ANSI Codes Description or Under Standing-4 (31-40)

31) – Separate Excitation device: It is a device that connects a circuit to separate excitation for starting. Such as Shunt field of Synchronous converter is connected to separate source of excitation.

32) – Sensitive Directional Power relay: It is a device which functions on a desire value of power flow in a given direction.

These relays will acts based on the power flow.

32RP: Reverse power relay:

When a generator is running with system/grid or in parallel with another generator, and torque given by prime mover is less than the torque required by generator, in this case to maintain grid frequency or synchronization generator will draw current from grid or from another generator side, and  will act to run as motor. It can be dangerous depending on the type of prime mover for generator. Prime mover can be Diesel Engine, Steam Turbine, gas Turbine, hydro turbine, Wind turbine etc.

CASE 1: Consider a generator driven by Steam Turbine. In this case when steam turbine is less efficient to drive the turbine or by some reason it gets tripped, then there is heavy cut off in the torque required to drive generator. Now in this condition Generator breaker must get trip in order to maintain system healthy.

Consider Generator breaker has not trip, now in order to maintain system stability at the grid frequency, generator will draw current from grid and hence generator will draw a power from grid instead delivering to grid, so it will terms as reverse power.

Now in condition of reverse power absorb by generator stator winding, generator will start to behave like an induction motor and rotor will rotate to match with synchronous speed in order to maintain grid frequency. In this case there will be heating of rotor end ring and outer wedges of the same which will in turn damage the rotor winding. This reverse power flow is restricted by tripping the generator on this condition. So GCB must have to trip to avoid such condition. This can be achieve by using REVRSE POWER RELAY.

CASE 2: Consider a wind turbine. This is driven by wind. During high wind situation rotor blades will rotate by the kinetic energy of wind, wind turbine converts this kinetic energy into mechanical energy and wind generator (which is asynchronous generator) will produce electricity.

Now in condition of low wind, generator will draw reverse power from grid (as asynchronous motor can run as induction motor) and will give un-necessary power losses. During No-Wind, generator will operate as motor drags rotor blades round shape as like huge fan, which is unwanted. How ever to minimize this electrical losses wind generator generally cuts-in during low wind or no wind conditions, and this can achieve by control system.

Now in the running condition, if there is a situation when wind pressure decreases and some instant wind pressure is too low that torque produced by wind turbine is too low to rotate generator, in this case it will draw reverse power from grid and generator will start running as asynchronous motor. It will lead loss of electric power and penalty or economic loss to Generation Company. So in order to avoid this situation reverse power relay will sense when generator will start drawing reverse power, and will cut off it from grid.

However this cutoff is done by thyristor switching which allows smooth and slower cut off, if direct cut off will be done it will create violent effects to whole grid.

32F: Low Forward Power

In the event of failure of excitation, there will be absence of field in generator stator. So in this case as there is no AVR present, turbine will be in over speed condition as and in the absence of field to generator no emf will be present and hence absence of field in Stator. Now generator will start working as induction generator (as rotor rotates at the speed corresponding to turbine output and it will induced field in stator) and active power generation will reduced and reactive power generation will increase.

This condition will be monitored by relay and after some time delay it will give trip as non urgent fault. How ever if this condition last longer it can lead to more reactive power generation and in turn motoring action of induction generator and it will act as reverse power.

33): Position Switch:

                It is a switch which makes or breaks contact main device or a device which has no device function number reaches a given position.

Like in switchgear, we will provide Test Service Position switch for rack-in/rack-out type breaker.

Or like in earth switch in switchgear.

34): MASTER SEQUENCE DEVICE:

                It is a device which determines the operating sequence of the major devices in a equipment during starting and stopping or during other sequential switching operation.

Example: Breaker or motor operated multi contacted auxiliary switch, or PLC, or Computer or DCS.

35) BRUSH OPERATING OR SLIPPING SHORT CIRCUITING DEVICE:

It is a device for shifting, raising, lowering a brushes of a machine of for short circuiting its slip rings or for engaging or disengaging the contacts of a mechanical rectifier.

36) POLARITY OR POLARIZING VOLTAGE:

                It is a device that operates or permits the operation of, another device on a predetermined polarity only, or verifies the presence of a polarizing voltage in equipment.

37) UNDER CURRENT OR UNDER POWER RELAY:

                It is a relay which functions when the current or power flow decreases below a predetermined value.

                This element is used to trip the generator or issue an alarm depending up n severity. How ever in case of expensive peaking units are operating at low power levels, this element is useful to issue an alarm.

