Essential Guide: Generator Protection Measures and Best Practices

Generator Protection Measures and Best Practices

Discover how to safeguard generators for reliable operation. Learn key principles, detect faults, and implement effective protection strategies. Optimize performance and ensure uninterrupted power supply. Join us on this educational journey to enhance generator reliability.

GENERATOR PROTECTION:PHILOSOPHY

• •STATOR EARTH FAULT PROTECTION
• •ROTOR EARTH FAULT PROTECTION
• •GT OVERFLUXING PROTECTION
• •OVERCURRENT PROTECTION
• •LOSS OF EXCITATION
• •GENERATOR DIFFERENTIAL PROTECTION
• •LOW FORWARD & REVERSE POWER PRTN
• •GEN BACK UP IMPEDENCE PROTECTION
• •GENERATOR POLE   SLIPPING
• •GENERATOR OVERVOLTAGE PROTECTION

STATOR EARTH FAULT PROTECTION

• Conventional unit type generator has the neutral earthed through a resistance loaded distribution type transformer. The load resistor designed to limit the ground fault current to a value less than the total charging currents of the capacitance to ground of generator winding. Resistance results in low ground fault currents(5A) for a solid phase to ground fault at the generator terminals. Currents of this magnitude may not cause serious damage to the core steel. For a single ground fault near the neutral end of the winding, there will be proportionately less voltage available to drive the current through the ground, resulting in lower fault current and a lower neutral bus displacement voltage. At the lower limit, a fault on neutral bus will result in no fault current or displacement voltage at all.
• Low magnitude fundamental (50Hz) ground currents may flow under normal conditions, possibly due to generator winding imbalances or due to faults on the HV side of the unit TRF or the secondary side of the GEN PT. Under these conditions the GEN shall not removed from service. To allow for these low magnitude earth fault currents, trip setting for over voltages in excess of 5-10% of the P-N voltages.
• So the possibilities of an undetected earth fault in the lower portion of the winding , where the corresponding neutral displacement voltage is below the earth fault relay trip setting, may arise. If an earth fault occurs & remains undetected , the probability of second fault occurring is greater. This second earth fault may result from the insulation deterioration caused by the transient overvoltage due to erratic low current & unsteady arcing at the first fault joint. The second earth fault yield currents of larger magnitudes. Therefore considered to provide 100% stator earth fault protection
• If the relay setting is too sensitive to sense the earth-fault currents of small magnitude, there is possibility of relay coil damage & ultimately the stator winding to ground capacitance is shorted or broken where ever there is the flow of heavy fault currents in the stator circuit. The diagram indicates that when the generator is working normally, stator winding ground capacitance is charged to the phase to ground voltage. This capacitor discharges suddenly through the relay coil & grounding resistor which may damage relay coil. When this happen, the stator winding capacitance gets shorted due to the flow of heavy currents. So the relay setting adjusted for the protection of only 85-90% of the winding so that it can withstand to the flow of capacitor discharge current & fault current.

OPERATING PRINCIPLE:  This relay detects neutral end earth faults with the aid of a coded signal of 12.5Hz( which is generated from the battery voltage or obtained by frequency divisions of the network voltage & fed to the circuit of the generator stator winding through the coupling transformer.

  During the operation without any earth fault a small current (ma) flows to the earth through the capacitance of the stator winding as well as through the capacitance of the galvanically connected circuits. With an earth fault these capacitance short circuited and the current increases . To exclude the disturbing effect of transient phenomenon , 12.5Hz signal coded.

OPERATING PRINCIPLE(EE): 3^rd harmonic scheme:    A.C generators in service produce certain magnitude of 3rd harmonic voltage. However no 3^rd harmonic  voltage appears across the phases of a star connected generator, though there exists a certain magnitude of 3^rd harmonic  voltage  b/w the respective phases & gnd of the m/c. This voltage causes currents to flow when the star point earthed. In fact under healthy conditions the 3^rd harmonic voltage generated by the m/c shared b/w the phase & ground capacitor impedance at the m/c terminals & the neutral-ground impedance.

•3^rd harmonic voltage produced can vary 1 to 3% of the rated terminal voltage. 3^rd harmonic voltage changes with machine loading. The ratio of these voltages remain relatively constant . When fault occurs at X on the winding, the voltage distribution undergoes change from that during healthy conditions.

•Fig shows the distribution of  VN3 & VL3  for a quiescent condition Q corresponding to an healthy conditions. Fig shows the variation with respect to fault position X. Relay measures continuously the difference in the scalar magnitudes of these quantities & trips if it exceeds the setting. When fault occurs at the neutral end, VN3 becomes zero.  Similarly when fault occurs at the line terminal, VL3 becomes zero.

