Protection 11

8/10/2019 Protection 11

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Generator & transformer

Protection

Hui Ren

Electrical Engineering Department

North China Electric Power University

Source: Power System Protection, Edited by the Electricity Training Association


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2014/11/4 HUI REN @ NCEPU100-2

Differential protection

overcurrent protectionEarth fault protection

Overload protection

Loss of field protection

Reverse power protection

Phaseto phase faults

interturn faults

earth faults

earth faults

Loss of excitation

overloading

Reverse power

Stator

faults

rotor

faults

overvoltage

Abnormal

conditionOvervoltage protection

Interturn fault protection

Generator faults, Abnormal

Condition and Protection


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2014/11/4 HUI REN @ NCEPU

100-3

Overall differential protection for phase-to-

phase faults

Overcurrent protection as backup

protection and protection for earth fault.


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(a) Basic differential

overcurrentrelay

(b) percentage

differential relay

Differential Relay


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Percentage-differential relaying for a wye-connected

generator.

Differential Relay


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Percentage-differential relaying for a delta-connectedgenerator.

Differential Relay


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Percentage-differential relay for

a generator and transformer unit


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100-8

Inter-turn fault protection

Inter-turn fault can not be detected by the Differentialprotection

Remaining clear of earth, no differential current except fora large current circulating the shorted turns.

Fault evolving and cleared by other protection If the faults occur in the stator slots, they quickly develop

into faults to earth, then cleared by the stator earth faultprotection.

risk If occur at the winding ends, may cause extensive damage

to the generator before the fault evolves to one detectableby other protection.


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High-resistance groundingAdvantages of High-resistance grounding

reduced thermal and mechanical stress inapparatus carrying ground fault current;

reduced shock, burn, and flash hazards topersonnel in the vicinity of a ground fault,

ability to control transient overvoltages due toarcing faults.

For these reasons, most generators usehigh-resistance grounding.

Stator Ground Protection


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drawbacks of High resistance grounding it complicates the detection of ground

faults on the stator winding, particularlywhen the fault is close to the neutral.

Due to the small fault current, thegenerator differential relay is insensitive toa ground fault.

The differential relay has troubledistinguishing the small fault current fromthe third harmonic current that also flows inthe neutral.



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100-12

Two methods of detecting a short circuit to ground

Overvoltagerelay



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100% of Stator ground protection



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Rotor earth-fault protection

Utilizing a high resistance connected

across the rotor circuit, the centre point of

which is connected to earth through the

coil of a sensitive relay.


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100-15

If the prime mover does not produce sufficient torque todrive the generator, the generator may operate as a motordriving the prime mover. Since this is a potentiallydangerous condition for steam turbines, gas turbines,engines, and some hydro turbines, protection is needed.

The simplest way to provide this protection is with adirectional relay that senses the real (or average) powerflow P, but is insensitive to the reactive flow Q. If the relaydetects P into the generator from the electrical system, thenthe prime mover may have insufficient input power and therelay should trip. The relay may be set to alarm and/or tripafter a time delay of a few seconds.

Reverse Power Relay


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100-16

Faults external to the generator are usually cleared quicklyby circuit protection, but failure of remote protection tooperate, or its associated circuit breaker to trip, wouldleave the faulted circuit connected to the genrator.

Phase to ground and phase to phase short circuits

unbalanced loads, and unsymmetrical (non-transposed)transmission lines are produce varying degrees of negativesequence current in the generatorso cause the rotoroverheating. Therefore, the negative sequence protection isone of the primary protection.

the negative sequence relay is a backup protection, since itprimarily protects the generator from faults external to theunit. Because of this, the relay must be coordinated withother relays in the system. Typical relay application is atime-overcurrent relay with a negative-sequence currentmeasuring network

Negative phase-sequence relay


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100-17

diagram of a negative phase sequence overcurrent relay

Negative Sequence Relay

Negative phasesequence current

measuring network


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Loss of excitation (field failure)

protection

Loss of excitation results in a generator losingsynchronism and running above synchronous speed.Operating as an induction generator, it would produceits main flux from wattless stator current drawn fromthe power system to which it was still connected.Excitation under these conditions requirescomponents of reactive current which may wellexceed the rating of the generator and so overload the

stator winding.


