Generator Protection
A
generator is subjected to electrical stresses imposed on the insulation
of the machine, mechanical forces acting on the various parts of the machine,
and temperature rise. These are the main factors which make protection
necessary for the generator or alternator.
Even when properly used, a machine in its perfect running condition does not
only maintain its specified rated performance for many years, but it does also
repeatedly withstand certain excess of overload.
Preventive measures must be taken against overloads and abnormal
conditions of the machine so that it can serve safely. Even ensuring an
efficient design, construction, operation, and preventive means of protection –
the risk of a fault cannot be completely eliminated from any machine. The
devices used in generator protection, ensure that when a fault arises,
it is eliminated as quickly as possible.
An electrical generator can be subjected to either an internal fault or
external fault or both. The generators are normally connected to an electrical power system, hence any fault occurred in the power system should also be cleared
from the generator as soon as possible otherwise it may create permanent damage
in the generator.
The number and variety of faults occur in a generator are huge. That is
why generator or alternator is protected with several protective schemes.
Generator protection is of both discriminative and non-discriminative type.
Great care is to be taken in coordinating the systems used and the settings
adopted to ensure that a sensitive, selective and discriminative generator
protection scheme is achieved.
Classification Generator tripping Scheme Class A, Class B & Class C
The generator Tripping classified according to
the fault.
Class A trip:
Class A trip involves a serious electrical fault like differential,
stator earth fault etc. and is considered to be the most dangerous in terms of
the shock on the unit. Since it involves serious electrical faults, connections
from both generator and the EHV bus is immediately switched off to limit the
damage at the fault point and also to isolate the healthy system. Hence the
unit (turbine, generator and boiler) has to be tripped.
Class B trip:
Class B primarily relates to mechanical problems. This results in
tripping of turbine followed by generator.
Class C trip:
Class C involves basically external system related problems like
frequency, over voltage etc. This does not involve instant tripping of the unit.
CPP unit operates on house load.
On this discussion, I try to explain the protection system through below
mentioned configuration of TG.
Generator Details
Active Power : 16000 kW
Power Factor :
0.8
Apparent Power : 20000 kVA
Rated Voltage : 11 kV
Full load
Current : 1050 A
Rotor Voltage : 53-194 V
Rotor Current : 196-514 A
Speed : 3000 RPM
Frequency : 50 Hz
- 1.
Differential Protection
ANSI Code: 87
Purpose
To detect difference of current flow between
line and neutral side of generator
Construction
of Relay
·
Instantaneous
attracted armature type
·
Percentage
biased
Causes of
Fault
Any unbalance in circuit (Localized faults)
Set Point
0.1 Amps (Left (R), Centre (Y), Right (B))
TG FLA @ 1050 A
CT Ratio: 1200/1 A
Differential @ 120 A
11.43 % of TG FLA
Remarks
- 2.
1st and 2nd Rotor Earth Fault
ANSI Code: 64
R
Purpose
To detect the earth fault occurred on the
rotor field winding circuit
Construction
of Relay
3 Methods
1. Potentiometer method
2. AC Injection method
3. DC Injection method
Causes of
Fault
A single earth fault in the field winding or
its associated circuits, therefore, gives rise to a negligible fault current
and does not represent any immediate danger. If however a second ground fault
should occur, heavy fault current and severe mechanical unbalance may quickly
arise and lead to serious damage. It is essential therefore that any occurrence
of insulation failure is discover and that the machine is taken out of service
as soon as possible. Normally the machine is tripped instantly on occurrence of
second rotor earth fault.
Set Point
1st Stage
64 R1
On Low fault current
Delay 2 Sec
2nd Stage
64 R2
On High Fault Current
Delay 2 Sec
Remarks
- 3.
Reverse Power
ANSI Code: 32
R
Purpose
To protect the alternator/generator from
motoring action when the turbine failed to give mechanical power.
Construction of Relay
1.
Lightweight non-magnetic aluminium disc between two soft laminated iron
core electromagnets, and fixed on a spindle running on low friction bearings.
2. The two magnetic fields which are out of
phase, produces eddy current in the aluminium disc, and this creates a torque
that tries to rotate the disc.
3. Under normal condition when power is
flowing as expected, the trip contacts of the relay are open, and the disc is
against a stop. If a reverse power starts to flow, the disc rotates in the
opposite direction, moves away from the stop and towards the trip contacts that
activates the trip circuit.
Causes of
Fault
1. Turbine output is lesser than the generator
no load losses.