ROTOR EARTH FAULT PROTECTION

•Failure of rotor winding insulation or single ground fault doesn’t cause flow of current since the rotor circuit is ungrounded. When the second EF occurs , that part of the winding is bypassed & the currents in the remaining portion may increase.

 This causes unbalance in the rotor and may cause mechanical as well as thermal stresses. Current unbalance increase with rotor current resulting in damage to the rotor. In some cases the vibrations have caused damage to the bearings & bending of rotor shaft.

 There are 3 methods to detect this fault:

1.Potentiometer method

2.A.c injection method

3.D.c injection method

A.C INJECTION METHOD:

RELAY OPERATION:  D.C INJECTION

GT OVERFLUXING PROTECTION

Over fluxing condition is most likely to arise when the m/c speed is rising towards or decreasing from synchronous speed . And a failure of both AVRs in automatic control, or incorrect oprn in manual control. This is rare but it can cause overfluxing of GT & UAT during commissioning tests or testing of the excitation equipment.

 Over fluxing results in

• Large increase in magnetizing current &  iron losses
• Increase in winding temperature
• Increase in transformer noise & vibration
• Overheating of non-laminated metal parts affected by stray fluxes .

This protection is valuable for the generator & step up transformer during start up/shutdown & also for system transformers during load shedding. Increase in the flux diverted from laminated core structure to steel structure parts. Core bolts get rapidly heated as they subject to this large flux & their insulation destroyed along with coil insulation.

From the transformer equation

    E =  4.44 B A  f  N                where    E = emf induced in transformer

           B = flux density                A = area of the core

           f  =  frequency                N = number of turns

So   B  is inversely proportional to  f    &    directly  proportional to  E.

So disproportional variations in these quantities  may give rise to core over fluxing. A transformer designed for a voltage limit of 1.2 per unit at rated frequency. It experiences over fluxing when it exceeds 1.2 pu.

Transformer shall operate with some degree of overvoltage with a corresponding  increase in frequency . But operation must not continued with a high voltage input at low frequency.

OP PRINCIPLE(EE):

The principle of the relay is to produce an alternating voltage which is proportional to the ratio of input voltage & frequency and to compare this with a fixed ref. Voltage. When the peak of the alternating voltage exceeds the fixed dc ref , the first timer started. At the end of the second timer cycle the second adjustable timer initiated.

To obtain the correct measurement the applied voltage V converted to a current by means of a R. This current V/R arranged to flow thro a capacitor C to produce an o/p voltage V/( 2*PI*f*R*C)

LOSS OF EXCITATION

Loss of excitation can occur as a result of

• Loss of field to the main exciter
• Accidental tripping of the field breaker
• Short circuits
• Poor brush contact in the exciter
• Loss of a.c supply to the excitation

What happens when generator loses excitation ?

Gen runs as an induction generator & runs as asynchronously(>the synchronous speed). Generator draws reactive power (VAR) from the grid .Stator current produces field, which is cut by the rotor & rotor magnetic field is set up (or the grid itself supplies reactive power to sustain the rotor magnetism)(that is the main flux is produced by wattless stator current drawn from the system). Generator still supplies  active power. Rotor takes 2 to 4 times the rated reactive power .This large reactive load suddenly thrown on the system causes widespread voltage reduction  & instability unless other power plants are large enough to supply  the deficiency of VARS immediately.

Operation  as an induction generator necessitates the flow of slip frequency current in the rotor, the current flowing in the amortisseur (or damper) winding and also in slot wedges and the surface of the solid rotor body. Excitation under these conditions requires a large reactive component. However the gen not designed to operate as induction generator. The damper windings not adequate to carry the rotor slip current.

So rotor overheats quickly from the induced currents flowing in the rotor iron & windings,particularly at the ends ( time required to reach dangerous currents=2 to 3 mints) & overloading of the stator winding takes place also.

The quantity which changes most when gen looses excitation the impedance measured at the stator terminals. On the LOE , the terminal voltage will begin to decrease & the current increases, resulting in a decrease of impedance & also a change in power factor.

OPERATING CHARACTERISTICS OF THE RELAY

•Fig shows the LOC characteristics for a typical large generator that is connected to a system through a stepup transformer having a .15 per unit impedance on the machine base. These characteristics are shown as a function of both initial machine loading and system impedance. The following discussion will consider the effect of initial generator loading and system impedance on the impedance locus, on the generator terminal voltage and on machine loading during a loss of excitation condition.

In cases the LOC characteristics will be plotted with respect to two relay settings: one setting will have a circle diameter of 1.0 per unit, the other will have a circle diameter equal to machine synchronous reactance. The offset, in both cases, will be equal to X’d/2. Fig shows LOE chars of a relay on an R-X diagram. On LOE , the equivalent generator impedance(apparent impedance) as viewed from the m/c terminals, goes to the negative X region , that is from 1 to 4rth quadrant. As noted in the diagram, curves (a), (b) and (c)show the impedance loc ii as a function of system impedance with the machine operating initially at or near full load.