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100-19

Loss of field (loss of excitation) relays arerecommended for generators. If the generator

loses its excitation, then it will draw its excitation

from the electrical system by drawing reactive

power as what an induction machine would,meaning the generator is supplying real power but

absorbing reactive power.

Excitation under these conditions requires

components of reactive current which may well

exceed the rating of the generator and so overload

the stator winding.

Loss of Field Protection


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100-20


Although loss of field can be damaging to the

generator, it is also a system problem that causes low

voltage and resulting in low reactive-power support

of other nearby generators. In some cases, systeminstability could occur because one machine lost its

field at a time when the remaining generators were

heavily loaded with reactive power.


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Introduction


Percentage differential protection Magnetizing inrush current

Overcurrent relays

Pressure relays

Conclusion

Transformer Protection Topic


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Fault, abnormal condition and protection


Overcurrent protection

Overload protection

Overfluxing protection

Overheating protection

Phase to phase faults

interturn faults

Phase to earth faults

Core fault

overfluxing

overheating

tank fault

Introduction


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100-23


Similar to that of generators, but

The differential protection system comparesh.v. and l.v. current, which are in a known

relationship under healthy conditions, ratherthan the same current entering and leavingthe protected apparatus, as for generatorprotection.

So, it is capable of detecting interturn shortcircuits, since these change the effectiveoverall transformation ratio of the powertransformer.


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100-24

c.t. connection requirements

Giving a through fault balance

No zero-sequence currents;

Phase shift due to the through transformer of

positive and negative-sequence currents mustbe compensated;

Effect of tap changing equipment upon the

overall transformer ratio.


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Two basic requirements that the differential-relay connectionsmust satisfy are:

(1) the differential relay must not operate for load or

external faults;

(2) the relay must operate for severe enough internal faults.

Differential ProtectionDifferential Protection


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100-26

Unbalanced currentunbalances between CTs during external faultsarising from an accumulation of unbalances forthe following reasons:

(1) tap-changing in the power transformer;

(2) mismatch between CT currents and relay tapratings;

(3) the difference between the errors of the CTs oneither side of the power transformer;

(4) the inrush current;

Differential Protection


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the CTs on any

wye winding of a

power transformer

should beconnected in delta,

and the CTs on

any delta winding

should be

connected in wye.

Differential Protection


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Connections of Two-winding transformer with differentialrelays.

Percentage (biased) Differential

Protection


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Percentage Differential

Protection


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100-30

Problems related to differential relaying of power

transformersdisturbance measurement Side effects 1 Side effects 2 Side effects 3

inrush Accurate estimationof the 2ndand the 5thharmonics takes

around one cycle.

Due to the magneticof the core, the 2nd

and the 5thharmonic

may be jeopardizing

relay security

The harmonicsmayblock a relay during

severe internal faults

due to saturation of CT

The means ofrestraining the

relay from

tripping during

external

faults,inrush and

overexcitation

may Limit the

relay speed ofoperation

overfluxing The 5thharmonic maybe present in internal

fault currents due to

saturation of CT

External fault The measuredcurrents display

enormous rate of

change and are often

significantly distorted

The CTs saturation

during external fault

may produce an

extra differential

signal

All the means of

preventing false tripping

during external faults

reduce to the

dependability of the

relay

Internal fault The internal currentmay be as low an

few percent of the

rated value

The security demandsunder inrush,

overexcitation and

external faults may limit

relay dependability


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the magnetizing inrush may be also caused by

(a) Initial magnetizing due to switching a

transformer in.

(b) occurrence of an external fault, voltagerecovery after clearing an external fault.

(c) when a phase-to-ground fault evolves into a

phase-to-phase-to-ground fault

(d) out-of-phase synchronizing of a connected

generator.