2. CT connection reversal
Set Point
5 %
0.8 MW
Delay: 0.6 Sec
- 4.
Low Forward Power
ANSI Code: 32
L
Purpose
Low forward Power protection occurs when the
generator is not able to produce the minimum output power. Typically, the
generator output is lesser than the no load losses, then the Low forward power
protection starts working.
Construction of Relay
·
Construction
similar to reverse power protection relay.
·
Usually
alarm activated.
·
Generator
breaker trip optional.
Causes of
Fault
1. Turbine’s input power is less or the
turbine is not able to produce the sufficient mechanical power
2. Generator work as synchronous condenser.
The generator only produces the reactive power in condenser mode
3. Insufficient excitation.
4. Generator breaker tripped due to some other
fault
Set Point
5 %
0.8 MW
Delay: 1
Sec
- 5.
Loss of Field/ Excitation
ANSI Code: 40
Purpose
Loss of field or excitation can be caused in
the generator due to excitation failure. Failure of excitation that is failure
of field system in the generator makes the generator run at a speed above the
synchronous speed. In that situation the generator or alternator becomes an
induction generator which draws magnetizing current from the system. Over
loading of the stator and overheating of the rotor due to continuous operation
of the machine in this mode may create problems in the system in long-run.
Construction
of Relay
The scheme comprises an offset mho relay, and
an instantaneous under voltage relay
a. Under Voltage Relay
·
Instantaneous
·
The
drop in system voltage is detected by an under voltage relay which is set to approximately 70 % of
normal rated system voltage.
b. Mho Relay (Admittance Relay)
·
While
on normal operation R and X will be positive (1st Quadrant
operation)
·
On
loss of excitation X becomes negative (4th Quadrant operation) as
shown in figure
·
When
the measure impedance falls into region, the relay function picked up and after
certain delay generator CB will trip.
·
Terminal
Voltage V ↓, Terminal current I ↑,Impedance Z ↓
Causes of
Fault
1. Field circuit open
2. Field circuit short
3. AVR Failure
4. Field breaker trip
5. Excitation supply fail
6. Brush contact issue
7. Slip ring issues
Set Point
c. Under Voltage Relay
·
110
V/ 77 V
·
7.7
kV (70 % Rated Voltage)
d. Mho Relay (Admittance Relay)
·
Z1:
7.5 Ohm
·
Z2:
149.4 Ohm
·
Delay:
2 Sec (40G1) + 4 Sec (40G2)
Remarks
- 6.
Negative Phase Sequence
ANSI Code: 50
Q
Purpose
To protect the generator from unbalance
loading and negative sequence component. The negative sequence current flows in
the generator winding due to phase to phase short circuit. The negative
sequence currents cause the overheating of generator’s winding.
Construction
of Relay
·
Under
balanced condition, the generator delivers equal currents in all three phase
either star connected or delta connected loads. Here the vector sum of negative
sequence current is zero, hence net unbalance current is zero.
·
The
unbalance may occur due to circuit breaker operation, circuit breaker failures
or system faults. Hence the negative sequence current starts flowing in the
stator winding; this creates additional flux in the air gap which rotates in opposite
direction to that of rotor synchronous speed.
·
If
the magnitude of this negative sequence current is high, then this opposite
direction potentially affects the normal operating direction of the turbine.
Causes of
Fault
Phase to Phase fault
Set Point
(I2/In)2t =
K1K3
In = 1 A
I2= Negative Sequence Current
K1= 10
K3=1
(I2/1)21 = 1*10
I22= 10
I2= 3.16
3.16*1200 = 3792 A (1200/1 A)
I2s Alarm= 80 %
I2s = 7.5 %
Remarks
- 7.
Over Current
ANSI Code: 51
Purpose
Which operates when the load current exceeds a pickup value.
Construction
of Relay
In
an over current relay, there would be essentially a current
coil. When normal current flows through this coil, the magnetic effect
generated by the coil is not sufficient to move the moving element of the
relay, as in this condition the restraining force is greater than deflecting
force. But when the current through the coil increases, the magnetic effect
increases, and after a certain level of current, the deflecting force generated
by the magnetic effect of the coil, crosses the restraining force. As a result,
the moving element starts moving to change the contact position in the relay.
Depending upon time of operation, there are
various types of Over Current relays, such as,
- Definite time over current
relay.
Used to co-ordinate over other definite time,
or instantaneous protection. Generally less sensitive (higher pickup) to
prevent operating for load inrush. Generally faster operating time.
- Inverse time over current
relay.
