Dr. K. V.

Vidyanandan
Power Management Institute, NTPC Ltd.
Noida, Delhi - NCR, India.
 Dr. K. V. Vidyanandan
Power Management Institute, NTPC Ltd.
Noida, Delhi - NCR, India.
 NEED FOR TURBINE PROTECTION

❖ A steam turbine being rotating equipment, operating at very high

temperature, pressure and speed, is subjected to enormous stresses.

❖ Deviations in various operating parameters of a turbine may lead to

costly damages of various components, resulting in long outages.

❖ Accordingly, a turbine needs to be tripped for the following reasons:

1. To protect the turbine from inadmissible operating conditions
2. To prevent damages in case of plant failure
3. To restrict occurrence of failure to a minimum

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GOVERNING OIL RACK (KWU- 200 MW)
TRIP GEAR AND REMOTE TRIP SOLENOID

Remote Trip Solenoids

Main Trip Gears

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TURBINE PROTECTION (KWU)

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TURBINE PROTECTIONS (KWU - 500 MW)
FEW TRIPPING SET-VALUES

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 GENERATOR MOTORING

❖ Motoring is the condition in which the turbine is driven by the

generator at rated speed with the stop/control valves closed.

❖ In this operating mode, longer blades of the turbine are heated up

due to blading windage.
❖ To prevent heat-up beyond permissible temperatures, motoring
must not be allowed to continue for longer than one minute.

❖ If the condenser vacuum is very poor, (i.e. condenser pressure is

high), motoring must not be allowed for more than 4 seconds.

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FIRE PROTECTION -1

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FIRE PROTECTION - 2

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MECHANICAL OVER-SPEED TRIP DEVICE

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OVER-SPEED TRIP DEVICE (KWU)

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OVER-SPEED TRIP DEVICE (KWU)

Overspeed trip bolts

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LOW VACUUM TRIP DEVICE

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THRUST BEARING TRIP DEVICE

1. Compression spring
2. Bearing pedestal
3. Piston
4. Valve body
5. Turbine shaft
6. Pawl
7. Torsion spring
8. Piston
9. Compression spring
10. Limit switch
11. Knob

a: Test Oil
c: Return Oil
u: Aux. Startup Oil
x: Aux. Trip Oil

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ELECT. TURBINE PROTECTIONS (660 MW)

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 Dr. K. V. Vidyanandan
Power Management Institute, NTPC Ltd.
Noida, Delhi - NCR, India.
 GENERATOR TRANSIENT BEHAVIOUR

❖ After the occurrence of a fault, the fault current reduces with time.

❖ This is due to the decrease in the electromagnetic energy stored in

the inductances of various windings of the machine.

❖ Since energy stored in a magnetic circuit cannot be altered quickly,

it takes some time to establish a new electrical field.

❖ These time intervals counted in ms are known as:

❖ Subtransient period
❖ Transient period
❖ Steady state period
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GENERATOR SHORT CIRCUIT CURRENTS

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 GENERATOR DYNAMICS DURING FAULT

With a short circuit at stator terminal,

❖ In the initial subtransient period, stator current increase to 8 - 11

times of rated, with one phase offsetting an equal amount.

❖ Within few ms, current decay to a transient value of 3 - 5 IRated.

❖ In tenths of a second, current decay to a relatively steady value.

❖ Coupled with this, the field current increases suddenly by 3 - 5

times and decays in tenths of a second.

❖ Stator voltage on the shorted phases drops to zero and remains

so until the short circuit isDcr.lKeV.a.Vrideyadna.ndan
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VALUES OF GENERATOR REACTANCES

Sipat unit-1: 660 MW

Xd" : 0.188 pu Direct-axis subtransient reactance

Xd' : 0.265 pu Direct-axis transient reactance

Xd : 2.0 pu Direct-axis synchronous reactance

X2 : 0.23 pu Negative-sequence reactance

X0 : 0.1 pu Zero-sequence reactance

T"d : 20 ms Sub-transient time constant

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 GENERATOR ABNORMAL CONDITIONS
Stator (due to over-load or loss of cooling)
❖ Overheating
Rotor (due to over-excitation, loss of cooling)
Stator (phase and ground faults)
❖ Winding faults
Rotor (ground faults and shorted turns)
❖ Over-voltage
❖ Loss of excitation
❖ Unbalanced current operation
❖ Over-speed and under-speed
❖ Motoring
❖ Out of step
❖ Inadvertent energization
❖ Sub-synchronous oscillations
❖ Non-synchronized connectionDr. K.V. Vidyanandan
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 CLASSES OF UNIT TRIP
CLASS OF BREAKERS TO BE TRIPPED UNDER VARIOUS CLASSES OF TRIPPING
TRIP
GCB SCHEME NON GCB SCHEME
(additional LV CB between Gen and
GT)
Class A A1: GCB,HVCB,UT LV CB, HVCB,UT LV CB, FIELD, TURBINE
FIELD, TURBINE (All the system tripped)
(All the system tripped)

