Large power transformers belong to a class of very expensive and vital components in electric power systems. If a power transformer experiences a fault, it is necessary to take the transformer out of service as soon as possible so that the damage is minimized. The costs associated with repairing a damaged transformer can be very high. The unplanned outage of a power transformer can also cost electric utilities millions of dollars. Consequently, it is of great importance to minimize the frequency and duration of unwanted outages. Accordingly, high demands are imposed on power transformer protective relays. The requirements include dependability (no missing operations), security (no false trippings), and speed of operation (short fault clearing time).
The operating conditions of power transformers do not, however make, the relaying task easy. Protection of large power transformers is perhaps the most challenging problem in the power system relaying area.
Advanced digital signal processing techniques and recently introduced Artificial Intelligence (AI) approaches to power system protection provide the means to enhance the traditional protection principles and facilitate faster, more secure and dependable protection for power transformers. Also, it is anticipated that in the near future more measurements will be available to transformer relays owing to both substation integration and novel sensors installed on power transformers. All this will change the practice for power transformer protection. This paper briefly reviews the state of the art, but is primarily devoted to discussion of new approaches and future directions in digital relaying for power transformers.
Figure 1 presents a general hardware configuration of a digital power transformer relay. The differential relaying principle is used for protection of medium and large power transformers. This superior approach compares currents at all the terminals of the protected transformer by computing and monitoring a differential (unbalance) current. The non-zero value of the differential signal indicates an internal fault. However, transformer operating conditions may introduce problems.
The operating criteria for transformer differential protection used to overcome the reported difficulties can be classified as:
• Principles broadly used in today’s products,
• Advanced numerical principles already invented but not broadly implemented,
• AI approaches already suggested but not sufficiently investigated.
The relaying methods applied in today’s products basically use current signals and limit the analysis to the fundamental frequency components and higher harmonics of those signals. The advanced numerical principles use more information including voltage signals as well as signal features other than just harmonics. The AI methods tend to utilize all the available information. As shown in the figure, the numerical complexity of an algorithm is the price to pay for processing more information.
Within this frame, the second or higher harmonics are used to prevent false tripping during magnetizing inrush conditions; the fifth harmonic is commonly used to restrain the differential relay during stationary overexcitation conditions; while the biased percentage characteristic is used to prevent false tripping during external faults. However, this traditional approach may not be able to deal with certain problems as revealed
From: Electrical Relays, Protection & Control Handbook, Vol 5, The Electricity Forum