What it is ?What it is ?
Why is required ?Why is required ?
How it will be provided ?How it will be provided ?
When it will be lost ?When it will be lost ?
What will happen if it lost ?What will happen if it lost ?
What are the Protection Schemes available?What are the Protection Schemes available?
LOSS OF EXCITATION - CONTENTS
• Generator is a rotating machine which converts
Mechanical Energy from Prime mover into Electrical
Basic Principle:Basic Principle:
• Basically Generator works on Faraday’s laws of Electro
Ist Law: Whenever a conductor placed in a rotating magnetic
field an EMF will be induced in that conductor.
IInd Law: The magnitude of the Induced EMF is Directly
Proportional to the rate of change of flux linkages.
E = K.L dØ/dt
L = Length of the magnetic flux lines K = Constant
dØ/dt = Rate of change of flux linkages
Creating the required Magnetic Field
strength in the Rotor winding of the
Generator by giving D.C supply which
when cut by conductors produces Voltage.
EXCITATION – WHAT IT IS ?
The system which is
used to Supply,
Control & Monitoring
of the D.C supply is
called the ExcitationExcitation
1) Basic requirement for the generation of
Magnetic FieldMagnetic Field in the air gap between the
Rotor and the Stator.
2) Results in the creation of the ““RotatingRotating
Magnetic fieldMagnetic field”” in the air gap.
3) To Regulate the Terminal Voltage.Terminal Voltage.
4) To control Reactive Power flowReactive Power flow and facilitate
the sharing of reactive load between the
machines operated parallel in the grid.
5) Enabling the Maximum utilization of
EXCITATION – WHY IS REQUIRED ?
WHAT IS REACTIVE POWER ?
• In AC power networks, while Active Power
corresponds to useful work, Reactive Power supports
voltage magnitudes that are controlled for system
Reliability, Voltage stability, and operational
• Reactive power is essential to “move Active Power”
through the Transmission and Distribution system to
• Reactive power is required to “maintain the Voltage”
to deliver Active Power (Watts) through transmission
(Apparent Power (S))2 = (Active Power (P))2 + (Reactive Power (Q))2
Reactive PowerReactive Power
• Reactive power does
not travel very far.
• Usually necessary to
produce it close to
the location where it
Analogy of understanding Reactive PowerAnalogy of understanding Reactive Power
Power Factor = Active power / Apparent power = Kw / kVA
= Active Power / (Active Power + Reactive Power)
= Kw / (Kw + kVAr)
The higher kVAr indicates Low Power Factor and vice versa.
True Power (MW)
EXCITATION – HOW IT WILL BE PROVIDED ?
Excitation systemExcitation system
(up to 110MW)
Brush less Excitation
1) Field Open Circuit (Field Current is zero).
2) Field Short Circuit (Field Current is too high).
3) AVR Control Failure.
4) Accidental Tripping of Field Breaker.
5) Loss of supply to the Main Exciter.
6) Poor brush contact in the Exciter.
7) Field Circuit Breaker latch failure.
8) Slip Ring flash over.
EXCITATION – WHEN IT WILL BE LOST ?
• The Generator delivers both Real and Reactive
Power to the grid. The Real power comes from the
Turbine while the Reactive power is due to FieldField
• When Field Excitation is lostField Excitation is lost while the Mechanical
Power remains intact, it would attempt to remain
synchronized by running as an InductionInduction
• As an Induction Generator, the machine speeds up
slightly above the synchronous speed and drawsdraws
ExcitationExcitation from the grid. ContdContd……..
EXCITATION–WHAT WILL HAPPEN IF IT LOST ?
Speed = Ns
Voltage = V ratedField Current = I f
Before Loss of ExcitationBefore Loss of Excitation
Speed > Ns
Voltage = V LOEField Current I f = Zero
Loss of Excitation conditionLoss of Excitation condition
• Operation as an Induction GeneratorInduction Generator necessitates the
flow of ““Slip frequencySlip frequency”” current in the rotor, the
current flowing in the Damper Winding and also in
the slot wedges and surface of the solid rotor body.
