We can divide alternator faults in two type -Internal fault, 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(newly added).
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 is discussed in a separate post.
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.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.
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.
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.
04. Over voltage protection of an alternator:
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.
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.
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 vectorically 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 foloow the above conception. See the secondaries 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 sen trip signal.
06. Stator inter turn protection of alternator
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.
The protection scheme to cover stator inter turn fault is simply basissing 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 secondaries shorted and primaries with individual coils. If some fault persists, then there will be current in the secondaries which will energize the trip circuit.
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 sysnchronism”.
To understand this fault we can have some brief theory. As in all electrical rotating machine, the common working principle is that there is some disalignment generated between two flux- rotor flux & stator flux. Thus a force is exerted to straighten this dis alignment-which is harnessed as working force.
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.
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.
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.
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(excess or less then generation). 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.
We can summarize over & under frequency in a mathematical concept to understand and memorize as below-
Production > Load demand = frequency high
Production < Load demand = frequency low
Production = Load demand = frequency stable.
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.