Generator Protection Relay Working Principle

A generator is exposed to temperature increases, mechanical forces operating on its numerous components, and electrical pressures placed on the machine's insulation. These are the primary reasons why the generator or alternator needs protection. Even when utilized correctly, a machine in optimal operating condition can sustain a certain amount of extra overload over time in addition to maintaining its rated performance for many years. Generators may experience external or internal issues. Due to the connection between generators and electrical power systems, it is crucial to promptly resolve any system issues to prevent irreversible harm to the generator. Since there are many different kinds of problems with generators, several safety measures are employed. This category includes both discriminatory and non-discriminatory strategies. For sensitive and selective generator protection, system and configuration coordination is essential.

Table of Contents

• What is Generator Protection?
• Types of Generator Protection
• Generator Protection Functions
• Protection against Insulation Failure
• Protection Against Abnormal Running Conditions
• Generator Protection Relay Working Principle
• Conclusion

What is Generator Protection?

Protecting generators from different electrical, mechanical, and thermal stresses is known as generator protection. To safeguard machines from overloads and unusual circumstances, preventive measures are required. Faults are inevitable even with effective design, construction, and operation. When problems arise, generator protection devices swiftly fix them.

Types of Generator Protection

There are two ways to classify the different types of protection used on the generator:

• Relays provide protection by identifying problems outside the generator.
• Protective relays are used to identify internal problems in generators.

In addition to protective relays that are directly connected to the generator and its transformer, there are devices that measure temperature for shaft bearings, stator windings, transformer windings, and transformer oil, as well as lightning arrestors, overspeed safety guards, and oil flow devices. Some of these safeguards are non-trip types, meaning they only sound an alarm in the event of an anomaly. Ultimately, the other safety measures control the generator's master tripping relay. It should be mentioned that no protective relay can stop a fault; instead, it merely alerts the user and shortens the fault's length to keep the generator from overheating and perhaps causing irreversible damage. In order to lessen the impact of lightning and other voltage surges on the generator, it is customary to install a surge capacitor, surge diverter, or both. This is done to prevent any unnecessary stresses in the generator. A brief discussion of the protection mechanisms typically used for generators is provided below.

Generator Protection Functions

1. For Insulation Failure:

• Differential
• Interturn Fault
• Stator Earth Fault
• Rotor Earth Fault

2. For Abnormal Running conditions

• Loss of excitation
• Unbalanced Loading
• Pole Sleeping
• Overfrequency or Overspeed
• Overvoltage
• Reverse Power or Forward Power
• Impedance or Overcurrent Back-up Protection

3. For Generator Transformer Protections

• Differential Protection
• Bias Test
• 2^nd Harmonics Restrained
• REF Protection

Protection against Insulation Failure

The generator's longitudinal differential protection is the primary safeguard against phase-to-phase or phase-to-earth faults in the stator winding. Interturn fault protection is the second most crucial stator winding protection method. Because insulation failure between points in the same phase winding, contained in the same slot, and between which a potential difference exists, quickly transforms into an earth fault and is detected by either the stator differential protection or the stator earth fault protection, this type of protection was once thought to be unnecessary. A generator has many conductors in each slot because it is made to generate a comparatively high voltage relative to its output. This type of safety is increasingly necessary for all large producing units as the generator's size and power increase.

Stator Earth Fault Protection

A current transformer is installed in the neutral-to-earth connection when the stator neutral is earthed via a resistor. When the generator is connected straight to the busbar, an inverse time relay is employed over the CT secondary. An instantaneous relay serves the same function when power is fed from a generator via a delta-star transformer.

Rotor Earth Fault Protection

A single earth fault does not cause a significant issue for the generator; however, if a second earth fault occurs, a portion of the field winding will short circuit, creating an imbalanced magnetic field in the system and potentially causing significant mechanical damage to the generator's bearings. There are three ways to identify the different kinds of rotor faults. The techniques are

• The potentiometer method
• AC injection technique
• DC injection technique

  

  Rotor Earth Fault Protection
  Image used courtesy of Circuitglobe

Protection Against Abnormal Running Conditions

Unbalanced Stator Loading Protection

In the stator circuit, unbalanced loading results in negative sequence currents. The generator's differential protection would immediately resolve any unbalancing that might have arisen from a stator winding malfunction. Depending on how the system's protective coordination works, the unbalancing may go unnoticed or may continue for a long time if it results from an external malfunction or unbalanced workload. Installing a negative phase sequence relay with the specifications to match the machine's withstand curve will then fix these issues.

Protection against Stator Overheating

The generator's stator winding may overheat as a result of overload. The stator winding overheats due to overloading, cooling system failure, and stator lamination insulation failure.

Temperature sensors that are installed at different locations throughout the stator winding detect the overheating. One arm of the Wheatstone Bridge circuit is made up of the temperature detector coils, which are typically resistance components. Smaller generators, typically those under 30 MW, typically have thermal relays installed in order to measure the current flowing in the stator winding rather than an embedded temperature coil.

Overspeed Protection

Although mechanical overspeed devices that run directly on the steam throttle valve or main step valve are commonly found on both steam and hydro turbines, it is uncommon for steam-driven sets to have an overspeed relay to support these devices. On hydroelectric units, however, it is regarded as best practice because the set is more likely to overspeed and the governor's response is quite delayed. When installed, the relay is often powered by the permanent magnet generator that controls the governor.

Backup up Protection of Generator

Highly regarded machines, such as alternators or synchronous generators, should always have backup protection. Relays for backup protection should be used to fix any issues that have arisen and not been resolved by the proper protection plan. Typically, overcurrent relays are employed for this function. The sustained fault current fed from the machine into an external fault is always less than the typical full load current since modern machines frequently have synchronous reactances greater than 100%.

Generator Protection Relay Working Principle

• To monitor various electrical variables, the relay receives inputs from a variety of sensors and transducers. Temperature, power, frequency, voltage, and current measurements are among these inputs.
• To ascertain whether any parameter is outside the permissible range, the relay compares the measured data with typical curves and pre-established criteria.
• Certain protective features, such as reverse power protection, overcurrent protection, overvoltage protection, under voltage protection, overfrequency protection, and others, are activated by the relay when it detects a defect or abnormal state.
• The SCADA system is utilized for proper monitoring. Alarms are also set for receiving early warning.

Conclusion

To ensure safe working conditions and proper functions, protection methods must be wisely implemented. Protective relays are used to detect both internal and external faults, ensuring comprehensive generator protection. They are the silent heroes and a strong weapon as well to safeguard the generator.