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Transmission transformers are important links in the bulk power system. They allow transfer of power from generation centers, up to the high-voltage grid, and to bulk electric substations for distribution through the subtransmission and distribution system to load centers. Transmission transformers are very expensive assets to purchase, transport, and install and they require long lead time and a great deal of planning for their purchase and installation. Transformers are often bottlenecks in the flow of power through the grid. However, they have high thermal mass and can be overloaded for short periods of time to alleviate stressed system conditions and contingencies. But, overloading carries risk of accelerated aging and premature failure. 2
Modern transformer relays include a comprehensive set of protective elements to protect transformers from faults and abnormal operating conditions with advanced features that provide greater security, dependability, sensitivity, and performance plus, monitoring and recording functions that can provide valuable information that enables getting to root cause of relay operations. 3
Using thermal models to monitor transformer overload recognizes that the hot-spot temperature of a transformer is a function of the starting temperature, thermal mass, and balance between the thermal energy entering the transformer and the thermal energy being dissipated by the transformer. For any given situation, the temperature versus time will vary. Simple overcurrent elements do not provide accurate overload protection and will quite often trip a transformer well before it is in danger of damage. Thermal monitoring can also alert maintenance personnel when transformer cooling equipment needs repair. An alarm element that asserts when there is a significant difference between calculated top-oil temperature and measured top-oil temperature can indicate that fans motors have failed or radiators are clogged with bird nests. One of the major assumptions of loading guides is that the forced cooling ratings of the transformer are available. 4
Loading guides for large power transformers relate operating temperature to an exponential aging function. System planning engineers make assumptions when creating contingency loading guides that attempt to balance the risk of excessive accelerated aging with the need to survive system contingencies over the life of the transformer. There are many factors that affect the actual temperatures and risk of damage during an event. Thermal monitoring of important bulk power transformers can provide real-time information to operators that allows them to judge the severity of the situation and time available to correct it before accelerated aging becomes excessive. This can improve optimization of available assets while managing risk. 5
In addition to thermal aging of insulation, transformers can be damaged by the large mechanical forces and heating caused by high currents during short circuits in the adjacent lines and buses. Damage is cumulative over time. The severity of each event varies with fault type, fault severity, and duration. Some transformers naturally lead a hard life with many through faults over time. Others may see very few through faults in the power system that they serve. Monitoring for this important contributor to premature transformer failure can increase visibility of at-risk transformers. This information can be used to help plan and prioritize system improvements, such as shielding overhead lines, improving high-speed protection coverage, or rearranging the system to lower fault current levels. 6
Modern relays include a comprehensive set of functions that can protect transformers from faults and abnormal operating conditions they may be subject to. We nearly always apply adequate protection to detect internal faults. But, do we routinely apply protection for through faults, overexcitation that can occur during system separations, and thermal overload? The advent of microprocessor-based protection has opened the doors to advancements in algorithms that simultaneously improve security, dependability, sensitivity, and speed. Adaptive restraint differential elements use a low slope for improved sensitivity, but switch to a higher slope during external faults to improve security from CT saturation. This provides optimization of both sensitivity and security. Percentage restrained differential elements with cross-harmonic blocking can be faster and more secure than differential elements with harmonic restraint. But, they can suffer reduced dependability when energizing a faulted transformer when an unfaulted phase may block the faulted phase element. On the other hand, percentage restrained elements with harmonic restraint can be more dependable for this case but are slower to operate. Advanced transformer protection relays can run both elements in parallel to get the advantages of each. 7
Detecting partial winding faults is a challenge that is unique to power transformers. When turns are shorted, the high current in the shorted turns is transformed by the autotransformer effect to a lower current at the terminals of the transformer where the differential protection senses current. When a turn-to-turn fault occurs, it may have to evolve to include more turns before being sensed. Modern transformer protection relays include advanced elements to improve sensitivity to partial winding faults and trip the transformer quickly, before the fault needs to evolve, and significantly reduce damage. The photo on the slide shows a turn-to-turn fault in the leads to the tap board of a transformer. The fault was detected and tripped by negative-sequence differential protection. The damage was limited to this punctured insulation paper and some carbon residue. 8
The traditional protection for partial winding faults has been the transformer sudden pressure relay (63SPR). This relay responds to the pressure wave caused by the tremendous amount of energy being dissipated at the location of the fault. Negative-sequence differential elements are not restrained by load flow through the transformer and can detect turn-to-turn faults anywhere in the windings. Restricted earth fault (REF) elements provide sensitivity to turn-to-ground faults near the neutral of the winding. This principle takes advantage of the fact that it can directly see the current flowing in the fault loop. 9
Isolated, low-burden, six-terminal, CT inputs with internal phase shift and zero-sequence compensation are game changers. Dedicated differentially connected CT circuits are no longer required, making it possible to use available CTs for multiple protection and metering purposes. This attribute makes it easy to improve protection to use dual differential protection. It also makes it possible to improve zones of protection when renovating a substation without having to add missing CTs. Auxiliary relays used to be required for contact multiplication to allow the relays to trip all of the breakers to clear the primary zone of protection. Programmable outputs allow a modern relay to direct trip everything to clear it s zone. This improves speed of fault clearing to further reduce damage and eliminates the auxiliary relay as a point of failure to improve reliability. Event reports record detailed information on waveforms and protective element responses to system disturbances, switching, and faults. This ready access to data has allowed us to more fully understand many transient phenomena that can challenge our protection and make improvements to settings and algorithms. We can now get to root cause of all operations and take action to prevent recurrence of misoperations. Event reports can save expense and speed system restoration time when the information is used to eliminate the significant work required to disconnect all of the buswork to isolate the transformer and then test the transformer prior to placing it back in service. 10
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