                Also this element can be used to trip the generator during orderly shutdown.

38) BEARING PROTECTIVE DEVICE:

                It is a device that functions on excessive bearing temperature or on another abnormal mechanical condition associated with the bearing, such as undue wear, which may eventually result in excessive bearing temperature.

39) MECHANICAL CONDITION MONITOR:

                It is a device that functions up on the occurrence of an abnormal mechanical condition (except conditions associated with bearing as covered under function 38), such as excessive vibration, eccentricity, expansion shock, tilting or seal failure.

40) Filed failure Relay (Loss of field or Loss of excitation).

                It is a relay that functions on a given or abnormally low value or failure of a machine field current, or an excessive value of the reactive component of armature current in an a-c machine indication abnormally low field excitation.

                A loss-of-field condition can occur due to an open circuit in the DC supply to the generator field windings, a short circuit in the field windings, or an inadvertent tripping of the excitation system circuit breaker. When a loss-of-field condition occurs, the high reactive current drawn from the power system by the generator can overload the stator windings.

                Partial or total loss of field on a synchronous generator is detrimental to the generator and the connected power system. The condition must be quickly detected and the generator isolated from the system to avoid generator damage. A loss-of-field condition that is not detected can have a devastating effect on the power system by causing a loss of reactive power support and a substantial reactive power drain. This reactive drain, when the field is lost on a large generator, can cause a substantial system voltage dip. When the generator loses its excitation, it operates as an induction generator, causing the rotor and amortisseur bar temperature to rapidly increase due to the slip-induced eddy currents in the rotor iron.

                The loss-of-field relay needs to be coordinated with the steady-state stability and the generator capability and under-excitation limiter. To control system high voltage, the generator may have to operate under-excited and absorb reactive power from the power system. This is especially true when the system breaks into islands during a major disturbance. It is important that the generator be able to absorb reactive power within its capabilities, as defined by the generator capability curve, to regulate system voltage. The generator under-excitation limiter must be set to maintain operation within the capability curve. The loss-of-field relay must be set to allow the generator to operate within its under-excited capability.

Source: http://www.hydroworld.com/index/display/article-display/353952/articles/hydro-review/volume-28/issue-2/feature-articles/system-protection/coordinating-generator-protection-and-controls-an-overview.html

ANSI Codes Description or undrestanding-3

21)- Distance Relay:

The relay which will function when the circuit admittance, impedance, or reactance increases or decreases beyond predetermined current limits.

Example:

Transmission lines are protected with this type of relay. They are covered in different zones of protection like 3 zones or 5 zones. And depending on the value set for each zone, protection will operate.

Question may arise here, why distance protection? Isn’t over current protection capable to take care of transmission line?

This can be understood by following figure. There are two different conditions shown. First condition is showing, fault at F1. So, total impedance will be Z= 5+4 = 9.

So on I= 110*1000/rt3*9= 7064 Amps.

Now consider another condition. As in this condition I=110*1000/(rt3*10) = 6358.

So over current range will be 6358 to 7064 Amps. This is unpractical.

In distance relay, relay will be operates on based on the impedance settings. So if impedance seen by relay is below set value, it will operate for that zone.

However, traditional relays don’t measure accurate impedance. These relays will measure fault voltage and compare it with the voltage derived from fault current and zone impedance setting. And it will determine whether fault is within zone or out of zone.

22) – Equalizer Circuit Breaker:

It is a circuit breaker that serves to control or to make and break equalizer or the current balancing connection for a machine field, or for regulating equipment in a multiple-unit installation.

23)- Temperature Control Device:

It is a device that functions to raise or lower the temperature of a machine or other apparatus, or of any medium, when its temperature falls blow or rises above, predetermined value.

Example: Thermostat in Switchgear assembly.  When temperature falls below set value, it will switch on heater and after reaching desired value it will disconnect heater circuit.

24)- Reserved for Future Application: OVER EXCITATION/OVER FLUXING Device

                It is a device which operates in response to the over fluxing of a machine.

                Example:

               

                Transformer Over-fluxing or generator over-fluxing problem

               

                Short Description:

                                Over excitation or over fluxing protection depends on ration of V/f (V= Voltage, & f = Frequency). So when Voltage increases or frequency decreases over excitation or over fluxing introduce.

                                During a power system emergency disturbance, for instance, a load rejection or load shedding, the transformers may be subjected to overvoltage or under frequency resulting in a condition usually called over excitation, also known as over fluxing.