Curves (d) and (e) show the loc ii at two values of system impedance with the machine initially at about 30% load. For the case of the machine operating at full load, all of the impedance loc ii terminate in an area to the right of the (-X) ordinate and will approach impedance values, which at the final steady-state slip, will be somewhat higher than the average of Voltage decreases & oscillates around an average of 0.5P.U , power op decreases & oscillates around 0.3P.U. & VAR go negative around –0.93P.U .When the m/c is operating at full load, on loss of EXT,it is damaging to the gen & system. Stator currents rise in excess of 2.0per unit.

The high current due to the fact that the resulting machine loading at a substantially reduced terminal voltage. Of course, the VAR drain from the system can depress system voltages and thereby affect the performance of other generators in the same station or elsewhere on a system. IN addition, the increased reactive flow across the system can cause tripping of transmission lines and thereby adversely affect system stability. other generators and interconnections could not withstand the additional reactive load imposed on the system.

For example, in 1951 a utility reported4 that loss of excitation on a 50 MW generator caused system wide instability, the tripping of interconnections and tie lines and over 100 breaker operations before the disturbance subsided.

Relay chars

Variation of active power

Since input mech power not changed ,but EM power becomes less than than input mech power that is braking EM torque on the gen is less than the input mech power . So the rotor accelerates & pt moves along the Curve 2. At pt A  EM  torque again balances input mech power. So the gen operates stably at power angle U2. If exctn continues to drop then U reaches to 90degs, then speed will increase above sync speed.. This is asynchronous operation. If the m/c is still connected to the network, the n/w supplies a 3-phase symmetrical current to gen stator winding to make it to establish an air gap magnetic field & induce the potential & current in the rotor , so as to produce an asynchronous torque.

2. VARIATION OF REACTIVE POWER: After the LOE, gen enters into the asynhronous operation. As the active power fluctuates, there is an alternating EM power with frequency 2f & the reactive power consumed by the gen also fluctuates accordingly.

GEN DIFFERENTIAL PROTECTION

PRINCIPLE :

This uses circulating  current differential relays in which vector difference b/w the current entering the winding & current leaving the winding is used for sensing.

DIFFERENTIAL PROTECTION(DP) is provided for the gens above 2MVA . DP does not sense inter turn faults & over loads but senses P-P &  P-E faults. The magnitude of the earth fault current depends upon the value of the reactance connected b/w neutral & earth : and the position of the earth fault in the gen. When the gen is earthed through impedance, a separate additional earth fault protection is necessary in addition to diff protection.  DP provides earth fault protection to about 85% of gen winding.

REVERSE POWER PROTECTION

•Generator motoring (reverse active power ) protection is designed for the prime mover rather than the generator. Steam turbines will overheat on low steam flow. ( in hydro turbines blade cavitation  occurs on low water flow).  Gen motor protection can be provided by the devices such as limit switches or exhaust-hood temperature detectors, but rev pr relay is recommended.

•On motoring ,real power will flow into the machine and as the field excitation is same as before  the reactive may either flow in or out of the machine. KW drawn by the generator would only  be 1 to 3% of the name plate rating.

NEGATIVE SEQUENCE PROTECTION

•This protects gen against unbalanced loads.Unbalanced loading is caused by an open circuit of 1-P external to gen & may persist for sufficient time.

•A 3-P balanced load produces a reaction field which is constant & rotates synchronously with the rotor field system. Any unbalanced condition can be resolved into PSC, NSC & ZSCs. PSC is similar to normal balanced load. ZSC produces no main armature reaction & no heating of rotor. NSC is similar to PSC except that the resulting reaction field rotates counter to the d.c field system & hence produces a flux which cuts the rotor at twice the rotational velocity, thereby inducing double frequency currents in the field system & rotor body. The resulting eddy currents are very large & cause severe heating of rotor & damage the rotor.

•Since the heating depends on the reaction field & hence also on the load current, a m/c can be assigned a continuous NS rating. NPS levels can be increased  depending upon the cooling levels. Unbalanced stator currents also cause severe vibrations & heating of stator.

•The length of time (T) that  a gen may be expected to operate with unbalanced stator currents without danger of being damaged can be expressed in the form of ;    I₂ inst NPS comp of stator current as a function of time.

GEN POLE SLIPPING

•Prolonged fault clearing time, low system voltage, weak field condition or some line switching operation may cause pole to slip. A gen can lose synchronism with power system, without failure of the excitation system , becoz of severe disturbance or operation at high load with leading power factor & relatively weak  field.

• Rotor oscillations cause vibrations in voltage,current powerfactor, and torque reversals. Loss of excitation relay provides some protection , but cannot be relied upon under all conditions.

