Setting the IDMTL Overcurrent Protection

Setting Guidelines

The IDMTL overcurrent protection Digital Module can be used to improve selectivity with upstream overcurrent protection provided by a fuse or standalone relay, for example, a medium voltage relay, without affecting selectivity with downstream low voltage devices.

Fuses and standalone relays often provide tripping curves which are different from the standard tripping curve offered by MicroLogic X long-time overcurrent protection. The setting guidelines give an indication of how the IDMTL tripping curves can be used to maintain improved selectivity with upstream devices.

Selectivity with Upstream Fuse Using HVF Tripping Curve

The following graph shows how the IDMTL HVF curve can be used to curtail the standard long-time overcurrent protection tripping curve to help improve selectivity with an upstream fuse.

Upstream fuse tripping curve

Compromised selectivity with standard long-time overcurrent protection tripping curve

Tripping curve with IDMTL HVF tripping curve

The following settings are recommended for the IDMTL overcurrent protection:

Setting	Value
IDMTL mode	ON
IDMTL action	Trip
IDMTL inhibit	OFF
IDMTL Ir	1.0 x In⁽¹⁾
IDMTL tr	1 s⁽¹⁾
IDMTL curve	HVF
IDMTL reset time type	Definite time
IDMTL reset time	0 s
(1) The setting proposed is consistent with commonly used high voltage fuses. The setting should be checked on a case-by-case basis.

The following settings are recommended for long-time overcurrent protection:

Ir ≤ Iz cable
tr = 24 s

NOTE: I²t OFF is recommended for short-time overcurrent protection. With I²t ON the long-time protection could trip faster than the short-time protection.

Selectivity with Upstream Relay Using DT Tripping Curve

Without IDMTL overcurrent protection, when the MasterPacT MTZ circuit breaker is downstream from a relay using a DT tripping curve, it would be necessary to drastically reduce the long-time overcurrent protection time delay, tr, or the short-time overcurrent protection time delay, tsd, to improve selectivity between the devices. The reduction would significantly reduce the setting options for downstream feeders and in some cases, would make downstream selectivity impossible.

The following graph shows how the IDMTL DT curve can be used to curtail the standard long-time overcurrent protection tripping curve to help improve selectivity with an upstream relay.

Upstream relay tripping curve

Compromised selectivity with standard long-time overcurrent protection tripping curve

Tripping curve with IDMTL DT tripping curve

The following settings are recommended for the IDMTL overcurrent protection:

Setting	Value
IDMTL mode	ON
IDMTL action	Alarm
IDMTL inhibit	OFF
IDMTL Ir	≤1.3 Iup
IDMTL tr	tsd < IDMTL tr ≤ tup -0.2 s
IDMTL curve	DT
IDMTL reset time type	Same as reset type of upstream relay
IDMTL reset time	Lower than reset time of upstream relay

Selectivity with Upstream Relay Using SIT or VIT Tripping Curve

As in the previous example, without IDMTL overcurrent protection, when the MasterPacT MTZ circuit breaker is downstream from a relay using an SIT or VIT tripping curve, it would be necessary to drastically reduce the long-time overcurrent protection time delay, tr, or the short-time overcurrent protection time delay, tsd, to improve selectivity between the devices. The reduction would significantly reduce the setting options for downstream feeders and in some cases, would make downstream selectivity impossible.

The following graph shows how the IDMTL SIT or VIT tripping curve can be used to curtail the standard long-time overcurrent protection tripping curve to help improve selectivity with an upstream relay.

Upstream relay tripping curve

Compromised selectivity with standard long-time overcurrent protection tripping curve

Tripping curve with IDMTL SIT or VIT tripping curve

The following settings are recommended for the IDMTL overcurrent protection:

Setting	Value
IDMTL mode	ON
IDMTL action	Trip
IDMTL inhibit	OFF
IDMTL Ir	≤1.3 Iup
IDMTL tr	tsd < IDMTL tr ≤ tup @ 6 x Iup
IDMTL curve	SIT or VIT, same as tripping curve of upstream relay
IDMTL reset time type	Same as reset type of upstream relay
IDMTL reset time	Lower than reset time of upstream relay

NOTE: When the calculated tripping time is lower than tsd, the IDMTL overcurrent protection tripping time is forced to the short-time overcurrent protection tripping time. Setting IDMTL tr > tsd is recommended to avoid a race between short-time overcurrent protection and IDMTL overcurrent protection for currents higher than Isd.

EIT Tripping Curve Application: Pre-alarm for Load-shedding

An example for the application of this tripping curve is as an adjustable pre-alarm for load shedding.

Tripping curve

IDMTL EIT pre-alarm curve

The following settings are recommended for the IDMTL overcurrent protection:

Setting	Value
IDMTL mode	ON
IDMTL action	Alarm
IDMTL inhibit	OFF
IDMTL Ir	0.9 Ir
IDMTL tr	tsd < IDMTL tr ≤ tr
IDMTL curve	EIT
IDMTL reset time type	Inverse time
IDMTL reset time	10 s