 E= 4.44*f*N*a*B

Where E is the sinusoidal root mean square voltage of the winding, f is the frequency in hertz, N is the number of turns of wire, a is the cross-sectional area of the core and B is the peak magnetic flux density in tesla. The value 4.44 collects a number of constants required by the system of units.

So from EMF equation it is clear that B is inversely proportional to frequency.

So in short when rated flux density in the transformer core exceeding the maximum allowed limit set by the equipment designer.

So term Over-fluxing is more appropriate than Over-excitation.

Now consider a generator, sudden full load rejection. So there will be Power frequency high voltage at nominal frequency. However this condition would not be sustained for prolong time. So there will not be any effect other than a problem related to transformer differential protection stability.

Consider, a generator in run up condition. AVR is in service and if field excitation is applied to early, then over voltage condition would be sustain and resulting in to over fluxing.

Or excitation still applied even though generator rundown.

It is important that VT reference for AVR and Over fluxing protection must be different.

25) – Synchronizing or Synchronism Check Device:

                It is a device which checks the synchronism condition before paralleling two A.C. devices. It will operate on when frequency, phase angle & voltage of these two A.C. Circuits will be equal, and it will permit to close the closing circuit.

Example: Paralleling of Two generators or connecting a generator with bus. 

                   It ensures when switching a line onto a Bus bar, that the stability of system is not endangered.

26) – Apparatus Thermal Device:

                It is a device that functions when the temperature of the shunt field or the amortiseur winding of a machine or that of load limiting or load shifting resistor or of a liquid or other medium, exceeds a predetermined value; or if the temperature of the protected apparatus, such as a power rectifier, or of any medium decrease below a predetermined value.

27) – Under voltage Relay:

                It is relay which functions on a given value of under voltage.

                When a load is connected suddenly to a source, the load will start to draw a current causing voltage drop. This may lead to under voltage condition. And if it will continue in the absence of UV protection, it may lead to system shutdown, or it may damage the equipment.

                For some equipments, UV protection is used for tripping purpose and for some equipment UV protection used for interlock &/or annunciation purpose.

Like in Motor, which is drawing high current from source, motor can get tripped from thermal overload relay in the sense of overload setting, but if source voltage is getting more dip, it will get trip for UV protection.

In generator, tripping for UV protection rarely applied, however it may be used for interlock purpose for another protection schemes, like field failure protection or inadvertent energisation protection or other cases where the abnormality is directly leads / connected to under voltage condition.

In transmission system, in the event of insufficient reactive power generation, which is necessary to maintain system profile, UV condition may arise. Such condition must be removed, in order to avoid possible condition of system voltage collapse.  But in such condition one should not trip generation. Generally UV occurs due to overloading or AVR failure or any other reason.

Even though if there is requirement of UV protection trip, it should be comprise with time delay, in order to avoid mal-operation or clearance of another cause behind UV.

27P: Phase under voltage Protection. UV relay will operates for the UV in phase only.

27N: Neutral/Ground Under voltage protection:  UV conditions in ground only.

27A: Auxiliary under Voltage: UV conditions arises in Auxiliary circuits, or the circuits used

28) – Flame Detector:

               

                It is a device that monitors the presence of pilot or main flame of such apparatus as a gas turbine or steam boiler.

29) – Isolating contactor: It is a device that used expressly for disconnecting one circuit from another for the purpose of emergency operation, maintenance or test.

30) – Annunciator Relay: It is a device which gives a number of separate visual indication of the function of protective devices.


Still Working on ANSI devices Description, Will publish it soon as soon as I will complete it.

ANSI Codes Description or undrestanding-2

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.

Synchronous speed: It is the speed of an AC induction motor at which motor should spin if induced magnetic field in rotor is exactly equals to the rotating magnetic field of stator.
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 90^o 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 30^o 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.

In the split-phase motor, the startup winding is designed with a higher resistance than the running winding. This creates an LR circuit which slightly shifts the phase of the current in the startup winding. When the motor is starting, the startup winding is connected to the power source via a set of spring-loaded contacts pressed upon by the stationary centrifugal switch. The starting winding is wound with fewer turns of smaller wire than the main winding, so it has a lower inductance (L) and higher resistance (R). The lower L/R ratio creates a small phase shift, not more than about 30 degrees, between the flux due to the main winding and the flux of the starting winding. The starting direction of rotation may be reversed simply by exchanging the connections of the startup winding relative to the running winding.
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.

http://www.brighthub.com/engineering/electrical/articles/44951.aspx

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.