P.3.73
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
The Alpha Plane extended security switchover logic uses the 87CCU Relay
Word bit to switch to more secure (less sensitive) settings. This recognizes that
compensation allows regular Alpha Plane settings to be more sensitive and
that loss of compensation results in a fallback to less sensitive, but more
secure, settings. See Extended Security Setting Switchover Logic for more
details.
You can use the 87CCB, 87CCD, and 87CCU Relay Word bits to control the
87L function accordingly when the quality of compensation changes in
response to LOP, stub bus, loss of a channel, or breaker position per
Figure 3.40.
It is important to recognize that all relays in the 87L scheme provide the same
quality in their performance regarding line charging current compensation. At
any given time, a number of relays in the scheme (assuming adequate
voltages) calculate and subtract portions of the total charging current. These
relays then add these portions to obtain the total charging current for the
scheme and provide full compensation for the differential current. The fully
compensated differential current appears at all master relays of the 87L
scheme regardless of whether a particular relay compensates at a given time.
Note, however, that the line charging current compensation quality bits
(87CCB, 87CCD, 87CCU) are only available in a given relay if the charging
current compensation is enabled in this relay. You can therefore consider
enabling the compensation at all relays of the scheme, regardless of whether
they have access to voltage. Terminals with no voltage will contribute nothing
to the compensation, but they will have information about the quality of
compensation in the entire 87L scheme. In such a case, the terminal(s) without
access to voltage would have E87LCC = Y and 87LINEV = OFF. Otherwise,
you should send the 87CCU bit using the user-programmable communications
bits (see User-Programmable 87L Communications Bits) to the relays with
disabled compensation and add this bit into the E87LPS, E87LQS, and
E87LGS SEL
OGIC control equations (see Table 3.24) so that all relays switch
over to the extended security Alpha Plane settings in the event of complete
loss of compensation.
Figure 3.40 Line Charging Current Compensation Control Logic
As Figure 3.41 shows, the relay with charging current compensation enabled
(E87LCC = Y) uses the voltage input as specified by the 87LINEV setting (Y
or Z voltage terminal) to calculate the line charging current in the time domain
for the phase currents ( = A, B, C). As 87L Theory of Operation explains, the
relay calculates the numerical derivative of the voltages (i
C
= C • dv/dt) and
0
Σ
Setting
E87LCC = Y
Relay
Word Bits
ESTUB
LOP
ALTV
87SLV
SPO
3PO
Setting
0
2 cyc
87MCC
(Multiplier for the Local
Charging Current)
Relay Word Bit
87CCC
Transmit to
Remote Relays
87CCC From
All Required
Channels
Relay
Word Bit
87CCB
87CCD
87CCU
Setting
87CCN
x
x = 87CCN
0 < x < 87CCN
x = 0
x
1
Setting
87CCLPT = B
87LINEV = OFF
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