Magnetizing Inrush Current


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100-32

When a transformer is switched-off, the

magnetizing voltage is taken away, the

magnetizing current goes to zero while

the flux follows the hysteresis loop of the

core. This results in certain residual flux

left in the core. When, afterwards, the

transformer is re-energized by analternating sinusoidal voltage, the flux

becomes also sinusoidal but biased by the

remaining flux. The residual flux may be

as high as 80-90% of the rated flux, and

therefore, it may shift the flux-currenttrajectories far above the knee-point of

the characteristic resulting in both large

peak values and heavy distortions of the

magnetizing current .

Inrush due to switching-in


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Characteristics of the inrush current

Include a large dc component.Include amounts of higher harmonics , mainly the second harmonic.

Typical Inrush Current


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0

10

20

30

40

50

60

70

2nd

3rd

4th

5th

6th

7th

Harmonic components of the

inrush current


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This is a classical way to restrain the relay from tripping during

magnetizing inrush conditions. As analyzed before, themagnetizing inrush current consists of certain amounts of higherharmonics, but the second harmonic always dominates. Generally,low levels of harmonics indicate internal fault and enabletripping, while high levels indicate inrush and restrain the relay.

For digital relays this may be written as:

Where, Id2is the amplitude of the second Harmonic in the

differential current;

Id1is the amplitude of the power frequency component in

the differential current;

Kis the restraint ratio of the second harmonic, and the

setting is about 0.15-0.2(15-20%).

Second Harmonic Restraint


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The shape, magnitude and duration of the inrushcurrent depend on several factors:

Size of a transformer

Impedance of the system from which a transformer

is energized

Magnetic properties of the core material

Remanence in the core

The moment when a transformer is switched in

How a transformer is switched in

Inrush Current


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Sample inrush currents in a three-phase wye-delta connected

transformers


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Inrush currents measured in separate phases of athree-phase transformer may differ considerably

because of the following:

The angle of the energizing voltages are

different in different phases.

When the delta-connected winding is switched-

in, magnetizing voltages are line voltages.

Depending on the core type and other conditions,only some of the core legs may get saturated.

Inrush in Three Phase

Transformer


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Due to the large and slowly decaying dc component, the inrushcurrent is likely to saturate the CTs even if the magnitude of the

current is comparatively small. When being saturated, a CT

introduces certain distortions to its secondary current. Due to

CTs saturation during inrush conditions, the amount of thesecond harmonic may drop considerably.

Saturation of current

transformers during inrush


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High current settingmust not operate

under emergency loading conditions

Slow operationtime setting may have to

be high in order to grade with other

overcurrent relays on the system

On large transformersas backup

protection for terminal faults, or unclearedl.v. system faults.


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Various types of mechanical relays such as suddenpressure relays and gas-accumulation relays havebeen used for transformer protection.

On the occurrence of an internal transformer fault,

the pressure inside the tank suddenly rises. Thesudden pressure relay senses this, but it isinsensitive to pressure changes that normallyoccur in a transformer during operation. In many

cases, the pressure relay will operate on aninternal fault that does not produce enough currentto trip the differential relays to avoid catastrophicfailures.

Pressure Relay


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Percentage differential protectionApplication: above 6300KVA

Magnetizing inrush current

The cause of magnetizing inrush currentThe characteristics of magnetizing inrush

current

The measures of distinguish inrush current


Application: external phase-fault protection

Conclusion


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No matter what the initial conditions are, when excitation is lost, the equivalent

generator impedance traces a path from the first quadrant into a region of the

fourth quadrant that is entered only when excitation is severely reduced or lost.By encompassing this region within the relay characteristic, the relay will

operate when the generator first starts to slip poles and will trip the field breaker

and disconnect the generator from the system before either the generator or the

system can be damaged.

the most common type of lose-of-

excitation relay is a directional-distance relay measuring the AC

current and voltage at the main

generator terminals. Figure shows

several loss-of-excitation

characteristics and the operating

characteristic of one type of loss-of

excitation relay on an R-X diagram.


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Protection 11