Now there are Two Possibilities:Now there are Two Possibilities:--
• Either the grid is able to meet the reactive power
demand Fully oror meet it Partially.
• If the grid is able to fully satisfy this demand for
reactive power, the machine continuous to deliver
active power of ‘PPeeMWMW’’ but draws reactive power of
‘QQLOELOE’’ MVA and there is no risk of instability.
• However, the Generator is not designed as an
Induction Machine, so “Abnormal Heatingbnormal Heating”” of the
Rotor and overloading of Stator winding will take
• If the Grid able to meet the Reactive Power
demand partially then this would be reflected by
fall of a Generator Terminal VoltageTerminal Voltage. The
Generator would be under excited.
• There are certain limits on the degree to which a
Generator can be operated within the Under
Reduced Excitation weakensweakens the magnetic coupling
between the Rotor and Stator.
If the coupling becomes too weak, the Turbine
output cannotcannot be fully converted into Electrical
form (Pa = Pm-Pe).
This leads to acceleration of Rotor, resulting
into increased ‘‘δδ’’..
Increased Rotor AngleRotor Angle force the Generator to
• Therefore, the operation in case of loss of
excitation must be quickly detected and
• If a generator is operating at full loadfull load
when it loses excitation, it will reach a
speed of 2% to 5%2% to 5% above normal.
• This over speed condition will be harmful
to Steam Turbine driven GeneratorsSteam Turbine driven Generators.
• If a Generator is operating at reduced
loading ((<< 30%),30%), the machine speed may
only be 0.1% to 0.2%0.1% to 0.2% above normal.
• When Excitation is lost, rotor current (If),
Internal voltage (E) and terminal voltage (Vt)
• Due to reduced voltage, Stator current
increases for the same ‘Pe’.
• As V/I ratio become smaller, the Generator
Positive Sequence ImpedancePositive Sequence Impedance (Z+) as measured
at its terminals will reduce and enter the 4th
Quadrant of the R-X plane.
- MW MW
Machine acts as an
Machine acts as an
Machine acts as an
Machine acts as an
+ R- R
POWER FLOW DIRECTION AND POWER FACTORPOWER FLOW DIRECTION AND POWER FACTOR
• The simplest method by which loss of excitation
can be detected is to monitor Field currentField current of
• If the filed current falls below a threshold, a
loss of field signal can be raised.
• A complicating factor in this protection is the
Slip Frequency CurrentSlip Frequency Current induced in the event of
loss of excitation and running as an Induction
LOSS OF EXCITATION – PROTECTION SCHEMES
• The quantity which changes most when a Generator
loses Field ExcitationField Excitation is the ImpedanceImpedance measured at
the Stator terminals.
• On loss of excitation, the terminal voltage begins to
decrease and the current begins to increase, resulting
in “Decrease of ImpedanceDecrease of Impedance””..
• The Loss of Excitation can be unambiguously detected
by a Mho relayMho relay located at the Generator terminals.
• In 1949, a Single Phase “Offset Mho Relay” was
introduced for the high speed detection of ““Loss ofLoss of
ExcitationExcitation”” in Synchronous Generators.
There are Five LOE protection schemes used today, namely,
1) R-X Scheme with Single and Double Relay Scheme
(Based on Generator terminal Impedance
2) R-X with Directional element Scheme (-do-).
3) G-B Scheme (Based on Generator terminal Admittance
4) P-Q Scheme (Based on Generator Active and Reactive
5) U-I Scheme (Based on the measurement of Phase Angle
difference between Phase Voltage and Current).
However, R-X Schemes is widely used in Power Systems.
• The diameter of the circle set equal to the
““Synchronous ReactanceSynchronous Reactance”” (Xd) and Offset
will be set equal to one - half of the
““Transient ReactanceTransient Reactance”” (X’d/2).
• This circle is operation zone for LOE relay.
