P.3.80
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
87L Differential
Applications With
In-Line Transformers
The relay allows 87L applications with in-line transformers. 87L Theory of
Operation briefly explains the implementation. In particular, the relay uses the
Alpha Plane operating principle to protect lines with in-line transformers,
provides for true differential harmonic measurements and allows for harmonic
blocking, harmonic restraint (or both) for security during magnetizing inrush
conditions.
For a better understanding of the transformer application, read 87L
Differential Elements which explains the basics of the relay 87L function that
apply to applications without and with in-line transformers. This section adds
information that relates specifically to applications with in-line transformers.
First, Transformer Winding Configuration Settings describes extra settings
related to ratings and connections of the in-line transformer windings. These
settings allow the relay to perform proper vector group compensation, zero-
sequence current balancing and ratio matching as per the principles of
transformer differential protection.
Second, Harmonic Blocking and Restraining Logic describes the harmonic
sensing, harmonic blocking and harmonic restraining logic and associated
settings for securing the 87L elements during transformer magnetizing inrush
and steady-state overexcitation conditions.
Third, 87LP Phase Differential Element With In-Line Transformers and 87LQ
Negative-Sequence Differential Element provide details of the phase (87LP)
and negative-sequence (87LQ) differential elements in applications with in-
line transformers. The 87LQ element provides high sensitivity not only for
high-resistive line faults, but also for internal transformer faults. Note that the
zero-sequence differential element (87LG) is not available in applications
with in-line transformers because of the nature of the application (power
transformers with grounded wye and zigzag windings do not balance zero-
sequence currents).
The relay allows for line charging current compensation and in-line
transformers in the same application. Charging Current Compensation
Settings With In-Line Transformers explains extra settings and precautions
related to combining the in-line transformer and charging current
compensation features in the same relay application (see Charging Current
Compensation Settings With In-Line Transformers for more charging current
compensation information).
Transformer Winding Configuration Settings
Relays must have current input terminals (W and X) configured for the 87L
application as explained in Configuration of the 87L Current Inputs.
The CT ratios must be provided via the CTRW and CTRX settings,
respectively. The two currents must be specified as inputs to the 87L zone of
protection via the 87CTWL and 87CTXL SEL
OGIC control equations. The
87CTPWL and 87CTPXL settings allow inverting polarities of the currents
from connected CTs if the CTs are not wired with compatible polarities within
the entire 87L zone of protection.
In applications with in-line transformers, each current terminal of the relay
has extra attributes that are provided as settings in order to ensure proper
compensation for the transformer.
These settings are listed in Table 3.30 and are described below.
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