Differential protection schemes play a critical role in safeguarding electrical power networks by detecting and isolating faults. These schemes are designed to provide fast and reliable protection to generators, transformers, motors, and other important equipment. In this text, we will explore the concept of differential protection, different schemes used, and their applications within high-voltage transmission and distribution systems.
Differential protection is based on the principle of comparing the current entering and leaving a specific zone, such as a generator or a transformer. If the currents are equal, no fault exists within the zone. However, if there is a fault, such as a short circuit or an internal fault in the protected equipment, the currents will be unbalanced, indicating the presence of a fault.
To implement differential protection, specialized devices called differential relays are employed. These relays measure the current flowing into and out of the protected zone and compare them using a mathematical subtraction operation. If the difference between the two currents exceeds a predetermined threshold, the differential relay initiates a trip signal to disconnect the faulty zone from the power network, preventing further damage.
Various differential protection schemes are commonly used in power networks, including percentage differential, harmonic restraint, and biased differential schemes.
- Percentage Differential Scheme:
In this scheme, the differential relay measures the percentage difference in current between the input and output of the protected zone. The percentage differential characteristic is defined as:
If the percentage differential exceeds a predetermined threshold, typically set between 5% and 20%, the relay initiates a trip signal.
Harmonic Restraint Scheme:
To improve the performance of differential protection, harmonics restraint is used. Harmonic currents, which may be present due to non-linear loads, can cause false differential signals. The harmonic restraint scheme ensures that differential protection operates only for fundamental frequency currents, while harmonic currents are ignored.Biased Differential Scheme:
The biased differential scheme is employed when the protected zone has significant inrush or overexcitation currents, which can cause false differential signals. To mitigate this issue, a bias current is added to the restraint coil of the differential relay. The bias current is proportional to the expected inrush or overexcitation current and helps prevent unwanted tripping during normal operating conditions.
These differential protection schemes find numerous applications in power networks. They are commonly used in generator protection, transformer protection, motor protection, and busbar protection.
Now, let’s consider an application example to illustrate the concept of differential protection. Suppose we have a 100 MVA transformer with a 230 kV/13.8 kV voltage ratio. We want to provide differential protection for this transformer to ensure its safe and reliable operation.
For transformer differential protection, a typical setting criterion is to restrict the maximum allowable differential current to 10% of the rated current. Therefore, the differential current setting will be:
Suppose the rated current of the transformer is 1000 A. Thus, the differential current setting would be 100 A.
To ensure the differential protection operates correctly, various settings and coordination parameters need to be configured for the differential relay. These include current transformer (CT) ratios, operating time delays, and harmonic restraint factors.
In practice, a protection engineer would perform a detailed relay coordination study considering system impedance, fault analysis, and relay characteristics to optimize these settings.
In conclusion, differential protection schemes play a vital role in safeguarding electrical power systems by detecting and isolating faults within protected equipment. These schemes utilize differential relays and mathematical comparison of currents to identify fault conditions. Different schemes, such as percentage differential, harmonic restraint, and biased differential, are used to enhance the performance and reliability of differential protection. By implementing appropriate settings and coordination parameters, differential protection provides fast and reliable protection for generators, transformers, motors, and other critical equipment in high-voltage transmission and distribution systems.