• As viewed from the machine terminals the
Relay will operate for any impedance
phasor that terminates inside the circular
IMPEDANCE MEASUREMENT (SINGLE ELEMENT)
• In normal operation condition, the Generator
generates Active and Reactive Power to the system
which means both ‘R’ and ‘X’ are positive and the
Terminal Impedance is located in the First Quadrant
in R-X plane.
• When the Excitation is lost, the Generator starts to
draws Reactive power from the system and ‘X’
becomes Negative from the LOE relay point of view.
• As a result, the Terminal Impedance loci in R-X plane
moves to the ‘Forth Quadrant’ and the endpoint of
terminal Impedance ranges between the sub Transient
Reactance and Synchronous Direst Axis reactance.
When the measured
Impedance falls into
the operating region,
the relay function will
be picked up and
after a certain Time
Delay to enhance the
security for power
A trip signal will be
sent to the GeneratorGenerator
Typical impedance loci on loss of Excitation
Locus of Apparent
Time = 0
Initially it’s a
After motoring action
Machine starts to work
As Induction Generator
• To limit system voltage, the Generators may have to
operate Under Excited and absorb VARS from the
• It is important that the Generator be able to do so
within its capabilities as defined by the Generator
• The Generator Under Excitation Limiter (UEL) must
be set to maintain operation within the capability
• The Loss Of Field Relay must be set to allow the
Generator to operate within its Under Excited
Impedance measurement (Double Element)
• This protection scheme applies Two offset MhoTwo offset Mho
ImpedanceImpedance circles by using the Generator Terminal
side Voltages and Stator Currents as input signals.
• The Offset-mho relay in the impedance plane has two
circles with a diameter of Direst Axis Transient
Reactance X’d and a Negative offset of X’d /2 for the
• And the diameter of ‘1.0’ (pu) and a Negative offset
of X’d /2 for the Inner Circle.
• Zones 1 and 2 are for detecting LOE with full load and
light load. The typical time delays for Zone - 1 & Zone
- 2 are about 0.1 s & 0.5–0.6 s.
ZoneZone -- 22
- R + R
Heavy load Light load
Steady stateSteady state
Stability limitStability limit
Machine operatingMachine operating
Limit in Leading PFLimit in Leading PF
ZoneZone -- 2 setting crosses2 setting crosses
Steady state stability limitSteady state stability limit
R-X with Directional Element Scheme
• It’s a combination of Two Impedance elements,
a Directional unit and an Under voltage unit
applied at the Generator Terminals.
• The Zone - 2 element is set to coordinate with
the Steady State Stability Limit. The top of the
Zone - 2 circle (positive offset) is set at the
System Impedance in front of the Generator.
• It will detect reduced or Loss of Excitation
condition, raise an alarm and if the
abnormality persists, Trips the Generator.
- R + R
During loss of field
Two zone Loss of field scheme with Directional unitTwo zone Loss of field scheme with Directional unit
Min Exciter Limiter
Machine capabilityMachine capability
Stage – 1 Relay details
+ R- R
Z1 = 2.17 Ω
Z2 = 12.25 Ω
CT Sec = 5A, PT Sec = 110V
Make – ALSTOM, Type = YCGF
Model – YCGF11AF1A
Stage 2&3 Relay details
+ R- R
Z1 = 3.0 Ω
Z2 = 14.88 Ω
CT Sec = 5A, PT Sec = 110V
Make : English Electric, Type = YCGF
Model – YCGF11AF1A5
Typical Relay setting calculations
Information required:Information required:--
PT Ratio : 22000 : 110 = 200 : 1
CT Ratio : 20000 : 5 = 4000 : 1
Transient Reactance (X’d) : 0.30 Ω (0.16 to 0.45 Ω)
Synchronous Reactance (Xd) : 2.50 Ω (2.0 to 3.90 Ω)
Generator Rating : 588 MVA
Generator Voltage : 21.0 KV
T = CT Ratio / PT Ratio : 4000 / 200 = 20
Base Ω (Pri) = KV2/MVA : 21 X 21 / 588 = 0.75 Ω
Base Ω (Sec) = T X Base ohms (Pri) : 20 x 0.75 = 15 Ω
X’d (Sec) = X’d x Base Ω (sec) : 0.30 x 15 = 4.50 Ω.
