Operator's Manual

ASCO 336 / 337 Operator’s Manual
Page 14 ASCO Power Technologies 381333-427 A
Protective relay functions
Most protective functions will open the CTTS contacts and not
the generator circuit breaker. There are two conditions that
will trip the generator circuit breaker: (1) short circuit or fault
currents detected by the circuit breakers’ internal protective
device and (2) if there is a loss of AC control power and a
protective system fault resulting in a protective relaying trip,
then the control system knows the CTTS will not operate and
will trip the generator circuit breaker.
Just losing AC control power is not a fatal problem: the
system will not shut down on loss of AC control power, it just
shifts the protective trips (like ANSI 32R, 32RV, 27, 59, 81,
etc.) to the generator circuit breaker.
Once the generator circuit breaker trips, however, operator
intervention will be required to close manually operated
circuit breakers in order to return the generator to service. For
more details, see Generator Circuit Breaker section.
Electrically closed circuit breakers may be controlled by the
control system: contact factory for details.
Various electrical protective functions may be programmed in
the controller. This section describes the function and the table
at the end of this section lists the factory test and default
values. Since every power system is unique, it is the
customer’s responsibility to set ALL protective functions to
the correct value for a specific installation. Normally this
requires a power system study, also the responsibility of
others.
Note: Since the current transformers are in the CTTS, any
faults between the generator and the CTTS must be detected
and protected by the generator circuit breaker.
Over current, ANSI 50/51. Time over current and
instantaneous over current protection may be set in the
controller, but primary protection for these faults is the
generator circuit breaker. The 50/51 elements in the controller
are factory set to alarm only.
There is a 4th CT input that may be used for one of these
functions:
1. Ground current detection and over current (50/51G) on
3P3W systems.
2. Neutral current detection and over current (50/51N) on
3P4W systems.
3. A single input differential (87G) protection. Requires
additional, field installed, CTs. Contact factory for details.
Reverse power, 32R, may be set to protect both the generator
and engine by detecting power flow into the generator.
Forward power, 32F, may be set to protect against overload.
Reverse Vars / Field loss, ANSI 40/32RV. Var flow into the
generator means the voltage of the generator is below the
voltage of the system or other generators. Reverse var flow is
also an indication of field loss that could indicate a problem
with the generator’s excitation system.
Over/under voltage, (59 and 27), Over/under frequency, (81O
and 81U) are primarily set to protect the loads, but overvoltage
may also damage the generator.
Negative sequence current (46) and voltage (47) may be used
to detect an open phase on systems using fuses.
Other protective functions are available, see controller
documentation.
ANSI Description
Trip/
Alarm
Factory Test Values
/ Default Settings
27 Under voltage T/A 90% for 10 sec
59 Over voltage T/A 110% for 10 sec
32R Reverse Power, power
flow into Generator
T/A 20% for 5 sec
32RV Reverse Vars, var flow
into Generator,
voltage below system
volts
T/A 50% for 2 sec
32F Forward Power, power
out of Generator
T/A 120% for 10 sec
46 Negative Sequence
Current or phase loss
detection
T/A 20% for 1 sec
47 Negative Sequence
Voltage or phase loss
detection
T/A 20% for 1 sec
81U Under Frequency T/A 90% for 10 sec
81O Over Frequency T/A 110% for 10 sec
50 Instantaneous Over
current
A 150% for 1 sec*
51 Time Over current A 125% for 5 sec*
*These protective functions must be provided by generator
circuit breaker. It is possible to program the controller to trip
on these functions. ASCO CTTSs are rated to interrupt up to 6
times rated frame current, so it is quite possible that the CTTS
could function as the over current trip device for the system
protection. However the current is being sensed at the CTTS
and the system relies upon the generator circuit breaker to
protect the power system between the generator output
terminals and the CTTS input.