The problem of mal operation of differential protection of power transformer due to the inrush magnetizing current has long considered as a challenging problem. Several types of protection relays have been used to solve the issue (basic relay, percentage relay, multi slop ….). Each of them has its advantage and its limits.
In this paper, a Digital differential relay has been developed and simulated. The logic used to distinguish between the inrush current and the internal fault is based on the theory of harmonic analysis. The behavior of the presented relay has been simulated versus various situations (inrush current, internal fault and external fault). The obtained results shows that the proposed algorithm provides a good discrimination and a fast action.
Index Terms— Algorithm, Differential, DSP, Digital, power transformer, harmonic analysis.
ONE of the most important and expansive element in power systems is the power transformer, which is used in different sizes, types, and connections. A power transformer functions as a node to connect two (02) different voltage levels. Therefore, the continuity of its operation is of vital importance in maintaining the reliability of power supply [1, 2]. However, transformers are expected to many internal and external stresses and the consequence of any failure can be very heavy in term of damage as well as in terms of operating losses . So a performing and secure protection is needed.
Differential protection is widely used to protect large power transformer against internal faults. Its principle of operation is based on the fact that if the difference between the entering and the leaving currents in the protected zone exceed a predetermined value; the relay transmits a trip signal to the circuit breaker and isolates the faulted zone.
Based on this principle, the relay must operate when, and only when, an internal fault occurs. However, certain phenomena (such as inrush current, CT saturation levels,
nonlinearities…) in power transformer can create a differential current between its two terminals, even without internal fault, causing thus mal tripping.
When a transformer is energized, magnetizing inrush current that ﬂows in one set of terminals may equal many times the transformer rating . This is the most challenging problem in power transformer differential protection.
Over the years, many different methods of preventing differential relay operation on inrush have been utilized :
- Deactivate the relay: since the magnetizing current has a damped nature, the simple way to avoid the mal tripping at the energizing moment was the deactivation of the relay until the death (disappearance) of the inrush current.
- Decrease the acting speed: Use slow-speed induction-type relays with long time and high current settings.
- Power differential method: This method is based on the idea that the average power drawn by a power transformer is almost zero on inrush, while...