A2 : GCB, FIELD, TURBINE

(Generator circuit tripped & Auxiliaries
charged from the grid through
GT&UT)
Class B GCB,FIELD BREAKER HVCB,UT LV CB, FIELD BREAKER.
Initiated by Turbine trip & Low
Forward /reverse power, to release
the trapped steam. Generator circuit
breaker tripped & Auxiliaries charged
from the grid through GT&UT)
Class C HVCB Dr. K.V. Vidyanandan HVCB
(Generator under House klvovaiddya)s@gmail.com (Generator under House load )
 MAJOR GENERATOR PROTECTIONS

21 Backup Impedance 60 Voltage Balance

24 Over-Excitation, Volt/Hz 64 Earth fault
32 Anti-Motoring 78 Out-of-Step
40 Loss of Field 81 Frequency (u/o) *
46 Current Unbalance (-ve seq) 86 Lockout Relay
51 Backup Over-current 87 Differential
59 Over-voltage 98 Pole slipping

* u: under
o: over

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 GENERATOR PROTECTION SCHEME

60 Voltage Balance 21 Backup Impedance

64 Earth fault 24 Volt/Hz
78 Out-of-Step 32 Anti-Motoring
81 Frequency 40 Loss of Field
87 Differential 46 Current Unbalance
98 Pole slipping 51 Overcurrent
Dr. K.V. Vidyanandan
99 Over fluxing kvvidyas@gmail.com 59 Overvoltage
 GENERATOR LOSSES

The efficiency of a large generator is about 98.75 %.

In units above 20 MW capacity, to dissipate the heat generated

inside the generator, pressurised H2 Gas and D.M. Water are used.
Dr. K.V. Vidyanandan
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 WINDING INSULATION LIMITS
Insulation systems are rated by standard NEMA (National Electrical
Manufacturers Association) classifications according to maximum
allowable operating temperatures as follows:
Temp. tolerance Max. temp. Allowable temp.
Material
Class allowed oC rise at full load oC

Organic materials such as Cotton, Silk,

A 105 60
Paper and certain Synthetic Fibres
Inorganic materials such as Mica, Glass
B 130 80
Fibres, Asbestos and Synthetic Fibres.
Similar to Class-B with high
F 155 105
temperature binders
Silicon Elastomers, Mica, Glass Fibre
H 180 125
Dr. K.V. Vidya nanadnandAsbestos with high temp. binders
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 THERMAL PROTECTION
Thermal protection for the generator stator core and windings is
provided for the following contingencies:
❖ Generator overload
❖ Failure of Cooling Systems and Sensors (RTD/Thermocouple)
❖ Localized hot spots due to core lamination insulation failures
❖ Localized or rapidly developing winding failures
These abnormalities are long term and not readily detected.
❖ In attended units, generator is rarely tripped on Hi temperature.
Alarm is provided so that operator can take corrective actions.

❖ In unattended stations, theDr.gKe. Vn

. Ve
i dyraanatnodarn may be tripped.
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 GENERATOR OVER-LOAD

❖ Over-loading of Generator is rare as the amount of power it deliver

is a function of turbine capacity, which is less than the generator.

E.g.: For 660 MW unit, PTurb = 660 MW, PGen = 777 MVA.

❖ Turbine output is regulated by the governor and its limiters.

❖ Even if a generator is over-loaded for a short period, its stator

current still can be regulated by adjusting the MVAR supply.

❖ However, sustained over-load or faults can cause over-current.

❖ Over-current causes overheating, which may damage the winding
insulation and also results Din.rKm.V.Veidcyhanaanndaincalstress over the windings.
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 GENERATOR OVER-LOAD PROTECTION

❖ Protection against stator over-load/over-current is usually provided

in the form of RTD/thermocouple embedded in the stator winding.

❖ As the rotor body temperature cannot be measured directly,

overheating of the rotor core is indirectly calculated by measuring
the rotor winding resistance.

