Functions :-
1. Thermal overload protection
2. Earth fault protection
3. Phase unbalance protection
4. Undercurrent protection
5. Number of starts protection
6. Circuit breaker fail protection
Whilst medium voltage 3-phase motors are very reliable and robust, modern designs operate much closer to their thermal limits and to give adequate protection sophisticated protection relays are required. In addition, increased industrial use of power electronics leads to corruption of power systems and unless specific equipment is installed to eliminate the corruption it can cause considerable rotor overheating. The relay has been designed to protect the motor against these phenomena as well as known abuses such as mechanical overload, stalling, single phasing, terminal box and cabling failures, and too frequent starts. The relay can be set to accurately mimic both the heating and cooling characteristics of the protected motor and consequently ensure that the thermal withstand of the machine is not exceeded, at the same time allowing full use of the motor’s thermal capability.
RTD inputs :-
RTD inputs may be selected from a number of types, namely 100W Platinum (standard type DIN 43760), 100W Nickel, 120W Nickel, 10W Copper or ‘other’. The named types have reasonably linear, known characteristics and the measured resistances are converted into temperatures between -50°C and +250°C on the display. Alarm and trip settings are also programmed in terms of temperature. If the ‘other’ type is selected then measurements and settings are displayed as resistances up to 350W.
Thermistor inputs :-
Thermistor inputs may be selected as either positive or negative temperature coefficient types (PTC or NTC). Thermistors are available with a wide range of generally non-linear characteristics and so settings and displays
are given in terms of resistance. Values between 100W and 30kW are accommodated. For PTC type devices, the protection operates when resistance is measured above the applied setting. For NTC type devices, the protection operates when resistance is measured below the applied setting.
Detector failure protection :-
Each active temperature input is monitored for short circuit and open circuit failure. A temperature input fail alarm
output is generated by a failure condition and the failed input is identified in the Instruments Menu. A failed device
will not give a trip or alarm output. Each device is individually addressable and can be disabled. This feature can
be disabled.
Detector gating :-
Further security is provided by allowing each temperature input to be AND gated with any other input. If this feature is selected then no trip will be issued unless both gated inputs detect temperature (or resistance) above the trip setting. The temperature input alarm outputs are not subject to gating.
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