Desired offset = X’d/2 : 4.50 / 2 = 2.250 Ω.
Xd (Sec) = Xd (pu) x Base Ω (sec) : 2.50 x 15 = 37.50 Ω
Diameter of circle = 37.50 Ω
Offset setting = 2.250 Ω
400 KV Bus -I
400 KV Bus -II
GT TRANS Y/∆
100% STATOR E/F (64G1)
& INTER TURN PROTN (95G)
400KV TEE PROT1/2
400 KV Bus -I
400 KV Bus -II
VT2:- AVR / EHG / SYNC
VT3:- PERFORMANCE TEST / AVR /EHG /
LOW FRWD /REV POWER RELAYS
Typical Generator Protection scheme (500MW)
ANSI/IEEE Standard Device Numbers
1 - Master Element
2 - Time Delay Starting or Closing Relay
3 - Checking or Interlocking Relay
4 - Master Contactor
5 - Stopping Device
6 - Starting Circuit Breaker
7 – Rate of Change Relay
8 - Control Power Disconnecting Device
9 - Reversing Device
10 - Unit Sequence Switch
11 – Multifunction Device
12 – Over speed Device
13 - Synchronous-speed Device
14 – Under speed Device
15 - Speed or Frequency-Matching
16 – Data Communications Device
20 - Elect. operated valve (SV)
21 - Distance Relay
23 - Temperature Control Device
24 – Volts per Hertz Relay
25 – Synchronizing Check Device
26 - Apparatus Thermal Device
27 – Under voltage Relay
30 - Annunciator Relay
32 - Directional Power Relay
36 - Polarizing Voltage Devices
37 - Undercurrent Relay
38 - Bearing Protective Device
39 - Mechanical Conduction
40 –Field failure Relay
41 - Field Circuit Breaker
42 - Running Circuit Breaker
43 - Selector Device
46 –Phase- Bal. Current Relay
47 - Phase-Bal. Voltage Relay
48 - Incomplete-Sequence Relay
49 - Transformer Thermal Relay
50 - Instantaneous Over current
51 - AC Time Over current Relay
52 - AC Circuit Breaker
53 – Field Excitation Relay
55 - Power Factor Relay
56 - Field Application Relay
59 – Over voltage Relay
60 - Voltage or Cur. Balance Relay
62 – Time-Delay Stopping / Opening
63 - Pressure Switch
64 - Ground Detector Relay
65 - Governor
66 – Notching or jogging device
67 - AC Directional OC Relay
68 - Blocking or “out of step” Relay
69 - Permissive Control Device
74 - Alarm Relay
75 - Position Changing Mechanism
76 - DC Over current Relay
78 - Phase-Angle Measuring Relay
79 - AC-Reclosing Relay
81 - Frequency Relay
83 - Automatic Selective Control or
84 - Operating Mechanism
85 – Pilot Communications, Carrier
or Pilot Wire Relay
86 - Lockout Relay
87 - Differential Protective Relay
89 - Line Switch
90 - Regulating Device
91 - Voltage Directional Relay
92 - Voltage and Power Directional
94 - Tripping or Trip-Free Relay
• An Induction Generator or Asynchronous Generator is a
type of AC Electrical Generator that uses the principles
of Induction motors to produce power.
• Induction Generators and motors produce electrical power
when their rotor is turned faster than the SynchronousSynchronous
• In Generator operation, a Prime mover (Turbine) drives
the rotor above the synchronous speed. The stator flux still
induces currents in the rotor, but since the opposing rotor
flux is now cutting the stator coils, an active current is
produced in stator coils and the motor now operates as a
Generator, sending power back to the Electrical Grid.
