Generator protection is different and complex as compared to other equipment and machines due to the reason that it is connected with 3 other systems simultaneously, a DC exciter circuitry for providing DC to field winding, a prime mover for a rotating rotor, and is synchronized with the grid. Also, generating systems consist of auxiliaries like heat water pumps and exhaust fans, etc. which are supplied power through the generator itself that is why it is never preferred to completely turn off the generator as it would be a time taking task to start the generator again. Also, it is not preferred to have a backup generator for auxiliaries as this would change the short circuit rating.
When it is required to cut the generator with the grid due to maintenance or fault, then the following steps should be taken:
Generators can have certain abnormal conditions and faults that may reduce their useful life and can cause some serious damage. These are discussed below along with their respective protection and preventive measures.
These conditions are related to the equipment connected with a generator like the prime mover, exciter, and grid.
It is further classified into two parts:
If the prime mover stops rotating and is not providing active power to the generator then the generator will act as a synchronous condenser (synchronous motor).
If you haven’t visited our previous blog on the synchronous condenser, then please click here.
In this condition, the generator tries to compensate its losses causing a lot of heat.
The Machine starts drawing small active power from the grid but continues to supply reactive power.
|A reverse power relay can be used, It consists of 2 coils, a voltage coil (PT) and a current coil (CT), both are energized by the same phase. They both induce magnetic fields in an Aluminum disk which under normal operation is restricted by stoppers but as the power flows in opposite direction, the Aluminum disk also rotates in the reverse direction and a trip mechanism gets activated.|
|Whenever a generator is disconnected from the grid, it tends to run at no load with a speed much higher than the rated speed known as ‘runaway speed’.||
Overspeeding results in an increase of frictional losses.
Governor which can vary the speed by changing frequency and fuel supply is used to overcome this scenario.
When the load on the three phases of the generator has high variation. For example, phase A consists of 20 KW load, phase B consists of 10 KW, and phase C consists of 5 KW, this refers to unbalance loading.
An unbalance loading results in a negative sequence current. The negative sequence current rotates in the opposite direction as compared to the rotor, this results in the induction of current in the core having a double frequency. Due to this, eddy current and hysteresis losses are increased resulting in a high amount of heating in the generator. (Rotor overheating)
A negative sequence relay can be used, It consists of 4 impedances and a relay operating coil between them, 3 CTs (1 from each phase) feeds current to these impedances, all impedances are chosen such that in case of negative sequence current, input from 2 CTs cancel out each other and remaining CT input energizes the relay coil and initiates tripping mechanism.
|Loss of excitation means there would be no availability of an exciter circuit. Due to this, the synchronous generator starts acting as an induction generator with a slightly higher speed that is it will draw reactive power Q from the grid and provides active power P.||
There are two scenarios that may occur:
1. The grid can compensate Q. in this case there would be large heating in the rotor and stator.
2. If the grid fails to provide Q, then there would be a chance of a blackout.
MHO relay is used to monitor field current to prevent loss of excitation.
an alarm can also be used here, and an effort can be made to restore the excitation
A generator consists of 3 phase windings connected in Y and a neutral wire. The following faults may have the probability to occur:
For the protection of phase and ground fault, a differential protection scheme is employed. This protection scheme differs from the transformer differential protection scheme:
This type of protection scheme is known as a longitudinal differential protection scheme.
Figure 1: connection diagram of the longitudinal differential protection scheme
The connection diagram of longitudinal protection is such that, for each phase red (R), yellow (Y), and blue (B) there are two current transformers (CT). One of the CT connected at the neutral point (at the generator) and the other at phase (switchgear). There are 4 pilot cables available, which are used for the connection of CTs with the relay coil (R1, R2, and R3).
The differential protection scheme works on the current matching principle that is when there is no fault, the current flowing through CT1 and CT2 will be the same, in this case, no current flows through the relay coil and there is no tripping. In case of phase or ground fault, the relay coil gets energized and the difference of current is non-zero. Thus, the relay issues a trip signal.
