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 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:
- Slowing boiler firing
- Fuel supply is minimized
- Running the generator at baseload
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Types of Sub
Generators can have certain abnormal conditions and faults that may reduce their useful life and can cause some serious damages. These are discussed below along with their respective protection and preventive measures.
Abnormal conditions in generator:
These conditions are related to the equipment connected with a generator like prime mover, exciter and grid.
It is further classified into two parts:
Loss of prime mover
Definition: 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).
Problem: 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.
Solution: 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.
If you haven’t checked out our previous blog on the Synchronous Condenser, then please click here.
Definition: Whenever a generator is disconnected from the grid, it tends to run at no load with speed much higher than rated speed known as ‘runaway speed’.
Problem: Over speeding results in an increase of frictional losses.
Solution: Governor which can vary the speed by changing frequency and fuel supply is used to overcome this scenario.
Definition: When the load on the three phases of the generator has high variation. For example, phase A consists of 20 KW load, phase B consist of 10 KW, and phase C consist of 5 KW, this refers to unbalance loading
Problem: An unbalance loading results in 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 which eddy current and hysteresis losses are increased resulting in a high amount of heating in the generator. (Rotor overheating)
Solution: 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
Definition: Loss of excitation means there would be no availability of an exciter circuit. Due to which, 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.
Problem: There are two scenarios which may occur:
1. The grid can compensate Q, in this case there would be large heating in rotor and stator.
2. If the grid fails to provide Q, then there would be a chance of blackout.
Solution: MHO relay 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.
Faults in generator:
The Stator faults can be categorized as:
- Phase & earth faults
- Longitudinal differential protection scheme
- Differential protection scheme using a balance resistor
- Modified differential protection
- Balanced earth fault protection
- Inter turn faults
To learn the detail of each faults with example and diagrams, click on “Stator Faults”.
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 which 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.
In 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 rotor fault, the positive potential of the external DC source appears across the terminals of relay and the protective relay operates.
You may check our previous blog focusing on fault analysis in power systems
It is very important to protect generators from all kinds of faults since they are one the most important and initial part 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.
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