Eaton Cutler-Hammer Digitrip 3000 Instructions For Installation, Operation And Maintenance page 26

Page 20
The adjustability and continuous current of the Digitrip
3000 Protective Relay are two factors that contribute sig-
nificantly to the great flexibility of the relay.
a) Adjustability: The adjustability of the relay permits
movement of its characteristic curve or parts of the curve.
This movement can be done in both a horizontal and ver-
tical direction on the time current grid. The actual shape
of the curve can be changed along with the curve move-
ment. This adjustability permits distinct curves to be
established that will better match the electrical protection
to the application need (Figures 3-3 through 3-8) Notice
that there is no horizontal movement of the ANSI and lEC
curve shapes. Only the point at which the relay starts to
time out moves along the curve shape.
b) Nominal Continuous Current: The Digitrip 3000's
nominal continuous primary current (I
the ratio of the selected current transformers. The current
transformer ratio must by set via the initial programming
of the relay. These settings must agree with the circuit
current transformers to which the relay is connected.
Therefore, I is established by the current transformer
n
ratio used and becomes the primary scale factor for the
trip functions and readouts.
Before proceeding with the curve explanation, it should
be noted that combining functional capabilities, such as
inverse time overcurrent, short delay and instantaneous,
is a coordination activity. The effects of one set of settings
on another setting should always be evaluated to deter-
mine if the results under all possible circumstances are
acceptable. This helps to avoid unexpected operations or
non-operations in the future. Such possibilities are high-
lighted at the end of this discussion as a reminder when
establishing relay characteristic parameters.
Inverse Time Overcurrent Protection
Inverse time overcurrent protection consists of a curve
shape pickup setting and an inverse time multiplier set-
ting. The inverse time overcurrent function offers eleven
possible curve shape types as previously described (Fig-
ure 3-1 and Table 3.1.). When programming the relay, this
will be the first choice to make. The curve shape and its
effect on the characteristic curve will be covered with the
time multiplier explanations.
The pickup setting establishes the current level pickup at
which the relay's inverse time overcurrent tripping func-
tion begins timing. If, after a predetermined amount of
time, the current condition that started the timing process
still exists, the relay's trip relay is energized. Pickup set-
tings can be adjusted from 0.20 to 2.20 times In. Refer to
Tables 2.2 and 2.3 for a complete set of available set-
) is established by
n
tings. Figure 3-3 graphically illustrates how the Inverse
Time Overcurrent Pickup portion of the overall curve can
be moved horizontally on the time current grid by means
of the pick-up settings. The Inverse Time Overcurrent
Pickup is represented by the dotted lines, while the rest
of the curve is represented by a solid line.
The Time Multiplier setting is used to select a predeter-
mined amount of time a sustained overload condition will
be carried before the breaker trips. For the Thermal
Curves, a value of (3 x I
the programmed time multiplier setting is fixed on the
curve. A wide range of time settings are available and
depend upon the curve shape selection. Refer to Tables
2.2 and 2.3 for a complete list of available time multiplier
settings. As Time Multiplier settings are varied, the Time
Multiplier portion of the overall curve is moved vertically
up or down on the time current grid. This movement is
also independent of the other portions of the curve. Fig-
ure 3-4 graphically illustrates the vertical time line move-
2
ment with an I t curve shape selection. Similar movement
occurs for the remaining curve shapes.
Short Time Protection
Short time (fault) protection responds to short circuit con-
ditions. Similar to the inverse time overcurrent function,
the short time function is comprised of a short time cur-
rent pickup setting and a short delay time setting. The
Short Delay pickup setting establishes the current level at
which the relay's short time tripping function begins tim-
ing. The Short Delay Time setting establishes the amount
of time a short-circuit will be carried before the protective
relay's trip relay is energized. As is the case with inverse
time overcurrent protection, short delay protection also
offers a range of settings for both pickup and time. Refer
to Tables 2.2 and 2.3 for available selections.
Two points should be made concerning the available
selections: 1) In Table 2.2 covering Short Delay Pickup
settings, "NONE" is one of the available selections. If
"NONE" is selected, the Short Delay function is disabled
and there will be no Short Delay protection. Also, if
"NONE" is selected, a Short Delay Time selection is not
offered. 2) There is no curve shape selection for the
Short Delay Time portion of the curve. A flat response
curve is automatic.
When a short delay pickup setting other than "NONE" is
selected, the Short Delay pickup and the Short Delay
Time portions of the overall curve are moved horizontally
and vertically in a similar manner to the inverse time pro-
tection functions. Refer to Figures 3-5 and 3-6 for graphic
illustrations of this movement.
I.B. 17555C
) is the reference point where
n
Effective November 1999