•The angular displacement of the rotor exceeds the stable limit & the rotor will slip a pole pitch. If the disturbance sufficiently removed by the time this has occurred, the m/c may regain synchronism, but if it doesn’t, it must be isolated from the system.

•Fluctuations in gen speed durin pole

•Field bkr opened making it run asynchronously & thereby removing the voilent power fluctuations from the system & corresponding mechanical torque oscillations from the m/c.

•The load should then be reduced to a low value, at which the set will probably resynchronize..  If it doesn’t work reclosing the field switch with the exc control  set to the minimum position will cause the set to synchronize smoothly.  Ohms relay will detect the impedance changes during the power swing & actuate the tripping sequence

GENERATOR CONSTRUCTIONAL DETAILS

• STATOR BODY
• STATOR CORE
• STATOR WINDING
• ROTOR
• BEARINGSSHAFT SEAL(RING TYPE)
• GAS COOLER

STATOR BODY

* Totally enclosed gas tight fabricated structure made of mild steel & austenetic steel , suitably ribbed internally to ensure high rigidity.

•Designed  mechanically to withstand internal pressure & forces as a result of unlikely event of explosion of hydrogen, air mixture without any residual deformation..

•H2 gas coolers housed longitudinally inside the stator body.

•Rigid end shields close the casing & support the fan shields & shaft seals made in 2 halves

STATOR CORE

*   made up of laminations, segmental warnish insulated punchings of electro technical sheet  steel with low loss factor to reduce magnetising & eddy current losses

•stampings arranged in an inter leaved manner in order to damp out the oscillations so that magnetic vibration of the stator core not transferred to the foundation through stator frame.

•Insulating paper press boards also put b/w the layer of stampings for additional insulation & to localize short circuit which may occur due to failure of varnish insulation.

•Corebars designed to provide elastic suspension of core in the stator

•Core consists of several packets separated by steel spacers for radial cooling of the core by H2

•Core held in pressed condition by means of heavy non magnetic steel press rings bolted to the ends of core bars

•To avoid heating of press rings due to end leakage flux 2 rings made of copper sheet used as flux shield

•The revolving magnetic field exerts a pull on the core resulting in revolving & elliptical deformation of the core which sets up a stator vibration at twice the system frequency known as double frequency vibrations.

•All gen cores featuring high dynamic vibration amplitudes are spring mounted in the stator frame to damper the transmission of double frequency vibrations to the foundations.

•Use of CRGO steel punchings can contribute to the reduction in weight of stator core because  high magnetic permeance of CRGO steels make the core work at comparitively high magnetic saturation ,without fear of excessive iron loss or too heavy demand for excitation ampere turns from generator rotor.

STATOR WINDING

3-PHASE,DOUBLE LAYER SHORT PITCHED ,BAR TYPE

Each slot accommodates 2 bars.

Bar taped with several layers of thermosetting epoxy mica tape.

To prevent corona discharge between insulation & the wall of the slot, the insulation in the slot portion coated with semiconductor varnish.

 Bus bars connected to bring out the three phases through bushings & 6 neutrals. The transposition  is of Roebel arrangement.

To make the stator body gas tight at the 2 ends, two end shields fitted with the help of bolts. End shield are of mild steel & ribbed to achieve rigidity.

Rotor comprises of the following

•Shaft

•Winding

•Wedges

•Retaining ring

•Fans

•Field lead connections

    High mechanical stresses resulting from the centrifugal forces due to rotation & short circuit torques call for a high quality , heat treated steel. The main constituents of the steel are chromium , molybdenum ,nickle & vanadium.

The conductors made of hard drawn silver bearing copper(low electrical resistance & high creep resistance so that coil deformations due to thermal cycling due to start & stop operations are minimum. Turns insulated  from each other by a layer of glass laminates. Coils insulated from rotor body by U-shaped glass laminates impregnated with epoxy varnish.

•For protection against the effect of centrifugal forces the winding secured in the slots by slot wedges. Wedges made from duralumin, an alloy of copper magnesium & aluminium .the wedges at the ends of the slot made from an alloy of chromium & copper . These wedges connected with damper segments under retaining ring for short circuiting induced shaft current. The overhang portion of field winding held by non magnetic austenitic steel forging of retaining ring against centrifugal forces. The centering  rings shrink fitted at the free end of the retaining ring. A spring ring used to prevent any relative movement between the retaining ring & centering ring.

•The generator cooling gas circulated by two single stage  axial flow propeller fans. The fans shrink fitted on either side of rotor body.   The rotor shaft supported on the pedestal type of bearings which have spherical seating to allow self alignment. The rotor winding solidly connected to the slip rings by means of field lead bars, current carrying bolts , field lead core bar and  flexible leads.