ABB RELION 670 Series Technical Manual page 132
Generator protection
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Also See for RELION 670 Series:
- Technical reference manual (1232 pages) ,
- Applications manual (944 pages) ,
- Manual (366 pages)
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Section 6
Differential protection
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 ground-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 ground-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 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 other 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 exists within the
differential zone on the delta side of the protected power transformer.
Removing the zero sequence current from the differential currents decreases to some extent the
sensitivity of the differential protection for internal ground-faults. In order to counteract this
effect to some degree, the zero sequence current is subtracted not only from the three
fundamental frequency differential currents, but from the bias current as well.
Restrained and unrestrained limits of the differential protection
The power transformer differential protection function uses two limits, to which actual
magnitudes of the three fundamental frequency differential currents are compared at each
execution of the function.
126
1MRK 502 066-UUS B
M13039-185 v8
M13039-194 v6
Technical manual
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