11)- Reserved for Future Application ( USBR Assigned –Control Power transformer) /Multifunction Device:
12)- Over Speed Device :
It is directly connected speed, which operates on machine over speed.
Overview: Generally Turbines are equipped with speed governors, so as to control their desired speed and so on control on the driving substance. Such as Steam for Steam Turbine, Water for Hydro turbine, gas for gas turbine, wind for wind turbine etc.
In addition with this speed governor there will be a shutdown or tripping system in order to control and maintain turbine from sever and disastrous conditions. As in the absence of proper control & insufficient over speed control can lead severe or disastrous damage to turbine as well to the surrounding.
The governor and over speed system may vary from machine to machine; it can be either mechanical, electrical, hydraulic, or combination.
The control of a turbine with a governor is essential, as turbines need to be run up slowly, to prevent damage while some applications (such as the generation of alternating current electricity) require precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an over speed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, and this can lead to big accident, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials. During normal operation in synchronization with the electricity network, power plants are governed with a five percent droop speed control this means the full load speed is 100% and the no-load speed is 105%. This is required for the stable operation of the network without hunting and drop-outs of power plants. Normally the changes in speed are minor. Adjustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a centrifugal governor (A centrifugal governor is a governor that controls the speed of an engine by regulating the amount of fuel (or working fluid) admitted, so as to maintain a near constant speed whatever the load or fuel supply conditions. It uses the principle of proportional control.) (Proportional control is how most drivers control the speed of a car. If the car is at target speed and the speed increases slightly, the power is reduced slightly, or in proportion to the error (the actual versus target speed), so that the car reduces speed gradually and reaches the target point with very little, if any, "overshoot", so the result is much smoother control than on-off control.)
Generally this is a basic system requirement for all power plants because the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication.
13) – Synchronous speed device:
It is a device which operates at approx. the synchronous speed of machine.
Example: Centrifugal Switch, Slip-Frequency Relay, Voltage Relay, under-current relay.
In split phase motor as motor is started up by magnetic field sets up by external field, so motor is started up on synchronous speed almost, as there will be a same rotating magnetic field and induced magnetic field of rotor. And after getting designed speed, say ¾ of synchronous speed, starting winding is disconnected by centrifugal switch. Centrifugal switch is acting as Synchronous speed switch.
Brief explaination:
Centrifugal Switch: A centrifugal switch is an electrical switch that operates using the centrifugal force created from a rotating shaft, most commonly that of an electric motor. The switch is designed to activate or de-activate as a function of the rotational speed of the shaft.
Perhaps the most common use of centrifugal switches is within single phase, split—phase induction motors. Here, the switch is used to disconnect the starting winding of the motor once the motor approaches its normal operating speed.
In Newton’s mechanics, the term centrifugal force is used to refer to one of two distinct concepts: an inertial force (also called a fictitious force) observed in a non-inertial reference frame and a reaction force.
Resistance split-phase motor induction motor
If an auxiliary winding of much fewer turns of smaller wire is placed at 90o electrical to the main winding, it can start a single phase induction motor. (Figure below) With lower inductance and higher resistance, the current will experience less phase shift than the main winding. About 30o of phase difference may be obtained. This coil produces a moderate starting torque, which is disconnected by a centrifugal switch at 3/4 of synchronous speed. This simple (no capacitor) arrangement serves well for motors up to 1/3 horsepower (250 watts) driving easily started loads.
The phase of the magnetic field in this startup winding is shifted from the phase of the mains power, allowing the creation of a moving magnetic field which starts the motor. Once the motor reaches near design operating speed, the centrifugal switch activates, opening the contacts and disconnecting the startup winding from the power source. The motor then operates solely on the running winding. The starting winding must be disconnected since it would increase the losses in the motor.
14)- Under Speed Device:
It is a device which functions when the machine speed fall below desired speed.
15) – Frequency or Speed Matching Device:
It is a device that functions to match and hold the frequency or speed of the machine or system equal to another machine or system or source.
Example: In Turbine Auxiliary governor panel, some speed controller will be there. This speed controller will raise or lower down the speed when and where required.
So this speed raising or lowering equipment are acting as speed matching devices.
Now as we know speed is directly depending up on the Frequency.
N= 120f/P for rotating machine N = RPM, F = Frequency & P = Number of pole.
So N is directly proportional to f.
Hence speed is directly depends on frequency or vice versa.
Consider a grid with 50Hz frequency. Now if there is reduction in system frequency, that means there is more demand of power requirement in grid what is being producing, so in view of that there must be increase in power production to meet this requirement, and this can achieve by developing more power or cut off the additional loads or by paralleling another machine to the grid.
But every time and every moment is not feasible to connect another machine (or may be all machines are connected), and its also not practical to cut down additional loads every time. So the solutions is only to increase the production can be achieved by more rotation of turbine, which will gives more torque to generator. And as generator converts torque into current, it will develop more power.
Rotation of turbine can be increased by increasing speed of turbine, and this can be achieved by increasing fuel input, which is controlled by some governor or other speed raiser equipment.
Similarly can be consider for speed lowering, when frequency increases requirement.
So this kind of equipment would be termed as Speed or frequency matching device.
16) – RESERVED FOR FUTURE APPLCIATION ( USBR ASSIGNED BATTERY CHARGING DEVICE)/ Data Communications Device
17) - Shunting or Discharge Device:
It is a switch which serves to open or close a shunting circuit around any piece of apparatus, other than that of resistor.
Like switching of a machine field, a machine armature, a capacitor, or a reactor
It is entirely different that of device no 6, explained earlier, which are into motor starting process. Also it doesn’t include device no. 42, used as running circuit breaker, like air contactor, or load resistor contactor used as device no 73, as it includes switch resistance device, or like switching of space heater, or power rectifier.
Example:
Consider capacitor start induction run motor.
As shown in figure, for starting of motor, capacitor is introduced in series with starter winding. So that Is (starting winding current) will lead V (voltage) applied across circuit. As Im current in main winding lags behind voltage (as it is inductive load) Is will lead Im by a greater phase angle.
When motor reaches its 75% of full speed, and it will disconnect starter winding as well as capacitor, for main winding connected in parallel to starting winding.
So as centrifugal switch disconnects shunting equipment from armature, it can be termed as shunting or discharge switch.
For clear view on capacitor run induction motor, following link can be useful.
18) - Accelerating or Decelerating Device:
It is device that is used to close or to cause the closing of circuits which are used to increase or decrease the speed of machine.
Example:
As in TAGP panel , Turbine Auxiliary and governor panel, there is use of Speed raiser & speed lower with the help of governor. However, for this action command to governor is given by some other means, it may be some power relays in case of auto mode, or may be through some switch in case of manual mode.
19)- Starting or Running Transition Contactor:
It is a device that operates to initiate or cause the automatic transfer of a machine from the starting to run power connection.
20) VALVE: is used in vacuum, air, gas, oil or similar line, when it is electrically operated, or has electrical accessories, such as auxiliary switches.
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