highest differential current regardless of the fault type. During normal through-load
operation of the power transformer, the bias current is equal to the maximum load
current from two (three) -power transformer windings.
The magnitudes of the common bias (restrain) current expressed in HV 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.
For application with so called "T" configuration, that is, two restraint CT inputs from
one side of the protected power transformer, such as in the case of breaker-and-a-half
schemes the primary CT ratings can be much higher than the rating of the protected
power transformer. In order to determine the bias current for such T configuration, the
two separate currents flowing in the T-side are scaled down to the protected power
transform level by means of additional settings. This is done in order to prevent
unwanted de-sensitizing of the overall differential protection. In addition to that, the
resultant currents (the sum of two currents) into the protected power transformer
winding, which is not directly measured is calculated, and included in the common
bias calculation. The rest of the bias calculation procedure is the same as in protection
schemes without breaker-and-a-half arrangements.
Optional Elimination of zero sequence currents
To avoid unwanted trips for external earth-faults, the zero sequence currents should be
subtracted on the side of the protected power transformer, where the zero sequence
currents can flow at external earth -faults.
The zero sequence currents can be explicitly eliminated from the differential currents
and common bias current calculation by special, dedicated parameter settings, which
are available for every individual winding.
Elimination of the zero sequence component of current 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
Section 6
1MRK502052-UEN B
Differential protection
122
Technical manual