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