❖ Thermal protection for rotor is provided in the form of

1. IDMT/ Definite Time relays
2. Excitation limiters

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GENERATOR OVER-LOAD PROTECTION

Turbine-generator short time thermal capability for balanced three-phase loading

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GENERATOR OVER-LOAD PROTECTION

Generator field short time thermal capability

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 PHASE-TO-PHASE FAULT PROTECTION

❖ Phase faults in stator windings can cause irrecoverable damage to

1. Insulation
2. Winding Conductor
3. Stator Core

❖ These faults may lead to torsional shock to shafts and couplings.

❖ Trapped flux within the machine can cause fault current to flow
for many seconds after the generator is tripped and the field
excitation is disconnected.

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 PHASE-TO-PHASE FAULT PROTECTION
❖ Primary protection for phase fault is by Differential Relay (87G).

❖ Differential relays can detect phase-to-phase faults, three-phase

faults, and double-phase-to-ground faults.

❖ With low-impedance grounding of the generator, some single

phase- to-ground faults can also be detected.

❖ Turn-to-turn faults in the same phase cannot be detected, since the

current entering and leaving the winding will be the same.

❖ Backup protection for phase faults in Gen., GT, GT-765kV bus, UATs,
is provided by a Diff. RelayD(r.8K.7V.VGidTya)naondranaphase distance relay (21).
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 DIFFERENTIAL PROTECTION (87 G)

❖ Differential protection a very sensitive unit level protection

provides high-speed tripping in case of stator winding short circuits.

❖ Needs two identical CTs per phase.

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DISADVANTAGE OF DIFFERENTIAL RELAY

❖ CTs do not transform their primary currents so accurately under

transient conditions after a short circuit in the system.

❖ This is due to slight differences in magnetic properties or

manufacturing inaccuracies.

❖ The difference in CT currents may be greater for large short-

circuit currents.

❖ Difference in CT currents may result in to false unit tripping, even

for external faults.
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 PERCENTAGE DIFFERENTIAL RELAY

Differential current in the operating coil = I1S − I2S

I1S + I2S
Equivalent current in the restraining coil =
2

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 PERCENTAGE DIFFERENTIAL RELAY

❖ Combines two coils, one for operation and other for restraining
❖ Operating coil is connected to the midpoint of the restraining coil
❖ Differential current in the operating coil is proportional to (I1 - I2)
❖ Current in the restraining coil is proportional to (I1 + I2)/2

If N be the total number of turns on the restraining coil, the total

ampere-turns of the restraining coil = I1(N/2) + I2(N/2), which is the
same as if (I1 + I2)/2 were to flow through the whole coil.

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INTER-TURN PROTECTION

Split Phase
Protection

Voltage Based

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 GENERATOR EARTH-FAULT

❖ Most of the generator winding faults begin as ground faults.

❖ This is because the insulation between coils of different phases in
a slot is twice as thick as the insulation between a coil and the
ground (stator core is groDur.nKd.V.eVdidy)a.nandan
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GENERATOR EARTH-FAULT PROTECTION

❖ Generator grounding through impedance can limit the earth-fault

current.
❖ Differential relays will not detect phase-to ground faults.
❖ The higher the grounding impedance, the less the fault current
magnitude and the more difficult it is to detect.
❖ A separate relay in the grounded neutral will provide sensitive
protection, since it can be set without regard to load current.

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 NEED FOR GENERATOR GROUNDING

❖ To limit over voltages on the generator under phase-to-ground

faults (i.e. to avoid neutral floating).

❖ To permit the application of suitable ground fault relaying.

❖ To detect less severe phase-to-ground faults before they

become phase-to-phase faults.

❖ To limit transient over-voltages.

❖ Provide ground source for other system protection.

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TYPES OF GENERATOR
GROUNDING

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TYPES OF GENERATOR
GROUNDING

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TYPES OF GENERATOR
GROUNDING

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 UNGROUNDED SYSTEM

❖ Under balanced load conditions, the Vn of an ungrounded

system will be close to ground potential.

❖ With a φ-G fault, the ground fault current will be small, but the
voltages on the un-faulted phases can reach up to L-L voltage.

❖ Over a period of time this breaks down the line-to-neutral

insulation and results in insulation failure.

❖ Ungrounded system operation is not recommended because of

the high probability of failures due to transient over-voltages.

Advantage of ungrounded system:

It can continue to operate under single L-G fault without much
damage to the equipment or power interruption to the loads.
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GENERATOR GROUNDING TRANSFORMER
Sipat unit-1, 660MW

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 TYPES OF EARTH-FAULT PROTECTION

❖ 95% Stator Ground

❖ 100% Stator Ground: 2 basic methods:
1. Third-harmonic voltage schemes
➢ 27 TH : Third Harmonic Neutral Under-voltage
➢ 59 T : Third Harmonic Over-voltage
➢ 59 TH : Third Harmonic Voltage Differential

2. Neutral injection scheme

➢ 64 S : Sub-harmonic Voltage Injection

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 95% EARTH-FAULT PROTECTION

❖ For a generator earth fault, the If flows in the primary of NGT.