Slow to trip at low currents. Faster to trip
at high fault currents. Used to co-ordinate over load protection, which may
have a high starting current. Generally the most sensitive (lowest amps
pickup), and slowest to operate.
Causes of
Fault
Overload
Set Point
a. Definite time Over Current
Delay = 10 Sec
Setting = 0.9 = 0.9
* 1200 = 1080 A
b. Inverse time Over Current
Setting=1=1*1200=
1200 A
Remarks
- 8.
Stand-by Earth Fault
ANSI Code: 64
S
Purpose
It is used to protect the system if the neutral current or
unbalance current reaches the predetermined value, then the standby earth fault
relay operates.
Construction
of Relay
It is a backup protection for restricted earth fault relay.
This relay operates when REF delayed operation, Heavy earth fault outside of
the REF protective Zone and all other earth faults. Simply we can say, it is a
standby protection of all other earth faults.
In field side, one current transformer will be installed at a
neutral side of the transformer or alternator. The current reference will be
taken from that CT. In panel side, an over current element will be directly
connected with the CT reference.
Consider the Current in the three phases are I1, I2 ,
and I3 the natural current is IN. During
Normal condition, there is zero current flow in the neutral.
Normal Condition: I1+I2+I3=IN=0
On faulty condition: IN=0
Causes of
Fault
L-G, L-L-G, L-L-L-G faults.
Set Point
a. Definite time Over Current (64 SA)
Alarm
Delay = 4 Sec
Setting = 0.11 =
0.11 * 1200 = 132 A
b. Inverse definite minimum time Over
Current (64 S)
Trip
Setting=0.2 =0.2*1200= 240 A
Remarks
- 9.
Under Frequency (1st& 2nd
stage)
ANSI Code: 81
U
Purpose
Under Frequency Protection is used to protect the generator when
the frequency drops below the operating frequency. It is a backup protection
for over fluxing (V/F) protection.
Construction
of Relay
The generator under frequency protection consists of two
stage tripping. Stage 1 trip command is given to grid circuit breaker and stage
2 trip commands are given to generator circuit breaker.
Causes of
Fault
Under frequency occurs due to turbine low speed, AVR failure,
diode failure, grid frequency fluctuation etc.
Set Point
a. Stage 1
Under Frequency =
48.5 Hz
PU/DO
(Differential) = 0.5
Delay= 2 Sec
b. Stage 2
Under Frequency =
47.5 Hz
PU/DO
(Differential) = 0.5
Delay= 2 Sec
- 10. Under voltage
ANSI Code: 27
Purpose
Under voltage fault protection is used to protect the
generator winding from low voltage operation. Under voltage protection sense
the phase to phase voltage of the generator/transformer using instrument
transformer (Potential transformer). When the voltage drops below the rated
voltage typically 80% under voltage protection will be activated
Construction
of Relay
The output from the generator’s potential transformer will be
given to the under voltage coil typically 110 Volts relay coil. In principle of
U/V coil, which do not trip the circuit breaker when the voltage across the PT
is high. When the voltage drops the pre-set value, the voltage coil operates
the circuit breaker.
Instantaneous type under voltage relay
Causes of
Fault
Under voltage fault occurs due to failed excitation, diode failure,
under frequency o turbine low speed, failed PT fuse etc
Set Point
110/88 V
80% Rated voltage
8.8 kV
Delay= 3 Sec
Make &
Type
English Electric VAGM
- 11. Over Voltage
ANSI Code: 59
Purpose
Over voltage protection is used to protect the Generator form
high voltage the power system must be isolated when the system voltage high.
Seviour overvoltage causes the winding or electrical insulation failure, over
fluxing (u/f), the over voltage protection can be considered as a backup to the
Volts-per-Hertz protection (Over fluxing).
Construction
of Relay
The overvoltage protection consists of two stage operation.
Stage 1 trip command will be given to the 110kV grid circuit breaker and stage
2 trip commands will be given to generator’s circuit breaker.
Instantaneous type over voltage relay
Causes of
Fault
Overvoltage occurs due to sudden load through off, elevated
grid synchronized voltage, AVR malfunctioning, power transformer taps changer
failure, lightning strike on the transmission line, turbine over speed etc.
Set Point
a. Stage 1
110/126.5
115%
Rated voltage
12.65
kV
Delay=
0.3 Sec
b. Stage 2
110/137.5
125%
Rated voltage
13.75
kV
Delay=
2 Sec
13. DC Fail Supervision
14. Turbine Oil Pressure Low
15. Generator Frame Temperature High
16. PT Fuse Failure
17. Emergency Trip
