NEUTRAL GROUNDING RESISTOR AND FAIL SAFE RELAY

The Indian Electricity Rule 116 is modified for mines. It says “The earth fall current should not be more than 50 Amp. In 3.3 KV or 6.6 KV system in mines”.

The suitable Neutral Grounding Resistance (NGR) should be installed betwseen neutral point (star point) and the ground to restrict the earth fault current to 50 Amps. In 3.3 KV system resistance of 38 ohms, 100 KW, 5 sec & in 6.6 KV system resistance of 76 ohms, 200 KW, 5 sec will be installed. By installing these resistance fault current can be restricted to maximum 50 Amps.
In outdoor distribution station there are many possibilities of damage to the neutral grounding connections due to various environmental effects causing open circuit or burning of NGR. This is a very dangerous continuity i.e. totally floating type. This may also result in failure of earth-leakage protection operating through neutral CT as the leakage current may not be able to flow through open circuit of neutral grounding connection.

To prevent this type of failure, Fail Safe Relays must be installed. The fail safe relay will continuously monitor the continuity of NGR and neutral grounding connections and will trip the feeder circuit breaker as and when any open circuit occurs in neutral grounding connection or in NGR. The fail safe relay will ensure fail safe operation of neutral grounding system.
FAIL SAFE RELAY (NGR-MONITORING AND PROTECTION RELAY)
A device called as Fail Safe Relay (NGR-Monitoring and Protection Relay) has been developed and made specifically for the fail safe condition. It has a highly insulated sensor. This sensor is connected to the neutral terminal of the transformer. The sensor has also the capacity to protect the relay from any sudden transient peak voltage.

This relay ensures the fail-safe function of restricted neutral grounding system. It continuously monitors through its sensor, the continuity of N.G.R. (or variation in resistance value of NGR) as well as the continuity of neutral grounding connections.  It trips the circuit breakers as and when any open circuit (or variation in resistance value) occurs in the N.G.R., in the neutral grounding connections or in the relay ground and sensor connections occur.

This relay provides earth fault protection also to save the N.G.R. The relay senses earth fult current by its sensor. When the fault current exceeds the limit, the relay operates to trip the circuit breaker. The relay by it’s own earth-fault feature protects the N.G.R. independently.
NGR MONITORING CUM PROTECTION REALY AND EARTH LEAKAGE RELAY THE TRIPPING CONDITIONS:
  1. Open Circuit of Grounding resistor.
  2. Open Circuit of Neutral Connection.
  3. Open circuit of Grounding connection.
  4. Open circuit of relay ground connection.
  5. Open circuit of sensor connection.
  6. Phase to earth fault.
Smart Fail-Safe Relay with Neutral Grounding Resistor Monitoring Relay,& Ground Fault Monitoring Relay
In the Neutral Grounding Resisitor systeam, continuous monitoring of Neutral Grounding Resistor is Must (as per DGMS) to achive Fail-safe working condition, Smart Fail-safe Relay (Continuosly Monitoring Relay of Neutral Grounding Resistor) works on the principle of the continuous monitoring Neutral Grounding Resistance and continuously monitors the connection between Neutral to Ground by injecting the low frequency signals through the sensor. The system has Specific indication for the different faults conditions. It also continuously monitor and indicate current passing through the Neutral to Grounding Resistor(NGR), with the facility to set trip value of current from 1 Amp. to 9.9 Amp. (as requaired by DGMS) and also the memory to hold the fault current and votage at the time of tripping the relay. The smart fail-safe relay is also check for the connections of the CT connected periodicaly (every hour) for the fullproof fail-safe operation.
  • Fail Safe Fault-Tripping Conditions in the Neutral Grounding Resistor Monitoring Relay:
    (a) NGR open circuit                   (b) Neutral open circuit                  (c) Ground open circuit
    (d) Sensot open circuit                (e) Phase to Earth Fault                (F) CT open circuit

  • i) LED INDICATIONS FOR:
    1) NGR Fault. (Red LED indication)
    2) Sensor Fault. (Yellow LED indication)
    3) Phase Fault. (Red LED indication)
    4) Set Adjustment mode, Push Button (Green Flashing LED)

    ii) Seven Segment Display:
    a) Voltage between Neutral point of transformer to earth.
    b) Current through the Resistance (NGR).

  • SENSOR: Suitable Sensor to suit the NGR-MR series 033, which gives the sensing signals to the
    monitoring relay. It is connected across from the transformer neutral and ground (earth).

  • SENSE VOLTAGE RANGE : 2 KV to 6.6 KV system.

  • Trip-set Adjust for let-through current Range: 2A - 10A
    Trip Current Setting: Through Set, Increment, Decrement push-button Keys in steps of 0.1
    Amps. (Min from 1 Amp to mas 9.9 Amp.)

  • TRIP DELAY- Time Range (Adjustable) : 100 mSec. - 2 Sec. in steps of 100 mSec.
    Trip Delay Time Setting: Through Set, Increment, Decrement push-button Keys in steps of 100 mSec. (min from 200 mili-second to max 2 second.)

  • AUDIO-BUZZER : Latching-type Fault Trip operation Audio Buzzer.

  • INDICATION : Latched actual fault Current in seven segment LED display in 2 Digit in Amp.
    Actual fault Voltage in seven segment LED display in 2 Digit in Kilo-Volts.

  • ACTUATOR-Relay: The Trip Relay actuates the singnal for trip circuit of main breaker in a main circuit breaker.

  • TESTING: The NGR-MR-033 has a built-in test circuit. The ideal test condition is with no voltage or current applied to the system's input, i.e. the transformer secondary isolated from the load and the NGR-MR-033 connected. By pressing a Test pushbutton a voltag is applied on the sensing resistor unit. This creates an out-of-balance condition between the voltage and current causing a NGR trip. This proves (test) the trip circuit is functioning. By resetting the relay after the TEST button has been released (test) proves all of the following:

    1) The Current Transformer (CT) is connected in the circuit.
    2) The current input circuitry is functioning.
    3) The voltage input circuitry is functioning.
No external power supply or test equipment is required. Test and Reset can be by means of the buttons
mounted on the cover.

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