GE Entellisys 4.0, Инструкция по применению

Страница: 20 / 43

38
Voltage Protective Relays, High Current Alarm
The Entellisys protection system provides for the
following additional optional voltage-related
protective functions:
• Overvoltage
• Undervoltage
• Phase loss
• Overfrequency
• Underfrequency
• Power reversal
• Synch check
In addition, the Entellisys system provides for an
optional high-current alarm function that may be
selected and set for each circuit breaker.
Each relay function also has the following settings
associated with it:
• Alarm-only relay activated
• Alarm-only relay pickup
• Alarm-only relay drop out
• Trip/Open relay activated
• Trip/Open relay pickup
• Trip/Open relay drop out
The Trip/Open selections are not available for the
high-current relay, which is intended only for
alarm or logic input use. All relay calculations are
performed at the CPU and not the Messengers.
Because many of the conditions identified by these
relays would not be cleared if the circuit breaker were
opened, the system is able to handle the situation
when a circuit breaker is re-closed and the condition
persists. The CPU monitors the open or not-open
status of a circuit breaker after the trip or open
command is issued. If after 180 cycles the circuit
breaker transitions to “not-open” status, and the
protective relay remains in pickup status, then the
trip/open command is reissued.
Many of the relays may be disabled via Flex Logic,
which allows a protective relay to be turned off by
a control sequence programmed in the CPU.
Table 7.12 provides a summary of the settings
available for each relay function. For a more
detailed explanation, see the descriptions below.
Undervoltage relay
Undervoltage protection compares the sensed voltage
against a threshold. In wye systems, the rms
phase-to-neutral voltage is used, while in delta
system the rms phase-to-phase voltage is used. The
relay has the following capabilities and characteristics:
• Fixed and inverse time curves
• Expanded pickup adjustment range from 10 to
95% of nominal in 1% increments, which allows
the undervoltage relay function to be used for
residual voltage detection when assigned to bus
potential transformers
• Delay setting range of 0.5 to 600 seconds in
increments of 0.5 seconds
• Selection of sensitivity to one, two, or all three
phases below the threshold
• Relay drop out at 103% of the set pickup level
• Blocking voltage enable setting
• The undervoltage function is disabled if the voltage
drops below the blocking voltage
• Ability to work with multiple potential transformers
in different topology configurations per topology
PT assignments
• Pickup accuracy of ±2%
If the fixed-time curve is selected, then the relay
function trips the selected circuit breaker or activates
the alarm if the monitored voltage is less than the
threshold for the specified time delay. The timing
accuracy is ±0.1 second.
The inverse-time curve relates the selected time
delay to the voltage under the pickup threshold as:
T
=
D/
(
1-V/V
pickup
)
where
T
is the operating time,
D
is the delay setting,
V
is the sensed voltage, and
V
pickup
is the pickup voltage.
Once the overvoltage threshold has been exceeded
and the relay starts timing, incrementing the count
will stop if the voltage reaches 100% of threshold
again and will start decrementing per the cooling
algorithm when the voltage reaches 103% of the
set threshold.
Overvoltage relay
The overvoltage relay function works like the
undervoltage relay, except that the thresholds are
voltages above the nominal system voltage and only
a fixed time-delay setting is available. The pickup
setting range is adjustable from 105% to 125% of
nominal in increments of 1%. The overvoltage
relay drops out of pickup when the sensed voltage
drops to 97% of the threshold setting. A cooling
function decrements the accumulator.
Section 7. Protection