There is a certain limitation in the above-mentioned protection scheme that is for the efficient working of this scheme, the relay coil should be placed in mid of the pilot cable (at an equipotential point) otherwise there could be a mal-operation of the relay.
But in some cases, the generator and switch gears are placed at a certain distance and as the relays are placed in switchgear, the relay coil is placed at an uneven distance. The relay might give a trip signal in normal conditions as well (due to unequal impedance).
A balancing resistor is connected with phase point CT for reliable working at the time of fault and no-fault. It is known as differential protection using a balancing resistor.
Fig 2: connection diagram of differential protection using balancing resistor
As the star point of the stator, the winding is grounded using some earthing resistance to reduce the effect of an earth fault. The fault which occurs close to the neutral point is not detected by the relay. So, it is usually in practice to protect 85% of the stator winding,
Due to the presence of earthing resistance, the sensitivity of detecting an earth fault by the above schemes decreases. For this purpose, a modified differential protection scheme is used.
Fig 3: connection diagram of modified differential protection
There are 2 phase relay coils (PA and PC) while an earth relay coil (ER) and a balance resistor (BR). In case of a phase fault, only the PC or PA will get energized, there will be no current at the earth relay. While the case of an earth fault, the earth relay gets energized and issues a trip.
For small generators, the above-mentioned schemes cannot be used for earth fault protection purposes, as the neutral point of these generators is connected internally, there is no provision for 3 CTs to connect at the neutral point. Instead of it, balance earth fault protection is used.
Fig 4: connection diagram of balanced earth fault protection
Inter turn faults in generators cannot be detected using the Buchholz relay as there is no oil tank present in the generator as the transformer does.
The transverse differential protection scheme is used for detecting an inter-turn fault in generators.
Fig 5: connection of transverse differential protection
In this scheme, each winding is split into two sub-windings, shown as S1 and S2. Its working mechanism is such that if there is no fault then equal current is divided in each sub winding like if the red phase has 7000 A current then its sub winding S1 and S2 would have 3500 A current.
In case of an inter-turn fault in any of the sub-winding current drops, for example, an inter-turn fault occurs in red phase S1 sub winding and its current now changes from 3500 A to 3000 A, due to the difference in current in both sub-winding (3500-3000), differential current flows through relay coil and relay issues a trip signal.
Rotor field winding fault includes dc field excitation short circuit fault, due to which secondary flux is generated which opposes primary flux distorting the main flux, this asymmetrical magnetic flux can potentially cause mechanical damage to bearings due to vibrations or permanent damage to machines that have very small rotor-stator clearance. These faults are protected using AC or DC injection method. However, the DC injection method is preferred more as the AC injection method has a leakage current problem.
Fig 6: DC Injection Method of Rotor Earth Fault Protection in Alternator
In the DC injection method, a DC voltage relay is connected with the positive terminal of a DC exciter, the negative terminal of the relay is connected with an external DC power source which is usually fed by an auxiliary transformer through bridge rectifier circuitry, the positive terminal of the bridge rectifier is grounded. In the case of a rotor fault, the positive potential of the external DC source appears across the terminals of the relay and the protective relay operates.
Check out our Power System Protection Fundamentals Course in which we briefly discussed "Types of protective relays & design requirements". We started with the introduction to the design and working mechanism of a Relay, based on a protection system. Then moved forward to the discussion on the factors that need to consider when designing a relay-based protection scheme. Then we introduced Overcurrent Relays, Directional Relays, Distance or Impedance Relays & Reverse Power Flow Relays in detail.
It is very important to protect generators from all kinds of faults since they are one the most important and initial parts of a power system hence any fault in a generator can lead to severe power abnormalities or even blackout, moreover, backup protection must always be present so that the equipment and rest of system are protected.
This concludes our topic of Generator protection; we hope our blog made it easier for you to understand this topic. Feel free to suggest or ask us any questions you might have in the comments section below.
About The Author
Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.