Generator Protection: An Overview of the Need and Application
Introduction:
Reliable and uninterrupted power generation is crucial for the stability and functioning of electrical power systems. Generators, being the heart of power plants, require robust protection schemes to detect and isolate faults promptly. The protection of generators ensures the safety of personnel working in power plants, prevents costly equipment damage, and helps maintain power system stability. In this article, we will provide an overview of generator protection, discussing the need for protection schemes and their application in power network transmission and distribution.
The Need for Generator Protection:
Generator protection aims to safeguard generators from various types of faults, such as short circuits, overloads, and abnormal operating conditions. Faults in generators can result from internal or external causes, such as insulation failures, winding faults, or faults in the power system. These faults can lead to severe damage to generators if not effectively detected and isolated. Therefore, generator protection schemes are designed to provide fast and reliable detection, accurate fault identification, and speed up fault clearing operations.
Generator Protection Schemes:
Generator protection schemes typically consist of a combination of protective relays, sensors, and communication systems. These schemes are designed to detect faults and initiate the necessary protective actions to mitigate their impact. The protection schemes for generators may include the following key elements:
Differential Protection: Differential protection is widely used to detect internal faults in generators. It compares the current entering and leaving the generator windings, and if any imbalance is detected, it signals the presence of a fault. Differential protection offers high sensitivity and fast fault detection for generator stator and rotor faults.
Overcurrent Protection: Overcurrent protection is employed to detect faults in the generator’s external circuit, such as feeder or transformer faults. This protection relies on the measurement of current levels and compares them against preset thresholds. If the current exceeds the set limits, the protection scheme operates to isolate the faulted section.
Loss of Field Protection: Loss of field protection is essential to prevent damage to generators due to the loss of excitation. It monitors the generator’s field current, and if a significant drop or complete loss is detected, the protection scheme acts to disconnect the generator from the system automatically.
Overvoltage and Undervoltage Protection: Overvoltage and undervoltage protection schemes monitor the voltage levels at the generator terminals. If abnormal voltage fluctuations are detected, the protection system becomes active to isolate the generator and prevent further damage.
Numerical Example Application:
Let’s consider an example of a 20 MVA generator connected to a power system. The protection scheme for this generator consists of a differential relay and an overcurrent relay. The relay settings are as follows:
- Differential Relay Setting: Primary current setting = 2 A, Secondary current setting = 5 A, Fault sensitivity = 10% of rated current, Time grading = 0.1 s
- Overcurrent Relay Setting: Overcurrent pickup setting = 1.5 times rated current, Time grading = 0.2 s
During normal operation, the generator’s rated current is 1000 A. In the event of a fault, the fault current rises to 5000 A.
Suppose a fault occurs in the generator winding due to insulation failure. The differential relay, with its fault sensitivity set at 10% of the rated current (100 A), will detect the fault as the relay current exceeds this threshold. The differential relay will operate within 0.1 seconds, initiating the disconnection of the generator from the system.
If the fault occurs in the external circuit, the overcurrent relay, with its pickup setting at 1.5 times the rated current (1500 A), will detect the fault. The overcurrent relay will operate within 0.2 seconds, isolating the faulted section of the system.
Conclusion:
Generator protection is crucial for ensuring the reliable and safe operation of power generation systems. Through the effective application of protection schemes, faults in generators can be promptly detected and isolated, minimizing damage to equipment, ensuring personnel safety, and maintaining power system stability. Generator protection schemes encompass various elements such as differential protection, overcurrent protection, loss of field protection, and overvoltage/undervoltage protection. These protection schemes are designed to meet the standards set by organizations such as the IEEE (e.g., IEEE C37.102) and IEC (e.g., IEC 60255) to ensure the reliable performance of power networks.