Major faults in an alternator and their protection.javid iqbal sodagar

2. We can divide alternator faults in two type
1 -Internal fault,
2-External fault.
There is internal Fault like
1.Stator Winding Fault (most severe),
2.Excitation circuit fault or failure of rotor field,
3.Unbalanced loading. On the other hand, there is external
fault like
3.Failure of prime Mover,
4.Over current,
5.Over / under Voltage,
6.Over Speed;
7.over / under frequency.
8. Loss of synchronism or pulling out of step .
9.Over or under frequency fault of alternator.

3. 01. Stator winding fault.
It is the most severe fault and considered most important for
alternator protection.
This vital protection of alternator stator is Differential
circulating current scheme or Mertz price differential
current protection scheme of alternator
02. Alternator Prime Mover failure fault or reverse power
fault-
What is prime mover in an alternator-We know about
prime mover. Its the mechanical system that rotates the
rotor in alternator or just in simple word runs the alternator.
Typical prime movers are diesel or gas engines, steam
turbine, wind – tidal force, water flow static force in hydro
electric plant with dam etc.

4. Now that we understand about prime mover, it is clearly
understandable that any time a prime mover might fail to
keep the alternator running. The root causes are so simple
like fuel flow shut up in gas /diesel engine, inadequate
water flow in hydro electric plants etc. The effect of prime
mover failure in an alternator– we can imagine the
situation, an alternator is coupled with prime mover
running, and it is directly connected to the grid or some
parallel electrical bus supplying power. When the prime
mover fails, it is still connected to the live electric bus. Then
the alternator fails to supply power but starts receiving
power from live electric bus. This time the synchronous
alternator takes power acts as an synchronous motor and
run the engines or turbine in uncontrolled high speed. This
is called reverse power.

5. Severity of reverse power: For gas turbine and any
hydroelectric system the turbine installed is capable of
running in very high speeds, so when high speed occurs
due to reverse power, any technician can decouple the
circuit breaker. But for diesel / engine system, the reverse
power is so dangerous, as the over speed limit of engines
re nominal like 120% of rated speed. As example our
plant’s Wartsila engines rated speed was 750 RPM, and its
high speed limit was 840/880 RPM.
Remedy -A reverse power relay is recommended. But this
relay should have time delay to avoid false trip in case of
short time system disturbance, phase swinging and
fluctuation in synchronization. Thus when a reverse
power problem occurs, the first thing is to decouple the
corresponding circuit breaker to disconnect the alternator
from live line. If it is stuck, then shut down the whole bus,
if that too is not possible then shut the whole power plant
for engine based power plant only.

6. 03. Over current fault on an alternator.
Causes of over current fault on an alternator is the Partial
breakdown of internal winding insulation, Overload on the
supply system. Over current protection for alternator is
unnecessary because of high internal impedance of
alternator, Modern design concept of alternator is to set high
internal impedance of alternator. Thus if over current fault
occurs, due to high internal impedance alternator can
withstand short circuit for a while. It can manually
disconnect from the bus.
False tripping on an alternator by over current protection
relay is need to be considered as this might disconnect
alternator from bus for some fault out side of plant. This will
cause interruption of continuous operation, so over current
relay used must have time delay.

7. 04. Over voltage protection of an alternator:
If we talk about over voltage of an alternator, its
internal over voltage. Over voltage fault of an
alternator is very rare, as the excitation current that
controls the voltage has closed supervision with AVR,
automatic voltage regulator system.
However, if somehow alternator suddenly
disconnected from the load, its speed exceeds rated
speed, over voltage might occur. So an over voltage
relay is recommenced to install where alternator rpm
is not closely supervised.

8. 05. Unbalanced loading of an Alternator
This is not a direct fault, just a symptom of some severe
faults. But we can summarize those faults in one fault
category to apply a common protection scheme for it. First
of all, unbalanced loading means there are different
current in three phases of alternator. In normal condition,
the difference in phase current varies +/- 5% , but when
this difference exceeds- then it is unbalanced condition.
As it is stated earlier that this unbalanced is not itself a
fault, does not apply any ham but the cause of unbalanced
is harmful like grounding of one phase, or short circuit of
phase to phase.

9. To sense and protect the unbalanced loading a simple
conception is used that the three phase current is equal in
magnitude but 120 degree separate by phase angle. So
when we vectornically sum up three phase current- the
result is zero. Thus we can apply some simple instrument
transformer to watch out the summation of three phase to
zero, if by some means there is some value other than zero
that is not in the range of tolerable, then this circuit can
send a trip signal.
The below circuit follow the above conception. See the
secondary's of three CT’s are shorted, so the sum of
normal phase currents is zero, no current in the trip coil.
By some instance if there is some unbalanced, then there
will be some current in the secondary, that will eventually
send trip signal.

10. Unbalanced loading fault protection of an
alternator with a protection relay.

11. 06. Stator inter turn protection of
alternator

12. This specific protection is for alternators with multiple
coils turns like in double phase winding used in large
steam turbine alternators. The concept is that this
protection scheme covers the stator coil inter turn to turn
short circuit, not coil to coil(phase to phase) or coil to
ground(phase to ground). Its like a local fault in one of
the coil.

13. The protection scheme to cover stator inter turn fault is
simply on the basis of the theory of differential theory. In
normal condition, the two coil or multi coils individually
carry same magnitude of current but with a phase
displacement. If this current is summed up in a common
circuit then the result will be zero. So we can just place
some CT’s with their secondary's shorted and primaries
with individual coils. If some fault persists, then there will
be current in the secondary's which will energize the trip
circuit

14. 07. Loss of Synchronism or pulling out of step fault of
alternator
This type of fault we used to see in power plant.
Suddenly one of alternator shut down and in
protection relay fault log display shows “Loss of
synchronism”.
Fp=resultant air-gap flux per pole
Fw = mmf of the dc field winding
Drf = electrical phase angle between
magnetic axes of Fp & Fw
As the prime-mover torque is increased, the magnitude of
Drf must increase until the electromechanical torque
balances the shaft torque. The readjustment process is
actually a dynamic one, requiting a change in the
mechanical speed of the rotor.

15. As can be seen from Fig, an increase in prime-mover torque will
result in a corresponding increase in the torque angle. When
Drf becomes 90 °, the electromechanical torque reaches its
maximum value, known as the pull-out torque. Any further
increase in prime-mover torque cannot be balanced by a
corresponding increase in synchronous electromechanical
torque, with the result that synchronism will no longer be
maintained and the rotor will speed up. This phenomenon is
known as loss of synchronism or pulling out of step.

16. As remedy– Under these conditions, the generator is
usually disconnected from the external electrical system by
the automatic operation of circuit breakers, and the prime
mover is quickly shut down to prevent dangerous over
speed. The value of the pull-out torque can be increased by
increasing either the field current or the resultant air-gap
flux. However, this cannot be done without limit; the field
current is limited by the ability to cool the field winding,
and the air-gap flux is limited by saturation of the machine
iron.

17. 08. Over frequency & Under frequency
fault and protection for alternator
The frequency has a constant stable value, defined by the
equation (Ns=120f/p). But frequency varies causing over
and under frequency fault. Over or under frequency has its
own severe effect on both the load side and generating
equipment side, We are talking here about the generating
set – alternator. The first thing we like to know why the over
or under frequency occurs. The frequency variation created
by the load demand. Say when a generating station
producing power that equals the net consumption plus the
loss, then the system will be stable. But when the load
demand is higher then production, that is production is low
then use/demand then frequency will be low, on the other
hand when the load demand is less then production or
production is excess then frequency will be higher.

18. The remedy or protection of over and under protection is
simple. When under frequency occurs that is more power
consumer then the covering capacity, then the only
solution is to effect load shedding or increase the power
generation. For over frequency the only solution is to
lower the power generation. A relay can be used to
automatically control the loading, unloading of generator
or effect the load shedding.