❖ A voltage across the resistor is developed which activates stator
earth fault relay 64 G2.
❖ 5% of the generator winding starting from neutral remains
unprotected because a fault in this portion will generate very low
voltage for the relay operaDtr.iKo.Vn. V.idyanandan
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100% EARTH-FAULT PROTECTION

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 100% EARTH-FAULT PROTECTION

Protection using 3rd harmonic voltage measurement is used for

stator ground faults on high-Z grounded generators.

Three possible earth fault protection schemes are available:

1. Use of a 3rd harmonic undervoltage at the neutral.

It will pick up for a fault at the neutral.

2. Use of a 3rd harmonic overvoltage at the terminals.

It will pick up for a fault near the terminal.

3. The most sensitive schemes are based on 3rd harmonic

differential relays that monitor the ratio of 3rd harmonic at
the neutral and the terminals.
Dr. K.V. Vidyanandan
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 NEUTRAL INJECTION SCHEME

❖ An AC voltage signal is applied at the neutral using an injection

transformer in series with NGT.

❖ This signal is at a sub-harmonic of normal frequency, for 60 Hz

system 15 Hz is used.

❖ Resulting 15 Hz current is determined by the Z of the injection

transformer, NGT and shunt capacitance of the stator circuit.

❖ The 15 Hz current is monitored using an overcurrent element.

❖ A stator ground fault will bypass winding capacitance and

increase the current initiating a trip.
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 NEUTRAL INJECTION SCHEME

Typical settings for 500 MW unit

• Trip : 1 KOhm / 1 sec
• Alarm : 10 KohDkvvidyas@gmail.com
mr.K/.V1.0Vidsyeacnandan
 ROTOR EARTH-FAULT PROTECTION

❖ A generator field circuit (field winding, exciter and FB) is a DC

circuit that need not be grounded.

❖ If a first earth fault occurs, no current will circulate and the

generator operation will not be affected.

❖ If a second ground fault at a different location occurs, a current

will flow that is high enough to cause damage to the rotor and
the exciter.

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 VOLTAGE DIVIDER METHOD
❖ Two lamps are connected across the field circuit with the mid
point is grounded; the lights will glow with equal brightness.

❖ If a ground occurs near one terminal, light connected to that

terminal will go off and the other will show increased brightness.

❖ As the fault is moved from the terminal towards winding centre,

difference in brightness between the two lamps will diminish.

❖ For a ground at the middle of the winding, there will be no

change in the lamp brightness from the ungrounded conditions,
thus, a ground at the winDdkvvidyas@gmail.com
r.iKn.Vg.Vmidyaindanpdaonintis not detectable.
ROTOR EARTH-FAULT PROTECTION

Using Low frequency injection method

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LOW FORWARD & REVERSE POWER RELAY

➢ To allow entrapped steam in the turbine to be utilized to avoid

damage of the turbine blade.

➢ To protect the machine from motoring action

➢ Trip under class B after a short time delay in case the turbine is
already tripped ( typ set at 2 sec)

➢ Trip under class A, after a long time delay if turbine is not

tripped (typically set at 10 -30 sec)

➢ Power setting typ 0.5 % of rated power

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 LOSS OF EXCITATION (40 G)

❖ Generator field failure results in acceleration of the rotor to super-

synchronous speed where it behaves as an Ind. Gen.

❖ During field failure, the air-gap flux is provided by a large

magnetising current drawn from the system, which may exceed the
generator rating and thus overload the stator.

❖ Super-synchronous speed results in slip frequency voltage in the

damper windings, resulting in heavy current flow and rotor heating.

Dr. K.V. Vidyanandan

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CAUSES OF LOSS OF EXCITATION

❖ AVR failure
❖ Loss of field to the main exciter
❖ Accidental tripping of the field breaker
❖ Short circuits in the field circuits
❖ Poor brush contact in the exciter
❖ Field circuit-breaker latch failure
❖ Loss of ac supply to the excitation system
❖ Slip ring flashover

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 DYNAMICS DURING FIELD FAILURE

❖ When excitation is lost, rotor current (If), internal voltage (E) and
terminal voltage (Vt) falls.

❖ Due to reduced voltage, stator current increases for the same Pe.

