Relay

This below is how the relay trigger manually. Now the relay already use by automatically based on temperature or pressure trigger to energize the equipment.



The basically principle work of relay consist thermal relay, voltage relay and current relay. In the power station, all the relay already classified as ANSI, for example Relay 86 Lock Out Relay, Relay 24 Voltage per hertz limitation and protection.


Realy 86 Lockout Relay
The lock out relay is used as a motor for safety device. The relay coil voltage is the same as the contactor coil volatge and the contact may be of the pilot duty type.

It is wired into the circuit with its normally closed contacts in series with the starter or contactor coil. The relay coil is wired parallel to the starter or contactor coil (figure 3-3). If any if the other safety devices, such as pressure switches, overloads or like, stop the motor, the lock out relay keeps it from restarting until it is reset by turning off the electricla power to it.

















In the transformer protection consist oil temperature, Bucholz Relay Operation, Transformer winding temperature, Pressure relief main tank and OLTC oil flow can be trigger to 86 lockout relay to open Generator Circuit Breaker (GMCB) and make unit trip.

For pressure relief main tank of transformer, you can see http://www.idpowerstation.blogspot.com/2010/04/pressure-relief-valves-for-transformer.html


Voltage Relay.
This relay may be recognize by its resistance coil wound with very fine wire. The coil of the voltage is connected in parallel with the starting winding of the motor. Voltage relay are normally used on high starting torque motors, but they may also used on low starting torque motor. The relay contact are normally closed.

This is normally a three terminal relay, but it may also have auxiliary (binding) terminal. The terminal numbered 1,2 and 5 are the operating terminals while 3, 4 and 6 are binding terminals, if these are included in the relay (figure 3-5).




When wiring the relay into the system, terminal 1 and the Run winding terminal on the motor are connected to the same supply line.

The starting capacitor is installed in this line (figure 3-6). Terminla 5 and the mototr common winding terminal are connected to the same supply line. Terminal 2 is connected to the motor starting winding terminal.




When power is first supplied to the mototr, te contacts of the starting relay are closed. In the closed position there is an electrical out-of-phase condition between the motor windings.

During the initial start up there is no back-EMF and maximum current is flowing. As the motor speed increase a back-EMF is produced causing an increase in voltage.

When the voltage is increased to the pick-up voltage of the motor starting relay, the armature will actuate opening the contact points. At this point the out-of-phase condition is relieved, allowing the motor to function under normal operating conditions.


Current Relay.
The coil (or thermal element) of this type of relay is connected electrically in series with the running winding of the motor. The coil of the current relay can be recognize by the low resistance, heavy wire with which it is wound. Current relays are used mostly on low starting torque motors. The contact of this relay are normally open.




Both three and four terminal relays are available. Some of them have binding terminal. Three terminla current relays have switch connection from L to S and the coil between L and M (figure 3-7). Four terminal relays have switch connection from 3 to S and the coil between 2 and 4 and M (figure 3-8).

Four terminal relays are often used with three terminal overloads and provide a convenient method of connecting starting capacitor.




When power to the motor is first turned on, the contact of the coil-type current relay are open. The high inrush or current through the electromagntic coil closes the contact, thereby producing the out-of-phase in the motor needed to start it.

As the back-EMF increases, the current decrease allowing the relay contact to open and remove the out-of-phase condition. The motor then operates as usual.




The thermal element (hotwire) type of current relay (figure 3-9) operates much the same as the voltage relay.

The only exception is that the main current is routed through the relay by use of a resistance wire.

The great generated during the maximum current inrush actuates a bimetal warp switch. the switch opens the contact and removes the out-of-phase condition in the motor.

Also, the main contact remain closed and open only in an overcurrent conditon to stop the motor.




A review of the counter-EMF in electric motors may be needed before this is completely understood.








Thermal relay
The thermal relay contact and terminal are the same as in the electromagnetic relay. The method of pulling the movable contact against the fixed contacts is the major difference between the electromagnetic and the thermal relay.




The thermal relay utilized a bimetal blade with a heater coil wound around it. The heater leads connect to the control power supply through the low voltage controller.

When the controller demands, the heater warms the bimetallic element, causing it to bend toward the stationary contact to complete the output circuit.

When the controller opens the control circuit to the heating element, the bimetal element cools and moves away from the stationary contact, breaking the circuit to the external device (figure 3-4).


Bucholz Relay.
a Buchholz relay is a safety device mounted on some oil-filled power transformers and reactors, equipped with an external overhead oil reservoir called a conservator. The Buchholz Relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment.















If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This flow of oil operates a switch attached to a vane located in the path of the moving oil. This switch normally will operate a circuit breaker to isolate the apparatus before the fault causes additional damage.


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Now continue to Relays on Power plant as ANSI.

Relay 24: Voltage per hertz limitation and protection, please visit http://idpowerstation.blogspot.com/2012/03/voltage-per-hertz-relay-24.html


Relay 49 50 51 : Over-current relay, please visit http://idpowerstation.blogspot.com/2012/04/over-current-relay-49-50-51.html

Relay 60FL : VT Fuse Loss Detection http://idpowerstation.blogspot.com/2012/07/relay-60fl-vt-fuse-loss.html

Relay 59G : Earth Fault Protection or Neutral Over Voltage Relay http://idpowerstation.blogspot.com/2012/08/relay-59g-earth-fault-protection.html