Long-Time Overcurrent Protection (L or ANSI 49RMS/51)

Presentation

Long-time overcurrent protection helps to protect cables, busbars, and busbar trunking against overloads, based on the true RMS current. It is implemented independently for each phase and for the neutral.

This protection function is an overcurrent time-dependent protection with thermal memory. It operates as a thermal image, using the heating and cooling model of a conductor. After tripping, the protection continues to integrate the cooling of the conductor.

This protection function can be used also for transformer or generator protection thanks to the wide range of settings offered.

Availability

Long-time overcurrent protection is available on:

oMicroLogic 2.0 X, 5.0 X, 6.0 X, and 7.0 X control units for IEC standard

oMicroLogic 3.0 X, 5.0 X, and 6.0 X control units for UL standard

o3-pole and 4-pole circuit breakers

Long-time overcurrent protection is powered by the current flowing through the internal current transformers of the circuit breaker and it does not require additional external power supply.

Operating Principle

Thermal Image

The control unit uses the calculation of a thermal image to evaluate the conductor heat rise and precisely monitor the thermal state of the conductors.

Example:

Comparison of the heat rise calculation without thermal image (diagram A) and with thermal image (diagram B):

 

0   Instantaneous current (cyclical) in the load

1   Conductor temperature

2   Thermal state calculated without thermal image (diagram A), with thermal image (diagram B)

3   Long-time overcurrent protection threshold

oControl unit without thermal image: On each current pulse, the control unit only considers the thermal effect on the pulse under consideration. No tripping occurs despite the build-up in conductor heat rise.

oControl unit with thermal image: The control unit adds the thermal effect of successive current pulses. Tripping occurs based on the actual thermal state of the conductor.

The thermal image function helps to protect cables and busbars from overheating in case of low-amplitude repetitive faults. Such faults can be due to repetitive motor starts, fluctuating load, intermittent ground faults, or subsequent closing after an electrical fault.

Traditional electronic protection does not protect against repetitive faults because the duration of each overload detected above the threshold setting is too short to trigger effective tripping. However, each overload involves a temperature rise in the installation. The cumulative effect of successive overloads can overheat the system.

Thanks to its thermal memory, the thermal image function remembers and integrates thermal heating caused by each overload detected above the threshold setting:

oBefore tripping, the integrated heating value reduces the associated time delay. The reaction of the control unit is closer to the real heating of the power network system.

oAfter tripping, the thermal function reduces the time delay when closing the circuit breaker on an overload.

The thermal memory works whatever the current value. It offers an accurate image of the cable or busbar thermal status. The time constant is the same for heating and cooling.

In the case of a control unit that is not supplied, the thermal memory is performed by a capacitor, which implies a fixed cooling time constant. The time constant is equivalent to a tr setting of 12 seconds.

Setting the Protection

The long-time overcurrent protection settings are:

oIr: long-time overcurrent protection threshold

otr: long-time overcurrent protection time delay

They can be set as follows:

oOn the MicroLogic X display screen, at Home > Protection > I Long time

oWith EcoStruxure Power Commission software (password-protected)

oWith EcoStruxure Power Device app (password-protected)

oBy sending a setting command using the communication network (password-protected).

Long-time overcurrent protection can be duplicated when dual settings are activated.

NOTE: To achieve the equivalent of the long-time rating plug OFF setting (available on MicroLogic trip units for MasterPact NT/NW devices), set the long-time protection settings as follows: Ir = 1; tr = 24 s.

Protection Settings

The tr long-time overcurrent protection time delay is given in cold-state conditions, and for a phase or neutral current equal to 6 x Ir.

When the current is higher than Isd or Ii, only short-time overcurrent protection and instantaneous protection are operational.

For more information, refer to the setting guidelines.

Tripping Time According to tr Time Delay

The tripping time according to tr time delay is given in cold-state conditions.

Protection Characteristics

The accuracy on the tr time delay is:

o-20% to 0% when tr > 2 s

o-25% to 0% when tr = 2 s

o-30% to 0% when tr < 2 s

Ir characteristics:

oI < 1.05 x Ir: no trip

oI > 1.2 x Ir: trip

Predefined Events

The function generates the following predefined events:

Predefined events cannot be modified by the user. For general information about events, refer to Event management.

Protection events are generated as follows:

oThe start event is generated when the protection picks up.

oThe operate event is generated when the protection time delay elapses.

oThe trip event is generated when the circuit breaker tripping voltage release (MITOP) activates.

Recommended Actions

Resetting a Trip Event

For information about resetting the circuit breaker after a trip due to an electrical fault, refer to the relevant document:

oMasterPact MTZ1 - Circuit Breakers and Switch-Disconnectors - User Guide

oMasterPact MTZ2/MTZ3 - Circuit Breakers and Switch-Disconnectors - User Guide