MASTER TRIP RELAY
A master trip relay is an auxiliary relay used in the control and relay panel of a substation or any plant. It is a very essential part of the control and protection scheme of any substation. It offers high reliability and high contact ratings with minimal internal consumption. It acts as an instantaneous switching element and provides galvanic separation to the main protection relays of the protection scheme. Master trip relay provides contact multiplication in tripping and signaling circuits of protective relays.
Table of Contents
Construction:
It is an instantaneous, plunger type, auxiliary relay with many contacts and interruptions. They are designed to operate in DC. The magnet system has a fixed core and a moving plunger-type armature which actuates the contact bar directly. When the coil is de-energized by a reset button, electrically or hand reset, the armature of the magnet and the contact bar are forced back to their initial position by spring action.
The contacts are arranged on two sides of the magnet which are clearly visible and easily accessible. The material used for the contact tip is hard silver or silver alloy. A field-weakening resistor is cut by means of a delayed NC contact on a special contact bar in series with the contact.
The operation signal pops out when the relay picks up. It has to be reset by pressing a button when the contact system drops off. The relay is designed for high-speed operation standard is operation in less than 15ms.
Operation:
The master trip relay operates on any signals from various protective relays connected to it. On receiving the input signal from any protective relay, it operates in a very short time and sends a pulse to the associated circuit breaker’s trip coil. Which immediately trips the circuit breaker saving the entire circuit from electrical fault.
Master trip relay cuts off/ isolates the electrical connection via NC contact of its input as it sets to send trip command, to save the protective relay from any malfunction in the circuit.
The NO contacts are used for indication and initiation of tripping command to the circuit breaker’s trip coil.
NC contact is also used in the circuit breaker’s closing circuit. As the master trip relay operates, its NC contact becomes open and hence the circuit breaker cannot be closed again until the master trip relay is reset.
NO contact is also used for LBB initiation.
Wiring Diagram for Master Trip Relay
When a protective relay is operated it sends a positive pulse to the relay 86 because of which the relay actuates. The operation of relay 86 isolates the protective relay by operation of the NC contact. The NO contact which closes on operation, maintains the actuated condition of the relay until the PB or press button is manually reset or via SCADA, which resets the relay back to normal condition. It is for this reason, that the master trip relay is also called a lockout relay.
Upon actuation, the auxiliary contacts of the master trip relay send the signal to trip circuits 1 and 2 of the circuit breaker, which then operates the circuit breaker isolating the faulty circuit.
The M1 DR and the M2 DR are the disturbance recorders of main 1 and main 2 protection units which are initiated by the signal from the relay 86.
The annunciation circuit is also initiated by the closure of auxiliary NO contact of the relay 86. The SER is the sequential events recorder in the RTU or BCU.
A connection is also taken from the auxiliary contact for the LBB protection initiation.
Some contacts may be kept free for future use if used contacts fail or burn out due to faulty circuit conditions.
COMPLIANCE
- Shall have fast operating attracted armature multi-contact relay coil assembly, operating time should be less than 15ms.
- Shall be provided with electrical reset as well as hand reset arrangement.
- Shall have cut off contact with the relay operating coil.
- Shall have a high burden and a high degree of mechanical stability.
High-burden relays are recommended in high-security circuit breaker tripping circuits. The high burden provides immunity against operation due to capacitance discharge currents. Low-burden relays are generally used when a number of simultaneous operations are to be initiated by a single protective relay having insufficient contacts of its own and where a series of connected operation indicators are not used.
Conclusion:
Master trip relay with ANSI code 86 is therefore a very important auxiliary relay used in substations/power stations for protection schemes. It provides protection to the main protection relays by electrically isolating them from the tripping circuit of the associated breaker. It serves as a common point for all protection device’s output, otherwise, all protection devices had to be hardwired with the circuit breaker’s trip-coil, which would have resulted in a clumsy and messy situation as far as maintenance and fault finding in the DC circuit is concerned.
FAQ’s
What is the ANSI code of the Master trip relay?
The ANSI code for it is 86.
Can the relay 86 sense faults?
No, it cannot sense any faults, it is basically a contact multiplier.
What is the function of Relay 86?
As the protective relays are expensive, therefore an intermediate auxiliary relay is required for the contact multiplication, which sends the trip signal from the protective relay to the trip circuit, to the annunciation circuit, etc.
The installation of relay 86 between the protective relay and the trip circuit of the circuit breaker saves the cost of additional wiring as in the absence of relay 86 all protective relays have to be weird with the trip circuit, which will also increase the chances of DC leakage.
Relay 86 provides isolation between the protective relay and the trip circuit so that the protective relay is not harmed in the occurrence of any fault in the trip circuit.
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