Electric Generator | Source: GE Grid Solutions

A generator fault is a fault condition that occurs within an electrical generator. Generators are used to convert mechanical energy into electrical energy and can be found in a wide variety of applications, from large power plants to small portable units. Like all electrical equipment, generators can experience faults that can affect their performance and reliability. 

Related Article: Fundamentals of Generator Protection

Here are some common types of generator faults:

  • Stator Faults - The stator is the stationary component of the generator that contains the winding that produces the electrical output. Stator faults can occur due to insulation breakdown, overheating, or physical damage. Common stator faults include short circuits, open circuits, and winding faults. 
  • Rotor Faults - The rotor is the rotating component of the generator that contains the magnetic field that interacts with the stator windings. Rotor faults can occur due to mechanical damage, overheating, or electrical problems. Common rotor faults include open circuits, short circuits, and ground faults. 
  • Bearing Faults - Bearings are used to support the rotating components of the generator and can experience faults due to wear, corrosion, or lack of lubrication. Bearing faults can lead to excessive vibration, noise, and can cause damage to other components in the generator. 
  • Excitation Faults - The excitation system is responsible for providing the magnetic field that interacts with the stator winding. Excitation faults can occur due to issues with the voltage regulator, the field winding, or the excitation transformer. Excitation faults can result in reduced output voltage or complete loss of output. 
  • Overload Faults - Generators can experience faults due to being overloaded or subjected to excessive electrical or mechanical stress. Overload faults can lead to overheating, insulation breakdown, and mechanical damage. 
  • Grounding Faults: Grounding faults can occur when a fault current flows from a phase conductor to ground. Grounding faults can cause damage to the generator and other equipment and can create a safety hazard for personnel.

To prevent generator faults, it is important to perform regular maintenance and inspections, including visual inspections, electrical tests, and vibration analysis. In addition, it is important to use protective devices such as circuit breakers, fuses, and protective relays to detect and isolate faults before they can cause damage to the generator or other equipment.

Advance Generator Fault Protection Using GE Technology

GE Multilin 889 Generator Protection System

100% Stator ground protection using the 3rd harmonic voltage differential (64TN). 

100% stator ground protection is an essential part of the protection scheme for electrical generators. A stator ground fault occurs when one or more of the phase windings of the stator comes into contact with the ground or a grounded component, which can lead to severe damage and potential failure of the generator. To detect stator ground faults, the third harmonic voltage differential method is commonly used.

The third harmonic voltage differential method is based on the principle that a ground fault in a three-phase system produces a third harmonic component in the voltage waveform. This component is not present in a balanced three-phase system,but is present in a system with a ground fault. The third harmonic voltage differential protection scheme uses this principle to detect and isolate stator ground faults.

In this protection scheme, the voltage of the third harmonic component of the voltage waveform is measured at two points in the system and the difference between the two measurements is calculated. This difference is then compared to a set threshold value. If the difference exceeds the threshold value, the system is tripped to isolate the faulted section of the electrical system.

The advantage of using the third harmonic voltage differential method is that it is sensitive to low-level stator ground faults and provides a fast response time. The method is also less susceptible to false tripping due to external interference or transients. The use of the third harmonic voltage differential method for 100% stator ground protection has been widely adopted in the industry due to its reliability and effectiveness.

100% stator ground protection is easy to apply using the 3rd harmonic voltage differential (64TN) in conjunction with 59X (auxiliary overvoltage). The 64TN protection function utilizes the fact that for a fault near the neutral, the 3rd harmonic residual voltage at the terminals of a machine increases, while the third harmonic voltage at the neutral decreases. The ratio of the third harmonic residual voltage to the neutral third harmonic may be nearly constant for all load conditions on a healthy machine without faults. This protection function requires phase VTs on the terminal side be connected as WYE. 

Generator Winding Protection | Source: GE Grid Solutions

The figure above illustrates various protection functions applied for protecting 100% generator stator winding. It also shows 3rd harmonic voltage distribution over the winding in healthy condition (no fault on the stator winding).

Related Article: Protection Relays in Power System

Neutral directional protection (67N) for parallel machine ground protection coordination.

For the application of low impedance grounded parallel generators, the 889 provides neutral directional (67N) protection function for a secure & selective protection scheme. As illustrated in figure below, for parallel synchronous generators, a ground fault in any one machine causes neutral overcurrent (51N) protection to operate in both generators. With enhanced 67N enabled in both the generators, the faulty generator relay would be identified, and the healthy machine would continue to run as 67N would not pickup although 51N indicates a fault condition. 

Therefore, 67N in conjunction with 51N in 889 provides secure and selective protection for parallel operation of synchronous generators connected to the same bus.

Neutral Directional Protection | Source: GE Grid solutions

The advantages of using the neutral directional protection scheme include:
  • Accurate detection and isolation of ground faults - The directional overcurrent relay provides accurate detection and isolation of ground faults in the system.
  • Fast response time - The protection scheme can respond quickly to ground faults and prevent damage to the generators and other equipment in the system.
  • Coordination with other protective devices - The neutral directional protection scheme can be coordinated with other protective devices to ensure reliable and effective protection of the parallel system.

Out-of-step Protection

The out-of-step element provides an out-of-step (loss-of-synchronism or pole slip) tripping function for generators. The element is simplified using a single blinder operating characteristic with an offset mho supervisory. The purpose of the supervisory mho is to prevent operation on stable swings that pass through both blinders and outside the mho characteristic. In addition, the out-of-step tripping feature allows a "MHO EXIT" trip mode to reduce stresses on the breaker. The figure below illustrates the single blinder characteristic with a supervisory mho. 

Out of Step Protection | Source: GE Grid Solutions

The advantages of using SMHO in generator protection include:
  • High-speed backup protection - SMHO provides high-speed backup protection for the stator winding, which is the most critical component of the generator.
  • Improved reliability - SMHO can improve the reliability of generator protection by providing a backup protection scheme that can detect faults that are not detected by other protective devices.
  • Coordinated protection - SMHO can be coordinated with other protective devices, such as overcurrent and differential relays, to provide reliable and effective protection for the generator.
  • Versatility - SMHO can be used in a wide range of generator applications, including large power plants and small portable units.

Generator protection is critical for ensuring the safe and reliable operation of electrical power systems. Generators are key components in the generation of electrical energy, and any faults or failures can result in significant damage to the generator and other electrical equipment. The protection schemes discussed, including stator ground fault protection, out of step protection, and supervisory mho protection, provide backup protection for the generator and can detect and isolate faults that other protective devices may miss. 

Proper maintenance, inspections, and the use of protective devices can prevent faults and failures, ensuring the efficient and safe operation of electrical power systems. Overall, generator protection is a crucial aspect of maintaining reliable and safe electrical power systems.


No comments: