Current Differential Relay
GARD 8000 SYS RFL Electronics
June 20, 2014 13-71 973.334.3100
13.5.2 ADDITIONAL FUNCTIONS
13.5.2.1 TEST MODE
The test mode is a relay function that blocks local tripping allowing commissioning and other
diagnostic tests to be run without actually tripping breakers. Given the test mode disables breaker
tripping it should be used with extreme caution. RFL recommends that the relay trip outputs be
manually bypassed during testing when possible.
While in test mode virtually all of the relay functions continue to operate as normal—currents are
measured, messages are passed between relays, trips are calculated, and SOE and oscillography data is
recorded. The trips are blocked from being sent to the breakers by the logic in the GARD controller.
This allows all of the functions of the relay to be stimulated and tested.
When the relay is in test mode a minor alarm will be issued.
13.5.2.2 PHASE SEQUENCE REPORTING
The relay checks the relative phase sequencing of the current inputs. All measurements are made with
respect to (local) phase A current. The measurements are status indicators only, not intended as
measurements of actual phase angles. The readings are expected to be used in a steady-state condition
with nominal currents applied to the relay (a minimum of 0.5A for 5A CTs, 0.1A for 1A CTs).
The test checks each phase to see if they are at 0, 120, or 240 degrees (relative to phase A). A
measurement within about ±11° will be accepted, if the measurement is outside this range or the
currents are too low “Error” will be displayed. Given the measurements are relative to phase A, phase
A will either be 0° or “Error”.
Each relay will perform the same measurement and report the result to the remote relay. Under normal
steady-state conditions the local and remote phase A currents will be 180° out of phase and each relay
will be taking measurements with phase A defined as 0°. In order to make the display of results more
useful, the relay uses the channel delay time and relay to relay current measurements to confirm the
two ends are out of phase and flips the remote measurement by 180°.
Thus the remote phase A current will be displayed as 180° (or “Error” if the remote end did not make a
measurement or the local and remote are not out of phase). The remaining phases will be displayed as
180° (remotely measured as 0°), 300° (remotely measured as 120°), or 60° (remotely measured as
240°), or “Error”. This provides an approximation of both the local and remote phasing and can
readily identify miswires or gross system abnormalities.
13.5.2.2.1 Entering Test Mode
When the user commands the relay to enter test mode the relay must not be tripped and there cannot be
any fault detectors (local or remote) active. The relay will then enter test mode, block the local trip
outputs, and issue a minor alarm. It will also send a message to the remote relay to place it in test
mode (this is sent repeatedly as a configuration).
The remote relay will also verify that there are no trips or fault detectors and then enter test mode. The
remote relay will send a return message confirming it has entered test mode, block the local trip
outputs, and issue a minor alarm. The user should confirm, using the status web pages, that both the
local and remote relays have entered test mode prior to performing any testing.
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