S&C Vista SD, Programming

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Appendix D
Coordination The S&C Overcurrent Control provides the flexibility required to meet the critical needs
of protective device coordination between upstream and downstream devices such as
cir cuit breakers and fuses. The example below shows how well the U.S. U4 time­overcur­
rent curve coordinates with both an upstream protective relay and downstream power
fuse. The reference document used for these curve families and their equations is IEEE
Stan dard C37.112­1996 IEEE Standard Inverse­Time Characteristic Equations for Over­
current Relays.
As shown in this instruction sheet and as seen on the overcurrent control’s user inter­
face, all pickup­current settings selections for E, K, Coordinating­Tap, Coordinating­Main,
ANSI/U.S. U1 through U5, and IEC C1 through C5 are expressed in terms of primary amperes.
However, in IEEE Standard C37.112­1996, U.S. and IEC curve families are expressed in
terms of “multiples of pickup” where 1.0 on this axis represents the pickup setting. The
standard uses this method of illustrating time versus current response because different
current transformer ratios may be used for each specific application. In Vista SD switch­
gear, the CT ratio is fixed at 660:1. Consequently, we will refer to the “multiples­of­pickup”
nomenclature for this example.
Based on the desired application, any of the 10 curve families can be selected to
meet the coordination requirements. Adjusting the minimum pickup setting in terms
of primary amperes moves the desired time­overcurrent curve to the left or right on
the TCC coordi nation plot. In addition to the minimum pickup setting, a “time dial”
feature is used to move the desired time­overcurrent curve up or down in time (in
seconds) to meet the spe cific coordination requirements.
Example:
Referring to the coordination plot in Figure 2 on page 30, a Vista SD switchgear assembly
with 600­ampere fault interrupt ers is placed into an electrical system where the upstream
protective device is controlled by an overcurrent relay whose phase time­overcurrent curve
family is set for a “CO­9” Very Inverse curve. The relay’s phase overcurrent pickup is set for
960 amperes with a “time dial” setting of 2. There are two curves shown that take into
account the operating time tolerance of the “CO­9” overcurrent relay. Downstream of the
switchgear assembly is an S&C SMU­20 power fuse with a 200K­ampere rating. Its mini­
mum melt and total clearing curves are shown on the TCC coordination plot.
It is desired to coordinate the switchgear assembly with the upstream and downstream
devices. A review of the ANSI/U.S. and IEC curve families resulted in the selection of the
ANSI/U.S. “U4” curve. See Figure 3 on page 31. Its curve shape is similar to the other two
devices’ curves. Using a TCC coordination program, it is possible to “fit” a time­overcurrent
curve “between” the curves of the upstream source­side and downstream load­side devices.
Note that the Vista SD curve selected is shown starting at 150% of the actual pickup setting
in primary amperes (880 amperes
×
1.5 = 1320 amperes). The Time Dial setting selection
of 0.8 was selected to maintain an acceptable coordinating time interval (CTI) between the
upstream protec tive relay phase setting, the power fuse, and the Vista SD unit. Because of
the extremely fine reso lution of pickup current and
Time Dial
settings selections within
the ANSI/U.S. and IEC curve families, it is obvious there are other Pickup Current and
Time Dial settings within the U4 family that could accomplish proper device coordina­
tion; in this example the S&C Overcurrent Control’s time­overcurrent curves are placed
equidistant between the upstream and downstream device time­overcurrent curves.
S&C Instruction Sheet 695-515 27