• The overall reactance of the Armature
winding is the sum of its Leakage
Reactance plus Fictitious Reactance,
which is known as Synchronous ReactanceSynchronous Reactance
• The Impedance of armature winding is
obtained by combining its Resistance and
its Synchronous Reactance. It is called
Synchronous ImpedanceSynchronous Impedance ‘‘ZsZs’’..
Synchronous Reactance and Impedance
• Synchronous Reactance determines steady-
state current. However, when a sudden change
from steady state occurs, such as short circuit,
other reactance's come into play. This happens
because the flux in the machine cannot change
• Sub-Transient Reactance determines maximum
instantaneous current. It lasts up to about 6
• Transient Reactance is a longer lasting
reactance determining current up to as much as
• Zero Sequence Reactance determines neutral
currents in grounding studies. It is also a factor
in determining neutral currents when third
harmonics are encountered.
• Negative Phase Sequence Reactance is used in
calculating line-to-line faults.
• Transient Reactance (X’d):- It is One of the Five
reactance figures frequently used by engineers
when comparing Generator capability with
load requirement, or when comparing one
Generator with another.
a)Sub transient Reactance = X’’d
b)Transient Reactance = X’d
c) Synchronous Reactance = Xd
Total Short Circuit Current
21 G GENERATOR BACK UP IMPEDANCE PROTECTION
40 G A / B FIELD FAILURE PROTECTION
46 G NEGATIVE SEQUENCE PROTECTION
DR DIGITAL FAULT & DISTRUBENCE RECORDER
98 G POLE SLIPPING PROTECTION
37 GA / GB - 32GA / GB LOW FORWARD / REVERSE POWER RELAYS
87 G GENERATOR DIFFERENTIAL PROTECTION
87 GT OVER ALL DIFFERENTIAL PROTECTION
87 T GENERATOR TRANSFORMER DIFFERENTIAL PROTECTION
87 UT UNIT AUXILLIARY TRANSFORMER DIFFERENTIAL PROTECTION
64 RUT UNIT AUXILLIARY TRANSFORMER RISTRICTED EF PROTECTION
51 NUT UNIT AUXILLIARY TRANSFORMER EARTH FAULT PROTECTION
51 UT UNIT AUXILLIARY TRANSFORMER OVER CURRENT PROTECTION
50 Z BREAKER FAILURE PROTECTION
87 HV TRANSFORMER HV WINDING + OVER HANGE DIFFERENTIAL PROTECTION
51 NGT GENERATOR TRANSFORMER BACK UP EARTH FAULT PROTECTION
99 GT GENERATOR TRANSFORMER OVER FLUXING PROTECTION
64 G2 95% STATOR EARTH FAULT PROTECTION
81 G UNDER FREQUENCY PROTECTION
51 G OVER VOLTAGE PROTECTION
27 G UNDER VOLTAGE PROTECTION
RELAY NUMBERS AND THEIR UNIVERSAL NOMENCLATURERELAY NUMBERS AND THEIR UNIVERSAL NOMENCLATURE
• Synchronous machine maintains constant flux.
When DC field current gets reduced (under
excited), to strengthen main field, it absorb
reactive power (draw current from AC supply
• In reverse, when DC field current gets increased
(over excited), to weaken main field, it deliver
reactive power to the bus bar.
• All these are controlled by magnetizing and
demagnetizing effect of Armature Reaction
• Generator Active Power output equation:
Pe = ----------- Sin δ
‘Pe’ = Active Power output to the system.
‘Eq’ = Gen int. vol. behind the d-axis Synch Reactance.
‘Us’ = Equivalent System Voltage.
‘Xs’ = Direct axis Synch Reactance.
‘δ’ = Angle between Eq and Us.
Pe α Eq, Us Sin δ.
• As the Generator internal
Voltage Eq is a function
of Field Voltage, the
Generator Active Power
output is a function of
Field Voltage as well.
• When the generator
operates at δ=90◦, any
increase of Mechanical
Power or decrease of
Electrical Power will lead
to Generator Loss of
Synchronism. Generator Active PowerGenerator Active Power
Vs Load AngleVs Load Angle