❖ Generator draws VAR from the power system to replace

excitation initially provided by the exciter.

❖ As V/I ratio becomes smaller, the generator positive sequence

impedance (Z+) as measured at its terminals will reduce & enter
the 4th quadrant of the R-X plane.

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 LOSS OF EXCITATION & ROTOR SPEED

❖ Reduced excitation weakens the magnetic coupling between the

rotor and stator.

❖ If the coupling becomes too weak, the turbine output cannot be

fully converted into electrical form, ( Pa = Pm – Pe ).

❖ This leads to acceleration of rotor, resulting in to increased δ.

❖ Increased rotor angle force the generator to lose synchronism.

% Load Rise in speed

Low load (< 30%) 0.1 to 0.2%
Full loDar.Kd.V.Vidyanan dan 2 to 5%
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PROTECTION FOR LOSS OF EXCITATION
3 types of protective devices are used for loss-of-field protection:
❖ Distance relays
❖ Reactive (or var) relay
❖ DC undercurrent relays

There are two types of distance relaying schemes:

❖ Mho element with –ve offset
❖ Mho element with +ve offset

For smaller units: single relay with characteristic diameter of Xd

and offset of Xd’/2, with time delay of 0.5 to 0.6 s
For high capacity units: one additional element with Z-diameter
of 1 pu is used to detect loss Dr.ofK.V.field between full load to 30% load.
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LOSS OF EXCITATION

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 ACCIDENTAL ENERGIZATION

❖ Protects against closing of the generator CB while machine is not

spinning or on barring gear

❖ Usually caused by
➢ Operator error
➢ Breaker flash-over
➢ Control circuit malfunction

❖ Two routes for back charging:

➢ Through GT
➢ Through UATs
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 ACCIDENTAL ENERGIZATION
❖ When a generator on turning gear is energized from a 3Ø source, it
will accelerate like an induction motor, draws both P and Q.

❖ The starting current as an Induction Motor may vary from 1 pu to

4 pu and will remain high during the acceleration period.

❖ The acceleration period last from tens of seconds to minutes.

❖ The high current induced in rotor will cause severe rotor damage.

❖ If the Gen. is accidentally backfed from the 6.6 kV bus through

UAT, the current will be ~ 0.1 - 0.2 pu. This may damage UAT.
Dr. K.V. Vidyanandan
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 UNBALANCED LOAD
During unbalance, either the voltages are not identical in magnitude
or the phase angle between them are not 120o, or both.

% unbalance is expressed as the max deviation from the average of

the 3-phase voltages divided by the average of the 3-phase voltages.

Causes: Unbalanced distribution of single-phase loads (e.g. railways),

untransposed overhead lines and unstable system neutral
NEGATIVE SEQUENCE PROTECTION (46)

❖ Protects generator from excessive heating in the rotor due to

unbalanced stator currents.

❖ Negative sequence component of stator current induces double

frequency current in rotor, causing heating.

❖ Rotor temperature rise proportion to I 22t .

❖ Relay 46 provide settings for this relationship in the form of a

constant, k = I22t.

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NEGATIVE SEQUENCE CAPABILITY

2
Type of Generator Permissible l2 .t
Salient pole generator 40
Cylindrical rotor generator
Indirectly cooled 30
Directly cooled (0-800 MVA) 10
Directly cooled (801-1600 MVA) As per curve
* As per ANSI C50.13-1989

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 POWER SWING
❖ System disturbances, such as tripping of a unit or line, will disturb
the balance between Pm and Pe.

❖ As a result, some units will tend to speed up and some will tend
to slow down.

❖ During this, the rotor angle swings around the final steady state
value and produces power swings resulting in heavy current flow.

❖ This is called Power Swing, resulting into Pole slipping.

❖ If this tendency is too great for any unit, it will loose

synchronism. Dr. K.V. Vidyanandan
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 POWER SWING BLOCKING

❖ In normal condition, ‘Z’ seen by the relay at Generator bus is the

Zload, which is far away from the Distance Relay operating zone.

❖ During line faults, the change in ‘Z’ is very fast (i.e. dZ/dt is fast)
and ‘Z’ moves towards the tripping zone of the mho relay.

❖ During power swing, ‘Z’ moves towards the tripping zone at a

slower rate (i.e. dZ/dt is slow).

❖ By measuring the time ‘Z’ takes to cross two selected points, dZ/dt
can be calculated and thus tripping or blocking can be decided.
❖ Normally blocking: dZ/dt >D3
.r K5.V-
. V4id0yanmandsa,nelse tripping.
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Dr. K. V. Vidyanandan
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